Understanding and Exploiting Transition Metal Complexes with Redox-active Ligands

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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.