Porphyrin ligands, structures

Ligands with more than one coordination site have been synthesized. A superstructured porphyrin ligand with an additional binding site for a second zinc ion was synthesized. The X-ray structure shows a bridging acetate ligand between the porphyrin cavity-bound zinc and the zinc bound to the appended tripodal chelator.778... 

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

Secondly, there is an indication that metal(III)-peroxo side-on complexes are in general in equilibrium with corresponding metal(II)-superoxo end-on species. The position of such equilibrium could depend on various factors as structural and electronic properties of the porphyrin ligand, coordination of an axial ligand trans to peroxide/superoxide, solvent medium, temperature and involvement of coordinated peroxide/superoxide in possible hydrogen bonding or electrostatic interactions. These are interesting questions which should be addressed in future studies. 

One of the two most common nonplanar deformations of the porphyrin ligand is the saddle conformation in which the pyrrole Cp—C bonds are displaced alternately above and below the mean 24-atom plane. The other is the ruffled conformation in which the Cmeso carbon atoms are displaced alternately above and below the mean 24-atom plane with concomitant twisting of the pyrrole rings. There are various measures for the extent of mffling for example, the C eso cross-ring distance decreases and the mean displacement of Cmeso from the mean 24-atom plane increases as the structures become more ruffled. The average M—N... 

Figure 8. Empirical correlations of Fe porphyrins XANES structure with spin state. A) The "Ligand Field Indicator Region" identified by Chance et al. (Reproduced with permission from reference 44. Copyright 1986, Journal of Biological Chemistry) B) Spin state sensitive bands identified by Oyanagi et al. Redrawn from data in Reference 45.

A few metalloproteins (enzymes) in which porphyrin ligands are involved. The structural features of the ligand systems, responsible for the enzyme activity. 

Hz IR (cm-1, nujol) iv=o 1700 cm-1). An X-ray crystal structure has confirmed the Rh(OEP)(CHO) structure. The porphyrin ligand clearly imparts special properties (stability, H NMR, IR) to the formyl ligand. However, the reason why the carbonylation of Rh(OEP)(H) should be more facile than the carbonylation of other metal hydrides is not yet apparent. 

This chapter deals with the provision of suitable starting materials for investigations in the noble metal porphyrin field (Sects. 2.1-2.4) and the establishment of their structures (Sect. 2.5). Most of these investigations are devoted to complexes with synthetic porphyrin ligands which are soluble in organic solvents [e.g. (OEP)- or (TPP) complexes Sect. 2.1]. Special mention is made to some novel porphycene derivatives (Sect. 2.2), to phthalocyanine systems (Sect. 2.3), and to water-soluble porphyrin complexes (Sect. 2.4). 

Despite the similarities existing among natural tetrapyrrolic macrocycles, e.g. they are all tetradentate equatorial N4 ligands, structural variations cause a fine modulation of the reactivity of the central metal atom. Thus it is most probably the corrin skeleton that enables the cobalt atom of vitamin B12 to carry out reactions impossible for a similar cobalt porphyrinate. 

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