Synthesis and carbene-transfer reactivity of dimeric nickel carbene cations supported by N-heterocyclic carbene ligands

Abstract

Dimeric nickel bridging-carbene complex salts [{(IPr)Ni}₂(μ-Cl)(μ-CR¹R²)][B(Ar^F)₄](6, R¹ = R² = Ph; 7, R¹ = H, R² = SiMe₃) are synthesized by reaction of {(IPr)Ni(μ-Cl)}₂ (3) with NaB(Ar^F)₄ and N₂CPh₂ or N₂CHSiMe₃, with elimination of N₂ and NaCl. The solid-state structure of 6 features an unsymmetric μ,η³-bonding motif for the bridging diphenylcarbene ligand in which one Ni center binds to the CPh₂ ligand in a π-fashion involving the ipso- and one ortho-carbon of a phenyl ring along with the carbene carbon. The solution structure (NMR) of 6 indicates a C_2v-symmetric structure even at −80 °C. The solid-state structure of 7 shows a (trimethylsilyl)carbene unit symmetrically disposed between the two nickel centers and bound through only the carbene-carbon atom. Diphenylcarbene-group transfer from 6 to carbon monoxide (3 equiv) gives OCCPh₂ and the dimeric Ni(I)-Ni(I) carbonyl complex [{(IPr)Ni(CO)}₂(μ-Cl)][B(Ar^F)₄] (8). Excess pivaloisocyanide reacts with 6 to afford ^tBuNCCPh₂, [(IPr)Ni(CN^tBu)₃][B(Ar^F)₄] (9), and (IPr)NiCl(CN^tBu) (10). Mesitylazide reacts with 6 to give the ketimine MesNCPh₂ and the bridging mesitylimido dimer [{(IPr)Ni}₂(μ-Cl)(μ-NMes)][B(Ar^F)₄] (5). A secondary reaction of diphenyldiazomethane with 6 to give Ph₂CCPh₂ and Ph₂CN–NCPh₂ prevents catalytic CPh₂-group transfer reactions from being realized in these systems.