Zeolite‑Y Encapsulated Metal Picolinato Complexes as Catalyst forOxidation of Phenol with Hydrogen PeroxideKusum 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 hasbeen 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 counterpartsare compared under microwave irradiation in the presence of H2O2 as mild oxidant. Catecholis obtained selectively in good yield when the catalytic oxidation is carried out usingbis(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 tobenzoquinone. Electron paramagnetic resonance (EPR) and cyclic voltammetry studiesreveal the existence of cis → trans isomerization in case of Cu(II) complex, which has beenfurther substantiated by DFT calculations. A plausible mechanism for the formation ofcatechol mediated by synthesized complexes has been provided by means of DFTcalculations from the energetics involved in the transformations.

1. INTRODUCTION

The contemporary interest in the selective oxidation of phenolto catechol, to a great extent, has been fueled by its utility innearly every sector of chemical industries including pharma-ceuticals, agrochemicals, flavors, polymerization inhibitors, andantioxidants.1 Worldwide, the catechol production from phenolinvolves two procedures: (i) ortho-formylation of phenolfollowed by subsequent oxidation, and (ii) oxidation of phenolsto o-quinones and subsequent reduction of the latter intocatechol. These multi step routes are often lengthy, energy-intensive, and generate a large number of oxidized, coupling,and polymerized products.2 To meet the increasing demand forcatechol and to satisfy environmental requirements, consid-erable efforts have been made for producing catechol by theone-step hydroxylation method using various homogeneousand heterogeneous transition metal complexes.3−5 The utilityof hydroxidation is mainly measured by several factors,including selectivity and ecological sustainability of theoxidation and the availability of the oxidant and catalyst.Among various oxidants, H2O2 of less than 60% concentrationhas been recognized as an ideal, clean, and green oxidizingagent.6−8

In recent years, it has been established that zeoliteencapsulated organometallic compounds and transition metalcomplexes, in addition to chiral metal complexes andbiocatalysts, can be highly selective and efficient catalysts. Asa consequence, the application of intrazeolite complexes israpidly gaining importance in organic transformations,complementing bio and organometallic catalysis.9 The stericconstrain imposed by the walls of the zeolite plays a vital role inmodifying the properties, viz., magnetic, electronic, and redoxbehavior of the encapsulated complexes.10−12 These changes in

the properties of the transition metal complexes uponencapsulation have led various researchers to develop newerheterogeneous catalyst and apply them in various organictransformations including the asymmetric synthesis. Besideshaving wide advantages of these heterogeneous catalysts overthe homogeneous counterparts, the microwave-assisted cata-lytic transformation in the presence of these hybrid catalysts arenow found to be most atom economic in comparison to theconventional methods.13 In the context of the growing generalinterest in microwave-assisted organic synthesis, we plan here,to perform selective oxidation of phenol to catechol by fewhomogeneous and zeolite-Y encapsulated transition metalpicolinato complexes under the influence of microwaveirradiation.The picolinato complexes of transitional metals have been

determined to exhibit a broad spectrum of physiological effectson the activity functions of both animal and plants organisms.14

Additionally, in recent years the complexes, especially ofvanadium,15 zinc,16 manganese,17 copper,18 chromium,19 andtungsten,20 are known to have in vitro insulin mimetic activityand in vivo antidiabetic ability. However, the potential of thesetype of complexes as catalyst in organic transformations is lessexplored.21 There are only few reports on the encapsulation ofpicolinato complexes into the cavity of zeolite-Y and on theirapplication as a heterogeneous catalyst.22 We report herein thefirst systematic experimental and theoretical studies on theselective oxidation of phenol under microwave irradiation by

Received: March 10, 2013Revised: May 9, 2013

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

2. EXPERIMENTAL SECTION2.1. 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 reportedprocedure.23 An aqueous solution of the free ligand is preparedby dissolving the free acid (picolonic acid) in a slight excess ofdilute sodium hydroxide solution and adjusting the pH to 6−7with dilute acid. The filtered ligand solution is then added to anaqueous solution of MCl2·xH2O in a 2:1 ligand−metal moleratio. The blue-violet solid for copper complex is washed withacetone and then ethanol, and finally dried in vacuum. Thepurple 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 NaYzeolite 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 usingbuffer tablets in order to prevent the precipitation of metal ionsas hydroxides. The slurry is then filtered, washed with distilledwater until the silver ion test for chloride is negative, and finallydried overnight in an oven at 200−250 °C to obtain Co2+-exchanged NaY as pink powder, Ni-exchanged NaY as palegreen powder, and Cu-exchanged zeolite NaY as light bluepowder.2.3. Encapsulation of Bis(picolinato) M(II) Complexes

in Metal Exchanged Zeolite-Y, [M(Pic)2]Y. The metal-exchanged zeolites (represented as Co2+−Y, Ni2+−Y, andCu2+−Y) are treated individually with stoichoimetric excess ofsodium salt of pyridyl 2-carboxylic acid in 50 mL deionizedwater. The mixture is refluxed for 48 h at 90 °C under constantstirring. On heating, the solid mass changed color from lightpink to dark pink in the case of cobalt, light green to sky blue inthe case of nickel, and from light blue to blue-violet in the caseof copper. The zeolite encapsulated complexes are then filtered,washed repeatedly with deionized water, and dried at roomtemperature under vacuum. The resultant products are furtherpurified by Soxhlet extraction using the sequence of solventsacetone, methanol, and finally with diethyl ether to remove anyunreacted species or species adsorbed on the surface of thezeolite crystallites. The color of the resultant solid does notchange even after repeated Soxhlet extraction. This observationgives a preliminary idea about the formation of complexesinside the cavity of zeolite-Y. The products are dried undervacuum 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]Ypowder.2.4. Catalytic Oxidation of Phenol. To carry out the

catalytic oxidation of phenol, the catalyst (15 mg) is first treatedwith a stoichiometric amount of 30% H2O2 and stirred for 10min in nitrogen atmosphere. To this a solution ofstoichiometric amount of phenol prepared in acetonitrile isadded, and the whole reaction mixture is subjected tomicrowave irradiation (280 W). The progress of the reactionis monitored by TLC and ultraviolet−visible (UV−vis)spectroscopy after an interval of 10 min. The solid catalyst isextracted by filtration, and the crude reaction mixture isquenched with saturated aqueous NH4Cl and extracted with

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

3. RESULTS AND DISCUSSION3.1. Experimental Section. 3.1.1. Elemental Analysis. In

order to confirm the presence of bis(picolinato) complexes ofCo(II), Ni(II), and Cu(II) inside zeolite-Y various physico-chemical and spectrochemical analyses are performed. At firstwe perform the elemental detection by energy dispersive X-rayspectroscopy (EDX) and determine the metal content in theencapsulated systems via UV−vis technique. The results of theelemental analyses obtained from the EDX study gives a Si/Alratio of 2.76, which corresponds to a unit cell formula Na52[(AlO2)52(SiO2)140] for parent NaY. The Si/Al ratio hasremained unchanged in all metal-exchanged zeolites, indicatingthe absence of dealumination during exchange process. Theamount of metal contents in the neat and the intrazeolitecomplexes are obtained by Vogels method24 and are found tobe less compared to the metal-exchanged zeolites. The decreasein the metal content during complex formation inside thezeolite cavity can be attributed to the participation of metal ionin the formation of co-ordination complexes inside the cavitiesof zeolite-Y.

3.1.2. X-ray Diffraction (XRD) Studies. The powder X-raydiffraction patterns of the zeolite NaY, metal-exchanged zeolitesand the encapsulated metal picolinato complexes are shown inFigure 1. Essentially similar diffraction patterns are noticed in

the encapsulated complexes and NaY, except that the zeoliteencapsulated [M(Pic)2] (M = Co, Cu, and Ni) complexes haveslightly weaker intensities. These observations indicate that theframework of the zeolite does not suffer any significantstructural changes during encapsulation. However, there aredifferences in the relative peak intensities of the 220 and 311reflections appearing at 2θ = 10 and 12°, respectively. For purezeolite-Y and for M2+-exchanged zeolite-Y, I220 > I311, but forthe encapsulated complexes, I311 > I220. This reversal inintensities has been empirically correlated with the presence ofa large complex within the zeolite-Y supercage.25 This change

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

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in the relative intensities may be associated with theredistribution of randomly coordinated free cations in zeolite-Y at sites II and I. The above observation may therefore beconstrued as evidence for the successful encapsulation of metalpicolinato 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 neatand encapsulated picolinato complexes are shown in Figure S1(Supporting Information). It is evident from the FTIR spectrathat the framework vibration band of zeolite-Y dominate thespectra below 1200 cm−1 for all samples. FTIR spectra of NaYand metal-exchanged zeolites (Figure S1a,b, respectively) showstrong zeolite lattice bands in the range 500−1200 cm−1. Thestrong and broad band at the region 1010−1045 cm−1 could beattributed to the asymmetric stretching vibrations of (Si/Al) O4units. The broad bands in the region 1650 and 3500 cm−1 aredue to lattice water molecules and surface hydroxylic groups,respectively. The parent NaY zeolite shows characteristicsbands at 556, 690, and 1010 cm−1 (Figure S1, curve a) and areattributed to T−O bending mode, symmetric stretching, andantisymmetric vibrations, respectively.26 These bands are notmodified during the ion exchange with metal cations (Co2+,Ni2+, and Cu2+ or by supporting the metal complexes (FigureS1, curve b). The absence of shifting of the characteristicvibrational bands of zeolite framework on metal exchanged orencapsulation of transition metal picolinato complexes impliesthat the zeolite framework has remained unchanged uponencapsulation of complexes. However, there is obviously adifference in the range of 1200−1600 cm−1 among the threeencapsulated complexes (see Table S1). It can also be observed

from Figure S1c−f that the IR bands of all encapsulatedcomplexes are weak due to their low concentration in thezeolite cage and thus can only absorb in the region where thezeolite matrix does not show any absorption band that lie in1200−1600 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 Theband at 966 cm−1 attributable to δ(O−H) acid of picolinicacid,27 is absent in both the neat and encapsulated systems. Theabsence of this peak further reveals the nonexistence ofextraneous picolinic acid on the external surface of zeolite-Yencapsulated complexes. Moreover, no additional bands areobserved that could ascribe the coordination modes to bedifferent from bidentate.The presence of similar peak positions in all the encapsulated

and neat crystalline complexes gives indirect evidence for thepresence 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 theeffect of zeolite matrix on the geometry of the picolinatocomplexes trapped in the zeolite supercages. Besides thesebands at 770 and 685 cm−1 assigned to the out-of-plane γ(CH)vibrations and out-of-plane ϕ(CC) ring deformation aroundthe pyridine get shifted to a lower wavenumber (Table S1) inthe case of the encapsulated complexes. This further suggeststhat the pyridine rings of [M(Pic)2]−Y in the supercage are

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 spectraof metal-exchanged zeolites are shown in the inset for the respective cases.

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located under different conditions. Such changes may be causedby distortion of the encapsulated complexes inside the zeolitesupercages or with a difference in coordination by the −OHgroups of the zeolite-Y.3.1.4. UV−Vis/Diffuse Reflectance Spectroscopy (UV−vis/

DRS). The UV−vis spectrum of the neat and encapsulatedpicolinato complexes are shown in Figure 2. The spectra of themetal exchanged zeolites are shown in the inset of therespective cases. The UV−vis spectrum of bis(picolinato)Co(II) complex (Figure 2a) shows two intense peak in theregion 267 and 368 nm due to intraligand and metal-to-ligandcharge transfer (MLCT) transitions, i.e., π→π* and dπ→pπ*,respectively. The highest energy d−d transition from the lower-lying fully occupied 3dx2−y2 orbital to the upper empty 3dyzorbital (2A1g →

2B1g, transition) at 432 nm is obscured by theabove MLCT transitions. The peak at 574 nm is again mainlydue to the 2A1g →

2B1g transition supporting the square-planargeometry of the [Co(Pic)2] complex. The electronic spectra areclearly distinct from both the well-known tetrahedral andoctahedral species. The tetrahedral [CoCl4]

2− has two featuresat around 650 (ε = 550) and 675 nm (590 M−1cm−1) with asmaller feature at 600 nm (360 M−1cm−1).28 The octahedral[M(H2O)6]

2+ complexes have distinct absorbance at 560 nmfor 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 tointraligand π→π*(Ag →B2u, 3b1u → 4b3g) transition. The highintense band at 331and 383 nm are the MLCT transitions (Ag→B2u, 4b2g → 3au) from the filled metal 3dyz and 3dzx orbital tothe two lowest-energy ligand-based π* orbital. The lowestenergy band at 427 nm is due to A1g →B1g, suggesting theformation of a square planar [Ni(Pic)2] complex.30 Thecopper(II) picolinato complex (Figure 2e) shows a sharppeak at 233 nm and two low intense peaks at 307 and 354 nm.The first highly intense peak can be attributed to intraligandπ→π* transition, and the other two are due to ligand to metalcharge transfer transition. In addition to these, it shows MLCTat 434 nm and a broad d−d (2B1g→

2E2g) transition at 562nm.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 casefrom the oxygen atom of the zeolite moiety to a tetra-coordinated 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 tocharge transfer transition originated from a transition fromoxygen to Ni2+. This indicates the presence of sometetrahedrally coordinated Ni2+ ions in the supercages. Thebands at about 388 and 725 nm are assigned to 3A2g→

3T1g (F)and 3A2g → 3T1g (P) transitions related to the octahedrallycoordinated Ni2+ ions mainly in the hexagonal prisms of thezeolite.33 The dehydrated Cu2+−Y sample exhibits absorptionband at 578 nm, characteristic of the e → t2 transition of theCu(II) (3d9) ion in the trigonal site. The intense UVabsorption component is attributed to charge transferexcitation. This spectral behavior of the Cu2+ exchangedzeolites indicates that Cu2+ ion maintains a pseudo-tetrahedralenvironment of the type (O1)3−Cu2+−L (where O = oxygen ofthe supercage of zeolite-Y).34

Figure 2b shows the DR-spectrum peak of [Co(Pic)2]Y at292, 344, 496, and 635 nm. The first intense peak is due to theπ→π* transition originated from the ligand system. The next

two low intense bands are due to n→π* transitions associatedwith the ligand and are being veiled by the MLCT associatedwith Co2+-exchanged zeolite. The other two bands are mainlydue to dπ→pπ*(MLCT) and d−d transition (2A1g →

2B1g). Theencapsulated 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 and1A1g →1B1g transitions, respectively. The DRS/UV−visspectrum of encapsulated copper complex is shown in Figure2f. The absorption peaks in the range 280−308 nm are due tointraligand transition. The bands at 366 and 426 nm are due toMLCT transition. The peaks at 503 and 602 nm can beattributed to MLCT and d−d transitions, respectively.The comparison of the absorption spectrum of the neat

complexes with those of the encapsulated complexesdemonstrates that encapsulation results in a blue shift in theelectronic transitions associated with the nickel complex. In thecase of the corresponding cobalt and copper complex, the peaksare found to be red-shifted under the influence of the zeolitematrix. This further gives an idea that all the three complexesdo not undergo a similar kind of structural change insidezeolite-Y. However, the presence of the similar electronictransitions in the encapsulated complexes in comparison to theneat complex in solution gives evidence for the formation ofcomplexes inside the supercage of zeolite-Y.

3.1.5. Cyclic Voltammetry Study. The cyclic voltammetrystudy of the intrazeolite complexes as a component of thezeolite-modified electrode (ZME) has gained interest in recentyears for probing the presence of redox active species inside thezeolite cavity.9a,10,35 In the case of the “ship-in-a-bottle”complexes, the electroactive complex already encapsulatedundergoes electron transfer within the zeolite cavities. Shaw etal.36 first proposed two possible mechanismsintrazeolite andextrazeolite mechanisms (eqs I and II, respectively)forelectron transfer associated with encapsulated transition metalcomplexes within the supercage of zeolite and surface-boundmetal complexes, respectively, based on ZMEs. Although, thesemechanistic assignments are controversial, there are variousreports demonstrating the redox behavior of transition metalcomplexes encapsulated in zeolite cavities.37−39

Intrazeolite mechanism:

+ + ↔ ++ − + − + +E n nC E nCezm

s zm n

z( )( )

( ) (I)

Extrazeolite mechanism:

+ ↔ +

+ ↔

+ + + +

+ − − +

E mC E mC

E n Ee

zm

s sm

z

sm

sm n

( ) ( ) ( )

( ) ( )( )

(II)

(where Em+ = electroactive probe, z = zeolite, s = solutionphase, and C+ = an electrolyte cation).The electrochemical data for the intrazeolite metal picolinato

complexes are obtained using modified glassy carbon as theworking electrode in dichloromethane (DCM) solution under ablanket of nitrogen containing 0.1 M tetrabutyl ammoniumphosphate (TBAP) as the supporting electrolyte. The electro-chemical behavior of the intrazeolite complexes are comparedwith the cyclic voltammogram of neat complexes taken insolution mode. The results reflect the redox activity ofintrazeolite complexes located in the supercages and held insufficient proximity to the electronic conductor.

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The cyclic voltammogram of neat complexes, metalexchanged zeolites, and those of zeolite-Y encapsulated onesare depicted in Figure 3a−i. The cyclic voltammogram of theneat 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 secondcycle. This indicates that both cis and trans isomers persist insolution for a smaller period of time. In the second cycle, the cisCu (II) complex undergoes a chemical isomerization reactionto form the corresponding trans isomer. The peak potentialobtained from the cyclic voltammogram of the encapsulatedCu(II) complex (Figure 3g) matches nicely with that of thetrans isomer of the neat complex. This further revealed the factthat cis−trans isomerism does not occur inside the zeolite cages.Additionally, these results are in accordance with our electronparamagnetic resonance (EPR) and theoretical studies(discussed latter). The redox potential values of theencapsulated complexes are quite different from those of

Cu2+−Y (Epa = 0.185 V; Epc = 0.489 V). The difference in theredox potential values of the [Cu(Pic)2]Y and Cu2+−Yindicates that the redox behavior of the encapsulated complexis not due to surface-bound species or due to species located atthe boundary site. Ganesan and Ramaraj40 also reported thatpolypyridyl metal complexes ([Ru(bpy) 3]

2+ and [Fe(bpy)3]2+)

synthesized inside the supercages of zeolite-Y shows electro-chemical behavior in 0.05 M H2SO4 but are electrochemicallyinactive in 0.1 M Na2SO4 or in other supporting electrolytessuch as LiNO3 or CsNO3. Thus the appearance of cyclicvoltammogram in the presence of TBAP ion supports that the[Cu(Pic)2]Y complex is formed on the internal cavity ofzeolite-Y, and it follows an intrazeolite electron transfer path.Moreover, the disappearance of the short-lifetime oxidationpeak in the encapsulated complexes also revealed thenonexistence of topological redox isomers either in the surfaceor in the interior part of the zeolite cavity. Domenech et al.41

observed similar redox behavior between the ZMEs of zeolite-

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

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Y-associated Mn(salen)N3 in aqueous media and the samecomplex in solution. They also reported the existence of thetopological isomer Mn(Salen)N3 complex and put forward theextrazeolite electron path. Thus the presence of redox behaviorin TBAP electrolyte and the nonexistence of redox isomer inthe case of the zeolite-Y encapsulated complex can be taken asevidence 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 couplewith values of Epc = −0.307 V and Epa = 1.14 V, and E1/2 =0.416 V (Figure 3b). This redox process, when associated witha cathodic peak, is the reduction of [CoII(Pic)2] to cobalt[CoII(Pic)2/Co], and, when associated with an anodic peak, isthe oxidation of deposited cobalt metal to the Co cation (Co/CoII). At different scan rates, there is no major change in E1/2values, clearly indicating that E1/2 is independent of scan rate.The corresponding zeolite-Y encapsulated Co(II) complexgives a quasi reversible couple with Epc = 0.322 V and Epa =0.677 V, E1/2 = 0.499 V. On encapsulation, the E1/2 valueshifted to positive value, and the peaks are broadened. Theshifting of the peak potential toward more positive values onencapsulation indicates the stabilization of Co(II) oxidationstate in zeolite cages. This change in the peak potential valuemay be attributed to axial interaction with the zeolite matrix,which influences the geometry of the complexes. The redoxpotential of a metal complex encapsulated in a given zeolite isdependent on the axial interaction of the metal complex with anO atom of the zeolite matrix and also distortion of the molecule

due to steric constraints. The peak broadening is observed afterencapsulation of the metal complexes in various zeolites. This isbecause of axial interaction of the metal complex with differenttypes of O atoms in a given zeolite, so that the metal complexexhibits different redox potentials at different places, leading topeak broadening. This result is in accordance with ourtheoretical results where we observed a change in geometricalparameters in the encapsulated complexes in comparison to theneat complexes (see Table S2). To ascertain whether differentelectrochemical responses are due to uncomplexed Co cationsor [Co(Pic)2] complex present on the external surface, werecorded a cyclic voltammogram for Co2+-exchanged zeoliteNaY. The cyclic voltammogram characteristics and the peakpotentials for Co2+-exchanged (Epa = 0.185 V and Epc = 0.489V; Figure 3e) are entirely different from those of theencapsulated [Co(Pic)2]Y complex. This indicates that the[Co(Pic)2] complex is encapsulated inside the zeolite matrixand not present on the external surface of NaY zeolite. Thisfurther suggests the intrazeolite electron transfer process.The neat nickel complex undergoes a reversible and a quasi-

reversible one-electron reductions process corresponding toNi(II)/Ni(I) and Ni(I)/Ni(0) couples, respectively, as shownin Figure 3c. The Ni(II/I) couple shows a cathodic reductionpeak 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 Vand the anodic peak at 1.08 V, indicating a highly quasireversible process. The corresponding zeolite encapsulatedNi(II) complex exhibits a broad cyclic voltammogram, and thepeak potential is shifted to more negative values in comparison

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.

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to the neat complex (Figure 3i). The redox potential values ofthe encapsulated [Ni(Pic)2]Y is different from those of theNi2+−Y-exchanged zeolite-Y (Epa = 0.173 V and Epc = 0.0115 V;Figure 3f), suggesting electron transport via the intrazeolitemechanism.3.1.6. Electron Spin Resonance (ESR) Analysis. The ESR

spectrum of the metal-exchanged zeolites, neat and zeolite-encapsulated transition metal complexes are shown in Figure4a−f. The ESR spectrum of copper-exchanged zeolite taken in aglycerine−water mixture in N2 atmosphere without calcinationshows 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 thereported g values obtained from theoretical ESR calculation fortetra-coordinated Cu2+ exchanged zeolite.42 When the sample iscalcinated by heating to 250 °C under vacuum, the room-temperature ESR spectrum (see Supporting Information FigureS2a) exhibited resolved copper hyperfine structure. The EPRsignals of neat Cu (II) complexes in the polycrystalline state areusually broadened due to dipolar and spin−spin exchangeinteractions. Consequently, structural information from thehyperfine and superhyperfine coupling interactions between theunpaired electron of copper and surrounding magnetic nucleiare lost, and the g values are not that of the molecular values.Encapsulation of complexes in the supercages of zeolites resultsin isolation and dilution of the paramagnetic complex in adiamagnetic aluminosilicate matrix and, hence, is expected toyield resolved signals. For one copper(II) species, the typical63,65Cu hyperfine structure quartet is expected due to theinteraction of the unpaired electron with the Cu nuclei (63,65Cu,I = 3/2). However, in the spectrum represented in Figure 4e forthe neat Cu−picolinato complex in addition to an intense Cuhyperfine structure quartet in the high-field part, there arefurther lines in the low field region that indicate the presence ofa second copper(II) species with a lower concentration. Thezeolite-encapsulated Cu- complex, however, shows a well-resolved hyperfine structure quartet. The presence of morespectral lines in the neat complex indicates the existence of cis−trans isomers in the neat complex. The appearance of morenumber of peaks of lower intensity at low field region indicatesthe cis−trans equilibrium in neat Cu−picolinato, and this resultis in accordance with our cyclic voltametric study. The g-tensorvalues of Cu(II) complex can be used to derive the groundstate. In square-planar complexes having unpaired electrons inthe dx2−y2 orbital gives

2B1g as the ground state with g∥ > g⊥ andin 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

complex and g∥ = 2.32 and g⊥= 2.10 for the encapsulatedcomplex), it is clear that g∥ > g⊥, which indicates that thestructure of the complex is square-planar and that the unpairedelectron 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 valueof 4.35. This g value is consistent with a high-spin d7 ion in theweak-field limit. The ESR spectrum of neat cobalt complexrecorded at 77 K is depicted in Figure 4a. The spectrumexhibits axial symmetry with well-resolved eight line patternscharacteristic of the 59Co (I = 7/2) hyperfine interaction withg⊥ = 2.95, and g∥ = 2.02, similar to those found for a variety oflow-spin Co(II) complexes.43 An identical spectrum is observedfollowing the reaction of the Co2+−Y zeolite with the picolinatoligand. Since the spectra are not clearly separated intoperpendicular and parallel components, it was not possible todirectly determine the anisotropic g and A values. The absenceof the broad signal at g = 4.35 in the case of both the neat andthe encapsulated complexes indicates the formation of a low-spin 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 thereported 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 zero-field splitting (D) is negligible, one would expect and observe asingle 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 forbiddentransitions.46 This observation has been found in the case of theNi-exchanged zeolite-Y (see Supporting Information FigureS2b) but not in the neat and the encapsulated complexes. Thisfurther indicates the formation of a low-spin square-planarcomplex of Ni(II) inside zeolite-Y.

3.1.7. X-ray Photoelectron Spectroscopy (XPS). Thelocation of the complexes in the zeolite cages can also beconfirmed by XPS as it provides information about the relativeconcentrations of elements in the surface ca. 40−50 Å thicklayers of the sample (ca. 1% of the crystal).47 The XPSmeasurements are carried out for various metal-picolinatosamples. It is found from comparison of the signal intensities ofthe M 2p level (M = Co, Ni and Cu) for the encapsulatedsamples and the metal exchanged samples that the encapsulatedcomplexes contain less concentration of the metal ions thanNaY metal exchanged samples. The results obtained are inaccordance with our EDX and UV−vis studies. The decrease inthe metal content in the encapsulated metal complexes can beattributed to the migration of noncomplexed metal ions under

Table 1. Binding Energy (eV) for Neat and Encapsulated Complexes

[M(Pic)2]·H2O [M(Pic)2]−Y M2+−Y

state Co Ni Cu Co Ni Cu Co Ni Cu

M2p3/2 782.4 851.6 932.4 782.6 851.7 932.5 782.5 851.5 931.3M2p1/2 785.2 877.4 955.2 786.3 878.0 955.3 785.0 877.2 955.1satellite 941.3 943.1 941.1O1s 531.4 531.4 531.4 531.4 530.1 529.9 530.2 531.4 531.4N1s 400.2 400.2 400.2 399.2 399.2 399.2C1s(C−C) 284.6 284.6 284.6 284.6 284.6 284.6(O−CO) 287.2 287.2 287.2 287.2 287.2 287.2Si2s 154.0 154.0 154.0 154.0 154.0 154.0Si2p 102.9 102.9 102.9 103.0 103.0 103.0Al2p 74.4 74.4 74.4 74.5 74.5 74.5Na1s 1073.2 1073.2 1073.1 1073.1 1073.0 1073.2

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the high-temperature synthesis conditions used in the flexibleligand method. In addition to the information about location ofthe complexes, some preliminary information about theoxidation states of the metal ion in both the neat and thezeolite encapsulated complexes can be obtained from the XPSdata. 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−CO 287.2 eV) and the only one kind of nitrogen atom(399.2 eV) are observed, Figure S3. The core level photo-electron peaks of neat and the encapsulated complexes as wellof the metal exchanged zeolites are assigned in Figure 5. Thefeatures, such as spin−orbit splitting and shakeup satelliteobtained from XPS studies can be used for identifying thetransition metals oxidation state.48 The presence of Cu2+/Cu1

species is confirmed by the Cu2p3/2 and Cu2p1/2 peaks at 932.4and 955.2 eV, respectively accompanied by a relatively lowintense satellite peak at 941.3 eV. The intense peak at 932.4 and932.5 eV in the case of neat and zeolite-Y encapsulated[Cu(Pic)2] complexes can be attributed to the presence ofCu1+, which is close to the one reported earlier for pure Cu2O(932.8 eV).49 It occurs during the acquisition time that X-rayirradiation from XPS caused reduction of the Cu-complexes.According to Batista et al.,50 only the Cu2+ species shows ashakeup satellite peak located about 10 eV higher than the Cu2p3/2 transition; this characteristic peak is used to differentiatebetween Cu2+ and reduced copper. Therefore, shakeup featuresobserved at 941.3 and 943.1 eV for the Cu 2p3/2 core levels inneat and encapsulated Cu-complexes, respectively, can beattributed to an open 3d9 shell of Cu2+. Similar to Cu-complexes, the presence of Co2+ and Ni2+ is confirmed by XPS,and the values are given in Table 1. The presence of Co2+ isconfirmed by the Co 2p3/2 peak at 782.4 eV in the case of neatcomplex and 782.6 eV in the case of encapsulated cobaltcomplex, [Co(Pic)2]Y.

51 The Co 2p3/2 binding energy is foundto be higher in the case of cobalt exchanged zeolite andencapsulated complex, and this may be due to the influence ofzeolite matrix on the effective nuclear charge. The near absenceof a strong shakeup structure indicates that the Co(II) is mainlydiamagnetic.52 The Ni 2p spectrum of the neat complex shows

a largely separated spin−orbit doublet with BEs of the Ni 2p3/2and Ni 2p1/2 core levels of 851.6 and 877.4 eV, respectively,whereas that of zeolite-Y encapsulated complex are 851.7 and878.0 eV, respectively. The value 851.6 eV can be attributed toreduced nickel species. The absence of shakeup satellitestructure confirms the diamagnetic nature of Ni(II). Thebinding energy for nickel(II) is lower than the usual Ni(II)maintaining octahedral geometry. This suggests the formationof square planar complex of nickel inside zeoliteY.51 It can beobserved from Table 1 that, in all the samples, the bindingenergies of Si2s, Al2p, and Na1s remain unchanged. However,small shifts toward higher energies for M2p and toward lowerenergy for O1s and N1s are observed in all the encapsulatedcomplexes. The high M2p3/2 binding energy found in[M(Pic)2]Y indicates the presence of picolinato complexesinside zeoliteY. This is attributed to the fact that uponencapsulation, the charge density on the metal centersdecreases, which could be due to the impairment of thedelocalization of the π-electrons of the ring caused by thedistortion 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 zeolite-Y is accomplished using a flexible ligand synthesis. The ligands,which are flexible enough to diffuse through the zeolitechannels, react with the pre-exchanged metal ions in the supercage to afford the encapsulated complexes. The productmaterial is purified by extensive Soxhlet-extraction with suitablesolvents to remove unreacted ligand and surface complexes.The samples did not change their color on purification,indicating that the complexation occurred in the cavities. SEmicrographs of bis(picolinato) complexes taken before andafter Soxhlet extraction are shown in Figure S4a−c,respectively, as representative cases. The SEM taken beforepurification shows the presence of some unreacted orextraneous particles on the external surface. In the SEM offinished products, no surface complexes are seen, and theparticle boundaries on the external surface of zeolite are clearlydistinguishable. This is much clearer from the surface plotshown in Figure S5a−c. The homogeneous surface morphologyobserved for the neat NaY zeolite and the samples after Soxhletextraction is shown in Figure S5 (a and c). The surface plot for

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.

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the samples before Soxhlet extraction however, are found to benonhomogeneous, indicating that the surface is being occupiedby extraneous complexes or the uncomplexed ligands. Thepresence of a similar surface morphology before and afterencapsulation into zeolite-Y confirms that encapsulation hasnot affected the surface crystallinity. Further, it indicates theefficiency of the purification procedure to effect completeremoval of extraneous complexes, leading to well-definedencapsulation in the cavity.3.1.9. Thermogravimetric Analysis (TGA). The TG patterns

of metal exchanged zeoliteY and neat and the encapsulatedcomplexes are shown in Figure S6a−i. All three metal-exchanged zeolite-Y’s show only single degradation at 190 °Cdue to loss of surface hydroxyl group and show no weight lossup 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 asimilar pattern. The neat complexes mainly show three weightlosses at 82, 196, and 248 °C. The first weight loss correspondsto loss of water of crystallization, and the other two weightlosses are due to partial sublimation and pyrolitic decom-position of the sample. The comparison of TGA for neatpicolinato complexes with that of the encapsulated one showsthat these complexes become more stable once they getembedded inside the cavity of zeolite-Y. In case of theencapsulated metal complexes, the weight losses due tosublimation and pyrolytic decomposition of the complexesextends up to 427 °C. These indicate that, on encapsulation,the thermal stability of the complexes are greatly enhanced andhence can be thermally treated without any decomposition.3.2. Theoretical Calculation. From our experimental

studies, it is evident that cobalt and nickel complexes do notundergo cis−trans isomerization, whereas the copper complex isfound to undergo such isomerization. Hence, to have an insightinto such an isomerization process, we performed densityfunctional thoery (DFT) calculation using the Gaussian03program at the PBE1/SDD level. The details of thecomputational methods are provided in the SupportingInformation. The isomerization process is shown in Figure

S7. The starting bis(picolinato) M(II) (M = Co, Ni, Cu)complexes have a distorted cis conformation where one of thephenyl groups is slightly twisted from the molecular plane. Onthe contrary, the trans conformation of the complexes haveperfectly planar phenyl rings. The transition-state geometriesinvolved in this isomerization process resemble more thereactant (Figure S7a), the cis form, rather than the product, thetrans form. The trans conformation of all the complexes arefound to be energetically more preferable. The energydifference between the cis and trans conformers for the Cu(II)complex is found to be the least (17.4 kcal mol−1), while thatfor Co(II) and Ni(II) complexes are found to be higher (23.6and 22.3 kcal mol−1, respectively). Interestingly, the transitionstate TSCu for the conversion of cis-bis(picolinato) Cu(II) tothe corresponding trans- geometry lies at only 7.2 kcal mol−1,while the transition state TSCo and TSNi lie almost twice andthrice that of TSCu, respectively (Figure S7b). Thus, it isevident from Figure S7 that the cis−trans isomerization for theCu(II) complex is energetically favorable. In other words, theremight be a possible equilibrium between the cis and trans formsof copper complex, while this possibility is less in the case of Coand Ni complexes. This is in tune with cyclic voltammetrymeasurement as well as EPR spectra for the complexes.Our experimental studies suggest that the metal complexes

do not maintain the same geometry on encapsulation intozeolite-Y; hence we also performed all-electron calculations onthe encapsulated complexes. The details of the computationalmethods are provided in the Supporting Information. Herein,we have considered only the trans isomers for all threecomplexes. The geometrical parameters obtained from VWN/DN level calculations for the neat and the encapsulatedcomplexes are provided in Table S2. The geometricalparameters such as bond length and bond angles have beencompared 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 metalcomplexes with square planar geometry are encapsulated inzeolite-Y, the bond length and the bond angle between the

Figure 6. Schematic representation of the HOMO and LUMO level of the neat and encapsulated phenanthroline complexes showing the change inthe 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.

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metal and the ligand molecule slightly changes in comparisonto those of the corresponding neat complexes. Quantumchemical calculations have proven that Si−O bonds in zeoliteshave covalent character.54 Valence electrons in zeolites aredistributed all over the framework as a partially delocalizedelectronic cloud. At relatively short distances between thecomplex molecule and the walls of the zeolite cavities, theelectron−electron repulsions will be operative, which will causethe bond length between the metal ion and the ligand moleculeto change.The pattern of the highest occupied molecular orbital

(HOMO) and the lowest unoccupied molecular orbital(LUMO) for the neat and the encapsulated complexes areshown in Figure 6. It is observed from Figure 6 thatencapsulation of the complexes into the zeolite frameworkincreases the energies of the HOMO and the LUMO orbitals incomparison to the neat complexes. However, the HOMO−LUMO gap on encapsulation in copper and cobalt complexes isfound to decrease, whereas that of in nickel complex is found toincrease in comparison to those for the free complexes. Amongall the systems, the HOMO energies of zeolite-Y encapsulated[CoII(Pic)2] and [CuII((Pic)2] complexes are higher incomparison to the other complexes, Table2. This indicates

transfer of electrons from these two complexes becomes muchmore feasible. And this has been reflected in the catalytic abilityof these two complexes toward the selective oxidation ofphenol to catechol.Applying the Koopmans’ theorem, global hardness and

softness values of the neat and encapsulated complexes arecalculated. The values are given in Table 2. It can be seen fromTable 2 that the global hardness values decrease onencapsulation into the zeolite cavities. According to themaximum hardness principle, the most stable structure hasmaximum hardness. So the encapsulated complexes withminimum η values will be comparatively less stable and hencemore reactive than the neat complexes. Furthermore, onencapsulation, the values of the Fukui functions calculatedusing the Hirshfeld population analysis are also found to differfrom those of the neat complexes. Table S3 presents the Fukuifunctions (FFs, f + and f −) for the selected metal atoms and thecoordinated nitrogen and oxygen atoms. It is seen in Table S3that the values of the Fukui functions at the Cu center increasesupon encapsulation. Inthe case of the cobalt complex, the f +

values remains approximately the same as that of the neatcomplex. However, the f − value decreases at the cobalt metalcenter. In the case of nickel complex, the Fukui function valuesalmost remain unchanged. These results further indicate thaton encapsulation, the copper center becomes a favorable siteeither for a nucleophilic or electrophillic attack. This furtherreflects the difference in the catalytic behavior of the twocomplexes. Additionally it has been found in our experimental

studies that the encapsulated Cu-complex serves as a bettercandidature for catalytic oxidation of phenol. The change in theenergies of the frontier orbital as well as the chemical behaviorof the complexes on encapsulation can be attributed to theinfluence of the zeolite matrix.

3.3. Catalytic Study. 3.3.1. Selective Oxidation of Phenolto Catechol. Selective oxidation of phenol is conducted toinvestigate the potential catalytic ability of neat andencapsulated picolinato complexes. In contrast to the largenumber of publications dealing with the selective oxidation ofphenol compounds in the presence of molecular oxygen orhydrogen peroxide, the number of examples reporting the useof picolinato complexes of transition metals to synthesizecatechol selectively as the desirable product is sparse.55 To thebest of our knowledge, this is the first example for selectiveoxidation of phenol to catechol under microwave irradiation-catalyzed metal picolinato encapsulated in zeolite-Y. Microwaveenabling selective organic transformations may have advantagesover 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 environ-mentally benign.We found that in presence of hydrogen peroxide, metal

picolinato complexes of copper and cobalt can selectivelyoxidize phenol to catechol. Upon encapsulation of thecomplexes within zeolite-Y, the catalytic activity of thecomplexes is found to enhance further. Nickel picolinatocomplexes are found to be inactive under identical conditions.No significant reaction is observed either in the absence ofperoxide or in the absence of the catalyst. The progress of thereaction is extremely slow in a nonpolar solvent, such astoluene and hexane. The reaction is found to proceed well inacetonitrile, dimethylformamide (DMF), and DCM withmaximum conversion in acetonitrile. Water and ammonia arenot chosen for such catalytic conversion, as they have strongtendency to form a hydrogen bond with phenol or, on theother hand, they may block the vacant co-ordination site in themetal complexes.56 So we carry out the entire catalytic reactionin a minimum amount of acetonitrile. The reaction completeswith moderate to high yield and selectivity under the influenceof microwave irradiation. The same reaction when carried outunder identical conditions in the absence of microwaveirradiation is found to be very sluggish as observed from thinlayer chromatography (TLC). Moreover, catechol is notobtained selectively even after 24h of stirring the reactionunder identical conditions. On microwave irradiation, catecholis obtained selectively up to 74% yield within 70 min. Thereduction of the reaction time is the result of suddenuncontrollable temperature growth of the reaction mixtureunder microwave irradiation, which in turn leads to the increaseof reaction rates. This indicates that microwaves couple directlywith the molecules of the entire reaction mixture, leading to arapid rise in the temperature, better homogeneity, and selectiveheating of polar molecule. Furthermore, catalytic oxidation didnot occur to a significant extent in the presence of metal salts orin the presence of metal-exchanged zeolites under identicalreaction conditions. However, the addition of a stoichiometricamount of 30% H2O2 results in a mixture of products. Theformation of the product is monitored through NMR, HPLCanalysis, and UV−vis spectroscopy. Figure S8 shows the 13CNMR spectra of a reaction mixture before being subjected tomicrowave irradiation and after 30 and 70 min of microwaveirradiation. As shown in Figure S8a, only four peaks at 158.5,

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] −4.919 −4.219 0.35 1.428[Co(Pic)2] −5.355 −4.206 0.574 0.870[Ni(Pic)2] −5.534 −3.739 0.897 0.557[Cu(Pic)2]Y −3.429 −3.920 0.245 1.855[Co(Pic)2]Y −3.761 −2.890 0.435 0.954[Ni(Pic)2]Y −4.572 −2.466 1.053 0.4748

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130.2, 121.4, and 115.9 ppm, assigned to the signals of phenol,are observed before microwave irradiation, revealing nohydroxylation of phenol in solution. The 13C NMR spectra ofthe reaction mixture taken after 30 min show two new peaks at123.8 and 119.1 ppm, as illustrated in Figure S8b, which areattributed to the 13C NMR signals of catechol. This indicatedthat phenol is oxidized to catechol gradually. When the solutionreacted for 70 min, an obvious peak at 146.7 ppm, assigned tothe signal of catechol, is observed in solution, whichdemonstrates that catechol is the main product (Figure S8c).The 13C NMR spectra of the reaction mixture taken afterincreasing the amount of peroxide indicates another signal at137.0 ppm corresponding to that of benzoquinone, as shown inFigure S8 d. From the HPLC analysis it is found that the caseof the reaction catalyzed by [Cu(Pic)2]Y and [Co(Pic)2]Yshows 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 thereaction catalyzed by neat complexes other than phenol andcatechol, we observe one more additional peak with a retentiontime of 3.373 min, corresponding to hydroquinone (seeSupporting Information, Figure S9). The reaction catalyzedby metal-exchanged zeolites and metal salts in the presence ofperoxides results in a number of peaks, indicating the mixtureof products. The wt (%) value of the catechol is found to behighest in the case of the reaction catalyzed by [Cu(Pic)2]Y andhence is considered to be better catalyst. Depending on theHPLC analysis, we monitored the progress of the reactioncatalyzed by [Cu(Pic)2]Y catalyst after every 10 min via UV−vis spectroscopy. From the UV−vis spectrum shown in Figure7a, it is clearly visible that as the reaction precedes, theabsorption intensity of the peak at 254 nm corresponding tophenol57 decreases, whereas that at 285 nm corresponding tothat of catechol58 increases. Moreover, we observe almost asymmetrical curve that indicates that almost all of the phenolthat diffuses into the cavities of zeolites has been selectivelyconverted to catechol. An increase in the amount of catalysts(keeping the amount of substrate) does not affect theconversion to a greater extent. However, on increasing theamount of hydrogen peroxide results, a new peak correspond-ing to that of benzoquinone as observed in the HPLC analysis.The UV−vis study also shows an additional peak above 300nm, and its absorption maxima is found to increase as thereaction proceeds (Figure 7b). In order to understand the effect

of peroxide concentration on catechol formation, we furtherchose 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 catecholformation. The reason for decreasing the percent of conversionof 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 balancebetween phenol and the products after the reaction are, ofcourse, not in reasonable agreement. To understand this fact,UV−vis absorption spectroscopy is applied to record theadsorption behavior of the phenol solution before and aftertreatment with the catalyst (Figure S10a−g). For this we treat10 mg of catalyst in 10 mL of acetonitrile containing 1 mmol ofphenol. The characteristic absorption of phenol at 254 nm ischosen for monitoring the adsorption process. It is found thatsome amount of phenol gets adsorbed on the surface orremains bound to the metal center via chelate formation.Among all the catalysts, the Ni-complex exhibited the highestadsorption capability for phenol. This accounts for the massimbalance and catalytic inactivity of nickel catalyst. Thecomparison of the turn over number (TON) based on metalcontent is presented in Table 3. It is observed from Table 3 thatthe [Cu(Pic)2]Y catalyst gives high yield with high TON.

3.3.3. Structure Activity Relationship. Quantitative use ofstructural and electronic parameters for rationalizing orpredicting properties of metal complexes and their catalyticand drug activity has received a great deal of attention.59 So, inorder to know the structural activity relationship between thestructural and electronic properties of the synthesized neat andencapsulated complexes with the TON, we have performedboth simple and multiple linear regression analyses. It can beseen from Figure S11 that the LUMO energy, the E1/2, LUMOenergy + E1/2, and metal oxygen bonds of the neat and theencapsulated [Co(Pic)2] and [Cu(Pic)2] complexes have asignificant correlation with the TON. The plot of LUMOenergies of the metal complexes against TON (Figure S11a)gives an r2 value of 0.89, which is considered to be highlysignificant as far as the regressional analysis is concerned. Thenegative slope in the equation suggests that the TON will beenhanced as the value of the LUMO energy becomes muchhigher lying. It has been explicitly found from our theoreticaland experimental study that the encapsulated complexes with

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 ofH2O2. (b) Effect of peroxide amount on the selective oxidation of phenol. Appearance of a second peak above 300 nm indicates the further oxidationof catechol to benzoquinone.

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higher lying LUMO shows high catalytic activity resulting inhigh TON. Similarly, the E1/2 and combination of E1/2 withLUMO energy gives the r2 values of 0.84 and 0.99, respectively.This further indicates that the redox potential value of thecatalyst highly influences the catalytic oxidation of phenol tocatechol, and one can manipulate the catalytic activity of thecomplexes just by tuning the redox potential values. Thepositive slope in the equation (Figure S11b,c) indicates that themore positive the E1/2 value, the higher the catalyticenhancement. This is in accordance with our cyclicvoltammetry study, where we have observed a positive shiftin E1/2 value on encapsulation. Besides the correlation of theelectronic properties of the catalyst with the TON, the bondbetween the metal cation and the oxygen atom of the picolinateligand shows very good correlation with the TON (FigureS11d). The correlation between the M−O bond length andTON directly implies that change in the M−O bond distancewill influence the catalytic ability of the picolinato complexes. Ithas been also confirmed from our mechanistic study (discussedbelow) that the M−O bond strongly participates in holding theperoxide moiety in the transition state geometry via O−H bondformation. Hence, any change in the M−O bond distance willstrongly influence the transition state geometries and willdirectly influence the catalytic cycle.3.3.4. Mechanism of Phenol Oxidation. The mechanisms of

the catalytic oxidation reaction have been the subject of intenseresearch, since many reactive species are involved, and theirroles vary depending on the catalytic systems. A plausiblemechanism for the conversion of phenol to catechol catalyzedby picolinato complexes of Co and Cu in the presence of H2O2is shown in Scheme 1. An active species ML2*(M = Co or Cu,L = 2-pyridine carboxylate) is first generated quickly in theML2−H2O2 buffer solution, then the intermediate ML2*S isgenerated by coordination of phenol to M2+. Finally thecatechol is obtained by transfer of oxygen from peroxide tophenol with the simultaneous release of H2O.

60,61 To establishthe possible path of reactions, 10−3 M solutions of neat cobaltand copper picolinato complexes are first dissolved separatelyin a minimum amount of acetonitrile and treated individuallywith a solution of 30% H2O2 prepared in acetonitrile. Spectralchanges in the electronic spectra of neat complexes on additionof hydrogen peroxide are monitored through UV−vis spec-

troscopy. Gradual addition of H2O2 to the solution of cobaltcomplex 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. Asshown in Figure S12b, in the case of copper complex, the peakintensity due to MLCT and d−d transitions are also found todiminish. The decrease in intensity of the MLCT and d−dbands on addition of peroxide may be attributed to the chargetransfer transition occurring from filled d-orbital of Co and Cuto the vacant π* orbital of H2O2 (Figure S13). These changesin the intensity of the MLCT and d−d transition band indicatethe interaction of hydrogen peroxide with Co(II) and Cu(II)metal centers. In order to gain insight into the mechanism ofphenol oxidation, we have further carried out DFT calculationon the probable transition (TS) and intermediate (Int) statesformed during the catalytic cycle. We examined relative

Table 3. Oxidation of Phenol by Metal Chlorides, MetalExchanged Zeolites, and Neat and Encapsulated PicolinatoComplexes under Microwave Irradiation in the Presence ofH2O2

yield (mmol)b

catalystM2+ inmmola

time(min) catechol HQ BQ TONc

CoCl2·6H2O 0.063 70 0.20 0.15 0.10 3CuCl2·2H2O 0.088 70 0.23 0.12 0.17 2.6Co2+−Y 0.054 70 0.21 0.20 0.30 3.8Cu2+−Y 0.067 70 0.23 0.18 0.32 3.4[Co(Pic)2] 0.040 70 0.52 0.10 13[Cu(Pic)2] 0.044 70 0.54 0.12 12[Co(Pic)2]Y 0.018 70 0.70 39[Cu(Pic)2]Y 0.012 70 0.74 62

aAmount of metal content in 15 mg of the catalyst. bIsolated yieldobtained by chromatographic separation. cTON = [amount ofcatechol(mmol)/metal atom(mmol) per 15 mg of catalyst]. HQ =hydroquinone; BQ = benzoquinone.

Scheme 1. A Plausible Catalytic Cycle for Conversion ofPhenol to Catechol in the Presence of Metal PicolinatoComplex and H2O2

a

aThe possible C−O bind formation and O−O bond breaking processis shown in the closed bracket.

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

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energies for conversion of ML2→TS1→ML2*→TS2→ML2S*→TS3→ catechol.Starting from the trans bis(picolinato) M(II) [M = Co, Ni,

and Cu] complex, we introduced hydrogen peroxide, whichimmediately leads to the formation of a weakly bound van derWaals complex [ML2].H2O2 (ML2*, Int-1) through TS1. Thisstep has a barrier (ΔE) of 8.8 and 8.0 kcal/mol for Cu and Cocomplexes, respectively (Figure 8). At the TS1, one of thehydrogen atoms of H2O2 moves toward the oxygen atom of thepicolinato ligand and one of the oxygens toward the metalcenter. The M···O and O···H distances are found to be 2.40and 3.32 Å, 2.91 and 1.73 Å, respectively, in the case of the Cu-complex and Co-complex (Figure 9). The optimized geo-metries of the intermediate states of cobalt and copper showthat the weakly bound species complexes are formed via theinteraction of the metal center with one of the oxygen atom ofthe peroxide moiety. The M···O and O···H distances are foundto 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 anegligible electron transfer between the H2O2 and ML2

complexes. Unlike Co and Cu picolinato complexes, Ni failedto form ML2* intermediates. This may be attributed to thespecial stability associated with the square planar 16-electronmetal complexes. However, addition of H2O2 to the squareplanar 15- and 17-electron picolinato complexes of Co and Cuare 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 forthe Co and Cu complexes, respectively. On phenol insertioninto the metal peroxide complex (ML2*, Int-2), the H2O2

moves away from the square plane toward the Pic ligand andfacilitates the formation of M−O (phenyl) linkage. This stepproceeds through TS2 and has a barrier of 7.3 and 8.5 kcal/molfor Cu and Co-complexes, respectively (Figure 8). However,

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.

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the peroxide is bound to the O-atom of the picolinato ligandthrough one of the H-atoms. At this stage, the O−H bonddistance is found to decrease from 1.67 to 1.58 in the case ofthe copper complex and from 1.69 to 1.64 Å in the case of thecobalt complex. The formation of ML2*S is found to be equallyfavorable for both complexes. The last step of the mechanisminvolves the transfer of oxygen from peroxide to phenol (TS3)leading to the formation of catechol and liberation of water andsubsequent regeneration of the square planar complex ML2.The last step of the mechanism is basically a C−H bondactivation process taking place by transfer of oxygen fromhydrogen peroxide to phenol. This step is the rate-determiningstep. This process proceeds via an intermolecular oxidation−reduction reaction between the substrate phenol and H2O2.Homolytic cleavage results in rapid dissociation to givecatechol, regenerating the complex in the catalytic cycle. Asimilar kind of mechanism has been recently proposed by Modiet al. and Liu et al.60,61 This step has a barrier of 8.2 and 21.3kcal/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 theCo- and Cu-complexes, respectively. It can be observed fromFigure 8 that the formation of ML2* and ML2*S is notenergetically costly in the case of Cu-complex. However, in caseof the cobalt complex, the last step, i.e., the transfer of oxygenfrom peroxide to phenol leading to catechol formation, involvesa high energy barrier, and this brings out a difference in the rateof 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 islikely that both cis and trans isomers of the Cu-complex maycoexist in the reaction mixture. However, addition of H2O2 tocis ML2 complex results in a ML2* similar to that obtained bystarting with the trans geometry. This further revealed that,even though both isomers may exist in solution, after additionof peroxide, the catalytic cycle will be governed by the stabletrans Cu-complex.

4. CONCLUSIONSIn summary, three square planar picolinato complexes ofCo(II), Ni(II) and Cu(II) are synthesize inside zeolite-Y andtheir encapsulation is ensured by different studies. Thespectroscopic and theoretical studies revealed that the wallsof the zeolite framework imparts space constrain to the metalcomplexes, which modifies their structural, electronic, andcatalytic behavior. Under microwave irradiation and in thepresence of hydrogen peroxide these metal complexes facilitatethe selective oxidation of phenol to catechol with moderate togood yield. UV−vis and DFT-based studies allow us toconclude that the H2O2 interacts with the transition metalcomplexes via an electron transfer process occurring from theoccupied metal d-orbital to the π*-orbital of H2O2. Out of thethree metal complexes, the bis(picolinato) Cu(II) and Co(II)being 15- and 17-electron species, respectively, participateactively in this electron back-donation process and hence showbetter catalytic activity in comparison to the corresponding 16-electron Ni(II) complex. In the case of the reaction catalyzedby neat metal complexes, catechol is found to be the majoroxidation product along with hydroquinone as the minorproduct. The selective oxidation of phenol to catechol is foundto enhance in the presence of the encapsulated complexes. Theamount of H2O2 is found to affect the selectivity of the reaction.UV−vis and HPLC analyses suggest that increasing theconcentration of hydrogen peroxide further oxidizes the

catechol to benzoquinone. A plausible catalytic cycle isproposed on the basis of UV−vis study and DFT calculation.Both the studies suggest that the reaction precedes via theformation of a [M(Pic)2]·H2O2 (M = Co and Cu)intermediate.

■ ASSOCIATED CONTENT*S Supporting InformationMaterials and physical measurement, details of computationalmethods, FTIR spectra, EPR spectra of calcined Cu2+ and Ni2+-exchanged zeolite, SEM, surface plot, TGA, cis−trans isomer-zation, 13C NMR of reaction mixture, HPLC analysis of thereaction catalyzed by neat and encapsulated complexes, phenoladsorption, structure−activity relationship plot, and UV−visand orbital picture showing the interaction of H2O2 with Cuand Co picolinato complexes. This information is available freeof charge via the Internet at http://pubs.acs.org

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ramesh@tezu.ernet.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the Department of Science and Technology,New Delhi, for financial support.

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