Volume 3 | Number 6 | June 2016 View Article Online View Journal | View Issue Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 01 February 2016. Downloaded 2021-10-01 7:30:22 PM. INORGANIC CHEMISTRY FRONTIERS http://rsc.li/frontiers-inorganic View Article Online INORGANIC CHEMISTRY FRONTIERS REVIEW Recent developments of iron pincer complexes for catalytic applications Cite this: Inorg. Chem. Front., 2016, 3, 741 Gerald Bauer and Xile Hu* Iron catalysis is attractive for organic synthesis because iron is inexpensive, abundant, and non-toxic. To Received 25th November 2015, control the activity and stability of an iron center, a large number of iron pincer complexes have been syn- Accepted 31st January 2016 thesized. Many such complexes exhibit excellent catalytic activity in a number of important organic reac- DOI: 10.1039/c5qi00262a tions such as hydrogenation, hydrosilylation, dehydrogenation, and carbon–carbon bond forming rsc.li/frontiers-inorganic reactions. In this review, recent examples of representative iron pincer catalysts are presented. 1. Introduction lysts, and examples include cytochrome P-450, peroxidases, various oxygenases, and hydrogenases. While numerous Complexes of precious metals, particularly those from the reports of iron-catalyzed organic reactions are known,1,2 Creative Commons Attribution 3.0 Unported Licence. platinum group, occupy a central place in homogeneous cataly- studies of well-defined iron catalysts are less common. Such sis. However, the high cost and potential toxicity of precious studies, however, are essential for the in-depth understanding metals make them less desirable for industrial applications. and further advancement of iron catalysis. Thus, homogeneous catalysis using more abundant and This review focuses on the catalytic applications of iron – cheaper first row transition metals has become a popular pincer complexes.3,6 8 Pincer ligands, in general, are triden- research theme. Iron catalysis is particularly desirable because tate, 6-electron donor ligands that enforce a meridional coordi- iron is inexpensive, readily available, and non-toxic. In biologi- nation geometry on the metal center. Although there is no cal systems iron-containing enzymes are widely used as cata- strict definition, a pincer ligand traditionally consists of a This article is licensed under a central benzene ring, which is 1,3-disubstituted with two che- lating side arms (Fig. 1). Both the central atom (Dc) and the Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), ISIC-LSCI, donors at the two arms (Ds) bind to the metal center, resulting 4,5,9–15 BCH 3305, 1015 Lausanne, Switzerland. E-mail: [email protected] in strong chelation. Replacement of the central aryl ring Open Access Article. Published on 01 February 2016. Downloaded 2021-10-01 7:30:22 PM. Gerald Bauer was born in 1984 Xile Hu was born in 1978 in in Tulln/Donau, Austria. He Putian, China. He received a B.S. studied ‘Technical Chemistry’ at degree from Peking University the University of Technology in (2000) and a Ph.D. degree from Vienna with semesters at the the University of California, San UCT in Prague and the Univer- Diego (2004; advisor: Prof. sity of Waterloo, ON, Canada Karsten Meyer). He carried out a during his bachelor studies. postdoctoral study at the Califor- Gerald received his M.Sc. in nia Institute of Technology 2010 under the joint supervision (advisor: Prof. Jonas Peters) of Prof. Karl Kirchner, UT before joining the École Polytech- Vienna, and Prof. Kazushi nique Fédérale de Lausanne Gerald Bauer Mashima, University of Osaka. Xile Hu (EPFL) as a tenure-track assist- Shortly afterwards, he joined the ant professor in 2007. He is cur- lab of Prof. Xile Hu at the EPFL working on the catalytic appli- rently an associate professor at the same institute. His research cations and mechanistic investigation of iron pincer complexes in interests span from organometallic chemistry, synthetic methodo- Kumada cross-coupling reactions, for which he was awarded his logies, and reaction mechanisms to bio-mimetic and bio-specu- PhD in 2015. lated coordination chemistry to electrocatalysis and artificial photosynthesis. This journal is © the Partner Organisations 2016 Inorg. Chem. Front.,2016,3,741–765 | 741 View Article Online Review Inorganic Chemistry Frontiers – their iron complexes are very active for polymerization,16 19 hydrogenation, and other alkene, alkyne, and ketone functionalization reactions. Several excellent reviews covering these iron complexes have been published recently.20,21 There- fore, they will not be reviewed here. Fig. 1 Generalized structure of pincer complexes.3–5 2. Neutral pyridine-based PNP ligand by a pyridinyl or diarylamine group leads to two major classes systems and their applications in iron of variants of the original pincer ligands. In addition to their catalysis strong chelating ability and structural rigidity, the advantage of pincer ligands is their diversity. By modifying the central The first iron PNP complexes were synthesized by Dahlhoff and side donors as well as the ligand backbone, it is possible and Nelson in 1971 by reacting 2,6-bis(diphenylphosphino- to synthesize an almost endless number of ligands with methyl)pyridine with FeX2 (X = Cl, Br, I, NCS). The resulting varying electronic and steric properties.5 This diversity is very complexes were in a high spin configuration and exhibited a attractive for homogeneous catalysis where systematic studies 5-coordinate geometry with a slightly distorted square pyrami- of metal–ligand combinations are desired. dal structure.6 This ligand framework became popular, and in In this review, iron complexes of pincer ligands based on addition to the variance of the substituents on the phosphorus the following frameworks are discussed: 2,6-disubstituted pyri- donors,5 the linker (L) that connects the phosphorus donors to dine, 1,3-disubstituted benzene, N,N-diarylamine, isoindoline the central pyridine ring could be modified to include not only 6,22–31 5,32–39 40,41 and bis(phosphinoethyl)amine. Only catalysis by pre-formed CR2, but also NR and O (Fig. 2). To differentiate Creative Commons Attribution 3.0 Unported Licence. iron complexes, but not in situ generated iron species, will be between the different linkers, these pincer ligands are abbre- “ L L ” presented. The examples are selected to give a representative, viated as P N P , where L = C, N, O, to represent CR2, NR, but not comprehensive overview of the developments in the and O, respectively. field. Mostly symmetrically substituted pincer ligands with a Several groups studied the influence of linkers on the elec- C2 or C2v symmetry are included. Unsymmetrically substituted tronic density of the metal centre using the IR frequencies of ligands and terpyridine-type ligands are not treated. Pyridine M–CO bonds as a probe (Fig. 3).26,34,40 The CO stretching fre- diimines (PDI) might be considered as pincer ligands, and quency of [(PNP)FeX2(CO)] (X = Cl, Br) decreases when the This article is licensed under a Open Access Article. Published on 01 February 2016. Downloaded 2021-10-01 7:30:22 PM. Fig. 2 Structure of pyridine-based PNP ligands and selected chiral and achiral substituents on the phosphorus.5 Fig. 3 CO stretching frequencies for 1, 2-cis, 2-trans, 3 and 4.26,34,40 742 | Inorg. Chem. Front.,2016,3,741–765 This journal is © the Partner Organisations 2016 View Article Online Inorganic Chemistry Frontiers Review linker is changed from O to NH and to CH2. This result mechanism of the catalysis with 10-BF4 was investigated by suggests that the electron donating character of the corres- DFT/B3LYP computations. It was found that the formation of ponding PNP ligands follows a similar order. ethyl 3-hydroxy-2-arylacrylates had a lower energy barrier com- An interesting feature of these PNP pincer complexes is pared to the formation of the β-keto ester. Hydrogen bonds – – their metal ligand cooperation. The slightly acidic CH2 and between the acidic N H of the ligand, the BF4 anion and the 46 NH linkers are easily deprotonated under basic conditions, EDA (N–H⋯F–BF2–F⋯EDA) seemed to play an important role. which causes a reversible dearomatization of the central pyri- 42 dine ring. Further details will be discussed below. 2.2 Iron(II) PNP pincer complexes for catalytic hydrogenation Chirik and co-workers reported that the dinitrogen cis-dihy- N N 2.1 Iron(II)P N P pincer complexes for the selective iPr C C dride complex [( P N P)Fe(H)2(N2)] (11, Fig. 6) catalyzed the formation of 3-hydroxyacrylates from benzaldehydes and ethyl hydrogenation of simple acyclic and cyclic alkenes.24 The diazoacetates hydrogenation of 1-hexene under 4 bar of H2 was achieved Aldehydes react with ethyl diazoacetates (EDA) in the presence using a 0.3 mol% catalyst loading in three hours and with a of Lewis acids (e.g. BF3, ZnCl2, AlCl3, SnCl2, GeCl2) to form conversion of more than 98%. The conversion for the hydro- β-keto esters.43 It was previously shown that the Lewis acid genation of cyclohexene was only 10%. Milstein and co- 5 iPr C C [(η -Cp)Fe(CO)2THF](BF4) catalyzed the reaction of benzal- workers developed a new iron pincer complex [( P N P)FeH dehydes with EDA to give β-hydroxy-2-aryl acrylates.44,45 (CO)Br] (12, Fig. 6).26 The complex was active for the hydrogen- More recently Kirchner and co-workers employed cis- ation of ketones under mild conditions. The reactions typically iPr N N − − [( P N P)Fe(CO)(CH3CN)2](X)2 (5-X;X=BF4 , BArF ) for the proceeded in an ethanolic solution with 0.05 mol% of 12 reaction of p-anisaldehyde with EDA (Fig.