A mononuclear iron(ii) complex: cooperativity, kinetics and activation energy of the solvent-dependent spin transition

Abstract

The system [FeL2](BF4)2 (1)-EtOH-H2O (L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine) shows a complicated balance between the relative stabilities of solvatomorphs and polymorphs of the complex [FeL2](BF4)2. New solvatomorphs, 1(LS)·EtOH·H2O and β-1(LS)·xH2O, were isolated in this system. They were converted into four daughter phases, 1(A/LS), 1(D/LS), 1(E/LS)·yEtOH·zH2O and 1(F/LS). On thermal cycling in sealed ampoules, the phases 1(LS)·EtOH·H2O and β-1(LS)·xH2O transform into the anhydrous phase 1(A/LS). The hysteresis loop width for the (A/LS) ↔ (A/HS) spin transition depends on the water and ethanol contents in the ampoule and varies from ca. 30 K up to 145 K. The reproducible hysteresis loop of 145 K is the widest ever reported one for a spin crossover complex. The phase 1(A/LS) combines the outstanding spin crossover properties with thermal robustness allowing for multiple cycling in sealed ampoules without degradation. The kinetics of the 1(A/LS) → 1(A/HS) transition is sigmoidal which is indicative of strong cooperative interactions. The cooperativity of the 1(A/LS) → 1(A/HS) transition is related to the formation of a 2D supramolecular structure of the phase 1(A/LS). The activation energy for the spin transition is very high (hundreds of kJ mol(-1)). The kinetics of the 1(A/HS) → 1(A/LS) transition can either be sigmoidal or exponential depending on the water and ethanol contents in the ampoule. The phases 1(D/LS) and 1(F/LS) show gradual crossover, whereas the phase 1(E/LS)·yEtOH·yH2O shows a reversible hysteretic transition associated with the solvent molecule release and uptake.

Contribution of Coulomb Interactions to a Two-Step Crystal Structure Phase Transformation Coupled with a Significant Change in Spin Crossover Behavior for a Series of Charged FeII Complexes from 2,6-Bis(2-methylthiazol-4-yl)pyridine

“…Recently the desolvation-induced phase transformations in the charged SCO compounds were reported. 46,47 Our finding can shed a light on elucidation of the mechanism of SCO behavior variations and phase transformations in charged SCO compounds.…”

“…Recently desolvation-induced phase transformations in charged SCO compounds were reported. 46,47 Our finding can shed light on elucidation of the mechanism of SCO behavior variations and phase transformations in charged SCO compounds. So far the introduction of intermolecular interactions between SCO molecules has been considered to be important to achieve the cooperativity in SCO transitions.…”

Solvatomorphism‐Induced 45 K Hysteresis Width in a Spin‐Crossover Mononuclear Compound

“…Such solvatomorphism has been reported in many compounds to affect the magnetic properties, especially on the [Fe(bpp) 2 ] 2 + family [6,7] and sometimes with remarkable changes in other bis-tridentate homoleptic complexes. [5] Crystal structure…”

“…Extended networks built on bridging ligandso ften exhibit thermalh ysteresis loops, up to 88 Kw ide. [4] However,t he widest hysteresis loops are found in mononuclear compounds, up to 140 Kw ide, [5] sometimes accompanied by structural phaset ransitions. The most cooperative compounds belongt oh omoleptic complexes based on tris-heterocyclic N,N,N-tridentate ligands, which is case with the bpp family (2,6-di-(1H-pyrazol-1-yl)pyridine (1-bpp) or 2,6di-(1H-pyrazol-3-yl)pyridine, (3-bpp), which is ah ighly documented family.…”

“…[6] In many cases, and in particular with the homoleptic bis-tridentate iron(II) complexes, solvatomorphs show very differents pin crossover properties. [5,7] This follows from the fact that the solventm olecules inserted in the crystal packing could favour,o rn ot, the intermoleculari nteractions.U sually,u pon solventr emoval, more cooperative transitions are observed. Complexes of the bpp family often show thermals pincrossover close to room temperature,o rw ith hysteresis loops.…”