Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine–Benzophenone Ligand in Alkyne Cyclotrimerization

Adaptive ligands,
which can adapt their coordination mode to the
electronic structure of various catalytic intermediates, offer the
potential to develop improved homogeneous catalysts in terms of activity
and selectivity. 2,2′-Diphosphinobenzophenones have previously
been shown to act as adaptive ligands, the central ketone moiety preferentially
coordinating reduced metal centers. Herein, the utility of this scaffold
in nickel-catalyzed alkyne cyclotrimerization is investigated. The
complex [(p‑tolL1)­Ni­(BPI)]
(p‑tolL1 = 2,2′-bis­(di­(para-tolyl)­phosphino)-benzophenone; BPI = benzophenone imine)
is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal
alkynes, selectively affording 1,2,4-substituted benzenes from terminal
alkynes. In particular, this catalyst outperforms closely related
bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction
pathway involving a hemilabile interaction of the CO unit
with the nickel center. This is further borne out by a comparative
study of the observed resting states and DFT calculations.