FORMATION OF DINUCLEAR (1,4,7-TRIAZACYCLONONANE)CHROMIUM(III) COMPLEXES WITH 3 HYDROXO OR ONE ACETAMIDATO AND 2 HYDROXO BRIDGES FROM THE AQUAHYDROXO MONOMERS IN ACETONITRILE

(tacn)(H2O)2(OH)][Cr(tacn)(H2O)(OH)2](CF3SO3)3.H2O (2), where tacn is 1,4,7-triazacyclononane, can be isolated by the addition of base to [C r(tacn)(H2O)3](CF3SO3)3. In acetonitrile 2 condenses nearly quantitati vely to give the trihydroxo-bridged dimer, [(tacn)Cr(OH)3Cr(tacn)]3+, in solution provided C(Cr) < 5-10 mM. At higher chromium concentration s other reactions compete, and from such solutions [(tacn)Cr(mu-OH)2(m u-CH3CONH)Cr(tacn)]I3. 3H2O (3) was isolated in relatively high yields . The cleavage of one of the bridges in the trihydroxo-bridged dimer w as studied in acetonitrile-water mixtures, and the rate constant was d etermined to be 0.0122(2) s-1 in water at 25.0-degrees-C. The acetamid ato-bridged complex is very robust in aqueous solution over a wide pH range, and it was possible to determine the acid dissociation constant for the deprotonation of one of the hydroxo bridges: - log(K(a)/M) = 12.8 in 1.0 M Na(Br,OH). For comparison the analogous acetato-bridged dimer was isolated, and it showed many similarities to 3. A single cry stal X-ray structure determination of 3 confirms the constitution. It crystallizes in the space group P2(1)2(1)2(1) with a = 9.972(2), b = 1 3.73 1(4) and c = 21.577(6) angstrom (at 122 K) and Z = 4. Using 4618 independent reflections the structure was refined to R(F(obs)) = 0.04 and R(w)(F2, all) = 0.14.