Influence of the Lewis Acidity of Gallium Atoms on the Reactivity of a Frustrated Lewis Pair: Experimental and Theoretical Studies

The reactivity of
the Ga/P-based frustrated Lewis pair (FLP) Mes2P–C­[C­(H)–Ph]–GatBu2 (3) is influenced
by the relatively
weak Lewis acidity of its Ga atom and differs significantly from that
of the analogous Al compound 1. The adduct of 3 with CO2 was only detectable at low temperature by NMR
spectroscopy. Benzaldehyde was coordinated only via a Ga–O
bond; the P atom was not involved. In contrast, a relatively persistent
adduct was formed with soft CS2 to yield a five-membered
GaCPCS heterocycle. Dehydrocoupling with H3B←NHMe2 afforded the dimeric amidoborane (H2B–NMe2)2, while an adduct with a GaCPBN heterocycle was
isolated with the sterically less shielded ammonia–borane H3B←NH3. The latter product was unstable in
solution and decomposed by H2 elimination and formation
of oligomeric BN compounds. Small quantities of 3 catalyzed
hydrogen transfer from H3B←NH3 to an
imine. The Lewis acidities of the Al/P- and Ga/P-based FLPs were examined
by experiments (Gutmann–Beckett method) and by calculation
of the fluoride ion affinity (including the B and In analogues). The
Al compound is the strongest Lewis acid; the Ga FLP is significantly
weaker but is a stronger F– acceptor in comparison
to the unknown analogues of B and In. These results reflect the different
reactivities of these FLPs and may help to develop FLPs with finely
adjusted properties.