Structure, stability, and interconversion barriers of the rotamers of cis-[Pt11Cl2(quinoline)2] and cis- [PtiiCl2(3-bromoquinoline)(quinoline)] from x-ray crystallography, NMR spectroscopy and molecular mechanics evidence

Reported are the preparations of cis-[PtCl₂(quinoline)₂] and cis-[PtCl₂(3-bromoquinoline)(quinoline)] and an investigation of the stabilities and interconversion of the rotamer forms of these complexes. Both head-to-head (HTH) and head-to-tail (HTT) rotamer forms are found in the crystal structure of cis-[PtCl₂(quinoline)₂]. The NOESY NMR spectrum of cis-[PtCl₂(quinoline)₂] in dmf-d7 at 300 K is consistent with conformational exchange brought about by rotation about the Pt-N(quinoline) bonds. H···H nonbonded distances between H atoms of the two different quinoline ligands were determined from NOESY data, and these distances are in accord with those observed in the crystal structure and derived from molecular mechanics models. Cis-[PtCl₂(3-bromoquinoline)(quinoline)] was prepared to alleviate the symmetry-imposed absence of inter-ring H2/H2 and H8/H8 NOESY cross-peaks for cis-[PtCl₂(quinoline)₂]. Molecular mechanics calculations on the complexes show the HTT rotamers to be 1-2 kJ mol^-1 more stable than the HTH forms, consistent with the ¹H spectra where the intensities of resonances for the two forms are approximately equal. Variable-temperature ¹H NMR spectra of cis-[PtCl₂-(quinoline)₂] in dmf-d7 indicate a rotational energy barrier of 82 ± 4 kJ mol^-1. Variable-temperature ¹H NMR spectra indicate that the Br substituent on the quinoline ring does not affect the energy barrier to interconversion between the HTT and HTH forms (79 ± 5 kJ mol^-1). The steric contribution to the rotation barrier was calculated using molecular mechanics calculations and was found to be ∼40 kJ mol^-1, pointing to a possible need for an electronic component to be included in future models.