Zerovalent N-heterocyclic carbene complexes of palladium and nickel dimethyl fumarate: Synthesis, structure, and dynamic behavior

Novel, well-defined, coordinatively unsaturated NHC palladium(0) and nickel(0) complexes have been synthesized and fully characterized. Mixed N-heterocyclic carbene-olefin complexes, M-0(NHC)(n)(DMFU)(m) (where M = Ni, Pd, NHC = 1,3-dimesitylimidazolin-2-ylidene, and DMFU = dimethyl fumarate; for Pd n = 1, m = 2 and n = 2, m = 1; for Ni n = 1, m = 1, 2 and n = 2, m = 1) have been prepared, including a dimeric Ni complex in which the DMFU acts as a bridging ligand, via the double bond and one carbonyl oxygen. The bis-NHC complexes of Ni-0 and Pd-0 undergo facile ligand exchange equilibria in solution in the presence of excess olefin (DMFU), leading to mono-NHC-M-0-(olefin) complexes. This exchange is driven by the formation of an NHC-olefin coupling product. The dimeric Ni complex is labile in solution and can be used in situ, to provide a source of the L-Ni-0 moiety. The complexes M-0(NHC)(n)(DMFU)(m) (M = Pd, n = 1, m = 2; M = Ni, n = 1, m = 1, 2 and n = 2, m = 1) could be isolated as pure crystalline compounds in the solid state, and molecular structures were obtained.