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.

Ag₂SO₄ and H₂SO₄ (Riedel de Haën) were used as purchased. The precursor salt Ba_0.50[Ag₂Cr(C₂O₄)₃]・5H₂O was prepared as described in the literature [15] .

Powder of Ag₂SO₄ (0.16 g, 0.5 mmol) was added in successive small portions to a stirred aqueous solution (50 mL) of Ba_0.50[Ag₂Cr(C₂O₄)₃]・5H₂O (1.38 g, 2 mmol) acidified with 2 drops of concentrated H₂SO₄. After 2 hours of stirring at 70˚C, the mixture was cooled to room temperature and a white precipitate of BaSO₄ was filtered off. The resulting indigo-blue solution was allowed to evaporate in a hood at room temperature. Elongated violet needle-shaped crystals suitable for X-ray diffraction were formed within ten days. Anal. Calcd. for C₆H_10.50Ag_2.50CrO₁₇ (676.30 g mol⁻¹): C, 10.66; H, 1.56%. Found: C, 10.51; H, 1.48%. IR (KBr, cm⁻¹): ν = 3587 (w), 3246 (w), 1623 (s), 1378 (s), 1258 (w), 899 (w), 801 (s), 543 (s), 474 (s). UV-Vis (H₂O solution, nm): 430; 570.

A violet crystal with dimensions 0.38 × 0.32 × 0.20 mm³ 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-de- tector. 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.

Reaction of Ag₂SO₄ with Ba_0.50[Ag₂Cr(C₂O₄)₃]・5H₂O in an aqueous solution of H₂SO₄ acid (pH ≈ 3) afforded the compound Ag_2.50H_0.50[Cr(C₂O₄)₃]・5H₂O (1) as violet needles. This acidic medium may be responsible for the easy formation of the title compound. Compound 1 is an air-stable material which does not melt up to 250˚C.

The FTIR spectrum of 1 (Figure 1) shows two broad bands at 3587 cm⁻¹ and 3246 cm⁻¹ which are assigned to water molecules of hydration within the channels. The bands centered at 1623 cm⁻¹, 1378 cm⁻¹ and 1258 cm⁻¹ are attributed to ν(C=O), ν(C?O) and C-C stretches respectively, as expected for oxalate ligands [21] [22] . The sharp bands observed at 543 cm⁻¹ and 474 cm⁻¹ can be assigned to the Cr?O and Ag?O vibrations respectively [12] [15] .

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 ⁴A_2g(F) " ⁴T_1g(F) and ⁴A_2g(F) " ⁴T_2g(F) (d-d) transitions within the octahedral complex ions [Cr(C₂O₄)₃]³⁻ contained in 1 respectively [12] [13] [14] [15] . Obviously, the

present electronic absorption spectrum is virtually super imposable with that reported since the spectral informations thus obtained solely relate to the [Cr(C₂O₄)₃]³⁻ species.

The TG curve of 1 depicted in Figure 3 evidences three distinct weight losses in the temperature range 50˚C - 360˚C, the first process being endothermic and the two others exothermic. In the temperature range 80˚C - 100˚C, compound 1 suffers a first weight loss of 5.86% (calc. 5.32%) corresponding the release of two out of five water molecules of hydration. A second weight loss of 23.08% (calc. 23.07%) occurs between 263˚C and 320˚C, corresponding to the decomposition of the framework with release of 5/2 CO₂ + 3 H₂O. From 320˚C to 360˚C, a third weight loss of 4.40% (calc. 4.58%) is consistent with the release of 1/2 CO₂ + 1/2 H₂O, the final residue being a mixture of Ag₂O and Cr₂O₃.

The solid state structure of 1, Ag_2.50H_0.50[Cr(C₂O₄)₃]・5H₂O, has been established by single-crystal X-ray diffraction. Compound 1 crystallizes, like many other members of open-framework silver-deficient oxalate chromate(III) salts [11] [13] [23] [24] , in the monoclinic space group C2/c. The environment of the metallic centers in 1 is illustrated in Figure 4. Each Cr^III site (Figure 4(a)) is hexacoordinated by the hydroxylic O atoms (O2, O3, O6, O7, O9, O12) of three oxalate ligands to give the well-established pseudo-octahedral [Cr(C₂O₄)₃]³⁻ complex anion. The three oxalate ligands of the [Cr(C₂O₄)₃]³⁻ complex bridge to the three crystallographycally different Ag atoms Ag1, Ag2, and Ag3. The Ag1 site (Figure 4(b)) experiences distorted pentacoordination geometry, being linked in a monodentate manner to three oxalate O atoms (O1, O4, O9) and chelated by the oxalate O atoms (O5, O8). By contrast, Ag2 and Ag3 sites (Figure 4(c), Figure 4(d)) are six-coordinate. The Ag2 ion sits on an inversion center of a strongly distorted (2 + 2 + 2) octahedron involving two chelating oxalate O atoms (O10, O11) and two monodentate oxalate O atoms (O6). The Ag3 ion is chelated by an oxalate through O1 and O4 atoms and bonded in a monodentate manner to the oxalate O atoms O10, O12, O24 and a nonbonding Ag ・・・ Ag3ⁱⁱⁱ separation of 3.173(3) Å completes the hexacoordination around this site. Relevant interatomic distances and angles are given in Table 2. The Ag-O bond lengths and O-Ag-O bond angles fit well within the ranges of previous results [11] [12] [13] [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. The structure of 1 can be best described as a silver-deficient oxalatochromate(III) salt where one of the three Ag⁺ ions is partially replaced by H⁺, the anionic building blocks which delineate tubular channels being [Ag_2.50Cr(C₂O₄)₃]^0.50−. Thus, it is obvious that the fractional (0.50 H⁺) proton balancing the charge of the anionic building block is attached to a water molecule within the channels. The compound is therefore better formulated as [H_0.50H₂O]@[Ag_2.50Cr(C₂O₄)₃]・4H₂O. The protonated and neutral water molecules included in the channels are hydrogen-bonded to each other and to surrounding oxalate O atoms. The five 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.

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-

Symmetry transformations used to generate equivalent atoms for (1): (i) ?x + 1/2, −y + 1/2, −z + 1; (ii) ?x + 1/2, y + 1/2, −z + 1/2; (iii) ?x + 1, y, −z + 3/2; (iv) −x, y, −z + 1/2; (v) x−1/2, y−1/2, z; (vi) ?x + 1/2, y−1/2, −z + 1/2; (vii) x, −y, z + 1/2.

do-tubes are well isolated from one another by thick walls. The nanochannel structure of 1 is reminiscent of aluminosilicate zeolites [25] , hybrid open- framework iron phosphate-oxalates [26] [27] , the family of compounds with a variable Ag-Cr-oxalate channel lattice, formulated as [(M_xAg_0.50-x)(H₂O)₃]@ [Ag_2.50Cr(C₂O₄)₃] (0 ≤ x ≤ 0.50; M = K, Cs, Ag) [11] , and most importantly, the compound, Ag_11.60H_0.40[Cr(C₂O₄)₃]₄・15H₂O [12] . On comparing the structures of Ag_11.60H_0.40[Cr(C₂O₄)₃]₄・15H₂O [12] and Ag_2.50H_0.50[Cr(C₂O₄)₃]・5H₂O (1), the two main differences seem to be: a) the composition of their lattice grids: [Ag_2.90Cr(C₂O₄)₃]^0.10− in the former compound (silver charge deficit: 0.10 offset by 0.10 H⁺) and [Ag_2.50Cr(C₂O₄)₃]^0.50− in the latter (silver charge deficit: 0.50 offset by 0.50 H⁺); b) their space groups: P2₁/n for the former compound and C2/c for the present case. It is worth noting that the equivalent charge from 0.50 H⁺ compensating the silver charge deficit of 0.50 in compound 1 is, to the best of

^aox = dianion oxalate ().

our knowledge, the highest value observed so far for this family of materials. Structural data and compositions of a series of host lattice networks for some selected nanochannel Ag(I)/Cr(III)-oxalate complexes are summarized in Table 3, illustrating that these compounds predominantly crystallize in the monoclinic system, space group C2/c. A channel lattice network with the chemical composition [Ag₂Cr(C₂O₄)₃]⁻ was found in the structure of the compound Ba_0.5Ag₂ [Cr(C₂O₄)₃]・5H₂O (silver deficit: 1) [15] , yet, with excess negative charge (1−) compensated not by protons, but rather by an equivalent cationic charge from 0.5 Ba²⁺. In light of this latter result, it now appears likely that the proton content in Ag-Cr-oxalate channel lattices can be adjusted to the stoichiometric formula Ag₂H[Cr(C₂O₄)₃]・nH₂O [13] . Such materials with protons residing within “hydrogen-bonded water molecules” in the channels are expected to exhibit very high proton conductivity [28] - [33] .