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(l-MeC-iV3)₂]²⁺ (1-MeC = 1-methylcytosine)

The two rotamers (head-tail, 1, and head-head, 2) of the bis(l-methylcytosine)complex of trans-(CH₃NH₂)₂Pt(II), have been crystallized as C10₄- (1, 2a) and PF₆- (2b) salts and characterized by X-ray crystal structure analysis and ’H 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(l-MeC’-iV3,yV4)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 rotam-ers. From comparison with the behavior of 1 and 2 in H₂0 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.