Angen Chem. Inr Ed. Engl. 25 (1986) No. 3 0 VCH Verlagsgesellschafi mbH. 0-6940 Wernheim, I986 0570-0833/86/0303-0259 $ 02 50/0

Dtw'ilctrtecl to Piofcrc.or Eimt Otto F i~t h e r 017 the> ol c ci\ioi? of h r~ 75th birt~7da1The coordination chemistry of metals in unusual oxidation states is a fascinating aspect of research in the field ofmetalloporphyrins. Of particular significance in this respect are oxoiron porphyrins containing tetravalent iron, since complexes of this sort occur as reactive intermediates in numerous biological and biomimetic oxidation processes."] Iron(iv) porphyrins, in which the metal is not present as part of a [Fe=O] (FRG) phyrinoids corrole["-the aromatic parent substance of corrin. the ring system of vitamin B,, --and iso~orrole['~ may enable the preparation of the corresponding neutral complexes, that is. types 2 and 4, respectively. This supposition appeared to be substantiated all the more so. as it was recently shown that stable iron(iv) complexes with ligands based on pentane-2,4-dione-bis(S-alky1isothiosemicarbazides)-which are also trianionic-could be synthesized.18,9] We have now found that octaethylcorrole 5 can be readily converted into the triad of stable complexes 7-9, all of which contain formally tetravalent iron."'] Furthermore, the existence of the iron corrole 10 (bearing an axial pyridine ligand) with a formal oxidation state of the metal of + 111 was corroborated.

No abstract

Polarized UV-visible absorption, emission, and magnetic circular dichroism of a series of porphycenes are reported and interpreted in terms of the classical perimeter model as well as semiempirical CNDO/S, INDO/S, and PPP calculations. Differences relative to porphyrins follow readily from their different topology. Two results are particularly striking: (i) unlike the soft MCD chromophores, porphyrins, the porphycenes are negative-hard chromophores and provide a clear example of the competition between the µ+ contributions and the µ" contributions to the B terms of the Soret bands and (ii) fluorescence polarization results for free-base porphycene suggest the existence of a fast intramolecular proton-transfer process in the first singlet excited state, which calls for a closer examination by time-resolved methods.

The electrochemistry of (OEC)M where M ) Mn, Co, Ni, or Cu and OEC is the trianion of 2, 3,7,8,-12,13,17,18-octaethylcorrole was investigated in dichloromethane, benzonitrile, or pyridine, and the oxidized compounds were characterized by UV-visible and/or ESR spectroscopy. The first two oxidations of the Co, Ni, and Cu corroles involve the reversible stepwise abstraction of 1.0 electron per two (OEC)M units and lead to [(OEC)M] 2 + and [(OEC)M] 2 2+ , which are assigned as π-π dimers containing oxidized corrole macrocycles and divalent central-metal ions on the basis of the electrochemical and spectroscopic data. The ESR spectrum of [(OEC)Cu] 2 + suggests the presence of one ESR-active Cu(II) center in the singly oxidized dimer. Further bulk electrooxidation of [(OEC)Cu] 2 + at potentials positive of the second oxidation results in the abstraction of a second electron from the dimeric unit and leads to a triplet ESR spectrum typical of a copper(II) dimer, from which a Cu-Cu distance of 3.88 Å is calculated. The ESR spectrum of [(OEC)Co] 2 + in frozen CH 2 Cl 2 at 77 K has a major line at g ⊥ ) 2.40 with a weak signal at g | ) 1.89 and is typical of a Co(II) ion. The doubly oxidized dimer, [(OEC)Co] 2 2+ , is ESR silent in CH 2 Cl 2 or PhCN, thus suggesting that the two unpaired electrons of the two Co(II) ions in [(OEC)Co] 2 2+ are coupled. The absolute potential difference between E 1/2 for generation of [(OEC)M] 2 + and [(OEC)M] 2 2+ can be related to the degree of interaction between the two (OEC)M units of the dimer and follows the order Co (∆E 1/2 ) 460 mV) > Ni (∆E 1/2 ) 260 mV) > Cu (∆E 1/2 ) 140 mV). No evidence is seen for dimerization of (OEC)Mn after oxidation to its Mn(IV) form in the first electron-transfer step, and the occurrence of this metal-centered reaction may be the reason for the absence of dimerization.

Until very recently the Ni and Cu corroles, already described in the sixties, were regarded as the MII complexes 1 (M = Ni, Cu). Some doubt arose about this interpretation after the existence of FeIV corroles demonstrated that corroles can stabilize metals in unusual oxidation states. Thorough physical studies have now shown that the metal atoms in the Ni and Cu corroles do in fact have the formal oxidation state +III (2).

Angew. Chem. lnt. Ed. Engl. 26 (1987) No. 9 Angen Chem. I n f . Ed. Engl. 26 (1987) No. 9 0 VCH Verlagsgesellschafi mbH. 0-6940 Weinheim. 1987 0570-0833/87/0909-0931 $ 02.50/0

Benzene oxide and the potential 8x-electron system oxepin exist in valence-