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 <i>S</i> = 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

A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies

Heating (THF)₂MnCl₂ in the presence of the pyridine-substituted bis­(imino)­pyridine ligand, ^PyEtPDI, allowed preparation of the respective dihalide complex, (^PyEtPDI)­MnCl₂. Reduction of this precursor using excess Na/Hg resulted in deprotonation of the chelate methyl groups to yield the bis­(enamide)­tris­(pyridine)-supported product, (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)­Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (<i>S</i> = ³/₂) Mn­(II) center by the Evans method and electron paramagnetic resonance spectroscopy. Furthermore, (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)Mn was determined to be an effective precatalyst for the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 2475 min^–1 when employed under solvent-free conditions. This optimization allowed for isolation of the respective alcohols and, in two cases, the partially reacted silyl ethers, PhSiH­(OR)₂ [R = Cy and CH­(Me)­(ⁿBu)]. The aldehyde hydrosilylation activity observed for (κ⁵-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-^PyEtPDEA)Mn renders it one of the most efficient first-row transition metal catalysts for this transformation reported to date