Ligand, axial

Reaction of free-base porphyrin compounds with iton(II) salts in an appropriate solvent results in loss of the two N—H protons and insertion of iron into the tetradentate porphyrin dianion ligand. Five-coordinate iton(III) porphyrin complexes (hemins), which usually have the anion of the iton(II) salt for the fifth or axial ligand, ate isolated if the reaction is carried out in the presence of air. Iron(II) porphyrin complexes (hemes) can be isolated if the reaction and workup is conducted under rigorously anaerobic conditions. Typically, however, iton(II) complexes are obtained from iton(III) porphyrin complexes by reduction with dithionite, thiolate, borohydtide, chromous ion, or other reducing agents. 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

Similarly, the corresponding nickel complex, based on ESR spectroscopy, should be formulated as a nickel(II) corrole-71-radical. The iron corroles exist in the oxidation state + 111 or + IV depending on the nature of additional axial ligands. 

The cobalt corroles are as expected isolated as neutral cobalt(III) complexes with an additional neutral axial ligand, which in most cases is triphenylphosphane or pyridine. 

Additional axial ligands can be linked to the center of phthalocyanines either by valence bonds as in the case of silicium phthalocyanine151 or by coordinative bonds as in the case of bis(pyridino)iron(II) phthalocyanine [PcFe(py)2].131353... 

Figure 6.8 Rationale for enantioselection in Mn-salen epoxida-tion. a) Initially proposed model (the axial ligand L is omitted for clarity), b) Recently proposed model based on calculation.

There is a slight dependence on the nature of the carboxylate group and upon the axial ligand, but they are not imposed by the steric requirements of the carboxylates. Some points are germane to this ... 

The effect of axial ligands on ground state properties of complexes with orbitally degenerate ground terms. G. A. Webb, Coord. Chem. Rev., 1969, 4,107-145 (151). 

Structural aspects and coordination chemistry of metal porphyrin complexes with emphasis on axial ligand binding to carbon donors and mono- and di-atomic nitrogen and oxygen donors. P. D. Smith, B. R. James and D. H. Dolphin, Coord. Chem. Rev., 1981,39, 31-75 (170). 

Organocobalt B models axial ligand effects on the structure and coordination chemistry of coba-loximes. N. Bresciani-Pahor, M. Forcohin, L. G. Marzilli, L. Randaccio, M. F. Summers and P. J. Toscano, Coord. Chem. Rev., 1985, 63,1 (263). 

FIGURE 1. Structures for Me2SCT(I) and MeSO" Me(II). The assignment of 0.5 spin density to each of the two axial ligands in I is only applicable to equivalent ligands in the Rundle formulation. Qualitatively, more spin density is expected to reside on the axial methyl than on the O group. Reproduced by permission of the authors from Reference 8. 

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