Tungsten(VI) Carbyne/Bis(carbene) Tautomerization Enabled by N-Donor SBA15 Surface Ligands: A Solid-State NMR and DFT Study

Article abstract of DOI:10.1002/anie.201605934

Designing supported well-defined bis(carbene) complexes remains a key challenge in heterogeneous catalysis. The reaction of W(≡CtBu)(CH₂tBu)₃with amine-modified mesoporous SBA15 silica, which has vicinal silanol/silylamine pairs [(≡SiOH)(≡SiNH₂)], leads to [(≡SiNH₂?)(≡SiO?)W(≡CHtBu)(CH₂tBu)₂] and [(≡SiNH₂?)(≡SiO?)W(=CHtBu)₂(CH₂tBu). Variable temperature,¹H–¹H 2D double-quantum,¹H–¹³C HETCOR, and HETCOR with spin diffusion solid-state NMR spectroscopy demonstrate tautomerization between the alkyl alkylidyne and the bis(alkylidene) on the SBA15 surface. Such equilibrium is possible through the coordination of W to the surface [(≡Si?OH)(≡Si?NH₂)] groups, which act as a [N,O] pincer ligand. DFT calculations provide a rationalization for the surface–complex tautomerization and support the experimental results. This direct observation of such a process shows the strong similarity between molecular mechanisms in homogeneous and heterogeneous catalysis. In propane metathesis (at 150 °C), the tungsten bis(carbene) tautomer is favorable, with a turnover number (TON) of 262. It is the highest TON among all the tungsten alkyl-supported catalysts.

Full text of DOI:10.1002/anie.201605934

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International Edition: DOI: 10.1002/anie.201605934
German Edition: DOI: 10.1002/ange.201605934
Heterogeneous Catalysis
Tungsten(VI) Carbyne/Bis(carbene) Tautomerization Enabled by N-
Donor SBA15 Surface Ligands: A Solid-State NMR and DFT Study
Anissa Bendjeriou-Sedjerari, Julien Sofack-Kreutzer, Yury Minenkov, Edy Abou-Hamad,
Bilel Hamzaoui, Baraa Werghi, Dalaver H. Anjum, Luigi Cavallo, Kuo-Wei Huang, and Jean-
Marie Basset*
ꢀ
ꢁ
=
Abstract: Designing supported well-defined bis(carbene)
complexes remains a key challenge in heterogeneous catalysis.
(tBuCH2)2W( CtBu)(SitBuPh2)
and
(tBuCH2) W(
CHtBu)2(SitBuPh2).[5] This tautomerization process occurs
through a-hydrogen transfer from a neopentyl to the carbyne
(at 287 K). Until now, such equilibrium was promoted by the
coordination of sterically bulky phosphine[6] on a classical
silica surface.[7] Our interest was to achieve a direct isolation
and characterization of a W bis(carbene) alkyl tautomeric
form as a SOMF-active metathesis carried out in propane.
ꢀ
The reaction of W( CtBu)(CH2tBu)3 with amine-modified
mesoporous SBA15 silica, which has vicinal silanol/silylamine
ꢀ
ꢀ
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
pairs [( SiOH)( SiNH2)], leads to [( SiNH2 )( SiO )W(
ꢀ
ꢁ ꢀ
ꢁ
=
CHtBu)(CH2tBu)2] and [( SiNH2 )( SiO )W( CHtBu)2-
(CH2tBu). Variable temperature, H–1H 2D double-quantum,
1
1H–13C HETCOR, and HETCOR with spin diffusion solid-
state NMR spectroscopy demonstrate tautomerization between
the alkyl alkylidyne and the bis(alkylidene) on the SBA15
surface. Such equilibrium is possible through the coordination
Recently, we reported the synthesis of SBA15 containing
adjacent SiNH2 and Si OH surface groups. Herein, we
describe the chemisorption of W( CtBu)(CH2tBu)3 on such
a surface, which plays the role of chelating [N,O] pincer
[8]
ꢀ
ꢀ ꢁ
ꢀ
ꢀ ꢁ
ꢀ ꢁ
of W to the surface [( Si OH)( Si NH2)] groups, which act
as a [N,O] pincer ligand. DFT calculations provide a ration-
alization for the surface–complex tautomerization and support
the experimental results. This direct observation of such
a process shows the strong similarity between molecular
mechanisms in homogeneous and heterogeneous catalysis. In
propane metathesis (at 1508C), the tungsten bis(carbene)
tautomer is favorable, with a turnover number (TON) of 262.
It is the highest TON among all the tungsten alkyl-supported
catalysts.
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
ligands,
leading
to
[( SiNH2 )( SiO )W( CHtBu)-
ꢀ
ꢁ ꢀ
ꢁ
=
(CH2tBu)2] (3a) and [( SiNH2 )( SiO )W( CHtBu)2-
(CH2tBu)] (3b) (Scheme 1).
O
ne of the objectives of surface organometallic chemistry is
to transfer the concepts of molecular chemistry to surface
chemistry.[1] The main strategy is to use surface organometal-
lic fragments (SOMF) to enter directly into the catalytic cycle
of a given reaction, leading to real predictive catalysis by
design.[1b] Through this approach, we proposed, for example,
that olefin metathesis should require a surface metallocar-
bene and alkane metathesis should require a metallocarbene
hydride or a metallocarbene alkyl.[1b,2] In the past, we
ꢀ
Scheme 1. Reaction of W( CtBu)(CH2tBu)3 (1) with a [N,O] chelating
SBA15 surface (2).
ꢀ
Reaction of W( CtBu)(CH2tBu)3 (1) with 2, in pentane at
508C leads to the formation of a tungsten complex covalently
bonded to silica in accordance with the gas-phase analysis
(0.9 ꢂ 0.1 equivalent of gaseous neopentane per W), a C/W
ratio of 16 is consistent with the theoretical value (15) for
a compound retaining 3 neopentyl-like species. Finally, W/N
and C/N ratios of 0.9 and 14.3, respectively, agree with the
expected value for either [( SiNH2 )( SiO )W( CHtBu)-
(CH2tBu)2] (3a) or [( SiNH2 )( SiO )W( CHtBu)2-
(CH2tBu)] (3b; Supporting Information, Table S1).
ꢀ
reported the grafting of W( CtBu)(CH2tBu)3 (1) on a partially
dehydroxylated silica surface (at 7008C) to yield the mono-
podal species: [ Si O W( CtBu)(CH2tBu)2].[3] This carbyne
was found to be inactive in propane metathesis[1b,4] and active
in propene metathesis, which led to some speculation on the
real mechanism. In molecular organometallic chemistry, Xue
et al. discovered that an equilibrium could occur between
ꢀ ꢁ ꢁ
ꢀ
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
ꢀ
ꢁ ꢀ
ꢁ
=
ꢀ
The FTIR bands characteristic of the starting SiNH2
moiety are found at 3532 [uas(NH2)], 3448 [us(NH2)], and
1550 cmꢁ1 [d(NH2)]. After reaction of 1 with 2, a total
consumption of isolated silanols [u(OH) at 3741 cmꢁ1] is
observed. In contrast, the characteristic bands of the silyl-
amine groups are still present but with a slight blue shift
(ꢁ5 cmꢁ1) at 3527 and 3442 for uas(NH2) and us(NH2),
respectively (Figure 1). Notably, a weak consumption of the
characteristic vibration bands of the silylamine group, uas-
(NH2) and us(NH2), is observed in accordance with the
presence of the weak u(NH) stretching bands at 3226 and
[*] A. Bendjeriou-Sedjerari, J. Sofack-Kreutzer, Y. Minenkov,
E. Abou-Hamad, B. Hamzaoui, B. Werghi, D. H. Anjum, L. Cavallo,
K. W. Huang, J. M. Basset
King Abdullah University of Science and Technology (KAUST)
Kaust Catalysis Center (KCC)
Thuwal, 23955-6900 (Saudi Arabia)
E-mail: jeanmarie.basset@kaust.edu.sa
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605934.
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Figure 2. 1H–1H double-quantum (DQ)/single-quantum (SQ) NMR
spectrum of 3 (acquired at 600 MHz with a 22 kHz MAS frequency,
32 scans per t1 increment, 5 s repetition delay, and 128 individual t1
increments).
Figure 1. FTIR spectra of 2 in black and 3 in red.
3180 cmꢁ1 (Supporting Information, Figure S3 and Sche-
me S1).[8a,9] This observation is consistent with the formation
of a small amount (< 10%) of a covalent bond between the
NH surface ligand and W.
ꢁ
ꢁ
ꢁ
[W CH2C(CH3)3], [W CH2C(CH3)3], and [W CH2C-
(CH3)3], respectively. Note that additional signals are
ꢀ
The coordination sphere of the tungsten species in 3 was
elucidated by various solid-state NMR studies. The 1H NMR
spectrum of 3 displays two intense signals at 0.9 and 1.1 ppm
assigned to the remaining neopentyl-like group (CH2 and
CH3). A weak signal at 2 ppm could be tentatively attributed
observed at 30 and 316 ppm, which can be assigned to [W
ꢁ
ꢀ ꢁ
C C(CH3)3] and [W C C(CH3)3], respectively, by compar-
ison with the 13C NMR spectrum of 2 in solution in deuterated
benzene (Supporting Information, Figure S2). Additionally,
the presence of a resonance at 227 ppm confirms the presence
ꢀ
to SiNH2 coordinated to W (Supporting Information, Fig-
ure S4).[10] Interestingly, the spectrum exhibits also a weak
signal at 6.8 ppm, which could be assigned to the proton of
a carbene (Figure 2 and the Supporting Information, S4 enlar-
gement).[5a]
1
2D H–1H double-quantum (DQ) solid-state NMR spec-
troscopy was performed to investigate the probable and rare
transformation of the d0-alkyl-alkylidyne (3a) to its alkyli-
dene tautomer (3b) upon grafting of 1. The DQ spectrum of 3
shows a strong auto-correlation peak on the 2:1 diagonal
centered at around 0.9 ppm in F2 and at 1.8 ppm in F1
(Figure 2). These resonances are assigned to the CH2 and CH3
moieties. The proton resonance at 0.9 ppm shows a weak
ꢀ ꢁ
ꢁ
correlation with the proton at 3.6 ppm assigned to Si NH
W
[4.5 ppm in F1: dH(CH3/CH2) + dH(NH) = 0.9 + 3.6]
establishing the identity of the minor species (Supporting
Information, Scheme S1, 3c and 3d ca. 10%). Furthermore,
the correlation at about 7.7 ppm corresponds to [dH(CH3) +
=
dH(W CH) = 0.9 + 6.8], which confirms the presence of the
bis(alkylidene) (3b).
To unambiguously confirm these results, 13C cross-polar-
ization magic-angle spinning (CP-MAS) and 1H–13C two-
dimensional (2D) heteronuclear correlation (HETCOR) with
1
short contact times (0.2 ms) and H–13C HETCOR with spin
diffusion using a long mixing time (tm = 100 ms) experiments
were performed.
Figure 3. Two-dimensional 1H–13C CP/MAS dipolar HETCOR spectrum
of 3 (acquired at 600 MHz proton frequency with an 8.5 kHz frequency,
4000 scans per t1 increment, a 4 s repetition delay, 32 individual t1
increments, and a 0.2 ms contact time).
The 13C CP-MAS spectrum of 3 reveals a clear pattern, as
six signals are identified (Figure 3). As observed previously,
the spectrum has signals at 32, 51, and 95 ppm assigned to the
2
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=
ꢁ
of metallocarbene species, [W CH C(CH3)3]. These results
unambiguously confirm the presence of a supported tungsten
d0-alkyl alkylidyne and
a
bis(alkylidene). The 1H–13C
HETCOR spectrum of 3 revealed, as expected, a correlation
between the alkylidene carbon resonance at 227 ppm and its
proton resonance at 6.8 ppm (Figure 3). Furthermore, a cross-
peak between the alkylidyne carbon at 316 ppm and the
proton of the alkylidene at 6.8 ppm was clearly observed in
1
1
the H–13C dipolar HETCOR with H spin diffusion period
(Figure 4), resulting from a chemical exchange process
between 3a and 3b.
Figure 5. Variable temperature 1H MAS NMR spectra of 3 (acquired at
600 MHz with a 15 kHz MAS frequency, a repetition delay of 5 s, and
8 scans).
To understand these results, we performed density func-
tional theory (DFT) calculations (see the Supporting Infor-
mation for computational details). We examined the
ꢀ ꢁ
3a’’Ð3b’’ equilibrium in the absence of the Si NH2 group,
that is, on an unmodified silica surface dehydroxylated at
2008C (Supporting Information, Figure S8). Practically all the
tested functionals predict the tungsten bis(neopentylidene)
(3b’) to be higher in Gibbs free energy compared to the
neopentyl-neopentylidyne tautomer (3a’) by approximately
5–6 kcalmolꢁ1, except for the M06 functional, which predicts
3a’ to be more stable by 8 kcalmolꢁ1. This underlines that the
equilibrium between the two tautomers on the classical silica
surface (SiO2–200), is strongly shifted towards the formation of
the neopentyl-neopentylidyne tungsten (3a’). This is in
accordance with the experimental evidence that the bis(neo-
pentylidene) species (3b’) is not detected by NMR after
reaction of tungsten tris-neopentyl-neopentylidyne on both
classical SiO2–200 and SiO2-700 surfaces.[3] The activation Gibbs
free-energy barrier associated with the tautomerization of 3a’
to 3b’ is quite high, approximately 27 kcalmolꢁ1, to be
overcome at room temperature (Table 1).
Figure 4. Two-dimensional 1H–13C spin diffusion HETCOR spectrum of
3 (acquired at 600 MHz proton frequency with an 8.5 kHz frequency,
4000 scans per t1 increment, a 4 s repetition delay, 64 individual t1
increments, and a mixing time tm =100 ms).
Finally, variable-temperature 1H solid state NMR studies
highlight the isomerization of 3a to 3b (Figure 5). Thus,
decreasing the temperature to 230 K results in the total
disappearance of the neopentylidene proton resonance
observed at 6.8 ppm at 300 K. In contrast, the same resonance
reappears with a higher intensity at 340 K, which indicates
that the neopentylidene tautomer 3b is favored at high
temperatures. At this temperature, the broad signal of the
corresponding carbene is split. This suggests that both syn and
anti isomers exist (Supporting Information, Scheme S2),
indicative of the rigid behavior of the two carbenes.[5a,d,6b,11]
Furthermore, the proton signal at 2 ppm (previously assigned
Next, we focused on examining the impact of a vicinal
ꢀ ꢁ
Si NH2 group on the 3aÐ3b equilibrium. The first question
ꢀ ꢁ
we addressed is the eventual coordination of the Si NH2
group to the W atom. To this end, we performed a conforma-
tional search (see the Supporting Information), which indi-
cated that for 3a, the most stable structure does not show a
ꢀ ꢁ
ꢀ ꢁ
to the Si NH2 group coordinated to W) is more intense at
Si NH2···W interaction (W–N distance of 3.85 ꢀ)
(Figure 6).
340 K, supporting the effect of coordination between N and
W atoms at high temperatures. This tendency is similar to
what was observed by Xue et al.[5] in molecular chemistry,
which shows the benefit of SOMC in transferring the concepts
of molecular chemistry to surface chemistry.
Indeed, the most stable structure of 3a involving the
ꢁ
formation of a W NH2 bond (a W–N distance of ca. 2.6 ꢀ) is
about 3 kcalmolꢁ1 higher in energy. Conversely, the most
stable structure of the 3b tautomer shows the NH2 group
coordinated to tungsten atom (W–N distance of 2.63 ꢀ). This
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Table 1: Activation and standard Gibbs free energy [kcalmolꢁ1] calcu-
lated for the equilibrium between tungsten neopentylidyne and its
bis(neopentylidene) tautomer on classical (3a’Ð3b’) and amine-
modified (3aÐ3b) silica surfaces at room temperature.
ꢀ ꢁ
the [ Si NH2!W] signal is weak. This trend is enhanced at
340 K, with a more rapid interconversion between 3a and 3b,
=
ꢁ
a more intense signal corresponding to [W CH C(CH3)3],
and the clearly visible characteristic broad signal of ( Si
ꢀ ꢁ
TPSS-D3 TPSSh-D3 PBE-D3 PBE0-D3 M06
NH2!W).
The catalytic performance of the tungsten bis(carbene)
alkyl (3) was evaluated in propane metathesis at 1508C (in
a batch reactor). In previous studies, the tungsten carbyne
Thermochemistry
DG(3a’/3b’)
DG(3a/3b)
DDG
Activation
DG°(3a’/3b’)
DG°(3a/3b)
DDG°
5.1
2.5
ꢁ2.6
5.4
3.3
ꢁ2.1
5.1
4.2
ꢁ0.9
5.9
5.3
ꢁ0.6
8.3
6.3
ꢁ2.0
ꢀ ꢁ ꢁ
ꢀ
supported on silica [( Si O )W( CtBu)(CH2tBu)2] was
inactive in propane metathesis, and the highest TON (121)
was obtained on tungsten hydride supported on silica–
alumina after 120 h.[4] As demonstrated by SS NMR spec-
troscopy and DFT calculations, the tungsten bis(carbene)
neopentyl tautomer is, at this temperature, favored. Conse-
quently, the catalytic results show that the tungsten bis-
(carbene) neopentyl is efficient in the conversion of propane
into methane (1.6%), ethane (60%), butane (21%), isobu-
tane (8%), and pentane (3%) with a TON of 197 after 24 h
and 262 with 26% conversion after 240 h.
26.7
24.2
ꢁ2.5
27.8
25.5
ꢁ2.3
25.5
23.1
ꢁ2.4
28.6
26.5
ꢁ2.1
31.1
28.4
ꢁ2.7
In conclusion, a direct observation of a-H exchange
between alkyl alkylidyne and a bis(alkylidene) was made,
which is an exciting step toward understanding the elemen-
tary steps in heterogeneous catalysis. It is interesting to note
that SOMC applied to [N,O] SBA15 surfaces plays an
important role in the stabilization of a tungsten bis(neopen-
tylidene). Indeed, the formation of the tungsten bis(carbene)
alkyl, favored at high temperatures on a [N,O] SBA15 surface,
leads to better catalytic activity in propane metathesis
compared to the classical silica-supported tungsten carbyne
bis(alkyl). The fact that a surface may play the role of an
external ligand for this transformation is novel and shows the
power of the surface organometallic chemistry concept.
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
Figure 6. Calculated geometries of [( SiNH2 )( SiO )W( CHtBu)-
ꢀ
ꢁ ꢀ
ꢁ
=
(CH2tBu)2] (3a) and of [( SiNH2 )( SiO )W( CHtBu)2(CH2tBu)]
(3b). Distances (N–W) are given in ꢀ. Si beige, O red, H white,
W blue, N dark blue, C gray, alkylidene H in 3b yellow.
finding is in agreement with a previous study, which focused
on the influence of PMe3 on a similar tautomerization
equilibrium in a W–bis(alkylidene) complex, in molecular
organometallic chemistry.[6a]
ꢀ ꢁ
In presence of the Si NH2 group all of the tested
Acknowledgements
functionals reduce the Gibbs free-energy difference between
species 3b and 3a by roughly 1–2 kcalmolꢁ1. Incidentally, the
enthalpic difference is reduced even more by the TPSS-D3
functional (by ca. 4 kcalmolꢁ1), which results in 3b being
enthalpically more stable than 3a by 1.4 kcalmolꢁ1 (see the
Supporting Information). This indicates that the interaction
This work received support from the King Abdullah Uni-
versity of Science and Technology (KAUST) Office of
Sponsored
Research
(OSR)
under
Award
No.
CRG_R2_13_BASS_KAUST_1.
ꢀ ꢁ
of the Si NH2 group with the W atom stabilizes 3b more
than 3a. The evidence that entropy effects destabilize 3b
relative to 3a is consistent with the more ordered structure of
Keywords: chelating ligands · a-H exchange · SBA15 silica ·
surface organometallic chemistry · tautomerization
ꢀ ꢁ
3b, due to the coordination of the Si NH2 group to the W
atom. Finally, all the functionals tested predict the Gibbs free
energy barrier for the 3a!3b rearrangement to be roughly
2 kcalmolꢁ1 lower in presence of the Si NH2 group.
ꢀ ꢁ
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exists on the [N,O] SBA15 surface. As the N–W interaction is
lost in the most stable structure of 3a, the corresponding
ꢀ ꢁ
resonance at 2 ppm ( Si NH2!W) is not present in the
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=
resulting in the presence of the signal corresponding to [W
CH C(CH3)3], but species 3a still dominates the surface, and
ꢁ
4
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Received: June 19, 2016
Published online: && &&, &&&&
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Communications
Heterogeneous Catalysis
A. Bendjeriou-Sedjerari,
J. Sofack-Kreutzer, Y. Minenkov,
E. Abou-Hamad, B. Hamzaoui, B. Werghi,
D. H. Anjum, L. Cavallo, K. W. Huang,
J. M. Basset*
&&&& — &&&&
Tungsten(VI) Carbyne/Bis(carbene)
Tautomerization Enabled by N-Donor
SBA15 Surface Ligands: A Solid-State
NMR and DFT Study
O brave HETCOR! Look how it binds!
Solid-state NMR was used to demon-
strate the tautomerization of the alkyl
alkylidyne and bis(alkylidene) forms of
a tungsten complex on a SBA15 silica
surface. Such equilibrium is possible
through the coordination of W to the
ꢀ ꢁ
ꢀ ꢁ
surface-adjacent [( Si OH) ( Si NH2)]
groups, which behave as a [N,O] pincer
ꢀ
ligand. {W}=W( CtBu)(CH2tBu)3.
6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!