Cobalt lawsone complexes: searching for new valence tautomers

Abstract

Bi-stable molecular systems presenting valence tautomerism are associated with the development of new functional materials, which can be used for applications in organic electric conductors, optoelectronic and molecular magnetic devices. The properties of these materials can be adjusted with slight chemical changes and can be induced by external stimuli. Typical examples of valence tautomer compounds are coordination complexes of Co and o-dioxolene ligands, notably quinone like ones. In the search for a new class of cobalt complexes presenting valence tautomerism we report herein the synthesis and characterization of five new coordination compounds of cobalt and 2-hydroxy-1,4-naphthoquinone (lawsone or shortly Law). Complexes [Co(Law)2(im)2]·C6H5CH3 (1), [Co(Law)2(py)2]·CH3OH (2), [Co(Law)2(phen)]·(C4H8O)2 (3), [Co(Law)2(2,2-bpy)]·C6H5CH3 (4) and [Co(Law)2(2,2-bpy)] (5) were synthesized and fully characterized by X-ray diffraction and EPR techniques in a wide range of temperatures and under illumination. Despite presenting similar molecular and geometry packing of the valence tautomer complexes of cobalt and o-dioxolenes, neither structural nor electronic evidence of valence tautomerism could be found in the Co and lawsone complexes.

Investigation of Cobalt(III)‐Tetrachlorocatechol Complexes as Models for Catechol‐Based Anticancer Prodrugs

“…The optimized structures and spin density for [Co II (TPA)(TCC)] and [Co II (py 2 en)(TCC)] are shown in Figure and indicate the HS configuration the most stable. Similar results were obtained under vacuum and water as solvent, with an elongation of the Co–O and Co–N bonds (Table ) for both HS and LS configurations, typically of cobalt(II) complexes , …”

“…Similar results were obtained under vacuum and water as solvent, with an elongation of the Co-O and Co-N bonds (Table 3) for both HS and LS configurations, typically of cobalt(II) complexes. [37,44] Considering that the dissociation of TCC leads to the formation of an aquo complex, [45] the most stable species found from [Co II (TPA)(TCC)] is a pentacoordinated high spin [Co(TPA)(H 2 O)] 2+ complex, with a single water molecule coordinated to the cobalt(II) center trans to the tertiary amine nitrogen atom. A second water molecule is interacting with the complex through hydrogen bond.…”

The synthesis, characterization and catecholase activity of dinuclear cobalt(ii/iii) complexes of an O-donor rich Schiff base ligand

“…[37][38][39][40][41][42] Co II/III complexes are also known to show valence tautomerism [43][44][45][46][47][48][49] in the presence of dioxolene type substrates. A number of Co III complexes reported to exhibit valence tautomerism bear a higher number of oxygen donors per metal ion [50][51][52] compared to nitrogen donors, which is the main difference in the design while attempting to mimic catechol oxidase activity using Co II/III complexes. The valence tautomerism of Co III ions with dioxolenes provide an indication that upon coordination to Co III catechol may transfer an electron under the right conditions rendering the metal centre reduced to Co II which would initiate the oxidation process of catechol.…”

The structure of the 1H-imidazol-3-ium lawsonate salt aided byab initiogas-phase calculations

“…Its anionic form, 1,4-dioxo-1,4-dihydronaphthalen-2-olate (lawsonate), has its negative charge partially delocalized over the carbonyl O atoms (O1 and O4 in the Scheme). Lawsone can also be used as a building block in the construction of molecular polymeric structures, since the phenol group at position 2 (O2) can be ionized and, with the O atom attached at position 1 (O1), forms a bidentate ligand, whereas the carbonyl O atom at position 4 (O4) can interact with a metallic cation and/or form hydrogen bonds with neighbouring molecules (Bustamante et al, 2012;Ribeiro et al, 2013).…”