Anionic Nanochanneled Silver-Deficient Oxalatochromate(III) Complex with Hydroxonium as Counter Ion: Synthesis, Characterization and Crystal Structure

Reaction of Ba0.50[Ag2Cr(C2O4)3]·5H2O with Ag2SO4 in an aqueous solution of sulfuric acid (pH ≈ 3) yielded the silver(I)/chromium(III) oxalate salt H0.50[Ag2.50Cr(C2O4)3]·5H2O (1). Compound 1 can be best described as an anionic silver-deficient oxalatochromate(III) complex [Ag2.50Cr(C2O4)3]0.5- with nanochannels containing hydrogen-bonded water molecules and protons. Thermal analyses show significant weight losses corresponding to the elimination of water molecules of crystallization followed by the decomposition of the network.

oxalate is one of the simplest connectors potentially able to bridge metal ions in the bidentate chelating manner. The use of molecular building units such as tris(oxalato)metalate(III) anions, [M III (C 2 O 4 ) 3 ] 3-, has opened up tremendous possibilities for research in this area, giving rise to new compounds exhibiting a variety of nanostructured features [9] [10].
In the course of the past few years, the tris-chelated chromium complex anion, [Cr(C 2  Recently, our research group reported a closely related channel lattice network with chemical composition [Ag 2.90 Cr(C 2 O 4 ) 3 ] 0.10− , where the silver charge deficit (0.10) per formula unit is solely compensated by an equivalent charge from 0.10 proton embedded amongst hydrogen-bonded water molecules within the channels [12]. Such systems with protons residing within "stagnating water streams" could be suitable to monitor proton transport processes in the solid state [12] [13] [14].
In the present work, we aimed to widen the scope of this family of nanochanneled coordination polymers, a special emphasis being set on increasing the amount of protons within the channels defined by the negative host lattice grid substantially. Herein, we report a novel open framework silver-deficient oxalatochromate(III) compound, Ag 2.50 H 0.50 [Cr(C 2 O 4 ) 3 ]·5H 2 O (1). Its host lattice grid has the chemical composition [Ag 2.50 Cr(C 2 O 4 ) 3 ] 0.50-, accusing a deficiency of 0.50 in Ag + ion per formula unit which is compensated by an equivalent charge from 0.50 H + , the highest amount of protons known so far for this family of materials.

Materials and Physical Measurements
Elemental analysis for carbon and hydrogen was performed on a Vario EL (Heraeus) CHNS analyzer. The infrared spectrum (4000 -400 cm −1 ) of the solid sample was recorded on a Perkin-Elmer 2000 FT-IR spectrometer as KBr disks. UV/Vis spectrum of the solution was measured on an AQUALYTIC spectrophotometer in the range 300 -800 nm. Thermal analyses (TGA and DSC) were performed with a Mettler Toledo TGA/SDTA 851 thermal analyzer. The powdered sample (ca. 15 mg) was heated in air atmosphere from 25˚C to 500˚C with a rate of 10˚C min −1 .

X-Ray Crystallography
A violet crystal with dimensions 0.38 × 0.32 × 0.20 mm 3 was taken directly out of the mother liquor, immersed in perfluorinated polyether, and fixed on top of a glass capillary. Graphite monochromated Mo Kα radiation (λ = 0.71073 Å) was used throughout. Intensity data were collected on a Siemens SMART CCD-detector. The temperature was set to 293 K. Data collection was performed by a hemisphere run taking 20 frames at intervals of 0.3˚ about ω. The data were corrected for Lorentz and polarization effects. A multi-scan absorption correction using the program SADABS [16] was performed for the compound. The structure was solved and refined by the direct methods and Fourier techniques. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. For structure solving and refinement the software package SHELXTL [17] [18] was used. The ORTEP-3 program [19] was used within the WinGX software package [20] to deal with the processed crystallographic data and artwork representations. Crystallographic data and refinement parameters are listed in Table 1.

Characterization of 1
The FTIR spectrum of 1 ( Figure 1 The UV-V is spectrum of 1 ( Figure 2) reveals two absorption bands at 430 nm (23,256 cm −1 ) and 570 nm (17,544 cm −1 ) which correspond to 4    present electronic absorption spectrum is virtually super imposable with that reported since the spectral informations thus obtained solely relate to the The TG curve of 1 depicted in Figure 3      [23] [24]. Figure 5 shows a lattice packing of the unit cell of 1 projected down [001] highlighting four halves of tubular channels filled with water molecules. of crystallization found in the crystal structure of 1 build water clusters inside the channels through hydrogen bonds shown as broken lines in Figure   6.

Crystal Structure of 1
A large cross-section of the three-dimensional structure packing of 1 projected down [001] is depicted in Figure 7, highlighting the fact that the pseu-