Komplexe mit makrocyclischen Liganden I. Mechanismus der Komplexbildung zwischen Ni²⁺ und 1,4,8,11‐Tetraazacyclotetradecan

Abstract

The kinetics of the formation of the NiII‐complex with 1,4,8,11‐tetraazacyclotetradecan (Cyclam) are studied by spectrophotometric and potentiostatic methods. The reaction of Ni2+ with the cyclic ligand is about 30000 times slower than with the open chain triethylenetetraamine (Trien). Therefore the rate determining step with Cyclam can not be the dissociation of the first coordinated water molecule, as it is usually in complexation reactions, but it must be the formation of the second coordinative bond. This is considered to be due to the more rigid structure of Cyclam in comparison with Trien.

Tailored adsorption of His6-tagged protein onto nickel(ii)–cyclam grafted mesoporous silica

“…SBA-15 is a mesoporous silicate generated from a non- 21 , in particular the tetra-Nsubstituted complexes 22,23 . Cyclam which is mono-tethered to the surface of SBA-15 displays a greater degree of metal uptake than di-or tetra-tethered cyclams 24 .…”

Modification of Mesoporous Silicates for Immobilization of Enzymes

“…Cyclams can bind to a range of transition metals, including nickel, often achieving very high thermodynamic and kinetic stability in terms of metal ion dissociation which has lead to applications in catalysis [62] and medicine [63,64]. Ni-cyclam complexes have been extensively studied [65], in particular the tetra-N-substituted complexes [66,67]. Mono-N-substituted cyclam they exist in a trans-III (S, S, R, R) (square-planar) configuration with two accessible coordination sites above and below the plane of the ring [68], sites which can facilitate attachment of a protein to the surface of the silicate through a His 6 -tag on the protein (Scheme 1).…”

Metal complexes with macrocyclic ligands. Part XX. Influence of coordinated ligands onto the complexation rate of Ni2+ with 1,4,8,11‐tetraazacyclotetradecane

“…The curves start with a plateau corresponding to the reactiv-ity of the completely formed complex NIL, and show with increasing log [L] a decreasing log kobs since a nonreactive 1 : 2 complex is formed. The pH dependence between 9 and 11 clearly shows that CyH' and at high pH Cy both are reactive. The other ligands gave log kJog [L] or log k,,,/pH profiles similar to the three types described above although sometimes one of the aspects is missing.…”

“…crocycles have slower complexation rates than what one would expect from analogous open-chain tetraamines or from solvent-exchange rates assuming that the Eigenmechanismus is effective [9]. These observations have given rise to a long discussion about the possible reasons for the lower reactivity of the protonated species CyH+ or CyH:' in H,O.…”

“…These observations have given rise to a long discussion about the possible reasons for the lower reactivity of the protonated species CyH+ or CyH:' in H,O. Thus 1 ) second bond formation as rate-determining step [9], 2) steric effects [lo], 3 ) electrostatic repulsion of the positively charged species M2+ and CyHi+ [5], and 4 ) conformational changes [2] have been put forward. In principle, each one or a combination of these effects can explain the lower reactivity of protonated macrocycles, but up to now it is not clear to which degree they are involved.…”