Organoscandium complexes supported by four different ancillary ligand frameworks were studied in the context of developing catalysts for CO2 hydrosilation. These include known B-diketiminate supported bis-alkyls, as well as novel anilido-imine and anilido-pyridine based scandium alkyls, incorporating previously published ligand frameworks. A new anilido-bypyridyl ligand set was designed and synthesized, based on insights garnered from studying the previous systems. For each system, a survey of reactivity with CO2 was undertaken, followed by a study of their activity as catalysts for CO2 hydrosilation, using trialkylsilanes as reductants and a B(C6F5)3 co-catalyst.
In all cases, facile insertions into alkyl-scandium bonds were observed, to give isolable carboxylate complexes. In the case of the B-diketiminate systems, exposure to an excess of CO2 led to transannular addition of a third equivalent of CO2 into the ligand framework, leading to ligand decoordination. A cationic complex was formed by abstraction of acetate with B(C6F5)3, which was found to active for CO2 hydrosilation to methane.
The carboxylate complexes of the anilido-imine and anilido-pyridine based complexes underwent no further deleterious reactivity when exposed to excess CO2. For the latter, L2ScR type carboxylates were preferentially formed, and generation of reactive cations was accomplished by addition of B(C6F5)3 and trialkylsilane, forming a silyl-ester adduct with hydridoborate counter anion. This ion pair was catalytically active, accomplishing ~85 turnovers in a period of 13 hours, however the catalytic activity was eventually quenched, likely due to the buildup of the bis-silyl acetal hydrosilation product which barred the approach and activation of CO2 at the metal center.
The anilido-bypyridine system was designed to be inert towards CO2 after ligation, but to preserve the favourable steric and electronic properties of the _-diketiminate framework. Three increasingly sterically demanding derivatives were prepared and installed on scandium. The bulkiest of these incorporated a pendant 3,5-di-tert-butylphenyl group, which underwent metallative C-H bond activation to form a dianionic tetradentate ligand. It was found to be a robust catalyst for CO2 hydrosilation, giving unprecendented selectivity for the bis-silyl acetal product. The mechanism of the conversion was studied, and was shown to involve alternating shuttling of borane and silylium across a coordinatively labile carboxylate group.