First bistable monocobalt complex with six free nitrogen donors: 3,5-Di-tert-butylbenzoquinonatocobalt complex containing triethanolaminetriisonicotinate

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.…”

“…The optimized data in the gaseous phase using a high-spin configuration in the calculations showed the transformation of the ls-Co III –Cat to the hs-Co II –SQ tautomer, as evidenced by significantly longer coordination bonds around the metal center that were comparable to the bond distances found in the reported structurally characterized hs-Co II –SQ systems (Figure S10). ,,, For both geometrical isomers, the low-spin configuration (ls-Co III –Cat) is more stable than the high-spin analogue (hs-Co II –SQ), and the energy difference between these states at the B3LYP/def2-TZVP level of theory is 4.4 kcal/mol (1539 cm –1 ) for 1 and 19.5 kcal/mol (6820 cm –1 ) for 2 . However, the small energy differences between the optimized data for the ls-Co III –Cat and hs-Co II –SQ isomers suggest that the transformation from the former to the latter is permissible with a small change in thermal energy.…”

“…1.34 Å). CC bond distances ranging between 1.36 and 1.44 Å are the signature for the catecholate state, whereas the semiquinonate state of the ligand has bond lengths in the range of 1.45–1.48 Å. − The average CO distances are 1.332 Å for 1 and 1.344 Å for 2 , and the average (O)­CC­(O)­distances are 1.419 Å for 1 and 1.420 Å for 2 , indicating the fully reduced catecholate state of the ligands. An empirical method developed by Brown in 2012 to determine the oxidation state of dioxolene ligands is useful for calculating a “metrical oxidation state” (MOS) by taking into account the CO and all of the ring CC bond distances, which corroborates the valence of the dioxolene ligand .…”

“…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.…”