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(ArF)4](6, R1 = R2 = Ph; 7, R1 = H, R2 = SiMe3) are synthesized by reaction of {(IPr)Ni(μ-Cl)}2 (3) with NaB(ArF)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 C2v-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(ArF)4] (8). Excess pivaloisocyanide reacts with 6 to afford tBuNCCPh2, [(IPr)Ni(CNtBu)3][B(ArF)4] (9), and (IPr)NiCl(CNtBu) (10). Mesitylazide reacts with 6 to give the ketimine MesNCPh2 and the bridging mesitylimido dimer [{(IPr)Ni}2(μ-Cl)(μ-NMes)][B(ArF)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.