Reversible 1,2-Addition of Water To Form a Nucleophilic Mn(I) Hydroxide Complex: A Thermodynamic and Reactivity Study

Abstract

(iPrPNHP)Mn(CO)2(OH) (2; iPrPNHP = HN{CH2CH2(PiPr2)}2) was formed from the reversible 1,2-addition of water to (iPrPNP)Mn(CO)2 (1; iPrPNP = the deprotonated, amide form of the ligand, –N{CH2CH2(PiPr2)}2). This reversible reaction was probed via variable-temperature NMR experiments, and the energetics of the 1,2-addition/elimination was found to be slightly exothermic (−0.8 kcal/mol). The corresponding manganese hydroxide was found to react with aldehydes, yielding the corresponding manganese carboxylate complexes (iPrPNHP)Mn(CO)2(CO2R), where R = H, methyl, phenyl. While no reaction between 1 and neat benzaldehyde was observed, in the presence of water, conversion to the corresponding manganese-bound benzoate with formation of H2 was observed. The catalytic oxidation of benzaldehyde by water without additives was unsuccessful due to strong product inhibition, with the manganese benzoate formed under a variety of reaction conditions. Upon addition of base, a catalytic cycle for the conversion of aldehyde to carboxylate and hydrogen can be devised.