Guest Tunable Structure and Spin Crossover Properties in a Nanoporous Coordination Framework Material

Near Room-Temperature Memory Devices Based on Hybrid Spin-Crossover@SiO2 Nanoparticles Coupled to Single-Layer Graphene Nanoelectrodes

The charge transport properties of SCO [Fe(Htrz)2 (trz)](BF4 ) NPs covered with a silica shell placed in between single-layer graphene electrodes are reported. A reproducible thermal hysteresis loop in the conductance above room-temperature is evidenced. This bistability combined with the versatility of graphene represents a promising scenario for a variety of technological applications but also for future sophisticated fundamental studies.

ESR study of molecular orientation and dynamics of TEMPO derivatives in CLPOT 1D nanochannels

The molecular orientations and dynamics of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical derivatives with large substituent groups at the 4-position (4-X-TEMPO) in the organic one-dimensional nanochannels within the nanosized molecular template 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) were examined using ESR. The concentrations of guest radicals, including 4-methoxy-TEMPO (MeO-TEMPO) or 4-oxo-TEMPO (TEMPONE), in the CLPOT nanochannels in each inclusion compound (IC) were reduced by co-including 4-substituted-2,2,6,6-tetramethylpiperidine (4-R-TEMP) compounds at a ratio of 1 : 30-1 : 600. At higher temperatures, the guest radicals in each IC underwent anisotropic rotational diffusion in the CLPOT nanochannels. The rotational diffusion activation energy, Ea , associated with MeO-TEMPO or TEMPONE in the CLPOT nanochannels (6-7 kJ mol(-1) ), was independent of the size and type of substituent group and was similar to the Ea values obtained for TEMPO and 4- hydroxy-TEMPO (TEMPOL) in our previous study. However, in the case in which TEMP was used as a guest compound for dilution (spacer), the tilt of the rotational axis to the principal axis system of the g-tensor, and the rotational diffusion correlation time, τR , of each guest radical in the CLPOT nanochannels were different from the case with other 4-R-TEMP. These results indicate the possibility of controlling molecular orientation and dynamics of guest radicals in CLPOT ICs through the appropriate choice of spacer. Copyright © 2016 John Wiley & Sons, Ltd.

Reversible quantitative guest sensing via spin crossover of an iron(ii) triazole

A new phenyl-triazole-pyrazine ligand, 4-p-tolyl-3-(phenyl)-5-(2-pyrazinyl)-1,2,4-triazole (tolpzph), was prepared in order to enforce pyrazine coordination of the iron(ii) centre in the resulting complex, [FeII(tolpzph)2(NCS)2]·THF (1·THF). Structure determinations carried out on this discrete mononuclear complex, 1·THF, at 273 K (mostly high spin) and 100 K (mostly low spin) demonstrate this was successful, and that spin crossover (SCO) occurred on cooling. Subsequent magnetic measurements on 1·THF revealed that it shows highly sensitive and reversible solvent-dependent SCO, with T1/2(1·THF) = 255 K vs. T1/2(1) = 212 K (with SCO of 1 more abrupt and occurring with a 4 K hysteresis loop), a drop of 43 K due to THF loss. This is reversible over at least 10 cycles of re-solvating with THF followed by re-drying, so 1 ↔ 1·THF can be considered an 'on-off' THF sensor, monitored by the T1/2 reversibly shifting (by 43 K). Furthermore, quantitative sensing of the fractional amount of THF present in 1·nTHF, 0 ≤ n ≤ 1, is demonstrated. Monitoring the T1/2 and using TGA to quantify n(THF) revealed a linear dependence (25 data points; Pearson r2 = 0.93): T1/2 = 41.1n(THF) + 219. Finally, 1 is also shown to take up CHCl3 [T1/2(1·CHCl3) = 248 K], with a logarithmic T1/2 dependence on the fractional amount of CHCl3 present (10 data points; Pearson r2 = 0.98): T1/2 = 27.0 log10[n(CHCl3)] + 243. This study is a proof of principle that a (multi-use) quantitative sensor material based on spin crossover is feasible.

Spin crossover properties of enantiomers, co-enantiomers, racemates, and co-racemates

Spin-crossover Puzzle: SCO co-enantiomers, racemates, and co-racemates were successfully assembled via cocrystallization of four prefabricated enantiomeric iron(ii) modules 4R, 4S, 5R and 5S.

Room-temperature switching of magnetic hysteresis by reversible single-crystal-to-single-crystal solvent exchange in imidazole-inspired Fe(ii) complexes

Molecular switch ON/OFF magnetic hysteresis around room temperature can be achieved by reversible single-crystal-to-single-crystal (SCSC) solvent exchange in imidazole-inspired dihydroquinazoline Fe(ii) complexes.

HCl chemisorption-induced drastic magneto-structural transformation in a layered cobalt–phosphonotriazolate coordination polymer

A stimuli-responsive magnetic coordination polymer was synthesized, which shows an antiferro- to ferromagnetic response to HCl vapour in a crystal-to-crystal transformation.

Neutral N-donor ligand based flexible metal–organic frameworks

This short review focuses on the flexibility aspect of MOFs based on neutral N-donor ligands with representative examples concerning the structural aspects and the subsequent properties induced by the reorganization of the frameworks.

The relationship between the strength of hydrogen bonding and spin crossover behaviour in a series of iron(III) Schiff base complexes

X-ray crystal structures and magnetic properties of an isostructural series of iron(III) Schiff base complexes with the general formula [Fe(L(5))(NCX)]·Solv (where H2L(5) = N,N'-bis(2-hydroxy-naphthylidene)-1,6-diamino-4-azahexane, X = S, Solv = tetrahydrofuran, 1a; X = S, Solv = methanol and 0.5 pyrazine, 1b; X = S, Solv = butanone, 1c; Solv = N,N'-dimethylformamide, X = S (1d) or X = Se (1d'); X = S, Solv = dimethyl sulfoxide, 1e) are reported. In the crystals, the individual [Fe(L(5))(NCX)] molecules are connected through weak C-H···O, C-H···π or C-H···S non-covalent contacts into 2D supramolecular networks, while the guest-solvent (Solv) molecules are trapped in the cavities between two adjacent layers, which are furthermore stabilized by N-H···O hydrogen bonds connecting the Solv oxygen atom with the amine group of the [Fe(L(5))(NCX)] molecule, with the N···O distances varying from 2.921(6) Å (in 1d') to 3.295(2) Å (in 1a). The magnetic properties of the complexes were tuned by the different Solv molecules and as a result of this, four new spin crossover (SCO) compounds with cooperative spin transitions are reported, which are accompanied by thermal hysteresis in two cases (1d and 1e): , T1/2 = 84 K; 1d, T1/2↓ = 232 K, T1/2↑ = 235 K and 1e, T1/2↓ = 127 K, T1/2↑ = 138 K. The role of the N-H···O hydrogen bonding in the occurrence and tuning of SCO was also computationally studied using a topological analysis, and also by evaluation of non-covalent interaction (NCI) indexes. Both theoretical approaches showed a clear relationship between the strength of the N-H···O hydrogen bonds and T1/2, as already inferred from X-ray structural and magnetic data.

Structural phase transition in a multi-induced mononuclear FeII spin-crossover complex

One mononuclear Fe^II complex displays a structural phase transition and a multi-induced spin-crossover behavior mediated by heat, light, pressure and solvent.

Cyanide-bridged bimetallic 3D Hoffman-like coordination polymers with tunable magnetic behaviour

Two isostructural cyanide-bridged bimetallic coordination polymers with a 3D doubly interpenetrated Hoffman-like network exhibit complete two-step spin-crossover behaviours.

Two-Dimensional Coordination Polymers with Spin Crossover Functionality

In the solid state, the propagation of spin crossover (SCO) information is governed by a complex interplay between inner and outer coordination sphere effects. In this way, lattice cooperativity can be enhanced through solid state packing interactions (i.e. hydrogen-bonding and π-stacking) and via coordinatively linking spin switching sites (i.e. coordination polymers). SCO framework materials have successfully provided an avenue for enhanced cooperativity and additional function as host–guest sensors via their potential porosity. In this review, we explore two-dimensional SCO coordination polymers: (1) spin crossover frameworks (SCOFs) consisting of (4,4) grids and (2) Hofmann-type materials where layers are separated by organic ligands. These families have each allowed the elucidation of important structure–function properties and provided a novel platform for molecular sensing applications. Towards advancing the field of infinite polymeric SCO materials, two-dimensional materials can offer flexible porosity, potentially leading to novel spin state-switching functionality.

Thermal Spin Crossover Behaviour of Two-Dimensional Hofmann-Type Coordination Polymers Incorporating Photoactive Ligands

Two spin crossover (SCO)-active 2D Hofmann-type framework materials, [Fe(3-PAP)2Pd(CN)4] (A) and [Fe(4-PAP)2Pd(CN)4] (B) containing the photoactive azo-benzene-type ligands 3-phenylazo-pyridine (3-PAP) and 4-phenylazo-pyridine (4-PAP) were prepared. These materials form non-porous Hofmann-type structures whereby 2D [FeIIPd(CN)4] grids are separated by 3- or 4-PAP ligands. The iron(ii) sites of both materials (A and B) undergo abrupt and hysteretic spin transitions with characteristic transition temperatures T1/2↓,↑: 178, 190 K (ΔT: 12 K) and T1/2↓,↑: 233, 250 K (ΔT: 17 K), respectively. Photo-magnetic characterisations reveal light-induced excited spin state trapping (LIESST) activity in both A and B with characteristic T(LIESST) values of 45 and 40 K. Although both free ligands show trans- to-cis isomerisation in solution under UV-irradiation, as evidenced via absorption spectroscopy, such photo-activity was not observed in the ligands or complexes A and B in the solid state. Structural analysis of a further non-SCO active isomer to B, [Fe(4-PAP)2Pd(CN)4]·1/2(4-PAP) (B·(4-PAP)), which contains free ligand in the pore space is reported.

Detection of host–guest interactions in clathrates of heterocyclic molecules adsorbed in a porous MOF with Cu2 cluster nodes via vibration spectra and magnetic properties

A 3-D MOF Cu₂(EBTC)(H₂O)₂·[G] was used to encapsulate benzopyrrole, benzofuran and benzothiophene molecules. The host–guest interactions were studied via vibration spectra and magnetic susceptibility.

Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers

This review gives a brief overview of single-crystal X-ray diffraction studies and single-crystal to single-crystal transformations of porous coordination polymers.

DPDS–DPS in situ transformation at room temperature via a 1,2-nucleophilic addition mechanism

A cleavage–reorganization reaction at room temperature has been detected in three metal organic coordination compounds synthesized from a DPDS [di(4-pyridyl)disulphide] ligand: Mn(NCS)₂(DPS)₄ (1), [Fe(NCS)₂(DPS)₂]·2H₂O (2) and Zn(NCO)₂(DPS) (3), [DPS = di(4-pyridyl)sulphide].