Synthesis and Characterization of Ion Pairs between Alkaline Metal Ions and Anionic Anti-Aromatic and Aromatic Hydrocarbons with π-Conjugated Central Seven- and Eight-Membered Rings

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop‒ and trop2−• anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11 ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.


Introduction
Transition metal complexes with 5H-dibenzo[a,d]cyclohepten-5-yl units (trivial name tropylidenyl = trop, see Scheme 1, right) as ligands are well established in the literature [1][2][3][4]. Their special properties give rise to complexes with extraordinary catalytic activities. For example, bis(trop)amine as ligand in d 8 -Rh(I) complex A (Scheme 1, top) provokes an unusual butterfly-type structure for tetracoordinated sixteen electron configured transition metal complexes and can act as a cooperating ligand [5]. Both factors contribute to the high activities in the hydrogenation of ketone derivatives [6,7] or the dehydrogenative coupling of alcohols [8,9] under very mild reaction conditions. In another example, complexes of type B with the redox and chemically non-innocent cooperative diazadiene ligand trop 2 dad (dad = diazadiene) [10] show an unprecedented high efficiency in dehydrogenation reactions of methanol or formaldehyde [10][11][12]. The olefinic double bond in On the other hand, compounds with main group element metals and trop-type ligands are very scarce [14]. We became especially interested in compounds which contain an alkaline metal ion and an anionic trop moiety because of their potential as building blocks for the synthesis of new trop-type ligands. Furthermore, these species, (M+)n[trop] n− , may form various forms of ion pairs with fascinating properties, and related species with reduced arenes as anions are interesting to explore on their own [15][16][17].
To the best of our knowledge, salts containing anionic 5H-dibenzo [a,d]cycloheptenides, [trop] n− (n = 1, 2) have never been isolated. Based on the 4n-Hückel-rule, the trop anion, C15H11 − , with its central 8π electron system flanked by two annulated benzo groups (giving a 16π electronsystem in total), should show anti-aromatic character. Indeed, when the trop anion is generated in situ in liquid On the other hand, compounds with main group element metals and trop-type ligands are very scarce [14]. We became especially interested in compounds which contain an alkaline metal ion and an anionic trop moiety because of their potential as building blocks for the synthesis of new trop-type ligands. Furthermore, these species, (M+) n [trop] n− , may form various forms of ion pairs with fascinating properties, and related species with reduced arenes as anions are interesting to explore on their own [15][16][17].
To the best of our knowledge, salts containing anionic 5H-dibenzo[a,d]cycloheptenides, [trop] n− (n = 1, 2) have never been isolated. Based on the 4n-Hückel-rule, the trop anion, C 15 Compound Ktrop can be further reduced with potassium graphite to give the ion triple K2trop, which, apart from two K + ions, contains the trop dianion radical trop 2−• ((b) in Scheme 2). K2trop was isolated as a dark green, micro-crystalline solid. We failed to prepare and isolate the dilithium and the disodium analogue so far. The synthesis of the disodium and the dipotassium derivatives of the dbcot 2− dianion, Na2dbcot or K2dbcot, is achieved in a straightforward manner by simply exposing dibenzo[a,e]cyclooctatetraene (dbcot) to elementary sodium or potassium graphite in anhydrous tetrahydrofuran ((c) in Scheme 2) as was likewise reported for the preparation of Li2dbcot [28,31].
The trop anion can be used as a ligand in transition metal complexes, and reaction with half an equivalent of [Rh2Cl2(cod)2] gives [Rh(trop)(cod)] as a dark red crystalline compound in moderate isolated yield (63%) ((d) in Scheme 2). In this complex, the d 8 -valence electron configured Rh(I) center is exclusively coordinated to hydrocarbon ligands and represents to the best of our knowledge the first fully characterized mononuclear Rh(I) heptatrienide species. Only very few related compounds such as bimetallic Rh complexes [34,35], a Rh(I) azulene complex [36] and a Rh(III) cycloheptatrienide complex have been reported [37]. In contrast, the reaction with the dianion radical containing salt K2trop did not yield any identifiable compound, likely due to the strongly reducing properties of the trop 2−• (vide infra). Compound Ktrop can be further reduced with potassium graphite to give the ion triple K 2 trop, which, apart from two K + ions, contains the trop dianion radical trop 2−• ((b) in Scheme 2). K 2 trop was isolated as a dark green, micro-crystalline solid. We failed to prepare and isolate the dilithium and the disodium analogue so far. The synthesis of the disodium and the dipotassium derivatives of the dbcot 2− dianion, Na 2 dbcot or K 2 dbcot, is achieved in a straightforward manner by simply exposing dibenzo[a,e]cyclooctatetraene (dbcot) to elementary sodium or potassium graphite in anhydrous tetrahydrofuran ((c) in Scheme 2) as was likewise reported for the preparation of Li 2 dbcot [28,31].

Characterization by NMR, EPR and X-Ray Diffraction Methods
The trop anion can be used as a ligand in transition metal complexes, and reaction with half an equivalent of [Rh 2 Cl 2 (cod) 2 ] gives [Rh(trop)(cod)] as a dark red crystalline compound in moderate isolated yield (63%) ((d) in Scheme 2). In this complex, the d 8 -valence electron configured Rh(I) center is exclusively coordinated to hydrocarbon ligands and represents to the best of our knowledge the first fully characterized mononuclear Rh(I) heptatrienide species. Only very few related compounds such as bimetallic Rh complexes [34,35], a Rh(I) azulene complex [36] and a Rh(III) cycloheptatrienide complex have been reported [37]. In contrast, the reaction with the dianion radical containing salt K 2 trop did not yield any identifiable compound, likely due to the strongly reducing properties of the trop 2−• (vide infra).

Characterization by NMR, EPR and X-ray Diffraction Methods
All diamagnetic compounds were fully characterized by NMR spectroscopy. The paramagnetic compound K 2 trop was analyzed by EPR spectroscopy. Single crystals of Litrop, Natrop, Ktrop, K 2 trop, Na 2 dbcot, K 2 dbcot and [Rh(trop)(cod)] were grown and investigated with X-ray diffraction methods in order to determine the structures experimentally. In addition, the structures of compounds Mtrop (M = Li − K) and [Rh(trop)(cod)] and their degree of anti-aromatic or aromatic character was evaluated by calculating the Nuclear Independent Chemical Shifts (NICSs) (DFT, PBE [38,39] and 6-311+G (df, pd) [39] in a continuum solvation model (THF) [40,41]).
All compounds containing the tropanion show strongly shielded signals for all protons in the 1 H NMR spectra. In the dimer of the trop radical [23], namely C 15 H 11 − C 15 H 11 = trop 2 [42] (entry 1, Table 1), which has an electronic structure reminiscent of the conjugated hydrocarbon cis-stilbene, the olefinic protons 1 H ol and the benzylic proton 1 H bz attached to the central seven-membered ring are observed in the normal range at δ = 7.06 ppm and δ = 4.73 ppm. In the ion pairs, Mtrop, these resonances are significantly shifted to lower frequencies to δ ( 1 H ol ) < 2.6 ppm and δ ( 1 H bz ) < 0.56 ppm (M = Li, Na, K, entries 2-4, Table 1). Also, the protons of the annulated benzo groups are strongly shifted to lower frequencies by about 3 ppm. The experimental structures of the contact ions pairs Li thf trop, Li dme trop, Na dme trop, K thf trop and K 18c6 trop were determined with X-ray diffraction methods using a suitable single crystal of every compound and are depicted in Figure 1. Selected bond distances and angles are listed in Tables 2 and 3. The most remarkable feature is the bending of the central seven-membered ring in the tropmoiety expressed by the fold angles Θ 1 and Θ 2 (Table 3). These angles vary strongly with the nature of the counter cation but also with the nature of the solvent molecules acting as co-ligands to M + . In Li thf trop, the [Li(thf) 3 ), in which no close contact between cation and anion occurs (shortest Li-C contact = 6.79 Å) and the tropanion adopts an almost flat structure (Θ 1 = 6 • ; Θ 2 = 3 • ). Likewise, in the structure of Natrop, which contains [Na(dme) 3 ] + , the contact between cation and anion is long (the distance between Na + and the centroid (cnt) of the central C 7 ring is 5.25 Å) and the tropanion is flat (Θ 1 = 6.5 • ; Θ 2 = 4.7 • ).  (18). Further selected structural parameters of all compounds are given in Table 2 and Table 3.  Figure 1 for clarification). c Centroid of C1, C2, C5, C6, C7. d Allylic centroid of C1′, C2′ and C8′.
The most planar structure for a tropanion is observed in K thf trop (Θ1 = 1.7°; Θ2 = 2.4°), which is best described as a close ion pair with a rather short distance between K1 and the centroid cnt1 of the  Tables 2 and 3.  Figure 1 for clarification). c Centroid of C1, C2, C5, C6, C7. d Allylic centroid of C1 , C2 and C8 .

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The most planar structure for a tropanion is observed in K thf trop (Θ 1 = 1.7 • ; Θ 2 = 2.4 • ), which is best described as a close ion pair with a rather short distance between K1 and the centroid cnt1 of the C 7 ring of 2.82 Å. The K + is coordinated to the C 7 ring in a slightly asymmetric manner as indicated by the unequal K-C1 (3.098 Å) and K-C4/C5 distances (3.407 Å; 3.346 Å). Furthermore, the potassium ion has an additional contact at about 2.97 Å to another tropunit, such that K + . . . trop − . . . K + . . . . trop − chains are formed in the solid state. Remarkably, in K 18c6 trop, the trop − anion shows larger fold angles (Θ 1 = 12.4 • ; Θ 2 = 12.3 • ). In this ion pair, K + is rather asymmetrically bound and the distance to the centroid of to the central C 7 ring is large (3.812 Å). Instead, K + binds closer to the allylic fragment C1, C7, C15 (K1-cnt2 3.303 Å) with a rather short contact to the benzylic carbon center C1 (3.02 Å) and longer ones to C7 (3.263 Å) and C15 (3.490 Å). It is highly speculative to which extent these features of the solid-state structures are retained in solution. But, the coupling constants 1 J HC of the benzylic carbon nucleus, 13 C bz , to the benzylic proton, 1 H bz , can be empirically correlated to the s-orbital character in the C bz -H bond. In a bent trop anion, the C bz -H bond will be closer to a sp 3 configuration (25% s, smaller 1 J HC ) that in a flat structure, where the C bz -H is closer to a sp 2 configuration (33% s, larger 1 J HC ). Indeed, 1 J HC obtained in deuterated tetrahydrofuran, [D 8 ]THF, as solvent (Table 1) is larger for the potassium salt than in the bent lithium salts, indicating that the solid-state structures are retained to a certain degree in solution.
At this point, it is interesting to compare these main group metal complexes of trop − with a "classical" organometallic complex such as [Rh(trop)(cod)]. In contrast to Mtrop, which shows strongly shielded 1 H NMR resonances for all protons, the signals for the 1 H nuclei at the benzo groups in [Rh(trop)(cod)] are in the same region (δ 1 H: 6.82-7.01 ppm) as in the reference compound trop 2 (δ 1 H: 6.58-7.25 ppm). Also, the chemical shifts of the benzo 13 C nuclei are in the normal range for arenes in between δ = 127 and 137 ppm. Only the olefinic and benzylic 1 H and 13 C NMR signals (δ 1 H ol , δ 1 H bz , δ 13 C ol , δ 13 C bz ) are significantly shifted to lower frequencies, indicating an interaction with the metal center, c.f. δ ( 13 C ol ) = 97.6 ppm, δ ( 13 C bz ) = 56.5 ppm in [Rh(trop)(cod)] vs. δ ( 13 C ol )~139 ppm and δ ( 13 C bz ) = 81.6-89.4 ppm in Mtrop, see Table 1). In solution, [Rh(trop)(cod)] is seemingly C 2v -symmetric which is not in agreement with the structure in solid state but can be explained by a dynamic phenomenon (vide infra). Complex [Rh(trop)(cod)] crystallizes as racemic mixture in the space group P2 1 /c. The structure of one of the enantiomers is depicted in Figure 1 and is very different from the Mtrop compounds. The Rh(I)(cod) fragment binds to C1, C2, C3, C4 and C5 at distances between 2.09 to 2.40 Å (see legend to Figure 1) of the central seven-membered ring in a way which is also observed in other metal [43,44] and specifically, Rh(I) pentadienide complexes [43,44]. This pentadienide-type interaction donates 6π electrons to the Rh(I)(cod) fragment which thereby reaches an 18-electron configuration. Metal-to-ligand electron back donation leads to a slight elongation of all involved C-C bonds by about 0.03 Å with respect to the distances in the trop anion in the Mtrop contact pairs (M = Li, Na, K). The most notable structural feature is the strongly bent conformation of the tropunit: the intersection angle between the non-coordinated benzo group and the benzopentadienide unit is θ 3 = 49.9(1) • (cf. Figure 1). The η 5 coordination mode and non-symmetric structure observed in the solid state is in contrast with the apparent C 2v symmetry of the complex in solution. We therefore assume that in solution, [Rh(trop)(cod)] underlies a dynamic phenomenon by which the rhodium center rapidly exchanges between the two possible η 5 binding sites of the central seven-membered ring, as shown in Scheme 2d. This process must have a very low activation barrier because even at low temperatures, this process is not frozen out on the NMR time scale. This coordination mode is in stark contrast to the one of the only other dibenzo[a,d]cycloheptenide metal complex C (Scheme 1d) [25,26]. Here, NMR data indicate that the Cr(0)(CO) 3 fragment binds in a η 6 fashion to one of the annulated benzo groups of the trop moiety which leads to C 1 symmetric structure. The 13 C chemical shifts of this coordinated benzo group are characteristically shifted to lower frequencies [δ( 13 C) = 70-100 ppm]. That is, in this case, the anti-aromatic π-electron system of tropis localized in a different way from the one in [Rh(trop)(cod)], namely into a 6π-electron system at a terminal arene bound to the metal and an uncoordinated 10π-electron system. The structures of the ion triples K 2 trop, Na 2 dbcot and K 2 dbcot with the trop 2-• dianion radical or the dbcot 2dianion respectively, are shown in Figure 2. Selected bond lengths and angles are listed in Tables 3 and 4. Molecules 2020, 25, x FOR PEER REVIEW 8 of 22 Figure 2. ORTEP plots of dianions at 50% ellipsoid probability (hydrogen atoms are omitted for clarity). K2trop: A co-crystallized DME-molecule is omitted for clarity. Na2dbot, K2dbot: The second chemically identical but crystallographically unique compound in the asymmetric unit is omitted for clarity. Selected bond lengths (Å) and angles (°) are shown in Tables 3 and 4.    Figure 2. ORTEP plots of dianions at 50% ellipsoid probability (hydrogen atoms are omitted for clarity). K 2 trop: A co-crystallized DME-molecule is omitted for clarity. Na 2 dbot, K 2 dbot: The second chemically identical but crystallographically unique compound in the asymmetric unit is omitted for clarity. Selected bond lengths (Å) and angles ( • ) are shown in Tables 3 and 4.    (4) 180.00 (5) 180.00 (4) a Two chemically identical, but crystallographically unique compounds are in the asymmetric unit. b The space group P2 1 /n leads to pairwise identical C-C bond lengths.
As expected, the reduction of Ktrop to the radical dianion K 2 trop leads to an overall elongation of all bonds within the central seven-membered ring (see increased average bond length C = C in Table 3). The average bond length difference (∆C = C in Table 3) becomes smaller and the C3-C4, C4-C5 and C5-C6 bond lengths' alternation is slightly diminished. The plots and structural data for Na 2 dbcot and K 2 dbcot are given in Figure 2 and Table 4 and are very similar to the ones reported for Li 2 dbcot (Scheme 1 and Table 3) [31]. The dbcot 2dianions in K 2 dbcot and Li 2 dbcot adopt a rather flat conformation while the one in Na 2 dbcot shows a slightly twisted conformation likely because of crystal packing forces (see dihedral angles ϕ 1 and ϕ 2 in Table 3). All C-C bond lengths are longer than 1.4 Å and there is very little bond length variation (see ∆C = C in Table 3).
The paramagnetic compound K 2 trop was further characterized by EPR spectroscopy. In tetrahydrofuran (THF) as solvent, the EPR spectrum of K 2 trop shows a rather complex hyperfine coupling pattern ( Figure 3, line a) which can be simplified by the addition of two equivalents of 18-crown-6. This indicates that complexation of K + by 18-crown-6 significantly elongates the distances between the cation and dianion radical, which leads to much smaller and hence non-detectable 39 K-hyperfine splittings. Indirect proof comes from a comparison of the structures K 2 trop, where one 18-crown-6 binds to every K + ion, and K 2 dbcot, where each K + ion is coordinated by four THF molecules. In the former the K + -Cnt distance is 2.79 Å while in the latter a shorter distance of 2.47 Å is observed (Cnt = centroid of the central ring). The less complex spectrum ( Figure 3, line b) was used to simulate the hyperfine pattern ( Figure 3, line c). The experimentally determined and calculated hyperfine coupling constants (HFCs) indicate delocalization of the unpaired electron over the whole dianion (see spin density plot in Figure 3). These data and the corresponding g-factor of 2.0064 are in fairly good agreement with the ones of the reported sodium analogue Na 2 trop [24] more than 50 years ago (g = 2.0027 and HFCs shown in Figure 3). tetrahydrofuran (THF) as solvent, the EPR spectrum of K2trop shows a rather complex hyperfine coupling pattern ( Figure 3, line a) which can be simplified by the addition of two equivalents of 18crown-6. This indicates that complexation of K + by 18-crown-6 significantly elongates the distances between the cation and dianion radical, which leads to much smaller and hence non-detectable 39 Khyperfine splittings. Indirect proof comes from a comparison of the structures K2trop, where one 18crown-6 binds to every K + ion, and K2dbcot, where each K + ion is coordinated by four THF molecules. In the former the K + -Cnt distance is 2.79 Å while in the latter a shorter distance of 2.47 Å is observed (Cnt = centroid of the central ring). The less complex spectrum ( Figure 3, line b) was used to simulate the hyperfine pattern (Figure 3, line c). The experimentally determined and calculated hyperfine coupling constants (HFCs) indicate delocalization of the unpaired electron over the whole dianion (see spin density plot in Figure 3). These data and the corresponding g-factor of 2.0064 are in fairly good agreement with the ones of the reported sodium analogue Na2trop [24] more than 50 years ago (g = 2.0027 and HFCs shown in Figure 3). The 1 H NMR spectra of the contact ion pairs M2dbcot (M = Na, K) show, in comparison to neutral dbcot, which is best described as a cyclic conjugated polyolefin, a diatopic ring current which leads to significantly de-shielded olefinic resonances, 1 Hol > 7 ppm (Table 1). Table 5 lists the calculated nuclear independent chemical shifts in the center of the rings, NICS(0), and 1 Å above and below the central plane of these, NICS(1) (DFT, PBE [38,39] 6-311+G(df, pd) [40]). The data for both six-membered rings A and C and the central seven-membered ring B is given. For comparison, also the data for the tropylium cation trop + are listed. As expected for an aromatic molecule with a 14 π-electron configuration, the NICSs data are strongly negative for all rings in trop + . On the other end of the scale, cyclobutadiene (CBD) is listed for comparison which is considered to be an archetypical anti-aromatic molecule with a 4 The 1 H NMR spectra of the contact ion pairs M 2 dbcot (M = Na, K) show, in comparison to neutral dbcot, which is best described as a cyclic conjugated polyolefin, a diatopic ring current which leads to significantly de-shielded olefinic resonances, 1 H ol > 7 ppm (Table 1). Table 5 lists the calculated nuclear independent chemical shifts in the center of the rings, NICS(0), and 1 Å above and below the central plane of these, NICS(1) (DFT, PBE [38] 6-311+G(df, pd) [39]). The data for both six-membered rings A and C and the central seven-membered ring B is given. For comparison, also the data for the tropylium cation trop + are listed. As expected for an aromatic molecule with a 14 π-electron configuration, the NICSs data are strongly negative for all rings in trop + . On the other end of the scale, cyclobutadiene (CBD) is listed for comparison which is considered to be an archetypical anti-aromatic molecule with a 4  Figures S1-S4)-going from Li to K, the observed bathochromic shift is for example well reproduced by the model compounds. In addition, the overall structural agreement between these model compounds and the experimentally determined structures given in Table 3 is very good (e.g., the calculated distances C5-M are listed in Supplementary  Table S1) and specifically, the decrease of the bent angles in the order Litrop m > Natrop m > Ktrop m is well reproduced. The NICS values of the contact ion pairs are very sensitive to the bending of the central seven-membered ring. The stronger the ring B is bent, as in Litrop m (θ 1 = 26.2 • , θ 2 = 13.5 • ), the smaller the NICS values are and the less pronounced the anti-aromatic character. On the contrary, when the bending becomes small such as in Ktrop m , the NICS values reach the ones of flat trop -. While it may be debated whether CBD is an anti-aromatic compound because of its rectangular distortion [46], Ktrop m can be truly considered as an anti-aromatic species which in the form of K thf trop can be isolated as a substance. This claim is further bolstered by the fact that the averaged experimental C=C bond distances listed in Table 3 do not differ much between trop + and K thf trop and the C=C bond length variation ∆C=C is modest (0.028 Å in trop + , vs. 0.049 Å in K thf trop), indicating only small structural distortions. This is in contrast to CBD, where this variation is significant (C-C 1.576 Å, C=C 1.332 Å, ∆C = C = 0.244 Å) [46]. CBD is an anti-aromatic compound because of its rectangular distortion [47], Ktrop m can be truly considered as an anti-aromatic species which in the form of K thf trop can be isolated as a substance. This claim is further bolstered by the fact that the averaged experimental C=C bond distances listed in Table 3 do not differ much between trop + and K thf trop and the C=C bond length variation ΔC=C is modest (0.028 Å in trop + , vs. 0.049 Å in K thf trop), indicating only small structural distortions. This is in contrast to CBD, where this variation is significant (C-C 1.576 Å, C=C 1.332 Å, ∆C=C = 0.244 Å) [47]. As already stated above, the tropanion as a ligand in a transition metal complex behaves very differently. The NICS data of the annulated benzo rings A and C in [Rh(trop)(cod)] are negative and indicate modest magnetic aromatic character. CBD is an anti-aromatic compound because of its rectangular distortion [47], Ktrop m can be truly considered as an anti-aromatic species which in the form of K thf trop can be isolated as a substance. This claim is further bolstered by the fact that the averaged experimental C=C bond distances listed in Table 3 do not differ much between trop + and K thf trop and the C=C bond length variation ΔC=C is modest (0.028 Å in trop + , vs. 0.049 Å in K thf trop), indicating only small structural distortions. This is in contrast to CBD, where this variation is significant (C-C 1.576 Å, C=C 1.332 Å, ∆C=C = 0.244 Å) [47].  in contrast to CBD, where this variation is significant (C-C 1.576 Å, C=C 1.332 Å, ∆C=C = 0.244 Å) [47]. As already stated above, the tropanion as a ligand in a transition metal complex behaves very differently. The NICS data of the annulated benzo rings A and C in [Rh(trop)(cod)] are negative and indicate modest magnetic aromatic character.

Evaluation of Anti-Aromaticity and Aromaticity by Calculation of NICS Values
The NICS data for the ion triple containing paramagnetic dianion radical trop 2-• are much smaller than for the mono-anions and indicate diminished anti-aromatic character in accord with conclusions derived from a comparison of the experimental structural data (the C=C bond lengths variation is slightly diminished in the less anti-aromatic dianion radical, see Table 3).
For completeness, the NICS(0) and NICS(1) data for the free dianion dbcot 2-and the model compounds M2dbcot m (M = Li, Na, K) are listed in Table 6. As expected, all rings A, B, and C show negative NICS values indicating aromatic character. Note that the structural parameters and the UV/Vis spectra of the ion triples are accurately reproduced by DFT-calculations ( Supplementary  Figures S5-S7). in contrast to CBD, where this variation is significant (C-C 1.576 Å, C=C 1.332 Å, ∆C=C = 0.244 Å) [47]. As already stated above, the tropanion as a ligand in a transition metal complex behaves very differently. The NICS data of the annulated benzo rings A and C in [Rh(trop)(cod)] are negative and indicate modest magnetic aromatic character.
The NICS data for the ion triple containing paramagnetic dianion radical trop 2-• are much smaller than for the mono-anions and indicate diminished anti-aromatic character in accord with conclusions derived from a comparison of the experimental structural data (the C=C bond lengths variation is slightly diminished in the less anti-aromatic dianion radical, see Table 3).
For completeness, the NICS(0) and NICS(1) data for the free dianion dbcot 2-and the model compounds M2dbcot m (M = Li, Na, K) are listed in Table 6. As expected, all rings A, B, and C show negative NICS values indicating aromatic character. Note that the structural parameters and the UV/Vis spectra of the ion triples are accurately reproduced by DFT-calculations ( Supplementary  Figures S5-S7).  As already stated above, the tropanion as a ligand in a transition metal complex behaves very differently. The NICS data of the annulated benzo rings A and C in [Rh(trop)(cod)] are negative and indicate modest magnetic aromatic character.
The NICS data for the ion triple containing paramagnetic dianion radical trop 2-• are much smaller than for the mono-anions and indicate diminished anti-aromatic character in accord with conclusions derived from a comparison of the experimental structural data (the C=C bond lengths variation is slightly diminished in the less anti-aromatic dianion radical, see Table 3).
For completeness, the NICS(0) and NICS(1) data for the free dianion dbcot 2and the model compounds M 2 dbcot m (M = Li, Na, K) are listed in Table 6. As expected, all rings A, B, and C show negative NICS values indicating aromatic character. Note that the structural parameters and the UV/Vis spectra of the ion triples are accurately reproduced by DFT-calculations ( Supplementary Figures S5-S7).

Electrochemistry
The electrochemical properties of trop2, Ktrop and dbcot were investigated to provide some understanding of the reactivity, especially with respect to mutual electron exchange processes. As previously reported, the cyclic voltammogram of dbcot shows two close lying reduction events at E° = −2.24 V and E° = −2.33 V vs. Fc/Fc + , resulting from two consecutive one-electron transfer steps to give a planar dbcot 2-dianion (Fc = ferrocene, Fc + = ferrocenium) [28,29]. The dbcot -• anion radical was detected by EPR at low temperatures but cannot be isolated. The formation of the trop 2-• dianion radical using Ktrop as starting material was investigated electrochemically and the cyclic

Electrochemistry
The electrochemical properties of trop2, Ktrop and dbcot were investigated to provide some understanding of the reactivity, especially with respect to mutual electron exchange processes. As previously reported, the cyclic voltammogram of dbcot shows two close lying reduction events at E° = −2.24 V and E° = −2.33 V vs. Fc/Fc + , resulting from two consecutive one-electron transfer steps to give a planar dbcot 2-dianion (Fc = ferrocene, Fc + = ferrocenium) [28,29]. The dbcot -• anion radical was detected by EPR at low temperatures but cannot be isolated. The formation of the trop 2-• dianion radical using Ktrop as starting material was investigated electrochemically and the cyclic

Electrochemistry
The electrochemical properties of trop 2 , Ktrop and dbcot were investigated to provide some understanding of the reactivity, especially with respect to mutual electron exchange processes. As previously reported, the cyclic voltammogram of dbcot shows two close lying reduction events at E • = −2.24 V and E • = −2.33 V vs. Fc/Fc + , resulting from two consecutive one-electron transfer steps to give a planar dbcot 2− dianion (Fc = ferrocene, Fc + = ferrocenium) [28,29]. The dbcot -• anion radical was detected by EPR at low temperatures but cannot be isolated. The formation of the trop 2−• dianion radical using Ktrop as starting material was investigated electrochemically and the cyclic voltammogram in a 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) solution in THF is presented in Figure 4. It shows two well-defined waves at −3.04 and −3.37 V vs. Fc/Fc + . The redox process at −3.04 V is fully reversible, as indicated by the linear dependence of the anodic and cathodic peak heights as function of ν 1/2 (Supplementary Figure S8). This reduction was determined to be a one-electron reduction and attributed to the reduction of tropto the trop 2−• dianion radical. The second process at −3.37 V is irreversible and no anodic wave is recorded upon reversal of the potential scan" even at high scan rates (this could correspond to the formation of the trianion which was reported for [C 7 H 7 ] 3- [47]). The dependence of the cathodic peak current height of this process with ν 1/2 is below the theoretical curve for a one electron Nernstian reaction (Supplementary Figure S9). In addition, the cathodic peak current decreases upon multiple cycling, while a new oxidation wave at -0.72 V appears, increasing upon multiple cycling (Supplementary Figure S10). This behavior is expected for a redox process coupled to a chemical reaction (EC mechanism) resulting in the deposition of the electrochemically generated species on the electrode. Indeed, a blue film formed after electrolysis on the electrode surface. The identity of that deposit could so far not be determined. The neutral hydrocarbon trop 2 shows a comparable behavior. Two consecutive redox waves at -2.86 and -3.29 V vs. Fc/Fc + (Figure 4) are observed in its cyclic voltammogram. The first occurs at a significantly more anodic potential than for the potassium salt Ktrop (∆E = 180 mV). The redox process at −2.86 V is fully reversible whereas the second redox wave at −3.29 V is irreversible. Chronoamperometry and linear sweep voltammetry data indicate that the first reduction process at −2.86 V is a 1 ereduction (see Supplementary for details, Figures S11-S12 and Table S2). In contrast to the reduction of trop -, the variation of the cathodic peak current height of this process with ν 1/2 follows the theoretical curve for a two-electron Nernstian reaction at low scan rates and a one-electron Nernstian reaction at high scan rates, suggesting an electron transfer-chemical reaction-electron transfer (ECE) mechanism (Supplementary Figure S9). The reversibility of the first reduction process at −2.86 V suggests therefore that the singly reduced radical trop 2 -• has a certain lifetime under electrochemical conditions. The ECE mechanism observed for the second reduction to the dianion trop 2 2suggests the cleavage of the C-C bond of trop 2 2-, forming two equivalents of the tropmono-anion, which are further reduced at these potentials to give the dianion radical trop 2−• . Hence, the electrochemical behavior of trop 2 can be summarized with the steps (1)-(4) indicated below: 2 trop -+ 2 e -→ 2 trop 2-• (4) Interestingly, cyclic voltammetry studies of trop2 in the presence of 10 equivalents of KPF6 showed anodically shifted redox waves, indicating that potassium ions contribute in stabilizing the trop anions and in facilitating the C-C bond cleavage of trop2 (Supplementary Figures S13-S14).
The electrochemical behavior of trop2 and Ktrop is reflected in the chemical reactivity of these species. First, the neutral hydrocarbon trop2 is cleanly converted with KC8 under cleavage of the central C-C bond to give Ktrop (step (a) in Scheme 3). This reduction is reversible and addition of the ferrocenium salt FcPF6 oxidizes Ktrop to reform trop2 (step (b) in Scheme 3). As already shown in Scheme 2, Ktrop is further reduced to K2trop with potassium graphite (see (c) in Scheme 2). The antiaromatic radical dianion salt K2trop is a strong reductant (E° = -3.04 V). Addition of half an equivalent trop2 oxidizes K2trop to give the mono-anion Ktrop, although heating to 60 °C is required ((d) in Scheme 2). Upon addition of half an equivalent of dbcot to K2trop, the former is fully reduced to the aromatic dianion K2dbcot ((e) in Scheme 2). When one equivalent of dbcot is used in the oxidation of the dianion radical trop 2-• , a mixture of K2dbcot (0.65 equivalents), aside 0.64 equiv. Ktrop and 0.16 equiv. of trop2 is obtained according to 1 H NMR spectroscopy. A featureless EPR signal (g = 2.00485) suggests that also K + [dbcot -• ] may be present [28,30,49]. Considering the stoichiometry of the reaction, we suspect that K + [dbcot -• ] (≈0.06 equiv.) is in a rapid equilibrium with the remaining neutral dbcot molecules (≈0.3 equiv.) which we could not detect by 1 H NMR spectroscopy. We also reacted two equivalents of the anti-aromatic When one equivalent of dbcot is used in the oxidation of the dianion radical trop 2-• , a mixture of K 2 dbcot (0.65 equivalents), aside 0.64 equiv. Ktrop and 0.16 equiv. of trop 2 is obtained according to 1 H NMR spectroscopy. A featureless EPR signal (g = 2.00485) suggests that also K + [dbcot -• ] may be present [28,30,49]. Considering the stoichiometry of the reaction, we suspect that K + [dbcot -• ] (≈0.06 equiv.) is in a rapid equilibrium with the remaining neutral dbcot molecules (≈0.3 equiv.) which we could not detect by 1 H NMR spectroscopy. We also reacted two equivalents of the anti-aromatic mono-anionic ion pair Ktrop with dbcot. Also, in this reaction, a product mixture consisting of K 2 dbcot (0.5 equiv.), unreacted Ktrop (0.9 equiv.) and trop 2 (0.5 equiv.) is obtained. EPR spectroscopy indicates that, very likely, K + [dbcot -• ] (g = 2.00565) is formed in this reaction as well (Supplementary Scheme S1).
Finally, the oxidation of the dianion salts K 2 trop and K 2 dbcot with dry oxygen was investigated at low temperature. Remarkably, the oxidation of the paramagnetic K 2 trop gives the hydrocarbon C 15 H 12 (suberene = tropH) as a major product (95%) and not as expected Ktrop or trop 2 , which is formed in only 5% yield ((f) in Scheme 3). On contrast, the reaction between oxygen and K 2 dbcot, which contains the diamagnetic and aromatic dbcot 2dianion, is very clean and gives the neutral hydrocarbon dbcot in almost quantitative yield ((g) in Scheme 3).

General Comments
All experiments were performed under an argon atmosphere using standard Schlenk and vacuum-line techniques or in a MBraun inert-atmosphere drybox (argon atmosphere). All reagents were used as received from commercial suppliers unless otherwise stated. The following compounds were synthesized according to literature procedure: KC 8 [50], dbcot [32] and [Rh 2 Cl 2 (cod) 2 ] [51]. THF, DME, diethyl ether, toluene and n-hexane were purified using an Innovative Technologies PureSolv system and stored over 4 Å molecular sieves. THF-d 8 , C 6 D 6 were distilled from sodium benzophenone ketyl. CDCl 3 was distilled from CaH 2 . Solution NMR spectra were recorded on Bruker Avance 500, 400, 300, 250 and 200 MHz spectrometers. The chemical shifts (δ) are expressed in ppm relative to SiMe 4 for 1 H and 13 C respectively. Coupling constants (J) are given in Hertz (Hz) as absolute values. The multiplicity of the signals is indicated as s, d, t, q, or m for singlets, doublets, triplets, quartets, or multiplets. If the data allowed it, the assignment was based on the IUPAC recommendations for fused polycyclic hydrocarbons (Scheme 4) [52]. Further abbreviations were used in the assignment: br for broadened signals, Ar for aromatic signals, quart for quaternary 13 C signals. EPR spectra were recorded by Dr. Reinhard Kissner on an X-band (9.50 GHz) Magnettech Miniscope 5000 EPR spectrometer with liquid nitrogen cooling. EPR spectra were simulated with Matlab R2016b, using EasySpin-5.2.11 package. UV/Vis spectra were recorded on a UV/Vis/NIR Lambda-19 spectrometer in a cell with a 2 mm path length. IR spectra were collected on a PerkinElmer Spectrum 2000 FT-IR-Raman spectrometer. Absorption bands are described as w, m or s for weak, medium or strong. Elemental analyses were performed by Peter Kälin in the Mikrolabor of the ETH Zürich. Melting points were determined with a Büchi melting-point apparatus and are not corrected. X-ray diffraction was performed at 100 K on an Oxford Xcalibur or Venture diffractometer with a CCD area detector and a molybdenum X-ray tube (0.71073 A). Using Olex 2 [53], the structure was solved by direct methods (SHELXS [54] or SHELXT [55]), followed by least-squares refinement against full matrix (versus F 2 ) with SHELXL [54]. All non-hydrogen atoms were refined anisotropically. The contribution of some hydrogen atoms, in their calculated positions, was included in the refinement using a riding model. Scheme 4. The assignment in the NMR spectra are based on these numbering schemes. EPR spectra were recorded by Dr. Reinhard Kissner on an X-band (9.50 GHz) Magnettech Miniscope 5000 EPR spectrometer with liquid nitrogen cooling. EPR spectra were simulated with Matlab R2016b, using EasySpin-5.2.11 package. UV/Vis spectra were recorded on a UV/Vis/NIR Lambda-19 spectrometer in a cell with a 2 mm path length. IR spectra were collected on a PerkinElmer Spectrum 2000 FT-IR-Raman spectrometer. Absorption bands are described as w, m or s for weak, medium or strong. Elemental analyses were performed by Peter Kälin in the Mikrolabor of the ETH Zürich. Melting points were determined with a Büchi melting-point apparatus and are not corrected. X-ray diffraction was performed at 100 K on an Oxford Xcalibur or Venture diffractometer with a CCD area detector and a molybdenum X-ray tube (0.71073 A). Using Olex 2 [53], the structure was solved by direct methods (SHELXS [54] or SHELXT [55]), followed by least-squares refinement against full matrix (versus F 2 ) with SHELXL [54]. All non-hydrogen atoms were refined anisotropically. The contribution of some hydrogen atoms, in their calculated positions, was included in the refinement using a riding model. Litrop: Method A: Lithium sand (0.348 g, 50.20 mmol, 2.3 equivalents) was suspended in dry tetrahydrofuran (10 mL) and placed in a cooling bath (propan-2-ol and dry ice). 5-Chloro-5H-dibenzo[a,d]cyclo-heptene (5.00 g, 22.06 mmol, 1.0 equivalent) was dissolved in dry tetrahydrofuran (30 mL). The resulting light yellow solution was added dropwise (during 25 min) to the brown suspension. The reaction mixture was allowed to warm to a.T. and a color change to dark red was observed under heat development. After two hours, the dark red liquid was concentrated to approximately 25 mL and filtered over a plug of celite. The resulting red solution was layered with dry n-hexane (71 mL) and stored in the freezer (−23 • C, 6 days). A first crop of green, crystalline needles was obtained (0.479 g, 1.26 mmol, yield: 2.5%; according to 1 H NMR spectroscopy, the compound contained 2.5 equivalents of THF). The supernatant was layered with dry n-hexane (30 mL) and stored in the freezer (−23 • C, 1 day) and a second crop was obtained (7.448 g, 19.68 mmol, 1 yield: 94.0%, overall yield: 95.0%).

Conclusions
A desilylation reaction using silyl trop derivative which proceeds by addition of KOtBu allowed the very clean synthesis of larger amounts of the salts of the trop anion, trop -, which contain alkali metal counter cations M + = Li, Na, K. With these at hand, an in-depth study on the electronic configuration of these conjugated hydrocarbons exclusively composed from sp 2 -valence electron hybridized carbon centers could be performed. As the Hückel counting rules imply, the 16π-electron configuration of troppredicts this to be anti-aromatic. This is indeed the case as NMR data and as well as calculated NICS data clearly show. A number of contact ion pairs, Mtrop, could be structurally characterized by X-ray diffraction methods. In these, ethereal molecules complete the coordination sphere around the M + cation. From tetrahydrofuran (THF), close ion pairs are obtained which show an intimate contact between cation and tropanion. In these, the tropanion is bent and its anti-aromaticity lowered. On the contrast, crystallization from solvents, which have a higher solvation energy for alkali cations, lead to separated ion pairs in which the tropanion is flatter and consequently, more anti-aromatic. Moreover, Li + shows a higher tendency to form contact ion pairs than the larger potassium cation with a significantly reduced charge density. This culminates in the isolation of [K(thf) 2 ][trop] ∞ , which forms a one-dimensional coordination polymer in the solid state with long K + tropdistances. This compound contains a flat tropanion which shows very little variation of the C=C bond lengths, strongly shielded NMR signals, strongly positive NICS values and as such, fulfils all formal criteria requested for an anti-aromatic compound. In this light, the ease of synthesis from trop silyl ethers and alcoholates is somewhat surprising, indicating that Mtrop salts are remarkably stable. The tropmonoanion can be further reduced to give salts M 2 trop with a paramagnetic dianion radical, trop 2-• , which was isolated and fully characterized as K 2 trop. In this species, the anti-aromaticity is reduced compared to the mono-anion. In a reaction between a Rh(I) halide complex and trop -, the complex [Rh(trop)(cod)] was prepared. Not unexpected, this pure organometallic 18 electron complex shows a very different structure in which the trop unit-especially its annulated benzo groups-is aromatized. This reaction also shows that trop anions may have potential as ligands for transition metal complexes and as a building block for main group element compounds. It remains to be seen in how far trop-type anions can be used as electron carriers related to the well-established use of arenes for that purpose [15].
Supplementary Materials: The following are available online: 1. DFT-Calculations; Table S1: Selected bond lengths [Å] for the calculated trop mono-and dianions; Table S2: Key parameters for the determination of the number of electrons; Figure S1