1410 - 1450 Gregory L Hillhouse

Carbene Group-Transfer Reactivity of Dimeric Nickel Complexes Supported by N-Heterocyclic Carbene Ligands

The facile preparation of bridging-carbene complexes of low-valent nickel and their ensuing group-transfer reactions with unsaturated organic substrates has been studied. Dimeric nickel bridging-carbene complex salts [{(IPr)Ni}₂(-Cl)(-CR,sup>1R²)][B(ArF)4](1, R¹=R²=Ph; 2, R¹= H, R²=SiMe₃) can be synthesized by reaction of {(IPr)Ni(-Cl)}₂ with NaB(ArF)4 and N2CPh2 or N₂=CHSiMe₃, with elimination of N,₂ and NaCl.

The solid-state structure of 1 features an unsymmetric h¹,h³-bonding motif for the bridging diphenylcarbene ligand in which one Ni center binds to the CPh₂ ligand in a p- fashion involving the ipso- and one ortho-carbon of a phenyl ring along with the carbene carbon. Solution spectra (NMR) of 1 indicate a C_2v-symmetric structure even at -80°C. The solid-state structure of 2 (the complex cation is shown in the Figure below) features a (trimethylsilyl)carbene unit symmetrically disposed between the two nickel centers and bound through only the carbene-carbon atom. 

Diphenylcarbene-group transfer from 1 to carbon monoxide (3 equiv) gives O=C=CPh₂ and the dimeric Ni(I)-Ni(I) carbonyl complex [{(IPr)Ni(CO)}₂(m-Cl)][B(Ar^F)₄] (3). Excess pivaloisocyanide reacts with 1 to afford ^tBuN=C=CPh₂, [(IPr)Ni(CN^tBu)₃][B(Ar^F)₄] (4), and (IPr)NiCl(CN^tBu) (5). Mesitylazide reacts with 1 to give the ketimine MesN=CPh₂ and the bridging mesitylimido dimer [{(IPr)Ni}₂(m-Cl)(m-NMes)][B(Ar^F)₄] (6). A secondary reaction of diphenyldiazomethane with 1 to give Ph₂C=CPh₂ and Ph₂C=N-N=CPh₂ prevents catalytic CPh₂-group transfer reactions from being realized in this system.