Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India.
Department of Chemistry, Indian Institute of Technology Indore, Indore 453552, India.
Acc Chem Res. 2021 Mar 2;54(5):1260-1273. doi: 10.1021/acs.accounts.0c00819. Epub 2021 Feb 10.
The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR, H-BR, (H-)BR) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.
元素-元素 σ 键与过渡金属（TM）的配位既是一种具有根本吸引力的结合模式，也是金属介导的键活化机制和催化的关键，特别是在热门的 C-H 活化领域。TM 对氢气（即 H-H）和硅烷（H-SiR）的σ 配合物进行了广泛的研究，后者作为烷烃（即 H-CR）的通常不稳定和不可分离的σ 配合物的模型很有意义。TM 对硼烷和硼酸盐（即 H-BR、H-BR、（H-）BR）的σ 配合物的研究则少一些，但同样与催化硼化反应有关，这些反应对有机合成具有很高的当前兴趣。我们的两个研究小组通过两种截然不同的方法对 σ-硼烷/硼酸盐配合物家族进行了重要的阐述：虽然 Ghosh 小组通常从富氢四配位硼物种（如硼酸盐）开始，而 Braunschweig 小组则从低价和/或低配位硼构建块开始。通过这两种方法，已经制备了含有一个或两个 σ 键合 B-H 配体的各种物种，其中一些具有额外的螯合给体位点。在过去的 2 年中，我们两个研究小组在 σ-硼烷/硼酸盐配合物方面的工作成果显著扩大，仅在 2019-2020 年就发表了九篇文章。最近 Braunschweig 小组的工作导致了首例六族金属的双（σ）-硼烷配合物的合成，以及一系列新型的双（σ）-硼烷和双（σ）-硼酸盐配合物的合成。钌和铱，前者是 Ru 五配位硼烯配合物的有用前体。最近 Ghosh 小组的工作揭示了硼氢化物与含氮/硫杂环和杂环组合产生的具有 σ（B-H）键合配体的结构的显著多样性。这些反应虽然在某些情况下产生了常规的三角烯型螯合产物，但更频繁地发生了具有不可预测结果的迷人重排。本综述旨在强调该领域研究进展的最近加速，特别是我们两个研究小组的独特但相关的方法和产生的配合物，以及其他小组的相关工作。
