Australian Journal of Chemistry Australian Journal of Chemistry Society
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Syntheses and Comparative Base Hydrolysis Reactions of Chlorocobalt(III) Complexes of Pendant-Arm Macrocyclic Polyamines and Polyamino Acids

Yakup Baran, Trevor W. Hambley, Geoffrey A. Lawrance and Eric N. Wilkes

Australian Journal of Chemistry 50(9) 883 - 888
Published: 1997


Chlorocobalt(III) complexes of the pendant-arm macrocycles 1,4,8,12-tetraazacyclopentadecane-10-carboxylate (2) and 1,5,9,13-tetraazabicyclo[11.2.2]heptadecane-7-carboxylate (4) have been prepared to complement the known complexes of 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (1) and 7-methyl-1,5,9,13-tetraazabicyclo[11.2.2]heptadecan-7-amine (3). The pairs (1),(3) and (2),(4) differ in replacement of pendant amine and methyl groups in the former pair by a pendant carboxylate group and hydrogen in the latter pair. The macromonocyclic ligands (1) and (2) form cis-chlorocobalt(III) complexes whereas the macrobicyclic ligands (3) and (4) (which contain an additional –CH2–CH2– ‘strap’ linking two amines) form trans-chlorocobalt(III) complexes, defined in terms of location of the pendant donor and the chloride donor. Chloride base hydrolysis kinetics varies with ligand {cis-[Co(1)Cl]2+, kOH 6700 dm3 mol-1 s-1 ; cis-[Co(2)Cl] +, kOH 1800 dm3 mol-1 s-1;trans-[Co(3)Cl]2+, kOH 3450 dm3 mol-1 s-1; trans-[Co(4)Cl]+, kOH 2250 dm3 mol-1 s-1 at 25°C}. Variations in rate constant are tied mainly to variations in activation enthalpy. Chloride hydrolyses occur with retention of configuration, but slow following isomerization can lead to products such as trans-[Co(2)(OH2)] (ClO4)2.2H2O, which crystallizes in the monoclinic space group P21/c, a 9·581(2), b 16·214(2), c 14·350(2) Å and β 94·66(1)°. The pendant carboxylate group and two adjacent secondary amines necessarily occupy an octahedral face, with the water molecule bound trans to the pendant carboxylate. The four Co–N distances range from 1·981(2) to 2·22(3) Å, and along with carboxylate Co–O (1·882(3) Å) and water Co-O (1·934(2) Å) distances are similar to usual distances in cobalt(III) complexes.

© CSIRO 1997

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