Fast atom bombardment mass spectrometry of cisplatin analogs

Cisplatin analogs of the type PtLACl2 and PtLALB are thermally unstable, non-volatile and highly insoluble. For these platinum coordination complexes, LA is a bidentate amine ligand and LB is a bidentate carboxylate ligand. Mass spectral data for structural elucidation of these compounds are absent in the literature because they are difficult to ionize. Nevertheless, a routine fast atom bombardment mass spectroscopic method has been developed utilizing the mixed solvent system of dimethyl sulfoxide:thioglycerol in a ratio of about 1:3 v/v. Using both positive and negative ionization modes, structurally significant ions were observed from representative molecules of the two named classes of compounds. [M-H]- ions were observed in both structural classes while [M + H]+ ions were observed only in the PtLALB class of compounds. Additional ions observed are rationalized in terms of the condensed-phase solution chemistry of the cisplatin analogs and the mixed solvent system when exposed to the fast atom beam. The two mechanisms causing ionization of the cisplatin analogs in the condensed phase appear to be: displacement of the ligands with dimethyl-sulfoxide and addition of chloride and the ionized solvents [dimentyl sulfoxide + H]+ and [thioglycerol - H]- to the cisplatin analogs. It is hypothesized that the addition reactions of the ionized solvents occur because of the differences in the basicity of the solvents and their reactivity in forming platinum(II)-sulfur bonds.