Ramakrishnan Srinivasan, Waldie Kate M, Warnke Ingolf, De Crisci Antonio G, Batista Victor S, Waymouth Robert M, Chidsey Christopher E D
Department of Chemistry, Stanford University , Stanford, California 94305, United States.
Department of Chemistry, Yale University , New Haven, Connecticut 06520-81087, United States.
Inorg Chem. 2016 Feb 15;55(4):1623-32. doi: 10.1021/acs.inorgchem.5b02556. Epub 2016 Feb 2.
The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298 K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.
氢化钌[RuH(CNN)(dppb)]（1；CNN = 2-氨基甲基-6-甲苯基吡啶，dppb = 1,4-双（二苯基膦基）丁烷）在环境条件下与二氧化碳迅速且不可逆地反应，生成相应的钌甲酸盐配合物2。相比之下，氢化钌1与丙酮可逆反应生成钌异丙醇盐，通过在氘代四氢呋喃中用¹H NMR测得反应自由能ΔG°(298 K) = -3.1 kcal/mol。利用校准到酮插入实验测量自由能的密度泛函理论（DFT）来评估和比较丙酮和二氧化碳插入1的Ru-氢键的机理和能量学。计算得到的丙酮插入反应坐标涉及通过从金属进行氢化物转移和从CNN配体上的N-H基团进行质子转移，将二氢逐步向外球转移到丙酮。相比之下，计算得到的二氧化碳插入的最低能量途径是通过最初将Ru-H氢化物转移到二氧化碳，然后将所得的N-H稳定化甲酸盐旋转为Ru-O键合的甲酸盐。DFT计算用于评估辅助配体对二氧化碳插入热力学的影响，结果表明，增加与氢化物配体顺式的配体的π酸度以及增加与它反式的配体的σ碱度会降低二氧化碳插入的自由能，这为设计能够实现二氧化碳和甲酸盐可逆、能量中性相互转化的金属氢化物体系提供了一种策略。
