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Different configurations of the tridentate amine ligand N,N′,N′′-trimethyldiethylenetriamine coordinated to platinum have been characterized both in the solid state and in solution. Four isomers are possible: two isomers have a mirror symmetry (R,S configurations at the terminal nitrogen atoms) and the terminal NMe's anti or syn with respect to the central NMe (anti-(R,S) and syn-(R,S) isomers, respectively), two more isomers have R,R and S,S configurations at the terminal nitrogen atoms and constitute a couple of enantiomers (rac denotes a 1:1 mixture of the two enantiomers). In the experimental conditions used, only one symmetrical isomer was isolated in the solid state (1) which, in solution, slowly isomerizes into a non-symmetrical isomer (2) and subsequently into a second symmetrical isomer (3). NMR and crystallographic studies have shown that the first species isolated (1) is the syn-(R,S) isomer having all three methyl groups on the same side of the platinum coordination plane, such a configuration allows steric interaction between bis-axial 1,3-substituents within the chelate rings to be reduced. This isomer is also the preferred one in water solution where the ratios 1:2:3 are approximately 52:36:12. In a separate work the guanine derivatives [Pt(Me3dien)G]n+ were used to investigate cis-amine-G intramolecular interactions without the complication arising from cis-G to cis-G interactions present in cis-PtA2G2 intrastrand crosslink models (A2 is two unidentate or one didentate amine ligand and G is a unidentate guanine derivative). [Pt(NO3){syn-(R,S)-Me3dien}](NO3) crystallizes in the orthorhombic space group P212121 with one molecule in the asymmetric unit. |
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