Nicolás Moliner

Critical temperature of the LIESST effect in iron(II) spin crossover compounds

Abstract The light-induced crossover in a series of iron(II) compounds has been investigated by irradiating the sample at 10 K with a Kr + laser coupled to an optical fiber within a SQUID cavity. The temperature dependence of the molar fraction of the light-induced metastable HS state has been recorded for 22 compounds. The critical LIESST temperature, T c (LIESST), has been defined as the temperature for which the light-induced HS information was erased in the SQUID cavity. The dependence of T c (LIESST) as a function of the thermal spin crossover temperature, T 1/2 , has been discussed. The effect of cooperation has been analyzed and the influence of horizontal and vertical displacements of the LS and HS potential wells has been discussed on the basis of the non-adiabatic multiphonon theory.

SPIN-CROSSOVER IN THE FE(ABPT)2(NCX)2 (X = S, SE) SYSTEM : STRUCTURAL, MAGNETIC, CALORIMETRIC AND PHOTOMAGNETIC STUDIES

The compounds [Fe(abpt) 2 (NCS) 2 ] ( 1 ) and [Fe(abpt) 2 (NCSe) 2 ] ( 2 ) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P 2 1 / n with Z =2, a =8.538(8), b =10.246(8), c =16.45(2) A, β =93.98(9)° for 1 and a =8.623(2), b =10.243(3), c =16.585(3) A, β =93.19(2)° for 2 . In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1 ) and NCSe − ( 2 ) groups in the trans -position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2 , respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H =5.8±0.5 ( 1 ) and 8.6±0.8 kJ mol −1 ( 2 ); Δ S =32.5±3 ( 1 ) and 38±4 J mol −1 K −1 ( 2 ). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2 , respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1 . The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.

Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe(abpt)2(NCX)2] (X=S, and Se): Synthesis,Structure and Magnetic Properties

The monomeric compounds [Fe(abpt 2(NCX)2(X = S (1), Se (2) and abpt = 4-amino- 3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P21/n space group with a = 11.637(2) A, b = 9.8021(14) A, c = 12.9838(12) A, β = 101.126(14)°, and Z=2 for 1, and a= 11.601(2) A, b = 9.6666(14) A, c = 12.883(2) A, β = 101.449(10)°, and Z = 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)2(NCX)21 units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(II) ion belong to the pyridine-N(1) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4kbar, where an incomplete thermal spin crossover appears at around T 1/2 = 65 K.. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T 1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively.

Direct monitoring of spin state in dinuclear iron(II) coordination compounds

So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high-spin (HS) and low-spin (LS) species have been deduced from magnetic susceptibility and zero-field Mossbauer spectroscopy data irrespective of whether they belong to LS–LS, LS–HS and HS–HS pairs. However, the distinction of pairs becomes possible if Mossbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.

Spin crossover in six-coordinate [Fe(L)2(NCX)2] compounds with L = DPQ = 2,3-bis-(2′-pyridyl)-quinoxaline, ABPT = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole and X = S, Se: synthesis, magnetic properties and single crystal studies

Abstract The iron(II) compounds of formulae [Fe(DPQ)2(NCS)2]·CO(CH)3)2(DPQ = 2,3-bis-(2′-pyridyl)-quinoxaline) (1) and [Fe(ABPT)2-(NCX)2] (ABPT = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) X = S (2) and Se (3) were synthesized and the crystal structure of 1 determined by X-ray diffraction methods. It crystallizes in the monoclinic system P2 1 /n, a = 12.057(2), b = 22.474(5), c = 15.004(7) A , β = 109.76(3)°, V = 3826 A 3 , Z = 4 and T = 293 K . The structure is made up of discrete [Fe(DPQ)2(NCS)2] units. Each metal atom is in a distorted FeN6 octahedral environment, the FeN bonds ranging from 2.013(8) A to 2.425(8) A. Variable-temperature magnetic susceptibility data in the temperature range 290–4.2 K revealed that 1 is high spin, in contrast to 2 and 3 which show a moderately cooperative high spin ↔ low spin (HS ↔ LS) transition, centred at T 1 2 = 186 K and 224 K for 2 and 3 respectively. The thermodynamic model of Slichter and Drickamer was applied to account for the magnetic data. The intermolecular interaction parameter, the enthalpy and entropy changes associated with the spin transition of 2 were estimated as Γ = 2.3 kJ mol−1, ΔH = 10.7 kJ mol−1 and ΔS = 58 J mol−1K−1 respectively.

Two-dimensional assembling of 4,4′-bipyridine and 4,4′-azopyridine bridged iron (II) linear coordination polymers via hydrogen bond

Novel two-dimensional polymers, [Fe(L 1 )(H 2 O) 2 (NCX) 2 ] · L 1 (L 1 = 4,4′-bipyridine (bipy)) ( 1, 2 ) and [Fe(L 2 )(CH 3 OH) 2 -(NCX) 2 ] · L 2 (L 2 = 4,4′-azopyridine (azpy)) ( 3 ) and X = S ( 1,3 ),Se ( 2 ), have been synthesized and characterized by X-ray crystallography. The structures reveal the formation of trans -L-bridged [Fe(NCX) 2 (Y) 2 ] where Y = H 2 O, CH 3 OH linear chains assembled into two-dimensional networks by hydrogen bonds between the uncoordinated ligand L and the coordinated solvent molecules.

Spin Crossover in Double Salts Containing Six- and Four-Coordinate Cobalt(II) Ions

The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz)2](dca)2 (1), [Co(tppz)2][Co(NCS)4]·MeOH (2), and [Co(tppz)2][Co(NCO)4]·2H2O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz)2]2+ entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz)2]2+ unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d7 electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being Cvib = 0.1367(9), λ = -168(2) cm-1, α = 1.12(1), Δ = 1626(15) cm-1, and gLS = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS)4]2- (2) and [Co(NCO)4]2- (3) with a 4A2 ground state overlaps with the spin crossover of the [Co(tppz)2]2+ entity, the abrupt decrease of the χMT product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: Cvib = 0.105(5), λ = -170(4) cm-1, α = 1.10(2), Δ = 1700(25) cm-1, gLS = 2.10(1), gHS = 2.27(1), and |D| = 3.80(2) cm-1 (2) and Cvib = 0.100(1), λ = -169(5) cm-1, α = 1.10(3), Δ = 1500(30) cm-1, gLS = 2.10(1), gHS = 2.28(1), and |D| = 4.30(2) cm-1 (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)4]2- anionic entity.

Metamagnetic Behavior in [M (tvp) (NCS)2] Coordination Polymers (M = Fe(II) and Co(II); tvp = 1,2-di-(4-pyridyl)-ethylene)

Abstract We report the synthesis and magnetic properties of two new coordination polymers of formula [M(tvp)(NCS)2] (M = Fe(II) and Co(II)). The magnetic data reveal the occurrence of metamagnetic behavior. Switching from bulk antiferromagnetic to ferromagnetic behavior occurs for magnetic fields greater than 1300 G (Fe(II)) and 450 G (Co(II)) at temperatures lower than 4 K. Both compounds are isomorphous. A molecular structure is suggested in the light of the magnetic properties and precedent data on related systems.

A bioinspired heterogeneous catalyst based on the model of the manganese-dependent dioxygenase for selective oxidation using dioxygen

A hybrid bioinspired material with manganese(II) complexes grafted on the surface of a mesostructured porous silica is investigated. The Mn sites mimic the manganese-dependent dioxygenase (MndD), which is an enzyme that catalyses the oxidation of catechol derivatives. The metal complexes were introduced in the silica using a dinuclear complex [Mn2L2(Cl)2(μ-Cl)2] as a precursor with a clickable ligand N,N′-bis[(pyridin-2-yl)methyl]prop-2-yn-1-amine (L). Azide moieties covalently grafted on MCM-41 type mesoporous silica were utilised to anchor the manganese complex through Huisgen cycloaddition using CuBr(PPh3)3 as a catalyst. A second functional group – trimethylsilyl or pyridine—was grafted on the silica to bring, together with nanopore size confinement, a similar metal environment as in MndD. The mesostructure of the materials was maintained after incorporation of the Mn complex. Catalytic oxidation of 3,5-di-t-butyl-catechol (3,5-DTBC) into quinone occurred without the need of an additional base when the metal complex was confined in the porous solid. In comparision, the oxidation of 4-t-butyl-catechol (4-TBC) that always required a basic media led to a total oxidation into the ortho-quinone, contrary to the molecular analogue.

Hexakis(dimethylformamide)iron(II) complex cation in hexahalorhenate(IV)-based salts: synthesis, X-ray structure and magnetic properties

Abstract Two iron(II)-rhenium(IV) compounds of general formula [FeII(dmf)6][ReIVX6] [X = Cl (1) and Br (2); dmf = N,N-dimethylformamide] have been prepared and characterized. X-ray powder diffraction measurements on samples of 1 and 2 support the same structure for both systems. The crystal structure of 1 was determined by single-crystal X-ray diffraction. 1 crystallizes in the triclinic system with space group Pī. Each iron(II) is six-coordinate and bonded to six oxygens from six dmf molecules building a distorted octahedral environment. Rhenium(IV) is six-coordinate by six halide anions in an almost regular octahedral geometry. The magnetic properties were investigated from variable-temperature magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2, whose experimental data were reproduced by a model of two isolated paramagnetic centers [S = 2 (FeII) and S = 3/2 (ReIV)] with large values of zero-field splitting (zfs) parameter.