Rigid <i>N</i>-Phosphino Guanidine P,N Ligands and Their Use in Nickel-Catalyzed Ethylene Oligomerization

Abstract

The rigid bidentate P,N-ligands 3a − d (phosphino substituent: a, Ph; b, mes; c, N- i -Pr 2; d, NPh 2 ), each with a 7-phosphino-1,5,7-triazabicyclo[4.4.0]dec-5-ene skeleton, are readily prepared in high yields and have been used in the preparation of the nickel complexes [NiBr 2 ( 3a − d )] ( 4a − d ). The derivatives 4a, d are both diamagnetic, while their counterparts 4b, c are paramagnetic with values of μ eff (300 K) of 2.05 and 2.10 μ B, respectively. The structure of 4a has been determined in the solid state by X-ray diffraction, which confirmed the κ 2 P,N coordination of 3a and the essentially square-planar geometry about Ni. In combination with EtAlCl 2 the Ni(II) complexes of 4b − d are active ethylene oligomerization initiators (C 2 H 4, 1 bar; Ni:Al = 1:14; toluene), affording varying mixtures of butenes, hexenes, and octenes (trace), depending on the nature of the P-donor, but with a reasonable selectivity toward C 4 with complex 4c . In contrast, under identical reaction conditions complex 4a gives rise to products resulting from a sequence of ethylene oligomerization and subsequent Friedel−Crafts alkylation of the toluene solvent. Notably, no activity toward ethylene (1 bar) was observed for 4 /MAO (Ni:Al = 1:15 or 1:500).