Pyridine macrocycles seven-coordination

Among many different complexes that have been synthesized in attempt to mimic the structure and/or functionality of SODs (16-22), the most active SOD mimetics known to date are seven-coordinate Mn(II) complexes with macrocyclic ligands derived from C-substituted pentaazacyclopentadecane [15]aneNs and its pyridine derivative (Scheme 4) (12d,16a,23-25). Some of them possess SOD activity that exceeds the one of native mitochondrial MnSOD, and are the first SOD mimetics which entered clinical trials (12d,16a,23,26-28). A few Fe(III) complexes with the same type of ligands have also been studied and they are one of the best iron-based SOD catalysts (18). It should be stressed that the decomposition of superoxide catalyzed by these complexes has been quantified by direct stopped-fiow method, in the presence of a substantial superoxide excess over catalyst, as a reliable method for determining true SOD activity (29). 

The electrochemistry of a number of such six-coordinate compounds [MnXL]+ and seven-coordinate compounds [MX2L] (with L = (203), R,R = Me and X = halide, water, triphenylphosphine oxide, imidazole, 1-methylimidazole or pyridine) has been investigated.551 The redox behaviour of these compounds was of interest because it was considered that the potentially -acceptor macrocycle (203 R = R = Me) may promote the formation of Mn° or Mn1 species or may yield a metal-stabilized ligand radical with the manganese remaining in its divalent state. For a number of macrocyclic ligand systems, it has been demonstrated that the redox behaviour can be quite dependent on axial ligation it was also of interest to study whether this was the case for the present systems. 

Examples of seven-coordination in pyridine-containing macrocycles have also been osberved. Thus the iron(TI) complexes [Fe(L)X,] xhLO (L = 126 X = halide or pseudohalide) are formed923 when the corresponding seven-coordinate iron(lll) complexes are reduced with aqueous sodium dithionite in the presence of excess NaX or by the direct template synthesis with 2,6-diacetylpyridine and the corresponding tetramine in the presence of iron(IJ). The triimine moiety gives rise to a strong... 

A review of expanded porphyrin hgands can be fonnd. The texaphyrins (8) can be considered as 22-tt-electron benzaimnlene systems with an IS-tt-electron delocahzation path, based on crystal stmctnre data as well as NMR. The cadminm complex of the macrocycle is fonnd to be planar with pentadentate coordination of the macrocycle to cadminm, which becomes seven-coordinate as a resnlt of axial coordination to two pyridine molecnles. The cavity is nearly circular with a center-to-nitrogen distance of 2.39 A. Because of the larger size of this macrocycle, metal ion coordination is generally seen with the larger transition metals and lanthanides. 

The last example, Figure 15, is a bromo chloro macrocyclic iron complex. Bromine is CIP priority 1, chlorine is 2, the imine nitrogens 3 and 4, the pyridine nitrogen 5, and the amine nitrogens 6 and 7. The complex is pentagonal bipyramidal-seven coordinate, PB-7, the atoms on the axis are priority numbers 12- and the priority numbers for the atom in the plane are given in lowest sequential order, 35476. This clockwise order is symbolized as C. 

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