X-ray structure and circular dichroism of pure rotamers of Bis[guanosine-5'-monophosphate(-1)](N,N,N'-N'-tetranethylcyclohexyl-1,2-diamine)phatinum(II)complexes that have R,R and S,S Configurations at the Asymmetric Diamine

摘要

The use fo a sterically hindered diamine ligand (Me_4DACH)has allowed for the first time,the isolation and characterization,both in the solid state (X-ray crystallography)and in solution (circular dichroism),of pure triangle openHT rotamers of [Pt(Me_4dach)(5'-GMP)_2](compounds 1 and 2 for R,R and S,S configurations of the Me_4DACH ligand,repectively).Comparison of the CD spectra obtained for each rotamer,which differ only in the chirality of the Me_4DACH ligand (R,R or S.S)or in the chirality of the HT conformations (triangle open or LAMBDA),allowed us to conclude that,in the 200-350 nm range,the contributions to the overal CD spectrum that stem from diamine chirality adn diamine-induced chirality of platium d--d transitions or form sugar chirality are negligible relative to the exciton chiral coupling that coccurs for pi-pi~* transitions of the cis guanines.Accurate molecular structures of 1 centre dot10 D_2O and 2 centre dot14 D_2O (conventional crystallographic agreement indexes R_1 convergent to 2.07% and 2.18%,respectively)revealed that the crystallized rotamers have a triangle openHT conformation that is in agreemtn with all previosuly reported -ray structures of [Pt(diamine)(uncleos(t)ide)_2] complexes.This conformation allows the 5'-phosphate to be located in proximity to the Me_4DACH ligand so that (P)O...HC(N)hydrodgen-bond interactions exists in both complexes.For both structures,the canting of te guanine planes on hte coordination plane is right-handed (R;canting angle (PHI)of 80.9 and 73.2,respectively);this indiates that the canting direction is driven by the HT conformation chirality (trangle open for both compounds)and not by the chirality of the carrier ligand (different for the two compounds).Density functional theory analysis of the conformational space as a function of PHI indicated a good agreement between the computed and experimental structures.The increase in energy for PHI values below 65 and 55 (for 1 adn 2,respectively)is mainly duet to the short intramolecular contacts between C(8)H and the cis N-Me groups on the same side of the platinum coordination plane.