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

Abstract

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(H 2O) 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(H 2O) 2(Ox) 2] suggests an apparent p K a of less than 1.0. Deprotonation of coordinated hydroxide occurs above pH of approximately 11 for both cis- and trans-[Pt(H 2O) 2(Ox) 2]. Analysis of 18O isotopic shifts in the 195Pt NMR spectra reveals the exchange of coordinated aqua– and oxalato–oxygens with H 2 18 O from the solvent. In 1 M HClO 4 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(H 2O) 2(Ox) 2]. The activation parameters for H 2O solvent exchange are Δ H ≠=115 (±5) kJ mol −1 and Δ S ≠=42 (±16) J mol −1 K −1. The dissociation of water from trans-[Pt(H 2O) 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(H 2O) 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 H 2 18 O solvent and a pH of approximately 5, isomerization occurs without the incorporation of H 2 18 O and is consistent with an intramolecular Bailar or Ray–Dutt twist.