Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India.
Inorg Chem. 2011 Mar 7;50(5):1775-85. doi: 10.1021/ic102195w. Epub 2011 Jan 18.
The present work deals with the isomeric complexes of the molecular composition [Ru(II)(trpy)(L)Cl] in 1 and 2 (trpy = 2,2':6',2''-terpyridine, L = deprotonated form of quinaldic acid, HL). Isomeric identities of 1 and 2 have been established by their single-crystal X-ray structures, which reveal that under the meridional configuration of trpy, O(-) and N donors of the unsymmetrical L are in trans, cis and cis, trans configurations, respectively, with respect to the Ru-Cl bond. Compounds 1 and 2 exhibit appreciable differences in bond distances involving Ru-Cl and Ru-O1/Ru-N1 associated with L on the basis of their isomeric structural features. In relation to isomer 2, the isomeric complex 1 exhibits a slightly lower Ru(II)-Ru(III) oxidation potential [0.35 (1), 0.38 (2) V versus SCE in CH(3)CN] as well as lower energy MLCT transitions [559 nm and 417 nm (1) and 533 nm and 378 nm (2)]. This has also been reflected in the DFT calculation where a lower HOMO-LUMO gap of 2.59 eV in 1 compared to 2.71 eV in 2 is found. The isomeric structural effect in 1 and 2 has also been prominent in their (1)H NMR spectral profiles. The relatively longer Ru-Cl bond in 1 (2.408(2) Å) as compared to 2 (2.3813(9) Å) due to the trans effect of the anionic O(-) of coordinated L makes it labile, which in turn facilitates the transformation of [Ru(II)(trpy)(L)(Cl)] (1) to the solvate species, Ru(II)(trpy)(L)(CH(3)CN) (1a) while crystallizing 1 from the coordinating CH(3)CN solvent. The formation of 1a has been authenticated by its single-crystal X-ray structure. However, no such exchange of "Cl(-)" by the solvent molecule occurs in 2 during the crystallization process from the coordinating CH(3)CN solvent. The labile Ru-Cl bond in 1 makes it a much superior precatalyst for the epoxidation of alkene functionalities. Compound 1 is found to function as an excellent precatalyst for the epoxidation of a wide variety of alkene functionalities under environmentally benign conditions using H(2)O(2) as an oxidant and EtOH as a solvent, while isomer 2 remains almost ineffective under identical reaction conditions. The remarkable differences in catalytic performances of 1 and 2 based on their isomeric structural aspects have been addressed.
本工作研究了分子组成 [Ru(II)(trpy)(L)]Cl 的顺反异构体配合物 1 和 2（trpy = 2,2':6',2''-三联吡啶，L = 喹哪啶酸的去质子形式，HL）。通过单晶 X 射线结构确定了 1 和 2 的顺反异构体身份，这表明在 trpy 的赤道构型下，O(-)和 L 的非对称 N 供体分别处于反式、顺式和顺式、反式构型，相对于 Ru-Cl 键。基于其顺反异构体结构特征，化合物 1 和 2 在涉及 Ru-Cl 和 Ru-O1/Ru-N1 的键长上表现出明显的差异。与异构体 2 相比，异构体配合物 1 表现出稍低的 Ru(II)-Ru(III)氧化电位 [0.35(1)，0.38(2) V 相对于 SCE 在 CH(3)CN 中]以及较低的 MLCT 跃迁能量 [559nm 和 417nm(1)和 533nm 和 378nm(2)]。这也反映在 DFT 计算中，其中在 1 中发现较低的 HOMO-LUMO 隙为 2.59 eV，而在 2 中为 2.71 eV。1 和 2 中的顺反异构体效应在它们的 (1)H NMR 光谱谱图中也很明显。由于配位 L 的阴离子 O(-)的反式效应，1 中相对较长的 Ru-Cl 键（2.408(2)Å）与 2 中的 2.3813(9)Å 相比不稳定，这反过来又促进了 [Ru(II)(trpy)(L)(Cl)]（1）转化为溶剂化物物种 [Ru(II)(trpy)(L)(CH(3)CN)]（Cl）（1a），同时从配位 CH(3)CN 溶剂中结晶 1。通过单晶 X 射线结构鉴定了 1a 的形成。然而，在从配位 CH(3)CN 溶剂中结晶时，2 中没有“Cl(-)”被溶剂分子交换。1 中不稳定的 Ru-Cl 键使其成为烯烃官能团环氧化的优异前催化剂。在环境友好的条件下，使用 H(2)O(2)作为氧化剂和 EtOH 作为溶剂，发现 1 可作为多种烯烃官能团环氧化的优良前催化剂，而在相同的反应条件下，异构体 2 几乎无效。根据它们的顺反异构体结构方面的差异，探讨了 1 和 2 在催化性能上的显著差异。
