Chirality Generated by Hindered Rotations in Platinum(II)-Pyridine Complexes

The reaction of trans-[Pt(DMSO)2MeCl] with 2-alkylsubstituted pyridines pyR affords a series of complexes of the type [Pt(pyR)(DMSO)MeCl]. These complexes are rare examples of organometallic square-planar complexes having four different ligands. Of the three possible geometric isomers, only the trans(N,S)-[Pt(pyR)(DMSO)MeCl] was observed in solution and isolated in the solid state. However, the lack of chirality typical to square planar geometry can be resolved in several ways one of which is described in this paper. 1H NMR studies reveal diastereotopicity of the DMSO methyls, as well as the CH2 protons in the ethyl- and neopentyl-pyridine complexes and the methyls in the isopropyl-pyridine derivative. This observation can be ascribed to restricted rotations around the Pt-N bond, thus resulting in the presence of planar chirality for these complexes. Additionally, large downfield shifts in the 1H NMR spectra of the alpha C-H protons in the pyridine substituents suggested the presence of PtMIDLINE HORIZONTAL ELLIPSISH-C interactions, confirmed via single crystal X-ray studies in the case of the isopropyl complex [Pt(pyip)(DMSO)MeCl] (4), which shows an apical PtMIDLINE HORIZONTAL ELLIPSISH interaction with a short 2.585 angstrom distance. DFT calculations shed light into the stability of these complexes and the influence of the pyridine-substituent on their chiral configuration. A series of Pt(II) complexes with 2-alkylsubstituted pyridines display planar chirality due to the influence of the pyridine substituent, which enhances the rotational barrier around the Pt-N bond generating a couple of enantiomers. The chiral conformation is also stabilized by the presence of an axial Pt & sdot;& sdot;& sdot;H-C interaction.image