Understanding and Exploiting Transition Metal Complexes with Redox-active Ligands

我们的研究项目集中在过渡金属配合物。我们正在为解决具有挑战性的问题做出根本性的贡献。重点是涉及氧化还原活性配体的过渡金属配合物。在过去，我们一直专注于一类特定的氧化还原活性配体，即二硫杂环戊烯。我们现在在我们的研究计划中包括其他配体，特别是配体的氧化还原活性行为仍然知之甚少的配体。虽然我们也将研究双金属（其中氧化还原活性源于H+/H-二分法），但一个特别重要的配体是Ar-N2（芳基偶氮）配体，根据金属的确切性质，它可以携带从+1（芳基重氮）到-1（芳基重氮基）的电荷。虽然芳基重氮阳离子通常容易分离（通常作为四氟硼酸盐）并且已经被用作芳基阳离子等价物超过一百年，但它们作为过渡金属配体的行为仍然没有完全理解。更好地理解芳基偶氮配体在这个时候是特别重要的，因为它们作为芳基阳离子等价物在交叉偶联反应（铃木偶联等）中的潜力变得越来越明显。我们专注于在交叉偶联反应中非常活跃的金属（最初是钯，后来是镍）。研究问题涉及：a）关键物质的表征B）结构c）所需氧化态的稳定d）N2损失的机制e）提高速率f）激活弱亲核试剂。首先出现的是基本问题，其次是综合适用性问题。作为一个非常重要但尚未解决的合成挑战的例子，以下问题突出：缺少但需要实现以下的方案：捕获作为刘易斯酸加合物的水性氟化物;在金属催化剂存在下，新鲜形成的刘易斯酸加合物与芳基重氮阳离子的干净反应，以高产率和优异纯度快速形成芳基氟化物。换句话说，需要一种高效的新型催化Balz-Schiemann反应。它将解决一个重要的问题：如果水性氟化物可以快速，干净地引入到任意的有机基板具有芳基残基，正电子发射断层扫描（PET）领域将大大推进。PET的应用受到有限（数量和范围）18F标签协议的阻碍。Balz-Schiemannn反应传统上既不快速也不高效，现有的金属催化剂导致改进不足。用于Balz-Schiemann反应的金属催化剂的合理设计是不存在的，但将来将是需要的。深入了解芳基重氮阳离子与金属的相互作用正在追求在我们的小组，并将导致显着的实际长期的进步。

Our research program focuses on transition metal complexes. We are making fundamental contributions to solving challenging problems. The emphasis is on those transition metal complexes that involve redox-active ligands. In the past, we have concentrated on one specific class of redox-active ligands, namely dithiolenes. We are now including other ligands in our research program, specifically ligands where the redox-active behavior of the ligand is still poorly understood. While we will be working on hydrides as well (where redox activity stems from the H+/H- dichotomy), a particularly important ligand is the Ar-N2 (arylazo) ligand, which, dependent on the exact properties of the metal, can bear a charge ranging from +1 (aryldiazonium) to -1 (aryldiazenido). While aryldiazonium cations are typically easily isolable (typically as tetrafluoroborate salts) and have been used as aryl cation equivalents for over one hundred years, their behavior as ligands toward transition metals is still incompletely understood. A better understanding of arylazo ligands is particularly important at this time, since their potential as aryl cation equivalents in cross-coupling reactions (Suzuki coupling and others) is becoming increasingly evident. We are concentrating on metals that are very active in cross-coupling reactions (initially palladium, later nickel). Research questions are related to: a) characterization of key species b) structures c) stabilization of desired oxidation states d) mechanism of N2 loss e) enhancing rates f) activating weak nucleophiles. Fundamental questions arise first, followed by questions of synthetic applicability. As an example of a very important and yet unmet synthetic challenge, the following problem stands out: a protocol is missing but needed that accomplishes the following: trapping of aqueous fluoride as a Lewis acid adduct; clean reaction of the freshly formed Lewis acid adduct with an aryldiazonium cation in the presence of a metal catalyst, to rapidly form the aryl fluoride in high yield and excellent purity. In other words, a highly efficient new catalytic version of the Balz–Schiemann reaction is needed. It will solve an important problem: if aqueous fluoride can be rapidly and cleanly introduced into arbitrary organic substrates having aryl residues, the field of positron emission tomography (PET) will be drastically advanced. Applications of PET are being held back by the limited (in number and scope) 18F labeling protocols. The Balz-Schiemannn reaction is traditionally neither fast nor efficient, and existing metal catalysis has led to insufficient improvements. Rational design of metal catalysts for the Balz-Schiemann reaction is nonexistent but will be needed in the future. Deeper understanding of the interaction of aryldiazonium cations with metals is being pursued in our group and will lead to significant practical long-term advances.