Pt(ii) complexes of unsubstituted guanine and 7-methylguanineElectronic supplementary information (ESI) available: Additional tables and figures. CCDC reference numbers 731729–731734 (1a, 1b, 3–6). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b912701a

In solution, unsubstituted guanine (GH2) is in a tautomeric equilibrium between 1H,7H-keto and 1H,9H-keto forms. Upon coordination with (dien)PtII, N9-bound mononuclear complex is preferentially formed. With excess (dien)PtII, the dinuclear complex [{Pt(dien)}2(GH-N7,N9)]3+is formed at basic pH. The X-ray crystal structure analysis of mononuclear complexes [Pt(dien)(GH2-N9)](ClO4)2·2.25H2O (1a), [Pt(dien)(GH2-N9)](Cl)(ClO4) (1b), and the dinuclear complex [{Pt(dien)}2(GH-N7,N9)](ClO4)3(3) are reported. The N7 linkage isomer [Pt(dien)(GH2-N7)]2+(2) could not be isolated and was tentatively assigned in solution. In HCl-acidic medium, the dinuclear complex 3dissociates to form N9 bound mononuclear complex (1) as the major product and presumably the N7 linkage isomer (2) as a minor side product. Species 1and 3have been identified by the pD dependence of the 1H NMR guanine resonances. pKavalues (calculated for H2O) for deprotonation of the N9 linkage isomer (1a, 1b) are 6.51 ± 0.01 and 10.53 ± 0.05 as well as 0.52 ± 0.07 and 8.55 ± 0.06 for the dinuclear complex [{Pt(dien)}2(GH-N7,N9)]3+(3). Ab initiocalculations have been performed for [Pt(dien)(GH2-N7)]2+, [Pt(dien)(GH2-N9)]2+and [{Pt(dien)}2(GH-N7,N9)]3+. They reveal that in gas phase the N7 linkage isomer is more stable than the N9 linkage isomer by ΔE= 8.4 kcal mol−1. Reactions with (dien)PtIIand cis-(NH3)2PtIIwere also carried out to study the metal binding behaviour of the ligand, 7-methylguanine (7-MeGH). The complexes [Pt(dien)(7-MeGH-N9)](NO3)(ClO4) (4), cis-[Pt(NH3)2(7-MeGH-N9)2]Cl2·2H2O (5) and cis,cis-[{PtCl(NH3)2}2(7-MeGH-N3,N9)]Cl2·3H2O (6) have been characterized by X-ray crystallography. pKavalues for 4are < 2 and 8.16 ± 0.04, and for 51.86 ± 0.45 and 8.00 ± 0.08 (average for deprotonation of two 7-MeGH ligands). Structural peculiarities of N9 platinated guanine ligands are discussed and pKavalues of Pt complexes with related guanine nucleobases are compared. [ABSTRACT FROM AUTHOR]