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

Carbon Dioxide Promoted H⁺ Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst

Heating a 1:1 mixture of (CO)₅MnBr and the phosphine-substituted pyridine diimine ligand, ^Ph2PPrPDI, in THF at 65 °C for 24 h afforded the diamagnetic complex [(^Ph2PPrPDI)­Mn­(CO)]­[Br] (<b>1</b>). Higher temperatures and longer reaction times resulted in bromide displacement of the remaining carbonyl ligand and the formation of paramagnetic (^Ph2PPrPDI)­MnBr (<b>2</b>). The molecular structure of <b>1</b> was determined by single crystal X-ray diffraction, and density functional theory (DFT) calculations indicate that this complex is best described as low-spin Mn­(I) bound to a neutral ^Ph2PPrPDI chelating ligand. The redox properties of <b>1</b> and <b>2</b> were investigated by cyclic voltammetry (CV), and each complex was tested for electrocatalytic activity in the presence of both CO₂ and Brønsted acids. Although electrocatalytic response was not observed when CO₂, H₂O, or MeOH was added to <b>1</b> individually, the addition of H₂O or MeOH to CO₂-saturated acetonitrile solutions of <b>1</b> afforded voltammetric responses featuring increased current density as a function of proton source concentration (<i>i</i>_cat/<i>i</i>_p up to 2.4 for H₂O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk electrolysis using 5 mM <b>1</b> and 1.05 M MeOH in acetonitrile at −2.2 V vs Fc^+/0 over the course of 47 min gave H₂ as the only detectable product with a Faradaic efficiency of 96.7%. Electrochemical experiments indicate that CO₂ promotes <b>1</b>-mediated H₂ production by lowering apparent pH. While evaluating <b>2</b> for electrocatalytic activity, this complex was found to decompose rapidly in the presence of acid. Although modest H⁺ reduction activity was realized, the experiments described herein indicate that care must be taken when evaluating Mn complexes for electrocatalytic CO₂ reduction