The principal objective of this project is to synthesize low valent manganese compounds and evaluate their potential to catalyze hydrosilylation and dehydrogenative silylation reactions. The former transformation is conducted to prepare value-added silicones from olefins and hydrosiloxane functionalities; however, its industrial implementation often requires the use of expensive and toxic platinum catalysts. Because manganese is an essential element that is Earth-abundant (950 mg/kg in Earth’s crust), catalysts that feature this metal can be considered promising replacements for platinum- and precious metal-based reagents. In 2014, we discovered that reduction of the bis(imino)pyridine (or pyridine diimine, PDI) compound (Ph2PPrPDI)MnCl2 allows for the isolation of (Ph2PPrPDI)Mn (shown below, J. Am. Chem. Soc. 2014, 136, 882-885).
Although the formal oxidation state of the metal in (Ph2PPrPDI)Mn is Mn(0), thorough electronic structure analysis has revealed that this compound features a Mn(II) center, where two electrons have been transferred to the redox non-innocent PDI chelate (J. Am. Chem. Soc. 2017, 139, 4901-4915). This reversible electron transfer allows the metal to retain its preferred Mn(II) oxidation state following Si-H activation, a feature that enables (Ph2PPrPDI)Mn to perform hydrosilylation reactions with exceptional activity (Acc. Chem. Res. 2017, 50, 2842-2852). Notably, at catalyst loadings as low as 0.01 mol%, the hydrosilylation of aldehydes and ketones using an equimolar quantity of phenylsilane proceeds in an exothermic fashion with turnover frequencies (TOFs) of greater than 1,000 min-1. While somewhat slower, (Ph2PPrPDI)Mn has also been shown to mediate the dihydrosilylation of formates and esters to yield a mixture of silyl ethers.
We have recently identified Mn catalysts that mediate olefin hydrosilylation and dehydrogenative silylation and our continued efforts will aim to prepare value-added polymers and reagents.
Our work in this area seeks to expand the scope of carbon dioxide utilization in large scale chemical synthesis by unveiling a molecular level understanding of the catalytic capture and incorporation of carbon dioxide from either flue gas or the atmosphere into value-added organic products. The benefits of accomplishing this challenge remain two-fold; advances in this area could lower overall carbon dioxide emissions and circumvent the current industrial demand for oxidative petroleum consumption. We are currently focused on preparing molybdenum complexes that can bind and reduce carbon dioxide using chemical reductants.
Several years ago, we discovered that the bis(imino)pyridine molybdenum complex (Ph2PPrPDI)Mo(CO) slowly catalyzes the hydrosilylation of aldehydes (Inorg. Chem. 2014, 53, 9357-9365). In order to prepare Mo catalysts that more efficiently reduce carbonyl moieties, including the C=O bonds of CO2, removal of the CO ligand was targeted. Adding I2 and heating the respective intermediate allowed for the isolation of [(Ph2PPrPDI)MoI][I]. Surprisingly, reduction of this compound resulted in chelate C-H bond activation to form the 7-coordinate hydride complex, (κ6-Ph2PPrPDI)MoH (shown below, Inorg. Chem. 2015, 54, 7506-7515).
In the presence of a reductant such as pinacolborane (HBPin), this complex has been shown to insert carbon dioxide and selectively reduce it into H3COBPin, which undergoes hydrolysis in water to yield methanol. This transformation has been achieved with TOFs of up to 40 h-1 at 90 °C, and current efforts in our laboratory are aimed at developing modified catalysts that are highly active for CO2 hydrofunctionalization at ambient temperature. Using related complexes, we have recently demonstrated the selective capture of CO2 from air and have uncovered CO2 reductive coupling pathways that may allow for the preparation of carbon chain extended organic products.
3) Earth-Abundant Metal Catalysts for Organic Transformations
This instability has complicated efforts to employ (Ph2PPrDI)Fe(N2) as a catalyst; however, applying the same ligand to Co has allowed for the preparation of hydride complex, (Ph2PPrDI)CoH (above, at middle). This compound was found to feature a redox non-innocent DI chelate, exhibit alkyne hydroboration TOFs of up to 900 h-1 at ambient temperature, and catalyze nitrile dihydroboration with modest activity at 60 °C (Chem. Commun. 2017, 53, 7333-7336). Adding the same ligand to Ni(COD)2 affords the 4-coordinate precatalyst, (Ph2PPrDI)Ni (above, at right). This compound exhibits carbonyl, ester, and alkyne hydrosilylation activity and has recently been made commercially available (Sigma Aldrich Catalog No. 798975). Ongoing efforts in our laboratory are aimed at expanding the synthetic utility of these catalysts and preparing second-generation variants.