Synthesis and Catalytic Property of Iron Pincer Complexes Generated

Article pubs.acs.org/Organometallics

Synthesis and Catalytic Property of Iron Pincer Complexes Generated by Csp3−H Activation Hua Zhao, Hongjian Sun, and Xiaoyan Li* School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27, 250199 Jinan, People’s Republic of China S Supporting Information *

ABSTRACT: When the diphosphinito PCP ligand (Ph2P(C6H4))2CH2 (1) was treated with Fe(PMe3)4 and FeMe 2 (PMe 3 ) 4 , the C sp 3 −H activation products [(Ph 2 P(C6H4))2CH]Fe(H)(PMe3)2 (2) and [(Ph2P(C6H4))(PhP(C 6 H 4 ) 2 )CH]Fe(PMe 3 ) 2 (3) were obtained at room temperature. The generation of product 3 underwent one Csp3−H and one Csp2−H bond activation process. The new iron hydride complex 2 showed good activity in the catalytic hydrosilylation of aldehydes and ketones by using (EtO)3SiH as the hydrogen source under mild conditions. Complexes 2 and 3 were characterized by spectroscopic methods and X-ray diffraction analysis.

I

iridium complex.9 In this paper, we report our recent progress in stoichiometric C−H bond activation by using an iron-based system under mild reaction conditions. The novel hydrido iron pincer complex 2 prepared from the reaction of (Ph2P(C6H4))2CH2 (1) with Fe(PMe3)4 was fully characterized ,while the reaction of 1 with FeMe2(PMe3)4 via both Csp3−H and Csp2−H bond activation delivered the pincer Fe(II) complex 3. The hydrido pincer iron(II) complex 2 could catalyze the hydrosilylation of aldehydes and ketones. The reaction of PCP pincer ligand (Ph2P(C6H4))2CH2 (1) with Fe(PMe3)4 in THF at room temperature resulted in the formation of hydrido pincer Fe(II) complex 2 via one oxidative addition process after 12 h with the substitution of two PMe3 ligands by two Ph2P− groups (eq 1). The hydrido Fe(II)

n the past few years, the widespread existence of hydrocarbons has caused activation studies on C−H bonds to become one of the most important fields in organic synthesis due to not only the inertness of the C−H bond but also the potential economic benefits in synthetic applications.1 The C− H bond acting as one “unfunctionalized” group, its cleavage often occurs under harsh conditions. Early efforts in C−H activation were successful by taking advantage of a directing group via chelation. This could enhance the reactivity of the C−H bond and result in its activation.2 Most work on C−H bond activation has been focused on the activations of Csp2−H bonds while there have been fewer reports on Csp3−H bond activation because, in general, the former is much easier than the latter.3 Because of the strongly chelating nature of PCP pincer ligands, it is easy to carry out the activation of intramolecular C−H bonds, and this has also attracted a substantial amount of interest.4 The PCP pincer complexes formed by transition metals could also be used as catalysts in homogeneous catalysis.5 In early work, precious-metal (Rh, Ir, Pd, Pt etc.) complexes were often chosen to conduct the study of Csp3−H bond activation. Reports on cheap and environmentally friendly iron complexes are rare.3b,6 Our group had been focusing on the Csp3−H bond activation of pincer ligands supported by Fe, Co, and Ni complexes. We explored the Csp3−H activation of (Ph2POCH2)2CH2 induced by iron and cobalt complexes under mild conditions.7 As a continuation of the study on Csp3− H bond activation, we tried to change the molecular skeleton of the pincer ligands. The diphosphinito PCP ligand (Ph2P(C6H4))2CH2 was introduced because it showed considerable inertness to precious metals, even under severe conditions.8 By changing the substituents on the phosphorus atom from aryl to alkyl, Warren realized a double C−H activation by using an © 2014 American Chemical Society

complex 2 was isolated as red crystals from Et2O at 0 °C in a yield of 65%. Complex 2 was stable at room temperature and could be handled in air in the solid state without noticeable decomposition even after 1 day. In the infrared spectrum of 2, one typical stretching band for the Fe−H bond was observed at 1938 cm−1. The 1H NMR spectrum of complex 2 in C6D6 gave evidence for the hydrido ligand at −12.42 ppm as a tdd peak due to coupling between the hydrido hydrogen and the four coordinated phosphorus Received: April 24, 2014 Published: July 1, 2014 3535

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atoms with coupling constants 2J(PH) of 75.4, 43.5, and 17.4 Hz (Supporting Information, Figure S1). The proton signal of Fe−CH appears at 4.67 ppm as one singlet. The signals of two PMe3 ligands are situated at 0.92 and 0.88 ppm. In the 31P NMR spectrum, two types of PMe3 groups and one kind of diphenylphosphanyl group appear at 25.0, 38.0, and 96.0 ppm with a relative integral ratio of 1 (dt):1 (dt):2 (bs). The doublet of triplets (dt) signals were formed by the PPMe3 atom with two identical P−PPh2 atoms and another PPMe3 atom. A broad singlet (bs) was found for the two P−PPh2 atoms, although this peak should be a triplet or quartet. The structure of 2 was confirmed by X-ray diffraction structure analysis (Figure 1). In the

Figure 2. Molecular structure of 3. Hydrogen atoms are omitted for clarity.

distorted-octahedral coordination sphere and makes up the equatorial plane P1, P2, P9, and C21 along with C7 and P4 located at the axial positions with the angle C7−Fe1−P4 = 172.5(3)°. In comparison with complex 2, complex 3 has three chelate rings. Two of them are five-membered rings formed by the [PCP]-pincer ligand. The third is a four-membered chelate ring formed by ortho metalation of one of the phenyls of the Ph2P− group via Csp2−H bond cleavage. These three chelate rings are in three different equatorial planes and are almost perpendicular to each other. In comparison with many reactions catalyzed by noble metals, less work on homogeneous catalysis with Fe, Co, and Ni has been disclosed, although their compounds are inexpensive. Currently, there are more and more reports on the use of iron catalysts in a variety of organic synthetic reactions.12 Hydrido iron complexes often act as important intermediates in the catalytic cycle. Some catalytic systems of hydrido iron complexes have been established in hydrogenation13 and hydrosilylation processes.14 Milstein explored iron-catalyzed reduction reactions using [PNP]-pincer iron complexes.15 It was confirmed that iron hydride complex 2 could be used in the reduction of aldehydes and ketones by using silane as the hydrogen source under mild conditions with a catalyst loading of 0.3−1 mol % (eq 3). The experiments showed that complex

Figure 1. Molecular structure of 2. Most hydrogen atoms are omitted for clarity.

molecular structure of 2, the iron atom is centered in a distorted-octahedral geometry caused by the small hydrido ligand. Therefore, the bond angle P4−Fe1−P7 (137.13(5)°) strongly deviates from 180°. The angle P4−Fe1−P7 bends toward the direction of the orientation of the hydrido ligand. The axial H1−Fe1−P9 is almost perpendicular to the equatorial plane formed by C19, P4, P7, and P10 . Both Fe1−P9 (2.252(1) Å) and Fe1−P10 (2.28(2) Å) are remarkably longer than both Fe1−P4 (2.176(1) Å) and Fe1− P7 (2.184(1) Å) due to the strong trans influence of the hydrido hydrogen (H1) and the carbanion (C19). The Fe1− H1 (1.56(4) Å) and Fe1−C19 distances (2.170(4) Å) are within the normal ranges for Fe−H (1.51−1.69 Å)10 and Fe− Csp37,11 bonds. Since the C19 atom is sp3 hybridized, not only are the two five-membered chelate rings not coplanar but also each five-membered chelate ring is nonplanar. The four chemical bonds linked to C19 (C19−Fe1, C19−H19, C19− C15, and C19−C20) must meet the requirements of bond angles for the sp3 hybridization of C19. Through the reaction of 1 with FeMe2(PMe3)4 in THF at room temperature, complex 3 was obtained after 12 h. 3 could be crystallized from n-pentane in a yield of 45% (eq 2).

2 as catalyst was necessary for this reduction reaction of benzaldehyde under the given conditions (Table 1). Interestingly, Ph3SiH and Et3SiH were not suitable for this catalytic system, while the reaction proceeded quantitatively with (EtO)3SiH as a reducing agent. The scope of the substrates was expanded to explore the catalytic hydrosilylation reactions (Table 2). Different aldehydes could be reduced to the corresponding alcohols with 0.3−1 mol % of catalyst 2 at 50 °C in THF in variable yields over different periods (Table 2). It can be seen from Table 1. Catalytic Activity of 2 for Hydrosilylation of PhCHOa

In the 1H NMR spectrum the signal of Fe−CH appears at 4.95 ppm as one broad singlet. In the 31P NMR spectrum, three types of signals appear at 91.2, 31.1, and 23.0 ppm with a relative integral ratio of 1:2:1. From the molecular structure of 3 (Figure 2) established by an X-ray diffraction study it could be concluded that complex 3 was formed by one Csp3−H and one Csp2−H bond activation process. The molecular structure of complex 3 confirms that the central iron atom is situated in a

entry

catalyst

silane

time (h)

conversion (%)b

1 2 3 4

2 2 2

(EtO)3SiH (EtO)3SiH Ph3SiH Et3SiH

24 1 24 24

0 >99 0 0

a

Conditions: PhCHO (1.0 mmol), silane (2.0 mmol), complex 2 (0.003 mmol), THF (1 mL), 50 °C. bDetermined by GC with ndodecane as an internal standard. 3536

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Table 2 that this catalytic system has a certain tolerance for both the electron-withdrawing group(s) and electron-donating group(s) on the phenyl ring. The substrates with the electrondrawing substituents (Table 2, entries 3−8 and 10) were reduced to the corresponding alcohols with a catalyst loading of 0.3 mol % in a few hours. The reduction of the substrates with electron-donating groups was slow and needed a longer reaction time and a higher catalyst loading to reach full conversion (Table 2, entries 2 and 11). In comparison with aldehydes, ketones are less reactive (Table 2). It was difficult to reach full conversions, though we had attempted to optimize the reaction conditions. For the ketones, there was no obvious trend similar to that for aldehydes. The experimental results indicate that the polarity of the carbonyl group has an effect on the hydrosilylation reactions. With a decrease in the polarity of carbonyl group, the reaction time increased and the reaction rate was reduced. In previous work on the iron-catalyzed hydrosilylation of aldehydes and ketones, the best results were reported by Tilley with 0.01−2.7 mol % of Fe{N(SiMe3)2}216 and Ph2SiH2 as the hydrogen source. In the hydrosilylation of benzaldehyde, the catalyst loading was 2.7 mol %. In comparison with the catalytic exploration by [POCOP]-pincer iron hydride using (EtO)3SiH as the hydrogen source,17 complex 2 in this work has better catalytic activity for the hydrosilylation of benzaldehyde at similar reaction temperatures because only 0.3% catalyst loading was needed. In summary, the pincer iron(II) hydrido complex 2 was synthesized by the reaction of the [PCP] ligand (Ph2P(C6H4))2CH2 (1) with Fe(PMe3)4. Complex 3 as one Csp3−H/ Csp2−H bond activation product was obtained by the reaction of 1 with FeMe2(PMe3)4. The molecular structures of complexes 2 and 3 were determined by X-ray diffraction. Importantly, complex 2 showed good activity in the catalytic hydrosilylation of aldehydes and ketones with (EtO)3SiH as a hydrogen source under mild conditions.

Table 2. Catalytic Hydrosilylation of Aldehydes and Ketonesa

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EXPERIMENTAL SECTION

General Procedures and Materials. All air-sensitive and volatile materials were handled by using standard vacuum techniques. Solvents were dried according to known procedures and freshly distilled prior to use. Literature methods were applied for the preparation of (Ph2P(C6H4))2CH2,8 Fe(PMe3)4,18a and FeMe2(PMe3)4.18b Infrared spectra (4000−400 cm−1) were recorded on an FT-IR instrument from Nujol mulls between KBr disks. NMR data were obtained on a NMR spectrometer at 300 MHz. Single-crystal X-ray diffraction data were collected using a CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. GC was carried out with ndodecane as an internal standard. All of the aldehydes and ketones were freshly distilled or recrystallized. Typical Procedure for the Syntheses of 2 and 3. Fe(PMe3)4 (396 mg, 1.1 mmol) or FeMe2(PMe3)4 (432 mg, 1.1 mmol) in 30 mL of THF was added to a solution of 1 (563 mg, 1.0 mmol) in 40 mL of THF. The mixture was stirred at room temperature for 12 h, and then the solvent was removed by vacuum. The solid residue was extracted with diethyl ether or n-pentane. 2 as red crystals was obtained from diethyl ether at 0 °C. 3 as red crystals was obtained from n-pentane at 0 °C. Analytical data can be found in the Supporting Information. General Procedure for the Catalytic Hydrosilylation. The general procedure was same as in our earlier work,19 with different catalyst loadings for some substrates.

a

Conditions: substrate (1.0 mmol), (EtO)3SiH (2.0 mmol), complex 2, THF (1 mL), 50 °C. bGC yield with n-dodecane as an internal standard. 3537

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures and CIF files giving crystallographic data and details of the X-ray study for 2 and 3, 1H NMR peaks of the hydrido hydrogen in 2 (Figure S1), analytical data and spectra for 2 and 3, and NMR data for alcohols. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for X.L.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support by the NSF of China (No. 21372143). We also thank Prof. Dieter Fenske and Dr. Olaf Fuhr (Karlsruhe Nano-Micro Facility (KNMF), KIT) for their kind assistance with the X-ray diffraction analysis.

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