Indirectly Connected Bis(N-Heterocyclic Carbene) Bimetallic Complexes: Dependence of Metal-Metal Electronic Coupling on Linker Geometry

Reaction of 1, 1',3,3'-tetra(tert-amyl)benzobis(imidazolylidene) (1) with 2 equiv of FcN(3) or FcNCS afforded bisadducts [(FcN(3))(2)(1)] (2) or [(FcNCS)(2)(1)] (3), respectively (Fc = ferrocene). To the best of our knowledge, these represent the first examples of complexes comprising metals indirectly connected to the carbene atoms of N-heterocyclic carbenes (NHCs) via their ligand sets. Cyclic and differential pulse voltammetry indicated that bis(NHC) I facilitated significant electronic coupling between ferrocene centers in 2 (Delta E = 140 mV), but not in 3. We believe the different degrees of electronic interaction are due to geometric factors: the triazene linker in 2 is nearly coplanar with the bis(NHC) scaffold, whereas the isothiocyanate linker is orthogonal, as determined by X-ray crystallography. Employing this indirect connection strategy should enable tuning of metal-metal interactions by simple alteration the organic linker between NHC and MLn fragments rather than complete redesign thereof. Given that NHC-reactive azide or isothiocyanate groups can be incorporated into both organic and inorganic compounds, this approach is envisioned to facilitate access to otherwise inaccessible catalysts and materials.