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

详细信息    查看全文

• 作者：Eric S. Wiedner ; Aaron M. Appel
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：June 18, 2014
• 年：2014
• 卷：136
• 期：24
• 页码：8661-8668
• 全文大小：428K
• 年卷期：v.136,no.24(June 18, 2014)
• ISSN：1520-5126

文摘

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^{鈥?/sup>, and e鈥?/sup> have been determined. A pK_a of 10.6 was estimated for [CpRe(PPh₃)(NO)(CHOH)]+ by measuring the pK_a for the analogous [CpRe(PPh₃)(NO)(CMeOH)]+. The hydride donor ability (螖G掳_H鈥?/sup>) of CpRe(PPh₃)(NO)(CH₂OH) was estimated to be 58.0 kcal mol鈥?, 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 (螖G掳_H2) to [CpRe(PPh₃)(NO)(CO)]+ (+15 kcal mol鈥?) 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 螖G掳_H2 can be varied by 11 kcal mol鈥? through variation of both the ancillary ligands and the metal.}