Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru–PNP Pincer Complexes

Ruthenium PNP complex 1a (RuH­(CO)­Cl­(HN­(C2H4Pi-Pr2)2)) represents
a state-of-the-art catalyst for low-temperature (<100 °C)
aqueous methanol dehydrogenation to H2 and CO2. Herein, we describe an investigation that combines experiment,
spectroscopy, and theory to provide a mechanistic rationale for this
process. During catalysis, the presence of two anionic resting states
was revealed, Ru–dihydride (3–) and Ru–monohydride (4–) that are deprotonated at nitrogen in the pincer ligand backbone.
DFT calculations showed that O- and CH- coordination
modes of methoxide to ruthenium compete, and form complexes 4– and 3–, respectively. Not only does the reaction rate
increase with increasing KOH, but the ratio of 3–/4– increases, demonstrating that the “inner-sphere”
CH cleavage, via CH coordination
of methoxide to Ru, is promoted by base. Protonation of 3– liberates H2 gas and
formaldehyde, the latter of which is rapidly consumed by KOH to give
the corresponding gem-diolate and provides the overall driving force
for the reaction. Full MeOH reforming is achieved through the corresponding
steps that start from the gem-diolate and formate. Theoretical studies
into the mechanism of the catalyst Me-1a (N-methylated 1a) revealed that CH coordination
to Ru sets-up CH cleavage and hydride delivery; a process
that is also promoted by base, as observed experimentally. However,
in this case, Ru–dihydride Me-3 is much more stable
to protonation and can even be observed under neutral conditions.
The greater stability of Me-3 rationalizes the lower
rates of Me-1a compared to 1a, and also
explains why the reaction rate then drops with increasing KOH concentration.

钌PNP配合物1a（RuH(CO)Cl(HN(C₂H₄Pi-Pr₂)₂)）是目前用于低温（<100 ℃）下甲醇水溶液脱氢制备氢气与二氧化碳的顶尖催化剂。本文报道了一项结合实验、光谱学与理论计算的研究，旨在阐明该反应的机理。催化过程中，我们发现体系存在两种阴离子型静息态：钳形配体骨架氮位点去质子化后的Ru-二氢化物（3⁻）与Ru-单氢化物（4⁻）。密度泛函理论（DFT, Density Functional Theory）计算表明，甲氧基与钌中心存在两种配位模式——O配位与CH配位，分别对应生成配合物4⁻与3⁻。不仅反应速率随氢氧化钾（KOH）浓度升高而提升，3⁻/4⁻的比值也同步增大，这证明通过甲氧基与钌中心的CH配位实现的“内球（inner-sphere）”C-H裂解过程可被碱促进。3⁻发生质子化后会释放氢气与甲醛，甲醛可迅速与氢氧化钾反应生成对应的偕二醇盐，这为整个反应提供了总驱动力。完整的甲醇重整过程可通过以偕二醇盐与甲酸盐为起始底物的后续步骤实现。针对甲基化修饰后的催化剂Me-1a（即N-甲基化的1a）的机理理论研究表明，其与钌中心的CH配位可触发C-H裂解与氢化物转移过程，该过程同样可被碱促进，与实验观测结果一致。但在此体系中，Ru-二氢化物Me-3的质子化稳定性显著提升，甚至可在中性条件下被观测到。Me-3更高的稳定性既可解释Me-1a的催化活性低于1a的原因，也能说明为何随着氢氧化钾浓度升高，该反应的速率会出现下降。