Exchange interaction mediated by O-H center dot center dot center dot O hydrogen bonds: Synthesis, structure, and EPR study of the paramagnetic organometallic carboxylic acid (eta(7)-C7H7)V(eta(5)-C5H4COOH)

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.