SYNTHESIS AND CHARACTERIZATION OF THE COBALT(III) COMPLEXES WITH TETRAAZA MACROCYCLIC LIGANDS HAVING STRATEGICALLY APPENDED FUNCTIONAL-GROUPS

The ligands L(I) and L(II)Hare 1,4,7,10-tetraazacyclododecanes having single, strategically placed, appended functional groups attached to ring nitrogens, m-toluonitrile and m-toluic acid, respectively. The functional groups are positioned so that they cannot chelate to the metal ion so long as the macrocycle remains fully bound, but they can interact with adjacent atomic centers, such as the protons of coordinated water molecules or the carbonyl groups of chelated amino acid esters. Whereas previous studies yielded dinuclear complexes for (L(II))-, this work has produced mononuclear octahedral complexes for both ligands: [CoL(I)Cl2]Cl-2H2O, [Co(L(II)H)Cl2]Cl.H2O. The new compounds were characterized by elemental analyses, molar conductance, infrared spectra, and UV-vis spectra, and the acid dissociation constants were determined in water for the dangling carboxyl group and the coordinated water molecules in the aquation product [Co(L(II)H)(H2O)2]3+ (pK1 = 2.57 +/- 0.15, pK2 = 7.25 +/- 0.16, and pK3 = 10.13 +/- 0.12). Rate studies, by spectrophotometry, on their aquation reactions revealed different behaviors (eqs 1 and 2); L = L(I) or L(II)): (1) [CoLCl2]+ + H2O --> [CoL(H2O)Cl]2+ + Cl-; (2) [CoL(H2O)Cl]2+ + H2O --> [CoL(H2O)2]3+ + Cl-. The rate of the first aquation step for [CoL(I)Cl2]+ in aqueous solution at 25-degrees-C is moderate, giving a value of k1 = 8.24 X 10(-3) s-1. In contrast, acid hydrolysis of [Co(L(II)H)Cl2]+ is very rapid. In order to determine the first aquation rate for [Co(L(II)H)Cl2]+ it was necessary to use a methanol-water medium and measure the rate by the stopped-flow technique; the observed rate constant at 25-degrees-C is 1.8 X 10(-2) s-1. The second aquation process is also accelerated relative to comparable complexes, but its more moderate rate facilitated detailed studies. Increasing H3O+ concentration retards the rates of reaction of the complexes of L(II)H, and the behavior is consistent with catalysis by the dangling carboxylate function. At 25-degrees-C, the rate-derived value for the pK of the carboxylic acid group is 3.39 and the rate constants for aquation of the protonated and deprotonated forms of the complex are k2 = 2.1 X 10(-4) s-1 and k2' = 9.68 X 10(-3) s-1, respectively. These studies reveal a strong intramolecular catalysis due to the dangling carboxylate function of (L(II))-. Aquation reactions for both [Co(L(II)H)Cl2]+ and [Co(L(II)H)(H2O)Cl]2+ are inhibited by acid with limiting rates in both acid and neutral solutions. The maximum observed catalyzed rates were 0.36 s-1 and 9.9 x 10(-3) s-1, respectively, and the total retardation observed was a factor of about 50 for the better-defined second aquation step, but much greater for the rapid first step.