High-Valent Metal-Oxo Coordination Complexes for Homogenous Water Oxidation

该项目旨在研究单核第一排过渡金属络合物，即锰、铁和钴络合物作为均相水氧化催化剂的潜力，并阐明O-O键形成的基本机制。我们的目标是通过应用两步法获得分子洞察力：(A)在定制的三角配体环境中合成和表征高价Mn-、Fe-和Co-oxo配合物，以及(B)它们的生成和氧化底物转化(包括水)和相关O-O键形成的反应性、机理和动力学研究。研究计划的两个步骤都是迭代方法的一部分：电子表征、反应性和机械学研究将深入了解结构-功能关系，并允许重新设计新的复合体世代。所有研究的基础将是合成定制的螯合配体及其相应的三角前体络合物，这是形成寿命可修正的活性物种的先决条件，可用于光谱和反应性研究。所提出的N-锚定三卡宾和/或三酚配体骨架的高度模块化合成具有足够的适应性，可以迅速改变二次配位环境。三脚架螯合剂的N-杂环卡宾和酚类配体由于具有稳定异常高(和低)金属氧化态的显著能力而被精心选择，并将通过在配体框架中引入额外的官能团来进一步优化。随着所有已提出的螯合剂和非血红素Fe-、Mn-，特别是Co-氧和-过氧基物种的合成方案的提出，我们希望揭示二次相互作用对氧化反应活性的影响，特别是对H2O/底物氧化和O-O键形成的所有方面的影响。详细的光谱研究将旨在区分反应结果，例如，2-e氧化成过氧化氢或4-e氧化生成O2，以及反应机理。为了获得水氧化的证据，将验证产品混合物中过氧化氢和/或氧气的生成。我们还将通过研究通过替代合成路线获得的化合物来补充光谱研究，例如，通过使用各种氧转移试剂。预计比较光谱分析将对中间体进行独立验证。在手术中，中间体的表征和动力学研究将通过时间分辨的UV-Vis电子吸收和红外振动光谱以及高分辨率冷冻喷雾质谱学和EPR光谱进行。结合使用实验、光谱和(含时)密度泛函理论，将有助于验证成键模型和机制，研究氧化性质，并创建返回到配体和络合物合成的信息反馈循环。

The project seeks to study the potential of mononuclear first-row transition metal complexes, namely manganese, iron, and cobalt complexes, as catalysts for homogenous water oxidation and to elucidate the fundamental mechanism of O-O bond formation. We will aim at gaining molecular insight by applying a two-step approach: (a) the synthesis and electronic structure characterization of high-valent Mn-, Fe- and Co-oxo complexes in custom-tailored, trigonal ligand environments and (b) reactivity-, mechanistic-, and kinetic studies of their generation and oxidative substrate transformations, including water, and related O-O bond formation. Both steps of the research plan are part of an iterative approach: The electronic characterization, reactivity, and mechanistic studies will create insight into the structure-function relationships and allow for the re-design of new complex generations. The basis for all research will be the synthesis of custom-tailored chelating ligands and their corresponding trigonal precursor complexes, prerequisite for the formation of reactive species with lifetimes amendable to spectroscopic and reactivity studies. The highly modular synthesis of the proposed N-anchored tris-carbene and/or tris-phenolate ligand framework is sufficiently adaptable to swiftly vary the secondary coordination environment. The N-heterocyclic carbene and phenolate ligands of the tripodal chelators are carefully chosen due to their remarkable ability to stabilize unusual high (and low) metal oxidation states, and will be further optimized by introducing additional functional groups to the ligand framework. With all proposed chelators and the synthetic protocols for non-heme Fe-, Mn-, and particularly Co-oxo and -peroxo species at hand, we hope to reveal the effects of secondary interactions on oxidative reactivity with special emphasis on all aspects affecting H2O/substrate oxidation and O-O bond formation. Detailed spectroscopic investigations will aim at distinguishing reaction outcome, e.g., 2-e- oxidation to H2O2 or 4-e- oxidation to yield O2, and reaction mechanisms. To obtain evidence for water oxidation, the generation of H2O2 and/or O2 in the product mixture will be verified. We will complement the spectroscopic investigations by also studying compounds obtained via alternative synthetic routes; e.g., by employing various oxygen transfer reagents. Comparative spectroscopy is expected to give independent verification of the intermediates. In operando characterization of intermediates and kinetic studies will be pursued by time-resolved UV-Vis electronic absorption and IR vibrational spectroscopy as well as high-resolution cryo-spray mass spectrometry and EPR spectroscopy. The combined use of experiment, spectroscopy, and (time-dependent) density functional theory will assist to validate bonding models and mechanisms, investigate oxidizing properties, and create informational feedback looping back to ligand- and complex synthesis.