Metalloporphyrins structure

Fig. 12. Schematic view of the possible orientations of a bound acetate group in metalloporphyrin structural chemistry. The oxo ligand of the Nb complex and the second acetate groups of the Zr and Hf derivatives are omitted for clarity...

Cullen. D. Meyer, E., Jr. Srivastava. T.S. Tsutsui. M. Unusual metalloporphyrins—Structure of product from reaction of dodecacarbonylruthenium with meso-tetra-phenylporphine—Dicarbonyltetraphenylporphinatomthe-nium (TT). J. Chem. Soc., Chem. Commun. 1972. 584-585. 

Acknowledgments. The authors gratefully acknowledge financial support from the National Institutes of Health (GM-28882) and Mallinckrodt Sensor Systems. The authors also wish to thank Professor John T. Groves, Department of Chemistry, Princeton University, for providing several of the metalloporphyrin structures used in these studies. 

Schmidtke H-H, Degan J (1989) A Dynamic Ligand Field Theory for Vibronic Structures Rationalizing Electronic Spectra of Transition Metal Complex Compounds. 71 99-124 Schneider W (1975) Kinetics and Mechanism of Metalloporphyrin Formation. 23 123-166... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

The above-described structures are the main representatives of the family of nitrogen ligands, which cover a wide spectrum of activity and efficiency for catalytic C - C bond formations. To a lesser extent, amines or imines, associated with copper salts, and metalloporphyrins led to good catalysts for cyclo-propanation. Interestingly, sulfinylimine ligands, with the chirality provided solely by the sulfoxide moieties, have been also used as copper-chelates for the asymmetric Diels-Alder reaction. Amide derivatives (or pyridylamides) also proved their efficiency for the Tsuji-Trost reaction. 

Relatively little work has been done on ORR catalysis by self-assembled mono-layers (SAMs) of metalloporphyrins. The advantages of this approach include a much better defined morphology, structure, and composition of the catalytic film, and the surface coverage, and the capacity to control the rate at which the electrons ate transferred from the electrode to the catalysts [CoUman et al., 2007b Hutchison et al., 1993]. These attributes are important for deriving the catal5d ic mechatfism. The use of optically transparent electrodes aUows characterization of the chemical... 

Fig. 18.17) when adsorbed on a graphite electrode, it manifests catalytic behavior comparable to that of series 2 metalloporphyrins. (b) The simplest structural motif that ensures the axial imidazole ligation of the Fe site [Khvostichenko et al., 2007 Yang et al., 2008]. 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

Metalloporphyrin reactivity and catalytic properties can also be controlled by modifications of the porphyrin core. Ni coordination chemistry of core-modified porphyrins has been reviewed.1791,1792 Several X-ray structures of four-, five-, and six-coordinate Ni11 complexes with oxa-, thia-, and dioxatetraarylporphyrins are known. 

Asymmetric induction by metalloporphyrin is affected not only by the structure of the ligand, but also by other factors the nature of the metal center, the oxidant used, and the donor ligand added.62,63 As shown in Figure 1, oxo-metalloporphyrins have been considered to be the active species in metalloporphyrin-catalyzed oxidation. In some oxidations, however, metal-oxidant adducts have been suggested as the real active species. 

Pharmaceutical discoveries are principally made by thoughtful structural variation on a lead compound which has been found, by chance or design, to have a certain amount of the desired activity. It is clear that with metalloporphyrins, there are additional structural variations to be had. In the first place, it is possible to vary the metal. It was realized at an early stage that inserting and varying the metal would modify PDT activity.61 The possibility also exists of structural changes in axial ligands in those metalloporphyrins which possess them. This structural variation occurs in the space immediately above and below the macrocycle, which is a space not readily accessible to controlled variation in metal-free compounds of this series. 

For a review of absorption and emission spectroscopy of porphyrins and metalloporphyrins, see Gouterman, M. Optical spectra and electronic structure of porphyrins and related rings. In The Porphyrins Dolphin, D., Ed., Academic Press New York 1978, Vol. Ill, pp 1-165. 

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