HETERONUCLEAR NUCLEOBASE COMPLEXES AS TOOLS FOR THE ISOLATION OF THE MINOR ROTAMER OF THE PARENT COMPOUND - SYNTHESIS AND CRYSTAL-STRUCTUREDETERMINATION OF HEAD-HEAD AND HEAD-TAIL FORMS OF 3NH2)(2)PT(1-MEC-N3)(2)](2-MEC=1-METHYLCYTOSINE)() (1)

The two rotamers (head-tail, 1, and head-head, 2) of the bis(1-methylc ytosine)complex of trans-(CH3NH2)(2)Pt(II), have been crystallized as ClO4- (1, 2a) and PF6- (2b) salts and characterized by X-ray crystal s tructure analysis and H-1 and Pt-195 NMR spectroscopy. In aqueous solu tion, 1 is preferred over 2 by 70:30. Upon slow crystallization from H 2O, 1 is obtained as the only product. Isolation of 2a and 2b has now been accomplished via formation of the heteronuclear derivative trans- [(CH3NH2)(2)Pt(1-MeC(-)-N3,N4)(2)Hg](2+), in which the deprotonated 1- methylcytosinato ligands (1-MeC(-)) are oriented head-head, precipitat ion of Hg(II) by thiourea, and rapid crystallization of the parent com pound. The solid state structures of 1 and 2b differ markedly in a num ber of aspects. Isolation of pure 1 and 2 permits a detailed study of the kinetics and thermodynamics of the interconversion of the 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 di stribution is seen which most likely relates to differences in H bondi ng between solvent and solute.