Single site silica supported tetramethyl niobium by the SOMC strategy: Synthesis, characterization and structure-activity relationship in the ethylene oligomerization reaction

Article abstract of DOI:10.1039/c7cc02585h

A silica supported tetramethyl niobium complex [(SiO)NbMe₄] 2 has been isolated by the surface alkylation of [(SiO-)NbCl₃Me] 1 with dimethyl zinc in pentane. 1 can be easily synthesized by grafting NbCl₃Me₂ onto the surface of partially dehydroxylated silica by the SOMC strategy. Precise structural analysis was carried out using FTIR, advanced solid state NMR, elemental analysis and mass balance techniques (gas quantification after treating 2 with degassed water). Complex 1 was found to be active in the ethylene oligomerization reaction, producing up to C₃₀, whereas to our surprise complex 2 selectively dimerized ethylene into 1-butene in the absence of a co-catalyst at the same conversion level.

Full text of DOI:10.1039/c7cc02585h

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DOI: 10.1039/C7CC02585H.
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Reactivity differences of Sc
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Single Site Silica Supported Tetramethyl Niobium by the SOMC
Strategy: Synthesis, Characterization and Structure-Activity
Relationship in Ethylene Oligomerization Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
[
a]
[a]
[a,b]
[a]
[a,c]
DOI: 10.1039/x0xx00000x
Ali Hamieh , Raju Dey , Bijan Nekoueishahraki , Manoja K. Samantaray , Yin Chen , Edy
[
a]
[a]
Abou-Hamad and Jean-Marie Basset*
www.rsc.org/
1
1
Silica supported Tetramethyl niobium complex [(≡SiO)NbMe
has been isolated by surface alkylation of [(≡SiO-)NbCl
dimethyl zinc in pentane. 1 can be easily synthesized by grafting complexes and isolate the key intermediate for alkane
of NbCl Me on to the surface of partially dehydroxylated silica by metathesis reaction and trimerization of ethylene.
4
] 2 pepared a variety of tungsten, [(≡SiO-)WMe
5
]
tantalum
2
1
v
22
23
3
Me] 1 with [(≡SiO-)TaMe
4
] ,[(≡SiO-)Ta Cl
2
Me
2
], and niobium
SOMC
3
2
the SOMC strategy. Precise structural analysis was carried out by Very recently, we developed surface alkylation technique
the FTIR, advance solid state NMR, elemental analysis and mass where a combination of surface coordination compounds and
balance techniques (gas quantification after treating 2 with their alkylation lead us to isolate metal alkyl fragments
2
4, 25
degassed water) . Complex 1 was found to be active in the (SOMF),
ethylene oligomerization to produce up to C30, whereas to our by homogeneous condition. Using this techniques we were
especially those that are difficult to synthesized
2
6
surprise complex 2 selectively dimerizes ethylene into 1-butene in able to synthesize [(≡SiO)W(=O)Me
]
whereas the synthesis
3
the absence of a co-catalyst at the same conversion levels.
of the homogeneous counterpart [W(=O)Me ] is not known in
4
2
7
the literature.
Me
Nb
Transition metal alkyl compounds are active intermediates in
-5
Cl
O
Cl
Cl
1
O
many catalytic reactions and are heavely used in industry,
though many of them have never been isolated or
OH
Si
O
O
DCM, rt, 2h
Si
+
CH4
O
+
NbCl Me
2
3
Si
O
O
O
Si
6
-8
O
O
O
O
characterized yet. These type of metal alkyl compounds
usually are highly unstable, and prone to decompose via β-H
abstraction or through bi-molecular pathway. Till date, only
O
SiO2-700
limited number of metal alkyl compounds of early transition Scheme 1: Grafting of NbCl Me on to the SiO
to form complex 1.
2-700
3
2
metals are isolated which do not poses β-H due to their poor
9
stability.
, 10
v
In a previous report we demonstrate that [(≡SiO-)Ta Cl Me ]
2
2
showed a high selectivity towards trimerization of ethylene
8
2
Suface organometallic chemistry (SOMC) allows isolation, into 1-hexene. On the other hand, although the homoleptic
characterizion and study of the catalytic properties of those tantalum and tungsten methyl SOMC complex was well
organometallic
compounds
(Surface
OrganoMetallic explored, corresponding niobium complexes are not reported
Fragments, SOMF) which are highly unstable at room earlier. Thus, we decided here to chose niobium complex for
1
1-13
temperature or below.
In addition, SOMC based catalysts our research. However, niobium alkyl compounds are less
2
9-31
are easily separated from the products which is highly studied, probably because of their higher instability.
To our
Another major advantage of SOMC based knowledge, homoleptic pentaalkyl or tetraalkyl niobiumhalides
1
4-16
preferable.
catalysts is the ability to isolate reaction intermediates, have not been isolated yet. Thus, we applied our suface
propose reaction mechanisms, and study the structure activity alkylation strategy to isolate well-defined alkyl niobium
12, 17-20
24, 26
relationship.
.
Recently, using this strategy we have complex.
In the present manuscript, we report the synthesis of silica
supported tetramethyl niobium complex [( SiO-)NbMe ] via
the surface methylation of niobium monomethyl [( SiO-
NbCl Me] SOMC complex. These surface niobium
≡
4
≡
)
3
(1)
compounds were systematically characterized by the classical
tools of SOMC.
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Niobium complex
1
3
was prepared by the grafting of To fully methylate compound 1 [(≡SiO-)NbCl Me] we used
(V)
Nb Cl
3
Me
2
on partially dehydoxylated silica (partial
under high vacuum
was reacted with SiO2-
00 at room temperature in dichloromethane or pentane for
two hours under argon flow (Scheme 1), followed by washing
dehydroxylation was carried out at 700
℃
-
5
(V)
(
3 2
10 bar) to form SiO2-700). Nb Cl Me
7
−
5
with pentane and drying under high vacuum (10 bar) in the
absence of light, giving as a pale yellow powder. The grafting
1
reaction leads to the release of one equivalent of methane and
the formation of a major product, well-defined niobium
complex [(≡SiO-)NbCl
propably as result of the reaction between the chloride of the
niobium complex and the silanol. The structure of was
confirmed by means of FTIR, elemental analysis and solid state
Me] 1 with traces of a by byproduct
3
1
1
13
NMR ( H, C, DQ, TQ, and HETCOR). Infrared spectrum showed
that the characteristic peak of free silanol in SiO at 3743
2
-700
−1
cm (Fig. 1) completely disappeared (Fig. 1, yellow) as a sign of
a quasi complete grafting.
1
1
H MAS NMR spectra of complex 1 b) 2D H− H DQ/SQ and
1
1
c) H− H TQ/SQ NMR spectra of complex 1 c) One-dimensional C
1
13
(
CP/MAS NMR spectra of complex 1 d) 2D CP/MAS HETCOR NMR
spectra of 1.
surface methylation technique with dimethylzinc to form full
alkylated complex [( SiO-)NbMe4]. Complex
SiO)NbCl Me] was taken in dichloromethane and then
≡
1
[(≡
3
reacted with 1.5 equivalents (with respect to niobium) of
dimethylzinc (solution in pentane) at room temperature for an
hour (Scheme 2). The product was washed three time with
-
pentane and then dried under high vacuum for 30 min(<10
5
bar) to form
2. The FTIR shows characteristic peak of the
niobium methyls by the increase of intensity of the bands in
-
1
-1
the range of 3000 cm ,
to an increase in the amount of methyl fragments(Fig. 1, red).
Elemental analysis of gave 2.59 %Nb, 1.28 % C, 1.32 % Cl,
.11 % Zn with a ratio of Nb/C =1/3.8 (Theoretical: Nb/C =1/4).
ν(C-H), and 1540 cm , δ(C-H) assigned
Elemental analysis of
a ratio of Nb:C:Cl=1:1.1:2.8 (Theoretical: Nb:C:Cl=1:1:3) where
SiO-)NbCl Me] represents the major product.
1 gave 2.69 %Nb, 0.38 % C, 2.87 % Cl with
2
[(≡
3
1
In addition, the structure was further proved by gas
quantification method [adding degassed water to 100 mg of
The complex
1
H magic-angle spinning (MAS) NMR spectra of 1 exhibit one
1
was fully characterized by solid state NMR. The
complex
4
2 releases 0.114 mmol of methane (4.2 CH /Nb) after
main signal at 2.5 ppm with a small shoulder at 2.0 ppm. All
the signals auto-correlate in the two-dimensional (2D) double-
quantum (DQ) and triple-quantum (TQ) proving that they both
analyzing gas samples by gas chromatography (GC)]. This can
be explained by the presence of [(
species.
4
≡SiO-)NbMe ] as a major
1
3
belong to a CH group (Fig. 2). Similarly, C CP/MAS NMR
3
spectrum of
1
displays only a single peak at 69 ppm
. The 2D
Additionally
SOMF) was elucidated by solid state NMR. The H-solid state
NMR spectrum of displays a main peak at 2.0 ppm along with
a small peak at 0.3 ppm(Fig. 3). This peak shows a strong
, formulation of surface organometallic fragment
HETCOR spectrum of
1
with short contact time (0.5 ms) shows
1
(
2
a clear correlation between the main methyl proton and the
carbon peak at 69 ppm, allowing the assignment of the
carbon–proton pairs to the individual methyl groups.
2
1
1
autocorrelation peak on the diagonal of both 2D H- H double
and triple quantum (DQ, TQ). Thus, the peak at 2.0 ppm can be
assigned to a methyl group.
Me
Nb
Me
Nb
Cl
Cl
Cl
Me
Me
Me
O
Si
O
O
O
Si
O
O
DCM, rt,
1h
+
1.5 ZnMe
2
+ 1.5 ZnCl
2
O
O
Si
Si
O
O
O
1
O
O
O
2
2
Scheme 2: Surface methylation of complex 1 by ZnMe to generate complex.
2
| J. Name., 2012, 00, 1-3
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1
3
Besides, C cross polarization magic-angle spinning (CP-MAS) the selectivity of both complexes
1
and
were deactivated likely due to the formation of polymer
shows a correlation between the carbon peak at 54 ppm on the active sites. This difference of selectivity between and
is likely due to different coordination sphere and their ability
to abstract the β-hydrogen from the growing chain.
2. The pre-catalysts 1
1
13
of
of
2
2
shows a single peak at 54 ppm, H- C HETCOR spectrum and 2
1
and the proton peak at 2.0 ppm(Fig. 3).
2
Table 1: Oligomerization of ethylene catalyzed with complex
[
(≡SiO)NbCl
3
Me] 1, complex [(≡SiO)NbMe
Me ].
4
] 2 and previously reported
tantalum complex [(≡SiO)TaCl
2
2
a
Complex
Butenes
Hexenes
Activity
Selectivity
Selectivity
(1-butene %) (1-hexen %)
b
c
[
(≡SiO)NbCl
(≡SiO)NbCl
3
3
Me]
38.8(82)
36.5(66)
1150
[
[
Me]
40.1(83)
91.9(94)
34.4(54)
7.8(54)
934
b
(≡SiO)NbMe4]
1050
c
[
[
[
(≡SiO)NbMe
(≡SiO)TaCl
Me ]
2
4
]
87.2(93)
9.6
10.5(45)
82.7
977
375
2
a] Turnovers mol(ethylene)/mol(Nb)/h
b] Ehylene pressure =50 bars
[c] Ethylene pressure=40 bars
[
1
Fig. 3: a) H MAS NMR spectra of complex
1
1
2
b) 2D H− H
c) One-
d) 2D
1
1
DQ/SQ and (c) H− H TQ/SQ NMR spectra of complex
2
1
3
In conclusion, we have succeeded to prepare the first niobium
tetramethyl complex on a silica support [(≡SiO-)NbMe
] using
SOMC-approach combined by surface alkylation tool. Complex
dimensional C CP/MAS NMR spectra of complex
CP/MAS HETCOR NMR spectra of complex
2
4
2
[
(≡SiO-)NbMe
NbCl Me]
4
]
2
was synthesized by alkylating [(≡SiO-
The alkylation reaction of
1 has been repeated with an excess
)
3
1
with dimethylzinc. Complexes 1 and 2 were fully
amount of dimethylzinc (5 equivalents compared to Nb)
keeping in mind to get fully converted tetramethyl niobium
complex. After analyzing the formed product, the proton peak
at 2.0 and the carbon peak at 54 are still intact, as well as new
proton and carbon peaks appears at 0.3 and -20 ppm as
characterized by advance solid state NMR, FTIR, elemental
analysis and mass balance. Finally, we reveiled that exhibits
2
good activity and high selectivity in the dimerization of
ethylene into 1-butene.
expected due to the formation of [(
≡SiO-)ZnMe] because of
This work was supported by funds from King Abdullah
University of Science and Technology (KAUST) office of
sponsored research (OSR).
excess use of dimethyl zinc (SI, Fig. S1).
After systematic characterization, the reactivity of the surface
niobium compounds was tested in ethylene oligomerization.
complex
toluene, followed by a high pressure introduction of ethylene
at constant pressure of 50 bars) at 100 . Using
1 or 2 was taken inside a special glass reactor in dry
Notes and references
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℃
1,
oligomerization of ethylene into olefins ranging between C4
and C32 (with a difference of two carbons between two
consecutive products). In this reaction C4 (38.8%) and
1
2
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We describe herein a facile approach to synthesize silica grafted niobium tetramethyl complex from its chloride analogue.
Those two complexes shows different selectivity in the ethylene oligomerization reaction, where the fully alkylated species is
highly selective toward the dimerization of ethylene.