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 [(η7-tropylium)vanadium(η5-cyclopentadienyl)] (1 • ) and appropriate subsequent reactions afforded trovacenyl carboxylic acid (η7-C7H7)V(η5-C5H4COOH) (2 • ) and its anhydride [(η7-C7H7)V(η5-C5H4CO)]2O (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 1H 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(η6-C6H5COOH)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 E1/2 (2 +/0) and the hyperfine coupling constant a(51V, 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(51V, 2 • ) versus pH.