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 p<i>K</i>_a of 10.6 was estimated for [CpRe­(PPh₃)­(NO)­(CHOH)]⁺ by measuring the p<i>K</i>_a for the analogous [CpRe­(PPh₃)­(NO)­(CMeOH)]⁺. The hydride donor ability (Δ<i>G</i>°_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 P_H₂. The free energy for H₂ addition (Δ<i>G</i>°_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^XM­(L)­(NO)­(CO)]⁺ (M = Re, Mn) complexes indicate that Δ<i>G</i>°_H₂ can be varied by 11 kcal mol^–1 through variation of both the ancillary ligands and the metal