Strong Cooperativeness in the Mononuclear Iron(II) Derivative Exhibiting an Abrupt Spin Transition above 400 K

Abstract

The spin crossover system, [Fe(bzimpy)(2)](ClO(4))(2).0.25H(2)O, was reinvestigated above room temperature (bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine). The system exhibits an abrupt low-spin to high-spin transition at T(c) = 403 K. Liberation of a fractional amount of water does not affect the spin crossover: the system is perfectly reversible with a hysteresis width of DeltaT = 12 K. The existence of the hysteresis at such high temperature determines that the lowest limit of the solid-state cooperativity parameter is J/k > 403 K despite long iron(II) separations (10 A). The high cooperativeness has been assigned to a perfect pi-stacking of the benzimidazole rings in the crystal lattice at a distance as short as 3.6 A. Variable-temperature IR data and the heat capacity measurements match well the magnetic data. The thermodynamic properties are DeltaH = 17 kJ mol(-)(1), DeltaS = 43 J K(-)(1) mol(-)(1), so that the entropy of the spin transition shows a considerable contribution from the molecular vibrations. A theoretical model has been applied in fitting the magnetic data along the whole hysteresis path. A statistical distribution of the cooperativity parameter led to the feature that angled walls of the hysteresis loop are well reproduced.

Spin-crossover thermal hysteresis and light-induced effect on iron(II) complexes with tripodal tris(2-pyridyl)methanol

“…As the crystal structure analysis revealed, the py groups in py 3 COH play an important role to construct a two-dimensional network with an aid of the p-p interaction. Such p-p interaction seems to serve as a channel of cooperative interaction, as found in various hysteretic SCO compounds; for example, [Fe(qnal)] 3+ (Hqnal = N-(8-quinolyl)-2-hydroxy-1-naphthaldimine) [18], the Fe and abpt complexes (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) [19], and [Fe(bzimpy) 2 ] 2+ (bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine) [20]. In the present work, the crystal structure of 1 0 has not yet been determined, because the crystalline form was collapsed on annealing.…”

Special Classes of Iron(II) Azole Spin Crossover Compounds

“…These structural features observed for [Fe(abpt) 2 (TCNQ) 2 ] involving pronounced and extended p-p stacking interactions lead to a duality with respect to its gradual spin crossover behaviour. It has generally been accepted that extended p-p interactions may lead to the occurrence of thermal hysteresis in mononuclear Fe(II) spin crossover compounds [62][63][64][65]. Clearly, the requirements responsible for cooperative Fe(II) spin crossover behaviour are not easy to define, since obviously [Fe(abpt) 2 (TCNQ) 2 ] represents an exception to this rule: in spite of the pronounced TCNQ p-p stacking interactions, the Fe(II) spin crossover displays at best weak cooperativity.…”

“…Therefore, it is important to acquire a detailed understanding of the factors likely to be responsible for this feature. It appears that the occurrence of thermal hysteresis in mononuclear Fe(II) spin crossover compounds may also be brought about by strong intermolecular interactions resulting from the presence of an important hydrogen-bonding network [85,86] or extended p-p interactions [62][63][64][65].…”

Direct Observation of Ordered High‐Spin–Low‐Spin Intermediate States of an Iron(III) Three‐Step Spin‐Crossover Complex

“…[11] It has been recognized that the spin transition strongly depends on cooperativity,w hich results from local structural distortions associated with the molecular spin-state change, coupled to long-range interactions of elastic origins in the crystalline solid. [13] Although afew multistep Fe II SCO complexes have been reported, Fe III complexes exhibiting multistep (above three steps) spin transitions with periodicity have not been reported. [13] Although afew multistep Fe II SCO complexes have been reported, Fe III complexes exhibiting multistep (above three steps) spin transitions with periodicity have not been reported.…”