Tetraazamacrocyclic

Cobalamin chemistry is the best-established area of cobalt biological chemistry. The 15-membered tetraazamacrocyclic corrin ring (incorporating four pyrrole residues) is the binding site for... 

Studies on the oxidative chemistry of Ni complexes with larger macrocycles, in particular with tetraazamacrocycles, are countless. Pulse radiolysis continues to be one of the methods used for... 

Aliphatic tetraazamacrocycles are well known to be effective in stabilizing Ni in higher oxidation states due to the very strong in plane -interactions which raise the energy of the (metallic) antibonding orbital from which the electron is extracted on oxidation. Table 1 lists characteristics... 

Other complexes with tetraaza macrocycles have been prepared by reaction of [Au(en)2]Cl3, ethylenediamine, or nitroethane and formaldehyde, although with nitroethane an acyclic ligand was also obtained (293).1715,1716 A gold(III) complex with a hexaaza macrocycle (1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) has been obtained by a transmetallation reaction from the nickel compound [NiL]2+ by reaction with [AuC14], 1717 The chemistry of tetraazamacrocycles in aqueous solution has been reported.1718... 

Bradley et al.109 have combined a p-Si photocathode and homogeneous catalysts (tetraazamacrocyclic metal complexes, which had been shown to be effective catalysts for C02 reduction at an Hg electrode110) to reduce the applied cathode potential. The catalysts showed111 reversible cyclic voltammetric responses in acetonitrile at illuminated p-Si electrodes at potentials significantly more positive (ca. 0.4 V) than those required at a Pt electrode, where the p-Si used had surface states in high density and Fermi level pinning112 occurred. Electrolysis of a C02-saturated solution (acetonitrile-H20-LiC104 1 1 0.1 M) in the presence of 180 mM... 

Tetraazamacrocyclic complexes131 of cobalt and nickel were found110 to be effective in facilitating the reduction of C02 at -1.3 to -1.6 V versus SCE (Table 8). An acetonitrile-water mixture and water were used as solvents, while in dry dimethylsulfoxide no catalytic reduction of C02 took place. Using an Hg electrode, both CO and H2 were produced, where total current efficiencies were greater than 90%. The turnover numbers of the catalysts were 2-9 h 1. The catalytic activity lasted for more than 24 h and the turnover numbers of the catalysts exceeded 100. A protic source was required to produce both CO and H2, and the authors suggested that both products may arise from a common intermediate, which is most likely a metal hydride. The applied potential for C02 reduction was further reduced by using illuminated p- Si in the presence of the above catalysts.111... 

Tinnemans et al.132 have examined the photo(electro)chemical and electrochemical reduction of C02 using some tetraazamacrocyclic Co(II) and Ni(II) complexes as catalysts. CO and H2 were the products. Pearce and Pletcher133 have investigated the mechanism of the reduction of C02 in acetonitrile-water mixtures by using square planar complexes of nickel and cobalt with macrocyclic ligands in solution as catalysts. CO was the reduction product with no significant amounts of either formic or oxalic acids... 

A number of novel dioxo variants of the tetraazamacrocycles have been synthesized and stability constants were measured with Cu2+.114 A few of these were further evaluated as chelating agents with 64Cu.115 Structural differences included ring size (from 12 to 14) and placement of the oxo groups. Of the six variations synthesized, only one, l,4,8,ll-tetraazacyclotetradecane-3-9-dione(14N402-3,9), readily formed a complex (Figure 9). 

A variety of other mono-A-substituted tetraazamacrocycles have been prepared (Lotz Kaden, 1978 Hediger Kaden, 1978 Keypour ... 

Ni2+ was very popular in the early days of the investigation of mechanisms of complex formation, since the time-scale for its reactions with simple ligands was so convenient for the then recently developed stopped-flow technique. However, interest has now moved on to other first-row cations, especially to Cu2+. A review of the kinetics and mechanisms of formation of tetraazamacrocyclic complexes concentrates on Ni2+ and Cu2+, and their reactions with cyclam and similar ligands (267). The tetra(4 -sulfonatophenyl)porphyrin complexes of Ni2+ and of Cu2+ react immeasurably slowly with cyanide, but their IV-methyl derivatives do react, albeit extremely slowly. The relevant time scales are hours for removal of Ni2+, months for the removal of Cu2+, by 10-4 M cyanide at pH 7.4 (268). 

Table 8 Formal electrode potentials (V vs. SCE) for the redox processes exhibited by the tetraazamacrocycle-Fe(II) complexes in MeCN solution...

As for d6-iron(II), d6-cobalt(III) also forms complexes with the 14-membered tetraazamacrocycles shown in Scheme 13. 

The most extensive electrochemical study of nickel complexes is probably that carried out on tetraazamacrocycle-Ni(II) derivatives, a few examples of which are reported in Scheme 17. 

It is noted that the (tetragonally distorted) octahedral Ni(II)-tetraazamacrocycle complexes of general formula trans[NiN4X2] are high spin (d8-dvz2dx-2dxi2d 1 dx2 y21). 

The effect of solvent on the charge-transfer bands of thiocyanato-tetraazamacrocycle manga-nese(III) complexes has been investigated and shown to be influenced by the donor and acceptor (H bonding) properties of the respective solvents." ... 

The first reported low-spin iron(III) semiquinonate complex contained the 3,5-di-t-butyl-l, 2-benzoquinonate radical anion and a tetraazamacrocycle (tazm). It reacts reversibly with acetonitrile to give [Fe (tazm)(MeCN)2] plus 3,5-di-t-butyl-l,2-benzoquinone. " The related 1,2-iminobenzosemiquinonate (ibsq) complexes [Fe(ibsq)2X] have 5 = 5/2 for X = C1 and 5 =3/2 for X = I the complex with X = Br is a mixed spin (5=5/2, 3/2) species. " ... 

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