Platinum Complexes with Only One Purine Ligand (Guanine, Deoxyguanine, or Adenine) Flanked by Two cis‐NH(CH3) Groups – Informative Models for Assessing the Interaction of Purine C6 Substituents with cis‐Amines

Abstract

Abstractsyn‐(R,S)‐Me3dienPtL complexes (Me3dien = N,N',N‐trimethyldiethylenetriamine; L = a guanine, a 6‐deoxyguanine, or an adenine derivative) represent useful models for assessing the influence of the purine C6 substituent on the rate of rotation about the Pt–N7 bond. Because of the unsymmetric nature of the syn‐(R,S)‐Me3dien carrier ligand with respect to the coordination plane (all N–Me groups on one side of this plane), two rotamers are possible; they are defined as endo or exo for the six‐membered ring of the purine on the same or opposite side of the coordination plane as the N–Me groups, respectively. The derivatives of guanine (Pen), 6‐deoxyguanine (deoxy‐Pen), and adenine (MeA) used are distinguished by their C6 substituent (O, H, or NH2, respectively). At ambient temperature the rate of rotation of the purine about the Pt–N7 bond is slow on the NMR time scale for all three complexes. However, for Pen and deoxy‐Pen, an increase of the temperature from 293 to 353 K led to coalescence of the 1H NMR signals, indicating fast interconversion between rotamers and allowing evaluation of the rate constants and activation parameters (ΔH≠ and ΔS≠) for rotation. In contrast, for MeA raising the temperature to 353 K caused only a very slight broadening of the NMR signals, thus precluding a kinetic analysis of this complex by NMR methods. The rate of interconversion between the rotamers was comparable for the Pen and deoxy‐Pen complexes, notwithstanding the greater bulk of the C6 substituent in the Pen complex. On the other hand, the far slower rate of rotation for MeA, as compared to Pen, cannot be explained solely on the basis of C6 substituent bulk. Activation parameters for rotation (average values for the two rotamers: ΔH≠ = 57 ± 4 and 108 ± 3 kJ·mol–1, ΔS≠ = –18 ± 11 and 170 ± 11 J·K–1·mol–1 for Pen and deoxy‐Pen, respectively) suggest a plausible explanation for the observed trend. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)