Stability Constants of Divalent Metal Chelates of 1, 4, 7-Triazacyclononane and Some Straight-chain Triamines

Abstract

Acid formation constants, and stability constants fer divalent metal chelates of 1, 4, 7-triazacyclononane (2, 2, 2), 2, 2 diaminodiethylamine (2, 2), N (2-aminoethyl) 1, 3-propanediamine (2, 3), and 3, 3 diaminodipropylamine (3, 3) were determined by the pH titration at 25. OCC and ionic strength of O.1. A (2, 2, 2) coordinates with most metal ions alone in the facial configuration and forms three combined chelate rings. This restriction on freedom should contribute to the stabilities of its metal chelates as shown in Table 2. Besides, nickel, cobalt, zinc and cadmium chelates of (2, 2, 2) have the highest stabilities among triamine chelates ever reported. However, copper (2, 2, 2) chelate has slightly lower value than that of (2, 2) chelate, becau e copper(II)can only be coordinated with two nitrogen atoms of (2, 2, 2). On the other hand, the formation rates of (2, 2, 2) chelates are generally very slow, perhaps due to the comformational effect. Systematic comparison of the fermation constants with straight-chain triamine chelates has also been done in Table 1 and 2. Among three acid formatien constants of each triamine, Ka3 is mostly af ected by the Iigand size, K vaules of(2, 2)and(2, 3)chelates are nearly comparable and that of (3, 3) chelate falls respecting each metal, but KML, values are simply decreased in the order of (2, 2), (2, 3), and (3, 3), except copper and zinc chelates.