Ethylene Dimerization and Oligomerization Using Bis(phosphino)boryl Supported Ni Complexes.

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type ( R PBP)NiBr (R = t Bu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol ethylene ·mol Ni -1 ·s -1 for the formation of butenes with 41(1)% selectivity for 1-butene using ( Ph PBP)NiBr, and 68 mol ethylene ·mol Ni -1 ·s -1 for butenes production with 87.2(3)% selectivity for 1-butene using ( t Bu PBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and ( t Bu PBP)NiBr as the precatalyst, ethylene oligomerization to form C 4 through C 20 products was achieved, while the use of ( Ph PBP)NiBr as the pre-catalyst retained selectivity for C 4 products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni-B unit of the ( R PBP)Ni catalysts is proposed as a key element of the Ni catalysis.