NMR Studies of Phenylbenzohydroxamic Acid and Kinetics of Complex Formation with Nickel(II)

The behavior of N-phenylbenzohydroxamic acid (PBHA) in organic solvents has been investigated by 1H and 13C NMR spectroscopy. Measurements in acetone at different temperatures and concentrations enable one to individualized two signals, in a 20/80 area ratio, which were ascribed to partition of PBHA between two isomers, HZ (cis) and HE (trans). The dependence of the low-intensity peak on concentration and temperature strongly suggests dimer formation. Since only the HZ form can give dimers, it may be concluded that in acetone PBHA is present mainly in the HE form. 13C NMR measurements in methanol yielded a 50/50 [HZ]/[HE] ratio. The equilibria and kinetics of complex formation in aqueous solutions between Ni(II) and PBHA were investigated by spectrophotometric and stopped -flow techniques at 25 C and 0.2 M ionic strength. Two reaction paths, involving the binding of Ni^2+ to the neutral PBHA and to its anion, were observed. The rate constants of the forward and reverse steps are k1 = (7.1 (plus-minus) 0.3) × 102 M^-1 s^-1 and k^-1 = (4.9 (plus-minus) 0.6) s^-1 for the step involving the undissociated PBHA and k2 = (5.5 (plus-minus) 0.7) × 10^4 M^-1 s^-1 and k^-2 = (1.2 (plus-minus) 0.1) s^-1 for the step involving the anion. The k2 value indicates that the PBHA anion reacts with Ni^2+ according to Eigenʹs mechanism and that in water the cis form prevails. The k1 value is lower by a factor of 13 compared to the value estimated on the basis of Eigenʹs mechanism, suggesting that at least 90% of the neutral ligand is present in a nonreactive conformation.