Spin-crossover behavior in cyanide-bridged iron(II)-gold(I) bimetallic 2D Hofmann-like metal-organic frameworks

The synthesis and characterization of new two-dimensional (2D) cyanide-bridged iron(II)-gold(I) bimetallic coordination polymers formulated, {Fe(3-Xpy)2[Au(CN)2]2} (py = pyridine; X = F (1), Cl (2), Br (3), and I (4)) and the clathrate derivative {Fe(3-Ipy)2[Au(CN)2]2}.1/2(3-Ipy) (5), are reported. The iron(II) ion lies in pseudoctahedral [FeN6] sites defined by four [Au(CN)2](-) bridging ligands and two 3-Xpy ligands occupying the equatorial and axial positions, respectively. Although only compounds 2 and 4 can be considered strictly isostructurals, all of the components of this family are made up of parallel stacks of corrugated {Fe[Au(CN)2]2}n grids. The grids are formed by edge sharing of {Fe4[Au(CN)2]4} pseudosquare moieties. The stacks are constituted of double layers sustained by short aurophilic contacts ranging from 3.016(2) to 3.1580(8) A. The Au...Au distances between consecutive double layers are in the range of 5.9562(9)-8.790(2) A. Compound 5, considered a clathrate derivative of 4, includes one-half of a 3-Ipy molecule per iron(II) atom between the double layers. Compound 1 undergoes a half-spin transition with critical temperatures Tc downward arrow = 140 K and Tc upward arrow = 145 K. The corresponding thermodynamic parameters derived from differential scanning calorimetry (DSC) are Delta H = 9.8 +/- 0.4 kJ mol(-1) and Delta S = 68.2 +/- 3 J K mol(-1). This spin transition is accompanied by a crystallographic phase transition from the monoclinic P2(1)/c space group to the triclinic P1 space group. At high temperatures, where 1 is 100% high-spin, there is only one crystallographically independent iron(II) site. In contrast, the low temperature structural analysis shows the occurrence of two crystallographically independent iron(II) sites with equal population, one high-spin and the other low-spin. Furthermore, 1 undergoes a complete two-step spin transition at pressures as high as 0.26 GPa. Compounds 2- 4 are high-spin iron(II) complexes according to their magnetic and [FeN6] structural characteristics. Compound 5, characterized for having two different iron(II) sites, displays a two-step spin transition with critical temperatures of Tc(1) = 155 K, Tc(2) downward arrow = 97 K, and Tc(2) upward arrow = 110 K. This change of spin state takes place in both sites simultaneously. All of these results are compared and discussed in the context of other {Fe(L) x [M(I)(CN)2]} coordination polymers, particularly those belonging to the homologous compounds {Fe(3-Xpy)2[Ag(CN)2]2} and their corresponding clathrate derivatives.     

Spin-crossover and liquid crystal properties in 2D cyanide-bridged Fe(II)-M(I/II) metalorganic frameworks

Novel two-dimensional heterometallic Fe(II)-M(Ni(II), Pd(II), Pt(II), Ag(I), and Au(I)) cyanide-bridged metalorganic frameworks exhibiting spin-crossover and liquid crystal properties, formulated as {FeL(2)[M(I/II)(CN)(x)](y)}·sH(2)O, where L are the ligands 4-(4-alkoxyphenyl)pyridine, 4-(3,4-dialkoxyphenyl)pyridine, and 4-(3,4,5-trisalkoxyphenyl)pyridine, have been synthesized and characterized. The physical characterization has been carried out by means of EXAFS, X-ray powder diffraction, magnetic susceptibility, differential scanning measurements, and Mo?ssbauer spectroscopy. The 2D Fe(II) metallomesogens undergo incomplete and continuous thermally induced spin transition at T(1/2) ≈ 170 K and crystal-to-smectic transition above 370 K.     

Spin-crossover behavior and electrical conduction property in iron(II) complexes with tetrathiafulvalene moieties

Iron(II) complexes with neutral and oxidized tetrathiafulvalene (TTF) moieties were prepared and X-ray crystallographic analyses, magnetic and electrical resistivity measurements suggested an interaction of spin transition and electrical conductivity.     

Spin-crossover in dimeric hydrogen-bonded iron(II) 2-(pyrazolyl)-pyridine and 2-(imidazolyl)-pyridine complexes

Five new hydrogen-bonded solvated iron(II) complexes of pyrazolyl- and imidazolyl-based N,N-chelating ligands have been synthesised. Water to ligand-NH hydrogen-bonded bridges occur in the pseudo-dimeric complexes {cis-[Fe(pypzH)₂(NCX)₂]₂(μ-OH₂)(H₂O)₂} · H₂O · MeOH (where X = S or Se), and in the chain complex {cis-[Fe(pypzH)₂(NCS)₂](μ-OH₂)}_n. A “half” spin-crossover (T_c = 125 K) was observed in the dimeric X = Se complex by means of magnetic measurements and no thermal hysteresis occurred between 4 and 300 K. The crystal structure at 123 K showed Fe–N distances consistent with the magnetism. Each Fe in the dimeric unit was structurally equivalent in the HS–LS state. Removal of the solvate molecules led to HS–HS behaviour over the temperature range 4–300 K. The pseudo-dimer with X = S also showed HS–HS behaviour as did the monomeric analogue cis-[Fe(pypzH)₂(NCS)₂]H₂O and a structurally different methanol-bridged dimer {cis-[Fe(pyimH)₂(NCS)₂]₂(μ-MeOH)₂} · 2MeOH (pypzH = 2-(1H-pyrazol-3-yl)-pyridine; pyimH = 2-(1H-imidazol-2-yl)-pyridine).     

Copper(II) camphorates with tunable pore size in metal-organic frameworks

Three new homochiral metal-organic coordination polymers [Cu₂camph₂dabco] · DMF•2H₂O, [Cu₂camph₂bipy] · 3DMF•2H₂O, and [Cu₂camph₂bpe] · 4DMF•2H₂O (H₂camph is (+)-camphoric acid, bipy is 4,4’-bipyridyl, bpe is trans-bis(4-pyridyl)ethylene) were synthesized by heating copper(II) nitrate, (+)-camphoric acid, and N-donor ligands of different length (dabco, bipy, bpe) in DMF and characterized by powder X-ray diffraction, IR spectroscopy, and TGA. The obtained compounds are isostructural with the previously reported porous zinc(II) camphorates.     

Polyrotaxane metal-organic frameworks (PMOFs)

Polyrotaxane metal-organic frameworks (PMOFs), a relatively rare branch of entangled networks, have received significant attention due to their unusual entanglement topologies. The PMOFs we described here are still at an early stage of development. This feature article summarizes the recent developments in structural types of PMOFs from our own group and others. We make some generalizations about the various classes of PMOFs, and develop the definitions and nomenclature of these entanglements, including classification into trivial and nontrivial polyrotaxanes, and limits on what constitutes a (nontrivial) polyrotaxane. Finally, the synthetic strategies toward the design and preparation of new PMOFs are elucidated.     

Spin-crossover phenomena in tris(trifluoronicotinoylacetonato) iron (III)

Tris(trifluoronicotinoylacetonato) iron(III) has been shown to exhibit spin-crossover phenomenon between ⁶A_1g and ²T_2g terms which are about 322 cm^?1 apart.     

2D conductive metal-organic frameworks for electronics and spintronics

Two-dimensional(2D) materials showcase great potentials in both fundamental research and technology development, thanks to their unique chemical and physical properties that are usually not available in corresponding bulk counterparts. As an emerging class of 2D materials, 2D conductive metal-organic frameworks(2D c-MOFs) exhibit the characteristics of pre-designable and tunable structures, excellent crystallinity, intrinsic porosity and superior conductivity. During the past decade, 2D c-MOFs have been rapidly developed in electronics, sensors, energy storage devices, etc. In this review, the electrical, magnetic and quantum properties of 2D c-MOFs are surveyed in detail. Their applications in semiconductor, metal, superconductor, topological insulator and porous magnet are highlighted. We envision that the combination of 2D c-MOFs with quantum materials could evoke rich physics, flexible chemistry and potential applications in both electronics and spintronics.     

Spin-crossover in an iron(III)-bispidine-alkylperoxide system

The iron(II) complex of a tetradentate bispidine ligand with two tertiary amines and two pyridine groups (L = dimethyl [3,7-dimethyl-9,9'-dihydroxy-2,4-di-(2-pyridyl)-3,7-diazabicyclo nonan-1,5-dicaboxylate]) is oxidized with tert-butyl hydroperoxide to the corresponding end-on tert-butylperoxo complex [Fe(III)(L)(OOtBu)(X)]n+ (X = solvent, anion). UV-vis, resonance Raman, and EPR spectroscopy, as a function of the solvent, show that this is a spin-crossover compound. The experimentally observed Raman vibrations for both low-spin and high-spin isomers are in good agreement with those computed by DFT.     

Spin-crossover in complexes of iron(II) carboranes with tris(pyrazol-1-yl)methane

Synthesis procedures for coordination compounds of iron(II) 1,5,6,10-tetra(R)-7,8-dicarba-nido-undecaborates(-1) (carboranes) with tris(pyrazol-1-yl)methane (HC(pz)₃) of the composition [Fe{HC(pz)₃}₂]A₂·nH₂O (A = (7,8-C₂B₉H₁₂)^? (I), (1,5,6,10-Br₄-7,8-C₂B₉H₈)^? (II), (1,5,6,10-I₄-7,8-C₂B₉H₈)^? (III), n = 0–2) are developed. The compounds are studied by static magnetic susceptibility in the temperature range of 160–500 K, electron (diffuse reflectance spectra), IR, and Mössbauer spectroscopy methods. It is shown that the complexes have high-temperature spin-crossover ¹ A ₁ ? ⁵ T ₂. Transition temperatures (T _c) for I–III are 370 K, 380 K, and 400 K respectively. Spin-crossover is accompanied by thermochromism (color change: pink ? white).     

Enhancing photocatalytic performance of metal-organic frameworks for CO2 reduction by a bimetallic strategy

Metal-organic frameworks（MOFs） as a type of crystalline heterogeneous catalysts have shown potential application in photocatalytic CO₂ reduction.However,MOF catalysts with high efficiency and selectivity are still in pursuit.Herein,by a bimetallic strategy,the catalytic performance of a Co-MOF for photocatalytic CO₂ reduction was enhanced.Specifically,the Co-MOF based on 4,5-dicarboxylic acid（H₃ IDC） and4,4’-bipydine（4,4’-bpy） can catalyze CO₂ reductio...     

Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni)

The synthesis and characterization of a series of three‐dimensional (3D) Hofmann‐like clathrate porous metal–organic framework (MOF) materials [Fe(bpac)M(CN)₄] (M=Pt, Pd, and Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin‐crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin‐crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)₄]}_n planar layers bridged by the bis‐monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7 % of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin‐crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π–π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host–guest interaction and the spin‐crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.     

Twelve-connected porous metal-organic frameworks with high H(2) adsorption

The twelve-connected metal-organic frameworks {[Ni(3)(OH)(L)(3)].n(solv)}(infinity) and {[Fe(3)(O)(L)(3)].n(solv)}(infinity) [LH(2) = pyridine-3,5-bis(phenyl-4-carboxylic acid)] have been prepared and characterised: these materials can be desolvated to form porous materials that show adsorption of H(2) up to 4.15 wt% at 77 K.     

Two new 3D metal-organic frameworks of nanoscale cages constructed by Cd(II) and conformationally-flexible cyclohexanehexacarboxylate

Hydrothermal reactions of Cd(NO3)2.4H2O with a,e,a,e,a,e-1,2,3,4,5,6-cyclohexanehexacarboxylic acid (H6L(I)) generate two new 3D MOFs with nanoscale cages, in which the L(I) ligand in the first MOF transforms its conformation to the e,e,e,e,e,e form while the L(I) ligand in the second MOF transforms its conformation to mixed e,e,e,e,e,e and e,e,e,e,a,a forms in a 1 : 3 ratio, showing the effect of the auxiliary sodium ion on stabilizing the intermediate conformations and on the construction of the heterometallic MOF structure.     

Synthesis, crystal structures and magnetic properties of single and double cyanide-bridged bimetallic Fe2(III)Cu(II) zigzag chains

The bimetallic complexes [[Fe(III)(phen)(CN)4]2Cu(II)(H2O)2].4H2O (1), [[Fe(III)(phen)(CN)4]2Cu(II)].H2O (2) and [[Fe(III)(bipy)(CN)4]2Cu(II)].2H2O (3) and [[Fe(III)(bipy)(CN)4]2Cu(II)(H2O)2].4H2O (4) (phen = 1,10-phenanthroline and bipy = 2,2'-bipyridine) have been prepared and the structures of 1-3 determined by X-ray diffraction. The structure of 1 is made up of neutral cyanide-bridged Fe(III)-Cu(II) zigzag chains of formula [[Fe(III)(phen)(CN)4]2Cu(II)(H2O)2] and uncoordinated water molecules with the [Fe(phen)(CN)4]- entity acting as a bis-monodentate bridging ligand toward two trans-diaquacopper(II) units through two of its four cyanide groups in cis positions. The structure of 2 can be viewed as the condensation of two chains of 1 connected through single cyanide-bridged Fe(III)-Cu(II) pairs after removal of the two axially coordinated water molecules of the copper atom. The structure of 3 is like that of 2, the main differences being the occurrence of bipy (phen in 2) and two (one in 2) crystallization water molecules. The crystals of 4 diffract poorly but the analysis of the limited set of diffraction data shows a chain structure like that of 1 the most important difference being the fact that elongation axis at the copper atom is defined by the two trans coordinated water molecules. 1 behaves as a ferromagnetic Fe(III)2Cu(II) trinuclear system. A metamagnetic-like behavior is observed for 2 and 3, the value of the critical field (Hc) being ca. 1100 (2) and 900 Oe (3). For H > Hc the ferromagnetic Fe(III)2Cu(II) chains exhibit frequency dependence of the out-of-phase ac susceptibility signal at T < 4.0 K. The magnetic behavior of 4 corresponds to that of a ferromagnetically coupled chain of low spin iron(III) and copper(II) ions with frequency dependence of the out-of-phase susceptibility at T < 3.0 K. Theoretical calculations using methods based on density functional theory (DFT) have been employed to analyze and substantiate the exchange pathways in this family of complexes.     

Spin crossover in tetranuclear cyanide-bridged iron(II) square complexes: a theoretical study

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.     

Extremely long axial Cu-N bonds in chiral one-dimensional zigzag cyanide-bridged Cu(II)(-)Ni(II) and Cu(II)(-)Pt(II) bimetallic assemblies

Preparations, crystal structures, and spectral and magnetic properties of two new chiral one-dimensional cyano-bridged coordination polymers, [Cu(II)L2]M(II)(CN)].2H2O (M(II) = Ni(II) (1) and Pt(II) (2), L = trans-cyclohexane-(1R,2R)-diamine) have been presented. Complex 1 crystallizes in the monoclinic P2(1) space group with a = 9.864(4) A, b = 15.393(8) A, c = 7.995(4) A, beta = 110.32(3) degrees , V = 1138.4(10) A3, and Z = 2, while 2 crystallizes in the monoclinic P2(1) space group with a = 9.899(3) A, b = 15.541(4) A, c = 8.102(2) A, beta = 111.02(2) degrees , V = 1163.6(5) A3, and Z = 2. The unique zigzag cyano-bridged chains along the crystallographic b axis consist of alternate chiral [CuL(2)]2+ cations and square-planar [M(CN)4]2- anions. One side of the axial Cu-N(triple bond C) bond distances are 2.324(6) and 2.34(1) A with Cu-N[triple bond]C angles of 137.8(6) degrees and 138.2(9) degrees for 1 and 2, respectively. On the other hand, the opposite side of the axial Cu-N(triple bond C) bond distances are 3.120(8) and 3.09(1) A with significantly large bent Cu-N[triple bond]C angles of 97.9(5) degrees and 96.8(7) degrees for 1 and 2, respectively. The novel axial bonding features of extremely long semi-coordination Cu-N bonds are attributed to coexistence of pseudo-Jahn-Teller elongation and electrostatic interaction in the unique zigzag cyano-bridged chains. The characteristic bonding features with overlap between small 3d (Ni(II)) or large 5d (Pt(II)) and 3d (Cu(II)) orbitals results in larger shifts in XPS peaks of not only Cu2p(1/2) and Cu2p(3/2) but also Ni2p(1/2) and Ni2p(3/2) for 1 than those of 2, which is also consistent with weak antiferromagnetic interactions with Weiss constants of -5.31 and -5.94 K for 1 and 2, respectively. The d-d, pi-pi*, and CT bands in the electronic, CD, and MCD spectra for 1 and 2 in the solid state at room temperature are discussed from the viewpoint of magneto-optical properties.     

Spin-crossover cobalt(II) compound with banana-shaped structure

A banana-shaped spin-crossover (SCO) cobalt(II) complex [Co(C16-terpy)2](BPh4)2 (1) with long alkyl chains, based on a terpyridine frame, was synthesized. Compound 1 exhibited very gradual SCO behavior and changes in the dielectric constant. This shows a way in which SCO materials can be used in electronic devices.     

Spin-crossover in coordination compounds of iron(II) with tris(pyrazol-1-yl)methane and cluster anions

Synthesis procedures for new coordination compounds of iron(II) with tris(pyrazol-1-yl)methane (HC(pz)₃), containing cluster anions in the outer sphere, of the composition [Fe{HC(pz)₃}₂][Mo₆Cl₁₄]?2H₂O (I), [Fe{HC(pz)₃}₂][Mo₆Br₁₄]?H₂O (II), and [Fe{HC(pz)₃}₂]₂[Re₆S₈(CN)₆]?2H₂O (III) are developed. The compounds are studied by static magnetic susceptibility, electronic, IR, and Mössbauer spectroscopic methods. The magnetochemical study shows that in the polycrystalline phases of all compounds the spincrossover ¹ А ₁ ? ⁵ Т ₂ is observed which is accompanied by thermochromism.