Platinum-Modified Adenines: Unprecedented Protonation Behavior Revealed by NMR Spectroscopy and Relativistic Density-Functional Theory Calculations

Two novel Pt^IV complexes of aromatic cytokinins with possible antitumor properties were prepared by reaction of selected aminopurines with K₂PtCl₆. The structures of both complexes, 9-[6-(benzylamino)­purine] pentachloroplatinate (IV) and 9-[6-(furfurylamino)­purine] pentachloroplatinate (IV), were characterized in detail by using two-dimensional NMR spectroscopy (¹H, ¹³C, ¹⁵N, and ¹⁹⁵Pt) in solution and CP/MAS NMR techniques in the solid state. We report for the first time the X-ray structure of a nucleobase adenine derivative coordinated to Pt^IV via the N9 atom. The protonation equilibria for the complexes in solution were characterized by using NMR spectroscopy (isotropic chemical shifts and indirect nuclear spin–spin coupling constants) and the structural conclusions drawn from the NMR analysis are supported by relativistic density-functional theory (DFT) calculations. Because of the presence of the Pt atom, hybrid GGA functionals and scalar-relativistic and spin–orbit corrections were employed for both the DFT calculations of the molecular structure and particularly for the NMR chemical shifts. In particular, the populations of the N7-protonated and neutral forms of the complexes in solution were characterized by correlating the experimental and the DFT-calculated NMR chemical shifts. In contrast to the chemical exchange process involving the N7–H group, the hydrogen atom at N3 was determined to be unexpectedly rigid, probably because of the presence of the stabilizing intramolecular interaction N3–H···Cl. The described methodology combining the NMR spectroscopy and relativistic DFT calculations can be employed for characterizing the tautomeric and protonation equilibria in a large family of transition-metal-modified purine bases.