(o -Hydroxyphenyl)methylphosphonic Acids: Spectrophotometric determination of their pKa values and of the deprotonation sequence

UV/VIS Absorption spectra of nitrosubsituted (o-hydroxyphenyl)methylphosphonic acids (o-(phosphonomethyl)phenols) were measured as a function of pH at 25° in 0.1M NaClo₄ solutions. Computational treatment of the whole set of optical density data between 200 and 500 nm resulted in the determination of the dissociation constants of these polyacids and also of the individual electronic spectra of all the species involved in the deprotonation sequence. The spectral behavior gives information on the structure of the anions formed and consequently the order of the subsequent deprotonation steps could be deduced. For the (2-hydroxy-3-nitro(or 5-nitro)phenyl)methylphosphonic acid and the 2-hydroxy-5-nitro-1,3-phenylenebis(methylphosphonic acid), the phenolic proton dissociates in the last step, while, in the case of (2-hydroxy-3,5-dinitrophenyl)methylphosphonic acid, the last dissociating proton comes from a P? OH group. An intermediate situation is found for (3-chloro-2-hydroxy-5-nitrophenyl)methylphosphonic acid. Generally, the deprotonation sequence is governed by intramolecular H-bonds involving the phenolic OH group.     

Octahedral and tetrahedral configuration changes of nickel(II) chloro complexes from spectrophotometric studies in aprotic solvents

Four NiII chloro complexes were identified spectrophotometrically in aprotic media such as DMSO, DMF and propylene carbonate for the first time. An original multiwavelength numerical treatment, based on a Marquardt method, results in the determination of the individual electronic spectra of the mononuclear chloro complexes and of their stability constants, the values of which indicate the presence of rather weak complexes in DMSO and DMF, much stronger in the less solvating propylene carbonate solvent. The overall stability constants calculated for the NiII chloro complexes are: 1=52, 2=140, 3=160, 4=210 in DMSO, 1=60, 2=110, 3=900, 4=15000 in DMF, 1=1.6×109, 2=6.4×1015, 3=2.7×1021, 4= 5.1×1026 in PC.The solvent effect is discussed in terms of the quantitative results obtained from this spectrophotometric study and reveals that the stability of the complexes is an inverse function of the donor properties of the solvent. This result illustrates the general behavior of complex formation in dipolar aprotic solvents, i.e. the substitution of one solvent molecule in the inner coordination sphere by the ligand is easier if the solvation strength of the solvent is lower. The solvent effect is also described in terms of the variation of shifts of the calculated absorption maxima for the individual electronic spectra of the chloro complexes. This result is used to identify clearly at which step of coordination the octahedral configuration of the nickel ion changes to the tetrahedral configuration, by comparison with the well-known structural properties of these complexes in the solid state.     

(o-Hydroxyphenyl)methylphosphonic acids: Synthesis and potentiometric determinations of their pKa Values

(o-Hydroxyphenyl)methylphosphonic acids are readily obtained from o-(bromomethyl)- or o-(hydroxymethyl)phenols and trialkyl phosphites. Subsequent hydrolysis leads to the corresponding phosphonic acids. For a series of such compounds, the pK_a values have been determined by potentiometry. Their dependence on additional substituents in the aromatic ring is discussed in terms of electronic and steric effects.     

Some aspects of the scope and limitations of derivative spectroscopy

Synthesised spectra are used to illustrate discussion of some relationships between recorded absorption profiles and their second and fourth derivative spectra. Limitations arising from the fortuitous overlap of a derivative peak with a neighbouring wing, and the possibilities of resolving spectra into their overlapping bands are also considered. The combined use of second and fourth derivative spectra to ascertain the correct number of bands within an observed profile is described. It is suggested that the practice of computing a least-squares fit of overlapping bands to a spectral profile be changed, because the minimisation achieved often produces a result involving excessive or negative absorbances: the spectral profile should be regarded as a boundary limit and any unaccounted (positive) absorbance then assessed as possible evidence for an additional band. An example is given, concerning the resolution of the spectrum of a thin, single crystal of uranium(IV) oxide at 77 K superimposed on an absorption edge. A comparison of the difference between the observed spectrum and the sum of its resolution into twelve overlapping bands, plus a similar comparison of their fourth derivative spectra, reveals a thirteenth band.     

Formation of cobalt(II), nickel(II) and copper(II) chloro complexes in alcohols and the Irving-Williams order of stabilities

Summary Co^II chloro complexes were studied in MeOH at 25 °C and at constant ionic strength of 1 mol dm^–3. Formation of three complexes is postulated for which the overall stability constants are calculated: log ₁ = 1.2, log ₂ = 1.7 and log ₃ = 1.4. The electronic spectra and the formation curves of the identified species are presented for the first time in this medium. The results are compared with those obtained in other alcohols and increasing stability with increasing molecular weight of the solvents is established. Further comparative study showed that the maximum stability of the chloro complexes is found with the Cu^II ion as the central atom. This confirms the Irving-Williams order of stabilities for the first transition metal complexes in this alcoholic medium and the result is explained in terms of the second ionization potential of the elements.