A series of 2-aminomethylazacycloalkanes was investigated as carrier ligands of platinum(II) to improve the efficacy of cis-diamminedichloroplatinum, CDDP, which is used in cancer treatment.1-3) The efficacy was found to depend on the size and absolute configuration of the azacycloalkane and the leaving ligands. In particular, platinum(II) complexes of R-2-aminomethylpyrrolidine (R-pyrda) and its S-isomer are superior anticancer agents than that of 2-aminomethylpiperidine (pipda). [4][5][6][7][8][9][10] The structure and numbering scheme of these diamines is shown in Fig. 1. The difference in the absolute configuration of pyrda also affects its efficacy, the Risomer is more efficacous in clinical tests. 10)In a previous report, we described the stereochemistry of coordinated S-pyrda and pipda coordinated to tetracyano Fe(II) and Co(III), which form regular octahedral complexes and reported that these two diamines assume different stereochemistries, when fused in two consecutive rings: cis-and trans-fusion for R-pyrda and pipda respectively. 11) Using NMR spectroscopy, a conformational study of platinum(II) complexes with pipda was undertaken two decades ago 12) and several studies using derivatives of its enantiomeric form have been reported recently in conjunction with the orientations of nucleosides that coordinate as the ligands.13-16) For the platinum(II) complex with pyrda, several X-ray studies have also been reported. 17,18) However, the conformations of these diamines in metal complexes in solution and in the crystal state may not be the same. Therefore we initiated a study of the conformations of chelates of these amines with platinum(II) and found that the pipda has the ability to assume two distinctly different conformations depending on the structure of the intramolecular ligand which is simultaneously coordinated to Pt(II). ExperimentalMaterial Potassium tetrachloroplatinate(II) was obtained from Wako. S-2-aminomethylpyrrolidine (S-pyrda) and 2-aminomethylpiperidine (pipda) were prepared according to a previously described method.11) Dichloro(2,2Ј-bipyridine)platinum(II) and dichloro(1,10-phenanthroline)platinum(II) were prepared according to the methods of Morgan and Burstall 19) and Hall and Plowman 20) respectively. Preparation of Platinum(II) Complexes. (2,2Ј-Bipyridine)(S-2-aminomethylpyrrolidine)platinum(II) Nitrate [Pt(bpy)(S-pyrda)](NO 3 ) 2 and (2,2Ј-Bipyridine)(2-aminomethylpiperidine)platinum(II) Nitrate