Exchange Interaction Mediated by O-H-O Hydrogen Bonds: Synthesis, Structure, and EPR Study of the Paramagnetic Organometallic Carboxylic Acid (h7-C7H7)V(h5-C5H4COOH)

C. Elschenbroich*, Olav Schiemann, O. Burghaus, K. Harms

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Abstract

Lithiation of trovacene [(eta(7)-tropylium)vanadium(eta(5)-cyclopentadienyl)] (1(.)) and appropriate subsequent reactions afforded trovacenyl carboxylic acid (eta(7)-C7H7)V(eta(5)-C5H4COOH) (2(.)) and its anhydride [(eta(7)-C7H7)V(eta(5)-C5H4CO)](2)O (3(..)). The deuterated acid 2(.)-d(1) was also prepared; 2(.), 2(.)-d(1), and 3(..) were characterized by X-ray structural analysis. The dimeric hydrogen bonded structure of 2(.) which is found in the crystal also prevails in aprotic, nonpolar solvents and gives rise to a biradical EPR spectrum from which the electron exchange coupling constant J = -0.011 cm(-1) can be determined via computer simulation. Thus, exchange interaction across eight bonds is observable even cm though the spacer contains weak hydrogen bonds. Considerably stronger exchange coupling (J = -0.345 cm(-1)) is displayed by the anhydride 3(..). The EPR spectrum of 2(.) in toluene is temperature dependent as expected for the equilibrium 2 (2(.)) reversible arrow (2(.))2 which leads to the superposition of monomer and dimer spectra at ratios governed by temperature. The enthalpy of dimerization can thus be derived; it amounts to -11.8 kcal mol(-1) for 2(.) and -15.2 kcal mol(-1) for 2(.)-d(1). The H-1 NMR spectrum of the radical 2(.) reveals the positive contact shift Delta delta=3 ppm for the carboxylic proton, relative to the chemical shift observed for the diamagnetic organometallic acid Cr(eta(6)-C6H5COOH)(2). Conceivably, the finite spin density an the carboxyl group, present in the monomer 2(.), contributes to the exchange interaction displayed by the hydrogen bonded dimer (2(.))(2). The redox potential E-1/2 (2(+/0)) and the hyperfine coupling constant a(V-51, 2(.)) are pH dependent. In the former case, a Nernst-Clark plot furnishes the pK(s) values of 4.4 for neutral 2(.) and 1.85 for the cation 2(+). A similar value, pK(s)(2(.)) = 3.7, is read off the plot a(V-51, 2(.)) versus pH.

Original languageEnglish
Pages (from-to)7452-7457
Number of pages6
JournalJournal of the American Chemical Society
Volume119
Issue number32
DOIs
Publication statusPublished - 13 Aug 1997

Keywords

  • PHOTOINDUCED ELECTRON-TRANSFER
  • METAL PI-COMPLEXES
  • BENZENE-DERIVATIVES
  • POLYNUCLEAR COMPLEXES
  • COPPER(II) COMPLEXES
  • MAGNETIC-PROPERTIES
  • TUNNELING PATHWAYS
  • SANDWICH COMPLEXES
  • CRYSTAL-STRUCTURE
  • CHEMISTRY

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