Communication pubs.acs.org/Organometallics
Unsymmetric Mononuclear and Bridged Dinuclear CoIII(cyclam) Acetylides Timothy D. Cook, Phillip E. Fanwick, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47906, United States S Supporting Information *
ABSTRACT: Reported herein are the preparation and characterization of a new trans-bis(acetylide) complex of CoIII(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane) bearing two different acetylide ligands (1) and the first example of an all-carbon (C4) bridged dinuclear Co species (2). Structural features of monomeric 1 and dimeric 2 are similar to those of the previously reported CoIII(cyclam) bis(acetylides), and the CC and C−C bond lengths of the butadiynediyl bridge in 2 conform to the acetylenic resonance form. The Co centers in the bridged complex 2 do not exhibit a measurable electronic coupling, as revealed by voltammetric measurements, and this behavior is rationalized through the DFT analysis of complexes 1 and 2.
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Scheme 1. Preparation of Unsymmetric Co(cyclam) Acetylides
ynthesis of metal acetylide compounds as potential molecular wires can be traced back to the early works of Nast1 and Hagihara.2 Recent decades have witnessed intensified exploration of metal acetylides as molecular wires,3,4 as well as optical limiting5 and photovoltaic materials.6 Bimetallic compounds with a polyynediyl bridge, namely [M]−C2n− [M], have been pursued by many groups around the world as prototypical wires, and examples with significant electronic delocalization include those based on Fe,7,8 Mn,9 Ru,10,11 and Re.12 Our and Lehn’s laboratories have contributed to this area by introducing diruthenium compounds as the capping group ([M]),13,14 and extensive charge delocalization has been observed both in bulk solution and on nanoscale devices.15 Recently, our interest in metal acetylide chemistry has shifted to 3d metal complexes based on the M(cyclam) unit (cyclam = 1,4,8,11-tetraazacyclotetradecane) with the reports of new Cr(III)16 and Fe(III) complexes,17 which significantly expands this new frontier of metal acetylide chemistry along with contributions from the laboratories of Wagenknecht,18,19 Nishijo,20 Shores,21 and Berben.22 Described in this contribution are the synthesis and characterization of new Co(cyclam) acetylide complexes depicted in Scheme 1, including an unprecedented Co−μ-C4−Co species (2). The starting material [Co(cyclam)(C2Ph)Cl]Cl was prepared via modification of a published procedure (see the Supporting Information).21 Complex 1 was prepared from the reaction between [Co(cyclam)(C2Ph)Cl]Cl and 1.8 equiv of LiC2SiMe3, where the inner-sphere Cl was displaced by the acetylide ligand. Complex 1 was isolated as a yellow crystalline material in a yield of 63% and authenticated through ESI-MS, 1 H NMR,23 and a single-crystal X-ray diffraction study.24 Complex 1 can be readily desilylated with K2CO3 in refluxing methanol. Aiming at complex 2, the desilylated product was subjected to the Glaser coupling reaction,25 but it was © 2014 American Chemical Society
unreactive under Hay, Eglinton, or Vogtle conditions (see the Supporting Information). It is likely that the Co(cyclam) unit is too bulky to allow for an η2 coordination to the acetylide ligand by the Cu catalyst. Subsequently, the preparation of 2 was attempted via anion metathesis using a dilithiated butadiynediyl. Treatment of a THF suspension of [Co(cyclam)(C2Ph)Cl]Cl with 0.50 equiv of LiC4Li resulted in a dark brown solution, from which complex 2 was obtained as orange-yellow crystals in 30% yield.26 Complex 2 was characterized unambiguously via ESI-MS, 1H NMR, and a single-crystal Xray diffraction study.24 It is interesting to note that the byproduct [Co(cyclam)(C2Ph)2]Cl was also formed in this Special Issue: Organometallic Electrochemistry Received: December 26, 2013 Published: February 25, 2014 4621
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reaction, indicating the metathesis of the −C2Ph moiety with −C4Li, followed quickly by reaction of the newly formed −C2Ph with the Cl group of the starting material. Complexes 1 and 2 both crystallized readily from polar solvents upon diffusion of diethyl ether and/or hexanes, and the solid materials appear to be stable toward ambient atmosphere. X-ray-quality crystals of 1 and 2 were obtained from the slow diffusion of hexanes into a concentrated chloroform solution and diethyl ether into a concentrated methanol solution, respectively. The molecular structure of 1, shown in Figure 1,
cumulenic or carbynic.3 The dominant contribution of the acetylenic resonance structure is further indicative of a lack of extensive π−dπ delocalization. In order to probe the electronic coupling between two Co centers in 2, electrochemical studies were performed on both 1 and 2. As shown in Figure 3, complex 1 undergoes an
Figure 1. ORTEP plot of 1+ at the 30% probability level. Hydrogen atoms, solvent molecules, and the Cl− counterion are omitted for clarity. Selected bond lengths (Å) and angle (deg): Co1−N1, 1.978(2); Co1−N2, 1.986(2); Co1−N3, 1.988(3); Co1−N4, 1.985(2); Co1−C1, 1.934(3); Co1−C4, 1.936(3); C1−C2, 1.210(4); C4−C5, 1.210(4); C2−C3, 1.444(4); C1−Co−C4, 179.52(12). Figure 3. Cyclic voltammograms of 1 (bottom) and 2 (top) in 0.2 M DCM solution of Bu4NPF6.
confirms both the pseudo-octahedral coordination of the Co center and the trans arrangement of the acetylide ligands. A comparison of the features of 1 with the previously published structure of [Co(cyclam)(C2Ph)2]+ reveals similar coordination geometries about the Co center, although 1 has significantly longer CC bond lengths than the latter (1.21 Å versus 1.11 Å), as well as shorter Co−C bond lengths (1.93 Å versus 2.00 Å).21 Complex 2 crystallizes in a monoclinic cell, and the asymmetric unit contains half of 22+, which is related to the other half by an inversion center bisecting the C4 bridge. The molecular structure of 22+, shown in Figure 2, confirms the
irreversible one-electron reduction. This behavior is similar to that of [Co(cyclam)(C2Ph)2]+ with an irreversible Co(+3/+2) couple21 but is in contrast to that of [Co(cyclam)(C2CF3)2]+ with a reversible Co(+3/+2) couple.19 Complex 1 also exhibits an irreversible couple at ca. 1.25 V, which is attributed to the one-electron oxidation to yield a Co4+ species that undergoes rapid ligand dissociation. Under the same conditions used to study 1, complex 2 displays an irreversible two-electron reduction wave. The Epc of 1 is shifted 24 mV cathodically with respect to that of 2, reflecting the electron deficiency of 2 due to the incorporation of a second Co3+ center as well as the lack of electron donation from the −SiMe3 group. The simultaneous reduction of both Co3+ centers in 2 indicates minimal coupling between the two centers mediated by the C4 bridge. This is in stark contrast with the strongly coupled M−C4−M systems reported for M = Fe,8 Ru,10 Re,12 Ru2,13 each of which displays well-separated stepwise one-electron oxidation (reduction) couples. On the other hand, lack of coupling was also observed for a Cr−C4−Cr species,27 which undergoes a two-electron reduction. To probe whether the appearance of a single reduction wave in 2 is the consequence of coalescence of two closely spaced one-electron waves, the method of Taube and Richardson was applied to yield a width at half-height of 96 mV, corresponding to a ΔEp value of 40 mV.28 From this analysis, [2]+ (CoII−C4−CoIII) is unambiguously a class I Robin−Day ion, wherein the two metal centers are essentially “valence-trapped”.29 In an effort to understand the absence of electronic coupling in 2, density functional theory calculations were performed on geometry-optimized structures of 1+ and 22+ at the B3LYP/ LanL2DZ level using the Gaussian 03 suite,30 with the respective crystal structures as the starting point for optimization. The optimized bond lengths and angles (Tables S1 and S2, Supporting Information) are in good agreement
Figure 2. ORTEP plot of 22+ at the 15% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions are omitted for clarity. Selected bond lengths (Å) and angle (deg): Co1−N1, 1.979(7); Co1− N2, 1.982(6); Co1−N3, 1.972(6); Co1−N4, 1.963(7); Co1−C1, 1.927(7); Co1−C4, 1.950(7); C1−C2, 1.219(9); C4−C5, 1.213(8); C2−C3, 1.42 (1); C5−C5′, 1.39(1); C1−Co1−C4, 178.6(2).
desired connectivity wherein two Co(cyclam)(C2Ph) units are bridged by the butadiynediyl ligand. The Co−C and CC distances are comparable to those observed in complex 1. Hence, addition of a second Co center does not lead to a significant alteration in π interactions between Co and CC moieties. Additionally, the C4C5 (1.213 Å) bond is much shorter than the C5−C5′ bond (1.39 Å), and they fall in the expected range for triple and single bonds, respectively, indicating an acetylenic form of the C4 bridge rather than 4622
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Figure 4. Molecular orbital diagrams for 1+ and 22+ from DFT calculations.
+1 of 22+ feature no contribution from the C4 bridge, explaining the lack of electron delocalization upon reduction of 2 observed in electrochemical studies. The striking similarity in orbital composition between 1+ and “half” of 22+ further explains the observed valence trapping of the two Co centers; electronically, the dimer is simply the sum of two monomers without discernible synergy between two Co centers. In conclusion, the novel unsymmetrically substituted Co bis(acetylide) 1 and the first all-carbon-bridged dinuclear Co species 2 were successfully prepared and structurally characterized. Voltammetric measurements of 2 revealed its Robin− Day class I nature, wherein the two Co centers display minimal electronic coupling. This atypical behavior for an M−C4−M system has been rationalized with DFT studies of 2 by the absence of substantial π−dπ interactions between the bridge and metal centers. The surprising lack of coupling in this Co− C4−Co species highlights the importance of the choice of both the metal centers and auxiliary ligands for mediating charge transfer in oligo-metal(polyynyl) systems, which should guide us in the search of effective wires based on 3d metals.
with the crystallographically determined values. The computed energies and contour plots of the most relevant MOs for the complexes 1+ and 22+ are given in Figure 4, while a detailed list of the optimized geometries and MOs is given in the Supporting Information. The LUMO and LUMO+1 of 1+ contain substantial contributions from the Co dx2−y2 and dz2 orbitals, respectively, corresponding to the empty eg set for a strong-field d6 ion. The HOMO and HOMO-2 contain respectively the Co dxz and dyz orbitals, part of the filled t2g set. Curiously, the HOMO-1 is a π orbital localized to the phenyl ring system. The Co dxy orbital, HOMO-9 (see the Supporting Information), is of substantially lower energy in comparison to the dx2−y2 and dz2 orbitals due to the lack of significant metal−ligand interaction. Of the five computed frontier orbitals, only HOMO and HOMO-2 exhibit any mixing between the Co d orbitals and the acetylide π orbitals. The HOMO is an antibonding combination of the Co dyz and the π(CC) of C2Ph, and the latter is extended onto the phenyl ring. The HOMO-2 consists of an antibonding combination of the dxz and the π(CC) of both acetylide ligands. These characteristics are consistent with the fact that the fill−fill type interactions are the dominant π-bonding modes in metal acetylide compounds, first established by Lichtenberger.31 It is clear from the contour plots that the frontier orbitals of the dimer 2 are the sums of two degenerate “halves”, each consisting of one Co(cyclam)(C2Ph)(C2−) unit. For each pseudodegenerate pair of molecular orbitals in 22+, there is a corresponding molecular orbital in 1+ of nearly identical origin. HOMO-5 is the only exception, which is based on the Co dxz orbital rather than dyz as in the HOMO-2 of 1+, but still features substantial mixing of the dxz orbital with the π(CC) orbitals of both acetylide ligands. Interestingly, the LUMO and LUMO
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, a CIF file, and an xyz file giving detailed synthetic procedures and relevant characterization data for complexes 1 and 2, computational details and relevant geometric parameters for the optimized structures of 1 and 2, X-ray crystallographic data for the structural determination of 1 and 2, and a full list of the authors of ref 30. This material is available free of charge via the Internet at http://pubs.acs.org. 4623
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Chem. 2008, 47, 11452−11454. Sun, C.; Turlington, C. R.; Thomas, W. W.; Wade, J. H.; Stout, W. M.; Grisenti, D. L.; Forrest, W. P.; VanDerveer, D. G.; Wagenknecht, P. S. Inorg. Chem. 2011, 50, 9354− 9364. (19) Sun, C.; Thakker, P. U.; Khulordava, L.; Tobben, D. J.; Greenstein, S. M.; Grisenti, D. L.; Kantor, A. G.; Wagenknecht, P. S. Inorg. Chem. 2012, 51, 10477−10479. Thakker, P. U.; Aru, R. G.; Sun, C.; Pennington, W. T.; Siegfried, A. M.; Marder, E. C.; Wagenknecht, P. S. Inorg. Chim. Acta 2014, in press. (20) 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. (21) Hoffert, W. A.; Kabir, M. K.; Hill, E. A.; Mueller, S. M.; Shores, M. P. Inorg. Chim. Acta 2012, 380, 174−180. (22) Berben, L. A. Toward acetylide- and N-hetercycle-bridged materials with strong electronic and magnetic coupling, Ph.D. Dissertation, University of California, 2005. (23) [Co(cyclam)(C2Ph)Cl]Cl (0.120 g, 0.29 mmol) was suspended in 5 mL of dry THF, to which was added LiC2TMS (0.52 mmol, 1.8 equiv). After solvent removal and purification on silica gel, the product was recrystallized from CHCl3/hexanes to give long yellow needlelike crystals (0.090 g, 63% based on Co). Anal. Found (calcd) for C23H43N4O2.5CoClSi (1·2.5H2O): C, 51.51 (51.34); H, 7.65 (8.05); N, 10.39 (10.41). (24) X-ray diffraction data for 1 were collected on a Nonius KappaCCD image plate diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 150 K. Crystal data for 1·CHCl 3 ·H 2 O: C24H41Cl4Co1N4OSi, fw 630.445, monoclinic, P21/n, a = 9.4442(2) Å, b = 26.1848(7) Å, c = 12.8480(3) Å, β = 97.6410(10)°, V = 3149.03(13) Å−3, Z = 4, Dcalcd = 1.330 g cm−3, R1 = 0.0489, wR2 = 0.1515. X-ray diffraction data for 2 were collected on a Rigaku Rapid II image plate diffractometer using Cu Kα radiation (λ = 1.54184 Å) at 150 K. Crystal data for 2·4MeOH: C44H70Cl2Co2N8O4, fw 963.863, monoclinic, P21/n, a = 11.9897(1) Å, b = 11.1484(8) Å, c = 18.9252(17) Å, β = 98.070(7)°, V = 2504.6(4) Å−3, Z = 2, Dcalcd = 1.283 g cm−3, R1 = 0.0862, wR2 = 0.2556. (25) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632−2657. (26) [Co(cyclam)(C2Ph)Cl]Cl (0.177 g, 0.41 mmol) was suspended in 6 mL of THF, to which was added LiC4Li (0.21 mmol, 0.5 equiv), prepared from the reaction of TMS−C4−TMS with 13 equiv of n-BuLi overnight. The solution was stirred overnight under N2 at room temperature. After removal of solvent and purification on silica gel, the product was recrystallized from MeOH/Et2O to give orange block crystals (0.052 g, 30% based on Co). Anal. Found (calcd) for C4H66Cl2Co2N8O4 (2·4H2O): C, 52.46 (52.69); H, 7.17 (7.30); N, 12.24 (12.29). (27) Egler-Lucas, C.; Blacque, O.; Venkatesan, K.; Lopez-Hernandez, A.; Berke, H. Eur. J. Inorg. Chem. 2012, 1536−1545. (28) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278− 1285. (29) Robin, M.; Day, P., In Adv. Inorg. Chem., Ed. Elsevier: 1968; Crutchley, R. J. Adv. Inorg. Chem. 1994, 41, 273−325. (30) Frisch, M. J., et al. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2003. (31) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am. Chem. Soc. 1993, 115, 3276−3285. 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 1057621) and the Purdue Research Foundation.
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