Preparation, structure, and interconversion kinetics of mono- and binuclear aquo(nitrilotriacetato)cobaltate(III) complexes

The role of hydroxo-bridged dinuclear species and the influence of “innocent” buffers in the reactivity of cis-[CoIII(cyclen)(H2O)2]3+ and [CoIII(tren)(H2O)2]3+ complexes with biologically relevant ligands at physiological pH

“…Similarly, when the solution obtained after 24 h at 50 °C is acidified to 1 M HClO 4 , the UV-Vis and 13 C NMR spectra of the cis-[Co(cyclen)(H 2 O) 2 ] 3+ species are obtained after 1 hour at room temperature. The formation of [{cis-Co(cyclen)} 2 (μ-OH) 2 ] 4+ dimeric units from the previous [{cis-Co(cyclen)(H 2 O)} 2 (μ-OH)] 5+ species 35,43,53 (see before) fully agrees both with the spectroscopic, pH and kinetic profiles associated with this slower process (24 h, 50 °C). It is also very relevant to indicate that leaching of Co III or cyclen to the medium is prevented during the entire process (Fig.…”

“…44 Unlike for some other model complexes, 52 this species fully reverts to the initial diaqua monomer in acidic medium, preserving the cis-{Co(cyclen){O} 2 } structural unit in a very fast process. Similarly, when the solution obtained after 24 h at 50°C is acidified to 1 M HClO 4 , the UV-Vis and 13 35,43,53 (see before) fully agrees both with the spectroscopic, pH and kinetic profiles associated with this slower process (24 h, 50°C). It is also very relevant to indicate that leaching of Co III or cyclen to the medium is prevented during the entire process ( Fig.…”

“…These changes can be considered to involve the bis-μ-OH formation between two cobalt units by comparison with similar systems. 43 Monitoring of the solution for extended periods at 50°C does not produce any further changes in the UV-Vis spectrum, thus corroborating the inert bis-μ-OH nature complex formed after the two step process. As expected, the full process is accelerated on increasing the pH.…”

“…The absence of mono-hydroxo-bridged species was already remarked for similar [Co(nta)(H 2 O) 2 ] derivatives. 43,44 Finally, it is extremely interesting to note that, contrary to [Co(tren)(H 2 O) 2 ] 3+ , for cis-[Co(cyclen)(H 2 O) 2 ] 3+ the use of MES at pH = 6.5 and 7.0, PIPES at pH = 7.0 and 7.5, or Tris at pH = 7.5 and 8.0 (Scheme 3) produces dramatic differences in the reactivity of the system. For the first fast reaction step the use of MES or PIPES leads to definite differences in the amplitude of the spectral changes when compared to those observed when HEPES had been used (Fig.…”

“…36,41,42 From these values it is evident that solutions prepared under physiologically relevant conditions will contain both diaquocomplexes and significant amounts of species with one or two coordinated OH − . However, according to the literature about related complexes with NH 3 , en or NTA, 35,[43][44][45] formation of simply-or double-OH − bridged dinuclear species must be also considered. Even so, although formation of a dinuclear complex has been found to be relevant in the Co III -cyclen catalysed hydrolysis of ethyl p-nitrophenyl phosphate, 46 the role of those dinuclear species remains essentially unexplored.…”