Group-IVa Porphyrins

Diamagnetic Ti porphyrins have been isolated and characterized as titanyl (Ti =0) complexes [Fuhrhop (71), Tsutsui (181), (183)]. An esr spectrum of a Tim state has been mentioned, but no detailed data are available on this oxidation state nor on the potential of the Ti111/ TiIV couple [Tsutsui (183)]. The normal electronic spectrum of titanyl OEP [573 (28,000), 535 (14,000) 405 (350,000)] [Fuhrhop (71)] is replaced by a hematin spectrum [582 (8,500) 480 (24,000) 359 (105,000)] [Fuhrhop (71)] and the fluorescence of the original titanyl compound disappears when the axial oxo group is protonated. The influence on the redox potential of the intramolecular n- dxz,yz charge transfer, which occurs similarly in the hematins (see IV.6), has not been investigated. 

Only diamagnetic metalloporphyrins with normal electronic spectra and oxidation states +4 with two axial ligands have been reported. [HfIV OEP (0 Ac)2 569 (42,500) 530 (25,000) 401 (420,000) ZrIV OEP(OAc) 2 567 (32,000) 529 (12,000) 400 (380,000)] [Buckler (19, Buckler (27)]. 

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