Inorganica Chimica Acta 362 (2009) 3715–3724

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Inorganica Chimica Acta

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Synthesis, characterization and catalytic oxidation of cyclohexane using a novelhost (zeolite-Y)/guest (binuclear transition metal complexes) nanocompositematerials

Masoud Salavati-Niasari a,b,*, Zohreh Salimi b, Mahdi Bazarganipour b, Fatemeh Davar b

a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iranb Department of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 December 2008Received in revised form 5 April 2009Accepted 17 April 2009Available online 3 May 2009

Keywords:Nanocomposite materialsZeolite encapsulationOctahydro-Schiff base complexOxidation of cyclohexaneNanopores of zeolite

0020-1693/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.ica.2009.04.028

* Corresponding author. Address: Institute of Nano SUniversity of Kashan, Kashan, P.O. Box 87317-51167,+98 361 5555 333; fax: +98 361 555 29 30.

E-mail address: salavati@kashanu.ac.ir (M. Salavat

Transition metal (M = Mn(II), Co(II), Ni(II) and Cu(II)) complexes with octahydro-Schiff base (H4-N4O4) =2,7,13,18-tetramethyl-3,6,14,17-tetraazatricyclo-[17.3.1.1]-tetracosa-1(23),2,6,8(24),9,11,13,17,19,21-decaene-9,11,20,22-tetraol; H4([H]8-N4O4) = 2,7,13,,18-tetramethyl-3,6,14,17-tetraazatricyclo-[17.13.1.1.]-tetracosa-1(23),8(24),9,11,19,21-hexane-9,11,20,22-tetraol) have been encapsulated in nanopores ofzeolite-Y; [M([H]8-N4O4)]@NaY; with Flexible Ligand Method (FLM) for the first time. The new Host-Guest Nanocomposite Materials (HGNM) was characterized by several techniques: chemical analysis,spectroscopic methods (DRS, FT-IR and UV/Vis), BET technique, conductometric and magnetic measure-ments. The catalytic activities for oxidation of cyclohexane with HGNM complexes are reported. Zeoliteencapsulated octahydro-Schiff base copper(II) complex; [Cu([H]8–N4O4)]@NaY; was found to be moreactive than the corresponding cobalt(II), manganese(II) and nickel(II) complexes for cyclohexane oxida-tion. The catalytic properties of the complexes are influenced by their geometry and by the steric envi-ronment of the active sites. HGNM are stable enough to be reused and are suitable to be utilized aspartial oxidation catalysts. The encapsulated catalysts systems; [M([H]8-N4O4)]@NaY; were more activethan the corresponding neat complexes; [M([H]8-N4O4))].

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The encapsulation of transition metal coordination complexesand organometallics within the voids of nanoporous zeolite has at-tracted attention since it provides a simple way of coupling thereactivity of the metal complex with the robustness and stereo-chemistry of the host zeolite [1–3]. Encapsulation provides a con-venient route for the heterogenization of homogeneous catalyticprocesses, and indeed, these compounds have found applicationin catalysis and gas purification [4–17]. These hybrid catalysts offerthe advantage of shape selectivity and site isolation, as provided bythe zeolite matrix, while retaining the solution reactivity of themetal complex. The ‘‘ship-in-a-bottle” complexes, which for stericreasons have to be assembled in situ by bringing the metal and li-gand species within the voids of the zeolite, are a fascinating classof encapsulated compounds [18]. Once assembled these complexescannot be removed without destroying the lattice. It has beenwidely recognized that space constraints imposed by the zeoliteas well as specific interactions with the zeolite framework can in-

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cience and Nano Technology,Islamic Republic of Iran. Tel.:

i-Niasari).

duce structural and functional modifications of the complex ascompared to its solution or solid-state properties, the most visiblemanifestation of which are changes in the reactivity and catalyticproperties. There are number of well-documented examples inwhich the catalytic activity of the encapsulated complex is eitherenhanced or more selective as compared to the same complex insolution [19–24].

Catalytic oxidation of C–H bonds in saturated hydrocarbons isone of the key steps in functionalizing hydrocarbons and rapidlybuilding functionality into a range of molecules [25]. This is be-cause alcohols and ketones, like cyclohexanone, are importantintermediate materials for the manufacture of many importantproducts, such as fiber, drugs and fragrance. However, most of suchoxidation reactions are still using homogeneous catalysts. From thesustainable and green chemistry point of view, heterogeneous cat-alysts would be attractive since they offer the advantages of easycatalyst separation, possible catalyst recycle and sometimes highactivity and selectivity. In this respect, encapsulation of transitionmetal complex in zeolites gained much interest in the last decade[26–33] since this process can give rise to the materials with bothhomogeneous-catalysis and heterogeneous-catalysis characters. Inaddition, degradation of the organic ligands, and dimerization ofthe transition metal complexes could occur during homogeneouscatalysis reactions, resulting in a reduction in the activity, and even

3716 M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724

irreversible deactivation. In contrast, upon encapsulation in zeo-lites, transition metal complex molecules are encaged and site-iso-lated, making these complexes stable, highly active and selectivefor the oxidation of alkanes, alkenes, alcohols and so on.

Encapsulation of Schiff base complexes could be considered asthe most extensively studied in the research field of ship-in-a-bot-tle materials as this type of complexes has a flexible conformationwith various geometries, viz. planar, umbrella type and steppedconfigurations, and as a consequence could generate different ac-tive site environments for various oxidation reactions. However,the amount of the encapsulated Schiff base complex is very limitedowing to the presence of relatively rigid C@N bond. In contrast,

MeOH

NaBH4

M(CH3COO)2.xH2O

M = Mn(II), Co(II),

H4[N4O4]

H4([H8]-N4O4)

[M2([H8]-N4O4)]

M(II)-NaY

[M2(N4O4)]

M(CH3COO)2.xH2O

CH3

OHOH

CH3

CH3

OHOH

CH3

NH

NH

N

H

NH

M M

CH3

NN

OO

CH3

CH3

NN

OO

CH3

H

H

H

H

CH3

OHOH

CH3

N

NN

CH3

OHOH

CH3

N

M M

CH3

N

OO

CH3

CH3

NN

OO

CH3

N

OH

O

CH3

NH2

NH2

Scheme 1. Schematic diagram illustrating the form

hydrogenation of C@N to C–N (Scheme 1), as expected, would in-crease N basicity and make the conformation of the complex moreflexible, and hence resulting in the more ready coordination to me-tal centers in a folded fashion, and thus the inclusion of more tran-sition metal complex molecules, or active sites without severeblockage of the channels in zeolitic matrix. In addition, it has beenshown that transition metal tetrahydro-salophen complex (M–[H4]salophen) is also much more active than [Cu–salophen] inthe homogeneous oxidation of cyclohexane [34]. This would makethe [M–([H4]salophen)]–Y be an effective heterogenous catalystfor the oxidation of such a type of organic substrate. Here, we reportthe synthesis and characterization of series of [M([H]8–N4O4)]–Y

Cu(II), Ni(II)

[M2([H8]-N4O4)]-NaY

M M

CH3

OO

CH3

CH3

OO

CH3

N

N

N

N

H

HH

H

OHOH(CH3CO)2O , ZnCl2

OH

O

CH3

ation of host-guest nanocomposite materials.

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724 3717

catalysts as well as their catalytic performance in the oxidation ofcyclohexane.

2. Experimental

2.1. Materials

All the solvents were purchased from Merck (pro analysis) andwere distilled and dried using molecular sieves (Linda 4 Å). Manga-nese(II), copper(II), nickel(II) and cobalt(II) acetate, resorcinol, ace-tic anhydride, ethylenediamine monohydrate, hydrochloric acid,zinc(II) chloride, sodium borohydride (NaBH4) and hydrogen per-oxide (H2O2) were obtained from Merck Co. Cyclohexane was dis-tilled under nitrogen and stored over molecular sieves (4 Å).Reference samples of cyclohexanol and cyclohexanone were dis-tilled and stored in the refrigerator. NaY with the Si:Al ratio of2.53 was purchased from Aldrich (Lot No. 67812).

2.2. Physical measurements

XRD patterns were recorded by a Rigaku D-max C III, X-ray dif-fractometer using Ni-filtered Cu Ka radiation. Elemental analyseswere obtained from Carlo ERBA Model EA 1108 analyzer. The metalcontents of the samples were measured by Atomic AbsorptionSpectrophotometer (AAS-Perkin–Elmer 4100–1319) using a flameapproach. After completely destroying the zeolitic framework withhot and concentrated HCl, sodium, aluminum and metal were ana-lyzed by AAS and SiO2 was determined by gravimetric analysis. Theproducts were analyzed by GC–MS on a Philips Pu 4400 gas chro-matograph mass spectrometer. Diffuse reflectance spectra (DRS)were registered on a Shimadzu UV/3101 PC spectrophotometerthe range 1500–200 nm, using MgO as reference. The stability ofthe supported catalyst was checked after the reaction by UV–Visand possible leaching of the complex was investigated by UV–Visin the reaction solution after filtration of the encapsulated zeolite.Nitrogen adsorption measurements were performed at 77 K usinga Coulter Omnisorb 100CX instrument. Nanopore volumes weredetermined by the t-method; a ‘‘monolayer equivalent area” wascalculated from the micropore volume [35,36]. XRD patterns wererecorded by a Rigaku D-max C III, X-ray diffractometer using Ni-fil-tered Cu Ka radiation. Conductance measurements with a Metr-ohm Herisau conductometer E 518. Magnetic moments werecalculated from magnetic susceptibility data obtained using aJohnson Matthey MK-1 magnetic susceptibility balance.

2.3. Synthesis of the Schiff base ligand, H4[N4O4]

The Schiff base, H4[N4O4] = 2,7,13,18- tetramethyl-3,6,14,17-tetraazatricyclo-[17.3.1.1]-tetracosa-1(23),2,6,8(24),9,11,13,17,19,21-decaene-9,11,20,22-tetraol, ligand were prepared in two steps.The first step was acetylating of resorcinol (5.01 g, 45.5 mmol)with acetic anhydride (9.29 g, 91.0 mmol) in presence of excesszinc(II) chloride (10.0 g, 73.4 mmol) at 140 �C in a paraffin oil path.The hot mixture was cooled to room temperature and poured onto140 ml 50% dilute hydrochloric acid. Orange precipitate wasformed and increased with standing until 1 h. The orange crudeproduct was obtained by filtration with suction, washed with dis-tilled water till the color of the filtrate is nearly colorless. Orangeneedle crystals were obtained by crystallization using either etha-nol or acetic acid/water mixture. The yield of 4,6-diacetylresorcinolwas 80%, m.p. 179 �C. The second step was the addition of a solu-tion of ethylenediamine monohydrate (en) (1.79 g, 22.9 mmol) inethanol (40 ml) to 4,6-diacetylresorcinol (4.45 g, 22.9 mmol) inethanol (40 ml). The solutions were refluxed for 3 h. An orange(H4[N4O4]) crystals were formed on cooling the solutions slowly

to room temperature and the precipitates were collected by filtra-tion, washed with ethanol then diethylether and finally air-dried.The yields were 78%, m.p. 210 �C. Scheme 1 illustrates the syn-thetic scheme of the Schiff base, H4[N4O4], ligand. Elemental andspectroscopic analysis of neat and zeolite encapsulated complexesconfirmed the molecular composition of ligand.

2.4. Preparation of [M(N4O4)]

The flask containing a stirred suspension of transition metal(II)acetate; M = Mn(II), Co(II), Ni(II), and Cu(II); (0.016 mol) in 100 mlmethanol (was purged with nitrogen), and then warmed to 50 �Cunder a nitrogen atmosphere. H4[N4O4]; (3.49 g, 0.008 mol) andNEt3 (4.46 ml, 0.032 mmol) were added in one portion, and theresulting suspension was then stirred and heated under reflux un-der a nitrogen atmosphere for 8 h. Then the mixture was cooledand filtered under reduced pressure. The collected solid waswashed with diethylether and dried in air to give colored crystal-line [M(N4O4)] which was purified by recrystallization fromchloroform.

2.5. Preparation of H4([H]8-N4O4)

Ligand; H4([H]8-N4O4) = 2,7,13,18-tetramethyl-3,6,14,17-tetra-azatricyclo-[17.13.1.1.]-tetracosa-1(23), 8(24),9,11,19,21-hexane-9,11,20,22-tetraol; was prepared by the following method:0.01 mol H4[N4O4] was dissolved in 60 ml methanol, followed bythe addition of 0.011 mol NaBH4 at ambient temperature. After2 h of stirring, the solvent was removed by distilling under vacuumconditions. The solid product was further washed with distilledwater and recrystallized from ethanol. The as-synthesizedH4([H]8–N4O4) Schiff base ligand were confirmed with 1H NMRand FT-IR spectroscopies. In the infrared (IR) spectra, a band attrib-uted to m(N–H) was observed at 3376 cm�1 for the H4([H]8–N4O4)ligand. IR spectroscopy also shows that the spectra of the formedtransition metal complexes were very similar to those of the corre-sponding H4([H]8–N4O4) Schiff base ligand except that the bandsarising from O–H stretching vibrations perturbed by the hydro-gen-bonded N atom disappeared.

2.6. Preparation of [M([H]8–N4O4)]

The procedures for the preparation of metal (M = Mn(II), Co(II),Ni(II) and Cu(II)) complexes are as follows: 0.01 mol H4([H]8-N4O4)ligand were dissolved in 50 ml ethanol, and then heated to boilingtemperature. This was followed by the drop-wise addition of asolution of 0.02 mol metal salt (Cu(CH3COO)2�H2O, Co(CH3-

COO)2�4H2O, Mn(CH3COO)2.4H2O and Ni(CH3COO)2�6H2O in125 ml ethanol under a nitrogen atmosphere. The resultant solu-tion was stirred and refluxed for 1 h. After cooling, the solid prod-uct was separated by filtration and recrystallized from CHCl3/petroleum ether.

2.7. Preparation of M(II)@NaY

M(II)@NaY was prepared by ion-exchange. Typically, 3.2 mmolof one type of the above-mentioned metal salt was first dissolvedin 400 ml deionized water. Then, 20 g NaY was added to the solu-tion, and further stirred for 24 h at ambient temperature. Then, thesolid fraction was filtered out, twice washed with deionized water,and dried at 120 �C for 12 h.

2.8. Preparation of nanocomposite materials; [M([H]8–N4O4)]@NaY

Encapsulation of metal complex was performed with the Flexi-ble Ligand Method (FLM). First, M(II)@NaY was uniformly mixed

Table 1Elemental analysis, vibrations parameters and some physical properties for Schiff-base and octahydro-Schiff base transition metal(II) complexes.

Sample Calculated (Found) KMa, X�1 cm2 M�1 leff (MB) IR (KBr, cm�1)

%C %H %N C/N %M tC@N tN–H tM–O tM–N

H4(N4O4) 66.04 (65.88) 6.47 (6.35) 12.84 (12.93) 5.14 (5.09) – – – 1585 – – –H4([H]8N4O4) 64.84 (64.63) 8.16 (7.98) 12.60 (12.78) 5.14 (5.06) – – – – 3376 – –[Mn2(N4O4)] 53.15 (53.01) 4.46 (4.31) 10.33 (10.42) 5.14 (5.09) 20.26 (20.13) 12 5.88 1635 – 440 426[Mn2([H]8–N4O4)] 52.37 (52.22) 5.86 (5.72) 10.18 (10.30) 5.14 (5.07) 19.96 (19.81) 17 5.90 – 3380 446 429[Co2(N4O4)] 52.38 (52.20) 4.40 (4.33) 10.18 (10.26) 5.14 (5.09) 21.42 (21.25) 14 5.13 1632 – 470 372[Co2([H]8–N4O4)] 51.62 (51.48) 5.78 (5.61) 10.03 (10.16) 5.14 (5.07) 21.11 (20.96) 19 5.16 – 3382 472 374[Ni2(N4O4)] 52.42 (52.33) 4.40 (4.35) 10.19 (10.28) 5.14 (5.09) 21.35 (21.21) 16 3.89 1630 – 499 357[Ni2([H]8–N4O4)] 51.67 (51.53) 5.78 (5.63) 10.04 (10.19) 5.14 (5.06) 21.04 (20.88) 20 3.92 – 3378 500 358[Cu2(N4O4)] 51.52 (51.36) 4.32 (4.24) 10.01 (10.17) 5.14 (5.05) 22.71 (22.57) 17 2.14 1628 – 460 350[Cu2([H]8–N4O4)] 50.78 (50.61) 5.68 (5.54) 9.87 (10.00) 5.14 (5.06) 22.39 (22.17) 22 2.16 – 3375 462 351

a In DMF solution.

3718 M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724

with an excessive amount of H4([H]8–N4O4) ligands (nligand/nmetal =3), and sealed into a round flask. The complexation was carried outunder high vacuum conditions for 24 h at the temperatures of220 �C. Uncomplexed ligands and the complex adsorbed on theexterior surface were removed by full Soxhlet extraction with ace-tone and methanol. The extracted sample was ion-exchanged with0.1 M NaCl aqueous solution to remove uncoordinated M2+ ions,followed by washing with deionized water until no Cl� anionscould be detected with AgNO3 aqueous solution.

2.9. Oxidation of cyclohexane

Aqueous solution of 30% H2O2 (2.28 g, 20 mmol), cyclohexane(0.84 g, 10 mmol) and catalyst (1.02 � 10�5 mol) were mixed in5 ml of CH3CN and the reaction mixture was heated at 70 �C withcontinuous stirring in an oil bath for 2 h. After filtration and wash-ing with solvent, the filtrate was concentrated and then subjectedto GC analysis.

3. Results and discussion

3.1. Synthesis and characterizations

The Schiff base and octahydro-Schiff base ligands were investi-gated by elemental analysis, infrared, UV–Vis, mass spectra and 1HNMR spectra. The physicochemical properties are listed in Table 1.The IR spectra of the ligands, Table 1 shows broad band in therange (3500–3340 cm�1) due to the stretching vibration of thephenolic groups. The broadness may be due to the intermolecularhydrogen bonding between the phenolic groups and the azome-thine groups. The band at 1264–1267 cm�1 is ascribed to the phe-nolic m(C–O) stretching vibrations while the strong bands observedat 1584–1602 cm�1 is assigned to the stretching vibrations of theazomethine group [37]. Mass spectra were performed forH4[N4O4] and H4[[H]8–N4O4], ligands to determine their molecularweights and fragmentation patterns. The molecular ion peaks were

Table 2Chemical composition and vibrations parameters (cm�1, KBr) of host guest nanocomposit

Sample C (%) H (%) N (%) C/N

NaY – – – –Mn(II)@NaY – – – –[Mn2([H]8–N4O4)]@NaY 5.39 1.68 0.92 5.86Co(II)@NaY – – – –[Co2([H]8–N4O4)]@NaY 5.37 1.65 1.07 5.01Ni(II)@NaY – – – –[Ni2([H]8–N4O4)]@NaY 5.33 1.60 1.06 5.03Cu(II)@NaY – – – –[Cu2([H]8–N4O4)]@NaY 5.28 1.63 1.07 4.94

observed at 436 and 444 m/e confirming their formula weights(F.W. 436 and 444) for H4[N4O4] and H4([H]8–N4O4) ligands,respectively. The 1H NMR spectrum of H4[N4O4] ligand in DMSO-d6, shows signals at d(ppm) 1.33 (s, CH3); 1.56 (s, CH3); 2.45 (s,CH3); 2.76 (s, CH3); 2.83 (s, 2CH2); 3.45 (s, 2CH2); 6.23 (s, C6H2);8.43 (s,, C6H2) and 17.36 (s, br, 4H, Ar–OH). The 1H NMR spectrumof H4[H]8–N4O4], in DMSO-d6 showed signals at d(ppm) 1.29 (s,CH3); 1.51 (s, CH3); 2.40 (s, CH3); 2.72 (s, CH3); 2.80 (s, 2CH2);3.42 (s, 2CH2); 3.54 (s, 2CH); 3.82 (s, 2CH); 6.21 (s, C6H2); 8.41(s, C6H2); 8.57 (s, br, 4H, NH) and 17.34 (s, br, 4H, Ar–OH).

Synthesis of complexes [M(N4O4)] and [M([H]8-N4O4)](M = Mn(II), Co(II), Ni(II) and Cu(II)) encapsulated in the nanocavityof zeolite-Y involved the exchange of transition metal(II) ions withsodium of NaY followed by reaction of metal exchanged zeolite-Y(M(II)@NaY) with H4([H]8-N4O4) in the molten state. The formationof octahydro-Schiff base ligand was confirmed by elemental anal-ysis and 1H NMR spectroscopy. Here, ligand entered into the nano-cavity of zeolite-Y due to its flexible nature and interacted withmetal ions. It is expected that the [M(N4O4)] and [M([H]8–N4O4)]complexes will be formed in the supercage as well as on the sur-face of the Y-zeolite. As the complex is neutral only a weak forcesuch as a van der Waals interaction is operative here to catch holdof the complex on the surface of the zeolite. The crude mass wassubjected to Soxhlet extraction in CHCl3 to remove excess ligandthat remained uncomplexed in the nanocavities of the zeolite aswell as located on the surface of the zeolite along with free[M([H]8–N4O4)]. The uncomplexed M(II) ions from the zeolitewas removed by exchanging back [M([H]8–N4O4)]–Y with aqueous0.01 M NaCl solution. Thus, presence of transition metal estimatedby AAS is only due to encapsulation of the complex. The resultingcatalyst was further characterized by recording its IR and elec-tronic spectra and X-ray powder diffraction pattern. All these stud-ies further supported the encapsulation of [M([H]8–N4O4)] insidethe supercages of the zeolite. Neat [M([H]8–N4O4)] complexes havealso been prepared by the reaction of H4([H]8–N4O4) with metalacetate in refluxing methanol for comparing its physico-chemical

e materials.

Si (%) Al (%) Na (%) M (%) Si/Al tN–H

21.76 8.60 7.50 – 2.53 –22.08 8.73 3.34 2.58 2.53 –21.50 8.50 5.30 2.47 2.53 337021.53 8.53 3.36 3.71 2.53 –21.45 8.48 5.32 2.45 2.53 337521.79 8.62 3.28 3.72 2.53 –21.40 8.46 5.31 2.46 2.53 337021.48 8.49 3.28 3.86 2.53 –21.32 8.43 5.29 2.44 2.53 3380

Fig. 1. FT-IR of spectra of H4[N4O4] (a), H4([H]8–N4O4) (b), [Cu2([H]8–N4O4)] (c), NaY(d) and [Cu2([H]8–N4O4)]@NaY (e).

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724 3719

properties with the encapsulated one. The formulation of theencapsulated complex is based on the neat [M([H]8–N4O4)]complexes.

The FLM, Scheme 1, leads to the encapsulation of manganese(II),cobalt(II), nickel(II) and copper(II) complexes of H4([H]8–N4O4) li-gand within the nanopores of zeolite. Elemental and chemical anal-ysis data confirmed the purity and stoichiometry of the neat andnanocavity encapsulated complexes (Tables 1 and 2). The chemicalanalyses of the samples reveal the presence of organic matter witha C/N ratio roughly similar to that for neat complexes. The mol ra-tios Si/Al obtained by chemical analysis for zeolites are presented(Table 2). The parent NaY zeolite has Si/Al molar ratio of 2.53which corresponds to a unit cell formula Na56[(AlO2)56(SiO2)136](Tables 1 and 2). The unit cell formulae of metal-exchanged zeo-lites show a metal dispersion of around 11 moles per unit cell(Mn(II)NaY, Na33.2Mn11.3[(AlO2)56(SiO2)136]�nH2O; Co(II)NaY, Na34-Co11[(AlO2)56(SiO2)136]�nH2O; Ni(II)NaY, Na33.8Ni11.1[(AlO2)56-(SiO2)136]�nH2O; Cu(II)NaY and Na34.4Cu10.8[(AlO2)56(SiO2)136]�nH2O).

The analytical data of each complex indicates M:C:H molar ra-tios almost close to those calculated for the dinuclear structure.However, the presence of minute traces of free metal ions in thelattice could be assumed as the metal content is slightly higherthan the stoichiometric requirement. Only a portion of metal ionsin metal-exchanged zeolite has undergone complexation and therest is expected to be removed on re-exchange with sodium chlo-ride solution. But, some of the sites in the zeolite lattice might beblocked from solution access by the encapsulated complexes. TheSi and Al contents in M(II)@NaY, [M([H]8–N4O4)]@NaY; are almostin the same ratio as in the parent zeolite. This indicates littlechanges in the zeolite framework due to the absence of dealumina-tion in metal ion exchange (Table 2).

A partial list of IR spectral data is presented in Tables 1 and 2.The intensity of the peaks in encapsulated complexes are, though,weak due to their low concentration in zeolite matrix, the dataspectra of encapsulated as well as their neat complexes showessentially similar bands (Fig. 1). Comparison of the spectra ofthese catalysts with the ligand provides evidence for the coordinat-ing mode of ligand in catalysts. The ligand H4([H]8–N4O4) exhibits abroad band in the 3400–3520 cm�1 due to extensive hydrogenbonding between phenolic hydrogen and nitrogen of N–H group.Absence of this band in the spectra of encapsulated complexesindicates the destruction of the hydrogen bond followed by thecoordination of phenolic oxygen after deprotonation. The sharpband appearing at 3380 cm�1 due to m(N–H), shifts to higher wavenumber and appears at 3380–3388 cm�1. This indicates theinvolvement of azomethine nitrogen in coordination. It is evidentthat framework vibration bands of zeolite Y dominate the spectrabelow 1200 cm�1 for all samples. The bands at 458, 564, 687 and767 as well as 990 and 1132 cm�1 are attributed to T–O bendingmode, double ring, symmetric stretching and asymmetric stretch-ing vibrations, respectively. No shift was observed upon inclusionof [M([H]8–N4O4)] complexes, further substantiating that zeoliteframework remains unchanged. In addition, the band due to thebending vibration of H2O molecules in zeolite lattices was also dis-played at 1636 cm�1 in all the spectra. It can be seen that all theprominent bands, assigned to C–O, C–N, and aromatic ring vibra-tions of ligand, were present at about 1255, 1290, 1396, 1450and 1505 cm�1 in the spectra of the [M(H]8-N4O4)]@NaY samples.The presence of multiple bonds at 3080 cm�1 in octaahydro-Schiffbase ligand and its complexes with slight shift suggests the pres-ence of CH2 group in ligand as well as its complexes.

The X-ray diffraction (XRD) patterns of encapsulated complexesare shown in Fig. 2. The encapsulated complexes exhibit similarpeaks to those of zeolite Y; except for a slight change in the inten-sity of the peaks, no new crystalline pattern emerges. These facts

confirmed that the framework and crystallinity of zeolite werenot destroyed during the preparation, and that the complexes werewell distributed in the cages. The relative peak intensities of the220, 311 and 331 reflections have been thought to be correlatedto the locations of cations. In zeolite Y, the order of peak intensityis in the order: I331 >> I220 > I311, while in encapsulated complexes,the order of peak intensity became I331 >> I311 > I220. The differenceindicates that the ion-exchanged Cu2+, which substitutes at thelocation of Na+, undergoes rearrangement during complexation.

Fig. 2. X-ray diffractograms of NaY (a), Cu(II)@NaY (b) and [Cu2([H]8–N4O4)]@NaY(c).

3720 M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724

The d–d transition in the electronic spectral data, measured inDMF solvent of the Ni(II) and Cu(II), Co(II) and Mn(II) complexes,is provided in Table 3. The three bands in all metal complexes lo-cated at 210–370 nm regions. The absorption band in the range360–370 nm could be assigned to both the C@N group [37]. Thebands appeared at 400�426 nm, could be assigned to charge trans-fer transition [37]. Electronic spectrum of the Ni(II) complexesshowed more intense doublet bands at 770 and 722 nm. Thesebands were more intense than the other electronic spectra of theother Ni(II) Schiff base complexes under investigation. These bandsmay be due to 3T1(P) 3T1(F) transition, which could occur in atetrahedral d8 arrangement [38,39]. Generally, this band is ex-

Table 3Electronic absorption bands (nm, in DMF solutions) and DRS of the Schiff base, octahydro

Sample 1L ? 1A (phenyl ring) p ? p* n ?

H4(N4O4) 220 375 410H4([H]8N4O4) 223 – 413[Mn2(N4O4)] 270 369 423[Mn2([H]8-N4O4)] 271 – 426[Mn2([H]8-N4O4)]@NaY 270 – 425[Co2(N4O4)] 264 370 418[Co2([H]8-N4O4)] 263 – 420[Co2([H]8-N4O4)]@NaY 264 – 419[Ni2(N4O4)] 261 370 400[Ni2([H]8-N4O4)] 262 – 402[Ni2([H]8-N4O4)]@NaY 260 – 401[Cu2(N4O4)] 263 367 423[Cu2([H]8-N4O4)] 265 – 425[Cu2([H]8-N4O4)]@NaY 264 – 424

pected to split by spin–orbital coupling to an extent which makesunambiguous assignments difficult. The magnetic moment for theformer complex is 3.92 lB, which may be due to either tetrahedralstructure [38,39]. The electronic spectrum of the complex[Mn2([H]8–N4O4)], run in DMF, presents a shoulder at 653 nm(Fig. 3). In view of the position and magnitude of the molar extinc-tion coefficient of this band, it can he assigned to the 6A1 ? 4T1

transition. In tetrahedral fields and in contrast with the spin tran-sitions which are forbidden, the Laporte transitions are allowedand the bands which originate are �100 times more intense thanin octabedral structures [40]. Another band, which is observed at426 nm in DMF solution, is attributed to the charge transfers.The remaining bands are typical for the transitions at the ligand.The estimated magnetic moment is l = 5.90 BM, a value which isin agreement with the expected one for a tetrahedral Mn(II) com-plex [40]. Electronic spectra of [Cu2([H]8–N4O4)] complex were re-corded in DMF solution over the range 400–700 nm (Table 3,Fig. 3). The visible spectra of the [Cu2([H]8–N4O4)] complex con-sists of a maximum or a broad shoulder around 609 nm, whichcan be assigned to the 2Eg ?

2T2g. The magnetic moment for theformer complex is 2.12 lB, which may be due to either tetrahedralor square planar structure [39,38]. The spectrum of the [Co2([H]8–N4O4)] exhibit two bands at 600–680 nm which are assigned tod M d transitions[39]. This geometry is confirmed by the valuesof the effective magnetic moment (Table 1).

The surface area and nanopore volume of the catalysts used inthe oxidation reaction are presented in Table 4. The encapsulationof metal complex reduced the surface area and adsorption capacityof zeolite. The lowering of the pore volume and surface area sup-ported the fact that [M([H]8–N4O4)]@NaY complexes are presentwithin the zeolite cages and not on the external surface.

3.2. Catalytic activity

The catalytic oxidation of cyclohexane was studied with[M2([H]8–N4O4)]@NaY (M = Mn(II), Co(II), Ni(II) and Cu(II)), usingH2O2 as the oxidant. Blank reactions performed over NaY zeoliteunder identical conditions show only negligible conversion indi-cating that zeolite host is inactive for oxidation. The H4[N4O4]and H4([H]8–N4O4) ligands alone in the absence of metal werenot catalytically active. Furthermore, H2O2 alone is unable to oxi-dize the substrates in the absence of any catalyst. In representativetests, zeolite complex was filtered out and the filtrate was analyzedfor metal content using atomic absorption spectrophotometry. Theabsence of metal ions in solution phase indicates that no leachingof complexes has occurred during reaction, as they are too intact inthe nanopores. At the end of reaction, the catalyst was separatedby filtrations, thoroughly washed with solvent and reused under

-Schiff base ligands, nanocomposite materials and their transition metal complexes.

p* and CT d M d transition d M d Transition assignment

– –– –642 4T1 6A1

653652668, 580 4A2g(F) 4T1g(F)4T1g(P) 4T1g(F)680, 600678, 598456, 707 3T1(P) 3T1(F)770, 722768, 720594 2T2g 2Eg

609607

Fig. 3. UV–Vis. and DRS spectra of [M2([H]8–N4O4)] and [M2([H]8–N4O4)]@NaY (L = [H]8–N4O4).

Table 4Surface area and pore volume data of Schiff base and octahydro-Schiff base metal(II)complexes encapsulated in nanopores of zeolite Y.

Sample Surface area (m2/g)a Pore volume (ml/g)b

NaY 545 0.31Mn(II)@NaY 535 0.30[Mn2([H]8-N4O4)]@NaY 183 0.09Co(II)@NaY 532 0.30[Co2([H]8-N4O4)]@NaY 185 0.10Ni(II)@NaY 528 0.30[Ni2([H]8-N4O4)]@NaY 187 0.11Cu(II)@NaY 532 0.30[Cu2([H]8-N4O4)]@NaY 180 0.08

a Surface area is the ‘‘monolayer equivalent area” calculated as explained in thereference 35 and 36.

b Calculated by the t-method.

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724 3721

similar conditions by AAS which showed no reduction in theamount of metal. These observations suggest that the oxidationsoccur due to the catalytic nature of the encapsulated ‘‘[H]8–N4O4”complexes and no significant role is played by either the zeolitesupport or free complexes.

[M2([H]8–N4O4)] and [M2([H]8–N4O4)]@NaY, the promising sam-ples from the screening test, was further evaluated for catalyticactivity for the oxidation of cyclohexane with H2O2 at various tem-peratures and the results are given in Tables 5 and 6 and Figs. 4and 5. The conversion in each case was found to increase withincreasing reaction temperature. [M2([H]8–N4O4)] as soluble cata-lysts are more prone to deactivation by the dimerization of activecenters, which is expected to be reduced by encapsulating them inzeolites. Site isolation and immobilization of homogeneous transi-tion metal catalysts within the nanocavity of zeolite carriers offersseveral practical benefits of heterogeneous catalysis, while retainingthe advantages of homogeneous catalytic reactions. Some of theattractive features of zeolite encapsulated catalysis include: (1) easyseparation of the catalysts from the reagents and reaction products;(2) simplification of methods to recycle expensive catalysts; (3) non-volatile and nontoxic characteristics to high molecular weight zeo-lite backbones; (4) minimization of certain catalyst deactivationpathways by site isolation. These attractive features of thesecatalysts could possibly help in developing high through putdiscovery applications as well as in developing continues catalyticprocesses for industrial scale synthesis. The activity of cyclohexaneoxidation decreases in the series [Cu2([H]8–N4O4)]@NaY >[Co2([H]8–N4O4)]@NaY > [Mn2([H]8–N4O4)]@NaY > [Ni2([H]8–N4-

O4)]@NaY (Table 6). The trend observed in Tables 5 and 6 can beexplained by the donor ability of ligand available in the complex

catalysts. As Wang and our work have pointed out recently, thekey point in the conversion of substrate to the products is thereduction of L–M(n + 1)+ to L–Mn+. This reduction to L–Mn+ is facil-itated with the ligands available around the metal cation [34,41].

The effect of transition metal complexes encapsulated in zeo-lite; [M2([H]8–N4O4)]@NaY; was studied on the oxidation of cyclo-hexane with hydrogen peroxide in CH3CN and the results areshown in Table 6. As shown in Table 6, oxidation has occurred withthe formation of cyclohexanol and cyclohexanone. Oxidation withthe same oxidant in the presence of Cu(II)@NaY was 10.6%. The in-crease of conversion from 10.6% to 40.3% compared to Cu(II)@NaYwith [Cu2([H]8–N4O4)]@NaY indicates that the existence of ligandhas increased the activity of the catalyst by a factor of 3.80. From

Table 5Oxidation of cyclohexane with H2O2 catalyzed by Schiff base and octahydro-Schiffbase transition metal complexes in CH3CN.

Catalyst Conversion (%) Selectivity (%)

Cyclohexanol Cyclohexanone

[Mn2(N4O4)]a 11.1 15.2 84.8[Mn2([H]8-N4O4)]a 17.2 21.4 78.6[Co2(N4O4)]a 30.4 36.5 63.5[Co2([H]8-N4O4)]a 38.5 41.2 58.8[Ni2(N4O4)]a 5.4 12.4 87.6[Ni2([H]8-N4O4)]a 7.9 16.1 83.9[Cu2(N4O4)]a 46.1 39.7 60.3[Cu2([H]8-N4O4)]a 53.5 44.6 55.4[Cu2([H]8-N4O4)]b 30.4 63.4 36.6[Cu2([H]8-N4O4)]c 43.9 40.2 59.8[Cu2([H]8-N4O4)]d 14.6 32.7 67.3

a Reaction condition: time, 2 h; cyclohexane, 10 mmol; catalyst, 1.02�10�5 mol;H2O2, 20 mmol; CH3CN, 5 ml.

b A similar procedure as system was carried out with 0.5�10–5 mol catalystinstead of 1.02�10�5 mol.

c A similar procedure as system a was carried out with 2.04�10�5 mol catalyst.d A similar procedure as system a was carried out with 4.08�10�5 mol catalyst.

Table 6Oxidation of cyclohexane with H2O2 catalyzed by HGNM in CH3CN.

Catalyst Conversion (%) Selectivity (%)

Cyclohexanol Cyclohexanone

[Mn2([H]8-N4O4)]@NaYa 8.7 68.4 31.6[Co2([H]8-N4O4)]@NaYa 18.6 70.3 29.7[Ni2([H]8-N4O4)]@NaYa 3.6 66.7 33.3[Cu2([H]8-N4O4)]@NaYa 40.3 59.3 40.7[Cu2([H]8-N4O4)]@NaYb 40.1 60.4 39.6[Cu2([H]8-N4O4)]@NaYc 39.4 61.7 38.3[Cu2([H]8-N4O4)]@NaYd 39.2 62.4 37.6[Cu2([H]8-N4O4)]@NaYe 20.7 64.7 35.3Cu(II)@NaYa 10.6 11.7 88.3

a Reaction condition: time, 2 h; cyclohexane, 10 mmol; catalyst, 1.02�10�5 mol;H2O2, 20 mmol; CH3CN, 5 ml.

b First reuse.c Second reuse.d Third reuse.e Poisoned.

0

20

40

60

80

100

[Mn(

[H8]

-N4O

4)]

[Co(

[H8]

-N4O

4)]

[Ni([

H8]-N

4O4)

]

[Cu(

[H8]

-N4O

4)]

Con

vers

ion

(%)

Conversion Cyclohexanol Cyclohexanone

Fig. 4. Conversion and oxidation products distribution in acetonitrile with neatoctahydro-Schiff base complexes in the oxidation of cyclohexane with hydrogenperoxide (Reaction condition: time, 2 h; cyclohexane, 10 mmol; catalyst;1.02 � 10�5 mol).

0

20

40

60

80

[Cu([H8]-N

4O4)]-NaY

[Ni([H8]-N

4O4)]-NaY

[Co([H8]-N

4O4)]-NaY

[Mn([H

8]-N4O4)]-N

aY

Con

vers

ion

(%)

Conversion Cyclohexanol Cyclohexanone

Fig. 5. Conversion and oxidation products distribution in acetonitrile with HGNMin the oxidation of cyclohexane with hydrogen peroxide (Reaction condition: time,2 h; cyclohexane, 10 mmol; catalyst, 1.02 � 10�5 mol).

3722 M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724

the indicated results in Table 6 it is evident that cyclohexanol isselectively formed in the presence of all HGNM as catalyst.

The amount of the encapsulated Schiff base complex is verylimited owing to the presence of relatively rigid C@N bond. In con-trast, hydrogenation of C@N to C–N (Scheme 1), as expected, wouldincrease nitrogen basicity and make the conformation of the com-plex more flexible, and hence resulting in the more ready coordina-tion to metal centers in a folded fashion, and thus the inclusion ofmore transition metal complex molecules, or active sites withoutsevere blockage of the channels in zeolitic matrix. In addition, ithas been shown that copper octahydro-Schiff base complex([Cu2([H]8–N4O4)]@NaY) is also much more active than other het-erogeneous oxidation (Table 6 and Fig. 5). This would make the[Cu2([H]8–N4O4)]@NaY be an effective heterogenous catalyst forthe oxidation of such a type of cyclohexane.

The recycle ability of encapsulated complexes; [Cu2([H]8–N4O4)]@NaY; has been tested in catalytic oxidation of cyclohexane(Table 6). In a typical experiment the reaction mixture after a con-tact time of 2 h was filtered and after activating the catalyst bywashing with acetonitrile and drying at ca. 120 �C, it was subjectedto further catalytic reaction under similar conditions. No apprecia-ble loss in catalytic activity suggests that complex is still presentin the nanocavity of the zeolite-Y. The filtrate collected after sepa-rating the used catalyst was placed into the reaction flask and thereaction was continued after adding fresh oxidant for another 2 h.

The gas chromatographic analysis showed no improvement in con-version and this confirms that the reaction did not proceed uponremoval of the solid catalyst. The reaction was, therefore, heteroge-neous in nature. No evidence for leaching of metal or decompositionof the catalyst complex was observed during the catalysis reactionand no metal could be detected by atomic absorption spectroscopicmeasurement of the liquid reaction mixture after each catalyticreaction. The IR spectrum of the solid catalyst after reuse was alsoidentical to the fresh catalyst. Also, the catalytic behavior of the sep-arated liquid was tested by addition of fresh cyclohexane to the fil-trates after each run. Execution of the oxidation reaction under thesame reaction conditions, as with catalyst, showed that the obtainedresults are the same as blank experiments.

The catalytic activities of encapsulated cobalt(II), manga-nese(II), nickel(II) and copper(II) complexes for the oxidation ofcyclohexane with hydrogen peroxide in acetonitrile arerepresented in Table 6 and Fig. 5. Zeolite Cu(II) complexes have

0

10

20

30

40

50

60

0.25 0.5 1 1.5 2 2.5 3

Time (h)

Con

vers

ion

(%)

Fig. 7. Effect of time on substrate conversions in the oxidation of cyclohexane withhydrogen peroxide in the presence of [Cu2([H]8–N4O4)]@NaY as catalyst. (Reactioncondition: time, 2 h; cyclohexane, 10 mmol; catalyst, 1.02 � 10�5 mol).

M. Salavati-Niasari et al. / Inorganica Chimica Acta 362 (2009) 3715–3724 3723

exhibited reasonably good activity, whereas Mn(II), Ni(II) andCo(II) complexes are weakly active. Zeolite Cu(II) complexes werethoroughly evaluated for catalytic efficiency at various conditions;the results are given in Table 6. Encapsulated Cu(II) complexes arefound to be active for the oxidations in the presence of H2O2 as ini-tiator. It is interesting to note that the catalytic performances ofzeolite Cu(II) complexes are comparable to the activity exhibitedby the encapsulated tetrahydro-salen complexes reported earlier[42]. Tetrahedrally distorted square-planar geometries of zeoliteCu(II) complexes may account for their higher activity as the com-plexes readily provide the vacant coordination sites for oxygenbinding.

Poison-tolerance of [Cu2([H]8–N4O4)]@NaY was indirectlyascertained by evaluating their activity for the oxidation of cyclo-hexane in the presence of traces of pyridine which acts as a poisonby coordinating irreversibly at the active site. The extent of deacti-vation of the catalyst, given by the loss in activity, could be relatedto the mobility of poison molecules at the active site. The activityresults given in Table 6 show that [Cu2([H]8–N4O4)]@NaY has expe-rienced a higher degree of deactivation (51.36% loss in activity). Itis widely believed that the stereochemical environment of the ac-tive site has a profound influence on the mobility of molecules atthe reaction center and hence on the poison-tolerance.

Effect of reaction temperature on cyclohexane conversion isillustrated in Fig. 6. As expected, cyclohexane conversion increasedwith increasing reaction temperature although no conversion wasdetected when the reaction temperature was below 30 �C. How-ever, high temperature led to a quick decomposition of H2O2, asa consequence, conversely lowering down the conversion. Theoptimum temperature was 60 �C. Fig. 7 shows that cyclohexaneconversion increased with the increase in the reaction time up to2 h. An attempt to further increase the cyclohexane conversionby prolonging the reaction time failed: about 95% of the addedH2O2 was consumed when the reaction was carried out for 2 h.

The results clearly suggest that [Cu2([H]8–N4O4)]@NaY effi-ciently catalyses conversion (40.3%) of cyclohexane to cyclohexa-nol and cyclohexanone with 59.3% and 40.7% selectivityrespectively. The more activity of H4([H]8–N4O4) system has clearlyarisen from the existence of electron donating ligand which facili-tate the electron transfer rate, a process that has previously ob-served by us in other oxidation reactions [43–50]. All conversionsefficiency with high selectivity obtained in this study is signifi-cantly higher than that obtained using copper containing porousand nonporous materials [43–50].

0

5

10

15

20

25

30

35

40

45

50

20 30 40 50 60 70 80Temperature (ºC)

Con

vers

ion

(%)

Fig. 6. Effect of temperature on substrate conversions in the oxidation ofcyclohexane with hydrogen peroxide in the presence of [Cu2([H]8–N4O4)]@NaY ascatalyst. (Reaction condition: time, 2 h; cyclohexane, 10 mmol; catalyst,1.02 � 10�5 mol).

4. Conclusion

Nanopores of zeolite-encapsulated Mn(II), Co(II), Ni(II) andCu(II) complexes of octahydro-Schiff base have been synthesizedusing the FLM. Encapsulated complexes exhibit fairly clear evi-dence in the physico-chemical (XRD, BET) and IR spectral charac-terization for the well-defined inclusion and distribution ofcomplexes in the nanopores of zeolite matrix. Tentative assign-ments are made for the geometry of complexes on the basis ofmagnetic moment, UV–Vis and DRS data. Nanopores of zeolite cop-per(II) complexe; [Cu2([H]8–N4O4)]@NaY; are reasonably good cat-alysts for the partial oxidation of cyclohexane with hydrogenperoxide, whereas Mn(II), Co(II) and Ni(II) are weakly active. Theencapsulated complexes are believed to be stable and reusabledue to the following reasons: (1) complexes are immobilized inthe nanocavities, (2) reduced formation of inactive oxo and peroxodimeric and other polymeric species in the nanocavities due to thesteric effects of zeolite framework and (3) the interaction of encap-sulated complexes with the zeolite lattice. This catalytic systemshowed high activity in oxidation of cyclohexane under mild reac-tion conditions. Encapsulated complex, HGNM, can be recoveredand reused without the loss of catalytic activity. The catalyticbehavior could be mainly attributed to the geometry of encapsu-lated complexes. Activity and poison-tolerance of the catalystsare dependent on the mobility of molecules, which is related tothe diffusion limitations due to the steric hindrance at the metalcenter. Encapsulated complexes can be recovered and reused with-out the loss of catalytic activity. To summarize, zeolite-encapsu-lated complexes have interesting catalytic potential particularlywith respect to the activity for partial oxidation and stability,and offer further scope to design efficient catalyst systems by anappropriate choice of guest and host materials.

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