The interaction of methylcobalamin with tetracyanoplatinate(II), tetrathiocyanoplatinate(II) and tetrachloroplatinate(II)

The interactions of methylcobalamin (CH₃-B₁₂) with Pt(CN)₄^2?, PtCl₄^2?, and Pt(SCN)₄^2? in aqueous solution were studied by UV-visible and ¹H NMR spectroscopy. Together with earlier results on the mechanism of the Pt(IV)-dependent methyl-transfer reaction from CH₃-B₁₂ to Pt(II), these studies suggest at least three Pt binding sites on CH₃-B₁₂. One site, which is occupied by all three complexes (K₁ = 4 X 10³ M^?1 for Pt(CN)₄^2? and 3 X 10³ M^?1 for PtCl₄^2?), is located on the CoCH₃ side of the corrin macrocycle, and is involved in the methyl-transfer process in the presence of a Pt(IV) complex. An additional site for Pt(SCN)₄^2? is the N-3 of the benzimidazole group, resulting in dissociation of this group from the cobalt. An additional site for Pt(CN)₄^2? has a binding constant of 16 M^1? and ¹H NMR changes indicate perturbation but not dissociation of the benzimidazole group. Only the first interaction is discerned for PtCl₄^2?.