José Sánchez Costa

Iron Spin-Crossover compounds: from fundamental studies to practical applications

Over the past five years, the spin-crossover (SCO) phenomenon has experienced a clear new lease of interest from the scientific community coinciding with the recent publication of remarkable new advances. This perspective paper describes five illustrative examples of SCO systems, published during the past twelve months, showing new aspects of the unique and very appealing behaviour of these molecular switches, which may find interesting applications in the near future.

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands.

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

Coupled Crystallographic Order-Disorder and Spin State in a Bistable Molecule: Multiple Transition Dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H(4)L, has led to an Fe(II) spin-crossover (SCO) complex, [Fe(H(4)L)(2)][ClO(4)](2)⋅H(2)O⋅2 (CH(3))(2)CO (1), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T(0.5) of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high-spin state may also be thermally trapped, exhibiting a very large T(TIESST) (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO(4)(-) and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe-N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order-disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising-like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.

Two-Step Spin-Transition Iron(III) Compound with a Wide [High Spin-Low Spin] Plateau

A new iron(III) coordination compound exhibiting a two-step spin-transition behavior with a remarkably wide [HS-LS] plateau of about 45 K has been synthesized from a hydrazino Schiff-base ligand with an N,N,O donor set, namely 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol (Hmph). The single-crystal X-ray structure of the coordination compound {[Fe(mph)(2)](ClO(4))(MeOH)(0.5)(H(2)O)(0.5)}(2) (1) determined at 150 K reveals the presence of two slightly different iron(III) centers in pseudo-octahedral environments generated by two deprotonated tridentate mph ligands. The presence of hydrogen bonding interactions, instigated by the well-designed ligand, may justify the occurrence of the abrupt transitions. 1 has been characterized by temperature-dependent magnetic susceptibility measurements, EPR spectroscopy, differential scanning calorimetry, and (57)Fe Mossbauer spectroscopy, which all confirm the occurrence of a two-step transition. In addition, the iron(III) species in the high-spin state has been trapped and characterized by rapid cooling EPR studies.

Crystallographic Evidence for Reversible Symmetry Breaking in a Spin‐Crossover d7 Cobalt(II) Coordination Polymer

This work was supported by the DST and CSIR, India. J.S.C. thanks the Spanish MICINN through CTQ2009-06959 for the research fellowship “Juan de la Cierva” (18-08-463B-750). J.R. also thanks the Spanish MICINN through CTQ2009-07264.

Magnetic Coupling between Copper(II) Ions Mediated by Hydrogen-Bonded (Neutral) Water Molecules

A new hydrogen-bonded dinuclear copper(II) coordination compound has been synthesized from the Schiff-base ligand 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp). The molecular structure of [Cu(2)(php)(2)(H(2)O)(2)(ClO(4))](ClO(4))(H(2)O) (1), determined by single-crystal X-ray diffraction, reveals the presence of two copper(II) centers held together by means of two strong hydrogen bonds, with O...O contacts of only 2.60-2.68 A. Temperature-dependent magnetic susceptibility measurements down to 3 K show that the two metal ions are antiferromagnetically coupled (J = -19.8(2) cm(-1)). This exchange is most likely through two hydrogen-bonding pathways, where a coordinated water on the first Cu, donates a H bond to the O atoms of the coordinated php at the other Cu. This strong O...H(water) bonding interaction has been clearly evidenced by theoretical calculations. In the relatively few related cases from the literature, this exchange path, mediated by a (neutral) coordinated water molecule, was not recognized.

Coexistence of Intramolecular Ligand-Mediated and Through Hydrogen-Bond Magnetic Interactions in a Chain of Dicopper(II) Units

The reaction of 2,8-dimethyl-5,11-bis(pyridin-2-ylmethyl)-1,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-diazecine (dimp) with copper(II) nitrate in water produces the compound [Cu(2)(dimp)(H(2)O)(2)(NO(3))(2)](NO(3))(2). The single-crystal X-ray structure shows the formation of hydrogen-bonded chains in the lattice that are formed by dicopper(II) units doubly connected by nitrate/water bridges. Within the one-dimensional chains, the Cu ions are separated by either intramolecular or intermolecular distances of 7.309(2) Å or 6.255(2) Å, respectively. The magnetic susceptibility data revealing weak antiferromagnetic exchange interactions between the copper(II) ions were interpreted by considering two possible models, namely, an isolated dinuclear and a 1-D chain picture. The latter leads to an alternation J(1) = -11.6 and J(2) = -3.0 cm(-1) along the chain. In order to clarify the relative strengths of the exchange couplings through hydrogen bonds and via the bridging dimp ligand, solution EPR studies and quantum chemical calculations were carried out. EPR studies unambiguously conclude on the existence of an exchange interaction J(a) mediated by the dinucleating dimp ligand, while the through-H coupling J(b) is physically absent in solution. On the basis of dinuclear units extracted from the X-ray data, J(a) was estimated around -5.0 cm(-1) from DFT-based calculations (M06 functional), whereas J(b) is negligible. In contrast, wave function configuration interaction calculations (DDCI) support a description where both inter- and intramolecular pathways coexist with a preeminent role of H bonds with J(a) = -2.8 and J(b) = -10.4 cm(-1). Not only are these values very consistent with the extracted set of parameters (J(1), J(2) = -11.6, -3.0 cm(-1)) but the possibility to generate leading exchange coupling through weak bonds is evidenced by means of wave function-based calculations.

Spin crossover composite materials for electrothermomechanical actuators

Composites of the spin crossover complex [Fe(trz)(H-trz)2](BF4) (H-trz = 1,2,4-4H-triazole and trz = 1,2,4-triazolato) dispersed in a poly(methylmethacrylate) (PMMA) matrix were synthesized and investigated for their spin crossover properties by optical reflectivity, Raman spectroscopy and calorimetry. These composite films were used to fabricate bilayer cantilevers that can perform efficient and tuneable mechanical actuation based on the spin transition. A prototype device that uses the spin transition phenomenon to convert electrical energy into mechanical motion through Joule heating is described. This device is used to perform oscillatory actuation driven by a modulated current. The ability to tune the performance of this electromechanical system is demonstrated by varying the working temperature, the applied ac current and its frequency.

Local Coordination Geometry and Spin State in Novel FeII Complexes with 2,6-Bis(pyrazol-3-yl)pyridine-Type Ligands as Controlled by Packing Forces: Structural Correlations

A substituted 2,6-bis(pyrazol-3-yl)pyridine (3-bpp) ligand, H(4)L, created to facilitate intermolecular interactions in the solid, has been used to obtain four novel Fe(II) complexes: [Fe(H(4)L)(2)](ClO(4))(2)⋅2 CH(3)NO(2)⋅2 H(2)O, [Fe(H(4)L)(H(2)LBF(2))](BF(4))⋅5 C(3)H(6)O (H(2)LBF(2) is an in situ modified version of H(4)L), [Fe(H(4)L)(2)](ClO(4))(2)⋅2 C(3)H(7)OH and [Fe(H(4)L)(2)](ClO(4))(2)⋅4 C(2)H(5)OH. Changing of spin-inactive components (solvents, anions or distant ligand substituents) causes differences to the coordination geometry of the metal that are key to the magnetic properties. Magnetic measurements show that, contrary to the previously published complex [Fe(H(4)L)(2)](ClO(4))(2)⋅H(2)O⋅2 CH(3)COCH(3), the newly synthesised compounds remain in the high-spin (HS) state at all temperatures (5-300 K). A member of the known family of Fe(II)/3-bpp complexes, [Fe(3-bpp)(2)](ClO(4))(2)⋅1.75 CH(3)COCH(3)⋅1.5 Et(2)O, has also been prepared and characterised structurally. In the bulk, this compound exhibits a gradual and incomplete spin transition near 205 K. The single-crystal structure is consistent with it being HS at 250 K and partially low spin at 90 K. Structural analysis of all these compounds reveals that the exact configuration of intermolecular interactions affects dramatically the local geometry at the metal, which ultimately has a strong influence on the magnetic properties. Along this line, the geometry of Fe(II) in all published 3-bpp compounds of known structure has been examined, both by calculating various distortion indices (Σ, Θ, θ and Φ) and by continuous shape measures (CShMs). The results reveal correlations between some of these parameters and indicate that the distortions from octahedral geometry observed on HS systems are mainly due to strains arising from intermolecular interactions. As previously suggested with other related compounds, we observe here that strongly HS-distorted systems have a larger tendency to remain in that state.

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.