Above Room Temperature Spin Transition in Thermally Stable Mononuclear Fe(III) Complexes

Abstract

Two solvent-free mononuclear Fe(III) complexes [Fe(L) 2 ]-NO 3 (1) and [Fe(L) 2 ]ClO 4 (2) have been synthesized by employing a new πconjugated azo-phenyl substituted ligand, 2-((E)-((2-(ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol (HL). The noncoordinated azophenyl part of the ligand adopts two different conformations which can exert a varied local distortion around the metal center affecting the spin crossover behavior. The magnetic data (2−450 K) reveal that complex 1 displays spin crossover above room temperature where the ligand is in linear form, while complex 2 shows an incomplete spin transition where the ligand adopts a skew form in the solid state. These complexes represent rare examples of high-temperature spin transition for mononuclear Fe(III) complexes with T 1/2 > 350 K with very high thermal stability. Presence of strong intermolecular interactions and solvent-free nature of the complexes leads to exceptional thermal stability up to 485 K (for 1) and 496 K (for 2) as revealed by thermogravimetric analysis. The magnetic data for complex 1 have been analyzed by employing an Ising-like model with vibrations yielding the enthalpy change ΔH and entropy change ΔS of the spin transition along with the critical temperature T 1/2 and the solid-state cooperativeness Γ. Spin crossover behavior of complex 1 has also been characterized by differential scanning calorimetry and electron paramagnetic resonance measurements. Ab initio calculations have been performed to analyze the difference in energies of the ground state and excited states of the complexes.

Unusual Magneto‐Structural Features of the Halo‐Substituted Materials [FeIII(5‐X‐salMeen)2]Y: a Cooperative [HS‐HS]↔[HS‐LS] Spin Transition

“…It is noteworthy that for a bis‐chelate octahedral iron(III) complex possessing a tridentate Schiff‐base ligand derived from N ‐alkylethylenediamine, the HS and LS states are reliably distinguishable by the size of the imine‐Fe‐amine chelate angle (θ im‐am :N im ‐Fe III ‐N am ) . Literature values of θ im‐am for the HS and LS complexes occur in the ranges ≈77–80° and ≈83–85°, respectively, which concur with those observed in the series [Fe III (5‐X‐salMeen) 2 ]ClO 4 ( 1 – 4 ) (Table ). To the best of our knowledge, this is the first crystallographic characterization of spin crossover of the type [HS‐HS]↔[HS‐LS] within the [Fe III (5‐X‐salRen) 2 ]Y family.…”

“…Selected geometric and distortion parameters for the coordination spheres of compounds 1 – 4 are presented in Table . Within the [Fe III (X‐salRen) 2 ] + system the literature values of the distances for the bonds Fe III −O ph , Fe III −N im and Fe III −N am in the HS state lie in the ranges 1.90–1.96, 2.06–2.14 and 2.17–2.25 Å, respectively, whereas the corresponding ones in the LS state are in the ranges 1.86–1.89, 1.91–1.94 and 2.02–2.06 Å, respectively . Evidently, both complex cations of compounds 1 – 4 exist in the HS state at room temperature; however, at 90 K one complex cation is HS whereas the other is LS (Table ).…”

“…X-band EPR spectroscopy revealed that all the complexes [Fe III (5-X-salMeen) 2 ]ClO 4 (1-4) are fully LS in frozen methanol solution at liquid nitrogen temperature and their EPR spectra are virtually indistinguishable from one another (see Experimental Sectioni nS upporting Information). LS Fe III EPR spectra tend to be either rhombic [28,35,42,49,53,60,62] or axial. [19,27,49,53,58] Those of complexes 1-4,r epresented by the spectrum of [Fe III (5-Cl-salMeen) 2 ]ClO 4 (2)[ Figure 11 b: g ?…”

“…A comparison of the structural analyses of the compounds [Fe III (5‐Cl‐salMeen) 2 ]ClO 4 ( 2 ) and [Fe III (5‐Cl‐salMeen) 2 ]BPh 4 ⋅ 2 MeOH ( 5⋅ 2MeOH) illustrates the combined influence of intermolecular forces and crystal packing on the magneto‐structural properties of SCO materials. Recently, a π‐conjugated photo‐isomerizable azo‐phenyl substituent was introduced in the para ‐position of the phenolic structural moiety of the salEen ligand to diversify and maximize the noncovalent interactions, leading to thermally stable iron(III) complexes exhibiting high‐temperature spin transitions. In another study, SCO in the compound [Fe(3‐OMe‐salEen) 2 ][Ni(dmit) 2 ] ⋅ MeOH and the non‐solvated compound [Fe(3‐OMe‐salEen) 2 ][Ni(dmit) 2 ] (dmit 2− =2‐thioxo‐1,3‐dithiole‐4,5‐dithiolato) occurred with T 1/2 values of 345 and 275 K, respectively, with the latter compound having lost noncovalent interactions involving the solvent …”

Ab Initio Methods in First‐Row Transition Metal Chemistry

“…While quantitative results with CASPT2 and NEVPT2 can be achieved when appropriate active spaces and basis sets are chosen, it is noteworthy to mention that even with small basis sets and minimal active spaces, CASPT2 or NEVPT2 in some cases can give results being close to experimental data. [160,161] However, the results likely benefit from fortuitous error cancellation and should be interpreted with care.…”