論文の公開元へFundamental Reactivity and Metal-Ligand Cooperation Behavior of Fe Complexes Bearing a Tetradentate PNNP Ligand
概要
In the history of organometallic chemistry, precious metal complexes have been mainly applied to homogeneous catalysis due to their inherent 2e redox process, which is often favored for organic synthesis.1 However, precious metal catalysts fail to fulfil the requirement for development of sustainable and green chemistry due to their high cost and potential toxicity.2 Thus, to substitute precious metal catalysts with those using more abundant and cheaper first row transition metals have become a widespread research subject in the field of homogeneous catalysis.3 Among 3d transition metal catalysts, iron is a preferable metal because it is most abundant (43200 ppm in the continental crust)4, low cost, and non-toxic. Apart from homogeneous catalysis in industry, iron-based enzymes function as catalysts in nature. Another aspect of iron catalyst is that they adopt various oxidation states ranging from –2 to +5 (rarely to +8) and various spin states such as low spin, intermediate spin, and high spin.5 Moreover, spin states easily change via spin crossover due to its small ligand field splitting. Although numerous reports on iron catalyzed reactions are known so far, numbers of studies on well-defined iron catalysts are still limited compared to those of precious metal catalysts due to their complicated paramagnetic nature,6 i.e. difficulty in identification, controlling the reactivity, etc. Thus, detailed reactivity investigation of iron complexes is still a big challenge in organometallic chemistry.
For the design of well-defined iron complexes, employing a strong-field ligand is effective. A strong-field ligand enlarges d-orbital splitting and stabilizes diamagnetic complexes in low-spin states. In this context, a pincer ligand is one of the most useful ligand candidates. Shaw and co-workers have reported the first pincer ligand in 1976 7a (Scheme 1) and the term ‘pincer’ was coined by van Koten in 1989.7b It was mainly applied on precious metals with exception of Ni (3d transition metal) metal in the early stage of its chemistry.8
The remarkable feature of these pincer ligands is that their structure can be optimized by tuning side-arm-linker such as CH2, NH, and O (Scheme 2). Due to this character, the pincer ligands were successfully applied in recent years in the precise design of iron complex.9 The first iron pincer complex PNP complexes were synthesized by Dahlhoff and Nelson in 1971 by the reaction of 2,6- bis(diphenylphosphinomethyl)pyridine with FeX2 (X = Cl, Br, I, NCS) (Scheme 3).8a Since then, iron pincer chemistry become a prime interest in the development of iron catalysts.
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