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^.-d₁ was also prepared; 2^., 2^.-d₁, 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 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^.-d₁. 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 E( 1/4 ) (2(+/0)) and the hyperfine coupling constant α(⁵¹V, 2^.) are pH dependent. In the former case, a Nemst-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 α(⁵¹V, 2^.) versus pH.