Immobilization of Pd catalysts on mesoporous silica for amine- and copper-free Sonogashira coupling reactions

Article abstract of DOI:10.1080/00397910802138546

Immobilization of catalysts on solid supports is a promising approach to combine the advantages of heterogeneous and homogeneous catalysts. Pd(PPh3)2Cl2, known as an extremely active homogeneous catalyst for the Sonogashira coupling reaction, has been immobilized on high-surface-area MCF (mesocellular foams)-type mesoporous silica powder modified with 3-aminopropyltriethoxysilane and subsequently with diphenylphosphine. The functionalized MCF-type silica and supported catalysts have been characterized by x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), elemental analysis, nitrogen sorption porosimetry, and scanning electron microscopy (SEM). Such supported Pd catalysts have proven to be useful recyclable reagents for copper- and amine-free Sonogashira coupling reactions of haloaromatic compounds with terminal alkynes. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802138546

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Immobilization of Pd Catalysts
on Mesoporous Silica for
Amine- and Copper-Free
Sonogashira Coupling Reactions
Chariklia Sotiriou-Leventis a , Xiaojiang Wang a
,
Sudhir Mulik a , Arumugam Thangavel a & Nicholas
Leventis a
a Department of Chemistry , Missouri University of
Science and Technology , Rolla, Missouri, USA
Published online: 11 Jul 2008.
To cite this article: Chariklia Sotiriou-Leventis , Xiaojiang Wang , Sudhir Mulik ,
Arumugam Thangavel & Nicholas Leventis (2008) Immobilization of Pd Catalysts
on Mesoporous Silica for Amine- and Copper-Free Sonogashira Coupling Reactions,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:14, 2285-2298, DOI: 10.1080/00397910802138546
To link to this article: http://dx.doi.org/10.1080/00397910802138546
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Synthetic Communications1, 38: 2285–2298, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802138546
Immobilization of Pd Catalysts on Mesoporous
Silica for Amine- and Copper-Free Sonogashira
Coupling Reactions
Chariklia Sotiriou-Leventis, Xiaojiang Wang, Sudhir Mulik,
Arumugam Thangavel, and Nicholas Leventis
Department of Chemistry, Missouri University of Science and Technology,
Rolla, Missouri, USA
Abstract: Immobilization of catalysts on solid supports is a promising approach
to combine the advantages of heterogeneous and homogeneous catalysts.
Pd(PPh3)2Cl2, known as an extremely active homogeneous catalyst for the
Sonogashira coupling reaction, has been immobilized on high-surface-area MCF
(mesocellular foams)–type mesoporous silica powder modified with 3-aminopro-
pyltriethoxysilane and subsequently with diphenylphosphine. The functionalized
MCF-type silica and supported catalysts have been characterized by x-ray photo-
electron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR),
elemental analysis, nitrogen sorption porosimetry, and scanning electron
microscopy (SEM). Such supported Pd catalysts have proven to be useful recycl-
able reagents for copper- and amine-free Sonogashira coupling reactions of
haloaromatic compounds with terminal alkynes.
Keywords: Mesoporous silica; Pd catalyst immobilization; Sonogashira coupling
INTRODUCTION
The Sonogashira reaction was originally reported in 1975.[1] Over the past
few decades, this palladium-catalyzed coupling of terminal acetylenes
with aryl and vinyl halides has emerged as one of the most important
and widely used carbon–carbon bond-forming reactions in organic
Received in the USA March 23, 2008
Address correspondence to Chariklia Sotiriou-Leventis, Department of
Chemistry, Missouri University of Science and Technology, Rolla, MO 65409,
USA. E-mail: cslevent@mst.edu
2285

2286
C. Sotiriou-Leventis et al.
synthesis.[2] The Sonogashira reaction also provides a powerful method
for the preparation of conjugated oligomers and polymers,[3] which show
interesting optical[4] and electronic properties.[5] Typical procedures for
the Sonogashira reaction utilize palladium as a catalyst, an amine as a
base, a phosphine as a ligand for palladium, and Cu(I) as a co-catalyst.[1,6]
In addition, Cu(I) mediates homocoupling of terminal alkynes in the
presence of oxidative agents or air.[7] To address this issue, a number of
modifications such as copper-free or amine- and copper-free systems have
been used. Most recently, ligand-, amine- and copper-free Sonogashira
reactions in dimethylformamide (DMF) have been reported.[8] Typically,
the expensive palladium complex catalyst is used homogeneously, which
makes the recovery of the metal tedious.[9] The immobilization of tran-
sition-metal catalysts on insoluble supports has received considerable
attention in recent years as a method of combining the practical advan-
tages of heterogeneous and homogeneous catalysts.[10] Heterogenization
allows for easy handling, retrieving, and recycling of the catalyst, while
homogenization offers excellent catalytic activity.[11] Polymer-supported
palladium-catalyzed coupling reactions have been reported[12], however
applications that involve the Sonogashira reaction have been less widely
investigated[13,14] and usually employ a polystyrene-based support
medium.[15] Recently, nanosized Pd(0)[9] and silica-based supported Pd-
complex catalyst for Sonogashira reaction have been reported.[16] No
information on the source of silica or the materials properties of the sup-
ported Pd catalyst were given. As to our knowledge, use of mesoporous sil-
ica as the catalyst support for Sonogashira reaction has not been reported.
Since the discovery of a new family of mesoporous silicas (MCM-41)
in 1992, ordered mesoporous silicas have been widely investigated
because of their high surface area, ordered pore structure of varying
morphologies, and controllable pore size, compared with conventional
microporous silica gels.[17] Such properties make these mesoporous mate-
rials very attractive for applications as catalyst supports, sensors, and
sorbents.[18] One of the most extensively studied member of the mesopor-
ous materials has been SBA-15, which combines large ordered mesopores
(up to 30 nm in diameter) and micro- and relatively thick walls.[19] Adding
a sufficient amount of 1,3,5-trimethylbenzene (TMB) induces a phase
transformation from SBA-15-type mesoporous silicas to MCFs (mesocel-
lular foams) composed of uniformly sized, large micron-size spherical cells
that are interconnected by uniform windows. The pores constitute a 3D
channel system, which makes MCF silicas promising candidates for
flow-through supports in heterogeneous catalysis. Recently, we have
reported the preparation of polymer nano-encapsulated monoliths with
improved mechanical properties using templated mesoporous silica.[20,21]
Herein, we report for the first-time immobilization of Pd catalyst on
powder MCF-type mesoporous silica, and we demonstrate its use in

Immobilization of Pd Catalysts
2287
copper- and amine-free Sonogashira reactions of phenyl acetylene with
various aryl iodides and bromides. The corresponding coupling products
are obtained in good yields, and the supported catalysts can be recovered
and recycled without significant decrease in reactivity.
RESULTS AND DISCUSSION
Preparation of the Supported Catalysts
Scheme 1 summarizes the procedure for the preparation of Pd catalysts
supported on MCF silica powders. The latter have been synthesized
according to Stucky’s method.[22]
Modified MCFs with different amine content (xx-G-N) were synthe-
sized by introducing controlled amounts of APTES.[23] ‘‘xx’’ denotes the
content of the amine groups present in the amine-modified MCF silica
and is defined as xx ¼ 100 ꢀ nAPTES =(nAPTES þ nTEOS), where TEOS is
tetraethoxysilane. Diphenylphosphine ligands were introduced into the
xx-G-N surface via the Mannich reaction.[24] Supported Pd catalysts
xx-G-Pd were prepared via ligand metathesis of (COD)PdCl2
(COD: 1,5-cyclooctadiene) with xx-G-P.[15]
Scheme 1. Preparation of supported catalysts based on MCF silica.
Materials Characterization
Characterization of the chemical identity of the materials was conducted
by XPS, FTIR, and elemental analysis, while the porous structure of the
supported catalyst was probed by nitrogen sorption porosimetry.
XPS
The incorporation of APTES on the MCF silica surface was confirmed
by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 1. Pure

2288
C. Sotiriou-Leventis et al.
Figure 1. X-ray photoelectron spectra of pure MCF silica (a) and 20-G-N(b).
MCF silica (Fig. 1a) shows the presence of peaks at binding energies
of 285, 151, 103, and 23 eV corresponding to C1s, Si2s, Si2p, and
O2s, respectively. A new peak at 398 eV corresponding to N1 s is seen
on 20-G-N (Fig. 1b). Also the higher intensity peak at 285 eV
corresponding to C1s (Fig. 1b) supports the conversion of Si-OH to
Si-O-(CH2)3-NH2 group.
FTIR
The incorporation of the amine group in the MCF silica can be further
confirmed by a weak peak in the FTIR spectrum of 20-G-N at
681 cmꢁ1 corresponding to N-H bend (Fig. 2b). It has been reported
that pure silica has three strong peaks at about 3747, 3680, and
3545 cm
.
ꢁ1 [25,26] However, these peaks are masked by a broad ꢁOH peak

Immobilization of Pd Catalysts
2289
Figure 2. IR spectra of MCF silica (a) and functionalized MCF silicas 20-G-N
(b), 20-G-P (c), and 20-G-Pd (d).
due to the hygroscopic nature of this material.[27] Compared with the
pure MCF silica, the increase in the width of this broad peak at
ꢂ3500 cmꢁ1 may be due to the symmetric N-H stretch at
3346 cmꢁ1 [25]
The presence of the phenyl groups from the diphenylpho-
.
sphine ligands in the 20-G-P and 20-G-Pd samples can be confirmed from
the peaks at 3057 cmꢁ1 (aromatic C-H stretch), 1437 cmꢁ1 (C C
=
aromatic ring stretch), and in the 600–760 cmꢁ1 region (out-of-plane ring
=
C C and C-H bends) as seen in Fig. 2c and 2d.
Elemental Analysis
Elemental analysis data for modified MCFs are given graphically in Fig. 3
and reveal the presence of N in xx-G-N, P in xx-G-P, and Pd in xx-G-Pd,
showing that the amine functionality, the diphenylphosphine ligand, and
Pd metal ions were successfully introduced on the MCF silica surface.
Figure 3 indicates also that the amount of N, P, and subsequently the
Pd content in the supported catalysts all level off at ꢂ10-G-(N, P, or
Pd). Therefore, in this study, we chose 5-G-Pd and 10-G-Pd for testing
the catalytic activity toward Sonogashira coupling reactions.

2290
C. Sotiriou-Leventis et al.
Figure 3. Elemental analysis data: Nitrogen content in amine-modified MCF
silica (a), phosphorus content in modified MCF silica with phosphine ligand
(b), and palladium content in supported catalysts (c).
Nitrogen Sorption Porosimetry
A typical N sorption isotherm is shown in Fig. 4. The shape is character-
istic for a mesoporous material with a Type IV isotherm and a H1
hysteresis loop. Table 1 summarizes the structural properties of APTES-
functionalized MCF silica (xx-G-N) in comparison with pure MCF silica,
characterized by N2 adsorption. As expected, the introduction of the
organic fragment leads to a somewhat decreased surface area and pore
volume.
Scanning Electron Microscopy (SEM)
SEMs of MCF silica at all stages of modification look identical, confirm-
ing that modification does not affect the microscopic structure of the
catalyst. Figure 5 shows typical SEM images at two different magnifica-
tions (A: 5,000ꢀ ; B: 45,000ꢀ ) of a sample terminally modified with Pd
(5-G-Pd) where the macroporosity of the MCF-type silica is clearly
evident (A) as well as the texture on the surface (B) that leads to the
mesopores.

Immobilization of Pd Catalysts
2291
Figure 4. N2 sorption isotherms of an APTES-functionalized MCF silica sample:
(A) adsorption; (B) Desorption.
Supported Catalysts in Sonogashira Reactions
Catalyst Comparison
We examined the catalytic activity of 5-G-Pd and 10-G-Pd in the
Sonogashira coupling reaction of phenylacetylene with 1-bromo-4-
nitrobenzene (Table 2, entry 1). The catalyst loading was 5 mol% Pd rela-
tive to the aryl halide substrate. The disappearance of phenylacetylene
was monitored as a function of time using gas chromatography (GC).
Results show that after 3 h, ꢂ80% of phenylacetylene had reacted using
either 5-G-Pd or 10-G-Pd as a catalyst, suggesting that both 5-G-Pd and
10-G-P have similar catalytic activity. Therefore, for economic reasons,
we chose 5-G-Pd as the supporting catalyst for all of the subsequent
Sonogashira reactions in this study (Table 2).
Table 1. Structural properties of APTES-functionalized and pure MCF silica
samples
Samples
SBET (m2=g)
Pore volume (cc=g)
Pore size, (nm)
MCF silica
5-G-N
570
459
367
386
445
1.69
1.59
1.2
1.3
1.45
11.9
13.8
13.1
13.4
13.0
10-G-N
15-G-N
20-G-N

2292
C. Sotiriou-Leventis et al.
Figure 5. Typical SEM images of 5-G-Pd samples (see Table 2) using a Hitachi
S4700 FESEM, at 10.0 kV and a 5.7-mm focal distance at two different
magnifications: (A) 5,000ꢀ ; (B) 45,000ꢀ .
Reactivity and Recycling Ability
We investigated the Sonogashira reaction using a representative group of
aryl iodides and bromides with phenylacetylene. The results are shown in
Table 2. For reporting reaction yields, products were isolated, purified on
preparative silica-gel thin-layer-chromatographic (TLC) plates, and char-
acterized by NMR. The electron-neutral, electron-rich aryl iodides
Table 2. Sonogashira coupling reaction of aryl halides with phenylacetylene
Reaction
time (h)
Product
yield (%)a
Entry
R
X
1
2
3
4
5b
6c
-NO2
-CN
-H
-Me
-Me
-Ph
-Br
-Br
-I
-I
-I
-Br
5
5
5
5
5
4
72
68
62
67
64
74
aIsolated yields.
bUsing Pd(PPh3)2Cl2 as a homogeneous catalyst.
cReaction was carried out at 80ꢃC, with a molar ratio of phenylacetylene to aryl
halide equal to 2.

Immobilization of Pd Catalysts
2293
Table 3. Recycling of the immobilized catalyst 5-G-Pda
Cycle
Yield (%)b
1
2
3
67
73
66
aUnder the conditions of entry 4, Table 2.
bIsolated yields.
(entries 3 and 4, Table 2) and electron-poor aryl bromides (entries 1 and
2, Table 2) reacted with phenylacetylene well, generating the correspond-
ing cross-coupling products. The reaction of electron-rich aryl bromide
(entry 6, Table 2) with phenylacetylene generates the product in good
yield at higher reaction temperature (80ꢃC) when the molar ratio of the
phenylacetylene to aryl bromide is increased from 1 to 2.
The catalytic activity of 5-G-Pd was compared with that of homo-
geneous catalyst Pd(PPh3)2Cl2 in the coupling reaction of 4-iodotoluene
with phenylacetylene (entries 4 and 5, Table 2). Interestingly, 5-G-Pd
exhibited a slightly higher activity than the homogeneous catalyst.
The recycling ability of the 5-G-Pd catalyst was also investigated
under the conditions of entry 4, Table 2. Thus, after the completion of
the reaction, the catalyst was recovered by filtration, rinsed with Et2O, dried
in vacuo at room temperature, and stored in a desiccator. In 2 to 3 days, a
new reaction was performed with fresh solvent and reactants under the same
conditions. The product yields are shown in Table 3 after each cycle. The
results indicate the supported 5-G-Pd catalyst is a highly efficient reusable
reagent for copper- and amine-free Sonogashira reactions.
CONCLUSIONS
The Pd complexes were first immobilized on the high surface area of
MCF silica, and the resulting materials were used as catalysts for
Sonogashira reactions. For this, -NH2 groups were successfully intro-
duced on the mesoporous surfaces of MCF silica by grafting with
APTES. The -NH2 group content can be controlled by adjusting the
amount of APTES, but it has an upper limit of 2.38 wt% N content of
the modified silica. The -PPh2 ligand and the Pd were successfully intro-
duced on the amine-modified MCF silica surface to yield supported
catalysts, which proved to be highly efficient reusable catalysts for cop-
per- and amine-free Sonogashira reactions. It is noted that the activity of
5-G-Pd is comparable to that of homogeneous Pd catalysts.

2294
C. Sotiriou-Leventis et al.
The Sonogashira reaction should be considered as a model, or a
point of departure for a new generation of flow-through monolithic
catalysts that will include acids and bases for classic C-C forming reac-
tions such as Michael additions and oxidative as well as chiral catalyses.
EXPERIMENTAL
General
Starting materials and solvents were purchased from Aldrich, Fluka, or
Acros and were used without further purification. Melting points were
uncorrected. 1H and 13C NMR spectra were recorded on a Varian Inova
400 NMR spectrometer and are reported as parts per million (ppm) with
the NMR solvent serving as a reference. Infrared spectra were recorded
on a Nicolet Magna IR model 750 spectrometer. Gas chromatographic
(GC) analyses were conducted on a Hewlett-Packard HP5890 series II
chromatograph equipped with a HP-1 (15 m ꢀ 0.25 mm ꢀ 0.25 mm film;
Hewlett-Packard) capillary column using 1,3,5-trimethyl benzene
(TMB) as an internal standard and nitrogen as a carrier gas. The injec-
tion temperature was 300ꢃC and the detection temperature was 325ꢃC.
The GC column was initially maintained at 100ꢃC for 1 min, and its tem-
perature was gradually increased to 250ꢃC at a rate of 30ꢃC=min. The
column temperature was maintained at 250ꢃC for 5 min. Elemental analy-
sis was performed by Prevalere Life Sciences, Inc. (Whitesboro, N.Y.).
Preparation of MCF (Mesocellular Foam) Silica[22]
Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene
oxide) triblock copolymer (EO20PO70EO20, Pluronic P123, Mav ¼ 5800,
Aldrich, 2.0 g, 0.4 mmol) was dissolved in 75 mL of aqueous HCl
(1.6 M, 120 mmol) at 40ꢃC, followed by the sequential addition of
NH4F (23 mg, 0.6 mmol) and 1,3,5-trimethylbenzene (TMB, 2.0 g,
17 mmol). After 1 h at 35–40ꢃC, tetraethoxysilane (TEOS, 4.4 g, 21 mmol)
was added. After 20 h at 35–40ꢃC, the cloudy mixture was kept at 100ꢃC
for 24 h without stirring. The filtered precipitate was dried in air and then
calcined at 500ꢃC for 12 h in air to yield a white powder (MCF silica).
Preparation of Amine-Modified MCF Silica[23]
To prepare 5-G-N, which means amine-grafted MCF silica with
APTES=(APTES þ TEOS) molar ratio equal to 0.05, APTES (0.52 mL,

Immobilization of Pd Catalysts
2295
2.19 mmol) was added dropwise to a suspension of MCF silica (2.50 g) in
anhydrous toluene (65 mL), under an N2 atmosphere. The resulting mix-
ture was refluxed at 115ꢃC for 24 h. After cooling, the suspension was fil-
tered, and the solid residue was washed successively with methanol,
acetone, and hexane (3 ꢀ 40 mL, each). The resulting solid was dried
under vacuum at room temperature, giving a white powder.
Preparation of PPh2-Modified MCF Silica from Amine-Modified
MCF Silica[24]
To prepare 5-G-P, diphenylphosphine (5.86 g, 31.46 mmol) was added,
under an N2 atmosphere, to a solution of paraformaldehyde (0.71 g,
23.60 mmol) in degassed anhydrous methanol (30 mL). The mixture
was stirred at 70ꢃC for 30 min and then cooled to room temperature, fol-
lowed by the addition of 5-G-N (2.00 g, 1.57 mmol with respect to NH2)
in degassed methanol (15 mL) and toluene (60 mL). The reaction mixture
was stirred at 70ꢃC for 2 h and then at room temperature for 16 h. The
product was isolated by filtration under an N2 atmosphere and washed
with methanol (100 mL). The residual solvent was removed in vacuo,
and the product was stored under N2.
Preparation of Supported Pd Catalyst from PPh2 Modified MCF Silica[15]
To prepare 5-G-Pd, (COD)PdCl2 (0.161 g, 0.565 mmol, 1.2 equiv.) was
added to diphenylphosphine modified MCF silica 5-G-P (1.80 g,
0.94 mmol based on P), dispersed in degassed CHCl3 (100 mL). The
reaction mixture was heated to 60ꢃC for 26 h. After cooling to room
temperature, the solid was collected by filtration under N2, washed suc-
cessively with CHCl3 (4 ꢀ 40 mL), EtOH (1 ꢀ 40 mL), and CH2Cl2
(3 ꢀ 40 mL), and dried in vacuo.
Typical Procedure for the Sonogashira Reaction using 5-G-Pd[8]
An oven-dried three-necked flask equipped with a magnetic stirring bar
was charged with Bu4NOAc (452 mg, 1.5 mmol), 5-G-Pd (202 mg,
0.050 mmol), and 1-bromo-4-nitrobenzene (202 mg, 1.0 mmol). The flask
was capped with a rubber septum, and then anhydrous DMF (3 mL) and
TMB (0.162 g, 1.35 mmol) were successively added. After 5 min of
stirring, phenylacetylene (0.154 mL, 1.4 mmol) was added. Stirring was
continued at room temperature under nitrogen (reaction times are shown
in Table 2). The reaction mixture was diluted with water (10 mL) and the
solid catalyst 5-G-Pd was removed by filtration. The filtrate was

2296
C. Sotiriou-Leventis et al.
extracted with diethyl ether (4 ꢀ 10 mL). The combined ether layers were
dried over anhydrous Na2SO4, filtered, concentrated, and purified on pre-
parative silica gel plates (20 ꢀ 20 cm, 1000 m, Analtech) using methylene
chloride=hexane (20=80; v=v) as eluent. All coupling product yields are
shown in Table 2.
Coupling Product Characterization
1-Nꢃitro-(4-phenylethynyl)benzene (Table 2, entry 1): mp 118–119ꢃC (lit.[28]
119 C); 1H NMR (400 MHz, CDCl3) d 8.20 (d, 2H, J ¼ 8.98 Hz), 7.65 (d,
2H, J ¼ 8.98 Hz), 7.56–7.53 (m, 2 H), 7.40–7.37 (m, 3H); 13C NMR
(100 MHz, CDCl3) d 146.8, 132.2, 131.7, 130.1, 129.2, 128.4, 123.5,
121.9, 94.6, 87.5.
Diphenylethyne-4-carbonitrile (Table 2, entry 2): mp 105–106ꢃC
(lit.[29] mp 106–108ꢃC); 1H NMR (400 MHz, CDCl3) d 7.63–7.61 (m,
4H), 7.48–7.44 (m, 2H), 7.39–7.36 (m, 3H); 13C NMR (100 MHz, CDCl3)
d 132.1, 132.1, 131.6, 128.7, 128.1, 128.0, 122.0, 118ꢃ.4, 111.2, 93.6, 87.6.
Diphenylacetylene (Table 2, entry 3): mp 59–60 C (lit.[28] mp 60ꢃC);
1H NMR (400 MHz, CDCl3) d 7.53–7.50 (m, 4H), 7.35–7.31 (m, 6H);
13C NMR (100 MHz, CDCl3) d 131.6, 128.3, 128.2, 123.3, 89.4.
4-(Phenylethynyl)toluene (Table 2, entries 4 and 5): mp 70–71ꢃC
(lit.[28] mp 70ꢃC);1H NMR (400 MHz, CDCl3) d 7.58–7.56 (m, 2H),
7.48 (d, 2H, J ¼ 7.9 Hz), 7.40–7.34 (m, 3H), 7.19 (d, 2 H, J ¼ 7.9 Hz),
2.40 (s, 3H),; 13C NMR (100 MHz, CDCl3) d 138.3, 131.5, 131.4, 129.1,
128.3, 128.0, 123.4, 120.1, 89.5, 88.7, 21.4.
4-(Phenylethynyl)-1,10-biphenyl (Table 2, entry 6): mp 161–163ꢃC
(lit.[9]mp 163–164ꢃC); 1H NMR (400 MHz, CDCl3) d 7.60–7.58 (m,
6H), 7.54–7.52 (m, 4H), 7.37–7.35 (m, 4H); 13C NMR (100 MHz, CDCl3)
d 140.8, 140.2, 132.1, 131.7, 131.7, 128.7, 128.5, 128.1, 127.1, 126.7, 123.2,
122.1, 90.0, 89.2.
ACKNOWLEDGMENTS
The authors thank the University of Missouri Research Board for
financial support.
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