Palladium, Platinum, Silver, and Gold Porphyrins

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]. 

The only reactions that do occur with these metalloporphyrins follow oxidations of the metals or the porphyrin in rings. They have been known for a long time and summarized in reviews on the primary redox processes of metalloporphyrins [292, 293] or electrochemistry in nonaqueous media [294]. 

The formation of dichloro(porphyrinato)platinum(IV) porphyrins was achieved by action of hydrogen peroxide in hydrochloric acid (Eq. 21). Apart from reduction to Pt(II), further reactions of the chlorides have not been observed. 

The analogous reactions of Pd(OEP) or Pd(TTP) did not result in formation of Pd(IV) porphyrins, but in excessive chlorination of the meso [295] or peripheral (p-pyrrole) positions [296] of the porphyrin rings. The oxidation of Pt(P) in the absence of hydrogen chloride did not lead to isolatable products. Of course, an oxoplatinum(IV) porphyrin would have been a very interesting compound. 

Due to poor chemistry and the stability of the porphyrin complexes containing the precious metals Ag, Au, Pd, and Pt, most papers dealing with these complexes are devoted to physical measurements, especially optical or photophysical investigations [23]. 

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