MONONUCLEAR-DINUCLEAR EQUILIBRIUM FOR THE PYRIDINE ADDUCTS OF CHROMIUM(II) SACCHARINATES

The azeotropic removal of water by pyridine from [Cr(Sac)2(H2O)4].2H2O, Sac = the anion of saccharine, gives a solution with a mixture of a mononuclear and dinuclear chromium(II) saccharinate, the relative concentration of which varies with changes in temperature. Appropriate workup of the pyridine solution has provided crystals of [Cr2(SaC)4(PY)2].2py (1) and [Cr(SaC)2-(py)3].2py (2). Both compounds have been characterized by X-ray crystallography at -60-degrees-C. The chromium-to-chromium bond length in 1 (2.5911 (8) angstrom) is one of the longest ones known of any dichromium(II) dimer. Compound 1 crystallizes in the monoclinic space group Ia, with a = 11.705 (2) angstrom, b = 21.868 (2) angstrom, c = 20.649 (2) angstrom, beta = 98.87 (1)degrees, V = 5222 (2) angstrom3, and Z = 4. The structure of the five-coordinated compound 2 is bipyramidal and is easily envisioned as the result of breaking the metal-to-metal bond in 1 and replacing the O-bonded saccharinate groups by pyridine molecules. Compound 2 crystallizes in the monoclinic space group C2/c, with a = 13.347 (2) angstrom, b = 26.134 (2) angstrom, c = 11.802 (2) angstrom, beta = 110.541 (7)-degrees, V = 3855 (1) angstrom3 and Z = 4. Both compounds possess interstitial pyridine molecules that occupy channels formed in the crystal. Under appropriate conditions 1 and 2 are reversibly interconvertable both in the solid state and in pyridine solution.