Unprecedented dinuclear Robson type macrocyclic complexes having two +iii metal ions in two compartments and the role of the diimino moiety on the stability of metal ion oxidation states

Abstract

The work in this investigation deals with the syntheses, characterization, crystal structures and catechol oxidase activity of four diphenoxo-bridged Co^IIICo^III compounds of composition [Co^IIICo^IIIL^Me–pn(N₃)₄]·6H₂O (1), [Co^IIICo^IIIL^Me–pn(N₃)₄]·4H₂O (2), [Co^IIICo^IIIL^Et–pn(N₃)₄]·3.5H₂O (3) and [Co^IIICo^IIIL^Et–pn(N₃)₄]·CH₃CN·2.5H₂O (4), and two diphenoxo-bridged Co^IIICo^II compounds of composition [Co^IIICo^IIL^Me–Me2pn(N₃)₃]·2H₂O (5) and [Co^IIICo^IIL^Et–Me2pn(N₃)₃]·H₂O (6). In these compounds, H₂L^Me–pn, H₂L^Et–pn, H₂L^Me–Me2pn and H₂L^Et–Me2pn are four Robson type tetraiminodiphenolate macrocyclic ligands, in which the superscript Me and Et indicate that the dialdehyde components are 4-methyl-2,6-diformyl-phenol and 4-ethyl-2,6-diformyl-phenol, respectively, while pn and Me2pn indicate that the diamine components are 1,3-diaminopropane and 2,2-dimethyl-1,3-diaminopropane, respectively. The Co^IIICo^III compounds 1–4 are further characterized by ¹H NMR spectra. Compounds 1 and 3 are prepared following a metal-templated synthesis, while compounds 2 and 4 are prepared by direct synthesis, which includes using a pre-isolated macrocycle, [H₄L^Me–pn](ClO₄)₂/[H₄L^Et–pn](ClO₄)₂, as a reactant. On changing the diamine component of the corresponding macrocycle from 1,3-diaminopropane to 2,2-dimethyl-1,3-diaminopropane, but otherwise following the similar reaction procedure (both direct and template), the Co^IIICo^II compounds [Co^IIICo^IIL^Me–Me2pn(N₃)₃]·2H₂O (5), [Co^IIICo^IIL^Et–Me2pn(N₃)₃]·H₂O (6), [Co^IIICo^IIL^Me–Me2pn(N₃)₃]·0.5MeCN·0.27H₂O (5A; Inorg. Chim. Acta, 2014, 412, 38) and [Co^IIICo^IIL^Et–Me2pn(N₃)₃]·MeCN (6A; Dalton Trans., 2013, 42, 4561) are formed, rather than Co^IIICo^III compounds. Like compounds 5A and 6A, all the compounds 1–6 show catechol oxidase activity (substrate: 3,5-di-tert-butyl catechol). The kinetic parameters for 1, 3 and 6 have been determined. Their K_cat values are, respectively, 212.6, 188.0 and 191.3 h⁻¹. ESI-MS (positive) spectra of some representative cases (with and without the substrate) have been recorded and the positive species have been assigned well. Unprecedented and surprising aspects of the composition/structure and properties of the complexes in this investigation have been discussed.