Thermochemical Insight into the Reduction of CO to CH_3OH with [Re(CO)]~+ and [Mn(CO)]~+ Complexes

摘要

To gain insight into thermodynamic barriers for reduction of CO into CH_3OH, free energies for reduction of [CpRe(PPh_3)(NO)(CO)]~+ into CpRe(PPh_3)(NO)(CH_2OH) have been determined from experimental measurements. Using model complexes, the free energies for the transfer of H~+, H~-, and e~- have been determined. A pK_a, of 10.6 was estimated for [CpRe(PPh_3)(NO)-(CHOH)]~+ by measuring the pK_a, for the analogous [CpRe(PPh_3)-(NO)(CMeOH)]~+. The hydride donor ability (ΔG~°_(H~-)) of CpRe-(PPh_3)(NO)(CH_2OH) was estimated to be 58.0 kcal mol~(-1), based on calorimetry measurements of the hydride-transfer reaction between CpRe(PPh_3)(NO)(CHO) and [CpRe(PPh_3)(NO)(CHOMe)]~+ to generate the methylated analogue, CpRe(PPh_3)(NO)(CH_2OMe). Cyclic voltammograms recorded on CpRe(PPh_3)(NO)-(CMeO), CpRe(PPh_3)(NO)(CH_2OMe), and [CpRe(PPh_3)(NO)(CHOMe)]~+ displayed either a quasireversible oxidation (neutral species) or reduction (cationic species). These potentials were used as estimates for the oxidation of CpRe(PPh_3)(NO)(CHO) or CpRe(PPh_3)(NO)(CH_2OH) or the reduction of [CpRe(PPh_3)(NO)(CHOH)]~+. Combination of the thermodynamic data permits construction of three-dimensional free energy landscapes under varying conditions of pH and P_(H_2). The free energy for H_2 addition (ΔG°_(H_2)) to [CpRe(PPh_3)(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 AG°_(h_2) can be varied by 11 kcal mol~(-1) through variation of both the ancillary ligands and the metal.