The effect of different organic solvent + water mixtures on the acid dissociation constants of protonated 5-amino-4-(arylazo)-3-methyl-1-phenylpyrazole derivatives were studied. The organic solvents used are methanol, ethanol, acetone, and dimethylformamide. The results obtained were discussed in terms of the solvent characteristics.The pKBH+ values of the dyes in question were determined and found to depend on both the amount and nature of the organic cosolvent. The decrease of the pKBH+ values as the amount of organic solvent in the medium is increased can be accounted for in terms of the high stabilization of the protonated form of the compounds by dispersion forces rather than by hydrogen bonding. Also the effect of solvent polarizability and hydrogen-bonding interaction on the pKBH+ values and thus on the spectra of the charge-transfer band observed have been discussed. The absolute pKBH+ value for each compound despite the presence of more than one basicity center has also been discussed.

A new series of chromium(II)-alkylphosphonates, Cr[RPO3]·H2O (R = CH3, C2H5), and chromium(II) ethylenebis(phosphonate), Cr2[O3P−(CH2)2−PO3]·3H2O, have been synthesized by reaction of CrCl2 and the corresponding phosphonic acid in water in the presence of urea. The (2-aminoethyl)phosphonic acid reacts with CrCl2 to give an unexpected compound of formula Cr[H3N−(CH2)2−PO3(Cl)]. All the compounds are sensitive to the air, except for the latter one. They were characterized by X-ray powder diffraction, thermogravimetry, and UV−visible and infrared spectroscopy, and their magnetic properties were studied by a SQUID magnetometer. The metal ion is in the high-spin d4 configuration, and it appears to have an octahedral coordination. Cr[CH3PO3]·H2O behaves as a “weak” ferromagnet, with a Nèel temperature of TN = 35 K, and it represents an example of an unusual kind of molecular-based magnetic material, having a finite zero-field magnetization. The thermal variation of the magnetic susceptibility of Cr2[O3P−(CH2)2−PO3]·3H2O follows the Curie−Weiss law, with a large negative Weiss constant of θ = −73 K. Below 100 K, the susceptibility increases until a peak at TN = 15 K is observed, and this is associated to the three-dimensional antiferromagnetic order. Cr[H3N−(CH2)2PO3(Cl)] is paramagnetic down to 5 K.