The Zintl ion [Pb10]2-: A rare example of a Homoatomic closo cluster

Article abstract of DOI:10.1002/anie.200503916

(Figure Presented) Closed encounters: A closed deltahedral homoatomic cluster, [Pb₁₀]^2-, is structurally characterized. When this empty cluster is considered together with the recently published filled cluster [Ni@Pb₁₀]^2-, a striking parallel to the fullerenes emerges: Zintl ions form polyhedral cages that can enclose a central atom without any major structural alterations.

Full text of DOI:10.1002/anie.200503916

Angewandte
Chemie
“Empty” Metal Clusters
DOI: 10.1002/anie.200503916
The Zintl Ion [Pb10]2ꢀ: A Rare Example of a
Homoatomic closo Cluster**
Annette Spiekermann, Stephan D. Hoffmann, and
Thomas F. Fässler*
Dedicated to Professor Hansgeorg Schnöckel
on the occasion of his 65th birthday
Wadeꢀs rules were originally developed to describe the
polyhedral structures of boranes (Figure 1).[1] Their applica-
tion to ligand-free polyhedra of the atoms of Groups 13–15 is
possible, but has been the subject of controversy. The isolobal
ꢀ
replacement of the B H units, which each donate two
[*] A. Spiekermann, Dr. S. D. Hoffmann, Prof. Dr. T. F. Fässler
Department Chemie
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-289-13186
E-mail: thomas.faessler@lrz.tum.de
[**] We would like to thank Christian Benda for his help with the
synthetic work.
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Figure 1. Deltahedral frameworks: closo-hydridoborates [BnHn]2ꢀ are
Figure 2. a) Bicapped-square-antiprismatic structure of the anion
[Pb10]2ꢀ (1a); thermal ellipsoids are set at 50% probability. For
comparison, the structures of b) [Ni@Pb10]2ꢀ (2a)[20] and
known for n=6–12; ligand-free homoatomic closo clusters [En]2ꢀ of
Group 14 elements are only known for n=5 (E=Si–Pb) and n=9
(E=Si).
c) [Pb9Mo(CO)3]4ꢀ (3a)[13] are also shown. Average interatomic Pb Pb
distances [Š] between the square planes of the antiprisms and the
capping atoms (d2; only one square face is capped with a Pb atom in
3a): 1a 3.074, 2a 3.094, 3a 3.051; between the square planes (d1): 1a
3.082, 2a 3.135, 3a 3.136; within the square planes (d3): 1a 3.310, 2a
3.405, 3a 3.320.
ꢀ
electrons to the skeletal framework of the polyhedral boranes,
with an atomE fromGroup 14 (E = Si, Ge, Sn, Pb) leads to an
electronic situation that is only roughly comparable to that of
the boranes. Each E atommust be described as having a
nonbonding electron pair with an outward orientation. The
lone-pair states of En clusters are relatively higher in energy,
with respect to the bonding skeletal orbitals, than are the
addition, two representatives of homoatomic cluster anions
enclosing an additional heteroatomhave recently been
characterized: the deltahedral clusters [Ni@Pb10]2ꢀ (2a;
Figure 2b)[20] and [Pt@Pb12]2ꢀ each contain an interstitial
transition-metal atom. In the latter compound, only weak
interactions occur between the orbitals of the central Pt atom
and those of the cluster atoms,[21] which leads to the
formulation [Pt0@Pb12]2ꢀ. Hence, the “empty” homoatomic
clusters of the Group 14 elements should also be stable, but
they have not yet been described. It has been speculated that
larger electron-precise clusters can only be formed with
endohedral atoms or templates.[22]
ꢀ
external B H bonds in borane clusters. The chemical avail-
ability of the free electron pairs is demonstrated for E = Ge
and Sn by the anions [{E[Cr(CO)5]}6]2ꢀ, in which each E atom
of the octahedral {E6}2ꢀ cluster framework is coordinated to a
{Cr(CO)5} group.[2,3] External ligands are also known to
influence the number of bonding skeletal orbitals in the
boranes. For example, replacing the borane H atoms with
more electronegative Cl (or Br) atoms in the 18-skeletal-
electron cluster B9Cl9 (or B9Br9) leads to a tricapped trigonal-
prismatic structure (Figure 1, n = 9) that is predicted for 20
bonding skeletal electrons according to Wadeꢀs rules.[4,5]
Furthermore, a number of the ligand-free clusters of
After oxidation of a solution of [Pb9]4ꢀ ions in ethylene
diamine with AuI in the formof [P(C 6H5)3AuCl] as the
oxidizing agent, the compound [{K(2.2.2-crypt)}2Pb10] (1;
2.2.2-crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-
[8.8.8]hexacosane) was isolated, allowing the first structural
characterization of a 10-atom closo cluster of a Group 14
element. During the redox reaction, some of the starting
material is oxidized to Pb0, which at the end of the reaction
forms a voluminous precipitate that surrounds the nearly
black crystals of 1 at the bottomof the reaction container
[Eq. (1)].
Group 14 elements have structures that deviate considerably
[6,7]
fromthe expected polyhedral structures.
The structural
characterization of numerous [E9]4ꢀ ions has revealed that in
many cases distortions from the expected nido polyhedral
structure (monocapped square antiprism) occur, as has also
been observed for the electron-deficient clusters [E9]3ꢀ.[7] The
unambiguous assignment of a framework structure to a given
number of bonding skeletal electrons is, therefore, not always
possible.[8] If the nido clusters [E9]4ꢀ are completed by a
transition-metal fragment, which according to the isolobal
concept, does not contribute any electrons to the polyhedral
framework, the resulting heteroatomic closo polyhedra are
nearly undistorted (with respect to the homoatomic contacts),
as observed for the structurally characterized compounds
[E9M(CO)3]4ꢀ (E = Sn, and M = Cr, Mo, W;[9–11] E = Pb, and
M = Cr, Mo (3a);[10,12,13] Figure 2c). The structure of the
recently described closo cluster [Si9]2ꢀ also deviates consid-
2 ½Pb9Š4ꢀ þ 6 Auþ ! ½Pb10Š2ꢀ þ 8 Pb0 þ 6 Au0
ð1Þ
Crystals of 1 and their intensely colored brown solutions
in ethylene diamine or dimethyl formamide (DMF) are
extremely sensitive to air and moisture. A qualitative energy
dispersive X-ray (EDX) analysis of the crystals indicates the
presence of K and Pb, but not Au.
erably fromthat of a tricapped trigonal prismwith
D3h
A single-crystal X-ray structure analysis of 1 reveals that
five symmetry-independent Pb atoms form a [Pb10]2ꢀ ion (1a;
Figure 2a) with a bicapped-square-antiprismatic structure of
nearly perfect D4d symmetry.[23] The presence of two [K(2.2.2-
crypt)]+ ions per cluster indicates that the deltahedron is a
dianion. As according to Wadeꢀs rules, each Pb atom
contributes two electrons to the cluster framework, the
[Pb10]2ꢀ polyhedron contains 22 bonding skeletal electrons
symmetry.[14]
At present, significantly more examples of closo-hydrido-
borates [BnHn]2ꢀ (n = 6–12)[15] than of the corresponding
homoatomic anions [En]2ꢀ, which are only known for n = 5
and E = Si, Ge, Sn, Pb,[16–19] or for n = 9 and E = Si,[14] have
been structurally characterized. Homoatomic [E6]2ꢀ ions have
only been isolated as derivatives stabilized by Lewis acids. In
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Chemie
(10 ” 2 + 2). Therefore, [Pb10]2ꢀ is the first empty 10-atom
closo cluster of a Group 14 element that has been unambig-
uously characterized.
Ar-filled glove box. The metallic compound of nominal composition
“K4Pb9” was synthesized by heating the elements at 3508C for 8 h in a
stainless-steel tube. X-ray diffraction analysis showed that, apart from
amorphous components, “K4Pb9” consists of a mixture of the phases
K4Pb9 and KPb. Ethylene diamine (Merck) was distilled over calcium
hydride and used immediately after collection. [P(C6H5)3AuCl] was
synthesized fromHAuCl 4 (ChemPur) and P(C6H5)3 (Merck), accord-
ing to a known procedure,[32] and dried under vacuumfor 8 h. The
cryptand 2.2.2-crypt (Merck) was also dried under vacuumfor 8 h.
The “K4Pb9” compound (303 mg, 0.15 mmol) and 2.2.2-crypt (162 mg,
0.4 mmol) were dissolved in ethylene diamine (4 mL) and stirred for
2 h. The red-brown solution was separated fromthe insoluble residue
by filtration onto [P(C6H5)3AuCl] (75 mg, 0.15 mmol), stirred for 1 h,
and collected after filtration through tightly packed glass wool. EDX
analysis (JEOL-SEM 5900LV) of the filtration residue indicated the
presence of Pb and Au. After 2 days, dark brown to black platelike
crystals formed in the dark red solution (ca. 15% yield). The crystals
of 1 are embedded in a voluminous Pb sponge. Attempts to obtain
207Pb NMR spectra of the intensely colored brown DMF solutions
that result fromthe digestion of the solid have not yet been successful.
ꢀ
The shortest Pb Pb contacts occur between the atoms of
opposite square planes in the 1a antiprism(3.068(1) ꢁ d1 ꢁ
3.093(2) Š, 1(d1) = 3.082 Š), as well as between these atoms
and the capping atoms (3.036(1) ꢁ d2 ꢁ 3.109(1) Š, 1(d2) =
ꢀ
3.074 Š). The Pb Pb distances within the square planes
(3.294(1) ꢁ d3 ꢁ 3.339(1) Š, 1(d3) = 3.310 Š) are considera-
bly longer; therefore, the [Pb10]2ꢀ cage can be described as a
slightly compressed bicapped square antiprism. The distance
sequence d1 ꢂ d2 < d3 is similar to that found in the [B10H10]2ꢀ
ion.[24] Moreover, the structure of 1a is comparable to that of
the binary cluster [Ni@Pb10]2ꢀ (2a) recently published by
Eichhorn et al. (Figure 2b).[20] With respect to the shortest
average bond length 1(d2)of 1a, the relative lengths d1:d2:d3
are 1.00:1:1.08 for 1a and 1.01:1.02:1.11 for 2a. The
endohedral nickel atomwidens the cage ever so slightly (by
less than 3% for d3). The similarity of the anions is also
reflected in the fact that 1 can be described as a defect variant
of [{K(2.2.2-crypt)}2NiPb10] (2).[20]
Received: November 4, 2005
Published online: April 19, 2006
Apart fromthe [Si 9]2ꢀ ion, which is significantly distorted
fromthe expected D3h symmetry, only two other closo clusters
with n > 5 have been described for the Group 14 elements.
However, owing to unclear charge distribution ([Ge9]2ꢀ), or
inadequate structural characterization ([Ge10]2ꢀ),[25] these
clusters do not allow a concrete correlation between the
number of bonding skeletal electrons and the polyhedral
structure. The anion described as closo-[Ge9]2ꢀ has a structure
that deviates significantly fromthe ideal tricapped-trigonal-
prismatic structure (one of the three edges of the prism is
considerably longer than the others) and is only found in salts
that also contain the [Ge9]4ꢀ ion.[26] As many of the
compounds containing two different E9 anions (E = Ge, Sn,
Pb) are paramagnetic,[27,28] the charge distribution
[Ge9]2ꢀ/[Ge9]4ꢀ is not certain. The only example of a 12-
atomcluster of a Group 14 element is found in the formof the
endohedrally filled anion [Pt@Pb12]2ꢀ.[21]
Keywords: cluster compounds · cryptands · lead · Wade’s rules ·
Zintl anions
.
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Thus, the [Pb10]2ꢀ ion is not only the first unambiguously
characterized representative of its class, but owing to its
similarity with the filled analogue [Ni@Pb10]2ꢀ, is also an
ꢀ
attestation that the Ni Pb bonds are not necessary for the
formation of a stable 10-atom Pb cluster. The existence of
main-group-element polyhedra in both empty and endohe-
drally filled states, had previously only been demonstrated for
the fullerenes.[29] The [Pb10]2ꢀ ion described herein offers
striking evidence that this characteristic is also encountered in
Zintl ions. Remarkably, clusters of the heaviest member of
Group 14, namely Pb, are involved, despite the fact that these
cages are considered to be far less rigid than the fullerenes.[30]
The existence of empty as well as filled Zintl anions of
Group 14 elements, thus, represents yet another striking
parallel between the fullerene cages and the polyhedral Zintl
ions.[21,22,31]
Experimental Section
All manipulations and reactions were performed in an Ar atmos-
phere, using standard Schlenk techniques. Solids were weighed in an
[23] a) Single crystals of 1 were fixed on glass capillaries. The
structure was solved by direct methods (SHELXS-97[23b]) and
refined by full-matrix least-squares calculations against F2
Angew. Chem. Int. Ed. 2006, 45, 3459 –3462
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3461

Communications
(SHELXL-97[23c]). All non-H atoms were refined with aniso-
tropic displacement parameters. Crystal size: 0.2 ” 0.1 ”
0.006 mm; cell parameters at 135 K: a = 24.682(2), b =
11.862(1), c = 21.271(2) Š, b = 97.23(1)8, V= 6178(1) Š3; space
group C2/c (No. 15), Z = 4, 1calcd = 3.121 gcmꢀ3, m = 27.322 mmꢀ1
,
numerical absorption correction (Tmin = 0.07, Tmax = 0.85); data
collection: Oxford Diffraction Xcalibur3 diffractometer, MoKa
radiation, 2qmax = 25.318, 34647 measured reflections, 5628
independent reflections,
Rint = 0.058; R1 = 0.059 and wR2 =
0.130 for I > 2s(I), R1 = 0.089 and wR2 = 0.145 for all data.
CCDC-288640 (1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
fromThe Cabmridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif;
b) G. M.
Sheldrick.
SHELXS-97, Programfor the Solution of Crystal Structures,
Universität Göttingen, 1997; c) G. Sheldrick. SHELXL-97,
Programfor the Refinement of Crystal Structures, Universität
Göttingen, 1997.
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[25] The structure of the only previously characterized compound
containing a 10-atom closo cluster of a Group 14 element,
[Ge10]2ꢀ [33]
has not been satisfactorily determined, owing to a
,
threefold rotational disorder of the cluster. Analysis of crystals
with similar cell parameters and symmetry has also led to an
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[26] C. H. E. Belin, J. D. Corbett, A. Cisar, J. Am. Chem. Soc. 1977,
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[30] 207Pb NMR spectra of the anions [Pb9]4ꢀ and [Ni@Pb10]2ꢀ show
only one signal: R. W. Rudolph, W. L. Wilson, F. Parker, R. C.
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[31] T. F. Fässler, Angew. Chem. 2001, 113, 4289; Angew. Chem. Int.
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[33] C. Belin, H. Mercier, V. Angilella, New J. Chem. 1991, 15, 931.
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