Pentaazamacrocyclic complexes

Fig. 8. Pustulated mechanism for superoxide dismutation by manganese pentaazamacrocyclic complexes (modified from Ref. 7b).

Mn(II)pentaazamacrocyclic complexes are least studied in terms of any other reactive oxygen or nitrogen species except superoxide, as the authors always stressed that they posses strict selectivity. However, we showed (43) that some representatives of this class do react with NO (vide infra) and suggested that a reaction is possible with peroxynitrite (44) as well. Figure 15 clearly shows that when present in solution, Mn... 

Scheme 4. Chemical structure of two representatives of Mn (pentaazamacrocyclic) complexes studied for the reaction with NO. 1, SOD active complex [Mn(II)(pyane)] 2, SOD inactive precursor of 1, [Mn(II)(pydiene)].

It should be emphasized that one of the reasons why most of the MnSOD mimics, in particular pentaazamacrocyclic complexes, have not been tested for the reaction with NO is because of the prevailing opinion that all of them do not have sufficiently high redox potential to reduce NO via outer-sphere electron transfer (redox potential of these MnSOD mimetics is >0.8 V, NHE) 47a). However, these complexes are generally prone to react with different monodentate ligands, and coordination of NO is quite feasible. Once NO coordinates, its redox potential shifts toward significantly more positive values, enabling an inner-sphere electron transfer resulting in the Mn (III)NO nitrosyl species. 

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. 

One of the main problems with studying nonporphyrin Mn complexes with SOD acitivity is that the UV-vis spectral properties are very limited and not well defined, so the need for combination of different other spectroscopic methods is needed. By combining the amperometric detection of NO consumption, with ATR-FTIR, EPR, and MS spectrometry, together with some analytical methods for detection of the products, we were able to detect all reaction steps behind the reaction of Mn(II) pentaazamacrocyclic SOD mimics and NO. The addition of SOD active [Mn(pyane)] and inactive [Mn(pydiene)] complexes (Scheme 4) into NO solution leads to immediate removal of NO. This process was followed by formation of three distinctive IR bands at 1840, 1653, and 1647cm assigned to Mn(II)NO+ and six- and seven-coordinate Mn(III)NO, respectively, as Mn(III) is not very stable in seven-coordinate geometry and an equilibrium between seven- and six-coordinate species exists in the solution (27). Consequently, when followed by EPR, the cycling between EPR active Mn(II) and EPR inactive Mn(III) could be observed (Fig. 16). 

Template tailoring of acyclic tetramines can also be carried out using formaldehyde and primary amines, amides and sulphonamides [190-195]. From the general reaction (2.93) pentaazamacrocyclic species [M(L161)] +, where M = Ni or Cu have been prepared. Details concerning the yield of each particular reaction and designation of substituent R are given in Tables 2-1 and 2-2, for nickel(II) and copper(II) complexes, respectively. 

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