Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Bioinspired Olefin cis-Dihydroxylation and Aliphatic C−H Bond Hydroxylation with Dioxygen Catalyzed by a Nonheme Iron Complex Sayanti Chatterjee,† Shrabanti Bhattacharya,† and Tapan Kanti Paine* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
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ABSTRACT: A mononuclear iron(II)-α-hydroxy acid complex [(TpPh,Me)FeII(benzilate)] (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate) of a facial tridentate ligand has been isolated and characterized to explore its catalytic efficiency for aerial oxidation of organic substrates. In the reaction between the iron(II)-benzilate complex and O2, the metal-coordinated benzilate is stoichiometrically converted to benzophenone with concomitant reduction of dioxygen on the iron center. Based on the results from interception experiments and labeling studies, different iron-oxygen oxidants are proposed to generate in situ in the reaction pathway depending upon the absence or presence of an external additive (such as protic acid or Lewis acid). The five-coordinate iron(II) complex catalytically cis-dihydroxylates olefins and oxygenates the C−H bonds of aliphatic substrates using O2 as the terminal oxidant. The iron(II) complex exhibits better catalytic activity in the presence of a Lewis acid.
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of sacrificial reductants26,34,35 the crisis of selective and catalytic systems prevail. In our endeavor to develop bioinspired catalysts for oxidations with O2, we have been pursuing systematic studies on the reductive activation of dioxygen by biomimetic iron(II) complexes using α-hydroxy acids as sacrificial reductants.36−40 The iron(II)−benzilate complexes have been reported to oxidize substrates through in situ generation of iron−oxygen oxidants (Scheme 1). Unfortunately, there is no direct experimental evidence yet for such iron−oxygen oxidants. We have proposed different iron−oxygen oxidants generated under different experimental conditions based on indirect evidence such as interception and mechanistic studies. Among the reported complexes, the iron(II)−benzilate complex of the TpPh2 (hydrotris(3,5-diphenylpyrazolyl)borate)) ligand exhibits better reactivity in stoichiometric oxidation compared to those supported by tetradentate ligands. However, the tendency of the ligand (TpPh2) to undergo intramolecular hydroxylation21,41 under oxidizing conditions is expected to make the iron complexes of this ligand less active for catalytic oxidation. Recently, we demonstrated that the iron(II) complex [(TpPh,Me)FeII(BF)] (BF = monoanionic benzoylformate, TpPh,Me = hydrotris(3-phenyl-5-methylpyrazolyl)borate) was capable of performing the catalytic aerobic oxidation of alcohols and oxygen atom transfer reactions without detectable
INTRODUCTION Selective oxyfunctionalization of hydrocarbons is an important step in the synthesis of useful materials in chemical industries.1 Traditional oxidants, such as heavy-metal oxides/oxo-hydroxides, chlorine, periodate, peroxides, and so on, are commonly used for oxidation of alkanes and alkenes.2 In these reactions, the oxidant-derived products are often hazardous and the reactions are not selective. Despite extensive research in this field,3,4 development of sustainable methods for oxidation reactions with high efficiency and selectivity still remains a challenge. In biological systems, metalloenzymes are involved in catalyzing these challenging oxidations under mild conditions using dioxygen as the terminal electron acceptor.5−9 Taking lessons from nature, bioinspired approaches toward development of sustainable oxidation catalysts have attracted considerable attention. While significant progress has been made in bioinspired catalysis using peroxides or peracids for the oxidation of hydrocarbon substrates by metal complexes,10−18 use of dioxygen in catalytic oxidations remains less explored. The reductive activation of dioxygen by bioinspired complexes and subsequent generation of metal− oxygen oxidants for substrate oxidation requires electron and proton sources.19−27 In biomimetic chemistry, cosubstrateassisted activation of O2 by transition metal complexes offers attractive alternatives for bioinspired catalysis.5,28−33 In spite of a few reports on oxygen-dependent transformation of hydrocarbon substrates by nonheme iron complexes in the presence © XXXX American Chemical Society
Received: May 17, 2018
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DOI: 10.1021/acs.inorgchem.8b01353 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Proposed Iron−Oxygen Oxidants Generated In Situ upon Reductive Activation of Dioxygen from [(TpPh2)FeII(benzilate)] Complex under Different Reaction Conditions36−38
intraligand hydroxylation.42 This motivated us to isolate the iron(II)−benzilate complex [(TpPh,Me)FeII(benzilate)] (1) and explore its efficiency in the aerobic oxidation of substrates. In the present investigation, we found that complex 1 not only displayed selective oxygenation of substrates under stoichiometric condition, but also exhibited catalytic oxidation of alkanes, alkenes and sulfides. The catalytic ability of 1 in the O2-dependent oxidation reactions and its comparison with that of [(TpPh2)FeII(benzilate)] (2; Chart 1) are presented in this manuscript.
Table 1. Crystallographic Data for [(TpPh,Me)FeII(benzilate)] (1) 1 empirical formula formula weight crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg volume, Å3 Z Dcalcd, Mg/m3 μ Mo Kα, mm−1 F(000) θ range, deg reflections collected reflns unique R(int) data (I > 2σ(I)) parameters refined goodness-of-fit on F2 R1 [I > 2σ(I)] wR2
Chart 1. Iron(II)−Benzilate Complexes of Facial N3 Ligands
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RESULTS AND DISCUSSION Synthesis and Characterization. Complex 1 was isolated from the reaction of KTpPh,Me with iron(II) chloride and sodium benzilate in a solvent mixture of dichloromethane and methanol at room temperature under nitrogen atmosphere (Experimental Section). The 1H NMR spectrum of 1 displaying paramagnetically shifted proton resonances in the range between +70 and −25 ppm supported the high spin nature of the complex in solution (Figure S1, SI). The ESImass spectrum of 1 in acetonitrile exhibits an ion peak at m/z 767.3 with the isotope distribution pattern calculated for [(TpPh,Me)Fe(benzilate) + H+] along with other mass fragments (Figure S2 and Experimental Section). The composition of the complex was further confirmed by single crystal X-ray diffraction studies (Table 1). The X-ray crystal structure of the neutral complex displays a five-coordinate iron center coordinated by the facial tridentate TpPh,Me ligand and the carboxylate oxygen atoms of benzilate monoanion (Figure 1). In the ternary complex, the iron center adopts a distorted square pyramidal coordination geometry. Two nitrogen donors (N1 and N3) from the TpPh,Me ligand, and two carboxylate oxygen atoms (O1 and O2) from benzilate constitute the basal plane of the distorted square pyramid (τ = 0.46)43 The apical position is occupied by N5 nitrogen from the TpPh,Me ligand with the Fe1−N5 distance of 2.076(6) Å (Table 2). The
C44H39BFeN6O3 766.47 triclinic P-1 11.878(4) 14.847(5) 15.937(6) 100.607(8) 100.605(8) 101.476(8) 2635.2(16) 2 0.967 0.322 800.96 1.34−28.44 12086 8208 0.0453 6826 504 1.153 0.0782 0.2132
average Fe−N bond length of 2.105 Å is comparable to those of other FeII(TpPh,Me) complexes,44 while the iron−oxygen distances (r(Fe1−O1), 2.338(6) and r(Fe1−O2), 2.020(5) Å) indicate asymmetric bidentate binding mode of carboxylate. The geometry of the five-coordinate iron center and the binding mode of benzilate is very similar to those of the reported complex of the TpPh2 ligand.36 The hydroxy group of benzilate remains noncoordinated in both the complexes. The Fe−O2(carboxylate) and the Fe−N1 bonds in 1 are slightly elongated compared to those in 2 (Table 2). Additionally, the phenyl rings at 3-position of each pyrazole ring in 1 are slightly tilted away from the metal center (Figure S3). These slight structural variations likely prevent the intraligand hydroxylation. Dioxygen Reactivity. The iron(II) complex (1) reacted with pure dioxygen in benzene to convert the colorless solution to light yellow (Figure S4). Time-dependent product analyses revealed the quantitative conversion of iron-coordinated B
DOI: 10.1021/acs.inorgchem.8b01353 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. Time-dependent 1H NMR (500 MHz, CDCl3, 295 K) spectra monitoring the formation of benzophenone in the reaction between complex 1 and O2.
mixture of methyl phenyl sulfoxide (93%) and methyl phenyl sulfone (4%; Figure S6). The oxidized solution of 1 after the reaction with thioanisole was found to be X-band EPR silent and exhibited paramagnetically shifted proton resonances in the NMR spectrum indicating the formation of an iron(II) species (Figure S7). Thus, the in situ formed species was a two-electron reduced iron−oxygen oxidant which after oxidation of thioanisole generated an iron(II) complex. Hammett analyses using 1:1 mixtures of thioanisole and different para-substituted thioanisoles (p-XC6H4SCH3, where X = NO2, Br, H, Me, OMe) resulted in a ρ value of +1.04 (Figure S8) supporting the nucleophilic nature of the iron− oxygen oxidant from 1. Nucleophilic iron−oxygen oxidants from H 2 O 2 have been reported to cis-dihydroxylate alkenes.15,45 Reaction of 1 with cyclohexene yielded around 80% cis-cyclohexane-1,2-diol as the only product (Figure S9). Cyclooctene afforded 86% cis-cyclooctane-1,2-diol (Figure S10) and 1-octene was selectively converted to octane-1,2diol with about 92% yield (Figure S11). Electron-deficient olefin, tert-butyl acrylate showed very high yield of the corresponding diol (96%; Figure S12). As observed with complex 2,37 an electrophilic oxidant was generated from 1 in the presence of Sc(OTf)3 (Figure S13). In the reaction condition, thioanisole was selectively oxidized to thioanisole oxide with the incorporation of one 18O atom from H218O into the sulfoxide product (Figure S14). The electrophilic oxidant selectively oxidized alkenes to the corresponding cis-diols, in which partial incorporation of oxygen atom from water took place (Figure 3, Scheme 2). The product profile and the results of labeling experiments clearly support that the oxidant from 1 and O2 with Sc3+ can exchange its oxygen atoms with water. Aliphatic C−H bonds were oxygenated with high selectivity by the electrophilic oxidant (Scheme 2). Cyclohexane formed cyclohexanol (60%) and cyclohexanone (5%) with high A/K (12) ratio (Figure S15). Labeling experiments with 16O2 and H218O showed about 38% incorporation of 18O into cyclohexanol (Figure S16). Adamantane and methylcyclohexane were selectively transformed to 1-adamantanol (70% yield; Figure S17) and 1-methylcyclohexanol (63% yield), respectively (Figure S18). Except for the electron-deficient alkene (tert-butyl acrylate), the yields of oxygenated products were found to be higher compared to that found with complex 2 under similar experimental conditions (Table 3).
Figure 1. ORTEP plot of [(TpPh,Me)FeII(benzilate)] (1). All the hydrogen atoms other than those on B1 and O3 are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for [(TpPh,Me)FeII(benzilate)] (1) and [(TpPh2)FeII(benzilate)] (2)
Fe(1)−N(1) Fe(1)−N(3) Fe(1)−N(5) Fe(1)−O(2) Fe(1)−O(1) C(1)−O(1) C(1)−O(2) C(1)−C(2) C(2)−O(3) N(1)−Fe(1)−O(1) N(1)−Fe(1)−O(2) N(3)−Fe(1)−O(1) N(3)−Fe(1)−O(2) N(5)−Fe(1)−O(1) N(5)−Fe(1)−O(2) O(1)−Fe(1)−O(2) N(1)−Fe(1)−N(3) N(1)−Fe(1)−N(5) N(3)−Fe(1)−N(5) τ
[(TpPh,Me) FeII(benzilate)] (1)
[(TpPh2)FeII(benzilate)] (2)39
2.147(7) 2.095(6) 2.076(6) 2.020(5) 2.338(6) 1.251(10) 1.275(9) 1.520(11) 1.430(10) 107.74(2) 131.22(2) 98.23(2) 134.94(2) 162.8(2) 104.3(2) 60.00(2) 91.57(2) 87.7(2) 88.5(2) 0.46
2.075(3) 2.082(3) 2.120(3) 2.008(3) 2.346(3) 1.248(4) 1.273(5) 1.534(5) 1.421(5) 98.23(10) 134.82(11) 107.88(10) 131.04(11) 162.16(11) 103.85(12) 59.83(10) 91.87(11) 88.75(11) 88.16(11) 0.46
benzilate to benzophenone in 45 min (Figure 2). The ESImass spectrum of the oxidized solution showed an ion peak at m/z 539.2 attributable to [(TpPh,Me)Fe]+ ion. Unlike that observed with 2, no peak corresponding to the iron complex of the hydroxylated form of TpPh,Me was observed in the mass spectrum of the oxidized solution of 1 (Figure S5, inset). The two-electron oxidative decarboxylation of benzilate to benzophenone suggests that dioxygen can undergo twoelectron reduction on the iron center. In the absence of any observable intermediate in the reaction pathway, the in situ formed iron−oxygen oxidant from 1 was intercepted using external substrates as probes (Table 3, Experimental Section). Complex 1 reacted with thioanisole (10 equiv) to afford a C
DOI: 10.1021/acs.inorgchem.8b01353 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 3. Reactivity of Iron(II)-Benzilate Complexes (1 and 2) of Monoanionic N3 Ligands toward Different Substratesa yield (%) of products with [(TpPh,Me)FeII(benzilate)] (1)c substrate thioanisole (10 equiv) cyclooctene (100 equiv) 1-octene (100 equiv) cyclohexene (100 equiv)
tert-butyl acrylate (100 equiv) cyclohexane (100 equiv) 1-methyl-cyclohexane (100 equiv) adamantane (50 equiv)
product(s) PhSOCH3 PhSO2CH3 cis-cyclooctane-1,2-diol cyclooctene oxide octane-1,2-diol 1,2-epoxy-octane cis-cyclohexane-1,2-diol 2-cyclohexen-1-one 2-cyclohexen-1-ol cyclohexene oxide tert-butyl 2,3dihydroxypropanoate cyclohexanol cyclohexanone 1-methyl-cyclohexanol 2-methyl-cyclohexanol 1-adamantanol 2-adamantanol 2-adamantanone
yield (%) of products with [(TpPh2)FeII(benzilate)] (2)b
without additive
with Sc(OTf)3
with PyHClO4
without additive
with Sc(OTf)3
with PyHClO4
93 ± 1 4±1 86 ± 1 0 92 ± 2 0 80 ± 1 0 0 0 96 ± 2
96 ± 2 0 87 ± 3 0 93 ± 1 0 82 ± 2 0 0 0 8±1
97 ± 1 0 0 58 ± 1 0 56 ± 2 0 31 ± 2 20 ± 2 5±1
84 7 80 0 85 0 60 0 0 0 96
90 0 80 0 85 0 60 0 0 0 10
90 0 0 70 0 67 0 35 22 5
0 0 10 ± 1 0 20 ± 2 4±1 5±1
60 ± 1 5±1 63 ± 2 0 70 ± 4 0 0
58 ± 3 6±1 60 ± 2 4±1 65 ± 4
