Boat versus chair conformation in N-methyl- and N,N′-dimethylpiperazine platinum(II) complexes studied by X-ray analysis.: A rare example of metal chelate piperazine:: cis-[PtCl2(Me2ppz)]: Part I

The square-planar platinum(II) complexes [PtCl3(H(2)Meppz)] (1), trans-[PtCl2-(H(2)Meppz)(2)](2+)2Cl(-). 2H(2)O (2), and cis-[PtCl2-(Me(2)ppz)] (3) (HMeppz = N-methylpiperazine, Me(2)ppz = N,N'-dimethylpiperazine) have been synthesized and characterized by IR, H-1 NMR and X-ray diffraction analysis. In complexes I and 2 the hexaatomic ring of N-methylpiperazine adopts a chair conformation with the N-CH3 and N-Pt bonds in equatorial position and the two N-H bonds in axial position. The coordination to platinum(II) always occurs through the unmethylated nitrogen atom while the methylated nitrogen is protonated, producing a positive charge on the outside surface of the molecule. In addition, complex 1 is present as an amphiionic species since =N+H-CH3 is balanced by -PtCl3-. In contrast, in complex 3 N,N'-dimethylpiperazine adopts a boat conformation stabilized by chelation of the nitrogen donors to platinum(II) so that complex 3 represents a rare example of piperazine chelated to platinum(II). An attempt is also made to correlate the biological activity of chair and boat piperazine complexes with their structural parameters. (C) 1998 Elsevier Science S.A. All rights reserved.