Exchange Interaction Mediated by O−H···O Hydrogen Bonds: Synthesis, Structure, and EPR Study of the Paramagnetic Organometallic Carboxylic Acid (η7-C7H7)V(η5-C5H4COOH)

Lithiation of trovacene [(η 7 - tro pylium) va nadium(η 5 - c yclop en tadi e nyl)] ( 1 • ) and appropriate subsequent reactions afforded trovacenyl carboxylic acid (η 7 -C 7 H 7 )V(η 5 -C 5 H 4 COOH) ( 2 • ) and its anhydride [(η 7 -C 7 H 7 )V(η 5 -C 5 H 4 CO)] 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 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 • ) ⇌ ( 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 1 H NMR spectrum of the radical 2 • reveals the positive contact shift Δδ = 3 ppm for the carboxylic proton, relative to the chemical shift observed for the diamagnetic organometallic acid Cr(η 6 -C 6 H 5 COOH) 2 . Conceivably, the finite spin density on 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 ( 51 V, 2 • ) are pH dependent. In the former case, a Nernst−Clark plot furnishes the p K s values of 4.4 for neutral 2 • and 1.85 for the cation 2 + . A similar value, p K s ( 2 • ) = 3.7, is read off the plot a ( 51 V, 2 • ) versus pH.