Macrocyclic complex

Much interest has also been expressed in tetra-azamacrocyclic compounds, due to their role in the natural reduction of C02 to CH4 by a nickel tetrapyrrole coenzyme found in methane-producing bacteria. Tinnemans et al. used Co(II) tetra-azamacrocyclic complexes with [Ru(bpy)3]2+ as the photosensitizer and ascorbic acid as the sacrificial electron donor in aqueous C02-saturated solutions at acidic pH [33]. Whilst the TON for the total observed products of CO and H2 exceeded 500, they were formed in a ratio of 0.27 1, respectively. 

Grant et al. studied a similar system using [Ni(cyclam)]2+ as the catalyst (cyclam = 1,4,8,11-tetra-azacyclo tetradecane), [Ru(bpy)3]2+ as the photosensitizer, and ascorbic acid as the sacrificial reductant [34], and observed a pH dependence on CO/H2 ratios, with the best ratio of 0.83 1 at pH 5. When Kimura etal. prepared pyridine derivatives of [Ni(cyclam)]2+ [35], the best complex, in C02-saturated ascorbate buffer at pH 5.1 and [Ru(bpy)3]2+ as the photosensitizer, produced 5.8-fold more CO than [Ni(cyclam)]2+. 

Mochizuki et al. synthesized a bimacrocydic Ni(II) complex, [6,6 -bi(5,7-dimethyl-l,4,8,ll-tetraazacyclotetradecane)]-dinickel(II) triflate [36]. In a C02-saturated aqueous solution at pH 4, with [Ru(bpy)3]2+ as the photosensitizer, and ascorbic acid as the sacrificial reductant, the CO/H2 ratio was 15 1 and the rate of CO production was approximately eightfold higher than for [Ni(cyclam)]2+. 

Matsuoka used a different photosensitizer, p-terphenyl, with a cobalt(III) cyclam as the catalyst [37-39], In a C02-saturated acetonitrile/methanol solution with either TEOA or TEA as the sacrificial reductant, the quantum efficiencies for CO and formic acid production were 15% and 10%, respectively, under 313 nm illumination. Again, however, the TONs and production rates for macrocyclic complexes were low. 

There is considerable interest in the use of non-noble metals for fuel-cell catalysis, which is not surprising when one considers the cost of the noble metal Pt (at well over 900 US per troy ounce) and its noble metal alloying additions. In a PEM fuel cell using pure hydrogen at the anode, Pt loading requirements down to 0.05 mg Pt cm are manageable. On the other hand, the eathode eomponent of the MEA still requires a loading of 1 mg Pt em in the best ease seenario, due to the slow kinetics of the ORR. 

This is a major reason why the majority of non-noble metal catalysis research has focused on the cathode part of the fuel cell system this work began about 1964 with a paper in Nature by Jasinski indicating that N4-metal chelates had electrochemical oxygen reduction capacity, and specifically with the discovery that C0N4 phthalocyanine is an oxygen reduction catalyst in alkaline solution [102, 

Subsequently, it was shown that metallic N4 phthalocyanine components were also active for the ORR in sulfuric acid solutions, with measured catalytic activity decreasing in the order Cu Ni Co Fe [103], 

The interaction of the oxygen molecule with the metal or metal chelate surface has been studied in some detail (e.g., [105]), and linear single metal interactions (i), single metal oxygen interactions (ii), and bridged interactions (iii) have been suggested and investigated [105]  

Obviously, the different types of interaction will lead to different kinetics and mechanisms, and different proportions of water to peroxide in the reaction product. Much has been written and speculated about these different interactions and the influence of metal d-orbital characteristics on the overall ORR. Reviews covering this aspect of the subject can be found in references such as [103] and [105]. On the whole, these conclusion can be drawn from the significant body of research on this topic (a) Fe and Co macrocyclic complexes appear to constitute the best catalysts for oxygen reduction (b) Ir complexes also appear to be quite active (c) the redox potential for the metal ion couple plays a major role in dictating the activity of the ORR, with optimal redox potentials existing for maximum activity (d) many of the non-noble metal catalysts lack the long-term stability needed for practical fuel cell applications and (e) heat treatment of supported non-noble metal catalysts tends to increase both their activity towards oxygen reduction and their stability, but optimum heat treatments achieve the best balance of the two. 

---

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

Porphyrin complexes have been the most intensively studied macrocyclic complexes of these metals [129]. They are formed in a wide range of oxidation states (II-VI) and they are, therefore, treated together under this heading, though most of the chemistry for ruthenium lies in the II-IV states. Octaethylporphyrin (OEP) complexes are typical. 

It has square planar coordination (Pd-N 2.010-2.017 A) similar to the value of 2.009 A in the tetraphenylporphyrin analogue, prepared by a similar route. As with nickel, macrocycle complexes can be made by in situ template... 

Reaction of iodine with Pt(phen)Cl2 gives compounds with the unusual stoichiometries Pt(phen)I (a = 5,6) these contain Pt(phen)I4 molecules and free iodine molecules in the lattice. Pt(bipy)I4 has also been made [172], Macrocyclic complexes of platinum(IV) are readily made by oxidation ... 

With a tridentate ligand Au(terpy)Cl3.H20 has, in fact, AuCl(terpy)2"1" with weakly coordinated chloride and water while Au(terpy)Br(CN)2 has square pyramidal gold(III) the terpyridyl ligand is bidentate, occupying the axial and one basal position [124]. Macrocyclic complexes include the porphyrin complex Au(TPP)Cl (section 4.12.5) cyclam-type macrocyclic ligands have a very high affinity for gold(III) [125],... 

Figure 4.24 Template synthesis of a gold(lll) macrocycle complex.

Compounds 139 are tris(oximehydrazone) derivatives with an iron(ll) ion in the center of the cavity [230]. Compound 140 (Fig. 38) has been known for 30 years [231, 232] and was prepared from a tris(2-aldoximo-6-pyridyl)phos-phine that is capped by a BF unit to encapsulate cobalt(ll), zinc(ll), nickel(ll), and iron(II). All four macrocyclic complexes were characterized later by a comparative X-ray crystallographic study [233-236]. 

Scheme 1. Formation of a macrocyclic complex between a diboronic acid and a saccharide...

Macrocyclic Complexes as Models for Nonporphine Metalloproteins Vickie McKee... 

In this section, the corrinoids, the other macrocyclic complexes, and the cyanides are dealt with separately (Sections A-C). The preparative organo-metallic chemist will be primarily interested in Sections B and C, whereas... 

Complexes such as [Ni(16)] are known to stoichiometrically interact with 02 to give 1 1 adducts and subsequently the autoxidized Ni111 species (compare Section 6.3.4.10.2(v)). 5 Such systems have been tested for the NiIII-catalyzed cleavage of DNA (see Section 6.3.4.10.2(v)). It has been suggested that Ni11 macrocycle complexes with rather low Nin/Nim reduction potentials can be active inhibitors of aldehyde autoxidation.156... 

The Ni—N bond distances, N—N bite distances, and N—M—N bite angles of Ni11 macrocyclic complexes depend on the coordination number of the metal ion and the type of macrocycle. These... 

Square planar Ni11 macrocyclic complexes are typically yellow, red, or brown in color and absorb around 400 500 nm, depending on the ligand structure. The octahedral Ni11 complexes absorb at 500A100 nm. 

Table 8 Electronic absorption spectra and electrochemical data for Ni11 macrocyclic complexes.

Electrochemical properties of the Ni11 macrocyclic complexes are related to the cavity size, the unsaturation, the degree of functionalization, and the subring moieties fused to the macrocycles.1467,1514,1581,1582... 

---