Conformational flexibility within the chelate rings of [Pt(en)(CBDCA-O,O′)], an analogue of the antitumour drug carboplatin: X-ray crystallographic and solid-state NMR studies

Abstract

The X-ray crystal structure of [Pt(en)(CBDCA-O,O′)] 1, an analogue of the anticancer drug carboplatin, shows that platinum has an approximate square-planar coordination. The crystals are orthorhombic with space group Pnma. The Pt-CBDCA chelate ring adopts a flattened-boat conformation, similar to that found for carboplatin, and the Pt-ethylenediamine chelate ring exhibits both δ and λ conformations with equal populations. Ethylenediamine chelate ring inversion was observed by ¹³C CP/MAS NMR spectroscopy. The exchange rate and the activation free energy (ΔG^‡) at 317 K were determined to be ca. 415 s^-1 and 62 kJ mol^-1, respectively. The CBDCA ligand appears to have a direct effect on the dynamics of the en ring. Conformational flexibility of the CBDCA ring is also discussed. Such dynamic processes within chelated platinum complexes could play a role in the biological recognition of anticancer complexes.