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(Me2DAP)] (Me2DAP
= 2,4-bis(methylamino)pentane) have been prepared and
studied
by 1H NMR spectroscopy, molecular mechanics/dynamics (MMD)
calculations, and X-ray crystallography. The
coordinated Me2DAP ligand has four asymmetric centers.
Complexes with an enantiomeric form of the ligand
(e.g., RR-Me2DAP, 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:∼0 for X = I- and of 7:3:1
for X = Cl-. Complexes with the meso ligand
(RS-Me2DAP)
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:  δ-chair (SRRR), λ-skew
(RRRR), fluxional chair (SRRS), fluxional skew
(SRSR),
and λ-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 δ-chair instead of skew conformation. X-ray structural
studies of the SRSR species (X = Cl- and X
=
I-) confirmed the δ-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.