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.