A novel oxalate-based three-dimensional coordination polymer showing magnetic ordering and high proton conductivity.

A novel three-dimensional (3D) coordination polymer with the formula (C3N2H5)4[MnCr2(ox)6]·5H2O (2), where ox = oxalate and C3N2H5 = imidazolium cation, is reported. Single crystal X-ray diffraction reveals that this porous coordination polymer adopts a chiral three-dimensional quartz-like architecture, with the guest imidazolium cations and water molecules being hosted in its pores. This novel multifunctional material exhibits both a ferromagnetic ordering at TC = 3.0 K, related to the host MnCr2 network, and high proton conductivity [1.86 × 10-3 S cm-1 at 295 K and 88% relative humidity (RH)] due to the presence of the acidic imidazolium cations and free water molecules. The similarity of the structure of compound 2 to that of the previously reported analogous compound (NH4)4[MnCr2(ox)6]·4H2O, (1), also allows us to analyse, to a certain extent, the effect of the acidity of the proton donating guest molecules on proton conduction properties. 2 hosts, in one-dimensional (1D) channels, imidazolium cations, which are more acidic than the ammonium ones in 1 and, as a consequence, 2 shows higher proton conduction than 1, highlighting the effect of the pKa of the proton donating guest molecules on proton conductivity.

Postsynthetic Approach for the Rational Design of Chiral Ferroelectric Metal-Organic Frameworks.

Ferroelectrics (FEs) are materials of paramount importance with a wide diversity of applications. Herein, we propose a postsynthetic methodology for the smart implementation of ferroelectricity in chiral metal-organic frameworks (MOFs): following a single-crystal to single-crystal cation metathesis, the Ca2+ counterions of a preformed chiral MOF of formula Ca6II{CuII24[(S,S)-hismox]12(OH2)3}·212H2O (1), where hismox is a chiral ligand derived from the natural amino acid l-histidine, are replaced by CH3NH3+. The resulting compound, (CH3NH3)12{CuII24[(S,S)-hismox]12(OH2)3}·178H2O (2), retains the polar space group of 1 and is ferroelectric below 260 K. These results open a new synthetic avenue to enlarge the limited number of FE MOFs.

Solvent-Dependent Self-Assembly of an Oxalato-Based Three-Dimensional Magnet Exhibiting a Novel Architecture.

The old but evergreen family of bimetallic oxalates still offers innovative and interesting results. When (Me4N)3[Cr(ox)3]·3H2O is reacted with Mn(II) ions in a nonaqueous solvent, a novel three-dimensional magnet of the formula [N(CH3)4]6[Mn3Cr4(ox)12]·6CH3OH is obtained instead of the one-dimensional compound obtained in water. This new material exhibits an unprecedented stoichiometry with a binodal (3,4) net topology and the highest critical temperature (TC = 7 K) observed so far in a manganese-chromium oxalate based magnet.

A tricky water molecule coordinated to a verdazyl radical-iron(II) complex: a multitechnique approach.

The first iron complexes of high-spin iron(II) species directly coordinated to verdazyl radicals, [Fe(II)(vdCOO)2(H2O)2]·2H2O (1; vdCOO(-) = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate) and [Fe(II)(vdCOO)2(D2O)2]·2D2O (2), were synthesized. The crystal structure of 1 was investigated by single-crystal X-ray diffraction at room temperature and at 90 K. The compound crystallizes in the P1 space group with no phase transition between 300 and 90 K. The crystals are composed of discrete [Fe(II)(vdCOO)2(H2O)2] complexes and crystallization water molecules. In the complex, two vdCOO(-) ligands coordinate to the iron(II) ion in a head-to-tail arrangement and two water molecules complete the coordination sphere. The Fe-X (X = O, N) distances vary in the 2.069-2.213 Å range at 300 K and in the 2.0679-2.2111 Å range at 90 K, indicating that the iron(II) ion is in its high-spin (HS) state at both temperatures. At 300 K, one of the coordinated water molecules is H-bonded to two crystallization water molecules whereas the second one appears as loosely H-bonded to the two oxygen atoms of the carboxylate group of two neighboring complexes. At 90 K, the former H-bonds remain essentially the same whereas the second coordinated water molecule reveals a complicated behavior appearing simultaneously as tightly H-bonded to two oxygen atoms and non-H-bonded. The (57)Fe Mössbauer spectra, recorded between 300 K and 10 K, give a clue to this situation. They show two sets of doublets typical of HS iron(II) species whose intensity ratio varies smoothly with temperature. It demonstrates the existence of an equilibrium between the high temperature and low temperature forms of the compounds. The solid-state magic angle spinning (2)H NMR spectra of 2 were recorded between 310 K and 193 K. The spectra suggest the existence of a strongly temperature-dependent motion of one of the coordinated water molecules in the whole temperature range. Variable-temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction (J(Fe-vd) = -27.1 cm(-1); H = -J(ij)S(i)S(j)) of the HS iron(II) ion and the radical spins with high g(Fe) and D(Fe) values (g(Fe) = 2.25, D(Fe) = +3.37 cm(-1)) for the HS iron(II) ion. Moreover, the radicals are strongly antiferromagnetically coupled through the iron(II) center (J(vd-vd) = -42.8 cm(-1)). These last results are analysed based on the framework of the magnetic orbitals formalism.

Solid-state magnetic switching triggered by proton-coupled electron-transfer assisted by long-distance proton-alkali cation transport.

Acidity of water molecules coordinated to Co ions in CoFe Prussian blue analogues (PBA) has been used to reversibly activate the Co(III)Fe(II) ↔ Co(II)Fe(III) electron transfer. The study of the structure and the electronic structure shows that the process implies an original PCET reaction between a solid-state porous coordination polymer and hydroxide ions in solution. The PCET reaction spreads throughout the solid network thanks to a long-range H(+) and Rb(+) transport within the pore channels of PBA taking advantage of the hydrogen-bonding network of zeolitic water molecules acting as proton wires.

Topological versatility of oxalate-based bimetallic one-dimensional (1D) compounds associated with ammonium cations.

A new family of oxalate-bridged chains of formula (C(1))[Mn(H(2)O)(3)Cr(ox)(3)]·H(2)O (1), (C(2))(4)[Mn(2)(H(2)O)(3)ClCr(2)(ox)(6)]Cl·H(2)O·2C(2)H(6)O (2a), (C(2))(4)[Co(2)(H(2)O)(3)ClCr(2)(ox)(6)]Cl·2H(2)O·2C(2)H(6)O (2b), [Mn(C(3))(H(2)O)(2)Cr(ox)(3)]·H(2)O (3), and (C(4))(4)[Mn(H(2)O){Cr(ox)(3)}(2)]·H(2)O (4) [C(1)(+) = tetramethylammonium, C(2)(+) = 4-N,N-dimethylaminopyridinium, C(3)(+) = 1-hydroxyethyl-4-N,N-dimethylamino-pyridinium, C(4)(+) = 1-hydroxyethyl-4-(4'-dimethylamino-α-styryl)-pyridinium, ox(2-) = oxalate] have been synthesized by self-assembly of the (C(n))(3)[Cr(ox)(3)] (n = 1-4) mononuclear compound and the chloride salts of the corresponding metal(II) ions. The crystal structures of the five chain compounds have been determined by single-crystal X-ray diffraction. Compounds 1 and 2 crystallize in the Pc and P2(1)/c centrosymmetrical space groups, respectively, whereas 3 and 4 crystallize in the C2cb and P1 noncentrosymmetrical space groups, respectively. Compounds 1, 2, and 3 adopt a zigzag chain structure while 4 exhibits a comb-like chain structure consisting of the repetition of the [Mn(H(2)O){Cr(µ-ox)(ox)(2)}{Cr(µ-ox)(2)(ox)}](4-) entities. Compound 3 displays large second-order optical nonlinearity. The magnetic properties of 1-4 have been investigated in the temperature range 2-300 K. Monte Carlo simulations on 1, 2a, 2b, and 3 provide a quantitative description of the magnetic properties indicating ferromagnetic interactions through the bis(bidentate) oxalate bridges [J = +0.55 cm(-1) (1), J = +1.02 cm(-1) (2a), J = +3.83 cm(-1) (2b), and J = +0.75 cm(-1) (3) using Hamiltonian H = -J(S(i)·S(j))]. On the other side, the fit of the magnetic susceptibility data of 4 by full-matrix diagonalization agrees with a ferromagnetic exchange interaction within the [Mn(H(2)O){Cr(µ-ox)(ox)(2)}{Cr(µ-ox)(2)(ox)}](4-) trinuclear units (J = +2.07 cm(-1)) antiferromagnetically coupled along the chain. Compound 2b exhibits a metamagnetic behavior, the value of the critical field being H(C) = 1000 G, due to the occurrence of weak interchain antiferromagnetic interactions.

Synthesis, crystal structure and magnetism of new salicylamidoxime-based hexanuclear manganese(III) single-molecule magnets.

Salicylamidoxime was used to synthesize 13 new polynuclear Mn(III) complexes. We present the crystallographic structures, the magnetic susceptibility and the magnetization measurements of eight of them (1-8) with the general formula [Mn(6)O(2)(H(2)N-sao)(6)(L)(2)(solvent)(4-6)] (L = carboxylate, chloride, 2-cyanophenolate; solvent = H(2)O, MeOH, EtOH, py). These complexes consist of two trinuclear {Mn(III)(3)(µ(3)-O)(H(2)N-sao)(3)}(+) cationic units linked together via two oximate and two phenolate oxygen atoms. All behave as single-molecule magnets, with the spin ground state varying from 4 to 12 and anisotropy energy barriers from 24 to 86 K, the latter being as high as the present record barrier in the Mn(6) complexes. DFT calculations were performed to compute the exchange magnetic coupling constants J between the metallic ions and to provide an orbital interpretation of exchange. Our results are in line with previously reported results with the parent salicylaldoxime derivatives. The Mn-N-O-Mn torsion angle appears as the main parameter controlling the J values. The critical angle where the exchange coupling between two Mn(III) switches from antiferromagnetic to ferromagnetic is 27°, less than the one found in related complexes with salicylaldoxime (30°). We propose a structural classification of the {Mn(6)} complexes in four classes depending on the coordination of the axial carboxylate. The work points out the structural flexibility of such systems, their sensitivity to solvent effects and their ability to achieve high anisotropy energy barriers by simple desolvation.

High proton conduction in a chiral ferromagnetic metal-organic quartz-like framework.

A complex-as-ligand strategy to get a multifunctional molecular material led to a metal-organic framework with the formula (NH(4))(4)[MnCr(2)(ox)(6)]·4H(2)O. Single-crystal X-ray diffraction revealed that the anionic bimetallic coordination network adopts a chiral three-dimensional quartz-like architecture. It hosts ammonium cations and water molecules in functionalized channels. In addition to ferromagnetic ordering below T(C) = 3.0 K related to the host network, the material exhibits a very high proton conductivity of 1.1 × 10(-3) S cm(-1) at room temperature due to the guest molecules.

Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged Fe(III)2M(II)2 (M=Mn, Ni, and Co) square complexes.

New heterobimetallic tetranuclear complexes of formula [Fe(III){B(pz)(4)}(CN)(2)(µ-CN)Mn(II)(bpy)(2)](2)(ClO(4))(2)·CH(3)CN (1), [Fe(III){HB(pz)(3)}(CN)(2)(µ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2a), [Fe(III){B(pz)(4)}(CN)(2)(µ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2b), [Fe(III){HB(pz)(3)}(CN)(2)(µ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3a), and [Fe(III){B(pz)(4)}(CN)(2)(µ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3b), [HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(Pz)(4)(-) = tetrakis(1-pyrazolyl)borate, dmphen = 2,9-dimethyl-1,10-phenanthroline, bpy = 2,2'-bipyridine] have been synthesized and structurally and magnetically characterized. Complexes 1-3b have been prepared by following a rational route based on the self-assembly of the tricyanometalate precursor fac-[Fe(III)(L)(CN)(3)](-) (L = tridentate anionic ligand) and cationic preformed complexes [M(II)(L')(2)(H(2)O)(2)](2+) (L' = bidentate α-diimine type ligand), this last species having four blocked coordination sites and two labile ones located in cis positions. The structures of 1-3b consist of cationic tetranuclear Fe(III)(2)M(II)(2) square complexes [M = Mn (1), Ni (2a and 2b), Co (3a and 3b)] where corners are defined by the metal ions and the edges by the Fe-CN-M units. The charge is balanced by free perchlorate anions. The [Fe(L)(CN)(3)](-) complex in 1-3b acts as a ligand through two cyanide groups toward two divalent metal complexes. The magnetic properties of 1-3b have been investigated in the temperature range 2-300 K. A moderately strong antiferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Mn(II) (S = 5/2) ions has been found for 1 leading to an S = 4 ground state (J(1) = -6.2 and J(2) = -2.7 cm(-1)), whereas a moderately strong ferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Ni(II) (S = 1) and Co(II) (S = 3/2) ions has been found for complexes 2a-3b with S = 3 (2a and 2b) and S = 4 (3a and 3b) ground spin states [J(1) = +21.4 cm(-1) and J(2) = +19.4 cm(-1) (2a); J(1) = +17.0 cm(-1) and J(2) = +12.5 cm(-1) (2b); J(1) = +5.4 cm(-1) and J(2) = +11.1 cm(-1) (3a); J(1) = +8.1 cm(-1) and J(2) = +11.0 cm(-1) (3b)] [the exchange Hamiltonian being of the type H = -J(S(i)·S(j))]. Density functional theory (DFT) calculations have been used to substantiate the nature and magnitude of the exchange magnetic coupling observed in 1-3b and also to analyze the dependence of the exchange magnetic coupling on the structural parameters of the Fe-C-N-M skeleton.

The fruitful introduction of chirality and control of absolute configurations in molecular magnets.

In this critical review, it is shown how the introduction of chirality and the control of the absolute configurations of chiral elements in molecular magnets allow obtaining enantiopure chiral magnets (ECM), an archetype of multifunctional materials. This task has been recognised as a major challenge for both chemists and physicists of molecular magnetism. To reach this goal, the former have combined the rational approaches towards molecular-based magnets and of enantiopure metal-organic frameworks. They have used enantiopure stable radicals, ligands from the chiral pool, enantiopure coligands associated with achiral connectors or enantioselective self-assembly to successfully reach their synthetic targets. They were motivated by the will to obtain suitable systems for the experimental demonstration of the influence of enantiomeric purity on the physico-chemical properties. This influence can be found in the magnetic properties themselves but, most interestingly, in the coexistence and interaction between the properties arising from controlled non-centrosymmetry. Thus the combination of natural circular dichroism, second harmonic generation or ferroelectricity with long-range magnetic ordering can give birth to new properties like magneto-chiral dichroism, magnetisation induced second harmonic generation or multiferroicity. The two former synergetic effects have already been demonstrated in enantiopure chiral magnets. The third one remains a challenging target that can be reached by adapting strategies developed towards enantiopure molecular ferroelectrics (119 references).

W-knotted chain {[Cu(II)(dien)]4[W(V)(CN)8]}(5+)∞: synthesis, crystal structure, magnetism, and theory.

The self-assembly of [Cu(II)(dien)(H(2)O)(2)](2+) and [W(V)(CN)(8)](3-) in aqueous solution leads to the formation (H(3)O){[Cu(II)(dien)](4)[W(V)(CN)(8)]}[W(V)(CN)(8)](2)·6.5H(2)O (1). The crystal structure of 1 consists of an unprecedented {[Cu(II)(dien)](4)[W(V)(CN)(8)]}(5+)(∞) chain of (2,8) topology, nonbridging [W(CN)(8)](3-) anions, and crystallization water molecules. The analysis of magnetic behavior of 1 was performed by the density functional theory (DFT) method and magnetic susceptibility measurements. The DFT broken symmetry approach gave two J(CuW) coupling constants: J(ax) = +2.9 cm(-1) assigned to long and strongly bent W-CN-Cu linkage, and the J(eq) = +1.5 cm(-1) assigned to short and less bent W-CN-Cu linkage, located at the axial and the equatorial positions of square pyramidal Cu(II) centers, respectively, in the hexanuclear {W(2)Cu(4)} chain subunit. The dominance of weak-to-moderate ferromagnetic coupling within the chain was confirmed by magnetic calculations. Zero-field susceptibility of the full chain segment {WCu(4)}(n) was calculated by a semiclassical analytical approach assuming that only one W(V) out of five ½ spins of the chain unit WCu(4) is treated as a classical commuting variable. The calculation of the field dependence of the magnetization was performed separately by replacing the same spin with the Ising variable and applying the standard transfer matrix technique. The intermolecular coupling between the chain segments and off-chain [W(CN)(8)](3-) entities was resolved using the mean-field approximation set to be of antiferromagnetic character. The magnetic coupling parameters are compared with those of other low dimensional {Cu(II)-[M(V)(CN)(8)]} systems.

A new family of oxime-based hexanuclear manganese(III) single molecule magnets with high anisotropy energy barriers.

A magneto-structural study of two salicylamidoxime-based {Mn(6)} single-molecule magnets revealed that their anisotropy energy barriers, which can reach the current record for d-transition metal complexes, are strongly dependent upon the precise arrangement of ligands and the solvation state.

Synthesis, crystal structure and magnetic properties of two oxalato-bridged dimetallic trinuclear complexes combined with a polar cation.

Two isostructural heterometallic trinuclear oxalato-bridged complexes of formula C(4)[MCr(2)(ox)(6)(H(2)O)(2)]·nH(2)O (C(+) = 4-aminopyridinium; ox(2-) = oxalate dianion; M(2+) = Mn(2+), n = 3, 1; M(2+) = Co(2+), n = 3.25, 2) have been synthesized by using direct self-assembly methods combining C(3)[Cr(ox)(3)] and the chloride salts of the corresponding metal ion. The crystal structures of both compounds have been resolved by single-crystal X-ray diffraction. They crystallize in the C2/c space group [a = 11.5113(15) Å, b = 20.250(3) Å, c = 21.810(4) Å, beta = 100.447(10) degrees, V = 5161.6(3) Å(3), and Z = 4 for 1, and a = 11.4334(16) Å, b = 20.243(2) Å, c = 21.805(3) Å, beta = 101.113(9) degrees, V = 4951.9(11) Å(3), and Z = 4 for 2]. The structures of 1 and 2 consist of discrete linear [MCr(2)(ox)(6)](4-) bimetallic trinuclear units, pyridinium cations and crystallization water molecules. The linear trinuclear unit is built from a central trans-diaquametal(II), linked to two Cr(ox)(3)](3-) entities by oxalate bridges. One of the oxalate ions is coordinated to the central metal ion whereas the other two oxalate ligands are non-bridging. In the crystal, intermolecular hydrogen bonds involving oxalate ligands, water molecules and pyridinium cations, build a complex three-dimensional network. Variable-temperature magnetic susceptibility measurements for 1 and 2 indicate a weak ferromagnetic interaction (J = +1.16 and +2.62/+2.70 cm(-1) for 1 and 2, respectively) between the two terminal Cr(III) (S(Cr) = 3/2) and the central high-spin Mn(II) (S(Mn) = 5/2) and Co(II) (S(Co) = 3/2) ions. The nature and the amplitude of the exchange interaction are rationalized using DFT calculations and orbital interpretations.

Photomagnetism in clusters and extended molecule-based magnets.

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Strong magneto-chiral dichroism in enantiopure chiral ferromagnets.

As materials science is moving towards the synthesis, the study and the processing of new materials exhibiting well-defined and complex functions, the synthesis of new multifunctional materials is one of the important challenges. One of these complex physical properties is magneto-chiral dichroism which arises, at second order, from the coexistence of spatial asymmetry and magnetization in a material. Herein we report the first measurement of strong magneto-chiral dichroism in an enantiopure chiral ferromagnet. The ab initio synthesis of the enantiopure chiral ferromagnet is based on an enantioselective self-assembly, where a resolved chiral quaternary ammonium cation imposes the absolute configurations of the metal centres within chromium-manganese two-dimensional oxalate layers. The ferromagnetic interaction between Cr(III) and Mn(II) ions leads to a Curie temperature of 7 K. The magneto-chiral dichroic effect is enhanced by a factor of 17 when entering into the ferromagnetic phase.

Six-fold oxygen-coordinated triplet (S = 1) palladium(II) moieties templated by tris(bipyridine)ruthenium(II) ions.

Tris(bipyridine)ruthenium(II) is used as a templating agent to insert palladium(II) into three-dimensional oxalate-based networks. The templated-assembly of [Ru(bpy)(3)][Pd(2)(ox)(3)] (Pd(2)) and [Ru(bpy)(3)][PdMn(ox)(3)] (PdMn) is described. The latter compound is structurally characterized by powder X-ray diffraction and X-ray absorption spectroscopy. These techniques reveal an unusual 6-fold oxygen environment around the Pd(II) atoms with two short (2.02 Angstrom) and four long (2.17 Angstrom) Pd-O distances. As stated by magnetometry, this environment is associated with a triplet ground state (S = 1) of the palladium(II) ion: when the temperature is decreased, the chiMT product shows a monotonous decrease from 5.54 cm(3) K mol(-1) at 300 K, a value which is slightly lower than the one expected for independent paramagnetic Pd(II) (S = 1, g = 2) and Mn(II) (S = 5/2, g = 2) ions. This thermal variation is due to antiferromagnetic exchange interactions between the two spin bearers. Nevertheless, no long-range magnetic order is detected down to 2 K. These results are confirmed by an analysis of the [MII(C(2)O(4))(3)](4-) (M = Ni, Pd, Pt) complex and of a [Pd(II){mu-(C(2)O(4))Mn(II)(OH(2))(4)}(3)](2+) tetranuclear model using density functional theory.

Coordination chemistry of the hexavacant tungstophosphate [H2P(2W12O48]12-: synthesis and characterization of iron(III) complexes derived from the unprecedented {P2W14O54} fragment.

Clusters which display the rare cubic Fe8 topology have been obtained by reaction of the metastable hexavacant polyoxotungstate [H2P2W12O48]12- with basic trinuclear metal acetates.