Synthesis and characterization of versatile MgO–ZrO₂ mixed metal oxide nanoparticles and their applications

Abstract

A heterogeneous, versatile nano-magnesia-zirconia or MgO–ZrO₂ (MZ) catalyst was prepared by an ultra dilution method. The as-synthesised catalyst was characterized by several analytical techniques such as XRD, particle size analysis, BET surface area, thermogravimetric analysis (TGA), differential thermal analysis (DTA), FT-IR spectroscopy, SEM (scanning electron microscope), TEM (transmission electron microscope) and XPS (X-ray fluorescence spectroscopy). The surface area is found to be 268 m² g⁻¹. The catalytic activity of MZ was tested for various important organic reactions such as cross-aldol condensation, N-benzyloxycarbonylation of amines, reduction of aromatic nitrocompounds, and synthesis of 1,5-benzodiazepines. It has been observed that for all reactions MZ shows a good catalytic activity. All corresponding products were obtained in good to excellent yield under mild conditions. The MgO-ZrO₂ catalyst can be prepared from inexpensive precursors, has high surface area, and is reusable and recyclable for all reactions.

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Introduction

In heterogeneous catalysis, mixed metal oxides (MMOs) play a very important role because of their versatile chemical and physical properties and wide applications in the field of catalysis and technology.^1–5

Mixed metal oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or supports. Mixed oxides are applied both for their acid–base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis.⁶

There is very much interest to understand the phenomena for the deposition of one oxide on another oxide. There are various methods for the preparation of MMOs such as sol–gel,⁷ wet impregnation,⁸ mechanochemical synthesis,⁹ hydrothermal method,¹⁰ and co-precipitation.¹¹

Recently Parida et al prepared nano-sized iron–cerium mixed oxides with diameters ranging from 10 to 20 nm by slow addition of a precipitating agent and reported their application as photocatalysts for dye degradation.¹² Nanometer coupled oxide ZnO–SnO₂ was prepared by An et al using the co-precipitation method, with slow addition of aqueous ammonia.¹³ Zhang and co-workers prepared nanosized ZnO–SnO₂ with high photocatalytic activity by slow addition of NaOH as a precipitating agent with vigorous stirring.¹⁴ Yoo and coworkers synthesized MgO–CeO₂ mixed oxide catalysts by co-precipitation using ionic liquid and slow addition of NaOH for dimethyl carbonate synthesis.¹⁵ The advantages of this slow or drop wise co-precipitation method are that the synthesized mixed metal oxides were obtained in nanosize and having high surface area.

Previously, we have synthesized the MgO–ZrO₂ catalyst by co-precipitation using magnesium nitrate [Mg (NO₃)₂·6H₂O] and zirconium oxychloride [ZrOCl₂·8H₂O] was found to have an excellent catalytic activity for the Knoevenagel condensation reaction, under solvent-free conditions.²² In this article, we have modified the co-precipitation technique with increased quantity of distilled water ten times, than the previous one, stirring speed 5000 RPM, while, it was 400–500 RPM in the previous method and catalyst calcinations also done by gradient heating for 10 h (ESI†). Due to a large quantity of distilled water, we called this method “ultra-dilution co-precipitation method”. Notably, the surface area of the catalyst is increased from 42.3 to 268.7 m² g⁻¹ and MgO–ZrO₂ particles were obtained in nano size (TEM). The prepared MgO–ZrO₂ catalyst was characterized by several analytical techniques such as XRD, FT-IR, TG-DTA, nitrogen adsorption, SEM-EDS, TEM and XPS. In continuation of our efforts to study the catalytic activity of heterogeneous catalysts as well as mixed metal oxides,¹⁶ the MgO–ZrO₂ catalyst was tested for various important organic reactions such as cross-aldol condensation, Cbz–protection of amines, reduction of aromatic nitro compounds and synthesis of 1,5-benzodiazepines.

Experimental

All commercial reagents were used as received unless otherwise mentioned. For analytical and preparative thin-layer chromatography, Merck, 0.2 mm and 0.5 mm Kieselgel GF 254 pre-coated were used, respectively.

Catalyst preparation

In continuation of our interest to explore the utility of the MZ catalyst in organic synthesis, we have successfully prepared the MgO–ZrO₂ catalyst, by an ultra dilution method as described below.

In a typical experiment, for the preparation of MgO–ZrO₂ an appropriate amount of magnesium nitrate [Mg(NO₃)₂·6H₂O] (3.10 g) and zirconium oxychloride [ZrOCl₂·8H₂O] (8.11 g) was dissolved together in a 2 L flask with 1 L deionized water. Dilute ammonia solution was added dropwise with vigorous stirring (RPM-5000) until the precipitation was complete (around 6 to 8 h and pH = 10.0). The resultant precipitate was filtered and washed with distilled water till free from chloride ions. The residue was dried for 24 h at 383 K in an oven and the obtained precipitate of metal hydroxides heated in a porcelain crucible progressively to 873 K for 10 hours (ESI†).

Catalyst characterization

After calcinations, the MZ catalyst was characterized by various analytical and spectroscopic techniques. The X-ray powder diffraction pattern was obtained using a conventional powder diffractometer (Philips 1050) using graphite monochromatized Cu-Kα radiation operating in Bragg–Brentano (θ/2θ) geometry. Nitrogen adsorption measurements were carried out at −196 °C using Micromeritics ASAP 2020. The total pore volume was evaluated from the amount of nitrogen adsorbed at the highest relative pressure of 0.99. The pore-size distribution was estimated by applying the BJH method to the desorption isotherm. Before each measurement, the samples were degassed at 4 ×10⁻³ mbar at 200 °C for 10–12 h. The particle size of the catalyst was determined on a computerized inspection system (Galai–Cis–1).

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) under a nitrogen atmosphere were carried out by using Seiko instruments Model-326. The thermal traces under an air (static) atmosphere were recorded employing Mettler Toledo star instruments. The Fourier transform infrared spectrum (FT-IR) of the sample was recorded on a Nicolet–360 FT-IR spectrometer using KBr pellets. We have also determined the strength of basic sites on catalysts by an indicator method.¹⁷ The basic site was found to be stronger than H_ = 26. Here H_ is the acidity function and is defined as a measure of ability of the basic solution to abstract a proton from an acidic solute.

Transmission Electron Microscopy (TEM) experiments were performed on a Hitachi 8100 microscope with a Rontec standard EDS detector and digital image acquisition. The scanning electron microscopy (SEM) analysis (FEI, Quanta 200) was carried out to study the surface morphology. The MgO–ZrO₂ fine powder was placed on a carbon stub and the images were recorded at 5–15 kV using an LFD detector under low vacuum.

XPS measurements were performed on a VSW XPS system with the Class 100 energy analyzer being a part of an experimental setup assembled for surface investigation. The spectra were taken in fixed analyzer transmission mode with the pass energy of 22 eV i.e. FAT 22. The analysis has been performed using the non-monochromatic Al K_α line (photon energy of 1486.3 eV) in order to enable recording of the most intensive Mg 1s XPS peak at about 1303 eV. MgO–ZrO₂ fine powder was prepared for XPS by pressing on an In (Indium) plate as a matrix in order to reduce the charging problems and provide a mechanical support. For the energy axis calibration Ag (110) and polycrystalline Au samples (previously cleaned by ion sputtering) were used. The energy was calibrated to the peak position of Ag 3d_5/2 (binding energy of 368.22 eV) and Au 4f_7/2 (binding energy of 83.96 eV) lines.

X-Ray diffraction analysis

The XRD spectrum of MgO–ZrO₂ is depicted in Fig. 1. Monoclinic ZrO₂ and cubic MgO phases were observed in powder diffraction patterns of MZ. A small shoulder diffraction peak was recognized as a tetragonal phase of ZrO₂ at 2θ = 30°. The XRD pattern shows the presence of both MgO and ZrO₂ in the sample.¹⁸

Fig. 1 XRD spectrum of MgO–ZrO₂.

Particle size analysis

The particle size of MgO–ZrO₂ is shown in Fig. 2. The MgO–ZrO₂ catalyst was ground to 100 mesh sizes. The particle size is in the range 0.5–0.7 μm.

Fig. 2 Particle size profile of MgO–ZrO₂.

FT-IR spectrum of MgO–ZrO₂

The Fourier transform infrared spectrum (FT-IR) of MgO–ZrO₂ shows a strong absorption at 470 cm⁻¹ due to the Zr–O vibration. The intense bands were observed at 3385 and 1630 cm⁻¹, it could be due to hydrated compounds.¹⁹

The FT-IR spectrum of MZ in the ν(OH) region shows one broad band around 3600–3700 cm⁻¹, this band corresponds to –OH stretching vibrations of surface hydroxyl groups.²⁰ The peaks observed at 1190 cm⁻¹ and 1440 cm⁻¹ could be due to Mg–O interaction²¹ (Fig. 3).

Fig. 3 FT-IR spectrum of MgO–ZrO₂.

BET surface area and pore size analysis

The textural characterization of MgO–ZrO₂ was performed by BET surface area, pore volume and average pore diameter measurements (Table 1). N₂ adsorption–desorption isotherms of the MgO–ZrO₂ sample are shown in Fig. 4.

Table 1 Nitrogen physisorption of MgO–ZrO₂

Catalyst	BET surface area/m^2 g^−1	Pore volume/cm^3 g^−1	Average pore diameter/Å
a Present work BET measurements.
MgO	26.1	0.25	42.5^22
ZrO2	18.1	0.14	38.3^22
MgO–ZrO2 (co-precipitation)	42.3	0.37	53.4^22
MgO–ZrO2 (ultradilution)	268.7	0.22	30.9^a

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Fig. 4 Nitrogen adsorption–desorption isotherm.

The system corresponds to a type IV nitrogen adsorption isotherm with the characteristic H1 type hysteresis at around P/P₀ = 0.40–0.80. The hysteresis loop formed is due to the capillary condensation within the mesopores and is indicative of well ordered uniform shape and size of mesopores.

The sharp increase in adsorption at higher pressures and the one step capillary condensation indicate the presence of large and uniform mesopores respectively. The surface area was 268.7 m² g⁻¹, while in our previous report²² it was only 42.3 m² g⁻¹. It could be due to slow precipitation in a bulk amount of water, with vigorous stirring (5000 RPM) and gradient heating of the hydroxides (ESI†).

TG-DTA profile of MgO–ZrO₂

The thermal stability of the prepared sample was investigated by a TGA/DTA method. The thermogram obtained for the uncalcined MgO–ZrO₂ is presented in Fig. 5. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were applied to the precursor hydroxide to determine the temperature for conversion of the precursors to MgO–ZrO₂. Both measurements were carried out from 30 °C to 650 °C at a heating rate of 5 °C min⁻¹. The analysis showed that the thermal decomposition consists of two major exothermic peaks in the differential thermal curve indicating that the hydroxides are thermally stable up to nearly 250 °C. The TG profile was characterized by two peaks of weight loss till 514 °C. The first one occurs at 30 °C to 277 °C (18.5% weight loss). The second weight loss occurred between (6.6% weight loss) 277 °C and 514 °C. The DTA profile showed two decompositions till 532 °C. First one occurs at 257 °C and the second decomposition at 532 °C.

Fig. 5 TG-DTA profile of MgO–ZrO₂.

TEM, SEM-EDS analysis

Transmission Electron Microscopy (TEM) experiments were performed on a Hitachi 8100 microscope with a Rontec standard EDS detector and digital image acquisition. From TEM and SEM images, it is clear that MgO–ZrO₂ particles were obtained in a nano size range and showed uniform-sized particles with somewhat spherical morphology with an average size range of 20–35 nm. TEM and SEM images are depicted in Fig. 6a–c. The EDS spectrum of the MgO–ZrO₂ catalyst clearly indicates the presence of Mg and Zr metals (Fig. 6d).

Fig. 6 (a) TEM images of MgO–ZrO₂ at 100 nm and (b) at 50 nm; (c) SEM image of MZ at 1 μm; (d) EDS spectrum of MgO–ZrO₂.

X-Ray photoelectron spectroscopy

The survey spectrum of MgO–ZrO₂ is shown in Fig. 7. The most intensive XPS and Auger lines are marked by arrows. Besides the main Zr, O and Mg lines (the last being of very small intensity) we also observed In lines (from the support) and also a C 1s line which is expected impurity.

Fig. 7 Survey spectrum of ZrO₂–MgO powder supported on In plate.

The chemical composition of the powder was determined from the characteristic XPS peak intensities of Zr, O and Mg i.e. Zr 3d, O 1s and Mg 1s. In order to improve the sensitivity, the lines were recorded with long acquisition times. As for the sensitivity factors, we use recommended values for the Mg K_α line,²³ which should differ from those for the Al K_α line up to 10%. This should give us fair estimation of the chemical surface composition of the sample.

The preliminary spectrum analysis shows that all peaks are systematically shifted towards higher binding energies, which we attributed to surface charging. The standard way to determine the energy shift due to charging is from the position of the C 1s peak, which should be at 284.9 eV (according to the NIST database) assuming that it originates from saturated hydrocarbons. The determined peak shift due to charging was 5.1 eV.

The XPS peaks of main interest i.e. O 1s, Zr 3d and Mg 1s are given in Fig. 8–10. Zr 3d and Mg 1s can be fitted to a single contribution while O 1s is best fitted to two contributions. The adopted peak model was a pseudo-Voigt profile taken as a product of Gaussian and Lorentzian profiles with the same position and the respective relative intensities 70 and 30 (GL30). The background subtraction has been performed using the approach of Shirley.²⁴ The identification of the contributions is performed using the NIST database available online.²⁵ Zr 3d lines are fitted to two contributions corresponding to 3d_5/2 and 3d_3/2 lines, respectively. Their intensity ratio is 59.97 : 40.03 which practically coincides with theoretically expected 60 : 40. FWHM of both peaks is 1.85 eV and their positions are 182.1 eV and 184.5 eV, respectively. The reported position of 3d_5/2 for ZrO₂ is in the range 182–184 eV, and the closest values to ours are reported in references.²⁶

Fig. 8 Zr 3d XPS line taken with the energy step of 0.1 eV and acquisition time window of 11 s.

Fig. 9 Mg 1s XPS line taken with the energy step of 0.1 eV and acquisition time window of 22 s.

Fig. 10 O 1s XPS line taken with the energy step of 0.1 eV and acquisition time window of 14 s.

The Mg 1s XPS line (Fig. 9) is fitted to a single contribution having the position of 1303.7 eV (the peak width is 2.2 eV FWHM) which fits well to 1303.9 eV reported for MgO.^27,30

As for the O 1s peak (Fig. 10), it was fitted to two contributions with the relative intensities 87.3 : 12.7. The peak positions are 530.0 eV and 532.2 eV while their widths are 2.1 eV. The range of reported positions for O 1s in ZrO₂ is 529.9–531.3 eV and the values 530.2 eV^28,29 and 529.9 eV³⁰ were reported to be closer to our values.

As for the additional contribution, and having in mind the analysis of other peaks, it is reasonable to attribute it to MgO. Indeed, the range of reported O 1s positions for MgO is 530–532.1 eV, and the value closest to our second contribution is reported³¹ at 532.1 eV.

Results and discussion

After characterization of the MgO–ZrO₂ (MZ) catalyst, we have tested its catalytic activity by carrying out several important organic reactions such as cross-aldol condensation, Cbz-protection of amines, catalytic transfer hydrogenation of nitro compounds and synthesis of 1,5-benzodiazepine. It has been observed that MZ gave excellent results for all types of reactions compared to its component oxides. The reactions are described as below.

Cross-aldol condensation

The cross-aldol condensation is a general method for the formation of a carbon–carbon bond in many classes of carbonyl compounds.³² Cross-aldol condensation of aromatic aldehydes with cyclic ketone is an important synthetic reaction for the preparation of α,α′-bis-cycloalkanones. These benzylidene derivatives are important precursors for the synthesis of bioactive pyrimidine derivatives.³³ They also play an important role as intermediates of perfumes,³⁴ pharmaceuticals, and agrochemicals. Considering these important applications of cross-aldol products in mind, we have tested the MZ catalyst for the cross-aldol condensation reactions of various aromatic aldehydes with cylcohexanone under solvent-free conditions at room temperature (Scheme 1).

Scheme 1 MgO–ZrO₂ catalyzed cross-aldol condensation under solvent-free conditions.

In order to study the effect of catalyst loading for cross-aldol reaction, we have carried out reaction between benzaldehyde and cyclohexanone under solvent-free conditions. The results are depicted in Fig. 11. It has been observed that 10 wt% of the catalyst is sufficient to catalyze the reaction to afford maximum yield (95%). We did not observe considerable increase in yield for 15 wt% and 20 wt% of the catalyst; however, 5 wt% of the MZ catalyst gives lower yield (55%). After study of this reaction, we have explored the catalytic activity of MZ for some other aldehydes. The result is depicted in Table 2.

Fig. 11 Influence of catalyst loadings.

Table 2 Cross-aldol condensation reaction of aromatic aldehydes with cyclohexanone^a

No.	Product	Isolated yield (%)	Melting point/°C
			Observed	Reported
a Reaction conditions—cyclohexanone = 5 mmol, aldehyde = 10 mmol, MgO–ZrO2 (10 wt% with respective of ketone), time = 30 min. Solvent-free.
1		95	116–118	117–118^35
2		96	161–162	161–163^35
3		94	163–165	164–165^35
4		97	176–178	177–178^35

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Under the solvent free reaction conditions, substituted aromatic aldehydes such as p-methoxy, p-nitrobenzaldehyde, and cinnamaldehyde worked well to give excellent yields of the corresponding products (94–97% yield).

We have also tested the reusability of MZ for the model reaction of benzaldehyde with cyclohexanone (Fig. 12). After completion of the reaction monitored by TLC, the catalyst was filtered off, washed with ethyl acetate several times, dried under vacuum at 60 °C for 6 hours and activated at 120 °C for 3 h and used for the next cycle. We observed that the yield of the corresponding product slightly decreased after the sixth cycle.

Fig. 12 Reusability profile of MZ for cross-aldol reaction.

General experimental procedure for cross-aldol condensation

To a mixture of cyclohexanone (5 mmol) and aldehyde (10 mmol), was added MgO–ZrO₂ (10 wt% with respect to ketone) and reactants were stirred at room temperature for 30 min under solvent-free conditions. The formation of product was monitored by thin layer chromatography. After completion of the reaction, the reaction mixture was dissolved in ethyl acetate and filtered off the catalyst from the reaction mixture and the solvent was concentrated and the product was purified by recrystallization in ethanol.

Cbz protection of amines

In a wide range of chemical conversions such as amino acid, peptide, glycopeptide, β-lactam, amino glycoside, and nucleoside synthesis, the nucleophilicity and basicity of the amino function can cause problems. The challenge of designing new amino protecting groups that are stable to a wide range of reaction conditions continues to attract a deal of attention.³⁶

The benzyloxycarbonyl (Cbz) protected amines are useful synthetic precursors for various pharmaceuticals and natural products.³⁷ This group is an important functionality for the protection of amines and amine derivatives, since it can be easily removed by catalytic hydrogenation without any side reactions and is stable to basic and most aqueous acidic conditions.³⁸

There are several methods available for the protection of amino groups as N-Cbz derivatives, which include LiHMDS as a base in THF-HMPA (hexamethyl phospharamide) solvent, β-cyclodextrin in aqueous medium, I₂ and La(NO₃)₃·6H₂O under solvent free conditions.³⁹ However, still there is scope to develop new methodology for Cbz protection of amines. The MgO–ZrO₂ catalyst was tested for benzyloxycarbonylations of amines under solvent-free conditions at room temperature (Scheme 2). A variety of amines worked well to give the corresponding N-Cbz products in excellent yield (Table 3).

Scheme 2 N-Benzyloxycarbonylation of amines under solvent-free conditions over MgO–ZrO₂.

Table 3 N-Benzyloxycarbonylation of amines under solvent-free conditions using the MgO–ZrO₂ catalyst^a

Entry	Amines	Product	Isolated yield (%)
a Reaction conditions amine = 1 mmol, Cbz-Cl = 1.2 mmol, MZ = 10 wt% with respective of amines, solvent-free, room temperature, time = 10 min for all reactions. All compounds are well characterized and reported in the literature.^40 b Yield after the fifth cycle. c 5 wt% catalyst used. d 15 wt% catalyst used.
1			94, 91^b
2			50^c
			95
			96^d
3			84
4			96

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This methodology provides a convenient method for the Cbz-protection of a wide range of amines such as aromatic (Table 3, entries 1 and 2), aliphatic (entry 3) and heterocyclic amines (entry 4). We have proposed the following mechanism for the benzyloxycarbonylation of aniline with Cbz chloride (Fig. 13). In first step, the –NH₂ group of aniline and Cbz-Cl is adsorbed on the catalyst⁴¹ as shown in I. It is well known that functional groups containing electronegative atoms such as N, O, and S are adsorbed on a metal or a Lewis acid site and hydrogen is adsorbed on oxygen of the catalyst.⁴² Then in the second step electrophilicity of carbonyl carbon of Cbz-Cl is increased and it generates an intermediate state II, which, finally yields the final product III along with liberation of HCl. The MgO–ZrO₂ catalyst participates in all these steps by weakening the chemical bonds of reactants and subsequently lowering the activation energy.

Fig. 13 Mechanism of Cbz protection of aniline with benzyloxycarbonyl chloride over MZ.

Typical experimental procedure

The corresponding amine (1 mmol) was added to benzyloxycarbonyl chloride (Cbz-Cl) (1.2 mmol) in the presence of MZ (10 wt% w.r.t. amine) and the reaction mixture was stirred under solvent-free conditions at room temperature for an appropriate time. After completion of the reaction, the product was extracted in ethyl acetate (3 × 5 mL). The combined organic layer was washed with brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to give a crude product which was purified by recrystallisation in ethanol, without any column purification.

Reduction of aromatic nitro compounds

The catalytic transfer hydrogenations of aromatic nitro compounds to the corresponding amines is an important step in the industrial synthesis of dyes, biologically active compounds, pharmaceuticals, rubber chemicals, and photographic and agricultural chemicals.⁴³ A wide range of methods have been developed for this purpose.⁴⁴

In general for the reduction reactions, which involve hazardous molecular hydrogen or Fe/HCl⁴⁵or Sn/HCl⁴⁶ catalytic transfer hydrogenation (CTH) employing hydrogen donors, e.g., propan-2-ol, is harmless, greatly selective, and environment friendly.⁴⁷ Various methods have been developed for the reduction of nitro compounds such as sodium borohydride/catalyst,⁴⁸ hydrazine catalyst,⁴⁹ Cu nanoparticles/HCOONH_4,⁵⁰ S₈/NaHCO_3,⁵¹ HI,⁵² and Sm/I_2.⁵³

Previously we have explored catalytic transfer hydrogenation reactions over heterogeneous catalysts,^54,55 and we herewith report MZ for catalytic transfer hydrogenation of aromatic nitro compounds. We have found that MZ is the best candidate for the catalytic transfer hydrogenation reactions of aromatic nitro compounds to the corresponding amines (Scheme 3).

Scheme 3 Reduction of aromatic nitro compound by using MgO–ZrO₂.

We have used KOH as a promoter and IPA (propan-2-ol or isopropyl alcohol) is a hydrogen source for the reduction reactions. However, in the absence of either IPA or KOH, we have not observed the corresponding product. The results of catalytic transfer hydrogenation reaction over MZ are depicted in Table 4.

Table 4 Catalytic transfer hydrogenations of aromatic nitro compounds in the presence of MgO–ZrO₂ to the corresponding amines^a

No.	Reactant	Product	Time/h	Isolated yield (%)	Isolated yield after 5th cycle
a Reaction conditions: nitrocompound (2 mmol), KOH pellets (2 mmol); catalyst (10 wt% with respect to starting materials); propan-2-ol (5 ml), reflux.
1			2	94	91
2			3	96	92
3			4	94	91
4			4	90	87

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The effect of reaction time was examined for the reduction of nitrobenzene and it was observed that MZ gives quantitative yield (94%) within 2 hours; however in 3 hours of time, the yield of aniline was not increased further (94%). Therefore, the reaction time 2 h was optimal for the reduction of nitrobenzene to aniline (Fig. 14).

Fig. 14 Influence of reaction time on % yield.

The effect of catalyst loading was also examined for the reduction of nitrobenzene. It was observed that 10 wt% catalyst is enough to give excellent yield for the reduction reaction, while further increase in the amount of catalyst such as 15 wt%, 20 wt% did not affect the yield of the product (Fig. 15). The further substrates scope is depicted in Table 4.

Fig. 15 Influence of catalyst wt%.

All aromatic nitro substituted compounds such as –Cl, –CH₃ and –OCH₃ are compatible under these reaction conditions and no other side products were observed.

We have proposed the following possible mechanism for hydrogen transfer reaction of nitroarenes to aromatic amines (Fig. 16). Previously, we have described the mechanism of catalytic transfer hydrogenation over γ-Fe₂O₃.⁵⁶ During adsorption of propan-2-ol the hydrogen from the –OH gets adsorbed as proton and hydrogen of C–H migrates via hydride transfer to the substrate. Hence the rate of reaction can show dependence on the strength of adsorption of both propan-2-ol and substrate. First nitrobenzene and isopropyl alcohol adsorbed on nano-MgO–ZrO₂I, then reduction of nitrobenzene with IPA as a hydrogen source generates an intermediate II with elimination of an acetone molecule, then intermediate III with elimination of a water molecule and finally aniline Vvia intermediate IV.

Fig. 16 Mechanism for reduction of nitroarenes to aromatic amines over MZ.

General procedure for reduction reaction

In a typical CTH reaction, KOH pellets (2 mmol) were dissolved in propan-2-ol (5 mL) to which the substrate (2 mmol) was added along with the catalyst. It was then refluxed for a few hours depending upon the nature of the substrate. After completion of the reaction monitored by thin layer chromatography, the reaction mixture was extracted in ethyl acetate and repeatedly washed with water (5 to 7 times) to remove KOH. The obtained aromatic amines were purified by preparative TLC using ethyl acetate and hexane as eluent.

Synthesis of 1,5-benzodiazepine

1,5-Benzodiazepines are generally synthesized by acid catalyzed condensation of o-phenylenediamines with ketones. A large number of catalytic processes are reported for the synthesis of benzodiazepines which include the use of various metal salts, CAN, heteropolyacids, ionic liquids etc.⁵⁷

1,5-Benzodiazepines are biologically important molecules and are extensively used clinically as analgesic, hypnotic, sedative and antidepressive agents.⁵⁸ Also, benzodiazepines and their derivatives are a useful functionality of bioactive compounds. They are applicable and are broadly used as anticonvulsant, anti-inflammatory, analgesic, hypnotic, sedative, and antidepressive agents.⁵⁹ Benzodiazepines are also valuable intermediates for the synthesis of fused ring compounds such as triazolo-, oxadiazolo-, oxazino-, and furano-benzodiazepines.⁶⁰

We have carried out reaction of orthophenylene diamine (OPDA) with various ketones by using MgO–ZrO₂ under solvent-free conditions at room temperature. It has been noted that all ketones such as aliphatic, cyclic and aromatic worked well under solvent-free conditions to obtain the corresponding benzodiazepines in good yield. The reaction of cyclohexanone and orthophenylene diamine (OPDA) is depicted in Scheme 4.

Scheme 4 MgO–ZrO₂ catalyzed synthesis of 1,5-benzodiazepine by using cyclohexanone and orthophenylene diamine.

The influence of time was examined for the reaction between cyclohexanone and OPDA, it was observed that 1.5 h was sufficient time for the synthesis of 1,5-benzodiazepine in excellent yield. However, at 2.0 h, there is no considerable increase in the yield of the corresponding product was observed (Fig. 17).

Fig. 17 Influence of time for the reaction of cyclohexanone with OPDA.

The feasibility of the catalyzed process was shown by using various substrates. Various ketones such as aliphatic and aromatic gave excellent yield with OPDA. The results are depicted in Table 5. We have studied the reusability of the MZ catalyst for all reactions; we have found that no considerable decrease in yield was observed even after five cycles.

Table 5 Synthesis of 1,5-benzodiazepine derivatives using MgO–ZrO₂ under solvent-free conditions at room-temperature^a

Entry	Ketone	Product	Isolated yield (%)	Yield (%) after 5th cycle	Melting point/°C
					Found	Reported
a Reaction conditions: OPDA (1 mmol), ketone (2.5 mmol), catalyst 20 wt% w.r.t. ketone, time=1.5 h.
1			97	95	135–136	137–139^61
2			95	93	137–138	137–139^61
3			92	89	136–137	138–139^61
4			93	90	150–151	151–152^61

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General procedure for the synthesis of 1,5-benzodiazepine

In a 10 mL round bottom flask orthophenylene diamine (1 mmol), ketone (2.5 mmol) and 20 wt% catalyst were added. The reaction mixture was stirred at room temperature. The progress of the reaction was monitored by TLC with hexane and ethyl acetate (5 : 1) as eluent. The reactions were completed in 1.5 h. After the completion of the reaction, reaction mixture was extracted with ethyl acetate and compounds were purified by recrystallisation in ethanol. All the products were known compounds and were characterized by FT-IR and ¹H NMR spectroscopic data.⁶¹

Conclusions

Nano-MgO/ZrO₂ was found to catalyze efficiently a variety of organic reactions such as cross-aldol condensation, Cbz-protection of amines, reduction of aromatic nitro compounds, and synthesis of 1,5-benzodiazepine. In cross-aldol condensation reactions, the corresponding products were obtained in excellent yield, under solvent-free conditions. The significant features of this process include high yields, ambient temperature, ease of product isolation and reusability of the catalyst. N-Benzyloxycarbonylation of amines worked well under solvent-free conditions to yield the corresponding N-Cbz products in quantitative yield. Reduction of aromatic nitro compounds to the corresponding amines worked well by using IPA as a hydrogen source and KOH as a promoter. Synthesis of 1,5-benzodiazepine derivatives using MgO–ZrO₂ under solvent-free conditions at room-temperature resulted in excellent yield of compounds. We believe that this versatile catalyst can also be useful for other important organic reactions and has great scope in catalysis technology. Further development and applications of MgO–ZrO₂ mixed metal oxide nanoparticles are under progress in our laboratory.

Acknowledgements

Manoj Gawande thanks to Foundation of Science and Technology, Portugal for the award of Research Fellowship SFRH/BPD/64934/2009 and financial support.

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c1cy00259g

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