A homochiral tartrate-bridged dinuclear chromium(III) complex anion with a resonance-assisted hydrogen bond for proton conduction.

The synthesis of a homochiral building block based on L-tartrate-chromium(III) complex anions is reported. The dinuclear complex anion, which contains two bridging L-tartrate ligands and one aromatic N-donor ligand coordinated to chromium(III) ions, exhibits a boat conformation in which intramolecular resonance-assisted hydrogen bonding is present. The sodium L-tartrate-chromium(III) compound with the formula Na[Cr2(bpy)2(L-tart)2H]·9H2O (1) crystallizes from a methanol-water solution as a high water content material in the monoclinic space group P2. The as-synthesized compound is only stable at high relative humidity and undergoes structural transformations during drying, which are accompanied by water loss. However, these transformations are reversible and upon wetting, the material returns to its high water content structure. Based on a combination of experimental techniques (PXRD, in situ ATR-FTIR and EPR spectroscopy), the structure of the complex anions appears to be insensitive to the humidity variable processes (wetting and drying). Due to the presence of several hydrogen acceptor and donor groups in the L-tartrate-chromium(III) complex anion, we investigated the proton transport properties of a sodium L-tartrate-chromium(III) compound by impedance spectroscopy under dry and wet conditions at different temperatures. Since the relative humidity affects the structural transformations in this system, it also has a large influence on the proton conductivity, which varies by up to four orders of magnitude depending on the degree of hydration. These results confirm that the proton conductivity can be tuned in flexible structures in which non-covalent interactions determine the crystal packing.

Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium Vanadyl Tartrate System.

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.

Light-Induced Intramolecular Electron Transfer in a 1D [CuFe] Coordination Polymer Containing the [Fe(C2O4)3]3- Core.

A one-dimensional (1D) ladder-like coordination polymer {NH4[{Cu(bpy)}2(C2O4)Fe(C2O4)3]·H2O}n (1; bpy = 2,2'-bipyridine) containing [Cu(bpy)(µ-C2O4)Cu(bpy)]2+ cationic units linked by oxalate groups of [Fe(C2O4)3]3- building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to [CuII(bpy)(µ-C2O4)CuII(bpy)]2+, leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO2.

From a square core to square opening: structural diversity and magnetic properties of the oxo-bridged [CrIIINbV] complexes.

Three heterometallic oxo-bridged compounds, [Cr2(phen)4(µ-O)4Nb2(C2O4)4]·2H2O (1; phen = 1,10-phenanthroline), [Cr2(terpy)2(H2O)2(µ-O)4Nb2(C2O4)4]·4H2O (2; terpy = 2,2';6',2''-terpyridine) and [Cr(terpy)(C2O4)(H2O)][Cr2(terpy)2(C2O4)2(µ-O)2Nb(C2O4)2]·3H2O (3), have been synthesized using a building block approach and characterized by IR spectroscopy, single-crystal and powder X-ray diffraction, magnetization measurements, and DFT calculations. The molecular structures of 1 and 2, crystallizing in P42212 and P21/n space groups, respectively, contain a square-shaped {Cr(µ-O)4Nb} unit, while that of complex salt 3 (P1[combining macron] space group) consists of a mononuclear cation containing CrIII and trinuclear anions in which two CrIII ions are bridged by a -O-NbV-O- fragment. Besides hydrogen-bonding patterns resulting in a 1D- or 3D-supramolecular arrangement in 1-3, an unusual intermolecular contact has been noticed between parallel oxalate moieties occurring due to the electrostatic attraction of electron-rich carbonyl oxygen and severely electron-depleted carbon atoms in the crystal packing of 2. The antiferromagnetic coupling observed in all three compounds, determined from magnetization measurements (J = -13.51(2), -8.41(1) and -7.44(4) cm-1 for 1, 2 and 3, respectively) and confirmed by DFT calculations, originates from two CrIII ions with spin 3/2 interacting through diamagnetic -O-NbV-O- bridge(s).

The influence of metal centres on the exchange interaction in heterometallic complexes with oxalate-bridged cations.

The reaction of bis(phenanthroline)metal(ii) cations (M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) with bis(oxalato)chromium(iii) anions in a water/ethanol solution gives rise to a series of compounds with oxalate-bridged cations, [{M(phen)2}2(µ-C2O4)][Cr(phen)(C2O4)2]2·4H2O [Mn2Cr2 (1), Co2Cr2 (2), Ni2Cr2 (3), Cu2Cr2 (4) and Zn2Cr2 (5)]. Their structural analysis reveals that all the prepared compounds crystallize in the triclinic system, space group P1[combining macron], having similar unit cell parameters, molecular structures and crystal packing features. All metal centres in 1-5 are octahedrally coordinated: M2+ in homodinuclear cations are coordinated with two phen molecules and one bridging oxalate ligand; Cr3+ in anions is coordinated with one phen ligand and two bidentate oxalate groups. The copper atom in Cu2Cr2 (4) exhibits a Jahn-Teller-distorted octahedral coordination. Owing to the considerable number of pyridyl groups present in 1-5 (from phen ligands) the crystal packing of cations and anions is driven by stacking interactions appearing in offset-face-to-face (OFF) and edge-to-face (EF) orientations. The hydrogen bonds between the anions and water molecules of crystallization form 1D ladder-like motifs. In addition to the single crystal X-ray diffraction studies, the characterization of the new complexes was accomplished by means of IR and UV/Vis spectroscopy and magnetization measurements on a SQUID magnetometer. The temperature dependence of magnetic susceptibility reveals different magnetic super exchange interactions taking place in homodinuclear oxalate-bridged cations depending on the transition metal centre (Mn2+, Co2+, Ni2+ and Cu2+). Oxalate ligands mediate the ferromagnetic coupling of Cu2+ metal cations in Cu2Cr2 (4), whereas in Mn2Cr2 (1), Co2Cr2 (2) and Ni2Cr2 (3), antiferromagnetic interactions are observed between Mn2+, Co2+ and Ni2+ cations, respectively. Also, relatively large zero-field splitting parameters for Cr3+ ions (from mononuclear anions), D ≈ 1 cm-1, were observed.

Experimental and Theoretical Investigation of the Anti-Ferromagnetic Coupling of CrIII Ions through Diamagnetic -O-NbV-O- Bridges.

The synthesis and properties of a novel hetero-tetranuclear compound [Cr2(bpy)4(µ-O)4Nb2(C2O4)4]·3H2O (1; bpy = 2,2'-bipyridine), investigated by single-crystal X-ray diffraction, magnetization measurements, IR, UV/visible spectroscopy, electron paramagnetic resonance (EPR; X- and Q-bands and high-field), and density functional theory (DFT) calculations, are reported. Crystal structure of 1 (orthorhombic Pcab space group) consists of a square-shaped macrocyclic {Cr2(µ-O)4Nb2} core in which CrIII and NbV ions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular CrIII···CrIII distances through the -O-NbV-O- bridges are 7.410(2) and 7.419(2) Å, while diagonal separation is 5.406(2) Å. The temperature dependence of magnetization M(T) evidences an anti-ferromagnetic ground state, which originates from a magnetic interaction between two CrIII ions of spin 3/2 through two triatomic -O-NbV-O- diamagnetic bridges. A spin Hamiltonian appropriate for polynuclear isolated magnetic units was used. The best-fitting curve for this model is obtained with the parameters gCr = 1.992(3), J = -12.77(5) cm-1, and |D| = 0.17(4) cm-1. The CrIII···CrIII dimer model is confirmed by EPR spectra, which exhibit a pronounced change of their shape around the temperature corresponding to the intradimer coupling J. The EPR spectra simulations and DFT calculations reveal the presence of a single-ion anisotropy that is close to being uniaxial, D = -0.31 cm-1 and E = 0.024 cm-1.