Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation

We recently reported a bis­(imino)­pyridine (or pyridine diimine, PDI) manganese precatalyst, (^Ph2PPrPDI)Mn (<b>1</b>), that is active for the hydrosilylation of ketones and dihydrosilylation of esters. In this contribution, we reveal an expanded scope for <b>1</b>-mediated hydrosilylation and propose two different mechanisms through which catalysis is achieved. Aldehyde hydrosilylation turnover frequencies (TOFs) of up to 4900 min^–1 have been realized, the highest reported for first row metal-catalyzed carbonyl hydrosilylation. Additionally, <b>1</b> has been shown to mediate formate dihydrosilylation with leading TOFs of up to 330 min^–1. Under stoichiometric and catalytic conditions, addition of PhSiH₃ to (^Ph2PPrPDI)Mn was found to result in partial conversion to a new diamagnetic hydride compound. Independent preparation of (^Ph2PPrPDI)­MnH (<b>2</b>) was achieved upon adding NaEt₃BH to (^Ph2PPrPDI)­MnCl₂ and single-crystal X-ray diffraction analysis revealed this complex to possess a capped trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone was added to <b>1</b>, radical transfer yielded (^Ph2PPrPDI<b>·</b>)­Mn­(OC<b>·</b>(Ph)­(CF₃)) (<b>3</b>), which undergoes intermolecular C–C bond formation to produce the respective Mn­(II) dimer, [(μ-<i>O</i>,<i>N</i>_py-4-OC­(CF₃)­(Ph)-4-H-^Ph2PPrPDI)­Mn]₂ (<b>4</b>). Upon finding <b>3</b> to be inefficient and <b>4</b> to be inactive, kinetic trials were conducted to elucidate the mechanisms of <b>1</b>- and <b>2</b>-mediated hydrosilylation. Varying the concentration of <b>1</b>, substrate, and PhSiH₃ revealed a first order dependence on each reagent. Furthermore, a kinetic isotope effect (KIE) of 2.2 ± 0.1 was observed for <b>1</b>-catalyzed hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6 was determined using <b>2</b>, suggesting <b>1</b> and <b>2</b> operate through different mechanisms. Although kinetic trials reveal <b>1</b> to be the more active precatalyst for carbonyl hydrosilylation, a concurrent <b>2</b>-mediated pathway is more efficient for carboxylate hydrosilylation. Considering these observations, <b>1</b>-catalyzed hydrosilylation is believed to proceed through a modified Ojima mechanism, while <b>2-</b>mediated hydrosilylation occurs via insertion