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, M0(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 Ni0 and Pd0 undergo facile ligand exchange equilibria
in solution in the presence of excess olefin (DMFU), leading to mono-NHC−M0−(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−Ni0 moiety. The complexes
M0(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.