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

Dissociation of coordinated water from trans-diaquabisoxalatoplatinum(IV) h as been proposed as the first step in the formation of one-dimensional mixe d valence polymers of oxalato-platinum complexes. Nuclear magnetic resonanc e spectroscopy utilizing Pt-195 and C-13 was used to study hydroxo-aqua pro tonation states and aqua ligand exchange rates for trans-diaquabisoxalatopl atinum(IV). The Pt-195 NMR chemical shift of trans-[Pt(H2O)(2)(Ox)(2)] reve als deprotonation near pH of approximately 3 for both coordinated waters. T he lack of a change in the Pt-195 NMR chemical shift for cis-[Pt(H2O)(2)(Ox )(2)] suggests an apparent pK(a) of less than 1.0. Deprotonation of coordin ated hydroxide occurs above pH of approximately 11 for both cis- and trans- [Pt(H2O)(2)(Ox)(2)]. Analysis of O-18 isotopic shifts in the Pt-195 NMR spe ctra reveals the exchange of coordinated aqua- and oxalato-oxygens with (H2 O)-O-18 from the solvent. In 1 M HClO4 at 298 K, O-18 exchange of oxalato- and aqua-oxygen ligands occurs with comparable rates of 7.0 (+/- 1.2) x 10( -6) s(-1) for both cis- and trans-[Pt(H2O)(2)(Ox)(2)]. The activation param eters for H2O solvent exchange are Delta H-not equal = 115 (+/- 5) kJ mol(- 1) and Delta S-not equal = 42 (+/- 16) J mol(-1) K-1. The dissociation of w ater from trans-[Pt(H2O)(2)(Ox)(2)] complex is too slow to be involved in t he mechanism of forming metal-metal bonded bisoxalatoplatinate-polymers, an d 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 1(95P)t NMR s pectroscopy to characterize the isomerization for Pt(IV) complexes with oxa lato, malonato, and 2-methylmalonato ligands. In (H2O)-O-18 solvent and a p H of approximately 5, isomerization occurs without the incorporation of (H2 O)-O-18 and is consistent with an intramolecular Bailar or Ray-Dutt twist. (C) 2000 Elsevier Science S.A. All rights reserved.