Must an N-Heterocyclic Carbene Be a Terminal Ligand

A “stand-alone” NHC carbene without a heterocyclic secondary donor has been crystallographically identified together with its terminal counterpart...
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Organometallics 2010, 29, 2403–2405 DOI: 10.1021/om100277f

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Must an N-Heterocyclic Carbene Be a Terminal Ligand? Xiaoyan Han,† Lip-Lin Koh,† Zhi-Pan Liu,*,‡ Zhiqiang Weng,*,†,§ and T. S. Andy Hor*,†, †

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Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore, Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, ‡Department of Chemistry, MOE Key Laboratory for Computational Physical Sciences, Fudan University, Shanghai 200433, People’s Republic of China, and §College of Chemistry and Chemical Engineering, Fuzhou University, Fujian 350108, People’s Republic of China Received April 7, 2010 Summary: A “stand-alone” NHC carbene without a heterocyclic secondary donor has been crystallographically identified together with its terminal counterpart, with both coordination isomers cocrystallized in a single crystal. Carbonyl is among the best known ligands that are at ease with both terminal and bridging modes.1 Bridging phosphine was first mooted by Braunstein et al.2 and later crystallographically verified by Werner et al.3 in a Rh2(μ-CPh2) system. Similar bridging phospholes in Cu2 have also been observed.4a Its detection has substantiated the proposal of intramolecular phosphine migration across the metals in a dimetallic system such as Fe2.4b Ligand mobility and dynamics in di- and polynuclear complexes, as well as on metal surfaces, is an important area of research, as it is central to the understanding of many fluxional processes, catalytic mechanism, surface modifications, etc.5 Although N-heterocyclic carbene (NHC) has been coined as a “phosphine mimic”,6 there is no crystallographic example, to our knowledge, of pure NHC bridging ligands among a large array of N,N, N,O, N,P, and N,S variants,7 although divalent carbenes such as diphenylmethylene8 are known to bridge CuI2 (e.g., A and B in Figure 1).9 The few established bridging

NHC ligands are invariably stabilized by a heterocyclic tether which serves as a secondary donor in Cu(I)10 and Ag(I)11 (e.g., C and D in Figure 1). Accordingly, reversible exchange between terminal and bridging NHC as a means of NHC migration in a di- or polynuclear metal core is so far unknown. We herein present the crystallographic evidence for not only a stand-alone NHC carbene as a bridging ligand but also its coexistence with its terminal form in a rare cocrystallization of two coordination isomers within a single crystal. This structural anomaly and a possible interconversion pathway have been further analyzed by DFT. Room-temperature condensation and ion-exchange reaction of molar equivalents of CuOtBu and N-allylbenzothiazolium iodide led to the isolation of [Cu2I2(AllBzThzylid)3] (1; AllBzThzylid = N-allylbenzothiazolin-2-ylidene) instead of the stoichiometrically dictated [CuI(AllBzThzylid)] or [Cu2(μ-I)2(AllBzThzylid)2]. X-ray diffraction analysis of a single crystal grown from the THF solution of 1 revealed cocrystallization of two chemically distinguishable coordination isomers: viz., [(AllBzThzylid)Cu(μ-I)2Cu(AllBzThzylid)2] (1a) and [(AllBzThzylid)Cu(μ-I)2(μ-AllBzThzylid)Cu(AllBzThzylid)] (1b) (Figure 2). Both complexes are dinuclear, with two iodide bridges between two Cu(I) centers, each of which carries a terminal NHC group. The coordination of the third carbene, however, differs for being terminal in 1a and bridging in 1b. The terminal Cu-C bonds (1.921(9)1.935(9) A˚ in 1a and 1.901(9)-1.926(9) A˚ in 1b) are within expectations in Cu(I) carbenes.12,13 The Cu-C bridge bonds in 1b (1.978(9) and 2.285(9) A˚) are understandably longer than their terminal counterparts. This bridging carbene carbon is also distinctly closer to one of the metals. Its bridging nature necessarily pulls the two metals closer from 2.753(2) A˚ in 1a to 2.412(2) A˚ in 1b such that it falls within the range of carbene-bridged complexes (e.g., 2.4635(7) A˚ in A,9a 2.4165(3) A˚ in B,9b and 2.356(1) A˚ in C10).14 There is no evidence of secondary interaction on the pendant olefin.

*To whom correspondence should be addressed. E-mail: andyhor@ nus.edu.sg (T.S.A.H.). (1) (a) Colton, R.; McCormick, M. J. Coord. Chem. Rev. 1980, 31, 1–52. (b) Bitterwolf, T. E. Coord. Chem. Rev. 2000, 206-207, 419-450. (c) Tran, N. T.; Dahl, L. F. Angew. Chem., Int. Ed. 2003, 42, 3533–3537. (2) (a) Bender, R.; Braunstein, P.; Dedieu, A.; Dusausoy, Y. Angew. Chem., Int. Ed. Engl. 1989, 28, 923–925. (b) Braunstein, P.; Boag, N. M. Angew. Chem., Int. Ed. 2001, 40, 2427–2433. (3) (a) Pechmann, T.; Brandt, C. D.; Werner, H. Angew. Chem., Int. Ed. 2000, 39, 3909–3911. (b) Werner, H. Angew. Chem., Int. Ed. 2004, 43, 938–954. (4) (a) Leca, F.; Lescop, C.; Rodriguez-Sanz, E.; Costuas, K.; Halet, J.-F.; Reau, R. Angew. Chem., Int. Ed. 2005, 44, 4362–4365. (b) Sun, H.; Gu, J.; Zhang, Z.; Lin, H.; Ding, F.; Wang, Q. Angew. Chem., Int. Ed. 2007, 46, 7498–7500. (5) (a) Muetterties, E. L. Science 1977, 196, 839–848. (b) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, W. R. Chem. Rev. 1979, 79, 91–137. (c) Lu, X. L.; Ng, S. Y.; Vittal, J. J.; Tan, G. K.; Goh, L. Y.; Hor, T. S. A. J. Organomet. Chem. 2003, 688, 100–111. . fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; (6) (a) O Herdtweck, E.; Scherer, W.; Mink, J. J. Organomet. Chem. 1993, 459, 177–184. (b) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev.  2009, 253, 862–892. (c) Campora, J.; de la Tabla, L. O.; Palma, P.; Alvarez, E.; Lahoz, F.; Mereiter, K. Organometallics 2006, 25, 3314–3316. (7) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122– 3172. (8) Etter, R. M.; Skovronek, H. S.; Skell, P. S. J. Am. Chem. Soc. 1959, 81, 1008–1009. (9) (a) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085– 10094. (b) Hofmann, P.; Shishkov, I. V.; Rominger, F. Inorg. Chem. 2008, 47, 11755–11762.

(10) Gischig, S.; Togni, A. Organometallics 2005, 24, 203–205. (11) (a) Garrison, J. C.; Simons, R. S.; Kofron, W. G.; Tessier, C. A.; Youngs, W. J. Chem. Commun. 2001, 1780–1781. (b) Garrison, J. C.; Simons, R. S.; Tessier, C. A.; Youngs, W. J. J. Organomet. Chem. 2003, 673, 1–4. (c) Catalano, V. J.; Malwitz, M. A. Inorg. Chem. 2003, 42, 5483–5485. (d) Liu, B.; Chen, W.; Jin, S. Organometallics 2007, 26, 3660–3667. (e) Zhang, X.; Xi, Z.; Liu, A.; Chen, W. Organometallics 2008, 27, 4401– 4406. (12) Venkatachalam, G.; Heckenroth, M.; Neels, A.; Albrecht, M. Helv. Chim. Acta 2009, 92, 1034–1045. (13) Diez-Gonzalez, S.; Nolan, S. P. Aldrichim. Acta 2008, 41, 43–51. (14) (a) Beck, J.; Str€ahle, J. Angew. Chem., Int. Ed. Engl. 1985, 24, 409–410. (b) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23, 3369–3371.

r 2010 American Chemical Society

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Figure 3. DFT calculated potential energy surface (unit: kJ/mol).

Figure 1. Selected known μ-carbene complexes of Cu(I) and Ag(I).

Figure 2. ORTEP view of 1a and 1b cocrystallized in 1 with 30% thermal ellipsoids. Selected bond lengths (A˚) and angles (deg): for 1a, Cu3 3 3 3 Cu4 = 2.753(2), Cu3-C31 = 1.935(9), Cu4C41 = 1.928(9), Cu4-C51 = 1.921(9), Cu4-I3 = 2.881(1), Cu4I4 = 2.798(1), C51-Cu4-C41 = 139.7(4); for 1b, Cu1 3 3 3 Cu2 = 2.412(2), Cu1-C1 = 1.901(9), Cu1-C21 = 1.978(9), Cu2-C11 = 1.926(9), Cu2-C21 = 2.285(9), Cu1-I1 = 2.926(1), Cu1-I2 = 2.752(1), C1-Cu1-C21 = 124.2(4), C21-Cu1-Cu2 = 61.8(3).

In order to create room for the NHC entry between the metals, the dihedral angle of the [Cu2I2] butterfly reduces sharply from 1a (Cu3-Cu4-I3/Cu3-Cu4-I4 = 135.1°) to 1b (Cu1Cu2-I1/Cu1-Cu2-I2 = 70.4°), thereby closing the two wingtips from 4.296 A˚ (I3...I4) in 1a to 4.071 A˚ (I1....I2) in 1b. The solution of 1 in CD2Cl2, which slowly decomposes in solution even under an inert atmosphere, gives only one set of NHC peaks in its 1H and 13C NMR spectra over the range 183-300 K. The resonances broaden but are not resolved at low temperatures. A possible explanation is a fast shuttle of the NHC ligand between the two metals through the bridging mode. There is no evidence of dissociation to yield terminal

carbenes, viz. 1 f [CuI(AllBzThzylid)] þ [CuI(AllBzThzylid)2], although similar cleavage was observed in the μ-CPh2 complexes.9a 1H NMR spectral analysis of the decomposition product of 1 only revealed a set of unidentified low-field resonances around δ 10.0 ppm which could be attributed to the dimerization of the carbene ligand.15 The Cu-C bridge bonds observed in 1b are significantly longer than in Cu(I) μ-CPh2 complexes such as A (1.922(4) and 1.930(4) A˚) and B (1.922(2) and 1.922(2) A˚). The μ-carbene carbon, as in C, is more likely to be sp2 than sp3 hybridized10 in a 3c-2e bridge-bond that enables the NHC carbon to traverse across two metals without any thermodynamic assistance through chelation or tether coordination. To provide a deeper understanding of the relationship between 1a and 1b and a possible interconversion path, we have performed density functional theory (DFT) analyses on the reaction kinetics. The transition state (TS) of the 1a to 1b conversion has been located. The DFT optimized structures are in good agreement with the X-ray structures (Figure 3). More importantly, the potential energy surface of 1a to 1b conversion is very flat. The bridged complex is marginally more stable than 1a by 6.75 kJ/mol, and the reaction barrier calculated with respect to the 1a state is 4.85 kJ/mol. These low values are within the typical DFT error bar and confirm the experimental finding that both 1a and 1b are stable and the migration of the third carbene group should be facile. This is especially so when the two metals are in close contact, and the process could equilibrate the trigonal-planar (16e) and tetrahedral (18e) configurations of the two Cu(I) centers across the bridge. The Mulliken bond order shows that there are indeed two Cu-C bridge bonds with one bond being shorter than the other (0.277 and 0.138), and this is consistent with the experimental finding. Theoretically, it is also possible to verify whether a fourth terminal NHC group can be attached to 1a, which would result in a more symmetrical dinuclear structure with saturated (18e) Cu(I). Our data suggest that binding of this extra NHC ligand is possible but weak (only 20 kJ/mol). Without additional stabilization forces, the fourth NHC ligand can readily escape at room temperature. In contrast, the binding of the first three NHC groups is rather strong, indicating that 1a and 1b are thermodynamically stable (the binding energy of a NHC ligand is 262.4 kJ/mol on CuI, 105.2 kJ/mol on [CuI(AllBzThzylid)], and 126.3 kJ/mol on [Cu2(μ-I)2(AllBzThzylid)2]). The ease of entry for the third carbene also explains why the stoichiometric 1:1 products were not observed. The central question is as follows: why is a bridging NHC group present here when the overwhelming majority of NHC carbenes are terminal ligands? Any possible contribution from (15) Hyunh, H. V.; Meier, N.; Pape, T.; Hahn, F. E. Organometallics 2006, 25, 3012–3018.

Communication

Figure 4. HOMO (left) and LUMO (right) molecular orbital contour plot of the NHC ligand.

the S atom in the heterocyclic ring can be discarded, because the bridging NNHC compound [Cu2(μ-I)2(μ-NNHC)(NHC)2] (NNHC represents the μ-NHC in 1b upon replacement of S by NH) can be similarly obtained from DFT calculations. For a better comparison, we have replaced the third NHC ligand with CO and PH3. The results suggest that carbonyl is a NHC-like group. It is stable in both the bridging and terminal positions with a small energy difference between the two isomers. In contrast, PH3 is only possible as a terminal ligand. These facts enable us to rationalize the presence of both bridging and terminal NHC groups as follows. It is well established that the bistability of a CO ligand in bridging and terminal positions can be attributed to its 2π* and 5σ frontier molecular oribitals. The electronic structure of the carbene C in NHC, which is spin-unpolarized, is in fact very close to that of CO. As shown in Figure 4, the HOMO of the NHC ligand (energy level ε = -5.08 eV) is 5σ-like, mainly concentrating on the carbene C, while the LUMO, being an antibonding orbital (ε = -1.81 eV), is 2π*-like. The HOMO and LUMO of [Cu2(μ-I)2(AllBzThzylid)2] situate between those of the NHC ligand (εHOMO = -4.08 eV, εLUMO = -2.51 eV), enabling the classical donation and backdonation interaction between the bridging NHC and [Cu2(μ-I)2(AllBzThzylid)2]. The bonding picture can be viewed by (16) Greenwood, N. N.; Earnshaw, A. In Chemistry of the Elements; Pergamon: Oxford, U.K., 1984; Chapter 28.2.3, p 1368.

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examining the difference in charge density before and after the bonding of the NHC ligand with [Cu2(μ-I)2(AllBzThzylid)2]. We found that the antibonding p orbitals perpendicular to the NHC plane obtain electrons, while the in-plane bonding orbitals lose electrons, which is also consistent with the significant C-S bond stretch in forming the bridging NHC (from 1.758 to 1.779 A˚). On the other hand, the dominant bonding mode of PH3 arises from the σ donation of the lone-pair electrons, and thus it is only stable as a terminal ligand. The bridging carbene ligand is a better π-acceptor and hence can benefit more from the stronger π backbonding as a result of the presence of an additional Cu(I). This makes the carbene more a carbonyl than a phosphine mimic in a dinuclear environment. The findings here suggested that we may have underestimated the ability of pure NHC carbenes to bridge two metals and perhaps to cross over the metals by a reversible bridge T terminal switch. These phenomena are even rarer than in phosphines because phosphorus, being larger, can cross over longer M 3 3 3 M separations, including nonM-M-bonded systems. Copper is among the smallest transition metals, with a covalent radius of 1.32 A˚, an ionic (Cuþ) Pauling radius of 0.96 A˚, and a Cu-Cu length of 2.556 A˚ in Cu metal.16 It thus provides probably the best candidate for the stabilization of a bridging NHC and a suitable platform for the NHC to hop across the metal centers. Such dynamic behavior could hold the key to the design of a new breed of Cu-NHC catalysts.17

Acknowledgment. We thank A*Star (R143-000-364305) and the MOE (R143-000-361-112) for support and G. K.Tan for X-ray assistance. Supporting Information Available: Text and a figure, table, and CIF file giving details of experimental preparations, calculation details, crystallographic data for complex 1, and the charge density difference plot of NHC with [Cu2(μ-I)2(AllBzThzylid)2]. This material is available free of charge via the Internet at http:// pubs.acs.org. (17) (a) Ohishi, T.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2008, 47, 5792–5795. (b) Selim, K. B.; Matsumoto, Y.; Yamada, K.-i.; Tomioka, K. Angew. Chem., Int. Ed. 2009, 48, 8733–8735.