The reactions between iron(III) and the phenols gallic acid, catechol, protocatechuic acid and pyrogallol have been studied in aqueous solution at 25�C (I 0.10 M KCl). From spectrophotometric and pH titration data it was established that (i) complexes with 1 : 1, 1 : 2 and 1 : 3 (M : L) stoichiometry are formed in oxygenated solution, (ii) the redox reaction given at low pH by these ligands Fe3+ + �phenol → Fe2+ + �quinone + H+ is not reversed by complexing of Fe3+ at higher pH, and (iii) the percentage composition of iron(II) and iron(III) in solutions of these ligands is a function of ER (ligand), the pH and age of solutions and the partial pressure of dissolved oxygen.

A series of spin-paired bivalent bipyridyl complexes of iron, ruthenium, and osmium, of the type [M(bipy)3]2+, [M(bipy)2X2]n+, [M(bipy)2XY]n+, [M(bipy)X4]n+, [M(bipy)(X2)2]n+, and [M(bipy)X2Y2]n+ (X and Y include a range of monodentate ligands and X2 a range of bidentate ligands) have been prepared. Their electronic spectra within the range 41000-7000 cm-1 have been recorded. All the complexes show two intraligand transitions which are present in free bipyridyl. The iron and ruthenium complexes have two metal oxidation charge-transfer bands, while the osmium complexes have a more complex charge-transfer pattern. Trends in the positions of the bands have been related to the bonding of the ligands X and Y to the metal ions. It is suggested that the energy of the metal t2 type orbitals relative to the ligand π and π* orbitals is the main factor in determining the intense electronic spectra of these complexes.

No abstract

The protonation reactions for catechin and epicatechin have been studied at 25�C, I 0.10M (KCl). Equilibrium constants Kn for the protonation steps of the anions L4- have been determined from potentiometric and/or spectrophotometric measurements. The logKn values for the epimers differed significantly only for logKl and logK4 (values for catechin 13.26 � 0.05, 11.26 � 0.06, 9.41 � 0.02, 8.64 � 0.01, and epicatechin 13.40 � 0.05, ll.23 � 0.06, 9.49 � 0.02, 8.72 � .01, n = 1-4 respectively). These differences are ascribed to specific solvation effects or ion associations between K+ and the ligand anions. Epimerization was studied by h.p.l.c., and was found to be minimal over the experimental time scale at pH c. 13.5. The ring on which each stepwise protonation occurs was elucidated by reference to log Kn values measured for the model compounds catechol, 4-methylbenzene-1,2-diol, protocatechuic acid and 3',4'-di-O-methylcatechin. The protonation sequence is B ring, A ring, B ring and A ring. In contrast to earlier results, it has been established that the epimerization of catechin or epicatechin at C2, and the formation of catechinic acid in basic solution, may proceed through a radical rather than ionic mechanism.

The equilibrium reactions of citrate ion with protons and aluminium(III) have been studied by potentiometric titration in aqueous solution, I 0.10 M KCl, 25°C. The protonation constants for citrate, log Ki, corrected for K+-citrate ion pairing, were determined as 5.90�0.02, 4.35�0.01 and 2.91�0.02, i = 1-3 respectively. The model which gave the best fit to experimental data for the aluminium-citrate equilibrium system (TL/TM > 5.3) included AlHL + ( logβ 11.02�0.02), AlL (8.35�0.23), AlL23- (13.40�0.35), Al(HL)L2- (17.36�0.11), AlL2H-14-[ pK (AlL23-)6.07�0.24] and Al(LH-1)25- [ pK (AlL2H-14-) 7.09�0.08]. 13C n.m.r.data established that ligand exchange in the aluminium(III) complexes is slow on the n.m.r . time scale. The spectra were consistent with deprotonation of the coordinated C(2)-OH group commencing between pH 5 and 6, corresponding to formation of the species designated AlL2H-14- and Al(LH-1)25-. The broad structure of the- CH2-, -COOH (central) and -COOH (terminal) resonances for coordinated citrate was indicative of coordination isomers and/or the presence of both mono and bis complexes.