Coinage metal aluminyl complexes: probing regiochemistry and mechanism in the insertion and reduction of carbon dioxide

The synthesis of coinage metal aluminyl complexes, featuring M-Al covalent bonds, is reported via a salt metathesis approach employing an anionic Al(i) ('aluminyl') nucleophile and group 11 electrophiles. This approach allows access to both bimetallic (1 : 1) systems of the type ((Bu3P)-Bu-t)MAl(NON) (M = Cu, Ag, Au; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) and a 2 : 1 di(aluminyl)cuprate system, K[Cu{Al(NON)}(2)]. The bimetallic complexes readily insert heteroallenes (CO2, carbodiimides) into the unsupported M-Al bonds to give systems containing a M(CE2)Al bridging unit (E = O, NR), with the mu-kappa(1)(C):kappa(2)(E,E ') mode of heteroallene binding being demonstrated crystallographically for carbodiimide insertion in the cases of all three metals, Cu, Ag and Au. The regiochemistry of these processes, leading to the formation of M-C bonds, is rationalized computationally, and is consistent with addition of CO2 across the M-Al covalent bond with the group 11 metal acting as the nucleophilic partner and Al as the electrophile. While the products of carbodiimide insertion are stable to further reaction, their CO2 analogues have the potential to react further, depending on the identity of the group 11 metal. ((Bu3P)-Bu-t)Au(CO)(2)Al(NON) is inert to further reaction, but its silver counterpart reacts slowly with CO2 to give the corresponding carbonate complex (and CO), and the copper system proceeds rapidly to the carbonate even at low temperatures. Experimental and quantum chemical investigations of the mechanism of the CO2 to CO/carbonate transformation are consistent with rate-determining extrusion of CO from the initially-formed M(CO)(2)Al fragment to give a bimetallic oxide that rapidly assimilates a second molecule of CO2. The calculated energetic barriers for the most feasible CO extrusion step (Delta G(double dagger) = 26.6, 33.1, 44.5 kcal mol(-1) for M = Cu, Ag and Au, respectively) are consistent not only with the observed experimental labilities of the respective M(CO)(2)Al motifs, but also with the opposing trends in M-C (increasing) and M-O bond strengths (decreasing) on transitioning from Cu to Au.

Automated summary

The synthesis of coinage metal aluminyl complexes via a salt metathesis approach allows the insertion of heteroallenes and formation of M(CE2)Al bridging units, with differences in crystal structure and reactivity observed for copper, silver, and gold. The mechanism of CO2 to CO/carbonate transformation involves extrusion of CO from M(CO)(2)Al fragment to form a bimetallic oxide that assimilates a second molecule of CO2, with energetic barriers consistent with experimental observations and trends in M-C and M-O bond strengths.