Intramolecular Agostic Interactions and Dynamics of a Methyl Group at a Preorganized Dinickel(II) Site

Alkyl nickel intermediates relevant to catalytic processes often feature agostic stabilization, but relatively little is known about the situation in oligonickel systems. The dinickel(I) complex K[LNi^I₂], which is based on a compartmental pyrazolato-bridged ligand L^3- with two β-diketiminato chelate arms, or its masked version, the dihydride complex [KL(Ni^II-H)₂] that readily releases H₂, oxidatively add methyl tosylate to give diamagnetic [LNi^II₂(CH₃)] (1) with d(Ni···Ni) ≈ 3.7 Å. Structural characterization shows that the methyl group in 1 is bound to one Ni^II and exhibits an intramolecular agostic interaction with the more distant Ni^II. This is supported spectroscopically (viz., a ν(C-H) stretch at 2658 cm^-1 and lowered ¹J_C-H of 114 Hz) and by DFT calculations, including topological analysis of the computed electron density for 1. NMR spectroscopy reveals very fast hopping of the CH₃ group between the two Ni^II ions, which according to DFT has a minute barrier of 4 kcal mol^-1 and proceeds via a planar CH₃ moiety in the transition state (Walden-like inversion). The alkylidene group in K[LNi₂(μ-CHSi(Me₃)₃)], obtained from the reaction of [KL(Ni-H)₂] with N₂CHSiMe₃, is symmetrically bridging. This work provides new insight into the stabilization and dynamics of alkyl ligands at dinickel sites with a constrained metal···metal distance.