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

Abstract

The spin-crossover (SCO) phenomenon is an active area of research. This paper describes the synthesis of an Fe−Ag Hofmann-type complex, {Fe(4methylpyrimidine) 2 [Ag(CN) 2 ] 2 }, which demonstrates a one-step SCO and single-layer Hofmann-type structure with Ag−N interactions and no Ag−Ag interactions, which is strikingly different from the previously synthesized complex {Fe(4-methylpyrimidine) 2 [Au-(CN) 2 ] 2 } that contains Au−Au interactions and no Au−N interactions. This difference can be explained in terms of the lack of relativistic effect in the Ag atoms and the different cooperative effects caused by their different structures. A scan-rate-dependent hysteresis is observed using magnetic measurement whereas not using 57 Fe Mossbauer spectroscopy, suggesting that the spin transition is relatively slow.

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

“…A nice similar example is complexes based on 4-methylpyrimidine of the composition [Fe­(4-methylpyrimidine) 2 [M I (CN) 2 ] 2 ] (where M I = Ag, Au), where the Ag analogue has a lower transition temperature than the Au analogue. In that case, the difference is also explained from the viewpoint of different crystal packing effects: the presence of M···M interactions, M···N contacts, and other structural features. , …”

“…In that case, the difference is also explained from the viewpoint of different crystal packing effects: the presence of M•••M interactions, M•••N contacts, and other structural features. 24,67 Because of spin−orbit interaction, the experimental value of χ M T for the HS state of both complexes is higher than the calculated value based on the spin of Fe(II) and equal to ∼3.1−3.5 cm 3 K mol −1 . These χ M T values are typical for most of the Fe(II) Hofmann-like clathrates reported previously (∼3.0−3.8 cm 3 K mol −1 ).…”

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

“…The first type is bridged by square planar tetra-coordinated divalent cyanometalate [M(CN) 4 ] 2− (M = Ni, Pd, Pt) 3–10 such as Fe(pz)[M(CN) 4 ]·H 2 O (M = Ni, Pd, Pt), 10 while the other is bridged by linear coordinated monovalent cyanometalate [M(CN) 2 ] − (M = Ag, Au). 11–16 The first complex possessing the latter type of structure is [Cd(4,4′-bpy) 2 {Ag(CN) 2 } 2 ], 17 while the second is [Cd(py) 2 {Ag(CN) 2 } 2 ], 18,19 which were both reported by Soma and Iwamoto, and therefore, the latter type of structures are denoted as “Soma–Iwamoto types”. Coordination polymer frameworks of Hofmann-type ML 2 M′(CN) 4 (M′ = Ni, Pd, Pt) structures are different from that of the Soma–Iwamoto type ML 2 [M′′(CN) 2 ] 2 (M′′ = Ag, Au).…”

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

“…Among the most well-studied spin-crossover frameworks (SCOFs) are a set of Hofmann-type 3D lattices with the general formula Fe II (pz)­[M II (CN) 4 ], composed of Fe­(II) tetracyanometallate layers (where M II = Ni, Pd or Pt) connected through apically coordinated pyrazine (pz) linkers. ,− Like other SCO systems, − the octahedral ligand field-induced d-orbital splitting energy of the open shell d 6 Fe sites is close to the electron pairing energy. This engenders parallel stability of the low-spin (LS) and high-spin (HS) states and a propensity to switch between the two in response to external stimuli (e.g., temperature or light).…”

“…Variable temperature magnetic susceptibility measurements clearly established the overall SCO behavior and in situ diffraction studies tracked the corresponding long-range structure changes in these SCOFs, yet these measurements do not fully capture local electronic and structural changes associated with spin transition at the Fe sites. Mossbauer spectroscopy is a valuable tool for studying Fe-based SCO systems, ,− but isotopic labeling, cryogenic temperatures, and/or very long data acquisition times are often required to produce the data quality needed for deeper electronic structure insight beyond spin state assignment, particularly for lower-concentration Fe samples.…”