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

Abstract

In view of the relevance of the reactivity of inert tetraamine Co^III complexes having two substitutionally active cis positions capable of interact with biologically relevant ligands, the study of the reaction of cis-[Co(cyclen)(H₂O)₂]³⁺ and [Co(tren)(H₂O)₂]³⁺ with chlorides, inorganic phosphate and 5′-CMP (5′-cytidinemonophosphate) has been pursued at physiological pH. The results indicate that, in addition to the actuation of the expected labilising conjugate-base mechanism, the formation of mono and inert bis hydroxo-bridged species is relevant for understanding their speciation and reactivity. The reactivity pattern observed also indicates the key role played by the “innocent” buffers frequently used in most in vitro studies, which can make the results unreliable in many cases. The differences between the reactivity of inorganic and biologically relevant phosphates has also been found to be remarkable, with outer-sphere hydrogen bonding interactions being a dominant factor for the process. While for the inorganic phosphate substitution process the formation of μ–η²-OPO₂O represents the termination of the reactivity monitored, for 5′-CMP only the formation of η¹-OPO₃ species is observed, which evolve with time to the final dead-end bis hydroxo-bridged complexes. The promoted hydrolysis of the 5′-CMP phosphate has not been observed in any of the processes studied.