Article pubs.acs.org/IC
Reductive Activation of O2 by Non-Heme Iron(II) Benzilate Complexes of N4 Ligands: Effect of Ligand Topology on the Reactivity of O2‑Derived Oxidant Biswarup Chakraborty, Rahul Dev Jana, Reena Singh, Sayantan Paria, and Tapan Kanti Paine* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India S Supporting Information *
ABSTRACT: A series of iron(II) benzilate complexes (1−7) with general formula [(L)FeII(benzilate)]+ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N4 ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA in 2), N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N 1 ,N 1 -dimethyl-N 2 ,N 2 -bis(6-methyl-2pyridylmethyl)ethane-1,2-diamine (6-Me2-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N1,N2-dimethyl-N1,N2-bis(2pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N1,N2dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1−3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron−oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4−6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O2-derived iron− oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-Me2-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.
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INTRODUCTION A large variety of biologically important oxidative transformations such as aliphatic and aromatic C−C bond cleavage, hydroxylation of alkanes and arenes, C−H bond halogenation, epoxidation and cis-dihydroxylation of olefins, oxidation of organosulfur compounds, etc. are catalyzed by non-heme iron enzymes using dioxygen as the terminal oxidant.1−5 In the catalytic cycles of these oxygenases, NADH or organic cofactors/substrates facilitate the reduction of O2, leading to the generation of iron−oxygen oxidants. The resulting metalbased oxidants carry out different oxidation/oxygenation reactions.1,6,7 Generation of different iron−oxygen intermediates through the reductive activation of dioxygen by synthetic iron(II) complexes in the presence of electron and proton donors has been documented.8−15 However, only a limited number of iron complexes for oxyfunctionalization of organic © XXXX American Chemical Society
substrates have been developed that use dioxygen directly in the presence of reductants.1,16−26 In bioinspired oxidation catalysis with metal-based catalysts and dioxygen, selective electron transfer remains a major challenge. With small-molecule systems, it is difficult to provide electrons selectively for the reduction of dioxygen without quenching the active oxidant. Uncontrolled electron transfers from reductants to metal−oxygen species result in nonselective oxidation of substrates. Therefore, most of the bioinspired catalysts developed so far have used peroxides or other oxidants to bypass the reduction process.27−42 Inspired by the reactions of the non-heme α-keto aciddependent enzymes,2,6 where α-keto acid cofactors bind to the Received: September 20, 2016
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DOI: 10.1021/acs.inorgchem.6b02282 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
TPA = tris(2-pyridylmethyl)amine, 6-Me3-TPA = tris(6methyl-2-pyridylmethyl)amine, iso-BPMEN = N1,N1-dimethylN 2 ,N 2 -bis(2-pyridylmethyl)ethane-1,2-diamine, 6-Me2 -isoB PM E N = N 1 , N 1 - d i m e t h y l - N 2 , N 2 -b i s ( 6 -m e t h y l - 2 pyridylmethyl)ethane-1,2-diamine, BPMEN = N1,N2-dimethylN1,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, 6-Me2-BPMEN = N1,N2-dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane1,2-diamine, and TBimA = tris(2-benzimidazolylmethyl)amine) (Chart 1). While the iron(II) benzilate complexes of TPA, 6Me3-TPA, iso-BPMEN, and 6-Me2-iso-BPMEN have a distorted-octahedral geometry,30,47 BPMEN and 6-Me2-BPMEN44 adopt a cis-α geometry in the corresponding six-coordinate iron(II) benzilate complexes. The ligand TBimA, on the other hand, stabilizes a five-coordinate complex with trigonalbipyramidal coordination geometry at the iron center. The iron−oxygen oxidants derived from the reactions between iron(II) complexes and O2 are intercepted by external substrates. The effect of ligand topology on the reactivity of O2-derived oxidants is discussed in this work.
metal center prior to dioxygen activation and selectively provide electrons to generate high-valent iron oxo species, we have been investigating the dioxygen reactivity of ternary metal complexes of polydentate supporting ligands and different reductants as coligands.20−23 Recently, we have reported an iron(II) benzilate complex of a facial tridentate N3 ligand, [(TpPh2)FeII(benzilate)], which reacts with O2 to form benzophenone in quantitative yield. The complex oxidizes sulfides and activates aliphatic C−H bonds of various substrates. In addition, the complex efficiently transforms alkenes to the corresponding cis-diols with the incorporation of both of the oxygen atoms of O2.20 In a subsequent study, the reactivity of a series of iron(II) α-hydroxy acid complexes of the same ligand toward various external substrates has been investigated.21 On the basis of interception and mechanistic studies, a nucleophilic iron−oxygen intermediate, generated upon 2e reduction of O2, has been proposed as the active oxidant. Metal complexes of tetradentate nitrogen donor ligands that provide two available cis sites for binding of oxidant have been extensively used in peroxide-dependent catalytic oxidation of substrates.29,30,38,42−48 The effect of ligand topology on the catalytic oxidation of olefins and alkanes by biomimetic iron(II) complexes has been reported.32,46,47,49,50 Therefore, iron complexes of N4 ligands likely allow the formation of iron− oxygen intermediates from O2 in the decarboxylation of αhydroxy acids. The tetradentate ligands shown in Chart 1 can
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RESULTS AND DISCUSSION Synthesis and Characterization. Reactions of equimolar amounts of iron(II) perchlorate, benzilic acid, triethylamine, and respective tetradentate ligand in methanol afforded light yellow iron(II) benzilate complexes. Iron(II) complexes of TPA and iso-BPMEN were isolated as tetraphenylborate salts by addition of sodium tetraphenylborate to the reaction solution (Scheme 1). The optical spectra of the complexes in acetonitrile display absorption bands in the region between 370 and 390 nm (Figure S1 in the Supporting Information). Room-temperature magnetic moment values for the complexes are found in the range of 4.8−5.2 μB, indicating the high-spin nature of the complexes. A higher magnetic moment for 2 indicates a large orbital contribution to the magnetic moment. All of the iron complexes exhibit broad and paramagneticaly shifted resonances of protons. The 1H NMR spectra of 1, 3, and 7 show resonance signals between 0 and 140 ppm (Figure S2 in the Supporting Information), whereas complexes 2, 4 and 6 exhibit proton resonances in the region between −68 and 115 ppm (Figure S3 in the Supporting Information). The number of peaks observed in the 1H NMR spectra of 5 and 6 indicate that the complexes have a C2-symmetric high-spin iron(II) center in contrast to complexes 1−4 with respective ligand topologies in solution.53 The 1H NMR spectrum of complex 7 bears a resemblance to that of [(TBimA)FeII(CH3CN)]2+ and other iron(II) complexes of the ligand, indicating a 3-fold symmetry of the complex in solution (Figure S4 in the Supporting Information).54 X-ray Crystal Structures. The structures of the complexes (1−3, 5, and 7) in the solid state have been established from single-crystal X-ray diffraction studies. Each of the monocationic complexes, except complex 7, contains a six-coordinate iron center surrounded by one N4 ligand and one monoanionic benzilate (Figures 1 and 2). The average Fe−N distances are found in the range of 2.056(2)−2.415(1) Å, which match well with the reported high-spin iron(II) complexes of the respective ligands (Table 1).53,55,56 The monoanionic benzilate is coordinated to the metal center of 1 through one of the carboxylate oxygens (O1) with an Fe1−O1 distance of 2.032(1) Å (Table 1). One methanol molecule binds at the sixth site, giving rise to distorted-octahedral coordination geometry at the metal center (Figure 1a). The pyridine
Chart 1. Tetradentate Ligands Used in This Study
engender different geometries of iron complexes.51,52 Similarly, the stabilities of iron−oxygen intermediates generated from iron(II) benzilate complexes and their reactivities toward external reagents are expected to vary depending upon the topology/geometry engendered by tetradentate ligands. To test this hypothesis, we have explored the dioxygen reactivity of iron(II) α-hydroxy acid complexes of different N4 ligands. Herein, we report the isolation and characterization of seven ternary iron(II) benzilate complexes, [(TPA)FeII(benzilate)(MeOH)](BPh4) (1), [(6-Me3-TPA)FeII(benzilate)](ClO4) (2), [(iso-BPMEN)FeII(benzilate)](BPh4) (3), [(6-Me2-isoB P M E N ) F e I I ( b e n z i l a t e ) ] ( C l O 4 ) ( 4 ) , [ (B P ME N )FeII(benzilate)](ClO4) (5), [(6-Me2-BPMEN)FeII(benzilate)](ClO4) (6), and [(TBimA)FeII(benzilate)](ClO4) (7) (where B
DOI: 10.1021/acs.inorgchem.6b02282 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Synthesis of Iron(II) Benzilate Complexes
Figure 2. ORTEP plots of the complex cations of (a) 3 and (b) 5 with 30% thermal ellipsoid parameters. The counterion and all H atoms except that on O3 have been omitted for clarity.
Figure 1. ORTEP plots of the complex cations of (a) 1 and (b) 2 with 30% thermal ellipsoid parameters. The counterion and all H atoms other than those on O4 and C57 in 1 and O3 in 2 are omitted for clarity.
complex, [(6-Me3-TPA)FeII(mandelate)](BPh4) (Table 1).57 While the hydroxy oxygen O3 and pyridine nitrogen N4 occupy the axial positions with an O3−Fe1−N4 angle of 165.77(4)° in 2, the axial positions are occupied by the carboxylate oxygen O1 and the amine nitrogen N2 with an O1−Fe1−N2 angle of 174.45(11)° in 3. On the other hand, in 5, two pyridine nitrogens (N1 and N4) are positioned at the axial sites with an N1−Fe1−N4 angle of 169.17(18)°. The solid-state packing of 5 allows an intermolecular hydrogen bond between the hydroxy hydrogen atom of O3 and the noncoordinated carboxylate oxygen O2 of a neighboring molecule. Complex 7 is a mononuclear five-coordinate iron complex, where the tetradentate ligand wraps the metal center with three imidazolyl nitrogens with an average Fe−Nimidazole distance of
nitrogen N4 and the oxygen atom O3 of methanol occupy the axial positions with an N4−Fe1−O3 angle of 167.82(5)°. An intermolecular hydrogen bond is observed between the noncoordinated hydroxy group of benzilate and the coordinated methanol molecule of another complex, forming a onedimensional hydrogen-bonded network structure. In 2, 3, and 5, benzilate is coordinated to the iron center in a bidentate chelating mode through the neutral hydroxy oxygen (O3) and the anionic carboxylate (O1) with Fe1−O1 and Fe1−O3 distances ranging between 2.013(9) and 2.074(5) Å and between 2.084(5) and 2.214(9) Å, respectively (Figures 1b and 2 and Table 1). The average Fe−N distance is longer in 2 and is closely comparable to a related iron(II) α-hydroxy acid C
DOI: 10.1021/acs.inorgchem.6b02282 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1−3, 5, and 7 bond distance/angle 1
2
3
5
7
Fe(1)−N(1) Fe(1)−N(2) Fe(1)−N(3) Fe(1)−N(4) Fe(1)−O(1) Fe(1)−O(3)
2.205(1) 2.245(1) 2.195(1) 2.169(1) 2.032(1) 2.184(1)
2.205(1) 2.179(1) 2.240(1) 2.266(1) 2.013(1) 2.214(1)
2.134(4) 2.248(3) 2.149(4) 2.247(3) 2.043(2) 2.140(2)
2.191(5) 2.217(5) 2.233(5) 2.183(5) 2.074(4) 2.084(4)
2.415(1) 2.090(1) 2.085(1) 2.056(1) 2.019(1)
O(1)−Fe(1)−N(1) O(1)−Fe(1)−N(2) O(1)−Fe(1)−N(3) O(1)−Fe(1)−N(4) O(1)−Fe(1)−O(3) N(1)−Fe(1)−N(2) N(1)−Fe(1)−N(3) N(1)−Fe(1)−N(4) N(2)−Fe(1)−N(3) N(2)−Fe(1)−N(4) N(3)−Fe(1)−N(4) O(3)−Fe(1)−N(1) O(3)−Fe(1)−N(2) O(3)−Fe(1)−N(3) O(3)−Fe(1)−N(4)
90.34(5) 166.73(5) 118.69(5) 97.33(5) 90.17(4) 76.55(5) 150.90(5) 80.36(5) 74.50(5) 78.69(5) 96.72(5) 90.05(5) 91.84(4) 87.98(5) 167.82(5)
102.62(4) 165.28(4) 101.01(4) 114.30(4) 74.26(4) 77.40(5) 154.97(5) 95.89(4) 77.64(5) 80.17(4) 81.73(5) 93.03(4) 91.02(4) 85.49(4) 165.77(4)
107.18(12) 174.45(11) 99.92(13) 94.49(11) 75.05(9) 76.28(15) 151.51(14) 95.56(14) 77.43(15) 80.75(12) 90.85(14) 89.10(11) 109.62(11) 89.52(12) 169.43(12)
98.50(16) 101.29(17) 167.61(16) 91.65(16) 76.33(15) 76.81(18) 93.87(17) 169.17(18) 80.88(17) 97.58(17) 75.96(17) 89.80(16) 165.99(16) 104.46(16) 96.29(16)
175.38(5) 104.74(5) 102.15(5) 108.77(5) 74.74(5) 75.03(5) 75.75(5) 127.93(5) 102.71(5) 109.55(5)
2.077(1) Å. However, the iron−Namine distance becomes elongated to 2.415(1) Å. Benzilate is coordinated to the metal center in a monodentate mode through one anionic carboxylate oxygen (O1) with an Fe1−O1 distance of 2.019(1) Å (Figure 3 and Table 1). The geometry adapted by the metal
Figure 4. Coordination geometry at the metal center of iron(II) benzilate complexes.
and analytical data leave no doubt about the composition of the complexes. The optimized geometry of 4 obtained from DFT calculations suggests a six-coordinate iron complex with bidentate binding of monoanionic benzilate similar to that in 3 (Table S1 and Figure S5 in the Supporting Information). Reactivity of the Iron(II) Benzilate Complexes. All of the iron(II) α-hydroxy acid complexes (1−7) are air-sensitive in solution. In reactions with O2, light yellow solutions of the complexes in acetonitrile readily convert to light orange. In the reaction between 1 and dioxygen, the intensity of the CT band at 392 nm increases slowly with the formation of a broad band at 326 nm (Figure 5). In the X-band EPR spectrum collected at 77 K, a gradual increment of the rhombic signal at g = 4.2 is observed. In addition, three weak signals at g = 2.39, 2.23, 1.88 are observed (Figure S6 in the Supporting Information). The signals in the EPR spectrum (Figure 5, inset) indicate the formation of a mixture of high-spin iron(III) and low-spin iron(III) species.59 The changes in optical and EPR spectra clearly suggest the oxidation of 1. Reactions of other iron(II) benzilate complexes with O2 exhibit similar spectral changes (Figures S7−S12 in the Supporting Information). Complex 1 reacts over a period of 2 h, and complex 3 takes about 1 h. The other complexes (2 and 4−7) require 2−6 h for complete
Figure 3. ORTEP plot of the complex cation of 7 with 30% thermal ellipsoid parameters. The counterion and all H atoms other than those on O3 and on selected nitrogen atoms have been omitted for clarity.
center is distorted trigonal bipyramidal (τ = 0.79),58 where the axial positions are occupied by an amine nitrogen (N1) and carboxylate oxygen (O1) with an N1−Fe1−O1 angle of 175.38(5)°. The linear tetradentate ligand BPMEN adopts a cis-α geometry in complex 5 (Figure 4). The presence of a fewer number of proton resonances in the NMR spectra of 5 and 6 (Figures S2 and S3 in the Supporting Information) strongly suggests that both the complexes retain their cis-α geometry in solution as well.52 In contrast, complex 7 possesses a fivecoordinate trigonal-bipyramidal (tbp) geometry (Figure 4). Although X-ray diffraction quality single crystals could not be isolated for 4 and 6 after several attempts, the spectroscopic D
DOI: 10.1021/acs.inorgchem.6b02282 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
quantitative formation of benzophenone in 4 h (Figure S17 in the Supporting Information). The iron−oxygen oxidants, formed in situ upon two-electron oxidative decarboxylation of iron(II) benzilate complexes, were intercepted in the reaction with different substrates (Scheme 2). When 10 equiv of thioanisole is added separately to the solutions of 1−7, thioanisole oxide is formed in each reaction. In addition, methyl phenyl sulfone is observed in low yield (
