The nature of the [Pt(bipy)2]2+ ion in aqueous alkaline solution: a new look at an old problem

It has been known since 1973 that addition of NaOH (aq) to an aqueous solution of [Pt(bipy)2](NO3)2·H2O gives rise to changes in both the solution UV/Vis and solution 1H and 13C NMR spectra of the complex. These spectral changes have been variously interpreted as arising either from attack of OH- at C-6 of a bipyridine ligand to form a covalent hydrate, or coordination of OH- to the Pt ion to form a higher coordinate complex. In this paper, we utilise 195Pt NMR spectroscopy for the first time to study this system. The broad 195Pt NMR peak due to [Pt(bipy)2]2+ at -2273 ppm decreases in intensity on sequential addition of less than stoichiometric amounts of NaOH (aq) and a significantly narrower peak appears at -2094 ppm in proportion to the amount of NaOH (aq) added. Only the latter peak is observed after addition of one mol equivalent of NaOH (aq). Neither NaCl (aq) nor Na2SO4 (aq) affect the 195Pt NMR spectrum of [Pt(bipy)2]2+ but addition of half a mol equivalent of Na2S2O3(aq) gives a peak ~430 ppm upfield of that due to [Pt (bipy)2]2+. The 195Pt chemical shift of the structurally characterised pseudo-five-coordinate complex [Pt(phen)2(CN)]+ is -2726 ppm. Our results rule out deprotonation of a water molecule weakly coordinated to [Pt(bipy)2]2+ by the added NaOH (aq) and are consistent with formation of a conformationally mobile pseudo-five-coordinate complex.