Zeolite-Y Encapsulated Metal Picolinato ... - ACS Publications

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Zeolite‑Y Encapsulated Metal Picolinato Complexes as Catalyst for Oxidation of Phenol with Hydrogen Peroxide Kusum K. Bania and Ramesh C. Deka* Department of Chemical Sciences, Tezpur University, Napaam-784028, Assam S Supporting Information *

ABSTRACT: A systematic experimental and density functional theory (DFT) study has been carried out on the selective oxidation of phenol to catechol by bis(picolinato) complexes of cobalt, nickel, and copper prepared in solution and encapsulated in zeolite-Y. The catalytic activities of the homogeneous catalysts and their heterogeneous counterparts are compared under microwave irradiation in the presence of H2O2 as mild oxidant. Catechol is obtained selectively in good yield when the catalytic oxidation is carried out using bis(picolinato) Cu(II) complex encapsulated in zeolite-Y. An ultraviolet−visible (UV−vis) spectrum shows that the increase in the amount of H2O2 further oxidizes catechol to benzoquinone. Electron paramagnetic resonance (EPR) and cyclic voltammetry studies reveal the existence of cis → trans isomerization in case of Cu(II) complex, which has been further substantiated by DFT calculations. A plausible mechanism for the formation of catechol mediated by synthesized complexes has been provided by means of DFT calculations from the energetics involved in the transformations.

1. INTRODUCTION The contemporary interest in the selective oxidation of phenol to catechol, to a great extent, has been fueled by its utility in nearly every sector of chemical industries including pharmaceuticals, agrochemicals, flavors, polymerization inhibitors, and antioxidants.1 Worldwide, the catechol production from phenol involves two procedures: (i) ortho-formylation of phenol followed by subsequent oxidation, and (ii) oxidation of phenols to o-quinones and subsequent reduction of the latter into catechol. These multi step routes are often lengthy, energyintensive, and generate a large number of oxidized, coupling, and polymerized products.2 To meet the increasing demand for catechol and to satisfy environmental requirements, considerable efforts have been made for producing catechol by the one-step hydroxylation method using various homogeneous and heterogeneous transition metal complexes.3−5 The utility of hydroxidation is mainly measured by several factors, including selectivity and ecological sustainability of the oxidation and the availability of the oxidant and catalyst. Among various oxidants, H2O2 of less than 60% concentration has been recognized as an ideal, clean, and green oxidizing agent.6−8 In recent years, it has been established that zeolite encapsulated organometallic compounds and transition metal complexes, in addition to chiral metal complexes and biocatalysts, can be highly selective and efficient catalysts. As a consequence, the application of intrazeolite complexes is rapidly gaining importance in organic transformations, complementing bio and organometallic catalysis.9 The steric constrain imposed by the walls of the zeolite plays a vital role in modifying the properties, viz., magnetic, electronic, and redox behavior of the encapsulated complexes.10−12 These changes in © XXXX American Chemical Society

the properties of the transition metal complexes upon encapsulation have led various researchers to develop newer heterogeneous catalyst and apply them in various organic transformations including the asymmetric synthesis. Besides having wide advantages of these heterogeneous catalysts over the homogeneous counterparts, the microwave-assisted catalytic transformation in the presence of these hybrid catalysts are now found to be most atom economic in comparison to the conventional methods.13 In the context of the growing general interest in microwave-assisted organic synthesis, we plan here, to perform selective oxidation of phenol to catechol by few homogeneous and zeolite-Y encapsulated transition metal picolinato complexes under the influence of microwave irradiation. The picolinato complexes of transitional metals have been determined to exhibit a broad spectrum of physiological effects on the activity functions of both animal and plants organisms.14 Additionally, in recent years the complexes, especially of vanadium,15 zinc,16 manganese,17 copper,18 chromium,19 and tungsten,20 are known to have in vitro insulin mimetic activity and in vivo antidiabetic ability. However, the potential of these type of complexes as catalyst in organic transformations is less explored.21 There are only few reports on the encapsulation of picolinato complexes into the cavity of zeolite-Y and on their application as a heterogeneous catalyst.22 We report herein the first systematic experimental and theoretical studies on the selective oxidation of phenol under microwave irradiation by Received: March 10, 2013 Revised: May 9, 2013

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dx.doi.org/10.1021/jp402439x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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diethyl ether (3−20 mL), and the combined organic phases are dried over Na2SO4. A sample is taken for high-performance liquid chroatography (HPLC) analysis, and the remaining mixture is evaporated and purified by column chromatography (EtOAc/petroleum ether, 1:3) to afford catechol.

bis(picolinato) complexes of cobalt, nickel, copper complexes and those encapsulated in zeolite-Y.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Bis(picolinato) M(II) [M = Co, Ni, and Cu] Complexes, [M(Pic)2]. The bis(picolinato) complexes [MII(Pic)2]·H2O are prepared according to the reported procedure.23 An aqueous solution of the free ligand is prepared by dissolving the free acid (picolonic acid) in a slight excess of dilute sodium hydroxide solution and adjusting the pH to 6−7 with dilute acid. The filtered ligand solution is then added to an aqueous solution of MCl2·xH2O in a 2:1 ligand−metal mole ratio. The blue-violet solid for copper complex is washed with acetone and then ethanol, and finally dried in vacuum. The purple and sky-blue complexes of cobalt and nickel, respectively, are prepared using the same procedure. 2.2. Preparation of M(II) [M = Co, Ni, and Cu] Exchanged Zeolites, M2+−Y. A mixture of 1 g of the NaY zeolite and 1 mmol of respective metal chlorides [CoCl2·6H2O = 0.237 g, NiCl2·6H2O = 0.237 g, and CuCl2·2H2O = 0.170 g] solution in water are stirred under reflux at 120 °C for 24 h. The pH of the solution is maintained within 3.0−3.5 using buffer tablets in order to prevent the precipitation of metal ions as hydroxides. The slurry is then filtered, washed with distilled water until the silver ion test for chloride is negative, and finally dried overnight in an oven at 200−250 °C to obtain Co2+exchanged NaY as pink powder, Ni-exchanged NaY as pale green powder, and Cu-exchanged zeolite NaY as light blue powder. 2.3. Encapsulation of Bis(picolinato) M(II) Complexes in Metal Exchanged Zeolite-Y, [M(Pic)2]Y. The metalexchanged zeolites (represented as Co2+−Y, Ni2+−Y, and Cu2+−Y) are treated individually with stoichoimetric excess of sodium salt of pyridyl 2-carboxylic acid in 50 mL deionized water. The mixture is refluxed for 48 h at 90 °C under constant stirring. On heating, the solid mass changed color from light pink to dark pink in the case of cobalt, light green to sky blue in the case of nickel, and from light blue to blue-violet in the case of copper. The zeolite encapsulated complexes are then filtered, washed repeatedly with deionized water, and dried at room temperature under vacuum. The resultant products are further purified by Soxhlet extraction using the sequence of solvents acetone, methanol, and finally with diethyl ether to remove any unreacted species or species adsorbed on the surface of the zeolite crystallites. The color of the resultant solid does not change even after repeated Soxhlet extraction. This observation gives a preliminary idea about the formation of complexes inside the cavity of zeolite-Y. The products are dried under vacuum and finally kept in a muffle furnace for 48 h at 50−55 °C to obtain anhydous [Co(Pic)2]Y as a dark brown powder, [Ni(Pic)2]Y as a sky-blue powder, and blue-violet [Cu(Pic)2]Y powder. 2.4. Catalytic Oxidation of Phenol. To carry out the catalytic oxidation of phenol, the catalyst (15 mg) is first treated with a stoichiometric amount of 30% H2O2 and stirred for 10 min in nitrogen atmosphere. To this a solution of stoichiometric amount of phenol prepared in acetonitrile is added, and the whole reaction mixture is subjected to microwave irradiation (280 W). The progress of the reaction is monitored by TLC and ultraviolet−visible (UV−vis) spectroscopy after an interval of 10 min. The solid catalyst is extracted by filtration, and the crude reaction mixture is quenched with saturated aqueous NH4Cl and extracted with

3. RESULTS AND DISCUSSION 3.1. Experimental Section. 3.1.1. Elemental Analysis. In order to confirm the presence of bis(picolinato) complexes of Co(II), Ni(II), and Cu(II) inside zeolite-Y various physicochemical and spectrochemical analyses are performed. At first we perform the elemental detection by energy dispersive X-ray spectroscopy (EDX) and determine the metal content in the encapsulated systems via UV−vis technique. The results of the elemental analyses obtained from the EDX study gives a Si/Al ratio of 2.76, which corresponds to a unit cell formula Na52 [(AlO2)52(SiO2)140] for parent NaY. The Si/Al ratio has remained unchanged in all metal-exchanged zeolites, indicating the absence of dealumination during exchange process. The amount of metal contents in the neat and the intrazeolite complexes are obtained by Vogels method24 and are found to be less compared to the metal-exchanged zeolites. The decrease in the metal content during complex formation inside the zeolite cavity can be attributed to the participation of metal ion in the formation of co-ordination complexes inside the cavities of zeolite-Y. 3.1.2. X-ray Diffraction (XRD) Studies. The powder X-ray diffraction patterns of the zeolite NaY, metal-exchanged zeolites and the encapsulated metal picolinato complexes are shown in Figure 1. Essentially similar diffraction patterns are noticed in

Figure 1. XRD pattern of (a) Pure zeolite-Y, (b) metal exchanged zeolite-Y, (c) [M(Pic)2]Y complexes (M = Co2+, Ni2+, and Cu2+).

the encapsulated complexes and NaY, except that the zeolite encapsulated [M(Pic)2] (M = Co, Cu, and Ni) complexes have slightly weaker intensities. These observations indicate that the framework of the zeolite does not suffer any significant structural changes during encapsulation. However, there are differences in the relative peak intensities of the 220 and 311 reflections appearing at 2θ = 10 and 12°, respectively. For pure zeolite-Y and for M2+-exchanged zeolite-Y, I220 > I311, but for the encapsulated complexes, I311 > I220. This reversal in intensities has been empirically correlated with the presence of a large complex within the zeolite-Y supercage.25 This change B



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Figure 2. UV−vis/DRS of (a) [Co(Pic)2], (b) [Co(Pic)2]Y, (c) [Ni(Pic)2], (d) [Ni(Pic)2]Y, (e) [Cu(Pic)2], and (f) [Cu(Pic)2]Y. The DRS spectra of metal-exchanged zeolites are shown in the inset for the respective cases.

from Figure S1c−f that the IR bands of all encapsulated complexes are weak due to their low concentration in the zeolite cage and thus can only absorb in the region where the zeolite matrix does not show any absorption band that lie in 1200−1600 cm−1. The characteristic vibrational bands that appeared in both neat and encapsulated complexes are assigned in Table S1. FTIR spectra of all the encapsulated M(Pic)2−Y complexes, M = Co, Ni, and Cu, exhibit a strong band at 1335−1343 cm−1 and 1445−1480 cm−1 characteristic of picolinate species.22 The band at 966 cm−1 attributable to δ(O−H) acid of picolinic acid,27 is absent in both the neat and encapsulated systems. The absence of this peak further reveals the nonexistence of extraneous picolinic acid on the external surface of zeolite-Y encapsulated complexes. Moreover, no additional bands are observed that could ascribe the coordination modes to be different from bidentate. The presence of similar peak positions in all the encapsulated and neat crystalline complexes gives indirect evidence for the presence of a bis(picolinato) complex inside the zeolite cage. The slight shifting of the peak positions in the ν(COO−), ν(ring), and ν(C−O) bands to wave numbers 1660, 1549, 1480, and 1335 cm−1, respectively, can be attributed to the effect of zeolite matrix on the geometry of the picolinato complexes trapped in the zeolite supercages. Besides these bands at 770 and 685 cm−1 assigned to the out-of-plane γ(CH) vibrations and out-of-plane ϕ(CC) ring deformation around the pyridine get shifted to a lower wavenumber (Table S1) in the case of the encapsulated complexes. This further suggests that the pyridine rings of [M(Pic)2]−Y in the supercage are

in the relative intensities may be associated with the redistribution of randomly coordinated free cations in zeoliteY at sites II and I. The above observation may therefore be construed as evidence for the successful encapsulation of metal picolinato complexes within the supercage of zeolite-Y. 3.1.3. Fourier Transformed Infrared Spectroscopy (FTIR). The FTIR spectra of NaY, metal exchanged zeolite-Y, and neat and encapsulated picolinato complexes are shown in Figure S1 (Supporting Information). It is evident from the FTIR spectra that the framework vibration band of zeolite-Y dominate the spectra below 1200 cm−1 for all samples. FTIR spectra of NaY and metal-exchanged zeolites (Figure S1a,b, respectively) show strong zeolite lattice bands in the range 500−1200 cm−1. The strong and broad band at the region 1010−1045 cm−1 could be attributed to the asymmetric stretching vibrations of (Si/Al) O4 units. The broad bands in the region 1650 and 3500 cm−1 are due to lattice water molecules and surface hydroxylic groups, respectively. The parent NaY zeolite shows characteristics bands at 556, 690, and 1010 cm−1 (Figure S1, curve a) and are attributed to T−O bending mode, symmetric stretching, and antisymmetric vibrations, respectively.26 These bands are not modified during the ion exchange with metal cations (Co2+, Ni2+, and Cu2+ or by supporting the metal complexes (Figure S1, curve b). The absence of shifting of the characteristic vibrational bands of zeolite framework on metal exchanged or encapsulation of transition metal picolinato complexes implies that the zeolite framework has remained unchanged upon encapsulation of complexes. However, there is obviously a difference in the range of 1200−1600 cm−1 among the three encapsulated complexes (see Table S1). It can also be observed C



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two low intense bands are due to n→π* transitions associated with the ligand and are being veiled by the MLCT associated with Co2+-exchanged zeolite. The other two bands are mainly due to dπ→pπ*(MLCT) and d−d transition (2A1g →2B1g). The encapsulated Ni−picolinato complex gives a number of peaks (Figure 2d). The first three bands are due to a ligand-based π→ π* transition, and the other two bands are due to MLCT and 1 A1g → 1B1g transitions, respectively. The DRS/UV−vis spectrum of encapsulated copper complex is shown in Figure 2f. The absorption peaks in the range 280−308 nm are due to intraligand transition. The bands at 366 and 426 nm are due to MLCT transition. The peaks at 503 and 602 nm can be attributed to MLCT and d−d transitions, respectively. The comparison of the absorption spectrum of the neat complexes with those of the encapsulated complexes demonstrates that encapsulation results in a blue shift in the electronic transitions associated with the nickel complex. In the case of the corresponding cobalt and copper complex, the peaks are found to be red-shifted under the influence of the zeolite matrix. This further gives an idea that all the three complexes do not undergo a similar kind of structural change inside zeolite-Y. However, the presence of the similar electronic transitions in the encapsulated complexes in comparison to the neat complex in solution gives evidence for the formation of complexes inside the supercage of zeolite-Y. 3.1.5. Cyclic Voltammetry Study. The cyclic voltammetry study of the intrazeolite complexes as a component of the zeolite-modified electrode (ZME) has gained interest in recent years for probing the presence of redox active species inside the zeolite cavity.9a,10,35 In the case of the “ship-in-a-bottle” complexes, the electroactive complex already encapsulated undergoes electron transfer within the zeolite cavities. Shaw et al.36 first proposed two possible mechanismsintrazeolite and extrazeolite mechanisms (eqs I and II, respectively)for electron transfer associated with encapsulated transition metal complexes within the supercage of zeolite and surface-bound metal complexes, respectively, based on ZMEs. Although, these mechanistic assignments are controversial, there are various reports demonstrating the redox behavior of transition metal complexes encapsulated in zeolite cavities.37−39

located under different conditions. Such changes may be caused by distortion of the encapsulated complexes inside the zeolite supercages or with a difference in coordination by the −OH groups of the zeolite-Y. 3.1.4. UV−Vis/Diffuse Reflectance Spectroscopy (UV−vis/ DRS). The UV−vis spectrum of the neat and encapsulated picolinato complexes are shown in Figure 2. The spectra of the metal exchanged zeolites are shown in the inset of the respective cases. The UV−vis spectrum of bis(picolinato) Co(II) complex (Figure 2a) shows two intense peak in the region 267 and 368 nm due to intraligand and metal-to-ligand charge transfer (MLCT) transitions, i.e., π→π* and dπ→pπ*, respectively. The highest energy d−d transition from the lowerlying fully occupied 3dx2−y2 orbital to the upper empty 3dyz orbital (2A1g →2B1g, transition) at 432 nm is obscured by the above MLCT transitions. The peak at 574 nm is again mainly due to the 2A1g →2B1g transition supporting the square-planar geometry of the [Co(Pic)2] complex. The electronic spectra are clearly distinct from both the well-known tetrahedral and octahedral species. The tetrahedral [CoCl4]2− has two features at around 650 (ε = 550) and 675 nm (590 M−1cm−1) with a smaller feature at 600 nm (360 M−1cm−1).28 The octahedral [M(H2O)6]2+ complexes have distinct absorbance at 560 nm for Co(II).29 The corresponding Ni(II) complex (Figure 2c) shows five characteristic absorption bands at 257, 295, 331, 383, and 427 nm. The first two high energy bands are due to intraligand π→π*(Ag →B2u, 3b1u → 4b3g) transition. The high intense band at 331and 383 nm are the MLCT transitions (Ag →B2u, 4b2g → 3au) from the filled metal 3dyz and 3dzx orbital to the two lowest-energy ligand-based π* orbital. The lowest energy band at 427 nm is due to A1g →B1g, suggesting the formation of a square planar [Ni(Pic)2] complex.30 The copper(II) picolinato complex (Figure 2e) shows a sharp peak at 233 nm and two low intense peaks at 307 and 354 nm. The first highly intense peak can be attributed to intraligand π→π* transition, and the other two are due to ligand to metal charge transfer transition. In addition to these, it shows MLCT at 434 nm and a broad d−d (2B1g→2E2g) transition at 562 nm.31 The diffuse reflectance (DR) spectrum of dehydrated Co2+exchanged zeolite NaY shows peak at 194 and 210 and 262 nm, which can be assigned to MLCT transition, in the present case from the oxygen atom of the zeolite moiety to a tetracoordinated Co2+ ion.32 The DR spectrum of the Ni2+exchanged zeolite-Y exhibits three bands at about 208, 288, 388, and 726 nm. The first two bands at 208 and 288 nm are due to charge transfer transition originated from a transition from oxygen to Ni2+. This indicates the presence of some tetrahedrally coordinated Ni2+ ions in the supercages. The bands at about 388 and 725 nm are assigned to 3A2g→ 3T1g (F) and 3A2g → 3T1g (P) transitions related to the octahedrally coordinated Ni2+ ions mainly in the hexagonal prisms of the zeolite.33 The dehydrated Cu2+−Y sample exhibits absorption band at 578 nm, characteristic of the e → t2 transition of the Cu(II) (3d9) ion in the trigonal site. The intense UV absorption component is attributed to charge transfer excitation. This spectral behavior of the Cu2+ exchanged zeolites indicates that Cu2+ ion maintains a pseudo-tetrahedral environment of the type (O1)3−Cu2+−L (where O = oxygen of the supercage of zeolite-Y).34 Figure 2b shows the DR-spectrum peak of [Co(Pic)2]Y at 292, 344, 496, and 635 nm. The first intense peak is due to the π→π* transition originated from the ligand system. The next

Intrazeolite mechanism: Ezm + + ne− + nC(+s) ↔ Ez(m − n) + + nC(+z)

(I)

Extrazeolite mechanism: Ezm + + mC(+s) ↔ E(ms)+ + mC(+z) E(ms)+ + ne− ↔ E((sm) − n) +

(II)

(where Em+ = electroactive probe, z = zeolite, s = solution phase, and C+ = an electrolyte cation). The electrochemical data for the intrazeolite metal picolinato complexes are obtained using modified glassy carbon as the working electrode in dichloromethane (DCM) solution under a blanket of nitrogen containing 0.1 M tetrabutyl ammonium phosphate (TBAP) as the supporting electrolyte. The electrochemical behavior of the intrazeolite complexes are compared with the cyclic voltammogram of neat complexes taken in solution mode. The results reflect the redox activity of intrazeolite complexes located in the supercages and held in sufficient proximity to the electronic conductor. D



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Figure 3. (a−c) Cyclic voltammograms for neat picolinato complexes of copper, cobalt, and nickel, respectively, taken in DCM using 0.1 M TBAP as the supporting electrolyte. (d−f) Cyclic voltammograms of metal-exchanged Cu2+−Y, Co2+−Y, and Ni2+−Y, respectively. (g−i) are for encapsulated complexes of copper, cobalt, and nickel, respectively. The cyclic voltammograms for metal-exchanged zeolites and encapsulated complexes are taken as ZMEs.

Cu2+−Y (Epa = 0.185 V; Epc = 0.489 V). The difference in the redox potential values of the [Cu(Pic)2]Y and Cu2+−Y indicates that the redox behavior of the encapsulated complex is not due to surface-bound species or due to species located at the boundary site. Ganesan and Ramaraj40 also reported that polypyridyl metal complexes ([Ru(bpy) 3]2+ and [Fe(bpy)3]2+) synthesized inside the supercages of zeolite-Y shows electrochemical behavior in 0.05 M H2SO4 but are electrochemically inactive in 0.1 M Na2SO4 or in other supporting electrolytes such as LiNO3 or CsNO3. Thus the appearance of cyclic voltammogram in the presence of TBAP ion supports that the [Cu(Pic)2]Y complex is formed on the internal cavity of zeolite-Y, and it follows an intrazeolite electron transfer path. Moreover, the disappearance of the short-lifetime oxidation peak in the encapsulated complexes also revealed the nonexistence of topological redox isomers either in the surface or in the interior part of the zeolite cavity. Doménech et al.41 observed similar redox behavior between the ZMEs of zeolite-

The cyclic voltammogram of neat complexes, metal exchanged zeolites, and those of zeolite-Y encapsulated ones are depicted in Figure 3a−i. The cyclic voltammogram of the neat Cu(II) complex shows two anodic peaks at −0.165 V and −0.061 V and a single cathodic peak at −0.266 V (Figure 3a). However, the first anodic peak vanishes during the second cycle. This indicates that both cis and trans isomers persist in solution for a smaller period of time. In the second cycle, the cis Cu (II) complex undergoes a chemical isomerization reaction to form the corresponding trans isomer. The peak potential obtained from the cyclic voltammogram of the encapsulated Cu(II) complex (Figure 3g) matches nicely with that of the trans isomer of the neat complex. This further revealed the fact that cis−trans isomerism does not occur inside the zeolite cages. Additionally, these results are in accordance with our electron paramagnetic resonance (EPR) and theoretical studies (discussed latter). The redox potential values of the encapsulated complexes are quite different from those of E



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Figure 4. Powder EPR spectra of (a) Co2+−Y , (b) [Co(Pic)2], (c) [Co(Pic)2]Y, (d) Cu2+−Y, (e) [Cu(Pic)2], and (f) [Cu(Pic)2]Y taken at 77 K.

due to steric constraints. The peak broadening is observed after encapsulation of the metal complexes in various zeolites. This is because of axial interaction of the metal complex with different types of O atoms in a given zeolite, so that the metal complex exhibits different redox potentials at different places, leading to peak broadening. This result is in accordance with our theoretical results where we observed a change in geometrical parameters in the encapsulated complexes in comparison to the neat complexes (see Table S2). To ascertain whether different electrochemical responses are due to uncomplexed Co cations or [Co(Pic)2] complex present on the external surface, we recorded a cyclic voltammogram for Co2+-exchanged zeolite NaY. The cyclic voltammogram characteristics and the peak potentials for Co2+-exchanged (Epa = 0.185 V and Epc = 0.489 V; Figure 3e) are entirely different from those of the encapsulated [Co(Pic)2]Y complex. This indicates that the [Co(Pic)2] complex is encapsulated inside the zeolite matrix and not present on the external surface of NaY zeolite. This further suggests the intrazeolite electron transfer process. The neat nickel complex undergoes a reversible and a quasireversible one-electron reductions process corresponding to Ni(II)/Ni(I) and Ni(I)/Ni(0) couples, respectively, as shown in Figure 3c. The Ni(II/I) couple shows a cathodic reduction peak at −0.76 V and the corresponding anodic peak at −0.82 V. The Ni(I)/Ni(0) couple shows the cathodic peak at −0.23 V and the anodic peak at 1.08 V, indicating a highly quasi reversible process. The corresponding zeolite encapsulated Ni(II) complex exhibits a broad cyclic voltammogram, and the peak potential is shifted to more negative values in comparison

Y-associated Mn(salen)N3 in aqueous media and the same complex in solution. They also reported the existence of the topological isomer Mn(Salen)N3 complex and put forward the extrazeolite electron path. Thus the presence of redox behavior in TBAP electrolyte and the nonexistence of redox isomer in the case of the zeolite-Y encapsulated complex can be taken as evidence for assuming that the electron transfer process in [Cu(Pic)2]Y proceeds via the intrazeolite mechanism. The cyclic voltammogram of the neat Co(II) complex taken in solution mode at a scan rate of 0.1 V shows a redox couple with values of Epc = −0.307 V and Epa = 1.14 V, and E1/2 = 0.416 V (Figure 3b). This redox process, when associated with a cathodic peak, is the reduction of [CoII(Pic)2] to cobalt [CoII(Pic)2/Co], and, when associated with an anodic peak, is the oxidation of deposited cobalt metal to the Co cation (Co/ CoII). At different scan rates, there is no major change in E1/2 values, clearly indicating that E1/2 is independent of scan rate. The corresponding zeolite-Y encapsulated Co(II) complex gives a quasi reversible couple with Epc = 0.322 V and Epa = 0.677 V, E1/2 = 0.499 V. On encapsulation, the E1/2 value shifted to positive value, and the peaks are broadened. The shifting of the peak potential toward more positive values on encapsulation indicates the stabilization of Co(II) oxidation state in zeolite cages. This change in the peak potential value may be attributed to axial interaction with the zeolite matrix, which influences the geometry of the complexes. The redox potential of a metal complex encapsulated in a given zeolite is dependent on the axial interaction of the metal complex with an O atom of the zeolite matrix and also distortion of the molecule F



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Table 1. Binding Energy (eV) for Neat and Encapsulated Complexes [M(Pic)2]·H2O

M2+−Y

[M(Pic)2]−Y

state

Co

Ni

Cu

Co

Ni

Cu

Co

Ni

Cu

M2p3/2 M2p1/2 satellite O1s N1s C1s(C−C) (O−CO) Si2s Si2p Al2p Na1s

782.4 785.2

851.6 877.4

782.6 786.3

851.7 878.0

851.5 877.2

531.4 400.2 284.6 287.2

531.4 399.2 284.6 287.2 154.0 102.9 74.4 1073.2

530.1 399.2 284.6 287.2 154.0 102.9 74.4 1073.2

932.5 955.3 943.1 529.9 399.2 284.6 287.2 154.0 102.9 74.4 1073.1

782.5 785.0

531.4 400.2 284.6 287.2

932.4 955.2 941.3 531.4 400.2 284.6 287.2

530.2

531.4

931.3 955.1 941.1 531.4

154.0 103.0 74.5 1073.1

154.0 103.0 74.5 1073.0

154.0 103.0 74.5 1073.2

complex and g∥ = 2.32 and g⊥= 2.10 for the encapsulated complex), it is clear that g∥ > g⊥, which indicates that the structure of the complex is square-planar and that the unpaired electron is predominantly in the dx2−y2 orbital. The ESR spectrum of Co2+−Y-exchanged zeolite, recorded with the sample at 77 K, shows a broad signal having a g value of 4.35. This g value is consistent with a high-spin d7 ion in the weak-field limit. The ESR spectrum of neat cobalt complex recorded at 77 K is depicted in Figure 4a. The spectrum exhibits axial symmetry with well-resolved eight line patterns characteristic of the 59Co (I = 7/2) hyperfine interaction with g⊥ = 2.95, and g∥ = 2.02, similar to those found for a variety of low-spin Co(II) complexes.43 An identical spectrum is observed following the reaction of the Co2+−Y zeolite with the picolinato ligand. Since the spectra are not clearly separated into perpendicular and parallel components, it was not possible to directly determine the anisotropic g and A values. The absence of the broad signal at g = 4.35 in the case of both the neat and the encapsulated complexes indicates the formation of a lowspin square planar [Co(Pic)2] complex. For nickel(II) (d8) ion, being a non-Krammer’s ion, ESR spectra is observable, generally at low temperatures. Most of the reported work concerns the study of this ion at 77 or 4 K.44,45 The g value is isotropic and close to 2.2. In general, if the zerofield splitting (D) is negligible, one would expect and observe a single ESR line. However, in most of the systems, D is nonzero, and hence more than one line has been observed. Generally, when zero-field splitting is large, one would expect forbidden transitions.46 This observation has been found in the case of the Ni-exchanged zeolite-Y (see Supporting Information Figure S2b) but not in the neat and the encapsulated complexes. This further indicates the formation of a low-spin square-planar complex of Ni(II) inside zeolite-Y. 3.1.7. X-ray Photoelectron Spectroscopy (XPS). The location of the complexes in the zeolite cages can also be confirmed by XPS as it provides information about the relative concentrations of elements in the surface ca. 40−50 Å thick layers of the sample (ca. 1% of the crystal).47 The XPS measurements are carried out for various metal-picolinato samples. It is found from comparison of the signal intensities of the M 2p level (M = Co, Ni and Cu) for the encapsulated samples and the metal exchanged samples that the encapsulated complexes contain less concentration of the metal ions than NaY metal exchanged samples. The results obtained are in accordance with our EDX and UV−vis studies. The decrease in the metal content in the encapsulated metal complexes can be attributed to the migration of noncomplexed metal ions under

to the neat complex (Figure 3i). The redox potential values of the encapsulated [Ni(Pic)2]Y is different from those of the Ni2+−Y-exchanged zeolite-Y (Epa = 0.173 V and Epc = 0.0115 V; Figure 3f), suggesting electron transport via the intrazeolite mechanism. 3.1.6. Electron Spin Resonance (ESR) Analysis. The ESR spectrum of the metal-exchanged zeolites, neat and zeoliteencapsulated transition metal complexes are shown in Figure 4a−f. The ESR spectrum of copper-exchanged zeolite taken in a glycerine−water mixture in N2 atmosphere without calcination shows a broad spectrum with g values gII = 2.39 and g⊥ = 2.09. The hyperfine spectrum characteristics of the Cu nucleus with I = 3/2 are not resolved. This result is in accordance with the reported g values obtained from theoretical ESR calculation for tetra-coordinated Cu2+ exchanged zeolite.42 When the sample is calcinated by heating to 250 °C under vacuum, the roomtemperature ESR spectrum (see Supporting Information Figure S2a) exhibited resolved copper hyperfine structure. The EPR signals of neat Cu (II) complexes in the polycrystalline state are usually broadened due to dipolar and spin−spin exchange interactions. Consequently, structural information from the hyperfine and superhyperfine coupling interactions between the unpaired electron of copper and surrounding magnetic nuclei are lost, and the g values are not that of the molecular values. Encapsulation of complexes in the supercages of zeolites results in isolation and dilution of the paramagnetic complex in a diamagnetic aluminosilicate matrix and, hence, is expected to yield resolved signals. For one copper(II) species, the typical 63,65 Cu hyperfine structure quartet is expected due to the interaction of the unpaired electron with the Cu nuclei (63,65Cu, I = 3/2). However, in the spectrum represented in Figure 4e for the neat Cu−picolinato complex in addition to an intense Cu hyperfine structure quartet in the high-field part, there are further lines in the low field region that indicate the presence of a second copper(II) species with a lower concentration. The zeolite-encapsulated Cu- complex, however, shows a wellresolved hyperfine structure quartet. The presence of more spectral lines in the neat complex indicates the existence of cis− trans isomers in the neat complex. The appearance of more number of peaks of lower intensity at low field region indicates the cis−trans equilibrium in neat Cu−picolinato, and this result is in accordance with our cyclic voltametric study. The g-tensor values of Cu(II) complex can be used to derive the ground state. In square-planar complexes having unpaired electrons in the dx2−y2 orbital gives 2B1g as the ground state with g∥ > g⊥ and in the dz2 orbital gives 2A1g as the ground state with g⊥ > g∥. From the observed values (g∥ = 2.21 and g⊥= 2.09 for the neat G



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Figure 5. XPS of (i) (a) Cu2+−Y, (b) [Cu(Pic)2], (c) [Cu(Pic)2]Y; (ii) (d) Co2+−Y, (e) [Co(Pic)2], (f) [Co(Pic)2]Y, and (iii) (g) Ni2+−Y, (h) [Ni(Pic)2], (i) [Ni(Pic)2]Y.

the high-temperature synthesis conditions used in the flexible ligand method. In addition to the information about location of the complexes, some preliminary information about the oxidation states of the metal ion in both the neat and the zeolite encapsulated complexes can be obtained from the XPS data. Table 1 lists the binding energies for M2p, O1s, N1s, C1s, Si2s, Si2p, Al2p, and Na1s in various metal picolinato complexes. In all the picolninate complexes (both neat and encapsulated) two different kinds of carbon atoms (C−C 284.6 eV and O− CO 287.2 eV) and the only one kind of nitrogen atom (399.2 eV) are observed, Figure S3. The core level photoelectron peaks of neat and the encapsulated complexes as well of the metal exchanged zeolites are assigned in Figure 5. The features, such as spin−orbit splitting and shakeup satellite obtained from XPS studies can be used for identifying the transition metals oxidation state.48 The presence of Cu2+/Cu1 species is confirmed by the Cu2p3/2 and Cu2p1/2 peaks at 932.4 and 955.2 eV, respectively accompanied by a relatively low intense satellite peak at 941.3 eV. The intense peak at 932.4 and 932.5 eV in the case of neat and zeolite-Y encapsulated [Cu(Pic)2] complexes can be attributed to the presence of Cu1+, which is close to the one reported earlier for pure Cu2O (932.8 eV).49 It occurs during the acquisition time that X-ray irradiation from XPS caused reduction of the Cu-complexes. According to Batista et al.,50 only the Cu2+ species shows a shakeup satellite peak located about 10 eV higher than the Cu 2p3/2 transition; this characteristic peak is used to differentiate between Cu2+ and reduced copper. Therefore, shakeup features observed at 941.3 and 943.1 eV for the Cu 2p3/2 core levels in neat and encapsulated Cu-complexes, respectively, can be attributed to an open 3d9 shell of Cu2+. Similar to Cucomplexes, the presence of Co2+ and Ni2+ is confirmed by XPS, and the values are given in Table 1. The presence of Co2+ is confirmed by the Co 2p3/2 peak at 782.4 eV in the case of neat complex and 782.6 eV in the case of encapsulated cobalt complex, [Co(Pic)2]Y.51 The Co 2p3/2 binding energy is found to be higher in the case of cobalt exchanged zeolite and encapsulated complex, and this may be due to the influence of zeolite matrix on the effective nuclear charge. The near absence of a strong shakeup structure indicates that the Co(II) is mainly diamagnetic.52 The Ni 2p spectrum of the neat complex shows

a largely separated spin−orbit doublet with BEs of the Ni 2p3/2 and Ni 2p1/2 core levels of 851.6 and 877.4 eV, respectively, whereas that of zeolite-Y encapsulated complex are 851.7 and 878.0 eV, respectively. The value 851.6 eV can be attributed to reduced nickel species. The absence of shakeup satellite structure confirms the diamagnetic nature of Ni(II). The binding energy for nickel(II) is lower than the usual Ni(II) maintaining octahedral geometry. This suggests the formation of square planar complex of nickel inside zeoliteY.51 It can be observed from Table 1 that, in all the samples, the binding energies of Si2s, Al2p, and Na1s remain unchanged. However, small shifts toward higher energies for M2p and toward lower energy for O1s and N1s are observed in all the encapsulated complexes. The high M2p3/2 binding energy found in [M(Pic)2]Y indicates the presence of picolinato complexes inside zeoliteY. This is attributed to the fact that upon encapsulation, the charge density on the metal centers decreases, which could be due to the impairment of the delocalization of the π-electrons of the ring caused by the distortion of the picoline when confined in the zeolite cavity.53 3.1.8. Scanning Electron Microscopy (SEM) Analysis. Formation of M(II) complexes with picolonic acid in zeoliteY is accomplished using a flexible ligand synthesis. The ligands, which are flexible enough to diffuse through the zeolite channels, react with the pre-exchanged metal ions in the super cage to afford the encapsulated complexes. The product material is purified by extensive Soxhlet-extraction with suitable solvents to remove unreacted ligand and surface complexes. The samples did not change their color on purification, indicating that the complexation occurred in the cavities. SE micrographs of bis(picolinato) complexes taken before and after Soxhlet extraction are shown in Figure S4a−c, respectively, as representative cases. The SEM taken before purification shows the presence of some unreacted or extraneous particles on the external surface. In the SEM of finished products, no surface complexes are seen, and the particle boundaries on the external surface of zeolite are clearly distinguishable. This is much clearer from the surface plot shown in Figure S5a−c. The homogeneous surface morphology observed for the neat NaY zeolite and the samples after Soxhlet extraction is shown in Figure S5 (a and c). The surface plot for H



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Figure 6. Schematic representation of the HOMO and LUMO level of the neat and encapsulated phenanthroline complexes showing the change in the HOMO−LUMO gap between (a) [Co(Pic)2] and [Co(Pic)2]Y, (b) [Ni(Pic)2] and [Ni(Pic)2]Y, and (c) [Cu(Pic)2] and [Cu(Pic)2]Y.

S7. The starting bis(picolinato) M(II) (M = Co, Ni, Cu) complexes have a distorted cis conformation where one of the phenyl groups is slightly twisted from the molecular plane. On the contrary, the trans conformation of the complexes have perfectly planar phenyl rings. The transition-state geometries involved in this isomerization process resemble more the reactant (Figure S7a), the cis form, rather than the product, the trans form. The trans conformation of all the complexes are found to be energetically more preferable. The energy difference between the cis and trans conformers for the Cu(II) complex is found to be the least (17.4 kcal mol−1), while that for Co(II) and Ni(II) complexes are found to be higher (23.6 and 22.3 kcal mol−1, respectively). Interestingly, the transition state TSCu for the conversion of cis-bis(picolinato) Cu(II) to the corresponding trans- geometry lies at only 7.2 kcal mol−1, while the transition state TSCo and TSNi lie almost twice and thrice that of TSCu, respectively (Figure S7b). Thus, it is evident from Figure S7 that the cis−trans isomerization for the Cu(II) complex is energetically favorable. In other words, there might be a possible equilibrium between the cis and trans forms of copper complex, while this possibility is less in the case of Co and Ni complexes. This is in tune with cyclic voltammetry measurement as well as EPR spectra for the complexes. Our experimental studies suggest that the metal complexes do not maintain the same geometry on encapsulation into zeolite-Y; hence we also performed all-electron calculations on the encapsulated complexes. The details of the computational methods are provided in the Supporting Information. Herein, we have considered only the trans isomers for all three complexes. The geometrical parameters obtained from VWN/ DN level calculations for the neat and the encapsulated complexes are provided in Table S2. The geometrical parameters such as bond length and bond angles have been compared with the available crystal structures for bis(picolinato) complexes of cobalt(II), nickel(II) and copper(II) and are found to be in good agreement. When these metal complexes with square planar geometry are encapsulated in zeolite-Y, the bond length and the bond angle between the

the samples before Soxhlet extraction however, are found to be nonhomogeneous, indicating that the surface is being occupied by extraneous complexes or the uncomplexed ligands. The presence of a similar surface morphology before and after encapsulation into zeolite-Y confirms that encapsulation has not affected the surface crystallinity. Further, it indicates the efficiency of the purification procedure to effect complete removal of extraneous complexes, leading to well-defined encapsulation in the cavity. 3.1.9. Thermogravimetric Analysis (TGA). The TG patterns of metal exchanged zeoliteY and neat and the encapsulated complexes are shown in Figure S6a−i. All three metalexchanged zeolite-Y’s show only single degradation at 190 °C due to loss of surface hydroxyl group and show no weight loss up to 700 °C, Figure S6g−i. The TGA of the neat [M(Pic)2] [M = Co(II), Ni(II), and Cu(II)] complexes almost shows a similar pattern. The neat complexes mainly show three weight losses at 82, 196, and 248 °C. The first weight loss corresponds to loss of water of crystallization, and the other two weight losses are due to partial sublimation and pyrolitic decomposition of the sample. The comparison of TGA for neat picolinato complexes with that of the encapsulated one shows that these complexes become more stable once they get embedded inside the cavity of zeolite-Y. In case of the encapsulated metal complexes, the weight losses due to sublimation and pyrolytic decomposition of the complexes extends up to 427 °C. These indicate that, on encapsulation, the thermal stability of the complexes are greatly enhanced and hence can be thermally treated without any decomposition. 3.2. Theoretical Calculation. From our experimental studies, it is evident that cobalt and nickel complexes do not undergo cis−trans isomerization, whereas the copper complex is found to undergo such isomerization. Hence, to have an insight into such an isomerization process, we performed density functional thoery (DFT) calculation using the Gaussian03 program at the PBE1/SDD level. The details of the computational methods are provided in the Supporting Information. The isomerization process is shown in Figure I



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studies that the encapsulated Cu-complex serves as a better candidature for catalytic oxidation of phenol. The change in the energies of the frontier orbital as well as the chemical behavior of the complexes on encapsulation can be attributed to the influence of the zeolite matrix. 3.3. Catalytic Study. 3.3.1. Selective Oxidation of Phenol to Catechol. Selective oxidation of phenol is conducted to investigate the potential catalytic ability of neat and encapsulated picolinato complexes. In contrast to the large number of publications dealing with the selective oxidation of phenol compounds in the presence of molecular oxygen or hydrogen peroxide, the number of examples reporting the use of picolinato complexes of transition metals to synthesize catechol selectively as the desirable product is sparse.55 To the best of our knowledge, this is the first example for selective oxidation of phenol to catechol under microwave irradiationcatalyzed metal picolinato encapsulated in zeolite-Y. Microwave enabling selective organic transformations may have advantages over conventional thermal reactions as it avoids the use of acid, bases, and other toxic reagents. Also, in terms of green context, such organic transformations are considered to be environmentally benign. We found that in presence of hydrogen peroxide, metal picolinato complexes of copper and cobalt can selectively oxidize phenol to catechol. Upon encapsulation of the complexes within zeolite-Y, the catalytic activity of the complexes is found to enhance further. Nickel picolinato complexes are found to be inactive under identical conditions. No significant reaction is observed either in the absence of peroxide or in the absence of the catalyst. The progress of the reaction is extremely slow in a nonpolar solvent, such as toluene and hexane. The reaction is found to proceed well in acetonitrile, dimethylformamide (DMF), and DCM with maximum conversion in acetonitrile. Water and ammonia are not chosen for such catalytic conversion, as they have strong tendency to form a hydrogen bond with phenol or, on the other hand, they may block the vacant co-ordination site in the metal complexes.56 So we carry out the entire catalytic reaction in a minimum amount of acetonitrile. The reaction completes with moderate to high yield and selectivity under the influence of microwave irradiation. The same reaction when carried out under identical conditions in the absence of microwave irradiation is found to be very sluggish as observed from thin layer chromatography (TLC). Moreover, catechol is not obtained selectively even after 24h of stirring the reaction under identical conditions. On microwave irradiation, catechol is obtained selectively up to 74% yield within 70 min. The reduction of the reaction time is the result of sudden uncontrollable temperature growth of the reaction mixture under microwave irradiation, which in turn leads to the increase of reaction rates. This indicates that microwaves couple directly with the molecules of the entire reaction mixture, leading to a rapid rise in the temperature, better homogeneity, and selective heating of polar molecule. Furthermore, catalytic oxidation did not occur to a significant extent in the presence of metal salts or in the presence of metal-exchanged zeolites under identical reaction conditions. However, the addition of a stoichiometric amount of 30% H2O2 results in a mixture of products. The formation of the product is monitored through NMR, HPLC analysis, and UV−vis spectroscopy. Figure S8 shows the 13C NMR spectra of a reaction mixture before being subjected to microwave irradiation and after 30 and 70 min of microwave irradiation. As shown in Figure S8a, only four peaks at 158.5,

metal and the ligand molecule slightly changes in comparison to those of the corresponding neat complexes. Quantum chemical calculations have proven that Si−O bonds in zeolites have covalent character.54 Valence electrons in zeolites are distributed all over the framework as a partially delocalized electronic cloud. At relatively short distances between the complex molecule and the walls of the zeolite cavities, the electron−electron repulsions will be operative, which will cause the bond length between the metal ion and the ligand molecule to change. The pattern of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the neat and the encapsulated complexes are shown in Figure 6. It is observed from Figure 6 that encapsulation of the complexes into the zeolite framework increases the energies of the HOMO and the LUMO orbitals in comparison to the neat complexes. However, the HOMO− LUMO gap on encapsulation in copper and cobalt complexes is found to decrease, whereas that of in nickel complex is found to increase in comparison to those for the free complexes. Among all the systems, the HOMO energies of zeolite-Y encapsulated [CoII(Pic)2] and [CuII((Pic)2] complexes are higher in comparison to the other complexes, Table2. This indicates Table 2. Calculated Energy of HOMO, LUMO (in eV), Global Hardness (η, in ev), and Softness (S, in eV) complexes

HOMO

LUMO

η

S

[Cu(Pic)2] [Co(Pic)2] [Ni(Pic)2] [Cu(Pic)2]Y [Co(Pic)2]Y [Ni(Pic)2]Y

−4.919 −5.355 −5.534 −3.429 −3.761 −4.572

−4.219 −4.206 −3.739 −3.920 −2.890 −2.466

0.35 0.574 0.897 0.245 0.435 1.053

1.428 0.870 0.557 1.855 0.954 0.4748

transfer of electrons from these two complexes becomes much more feasible. And this has been reflected in the catalytic ability of these two complexes toward the selective oxidation of phenol to catechol. Applying the Koopmans’ theorem, global hardness and softness values of the neat and encapsulated complexes are calculated. The values are given in Table 2. It can be seen from Table 2 that the global hardness values decrease on encapsulation into the zeolite cavities. According to the maximum hardness principle, the most stable structure has maximum hardness. So the encapsulated complexes with minimum η values will be comparatively less stable and hence more reactive than the neat complexes. Furthermore, on encapsulation, the values of the Fukui functions calculated using the Hirshfeld population analysis are also found to differ from those of the neat complexes. Table S3 presents the Fukui functions (FFs, f + and f −) for the selected metal atoms and the coordinated nitrogen and oxygen atoms. It is seen in Table S3 that the values of the Fukui functions at the Cu center increases upon encapsulation. Inthe case of the cobalt complex, the f + values remains approximately the same as that of the neat complex. However, the f − value decreases at the cobalt metal center. In the case of nickel complex, the Fukui function values almost remain unchanged. These results further indicate that on encapsulation, the copper center becomes a favorable site either for a nucleophilic or electrophillic attack. This further reflects the difference in the catalytic behavior of the two complexes. Additionally it has been found in our experimental J



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Figure 7. (a) UV−vis spectrum showing the conversion of phenol to catechol after an interval of 10 min catalyzed by [Cu(Pic)2]Y in the presence of H2O2. (b) Effect of peroxide amount on the selective oxidation of phenol. Appearance of a second peak above 300 nm indicates the further oxidation of catechol to benzoquinone.

130.2, 121.4, and 115.9 ppm, assigned to the signals of phenol, are observed before microwave irradiation, revealing no hydroxylation of phenol in solution. The 13C NMR spectra of the reaction mixture taken after 30 min show two new peaks at 123.8 and 119.1 ppm, as illustrated in Figure S8b, which are attributed to the 13C NMR signals of catechol. This indicated that phenol is oxidized to catechol gradually. When the solution reacted for 70 min, an obvious peak at 146.7 ppm, assigned to the signal of catechol, is observed in solution, which demonstrates that catechol is the main product (Figure S8c). The 13C NMR spectra of the reaction mixture taken after increasing the amount of peroxide indicates another signal at 137.0 ppm corresponding to that of benzoquinone, as shown in Figure S8 d. From the HPLC analysis it is found that the case of the reaction catalyzed by [Cu(Pic)2]Y and [Co(Pic)2]Y shows two peaks with retention times of 3.684 and 2.265 min, corresponding to those of phenol and catechol, respectively (see Supporting Information Figure S9). In the case of the reaction catalyzed by neat complexes other than phenol and catechol, we observe one more additional peak with a retention time of 3.373 min, corresponding to hydroquinone (see Supporting Information, Figure S9). The reaction catalyzed by metal-exchanged zeolites and metal salts in the presence of peroxides results in a number of peaks, indicating the mixture of products. The wt (%) value of the catechol is found to be highest in the case of the reaction catalyzed by [Cu(Pic)2]Y and hence is considered to be better catalyst. Depending on the HPLC analysis, we monitored the progress of the reaction catalyzed by [Cu(Pic)2]Y catalyst after every 10 min via UV− vis spectroscopy. From the UV−vis spectrum shown in Figure 7a, it is clearly visible that as the reaction precedes, the absorption intensity of the peak at 254 nm corresponding to phenol57 decreases, whereas that at 285 nm corresponding to that of catechol58 increases. Moreover, we observe almost a symmetrical curve that indicates that almost all of the phenol that diffuses into the cavities of zeolites has been selectively converted to catechol. An increase in the amount of catalysts (keeping the amount of substrate) does not affect the conversion to a greater extent. However, on increasing the amount of hydrogen peroxide results, a new peak corresponding to that of benzoquinone as observed in the HPLC analysis. The UV−vis study also shows an additional peak above 300 nm, and its absorption maxima is found to increase as the reaction proceeds (Figure 7b). In order to understand the effect

of peroxide concentration on catechol formation, we further chose three molar ratios of phenol:H2O2, viz., 1:1, 1:2, and 1:3. The maximum yield is found when the molar ratio is 1:2. However, increasing the molar ratio to 1:3 retards the catechol formation. The reason for decreasing the percent of conversion of phenol might be due to dilution of the reaction mixture, since 30% H2O2 has a considerable amount of water. Therefore, the 1:2 molar ratio is considered to be optimum. 3.3.2. Phenol Adsorption Study. The material balance between phenol and the products after the reaction are, of course, not in reasonable agreement. To understand this fact, UV−vis absorption spectroscopy is applied to record the adsorption behavior of the phenol solution before and after treatment with the catalyst (Figure S10a−g). For this we treat 10 mg of catalyst in 10 mL of acetonitrile containing 1 mmol of phenol. The characteristic absorption of phenol at 254 nm is chosen for monitoring the adsorption process. It is found that some amount of phenol gets adsorbed on the surface or remains bound to the metal center via chelate formation. Among all the catalysts, the Ni-complex exhibited the highest adsorption capability for phenol. This accounts for the mass imbalance and catalytic inactivity of nickel catalyst. The comparison of the turn over number (TON) based on metal content is presented in Table 3. It is observed from Table 3 that the [Cu(Pic)2]Y catalyst gives high yield with high TON. 3.3.3. Structure Activity Relationship. Quantitative use of structural and electronic parameters for rationalizing or predicting properties of metal complexes and their catalytic and drug activity has received a great deal of attention.59 So, in order to know the structural activity relationship between the structural and electronic properties of the synthesized neat and encapsulated complexes with the TON, we have performed both simple and multiple linear regression analyses. It can be seen from Figure S11 that the LUMO energy, the E1/2, LUMO energy + E1/2, and metal oxygen bonds of the neat and the encapsulated [Co(Pic)2] and [Cu(Pic)2] complexes have a significant correlation with the TON. The plot of LUMO energies of the metal complexes against TON (Figure S11a) gives an r2 value of 0.89, which is considered to be highly significant as far as the regressional analysis is concerned. The negative slope in the equation suggests that the TON will be enhanced as the value of the LUMO energy becomes much higher lying. It has been explicitly found from our theoretical and experimental study that the encapsulated complexes with K



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Table 3. Oxidation of Phenol by Metal Chlorides, Metal Exchanged Zeolites, and Neat and Encapsulated Picolinato Complexes under Microwave Irradiation in the Presence of H2O2

Scheme 1. A Plausible Catalytic Cycle for Conversion of Phenol to Catechol in the Presence of Metal Picolinato Complex and H2O2a

yield (mmol)b 2+

catalyst

M in mmola

time (min)

catechol

HQ

BQ

TONc

CoCl2·6H2O CuCl2·2H2O Co2+−Y Cu2+−Y [Co(Pic)2] [Cu(Pic)2] [Co(Pic)2]Y [Cu(Pic)2]Y

0.063 0.088 0.054 0.067 0.040 0.044 0.018 0.012

70 70 70 70 70 70 70 70

0.20 0.23 0.21 0.23 0.52 0.54 0.70 0.74

0.15 0.12 0.20 0.18 0.10 0.12

0.10 0.17 0.30 0.32

3 2.6 3.8 3.4 13

12 39 62

a Amount of metal content in 15 mg of the catalyst. bIsolated yield obtained by chromatographic separation. cTON = [amount of catechol(mmol)/metal atom(mmol) per 15 mg of catalyst]. HQ = hydroquinone; BQ = benzoquinone.

higher lying LUMO shows high catalytic activity resulting in high TON. Similarly, the E1/2 and combination of E1/2 with LUMO energy gives the r2 values of 0.84 and 0.99, respectively. This further indicates that the redox potential value of the catalyst highly influences the catalytic oxidation of phenol to catechol, and one can manipulate the catalytic activity of the complexes just by tuning the redox potential values. The positive slope in the equation (Figure S11b,c) indicates that the more positive the E1/2 value, the higher the catalytic enhancement. This is in accordance with our cyclic voltammetry study, where we have observed a positive shift in E1/2 value on encapsulation. Besides the correlation of the electronic properties of the catalyst with the TON, the bond between the metal cation and the oxygen atom of the picolinate ligand shows very good correlation with the TON (Figure S11d). The correlation between the M−O bond length and TON directly implies that change in the M−O bond distance will influence the catalytic ability of the picolinato complexes. It has been also confirmed from our mechanistic study (discussed below) that the M−O bond strongly participates in holding the peroxide moiety in the transition state geometry via O−H bond formation. Hence, any change in the M−O bond distance will strongly influence the transition state geometries and will directly influence the catalytic cycle. 3.3.4. Mechanism of Phenol Oxidation. The mechanisms of the catalytic oxidation reaction have been the subject of intense research, since many reactive species are involved, and their roles vary depending on the catalytic systems. A plausible mechanism for the conversion of phenol to catechol catalyzed by picolinato complexes of Co and Cu in the presence of H2O2 is shown in Scheme 1. An active species ML2*(M = Co or Cu, L = 2-pyridine carboxylate) is first generated quickly in the ML2−H2O2 buffer solution, then the intermediate ML2*S is generated by coordination of phenol to M2+. Finally the catechol is obtained by transfer of oxygen from peroxide to phenol with the simultaneous release of H2O.60,61 To establish the possible path of reactions, 10−3 M solutions of neat cobalt and copper picolinato complexes are first dissolved separately in a minimum amount of acetonitrile and treated individually with a solution of 30% H2O2 prepared in acetonitrile. Spectral changes in the electronic spectra of neat complexes on addition of hydrogen peroxide are monitored through UV−vis spec-

a The possible C−O bind formation and O−O bond breaking process is shown in the closed bracket.

Figure 8. Simple energy profile plot for the proposed catalytic cycle.

troscopy. Gradual addition of H2O2 to the solution of cobalt complex shows a decrease in intensity of the peaks at 388, 437, and 574 nm. The spectral changes are shown in Figure S12a. Three isosbestic points are found at 532, 446, and 355 nm. As shown in Figure S12b, in the case of copper complex, the peak intensity due to MLCT and d−d transitions are also found to diminish. The decrease in intensity of the MLCT and d−d bands on addition of peroxide may be attributed to the charge transfer transition occurring from filled d-orbital of Co and Cu to the vacant π* orbital of H2O2 (Figure S13). These changes in the intensity of the MLCT and d−d transition band indicate the interaction of hydrogen peroxide with Co(II) and Cu(II) metal centers. In order to gain insight into the mechanism of phenol oxidation, we have further carried out DFT calculation on the probable transition (TS) and intermediate (Int) states formed during the catalytic cycle. We examined relative L



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Figure 9. The optimized geometries of the possible transition and intermediate states involved in the conversion of phenol to catechol: (a) CuL2, (b) CoL2 , (c) Cu-TS1, (d) Co-TS1, (e) CuL2*, (f) CoL2*, (g) Cu-TS2, (h) Co-TS2, (i) CoL2*S, (j) CuL2*S, (k) Cu-TS3, and (l) Co-TS3.

energies for conversion of ML 2 →TS1→ML 2 *→TS2→ ML2S*→TS3→ catechol. Starting from the trans bis(picolinato) M(II) [M = Co, Ni, and Cu] complex, we introduced hydrogen peroxide, which immediately leads to the formation of a weakly bound van der Waals complex [ML2].H2O2 (ML2*, Int-1) through TS1. This step has a barrier (ΔE) of 8.8 and 8.0 kcal/mol for Cu and Co complexes, respectively (Figure 8). At the TS1, one of the hydrogen atoms of H2O2 moves toward the oxygen atom of the picolinato ligand and one of the oxygens toward the metal center. The M···O and O···H distances are found to be 2.40 and 3.32 Å, 2.91 and 1.73 Å, respectively, in the case of the Cucomplex and Co-complex (Figure 9). The optimized geometries of the intermediate states of cobalt and copper show that the weakly bound species complexes are formed via the interaction of the metal center with one of the oxygen atom of the peroxide moiety. The M···O and O···H distances are found to be 2.22 Å and 1.69 Å, 2.31 Å and 1.67 Å in Co and Cu

complexes, respectively (Figure 9). Natural bond orbital (NBO) analysis shows that in both systems, there occurs a negligible electron transfer between the H2O2 and ML2 complexes. Unlike Co and Cu picolinato complexes, Ni failed to form ML2* intermediates. This may be attributed to the special stability associated with the square planar 16-electron metal complexes. However, addition of H2O2 to the square planar 15- and 17-electron picolinato complexes of Co and Cu are not prohibited, and, hence, they can easily react with H2O2 to form [M(Pic)2]·H2O2. The addition of H2O2 to ML2 is calculated to be thermodynamically favorable by 14.5 and 13.3 kcal mol−1 for the Co and Cu complexes, respectively. On phenol insertion into the metal peroxide complex (ML2*, Int-2), the H2O2 moves away from the square plane toward the Pic ligand and facilitates the formation of M−O (phenyl) linkage. This step proceeds through TS2 and has a barrier of 7.3 and 8.5 kcal/mol for Cu and Co-complexes, respectively (Figure 8). However, M



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catechol to benzoquinone. A plausible catalytic cycle is proposed on the basis of UV−vis study and DFT calculation. Both the studies suggest that the reaction precedes via the formation of a [M(Pic) 2 ]·H 2 O 2 (M = Co and Cu) intermediate.

the peroxide is bound to the O-atom of the picolinato ligand through one of the H-atoms. At this stage, the O−H bond distance is found to decrease from 1.67 to 1.58 in the case of the copper complex and from 1.69 to 1.64 Å in the case of the cobalt complex. The formation of ML2*S is found to be equally favorable for both complexes. The last step of the mechanism involves the transfer of oxygen from peroxide to phenol (TS3) leading to the formation of catechol and liberation of water and subsequent regeneration of the square planar complex ML2. The last step of the mechanism is basically a C−H bond activation process taking place by transfer of oxygen from hydrogen peroxide to phenol. This step is the rate-determining step. This process proceeds via an intermolecular oxidation− reduction reaction between the substrate phenol and H2O2. Homolytic cleavage results in rapid dissociation to give catechol, regenerating the complex in the catalytic cycle. A similar kind of mechanism has been recently proposed by Modi et al. and Liu et al.60,61 This step has a barrier of 8.2 and 21.3 kcal/mol for the Cu- and Co-complexes, respectively (Figure 8) and is found to be favorable by 33.3 and 34.4 kcal mol−1 for the Co- and Cu-complexes, respectively. It can be observed from Figure 8 that the formation of ML2* and ML2*S is not energetically costly in the case of Cu-complex. However, in case of the cobalt complex, the last step, i.e., the transfer of oxygen from peroxide to phenol leading to catechol formation, involves a high energy barrier, and this brings out a difference in the rate of the catalytic oxidation mediated by the two complexes. In addition to the trans isomer, we also studied the catalytic cycle starting with the cis geometry of the Cu-complex, as it is likely that both cis and trans isomers of the Cu-complex may coexist in the reaction mixture. However, addition of H2O2 to cis ML2 complex results in a ML2* similar to that obtained by starting with the trans geometry. This further revealed that, even though both isomers may exist in solution, after addition of peroxide, the catalytic cycle will be governed by the stable trans Cu-complex.

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ASSOCIATED CONTENT

* Supporting Information S

Materials and physical measurement, details of computational methods, FTIR spectra, EPR spectra of calcined Cu2+ and Ni2+exchanged zeolite, SEM, surface plot, TGA, cis−trans isomerzation, 13C NMR of reaction mixture, HPLC analysis of the reaction catalyzed by neat and encapsulated complexes, phenol adsorption, structure−activity relationship plot, and UV−vis and orbital picture showing the interaction of H2O2 with Cu and Co picolinato complexes. This information is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Science and Technology, New Delhi, for financial support. REFERENCES

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