Macrocyclic tetraaza ligands

In addition, iron(II) complexes of tetraaza macrocyclic ligands 17-20 were encapsulated within the nanopores of zeolite-Y and were used as catalysts for the oxidation of styrene with molecular oxygen under mild conditions (Scheme 9) [57]. 

Scheme 9 Tetraaza macrocyclic ligands for supported iron catalysts...

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. 

Halo-alkenes are common pollutants. Therefore, there is an ongoing study on plausible approaches to the dehalogenation of halo-alkanes. One of these approaches involves their electrocatalytic reduction. NinL2 + (L = a tetraaza macrocyclic ligand) complexes were proposed as plausible electrocatalysts (150). A pulse radiolytic study on the mechanism and kinetics of the reaction ... 

The 12-membered tetraaza-macrocyclic ligand with four pendant pyrazole groups, L6, was synthesized by the reaction of l-(hydroxy-methyDpyrazole with 1,4,7,10-tetraazacyclododecane in MeCN. The Ni(II) complex [NiLe]I2 formed a distorted octahedral structure with... 

The Ni(II) complexes of 14-membered tetraaza macrocyclic ligands, cyclam, Lie, L18, and Lie show catalytic activity in H20 or aqueous MeCN. The total mole-for-mole yields of CO and H2 are ca. 1 in most cases. The [Ni(cyclam)]2+ complex is a very effective and selective catalyst for the electrochemical reduction of C02 to CO relative to the reduction of water to H2 in aqueous solution when it is adsorbed onto mercury. The CO/H2 product ratio is >100 for [Ni(cyclam)]Cl2 (79). It is suggested that the greater selectivity for the electroreduction of C02 compared with water is related to the size of the macrocyclic ligand... 

The restricted geometry of the tetraaza macrocyclic ligands favors the formation of low-spin, square-planar nickel(II). An equilibrium exists between the blue, high-spin, six-coordinate form and the yellow, substitution-inert, square-planar form [Eq. (1)]. Intercoversion... 

The thermodynamic and kinetic stabilization ofnickel(III) macrocyclic complexes by axial coordination has prompted a number of new approaches. Studies with tetraaza macrocyclic ligands with pendant donors acting as potential fifth ligands have had some success (96, 99,100). Oxidation of [Ni"[14]aneN4CH2CH2py]2+ in aqueous solution... 

Like other tetraaza metallo(I) complexes, the nickel(I) macrocyclic ions are powerful and labile reducing agents. A point of some interest in these systems is to design a complex couple for which the nickel(I) state is accessible at reasonable potentials. Provided the tetraaza macrocyclic ligand maintains close to planar microsymmetry, reorganizational barriers for a low-spin d8-d9 system might be expected to be small (194). 

The energetics of isomer prediction using molecular mechanics is discussed in detail in Chapter 7. One of the results of such a study is the structure of each of the isomers.The archetypal studies in this field relate to the complexes [Co(dien)2]3+ (dien = 3-azapentane-l,5-diamine see Chapter 7). Other important studies include those on macrocyclic ligands (see also Chapter 8). Tetraaza macrocyclic ligands, for example, can adopt a series of configurational isomers, and these have been the subject of numerous molecular mechanics calculations. Consider an equatorially coordinated tetraaza macrocylce. Each of the amine groups can coordinate with the amine proton or substituent disposed above or below the coordination plane. How many isomers result depends on the symmetry of the macrocycle. For example, in the classic case of cyclam (cyclam - 14-ane-N4 = 1,4,8,11-tetraazacyclotetradecane) there are five isomers[12] and these are shown schematically in Fig. 6.3. It is not always possible to prepare or separate all of these isomers and, therefore, in many cases only a minority have been structurally characterized. Thus, the energy-minimized structures represent the best available three-dimensional representations of the other isomers. 

Table 8.2. Cavity size for the tavu-III conformers of tetraaza macrocyclic ligands, calculated by molecular mechanics.

Table 9.3. In-plane ligand field splitting (Dq ) or transition energies (cm ) for square planar trans-octahedral transition metal complexes with tetraaza macrocyclic ligands 1 .

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

Figure 6-16. Co-ordination of the four nitrogen atoms of a tetraaza macrocyclic ligand to a metal results in a restricted inversion at each nitrogen. The filled circles represent a substituent lying above the plane of the paper, and the open circles one lying below the plane. In the case of cyclam, with hydrogen substituents, the barriers to inversion are relatively low, but with bulkier substituents on the nitrogen the different diastereomers are readily isolable. Note also that the isomers labelled II and V are chiral and will exist in two enantiomeric forms.

Figure 6-18. The condensation of the [Ni(en)3]2+ salts with acetone yields nickel(n) complexes of a new tetraaza macrocyclic ligand.

Several Co complexes with chelating nitrogen ligands catalyze oxidation reactions, e.g. [CoII(bipy)2]2+ activates 02 and oxidizes N-methyl-anilines, benzyl alcohols, and aldehydes. In the absence of organic substrates 02 is reduced to H202.M Cobalt(II) complexes of tetraaza macrocyclic ligands (N4) reversibly form 02 adducts [(N4)CoOO]2+ which are rapidly reduced to [(N4)CoOOH]2+ these species are involved in the electro-reduction of 02 to H202.23... 

A large number of complexes of tetraaza macrocyclic ligands is known. These ligands have from 12 to 16 atoms in the ring, and the most common structure is a 14-membered ring. The simplest of the ligands is 1,4,8,11-tetraazacyclo-tetradecane ... 

An extension of the template reaction mentioned above [involving condensation of the Cu(II) complex of a tetradentate amine with formaldehyde and a nitroalkane] was employed to prepare the Cu(II) complexes of the series of bis(tetraaza macrocyclic) ligands given by 45 (79). Reduction of the nitro groups in these species with Zn/HCl produced the corresponding metal-free bis(pendant amino) derivatives (46). 

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