Syntheses, Structures and Reactivity of Metal Complexes of Trindane, Trindene, Truxene, Decacyclene and Related Ring Systems: Manifestations of Three-Fold Symmetry

The triple condensation of cyclopentanone or indanone to trindane (C15H18) or truxene (C27H18), respectively, provides convenient access to molecular skeletons on which major fragments of the prototypical fullerene C60 can be assembled. In particular, early approaches (both organic and organometallic) towards sumanene, as well as the final successful synthesis, are described. Organometallic derivatives of trindane have been prepared in which Cr(CO)3, Mo(CO)3, [Mn(CO)3]+ or [(C5H5)Fe(CO)2]+ are η6-bonded to the central arene ring. The debromination of hexabromotrindane yields trindene, which forms a tri-anion to which as many as three organometallic fragments, such as Mn(CO)3, W(CO)3Me, or Rh(CO)2, may be attached. Truxene forms complexes whereby three metal fragments can bind either to the peripheral arene rings, or to the five-membered rings, and these can be interconverted via η6 ↔ η5 haptotropic shifts. Truxene also forms a double-decker sandwich with Ag(I) bridges, and decacyclene, C36H18, forms triple-decker sandwiches bearing multiple cyclopentadienyl-nickel or -iron moieties. The organic chemistry of trindane has been investigated, especially with respect to its unexpectedly complex oxidation products, which were only identified unambiguously via X-ray crystallography. The three-fold symmetric trindane framework has also been used as a template upon which a potential artificial receptor has been constructed. Finally, the use of truxene and truxenone derivatives in a wide range of applications is highlighted.


Introduction
The cyclopentadienide fragment is a component of many polycyclic systems, and their metal complexes are very numerous [1].We focus here on the syntheses, structures and reactivity of three-fold symmetric ligands, such as trindane or trindene, and related frameworks whereby the molecular periphery has been augmented via the incorporation of additional benzo rings, as in truxene or decacyclene.

Trindane Complexes of Chromium, Molybdenum, Manganese and Iron
The earliest report of an organometallic derivative of 1 appears to describe the formation of a series of cationic complexes of the type [ 99m Tc(arene)2] +, , where the arene was C6H6, C6Me6, C6Et6, indane, trindane, etc. [10].These materials were injected into rats to study the biodistribution of radioactive technetium to test their viability as myocardial imaging agents.These studies were, out of necessity, carried out on trace quantities of material, and no analytical or spectroscopic data were reported for [ 99m Tc(trindane)2] + [PF6] − , 5. In terms of the first fully characterised derivatives, complexes of the type (η 6 trindane)MLn, where MLn = Cr(CO)3, 6, Mo(CO)3, 7, [Mn(CO)3] + , 8, or Fe(C5H5) + , 9, were each prepared via the reaction of trindane with an appropriate organometallic precursor (Scheme 2) [11].
(η 6 -Trindane)Cr(CO)3, 6, forms yellow crystals, the structure of which appears in Figure 1, and reveals that the tripodal moiety is oriented such that the three carbonyl ligands are staggered with respect to the cyclopentenyl rings.These five-membered rings adopt envelope conformations such that the three methylene "wingtips" are folded in an endo fashion relative to the metal [11].This may be compared to the structures of the previously known molecules [tris(cyclohexene)benzene]MLn, where MLn = Cr(CO)3 or [Mn(CO)3] + , 10, which also crystallise in a staggered tripodal orientation, but their peripheral six-membered rings exhibit conventional half-chair conformations [12].

Organometallic Derivatives of Trindane 2.1. Trindane Complexes of Chromium, Molybdenum, Manganese and Iron
The earliest report of an organometallic derivative of 1 appears to describe the formation of a series of cationic complexes of the type [ 99m Tc(arene) 2 ] +, , where the arene was C 6 H 6 , C 6 Me 6 , C 6 Et 6 , indane, trindane, etc. [10].These materials were injected into rats to study the biodistribution of radioactive technetium to test their viability as myocardial imaging agents.These studies were, out of necessity, carried out on trace quantities of material, and no analytical or spectroscopic data were reported for [ 99m Tc(trindane) 2 ] + [PF 6 ] − , 5. In terms of the first fully characterised derivatives, complexes of the type (η 6 -trindane)ML n , where ML n = Cr(CO) 3 , 6, Mo(CO) 3 , 7, [Mn(CO) 3 ] + , 8, or Fe(C 5 H 5 ) + , 9, were each prepared via the reaction of trindane with an appropriate organometallic precursor (Scheme 2) [11].

Trindane Complexes of Chromium, Molybdenum, Manganese and Iron
The earliest report of an organometallic derivative of 1 appears to describe the formation of a series of cationic complexes of the type [ 99m Tc(arene)2] +, , where the arene was C6H6, C6Me6, C6Et6, indane, trindane, etc. [10].These materials were injected into rats to study the biodistribution of radioactive technetium to test their viability as myocardial imaging agents.These studies were, out of necessity, carried out on trace quantities of material, and no analytical or spectroscopic data were reported for [ 99m Tc(trindane)2] + [PF6] − , 5. In terms of the first fully characterised derivatives, complexes of the type (η 6 trindane)MLn, where MLn = Cr(CO)3, 6, Mo(CO)3, 7, [Mn(CO)3] + , 8, or Fe(C5H5) + , 9, were each prepared via the reaction of trindane with an appropriate organometallic precursor (Scheme 2) [11].
(η 6 -Trindane)Cr(CO)3, 6, forms yellow crystals, the structure of which appears in Figure 1, and reveals that the tripodal moiety is oriented such that the three carbonyl ligands are staggered with respect to the cyclopentenyl rings.These five-membered rings adopt envelope conformations such that the three methylene "wingtips" are folded in an endo fashion relative to the metal [11].This may be compared to the structures of the previously known molecules [tris(cyclohexene)benzene]MLn, where MLn = Cr(CO)3 or [Mn(CO)3] + , 10, which also crystallise in a staggered tripodal orientation, but their peripheral six-membered rings exhibit conventional half-chair conformations [12].(η 6 -Trindane)Cr(CO) 3 , 6, forms yellow crystals, the structure of which appears in Figure 1, and reveals that the tripodal moiety is oriented such that the three carbonyl ligands are staggered with respect to the cyclopentenyl rings.These five-membered rings adopt envelope conformations such that the three methylene "wingtips" are folded in an endo fashion relative to the metal [11].This may be compared to the structures of the previously known molecules [tris(cyclohexene)benzene]ML n , where ML n = Cr(CO) 3 or [Mn(CO) 3 ] + , 10, which also crystallise in a staggered tripodal orientation, but their peripheral six-membered rings exhibit conventional half-chair conformations [12].It was noteworthy that in the mass spectra of the neutral complexes of ( dane)M(CO)3, where M = Cr or Mo, peaks with m/z values corresponding to those o dane)2M2(CO)3] + were observed, leading to the speculation of the formation of bridged systems of the type [(η 6 -trindane)M(µ-CO)3M(η 6 -trindane)] + , 11, entirely gously to known 30-electron systems such as (η 5 -C5Me5)Re(µ-CO)3Re(η 5 -C5Me5) [1 Trindane itself exhibits a very simple 1 H NMR spectrum, i.e., a triplet (12H) benzylic protons and a quintet (6H) for the wingtip methylene groups.The incorp of a π-complexed organometallic fragment renders the faces of the trindane liga equivalent, thus giving rise to four 1 H NMR environments in complexes such as 6 th 9.The benzylic protons (each 6H) are readily distinguished from the resonances wingtip methylenes (each 3H) by their relative intensities, but their assignment to endo positions is less trivial.However, the X-ray data for the chromium complex, vide a rational means of assigning these resonances.Figure 2 illustrates the dihed gles between a pair of benzylic hydrogens and those of the neighbouring wingtip ylene group within a five-membered ring.In all cases, the endo-benzylic hydrogen adjacent exo-wingtip hydrogen make a dihedral angle of approximately 90°, which ical of di-equatorial interactions, which, in accordance with the Karplus relations lating dihedral angles to 3 JH-H values, exhibit a rather small vicinal coupling consta Hz).In contrast, the diaxial interaction between the exo-benzylic hydrogen and it wingtip counterpart leads to a noticeably larger 3 JH-H value (~12 Hz), thus allo straightforward assignment of all the resonances.It was noteworthy that in the mass spectra of the neutral complexes of (η 6 -trindane)M(CO) 3 , where M = Cr or Mo, peaks with m/z values corresponding to those of [(trindane) 2 M 2 (CO) 3 ] + were observed, leading to the speculation of the formation of triple-bridged systems of the type [(η 6 -trindane)M(µ-CO) 3 M(η 6 -trindane)] + , 11, entirely analogously to known 30-electron systems such as (η 5 -C 5 Me 5 )Re(µ-CO) 3 Re(η 5 -C 5 Me 5 ) [13].
Molecules 2023, 28, x FOR PEER REVIEW It was noteworthy that in the mass spectra of the neutral complexes of (η dane)M(CO)3, where M = Cr or Mo, peaks with m/z values corresponding to those of dane)2M2(CO)3] + were observed, leading to the speculation of the formation of bridged systems of the type [(η 6 -trindane)M(µ-CO)3M(η 6 -trindane)] + , 11, entirely gously to known 30-electron systems such as (η 5 -C5Me5)Re(µ-CO)3Re(η 5 -C5Me5) [13 Trindane itself exhibits a very simple 1 H NMR spectrum, i.e., a triplet (12H) f benzylic protons and a quintet (6H) for the wingtip methylene groups.The incorpo of a π-complexed organometallic fragment renders the faces of the trindane liga equivalent, thus giving rise to four 1 H NMR environments in complexes such as 6 th 9.The benzylic protons (each 6H) are readily distinguished from the resonances f wingtip methylenes (each 3H) by their relative intensities, but their assignment to endo positions is less trivial.However, the X-ray data for the chromium complex, 6 vide a rational means of assigning these resonances.Figure 2 illustrates the dihedr gles between a pair of benzylic hydrogens and those of the neighbouring wingtip ylene group within a five-membered ring.In all cases, the endo-benzylic hydrogen a adjacent exo-wingtip hydrogen make a dihedral angle of approximately 90°, which ical of di-equatorial interactions, which, in accordance with the Karplus relationsh lating dihedral angles to 3 JH-H values, exhibit a rather small vicinal coupling constan Hz).In contrast, the diaxial interaction between the exo-benzylic hydrogen and its wingtip counterpart leads to a noticeably larger 3 JH-H value (~12 Hz), thus allow straightforward assignment of all the resonances.
Trindane itself exhibits a very simple 1 H NMR spectrum, i.e., a triplet (12H) for the benzylic protons and a quintet (6H) for the wingtip methylene groups.The incorporation of a π-complexed organometallic fragment renders the faces of the trindane ligand inequivalent, thus giving rise to four 1 H NMR environments in complexes such as 6 through 9.The benzylic protons (each 6H) are readily distinguished from the resonances for the wingtip methylenes (each 3H) by their relative intensities, but their assignment to exo or endo positions is less trivial.However, the X-ray data for the chromium complex, 6, provide a rational means of assigning these resonances.Figure 2 illustrates the dihedral angles between a pair of benzylic hydrogens and those of the neighbouring wingtip methylene group within a five-membered ring.In all cases, the endo-benzylic hydrogen and its adjacent exo-wingtip hydrogen make a dihedral angle of approximately 90 • , which is typical of di-equatorial interactions, which, in accordance with the Karplus relationship relating dihedral angles to 3 J H-H values, exhibit a rather small vicinal coupling constant (4-5 Hz).In contrast, the diaxial interaction between the exo-benzylic hydrogen and its endo-wingtip counterpart leads to a noticeably larger 3

Trindane Complexes of Ruthenium
An arene exchange reaction between [(p-cymene)RuCl2]2 and molten trind attempted and, gratifyingly, [(trindane)RuCl2]2 was produced in a 85% yield; its s appears in Figure 3.In the solid state, the complex adopts the arrangement whe two ruthenium atoms are linked by two bridging chlorines, as in 12a, and each a sesses a terminally bonded chlorine.However, in solution, 1 H-1 H and 1 H- 13 C two sional NMR data clearly reveal the existence of a second species, the triple-bridg isomer [(η 6 -trindane)Ru(µ-Cl)3(η 6 -trindane)]Cl, 12b, formed via the loss of a chlo and.These isomers are in a temperature-dependent equilibrium such that the rati to 12b is 30:70 at −50 °C in CD2Cl2, but at room temperature in nitromethane C (which would surely favour the ionised species), this ratio changes dramatically [14].We note parenthetically that hexaethylbenzene (HEB) may be thought of as restricted" trindane in which the wingtip methylene groups of the cyclopentenyl r now untethered, thus leaving them free to rotate.It was shown that the analogou nium complex of HEB, i.e., [(HEB)RuCl2]2, also exists as an interconverting dou triple-bridged species, but in this case both the double-bridged neutral species, 13

Trindane Complexes of Ruthenium
An arene exchange reaction between [(p-cymene)RuCl 2 ] 2 and molten trindane was attempted and, gratifyingly, [(trindane)RuCl 2 ] 2 was produced in a 85% yield; its structure appears in Figure 3.In the solid state, the complex adopts the arrangement whereby the two ruthenium atoms are linked by two bridging chlorines, as in 12a, and each also possesses a terminally bonded chlorine.However, in solution, 1 H-1 H and 1 H- 13 C two-dimensional NMR data clearly reveal the existence of a second species, the triple-bridged ionic isomer [(η 6 -trindane)Ru(µ-Cl) 3 (η 6 -trindane)]Cl, 12b, formed via the loss of a chloride ligand.These isomers are in a temperature-dependent equilibrium such that the ratio of 12a to 12b is 30:70 at −50

Trindane Complexes of Ruthenium
An arene exchange reaction between [(p-cymene)RuCl2]2 and molten trindane was attempted and, gratifyingly, [(trindane)RuCl2]2 was produced in a 85% yield; its structure appears in Figure 3.In the solid state, the complex adopts the arrangement whereby the two ruthenium atoms are linked by two bridging chlorines, as in 12a, and each also possesses a terminally bonded chlorine.However, in solution, 1 H-1 H and 1 H-13 C two-dimensional NMR data clearly reveal the existence of a second species, the triple-bridged ionic isomer [(η 6 -trindane)Ru(µ-Cl)3(η 6 -trindane)]Cl, 12b, formed via the loss of a chloride ligand.These isomers are in a temperature-dependent equilibrium such that the ratio of 12a to 12b is 30:70 at −50 °C in CD2Cl2, but at room temperature in nitromethane CD3NO2 (which would surely favour the ionised species), this ratio changes dramatically to 1:10 [14].We note parenthetically that hexaethylbenzene (HEB) may be thought of as an "unrestricted" trindane in which the wingtip methylene groups of the cyclopentenyl rings are now untethered, thus leaving them free to rotate.It was shown that the analogous ruthenium complex of HEB, i.e., [(HEB)RuCl2]2, also exists as an interconverting double-and triple-bridged species, but in this case both the double-bridged neutral species, 13a, analogous to 12a, and also the D3h-symmetric cation, 13b, analogous to 12b, together with its [C5(CO2Me)5] − counter-anion, have been characterised via X-ray crystallography (Figure 4) [15][16][17].We note parenthetically that hexaethylbenzene (HEB) may be thought of as an "unrestricted" trindane in which the wingtip methylene groups of the cyclopentenyl rings are now untethered, thus leaving them free to rotate.It was shown that the analogous ruthenium complex of HEB, i.e., [(HEB)RuCl 2 ] 2 , also exists as an interconverting doubleand triple-bridged species, but in this case both the double-bridged neutral species, 13a, analogous to 12a, and also the D 3h -symmetric cation, 13b, analogous to 12b, together with its [C 5 (CO 2 Me) 5 ] − counter-anion, have been characterised via X-ray crystallography (Figure 4) [15][16][17].When [(trindane)RuCl2]2, 12, was allowed to react with excess trindane in the presence of AgBF4, the sandwich compound [(η 6 -trindane)2Ru] 2+ 2[BF4] − , 14, was isolated and fully characterised spectroscopically [14].This molecule is, of course, the ruthenium analogue of the [ 99m Tc(trindane)2] + [PF6] − complex, 5, claimed, but never unambiguously identified, in the medicinal study of potential radio-labelled myocardial imaging agents noted above [10].Finally, it was found that treatment of 12 with trimethyl phosphite delivered the anticipated monomeric species (η 6 -trindane)RuCl2[P(OMe)3], 15, the structure of which is shown in Figure 5.

Organometallic Derivatives of Trindene
As noted above, Katz and Ślusarek prepared the trindene trianion, 4, which was then allowed to react with ferrous chloride to form bis(trindene)diiron, 16, as a red-brown material, along with traces of a tri-iron complex [8].NMR data suggested that that the hydrocarbon rings in 16 are disposed as anti, as depicted in Scheme 3. Almost four decades later, the first triple-metallocene derivative of trindene was prepared via the exchange reaction of the potassium salt of 4 with [Fe(C5H5)(η 6 -fluorene)]PF6 to yield the syn,syn,syn-and syn,syn,anti-isomers of (η 5 :η 5 :η 5 -trindenyl)[Fe(C5H5)]3, 17a and 17b, respectively, in a 1:3 ratio.The latter molecule was characterised via X-ray

Organometallic Derivatives of Trindene
As noted above, Katz and Ślusarek prepared the trindene trianion, 4, which w allowed to react with ferrous chloride to form bis(trindene)diiron, 16, as a red-bro terial, along with traces of a tri-iron complex [8].NMR data suggested that that the carbon rings in 16 are disposed as anti, as depicted in Scheme 3. Almost four decades later, the first triple-metallocene derivative of trindene w pared via the exchange reaction of the potassium salt of 4 with [Fe(C5H5)(η 6 -fluore to yield the syn,syn,syn-and syn,syn,anti-isomers of (η 5 :η 5 :η 5  Almost four decades later, the first triple-metallocene derivative of trindene was prepared via the exchange reaction of the potassium salt of 4 with [Fe(C 5 H 5 )(η 6 -fluorene)]PF 6 to yield the syn,syn,synand syn,syn,anti-isomers of (η 5 :η 5 :η 5 -trindenyl)[Fe(C 5 H 5 )] 3 , 17a and 17b, respectively, in a 1:3 ratio.The latter molecule was characterised via X-ray crystallography (Figure 6), which clearly showed a notable deviation of two five-membered rings in the trindene skeleton from planarity caused by the need to relieve repulsive non-bonding interactions between the syn ferrocenyl groups [18].
Molecules 2023, 28, x FOR PEER REVIEW 6 of 26 crystallography (Figure 6), which clearly showed a notable deviation of two five-membered rings in the trindene skeleton from planarity caused by the need to relieve repulsive non-bonding interactions between the syn ferrocenyl groups [18].The organometallic chemistry of trindene was considerably extended by Lynch and Rheingold, who successfully characterised a series of trimetallic derivatives bearing manganese-or rhenium-tricarbonyl fragments.The treatment of dihydro-1H-trindene, 3, with KH and either Mn(CO)3(py)2Br (py = pyridine) or [Re(CO)3(THF)Br]2 delivered (η 5 :η 5 :η 5trindenyl)[M(CO)3]3, where M = Mn or Re, 18 or 19, respectively (Scheme 4).As was found in 17b, the X-ray crystal structure of 19 (Figure 7) revealed that the metals were situated in a syn,syn,anti-fashion such that the five-membered rings bearing the cis rhenium units were bent away from each other and out of the central plane by ~10°, and that the Re(CO)3 groups exhibited maximal staggering of their carbonyl ligands [19].It was also established that when only two metal carbonyl moieties were introduced, they adopted a trans geometry, 20, thus facilitating the introduction of a third different substituent, thereby forcing the cis disposition of the heterometals, as in the (trindenyl)Re2Rh(CO)8 complex, 21.The organometallic chemistry of trindene was considerably extended by Lynch and Rheingold, who successfully characterised a series of trimetallic derivatives bearing manganese-or rhenium-tricarbonyl fragments.The treatment of dihydro-1H-trindene, 3, with KH and either Mn(CO) 3 (py) 2 Br (py = pyridine) or [Re(CO) 3 (THF)Br] 2 delivered (η 5 :η 5 :η 5trindenyl)[M(CO) 3 ] 3 , where M = Mn or Re, 18 or 19, respectively (Scheme 4).As was found in 17b, the X-ray crystal structure of 19 (Figure 7) revealed that the metals were situated in a syn,syn,anti-fashion such that the five-membered rings bearing the cis rhenium units were bent away from each other and out of the central plane by ~10 • , and that the Re(CO) 3 groups exhibited maximal staggering of their carbonyl ligands [19].It was also established that when only two metal carbonyl moieties were introduced, they adopted a trans geometry, 20, thus facilitating the introduction of a third different substituent, thereby forcing the cis disposition of the heterometals, as in the (trindenyl)Re 2 Rh(CO) 8 complex, 21.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 26 crystallography (Figure 6), which clearly showed a notable deviation of two five-membered rings in the trindene skeleton from planarity caused by the need to relieve repulsive non-bonding interactions between the syn ferrocenyl groups [18].The organometallic chemistry of trindene was considerably extended by Lynch and Rheingold, who successfully characterised a series of trimetallic derivatives bearing manganese-or rhenium-tricarbonyl fragments.The treatment of dihydro-1H-trindene, 3, with KH and either Mn(CO)3(py)2Br (py = pyridine) or [Re(CO)3(THF)Br]2 delivered (η 5 :η 5 :η 5trindenyl)[M(CO)3]3, where M = Mn or Re, 18 or 19, respectively (Scheme 4).As was found in 17b, the X-ray crystal structure of 19 (Figure 7) revealed that the metals were situated in a syn,syn,anti-fashion such that the five-membered rings bearing the cis rhenium units were bent away from each other and out of the central plane by ~10°, and that the Re(CO)3 groups exhibited maximal staggering of their carbonyl ligands [19].It was also established that when only two metal carbonyl moieties were introduced, they adopted a trans geometry, 20, thus facilitating the introduction of a third different substituent, thereby forcing the cis disposition of the heterometals, as in the (trindenyl)Re2Rh(CO)8 complex, 21.   crystallography (Figure 6), which clearly showed a notable deviation of two five-membered rings in the trindene skeleton from planarity caused by the need to relieve repulsive non-bonding interactions between the syn ferrocenyl groups [18].The organometallic chemistry of trindene was considerably extended by Lynch and Rheingold, who successfully characterised a series of trimetallic derivatives bearing manganese-or rhenium-tricarbonyl fragments.The treatment of dihydro-1H-trindene, 3, with KH and either Mn(CO)3(py)2Br (py = pyridine) or [Re(CO)3(THF)Br]2 delivered (η 5 :η 5 :η 5trindenyl)[M(CO)3]3, where M = Mn or Re, 18 or 19, respectively (Scheme 4).As was found in 17b, the X-ray crystal structure of 19 (Figure 7) revealed that the metals were situated in a syn,syn,anti-fashion such that the five-membered rings bearing the cis rhenium units were bent away from each other and out of the central plane by ~10°, and that the Re(CO)3 groups exhibited maximal staggering of their carbonyl ligands [19].It was also established that when only two metal carbonyl moieties were introduced, they adopted a trans geometry, 20, thus facilitating the introduction of a third different substituent, thereby forcing the cis disposition of the heterometals, as in the (trindenyl)Re2Rh(CO)8 complex, 21.In these systems, two of the metal fragments are necessarily positioned proximate to each other on the same face, and this feature has been exploited in a series of trimolybdenum or tri-tungsten complexes.When the trindene trianion, 4, was treated with Mo(CO) 6 and then with methyl iodide, the major product was syn,syn,anti-(η 5 :η 5 :η 5trindenyl)[Mo(CO) 3 Me] 3 , 22, along with a lower quantity of a material, 23, in which the two adjacent molybdenum atoms had lost their methyl substituents and formed a metal-metal bond.The tungsten congener behaved similarly, and the structure of the corresponding benzyl analogue, 24, possessing a tungsten-tungsten linkage was verified via X-ray crystallography, as shown in Figure 8 [20].

FOR PEER REVIEW 7 of 26
In these systems, two of the metal fragments are necessarily positioned proximate to each other on the same face, and this feature has been exploited in a series of tri-molybdenum or tri-tungsten complexes.When the trindene trianion, 4, was treated with Mo(CO)6 and then with methyl iodide, the major product was syn,syn,anti-(η 5 :η 5 :η 5 -trindenyl)[Mo(CO)3Me]3, 22, along with a lower quantity of a material, 23, in which the two adjacent molybdenum atoms had lost their methyl substituents and formed a metal-metal bond.The tungsten congener behaved similarly, and the structure of the corresponding benzyl analogue, 24, possessing a tungsten-tungsten linkage was verified via X-ray crystallography, as shown in Figure 8 [20].The electrochemical behaviour of a number of these trimetallic derivatives of trindene was studied via cyclic voltammetry (CV).It was found that the complexes (trindenyl)(MLn)3, where MLn = Mn(CO)3, 18, or Rh(1,5-cyclooctadiene), 25, showed three welldefined, reversible one-electron oxidation reactions.In the rhodium case, the observed formal potentials are −0.33,−0.16 and +0.43 V, for the couples 25-25 + , 25 + -25 2+ and 25 2+ -25 3+ , respectively, and this abnormally large separation in potentials has been interpreted in terms of a Class III (totally delocalised) system [21].There is also a report of the trirhodium complex, (η 5 :η 5 :η 5 -trindene)[Rh(1,5-COD)]2Rh(CO)2, 26, whose structure is shown in Figure 9 [22].Since the two Rh(COD) fragments occupy anti positions, one can assume that they were coordinated initially, and the Rh(CO)2 unit was added subsequently.The CV scan of the triferrocenyl system 17b proceeds in two well-defined and reversible (chemically and electrochemically) one-electron steps with values of 0.23 and 0.61 V, for the first and second oxidations; however, this is followed by a chemically irreversible process (0.94 V) showing that the trication precipitates on the Au electrode [18].Finally, we note that the charge transfer properties of multi-ferrocenyl dihydro-1H-trindenes, 27, The electrochemical behaviour of a number of these trimetallic derivatives of trindene was studied via cyclic voltammetry (CV).It was found that the complexes (trindenyl)(ML n ) 3 , where ML n = Mn(CO) 3 , 18, or Rh(1,5-cyclooctadiene), 25, showed three well-defined, reversible one-electron oxidation reactions.In the rhodium case, the observed formal potentials are −0.33,−0.16 and +0.43 V, for the couples 25-25 + , 25 + -25 2+ and 25 2+ -25 3+ , respectively, and this abnormally large separation in potentials has been interpreted in terms of a Class III (totally delocalised) system [21].There is also a report of the tri-rhodium complex, (η 5 :η 5 :η 5 -trindene)[Rh(1,5-COD)] 2 Rh(CO) 2 , 26, whose structure is shown in Figure 9 [22].Since the two Rh(COD) fragments occupy anti positions, one can assume that they were coordinated initially, and the Rh(CO) 2 unit was added subsequently.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 26 In these systems, two of the metal fragments are necessarily positioned proximate to each other on the same face, and this feature has been exploited in a series of tri-molybdenum or tri-tungsten complexes.When the trindene trianion, 4, was treated with Mo(CO)6 and then with methyl iodide, the major product was syn,syn,anti-(η 5 :η 5 :η 5 -trindenyl)[Mo(CO)3Me]3, 22, along with a lower quantity of a material, 23, in which the two adjacent molybdenum atoms had lost their methyl substituents and formed a metal-metal bond.The tungsten congener behaved similarly, and the structure of the corresponding benzyl analogue, 24, possessing a tungsten-tungsten linkage was verified via X-ray crystallography, as shown in Figure 8 [20].The electrochemical behaviour of a number of these trimetallic derivatives of trindene was studied via cyclic voltammetry (CV).It was found that the complexes (trindenyl)(MLn)3, where MLn = Mn(CO)3, 18, or Rh(1,5-cyclooctadiene), 25, showed three welldefined, reversible one-electron oxidation reactions.In the rhodium case, the observed formal potentials are −0.33,−0.16 and +0.43 V, for the couples 25-25 + , 25 + -25 2+ and 25 2+ -25 3+ , respectively, and this abnormally large separation in potentials has been interpreted in terms of a Class III (totally delocalised) system [21].There is also a report of the trirhodium complex, (η 5 :η 5 :η 5 -trindene)[Rh(1,5-COD)]2Rh(CO)2, 26, whose structure is shown in Figure 9 [22].Since the two Rh(COD) fragments occupy anti positions, one can assume that they were coordinated initially, and the Rh(CO)2 unit was added subsequently.The CV scan of the triferrocenyl system 17b proceeds in two well-defined and reversible (chemically and electrochemically) one-electron steps with values of 0.23 and 0.61 V, for the first and second oxidations; however, this is followed by a chemically irreversible process (0.94 V) showing that the trication precipitates on the Au electrode [18].Finally, we note that the charge transfer properties of multi-ferrocenyl dihydro-1H-trindenes, 27, have been investigated [23].The CV scan of the triferrocenyl system 17b proceeds in two well-defined and reversible (chemically and electrochemically) one-electron steps with values of 0.23 and 0.61 V, for the first and second oxidations; however, this is followed by a chemically irreversible process (0.94 V) showing that the trication precipitates on the Au electrode [18].Finally, we note that the charge transfer properties of multi-ferrocenyl dihydro-1H-trindenes, 27, have been investigated [23].

Metal Complexes of Truxene
Using the analogy of the self-condensation of cyclopentanone to form trindane, the analogous reaction of indanone yields C 3h -symmetric truxene, C 27 H 18 , 28.It was first obtained accidentally by Hausmann in 1889 [24] during the synthesis of indanone from 3-phenylpropanoic acid, but the mechanism involving the intermediate tetracyclic ketone shown in Scheme 5 was only elucidated many years later [25].

Metal Complexes of Decacyclene
Decacyclene, C36H18, 32, was first obtained by Dziewonski in Krakow, Poland upon the dehydrogenation of acenaphthene with elemental sulphur at 205-295 ° differs from truxene in terms of the incorporation of additional peripheral benz thus apparently recovering the D3h symmetry of its trindenyl central core.How X-ray structure of decacyclene revealed it to be a shallow molecular propeller of metry as the result of non-bonded repulsions between hydrogens of the peripher thalene groups.This feature is clearly illustrated in the side view of 32 depicted i 11.Interestingly, crystals of decacyclene also have a helical morphology, which pronounced when grown from solutions in organic solvents [29].

Metal Complexes of Decacyclene
Decacyclene, C 36 H 18 , 32, was first obtained by Dziewonski in Krakow, Poland, in 1903 upon the dehydrogenation of acenaphthene with elemental sulphur at 205-295 • C [28].It differs from truxene in terms of the incorporation of additional peripheral benzo rings, thus apparently recovering the D 3h symmetry of its trindenyl central core.However, the X-ray structure of decacyclene revealed it to be a shallow molecular propeller of D 3 symmetry as the result of non-bonded repulsions between hydrogens of the peripheral naphthalene groups.This feature is clearly illustrated in the side view of 32 depicted in Figure 11.Interestingly, crystals of decacyclene also have a helical morphology, which is most pronounced when grown from solutions in organic solvents [29].

The Pyrolytic Approach
Although C60, with its "soccer-ball" icosahedral (Ih) symmetry, is preparable via the evaporation of graphite under appropriate conditions, and is now commercially available, attempts to develop logical stepwise synthesis continue to be explored.Two major components of the C60 skeleton are corannulene C20H10, 38, which possesses a central pentagon surrounded by five six-membered rings, and sumanene C21H12, 39, which has alternating five-and six-membered rings around a central hexagon; both are illustrated in Figure 14.
Corannulene is now available in kilogram quantities, thanks to the pioneering contributions of Larry Scott (in Nevada) and Jay Siegel (in Zurich), and this work has been comprehensively reviewed [33][34][35][36].However, in 1997, when the organometallic chemistry of trindane was still in its early stages, no route to sumanene had been reported, and success was only achieved some years later (see below).

The Pyrolytic Approach
Although C60, with its "soccer-ball" icosahedral (Ih) symmetry, is preparable via the evaporation of graphite under appropriate conditions, and is now commercially available, attempts to develop logical stepwise synthesis continue to be explored.Two major components of the C60 skeleton are corannulene C20H10, 38, which possesses a central pentagon surrounded by five six-membered rings, and sumanene C21H12, 39, which has alternating five-and six-membered rings around a central hexagon; both are illustrated in Figure 14.
Corannulene is now available in kilogram quantities, thanks to the pioneering contributions of Larry Scott (in Nevada) and Jay Siegel (in Zurich), and this work has been comprehensively reviewed [33-36].However, in 1997, when the organometallic chemistry of trindane was still in its early stages, no route to sumanene had been reported, and success was only achieved some years later (see below).

Towards Sumanene 6.1. The Pyrolytic Approach
Although C 60 , with its "soccer-ball" icosahedral (I h ) symmetry, is preparable via the evaporation of graphite under appropriate conditions, and is now commercially available, attempts to develop logical stepwise synthesis continue to be explored.Two major components of the C 60 skeleton are corannulene C 20 H 10 , 38, which possesses a central pentagon surrounded by five six-membered rings, and sumanene C 21 H 12 , 39, which has alternating five-and six-membered rings around a central hexagon; both are illustrated in Figure 14.Corannulene is now available in kilogram quantities, thanks to the pioneering contributions of Larry Scott (in Nevada) and Jay Siegel (in Zurich), and this work has been comprehensively reviewed [33][34][35][36].However, in 1997, when the organometallic chemistry of trindane was still in its early stages, no route to sumanene had been reported, and success was only achieved some years later (see below).The bowl-shaped C3v-symmetric molecule sumanene, with its central six-membered ring surrounded by alternating five-and six-membered rings, was first postulated by Mehta in 1993; its name is derived from the Sanskrit word suman, which translates into "flower", whereby the ring edges are considered to resemble petals.The earliest synthetic approach (Scheme 9) involved the palladium-mediated thermolysis of C3h-symmetric 1,5,9-trimethyltriphenylene, 40, in an attempt to bring about multiple dehydrogenations and subsequent ring closures; however, only the mono-bridged product, 41, was formed.Under even more forceful conditions, the flash vacuum pyrolysis (FVP) of 1,5,9-tri(bromomethyl)triphenylene, 42, led to the double-bridged species 43, which was characterised via X-ray crystallography [37].These experimental observations validate the computational results of Priyakumar and Sastry, which revealed that all these cyclopentenation steps lead to an increase in strain, with the third endothermic step leading to an increase of more than 50 kcal/mol, making the final ring closure almost impossible.In contrast, starting from trindane (thereby incorporating the five-membered rings early on in the procedure) and then generating the six-membered peripheral rings subsequently is a thermodynamically favoured process [38,39].

The Proposed Organometallic Approach
Retrosynthetic analysis (Scheme 10) suggested a strategy starting from the cationic species [(trindane)Mn(CO)3] + , 8, whereby multiple deprotonation at the benzylic positions with subsequent attack by an electrophile could lead to a system, 44, bearing six bromomethyl substitutuents.The attachment of the organometallic fragment to the central arene ring would be expected to block that face and thereby favour exo attack by incoming electrophiles.Ring closure to form the bromosulfide 45, oxidation to the corresponding sulfone, followed by a Ramberg-Bäcklund rearrangement (a standard technique for the formation of small rings [40]) of the triple halosulfone, 46, should result in the elimination of The bowl-shaped C 3v -symmetric molecule sumanene, with its central six-membered ring surrounded by alternating five-and six-membered rings, was first postulated by Mehta in 1993; its name is derived from the Sanskrit word suman, which translates into "flower", whereby the ring edges are considered to resemble petals.The earliest synthetic approach (Scheme 9) involved the palladium-mediated thermolysis of C 3hsymmetric 1,5,9-trimethyltriphenylene, 40, in an attempt to bring about multiple dehydrogenations and subsequent ring closures; however, only the mono-bridged product, 41, was formed.Under even more forceful conditions, the flash vacuum pyrolysis (FVP) of 1,5,9-tri(bromomethyl)triphenylene, 42, led to the double-bridged species 43, which was characterised via X-ray crystallography [37].The bowl-shaped C3v-symmetric molecule sumanene, with its central six-membered ring surrounded by alternating five-and six-membered rings, was first postulated by Mehta in 1993; its name is derived from the Sanskrit word suman, which translates into "flower", whereby the ring edges are considered to resemble petals.The earliest synthetic approach (Scheme 9) involved the palladium-mediated thermolysis of C3h-symmetric 1,5,9-trimethyltriphenylene, 40, in an attempt to bring about multiple dehydrogenations and subsequent ring closures; however, only the mono-bridged product, 41, was formed.Under even more forceful conditions, the flash vacuum pyrolysis (FVP) of 1,5,9-tri(bromomethyl)triphenylene, 42, led to the double-bridged species 43, which was characterised via X-ray crystallography [37].These experimental observations validate the computational results of Priyakumar and Sastry, which revealed that all these cyclopentenation steps lead to an increase in strain, with the third endothermic step leading to an increase of more than 50 kcal/mol, making the final ring closure almost impossible.In contrast, starting from trindane (thereby incorporating the five-membered rings early on in the procedure) and then generating the six-membered peripheral rings subsequently is a thermodynamically favoured process [38,39].

The Proposed Organometallic Approach
Retrosynthetic analysis (Scheme 10) suggested a strategy starting from the cationic species [(trindane)Mn(CO)3] + , 8, whereby multiple deprotonation at the benzylic positions with subsequent attack by an electrophile could lead to a system, 44, bearing six bromomethyl substitutuents.The attachment of the organometallic fragment to the central arene ring would be expected to block that face and thereby favour exo attack by incoming electrophiles.Ring closure to form the bromosulfide 45, oxidation to the corresponding sulfone, followed by a Ramberg-Bäcklund rearrangement (a standard technique for the formation of small rings [40]) of the triple halosulfone, 46, should result in the elimination of These experimental observations validate the computational results of Priyakumar and Sastry, which revealed that all these cyclopentenation steps lead to an increase in strain, with the third endothermic step leading to an increase of more than 50 kcal/mol, making the final ring closure almost impossible.In contrast, starting from trindane (thereby incorporating the five-membered rings early on in the procedure) and then generating the six-membered peripheral rings subsequently is a thermodynamically favoured process [38,39].

The Proposed Organometallic Approach
Retrosynthetic analysis (Scheme 10) suggested a strategy starting from the cationic species [(trindane)Mn(CO) 3 ] + , 8, whereby multiple deprotonation at the benzylic positions with subsequent attack by an electrophile could lead to a system, 44, bearing six bromomethyl substitutuents.The attachment of the organometallic fragment to the central arene ring would be expected to block that face and thereby favour exo attack by incoming electrophiles.Ring closure to form the bromosulfide 45, oxidation to the corresponding sulfone, followed by a Ramberg-Bäcklund rearrangement (a standard technique for the formation of small rings [40]) of the triple halosulfone, 46, should result in the elimination of HBr and SO 2 and the direct formation of the new alkene moieties, leaving only a final dehydrogenation step.
HBr and SO2 and the direct formation of the new alkene moieties, leaving only a final dehydrogenation step.This approach was predicated on earlier work by Astruc [41] on the multiple successive deprotonations at benzyl positions in [(η 6 -C6Me6)Fe(C5H5)] + , 47, to initially form (η 5 -Me5C6=CH2)Fe(C5H5), 48, which reacted with a range of electrophiles.This concept was further developed by Eyman [42] using the isoelectronic manganese system [(η 6 -C6Me6)Mn(CO)3] + , 49, as exemplified in Scheme 11.Moreover, it was found that substitution of a carbonyl ligand by a phosphine, as in [(η 6 -C6Me6)Mn(CO)2PMe3] + , increases the electron density on manganese and enhances the nucleophilicity of the exocyclic methylene group [43].To test this approach, [(trindane)Fe(C5H5)] + , 9, was deprotonated by t-BuOK in the presence of excess methyl iodide and yielded a product mixture in which up to twelve benzylic positions had been substituted, as shown in Scheme 12.The analogous reaction with allyl bromide also delivered the dodeca-substituted material.This approach was predicated on earlier work by Astruc [41] on the multiple successive deprotonations at benzyl positions in [(η 6 -C 6 Me 6 )Fe(C 5 H 5 )] + , 47, to initially form (η 5 -Me 5 C 6 =CH 2 )Fe(C 5 H 5 ), 48, which reacted with a range of electrophiles.This concept was further developed by Eyman [42] using the isoelectronic manganese system [(η 6 -C 6 Me 6 )Mn(CO) 3 ] + , 49, as exemplified in Scheme 11.Moreover, it was found that substitution of a carbonyl ligand by a phosphine, as in [(η 6 -C 6 Me 6 )Mn(CO) 2 PMe 3 ] + , increases the electron density on manganese and enhances the nucleophilicity of the exocyclic methylene group [43].
HBr and SO2 and the direct formation of the new alkene moieties, leaving only a final dehydrogenation step.This approach was predicated on earlier work by Astruc [41] on the multiple successive deprotonations at benzyl positions in [(η 6 -C6Me6)Fe(C5H5)] + , 47, to initially form (η 5 -Me5C6=CH2)Fe(C5H5), 48, which reacted with a range of electrophiles.This concept was further developed by Eyman [42] using the isoelectronic manganese system [(η 6 -C6Me6)Mn(CO)3] + , 49, as exemplified in Scheme 11.Moreover, it was found that substitution of a carbonyl ligand by a phosphine, as in [(η 6 -C6Me6)Mn(CO)2PMe3] + , increases the electron density on manganese and enhances the nucleophilicity of the exocyclic methylene group [43].To test this approach, [(trindane)Fe(C5H5)] + , 9, was deprotonated by t-BuOK in the presence of excess methyl iodide and yielded a product mixture in which up to twelve benzylic positions had been substituted, as shown in Scheme 12.The analogous reaction with allyl bromide also delivered the dodeca-substituted material.To test this approach, [(trindane)Fe(C 5 H 5 )] + , 9, was deprotonated by t-BuOK in the presence of excess methyl iodide and yielded a product mixture in which up to twelve benzylic positions had been substituted, as shown in Scheme 12.The analogous reaction with allyl bromide also delivered the dodeca-substituted material.In an attempt to hinder the approach by an electrophile to the endo face of the polycyclic ligand, the cyclopentadienyl-iron fragment was replaced by the more voluminous tricarbonylmanganese moiety.However, when [(trindane)Mn(CO)3] + , 8, was deprotonated by t-BuOK in the presence of excess allyl bromide, a red crystalline product, obtained in 17% yield after chromatographic separation, was identified, surprisingly, as (trindane)Mn(CO)2Br, 50 (Scheme 13).This material is also readily prepared, in a 93% yield, via the reaction of 8 with trimethylamine N-oxide (to bring about the loss of a carbonyl ligand) and a subsequent reaction with n-Bu4N + Br − .Likewise, the reaction of 8 with t-BuOK and methyl iodide produced (trindane)Mn(CO)2I, 51 (15%).The X-ray crystal structures of 50 and 51, shown in Figure 15, reveal that, as in (trindane)Cr(CO)3, 6, the tripodal moiety is oriented such that its ligands are staggered with respect to the cyclopentenyl rings, which adopt envelope conformations with endo-folded wingtip methylene groups [44].This contrasts the behaviour of [(C6Me6)Mn(CO)3] + , for which the displacement of a carbonyl ligand to form (C6Me6)Mn(CO)2X, where X = Cl, Br or I, requires either photolysis or a reaction with Me3NO in the presence of NaX [45].It is also noteworthy that the treatment of (C6Me6)Mn(CO)2Cl with t-BuLi yields (C6Me6)Mn(CO)2H, 52, apparently via the elimination of isobutene from the presumed tert-butyl intermediate (Scheme 14).However, this hydride, which is sufficiently stable for its structure to be corroborated via X-ray In an attempt to hinder the approach by an electrophile to the endo face of the polycyclic ligand, the cyclopentadienyl-iron fragment was replaced by the more voluminous tricarbonylmanganese moiety.However, when [(trindane)Mn(CO) 3 ] + , 8, was deprotonated by t-BuOK in the presence of excess allyl bromide, a red crystalline product, obtained in 17% yield after chromatographic separation, was identified, surprisingly, as (trindane)Mn(CO) 2 Br, 50 (Scheme 13).This material is also readily prepared, in a 93% yield, via the reaction of 8 with trimethylamine N-oxide (to bring about the loss of a carbonyl ligand) and a subsequent reaction with n-Bu 4 N + Br − .Likewise, the reaction of 8 with t-BuOK and methyl iodide produced (trindane)Mn(CO) 2 I, 51 (15%).The X-ray crystal structures of 50 and 51, shown in Figure 15, reveal that, as in (trindane)Cr(CO) 3 , 6, the tripodal moiety is oriented such that its ligands are staggered with respect to the cyclopentenyl rings, which adopt envelope conformations with endo-folded wingtip methylene groups [44].In an attempt to hinder the approach by an electrophile to the endo face of t cyclic ligand, the cyclopentadienyl-iron fragment was replaced by the more volu tricarbonylmanganese moiety.However, when [(trindane)Mn(CO)3] + , 8, was d nated by t-BuOK in the presence of excess allyl bromide, a red crystalline prod tained in 17% yield after chromatographic separation, was identified, surprisi (trindane)Mn(CO)2Br, 50 (Scheme 13).This material is also readily prepared, i yield, via the reaction of 8 with trimethylamine N-oxide (to bring about the loss bonyl ligand) and a subsequent reaction with n-Bu4N + Br − .Likewise, the reaction o t-BuOK and methyl iodide produced (trindane)Mn(CO)2I, 51 (15%).The X-ray structures of 50 and 51, shown in Figure 15, reveal that, as in (trindane)Cr(CO tripodal moiety is oriented such that its ligands are staggered with respect to th pentenyl rings, which adopt envelope conformations with endo-folded wingtip me groups [44].This contrasts the behaviour of [(C6Me6)Mn(CO)3] + , for which the displacem carbonyl ligand to form (C6Me6)Mn(CO)2X, where X = Cl, Br or I, requires either ph or a reaction with Me3NO in the presence of NaX [45].It is also noteworthy that t ment of (C6Me6)Mn(CO)2Cl with t-BuLi yields (C6Me6)Mn(CO)2H, 52, apparently elimination of isobutene from the presumed tert-butyl intermediate (Scheme 14 In an attempt to hinder the approach by an electrophile to the endo face of the polycyclic ligand, the cyclopentadienyl-iron fragment was replaced by the more voluminous tricarbonylmanganese moiety.However, when [(trindane)Mn(CO)3] + , 8, was deprotonated by t-BuOK in the presence of excess allyl bromide, a red crystalline product, obtained in 17% yield after chromatographic separation, was identified, surprisingly, as (trindane)Mn(CO)2Br, 50 (Scheme 13).This material is also readily prepared, in a 93% yield, via the reaction of 8 with trimethylamine N-oxide (to bring about the loss of a carbonyl ligand) and a subsequent reaction with n-Bu4N + Br − .Likewise, the reaction of 8 with t-BuOK and methyl iodide produced (trindane)Mn(CO)2I, 51 (15%).The X-ray crystal structures of 50 and 51, shown in Figure 15, reveal that, as in (trindane)Cr(CO)3, 6, the tripodal moiety is oriented such that its ligands are staggered with respect to the cyclopentenyl rings, which adopt envelope conformations with endo-folded wingtip methylene groups [44].This contrasts the behaviour of [(C6Me6)Mn(CO)3] + , for which the displacement of a carbonyl ligand to form (C6Me6)Mn(CO)2X, where X = Cl, Br or I, requires either photolysis or a reaction with Me3NO in the presence of NaX [45].It is also noteworthy that the treatment of (C6Me6)Mn(CO)2Cl with t-BuLi yields (C6Me6)Mn(CO)2H, 52, apparently via the elimination of isobutene from the presumed tert-butyl intermediate (Scheme 14).However, this hydride, which is sufficiently stable for its structure to be corroborated via X-ray This contrasts the behaviour of [(C 6 Me 6 )Mn(CO) 3 ] + , for which the displacement of a carbonyl ligand to form (C 6 Me 6 )Mn(CO) 2 X, where X = Cl, Br or I, requires either photolysis or a reaction with Me 3 NO in the presence of NaX [45].It is also noteworthy that the treatment of (C 6 Me 6 )Mn(CO) 2 Cl with t-BuLi yields (C 6 Me 6 )Mn(CO) 2 H, 52, apparently via the elimination of isobutene from the presumed tert-butyl intermediate (Scheme 14).However, this hydride, which is sufficiently stable for its structure to be corroborated via X-ray crystallography [46], is more conveniently prepared via the reaction of (C 6 Me 6 )Mn(CO) 2 I with (n-Bu) 4 N + [BH 4 ] − ; furthermore, 52 reacts readily with CCl 4 or CHCl 3 , but not with CH 2 Cl 2 to form (C 6 Me 6 )Mn(CO) 2 Cl [47].
In light of these data, one can postulate a mechanism for the formation o dane)Mn(CO)2I upon the treatment of 51 with t-BuOK in the presence of methyl Since [(C6Me6)Mn(CO)3] + is known to react with methanol to form the rather unstab (C6Me6)Mn(CO)2CO2Me [48], one can readily envisage the formation of the tert-buty 53, which readily eliminates isobutene and carbon dioxide via a very favourable six bered transition state to produce (trindane)Mn(CO)2H, 54, as in Scheme 15.Treatm 8 with t-BuOK in CH2Cl2 furnished (trindane)Mn(CO)2Cl, suggesting that 54 is m active than (C6Me6)Mn(CO)2H towards alkyl halides.Since numerous attempts to prepare 54 via the treatment of (trindane)Mn( with (n-Bu)4N + [BH4] − , or to detect the hydride signal via NMR were unsuccessful tempt was made to trap the purported metal hydride either as (trindane)Mn(CO)( or as the formyl complex (trindane)Mn(CO)(CHO)(PR3).When the cation 8 and tert were treated with trimethyl phosphite in THF and kept at 40 °C for 20 h, the pr isolated after chromatographic separation were yellow crystalline materials 55 and 1 H and 13 C NMR data of which indicated that the three-fold symmetry of the tr ligand had been broken, as shown in Scheme 16.These products were unambig identified via X-ray crystallography as η 5 -indenyl complexes (Figure 16) in wh manganese had migrated from the central arene onto a five-membered ring that h dently lost three hydrogens.The reaction with triphenylphosphine behaves analo to give 57 [49].In light of these data, one can postulate a mechanism for the formation of (trindane) Mn(CO) 2 I upon the treatment of 51 with t-BuOK in the presence of methyl iodide.Since [(C 6 Me 6 )Mn(CO) 3 ] + is known to react with methanol to form the rather unstable ester (C 6 Me 6 )Mn(CO) 2 CO 2 Me [48], one can readily envisage the formation of the tert-butyl ester, 53, which readily eliminates isobutene and carbon dioxide via a very favourable six-membered transition state to produce (trindane)Mn(CO) 2 H, 54, as in Scheme 15.Treatment of 8 with t-BuOK in CH 2 Cl 2 furnished (trindane)Mn(CO) 2 Cl, suggesting that 54 is more reactive than (C 6 Me 6 )Mn(CO) 2 H towards alkyl halides.
In light of these data, one can postulate a mechanism for the formation of (trindane)Mn(CO)2I upon the treatment of 51 with t-BuOK in the presence of methyl iodide.Since [(C6Me6)Mn(CO)3] + is known to react with methanol to form the rather unstable ester (C6Me6)Mn(CO)2CO2Me [48], one can readily envisage the formation of the tert-butyl ester, 53, which readily eliminates isobutene and carbon dioxide via a very favourable six-membered transition state to produce (trindane)Mn(CO)2H, 54, as in Scheme 15.Treatment of 8 with t-BuOK in CH2Cl2 furnished (trindane)Mn(CO)2Cl, suggesting that 54 is more reactive than (C6Me6)Mn(CO)2H towards alkyl halides.Since numerous attempts to prepare 54 via the treatment of (trindane)Mn(CO)2Br with (n-Bu)4N + [BH4] − , or to detect the hydride signal via NMR were unsuccessful, an attempt was made to trap the purported metal hydride either as (trindane)Mn(CO)(PR3)H, or as the formyl complex (trindane)Mn(CO)(CHO)(PR3).When the cation 8 and tert-BuOK were treated with trimethyl phosphite in THF and kept at 40 °C for 20 h, the products isolated after chromatographic separation were yellow crystalline materials 55 and 56, the 1 H and 13 C NMR data of which indicated that the three-fold symmetry of the trindane ligand had been broken, as shown in Scheme 16.These products were unambiguously identified via X-ray crystallography as η 5 -indenyl complexes (Figure 16) in which the manganese had migrated from the central arene onto a five-membered ring that had evidently lost three hydrogens.The reaction with triphenylphosphine behaves analogously to give 57 [49].Since numerous attempts to prepare 54 via the treatment of (trindane)Mn(CO) 2 Br with (n-Bu) 4 N + [BH 4 ] − , or to detect the hydride signal via NMR were unsuccessful, an attempt was made to trap the purported metal hydride either as (trindane)Mn(CO)(PR 3 )H, or as the formyl complex (trindane)Mn(CO)(CHO)(PR 3 ).When the cation 8 and tert-BuOK were treated with trimethyl phosphite in THF and kept at 40 • C for 20 h, the products isolated after chromatographic separation were yellow crystalline materials 55 and 56, the 1 H and 13 C NMR data of which indicated that the three-fold symmetry of the trindane ligand had been broken, as shown in Scheme 16.These products were unambiguously identified via X-ray crystallography as η 5 -indenyl complexes (Figure 16) in which the manganese had migrated from the central arene onto a five-membered ring that had evidently lost three hydrogens.The reaction with triphenylphosphine behaves analogously to give 57 [49].
isolated after chromatographic separation were yellow crystalline materials 55 and 56, the 1 H and 13 C NMR data of which indicated that the three-fold symmetry of the trindane ligand had been broken, as shown in Scheme 16.These products were unambiguously identified via X-ray crystallography as η 5 -indenyl complexes (Figure 16) in which the manganese had migrated from the central arene onto a five-membered ring that had evidently lost three hydrogens.The reaction with triphenylphosphine behaves analogously to give 57 [49].

The Synthesis of Sumanene
The synthesis of sumanene (Scheme 19) was finally achieved by Sakurai, Daiko and Hirao, who prepared a bromo-lithio derivative of norbornadiene, 64, that was converted into the corresponding tin compound 65.The copper-mediated trimerisation of 65 gave a 3:1 mixture of anti and syn benzotris(norbornadiene), 66, the latter of which underwent metathesis with the Grubbs catalyst to produce hexahydrosumanene, 67.The process was completed upon oxidation with DDQ to deliver sumanene, 39, with no need for a flash vacuum pyrolysis step [53].This work has since been comprehensively reviewed recently [54,55].

From Decacyclene
As we know, the first well-characterised fullerene, C60, is now commercially available, but efforts towards a rational stepwise synthetic procedure continue to unfold [36].As noted above, the chemistry of corannulene and sumanene have been well explored and numerous derivatives have been reported.Nevertheless, other elegant approaches based on three-fold symmetry have been disclosed.In particular, we note work by Scott, a major pioneer in the fullerene field, starting from decacyclene (Scheme 20).Having successfully prepared 8-chloro-1(2H)-acenaphthylenone, 68, this was trimerised using TiCl4 to produce 3,9,15-trichlorodecacyclene, 69, which upon FVP treatment at 1100 °C delivered a geodesic dome in a 27% yield with the formula C36H12, representing 60% of C60, and was given the

The Synthesis of Sumanene
The synthesis of sumanene (Scheme 19) was finally achieved by Sakurai, Daiko and Hirao, who prepared a bromo-lithio derivative of norbornadiene, 64, that was converted into the corresponding tin compound 65.The copper-mediated trimerisation of 65 gave a 3:1 mixture of anti and syn benzotris(norbornadiene), 66, the latter of which underwent metathesis with the Grubbs catalyst to produce hexahydrosumanene, 67.The process was completed upon oxidation with DDQ to deliver sumanene, 39, with no need for a flash vacuum pyrolysis step [53].This work has since been comprehensively reviewed recently [54,55].

The Synthesis of Sumanene
The synthesis of sumanene (Scheme 19) was finally achieved by Sakurai, Daiko and Hirao, who prepared a bromo-lithio derivative of norbornadiene, 64, that was converted into the corresponding tin compound 65.The copper-mediated trimerisation of 65 gave a 3:1 mixture of anti and syn benzotris(norbornadiene), 66, the latter of which underwent metathesis with the Grubbs catalyst to produce hexahydrosumanene, 67.The process was completed upon oxidation with DDQ to deliver sumanene, 39, with no need for a flash vacuum pyrolysis step [53].This work has since been comprehensively reviewed recently [54,55].

From Decacyclene
As we know, the first well-characterised fullerene, C60, is now commercially available, but efforts towards a rational stepwise synthetic procedure continue to unfold [36].As noted above, the chemistry of corannulene and sumanene have been well explored and numerous derivatives have been reported.Nevertheless, other elegant approaches based on three-fold symmetry have been disclosed.In particular, we note work by Scott, a major pioneer in the fullerene field, starting from decacyclene (Scheme 20).Having successfully prepared 8-chloro-1(2H)-acenaphthylenone, 68, this was trimerised using TiCl4 to produce 3,9,15-trichlorodecacyclene, 69, which upon FVP treatment at 1100 °C delivered a geodesic dome in a 27% yield with the formula C36H12, representing 60% of C60, and was given the As we know, the first well-characterised fullerene, C 60 , is now commercially available, but efforts towards a rational stepwise synthetic procedure continue to unfold [36].As noted above, the chemistry of corannulene and sumanene have been well explored and numerous derivatives have been reported.Nevertheless, other elegant approaches based on three-fold symmetry have been disclosed.In particular, we note work by Scott, a major pioneer in the fullerene field, starting from decacyclene (Scheme 20).Having successfully prepared 8-chloro-1(2H)-acenaphthylenone, 68, this was trimerised using TiCl 4 to produce 3,9,15trichlorodecacyclene, 69, which upon FVP treatment at 1100 • C delivered a geodesic dome in a 27% yield with the formula C 36 H 12 , representing 60% of C 60 , and was given the trivial name Circumtrindene, 70 [56].The structure was confirmed via X-ray crystallography [57], and Figure 17

The Designed Stepwise Synthesis of C60
The three-fold cyclisation of ketones, such as that of cyclopentanone to trindane, or indanone to truxene, is mirrored by the route to trichlorodecacyclene, 69, from 8-chloroacenaphthylenone, 68.Continuing with this approach, an even more audacious experiment by Scott and de Meijere sought to prepare C60 directly via the trimerisation of a C20 ketone.As shown in Scheme 21, the treatment of the pentacyclic ketone 71 with TiCl4 in refluxing o-dichlorobenzene led to the C3h-symmetric molecule, 72, with the formula C60H27Cl3.They were hoping to effect multiple dehydrohalogenations and dehydrogenations to bring about a "stitching together" of the three arms of the molecule via FVP at 1100 °C, thus forming C60 directly.Gratifyingly, this strategy was successful, and the only fullerene formed was C60; though in a very low yield, it was sufficient to be unambiguously characterised via mass spectrometry [58].

The Designed Stepwise Synthesis of C60
The three-fold cyclisation of ketones, such as that of cyclopentanone to trindane, or indanone to truxene, is mirrored by the route to trichlorodecacyclene, 69, from 8-chloroacenaphthylenone, 68.Continuing with this approach, an even more audacious experiment by Scott and de Meijere sought to prepare C60 directly via the trimerisation of a C20 ketone.As shown in Scheme 21, the treatment of the pentacyclic ketone 71 with TiCl4 in refluxing o-dichlorobenzene led to the C3h-symmetric molecule, 72, with the formula C60H27Cl3.They were hoping to effect multiple dehydrohalogenations and dehydrogenations to bring about a "stitching together" of the three arms of the molecule via FVP at 1100 °C, thus forming C60 directly.Gratifyingly, this strategy was successful, and the only fullerene formed was C60; though in a very low yield, it was sufficient to be unambiguously characterised via mass spectrometry [58].

The Designed Stepwise Synthesis of C60
The three-fold cyclisation of ketones, such as that of cyclopentanone to trindane, or indanone to truxene, is mirrored by the route to trichlorodecacyclene, 69, from 8-chloroacenaphthylenone, 68.Continuing with this approach, an even more audacious experiment by Scott and de Meijere sought to prepare C60 directly via the trimerisation of a C20 ketone.As shown in Scheme 21, the treatment of the pentacyclic ketone 71 with TiCl4 in refluxing o-dichlorobenzene led to the C3h-symmetric molecule, 72, with the formula C60H27Cl3.They were hoping to effect multiple dehydrohalogenations and dehydrogenations to bring about a "stitching together" of the three arms of the molecule via FVP at 1100 °C, thus forming C60 directly.Gratifyingly, this strategy was successful, and the only fullerene formed was C60; though in a very low yield, it was sufficient to be unambiguously characterised via mass spectrometry [58].The reported organic chemistry of trindane has focused primarily on its reactivity when treated with different oxidants.Ruthenium trichloride and sodium periodate, in situ, generate a ruthenium(VIII) species, "RuO 4 ", which reacted with trindane to bring about an unanticipated carbohydrate-like product, 72, the structure of which was elucidated via X-ray crystallography.One can envisage a mechanism whereby one of the double bonds in the aromatic ring is doubly hydroxylated to yield 73, and the subsequent cleavage of a second double bond forms a diketone that upon further oxidation and hydrolysis rearranges to deliver the final product (Scheme 22) [59].

Oxidation of Trindane
The reported organic chemistry of trindane has focused primarily on its reactivity when treated with different oxidants.Ruthenium trichloride and sodium periodate, in situ, generate a ruthenium(VIII) species, "RuO4", which reacted with trindane to bring about an unanticipated carbohydrate-like product, 72, the structure of which was elucidated via X-ray crystallography.One can envisage a mechanism whereby one of the double bonds in the aromatic ring is doubly hydroxylated to yield 73, and the subsequent cleavage of a second double bond forms a diketone that upon further oxidation and hydrolysis rearranges to deliver the final product (Scheme 22) [59].In contrast, the ozonolysis of trindane led to 74, which again was only unequivocally characterised via X-ray crystallography (Scheme 23).Under these conditions, the initial cleavage of two double bonds of the central aromatic ring forms the tetraketone, 75, that is in equilibrium with its enol tautomer, 76, and cyclisation leads to the final product.The retention of the C15 periphery in 74 resembles the structure of some natural products, such as ginkgolides, and demonstrates that the cleavage of the π-bond endo to the cyclopentane present in trindane provides a simple route to complex natural products [60].More recent work has revealed that the result of the Ru(VIII) oxidation of the next higher homologue of trindane, dodecahydrotriphenylene, 77, which is readily prepared via the cyclisation of cyclohexanone, differs from that found with trindane.In this case, the central aromatic ring remains intact and the products arise from benzylic oxidation to the mono-, di-and tri-ketones 78, 79 and 80, respectively, depending on the length of the reaction time (Scheme 24) [61].Interestingly, the products maintain their directionality such that the ketones are arranged unidirectionally (note that the cyclohexanone rings adopt their normal non-planar geometry).In contrast, the ozonolysis of trindane led to 74, which again was only unequivocally characterised via X-ray crystallography (Scheme 23).Under these conditions, the initial cleavage of two double bonds of the central aromatic ring forms the tetraketone, 75, that is in equilibrium with its enol tautomer, 76, and cyclisation leads to the final product.The retention of the C 15 periphery in 74 resembles the structure of some natural products, such as ginkgolides, and demonstrates that the cleavage of the π-bond endo to the cyclopentane present in trindane provides a simple route to complex natural products [60].

Oxidation of Trindane
The reported organic chemistry of trindane has focused primarily on its reactivity when treated with different oxidants.Ruthenium trichloride and sodium periodate, in situ, generate a ruthenium(VIII) species, "RuO4", which reacted with trindane to bring about an unanticipated carbohydrate-like product, 72, the structure of which was elucidated via X-ray crystallography.One can envisage a mechanism whereby one of the double bonds in the aromatic ring is doubly hydroxylated to yield 73, and the subsequent cleavage of a second double bond forms a diketone that upon further oxidation and hydrolysis rearranges to deliver the final product (Scheme 22) [59].In contrast, the ozonolysis of trindane led to 74, which again was only unequivocally characterised via X-ray crystallography (Scheme 23).Under these conditions, the initial cleavage of two double bonds of the central aromatic ring forms the tetraketone, 75, that is in equilibrium with its enol tautomer, 76, and cyclisation leads to the final product.The retention of the C15 periphery in 74 resembles the structure of some natural products, such as ginkgolides, and demonstrates that the cleavage of the π-bond endo to the cyclopentane present in trindane provides a simple route to complex natural products [60].More recent work has revealed that the result of the Ru(VIII) oxidation of the next higher homologue of trindane, dodecahydrotriphenylene, 77, which is readily prepared via the cyclisation of cyclohexanone, differs from that found with trindane.In this case, the central aromatic ring remains intact and the products arise from benzylic oxidation to the mono-, di-and tri-ketones 78, 79 and 80, respectively, depending on the length of the reaction time (Scheme 24) [61].Interestingly, the products maintain their directionality such that the ketones are arranged unidirectionally (note that the cyclohexanone rings adopt their normal non-planar geometry).More recent work has revealed that the result of the Ru(VIII) oxidation of the next higher homologue of trindane, dodecahydrotriphenylene, 77, which is readily prepared via the cyclisation of cyclohexanone, differs from that found with trindane.In this case, the central aromatic ring remains intact and the products arise from benzylic oxidation to the mono-, di-and tri-ketones 78, 79 and 80, respectively, depending on the length of the reaction time (Scheme 24) [61].Interestingly, the products maintain their directionality such that the ketones are arranged unidirectionally (note that the cyclohexanone rings adopt their normal non-planar geometry).The reported organic chemistry of trindane has focused primarily on its reactivity when treated with different oxidants.Ruthenium trichloride and sodium periodate, in situ, generate a ruthenium(VIII) species, "RuO4", which reacted with trindane to bring about an unanticipated carbohydrate-like product, 72, the structure of which was elucidated via X-ray crystallography.One can envisage a mechanism whereby one of the double bonds in the aromatic ring is doubly hydroxylated to yield 73, and the subsequent cleavage of a second double bond forms a diketone that upon further oxidation and hydrolysis rearranges to deliver the final product (Scheme 22) [59].In contrast, the ozonolysis of trindane led to 74, which again was only unequivocally characterised via X-ray crystallography (Scheme 23).Under these conditions, the initial cleavage of two double bonds of the central aromatic ring forms the tetraketone, 75, that is in equilibrium with its enol tautomer, 76, and cyclisation leads to the final product.The retention of the C15 periphery in 74 resembles the structure of some natural products, such as ginkgolides, and demonstrates that the cleavage of the π-bond endo to the cyclopentane present in trindane provides a simple route to complex natural products [60].More recent work has revealed that the result of the Ru(VIII) oxidation of the next higher homologue of trindane, dodecahydrotriphenylene, 77, which is readily prepared via the cyclisation of cyclohexanone, differs from that found with trindane.In this case, the central aromatic ring remains intact and the products arise from benzylic oxidation to the mono-, di-and tri-ketones 78, 79 and 80, respectively, depending on the length of the reaction time (Scheme 24) [61].Interestingly, the products maintain their directionality such that the ketones are arranged unidirectionally (note that the cyclohexanone rings adopt their normal non-planar geometry).

Synthesis of a Potential Artificial Receptor
In a particularly elegant example that takes advantage not only of the three-fold symmetry of trindane, but also of the ability of an organometallic fragment to block one face of the arene, these concepts were exploited to investigate the construction of artificial receptors.As illustrated in Scheme 25, the hexaester 81 was prepared from 1,3,5-tris(bromomethyl)-2,4,6-tris(chloromethyl)benzene via a reaction with the sodium enolate of diethyl malonate, whereupon saponification, decarboxylation and esterification produced a mixture of cis,cis,cisand cis,cis,trans-trindane-2,5,8-tricarboxylic ester, 82.However, the treatment of the triester with either Cr(CO) 6 or Mo(CO) 6 to form 83 allowed the isolation of the all-syn isomer in a good yield.A subsequent reaction with LDA and benzyl bromide proceeded exclusively via exo attack to form the desired C 3v -symmetric product.The subsequent removal of the metal carbonyl tripod with iodine to form 84, and further elaboration led to the urea derivative, 85, which was investigated for its anion-binding capability [62].

Synthesis of a Potential Artificial Receptor
In a particularly elegant example that takes advantage not only of the three-fold symmetry of trindane, but also of the ability of an organometallic fragment to block one face of the arene, these concepts were exploited to investigate the construction of artificial receptors.As illustrated in Scheme 25, the hexaester 81 was prepared from 1,3,5-tris(bromomethyl)-2,4,6-tris(chloromethyl)benzene via a reaction with the sodium enolate of diethyl malonate, whereupon saponification, decarboxylation and esterification produced a mixture of cis,cis,cis-and cis,cis,trans-trindane-2,5,8-tricarboxylic ester, 82.However, the treatment of the triester with either Cr(CO)6 or Mo(CO)6 to form 83 allowed the isolation of the all-syn isomer in a good yield.A subsequent reaction with LDA and benzyl bromide proceeded exclusively via exo attack to form the desired C3v-symmetric product.The subsequent removal of the metal carbonyl tripod with iodine to form 84, and further elaboration led to the urea derivative, 85, which was investigated for its anion-binding capability [62].

Applications of Truxene and Truxenone
Truxene, 28, with its planar C3h symmetry, is an important building block for dendrimers, organic frameworks, and star-shaped, cage-like and porous molecules [63][64][65].An early example of its application was its use in liquid crystals [66], but it has more recently found widespread utility in materials science [67], and in polymer chemistry [68], as a component of the stationary phase in capillary gas phase chromatography [69], as an organic sensitiser, or as a potential hole transport material in perovskite solar cells [70,71].

Applications of Truxene and Truxenone
Truxene, 28, with its planar C 3h symmetry, is an important building block for dendrimers, organic frameworks, and star-shaped, cage-like and porous molecules [63][64][65].An early example of its application was its use in liquid crystals [66], but it has more recently found widespread utility in materials science [67], and in polymer chemistry [68], as a component of the stationary phase in capillary gas phase chromatography [69], as an organic sensitiser, or as a potential hole transport material in perovskite solar cells [70,71].
Truxenone, 86, was originally prepared directly via the acid-catalysed triple condensation of indane-1,3-dione [72,73], but the more recent five-step synthesis starting from 2-methylacetophenone (Scheme 26) proceeds under milder conditions and is more tolerant to sensitive substituents [74].The ready availability of truxenone has led to the syntheses of a very large number of functionalised derivatives, many of which have been characterised via X-ray crystallography.A common feature that emerges in such structural determinations is the loss of planarity caused by steric crowding between the dicyanomethylene substituents at positions 5, 10 and 15 and the neighbouring peripheral aromatic rings.Typically, as shown in Figure 18, truxene maintains its C3h symmetry [75], whereas in the tris(dicyanomethylidene) system, 87, prepared via Knoevenagel condensation from truxenone (Scheme 27), the horizontal plane was broken, and the propeller-type molecule adopted a C3 symmetry and was chiral [76].Truxenone has found applications in many areas ranging from organic voltaics [77], and n-type semiconductors [78], to serving as the framework for the cathode in a solid-state lithium-ion battery [79].The ready availability of truxenone has led to the syntheses of a very large number of functionalised derivatives, many of which have been characterised via X-ray crystallography.A common feature that emerges in such structural determinations is the loss of planarity caused by steric crowding between the dicyanomethylene substituents at positions 5, 10 and 15 and the neighbouring peripheral aromatic rings.Typically, as shown in Figure 18, truxene maintains its C 3h symmetry [75], whereas in the tris(dicyanomethylidene) system, 87, prepared via Knoevenagel condensation from truxenone (Scheme 27), the horizontal plane was broken, and the propeller-type molecule adopted a C 3 symmetry and was chiral [76].Truxenone has found applications in many areas ranging from organic voltaics [77], and n-type semiconductors [78], to serving as the framework for the cathode in a solid-state lithium-ion battery [79].The ready availability of truxenone has led to the syntheses of a very large number of functionalised derivatives, many of which have been characterised via X-ray crystallography.A common feature that emerges in such structural determinations is the loss of planarity caused by steric crowding between the dicyanomethylene substituents at positions 5, 10 and 15 and the neighbouring peripheral aromatic rings.Typically, as shown in Figure 18, truxene maintains its C3h symmetry [75], whereas in the tris(dicyanomethylidene) system, 87, prepared via Knoevenagel condensation from truxenone (Scheme 27), the horizontal plane was broken, and the propeller-type molecule adopted a C3 symmetry and was chiral [76].Truxenone has found applications in many areas ranging from organic voltaics [77], and n-type semiconductors [78], to serving as the framework for the cathode in a solid-state lithium-ion battery [79].The ready availability of truxenone has led to the syntheses of a very large number of functionalised derivatives, many of which have been characterised via X-ray crystallography.A common feature that emerges in such structural determinations is the loss of planarity caused by steric crowding between the dicyanomethylene substituents at positions 5, 10 and 15 and the neighbouring peripheral aromatic rings.Typically, as shown in Figure 18, truxene maintains its C3h symmetry [75], whereas in the tris(dicyanomethylidene) system, 87, prepared via Knoevenagel condensation from truxenone (Scheme 27), the horizontal plane was broken, and the propeller-type molecule adopted a C3 symmetry and was chiral [76].Truxenone has found applications in many areas ranging from organic voltaics [77], and n-type semiconductors [78], to serving as the framework for the cathode in a solid-state lithium-ion battery [79].

Concluding Remarks
The three-fold D 3h symmetry of trindane, 1, or of the trindene trianion, 4, provides a versatile framework in which chemical structure and reactivity can be controlled.When an organometallic fragment, such as M(CO) 3 , is π-complexed to one face of trindane, it not only enhances the acidity of the exo-benzylic hydrogens, but also protects that face from approach by electrophiles.However, nucleophilic attack on the metal carbonyl ligands, with the elimination of specific fragments (such as isobutene or CO 2 ), can bring about the formation of a metal-hydride linkage that opens up other reaction possibilities, as exemplified in Scheme 16.When the trindene system bears three π-complexed organometallic units, at least two of them must be sited adjacently on the same face, thereby enhancing the likelihood of electronic interaction between the metals, or even the development of a formal metal-metal bond, as in Figure 8.The addition of three peripheral benzo rings, as in truxene, maintains the three-fold symmetry of the system but also offers different types of coordination sites whereby the organometallic moiety can bind in an η 5 or η 6 fashion.Moreover, protonationdeprotonation sequences can bring about haptotropic shifts between these two situations, as exemplified in Scheme 6.
The formation of decacyclene apparently regains the D 3h symmetry of trindenyl, but this is not in fact the case.Non-bonded repulsions between hydrogens of neighbouring naphthyl groups cause the molecule to adopt a chiral (C 3 ) propeller geometry that is even more evident when organometallic units are attached to opposite faces of the molecule.
It has long been hoped that functionalised versions of these polycyclic frameworks could lead towards a rational stepwise route to fullerenes, or at least to sizeable portions of the C 60 skeleton.This approach was initially used in early attempts to prepare sumanene, and one spectacular example of success has been the controlled coupling of adjacent peripheral naphthyl rings in decacyclene to generate circumtrindene, C 36 H 12 , which contains 60% of the C 60 framework.
Finally, one can only marvel at the complexity of the oxidation products of a molecule as simple as trindane, and the multitude of important applications, such as liquid crystals, optoelectronics, solar cell technology or solid-state batteries, in which truxene or truxenone plays such a crucial role.

Scheme 9 .
Scheme 9. Mehta's pioneering work on a potential route to sumanene.

Scheme 9 .
Scheme 9. Mehta's pioneering work on a potential route to sumanene.

Scheme 9 .
Scheme 9. Mehta's pioneering work on a potential route to sumanene.

Scheme 18 .
Scheme 18. Proposed mechanism for the formation of the rearrangement products 55, 56 and 57.

Scheme 18 .
Scheme 18. Proposed mechanism for the formation of the rearrangement products 55, 56 and 57.

Figure 17 .
Figure 17.Circumtrindene shown as 60% of the C60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).

Figure 17 .
Figure 17.Circumtrindene shown as 60% of the C60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).

Figure 17 .
Figure 17.Circumtrindene shown as 60% of the C 60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).

Figure 17 .
Figure 17.Circumtrindene shown as 60% of the C60 skeleton; (left) the purple bonds indicate the final ring closures, and (right) the X-ray crystal structure of circumtrindene 70 (CSD ID: NARZEL).

Scheme 25 .
Scheme 25.Construction of a C 3v -symmetric potential artificial receptor.