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

Yusaku Suenaga; Takuto Mibu; Takashi Okubo; Masahiko Maekawa; Takayoshi Kuroda-Sowa; Kunihisa Sugimoto

doi:10.1039/d1dt00995h

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

Dalton Trans. 2021 Jul 21;50(28):9833-9841. doi: 10.1039/d1dt00995h.

Authors

Yusaku Suenaga¹, Takuto Mibu¹, Takashi Okubo², Masahiko Maekawa³, Takayoshi Kuroda-Sowa¹, Kunihisa Sugimoto⁴

Affiliations

¹ Department of Science, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. suenagay@chem.kindai.ac.jp.
² Department of Science, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. suenagay@chem.kindai.ac.jp and Research Institute of Science and Technology, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan.
³ Research Institute of Science and Technology, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan.
⁴ Diffraction & Scattering Division, Japan Synchrotron Radiation Research Institute(JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan and Institute for Integrated Cell-Material Sciences (iCeMS), Institute for Advance Study (KUIAS), Kyoto University, Kyoto, Japan.

PMID: 34190272
DOI: 10.1039/d1dt00995h

Abstract

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.