Pentaaza macrocycles

The functional groups can be attached to the macrocyclic ligands by using methods similar to Eqs. (1) and (2). The Ni(II) complexes of pentaaza macrocyclic ligands, 3b-3g, in which functional groups are appended at the bridgehead nitrogen atom, are prepared by the template reaction of [Ni(2,3,2-tet)]2+ and formaldehyde with carboxamide or sulfonamide in the presence of base. The complexes 3b and 3c are... 

The Ni(II) complexes l-3g with hexaaza and pentaaza macrocyclic ligands as well as the Ni(II) cyclam complex display a low-spin square-planar and high-spin octahedral complex interconversion in aqueous solution (7, 12-14). However, complexes l-3a favor formation of the high-spin form upon addition of acid, which is in contrast to the cyclam complex. The protonation of tertiary nitrogen atoms at the bridgehead position facilitates the axial coordination of anion or water because of hydrogen-bonding between them 12, 14a). 

A summary of nickel(H) complexes formed by representative saturated polyaza macrocycles is reported in Table 103, together with a concise description of the synthetic procedures and some of their physicochemical properties. Most studies are concerned with nickel(II) complexes with tetraaza macrocycles, but examples of complexes with triaza and pentaaza macrocycles are not rare. 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

In a manner similar to that used to prepare 112, the reaction of the diformyl-tripyrrane 114 with o-phenylenediamine was found by Sessler and coworkers to result in the synthesis of a pentaazamacrocycle 115 (Scheme 13) [59], An X-ray structure of a derivative of 115 is shown in Fig. 19. Unfortunately, no structurally characterized metal complexes of 115 have been reported to date. However, oxidation of 115 in the presence of cadmium(II) was found to give the aromatic pentaaza-macrocycle metal complex 116, which has been characterized by X-ray diffraction [60]. The properties and chemistry of these tripyrroledimethine-derived texaphyrins is reported in the next section. 

Transition metal complexes of the larger polyaza macrocyclic ligands have been less extensively studied than for the smaller ring systems. For the pentaaza macrocycles, [ISJaneNs with ethylene bridges appears to form the most stable complexes with most metal ions. Structural data for a variety of pentaaza macrocyclic complexes have been reviewed. The N-H bonds as well as the different sized chelate rings must be considered in calculating the... 

Low-molecular-weight catalysts which mimic a natural enzymic function syn-Zymes) have potential utility for the treatment of diseases characterized by the overproduction of a potentially deleterious metabolic by-product or foreign gene product. The discovery and development of pentaaza macrocyclic ligand complexes of manganese(II) as functional mimics of superoxide dismutase (SOD) enzymes and their potential utility as human pharmaceutical agents is described. 

Vanadium compounds, mainly square pyramidal complexes of the type [0=V(0—R—0)2], act as insulin mimetics with potential in diabetes control if toxicity issues can be overcome one is in use as an orally-administered agent in animals. A pentagonal bipyramidal complex of manganese(II), CMnC A ], where N5 is a pentaaza macrocycle with four amine nitrogen donors and one pyridine nitrogen donor, acts as a... 

A common drawback with macrocyclic ligands is their slow complexation rate (Jang et al. 1999). A way to overcome this problem is to increase the ring size. Following this idea, two larger chelators have been developed, one based on a pentaaza macrocycle (PEPA) and the other on a hexaaza macrocycle (HEHA) O Fig. 45.3). Both these chelators show much faster labeling kinetics with some radiometals (by a factor of about 100 times faster). Still, they have slower labeling kinetics than the acyclic chelator DTPA (by a factor of about 10) (O Table 45.4). 

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