The cis/trans-isomerism on cobalt(III) complexes with 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid)

The X-ray structure determinations show that 1,4,8,11-tetraazacyclotetradec ane-1,8-bis(methylphosphonic) acid H4L forms both cis- and trans-O,O isomer ic complexes with Co(III). Orientation of the other donor atoms indicates t he formation of cis-O1,O2-trans-N1,N8-cis-O1,N4-[Co(HL)], The kinetically p referred cis-isomer is easily formed due to the strong hydrogen bond betwee n the neighboring phosphonic acid groups. The presence of the hydrogen bond in aqueous solution is confirmed by the value of pK(a) which increases by four orders of magnitude in comparison with the tl-ans-isomer. Formation of the trans-[Co(HL)] occurs only after long heating of a Co(II) solution wit h the ligand under inert atmosphere before oxidation, which is explained by insertion of the Co(II) ion in the plane of nitrogen atoms of 1,4,8,11-tet raazacyclotetradecane. (C) 2001 Elsevier Science B.V. All rights reserved.