Exploring the Optical and Energetic Properties of a Co(II)-Based Mixed Ligand MOF.

Due to their remarkable properties, including remarkable porosity and extensive surface area, metal-organic frameworks (MOFs) are being investigated for various applications. Herein, we report the first Co(II)-based mixed ligand MOF, formulated Co4(HTrz)2(d-cam)2.5(µ-OH)3. Its 3D structure framework is composed of helical chains {[Co4(µ3-HTrz)4]8+}n connected by d-camphorate ligand building blocks and featured as an extended structure in an AB-AB fashion. The investigated compound displays a wide absorption range across the visible spectrum, characterized by an optical gap energy of 3.7 eV, indicating its semiconducting nature and efficient sunlight absorption capabilities across various wavelengths. The electrochemical performance demonstrated an excellent reversibility, cyclability, structural stability, as well as a specific capacity of up to 100 cycles at a scan rate of 0.1 mV·s-1 and a current density of 50 mA·g-1. Thus, it showcases its ability to retain the capacity over numerous charge-discharge cycles. Additionally, the investigated sample displayed an impressive rate capability during the Li-ion charge/discharge process. Therefore, the material's remarkable electrochemical properties can be ascribed to the synergistic effects of its large specific surface area of 348.294 m2·g-1 and well-defined pore size distribution of 20.448 Å, making it a promising candidate for high-performance Li-ion batteries.

Multiscale Approach of Spin Crossover Materials: A Concept Mixing Russian Dolls and Domino Effects.

The spin crossover (SCO) phenomenon corresponds to a modification that originates at the atomic scale. However, the simple consideration of the transformations that occur following the SCO at this scale or in its close vicinity does not allow anyone to truly understand, anticipate and thus take advantage of what happens at the scale of the material, and even less at the device one. As the fruit of years of work and experience on this phenomenon, we formalize here the concept of the multiscale understanding of SCO. Clearly, the deflagration generated by the initial impressive atomic modification on all the physical scales of the solid must be understood in terms of structure-properties relationships that fit together, like Russian dolls, and propagate according to a kind of domino effect. Each scale can both give different and independent consequences from those of the other scales but at the same time can influence those of a larger or smaller scale, the whole being imperatively to take into account. The concept appears well illustrated by the volume modification, always the same at the atomic level but drastically different and adaptable, in amplitude and sense, at any other physical scale. This approach results in a much wider range of potential applications than the atomic level alone initially suggests, including one serious path to shape memory materials.

Unprecedented Reverse Volume Expansion in Spin-Transition Crystals.

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)2 (NCS)2 ] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials.

Photoinduced Mo-CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping.

The photoinduced properties of the octacoordinated complex K4 MoIV (CN)8 ⋅2 H2 O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)7 species originating from the light-induced cleavage of one Mo-CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]3 [MoIV (CN)7 ]⋅3 CH3 CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.

Variation of M···H-C Interactions in Square-Planar Complexes of Nickel(II), Palladium(II), and Platinum(II) Probed by Luminescence Spectroscopy and X-ray Diffraction at Variable Pressure.

Luminescence spectra of isoelectronic square-planar d8 complexes with 3d, 4d, and 5d metal centers show d-d luminescence with an energetic order different from that of the spectrochemical series, indicating that additional structural effects, such as different ligand-metal-ligand angles, are important factors. Variable-pressure luminescence spectra of square-planar nickel(II), palladium(II), and platinum(II) complexes with dimethyldithiocarbamate ({CH3}2DTC) ligands and their deuterated analogues show unexpected variations of the shifts of their maxima. High-resolution crystal structures and crystal structures at variable pressure for [Pt{(CH3)2DTC}2] indicate that intermolecular M···H-C interactions are at the origin of these different shifts.

Design and Study of Structural Linear and Nonlinear Optical Properties of Chiral [Fe(phen)3]2+ Complexes.

The dependence of nonlinear optical properties upon the spin state in molecular switches is still an unexplored area. Chiral [Fe( phen)3]2+ complexes are excellent candidates for those studies because they are expected to show nonlinear optical properties of interest and at the same time show photoconversion to a short-lived metastable high-Spin state by ultrafast optical pumping. Herein, we present the synthesis, crystallographic, and spectroscopic comparison of chiral [Fe( phen)3]2+ complexes obtained with chiral anions, a new lipophilic derivative of the D2-symmetric (As2(tartrate)2)2-, and D3-symmetric tris(catechol)phosphate(V) (TRISCAT), tris(catechol)arsenate(V) (TRISCAS), and 3,4,5,6-tetrachlorocatechol phosphate(V) (TRISPHAT). Complexes [Fe( phen)3]( rac-TRISCAT)2 (2) and [Fe( phen)3](X-TRISCAS)2 (X = rac (3), Δ (4), Λ (5)) were found to be isomorphous in the R32 Sohncke space group with twinning by inversion correlated with the starting chiral anion optical purity. The structures show the [Fe( phen)3]2+ complex interacting strongly along its 3-fold axis with two anions. Only the structure of a [Fe( phen)3]( rac-TRISPHAT)2 solvate (6) could be obtained, which showed no particular anion/cation interaction contrary to what was observed previously in solution. The [Fe( phen)3](X-As2(tartrate)2) (X = Δ (7), Λ (8), and racemic mixture (9)) crystallizes in enantiomorphic space groups P3121/ P3221 with the same solid-state packing. Dichroic electronic absorption studies evidenced racemization for all chiral complexes in solution due to ion pair dissociation, whereas the asymmetric induction is conserved in the solid state in KBr pellets. We evidenced on chiral complexes 4 and 5 strong nonlinear second harmonic generation, the intensity of which could be correlated with the complex electronic absorption.

Multiscale experimental and theoretical investigations of spin crossover Fe(II) complexes: examples of [Fe(phen)2(NCS)2] and [Fe(PM-BiA)2(NCS)2].

For spin crossover (SCO) complexes, computation results are reported and confirmed with experiments at multiscale levels of the isolated molecule and extended solid on the one hand and theory on the other hand. The SCO phenomenon which characterizes organometallics based on divalent iron in an octahedral FeN6-like environment with high spin (HS) and low spin (LS) states involves the LS/HS switching at the cost of small energies provided by temperature, pressure or light, the latter connected with Light-Induced Excited Spin-State Trapping (LIESST) process. Characteristic infra red (IR) and Raman vibration frequencies are computed within density functional theory (DFT) framework. In [Fe(phen)2(NCS)2] a connection of selected frequencies is established with an ultra-fast light-induced LS → HS photoswitching mechanism. In the extended solid, density of state DOS and electron localization function (ELF) are established for both LS and HS forms, leading to characterizion of the compound as an insulator in both spin states with larger gaps for LS configuration, while keeping molecular features in the solid. In [Fe(PM-BiA)2(NCS)2], by combining DFT and classical molecular dynamics, the properties and the domains of existence of the different phases are obtained by expressing the potential energy surfaces in a short range potential for Fe-N interactions. Applying such Fe-N potentials inserted in a classical force field and carrying out molecular dynamics (MD) in so-called "semi-classical MD" calculations, lead to the relative energies of HS/LS configurations of the crystal and to the assessment of the experimental (P, T) phase diagram.

A Nickel-Based Semiconductor Hybrid Material with Significant Dielectric Constant for Electronic Capacitors.

A novel semiconducting Ni(II)-based hybrid material with the formula (C7H12N2) NiCl4, which exhibits interesting optical and electrical properties, is reported. The crystal structure was investigated using SCXRD, whereas physical properties were studied by means of thermal analysis, Ft-Infrared, optical, and electrical measurements. Its crystal packing is formed through organic rings surrounded by inorganic [NiCl4]2- tetrahedral and stacked along the a-crystallographic axis. This arrangement is stabilized by a dense network of intermolecular hydrogen bonds. The investigated compound displayed a wide absorption range across the visible spectrum, characterized by an optical gap energy of 2.64 eV, indicating its semiconducting nature and efficient sunlight absorption capabilities across various wavelengths. Such features are of utmost importance in achieving a high energy conversion efficiency in solar cell applications. Further analyses of the thermal behavior using differential scanning calorimetry revealed a single-phase transition occurring at around 413 K, which was further confirmed through electrical measurements. A deep investigation of the electric and dielectric performances demonstrated a significant dielectric constant (ε' ∼ 104) at low frequencies and low dielectric loss at high frequencies. Thus, it highlights its exceptional dielectric potential, particularly in applications related to electronic capacitors.

Rational design of a photomagnetic chain: bridging single-molecule magnets with a spin-crossover complex.

The spin-crossover complex [Fe(LN5)(CN)2]·H2O (1, LN5 = 2,13-dimethyl-3,6,9-12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), reported previously by Nelson et al. in 1986, was reinvestigated, and its structure determined by single crystal X-ray diffraction for the first time. The reaction between [Mn(III)(saltmen)(H2O)](+) and this photomagnetic linker yielded the trinuclear molecular complex [{Mn(saltmen)}2FeHS(LN5)(CN)2](ClO4)2·0.5CH3OH (2) and the one-dimensional compound [{Mn(saltmen)}2FeLS(LN5)(CN)2](ClO4)2·0.5C4H10O·0.5H2O (3) depending on the addition order of the reagents (HS: High-Spin; LS: Low-Spin). Compound 3 exhibits a wave-shaped chain structure built from the assembly of the trinuclear [Mn(III)-NC-Fe(II)] motif found in 2. Static magnetic measurements revealed the existence of antiferromagnetic Mn(III)···Fe(II) (Fe(II) HS, S = 2) interactions in the trinuclear entity of 2 via the cyanido bridge leading to an ST = 2 ground state. In the case of 3, concomitant ferromagnetic and antiferromagnetic exchange interactions are found along the chain due to the presence of two crystallographically independent {Mn2(saltmen)2} units, which behave differently as shown by the magnetic susceptibility analysis, while the Fe(II) (LS, S = 0) cyanido-bridging moiety is isolating these dinuclear Mn(III) units. ac susceptibility experiments indicated slow relaxation of the magnetization arising from the ferromagnetically coupled [Mn2] units (τ0 = 1.1 × 10(-7) s and Δ(eff)/k(B) = 13.9 K). Optical reflectivity and photomagnetic properties of 1 and 3 have been investigated in detail. These studies reveal that the photomagnetic properties of 1 are kept after its coordination to the acceptor Mn(III)/saltmen complexes, allowing in 3 to switch "on" and "off" the magnetic interaction between the photoinduced Fe(II) HS unit (S = 2) and the Mn(III) ions. To the best of our knowledge, the compound 3 represents the first example of a coordination network of single-molecule magnets linked by spin-crossover units inducing thermally and photoreversible magnetic and optical properties.

Synergy between polymorphism, pressure, spin-crossover and temperature in [Fe(PM-BiA)2(NCS)2]: a neutron powder diffraction investigation.

The pressure dependencies of the lattice parameters of the spin transition compound [Fe(PM-BiA)2(NCS)2] have been derived from neutron powder diffraction measurements at low temperature. The study of the compound [Fe(PM-BiA)2(NCS)2]-pI has first confirmed the atypical spin crossover behaviour under pressure of this compound that shows a pressure induced structural transition inducing the transformation into a different polymorph, [Fe(PM-BiA)2(NCS)2]-pII. This phenomenon avoids a first-order spin transition in favour of continuous transition around 0.75 GPa at ambient temperature. Low temperature measurements under pressure up to 1.07 GPa allowed us not only to describe the spin-crossover for both polymorphs but also to reach phase-diagram regions where both polymorphs co-exist in different spin-states. Finally, the reversibility of the structural variations has been demonstrated.

Spin crossover complexes [Fe(NH2trz)3](X)2·nH2O investigated by means of polarized Raman scattering and DFT calculations.

Vibrational spectra of the spin crossover (SCO) polymers [Fe(NH2trz)3](X)2·nH2O where NH2trz = 4-NH2-1,2,4-triazole and X = Cl, Br, BF4, and NO3 have been analyzed. Our results show that the anions and water molecules have no significant influence on the vibrational properties of the Fe(NH2-trz)3 polymer chains. A detailed study of the nitrate derivative, based on the DFT analysis of the polarized spectra of single crystals, has been undertaken to propose the normal mode assignment of the Raman peaks in the low spin state of the compound. Changes in the Raman spectra in the high spin state could therefore be analyzed and interpreted by several Raman bands identified as molecular probes of the SCO phenomenon. Various factors (laser power, humidity, pressure) that influence the transition temperatures and the hysteresis loops have been identified and adjusted for obtaining reliable measurements. We demonstrate in particular that all the techniques used to probe the phase transition process give comparable results providing that the sample environment is well controlled.

Poly[diaqua-(µ(4)-carboxyl-atomethyl-phospho-nato)(µ(4)-carb-oxy-methyl-phospho-nato)penta-deca-methyl-penta-tin(IV)].

The central Sn(IV) atom of the penta-nuclear title complex, {[Sn(CH(3))(3)](3)O(2)C(CH(2))PO(3)[Sn(CH(3))(3)(H(2)O)](2)HO(2)C(CH(2))PO(3)}, is located on a twofold rotation axis; due to symmetry, the H atom of the carboxyl group of the anion is disordered with a site occupancy of 0.5. The central Sn(IV) atom is bonded to three methyl groups (one of which is disordered about the twofold rotation axis) and is symmetrically trans coordinated by two phospho-nate groups with Sn-O = 2.2665 (12) Šwhile the other SnMe(3) residues are asymmetrically trans coordinated with Sn-O = 2.1587 (12) and 2.3756 (13) Šfor one residue and Sn-O = 2.1522 (12) and 2.4335 (12) Šfor the other; the Sn-O distances involving two O atoms trans to carboxyl-ate are longer than those trans to phospho-nate groups. The Sn-C distances lie in a very narrow range [2.112 (2)-2.133 (3) Å]. The oxyanion behaves as a tetra-coordinating ligand. The bridging mode of the latter leads to the formation of layers parallel to (001) that are inter-connected by O-H⋯O and C-H⋯O hydrogen bonds.

Benzyl-triphenyl-phospho-nium dichlorido-triphenyl-stannate(IV).

The crystal structure of the title salt, (C25H22P)[Sn(C6H5)3Cl2] or (PhCH2PPh3)[SnPh3Cl2], consists of [PhCH2PPh3](+) cations and [SnPh3Cl2](-) anions in which the Sn(IV) atom is linked to two Cl atoms and three phenyl groups in a trigonal-bipyramidal geometry, with the Cl atoms in trans positions. The cation adopts a tetra-hedral geometry. In the crystal, the cations and the anions are connected by C-H⋯Cl hydrogen bonds, leading to an infinite chain propagating along the c direction.

Lattice Defects in Sub-Micrometer Spin-Crossover Crystals Studied by Electron Diffraction.

Spin-crossover particles of [Fe(Htrz)2trz](BF4) with sizes of some hundred nanometers are studied by in situ electron microscopy. Despite their high radiation sensitivity, it was possible to analyze the particles by imaging and diffraction so that a detailed analysis of crystallographic defects in individual particles became possible. The presence of one or several tilt boundaries, where the tilt axis is the direction of the polymer chains, is detected in each particle. An in situ exposure of the particles to temperature variations or short laser pulses to induce the spin crossover shows that the defect structure only changes after a high number of transformations between the low-spin and high-spin phases. The observations are explained by the anisotropy of the atomic architecture within the crystals, which facilitates defects between weakly linked crystallographic planes.

Photomagnetism of a sym-cis-dithiocyanato iron(II) complex with a tetradentate N,N'-bis(2-pyridylmethyl)1,2-ethanediamine ligand.

A comprehensive study of the magnetic and photomagnetic behaviors of cis-[Fe(picen)(NCS)(2) ] (picen = N,N'-bis(2-pyridylmethyl)1,2-ethanediamine) was carried out. The spin-equilibration was extremely slow in the vicinity of the thermal spin-transition. When the cooling speed was slower than 0.1 K min(-1), this complex was characterized by an abrupt thermal spin-transition at about 70 K. Measurement of the kinetics in the range 60-70 K was performed to approach the quasi-static hysteresis loop. At low temperatures, the metastable HS state was quenched by a rapid freezing process and the critical T(TIESST) temperature, which was associated with the thermally induced excited spin-state-trapping (TIESST) effect, was measured. At 10 K, this complex also exhibited the well-known light-induced excited spin-state-trapping (LIESST) effect and the T(LIESST) temperature was determined. The kinetics of the metastable HS states, which were generated from the freezing effect and from the light-induced excitation, was studied. Single-crystal X-ray diffraction as a function of speed-cooling and light conditions at 30 K revealed the mechanism of the spin-crossover in this complex as well as some direct relationships between its structural properties and its spin state. This spin-crossover (SCO) material represents a fascinating example in which the metastability of the HS state is in close vicinity to the thermal spin-transition region. Moreover, it is a beautiful example of a complex in which the metastable HS states can be generated, and then compared, either by the freezing effect or by the LIESST effect.

One-step vs stepwise immobilization of 1-D coordination-based Rh-Rh molecular wires on gold surfaces.

Reaction of dimeric [Rh(II)(2)(phen)(2)(µ-OAc)(2)(MeCN)(2)](BF(4))(2) (phen =1,10-phenanthroline) with pyrazine (pz) in a 1:2 ratio leads to the new 1-D metal-metal-bonded coordination oligomer {[Rh(II)(2)(phen)(2)(µ-OAc)(2)(pz)](BF(4))(2)}(n) (Rh-Rhpz)(n) (1), where each Rh atom of the dimeric unit (Rh-Rh) is coordinated in the equatorial plane to a nitrogen atom of a rigid and linear bifunctionalized organic linker (pz). Single X-ray diffraction analysis reveals the 1-D straight oligomeric chain structure (molecular wire, MW) consists of alternating (Rh-Rh) units and pz linking ligands with free BF(4)(-) as counteranions, and each metal center has a slightly distorted octahedral arrangement. The presence of accessible labile MeCN groups on both ends of these MWs ("free ends") enables functionalization of a 4-mercaptopyridine-gold coordinating platform (Au/MP) to form in one step a layer of coordination oligomer (Au/MP(Rh-Rhpz)(n); n ≈ 50). Furthermore (Rh-Rhpz)(n) (n = 1-6) MWs were grafted to Au/MP surfaces by a conventional step-by-step assembly construction involving coordination reactions between the Rh dimer ([Rh(2)(phen)(2)(µ-OAc)(2)(MeCN)(2)](BF(4))(2) (2)) and pz. A detailed physicochemical study (UV-vis, RAIR, QCM-D, ellipsometry, contact angle measurements, as well as impedance spectroscopy and cyclic voltammetry) has been made during both assembly methods to characterize the resulting surface-anchored coordination molecular wire (CMW) layers (Au/MP(Rh-Rhpz)(n)). The results indicate that the immobilized molecular assemblies (MAs) were successfully fabricated using both methods of assembly. The efficiency of the two methods is discussed.

Syntheses, structures, and magnetic properties of a novel mer-[(bbp)Fe(III)(CN)3](2-) building block (bbp: bis(2-benzimidazolyl)pyridine dianion) and its related heterobimetallic Fe(III)-Ni(II) complexes.

A new symmetrical tricyanide building block mer-[Fe(bbp)(CN)3](2-) [1; bbp = bis(2-benzimidazolyl)pyridine dianion] has been prepared and structurally and magnetically characterized. It forms a new low-spin meridionally capped {Fe(III)L(CN)3} fragment with the tridentate bbp ligand. The reaction of 1 with Ni(II) salts in the presence of various ancillary ligands affords several new cyanido-bridged complexes: a trinuclear complex {[Ni(ntb)(MeOH)]2[Fe(bbp)(CN)3][ClO4]2}·2MeOH (2), a tetranuclear compound {[Ni(tren)]2[Fe(bbp)(CN)3]2}·7MeOH (3), and a one-dimensional heterobimetallic system: {[Ni(dpd)2]2[Fe(bbp)(CN)3]2}·9MeOH·3H2O (4) [ntb = tris(2-benzimidazolylmethyl)amine, tren = tris(2-aminoethyl)amine, and dpd = 2,2-dimethyl-1,3-propanediamine]. The structural data shows that 2 is a linear complex in which a central Fe(III) ion links two adjacent Ni(II) ions via axial cyanides, while 3 is a molecular square that contains cyanido-bridged Ni(II) and Fe(III) ions at alternate corners. Complex 4 is a one-dimensional system that is composed of alternating cyanido-bridged Ni(II) and Fe(III) centers. Compounds 2-4 display extensive hydrogen bonding and moderately strong π-π stacking interactions in the solid state. Magnetic studies show that ferromagnetic exchange is operative within the Fe(III)LS(µ-CN)Ni(II) units of 2-4.

High-pressure spin-crossover in a dinuclear Fe(II) complex.

The effect of pressure on the dinuclear spin crossover material [{Fe(bpp)(NCS)(2)}(2)(4,4'-bipy)]·2MeOH (where bpp = 2,6-bis(pyrazol-3-yl)pyridine and 4,4'-bipy = 4,4'-bipyridine, 1) has been investigated with single crystal X-ray diffraction and Raman spectroscopy using diamond anvil cell techniques. The very gradual pressure-induced spin crossover occurs between 7 and 25 kbar, and shows no evidence of crystallographic phase transitions. The pressure-induced spin transition leads to a complete LS state which is not thermally accessible. This structural evolution under pressure is in stark contrast to the previously reported thermal spin crossover behaviour, in which a symmetry-breaking, purely structural phase transition results in only partial conversion to the low spin state. This observation is attributed to the symmetry-breaking phase transition becoming unfavourable under pressure.

catena-Poly[bis-(dibenzyl-ammonium) [[dichloridomercurate(II)]-µ-sulfato-κO:O']].

The structure of the title compound, (C(14)H(16)N)(2)[HgCl(2)(SO(4))], consists of an infinite chain propagating along the c direction, containing Hg(II) ions tetra-coordinated by two bridging O atoms of bis-monodentate sulfate anions and two chloride ligands. In the the crystal, N-H⋯O hydrogen bonding between the cations and the anionic chains consolidates the packing. The crystal structure was determined from an inversion twin with approximately equal twin domains.

Bis(1,1-dimethyl-guanidinium) tetra-aqua-dimethyl-tin(IV) bis-(sulfate).

Single crystals of the title salt, (C(3)H(10)N(3))(2)[Sn(CH(3))(2)(H(2)O)(4)](SO(4))(2), formed concomitantly with the already known [Sn(CH(3))(3)](2)SO(4)·2H(2)O. In the title structure, the Sn(IV) atom displays a slightly distorted octa-hedral coordination geometry defined by four O water atoms in the equatorial positions and two methyl groups in the axial positions. In the crystal, various O-H⋯O and N-H⋯O hydrogen-bonding inter-actions between the organic cation and the coordinated water mol-ecules as donors and the sulfate O atoms as acceptors result in a three-dimensional structure. The Sn(IV) atom is located on an inversion centre, resulting in half of the complex metal cation being in the asymmetric unit.