Amido-bridged dinuclear chromium(III) complexes with 1,4,7-triazacyclononane

The chemical properties of the amido-bridged chromium(III) complexes [(tacn )Cr(mu-NH2)(3)Cr(tacn)](3+) (1) (and its ammine analogue) and [(tacn)Cr(mu- NH2)(2)(mu OH)Cr(tacn)](3+) (4) (tacn = 1,4,7-triazacyclononane) have been investigated in aqueous solution (1 M NaClO, at 25 degrees C) with the emph asis on the cleavage of the amido bridges, and the cleavage and formation o f the hydroxo bridge. The cleavage products (tacn system) are new doubly br idged dimers with two amido bridges, or one amido bridge and one hydroxo br idge, which were isolated as perchlorate salts. The compounds 1 and 4 are extremely inert towards amido-bridge cleavage in neutral and basic solution. In acid solution the rate constant for the irre versible cleavage of one of the amido bridges follows the expression k(obs) =k[H+] with k being ca. 2 x 10(-3) M-1 s(-1) at 25 degrees C (1 x 10 M-1 s (-1) for the ammine analogue of 1). The cleavage of the hydroxo bridge in 4 is reversible with kobs falling from 10(-2) to 10(-6) s(-1) as pH increase s from 0 to 13. All the parameters of the reaction scheme have been determi ned, showing, e.g., that the equilibrium ratio between 4 and cis-[(H2O)(tac n)Cr(mu-NH2)(2)Cr(tacn)(OH)](3+) is ca. 200, in contrast to the trihydroxo- bridged analogue, which is unstable. The doubly bridged dimers with one non -bridging NH3 ligand lose this ligand,presumably by an intramolecular mecha nism, with a rate constant of ca. 10(-5) s(-1) (pH dependent) to produce tr iply and doubly bridged dimers.