Heterospin complexes based on cobalt semiquinolate with nitroxides

Abstract

Heterospin complexes based on cobalt semiquinolate with stable nitroxides of composi tion [Co(SQ) 2 (NIT) 2 ]•8CH 2 Cl 2 , [Co(SQ) 2 (NIT) 2 ]•4MeCN, [Co(SQ) 2 (IN) 2 ]•5Me 2 CO, [Co(SQ) 2 (IN) 2 ]•4MeCN, and [Co(SQ) 2 (IN) 2 ]•2CH 2 Cl 2 •C 6 H 14 , where SQ is 3,6 di tert butyl o benzosemiquinolate, NIT is 4,4,5,5 tetramethyl 2 (1 methyl 1H imidazol 5 yl) 4,5 di hydro 1H imidazole 3 oxide 1 oxyl, and IN is the imine analog of NIT, viz., 4,4,5,5 tetra methyl 2 (1 methyl 1H imidazol 5 yl) 4,5 dihydro 1H imidazole 1 oxyl, were synthesized. The characteristic feature of these compounds is that they contain two different types of para magnetic organic ligands. The X ray diffraction study showed that the solid phases of the complexes have a molecular structure. According to the data from thermomagnetic analysis (2-350 K), all complexes exhibit effects of valence tautomerism at ~20 °C. It was found that incorporated solvent molecules have an effect on the magnetic anomaly temperature.The design of new types of magnetically active com pounds requires the development of methods for the di rected construction of highly efficient exchange coupled multispin ensembles. 1-15 One of approaches to the syn thesis of such ensembles is based on the use of heterospin compounds, viz., transition metal complexes with organic radicals, 15-32 whose molecules contain paramagnetic cen ters of different nature with unpaired d (or f) and p elec trons. Metal complexes with nitroxides 15-33 and o semi quinone (o iminosemiquionone) metal complexes 34-37 comprise the most representative classes of such com pounds. Many complexes belonging to these classes can be magnetized in the solid state. 15,16,18-20,25,28-32 Known synthetic approaches provide a way to prepare magneti cally active compounds containing simultaneously a stable organic radical and two or more paramagnetic metal centers. 28,31-33, 38 Both antiferro and ferromagnetic exchange interac tions can occur between the magnetic centers in heterospin transition metal complexes with o semiquinolate ligands. The geometry of the complexes is one of the factors that influence the character of these interactions, and the mag netic properties of the complexes can be controlled by varying the geometric parameters. The addition of neutral ligands to square planar low spin bis o semiquinone Ni II complexes affords high spin five or six coordinate com plexes. 39-41 Recently, it has been shown 41 that the reac tion of copper(II) bis(3,6 di tert butyl o benzosemiquino late) [Cu(SQ) 2 ] with neutral aza ligands is accompanied by the transformation of the geometry resulting in a con siderable change in the ratio of the ferro to antiferromag netic exchange in the [Cu(SQ) 2 ] matrix. In the cited study, the first heterospin Cu II complex with paramagnetic ligands of different nature, viz., the o semiquinone radical anion and stable 2 imidazoline nitroxide, was isolated and characterized. 41 In continuation of these investigations, we synthesized first heterospin complexes based on cobalt o semiquinola...

Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism

“…The trans angles ranging from 177.7(2)° to 178.2(2)° deviate slightly from ideal angle of 180° in 1 , supporting the above observation. The bond distances CoN­(pyridine) [1.948(5)–1.951(6) Å in 1 and 1.958(5) Å in 2 ] and CoO­(dioxolene) [1.887(4)–1.921(5) Å in 1 and 1.892(3) Å in 2 ] strongly indicate the Co III catecholate charge distribution in these complexes and are in accordance with the values from the literature for cobalt­(III) complexes with dioxolene ligands. − The CO and ring CC (bearing catecholate OH groups) bond distances of the coordinated o -dioxolene ligand are helpful for distinguishing the different oxidation states of the redox-active dioxolene ligand. The semiquinone states of such ligands usually have shorter CO distances (ca.…”

“…This could be one of the reasons that the isolation of the cis isomer becomes possible in mononuclear cobalt bis­(dioxolene) systems, thereby indicating that alcoholic solvents could be better choice for the possible isolation of the related kinetically controlled cis analogues. However, cobalt complexes with dioxolene ligands are well-known, and the literature results suggest that all of them so far were isolated exclusively in the trans configuration when monodentate N-donor ancillary ligands were employed. − The present report is unique in the cobalt dioxolene system in that both the cis and trans isomers were isolated in reasonable yields depending on the judicial choice of the solvents used for the synthesis.…”

“…Whereas the compound recrystallized from acetone yielded a sufficiently open structure to allow the conformational change of the py 2 O coligand, thereby favoring the VT transition associated with the hysteresis effect, the recrystallization product of the same compound from toluene is composed of a tight-packed layered structure that prevents this conformational change and, therefore, the VT transition . There are also several examples where intermolecular interactions or packing effects, in addition to intrinsic properties, clearly influence the VT transitions. − It is worth noting that, although numerous bis­(dioxolene)-chelated mononuclear cobalt complexes with monodentate N-donor ancillary ligands have been reported, none of them were characterized in the cis configuration. − However, some coordination polymers involving cis-coordinated bis­(dioxolene)-chelated cobalt with N-donor bridging ligands are known in the literature. − Although the donor sites around the metal centers are identical in the cis and trans isomers, their geometric difference causes different orientations of the ligand-based magnetic orbitals in these isomers that can yield distinct electronic properties. Therefore, to gain insight into the electronic states of such isomeric compounds and the effects of the packing and intermolecular interactions on the VT transition in these geometrical isomers, it remains a challenge to isolate them for cobalt dioxolene compounds.…”

Bis-o -Benzosemiquinonato Cobalt(II) and Nickel(II) Complexes with Neutral N-Heterocyclic Carbene Ligand: Synthesis, Structure and Magnetic Properties

“…The second synthetic way (Scheme , equation Ib) is the redox‐substitution of o ‐benzosemiquinonato ligand with NHC from cobalt coordination sphere in tris(3,6‐di‐ tert ‐butyl‐ o ‐benzosemiquinonato)cobalt(III) (3,6‐SQ) 3 Co. This method is often used for the synthesis of o ‐benzosemiquinonato cobalt complexes with neutral ligands because (3,6‐SQ) 3 Co exchanges easily one o ‐benzosemiquinonato ligand . However this reaction proceeds sluggishly in comparison with previous method by the reason of equilibrium between (3,6‐SQ) 3 Co and (3,6‐SQ) 2 Co+3,6‐Q and the competitive reactions between 3,6‐Q and neutral ligand.…”

“…The six‐membered rings C(1‐6) and C(1 A‐6 A) of chelating ligands demonstrate quinoid‐type distortion that also points to the radical‐anion nature of these ligands. The Co(1)‐O(1), Co(1)‐O(2) and Co(1)‐O(1 A), Co(1)‐O(2 A) bond length (2.0001(8), 2.0582(8) respectively) are typical for Co(II) high‐spin complexes …”