A new spin crossover FeII coordination environment in a two-fold interpenetrated 3-D Hofmann-type framework material

Anion Modified Spin Crossover in [Fe(qsal-4-F)]+ Complexes with a 4-Position Substituted Qsal Ligand

Four iron(III) complexes, [Fe(qsal-4-F)₂]Y·sol (Hqsal-4-F = 4-fluoro-N-(8-quinolyl)salicylaldimine; Y = NO₃^-, sol = 0.91MeOH·0.57H₂O (1_NO3); Y = PF₆^- (2_PF6); Y = BF₄^- (3_BF4); Y = OTf^-, sol =1.5MeOH (4_OTf)), with a new 4-position substituted qsal type ligand Hqsal-4-F have been synthesized and structurally and magnetically characterized. Complexes 1_NO3-3_BF4 consist of 1D chains formed by the [Fe(qsal-4-F)₂]⁺ cations connected by π-π and C-H···O interactions, which are further linked by more weak interactions to form 2D layers and 3D networks. On the other hand, complex 4_OTf has a structure of nearly isolated 1D column where the [Fe(qsal-4-F)₂]⁺ cations are connected by π-π, C-H···π, and C-F···π interactions. Magnetic studies revealed the occurrence of two-step symmetry-breaking SCO in 1_NO3 and two-step gradual SCO in 2_PF6. Complex 3_BF4 undergoes a gradual SCO, whereas 4_OTf remains almost high-spin. The smaller anions tend to stabilize the low-spin state, while larger anions tend to stabilize the high-spin state. In addition, the intermediate spin state of 1_NO3 could be thermally trapped by quenching from the high temperature, thereby kinetically suppressing the spin transition to the full low-spin state. This work represents a good example that the position of the substituent and the anions plays critical roles in the preparation of SCO materials with tunable properties.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)₂Pd(CN)₄]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H₂O,EtOH (A·H₂O,Et), and H₂O (A·H₂O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H₂O,Et and an abrupt single-step SCO for A·H₂O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H₂O,Et characterized by a unique Fe^II center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H₂O shows a triclinic symmetry with two inequivalent Fe^II centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework

The interplay of host-guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [Fe^IIPd(CN)₄(L)₂]·1.3MeOH (1·1.3MeOH; L = methyl isonicotinate). Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1·1.3MeOH) and partially desolvated (1·0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the Fe^II sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet (¹Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host-guests interactions provides a great opportunity to design potential chemosensors.

Peculiar Spin-Crossover Behavior in the 2D Polymer K[FeIII(5Cl-thsa)2]

A potassium salt of the N₂S₂O₂-coordination Fe(III) anion K[Fe(5Cl-thsa)₂] (1) (5Cl-thsa - 5-chlorosalicylaldehyde thiosemicarbazone) is synthesized and characterized structurally and magnetically over a wide temperature range. Two polymorphs of salt 1 characterized by the common 2D polymer nature and assigned to the same orthorhombic Pbcn space group have been identified. The molecular structure of the minor polymorph of 1 was solved and refined at 100, 250, and 300 K is shown to correspond to the LS configuration. The dominant polymorph of 1 features K⁺ cations disordered over a few crystallographic sites, while the minor polymorph includes fully ordered K⁺ cations. The major polymorph exhibits a complete three-step cooperative spin-crossover transition both in the heating and cooling modes: The first step occurs in a temperature range from 2 to 50 K; the second abrupt hysteretic step occurs from 200 to 250 K with T_1/2 = 230 K and a 6 K hysteresis loop. The third gradual step occurs from 250 to 440 K. According to ⁵⁷Fe Mössbauer, XRPD, and EXAFS data, the spin-crossover transition for the dominant polymorph is quite peculiar. Indeed, the increase in the HS concentration by 57% at the second step does not result in the expected significant increase in the iron(III)-ligand bond lengths. In addition, the final step of the spin conversion (Δγ_HS = 26%) is associated with a structural phase transition with a symmetry lowering from the orthorhombic (Pbcn) to the monoclinic (P2₁/n) space group. This nontrivial phenomenon was investigated in detail by applying magnetization measurements, electron spin resonance, ⁵⁷Fe Mössbauer spectroscopy, and DFT calculations. These results provide a new platform for understanding the multistep spin-crossover character in the Fe(III) thsa-complexes and related compounds.

Spin-Crossover 2-D Hofmann Frameworks Incorporating an Amide-Functionalized Ligand: N-(pyridin-4-yl)benzamide

Two analogous 2-D Hofmann-type frameworks, which incorporate the novel ligand N-(pyridin-4-yl)benzamide (benpy) [FeII(benpy)2M(CN)4]·2H2O (M = Pd (Pd(benpy)) and Pt (Pt(benpy))) are reported. The benpy ligand was explored to facilitate spin-crossover (SCO) cooperativity via amide group hydrogen bonding. Structural analyses of the 2-D Hofmann frameworks revealed benpy-guest hydrogen bonding and benpy-benpy aromatic contacts. Both analogues exhibited single-step hysteretic spin-crossover (SCO) transitions, with the metal-cyanide linker (M = Pd or Pt) impacting the SCO spin-state transition temperature and hysteresis loop width (Pd(benpy): T½↓↑: 201, 218 K, ∆T: 17 K and Pt(benpy): T½↓↑: 206, 226 K, ∆T: 20 K). The parallel structural and SCO changes over the high-spin to low-spin transition were investigated using variable-temperature, single-crystal, and powder X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry. These studies indicated that the ligand–guest interactions facilitated by the amide group acted to support the cooperative spin-state transitions displayed by these two Hofmann-type frameworks, providing further insight into cooperativity and structure–property relationships.