Effects of Both Methyl and Pyrimidine Groups in Fe-Ag Spin-Crossover Hofmann-Type Complex {Fe(4-Methylpyrimidine)2[Ag(CN)2]2}

Soma-Iwamoto-type SCO complex Fe(quinazoline)2[Au(CN)2]2 using the quinazoline-type ligand

The Hofmann-type complex and Soma-Iwamoto-type complex are cyano-bridged coordination polymers and both have been widely researched. Now we have synthesized a novel Soma-Iwamoto-type complex, namely Fe(quinazoline)2[Au(CN)2]2. This complex shows the Soma-Iwamoto-type bilayer with Au-Au interactions and a SCO phenomenon with a gradual change of magnetic susceptibility. Fe(H2O)2(quinazoline)2[Au(CN)2]2 has also been synthesized and crystallized, and has been found to be a mononuclear complex with hydrogen-bonding network interactions.

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.

Guest-triggered "Soma-Iwamoto-type" penetration complex {Fe(4-methoxypyrimidine)2[M(CN)2]2}·Guest (M = Ag, Au)

Many laboratories have performed extensive studies on the spin-crossover (SCO) phenomenon. Herein, we synthesized novel "Soma-Iwamoto type" complexes containing a 4-methoxypyrimidine (4-MeOPmd) ligand, i.e., {Fe(4-MeOPmd)₂[Ag(CN)₂]₂} (1Ag), {Fe(4-MeOPmd)₂[Au(CN)₂]₂} (1Au), and {Fe(4-MeOPmd)₂[Ag(CN)₂]₂·0.25(4-MeOPmd)} (1Ag·0.25G). Moreover, we synthesized a 4,6-dimethoxypyrimidine (4,6-diMeOPmd) clathrate complex, i.e., {Fe(4-MeOPmd)₂[Ag(CN)₂]₂·0.25(4,6-diMeOPmd)} (2Ag·0.25G) and {Fe(4-MeOPmd)₂[Au(CN)₂]₂·0.25(4,6-diMeOPmd)} (2Au·0.25G). Some of these complexes, i.e., 1Ag·0.25G, 2Ag·0.25G, and 2Au·0.25G, demonstrated penetrated structures, which are very interesting given that they have rarely been reported to date. 1Au and 1Ag did not show the SCO phenomenon, 1Ag·0.25G showed a gradual multi-step SCO phenomenon, and 2Au·0.25G and 2Ag·0.25G showed an abrupt half SCO phenomenon.

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study

The porous Hoffman-type 3D lattice Fe(pz)[Ni^II(CN)₄] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Two-Step Spin Crossover in Hofmann-Type Coordination Polymers [Fe(2-phenylpyrazine)2{M(CN)2}2] (M = Ag, Au)

Two 2D Hofmann-type complexes of the composition [Fe(Phpz)₂{M(CN)₂}₂] (where Phpz = 2-phenylpyrazine; M = Ag, Au) have been synthesized, and their spin-crossover (SCO) behavior has been thoroughly characterized. Single-crystal X-ray analysis reveals that these complexes contain a crystallographically unique Fe(II) center surrounded by two axial Phpz ligands and four equatorial cyanide [M(CN)₂]^- bridges. It is shown that, using of a ligand with two aromatic rings, an advanced system of weak supramolecular interactions (metal-metal, C-H···M, and π···π stacking contacts) is realized. This ensures additional stabilization of the structures and the absence of solvent-accessible voids due to dense packing. Both complexes are characterized by a highly reproducible two-step SCO behavior, as revealed by different techniques (superconducting quantum interference device magnetometry, optical microscopy, etc.). Research shows the exceptional role of the presence of various supramolecular interactions in the structure and the influence of the bulky substituent in the ligand on SCO behavior. Moreover, the perspective of substituted pyrazines for the design of new switchable materials is supported by this work.