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

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) A], 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) A, comparable to those found in crystals of the PtCl 2(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.