Thermochemical insight into the reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ complexes

Abstract

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(PPh 3)(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(PPh 3)(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(PPh 3)(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)(CH 2OH) 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°H 2) 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°H 2 can be varied by 11 kcal mol-1 through variation of both the ancillary ligands and the metal. © 2014 American Chemical Society.