Thermally driven intramolecular charge transfer in an oxo-molybdenum dithiolate complex
Thermally driven intramolecular charge transfer in an oxo-molybdenum dithiolate complex
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DOI：
10.1021/ja0100470
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发表时间：
2001-10-24
影响因子：
15
通讯作者：
Kirk, ML
中科院分区：
文献类型：
--
作者：
Helton, ME;Gebhart, NL;Kirk, ML
在过渡金属离子和氧化还原活性配体之间可能发生的协同相互作用形成了无数复杂现象的基础，包括容易的氧化还原行为，1-3酶催化，4，5新的光学性质，6，7和价态互变异构。8-20一个明智的选择的金属和配体允许可访问的低位电子状态具有相当大的电荷转移字符的发生。这种金属配体的氧化还原相互作用，原则上，可以利用构建新的分子和分子为基础的多性能材料。基本目标是能够控制这些不同的属性，并最终通过外部扰动在它们之间“切换”。这对全光分子开关和分子RAM器件的发展是极其重要的。具有氧化还原活性配体的过渡金属配合物所表现出的最迷人的性质之一是价态互变异构现象（氧化还原异构现象）。迄今为止，所有显示价态互变异构的络合物都利用烯-1，2-二醇酯（二氧戊环）供体8、9或它们的席夫碱变体10、11来促进利用儿茶酚合/半醌合对的热诱导的分子内氧化还原反应。已经指出，这种氧化还原相互转化依赖于ML键合方案中的低程度的共价性。[9]这个最初的修饰词是用来描述一个真正的金属-配体电子转移过程，其中一个完整的电子等价物在金属和配体之间转移。据推测，金属和配体波函数必须局部化，以促进离散的氧化还原相互转换。然而，与相应的金属二醇盐相比，金属二硫醇盐具有高度离域的成键异构性21 -24，因此这类分子的价态互变异构现象尚未被预期。在这里，我们显示了氧代钼二硫纶配合物[MoO（qdt）2]，（qdt）喹喔啉-2，3-二硫纶）2]的变温吸收数据，这清楚地揭示了该化合物的高度热致变色性质。虽然所观察到的热致变色可以在价态互变异构的背景下解释，但我们更倾向于更一般的描述，即热驱动的分子内电荷转移，因为这更准确地反映了Mo-S键的高度共价性质21 -24和qdt配体的极端非无辜性。25，26在此，我们讨论了[MoO（qdt）2]-在吡喃蝶呤酶的催化循环中基于分子的开关装置和电荷再分配方面的独特性质。30）深蓝色Mo（IV）化合物（PPh 4）2 [MoO（qdt）2]的溶液产生单电子氧化的[MoO（qdt）2]-。27瓶绿色[MoO（qdt）2]-是高度热致变色的，在浸入液氮中时几乎瞬间变成深橙子。[28]在图1所示的变温电子吸收光谱中，观察到的热致变色的程度非常明显。89 K吸收光谱在14 750和18 000 cm-1处显示出两个强烈的低能电荷转移特征，这些吸收带是我们迄今为止所研究的所有[MoVO（dithiolate）2]-化合物的特征。24磁性圆二色性的观察
The synergistic interactions that can occur between transition metal ions and redox active ligands form the basis for a myriad of complex phenomena including facile redox behavior, 1-3 enzymatic catalysis, 4, 5 novel optical properties, 6, 7 and valence tautomerism. 8-20 A judicious choice of metal and ligand allows for the occurrence of accessible low-lying electronic states possessing considerable charge-transfer character. This metalligand redox interplay can, in principle, be exploited to construct novel molecular and molecule-based multiproperty materials. The fundamental goal is the ability to control these various properties and ultimately “switch” between them via an external perturbation. This is extremely important in the development of all-optical molecular switches6 and molecular RAM devices. One of the most fascinating properties displayed by transition metal complexes possessing redox active ligands is valence tautomerism (redox isomerism). To date all of the complexes which display valence tautomerism have utilized ene-1, 2-diolate (dioxolene) donors, 8, 9 or their Schiff-base variants, 10, 11 to facilitate a thermally induced intramolecular redox reaction utilizing the catecholato/semiquinonato couple. It has been stated that this redox interconversion is dependent upon a low degree of covalency in the ML bonding scheme. 9 This initial qualifier for complexes which may display“valence tautomerism” was meant to describe a true metal-ligand electron-transfer process, where a full electron equivalent is transferred between the metal and ligand. Presumably, the metal and ligand wave functions must be localized to facilitate a discrete redox interconversion. However, metallodithiolates possess a highly delocalized bonding description21-24 compared to the corresponding metallodiolates and therefore valence tautomerism has not been anticipated for this class of molecules. Here we show variable-temperature absorption data for the oxomolybdenum dithiolate complex [MoO (qdt) 2],(qdt) quinoxaline-2, 3-dithiolate) 2], which clearly reveals the highly thermochromic nature of this compound. Although the observed thermochromism may be explained within the context of valence tautomerism we prefer a more general description, namely thermally driven intramolecular charge transfer, as this reflects more accurately the highly covalent nature of the Mo-S bonds21-24 and the extreme noninnocence of the qdt ligand. 25, 26 Herein, we discuss the unique properties of [MoO (qdt) 2]-in terms of molecule-based switching devices and charge redistribution in the catalytic cycle of pyranopterin enzymes.Substoichiometric addition of ferrocenium hexafluorophosphate to a dry toluene: DMF (70: 30) solution of the dark blue Mo (IV) compound (PPh4) 2 [MoO (qdt) 2] generates the one-electron oxidized [MoO (qdt) 2]-. 27 Bottle green [MoO (qdt) 2]-is highly thermochromic, turning dark orange almost instantaneously upon submersion into liquid nitrogen. 28 The extent of the observed thermochromism is clearly evident in the variable-temperature electronic absorption spectra shown in Figure 1. The 89 K absorption spectrum displays two intense low-energy chargetransfer features at 14 750 and 18 000 cm-1, and these bands are characteristic of all the [MoVO (dithiolate) 2]-compounds we have studied to date. 24 The observation of magnetic circular dichroism