Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)-Benzilate

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)Benzilate Complex Immobilized on Gold Nanoparticles Debobrata Sheet,† Abhijit Bera, Rahul Dev Jana, and Tapan Kanti Paine* School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 03/27/19. For personal use only.

S Supporting Information *

ABSTRACT: An iron(II)-benzilate complex [(TPASH)FeII(benzilate)]ClO4@C8Au (2) (TPASH = 11-((6-((bis(pyridin-2ylmethyl)amino)methyl)pyridin-2-yl)methoxy)undecane-1-thiol) immobilized on octanethiol stabilized gold nanoparticles (C8Au) of core diameter less than 5 nm has been prepared to evaluate its reactivity toward O2-dependent oxidations compared to a nonimmobilized complex [(TPA-O-Allyl)FeII(benzilate)]ClO4 (1a) (TPA-O-Allyl = N-((6-(allyloxymethyl)pyridin-2yl)methyl)(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine). X-ray crystal structure of the nonimmobilized complex 1a reveals a six-coordinate iron(II) center in which the TPA-O-Allyl acts as a pentadentate ligand and the benzilate anion binds in monodentate fashion. Both the complexes (1a and 2) react with dioxygen under ambient conditions to form benzophenone as the sole product through decarboxylation of the coordinated benzilate. Interception studies reveal that a nucleophilic ironoxygen intermediate is formed in the decarboxylation reaction. The oxidants from both the complexes are able to carry out oxo atom transfer reactions. The immobilized complex 2 not only performs faster decarboxylation but also exhibits enhanced reactivity in oxo atom transfer to sulfides. Importantly, the immobilized complex 2, unlike 1a, displays catalytic turnovers in sulfide oxidation. However, the complexes are not efficient to carry out cis-dihydroxylation of alkenes. Although the immobilized complex yields a slightly higher amount of cis-diol from 1-octene, restricted access of dioxygen and substrates at the coordinatively saturated metal centers of the complexes likely makes the resulting iron-oxygen species less active in oxygen atom transfer to alkenes. The results implicate that surface immobilized nonheme iron complexes containing accessible coordination sites would exhibit better reactivity in O2-dependent oxygenation reactions.

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progress in recent years.4,17−22 The accumulated mechanistic information from those bioinspired studies provide valuable information in designing catalytic systems. Moreover, some of the reported bioinspired complexes allowed isolation and characterization of iron-oxygen intermediates.14,15,23−27 However, generation of reactive hydroxyl radical and subsequent formation of nonselective products remains a matter of concern in peroxide-dependent catalysis.28 Inspired by the enzymatic reactions where organic cofactorassisted activation of dioxygen activation results in the generation of highly reactive metal-based oxidants,12,29−31 attempts have been made to use iron complexes together with different reductants and O2 in performing oxidation chemistry.15,32,33 While O2-derived iron-oxygen species have been trapped and characterized in a few cases,34−40 dioxygen-

INTRODUCTION The development of sustainable methods for selective and efficient C−H and CC bond oxidations is one of the great challenges in chemistry.1,2 To address these challenges, different approaches are being explored to develop methodologies for catalytic hydroxylation of aliphatic C−H bonds and epoxidation/cis-dihydroxylation of CC bonds.3−5 Nature employs dioxygen-activating iron enzymes to catalyze these oxidations in biological systems.6−13 In the reductive activation of dioxygen by iron enzymes, different iron-oxygen intermediates such as iron(III)-superoxide, iron(III)-(hydro)peroxide or high-valent iron-oxo are generated.14,15 Depending upon enzymatic functions, these intermediates serve as active oxidants in catalytic cycles. The intriguing reactions catalyzed by iron enzymes have attracted considerable attention in bioinspired oxidation catalysis using synthetic iron complexes.4,16 Iron-based oxidation catalysis of hydrocarbon substrates with peracids or peroxides has made significant © XXXX American Chemical Society

Received: November 25, 2018

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DOI: 10.1021/acs.inorgchem.8b03288 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry dependent iron catalysis in the oxidation of aliphatic and olefin substrates is less explored.41−44 In that direction, our research group has been developing nonheme iron(II)-α-hydroxy acid complexes of polydentate ligands to activate dioxygen for performing oxidations of different substrates.45−48 In those reactions, α-hydroxy acids serve as reductants for the reduction of O2 on the iron center. We have shown the use of benzilate ion as a sacrificial reductant to carry out catalytic oxidations of aliphatic substrates including the oxygenation of the C−H bond and epoxidation/cis-dihydroxylation of olefins.49 In bioinspired oxidations with small molecule iron complexes, formation of thermodynamically stable iron(III) species or oxidative degradation of complexes limit the efficiency of the complexes.33,49−52 Thus, one of the major challenges in oxidation catalysis is to inhibit the formation of inactive species. One approach to enhance the catalytic efficiency is to immobilize the catalyst on a surface. Transition metal catalysts have been shown to exhibit better activity upon immobilization on thiol-protected gold nanoparticles.53−57 Recently, we reported a 10-fold increase in the rate of reaction with dioxygen by immobilizing an iron(II)-benzoylformate complex on octanethiol stabilized gold nanoparticles.58 The immobilized iron complex reacted with dioxygen, leading to the generation of an oxidant, which could oxidize various substrates. Furthermore, the immobilized complex catalyzed the conversion of sulfides to sulfoxides in the presence of excess benzoylformate and substrates, most likely by preventing the formation of inactive iron(III) species during the reaction. With this understanding that immobilizing an iron complex on gold nanoparticles could reduce the probability of self-decay of active oxidants, we have extended our investigation to immobilize a nonheme iron(II)-benzilate complex on gold nanoparticles (AuNPs) and explored its ability to perform O2-dependent oxidations. As a result of our investigation, we report herein the synthesis and characterization of an iron(II)-benzilate complex [{(TPASH)FeII(benzilate)}ClO4]@C8Au (2) of a nitrogen rich polydentate ligand (TPASH) immobilized on octanethiolate protected gold nanoparticles. Analogous nonimmobilized complexes, [(TPAO-Allyl)FeII(benzilate)]ClO4 [TPA-O-Allyl = N-((6-(allyloxymethyl)pyridine-2-yl)methyl)(pyridine-2-yl)-N-(pyridine-2-ylmethyl)methanamine] (1a) and [(TPASH)FeII(benzilate)]ClO4 [TPASH = 11-((6-((bis(pyridine-2-ylmethyl)amino)methyl)pyridine-2-yl)methoxy)undecane-1-thiol] (1b), were isolated to compare their reactivity with the immobilized complex. The impact of immobilization on the reactivity pattern of the in situ generated iron-oxygen oxidant from 2 in substrate oxidation is presented here.

Scheme 1. Synthesis of Iron(II) Complexes, 1 and 1a

benzilate complex [(TPASH)FeII(benzilate)]ClO4 (1b) was prepared in the reaction of TPASH, iron(II) perchlorate hydrate, and sodium benzilate in a dichloromethane-methanol solvent mixture (Scheme 2). Octanethiol stabilized gold nanoparticles (C8Au) of 2−5 nm diameter were prepared following the reported procedure.59,60 For effective immobilization of the iron(II)−benzilate complex on AuNPs, two methods were explored. The first method involved an initial place-exchange reaction of the TPASH ligand with C8Au in dichloromethane as reported earlier.58 The reaction between TPASH@C8Au and iron(II) perchlorate hydrate in a dichloromethane−methanol mixture afforded the immobilized iron(II) complex, which, upon treatment with sodium benzilate, yielded [{(TPASH)FeII(benzilate)}ClO4]@ C8Au (2) (Scheme 3a). In another method, place-exchange of complex 1b on C8Au in dichloromethane under an inert atmosphere formed the immobilized complex 2 (Scheme 3b). Although the purification of the immobilized complex through removal of impurities such as unreacted sodium benzilate or metal salts was easier in the latter, the reprecipitation step in method (b) required large volume of solvents. Therefore, method (a) was preferred for the synthesis of 2. To understand the binding mode of the ligand, single crystals of 1 and 1a were grown for X-ray diffraction studies. Xray quality single crystals of 1 were obtained by diffusion of diethyl ether into a dichloromethane solution of the complex at −30 °C. The solid state structure of 1 shows a mononuclear seven-coordinate iron(II) center coordinated by the TPA-OAllyl ligand and two triflate anions (Figure 1a). The supporting ligand coordinates to the iron center through four nitrogens and the oxygen atom of the allylic ether group. The rest of the coordination sites are occupied by two triflate anions with the Fe1−O2 and Fe1−O3 distances of 2.257(2) Å and 2.278(2) Å, respectively (Table 1). The seven-coordinate complex adopts slightly distorted capped trigonal prismatic (CTP) geometry at the iron center.61 The quadrilateral face of the trigonal prism is constituted by N2, N1, O1 from the ligand, and O2 oxygen atom from one triflate anion. The N3 and N4 nitrogens of the ligand make up the remaining edge of the trigonal prism. The quadrilateral face of the trigonal prism is capped by the oxygen atom O3 of another triflate. The iron−N(pyridine) bonds are very similar to those observed in related high-spin iron(II) complexes, but the Fe−N(amine) bond is quite longer than the Fe−N(pyridine) bonds reported for analogous complexes of tripodal ligands.62,63 The coordination of the allylic oxygen likely prevents the amine nitrogen to bind strongly. The iron−

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RESULTS AND DISCUSSION Synthesis and Characterization. The iron(II)-benzilate complex [(TPA-O-Allyl)FeII(benzilate)]ClO4 (1a) was synthesized from the precursor iron(II)-triflate complex [(TPAO-Allyl)FeII(CF3SO3)2] (1). The latter was prepared by mixing equimolar amounts of iron(II)-triflate and TPA-OAllyl in dichloromethane under a nitrogen atmosphere (Scheme 1). The reaction of 1 with sodium benzilate, followed by metathesis with NaClO4, yielded complex 1a. Complex 1a may also be isolated from the reaction of TPA-O-Allyl, iron(II) perchlorate hydrate, and sodium benzilate in a dichloromethane-methanol solvent mixture (Scheme 1). The alkanethiol terminated tripodal ligand (TPASH) was synthesized following the reported procedure.58 The iron(II)B


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Inorganic Chemistry Scheme 2. Synthesis of Iron(II)-benzilate Complex (1b)

Scheme 3. Synthesis of the Immobilized Complex [{(TPASH)FeII(benzilate)}ClO4]@C8Au (2)

Figure 1. ORTEP plots of (a) complex 1 and (b) the cationic part of complex 1a.

(Figure 1b). The iron center is ligated by four pyridine nitrogens and one allylic oxygen atom from the ligand (TPAO-Allyl). Thus, TPA-O-Allyl acts a pseudo-pentadentate ligand. This binding mode of the supporting ligand forces the benzilate to coordinate in monodentate mode through one of the carboxylate oxygens (O1) with the Fe1−O1 distance of 2.035(1) Å. (Table 2). The iron−nitrogen bond (Fe−N) distances are in a range of 2.161(2)−2.251(2) Å, which are comparable to those of reported high-spin iron(II) complexes of polypyridyl ligands. The pyridine nitrogen donors, N3 and N2, occupy the axial positions with the N2−Fe1−N3 angle of 152.78(6)° (Table 2). Similar to that observed in 1, the Fe1− O4 distance of 2.313(1) Å in 1a is quite long, indicating a

oxygen (allylic ether) distance (Fe1−O1 = 2.300(2) Å) is the longest bond among all the Fe−O bonds in the complex, suggesting a weak bonding interaction. The high-spin nature of the complex is evident from the paramagnetically shifted proton resonances in the NMR spectrum (Figure S1, Supporting Information). Furthermore, the 19F NMR spectrum of the complex in acetonitrile reveals that the triflate groups are replaced by solvent molecules (Figure S2, Supporting Information). X-ray quality single crystals of 1a·CH2Cl2 were grown from a solvent mixture of dichloromethane and hexane. The X-ray structure reveals a six-coordinate mononuclear complex with distorted octahedral coordination geometry at the iron center C


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Inorganic Chemistry Table 1. Selected Bond Distances (Å) and Angles (deg) for [(TPA-O-Allyl)FeII(OTf)2] (1) Fe1−N1 Fe1−N3 Fe1−O1 Fe1−N4 N1−Fe1−N2 N3−Fe1−O2 N1−Fe1−O3 N2−Fe1−O3 N1−Fe1−O1 N2−Fe1−O1 O1−Fe1−O3 N1−Fe1−N4 O3−Fe1−N4 O1−Fe1−N4

2.2014(17) 2.1870(17) 2.3003(14) 2.3623(17) 108.47(6) 80.94(6) 88.66(6) 79.41(6) 85.50(6) 157.43(6) 83.40(5) 72.10(6) 138.59(6) 129.21(6)

Fe1−N2 Fe1−O2 Fe1−O3 N2−Fe1−N3 N1−Fe1−O2 N2−Fe1−O2 N3−Fe1−O3 O2−Fe1−O3 N3−Fe1−O1 O2−Fe1−O1 N3−Fe1−N4 N2−Fe1−N4 O2−Fe1−N4 N1−Fe1−N3

2.2030(17) 2.2569(14) 2.2783(15) 120.18(6) 161.47(6) 83.70(6) 150.55(6) 79.73(5) 71.19(6) 78.91(5) 70.82(6) 72.95(6) 125.82(5) 103.53(6)

Figure 2. UV−vis spectra of the iron(II)-benzilate complexes, C8Au and TPASH@C8Au, in acetonitrile at 298 K.

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Complex 1a·CH2Cl2 Fe1−N1 Fe1−N3 Fe1−O1 C14−O1 N2−Fe1−O4 N4−Fe1−O1 N2−Fe1−N4 N3−Fe1−N2 O1−Fe1−N1 N2−Fe1−N1 O1−Fe1−O4 N3−Fe1−O4

2.2519(16) 2.1868(17) 2.0357(14) 1.272(2) 99.65(6) 149.80(6) 91.95(6) 152.78(6) 132.45(6) 76.80(6) 79.64(5) 105.86(6)

Fe1−N2 Fe1−N4 Fe1−O4 C14−O2 N1−Fe1−O4 O1−Fe1−N2 N3−Fe1−O1 N4−Fe1−N3 N4−Fe1−N1 N3−Fe1−N1 O4−Fe1−N4

broad shoulders observed at 315 and 376 nm likely arise from the pyridine-donor ligand as observed in FeII−TPA complexes. The optical spectral data thus support further the immobilization of the TPASH-FeII-benzilate complex on AuNP in 2. The changes in the absorption spectrum reflect the alteration of the electronic properties of C8Au upon interaction between the thiolate group of TAPSH and AuNP. The NMR spectra of 1a, 1b, and 2 display paramagnetically shifted proton resonances indicating the high-spin nature of the iron(II) center (Figure 3 and Figure S6, Supporting

2.1692(17) 2.1610(17) 2.3133(14) 1.243(2) 147.81(6) 97.05(6) 96.73(6) 87.79(6) 77.67(6) 76.57(6) 70.43(6)

weak bonding between the iron(II) center and the etheral oxygen from the ligand. The binding motif of benzilate is similar to that observed in the iron(II)-benzilate complex of the TPA ligand.64 Similar monodentate binding of benzilate has also been observed for the iron(II) complexes of tetradentate TBimA ligand. However, benzilate has been reported to bind in a bidentate fashion when 6-Me3TPA ligand is employed.64 The ESI-mass spectra (in positive-ion mode, acetonitrile) of 1a and 1b display ion peaks at m/z 643.02 and at m/z 789.30 with the isotope distribution patterns calculated for [(TPA-OAllyl)Fe(benzilate)] + (calculated: m/z 643.20) and [(TPASH)Fe(benzilate)]+ ions (calculated: m/z 789.31), respectively (Figures S3 and S4, Supporting Information). The FTIR spectrum of 1a exhibits υ(COO) bands around 1629 and 1606 cm−1, indicating asymmetric binding of the carboxylate group of benzilate. The perchlorate anion displays sharp bands at 1116, 1083, and 628 cm−1 (Figure S5a, Supporting Information). The functionalized complex on AuNPs (2) shows IR bands at 1116 and 698 cm−1 attributed to perchlorate counteranion. Additionally, the presence of two peaks at 1606 and 1629 cm−1 from the carboxylate group supports the immobilization of the iron-benzilate complex on octanethiolate stabilized C8Au gold nanoparticles (Figure S5b, Supporting Information). The optical spectrum of complex 1a in acetonitrile displays broad bands at 321 nm (1905 M−1 cm−1) and 374 nm (1695 M−1 cm−1) (Figure 2). The immobilized complex 2 shows the surface plasmon (SP) band at around 520 nm, which is slightly shifted than that in C8Au (Figure 2).65,66 Additionally, two

Figure 3. 1H NMR spectra (500 MHz, CD3CN, 295 K) of (a) 1a and (b) 2.

Information). The assignment of the proton signals was performed on the basis of 1H NMR spectra reported for related iron(II) complexes.50,58,64 In 1a and 1b, the downfield shifted peaks at 48 and 45 ppm can be assigned to β and β′ protons of the pyridine rings of TPA-O-Allyl ligand or the TPASH ligand. For 2, these peaks appear as broad signals near the 45−47 ppm region. The resonance for γ-protons of the pyridine rings are observed at 17 and 13 ppm for 1a and 2, respectively. The TEM image of the immobilized complex 2 indicates fairly monodisperse particle size distribution with the average diameter between 2 and 5 nm, suggesting that no drastic change in the core dimension takes place upon immobilization of the iron(II)-benzilate complex on the nanoparticle surface (Figure 4). EDAX measurement shows the presence of Au, C, O, S, N, Fe, and Cl in the immobilized complex. D


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Inorganic Chemistry

Figure 4. TEM images of (a) C8Au and (b) immobilized complex 2.

reported for thiolate protected gold nanoparticles (Figure 5d).70,71 The absence of any shoulder or peak from S(2p3/2) in the region greater than 163 eV discards the presence of any unbound thiols or oxidized sulfur species. The binding energy for oxygen (1s) is observed at 532.6 eV (Figure 5e) along with the peaks for Fe(2p) (Figure 5f). Thus, the XPS results strongly support that C8Au gold nanoparticles have been successfully functionalized with the TPASH-iron(II)-benzilate complex. Reactivity of the Complexes. The yellow solution of complex 1a in acetonitrile gradually changes to orange upon exposure to O2. In the reaction, intensities of the CT bands at 321 and 374 nm increase slowly over a period of 10 h (Figure 6a). The oxidized solution after the reaction shows a broad band at around 326 nm and a shoulder at 510 nm. The absence of paramagnetically shifted resonances in the 1H NMR spectrum suggests the formation of a diiron(III)-μ-oxo bridged species similar to that observed with analogous iron complexes of N4 ligands (Figure S10, Supporting Information).50 The spectral change of 2 in a dichloromethane-acetonitrile (1:4) mixture upon exposure to dioxygen is not remarkable considering the low concentration of the immobilized complex. However, the oxidation of iron(II) species on AuNPs, indicated by the gradual increase in the UV band near 320 nm (Figure 6b), is established from the 1H NMR analysis of the organic product after the reaction with dioxygen (vide infra). The surface plasmon (SP) band at 520 nm remains unaltered during the oxidation process, indicating the stability of the functionalized gold nanoparticles. The organic products extracted from the oxidized solution were analyzed by 1H NMR spectroscopy and GC-mass spectrometry. In the case of complex 1a, time-dependent 1H NMR spectra reveal quantitative formation of benzophenone in 16 h (Figure 7a). It is well established that iron(II)-αhydroxy acid complexes undergo oxidative decarboxylation to form the corresponding carbonyl compounds.45,46,64 However,

Thermogravimetric analysis (TGA) of C8Au reveals an initial mass loss of 20% at 200 °C attributable to the desorption of octanethiol from the surface of AuNPs (Figure S7, Supporting Information).58,67 The TGA plot of 2 displays 14% mass loss at 182 °C due to the desorption of octanethiols from the core. Further degradation (mass loss of 12%) takes place at 272 °C with the desorption of the immobilized iron(II)-benzilate complex from the gold core (Figure S7, Supporting Information). An overall degradation of approximately 1.2 mg of complex out of 10 mg of 2 is estimated to be 0.0013 mmol of [(TPASH)FeII(benzilate)]ClO4 complex (MW of 889.28 g/mol) on AuNP. The XRD spectra obtained for the nanoparticles shows diffraction patterns similar to fcc Au; the planes (111), (200), (220), and (311) reflections are in general clearly visible in Au particles of size more than 6 nm.68 As the gold core dimension decreases, the intensity of the planes is reduced gradually and resolving all the planes becomes difficult. In fact, it is observed that the peaks are broad and the (200) reflection is seen as a shoulder of the (111) peak. These data suggest that the average core diameter of the AuNPs is below 6 nm (Figure S8, Supporting Information). Similar spectral patterns observed for all the nanoparticles clearly suggest that the crystalline surface of the gold nanoparticles is not destroyed upon surface modification. In the XPS analysis of 2, the Au 4f doublet is observed at 83.74 eV [Au(4f7/2)] and 87.43 eV [Au(4f5/2)] with the interpeak distance of ∼3.67 eV supporting the presence of Au(0) in the immobilized complex (Figure 5a).69 Signals for Au(4d5/2) and Au(4d3/2) are also observed at 336 and 355 eV, respectively (Figure S9, Supporting Information). The binding energy for the C(1s) singlet is observed at 285.61 eV (Figure 5b). The high-resolution scan in the region 390−408 eV shows a peak at 400.08 eV corresponding to N(1s) (Figure 5c).70 The high resolution spectrum reveals a peak at 162.22 eV due to the S(2p3/2) state, which is consistent with the values E


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Inorganic Chemistry

Figure 5. High resolution XPS spectra of 2. (a) Au(4f) doublet, (b) C(1s), (c) N(1s), (d) S(2p3/2), (e) O(1s), (f) Fe(2p).

Figure 6. UV−vis spectral changes of (a) 1a (0.5 mM in acetonitrile), inset: time trace for 321 nm band, and (b) 2 (in dichloromethaneacetonitrile (1:4) mixture) during the reaction with dioxygen at 298 K.

saturated. On the contrary, the immobilized complex 2 only takes 5 h for complete decarboxylation of benzilate to afford benzophenone. The loading of TPASH calculated from the 1H NMR spectrum is approximately 0.0012 mmol in 10 mg of the TPASH@C8Au nanoparticles. In the synthesis of 2, excess

the rate of decarboxylation of 1a is much slower compared to the iron(II)-α-hydroxy acid complexes of facial N3 or tripodal N4 ligands. This slow reaction could be attributed to the coordination of the etheral oxygen to the iron center in complex 1a, which makes the iron center coordinatively F


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Inorganic Chemistry

Figure 7. Time-dependent 1H NMR spectra (500 MHz in CDCl3 at 298 K) of organic products formed in the reaction of (a) 1a and (b) 2 with dioxygen. Peaks marked with * originate from residual solvent.

Scheme 4. Reactivity of 1a and 2 toward Thioanisole

Scheme 5. Interception of Active Oxidants from Complex 1a and 2 in the Reaction with Dioxygen and Alkenesa

a

The values within brackets indicate the yields of products in the reaction with 2.

enhancement of the reaction rate, dioxygen reactivity of 1b was investigated. Complex 1b takes about 15 h for quantitative conversion of benzilate to benzophenone (Figures S11 and S12, Supporting Information). While the rate of decarboxylation of 2 is faster than 1b, the reaction is slower than the immobilized iron(II)-benzoylformate complex of the same ligand (TPASH).58 Although the allylic ethereal oxygen of the TPASH ligand can weakly coordinate to the iron(II) center in both 2 and [(TPASH)FeII(BF)]@C8Au, the ligand likely binds in a tetradentate (N4) mode in the presence of dioxygen. In [(TPASH)FeII(BF)]@C8Au, the BF binds in a planar bidentate mode, whereas nonplanar benzilate with two phenyl rings engenders steric bulk at the iron center, resulting in slow reaction with dioxygen. To intercept the iron-oxygen oxidants generated in the reactions between iron(II)-benzilate complexes and dioxygen, external substrates such as sulfides, 4-bromobenzaldehyde,

iron(II) salt was used to ensure quantitative complexation with the immobilized ligands. The amount of benzophenone (0.0011 mmol/10 mg of 2) formed in the reaction with O2 support that all the immobilized ligands (TPASH@C8Au) in 2 are in metalated form with the Fe(II)-benzilate unit (Figure 7b). Thus, the amount of metalated ligand on AuNP calculated from the NMR study is nearly similar to that estimated from TGA data (vide supra). A number of control experiments were performed to establish the inertness of octanethiolate stabilized gold nanoparticles C8Au toward dioxygen (see the Experimental Section). The results of control experiments support that the immobilization of the iron(II)-benzilate complex on octanethiolate gold nanoparticles increases the rate to a certain extent than the nonimmobilized complex. To explore if a long aliphatic chain terminated at the N4 ligand prevents the coordination of allylic ethereal oxygen for G


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Inorganic Chemistry

labeling experiment using 16O2 and H218O with 2 reveals no labeled oxygen incorporation into thioanisole, supporting that the metal-oxygen oxidant does not exchange its oxygen atom with water. While benzylic C−H bonds of toluene or ethylbenzene are not activated by the complexes, both 1a and 2 can oxidize fluorene and 9,10-dihydroanthracene to fluorenone and anthracene, respectively (Table 3, and Figures S20 and S21, Supporting Information). Benzyl alcohol affords benzaldehyde as a sole product with both 1a and 2. In all the reactions, complex 2 exhibits higher oxidizing ability than complex 1a or the immobilized iron(II)-benzoylformate complex [(TPASH)FeII(BF)]@C8Au.58 The highly ordered octanethiols on C8Au provides nearly a spherical (3D) morphology and larger surface area that promotes greater access of the reactants, resulting in enhanced reactivity of 2. Both the complexes react with 4-bromobenzaldehyde (10 equiv) to form a mixture of 4-bromobenzoic acid and 4bromobenzyl alcohol (Table 3 and Figure S22, Supporting Information). The formation of benzyl alcohol from benzaldehyde via Cannizzaro reaction has been reported for the iron(II)-mandelate complex of Tp Ph2 ligand. 46 A nucleophilic iron-oxygen oxidant, generated upon reductive activation of dioxygen by the iron(II)-mandelate complex, has been proposed to carry out the conversion of aldehyde to the corresponding acid and alcohol. The above experimental results suggest that the iron-oxygen species derived from 1a bears similarity to that proposed for [(TpPh2)FeII(benzilate)] and [(TpPh2)FeII(mandelate)] complexes.46 Thus the formation of sulfoxide and sulfone from sulfides, and 4bromobenzoic acid from 4-bromobenzaldehyde, suggest that, with both 1a and 2, the oxygen-derived oxidants have nucleophilic character. Hammett analyses with complex 1a for competitive oxidations of various para-substituted thioanisoles showing a ρ value of 0.26 further corroborate the nucleophilic nature of the oxidant (Figure S23, Supporting Information). Nucleophilic iron-oxygen oxidants from iron(II)-α-hydroxy acid complexes have been reported to carry out the cisdihydroxylation of alkenes with the incorporation of both the oxygen atoms of O2 into a diol product.45−47,49 The reaction of 1a with styrene affords benzoic acid (12%) and 1-phenylethane-1,2-diol (2%) (Scheme 5, Figure S24, Supporting Information). Very small amounts of oxidized products ciscyclohexane-1,2-diol (6%) and cycloctane-1,2-diol ( 2σ(I)] wR2

C24H24FeN4O7S2F6 714.44 monoclinic P2(1)/c 14.8561(11) 10.0970(7) 19.1813(14) 90 92.874(2) 90 2873.6(4) 4 1.651 0.760 1456 1.37−26.00 28086 5641 0.0244 5195 397 1.053 0.0339 0.0951

C36H35FeN4O8Cl·CH2Cl2 827.90 monoclinic P2(1)/c 8.8486(6) 22.1933(14) 19.4135(12) 90 101.556(2) 90 3735.1(4) 4 1.472 0.676 1712 1.410−32.349 11920 9223 0.0774 9223 479 1.280 0.0545 0.1816

Solvents were distilled and dried before use. Preparation and handling of air-sensitive materials were carried out under an inert atmosphere in a glovebox. Air-sensitive complexes were prepared and stored in an inert atmosphere glovebox. The ligands, TPA-O-Allyl and TPASH, were synthesized according to the protocol reported in the literature.58 Caution! Metal perchlorates are potentially explosive. Only small quantities of these salts were used with care!72 Fourier transform infrared spectroscopy on KBr pellets was performed on a Shimadzu FT-IR 8400S instrument. Elemental analyses were performed on a PerkinElmer 2400 series II CHN analyzer. Electro-spray ionization mass spectra were recorded with a J


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Inorganic Chemistry

under high vacuum. The organic products were then analyzed by 1H NMR spectroscopy or by GC-MS. The oxidation reaction of the immobilized complex 2 (15 mg) in the presence of various substrates was carried out in acetonitrile under the same experimental condition as described for complex 1a. For analyzing organic products derived from olefins and alcohols in GC-MS, another set of reactions were carried out in which the complex 1 (0.02 mmol) was dissolved in 2 mL of dioxygen saturated dry acetonitrile. To the solution, external reagents 100 equiv (2 mmol) of alkenes or 10 equiv of alcohols (0.2 mmol) was added. Dioxygen gas was purged through the solution for 5 min, and the reaction solution was allowed to stir at room temperature for 16 h in closed condition. The solution was then passed through a 15 cm column with [10 cm silica (60−120 mesh size)] using dichloromethane as eluent. The combined organic phase was then analyzed by GC-mass spectrometry. The reaction with ethylbenzene was performed in a benzene-acetonitrile mixture. Thioanisole oxide was quantified by comparing the peak area of the three protons −CH3 (δ 2.74 ppm) with four ortho-protons (δ 7.8 ppm) of benzophenone. Quantification of the substituted thioanisole oxide was done by comparing the peak area of the three protons −CH3 of the substituted thioanisole oxide (δ 2.80−2.72 ppm) with the area integration of the four ortho-protons of benzophenone. Dibenzothiophene oxide was quantified by comparing the peak area of the two ortho-protons (δ 8.04 ppm) with four ortho-protons of benzophenone. The products derived from dihydroanthracene (DHA) and fluorene were analyzed by GC-MS and quantified using calibration curves obtained with authenticated compounds. Interception Studies with Dimethyl Sulfoxide and Dimethyl Sulfide. Complex 1a (0.02 mmol) was dissolved in dry acetonitrile (10 mL). To the solution was added dimethyl sulfoxide (10 equiv, 0.2 mmol) or dimethyl sulfide (50 or 100 equiv, 1 or 2 mmol). Dry dioxygen gas was bubbled through the solution for 5 min, and the solution was stirred at room temperature under an oxygen atmosphere for 1.5 h. The solvent was then removed from the reaction mixture, and distilled benzene (2 mL) was added to dissolve the residue. A slight excess of sodium dithionite (0.04 mmol) was then added to the benzene solution, followed by addition of D2O (1 mL), and the resulting solution was stirred for 15 min. To the solution was added 1,10-phenanthroline monohydrate (0.06 mmol), and the solution was stirred for an additional 30 min. The D2O layer was then collected and analyzed by 1H NMR spectroscopy. 1H NMR (500 MHz, D2O) data of dimethyl sulfone: δ 3.17 (s, 6H) ppm; dimethyl sulfoxide: δ 2.78 (s, 6H) ppm. The oxidation of dimethyl sulfide/dimethyl sulfoxide by the immobilized complex 2 (15 mg) was carried out in acetonitrile under similar experimental conditions as described for complex 1a. Catalytic Experiments. The experimental procedure for the catalytic reactions was the same as that mentioned above for individual substrates except that the amounts of benzilate and substrates were varied. The catalytic experiments were carried out using 0.02 mmol of complex 1a or 15 mg of complex 2 (0.0015 mmol of TPASH-iron(II)-benzilate) in an acetonitrile-benzene mixture. The products were analyzed by 1H NMR or by GC-mass spectrometry. 1 H NMR (500 MHz, CDCl3) Data of the Oxidized Products. Benzophenone: δ 7.80 (d, 4H), 7.56 (t, 2H), 7.48 (t, 4H). Thioanisole oxide: δ 7.66 (m, 2H), 7.52 (m, 3H), 2.74 (s, 3H). 4Bromobenzoic acid: δ 8.00 (d, 2H), 7.61 (d, 2H). 4-Bromobenzyl alcohol: δ 7.45 (d, 2H), 7.22 (d, 2H), 4.66 (d, 2H). Methyl phenyl sulfone: δ 7.96 (d, 2H), 7.60 (m, 3H), 3.06 (s, 3H). 1-Phenylethane1,2-diol: δ 7.37 (m, 4H), 7.30 (m, 1H), 4.85 (m, 1H), 3.80 (m, 1H), 3.70 (m, 1H). Octane-1,2-diol: δ 3.80−3.63 (m, 2H), 3.49 (m, 1H), 1.44−1.40 (m, 2H), 1.30−1.24 (m, 6H), 0.92−0.86 (m, 3H) ppm. cis-Cyclohexane-1,2-diol: δ 3.77−3.82 (m, 2H) 3.58 (br s, 2H), 1.73−1.82 (m, 2H), 1.51−1.64 (m, 4H), 1.27−1.35 (m, 2H) ppm. X-ray Crystallographic Data Collection and Refinement and Solution of Structures. X-ray single-crystal data for 1 and 1a were collected at 150 K using Mo Kα (λ = 0.7107 Å) radiation on a SMART-APEX diffractometer equipped with a CCD area detector. Data collection, data reduction, structure solution, and refinement

1352(w), 1112(s), 1099(s), 1016(w), 765(m), 675(m), 626(w). ESIMS (positive ion mode, CH3CN): m/z 789.30 (20%, ([(TPASH)Fe(benzilate)]+). UV−vis (in acetonitrile): 374 nm (1300 M−1 cm−1). 1 H NMR (500 MHz, CD3CN, 295 K): δ 134.65, 80.87, 70.98, 57.14, 50.22−48.84, 23.98, 18.62, 12.30, 9.23, 8.78, 7.78, 4.90, −35.19 ppm. [{(TPASH)FeII(benzilate)}ClO4]@C8Au (2). Method A. A methanolic solution (8 mL) of sodium benzilate (0.015 g, 0.06 mmol) was added to a solution of [(TPASH)FeII]@C8Au gold nanoparticles (0.100 g) dissolved in an acetonitrile-dichloromethane mixture (50 mL) and vigorously stirred for 12 h. The solvent was removed under reduced pressure, and the residue was washed twice with 5 mL of methanol and decanted off. Dichloromethane (30 mL) was added to the black residue and filtered to remove any insoluble impurity. The filtrate was then dried to obtain a black solid. Yield: 0.60 g. IR (cm−1): 3434(br), 2923(s), 2852(w), 1745(m), 1629(s), 1606(s), 1444(m), 1364(w), 1145(w), 1116(s), 1087(s), 1050(m), 756(m), 698(m), 626(w). UV−vis (in dichloromethane): 314 nm, 374 and 518 nm. 1H NMR (500 MHz, CD3CN, 295 K): δ 46.18, 40.15, 16.30, 12.26, 8.35, 7.02, 6.50, 6.09, 4.37, 2.80 ppm. Method B. A solution of [(TPASH)FeII(benzilate)]ClO4 (0.100 g, 0.11 mmol) dissolved in dichloromethane (25 mL) was added to the dichloromethane solution (15 mL) of octanethiol stabilized gold nanoparticles (C8Au) (0.080 g). (Note: C8AuNP were dried in vacuum to ensure complete removal of adsorbed dioxygen if present in the nanoparticles). The reaction was vigorously stirred for 48 h at room temperature inside the glovebox. The solvent was evaporated to complete dryness, and the residue was suspended in methanol (30 mL). The methanolic solution was then kept for 24 h at −10 °C for complete precipitation. The supernatant was decanted off, and the process was repeated several times until a colorless supernatant was obtained. The process was extremely important to remove insoluble impurities of the nonimmobilized metal complex, excess desorbed ligands, and organic compounds. The dark precipitate was collected and dried in vacuum for 30 min to yield the desired metal complex immobilized on the surface of gold nanoparticles. IR (cm−1): 3433(br), 2923(s), 2852(w), 1746(m), 1635(s), 1606(s), 1446(m), 1384(w), 1145(w), 1118(s), 1081(s), 1049(m), 756(m), 698(m). Reaction with Dioxygen. A solution of the complex 1a or 1b (0.02 mmol) in 10 mL of dioxygen saturated CH3CN was allowed to stir at room temperature. After the reaction, the solvent was removed under reduced pressure and the residue was treated with 10 mL of 3 M HCl. The organic products were extracted with diethyl ether, and the organic phases were washed with brine solution. The combined organic part was dried over Na2SO4 and the solvent was removed to dryness. The organic products were analyzed by 1H NMR spectroscopy or by GC-MS. The oxidation reaction of the immobilized complex 2 (15 mg) was carried out in acetonitrile under the same experimental condition as described for complex 1a and 1b. Control Experiments. (a) C8Au (10 mg) was treated with sodium benzilate (0.005 mmol) in dichloromethane for 24 h; (b) C8Au (25 mg) was treated with NEt3 (0.005 mmol) and benzilic acid (0.005 mmol) in dichloromethane and acetonitrile for 24 h; and (c) a mixture of C8Au (25 mg), sodium benzilate (0.005 mmol), and Fe(ClO4)2·xH2O (0.005 mmol) was allowed to react with O2 in dichloromethane and acetonitrile for 24 h. In all the three cases, decarboxylation of benzilic acid was not observed. Increase in the reaction rate was also not observed when complex 1a (0.01 mmol) was allowed to react with O2 in the presence of C8Au (15 mg). Interception Studies with External Substrates. Complex 1a (0.02 mmol) was dissolved in 10 mL of dry acetonitrile. To the solution, the required amount of external substrate was added. The solution was then saturated with dioxygen by purging dioxygen gas for 5 min, and the reaction was allowed to continue for desired amount of time at room temperature under a dioxygen atmosphere. The reaction solution was then dried by rotary evaporator, and the residue was treated with 3 M HCl solution (10 mL). The organic products were extracted with diethyl ether (3 × 20 mL), and the organic layer was washed with brine solution (2 × 20 mL). The combined organic phase was then dried over Na2SO4, and the solvent was removed K


Article

Inorganic Chemistry were carried out using the software package of APEX II.73 The structure was solved by direct method and subsequent Fourier analyses and refined by the full-matrix least-squares method based on F2 with all observed reflections.74 The non-hydrogen atoms were treated anisotropically. Crystallographic data of the complexes are presented in Table 4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03288. Spectroscopic data (PDF) Accession Codes

CCDC 1421764 and 1421786 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tapan Kanti Paine: 0000-0002-4234-1909 Present Address †

Department of Chemistry, Presidency University, 86/1 College Street, Kolkata 700073, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.K.P. gratefully acknowledges the Science and Engineering Research Board (SERB), India, for the financial support (Project: EMR/2014/000972). D.S. and R.D.J. thank the Council of Scientific and Industrial Research (CSIR), India, for research fellowships. D.S. is thankful to Presidency University for providing the FRPDF fund. A.B. acknowledges the University Grants Commission (UGC), India, for a research fellowship.

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Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)-Benzilate

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