Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst

The formally zero-valent Mn dimer [(Ph2PEtPDI)­Mn]2 has been synthesized upon reducing (Ph2PEtPDI)­MnCl2 with excess Na/Hg. Single crystal X-ray diffraction analysis has revealed that [(Ph2PEtPDI)­Mn]2 possesses a κ4-PDI chelate about each Mn center, as well as η2-imine coordination across the dimer. The chelate metrical parameters suggest single electron PDI reduction and EPR spectroscopic analysis afforded a signal consistent with two weakly interacting S = 1/2 Mn centers. At ambient temperature in the absence of solvent, [(Ph2PEtPDI)­Mn]2 has been found to catalyze the hydrosilylation of aldehydes at loadings as low as 0.005 mol % (0.01 mol % relative to Mn) with a maximum turnover frequency of 9,900 min–1 (4,950 min–1 per Mn). Moreover, the [(Ph2PEtPDI)­Mn]2-catalyzed dihydrosilylation of formates has been found to proceed with turnover frequencies of up to 330 min–1 (165 min–1 relative to Mn). These metrics are comparable to those described for the leading Mn catalyst for this transformation, the propylene-bridged variant (Ph2PPrPDI)­Mn; however, [(Ph2PEtPDI)­Mn]2 is more easily inhibited by donor functionalities. Carbonyl and carboxylate hydrosilylation is believed to proceed through a modified Ojima mechanism following dimer dissociation.