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 [Cp(X)M(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.