Haptotropism in a Nickel Complex with a Neutral, π-Bridging cyclo-P4 Ligand Analogous to Cyclobutadiene

Abstract

Dedicated to Professor Manfred Scheer on the occasion of his 65th birthday. The reaction of (1)Ni(η2-cod), 2, incorporating a chelating bis(N-heterocyclic carbene) 1, with P4 in pentane yielded the dinuclear complex [(2)Ni]2(μ2,η2 : η2-P4), 3, formally featuring a cyclobutadiene-like, neutral, rectangular, π-bridging P4-ring. In toluene, the butterfly-shaped complex [(1)Ni]2(μ2,η2 : η2-P2), 4, with a formally neutral P2-unit was obtained from 2 and either P4 or 3. Computational studies showed that a haptotropic rearrangement involving two isomers of the μ2,η2 : η2-P4 coordination mode and a low-energy μ2,η4 : η4-P4 coordination mode, as previously predicted for related nickel cyclobutadiene complexes, could explain the coalescence observed in the low-temperature NMR spectra of 3. The insertion of the (1)Ni fragment into a P−P bond of P7(SiMe3)3, forming complex 5 with a norbornane-like P7 ligand, was also observed.