New linker systems for superior immobilized catalysts

Article abstract of DOI:10.1039/c0nj00482k

The rigid linkers E(p-C₆H₄PPh₂) ₄ (E = Si, Sn) have been synthesized from the corresponding precursors E(p-C₆H₄Br)₄ and immobilized by forming one (E = Si) or three (E = Sn) phosphonium groups that are attached to the silica support by electrostatic interactions. The remaining unquaternized phosphines are coordinated to Wilkinson-type Rh complexes. The catalyst immobilized by the rigid scaffold with three surface-bound phosphonium groups (E = Sn) displays superior catalytic activity and lifetime with respect to the hydrogenation of dodecene. This catalyst can be recycled 30 times in a batchwise manner under standardized conditions. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c0nj00482k

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LETTER
www.rsc.org/njc | New Journal of Chemistry
New linker systems for superior immobilized catalystsw
ab
Bjorn Beele, Johannes Guenther,a Melanie Perera,a Michaela Stach,a
¨
Thomas Oeserb and Janet Blumel*a
¨
Received (in Gainesville, FL, USA) 23rd June 2010, Accepted 27th July 2010
DOI: 10.1039/c0nj00482k
The rigid linkers E(p-C6H4PPh2)4 (E = Si, Sn) have been
synthesized from the corresponding precursors E(p-C6H4Br)4
and immobilized by forming one (E = Si) or three (E = Sn)
phosphonium groups that are attached to the silica support
by electrostatic interactions. The remaining unquaternized
phosphines are coordinated to Wilkinson-type Rh complexes.
The catalyst immobilized by the rigid scaffold with three surface-
bound phosphonium groups (E = Sn) displays superior catalytic
activity and lifetime with respect to the hydrogenation of
dodecene. This catalyst can be recycled 30 times in a batchwise
manner under standardized conditions.
Immobilized species are important in such diverse areas as
combinatorial chemistry,1 solid-phase synthesis,2 chromato-
graphy,3 and catalysis.4,5 Immobilized catalysts4,5 are of
immense academic and industrial interest because they can
combine the advantages of homogeneous with those of hetero-
geneous catalysts. Immobilized catalysts are highly active and
selective, while they can easily be separated from the reaction
mixture and reused for example in a batchwise manner.4,5 The
most favorable support materials are silica and neutral
alumina,3,6 and bifunctional phosphines, such as the chelate
phosphine Ph2P(CH2)3PPh(CH2)3Si(OEt)3, are popular
linkers.6,7 The latter have led to very successful immobilized
nickel8 and rhodium9 catalysts for the cyclotrimerization of
acetylenes and olefin hydrogenation, respectively.
However, as demonstrated in previous work, when flexible
Scheme 1 Syntheses of 3 and 4, the immobilized linkers 3i and 4i, and
the catalysts 5i and 6i.13
phosphine linkers with high surface coverages are used catalyst
deactivation by dimerization can occur.9 The catalysts can be
‘‘diluted’’ on the surface to prevent dimerization,9 but increasing
the amount of bulk material in this way is not desirable from
an industrial point of view. Additionally, deactivation of the
catalysts or linker destruction by reaction with the oxide
surface10 remains possible.
The silane Si(p-C6H4Br)4 (1) (Scheme 1) has been synthesized
by the method described by Wuest et al.,11 slightly modified as
outlined for the stannane Sn(p-C6H4Br)4 (2).12
Although Si(p-C6H4Br)4 (1) is used extensively as the
core building block for constructing networks for various
applications,11 its 29Si NMR data and crystal structure have
not yet been reported. The value d(29Si) = À13.76 ppm for 1
in CDCl3 solution corresponds very well to the value of SiPh4
with À13.98 ppm.14 In the solid-state NMR15 spectrum of
polycrystalline 1 (Fig. 1) without rotation, a wideline signal
with the moderate CSA span (d11 À d33)15a of 14 ppm and the
CSA values d11 = À8, d22 = À15, and d33 = À22 ppm are
obtained. Two isotropic lines at d = À14.3 and À15.0 ppm are
visible at 4 kHz rotational speed. This is corroborated by the
single crystal X-ray analysis,16 where two crystallographically
independent molecules generate the asymmetric unit (Fig. 1).
Due to the tetragonal symmetry one fragment lies upon a
fourfold rotation axis, the other one is placed upon a twofold
rotation axis.
Here, we present a new rigid linker scaffold that allows the
clean immobilization of homogeneous catalysts on oxide
supports. This linker scaffold prevents catalyst dimerization
or contact with the support surface and leads to unprece-
dented lifetimes and recyclabilities.
a Department of Chemistry, Texas A&M University,
P.O. Box 30012, College Station, Texas 77842-3012, USA.
E-mail: bluemel@tamu.edu
b Institute of Organic Chemistry, University of Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
w Electronic supplementary information (ESI) available: Experimental
data of 3, 4, and 3i to 6i. CCDC 742605 and 742606. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0nj00482k
c
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Fig. 2 119Sn wideline (bottom spectrum) and MAS (top two) NMR
spectra of polycrystalline 2 (asterisks denote rotational sidebands),
and unit cell of a single crystal of 2 (view along the crystallographic
a-axis).17
Fig. 1 29Si wideline (top) and MAS (bottom) NMR spectra of
polycrystalline 1, and unit cell of a single crystal of 1 (view along
the crystallographic a-axis).16
As compared to 1, polycrystalline Sn(p-C6H4Br)4 (2)12
shows a rather large CSA span15a of (d11 À d33) = 40 ppm
in its 119Sn wideline spectrum (Fig. 2), as expected for a
heavier nucleus.15a The CSA values d11 = À92, d22 = À129,
and d33 = À133 ppm are obtained. Only one isotropic line is
visible in the 119Sn MAS spectra of 2 (Fig. 2). This is in
correspondence with the single crystal X-ray analysis17 (Fig. 2),
which shows that all Sn nuclei are magnetically equivalent.
For both, the tetraphenylsilane and -stannane systems, the
intramolecular distance between the bound catalytic centers
should be large enough to prevent their dimerization, as the
average intramolecular BrÁ Á ÁBr distances of 10.53 A for 1 and
11.07 A for 2 show.
Fig. 3 31P MAS NMR spectrum of 4i. nrot = 4 kHz.
The Rh catalysts 5i and 6i (Scheme 1) have been obtained
via ligand exchange by stirring 3i and 4i with a slight excess of
Wilkinson’s catalyst, and removing the surplus ClRh(PPh3)3
and PPh3 by washing with toluene. Both catalysts 5i and 6i
have been tested with respect to the hydrogenation of
1-dodecene, a catalytic reaction that allows the comparison
with ample previous data9 under the applied standard conditions
described earlier.9c
The phosphines 3 and 4 have been synthesized in yields of 55
and 79% by Br/Li exchange, followed by reaction with ClPPh2
(Scheme 1).13 An alternative synthesis route via Br/Li
exchange of p-BrC6H4PPh2 and reaction with SiCl4 increases
the yield of 3 to 70%.13
The silane 3 has been immobilized selectively via one
phosphonium group on silica18 to give 3i (Scheme 1) according
to the procedure explored in general,19 and communicated for
rigid tetraphosphines previously.12 In the meantime, we found
reaction conditions that also allow us to bind the rigid tetra-
phosphines to silica via three phosphonium groups.
Since the support material settles down quickly after each
catalytic run, 5i and 6i could be recycled by decanting the
supernatant and washing the solid thoroughly with toluene.
Catalyst 5i could be recycled 13 times before the consumption
of H2 was no longer 100% within 100 h. This activity
and lifetime scenario is comparable to cases where complexes
are immobilized via flexible chelate linkers such as
Ph2P(CH2)3PPh(CH2)3Si(OEt)3.9 Therefore, if the scaffold is
only bound via one phosphonium group, the active catalytic
centers might still get deactivated by contact with the reactive
silica surface.
4i (Scheme 1) can be generated by adding a 100-fold excess
of Cl(CH2)3Si(OEt)3 to 4 and SiO2 and stirring at 90 1C for
5 days. The quantitative 31P MAS spectrum12,15d (Fig. 3),
recorded with a long relaxation delay of 10 s, proves the
number of binding sites per linker molecule by the intensity
ratio of ca. 3 : 1 for the phosphonium and the phosphine
signals at 23.8 and À5.2 ppm.
c
2730 New J. Chem., 2010, 34, 2729–2731 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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J. Blumel, Chem. Commun., 2005, 1481.
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Catal., 2003, 345, 589; (b) K. D. Behringer and J. Blumel, Chem.
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Commun., 1996, 653; (c) S. Reinhard, K. D. Behringer and
J. Blumel, New J. Chem., 2003, 27, 776.
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9 (a) C. Merckle and J. Blumel, Adv. Synth. Catal., 2003, 345, 584;
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(c) C. Merckle, S. Haubrich and J. Blumel, J. Organomet. Chem.,
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2001, 627, 44.
10 T. Posset, F. Rominger and J. Blumel, Chem. Mater., 2005, 17, 586.
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11 J.-H. Fournier, X. Wang and J. D. Wuest, Can. J. Chem., 2003, 81,
Fig.
4 Batchwise recycling of 6i after the hydrogenation of
1-dodecene.9 Ratio dodecene/Rh: 100/1; concentration of dodecene
376.
12 Y. Yang, B. Beele and J. Blumel, J. Am. Chem. Soc., 2008, 130,
3771.
in toluene: 0.1 mol lÀ1; pressure 1.1 bar; reaction temperature 25 1C.
¨
13 Analytical data see ESIw.
In the case of catalyst 6i, which is bound by a ligand
platform with three phosphonium groups, no flexibility should
be left. Therefore, no catalyst deactivation by dimerization or
contact with the support surface should occur. Accordingly, 6i
showed unprecedented activity and lifetime, and could be
recycled for the record number of 30 times under the standard
conditions9c (Fig. 4). The 31P CP/MAS spectra before and
after catalysis show the same signals (see ESIw), in analogy
to previously studied successful catalysts with different linker
systems.9b
14 R. H. Cragg and R. D. Lane, J. Organomet. Chem., 1984, 277, 199.
15 (a) T. M. Duncan, A Compilation of Chemical Shift Anisotropies,
Farragut Press, Chicago, IL, 1990; (b) C. A. Fyfe, Solid-State
NMR for Chemists, C.F.C. Press, Guelph, Canada, 1983;
(c) M. J. Duer, Introduction to Solid-State NMR Spectroscopy,
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¨
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16 X-Ray data and parameters of 1: colourless crystal (irregular),
dimensions 0.20 Â 0.14 Â 0.10 mm3, crystal system tetragonal,
ꢀ
space group I4, Z = 6, a = b = 13.6296(1) A, c = 18.9768(2) A,
a = b = g = 901, V = 3525.24(5) A3, r = 1.843 g cmÀ3, T = 200(2)
K, ymax = 24.111, radiation MoKa, l = 0.71073 A, 0.31o-scans
with CCD area detector, covering a whole sphere in reciprocal
space, 13 920 reflections measured, 2791 unique (Rint = 0.0490),
2482 observed (I >2s(I)), intensities were corrected for Lorentz
and polarization effects, an empirical absorption correction was
applied using SADABS based on the Laue symmetry of the
reciprocal space, m = 6.91 mmÀ1, Tmin = 0.34, Tmax = 0.55,
structure solved by direct methods and refined against F2 with a
Full-matrix least-squares algorithm using the SHELXTL (6.12)
software package, 197 parameters refined, hydrogen atoms were
treated using appropriate riding models, Flack absolute structure
parameter 0.037(16), goodness of fit 1.02 for observed reflections,
final residual values R1(F) = 0.036, wR(F2) = 0.090 for observed
reflections, residual electron density À0.66 to 0.49 eAÀ3. CCDC
742605 contains the supplementary crystallographic data for this
structurew.
In summary, it has been demonstrated that catalysts with
rigid linker scaffolds are easy to synthesize and analyze. These
linkers prevent the dimerization of the catalytically active
metal centers and their deactivation by contact with the
reactive support surface. Especially catalysts surface-bound
by tetrahedral systems with three phosphonium binding sites
per linker show unprecedented activities and lifetimes.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under CHE-0911207. Furthermore we
thank The Welch Foundation (A-1706), Texas A&M University,
and DFG (SFB 623) for support.
17 X-Ray data and parameters of 2: colourless crystal (irregular),
dimensions 0.24 Â 0.08 Â 0.04 mm3, crystal system triclinic, space
ꢀ
group P1, Z = 2, a = 9.7752(6) A, b = 11.2894(6) A, c =
11.8628(7) A, a = 70.695(1)1, b = 74.850(1)1, g = 79.261(2)1, V =
1185.4(1) A3, r = 2.081 g cmÀ3, T = 200(2) K, ymax = 22.461,
radiation MoKa, l = 0.71073 A, 0.31 o-scans with CCD area
detector, 8197 reflections measured, 3093 unique (Rint = 0.0493),
2361 observed (I >2s(I)), intensities were corrected for Lorentz
and polarization effects, an empirical absorption correction was
applied using SADABS based on the Laue symmetry of the
reciprocal space, m = 7.83 mmÀ1, Tmin = 0.26, Tmax = 0.74,
structure solved by direct methods and refined against F2 with a
Full-matrix least-squares algorithm using the SHELXTL (6.12)
software package, 262 parameters refined, hydrogen atoms were
treated using appropriate riding models, goodness of fit 1.01 for
observed reflections, final residual values R1(F) = 0.043, wR(F2) =
0.101 for observed reflections, residual electron density À0.77 to
1.46 eAÀ3. CCDC 742606 contains the supplementary crystallo-
graphic data for this structurew.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010 New J. Chem., 2010, 34, 2729–2731 2731