Photochemical Organic Oxidations and Dechlorinations with a μ-Oxo Bridged Heme/Non-Heme Diiron Complex

Abstract

Steady state and laser flash photolysis studies of the heme/non-heme mu-oxo diiron complex [((6)L)Fe(III)-O-Fe(III)-Cl](+) (1) have been undertaken. The anaerobic photolysis of benzene solutions of 1 did not result in the buildup of any photoproduct. However, the addition of excess triphenylphosphine resulted in the quantitative photoreduction of 1 to [((6)L)Fe(II)...Fe(II)-Cl](+) (2), with concomitant production by oxo-transfer of 1 equiv of triphenylphosphine oxide. Under aerobic conditions and excess triphenylphosphine, the reaction produces multiple turnovers (approximately 28) before the diiron complex is degraded. The anaerobic photolysis of tetrahydrofuran (THF) or toluene solutions of 1 likewise results in the buildup of 2. The oxidation products from these reactions included gamma-butyrolactone (approximately 15%) for the reaction in THF and benzaldehyde (approximately 23%) from the reaction in toluene. In either case, the O-atom which is incorporated into the carbonyl product is derived from dioxygen present under workup or under aerobic photolysis conditions. Transient absorption measurements of low-temperature THF solutions of 1 revealed the presence of an (P)Fe(II)-like [P = tetraaryl porphyrinate dianion] species suggesting that the reactive species is a formal (heme)Fe(II)/Fe(IV)=O(non-heme) pair. The non-heme Fe(IV)=O is thus most likely responsible for C-H bond cleavage and subsequent radical chemistry. The photolysis of 1 in chlorobenzene or 1,2-dichlorobenzene resulted in C-Cl cleavage reactions and the formation of [[((6)L)Fe(III)-Cl...Fe(III)-Cl](2)O](2+) (3), with chloride ligands that are derived from solvent dehalogenation chemistry. The resulting organic products are biphenyl trichlorides or biphenyl monochlorides, derived from dichlorobenzene and chlorobenzene, respectively. Similarly, product 3 is obtained by the photolysis of benzene-benzyl chloride solutions of 1; the organic product is benzaldehyde (approximately 70%). A brief discussion of the dehalogenation chemistry, along with relevant environmental perspectives, is included.

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

“…Density functional (DFT) methods support the assignment of the μ‐oxido diiron(III) complex 2 a (Figure 1) as the photochemically reactive species with photoreduction proceeding via a [(L)Fe IV =O] 2+ intermediate analogous to that reported for the heme‐based systems 21, 22, 23. [(L)Fe IV =O] 2+ ( 4 ) is itself photoreactive, as we have shown recently 25.…”

“…Photoreduction in such systems is irreversible and accompanied by ligand oxidation (e.g., CO 2 formation from carboxylate ligands), and hence Fe III complexes are of limited use in the photocatalytic oxidation of organic substrates. Notable exceptions (see below) are to be found in the reports of Richman,21, 22 Karlin,23 and co‐workers on the photochemistry of μ‐oxido‐bridged diiron(III) complexes.…”

“…The photochemically driven oxidation of an Fe II complex together with the earlier reports of photoreduction of Fe III complexes raises the possibility that a fully light driven photocatalytic oxidation cycle can be achieved without the need for a separate photosensitiser, dor example, [Ru(bpy) 3 ] 2+ . However, simple non‐heme Fe III systems lack the distinct photophysics and chromophoric properties of the heme unit present in the systems of Richman,21, 22 Karlin,23 and co‐workers, and hence it would seem unlikely that a fully non‐heme Fe III complex would show similar photoreactivity. …”

“…In the absence of substrates with weak C−H bonds, the quantum yield for the reduction was negligible due to rapid recombination of the Fe IV =O/Fe II centers to the Fe III −O−Fe III state. Karlin and co‐workers23 have shown that photocatalytic oxidation and aromatic dehalogenation are possible with turnover by using a nonsymmetric dinuclear Fe III complex based on a non‐heme Fe III unit and an Fe III porphyrin, which were bridged by both a μ‐oxido unit and a covalent link between the heme and non‐heme ligands. As in the double iron(III) porphyrin systems,34 an intermediate Fe IV =O/Fe II species was observed by flash photolysis.…”

Oxidations catalyzed by osmium compounds. Part 1: Efficient alkane oxidation with peroxides catalyzed by an olefin carbonyl osmium(0) complex

“…Catalytic system FeCl 3 -H 2 O 2 is also not efficient in alkane oxidation in acetonitrile [42]. All these processes proceed with formation of free hydroxyl radicals [48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. It should be also noted that oxidations of alkanes, RH, with participation of cytochrome P450 and MMO are believed to include in the crucial step an abstraction of a hydrogen atom to produce an alkyl radical, R Å , which is involved into subsequent transformations.…”