Redox Transformations of Sulfane Sulfur at Zinc Thiolate Complexes and Other Metal Cations

This dissertation details investigations into the reactivity between Zn-S bonds in molecular zinc thiolate complexes and elemental sulfur. With spectroscopic, voltammetric, and crystallographic evidence, changes in both metal cation with benzenethiolate anion and the ligand environment of zinc thiolate complexes were observed to influence whether sulfur was inserted, resulting in zinc polysulfanide products, or reduced via thiolate oxidation.
Chapter 2 describes the comparison of nucleophilic reactivity between synthetic zinc dithiolate and zinc tetrasulfanide complexes. Comparing rates of catalytic thiolate-disulfide exchange as well as alkylation kinetics demonstrates that sulfane sulfur decreases the nucleophilic reactivity of the less basic tetrasulfanide moiety when compared to that of the zinc thiolate.
Chapter 3 describes the study of the shifts induced by redox-inert metal cations in the equilibria that results from reaction between thiolate anion and elemental sulfur, forming distributions of polysulfanide anions, polysulfide anions, and organic disulfide species. Spectroscopic characterization was used to track solution-phase speciation of reduced sulfur species and thiolate oxidation to organic disulfide. Electrochemical analysis reveals changes in equilibria in the presence of metal cations results from metal-sulfur interactions which shift the sulfur reduction and thiolate oxidation half-reaction potentials. Together with computational studies, the stability of strong, covalent metal polysulfide interactions of more highly-charged cations over those of metal thiolates or metal polysulfanides is shown to promote sulfur reduction and thiolate oxidation pathways over sulfur catenation at thiolate.
Chapter 4 describes the effects of ligand environment in zinc thiolate complexes on their reactivity with sulfur. Modulation of thiolate-containing ligand across a series of 4- and 5-coordinate platforms suggests a dissociative mechanism of thiolate nucleophilicity dependent on ligand electronics and chelation. More electron-rich and monodentate thiolate donors were observed to favor sulfur reduction and thiolate oxidation while electron-poor, chelating thiolate moieties selectively insert sulfur to form zinc polysulfanide complexes.

本论文针对分子硫醇锌（zinc thiolate）配合物中的锌-硫（Zn-S）键与单质硫（elemental sulfur）之间的反应活性展开了系统研究。借助光谱学、伏安法与晶体学证据，研究团队观察到：金属阳离子与苯硫醇根阴离子（benzenethiolate anion）的配对方式变化，以及硫醇锌配合物的配体环境（ligand environment）改变，均会影响硫的反应路径——既可能发生硫插入反应生成多硫醇锌（zinc polysulfanide）产物，也可能通过硫醇盐氧化实现硫还原。
第二章对比了合成二硫醇锌（zinc dithiolate）配合物与四硫醇锌（zinc tetrasulfanide）配合物的亲核反应活性（nucleophilic reactivity）。通过对比催化硫醇盐-二硫键交换（catalytic thiolate-disulfide exchange）速率与烷基化动力学（alkylation kinetics），研究证实：相较于硫醇锌配合物，连硫硫原子（sulfane sulfur）会降低碱性更弱的四硫醇盐基团（tetrasulfanide moiety）的亲核反应活性。
第三章探究了氧化惰性金属阳离子（redox-inert metal cation）引发的平衡偏移——该平衡由硫醇根阴离子与单质硫反应生成，产物为多硫醇根阴离子（polysulfanide anion）、多硫化物阴离子（polysulfide anion）与有机二硫化物物种（organic disulfide species）的分布体系。本研究采用光谱表征追踪还原硫物种的溶液相物种分布（solution-phase speciation）以及硫醇盐氧化为有机二硫化物的过程。电化学分析表明，金属阳离子存在时的平衡变化源于金属-硫相互作用，该作用改变了硫还原与硫醇盐氧化的半反应电位（half-reaction potential）。结合计算研究（computational study）结果可知：相较于硫醇盐或多硫醇锌配合物，高电荷金属阳离子所形成的强共价金属多硫化物相互作用（covalent metal polysulfide interaction）更稳定，这会促使反应沿硫还原与硫醇盐氧化路径进行，而非在硫醇盐位点发生硫链合（sulfur catenation）反应。
第四章探究了硫醇锌配合物的配体环境对其与硫反应活性的影响。对一系列四配位与五配位平台上的含硫醇盐配体进行调控的实验表明，硫醇盐的亲核性解离机制取决于配体的电子效应与螯合作用（chelation）。实验发现，电子云密度更高的单齿硫醇盐供体更倾向于发生硫还原与硫醇盐氧化反应，而电子云密度较低的螯合型硫醇盐基团则选择性地发生硫插入反应，生成多硫醇锌配合物。