Impact of Substituent Variation on the Presence of Thermal Spin Crossover in a Series of Mononuclear Iron(III) Schiff Base Complexes with Terminal Pseudohalido Co‐ligands

Abstract

A series of novel iron(III) complexes of the general formula [Fe(L)X] (where L is a dianion of pentadentate Schiff base ligand N,N'-bis({2-hydroxy-3,5-dimethylphenyl}phenyl)methylidene-1,6-diamino-3-azapentane=H L1 for 1 and 2; N,N'-bis({2-hydroxy-3-ethoxyphenyl}methylidene)-1,6-diamino-3-azapentane=H L2 for 3 and 3⋅C H O) and X is terminal pseudohalido ligand (X=N for 1, X=NCS for 2, and X=NCSe for 3 and 3⋅C H O) were synthesized and thoroughly characterized. Magnetic measurements revealed the above room temperature spin crossover for isomorphic complexes 1 and 2 (T =441 K and T =435 K, respectively), whereas the solvent-free complex 3 showed a half complete spin crossover (T =250 K), which was detected by variable temperature crystallography as well. On the other hand, solvated complex 3⋅C H O exhibited permanent high spin state behaviour and either recrystallization or in situ thermal desolvation converts 3⋅C H O to solvent-free and spin-crossover-active form 3. Magnetic properties of all the reported complexes were also supported by EPR spectroscopy experiments and in addition, DFT and ab initio calculations were employed for the evaluation of the g-factor and zero field splitting parameters.

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

“…The room temperature values of χ M T for 1 – 4 , which occur in the range 4.06–4.28 cm 3 K mol −1 , are consistent with the spin‐only value for HS iron(III) (χ M T ≈4.38 cm 3 K mol −1 ). They also concur with literature values in the range 3.80–4.50 cm 3 K mol −1 for closely related ferric spin‐crossover materials; although some of these reported values are somewhat low . The spin conversion in 1 – 4 , evidenced by the decrease in the values of χ M T with temperature, culminates in a plateau with a width of 115 K (140‐25 K) for 2 – 4 and 85 K (110‐25 K) for 1 .…”

“…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 ?…”

“…[Fe III (5‐Br‐ salEen) 2 ]Cl ⋅ 0.5 H 2 O exists at 150 K as two independent complex cations in the crystallographic asymmetric unit: one in the HS state and the other in the LS state, a magnetic feature not crystallographically observed previously in this system. However, structural evidence of such [HS‐LS] coexistence has been reported for a mere handful of other types of iron(III)‐based SCO complexes, generally those whose spin transition exhibits a [HS‐LS] plateau. Unfortunately, room‐temperature magnetic and crystallographic data on [Fe III (5‐Br‐ salEen) 2 ]Cl ⋅ 0.5 H 2 O are not available, nor a spin‐transition curve for this compound.…”

Recent developments in penta-, hexa- and heptadentate Schiff base ligands and their metal complexes

“…Particularly, when the pentadentate ligand lies in octahedral complexes, the spin crossover behavior strongly depends on the nature of the sixth simple small ligand [103]. Though plenty of pentadentate Schiff base ligands and their metal complexes have been reported [95,[104][105][106][107][108][109][110][111][112][113][114][115], the pentadentate Schiff base ligand and their metal complexes are still rare and expected to be given more attention.…”

[Fe(abpt)2(NCSe)2] polymorph A: structural studies into the spin crossover behaviour

“…24,25 Insights into spin crossover materials and their behaviour are coming from a range of techniques including magnetic data, 13,26 X-ray crystallography, 11,27 spectroscopic techniques including UV-Vis, 26,28 Resonant Ultrasound, 29 Mossbauer 11,30 and X-ray photoelectron spectroscopy. 27 Given the complexity of spin crossover behaviour, 26,31 it is important to study such systems in detail to further our understanding and allow the customised design of spin crossover materials for desired applications. Spin crossover behaviour has been identified in various Fe(II) complexes containing 3,5-bis(pyridin-2-yl)-1,2,4-triazole derivatives with trans NCS or NCSe ligands: for example, trans- 34 (pldpt = N 4pyrrol-3,5-di(2-pyridyl)-1,2,4-triazole), which has both cis and trans coordinated (2:1 ratio) NCS ligands around the Fe centre, displays interesting spin crossover behaviour.…”