Photochemistry of Metalloporphyrin Carbene Complexes

Abstract

The photochemistry of metalloporphyrins and other tetrapyrrolic systems continues to attract considerable interest.1 There have been numerous studies on the photoactivity of various porphyrin complexes with single-bonded axial ligands, including halogens, oxo-anion species, molecular oxygen,2 and a variety of metals, including chromium, manganese, and iron.3 The photochemistry of metalloporphyrins with multiple-bonded axial ligands has not been explored. We report here the photochemistry of several iron porphyrin carbene and vinylidene complexes. Irradiation of these complexes with visible light cleaves the iron-carbon double bond, resulting in a four-coordinate iron(II) porphyrin and a free carbene. This chemistry is unique in the area of transition metal carbene or alkylidene compounds. Porphyrins with multiple-bonded axial ligands are especially relevant to both biological and catalytic oxidative processes.4 Iron porphyrin carbene and nitrene species are isoelectronic with a terminal iron-oxo complex, the proposed intermediate of the cytochrome P-450 monooxygenases and its synthetic analogs. Most recently, the catalytic cyclopropanation of olefins by iron porphyrins using ethyl diazoacetate was thought to involve a porphyrin carbene complex.5 We prepared Fe(TPP)CCl2, Fe(TPP)CBr2, Fe(TPP)CClF, and Fe(TPP)CC(p-C6H4Cl)2 under anaerobic conditions following Mansuy’s procedure using iron metal as the reductant (TPP ) 5,10,15,20-tetraphenylporphyrinate).6 These compounds were further purified via crystallization from benzene/pentane under argon in an inert atmosphere box and then fully characterized.7 To examine the photochemistry of these complexes, degassed benzene solutions (∼1 × 10-4 M) were irradiated with a 300 W Xe arc lamp, which was filtered to remove both infrared and ultraviolet (<360 nm) light in order to prevent sample heating and porphyrin bleaching, respectively. In addition, the photolysis cells were thermostated to an internal solution temperature of 20 °C. In all cases, irradiation of the parent compounds cleanly produced a new porphyrin identified as Fe(TPP) (λmax ) 419, 442, and 537 nm), as shown in Figure 1 for Fe(TPP)CCl2. Both the Soret and Q-bands of the parent compounds are photoactive.8 The formation of Fe(TPP) was confirmed by addition of pyridine after photolysis of the carbene or vinylidene complexes, yielding the bispyridine complex, Fe(TPP)(py)2. Also as expected, exposure of photolysed solutions to oxygen resulted in the formation of the μ-oxo dimer species, [Fe(TPP)]2O. The production of Fe(TPP) in this reaction is indicative of homolytic cleavage of the metal-carbon double bond and production of a carbene, as in the following reaction: