Mechanically activated solid-state synthesis of flavones by high-speed ball milling

Article abstract of DOI:10.1080/00397910902898551

An efficient, mechanically activated solid-state synthesis of flavones from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones using high-speed ball milling is described. This method has notable advantages in terms of good yield, short reaction time, and neat conditions.

Full text of DOI:10.1080/00397910902898551

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Mechanically Activated Solid-
State Synthesis of Flavones by
High-Speed Ball Milling
Xingyi Zhu a , Zhenhua Li a , Qiangfeng Shu a ,
Chenfeng Zhou a & Weike Su a
a Key Laboratory of Pharmaceutical Engineering of
Ministry of Education, College of Pharmaceutical
Sciences, Zhejiang University of Technology ,
Hangzhou, China
Published online: 29 Oct 2009.
To cite this article: Xingyi Zhu , Zhenhua Li , Qiangfeng Shu , Chenfeng Zhou & Weike
Su (2009) Mechanically Activated Solid-State Synthesis of Flavones by High-Speed Ball
Milling, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 39:23, 4199-4211, DOI: 10.1080/00397910902898551
To link to this article: http://dx.doi.org/10.1080/00397910902898551
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Synthetic Communications1, 39: 4199–4211, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902898551
Mechanically Activated Solid-State Synthesis of
Flavones by High-Speed Ball Milling
Xingyi Zhu, Zhenhua Li, Qiangfeng Shu,
Chenfeng Zhou, and Weike Su
Key Laboratory of Pharmaceutical Engineering of Ministry of Education,
College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, China
Abstract: An efficient, mechanically activated solid-state synthesis of flavones
from 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones using high-speed ball milling
is described. This method has notable advantages in terms of good yield, short
reaction time, and neat conditions.
Keywords: Ball milling, flavones, solid-state synthesis
Flavones are a class of naturally occurring compounds with various
biological activities.[1] One of the most commonly used methods for the
synthesis of flavones is the cyclodehydration of 1-(2-hydroxyphenyl)-
3-aryl-1,3-propanediones. Many of these procedures use strong acids
such as H2SO4,[2] HCl,[3] HBr, or HI[4] as the reagent or use H3PMo12O40
ꢀ
nH2O=SiO2,[5] nonaqueous cation-exchange resin,[6] or supported
trifluoromethanesulfonic acid[7] as the catalyst. All of these require high
temperatures or long times to complete the reactions. Other methods,
including using Br2 or I2 under irradiation conditions[8] and using ionic
liquid [EtNH3]NO3,[9] CuCl2,[10] or montmorillonite K-10 clay[11] as
the catalyst by microwave-assisted synthesis, have been reported with
different yields.
Received November 24, 2008.
Address correspondence to Weike Su, Key Laboratory of Pharmaceutical
Engineering of Ministry of Education, College of Pharmaceutical Sciences,
Zhejiang University of Technology, Hangzhou 310014, China. E-mail: suweike@
zjut.edu.cn
4199

4200
X. Zhu et al.
Scheme 1. Synthesis of flavones by high speed ball milling.
In recent years, solvent-free chemical synthesis has developed into a
powerful methodology because it reduces the toxic waste produced and is
less harmful to the environment. Mechanically activated solid-state reac-
tions are solvent-free reactions conducted by grinding or ball milling.
High-speed ball milling (HSBM) is an attractive, mechanically activated
method that has started to gain attention. It is often used for milling
minerals into fine particles and for the preparation and modification of
inorganic solids.[12] In the field of synthetic organic chemistry, many
applications also have been found, such as Heck-type cross-couplings,[13]
asymmetric aldol reaction,[14] Baylis–Hillman reaction,[15] reduction of
esters,[16] synthesis of fluoroaromatic compounds,[17] and protection of
diamines, anthranilic acid, diols, and polyols.[18] However, the applica-
tion of HSBM to the cyclodehydration of 1-(2-hydroxyphenyl)-
3-aryl-1,3-propanediones has not been reported.
It is known that KHSO4 is an efficient reagent for different func-
tional group transformations under heterogeneous conditions.[19]
Because it is stable and inexpensive, together with its high efficiency in
heterogeneous reaction, the reagent has received much attention in
organic synthesis. Herein, we report the efficient synthesis of flavones
by cyclodehydration of 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones
in the presence of KHSO4 by HSBM (Scheme 1).
RESULTS AND DISCUSSION
Our initial investigation focused on the cyclodehydration of 1-(2-
hydroxyphenyl)-3-phenyl-1,3-propanedione. To prevent ‘‘overheating’’
of the reaction mixture, a milling cycle with a rotational speed of
1290 rpm for 5 min followed by a 5-min pause was specified. This
cycle could be repeated until the completion of the reaction. Different
amounts of the catalyst were tested to seek the optimum conditions.
As shown in Table 1, the best result with 94% yield was achieved
when 20 mol% KHSO4 was used as catalyst (Table 1, entry 3). Com-
parable yields could also be obtained by employing 50 mol% and

Mechanically Activated Solid-State Synthesis
4201
Table 1. Effect of KHSO4 amount on the cyclodehydration of
1-(2-hydroxyphenyl)-3-phenyl-1,3-propanedione under HSBM
conditionsa
Entry
KHSO4 (mol%)
Time (min)
Yields (%)b
1
2
3
4
5
50
30
20
5
5
5
5
10
25
95
94
94
88
62
5
aReactions were run with 10 mmol of 1-(2-hydroxyphenyl)-
3-phenyl-1,3-propanedione, catalyst and 10 g silica gel.
bIsolated yield.
30 mol% KHSO4 (Table 1, entries 1 and 2), but it is obvious that
the use of an extra amount of catalyst was unnecessary. While using
the lesser amounts of catalyst, longer reaction times and relatively
poorer yields were observed in low catalyst loading (Table 1, entries
4 and 5).
In light of the good results obtained, several structurally diverse
1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones were subjected to the opti-
mized conditions for the preparation of flavones to explore the generality
of the reaction. The results are listed in Table 2. It is worth noting that
electron-withdrawing and electron-donating groups on the aromatic ring
did not seem to affect the reaction significantly either in the yield of pro-
duct or the rate of the reaction.
In a typical experiment, after the reaction was completed, KHSO4
with silica gel was collected and dried at 120ꢁC for 4 h to be re-
generated. The reusability of the catalyst was examined with the 1-(2-
hydroxyphenyl)-3-phenyl-1,3-propanedione and resulted in 94, 92, 92,
and 88% yields over four cycles (Table 3).
In conclusion, we have developed a novel and comparatively green
strategy for the synthesis of flavones by HSBM. Good yields, short reac-
tion times, and neat conditions are the notable advantages of this
method. Catalyst recovery and reusability is another feature of this
method. We believe that this procedure has provided a better scope for
the synthesis of flavones. The application of HSBM to other synthetic
reactions is now under investigation.

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X. Zhu et al.
Table 2. Synthesis of flavones catalyzed by KHSO4 under HSBM conditionsa
Time Yields
(min) (%)b
Entry
1
Substrate
Product
5
5
94
93
2
3
4
5
6
7
8
9
10
15
10
5
93
90
90
94
93
91
96
5
5
5
(Continued )

Mechanically Activated Solid-State Synthesis
4203
Table 2. Continued
Time Yields
(min) (%)b
Entry
10
Substrate
Product
5
15
15
15
10
97
88
90
89
91
11
12
13
14
aReactions were run with 10 mmol of substrate, 1 mmol of KHSO4, and 10 g
silica gel.
bIsolated yield.
Table 3. Reuse of KHSO4 on the synthesis of flavone by
HSBMa
Yield of
1a (%)b
Run
Time (min)
1
2
3
4
5
5
5
10
94
92
92
88
aReactions were run with 10 mmol of substrate,
1 mmol of KHSO4, and 10 g silica gel.
bIsolated yield.

4204
X. Zhu et al.
EXPERIMENTAL
1-(2-Hydroxyphenyl)-3-aryl-1,3-propanediones were prepared by pro-
cedures reported previously.[20] Other reagents were from commercial
sources. The HSBM experiments were conducted in a planetary-
centrifugal mill AGO-2 (acceleration: 60 g; volume of one drum: 35 ml;
diameter of steel ball: 5 mm; weight of balls: 75 g). Melting points (mp)
were recorded on a WRS-1B digital melting-point apparatus and are
uncorrected. Infrared (IR) spectra were recorded in centimetersꢂ1 on a
Thermo Nicolet Avatar 370 spectrophotometer. 1H NMR and 13C
NMR spectra were obtained on a Varian Mercury Plus 400 spectrometer
using CDCl3 as the solvent with tetramethylsilane (TMS) as the internal
standard. Mass spectra (MS) were measured in m=z with a Finnigan
Trace DSQ spectrometer. Elemental analyses were measured on a
Thermo Finnigan Flash EA 1112 elemental analyzer.
Typical Procedure for Synthesis of Flavones
The following components were added to the reaction vessels: 1-(2-
hydroxyphenyl)-3-aryl-1,3-propanedione (10 mmol), KHSO4 (0.27 g,
2 mmol), and silica gel (10 g, 200–300 mesh). Then, stainless steel balls
were added, and the vessel was closed with lid and gasket. The ball mill
was then run at a milling cycle with a rotational speed of 1290 rpm for
5 min followed by a 5-min pause. During the pause, a portion of the
mixture was taken out and dissolved in CH2Cl2 to monitor the
progress of the reaction using thin-layer chromatography (TLC).
This cycle was repeated until the reaction was completed. The mixture
was washed off the vessel using CH2Cl2 (20 ml) and then was filtered.
The filtrate was dried (MgSO4) and evaporated. The crude product
was purified by column chromatography to provide the corresponding
product.
Data
2-Phenyl-4H-chromen-4-one (1a)
White solid: mp 96–97ꢁC (lit.[21] 96–97ꢁC). IR (KBr): 1646, 1605, 1568,
1
1128, 768. H NMR (400 MHz, CDCl3): 8.24 (d, J ¼ 8.0 Hz, 1H, ArH),
7.94 (d, J ¼ 8.0 Hz, 2H, ArH), 7.71 (t, J ¼ 8.0 Hz, 1H, ArH), 7.53–7.60
(m, 4H, ArH), 7.43 (t, J ¼ 8.0 Hz, 1H, ArH), 6.86 (s, 1H, CH). 13C
NMR (100 MHz, CDCl3): 163.33, 156.29, 133.77, 131.75, 131.59,

Mechanically Activated Solid-State Synthesis
4205
129.03, 126.27, 125.68, 125.21, 120.11, 118.17, 107.64. EI-MS: 222 (100,
Mþ), 194 (60), 165 (9), 120 (55), 92(43).
2-(4-Chlorophenyl)-4H-chromen-4-one (1b)
White solid: mp 185–187ꢁC (lit.[22] 185–188ꢁC). IR (KBr): 1641, 1466,
1
1090, 828, 772. H NMR (400 MHz, CDCl3): 8.21 (d, J ¼ 8.0 Hz, 1H,
ArH), 7.84 (d, J ¼ 8.0 Hz, 2H, ArH), 7.70 (t, J ¼ 8.0 Hz, 1H, ArH), 7.54
(d, J ¼ 8.0 Hz, 1H, ArH), 7.48 (d, J ¼ 8.0 Hz, 2H, ArH), 7.42
(t, J ¼ 8.0 Hz, 1H, ArH), 6.78 (s, 1H, CH). 13C NMR (100 MHz, CDCl3):
178.08, 162.09, 156.06, 137.79, 133.81, 130.15, 129.29, 127.47, 125.65,
125.27, 123.84, 117.95, 107.60. EI-MS: 258 (36, Mþ þ 2), 256 (100,
Mþ), 230 (14), 228 (46), 221 (15), 120 (67), 92 (32).
6-Chloro-2-(4-chlorophenyl)-4H-chromen-4-one (1c)
White solid: mp 226–227ꢁC (lit.[23] 226–227ꢁC). IR (KBr): 1658, 1492,
1
1438, 1092, 906, 831, 664. H NMR (400 MHz, CDCl3): 8.19 (s, 1H,
ArH), 7.85 (d, J ¼ 8.0 Hz, 2H, ArH), 7.64–7.67 (m, 1H, ArH),
7.51–7.54 (m, 3H, ArH), 6.79 (s, 1H, CH). 13C NMR (100 MHz, CDCl3):
176.98, 162.51, 154.49, 138.21, 134.09, 131.40, 129.90, 129.47, 127.57,
125.25, 124.94, 119.75, 107.63. EI-MS: 290 (10, Mþ), 293 (2, Mþ þ 2),
262 (100), 199 (60), 163 (87).
6,8-Dibromo-2-(4-chlorophenyl)-4H-chromen-4-one (1d)
Yellow solid: mp 248–251ꢁC (lit.[24] 249–250ꢁC). IR (KBr): 1649, 1612,
1
1492, 1092, 839, 699. H NMR (400 MHz, CDCl3): 8.29 (s, 1H, ArH),
8.04 (s, 1H, ArH), 7.93 (d, J ¼ 8.0 Hz, 2H, ArH), 7.53 (d, J ¼ 8.0 Hz,
2H, ArH), 6.84 (s, 1H, CH). 13C NMR (100 MHz, CDCl3): 176.21,
162.51, 151.74, 139.47, 138.63, 129.63, 129.41, 127.89, 127.74, 126.00,
118.73, 118.11, 113.03, 107.29. EI-MS: 414 (100, Mþ), 416 (74,
Mþ þ 2), 278 (98), 250 (11), 197 (16), 163 (10), 136 (15).
2-(4-Chlorophenyl)-7-methyl-4H-chromen-4-one (1e)
Yellow solid: mp 179–180ꢁC. IR (KBr): 1638, 1490, 1410, 1092, 906, 827,
1
813. 477. H NMR (400 MHz, CDCl3): 8.05 (d, J ¼ 8.0 Hz, 1H, ArH),
7.77–7.80 (m, 2H, ArH), 7.43–7.46 (m, 2H, ArH), 7.29 (s, 1H, ArH),
7.19 (d, J ¼ 8.0 Hz, 1H, ArH), 6.70 (s, 1H, ArH), 2.47 (s, 3H, ArH).

4206
X. Zhu et al.
13C NMR (100 MHz, CDCl3): 177.89, 161.60, 156.05, 145.11, 137.55,
130.11, 129.14, 127.24, 126.66, 125.25, 121.46, 117.63, 107.36, 21.69.
EI-MS: 270 (5, Mþ), 242 (100), 178 (90), 152 (70), 78 (6). Anal. calc.
for C16H11ClO2: C, 70.99; H, 4.10. Found: C, 70.92; H, 4.04.
6-Chloro-2-p-tolyl-4H-chromen-4-one (1e)
Yellow solid: mp 183–184ꢁC (lit.[24] 183–184ꢁC). IR (KBr): 1641, 1613,
1
1465, 1436, 1358, 816, 670. H NMR (400 MHz, CDCl3): 8.16 (s, 1H,
ArH), 7.78 (d, J ¼ 8.0 Hz, 2H, ArH), 7.61 (d, J ¼ 8.0 Hz, 1H, ArH),
7.50 (d, J ¼ 8.0 Hz, 1H, ArH), 7.31 (d, J ¼ 8.0 Hz, 2H, ArH), 6.79 (s,
1H, CH), 2.43 (s, 1H, CH3). 13C NMR (100 MHz, CDCl3): 177.08,
163.79, 154.46, 142.55, 133.76, 131.02, 129,78, 128.45, 126.18, 125.07,
119.72, 106.75, 21.51. EI-MS: 270 (100, Mþ), 272 (35), 255 (45), 242
(20), 154 (55), 116 (31), 115 (47).
2-p-Tolyl-4H-chromen-4-one (1f)
Yellow solid: mp 108–111ꢁC (lit.[25] 108–111ꢁC). IR (KBr): 1637, 1465,
1371, 1227, 817, 752, 634. 1H NMR (400 MHz, CDCl3): 8.21 (d,
J ¼ 8.0 Hz, 1H, ArH), 7.78 (s, 2H, ArH), 7.65 (t, J ¼ 4.0 Hz, 1H, ArH),
7.52 (s, 1H, ArH), 7.27 (s, 1H, ArH), 7.38 (t, J ¼ 8 Hz, 2H, ArH), 6.76
(s, 1H, CH), 2.40 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): 178.26,
163.37, 156.05, 142.10, 133.49, 129.61, 128.73, 126.04, 125.49, 124.96,
123.83, 117.92, 106.76, 21.38. EI-MS: 236 (100, Mþ), 221 (44), 120
(36), 115 (37), 104 (10), 92 (23).
6-Methyl-2-phenyl-4H-chromen-4-one (1g)
Yellow solid: mp 121–122ꢁC (lit.[23] 122ꢁC). IR (KBr): 1638, 1615, 1482,
1
1359, 1044, 883, 814, 778. H NMR (400 MHz, CDCl3): 8.00 (s, 1H,
ArH), 7.90 (d, J ¼ 8.0 Hz, 2H, ArH), 7.43–7.52 (m, 5H, ArH), 6.79 (s,
1H, CH), 2.45 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): 178.46,
163.14, 154.43, 135.10, 134.90, 131.78, 131.42, 128.92, 126.16, 124.93,
123.51, 117,76, 107.31, 20.85. EI-MS: 236 (100, Mþ), 208 (30), 134
(71), 106 (17), 78 (12).
6-Methyl-2-p-tolyl-4H-chromen-4-one (1h)
White solid: mp 151–152ꢁC (lit.[23] 150–151ꢁC). IR (KBr): 1643, 1613,
1483, 817. 1H NMR (400 MHz, CDCl3): 8.00 (s, 1H, ArH), 7.80

Mechanically Activated Solid-State Synthesis
4207
(d, J ¼ 8.0 Hz, 2H, ArH), 7.44–7.50 (m, 2H, ArH), 7.31 (d, J ¼ 8.0 Hz, 2H,
ArH), 6.77 (s, 1H, CH), 2.46 (s, 3H, CH3), 2.43 (s, 3H, CH3). 13C NMR
(100 MHz, CDCl3): 178.49, 163.42, 154.50, 142.06, 135.03, 134.80,
129.70, 129.08, 126.16, 125.01, 123.64, 117.77, 106.81, 21.45, 20.88.
EI-MS: 250 (100, Mþ), 235 (30), 222 (25), 134 (80), 115 (17), 106 (17),
78 (10).
2-(4-Methoxyphenyl)-4H-chromen-4-one (1i)
White solid: mp 157–159ꢁC (lit.[21] 157–158ꢁC). IR (KBr): 1649, 1608,
1465, 1380, 1133, 827, 767. 1H NMR (400 MHz, CDCl3): 8.21 (d,
J ¼ 8.0 Hz, 1H, ArH), 7.85 (d, J ¼ 8.4 Hz, 2H, ArH), 7.65–7.69 (m, 1H,
ArH), 7.53 (d, J ¼ 8.0 Hz, 1H, ArH), 7.40 (t, J ¼ 7.2 Hz, 1H, ArH),
6.98–7.01 (m, 2H, ArH), 6.72 (s, 1H, CH), 3.87 (s, 1H, CH3). 13C
NMR (100 MHz, CDCl3): 178.22, 163.25, 162.28, 156.03, 133.45,
127.86, 125.50, 124.95, 123.80, 120.08, 117.86, 114.33, 106.01, 55.39.
EI-MS: 252 (100, Mþ), 237 (7), 221 (15), 209 (13), 132 (63).
6-Methoxy-2-(4-methoxyphenyl)-4H-chromen-4-one (1j)
White solid: mp 196ꢁC (lit.[26] 194–195ꢁC). IR (KBr): 1649, 1608, 1515,
1
1466, 1382, 1268, 1195, 1026, 827, 768. H NMR (400 MHz, CDCl3):
7.88 (d, J ¼ 8.0 Hz, 2H, ArH), 7.61 (d, J ¼ 4.0 Hz, 1H, ArH), 7.45 (d,
J ¼ 8.0 Hz, 1H, ArH), 7.29 (d, J ¼ 4.0 Hz, 1H, ArH), 7.03 (d, J ¼ 8.0 Hz,
Hz, 2H, ArH), 6.76 (s, 1H, CH), 3.92 (s, 3H, OCH3), 3.90 (s, 3H, OCH3).
13C NMR (100 MHz, CDCl3): 178.30, 163.28, 162.36, 156.95, 151.04,
127.97, 124.54, 124.16, 123.59, 119.39, 114.47, 105.52, 104.91,
55.96, 55.51. EI-MS: 282 (100, Mþ), 252 (22), 150 (98), 132 (23), 107
(28), 107 (28).
6-Nitro-2-phenyl-4H-chromen-4-one (1k)
Pale yellow solid: mp 194–195ꢁC (lit.[27] 193ꢁC). IR (KBr): 1646, 1610,
1
1454, 1340, 1135, 922, 841, 772, 679, 627. H NMR (400 MHz, CDCl3):
9.12 (s, 1H, ArH), 8.52 (d, J ¼ 8.0 Hz, 1H, ArH), 7.93–7.95 (m, 2H,
ArH), 7.74 (d, J ¼ 4.0 Hz, 1H, ArH), 7.55–7.63 (m, 3H, ArH), 6.90 (s,
1H, CH). 13C NMR (100 MHz, CDCl3): 176.67, 164.15, 159.08,
144.86, 132.39, 130.78, 129.30, 128.15, 126.45, 124.11, 122.52, 119.84,
107.89. EI-MS: 267 (100, Mþ), 239 (17), 221 (22), 165 (32), 139 (18),
102 (61).

4208
X. Zhu et al.
2-(4-Nitrophenyl)-4H-chromen-4-one (1l)
Pale yellow solid: mp 241–243ꢁC (lit.[23] 242–244ꢁC). IR (KBr): 1659,
1520, 1467, 1346, 1130, 857, 750, 692. 1H NMR (400 MHz, CDCl3):
8.11–8.40 (m, 5H, ArH), 7.48–7.76 (m, 3H, ArH), 6.92 (s, 1H, CH).
13C NMR (100 MHz, CDCl3): 177.93, 160.56, 156.18, 149.46, 137.66,
134.31, 127.21, 125.88, 125.75, 124.23, 123.96, 118.19, 108.64. EI-MS:
267 (25, Mþ), 236 (100), 208 (25), 166 (9), 115 (13), 92 (12).
6-Methy-2-(4-nitrophenyl)-4H-chromen-4-one (1m)
Yellow solid: mp 276–278ꢁC (lit.[23] 275–277ꢁC). IR (KBr): 1639, 1617,
1
1522, 1343, 1138, 850, 823, 691. H NMR (400 MHz, CDCl3): 8.39 (d,
J ¼ 8.0 Hz, 2H, ArH), 8.11 (d, J ¼ 8.0 Hz, 2H, ArH), 8.03 (s, 1H, ArH),
7.50–7.58 (m, 2H, ArH), 6.90 (s, 1H, CH), 2.49 (s, 3H, CH3). 13C
NMR (100 MHz, CDCl3): 178.16, 160.43, 154.52, 149.43, 137.82,
135.88, 135.56, 127.19, 125.27, 124.22, 123.73, 117.89, 109.53, 20.96.
EI-MS: 281 (100, Mþ), 253 (20), 134 (48), 106 (12).
7-Hydroxy-2-phenyl-4H-chromen-4-one (1n)
White solid: mp 240–242ꢁC (lit.[28] 240ꢁC). IR (KBr): 3118, 2593, 1626,
1
1552, 1512, 1454, 1384, 1166. H NMR (400 MHz, CDCl3): 10.84 (s,
1H, OH), 8.06–8.08 (m, 2H, ArH), 7.90 (d, J ¼ 6.8 Hz, 1H, ArH),
7.56–7.60 (m, 3H, ArH), 7.02 (d, J ¼ 2.0 Hz, 1H, ArH), 6.94 (dd,
J ¼ 2.0 Hz, 7.0 Hz, 1H, ArH), 6.91 (s, 1H, CH). 13C NMR (100 MHz,
CDCl3): 176.33, 162.74, 161.91, 157.48, 131.48, 131.29, 129.03, 126.50,
126.13, 116.15, 115.04, 106.62, 102.52. EI-MS: 238 (100, Mþ), 237 (22),
210 (82), 136 (21), 108 (24).
ACKNOWLEDGMENT
We are grateful to the Natural Science Foundation of China (Nos.
20476098 and 20876147) for financial help.
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Mechanically Activated Solid-State Synthesis
4209
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