Bismuth pyrostannate, Bi2Sn2O7, from the first structurally characterized heterobimetallic Bi:Sn alkoxides

Article abstract of DOI:10.1021/ic202707p

The first heterobimetallic Bi:Sn alkoxide complexes [Bi ₂SnO(OCH(CF₃)₂)₅(O ^tBu)₃(THF)] (1) and [BiSnO(OCH(CF₃) ₂)₃(O^tBu)₂]₂ (2) are described. The complexes were obtained through mixing and heating equimolar quantities of the component alkoxides, Bi(OCH(CF₃)₂) ₃ and Sn(O^tBu)₄, under solvent-free conditions (1) and in THF (2). The solid-state structures were determined by single crystal X-ray diffraction showing ligand redistribution from Bi(III) to Sn(IV) in the two molecular species. Compound 2 behaves as a single-source precursor for the thermolytic formation of bismuth pyrostannate, Bi₂Sn ₂O₇.

Full text of DOI:10.1021/ic202707p

Communication
pubs.acs.org/IC
Bismuth Pyrostannate, Bi2Sn2O7, from the First Structurally
Characterized Heterobimetallic Bi:Sn Alkoxides
Philip C. Andrews,* Peter C. Junk, Iryna Nuzhnaya, and Dominique T. Thielemann
School of Chemistry, Monash University, Clayton, Melbourne, Vic. 3800, Australia
S
* Supporting Information
alkali and alkaline earth metals.3,14−17 To date, there is only one
example in the literature of a complex containing bismuth(III)
with a second different p-block metal, namely, [Bi{Al-
(OiPr)4}2(OiPr)]. However, while the composition is reported,
the solid-state structure has not been established.18 The
majority of heterobimetallic complexes are composed of
carboxylates or diketonates as O-bound ligands and contain
d-block metals alongside bismuth.9
Herein, we report the synthesis, structural characterization,
and thermolysis of two heterobimetallic Bi:Sn alkoxide
complexes: [Bi2SnO(OCH(CF3)2)5(OtBu)3(THF)] (1) and
[BiSnO(OCH(CF3)2)3(OtBu)2]2 (2), both derived from the
equimolar combination of the component metal alkoxides,
Bi(OCH(CF3)2)3 and Sn(OtBu)4 (Scheme 1). The two
complexes, however, were synthesized using two different
synthetic routes.
ABSTRACT: The first heterobimetallic Bi:Sn alkoxide
complexes [Bi2SnO(OCH(CF3)2)5(OtBu)3(THF)] (1)
and [BiSnO(OCH(CF3)2)3(OtBu)2]2 (2) are described.
The complexes were obtained through mixing and heating
equimolar quantities of the component alkoxides, Bi-
(OCH(CF3)2)3 and Sn(OtBu)4, under solvent-free con-
ditions (1) and in THF (2). The solid-state structures
were determined by single crystal X-ray diffraction
showing ligand redistribution from Bi(III) to Sn(IV) in
the two molecular species. Compound 2 behaves as a
single-source precursor for the thermolytic formation of
bismuth pyrostannate, Bi2Sn2O7.
in and bismuth alkoxides are important precursors in the
T
formation of their respective oxide films, SnO2 and Bi2O3,
and their incorporation in ternary materials.1 While the
homometallic oxides have chemical and physical features that
make them important in modern materials chemistry,2,3 much
recent interest has come to focus on the properties and utility
of heterometallic systems.4 These systems allow for greater
variation in the physical, structural, electronic, and optical
properties of the materials and can accommodate greater
specificity in their application. Bismuth pyrostannate Bi2Sn2O7
is a desirable ternary oxide5 showing good catalytic activity and
utility in a wide array of organic transformations and can act as
a selective CO-sensing device.
a
Scheme 1
a
Reaction conditions: (i) solvent-free, 60 °C, N2(g), 30 min, THF and
toluene extraction; (ii) THF, 60° C, N2(g), 30 min, hexane and
toluene extraction.
Compound 1 was obtained from the treatment of Bi(OCH-
(CF3)2)3 (mp 52 °C) with Sn(OtBu)4 (mp 42 °C) under
solvent-free conditions, heating to 60 °C under N2 for 30 min.
This produced a white solid from a molten mixture, which was
washed with hexane to remove unreacted material. Ultimate
extraction of the solid with hot toluene gave a clear yellow
solution. Colorless crystals of 1 were readily obtained on
allowing the solution to cool slowly to room temperature. The
origin of the preference for a Bi:Sn ratio of 2:1 is not yet known
but may be due to differences in ligand lability and the ease of
M−OR decomposition and/or dissociation.
Compound 2 was synthesized in a more conventional way,
combining THF solutions of the Bi and Sn alkoxide precursors
and heating to 60 °C for 30 min. Removal of THF under
vacuum conditions gave a viscous yellow oil which was washed
with n-hexane and taken up in hot toluene. As with 1, single
While the chemistry of Sn(OR)4 species and their application
in the deposition of thin films is now relatively well
established,6 that of homometallic Bi(OR)3 is somewhat more
limited, mostly stemming from difficulties in synthesis,
solubility, and volatility.3,7,8 These difficulties are amplified in
the exploration of bismuth-containing heterobimetallic sys-
tems.9 Heterobimetallic single-source precursors have been
long touted as being ideal for the controlled formation of
ternary oxide materials. However, the lack of success in their
formation and isolation means that the predominant method
for oxide formation remains the premixing of two discrete
homometallic species, with accompanying problems of
solubility, stoichiometry, and mismatching of reactivity and
decomposition profiles.
Despite alkoxide-based heterobimetallic complexes being
identified and targeted as the best and most flexible systems for
oxide formation under mild conditions,10,11 the molecular
compounds remain elusive. Very few heterobimetallic bismuth
alkoxides have been reported in the literature, and even less
characterized in the solid state.3,12,13 Of these, most involve the
Received: May 31, 2011
Published: January 4, 2012
© 2012 American Chemical Society
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crystals of 2 were obtained by allowing the solution to cool to
room temperature.
three metal atoms are bridged around the perimeter by three
alkoxy groups: one OCH(CF3)2 group bridges Sn and Bi atoms
with bond distances of Sn(1)−O(5) = 2.074(7) Å and Bi(2)−
O(5) = 2.662(7) Å, and one OtBu group bridges Sn and Bi
atoms with bond distances of Sn(1)−O(6) = 2.062(7) Å and
Bi−O(6) = 2.337(6) Å. A second OtBu group straddles the two
Bi atoms with one short bond, Bi(2)−O(9) = 2.185(6) Å, and
one longer bond, Bi(1)−O(9) = 2.344(6) Å. The Sn and two
Bi atoms are linked by a central asymmetrically bonded μ3-O
atom with distances of Sn(1)−O(1) = 2.089(6) Å, Bi(1)−O(1)
= 2.176(5) Å, and Bi(2)−O(1) = 2.092(6) Å.
The coordination environment around Sn(1) is distorted
octahedral with bond angles in the range of 76.5(2)−169.1(3)°.
In addition to the two bridging alkoxy groups, the Sn atom has
three terminal coordinated OCH(CF3)2 groups with an average
Sn−O bond distance of 2.08 Å, which is more consistent with
distances observed in Sn(II) complexes, e.g., [Sn(OCH-
(CF)3)2·(HNMe2)].19
The two Bi atoms have distinctive coordination environ-
ments. The four-coordinate Bi(2) atom has distorted trigonal
bipyramidal geometry, with bond angles in the range of
64.8(2)−130.6(2)°. In addition to the two bridging alkoxy
groups and the central O atom, it coordinates to a terminal
OtBu ligand at a bond distance of Bi(2)−O(8) = 2.037(6) Å.
The penta-coordinated Bi(1) atom has a distorted octahedral
geometry with bond angles in the range of 69.3(2)−166.8(2)°.
In addition to the two definite bridging alkoxy ligands and the
bond with the central O atom, Bi(1) bonds with a terminal
OCH(CF3)2 ligand at a distance of Bi(1)−O(7) = 2.092(6) Å,
and with the single terminal THF molecule, Bi(1)−O(10) =
2.710(7) Å. It expands its coordination environment to six if
the longer, weaker interaction between Bi(1) and O(2), 3.263
Å, is also taken into account.
The observed bridging and terminal Bi−O bond distances
are largely consistent with those observed in related
homometallic bismuth compounds.20,21 For example, in
[Bi2(μ3-O)(OCH(CF3)2)2(μ-OCH(CF3)2)2(THF)]2,21 the ter-
minal Bi−OCH(CF3)2 distances are 2.082(6) and 2.133(6) Å,
while the bridging ligands show a more varied bonding pattern
with distances ranging from 2.219(7)−2.702(7) Å. The average
distance from the three Bi atoms in this complex to the central
μ3-O is 2.14 Å.
The isolated yields of crystals of 1 and 2 were 26% and 42%,
respectively. Both compounds have relatively low melting
points; 1 melts in the range 114 −117 °C and 2 from 105 to
107 °C. Single crystal X-ray diffraction studies show that 1 is a
heteroleptic trinuclear system of formula [Bi2SnO(OCH-
(CF3)2)5(OtBu)3(THF)], containing one coordinating THF
molecule, while 2 is a binuclear complex [BiSnO(OCH-
(CF3)2)3(OtBu)2]2·C7H8, accommodating one toluene mole-
cule in the crystal lattice. The elemental analysis of 1 is
consistent with the crystal structure, while for 2 it is consistent
with the loss of the noncoordinating toluene molecule. CCDC
827443 and 827444 contain the supplementary crystallographic
data for this paper. These data (excluding structure factors) can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
A significant problem in forming isolable heterobimetallic
alkoxides is the high lability of the ligands, combined with the
different electronic and physical properties of the metals.
Solution-state NMR spectroscopic studies on 1 and 2
conducted in THF-d8 and C6D6 exemplify this problem. The
spectra show evidence of the compounds undergoing dynamic
rearrangement in solution, forming species of differing
nuclearity, substitution, and thus composition. This is also
evident in the single crystal X-ray structure of 2, which displays
a dual site occupancy by OtBu and OCH(CF3)2 at a single
1
bridging position (see below). For both 1 and 2, the H and
13C{1H} NMR spectra at room temperature show numerous
resonances corresponding to the C(CH3)3 and OCH(CF3)2
groups, though the spectra in THF-d8 show greater signal
averaging than those obtained in C6D6 (these data are provided
in the Supporting Information). The 19F NMR spectrum for 1
has a single dominant doublet at δ −74.72 and for 2 at δ
−75.85. The 119Sn NMR spectra revealed one singlet for each
of the title compounds, at δ −249.3 for 1 and δ −701.4 for 2.
The NMR spectra, coupled with elemental analyses, indicate
that compounds of composition consistent with 1 and 2 are
formed reproducibly.
Compound 1 crystallizes in space group P21/n with the
whole molecule comprising the asymmetric unit (Figure 1).
Compound 2 crystallizes in the triclinic crystal system P1
with two independent half molecules (2A and 2B) in the
asymmetric cell. The central planar Bi2O2 parallelogram in each
molecule resides on an inversion center. While the atoms in
one half-molecule (A) could be satisfactorily refined, the other
half-molecule (B) suffers from significant disorder in both its
OCH(CF3)2 groups. This is attributed to high thermal motion
and in having one of the metal bridging sites occupied 50:50 by
OCH(CF3)2 and OtBu groups. The remaining bridging OtBu
group shows no disorder or partial occupancy, and the structure
of 2A is presented in Figure 2.
̅
The bond lengths in the central Bi2O2 ring are Bi(1)−O(1) =
2.111(8) and Bi(1)−O(1′) = 2.190(7) Å. The Sn atoms then
adopt a trans orientation relative to the ring, with each
emanating from separate ring O atoms at an angle of 119.6(3)°
and bonding at a distance of Sn(1)−O(1) = 2.100(7) Å. One
bridging and two terminal OCH(CF3)2 groups, and two
bridging OtBu groups, contribute to a distorted octahedral
coordination environment around the Sn atom. The terminal
bond distances, Sn(1)−O(5) and Sn(1)−O(6), are essentially
identical at 1.988(8) and 1.994(8) Å, respectively. The bridging
Figure 1. Crystal structure of trinuclear complex [Bi2SnO(OCH-
(CF3)2)5(OtBu)3(THF)] (1). Ellipsoids are shown at 50% probability.
All hydrogen atoms are omitted for clarity.
The asymmetric unit consists of a trinuclear metal core
incorporating a hexa-coordinate Sn atom, and penta- and tetra-
coordinate Bi atoms, Bi(1) and Bi(2), respectively. The
intermetallic distances are Sn(1)···Bi(1) = 3.304 Å,
Sn(1)···Bi(2) = 3.516 Å, and Bi(1)···Bi(2) = 3.513 Å. These
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groups move away from the Bi(III) centers to the more
electropositive Sn(IV) centers. The concomitant formation of
the oxo-species, and by inference the mechanism of formation
of the final ternary oxide species, was investigated using GC-MS
(see Supporting Information). The alkoxides were mixed, dried
in vacuo, and heated under an atmosphere of dry argon gas to
100 °C for 1 h. Head-space GC-MS analysis of the volatile
components showed them to be unambiguously iso-butylene,
tBuOH, and (CF3)2CHOH. This is consistent with known
decomposition pathways of highly branched metal alkoxides.22
Figure 2. Crystal structure of [BiSnO(OCH(CF3)2)3(OtBu)2]2·C7H8,
2A. Ellipsoids are shown at 30% probability. All hydrogen atoms and
the lattice toluene molecule are omitted for clarity.
ASSOCIATED CONTENT
* Supporting Information
■
S
Syntheses, analyses, and crystallographic data, including CIFs.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OCH(CF3)2 group forms a much shorter bond to tin, Sn(1)−
O(4) = 2.052(8) Å, than it does to bismuth, Bi(1′)−O(4) =
2.813 Å. One of the bridging OtBu groups forms a shorter bond
with tin than with bismuth, Sn(1)−O(2) = 2.075(9) and
Bi(1)−O(2) = 2.454(8) Å, while the opposite is the case for
the other, Sn(1)−O(3) = 2.207(8) and Bi(1)−O(3) =
2.157(8) Å. Most likely, this is due to steric effects. If the
stereochemically active lone pair on Bi(1) is included in the
coordination environment, then the Bi center adopts a highly
distorted octahedral arrangement. There are four acute angles
involving Bi(1) and O(1−3) ranging from 62.2(3) to 75.8(3)°
and a single obtuse angle of 147.8(3)° for O(2)−Bi(1)−O(2).
Thermal decomposition profiles of 1 and 2 were studied via
thermogravimetric analysis (TGA). Unfortunately, the final
weight-percent values did not allow for an unambiguous
assignment to a certain oxide or oxyfluoride species (see the
Supporting Information) though energy dispersive spectrosco-
py (EDS) experiments on the residues were consistent with the
Bi:Sn ratios of 1 and 2.
AUTHOR INFORMATION
Corresponding Author
*Ph.: +61 3 9905 5509. Fax: +61 3 99054597. E-mail: phil.
andrews@monash.edu.
■
ACKNOWLEDGMENTS
We thank the Australian Research Council, Monash University,
and Smartprint CRC for financial support.
■
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