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Phil. Trans. R. Soc. A (2010) 368, 1265–1284
doi:10.1098/rsta.2009.0270
REVIEW
Polyhedral nine-atom clusters of tetrel elements
and intermetalloid derivatives
B Y S ANDRA S CHARFE AND T HOMAS F. F ÄSSLER*
Department of Chemistry, Technische Universität München, Munich, Germany
Homoatomic polyanions have the basic capability for a bottom-up synthesis of
nanostructured materials. Therefore, the chemistry and the structures of polyhedral
nine-atom clusters of tetrel elements [E9 ]4− is highlighted. The nine-atom Zintl ions
are available in good quantities for E = Si–Pb as binary alkali metal (A) phases of the
composition A4 E9 or A12 E17 . Dissolution or extraction of the neat solids with aprotic
solvents and crystallization with alkali metal-sequestering molecules or crown ethers leads
to a large variety of structures containing homoatomic clusters with up to 45 E atoms.
Cluster growth occurs via oxidative coupling reactions. The clusters can further act as
a donor ligand in transition metal complexes, which is a first step to the formation of
bimetallic clusters. The structures and some nuclear magnetic resonance spectroscopic
properties of these so-called intermetalloid clusters are reviewed, with special emphasis
on tetrel clusters that are endohedrally filled with transition metal atoms.
Keywords: Zintl ions; intermetalloid cluster; polyhedron; wade rules; nano-materials;
main group element
1. Introduction
In 1891, Joannis described the beginning of the fascinating field of the chemistry
of polyanions of main group elements in Comptes Rendues: ‘Lorsque l’on met
une baguette de plomb pur en excés, en presence du sodammonium, on constate
que la liquour mordorée ne trade pas à venir bleue au contact du plomb, puis
verde’ (Joannis 1891). This report debuted the first surveillance of the green
polyanion—later determined as [Pb9 ]4− —from the reaction of Na and Pb in
liquid ammonia and opened the important field of deltahedral anions. These
homoatomic clusters are not only fascinating because of the aesthetic simplicity
of their polyhedral structures, but comprise an enormous synthetic potential.
The investigation of the versatile chemical reactivity of the anions revealed their
capability as precursors for the synthesis of nanostructured materials or large,
intermetalloid clusters.
*Author for correspondence ([email protected]).
One contribution of 13 to a Theme Issue ‘Metal clusters and nanoparticles’.
1265
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S. Scharfe and T. F. Fässler
2. Homoatomic clusters of tetrel elements
(a) Brief review of homoatomic polyanions
Since Joannis, the successive research work of Kraus, Smyth and Zintl (Smyth
1917; Kraus 1924; Zintl & Harder 1931) established the existence of highly
charged element particles. The imagination of tetrel element polyanions was
finally verified by the crystal structure determination of a [Sn9 ]4− ion in 1976
(Kummer & Diehl 1970) and ever since has developed into a fast growing research
field (Corbett 1985; Fässler 2001).
The smallest deltahedral clusters [E4 ]4− were first observed in alloys of alkali
metals (A = Na, K, Rb, Cs) and tetrels (E = Si, Ge, Sn, Pb) in a 1 : 1 ratio,
with NaPb being the first reported representative (Marsh & Shoemaker 1953).
Applying a salt-like description (Zintl–Klemm–Busmann concept) to these AE
phases (Zintl 1939; Klemm 1958), the valence electron of the alkali metal is
formally transferred to an anionic substructure. According to the (8 − N ) rule, the
tetrel elements form P4 -analogous tetrahedral clusters [E4 ]4− , which are retained
in a liquid ammonia solution of RbPb as shown by Korber and co-workers, who
recently obtained the ammoniate Rb4 Pb4 (NH3 )2 (Wiesler et al. 2009).
In order to promote the crystal growth of compounds containing polyanions,
alkali metal ion-sequestering agents such as [2.2.2]crypt (= 4,7,13,16,21,24hexaoxa-1,10-diazabicyclo-[8.8.8]hexacosane) (Corbett & Edwards 1977) or
alkali metal ion-complexing molecules such as [18]crown-6 (= 1,7,10,13,16hexaoxacyclooctadecane) and its derivatives (Fässler & Hoffmann 1999a) were
introduced.
The crystal structures of the D3h -symmetric [Sn5 ]2− and [Pb5 ]2− anions were
elucidated as long ago as 1977 (Edwards & Corbett 1977) and [Ge5 ]2− was
discovered 20 years later (Campbell & Schrobilgen 1997). They were obtained
from ethylenediamine solutions of A–E alloys with different ratios of A : E under
the stoichiometric deficiency of [2.2.2]crypt. For the synthesis of [Si5 ]2− , the
binary compound Rb12 Si17 was extracted with liquid ammonia in the presence
of [2.2.2]crypt and subsequently treated with triphenylphosphine (Suchentrunk &
Korber 2006). Intermetallic compounds of the general composition A12 E17 contain
tetrahedral [E4 ]4− and [E9 ]4− polyanions in the ratio 2 : 1 (A = Na, K, Rb, Cs;
E = Si, Ge, Sn) (von Schnering et al. 1997; Hoch et al. 2003).
The largest unfilled representative of a homoatomic tetrel element cluster is
the 10-vertices closo cluster [Pb10 ]2− , which was obtained after the oxidation
of an ethylenediamine solution of K4 Pb9 with PPh3 Au(I)Cl (Spiekermann
et al. 2006).
Homoatomic, ligand-free tetrel clusters with six, seven, eight or more than 10
vertices have not been detected in solutions so far. Octahedral [E6 ]2− clusters
have been isolated only by an organometallic synthetic route as transition metalstabilized species [{EM(CO)5 }6 ]2− (E = Ge, Sn; M = Cr, Mo, W) consisting of
an almost perfect E6 octahedron with each E atom coordinated to an M(CO)5
fragment, as shown for the first time for E = Sn and M = Cr (Schiemenz &
Huttner 1993). However, the anion [{SnCr(CO)5 }6 ]2− was produced when
Na2 [Cr2 (CO)10 ] was treated with SnCl2 in the presence of bipy (bipy = 2,2 bipyridine) (Kircher et al. 1998) and not from Zintl ion solutions. From
gas-phase experiments as well as from theoretical investigations, a particular
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Review. Clusters of tetrel elements
1267
stability is expected for the stannaspherene [Sn12 ]2− and the plumbaspherene
[Pb12 ]2− , although these species have not been obtained in solid state up to now
(Cui et al. 2006).
The structures of tetrahedral [E4 ]4− , trigonal bipyramidal [E5 ]2− and bicapped
quadratic antiprismatic [E10 ]2− clusters strictly follow Wade’s rules corresponding
to a 12 skeletal electron (ske) nido, 12 ske closo and 22 ske closo cluster,
respectively (Wade 1976). Each group 14 atom possesses four valence electrons,
two of which remain as one ‘lone pair’ at each vertex equivalent to the covalent
exo-BH bonds in boranes.
Nine-atom Zintl clusters of tetrel elements are most frequently examined and
best described. They display a complex and exciting structural flexibility, which
is described in detail in the following section.
(b) Nine-atom clusters
Nine-atom clusters are prominent in the intermetallic compounds A4 E9
(A = Na, K, Rb, Cs; E = Ge, Sn, Pb), of which Cs4 Ge9 was the first one
described in solid state by Sevov and co-workers in 1997 (Quéneau & Sevov
1997). The crystals often show twinning problems and disordered and distorted
E9 clusters (Ponou & Fässler 2007). Extraction of these intermetallic compounds
with polar aprotic solvents such as ethylenediamine (en), liquid ammonia and
dimethylformamide (dmf) leads to highly concentrated and intensively coloured
solutions of the nine-atom clusters. However, all reported experiments for the
isolation of nine-atom silicon clusters from solutions started from A12 Si17 (A = K,
Rb or K/Rb) Zintl phases (Goicoechea & Sevov 2006b; Sevov & Goicoechea 2006).
The nine-atom deltahedral closo cluster (n = 9) corresponds to a tricapped
trigonal prism I with D3h point group symmetry (figure 1) if 2n + 2 = 20 ske are
available for the cluster skeleton bonding. Hence, clusters with a twofold negative
charge [E9 ]2− are appropriate to adopt a closo structure. Consequently, [E9 ]4−
is on a par with a nido-type cluster owing to 2n + 4 = 22 ske forming a C4v symmetric monocapped square anti-prism IV (figure 1). The shape of a 21 ske
cluster [E9 ]3− therefore should appear between these two boundary structures.
Distortions forming the boundary structures I and IV are also observed for
22 ske clusters [E9 ]4− in neat solids A4 E9 as well as in clusters crystallized
from solution. Since the lowest unoccupied molecular orbital (LUMO) of the
hypothetical ideal D3h -symmetric [E9 ]2− cluster has anti-bonding character along
the trigonal prism heights, the addition of further electrons (formal reduction)
leads to a prism elongation, and distorted C2v -symmetric trigonal prisms with one
(III), two (II) or three elongated heights result (figure 1). One strongly elongated
height leads—after relaxation of all atomic positions—to structure IV, while
three equally elongated bonds preserve the D3h symmetry of the cluster. The two
examples [K([18]crown-6)]3 K[Sn9 ] (Fässler & Hoffmann 1999a) and [K([18]crown6)]4 [Pb9 ] (Fässler & Hoffmann 1999b) show that, in spite of their unambiguously
determined fourfold negative charge, E9 clusters can occur either with perfect
C4v -symmetric nido structures or with those closer to D3h symmetry (figure 2).
The energy barrier for the intermolecular conversion between the D3h -closo and
the C4v -nido structure is indeed very low (Rosdahl et al. 2005), and 119 Sn and
207
Pb nuclear magnetic resonance (NMR) experiments on [Sn9 ]4− and [Pb9 ]4−
solutions show only a single resonance indicating a fast atom exchange on the
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1268
S. Scharfe and T. F. Fässler
[E4]4–
[E5]2–
Td
D3h
[E9]x–
D3h (I)
C2v (II)
C2v (III)
C4v (IV)
[E10]2–
D4d
Figure 1. Structures and point groups of known homoatomic tetrel clusters.
(a)
(b)
Figure 2. Structural details of (a) [K([18]crown-6)]4 [Pb9 ] (Fässler & Hoffmann 1999b) with C4v
symmetry and (b) [K([18]crown-6)]3 K[Sn9 ] (Fässler & Hoffmann 1999a) with C2v symmetry.
NMR time scale (Rudolph et al. 1978). The results of a large number of crystal
structure determinations of [E9 ]4− -containing representatives (E = Si, Ge, Sn,
Pb) obtained from liquid ammonia and ethylenediamine solutions reveal the
large flexibility of the polyhedral cages (Fässler 2001). Careful examination of
the atomic displacement parameters of the E atoms in many cases shows that
the static structure usually obtained by this method does not allow a distinct
assignment to one of the boundary structures (Rosdahl et al. 2005). In particular,
the cluster charge cannot be clearly assigned on the basis of the appearing
structures.
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Review. Clusters of tetrel elements
1269
Structures of [E9 ]3− with a singly occupied LUMO of [E9 ]2− anions should
integrate between the closo and the nido form, and adopt a C2v -symmetric shape
that derives from a tricapped trigonal prism with one or two elongated prism
heights (figure 1; II and III). Indeed, all these [E9 ]3− units adopt the shape
of a distorted tricapped trigonal prism tending towards C2v symmetry, except
for [Sn9 ]3− , which surprisingly retains almost perfect D3h symmetry with three
elongated prism heights. Nine-atom clusters with a threefold negative charge
[E9 ]3− were isolated from solution for all heavier group 14 elements (E = Si
to Pb) exclusively as [A+ ([2.2.2]crypt)] salts (Fässler et al. 2000; Fässler 2001;
Goicoechea & Sevov 2004; Yong et al. 2005b). Bonding interactions between the
anionic and the cationic units do not occur in these compounds. Cation–anion
contacts stabilize higher charged clusters [E9 ]4− or support the formation
of dimers. An [E9 ]2− cluster was unambiguously isolated and structurally
characterized only in the case of K([18]crown-6)]2 Si9 (py) (py = pyridine) for
E = Si. This compound is obtained again by the extraction of K12 Si17 . The
closo-[Si9 ]2− cluster noticeably deviates from an ideal D3h -symmetric structure
expected according to its 20 ske. The [Si9 ]2− cluster interacts with two K
counterions via two deltahedral cluster faces, and the shape of the cluster is
best described as a distorted tricapped trigonal prism with one elongated prism
height (Goicoechea & Sevov 2005c).
Powdered samples of [K+ ([2.2.2]crypt)] salts of [E9 ]3− show anisotropic electron
paramagnetic resonance (EPR) signals with increasing line widths from Ge to
Pb. Magnetic measurements of [K([2.2.2]crypt)]6 E9 E9 (en)1.5 (tol)0.5 (E = Sn, Pb;
tol = tolyl) with one ordered and one disordered E9 cluster per asymmetric unit
reveal that only 50 per cent of the clusters are paramagnetic. In consequence, a
threefold negative charge was assigned to the ordered clusters and a superposition
of diamagnetic [E9 ]2− and [E9 ]4− species was assumed for the disordered (Fässler &
Schütz 1999; Fässler et al. 2000).
3. Reactions of E9 clusters
(a) Oxidative coupling reactions
The formation of stable radicals [E9 ]3− also offers the possibility of a cluster
dimerization. To date, such oxidative coupling reactions have been reported
solely for Ge9 clusters (figure 3), which mostly contain isomer A (Sevov &
Goicoechea 2006).
Two different isomers (A and B) were observed in K6 (Ge9 –Ge9 )(dmf)12
(Nienhaus et al. 2006). Conformer A is the result of linking two nido-shaped
Ge9 clusters via an exo-bond at one atom of the open rectangular cluster face.
The shape of the clusters deviates slightly from C4v symmetry, and in general the
diagonal of the open face that points towards the exo-bonded atom is significantly
shorter than the second diagonal. The interconnecting exo-bond is collinear with
the shorter diagonal of the open rectangular face of each cluster. In B, the
connecting exo-bond between the Ge9 clusters approximately points to both
cluster centres. The shape of these clusters derives from a tricapped trigonal prism
with two strongly elongated prism heights leading to two rectangular cluster faces
according to II (two elongated heights). The connection occurs through the Ge
atom that links the related rectangular faces.
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S. Scharfe and T. F. Fässler
(a)
(b)
A
B
(c)
Figure 3. (a,b) Two different dimeric [Ge9 –Ge9 ]6− units with the next four coordinated alkali
metal atoms (Nienhaus et al. 2006). (c) Polymeric ∞1 {[Ge9 ]2− } with coordinated alkali metal atoms
(Downie et al. 2000).
In all cases several alkali cations coordinate with the atoms of [Ge9 –Ge9 ]6− .
Consequently, the complete encapsulation of the alkali metal atoms by the
sequestering agent seems to be the crucial factor for the stabilization of free
cluster radicals in the crystals.
Further oxidation formally leads to [Ge9 ]2− species, which in principle can be
interconnected in two ways. (i) In analogy to the formation of dimers, a further
linkage leads to a one-dimensional cluster polymer 1∞ {[Ge9 ]2− } (figure 3c) with
two external two-electron–two-centre bonds at each cluster (Downie et al. 2000).
The connection proceeds via two opposite vertex atoms of the open faces of the
nido clusters. In the structure of [K([18]crown-6)]2 {1∞ [Ge9 ]2− }(en), one K atom of
the two [K([18]crown-6)] units caps a triangular cluster face, while the other one
serves as spacer, causing a separation of the polymeric chains of more than 14 Å.
(ii) An alternative connection between formal [Ge9 ]2− clusters occurs in the linear
trimeric anion [Ge9 =Ge9 =Ge9 ]6− (Ugrinov & Sevov 2002; Yong et al. 2005a) and
the linear tetrameric anion [Ge9 =Ge9 =Ge9 =Ge9 ]8− (Ugrinov & Sevov 2003; Yong
et al. 2004). In these units (figure 4), the cluster contacts are in the range from
2.546 Å to 2.752 Å and thus exceed the length of typical Ge–Ge single bonds
as found in Ge9 –Ge9 dimers (mean 2.477 Å) or elemental germanium (2.445 Å).
Further, the two almost parallel exo-bonds lead to a very unusual coordination of
the corresponding Ge atoms with Ge–Ge–Ge bond angles of approximately 90◦ .
Quantum chemical calculations demonstrate that the exo-bonds participate in a
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Review. Clusters of tetrel elements
(a)
[Ge9
Ge9
Ge9]6–
(b)
[Ge9
Ge9
Ge9
Ge9]8–
Figure 4. Oligomeric structures of linked Ge9 clusters including coordinated alkali metal
atoms A in (a) [K([18]crown-6)]6 [Ge9 =Ge9 =Ge9 ](en)3 (tol) (Yong et al. 2005a) and also in
(b) [Rb([18]crown-6)]8 [Ge9 =Ge9 =Ge9 =Ge9 ](en)2 (Ugrinov & Sevov 2003).
delocalized electron system that comprises the whole anion (Yong et al. 2004). The
individual clusters in [Ge9 =Ge9 =Ge9 ]6− and [Ge9 =Ge9 =Ge9 =Ge9 ]8− adopt the
shape of tricapped trigonal prisms, and since they are interconnected collinearly
to two of their three prism heights, the clusters are elongated along these heights
towards a C2v -symmetric polyhedron (see figure 1, II). It is worth mentioning
that most of the dimeric, oligomeric and polymeric cluster compounds can also
be obtained from solutions without the addition of any oxidation agent.
(b) Homoatomic E9 clusters bonded to d-block metals
Structural units with less than nine tetrel element atoms that serve as ligands
for d-block elements are reported rarely in the literature. As already pointed out,
the anion [{SnCr(CO)5 }6 ]2− contains the octahedral subunit [Sn6 ]2− in which
each tin atom coordinates to a Cr(CO)5 fragment (figure 5a; Kircher et al. 1998).
This example shows that the extended Wade’s rules for pure main group element
clusters are obeyed when the four valence electrons of each Sn atom are considered
as two ske and one electron ‘lone pair’. The ‘lone pair’ completes the 18-electron
configuration of the transition metal. In consequence, the central unit corresponds
to a 14-ske closo cluster (6×2 + 2 including the two negative charges). The ‘lone
pair’ points to the outside of the cluster and shows nicely that it can be treated
as the external B–H bond in boranes in the skeletal electron pair formalism
(Wade’s rules).
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S. Scharfe and T. F. Fässler
(a)
M
(b)
(c)
M
M
M
(d)
(e)
Ir
(f)
Cu
Zn
Figure 5. Examples of coordination compounds with Zintl ions: (a) [{SnCr(CO)5 }6 ]2−
(Schiemenz & Huttner 1993); (b) [Pb5 {Mo(CO)3 }2 ]4− (Yong et al. 2005d); (c) [E9 –M(CO)3 ]4−
(E = Sn, Pb; M = Cr, Mo, W) (Kesanli et al. 2001; Yong et al. 2005c); (d) [Sn9 –Ir(cod)]3− (Wang
et al. in press); (e) [Ge9 –Cu(Pi Pr)3 ]3− (Scharfe & Fässler in press); and (f ) [E9 –Zn(C6 H5 )]3−
(E = Si to Pb) (Sevov & Goicoechea 2006).
An anionic Pb5 unit has been stabilized in [Pb5 {Mo(CO)3 }2 ]4− synthesized
from an ethylenediamine solution of [Pb9 ]4− and (h6 -mesityl)Mo(CO)3 (figure 5b).
In this molecule, a planar Pb5 ring binds to two {Mo(CO)3 } fragments in a
h5 fashion (Yong et al. 2005d). Density functional theory (DFT) calculations
support that the anion is best described formally as a complex of an aromatic
2p electron system [Pb5 ]2− and two [Mo(CO)3 ]1− moieties. As mentioned in
§2(a), a ‘naked’ [Pb5 ]2− unit was obtained from ethylenediamine solutions of
[Pb9 ]4− as [K+ ([2.2.2]crypt)] salt, and adopts—in contrast to the Pb5 ring
in [Pb5 {Mo(CO)3 }2 ]4− —a closo D3h -symmetric trigonal bipyramidal structure
(figure 1; Edwards & Corbett 1977).
Owing to their facile accessibility, soluble homoatomic [E9 ]4− Zintl ions
(E = Ge, Sn, Pb) offer an excellent possibility for heteroatomic cluster formation.
The elements are fused at high temperatures, in most cases, with K in metal
ampoules in the ratio of K : E = 4 : 9. The resulting K4 E9 phases are soluble
in liquid ammonia, and—more conveniently—in ethylenediamine or dmf, as
introduced in 1970 (Kummer & Diehl 1970) and 2006 (Nienhaus et al. 2006),
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Review. Clusters of tetrel elements
1273
respectively. A simple and efficient one-pot synthesis for Sn9 and Pb9 clusters is
the reduction of the corresponding elements with Na or K in liquid [18]crown-6
at 40◦ C (Fässler & Hoffmann 1999a), or by applying electrochemical methods
(Pons et al. 1981; Eisenmann 1993).
The first representative of E9 complexes in which the nido cluster coordinates
with four atoms of its open face to a transition metal of group 6 [(h4 E9 )M(CO)3 ]4− (E = Sn, Pb; M = Cr, Mo, W) was already presented in 1988
with E = Sn and M = Cr (Eichhorn & Haushalter 1988) and the whole series
was completed in 2005 (Eichhorn & Haushalter 1990; Kesanli et al. 2001;
Campbell et al. 2002; Yong et al. 2005c; figure 5c). The isomerization of the
resulting closo clusters with h4 coordination leads to h5 -E9 complexes analogous
to the above-mentioned [(h5 -Pb5 ){Mo(CO)3 }2 ]4− unit (figure 5b). Recently, other
complexes involving d-block elements have been isolated: [Sn9 –Ir(cod)]3− (cod =
cyclooctadiene) (Wang et al. in press), [Ge9 –Cu(Pi Pr)3 ]3− (Scharfe & Fässler
in press), [E9 –Zn(C6 H5 )]3− (E = Si–Pb) (Sevov & Goicoechea 2006) and [Sn9 –
Cd(Snn Bu3 )]3− (Zhou et al. 2009a). All these compounds exclusively adopt an h4
coordination in the solid state (figure 5d–f ).
According to the isolobal concept, M(CO)3 fragments with M = Cr, Mo and
+
(M = Zn, Cd and R = C6 H5 and
W as well as Ir(cod)+ , CuPR+
3 and MR
Sn(alkyl)3 ) units act as zero electron building blocks. Therefore, and based on
the Wade/Mingos rules, the cluster expansion by a transition metal fragment
corresponds to the transformation of a nine-atom nido cluster to a 10-atom
closo cluster with 2×9 + 4 = 22 ske and 2×10 + 2 = 22 ske, respectively, since
the number of cluster vertices is increased by one but the number of delocalized
skeleton electrons remains unchanged.
(c) Ligand-free heteroatomic cluster: intermetalloids
The first ligand-free complex of a Zintl ion with a d-block element was obtained
as a polyanionic polymer chain {1∞ [HgGe9 ]}2− (figure 6a; Nienhaus et al. 2002).
Ever since, the formation of heterometallic ligand-free anions allows an approach
of intermetallic phases from a bottom-up synthesis using E9 Zintl clusters as
building blocks. In {1∞ [HgGe9 ]}2− Ge–Hg contacts covalently bridge the Ge9 nido
clusters. Each exo-bond of the [Ge9 ]4− nido cluster reduces the charge by one.
With two exo-bonds, the clusters carry a charge of −2.
The anion [(h4 -Ge9 )Cu(h1 -Ge9 )]7− (figure 6b) is closely related to [(h4 -Ge9 )Cu–
i
P( Pr)3 ]3− (figure 5e; Scharfe & Fässler in press). Both are synthesized from
K4 Ge9 and i Pr3 PCu(I)Cl, and [(h4 -Ge9 )Cu(h1 -Ge9 )]7− represents a rare example
where a homoatomic E9 cluster acts as a two-electron s-donor ligand similar to
[{SnCr(CO)5 }6 ]2− (Schiemenz & Huttner 1993).
Similar reactions starting from the analogous gold compound Ph3 PAu(I)Cl
lead to the related cluster [Ge9 Au3 Ge9 ]5− shown in figure 6c (Spiekermann et al.
2007b), in which three singly positively charged Au atoms bridge two Ge9
clusters via triangular Ge faces. As a result, the Au atoms form a triangle
that is almost coplanar with the two deltahedral faces of the interconnected
[Ge9 ]4− clusters. Rather short Au–Au contacts hint at aurophilic interactions
(Schmidbaur 1999). The positive charge of the Au atoms was confirmed by DFT
calculations.
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S. Scharfe and T. F. Fässler
(a)
Hg
n
(b)
(c)
Cu
Au
(d)
Au
Figure 6. Zintl ions with ligand-free d-block metal atoms: (a) {1∞ [HgGe9 ]}2− (Nienhaus et al. 2002);
(b) [(h4 -Ge9 )Cu(h1 -Ge9 )]7− (Scharfe & Fässler in press); (c) [Ge9 Au3 Ge9 ]5− (Spiekermann et al.
2007b); and (d) [Au3 Ge45 ]9− (Spiekermann et al. 2007a).
A combination of the oxidative coupling reaction described in §3(a) and the
complexation of the resulting polyanion reported in §3(b) leads to larger clusters
reaching the nanometre scale. [Au3 Ge45 ]9− was found as a by-product of the
reaction of ethylenediamine solutions of K4 Ge9 and Ph3 PAu(I)Cl (figure 6d;
Spiekermann et al. 2007a). By changing the reaction conditions, the yield of this
fascinating large cluster can be raised. Within this anion, four deltahedral Ge9
clusters are covalently connected by a unit of nine Ge atoms. The cluster displays
a variety of bonding schemes reminiscent of the structural chemistry of boranes:
multi-centre bonds arise in the deltahedral clusters, and covalent two-centre–
two-electron bonds link them together. Long Ge–Ge contacts within a triangle
of five-coordinated Ge atoms shown as dashed lines in figure 6d correspond to
three-centre–two-electron bonds. Furthermore, five-membered Ge faces resemble
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Review. Clusters of tetrel elements
(a)
(b)
(c)
Cu
(d)
M
Ni
Co
Figure 7. Representative Zintl ions with one endohedral metal atom: (a) [Cu@Eq ]3− (Scharfe et al.
2008); (b) [Ni@Pb10 ]2− (Esenturk et al. 2005)); (c) [M@Pb12 ]2− (Esenturk et al. 2006a); and
(d) [Co@Ge10 ]3− (Wang et al. 2009).
a structural motif found in three-dimensional solids, such as alkali metal and
alkaline earth metal germanides with clathrate structures. Ge–Au contacts in
[Au3 Ge45 ]9− confine the cluster to a more or less spherical shape. Interestingly,
[Au3 Ge45 ]9− exhibits no Au–Au contacts, despite the presence of a largely negative
cluster charge (Spiekermann et al. 2007a).
(d) Endohedrally filled clusters: intermetalloids
Backing up Schnöckel’s term ‘metalloid clusters’ (Schnepf & Schnöckel 2002),
the expression ‘intermetalloid clusters’ was introduced (Fässler & Hoffmann 2004)
for clusters consisting exclusively of atoms of at least two different (semi)metallic
elements. The complexes in §3(c) can already be seen as intermetalloids; however,
high coordination numbers as observed in intermetallic compounds are better
realized in endohedrally filled clusters. Such compounds might even topologically
exhibit similar structural motifs as found in related intermetallic compounds.
Endohedral, filled clusters—as shown in figure 7—can be formed by stripping
off the ligands in those complexes depicted in figure 5. The smallest transition
metal-filled Zintl clusters were found in [Ni@Ge9 ]3− (Sevov & Goicoechea 2006)
and [Cu@E9 ]3− for E = Sn and Pb (Scharfe et al. 2008; figure 7a). [Ni@Ge9 ]3−
was obtained from reactions of K4 Ge9 with Ni(cod)2 , whereas [Cu@E9 ]3−
resulted from reactions of Cu–MeS with the respective Zintl phases K4 Sn9 and
K4 Pb9 in dmf. In these compounds, the E9 cluster skeleton is retained, and
a transition metal is incorporated into it. Although the structure refinement
of [Ni@Ge9 ]3− suffers from an intense disorder, the cluster apparently adopts
the shape of a strongly distorted tricapped trigonal prism with unequally
elongated prism heights in the range of 3.089–3.560 Å. In [Cu@E9 ]3− , the
cluster atoms also build a tricapped trigonal prism, but with equally elongated
heights (Scharfe et al. 2008), and maximization of the strength of nine Cu–E
interactions leads to an almost perfect spherical shape of the nine-atom
cluster and not to the nido-type structure typical of 22 skeleton electrons
[E9 ]4− clusters. The prism heights in this anion differ from each other by
at most 5 per cent (E = Sn) or 6 per cent (E = Pb), but they are extended
by 17 per cent and 15 per cent, respectively, compared to empty clusters
with a similar skeleton shown in figure 1d. As a result of this elongation,
Phil. Trans. R. Soc. A (2010)
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1276
S. Scharfe and T. F. Fässler
(a)
Cu
(c)
35.7
(b)
22.6
286 Hz
7.7
1.6
0.2
δ
85 Hz
–1436
–1438
–1440
δ (ppm)
280 Hz
–1442
–1444
–324
–328 –332
δ (ppm)
–336
Figure 8. NMR spectra of [Cu@Sn9 ]3− . (a) Spherical shape of the intermetalloid cluster.
(b) Experimental 119 Sn NMR spectrum (d = −1440 ppm, 1 : 1 : 1 : 1 quartet, J (119 Sn–63/65 Cu)
= 286 Hz, two satellites with J (119 Sn–117 Sn) = 85 Hz). (c) Experimental (d = −332 ppm,
J (119 Sn–63 Cu) = 280 Hz) and simulated (insert) 63 Cu NMR spectrum. The spectra are recorded in
acetonitrile at room temperature. Simulations account for coupling to various 119/117 Sn isotopomers
[Cu@119/117 Snx Sn(9−x) ]3− (with x = 0, 1, . . . , 4) (Scharfe et al. 2008).
the Cu–E distances vary in a very narrow range of ±0.05 Å with slightly
longer contacts to the capping atoms. The clusters can be considered as
paramagnetic [Ni0 @(Ge9 )3− ] as confirmed by EPR measurements (Sevov &
Goicoechea 2006), and [Cu+ @(E9 )4− ] reveals sharp 119 Sn, 207 Pb and 63 Cu NMR
signals as expected for diamagnetic compounds (Scharfe et al. 2008). NMR
experiments on [Cu@E9 ]3− in solution again nicely illustrate the structural
flexibility of the E9 clusters. The 119 Sn NMR spectrum of [Cu@Sn9 ]3− at room
temperature (figure 8b) displays a single resonance, which indicates that all
Sn atoms are time-averaged in solution. At room temperature, [Cu@Sn9 ]3−
also provides one sharp 63 Cu NMR (figure 8c). The high-resolution satellite
pattern permits the rare determination of the 63 Cu–119 Sn coupling constant. The
observed resonance frequency of −332 ppm in [Cu@Sn9 ]3− is in the range of other
Cu(I) compounds, and therefore the Cu atom is assumed to have the formal
Phil. Trans. R. Soc. A (2010)
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Review. Clusters of tetrel elements
1277
oxidation state +1. The charge of Cu(I) was also confirmed by DFT calculations.
Both the 119 Sn and 63 Cu NMR spectra do not change upon cooling to −40◦ C
(Scharfe et al. 2008).
The endohedral 10-vertices cluster [Ni@Pb10 ]2− (figure 7b) was found from
the reaction of Ni(cod)2 with K4 Pb9 in ethylenediamine (Esenturk et al. 2005).
Herein, the Ni atom occupies the centre of a bicapped square anti-prism as
predicted from DFT calculations. 207 Pb NMR studies again revealed a fast atom
exchange of the cluster in solution, since only one (broad) signal was detected at
−996 ppm (Esenturk et al. 2005).
As a minor by-product from the reaction of Ni(cod)2 with K4 Pb9 , the
icosahedral cluster [Ni@Pb12 ]2− was found (figure 7c; Esenturk et al. 2006a).
For the heavier homologues Pd and Pt, the isostructural clusters [Pd@Pb12 ]2−
and [Pt@Pb12 ]2− were obtained from the reaction of K4 Pb9 with Pd(PPh3 )4 and
Pt(PPh3 )4 , respectively (Esenturk et al. 2004, 2006a). Apparently, nine-atom
clusters must be oxidized during the reaction, which is ascribed to cod and/or
PPh3 . A comparison of solid-state structures of [M@Pb12 ]2− confirms that the
smaller the transition metal M is, the more strongly distorted is the Pb skeleton.
While the interstitial Pt atom is surrounded by an almost perfect icosahedron
with virtually equal Pb–Pb distances, these contacts vary over a larger range
with increasing variances for clusters with interstitial Pd and Ni atoms. The
cluster distortion manifests in the distribution of the M–Pb distance, which
increases from Pt to Ni. The arising anisotropy certainly indicates the instability
of the cluster, whereas for M = Ni the 10-vertices cluster is clearly favoured
(Esenturk et al. 2006a). In solution, only one 207 Pb NMR resonance was found for
[M@Pb12 ]2− (Ni: 1167 ppm, Pd: 1520 ppm, Pt: 1780 ppm), which either reflects
the high symmetry of the cluster, or can be ascribed to a fluxional behaviour
(Esenturk et al. 2006a).
Recently, we succeeded in the synthesis of an endohedral 12-atom tin cluster
[Ir@Sn12 ]3− (Wang et al. in press). It is apparently formed in a stepwise reaction
that starts from an ethylenediamine solution of K4 Sn9 and [IrCl(cod)]2 . At first,
the Ir(cod)-capped Sn9 cluster is formed (figure 5d). If the ethylenediamine
solution of this 10-vertices cluster is heated to 80◦ C, it is—under the loss of
the cod ligand—transformed into [Ir@Sn12 ]3− . This reaction is accelerated by the
addition of PPh3 or dppe (1,2-bis(diphenylphosphino)ethane). The cluster can
be formulated as [Ir1− @(Sn12 )2− ], and the cluster skeleton fulfils Wade’s rules.
The 12 tin atoms form an almost perfect icosahedron, with Sn–Sn distances in a
slightly larger range than those found for the Pb–Pb contacts in [Pt@Pb12 ]2− . No
analogous germanium compound displaying a metal atom-centred Ge12 cluster
has been reported to date.
The family of intermetalloid clusters was recently expanded to electronpoor transition elements such as Co, and a non-deltahedral structure much
more similar to structures observed in intermetallic compounds was found.
The anion [Co@Ge10 ]3− (figure 7d) with an unprecedented structure emerged
during the reaction of K4 Ge9 with Co(C8 H12 )(C8 H13 ) (Wang et al. 2009).
Although all the tetrel atom clusters obtained from solution so far have had
deltahedral structures, the germanium atoms in [Co@Ge10 ]3− build a D5h symmetric pentagonal prism that comprises no triangular faces at all. This
unprecedented cluster shape indicates that neither its electron count nor its
bonding situation follow conventional rules. From Wade’s rules a D4d -symmetric
Phil. Trans. R. Soc. A (2010)
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1278
S. Scharfe and T. F. Fässler
(b)
(a)
Ni
Pt
Figure 9. Representative Zintl ion complexes with an endohedral metal atom: (a) [(Ni@Sn9 )Ni–
CO]3− and (b) [(Pt@Sn9 )Pt–PPh3 ]3− (Kesanli et al. 2002).
bicapped square anti-prism would have been forecast for the shape of this species,
and DFT calculations performed for [Co@Ge10 ]3− revealed that the potential
surface for this cluster is rather flat, with an energy difference of 13.3 kcal mol−1
between the stationary points. Therefore, it can easily transform between
the favoured D5h -symmetric and the energetically unpropitious D4d -symmetric
arrangement (Wang et al. 2009). The same calculations for [Ni@Ge10 ]2− revealed
an energetic minimum for the D4d geometry, while the pentagonal prism
corresponds to a ground state of 5.33 kcal mol−1 (King et al. 2008; Wang
et al. 2009). Thus for these clusters the global minimum cannot be determined
indisputably, whereas the empty [Ge10 ]2− unit preferentially adopts a bicapped
square anti-prism (King et al. 2006), which was also found for [Cu@Ge10 ]−
and [Zn@Ge10 ] (King et al. 2008). Natural bond orbital (NBO) analyses of
[Co@Ge10 ]3− pointed to a natural charge of −1.05 for the incorporated Co atom
and consequently a d10 electron configuration (Wang et al. 2009). Interatomic
Co–Ge and Ge–Ge distances in [Co@Ge10 ]3− resemble the bonding situation in
the binary intermetallic compound CoGe2 . In the latter, the Co atom is located in
a distorted cubic cavity of eight Ge atoms. Recently, the formation of the cluster
[Fe@Ge10 ]3− was reported in a similar reaction of Fe(2,6-MeS2 C6 H3 )2 with K4 Ge9
(Zhou et al. 2009b). Owing to the odd number of electrons, the anion should be
paramagnetic; however, no magnetic data have been reported so far.
Precursors for larger intermetalloid units consisting of 17 or 18 tetrel atoms
are given by [(h4 -Ge9 )Cu(Pi Pr)3 ]3− (Scharfe & Fässler in press) as shown
in figure 5e, and also by a class of E9 clusters that are filled and are
simultaneously capped by a transition metal, such as the anions [(Ni@Sn9 )Ni–
CO]3− and [(Pt@Sn9 )Pt–PPh3 ]2− (Kesanli et al. 2002) shown in figure 9a,b,
respectively, as well as by modified Ge9 clusters [(Ni@Ge9 )Ni–R] (R = CO,
C–C–Ph, en) (Sevov & Goicoechea 2006). Assuming that a ligand exchange in
[Ge9 –Cu(Pi Pr)3 ]3− as discussed above also takes places in [(Pt@Sn9 )Pt–PPh3 ]2− ,
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1279
Review. Clusters of tetrel elements
(a)
(b)
(c)
(d)
Pd
Ni
Pt
Ni
Figure 10. Representative intermetalloid clusters with two endohedral metal atoms: (a)
[Ni2 @Sn17 ]4− (Esenturk et al. 2006b); (b) [Pt2 @Sn17 ]4− (Kesanli et al. 2007); (c) [Ni3 @Ge18 ]4−
(Goicoechea & Sevov 2005a); and (d) [Pd2 @E18 ]4− (Goicoechea & Sevov 2005b; Sun et al. 2007).
the formation of a cluster species that includes two transition metals and 18
tetrel atoms (see figure 10d) could be achieved. Four new clusters of higher
nuclearity, [Ni2 @Sn17 ]4− (Esenturk et al. 2006b), [Pt2 @Sn17 ]4− (Kesanli et al.
2007), [Pd2 @Ge18 ]4− (Goicoechea & Sevov 2005b) and [Pd2 @Sn18 ]4− (Kocak
et al. 2008), were synthesized in ethylenediamine solution from Ni(cod)2 ,
Pt(PPh3 )4 and Pd(PPh3 )4 , respectively (figure 10a,b and d). Cluster formation
requires the partial oxidation of the Sn9 clusters along with the reduction
of cyclooctadiene probably also involving the solvent ethylenediamine. In
[Ni2 @Sn17 ]4− , two [Ni@Sn9 ]2− subunits are connected via one apex-like, central
Sn atom that participates in both Sn9 clusters (figure 10a; Esenturk et al.
2006b). The cluster shapes deviate significantly from deltahedral structures.
With a more open shape, [Ni2 @Sn17 ]4− displays D2d point group symmetry,
and the central Sn atom is surrounded by a pseudo-cube of eight Sn atoms.
The high coordination number of the central Sn atom causes remarkably
long Sn–Sn contacts that are more common in solid-state compounds such
as BaSn5 , in which the Sn atoms even reach a 12-fold coordination by other
tin atoms (Fässler et al. 2001). The dynamic behaviour of this cluster was
examined by temperature-dependent 119 Sn NMR experiments in dmf solutions
(Esenturk et al. 2006b). At a temperature of −64◦ C, four resonances appear
in the 119 Sn NMR spectrum (−1713, −1049, −1010 and +228 ppm). They
are assigned to the different cluster atoms via peak intensity analysis, and
the central Sn atom generates the most deshielded signal. At 60◦ C only one
signal is observed at −1167 ppm, indicating fast atom exchange reactions.
In contrast, the analogous [Pt2 @Sn17 ]4− unit (figure 10b) generates only one
119
Sn NMR resonance over the whole temperature range (−742.3 ppm) and
is apparently fluxional in dmf solution even at −60◦ C (Kesanli et al. 2007).
The solid-state structure of [Pt2 @Sn17 ]4− differs significantly from that of the
isoelectronic [Ni2 @Sn17 ]4− . In the former, the Sn17 cluster encloses two Pt atoms
at a distance of 4.244 Å, which does not indicate bonding Pt–Pt contacts
(Kesanli et al. 2007).
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1280
S. Scharfe and T. F. Fässler
A similar ellipsoid structure was found for the clusters [Pd2 @E18 ]4− (E = Ge,
Sn) in which two Pd0 atoms are encapsulated by 18 tetrel atoms that form a
prolate deltahedral cluster (Goicoechea & Sevov 2005b; Sun et al. 2007). These
units are the largest endohedral tetrel atom clusters with a completely closed
cluster shell discovered up to now. The interstitial Pd atoms with d10 electron
configuration approximately occupy the ellipse foci. Even though the Pd–Pd
distance of 2.831 Å is shorter in the smaller Ge cage than in the Sn cage (3.384 Å),
no bonding Pd–Pd contacts are to be considered. The [Pd2 @Sn18 ]4− cluster is
fluxional at room temperature in ethylenediamine solution, with only one 119 Sn
NMR resonance at −751.3 ppm.
The intermetalloid cluster [Ni3 @Ge18 ]4− displays a somewhat similar structure
where a linear trimer of Ni atoms was found to bind to two separated, widely
opened Ge9 clusters (Goicoechea & Sevov 2005a). These two Ge9 units feature
the same shape as one of the two units found in [Pd2 @E18 ]4− . Although the Ge
atoms of the two cluster units are not connected, the relative orientation of the
atoms of each open face is already staggered as required for an 18-vertices cage
(Sevov & Goicoechea 2006).
4. Summary
Zintl ions have the basic capability for a bottom-up synthesis of nanostructured
materials. The known examples show that such anionic clusters undergo oxidative
addition on the one hand, but can also add step-by-step transition metals on the
other. Up to now homoatomic clusters, homoatomic complexes and endohedral
clusters derived from Zintl tetrel anions reach the size of 36 Ge atoms in
[(Ge9 )4 ]8− , 45 Ge atoms in [Au3 Ge45 ]8− and 18 E atoms in [Pd2 @E18 ]4− (E = Ge
and Sn), respectively. Still there is a necessity for a better understanding of the
reactivity and the cluster growth mechanisms, since oxidative coupling reactions
often proceed even without the addition of an oxidation reagent. Although the
chemistry of highly charged anions is still limited to a few polar and aprotic
solvents, its high potential has become evident, and has developed into a rapidly
growing field.
The authors thank Dr A. Schier for revising the manuscript and the German National Science
Foundation (Deutsche Forschungsgemeinschaft) for their financial support.
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