Source: https://pubs.rsc.org/en/content/articlehtml/2018/dt/c8dt02919a
Timestamp: 2019-04-20 05:11:21+00:00

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Three phenoxyimine Fe(III)Cl complexes bearing electronically diverse -Cl, -H or -tBu substituents in the ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear structures with pentacoordinate iron centres and trigonal bipyramidal geometries. All three complexes demonstrated excellent catalytic activities towards CO2/epoxide coupling to selectively form cyclic carbonates, with catalyst activity correlating with the electron withdrawing nature of the ortho-substituent (Cl > H > tBu) and thus the Lewis acidity of the metal centre. The chloro-substituted complex displayed remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO2, reaching turnover frequencies (TOF) up to 760 h−1 in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO2 pressure. Importantly, the catalyst is also very robust, functioning with high substrate loading, under air or in the presence of water. The substrate scope was successfully extended to other terminal epoxides including epichlorohydrin (TOF = 900 h−1) and to the more challenging internal epoxide, cyclohexene oxide (TOF = 80 h−1). These are amongst the highest TOF values reported for an iron CO2/epoxide coupling catalyst to date.
Scheme 1 General structure of a bis(phenoxyimine) complex vs. a salen complex.
We have had a longstanding interest in iron-mediated catalysis thanks to its abundance, low cost, and low toxicity.17,18 In particular, Fe3+ compounds offer a user-friendly route into catalysis, due to their air- and moisture-stability which enables convenient synthesis and handling. Only a few examples of mono- or bi-metallic Fe(III) complexes have been reported as catalysts for CO2/epoxide couplings and in most cases high catalyst loadings were required to achieve good conversions.16,19–27 Uniting these joint ligand and metal benefits, we targeted a series of phenoxyimine Fe(III) chloride complexes as robust and flexible catalysts for epoxide/CO2 coupling.
Three phenoxyimine ligand precursors were targeted, bearing ortho-substituents of H (L1, Scheme 2), tBu (L2) or Cl (L3). Accordingly, all three ligands were synthesised in excellent yields (95–99%) via the straightforward condensation of methyl amine and mono-substituted salicylaldehydes (Scheme 2).28 Confirming the synthesis of L1, L2 and L3, no aldehyde resonances were present in the 1H NMR spectra, and diagnostic imine resonances were observed (CDCl3 solvent, L1, δ = 8.33; L2, δ = 8.35; L3, δ = 8.30). Complexes C1–C3 were synthesised through deprotonation of pro-ligands L1–L3 using NaH (1.1 eq.) to form the corresponding Na-phenolates. Subsequent reaction with anhydrous FeCl3 (0.5 eq.) in THF solvent at room temperature gave an immediate colour change from yellow to dark purple, which indicated the formation of the bis-ligated Fe(III) chloride complexes C1–C3. After purification via filtration to remove residual NaCl, the products were isolated in high yields (74–98%, Scheme 2) as dark purple/brown solids. As the paramagnetically shifted 1H NMR spectra of C1–C3 provided limited information, the stoichiometry was confirmed via high resolution mass spectrometry and elemental analysis.
Scheme 2 Synthesis of pro-ligands L1–L3 and Fe(III) phenoxyimine complexes C1–C3.
Fig. 1 Molecular structure of C1 with ellipsoids set at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Fe1–Cl1 2.2713(5), Fe1–O1 1.8758(9), Fe1–O1′ 1.8757(9), Fe1–N1 2.111(1), Fe1–N1′ 2.111(1). Selected bond angles (°): O1–Fe1–Cl1 119.50(3), O1′–Fe1–Cl1 119.50(3), O1′–Fe1–O1 121.00(6), O1′–Fe1–N1′ 90.29(4), O1–Fe1–N1′ 88.46(4), O1–Fe1–N1 90.28(4), O1′–Fe1–N1 88.46(4), N1′–Fe1–Cl1 91.28(3), N1–Fe1–Cl1 91.28(3), N1–Fe1–N1′ 177.44(5).
Fig. 2 Molecular structure of C2 with ellipsoids set at the 50% probability level. Hydrogen atoms and co-crystallised solvent have been omitted for clarity. Selected bond lengths (Å): Fe1–Cl1 2.245(2), Fe1–O1 1.880(5), Fe1–O2 1.881(4), Fe1–N1 2.103(6), Fe1–N2 2.108(6). Selected bond angles (°): O1–Fe1–Cl1 116.1(2), O2–Fe1–Cl1 119.4(2), O2–Fe1–O1 124.6(2), N1–Fe1–Cl1 93.6(2), N1–Fe1–O1 87.5(2), N1–Fe1–O2 87.5(2), N2–Fe1–Cl1 96.8(2), N2–Fe1–O1 88.5(2), N2–Fe1–O2 86.9(2), N2–Fe1–N1 169.7(2).
Fig. 3 Molecular structure of C3 with ellipsoids set at the 50% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å): Fe1–Cl1 2.2517(6), Fe1–O1 1.871(2), Fe1–N1 2.092(2), Fe1–O2 1.889(2), Fe1–N2 2.104(2). Selected bond angles (°): O1–Fe1–N1 88.64(7), O1–Fe1–Cl1 117.12(5), O1–Fe1–O2 121.78(7), O1–Fe1–N2 89.43(7), N1–Fe1–Cl1 92.25(5), N1–Fe1–N2 174.92(7), O2–Fe1–N1 89.00(7), O2–Fe1–Cl1 121.11(5), O2–Fe1–N2 88.02(7), N2–Fe1–Cl1 92.81(5).
The C–O bond lengths of complexes C1, C2 and C3 are short [1.328(1) Å, 1.318(8) Å and 1.321(3) Å, respectively] in comparison to the related protonated ligand bearing a naphthalene substituent on the imine N [C–O, 1.354(2) Å].39 These short C–O bonds are indicative of resonance delocalisation of the anionic charge on the phenoxide ligand in complexes C1–C3. Furthermore, the C–C bond lengths of the (O–)C C–C( N) scaffold within C1–C3 lie between the expected bond lengths for C(aromatic) C(aromatic) double bonds and C(aromatic)–C single bonds (1.40 and 1.52 Å, respectively),40 suggestive of resonance delocalisation through the phenoxyimine moiety. This resonance delocalisation is most pronounced for the electron donating tBu-substituted complex C2 [C1–C6, 1.434(9); C6–C7, 1.43(1) Å], followed by the unsubstituted CH analogue C1 [C1–C6, 1.417(2); C6–C7, 1.447(2) Å] and the electron withdrawing Cl analogue C3 [C1–C6, 1.411(3); C6–C7, 1.450(3) Å].
To test the catalytic activity of complexes C1–C3 towards CO2/epoxide coupling, propylene oxide (PO) was selected as a benchmark substrate, as it has previously been studied with other iron catalysts.16 Initially, complex C2 was tested under solvent free conditions using 0.1 mol% catalyst loading with 0.1 mol% tetrabutylammonium iodide (TBAI) as a co-catalyst (vs. PO), using 20 bar pressure of CO2 at 120 °C. 1H and 13C NMR studies of the crude mixture revealed that the catalysts were selective towards the formation of cyclic propylene carbonate (Fig. S8†).41 Under these conditions, C2 successfully achieved 99% conversion to cyclic propylene carbonate in 12 hours (Table 1, entry 2). Control reactions testing only the Fe-complex C2 (entry 4) or only co-catalyst TBAI (entry 5) showed that both components individually display low activity, however, their combination displayed a synergistic effect with a significantly higher conversion achieved (cf. entries 1 and 3). A systematic optimisation of the reaction conditions was subsequently performed. Catalyst C2 displayed tolerance towards an increased substrate loading of 1 : 2000 ([Fe] : [PO]), reaching a high TOF value of 400 h−1 (Table 1, entry 6). Doubling the co-catalyst ratio from 1 to 2 equivalents (vs. catalyst C2) further improved the turnover frequency of the catalyst system, from 400 h−1 to 480 h−1 (Table 1, entries 6 and 8, respectively). Finally, tetrabutylammonium bromide (TBABr) was investigated as a co-catalyst, which resulted in a modest improvement in the catalytic activity (entry 12, TOF = 510 h−1). Kinetic studies were performed and confirmed a linear, first-order relationship between substrate conversion and time, highlighting the catalyst stability under the reaction conditions tested (Fig. 4). All three Fe(III) phenoxyimine complexes were subsequently screened for CO2/PO coupling using the optimised reaction conditions (Table 1, entries 11–13).
Fig. 4 Kinetic plot for the synthesis of propylene carbonate using C2 with 2000 equivalents of substrate.
Conditions: 100 ml stainless steel autoclaves, 20 bar CO2 pressure, 120 °C, neat. Conversion was determined using 1H NMR spectra of crude reaction mixtures.a The reaction was carried out without an iron complex, 2 equivalents of TBAI were added per 1000 equivalents of epoxide.b 100 equivalents H2O/[Fe] was added.c The reaction was carried out under air.d The reaction was carried out using unpurified propylene oxide (99%).
In addition to their high catalytic activity, it is particularly noteworthy that C1, C2 and C3 are all air-stable complexes. When the coupling reaction was set up under air, ortho-chloro catalyst C3 still displayed high activities towards cyclic propylene carbonate formation, with only a minor reduction in the TOF value observed (entry 13, TOF = 760 h−1; entry 15, TOF = 650 h−1). Furthermore, catalyst system C3 also displayed some tolerance towards water; the addition of 100 equivalents of water (vs. catalyst C3) still gave high TOF values of 430 h−1 (entry 14). Catalyst C3 is noteworthy as it demonstrates high catalytic activities even at low catalyst loadings of 0.01 mol% (1 : 10 000 [Fe] : [PO]), displaying a TOF of up to 550 h−1 after 2 hours (entry 16), slowing only after 8 hours (Fig. S12†). The robustness of C3 was further demonstrated when high conversion was maintained using 10 000 equivalents of unpurified (wet) PO (entry 18). The water tolerance of catalyst C3 was supported by FT-IR spectroscopic studies (refer to ESI† for further details).
Conditions: 100 ml stainless steel autoclaves, 20 bar CO2 pressure, 120 °C, 2 hours, neat. Conversion was determined using 1H NMR spectra of crude reaction mixtures.
It has previously been proposed that the metal geometry is of key importance for the conversion of sterically congested internal epoxides to the corresponding cyclic carbonates, with complexes bearing a trigonal bipyramidal geometry around the metal centre typically showing greater success.1,51 Complexes C1–C3 all display a distorted trigonal bipyramidal geometry, in contrast to the square pyramidal geometries often observed with Fe(III) salen complexes.52–55 The flexible coordination modes available when using phenoxyimine ligands may present an advantage over the well-established salen analogues. These findings suggest that phenoxyimine ligand supported metal complexes have significant potential for a broad scope of CO2/epoxide coupling reactions.
In conclusion, three new iron(III) phenoxyimine complexes were synthesised and tested in CO2/epoxide coupling reactions. All three earth-abundant, air-tolerant complexes display excellent performance in the selective catalytic coupling of CO2/epoxides, when activated by the presence of a tetrabutylammonium bromide or iodide co-catalyst. These results highlight that for this class of catalysts, the activity is influenced by the presence of electron donating or electron withdrawing ortho-substituents on the phenoxyimine ligand. The highest catalyst activities were achieved using the electron withdrawing chloro-substituted complex C3, with TOF values up to 900 h−1. Importantly, the complexes are very robust, tolerating the presence of both air and water, and very flexible, selectively forming the cis isomer of cyclohexene carbonate from the sterically challenging secondary epoxide. These iron catalyst precursors offer significant potential as stable, robust and selective catalysts for the valorisation of carbon dioxide.
Data for L2: (1.86 g, 97%) 1H NMR (500 MHz, CDCl3) δ 14.11 (s, 1H, OH), 8.35 (s, 1H, HC N), 7.31 (d, J = 7.7 Hz, 1H, ArH), 7.10 (d, J = 7.6 Hz, 1H, ArH), 6.80 (t, J = 7.6 Hz, 1H, ArH), 3.48 (d, J = 1.5 Hz, 3H, NCH3), 1.44 (s, 9H, CCH3). 13C NMR (126 MHz, CDCl3) δ 166.89 (C N), 160.51 (C-OH), 137.43, 129.32, 129.13, 118.75, 117.63 (Ar-C), 45.72 (N-CH3), 34.83 (CCH3), 29.32 (CCH3). HRMS (EI): m/z [M]+ 191.1319 calculated [M]+ 191.1310.
Data for L3: (1.10 g, 99%) 1H NMR (500 MHz, CDCl3) δ 14.51 (s, 1H, OH), 8.29 (s, 1H, HC N), 7.39 (d, J = 7.9 Hz, 1H, ArH), 7.14 (d, J = 7.7 Hz, 1H, ArH), 6.77 (t, J = 7.8 Hz, 1H, ArH), 3.49 (d, J = 1.5 Hz, 3H, CH3). 13C NMR (126 MHz, CDCl3) δ 165.8 (C N), 158.8 (C-OH), 132.7, 129.6, 122.2, 119.3, 118.1 (Ar-C) 44.9 (CH3). HRMS (EI): m/z [M]+ 169.0315 calculated [M]+ 169.0294.
To a solution of pro-ligand (L1–L3) in THF, 1.1 equivalents of NaH was gradually added and the mixture was stirred at ambient temperature for 1 hour. A solution of anhydrous FeCl3 (0.5 eq.) in THF was added dropwise to afford a dark coloured mixture that was stirred for a further 16 hours at room temperature. Solvents were evaporated in vacuo and the crude product was taken up in toluene. The NaCl by-product was removed by filtration through Celite and the filtrate was dried in vacuo. The crude products were washed three times with hexane to afford dark purple-brown powders. Single crystals suitable for X-ray diffraction analysis were obtained via slow evaporation of DCM or toluene.
Data for C1 (0.42 g, 81%) HRMS (EI): m/z [M]+ 359.0205 calculated [M]+ 359.0250 elemental analysis calculated for C16H16ClFeN2O2: C, 53.4; H, 4.5; N, 7.8. Found: C, 53.3; H, 4.65; N, 7.6.
Data for C2 (2.11 g, 98%) HRMS (EI): m/z [M]+ 471.1523 calculated [M]+ 471.1502 elemental analysis calculated for C24H32FeN2O2: C, 61.1; H, 6.84; N, 5.9. Found: C, 60.2; H, 6.2; N, 6.0.
Data for C3 (0.49 g, 74%) HRMS (EI): m/z [M]+ 426.9474 calculated [M]+ 426.9475 elemental analysis calculated for C16H14Cl3FeN2O2: C, 44.85; H, 3.3; N, 6.5. Found: C, 44.7; H, 3.35; N, 6.5.
Reactions were carried out in 100 mL stainless steel autoclaves equipped with a magnetic stirrer, heating mantle (controlled by a thermocouple) and a pressure gauge. Complex C1, C2 or C3 (10.0 μmol) and co-catalyst (TBAI or TBABr) were placed into the autoclave and purged with argon. The relevant epoxide (2000 eq.) was added and the autoclave was subsequently pressurised with 20 bar of CO2. The autoclave was heated to 120 °C for 24 hours, then rapidly cooled in an ice bath for 1 hour and slowly vented. An aliquot of the crude reaction mixture was immediately taken up in CDCl3 and analysed via1H NMR spectroscopy, to determine epoxide conversion via integration of product resonances against starting material resonances (refer to ESI† for further details).
Single crystal X-ray diffraction data were collected on an Rigaku Oxford Diffraction SuperNova diffractometer fitted with an Atlas CCD detector with Mo-Kα radiation (λ = 0.7107 Å) or Cu-Kα radiation (λ = 1.5418 Å). Crystals were mounted under Paratone on MiTeGen loops. The structures were solved by direct methods using SHELXS or SHELXT and refined by full-matrix least-squares on F2 using SHELXL interfaced through Olex2.57,58 Molecular graphics for all structures were generated using Diamond.
We gratefully acknowledge the University of Edinburgh for funding.
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