The Synthesis, Lewis Acidity and Catalytic Activity of Bis(catecholato)germanes
Abstract
Main group Lewis acids have been shown to be viable alternatives to state-of-theart transition metal catalysts. While extensive research into a variety of p-block Lewis acids have been reported, the field of germanium Lewis acid chemistry has been described as “almost non-existent”. A variety of bis(catecholato)germane derivatives have been synthesized. The Lewis acidity of these compounds was analyzed by the Gutmann-Beckett and fluoride ion affinity methods demonstrating the high Lewis acidity of these complexes. The bis(catecholato)germanes were utilized as Lewis acid catalysts for the hydrosilylation of aldehydes, the hydroboration of alkynes, Friedel-Crafts alkylation of alkenes, and the oligomerization of styrene derivatives. Notably, the use of donor additives resulted in tunable product selectivity in the oligomerization of αmethylstyrene comparable to the selectivity that can be achieved using transition metal catalysts. The mechanism of catalysis by bis(catecholato)germanes in the oligomerization of α-methylstyrene was examined using variable time normalization analysis, Hammett analysis, and density functional theory calculations of Gibbs free energies of key intermediates, revealing the species with only one donor solvent ligated to the germanium centre is the active catalyst species. Finally, the use of bis(catecholato)germanes as potential Lewis acid components in frustrated Lewis pairs was investigated. The reactivity of the bis(catecholato)germane complexes with various bulky bases and supporting DFT calculations revealed the formation of Lewis adducts. The reactivity of the Lewis pairs was explored; however, no small molecule activation or catalysis, typical reactivity of frustrated Lewis pairs, was observed. While Lewis pairs derived from bis(catecholato)germanes do not exhibit any reactivity typical of frustrated Lewis pairs, weak donor complexes of bis(catecholato)germanes are highly Lewis acidic and capable of facilitating several reactions as a Lewis acid catalyst. Previous work in the literature has shown the potential of a green, solvent free synthesis of the bis(catecholato)germanes and catalytic activity using water as a solvent, demonstrating the principles of green chemistry. The green chemistry of bis(catecholato)germanes, in conjunction with their high catalytic activity in a variety of reactions and their tuneability to influence product selectivity, illustrate that bis(catecholato)germanes are exciting alternatives to transition metal catalysts and should be explored further.