Abstract
The tetra-aza macrocycle, C-rac-1,5,8,12-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, L a (1), has been prepared from the reduction of 1,5,8,12-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene by NaBH 4 in methanol. The cobalt(III) complex, [CoCl 2 L a ](ClO 4 ) (2) has been prepared from CoCl 2 as well as Na 3 [Co(CO 3 ) 3 ] with ligand 1 and subsequent treatment with concentrated hydrochloric acid. The spectral features of complex 2 are discussed. Ligand 1 and complex 2 have been characterized by single crystal structural analyses. The X-ray crystal structure determination showed that complex 2 has a distorted cis-octahedral geometry with N 4 Cl 2 chromophore. The aquation kinetics of complex 2 has been studied over a temperature range and activation parameters determined. The results indicate a dissociative activation with retention of configuration. The effects of the two N-methyl groups and the stability of the four chelate rings in cis-[CoCl 2 L a ] lead to an extremely small tendency to unfold the macrocyclic ligand. Consequently the values of k aq , ΔH ≠ and ΔS ≠ for the aquation reaction of cis-[CoCl 2 L a ] + are much smaller than the values of k aq , ΔH ≠ and ΔS ≠ , respectively, for the other cis-Co(III) complexes reported so far.