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 (RPBP)­NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 molethylene·molNi –1·s–1 for the formation of butenes with 41(1)% selectivity for 1-butene using (PhPBP)­NiBr, and 68 molethylene·molNi –1·s–1 for butenes production with 87.2(3)% selectivity for 1-butene using ( tBuPBP)­NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and ( tBuPBP)­NiBr as the precatalyst, ethylene oligomerization to form C4 through C20 products was achieved, while the use of (PhPBP)­NiBr as the pre-catalyst retained selectivity for C4 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 (RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.