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Dalton Transactions

The international journal for inorganic, organometallic and bioinorganic chemistry



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Dalton Trans., 2010, 39, 73 - 84, DOI: 10.1039/b912701a

Pt(II) complexes of unsubstituted guanine and 7-methylguanine

Deepali Gupta, Ralf Nowak and Bernhard Lippert

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 3 dissociates to form N9 bound mononuclear complex ( 1) as the major product and presumably the N7 linkage isomer ( 2) as a minor side product. Species 1 and 3 have been identified by the pD dependence of the 1H NMR guanine resonances. pKa values (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 initio calculations 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 E = 8.4 kcal mol-1. Reactions with (dien)PtII and cis-(NH3)2PtII were 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. pKa values for 4 are < 2 and 8.16 ± 0.04, and for 5 1.86 ± 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 pKa values of Pt complexes with related guanine nucleobases are compared.

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