Hydrosilylation of Aldehydes and Formates Using a Dimeric Manganese Precatalyst

The formally zero-valent Mn dimer [(<sup>Ph2PEt</sup>PDI)­Mn]<sub>2</sub> has been synthesized upon reducing (<sup>Ph2PEt</sup>PDI)­MnCl<sub>2</sub> with excess Na/Hg. Single crystal X-ray diffraction analysis has revealed that [(<sup>Ph2PEt</sup>PDI)­Mn]<sub>2</sub> possesses a κ<sup>4</sup>-PDI chelate about each Mn center, as well as η<sup>2</sup>-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 <i>S</i> = <sup>1</sup>/<sub>2</sub> Mn centers. At ambient temperature in the absence of solvent, [(<sup>Ph2PEt</sup>PDI)­Mn]<sub>2</sub> 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<sup>–1</sup> (4,950 min<sup>–1</sup> per Mn). Moreover, the [(<sup>Ph2PEt</sup>PDI)­Mn]<sub>2</sub>-catalyzed dihydrosilylation of formates has been found to proceed with turnover frequencies of up to 330 min<sup>–1</sup> (165 min<sup>–1</sup> relative to Mn). These metrics are comparable to those described for the leading Mn catalyst for this transformation, the propylene-bridged variant (<sup>Ph2PPr</sup>PDI)­Mn; however, [(<sup>Ph2PEt</sup>PDI)­Mn]<sub>2</sub> is more easily inhibited by donor functionalities. Carbonyl and carboxylate hydrosilylation is believed to proceed through a modified Ojima mechanism following dimer dissociation.