Four-step iron(ii) spin state cascade driven by antagonistic solid state interactions

Abstract

A rare four-step spin crossover transition has been attained in a two-dimensional Hofmann-type material through the presence of an array of antagonistic host–host and host–guest interactions.

Guest‐Switchable Multi‐Step Spin Transitions in an Amine‐Functionalized Metal–Organic Framework

“…The 1‐ ii goes back to four unique Fe II sites. However, unambiguous identification of HS and LS sites are unable to be realized, which are due to very narrow pseudo plateau observed magnetically and slight temperature fluctuation during X‐ray data collection . The 1‐ i reverts to C 2/ c with one unique HS Fe II site, indicating re‐entrant behavior .…”

“…The first is the three‐dimensional (3D) Hofmann‐type metal–organic framework (MOF) [Fe(bipydz){Au(CN) 2 } 2 ]⋅4 EtOH (bipydz=3,6‐bis(4‐pyridyl)‐1,2‐diazine), which shows five long‐range ordering spin states with the HS fractions ( γ HS ) of 1, 2/3, 1/2, 1/3, and 0 . The other is 2D Hofmann‐type compounds [Fe 3 (saltrz) 6 {M(CN) 4 } 3 ]⋅8 H 2 O (saltrz=( E )‐2‐(((4 H ‐1,2,4‐triazol‐4‐yl)imino)methyl)phenol, M=Pt II , Pd II ) with γ HS of 1, 5/6, 4/6, 1/6, and 0, in which long‐ and short‐range magnetostructural orderings are alternated for five spin states . Although the sequences of γ HS =1↔3/4↔1/2↔1/4 and γ HS =1↔1/2↔1/4↔0 had been, respectively found in the Fe 4 grid and [Fe(bztrz) 2 {Pd(CN) 4 }]⋅≈2 H 2 O (bztrz=( E )‐1‐phenyl‐ N ‐(1,2,4‐triazole‐4‐yl)‐methanimine), four‐step SCO behavior with the intermediate states of γ HS =3/4, 1/2, and 1/4 has not been documented.…”

“…According to the elastic model, the competition between antiferro‐ and ferro‐elastic interactions is required to stabilize the intermediate states ,,. The octahedral distortion parameters Σ for the HS Fe II sites hint the presence of antiferro‐elastic interaction: 15.1° ( 1‐ i )→15.5° ( 1‐ iii )→16.9° ( 1‐ iv ), in which Σ values increase with cooling .…”

Hysteretic Two‐Step Spin‐Crossover Behavior in Two Two‐Dimensional Hofmann‐Type Coordination Polymers

“…Heretofore, a handful of complete two‐step SCO coordination polymers (CPs) with structural phase transitions have been observed and these examples form ordered HS‐LS pairs with various ratios in the IP region . Moreover, a series of Hofmann‐type CPs [L=4,4′‐di(pyridylthio)methane, ( E )‐2‐(((4H‐1,2,4‐triazol‐4‐yl)imino)methyl)phenol, ( E )‐1‐phenyl‐ N ‐(1,2,4‐triazol‐4‐yl)‐methanimine, 3,6‐bis(4‐pyridyl)‐1,2‐diazine] displaying three of four stepped SCO where each spin state adopts different long‐range ordering of LS and HS sites yielding inequivalent Fe II centers for the intermediate states through the phase transition, and each ordering step has been structurally characterized.…”

Modification of Cooperativity and Critical Temperatures on a Hofmann-Like Template Structure by Modular Substituent

“…Since we reported the first Hofmann like SCO coordination polymer {Fe(py) 2 [Ni(CN) 4 ]} n (py = pyridine) [3], many derived types of {Fe II (L) 1~2 [M II (CN) 4 ]} n [4][5][6][7][8][9] and {Fe II (L) 1~2 [M I (CN) 2 ] 2 } n [10][11][12][13][14][15][16][17][18][19][20][21] (M I = Cu, Ag, or Au, M II = Ni, Pd, or Pt, L = pyridine derivatives) have been developed. These compounds show a template 2-dimentional (2-D) sheet structure because of their strongly determinate self-assembly process in which they link octahedral metal centers through the N atoms of the bidentate [Au(CN) 2 ] − unit.…”