Isomerization of Binuclear Amidate-Bridged Platinum (II) Amine Complexes: A 19SPt NMR Investigation

Thomas V. O'Halloran*, Stephen J. Lippard

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

46 Scopus citations

Abstract

195Pt NMR spectroscopy has been used to identify and follow the reversible stereochemical rearrangement of the head-to-head (HH) isomer of [Pt2(en)2(C5H4NO)2](NO3)2, where en = ethylenediamine and C5H4NO” is the a-pyridonate anion, to form the head-to-tail (HT) isomer in aqueous solution. Kinetic studies of the reaction established the rate law to be -d(HH)/dr - d(HT)/df = fcf[HH] - &r[HT], an approach-to-equilibrium expression for which K^ = kf/kT and kohs= kt+ kT. For a typical run at 39 °C and pH 6.5 in 0.04 M phosphate buffer, k{~ 1A (2) X 10-5s-12, kr= 13.9 (2) X 10'5s-1, and K = 0.53. Temperature-dependent rate measurements revealed AHr and AS* to be 114 (5) kJ mol-1and 40 (10) J mol-1K_1, respectively, values characteristic of a dissociative rate-determining step in the reaction mechanism. The suggestion that the isomerization is dissociatively activated is supported by the failure of DMSO to accelerate the rate, as would be expected for an associative mechanism, and the fact that the m-diammineplatinum(II) analogue isomerizes at least 70 times slower than the sterically hindered ethylenediamine complex. The latter result is ascribed to steric strain in the [Pt2(en)2(C5H4NO)2]2+ complex, a consequence of nonbonded interactions between the hydrocarbon chains of the ethylenediamine ligands in two adjacent coordination planes. The isomerization reaction was shown to be intramolecular by the failure of added 6-methyl-a-pyridone or [Pt(en)(H2O)2]2+ to affect the rate of product distribution. A mechanism is presented in which Pt-N(a-pyridonate) bond breaking is postulated to occur in the HH HT rearrangement process.

Original languageEnglish (US)
Pages (from-to)1289-1295
Number of pages7
JournalInorganic chemistry
Volume28
Issue number7
DOIs
StatePublished - Apr 1 1989

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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