A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes withC2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

Abstract

The tetrapodal pentaamine ligand 2,6-bis(1′,3′-diamino-2′-methylprop-2′yl)pyridine (pyN4, 1) provides square-pyramidal coordinated cobalt(II) building blocks, which have been used in the synthesis of the singly-bridged dicobalt(III) µ2-η1:η1-peroxo and superoxo complexes [(1)Co−O2−Co(1)]4+/5+, isolated as the chloride, bromide, and mixed chloride/dithionate and bromide/dithionate salts. The peroxo complex is accessible by the classical route involving oxygenation of [(pyN4)CoII], but has also been obtained from a cobalt(III) precursor in air, which implies that the pentaamine 1 acts as a multi-electron reductant. Oxidation of the peroxo complex with chlorine generated in situ from an HCl/H2O2 mixture (H2O2 derived from partial peroxo complex hydrolysis) generates the superoxo complex. Both cations have highly symmetrical solid state structures, locked in transoid Co−O−O−Co conformations by two pairs of intramolecular hydrogen bonds. Each involves two protons in the equatorial Co(NH2R)4 plane of one half of the molecule and the bridge oxygen atom in the other half. A significant difference between the two structures is the orientation of the O2 bridge, which is coplanar with the pyridine rings of the coordination caps in the peroxo complex and at right angles in the superoxo complex. The reactivities of the complexes in acidic and basic media have been explored, and the mononuclear bromo complex [(1)CoBr]2+ was isolated in one of the products. Dithionite, intended as an external reducing agent in the reaction of Na3[CoIII(CO3)3] with 1, instead yields the S-sulfito complex cation [(1)Co(SO3)]+, by disproportionation of S2O42− to give SO32− and S2−. All complexes have been characterised by 1H, 13C NMR, IR, Raman, UV/Vis, and EPR spectroscopy (as applicable), elemental analysis and X-ray structure determination, and the cyclic voltammetry parameters of the peroxo/superoxo pair of complexes have been determined.