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DOI: 10.1039/C4RA03245D (Paper) RSC Adv., 2014, 4, 30081-30089

Sol–gel synthesis of highly pure α-Al2O3 nano-rods from a new class of precursors of salicylaldehyde-modified aluminum(III) isopropoxide. Crystal and molecular structure of [Al(OC6H4CHO)3]†

Anita Raj Sanwariaa, Meena Nagara, Rakesh Bohra*a, Archana Chaudharyb, Shaikh M. Mobinb, Pradeep Mathurb and Banwari L. Choudharyc
aDepartment of Chemistry, University of Rajasthan, Jaipur-302004, India. E-mail: rkbohrachem@gmail.com
bDiscipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Indore 452017, India
cUSIC, University of Rajasthan, Jaipur-302004, India

Received 10th April 2014 , Accepted 4th June 2014

First published on 9th June 2014

Abstract

Reactions of aluminum isopropoxide with salicylaldehyde in 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3 molar ratios in anhydrous benzene yield complexes of the type [(OPri)3−nAl(OC6H4CHO)n] {where n = 1(1); n = 2 (2); n = 3 (3)}. All the products are yellow solids and are soluble in common organic solvents. They were characterized by elemental analysis, ESI-mass spectrometry, FT-IR and 1H, 13C and 27Al NMR studies. The ESI-mass spectral studies indicate dimeric nature for (1) and (2) and monomeric nature for compound (3). Crystal and molecular structures of [Al(OC6H4CHO)3] (3) suggest that salicylaldehyde ligands bind to the metal in a side-on dihapto η2-(O,O) manner, leading to the formation of a hexa-coordinated environment around the aluminum atom. Powder XRD, SEM image, and EDX analysis appear to indicate formation of nano-sized rods for precursor (3). Sol–gel hydrolysis of all the precursors Al(OPri)3, (1), (2) and (3) followed by sintering at 1100 °C yielded α-Al2O3 (a), (b), (c) and (d), respectively. The powder X-ray diffraction patterns and the SEM images of all the oxides exhibit nano-sized microcrystalline morphology for (a) and mixed platelike nano-rod morphology for (b), (c) and (d). The EDX and TEM studies of (d) also corroborate the formation of α-Al2O3. The IR spectral studies of all the oxides indicate the formation of pure α-alumina, a versatile ceramic oxide known for exhibiting a wide variety of applications in engineering and biomedical areas.

1. Introduction

Corundum (α-Al2O3) is one of the most important ceramic materials due to its high strength at elevated temperatures, hardness, high melting point, thermal conductivity, chemical inertness, large band-gap, abrasion resistance and thermal shock resistance properties.1–7 Such excellent properties make it the best quality ceramic material for a wide range of applications in engineering (electronics; optoelectronics; reinforcement filler in composites; cutting tools; spark plugs; thermoluminescent dosimeters and uses in refractory materials),8–15 biomedical areas (bio-ceramic materials; inert ceramic materials in medicine and dentistry)16,17 as well as in modern surgery (total artificial hip replacement; knee joint implants and bone screws).18,19

Alpha-alumina nano-structures such as nanotubes, nanowires, nanobelts, nanoplatelets, and spherical nanoparticles have been receiving increasing attention.20–24 Reported synthetic methods for nano-sized α-Al2O3 are chemical vapor deposition (CVD),21 combustion chemical deposition (CCD),22 atomic layer deposition (ALD),28 spray pyrolysis,27 solution combustion synthesis (SCS),26 and sol–gel method using different organic precursors: aluminum secondary butoxide, aluminum nitrate or aluminum isopropoxide.25,29–31

It is interesting to mention here that aluminum(III) complexes of 8-hydroxyquinoline like Alq3 have been widely used as a precursor for the fabrication of organic electroluminescent diodes.32–34 Although formation of δ-Al2O3 from 8-hydroxyquinoline modified aluminum(III) isopropoxide has recently been reported,35 little is known about the formation of α-Al2O3 from chemically modified aluminum(III) alkoxide precursors.36

Herein, we report sol–gel synthesis of highly pure α-Al2O3 nano-rods from a new class of salicylaldehyde-modified aluminum(III) isopropoxide precursors and crystal and molecular structure of [Al(OC6H4CHO)3] (3).

2. Results and discussion

2.1 Synthesis and characterization of aluminium(III) isopropoxide derivatives with salicylaldehyde (HOC6H4CHO)

Reactions of aluminum(III) isopropoxide with salicylaldehyde (HOC6H4CHO) in 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 1[thin space (1/6-em)]:[thin space (1/6-em)]2 and 1[thin space (1/6-em)]:[thin space (1/6-em)]3 molar ratios in anhydrous benzene yield precursors of the following type (Scheme 1):
image file: c4ra03245d-s1.tif
Scheme 1
where n = 1(1); n= 2(2); n= 3(3).

The above reactions were quantitative and their progress was monitored by determining the liberated 2-propanol in benzene-2-propanol azeotrope by oxidimetric method.37 All the three products (1), (2) and (3) were yellow solids, soluble in common organic solvents. The products were characterized by FT-IR and 1H, 13C and 27Al NMR spectral studies. Synthetic and elemental analyses of all these complexes are summarized in Table 1. The ESI mass spectral studies indicate dimeric nature for (1) and (2) and monomeric nature for (3) (Table 2).

Table 1 Synthetic and analytical data of aluminium(III) isopropoxide derivatives (1–3)
C* Reactants (a) = Al(OPri)3, (b) = HOC6H4CHO Nature of the product Mol. ratio Yield % Found (cal.) in g Analysis % found (cal.) MP °C
Lib. PriOH OPri Al C H
(1) (a) 2.57 g, 12.58 mmol Yellow solid 1[thin space (1/6-em)]:[thin space (1/6-em)]1 99.85 0.748 0.11 10.0 58.52 7.10 ∼110
(b) 1.54 g, 12.61 mmol (0.75) (0.13) (10.1) (58.62) (7.13)
(2) (a) 2.81 g, 13.75 mmol Yellow solid 1[thin space (1/6-em)]:[thin space (1/6-em)]2 99.76 1.63 0.09 8.00 61.89 5.20 ∼150
(b) 3.36 g, 27.51 mmol (1.65) (0.10) (8.21) (62.14) (5.17)
(3) (a) 1.66 g, 8.12 mmol Yellow solid 1[thin space (1/6-em)]:[thin space (1/6-em)]3 99.86 1.44 — 6.81 62.58 3.94 ∼189
(b) 2.97 g, 24.34 mmol (1.46)   (6.88) (64.35) (3.83)

Table 2 Fragmented molecular ion m/z values of (1) to (3)
Complexes Fragmented ions m/z values
(3) [Al(OCHOC6H4)(OCHOC6H4)(OCHOC6H4)]+ 389
[Al(OCHOC6H4)(OCHOC6H4)H2]+ 270
[Al(OCHOC6H4)(O2CH5)H2]+ 198
[Al(OCHOC6H4)(OC)H2]+ 177
[Al(OCHOC6H4)(O)H]+ 164
[Al(OCHOC6H4)H]+ 148
[Al(OCHC4H3)]+ 106
[Al(OC)]+ 54
(2) [(OCHOC6H4)2Al(μ-OC3H7)2Al(OCHOC6H4)2]+ 655
[(OCHOC6H4)2Al(μ-OC3H7)2Al(OCHOC6H4)]+ 534
[(OCHOC6H4)2Al(μ-OC3H7)2Al(OC6H5)]+ 506
[(OCHOC6H4)2Al(μ-OC3H7)2Al(OC2H)]+ 454
[(OCHOC6H4)2Al(μ-OC3H7)2Al(O)]+ 429
[(OCHOC6H4)Al(μ-OC3H7)2Al(O)]+ 308
[(OC6H5)Al(μ-OC3H7)2Al(O)]+ 379
[(OC2H)Al(μ-OC3H7)2Al(O)]+ 228
[(OH)Al(μ-OC3H7)2Al(O)]+ 203
[(OH)Al(μ-OC3H7)Al(O)]+ 145
(1) [(OC3H7)2Al(μ-OC3H7)2Al(OCHOC6H4)2]+ 531
[(OC3H7)2Al(μ-OC3H7)2Al(OCHOC6H4)2]+ 410
[(OC3H7)2Al(μ-OC3H7)2Al(OC6H5)]+ 382
[(OC3H7)2Al(μ-OC3H7)2Al(OC2H)]+ 330
[(OC3H7)2Al(μ-OC3H7)2Al(O)]+ 299
[(O2C3H7)Al(μ-OC3H7)2Al(O)]+ 256
[(O)2Al(μ-OC3H7)2Al(O)]+ 213
[(O)2Al(μ-OC3H7)Al(O)]+ 176

2.1.1 IR and NMR spectra. The IR and 1H and 13C NMR spectral data of all these derivatives are summarized in Table 3.
Table 3 FT-IR spectral data (cm−1) and (1H and 13C NMR) spectral data (δ ppm) of (1), (2) and (3)a
C* FT-IR spectral data Isopropoxy Salicylaldehyde moiety
Compound νC–O νC–H C=C νAr–O νAl–O νCHO νAl–O–Al
a s = strong, m = medium, w = weak, C* = compound; s = singlet; d = doublet; m = multiplet; sept. = septet.
(1) [Al(OPri)2(OC6H4CHO)]2 1000 s 3049 w 1603 s 1125 m 643 m 1660 s 755 w
(2) [Al(OPri)(OC6H4CHO)2]2 1001 s 3048 w 1604 s 1128 m 645 m 1650 s 756 w
(3) [Al(OC6H4CHO)3] — 3047 w 1605 s 1131 m 644 m 1640 s —
C* 1H NMR spectral data
Isopropoxy moiety Salicyladehyde moiety

image file: c4ra03245d-u1.tif
CH3– −OCH< CHO OH Aromatic protons
(1) 1.30–1.48, d (24H) 4.00–4.25 sept (4H) 11.02 s (1H) — 6.98–7.58 m (4H)
(2) 1.11–1.33, d (12H) 4.05–4.29 sept (2H) 11.01 s (2H) — 6.98–7.58 m (8H)
(3) — — 11.00 s (3H) — 6.98–7.58 m (12H)
C* 13 C-NMR spectral data
Isopropoxy moiety Salicylaldehyde moiety

image file: c4ra03245d-u2.tif
CH3 –OCH< C1 C3 and 2 Others C4–7
(1) 25.31–27.95 63.05–66.13 195.00 132.71–137.58 114.14–127.70
(2) 26.10–27.75 64.69–66.02 194.99 132.71–138.57 113.45–128.70
(3) — — 193.01 132.71–139.55 113.84–129.70

In the IR spectra, broad stretching vibrations at ∼3145 cm−1 due to the –OH group of the free ligand disappeared, suggesting the deprotonation of the –OH group and concomitant formation of an Al–O bond.35,37 Bands in the region 643–645 cm−1 may be assigned to ν(Al–O) stretching vibrations.33,34 The medium intensity band observed in the region of 1000–1003 cm−1 is assigned to ν(C–O) of the isopropoxy group. The Al–O–Al vibrations have been observed in the region 755–756 cm−1.38–40 The IR spectrum of the salicylaldehyde ligand exhibits an aldehydic ν(C–O) stretching band at about 1670 cm−1, which undergoes a downfield shift in precursors (1), (2) and (3) at ∼1660, 1650 and 1640 cm−1, respectively, indicating coordination of the aldehydic group with the aluminum atoms in the precursors.

In the 1H NMR spectra, the signal due to the hydroxyl proton of the free ligand at 8.80 ppm was found to be absent in all these derivatives, suggesting the formation of Al–O bond by deprotonation of the –OH group of the free ligand. The methine protons of the bridging and terminal isopropoxy groups overlapped to give a multiplet in the range 4.0–4.29 ppm. Similarly, the methyl protons of the bridging isopropoxy groups appear at 1.11–1.37 ppm as broad doublets and the terminal isopropoxy groups appear at 1.46–1.48 ppm, also as doublets with different intensities, indicating the nonequivalent nature of the bridging and terminal isopropoxy groups in (1) and (2).

In the 13C NMR spectra of all the complexes, the distribution of new resonance lines (114.14–195.01 ppm) of the ligand moiety indicates bonding between aluminium(III) and salicaldehyde. The characteristic chemical shifts values of the isopropoxy groups were at 25.31–27.95 ppm and 63.05–66.07 ppm assignable to methyl and methine carbons, respectively. Similarly, two signals each for methyl and methine carbons of the bridging and terminal isopropoxy groups were observed in compounds (1) and (2), suggesting the nonequivalent nature of the isopropoxy groups in these derivatives.

The 27Al NMR spectrum of (1) [Fig. 1A] at room temperature exhibits broad signals at ∼75.55 ppm, indicating the presence of tetra-coordination around aluminum(III) atoms39–42 in solution. The 27Al NMR spectrum of (2) [Fig. 1B] exhibits two signals at ∼3.5 and ∼70.99 ppm, indicating the presence of both hexa- and tetra-coordinated aluminum atoms in solution. In the absence of a single crystal X-ray diffraction study, it is not possible to comment on the exact molecular structure of such derivatives, yet the above studies do suggest the possibility of a binuclear structure containing four-coordination around both aluminum(III) atoms in (1) [Fig. 2I] and binuclear structure containing six- and four-coordination around aluminum(III) atoms in (2) [Fig. 2(II)]. The 27Al NMR spectrum of (3) [Fig. 1C] exhibits a signal at ∼3.88 ppm, indicating that six-coordination around the aluminum species is present in solution [Fig. 2(III)].


image file: c4ra03245d-f1.tif
Fig. 1 A, B and C are 27Al NMR spectra at room temperature of precursors (1), (2) and (3), respectively.

image file: c4ra03245d-f2.tif
Fig. 2 I, II and III are proposed structures of precursors (1), (2) and (3), respectively.
2.1.2 Single crystal X-ray analyses of [Al (OC6H4CHO)3] (3). Precursor (3) is monoclinic and crystallizes in space group P21/n. The three salicylaldehyde ligands bind the metal in a dihapto η2-(O,O) manner leading to the formation of a hexa-coordinated environment around the aluminum atom. The selected bond lengths and bond angles are summarized in Table 4. The Al–O bond lengths vary between 1.841 Å and 1.914 Å. The cis O–Al–O angles range from 87.39° to 94.53° while the trans O–Al–O angles vary from 176.10(11)° to 177.73(10)°, indicating that the aluminum atom in the complex occurs in a distorted octahedral environment. The structure indicates that the AlO6 core is constructed from six oxygen atoms of three salicylaldehyde ligands (Fig. 3). Aluminum content was found to be 10% from the chemical analysis of (3). The metal content is in agreement with the calculated value of 10.1% for the above structure. Considering the structure and aluminum content, the formula can be fixed as [Al(OC6H4CHO)3] and the crystal and molecular structure is shown in Fig. 3.
Table 4 Selected bond lengths and bond angles in [Al(OC6H4CHO)3] (3)
Bond lengths (Å)
Al(1)–O(6) 1.836(2) Al(1)–O(5) 1.914(2)
Al(1)–O(1) 1.841(2) Al(1)–O(2) 1.936(2)
Al(1)–O(3) 1.846(2) Al(1)–O(4) 1.947(3)
O(1)–C(1) 1.313(3) O(4)–C(14) 1.267(4)
O(2)–C(7) 1.255(3) O(5)–C(21) 1.261(4)
O(3–C(8) 1.300(4) O(6)–C(15) 1.323(3)
C(1)–C(2) 1.400(4) C(1)–C(6) 1.434(4)
C(2)–C(3) 1.374(4) C(4)–C(5) 1.349(4)
C(5)–C(6) 1.415(4) C(3)–C(4) 1.396(4)
C(6)–C(7) 1.405(4)    
Bond angles (°)
O(6)–Al(1)–O(1) 94.53(11) O(6)–Al(1)–O(3) 176.10(11)
O(1)–Al(1)–O(3) 88.89(10) O(6)–Al(1)–O(5) 91.76(9)
O(1)–Al(1)–O(5) 92.07(10) O(3)–Al(1)–O(5) 90.01(10)
O(6)–Al(1)–O(2) 88.25(9) O(1)–Al(1)–O(2) 91.69(9)
O(3)–Al(1)–O(2) 89.76(9) O(5)–Al(1)–O(2) 176.23(11)
O(6)–Al(1)–O(4) 87.52(10) O(1)–Al(1)–O(4) 177.73(10)
O(3)–Al(1)–O(4) 89.03(10) O(5)–Al(1)–O(4) 88.85(10)
O(2)–Al(1)–O(4) 87.39(9) C(1)–O(1)–Al(1) 125.68(19)
C(7)–O(2)–Al(1) 122.23(18) C(8)–O(3)–Al(1) 129.92(19)
C(14)–O(4)–Al(1) 127.1(2) C(21)–O(5)–Al(1) 126.7(2)
C(15)–O(6)–Al(1) 129.74(19) O(1)–C(1)–C(2) 121.3(3)
O(1)–C(1)–C(6) 121.5(3) O(2)–C(7)–C(6) 126.9(3)
O(2)–C(7)–H(7) 116.5 O(3)–C(8)–C(9) 120.4(3)
O(3)–C(8)–C(13) 122.8(3) O(4)–C(14)–C(13) 125.9(3)
O(4)–C(14)–H(14) 117.0 O(5)–C(21)–C(20) 126.5(3)
O(5)–C(21)–H(21) 116.7 O(6)–C(15)–C(16) 119.9(3)


image file: c4ra03245d-f3.tif
Fig. 3 Crystal structure/ORTEP plot of the molecule [Al(OC6H4CHO)3] (3) showing the atom labeling scheme (displacement ellipsoids are drawn at the 50% probability level, except for the H atoms which are shown as circles of arbitrary radius).

A comparison with the known corresponding crystal structures of Al(acac)3 and Alq3 (where acacH = acetylacetone and qH = 8-hydroxyquinoline)43,44 suggests that all the bond lengths and bond angles are in the expected range.

2.1.3 XRD, SEM and EDX of [Al(OC6H4CHO)3] (3). The X-ray powder diffraction spectrum of precursor (3) dried at 100 °C shown in Fig. 4 exhibits the crystalline nature of the product. In the X-ray powder diffraction spectrum, the dominant peaks are at 2θ = 11.12°, 12.79°, 14.18°, 17.62°, 20.22°, 23.08°, 25.39°, 26.93°, 30.24°, 34.01°, 35.94°, 38.70°, 40.37°, 42.62°, 45.14° and 46.65°. Their calculated inter-planar distances are 7.94, 6.91, 6.23, 5.02, 4.38, 3.84, 3.49, 3.30, 2.95, 2.63, 2.49, 2.32, 2.23, 2.11, 2.00 and 1.94, corresponding to diffraction from (101), (012), (003), (020), (120), (122), (−115), (030), (−224), (231), (040), (−403), (−333), (−415), (−219), and (−407) planes, respectively. The average particle size (∼40 nm) was calculated using the Debye–Scherrer equation45 from the major reflections {(012), (020), (120), (122)}. The SEM image of (3) indicates formation of nano-sized rods like morphology and EDX analysis [Fig. 4] corroborates the molecular formula [Al(OC6H4CHO)3].
image file: c4ra03245d-f4.tif
Fig. 4 (I) SEM image, (II) EDX and (III) XRD spectrum of the precursor [Al(OC6H4CHO)3] (3) dried at ∼100 °C.
2.1.4 TGA and DSC curves of precursors (1–3). Thermogravimetric and DSC curves of precursors (1–3) are shown in Fig. 5. It appears that precursors (1) and (2) are thermally less stable compared to (3). The onset of decomposition begins at different temperatures for all precursors (1–3) indicating different degrees of thermal stability. TGA curves show multistep decomposition in all the complexes. The appearance of multiple weight loss steps in the curves of all the three derivatives indicates multiple thermal events due to organic pyrolysis under nitrogen atmosphere and the decomposition appears to be almost completed at about 900 °C. Formation of pure Al2O3 does not appear to be the final product. The DSC curves show that the decomposition reaction is exothermic at ∼500 °C and endothermic at ∼650 °C & 800 °C.
image file: c4ra03245d-f5.tif
Fig. 5 (a) TGA and (b) DSC curves of prepared precursors (1), (2) and (3).

2.2 Hydrolytic studies of (1–3) by the sol–gel technique

Schematic representation of the hydrolytic process of Al(OPri)3, (1), (2) and (3) are shown in Fig. 6.
image file: c4ra03245d-f6.tif
Fig. 6 Proposed mechanism for the formation of α-Al2O3 nano-particle/rod.

In order to control the size of the desired material, initially the precursors were subjected to small hydrolysis ratio (one drop of a mixture of 1 mL water and 4 mL anhydrous 2-propanol) during stirring.46 Finally, excess water was added after an interval of 6–8 h and stirred for 2 days.

Thus, the hydrolysis of the precursors Al(OPri)3 and (1–3) followed by sintering at 1100 °C for 2 h affords formation of alumina (a) from Al(OPri)3, (b) from (1), (c) from (2) and (d) from (3).

2.3 Material characterization

The XRD analysis showed the most stable phase, α-Al2O3 occurred dominantly in all the oxides at 1100 °C. These were characterized by FT-IR, XRD, SEM, EDX, TEM and SAED pattern analyses.

The XRD patterns of oxides (a)–(d) obtained from sol–gel hydrolysis followed by sintering (1), (2) and (3), respectively, at 1100 °C for 3 h of Al(OPri)3, are presented in Fig. 7. Peak positions in all oxides (α-Al2O3) at 2θ = 25.34°, 34.89°, 37.49°, 43.09°, 52.30°, 57.23°, 59.50°, 61.05°, 66.26° and 67.93° correspond to diffraction from the (012), (104), (110), (113), (024), (116), (211), (122), (214) and (300) planes. Comparisons of these patterns with the reported pattern of α-Al2O3 [JCPDS # 46-1212] have been made, which suggest the formation of high purity α-alumina (hexagonal rhomb-centered phase with a = 4.758 Å, b = 4.758 Å, c = 12.99 Å). The sizes of the α-Al2O3 obtained from the modified precursors evaluated from the Debye–Scherrer equation45 are ∼16 nm (a), ∼14 nm (b), ∼12 nm (c) and ∼11 nm (d).


image file: c4ra03245d-f7.tif
Fig. 7 Combined XRD spectra of nano-crystalline α-Al2O3 (a–d).

Scanning electron microscopic (SEM) images of α-Al2O3 (a–d), shown in Fig. 8, indicate the nano-sized microcrystalline morphology for (a) and mixed platelike rods morphology for (b–d). Further characterization of the nanostructures was carried out using transmission electron microscopy and the selected area electron diffraction (SAED) pattern as shown in Fig. 9. The TEM image of representative oxide (d) reveals that the nano-rods of α-Al2O3, size with average range of about 10–15 nm. The SAED pattern {d = 3.4 Å, corresponding plan (024)} can be indexed as a hexagonal rhomb-centered α-Al2O3 and is in complete agreement with the XRD observations; thus, it is evident that each individual α-Al2O3 nano-rod is a single crystal. The EDX and quantitative weight % results of α-Al2O3 (Fig. 9) indicate the formation of pure α-alumina.


image file: c4ra03245d-f8.tif
Fig. 8 SEM images of nano-crystalline α-Al2O3, (a–d).

image file: c4ra03245d-f9.tif
Fig. 9 (i) EDX, (ii) Quantitative weight % results, (iii) TEM image and (iv) TEM image with corresponding SAED pattern of nano-crystalline α-Al2O3.

IR absorption spectra of the α-Al2O3 oxides show a large band between 750 and 400 cm−1, which is a characteristic absorption band of transition alumina and the stable phase of α-Al2O3; it is attributed to the stretching vibration of the Al–O–Al bond.47–49 Significant bands at ∼600, ∼500 and ∼450 cm−1 were observed in the spectra, which are recognized to be the characteristic absorption bands of α-Al2O3.50

3. Experimental

3.1 Materials and methods

All the experimental manipulations (except hydrolysis reactions) were carried out under strictly anhydrous conditions. Solvents and reagents were dried and purified by conventional methods and distilled/sublimed prior to use.51 Due precautions were taken to handle hazardous chemicals like benzene. Aluminium(III) isopropoxide was synthesized and purified as reported in the literature.52 Infrared spectra (4000–400 cm−1) were recorded as dry KBr pellets on a SHIMADZU FT-IR 8400 spectrometer. The 1H and 13C NMR data were collected on a JEOL FX 300 FT NMR spectrometer at 300.4 and 75.45 MHz frequencies for 1H and 13C NMR, respectively, in a solution of CDCl3 using TMS as internal standard. The 27Al NMR spectral studies were carried out in CDCl3 using aluminum nitrate as an external reference in aqueous solution at 100 MHz. Thermo-gravimetric analyses were performed on a Mettler Toledo thermal analysis system with a heating rate of 0–900/10 °C under a nitrogen atmosphere. ESI mass spectra were performed on an Agilent 1100 LC/MSD SL quadrupole mass spectrometer (Agilent LC/MSD API-Electrospray SL G2708DA). The XRD patterns were recorded on a PANalytical make X'Pert PRO MPD diffractometer (model 3040). SEM and EDX were performed on a Carl-Zeiss (30 keV) make and model EVO 18. TEM was carried out on FEI Technai (200 keV) G2 S-T win. The C, H, N analyses were performed on a Thermo Scientific Flash 2000.

3.2 Preparation of [Al(OC6H4CHO)3] (3)

An anhydrous benzene solution of salicylaldehyde (2.97 g, 24.34 mmol) was added to an anhydrous benzene solution of Al(OPri)3 (1.66 g, 8.12 mmol). The contents were refluxed for ∼4 h on a fractionating column and completion of the above reaction was monitored by estimating the liberated 2-propanol collected azeotropically with benzene. The resulting mixture was concentrated under vacuum to give a dark yellow solid (3.17 g, 99.36% yield). The other derivatives (1) and (2) were prepared by the above similar procedure.

3.3 Hydrolysis of [Al(OPri)2(OC6H4CHO)]2 (1)

3.2 g of precursor (1) was dissolved in anhydrous 2-propanol (∼30 mL). To this yellow colored clear solution, a drop of a mixture of distilled water–2-propanol (1 mL water and 4 mL anhydrous 2-propanol) was initially added and gelation occurred immediately. An excess of the stoichiometric amount (0.43) of water was then added in small lots with continuous stirring for 4–5 h to ensure complete hydrolysis. The mixture was further stirred for 2 days at room temperature. A yellow colored gel was obtained. It was dried in a preheated oven (at >100 °C), to yield a yellow powder. In order to remove the organic/inorganic impurities, the powder was washed several times with distilled water and then with a mixture of n-hexane and acetone (1[thin space (1/6-em)]:[thin space (1/6-em)]1). The resulting powder was sintered at 1100 °C for 2 h in a muffle furnace to give a pure white colored powder, which was characterized by powder XRD as α-alumina. Hydrolyses of the other precursors Al(OPri)3, (2) and (3) were carried out by the above similar procedure.

3.4 Crystal and molecular structure of [Al(OC6H4CHO)3] (3)

When 2.29 g of (3) was subjected to recrystallization in a mixture of dichloromethane and n-hexane, well-shaped yellow shiny single crystals were obtained, which were then covered in Paratone-N and placed rapidly into the cold N2 stream of the Cryo-Flex low temperature device.

Single crystal X-ray diffraction data were collected on an Agilent technologies supernova X-ray diffraction system. Data were collected at 150(2) K using graphite-monochromated Mo-Kα radiation (λα = 0.71073 Å). The strategy for the data collection was evaluated by using the CrysAlisPro CCD software. The data were collected by the standard 'phi-omega scan' techniques and were scaled and reduced using CrysAlisPro RED software. The structures were solved by direct methods using SHELXS-97 and refined by full matrix least squares with SHELXL-97, refining on F2.

The positions of all the atoms were obtained by direct methods. All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2 × Ueq of their parent atoms. Molecular drawings were obtained using the program ORTEP (Fig. 3). The crystal and refinement data are summarized in Table 5 for [Al(OC6H4CHO)3] (3).

Table 5 Crystal data and structure refinement for [Al(OC6H4CHO)3] (3)
Complex [Al(OC6H4CHO)3] (3)
Empirical formula C21H15AlO6
Formula weight 390.31
Temperature (K) 150(2)
Wavelength (Å) 0.71073
Crystal system Monoclinic
Space group P21/n
Unit cell dimensions a (Å) 9.5167(5)
b (Å) 9.9644(3)
c (Å) 19.0736(7)
α° 90
β° 100.406(4)
γ° 90
Volume (Å3) 1778.97(13)
Z 4
Dcalc (mg m−3) 1.457
Absorption coefficient (mm−1) 0.152
F(000) 808
Crystal size (mm3) 0.33 × 0.26 × 0.21
θ range for data collection (°) 2.98 to 25.00
Limiting indices −11 <= h <= 11, −11 <= k <= 11, −2 <= l <= 22
Reflections collected/unique 13[thin space (1/6-em)]807/3132
Independent reflections [R(int)] 0.0962
Completeness to θ = 25.00 99.8%
Absorption correction Semi-empirical from equivalents
Maximum and minimum transmission 0.9688 and 0.9516
Refinement method Full-matrix least-squares on F2
Data/restraints/parameters 3132/0/253
Goodness-of-fit on F2 1.015
Final R indices [I > 2σ(I)] R1 = 0.0552, wR2 = 0.1166
R indices (all data) R1 = 0.0857, wR2 = 0.1340
Largest diffraction peak and hole (e Å−3) 0.312 and −0.253

4. Conclusions

The nano-shaped rods of precursor [Al(OC6H4CHO)3] (3) are easily formed at ∼100 °C. This observation is similar to that reported for Alq3,34,35 the most commonly used material for OLED applications, suggesting the possibility of application of [Al(OC6H4CHO)3] (3) in this direction. Highly pure α-Al2O3 material was obtained from salicylaldehyde modified aluminum(III) isopropoxide precursors (1–3) through a sol–gel method. The salicylaldehyde modification of aluminum(III) isopropoxide appears to control the particle size, shape and purity. The α-Al2O3 exhibits a wide verity of applications in engineering and biomedical sciences.

Acknowledgements

We are grateful to DST and CSIR New Delhi for financial support. We are highly thankful to the Department of Physics and USIC, UOR for carrying out XRD, EDX, SEM and TEM analyses and Therachem Research Medilab, Jaipur for ESI mass analyses.

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Footnote

† CCDC 987154. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra03245d
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