Anomalous Hydroboration at a Four-Coordinate Ni(II) Diphenylvinylphosphine Complex: Geometric Impacts on Solution Equilibria

For coordination compounds, reliable
control of metal
geometry
and spin state is of great interest, permitting control of reactivity
and selectivity. As tuneable outputs, these observables are adjusted
through thoughtful choice of ligand. This report examines a family
of monophosphine-ligated nickel(II) dihalide compounds containing
secondary coordination sphere borane groups. Interestingly, anomalous
hydroboration of trans-[NiII(X)2(PPh2Vin)2] (Vin = vinyl) was witnessed upon
treatment with HBCy2 (Cy = cyclohexyl), resulting in halide-directed
Markovnikov hydroboration; these branched products were observed to
be diamagnetic in solution. This outcome is in contrast with the linear
ligand variants, which were prepared independently and show solution
paramagnetism. Together, this report details an unusual hydroboration
outcome and how this anomalous reactivity gives way to unique solution
behavior of the resulting nickel(II) products simply by virtue of
linear or branched secondary coordination sphere incorporation.