SYNTHESIS, STRUCTURE, AND SPECTROSCOPIC CHARACTERIZATION OF BISMUTH CITRATE COMPOUNDS AND BISMUTH-CONTAINING ULCER HEALING AGENT COLLOIDAL BISMUTH SUBCITRATE (CBS) .3. CRYSTAL AND SOLUTION STRUCTURES OF K(NH4)[BI2(CIT)2(H2O)2](H2O)X (X=2,4)

Four new stoichiometrically closely-related bismuth complexes with citrate (cit4-) are synthesized by the reaction of bismuth citrate Bi3+ (Hcit3-) and potassium hydroxide in the presence of ammonia, they have also been obtained by recrystallization of commercially available colloidal bismuth subcitrate (CBS; a well-known ulcer healing agent). Compounds K(NH4)[Bi2(cit4-)2(H2O)2](H2O)4 (3a) and K(NH4)[Bi2(cit)2(H2O)2](H2O)4 (6) isolated as a mixture from aqueous solution containing bismuth citrate, potassium, and ammonium have been fully characterized by using three-dimensional X-ray analysis. Two other compounds K0.8(NH4)0.2Bi(cit)(H2O) (2c) and K0.6(NH4)0.4Bi(cit)-(H2O)2 (3b), which were isolated from CBS aqueous solutions, have been characterized on the basis of elemental analyses and spectroscopic data only. Compound 3a crystallizes in the space group C2/c with a = 16.860(4) angstrom, b = 12.395(2) angstrom, c = 10.328(3) angstrom, beta = 91.79(2)degrees, V = 2157.2 angstrom3, and Z = 8. The asymmetric unit, which contains a Bi(cit4-)(H2O) unit, is linked to another by the symmetry operation (1 - x, y, 1/2 - z) to form a dinuclear subunit with a Bi-Bi distance of 6.11 angstrom. The dinuclear subunit is paired to itself by an inversion center to form a tetranuclear aggregate. The resultant units are further interlinked by citrate bridges and hydrogen bond bridges to result in a complicated polymeric network. Compound 6 crystallizes in the space group P2(1)/n with a = 10.923(2) angstrom, b = 15.424(2) angstrom, c = 15.037(2) angstrom, beta = 105.67(1)degrees, V = 2439.3 angstrom3, and Z = 4. In this compound the dinuclear unit [Bi2(cit4-)2(H2O)2]2- from the asymmetric unit functions as the building block for the polymeric architecture. Although the mode of citrato bridging to yield the tetranuclear unit is the same as that of 3a, the different asymmetric unit structure compared to that of 3a is eventually reflected in the slight difference in the two polymeric network structures. The solution behavior of CBS samples has been investigated by using H-1 and C-13 NMR spectroscopy. At low pH (<7), rapid ligand exchange is observed and all citrates are averaged on the NMR time scale. At high pH the citrate are hardly coordinated to Bi(III) anymore. The behavior is the same as that of the previously reported ''[(cit4-)BiBi(cit4-)]2- aggregated polymer'', but different from that of the ''[Bi6O4(OH)(Cit)3(H2O)3]3- aggregated polymer''. On the basis of a comparison of the CBS solution behavior with that of CBS model complexes, it was concluded that solid samples of CBS which are amorphous could be formed by aggregation of the dinuclear building block [(cit4-)BiBi(cit4-)]2- through citrato bridges and hydrogen bonding.