Macrocyclic ligands, stabilization

When 2-aminobenzaldehyde was heated in absolute ethanol in the presence of nickel nitrate, the tridentate and tetradentate macrocyclic complexes (7 and 8, respectively) were isolated.17 The presence of copper(II) nitrate produced only the latter (8).17 Although the monomer has two functional groups and the oligomers have reactive azomethine groups, the formation of macrocyclic ligands stabilized by a metal halts further polymerization. 

This system has characteristic in ingeneous combination of a redox pump and the selective complexation of cation by the macrocyclic ligand. It must be noted that this new system is very promising from the point of view of extending the scope of the selection of the cation carriers, since any carrier can be employed so long as it has selectivity for a special cation and has enough stability toward redox system. 

Figure 3.96 Macrocyclic ligands used to stabilize palladium(III) and platinum(III).

The intermediate-spin ground state of the ferric compounds published by Jager and coworkers is also stabilized by a N4-macrocyclic ligand, [N4] which exist in different varieties of substitutions. The apical ligands are weakly coordinating halides or pseudohalides, such as iodide in the case of [Fe [N4]l] (20) [68]. The electronic structure was elucidated by EPR, Mbssbauer and DFT studies. 

Fig. 18. Plot of change in complex stability, Alogf , that occurs for the pairs of ligands cryptand-2,2,2 and EN ( ), and 18-aneN204 and EN (O), as a function of metal ion radius (20). The diagram shows how neutral oxygen donors tend to stabilize the complexes of large relative to small metal ions in macrocyclic ligands. Data from Ref. (11).

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

Thioether ligand complexes are not very common, however, studies since the mid 1990s have dramatically increased the number of structurally characterized examples. Almost all of these compounds are with chelate or macrocyclic ligands that will stabilize thioether coordination. Mixed donor multidentate ligands containing thioether donors will be discussed in Section 6.8.11. 

Thermodynamic aspects of the interaction of metal ions with macrocyclic ligands have been well studied. In many instances such studies have involved a comparison of the behaviour of cyclic ligand systems with that of their open-chain analogues. In this manner, information concerning the thermodynamic consequences arising from the cyclic nature of the macrocyclic ligand has been obtained. Frequently these studies have been restricted to stability constant (log K) measurements and, for such studies, a variety of techniques has been employed (Izatt etal., 1985). 

Table 6.1 summarizes the thermodynamic parameters relating to the macrocyclic effect for the high-spin Ni(n) complexes of four tetraaza-macrocyclic ligands and their open-chain analogues (the open-chain derivative which yields the most stable nickel complex was used in each case) (Micheloni, Paoletti Sabatini, 1983). Clearly, the enthalpy and entropy terms make substantially different contributions to complex stability along the series. Thus, the small macrocyclic effect which occurs for the first complex results from a favourable entropy term which overrides an unfavourable enthalpy term. Similar trends are apparent for the next two systems but, for these, entropy terms are larger and a more pronounced macrocyclic effect is evident. For the fourth (cyclam) system, the considerable macrocyclic effect is a reflection of both a favourable entropy term and a favourable enthalpy term. 

Figure 6.2. A comparison of the stabilities of the Cu(n) complexes of open-chain and macrocyclic ligands values are for water at 25 °C with / = 0.1 or 0.5 mol dm-3.

In the promotion of less-common oxidation states, much attention has been focused on the redox behaviour of transition metal ions such as nickel and copper although many other metal types have also had unusual oxidation states stabilized by macrocyclic ligands. However, within the limitations of a single chapter it is not possible to attempt a wide ranging... 

Macrocyclic ligands have also been effective in stabilizing less-common, lower oxidation states for a variety of metal ions. In particular, a considerable number of investigations have been concerned with the formation of stable Ni(i), Cu(i), Fe(i) and Co(i) species. 

A second possibility is that the metal center remains intact, but the macrocycle ligands react with each other. In macrocycles treated in the absence of a support there is evidence that polymerization of the macrocycles occurs.76,111 Likewise, in the presence of a carbon black support, such polymerization could occur during pyrolysis and could possibly affect activity and stability for similar reasons to the ones mentioned in the previous paragraph.76,92 However, for a treatment above 400 °C (which produces a more active material) the macrocycle polymer is thought to decompose.92 Another possibility is that the heat treatment helps disperse the macrocycles on the support surface and leads to strong chemisorption rather than physisorption.110... 

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