Tetraazamacrocycles, metal complexes

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... 

Haines, R.I. Pendant arm tri- and tetraazamacrocycles synthesis, structure and properties of their first-row transition metal complexes, Rev. Inorg. Chem. 21 (2001), 165-205. 

Alkali metals used to reduce the Co(II) are essential as ions to give CO2 binding to Cod). A large number of Co-7 HC)-C02 complexes have been prepared by this method. The kinetics and equilibrium of CO2 binding to a series of tetraazamacrocycle Cod) complexes,... 

Such hydrido intermediates have been detected in the reactions of Cr2 +, 120,121 Cu +, 122 Fe2 +,123 and Ti3 +.124 The rates of formation of the hydrido species conform to the usual reaction patterns for Id substitution kinetics. An example of H-atom addition to metal complexes is provided by the reaction of [Ni(cyclam)]2 + and a Co (II) tetraazamacrocycle.82,125... 

Macrocycles within macrocycles were synthesized where tetraazamacrocycles and their metal complexes are encapsulated within CB[8]. The 1 1 inclusion complexes cyclen4HCl-CB[8], cyclam-4HCl-CB[8], and [Cu(cyclen)(H20)](N03)2-CB[8] (Fig. 3b) were characterized by X-ray crystallography. An electrochemical study shows that the encapsulation of Cu(cyclen) in CB[8] increases the stability of the Cu(I) state but considerably... 

Addition of pendant donor groups increases the coordination number of the ligand-metal complex. Hence, consideration of the ability of the target ion to accommodate the increased coordination number is necessary. For example, Hancock and Martell (13) review the effect of addition of four N-acetate pendant groups to tetraazamacrocycles. Metals such as Ca that can accommodate a coordination number of eight experience a dramatic increase in complex stability, whereas metals that cannot accommodate the increased coordination (e.g. Cu ) experience a decrease in complex stability. 

The slowness of formation and dissociation of metal complexes of rigid tetraazamacrocycles such as porphyrins and phthalocyanines has long been recognized, but is still being investigated and quantified. Recent examples of kinetic studies in this area include the displacement of tetrakis(4-sulfonato-phenyl)porphyrin from Gd " by edta, " of tetrakis(iV-methyl-4-pyridyl) porphyrin from Ln " (Ln = Yb, Dy, Gd, Sm, Nd) again by edta, and of dechelation of tetraphenylporphyrin from acetato-, chloro-, and bromo-derivatives of Sc " and y " ", of Eu , Gd ", and and of Ti " "... 

CONTENTS Preface. George W. Gokel. Cryptophanes Receptors for Tetrahedral Molecules, Andre Collett, Jean-Pierre Dutasta and Benedict Lozach. Inclusion Polymerization in Steroidal Canal Complexes, Kiichi Takemoto, Mikiji Miyata. Functionalized Tetraazamacrocycles Ligands with Many Aspects, Thomas A. Kaden. Calixarenes as the Third Supramolecular Host, Seiji Shinkai, Kyushu University, Japan. Fluorescent Chemosensors for Metal and Non-Metal Ions in Aqueous Solutions Based on the Chief Paradigm, Anthony W. Czamik. Index. 

In comparison with some other metal ions, limited synthetic work has been carried out on chromium(III) complexes with macrocyclic ligands. The vast majority of reported complexes involve tetraazamacrocycles. 

An area of increasing interest is the selective complexation of Sn2+ and more particularly Pb2+ for the treatment of heavy metal poisoning. Molecular mechanics has been extensively applied to the problem of metal ion selectivity (see Chapter 8) but there have been few studies of lead or tin complexes. The fit of Sn2+ to 18-crown-6 has been considered12811, as has the size selectivity of tetraazamacrocycles with respect to Pb2+ binding131. The binding of Pb2+ to porphyrin-1 has been modeled, though in this case the point of interest was the structural deformations caused by the metal cation11901. 

Figure 6-32. The condensation of 2,6-diacetylpyridine with the diamine in the presence of nickel(n) or other first-row transition metal dications leads to complexes of the tetraazamacrocycle 6.33.

A number of 14-membered tetraazamacrocyclic complexes serve as catalysts for photochemical and electrochemical CO2 reduction. [CoHMD(H20)](C104)2 (HMD = 5,7,7, 12,14,14-hexamethyl-l,4,8,1 l-tetraaza-cyclotetradeca-4,11-diene) [ 1,2] and Ni(cyclam)Cl2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) [3] have been used as electrocatalysts for the reduction of CO2 in H2O or aqueous CH3CN. The ratio for CO/H2 production is 1 for [CoHMD(H20)](C104)2 and >100 for Ni(cyclam)Cl2. Metal(I) complexes, metal(III) hydride complexes, and metallocarboxylates such as [Ni (cyclam)(C02 )] are postulated as intermediates in the electro- and photo-chemical CO2 reduction [4]. 

Complexes of metals with tetraazamacrocycles possessing pendant arms are currently being considered as models for biologically relevant systems. Such species generally possess a metal ion bound to... 

King, G., S.J. Higgins, and N. Price (1992). Novel conducting polymers incorporating covalently bound metal-tetraazamacrocycle complexes. Analyst 117, 1243-1246. 

Higgins, S.J., T.J. Pounds, and PA. Christensen (2001). Synthesis and electro (co)polymerization of novel thiophene- and 2,2 5, 2"-terthiophene-functionalized metal-tetraazamacrocycle complexes, and electrochemical and spectroelectrochemi-cal characterization of the resulting polythiophenes. J. Mater Chem. 11, 2253-2261. 

Tetraazamacrocyclic ligands incorporating an intracyclic terpyridine chromophore 39 was described [106, 107], as expected the absorption maxima (335 nm) was more advantageous compared to 38. The physical characteristics were described X (H2O) = 1.06 ms, luminescence decays indicated that no water molecule is coordinated to the metal ion suggesting that the nine binding sites provided by the ligand are coordinating the lanthanide. No decomposition of these complexes was observed after 1 day in... 

Thus, in examples of synthesis of tetraazamacrocyclic compounds on nickel ions (mainly) and on copper(II), square-planar system assembling is realised [38,47-49]. Guanidium ion promotes building of 27-crown-9-ether [60]. o-Aminobenzaldehyde self-condensation on the copper ion as matrix results exclusively in forming a complex with TAAB [67]. When using nickel(II) or cobalt(II) as templates under the same conditions, two types of macrocyclic azomethine systems - TAAB and TRI - may be synthesised [67-71]. Macrocychsation of phthalonitrile on the anisotropic matrix 0=U=0 ends with obtaining the so-called superphthalocyanine product [U02(L29)] [72, 73], rather than with the isolation of the corresponding complex with the phthalocyanine (Pc), as observed for other metal ions (Eq. 1.18) [11,74]. 

Completely saturated tetraazamacrocyclic compounds and their complexes with metals are usually obtained by non-template procedures [159]. Nevertheless, there are two often-used template strategies which show the effectiveness of using reactions on matrices for assembling such systems ... 

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