Tetrahedral Sn-silsesquioxane: Synthesis, characterization and catalysis

Article abstract of DOI:10.1039/c4cc07897g

A tetrahedral stannasilsesquioxane complex was synthesized as a racemic mixture using Sn(OⁱPr)₄ and silsesquioxanediol, and its structure was confirmed with X-ray crystallography, NMR, and EXAFS. The complex was a Lewis acid, and both anti and syn-binding with Lewis bases were possible with the formation of octahedral Sn complexes. It was also a Lewis acid catalyst active for epoxide ring opening and hydride transfer.

Full text of DOI:10.1039/c4cc07897g

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Tetrahedral Sn–silsesquioxane: synthesis,
characterization and catalysis†
Evgeny V. Beletskiy,a Zhongliang Shen,a Mark V. Riofski,a Xianliang Hou,ab
James R. Gallagher,c Jeffrey T. Miller,c Yuyang Wu,d Harold H. Kung*a and
Mayfair C. Kung*a
Cite this: Chem. Commun., 2014,
50, 15699
Received 7th October 2014,
Accepted 27th October 2014
DOI: 10.1039/c4cc07897g
www.rsc.org/chemcomm
A tetrahedral stannasilsesquioxane complex was synthesized as a to fructose isomerization,12,13 and transformation of 5-(hydroxy-
racemic mixture using Sn(OiPr)4 and silsesquioxanediol, and its methyl)furfural for biofuel processing.14 This could be due to the
structure was confirmed with X-ray crystallography, NMR, and difficulties in synthesizing tetrahedral (Td) stannasilsesquioxane.
EXAFS. The complex was a Lewis acid, and both anti and syn- Synthesis starting with SnCl4 were reported to be unsuccessful,15
binding with Lewis bases were possible with the formation of even though well-defined Td metallasilsesquioxanes could be
octahedral Sn complexes. It was also a Lewis acid catalyst active formed from TiCl4 and ZrCl4 under similar conditions.16 To
for epoxide ring opening and hydride transfer.
our knowledge, one Td corner-capped CH3-stannasilsesquioxane
was synthesized successfully using CH3SnCl3,17 although an
Polyhedral oligomeric silsesquioxane (POSS) are cage-structured octahedral, dinuclear byproducts could be easily formed
siloxanes with aliphatic or aromatic groups attached to Si located (Fig. S1a–e, ESI†). However, the difficulty in displacing the stable
at the vertices of the cage.1 An incompletely condensed POSS ancillary methyl ligand makes this Td CH3-stannasilsesquioxane
possesses reactive silanols that can ligate metal cations effectively,2 unsatisfactory as a model for Sn-beta. Since single site Td metal–
such as Ti,3,4 V,5 Al,6 and Ce7 to form a wide variety of metalla- oxo Lewis acid is an important class of catalyst,18 the synthesis and
silsesquioxanes which have been used as analogues of silica- characterization of Td stannasilsesquioxane with only siloxane
supported or zeolitic heterogeneous catalysts.8 To be a realistic ligands should be interesting. Here we report a successful
structural analogue of an active site in zeolite, the metal center of synthesis scheme and the results of adsorptive and catalytic
the metallasilsesquioxane should be tetrahedrally coordinated properties of the Td Sn–oxo complex.
with three of the four ligands being siloxy bonds, and the fourth
either a siloxy or an ancillary ligand that can be easily displaced incompletely condensed silsesquioxane 1, (c-C6H11)7Si7O9(OH)3,
during reaction.8
was converted to a disilanol with dimethylvinylchlorosilane via
The Sn precursor in our scheme was Sn(OiPr)4 (Scheme 1). An
Although Ti and Sn complexes share many similar properties, heterofunctional condensation following the method of Feher
and titanasilsesquioxanes have been extensively investigated, et al.19 and Sakugawa et al.20 Triethylamine was added as a catalyst
there is yet no similar investigation of Sn–silsesquioxanes, and to remove the HCl byproduct as the salt Et3NÁHCl.19 2 was
even though Sn-beta catalyzes many important reactions formed in good yield (480%), and its structure and purity were
such as Baeyer–Villiger reaction,9 Meerwein–Ponndorf–Verley– confirmed with NMR (Fig. S2a–c, ESI†), ESI-MS, and crystal
Oppenauer redox equilibration,10 Diels–Alder reaction,11 glucose structure (Fig. S2d, ESI†). Addition of 0.5 eq. of Sn(OiPr)4 to 2 at
100 1C afforded a white powder, complex 3. This synthesis scheme
could be used with other silsesquioxanes with the general formula
of R7Si7O9(OH)3. When R is i-butyl, the product 3b, an i-butyl
a Chemical and Biological Engineering Department, Northwestern University,
Evanston, IL. 60208, USA. E-mail: hkung@northwestern.edu,
m-kung@northwestern.edu
b Northwestern Polytechnical University, Xi’an 710072, P. R. China
c Argonne National Laboratory, Chemical Sciences and Engineering Division,
Argonne, IL. 60429, USA
d Chemistry Department, Northwestern University, Evanston, IL. 60208, USA
† Electronic supplementary information (ESI) available: Experimental details of
synthesis and characterization, 1H, 13C, 29Si, 119Sn NMR spectra of 3, complexes of
3 with bases, and (POSS-iBu7–Sn–Me)2ÁEt2NHÁH2O, ESI-MS and EXAFS and XANES
data of 3, and X-ray structures of 2 and (POSS–iBu7–Sn–Me)2ÁEt2NHÁH2O. See DOI:
Scheme 1 Synthesis of complex 3, [(c-C6H11)7Si7O11(OSiC2H3Me2)]2Sn.
Chem. Commun., 2014, 50, 15699--15701 | 15699
10.1039/c4cc07897g
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homolog of 3 was a viscous yellow oil. In this synthesis scheme, the no 1H or 13C NMR peaks of free amine in solution were detected up
spatial orientation of the disilanol in the silsesquioxane was found to 2 eq. Et2NH, which indicated strong, stoichiometric binding of
to be important. If instead of 2, another disilanol silsesquioxane two Et2NH to each 3 (Fig. S6a–c, ESI†). After addition of 1 eq. Et2NH,
R8Si8O11(OH)2 in which the OH groups pointed at different corners 1H and 13C NMR indicated the presence of unperturbed 3 as well
(Fig. S3b, ESI†), its reaction with Sn(OiPr)4 generated multiple as appearance of a new species whose peaks were slightly shifted
octahedral products (Fig. S3a, ESI†).
from 3. 119Sn NMR indicated co-existence of 3 and a new, octahedral
Comparison of the 1H and 13C NMR spectra of 3 (Fig. S4a and species that resonated at À706 ppm (Fig. S6d, ESI†). The ratio of Oh
b, ESI†) with those of POSS (Fig. S2a and b, ESI†) showed much to Td species was 0.88. No other signals were generated, suggesting
similarity, suggesting retention of the silsesquioxane structural no intermediate pentacoordinated species.
unit. Because the Sn center in 3 was chiral (the symmetry plane in
At 2 eq. Et2NH, complete conversion of Td to Oh Sn was achieved.
2 was lost), its 13C NMR spectrum showed seven CH resonances With the absence of unbound amine or unreacted 3, the 1H and 13
C
for the cyclohexyl groups in the 23–25 ppm region (Fig. S6b, ESI†). NMR spectra became those of the new compound. Single crystal X-ray
In addition, because of their diastereotopic nature, the two methyl structure of this compound (Fig. 1) showed the two amines in the anti
groups in –SiC2H3Me2 split into separate peaks in both 1H and 13
C
position and increased Sn–O bond distances to 1.98 and 2.00 Å. The
NMR spectra. All 8 Si in each of the POSS unit became distinct elongated Sn–O bond indicated decreased backbonding to the oxygen
and eight 29Si signals of equal integrated areas (Æ20%) were atoms of the POSS ligand upon binding of Et2NH. The Sn center was
observed (Fig. S4c, ESI†). The 119Sn NMR spectrum showed a no longer a chiral center in this complex, and the distinct methyl
resonance at À438 ppm (Fig. S4d, ESI†), consistent with Td Sn.21 peaks in 13C NMR were replaced by a single peak (Fig. S6b, ESI†).
This Td species was the predominant product, but occasionally
Complete conversion of Td to Oh Sn was also observed after
another 119Sn NMR signal at À330 ppm was detected. This resonance adding 1 eq. ethylenediamine, as indicated by a sharp 119Sn NMR
disappeared upon prolonged heating of the sample at 110 1C in signal at À665 ppm (Fig. S7c, ESI†). Contrary to Et2NH, the
toluene, and was never observed when the vinyl groups in 2 were bidentate nature of ethylenendiamine favoured syn-binding, which
replaced by methyl groups as in (c-C6H11)7Si7O10(SiMe3)(OH)2. A was confirmed by the X-ray crystal structure of the bound complex
parent peak at B2230 m eÀ1 with the expected isotope distribution (Fig. 1). In this structure, the Sn center remained chiral, and the
was detected in ESI-MS (Fig. S4e, ESI†). Single crystals of 3 of a quality asymmetry of the two POSS-ligands was retained. This was indi-
1
satisfactory for X-ray structural determination were obtained by slow cated by the split methyl peaks in the H and 13C NMR (Fig. S7a
addition of acetone to a chloroform solution of 3 in a J Young tube and b, ESI†). Interestingly, it appeared that only one of three
with subsequent crystallization over a 20 h period. The structure possible isomers was formed, and ethylenediamine coordinated
derived from such a single crystal (Fig. 1) showed a nearly perfect to Sn in the ‘‘pocket’’ formed by the –OSiC2H3Me2 substituents.
tetrahedral coordination of Sn to two silsesquioxane units via four This could be due to steric hindrance from the cyclohexyl groups in
siloxy bonds (Fig. 1, +O–Sn–O 109 Æ 31). The Sn–O bond distance other isomers.
was determined as 1.94 Å, which was similar to the 1.91 Å determined
A distinctly different behaviour was observed with triethylamine.
for the tetrahedral Sn in Sn-beta22 and significantly shorter than Addition of triethylamine to 3 caused merging of the two methyl
2.05 Å of hexa-coordinated SnO2. Extended X-ray absorption fine group signals of –SiC2H3Me2 in both 1H and 13C NMR spectra
structure (EXAFS) analysis of 3 also confirmed the Td coordination of (Fig. 2), and broadening and/or disappearance of four of the methine
Sn with a Sn–O bond distance of 1.93 Å (Table S3 and Fig. S5, ESI†), carbon peaks in the 13C spectra (Fig. S8a, ESI†), and four of the Si
and the absorption edge was consistent with Sn(IV) (Table S2, ESI†). peaks in the 29Si (Fig. S8b, ESI†). This fluxional behaviour23,24 was
Ligation to the Sn in 3 was investigated with external ligands of attributed to interaction of Et3N with 3, which led to rapid racemiza-
different basicity and bulkiness. In a mixture of diethylamine and 3, tion of the chiral Sn center and exchange of the positions of the
methyl groups. However, the binding of this amine with Sn was weak
and rapidly reversible as indicated by the broad NMR peaks of the
unbound amine at low equivalents of added amine (Fig. S8c, ESI†).
Fig. 1 X-ray crystal structures of 3 showing tetrahedral coordination of
Sn, 3Á(Et2NH)2, and 3Á(H2NCH2CH2NH2). Cyclohexyl groups are not shown
Fig. 2 The methyl NMR signals of SiC2H3Me2 of 3 with 0 (bottom) or 2 eq.
(top) triethylamine. Left: 1H NMR right: 13C NMR.
for clarity. Ellipsoids show 50% of the occupancy.
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A similar behaviour was observed with a bulky secondary base the complex unstable in an aqueous medium. The vinyl groups in the
(2,2,6,6-tetramethyl piperidine) or a less basic alcohol (PhCH2OH). It complex offer opportunities to heterogenize the complex by hydro-
is likely that steric hindrance of 3 contributed to the weak binding of silylation, which will be a topic for future studies.
these ligands. The steric barrier however can be overcome when a very
This work was supported by the Department of Energy, Basic
strong base was used. Sterically bulky 1,8-diazabicyclo[5.4.0]undec-7- Energy Sciences, grant no. DE-FG02-01ER15184 and also based
ene (DBU), with a pKb 6 times higher than that of triethylamine,25 on work supported as part of the Institute for Atom-efficient
bound to Sn in a 1 : 1 stoichiometry as indicated by a complete Chemical Transformations (IACT), an Energy Frontier Research
conversion of 3 to a new species with a one equivalent of the base. Center funded by the U.S. Department of Energy, Office of
Even in the presence of two equivalents of DBU, only one molecule Science, Office of Basic Energy Sciences. Use of the Advanced
would bind to 3, resulting in a pentacoordinated complex with a 119Sn Photon Source was supported by the U.S. Department of
NMR signal at À596 ppm (Fig. S9c, ESI†). 1H and 13C NMR spectra Energy, Office of Basic Energy Sciences, under contract No.
indicated loss of asymmetry (Fig. S9a and b, ESI†), which suggests DE-AC02-06CH11357. MRCAT operations are supported by the
symmetrical square pyramidal configuration of the complex, as Department of Energy and the MRCAT member institutions.
opposed to an alternative trigonal bipyramidal.
X.H. acknowledges China Scholarship Council. This work made
The observed behaviour of bases and benzyl alcohol indicated use of the IMSERC facility at Northwestern University.
that Sn in 3 was a Lewis acid and accessible. In fact, the remark-
able e-withdrawing tendency of the POSS ligands was expected to
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33–40.
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Chem. Commun., 2014, 50, 15699--15701 | 15701