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-tetraazacyclotetradecane-1,8-bis(methylphosphonic) acid H4L forms both cis- and trans-O,O isomeric complexes with Co(III). Orientation of the other donor atoms indicates the formation of cis-O1,O2-trans-N1,N8-cis-O1,N4-[Co(HL)]. The kinetically preferred cis-isomer is easily formed due to the strong hydrogen bond between the neighboring phosphonic acid groups. The presence of the hydrogen bond in aqueous solution is confirmed by the value of pKa which increases by four orders of magnitude in comparison with the trans-isomer. Formation of the trans-[Co(HL)] occurs only after long heating of a Co(II) solution with 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-tetraazacyclotetradecane.