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

The rigid bidentate P,N-ligands 3a−d (phosphino substituent: a, Ph; b, mes; c, N-i-Pr2; d, NPh2), 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 [NiBr2(3a−d)] (4a−d). The derivatives 4a,d are both diamagnetic, while their counterparts 4b,c are paramagnetic with values of μeff(300 K) of 2.05 and 2.10 μB, respectively. The structure of 4a has been determined in the solid state by X-ray diffraction, which confirmed the κ2P,N coordination of 3a and the essentially square-planar geometry about Ni. In combination with EtAlCl2 the Ni(II) complexes of 4b−d are active ethylene oligomerization initiators (C2H4, 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 C4 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).