Gauging Radical Stabilization with Carbenes

• Published in: Angewandte Chemie 2022-09, Vol.134 (37), p.n/a
• Main Authors: Breitwieser, Kevin, Bahmann, Hilke, Weiss, Robert, Munz, Dominik
• Format: Article
• Language: English
• Subjects: Carbenes; Chemistry; Coordination compounds; Density Functional Calculations; Ligands; Metal complexes; Molecular Orbitals; N-Heterocyclic Carbene; Radicals; Stabilization; Transition metal compounds
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202206390

Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence. Radical stabilization by carbenes is explored computationally. A donor/acceptor frontier orbital picture based on the allyl radical is presented, and allows one to understand and predict radical stabilization in organic radicals, carbene complexes of the transitions metals, as well as of s‐ and p‐block elements.