Pt ^II coordination to N1 of 9-methylguanine: Why it facilitates binding of additional metal ions to the purine ring

The preparation and X-ray crystal structure analysis of {trans-[Pt(MeNH 2) 2(9-MeG-N1) 2]}·{3 K 2[Pt(CN) 4]}·6 H 2O (3 a) (with 9-MeG being the anion of 9-methylguanine, 9-MeGH) are reported. The title compound was obtained by treating [Pt(dien)(9-MeGH-N7)] 2+ (1; dien=diethylenetriamine) with trans-[Pt(MeNH 2) 2(H 2O) 2] 2+ at pH 9.6, 60 °C, and subsequent removal of the [(dien)Pt II] entities by treatment with an excess amount of KCN, which converts the latter to [Pt(CN) 4] 2-. Cocrystallization of K 2[Pt(CN) 4] with trans-[Pt(MeNH 2) 2(9-MeG-N1) 2] is a consequence of the increase in basicity of the guanine ligand following its deprotonation and Pt coordination at N1. This increase in basicity is reflected in the pK a values of trans-[Pt(MeNH 2) 2(9-MeGH-N1) 2] 2+ (4.4±0.1 and 3.3±0.4). The crystal structure of 3 a reveals rare (N7,O6 chelate) and unconventional (N2,C2,N3) binding patterns of K + to the guaninato ligands. DFT calculations confirm that K + binding to the sugar edge of guanine for a N1-platinated guanine anion is a realistic option, thus ruling against a simple packing effect in the solid-state structure of 3 a. The linkage isomer of 3 a, trans-[Pt(MeNH 2) 2(9-MeG-N7) 2] (6 a) has likewise been isolated, and its acid-base properties determined. Compound 6 a is more basic than 3 a by more than 4 log units. Binding of metal entities to the N7 positions of 9-MeG in 3 a has been studied in detail for [(NH 3) 3Pt II], trans-[(NH 3) 2Pt II], and [(en)Pd II] (en=ethylenediamine) by using 1H NMR spectroscopy. Without exception, binding of the second metal takes place at N7, but formation of a molecular guanine square with trans-[(Me 2NH 2)Pt II] cross-linking N1 positions and trans-[(NH 3) 2Pt II] cross-linking N7 positions could not be confirmed unambiguously, despite the fact that calculations are fully consistent with its existence. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.