A Rare Angular Trinuclear Mixed Valence Cobalt(III-II-III) Complex With Azido Bridges And Salpn-Type Schiff-Base Ligand: Synthesis, Crystal Structure And DFT Study

We describe herein a novel angular trinuclear Co(III)–Co(II)–Co(III) mixed-valence Schiff base complex of formula [{(μ-L)(μ-N₃)Co^III(N₃)}₂Co^II(H₂O)₂]⋅MeOH⋅1.5H₂O (1), obtained by aerial oxidation of [Co^IIL] (2) precursor, where H₂L = N,N′-bis(5-bromosalicylidene)-1,3-diaminopropane. The crystal structure of complex 1 reveals that the two terminal cobalt(III) ions are connected to the central cobalt(II) ion through single phenoxo- and single end-on azido bridges giving rise to this unique bent configuration to the (Co₃}-core. The oxidation states and geometries around the cobalt atoms in the complexes are confirmed by spectroscopic studies and magnetic moment measurements, which are strongly supported by NBO analysis. It is found that the quartet spin state of the title complex has the lowest energy, both for the X-ray and optimized geometries. The HOMO/LUMO gap values calculated in implicit methanol indicate high stability of complex 1 in solution. The substantial value for the hydrogen bond energy is considered as a major contributing factor to the stabilization of complex 1. We have provided insight in the electronic structural features, including charge and spin distribution, of complex 2 based on the theoretical investigations of the optimized structure. Detailed study on facile synthesis enables us to propose a tentative mechanism of the reaction pathway.