Synthesis and Structures of Nickel Complexes with a PN-Chelate Phosphaalkene Ligand

Abstract

Phosphaalkenes with a P═C bond possess extremely strong π-accepting ability, often providing transition metal complexes with interesting structures and properties. This paper describes unique structures of nickel complexes coordinated with a PN-chelate phosphaalkene ligand (PEP = 2-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). The PEP ligand combines with [NiBr2(dme)] (dme = 1,2-dimethoxyethane) in toluene to afford [Ni(Br)(μ-Br)(PEP)]2 (1), which reacts with R2Mg(thf)2 in Et2O or THF to form three types of nickel complexes depending on the R groups and reaction conditions. The reaction with Ph2Mg(thf)2 produces a Ni(I) dimer bridged with two μ-Br ligands, [Ni(μ-Br)(PEP)]2 (2). Treatment of 1 with R2Mg(thf)2 (R = Me, Me3SiCH2) at −35 °C leads to dialkyl complexes [NiR2(PEP)] (3 and 4), with a significantly distorted square planar configuration. DFT calculations support the occurrence of effective π-back-bonding between Ni to PEP to cause the structural distortion. On the other hand, the reaction of 1 with R2Mg(thf)2 (R = Me3SiCH2) conducted at a low temperature of −78 °C forms an aryl bromide complex of the formula [Ni(Mes*)(Br)(PEP*)] (5; Mes* = 2,4,6-tBu3C6H2), in which the Mes* group originally bonded to the phosphorus atom of PEP is shifted to nickel; instead, the phosphorus atom is substituted with the R group to form the PEP* ligand. The temperature-dependent formation of 4 or 5 is rationalized by considering a common five-coordinate intermediate.