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Studies on the coordination chemistry of methylated xanthines and their imidazolium salts. Part 1: Benzyl derivatives

Abstract:

New imidazolium salts derived from the natural methylated xanthines theophylline, theobromine and caffeine, namely 1,3-dimethyl-9-benzylxanthinium bromide (tphBzBr, 1a), 3,7-dimethyl-9-benzylxanthinium bromide (tbrBzBr, 2a) and 1,3,7-trimethyl-9-benzylxanthinium bromide (caffBzBr, 3a), are reported. Also, the disubsti- tuted analog of 1a, 1,3-dimethyl-7,9-dibenzylxanthinium bromide (tphBz2Br, 1a′) was identified and characterized by NMR. The coordination chemistry of ligands 1a-3a toward palladium, and some theoretical aspects of the unmodified theophylline, theobromine and caffeine are studied. Our results prove that the theophylline derivative has the thermodynamic tendency to form N-bonded species, even when an equilibrium between the Pd-NHC and the "theophyllinate" was observed spectroscopically, due to the anisotropy of the NHC ligand. To confirm the N- coordination, the solid state structure of the new "theophyllinate" species PdBr2(tphBz- H)2 (4), derived from 1a, was determined by X-ray diffraction. The analog with theobromine, ligand 2a, coordinates to palladium via N1, in an analogous manner to 1a, and a mixture of the cis/trans isomers of its palladium complex is obtained. On the other hand, since there is no possibility of N-coordination in 3a, this caffeine derivative forms a Pd-NHC compound after deprotonation with a strong base. Both the theoretical results and the experimental evidence are in accordance, in terms of the predicted coordination sites or possibility of modification of the selected methylated xanthines to obtain new ligands. © Springer Science+Business Media B.V. 2009.