Pressure-induced hysteresis in the high spin [Formula: see text] low spin transition in bis(2,4-bis(pyridin-2-yl)thiazole) iron(II) tetrafluoroborate

Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale

Two acyclic tetradentate Schiff base ligands, HL^X-OH (X = H and Br), were synthesised by 2:1 condensation of either 2-pyridinecarboxaldehyde or 5-bromo-2-pyridinecarboxaldehyde and 1,3-diamino-2-propanol and then used to prepare six mononuclear complexes, [Fe^II(HL^X-OH)(NCE)₂], with three different NCE co-ligands (E = BH₃, Se, and S). The apparent solution spin crossover switching temperature, T_1/2, of these 6 complexes, determined by Evans method NMR studies, is tuned by several factors: (a) substituent X present at the 5 position of the pyridine ring of the ligand, (b) E present in the NCE co-ligand, (c) solvent employed (P'), and (d) potentially also by speciation effects. In CD₃CN, for the pair of NCE = NCBH₃ complexes, when X = H, the complex is practically LS (extrapolated T_1/2 ∼624 K), whereas when X = Br, it is far lower (373 K), which implies a higher field strength when X = H than when it is Br. The same trend, X = H results in a higher apparent T_1/2 than X = Br, is seen for the other two pairs of complexes, with E = Se (429 > 351 K, ΔT_1/2 = 78 K) or S (361 > 342 K, ΔT_1/2 = 19 K). For the family of three X = Br complexes, the change of E from BH₃ (373 K) to Se (351 K) to S (342 K) leads to an overall ΔT_1/2(apparent) = 31 K, whereas the decreases are far more pronounced in the X = H family (BH₃ ∼624 > Se 429 > S 361 K). Changing the solvent used from CD₃CN to (CD₃)₂CO and CD₃NO₂, for [Fe^II(HL^Br-OH)(NCE)₂] with either E = BH₃ or S, revealed excellent, and very similar, positive linear correlations (R² = 0.99) of increasing solvent polarity index P' (from 5 to 7) with increasing apparent T_1/2 of the complex (E = BH₃ gave T_1/2 300 < 373 < 451 K , ΔT_1/2 = 151 K; E = S gave T_1/2 288 < 342 < 427 K, ΔT_1/2 = 147 K). Several other solvent parameters were also correlated with the apparent T_1/2 of these complexes (R² = 0.74-0.96). Excellent linear correlations (R² = 0.99) are also obtained with the coordination ability (a^TM) of the three NCE co-ligands with the apparent T_1/2 in both families of compounds, [Fe^II(HL^X-OH)(NCE)₂] where X = H or Br. The ¹⁵N NMR chemical shifts of the nitrogen atom in the three NCE co-ligands (direct measurement) show modest correlations (R² = 0.74 for L^H-OH family and 0.80 for L^Br-OH family) with the apparent T_1/2 values of the corresponding complexes.

A Family of Heterobimetallic Cubes Shows Spin-Crossover Behaviour Near Room Temperature

Using 4-(4'-pyridyl)aniline as a simple organic building block in combination with three different aldehyde components together with metal(II) salts gave three different Fe8 Pt6 -cubes and their corresponding Zn8 Pt6 analogues by employing the subcomponent self-assembly approach. Whereas the use of zinc(II) salts gave rise to diamagnetic cages, iron(II) salts yielded metallosupramolecular cages that show spin-crossover behaviour in solution. The spin-transition temperature T1/2 depends on the incorporated aldehyde component, giving a construction kit for the deliberate synthesis of spin-crossover compounds with tailored transition properties. Incorporation of 4-thiazolecarbaldehyde or N-methyl-2-imidazole-carbaldehyde yielded cages that undergo spin-crossover around room temperature whereas the cage obtained using 1H-4-imidazolecarbaldehyde shows a spin-transition at low temperatures. Three new structures were characterized by synchrotron X-ray diffraction and all structures were characterized by mass spectrometry, NMR and UV/Vis spectroscopy.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [Fe^IIPd(CN)₄(R-1,2,4-trz)₂]·nH₂O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [Fe^IIPd(CN)₄(thiome)₂]·2H₂O (A·2H₂O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H₂O due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in A·2H₂O (0-0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in A·2H₂O under hydrostatic pressure compared to a one-step SCO in A at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities.