Computational Studies of Ambient Catalytic Dinitrogen Reduction by Electropositive Metal Tetraphenolate Complexes

全世界每年将氮气工业催化转化为数百万吨氨，这是生产药品、塑料、精细化学品和化肥的起点。后者使大约30亿人的生活得以实现，并实现了联合国全球可持续发展目标2：零饥饿。将氮气和氢气转化为氨的高压/高温Haber-Bosch(HB)工艺，称为氮气还原反应(N2RR)，经过了完美的优化，但仍然非常耗能。小规模、低能量的N2RR反应，包括生成氨以外的产物，将是对HB工艺的补充。它们还将通过允许与世隔绝的社区生产自己的化肥或胺来改善能源正义，并有可能促进外部世界的粮食生产。此外，氨具有取代化石燃料作为能源载体的潜力，因为它是能源密集型的，并与当前的基础设施和燃料电池技术兼容。然而，将其纳入可再生技术需要进一步的了解和更好的催化剂。加州大学伯克利分校的Polly Arnold教授是这项提议的项目合作伙伴，他最近报告了可以在常温常压下将氮转化为氨的分子铀和钍配合物的合成和表征，以及首次用任何金属催化将氮氮转化为仲硅胺。她现在已经将这项工作扩展到了Ce和Sm类似物--第一种非放射性的f-块n2rr催化剂。所有这些分子都有两个金属原子，由两个四苯酚芳烃(MTP)配体固定在适当的位置。阿诺德还用钛和锆合成了d-嵌段类似物，这同样可以催化氮气转化为仲硅胺，以及含有两种金属但只有一种mtp配体的铀和稀土化合物，其中一些也是有效的催化剂。阿诺德的实验室正在进行优化这种化学成分的工作，并将其扩展到其他金属，包括非常丰富的S-块元素钙、锶和钡。这项拟议的研究是首席研究员Kaltsoyannis实验室的一项全面的计算量子化学计划，旨在与Arnold正在进行的用于将氮转化为氨和仲胺或叔胺的新型双金属均相催化剂的实验开发进行协同联系和指导，特别强调提供详细的机理和电子结构洞察。Kaltsoyannis和Arnold在以前的许多项目上进行了合作，并在f和d块化学方面做出了重要和广受欢迎的贡献。阿诺德早些时候关于铀和钍N2RR催化剂的报告包括对反应机理的量子化学分析，实验和计算的结合对这项工作的成功至关重要。实验和理论的结合将产生新的N2RR化学和催化剂，以及对机理和电子结构的深入了解。它将解开碱金属还原剂、mtp配体和Lewis酸性f-、d-和S-块金属的作用，以了解电子到金属结合氮的路径及其随后的行为。通过这样做，它也将对理解d和f轨道在成键中的作用做出基本贡献，包括这些轨道在f块化学中的相互作用。还预计，目标化合物在催化方面的重大成功将刺激正电d嵌段催化领域的进一步发展。

The worldwide industrial catalytic conversion of nitrogen to millions of tons of ammonia per annum is a starting point for the production of pharmaceuticals, plastics, fine chemicals and fertiliser. The latter has enabled the lives of around 3 billion people and addresses the United Nations global sustainable development goal #2: zero hunger. The high pressure/temperature Haber-Bosch (HB) process that converts nitrogen and hydrogen to ammonia, known as the nitrogen reduction reaction (N2RR), is perfectly optimised but still very energy intensive. Small scale, low-energy N2RR reactions, including to products other than ammonia, would be complementary to the HB process. They would also improve energy justice by allowing isolated communities to generate their own fertilisers or amines, and potentially facilitate off-world food production. Furthermore, ammonia has the potential to replace fossil fuels as an energy carrier, as it is energy dense and compatible with current infrastructures and fuel cell technologies. However, its incorporation into renewable technologies demands further understanding and better catalysts.Professor Polly Arnold, from the University of California at Berkeley and the Project Partner on this proposal, has recently reported the synthesis and characterisation of molecular uranium and thorium complexes that can convert nitrogen to ammonia at room temperature and pressure, and the first catalytic conversion of dinitrogen into a secondary silylamine by any metal. She has now extended this work to cerium and samarium analogues - the first non-radioactive f-block N2RR catalysts. All of these molecules feature two metal atoms, held in place by two tetraphenol-arene (mTP) ligands. Arnold has also synthesised d-block analogues using titanium and zirconium, which again can effect catalytic conversion of nitrogen to secondary silylamines, and uranium and lanthanide compounds containing two metals but only one mTP ligand, some of which are also effective catalysts. Work is ongoing in Arnold's laboratories to optimise this chemistry, and extend it to other metals, including the very abundant s-block elements calcium, strontium and barium. The proposed research is a comprehensive programme of computational quantum chemistry in the laboratory of Principal Investigator Kaltsoyannis to link synergistically with, and guide, Arnold's ongoing experimental development of new bimetallic homogeneous catalysts for the conversion of nitrogen to ammonia, and secondary or tertiary amines, with particular emphasis on furnishing detailed mechanistic and electronic structural insight. Kaltsoyannis and Arnold have collaborated on many previous projects, and have made important and well-received contributions to f and d block chemistry. Arnold's earlier report of uranium and thorium N2RR catalysts included quantum chemical analysis of the reaction mechanism, and the combination of experiment and computation was essential to the success of that work. The proposed combination of experiment and theory will yield new N2RR chemistry and catalysts, and deep insight into both mechanism and electronic structure. It will deconvolute the roles of the alkali metal reductant, mTP ligand and the Lewis acidic f-, d- and s-block metals to understand the path of the electrons to the metal bound nitrogen, and their subsequent behaviour. In so doing, it will also make fundamental contributions to understanding the role of d and f orbitals in bonding, including the interplay of these orbitals in f block chemistry. It is also anticipated that significant success in catalysis arising from the target compounds will stimulate further advances in the field of electropositive d block catalysis.