Unprecedented structural variations in trinuclear mixed valence Co(<scp>ii</scp>/<scp>iii</scp>) complexes: theoretical studies, pnicogen bonding interactions and catecholase-like activities

Three new mixed valence trinuclear Co(II/III) compounds cis-[Co3L2(MeOH)2(N3)2(μ1,1-N3)2] (1), trans-[Co3L2(H2O)2(N3)2(μ1,1-N3)2]·(H2O)2 (2) and [Co3L(R)2(N3)3(μ1,3-N3)] (3) have been synthesized by reacting a di-Schiff base ligand (H2L) or its reduced form [H2LR] (where H2L= N,N'-bis(salicylidene)-1,3-propanediamine and H2LR= N,N'-bis(2-hydroxybenzyl)-1,3-propanediamine) with cobalt perchlorate hexahydrate and sodium azide. All three products have been characterized by IR, UV-Vis and EPR spectroscopies, ESI-MS, elemental, powder and single crystal X-ray diffraction analyses. Complex 2 is an angular trinuclear species in which two terminal octahedral Co(III)N2O4 centers coordinate to the central octahedral cobalt(II) ion through μ2-phenoxido oxygen and μ1,1-azido nitrogen atoms along with two mutually cis-oxygen atoms of methanol molecules. On the other hand, in linear trinuclear complex , in addition to the μ2-phenoxido and μ1,1-azido bridges with terminal octahedral Co(III) centres, the central Co(II) is bonded with two mutually trans-oxygen atoms of water molecules. Thus the cis-trans configuration of the central Co(II) is solvent dependent. In complex 3, the two terminal octahedral Co(III)N2O4 centers coordinate to the central penta-coordinated Co(II) ion through double phenoxido bridges along with the nitrogen atom of a terminal azido ligand. In addition, the two terminal Co(III) are connected through a μ1,3-azido bridge that participates in pnicogen bonding interactions (intermolecular N-N interaction) as an acceptor. Both the cis and trans isomeric forms of 1 and 2 have been optimized using density functional theory (DFT) calculations and it is found that the cis configuration is energetically more favorable than the trans one. However, the trans configuration of 2 is stabilized by the hydrogen bonding network involving a water dimer. The pnicogen bonding interactions have been demonstrated using MEP surfaces and CSD search which support the counter intuitive electron acceptor ability of the μ1,3-azido ligand. Complexes 1-3 exhibit catecholase-like activities in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone. Kinetic data analyses of this oxidation reaction in acetonitrile reveal that the catecholase-like activity follows the order: 1 (kcat = 142 h(-1)) > 3 (kcat = 99 h(-1)) > 2 (kcat = 85 h(-1)). Mechanistic investigations of the catalytic behaviors by X-band EPR spectroscopy and estimation of hydrogen peroxide formation indicate that the oxidation reaction proceeds through the reduction of Co(III) to Co(II).