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)
C. Elschenbroich et al., 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), Journal of the American Chemical Society, 119(32), 1997, pp. 7452-7457
Lithiation of trovacene ta(7)-tropylium)vanadium(eta(5)-cyclopentadien
yl)] (1(.)) and appropriate subsequent reactions afforded trovacenyl c
arboxylic acid (eta(7)-C7H7)V(eta(5)-C5H4COOH) (2(.)) and its anhydrid
e [(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 characterize
d by X-ray structural analysis. The dimeric hydrogen bonded structure
of 2(.) which is found in the crystal also prevails in aprotic, nonpol
ar solvents and gives rise to a biradical EPR spectrum from which the
electron exchange coupling constant J = -0.011 cm(-1) can be determine
d via computer simulation. Thus, exchange interaction across eight bon
ds is observable even cm though the spacer contains weak hydrogen bond
s. Considerably stronger exchange coupling (J = -0.345 cm(-1)) is disp
layed by the anhydride 3(..). The EPR spectrum of 2(.) in toluene is t
emperature dependent as expected for the equilibrium 2 (2(.)) reversib
le arrow (2(.))2 which leads to the superposition of monomer and dimer
spectra at ratios governed by temperature. The enthalpy of dimerizati
on 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 carb
oxylic proton, relative to the chemical shift observed for the diamagn
etic organometallic acid Cr(eta(6)-C6H5COOH)(2). Conceivably, the fini
te spin density an the carboxyl group, present in the monomer 2(.), co
ntributes 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.