Heteronuclear Nucleobase Complexes as Tools for the Isolation of the Minor Rotamer of the Parent Compound: Synthesis and Crystal Structure Determination of Head-Head and Head-Tail Forms of trans-[(CH3NH2)2Pt(1-MeC-N3)2]2+ (1-MeC = 1-methylcytosine)

Abstract

The two rotamers (head-tail, 1, and head-head, 2) of the bis(1-methylcytosine)complex of trans-(CH3NH2)2Pt(II), have been crystallized as ClO4- (1, 2a) and PF6- (2b) salts and characterized by X-ray crystal structure analysis and 1H and 195Pt NMR spectroscopy. In aqueous solution, 1 is preferred over 2 by 70:30. Upon slow crystallization from H2O, 1 is obtained as the only product. Isolation of 2a and 2b has now been accomplished via formation of the heteronuclear derivative trans-[(CH3NH2)2Pt(1-MeC--N3,N4)2Hg]2+, in which the deprotonated 1-methylcytosinato ligands (1-MeC-) are oriented head-head, precipitation of Hg(II) by thiourea, and rapid crystallization of the parent compound. The solid state structures of 1 and 2b differ markedly in a number of aspects. Isolation of pure 1 and 2 permits a detailed study of the kinetics and thermodynamics of the interconversion of the two rotamers. From comparison with the behavior of 1 and 2 in H2O on the one hand and DMSO and DMF on the other a clear solvent effect on the rotamer distribution is seen which most likely relates to differences in H bonding between solvent and solute.