A nuclear magnetic resonance investigation of water exchange reactions and trans to cis isomerization for diaqua and dihydroxobis(oxalato) and bis(malonato)platinum(IV) complexes

Dissociation of coordinated water from trans-diaquabisoxalatoplatinum(IV) has been proposed as the first step in the formation of one-dimensional mixed valence polymers of oxalato–platinum complexes. Nuclear magnetic resonance spectroscopy utilizing 195Pt and 13C was used to study hydroxo–aqua protonation states and aqua ligand exchange rates for trans-diaquabisoxalatoplatinum(IV). The 195Pt NMR chemical shift of trans-[Pt(H2O)2(Ox)2] reveals deprotonation near pH of approximately 3 for both coordinated waters. The lack of a change in the 195Pt NMR chemical shift for cis-[Pt(H2O)2(Ox)2] suggests an apparent pKa of less than 1.0. Deprotonation of coordinated hydroxide occurs above pH of approximately 11 for both cis- and trans-[Pt(H2O)2(Ox)2]. Analysis of 18O isotopic shifts in the 195Pt NMR spectra reveals the exchange of coordinated aqua– and oxalato–oxygens with H2 18O from the solvent. In 1 M HClO4 at 298 K, 18O exchange of oxalato– and aqua–oxygen ligands occurs with comparable rates of 7.0 (±1.2)×10−6 s−1 for both cis- and trans-[Pt(H2O)2(Ox)2]. The activation parameters for H2O solvent exchange are ΔH≠=115 (±5) kJ mol−1 and ΔS≠=42 (±16) J mol−1 K−1. The dissociation of water from trans-[Pt(H2O)2(Ox)2] complex is too slow to be involved in the mechanism of forming metal–metal bonded bisoxalatoplatinate-polymers, and a pH dependent Pt(II)–Pt(IV) oxidative mechanism for polymer formation is more likely to occur. While determining exchange rates for trans-[Pt(H2O)2(Ox)2], we observed a trans to cis isomerization. We used 195Pt NMR spectroscopy to characterize the isomerization for Pt(IV) complexes with oxalato, malonato, and 2-methylmalonato ligands. In H2 18O solvent and a pH of approximately 5, isomerization occurs without the incorporation of H2 18O and is consistent with an intramolecular Bailar or Ray–Dutt twist.