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.