Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan.
Chemistry. 2011 Jun 6;17(24):6652-62. doi: 10.1002/chem.201003522. Epub 2011 May 3.
The pterin-coordinated ruthenium complex, Ru(II) (dmdmp)(tpa) (1) (Hdmdmp=N,N-dimethyl-6,7-dimethylpterin, tpa=tris(2-pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex (2) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru(II) center to give an intermediate complex, Ru(dmdmp)(tpa)(acetone) (I), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (ϕ) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2, as a result of the rotation of the dmdmp(2-) ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first-order kinetics, and the rate constant at 298 K was determined to be 5.83×10(-5) s(-1). The activation parameters were determined to be ΔH(≠) =81.8 kJ mol(-1) and ΔS(≠) =-49.8 J mol(-1) K(-1). The negative ΔS(≠) value indicates that this reaction involves a seven-coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2, without going back to starting complex 1, and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1. The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first-order process in acetonitrile were determined to be ΔH(≠) =59.2 kJ mol(-1) and ΔS(≠) =-147.4 kJ mol(-1) K(-1). The largely negative ΔS(≠) value suggests the involvement of a seven-coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru(II) center is a determinant factor in the photoisomerization of the Ru(II)-pterin complex.
蝶啶配位钌配合物[Ru(II)(dmdmp)(tpa)]⁺(1)(Hdmdmp=N,N-二甲基-6,7-二甲基蝶啶，tpa=三(2-吡啶甲基)胺)可有效地进行光致变色异构化。对异构配合物(2)进行了全谱表征，揭示了蝶啶配体明显的 180°拟反转。在丙酮溶液中用光照射 1，用 460nm 的入射光照射，导致 tpa 配体的一个吡啶甲基臂从 Ru(II)中心解离，得到中间配合物[Ru(dmdmp)(tpa)(丙酮)]²⁺(I)，同时伴随着结构变化和溶剂分子的配位以占据空位点。该光反应的量子产率(ϕ)被确定为 0.87%。随后，中间物 I 经热过程生成异构配合物 2，这是由于 dmdmp(2-)配体的旋转和吡啶基团通过结构变化重新配位的结果。热过程遵循一级动力学，在 298 K 下确定的速率常数为 5.83×10⁻⁵ s⁻¹。确定的活化参数为ΔH(≠)=81.8 kJ·mol⁻¹和ΔS(≠)=-49.8 J·mol⁻¹·K⁻¹。负的ΔS(≠)值表明该反应在过渡态涉及七配位络合物(即，交换缔合机制)。该反应最独特的一点是，光解的吡啶甲基基团的重新配位仅从一个方向发生，生成异构体 2，而不会回到起始配合物 1，因此反应以 100%的转化率进行。在加热 2 在乙腈中的溶液时，异构体 2 变回起始配合物 1。该逆向反应强烈依赖于溶剂：异构体 2 非常稳定，在乙腈中难以回到 1；然而，在乙腈中加热时，2 可以顺利转化为 1。在乙腈中一级过程的活化参数确定为ΔH(≠)=59.2 kJ·mol⁻¹和ΔS(≠)=-147.4 kJ·mol⁻¹·K⁻¹。负的ΔS(≠)值表明，在过渡态中涉及到具有强配位乙腈分子的七配位物种。因此，溶剂分子与 Ru(II)中心的配位强度是决定 Ru(II)-蝶啶配合物光致异构化的因素。
