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

Abstract

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 confirmed a syn-anti conformational equilibrium exhibited in the solution of 3. The variable-temperature NMR for complexes 3 are consistent with restricted rotation about the nickel−carbene carbon bond. Plots of Keq vs 1/T are obtained with calculated ΔH° of 6.03 and 7.61 kJ/mol and ΔS° of 16.1 and 24.1 J/mol·K in toluene-d8 for 3-Br and 3-Cl, respectively, indicating that syn−anti conformers of 3 are more stable. The outcome of the theoretical calculation is in line with the experimental result.