Synthesis and characterization of the heterometallic aggregate Pb2Al5(μ3-O)(μ4-O)(μ-OiPr)9(OiPr)3(μ-OAc)3

Article abstract of DOI:10.1002/(sici)1099-0682(199908)1999:8<1291::aid-ejic1291>3.0.co;2-l

A novel heterometallic aggregate Pb₂Al₅(μ₃-O)(μ₄-O)(μ- OiPr)₉(OiPr)₃(μ-OAc)₃ obtained from the interaction of Pb(OAc)₂ and Al(O-iPr)₃ is the first structurally characterized example based on lead and aluminum. This compound has been isolated in high yield and examined by ¹H-, ¹³C-, and ²⁷Al NMR, and in the solid state by X-ray crystallography.

Full text of DOI:10.1002/(sici)1099-0682(199908)1999:8<1291::aid-ejic1291>3.0.co;2-l

FULL PAPER
Synthesis and Characterization of the Heterometallic Aggregate
Pb2Al5(µ3-O)(µ4-O)(µ-OiPr)9(OiPr)3(µ-OAc)3
Ashutosh Pandey,[a] Vishnu D. Gupta,*[a] and Heinrich Nöth[b]
Keywords: Aluminum / Lead / O ligands / Alkoxy carboxylates / Structure elucidation
A
novel heterometallic aggregate Pb2Al5(µ3-O)(µ4-O)(µ-
example based on lead and aluminum. This compound has
been isolated in high yield and examined by 1H-, 13C-, and
27Al NMR, and in the solid state by X-ray crystallography.
OiPr)9(OiPr)3(µ-OAc)3 obtained from the interaction of
Pb(OAc)2 and Al(O–iPr)3 is the first structurally characterized
was obtained in a high and reproducible yield, which ex-
cludes formation by accidental hydrolysis.
Introduction
The use of heterometallic alkoxides[1Ϫ3] as precursors for
mixed metal oxides has increased significantly in recent
times, with efforts directed towards understanding the re-
lationship, with regard to both structure and properties, be-
tween the precursor and the final material.[4] Structural
data, even for heterobimetallic alkoxides,[1] is still scarce.
Lead and aluminum are important components of many
such materials.[5]
In the IR spectrum of the compound the difference of
150 cmϪ1 in the ν(CO2) absorption frequency of the asym-
metric (1610 cmϪ1) and symmetric (1460 cmϪ1) bands is
indicative of the acetate group acting as bidentate ligand.
The absorptions observed at 1090 cmϪ1 and 1020 cmϪ1 ac-
count for the presence of terminal and bridging isopropoxy
groups, respectively.
1
At ambient temperature H- and 13C-NMR spectra of
The difficulty in preparing lead(II) alkoxides[6] has been
overcome by using commonly accessible salts such as ni-
trate,[7] acetate,[8] halides, hydroxides, or β-diketonates.[9][10]
In general metal acetates are the most common precursors
associated with metal alkoxides. Structural features of the
products[11] obtained by treating metal acetates with metal
alkoxides show that the acetate ligand is bridging, or
bridge-chelating, indicating the role of the carboxylate
group in maintaining the stoichiometry during the various
stages of the transformation that leads to mixed metal
oxides.
the compound are not structurally informative. However,
the 27Al-NMR spectrum shows a broad peak (δ ϭ 32Ϫ58)
centered at δ ϭ 42.24, which is at a position characterisitic
of both octahedral and tetrahedral symmetries around the
aluminum centers.[13]
The X-ray crystallography and molecular structure of the
compound is given in Figure 1 and the core is shown in
Figure 2.
This work reports on the synthesis and characterization
of a heterobimetallic aggregate, Pb2Al5(µ3-O)(µ4-O)(µ-OiPr)9-
(OiPr)3(µ-OAc)3, based on lead and aluminum. This com-
pound represents a unique example in which aluminum is
present in three different coordination environments. The
pentacoordination around the lead center observed in this
system is rare in molecular compounds, and has been
reported to be found in high-Tc superconductors.[12]
Figure 1. Molecular structure of Pb2Al5(µ3-O)(µ4-O)(µ-OiPr)9-
(OiPr)3(µ-OAc)3; thermal ellipsoids are represented with 25% prob-
ability; hydrogen atoms have been omitted for clarity
Results and Discussion
Reactions between metal alkoxides and metal acetates
have generally been reported to give oxo products by the
elimination of an ester.[11] Simple addition products[8] have
also been reported in room-temperature reactions. How-
ever, under refluxing conditions the formation of oxo spec-
ies is spontaneous. The product Pb2Al5(O)2(OiPr)12(OAc)3
Selected bond dimensions are summarized in Table 1.
The compound has a Pb2Al5 unit linked by acetato, isopro-
poxo, and oxo groups, making a complex and interesting
structure. There are two tetrahedral aluminum atoms
[Al(5), Al(4)] which differ environmentally, two trigonal bi-
pyramidal aluminum centers [Al(2) and Al(1)], and one
which is octrahedrally surrounded [Al(3)]. This constitutes
the first example of a system containing the same metal
atom in three different coordination environments. Both the
lead atoms are pentacoordinate, having different geometries
[a]
Department of Chemistry, Faculty of Science, Banaras Hindu
University,
Varanasi 221005, India
[b]
Institute of Inorganic Chemistry, University of Munich,
Meiserstraße 1, D-80333 Munich, Germany
Eur. J. Inorg. Chem. 1999, 1291Ϫ1293
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ1948/99/0808Ϫ1291 $ 17.50ϩ.50/0
1291

A. Pandey, V. D. Gupta, H. Nöth
FULL PAPER
The coordination around Al(5) is completed by one ter-
minal and three doubly bridging OiPr groups. The angles
subtended by O(20) are normal tetrahedral angles, while the
acute angle [O(18)ϪAl(5)ϪO(17), 92.1°] due to the four-
membered ring [ϪPb(2)ϪO(17)ϪAl(5)ϪO(18)Ϫ] leads to a
widening of the other two angles (average 117.8°). The se-
cond tetrahedral aluminum atom, Al(4), has two terminal
and two µ-OiPr groups. Four of the tetrahedral angles are
almost normal. The larger deviations in case of the angles
O(15)ϪAl(4)ϪO(12) [83.2(2)°] and O(14)ϪAl(4)ϪO(15)
[119.1(3)°] are caused by two isopropoxy groups bridging
to the octahedral Al(3). The former is quite comparable to
the corresponding angle (83.1°) in [Al(OiPr)3]4.[14]
Figure 2. Plot of the core of Pb2Al5(µ3-O)(µ4-O)(µ-OiPr)9(OiPr)3-
(µ-OAc)3
Al(2) and Al(1) are pentacoordinate with similar environ-
ments. The coordination around Al(1) is completed by two
µ-OiPr groups bridging with Pb(1) and Pb(2), and two oxo
groups µ3-O(11) and µ4-O(7) and one bidentate acetate
group bridging to Al(3). Al(1) and Al(2) almost lie in the
plane constituted by O(11), O(10), O(8) and O(9), O(11),
O(16), respectively. Bond angles at the pentacoordinated Al
atoms to the axial oxygen atoms are close to linear, e.g.
O(6)ϪAl(2)ϪO(7) ϭ 178.9°. Octahedral Al(3), along with
the coordination by oxo functionality O(1), is linked by
three bridging OAc groups, two aluminum atoms [Al(1) and
Al(2)] and one lead atom [Pb(1)], and two µ-OiPr groups
with Al(4), allowing the metal center to reach its most com-
mon coordination number six. Due to the complexity of the
structure the angles for the octahedron range from
74.9(2)Ϫ100.1(2)°.
but each essentially that of a distorted teragonal pyramid
with a lone pair.
Table 1. Selected bond dimensions for compound Pb2Al5(O)2(Oi-
Pr)12(OAc)3
˚
Bond lengths[A]
Pb(1)ϪO(7)
Pb(1)ϪO(10)
Pb(1)ϪO(20)
Pb(2)ϪO(17)
Pb(2)ϪO(8)
Al(1)ϪO(16)
Al(1)ϪO(11)
Al(1)ϪO(7)
Al(2)ϪO(10)
Al(2)ϪO(6)
Al(3)ϪO(11)
Al(3)ϪO(5)
Al(3)ϪO(12)
Al(4)ϪO(14)
Al(4)ϪO(15)
Al(5)ϪO(19)
Al(5)ϪO(18)
O(1)ϪC (1)
2.250(4)
2.454(5)
2.686(5)
2.485(5)
2.487(5)
1.790(5)
1.822(5)
1.920(5)
1.797(5)
1.869(5)
1.825(5)
1.920(5)
1.942(5)
1.694(6)
1.784(5)
1.694(6)
1.757(5)
1.228(9)
Pb(1)ϪO(9)
Pb(1)ϪO(1)
Pb(2)ϪO(7)
Pb(2)ϪO(18)
Pb(2)ϪO(16)
Al(1)ϪO(9)
Al(1)ϪO(4)
Al(2)ϪO(8)
Al(2)ϪO(11)
Al(2)ϪO(7)
Al(3)ϪO(2)
Al(3)ϪO(3)
Al(3)ϪO(15)
Al(4)ϪO(13)
Al(4)ϪO(12)
Al(5)ϪO(20)
Al(5)ϪO(17)
O(2)ϪC(1)
2.440(4)
2.535(5)
2.234(4)
2.476(5)
2.491(4)
1.808(5)
1.878(5)
1.788(5)
1.806(4)
1.909(5)
1.857(5)
1.926(5)
1.960(5)
1.717(5)
1.787(5)
1.747(6)
1.763(5)
1.295(9)
Among the AlϪOiPr bond lengths the terminal AlϪO
˚
distances (average 1.701 A) of the tetrahedral Al(4) and
Al(5) are the shortest. In the bridging distances the pattern
˚
is AlϪO (tetrahedral) (average 1.767 A) < AlϪO (trigonal-
˚
bipyramidal (average 1.797 A) < AlϪO (octahedral) (aver-
˚
age. 1.951 A). These are well within the range already re-
ported in the literature.[14Ϫ16] The aluminumϪoxygen bond
lengths due to the bridging acetate groups are almost equal,
whereas that of the bridging Pb(1) is somewhat shorter (see
Table 1) The coordination around Pb(1) is completed by
three µ-OiPr groups bridging Al(1), Al(2), and Al(5), and a
bidentate acetate group bridging Al(3) and µ4-O(7). Pb(2)
is surrounded by four bridging oxygen atoms of isopropoxy
groups, two with Al(5) and one each with Al(1) and Al(2)
and µ4-O(7). Thus Pb2O9 essentially constitutes a bitetra-
gonal-pyramidal geometry. The tetragonal angles for Pb(1)
range from 73.2(2)° [O(9)ϪPb(1)ϪO(1)] to 113.5(2)°
[O(10)ϪPb(1)ϪO(20)], while for Pb(2) the range is 61.9(2)°
[O(17)ϪPb(2)ϪO(18)] to 94.2(2)° [O(18)ϪPb(2)ϪO(8)].
This difference is obviously a consequence of the different
degree of constraint imposed by the coordinating groups.
For Pb(1) the leadϪoxygen distances cover the unusually
Bond angles [°]
O(7)ϪPb(1)ϪO(9)
65.1(2)
O(7)ϪPb(1)ϪO(10) 65.2(2)
O(7)ϪPb(1)ϪO(1) 102.6(2)
O(9)ϪPb(1)ϪO(10) 116.5(2)
O(9)ϪPb(1)ϪO(1)
73.2(2)
O(10)ϪPb(1)ϪO(1) 81.7(2)
O(9)ϪPb(1)ϪO(20) 108.0(2)
O(1)ϪPb(1)ϪO(20) 160.4(2)
O(7)ϪPb(2)ϪO(18) 83.4(2)
O(7)ϪPb(1)ϪO(20) 95.3(2)
O(10)ϪPb(1)ϪO(20) 113.5(2)
O(7)ϪPb(2)ϪO(17) 82.75(2)
O(17)ϪPb(2)ϪO(18) 61.9(15)
O(17)ϪPb(2)ϪO(8) 142.2(2)
O(7)ϪPb(2)ϪO(16) 65.73(2)
O(18)ϪPb(2)ϪO(16) 142.4(2)
O(7)ϪPb(2)ϪO(8)
64.82(2)
O(18)ϪPb(2)ϪO(8) 94.2(2)
O(17)ϪPb(2)ϪO(16) 92.12(2)
O(8)ϪPb(2)ϪO(16) 91.6(2)
O(16)ϪAl(1)ϪO(4)
O(11)ϪAl(1)ϪO(4)
O(9)ϪAl(1)ϪO(7)
O(4)ϪAl(1)ϪO(7)
O(10)ϪAl(2)ϪO(6)
O(8)ϪAl(2)ϪO(7)
O(11)ϪAl(2)ϪO(7)
O(11)ϪAl(3)ϪO(2)
O(11)ϪAl(3)ϪO(3)
93.3(2)
97.1(2)
85.3(2)
178.5(2)
94.6(2)
86.7(2)
82.1(2)
100.1(2)
94.6(2)
O(9)ϪAl(1)ϪO(4)
O(16)ϪAl(1)ϪO(7)
O(11)ϪAl(1)ϪO(7)
O(8)ϪAl(2)ϪO(6)
O(11)ϪAl(2)ϪO(6)
O(10)ϪAl(2)ϪO(7)
O(6)ϪAl(2)ϪO(7)
O(11)ϪAl(3)ϪO(5)
O(5)ϪAl(3)ϪO(3)
O(5)ϪAl(3)ϪO(15)
O(12)ϪAl(3)ϪO(15) 74.9(2)
O(14)ϪAl(4)ϪO(12) 112.9(3)
O(19)ϪAl(5)ϪO(18) 118.1(3)
O(18)ϪAl(5)ϪO(17) 92.1(2)
95.1(2)
87.8(2)
81.44(2)
92.6(2)
97.5(2)
86.5(2)
178.9(2)
96.0(2)
169.3(2)
85.7(2)
˚
wide range from 2.250(4) to 2.686(5) A. However, for Pb(2)
˚
the range [2.234(4)Ϫ2. 491(4) A] is comparable to re-
ported[8] values.
O(11)ϪAl(3)ϪO(15) 168.7(2)
O(3)ϪAl(3)ϪO(15) 84.0(2)
The coordination geometry of the µ3-O(11) group is al-
most planar (Σangles ϭ 357.6°) while that around µ4-O(7)
is distorted tetrahedrally with angles ranging from
94.0(2)Ϫ138.6(2)°. This distortion is not as severe as ob-
O((14)ϪAl(4)ϪO(15) 119.1(3)
O(15)ϪAl(4)ϪO(12) 83.2(2)
O(19)ϪAl(5)ϪO(17) 117.6(3)
1292
Eur. J. Inorg. Chem. 1999, 1291Ϫ1293

Heterometallic Aggregate Pb2Al5(µ3-O)(µ4-O)(µ-OiPr)9(OiPr)3(µ-OAc)3
FULL PAPER
ting rotational disorder. Calculations with split positions did not
improve the result significantly. Additional data (except structure
factor tables) have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC-
104505. Copies may be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.)
ϩ 44-1223/336-033; E-mail: deposit@cam.ac.uk].
served in the case of Pb2Ti2(µ4-O)(µ3-ΟiPr)2(µ-OiPr)4(O-
[17]
iPr)4
where the angles around µ4-O range from 95.0(5)
to 163.0(7)°.
For the two acetato moieties, bridging aluminum atoms,
˚
the CϪO distances (average 1.259 A) are almost similar.
˚
However, some asymmetry [C(1)ϪO(1) 1.228(9) A,
˚
C(1)ϪO(2) 1.295(9) A] is introduced for the third one at-
Pb2Al5(O)2(OiPr)12(OAc)3: A suspension of lead acetate (2.41 g,
7.41 mmol) in toluene (ca. 25 mL) was added dropwise to a solu-
tion of aluminum isopropoxide (3.02 g, 14.8 mmol) in toluene (ca.
30 mL) with stirring at room temp. (25°C). The reaction mixture
was heated slowly by raising the bath temperature to 100°C. At
this stage all the acetate had dissolved and the stirring was con-
tached to lead and aluminum atoms.
Conclusions
The first structurally characterized heterometallic aggregate based
on lead and aluminum is found to have a high degree of asymmetry. tinued for 1 h while maintaining the bath temp. A very small quan-
It is the first reported occurence of the presence of the three ident-
ical metal centers (aluminum) in three different coordination en-
tity of solid suspension was filtered off and the solution concen-
trated to 20 mL and kept at Ϫ15°C to afford a white crystalline
vironments. The asymmetry is also reflected by the fact that the product (4.51 g, yield 83%). Ϫ C42H93Al5O20Pb2 (1467.49): calcd.
pair of tetrahedral aluminum atoms and the tetragonal-pyramidal
lead centers also differ in their coordinating groups.
Pb 28.25, Al 9.19, OiPr 48.29; found Pb 28.50, Al 8.91, OiPr 48.19.
Ϫ
2.00 [s, CH3 (OAc)]. Ϫ 13C NMR: δ ϭ 174.72, 177.37, 179.50 [CO
(OAc)], 64.11, 64.31 (CH ), 27.31 and 27.69 [CH3 (OiPr and OAc)].
Ϫ Single crystals were grown from toluene at Ϫ15°C.
1H NMR (25°C): δ ϭ 1.24, [d, CH3 (OiPr)], 4.42 (sept., CH),
Experimental Section
General: The reaction was performed under dry argon using stand-
ard Schlenck and glove-box techniques. Toluene and CDCl3 were
dried by standard procedures and stored over molecular sieves.
Aluminum isopropoxide (Aldrich) was distilled prior to use. Anhy-
drous lead acetate was obtained by refluxing Pb(OAc)2 • 3 H2O
(BDH) with acetic anhydride (Qualigens). Ϫ IR: JASCO FT IR-
5300; Nujol mulls between NaCl plates. Ϫ 1H,13C, and 27Al NMR:
JEOL-FX 90Q spectrometer (CDCl3); SiMe4 internal reference for
1H- and 13C-NMR spectra, [Al(H2O)6]3ϩ external reference for 27Al
spectrum. Lead was estimated as lead chromate and aluminum as
aluminum oxinate. OiPr analysis was done by the method described
by Bradley et al.[18]
Acknowledgments
A. P. is grateful to U. G. C., New Delhi for financial support.
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Crystallographic Data: C42H92Al5O20Pb2; molecular mass 1466.44;
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˚
a ϭ 13.1182(3), b ϭ 23.6499(4), c ϭ 20.l6104(3) A; β ϭ 96.885(1)°,
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3
˚
V ϭ 6348.1(2) A , monoclinic system, space group P21/n, Z ϭ 4,
d ϭ 1.534·10Ϫ9 g/cmϪ3, µ ϭ 5.427 mmϪ1, 2θ ϭ 2Ϫ49.42°, index
range Ϫ15 < h < 15, Ϫ22 < k < 27, Ϫ24 < l < 24, max./min.
transmission: 0.483/0.283, measured/unique/observed reflections:
29924/9671/7677 (Rint ϭ 4.46%), no. of variables: 622, R1 ϭ 0.0411,
wR2 (all data) ϭ 0.0970, weight (x/y) ϭ 0.0442/12.2106, Goodness-
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˚
of-fit ϭ 1.090, ∆ρ ϭ 1.411e/A. The weighting scheme is w
ϭ
1950, 3450Ϫ3454.
Received October 21, 1998
[I98362]
σ2Fo ϩ (xP)2 ϩ yP with P ϭ (Fo ϩ 2 Fo2)/3. It should be noted
2
2
that atoms C(38) and C(39) have large thermal parameters indica-
Eur. J. Inorg. Chem. 1999, 1291Ϫ1293
1293