Understanding and Exploiting Transition Metal Complexes with Redox-active Ligands

我们的研究重点是过渡金属配合物。我们正在为解决具有挑战性的问题作出根本性的贡献。重点是那些涉及氧化还原活性配体的过渡金属配合物。在过去，我们专注于一类特定的氧化还原活性配体，即二噻吩。我们现在正在将其他配体纳入我们的研究计划，特别是配体的氧化还原活性行为仍然知之甚少的配体。虽然我们也将研究氢化物（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（芳偶氮）配体，根据金属的确切性质，它可以承受从+1（芳基重氮）到-1（芳基重氮）的电荷。虽然芳基重氮离子通常很容易分离（通常是四氟硼酸盐），并且已经作为芳基阳离子的等价物使用了一百多年，但它们作为过渡金属配体的行为仍然不完全清楚。更好地了解芳基偶氮配体在这个时候尤为重要，因为它们在交叉偶联反应（铃木偶联和其他偶联）中作为芳基阳离子等价物的潜力越来越明显。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)理想氧化态的稳定d) N2损失机制e)提高速率f)激活弱亲核试剂。首先出现基本问题，然后是综合适用性问题。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：缺少一种方案，但需要实现以下目标：捕获作为路易斯酸加合物的氟水；在金属催化剂的存在下，新生成的路易斯酸加合物与芳基重氮离子发生清洁反应，快速生成收率高、纯度高的氟化芳基。换句话说，需要一种高效的新催化版本的巴尔兹谢曼反应。它将解决一个重要的问题：如果能快速、干净地将含水氟化物引入到含有芳基残基的任意有机基质中，正电子发射断层扫描（PET）领域将得到极大的发展。PET的应用被有限的（在数量和范围上）18F标签协议所阻碍。传统上，balz - schiemann反应既不快也不高效，现有的金属催化也没有带来足够的改进。目前尚不存在合理设计用于Balz-Schiemann反应的金属催化剂，但未来将需要这种催化剂。我们小组正在深入了解芳基重氮离子与金属的相互作用，并将导致重大的实际长期进展。

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 BalzSchiemann 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.