An Azoaromatic Ligand as Four Electron Four Proton Reservoir

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

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex Rajib Pramanick,† Rameswar Bhattacharjee,Δ,‡ Debabrata Sengupta,†,‡ Ayan Datta,Δ and Sreebrata Goswami*,† †

Department of Inorganic Chemistry and ΔDepartment of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India S Supporting Information *

ABSTRACT: Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redoxinactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl2 (1a) along with the two hydrogenated complexes of 1a, ZnH2LCl2 (1b), and ZnH4LCl2 (1c) are reported. It has been argued that the reversible azohydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O2 and para-benzoquinone to H2O2 and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

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INTRODUCTION Synthetic compounds, which can act as electroprotic storage, are of high demand in present-day research, because these are important for their use in energy-related issues such as hydrogen production from alcohol (liquid fuel),1−9 activation of small molecules as O2 and CO2,10−12 and many more useful chemical reactions.2,13−30 Synthesis of such molecules are challenging but, so far, limited in the literature. A few recent examples of this class of compounds include the ruthenium(II) complex of a diazadiene ligand,31 bis(formazanate)zinc complex,32 and titanium(IV) complex of pyrrole-centered pincer ligand33 for their reversible electron and/or proton storage ability (Figure 1a). Herein we introduce an example of zinc(II) complex of a suitably designed bis-azoaromatic ligand L (L = 2,6-bis(phenylazo)pyridine),34 which acts as an efficient catalyst for dehydrogenation of alcohols in the presence of molecular oxygen.35 Here the ligand acts as a 4(e− + H+) reservoir, and the metal ion behaves as an oxophilic Lewis acid center (Figure 1b). In the catalytic cycle, hydrogen peroxide is produced36−38 by partial reduction of oxygen. This protocol is an alternative to the conventional ones and resembles39,40 the functional activity of zinc alcohol dehydrogenase. The possibility of participation of an azo function in alcohol oxidation was initially proposed by Marko et al. in 1996,41 which was substantiated recently by us42 and others.43−45 Also, we note here that, in literature, there exists only one report46 on aerobic oxidation of primary alcohols using a Zn(II) complex of © XXXX American Chemical Society

a bis-aminophenolate ligand as the catalyst. Galactose oxidase type of mechanism for the copper analogue was established with the involvement of CuII/CuI redox couple; however, the mechanism for the zinc(II) complex remained unsettled.

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RESULTS AND DISCUSSION Isolation and Characterizations of the Zinc Complex. The complex ZnLCl2 (1a) was obtained in a high yield (>80%) from the reaction between an equimolar quantity of the pincer ligand L and ZnCl2 in methanol (Figure 2 and cf. Experimental Section). The complex 1a is sparingly soluble in most of the common organic solvents, such as toluene, dichloromethane, acetonitrile, methanol, tetrahydrofuran, etc., but is freely soluble in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Single-crystal X-ray diffraction (XRD) studies47 reveal that the zinc(II) ion resides in a distorted trigonal bipyramidal coordination sphere in which two N(azo) atoms (N1 and N5) occupy the apical positions. The N1−Zn−N5 bond angle is 141.20(12)°, while the N3 atom (pyridine nitrogen) and two coordinated chlorides occupy the basal plane (Figure 3). The dN−N bond lengths of 1a (both are 1.260(5) Å) indicate neutral azo coordination (−NN−), which is supported34,42,48−54 by the appearance of characteristic νNN stretching frequency at 1417 cm−1 (Figure S12). It is C2Received: January 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Selected examples of electron/proton storage systems. (b) Schematic representation of our work: aerobic alcohol dehydrogenation by a zinc complex of a bis-azoaromatic ligand.

ation reaction. The results prompted us to investigate the above catalytic reaction using the corresponding zinc(II) complex 1a as a catalyst to establish exclusive ligand participation, where metal center acts only as a template. A mixture comprised of alcohol (1 mmol), KtBuO (0.10 mmol), zinc dust (2 mmol), and 1a (5 mol %) was stirred at 333 K in dry toluene in an oxygen atmosphere for 24 h. The products were purified using preparative thin layer chromatography. Different primary and secondary alcohols were tested, and the resulting carbonyl compounds (Chart 1) were isolated and characterized by NMR spectroscopy.

Figure 2. Synthesis of complex 1a.

Chart 1. Catalytic Aerobic Dehydrogenation of Alcohols Using the Zinc Complex 1a

Figure 3. Molecular view of complex 1a.

symmetric and shows four 1H NMR resonances (in DMSO-d6) in the standard range for a diamagnetic complex (Figure S5). In DMF−acetonitrile (1:10) solvent mixture, the complex 1a exhibits two reduction waves at −0.20 and −0.78 V (Figure S4a) versus Ag/AgCl. One-electron reduction of 1a at −0.45 V by exhaustive electrolysis produced a green solution, [1a]−, which displayed a single line electron paramagnetic resonance (EPR) spectrum with minor nitrogen hyperfine coupling at giso = 1.993 signifying ligand reduction (Figure S20). The isotropic simulation parameters for [1a]− are shown in Figure S20, which are in agreement with the ligand-based EPR spectrum with the magnetic hyperfine interaction of the 14N I = 1 nuclei. Aerobic Dehydrogenation of Alcohols by 1a and Electroprotic Storage. Recently we reported42 a nickel(II) complex of the neutral azo-aromatic ligand L, which follows the biomimicking pathway for catalytic dehydrogenation reactions of different alcohols. Notably, the ligand-based azo-hydrazo (2e+ + 2H+) redox couple accomplished the above dehydrogenB

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Now a second reaction mixture of identical composition, as above, was allowed to react beyond 30 min. The intermediate blue solution gradually became pale yellow in nearly 1 h. Usual workup and crystallization of the resultant mixture produced the fully hydrogenated compound 1c in a high yield (84%). Single-crystal XRD analyses of 1c reveal that it is also a pentacoordinated Zn(II) complex composed of H4L ligand and two chloride ions. Here both dN−N bond lengths are elongated at 1.398(3) Å indicating N−N single bond (Figure 4b). For comparison, selected bond parameters of ZnLCl2 (1a), Zn(H2L)Cl2 (1b), and Zn(H4L)Cl2 (1c) are collected in Table 1.

To examine the catalytic reactions, we investigated the dependency of the critical parameters, such as catalyst loading, base concentration, the presence of zinc dust, temperature, and O2 (Tables S2 and S3) on the reactions. A blank reaction in the absence of the catalyst or zinc dust did not provide any conversion. To gain an insight into the above catalytic reaction, we followed a typical stoichiometric reaction using 2-propanol both as substrate and solvent. Accordingly, a reaction mixture comprised of 2-propanol (5 mL), complex 1a (100 mg, 0.23 mmol), and 2 equiv of zinc dust (31 mg, 0.47 mmol) was stirred at room temperature (298 K) in a glovebox. The zinc compound is sparingly soluble in 2-propanol. The mixture upon stirring with zinc dust produced a clear green solution within 10 min, which displayed an identical EPR spectrum to that of the coulometrically generated reduced complex (vide supra). Upon further progression of the reaction, the mixture gradually became blue in nearly 30 min. The blue compound persists for a limited period in inert conditions. We isolated it by rapid precipitation inside a glovebox and attempted its characterization by EPR and IR spectroscopic methods. The complex displayed a single-line EPR spectrum at g = 1.992 at 77 K (Figure S21) signifying that it is a ligand-centered radical compound. Its IR spectrum consists of a characteristic doublet N−H stretching modes at 2925 and 2854 cm−1 and a N−N single bond stretching mode at 1330 cm−1 indicating that the blue complex contains both hydrazo and azo anion radical functions. We also attempted isolation of this blue complex by crystallization. The blue color reaction mixture (see the Experimental Section for details) in acetonitrile was layered over toluene in a tightly capped crystal tube and left at 253 K (refrigerator) in a glovebox for three weeks. During this period, the blue solution turned red, and a highly crystalline complex, 1b, deposited in the tube in moderate yield (46%). Singlecrystal XRD analysis and spectral studies revealed that 1b is a pentacoordinate Zn(II) complex (Figure 4a) of a new tridentate ligand, H2L (partially hydrogenated L); the remaining two positions are occupied by two chloride ions.

Table 1. Comparison of Bond Lengths and Bond Angles of 1a, 1b, and 1c bond Zn1−Cl1 distances Zn1−Cl2 Zn1−N1 Zn1−N3 Zn1−N5 N1−N2 N4−N5 bond N1−Zn−N3 angles N5−Zn−N3 Cl1−Zn−Cl2/ Cl1−Zn−Cl1′ a

1a

1b

1ca

2.2193(14)

2.2186(13)

2.2187(9)

2.2159(15) 2.340(4) 2.059(3) 2.360(3) 1.260(5) 1.260(5) 70.62(12)

2.2313(14) 2.419(3) 2.022(2) 2.309(3) 1.401(3) 1.262(3) 73.53(9)

2.2187(9)a 2.340(2) 2.019(3) 2.340(2)a 1.398(3) 1.398(3)a 75.72(5)

70.61(13) 120.58(5)

72.45(8) 116.04(3)

75.72(5)a 121.10(3)

Symmetry-generated structure.

Complex 1c has a crystallographic C2 symmetry axis, and thus half of the molecule is magnetically identical with the other half. Accordingly, only half of the possible proton resonances are observed in its 1H NMR spectrum. Additionally, two broad N−H signals appear at δ = 7.74 and 7.62 ppm (Figure S8). In the IR spectrum, the νN−N mode appears at 1325 cm−1, while the νN−H stretching frequencies42 appear at 3332 and 3217 cm−1(Figure S14). To ascertain the source of hydrogen atoms in the hydrogenated complexes 1b and 1c, we repeated the above chemical reductions in 2-propanol-d8 (C3D7-OD). We failed to arrest the intermediate complex, 1b-d2; however, we could trap the deuterated bis-hydrazo complex, 1c-d4, from the reaction mixture. Formation of complex 1c-d4 was confirmed by 1H NMR spectroscopy (Figure S10). The spectrum is identical to that of 1c, except that two N−H resonances are absent and two broad resonances, assignable to N-D resonances (2H NMR), appear at δ = 8.18 and 7.37 ppm (Figure S11). The above compound also shows two νN‑D stretching vibrations at 2254 and 2127 cm−1 (Figure S15). Reduction of Oxygen and para-Benzoquinone by Hydrogenated Complexes. The two hydrogenated complexes Zn(H2L)Cl2 (1b) and Zn(H4L)Cl2 (1c) are hyperreactive, and both of these regenerate complex 1a readily upon exposure to oxygen. During the reaction, oxygen is hydrogenated via sequential electron and proton transfer to produce one and two moles of H2O2 from 1b and 1c, respectively. Spontaneous aerobic oxidation of complex 1c to complex 1a via complex 1b was monitored by UV−vis spectroscopy (Figure 5). Quantification of the above transformation was made by 1H NMR spectroscopy using 1,4-dibromobenzene as an internal standard. Upon exposure to oxygen, the pale yellow solution of

Figure 4. Molecular views of (a) 1b and (b) 1c.

Thus, one of the two azo functions of the coordinated ligand in 1b is reduced to a hydrazo function, causing considerable elongation of the dN−N bond. Accordingly, two completely different dN−N bond lengths 1.262(3) Å (azo) and 1.401(3) Å (hydrazo) are observed in 1b.53 The compound shows a resolved 1H NMR spectrum consisting of all 13 possible proton resonances. Two broad singlets, attributable to N−H resonances, appeared at 8.32 and 6.90 ppm (Figure S7). The IR spectrum of 1b contains two νN−N stretching frequencies at 1311 cm−1 (hydrazo) and 1417 cm−1 (azo) (Figure S13); νN−H vibrational modes appear at 2924 and 2855 cm−1. C

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soluble in common organic solvents. So it was not possible to follow the reaction kinetics of catalytic alcohol dehydrogenation reaction. However, on the basis of the above stoichiometric reactions, isotope labeling experiments, and isolation of hydrazo complexes (1b and 1c), we discuss briefly the plausible mechanistic pathways for the catalytic dehydrogenation of alcohols. First, the above catalytic reaction does not proceed in the absence of Zn dust. Moreover, it has been observed that Zn dust brings about chemical reduction of the complex 1a freely to produce [1a]− (vide supra). Thus, we presume that the reduced complex is the active catalyst for alcohol oxidation. We also note here that the cyclic voltammogram of 1a in DMF− methanol (1:10) solvent mixture displays an irreversible reduction wave at −0.45 V (Figure S4b). The irreversible reduction is attributed to loss of coordinated chloride. Similar electrochemical behavior is well-documented58−61 in several metal-chloro complexes. On the basis of our previous results on the nickel catalyst it is presumed that subsequent to the generation of azo-anion radical there is α-C-H atom transfer42 (HAT) from the substrate. This may occur following the two pathways: (i) outer-sphere HAT to [1a]− from uncoordinated alcohol or (ii) inner-sphere HAT following substitution of chloride ion. We do not have any direct experimental evidence to ensure either of the above possibilities. However, we tried a similar catalytic reaction using a coordinatively saturated ligand radical complex, [Fe(L)(L·−)](ClO4),34 as catalyst. Even at high catalyst loading (20%) and harsh reaction conditions (ca. 393 K), it failed to oxidize 4-chloro benzyl alcohol. We consider it as an indication that the catalytic reaction in the present case follows an innersphere HAT pathway55,56,62 as it happens in the corresponding nickel catalyst.42 In the subsequent step, the coordinated ketyl radical gets intramolecularly oxidized via concomitant reduction of another azo-function and generates an intermediate II. Similar intramolecular pathway was previously reported46,55,63 in galactose oxidase model systems. At this point, there may be two possible ways for further propagation of the reaction: (i) aerobic oxidation of II for regeneration of the catalyst (following path A) or (ii) a second HAT leading to the formation of a bishydrazo complex (following path B). Path A is somewhat similar to that occurring in the case of nickel catalyst, but path B produces a bis-hydrazo intermediate V via second HAT to N5 (Figure 7). To have further insight, we performed IR spectroscopic studies during the catalytic reaction. After 4 h, a portion of the reaction mixture was subjected to IR spectral measurement. Clearly, two sets of distinct N−H stretching frequencies were observed: the IR modes at 2881 and 2937 cm−1 characterize the monohydrazo complex, whereas IR modes appeared at 3139 and 3221 cm−1 characterize the bishydrazo complex (Figure S16). For comparison, in the isolated complexes 1b and 1c the N−H stretching modes appear at 2924, 2855 cm−1 and at 3332, 3217 cm−1 respectively. Furthermore, we note that the two isolated hydrogenated complexes 1b and 1c both are equally good catalysts for oxidation of 4-chlorobenzyl alcohol under identical reaction conditions. This is as expected, since these two hydrogenated complexes, upon exposure to air, produce the precatalyst (1a) quantitatively (vide supra). In spite of all the above, we are not in a position to disprove either of the above-noted pathways for the regeneration of catalyst.

Figure 5. Regeneration of complex 1a from complex 1c via the formation of complex 1b in the air.

1c in dimethyl sulfoxide-d6 (DMSO-d6) turned brown almost instantaneously. The resultant spectrum indicates that the reaction has produced55,56 1a and 2 mol of H2O2 quantitatively (Figure 6). A similar experiment with 1,4-benzoquinone

Figure 6. 1H NMR spectral studies of the regeneration of 1a from 1c by O2. Experiments were performed at room temperature using DMSO-d6 as the solvent. (a) The spectrum of 1c under inert conditions. (b) The spectrum of the resultant solution after bubbling O2 into the solution of 1c. (inset) 1H NMR spectrum of 1a in DMSOd6.

formed 1,4-hydroquinone57 quantitatively (Figure S22). The rate of the transformation from complex 1b to 1a was checked in the presence of oxygen saturated methanol solvent. It was found that in O2 saturated methanol solution the rate of oxygen reduction follows a pseudo-first-order reaction with respect to [1b] and the rate constant of transformation is kexpt = 4.5 × 10−8 min−1 (Figure S23). Notably, the formation of H2O2 is also reported to be kinetically facile36,38 over water during the reduction of oxygen. We also performed the similar stoichiometric transformation using 1c-d4. In this case, D2O2 was detected by 2H NMR spectroscopy (Figure S24). Plausible Mechanism of Alcohol Oxidation Reaction. Unlike the Ni-analogue,42 the zinc complex 1a is only sparingly D

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Proposed catalytic cycle for alcohol dehydrogenation reaction.

have generated potential scopes of designing energy storage system65 using azo-hydrazo redox couple.66

The above catalytic reaction produces hydrogen peroxide (H 2O2 ) and apperantly superoxide (O 2·−). Superoxide formation by oxidation of intermediate II (path A) is bit intuitive, while its formation from ketyl radical (path B) has been well-documented65,66 in the literature. We wish to add here that in the corresponding nickel complex catalyzed reaction the ketyl radical has at best fleeting exsistence.42 It is likely that the oxidation of the intermediate III is brought about by O2, though the participation of resting state(s) of the catalyst for the oxidation of ketyl radical also cannot be ruled out. By the use of spin trapping agent 5,5-dimethyl-1-pyrrolineN-oxide (DMPO), we could detect superoxide ion in the reaction mixture: a complex EPR spectrum showing the signature for the resonances DMPO-OOH was observed (Figure S17).64 Formation of H2O2 during the reaction was chemically detected by standard UV−vis spectroscopic technique42 (cf. Experimental Section for details, Figure S18).

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EXPERIMENTAL SECTION

Materials. Anhydrous ZnCl2 was purchased from Merck, India. Different alcohols and deuterated alcohols, used for catalysis, were purchased from Sigma-Aldrich or Alfa Aesar. Zinc dust was purchased from Sigma-Aldrich. All solvents were dried before use. Tetrabutylammonium perchlorate was prepared and recrystallized following reported procedure.67 Caution! Perchlorate salts must be handled with care and appropriate safety precautions. Physical Measurements. UV−vis spectra were recorded using a PerkinElmer Lambda 950 spectrophotometer. A PerkinElmer 783 spectrophotometer was used for obtaining infrared spectra. 1H NMR spectra were recorded using a Bruker Avance 400 or 500 MHz spectrometer, where SiMe4 was used as the internal standard. A PerkinElmer 2400 elemental analyzer was used to collect microanalytical data (C, H, N). All electrochemical measurements were performed using a personal computer (PC)-controlled PAR model 273A electrochemistry system. Cyclic voltammetric experiments were performed in 10:1 acetonitrile/dimethylformamide solvent mixture containing supporting electrolyte, 0.1 M Bu4NClO4, under the nitrogen atmosphere. Three electrodes configuration comprising Ag/ AgCl as the reference electrode, glassy carbon as working electrode, and Pt as a counter electrode. E1/2 for the ferrocenium-ferrocene couple under our experimental conditions was 0.39 V. X-band EPR spectra were recorded with a JEOL JES-FA200 spectrometer. X-ray Crystallography. Crystallographic data for complexes 1a, 1b, and 1c are collected in Table S1. Suitable crystals of complex 1a

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CONCLUSION In summary, we have introduced a zinc(II) complex of a bisazoaromatic ligand that is capable of bringing about aerobic dehydrogenation of alcohols with high catalytic activities. Two novel hydrogenated zinc complexes are isolated and fully characterized. These hydrogenated complexes behave as excellent electroprotic reservoirs and hydrogenate O2 and 1,4benzoquinone spontaneously. The results, reported herein, thus E

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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d6): δ 160.64 (2C), 150.40 (2C), 139.65 (1C), 129.40 (4C), 118.39 (2C), 112.28 (4C), 95.67 (2C). Isolation of the [Zn(D4L)Cl2], 1c-d4. Complex 1c-d4 was isolated following the same procedure as described that for the complex 1c except that 2-propanol-d8 (C3D7-OD) was used instead of 2-propanol. After filtration through a G4-sintered glass funnel, a colorless product was precipitated from the reaction mixture using n-hexane as precipitating solvent. Finally, the compound was crystallized by slow diffusion of the acetonitrile solution of the compound into toluene at 253 K. The yield and spectral characterization of the product are as follows: Yield: 75%. 1H NMR (500 MHz, DMSO-d6): δ 7.09 (m, 5H), 6.63 (m, 6H), 5.90 (d, J = 10.0 Hz, 2H). 2H NMR (500 MHz, CH3CN) δ 8.18 (s, 2H), 7.37 (s, 2H). 1H NMR spectrum of the isolated 1c-d4 complex is shown in Figure S10. Notably, the two N-D resonances of 1c-d4 were absent in the 1H NMR spectrum, while the rest are identical to that of 1c. However, 2H NMR spectrum of the complex 1c-d4 in acetonitrile solvent showed only two broad resonances at 8.18 and 7.37 ppm (Figure S11). Cyclic Voltammetry of Complex 1a. Cyclic voltammetry of the complex 1a in 1:10 DMF/acetonitrile solution in the presence of 0.1 (M) tetrabutylammonium perchlorate (TBAP) exhibits two singleelectron reduction waves at −0.20 and −0.78 V (Figure S4a) versus Ag/AgCl reference electrode. The first response is quasi-reversible, whereas the second one is irreversible. The reduction waves are attributed to the reduction of the coordinated ligand. Exhaustive electrolysis of 1.182 × 10−3 M solution (acetonitrile/DMF = 1:10) of 1a containing 0.1 M TBAP was performed at −0.45 V: (Qcal = 2.85 C and Qexpt = 3.0 C; Qcal/Qexpt = 0.95). However, the cyclic voltammetry of 1a in DMF−methanol (1:10) solvent mixture in the presence of 0.1 M TBAP displays an irreversible wave at −0.45 V versus Ag/AgCl reference electrode. General Procedure for Catalysis. All the catalytic reactions were performed following a general procedure. In a typical reaction, a mixture of 1 mmol alcohol, 5.0 × 10−2 mmol catalyst 1a (21.2 mg), 0.1 mmol KtOBu (11.2 mg), and 2.0 mmol zinc dust (130 mg) was stirred in 10 mL of dry toluene in a round-bottom flask fitted with a condenser at 333 K under oxygen atmosphere (oxygen balloon). The stirring was continued for 24 h. The crude oxidized products, thus obtained, were filtered and purified on preparative UV-active silica (GF-254) thin-layer chromatography (TLC) plate using hexane/ dichloromethane (10:1) as eluent. Chart 1 collects the yields of the corresponding oxidized products. Regeneration of Complex 1a from Complex 1c via Formation of Complex 1b. Complex 1c is air-sensitive and reverts to complex 1a via the formation of complex 1b (Figure 5). 1.1 × 10−4 M acetonitrile solution of complex 1c was stirred in the air. The initial pale yellow solution became reddish-brown via a red intermediate. Time-dependent UV−vis spectra indicate that complex 1c is quantitatively converted to complex 1a via the formation of complex 1b. A similar experiment starting from preformed 1b in air produces 1a quantitatively. 1 H NMR Spectroscopic Study for Reduction of O2 to H2O2. The transformation of 1c to 1a was monitored by 1H NMR spectroscopically. The conversion is fast and quantitative in the presence of positive pressure of oxygen. Complex 1c along with the internal standard 1,4 dibromobenzene (1:1) were dissolved in the DMSO-d6 solvent in a glovebox, and 1H NMR spectrum of the sealed tube was recorded. Subsequently, oxygen gas was purged into the above mixture. The solution became reddish-brown almost instantaneously. 1H NMR spectrum of the resultant solution indicated quantitative regeneration of 1a. An additional signal at 10.22 ppm (integration, 4H) appeared, which confirmed the formation of 2 equiv of H2O2 during the transformation. The spectral changes are shown in Figure 6. 1 H NMR Spectroscopic Study for Reduction of p-Benzoquinone to p-Hydroquinone. To a solution of 1c (30 mg, 0.07 mmol) in 4 mL of DMSO-d6, para-benzoquinone (30.3 mg, 0.28 mmol) was added, and the mixture was stirred for nearly 10 min in an inert condition (glovebox). 1H NMR spectrum of the resultant mixture was recorded. The spectrum of the resultant mixture indicates that

for X-ray diffraction analysis were grown by the slow diffusion of a dichloromethane solution of the complex into hexane, while those for complexes 1b and 1c were obtained by slow diffusion of their acetonitrile solution into toluene at 253 K. All crystallographic data were collected on a Bruker SMART APEX-II diffractometer, equipped with graphite-monochromatic Mo Kα radiation (λ = 0.710 73 Å), and were corrected for Lorentz polarization effects. Data for 1a: a total of 41 269 reflections were collected, of which 3250 were unique (Rint = 0.116). Data for 1b: a total of 19 981 reflections were collected, of which 4121 were unique (Rint = 0.042). Data for 1c: a total of 13 164 reflections were collected, of which 2157 were unique (Rint = 0.044). These satisfy the I > 2σ(I) criterion and were used in subsequent analysis. The structures were solved by employing the SHELXS-2014 program package68 and were refined by full-matrix least-squares based on F68 (SHELXL-2014).69 All hydrogen atoms were added at calculated positions. Synthesis of Ligand L. The ligand 2,6-bis(phenylazo)pyridine (L) was synthesized following a reported procedure.42 Synthesis of [ZnLCl2], 1a. A mixture of equimolar quantities of anhydrous ZnCl2 (100 mg, 0.734 mmol) and ligand L (215 mg, 0.749 mmol) in 10 mL of dry methanol solvent was stirred at 298 K for 2 h. A reddish-brown precipitate was collected by filtration. It was washed thoroughly with diethyl ether and dried under vacuum. The compound is sparingly soluble in dichloromethane. The suspension of the compound in dichloromethane was filtered through Whatman 41 filter paper, and the filtrate was slowly diffused into hexane for generating suitable crystals for single-crystal X-ray diffraction analysis. Yield: 85%. The complex 1a is sparingly soluble in most of the common organic solvents such as toluene, dichloromethane, acetonitrile, methanol, tetrahydrofuran, etc., but it is freely soluble in DMF and DMSO. Anal. Calcd for C17H13N5Cl2Zn: C 48.20, H 3.09, N 16.53; Found C 48.63, H 3.14, N 16.42%. UV−vis (DMF/CH3CN, 1:10): λ [nm] (ε, M−1 cm−1) = 225(13 938), 325(18 552), 450(1075). IR (KBr disk, cm−1): ν (N−N): 1417.58 cm−1. 1H NMR (500 MHz, DMSO-d6): δ 8.30 (m, 1H), 8.02 (m, 4H), 7.93 (d, J = 5.0 Hz, 2H), 7.67 (m, 6H). 13C NMR (125 MHz, DMSO-d6): δ 162.19 (1C), 151.80 (2C), 141.61 (2C), 132.83 (2C), 129.67 (4C), 123.11 (4C), 115.45 (2C). Isolation of the [Zn(H2L)Cl2], 1b. To a suspension of complex 1a (100 mg, 0.23 mmol) in 5 mL of dry and deoxygenated 2-propanol, zinc dust (31 mg, 0.47 mmol) was added in an inert atmosphere. The mixture was stirred at room temperature. The color of the solution changes from reddish-brown to blue in 30 min, which was filtered through a G4-sintered glass funnel to remove the insoluble mass. The filtrate was evaporated under vacuum maintaining inert conditions (glovebox). The dark residue was crystallized via slow diffusion of acetonitrile solution into toluene at 253 K in a tightly capped crystal tube. Red crystals grew in approximately three weeks. Yield: 46% after crystallization. Anal. Calcd for C17H15N5Cl2Zn: C 47.97, H 3.55, N 16.45; Found C 48.09, H 3.72, N 16.41%. UV−vis (CH3CN): λ [nm] (ε, M−1 cm−1) = 230(12 287), 315(12 016), 410(2516). IR (KBr disk, cm−1): ν (N−N): 1417 (azo), 1311.50 (hydrazo) cm−1. 1H NMR (500 MHz, CD3CN): δ 8.39 (d, J = 5.0 Hz, 2H), 8.32 (s, 1H), 8.13 (t, J = 5.0 Hz, 1H), 7.77 (d, J = 5.0 Hz, 1H), 7.63 (m, 3H), 7.34(t, J = 10.0 Hz, 2H), 7.25(d, J = 10.0 Hz, 1H), 7.12(m, 3H), 6.9 (s, 1H). Isolation of the [Zn(H4L)Cl2], 1c. Complex 1c was isolated following the same procedure as described that for the complex 1b, except that the reaction was allowed for a longer period, 1 h. The initial reddish-brown color of the suspension of 1a became pale yellow during this period. The resultant solution was filtered through a G4sintered glass funnel, and the filtrate was evaporated under vacuum. The crude mass was then purified by slow diffusion of an acetonitrile solution of the complex into toluene at 253 K. Colorless crystals of 1c were isolated in 84% yield. Anal. Calcd for C17H17N5Cl2Zn: C 47.74, H 4.01, N 16.38; Found C 47.85, H 3.15, N 16.31%. UV−vis (CH3CN): λ [nm] (ε, M−1 cm−1) = 240(14 696), 315(7956), 390(730). IR (KBr disk, cm−1): ν (N−N): 1325.01 cm−1. 1H NMR (500 MHz, DMSO-d6): δ 7.74 (s, 2H), 7.62 (s, 2H), 7.08 (m, 5H), 6.61 (m, 6H), 5.87 (d, J = 10.0 Hz, 2H).13C NMR (125 MHz, DMSOF

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Author Contributions

complex 1c quantitatively converted into complex 1a; 2 equiv of parabenzoquinone were reduced to produce 2 equiv of para-hydroquinone, and 2 equiv of para-benzoquinone remained unreacted. The spectrum is shown in Figure S22. Detection of Hydrogen Peroxide during the Catalytic Reactions. In the catalytic oxidation reaction of alcohol H2O2 is formed as a byproduct, which is also characterized by UV−vis spectroscopy (Figure S18) by monitoring the characteristic peak of I3− at λmax = 345 nm; ε = 26 000 M−1 cm−1 upon reaction with KI. According to the reported procedure,42 into a round-bottom flask containing 1 mmol of 4-chlorobenzyl alcohol (142.5 mg) in 10 mL of dry toluene was mixed 0.05 mmol catalyst 1a, 0.1 mmol KtOBu, and 2.0 mmol zinc dust, and the mixture was stirred at room temperature for 6 h. An equal volume of water was added subsequently to the reaction mixture; the aqueous part was then separated using a separating funnel. The separated aqueous layer was then acidified with dilute H2SO4 to pH = 2 to stop further oxidation. To it, 1 mL of a 10% solution of KI and three drops of a 3% solution of ammonium molybdate were added. Hydrogen peroxide oxidizes I− to I2, which reacts with an excess of I− to form I3−. The chemical reactions are as follows: (i) H2O2 + 2I− + 2H+→ 2H2O + I2, (ii) I2 (aq) + I−→ I3− (Figure S18).70−72 The absorbance for I3− increases with increasing concentrations of acid. Ammonium molybdate catalyzes the oxidation of I− to I2. IR Spectroscopic Studies of the Reaction Mixture. A mixture of 1 mmol alcohol, 5.0 × 10−2 mmol catalyst 1a (21.2 mg), 0.1 mmol KtOBu (11.2 mg), and 2.0 mmol zinc dust (130 mg) was stirred in 10 mL of dry toluene in a round-bottom flask fitted with a condenser at 333 K under an oxygen atmosphere (oxygen balloon). The stirring was continued for 4 h. A portion of the solution was collected, filtered, and subjected to IR spectral analysis. We observed two distinct IR modes (N−H doublets) at 2881 and 2937 cm−1 that correspond to monohydrazido complex, and the peaks at 3139 and 3221 cm−1 correspond to bishydrazido complex (Figure S16).

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‡

R.B. and D.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Department of Science and Technology (DST), India, SR/S2/JCB-09/2011, EMR/2014/ 000520. S.G. sincerely thanks DST-SERB for J. C. Bose fellowship. A.D. thanks DST-SERB and BRNS for the partial support. R.P. and R.B. are thankful to the Council for Scientific and Industrial Research, India, and D.S. is thankful to Indian Association for the Cultivation of Science for their fellowship support.

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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.8b00034. Crystallographic data, optimization table of catalyst loading and nature of bases for catalytic alcohol dehydrogenation of 4-chlorobenzyl alcohol, optimization table of mol % of KtBuO, solvent, involvement of oxygen and temperature on catalytic alcohol dehydrogenation on 4-chlorobezyl alcohol, ORTEP illustrations of complexes 1a, 1b, and 1c, cyclic voltammograms, spectra obtained by 1H NMR, 13C NMR, IR, EPR, 2H NMR, kinetic studies for stoichiometric oxygen reduction (PDF) Accession Codes

CCDC 1548298−1548300 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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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rajib Pramanick: 0000-0001-8876-7238 Debabrata Sengupta: 0000-0001-7212-3761 Ayan Datta: 0000-0001-6723-087X Sreebrata Goswami: 0000-0002-4380-5656 G

DOI: 10.1021/acs.inorgchem.8b00034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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