Influence of Zeolite Framework on the Structure, Properties, and

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Influence of Zeolite Framework on the Structure, Properties, and Reactivity of Cobalt Phenanthroline Complex: A Combined Experimental and Computational Study Kusum K. Bania and Ramesh C. Deka* Department of Chemical Sciences, Tezpur University, Napaam, Tezpur - 784 028, Assam, India ABSTRACT: Bis(1,10-phenanthroline) cobalt(II) complexes have been encapsulated within the supercage of zeolite
NaY by reacting Co2þ exchanged NaY with the flexible phenanthroline ligand that diffuses into the cavities. The hybrid material obtained has been characterized by elemental analysis, SEM, powder XRD, FTIR, UV
vis, cyclic voltammetry, and EPR techniques. The difference in UV
vis spectra and redox properties between the neat and intrazeolite complexes suggest that the metal complex undergoes distortion inside the zeolite matrix and this result is being supported by density functional study where we found a change in structural parameters, position of the HOMO and LUMO energies, electrophilicity, and global hardness. Density functional theory based local reactivity descriptors such as Fukui functions and local softness are calculated to investigate the most probable active sites of the complexes.

1. INTRODUCTION Over the past decade, the encapsulation of transition metal coordination complexes and organometallics within the voids of microporus zeolite has attracted attention, since it provides a simple way of coupling the reactivity of the metal complex with the robustness and stereochemistry of the host zeolite.1
3 Rapid progress is being made in developing encapsulated base catalysts, as they are found to have wide applications in heterogeneous and homogeneous catalytic processes.4
9 In addition to the application of these hybrid materials in catalytic reactions, they are now being well studied to mimic the biosystem. Metal complexes encapsulated in zeolites can mimic metalloenzymes and therefore they are being termed as zeozymes.10
12 Recently, hydroxo bridged dinuclear cupric complexes of phenanthroline encapsulated in various mesoporous silicas were reported to mimic the catalytic activity of catechol oxidases.13 The tris(phenanthroline) complex of cobalt is being well recognized to interact with DNA via intercalation or groove binding.14 A novel illustration is one from electrochemistry, which has been applied in monitoring the binding of Co(Phen)33þ to DNA.15 There are studies on the steric effect imposed by the zeolite framework on such complexes by M€ossbauer spectroscopy.16 However, to the best of our knowledge, no study has been reported on the change of redox potential of the bis(1,10-phenanthroline) complex of cobalt(II) encapsulated inside zeolite. In the present investigation, we report for the first time the encapsulation of Co(Phen)22þ into the cavities of Y zeolite and the consequent synergistic effect of encapsulation on the electronic properties and reactive sites of the encapsulated complex. Density functional theory (DFT) methods have been extensively used to understand the structure, electronic properties, and chemical reactivity of zeolitic materials.17,18 Here, we perform DFT calculations to r 2011 American Chemical Society

determine the structural and electronic changes of the complex upon encapsulation. We also calculated DFT based global and local reactivity descriptors for both neat and encapsulated complexes to investigate the effect of the zeolite framework on the reactivity of the system.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Experimental Method. 2.1.1. Materials and Physical Measurements. Na
Y zeolites are purchased from HiMedia

Laboratories Pvt. Ltd. The chemicals used for the synthesis of the cobalt phenanthroline complex are cobalt chloride hexahydrate and 1,10-phenanthroline (analytical grade (AR), E-Merck). All the solvents are refined prior to use. Powder X-ray diffraction (XRD) patterns are recorded on a Shimadzu XD-D1 powder X-ray diffractometer using Cu KR radiation (λ = 1.542 Å) in the 2θ range 5
50
at a scanning rate of 2
/min. The electronic absorption spectra are measured using a Hitachi U-3400 spectrophotometer with a diffuse reflectance apparatus equipped with an integrating sphere of 60 mm inner diameter. Monochromatic light is used in the whole spectral region in order to minimize the effect of fluorescence. The samples are mixed with BaSO4 powder finer than 200 mesh. The samples are held between two quartz plates with a spacing of 2 mm. The infrared spectra in the range 450
4000 cm
1 are recorded on a Perkin-Elmer Spectrum 2000 FTIR spectrometer using a DRIFT accessory. The spectra of the zeolite-encapsulated complex are recorded against a zeolite background, at 100
C after 1 h of evacuation at Received: January 13, 2011 Revised: April 7, 2011 Published: April 27, 2011 9601

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2 Torr. The spectrum of the free CoPhen is recorded as KBr pellets. Electron spin resonance (ESR) spectra of neat and encapsulated complexes are recorded in appropriate solvents with a Varian E-112 spectrometer at liquid nitrogen temperature (77 K). The SEM and elemental analyses are performed by using JEOL JSM-6390 LV at an accelerated voltage of 5
10 kV. The samples are deposited on a brass holder and sputtered with platinum. Thermogravimetric and differential thermal analyses are performed on simultaneous TG-DTA thermo analyzer, Mettler Toledo, with a Pt crucible, Pt/Pt
Rh 13% thermocouples and flow rate of the controlling gas (air) of 20 mL/min. The cyclic voltammograms of neat and encapsulated complexes are recorded on a Wenking potentioscan (model POS73) with a digital recorder, and 0.1 M phosphate buffer is used as the supporting electrolyte. The working electrode is prepared by taking a 1:1 weight ratio of neat or encapsulated metal complexes in 1 mL of water. This suspension is ultrasonicated for 15 min. A 10 μL portion of this dispersion is coated on glassy carbon; 5 μL of 5% styrene (as binder from Aldrich) is then added on this coating, and it is dried. The glassy carbon electrode is used as the working electrode and Ag/AgCl/KCl (saturated) is used as the reference electrode. The cyclic voltammogram of the neat complex is taken in solution mode, using 0.01 M of the metal complex in a phosphate buffer. 2.1.2. Preparation of the Bis(1,10-phenanthroline) Cobalt(II) Complex, (Co(C12H8N2)2Cl2Co(Phen). To a well-stirred solution of 1,10-phenanthroline (0.396 g, 2 mmol) in methanol (10 mL), a solution of cobalt chloride hexahydrate (0.237 g, 1 mmol) in methanol (5 mL) is added. The resulting homogeneous solution is stirred at room temperature for 4 h. The solution is filtered, and the filtrate on standing led to crystallization of the product. The deep brown crystal so obtained is washed with ethanol and diethyl ether and dried at room temperature. 2.1.3. Preparation of Cobalt Exchange Zeolites. A mixture of 1 g of sodium Y zeolite with 100 mL of 0.1 M cobalt chloride hexahydrate in demineralized water is refluxed for 48 h at 250
270
C under constant stirring. The slurry is then filtered and washed with excess water to remove any chloride ions. The resulting solid is washed with deminerilized water and dried overnight under a vacuum. 2.1.4. Encapsulation of Cobalt Phenanthroline Complex in Na
Y Zeolite, CoPhen
Y. The encapsulated Co phenanthroline (CoPhen) complex is prepared by mixing dry Co2þ exchanged zeolite Y with a stoichiometric excess of 1,10-phenanthroline. The mixture is refluxed under constant stirring for 48 h at 250
270
C. On heating, the solid mass changed color from pink to blue. The product is crushed and purified by Soxhlet extraction using a sequence of solvents, viz., acetone, methanol, and diethyl ether, to remove any unreacted species or species adsorbed on the surface of the zeolite crystallites and finally dried under a vacuum. 2.2. Computational Method. All the density functional calculations are carried out using the DMol3 program19 with VWN correlation functional and double numeric (DN) basis set. We performed all electron calculations on both the neat and encapsulated cobalt phenanthroline complexes. The zeolite cluster is generated by taking 40 tetrahedral units (40T) of faujasite structure around the supercage, saturating them with hydrogen atoms. Initially, the framework Si and O atoms of the clusters are held fixed at their crystallographic positions and all the terminal H atoms are optimized. Following L€owenstein’s rule, two silicon atoms of the six-member ring are replaced with two aluminum atoms. The gas phase optimized cobalt phenanthroline complex is

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then encapsulated inside the supercage at several orientations. The two negative charges generated in the cluster are compensated by the two positive charges of the complex. While optimizing the clusters, terminated hydrogen atoms are held fixed at their initially optimized positions. The most stable structure is considered for further analysis. We calculated the chemical potential (μ) and global hardness (η) of the complexes using Koopmans’ theorem,20 as given below: μ¼

ELUMO þ EHOMO 2

ð1Þ

η¼

ELUMO
EHOMO 2

ð2Þ

where ELUMO is the energy of the lowest unoccupied molecular orbital and EHOMO is the energy of the highest occupied molecular orbital. The global electrophilicity as introduced by Parr et al.21 can be defined as ω¼

μ2 2η

ð3Þ

The global softness S is defined as the inverse of the global hardness η S¼

1 2η

ð4Þ

Using a finite difference approximation, we calculated the condensed Fukui functions22 of an atom k in a molecule with N electrons as fk þ ¼ ½qk ðN þ 1Þ
qk ðNÞ

ðfor nucleophilic attackÞ ð5Þ

fk
¼ ½qk ðNÞ
qk ðN
1Þ

ðfor electrophillic attackÞ ð6Þ

fk 0 ¼ ½qk ðN þ 1Þ
qk ðN
1Þ=2

ðfor radical attackÞ ð7Þ

where qk(N), qk(N þ 1), and qk(N
1) are the charges of the kth atom for N, N þ 1, and N
1 electron systems, respectively. The local softness value of an atom k can be defined as the product of the Fukui function and the global softness. The local softness values of selected atoms are calculated using eqs 8
10. sk þ ¼ ½qk ðN þ 1Þ
qk ðNÞS

ð8Þ

sk
¼ ½qk ðNÞ
qk ðN
1ÞS

ð9Þ

sk 0 ¼ ½qk ðN þ 1Þ
qk ðN
1Þ=2

ð10Þ

23

Recently, Chattaraj et al. have defined a generalized concept of philicity associated with a site k in a molecule as ωRk ¼ ωfkR

ð11Þ

where R = þ,
, and 0 represent nucleophilic, electrophilic, and radical attacks, respectively. We used Hirshfeld population analysis (HPA) to calculate local reactivity parameters. 9602

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Table 1. Elemental Data Analysis (in %) sample

Si

Al

Na

NaY

18.35

16.57

7.55

CoY

22.96

16.44

2.27

4.16

CoPhenY

17.60

16.02

6.12

1.70

15.18

7.12

0.03

2.64

77.70

11.28

8.38

CoPhen(neat)

Co

C

N

Cl

Figure 2. FTIR spectra of (a) NaY zeolite, (b) cobalt exchanged NaY, (c) CoPhen complex (neat), and (d) CoPhenY.

Figure 1. X-ray diffraction (XRD) pattern of (a) NaY, (b) Co2þ exchanged NaY, and (c) CoPhenY.

3. RESULTS AND DISCUSSION 3.1. Experimental Section. 3.1.1. Elemental Analysis. We first report the results of our elemental analysis. The elemental data of zeolite host and zeolite encapsulated complex are given in Table 1. It is seen from Table 1 that the percentage of aluminum has remained unchanged in cobalt exchanged zeolite Y, indicating the absence of dealumination during the ion-exchange process. The initial metal loading in the zeolite lattice is 4.16%, whereas the metal contents are considerably lower in the zeolite Y encapsulated complex. This decrease in metal content could be partly attributed to the formation of complexes inside the cavities which leaches out some of the metal ions. The metal, C, and N contents indicate a ligand/metal molar ratio of around 2, indicating cobalt to have 1:2 coordinations. 3.1.2. X-ray Diffraction Studies. XRD patterns of Na
Y, Co2þ exchanged zeolite, and the encapsulated CoPhen
Y are shown in Figure 1. Essentially similar diffraction patterns are noticed in the encapsulated CoPhen
Y and Na
Y, except the zeolite with encapsulated CoPhen has slightly weaker intensity. These observations indicate that the framework of the zeolite does not suffer any significant structural changes during encapsulation. There are, however, differences in the relative peak intensities of the 220 and 311 reflections appearing at 2θ = 10 and 12
, respectively. For pure zeolite Y and Co exchanged zeolite Y, I220 > I311, whereas, for the encapsulated complex, it is observed

that I311 > I220. This reversal in intensities has been observed in encapsulated complexes and empirically correlated to the presence of a large complex within the zeolite Y supercage.24 The above observation may therefore be construed as evidence for the successful encapsulation of Co phenanthroline complex within the supercage. 3.1.3. Infrared Spectroscopy (FTIR). The FTIR spectra of Na
Y, Co2þ
Y, and neat and encapsulated Co complexes are shown in Figure 2. IR spectra of Na
Y zeolite and metal exchanged zeolites show strong zeolite bands in the region 450
1200 cm
1. The strong band and broad band at 1000 cm
1 can be attributed to the asymmetric stretching vibration of (Si/Al)O4 units. The broad bands at 1650 and 3500 cm
1 are due to lattice water molecules and the surface hydroxylic group. The parent Na
Y zeolite shows characteristic bands at 463, 713, 786, 1021, and 1130 cm
1 (Figure 2, curve a), and these bands are not modified following the ion exchange with Co2þ or by supporting the cobalt complex (curves b and c), which further implies that the zeolite framework has remained unchanged upon encapsulation of the complex. The IR bands of the encapsulated complex are weak due to their low concentration in the zeolite cage and thus can only absorb in the region 1200
1600 cm
1. The IR spectra of the neat CoPhen complex show major bands at 1625 (CdC), 1509, 1417 (CdN), 846 (νC
H benzene ring), and 720 cm
1 (νC
H pyridine ring) for the coordinated Phen ligands. Similar frequencies are also observed in the case of CoPhen
Y with a little shifting of the bands at 1509 and 1417 cm
1 to higher frequencies (1522 and 1431 cm
1, respectively), indicating nitrogen coordination inside the cavity of the zeolite framework. The result shows that CoPhen complex is indeed present in the cavity of the zeolite. 3.1.4. Diffuse Reflectance Spectra. The formation and encapsulation of Co complex inside zeolite Y are further confirmed by the diffuse reflectance spectra (DRS) shown in Figure 3. It is evident from Figure 3a and b that, for the Co2þ
NaY, two peaks at 198 and 206 nm are observed in the DRS spectrum which can be assigned to ligand-to-metal charge transfer transition, in the present case from the oxygen ligand to a tetracoordinated cobalt Co2þ ion. In Figure 3c, the DRS spectrum of CoPhen 9603

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Figure 5. EPR spectra of (a) CoPhen neat complex and (b) CoPhenY.

Figure 3. UV
vis DR spectra of the samples (a) Co
NaY before calcination, (b) Co
NaY after calcination, (c) CoPhen
NaY, and (d) CoPhen (Neat complex).

Figure 4. Cyclic voltammogram of (a) CoPhen (neat) and (b) CoPhenY.

encapsulated in zeolite shows two additional broad bands at 222 and 288 nm which are not present in the Co2þ-exchanged zeolite Y sample. The spectrum of free Co phenanthroline is shown in Figure 3d. The bands at 227 and 267 nm belong to π f π* or n f π* orbital transition of the Phen ligand. The results show that the π f π* transition of the phenanthroline ligand is red-shifted upon encapsulation of cobalt phenanthroline in NaY zeolite. A possible origin of the spectral differences on encapsulation is that the CoPhen encapsulated in zeolite Y might not maintain the same electronic structure as that in the neat complex but may undergo slight distortion inside a constrained zeolite environment. 3.1.5. Cyclic Voltammetry. Cyclic voltammetry provides information on the nature of intrazeolite complexes that may not be readily apparent from spectroscopic studies. The voltammogram of a neat complex in solution mode (0.1 M phosphate buffer) shows a couple of peaks with values of Epc = 283 mV and Epa = 193 mV (Figure 4a). This redox process, when associated with a cathodic peak, is the reduction of cobalt(II) phenanthroline to cobalt(I) and when associated with an anodic peak is the oxidation of Co(I) to Co(II). This shows that the redox process is quasi-reversible in nature. On encapsulation in zeolite Y, the reduction potential is shifted toward more positive Epc = 0.422 mV and Epa = 211 mV and peaks are broadened (Figure 4b). The shifting of peak potential toward more positive values with respect to scan rate upon encapsulation indicates that the zeolite

matrix favors reduction of Co(II) to Co(I). A similar kind of shift in the anodic potential toward more positive values was reported for the interaction of the cobalt(III) phenanthroline complex with DNA.15 The alteration of peak potential indicates that the metal complex is encapsulated inside the zeolite matrix and not present on the external surface. The shifting of the redox potential and the peak broadening of an encapsulated metal complex are due to the interaction of the metal complex with the walls of the zeolite matrix. Because of the partial covalent character of the aluminosilicate crystals, electrons are not localized on the framework atoms; rather, they are partially delocalized.25 When a metal complex interacts with an active site, it will perturb all the active sites present within the zeolite so that the complex will have different interaction energies and altered redox potential at different places in the zeolite. The shifting of redox potential of the metal complex when encapsulated in zeolite may correlate well with the changes in energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of metal complexes. 3.1.6. Electron Spin Paramagnetic Study. Powder EPR spectra of neat CoPhen complex and the encapsulated one are obtained with microwaves in the 9442
10000 MHz region at liquid N2 temperature with fields corresponding to about 50
500 mT. Figure 5 shows the recorded spectra of the neat and encapsulated complexes. As expected, for axially distorted octahedral Co2þ ions, the broad single line having a g value of 4.23 corresponds to the high spin d7 ion in the weak field region. In the neat complex, a g value of 1.90 corresponds to the unpaired electron present in the 2p orbital of the chloride ion. In the case of the encapsulated complex, EPR shows hyperfine lines which may be due to the interaction of the 57Co (I = 7/2) nucleus with its own nucleus. This is comparable to that observed in other tetracoordinate complexes of high spin Co2þ.26 The splitting of spectroscopic states of high-spin Co2þ complex inside the zeolite framework may also be due to the combined effects of the symmetry of the crystal field and spin
orbit coupling. The presence of the hyperfine spectrum in the encapsulated complex in the low field region gives more evidence of encapsulation of a metal complex inside the supercage of zeolite Y. 3.1.7. Scanning Electron Microscopy (SEM) Analysis. Encapsulation of the Co(II) complex of phenanthroline ligand in Y zeolite is accomplished by using a flexible ligand synthesis scheme. The ligands, which are flexible enough to diffuse through the zeolite channels, react with the pre-exchanged metal ions in the supercage to afford an encapsulated complex. The product material is purified by extensive Soxhlet extraction with suitable 9604

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Table 2. Selected Bond Distances (in Å) and Bond Angles (in deg) of the Optimized Neat Complex, CoPhen, and Zeolite Encapsulated Cobalt Phenanthroline Complexes, CoPhenY bond distance

Figure 6. SE micrographs of CoPhen
Y before (A) and after (B) Soxhlet extraction.

bond angles

bonds

CoPhen

CoPhenY

angles

CoPhen

CoPhenY 83.4

Co
N1

1.903

1.910

— N1
Co
N2

85.0

Co
N2

1.908

1.971

— N1
Co
N3

93.6

90.8

Co
N3

1.909

1.914

— N1
Co
N4

178.1

170.3

Co
N4

1.902

1.893

— N2
Co
N3

91.9

100.4

Co
Cl1 Co
Cl2

2.254 2.256

— N2
Co
N4 — N4
Co
N3

93.6 85.1

89.0 84.6

Figure 8. Positions of the (a) HOMO and (b) LUMO in the neat CoPhen complex.

Figure 7. Thermogravimetric analysis (TGA) results for (a) NaY, (b) CoNaY, (c) CoPhenY, and (d) CoPhen complex (neat).

solvents to remove unreacted ligand and surface complexes. The sample colors do not change upon purification, indicating that the complexation has occurred in the cavities and that the resulting complex does not diffuse out through the zeolite channels. SE micrographs of CoPhen
Y taken before and after purification are shown in Figure 6. In the SEM taken before purification, the complex deposited on the external surface is available, whereas, in the SEM of the finished product, no surface complex is seen and the particle boundaries on the external surface of zeolite are clearly distinguishable. These micrographs reveal the efficiency of the purification procedure to effect complete removal of extraneous complexes, leading to well-defined encapsulation in the cavity. 3.1.8. Thermal Analysis. The TG patterns of parent NaY, cobalt exchange zeolite (CoY), CoPhenY, and CoPhen (neat) are displayed in Figure 7. The comparison of thermogravimetric analysis for CoPhen and CoPhen Na
Y shows that the neat complex has four weight loss steps at about 100, 280, 410, and 610
C. On the basis of the weight changes, the first weight loss step corresponds to the loss of a water molecule as an endothermic phenomenon and the second weight loss step may be related to the loss of Cl2. There is a sharp weight change at 410
C which is attributed to loss of two phenanthroline groups. The weak peak at 610
C may be due to sublimation of CoC2. However, for the corresponding encapsulated complex, the weight loss extends up to 560
C, which indicates that the thermal stability is greatly enhanced and there is no peak at 100
120
C, indicating the absence of water of crystallization. This gives another piece of strong evidence for the inclusion of CoPhen in Na
Y. On the

basis of thermal analysis data, we may conclude that zeolite encapsulated CoPhen may be treated thermally without any significant decomposition. 3.2. Theoretical Calculation. The selected geometric parameters of the optimized neat complex and the zeolite encapsulated complex at the VWN/DN level of calculation are provided in Table 2. It is seen from Table 2 that the DFT calculations with the VWN functional and DN basis set can be used to predict the structure of the neat complex quite reasonably. The optimized geometry of the neat complex has an octahedral structure where the ligands are laying cis to each other, which is in agreement with X-ray crystal structure.27 The bond length between the metal and the ligand molecule slightly changes upon encapsulation. Quantum chemical calculations have proven that Si
O bonds in zeolites have covalent character.28 Valence electrons in zeolites are distributed all over the framework atoms as a partially delocalized electron cloud. At relatively short distances between the complex molecule and the walls of the zeolite cavities, the electron
electron repulsions will be operative, which will cause the bond length between the Co ion and the ligand molecule to change. The energy of the HOMO level for the neat complex is
5.092 eV, and that of the LUMO is
3.262 eV. In the neat complex, the HOMO lies at chlorine atoms, whereas the LUMO lies at the carbon atoms of the phenanthroline ring (Figure 8). On the encapsulated complex, both the HOMO (
5.112 eV) and LUMO (
3.947 eV) lie lower in energy compared to the neat complex (Figure 9). The encapsulated complex approaches site II (SII) of the zeolite cluster along the pseudo
C3 axis of the sixmember ring, which may be the reason for the HOMO of the encapsulated complex to lie in that direction. The change in energy of the HOMO and LUMO upon encapsulation can be correlated with the change in the redox potential. Since the LUMO is low lying in the case of the encapsulated complex and is stabilized by an energy value of
0.685 eV, it has the ability to 9605

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Table 4. Fukui Functions, Local Philicity Indices, and Local Softness for the Selected Atoms of the Neat CoPhen Complex atoms of CoPhen N N Co

fþ κ

f
κ

ωþ κ

ω
κ

sþ κ

s
κ

0.013

0.013

0.104

0.104

0.007

0.007

0.012 0.011 0.088 0.096 0.006 0.007
0.002
0.002
0.016
0.016
0.001
0.001

N

0.012

0.011

0.088

0.096

0.006

0.007

N

0.013

0.013

0.104

0.104

0.007

0.007

Cl

0.040

0.040

0.320

0.320

0.022

0.022

Cl

0.040

0.040

0.320

0.320

0.022

0.022

Table 5. Fukui Functions, Local Philicity Indices, and Local Softness for the Selected Atoms of the Encapsulated CoPhen Complex, CoPhen
Y atoms of CoPhen
Y

Figure 9. Positions of the (a) HOMO and (b) LUMO in the encapsulated CoPhen complex.

N

Table 3. Calculated Hardness (η, in ev), Chemical Potential (μ, in eV), Electrophilicity Index (ω in eV), and Global Softness (S, in eV) complex

μ

CoPhen


4.1780

CoPhenY


4.5290

η

ω

S

0.9165

7.9991

0.5455

0.5825

5.9741

0.8583

accept an electron and thereby has the tendency to undergo reduction, and this is clear from the cyclic voltammetric study. One of the aims of the current study is to obtain a reactivity oriented description of the molecular systems in terms of the reactivity index. We calculate both the global and local descriptors to understand the reactivity of the molecular systems. 3.2.1. Global Descriptors. The values of chemical hardness (η), chemical potential (μ), and electrophilicity index (ω) computed using the DN basis set in connection with the VWN exchange
correlation functional for the neat complex and the encapsulated one are given in Table 3. According to the maximum hardness principle (MHP),29,30 the most stable structure has the maximum hardness. Thus, the neat complex with maximum hardness is more stable compared to the encapsulated one. This complex has the maximum value of ω and the maximum value of μ and is less reactive, whereas the encapsulated complex with the minimum value of electrophilicity (ω) and chemical potential (μ) is more reactive. 3.2.2. Local Descriptors. Table 4 presents the Fukui functions
þ
(FFs, fþ κ and fk ), local softness (sκ and sκ ), and electrophilicty þ
index (ωκ and ωκ ) of the most important atoms, namely, cobalt, nitrogen, and chlorine, calculated using the Hirshfeld population analysis (HPA) scheme of the neat cobalt complex, and Table 5
contains these values for the encapsulated one. The f
k , sκ , and
ωκ values are analyzed in the context of an electrophillic attack,

and the larger value of f
k , sκ , and ωκ corresponds to the most probable reaction site available for receiving an electrophile. We found, as may be seen in Tables 4 and 5, for the neat CoPhen complex the most preferred site for the attack of an electrophile is clearly predicted on the two chlorine atoms having the highest

positive value of f
k , sκ , and ωκ and the least susceptible site for an electrophilic attack is the central cobalt metal ion with a

fþ κ

f
κ

ωþ κ

ω
κ

sþ κ

s
κ


0.002
0.002
0.012
0.012
0.002
0.002

N

0.011

0.009

0.054

0.066

0.008

0.009

Co

0.102

0.098

0.585

0.609

0.084

0.088

N N

0.020 0.011

0.021 0.013

0.125 0.078

0.119 0.066

0.018 0.011

0.017 0.009


negative f
κ value. A negative value of fκ is obtained in the case of the neat complex because the Fco value for the cation is higher than the Fco value for the ground state (eq 6). In the context of the rationalization of Li and Evans,31 a minimum f
κ value for cobalt would be an indicator that the nitrogen atom of the phenanthroline ring maximizes the hardness of the cobalt atom. However, on encapsulation into the framework of zeolite Y, the value of f
κ is altered with the cobalt metal ion having the most positive value of f
. κ The values of f
, k s
, κ and ω
κ suggest that the encapsulation modifies the reaction site for an electrophillic attack from that of the neat complex. Presently, we are using the zeolite encapsulated complex as a base catalyst for nitroaldol reaction which will be published elsewhere.

4. CONCLUSIONS The Y zeolite encapsulated Co(II) complex of phenanthroline ligand has been synthesized using the flexible ligand method. The encapsulated complex exhibits fairly clear evidence in the physiochemical and spectrochemical characterization for the welldefined inclusion and distribution of complex inside the zeolite matrix. The results of the spectrochemical study and the DFT calculations support that the encapsulated complex undergoes distortion under the influence of a constrained zeolite framework. The difference in the electrochemical response and the peak broadening in cyclic voltammetry upon encapsulation of a metal complex is studied in correlation with the change in the positions of the HOMO and LUMO levels of the molecular systems. Evaluation of the reactivity indexes (e.g., Fukui functions) suggests that the cobalt atom is the most susceptible site for both nucleophilic and electrophilic attack. ’ ACKNOWLEDGMENT The authors thank Prof. B. Viswanathan, IIT Madras, for fruitful discussion and the Department of Science and Technology, New Delhi, for financial support. 9606

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dx.doi.org/10.1021/jp2003672 |J. Phys. Chem. C 2011, 115, 9601–9607