Alternative Route Triggering Multistep Spin Crossover with Hysteresis in an Iron(II) Family Mediated by Flexible Anion Ordering

Impact of flexible hexyl chain ordering in a mononuclear spin crossover iron(III) complex

A novel FeIII complex [Fe(Hex-tnal)2]BPh4 (1) with a tridentate N2O ligand having an n-hexyl chain, Hex-Htnal (=1-((((1-hexyl-1H-1,2,3-triazol-4-yl)methyl)imino)methyl)naphthalen-2-ol), is reported. Temperature-dependent magnetic susceptibility measurements revealed that 1 exhibits a two-step spin crossover (SCO) transition in the 400-10 K temperature range, including an unusual gradual χMT change above RT (300-345 K) and a hysteretic χMT jump in a narrow temperature range of 345-357 K. These behaviors were also characterized by differential scanning calorimetry. Variable-temperature single-crystal X-ray diffraction studies revealed that the order-disorder transition and conformational change of the hexyl chains and the symmetry change associated with the re-entrant structural phase transition, namely triclinic P1̄ (100-275 K) ↔ monoclinic C2/c (296-340 K) ↔ triclinic P1̄ (360 K), are coupled to variations in intermolecular interactions and the N4O2 coordination environment, resulting in the occurrence of the unusual two-step SCO transition of 1. This study demonstrates that the flexible motion of alkyl substituents in the supramolecular lattice influences the occurrence of anomalous magnetic switching properties.

Investigation of the unique magnetic behaviours of isomers in a 1,2-dithiooxalato-bridged diiron(II) complex

1,2-Dithiooxalate (dto) can be employed as a bridging ligand and it exhibits symmetric (O,S-chelation) or asymmetric (O,O- and S,S-chelation) coordination forms. In this study, we prepared a novel dto-bridged diiron(II) complex, [{Fe(TPA)}2(μ-dto)](ClO4)2 (1), where TPA is tris(2-pyridylmethyl)amine. Interestingly, the bridging dto ligand exhibited not only the asymmetric form but also a linkage isomer and a diastereomer within the same crystal. Notably, the three isomers of 1 exhibited different magnetic properties, resulting in a multi-step spin crossover behaviour.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [Fe^II(L^npdtz)₂(NCX)₂] (X = S; Se), with L^npdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Thermosalience coupled to abrupt spin crossover with dynamic ligand motion in an iron(II) molecular crystal

Here we report an iron(II) molecular crystal that show thermosalient effect (crystal jumping) coupled to cooperative high-spin (HS) to low-spin (LS) spin crossover (SCO). The new iron(II) compound [Fe(LPh,Et)2(NCS)2] (LPh,Et...

Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands

Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L¹) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L¹)₄(H₂O)₂][ClO₄]₂ and [Fe(NCS)₂(L¹)₂(H₂O)₂]·L¹, containing L¹ bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 4⁴ grid structures through O-H···N hydrogen bonding; a solvate of a related 4⁴ coordination framework, catena-[Cu(μ-L¹)₂(H₂O)₂][BF₄]₂, is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L²) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L³) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L³)₂][ClO₄]₂ are low-spin and diamagnetic in the solid state and in solution, while [Fe(L²)₂][ClO₄]₂ and [Co(L³)₂][BF₄]₂ are fully high-spin. Treatment of L³ with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L⁴) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L⁵). While salts of [Fe(L⁵)₂]²⁺ are low-spin in the solid state, [Fe(L⁴)₂][ClO₄]₂·H₂O is high-spin, and [Fe(L⁴)₂][ClO₄]₂·3MeNO₂ exhibits a hysteretic spin transition to 50% completeness at T_1/2 = 128 K (ΔT_1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects "spin-state frustration" of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L¹-L³ are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.

Crystal structure of (N 1,N 3-bis-{[1-(4-meth-oxy-benz-yl)-1H-1,2,3-triazol-4-yl]methyl-idene}-2,2-di-meth-yl-propane-1,3-di-amine)-bis-(thio-cyanato)-iron(II)

The unit cell of the title compound, [FeII(NCS)2(C29H32N8O2)], consists of eight charge-neutral complex mol-ecules. In the complex mol-ecule, the tetra-dentate ligand N 1,N 3-bis-{[1-(4-meth-oxy-benz-yl)-1H-1,2,3-triazol-4-yl]methyl-ene}-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole and aldimine groups. Two thio-cyanate anions, coordinated through their N atoms, complete the coordination sphere of the central Fe ion. In the crystal, neighbouring mol-ecules are linked through weak C⋯C, C⋯N and C⋯S inter-actions into a one-dimensional chain running parallel to [010]. The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H (37.5%), H⋯C/C⋯H (24.7%), H⋯S/S⋯H (15.7%) and H⋯N/N⋯H (11.7%). The average Fe-N bond distance is 2.167 Å, indicating the high-spin state of the FeII ion, which does not change upon cooling, as demonstrated by low-temperature magnetic susceptibility measurements.

Crystal structure of {N 1,N 3-bis-[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl-idene]-2,2-di-methyl-propane-1,3-di-amine}-bis-(thio-cyanato)-iron(II)

The unit cell of the title compound, [FeII(NCS)2(C19H32N8)], consists of two charge-neutral complex mol-ecules. In the complex mol-ecule, the tetra-dentate ligand N 1 ,N 3-bis-[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl-ene]-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole and aldimine groups. Two thio-cyanate anions, also coordinated through their N atoms, complete the coordination sphere of the central Fe ion. In the crystal, neighbouring mol-ecules are linked through weak C-H⋯C/S/N inter-actions into a three-dimensional network. The inter-mol-ecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H 50.8%, H⋯C/C⋯H 14.3%, H⋯S/S⋯H 20.5% and H⋯N/N⋯H 12.1%. The average Fe-N bond distance is 2.170 Å, indicating the high-spin state of the FeII ion, which does not change upon cooling, as demonstrated by low-temperature magnetic susceptibility measurements. DFT calculations of energy frameworks at the B3LYP/6-31 G(d,p) theory level were performed to account for the inter-actions involved in the crystal structure.

Structural, magnetic and Mössbauer spectroscopic studies of the [Fe(3-bpp)2](CF3COO)2 complex: role of crystal packing leading to an incomplete Fe(ii) high spin ⇋ low spin transition

Mononuclear complex [Fe(3-bpp)₂](CF₃COO)₂ exhibits a thermal (HS + HS) ⇋ (HS + LS) transition at ∼226 K which is not associated with any crystallographic transition.

Crystal structure of {N 1,N 3-bis-[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl-idene]-2,2-di-methyl-propane-1,3-di-amine}bis-(thio-cyanato-κN)iron(II)

The unit cell of the title compound, [FeII(NCS)2(C25H28N8)], consists of two charge-neutral complex mol-ecules related by an inversion centre. In the complex mol-ecule, the tetra-dentate ligand N 1,N 3-bis-[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl-ene]-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole moieties and aldimine groups. Two thio-cyanate anions, coordinating through their N atoms, complete the coordination sphere of the central ion. In the crystal, neighbouring mol-ecules are linked through weak C-H⋯π, C-H⋯S and C-H⋯N inter-actions into a two-dimensional network extending parallel to (011). The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H (35.2%), H⋯C/C⋯H (26.4%), H⋯S/S⋯H (19.3%) and H⋯N/N⋯H (13.9%).