Twisted coordination mode of bis(N-heterocyclic carbene) ligands in octahedral geometry of group 6 transition metal complexes: Synthesis, structure, and reactivity

On treatment of bis(imidazolium) salts bound by o-xylylene, propylene, and ethylene linkers with two moles of LiBEt3H, the corresponding BEt3-adducts of bis-NHCs, (Et3B·ImR)2E (Im = imidazole; R = Me, iPr; E = o-xylylene, propylene, ethylene) (2), were obtained. Reaction of [Mo(CO)6] with compound 2 afforded the carbene complex, [Mo(CO)4(bis-NHC)] (3-Mo), in a good yield. Tungsten and chromium analogs of 3-Mo were obtained from [M(CO)4(η4-norbornadiene)] (M = W, Cr). The X-ray analyses and NMR measurements of these complexes revealed that the bis-NHC ligand adopts a twisted conformation in an octahedral geometry and thus complexes 3 showed a C2-symmmetric structure. In a reaction of 3-Mo with trimethylphosphite, a CO/P(OMe)3 substitution reaction took place to give fac-[Mo(CO)3(bis-NHC){P(OMe)3}] (4-Mo). The formation of the fac-form was found to be caused by a strong electron donor ability of the NHC ligand. The electronic features of the bis-NHC ligand were investigated by X-ray analysis, CO stretching frequency, and cyclic voltammetry of the complex 3-Mo. Furthermore, we estimated the donor ability of the bis-NHC ligand by comparing with those of 2,2′-bipyridine and 1,2-bis(diphenylphosphino)ethane. Density functional calculations (B3LYP/DGDZVP) showed that the C2-symmetric structure of o-xylylene-bridged 3-Mo having N-methyl azole rings was more stable than a Cs-symmetric structure by ΔG = 6.69 kcal mol−1.