Group-VI a Porphyrins

Porphyrins have been prepared in solid form and show a normal spectrum in solution [Cr MESO 572 (4,400) 516 (1,350) 415 (47,000)] [Tsutsui (183)]. The magnetic susceptibility of the solid was found to be 2.84 BM, in solution 4.95 BM. The latter value corresponds to four unpaired electrons the low susceptibility in the solid state has been explained by intermolecular interactions. 

Porphyrins are easily formed by air oxidation from the Cr11 compounds [Tsutsui (183)], or directly from porphyrins and Crm acetylacetonates [.Buckler (79)], and show a normal spectrum with broadened bands in the visible but a complicated Soret band which somewhat resembles the Mn111 spectrum [Crm (Br) OEP 576 (4,300) 546 (9,800) 457 (43,000) 444 (45,000) 370 (51,000) 354 (47,000)] [Buckler (19)]. 

Porphyrins have not yet been obtained in pure form either in solution or in solid form, but their existence is thought probable because in voltammetric measurements an extra oxidation step is found at 1.0 V [Radish [109)]. 

Only with iron has the redox chemistry in the porphyrin series been worked out in more detail than with manganese. It is mainly the groups of M. Calvin and L. J. Boucher who have investigated these metalloporphyrins, and extensive reviews are available from both [Boucher [11), Calvin [22)]. The interest of these complexes lies in the fact that manganese is always found in photosynthetically active plant material and seems to be active in the oxidation of water [Park [138)]. Because of 

Mn11 porphyrins are obtained in solution by reduction of Mnm complexes with sodium dithionite in the presence of pyridine [Calvin (23), Loach (124)], or in the solid state as a thin layer by evaporation of Mn111 porphyrins in high vacuum [Calvin (22), Edwards (52)]. No preparation of crystalline materials has been reported so far. 

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