Thermodynamic Analysis of Metal–Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition

The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)­hydrogenation catalyst, (PNP)­Ru–Cl (PNP = 2,6-bis­(di-tert-butylphosphinomethyl)­pyridine; Ru = Ru­(CO)­(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)­Ru, with various H–X species (to give (PNP)­Ru–X; X = H, OH, OMe, OCHO, OC­(O)­NMe2) reveal that formic acid equilibrium is approximately temperature-independent; relative to H2, it is more favored at elevated temperatures. We also measure the hydricity of (PNP)­Ru–H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal–ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.

在胺类存在下将二氧化碳（CO₂）加氢制备甲酸盐、甲酰胺与甲醇（methanol, MeOH）的路径，是优化碳捕获与碳循环流程的极具前景的方案。要实现这一目标，阐明催化剂设计对选择性与催化性能的影响至关重要。本文系统开展了(PNP)-Ru-Cl型（脱氢）加氢催化剂的全面热化学分析，其中PNP = 2,6-双(二叔丁基膦甲基)吡啶；Ru = Ru(CO)(H)，并将所得结果与催化剂性能进行关联。尽管已知该催化剂可在弱碱作用下将CO₂加氢为甲酸盐，但我们的研究表明，在强碱条件下可生成甲醇。与pKa（酸解离常数）的测定结果一致，对强碱的需求表明，六配位钌物种的去质子化是生成甲醇的催化循环的核心步骤。我们的研究还显示，生成的甲醇浓度与催化剂浓度无关，这与依赖甲醇浓度而非当量的失活路径相符。我们对脱芳香化同系物(*PNP)-Ru与各类H-X物种（以生成(PNP)-Ru-X；X = H、OH、OMe、OCHO、OC(O)NMe₂）开展了温度依赖性平衡研究，结果表明甲酸平衡几乎不受温度影响；相较于氢气（H₂），其在高温下更易自发进行。我们还测定了四氢呋喃（tetrahydrofuran, THF）中(PNP)-Ru-H的氢化物亲合性，并阐明了底物的后续配位如何影响表观氢化物亲合性。本研究的结论可广泛应用于加氢与脱氢催化领域，尤其是那些可在催化剂上发生金属-配体协同作用（metal–ligand cooperativity, MLC）的体系。这些研究结果为未来的催化剂对比研究提供了基准参照，并将对合理催化剂设计产生积极推动作用。