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

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.