SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF PYRAZOLE-BRIDGED METALLA-BIS(DICARBOLLIDE) DERIVATIVES OF COBALT, NICKEL, COPPER, AND IRON - MODELS FOR VENUS FLYTRAP CLUSTER REAGENTS

The synthesis and characterization of a family of pyrazole-bridged metallacarborane clusters is described. These species serve as nonradioactive models for the corresponding radio-transition-metal carriers which are potentially useful for the antibody-mediated gamma-imaging or beta-therapy of tumors (Venus flytrap clusters). Both monofunctional and bifunctional chelate precursors were prepared from the reactions of the anions of pyrazole and 4-carbomethoxypyrazole, respectively, with 2 equiv of closo-1,8-C2B9H11. In each instance, meso- and dl-isomers of the pyrazole-bridged nido-carboranes were obtained. The diastereomers were then converted, in the presence of stoichiometric amounts of aqueous base, to the formal 7,9-bis(dicarbollide) anions. The metal ions Co3+, Ni3+, Cu3+, and Fe3+ were incorporated into the unsubstituted pyrazole derivative in aqueous media at pH 12-14, resulting in the mixed meso- and dl-metallacarborane derivatives of 7, 8, 9, and 10, respectively. The carbomethoxy pyrazole dicarbollide derivative was complexed with Co3+ and Co-57(3+) in a similar manner to produce mixed meso- and dl-cobaltacarborane derivatives of 11 and 12, respectively. All diastereomeric metallacarborane derivatives were separated by column chromatography or HPLC techniques and characterized by spectroscopic and electrochemical techniques. The molecular structures of dl-Co3+ (7a), dl-Ni3+ (8a), meso-Ni3+ (8b), meso-Cu3+ (9b), dl-Fe3+ (10a), and meso-Co3+ (11b) have been determined from single-crystal X-ray diffraction experiments, and the structures of the diastereomeric complexes are correlated with H-1 and B-11 FT NMR spectra. dl-7a crystallizes in the monoclinic space group P2(1)/c with a = 10.618 (1) angstrom, b = 13.366 (1) angstrom, c = 14.326 (2) angstrom, beta = 109.925 (3)-degrees, V = 1901 angstrom3, and Z = 4. dl-8a crystallizes in the orthorhombic space group Pnma with a = 13.6874 (7) angstrom, b = 15.8587 (8) angstrom, c = 11.2429 (6) angstrom, V = 2447 angstrom3, and Z = 4. meso-8b crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 7.2139 (5) angstrom, b = 13.404 (1) angstrom, c = 19.768 (2) angstrom, V = 1908 angstrom3, and Z = 4. meso-% crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 7.1854 (8) angstrom, b = 13.434 (2) angstrom, c = 19.980 (2) angstrom, V = 1925 angstrom3, and Z = 4. dl-10a crystallizes in the monoclinic space group P1(1)/c with a = 10.6926 (5) angstrom, b = 13.3320 (7) angstrom, c = 14.3288 (7) angstrom, beta = 110.081 (1)-degrees, V = 1952 angstrom3, and Z = 4. meso-11b crystallizes in the triclinic space group P1BAR with a = 9.966 (2) angstrom, b = 10.475 (2) angstrom, c = 17.134 (3) angstrom, alpha = 79.957 (7)-degrees, beta = 77.882 (7)-degrees, gamma = 74.732 (7)-degrees, V = 1674 angstrom3, and Z = 2. Spectroscopic and structural data for the bridged complexes are compared with those for the corresponding unsubstituted metallacarborane derivatives.