The [Pt IV Cl 6] 2−/thiamine system. Crystal structures of (H-thiamine)[PtCl 6]·H 2O and (H-thiamine) 2[PtCl 6]Cl 2·2H 2O: host—guest-like anion coordination

Abstract

The reaction of PtCl 4 with (H-thiamine)Cl 2 and (H-thiamine)(ClO 4) 2 in water produced a 1:1 salt (H-thiamine)[PtCl 6]·H 2O ( 1) and a 2:1 double salt (H-thiamine) 2[PtCl 6]Cl 2·2H 2O ( 2), respectively, whose crystal structures have been determined by X-ray diffraction. In both compounds, the Pt(IV) atom does not directly bind to thiamine but is linked as an octahedral [PtCl 6] 2− anion to a cationic thiamine; a [PtCl 6] 2− ion bridges between the thiazolium and the pyrimidinium moieties of the same thiamine molecule, through (i) a hydrogen bonding between the C(2)H of the thiazolium ring and the Cl − of the [PtCl 6] 2− ion and (ii) an electrostatic interaction between the pyrimidinium ring plane and the same 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′α)H of the pyrimidinium ring and the Cl − of the other [PtCl 6] 2− ion in 1 or the free Cl − ion in 2 and (ii) an electrostatic contact between the thiazolium plane and the same Cl −. Biological relevance of this host—guest-like anion coordination is mentioned in brief as a possible model for the substrate fixation to coenzyme and/or the coenzyme fixation to apoenzyme. Results are also compared with those of the [Pt IICl 4] 2−/thiamine system (R.E. Cramer, R.E. Kirkup and M.J.J. Carrie, Inorg. Chem., 27 (1988) 123).