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The reactions of Pd(II) ions with a series of chelate-tethered derivatives of adenine and guanine have been studied and reveal a difference in the reactivity of the purine bases. Reactions of PdCl2(MeCN)2 and A-alkyl-enH x Cl (alkyl = propyl or ethyl, A adenine, en = ethylenediamine) yield the monocationic species PdCl(A-N3-Et-en)+ (1) and PdCl(A-N3-Pr-en)+ (2). Both involve co-ordination at the minor groove site N3 of the nucleobase as confirmed by single-crystal X-ray analysis. Reactions with the analogous G-alkyl-enH x Cl derivatives (G=guanine, alkyl = ethyl or propyl) were more complex with a mixture of species being observed. For G-Et-en HCI a product was isolated which was identified as PdCl(G-C8-Et-en)+ (3). This compound contains a biomolecular metal-carbon bond involving C8 of the purine base. Crystallography of a product obtained from reaction of G-Pr-enH x Cl and Pd(MeCN)4NO32 reveals an octacationic tetrameric complex (4), in which each ligand acts to bridge two metal ions through a combination of a tridentate binding mode involving the diamine and N3 and monodentate coordination at N7.
Price et al. (Fri,) studied this question.