Synthesis and Structure of trans-Dichloro-tetra(pyrazole)-gallium(III) Chloride and Tetrachlorogallate(III)

There is current interest in the bioinorganic chemistry of gallium owing to the extensive use of the element in medicine both as a therapeutic and diagnostic agent [1]. After numerous studies of simple gallium(III) salts [2, 3] recent investigations have focussed on coordination compounds of gallium halides with bio-relevant ligands including also many of the common nitrogen heterocycles [3, 4]. These complexes show high water solubility in the near-physiological pH range and good carrier properties for gallium. Among these nitrogen heterocycles, pyrazole has attracted special attention because of its versatile donor properties for the alkaline earth and earth metals of groups II and XIII. Surprisingly, in the previous studies only a pyrazole complex of GaCl3 with a 2:1 ligandto-metal ratio has been isolated [4]. This deficit of pyrazol ligands leads to a ligand distribution [GaCl2(pyrazole)4][GaCl4] (1) with a tetrachlorogallate(III) counterion. However, the anion is susceptible to hydrolysis giving rise to galenic complications and general limitations to applications. In the present work we report the preparation and the determination of the structure of the complex obtained from reactions with an excess of pyrazole. The product has the stoichiometry [GaCl2(pyrazole)4]Cl (2), and its crystals are free of solvate molecules. The tetrachlorogallate(III) salt [GaCl2(pyrazole)4] GaCl4ÿ (1) was also obtained as single crystals and its structure determined. Treatment of a solution of anhydrous GaCl3 in diethyl ether with a solution of five equivalents of