Elham M. Abd-Alla

Charge Transfer Molecular Complexes of Some Pyrazoles with σ‐ and π‐Electron Acceptors

Charge‐transfer (CT) complexes of some pyrazoles with iodine as a σ‐electron acceptor and with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ), tetracyanoethylene (TCNE), chloranil (CHL) as a π‐electron acceptors were investigated spectroscopically. The spectral characteristics and stability constants of the formed 1: 1 CT complexes were examined and discussed in terms of the donor molecular structure, the nature of the electron acceptor as well as the solvent polarity. It is deduced that the complexes with iodine exist in the ionic structure (pyrazole)2 I+ · I3 − whereas the complex with DDQ is mainly in the dissociated state (radical cation and anion; pyrazole· +, DDQ· −). On the other hand pyrazoles‐TCNE, –CHL complexes are essentially of the nonbonding type (pyrazole‐TCNE, –CHL). Moreover, it is concluded that the solvent polarity plays an important role in determining the stability of these kinds of CT complex. The ionization potentials of the donors were calculated based on the CT transition energies of their complexes.

MEDIUM EFFECTS ON THE ACID DISSOCIATION CONSTANTS OF SOME DIIMINE HETEROCYCLIC BASES

The acid dissociation constants of the protonated form of diimine heterocyclic compounds (imidazole, 2-methylimidazole, 2,2′-bipyridyl, and 1,10-phenanthroline) were determinedpH-meterically in aqueous media containing different amounts of organic solvents, viz. amphiprotic (methanol, ethanol), dipolar aportic (DMSO), and low basic aprotic (acetonitrile) at 25±0.1 °C. It was observed that by increasing the amount of alcohol orDMSO in the aqueous medium thepKa, values of the investigated compounds decreased. On the other hand, thepKa values increased as the amount of acetonitrile in the medium was increased. These results are discussed in terms of various solvent characteristics. It is concluded that solvent effects, viz. differences in stabilization of the free base by dispersion forces and of the proton by its interaction with solvent molecules in amphiprotic or dipolar aprotic solvent-aqueous media relative to that in pure aqueous one, as well as the basicity effect of acetonitrile play a vital role in the ionisation equilibria of the investigated compounds.ZusammenfassungDie Säuredissoziationskonstanten der protonierten Formen der heterocyclischen Dimin-Basen Imidazol, 2-Methylimidazol, 2,2′-Bipyridyl und 1,10-Phenanthrolin wurdenpH-metrisch in wäßrigem Medium mit einem Anteil an organischen Lösungsmitteln bei 25±0.1 °C bestimmt. Dabei wurden als amphiprotische Zusätze Methanol und Ethanol, als dipolar aprotisches organisches LösungsmittelDMSO und als aprotisches niederer Basizität Acetonitril ausgewählt. Es wurde beobachtet, daß bei ansteigenden Konzentrationen an Alkohol oderDMSO diepKa-Werte der untersuchten Verbindungen kleiner werden. Andererseits bewirkt Acetonitril ein Ansteigen derpKa-Werte. Die Ergebnisse werden mittels verschiedener Lösungsmittelparameter diskutiert. Lösungsmitteleffekte wie Stabilisierung der freien Base mittels Dispersionskräften, Protonwechselwirkungen in den verschiedenen Lösungsmittelgemischen und Basizitätseffekte des Acetonitril spielen dabei eine entscheidende Rolle.

Complex Formation Reactions of Dimethyltin(IV) with Some Zwitterionic Buffers

Equilibrium studies in aqueous solution are reported for dimethyltin(IV) complexes of zwitterionic buffers, such as bicine and tricine (L). Stoichiometry and stability constants for the complexes formed were determined at 25°C and ionic strength 0.1M NaNO3. The results showed the best fit of the titration curves were obtained for complexes MLH, ML, ML2, MLH-1, and MLH-2 with the hydrolysis products of the dimethyltin(IV) cation. The bonding sites of the dimethyltin(IV) complexes with bicine and tricine at different pH were characterized in the solid state by elemental analyses, FTIR, and TG analysis. The molecular formula of the complexes synthesized at pH=3.0 is [(CH3)2Sn(L)(H2O)]Cl while in neutral and alkaline media the hydrolytic species are formed.

Divalent transition metal ion mixed ligand complexes with aliphatic dicarboxylic acids and imidazoles

SummaryMixed ligand metal complexes of CoII, NiII and CuII with dicarboxylic aliphatic acids, H2L (succinic, malic and tartaric) as primary ligands and with imidazoles, L′ (imidazole and 2-methylimidazole) as secondary ligands were prepared and characterized. MLL′2 and ML′4 molecular formulae were suggested for these complexes were Formation constants of the different complexes were determined pH-metrically at T = 25 ± 0.1°C and μ = 0.1 mol dm−3 (NaClO4). The stability of the mixed ligand complexes increased as the effective basicity of the dicarboxylic aliphatic acid anion increased, namely, tartarate < malate < succinate acid.

Thermal decomposition study of cobalt(II)-mixed ligand complexes with aliphatic or aromatic dicarboxylic acids and imidazole

Abstract The thermal decomposition study of Co(II)–malate, tartarate and phthalate complexes with imidazole was monitored by TG, DTG and DTA analysis in static atmosphere of air. The complexes and their calcination products were characterized by IR spectroscopy. The decomposition course and steps were analyzed and the kinetic parameters of the non-isothermal decomposition were calculated. The results revealed that the decomposition processes of these complexes are the best described by a random nucleation mechanism. The stability order found for these complexes follows the trend tartarate>phthalate>malate in terms of the dicarboxylic acid ligands.

CHARGE-TRANSFER COMPLEXES OF SOME HETEROARYLTHIOUREA DERIVATIVES WITH π-ACCEPTORS

Abstract Spectral characteristics and thermodynamic properties of charge-transfer molecular complexes of some heteroarylthioureas with π-acceptors DDQ, TCNE and CHL are investigated and discussed. It is deduced that the formed CT complexes are of n-π kind. Moreover, 1:1 solid CT complexes are synthesized and characterized.