Structure, stability, and interconversion barriers of the rotamers of cis-[(PtCl2)-Cl-II(quinoline)(2)] and cis-[(PtCl2)-Cl-II(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 in terconversion of the rotamer forms of these complexes. Both head-to-head (H TH) 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(quinol ine)(2)] in dmf-d(7) at 300 K is consistent with conformational exchange br ought about by rotation about the Pt-N(quinoline) bonds. H . . .H nonbonded distances between H atoms of the two different quinoline Ligands were dete rmined from NOESY data, and these distances are in accord with those observ ed in the crystal structure and derived from molecular mechanics models. ci s-[PtCl2(3-bromoquinoline)(quinoline)] was prepared to alleviate the symmet ry-imposed absence of inter-ring H2/H2 and H8/H8: NOESY cross-peaks for cis -[PtCl2(quinoline)(2)]. Molecular mechanics calculations on the complexes s how the HTT rotamers to be 1-2 kJ mol(-1) more stable than the HTH forms, c onsistent with the H-1 spectra where the intensities of resonances for the two forms are approximately equal. Variable temperature H-1; NMR spectra of cis-[PtCl2- (quinoline)(2)] in dmf-d(7) indicate a rotational energy barri er of 82 +/- 4 kJ mol(-1). Variable-temperature H-1 NMR spectra indicate-th at the Br substituent on the quinoline ring does not affect the energy barr ier to interconversion between the HTT and HTH forms (79 +/- 5 kJ mol(-1)). The steric contribution to the rotation barrier was calculated using molec ular mechanics calculations and was found to be similar to 40 kJ mol(-1), p ointing to:a possible need for an electronic component to be included in fu ture models.