An Organic Spin Crossover Material in Water from a Covalently Linked Radical Dyad

A covalently linked viologen radical cation dyad acts as a reversible thermomagnetic switch in water. Cycling between diamagnetic and paramagnetic forms by heating and cooling is accompanied by changes in optical and magnetic properties with high radical fidelity. Thermomagnetic switches in water may eventually find use as novel biological thermometers and in temperature-responsive organic materials where the changes in properties originate from a change in electronic spin configuration rather than a change in structure

Time-resolved single-crystal X-ray crystallography

In this chapter the development of time-resolved crystallography is traced from its beginnings more than 30 years ago. The importance of being able to “watch” chemical processes as they occur rather than just being limited to three-dimensional pictures of the reactant and final product is emphasised, and time-resolved crystallography provides the opportunity to bring the dimension of time into the crystallographic experiment. The technique has evolved in time with developments in technology: synchrotron radiation, cryoscopic techniques, tuneable lasers, increased computing power and vastly improved X-ray detectors. The shorter the lifetime of the species being studied, the more complex is the experiment. The chapter focusses on the results of solid-state reactions that are activated by light, since this process does not require the addition of a reagent to the crystalline material and the single-crystalline nature of the solid may be preserved. Because of this photoactivation, time-resolved crystallography is often described as “photocrystallography”. The initial photocrystallographic studies were carried out on molecular complexes that either underwent irreversible photoactivated processes where the conversion took hours or days. Structural snapshots were taken during the process. Materials that achieved a metastable state under photoactivation and the excited (metastable) state had a long enough lifetime for the data from the crystal to be collected and the structure solved. For systems with shorter lifetimes, the first time-resolved results were obtained for macromolecular structures, where pulsed lasers were used to pump up the short lifetime excited state species and their structures were probed by using synchronised X-ray pulses from a high-intensity source. Developments in molecular crystallography soon followed, initially with monochromatic X-ray radiation, and pump-probe techniques were used to establish the structures of photoactivated molecules with lifetimes in the micro- to millisecond range. For molecules with even shorter lifetimes in the sub-microsecond range, Laue diffraction methods (rather than using monochromatic radiation) were employed to speed up the data collections and reduce crystal damage. Future developments in time-resolved crystallography are likely to involve the use of XFELs to complete “single-shot” time-resolved diffraction studies that are already proving successful in the macromolecular crystallographic field.</p

Highly Porous Cyanometallic Spin-Crossover Frameworks Employing Pyridazino[4,5-d]pyridazine Bridge

Single crystals of two spin-crossover (SCO) cyanometallic coordination polymers based on the pyridazino[4,5-d]pyridazine ligand (pp) of the composition [Fe(pp)M(CN)4]∙G (where M = Pd, Pt; G = guest molecules) were obtained by a slow diffusion technique. A single-crystal X-ray analysis showed that both compounds adopted the structure of porous 3D frameworks, consisting of heterometallic cyano-bridged layers and interlayer pillar pp ligands, with a total solvent accessible volume of ca. 160 Å3 per iron(II) ion (about 37% of the unit cell volume). These frameworks displayed hysteretic SCO behaviour with T1/2 of 150/190 K (heating/cooling) for Pd complex and 135/170 K (heating/cooling) for Pt complex, which was confirmed by variable-temperature SCXRD experiments. This research shows the perspective of using pp ligand for building porous MOFs with spin transitions

Pyridinium bis(pyridine-κN)tetrakis(thiocyanato-κN)ferrate(III) -pyrazine-2-carbonitrile-pyridine (1/4/1)

In the title compound, (C5H6N)[Fe(NCS)4(C5H5N)2]- 4C5H3N3C5H5N, the FeIII ion is located on an inversion centre and is six-coordinated by four N atoms of the thiocyanate ligands and two pyridine N atoms in a trans arrangement, forming a slightly distorted octahedral geometry. A half-occupied H atom attached to a pyridinium cation forms an N—HN hydrogen bond with a centrosymmetrically-related pyridine unit. Four pyrazine-2-carbonitrile molecules crystallize per complex anion. In the crystal, – stacking interactions are present [centroid–centroid distances = 3.6220 (9), 3.6930 (9), 3.5532 (9), 3.5803 (9) and 3.5458 (8) A˚ ].peerReviewe

Solvatomorphism, polymorphism and spin crossover in bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(II)

International audienceSolvatomorphism and polymorphism play a crutial role in defining charachteristics of spin crossover (SCO) in coordination compounds. In this paper we describe a detailed characterization of the SCO bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(II) complex ([Fe(HB(1,2,3-tz)3)2]) 1. Five solvatomorphs of 1∙2Solv (where Solv = DMSO, MeOH, CHCl3), 1∙7DMSO and 1∙2DMF∙H2O were obtained ¬via recrystallization from the respective organic solvents. Solvatomorphs 1∙2MeOH and 1∙2CHCl3 undergo single-crystal-to-single-crystal transformations leading to 1 upon heating above 313 K. Single-crystal X-Ray diffraction analysis revealed π-π stacking and C–H···N contacts between molecules of the complex both in the solvated and desolvated crystals. Spin crossover and thermal properties of 1 and its solvates were investigated by means of calorimetry, thermogravimetry, Mössbauer spectroscopy, variable temperature magnetic susceptibility, Raman spectroscopy and powder X-Ray diffraction measurements. The anhydrous [Fe(HB(1,2,3-tz)3)2] complex, obtained via dehydration of the dodecahydrate, undergoes structural phase transitions upon the first heating in the 303-493 K range followed by reversible, gradual spin crossover upon further thermal cycling, with a transition temperature of T1/2 = 373 K

Room temperature magnetic detection of spin switching in nanosized spin-crossover materials

Building a better SQUID: A prototype for a SQUID-like magnetometry device for the indirect detection of room-temperature switching in spin-crossover nanoparticles has been developed and used in the study of [Fe(hptrz) 3](OTs)2 (hptrz=4-heptyl-1,2,4-triazole, OTs=p-toluenesulfonyl) nanoparticles, as a proof of concept for this novel micromagnetometry approach. The method provides significant benefits over conventional SQUID and nano-SQUID techniques. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim

Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established