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

Abstract

The behavior of N-phenylbenzohydroxamic acid (PBHA) in organic solvents has been investigated by (1)H and (13)C 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. (13)C 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 degrees 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 k(1) = (7.1 +/- 0.3) x 10(2) M(-)(1) s(-)(1) and k(-1) = (4.9 +/- 0.6) s(-)(1) for the step involving the undissociated PBHA and k(2) = (5.5 +/- 0.7) x 10(4) M(-)(1) s(-)(1) and k(-2) = (1.2 +/- 0.1) s(-)(1) for the step involving the anion. The k(2) value indicates that the PBHA anion reacts with Ni(2+) according to Eigen's mechanism and that in water the cis form prevails. The k(1) value is lower by a factor of 13 compared to the value estimated on the basis of Eigen's mechanism, suggesting that at least 90% of the neutral ligand is present in a nonreactive conformation.

Theoretical investigation of the conformational behaviour ofN-phenylbenzohydroxamic acid in solution

“…Experimentally, 30 in acetone solution at 85°C using a 0.29 M concentration of substrate, the difference in chemical shift between the H-5 signals for the two isomers is only ca 0.1 ppm, whereas difference of 2.5 ppm is found for the C-1 carbonyl signals at 90°C. 30 The intensity ratio of the proton signals around 10 ppm is 80 : 20 and their shifts, as a function of temperature and concentration, were used to assign them to the different isomers.…”

“…30 The inclusion of one explicit acetone molecule in our model allows for a semiquantitative rationalization of the experimental findings. First, we note that the formation of the acetone-PBHA cluster is always thermochemically favoured (G formation ³ 4 kcal mol 1 ) for both the cis and the trans forms both in the gas phase and in solution.…”

“…Amongst all possible arrangements, the formation of a dimer of cis isomers, such as that depicted in Fig. 3, was proposed experimentally 30 simply because that is the most symmetrical arrangement maximizing H-bonding (two H-bonds) without inclusion of direct solute-solvent interactions. Therefore, we decided to compute only this dimeric form to check if the experimental hypothesis of the formation of such a dimer could be consistent with the shift observed in the H-5 NMR signal as a function of temperature (see below).…”

“…30 Here we will compare the computed NMR spectra for both the isomers and the dimer with the experimental data. It should be kept in mind that the experimental spectrum always results from the simultaneous presence of both isomers and of the dimers, in different ratios, depending on the temperature and the concentration, whereas the computed spectra are those of the ideally isolated species.…”

“…Conformational behaviour of N-phenylbenzohydroxamic acid S49 using combined NMR and spectrophotometric/stoppedflow experiments. 30 In particular, a detailed 1 H and 13 C NMR characterization of PBHA in organic solvents, including both temperature and concentration effects on NMR spectra, was performed. 30 These studies provided experimental evidence of the coexistence of the two isomers in acetone solution, gave an estimate of their relative stability and suggested the formation of dimers.…”