Tetramines complexes, thermodynamics

Table 2 Thermodynamic Date for Complexes of Cyclic Tetramines ...

According to the modular approach, components of the fluorosensor can be changed at will. For instance, it could be of some interest to replace the quad-ridentate receptors of systems 4 and 5 by their cyclic counterparts, to obtain 6 and 7 [8]. The reason of the interest is that, ceteris paribus, cyclic ligands form more stable metal complexes than their open-chain analogues (the thermodynamic macrocyclic effect [12]). The tetramine receptor in 6 has the skeleton of the classical 14-membered macrocycle cyclam, whereas the receptor subunit of 7 refers to the other well-known object of macrocyclic chemistry dioxocyclam. 

A convenient way to append a transition metal to any organic molecule is making use of a cyclam ring. This 14-membered tetramine macrocycle incorporates 3d metal ions, to give complex species that are very stable both in a thermodynamic and a kinetic sense. Moreover, the firmly encircled cation keeps or enhances the interesting features typically presented by transition metals redox activity through fast and reversible one-electron transfer processes, affinity towards ligands (which will go to occupy the two axial positions left available by the co-planarly coordinated tetra-aza macrocycle). Thus, on reaction of 9-bromomethylanthracene with an excess of cyclam, the two-component system 4 was obtained (cyAn). 

Barriers to rotation around the Cca —N bonds have been determined experimentally for diaminocarbenes (3) and (4) and their protonated and lithiated counterparts the possible involvement of lithium or a proton in the dimerization of these acyclic diaminocarbenes was also reported. A computational study of the dimerization of diaminocarbenes has been performed via rate constant calculations using general transition-state theory calculations. Such a dimerization has been shown to be a rapid equilibrium between the carbenes and the tetra-A-alkyl-substituted enetetramines (5), by characterization of metathesis products when two different tetramines were mixed. The thermodynamic parameters of this Wanzlick equilibrium have been determined for the A-ethyl-substituted compound the enthalpy of dissociation has been evaluated at 13.7kcalmol and the entropy at 30.4calmor K . Complex-ation of diaminocarbenes by alkali metals has been clearly established by a shift of the C NMR signal from the carbene carbon of more than 5 ppm. ... 

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