Metalla-NHCs: Modular, Redox-Active Ligands for Next Generation Catalysts

With the support of the Chemical Synthesis (SYN) Program in the Division of Chemistry, Professor Ian Tonks of the University of Minnesota – Twin Cities will study the design and synthesis of new metal-based carbene ligands called metalla-NHCs (N-heterocyclic carbenes) that can be used for switchable catalytic reactions. A key innovation of this work is that these ligands are the first examples of structures that incorporate metals into the cyclic backbone of a free carbene, the latter being a typically highly reactive form of carbon. These new ligands can directly communicate electronic changes to a catalytically active metal center, potentially allowing organometallic chemists to develop new catalysts with unprecedented levels of precision control of reaction outcomes. Fundamental studies into new classes of tunable metalla-NHC and related ligands are critical for continued innovation in the global chemical catalysis field. Additionally, innovation in student-led bottom-up chemical safety efforts has been an integral part of the Tonks Research Group since its inception, and this grant will support translating locally-successful safety initiatives to the national stage by leveraging The Safety Net, a safety website jointly founded by the University of Minnesota – Twin Cities and the University of North Carolina – Chapel Hill. Specifically, Professor Tonks will expand their use of Learning Experience Reports, examples of near-misses or accidents in academic labs, into a publicly available (and editable) database. This database will be useful for new researchers as they learn to carry out hazard assessments in chemistry laboratories.

The overarching goal of this project is to capitalize on the Tonks Group's report of the first example of a free metallacarbene complex containing a dicyclopentadienyltitanium moiety. This proposal aims to synthesize a suite of other early transition metal metallacarbene complexes using the synthetic strategies developed for the titanium analogue. Specifically, the propensity of low valent early transition metals to react with carbodiimides will be explored, where the steric and electronic requirements for kappa-2 versus eta-2 bonding of the carbodiimide will be delineated. Further, the deprotonation of metal-formamidine complexes will be used to access derivatives that cannot be accessed via reductive routes. The electronic structure of these metallacarbene complexes will be studied in depth through combined spectroscopic and electrochemical techniques, as well as computational modeling. Finally, the coordination chemistry of the metalla-NHC complexes will be developed, with a specific focus on using them as strong, redox active dative ligands for other catalytically active metal centers. Ultimately, this research is focused on synthesizing new suites of redox active ligands for catalysis applications such as polymerizations, and elucidating metal-metal interactions in subsequent bimetallic complexes with an eye toward switchable catalytic applications.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.