Thermochemical Insight into the Reduction of CO to CH3OH with [Re(CO)]+ and [Mn(CO)]+ Complexes
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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 [CpXM(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.
为了明晰CO还原制备甲醇（CH3OH）过程中的热力学能垒，本研究通过实验测定得到了[CpRe(PPh3)(NO)(CO)]+还原为CpRe(PPh3)(NO)(CH2OH)的吉布斯自由能变化。借助模型配合物，研究人员测定了H+、H–与e–转移过程的吉布斯自由能。通过类比测定[CpRe(PPh3)(NO)(CMeOH)]+的酸解离常数（pKa），估算得到[CpRe(PPh3)(NO)(CHOH)]+的pKa为10.6。基于CpRe(PPh3)(NO)(CHO)与[CpRe(PPh3)(NO)(CHOMe)]+之间的氢负离子转移反应的量热测试结果，本研究估算得到CpRe(PPh3)(NO)(CH2OH)的氢负离子给体能力（ΔG°H–）为58.0 kcal mol–1，该反应生成了甲基化类似物CpRe(PPh3)(NO)(CH2OMe)。对CpRe(PPh3)(NO)(CMeO)、CpRe(PPh3)(NO)(CH2OMe)以及[CpRe(PPh3)(NO)(CHOMe)]+开展循环伏安法测试，结果显示其分别表现出准可逆氧化（中性物种）或还原（阳离子物种）行为。上述电位被用于估算CpRe(PPh3)(NO)(CHO)或CpRe(PPh3)(NO)(CH2OH)的氧化过程，以及[CpRe(PPh3)(NO)(CHOH)]+的还原过程。整合所有热力学数据后，可构建出不同pH与H2分压（PH2）条件下的三维吉布斯自由能图谱。[CpRe(PPh3)(NO)(CO)]+的H2加成自由能（ΔG°H2）为+15 kcal mol–1，该步骤被确定为CO还原过程中最关键的热力学阻碍因素。针对一系列[CpXM(L)(NO)(CO)]+（M=Re、Mn）配合物的密度泛函理论（DFT, Density Functional Theory）计算结果表明，通过同时改变辅助配体与金属中心，ΔG°H2的可调变范围可达11 kcal mol–1。
