Cyclam linked systems

There are a number of reports of studies involving dinucleating ligands incorporating linked subunits of the 14-membered ring, cyclam (or its derivatives). Linked systems based on cyclam are expected to 3ueld complexes with d" metals that show elevated kinetic and thermodynamic stabilities, because the latter are well-documented properties of the complexes of cyclam (2). 

A range of three-ring, linked aza macrocycles incorporating 9-membered 1,4,7-triazacyclononane (tacn), 12-membered cyclen and 14-membered cyclam and its derivatives have been reported but the number is much less than for the corresponding systems incorporating two linked macrocycles systems containing four or more linked rings are quite rare. 

The interaction of the linearly linked tris-cyclam derivative 13 with Ni(II), Cu(II), Zn(II), Cd(II), and Pd(II) has been investigated [32], As for the above tri-branched systems, all five metals yield solid complexes in which the metal ligand stoichiometry is 3 1 with, for Cu(II), a spectrophotometric titration also confirming the formation of a complex of this stoichiometry in acetonitrile. Cyclic voltammograms of both the Ni(II) (low-spin) and Cu(II) complexes both yield evidence for the presence of M(II)/M(III) as well as M(I)/M(II) couples in acetonitrile. 

Thus, the two tetramine receptors cyclam and tren were linked by a 1, 4-xylyl spacer to give system 14, tren-cyclam. The heterodimetallic complex [Cu(II)(tren)-Ni(II)(cyclam]4+ operates chloride anion translocation between copper and nickel centers, through the Ni(II)/Ni(III) couple, in an MeCN solution [36]. In particular, when 1 equivalent of CP is added to an MeCN solution of the heterodimetallic complex [with the nickel center in the Ni(II) state], the anion goes on the Cu(II) ion. In particular, since the log K value for the 1 1 adduct formation equilibrium is 5.7, in a lO-3 M solution of the heterodimetallic complex, 95% of the CP ion is bound to Cu(II), while the remaining 5% is dispersed in the solution. The solution has a blue-green color, due to an absorption band centered at 460 nm, pertinent to the [Cu(II)(tren]2+ fragment of the heterodimetallic complex [Cl-to-Cu(II) CT transition). If in an... 

The copper-nickel couple works well also in the heterodimetallic system based on molecule 15 [37]. Here, a 2, 2,-bipyridine fragment is covalently linked to the carbon backbone of a [Ni(II) (cyclam) ]2+ subunit. Two bpy fragments coordinate a Cu(II) center, giving rise to the trimetallic system 16... 

As an improvement to the 16 + 23 system, 23 was replaced by 24, which has a pyridinium cation as an electron acceptor covalently linked to the cyclam [41]. The photocatalytic performance of a mixed system of 16, 24 and H2A was better than that of the 16 + 23 system TNh2 and TNco were 22 and 2, respectively. Because the reduction potential of the pyridinium cation unit is more positive than that of [Ni (cyclam)] unit, the pyridinium unit is supposed to function as the initial electron acceptor (17). 

If, for some reasons, a two-electron redox system is desired, in which the electrons are exchanged at distinctly different potentials, two different fragments X and Y, each one active through a one-electron process, should be linked. At this stage, the reader does not need to be convinced that ferrocene and [Ni (cyclam)]2+ could be reasonable candidates to play the roles of X and Y fragments. 

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