Iron Complexes in Organic Chemistry

Abstract

Catalysis is an important field in both academic and industrial research because it leads to more efficient reactions in terms of energy consumption and waste production. The common feature of these processes is a catalytically active species which forms reactive intermediates by coordination of an organic ligand and thus decreases the activation energy. Formation of the product should occur with regeneration of the catalytically active species. The efficiency of the catalyst can be described by its turnover number, providing a measure of how many catalytic cycles are passed by one molecule of catalyst.For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions.Many transition metals have been applied as catalysts for organic reactions [1]. So far, iron has not played a dominant role in catalytic processes. Organoiron chemistry was started by the discovery of pentacarbonyliron in 1891, independently by Mond [2] and Berthelot [3]. A further milestone was the report of ferrocene in 1951 [4]. Iron catalysis came into focus by the Reppe synthesis [5]. Kochi and coworkers published in 1971 their results on the iron-catalyzed crosscoupling of Grignard reagents with organic halides [6]. However, cross-coupling reactions became popular by using the late transition metals nickel and palladium. More recently, the increasing number of reactions using catalytic amounts of iron complexes indicates a renaissance of this metal in catalysis. This chapter describes applications of iron complexes in organic chemistry and thus paves the way for an understanding of iron catalysis.

Stable and Easily Handled FeIII Catalysts for Hydrosilylation of Ketones and Aldehydes

“…Early reports of the catalytic hydrosilylation of carbonyl compounds have been dominated by the use of noble metal complexes. [7] Furthermore, in many cases the most active catalysts are supported by complex ligands for which the synthetic efforts needed to prepare the ligand can largely offset the economic advantage of using iron. As the second most abundant metal on earth, iron has long been regarded as one of the most promising candidates to replace noble metal catalysts [5] for the hydrosilylation of carbonyl functionalities.…”

“…[4] However, the high cost, potential toxicity and scarcity of these metals make it important to find catalysts that contain metals that are inexpensive, readily available, non-toxic and environmentally benign. [7] Therefore, it is of synthetic importance to develop Fe III complexes supported by simple ligands that are effective in academic and industrially relevant reactions. Indeed, a number of reports have detailed this particular type of reaction with iron complexes used as catalysts.…”

“…[6f,6g] However, most Fe II compounds are sensitive to air and easily oxidised to the Fe III analogues, increasing the complexity of translating these catalysts into industrial processes. [7] Furthermore, in many cases the most active catalysts are supported by complex ligands for which the synthetic efforts needed to prepare the ligand can largely offset the economic advantage of using iron. Fe III complexes are typically much more stable than Fe II complexes and are the most widespread iron species.…”

“…Fe III complexes are typically much more stable than Fe II complexes and are the most widespread iron species. [7] Therefore, it is of synthetic importance to develop Fe III complexes supported by simple ligands that are effective in academic and industrially relevant reactions. In this contribution, we report the successful development of an iron(III)-catalysed hydrosilylation of both ketones and aldehydes by using amine-bis(phenolate) iron(III) complexes originally developed for cross-coupling [8] and atomtransfer radical polymerisation.…”

Metabolically programmed iron chelators

“…However, the low solubility of Fe(III) hydroxide, the predominant form of the metal in the biosphere [2], necessitated the development of sophisticated iron storage and transport systems by both prokaryotes and eukaryotes. Microorganisms utilize low molecular weight, ferric iron-specific ligands, siderophores [3–7]; eukaryotes tend to employ proteins to transport and store iron [8,9].…”