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Cyclic Alumosiloxanes and Alumosilicates: Exemplifying the
Loewenstein Rule at the Molecular Level†
Sandra Gonzꢀalez-Gallardo,‡ Vojtech Jancik,§,^ Alma A. Delgado-Robles,§,‡ and Mꢀonica Moya-Cabrera*,§,^
§Centro Conjunto de Investigaciꢀon en Química Sustentable, UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Toluca, Estado de
Mꢀexico 50200, Mꢀexico
‡Instituto de Química, Universidad Nacional Autꢀonoma de Mꢀexico, Circuito Exterior, Ciudad Universitaria, 04510 Mꢀexico D.F., Mꢀexico
S Supporting Information
b
Compound 1 reacts readily with 2 equiv of the appropriate
silanediol R2Si(OH)2 (R = Ph, OtBu) at 0 ꢀC to yield the
ABSTRACT: The cyclic alumosiloxane [{LAl(μ-O)(Ph2Si)-
(μ-O)}2] (3) and alumosilicate [{LAl(μ-O){(tBuO)2Si}-
corresponding alumosiloxane [{LAl(μ-O)(Ph2Si)(μ-O)}2] (3)
or alumosilicate [{LAl(μ-O){(tBuO)2Si}(μ-O)}2] (4) in low
(μ-O)}2] (4) were obtained by reaction of the appropriate
R2Si(OH)2 precursor (R = Ph, OtBu) with [{LAl(H)}2(μ-O)]
yield. Compounds 3 and 4 are also attainable in high yield by
(1), providing a nice illustration of the Loewenstein rule at work
at the molecular level.
using the aluminum hydride 2 as the starting material instead of 1
(Scheme 1).
The cleavage of the AlꢀOꢀAl bridge, required to form the
cyclic Al2O4Si4, proceeds with the concomitant formation of
1 mol of water (Scheme 2). Furthermore, the reaction of 1 with
R2Si(OH)2 (R = Ph, OtBu) leads to AlꢀOꢀAl cleavage products
(AlꢀOꢀSi) rather than the expected six-membered ring pro-
ducts [{(LAl)2(μ-O)}(SiR2)(μ-O)2], providing a nice illustra-
tion of the Loewenstein rule at the molecular level.
any minerals found in nature are alumosilicates, including
M
zeolites, which are built by small ring structures made of
tetrahedral silicon and aluminum cations linked by two-coordi-
nate oxygen atoms. Structural features such as enclosed large
cavities give zeolites useful properties for a wide range of
applications, such as catalysis, ion exchange, and adsorbents.1
Therefore, the structures and properties of alumosilicate materi-
als have been extensively studied, and the synthesis of molecular
analogues has been pursued for their value as suitable molecular
models.2 The isolation of such species is important because
soluble alumosilicates are easily characterized by means of all
spectroscopic techniques available to the synthetic chemist,
adding to the understanding of the properties of zeolite materials.
A considerable number of molecular alumosiloxanes and
alumosilicates have been obtained using aluminum halogenides,
chalcogenides, hydrides, and organometallic compounds as starting
materials, by reacting them with the appropriate RnSi(OH)4ꢀn
precursor.3
Furthermore, one of the fundamental principles applicable to
the structures of alumosilicate solids is the Loewenstein rule.
This rule states that “whenever two tetrahedra are linked by one
oxygen bridge, the center of only one of them can be occupied by
aluminum; the other center must be occupied by silicon”.4
Theoretical calculations predict that the formation of AlꢀOꢀAl
bridges in the small rings that form the framework of zeolite
materials is energetically unfavorable.5 In this regard, the cleavage
of SiꢀOꢀSi linkages by reaction with aluminum compounds has
been previously observed.3a,g However, no such cleavage for
AlꢀOꢀAl moieties by reaction with RnSi(OH)4ꢀn has been
reported to date.
In the electron impact ionization mass spectrometry spectra of
3 and 4, only the peaks corresponding to half of the molecule
1
could be observed at m/z 574 and 566, respectively. Both H
NMR spectra exhibit two sets of signals in a 1:1 ratio: one set
corresponds to the β-diketiminate ligand, while the other set is
assigned to the R2Si fragments, 6.84 and 6.99 ppm for 3 and 1.25
ppm for 4.
The signal at ꢀ50 ppm in the 29Si NMR spectrum of 3 is
indicative of a silicon atom bonded to two carbon and two oxygen
atoms, whereas the corresponding spectrum of 4 exhibits a signal
at ꢀ113 ppm, characteristic of a SiO4 unit.7 On the other hand, no
such signals could be observed for 3 and 4 in their 27Al NMR
spectra, even after increasing the relaxation time or lowering the
measurement temperature (ꢀ70 ꢀC) of the experiments.
A 1H NMR (C6D6) follow-up of the preparation of 3 reveals
the presence of unreacted 1 (4.95 ppm), the desired alumosilox-
ane 3 (5.04 ppm), and a mixture of the products of the stepwise
hydrolysis of 1: [{LAl(H)}(μ-O){(OH)AlL}][{LAl(OH)}2-
(μ-O)] and LH, as indicated by the characteristic signals for
the γ proton of the β-diketiminate backbone, consistent with the
presence of H2O as a reaction byproduct (see the Supporting
Information),
Colorless crystals of 3 and 4 were obtained from concentrated
tetrahydrofuran solutions at room temperature. In both com-
pounds, the aluminum atoms are tetracoordinate and exhibit
distorted tetrahedral geometries (Figures 1 and S1 in the Support-
ing Information). The AlꢀO bond lengths in 3 [1.707(2) and
1.714(2) Å] and 4 [1.704(2), 1.705(2), 1.704(2), and 1.713(2) Å]
are comparable with each other and are slightly shorter than
In this regard, we recently reported on the preparation of the
alumoxane hydride [{LAl(H)}2(μ-O)] (1; L = HC[(CMe)N-
(2,4,6-Me3C6H2)]2ꢀ),6 which contains an AlꢀOꢀAl linkage. As
an extension of our research, we studied the reactivity of 1 as well
as the aluminum hydride [LAlH2] (2) with selected silanediols
R2Si(OH)2 (R = Ph, OtBu).
Received: February 15, 2011
Published: April 12, 2011
r
2011 American Chemical Society
4226
dx.doi.org/10.1021/ic200313k Inorg. Chem. 2011, 50, 4226–4228
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