Molecular and electronic structure of an azidocobalt(iii) complex derived from X-ray crystallography, linear spectroscopy and quantum chemical calculations

The photochemistry of transition-metal azides is remarkably complex and can involve multiple competing pathways leading to different fragmentation patterns. Therefore, an in-depth study of such rich photochemistry requires a thorough prior understanding of the molecular and electronic structures of these complexes. To this end, stationary (i.e. linear) spectroscopies in the ultraviolet-to-visible (UV/vis) and the mid-infrared (MIR) spectral regions are most often employed. Here, we investigate the electronic and vibrational spectroscopies of the cationic diazidocobalt(III) complex, trans-[Co(cyclam)(N3)2]+, in liquid dimethyl sulfoxide (DMSO) solution and interpret the experimental data in terms of detailed quantum chemical calculations. The X-ray crystallography reveals a Ci-symmetric molecular structure of the complex whereas in liquid solution, evidence for symmetry breakage with loss of the inversion center of the ligand sphere is found from both, the UV/vis and MIR-data. This interpretation is fully corroborated by a stereochemical and conformational analysis of the complex using ab initio calculations involving nuclear degrees of freedom of both, the equatorial cyclam ancillary ligand and the two axial azido ligands.