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

Abstract

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.

Pressure-Induced Hysteretic and Abrupt Spin Transition in Amine Functionalized Isoquinoline-Based Two-Dimensional FeII Hofmann Frameworks

“…Additionally, the octahedron distortion parameter (∑), defined as the sum of |90° – θ| for the 12 cis-N–Fe–N angles in the coordination octahedron, has increased from 2.950° (100 K) to 15.351° (235 K), which is in agreement with the SCO behavior from the LS state to HS state (Table ). Further, the trigonal distortion parameter (Θ = is the sum of the deviation of 24 unique torsional angles between the ligand atoms on opposite triangular faces of the octahedron viewed along the pseudo-3-fold axis (θ i ) from 60°) and length deviation (ζ = the average of the sum of the deviation of 6 unique metal–ligand bond lengths around the central metal atom ( d i ) from the average value ( d mean )) have increased their values significantly from 20.882° to 48.571° and 0.204 to 0.205 Å at 100 and 235 K respectively (Table ). Thus, the result suggests that the geometry around the coordination SCO centers is more distorted in a high spin than in a low spin state.…”

“…To tune SCO profiles, pyridine-, pyrazine-, and triazole-based − ligands have been used extensively, but recently the fused bicyclic azine-based ligand came into the picture in SCO-based materials. , Till now there are only two reports where the isoquinoline-based ligand is used in a 2D Hofmann clathrate [Fe­(isoq) 2 {Ag­(CN) 2 } 2 ] which exhibits a light induced excited spin state trapping (LIESST) effect and tristability for its spin states over a large temperature range, containing three inequivalent Fe II sites, with the sequence of γ HS = 1 ↔ γ HS = 3/4 ↔ γ HS = 1/4 . Similarly, Real and co-workers have used the same ligand with square planar M­(CN) 4 2– (M = Pt and Pd) to achieve SCO behavior.…”

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

“…For example, despite there being only one crystallographically distinct Fe II center, Hofmann layer distortion is apparent in the layer undulation and presence of two distinct ligand pairings: one with closely associated ligands and host–host interactions and the other with wider spaced ligands, where the guest molecules reside and where host–guest interactions exist. This general framework motif is relatively common in 2-D Hofmann frameworks containing functionalized 1,2,4-triazole ligands, such as [Fe II Pd­(CN) 4 (bztrz) 2 ]·EtOH,H 2 O and [Fe II Pd­(CN) 4 (thtrz) 2 ]·EtOH,H 2 O (where thtrz = N -thiophenylidene-4 H -1,2,4-triazol-4-amine), and have also been reported for some 2-D Hofmann frameworks with functionalized pyridyl ligands. ,, Here, as for the 1,2,4-triazole analogues, the layer undulation with distinct ligand spacings is driven by and supported by the presence of both host–host and host–guest interactions. Indeed, this is one of the reasons why guest molecule manipulation is possible in these frameworks, as the host–host interactions act as a support for the 2-D structures to give them a pseudo-3-D stability. ,,,,, Highlighting this, removal of the ethanol guest molecules from A ·H 2 O,Et is achieved in a single-crystal to single-crystal transformation.…”

Effect of intermolecular anionic interactions on spin crossover of two triple-stranded dinuclear Fe(ii) complexes showing above room temperature spin transition

“…8) leading to the stabilization of the intermediate [HS-LS] state (for a complete transition from LS to HS, 9-10% volume expansion is generally expected). 51 Moreover, the octahedral distortion parameters (calculated in Octadist tool 48 ) near room temperature shows a noteworthy variation for only one Fe1(II) center (62.96°to 76.41°) and a very slight variation for the other Fe2(II) center (62.50°to 67.57°). As the increase in distortion parameter stabilizes the HS state, the above values suggest that one Fe(II) is in HS and another is in the LS state, stabilizing the intermediate [HS-LS] state.…”