A Reversible Hydrogen-Bond Isomerization Triggered by an Abrupt Spin Crossover near Room Temperature.

The spin crossover salt [Fe(bpp)2 ](isonicNO)2 ⋅ 2.4 H2 O (1⋅2.4 H2 O) (bpp=2,6-bis(pyrazol-3-yl)pyridine; isonicNO=isonicotinate N-oxide anion) exhibits a very abrupt spin crossover at T1/2 =274.4 K. This triggers a supramolecular linkage (H-bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behavior reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1, a material that also exhibits spin crossover coupled to H-bond isomerization, leading to strong kinetic effects in the thermomagnetic properties.

A Ferroelectric Iron(II) Spin Crossover Material.

A dual-function material in which ferroelectricity and spin crossover coexist in the same temperature range has been obtained. Our synthetic strategy allows the construction of acentric crystal structures in a predictable way and is based on the high directionality of hydrogen bonds. The well-known iron(II) spin crossover complex [Fe(bpp)2 ]2+ (bpp=2,6-bis(pyrazol-3-yl)pyridine), a four-fold noncentrosymmetric H-bond donor, was combined with a disymmetric H-bond acceptor such as the isonicotinate (isonic) anion to afford [Fe(bpp)2 ](isonic)2 ⋅2 H2 O. This low-spin iron(II) compound crystallizes in the acentric nonpolar I4‾ space group and shows piezoelectricity and SHG properties. Upon dehydration, it undergoes a single-crystal to single-crystal structural rearrangement to a monoclinic polar Pc phase that is ferroelectric and exhibits spin crossover.

Cobalt Clusters with Cubane-Type Topologies Based on Trivacant Polyoxometalate Ligands.

Four novel cobalt-substituted polyoxometalates having cobalt cores exhibiting cubane or dicubane topologies have been synthesized and characterized by IR, elemental analysis, electrochemistry, UV-vis spectroscopy, X-ray single-crystal analysis, and magnetic studies. The tetracobalt(II)-substituted polyoxometalate [Co4(OH)3(H2O)6(PW9O34)](4-) (1) consists of a trilacunary [B-α-PW9O34](9-) unit which accommodates a cubane-like {Co(II)4O4} core. In the heptacobalt(II,III)-containing polyoxometalates [Co7(OH)6(H2O)6(PW9O34)2](9-) (2), [Co7(OH)6(H2O)4(PW9O34)2]n(9n-) (3), and [Co7(OH)6(H2O)6(P2W15O56)2](15-) (4), dicubane-like {Co(II)6Co(III)O8} cores are encapsulated between two heptadentate [B-α-PW9O34](9-) (in 2 and 3) or [α-P2W15O56](15-) (in 4) ligands. While 1, 2, and 4 are discrete polyoxometalates, 3 exhibits a polymeric, chain-like structure that results from the condensation of polyoxoanions of type 2. The magnetic properties of these complexes have been fitted according to an anisotropic exchange model in the low-temperature regime and discussed on the basis of ferromagnetic interactions between Co(2+) ions with angles Co-L-Co (L = O, OH) close to orthogonality and weakly antiferromagnetic interactions between Co(2+) ions connected through central diamagnetic Co(3+) ion. Moreover, we will show the interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates).

Construction of a general library for the rational design of nanomagnets and spin qubits based on mononuclear f-block complexes. The polyoxometalate case.

This paper belongs to a series of contributions aiming at establishing a general library that helps in the description of the crystal field (CF) effect of any ligand on the splitting of the J ground states of mononuclear f-element complexes. Here, the effective parameters associated with the oxo ligands (effective charges and metal-ligand distances) are extracted from the study of the magnetic properties of the first two families of single-ion magnets based on lanthanoid polyoxometalates (POMs), formulated as [Ln(W5O18)2](9-) and [Ln(ß2-SiW11O39)2](13-) (Ln = Tb, Dy, Ho, Er, Tm, Yb). This effective CF approach provides a good description of the lowest-lying magnetic levels and the associated wave functions of the studied systems, which is fully consistent with the observed magnetic behavior. In order to demonstrate the predictive character of this model, we have extended our model in a first step to calculate the properties of the POM complexes of the early 4f-block metals. In doing so, [Nd(W5O18)2](9-) has been identified as a suitable candidate to exhibit SMM behavior. Magnetic experiments have confirmed such a prediction, demonstrating the usefulness of this strategy for the directed synthesis of new nanomagnets. Thus, with an effective barrier of 51.4 cm(-1) under an applied dc field of 1000 Oe, this is the second example of a Nd(3+)-based single-ion magnet.

Hydrogen-bonded assemblies of iron(II) spin crossover complexes.

The paper reports on the synthesis, crystal structure, thermal and magnetic properties of spin crossover (SCO) salts containing the [Fe(bpp)2]2+ cation (bpp = 2,6-bis(pyrazol-3-yl)pyridine) and different rigid polycarboxylate anions, such as anthracene-9,10-dicarboxylate (ADC), benzene-1,3,5-tricarboxylate (BTC) and biphenyl-4,4'-dicarboxylate (BPDC). Compound [Fe(bpp)2](ADC)·9H2O (1) shows a porous hydrogen-bonded structure with water molecules sitting in the channels. It contains low-spin (LS) Fe2+ cations that undergo crossover to the high-spin (HS) state upon dehydration. Anhydrous 1 remains HS on cooling at low temperatures. A similar magnetic behaviour is obtained for the partially protonated BTC salt [Fe(bpp)2](HBTC)·5H2O (2), showing a spin change concomitant with dehydration to a HS phase that undergoes gradual and partial SCO on cooling, affecting 25% of the Fe2+ cations. Instead, the BPDC salt [Fe(bpp)2](BPDC)·5H2O (3) has a ground HS state in its fully hydrated form.

The series of molecular conductors and superconductors ET4[AFe(C2O4)3]·PhX (ET = bis(ethylenedithio)tetrathiafulvalene; (C2O4)2- = oxalate; A+ = H3O+, K+; X = F, Cl, Br, and I): influence of the halobenzene guest molecules on the crystal structure and superconducting properties.

An extensive series of radical salts formed by the organic donor bis(ethylenedithio)tetrathiafulvalene (ET), the paramagnetic tris(oxalato)ferrate(III) anion [Fe(C(2)O(4))(3)](3-), and halobenzene guest molecules has been synthesized and characterized. The change of the halogen atom in this series has allowed the study of the effect of the size and charge polarization on the crystal structures and physical properties while keeping the geometry of the guest molecule. The general formula of the salts is ET(4)[A(I)Fe(C(2)O(4))(3)]·G with A/G = H(3)O(+)/PhF (1); H(3)O(+)/PhCl (2); H(3)O(+)/PhBr (3), and K(+)/PhI (4), (crystal data at room temperature: (1) monoclinic, space group C2/c with a = 10.3123(2) Å, b = 20.0205(3) Å, c = 35.2732(4) Å, ß = 92.511(2)°, V = 7275.4(2) Å(3), Z = 4; (2) monoclinic, space group C2/c with a = 10.2899(4) Å, b = 20.026(10) Å, c = 35.411(10) Å, ß = 92.974°, V = 7287(4) Å(3), Z = 4; (3) monoclinic, space group C2/c with a = 10.2875(3) Å, b = 20.0546(15) Å, c = 35.513(2) Å, ß = 93.238(5)°, V = 7315.0(7) Å(3), Z = 4; (4) monoclinic, space group C2/c with a = 10.2260(2) Å, b = 19.9234(2) Å, c = 35.9064(6) Å, ß = 93.3664(6)°, V = 7302.83(18) Å(3), Z = 4). The crystal structures at 120 K evidence that compounds 1-3 undergo a structural transition to a lower symmetry phase when the temperature is lowered (crystal data at 120 K: (1) triclinic, space group P1 with a = 10.2595(3) Å, b = 11.1403(3) Å, c = 34.9516(9) Å, α = 89.149(2)°, ß = 86.762(2)°, γ = 62.578(3)°, V = 3539.96(19) Å(3), Z = 2; (2) triclinic, space group P1 with a = 10.25276(14) Å, b = 11.15081(13) Å, c = 35.1363(5) Å, α = 89.0829(10)°, ß = 86.5203(11)°, γ = 62.6678(13)°, V = 3561.65(8) Å(3), Z = 2; (3) triclinic, space group P1 with a = 10.25554(17) Å, b = 11.16966(18) Å, c = 35.1997(5) Å, α = 62.7251(16)°, ß = 86.3083(12)°, γ = 62.7251(16)°, V = 3575.99(10) Å(3), Z = 2; (4) monoclinic, space group C2/c with a = 10.1637(3) Å, b = 19.7251(6) Å, c = 35.6405(11) Å, ß = 93.895(3)°, V = 7128.7(4) Å(3), Z = 4). A detailed crystallographic study shows a change in the symmetry of the crystal for compound 3 at about 200 K. This structural transition arises from the partial ordering of some ethylene groups in the ET molecules and involves a slight movement of the halobenzene guest molecules (which occupy hexagonal cavities in the anionic layers) toward one of the adjacent organic layers, giving rise to two nonequivalent organic layers at 120 K (compared to only one at room temperature). The structural transition at about 200 K is also observed in the electrical properties of 1-3 and in the magnetic properties of 1. The direct current (dc) conductivity shows metallic behavior in salts 1-3 with superconducting transitions at about 4.0 and 1.0 K in salts 3 and 1, respectively. Salt 4 shows a semiconductor behavior in the temperature range 300-50 K with an activation energy of 64 meV. The magnetic measurements confirm the presence of high spin S = 5/2 [Fe(C(2)O(4))(3)](3-) isolated monomers together with a Pauli paramagnetism, typical of metals, in compounds 1-3. The magnetic properties can be very well reproduced in the whole temperature range with a simple model of isolated S = 5/2 ions with a zero field splitting plus a temperature independent paramagnetism (Nα) with the following parameters: g = 1.965, |D| = 0.31 cm(-1), and Nα = 1.5 × 10(-3) emu mol(-1) for 1, g = 2.024, |D| = 0.65 cm(-1), and Nα = 1.4 × 10(-3) emu mol(-1) for 2, and g = 2.001, |D| = 0.52 cm(-1), and Nα = 1.5 × 10(-3) emu mol(-1) for 3.

Synthesis and physical properties of K4[Fe(C5O5)2(H2O)2](HC5O5)2·4H2O (C5O5(2-) = croconate): a rare example of ferromagnetic coupling via H-bonds.

The reaction of the croconate dianion (C(5)O(5))(2-) with a Fe(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe(II)-croconate complex without additional coligands, K(4)[Fe(C(5)O(5))(2)(H(2)O)(2)](HC(5)O(5))(2)·4H(2)O (1). 1 crystallizes in the monoclinic P2(1)/c space group and presents discrete octahedral Fe(II) complexes coordinated by two chelating C(5)O(5)(2-) anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of trans-diaquabis(croconato)ferrate(II) complexes and layers containing the monoprotonated croconate anions, HC(5)O(5)(-), and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong-moderate being the O···O bond distance (2.771(2) Å) typical of moderate H-bonds and the O-H···O bond angle (174(3)°) typical of strong ones. This H-bond interaction leads to a quadratic regular layer where each [Fe(C(5)O(5))(2)(H(2)O)(2)](2-) anion is connected to its four neighbors in the plane through four equivalent H-bonds. From the magnetic point of view, these connections lead to an S = 2 quadratic layer. The magnetic properties of 1 have been reproduced with a 2D square lattice model for S = 2 ions with g = 2.027(2) and J = 4.59(3) cm(-1). This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (j) through a molecular field approximation with g = 2.071(7), J = 2.94(7) cm(-1), and j = -0.045(2) cm(-1) (the Hamiltonian is written as H = -JS(i)S(j)). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe(II) ions, it confirms the presence of weak ferromagnetic Fe-Fe interactions through H-bonds in compound 1 which represents one of the rare examples of ferromagnetic coupling via H-bonds.

Single-molecule magnetic behavior in a neutral terbium(III) complex of a picolinate-based nitronyl nitroxide free radical.

The terdentate anionic picolinate-based nitronyl nitroxide (picNN) free radical forms neutral and robust homoleptic complexes with rare earth-metal ions. The nonacoordinated Tb(3+) complex Tb(picNN)(3)·6H(2)O is a single-molecule magnet with an activation energy barrier Δ = 22.8 ± 0.5 K and preexponential factor τ(0) = (5.5 ± 1.1) × 10(-9) s. It shows magnetic hysteresis below 1 K.

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 (T1/2 = 180 K) and a quantitative LS to HS photomagnetic conversion at low temperatures, with a T(LIESST) value of 61 K.

Electronic and magnetic study of polycationic Mn(12) single-molecule magnets with a ground spin state S = 11.

The preparation, magnetic characterization, and X-ray structures of two polycationic Mn(12) single-molecule magnets [Mn(12)O(12)(bet)(16)(EtOH)(4)](PF(6))(14).4CH(3)CN.H(2)O (1) and [Mn(12)O(12)(bet)(16)(EtOH)(3)(H(2)O)](PF(6))(13)(OH).6CH(3)CN.EtOH.H(2)O (2) (bet = betaine = (CH(3))(3)N(+)-CH(2)-CO(2)(-)) are reported. 1 crystallizes in the centrosymmetric P2/c space group and presents a (0:2:0:2) arrangement of the EtOH molecules in its structure. 2 crystallizes in the noncentrosymmetric P4 space group with two distinct Mn(12) polycations, [Mn(12)O(12)(bet)(16)(EtOH)(2)(H(2)O)(2)](14+) (2A) and [Mn(12)O(12)(bet)(16)(EtOH)(4)](14+) (2B) per unit cell. 2A and 2B show a (1:1:1:1) distribution of the coordinated solvent molecules. Interestingly, bond valence sum calculations extracted from X-ray diffraction data indicate the presence of two Mn(2+) ions in the Mn(12) core for both 1 and 2. This finding is confirmed by X-ray absorption spectroscopy (XAS) measurements. A complete magnetic characterization, including subkelvin micro-SQUID magnetometry and inelastic neutron scattering (INS) measurements, permits to extract the parameters of the giant spin Hamiltonian of these polycations. Compared with the archetypal Mn(12) acetate, an increase in the value of the ground spin state from S = 10 to S = 11 together with a decrease in the effective energy barrier, is observed for 1 and 2. Such a result is consistent with the reduction of two Mn(3+) to the less anisotropic Mn(2+) ion in the structures.

Interplay between spin crossover and proton migration along short strong hydrogen bonds.

The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T 1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T 2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T 1(LIESST) = 68 K and T 2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.

Metal complexes of a picolinate-based nitronyl nitroxide free radical.

A nitronyl nitroxide free radical containing an appended picolinate moiety was synthesized. The resulting tridentate ligand picNN forms neutral mononuclear metal complexes of formula [M(picNN)(2)].3H(2)O (M = Mn, Co, Ni, Zn). These compounds are isostructural and crystallize in the orthorhombic Pnna space group. The metal complexes have a C(2) symmetric structure, with the metal centers lying on the binary axis and surrounded by two equivalent picNN radicals. The magnetic properties of this family of compounds indicate the presence of very strong metal-radical exchange interactions, ranging from J(Ni-rad) = -193 cm(-1) to J(Mn-rad) = -98 cm(-1). Relatively weak (J(rad-rad) = -15 cm(-1)) through-space magnetic interactions between free radicals coordinated to the same metal center were also evidenced in the study of the diamagnetic zinc(II) complex. Complexation with Cu(2+) affords the carboxylate-bridged tetranuclear copper(II) complex [Cu(4)(picNN)(4)(H(2)O)(4)](ClO(4))(4).4H(2)O having eight interacting S = 1/2 spins in a cyclic topology. The antiferromagnetic copper-radical exchange interaction (J(Cu-rad) = -268 cm(-1)) is one of the largest ever reported.

Magnetostructural correlations in Cu(II)-NC-W(V) linkage: the case of [Cu(II)(diimine)]2+-[W(V)(CN)8]3- 0D assemblies.

We report on the syntheses, crystal structures, and magnetic properties of two cyano-bridged molecular assemblies: [Cu(II)(phen)(3)](2){[Cu(II)(phen)(2)](2)[W(V)(CN)(8)](2)}(ClO(4))(2) x 10 H(2)O (phen = 1,10-phenanthroline) (1) and {[Cu(II)(bpy)(2)](2)[W(V)(CN)(8)]} {[Cu(II)(bpy)(2)][W(V)(CN)(8)]} x 4 H(2)O (bpy = 2,2'-bipyridyl) (2). Compound 1 consists of cyano-bridged [Cu(II)(2)W(V)(2)](2-) molecular rectangles and isolated [Cu(II)(phen)(3)](2+) complexes. The molecular structure of 2 reveals cyano-bridged trinuclear [Cu(II)(2)W(V)](+) and dinuclear [Cu(II)W(V)](-) ions. Magnetic interactions in 1 are interpreted in terms of the model of a tetranuclear moiety consisting of two ferromagnetic Cu(II)-NC-W(V) units (J(1) = +39(4) cm(-1)) interacting antiferromagnetically (J(2) = -1.6(4) cm(-1)). The analysis of the magnetic response of 2 reveals ferromagnetic interaction within the [Cu(II)-NC-W(V)-CN-Cu(II)](+) and [Cu(II)-NC-W(V)](-) isolated units, characterized by J = +35(7) cm(-1) and antiferromagnetic coupling between them (4J' = -0.30(8) cm(-1)). The discussion of the magnetic behavior and the correlation of the J(CuW) parameters with the geometry of cyano bridges at copper(II) centers of 1 and 2 is based on the DFT calculations, which yield J(av) = +13.2 cm(-1) for 1 and J = +31 cm(-1) for 2. The domination of the ferromagnetic Cu(II)-NC-W(V) interaction in 1 and 2 originates from the mutual orthogonality of natural magnetic orbitals in the case of coordination of cyano bridges at the equatorial sites of Cu(II) moieties.

Molecular conductors based on the mixed-valence polyoxometalates [SMo12O40]n- (n = 3 and 4) and the organic donors bis(ethylenedithio)tetrathiafulvalene and bis(ethylenedithio)tetraselenafulvalene.

The synthesis, crystal structure, and physical characterization of two new radical salts formed by the organic donors bis(ethylenedithio)tetrathiafulvalene (ET) and bis(ethylenediseleno)tetrathiafulvalene (BETS) and the Keggin polyoxometalate (POM) [SMo(12)O(40)](n-) are reported. The salts isolated are ET(8)[SMo(12)O(40)] x 10 H(2)O (1) (crystal data: (1) monoclinic, space group I2/m with a = 13.9300(10) A, b = 43.467(3) A, c = 13.9929(13) A, beta = 107.979(6) degrees, V = 8058.9(11) A(3), Z = 2) and BETS(8)[SMo(12)O(40)] x 10 H(2)O (2) (crystal data: monoclinic, space group I2/m with a = 14.0878(2) A, b = 44.1010(6) A, c = 14.0930(2) A, beta = 106.739(3) degrees, V = 8384.8 A(3), Z = 2). Both compounds are isostructural and consist of alternating layers of the organic donors (with an alpha or theta(42+40) packing mode) and POM anions. The structural data, as well as the magnetic susceptibility, ESR measurements, and band structure calculations, indicate that the Keggin POMs have been reduced by one electron in 1 and by two electrons in 2, leading to the POMs [SMo(12)O(40)](3-) and [SMo(12)O(40)](4-) in 1 and 2, respectively. At ambient pressure 1 is a classical semiconductor with a room temperature conductivity of about 1 S cm(-1) and an activation energy of about 130 meV. Salt 2 also exhibits an activated behavior although it does not follow a classical semiconducting regime. The conductivity of 2 under applied pressure shows an enhanced conductivity, in agreement with the analysis of the electronic structure, although no metallic behavior is detected below about 10 kbar.

Unravelling the spin-state of solvated [Fe(bpp)2]2+ spin-crossover complexes: structure-function relationship.

This paper reports firstly the syntheses, crystal structures, and thermal and magnetic properties of spin crossover salts of formulae [Fe(bpp)2]3[Cr(CN)6]2·13H2O (1) and [Fe(bpp)2][N(CN)2]2·H2O (2) (bpp = 2,6-bis(pyrazol-3-yl)pyridine) exhibiting hydrogen-bonded networks of low-spin [Fe(bpp)2]2+ complexes and [Cr(CN)6]3- or [N(CN)2]- anions, with solvent molecules located in the voids. Desolvation of 1 is accompanied by a complete low-spin (LS) to a high-spin (HS) transformation that becomes reversible after rehydration by exposing the sample to the humidity of air. The influence of the lattice water on the magnetic properties of spin-crossover [Fe(bpp)2]X2 complex salts has been documented. In most cases, it stabilises the LS state over the HS one. In other cases, it is rather the contrary. The second part of this paper is devoted to unravelling the reasons why the lattice solvent stabilises one form over the other through magneto-structural correlations of [Fe(bpp)2]2+ salts bearing anions with different charge/size ratios (Xn-). The [Fe(bpp)2]2+ stacking explaining these two different behaviours is correlated here with the composition of the second coordination sphere of the Fe centers and the ability of these anions to form hydrogen bonds and/or π-π stacking interactions between them or the bpp ligand.

Large Magnetic Polyoxometalates Containing the Cobalt Cubane '[CoIIICo 3II (OH)3(H2O)6-m(PW9O34)]3-' (m = 3 or 5) as a Subunit.

A synthetic procedure is presented to construct new magnetic polyoxometalates (POMs) containing one or two subunits of '[CoIIICo 3II (OH)3(H2O)6-m(PW9O34)]3-' (m = 3 or 5). The substitution of the water ligands present in these subunits by oxo or hydroxo ligands belonging to other POM fragments, gives rise to four, larger POM anions: [Co7(OH)6(H2O)6(PW9O34)2]9- (2), [Co7(OH)6(H2O)4(PW9O34)2] n9n- (2'), [Co11(OH)5(H2O)5(W6O24)(PW9O34)3]22- (3) and [{Co4(OH)3(H2O)(PW9O34)}2{K⊂(H2W12O41)2}{Co(H2O)4}2]17- (4). The crystal structures, magnetic characterization and stabilities in aqueous solutions of these POM derivatives are also presented.

Light-induced decarboxylation in a photo-responsive iron-containing complex based on polyoxometalate and oxalato ligands.

A new photoresponsive molecular polyanion in which two Fe(iii) ions are simultaneously coordinated by two [A-α-PW9O34]9- polyoxometalate units and two oxalato ligands has been obtained. When irradiated with UV light its potassium salt, 1, exhibits a remarkable photocoloration effect, attributable to the partial reduction of the POM units to give rise to a mixed-valence species. The photoinduced process is intramolecular and involves electron transfer from the oxalato ligands, which partially decompose releasing CO2, towards the Fe(iii) and the POM. This mechanism has been confirmed by DRS, IR, XPS and Mössbauer spectroscopy, magnetism and elemental analysis. An analogous derivative of 1 containing malonato ligands does not exhibit such photoactive behaviour, which is evidence that the oxalate ligand is essential for the photoactivity of 1. To our knowledge, 1 represents the first POM-based compound in which the photocoloration effect does not require the presence of intermolecular short interactions.

A chirality-induced alpha phase and a novel molecular magnetic metal in the BEDT-TTF/tris(croconate)ferrate(III) hybrid molecular system.

The novel paramagnetic and chiral anion [Fe(C5O5)3]3- has been combined with the organic donor BEDT-TTF (= ET = bis(ethylenedithio)tetrathiafulvalene) to yield the first chirality-induced alpha phase and a paramagnetic metal.

A decacobalt(ii) cluster with triple-sandwich structure obtained by partial reductive hydrolysis of a pentacobalt(ii/iii) Weakley-type polyoxometalate.

Partial reductive hydrolysis of a penta-CoII/III cluster [Co(H2O)2(CoIIIW9O34)(PW9O34)]12- (1) leads to the formation of [Co2{Co3(H2O)(Co(OH)2W7O26)(PW9O34)}2]22- (2). This polyoxometalate is made up of two capping [PW9O34]9- units and two bridging [W7O26]10- units that assemble to encapsulate a novel deca-CoII cluster core comprising octahedral and tetrahedral CoII ions.

Hydrogen-bonded networks of [Fe(bpp)2]2+ spin crossover complexes and dicarboxylate anions: structural and photomagnetic properties.

The paper reports the syntheses, crystal structures, thermal and (photo)magnetic properties of spin crossover salts of formula [Fe(bpp)2](C6H8O4)·4H2O (1·4H2O), [Fe(bpp)2](C8H4O4)·2CH3OH·H2O (2·2MeOH·H2O) and [Fe(bpp)2](C8H4O4)·5H2O (2·5H2O) (bpp = 2,6-bis(pyrazol-3yl)pyridine; C6H8O4 = adipate dianion; C8H4O4 = terephthalate dianion). The salts exhibit an intricate network of hydrogen bonds between low-spin iron(ii) complexes and carboxylate dianions, with solvent molecules sitting in the voids. Desolvation is accompanied by a low-spin (LS) to high-spin (HS) transformation in the materials. The dehydrated phase 2 undergoes a two-step transition with a second step showing thermal hysteresis (T1/2↑ = 139 K and T1/2↓ = 118 K). 2 displays a quantitative LS to HS photomagnetic conversion, with a T(LIESST) value of 63 K.