Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States.
J Am Chem Soc. 2012 Sep 26;134(38):15743-57. doi: 10.1021/ja302937q. Epub 2012 Sep 11.
Despite the fundamental importance of the hydricity of a transition metal hydride (ΔG(H–)°(MH) for the reaction M–H → M+ + H–) in a range of reactions important in catalysis and solar energy storage, ours (J. Am. Chem. Soc.2009, 131, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru(II) hydride complexes previously studied in water. For [Ru(η(6)-C6Me6)(bpy)H]+ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru(η(6)-C6Me6)(bpy)H]+ pKa = 22.5 ± 0.1 and the Ru(η(6)-C6Me6)(bpy)(NCCH3)(1/0) electrochemical potential (−1.22 V vs Fc+/Fc). For [Ru(tpy)(bpy)H]+ (tpy = 2,2′:6′,2″-terpyridine) we utilized organic hydride ion acceptors (A+) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined K for the hydride transfer reaction, S + MH+ + A+ → M(S)2+ + AH (S = CD3CN, MH+ = [Ru(tpy)(bpy)H]+), by 1H NMR measurements. Equilibration of initially 7 mM solutions was slow--on the time scale of a day or more. When E°(H+/H–) is taken as 79.6 kcal/mol vs Fc+/Fc as a reference, the hydricities of [Ru(η(6)-C6Me6)(bpy)H]+ and [Ru(tpy)(bpy)H]+ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The pKa estimated for [Ru(tpy)(bpy)H]+ in acetonitrile is 32 ± 3. UV–vis spectroscopic studies of [Ru(η(6)-C6Me6)(bpy)]0 and [Ru(tpy)(bpy)]0 indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water.
尽管过渡金属氢化物的亲水性（ΔG(H–)°(MH)对于反应 M–H → M+ + H–）在催化和太阳能存储中一系列重要反应中具有根本重要性，但我们的（J. Am. Chem. Soc.2009, 131, 2794）是仅有的在水溶剂中报告的值，并且没有基础可以将这些值与已经为乙腈溶剂确定的更广泛范围进行比较，特别是。因此，我们使用了各种方法来确定先前在水中研究过的 Ru(II)氢化物配合物在乙腈中的亲水性值。对于 [Ru(η(6)-C6Me6)(bpy)H]+（bpy = 2,2′-联吡啶），我们使用了基于评估 [Ru(η(6)-C6Me6)(bpy)H]+ 的酸度的热力学循环 pKa = 22.5 ± 0.1 和 [Ru(η(6)-C6Me6)(bpy)(NCCH3)(1/0)]（2+/0）电化学势（-1.22 V 对 Fc+/Fc）。对于 [Ru(tpy)(bpy)H]+（tpy = 2,2′:6′,2″-三联吡啶），我们利用了具有特征亲水性的有机氢化物离子受体（A+），其亲水性源自咪唑鎓阳离子和吡啶鎓阳离子，并通过 1H NMR 测量确定了氢转移反应的 K，S + MH+ + A+ → M(S)2+ + AH（S = CD3CN，MH+ = [Ru(tpy)(bpy)H]+）。最初为 7 mM 溶液的平衡很慢——在一天或更长时间的时间范围内。当 E°(H+/H–) 取为 79.6 kcal/mol 对 Fc+/Fc 作为参考时，[Ru(η(6)-C6Me6)(bpy)H]+ 和 [Ru(tpy)(bpy)H]+ 的亲水性分别估计为 54 ± 2 和 39 ± 3 kcal/mol，分别为在乙腈中，与分别在水介质中发现的 31 和 22 kcal/mol 值进行比较。在乙腈中估计的 [Ru(tpy)(bpy)H]+ 的 pKa 为 32 ± 3。[Ru(η(6)-C6Me6)(bpy)]0 和 [Ru(tpy)(bpy)]0 的紫外可见光谱研究表明，它们分别含有还原的 bpy 和 tpy 配体。这些结论得到了 DFT 电子结构结果的支持。比较乙腈和亲水性值表明，将氢化物配合物转移到水中会使亲水性范围变平或压缩。
