NMR and X-ray Structural Characterization of a Cisplatin Analogue Able To Slow Down the Pt−N7 Rotation of a Coordinated Guanine Base by a Billion-Fold Times: 2,2′-Bipiperidine(dimethylmalonato)platinum(II) Complex

Abstract

The synthesis and the NMR and X-ray structural characterization of a cisplatin analogue designed to reduce the Pt−N7 rotation of a coordinated guanine base by a billion times are reported. The [Pt(dmm){(±)-bip}] (dmm = dimethylmalonato; bip = 2,2′-bipiperidine) complex crystallizes in the C2/m space group, which contemplates a mirror plane bisecting the bip and dmm ligands. Because the bip moiety (R,R or S,S configuration at the 2,2′-carbon atoms) does not have planes of symmetry, the requirements of the crystal symmetry are satisfied by a statistical disorder made of bip molecules of R,R or S,S configurations alternating at the same crystallographic site. Such an unexpected arrangement has been permitted by a “quasi planarity” of the bip ligand [maximum deviation from the mean plane through the C and N atoms of 0.2927(9) Å], which allows bip molecules of different chiralities to fit in the same space. The bip array of heavy atoms is overlaid, from both sides, by a layer of “quasi axial” (C)H and (N)H atoms (six per side). Those on one side are hydrogen-bonded to the dmm oxygen atoms of another complex molecule joined in a pair. The distance between the average platinum coordination planes is as short as 3.498(1) Å, comparable to those found in crystals of the [PtCl₂(bipy)] complex (bipy = 2,2′-bipyridine) and of graphite, in which, however, all atoms of each unit are rigorously coplanar and there are no out-of-plane hydrogen atoms. The NMR data show a net chemical shift separation between geminal methylene protons, with the “quasi axial” protons being always at higher field with respect to the “quasi equatorial” ones. This is in accordance with a rigid bip ligand frame and the inability of the bip methylene protons adjacent to the coordinated nitrogen to rotate away from a cis-G base (G = guanine) during G rotation around the Pt−N7 bond.