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

((PNP)-P-iPr-P-H)Mn(CO)(2)(OH) (2; (PNP)-P-iPr-P-H = HN{CH2CH2((PPr2)-Pr-i)}(2)) was formed from the reversible 1,2 addition of water to ((PNP)-P-ipr)Mn(CO)(2) (1; (PNP)-P-iPr = the deprotonated, amide form of the ligand, N-{CH2CH2(PiPr(2))}(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 ((PNP)-P-iPr-P-H)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 H-2 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.