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

Addition of BF3·OEt2 to ethereal solutions of the Ni(II) β-diketiminates [Me2NN]Ni(R)(2,4-lutidine)
(R = Et (1), Pr (2)) allows the isolation of the neutral β-H agostic monoalkyls [Me2NN]Ni(R) (R = Et (3), Pr
(4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni−Cα−Cβ angles with short Ni−Cβ distances,
indicating structures along the β-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 [Me2NN]Ni(CHMe2) (4b).
Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from β-H elimination,
in-plane rotation of the β-CH3 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
β-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear
β-H agostic propyl isomer 4a as the ground state with the branched β-H agostic isomer 4b slightly higher
in energy. NMR studies and calculations show that the β-agostic 3 reluctantly coordinates ethene and that
3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect KH/KD =
1.3(2) measured for the loss of 2,4-lutidine from 1 to form β-agostic 3 was also examined by DFT calculations.