Synthesis and characterization of tantalum silsesquioxane complexes

Article abstract of DOI:10.1039/c2dt32601a

Tantalum polyhedral oligosilsesquioxane (POSS) complexes have been synthesised and characterized. X-ray structures of these complexes revealed that the coordination number of the tantalum center greatly affects the cube-like silsesquioxane framework. The

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Synthesis and Characterization of Tantalum Silsesquioxane Complexes
Pascal Guillo,a,b Meg E. Fasulo,a Michael I. Lipschutza and T. Don Tilley*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
three doublets at 1.19, 1.12 and 1.09 ppm for the iꢀbutyl ligands
50 (3:3:1 H).
5
Tantalum polyhedral oligosilsesquioxane (POSS) complexes
have been synthesised and characterized. X-ray structures of
these complexes revealed that the coordination number of the
tantalum center greatly affects the cube-like silsesquioxane
framework.
10
Over the past several years, singleꢀsite, supported tantalum
species have been observed to catalyze a number of interesting
and noteworthy transformations. For example, Basset and coꢀ
workers have developed silicaꢀsupported tantalum hydride
catalysts for alkane metathesis,1 and chiral tantalum catalysts for
15 enantioselective epoxidations.2 This laboratory has found that
supported tantalum species are highly efficient alkene
epoxidation catalysts that can employ hydrogen peroxide as the
oxidant.3 With growing interest in singleꢀsite tantalum catalysts
for various transformations, it is important to develop molecular,
20 structural models for such species and probe their spectroscopic
and structural properties. In this way it may be possible to
establish structureꢀactivity relationships, which are crucial for the
design and development of more efficient catalysts. Along these
lines, molecular models for silicaꢀbound metal centers have been
25 based on calixarene4 and siloxide5 ligands, but most interest has
Scheme 1 Synthesis of 2
Treatment of 2 with 1.2 equiv of methyl lithium in diethyl
ether at ꢀ30 °C resulted in formation of 3 in 94% (Scheme 2). In
55 addition to the expected resonances for the Cp* and iꢀbutyl
groups, the 1H NMR spectrum of 3 displays a singlet at 0.71 ppm
for the methyl ligand. The chloride ligand of 2 can also be
displaced by the "lessꢀcoordinating" trifluoromethanelsulfonate
(OTfꢀ) and BArF ([B(C6F5)4]ꢀ) anions, to give
4 and 5,
60 respectively. Complex 4 was obtained as a white powder in
quantitative yield by the reaction of 2 with 1 equiv of silver
trifluoromethanesulfonate (AgOTf) in toluene at room
temperature. The 19F NMR spectrum of 4 displays a singlet at
75.6 ppm, confirming substitution of the chloride ligand by
65 triflate. In order to access a tantalasilsesquioxane cation, 2 was
centered
on
incompletely
condensed
polyhedral
oligosilsesquioxane (POSS) ligands, which have been studied as
close analogues of the surface silanol groups present on silica.6,7
While many metalꢀPOSS derivatives have been prepared and
30 studied (e.g., with Ti, Zr, Hf, V, Cr, Mo, Re, Fe, Rh, Pt, Au, Zn,
Al, Ga, and Sn) as structural molecular models and as related
homogeneous catalysts for some of them,8 there are only two
reports in the literature of Ta silsesquioxane complexes which
differ substantially in their structures.9,10 While the complex
35 described by Edelmann and coꢀworkers appears to possess a
cubeꢀlike, cage structure with tantalum as part of the cage, the
system described by Basset and coꢀworkers possesses a Ta that is
coordinated to a completely condensed POSS. However, no Xꢀ
ray structural characterization of tantalasilsesquioxane complexes
40 has so far been reported. In this contribution we describe the
synthesis and characterization of four new tantalasilsesquioxane
.
treated with 1.15 equiv of LiB(C6F5)4 2Et2O in dichloromethane
to afford 5 as a white powder in 85% yield after crystallization
from acetonitrile at ꢀ30 °C. After removal of the residual solvent
under vacuum, the H NMR spectrum of 5 in C6D6 displays only
1
70 resonances for the iꢀbutyl and Cp* groups. No solvent appears to
be coordinated to the Ta (Fig. S1). Moreover, the 19F NMR
spectrum displays 3 singlets between ꢀ130 ppm and ꢀ165 ppm,
confirming the presence of the BArF anion. As has been observed
for Ti silsesquioxane complexes with the same silsesquioxane
75 backbone, the 29Si{1H} NMR spectrum of the complexes 3, 4 and
5 display three signals between ꢀ60 and ꢀ70 ppm.12 However,
only two resonances at ꢀ63.3 ppm and ꢀ66.9 ppm are observed in
the 29Si{1H} NMR spectrum of the complex 2, likely due to the
overlap of two signals.
complexes, along with their Xꢀray crystal structures.
11
The
reaction
of
Cp*TaCl4
(Cp*
=
pentamethylcyclopentadienyl) with 1 equiv of the silsesquioxane
45 1 in toluene at 110 °C for 12 h produced complex 2, isolated as a
white powder in 79% yield after dropwise addition of acetonitrile
into a solution of 2 in toluene (Scheme 1).† The 1H NMR
spectrum displays a singlet at 2.10 ppm for the Cp* moiety and
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30 Fig. 1 Molecular structure of 2 displaying thermal ellipsoids at the 50%
probability level. Hꢀatoms and iꢀbutyl groups have been omitted for
clarity.
Scheme 2 Synthesis of 3ꢀ5. i) MeLi (1.6 M in diethyl ether), diethyl ether,
.
ꢀ30 °C to RT, 3 h; ii) AgOTf, toluene, RT, 3 h; iii) LiB(C6F5)4 2Et2O,
dichloromethane, ꢀ30 °C to RT, 3 h.
5
Single crystal Xꢀray diffraction studies of the new
tantalasilsesquioxane complexes 2ꢀ5 confirmed the structures
proposed in Schemes 1 and 2. Crystals of 2 and 3 suitable for Xꢀ
ray diffraction were obtained from saturated solutions of the
compounds in hexanes and pentane, respectively, at ꢀ30 °C. Slow
Fig. 2 Molecular structure of 3 displaying thermal ellipsoids at the 50%
35 probability level. Hꢀatoms and iꢀbutyl groups have been omitted for
clarity.
10 evaporation of a solution of 4 in benzene at 20 °C afforded
crystals of 4, and crystals of 5 were obtained by slow diffusion of
pentane into a solution of 5 in toluene at ꢀ30 °C. The structures of
these complexes are represented in Fig. 1ꢀ4 and selected bonds
and angles are summarized in Table 1.
15
The crystal structures of 4 and 5 confirmed that the triflate
anion coordinates to the Ta center, while the BArF anion is not
coordinated. In general, a fourꢀlegged piano stool geometry for
the tantalum center (compounds 2ꢀ4) leads to a distorted cubeꢀ
like structure, whereas the threeꢀlegged piano stool structure of 5
20 gives
a regular, cubeꢀlike silsesquioxane framework. For
complexes 2ꢀ5, the TaꢀO(Si) distances are between 1.843(4) and
1.9694(18) Å and are similar to the TaꢀO(Si) bond distances
(1.806ꢀ2.041 Å) previously reported for tantalum complexes with
ꢀOSi(OtBu)3,3a ꢀOSitBu3,13 ꢀOSiPh314 or ꢀOSitBuPh215 ligands.
Fig. 3 Molecular structure of 4 displaying thermal ellipsoids at the 50%
probability level. Hꢀatoms, iꢀbutyl groups and a C6H6 molecule have been
40 omitted for clarity.
25
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Table 1 Selected bond lengths [Å] and angles [°] for 2ꢀ5
2
Cl(1)ꢀTa(1)
O(1)ꢀTa(1)
O(2)ꢀTa(1)
O(3)ꢀTa(1)
2.4624(9)
1.947(3)
1.939(2)
1.929(3)
O(9)ꢀSi(6)ꢀO(10)
O(9)ꢀSi(6)ꢀO(11)
109.37(14)
108.14(14)
O(10)ꢀSi(6)ꢀO(11)
O(3)ꢀTa(1)ꢀO(1)
O(3)ꢀTa(1)ꢀO(2)
O(2)ꢀTa(1)ꢀO(1)
O(3)ꢀTa(1)ꢀCl(1)
O(2)ꢀTa(1)ꢀCl(1)
O(1)ꢀTa(1)ꢀCl(1)
Si(1)ꢀO(1)ꢀTa(1)
Si(3)ꢀO(2)ꢀTa(1)
Si(2)ꢀO(3)ꢀTa(1)
108.86(14)
140.02(11)
86.66(10)
85.06(10)
83.67(8)
148.70(8)
83.49(8)
117.13(14)
169.99(17)
146.91(16)
3
C(1)ꢀTa(1)
O(1)ꢀTa(1)
O(2)ꢀTa(1)
O(3)ꢀTa(1)
C(2)ꢀTa(1)
C(3)ꢀTa(1)
C(4)ꢀTa(1)
C(5)ꢀTa(1)
C(6)ꢀTa(1)
2.275(2)
1.9318(18)
1.9694(18)
1.9433(18)
2.453(3)
2.490(3)
2.455(3)
2.425(3)
2.406(3)
O(12)ꢀSi(6)ꢀO(7)
O(12)ꢀSi(6)ꢀO(8)
O(7)ꢀSi(6)ꢀO(8)
O(1)ꢀTa(1)ꢀO(3)
O(1)ꢀTa(1)ꢀO(2)
O(3)ꢀTa(1)ꢀO(2)
O(1)ꢀTa(1)ꢀC(1)
O(3)ꢀTa(1)ꢀC(1)
O(2)ꢀTa(1)ꢀC(1)
Si(1)ꢀO(1)ꢀTa(1)
Si(3)ꢀO(2)ꢀTa(1)
Si(4)ꢀO(3)ꢀTa(1)
111.42(11)
107.66(10)
107.99(10)
131.56(8)
85.50(8)
84.06(8)
81.44(8)
80.81(9)
145.08(8)
143.54(11)
177.12(13)
129.66(11)
Fig. 4 Molecular structure of cation 5 displaying thermal ellipsoids at the
50% probability level. Hꢀatoms, iꢀbutyl groups and [B(C6F5)4]ꢀ have been
omitted for clarity.
5
The coordination number of the tantalum center greatly affects
the structure of the cubeꢀlike silsesquioxane framework, as can be
seen in examination of the TaꢀOꢀSi and OꢀTaꢀO angles. For 5, the
three TaꢀOꢀSi angles are similar (145.0(3)ꢀ153.3(3)°) whereas
three distinct angles are observed in the structures of 2ꢀ4. For 2ꢀ4,
10 the angle for the TaꢀOꢀSi linkage that is trans to the anionic
donor ligand (Cl, Me or OTf) outside the cage is greatest
(165.4(5)ꢀ177.1(1)°). In each complex, one of the remaining Taꢀ
OꢀSi angles is intermediate in value (143.5(1)ꢀ147.0(5)°), while
the other is more acute (117.1(1)ꢀ133.1(5)°). Literature values for
15 TaꢀOꢀSi angles in tantalum siloxide complexes vary between 162
and 175°,3a,13,14,15 whereas in complexes 2ꢀ5, the TaꢀOꢀSi angles
are distributed over a wider range (117ꢀ177°). This phenomenon
is probably due to a combination of electronic and steric effects
enforced by the TaꢀCp*X fragment in the cubeꢀlike framework.
20 For 5, the three OꢀTaꢀO angles have similar values (102.1(2)ꢀ
105.4(2)°). Similar OꢀMꢀO (M = Zr, Hf) angles have previously
been reported for isoelectronic ZrꢀPOSS16 (103.4(6)°) and Hfꢀ
POSS17 (102.7(2)°) compounds with the same threeꢀlegged piano
stool geometry. To the best of our knowledge, no other cationic
25 tantalum complex with a threeꢀlegged piano stool geometry has
been reported in the literature.
The OꢀTaꢀX (X = O, Cl, Me, OTf) angles for 2ꢀ4 exhibit two
distinctly different values. In general, the transꢀlike OꢀTaꢀX angle
involving the donor atom outside the cage (X) is larger (145ꢀ
30 153°) than the other trans OꢀTaꢀO angle (125ꢀ140°). This
structural feature is commonly observed for the fourꢀlegged piano
stool coordination geometry.18 The remaining bond angles
associated with cis arrangements of the silsesquioxane and X
donor atoms are rather acute (ca. 80ꢀ90° for the OꢀTaꢀO angles)
35 as expected, with the cis OꢀTaꢀX angles being smaller.
4
O(1)ꢀTa(1)
O(4)ꢀTa(1)
O(5)ꢀTa(1)
O(6)ꢀTa(1)
2.223(8)
1.894(8)
1.912(7)
1.895(8)
O(9)ꢀSi(6)ꢀO(13)
O(9)ꢀSi(6)ꢀO(12)
O(13)ꢀSi(6)ꢀO(12)
O(4)ꢀTa(1)ꢀO(6)
O(4)ꢀTa(1)ꢀO(5)
O(6)ꢀTa(1)ꢀO(5)
O(4)ꢀTa(1)ꢀO(1)
O(6)ꢀTa(1)ꢀO(1)
O(5)ꢀTa(1)ꢀO(1)
Si(3)ꢀO(4)ꢀTa(1)
Si(2)ꢀO(5)ꢀTa(1)
Si(1)ꢀO(6)ꢀTa(1)
109.2(4)
109.5(4)
108.2(4)
125.2(3)
88.9(3)
89.0(3)
77.3(3)
80.4(3)
152.7(3)
133.1(5)
165.4(5)
147.0(5)
5
O(1)ꢀTa(1)
O(12)ꢀTa(1)
O(4)ꢀTa(1)
1.855(5)
1.843(4)
1.867(4)
O(3)ꢀSi(3)ꢀO(6)
O(3)ꢀSi(3)ꢀO(10)
O(6)ꢀSi(3)ꢀO(10)
O(4)ꢀTa(1)ꢀO(12)
O(4)ꢀTa(1)ꢀO(1)
O(12)ꢀTa(1)ꢀO(1)
Si(1)ꢀO(1)ꢀTa(1)
Si(5)ꢀO(4)ꢀTa(1)
Si(7)ꢀO(12)ꢀTa(1)
109.2(3)
108.8(2)
108.0(3)
102.14(18)
105.4(2)
104.34(18)
152.1(3)
145.0(3)
153.3(3)
To evaluate the influence of a neutral donor ligand in the
40 coordination sphere of the tantalum, a saturated acetonitrile
solution of 5 was cooled to ꢀ30 °C to provide colorless crystals of
the acetonitrile adduct 5(CH3CN) (Fig. 5). Selected bond lengths
and angles are summarized in Table 2.
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(Table 1)). Structural and spectroscopic characteristics of the
complex model 3 are in good agreement with the data for the
grafted Ta analog and thus confirm the validity of such molecular
models in structural studies of analogous, grafted systems.
40
In summary, four new tantalasilsesquioxane complexes have
been synthesized and characterized. To the best of our
knowledge, the crystal structures of these compounds are the first
for polyhedral oligosilsesquioxane complexes with a Ta center.
Complexes 2ꢀ4 display a fourꢀlegged piano stool geometry
45 around the tantalum center whereas 5 possesses a threeꢀlegged
piano stool arrangement. These complexes are relevant structural
molecular models for Cp*Ta fragments grafted onto a silica
surface. Further modifications to the structures of these
tantalasilsesquioxane complexes are currently being investigated.
50
Fig. 5 Molecular structure of cation 5(CH3CN) displaying thermal
ellipsoids at the 50% probability level. Hꢀatoms, iꢀbutyl groups,
[B(C6F5)4]ꢀ and a CH3CN molecule have been omitted for clarity.
Acknowledgments
5
Table 2 Selected bond lengths [Å] and angles [°] for 5(CH3CN)
This work was supported by the Director, Office of Science,
Office of Basic Energy Sciences of the US Department of Energy
under contract No. DEꢀAC02ꢀ05CH11231. The authors are
55 grateful to Dr. Frank J. Feher for helpful discussions.
O(1)ꢀTa(1)
O(2)ꢀTa(1)
O(3)ꢀTa(1)
Ta(1)ꢀN(1)
1.897(8)
1.917(7)
1.879(7)
2.255(9)
O(13)ꢀSi(6)ꢀO(5)
O(13)ꢀSi(6)ꢀO(12)
O(5)ꢀSi(6)ꢀO(12)
O(3)ꢀTa(1)ꢀO(2)
O(3)ꢀTa(1)ꢀO(1)
O(1)ꢀTa(1)ꢀO(2)
O(3)ꢀTa(1)ꢀN(1)
O(1)ꢀTa(1)ꢀN(1)
O(2)ꢀTa(1)ꢀN(1)
Si(1)ꢀO(1)ꢀTa(1)
Si(4)ꢀO(2)ꢀTa(1)
Si(8)ꢀO(3)ꢀTa(1)
110.0(4)
108.6(4)
107.9(4)
123.5(3)
90.4(3)
90.4(3)
76.2(3)
Notes and references
a Department of Chemistry, University of California, Berkeley, CA 94720-
1461, USA; E-mail: tdtilley@berkeley.edu
151.5(3)
76.9(3)
161.9(4)
141.0(4)
146.1(4)
b Chemical Sciences Division,Lawrence Berkeley National Laboratory, 1
60 Cyclotron Road, Berkeley, CA 94720, USA.
†Electronic Supplementary Information (ESI) available: General
1
experimental details, synthetic procedures for compounds 2ꢀ5, H NMR
of 5 (Table S1) and crystallographic details (Table S1ꢀS5). CCDC
897182ꢀ897186. For ESI and crystallographic data in CIF or other
65 electronic format see DOI: 10.1039/b000000x/
Notably, as for 2ꢀ4, the cis OꢀTaꢀX (X = N(CH3CN)) angles
are acute (76.3(3) and 76.9(3)°) whereas the two cis OꢀTaꢀO
angles are close to 90°. For the trans OꢀTaꢀX (X = O, N) angles,
10 as expected, a behavior similar to that for 2ꢀ4 was observed with
a OꢀTaꢀN angle larger than the OꢀTaꢀO angle (151.5(3) vs.
123.5(3)°). Removal of the solvent under vacuum results in loss
1
a) V. Vidal, A. Théolier, J. ThivolleꢀCazat and J. M. Basset, Science
1997, 276, 99; b) S. Soignier, M. Taoufik, E. Le Roux, G. Saggio, C.
Dablemont, A. Baudouin, F. Lefebvre, A. de Mallmann, J. Thivolleꢀ
Cazat, J. M. Basset, G. Sunley and B. M. Maunders, Organometallics
2006, 25, 1569; c) O. Maury, L. Lefort, V. Vidal, J. ThivolleꢀCazat
and J. M. Basset, Angew. Chem. Int. Ed. 1999, 38, 1952; d) E. Le
Roux, M. Chabanas, A. Baudouin, A. de Mallmann, C. Copéret, E.
A. Quadrelli, J. ThivolleꢀCazat, J. M. Basset, W. Lukens, A. Lesage,
L. Emsley and G. J. Sunley, J. Am. Chem. Soc. 2004, 126, 13391; e)
J. M. Basset, C. Copéret, D. Soulivong, M. Taoufik and J. Thivolleꢀ
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a) D. Meunier, A. Piechaczyk, A. de Mallmann and J.M. Basset,
Angew. Chem. Int. Ed. 1999, 38, 3540; b) D. Meunier, A. de
Mallmann and J. M. Basset, Top. Catal. 2003, 23, 183.
a) R. L. Brutchey, C. G. Lugmair, L. O. Schebaum and T. D. Tilley,
J. Catal. 2005, 229, 72; b) D. A. Ruddy and T. D. Tilley, Chem.
Commun. 2007, 3350; c) D. A. Ruddy and T. D. Tilley, J. Am. Chem.
Soc. 2008, 130, 11088; d) P. J. Cordeiro and T. D. Tilley, Langmuir
2011, 27, 6295.
a) C. Floriani and R. FlorianiꢀMoro, Adv. Organomet. Chem. 2001,
47, 167; b) C. Floriani, Chem. Eur. J. 1999, 5, 19.
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1
70
75
80
85
90
95
of the coordinated acetonitrile ligand, as indicated by H NMR
spectroscopy (benzeneꢀd6), and elemental analysis. Thus, the
15 acetonitrile appears to be very weakly bound and due to its high
lability, further characterization of 5(CH3CN) was precluded.
To evaluate the validity of complexes 2ꢀ5 as molecular models
for silicaꢀbound tantalum centers with a Cp* ligand, a comparison
was made between 3 and a similar, previously reported surfaceꢀ
20 bound Ta species. Basset and coꢀworkers investigated the
grafting of Cp*TaMe4 on silica partially dehydroxylated at 700
°C, and determined the activity of the grafted complex toward
alkane metathesis.1d While for complex 3, the Ta is coordinated
by one Cp*, one methyl and three siloxide ligands, the grafted
2
3
25 material
possesses
a
Cp*TaMe3(OSiꢀsilica)
structure.
Interestingly, the 13C NMR spectrum of TaCp*Me4 displays a
signal at 74 ppm assigned to the methyl ligands whereas both
complex 3 and the grafted system display more upfield signals at
53.6 and 58 ppm, respectively. Comparison of the bond distances
30 for 3 and the grafted system is also informative. The EXAFS data
for the silicaꢀsupported Ta species indicates a TaꢀO(Si) distance
of 1.931 Å, a TaꢀC(Me) distance of 2.142 Å and 2.456 Å for the
TaꢀC(Cp*) distances. For 3, the bond distances are in the same
range (1.932(2)ꢀ1.969(2) Å for TaꢀO(Si), 2.275(2) Å for the Taꢀ
35 C(Me) and 2.406(3)ꢀ2.490(3) Å for the TaꢀC(Cp*) distances
4
5
6
7
a) F. J. Feher, D. A. Newman and J. F. Walzer, J. Am. Chem. Soc.
1989, 111, 1741; b) F. J. Feher and T. A. Budzichowski, Polyhedron
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Chandrasekhar, R. Boomishankar and S. Nagendran, Chem. Rev.
2004, 104, 5847; c) D. B. Cordes, P. D. Lickiss and F. Rataboul,
Chem. Rev. 2010, 110, 2081.
4
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8
9
See E. A. Quadrelli and J. M. Basset, Coord. Chem. Rev. 2010, 254,
707 for a recent review.
M. Chabanas, E. A. Quadrelli, B. Fenet, C. Copéret, J. Thivolleꢀ
Cazat, J. M. Basset, A. Lesage and L. Emsley, Angew. Chem. Int. Ed.
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Table of contents for
Synthesis and Characterization of Tantalum Silsesquioxane
Complexes
Pascal Guillo,a,b Meg E. Fasulo,a Michael I. Lipschutza and T. Don Tilley*a,b
X-ray structures of four new Ta-POSS complexes reveal that the coordination number of the
tantalum greatly affects the cube-like silsesquioxane framework.

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