Spin-crossover behavior in two new supramolecular isomers

New cyanido-bridged iron(II) spin crossover coordination polymers with an unusual ladder-like topology: an alternative to Hofmann clathrates

Two new cyanido-bridged {FeIIMII} double chains were obtained by reacting cyanido anions [M(CN)4]2- with complex cations [FeII(tptz)]2+ (preformed in situ by mixing a hydrated tetrafluoroborate salt of iron(II) and a tptz ligand, tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) having the general formula [FeII(tptz)MII(CN)4]·2H2O·CH3CN, where M = Pd (1) or Pt (2). Additionally, two molecular complexes formulated as [FeII(tptz)2][MII(CN)4]·4.25H2O, where M = Pd (3) or Pt (4), were subsequently obtained from the same reaction, as secondary products. Single crystal X-ray analysis revealed that 1 and 2 are isostructural and crystallize in the P-1 triclinic space group. Their structure consists of a double-chain with a ladder-like topology, in which cyanido-based [M(CN)4]2- metalloligands coordinate, through three CN- ligands and three [FeII(tptz)]2+ complex cations. Compounds 3 and 4 are also isostructural and crystallize in the P1̄ triclinic space group, and the X-ray structural data show the formation of [FeII(tptz)2]2+ and [MII(CN)4]2- ionic units interconnected through H-bonds and π⋯π stacking supramolecular interactions. The static DC magnetic measurements recorded in the temperature range of 2-300 K showed that 1 and 2 exhibit incomplete spin transition on cooling, which is also confirmed by single crystal XRD analysis and Mössbauer spectroscopy. Compounds 3 and 4 are diamagnetic, most likely due to the encapsulation of Fe(II) in a tight pocket formed by two tptz ligands that preserve the low-spin state in the temperature range of 2-400 K.

Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II)

The first examples of azole-triazole Rat ligands, bidentate L^4NMeIm(3-(1-methyl-1H-imidazol-4-yl)-5-phenyl-4-(p-tolyl)-4H-1,2,4-triazole) and L^4SIm (4-(5-phenyl-4-(p-tolyl)-4H-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of azine-triazoles. The tris-L Fe^II complexes of the azine-triazoles are consistently low spin (LS). As intended, these new azole-triazole ligands provide lower field strengths, resulting in high-spin (HS) [Fe^II(L^4NMeIm)₃](BF₄)₂ (1·4H₂O) and spin crossover (SCO) active [Fe^II(L^4SIm)₃](BF₄)₂ (2·0.5H₂O). Single-crystal structure determinations revealed that at 100 K 1·solvents is HS whereas 2·solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex 1·4H₂O remains HS while the thiazole-triazole complex 2·0.5H₂O undergoes a two-step gradual SCO (T_1/2 approximately 275 and 350 K). Variable-temperature Evans method NMR studies of 2, in five different solvents (CD₃NO₂, CD₃CN, CD₃COCD₃, CD₂Cl₂, and CDCl₃) gave T_1/2 values in a relatively narrow range, 214-259 K. These T_1/2 values did not correlate with the solvent polarity index P' (R² = 0.25) but did correlate with the solvent basicity parameter SB (R² = 0.90). Variable-temperature UV-vis studies on a golden yellow CH₃CN solution of 2, with monitoring of the d-d transition at 530 nm (ε = 39 L mol^-1 cm^-1 at 293 K) while the solution was heated from 253 to 303 K, showed that the high-spin fraction increased from 0.51 to 0.77. Cyclic voltammetry studies in CH₃CN revealed a Fe(III)/Fe(II) redox process that was reversible for 1 and irreversible for 2, with significant tuning of the E_pa value: the methylimidazole-triazole complex 1 is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex 2 (0.68 V; both vs 0.01 M Ag/AgNO₃).

Temperature-dependent hysteretic two-step spin crossover in two-dimensional Hofmann-type compounds

Two 2D Hofmann-type compounds [Fe^II(ppe)₂M^II(CN)₄]·3H₂O [ppe = 1-(2-pyridyl)-2-(4-pyridyl)ethylene; M = Pd for 1 and Pt for 2] have been synthesized. Both of them show complete two-step hysteretic SCO transitions HS^1.0 ⇌ HS^0.6–0.5LS^0.4–0.5 ⇌ LS^1.0.

Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions

Spin-crossover clusters with iron(II) high nuclearity are rare. By using 3,5-bis(pyridin-2-yl)-1,2,4-triazole (bptH) as the ligand with two bidentate chelating sites, we successfully obtained three pentanuclear iron(II) cluster helicate compounds [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-Br)(μ-bpt)(NCS)4(H2O)]·2H2O·C2H5OH (1), [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-bpt)2(NCS)4] (2), and [{FeII(μ-bpt)3}2FeII3(μ3-O)]I2·2C2H5OH (3). Research on single-crystal structure and magnetism has indicated that tuning the spin state of two iron(II) ions in axial direction is successfully realized by regulating the different counter anions: one apical [FeII(bpt)3]− unit exhibits spin-crossover behavior while the other [FeII(bpt)3]− unit remains in low spin state in 1, both apical ions are of high spin states in 2, and are of low spin states in 3.

Polymorphism of a chiral iron(ii) complex: spin-crossover and ferroelectric properties

A chiral iron(ii) complex with two polymorphs, one that undergoes a gradual spin crossover, whilst the other remains in a high-spin state but shows a typical ferroelectric feature, is reported.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group

Two iron(ii) structural isomers with only one methyl position difference in their ligands exhibit different spin states due to the competition of the electronic effect and steric crowding.

A ladder-type iron(ii) coordination polymer with enhanced spin-crossover behavior

A ladder-type 1D coordination polymer with enhanced spin-crossover behaviour is reported. It showed an abrupt spin transition above room temperature with a 3 K hysteresis (T_1/2↓ = 335 K andT_1/2↑ = 338 K).

Solvent and anion facilitated conformational changes in benzylamine substituted thiazolamine

Solvent and anion guided conformational adjustments in the syn–anti–syn form of N,N′-(1,4-phenylene-bis(methylene))-bis(5-methylthiazol-2-amine) and the utility of nitrate ions in stabilizing the anti–anti–anti form are established.

A series of multidimensional MOFs incorporating a new N-heterocyclic building block: 5,5′-di(pyridin-4-yl)-3,3′-bi(1,2,4-triazole)

By using a new N-heterocyclic building block: 4,4′-H₂dbpt, six novel coordination polymers with diversiform connectivity from one to three dimensional were constructed and characterized by X-ray crystallography successfully.

Spin crossover behaviour in one-dimensional FeIIcompounds based on the [M(CN)4]2−(M = Pd, Pt) units

Four one-dimensional heterobimetallic spin crossover coordination polymers {Fe(L)₂[M(CN)₄]}_n(M = Pd^IIand Pt^II) have been synthesized and characterized.

Enhanced spin-crossover behavior mediated by supramolecular cooperative interactions

Three one-dimensional (1D) hetereobimetallic coordination polymers [Fe(II)(L)2(AgCN)2]·Solv (L = bpt(-), 1; L = Mebpt(-), Solv = 1.75EtOH, 2; L = bpzt(-), 3) with in situ generated AgCN species were synthesized by solvothermal reactions of Fe(II) salt, K[Ag(CN)2], and the corresponding ligands [bptH = 3,5-bis(pyridin-2-yl)-1,2,4-triazole, MebptH = 3-(3-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, and bpztH = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole]. They were further characterized by X-ray crystallography, magnetic and photomagnetic measurements, and differential scanning calorimetry. Single-crystal X-ray analyses show that they are isostructural with 1D zigzag chain structures with rhombus {Fe2Ag2} units, in which the substituted bpt(-) ligand connects the Fe(II) ion and AgCN species in a cis bridging mode. Then the zigzag chains are packed into three-dimensional supramolecular structures by π···π interactions. Most importantly, weak Ag···N interactions (2.750 Å at 150 K) between the π-stacked neighboring chains present in complex 3. Magnetic susceptibility measurements exhibit that complex 1 displays characteristic paramagnetic behavior in the temperature range investigated. Complex 2 undergoes a gradual spin-crossover (SCO) with critical temperatures T(1/2)↓ = 232 K and T(1/2)↑ = 235 K, whereas 3 exhibits an abrupt SCO with critical temperatures T(1/2)↓ = 286 K and T(1/2)↑ = 292 K. The magnetostructural relationships suggest that the magnetic behaviors can be modulated from paramagnetic behavior to abrupt and hysteretic SCO near room temperature through adjustment of the electronic substituent effect and intermolecular interactions.

Cyanide-bridged bimetallic 3D Hoffman-like coordination polymers with tunable magnetic behaviour

Two isostructural cyanide-bridged bimetallic coordination polymers with a 3D doubly interpenetrated Hoffman-like network exhibit complete two-step spin-crossover behaviours.