Reactions Triggered by Lewis Acidic Organoaluminum Species. In: Woodward S, Dagorne S, editors. Modern Organoaluminum Reagents. Berlin: Springer Berlin Heidelberg; 2013. pp. 187-214. [DOI: 10.1007/3418_2012_37]

Cooperative Heterobimetallic CO2 Activation Involving a Mononuclear Aluminum(II) Intermediate

Molecular chemistry of aluminum most commonly involves AlIII ions due to their noble gas electronic configurations. In contrast, the chemistry of AlII ions is underexplored and may contain undiscovered reaction manifolds. Here, we report the CO2 activation chemistry of a transient AlII intermediate supported by a chelating, dianionic ligand and investigate the electronic structure details and reaction mechanisms required to access this reactivity. We found that a heterobinuclear complex, (NON)Al-FeCp(CO)2 (1), undergoes Al-Fe bond homolysis at ambient conditions to reveal the [(NON)Al]•/[CpFe(CO)2]• radical pair in situ. The presence of predominantly Al-centered spin density (i.e., an AlII ion) within this radical pair was established by quantum-chemical calculations and with experiments in which radical scavengers (TEMPO, benzophenone) induce Al-Fe bond homolysis. Exposure of 1 to CO2 atmosphere resulted in insertion of CO2 into the Al-Fe bond. This net 2-electron CO2 reduction process was computationally modeled using density functional theory and direct dynamics simulations, revealing that reduction involves two 1-electron steps and, thus, depends on stabilization of high-energy [CO2]•- by coordination to aluminum. This mechanism for CO2 activation is unexpected given the canonical predisposition of CO2 for multielectron reduction processes and demonstrates the possibility of discovering new reaction profiles using earth-abundant elements in unusual oxidation states.

Heterometallic aluminates: alkali metals trapped by an aluminium aryloxide claw

A series of heterometallic aluminium-alkali metal species [AlMMe2{2,6-(MeO)2C6H3O}2]n have been isolated for lithium, sodium and potassium. These compounds can be generated by the reaction of [AlMe2{2,6-(MeO)2C6H3O}]2 with the metallated phenol [M{2,6-(MeO)2C6H3O}]n or through the reaction of the mixture of AlMe3 and the appropriate alkali metal alkyl base with two equivalents of 2,6-dimethoxyphenol. In the heterometallic species obtained, the {AlMe2{2,6-(MeO)2C6H3O}2}(-) moiety is observed and could be described as a claw which fixes the alkali ion by the phenoxide oxygen atoms while the methoxy groups help to stabilize their coordination sphere. All compounds have been characterized by NMR spectroscopy and X-ray diffraction methods. Catalytic studies reveal that these compounds are active in ring-opening polymerization of L-lactide.