Structure and Dynamics of Neutral β-H Agostic Nickel Alkyls:  A Combined Experimental and Theoretical Study

Addition of BF(3).OEt(2) to ethereal solutions of the Ni(II) beta-diketiminates [Me(2)NN]Ni(R)(2,4-lutidine) (R = Et (1), Pr (2)) allows the isolation of the neutral beta-H agostic monoalkyls [Me(2)NN]Ni(R) (R = Et (3), Pr (4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni-C(alpha)-C(beta) angles with short Ni-C(beta) distances, indicating structures along the beta-H elimination pathway. Positional disorder of the alkyl group in the X-ray structure of 4 corresponds to partial (22%) occupancy by the secondary alkyl [Me(2)NN]Ni(CHMe(2)) (4b). Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from beta-H elimination, in-plane rotation of the beta-CH(3) group, and a tetrahedral triplet structure for 3 that were investigated by density functional theory calculations at the Becke3LYP/6-31G level of theory without simplifications on the beta-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear beta-H agostic propyl isomer 4a as the ground state with the branched beta-H agostic isomer 4b slightly higher in energy. NMR studies and calculations show that the beta-agostic 3 reluctantly coordinates ethene and that 3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect K(H)/K(D) = 1.3(2) measured for the loss of 2,4-lutidine from 1 to form beta-agostic 3 was also examined by DFT calculations.