A series of tetrabromocatecholate chelated cobalt(iii) complexes with various N-donor ancillary ligands: syntheses, crystal structures, co-crystallization, thermally induced valence tautomerism and electrochemical studies

Abstract

Nine new tetrabromocatechol chelated mononuclear cobalt(III) complexes with various nitrogen-donor ancillary ligands, [Co(en)₂(Br₄Cat)]ClO₄·0·5H₂O (1) [Br₄CatH₂ = tetrabromocatechol, en = ethylenediamine], [Co(pn)₂(Br₄Cat)]ClO₄ (2) [pn = propylenediamine], [Co(dpd)₂(Br₄Cat)]NO₃·CH₃OH (3) [dpd = 2,2′-dimethyl-1,3-propanediamine], [Co(amp)₂(Br₄Cat)]ClO₄ (4) [amp = 2-aminomethylpyridine], [Co(aqn)₂(Br₄Cat)]NO₃·CH₃OH (5) [aqn = 8-aminoquinoline], [Co(phen)₂(Br₄Cat)]ClO₄·0·5CH₃OH (6) [phen = phenanthroline], [Co(bpy)₂(Br₄Cat)]NO₃·Br₄CatH₂·2CH₃OH (7) [bpy = 2,2′-bipyridine], [Et₃NH][Co(pym)₂(Br₄Cat)₂]·H₂O (8) [pym = pyrimidine] and [Co(tren)(Br₄Cat)]NO₃·CH₃OH·H₂O (9) [tren = tris(2-aminoethyl)amine], are reported. All the complexes were characterised by various spectroscopic techniques and electrochemical methods, and their structures have been elucidated by X-ray crystallography. Structural analyses reveal that a 1 : 1 cobalt-dioxolene adduct is the exclusive product with bi- or tetradentate ancillary ligands, while for the monodentate pyrimidine ligand 2 : 1 cobalt-dioxolene is the only outcome. Importantly, an unusual co-crystallization of complex 7 with one molecule of tetrabromocatechol was observed in the crystal lattice. The electrochemical studies of all the complexes disclose quasi-reversible redox processes due to the o-dioxolene ligands, in addition to reductive responses corresponding to the reduction of the cobalt(III) centres to cobalt(II). Although in the solid state all the complexes are diamagnetic, complexes 5 and 8 undergo thermally induced valence tautomerism in solution associated with the drastic change in the optical absorption properties of the complexes in the UV-vis spectroscopy.