Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst

The formally zero-valent Mn dimer [((PDI)-P-Ph2PEt)Mn](2) has been synthesized upon reducing ((PDI)-P-ph2PEt)MnCl2 with excess Na/Hg. Single crystal X-ray diffraction analysis has revealed that ((PDI)-P-Ph2PEt)Mn-2 possesses a K-4-PDI chelate about each Mn center, as well as eta(2)-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, [((PDI)-P-Ph2PEt)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 [((PDI)-P-Ph2PEI)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 ((PDI)-P-Ph2PPr)Mn; however, [((PDI)-P-Ph2PEt](2) is more easily inhibited by donor functionalities. Carbonyl and carboxylate hydrosilylation is believed to proceed through a modified Ojima mechanism following dimer dissociation.