Ligand Modifications Produce Two‐Step Magnetic Switching in a Cobalt(dioxolene) Complex

Abstract

Mononuclear monodioxolene valence tautomeric (VT) cobalt complexes typically exist in their low spin (l.s.) CoIII(cat2‒) and high spin (h.s.) CoII(sq•‒) forms (cat2‒ = catecholato, and sq•‒ = seminquinonato forms of 3,5‐di‐tBu‐1,2‐dioxolene), which reversibly interconvert via temperature‐dependent intramolecular electron transfer. Typically, the remaining four coordination sites on cobalt are supported by a tetradentate ligand whose properties influence the temperature at which VT occurs. We report that replacing one chelating pyridyl arm of tris(2‐pyridylmethyl)amine (tpa) with a weaker field ortho‐anisole moiety facilitates access to a third magnetic state, and examine a series of related complexes. Variable temperature crystallographic, magnetic, calorimetric, and spectroscopic studies support that this third state is consistent with l.s. CoII(sq•‒). Thus, our ligand modifications not only provide access to the VT transition from l.s. CoIII(cat2‒) to l.s. CoII(sq•‒), but at higher temperatures, the complex undergoes spin crossover from l.s. CoII(sq•‒) to h.s. CoII(sq•‒), representing the first example of two‐step magnetic switching in a mononuclear monodioxolene cobalt complex. We hypothesize that ligand dynamicity may facilitate access to the rarely observed l.s. CoII(sq•‒) intermediate state, suggesting a new design criterion in the development of stimulus‐responsive multi‐state molecular switches.

Molecular Twist‐Induced Single‐Crystal Isomerization and Valence Tautomeric Transitions in a Cobalt‐Dioxolene Complex

“…Notably, Olshansky's recent report emphasized the significance of the LS-Co II -Sq *À /Sq *À intermediate state in driving a two-step VT transition. [9] However, our calculated results reveal that the LS-Co II -Sq *À /Sq *À electromer has a higher relative energy than the others, making it unlikely to be observed, as its formation from the closest energy electromer HS-Co II -Sq *À /Sq *À would require a decrease in system entropy with increasing temperature, which is improbable (Table S6, Figure S20-22).…”

“…By this criterion, assigning the electronic state at 270 K to [LS-Co II (pycz) 2 (Sq)(Sq)] seems tempting. [9] However, this assignment can be readily ruled out by XAS spectroscopy (Figure S8), UV/Vis-NIR (Figure S9), IR (Figure S10), EPR (Figure S11) and DFT calculations (see below). Ligand oxidation state analyses together with BVS calculations also suggest the absence of a [LS-Co II (pycz) 2 -(Sq)(Sq)] intermediate state (Table S2).…”

“…Subsequently, a sharp decline in c M T occurs, resulting in a value of 0.75 cm 3 mol À 1 K at 250 K, which undergoes only a slight decrease to 0.59 cm 3 mol À 1 K at 100 K. The two-step VT transition is reproducible during the subsequent heating cycle, featuring a small hysteresis loop of 2 K-wide between 250 and 270 K. The two-step VT transition is confirmed through differential scanning calorimetric (DSC) measurements (Figure S3), which reveal two sets of endothermic and exothermic peaks that indicate the occurrence of two distinct thermal events. To the best of our knowledge, such pronounced two-step VT behavior represents the second example in mononuclear VT complexes, [9] as it typically requires dinuclear systems. [2b-e] It is worth mentioning that the two-step feature can be observed for either 1-trans•S or 1-trans during the second measuring cycle (Figure S4).…”

Spectroscopic and thermodynamic characterization of a cobalt-verdazyl valence tautomeric system. influence of crystal structure, solvent and counterion

“…These parameters are more consistent with a low spin Co 2+ center, but this is inconsistent with the geometric parameters determined at 150 K, and more crucially the EPR spectrum at 5 K. 10,24 Several recent publications have provided evidence for low spin cobalt ions in cobalt oxolene valence tautomers. [6][7][8] A quartet state can arise from ferromagnetic coupling of a low spin Co 2+ with the two radicals, and thus our data suggests an equilibrium population of two distinct quartet states. Alternatively this could be a manifestation of the spinmerism phenomenon described by Robert et al 23 Initial density functional calculations on the quartet state of [Co(dipyvd) 2 ] 2+ using the B3LYP functional favor the low spin Co 2+ geometry as an energy minimum, but since the results of DFT calculations on VT systems are critically dependent upon the functional used, and the calculations must also take into account intermolecular interactions, 28 we do not put much weight on this result; rather, we consider the complexity of this system requires a separate, dedicated computational study.…”

“…2 In many of these cases, electron transfer also results in spin crossover at the metal ion, though for some recent examples the two processes are separate. [6][7][8] Spin crossover at the metal ion results in relatively large changes in bond length but changes in the ligand geometry are relatively small. Recent studies have explored other ligand systems in order to better control valence tautomerization and related geometry changes.…”