Thioether macrocyclic complexes copper

The value of log K for the copper complex of 6.24 is 4.3, whilst for that of 6.23 it is 1.97. The macrocyclic complex is thus about 100 times more stable than the open-chain complex, and this is presumably due to the macrocyclic effect. In this case, thermodynamic measurements have shown that Afor the macrocyclic and open-chain complexes are almost identical, and so the macrocyclic effect is due almost entirely to the entropy term. However, even with these ligands the involvement of solvation may not be neglected entirely. The stability values given above are for the complexes in aqueous solution if the measurements are repeated in 80 % aqueous methanol, the value of log K for the formation of the macrocyclic complex is only 3.5. A hole-size effect (section 6.6) is also apparent if we move to the larger thioether macrocycle 6.26. For the formation of the copper complex of 6.26 (again in 80 % aqueous methanol) log K is found to be 0.95. 

To gain insight into electron-transfer kinetics of copper(II/I) complexes of macrocyclic thioethers,446,447 Rorabacher and co-workers reported448,4 7 structures of five complexes (complexes (548)-(552)). 

It has been recognized that sulfur donors aid the stabilization of Cu(i) in aqueous solution (Patterson Holm, 1975). In a substantial study, the Cu(ii)/Cu(i) potentials and self-exchange electron transfer rate constants have been investigated for a number of copper complexes of cyclic poly-thioether ligands (Rorabacher et al., 1983). In all cases, these macrocycles produced the expected stabilization of the Cu(i) ion in aqueous solution. For a range of macrocyclic S4-donor complexes of type... 

The binding of cyclic thioethers to metal centers has also led to the isolation of complexes in which the coordinative properties of the ligand do not lit the stereochemical preferences of the metal ion(s) (188), Thus, a series of macrocyclic thioether complexes incorporating unusual stereochemistries and/or oxidation states has been generated (188). This is linked to the biological activity of the blue copper proteins and model systems in which the coordination geometry about Cu(II) is strained [in an entatic state (.212,221)] such that the Cu(II)/(I) couple occurs at a particularly positive potential that is, the Cud) state is stabilized. The ability of cyclic thioethers to modify their coordination properties is inherent in this approach (76,108,111). 

Macrocyclic polythioethers form coordination compounds with many transition metal ions [55] and, owing to their moderate rr-acidity (intermediate between that of amines and phosphines), can exert a stabilizing effect on lower oxidation states of the encircled metal [56]. Copper complexes of thiacrowns have been widely investigated, even in view of the fact that they can be considered convenient models in the study of redox properties of cuproproteins (systems whose active site is a copper center bound to the thioether groups of methionine residues [57]). 

Bernardo etal. [51] subsequently determined the potentials of a series of copper(II/I) complexes formed with 14-membered macrocyclic quadridentate ligands involving amine nitrogen and thioether sulfur donor atoms and found that the substitution of each sulfur donor atom by an amine nitrogen resulted in an average decrease of 0.3 V in the Cu(II/I) potential. Since the Afcu L values had been determined previously for these systems [99], Bernardo et al. were able to calculate the A (jji values using Eq. (9). The latter values proved to be nearly constant, indicating that Cu(I) does not discriminate... 

Finally, some work on macrocyclic thioethers was able to show that while a metal-free tetraester of a functionalised [14]aneS4 diol was non-mesomorphic as either its cis or trans isomer, complexation to palladium(II) stiffened the whole molecular unit and allowed differentiation between the non-mesomorphic, nonlinear cis complex ((312) y = l Cr 258 I), and the mesomorphic, linear trans complex ((312) y = 1 Cr 312 (N 272) Substituted with shorter lateral arms, the palladium salts (Pd(Bp4)2) exhibited monotropic SmC phases ((312) y = 0 Cr 222 (SmC 164) I). Difunctionalized tru 5-[16]aneS4 diol palladium ((313) MX = Pd(BF4)i, y = 0,l) also exhibited monotropic phase (SmC and N, respectively for y = 0 and 1), whereas the copper derivative ((313) y = 0, MX = Cu(PF6)) was surprisingly devoid of mesomorphism. Note that some related aza macrocyclic metal complexes as potential metallomesc ens were also studied, but it appeared that none of them were found to be mesomorphic. ... 

---