Syntheses, structure and properties of dinuclear Co complexes with bis(catecholate) ligands – Effect of a quinoline ring in the terminal group

Valence tautomeric transformation in the [CrCo] compound: exploration of cooperative interactions

A crystal containing the heterometallic Cr-ligand-Co cluster with an unpaired electron on the ligand as a structural unit is examined. The developed model which describes the magnetic and polarizability characteristics of this crystal takes into account that the electron residing on the ligand can be transferred to the Co-ion, thus converting the diamagnetic ls-Co^III ion into the paramagnetic hs-Co^II one. Since this transformation is accompanied by electron density redistribution and elongation of the Co-N bond lengths, the vibronic interaction of the Co-ion with totally symmetric displacements of the nearest surroundings and cooperative dipole-dipole and electron-deformational interactions are accounted for as well. The exchange interactions between the Cr^III ion and the electron localized on the ligand as well as in the Cr^III-hs-Co^II pair are also included in consideration; the parameters of these interactions are estimated within the framework of the DFT method. Bistability in the magnetic and polarization characteristics is predicted for certain strengths of intra- and intercluster interactions in the crystal under study. Within the framework of the developed approach an explanation of the observed magnetic properties of the [Cr(SS-cth)(Co(RR-cth)(μ-dhbq))](PF₆)₂Cl crystal is given.

Dinuclear cobalt complexes with a redox active biphenyl bridging ligand [Co2(BP)(tqa)2](PF6)2 (H4BP = 4,4'-bis(3-tert-butyl-1,2-catechol), tqa = tris(2-quinolylmethyl)amine): structure and magnetic properties

The biscatechol, H4BP (4,4'-bis(3-tert-butyl-1,2-catechol)) that can directly connect two redox active catechol moieties was synthesized. Also, tris(2-pyridylmethyl)amine (tpa), bis(2-pyridylmethyl)(2-quinolylmethyl)amine (bpqa), (2-pyridylmethyl)bis(2-quinolyl methyl)amine (pbqa), and tris (2-quinolylmethyl)amine (tqa) were synthesized as terminal ligands of the tetracoordinated tripod. In total, five different dinuclear Co complexes were synthesized from H4BP with various terminal ligands as follows, [Co2(BP)(tpa)2](PF6)2 (1), [Co2(BP)(tpa)2](PF6)3 (2), [Co2(BP)(bpqa)2](PF6)2 (3), [Co2(BP)(pbqa)2](PF6)2 (4), and [Co2(BP)(tqa)2](PF6)2 (5). After a one-electron oxidation reaction of complex (1), complex (2), was isolated as a mixed valence state lsCoIII-[SQ-Cat]-lsCoIII, with an absorption intensity of about 1370 nm (intervalence charge transfer (IVCT) bands) in CH3CN solution. In addition, an investigation of the magnetic properties of the dinuclear Co complex (3) with SQUID showed that the χMT value gradually increased as the temperature increased from 280 to 380 K. Studies in the solid and solution states using electronic spectra, cyclic voltammetry and SQUID for the above complexes provide clear evidence for three different charge distributions: complexes (1) and (3) are CoIII-[Cat-Cat]-CoIII, complex (2) is CoIII-[Sq-Cat]-CoIII, complexes (4) and (5) are CoII-[Sq-Sq]-CoII. Of the five cobalt dinuclear complexes, only complex (3) shows evidence of the temperature dependence of the charge distribution, displaying a thermally induced valence tautomeric transition from the lsCoIII-[Cat-Cat]-lsCoIII to hsCoII-[Sq-Sq]-hsCoII in both solid and solution states. However, this valence tautomeric step is incomplete at 380 K, with the χMT value of hsCoII-[Sq-Sq]-hsCoII. This suggests that the steric hindrance of the quinolyl rings around the Co ion produces a coordination atmosphere that is weaker than that observed with pyridyl rings, which facilitates a change in the CoIII ions to CoII.