Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols

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Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols Debabrata Sengupta,† Rameswar Bhattacharjee,Δ Rajib Pramanick,† Santi Prasad Rath,† Nabanita Saha Chowdhury,† Ayan Datta,*,Δ and Sreebrata Goswami*,† †

Department of Inorganic Chemistry and ΔDepartment of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India S Supporting Information *

ABSTRACT: Design of an efficient new catalyst that can mimic the enzymatic pathway for catalytic dehydrogenation of liquid fuels like alcohols is described in this report. The catalyst is a nickel(II) complex of 2,6-bis(phenylazo)pyridine ligand (L), which possesses the above requisite with excellent catalytic efficiencies for controlled dehydrogenation of alcohols using ligand-based redox couple. Mechanistic studies supported by density functional theory calculations revealed that the catalytic cycle involves hydrogen atom transfer via quantum mechanical tunneling with significant kH/kD isotope effect of 12.2 ± 0.1 at 300 K. A hydrogenated intermediate compound, [NiIICl2(H2L)], is isolated and characterized. The results are promising in the context of design of cheap and efficient earth-abundant metal catalyst for alcohol oxidation and hydrogen storage.

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INTRODUCTION A continuous increase in demand for clean and renewable energy has reintroduced an interest in hydrogen as a form of chemical energy, although storage of hydrogen is a demanding task.1−5 Consequently as a genuine alternative, liquid organic hydrogen carriers like alcohols are now under scan because of their easy storage, transport, and high hydrogen content.2,6 In this context, development7 of base metal complex as catalysts for selective dehydrogenation of alcohols is of interest. During the past few years, there have been attempts of using transitionmetal complexes of redox-active ligands as dehydrogenation catalysts, where the transition metal hydride is a key intermediate for dehydrogenation reaction, and the respective ligand acts as a supporting backbone.8−10 Recent investigations by Milstein, Beller, and others on the metal complexes bearing cooperative pincer ligands have led to the generation of efficient catalysts for dehydrogenation reaction.7,11−16 They have shown that the PNP/PNN ligand serves as a cooperative ligand and that hydrogen can be added reversibly across the M−N bond.10,12 Grützmacher et al. have introduced another novel ligand-based protocol,17−19 which resulted in successful oxidation of methanol into CO2 and H2 using Ru or Ir metal complex catalysts. In the Grützmacher catalyst, dehydrogenation of methanol is brought about via complete reduction (4e− + 4H+) of the two imine chromophores of the ligand. Another interesting protocol of using nitroxyl radical or azodicarboxylate as cocatalyst has developed high promise for copper-catalyzed aerobic dehydrogenation.20,21 The above propositions have generated genuine possibilities22,23 of designing complex catalysts of suitable ligands that will be capable of shifting the overall redox exclusively from metal to ligand center. As an outcome, it is envisaged21 that the use of azo−hydrazo couple (2e− + 2H+) would be a potential candidate to perform the © XXXX American Chemical Society

above proposition further. Engineering of metal complex catalysts of suitable azo aromatic ligands for alcohol dehydrogenation is evidently a challenge, which is the primary objective of this work. Some representative examples of catalytic alcohol dehydrogenation are shown in Scheme 1. Herein, we wish to disclose a model nickel catalyst of a pyridyl containing bis-azo aromatic pincer ligand to address the issue of reversible hydrogen storage via selective partial alcohol dehydrogenation using azo−hydrazo couples. In the present context it is worth noting that selective partial dehydrogenation of alcohols is presently emphasized because of no CO2 or CO emission. In the overall process, the two azo functions of the ligand are found to be operative in the dehydrogenation reaction, while the metal ion remains as a spectator that acts only as a template to hold the ligand(s) and substrate. The key point in this metal redox free alcohol dehydrogenation is that the model catalyst is capable of dehydrogenation of both primary and secondary alcohols following galactose oxidase pathway and also generates a genuine prospect24,25 toward the application of this class of systems in energy generation.

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RESULTS AND DISCUSSION Catalyst Design and Isolation. The catalyst here is a distorted octahedral nickel(II) complex, [NiIICl2L(H2O)] (1), as shown in Figure 1, which consists of a tridentate N,N,Ndonor ligand (L = 2,6-bis(phenylazo)pyridine), two chlorides (Cl−), and a H2O molecule in the coordination sphere. The compound is paramagnetic with two unpaired spins, μeff (300 K) = 2.45 μB. The ligand was chosen because of its known Received: May 31, 2016

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DOI: 10.1021/acs.inorgchem.6b01310 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Different Classes of Compounds Utilized in Dehydrogenation Reactions of Alcoholsa

a

Some represented examples are shown, Type I: Baeckvall, J.-E. Modern Oxidation Methods, 2nd ed.; Wiley-VCH, 2010. Type II: Gunanathan, C.; Milstein, D. Chem. Rev. 2014, 114, 12024−12087.

Figure 1. (a) Line drawing of the ligand 2,6-bis(phenylazo)pyridine (L) and the synthesis of the catalyst, 1. (b) Molecular view of the catalyst, 1. Selected bond lengths (Å): Ni1−Cl1, 2.335(3), Ni1−O1, 2.101(4), Ni1−N1, 2.303(4), Ni1−N3, 1.998(4), N1−N2, 1.256(4).

ability26 to transfer multiple electrons and protons, and the catalyst design was for a labile system to enable coordination of incoming substrates in the intermediate steps. Its synthesis involved a chemical reaction between equimolar quantities of L and hydrated nickel chloride salt in tetrahydrofuran (THF). A dark greenish-brown product of 1 was obtained in a nearly quantitative yield (isolated yield, 90%). Usual chemical characterizations reveal that the compound is a mixed ligand nickel(II) complex of above composition. Synthesis of 1 and its complete characterization are collected in Experimental Section. Single-crystal X-ray analysis of the nickel complex has revealed that it is an octahedral complex, which is shown in Figure 1b. The tridentate ligand L binds Ni(II) in a mer fashion using a pair of azo-N and a pyridine-N; two coordinated chloride ligands are mutually trans. A water molecule occupies the sixth coordination site. The dN−N bond length in the complex is 1.256(4) Å and indicative of neutral azo- (−N N−) coordination.26 The NN stretching frequency in the compound 1 appeared at 1398 cm−1, which also supports27 the neutral form of the azo chromophores of the coordinated ligand.

Cyclic voltammetric experiment of the complex 1 in dichloromethane solution using tetrabutylammonium perchlorate (TBAP) as supporting electrolyte exhibits four single electron transfer at −0.05, −0.28, −1.04, and −1.3 V (Figure S1) versus the Ag/AgCl reference electrode. The wave at the least cathodic potential is reversible, whereas other responses are quasi-reversible. The cyclic voltammogram of the free L showed26 two reductions due to L → [L●]− → [L●●]2− at −1.07 and 1.39 V, successively. Two more possible reductions, [L●●]2− → [L●]3− → [L]4−, were not however observed under our experimental conditions. Catalysis and Mechanism. It is well-known28−32 that the metal complexes of neutral azo ligands are susceptible to reductions and can be utilized to perform several unusual chemical reactions. We thus set out to study aerial catalytic oxidation of alcohols using the above complex. The complex 1 was found to bring about partial dehydrogenation of both primary and secondary alcohols catalytically into their corresponding aldehydes or ketones, respectively, in low catalyst loading (0.1−0.5 mol %). In a typical reaction, a mixture of 1 mmol of alcohol, 1 × 10−3 mmol of catalyst (1 mL from 100 mL stock solution containing 43 mg of catalyst), 1 × B


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Inorganic Chemistry 10−3 mmol of KtBuO (1 mL from 100 mL stock solution containing 11 mg of KtBuO), and 1.5 mmol of zinc was stirred in 10 mL of dry THF at room temperature under a positive pressure of oxygen (using oxygen-filled balloon; see Table 1). The stirring was continued for 2−4 h depending upon the substrate. Table 2 collects the yields of corresponding oxidized products.

Table 2. [NiIICl2L(H2O)] (1) Catalyzed Oxidation of Alcohols into Ketones/Aldehydesa,b

Table 1. Optimization of [NiIICl2L(H2O)] (1) Catalyzed Oxidation of 1-Phenylethanol into Acetophenonea catalyst 1 1 1 1 free ligand NiCl2

base

time (h)

isolated yield (%)

KtBuO NaOHb NaOMeb

3 3 3 3 3 3 3

81 76 75 73 NRd NRd NRd

KtBuOc KtBuOc KtBuOc

a

Reaction condition: [1] = 0.001 mmol, [substrate] = 1 mmol, [KtBuO] = [catalyst,1], Zn dust = 1.5 mmol (1.5 equiv with respect to substrate), 10 mL of solvent (dried). The reactions were studied in positive pressure of O2, bDMSO was added for a making clear solution. c[KtBuO] = 0.001 mmol. dNR = No reaction.

To understand the courses of the above oxidation reaction, kinetics measurements on the dehydrogenation of 1-phenylethanol were made as a function of the catalyst concentration using KtBuO as a base in a catalytic quantity. Increase in the concentration of 1 over the range from 0.8 × 10−4 to 1.5 × 10−4 times with respect to alcohol resulted in a linear rise in the rate kobs for oxidation of alcohol (Figure S2). Moreover, the kobs values were found to be dependent linearly on the substrate concentration as well over a range of 80−150 times of alcohol with respect to catalyst (see Figure S3). Thus, the rate law for the above dehydrogenation reaction is as in the following eq 1, which indicates alcohol (substrate) binding to the catalyst at the rate-limiting state.21,33 rate = k[1][1‐phenylethanol]

(1)

The reaction does occur in the absence of base. However, it is found that addition of catalytic quantity of base augments the yields of the products by ∼5−10% (Table 1). It may only be due to proton abstraction from alcohol to initiate the first catalytic cycle. In subsequent cycles the reaction proceeds via proton abstraction by one of the intermediates (see later). The catalytic cycle began with single electron reduction of the catalyst by zinc metal. To quantify the single electron transfer step, stoichiometric reactions in an inert atmosphere were monitored by mixing equimolar quantities of 1 and 1,1diphenyl carbinol and varied amounts of cobaltocene (CoCp2), 0.25−1.5 equiv with respect to 1. A maximum yield of benzophenone was obtained when equimolar quantities of 1 and CoCp2 were used. To trap an intermediate, if any, a stoichiometric reaction between the compound 1 and 2-propanol in dry THF medium using 1.5 equiv of zinc metal in an inert condition was attempted. An intense blue compound 2 with S = 1 (μeff (300K) = 2.35 μB) was isolated, which upon crystallization yielded fine crystals. The compound is sensitive to oxidation; reversal of 2→1 is fast and quantitative in positive pressure of oxygen (Figure S5). Generation of H2O2 from the above reaction is identified chemically from a bulk experiment in air

a

Reaction condition: [1] = 0.001−0.005 mmol, [substrate] = 1 mmol, [KtBuO] = [catalyst, 1], [Zn dust] = 1.5 mmol (1.5 equiv with respect to substrate), 10 mL of solvent (dried). The reactions were studied in positive pressure of O2. bDetails of reaction condition of each reaction are collected in Supporting Information, Table S2. cIsolated yield. d GCMS yield. eDetected from NMR spectroscopy from the reaction mixture; due to high volatility absolute yield of the product is not obtained.

(see Supporting Information for details and Figure S6). X-ray structure determination of 2 revealed a distorted trigonal bipyramidal geometry that consisted of two axial chlorine (Cl) coordination along with an azo−hydrazo tridentate coordination of the ligand (Figure 2b). Most notable part of this complex is that one of the two azo functions of the ligand L in catalyst 1 underwent hydrogenation to produce an unsymmetrical ligand containing an azo and a hydrazo function. Accordingly the two dN−N bond lengths are entirely different: C


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Inorganic Chemistry

Figure 2. (a) Isolation of complex 2 in deaerated condition. (b) Molecular view of the complex 2. Selected bond lengths (Å): Ni1−Cl1, 2.212(2), Ni1−Cl2, 2.230(2), Ni1−N1, 2.310(6), Ni1−N3, 2.015(5), Ni1−N5, 2.411(5), N1−N2, 1.270(8), N4−N5, 1.402(8). (c) Isotope labeling experiments: N−H stretching frequencies of complex 2, isolated from cyclohexanol-H (blue line) and cyclohexanol-D (red line).

−1.1 kcal/mol, resulting in an activation electronic energy barrier of 0.4 kcal/mol. Therefore, TS1 is an early transition state and possesses minimal structural distortion. In the optimized geometry of the intermediate VI the bond length dN1−N2 = 1.32 Å is significantly longer than dN4−N5 = 1.25 Å indicating an azo−anion radical and neutral azo oxidation states, respectively.26 That the unpaired electron is localized primarily on azo−anion radical function bearing N1−N2 is also verified by natural bond orbital analysis (see Experimental Section for details, Figure S23). Experimentally, the electron paramagnetic resonance (EPR) spectrum of the chemically reduced intermediate [1]− showed an isotropic spectrum (Supporting Information, Figure S4). In the subsequent steps, hydrogen atom transfer (HAT) from the β-C−H of the attached 2-propanol33,40 to the adjacent electron-rich nitrogen atom (N1) of the azo group occurs with the fast release of acetone, which has a barrier of 25.7 kcal/mol (TS2). This ratelimiting step is computed to be highly exergonic (ΔG = −15.4 kcal/mol). The structure of TS2 is depicted in Figure 3, and the distance of the migratory hydrogen atom from both carbon and nitrogen is found to be similar ∼1.32 Å. This HAT step is expected to have a significant contribution from quantum mechanical tunneling (QMT) under ambient conditions, as the barrier is rather narrow.41 For verification with the experimental data, we also computed kinetic isotope effect (KIE) for cyclohexanol. The free energy of activation in the latter case is 25.3 kcal/mol, which clearly indicates that replacement of 2propanol by cyclohexanol does not alter the mechanistic pathway discussed above. Experimentally the observed cyclohexanol-H/cyclohexanol-D KIE is 12.2 ± 0.1 at 300 K, which is in excellent agreement with a QMT model (TST/Eckart computed KIE = 12.9 using TheRate program42) and is significantly larger than the transition-state theory (TST; computed classical KIE, 4.2). The effect is more prominent at

1.402(8) Å, for hydrazo30 and 1.270(8) Å for azo groups,26 respectively (Figure 2). The hydrazo nitrogens are sp 3 hybridized, whereas that of the azo function is sp2 hybridized. IR spectrum of 2 isolated from cyclohexanol-H substrate displayed νN−H at 3220 and 3070 cm−1.34 To ascertain the source of hydrogen in the hydrazo function, an isotope labeling experiment was performed similarly using cyclohexanol-D in place of cyclohexanol-H. A similar compound was isolated, which showed two νN‑D stretches at 2335 and 2205 cm−1 (Figure 2c).34 So we conclude that dehydrogenation of alcohol in the above catalytic cycle proceeds with subsequent hydrogenation of one of the azo functions of coordinated L. The experimental data on reaction mechanism has been supported well by DFT calculation (see below). DFT (SMD35/M0636/TZVP,37 LANLTZ(f)38 (Ni), implemented in Gaussian 09,39see Supporting Information for details) calculated ground-state structures for the two isolated complexes, 1 and 2, match well with the experimental values (see Supporting Information, Figures S18 and S19). The first step is a single electron reduction (intermediate I → intermediate II). Notably, the Ni−Cl bond lengths are elongated in II (Ni1−Cl1 = 2.49 Å, Ni1−Cl2 = 2.36 Å) and in 1 (Ni1−Cl1 = 2.40 Å, Ni1−Cl2 = 2.32 Å). The next step is dissociation of one Cl− from the anionic complex, [1]− resulting in a penta-coordinated complex, III. The formation of III is though highly endergonic in the gas phase (ΔG = 33.5 kcal/mol); it becomes less endergonic (ΔG = 7.7 kcal/mol) in THF. Experimentally, the yield of products in hexane and toluene is significantly lower than that in THF (Table S2). In the first cycle, the intermediate complex III undergoes deprotonation by the external base, tert-butoxide, to form of a relatively stable intermediate VI via an intermediate complex V. The total distortion energy associated with IV → TS1 is 1.5 kcal/mol, while the interaction energy in TS1 is found to be D


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Inorganic Chemistry

Figure 3. Computed reaction profile with Arrhenius plot. (a) Gibb’s free energy profiles computed for the key steps involved in the catalytic cycle. The free energy changes (in kcal/mol) obtained from both the condensed phase (ΔGsol) and gas phase (ΔGgas). (b) DFT-computed optimized geometries of all the transition states involved in the reaction. (c) Arrhenius plot for primary KIE for HAT from cyclohexanol to the azo group of compound 6 without QMT effects (TST calculations, black dash line) and with QMT corrections (Eckart corrections for TST calculations, red solid line). The experimental KIE data at T = 283, 293, 300 K is shown as a blue filled circle.

S21, shows the unpaired electron is delocalized over azo functions with a minimal contribution of Ni d-orbital. Intramolecular oxidation of ketyl radical with concomitant reduction of unreduced azo function is expected to be the preferred pathway. Alcohol oxidation by the use of the model complexes primarily of copper(II) have been studied extensively; however, the primary difference in the present case is that hydrogen atoms of alcohol are added across the coordinated azo function resulting in the formation of a hydrazo complex.33,47,48 Participation of an azo function in catalytic alcohol oxidation was proposed primarily by Markó et al. using azo-dicarboxylate as a cocatalyst.20,48 This example, as far as we know, is the most prominent proof (achieved by isolation of a hydrazo intermediate) of the involvement of azo−hydrazo couple in alcohol dehydrogenation. Oxidation of the intermediate VII by aerial oxygen results in the azo−hydrazo intermediate VIII through an endergonic process (ΔG = 21.9 kcal/mol). The subsequent cycle begins with the addition of another alcohol to VIII in a stepwise fashion to produce the dehydrogenated penta-coordinated complex (XII). The intermediate complex 2 is crystallized from the mixture as NiCl2(LH2) by a simple substitution of alkoxide in XII by Cl−. In the final step,

lower temperatures as is evident in the curvature of Arrhenius plot of the KIE (see Figure 3c). To confirm it further, two sets of experiments were performed at 283 and 293 K; the observed KIE are 17.76 ± 0.2 and 14.42 ± 0.25, which are in excellent agreement with TST/Eckart calculated KIE = 18.7 and 14.9, respectively.43 Hydrogen atom transfer via five-membered chelate ring in GO model complex was first demonstrated33 by Tolman and others. The rate-determining step for such transfer is associated with large KIE. The following steps in the catalytic cycle involve oxidation of the ketyl radical with subsequent rejection of ketone. We propose that the above oxidation process is intramolecular and brought about by the concomitant reduction of the other unreduced azo function of L (intermediate VII).33,44,45 Intramolecular oxidations of ketyl radicals in galactose oxidase (GO) model has been previously established.46 To support our proposition further DFT studies on competing pathways like intramolecular versus intermolecular oxidation and the reduction of the unreduced azo function versus that of the nickel(II) center were also considered. The possibility of intermolecular oxidation is ruled out, as the ketyl radical is unstable along the potential energy surface and should, therefore, have at best fleeting existence (Supporting Information, Movie). A representation of singly occupied molecular orbital of VII, illustrated in Figure E


Inorganic Chemistry

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CONCLUSION The present results on combined experimental and theoretical studies highlight the use of azo−hydrazo (2e− + 2H+) couple in pure ligand-mediated catalytic dehydrogenation of alcohols. Metal ion of the catalyst behaves innocently and acts as a template. This protocol is a classic shift from the conventional ones in many respects and has generated scopes of designing efficient catalysts with earth-abundant 3d metal ions for their possible application in DAFCs. Mechanistic studies have clearly established that the dehydrogenation process proceeds via H atom transfer, which is more similar to what happens in enzymatic GO pathway. Our work on similar dehydrogenation reaction using a closely related zinc complex shows very high prospect, which will be reported in due course.

oxidation of XII regenerates the catalyst with the formation of H2O2 (ΔG = 1.0 kcal/mol). Thus, the complete catalytic cycle, based on the experimental data and supported by DFT calculation, is as in Figure 4. Here

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EXPERIMENTAL SECTION

Materials. [Ni(H2O)6]Cl2 was purchased from Merck, India. Different substituted alcohols were purchased from Sigma-Aldrich and Arora-Matthey Limited. All other reagents and chemicals were obtained from commercial sources and used without further purifications. Solvents were dried and deoxygenated before use. Tetrabutylammonium perchlorate was prepared and recrystallized as reported earlier.49 Caution! Perchlorates have to be handled with care and appropriate safety precautions. Physical Measurements. A PerkinElmer Lambda 950 spectrophotometer with Unisoku CoolSpeK UV USP-203-A attachments was used to record variable-temperature UV−vis spectra. Infrared spectra were obtained using a PerkinElmer 783 spectro-photometer. 1H NMR spectra were recorded on a Bruker Avance 400 MHz or 500 MHz spectrometer, and SiMe4 was used as the internal standard. A PerkinElmer 240C elemental analyzer was used to collect microanalytical data (C, H, N). All electrochemical measurements were performed using a PC-controlled PAR model 273A electrochemistry system. Cyclic voltammetric experiments were performed in dichloromethane solution containing supporting electrolyte, 0.1 M Bu4NClO4 under the nitrogen atmosphere using a Ag/AgCl reference electrode, a Pt disk working electrode, and a Pt wire auxiliary 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. Room-temperature magnetic moment measurements for complexes 1 and 2 were performed with Gouy balance (Sherwood Scientific, Cambridge, U.K). X-ray Crystallography. Crystallographic data for compounds 1 and 2 were collected in Table S1. Suitable X-ray quality crystals of complexes 1 and 2 were obtained by the slow diffusion of a dichloromethane solution of the complex into hexane. All data were collected on a Bruker SMART APEX-II diffractometer, equipped with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å), and were corrected for Lorentz polarization effects. Data for 1: a total of 10 954 reflections were collected, of which 1744 were unique (Rint = 0.050), satisfying the I > 2σ(I) criterion, and were used in subsequent analysis. Data for 2: A total of 20 125 reflections were collected, of which 3031 were unique (Rint = 0.164). The structures were solved by employing the SHELXS-97 program package50 and were refined by full-matrix least-squares based on F2 (SHELXL-97).51 All hydrogen atoms were added in calculated positions. Synthesis. The ligand L was synthesized52−54 and purified by following a procedure slightly modified from that for 2-(phenylazo)pyridine; 2,6-diaminopyridine was used in place of 2-aminopyridine in a 1:2 molar proportion. 2,6-Diaminopyridine (1 g) was dissolved in 20 mL of pyridine and mixed with 2 g of nitrosobenzene and 10 mL of 60% aqueous sodium hydroxide. The reaction mixture was refluxed for 12 h. The dark red solution was cooled, diluted with water, and extracted with toluene. The crude mass, obtained by evaporation of the solvent in vacuum, was purified by preparative thin-layer chromatography (TLC) using toluene as eluent. Yield: 30%. Synthesis of [NiIICl2L(OH2)], 1. 287 mg of L, dissolved in moist THF, was added dropwise to a wet THF solution containing 237 mg

Figure 4. Proposed mechanism for the alcohol dehydrogenation reaction.

the cycle is entirely controlled by the ligand redox, without any mass loss and no involvement of metal redox. The individual steps of the catalytic cycle along with their optimized structures are collected in Supporting Information (Figure S22). Furthermore, the above catalytic cycle also is operative with equal ease in the presence of 1,4-benzoquinone as an oxidant.16,18,24 Thus, catalytic dehydrogenation of 1,1-diphenyl carbinol was successfully achieved (in 80% yield) in deaerated conditions. The two products benzophenone and 1,4-hydroquinone both were detected by gas chromatography mass spectrometry (GCMS; Figure S7). To quantify the oxidative equivalent of the oxidant we used 2 mol equivalent of benzoquinone to perform a catalytic reaction in deaerated conditions. GCMS analysis of the resultant products indicated that 0.5 mol equivalent of benzoquinone remained unused and 1.5 mol equivalent of 1,4-hydroquinone was formed. In an attempt of decoupling of electron(s) and proton(s) transfer events (direct alcohol fuel cell (DAFC) requirement)8,24 we successfully performed the dehydrogenation reactions of the compound 2 using ferrocenium hexafluorophosphate (FcPF6) as the oxidant in an alkaline medium (see Supporting Information for details). The product benzophenone was isolated in 72% yield after 4 h of stirring with 5 mol % catalyst loading under deaerated conditions. F


Article

Inorganic Chemistry of [Ni(H2O)6]Cl2 with constant stirring for 4 h. During this period, the color of the solution changed from orange to dark green. The crude mass, obtained by evaporation of the solvent in vacuum, was purified by fractional crystallization from dichloromethane and ether solvent mixture. The precipitate was recrystallized by slow diffusion of a dichloromethane solution of the compound into hexane. Its yield and characterization data are as follows: Green colored solid. Yield: 90%. Anal. Calcd for C17H15Cl2N5NiO: C, 46.95; H, 3.48; N, 16.10, Found C, 46.76; H, 3.50; N, 16.05%. UV−vis (CH2Cl2): λ [nm] (ε, M−1 cm−1) = 255(17 320), 325(15 100), 395(13 180). IR (KBr disk, cm−1): ν (NN): 1398 cm−1. General Procedure for Catalysis. The catalytic reactions were performed following a general procedure. In a round-bottom flask containing 1 mmol substrate in dry THF medium was mixed with 0.001 mmol of catalyst (1 mL from 100 mL of stock solution containing 40 mg of catalyst), 0.001 mmol of KtBuO (1 mL from 100 mL stock solution containing 11 mg of KtBuO) and 1.5 mmol of zinc was stirred at 300−323 K in the presence of continuous bubbling of oxygen (using oxygen-filled balloon). The stirring was continued for 2−4 h depending upon substituent. The crude product, thus obtained, was purified on preparative alumina TLC plate using hexane as eluent. Higher concentration of catalyst and relatively higher temperatures and reaction time were needed for primary alcohols. The yields are collected in Table 2, and spectral characterizations of the products are as follows: Isolation of [NiCl2(LH2)], 2. In a glovebox, 1 mmol of the complex 1 was mixed with the equimolar quantity of 2-propanol and 1.5 mmol zinc in dry and deoxygenated THF solvent. The solution was stirred at 300 K for 3 h. The color of the solution became dark blue. The solution was evaporated under vacuum, and the crude product was purified by rapid precipitation from dichloromethane solution of the complex by hexane. Finally, the compound was crystallized by slow diffusion of the dichloromethane solution of complex 2 into hexane. The yield and spectral characterization of the product are as follows: Blue colored solid. Yield: 85%. UV−vis (CH2Cl2): λ [nm] (ε, M−1 cm−1) = 250(19 460), 315 (10 660), 402sh(3390), 605 (1165). IR (KBr disk, cm−1): ν (NN): 1399, ν (N−N): 1167, ν (N−H): 3220, 3070. Because of its high air sensitivity, reproducible C, H, N data for complex 2 were not obtained. Kinetic and Isotope Labeling Experiments. Isolation of [NiCl2(LD2)]. In an argon atmosphere Schlenk line, 1 mmol of the complex 1 was mixed with the equimolar quantity of cyclohexanol-D and 1.5 mmol zinc in dry and deoxygenated THF solvent. The solution was stirred at 300 K for 6 h. The color of the solution changed from green to dark blue. The solution was filtered; solvent was evaporated under vacuum. The IR spectrum of the resultant compound showed charecteristic streching due to N−D bonds. IR (KBr disk, cm−1): ν (NN): 1405, ν (N−N): 1172, ν (N−D): 2335, 2205. A similar experiment was also performed with cyclohexanone-H for comparison. The isolated compound was similar to the previously noted complex 2 (isolated from 2-propanol). Complex 2 to 1 Conversion. The complex 1 is regenerated from compound 2 upon exposure to air. The regeneration was fast at room temperature. However, we were able to monitor the transformation, spectroscopically at a low temperature (263 K) in acetonitrile solvent. A 1.1 × 10−4 molar acetonitrile solution of the complex 2 was continuously stirred in the air, and the spectral changes were monitored by UV−vis spectroscopy (Figure S5). The transformation was found to be quantitative. Detection of Hydrogen Peroxide during the Catalytic Reactions. The catalytic reactions produced H2O2, which was detected by following the gradual development of the charecteristic band for I3− spectro-photometrically (λmax= 350 nm; ε = 26 000 M−1 cm−1), upon reaction with I−. In a round-bottom flask containing 1 mmol of 1,1-diphenyl carbinol in 10 mL of dry THF was mixed with 0.005 mmol of catalyst, 0.005 mmol of KtBuO, and 1.5 mmol of zinc, and the mixture was stirred at 300 K for 1 h. An equal volume of water was added subsequently to the reaction mixture and was extracted with dichloromethane to remove the leftover reactants and products from

the reaction mixture. The separated aqueous layer was then acidified with 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 excess I− to form I3− according to the following chemical reactions: (i) H2O2 + 2I− + 2H+ → 2H2O + I2, (ii) I2(aq) + I− → I3− (Figure S6).55−57 The reaction rate was slow but increases with increasing concentrations of acid; ammonium molybdate solution catalyzes the oxidation reaction. Catalytic Reactions Using Benzoquinone as the Oxidant. In a round-bottom flask containing 1 mmol of 1,1-diphenyl carbinol in dry THF medium was mixed with 0.001 mmol of catalyst, 0.001 mmol of KtBuO, and 2 mmol of benzoquinone. The mixture was stirred for 4 h at 300 K in an inert atmosphere glovebox and then was extracted with ethyl acetate. After evaporation of the solvent the residue was dissolved in acetonitrile and subjected to GCMS analysis. Figure S7 shows the chromatogram, which identifies both benzophenone and hydroquinone (generated from benzoquinone) conclusively. GCMS analysis indicated that 0.5 mol equiv of benzoquinone remained unused and that 1.5 mol equiv of 1,4-hydroquinone was detected. Catalytic Reactions using Ferrocenium Hexafluorophosphate as Oxidant. In a round-bottom flask containing 1 mmol of 1,1-diphenyl carbinol in dry THF medium was mixed with 0.05 mmol of catalyst, 0.05 mmol of KtBuO, and 3.5 mmol of ferrocenium hexafluorophosphate. The mixture was stirred for 4 h at 300 K in an inert atmosphere glovebox and then was extracted with ethyl acetate. After evaporation of the ethyl acetate, the residue was dissolved in acetonitrile, filtered through silica gel, and subjected to GCMS analysis. Up to 72% of benzophenone was detected by GCMS. Computational. All geometry optimization and vibrational frequency calculations were performed in the gas phase at M0636/631+G(d),58 LANL2DZ (Ni)59 level of theory using Gaussian 09.39 For simplicity this method is represented as BS1. For complexes having unpaired electrons, the unrestricted version of DFT was employed. The stationary point is confirmed by the absence any imaginary frequency, and the transition states were confirmed by one unique imaginary frequency. For further verification of the transition state, intrinsic reaction coordinate (IRC) calculations were performed. Additional single-point calculations (at the BS1 structures) for the full catalytic cycle were performed using the M06/TZVP,37 LANLTZ(f) (Ni)38 for better estimation of electronic energy, which we represent as BS2. The role of solvation by THF was explored by an implicit PCM solvation model (SMD).35 All free energy calculations were performed at SMD/BS1//BS2 incorporating zero point energy (ZPE) correction at 298 K. The role of QMT on the ZPE-corrected potential energy surface was incorporated within the Eckart tunneling model using TheRate42 program. To this regard, the one-dimensional Eckart analytical potential energy function was fitted to the ZPE-corrected energies of the reactant, product, and concerned transition-state along the IRC. Replacement of the corresponding H atom by D atom and scaling the harmonic frequencies by mass of D provides the primary KIE.

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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.6b01310. Kinetics details with plots, EPR spectrum of intermediate, NMR, and GCMS spectrum of isolated ketones/ aldehydes and theoretical details. (PDF) Ketyl radical is unstable along the potential energy surface. (AVI) CCDC Nos. 1469110 and 1469111 contain the supplementary crystallographic data. (CIF) G


Article

Inorganic Chemistry

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(15) Broere, D. L. J.; Plessius, R.; van der Vlugt, J. I. New avenues for ligand-mediated processes - expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands. Chem. Soc. Rev. 2015, 44, 6886−6915. (16) Matsumoto, T.; Chang, H.-C.; Wakizaka, M.; Ueno, S.; Kobayashi, A.; Nakayama, A.; Taketsugu, T.; Kato, M. NonpreciousMetal-Assisted Photochemical Hydrogen Production from orthoPhenylenediamine. J. Am. Chem. Soc. 2013, 135, 8646−8654. (17) Donati, N.; Koenigsmann, M.; Stein, D.; Udino, L.; Gruetzmacher, H. Iridium aminyl radical complexes as catalysts for the catalytic dehydrogenation of primary hydroxyl functions in natural products. C. R. Chim. 2007, 10, 721−730. (18) Koenigsmann, M.; Donati, N.; Stein, D.; Schoenberg, H.; Harmer, J.; Sreekanth, A.; Gruetzmacher, H. Metalloenzyme-inspired catalysis: selective oxidation of primary alcohols with an iridiumaminyl-radical complex. Angew. Chem., Int. Ed. 2007, 46, 3567−3570. (19) Rodriguez-Lugo, R. E.; Trincado, M.; Vogt, M.; Tewes, F.; Santiso-Quinones, G.; Gruetzmacher, H. A homogeneous transition metal complex for clean hydrogen production from methanol-water mixtures. Nat. Chem. 2013, 5, 342−347. (20) Marko, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch, C. J. Copper-catalyzed oxidation of alcohols to aldehydes and ketones: an efficient, aerobic alternative. Science 1996, 274, 2044−2046. (21) McCann, S. D.; Stahl, S. S. Mechanism of Copper/ Azodicarboxylate-Catalyzed Aerobic Alcohol Oxidation: Evidence for Uncooperative Catalysis. J. Am. Chem. Soc. 2016, 138, 199−206. (22) Solis, B. H.; Maher, A. G.; Dogutan, D. K.; Nocera, D. G.; Hammes-Schiffer, S. Nickel phlorin intermediate formed by protoncoupled electron transfer in hydrogen evolution mechanism. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 485−492. (23) Dempsey, J. L. Ligand steals spotlight from metal to orchestrate hydrogen production. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 478− 479. (24) Bonitatibus, P. J.; Rainka, M. P.; Peters, A. J.; Simone, D. L.; Doherty, M. D. Highly selective electrocatalytic dehydrogenation at low applied potential catalyzed by an Ir organometallic complex. Chem. Commun. 2013, 49, 10581−10583. (25) Kariya, N.; Fukuoka, A.; Ichikawa, M. Direct PEM fuel cell using ″organic chemical hydrides″ with zero-CO2 emission and lowcrossover. Phys. Chem. Chem. Phys. 2006, 8, 1724−1730. (26) Ghosh, P.; Samanta, S.; Roy, S. K.; Demeshko, S.; Meyer, F.; Goswami, S. Introducing a New Azoaromatic Pincer Ligand. Isolation and Characterization of Redox Events in Its Ferrous Complexes. Inorg. Chem. 2014, 53, 4678−4686. (27) Sengupta, D.; Saha Chowdhury, N.; Samanta, S.; Ghosh, P.; Seth, S. K.; Demeshko, S.; Meyer, F.; Goswami, S. Regioselective ortho Amination of Coordinated 2-(Arylazo)pyridine. Isolation of Monoradical Palladium Complexes of a New Series of Azo-Aromatic Pincer Ligands. Inorg. Chem. 2015, 54, 11465−11476. (28) Bandyopadhyay, P.; Bandyopadhyay, D.; Chakravorty, A.; Cotton, F. A.; Falvello, L. R.; Han, S. Two novel examples of hydroxylation of aromatic rings in coordination chemistry. J. Am. Chem. Soc. 1983, 105, 6327−9. (29) Samanta, S.; Ghosh, P.; Goswami, S. Recent advances on the chemistry of transition metal complexes of 2-(arylazo)pyridines and its arylamino derivatives. Dalton Trans. 2012, 41, 2213−2226. (30) Sengupta, D.; Ghosh, P.; Chatterjee, T.; Datta, H.; Paul, N. D.; Goswami, S. Ligand-Centered Redox in Nickel(II) Complexes of 2(Arylazo)pyridine and Isolation of 2-Pyridyl-Substituted Triaryl Hydrazines via Catalytic N-Arylation of Azo-Function. Inorg. Chem. 2014, 53, 12002−12013. (31) Ghosh, A. K.; Majumdar, P.; Falvello, L. R.; Mostafa, G.; Goswami, S. Novel Examples of Simultaneous Reductive Azo Cleavage and Oxidative Aromatic Ring Amination in Rhodium Complexes of 2(Arylazo)pyridine. Organometallics 1999, 18, 5086−5090. (32) Santra, B. K.; Thakur, G. A.; Ghosh, P.; Pramanik, A.; Lahiri, G. K. A Novel Example of Metal-Mediated Aromatic Thiolation in a Ruthenium Complex. Crystal Structure of RuII(SC6H4N:NC5H4N)2. Inorg. Chem. 1996, 35, 3050−2.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (S.G.) *E-mail: [email protected]. (A.D.) 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, and Council of Scientific and Industrial Research (CSIR) funded projects, SR/S2/JCB-09/ 2011, EMR/2014/000520 and 01(274)/13/EMR-II, respectively. S.G. and D.S. sincerely thank DST-SERB for a J. C. Bose fellowship and a research fellowship, respectively. A.D. thanks DST-SERB and BRNS for partial support. R.B., R.P., and S.P.R. are thankful to the CSIR for their fellowship support. Crystallography was performed at the DST-funded National Single Crystal Diffractometer facility at the Department of Inorganic Chemistry, IACS. We thank Prof. S. Bhattacharya of Jadavpur Univ. for providing GC-MS data. Computational facility of the IACS-CRAT initiative is duly acknowledged. We are also thankful to Dr. S. K. Roy for valuable discussion on mechanism.

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Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols

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