~(35/37)Cl and ~(16/18)O isotope resolved ~(195)Pt NMR: Unique spectroscopic 'fingerprints' for unambiguous speciation of [PtCl _n(H _2O) _(6-n)] ~(4-n) (n = 2-5) complexes in an acidic aqueous solution (Review)

摘要

At high magnetic fields the 128.8 MHz ~(195)Pt NMR of all the species in the series [PtCl _n(H _2O) _(6-n)] _(4-n) (n = 2-6) display unique ~(35/37)Cl isotope effects resulting in a unique 'fine-structure' of each individual resonance, which constitutes an unambiguous spectroscopic 'fingerprint' characteristic of the structure of the octahedral platinum(iv) complex, provided ~(195)Pt NMR are recorded at optimum magnetic field homogeneity and carefully controlled temperature (293 ± 0.1 K). The detailed ~(195)Pt resonance fine-structure observed experimentally can readily be accounted for by an isotopologue and isotopomer model for each complex, showing particularly noticeable differences between stereoisomer pairs such as the cis/trans- and fac/mer-complexes. Moreover partial isotopic ~(18)O enrichment of the coordinated water molecules in the series [Pt ~(35/37)Cl _n(H _2 ~(16/18)O) _(6-n)] ~(n-2) (n = 2-6) confirms this model. This technique can thus be considered a novel, direct spectroscopic method of chemical speciation of appropriate platinum(iv) complexes in solution without reference to accurate chemical shifts of authentic members of such a series. These effects are interpreted qualitatively in terms of the high sensitivity of ~(195)Pt NMR shielding to very small and subtle Pt- ~(35/37)Cl and Pt- ~(16/18)OH _2 bond displacements. Preliminary work shows this also applied to the corresponding bromido-complexes.