Kinetics of the acid dissociation of cyclic and open-chain tetramine complexes of cobalt(II): general acid catalysis with co-ordinating phosphate—citric acid buffers

Abstract

The kinetics of dissociation of the cobalt(II) complexes of the quadridentate ligands 1.4.8.11-tetraazacyclotetradecane (cyclam), triethylenetetramine (trien) and 2,2′,2″-triaminotriethylamine (tren) was followed spectrophotometrically in the ranges 10 < T < 50 °C and 0.8 < pH < 4.0 in perchloric acid and Mcllvaine phosphate–citrate buffer system, an an ionic strength I= 1.0 mol dm^–3, NaClO₄. The complexes were prepared in situ in preaerated solutions under a nitrogen atmosphere, by the addition of a 10% excess of the ligand in the form of the free base to a solution of CoCl₂·6H₂O. The ligand dissociation reaction was then initiated by the addition of perchloric acid or Mcllvaine phosphate-citrate buffer. No dissociation was observed for the cyclam complex in perchloric acid media. It was, however, observed in the buffer and obeyed biphasic kinetics comprising two consecutive first-order steps. The action of the phosphate and/or citrate is attributed to their complexing ability and to their association through hydrogen bonding to the axial aqua ligand, thus bringin the proton closer to the dissociating nitrogen and hence catalysing the dissociation. The dissociation kinetics of the open-chain unbranched trien and the tripod tren was observed in perchloric acid as well as the Mcllvaine buffer system. Except when too fast to follow by conventional spectrophotometry, the reaction obeyed biphasic kinetics comprising two consecutive first-order steps. The tren complex dissociates at a rate 5–10 times faster than that of trien, the first step being too fast to follow except at 10 °C in the buffer system. The observed rate dependence is explained on the basis of mechanisms involving solvation, specific acid catalysis and general acid catalysis. The cyclam complex dissociates at a rate 5–30 times slower than those of the two open-chain complexes. This is attributed to the stabilization due to the hindered rotation of the dissociating nitrogen (entropic effect) and to the higher ligand-field stabilization of the macrocycle (enthalpic effect). Mechanisms covering the entire range of pH studied and conforming to the observed rate laws are given.