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Spherical Cluster Comprising a Four- and Six-Membered-Ring Motif.

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Fullerene-Like Molecules
DOI: 10.1002/anie.200503511
Spherical Cluster Comprising a Four- and SixMembered-Ring Motif**
Brian P. Johnson, Fabian Dielmann, Gbor Balzs,
Marek Sierka, and Manfred Scheer*
Dedicated to Professor Malcolm Chisholm
on the occasion of his 60th birthday
The synthesis, structures, and properties of giant spherical
molecules make up a challenging field in contemporary
chemistry, which unifies all fields of organic/supramolecular,
inorganic, and coordination chemistry as well as organometallic chemistry. The seminal discovery of buckminsterfullerene C60[1, 2] not only established a new carbon allotrope but
also unveiled a novel mode of icosahedral symmetry in threedimensional carbon-based molecules. This generated a theoretical discussion and experimental search for additional
fullerenes and fullerene-type molecules that exhibit modes of
high symmetry. Theoretical calculations predicted that,
except for C50, C60, and C70, stable fullerenes Cn (n = 20–70)
would generally be expected to exhibit low symmetry.[3] For
smaller fullerenes Cn (n < 60), C32 was predicted by Kroto as
one of the few fullerenes displaying exceptional stability in
this series, and thus, 32 represents one of the so-called “magic
numbers.”[4] However, for C32, only low symmetry can be
envisioned for molecules that contain few or no fourmembered rings,[5] which contribute in fullerene chemistry
to high strain energy and render their cage compounds too
unstable for formation and isolation. For the most highly
symmetric version of C32 with Oh symmetry, eight fourmembered rings are required.[6]
Our group has been interested in employing Pn-ligand
complexes for the synthesis of 1D and 2D coordination
polymers as well as spherical molecules. We recently reported
[*] B. P. Johnson, F. Dielmann, Dr. G. Balzs, Prof. Dr. M. Scheer
Institut f'r Anorganische Chemie der Universit.t Regensburg
93040 Regensburg (Germany)
Fax: (+ 49) 941-943-4439
Dr. M. Sierka
Institut f'r Chemie der Humboldt-Universit.t zu Berlin
Unter den Linden 6
10099 Berlin (Germany)
Fax: (+ 49) 30-2093-7136
[**] This work was comprehensively supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
We thank Prof. Dr. H. Eckert (Univ. M'nster) for recording the
P MAS NMR spectrum of the title compound. M. Sierka gratefully
acknowledges Prof. Joachim Sauer and the Humboldt-Universit.t
zu Berlin for providing computing facilities.
Supporting information for this article is available on the WWW
under or from the author.
Angew. Chem. Int. Ed. 2006, 45, 2473 –2475
the use of [Cp*Fe(h5-P5)] (Cp* = h5-C5Me5) in the
synthesis of the soluble, fullerene-like cage compounds
[{Cp*Fe(h5 :h1:h1:h1:h1:h1-P5)}12{CuX}10{Cu2X3}5{Cu(CH3CN)2}5] (1 a: X = Cl;[7] 1 b: X = Br[8]), which each possess
a 90-atom core consisting entirely of inorganic atoms. Herein,
we report the synthesis and characterization of [{Cp’’Ta(CO)2(h4 :h1:h1:h1:h1-P4)}6{CuCl}8] (2) (Cp’’ = h5-C5H3tBu2) as
the first spherical molecule which is based on four- and sixmembered rings and possesses 32 inorganic core atoms. The
core of compound 2 exhibits Oh-like symmetry, which has not
yet been attained in fullerene chemistry.
We have shown that the Pn-ligand complexes
[{CpMo(CO)2}2(m,h2-P2)] and [Cp*Fe(h5-P5)] can be
employed as connecting moieties between late-transitionmetal cations to form extended structures. Reactions of the
former complex with copper(i) and silver(i) centers result in
the formation of straight-chain or zigzag polymeric structures,[9, 10] while reactions of the latter with copper(i) halides
form linear polymers of the type [CuCl{Cp*Fe(h5 :h1:h1-P5)}]1
as well as two-dimensional polymers of the type [CuX{Cp*Fe(h5 :h1:h1:h1-P5)}]1 (X = Br, I).[11] We found that tweaking the reaction conditions and stoichiometry of the [Cp*Fe(h5-P5)] systems led to the formation of soluble, supramolecular assemblies, as evidenced by the isolation of 1. As a
continuation of these investigations, we have now incorporated the cyclo-P4 complex [Cp’’Ta(CO)2(h4-P4)] (3)[12] to
examine whether a cyclo-P4 unit can serve as a viable source
of stable four-membered phosphorus rings for building
spherical molecules.
Upon layering a solution of CuCl in a mixture of MeCN
and CH2Cl2 onto a yellow solution of 3 in CH2Cl2 (CuCl/3 in
4:3 stoichiometric ratio), the nearly immediate formation of 2
as bright-orange microcrystals was observed. After about five
days, the diffusion was complete, and 2 could be isolated in
analytically pure form in 66 % yield. To obtain larger cubic
crystals of 2 suitable for single-crystal X-ray analysis, slower
diffusion techniques were applied (see Experimental Section). Compound 2 crystallizes with one molecule of CH2Cl2
per formula unit in the trigonal space group R3̄.[13] In the
molecular structure (Figure 1 a), the P atoms of each cyclo-P4
ring are coordinated to the Cu atoms in a 1,2,3,4 mode, and
each {CuCl} unit is bound by three {Cp’’Ta(CO)2(h4-P4)} units
with each Cu atom in approximate tetrahedral geometry.
The overall structure resembles an extended cube in
which the six faces are represented by the cyclo-P4 rings, and
the twelve edges are replaced by six-membered Cu2P4 rings in
boat conformation, thus resulting in a closed-cage framework
consisting exclusively of alternating four- and six-membered
rings. The structural motif of the core in 2 parallels that of the
Td-symmetrical cages considered by theoretical calculations
of the Group 13/15 oligomers [HMEH]16 (M = B, Al, Ga, In;
E = N, P, As),[14] but to our knowledge, no such motif has been
observed in an isolated compound.
In the crystal structure, 2 is centered on the crystallographic C3 axis, which extends through two opposing {CuCl}
units and through the center of the inner cavity. A view of the
structure along this axis is shown in Figure 1 b. The symmetry
of the core is distorted-octahedral, whereby a slight deviation
from this symmetry arises from the small variations in PP
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) Molecular structure of 2. b) View of 2 along the crystallographic C3 axis. c) Space-filling model of the core of 2. The H atoms (a, b) as
well as the Cp’’Ta(CO)2 fragments and Cl atoms (c) have been omitted.
bond lengths within each cyclo-P4 ligand. The planar cyclo-P4
ligands in the free complex 3 are found in approximate kite
geometry (d(PP) = 2.162 J (av)). In 2, the cyclo-P4 ligands
remain planar (maximal deviation of the atoms from the best
plane: 0.02 J), and the kite conformation is maintained. All
the PP bonds are slightly elongated in comparison to those
in the free complex 3, thus resulting in an average PP bond
length in 2 (d(PP) = 2.174 J (av)) that is over 0.01 J longer
than that in 3. The P-P-P bond angles in 2 show only minor
changes from complex 3. The inner cavity of 2 possesses an
approximate cube shape, with the Cu atoms representing the
corners and the cyclo-P4 ligands representing the faces. This
cavity possesses a diagonal of 0.60 nm (Cu to Cu) and a
distance of 0.44 nm between the opposing faces (P to P).[15]
The maximum outside diameter of 2.19 nm reflects the
bulkiness of the tBu groups of the Cp’’ rings. The spherical
cluster core of 32 atoms (space-filling model in Figure 1 c)
possesses an outer diameter of 1.16 nm. For comparison, the
fullerene cage C60 carries outer dimensions of 0.7 nm, while
the nanoball 1 a has an inner-cavity diameter of 1.25 nm and
an outer diameter of 2.13 nm.
Compound 2 is insoluble in common organic solvents,
including more polar solvents such as acetonitrile, acetone,
THF, DME, and 1,2-dichlorobenzene. Attempts to dissolve 2
in DMF resulted in decomposition of the spherical structure
to yield free 3, as evidenced by 1H and 31P NMR spectroscopy.
In the IR spectrum of 2, the CO stretching frequencies display
a significant shift to higher wave numbers (3: 1952, 1983 cm1;
2: 1974, 2040 cm1), which can be attributed to the more rigid
nature of the overall structure. The ESI mass spectrum[16]
shows only partially maintained coordination fragments
[{Cp’’Ta(CO)2(h -P4)}2Cu2Cl]+ cations as well as the disproportionation fragment [{Cp’’Ta(CO)2(h4-P4)}4Cu]+.
Density functional theory (DFT) calculations were performed to address the origin of distortion of the core of 2 from
perfect octahedral symmetry. Calculations on the Oh-symmetric cation [{Ta(h4 :h1:h1:h1:h1-P4)}6{CuCl}8]6+, neutral
[{Ta(h4 :h1:h1:h1:h1-P4)}6{CuCl}8], and [{ClCo(h4 :h1:h1:h1:h1P4)}6{CuCl}8], in which Cp’’ and Ta(CO)2 are replaced by Cl
and Co, respectively, yield in all cases nondegenerate ground
states. This result suggests that the first-order Jahn–Teller
effect is an unlikely reason for the distortion. To investigate
the influence of the ligand arrangement on the structure of 2,
we performed calculations on the original complexes 3 and 2,
as well as on a series of model complexes (see the Supporting
Information). For 3, the optimized structure shows a staggered orientation of the cyclo-P4 and CO ligands, in agreement with experimental data. This orientation and the kite
distortion of the cyclo-P4 ligand from square geometry are a
result of orbital interactions similar to those discussed by Chu
and Hoffmann for cyclo-C4H4 complexes.[17] This interpretation is confirmed by our calculations on the hypothetical
complex [Cp*Co(h4-P4)], in which Cp* and P4 rings are
oriented parallel and the cyclo-P4 ligand shows almost perfect
square geometry (d(PP): 2.185 J, a(P-P-P): 89.68 and
90.48). From calculations on model complexes, we have also
found that the {CuCl} units provide quite flexible linking
between the cyclo-P4 rings. Calculations on complexes in
which the Cp’’ ligands in 3 and 2 are replaced by Cp
(hypothetical complexes 3’ and 2’) leave the structure of the
cyclo-P4 rings and of the core of 2 virtually unchanged.
However, in 2’, the less bulky 3’ units can be rotated by 908
around the axes perpendicular to the P4 units, thus leading to
two D3d-symmetric isomers 2’a and 2’b. The isomers are only 7
and 25 kJ mol1, respectively, less stable than 2’, but the kite
geometry of the P4 rings and their mutual arrangement with
the CO and Cp ligands are preserved. Thus, the CuP
linkages are flexible enough to adjust to the rotation of the
relatively stiff cyclo-P4 units, and essentially preserve the
shape of the building units of 2’.
The results have shown that [Cp’’Ta(CO)2(h4-P4)] (3) can
be employed in the coordination chemistry with copper(i)
halides for the assembly of inorganic, spherical molecules,
thus producing a novel 32-atom core. The use of the P4 ligands
offers a stable source of four-membered rings for the
formation of spherical aggregates, whereas no precedence
has been found for high-symmetry carbon structures bearing
four-membered rings. The distorted-octahedral structure of 2
is a result of the nonparallel arrangement of the Cp’’ and CO
ligands relative to the cyclo-P4 ligands, as well as the flexibility
of the {CuCl} linkages.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2473 –2475
Experimental Section
All reactions were performed under dry argon with standard vacuum,
Schlenk, and glove-box techniques. Solvents were purified and
degassed by standard procedures. Commercial-grade chemicals
were used without further purification. Complex 3 was prepared by
a literature method.[12]
2: A Schlenk tube was charged with a solution of 3 (70 mg,
0.13 mmol) in CH2Cl2 (2 mL). Onto this solution was layered a
mixture of CH2Cl2/MeCN (4 mL/2 mL)and subsequently a solution of
CuCl (17 mg, 0.17 mmol) in CH2Cl2/MeCN (1 mL/1 mL). The doubly
layered system was allowed to stand in an undisturbed area, and
orange crystals of 2 formed within 24 h. The mixture was allowed to
stand 5 days to ensure thorough diffusion, and the mother liquor was
decanted away. The crystals were washed once with MeCN (2 mL)
and twice with CH2Cl2 (2 mL), and dried under vacuum. Yield: 59 mg
(66 %); m.p. 196 8C (decomp., discoloration to black); IR (KBr
pellet): ñCO = 2040 (vs, br), 1974 cm1 (sh); MS (ESI, DMF): m/z (%):
2215.4 (99) [{Cp’’Ta(CO)2P4}4Cu]+, 1239.0 (9.0) [{Cp’’Ta(CO)2P4}2Cu2Cl]+, 1139.1 (100) [{Cp’’Ta(CO)2P4}2Cu]+. Elemental
analysis (%) calcd for C91H128O12Cl10Cu8P24Ta6 (4105.89): C 26.62, H
3.14; found: C 26.74, H 3.19.
DFT calculations were performed by using the TURBOMOLE
program package.[18] The BP86 exchange-correlation functional[19]
was used along with the triple-zeta plus polarization (TZVP) basis
set on all atoms.[20] To speed up the calculations, the Coulomb part was
evaluated by using the MARI-J method[21] and the triple-zeta plus
double set of polarization functions (TZVPP) auxiliary basis sets on
all atoms.[22] Quasi-relativistic pseudopotentials were used for Ta.[23]
Received: October 4, 2005
Published online: March 10, 2006
Keywords: cluster compounds · density functional calculations ·
fullerenes · phosphorus · tantalum
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[13] The crystal structure analysis of 2 was performed on a STOE
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structure was solved by direct methods with the program
SHELXS-97,[24a] and full-matrix least-squares refinement on F2
Angew. Chem. Int. Ed. 2006, 45, 2473 –2475
in SHELXL-97[24b] was performed with anisotropic displacements for non-H atoms. The H atoms were fixed at idealized
positions and refined isotropically by using the riding model. The
solvent molecules are disordered and fill the space between the
clusters of 2. The carbon atom of the CH2Cl2 molecule (C16)
occupies one position on the crystallographic threefold-symmetry axis, whereas each chlorine atom is disordered over three
positions. The additional symmetry operations for the chlorine
atoms of the dichloromethane molecule were defined by the
EQIV instruction. The Uij components of C16, Cl3, and Cl4 were
restrained by the SIMU instruction, and the CCl distances were
fixed to 1.65 J by the DFIX instruction. A free refinement of the
occupancy of C16, Cl3, and Cl4 converged to a value of 0.16667,
at which they were fixed. To calculate the H-atoms on C16, the
symmetry-generated chlorine atoms were freed from C16 by the
FREE instruction. 2·CH2Cl2 : C91H128Cl10Cu8O12P24Ta6, Mr =
4105.73, crystal dimensions 0.08 T 0.08 T 0.04 mm3, trigonal,
space group R3̄ (No. 148); a = b = 22.845(3), c = 22.658(5) J,
T = 150(1) K, Z = 3, V = 10 241(3) J3, 1calcd = 1.997 Mg m3,
m(MoKa) = 6.526 mm1, 22 248 reflections collected, 4007
unique reflexes (Rint = 0.0756, 2 qmax = 508), 3285 observed with
Fo = 4 s (Fo); 250 parameters, R1 = 0.0365, wR2(all data) =
0.0854. CCDC-281812 contains the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
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The inner diameters are calculated as geometrically opposing
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0.18 nm; Cu, 0.14 nm. The outer diameter is taken as twice the
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H (2 T 0.12 nm). For the inner cavity, the term “diameter” is
defined here as the diameter of the largest spherical form that is
geometrically allowed inside the cavity by the given atoms.
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clusters, comprising, four, membered, ring, motiv, six, spherical
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