Actinide Polyoxo Chemistry

铀是自然产生的最重的元素，是核废料的主要成分。在空气和环境中，它会形成一种叫做铀酰化合物的二氧化盐，这些二氧化盐都是基于一个带双重电荷的线性O=U=O基团。这些化合物非常易溶，是有问题的环境地下水污染物。U=O键在化学上也非常牢固，几乎不倾向于参与在生物和工业环境中具有化学和催化用途的过渡金属二氧化类似物所特有的无数反应。由于相对论效应，钍，核废料的另一种成分，以及由于扩散风险较低而引起兴趣的潜在核燃料，也不具有直接的，可预测的化学性质，并且是一种非常软的+4金属离子。人们对其分子氧化物的行为知之甚少，尽管从气相研究中可以看出，氧的结构与其他锕系离子完全不同。铀的人造和高放射性邻居镎形成像铀一样的线性O=Np=O指示，但由于额外的f电子，表现出更多的氧原子反应性。在核废料中，当氧基团与另一种金属二氧正离子结合时，与U、Np和Pu形成阳离子-阳离子配合物，使混合物的行为更难预测。然而，通过在铀酰离子上添加一个电子，我们和其他人近年来已经证明，单个还原的铀酰可以为更重的锕系基提供一个更好的氧反应模型。由于从地下水中沉淀铀的途径涉及到最初的单电子还原到水不稳定的中间体，这些稳定的U(V)铀酰配合物是了解铀如何沉淀的潜在重要模型。我们的工作是揭示类似于过渡金属氧化学中所见的锕离子反应性，重点是使用刚性有机配体框架来暴露其中一个氧原子。我们最近报道了一个更小的，更受约束的大环，可以结合一个或两个铀或钍阳离子，到目前为止，在较低的氧化态。这也使我们能够研究金属-配体和金属-金属相互作用中的共价。我们将利用这两种刚性配体所提供的控制力来制造一系列具有新几何形状的锕系氧配合物。有些项目，包括更深奥的化学项目，最初预计纯粹是出于学术兴趣，是研究人员培训的重要组成部分。一些反应将与环境和废物相关的分子过程更相关，包括质子、电子和氧基的重排、转移和抽象。有关这些新配合物的反应性的结果将有助于我们更好地了解核废料和环境中发现的更复杂的金属氧系统。我们将研究最活跃的锕系氧配合物对碳氢化合物C-H键的切割，在纯碳氢化合物底物上工作，但认识到铀酰光解引起的有机污染物破坏的相关性。在国际电联（卡尔斯鲁厄）的欧盟超铀研究联合研究中心工作，我们还将研究这些配合物的镎类似物。这些系统的分子性也将使单金属和双金属配合物的磁性比固态化合物更容易理解和建模。国际电联的专家将能够确定这两种金属是否通过中心氧原子或甚至通过配体pi系统进行通信。我们还将向卡尔斯鲁厄核废料处理研究所和美国洛斯阿拉莫斯国家实验室的合作者提供样品，以获得XAS数据，从而可以研究价轨道、金属-金属距离/相互作用（来自EXAFS）和共价（来自配体边缘XAS）。

Uranium, the heaviest naturally occurring element, is the main component of nuclear waste. In air, and in the environment, it forms dioxide salts called uranyl compounds, which are all based around a doubly charged, linear O=U=O group. These compounds are very soluble and are problematic environmental groundwater contaminants. The U=O bonds are also extraordinarily chemically robust and show little propensity to participate in the myriad of reactions that are characteristic of transition metal dioxide analogues which have chemical and catalytic uses in both biological and industrial environments. Due to relativistic effects, thorium, another component of nuclear waste, and a potential nuclear fuel of interest due to the lower proliferation risk, also does not have straightforward, predictable chemistry, and is a remarkably soft +4 metal ion. The behaviour of its molecular oxides is poorly understood, although tantalising glimpses of what might be possible come from gas phase studies that suggest oxo structures completely unlike the other actinyl ions. Uranium's man-made and highly radioactive neighbour neptunium forms linear O=Np=O dications like uranium, but due to the extra f-electron, shows much more oxygen atom reactivity. In nuclear waste, cation-cation complexes form with U, Np, and Pu when the oxo groups bind to another metal dioxo cation, making the behaviour of the mixtures harder to predict. However, by adding an electron to the uranyl ion, we and others have shown in recent years that the singly reduced uranyl can provide a more oxo-reactive, better model for the heavier actinyls. Since the route for precipitating uranium from groundwater involves an initial one-electron reduction to an aqueous-unstable intermediate, these stable U(V) uranyl complexes are potentially important models for understanding how uranium is precipitated. Our work to uncover actinyl ion reactivity similar to that seen in transition metal oxo chemistry has focused on using a rigid organic ligand framework to expose one of the oxygen atoms. We have most recently reported a smaller, more constrained macrocycle that can bind one or two uranium or thorium cations, so far in the lower oxidation states. This also allowed us to look at covalency in the metal-ligand and metal-metal interactions. We will use the control afforded by these two rigid ligands to make a series of actinide oxo complexes with new geometries. Some, including more chemically esoteric projects, are initially anticipated to be purely of academic interest, and an important part of researcher training. Some of the reactions will have more relevance to environmental and waste-related molecular processes, including proton, electron, and oxo group rearrangement, transfer, and abstraction. Results concerning the reactivity of these new complexes will help us better understand the more complex metal oxo systems found in nuclear wastes and the environment.We will look at hydrocarbon C-H bond cleavage by the most reactive actinide oxo complexes, working on pure hydrocarbon substrates, but recognising the relevance to the destruction of organic pollutants induced by photolysis of uranyl. Working at the EU Joint research centre for transuranic research at the ITU (Karlsruhe), we will also study the neptunium analogues of these complexes. The molecularity of these systems will also make the magnetism of mono- and bimetallic complexes easier to understand and model than solid-state compounds. The experts at the ITU will be able to identify whether the two metals communicate through a central oxo atom or even through ligand pi-systems. We will also provide samples to collaborators at the INE (institute of nuclear waste disposal), Karlsruhe and Los Alamos National Labs, USA, to obtain XAS data that allow the study of the valence orbitals, metal-metal distances/interactions (from the EXAFS) and covalency (from the ligand edge XAS).