Metalloporphyrin complex geometry

Fig. 3 Cartoon representation of various idealized geometries in metalloporphyrin complexes...

This chapter has reviewed certain experimental results and computational studies involving some of the metalloporphyrins (Fe(II)P, Co(II), and others). The present investigation also explored the accuracy of several DFT methods. The geometries of MP-XO complexes and XO binding energy were found to depend very strongly on the functional and basis set used. In many cases, model systems should be described at least with a triple- quality basis set. 

Pd(II), Pt(II), and Au(III) ions which strongly prefer square planar coordination geometry in their d8 configuration, but also the Ag(II) ion in its d9 configuration, are practically devoid of any coordination chemistry in their porphyrin complexes, apart from intermediates that may be observed during metalloporphyrin formation and a few reactions which have already been discussed in previous reviews [7, 8]. 

Metalloporphyrins can catalyze the hydroxylations of solvent species such as cyclohexane. From our studies with cyclodextrin dimers, we concluded that by attaching cyclodextrin rings to metalloporphyrins we should be able to bind substrates in water and achieve selective hydroxylations directed by the geometries of the complexes. This was successful. 

The geometry of metalloporphyrins and other tetrapyrroles have been studied in detail by molecular mechanics. The effect of (i) the size of the metal ion, (ii) axial ligation by planar ligands, such as imidazoles, (iii) the phenyl group orientation in tet-raphenyl porphinato complexes, and (iv) the flexibility of the porphyrin macrocycle,... 

With almost all of the conceivable coordination chemistry of the expanded porphyrins still left to be explored, it cannot be over-stres that the potential for new chemistry is enormous. This is i rticularly true when account is made of the fact that the chemistry of the metalloporphyrins has played a dominant role in modern inorganic chemistry. What with the possibility to enhance the stability of imusual coordination geometries (and, perhaps oxidations states) and the ability to form stable coordination complexes with a variety of unusual cations including those of the lanthanide and actinide series, the potential for new inorganic and organometallic discoveries are almost unlimited. For instance, as with the porphyrins, one may envision linear arrays of stacked expanded porphyrin macrocycles which may have unique conducting properties and/or which could display beneficial super- or semiconducting capabilities. Here, of course, the ability to coordinate not only to cations but also to anions could prove to be of tremendous utility. 

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