Development of Nonclassical Photoprecursors for Rh₂ Nitrenes

Characterization of reactive intermediates in C-H functionalization is challenging due to the fleeting lifetimes of these species. Synthetic photochemistry provides a strategy to generate post-turnover-limiting-step intermediates in catalysis under cryogenic conditions that enable characterization. We have a long-standing interest in the structure and reactivity of Rh₂ nitrene intermediates, which are implicated as transient intermediates in Rh₂-catalyzed C-H amination. Previously, we demonstrated that Rh₂ complexes bearing organic azide ligands can serve as solid-state and in crystallo photoprecursors in the synthesis of transient Rh₂ nitrenoids. Complementary solution-phase experiments have not been available due to the weak binding of most organic azides to Rh₂ complexes. Furthermore, the volatility of the N₂ that is evolved during in crystallo nitrene synthesis from these precursors has prevented the in crystallo observation of C-H functionalization from lattice-confined nitrenes. Motivated by these challenges, here we describe the synthesis and photochemistry of nonclassical nitrene precursors based on sulfilimine ligands. Sulfilimines bind to Rh₂ carboxylate complexes more tightly than the corresponding azides, which has enabled the full solid-state and solution-phase characterization of these new complexes. The higher binding affinity of sulfilimine ligands as compared with organic azides has enabled both solution-phase and solid-state nitrene photochemistry. Cryogenic photochemical studies of Rh₂ sulfilimine complexes confined within polystyrene thin films demonstrate that sulfilimine photochemistry can be accomplished at low temperature but that C-H amination is rapid at temperatures compatible with N═S photoactivation. The potential of these structures to serve as platforms for multistep in crystallo cascades is discussed.