195Pt NMR chemical shift trend analysis as a method to assign new Pt(IV)-halohydroxo complexes

A 195Pt NMR spectroscopy study of the speciation of [PtCl6]2-, [PtBr6]2-, and the mixed [PtCl6-mBrm]2- (m=0-6) anions in aqueous medium after hydroxide ion substitution of coordinated halide ions has been carried out under dynamic conditions. Of the 56 possible [PtCl6-m-nBrm(OH)n]2- (m, n=0-6) complex anions in solution under dynamic conditions, the relative chemical shifts delta(195Pt) of 52 observable species have been assigned, 33 of which had not been reported previously. The assignment of all these species including the possible stereoisomers is facilitated by systematic linear relationships between the delta(195Pt) increments resulting from substitutions of the halide ions by OH- ions. Under dynamic conditions, the relative concentration (as indicated by resonance intensities) of the [PtCl6-m-nBrm(OH)n]2- (m, n=0-6) species was found to be largely determined by the trans effects on ligand substitution reactions, leading to the transient appearance of previously unobserved complexes. Only the 2c2c2t and 2t2t2t isomers of the ternary [PtCl2Br2(OH)2]2- complex, the 3m12t [PtCl3Br(OH)2]2- and the 13m2t [PtClBr3(OH)2]2- anions could not be observed under our conditions, although their delta(195Pt) are confidently predictable from the linear trend analysis. This chemical shift trend analysis constitutes a powerful method for the virtually unambiguous assignment of 195Pt chemical shifts for such complex anions undergoing slow ligand exchange on the NMR time scale, as facilitated by the strikingly good correlations of delta(195Pt) for the species as a function of the number of hydroxide ions in the coordination sphere.