β-oxo-δ-diimine Ni Complexes: A Comparison of Tautomeric Active Species in Ethylene Polymerization Catalysis

Chiu, H.-C.; Pearce, A. J.; Dunn, P. L.; Cramer, C. J.; Tonks, I. A.
Organometallics 2016, 35, 2076 (doi:10.1021/acs.organomet.6b00256).

A series of mono- and bimetallic Ni alkyl complexes of a β-oxo-δ-diimine (BODDI) ligand are reported. The monometallic complexes have a second binding pocket, of which the free "arm" can exist as either an enamine (e.g., 8, BODEI, β-oxo-δ-enamineiminato) or imine (e.g., 3, BODII, β-oxo-δ-imineiminato) tautomer. The identity of the tautomer in the secondary Ni coordination sphere has a significant effect on ethylene polymerization behavior: the enamine tautomer, which hydrogen bonds to the central O atom and is in conjugation with the N,O backbone chelate, is significantly more electron rich and yields much lower molecular weight polymer than the imine tautomer, which rotates away from Ni to a distal position and has little effect on polymerization. Deprotonation of the second binding pocket with M(HMDS) (M = Li, Na, K) yields the Ni-alkali metal heterobimetallic complexes 3Li, 3Na, and 3K. The deprotonated alkali metal enamides display ethylene polymerization behavior similar to the neutral imine complex because the enamide arm can also distally rotate to minimize interaction with the Ni coordination sphere upon activation.