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

Abstract

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.

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

“…The largest Raman shift between the different spin states was observed in the range of ν CN = ∼2160−2210 cm −1 , which agrees well with literature reports on the Hofmann framework. 71,72 The well-resolved doublet peaks corresponding to the symmetric and asymmetric stretching modes of the CN groups in the 2D Hofmann layer 54 (Figure 6) shift to lower wavenumber, while the spin states change from 60% LS/ 40% HS state (red, 100 K) to 100% HS state (orange, 300 K) with an increase in the temperature. The relative intensity of the band peaks also decreased with a change in the spin state from a fractional HS/LS state (green, 140 K) to a completely HS state (red, 300 K) (Figure S21).…”

Modulation of the spin transition in 2D Hofmann frameworks via π⋯π stacking between the axial 2,5-dipyridyl-1,3,4-oxadiazoles

“…5–11 Among them, Fe 2+ Hofmann-type SCO frameworks are one of the most notable systems. 6,10–35 These frameworks generally contain neutral bimetallic layers consisting of Fe 2+ ions and [M(CN) 4 ] 2− anions (M = Pt 2+ , Pd 2+ , Ni 2+ ) in the equatorial plane, which allow a large number of linear N-donor ligands to coordinate axially to the Fe 2+ centers in order to produce 2D/3D Hofmann SCO materials. 10 If the linear exo -bidentate ligands are used as pillars, 3D porous Hofmann SCO frameworks can be constructed, 11 while 2D Hofmann SCO networks will be built if the axial ligands are merely monodentate.…”

“…12 Since the monodentate ligand is always more readily available than the bidentate one, the 2D Hofmann SCO networks can be more easily constructed, which will provide a good opportunity for exploring the influence of intermolecular interactions (hydrogen bonding, π⋯π stacking and interdigitated effect) on the SCO behavior. 12–35 So far, most of the monodentate axial ligands used in 2D Hofmann SCO networks are substituted pyridine derivatives, owing to their cheap and easy large-scale preparation. Moreover, pyridine derivatives modified with small substituent groups (–F, –Cl, –Br, –I, –NH 2 , etc. )…”

“…often possess suitable ligand-field strength for spin transition in 2D Fe 2+ Hofmann SCO frameworks. 12–30 Thus, by rational choice of substituted pyridines as axial ligands, diverse spin transition modes have been observed in 2D Hofmann SCO networks. For instance, by use of pyridine (py) as an axial ligand, Prof. Kitazawa et al in 1996 discovered the first 2D Hofmann SCO framework [Fe(py) 2 Ni(CN) 4 ] with a hysteresis loop of 9 K ( T 1/2 ↓ = 186 K and T 1/2 ↑ = 195 K).…”

Coordination Polymers and Metal-Organic Frameworks: Structures and Applications—A Themed Issue in Honor of Professor Christoph Janiak on the Occasion of His 60th Birthday

“…Cimpoesu, Ferbinteanu and coworkers describe a series of pyrazolato-bridged, multinuclear copper(II) coordination compounds; the magnetic properties of these Cu 3 , Cu 6 and Cu 7 -containing species are dominated by the strong antiferromagnetism across the pyrazolate bridges [9]. Neville and coworkers have investigated the use of the N-(pyridin-4-yl)benzamide ligand in {Fe(II)Pd(II)} and {Fe(II)Pt(II)}-containing 2D Hofmann-type frameworks to facilitate spin crossover cooperativity through hydrogen bonding involving the amide moiety [10]. A departure from the main theme of the issue comes with an investigation of donor-acceptor Stenhouse adducts.…”