This strongly suggests single coordination of 1a to Fe(HMDS)2. Upon leaving the mixture of 1a with one equivalent of Fe(HMDS)2 for 24 h, full conversion to a new species was observed, as determined by formation of new paramagnetic signals in the 1 H NMR spectrum (Fig. ). Specifically, the signal for the HMDS group shifted to -1 ppm. Upon opening the reaction container, pressure release was noted, which was attributed to release of dinitrogen indicating transient formation of a putative nitrene 3, and eventually the cyclized amine in the iron coordination sphere, i.e. complex 4 (cf. Fig. ). Using 2 equivalents of 1a resulted in the same signals in the 1 H NMR spectrum. No unbound 1b was detected, indicating a fast exchange between coordinated and uncoordinated 1b. Interestingly when using only 0.5 eq. azide 1a with Fe(HMDS)2 for 24 h, the same new paramagnetic signals are observed in 1 H NMR spectroscopy at -1 ppm, and in addition also a broad signal at 59 ppm in approximately a 1:1 relative ratio (Fig. ), which was attributed to Fe(HMDS)2. This 1:1 stoichiometry of Fe-bound product and Fe(HMDS)2 suggests that migration of 1b from one Fe(HMDS)2 unit to another is slow on the NMR time scale. We note that this behavior is incompatible with a dissociative mechanism for product release from the metal center. To elucidate the structure of 4, single crystals suitable for X-ray diffraction were grown by slow evaporation of pentane at -30 °C of a 1:1 reaction mixture of Fe(HMDS)2 and 1a. The structure of the colorless crystals displays a trigonal coordination geometry around the iron, comprised of two HMDS moieties and one cyclized amine product (Fig. ). The N-H hydrogen of the cyclized amine was located from the residual electron density on the Fourier difference map and refined using a riding model.