Two Quadruply Bonded Amino Acid Complexes of Dichromium(II): Structures of Tetrakis(glycine)tetrabromodichromium(II) Tetrahydrate and Tetrakis(glycine)tetrachlorodichromium(II) Trihydrate

Glycine in oxygen-free aqueous acid solutions of chromous halides coordinatesin its zwitterionic form, ^-O₂CCH₂NH₃⁺, to give carboxylate-bridged Cr₂⁴⁺ complexes. These complexes precipitate from concentrated solutions as air-stable hydrated purple crystallinesalts of the general formula Cr₂(O₂CCH₂NH₃)₄X₄-nH₂O, and the structures of two of these compounds (1, X = Br,n = 4; 2, X = Cl,n = 3) were investigated by single-crystal X-ray diffraction techniques. Crystals of 1 belong to the triclinic space group PĪ with 1 formula weight in a cell of dimensions a = 8.455 (1) Å, b = 11.340 (2) Å, c = 6.799 (1)Å, α = 98.74 (3)°, β = 104.38 (2)°, γ = 77.68 (3)°, and V = 613 (1) Å3. Least-squares refinement of 1751 reflections with I > 3σ(I) produced final discrepancy factors of R₁ = 0.036 and R₂ = 0.042. The structure consists of centrosymmetric [Cr₂(O₂CCH₂NH₃)₄]⁴⁺ cations approximating Dih symmetry and containing a Cr-Cr bond of 2.513 (1) Å in length, axial Br^- ions at 2.736 (1) Å from the Cr atoms and nearly collinear to the Cr-Cr bond, and another unique Br^- ion and two unique water molecules that reside on general positions in the unit cell. The zwitterionic glycine molecules form intramolecular hydrogen bonds averaging 2.69Å in length between the terminal nitrogen atom of the NH₃⁺ group and one of the bridgingcarboxylate oxygen atoms. Compound 2 crystallizes in the monoclinic space group P2₁/c with 2 formula units in a cell with dimensions a = 8.325 (1) Å, b =9.307 (1) Å, c = 14.675 (2) Å,β 103.27 (3)°, and V = 1107(1) ų. This compound is isomorphous to the corresponding molybdenum complex. Refinement of the structure by least-squares methods using 1187 reflections with I> 3σ (I) gave final discrepancy indices of R₁ = 0.062 and R₂ = 0.072. The [Cr₂(O₂CCH₂NH₃)₄]⁴⁺ cations reside on crystallographic centers of inversion with a slightly longer Cr-Cr bond length than in 1 with a value of 2.524 (1) Å. One of the two Cl^- ions in the asymmetric unit occupies the axial position of the centrosymmetric dichromium cation with a Cr-Cl bond distance of 2.581 (1) Å. The other Cl^- ion resides on a general position in the unit cell with hydrogen-bonding interactions between the NH₃⁺ groups of the glycine ligands and the water molecules that also are found in general positions in the unit cell. As was seen in 1 intramolecular hydrogen bonding occurs within the glycine ligands in 2 with an average N. O distance of 2.70 Å.