Development of Nonclassical Photoprecursors for Rh-2 Nitrenes

Characterization of reactive intermediates in C-Hfunctionalizationis challenging due to the fleeting lifetimes of these species. Syntheticphotochemistry provides a strategy to generate post-turnover-limiting-stepintermediates in catalysis under cryogenic conditions that enablecharacterization. We have a long-standing interest in the structureand reactivity of Rh-2 nitrene intermediates, which areimplicated as transient intermediates in Rh-2-catalyzedC-H amination. Previously, we demonstrated that Rh-2 complexes bearing organic azide ligands can serve as solid-stateand in crystallo photoprecursors in the synthesisof transient Rh-2 nitrenoids. Complementary solution-phaseexperiments have not been available due to the weak binding of mostorganic azides to Rh-2 complexes. Furthermore, the volatilityof the N-2 that is evolved during in crystallo nitrene synthesis from these precursors has prevented the in crystallo observation of C-H functionalizationfrom lattice-confined nitrenes. Motivated by these challenges, herewe describe the synthesis and photochemistry of nonclassical nitreneprecursors based on sulfilimine ligands. Sulfilimines bind to Rh-2 carboxylate complexes more tightly than the correspondingazides, which has enabled the full solid-state and solution-phasecharacterization of these new complexes. The higher binding affinityof sulfilimine ligands as compared with organic azides has enabledboth solution-phase and solid-state nitrene photochemistry. Cryogenicphotochemical studies of Rh-2 sulfilimine complexes confinedwithin polystyrene thin films demonstrate that sulfilimine photochemistrycan be accomplished at low temperature but that C-H aminationis rapid at temperatures compatible with N US photoactivation.The potential of these structures to serve as platforms for multistep in crystallo cascades is discussed. The synthesis of dibenzothiophene-derived sulfilimine ligandsand their coordination with Rh(2)esp(2) is described.The coordination of the sulfilimine ligands with Rh-2(II,II)complexes is characterized by UV-vis, IR, NMR spectroscopy,and X-ray crystallography. Photochemically promoted nitrogen grouptransfer (NGT) chemistry is observed in both solution-phase and solid-statephotolyses, demonstrating the potential of sulfilimine ligands asnonclassical nitrene photoprecursors.