Macrochelation, cyclometallation and G-quartet formation: N-3- and C-8-bound Pd-II complexes of adenine and guanine

The reactions of Pd-II ions with a series of chelate-tethered derivatives o f adenine and guanine have been studied and reveal a difference in the reac tivity of the purine bases. Reactions of [PdCl2(MeCN)(2)] and A-alkyl-enH . Cl (alkyl=propyl or ethyl, A=adenine, en=ethylenediamine) yield the monoca tionic species [PdCl(A-N3-Et-en)](+) (1) and [PdCl(A-N3-Pr-en)](+) (2). Bot h involve co-ordination at the minor groove site N3 of the nucleobase as co nfirmed by single-crystal X-ray analysis. Reactions with the analogous G-al kyl-enH . Cl derivatives (G=guanine, alkyl = ethyl or propyl) were more com plex with a mixture of species being observed. For G-Et-enH . Cl a product was isolated which was identified as [PdCl(G-C8-Et-en)](+) (3). This compou nd contains a biomolecular metal-carbon bond involving C8 of the purine bas e. Crystallography of a product obtained from reaction of G-Pr-enH . Cl and [Pd(MeCN)(4)][NO3](2) reveals an octacationic tetrameric complex (4), in w hich each ligand acts to bridge two metal ions through a combination of a t ridentate binding mode involving the diamine and N3 and monodentate coordin ation at N7.