Facile and efficient synthesis of the surface tantalum hydride (≡SiO)2TaIIIH and tris-siloxy tantalum (≡SiO)3TaIII starting from novel tantalum surface species (≡SiO)TaMe4 and (≡SiO)2TaMe 3

Article abstract of DOI:10.1021/om4012196

By grafting of TaMe₅ (1) on the surface of silica partially dehydroxylated at 500 C (silica₅₀₀), a mixture of (≡SiO)TaMe₄ (2a; major, 65 ± 5%) and (≡SiO) ₂TaMe₃ (2b; minor, 35 ± 5%) was produced, which has been characterized by microanalysis, IR, and SS NMR (¹H, ¹³C, ¹H-¹³C HETCOR, proton double and triple quantum). After grafting, these surface organometallic compounds are more stable than the precursor TaMe₅. Treatment of 2a,b with water and H ₂ resulted in the formation of methane in amount of 3.6 ± 0.2 and 3.4 ± 0.2 mol/grafted Ta, respectively. 2a,b react with H₂ (800 mbar) to form (≡SiO)₂TaH. After (≡SiO) ₂TaH was heated to 500 C under hydrogen or vacuum, [(≡SiO) ₃Ta][≡SiH] was produced, and the structure was confirmed by IR, NMR, and EXAFS. Considering the difficulty of the previous preparation method, these syntheses represent a facile and convenient way to prepare tantalum surface species (≡SiO)₂TaH and (≡SiO)₃Ta via the intermediate of the new surface organometallic precursors: (≡SiO)TaMe₄/(≡SiO)₂TaMe₃. (≡SiO)₂TaH and (≡SiO)₃Ta exhibit equal reactivities in alkane metathesis and ethylene polymerization in comparison to those in previous reports.

Full text of DOI:10.1021/om4012196

Article
pubs.acs.org/Organometallics
Facile and Efficient Synthesis of the Surface Tantalum Hydride
(SiO)2TaIIIH and Tris-Siloxy Tantalum (SiO)3TaIII Starting from
Novel Tantalum Surface Species (SiO)TaMe4 and (SiO)2TaMe3
Yin Chen,† Samy Ould-Chikh,† Edy Abou-Hamad,† Emmanuel Callens,† Janet C. Mohandas,†
Syed Khalid,‡ and Jean-Marie Basset*,†
†Physical Sciences and Engineering, KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Saudi Arabia
‡Brookhaven National Laboratory, 75 Brookhaven Avenue, Upton, New York 11973-5000, United States
S
* Supporting Information
ABSTRACT: By grafting of TaMe5 (1) on the surface of silica partially
dehydroxylated at 500 °C (silica500), a mixture of (SiO)TaMe4 (2a; major,
65 5%) and (SiO)2TaMe3 (2b; minor, 35 5%) was produced, which
1
has been characterized by microanalysis, IR, and SS NMR (1H, 13C, H−13C
HETCOR, proton double and triple quantum). After grafting, these surface
organometallic compounds are more stable than the precursor TaMe5.
Treatment of 2a,b with water and H2 resulted in the formation of methane in
amount of 3.6 0.2 and 3.4 0.2 mol/grafted Ta, respectively. 2a,b react
with H2 (800 mbar) to form (SiO)2TaH. After (SiO)2TaH was heated
to 500 °C under hydrogen or vacuum, [(SiO)3Ta][SiH] was produced,
and the structure was confirmed by IR, NMR, and EXAFS. Considering the
difficulty of the previous preparation method, these syntheses represent a
facile and convenient way to prepare tantalum surface species (SiO)2TaH
and (SiO)3Ta via the intermediate of the new surface organometallic precursors: (SiO)TaMe4/(SiO)2TaMe3. (
SiO)2TaH and (SiO)3Ta exhibit equal reactivities in alkane metathesis and ethylene polymerization in comparison to those in
previous reports.
prepared from (SiO)2Ta(H)x is active for ethylene polymer-
INTRODUCTION
■
ization.23 However, the synthesis of the surface compound (
SiO)Ta(CHCMe3)(CH2CMe3)2, a crucial precursor for the
Ta hydrides, is very tedious.2,24
Immobilization of organometallic complexes on solid supports,
such as oxides, zeolites, and metals, allows the preparation of
relatively well-defined surface species acting as highly selective
heterogeneous catalysts,1,2 which are commonly regarded as
“single site” catalysts. Some extremely unstable species can be
isolated effectively by this strategy: for example, immobilized
tantalum species on partially dehydroxylated silica (silica700),
(SiO)Ta(CHCMe3)(CH2CMe3)2,3 can leads to sup-
ported tantalum hydride (SiO)2Ta(H)x (x = 1−3, depending
on preparation conditions) upon hydrogenolysis. The highly
electron deficient nature of similar hydride species with M =
Ti,4 Zr,5 Hf,6 Ta,7 W8 gives rise to unusual catalytic activity: for
example, in the low-temperature hydrogenolysis of acyclic
alkanes or polyolefins (Ziegler−Natta depolymerization) via
C−H and C−C bond activation and cleavage.9 More
interestingly, the supported tantalum hydrides catalyze the
metathesis of alkanes in which acyclic alkanes are transformed
into their lower and higher homologues.1,10,11
We have been inspired by our recent syntheses of various
monopodal silica surface complexes that we found to be stable
on surfaces: (SiO)TaMe2Cl2,25 (SiO)WMe5,26 and (
SiO)Ta(CH2)Cl2.27 All of these compounds contain a
methyl ligand which, in contrast to the neopentyl or
neopentylidene ligand, does not contain β-H or γ-H. Therefore,
we considered that the methyl ligand could be another option
to develop surface tantalum chemistry. Thus, we decided to
prepare the silica-supported (SiO)TaMe4 using TaMe5 as the
starting material. Although this molecular compound is not
stable and readily decomposes at room temperature via a
multimolecular pathway,28 we expected that its stability could
be improved due to immobilization.
Herein, we report the preparation of novel well-characterized
surface tantalumV methyl complexes (SiO)TaMe4 (2a) and
(SiO)2TaMe3 (2b) by the reaction of TaMe5 with silica
partially dehydroxylated at 500 °C (silica500). These surface
In addition, (SiO)2Ta(H)x (x = 1−3) favors activation of
the NN bond in dinitrogen12−14 and the C−H and C−C
bonds in alkanes,15−20 promoting alkane metathesis, the
nonoxidative coupling of methane,18 and cleavage of alkanes
by methane.10 The surface TaIII species (SiO)3Ta21,22
Received: December 20, 2013
Published: February 5, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of (SiO)TaMe4/(SiO)2TaMe3 (2a,b)
1
Figure 1. (a) 600 MHz H MAS NMR spectrum of (SiO)TaMe4/(SiO)2TaMe3 (2). (b, c) Two-dimensional contour plots of the double
(DQ)- and triple-quantum (TQ) proton solid-state NMR correlation spectra of 2. (d) 13C CP-MAS NMR of 2. (e) Contour plot of the aliphatic
1
region of the H−13C HETCOR spectrum of 2.
1
organotantalum complexes are easily transformed into (
SiO)2TaH and (SiO)3Ta by controlled hydrogenolysis.
NMR Study. The H-MAS NMR spectrum of 2 displays a
main signal at 0.9 ppm, as well as two peaks of weak intensity at
1.9 and 0.0 ppm; these two peaks can be assigned respectively
to the small amount of unreacted silanols on the surface (1.90
ppm) and a trace amount of gaseous methane or SiMe25 (0.0
ppm; Figure 1a). In order to get a better signal to noise ratio 2*
(13C 97% enriched) was synthesized according to the same
protocol starting from 13CH3I. The 13C CP/MAS NMR
spectrum of 2* displays only one signal at 72 ppm, and
RESULTS AND DISCUSSION
■
TaMe5 was synthesized according to a modified procedure (see
NMR in the Supporting Information).28 An essentially pure
pentane solution of TaMe5 reacted with Aerosil500 in situ to
afford a grayish powder product which was identified as a
mixture of (SiO)TaMe4 (2a) and (SiO)2TaMe3 (2b)
(Scheme 1).
1H−13C HETCOR NMR shows a correlation of the H signal
1
at 0.9 ppm with the 13C signal at 72 ppm (Figure 1e). Proton
double (DQ)- and triple-quantum (TQ) autocorrelation
spectra with 22 kHz MAS (Figure 1b,c) support the hypothesis
that the signal belongs to the methyl group, as autocorrelation
peaks are observed on the diagonal of the 2D DQ and TQ
spectrum (0.9 ppm in F2 and 1.8, 2.7 ppm in F1, respectively).
However, these NMR results did not show a difference between
2a and 2b.
Upon adsorption of the pentane solution of TaMe5, the
characteristic IR bands of the silanol groups at 3747 cm−1
(sharp) and 3646 cm−1 (broad) disappear (Figure S2,
Supporting Information), a new broad band at 2932 cm−1
appears which is assigned to the ν(C−H) band of the methyl
groups, and a band at 1397 cm−1 belongs to the δ(C−H)
vibrations of the same ligand. Elemental analysis gives 7.85%
Ta, 1.90% C, and 0.48% H with the Ta/C/H ratio = 1.0/(3.6
With 13C CP-MAS NMR we observed room-temperature
decomposition of 2, since a broad peak at 50−70 ppm appeared
(Figure S3, Supporting Information); GC analysis shows
methane as the main product in the gas phase, and more
methane is produced upon hydrolysis. As we know, on the
surface of silica500 the concentration of silanol groups is around
0.3)/(11.0
1.0) (theoretical ratio for the monopodal
species1.0/4.0/12.0), which indicates that 96% of the silanols
are consumed and the compound is not a purely monopodal
species. The structure of this compound has been further
confirmed by NMR spectroscopy29 and mass balance.
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Scheme 2. Synthesis of (SiO)2TaH and (SiO)3Ta and the Corresponding PMe3-Coordinated Compounds
2 OH/nm2 30 and the silanol groups are close in space (average
distance around 7 Å on the basis of calculations), so that 2
possibly decomposed by a surface “bimolecular” pathway.
However, in comparison to the case for TaMe5, which
decomposes readily at room temperature in a few hours, the
grafted species are much more stable.
Hydrolysis of (SiO)TaMe4/(SiO)2TaMe3. When 2a,b
were treated with water vapor (13 mbar), the solid turned white
rapidly and the IR band at 1800 cm−1 region completely
disappeared; methane was found as the main product in the gas
phase, and the gas analysis revealed that methane was produced
in an amount of 1.52 0.15 mmol/(g of (SiO)TaMe4/(
SiO)2TaMe3) (Ta/C ratio 1.0/(3.6 0.3)), which agrees well
with the data from elemental analysis and suggests a (
SiO)TaMe4/(SiO)2TaMe3 ratio (%) of (65 5):(35 5).
Hydrogenolysis of (SiO)TaMe4/(SiO)2TaMe3. (
SiO)TaMe4/(SiO)2TaMe3 readily react with H2 at 80 °C
(Scheme 2), after treatment under H2 for 1 h (500 mbar, 80
°C); the characteristic ν(Ta−H) band around 1830 cm−1 is
identical to the IR band of Ta−H previously observed by us,5
and simultaneously the intensity of the ν(C−H) bands at 2900
cm−1 decreases (Figures S4 and S5, Supporting Information).
At 150 °C, the reaction was complete within 3 h. Methane was
found as the main product in the gas phase, and gas analysis
revealed 1.42 0.2 mmol of methane produced/g of 2 (Ta/C
ratio 1.0/(3.4 0.3)), which matches previous results.
Preparation of (SiO)2TaH. When 2a and 2b were
treated at 200 °C with hydrogen (800 mbar) for 1 h, followed
1
by evacuation at 10−5 mbar for another /2 h, the tantalum
hydride was produced, which has the characteristic ν(Ta−H)
IR band at 1830 cm−1, and a new band at 2269 cm−1 for the
Si−H produced in the reaction is also observed. The band at
1830 cm−1 disappeared after a H2−D2 exchange, at which point
a new band ν(Ta−D) at ca. 1320 cm−1 appeared (Figure 2a).
Figure 2. (a) IR spectrum of (SiO)2TaH prepared from 2 (green),
spectrum after D2 exchange (purple), and subtraction spectrum (red).
(b) IR spectrum of (SiO)2TaH (purple), spectrum after addition of
PMe3 (green), and subtraction spectrum (red).
[Ta]sH + CH3I → [Ta]sI + CH4
This compound was identified by CH3I titration and methane
evolution, as halogenated hydrocarbons are known to exhibit a
high reactivity toward early-transition-metal hydrides.6 The
treatment of 3 with CH3I vapor (13 mbar; 25 °C) for 10 min
resulted in the complete disappearance of its IR band at 1830
cm−1 and methane evolution to the gas phase. Gas analysis
revealed that methane is produced in an amount of 0.41 0.04
mmol/g of 3 (Ta/H = 0.96 0.1) (on the basis of the data
from elemental analysis). This indicates the presence of
tantalum monohydride on the silica surface, denoted as (
SiO)2TaIIIH.
After treatment of (SiO)2TaH with PMe3 (13 mbar) at
room temperature for a few minutes, IR spectra showed that
the band for ν(Ta−H) at 1830 cm−1 disappeared and a new
peak appeared at 1304 cm−1 (Figure 2b). The 31P CP-MAS
solid-state NMR spectrum showed a main broad peak at −13.0
ppm for (SiO)2TaIIIH(PMe3) (Figure 3 and Figures S7a and
S8a (Supporting Information); a very small peak at 6 ppm may
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Figure 3. 1D 31P CP-MAS NMR spectrum of (SiO)2TaIIIH(PMe3).
contribute to a trace amount of (SiO)2TaVH3(PMe3)), which
coincide with those in a previous report.31
Alkane Metathesis with (SiO)2TaH (3). Under the
same conditions as reported before,15−20 we tested the
reactivity of (SiO)2TaH as prepared in alkane metathesis
in a batch reactor (Table 1). When propane (420 equiv) was
Table 1. Alkane Metathesis with (SiO)2TaH as Prepared
product selectivity, %
a
b
alkane
TON
C1
C2
C3
C4
C5
C6
Figure 4. (a) 13C and (b) 31P CP-MAS NMR spectra of (
propane
butane
63 (14.1)
79 (17.7)
15.4
1.7
39.1
13.4
36.3
7.8
SiO)3TaIII(PMe3).
37.2
31.3
10.8
a
After 120 h of reaction in a batch reactor (Ta/alkane ratio ∼420, 150
b
°C, 0.95 atm). TON is expressed in (mol of alkane transformed)/
Scheme 3. Reaction between (SiO)3Ta and 2-Pentyne
(mol of Ta). The values in parentheses are conversions.
brought into contact with 3 at 150 °C in a batch reactor, it was
converted mainly into methane (15.4%), ethane (39.1%),
butanes (36.3%), and pentanes (7.8%), with a TON of 63 after
120 h (more information can be found in Table S2, Supporting
Information). When the reaction was carried out with butane,
the TON was found to be 79. These results demonstrate that
the tantalum hydride prepared from the TaMe5 precursor has a
reactivity and selectivity toward alkane metathesis equivalent to
those previously reported.
Preparation of (SiO)3Ta. After vacuum or hydrogen
treatment at 200−500 °C (50 °C/h, Scheme 2) for 10 h, (
SiO)2TaHx is converted into (SiO)3TaIII. The intensity of IR
band of ν(Ta−H) at 1830 cm−1 decreases with time, while the
intensity of the ν(Si−H) band at 2170 cm−1 increases (Figures
S4 and S6, Supporting Information).21
The coordinated complex (SiO)3Ta(PMe3) was also
synthesized and characterized by NMR. The 13C NMR
spectrum shows a sharp peak at 10 ppm, and the 31P NMR
spectrum shows a main broad peak at −23.2 ppm (Figure 4 and
and Figures S7b and S8b (Supporting Information))23 instead
of the peak at −13.0 ppm of (SiO)2TaIIIH(PMe3).
Upon the hydrolysis of the complex formed by reacting (
SiO)3Ta with 2-pentyne, cis-2-pentene is evolved as the major
product at a ratio of 5:1 to trans-2-pentene (Scheme 3). This
further supports the TaIII species on the silica surface by
analogy with that found by Wolczanski et al. for Ta-
(OSilox)3.23,32
The tantalum LIII-edge EXAFS signal is acquired for the (
SiO)3Ta as prepared. As we know 2 is possibly decomposed by
a surface “bimolecular” pathway, we analyzed the EXAFS data
of 3 with increased k range to identify any possible cluster
formed on the surface, since the detection of Ta−Ta
contributions with a bond distance ranging from 2.9 to 3.1 Å
is best resolved when k values are higher than 10.5 Å−1 (for a
tantalum cluster supported on silica33). Thus, the value of k
range we used was up to 12.6 Å−1, and no contribution
attributed to the presence of a Ta−Ta bond could be found,
illustrating that the formation of a tantalum cluster is negligible
in the preparation of (SiO)3Ta. Analysis results indicate a
first coordination sphere consisting of 2.9 0.4 oxygen atoms
in the vicinity of tantalum with a Ta−O distance of 1.88 0.01
Å (Figure 5 (bottom), Table 2). This is in agreement with the
previously reported formation of a tris-siloxy tantalum
complex.21
The reactivity of (SiO)3Ta as prepared was tested in
ethylene polymerization, under 50 bar of ethylene at 70 °C;
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(SiO)TaMe2Cl2 and (SiO)WMe5, and provides a
possibility to study the decomposition of the unstable
methylmetal compounds.
Finally, the methyl ligand seems to be much more interesting
than the neopentyl ligand, due to the relative simplicity of the
synthesis and easy characterization; the use of a cheap and
commercially available starting material also facilitates the
inexpensive and immediate preparation.
EXPERIMENTAL SECTION
■
General Procedures. All experiments were carried out under an
inert atmosphere, using Schlenk and glovebox techniques for the
organometallic synthesis. For the synthesis and treatments of the
surface species, experiments were carried out using high-vacuum-line
(10−5 mbar) and glovebox techniques. Infrared spectra (transmission)
were recorded on a Nicolet Magna 6700 FT spectrometer equipped
with a cell under a controlled atmosphere. Elemental analyses were
performed at the Mikroanalytisches Labor Pascher in Germany (Table
S1, Supporting Information). Liquid NMR spectra were recorded on
Bruker Avance 500 spectrometers, and solid state NMR spectra were
recorded under MAS on a Bruker Avance 400 instrument. All chemical
shifts were given relative to the residual H or 13C resonance in the
1
deuterated solvents.
Preparation of Compounds. TaMe5 (1). Under an argon
atmosphere, TaCl5 (359 mg, 1 mmol) was placed in one compartment
of a chilled (−30 °C) double-Schlenk tube with 15 mL of dry pentane;
after the mixture was stirred for 5 min, 200 mg of MeLi powder (60%
purity based calibration, ca. 5.5 equiv) was added into the suspension
in small portions. The reaction mixture was stirred at −20 to −10 °C
for 2 h; the suspension became black, and after it stood for a few
minutes, it separated into a bright yellow solution and a dark gray
precipitate. The solution was filtered into the other compartment of
the double Schlenk to give a pure solution of TaMe5 (0.027 mmol/mL
based on gas analysis of methane produced in hydrolysis of a 1 mL
1
solution, yield 41%). H NMR (CD2Cl2, −10 °C): δ 0.88 (s, 15H).
1
1
13C NMR (CD2Cl2, −10 °C): δ 82.0. H−13C COSY: H 0.88 ppm
correlated with 13C 82.0 ppm.
Figure 5. Imaginary part of the back Fourier transforms (top) and
Fourier transforms (bottom) of the EXAFS k2 χ(k) functions for (
SiO)3TaIII. Experimental data are given as open white squares, while
the solid red lines are the fit results conducted with a k range of 3.4 up
to 9.8 Å−1 and with an R range of 1 up to 2.3 Å.
(SiO)TaMe4/(SiO)2TaMe3 (2). In a chilled (−30 °C) double-
Schlenk tube, silica Aerosil500 (0.8 g of Aerosil200 was dehydroxylated
at 500 °C for 15 h under 10−5 mBar vacuum) was placed in one
compartiment, followed by chilled TaMe5 solution produced in the
first step from 1 mmol of TaCl5. The reaction mixture was stirred at
−20 °C for 1 h, and the silica became grayish; after filtration and
washing three times with chilled pentane, the product was dried in
vacuo for 1 h at −20 °C. Then, it was dried in vacuo (10−5 mBar) for
an additional 1 h to afford a grayish powder. Elemental analysis gave
Ta 7.85%, C 1.92%, H 0.48%, Cl <0.1%.
(SiO)2TaH (3) and (SiO)2TaH(PMe3) (4). 2 (1.0 g) was placed
in a 150 mL Schlenk tube and heated to 200 °C under H2 (800 mbar)
for 1 h and under vacuum for another 1/2 h to afford a brown powder.
Elemental analysis gave 7.97 wt % of tantalum with trace amounts of
carbon and hydrogen belonging to the transfer of methyl groups to the
silica surface. 3 (100 mg) was placed in a Schlenk flask through a
septum, and after evacuation (10−5 mbar), PMe3 (10 μL) was added
via syringe at room temperature. After standing for 1 h, the excess
PMe3 was removed in vacuo to yield 4 as a black solid. Elemental
analysis gave Ta 7.72%, C 2.48%, H 0.56%, P 1.60%.
Table 2. EXAFS Parameters for (SiO)3Ta
neighboring atom of
Ta
no. of
atoms
Debye−Waller factor
(σ/Å)
distance (Å)
1.88 0.01
Ta−O
2.9 0.4
0.008 0.001
a
R factor 0.002 (Δk = 3.4−9.8 Å−1); energy shift ΔE0 = −1.81 (2) eV;
2
amplitude factor So = 1.04.
polyethylene was produced with an efficiency of 78 (kg of PE)/
((mol of Ta) h), which is equal to that in a previously reported
case.23
CONCLUSION
■
Alkane Metathesis with 3. A 100 mg portion of 3 was placed in a
400 mL Schlenk batch reactor, and after evacuation to 10−5 mbar, 0.95
atm of dry propane or butane was added. The reactor was heated to
150 °C for 120 h, and the products were analyzed by GC.
These results show a simple and efficient way to prepare surface
organometallic compounds of tantalum. The fact that we can
obtain identical data in both spectroscopic and catalytic results
is a proof of the reliability of surface organometallic chemistry
to prepare well-defined surface compounds for catalysis, and we
are able to prepare the same surface compound from different
precursors.
(SiO)3TaIII (5) and (SiO)3TaIII(PMe3) (6). A 0.5 g portion of 3
was placed in a 100 mL Schlenk tube and heated under vacuum or H2
(800 mbar) to 500 °C to afford a dark brown powder of 5. Elemental
analysis gave 7.70 wt % of tantalum with trace amounts of carbon and
hydrogen. 6 was prepared as described for 4. Elemental analysis gave
Ta 7.62%, C 2.42%, H 0.57%, P 1.53%. After a pellet of 5 was heated
to 120 °C with dry 99.9% D2 for 2 h, the IR spectrum stayed exactly
This study confirms that the unstable metal methyl
compounds become more stable in the monomolecular state
after grafting on the silica surface, as we already observed with
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the same (Figure S4, Supporting Information); the subtraction
spectrum did not show any ν(Ta−H) band.
flange, which was held to the top of the cross. The remaining two sides
of the cross were closed with KB flanges with holes where Kapton
windows were pasted at the center for X-rays to pass through. X-ray
absorption spectra were acquired at the National Synchrotron Light
Source (NSLS) (Beamline X-18b), at Brookhaven National Labo-
ratory, Upton, NY, USA, at room temperature at the tantalum LIII
edge, in the transmission mode with a double-crystal Si(111)
monochromator. The flux at X18b is 1 × 1010 photons/s at 100 mA
and 2.8 GeV, and the usable energy range is 5.8−40 keV. The
calibration was done using tantalum foil of 5 μm thickness. The
spectra were recorded between 9750 and 11000 eV with a 2.0 eV step.
The spectra analyzed were the result of the averaging of three such
acquisitions, and it was carefully checked that the results obtained were
comparable and reliable, since for each sample no evolution could be
detected by comparing the spectra between the first and last
acquisitions.
Ethylene Polymerization with 5. 5 (25 mg) and dry toluene (5
mL) were placed in a glass vial equipped with a magnetic stirrer. The
glass vial was sealed with a rubber cap under argon. Then the vial was
put into an ILS Premex parallel reactor system and stirred at 70 °C,
after the reactor was purged with N2 (99.999%) three times, ethylene
(99.95%) was introduced into the system with 50 bar pressure. After
the mixture was stirred for 1 h, the reaction was stopped and the
reactor was cooled to −5 °C to condense all the volatile components.
The liquid products were characterized by GC, and the PE produced
was isolated by filtration.
Reaction of 5 with 2-Pentyne. A 100 mg portion of 5 was placed in
a Schlenk flask with a septum, and the flask was evacuated to 10−5
mbar. Then, 10 μL of 2-pentyne was added by syringe. After standing
for 1 h at room temperature, 50 μL of H2O was added. The gas-phase
products in the flask were analyzed by GC; 55% of 2-pentyne
(corresponds to 1 equiv of the TaIII species) was consumed, and cis-2-
pentene and trans-2-pentene were found as the products in a ratio of
5:1.
XAS data were analyzed using the HORAE package, a graphical
interface to the AUTOBK and IFEFFIT code.37 XANES and EXAFS
spectra were obtained after performing standard procedures for pre-
edge subtraction, normalization, polynomial removal, and wave vector
conversion. Determination of the energy level (E0) was performed at
the first inflection point of the edge. For each atomic shell, the
following structural parameters were adjusted: coordination number
(N), bond length distance (R), and the so-called Debye−Waller factor
via the mean-square relative displacement (σ2) of the considered bond
length. The parameter ΔE0, which accounts for the difference between
the experimental absorption-edge energy and its estimate made by the
Solid-State Nuclear Magnetic Resonance. All of the solid-state
NMR spectra were recorded on a Bruker AVANCE III spectrometer
1
operating at 400, 100, and 162 MHz resonance frequencies for H,
13C, and 31P respectively, with a conventional double-resonance 4 mm
CPMAS probe. The samples were introduced under argon into
zirconia rotors, which were then tightly closed. The spinning
1
frequency was set to 17 kHz for H and 10 kHz for 13C and 31P
2
code, was also fitted. The amplitude factor (S0 ) was fitted to the
spectra. NMR chemical shifts are given with respect to TMS or H3PO4
as external references. For CP/MAS 13C NMR, the following sequence
was used: 90° pulse on the proton (pulse length 2.4 s), then a cross-
polarization step with a contact time of typically 1 ms, and finally
acquisition of the 13C signal under high-power proton decoupling. The
delay between scans was set to 5 s, to allow the complete relaxation of
the 1H nucleus, and the number of scans was 20000 for 1D 13C NMR.
An apodization function (exponential) corresponding to a line
broadening of 100 Hz was applied prior to Fourier transformation.
For CP/MAS 31P NMR, 90° pulse on the proton and a cross-
polarization step with a contact time of 2 ms were used. The delay
between the scans was set to 5 s, no spectral smoothing was employed
prior to Fourier transformation, and the number of scans was 15000
for phosphine. For 2D HETCOR multiple-quantum spectra were
recorded on a Bruker DSX-600 spectrometer with a conventional
double-resonance 3.2 mm CPMAS probe. 1H−13C heteronuclear
correlation solid-state NMR was performed according to the following
scheme: 90° proton pulse, t1 evolution period, cross-polarization (CP)
to carbon spins, and detection of carbon magnetization under TPPM
decoupling.34,35 For the cross-polarization step, a ramped radio
frequency (rf) field centered at 75 kHz was applied to the protons,
while the carbon rf field was matched to obtain the optimal signal. A
total of 32 t1 increments with 2000 scans each were collected. The
sample spinning frequency was 10 kHz. Two-dimensional double-
quantum (DQ) and triple-quantum (TQ) spectra were recorded with
the following scheme:36 excitation of DQ coherences, t1 evolution, Z
filter, and detection. The spectra were recorded in a rotor
synchronized fashion in t1; that is, the t1 increment was set equal to
one rotor period (4.545 μs). One cycle of the standard back-to-back
(BABA) recoupling sequence was used for the excitation and
reconversion period. Quadrature detection in w1 was achieved using
the States-TPPI method. A spinning frequency of 22 kHz was used.
The 90° proton pulse length was 2.5 μs, while a recycle delay of 5 s
was used. A total of 128 t1 increments with 32 scans each were
recorded.
EXAFS spectrum obtained for the tantalum(V) methoxide (Ta-
(OMe)5) reference compound, the molecular structure of the
compounds being a dimeric molecule with two [TaO6] octahedra
sharing a common edge.38 S0 was found to be equal to 1.04 (see the
2
Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables and figures giving IR, NMR, and other characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.-M.B.: jeanmarie.basset@kaust.edu.sa.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are indebted to Prof. Lyndon Emsley for his
enthusiastic discussions and suggestions. We also thank King
Abdullah University of Science and Technology for generous
research support.
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NOTE ADDED AFTER ASAP PUBLICATION
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In the version of this paper published on February 5, 2014, an
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