Metal Complexes of Theophylline-7-acetic Acid. Crystal Structure of a Nickel(ii) Compound Containing Non-coordinated Theophylline-7- Acetate Ion

-The solids obtained by interaction of theophylline-7-acetic acid with the metal ions nickel(II), cobalt(II) and paUadium(II) have been studied by standard thermal and spectroscopic methods. The structure of the compound formed with nickel(II), hexaaqua nickel(II) theophylline-7-acetate tetrahydrate, has been solved by X-ray diffraction, this being the first example of a metal salt of an anionic purine derivative in which there is not a direct metal-purine bond. Powder X-ray diffraction demonstrates that the cobalt compound is isostructural to the nickel one. A trans-square planar structure with the ligand bound to the palladium via N(9) is proposed for the palladium compound [PdC12(theophylline-7-acetic acid)z]. The study of the behaviour of theophylline (1,3dimethyl-2,6-dioxo-purine) and its derivatives as ligands can be useful for modelling metal interactions involving the purine base of nucleic acids, since theophylline may be considered as a model compound for the nucleoside guanosine. N(7) has been found as the main coordination site in most of the crystal structures solved for metal complexes of this ligand, l'z with 0(6) usually involved in hydrogen bond interactions or weak interaction with the metal, as also happens with the copper complex of a theophylline derivative recently described by Colacio et al. 3 [distances Cu--N(7) and Cu--O(6), 1.999(2) and 2.825(2) A]. There are two cases in which the distance metal0(6) is short enough to be considered as a clear * Author to whom correspondence should be addressed. 61 N(7)--O(6) chelate, theophylline being bound in this way to titanocene 4 [Ti--O(6)= 2.211(3) /~] and to trimethylplatinum 5 [Pt--O(6) = 2.34(2) A] ; in the latter an additional interaction to a neighbouring platinum atom via N(9) defines hexameric units. Finally, an unusual N(9) monodentate coordination has been found in the compound [Rh (acetato)z(theophylline)]2.6 Under sufficiently acidic conditions, protonation of theophylline in N(9) may occur and three crystal structures have been reported including this cation and tetrachloroplatinate(II), trichloro(theophylline)platinate(II) and tetrabromopalladate(II) as counteranions. 7'8 Theophylline-7-acetic acid (TPAH) is an interesting theophylline derivative, since the N(7) position is blocked. Extended Hfickel MO calculations on this ligand point to N(9) as the most probably binding site. 9 Several palladium(II) complexes with 612 J. M. SALAS et al. the T P A anion have been described and dimeric structures with the ligand bridging via N(9) and the carboxylate group have been proposed. 9 The present paper is devoted to the thermal, spectroscopic and structural studies of the solids obtained by the interaction of theophylline-7-acetic acid with the metal ions nickel(II), cobalt(II) and palladium(II). The crystal structure of the nickel compound, involving non-coordinated T P A anions, is described. E X P E R I M E N T A L Preparation of the complexes Theophylline-7-acetic acid (TPAH) was purchased from Lancaster Synthesis. Its crystalline monohydrate is easily obtained by recrystallization. Hexaaqua nickel(II) theophylline-7-acetate tetrahydrate (I) Ni(NO3)2' 6H20 (0.5 mmol, 0.145 g) was dissolved in the minimum amount of water and added to 15 cm 3 of an aqueous solution of TPAH (1 mmol, 0.238 g) containing 1 cm 3 of concentrated ammonia. A pale green precipitate is immediately obtained, that was filtered off. From the mother liquor, after 1 day at room temperature, pale green crystals of the same compound, suitable for X-ray diffraction were isolated. Found : C, 30.4 ; H, 5.4 ; N, 16.0. Calc. for C18H38NsOIsNi : C, 30.4; H, 5.4; N, 16.1%. Hexaaqua cobalt(II) theophylline-7-acetate tetrahydrate (II) The same procedure described previously for the analogous nickel compound was performed, starting with COC12"6H20. Pale pink crystals were obtained. Found: C, 30.5; H, 5.4; N, 16.0. Calc. for CI8H38NsOI8Co : C, 30.3; H, 5.3; N, 15.7%. Dichloro bis(theophylline-7-acetic acid) palladium(II) (III) [NH4]2[PdCI4] (1 mmol, 0.248 g) dissolved in 20 cm 3 of water was added to 40 cm 3 of an aqueous solution containing TPAH (2 mmol, 0.476 g) and 1.5 cm 3 of concentrated HC1. A yellow precipitate immediately appeared, which was washed with cold water and air dried. Found: C, 32.7; H, 3.0; N, 17.0. Calc. for CIsHz0N808PdC12: C, 33.0; H, 3.1; N, 17.1%. Physical measurements Microanalyses of carbon, hydrogen and nitrogen were performed with a Perkin-Elmer 240C analyser at the Technical Services of the University of Granada; IR spectra were recorded on a Perkin-Elmer 983G spectrometer, using KBr and polyethylene as dispersing agents. Diffuse reflectance spectra were obtained on a Pe rk i~Elmer Lambda 19 spectrometer. TG and DSC studies were carried out on a Mettler TA-3000 instrument equipped by a Mettler TG-50 thermobalance and a DSC-20 differential calorimeter, using sample weights ranging from 1.6 to 16 mg, with heating rates of 20 and 2°C min 1 (TG) and 10 and 2°C min -1 (DSC) in an atmosphere of pure air.