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

Dimeric nickel bridging-carbene complex salts [{(IPr)Ni}(2)(mu-Cl)(mu-(CRR2)-R-1)][B(Ar-F)(4)](6, R-1 = R-2 = Ph; 7, R-1 = H, R-2 = SiMe3) are synthesized by reaction of {(IPr)Ni(mu-Cl)}(2) (3) with NaB(Ar-F)(4) and N2CPh2 or N2CHSiMe3, with elimination of N-2 and NaCl. The solid-state structure of 6 features an unsymmetric mu, eta(3)-bonding motif for the bridging diphenylcarbene ligand in which one Ni center binds to the CPh2 ligand in a pi-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 degrees 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 O=C=CPh2 and the dimeric Ni(I)-Ni(I) carbonyl complex [{(IPr)Ni(CO)}(2)(mu-Cl)][B(Ar-F)(4)] (8). Excess pivaloisocyanide reacts with 6 to afford (BuN)-Bu-t=C=CPh2, [(IPr)Ni((CNBu)-Bu-t)(3)][B(Ar-F)(4)] (9), and (IPr)NiCl((CNBu)-Bu-t) (10). Mesitylazide reacts with 6 to give the ketimine MesN=CPh2 and the bridging mesitylimido dimer [{(IPr)Ni}(2)(mu-Cl)(mu-NMes)][B(Ar-F)(4)] (5). A secondary reaction of diphenyldiazomethane with 6 to give Ph2C=CPh2 and Ph2C=N-N=CPh2 prevents catalytic CPh2-group transfer reactions from being realized in these systems.