Synthesis and characterization of an Al693- cluster with 51 naked Al atoms: Analogies and differences to the previously characterized Al772- cluster

Article abstract of DOI:10.1021/ic0104297

A disproportionation process of a metastable AlCl solution with a simultaneous ligand exchange - Cl is substituted by N(SiMe₃)₂ - leads to a [Al₆₉{N(SiMe₃)₂}₁₈]^3- cluster compound that can be regarded as an intermediate on the way to bulk metal formation. The cluster was characterized by an X-ray crystal structural analysis. Regarding its structure and the packing within the crystal, this metalloid cluster with 4 times more Al atoms than ligands is compared to the [Al₇₇N(SiMe₃)₂}₂₀]^2- cluster that has been published four years ago. Although there is a similar packing density of the Al atoms in both clusters as well as in Al metal, the X-ray structural analysis shows significant differences in topology and distance proportions. The differences between these - at a first glance almost identical - Al clusters demonstrate that results of physical measuring, e.g., of nanostructured surfaces which carry supposedly identical cluster species, have to be interpreted with great caution.

Full text of DOI:10.1021/ic0104297

4830
Inorg. Chem. 2001, 40, 4830-4838
3-
Synthesis and Characterization of an Al69 Cluster with 51 Naked Al Atoms: Analogies
and Differences to the Previously Characterized Al77 Cluster
2-
H. Ko1hnlein, A. Purath, C. Klemp, E. Baum, I. Krossing, G. Sto1sser, and H. Schno1ckel*
Institut fu¨r Anorganische Chemie, Universita¨t Karlsruhe (TH), Engesserstrasse,
Geb. 30.45 D-76128 Karlsruhe, Germany
ReceiVed April 24, 2001
A disproportionation process of a metastable AlCl solution with a simultaneous ligand exchangesCl is substituted
by N(SiMe3)2sleads to a [Al69{N(SiMe3)2}18]3- cluster compound that can be regarded as an intermediate on the
way to bulk metal formation. The cluster was characterized by an X-ray crystal structural analysis. Regarding its
structure and the packing within the crystal, this metalloid cluster with 4 times more Al atoms than ligands is
compared to the [Al77N(SiMe3)2}20]2- cluster that has been published four years ago. Although there is a similar
packing density of the Al atoms in both clusters as well as in Al metal, the X-ray structural analysis shows
significant differences in topology and distance proportions. The differences between thesesat a first glance
almost identicalsAl clusters demonstrate that results of physical measuring, e.g., of nanostructured surfaces which
carry supposedly identical cluster species, have to be interpreted with great caution.
Introduction
co-condensate is collected in a flask and can be stored at
-80 °C for a few months. The thermodynamically favored
disproportionation of aluminum(I), respectively gallium(I) ha-
lides to the metal and the trihalide can be kinetically controlled
by varying either the halide, the donor or the temperature. The
thus obtained metastable AlI and GaI halide solutions showed
to be ideal precursors for the synthesis of interesting subvalent
Al and Ga compounds: intermediates (on the way to metal
formation) with newly tied M-M bonds can be stabilized by
metathesis reactions replacing the halide by a bulky fragment.
Whereas metalloid7 Ga clusters were synthesized by a reaction
of gallium(I) halide solutions with different ligands, e.g.,
Si(SiMe3)3(Hypersilyl),8 C(SiMe3)3(Trisilyl),9 or N(SiMe3)2,10
the latter substituent seems to be especially suited to stabilize
Since many years subhalides of the 13th group elements
boron, indium, and thallium have been well-established. In the
case of indium and thallium, these subhalides can be synthesized
without problems.1 Already 50 years ago, molecular aluminum
monohalides were successfully characterized at high tempera-
tures in the gas phase.2 About 30 years ago, they were
characterized as monomers and dimers in matrix experiments,3
and only in the past decade they were synthesized on a
preparative scale.4 The reason for this late development iss
among other problemssthe instability of the AlI and GaI halides
toward disproportionation. Gaseous MX high-temperature mol-
ecules (M ) Al, Ga; X ) Cl, Br, I) which can be synthesized,
e.g., by passing HX gas over the liquid metal at temperatures
near 1000 °C and at pressures of about 10-3 mbar, are
thermodynamically stable only under these conditions. Keeping
these problems in mind, our group applied the co-condensation
technique5 in order to synthesize AlX species in gram-scales.6
To avoid disproportionation when only slowly cooling, the MX
species are quenched immediately after their formation on a
surface which is cooled by liquid nitrogen together with a
suitable donor-containing solvent mixture, e.g. toluene with
THF, NEt3 or Et2O. Upon thawing of the solvent matrix, the
(7) In 1966, F. A. Cotton defined clusters of metal atoms as molecules,
where two or more metal atomssapart from being bonded to other
nonmetal atomssare also bonded to each other. This description
applies to numerous compounds with different bonding relations as
Fe2(CO)9, [Au39(PPh3)14C6]2+, [HNi38(CO)42Cl6]5-, or Pd145(CO)x-
(PEt3)30. For clusters such as the aforementioned AlmRn species (with
m > n), whose common characteristic is that the number of direct
metal-metal interactions is larger than the number of metal-ligand
interactions (2e2c), we introduced the new term metalloid (metal-like)
cluster to draw a more distinct boundary to Cotton’s metal atom
clusters. References: (a) Metal atom clusters: Cotton, F. A. Q. ReV.
Chem. Soc. 1966, 389. (b) Metalloid clusters: Schnepf, A.; Sto¨sser,
G.; Schno¨ckel, H. J. Am. Chem. Soc 2000, 122, 9178. Ref 11b. (c)
Fe2(CO)9: Cotton, F. A.; Troup, J. M. J. Chem. Soc., Dalton Trans.
1974, 800. (d) [Au39(PPh3)14C6]2+: Teo, B. K.; Shi, X.; Zhang, H. J.
Am. Chem. Soc. 1992, 114, 2743. (e) [HNi38(CO)42Cl6]5-: Ceriotti,
A.; Fait, A.; Longoni, G.; Piro, G. J. Am. Chem. Soc. 1986, 108, 8091.
(f) Pd145(CO)60(PEt3)30: Tran, N. T.; Powell, D. R.; Dahl, L. F. Angew.
Chem., Int. Ed. 2000, 112, 4287; 39, 4121.
* To whom correspondence should be sent. Fax: (+49)721-608-4854.
E-mail: Hansgeorg.Schnoeckel@chemie.uni-karlsruhe.de.
(1) Morrison, J. A. Chem. ReV. 1991, 91, 35.
(2) (a) Klemm, H.; Voss, E.; Geiersberger, K. Z. Anorg. Allg. Chem. 1948,
256, 15, 24. (b) Huber, P.; Herzberg, G. Molecular Structure IV,
Constants of Diatomic Molecules; Van Nostrand, Reinhold: New
York, 1979; pp 18-22, 26.
(3) (a) Schno¨ckel, H. Z. Anorg. Allg. Chem. 1976, 424, 203. (b) Schno¨ckel,
H. J. Mol. Struct. 1978, 50, 267.
(8) (a) Ga22[Si(SiMe3)3]8: Schnepf, A.; Weckert, E.; Linti, G.; Schno¨ckel,
H. Angew. Chem., Int. Ed. 1999, 111, 3578; 38, 3381. (b) [Ga26{Si-
(SiMe3)3}9][Li(THF)4]2: Rodig, A.; Linti, G. Angew. Chem., Int. Ed.
2000, 112, 3076; 39, 2952. (c) [Ga9{Si(SiMe3)3}6][Li(THF)4]: Ko¨stler,
W.; Linti, G. Angew. Chem., Int. Ed. Engl. 1997, 109, 2758; 36, 2644.
(9) (a) Ga4[C(SiMe3)3]4: Uhl, W.; Hiller, W.; Layh, M.; Schwarz, W.
Angew. Chem., Int. Ed. Engl. 1992, 104, 1378; 31, 1364. (b) Ga8-
[C(SiMe3)3]6: Schnepf, A.; Ko¨ppe, R.; Schno¨ckel, H. Angew. Chem.,
Int. Ed. 2001, 113, 1287; 40, 1251.
(4) (a) Tacke, M.; Schno¨ckel, H. Inorg. Chem. 1989, 28, 2895. (b) Al4-
Br4‚4NEt3: Mocker, M.; Robl, C.; Schno¨ckel, H. Angew. Chem., Int.
Ed. Engl. 1994, 106, 1860; 33, 1754. (c) Al4I4‚4D: Ecker, A.;
Schno¨ckel, H. Z. Anorg. Allg. Chem. 1998, 624, 813. (d) Ga8I8‚
6NEt3: Doriat, C.; Friesen, M.; Baum, E.; Ecker, A.; Schno¨ckel, H.
Angew. Chem., Int. Ed. Engl. 1997, 109, 2057; 36, 1969.
(5) Timms, P. L. AdV. Inorg. Chem. Radiochem. 1972, 14, 142.
(6) Dohmeier, C.; Loos, D.; Schno¨ckel, H. Angew. Chem., Int. Ed. Engl.
1996, 108, 141; 35, 129.
(10) [Ga84{N(SiMe3)2}20]4-: Schnepf, A.; Schno¨ckel, H. Angew. Chem.
2001, 113, 734; Angew. Chem., Int. Ed. Engl. 2001, 40, 712.
10.1021/ic0104297 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/17/2001

3-
An Al69 Cluster
Inorganic Chemistry, Vol. 40, No. 19, 2001 4831
the respective Al compounds: Thus, LiN(SiMe3)2 reacts with
ether-stabilized AlI solution to the Al77R202- [R ) LiN(SiMe3)2]
cluster and to the partly substituted Al14I6R62- cluster; with AlCl
structurally characterized as an intermediate stage of the
disproportionation of AlI compounds to aluminum metal and
AlIII compounds. The synthesis of 1 succeeded using AlCl‚OEt2
solution and LiR [R ) N(SiMe3)2] under similar reaction
conditions applied for the synthesis of the Al77 cluster starting
from AlI‚OEt2 and LiR. A conceivable total reaction for the
Al69 cluster synthesis is formulated in eq 1:
-
-
solution the Al7R6 cluster and the Al12R8 cluster were
synthesized as intermediates11 on the way to aluminum metal.
Experimental Section
Preparation of LiN(SiMe3)2. Donor-free lithium-bis(trimethylsilyl)-
amide was synthesized by metalation of bis(trimethylsilyl)amine with
a solution of n-butyllithium in pentane.12 The product was recrystallized
several times from pentane. 1H NMR (C6D6) 0.13 ppm, 7Li NMR (C6D6)
0.89 ppm, 13C NMR (C6D6) 5.85 ppm, 29Si NMR (C6D6) -10.0 ppm.
Synthesis of 1a. A total of 40 mmol gaseous AlCl were co-
condensed with 60 mL toluene and 20 mL diethyl ether at -196 °C.
Next, 9.5 mL of the ca. 0.29 molar (2.75 mmol) dark reddish-brown
AlCl‚Et2O solution was added to 500 mg (3.00 mmol) donor free
LiN(SiMe3)2 at 25 °C. After 1 h the amide dissolved and LiI
precipitated, which was then filtered. For 1.5 h the solution was heated
to 60 °C. Apart from the precipitation of elemental aluminum we
obtained 1a as dark reddish-brown sticks which chemically and
mechanically proved to be extremely instable. Yield of crystalline solid
material: 5% (8 mg, 0.0014 mmol).
Synthesis of 1b. A solution of AlCl (10 mL, 2.90 mmol) in toluene/
Et2O (3:1) (cf. 1a) was added to 485 mg (2.90 mmol) LiN(SiMe3)2 at
-78 °C. The reacting solution was warmed to room temperature within
1 day. After 1 h of heating at 60 °C LiCl precipitated which was
subsequently filtered. After storage at +60 °C for 2 months, elemental
aluminum as well as 1b which appeared as dark reddish-brown cubes
crystallized from the solution. Yield of crystalline solid material: 7%
(10 mg, 0.0020 mmol).
+1
96 AlCl‚Et2O + 18 LiR f [+A0.2l1769R18]3-[Li(OEt2)4]3
+
+
+3
27 AlCl3‚Et2O + 15 LiCl‚3Et2O + 12 Et2O (1)
From different runs of the reaction two species of 1 could be
isolated. The X-ray structural analysis of the black crystalline
products showed the compounds to be [Al69{N(SiMe3)2}18]-
[Li(OEt2)4]3‚6 toluene (1a) and [Al69{N(SiMe3)2}18][Li(OEt2)3]2-
[Li(OEt2)4]‚n toluene (1b).14 The cluster anion which is structur-
ally identical in 1a and 1b15 is shown in Figure 1a.
In Figure 1b the corresponding Al-Al bonds between the
Al shells as well as the N(SiMe3)2 groups have been omitted
for a better visualization of the shell-like structure.
At first glance, the structure of the cluster compound
[Al77{N(SiMe3)2}20][Li2I(OEt2)5]2‚2 toluene (2; Figure 2), which
was synthesized in our group in 1996, seems to strongly
resemble to 1.
A common characteristic of both nanosized clusters 1 and 2
is the metal frame of three shells, the outer shell consisting of
AlR units. The diameter of the Al69 unit amounts to 1.27 nm,
that of the Al77 to 1.30 nm. Considering this aspect in the
empirical formula, 1 can be regarded as [Al51(AlR)18]3- and 2
as [Al57(AlR)20]2-. Due to the different number of metal atomss
however a similar Al/R ratio (1: 3.83, 2: 3.85) is observeds
the geometries of the cluster frames differ already in the inner
shell.
X-ray Crystallography. The data set of 1a was collected using a
NONIUS KAPPA CCD diffractometer with graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å) at 200 K.13 In the case of the species
1b we used a STOE IPDS diffractometer with the same X-ray source.
The structures were solved by direct methods (Shelxs 97). A total of
1092 and 2098 parameters for 1a and 1b, respectively, were refined
by full matrix least squares against F2 (Shelxl 97) with anisotropic
thermal parameters for all non-hydrogen atoms. H atoms were refined
on calculated positions according to the riding model.
Comparison of the Al Shells for 1 and 2. In 1 and 2 the
central Al atom is surrounded by 12 Al atoms, which for
their part are also coordinated by 9 atoms (cf. Tables 3 and
4). The decahedral shell of 1 (Figure 1a) exhibits a distorted
D5h symmetry, with the five-membered rings not being in
exactly eclipsed position to each other and also having different
edge lengths (distances from 2.717 to 3.232 Å). The coordina-
Table 1. Crystallographic Data for 1a
chemical formula:
120H372Al69Li3N18O12Si36
formula weight:
C
5482.34 g mol-1
,
a ) 29.1720(6) Å
b ) 37.6650(8) Å
c ) 31.5020(6) Å
â ) 96.107(3)°
V ) 34416 (12) Å3
Z ) 4
space group: C 2/c (no. 7)
T ) -100 °C
λ ) 0.71073 Å
(11) (a) Al77R202-: Ecker, A.; Weckert, E.; Schno¨ckel, H. Nature 1997,
Dcalc) 1.064 g cm-3
µ ) 3.40 cm-1
2-
387, 379. (b) Al14I6R6
: Ko¨hnlein, H.; Sto¨sser, G.; Baum, E.;
Mo¨llhausen, E.; Huniar, U.; Schno¨ckel, H. Angew. Chem. 2000, 112,
828; Angew. Chem., Int. Ed. 2000, 39, 799. (c) Al7R6-: Purath, A.;
Ko¨ppe, R.; Schno¨ckel, H. Angew. Chem., Int. Ed. 1999, 111, 3114;
38, 2926; (d) Al12R8-: Purath, A.; Schno¨ckel, H. Chem. Commun.
1999, 1933.
Ra ) 0.1296
Rwb ) 0.4275
2
a R ) (∑ | |Fo| - |Fc| |)/(∑ |Fo|). b Rw ) [(∑ w(Fo - Fc2)2)]1/2
;
2
w ) 1/(σ2(Fo ) + (0.115)P2 + 0.4631P).
(12) Wannagat, U.; Niederpru¨m, H. Chem. Ber. 1961, 94, 1540.
(13) Since the single crystals of 1a are extremely sensitive to air and
moisture the measuring was performed by the group of R. Minkwitz,
University of Dortmund. Their special technique for the cooled transfer
of sensitive crystals from the reaction flask to the diffraction
goniometer head enabled successful crystal mounting.
Table 2. Crystallographic Data for 1b
chemical formula:
117H333Al69Li1.5N18O5Si36
formula weight:
4956.28 g mol-1
C
(14) According to the X-ray structural analysis 1a and 1b differ in the
number of toluene molecules and Li+cations: In 1b only 1.5 toluene
molecules and 1.5 of three cations could be properly detected. To
elucidate the charge of the anion the compounds were investigated
by ESR spectroscopy. The lack of any signal indicates the total number
of electrons within the cluster anion to be even. Since a neutral Al69R18
cluster would have an odd electron number the stoichiometry of the
species 1a with three [Li(OEt2)4]+ cations per cluster anion is consistent
with the result of the ESR experiment. Considering the fact that 1a
and 1b both possess structurally identical cluster anions, we conclude
that in both compounds an anion-to-cation ratio of 1:3 was realized.
Obviously the remaining 1.5 cations in 1b are “smeared” over the
remaining gaps.
a ) 28.6252(18) Å
b ) 28.4753(13) Å
c ) 43.5454(24) Å
â ) 107.328(7)°
V ) 33883(4) Å3
Z ) 4
space group: P 21/c (no. 14)
T ) -73 °C
λ ) 0.71073 Å
Dcalc ) 0.972 g cm-3
µ ) 3.40 cm-1
Ra ) 0.1152
,
Rwb ) 0.2907
a R ) (∑ | |Fo| - |Fc| |)/(∑ |Fo|). b Rw ) [(∑ w(Fo - Fc2)2)]1/2
;
2
w ) 1/(σ2(Fo ) + (0.115)P2 + 0.4631P).
2
Results
(15) Therefore we will not distinguish between both species in the
following, except for observations made concerning the packing of
ions.
Here we discuss a remarkable metalloid aluminum cluster
anion, [Al69{N(SiMe3)2}18]3- (1), which could be isolated and

4832 Inorganic Chemistry, Vol. 40, No. 19, 2001
Ko¨hnlein et al.
Figure 2. (a) Molecular structure of the anion of 2 (methyl groups
excluded). (b) Shell-like Al skeleton of 2 (from central atom to
exterior): 1st shell, 12 Al atoms; 2nd shell, 44 Al atoms; 3rd shell, 20
Al atoms.
Figure 1. (a) Molecular structure of the anion of 1 (methyl groups
excluded). (b) Shell-like Al frame of 1 (from central atom to the
exterior): 1st shell, 12 Al atoms; 2nd shell, 38 Al atoms; 3rd shell, 18
Al atoms.
Table 3. Al-Al Distances within and between (e.g., 1 f 2) the
Al69 Cluster Shells
tion polyhedron can be described as a twice capped distorted
pentagonal prism with an idealized 5-fold axis runnig through
the Al atoms 4 and 4′. The actual symmetry element is a
C2-axissparallel to the crystallographic b-axissrunning
through the middle of the Al 6-6′bond, the central Al atom
and the center of the plane built up by the Al atoms 2, 3, 2′,
and 3′.
shell
center
1
1 f 2
2
2 f 3
3
atoms
1
12
12
9b
38
7c
18
c.n.a
4
2
(4)d
dminimal (Å)
dmaximal (Å)
daverage (Å)
2.681 2.708 2.650
2.894 2.991 2.942
2.783 2.782 2.783
2.607 2.543
2.968 2.883
2.797 2.683
(4.663)
(5.700)
(5.103)
a c.n. ) coordination number. Number of the next atoms surrounding
the central atom and forming the 1st shell (“Center”; column 2),
coordination of each of the atoms in the 1st shell by the atoms in the
2nd shell (“1 f 2”; column 4) etc. For the coordination within the 1st
and 2nd shell (column 3 and 5, respectively) a total c.n. is given. All
numbers represent average values (except for column 2). b The total
c.n. of an atom in the 1st and 2nd shell is the sum of the next atoms
within the same shell and the next atoms of the neighboring shells.
Total c.n. ) c.n.1‚shellfcentral atom + c.n.1‚shell + c.n.1‚f2‚shell ) 1 + 4 + 4
) 9. c Total c.n. ) c.n.2‚f1‚shell + c.n.2‚shell + c.n.2‚f3‚shell ) 1.5 + 3.5
+ 2 ) 7. d There is no significant bonding between the pairs of Al
atoms within the 3rd shell. Only atoms with a distance ranging from
2.5 to 3.0 Å from a particular atom were included and considered for
the coordination number.
The inner shell of 2 can be described as a distorted
icosahedron. Due to the fact that the triangles forming the
polyhedron are not equilateral, the symmetry is reduced to Ci.
The second shell of 1 (Figure 4a) contains 38 Al atoms. Each
of the Al atoms achieves an average coordination number of 7.
The polyhedron in this figure is tilted by 90° in comparison to
Figure 2.
Characteristic elements of this shell are the exposed Al atoms
situated opposite to each other (atoms 14 and 14′). Each of these
Al atoms is the intersection point of two triangular and two
quadrangular planes. The 2-fold axis points through the
quadrangular plane formed by the Al atoms 16, 16′, 17, and
17′ and through the edge between the Al atoms 24 and 24′.
Primarily, the shell is formed by an alternating connection of
triangles and quadrangles. However, there are also 6 pentagons
with larger Al-Al distances (maximum distance 4.863 Å
between the Al atoms 16 and 18 in contrast to 4.085 Å between
10 and 12). This connection pattern enables the construction of
a shell consisting of only 38 Al atoms around the inner 12-
numbered shell.
almost exclusively constructed of triangles and quadrangles.
Moreover, four vertexes are pointing to the interior (Al atoms
13, 13′, 21, 21′). The average Al-Al bond lengths are as
expected shorter than those in 1 (cf. Tables 3 and 4).
Within the outer shell of 1 and 2 all Al atoms show a 4-fold
coordination. However, this is only a topological description.
Shell 3 of 1 consists of 18 Al atoms (Figure 5a), the 2-fold
axis of symmetry in this representation runs through Al atoms
27 and 28. In 2 (Figure 5b) the ligand-bearing shell is formed
by 20 Al atoms.
In 2 (Figure 4b) the situation is different; 44 Al atoms are
surrounding the first shell of similar size as in 1, thus, a different
coordination of the atoms should result: the second shell is

3-
An Al69 Cluster
Inorganic Chemistry, Vol. 40, No. 19, 2001 4833
Table 4. Al-Al Distances within and between (e.g., 1 f 2) the
Al77 Cluster Shells
shell
center
1
1 f 2
2
2 f 3
3
atoms
1
12
9a
44
7b
20
c.n.
12
4
2
(4)c
dminimal (Å)
2.674
2.693 2.638
2.973 2.991
2.795 2.818
2.564 2.565
2.999 2.852
2.756 2.688
(4.905)
(5.170)
(5.019)
d
maximal (Å) 2.870
daverage (Å)
2.762
a Total c.n. ) c.n.1‚shellfcentral atom + c.n.1‚shell + c.n.1‚f2‚shell ) 1 + 4
+ 4 ) 9. b Total c.n. ) c.n.2‚f1‚shell + c.n.2‚shell + c.n.2‚f3‚shell ) 1 + 4
+ 2 ) 7. c There is no significant bonding between the pairs of Al
atoms within the 3‚shell. Only atoms with a distance ranging from 2.5
to 3.0 Å from a particular atom were included and considered for the
coordination number.
Figure 4. (a) 2nd shell (38 Al atoms) of 1. (b) 2nd shell (44 Al atoms)
of 2 (each central Al atom in black).
second shell, 2 has shorter Al-Al distances than 1, since in 1
the 12-membered shell is surrounded by only 38 Al atoms,
whereas in 2 there are 44 Al atoms which are more densely
packed. In both clusters, the shortest bonds are found between
the 2nd and 3rd shells. Thus, the bonding relations in the outer
sphere are similar to those of subvalent Al compounds with a
smaller Al/R ratio,16 where the average oxidation number of
aluminum is closer to +1 (1, +0.217; 2, +0.234).
The arrangement of the shells of 1 and 2 in the space-filling
model (Figure 6) shows two significant gaps in the second shell
of 1 which allow a closer look on the first shell: In the case of
2 the inner shell is completely covered.
A possible reason for this difference might be the fact that
the cavity between the five atoms of the second shell of 1 is
larger (distances between opposite Al atoms: 4.807-4.863 Å)
than the interstice between the four Al atoms which are covered
by an atom of the third shell (3.906-4.009 Å). However, the
space-filling cluster model including the N(SiMe3)2 groups
(Figure 7) demonstrates clearly that the cluster core of 1 is
almost completely protected by the ligand shell.
Figure 3. Central Al atom (black) and 1st shell (12 Al atoms) of 1 (a)
and 2 (b), each with the same orientation as in the corresponding
comprehensive view (1, Figure 1; 2, Figure 2).
In Table 3, the Al-Al distances for 1 within and between
the shells are summarized; for comparison the corresponding
values of 2 are given in Table 4.
While the average Al-Al distances between the aluminum
atoms encapsulated within the 2nd shell (i.e., including the center
and the 1st shell) are almost identical in 1, this does not hold
for 2 where the distances slightly increase starting from the
center to the second shell. This is surprising considering the
more symmetrical arrangement of 2, where one would rather
expect similar bond lengths within the inner shells. Within the
(16) Compare the Al-Al bonding lengths (Å) in [Al7{N(SiMe3)2}6]-, 2.540
and 2.737 (ref 11c); [Al12{N(SiMe3)2}8]2-, 2.542-2.759 (ref 11d);
Al12(AlBr2 ‚THF)10 ‚ 2THF: 2.526-2.762; reference: Klemp, C.;
Ko¨ppe, R.; Weckert, E.; Schno¨ckel, H. Angew. Chem., Int. Ed. 1999,
111, 1851; 38, 1739.

4834 Inorganic Chemistry, Vol. 40, No. 19, 2001
Ko¨hnlein et al.
Figure 5. (a) 3rd shell (18 Al atoms) of 1. (b) 3rd shell (20 Al atoms)
of 2 (each central Al atom in black).
Figure 6. Space-filling model of 1 (a) and 2 (b). Only Al atoms are
shown.
Comparison of the Arrangement of Ions in 1a, 1b, and 2
3-
in the Crystal. Figure 8a shows the arrangement of the Al69
anions, the Li+ cations, and the solvent molecules in the crystal
of 1a.
The packing of anions reminds of the hexagonal closest
packing. Although distortion leads to two longer distances
(29.172 Å) within a layer, this description appears to us to be
more appropriate than the description as a primitive hexagonal
arrangement shown in dashed lines in Figure 8a. In the
hexagonal closest packing, the distances of 15.754 Å (31.507/
2) between the layers (Figure 8b) are significantly shorter than
the distances in the primitive hexagonal arrangement where the
shortest distance is 23.820 Å (Figure 8c). In this latter case,
however, the layers are displaced distinctly: Therefore the
distances from the central Al atom of one layer to the Al atoms
of the two vicinal layers vary from 22.669 to 29.172 Å. Turning
to the anion closest packing (Figure 8b), the layers are strictly
parallel to each other and are placed exactly above each other.
In the following, we therefore refer to 1a as being a distorted
hexagonal closest packing.
Figure 7. Space-filling model of [Al69{N(SiMe3)2}18]3-
.
two [Li(OEt2)4]+ cations. They exhibit an inversion center which
lies in the square plane of the octahedron, there is also room
for four toluene molecules (Figure 9a). The “tetrahedral”
interstices in hexagonal closest packings always appear as pairs
of tetrahedra sharing one face to give trigonal bipyramids. A
cation is located in the center of each bipyramid; each of the
tetrahedral interstices contains a toluene molecule which is not
exactly located in the tetrahedron center but skipped to one of
The “coordination number” between the cluster unit centers
can be described as 10+4, because there are 10 shorter distances
(av 22.746 Å) and 4 longer ones (av 27.661 Å).
3-
Due to the distortion of the Al69 anion packing the
octahedral interstices are larger and offer sufficient space for

3-
An Al69 Cluster
Inorganic Chemistry, Vol. 40, No. 19, 2001 4835
Figure 8. Arrangement of anions (ligands excluded), cations (ethyl groups excluded), and toluene molecules (H atoms excluded) of 1a in the
crystal. (a) View parallel to the layers of the distorted hexagonal closest packing, broken line: coordination of anions within a single layer seen as
primitive hexagonal packing. Coordination pattern with hexagonal closest (b) and primitive hexagonal (c) anion packing. Distances (Å): 1, 20.399;
2, 22.669; 3, 23.023; 4, 23.820; 5, 26.149; 6, 29.172; 7, 31.507.
the triangular planes (Figure 9b). The cations in the trigonal
bipyramidal interstices are separated by 29.192 and 23.820 Å,
corresponding to the parallel connections between the anions.
The distances within each cation pair in the octahedral interstices
amount to 11.179 Å. With a closest packing having one
octahedral and two tetrahedral interstices per packing molecule,
1a reaches the stoichiometric ratio solvent/cations/anions of 6:3:
1. The cations form chains along the stapling direction of the
anions (cf. Figure 8a), the distances are identical to those
between the anion layers (23.820 Å).
than three simple [Li(OEt2)4]+ cations. Due to the different
coordination and charge of the cluster anions in 1a and 2 a
discussion of distances between the cluster anions seems
problematic.
The crystal packing of Al69 anions in 1b (Figure 11b) is
similar to the arrangement of gallium atoms in â-Ga metal
(Figure 11a) with regard to topology and coordination pattern.
The anions in 1bslike the atoms in â-galliumsform ladders
along the c-axis and are stapled on top of each other along the
b-axis in a zigzag pattern. These ladder layers are staggered
with respect to the a-axis. If we remove the “ladder sides” in a
thought experiment, irregular edge-connected hexagons would
be present. The cluster anions reach a “coordination number”
of 8+6, the shorter distances ranging from 20.273 to 22.736 Å
and the longer distances ranging from 27.495 to 28.625 Å; the
average value amounts to 24.501 Å (Figure 11c). The cations
form chains within canals along the b-direction, the distances
within the chains being 28.475 Å; between the chains the
distances are 18.715 Å. One part of the detected Li+ cations
are positioned in the interstitial lattice sites between the anion
“ladder steps”, another part of the Li+ cationssreferring to an
anion “hexagon” sis located alternatingly on the right and left
sides of an anion.15
2-
In the primitive hexagonal packing of 2 (Figure 10) the Al77
anions reach a “coordination number” of 8+4: There are 8
shorter distances (av 22.005 Å) and 4 longer distances (av
27.241 Å). The iodo-bridged Li2I+ cations are located in the
hexagonal interstices of the anion packing. Additionally, there
are two “halves” of toluene molecules per interstice. A
comparison of the average distances between the anions to the
average distances of 1a shows that 2 is obviously closer packed
(23.750 Å) than 1a (24.150 Å). The distances within the layers
are shorter (2, 22.329 Å; 1a, 25.604 Å), however, in 2 there
are longer distances between the layers (2, 21.034 Å; 1a,
15.754 Å). Furthermore, the smaller number of solvent mol-
ecules per anion points to a closer packing of 2. On the other
hand, two iodo-bridged [Li2I(OEt2)5]+ cations need more space

4836 Inorganic Chemistry, Vol. 40, No. 19, 2001
Ko¨hnlein et al.
Figure 9. Arrangement of the cations in the octahedral (a) and
tetrahedral interstices (b) of the distorted hexagonal closest packing of
anions of 1a in the crystal.
Discussion and Outlook
The differences between 1 and 2 are discussed taking into
account the following points: (a) The environment of the central
Al atoms, (b) the coordination numbers and distances within
the individual shells, and (c) the mechanisms of cluster
formation.
Figure 10. Arrangement of ions of 2 (N(SiMe3)2 and ethyl groups
excluded) within the crystal. (a) View along the stapling direction; (b)
coordination pattern with distances (Å) of 1, 21.043; 2, 21.707; 3,
22.154; 4, 23.125; 5, 26.968; 6, 27.514.
(a) One remarkable difference between 1 and 2 arises from
the coordination of the central Al atoms. The first shell of 1
displays a distorted decahedral structure (distorted D5h sym-
metry). However, the respective shell of 2 can be described as
icosahedral (distorted cuboctahedron). This difference within
the cluster cores is associated with a topologically modified
periphery which is shown in the space-filling models in Figure
6. The different arrangements within the cores comes not
unexpected, since according to quantum chemical calculations
the energetic difference between large, topologically different
Al clusters is very small, i.e., approximately (1% for Al55 in
various geometries.17 However, a different topologysas between
1 and 2smay cause different electronic situations which are
more likely to influence the various physical (especially
anisotropical) properties. Therefore, an X-ray structural analysis
constitutes the only suitable basis for the accurate interpretation
of the physical properties. Thus, one has to be cautious when
interpreting the physical effectssmeasured by the usual methods
of modern nanotechnologyswhich are based on the structuring
of a surface with an apparently homogeneous cluster species,
e.g. the Au55 cluster,18 which has not been characterized by
X-ray structural analysis so far.
(b) In the following, the difference between the 2nd shell
(38 and 44 Al atoms) in 1 and 2 will be discussed. According
to MACKAY a regular shell built around an icosahedron would
lead to a 42-membered shell.19 This might not be expected for
1 and 2, since in both cases the inner shell deviates from the
icosahedral symmetry. Therefore, our special interest was
focused on the volume difference (see below) between the naked
clusters Al51 (1+12+38 from 1) and Al57 (1+12+44 from 2),
respectively. In Tables 3 and 4 the tendency to decreasing Al-
Al bond lengths from the center to the surface becomes evident.
The shift of electron density of the Al atoms of the third shell
to the nitrogen atoms of the ligands leads to a decrease of the
Al-Al bond lengths between these Al atoms and the “naked”
Al atoms of the 2nd shell. The electronic influence of the ligands
weakens when approaching the cluster center, i.e., the formal
oxidation state of the aluminum atoms approaches the value 0
and the distances become more metal-like.20 However, the
arrangement of the “naked” Al atoms in both inner shells differs
distinctly from the orientation of the crystalline metal. This can
easily be demonstrated by geometrical comparison to a hypo-
thetical Al55 fragment from solid cubic face centered aluminum
metal (Figure 12).
(17) Binding energy per atom [eV] for a neutral Al55 cluster in different
symmetries: 2.678 (Oh), 2.652 (Ih). Reference: Ahlrichs, R.; Elliott,
S. D. Phys. Chem. Chem. Phys. 1999, 1, 13.
(18) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis;
G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634.
(19) Mackay, A. L. Acta Crystallogr. 1962, 15, 916.
(20) The average Al-Al distances between central atom and 1st shell are
2.783 Å (1) and 2.762 Å (2) and the shortest Al-Al distance in Al
metal is 2.855 Å.

3-
An Al69 Cluster
Inorganic Chemistry, Vol. 40, No. 19, 2001 4837
Figure 12. Ball-and-stick models and space-filling models of various
Aln clusters. (a) Shells 1+2 of 1 Al51 (1+12+38 atoms). (b) Shells
1+2 of 2: Al57 (1+12+44 atoms). (c) Hypothetical Al55 cluster section
from the fcc-lattice of Al metal. For Al51 and Al57 the different diameters
are shown in the illustration.
(29.61 Å) to Al57 (29.51 Å3) and Al55 (29.21 Å3). Thus,
3-
3-
3-
the Al55 cluster representing a section of the metal has the
(22) To calculate the volume of the clusters we used the experi-
mentally determined coordinates of the naked aluminum cores (-3
charge), i.e., Al51
(from [Al69{N(SiMe3)2}18]3-), Al57
(from
3-
3-
[Al77{N(SiMe3)2}20]2-) and Al553- (from the fcc-lattice of Al metal).
These coordinates were used as an input for a HF/3-21G* single point
calculation (GAUSSIAN98). Using the thereby obtained gas-phase
SCF-orbitals the IPCM solvation model (Isodensity Surface Polarized
Continuum Model) than constructed an iso(electron)density surface
around the Aln3- (n ) 51, 55, 57) trianions (isodensity value ) 0.0004
e‚Å-3). In the output file the within this surface enclosed volume is
given. This volume was used for the discussion. References: (a)
IPCM: Foresman, J. B.; Keith, T. A.; Wiberg, K. B.; Snoonian J.;
Frisch, M. J. J. Phys. Chem. 1996, 100, 16098. (b) GAUSSIAN98:
Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I., Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W., Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.3; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Figure 11. (a) Section of the lattice of metallic â-gallium. Arrangement
of ions of 1b (N(SiMe3)2, ethyl groups and solvent molecules excluded)
within the crystal, view along the crystallographic b-axis; (b) the “ladder
sides” (cf. text) are marked by dashed lines; (c) coordination pattern
in the orientation as in (b) with distances (Å) of 1, 20.273; 2, 21.626;
3, 21.790; 4, 22.226; 5, 22.736; 6, 27.495; 7, 28.376; 8, 28.475; 9,
28.625.
This description sugggests that the Al51 (shells 1+2 of 1)
and Al57 (shells 1+2 of 2) cluster cores are more densely packed
than in the solid metal.21 To give a reliable statement about the
relative density of Al atoms in the cluster frame, the volumes
of the three clusters were calculated each with a charge of -3.22
As expected, the cluster volume grows with increasing number
of atoms, but we can observe a slight decrease in the atomic
3-
volume (cluster volume/number of atoms) from Al51
(21) The diameter of constructed Al55 (1+12+42 atoms) is 11.42 Å, the
average diameter of Al51 (1+12+38 atoms) is 10.12 Å and that of
Al57 (1+12+44 atoms) is 10.47 Å.

4838 Inorganic Chemistry, Vol. 40, No. 19, 2001
Ko¨hnlein et al.
highest density. In the same order we find an increase of the
average coordination number23 as well as of the average
Al-Al distances.20 According to the pressure-coordination rule
and the pressure-distance paradoxon24 this tendency could be
to 1 if an AlCl solution is used, whereas 2 is obtained with an
AlI solution. A close look at the arrangement of ions within
the crystals of 1 and 2 shows that 2 only crystallizes if
iodobridged Li2I+ cations are offered to the cluster anions, as
these cations fit best into the interstices of the primitive
hexagonal packing of the Al77R202- anions. In this case, iodide
is required as an halide in order to enable an arrangement of
ions within the Al77 cluster compound that is favorable for
crystal formation. Should 1 and 2 be formed in a similar
pathway, it appears possible that the disproportionation process
from “AlI” and “AlCl”, respectively, to Al metal (and AlIII
species) would initially lead to the compound with the highest
3-
3-
expected: Taking the Al51 and Al57 cluster fragments as
examples, the transition to metal aluminum can be described
as follows: by exerting pressure, the Al atoms in the 2nd shell
are squeezed to a smaller volume. This necessarily leads to a
simultaneous increase of the coordination number and of the
average Al-Al distances. And yet, the density is larger in
metallic aluminum as the influence of the higher coordination
number overcompensates the interatomic distances.25 The
precipitation of elemental aluminum which we observed in the
course of our experiments can be interpreted as the product of
a progressing disproportionation of 1 and 2. During this process
the volume of the Al packing changes only slightly, Al atoms
must “only” be reorganized which obviously does not require
a high activation energy, since the reaction temperature is below
60 °C.
2-
average oxidation number in aluminum, i.e., the Al77R20
cluster (2, +0.234) which will crystallize under the appropriate
conditions, e.g., Li2I+ as countercation, temperature, and
concentration of the solution. With an AlCl solution the system
does not fall into the energy mould of 2, but the reaction will
3-
proceed until the Al69R18 cluster (average oxidation number
of Al atoms in 1: +0.217) is formed, which provides “suitable”
cations for crystal formation.
(c) For the stabilization of subvalent aluminum compounds
which are synthesized as intermediates during the dispropor-
tionation of AlI speciesswith oxidation numbers between +1
and 0sthe N(SiMe3)2 substituent proved to be very successful.
Although five crystalline intermediates with this substituent were
Conclusion
Due to its similarity to the Al77 cluster which for the first
time was synthesized four years ago, the isolation of an Al69
cluster provides the opportunity of a comparison between two
of the largest metalloid clusters29 regarding to their chemical
structures: both clusters give single crystals which can be
structurally characterized by X-ray diffraction. Although similar
to a large extent, both clusters demonstrate significant differ-
ences in their topologies as well as in their bonding relations
which would not have been detected by using the common
physical methods of nanotechnology. Nevertheless, the structural
results are not sufficient to answer questions pertaining the
circumstances which are crucial for the formation of either 1
or 2, and also to their formation mechanism. For this purpose,
further intermediates synthesized on the way from AlI species
to Al metal under determined conditions have to be structurally
characterized and be discussed also in regard to their formation
process.
-
-
characterized so farsAl7R6 (ref 11c), Al12R8 (ref 11d),
Al14I6R62- (ref 11b), Al69R183- (1), and Al77R202- (2, ref 11a)s
more intermediates are required to establish a formation
mechanism for 1 and 2. There is also a fundamental problem:
the disproportionation products formed in the system AlX‚OEt2
(X ) Cl, I)/LiN(SiMe3)2)/toluene upon increasing temperature
can only be structurally characterized when they crystallize from
the solution. Due to the lack of suitable techniques for detection
of the dissolved species, only a hypothetical formation mech-
anism can be formulated, based on the above-mentioned crystal
structures and further experimental observations.
During the cluster formation, disproportionation steps (3 AlX
f 2 Al + AlX3)26 which regulate the growing of the cluster
frame as well as metathesis reactions (AlX + LiR f AlR +
LiX),27 which are responsible for the structure of the ligand
shell, have to occur. The growth of the AlnRm species (with
n > m), for instance, could probably proceed by inserting “AlX”
units into existing Al-Al bonds with simultaneous separation
of AlX3 species.28 Once the ligand shell is completed, the cluster
core is completely protected, and, therefore, any further reaction
by inserting AlX units is impossible.
Acknowledgment. We especially thank F. Neikes and Dr.
A. Kornath, working group of Prof. Dr. R. Minkwitz, Institute
of Inorganic Chemistry, University of Dortmund, for the special
efforts they undertook to investigate the extremely sensitive
single crystals by X-ray diffraction measurements. We thank
Dr. B. Pilawa, Institute of Physics, University of Karlsruhe, for
performing the ESR measurements. Furthermore we thank Dr.
H.-J. Himmel for helpful discussions. We are grateful to the
Deutsche Forschungsgemeinschaft and the Fonds der Chemichen
Industrie for financial support.
This hypothetical mechanism of growth, however, provides
no clue to the question, why the N(SiMe3)2 substituent will react
(23) Coordination number in the 2nd shell of 1 and 2, 7; coordination
number in Al metal, 12.
(24) Mu¨ller, U. Anorganische Strukturchemie; B. G. Teubner: Stuttgart,
1992; p 135.
Supporting Information Available: An X-ray crystallographic file
in CIF format for the structure of [Al69{N(SiMe3)2}18][Li(OEt2)4]3‚6
toluene (1a) and [Al69{N(SiMe3)2}18][Li(OEt2)3]2[Li(OEt2)4]‚n toluene
(1b). This material is available free of charge via the Internet at
http://pubs.acs.org.
(25) In an analogous way, an expansion of a metallic Al lattice should
lead to a new Al modification with smaller coordination numbers,
but also with shorter Al-Al distances and all in all with a larger atomic
volume. Reference: Al12(AlCl2)10‚12D (D ) THF, THP): Klemp, C.;
Bruns, M.; Gauss, J.; Ha¨ussermann, U.; Jansen, M.; Sto¨sser, G.; van
Wu¨llen, L.; Schno¨ckel, H. J. Am. Chem. Soc., accepted for publication.
(26) A significant difference between either AlX solution applied consists
of solvated AlCl being less kinetically stable compared to AlI
concerning the disproportionation reaction which finally ends up with
the formation of Al metal and the AlX3.
IC0104297
(28) In the course of synthesis of this type the average oxidation numbers
within the cluster lattice increase and decrease with each individual
step until reaching a final value of 0, i.e., for metal and trihalide,
respectively.
(29) With respect to the total number of naked, nonligand-bearing atoms,
compound 1 with 51 Al atoms is comparable with 2 (57 Al atoms, ref
11a), the Pd145 species (55 Pd atoms, ref 7f), or the Ga84 species
containing 64 naked Ga atoms (ref 10).
(27) With AlCl, the metathesis reaction will also proceed faster than with
AlI. Therefore sand also due to the lesser disproportionation stabilitys
-
-
from AlCl solution the intermediates Al7R6 (ref 11c) and Al12R8
(ref 11d) are obtained already at T < 0 °C whereas AlI solution only
2-
reacts at T > 50 °C to Al14I6R6 (ref 11b).