Jordan et al. / Electroreduction of Porphyrins and Metaltoporphyrins 3857 result from increased covalency in the (MesCp)zMn system. ConclusionsOur magnetic studies of decamethylmanganocene indicate that permethylation of the C p ring results in exclusively lowspin behavior, in contrast to other manganocenes where high-spin states are thermally populated. From this result we conclude that the ligand field strength of the C p ring is significantly enhanced by the complete replacement of the hydrogens with electron-donating methyl groups. The low-spin configuration of (Me5Cp)zMn renders it inert toward ring displacement and hydrolysis but the complex does undergo reversible oxidation and reduction to give low-spin 16-and 18-electron species for which no analogues exist in the other manganocenes. Further studies on other first-row transitionmetal decamethylmetallocenes are now in progress.

The sediments that now comprise the Messel shale accumulated 47 +/- 2 million years ago in anaerobic waters at the bottom of a lake. Subsequent depths of burial have not exceeded 300 m, nor has the temperature of the shale exceeded 40 degrees C. Contents of organic carbon reach 25%, and preservation of molecular structures has been excellent. Sixteen different geoporphyrins, including three derived from bacteriochlorophylls of the d series and thus indicative of the existence in the lake of an anaerobic photic zone, have been isolated and identified. Here, we show that the carbon isotopic compositions of these and other biomarkers allow identification of specific sources for some materials and reconstruction of carbon flows within the lake and its sediments. The 13C content of organic matter synthesized by lacustrine primary producers can be estimated from the observed 13C content of the geoporphyrins derived from their chlorophylls. Total organic material in the shale is depleted in 13C by six parts per thousand relative to that input. This difference cannot be explained by selective loss of components enriched in 13C, nor, as shown by isotopic compositions of other biomarkers, by inputs from land plants surrounding the lake or from methanogenic bacteria.

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The preparation of "homoporphyrin" derivatives-porphyrins possessing a two-carbon meso bridge-is reported. These new tetrapyrrolic macrocycles were obtained from N-CHzCOzEt-porphyrins via nickel(I1) salt catalyzed rearrangement, The reaction involves formation of intermediate nickel(I1) cationic complex and cyclization to an aziridine fused to a pyrrole, followed by migration of a C-C bond to the meso position and electrocyclic ring-opening of the resultant cyclopropanic intermediate. The stereospecificity of this rearrangement is high. These homoporphyrins derivatives show a ring inversion at 1 IO0 with a barrier of ca. 30 kcal mol-'.The aromaticity of the porphin nucleus 1 has been well established both by its chemical and physical propertie~.l-~ The macrocyclic conjugated system can thus be considered in some ways as homologous to benzene, as are [4n + 21 ann~l e n e s .~The idea of comparing porphyrins lacking a bridge carbon, namely corroles (derivatives of 2), with cyclopentadiene has been well illustrated by the ease of formation of anions from corroles or metallo~orroles.'.~~~ In order to complete the comparison, it only remained to prepare and study the chemical behavior of the "homoporphyrin" system 3. This new tetrapyrrolic macrocycle should display 1 2 3 chemical properties related, to a certain extent, to those of cycloheptatriene and the corresponding heterocyclic systems, as azepine. At the time we obtained our results on homoporphyrins chemistry, the only known compounds possessing the required skeleton was the azahomoporphyrin 4 prepared by G~-i g g .~ Although well characterized, Grigg's compound was found to be unstable, undergoing ring contraction to porphyrin 5 either upon heating (M = H2) or treatment with a divalent cation (M = Zn, Cu). This reaction is similar to the ring contraction of N-carbethoxyazepine. 8 We found that the nickel(I1) promoted rearrangement of N-substituted porphyrins led to homoporphyrin derivatives in good yield, and therefore we undertook a study of the mechanistic details of that new reaction and of the thermal behavior of homop~rphyrins.~ Callot. ResultsSynthesis of Homoporphyrins 7 and 8. The substituted porphyrin 6 was prepared according to the literature procedureI0 and treated with a mixture of Ni(I1) bis (acety1)acetonate and Ni(I1) carbonate in 1,2-dichloroethane. The resultant mixture consisted of three major compounds: homoporphyrins 7 (57%) and 8 (4%) in addition to the known tetraphenylporphinatonickel(I1) (NiTPP) (8%). Compounds 7 and 8 were very similar (infrared, mass spectra, HFfi N HN ChHi CfiK 6 7, R = H, R -CO,C,H, (endo) 8, R = CO?C,H,; R = H (exo)visible spectra) except for the 'H N M R and chromatographic data. Visible spectra of both isomers were compared with those of unsaturated tetrapyrrolic derivatives whose conjugation has been interrupted, e.g., azahomoporp h y r i n~,~ bilatrienes,' I and phlorins.'* The presence of two major absorption bands at ca. 450 and 680 nm could be related to a certain amount of deformation of the macrocycle.I3...