A comparative study on structures and theoretical models of Rh azido complexes

The syntheses of PF6− salt of six-coordinate Rh(III) azido complexes containing polypyridyl ligands have been accomplished by treatment of the corresponding Rh(III) chloride with NaN3 in EtOH–H2O followed by the addition of NH4PF6. The complexes have been characterized by UV–vis, FTIR, 1H and 14N-NMR spectroscopy along with elemental analysis. The X-ray crystal structures of compounds [Rh(tpy)(phen)(N3)](PF6)2 (1·(PF6)2, tpy: 2,2′:6′,2″-terpyridine, phen: 1,10-phenanthroline) and [Rh(tpy)(bpy)(N3)](PF6)2 (2·(PF6)2, bpy: 2,2′-bipyridine) confirm that a rhodium atom is six-coordinate and the tpy and bpy or phen ligands are orthogonal and the RhN3 distances are 2.038(7) and 2.048(6) Å, respectively. The two NN distances for the coordinated azide in complex 1·(PF6)2 are identical with distances of 1.17 (1) and 1.17 (1) Å. However, the two NN distances for the azido group in the complex 2 may or may not be inequivalent with 1.170(8) and 1.15(1) Å. The bond angles formed by the azide group, Rh, and the axially-positioned nitrogen atom are 172.1(3) and 173.5(8)°. In order to answer the question as to whether the two NN distances in 2 would be expected to be different, calculations were carried out at HF/LANL2DZ and HF/SDD levels of theory. The results indeed predict the NN distances in 2 to be different.