catalysts Pentamethylcyclopentadienyl Molybdenum(V) Complexes Derived from Iodoanilines: Synthesis, Structure, and ROP of ε -Caprolactone

: The reaction of [Mo( η -C 5 Me 5 )Cl 4 ] with the ortho -, meta -, or para -iodo-functionalized anilines 2-IC 6 H 4 NH 2 , 3-IC 6 H 4 NH 2 , 4-IC 6 H 4 NH 2 yields imido or amine products of the type [Mo( η C 5 Me 5 )Cl 2 (IC 6 H 4 N)] (2-I, 1 , 3-I, 3 , 4-I, 5 ) or [Mo( η -C 5 Me 5 )Cl 4 (IC 6 H 4 NH 2 )] (3-I, 2 , 4-I, 4 ), respectively, depending on the reaction stoichiometry/conditions; we were unable to isolate an amine complex of the 2-I derivative. The reaction of [Mo( η -C 5 Me 5 )Cl 4 ] with one equivalent of 2-I,4-FC 6 H 3 NH 2 in the presence of Et 3 N afforded [Mo( η -C 5 Me 5 )Cl 2 (2-I,4-FC 6 H 3 N)] · MeCN ( 6 · MeCN), which, upon exposure to air, afforded the Mo(VI) imido complex [Mo( η -C 5 Me 5 )Cl 3 (2-I,4-FC 6 H 3 N)] ( 7 ). For comparative studies, the structure of the aniline (C 6 H 5 NH 2 )-derived complex [Mo( η -C 5 Me 5 )Cl 2 (2-C 6 H 3 N)] ( 8 ) has also been prepared. The molecular structures of 1 – 8 have been determined and reveal packing in the form of zig-zag chains or ladders. The complexes catalyze, in the presence of benzyl alcohol under N 2 , the ring-opening polymerization (ROP) of ε -caprolactone affording relatively low molecular weight products. The MALDI-ToF spectra indicate that a number of polymer series bearing a variety of end groups are formed. Conducting the ROPs as melts or under air results in the isolation of higher molecular weight products, again bearing a variety of end groups. Kinetic studies reveal the aniline-derived imido complex 8 performs best, whilst a meta -iodo substituent and a Mo(V) centre are also found to be beneﬁcial. The structures of the side products 2-IC 6 H 4 NH 3 Cl and 3-IC 6 H 4 NH 3 Cl are also reported.


Introduction
Molybdenum, and to a lesser extent tungsten, organoimido chemistry has been of interest for a number of decades now, given its relevance to a number of areas, particularly olefin metathesis [1]. Although the literature on imido-containing compounds is vast, reports concerning the synthesis of functionalized imido complexes are less widespread [2]. We have previously investigated the chemistry of [W(η-C 5 Me 5 )Cl 4 ] as an entry point to new half-sandwich species and have reported the structures of a number of products, including the diamido complex {W(η-C 5 Me 5 )Cl 2 [1,2-(HN) 2 C 6 H 4 ]} [3], as well as imido, hydrazido, amino acid derived chelates [4], and functionalized 6,12-epiiminodibenzo[b,f][1.5]diazocine ligands [5]. The complex [Mo(η-C 5 Me 5 )Cl 4 ] is prepared by a similar procedure to its tungsten analogue [6], and its chemistry is similarly relatively unexplored [7][8][9][10][11][12]. We and others have also been exploring the potential of molybdenum-based complexes as catalysts for the ring opening polymerization (ROP) of cyclic esters [13][14][15][16][17][18][19][20][21][22]. We were attracted to the use of iodo-substituted organomido groups as a stabilizing motif given their tendency to be involved in intermolecular bonding in the solid state. Such interactions are useful for crystal engineering, halogen-catalyzed reactions, and even in drug design [23][24][25]. Furthermore, early transition metal imido complexes are known to be quite reactive [26].

2-Iodo Complexes
The interaction of [Mo(η-C5Me5)Cl4] with two equivalents of 2-iodoaniline, 2-IC6H4NH2 in refluxing toluene afforded, following work-up in acetonitrile, the imido complex [Mo(η-C5Me5)Cl2(2-IC6H4N)] (1) in good yield (ca. 76%). Small, single crystals, suitable for an X-ray structure determination using synchrotron radiation, were obtained on prolonged standing (2-3 days) at ambient temperature. The molecular structure is shown in Figure 1, with selected bond lengths and angles given in the caption. The asymmetric unit contains one molecule of the molybdenum complex. The geometry of the molecule is a three-legged piano stool, typical of such organometal imido dichlorides [11]. The distance from the Cp* ring centroid to Mo(1) is 2.045(3) Å , whilst C(1) > C(5) are co-planar within 0.021 Å . Meanwhile, the methyl groups all tilt away from the metal, and of these, the most marked is that for C(7), which is under N(1), and C(10) under Cl (1). Bonds Mo(1)-C(4) and Mo(1)-C (5) trans to N are longer than those trans to the Cl ligands. The ring slippage can be measured by the τ value [3], which, here, is 3.5°. The organoimido ligand is somewhat bent at 159.8(4)°, but is still considered linear, albeit at the lower end [27].
In the packing of 1, there are some weak I(1)•••Cl (2) halogen bond interactions at 3.738 Å which result in zig-zag chains parallel to b (Figure 2). For alternative views of 1, see Figure S1.
The side product in this reaction is the salt [2-I-NH3C6H4] + Cl − . It, too, has an interesting structure, involving a number of intermolecular interactions. In the asymmetric unit, there is one cation/anion pair, in which all three of the NH protons are involved in strong H-bonds to the chloride anion. Unlike in [2-I-NH3C6H4] + Cl − vide infra, there are no I•••I halogen-halogen interactions, instead there are Cl•••I interactions at 3.306 Å . There are no π•••π interactions; at best the molecules are highly slipped, forming a layered structure Scheme 1. Complexes 1-8 prepared herein.

2-Iodo Complexes
The interaction of [Mo(η-C 5 Me 5 )Cl 4 ] with two equivalents of 2-iodoaniline, 2-IC 6 H 4 NH 2 in refluxing toluene afforded, following work-up in acetonitrile, the imido complex [Mo(η-C 5 Me 5 )Cl 2 (2-IC 6 H 4 N)] (1) in good yield (ca. 76%). Small, single crystals, suitable for an X-ray structure determination using synchrotron radiation, were obtained on prolonged standing (2-3 days) at ambient temperature. The molecular structure is shown in Figure 1, with selected bond lengths and angles given in the caption. The asymmetric unit contains one molecule of the molybdenum complex. The geometry of the molecule is a three-legged piano stool, typical of such organometal imido dichlorides [11]. The distance from the Cp* ring centroid to Mo(1) is 2.045(3) Å, whilst C(1) > C(5) are co-planar within 0.021 Å. Meanwhile, the methyl groups all tilt away from the metal, and of these, the most marked is that for C(7), which is under N(1), and C(10) under Cl (1). Bonds Mo(1)-C(4) and Mo(1)-C (5) trans to N are longer than those trans to the Cl ligands. The ring slippage can be measured by the τ value [3], which, here, is 3.5 • . The organoimido ligand is somewhat bent at 159.8(4) • , but is still considered linear, albeit at the lower end [27].
In the packing of 1, there are some weak I(1)···Cl (2) halogen bond interactions at 3.738 Å which result in zig-zag chains parallel to b (Figure 2). For alternative views of 1, see Figure S1.
The side product in this reaction is the salt [2-I-NH 3 C 6 H 4 ] + Cl − . It, too, has an interesting structure, involving a number of intermolecular interactions. In the asymmetric unit, there is one cation/anion pair, in which all three of the NH protons are involved in strong H-bonds to the chloride anion. Unlike in [2-I-NH 3 C 6 H 4 ] + Cl − vide infra, there are no I···I halogen-halogen interactions, instead there are Cl···I interactions at 3.306 Å. There are no π···π interactions; at best the molecules are highly slipped, forming a layered structure with alternating hydrophilic ionic and hydrophobic aromatic layers. The ionic layers are in the b/c plane, see Figure S2.

3-Iodo Complexes
Treatment of [Mo(η-C5Me5)Cl4] with one equivalent of the meta aniline 3-IC6H4NH2 at ambient temperature led, following work-up, to isolation of the amine complex [Mo(η-C5Me5)Cl4(3-IC6H4NH2)]•MeCN (2•MeCN). The IR spectrum of 2 contains two sharp (weak) stretches in the N-H region, at 3415 and 3328 cm −1 , characteristic of the NH2 group. As for 1 and 3 (see below), the 1 H NMR spectrum is broad and uninformative consistent with the presence of the paramagnetic Mo(V) centre. The molecular structure of 2•MeCN is shown in Figure 3, with selected bond lengths and angles given in the caption. The asymmetric unit contains one molecule of the molybdenum complex and a solvent (MeCN) molecule of crystallisation. The geometry at the metal is best described as distorted octahedral with the Mo ion 0.5783(18) Å out of the Cl4 plane. The Mo to Cp* ring centroid distance is 2.078(6) Å and all the methyl groups bend substantially away from the C5 aromatic ring, by 0.14-0.22(2) Å ; the τ value is 3.3°. In contrast to 1, here the anilinederived ligand maintains the amine group, hence the elongated Mo-N bond length at 2.322(10) Å .  with alternating hydrophilic ionic and hydrophobic aromatic layers. The ionic layers are in the b/c plane, see Figure S2.

3-Iodo Complexes
Treatment of [Mo(η-C5Me5)Cl4] with one equivalent of the meta aniline 3-IC6H4NH2 at ambient temperature led, following work-up, to isolation of the amine complex [Mo(η-C5Me5)Cl4(3-IC6H4NH2)]•MeCN (2•MeCN). The IR spectrum of 2 contains two sharp (weak) stretches in the N-H region, at 3415 and 3328 cm −1 , characteristic of the NH2 group. As for 1 and 3 (see below), the 1 H NMR spectrum is broad and uninformative consistent with the presence of the paramagnetic Mo(V) centre. The molecular structure of 2•MeCN is shown in Figure 3, with selected bond lengths and angles given in the caption. The asymmetric unit contains one molecule of the molybdenum complex and a solvent (MeCN) molecule of crystallisation. The geometry at the metal is best described as distorted octahedral with the Mo ion 0.5783(18) Å out of the Cl4 plane. The Mo to Cp* ring centroid distance is 2.078(6) Å and all the methyl groups bend substantially away from the C5 aromatic ring, by 0.14-0.22(2) Å ; the τ value is 3.3°. In contrast to 1, here the anilinederived ligand maintains the amine group, hence the elongated Mo-N bond length at 2.322(10) Å .

3-Iodo Complexes
Treatment of [Mo(η-C 5 Me 5 )Cl 4 ] with one equivalent of the meta aniline 3-IC 6 H 4 NH 2 at ambient temperature led, following work-up, to isolation of the amine complex [Mo(η-C 5 Me 5 )Cl 4 (3-IC 6 H 4 NH 2 )]·MeCN (2·MeCN). The IR spectrum of 2 contains two sharp (weak) stretches in the N-H region, at 3415 and 3328 cm −1 , characteristic of the NH 2 group. As for 1 and 3 (see below), the 1 H NMR spectrum is broad and uninformative consistent with the presence of the paramagnetic Mo(V) centre. The molecular structure of 2·MeCN is shown in Figure 3, with selected bond lengths and angles given in the caption. The asymmetric unit contains one molecule of the molybdenum complex and a solvent (MeCN) molecule of crystallisation. The geometry at the metal is best described as distorted octahedral with the Mo ion 0.5783(18) Å out of the Cl 4 plane. The Mo to Cp* ring centroid distance is 2.078(6) Å and all the methyl groups bend substantially away from the C 5 aromatic ring, by 0.14-0.22(2) Å; the τ value is 3.3 • . In contrast to 1, here the aniline-derived ligand maintains the amine group, hence the elongated Mo-N bond length at 2.322(10) Å. The Mo complex molecules form H-bonded zig-zag ladders in the crystallographic a direction. Each complex forms four strong H-bonds, two as donor and two as acceptor. These are via two independent N-H•••Cl′ H-bonds (see Figure 4). There are also two weaker, supporting, aromatic ortho-C-H•••Cl interactions along the ladders. Otherwise, there are only weak interactions involving C-H hydrogens between ladders. The MeCN solvent molecule of crystallization forms a weak N•••I interaction at 3.22 Å (see Figure S3). As for 1, use of three equivalents under reflux conditions affords an imido complex, namely [Mo(η-C5Me5)Cl2(3-I-NC6H4)] (3), see Figure 5. In the asymmetric unit, there is one molecule of 1. The distance from Mo(1) to the Cp* ring centroid is 2.0233(7) Å . All the methyl groups bend a little away from the metal relative to the C5 ring by between 0.06-0.12 Å , whilst the C(3) & C(4) bond lengths to Mo(1) are the longest because they are trans to N(1) ; the τ value is 3.9°. The Mo complex molecules form H-bonded zig-zag ladders in the crystallographic a direction. Each complex forms four strong H-bonds, two as donor and two as acceptor. These are via two independent N-H···Cl H-bonds (see Figure 4). There are also two weaker, supporting, aromatic ortho-C-H···Cl interactions along the ladders. Otherwise, there are only weak interactions involving C-H hydrogens between ladders. The MeCN solvent molecule of crystallization forms a weak N···I interaction at 3.22 Å (see Figure S3).  The Mo complex molecules form H-bonded zig-zag ladders in the crystallographic a direction. Each complex forms four strong H-bonds, two as donor and two as acceptor. These are via two independent N-H•••Cl′ H-bonds (see Figure 4). There are also two weaker, supporting, aromatic ortho-C-H•••Cl interactions along the ladders. Otherwise, there are only weak interactions involving C-H hydrogens between ladders. The MeCN solvent molecule of crystallization forms a weak N•••I interaction at 3.22 Å (see Figure S3). As for 1, use of three equivalents under reflux conditions affords an imido complex, namely [Mo(η-C5Me5)Cl2(3-I-NC6H4)] (3), see Figure 5. In the asymmetric unit, there is one molecule of 1. The distance from Mo(1) to the Cp* ring centroid is 2.0233(7) Å . All the methyl groups bend a little away from the metal relative to the C5 ring by between 0.06-0.12 Å , whilst the C(3) & C(4) bond lengths to Mo(1) are the longest because they are trans to N(1) ; the τ value is 3.9°. As for 1, use of three equivalents under reflux conditions affords an imido complex, namely [Mo(η-C 5 Me 5 )Cl 2 (3-I-NC 6 H 4 )] (3), see Figure 5. In the asymmetric unit, there is one molecule of 1. The distance from Mo(1) to the Cp* ring centroid is 2.0233(7) Å. All the methyl groups bend a little away from the metal relative to the C 5 ring by between 0.06-0.12 Å, whilst the C(3) & C(4) bond lengths to Mo(1) are the longest because they are trans to N(1); the τ value is 3.9 • .  Figure S5).

4-Iodo Complexes
Extension of this chemistry to the para iodoaniline 4-IC6H4NH2 led, following the conditions used for 2, to isolation of the amine complex [Mo(η-C5Me5)Cl4(4-IC6H4NH2)] (4). The IR spectrum of 4 contains two weak stretches in the N-H region, at 3318 and 3286 cm −1 , characteristic of the NH2 group. Unlike 2, this complex crystallizes without any solvent of crystallization. The molecular structure of 4 is shown in Figure 6, with selected bond lengths and angles given in the caption. The distorted octahedral Mo ion lies 0.5750(5) Å out of the Cl4 plane. The Mo(1) to Cp* ring centroid distance is 2.0826(15) Å (cf 2.078(6) in amine complex 2), and the methyl carbons are pushed between 0.173-0.215(6) Å away from the Cp* ring plane in the direction away from the metal ion; the τ value is 3.3°. As in 2, the aniline-derived ligand is an amine with Mo-N at 2.340(3) Å .
As in 2, molecules form zig-zag ladders via strong N-H•••Cl H-bonds in the b direction, with each molecule forming four such interactions, two as donor and two as acceptor (see Figure 7). The location of the iodine atom in either the meta or para position allows the ladder motif to form. In addition, in 4, there are some weak aromatic C-H•••X interactions either along ladders or between ladders. These involve all four of the hydrogens on the halogenated ring ( Figure S6).
Use of three equivalents of 4-iodoaniline in refluxing toluene led, on work-up, to the imido complex [Mo(η-C5Me5)Cl2(p-NC6H4I)] (5) as dark prisms in ca. 60% isolated yield. In the crystal structure there is one molecule in the asymmetric unit ( Figure 8). The distance from Mo(1) to the Cp* ring plane is 2.034(2) Å , which compares favorably with the other imido complexes 1 (2.045(3) Å ) and 3 (2.0233(7) Å ). The Me groups all point a little away from the metal relative to the Cp* ring plane, with C(12) pushed further away than There are a number of weak (Me)C-H···I/Cl interactions between molecules of 3. The molecules pack in weakly-bound layers in the b/c plane ( Figure S4). The shortest, and only feasible halogen-halogen interaction is Cl(1)···I(1 ) at 3.464 Å.
For the secondary product, namely [3-I-NH 3 C 6 H 4 ] + Cl − , there is one cation/anion pair in the asymmetric unit. Within the salt, the ions form strongly H-bonded stacks/layers via + N-H···Cl − interactions in the a and b directions. Among the NH hydrogen atoms, two form fairly strong, single N-H···Cl H-bonds, while the last is bifurcated to two different Clions, and, hence, all these are notably weaker. Moreover, in the b direction, there are zig-zag I···I interactions at 3.890 Å, and overall a 3D supramolecular network is formed ( Figure S5).

4-Iodo Complexes
Extension of this chemistry to the para iodoaniline 4-IC 6 H 4 NH 2 led, following the conditions used for 2, to isolation of the amine complex [Mo(η-C 5 Me 5 )Cl 4 (4-IC 6 H 4 NH 2 )] (4). The IR spectrum of 4 contains two weak stretches in the N-H region, at 3318 and 3286 cm −1 , characteristic of the NH 2 group. Unlike 2, this complex crystallizes without any solvent of crystallization. The molecular structure of 4 is shown in Figure 6, with selected bond lengths and angles given in the caption. The distorted octahedral Mo ion lies 0.5750(5) Å out of the Cl 4 plane. The Mo(1) to Cp* ring centroid distance is 2.0826(15) Å (cf 2.078(6) in amine complex 2), and the methyl carbons are pushed between 0.173-0.215(6) Å away from the Cp* ring plane in the direction away from the metal ion; the τ value is 3.3 • . As in 2, the aniline-derived ligand is an amine with Mo-N at 2.340(3) Å.
As in 2, molecules form zig-zag ladders via strong N-H···Cl H-bonds in the b direction, with each molecule forming four such interactions, two as donor and two as acceptor (see Figure 7). The location of the iodine atom in either the meta or para position allows the ladder motif to form. In addition, in 4, there are some weak aromatic C-H···X interactions either along ladders or between ladders. These involve all four of the hydrogens on the halogenated ring ( Figure S6). the other four due to the location of the imido ligand; the τ value is 4.8°. The Mo(1)-C(9)/C(10) distances are rather longer than the other three due to the trans influence at N.     the other four due to the location of the imido ligand; the τ value is 4.8°. The Mo(1)-C(9)/C(10) distances are rather longer than the other three due to the trans influence at N.    Use of three equivalents of 4-iodoaniline in refluxing toluene led, on work-up, to the imido complex [Mo(η-C 5 Me 5 )Cl 2 (p-NC 6 H 4 I)] (5) as dark prisms in ca. 60% isolated yield. In the crystal structure there is one molecule in the asymmetric unit ( Figure 8). The distance from Mo(1) to the Cp* ring plane is 2.034(2) Å, which compares favorably with the other imido complexes 1 (2.045(3) Å) and 3 (2.0233(7) Å). The Me groups all point a little away from the metal relative to the Cp* ring plane, with C(12) pushed further away than the other four due to the location of the imido ligand; the τ value is 4.8 • . The Mo(1)-C(9)/C(10) distances are rather longer than the other three due to the trans influence at N.
In the packing of 5, there are halogen bond interactions between I(1) and Cl(2 ) on a neighboring molecule at a distance of 3.431 Å. This gives rise to zig-zag chains propagating in the c direction ( Figure S7).

Use of 2-I,4-FC
Reactions using this aniline proved to be more sensitive than others employed herein. It was found that to avoid oxidation (see 7), it was better to mix [Mo(η-C 5 Me 5 )Cl 4 ] with one equivalent of 2-I,4-FC 6 H 3 NH 2 in the presence of Et 3 N in toluene at ambient temperature. Work-up as before (i.e., extraction into MeCN) afforded orange/brown crystals on standing. The molecular structure of [Mo(η-C 5 Me 5 )Cl 2 (2-I,4-FC 6 H 3 N)]·MeCN (6·MeCN) is shown in Figure 9, with selected bond lengths and angles given in the caption. This is the asymmetric unit. The Mo(1) to ring centroid distance is 2.0375(7) Å. All of the Me groups point away from the Mo(V) ion, with C(15) furthest displaced due to the proximity of the large imido ligand; the τ value is 4.4 • . As seen for the other complexes, C(7) and C(8) are notably further from the Mo than the other three C atoms in the ring due to the trans influence of the N atom. In the packing of 5, there are halogen bond interactions between I(1) and Cl(2′) on a neighboring molecule at a distance of 3.431 Å . This gives rise to zig-zag chains propagating in the c direction ( Figure S7).

Use of 2-I,4-FC6H3NH2
Reactions using this aniline proved to be more sensitive than others employed herein. It was found that to avoid oxidation (see 7), it was better to mix [Mo(η-C5Me5)Cl4] with one equivalent of 2-I,4-FC6H3NH2 in the presence of Et3N in toluene at ambient temperature. Work-up as before (i.e., extraction into MeCN) afforded orange/brown crystals on standing. The molecular structure of [Mo(η-C5Me5)Cl2(2-I,4-FC6H3N)]·MeCN (6·MeCN) is shown in Figure 9, with selected bond lengths and angles given in the caption. This is the asymmetric unit. The Mo(1) to ring centroid distance is 2.0375(7) Å . All of the Me groups point away from the Mo(V) ion, with C(15) furthest displaced due to the proximity of the large imido ligand; the τ value is 4.4°. As seen for the other complexes, C(7) and C(8) are notably further from the Mo than the other three C atoms in the ring due to the trans influence of the N atom.
Molecules of 6·MeCN pack in H-bonded tapes in the b direction. There is an intermolecular I(1)•••Cl(2′) halogen bond with separation 3.486 Å . For an alternative view of 6·MeCN and different views of the packing, see Figure S8. Oxidized product: Consistent use of three equivalents of 2-I,4-FC6H3NH2 afforded, following work up, the diamagnetic complex [Mo(η-C5Me5)Cl3(2-I,4-F-NC6H3)] (7). Presumably, here the complex has been oxidized by adventitious exposure to the atmosphere resulting in the formation of a Mo(VI) centre. We note that the complex [W(η-C5Me5) (NC6F5)Cl3] has been isolated from exposure of [W(η-C5Me5)(NC6F5)Cl2] to air [11]. The molecular structure of 7 is shown in Figure 10, with selected bond lengths and angles given in the caption. The geometry is a four-legged piano stool in which the Cp* centroid lies 2.071(3) Å from Mo(1) and is considerably slipped with a large variation in Mo(1)-C bond lengths from 2.300(6) Å for C(5) to 2.546(7) Å for C(2), which lies trans to N(1), itself having a strong trans influence. All five methyl groups are pushed away from the aromatic C5 plane, with C(7) less affected than the four others. The displacements away from the C5 plane (Å ) are 0.166(12) C(6), 0.052(12) C(7), 0.185(12) C(8), 0.139(12) C(9), 0.176(12) C(10). There is a degree of variation, i.e., localization in the C5 C-C distances with C(1)-C(5) and C(3)-C(4), Molecules of 6·MeCN pack in H-bonded tapes in the b direction. There is an intermolecular I(1)···Cl(2 ) halogen bond with separation 3.486 Å. For an alternative view of 6·MeCN and different views of the packing, see Figure S8.
Oxidized product: Consistent use of three equivalents of 2-I,4-FC 6 H 3 NH 2 afforded, following work up, the diamagnetic complex [Mo(η-C 5 Me 5 )Cl 3 (2-I,4-F-NC 6 H 3 )] (7). Presumably, here the complex has been oxidized by adventitious exposure to the atmosphere resulting in the formation of a Mo(VI) centre. We note that the complex [W(η-C 5 Me 5 ) (NC 6 F 5 )Cl 3 ] has been isolated from exposure of [W(η-C 5 Me 5 )(NC 6 F 5 )Cl 2 ] to air [11]. The molecular structure of 7 is shown in Figure 10, with selected bond lengths and angles given in the caption. The geometry is a four-legged piano stool in which the Cp* centroid lies 2.071(3) Å from Mo(1) and is considerably slipped with a large variation in Mo(1)-C bond lengths from 2.300(6) Å for C(5) to 2.546(7) Å for C (2), which lies trans to N(1), itself having a strong trans influence. All five methyl groups are pushed away from the aromatic C 5 plane, with C(7) less affected than the four others. The displacements away from the C 5 plane (Å) are 0.166(12) C(6), 0.052(12) C(7), 0.185(12) C(8), 0.139(12) C(9), 0.176(12) C(10). There is a degree of variation, i.e., localization in the C 5 C-C distances with C(1)-C(5) and C(3)-C(4), being longer at ca. 1 In the packing ( Figure S9), there are a number of weak C-H•••Cl interactions. The F•••I distance at 3.390 Å suggests weak halogen bonding. The molecules pack in layers with Cp*Mo units together and the halogen-bonded imido ligands together.

Use of Aniline
For comparative catalytic studies, we also prepared the complex [Mo(η-C5Me5)Cl2(NC6H5)] via the use of three equivalents of the parent aniline. As for the other derivatives isolated above, single crystals suitable for an X-ray diffraction study were grown from a saturated solution of acetonitrile. There is one molecule of the complex in the asymmetric unit, which adopts a piano-stool conformation ( Figure 11). The Cp* ligand is disordered over two sets of positions related by a ca. 22° rotation. The Mo(1) to Cp* ring plane distance is 2.013(8) Å for the major component and 2.058(17) Å for the minor component. For the major component, all the Cp* Me groups are bent somewhat away from the ring plane relative to the metal, but C(14) is notably more displaced away than the other four, presumably due to the proximity of the imido ligand 'below'. The statistics are less reliable for the minor component. The bond lengths Mo(1)-C(11)/C(11X)/C(7X) are notably longer than the other Mo-C distances, being positioned trans to the imido nitrogen N; the τ values are 3.2° and 3.6° for the major and minor components, respectively. The packing of 8 is shown in Figure S10.  In the packing ( Figure S9), there are a number of weak C-H···Cl interactions. The F···I distance at 3.390 Å suggests weak halogen bonding. The molecules pack in layers with Cp*Mo units together and the halogen-bonded imido ligands together.

Use of Aniline
For comparative catalytic studies, we also prepared the complex [Mo(η-C 5 Me 5 )Cl 2 (NC 6 H 5 )] via the use of three equivalents of the parent aniline. As for the other derivatives isolated above, single crystals suitable for an X-ray diffraction study were grown from a saturated solution of acetonitrile. There is one molecule of the complex in the asymmetric unit, which adopts a piano-stool conformation ( Figure 11). The Cp* ligand is disordered over two sets of positions related by a ca. 22 • rotation. The Mo(1) to Cp* ring plane distance is 2.013(8) Å for the major component and 2.058(17) Å for the minor component. For the major component, all the Cp* Me groups are bent somewhat away from the ring plane relative to the metal, but C(14) is notably more displaced away than the other four, presumably due to the proximity of the imido ligand 'below'. The statistics are less reliable for the minor component. The bond lengths Mo(1)-C(11)/C(11X)/C(7X) are notably longer than the other Mo-C distances, being positioned trans to the imido nitrogen N; the τ values are 3.2 • and 3.6 • for the major and minor components, respectively. The packing of 8 is shown in Figure S10. ponent. For the major component, all the Cp* Me groups are bent somewhat away from the ring plane relative to the metal, but C(14) is notably more displaced away than the other four, presumably due to the proximity of the imido ligand 'below'. The statistics are less reliable for the minor component. The bond lengths Mo(1)-C(11)/C(11X)/C(7X) are notably longer than the other Mo-C distances, being positioned trans to the imido nitrogen N; the τ values are 3.2° and 3.6° for the major and minor components, respectively. The packing of 8 is shown in Figure S10.

Ring Opening Polymerization (ROP) Studies
Based on our previous molybdenum ROP studies [20][21][22], we selected the conditions of 130 • C with a ratio of ε-CL to complex of 500:1 in the presence of one equivalent of benzyl alcohol over 24 h under N 2 . Data for the runs are presented in Table 1, and it can be seen that at ambient temperature, low molecular weight oily products are formed with good control (PDI < 1.25). End group analysis by 1 H NMR spectroscopy (e.g., Figure S11 for entry 7, Table 1) is consistent with the presence of a BnO end group, which indicates that the polymerization proceeds through a coordination-insertion mechanism. Interestingly, despite the narrow PDI values, MALDI-TOF spectra revealed at least five series of ions corresponding to sodiated PCL. For example, in Figure 12 (using PCL from entry 2, Table 1; for the full spectrum, see Figure S12), for each group of the five species, the end groups, from lowest to highest mass, very likely correspond to BnO-/-H (n-1 compared with the rest of the group), no end groups, H-/-OH end groups, MeO-/-H, and the artifact NaO-/-H. On increasing the temperature from ambient to 70 • C, the molecular weight increased (by more than 6-fold in the case of 2, entries 5 v 6, Table 1), although this was generally at the cost of control. On further increasing the temperature to 130 • C, there was a further increase in polymer molecular weight, together with an increase in the PDI. The presence of a high oxidation state appears beneficial for affording a high molecular weight product given that use of the molybdenum(VI) precursor 7, afforded the highest molecular weight product, albeit with the worst control (entry 16, Table 1).
However, if the runs were conducted as melts (Table 2), all the systems (except for the use of 2) afforded higher molecular weight products versus runs conducted in solution. Runs employing the amine species 2 (entry 2, Table 2) and 4 (entry 4, Table 2) afforded the lowest molecular weight products. Analysis of the MALDI-TOF spectra again indicated the presence of multiple species, e.g., for the PCL from entry 2 of Table 2, the same five polymer series as identified above were present but in different relative intensities (see Figure 13; for the full spectrum, see Figure S13).   Table 1).
However, if the runs were conducted as melts (Table 2), all the systems (except for the use of 2) afforded higher molecular weight products versus runs conducted in solution. Runs employing the amine species 2 (entry 2, Table 2) and 4 (entry 4, Table 2) afforded the lowest molecular weight products. Analysis of the MALDI-TOF spectra again indicated the presence of multiple species, e.g., for the PCL from entry 2 of Table 2, the same five polymer series as identified above were present but in different relative intensities (see Figure 13; for the full spectrum, see Figure S13).   Table 1).   Table 2). Figure 13. Close up of MALDI-TOF spectrum of PCL (entry 2, Table 2).
Good conversions were also observed on conducting the runs under air at 130 • C for 24 h (Table 3). Molecular weights were far higher than those observed under N 2 when using 1-5, whilst that for 7 (entry 7, Table 3) was far lower. At ambient temperature under air, the products were low molecular weight oily products (e.g., entry 8, Table 3). In the MALDI-TOF spectra, there is one dominant series corresponding to no end groups, with two minor series likely corresponding to BnO-/-H and H-/-OH series; the former of these two starts to become more dominant at higher mass (see Figure 14, entry 5, Table 3; for the full spectrum, see Figure S14).

Kinetics
Kinetic studies for the imido complexes 1, 3, 5, and 8 ( Figure 15), conducted using the ratio 500:1:1 ([CL]:[Cat]:[BnOH]) revealed the rate trend 8 > 3 > 1 > 5. This suggests the presence of either a metaor an ortho-iodo substituent is beneficial to the rate of conversion for the iodo-bearing systems, whilst the best rate was observed for the aniline-derived system. 24 h (Table 3). Molecular weights were far higher than those observed under N2 when using 1-5, whilst that for 7 (entry 7, Table 3) was far lower. At ambient temperature under air, the products were low molecular weight oily products (e.g., entry 8, Table 3). In the MALDI-TOF spectra, there is one dominant series corresponding to no end groups, with two minor series likely corresponding to BnO-/-H and H-/-OH series; the former of these two starts to become more dominant at higher mass (see Figure 14, entry 5, Table 3; for the full spectrum, see Figure S14).  Table 3).

Kinetics
Kinetic studies for the imido complexes 1, 3, 5, and 8 ( Figure 15), conducted using the ratio 500:1:1 ([CL]:[Cat]:[BnOH]) revealed the rate trend 8 > 3 > 1 > 5. This suggests the presence of either a meta-or an ortho-iodo substituent is beneficial to the rate of conversion for the iodo-bearing systems, whilst the best rate was observed for the aniline-derived system.  Table 3). Comparing the kinetics for complexes 6 and 7 ( Figure 16) suggests that a molybdenum(V) centre is beneficial to the rate of conversion versus molybdenum(VI).
For the amine complexes 2 and 4, kinetics ( Figure 17) revealed, as for the imido complexes, that a meta-rather than a para-iodo group is beneficial for the rate of conversion. Note complex 8 exhibits a slightly better rate than 2 (see Figure S15); prior to screening, sample 2 was dried in vacuo for >2 h to remove the acetonitrile of crystallization.  Comparing the kinetics for complexes 6 and 7 ( Figure 16) suggests that a molybdenum(V) centre is beneficial to the rate of conversion versus molybdenum(VI).
For the amine complexes 2 and 4, kinetics ( Figure 17) revealed, as for the imido complexes, that a metarather than a para-iodo group is beneficial for the rate of conversion. Note complex 8 exhibits a slightly better rate than 2 (see Figure S15); prior to screening, sample 2 was dried in vacuo for >2 h to remove the acetonitrile of crystallization.
Comparing the kinetics for complexes 6 and 7 (Figure 16) suggests that a molybdenum(V) centre is beneficial to the rate of conversion versus molybdenum(VI).
For the amine complexes 2 and 4, kinetics ( Figure 17) revealed, as for the imido complexes, that a meta-rather than a para-iodo group is beneficial for the rate of conversion. Note complex 8 exhibits a slightly better rate than 2 (see Figure S15); prior to screening, sample 2 was dried in vacuo for >2 h to remove the acetonitrile of crystallization. Comparing the kinetics for complexes 6 and 7 (Figure 16) suggests that a molybdenum(V) centre is beneficial to the rate of conversion versus molybdenum(VI).
For the amine complexes 2 and 4, kinetics ( Figure 17) revealed, as for the imido complexes, that a meta-rather than a para-iodo group is beneficial for the rate of conversion. Note complex 8 exhibits a slightly better rate than 2 (see Figure S15); prior to screening, sample 2 was dried in vacuo for >2 h to remove the acetonitrile of crystallization. An overall analysis of the kinetics for the systems herein reveals the rate trend 8 > 2 > 3 > 6 ≈ 1 > 7 > 5 > 4. Thus, the most active catalyst systems amongst the 'functionalized' systems are those bearing a meta iodo substituent, which is more likely influenced by the electronics of the system rather than the sterics. The near linear relationships above imply the polymerizations follow a first-order dependence on the monomer concentration.
IR spectra (nujol mulls, KBr windows) were recorded on a Nicolet Avatar 360 FT IR spectrometer (Thermo Nicolet Corporation, Madison, WI, USA); 1 H NMR spectra were recorded at 400.2 MHz at room temperature on a on a JEOL ECZ 400S spectrometer (JEOL Ltd., Tokyo, Japan). The 1 H NMR spectra were calibrated against the residual protio-impurity of the deuterated solvent; chemical shifts are given in ppm (δ). Elemental analyses were performed by the elemental analysis service in the Department of Chemistry at the University of Hull, OEA Labs Ltd. (Devon, UK), or London Metropolitan University. The precursor [Mo(η-C 5 Me 5 )Cl 4 ] was prepared by the literature method [6]. All other chemicals were purchased from Sigma Aldrich or TCI UK.
The mass spectra of the complexes 1-8 were run on a Bruker Maxis Impact HD Mass spectrometer at the University of Hull in ESI positive mode, or at the National Mass Spectrometry Facility at Swansea (UK), using an atmospheric solids analysis probe (ASAP). To [Mo(η-C 5 Me 5 )Cl 4 ] (1.12 g, 3.00 mmol) and 2-I,4-FC 6 H 3 NH 2 (0.36 mL, 3.0 mmol) in a Schlenk flask was added toluene (20 mL) and triethylamine (0.86 mL, 6.2 mmol). The system was stirred for 12 h, and then the volatiles were removed. The residue was extracted into MeCN (40 mL) and on standing for 24 h at ambient temperature, the complex 6·MeCN formed as dark orange crystals. Yield 1.06 g, 61%. C 16

ROP of ε-Caprolactone (ε-CL)
All polymerizations were carried out in Schlenk tubes under nitrogen atmosphere unless otherwise stated. ε-CL was polymerized using complexes 1-8 in the presence of BnOH (0.1 M in toluene) as a co-initiator. Complexes were weighed out in the glove box and then initiator and monomer were added to the flask successively via syringe. The molar ratio of monomer/catalyst/BnOH ([CL]/[Cat]/[BnOH]) used was 500:1:1. The reaction mixture was then placed into an oil bath, preheated to the required temperature. The reaction was quenched by the addition of an excess of glacial acetic acid (0.2 mL), then the reaction solution was poured into cold methanol (20 mL). The reaction conversion was monitored by 1 H NMR (400 MHz, CDCl 3 , 25 • C) spectroscopic studies. The resulting polymer was washed several times with methanol, collected on filter paper and then dried under vacuum to constant weight at 40 • C. GPC (in THF) were used to determine molecular weights (M n and PDI) of the polymer products.

Polymerization Kinetics
Kinetic experiments were carried out following the previous polymerization method. At regular time intervals, 0.05 mL aliquots were removed, quenched with wet CDCl 3 (1 mL), and analysed by 1 H NMR spectroscopy.

Polymer Samples Preparation for MALDI-TOF
All samples were dissolved in THF, as was the matrix. Data was acquired using a dithranol matrix and NaTFA additive, where the matrix, sample, and additive solutions were mixed together in a 5:1:0.1 ratio. Then, 0.5 µL of the mixture solution was spotted onto the MALDI target and left to air-dry prior to analysis.

Crystal Structure Determinations
In all cases, crystals suitable for an X-ray diffraction study were grown from a saturated MeCN solution at 0 • C or ambient temperature. Compounds 3 and 3I co-crystallized from the same vial. All (except 1) single crystal X-ray diffraction data were collected at the UK National Crystallography service using Rigaku Oxford Diffraction ultra-high intensity instruments employing modern areas detectors. For 1 diffraction data were collected using silicon 111-monochromated synchrotron radiation at Daresbury Laboratory Station 9.8. In all cases, standard procedures were employed for integration and processing of data.
Complex 2 was refined as a two-component non-merohedral twin with 180 • rotation about direct and reciprocal axes 0 0 1 with the major:minor component ratio: 56.2:43.8(2)%. For 7, there was some evidence of unresolved twinning from some largish residual electron density peaks and F obs > F calc for many reflections.
Crystal structures were solved using dual space methods implemented within SHELXT [28]. The completion of these structures was achieved by performing least squares refinement against all unique F 2 values using SHELXL-2018 [29]. Table 4 contains the crystallographic data for 1-8, 2-I NH 3 Cl and 3-I NH 3 Cl. CCDC 2122492-2122501 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed on 6 December 2021).

Conclusions
In conclusion, we have utilized iodoanilines to access and stabilize zig-zag chains or ladders incorporating organometallic molybdenum fragments. Amine or organoimido species can be accessed depending on conditions for the 3-and 4-iodoanilines; for the 2-iodoaniline, only an imido product could be isolated. Use of 4-fluoro-2-idoaniline was more sensitive, and both Mo(V) and Mo(VI) imido complexes were accessible, the latter via adventitious oxidation. All structures exhibit a variety of intermolecular interactions. In terms of ROP, the substituent pattern of the iodo substituents strongly influences the polymerization rate. A meta-iodo substituent is favoured, and it is assumed that the presence of this electron withdrawing group promotes the ability of the metal to perform a nucleophilic attack at the carbonyl of the ε-caprolactone.