On the Viability of Catalytic Turnover via Al-O/B-H Metathesis: The Reactivity of β-Diketiminate Aluminium Hydrides towards CO2 and Boranes.

Abstract

A series of β-diketiminate stabilized aluminium hydrides of the type (Nacnac)Al(R)H has been synthesized offering variation in the auxiliary R substituent and in the Nacnac backbone. These show significant variation in the nature of the Al-H bond: electron-donating R groups give rise to weaker (and presumably more hydridic) Al-H bonds, leading to enhanced rates of reactivity towards CO2 . The resulting κ1 -formate complexes (Nacnac)Al(R){OC(O)H} have been isolated and their reactivity towards B-H-containing reductants probed. In the case of HBpin no reaction is observed (even under forcing conditions), while the more reactive boranes HBcat and {H(9-BBN)}2 ultimately yield boryloxy complexes of the type (Nacnac)Al(R)(OBX2 ) (X2 =cat, 9-BBN). However, no hint of Al-O/B-H metathesis is observed even under forcing conditions. With BH3 ⋅SMe2 the major product is a related boryloxy system, although (uniquely) in this case a minor product is observed which contains an Al-H bond. We hypothesize that (Nacnac)Al(R)(κ2 -BH4 ) is formed (despite the unfavourable thermodynamics of Al-O/B-H metathesis) due to the additional driving force provided by coordination of the strongly Lewis acidic BH3 unit to the Al-H bond. That said, we find that (unlike the analogous gallium systems) no catalytic turnover can be achieved in the reduction of CO2 by boranes mediated by these aluminium hydrides.