Dimeric Complexes of CoIII(cyclam) with a Polyynediyl Bridge

Feb 6, 2015 - Voltammetric analysis revealed weak Co–Co interaction through the bridge, which is attenuated by polyyne length. The orbital origin of...
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Dimeric Complexes of CoIII(cyclam) with a Polyynediyl Bridge Timothy D. Cook, Sean N. Natoli, Phillip E. Fanwick, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Described in this work are the preparation and characterization of a family of polyynediyl-bridged (μ-C2n) dimers of CoIII(cyclam)Cl (cyclam = 1,4,8,11-tetraazacycloctetradecane) for n = 2−4. The complexes are robust to ambient conditions and are prepared under mild conditions without requiring anaerobic or anhydrous environments. Voltammetric analysis revealed weak Co−Co interaction through the bridge, which is attenuated by polyyne length. The orbital origin of the Co−Co interaction has been rationalized through DFT analysis. Scheme 1. Synthesis of Complexes [1]2+, [2]2+, and [3]2+ a

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etal acetylide compounds have been extensively investigated as potential molecular wires1,2 since the pioneering works by Nast3 and Hagihara.4 More recently, the possible applications of these materials have been expanded from simple charge transfer “wires” to photovoltaic and optical power limiting materials.5 Metal-capped polyynediyl compounds, [M]−C2n− [M], serve as simple synthetic surrogates of oligomeric metallapolyynes, which allow convenient investigation of the properties of these desirable materials without the need for construction of complex oligomeric assemblies. Indeed, a large number of [M]− C2n−[M] compounds have been studied for their propensity to delocalize charge, with those based on Fe,6 Mn,7 Ru,8 Ru2,9 W,10 and Re11 demonstrating substantial metal−metal communication through the butadiynediyl bridge. While there are abundant examples of [M]−C2n−[M] systems based on 4d or 5d metals, examples based on redox-active 3d metal elements remain rare, with the exception of Fe. Recently, our interest in metal acetylide chemistry has shifted from those based on diruthenium12 to 3d metal complexes based on the M(cyclam) unit (cyclam = 1,4,8,11-tetraazacyclotetradecane) with the reports of new mononuclear Cr(III)13 and Fe(III) complexes,14 which significantly expands this new frontier of 3d metal acetylide chemistry along with contributions from the laboratories of Wagenknecht,15,16 Nishijo,17 Shores,18 and Berben/Long.19 Notably, [M]−C2n−[M] systems containing 3d metal end groups had only been constructed for M = Fe,6,20 Mn,7 Cr21 prior to our investigation, all of which necessitate air-sensitive synthetic steps. Described in this contribution are the synthesis of a series of polyynediyl complexes capped with CoIII(cyclam)Cl end groups (Scheme 1) under mild and aerobic conditions and their characterizations. Our laboratory recently reported the preparation of the first Co−C2n−Co type compound via inner-sphere salt metathesis between 2 equiv of [Co(cyclam)(C 2Ph)Cl]Cl and 1,4dilithiobutadiyne (LiC4Li).22 In this contribution, the synthesis of a series of Co(III) dimers with polyynediyl bridges is achieved without necessitating anaerobic/anhydrous conditions or pyrophoric reagents; the syntheses are carried out in flasks open to ambient atmosphere. As shown in Scheme 1, the μ-C4 © XXXX American Chemical Society

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Conditions: (i) 0.5 equiv of Me3SiC2nSiMe3, Et3N, MeOH/THF, reflux, 24 h; (ii) 1.9 equiv of Me3SiC4SiMe3, Et3N, MeOH/THF, reflux, 24 h; (iii) CuCl/TMEDA, O2, MeOH/acetone, 4 h.

and μ-C6 dimers (1 and 2) are readily accessible by refluxing the appropriate bis(trimethylsilyl)alkyne with [Co(cyclam)Cl2]Cl in a one-pot deprotection/metalation reaction.23 After purification on silica gel and recrystallization, complexes 1 and 2 were isolated as red-orange crystalline solids in yields of 57% and 34%, respectively. However, attempts to prepare the C2-bridged dimer via analogous reactions with TMSC2TMS and HC2TMS were unsuccessful, yielding a mixture of [Co(cyclam)(C2TMS)Cl]Cl and [Co(cyclam)(C2H)Cl]Cl and unreacted [Co(cyclam)Cl2]Cl. The failure of these reactions is likely due to the steric demand of confining two Co(cyclam) units with a single acetylide bridge. With the success of attaining complexes 1 and 2, the complex with a C8 bridge, namely 3, became a natural target. Given the instability of octatetrayne under reflux conditions, an alternative Received: December 11, 2014

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DOI: 10.1021/om501272p Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics route to 3 was sought: the Glaser reaction of [Co(cyclam)(C4H)Cl]Cl. The μ-C8 dimer 3 was successfully prepared by employing standard Hay conditions (Scheme 1) and isolated as a red-orange crystalline solid in 53% yield. 24 The three polyynediyl-bridged dimers are insoluble in common solvents other than water and methanol, with the exception of 1, which is slightly soluble in acetonitrile. The solubility decreases markedly with the extension of the polyyne bridge. All complexes have been authenticated through ESI-MS and single-crystal X-ray diffraction studies and appear to be stable under ambient laboratory conditions. X-ray-quality crystals were readily obtained for 1−3 through slow diffusion of diethyl ether (1 and 2) or ethyl acetate (3) into a concentrated methanolic solution of the complex, and the structures of 1 and 2 were solved and successfully refined.25 The molecular structure of 1 (Figure 1) verifies the expected

Figure 2. ORTEP plot of 22+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg) for one of the two independent molecules in the unit cell: Co11−N11, 1.975(3); Co11− N12, 1.979(2); Co11−N13, 1.984(3); Co11−N14, 1.982(3); Co11− Cl11, 2.3041(8); Co11−C11, 1.874(3); C11−C12, 1.220(4); C12− C13, 1.376(4); C13−C13′, 1.203(6); Cl11−Co11−C11, 177.51(10).

The electrochemical behavior of 1−3 was examined to gain insight into their electronic structure and establish the extent of Co−Co interaction through the polyynediyl bridge. Previous analysis of voltammetric data of the Co(3+/2+) couple in [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2 indicated the lack of Co−Co interaction, which was traced to the localization of frontier orbitals (namely HOMO and HOMO-1) on the peripheral phenylacetylene moieties instead of the polyynediyl bridge.22 It was hypothesized that replacing the two axial phenylacetylide ligands with a weak π donor such as chloride would reduce electron localization to the terminal axial ligands and enhance the contribution of the bridging polyynediyl in the frontier orbitals, hence strengthening the Co−Co interaction. Given the insoluble nature of [2]Cl2 and [3]Cl2 in aprotic solvents and the narrow electrochemical potential window of methanol, counteranion metathesis was performed with a large excess of the appropriate Na salt in MeOH to precipitate [2](BPh4)2, [2](PF6)2, and [3](PF6)2, which are sufficiently soluble in acetonitrile for voltammetric analysis. Though [1]Cl2 is soluble in acetonitrile, [1](PF6)2 was also prepared for the sake of consistency in differential pulse voltammetry (DPV) analysis. As shown in Figure 3, the cyclic voltammogram of [1]Cl2 displays two irreversible reductive peaks in the potential window allowed by acetonitrile. The wave at Epc = −1.55 V (A) is assigned to the reduction of both Co3+ centers to Co2+ and the wave at Epc = −1.92 V (B) to the further reduction of both Co2+ centers to Co+. The Epc(A) value for [1]Cl2 (−1.55 V) is significantly more anodic than that of the bis-acetylide [{Co(cyclam)(C2Ph)}2(μC4)]Cl2 (−1.98 V), reflecting the increased electron deficiency of [1]Cl2. This relative electron deficiency is likely responsible for the appearance of the Co(2+/1+) couple in the acetonitrile potential window, which was not observed for [Co(cyclam)(C2R)2]+ type compounds.16 Wave A appears to be much broader than what is expected for a simple simultaneous two-electron reduction. Instead, A is likely the result of two closely spaced one-electron reductions (A1 and A2) as depicted in Scheme 2, which is indicative of some degree of Co−Co coupling mediated by the polyynediyl bridge. Further qualitative assessment of such coupling in compounds 1−3 was attempted on the basis of the method of Taube and Richardson.26 Under the conditions prescribed by Taube and Richardson, the width at half height of the first reductive peak for [1](PF6)2 was measured as 120 mV, corresponding to a ΔEp value of 57 mV. Similar analysis of the differential pulse voltammograms under identical conditions revealed ΔEp values of 51 mV for [2](PF6)2 and 49 mV for [3](PF6)2. The weak Co−

Figure 1. ORTEP plot of 12+ at 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1−N1, 1.981(3); Co1−N2, 1.980(4); Co1−N3, 1.968(3); Co1−N4, 1.978(3); Co2−N5, 1.981(3); Co2−N6, 1.974(3); Co2−N7, 1.967(3); Co2−N8, 1.990(4); Co1− Cl1, 2.3100(11); Co2−Cl2, 2.3173(11); Co1−C1, 1.878(4); Co2−C4, 1.890(4); C1−C2, 1.208(6); C2−C3, 1.388(5); C3−C4, 1.201(5); Cl1−Co1−C1, 177.25(13); Cl2−Co2−C4, 177.49(12).

connectivity, wherein the butadiynediyl moiety bridges the two Co centers in the position trans to the Cl ligands. The Co centers clearly exhibit pseudo-octahedral coordination geometry, with four nitrogens of cyclam constituting the equatorial plane. The carbon−carbon distances in the butadiynediyl moiety are consistent with those observed for [{Co(cyclam)(C2Ph)}2(μC4)]Cl2 previously reported,22 with C−C and CC bond lengths falling in the expected range for single and triple bonds, respectively. This suggests that the shift in donation by changing axial ligands from −C2Ph to −Cl does not promote an increased contribution of the cumulenic or carbynic resonance forms of the bridging acetylide.1 Structural features similar to those of 1 are observed for 2 (Figure 2) with the hexatriynediyl moiety exhibiting bond distances characteristic of the canonical acetylenic resonance form. Unlike 1, 2 possesses an inversion center bisecting the C6 bridge, and the unit cell contains two independent molecules. Notably, complexes 1 and 2 contain Co−C bonds (1.878(4) and 1.874(3) Å, respectively) shorter than those observed for [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2 (1.95(7) Å). The shortening of Co−C bonds is the result of trans influence: Cl− is a significantly weaker σ donor than phenylacetylide, which enables a much stronger trans-Co−C bond. Both of the structures exhibit sigmoidal rather than concave curvature, which was also observed for [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2. Though the quality of the crystal of 3 was too low to yield a well-refined structure, the connectivity and overall features of 3 (Supporting Information) are in agreement with those of 1 and 2. B

DOI: 10.1021/om501272p Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

Figure 4. Molecular orbital diagrams for 12+, 22+, and 32+ from DFT calculations. The isovalue of the contour plots was set at 0.02.

reflected in the cyclic voltammetry as Ep,c(A) becomes less negative with increasing bridge length. The contrast in the composition of frontier orbitals in 12+ in comparison to [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ helps to explain the contrast in ΔE1/2 observed upon the first reduction of the two dimers. 12+ displays substantial π−dπ interactions between the Co centers and the C4 bridge in both the HOMO and HOMO-1, which is expected to promote electron delocalization across the bridge. However, this conjugation in occupied orbitals is more suggestive of mixed valency upon oxidation, as reduction would populate the LUMO and not significantly affect HOMO and HOMO-1. Thus, the contribution to delocalization from these orbitals is likely to be minimal. LUMO and LUMO+1 contain some contribution from the bridge, especially in 1; however, these orbitals are σ-conjugated rather than π-conjugated, and as such delocalization upon reduction is expected to be minimally supported by these orbitals. In contrast, [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ has π−dπ contributions buried in the HOMO-4 and HOMO-5. At such lower energies, these orbitals are less likely to engender communication that is observable by voltammetric methods. Further, the LUMO and LUMO+1 of [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ contain no contribution from the polyyne bridge and are completely localized to the Co(cyclam) end groups. In summary, a series of polyynediyl-bridged dimers with Co end groups (Co−C2n−Co) were synthesized under mild conditions, leaving terminal −Cl groups to allow for further functionalization or incorporation into existing frameworks. Compounds 2 and 3 represent the first hexatriynediyl- and octatetraynediyl-bridged dinuclear Co species. Cyclic and differential voltammograms seem to indicate a modest Co−Co coupling in compounds 1−3. This behavior is in contrast to that of [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2, which lacked any observable Co−Co coupling, highlighting the sensitivity of M−C2n−M systems to the nature of ligands trans to the polyynediyl. The presence of labile chloride end groups allows for facile incorporation into larger systems, allowing the moiety to serve as an attenuator of conductivity or simply as a rigid linear linker.

Figure 3. Cyclic voltammograms of [1]Cl2 (top) and [2](BPh4)2 (middle) in 0.2 M MeCN solutions of Bu4NPF6 and [3](PF6)2 (bottom) in a 0.1 M MeCN solution of Bu4NBF4.

Scheme 2. Possible Pathways for the Reduction Wave A in 1− 3

Co coupling gradually decreases as the length of polyynediyl bridges increases, on the basis of this rough comparison. To rationalize the variation in Co−Co interaction among compounds 1−3 and the previously studied [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2, density functional theory calculations were performed on geometry-optimized structures of 12+, 22+, and 32+ at the B3LYP/LANL2DZ level using the Gaussian03 suite.27 Bond lengths and angles obtained from the optimization are in good agreement with the crystallographically determined parameters (Supporting Information). The computed contour plots and energy levels for the frontier molecular orbitals are shown in Figure 4. For 12+, 22+, and 32+, the LUMO and LUMO+1 are primarily based on the dz2 orbital, part of the unoccupied eg set for a strongfield d6 ion. These unoccupied orbitals also exhibit some σ conjugation onto the polyyne bridge, which decreases with extension of the bridge. The HOMO and HOMO-1 contain the Co dyz and dxz orbitals, as part of the filled t2g set. Both the HOMO and HOMO-1 feature substantial contribution from the antibonding combination of π(CC) orbitals of the polyynediyl bridge, a characteristic that is essential for through-bridge Co− Co interaction. 28 In keeping with the observation of Lichtenberger for metal−acetylide complexes, all calculated πbonding interactions are dominated by mixing of a filled d orbital on Co with filled π(CC) orbitals.28,29 The HOMO-2 and HOMO-3 are predominately based on the Cl lone pair (py) orbitals, with some contribution from the Co dyz orbitals. Likewise, the HOMO-4 and HOMO-5 are mostly the Cl px lone pairs. As the polyynediyl bridge lengthens, Eg decreases, which is

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF and xyz files giving detailed synthetic procedures and relevant characterization data for complexes 1−3, differential pulse voltammograms of complexes C

DOI: 10.1021/om501272p Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics [1](PF6)2−[3](PF6)2, computational details and relevant geometric parameters for the optimized structures of 1−3, X-ray crystallographic data for the structural determination of 1 and 2, all computed molecule (1−3) Cartesian coordinates in a format for convenient visualization, and a full list of the authors of ref 27. This material is available free of charge via the Internet at http:// pubs.acs.org.

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(16) Thakker, P. U.; Aru, R. G.; Sun, C.; Pennington, W. T.; Siegfried, A. M.; Marder, E. C.; Wagenknecht, P. S. Inorg. Chim. Acta 2014, 411, 158−164. (17) Nishijo, J.; Judai, K.; Numao, S.; Nishi, N. Inorg. Chem. 2009, 48, 9402−9408. Nishijo, J.; Judai, K.; Nishi, N. Inorg. Chem. 2011, 50, 3464−3470. Nishijo, J.; Enomoto, M. Inorg. Chem. 2013, 52, 13263− 13268. (18) Hoffert, W. A.; Kabir, M. K.; Hill, E. A.; Mueller, S. M.; Shores, M. P. Inorg. Chim. Acta 2012, 380, 174−180. (19) Berben, L. A. Toward acetylide- and N-heterocycle-bridged materials with strong electronic and magnetic coupling. Ph.D. Dissertation, University of California, Berkeley, cA, 2005. Berben, L. A.; Kozimor, S. A. Inorg. Chem. 2008, 47, 4639−4647. (20) Le Narvor, N.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995, 117, 7129−7138. Lissel, F.; Fox, T.; Blacque, O.; Polit, W.; Winter, R. F.; Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2013, 135, 4051−4060. (21) Egler-Lucas, C.; Blacque, O.; Venkatesan, K.; Lopez-Hernandez, A.; Berke, H. Eur. J. Inorg. Chem. 2012, 1536−1545. (22) Cook, T. D.; Fanwick, P. E.; Ren, T. Organometallics 2014, 33, 4621−4624. (23) Compound 1: [Co(cyclam)Cl2]Cl (0.500 g, 1.37 mmol) was dissolved in 50 mL of MeOH, to which was added TMSC4TMS (0.133 g, 0.684 mmol) in 5 mL of THF and Et3N (3.5 mL). The solution was refluxed for 24 h. After removal of solvents and purification on silica gel, the product was recrystallized from MeOH/Et2O to give 0.275 g of dark red-orange needles (56.8% based on Co). Anal. Found (calcd) for C24H56N8O4Co2Cl4 (1·4H2O): C, 37.37 (36.94); H, 7.05 (7.23); N, 14.00 (14.36). Compound 2: [Co(cyclam)Cl2]Cl (0.070 g, 0.19 mmol) was dissolved in 15 mL of MeOH, to which was added TMSC6TMS (0.020 g, 0.092 mmol) in 2 mL of THF and Et3N (0.6 mL). The resultant solution was refluxed for 24 h. After removal of solvents and purification on silica gel, the product was recrystallized from MeOH/ Et2O to give 0.024 g of dark red-orange needles (34.3% based on Co). Anal. Found (calcd) for C24H56N8O4Co2Cl4 (2·5H2O): C, 38.38 (37.97); H, 6.48 (7.11); N, 12.66 (13.33). (24) Compound 3: to a suspension of [Co(cyclam)(C4H)Cl]Cl (0.085 g, 0.22 mmol) in 10 mL of MeOH was added Cu2+(TMEDA) prepared from ca. 1 mmol of CuCl in 5 mL of acetone. The reaction mixture was stirred for 4 h with continuous O2 bubbling. After removal of solvents, rinsing with DCM, and purification on silica gel, the product was recrystallized from MeOH/EtOAc to yield 0.045 g of thin redorange plate crystals (53.2% based on Co). Anal. Found (calcd) for C24H56N8O4Co2Cl4 (3·2H2O): C, 43.25 (42.44); H, 6.57 (6.61); N, 13.25 (14.14). (25) X-ray diffraction data for 1 and 2 were collected on a Nonius KappaCCD image plate diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 150 K. Crystal data for 1: C24H48Cl4Co2N8, fw 708.376, P1̅, a = 8.0203(3) Å, b = 13.1933(7) Å, c = 17.3828(9) Å, α = 79.04(4)°, β = 81.03(3)°, γ = 72.31(2)°, V = 1710.95(14) Å3, Z = 2, Dcalcd = 1.375 g cm−3, R1 = 0.0582, wR2 = 0.1294. Crystal data for 2·4MeOH: C30H64Cl4Co2N8O4, fw 860.567, P1̅,, a = 10.7889(6) Å, b = 14.3914(9) Å, c = 15.0470(9) Å, α = 63.56(3)°, β = 73.33(3)°, γ = 75.37(3)°, V = 1982.64(2) Å−3, Z = 2, Dcalcd = 1.442 g cm−3, R1 = 0.0531, wR2 = 0.1562. (26) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278−1285. (27) Frisch, M. J., et al. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2003. (28) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am. Chem. Soc. 1993, 115, 3276−3285. (29) Lichtenberger, D. L.; Renshaw, S. K.; Wong, A.; Tagge, C. D. Organometallics 1993, 12, 3522−3526.

AUTHOR INFORMATION

Corresponding Author

*E-mail for T.R.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation (CHE 1362214). REFERENCES

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DOI: 10.1021/om501272p Organometallics XXXX, XXX, XXX−XXX