On the Viability of Catalytic Turnover via Al−O/B−H Metathesis: The Reactivity of β‐Diketiminate Aluminium Hydrides towards CO 2 and Boranes

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 CO 2 . The resulting κ 1 ‐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)(OBX 2 ) (X 2 =cat, 9‐BBN). However, no hint of Al−O/B−H metathesis is observed even under forcing conditions. With BH 3 ⋅ SMe 2 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)(κ 2 ‐BH 4 ) 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 BH 3 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 CO 2 by boranes mediated by these aluminium hydrides.