A Hydrazoic Acid Adduct of Rh 2 : Synthesis, Characterization, and Imidogen-Transfer Photochemistry

Metal-imidogen (i.e., M-NH) are proposed to mediate bond cleavage and construction in an emerging family of 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 (Keq = 1100 ± 100 M-2 at 243 K). Photolysis of Rh2(esp)2(HN3)2 in the presence of olefinic substrates results in stereospecific imidogen transfer to afford N-H aziridines. Cryogenic photolysis (77 K) enabled observation of a non-steady state intermediate, which we assign to be a triplet Rh2-NH complex. 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 catalytic NH transfer, and demonstrate a photochemical platform to study NH-transfer catalysis.