Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes

To gain insight into thermodynamic barriers for reduction of CO into CH3OH, free energies for reduction of [CpRe­(PPh3)­(NO)­(CO)]+ into CpRe­(PPh3)­(NO)­(CH2OH) have been determined from experimental measurements. Using model complexes, the free energies for the transfer of H+, H–, and e– have been determined. A pKa of 10.6 was estimated for [CpRe­(PPh3)­(NO)­(CHOH)]+ by measuring the pKa for the analogous [CpRe­(PPh3)­(NO)­(CMeOH)]+. The hydride donor ability (ΔG°H–) of CpRe­(PPh3)­(NO)­(CH2OH) was estimated to be 58.0 kcal mol–1, based on calorimetry measurements of the hydride-transfer reaction between CpRe­(PPh3)­(NO)­(CHO) and [CpRe­(PPh3)­(NO)­(CHOMe)]+ to generate the methylated analogue, CpRe­(PPh3)­(NO)­(CH2OMe). Cyclic voltammograms recorded on CpRe­(PPh3)­(NO)­(CMeO), CpRe­(PPh3)­(NO)­(CH2OMe), and [CpRe­(PPh3)­(NO)­(CHOMe)]+ displayed either a quasireversible oxidation (neutral species) or reduction (cationic species). These potentials were used as estimates for the oxidation of CpRe­(PPh3)­(NO)­(CHO) or CpRe­(PPh3)­(NO)­(CH2OH) or the reduction of [CpRe­(PPh3)­(NO)­(CHOH)]+. Combination of the thermodynamic data permits construction of three-dimensional free energy landscapes under varying conditions of pH and PH2. The free energy for H2 addition (ΔG°H2) to [CpRe­(PPh3)­(NO)­(CO)]+ (+15 kcal mol–1) was identified as the most significant thermodynamic impediment for the reduction of CO. DFT computations on a series of [CpXM­(L)­(NO)­(CO)]+ (M = Re, Mn) complexes indicate that ΔG°H2 can be varied by 11 kcal mol–1 through variation of both the ancillary ligands and the metal.