Kok-Peng Ang

Synthesis and characterisation of copper(II), nickel(II) and palladium(II) complexes of some schiff bases of dehydroacetic acid

SummaryTri-and quadri-dentate Schiff bases have been synthesized from the reaction of dehydroacetic acid with diamines, aminoacids, aminophenols and aminoalcohols. The copper(II) and some nickel(II) and palladium(II) chelates of these ligands as well as copper(II) complexes of bidentate Schiff bases of dehydroacetic acid with anilines have been prepared and characterised by electronic, i.r. and n.m.r. spectral measurements and magnetic moments.

Unsymmetrical bis-Schiff bases derived from ethylenediamine, dehydroacetic acid and another aldehyde/ketone and the preparation and characterisation of their copper(II) and nickel(II) complexes

SummaryPartial displacement of the symmetrical bis-Schiff bases (1)–(4) of ethylenediamine and salicyladehyde, o-hydroxyacetophenone, acetylacetone or benzoylacetone by dehydroacetic acid have led to the formation and isolation of the unsymmetrical bis-Schiff base ligands (6)–(9). Attempted partial displacements of the symmetrical bis-Schiff base (5) of dehydroacetic acid or (1)–(4) by each of the other four carbonyl compounds have been unsuccessful. The Ni(II) and Cu(II) complexes of (6)–(9) have been prepared and characterized.

(Z)- and (E)-5-Arylmethylenehydantoins: spectroscopic properties and configuration assignment

Both (Z)- and (E)-5-arylmethylene-1-methylhydantoins have been obtained directly in substantial proportions from condensations of 1-methylhydantoin with aromatic aldehydes. However, the products of similar condensations of hydantoin or 3-methylhydantoins consist almost entirely of the Z-isomers. The configurations of all the 5-arylmethylenehydantoins prepared can be unambiguously assigned by consideration of their 1H and 13C n.m.r., i.r., and u.v. spectra. The mass spectra of the isomers show similar fragmentation patterns.

Thermal, spectroscopic, and structural properties of aqua(malonato-O,O′)(1,10-phenanthroline)copper(II) hydrate (1/1.5)

Aqua(malonato-O,O′)(1,10-phenanthroline)copper(II) crystallizes in the triclinic space group P with a= 11.560(4), b= 11.734(5), c= 12.491(2)A, α= 92.12(2), β= 105.50(2), γ= 109.28(3)°, and Z= 2. The structure consists of two independent monomeric [Cu(phen)(mal)(H2O)] molecules with three water molecules in the lattice. The CuN2O2O′ chromophore is distorted square pyramidal with a symmetrically co-ordinated phen molecule (mean Cu–N 2.018 A) and a symmetrically co-ordinated malonate ion (mean Cu–O 1.914 A) in the plane of the square pyramid and a water molecule at 2.266 A. The relatively short Cu–O distance has been attributed to phen being a stronger π-acid. The compound and its several related complexes show complete loss of water molecules at ca. 100 °C followed by stepwise degradation of the organic ligands. Thermogravimetric analysis and visible reflectance spectral results are discussed in relation to the known structure of the title compound.

Thermodynamics of ionization of 2,4-dinitrophenol in water-dimethylsulfoxide solvents

The ionization constants of 2,4-dinitrophenol were measured in water-dimethylsulphoxide solvent mixtures at five temperatures ranging from 20 to 40°C. The enthalpy and entropy contributions to the ionization process are discussed. The results indicate that in water-rich solvent mixtures the ionization process is controlled by the entropy factor while in the dimethylsulphoxide-rich solvent mixtures it is controlled by the enthalpy factor.

Oxygenation of the cobalt(II) complex of an unsymmetrical bis-Schiff base

SummaryThe cobalt(II) complex of an unsymmetrical bis-Schiff base. Co(dha, salen), has been prepared. In the presence of pyridine, it forms both monomeric superoxo and dimeric peroxo adducts with dioxygen. At −10°C and lower temperatures, the adduct formed in pyridine solution is mainly superoxo as shown by electronic and e.s.r. spectra as well as volumetric measurement of oxygen uptake. The solid adduct precipitated by addition of toluene is predominantly peroxo as shown by elemental analysis, i.r. spectrum, magnetic and thermogravimetric measurements. Comparison of the oxygen affinities of this complex and of the symmetrical Co(salen) is discussed.

The dissociation constant ofm-nitroanilinium ion in water-sulfolane solvents at 25°C

The dissociation constants ofm-nitroanilinium ion have been determined in water and nine water-sulfolane mixtures at 25°C. The behavior is compared to results for several other aqueous solvent mixtures.

Ionization constants of 5-arylmethylenehydantoins in 80%(w/w) dimethyl sulphoxide–water at 25 °C

The pK values of (Z)-5-arylmethylenehydrantoins, (Z)-5-arylmethylene-3-methylhydantoins, and (Z)- and (E)-5-arylmethylene-1-methylhydantoins with various para-substituents in the phenyl ring have been measured. The effects of structure and stereochemistry on acidity are discussed. For each series, a good linear correlation is obtained between the pK values and Hammett constants σ and σ– in spite of the considerable distance of the site of deprotonation from the phenyl ring. The ρ values show that substituent effects are significantly greater on the acidity of N(1)–H than on that of N(3)–H, and are also greater for the E- than the Z-isomers of the 5-arylmethylene-1-methylhydantoins.

Mixed-ligand complex-formation of (2,2′,2″-terpyridine) copper(II) with monosubstituted phenolates

SummaryThe copper(II)-phenolate oxygen interaction in several mixed-ligand complexes of copper(II) containing a 2,2′,2″-terpyridyl and a phenolate has been studied by i.r., electronic, e.p.r. spectroscopies and conductometric and polarographic measurements. Electronic spectral results demonstrate the dependence of such interaction on the nature of the phenol as well as the solvent. Partial dissociation of the phenolate from the complex was detected in methanol at low concentrations (≤1×10−4M). In dimethyl sulphoxide the complexes remain intact. The X-band e.p.r. spectral data obtained at ambient temperature and at 77 K are consistent with the results from the electronic spectral and conductometric study.

Ionization constants of some Schiff's bases in 80%(w/w) dimethyl sulphoxide–water solvent at 25 °C

pK Determination in 80%(w/w) dimethyl sulphoxide–water solvent at 25 °C has been used as a means of studying the tautomerism in a series of Schiffs bases derived from some C-acylhydroxypyrones. While their much lower acidities compared with the corresponding parent compounds indicate a predominance of ketoamine tautomers, their higher acidities compared with amides and other nitrogen acids suggest significant contributions from dipolar structures. Correlation of the wide range of pK values with the structures is discussed.