Understanding and Exploiting Transition Metal Complexes with Redox-active Ligands

我们的研究项目集中在过渡金属络合物上。我们正在为解决具有挑战性的问题作出根本性贡献。重点是涉及氧化还原活性配体的过渡金属络合物。在过去，我们集中在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，特别是那些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-N 2(芳偶氮)配体，它可以携带从+1(芳基重氮)到-1(芳基重氮)的电荷，这取决于金属的确切性质。虽然芳基重氮阳离子通常很容易分离(通常为四氟硼酸盐)，并被用作芳基阳离子的等价物已有一百多年，但它们作为过渡金属配体的行为仍未完全了解。此时，更好地了解芳偶氮配体尤为重要，因为它们在交叉偶联反应(铃木偶联和其他反应)中作为芳基阳离子等价物的潜力正变得越来越明显。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)关键物种的表征；b)结构；c)所需氧化态的稳定性；d)氮气损失的机理；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：缺少一个方案，但需要完成以下工作：将氟化水溶液作为Lewis酸加合物捕获；在金属催化剂的存在下，新形成的Lewis酸加合物与芳基重氮阳离子干净地反应，以高产率和极高的纯度快速生成芳基氟。换句话说，需要一种高效的新催化版本的BalzSchiemann反应。这将解决一个重要的问题：如果能够快速、清洁地将氟水溶液引入含有芳基残基的任意有机底物中，正电子发射断层扫描(PET)领域将得到极大的发展。PET的应用正受到有限的(在数量和范围上)18F标记协议的阻碍。传统上，Balz-Schiemman反应既不快也不高效，现有的金属催化导致了不足的改进。为Balz-Schiemann反应设计合理的金属催化剂是不存在的，但在未来将是需要的。我们小组正在寻求对芳基重氮阳离子与金属相互作用的更深层次的了解，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.