Factors influencing the configuration and conformation of diamine chelate rings in platinum(II) compounds: 2,4-bis(methylamino)pentane complexes

Complexes of the type [PtX2(Me(2)DAP)] (Me(2)DAP = 2,4-bis(methylamino)pentane) have been prepared and studied by H-1 NMR spectroscopy, molecular mechanics/dynamics (MMD) calculations, and X-ray crystallography. The coordinated Me(2)DAP ligand has four asymmetric centers. Complexes with an enantiomeric farm of the ligand (e.g., RR-Me(2)DAP, RR being the configurations of the two asymmetric carbons) have three possible configurations, namely, SRRR, RRRR, and SRRS at N, C, C, and N, respectively. Indeed these three isomers were obtained in respective ratios of 9:1:similar to 0 for X = I- and of 7:3:1 for X = Cl-. Complexes with the meso ligand (RS-Me(2)DAP) have four possible configurations, i.e., SRSR, RRSS, RRSR, and SRSS at N, C, C, and N, respectively (the latter two constitute an enantiomeric pair), Only the two symmetrical isomers were obtained in the ratios of 9:1 for X = I-and 1:1 for X = Cl-. In addition, the preferred chelate ring conformations in solution (CDCl3) were determined to be the following: delta-chair(SRRR), lambda-skew (RRRR), fluxional chair (SRRS), fluxional skew (SRSR), and delta-chair (RRSS). This information was used to assess the contributions of intra- and interligand interactions to determine the conformational stability. MMD calculations employing the AMBER force field [as modified by Yao et al. (Yao, S.; Plastaras, J. P.; Marzilli, L. G. Inorg. Chem. 1994, 33, 6061) and extended to include parameters for the Cl- ligands] provided minimum-energy structures for all five X = Cl- complexes. These structures agreed well with the experimentally determined solution conformations except for SRSR, which had a lowest energy delta-chair instead of skew conformation. X-ray structural studies of the SRSR species (X = Cl- and X = I-) confirmed the delta-chair conformation. The results suggest that an equatorial N-methyl group has nonbonded clashes with the cis chloride ligand; therefore, axial N-methyl groups are favored. However, in solution, the solvent and entropy (connected with fluxionality of conformers) are factors which, to some degree, can overcome the interligand steric interaction.