THE [PT(IV)CL6]2- THIAMINE SYSTEM - CRYSTAL-STRUCTURES OF (H-THIAMINE)[PTCL6].H2O AND (H-THIAMINE)2[PTCL6]CL2.2H2O - HOST GUEST-LIKE ANION COORDINATION

The reaction of PtCl4 With (H-thiamine)Cl2 and (H-thiamine)(ClO4)2 in water produced a 1:1 salt (H-thiamine)[PtCl6].H2O (1) and a 2:1 double salt (H-thiamine)2[PtCl6]Cl2.2H2O (2), respectively, whose crystal st ructures have been determined by X-ray diffraction. In both compounds, the Pt(IV) atom does not directly bind to thiamine but is linked as a n octahedral [PtCl6]2- anion to a cationic thiamine; a [ptCl6]2- ion b ridges between the thiazolium and the pyrimidinium moieties of the sam e thiamine molecule, through (i) a hydrogen bonding between the C(2)H of the thiazolium ring and the Cl- of the [PtCl6]2- ion and (ii) an el ectrostatic interaction between the pyrimidinium ring plane and the sa me Cl-. There is another type of anion bridge between the pyrimidinium and the thiazolium rings of the same thiamine molecule, through (i) a hydrogen bonding between the amino N(4'alpha)H of the pyrimidinium ri ng and the Cl- of the other [PtCl6]2- ion in 1 or the free Cl- ion in 2 and (ii) an electrostatic contact between the thiazolium plane and t he same Cl-. Biological relevance of this host-guest-like anion coordi nation is mentioned in brief as a possible model for the substrate fix ation to coenzyme and/or the coenzyme fixation to apoenzyme. Results a re also compared with those of the pt(II)Cl4]2-/thiamine system (R.E. Cramer, R.E. Kirkup and M.J.J. Carrie, Inorg. Chem., 27 (1988) 123).