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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. [ABSTRACT FROM AUTHOR]

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 {CrIII2(μ-O)4NbV2} unit, while that of complex salt 3 (P1̅ 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). [ABSTRACT FROM AUTHOR]

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 P1̅, 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. [ABSTRACT FROM AUTHOR]