The Effect of an Active Guest on the Spin Crossover Phenomenon

A luminescent Pt2Fe spin crossover complex

A heterotrinuclear luminescent [Pt₂Fe] spin crossover (SCO) complex was developed, synthesized, and investigated.

A ladder-type iron(ii) coordination polymer with enhanced spin-crossover behavior

A ladder-type 1D coordination polymer with enhanced spin-crossover behaviour is reported. It showed an abrupt spin transition above room temperature with a 3 K hysteresis (T_1/2↓ = 335 K andT_1/2↑ = 338 K).

Guest induced hysteretic tristability in 3D pillared Hofmann-type microporous metal–organic frameworks

Hofmann-type SCO coordination polymers with the formula {Fe(C₄H₄N₂)[M(CN)₄]}·(C₄H₄N₂)·1.2H₂O, where M = Pt(ii), Pd(ii) displaying well-resolved hysteretic multistep thermally induced spin transition.

Measuring and Reporting Electrical Conductivity in Metal-Organic Frameworks: Cd2(TTFTB) as a Case Study

Electrically conductive metal-organic frameworks (MOFs) are emerging as a subclass of porous materials that can have a transformative effect on electronic and renewable energy devices. Systematic advances in these materials depend critically on the accurate and reproducible characterization of their electrical properties. This is made difficult by the numerous techniques available for electrical measurements and the dependence of metrics on device architecture and numerous external variables. These challenges, common to all types of electronic materials and devices, are especially acute for porous materials, whose high surface area make them even more susceptible to interactions with contaminants in the environment. Here, we use the anisotropic semiconducting framework Cd₂(TTFTB) (TTFTB^4- = tetrathiafulvalene tetrabenzoate) to benchmark several common methods available for measuring electrical properties in MOFs. We show that factors such as temperature, chemical environment (atmosphere), and illumination conditions affect the quality of the data obtained from these techniques. Consistent results emerge only when these factors are strictly controlled and the morphology and anisotropy of the Cd₂(TTFTB) single-crystal devices are taken into account. Most importantly, we show that depending on the technique, device construction, and/or the environment, a variance of 1 or even 2 orders of magnitude is not uncommon for even just one material if external factors are not controlled consistently. Differences in conductivity values of even 2 orders of magnitude should therefore be interpreted with caution, especially between different research groups comparing different compounds. These results allow us to propose a reliable protocol for collecting and reporting electrical properties of MOFs, which should help improve the consistency and comparability of reported electrical properties for this important new class of crystalline porous conductors.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [Fe^IIPd(CN)₄(R-1,2,4-trz)₂]·nH₂O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [Fe^IIPd(CN)₄(thiome)₂]·2H₂O (A·2H₂O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H₂O due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in A·2H₂O (0-0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in A·2H₂O under hydrostatic pressure compared to a one-step SCO in A at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities.

Strong Cooperative Spin Crossover in 2D and 3D FeII-MI,II Hofmann-Like Coordination Polymers Based on 2-Fluoropyrazine

Self-assembling iron(II), 2-fluoropyrazine (Fpz), and [M^II(CN)₄]^2- (M^II = Ni, Pd, Pt) or [Au^I(CN)₂]^- building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ₄-[M^II(CN)₄]^2- groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe(Fpz)₂([M^II(CN)₄]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by T_1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe(Fpz)[Pt(CN)₄]}·1/2H₂O and {Fe(Fpz)[Au(CN)₂]₂}, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine.

High-temperature spin crossover in two solvent-free coordination polymers with unusual high thermal stability

Two solvent-free two-dimensional (2D) coordination polymers, (2)∞[Fe(ptim)2] (1) and (2)∞[Fe(ptpy)2] (2) (Hptim = 2-(5-(4-(1H-imidazol-1-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine; Hptpy = 2-(5-(4-(pyridin-3-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine), have been successfully prepared by solvothermal reactions. Their iron atoms are bridged by the corresponding multidentate anionic ligands into dense neutral structures. The magnetic data reveal that complexes 1 and 2 are rare examples exhibiting reversible one-step high-temperature spin crossover behaviors with spin transition temperatures of 419 and 416 K, respectively. Moreover, these structures also display remarkable thermal stability up to 714 K (for 1) and 690 K (for 2), which are confirmed by thermogravimetric and variable-temperature powder X-ray diffraction analyses.

3,3'-di(pyrazinamoyl)-2,2'-bipyridine: rational ligand design for the self-assembly of a 1-D coordination polymer

A new ligand, 3,3'-di(pyrazinamoyl)-2,2'-bipyridine, L3H2 has been prepared and is comprised of distinct binding domains capable of facilitating the formation of coordination polymers. Reaction of L3H2 with [Cu2(OAc)4(H2O)2] in the presence of Et3N·HCl affords the complex {[Cu6(L3)2(Cl)2(OAc)6]·2H2O·2MeOH}n (1) in which the [L3](2-) anion binds two unique copper ions in a bis-tridentate fashion via amide and pyridyl N atoms. This bimetallic unit is linked to an equivalent unit via a pair of 1,3-acetato and μ-chloro bridges to afford a tetrameric [Cu4(L3)2Cl2(OAc)2] core. Coordination of one of the pyrazole substituents from each ligand to a third copper centre of a rigid paddlewheel [Cu2(OAc)4] unit leads to a unique 1-D polymeric structure. Magnetic studies of 1 reveal that the magnetism can be described in terms of two sets of exchange interactions; strong antiferromagnetic interactions between the two copper centres within the "paddle-wheel" (J/k = -190 K) and a combination of weaker ferro- and antiferro-magnetic interactions within the tetrameric core (J'/k = -5.7 K and J''/k = +2.1 K).

Dynamic structural transformations of coordination supramolecular systems upon exogenous stimulation

This feature article comments on the dynamic structural transformations of coordination supramolecular systems, which can be triggered by exposure to various exogenous stimuli such as concentration, temperature, light and mechanical force, as well as their synergic effect.

In air a spin crossover active iron(ii) complex of amine/NCBH3− ligands is converted to a low spin complex of imine/CN− ligands

A spin crossover Fe^II complex is in situ converted to a low spin complex in the presence of air, induced by the oxidative dehydrogenation of the tetradentate ligand and the generation of CN⁻ from NCBH₃⁻.

Guest-containing supramolecular isomers of silver(i) 3,5-dialkyl-1,2,4-triazolates: syntheses, structures, and structural transformation behaviours

Using benzene as a template, a series of guest-containing silver(i) 3,5-dialkyl-1,2,4-triazolate isomers showing different structural transformation behaviours have been synthesized.

Spin crossover behaviour in one-dimensional FeIIcompounds based on the [M(CN)4]2−(M = Pd, Pt) units

Four one-dimensional heterobimetallic spin crossover coordination polymers {Fe(L)₂[M(CN)₄]}_n(M = Pd^IIand Pt^II) have been synthesized and characterized.

Exceptional sensitivity to the synthetic approach and halogen substituent for Zn(ii) coordination assemblies with 5-halonicotinic acids

The significant influence of the synthetic approach and halogen substituent on Zn(ii) coordination assemblies with 5-halonicotinic acids has been demonstrated.

Spin-crossover behavior in two new supramolecular isomers

Two spin-crossover (SCO) supramolecular isomers formulated as [Fe(Mebpt){Au(CN)2}]n·xH2O (1, x = 0, and 2·H2O, x = n, MebptH = 3-(5-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole) have been successfully isolated and characterized by single-crystal X-ray crystallography, thermogravimetric analysis, differential scanning calorimetry, variable-temperature powder X-ray diffraction, and magnetic measurements. Structural analysis reveals that 1 is a two-dimensional coordination layer and 2·H2O is a one-dimensional coordination ladder structure, in which the Mebpt(-) ligands are coordinated in trans and cis bridging mode in 1 and 2·H2O, respectively. Dramatically, their SCO properties are further influenced by the octahedral coordination configuration of Fe(II). In 1, only the Fe(II) centers in trans configuration exhibit spin transition (Tc = 303 K), while in 2·H2O and the dehydrated 2, gradual two-step SCO (Tc1 = 235 K, Tc2 = 313 K) and one-step SCO behaviors (Tc = 315 K) occur, respectively.

Spin crossover intermediate plateau stabilization in a flexible 2-D Hofmann-type coordination polymer

The abrupt and hysteretic two-step spin crossover in a new triazole-based 2-D Hofmann-type complex shows a record breaking 120 K intermediate plateau (IP) region stabilized by negative cooperative interactions.

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.

Coligand-directed synthesis of five Co(ii)/Ni(ii) coordination polymers with a neutral tetradentate ligand: syntheses, crystal structures, and properties

The solvothermal reactions afforded five coordination polymers with different coligands, and the coligand-directed synthesis has a great effect on the final structures.

Structures and properties of coordination polymers involving asymmetric biphenyl-3,2′,5′-tricarboxylate

The conformation stability and coordination modes of an asymmetric polycarboxylate ligand (H₃bptc) and the properties of its compounds have been investigated.