Thermal- and Photoinduced Spin-State Switching in an Unprecedented Three-Dimensional Bimetallic Coordination Polymer

Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Abrupt Spin Transition in a Heteroleptic Fe(II) Complex with Pendant Naphthalene Functionality

A heteroleptic spin-crossover (SCO) complex, [Fe(tpma)(xnap-bim)](ClO₄)₂ (1; tpma = tris(2-pyridylmethyl)amine, xnap-bim = 8,15-dihydrodiimidazo[1,2-a:2',1'-c]naphtho[2,3-f][1,4]diazocine), has been obtained by reacting a Fe(II) precursor salt with tetradentate tpma and bidentate xnap-bim ligands. Depending on crystallization conditions, two different solvates have been obtained, 1·2.25py·0.5H₂O and 1·py. The former readily loses the interstitial solvent to produce either a powder sample of 1 upon filtration or crystals of 1 if the solvent loss is slowed by placing the crystals of 1·2.25py·0.5H₂O in diethyl ether. In contrast, 1·py exhibits higher stability toward solvent loss. The crystal packing of both solvates and of the solvent-free structure features double columns of [Fe(tpma)(xnap-bim)]²⁺ cations formed by efficient π-π interactions between the pyridyl groups of tpma ligands, as well as by stacks supported by π-π interactions between interdigitated naphthalene fragments of xnap-bim ligands. While both solvates show a gradual SCO between the high-spin (HS) and low-spin (LS) states of the Fe(II) ion, solvent-free complex 1 exhibits an abrupt spin transition centered at 127 K, with a narrow 2 K thermal hysteresis. Complex 1 also shows a light-induced excited spin state trapping effect, manifested as LS → HS conversion upon irradiation with white light at 5 K. The metastable HS state relaxes to the ground LS state when heated above 65 K.

2D/3D spin crossover porous coordination polymers based on isomeric tetrapyridyl benzene ligands

By employing cyanoborohydride and isomeric tetrapyridyl benzene ligands, a new three-dimensional (3D) [Fe(3-tpb)(NCBH3)2] (1·2H2O, 3-tpb = 1,2,4,5-tetra(pyridin-3-yl)benzene) and a two-dimensional (2D) [Fe(4-tpb)(NCBH3)2] (2·2-NapSMe and 2·H2O·3DMF, 4-tpb = 1,2,4,5-tetra(pyridin-4-yl)benzene, 2-NapSMe =...

A New Systematic Construction of Novel Three-Dimensional Spin Crossover Coordination Polymers Based on the [AgI 2(CN)3] Building Unit

New three-dimensional spin crossover (SCO) coordination polymers systematically constructed by the novel building unit [AgI 2(CN)3], FeII(3-Br-5-CH3pyridine)2[AgI 2(CN)3][AgI(CN)2] (1), FeII(3-Br-5-Clpyridine)2[AgI 2(CN)3][AgI(CN)2] (2), and FeII(3,5-Brpyridine)2[AgI 2(CN)3][AgI(CN)2] (3), have been synthesized and characterized. The bismonodentate binuclear [Ag2(CN)3]- and mononuclear [AgI(CN)2]- units and FeII atoms assemble to form a 3D network structure. The structures of 1-3 are crystallographically identical, which made up the triply interpenetration combined with complicated intermolecular interactions including Ag···Ag, Ag···X (pyridine substituents) and π-stacking interactions. Magnetic and differential scanning calorimetry studies were performed for 1-3. These compounds display a similar SCO behavior, while the critical temperatures (T c) are shifted by the substituent effect. Due to the identical structures of 1-3, the order of T c clearly corresponds with the Hammett constant.

Temperature dependence of desolvation effects in hydrogen-bonded spin crossover complexes

The synthesis, crystal structure and (photo)magnetic properties of the anhydrous spin crossover salt of formula [Fe(bpp)₂](C₆H₈O₄) (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; C₆H₈O₄ = adipate dianion), obtained by desolvation at 400 K of the original tetrahydrate in a single-crystal-to-single-crystal (SC-SC) transformation, are reported. This work offers a comparison between this compound and the previously reported hydrated material ([Fe(bpp)₂](C₆H₈O₄)·4H₂O, 1·4H₂O), highlighting the significance of the thermal conditions used in the dehydration-rehydration processes. In both compounds, a hydrogen-bonded network between iron(ii) complexes and adipate anions is observed. The original tetrahydrate (1·4H₂O) is low-spin and desolvation at 450 K triggers a low-spin (LS) to high-spin (HS) transition to an amorphous phase that remains stable over the whole temperature range of study. Surprisingly, the dehydrated compound at 400 K (1) keeps the crystallinity, undergoes a partial spin crossover (T_1/2 = 180 K) and a quantitative LS to HS photomagnetic conversion at low temperatures, with a T(LIESST) value of 61 K.

A novel two-step Fe-Au type spin-crossover behavior in a Hofmann-type coordination complex {Fe(4-methylpyrimidine)2[Au(CN)2]2}

A Hofmann-type two-step spin-crossover (SCO) complex with a pyrimidine derivative ligand, Fe(4-methylpyrimidine)2[Au(CN)2]2, was synthesized, and this complex shows a two-step SCO phenomenon in the intermediate state. Symmetry breaking also occurs in the intermediate state. These results reveal three spin states within the complex for high-spin (HS), HS0.5LS0.5, and low-spin (LS).

Advances in ligand-unsupported argentophilic interactions in crystal engineering: an emerging platform for supramolecular architectures

Ligand-unsupported argentophilic interactions are described, emphasizing reticular design and recent developments in advanced synthetic strategies, and highlighting diverse interactions that are useful for the construction of unmitigated networks.

Spin crossover in 2D iron(ii) phthalazine cyanometallic complexes

Two new spin-crossover analogues of Hofmann clathrates based on a bicyclic ligand phthalazine have been synthesized.

Spin-crossover in an organic–inorganic hybrid perovskite

The first spin-crossover complex with an organic–inorganic hybrid perovskite structure is reported, which displays three-step spin-crossover, light-induced excited spin-state trapping and spin-state dependent fluorescence properties.

Photo-switched magnetic coupling in spin-crossover complexes

This feature article summarizes the recent progress in the magnetically coupled spin-crossover (SCO) complexes. The photo-switched molecular nanomagnet property, long range magnetic ordering, and the perspectives of SCO complexes are also presented.

Synthesis, Structure, and Photomagnetic Properties of a Hydrogen-Bonded Lattice of [Fe(bpp)2]2+ Spin-Crossover Complexes and Nicotinate Anions

In this paper, we report on the synthesis, crystal structure, and photomagnetic properties of the spin-crossover salt of formula [Fe(bpp)2](C6H4NO2)2·4H2O (1·4H2O) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; C6H4NO2− = nicotinate anion). This compound exhibits a 3D supramolecular architecture built from hydrogen bonds between iron(II) complexes, nicotinate anions, and water molecules. As synthesized, the hydrated material is low-spin and desolvation triggers a low-spin (LS) to high-spin (HS) transformation. Anhydrous phase 1 undergoes a partial spin crossover (T1/2= 281 K) and a LS to HS photomagnetic conversion with a T(LIESST) value of 56 K.

Switchable Spin-Crossover Hofmann-Type 3D Coordination Polymers Based on Tri- and Tetratopic Ligands

Fe^II spin-crossover (SCO) coordination polymers of the Hofmann type have become an archetypal class of responsive materials. Almost invariably, the construction of their architectures has been based on the use of monotopic and linear ditopic pyridine-like ligands. In the search for new Hofmann-type architectures with SCO properties, here we analyze the possibilities of bridging ligands with higher connectivity degree. More precisely, the synthesis and structure of {Fe^II(L^N3)[M^I(CN)₂]₂}·(Guest) (Guest = nitrobenzene, benzonitrile, o-dichlorobenzene; M^I = Ag, Au) and {Fe^II(L^N4)[Ag₂(CN)₃][Ag(CN)₂]}·H₂O are described, where L^N3 and L^N4 are the tritopic and tetratopic ligands 1,3,5-tris(pyridin-4-ylethynyl)benzene and 1,2,4,5-tetrakis(pyridin-4-ylethynyl)benzene. This new series of Hofmann clathrates displays thermo- and photoinduced SCO behaviors.

Designing anisotropic cyanometallate coordination polymers with unidirectional thermal expansion (TE): 2D zero and 1D colossal positive TE

By design, zero thermal expansion of Cu(H₂O)₂[PtX₂(CN)₄] is observed in the two dimensional cyanometallate sheet (left) and colossal thermal expansion in the inter-sheet direction (right).

Reversible crystal-to-crystal transformation from a trinuclear cluster to a 1D chain and the corresponding spin crossover (SCO) behaviour change

Reversible crystal-to-crystal transformation between a linear trinuclear Fe(ii) complex and a 1D chain, and the SCO behaviour change are reported.

The role of symmetry breaking in the structural trapping of light-induced excited spin states

Six isostructural compounds show differing agreement with the correlation T(LIESST) = T₀ − 0.3T_1/2 (red line; LIESST = Light-Induced Excited Spin-State Trapping). This reflects crystallographic symmetry breaking under LIESST conditions, exhibited by one of the compounds but not by two others.

Synthesis and crystal structure of a cyanido-bridged copper(II)–silver(I) bimetallic complex containing a trimeric {[Ag(CN)2]−}3 anion, [Cu(Dach)2-Ag(CN)2-Cu(Dach)2][Ag(CN)2]3 (Dach=cis-1,2-diaminocyclohexane)

A new cyanido-bridged bimetallic copper(II)–silver(I) complex, [Cu(Dach)2-Ag(CN)2-Cu(Dach)2][Ag(CN)2]3 (1) (Dach=cis-1,2-diaminocyclohexane), has been prepared and characterized by IR spectroscopy and single crystal X-ray analysis. The crystal structure of complex 1 consists of a centrosymmetric trinuclear cation and three non-coordinated [Ag(CN)2]− ions assembled in the form of a trimer, {[Ag(CN)2]−}3. The complex ion is made up of two [Cu(Dach)2]2+ units connected by a bridging [Ag(CN)2]− anion. The copper atom in the complex ion assumes a distorted square pyramidal geometry with CN− nitrogen atoms occupying the apical position. The trinuclear units are joined by hydrogen bonding to form irregular chains.

Two-step spin crossover behaviour in the chiral one-dimensional coordination polymer [Fe(HAT)(NCS)2]∞

Two-step spin crossover behaviour with large stabilisation of the intermediate state in a chiral one-dimensional coordination polymer.

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.

Temperature- and pressure-dependent structural study of {Fe(pmd)2[Ag(CN)2]2}n spin-crossover compound by neutron Laue diffraction

The effect of pressure (up to 0.17 GPa) on the spin-crossover compound {Fe(pmd)2[Ag(CN)2]2}n [orthorhombic isomer (II), pmd = pyrimidine] has been investigated by temperature- and pressure-dependent neutron Laue diffraction and magnetometry. The cooperative high-spin ↔ low-spin transition, centred at ca 180 K at ambient pressure, is shifted to higher temperatures as pressure is applied, showing a moderate sensitivity of the compound to pressure, since the spin transition is displaced by ca 140 K GPa(-1). The space-group symmetry (orthorhombic Pccn) remains unchanged over the pressure-temperature (P-T) range studied. The main structural consequence of the high-spin to low-spin transition is the contraction of the distorted octahedral [FeN6] chromophores, being more marked in the axial positions (occupied by the pmd units), than in the equatorial positions (occupied by four [Ag(CN)2](-) bridging ligands).