Photoswitching in P-Centered Biradicaloid– and Tetraradicaloid–Group 4 Metallocene Hybrids

Abstract Biradical(oid)s are key intermediates in the formation and cleavage of chemical bonds, and their application as molecular switches is of particular interest. In contrast, tetraradical(oid)s remain far less explored, although they may, for example, consist of two biradical(oid) units. Herein, we report the synthesis of phosphorus-centered bi- and tetraradicaloids coordinated to group 4 metallocene fragments of titanium and zirconium. The target compounds were obtained via ring-expansion reactions of metallocene-substituted isonitriles with one or two equivalents of the biradicaloid [P(μ-NTer)]2, affording group 4 metallocene complexes bearing one or two bridged P-centered, five-membered heterocyclic biradicaloid units. These systems function as mono- and double-molecular switches, respectively, each based on a biradicaloid motif. Upon irradiation with light (638 nm), the biradicaloid heterocycles undergo transannular bond formation in one or both five-membered heterocyclic biradicaloid units, resulting in the generation of housane-type species. The photoinduced switching behavior as well as CO2 activation were investigated in solution by NMR and UV/Vis spectroscopy. Quantum-chemical calculations were further performed to examine the design of the isonitrile ligands, the mechanism of their insertion reactions, and the electronic structure and photoswitching behavior, supporting the experimental findings.