Magnetically Recyclable Nano-Fe 2O 3-Catalyzed Chemoselective Synthesis and Antioxidant Activity of Diethyl (3-((5-Aryl-1 H-1,2,4-triazol-3-yl)thio)propyl) phosphonates

Article abstract of DOI:10.1080/00397911.2014.918146

An efficient, green, and chemoselective S-alkylation of 5-aryl-1H-1,2,4-triazole-3-thiones with diethyl (3-bromopropyl)phosphonate in water, catalyzed by nano-Fe2O3 under ligand-and base-free conditions, is reported. Clean reaction, less expensive catalyst, excellent yields, and easy workup are the advantages of the present method. The catalyst can be easily collected by a magnet and recycled without significant loss in catalytic activity. The newly synthesized compounds were screened for their antioxidant property by using 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging assay. The majority of the compounds exhibited good antioxidant activity.

Full text of DOI:10.1080/00397911.2014.918146

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Magnetically Recyclable Nano-Fe2O3-
Catalyzed Chemoselective Synthesis
and Antioxidant Activity of Diethyl
(3-((5-Aryl-1H-1,2,4-triazol-3-
yl)thio)propyl)phosphonates
M. S. R. Murty a , Mohana Rao Katiki a , B. Ramalingeswara Rao a
, Jagadeesh Babu Nanubolu b , Sudheer Kumar Buddana c & R. S.
Prakasham c
a Medicinal Chemistry and Pharmacology Division, Discovery
Laboratory , CSIR–Indian Institute of Chemical Technology ,
Hyderabad , India
b Centre for X-Ray Crystallography, CSIR–Indian Institute of Chemical
Technology , Hyderabad , India
c Bioengineering and Environmental Sciences , CSIR–Indian Institute
of Chemical Technology , Hyderabad , India
Accepted author version posted online: 23 May 2014.Published
online: 03 Jul 2014.
To cite this article: M. S. R. Murty , Mohana Rao Katiki , B. Ramalingeswara Rao , Jagadeesh
Babu Nanubolu , Sudheer Kumar Buddana & R. S. Prakasham (2014) Magnetically Recyclable
Nano-Fe2O3-Catalyzed Chemoselective Synthesis and Antioxidant Activity of Diethyl (3-((5-
Aryl-1H-1,2,4-triazol-3-yl)thio)propyl)phosphonates, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 44:18, 2724-2737, DOI:
10.1080/00397911.2014.918146
To link to this article: http://dx.doi.org/10.1080/00397911.2014.918146
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Synthetic Communications1, 44: 2724–2737, 2014
Copyright # CSIR–Indian Institute of Chemical Technology
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.918146
MAGNETICALLY RECYCLABLE NANO-Fe2O3-
CATALYZED CHEMOSELECTIVE SYNTHESIS AND
ANTIOXIDANT ACTIVITY OF DIETHYL (3-((5-ARYL-1H-
1,2,4-TRIAZOL-3-YL)THIO)PROPYL)PHOSPHONATES
M. S. R. Murty,1 Mohana Rao Katiki,1
B. Ramalingeswara Rao,1 Jagadeesh Babu Nanubolu,2
Sudheer Kumar Buddana,3 and R. S. Prakasham3
1Medicinal Chemistry and Pharmacology Division, Discovery Laboratory,
CSIR–Indian Institute of Chemical Technology, Hyderabad, India
2Centre for X-Ray Crystallography, CSIR–Indian Institute of Chemical
Technology, Hyderabad, India
3Bioengineering and Environmental Sciences, CSIR–Indian Institute of
Chemical Technology, Hyderabad, India
GRAPHICAL ABSTRACT
Abstract An efficient, green, and chemoselective S-alkylation of 5-aryl-1H-1,2,4-triazole-3-
thiones with diethyl (3-bromopropyl)phosphonate in water, catalyzed by nano-Fe2O3 under
ligand- and base-free conditions, is reported. Clean reaction, less expensive catalyst, excellent
yields, and easy workup are the advantages of the present method. The catalyst can be easily col-
lectedbyamagnetandrecycledwithoutsignificantlossincatalyticactivity. Thenewlysynthesized
compounds were screened for their antioxidant property by using 1,1-diphenyl-2-picrylhydrazyl
(DPPH) scavenging assay. The majority of the compounds exhibited good antioxidant activity.
Keywords Antioxidant activity; C–S bond formation; magnetic separation; nano-Fe2O3;
phosphonate; 1,2,4-triazole-3-thione
INTRODUCTION
Nanoparticles of metals and metal oxides have been extensively used as cata-
lysts in many organic transformations.[1] Among the various nanomaterials, mag-
netic nanoparticles (MNPs) have attracted growing interest because of their
Received February 10, 2014.
Address correspondence to M. S. R. Murty, Medicinal Chemistry and Pharmacology Division,
Discovery Laboratory, CSIR–Indian Institute of Chemical Technology, Uppal Road, Hyderabad
500007, India. E-mail: msrmurty@ymail.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2724

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2725
unique properties. MNPs have wide range of applications,[2] including magnetic
fluids, magnetically assisted drug delivery, magnetic resonance imaging (MRI),
and magnetic separation of biomolecules. The advent of magnetic separation is an
efficient, practical, and significant alternative method to filtration or centrifugation
as it prevents loss of the catalyst and increases reusability.[3] Moreover, MNPs have
high surface area, low toxicity, and remarkable chemical and mechanical stability. In
this context, magnetic nano-iron-oxide particles have gained significant attention
because of their unique physicochemical characteristics and interesting catalytic
activities.
Catalytic methods for the C–S bond formation have paramount importance in
synthetic organic chemistry and the polymer industry in numerous ways and find
wide applications in medicinal and material research.[4] The efficient and selective
construction of C–S bond formation is much less explored compared to C–N,
C–O, and C–P bonds. This is due to the poisoning of the metal catalyst in view of
the strong coordination properties of organosulfur compounds.[5] This limitation
led to increased interest in the field of transition-metal-catalyzed organosulfur
chemistry. In this regard we attempted the C–S bond formation with readily avail-
able and inexpensive nano-iron(III)-oxide (Fe2O3).
Organophosphorus compounds have attracted widespread attention because of
their ubiquity in biological systems and their potential to serve as novel pharmaceu-
ticals and agricultural chemicals and in the development of a large number of new
chemical entities (NCEs) of commercial significance.[6] The well-known class of
phosphoro-organic alkylating drug cyclophosphamide[7] was used in the treatment
of cancer chemotherapy. The alkylating properties of phosphonates have been well
established in various hetrocycles such as pyrrolobenzodiazepines,[8] coumarins,[9]
and chromones[10] possessing anticancer activity. In addition, the bezoxazolone
phosphonate derivatives[11] are found to exhibit antimalarial activity. The
mercapto-substituted 1,2,4-triazole ring is present in various molecules with assorted
pharmacological properties such as antibacterial,[12] anti-inflammatory,[13] antifun-
gal,[14] anticancer,[15] anti-HIV,[16] and antioxidant[17] activities. In view of these
interesting activities and the importance of mercapto-1,2,4-triazoles and phospho-
nate compounds, we report the synthesis of novel diethyl (3-((5-aryl-1H-
1,2,4-triazol-3-yl)thio)propyl)phosphonates using nano catalyst.
The organic reactions in aqueous medium using recoverable catalysts has
gained attention in recent years.[18] Water is a preferred solvent medium compared
to the other organic solvents because it is readily available, safe, nontoxic, nonflam-
mable, and environmentally benign. Herein we report the environmentally friendly
and magnetically separable nano-Fe2O3 as a highly efficient heterogeneous catalyst
Scheme 1. Synthesis of diethyl (3-((5-aryl-1H-1,2,4-triazol-3-yl)thio)propyl)-phosphonates.

2726
M. S. R. MURTY ET AL.
for the chemoselective synthesis of diethyl (3-((5-aryl-1H-1,2,4-triazol-3-yl)thio)-
propyl)phosphonates in aqueous medium under ligand- and base-free reaction
conditions (Scheme 1). In addition, the newly synthesized compounds were evaluated
for their antioxidant activity.
RESULTS AND DISCUSSION
Initially, the reaction between 5-phenyl-1H-1,2,4-triazole-3-thione (1a, 1 mmol)
and diethyl (3-bromopropyl)phosphonate (2, 1 mmol) was used as a model system to
identify reaction parameters that would provide optimal results. In the absence of
the catalyst, only a trace amount of product would be formed even after a prolonged
reaction time (Table 1, entry 1). Then, we focused our attention on using various
nanocatalysts, which might help to reduce the reaction time and improve the yield
of the product (Table 1, entries 2–6). Among the nanocatalysts tested, nano-Fe2O3
catalyzed the reaction most efficiently (Table 1, entry 6). To check the solvent effect
on the outcome of the reaction, the model reaction was carried out with 10 mol% of
nano-Fe2O3 in various solvents such as H2O, PEG-400, C6H5CH3, CH3CN,
dimethylformamide (DMF), and dimethylsulfoxide (DMSO) (Table 1, entries
6–11). It was observed that water was the most effective solvent in which the reaction
proceeded smoothly giving the maximum yield (Table 1, entry 6).
Table 1. Optimization of reaction conditionsa
Catalyst loading
(mol%)
Temparature
(ꢀC)
No.
Nano catalyst
Solvent
Time (h)
Yieldb (%)
1
2
No Catalyst
CuO
NiO
–
H2O
H2O
24
1
80
80
trace
61
49
56
43
92
78
57
69
60
64
79
84
21
45
10
10
10
10
10
10
10
10
10
10
2
3
H2O
H2O
1
80
80
4
TiO2
ZnO
1
5
H2O
H2O
1
80
80
6
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
Fe2O3
1
7
PEG-400
C6H5CH3
CH3CN
DMF
DMSO
H2O
1
80
8
1
reflux
reflux
120
120
80
9
1
10
11
12
13
14
15
1
1
1
5
H2O
H2O
1
80
10
10
24
1
rt
50
H2O
aReaction conditions: 5-phenyl-1H-1,2,4-triazole-3-thione (1a, 1 mmol), diethyl (3-bromopropyl)-
phosphonate (2, 1 mmol), and catalyst in solvent (3 mL) at given temperature were stirred for given time.
bIsolated yield.

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2727
Scheme 2. Synthesis of 5-aryl-1H-1,2,4-triazole-3-thiones.
The quantity of the catalyst played a vital role in synthesizing the product. An
increase in the amount of nano-Fe2O3 from 2 to 10 mol% increased the yield of the
desired product (79 to 92%, Table 1, entries 6, 12, 13). Furthermore, during the opti-
mization of the reaction condition, the relation between the yields of the model reac-
tion with temperature was also studied. We carried out the reaction at room
temperature, 50 ꢀC and 80 ꢀC using 10 mol% of nano-Fe2O3 in water (Table 1, entries
6, 14, and 15). At lower temperatures the reaction proceeds slowly, giving lower yields,
and maximum yield of the product was observed at 80 ꢀC. Further increase of tempera-
ture did not improve the yield of the product. The overall results showed 10 mol%
nano-Fe2O3 at 80 ꢀC in aqueous medium was superior for this transformation.
5-Aryl-1H-1,2,4-triazole-3-thiones 1a–l were synthesized by sequence of reac-
tions from corresponding aryl carboxylic acids[19,20] (Scheme 2). The mercapto tria-
zoles 1a–l formed can exist in thione-thiol tautomeric forms (I and II). The thione
form (II) was found to be predominant on the basis of spectral analysis, which is
in agreement with the literature.[20] The infrared (IR) absorption bands in 1a–l
appeared at about 1250–1160 cmꢁ1 indicative of the thione form(II), and no absorp-
tions around 2600–2550 cmꢁ1 due to thiol form (I). Diethyl (3-bromopropyl)pho-
sphonate 2 was achieved by Arbuzov reaction of 1,3-dibromopropane with triethyl
phosphate[21] (Scheme 3). The reaction of 5-aryl-1H-1,2,4-triazole-3-thiones 1a–l
with diethyl (3-bromopropyl)phosphonate 2 in the presence of nano-Fe2O3 in water
proceed chemoselectively to give diethyl (3-((5-aryl-1H-1,2,4-triazol-3-l)thio)propyl)-
phosphonates 3a–l (Scheme 1). The 1H NMR spectra of the compounds 3a–l showed
a triplet at d 3.28–3.24 ppm for methylene protons attached to sulfur atom of the
triazole ring. The 13C NMR spectra of triazole derivatives displayed a doublet at
1
d 32.7–32.4 ppm due to S–CH2. The presence of these signals in H NMR and 13C
NMR spectra of the synthesized compounds is strong evidence of C–S bond forma-
tion. Thus, under the present reaction condition compound 3 is formed as the only
product in a chemoselective manner and the compound 4 is not formed (Scheme 4).
Scheme 3. Synthesis of diethyl (3-bromopropyl)phosphonate.

2728
M. S. R. MURTY ET AL.
Scheme 4. Approaches and chemoselectivity for the synthesis of diethyl (3-((5-aryl-1H-1,2,4-triazol-3-
yl)thio)propyl)phosphonates.
Theoretically three tautomeric forms are possible for the synthesized 1,2,
4-triazoles 3a–l (Fig. 1). Considering 3g as an example, we investigated the tautomer-
ism of these triazoles through single-crystal X-ray diffraction analysis. Crystallo-
graphic data for 3g have been deposited with the Cambridge Crystallographic
Data Centre with the deposition number CCDC 965121. As shown in ORTEP dia-
gram, the compound 3g exhibits a N1–H tautomer, which is in consonance with the
optimized structure shown in Fig. 2. It is worth mentioning that the hydrogen atom
attached to nitrogen atom N3 (as represented in Fig. 3) involves in the formation
of an intermolecular hydrogen bonding with the phosphonate oxygen O1
(N3–H ꢂꢂꢂ O1), through which these single molecules are linked into an infinite chain
(Fig. 4, Table 2). The formation of hydrogen bonding of N3–H further stabilizes the
N1–H tautomer.
Figure 1. Tautomeric forms for the substituted 1,2,4-triazoles.
Figure 2. Optimized structure of N1–H tautomer of compound 3g.

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2729
Figure 3. ORTEP diagram of the compound 3g with the arbitrary atom-numbering scheme. Displacement
ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radius.
Using the optimized reaction conditions, the generality of the present method-
ology was investigated for the synthesis of diethyl (3-((5-aryl-1H-1,2,4-triazol-3-
yl)thio)propyl)phosphonates by using a variety of 5-aryl-1H-1,2,4-triazole-3-thiones
Figure 4. Crystal packing diagram showing the hydrogen bond (N3–H ꢂꢂꢂ O1) between triazole ring N3–H
donor and phosphonate ester O1 acceptor.
˚
Table 2. Selected hydrogen-bonding geometries for 3g (in A and deg)
˚
D–H (A)
˚
H–A (A)
˚
D–H ꢂꢂꢂ A
D ꢂꢂꢂ A (A)
D-H ꢂꢂꢂ A (deg)
N3–H ꢂꢂꢂ O1
0.81 (3)
1.95 (3)
2.757 (3)
174 (3)
Notes. D, hydrogen bond donor; A, hydrogen bond acceptor.

2730
M. S. R. MURTY ET AL.
Table 3. Nano-Fe2O3-catalyzed C–S bond formation of 5-aryl-1H-1,2,4-triazole-3-thione with
diethyl(3-bromopropyl)phosphonatea
Yieldb
(%)
No.
1
Triazole
Product
92
2
3
4
5
90
88
91
87
6
86
(Continued )

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2731
Table 3. Continued
Yieldb
(%)
No.
Triazole
Product
7
90
8
9
92
86
10
88
11
12
90
91
aReaction conditions: 5-aryl-1H-1,2,4-triazole-3-thiones (1 mmol), diethyl (3-bromopropyl)phosphonate
(1 mmol), and nano-Fe2O3 (10 mol%) in water (3 mL) at 80 ꢀC were stirred for 1 h.
bIsolated yield.
with diethyl (3-bromopropyl)phosphonate (Table 3). The triazoles with both
electron-donating and electron-withdrawing substituents displayed high reactivity
and generated the desired products in excellent yields. The results given in Table 2

2732
M. S. R. MURTY ET AL.
shows that all reactions were very clean and the substitution groups did not play sig-
nificant roles in governing the reactivity of the substrate to give the corresponding
products. Though the thiol alkylation under conventional method using base in
organic solvents is a common organic reaction (method B), the novelty of our work
is the alkylation of thiol using magnetically separable nano-Fe2O3 under ligand- and
base-free conditions in aqueous medium, a green chemical approach (method A).
Thus in both the methods A and B only the S alkylated product was formed
(Scheme 4), which was confirmed by NMR and x-ray crystallographic data.
Recyclability of Nano-Fe2O3 Catalyst
The recyclability of nano-Fe2O3 was examined using 5-phenyl-1H-
1,2,4-triazole-3-thione with diethyl (3-bromopropyl)phosphonate as model reaction.
After completion of the reaction, the reaction mixture was cooled to room tempera-
ture and the catalyst was recovered magnetically, washed with methanol, and dried
at 60 ꢀC for 30 min. A new reaction was then performed with fresh reactants, under
the same conditions. The nano-Fe2O3 catalyst was reused for five times without any
change in activity (Fig. 5).
The X-ray diffraction (XRD) patterns of nano-Fe2O3 catalyst before and after
the reaction are displayed in Fig. 6. No substantial variation was observed in the
XRD patterns of fresh and used catalyst, revealing the retention of the original
Figure 5. Magnetic separation and recycling of nano-Fe2O3. Reaction conditions: 5-phenyl-1H-1,2,4-
triazole-3-thione (1a, 1 mmol), diethyl (3-bromopropyl)phosphonate (2, 1 mmol), and nano-Fe2O3
(10 mol%) in water (3 mL) at 80 ꢀC were stirred for 1 h. Product yield is isolated yield.

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2733
Figure 6. X-ray diffraction patterns of (a) native nano-Fe2O3 and (b) nano-Fe2O3 after fifth cycle.
structure of nano-Fe2O3 in the used catalyst. The scanning electron microscope
(SEM) image of the catalyst taken after the fifth cycle of the reaction does not show
a significant change in the morphology and the size of the catalyst, which indicates
the retention of the catalytic activity after recycling (Fig. 7).
Antioxidant Activity
Newly synthesized compounds (3a–l) were evaluated for their antioxidant
activity (in vitro) by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical method against
the ascorbic acid, which is considered as standard. The relative activity of the com-
pounds was mentioned with reference to antioxidant activity of ascorbic acid. The
results of free radical scavenging activity of the synthesized compounds are repre-
sented in Fig. 8. The antioxidant activity of the title compounds differ from each
other due to the substituents on phenyl ring attached to the triazole moiety. The anti-
oxidant activity seems to be guided by the electron-releasing and electron-
withdrawing nature of the substituents. Compounds 3b, 3c, and 3d were the strong
free radical scavengers among the compounds tested, with the levels of inhibition of
Figure 7. SEM images of (a) nano-Fe2O3 before use and (b) the same nano-Fe2O3 after the fifth cycle.

2734
M. S. R. MURTY ET AL.
Figure 8. Percentage of antioxidant activity of the synthesized compounds against ascorbic acid.
54.16%, 54.13%, and 55.58% respectively, followed by 3i–3l with the levels of inhi-
bition between 42.39% and 45.97%. The structure–activity relationship studies reveal
that the free radical scavenging activity of the triazole derivatives is due to the pres-
ence of methyl group on ortho, meta, and para positions of the phenyl ring (3b, 3c,
and 3d). The replacement of the methyl group with stronger releasing groups such as
methoxy, t-butyl, and n-butoxy groups resulted in decrease in activity. These results
revealed that an optimum electron density is inevitable for a compound to gain sig-
nificant activity. The halogens such as chloro and bromo substituents on the phenyl
ring were also responsible for antioxidant activity, while the fluoro substituent
showed reduced activity due to the strong electronegativity.
EXPERIMENTAL
Typical Procedure for the Synthesis of Diethyl (3-((5-Phenyl-1H-
1,2,4-triazol-3-l)thio)propyl)phosphonate 3a
Nano-Fe2O3(10 mol%, 16mg) was added to a mixture of 5-phenyl-1H-1,2,
4-triazole-3-thiones (1a, 1.0 mmol) and diethyl (3-bromopropyl)phosphonate (2,
1.0 mmol) in water (3 mL). The reaction mixture was warmed to 80 ꢀC and allowed
to stir for 1 h. The progress of the reaction was monitored by thin-layer chromato-
graphy (TLC). After completion of the reaction indicated by TLC, the reaction mixture
was cooled to room temperature. The catalyst was separated with the aid of external
magnet and the resulting reaction mixture was extracted with ethyl acetate (3 ꢃ 10 mL).
The combined organic layer was dried over anhydrous Na2SO4. The solvent was
evaporated under vacuum to give the crude product, which was purified by column
chromatography using ethyl acetate = hexane (8:2) as eluent to get pure compound.
Spectroscopic Data for Compound 3a
1
Yield 92%. Brown oil. H NMR (500 MHz, CDCl3) d (ppm): 8.08–8.02 (m,
2H); 7.45–7.34 (m, 3H); 4.18–4.05 (m, 4H); 3.26 (t, J ¼ 6.8 Hz, 2H); 2.16–1.93 (m,
4H); 1.30 (t, J ¼ 7.0 Hz, 6H). 13C NMR (75 MHz, CDCl3): d (ppm) 158.9, 129.5,

SYNTHESIS OF TRIAZOLE PHOSPHONATES
2735
128.5, 126.3, 61.9 (d, J ¼ 6.3 Hz), 32.7 (d, J ¼ 17.2 Hz), 24.5, 23.4, 22.7, 16.3
(d, J ¼ 5.4 Hz). IR (KBr, cmꢁ1): 3421, 2986, 1556, 1264, 1027. ESI–HRMS (m=z):
[M þ Na]þ. calcd. for C15H22O3N3NaPS: 378.10117; found: 378.10114.
Biology
Newly synthesized compounds 3a–l were evaluated for their antioxidant
activity by the DPPH radical method.[22] Freshly prepared DPPH (0.004% in meth-
anol) was reacted with the test compounds in known concentration (1 mg=mL) and
incubated for 30 min, and the reduction of color complexation against standard
ascorbic acid was recorded at 517 nm on an ultraviolet–visible spectrometer (Electro
420 series spectrophotometer). Free radical inhibition (I%) was calculated as
Ablank ꢁ Asample
Ið%Þ ¼
ꢃ 100
Ablank
where Ablank is the absorbance of the control reaction (containing all reagents
except the test compound) and Asample is the absorbance of the test sample.
CONCLUSION
In summary, nano-Fe2O3 has been shown to be a versatile, efficient, and non-
toxic heterogeneous catalyst for the C–S bond formation under ligand- and base-free
conditions in aqueous medium. By using this catalytic system, we have demonstrated
a novel method for the chemoselective synthesis of diethyl (3-((5-aryl-1H-
1,2,4-triazol-3-yl)thio)propyl)phosphonates in excellent yields. The highlights of this
method are safety, minimization of chemical wastes, good yields, simple experi-
mental procedure, short reaction times, and agreement with green chemistry proto-
cols. Importantly, the catalyst can be easily recovered and reused without substantial
loss in activity. The synthesized compounds (3a–l) were evaluated for antioxidant
property by free radical scavenging method and the triazole derivatives with methyl
group on ortho, meta, and para positions of the phenyl ring (3b, 3c, and 3d) exhibited
good antioxidant activity.
FUNDING
We are grateful to the Council of Scientific and Industrial Research (CSIR),
New Delhi, for the fellowship to M. R. K. and the University Grants Commission
(UGC), New Delhi, for the fellowship to B. R. R. The authors thank CSIR, New
Delhi, for financial support as a part of XII five-year plan program under title
ORIGIN (CSC-0108).
SUPPORTING INFORMATION
1
Full experimental details, spectral data of the products, and H NMR, 13C
NMR, and HRMS spectra of all the new compounds including X-ray crystallo-
graphic data can be accessed on the publisher’s website.

2736
M. S. R. MURTY ET AL.
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