Photoinduced de(thio)carboxylation dynamics monitored by time-resolved mid-IR-to-near-UV spectroscopies. From transition metals to main-group elements

3d过渡金属（TM）草酸盐的光化学对自然和技术中的各种过程具有重要意义。在过去，本项目致力于利用超快非线性激光光谱技术阐明这类物质的配体到金属电荷转移激发引起的光化学过程。第一个资助期的主要发现是：(1)当与TM结合时，双齿草甘膦配体容易发生异裂的cc键裂解，从而形成中性的CO2，迅速离开配体球体，而碳离子阴离子（CO2^2）仍然作为氧化还原活性配体结合在金属中心。（II）这种光反应模式建立了进入tm - co2配合物化学的入口。（III）对于高自旋体系，发现了一种初级产物配合物——草酸铁（ferrioxalate），该配合物具有特殊的弯曲，o端结合的CO2自由基阴离子配体，该配体与Fe(II)-中心具有铁磁性偶联。（IV）对于低自旋单氮化铁（III）模型配合物，初级产物具有CO2的经典侧对结合模式。(5)前人研究的结果有力地表明，co2结合模式可以通过自旋来控制，因此可以通过对TM配体球的精心设计来控制。由于其与二氧化碳活化相关的主题和受生物碳固定的启发，该继续项目现在打算扩展其研究方向：(1)光诱导主族元素配位化合物的脱羧，特别是Al和Si草酸盐。为了进一步了解所得到的开壳配体的性质及其与配位中心的相互作用，该项目还将扩大对与TMs和主基团元素连接的复杂双功能羧酸配体相关的光化学的研究。因此，重点放在(2)光诱导的TMs的高二羧酸盐和氨基羧酸盐的脱羧化和(3)光诱导的硫代草酸盐的脱（硫）羧化，所有这些都附着在TMs和主基团元素上。初步的量子化学计算表明，s配位硫代草酸盐的co2损失可以产生末端硫代配体。因此，这个项目可能为研究晚期tm -硫化物开辟了一条优雅的途径——这是一类极其罕见的配位化合物。所有这三条研究线都将在中红外、可见光和近紫外光谱区域再次使用超快非线性激光光谱来解决。

The photochemistry of 3d-transition metal (TM) oxalates is of significant importance for a variety of processes in nature and technology. In the past, this project was dedicated to elucidating the photochemical processes induced upon ligand-to-metal charge-transfer excitation of such species using ultrafast non-linear laser spectroscopy. The major findings during the first funding period are: (I) When attached to a TM, the bidentate oxalato ligand becomes susceptible to heterolytic CC-bond cleavage leading to the formation of a neutral CO2 that departs promptly from the ligand sphere and a carbonite anion (CO2^2─) that remains bound to the metal center as a redox-active ligand. (II) This photoreactivity pattern establishes an entry into the chemistry of TM-CO2-complexes. (III) For the high-spin system, ferrioxalate, a primary product complex was discovered, which features an exceptional bent, O-end-on-bound CO2●─ radical anion ligand that is ferromagentically coupled to an Fe(II)-center. (IV) For a low-spin monooxalato-iron(III) model complex, the primary product features the classical side-on binding mode of CO2. (V) The results obtained in previous project strongly suggest that the CO2-binding mode can be controlled by the spin and hence, by a careful design of the TM’s ligand sphere.Driven by its relevance for the topic of CO2-activation and inspired by biological carbon fixation, the continuation project now intends to expand ist research toward (1) the photo-induced decarboxylation of coordination compounds of main-group elements, specifically of Al and Si oxalates. To learn more about the nature of the resultant open-shell ligands and their interaction with the coordinating center, the project shall also expand ist research towards the photochemistry associated with intricate bifunctional carboxylate ligands attached to TMs and main-group elements. An emphasis is therefore laid here on (2) the photo-induced decarboxylation of higher dicarboxylates and amino carboxylates of TMs and on (3) the photo-induced de(thio)carboxylation of the thiooxalates, all of which attached to TMs and main-groups elements. Preliminary quantum-chemical calculations suggest that the CO2-loss of an S-coordinated thiooxalate can generate a terminal sulfido ligand. This project may thus open up an elegant route to late-TM-sulfides – a class of coordination compounds whose isolation is exceedingly rare. All three lines of research shall be tackled again using ultrafast non-linear laser spectroscopy in the mid-infrared, the visible, and the near ultraviolet spectral regions.