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Chemoselective Supported Ionic Liquid Phase (SILP) Aldehyde Hydrogenation Catalyzed by an Fe(II) PNP Pincer Complex Julian Brünig, Zita Csendes, Stefan Weber, Nikolaus Gorgas, Roland Bittner, Andreas Limbeck, Katharina Bica, Helmuth Hoffmann, and Karl Kirchner ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04149 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018
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ACS Catalysis
Chemoselective Supported Ionic Liquid Phase (SILP) Aldehyde Hydrogenation Catalyzed by an Fe(II) PNP Pincer Complex Julian Brünig,1,‡ Zita Csendes,1,‡ Stefan Weber,1 Nikolaus Gorgas,1 Roland W. Bittner,1 Andreas Limbeck,2 Katharina Bica,1 Helmuth Hoffmann,*, 1 and Karl Kirchner*,1 1
2
Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, AUSTRIA).
ABSTRACT: A base-tolerant supported ionic liquid phase (SILP) system containing a well-defined hydride Fe(II) PNP pincer complex has been prepared, structurally characterized and used as catalyst in the hydrogenation of aldehydes to alcohols. The new SILP catalyst, with the optimum pore filling, was highly active exhibiting TONs and TOFs of up to 1000 and 4000 h-1, respectively, under mild conditions (25 oC, 10-50 bar H2 pressure) without significant leaching of both the complex and the IL.
KEYWORDS: Hydrogenation, aldehydes, iron, pincer complexes, supported ionic liquid phase The catalytic hydrogenation of carbonyl compounds is an environmentally benign and economic method to obtain alcohols which are a valuable feedstock for a large number of fine and bulk chemicals.1 Accordingly, many highly active homogeneous catalysts based on both precious and non-precious metals have been developed for this purpose over the years.2,3 However, a major issue to be solved is still selectivity such as hydrogenation of carbonyl compounds in the presence of other reducible functional groups.4 In particular, catalysts which exhibit full selectivity for aldehydes over ketones and/or alkenes are of practical importance for the synthesis of flavors,5 fragrances5 and pharmaceuticals.6 One of the most efficient chemoselective catalyst for the hydrogenation of aldehydes in the presence of ketones is the ruthenium catalyst [Ru(en)(dppe)(OCOtBu)2] (dppe = 1,2-bis(diphenylphosphino)ethane, developed by Dupau and co-workers.7 In recent years, considerable progress has been made in the development of chemoselective hydrogenation catalysts based on earth abundant, and thus inexpensive, nonprecious metals.8-11 In particular, iron-based systems proved to be highly selective for the reduction of aldehydes in the presence of other carbonyl moieties and C=C double bonds.12-15 We recently reported the application of [Fe(PNPMe-iPr)(CO)(H)(Br)] (1) and [Fe(PNPMeiPr)(CO)(H)2] (2) as catalysts for the chemoselective hydrogenation of aldehydes to yield alcohols.15 The dihydride complex 2, rapidly formed from 1 in the presence of H2 and base, is the key catalyst. These catalysts were found to be among of the most efficient base metal cata-
lysts reported to date. The remarkable substrate selectivity was recently explained by the relative stability of alkoxide intermediates formed upon aldehyde insertion into a Fe-H bond of 2 based on state-of the-art DFT calculations.16
Scheme 1. A New Iron-Based SILP Catalyst. A major drawback of homogeneous catalysis is the need for separation of the (often rather toxic) catalysts at the end of the process. These shortcomings can be eliminated by direct immobilization of a sensitive homogeneous catalyst on a support, which often results in partial or complete loss of activity, since the active species suffers a modification of its chemical and physical properties. An alternative benign and efficient approach is the use of supported ionic-liquid phase (SILP) catalysts,17,18 where a homogeneous catalyst is dissolved in an ionic liquid (IL) and impregnated on a porous support
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material. The chemical nature of the homogeneous catalyst is preserved, yet dissolved in a separate phase, which can easily be separated after the reaction, reused or applied in a continuous flow process with high long-term stability.19 SILP catalysts are therefore considered as sustainable catalysts, since they combine the advantages of heterogeneous and homogeneous catalysis and make use of minimum amounts of IL/catalyst solutions in a highly efficient manner.20 While the SILP concept is particularly well established for gas phase reactions such as hydroformylations and implemented even in large scale, its application in liquid-liquid biphasic processes poses additional problems due to ionic liquid and/or catalyst leaching into the organic co-solvent.21 In this work, we report on an iron SILP system which is highly active for the chemoselective hydrogenation of aldehydes to alcohols (Scheme 1). The pre-catalyst is the well-defined hydride complex 1 which is dissolved in IL 1butyl-2,3-dimethyl-imidazolium bis(trifluoromethylsulfonyl) ([bm2im][NTf2]) and applied as a thin film coated onto powdered silica as the support. The silica gel (SG) itself is functionalized with 1,2dimethyl-3-(3-trimethoxysilylpropyl)imidazoliumbis(trifluoro-methylsulfonyl) imide ([TMSpm2im][NTf2]) in order to avoid side reactions with the active acidic OH groups and the iron catalyst and to create an “IL-philic” surface as protection against IL/catalyst leaching. To the best of our knowledge, this is the first example of a welldefined Fe(II) complex containing SILP system for the chemoselective hydrogenation of aldehydes to afford alcohols. The hydride ligand in 1 as well as the hydride ligands of the in the in situ generated active catalyst 2 are strongly basic and thus sensitive to acidic functionalities such as the H atom in the 2-position of 1,3-dialkylimidazolium ions and the OH groups of the silica gel. To avoid catalyst decomposition by these groups, the silica gel was coated with the 1,2,3-alkylated imidazolium chloride [TMSpm2im][Cl] yielding SILP[Cl]. Subsequently, the chloride ions were exchanged to non-coordinating NTf2ions resulting in the formation of SILP[NTf2]. This material was characterized by elemental analysis, 13C{1H}- and 29 Si{1H}-CP-MAS NMR, FT-IR, and N2 adsorptiondesorption techniques. From elemental analysis, the nitrogen content yielded 0.64 mmol [TMSpm2im][Cl] per gram of SILP[NTf2] corresponding to a surface coverage of 1.6 molecules per nm2. 29 Si{1H}-CP-MAS NMR spectroscopy confirmed that the organic moiety is covalently grafted to the silica surface (see Supporting Information). The infrared spectrum of SILP[NTf2] also confirmed that free silanol groups of the silica gel reacted with the silanized IL, since the isolated
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OH stretching vibration at 3748 cm-1 was no longer visible in the spectrum of SILP[NTf2] (see ESI). SILP[NTf2] was impregnated with 10, 20, 30, and 40 wt% of 1 dissolved in [bm2im][NTf2] producing SILP10, SILP20, SILP30, and SILP40, respectively. Structural parameters derived from N2 adsorption-desorption isotherms showed that the BET surface area and the pore volume decreased after IL functionalization due to the coating of the silica surface and the closure of micropores and further upon loading with IL and catalyst due to the filling of the pores (Table 1). The pore filling degree (α) increases with the IL loading up to 100% for SILP40. The catalytic performance of the new SILP materials containing the Fe(II) catalyst 1 was first investigated in the hydrogenation of 4fluorobenzaldehyde to 4-fluorobenzyl alcohol in the presence of 5 mol% of DBU (1,8-diaza-bicyclo[5.4.0]undec-7ene) as base in n-heptane at 25oC. The results are presented in Table 2. SILP20 was found to have the optimum pore filling degree catalyzTable 1. Structural Parameters Calculated from the N2 Adsorption-Desorption Isotherms. Sample
Pore b volume 3 (cm /g)
SG
BET surface a area 2 (m /g) 303
0.68
Average pore diamc eter (nm) 7.2
SILP[NTf2]
240
0.39
4.7
SILP10
133
0.23
4.5
19
SILP20
58
0.13
5.1
43
SILP30
15
0.03
5.2
74
SILP40 a
too low to measure
α d (%)
~100
b
Calculated by the BET equation. BJH pore desorption c d volume. Desorption average pore diameter. Pore filling degree (IL volume/pore volume).
ing this reaction most efficiently, while all other SILPs were found to be less efficient or completely inactive (Table 2, entries 1-4). No conversion was obtained without catalyst. Accordingly, we focused on the reactivity of SILP20. With a catalyst loading of 0.5 mol% under 10 bar H2 pressure, quantitative conversion was reached within 17 min (Table 2, entry 2). Increasing the hydrogen pressure to 20 and 50 bar reduced the reaction times to 13 and 8 min, respectively (Table 2, entries 5 and 6). Lowering the catalyst loading to 0.1 mol% at 10 bar H2 pressure resulted in 85% conversion after 90 min (Table 2, entry 7). With the same catalyst loading but under
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ACS Catalysis
Table 2. Hydrogenation of 4-Fluorobenzaldehyde with Catalyst 1 under SILP, Homogeneous, and Biphasic Conditions.a Me
H
N
cat, H2
O
OH
DBU (5 mol%) n-heptane, 25oC
F
entry
conditions
N
S/C
PiPr2 Fe
N PiPr2
F
Me
P
time
Yield
(bar)
(min )
(%)
CO
1
Br
b
TON
TOF -1
(h )
1
SILP10
200
10
75
16
32
26
2
SILP20
200
10
17
>99
200
706
3
SILP30
200
10
75
7
14
11
4
SILP40
200
10
75
3
6
5
5
SILP20
200
20
13
>99
200
923
6
SILP20
200
50
8
>99
200
1500
7
SILP20
1000
10
90
85
850
567
8
SILP20
1000
50
15
>99
1000
4000
9
homogeneous
200
10
6
>99
200
2000
10
biphasic
200
10
12
>99
200
1000
a
Conditions: entry 1: 2 mmol substrate, 1000 mg SILP10 (5 mg 1, 95 mg IL, 900 mg SILP[NTf2]); entry 2,5,6: 2 mmol substrate, 500 mg SILP20 (5 mg 1, 95 mg IL, 400 mg SILP[NTf2]); entry 3: 2 mmol substrate, 333 mg SILP30 (5 mg 1, 95 mg IL, 233 mg SILP[NTf2]); entry 4: 2 mmol substrate, 250 mg SILP40 (5 mg 1, 95 mg IL, 150 mg SILP[NTf2]); entry 7,8: 10 mmol substrate, 500 mg SILP20 (5 mg 1, 95 mg IL, 400 mg SILP[NTf2]); entry 10: 2 mmol substrate, 5 mg 1 dissolved in 255 mg IL; all reactions were o b 19 1 performed with 2 mL of n-heptane and 5 mol% of DBU at 25 C. Determined by F{ H} NMR spectroscopy.
a hydrogen pressure of 50 bar, quantitative conversion was already reached after 15 min corresponding to a TON and TOF of 1000 and 4000 h-1, respectively (Table 2, entry 8). After the reaction, the solid SILP20 could be easily separated from the solution. The liquid reaction mixtures were analyzed with ICP-MS for iron leaching, yielding in the worst case merely 0.125±0.038 mol% Fe with respect to the initial total loading of the iron catalyst. Based on 19 1 F{ H} NMR spectroscopy, there was no evidence for leaching of the IL into the n-heptane solution as no signals of the NTf2- anion could be detected. For comparison, the reaction was also performed under homogeneous and biphasic reaction conditions with 0.5 mol% catalyst loading and a hydrogen pressure of 10 bar (Table 2, entries 9 and 10). SILP20 is only slightly less active than the homogeneous and biphasic systems reach-
ing comparable TONs but reduced TOFs. It eliminates, however, their main drawbacks, i.e., difficult separation and large amounts of IL, making the SILP catalyst more sustainable.
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Figure 1. Catalyst Recycling in the Hydrogenation of 4Fluorobenzaldehyde by Repeated Addition of Aldehyde. Conditions: 500 mg SILP20, 2 mL n-heptane, 2 mmol substrate, 0.5 mol% 1, 5 mol% DBU, 25 oC, 50 bar H2. Moreover, it was possible to recycle the catalyst by repeated addition of aldehyde after each reaction cycle (Figure 1). This procedure was carried out four times every 7 minutes and yielded a TON of 1000 and a TOF of 1714. These results hold great promise for a continuous flow operation of this catalytic system.22 Table 3. Hydrogenation of Aldehydes A1-A20 Catalyzed by Catalyst 1 Under SILP20 Conditions.a
industry (Table 3, A14-A20), as well as the challenging α,β-unsaturated substrate cinnamaldehyde (Table 3, A13) were not hydrogenated. Importantly, 4acetylbenzaldehyde was hydrogenated only at the aldehyde moiety (Table 3, A10). This again emphasizes the high selectivity of the present system. In conclusion, we have developed a novel Fe(II) PNP pincer complex based SILP system, which was fully characterized and used as a hydrogenation catalyst for the chemoselective hydrogenation of aromatic and aliphatic aldehydes to give alcohols under mild conditions (0.1-0.05 mol% catalyst, 25oC, H2 50 bar pressure) without significant leaching of the catalyst. The newly developed SILP system shows high potential as an efficient catalyst under continuous flow conditions which will be the subject of future studies in our groups.
ASSOCIATED CONTENT * Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details of the synthesis of all materials 13 1 29 1 including solid state C{ H} and Si{ H} NMR spectra (PDF)
AUTHOR INFORMATION Corresponding Authors *E-mail:
[email protected] *E-mail:
[email protected].
ORCID Karl Kirchner: 0000-0003-0872-6159 a
Conditions: 500 mg SILP20, 2 mL n-heptane, 2 mmol subo strate, 0.5 mol% 1, 5 mol% DBU, 25 C, 50 bar H2, 1 h. Yields 1 were determined by calibrated GC/MS and H NMR spectroscopy with mesitylene as internal standard. In all cases yields >98% were observed.
Author Contributions ‡
These authors contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS To demonstrate the general applicability of catalyst SILP20 the scope has been tested with several substrates as shown in Table 3. The catalytic experiments were conducted in the presence of 0.5 mol% of catalyst at 25°C under 50 bar hydrogen pressure in the presence of 5 mol% DBU to ensure quantitative conversion for all substrates in a reasonable reaction time (1 h). Under these reaction conditions, very good results could be obtained for aromatic and heteroaromatic aldehydes bearing both electron withdrawing halogen substituents such as F, Cl, and Br in 4-halobenzaldehydes or electron donating groups such as OMe in 4-anisaldehyde (Table 3, A1-A5). High chemoselectivity was achieved with both conjugated and non-conjugated C=C double bonds. In fact, C=C double bonds in aliphatic aldehydes such as citronellal, lyral, or scentenal, which are used in the flavor and fragrance
JB, ZC, NG and KK thank the Austrian Science Fund (FWF) for the financial support through projects M 2068-N28 (ZC) and P 28866-N34 (JB, NG, KK).
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(22) (a) Datta, R.; Rydant, J.; Rinker, R. G. J. Catal. 1985, 95, 202-208. (b) Rony, P. R. J. Catal. 1969, 14, 142-147.
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