Spin crossover-induced colossal positive and negative thermal expansion in a nanoporous coordination framework material.

External control over the mechanical function of materials is paramount in the development of nanoscale machines. Yet, exploiting changes in atomic behaviour to produce controlled scalable motion is a formidable challenge. Here, we present an ultra-flexible coordination framework material in which a cooperative electronic transition induces an extreme abrupt change in the crystal lattice conformation.

Spray-Drying to Get Spin-Crossover Materials.

Spin-crossover (SCO) triazole-based coordination polymers can be synthesized by micelle techniques, which almost always lead to rod-shaped nanoparticles. In order to notably reach new morphologies, we explore here the potentiality of the spray-drying (SD) method to get SCO materials. Three SCO coordination polymers and a mononuclear complex are investigated.

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

A four-stepped cascade of Fe(ii) high spin (HS) to low spin (LS) states is demonstrated in a family of 2-D Hofmann materials, [Fe(saltrz)(M(CN))]·8(HO) (M = Pd ( ), Pt ( ); saltrz = ()-2-(((4-1,2,4-triazol-4-yl)imino)methyl)phenol). Alongside the fully HS and LS Fe(ii) states, fractional spin state stabilization occurs at HS/LS values of 5/6, 2/3, and 1/6. This unconventional spin state periodicity is driven by the presence of multiple spin crossover (SCO) active Fe(ii) sites which are in subtly distinct environments driven by a network of antagonistic host-host and host-guest interactions.

Stereochemistry for engineering spin crossover: structures and magnetic properties of a homochiral vs. racemic [Fe(N3O2)(CN)2] complex.

The Schiff-base condensation of the R,R-(+)-diamine () with 2,6-diacetyl pyridine in the presence of Fe(II) affords the macrocyclic complex [Fe(dpN3O2)(CN)2] () (dp = diphenyl) with ligand centred chirality comprising of a 1 : 1 mixture of LS 6- and HS 7-coordinate Fe(II) centres. Variable temperature magnetic susceptibility and Mössbauer studies reveal that () undergoes an incomplete thermal SCO transition with a T1/2 = 250 K as well as a LIESST effect. In contrast its racemic counterpart () comprises of mostly LS Fe(II) and exhibits no LIESST properties.

Ultrafast light-induced spin-state trapping photophysics investigated in Fe(phen)2(NCS)2 spin-crossover crystal.

Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption.

Sequential Activation of Molecular Breathing and Bending during Spin-Crossover Photoswitching Revealed by Femtosecond Optical and X-Ray Absorption Spectroscopy.

We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations.

A new family of diamagnetic macrocyclic Fe(II) compounds exhibiting the LIESST effect at high temperatures.

The photomagnetic properties of a new Fe(II) macrocyclic family [Fe(L(xyz)N5)(CN)2]·nH2O were investigated with respect to the T(LIESST) versus T(1/2) relationship. These compounds are diamagnetic below 400 K with T(LIESST) values above 100 K, which indicates that this family presents an unconventional LIESST effect that is much higher than the expected T0 = 180 K line for macrocyclic complexes.

Thermal- and light-induced spin-crossover bistability in a disrupted Hofmann-type 3D framework.

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe(II)M(II)(CN)4] (M(II) = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe(II)Pt(II)(CN)4] to form the material [Fe(H2O)2Fe(DPSe)2(Pt(CN)4)2]·3EtOH (DPSe = 4,4'-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron(II) environments, [Fe(II)N6] (Fe1) and [Fe(II)N4O2] (Fe2), (2) as a consequence of axial water coordination to Fe2, there are "holes" in the [Fe(II)Pt(II)(CN)4] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group.

An investigation of photo- and pressure-induced effects in a pair of isostructural two-dimensional spin-crossover framework materials.

Two new isostructural iron(II) spin-crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)2(NCX)2] (dpms = 4,4'-dipyridylmethyl sulfide; X = S (SCOF-6(S)), X = Se (SCOF-6(Se))) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF-6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high-spin (HS) to low-spin iron(II) sites over the temperature range 300-4 K (T1/2 = 75 K).

Solvate-dependent spin crossover and exchange in cobalt(II) oxazolidine nitroxide chelates.

Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3.

The role of iron(II) dilution in the magnetic and photomagnetic properties of the series [Fe(x)Zn(1-x)(bpp)â‚‚](NCSe)â‚‚.

The effects of metal dilution on the spin-crossover behavior of iron(II) in the mixed crystal series [Fe(x)Zn(1-x)(bpp)2](NCSe)2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) have been studied using magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. For each mixed-crystal system, the thermal spin transition temperature, T(1/2), and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T(1/2) decreases with the metal dilution while T(LIESST) remains unchanged.

Spin-state ordering on one sub-lattice of a mononuclear iron(III) spin crossover complex exhibiting LIESST and TIESST.

The two-step spin crossover in mononuclear iron(III) complex [Fe(salpm)2 ]ClO4 ⋅0.5 EtOH (1) is shown to be accompanied by a structural phase transition as concluded from (57) Fe Mössbauer spectroscopy and single crystal X-ray diffraction, with spin-state ordering on just one of two sub-lattices in the intermediate magnetic and structural phase. The complex also exhibits thermal- and light-induced spin-state trapping (TIESST and LIESST), and relaxation from the LIESST and TIESST excited states occurs via the broken symmetry intermediate phase.

The spin state of a molecular adsorbate driven by the ferroelectric substrate polarization.

The spin state of [Fe(H2B(pz)2)2(bipy)] thin films is mediated by changes in the electric field at the interface of organic ferroelectric polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE). Signatures of the molecular crossover transition are evident in changes in the unoccupied states and the related shift from diamagnetic to paramagnetic characteristics. This may point the way to the molecular magneto-electric effect on devices.

Spin crossover complexes [Fe(NH2trz)3](X)2·nH2O investigated by means of polarized Raman scattering and DFT calculations.

Vibrational spectra of the spin crossover (SCO) polymers [Fe(NH2trz)3](X)2·nH2O where NH2trz = 4-NH2-1,2,4-triazole and X = Cl, Br, BF4, and NO3 have been analyzed. Our results show that the anions and water molecules have no significant influence on the vibrational properties of the Fe(NH2-trz)3 polymer chains. A detailed study of the nitrate derivative, based on the DFT analysis of the polarized spectra of single crystals, has been undertaken to propose the normal mode assignment of the Raman peaks in the low spin state of the compound.

Crown-linked dipyridylamino-triazine ligands and their spin-crossover iron(II) derivatives: magnetism, photomagnetism and cooperativity.

The syntheses, crystallography and magnetic properties of a series of compounds of formula trans-[Fe(II)(L(1))2(NCX)2] (X = S, Se, BH3 (1-3)), cis-[Fe(II)(L(2))(NCX)2]·CH2Cl2 (X = S, Se, BH3 (4-6)) and trans-[Fe(II)(L(3))(NCX)2]n (X = S, Se (7-8)) are described (L(1) = 6-chloro-N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(2) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(3) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2),N(4),N(4)-tetra(pyridin-2-yl)-1,3,5-triazine-2,4-diamine)). The magnetostructural properties of 1-8 have been probed in detail by variable temperature magnetic measurements and crystallographic methods. 1-6 display mononuclear structures while 7 and 8 form 1-D chain structures.

Synergy between polymorphism, pressure, spin-crossover and temperature in [Fe(PM-BiA)2(NCS)2]: a neutron powder diffraction investigation.

The pressure dependencies of the lattice parameters of the spin transition compound [Fe(PM-BiA)2(NCS)2] have been derived from neutron powder diffraction measurements at low temperature. The study of the compound [Fe(PM-BiA)2(NCS)2]-pI has first confirmed the atypical spin crossover behaviour under pressure of this compound that shows a pressure induced structural transition inducing the transformation into a different polymorph, [Fe(PM-BiA)2(NCS)2]-pII. This phenomenon avoids a first-order spin transition in favour of continuous transition around 0.

Stimuli responsive hybrid magnets: tuning the photoinduced spin-crossover in Fe(III) complexes inserted into layered magnets.

The insertion of a [Fe(sal2-trien)](+) complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHCl3 (1·CHCl3), [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHBr3 (1·CHBr3), and [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CH2Br2 (1·CH2Br2). The three compounds crystallize in a 2D honeycomb anionic layer formed by Mn(II) and Cr(III) ions linked through oxalate ligands and a layer of [Fe(sal2-trien)](+) complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and Mössbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.

Thermal spin crossover and LIESST effect observed in complexes [Fe(L(Ch))2(NCX)2] [L(Ch) = 2,5-di(2-pyridyl)-1,3,4-chalcadiazole; Ch = O, S, Se; X = S, Se, BH3].

Two new complexes belonging to the family [Fe(II)(L)2(NCX)2] have been synthesized and structurally characterized. Complexation of the ligand L(O) = 2,5-di(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe(II)(L(O))2(NCS)2] (1) with the common s-trans and the rarely observed s-cis conformation. No thermal spin crossover (SCO) was observed for the amorphous bulk material of the mixture.

Biphenyl bridged hexadentate N6-ligands--a rigid ligand backbone for Fe(II) spin crossover complexes.

The novel hexadentate nitrogen based ligand N,N'-bis-(2-(1H-pyrazol-1-yl)pyridine-6-ylmethyl)-2,2'-biphenylenediamine (3) was synthesized and used for the preparation of iron Spin Crossover (SCO) complexes [Fe(3)][BF4]2 (4) and [Fe(3)][ClO4]2 (5), which differ only by the respective counter ion. These complex salts show different spin transition temperatures T(1/2) (135 and 157 K, respectively). This effect was studied by the investigation of the solid state structure of different low- and high-spin isomers.

One-step vs stepwise immobilization of 1-D coordination-based Rh-Rh molecular wires on gold surfaces.

Reaction of dimeric [Rh(II)(2)(phen)(2)(μ-OAc)(2)(MeCN)(2)](BF(4))(2) (phen =1,10-phenanthroline) with pyrazine (pz) in a 1:2 ratio leads to the new 1-D metal-metal-bonded coordination oligomer {[Rh(II)(2)(phen)(2)(μ-OAc)(2)(pz)](BF(4))(2)}(n) (Rh-Rhpz)(n) (1), where each Rh atom of the dimeric unit (Rh-Rh) is coordinated in the equatorial plane to a nitrogen atom of a rigid and linear bifunctionalized organic linker (pz). Single X-ray diffraction analysis reveals the 1-D straight oligomeric chain structure (molecular wire, MW) consists of alternating (Rh-Rh) units and pz linking ligands with free BF(4)(-) as counteranions, and each metal center has a slightly distorted octahedral arrangement. The presence of accessible labile MeCN groups on both ends of these MWs ("free ends") enables functionalization of a 4-mercaptopyridine-gold coordinating platform (Au/MP) to form in one step a layer of coordination oligomer (Au/MP(Rh-Rhpz)(n); n ≈ 50).

Photomagnetism of a sym-cis-dithiocyanato iron(II) complex with a tetradentate N,N'-bis(2-pyridylmethyl)1,2-ethanediamine ligand.

A comprehensive study of the magnetic and photomagnetic behaviors of cis-[Fe(picen)(NCS)(2) ] (picen = N,N'-bis(2-pyridylmethyl)1,2-ethanediamine) was carried out. The spin-equilibration was extremely slow in the vicinity of the thermal spin-transition. When the cooling speed was slower than 0.

Antagonism between extreme negative linear compression and spin crossover in [Fe(dpp)(2)(NCS)(2)]â‹…py.

A scissor-like geometric mechanism is responsible for the strongest negative linear compression effect yet observed in a molecular material, [Fe(dpp)(2)(NCS)(2)]â‹…py (see picture; dpp=dipyrido[3,2-a:2'3'-c]phenazine), C gray, N blue, S yellow, Fe red). The same mechanism is also responsible for suppressing the high-spin to low-spin transition under pressure.

High-pressure spin-crossover in a dinuclear Fe(II) complex.

The effect of pressure on the dinuclear spin crossover material [{Fe(bpp)(NCS)(2)}(2)(4,4'-bipy)]·2MeOH (where bpp = 2,6-bis(pyrazol-3-yl)pyridine and 4,4'-bipy = 4,4'-bipyridine, 1) has been investigated with single crystal X-ray diffraction and Raman spectroscopy using diamond anvil cell techniques. The very gradual pressure-induced spin crossover occurs between 7 and 25 kbar, and shows no evidence of crystallographic phase transitions. The pressure-induced spin transition leads to a complete LS state which is not thermally accessible.

Photomagnetic studies on spin-crossover solid solutions containing two different metal complexes, [Fe(1-bpp)(2)](x)[M(terpy)2](1-x)[BF4]2 (M = Ru or Co).

The photomagnetic properties of two series of spin-crossover solid solutions, [Fe(1-bpp)(2)](x)[Ru(terpy)(2)](1-x)(BF(4))(2) and [Fe(1-bpp)(2)](x)[Co(terpy)(2)](1-x)(BF(4))(2) (1-bpp = 2,6-bis[pyrazol-1-yl]pyridine), have been investigated. For all the materials, the evolution of the T(LIESST) value, the high-spin → low-spin relaxation parameters and the LITH loops were thoroughly studied. Interestingly in the Fe:Co series, along the photo-excitation, cobalt ions are concomitantly converted from low-spin to high-spin states with the iron centres, and also fully relax after light excitation.