Correlations between ligand field Δ o, spin crossover T 1/2 and redox potential E pa in a family of five dinuclear helicates

Triptycene-based diiron(II) mesocates: spin-crossover in solution

Triptycene-based diiron(II) and dizinc(II) mesocates were obtained using a novel rigid ligand with two pyridylbenzimidazole chelating units fused into the triptycene scaffold. Studies on the diiron(II) assembly in solution showed that the complex undergoes thermal-induced one-step spin-crossover with T1/2 at 243 K (Evans method).

54 K Spin Transition Temperature Shift in a Fe6L4 Octahedral Cage Induced by Optimal Fitted Multiple Guests

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M6L4, M = ZnII, CoII, FeII, and NiII) assembled from a tritopic tridentate ligand L with metal ions. The utilization of FeII ion leads to the successful creation of the Fe6L4-type SCO cage. Host-guest studies of these M6L4 cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T1/2 of Fe6L4 is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br4spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4- < Br4spiro4- < thea4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4- increase the ligand field strength relative to spiro4-, while the stronger ligand field provided by thea4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.

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.

Binuclear spin-crossover [Fe(bt)(NCS)2]2(bpm) complex: A study using first principles calculations

The spin-crossover [Fe(bt)(NCS)₂]₂(bpm) complex is studied using spin-polarized density functional theory within the generalized gradient approximation, the Hubbard U and the weak van der Waals interactions in conjunction with the projector augmented wave method in its molecular and periodic arrangements. It is shown that the considered complex has three magnetic configurations [high spin state (HS)-HS, HS-low spin state (LS), and LS-LS] corresponding to those observed experimentally after two transition temperatures T_c ⁽¹⁾ of 163 K and T_c ⁽²⁾ of 197 K. For the HS-HS magnetic state, we found that the two Fe centers are antiferromagnetically coupled for both molecular and periodic structures in good agreement with the experimental observations. Our results show that the computed total energy difference between the magnetic state configurations of the considered Fe₂ complex is significantly smaller compared to those reported in the literature for other mono- or binuclear compounds.

Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L¹ ), pyrid-2,6-diyl (L² ), benzene-1,2-dimethylenyl (L³ ) and propane-1,3-diyl (L⁴ ) linkers between the tridentate metal-binding domains. L¹ and L² form [Fe₂ (μ-L)₂ ]X₄ (X^- =BF₄ ^- or ClO₄ ^- ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe₂ (μ-L¹ )₂ ][BF₄ ]₄ exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe₂ (μ-L² )₂ ]⁴⁺ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L¹ -L³ vary with the ligand linker group, by mass spectrometry and ¹ H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe₂ (μ-L¹ )₂ ]⁴⁺ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

The substituent effect on the spin-crossover behaviour in a series of mononuclear Fe(ii) complexes from thio-pybox ligands

The correlation of the SCO temperature and substituent electronegativity of ligands is observed and discussed for a family of [Fe(thio-pybox)2]2+ complexes.

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...

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.