Synthesis, characterization, and imidogen photochemistry of a hydrazoic acid adduct of Rh2

Abstract

Metal–imidogen (i.e., M–NH) intermediates are proposed in metal-catalyzed NH-transfer reactions. Transition metal complexes of hydrazoic acid (HN₃) could serve as precursors to these transient intermediates; however, the coordination chemistry of HN₃ is essentially unknown. Here, we report the synthesis and characterization of Rh₂(esp)₂(HN₃)₂, the first crystalline transition metal complex of HN₃ (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropionate). Temperature-dependent multi-nuclear NMR, UV-vis, and IR spectroscopies demonstrate that HN₃ is a weakly coordinating ligand (K_eq = 1100 ± 100 M⁻² at 243 K). Cryogenic photolysis (77 K) enabled observation of a non-steady state intermediate, which we assign to be a triplet Rh₂–NH complex, that engages in olefin aziridination chemistry. The nitrene photochemistry of Rh₂(esp)₂(HN₃)₂ was corroborated by in crystallo synthesis and cryogenic spectroscopic characterization of a family of N-aryl nitrene complexes. Together, these results establish the coordination chemistry of HN₃, confirm the triplet ground state of the Rh₂–NH intermediate responsible for NH transfer, and demonstrate a photochemical platform to observe intermediates in NH-transfer catalysis.