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 FcN3 or FcNCS afforded bisadducts [(FcN3)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) 1 facilitated significant electronic coupling between ferrocene centers in 2 (ΔE = 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.