Kinetic and Thermodynamic Study of Syn−Anti Isomerization of Nickel Complexes Bearing Amino-Linked N-Heterocyclic Carbene Ligands: The Effect of the Pendant Arm of the NHC

The mono- (2) and bis(amino-NHC) (3) nickel halide complexes can be prepared in moderate yield by reacting Ni(PPh3)2X2 with the amino pendant linked NHC 1, [C{(MES)N(CHCH)N(CH2CH2NHtBu)}], or its lithium bromide carbene adduct 1-LiBr. The mono(amino-carbene) nickel complexes (2) are paramagnetic species bearing a tetrahedrally distorted Ni center featuring a bidentate coordination mode via NHC and amine dative bonds. In contrast, the bis(amino-carbene) nickel complexes (3) are proton NMR active, consisting of a square-planar geometry with two trans NHC moieties. Both complexes 2 and 3 contain nickel−carbene bonds that are comparable to the literature value. Two forms of isomeric products for 3 are observed in the solution structure, but only solid structural syn−trans isomers are obtained, despite numerous attempts using various solvents with different polarity. On the basis of the powder X-ray analysis and various spectroscopy and theoretical results, we have ruled out a cis−trans isomerization, but conf...