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A 3D heterometallic oxalate-bridged coordination polymer [CuIIFeII2(H2O)(terpy)(C2O4)3]n (terpy = 2, 2′:6′, 2″-terpyridine) (1) was investigated both as photocatalyst for the organic dye removal and as a single-source precursor for the preparation of the copper ferrite (CuFe2O4) nanocrystals by thermal processing. The dual functionality of 1 was supported by the degradation of aqueous solutions of rhodamine B (RhB) and methylene blue (MB) solutions under visible (Vis) and ultraviolet (UV) light irradiation, powder X-ray diffraction data collection at room temperature, and the optical and scanning electron microscopy analyses. A close inspection of the X-ray diffraction patterns unveiled qualitative and quantitative information on the phase composition obtained after the single-source molecular precursor route to spinel oxide. By optimizing the temperature levels and setting the controlled heating rate at 6 h of holding time, the phase composition of thermal processing of 1 was evaluated—thermal treatment of 1 at 950◦C for 6 h and a heating/cooling rate of 10◦C min−1 resulted in the formation of solely tetragonal spinel phase of CuFe2O4, whereas the formation of both tetragonal and cubic CuFe2O4 phases was observed at 950◦C by the heating rate of 30◦C min−1. To obtain the high-temperature cubic CuFe2O4 oxide, compound 1 was heated and then quenched at 925◦C, which led to the formation of the cubic spinel ferrite as the main crystalline oxide phase. Moreover, the photocatalytic properties of the t-CuFe2O4 spinel were investigated under the same conditions as for 1. The optical bandgap energies were estimated from UV–Vis absorption spectra for both metal oxide and precursor powder.

In recent years, great efforts have been made to develop new metal-organic frameworks (MOFs) and coordination polymers (CPs) with photocatalytic potential, especially with respect to the development of photocatalysts for organic pollutant degradation, CO2 reduction, and water splitting for H2 production. Compared to conventional metal oxide semiconductors, well- defined and tailored structures of MOFs offer great advantages for understanding the structure- activity relationship. Their high porosity enables the exposure of active sites for catalysis and facilitates the transport of substrates by creating short migration paths for charge carriers prior to their reaction with substrates, thus improving electron-hole separation.[1] The use of heterometallic complexes or frameworks containing more than one metal as single source precursors provides simplified synthetic routes through one- step thermal decomposition to form mixed metal oxide materials. The advantage of a solid phase transition is the retention of the elemental composition defined by the molecular precursor with only a loss of volatile decomposition products – allowing excellent stoichiometric control of the intermetallic ratio in the oxide products. It is known that the release of organic gases affects the particle size and porosity of the produced oxide ; a higher content of organics in the starting materials allows larger porosity and smaller particle sizes in the decomposition products. In addition, control of the heating rate is important to control decomposition and avoid melting/vaporisation of molecular species that can occur with rapid heating. High temperatures also determine the crystallinity, particle size and porosity of the produced materials, limiting the ability to develop specific properties for applications. By using lower synthesis temperatures, oxides with small particle sizes and large surface areas can be produced.[2] Since it is known that Fe–O clusters can be directly excited by visible light[3], leading to more efficient use of solar energy, and that compounds with iron are much cheaper due to the abundant Fe element on Earth, an oxalate-bridged heterometallic coordination polymer [CuII(H2O) (terpy)FeII2(C2O4)3]n (terpy = 2, 2′:6′, 2′′- terpyridine) with a three-dimensional (3D) network[4], was tested as a photocatalyst for the photodegradation of Rhodamine B (RhB) and methylene blue (MB). The optical band gap and photocatalytic activities were studied in detail. This 3D network [CuIIFeII2(H2O)(terpy)(C2O4)3]n was also tested as a single molecular precursor for the preparation of spinel oxide CuFe2O4 due to the suitable metal ratio. The oxalate group, C2O42-, easily decomposes at low temperatures into gaseous CO2 and CO, and therefore the oxalate‐ based solids can serve as suitable precursors for oxides. The (micro)structural, optical and photocatalytic properties of the (nano)crystalline spinel CuFe2O4 samples, prepared for the first time by a simple molecular precursor‐to‐material route, were presented.

In ferroelectric materials, spontaneous electric polarization can be reversibly switched under the external electric field. While homochirality provides an important bridging effect between structure and ferroelectric properties, the discovery of new ferroelectrics is often random rather than strategically designed.[1] Here we report on the targeted synthesis of two metal- organic complex salts containing bicyclic amine cations and bis(oxalato)chromium(III) anions: (DABCO)[Cr(phen)(C2O4)2]2·9H2O (DABCO = 1, 4- diazabicyclo[2.2.2]octane ; phen = 1, 10- phenanthroline) and (COOH-OH-ABCO)[Cr(bpy) (C2O4)2]·H2O (COOH-OH-ABCO = 3-hydroxy-1- azabicyclo[2.2.2]octane-3-carboxylic acid ; bpy = 2, 2′-bipyridine). Complex salts were prepared using the mechanochemical synthesis and recrystallized from aqueous solutions to give single crystals. Structural and dielectric properties of these compounds were studied by combination of single-crystal X-ray diffraction, thermal analysis, infrared and impedance spectroscopy, and polarization−voltage measurements. Newly prepared azabicyclic cation COOH-OH-ABCO+ was characterized using 1H and 13C NMR spectroscopy. Interesting optical phenomena of pleochroism was observed in (DABCO)[Cr(phen) (C2O4)2]2·9H2O, where crystals change color from pink to orange if rotated under plane-polarized light.

Ferroelectric materials exhibit spontaneous electric polarization that can be reversibly switched under external applied electric field. In recent years, ferroelectrics have been extensively investigated, showing promising applications in memory devices, capacitors, microactuators, etc. [1] In attempt to design new molecular ferroelectric crystals, two metal-organic complex salts containing bicyclic amine cations, O-ABCO+ (1-azabicyclo[2.2.2]octan-3-one) and COOH-OH-ABCO+ (3-hydroxy-1-azabicyclo[2.2.2]octane-3-carboxylic acid), and bis(oxalato)chromium(III) anions, [Cr(bpy)(C2O4)2]– (bpy = 2, 2′-bipyridine), were prepared using mechanochemical synthesis. Single crystals of prepared complex salts were obtained by slow evaporation of their aqueous solutions, which allowed structure determination from single- crystal X-ray diffraction analysis. Thermal analysis showed that water molecules of crystallization are retained in the structure up to ~100 °C. Temperature dependent dielectric studies were performed in -90–100 °C temperature range. For compound (O-ABCO)[Cr(bpy)(C2O4)2]·H2O dielectric constant and loss increase while increasing the temperature and decrease toward higher frequencies, pointing to its semiconducting nature. Polarization–voltage measurements were conducted to further study the observed dielectric anomalies in this sample.

Solid–solid phase transition compounds with electric, magnetic, and thermally switchable states usually provide an effective strategy for the design of functional materials such as ferroelectric materials, nonlinear-optical switches, and switchable dielectric devices. Among these properties, ferroelectric behaviour is particularly desirable because it is possible to switch rapidly between different states by an external electric field. The most widely used materials for ferroelectric applications are mainly pure inorganic oxides such as BaTiO3, LiNbO3, and PbZr1–xTixO3, due to their superior electric properties, high thermal stability and phase transition temperature. Recent studies have shown that ferroelectric metal-organic molecule based materials are a promising candidates to replace the predominant lead-based ferroelectrics. [1, 2] The strategy in design of molecular ferroelectrics frequently relies on rotating polar globular ionic molecules, such as 1- azabicyclo[2.2.2]octane (ABCO), as they can be readily reoriented in response to an applied electric field and induce ferroelectric polarization. In attempt to prepare new molecular ferroelectric systems different bicyclic amine cations, including simple derivatives of globular ABCO molecule (O-ABCO = 1-azabicyclo[2.2.2]octan- 3-one ; COOH-OH-ABCO = 3-hydroxy-1- azabicyclo[2.2.2]octane-3-carboxylic acid) and more complex chiral natural alkaloids with ABCO fragment (Q= quinine ; Cnd = cinchonidine) have been used in reactions with chlorocobaltate(II) anions. A series of complexes of the formula ACoCl3 (A = O-ABCO, COOH-OH-ABCO, Q, Cnd) has been prepared. By using the combined techniques of variable-temperature powder X-ray structural analyses, differential scanning calorimetry, impedance spectroscopy, and polarization-voltage measurements, we revealed exciting structural and electric properties in this series of compounds.

Ferroelectrics are ubiquitous materials used in daily life and industrial production, as memories, transducers, capacitors, sensors, and so forth. The main feature of ferroelectrics is spontaneous polarization that can be reversed by the application of an electric field larger than the coercive field. By far the most studied and used electroceramic with ferroelectric properties is lead zirconate titanate, which is nowadays becoming an increasingly serious problem in terms of the environment, costs, and energy.[1] Due to the overall superior performance of lead-based ferroelectrics finding alternatives has proven to be challenging. Herein preparation of two new lead-free ferroelectric systems will be presented, both based on chemical solution synthesis. First, the block copolymer–templated mesostructured LiNbO3 thin films prepared by evaporation induced self-assembly will be presented and their ferroelectric performance will be discussed in terms of different device construction, ie. top and bottom electrodes.[2] The efficiency of ferroelectric materials is related to the way they are prepared because characteristics such as chemical purity, crystallinity, and morphology are important in the obtained materials. Porous electroceramics typically exhibit lower polarizability than their dense counterparts, but they offer much more possibilities for structuring, especially when it comes to producing multifunctional and multicomponent systems and tuning/tailoring of properties. Another approach for the preparation of new ferroelectric systems relies on the chemical design of molecular compounds. Chirality that certain molecules possess is a very convenient feature especially important for the design of polar structures. Using this strategy we have chosen naturally occurring chiral alkaloids of the Cinchona plants: quinine and cinchonidine, in reactions with chlorocobaltate(II) anions to prepare complexes of the formula ACoCl3 (A = alkaloid), that reveal exciting structural and electric properties.

The perovskite family of oxides is very attractive area of research in material science community because of the diversity of their physical and chemical characteristics, which offers opportunity to explore new functionalities and potential applications, such as photocatalysis, light harvesting, superconductivity, magnetoresistance, ferroelectric, multiferroic or dielectric performance. In general complex perovskite oxides with A2B'B''O6 or A3B'B''B'''O9 formula, where A are alkali, alkaline earth or rare earth elements, while the B', B'' and B''' cations can be any transition element, have crystal structure that consists of the corner sharing B'O6, B''O6 and B'''O6 octahedra, and A atoms located in the space between the octahedra. A variety of devices and methods have previously been used to prepare and characterize perovskite compounds, however most common method of their preparation is solid state reaction at high temperatures. Herein we propose novel pathway for the synthesis of perovskite-type oxides through molecular precursor-to-material approach, which uses heterometallic complexes as precursors for oxides. Thermal decomposition of the mixture of the well-defined and structurally characterized heterometallic oxalate-based compounds {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O and [Ni(bpy)3]2[TaO(C2O4)3]Cl·12H2O leads to highly crystalline Ba3(NiNbTa)O9 oxide. We have also shown that Ba2NbCrO6 perovskite-type oxide can be obtained in thermal treatment of {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O and (NH4)3[Cr(C2O4)3]·3H2O. The phase formation and structural ordering of oxides obtained by modified molecular precursor- to-material method has been investigated through a combination of thermal analysis (TGA and DTA), powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. We have shown that ratio of metals can be finely tuned with the careful choice of molecular precursors as well as by appropriate amount of initial heterometallic complexes taken for the thermal decomposition reaction. The preparation strategy described here could be extended for the synthesis of various other complex perovskites at relatively mild conditions.

Metal–organic coordination polymers may exhibit a variety of specific properties including porosity, conductivity or luminescence and have potential applications in various fields from heterogeneous catalysis, gas storage and separation to supramolecular magnetism. Due to the diversity of metal species and ligands, coordination geometry, guests inside the pores, and supramolecular arrangements, vastly coordination polymers with various structures and pores have been synthesized and characterized. Since their discovery, oxalate-based extended networks of a general formula [MaII/IIIMbI/II(C2O4)]2n−/n− have attracted a lot of attention because the anionic networks provide magnetic ordering and are able to host a wide variety of functional cations, leading to a rational synthesis of magnetic multifunctional materials. The possibility of using coordination polymers through the thermal decomposition process as molecular precursors in the synthesis of nanomaterials has been considered only recently. This method of obtaining oxide materials, compared with conventional methods, has several advantages. The C2O42− group easily decomposes to gaseous CO2 and CO at low temperatures, and hence, heterometallic oxalate complexes have already been used for the single-step preparation of mixed metal oxides.[1] As a continuation of our magneto-structural studies concerning the polynuclear transition-metal complexes, heterometallic 3D oxalate-bridged polymers {; ; ; ; [A(bpy)3][M2(C2O4)3]·H2O}; ; ; ; n (A = Co2+ or Cu2+ ; M = Mn2+ or Fe2+ ; bpy = 2, 2′-bipyridine) have been prepared and, in addition to the single-crystal X-ray study, characterized by magnetization measurements. Utilizing the preparation of the oxide materials via thermal decomposition, these polymers have been tested as molecular precursors for the formation of spinel oxides CoMn2O4, CuMn2O4 and CuFe2O4. This conversion was explored by thermal analysis, powder X-ray diffraction and magnetic susceptibility measurement. The changes in preparation conditions, namely, temperature of the heat treatment, cause thermally induced cation redistribution within the spinel lattice and an increase of the particle sizes, acting ultimately on the magnetic behavior of the prepared oxides.

International audience; Herein, we present a detailed X-ray charge density study of the electron delocalisation in five species of the chloranilic acid: neutral molecule, mono- and dianion, and two chelating modes – bidendate and (bis) bidentate. The experiments provide the electron density at the bond critical points, which yields an accurate measure of bond order (and therefore electron delocalisation), and complements previous literature and our data on bond lengths extracted from crystal structures and infrared spectra. Mapping of the electrostatic potential indicates electron-rich and electron-poor areas in the molecule, corresponding to single (electron-poor), double and delocalised bonds (electron-rich) that can explain stacking interactions of quinoid rings in crystal packing.

Spin-glass is a disordered magnetic state, where the spins of the component atoms are not aligned in a regular pattern. Their randomness basically comes from the competition between different magnetic interactions. Mathematical complexity of spin-glass structures is the main reason why complete microscopic theory of such systems still doesn't exist, while there are many phenomena related to it, observable if proper material system is found. [1] Chromium niobate (CrNbO 4 ) of the rutile-type structure was synthesized by pyrolysis of the oxalate-based heterobimetallic complex molecular precursor [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (bpy = 2, 2'-bipyridine). [2] Compared to the conventional solid state reactions, this represents a simple one-step route to synthesize pure material and at lower temperatures. Magnetic behaviour of this oxide was investigated by SQUID magnetometer. Magnetization indicates occurrence of the spin-glass state at 9.3 K. Spins of Cr 3+ ions dominantly interact antiferromagnetically, while hysteresis curves suggest some additional interactions consistent with spin-glass structure, which is confirmed with other magnetic parameters, too. The most interesting is the non-ergodic behaviour observed through the specific relaxation of magnetization and peculiar memory effect. Ability to see all these phenomena might be found in the way of synthesis on the molecular level. Similar behaviour was observed in CrTaO 4 , but below 3.5 K, and even more interesting in some multiferroic material [3]. As CrNbO 4 and CrTaO 4 have specific electric and catalytic properties, new opportunities arise if spin-glass phase is combined with other spin-dependent processes in such multi-functional materials. [1] J. A. Mydosh, Spin glasses: an experimental introduction, Taylor-Francis, London, 1993. [2] L. Androš Dubraja et. al., Single-step preparation of rutile-type CrNbO 4 and CrTaO 4 oxides from oxalate precursors–characterization and properties, Accepted in J. Am. Ceram Soc. 2019. [3] Z. Jagličić et. al., Appl. Phys. Lett. 102 (2013) 242410.

Mixed-metal oxides are an important class of advanced materials due to their chemical stability, low cost, low toxicity, large scale of useful photophysical properties and wide range of technological applications. It is known that the microstructure of oxide materials can be tuned in part by changing the synthesis methods. Full control over particles dimensions and morphology is also prerequisite for the photocatalytic applications of oxide materials where large surface area promoting substrate adsorption at the surface and increasing the number of active sites, is considered to be an imperative. However, in general, synthesis approaches for obtaining highly crystalline materials utilize high temperature treatments, which often lead to the increase of particle size and consequently the decrease of the surface area. Thus, it is still challenging to attain both high crystallinity and high surface area in the same synthesis. Due to these limitations, in recent times, heteropolynuclear systems have been used as molecular precursors for the preparation of the mixed-metal oxides by their thermal decomposition. It has been observed that the use of a well-defined precursor can produce crystalline oxide materials under conditions that are significantly milder than those applied in traditional solid-state synthesis. Also, the single-source precursors provide better control over the stoichiometry of the metal ions in the final products as well as the homogeneity of the materials due to the mixing of the metals at the molecular level, and offer kinetically attractive low temperature decomposition. An effective molecular precursor-to-material conversion requires a reasonable choice of ligands. The nature of the ligand employed endows the precursor with a clean, low-temperature thermolysis leading to the phase-pure target material. Importantly, low cost of the ligand makes heterometallic precursor highly attractive to industrial applications. One of the suitable ligands for the synthesis of molecular precursors for oxides is the oxalate dianion, C2O42-, due to its easily decomposition on gaseous CO2 and CO at low temperatures. Another advantage of this ligand is low cost. However, heterometallic complexes do not always contain the appropriate stoichiometry for the formation of the desired single phase oxide by thermal degradation in one step. Also, it is not easy to prepare coordination compound with three or more different metals (in an appropriate ratio), having also suitable coordinated ligand. So, sometimes multimetallic oxides could be prepared by mixing two or more agreeable precursors in appropriate ratio prior to thermal decomposition. In our group we explore the ability of the heterometallic oxalate‐based complexes to act as the single-source precursors for the formation of the mixed‐metal oxides by heat treatment. We focused on the structural, optical, photocatalytic, and magnetic properties of the (nano)crystalline oxides prepared for the first time by facile molecular precursor‐to‐material route, investigated by powder X‐ray diffraction, electron microscopes, UV‐vis diffuse reflectance spectroscopy and magnetization measurements, where long‐term heating at high temperatures and repeated grinding procedures are successfully bypassed. References [1] J. Popović, M.Jurić, D. Pajić, M. Vrankić, J.Zavašnik, J. Habjanič, Inorg. Chem. 56 (2017) 3983-3989 [2] L. Androš Dubraja, D. Pajić, M.a Vrankić, J. Dragović, M. Valant, M.Benčina, M. Jurić, J. Am. Ceram. Soc. (2019) DOI:10.1111/jace.16521

Beside their applications in catalysis, photoluminescence, magnetism, gas storage and separation, metal-organic coordination systems have recently been used as molecular precursors for the preparation of the mixed metal oxides by their thermal decomposition. It has been observed that the use of a well-defined heterometallic precursor can produce crystalline oxide materials under conditions that are significantly milder than those applied in traditional solid-state synthesis. Moreover, the single-source precursors provide better control over the stoichiometry of the metal ions in the final products as well as the homogeneity of the materials due to the mixing of the metals at the molecular level. The existence of bridging or chelating ligands in the precursors prevents metal separation during oxide formation. The C2O42− anion easily decomposes to the vapour phases CO2 and CO, by the low- temperature routes, and hence, heterometallic oxalate complexes are very convenient for the preparation of mixed metal oxides [1]. Heterometallic oxalate complexes do not always contain the appropriate stoichiometry for the formation of the desired single phase mixed-metal oxide. So, we have tested whether the multimetallic oxides could be prepared by mixing two or more different oxalate precursor in various ratios prior to thermal decomposition [2]. A highly crystalline materials Ba2CrNbO6 were obtained after thermal decomposition of the mixture of the well-defined and structurally characterized oxalate-based compounds {; ; ; Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ·H2O [3] and (NH4)3[Cr(C2O4)3]·3H2O taken in equimolar ratio and grinded in agate mortar prior to heating treatment. Following the same strategy, oxide NiNb2O6 was prepared from the mixture of [Ni(bpy)3]2[NbO(C2O4)3]Cl·12H2O [4] and (NH4)3[NbO(C2O4)3]·H2O [3] complexes, taken in a 1 : 3 ratio. The phase formation and structural ordering of oxides obtained by modified molecular precursor-to-material method has been investigated through a combination of thermal analysis (TGA and DTA) and powder X-ray diffraction.

Properties of the mixed-metal oxides could be highly affected by the effect of crystallinity, particle size, phase composition and morphology. These can be tuned in part by changing the synthesis methods. The possibility of using metal– organic coordination systems through the thermal decomposition process as molecular precursors in the synthesis of nanomaterials with high surface and specific morphology has been considered only recently. It has been observed that the use of a well-defined heterometallic precursor can produce crystalline oxide materials under conditions that are significantly milder than those applied in traditional solid-state synthesis. Also, the single-source precursors provide better control over the stoichiometry of the metal ions in the final products as well as the homogeneity of the materials due to the mixing of the metals at the molecular level. The existence of bridging or chelating ligands in the precursors prevents metal separation during oxide formation. For example, the C2O42− anion easily decomposes to the vapour phases CO2 and CO, by the low-temperature routes, and hence, heterometallic oxalate complexes are very convenient for the preparation of mixed metal oxides.[1] Most of the perovskite compounds that have potential technological interests are not simple systems, but rather ternary oxides such as A(B'B'')O3. Heterometallic oxalate complexes do not always contain the appropriate stoichiometry for the formation of the desired single phase oxide. So, we have tested whether the multimetallic oxides containing two or more metals could be prepared by mixing two or more different oxalate precursor in various ratios prior to thermal decomposition.[2] A highly crystalline materials Ba(M1/3−xMx'NbV2/3)O3 [M = NiII, M' = CoII ; x = 0−1/3] were obtained after thermal decomposition of the mixture of the well-defined and structurally characterized heterometallic oxalate-based compounds Ba2(H2O)5[NbO(C2O4)3]HC2O4}; ; ; ; ; ·H2O, [1] [Ni(bpy)3]2[NbO(C2O4)3]Cl·12H2O[3] and [Co(bpy)3]2[NbO(C2O4)3]Cl·12H2O, [3] grinded in an agate mortar in different ratios. The phase formation and structural ordering of the tri- or tetrametallic perovskite oxides obtained by this modified molecular precursor route have been characterized by powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The magnetic properties of newly prepared materials which adopt the disordered cubic structure (Pm3 ̅m space group) with random distribution of B' and B'' ions, have been also investigated.

With the aim to control the magnetic properties of the heteropolynuclear complexes, Lt–M–Lb–M'–Lt (Lb = bridging ligand, Lt = terminal ligand), many strategies have been considered focusing on alternating the chemical nature of the metal center (M and/or M') and bridging ligands (Lb) and their intramolecular arrangement (Lb–M and Lb–M' bond lengths and M–Lb–M' bond angles). In particular, the delocalization between the magnetic centers can be improved by using long conjugated bridges, which lead to low energy ππ* excitations, small diamagnetic spin linkers such as Cl−, O2−, or closed-shell metal ions, which allow a direct M–M magnetic coupling. Thus, the main parameter which enables the control of magnetic properties is the strength of the orbital overlap between the metal and ligand. For the synthesis of novel and more diversified polynuclear species we often use the tris(oxalato)oxoniobate(V) anion, [NbO(C2O4)3]3− as a building block toward other metal centers. A special feature of this anion is that it can form heterometallic complexes through the bridging oxalate (μ-C2O4) and/or oxo (μ-O) group. In our previous work we reported compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (bpy = 2, 2'-bipyridine) which crystallizes in the orthorhombic Pcab space group and contains a square shaped {; ; ; Cr2(μ-O)4Nb2}; ; ; core, exhibiting relatively strong antiferromagnetic coupling between two CrIII ions mediated by two –O–NbV–O– diamagnetic bridges [1]. The occurrence of this unexpected magnetic exchange interaction between two paramagnetic centres (separated more than 7.4 Å) inspired us to investigate how the change of the aromatic N-donor ligands will affect the structural and magnetic properties of new compounds. From the reaction of aqueous solutions of tris(oxalato)oxoniobate(V) anions and bis(phenathroline)chromium(III) cations, compound [Cr2(phen)4(μ-O)4Nb2(C2O4)4]·2H2O (1) has been obtained (phen = 1, 10-phenanthroline), crystallizing in tetragonal P42212 space group. Using tridentate 2, 2′:6′, 2′′-terpyridine (terpy) instead of bidentate ligands in similar procedure, compounds [Cr2(terpy)2(H2O)2(μ-O)4Nb2(C2O4)4]·4H2O (2) and [Cr(terpy)(C2O4)(H2O)][Cr2(terpy)2(C2O4)2(μ-O)2Nb(C2O4)2]·3H2O (3) have been isolated (monoclinic P21/n and triclinic space groups, respectively). Heteropolynuclear compounds 1–3 have been investigated by IR spectroscopy, single-crystal X-ray diffraction and magnetization measurements.

The reaction of bis(phenanthroline)metal(II) cations (M = Mn2+, Co2+, Ni2+, Cu2+, Zn2+) with bis(oxalato)chromium(III) anions in water/ethanol solution gives rise to a series of compounds with oxalate-bridged cations, [{; ; ; ; M(phen)2}; ; ; ; 2(μ- C2O4)]1/2[Cr(phen)(C2O4)2]·2H2O [MnCr, CoCr, NiCr, CuCr, ZnCr]. The temperature dependence of magnetic susceptibility χ(T) indicates different magnetic super exchange interactions taking place in homodinuclear oxalate-bridged cations containing Mn2+, Co2+, Ni2+ and Cu2+. Oxalate ligands mediate ferromagnetic coupling of Cu2+ metal cations in CuCr with J = 3.45cm-1, whereas in MnCr, CoCr, NiCr, antiferromagnetic interactions are observed between Mn2+, Co2+ and Ni2+ cations, with J = - 2.48cm-1, -9.61cm-1 and -23.89cm-1 respectively. Presence of large zero- field splitting parameters for Cr3+ ion (from mononuclear anion) indicates significant influence of octahedral distortions on magnetic properties of the investigated compounds. Own software is developed in order to fit all magnetic parameters at the same time.

Magnetic behavior of novel heterotetranuclear compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (1 ; bpy = 2, 2’-bipyridine) was investigated by magnetization measurements, EPR (X-, Q-band and and high-field) spectroscopy and DFT calculations. Results of M(T) measurements show antiferromagnetic interaction of CrIII ions through two diamagnetic bridges -O-NbV -O- with parameter of interaction J = −12.77cm−1 and ZFS parameter D = −0.17cm−1. The EPR spectra simulations and DFT calculations reveal the presence of a single-ion anisotropy that is close to being uniaxial, D = −0.31cm−1 and E = 0.024cm−1. [1] [1] Project FerMaEl (UIP-2014-09- 8276) financed by the Croatian Science Foundation

Mixed-metal oxides are an important class of advanced materials, due to their stability, low cost, low toxicity, useful photophysical properties and wide range of technological applications. They are mostly utilized as catalysts and structural ceramics, but recently, the use of mixed-metal oxides as sensors, actuators, and smart materials has also been explored. It is known that the effect of crystallinity, particle size, structure and morphology of these materials could highly affect their properties. These can be tuned in part by changing the synthesis methods. The possibility of using coordination polymers through the thermal decomposition process as molecular precursors in the synthesis of nanomaterials with high surface and specific morphology has been considered only recently. This method of obtaining oxide materials, compared with conventional methods, has several advantages: (i) the obtained material is more homogeneous because the metals are mixed at the molecular level ; (ii) the resulting materials have relatively high specific surface areas because the crystalline oxides are formed under significantly milder conditions than those in, for instance, solid-state reaction processes ; (iii ) the existence of bridging or chelating ligands in the precursors prevents metal separation during oxide formation ; (iv) the re is much greater control of the metal stoichiometry in the final oxide. The C2O42− group easily decomposes to gaseous CO2 and CO at low temperatures, and hence, the oxalate-based solids can serve as a convenient source of oxides [1]. Utilizing the preparation of the oxide materials via thermal decomposition, several oxalate-based compounds were tested as molecular precursors. Discrete heterotetranuclear oxo -bridged compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (1 ; bpy = 2, 2'-bipyridine) showed to be a good candidate for molecular precursor-to-material conversion, yielding the rutile-type CrNbO4 oxide after heat treatment at 900°C. The thermal processing of heterodimetallic one-dimensional (1D) compound {; ; ; ; ; ; [CaCr2(bpy)2(C2O4)4]·0.83H2O}; ; ; ; ; ; n(2) proved to be a simple, one-step synthesis route for the preparation of the β-CaCr2O4 phase at 1100 °C in nitrogen flow. The r.t. structure of β-CaCr2O4 is isomorphic with calcium ferrite, unlike most chromate structures, which usually crystallize as spinel oxides. A three-dimensional (3D) oxalate-based coordination polymer {; ; ; ; ; ; [Co(bpy)3][Mn2(C2O4)3]·H2O}; ; ; ; ; ; n(3) was used as a single-source precursor for the formation of spinel oxide CoMn2O4 heating at 800 °C. The conversion via thermal decomposition of compounds 1–3 was explored by thermal analysis (TGA and DTA), IR spectroscopy and powder X-ray diffraction (PXRD). References [1] M. Jurić, J. Popović, A. Šantić, K. Molčanov, N. Brničević and P. Planinić, Inorg. Chem. 52 (2013) 1832-1842.

Chloranilic acid is a quinoid compound with very diverse chemical behaviour: it is a good ligand, proton donor and acceptor, and also able to form unusually strong π-interactions. The reason for its versatility is its especially malleable π-electron system, which can adopt various degrees of delocalisation, depending on ionisation and molecular environment. Therefore, its neutral form is a typical para-quinone, while its dianion has two delocalised systems separated by two single C-C bonds. The mono anion, however, has a delocalised system over a half of the ring, while the other half has distinguishable single and double bonds. It can also bind to metals either as a bridging (bis)bidentate ligand, which has a dianion-like structure or as a terminal bidentate ligand with an ortho-quinoid structure. Differences in molecular geometry, related to changes of electronic structure, affect significantly IR spectra: its C-C and C-O stretching bands display a variety of red- and blue-shifts. Herein, we present a detailed X-ray charge density study of the electron delocalisation in all five different forms of the chloranilic acid. Electron density at the bond critical points is a more accurate measurement of bond order (and therefore electron delocalisation) which complements previous data on bond lengths and IR bands. Also, mapping of the electrostatic potential indicates electron-rich and electron-poor areas in the molecule, corresponding to single (electron-poor), double and delocalised bonds (electron-rich).

The aqueous chemistry of tantalum is mainly related to certain forms of carboxylates, such as oxalate, tartarate, citrate, glycolate, lactate and polyaminocarboxylate, as well as peroxide compounds. Usually these species have been utilized as starting materials for the sol-gel or polymeric precursor methods for the preparation of oxide materials. Structurally defined heterometallic complexes of tantalum obtained from aqueous solutions are very scare, despite of great potential to produce mixed-metal oxides via thermal decomposition. Metal tantalates represent very common functional inorganic materials with various properties and applications, in particular photocatalytic and dielectric. It is known that the effect of crystallinity, particle size and morphology of oxides can highly affect their photocatalytic activity. These factors can be tuned in part by changing the synthesis method. It is widely recognized that a large surface area of photocatalyst is desired, since it can promote substrate adsorption to the surface and increase the number of active sites for catalysis. However, in order to obtain materials of good crystallinity high temperature treatments are often employed, inducing increase of the particle size and decrease of the surface area. Thus, it is difficult to harmonize both in the one-step synthesis approach. The thermal decomposition of heterometallic complexes give rise to (nano)crystalline oxides with relatively high specific surface area due to milder conditions than those employed in traditional solid-state syntheses. Also, due to the mixing of metal ions at the molecular level better control of metal stoichiometry and higher homogeneity is achieved. Relatively low thermal stability of the heterometallic oxalate compounds (oxalate ligand decomposes at  400 °C to gaseous CO2 and CO) makes them suitable for the use as molecular precursors for the mixed-metal oxides. As a continuation of our previous studies on the oxalate chemistry of tantalum, we have prepared a series of heterometalllic compounds of the formula [M(phen)3][Ta(OC2H5)(C2O4)3]·H2O [M = Fe, Co, Ni ; phen = 1, 10-phenanthroline] that were used as precursors for mixed-metal oxides: FeTaO4, CoTa2O6, Co4Ta2O9 and NiTa2O6. Up to now, such mixed-metal oxides have been synthesized by the solid-state reactions that required long-term heating at relatively high temperatures and repeating grinding procedures. The influence of heating treatment in studied Fe-Ta, Co-Ta and Ni-Ta systems on the composition and crystalline size of oxides was examined by powder X-ray diffraction. Microstructural characterization showed that all thermal decomposition residues were nanocrystalline, in general indicating that this method is very convenient way for obtaining mixed-metal oxides.

Some crystals jump during the phase transition. These thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellance. Yet, due to exclusivity and individuality of the phenomenon, all present investigations have not yet resulted in the full elucidation of this mechanism, let alone enabled us to predict necessary prerequisites for its existence so we aimed our research towards new materials obtained by careful modification of compounds in which the effect was noticed. It is to be expected that this exotic and unexpected effect will be extremely sensitive to subtle changes in the chemical composition (change of the anion, change of the functional groups, change of the position of the functional group inside the aromatic ring) and these investigations will enable determination of necessary chemical/structural parameters so as to arise the thermosalient effect in the system. No matter if these compounds will be thermosalient in nature, or not, comparison between original systems and their derivatives will certainly provide new and helpful insight into elucidation of thermosalient behavior and offer new guidelines in the sense of targeted fabrication of new thermosalient materials. One of the first systematic studies of this fascinating effect, which paved a way towards its final elucidation, was conducted on the anticholinergic agent oxitropium bromide.2 It was shown that the unit-cell distortion is accompanied by a conformational change of the oxitropium cation, which triggers increased separation between the ion pairs in the lattice at nearly identical separation between the cation and the anion within each ion pair. At the molecular level, the cation acts as a molecular shuttle composed of two rigid parts (epoxy- aza- tricyclic-nonyl portion and phenyl ring) that are bridged by a flexible ester linkage. The structure of the rigid, inert aza-tricyclic portion remains practically unaffected by the temperature, suggesting a mechanism in which the large, thermally accumulated strain is transferred over the ester bridge to the phenyl ring, which rotates to trigger the phase transition. In an attempt aimed at the targeted fabrication of thermosalient materials, a series of oxitropium salts was prepared. Oxitropium bromide was used as a starting reagent for preparation of several other oxitropium salts. Precursor compound was dissolved in minimal amount of water and the appropriate lead(II) salt was added under stirring conditions. After period of 1 h lead(II) bromide, which precipitated during the stirring, was removed by filtration. One of the prepared systems, oxitropium nitrate, showed thermosalient behaviour during observation on the hot-stage microscope and exhibited joyful jumps during heating (around 134ºC) and cooling (around 98ºC). Detailed structural (high temperature in-situ XRPD), thermal (DSC and TGA), spectroscopic (FT-IR), mechanistic and theoretical (DFT using state-of-the- art nonempirical vdW-DF-cx functional) study was conducted which revealed much information about the thermosalient effect in this system which are necessary to reach the final goal of understanding the nature and mechanism of this fascinating phenomenon.

The area of heteropolynuclear metal complexes has attracted considerable interest not only due to their different architectures and topologies, but also for their potential applications in various fields from heterogeneous catalysis, gas storage and separation to supramolecular magnetism. Although the most used metals for the preparation of these complexes are the 3d first row transition, there are also many systems containing two or more different 3d and 4d metal ions. Diamagnetic ions such as Cl−, O2−, due to their small size, or closed-shell metal ions are usually good spin linkers, and generally cause considerable delocalization between magnetic centres. Nevertheless, the degree of delocalization depends strongly on the overlap between the magnetic orbitals on the metal and valence orbitals of ligands. Molecular ions such as oxalate, acetate, azide, etc., and conjugated bridges display low-lying excited valence states and produce larger spin and charge polarization effects. In the search for suitable synthetic routes leading to novel and more diversified polynuclear species, we have introduced the tris(oxalato)oxoniobate(V) anion, [NbO(C2O4)3]3− as new building block toward different metal centres. Special feature of this anion is that it can form heterometallic complexes through the bridging oxo (μ-O) or oxalate group (μ-C2O4). A novel heterotetranuclear oxo-bridged compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (1) (bpy = 2, 2′-bipyridine) has been prepared and, in addition to the single-crystal X-ray study, characterized by magnetization measurements supported by DFT calculation. Compound 1 containing a square shaped {; ; ; ; Cr2(μ-O)4Nb2}; ; ; ; core exhibits relatively strong antiferromagnetic coupling between two chromium(III) ions mediated by two triatomic –O–NbV–O– diamagnetic bridges. In many cases, the magnetic interaction in the polynuclear complexes, based on metal centres and bridging ligands can be predicted ; in compound 1 any significant exchange coupling has not expected due to long intramolecular distances of ~7.4 Å between CrIII centres bridged by three diamagnetic atoms. In order to examine the role that metal centres, coordination sphere and crystal packing plays in modulating the magnetic properties of compound 1, additional new systems [CrIIINbV] of similar topology containing other aromatic ligands as 1, 10-phenathroline (phen) or terpyridine (terpy) have been prepared and structurally investigated: [Cr2(phen)4(μ-O)4Nb2(C2O4)4]·2H2O (2) and [Cr2(terpy)2(C2O4)2(μ-O)2Nb(C2O4)2][Cr(terpy)(C2O4)(H2O)]·3H2O (3). Exchange interaction between paramagnetic metal centres bridged by diamagnetic metal ion are known but are very rare, so the full comprehension of how the unpaired electrons of the metal centres feel each other through diamagnetic transition metal atom is still challenge.

The simultaneous presence of ferromagnetism and ferroelectricity in a single phase and possible spin-phonon couplings in these systems offers the appealing opportunity to design unconventional devices, such as multiple-state memory elements and electric-field controlled magnetic sensors. Mixed double perovskites, due to its structural diversity, represent a very promising platform in a quest for new multiferroics. A new synthesis route for the preparation of mixed metal oxides crystallizing in the double perovskite structural type will be presented and discussed. This procedure includes the thermal decomposition of the mixture containing two heterometallic molecular precursors (Ba-Nb oxalate- and Ni-Ta oxalate-based complexes!), in a specific ratio. A detailed X-rays powder diffraction study has been carried out in order to examine structural features of prepared oxide material containing two metals on both crystallographic sites (A and B) within perovskite lattice. The effect of thermal decomposition temperature on the microstructural parameters will be discussed as well.

There is a constant quest for novel metal-organic complexes with potential application in catalysis, conductivity and magnetism ; fields that are crucial for the development of modern materials science. While designing metal–organic coordination systems with specific properties, special attention is paid to the nature of metal ions and the geometry, flexibility and coordination preferences of the bridging ligands. Due to variety of coordination modes, rigidity and good donor ability of oxalate dianion, the oxalate chemistry of various transition metals was a subject of numerous studies. Moreover, this ligand is frequently used for the design and synthesis of molecule-based magnets, since it can mediate the magnetic exchange interaction between metal centres, when coordinated as a bridge between two paramagnetic metal atoms. (Clemente-León et al., 2011) A facile approach for the preparation of heteropolynuclear species with predetermined structures and spin topologies, known as “complexes as ligands” or “building-blocks chemistry”, involves the use of anionic mononuclear complexes as ligands towards a second metal ion. Good control over the reaction products is achieved by utilizing stable anionic complexes which are inert to ligand substitution, such as anionic chromium(III) complexes with potentially bridging oxalate groups. Moreover, the fact that chromium(III) ion is a spin carrier (S = 3/2) makes anionic chromium(III) complexes suitable for the preparation of heterometallic compounds with interesting magnetic features. (Marinescu et al., 2011) Following this rational synthesis approach we have focused on the use of [Cr(phen)(C2O4)2]– anion as building block (phen = 1, 10-phenanthroline) ; a versatile tecton in designing heterometallic oxalate-bridged compounds. Reaction of aqueous solutions of [Cr(phen)(C2O4)2]– anions with bis(phenanthroline)metal(II) cations (M = Mn, Co, Ni, Cu, Zn) resulted in a series of five new compounds 1–5, of general formula [{;M(phen)2};2(μ-C2O4)]0.5[Cr(phen)(C2O4)2]·2H2O [M2+ = Mn (1), Co (2), Ni (3), Cu (4), Zn (5)]. In addition to the single crystal X-ray diffraction studies, characterization of the new complexes was accomplished by means of IR and UV/Vis spectroscopy and magnetic measurements on SQUID magnetometer. References: Clemente-León, M., Coronado, E., Martí-Gastaldo, C. & Romero, F. M. (2011). Chem. Soc. Rev., 40, 473–497. Marinescu, G., Andruh, M., Lloret, F. & Julve, M. (2011). Coord. Chem. Rev. 255, 161–185.