Titanium-based molecular squares and rectangles: Syntheses by self-assembly reactions of titanocene fragments and aromatic N-heterocycles

Article abstract of DOI:10.1002/chem.200400880

This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyraz

Full text of DOI:10.1002/chem.200400880

FULL PAPER
Titanium-Based Molecular Squares and Rectangles: Syntheses by Self-
Assembly Reactions of Titanocene Fragments and Aromatic N-Heterocycles
[a]
Susanne Kraft, Edith Hanuschek, Rꢀdiger Beckhaus,* Detlev Haase, and Wolfgang Saak
Abstract: This paper reports on the po-
tential of titanium compounds as build-
ing blocks for supramolecular polygons.
Self-assembly reactions of low-valent
titanocene units and N-heterocyclic
bridging ligands lead to novel titanium-
based supramolecular squares. Pyrazine
can be synthesized by stepwise reduc-
tion of the titanocene dichlorides
[Cp TiCl ] and [(tBuCp) TiCl ] and
consecutive coordination of two differ-
ent bridging ligands. The resulting com-
plexes are the first examples of molec-
ular rectangles containing bent metal-
locene corner units. Single-crystal X-
ray analyses of the tetranuclear com-
pounds revealed the geometric proper-
ties of the molecular polygons in the
solid state. Comparison of bond lengths
and angles in coordinated and free li-
gands reveals the reduced state of the
bridging ligand in the low-valent titani-
um compounds. The syntheses and
properties of these novel, highly air-
and moisture-sensitive compounds are
discussed.
2
2
2
2
(
(
3), 4,4’-bipyridine (4), and tetrazine
5) were used as bridging ligands, and
2
the acetylene complexes [Cp Ti{h -
C (SiMe ) }] (1) and [(tBuCp) Ti{h -
2
Keywords: bridging ligands · nitro-
gen heterocycles · self-assembly ·
supramolecular chemistry · titanium
2
2
3
2
2
C (SiMe ) }] (2) as sources of titano-
2
3 2
cene fragments. Molecular rectangles
Introduction
sition metals, only a few attempts have been made to use
the well-defined coordination modes and reducing proper-
ties of early transition metals.
[19,20]
The design of highly ordered supramolecular structures has
attracted more and more interest in the last few decades.
The concept of self-assembly occupies a key position in this
field, and a multitude of supramolecular compounds have
been synthesized by combining simple building blocks to
The formation of molecular squares and rectangles re-
quires 908 angles at the vertices, as are typical of square-
[
1]
planar or octahedral late transition-metal species. Hence,
only a few examples are known with distorted tetrahedral
[1–3]
[21–23]
form two- and three-dimensional structures.
Owing to
geometries at the corners.
their electronic and steric versatility, aromatic N-heterocy-
cles play a prominent role as classical ligands in coordina-
In the course of our studies on the reactions of low-valent
titanium compounds and aromatic N-heterocycles, we suc-
ceeded in synthesizing various tetranuclear complexes, for
which single-crystal X-ray structure analyses confirmed
square and rectangular molecular structures. Here we report
on the syntheses of these novel self-assembled polynuclear
titanium complexes and their properties.
[4,5]
tion compounds,
as bridging ligands in binuclear deriva-
[6–8]
tives,
and as building blocks for supramolecular com-
[9–17]
pounds.
In addition to their ability to connect metal cen-
ters by forming ligand–metal bonds, they provide the oppor-
tunity of backbonding and thereby may affect
p
[18]
delocalization and transport of electrons. Compared with
the highly developed supramolecular chemistry of late tran-
Results and Discussion
[
a] Dr. S. Kraft, E. Hanuschek, Prof. Dr. R. Beckhaus, D. Haase, W. Saak
Institut fꢀr Reine und Angewandte Chemie
Fakultꢁt fꢀr Mathematik und Naturwissenschaften
Carl von Ossietzky Universitꢁt Oldenburg
Postfach 2503, 26111 Oldenburg (Germany)
Fax : (+49)441-798-3581
We recently reported on the reaction of the excellent titano-
2
[24]
cene precursor [Cp Ti{h -C (SiMe ) }] (1)
with pyrazine
3), which led to the formation of the first structurally char-
acterized molecular square with titanocene(ii) corner units,
2
2
3 2
(
[21]
[
{Cp Ti(m-C H N )} ]. By using different metal complexes
2 4 4 2 4
E-mail: ruediger.beckhaus@uni-oldenburg.de
(
1, 2) and ligands (3, 4, 5) as starting materials, further neu-
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
tral molecular squares with titanocene corner units can be
Chem. Eur. J. 2005, 11, 969 – 978
DOI: 10.1002/chem.200400880
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
969

synthesized. Scheme 1 shows the formation of molecular
squares 6–9, which were characterized by single-crystal X-
ray analysis, elemental analysis, and IR spectroscopy. All
compounds are intensely colored and highly sensitive to air
and moisture.
2
The reaction of [Cp Ti{h -C (SiMe ) }] (1) with 4,4-bipyri-
2
2
3 2
dine (4) in toluene leads after a few minutes to a color
change from yellow to dark blue, and after 48 h at 608C
dark blue crystals of 6 can be isolated in yields of about
50%. Tetrazine-bridged complex 7 can be isolated from a
dilute reaction solution of 1 and tetrazine (5) in toluene
after 48 h as a crystalline solid. The dark blue crystals of
both complexes show an intense metallic luster. They are
only sparingly soluble in aliphatic and aromatic solvents and
ethers and do not melt below 2508C. In the mass spectra
(
EI, 70 eV) no molecular peaks could be observed. Owing
to their low solubilities, recrystallization was not possible.
Therefore, suitable single crystals for X-ray analysis were
grown from the reaction solutions.
Single crystals of 6 can be obtained at 608C from toluene
and in better quality from tetralin at room temperature. The
molecular structure of 6, crystallized from tetralin (6a), is
shown in Figure 1. Complex 6a crystallizes in the space
Figure 1. Structure of 6a (50% probability, without H atoms). Selected
bond lengths [ꢃ] and angles [8]: Ti1ÀN1 2.2132(17), Ti1ÀN2a 2.1976(17),
Ti1ÀCt1 2.086, Ti1ÀCt2 2.092, N1ÀC1 1.358(3), N1ÀC1 1.374(3), N2ÀC6
1.363(3), N2
ÀC10 1.370(3), C1ÀC2 1.366(3), C2ÀC3 1.418(3), C3ÀC4
1
1
.418(3), C3ÀC8 1.424(3), C4ÀC5 1.366(3), C6ÀC7 1.368(3), C7ÀC8
group P4 /n with four solvent molecules per tetramer. The
2
.424(3), C8ÀC9 1.423(3), C9ÀC10 1.364(3); N1-Ti-N2a 84.83(6), Ct1-Ti-
metal atoms are coordinated tetrahedrally by two Cp ligands
and two heterocycles. As the titanium atoms are located in
one plane the complex forms a nearly perfect square with
the bent metallocene moieties as corner units.
Ct2 132.39. Ct1=ring centroid of C11–C15, Ct2=ring centroid of C16–
C20; symmetry transformation for the generation of equivalent atoms a:
Ày+1/2, x, Àz+1/.
Single crystals of tetrazine-bridged complex 7 can be
grown from dilute reaction mixtures in toluene. Figure 2
shows the molecular structure of 7. Complex 7 crystallizes in
molecules. In contrast to the analogous tetranuclear pyra-
[21]
zine-bridged complex [{Cp Ti(m-C H N )} ] (10), tetrazine
2
4
4
2
4
the space group P42 c, and the crystal contains no solvent
complex 7 does not really form a molecular square, since
1
Scheme 1. Reactions of titanocene complexes 1 and 2 with pyrazine (3), bipyridine (4), and tetrazine (5).
970
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Titanium-Based Molecular Squares and Rectangles
FULL PAPER
Figure 3. Structure of 8 in the crystal (50% probability, without H
atoms). Selected bond lengths [ꢃ] and angles [8]: Ti1ÀN8 2.132(3), Ti1À
N1 2.186(3), Ti1ÀCt1 2.115, Ti1ÀCt2 2.118, N1ÀC19 1.378(4), N1ÀC22
Figure 2. Structure of 7 in the crystal (50% probability, without H
atoms). Selected bond lengths [ꢃ] and angles [8]: Ti1ÀN1 2.028(5), Ti1À
N3a 2.132(5), Ti1ÀCt1 2.086, Ti1ÀCt2 2.075, N1ÀC1 1.377(7), N1ÀN2
1
.381(4), N2ÀC21 1.388(4), N2ÀC20 1.391(4), C19ÀC20 1.352(4), C21À
C22 1.359(4); N8-Ti1-N1 84.30(10), Ct1-Ti1-Ct2 133.49, N2-Ti2-N3
5.08(10).
1
1
.420(5), N2ÀC2 1.305(7), N3ÀC2 1.337(7), N3ÀN4 1.412(7), N4ÀC1
8
.298, N1-Ti1-N3a 88.84(19), Ct1-Ti-Ct2 130.24. Ct1=ring centroid of
C3–C7, Ct2=ring centroid of C8–C12; symmetry transformation for the
generation of equivalent atoms a: Ày+1, x+1, Àz+2.
the four titanium atoms do not lie in one plane but rather
form a tetrahedron. As is mostly observed with tetrazine,
the heterocycle coordinates as a bis(monodentate) ligand,
[7]
similar to pyrazine, and not as a bis(bidentate) ligand.
To obtain analogous complexes with higher solubilities
2
[
(tBuCp) Ti{h -C (SiMe ) }] (2) was used as the source of a
2
2
3 2
titanocene fragment with bulky substituted Cp ligands. The
reactions of 2 with pyrazine and bipyridine proceed more
slowly but show the same color changes to violet and blue
2
that are observed with [Cp Ti{h -C (SiMe ) }] (1). If the re-
2
2
3 2
actions with pyrazine and 4,4’-bipyridine are carried out in
n-hexane, crystals of 8 and 9 can be isolated from the reac-
tion mixture in yields of 65 and 79%, respectively. Com-
pared to the analogous complexes with unsubstituted Cp li-
gands they show considerably increased solubility in aromat-
ic solvents and THF. Furthermore, they have lower melting
points (8: 197–2008C, 9: 203–2068C), but again no molecu-
lar peaks could be observed in the mass spectra (EI, 70 eV).
Single crystals of 8 could be obtained from n-hexane;
single crystals of 9 were grown by slow diffusion of n-
hexane into a THF solution. Figures 3 and 4 show the mo-
lecular structures of 8 and 9. Complex 8 crystallizes in the
space group P2 /n, and the crystal contains two n-hexane
1
molecules per molecular square. Compound 9 crystallizes in
¯
the space group P1 and contains 11 molecules of THF per
Figure 4. Structure of 9 in the crystal (50% probability, without H
ÀN8 2.19(3), Ti1ÀN1
tetranuclear unit. Both complexes show a more or less
square configuration. The sterically demanding tBu groups
take up nearly the same position in both complexes. The
TiÀN distances in 6, 8, and 9 lie at the upper limit for TiÀN
atoms). Selected bond lengths [ꢃ] and angles [8]: Ti1
2
.22(4), Ti1ÀCt1 2.039, Ti1ÀCt2 2.104 Ti2ÀN2 2.19(4), N1ÀC23 1.37(5),
N1ÀC19 1.37(5), N2ÀC24 1.38(6), N2ÀC28 1.38(5), C19ÀC20 1.36(6),
C20ÀC21 1.42(6), C21ÀC26 1.42(6), C21ÀC22 1.43(6), C22ÀC23 1.37(6),
C24 C25 1.36(7), C25 C26 1.43(6), C26 C27 1.43(6), C27 C28 1.35(7);
N8-Ti1-N1 83.7(13), Ct1-Ti1-Ct2 134.18.
À
À
À
À
bonds and correspond to values expected for titanium-coor-
Chem. Eur. J. 2005, 11, 969 – 978
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R. Beckhaus et al.
[
21]
dinated N heterocycles. Bond lengths and angles of the ti-
tanocene units correspond to known values for tetrahedral
coordination geometry.
was used as soluble reducing agent, and sparingly soluble
rectangle 14 precipitated from the reaction mixture. To in-
hibit dissociation of complexes 12 and 13 and avoid ex-
change of the ligands during reduction, the reaction was car-
ried out at À788C.
The successful syntheses of molecular squares with bridg-
ing ligands 3–5 led us to attempt to synthesize a mixed-
bridge complex that contains bridging ligands of different
lengths and forms a molecular rectangle. Only a few molec-
ular rectangles are hitherto known, because most attempts
to synthesize them in one-step reactions resulted in the pre-
ferred formation of the two homobridged molecular
If pathway b) in Scheme 2 is used and 12 is reduced in
presence of pyrazine, the reaction does not lead to the rec-
tangular complex 14; instead, formation of molecular square
[26]
10 is accompanied by a further product (probably 6).
However, if 13 is reduced in the presence of 4,4’-bipyridine
(pathway a), 14 can be isolated as needle-shaped blue violet
crystals with an intense metallic luster. This complex was
characterized by X-ray analysis, elemental analysis, and IR
spectroscopy. The synthesis of 14 can be further simplified
[3,14]
squares.
Therefore, a reaction with two subsequent steps
was used to coordinate the two different ligands to the tita-
nocene moiety. Scheme 2 shows possible synthetic routes
starting from titanocene(iii) chloro complex 11.
such
that,
starting
from
[
Cp TiCl ], neither 11 nor 12
2
2
must be isolated and the molec-
ular rectangle is easily accessi-
ble from simple, commercially
available starting materials. If
titanocene dichloride is reduced
in the presence of pyrazine with
one equivalent of lithium naph-
thalenide, and then a solution
of 4,4’-bipyridine and the
second equivalent of lithium
naphthalenide are added after
cooling to À788C, 14 can be
isolated in 45% yield. Crystalli-
zation from THF yielded crys-
tals of 14 that were suitable for
X-ray diffraction. The molecu-
lar structure of 14 is shown in
Figure 5.
Compound 14 crystallizes
from THF in the trigonal space
group P3 21 with two solvent
1
molecules per tetranuclear mol-
ecule in the crystal. Each tita-
Scheme 2. Possible synthetic routes to molecular rectangle 14.
nocene unit is coordinated by a
pyrazine and a 4,4’-bipyridine
In the first reaction step the first bridging ligand is coordi-
ligand, and with the planar configuration of the four titani-
um atoms a rectangular geometry results for the complex.
The efficient synthesis of 14 requires the absence of free
pyrazine (3). If 14 is treated with 3, 10 is formed by ligand
nated between two [Cp TiCl] units whose last coordination
2
site is blocked by the chloro ligand. This reaction could be
carried out successfully with pyrazine and bipyridine, and
complexes 12 and 13 could be isolated as green crystals in
yields of 55 and 43%, respectively, and characterized by X-
[26]
exchange.
Therefore, only pathway a) is successful. On
the other hand no ligand exchange reactions occur between
14 and 4. Disproportionation of 14 itself to 6 and 10 appa-
rently do not take place.
[25a]
ray analysis, IR spectroscopy, and elemental analysis.
In
the mass spectra of 12 and 13 only peaks of the free ligands
and [Cp TiCl] are observed, and this indicates low stability
of the dimeric compounds. For similar monomeric com-
A similar, but more soluble, rectangular complex 15 can
be obtained by the same procedure by using [(tBuCp) TiCl ]
2
2
2
pounds [Cp TiClL] (L=pyridine, PPhMe ) complete dissoci-
instead of [Cp TiCl ] as starting material. After evaporating
2 2
2
2
ation into the ligand and 11 was observed at higher temper-
the solvent from the reaction mixture and dissolving the res-
idue in toluene, 15 can be obtained by filtration to remove
LiCl and subsequent addition of n-hexane. Again the blue-
violet crystals show an intense metallic luster. Single crystals
of 15 can be grown by recrystallisation from benzene, from
[25b]
ature (1308C in vacuo for [Cp TiClPPhMe ]).
In the
2
2
second step abstraction of the chloro ligand by reduction of
titanocene(iii) complexes 12 and 13 and coordination of the
second bridging ligand take place. Lithium naphthalenide
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Titanium-Based Molecular Squares and Rectangles
FULL PAPER
can be removed by drying the
crystalline solid in vacuum. In
6
a the tetralin molecules are lo-
cated in channels that are
formed by the molecular
squares. Figure 7 shows a larger
section of the structure of 6a
including the solvent molecules.
The importance of the sol-
vent molecules for the solid
state structure and the relative-
ly great conformational free-
dom of the tetranuclear com-
pounds is shown by the struc-
tures of 6 obtained by crystalli-
zation from tetralin (6a) and
toluene (6b). Figure 8 shows
the configuration of the four ti-
tanium atoms in 6a and 6b
from the side view onto the tet-
Figure 5. Structure of 14 in the crystal (50% probability, without H atoms). Selected bond lengths [ꢃ] and
angles [8]: Ti1ÀN1 2.166(4), Ti1ÀN4a 2.215(4), Ti1ÀCt1 2.111, Ti1ÀCt2 2.068, Ti2ÀCt3 2.095, Ti2ÀCt4 2.094
Ti2ÀN2 2.128(4), Ti2ÀN3 2.220(4), N1ÀC13 1.375(6), N1ÀC11 1.385(6), N2ÀC14 1.379(6), N2ÀC12 1.396(6),
ÀC29 1.353(6), N3ÀC25 1.359(6), N4ÀC30 1.352(7), N4ÀC34 1.356(6), C11ÀC12 1.359(7), C13ÀC14 1.351(7), ramers.
N3
C25ÀC26 1.371(7), C26ÀC27 1.426(7), C27ÀC28 1.425(7), C27ÀC32 1.432(7), C28ÀC29 1.359(7), C30ÀC31
The monocyclic bridged com-
plexes 7 and 8 also exhibit dif-
ferent configurations (Figure 9).
The difference becomes visible
in the arrangement of the tita-
1
8
.347(7), C31ÀC32 1.427(7), C32ÀC33 1.411(7), C33ÀC34 1.363(7); N1-Ti1-N1a 83.68(15), N2-Ti2-N3
4.10(15), Ct1-Ti-Ct2 131.48, Ct3-Ti2-Ct4 131.00. Ct1=ring centroid of C1–C5, Ct2=ring centroid of C6–C10,
Ct3=ring centroid of C15–C19, Ct4=ring centroid of C20–C24; symmetry transformation for the generation
of equivalent atoms a: xÀy+1, Ày+2, Àz+2/3.
nium centers. Whereas in 8 all
titanium atoms lie in one plane, a “butterfly-like” arrange-
ment is found for 7.
¯
which 15 crystallizes in the space group P1. The coordina-
tion geometry of 15 shows no significant differences to the
above-discussed structures of
the other tetrameric complexes.
The molecular structure of 15 is
shown in Figure 6. The TiÀN
distances to the pyrazine
bridges in 14 and 15 are 0.05–
0
.09 ꢃ shorter than the TiÀN
distances of the titanium–bipyr-
idine bond.
In contrast to most of the
known molecular rectangles
[3]
with basically different sides,
the titanium-based compounds
4 and 15 contain two bridging
1
ligands of similar type. Molecu-
lar rectangles with pyrazine and
4,4’-bipyridine bridges and octa-
hedrally coordinated rhenium
[27]
corners
exhibit comparable
L-M-L angles (83.58, 14: 83.9 8,
15: 83.8) and cavity sizes (7.21ꢄ
11.44 ꢃ, 14: 7.20ꢄ11.52 ꢃ, 15:
7
.22ꢄ11.38 ꢃ). However, in 14 Figure 6. Structure of 15 in the crystal (50% probability, without H atoms). Selected bond lengths [ꢃ] and
and 15 the rectangular geome- angles [8]: Ti1
try is realized by tetrahedrally
coordinated corner atoms.
ÀN8 2.121(5), Ti1ÀN1 2.204(5), Ti1ÀCt1 2.104, Ti1ÀCt2 2.093, Ti2ÀN2 2.153(5), Ti2ÀN3 2.155(6),
Ti2ÀCt3 2.100, Ti2ÀCt4 2.088, N1ÀC19 1.364(8), N1ÀC23 1.379(7), N2ÀC28 1.391(8), N2ÀC24 1.411(7), N3À
C47 1.367(8), N3ÀC49 1.385(7), N4ÀC50 1.396(8), N4ÀC48 1.402(7), C19ÀC20 1.397(8), C20ÀC21 1.410(8),
C21ÀC22 1.422(8), C21ÀC26 1.431(7), C22ÀC23 1.390(8), C24ÀC25 1.377(8), C25ÀC26 1.416(8), C26ÀC27
Except for 7 all complexes
À
À
À
1
.422(8), C27 C28 1.380(8), C47 C48 1.346(9), C49 C50 1.346(9); N8-Ti1-N1 84.82(18), Ct1-Ti1-Ct2 134.66,
contain solvent molecules that N2-Ti2-N3 82.51(19), Ct3-Ti2-Ct4 134.36.
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R. Beckhaus et al.
Although all N-Ti-N angles lie in the relatively small
range between 83.2 and 86.68 the complexes exhibit quite
different conformations, as shown in Figures 8 and 9. This is
indicated by the different Ti-Ti-Ti angles and their sum. If
all corner atoms lie in the same plane a sum of 3608 results
for the quadrangle, whereas any distortion of the planar
configuration leads to a decrease in the sum. Table 1 lists
the average values for the N-Ti-N angles, the Ti-Ti-Ti angles
and their sum and the TiÀTi distances for all complexes. The
sums of the Ti-Ti-Ti angles show that 6a, 8, 9, 14, and 15
Table 1. Selected geometric data of complexes 6a, 6b, 7, 8, 9, 14, and 15.
TiÀTi
ꢃ]
N-Ti-N [8]
[8]
Ti-Ti-Ti [8]
[8]
ꢀTi-Ti-Ti [8]
[8]
[
6
6
7
8
9
1
a
b
11.586
11.500
6.691
7.203
11.521
7.203
84.83(6)
83.4(4)
84.3(3)
84.23(10)
83.45(13)
83.89(15)
89.75
68.05
71.24
89.82
88.15
89.37
359.0
272.2
285.0
359.3
352.6
357.5
4
5
1
1.516
7.219
1.380
1
83.79(18)
89.95
359.8
1
Figure 7. Solid-state structure of 6a with tetralin molecules.
Figure 8. Side view of the structure of 6a (from tetralin) and 6b (from toluene). Cp rings are omitted for clarity.
Figure 9. Side view of 8 (left) and 7 (right).
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Titanium-Based Molecular Squares and Rectangles
FULL PAPER
adopt a nearly planar configuration, whereas 6b and 7 are
distorted from planarity.
Table 2. Bond lengths and angles for free, reduced and coordinated 4,4’-
biypridine.
C’ÀC’ [ꢃ]
Twist angle [8]
As the titanocene fragment has proved to reduce some ar-
[21]
omatic N heterocycles by CÀC coupling reactions and CÀ
[28]
H and CÀF bond cleavage, as well as forming complexes
1.4842(19)
1.4895(18)
34.39(6)
18.14 (8)
0
with stable heterocyclic radical anions by electron trans-
[
29,30]
fer,
it seems reasonable to assume electron transfer to
the ligand also for these reactions and some proof for this is
given by the molecular structures in the solid state.
1
.381(3)
In numerous complexes of low-valent metals and hetero-
cyclic ligands changes in bond lengths and angles that are
sensitive to transfer of electrons have been used to evidence
the electronic structure. With tetrazine derivatives as bridg-
ing ligands, for example, lengthening of the NÀN bonds by
6a
9
1.424(3)
1.425(30)
1.432(7)
1.438(7)
4.23
7.60
4.89
9.80
1
1
4
5
0
.068 and 0.073 ꢃ, observed by Kaim et al., indicated the
[
31,32]
formation of a radical anion in two copper complexes.
dicate for the 4,4’-bipyridine complexes an electronic struc-
ture with a radical ligand and titanium centers in the oxida-
tion state +III. A change in bond lengths towards those of
the reduced species is also observed in the lengthening of
0
In an Fe complex of 2,2’-bipyridine, electron transfer to the
heterocycle leads to a decrease in the bond length between
II
the two pyridyl rings by 0.083 ꢃ, and in a dimeric Yb com-
[36]
plex of 2,2’-bipyrimidine with a dianionic ligand, a shorten-
the NÀN bond in tetrazine from 1.321 ꢃ in free tetrazine
[
6,33,34]
ing of 0.142 ꢃ was observed.
Scheme 3 shows the two
to 1.416(6) ꢃ in 7. Another indication for the reduction of
the ligand is given by the change in the conformation of the
ligand in 7, which no longer ex-
hibit the planarity of the free
ligand (Figure 2).
[7,8]
reduction steps for tetrazine and bipyridine.
The electronic structure of
the radical complexes formed
by electron transfer from titani-
um to N-heterocyclic ligands
has been thoroughly investigat-
ed for 2,2-bipyridine complexes,
which show a singlet ground
state and a thermally accessible
Scheme 3. Two-electron reductions of tetrazine and bipyridine.
[29,30,37]
triplet state.
Similar to
monomeric 2,2’-bipyridine com-
plexes, for 4,4’-bipyridine complex 6, antiferromagnetic be-
havior is observed for the temperature dependence of the
Two-electron reduction of tetrazine leads to a single bond
between the two nitrogen atoms, and reduction of 4,4-bipyri-
dine to a double bond between the two aromatic rings and
an angle of 08 between the two pyridyl rings. These bonds
and angles show the greatest changes on reduction. To com-
pare our data to the values of the free ligand and a fully re-
duced species, the crystal structures of free 4,4-bipyridine
[38]
magnetic susceptibility. Owing to the special geometry of
[39,40]
the frontier orbitals in metallocene units,
the 2,2’-bipyri-
dine can not act as a classical p-acceptor ligand but forms
complexes with two remote electrons in interaction that are
not diamagnetic like the p-acceptor complexes of titanocene
with carbonyl or phosphane ligands. If an overlap of metal
and ligand orbitals is possible, a singlet ground state results
(
4) and two-electron-reduced bis(trimethylsilyl)dihydro-4,4’-
[25a]
[35]
bipyridine (16)
were determined. The asymmetric units
[37]
of both structures contain two independent molecules, and
for 4,4-bipyridine the important values are given for both,
since the angles between the pyridyl rings differ significantly
in the structures of the two molecules. In 16 no significant
differences exist, so here the average is given. Table 2 lists
the angles and bond lengths between the two pyridyl rings
in 4,4’-bipyridine for the free ligand, for the two-electron-re-
duced species 16 and for the titanium-coordinated ligands.
For the coordinated ligands these values show a signifi-
cant shift towards those of the two-electron-reduced species.
Nevertheless, the decrease is less than that observed in an
ytterbium complex that contains bipyrimidine as a dianionic
for the two remote electrons. This situation occurs if the
ligand orbitals lie in the L-Ti-L plane. For the overlap with
the p* orbitals of the heterocycle in the tetrameric com-
plexes it is thus important how far the ligand is rotated
around the TiÀN bond.
Tetrazine complex 7 shows two different conformations of
the bridging ligands towards the titanium atoms. The two re-
spective bond lengths differ in a characteristic manner, as is
[41]
also known for dp–pp interactions in titanium amides.
With the ligand in the straight position (angle of N-N-C
plane of 5 to N-Ti-N 55.328) an overlap of metal acceptor
and ligand donor orbitals becomes possible, and in this case
a short TiÀN bond of 2.028(5) ꢃ is found, while the other
[6]
ligand. So the changes in bond lengths and angles may in-
Chem. Eur. J. 2005, 11, 969 – 978
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
975

R. Beckhaus et al.
bond length of 2.132(5) ꢃ is considerably longer (angle of
N-N-C plane of 5 to N-Ti-N 12.308; Figure 10).
(75 MHz, C
6
D
6
, 303 K): d=1.48 (SiCH
).
3 3 3 3 3
), 31.1 (C(CH ) ), 32.7 (C(CH ) ),
1
13.5, 117.4, 144.3 ppm (C H
5 4
Details concerning 4, 12, 13, and 16 are given in the Supporting Informa-
tion.
Owing to the low solubility of 6, 7, and 14, NMR spectra
are not available. The more soluble complexes 8, 9, and 15
6
(
6
: Solutions of 1 (750 mg, 2.15 mmol) in toluene (60 mL) and of 4
336.2 mg, 2.15 mmol) in toluene (40 mL) were combined and heated at
08C for 48 h without stirring. The solution turned blue, and dark blue
1
exhibit sharp H NMR signals of the tBu groups (d=1.16
(
9); 1.12 (8); 1.19, 1.44 (15)). Further signals show more or
less pronounced broadening and chemical shifts over a wide
range, as is characteristic for nondiamagnetic compounds.
crystals with a metallic luster separated and were isolated in 50% yield
by decanting, washing with n-hexane and drying under vacuum. M.p.
[38]
>
2508C; IR (KBr): n˜ =2971 (m), 2901 (m), 2855 (m), 1661 (w), 1616 (s),
1
8
431 (s), 1375 (m), 1271 (s), 1190 (s), 1022 (m), 1036 (m), 968 (s), 941 (s),
01 (w), 734 (w), 737 (m), 613 (m), 606 (m), 417 (m), 399 (w), 336 cm
À1
(
72 8 4
m); elemental analysis (%) calcd for C80H N Ti : C 71.86, H 5.43, N
8
.38; found: C 72.06, H 5.51, N 8.30.
7
: Solutions of 1 (750 mg, 2.15 mmol) in toluene (60 mL) and of 5
(
336.2 mg, 2.15 mmol) in toluene (10 mL) were combined. The solution
turned blue, and tiny crystals with a metallic luster separated after 2–
days. After decanting the mother liquor, washing with n-hexane and
drying under vacuum, 7 was isolated in 15% yield. M.p. >2508C; IR
KBr): n˜ =3104 (w), 2969 (w), 1639 (w), 1533 (s), 1476 (m), 1442 (w),
5
(
1
395 (w), 1307 (m), 1193 (w), 1102 (w), 1074 (w), 1014 (m), 939 (s), 806
À1
(
s), 730 cm (w); elemental analysis (%) calcd for C56
H
56
N
16Ti
4
: C 55.41,
H 4.65, N 21.54; found: C 55.19, H 4.63, N 21.38.
8
: Solutions of 2 (400 mg, 0.868 mmol) in n-hexane (15 mL) and of 3
Figure 10. Positions of the two ligands at the titanium atoms and respec-
tive bond lengths [ꢃ] in 7.
(70 mg, 0.868 mmol) in n-hexane (10 mL) were combined and heated to
608C. The solution turned violet, and dark violet crystals separate after a
few hours. After decanting the mother liquor and drying in vacuum, 8
was isolated in 65% yield. M.p. 197–2008C; IR (KBr): n˜ =2957 (s), 2900
(
(
m), 2862 (m), 1618 (m), 1485 (w), 1460 (m) 1360 (m), 1280 (m), 1270
m), 1160 (w), 1048 (w), 1013 (s), 859 (w), 803 (m), 772 (s), 684 cm (w);
Conclusions and Outlook
À1
120 8 4
elemental analysis (%) calcd for C88H N Ti : C 71.35, H 8.16, N 7.56;
The reactions and compounds discussed in this paper show a
great and hitherto unused potential of early transition-metal
compounds and especially bent titanocene units as building
blocks in self-assembly reactions. A variety of molecular
squares with different Cp ligands and different bridging li-
gands are easily accessible by reaction of titanocene precur-
sors, especially titanocene acetylene complexes, with N-het-
erocyclic ligands. In contrast to most of the cationic, water-
soluble molecular polygons of late transition metals the tita-
nium complexes are neutral and highly sensitive to air and
moisture. Particularly the facile syntheses of the first titani-
um-containing molecular rectangles from easily available
starting materials demonstrate the opportunities provided
by the use of titanocene building blocks. A novel class of
molecular squares and rectangles containing tetrahedrally
coordinated corner units is presented. Further studies may
provide an insight into the interesting electronic and mag-
netic properties of these complexes. Furthermore these in-
vestigations open up new vistas onto the synthesis of higher
aggregated, for example, three-dimensional, compounds
with titanium units on the basis of self-assembly chemistry.
found: C 71.71, H 8.33, N 7.45.
9: Compounds 2 (400 mg, 0.868 mmol) and 4 (135.6 mg, 0.868 mmol)
were dissolved in n-hexane (50 mL) by heating. The solution turned blue,
and tiny needle-shaped crystals with a metallic luster separated immedi-
ately. After decanting the mother liquor, washing with n-hexane and
drying in vacuum, 9 was isolated in 79% yield. M.p. 203–2068C; IR
(KBr): n˜ =2958 (s), 2900 (m), 2863 (w), 1660 (w), 1600 (s), 1478 (w), 1459
(w), 1405 (w), 1360 (w), 1277 (w), 1205 (m), 1049 (w), 1015 (m), 959 (s),
À1
7
80 (m), 734 (s), 669 (m), 613 cm (w); elemental analysis (%) calcd for
C
112
H
136
N
8
Ti
4: A solution of lithium naphthalenide in THF (10 mL, 0.4m, 4 mmol)
was added dropwise with vigorous stirring to a solution of [Cp TiCl
996 mg, 4 mmol) and 3 (160.2 mg, 2 mmol) in THF(50 mL). The result-
ing dark green solution was cooled to À788C, and a solution of 4
312.4 mg, 2 mmol) and a further 10 mL of lithium naphthalenide in THF
4
: C 75.33, H 7.68, N 6.27; found: C 74.80, H 7.43, N 6.42.
1
2
2
]
(
(
were added. The solution turned blue-violet. After filtration 14 crystal-
lized as needle-shaped crystals with an intense metallic luster. After de-
canting the mother liquor and washing with n-hexane, 14 was isolated in
45% yield. M.p. 225–2308C; IR (KBr): n˜ =3088 (w), 1597 (s), 1437(w),
1271 (w), 1204 (m), 1065 (w), 1013 (m), 963 (m), 797 (s), 615 cm (w);
elemental analysis (%) calcd for C68
found C 68.78, H 5.54, N 9.42.
À1
64 8 4
H N Ti : C 68.94, H 5.44, N 9.46;
1
5: A solution of lithium naphthalenide in THF (5 mL, 0.4m, 2 mmol)
was added dropwise with vigorous stirring to a solution of [(tBuCp)2-
TiCl ] (722.38 mg, 2 mmol) and 3 (80.09 mg, 1 mmol) in 25 mL THF. The
2
resulting brown solution was cooled to À788C, whereby the color
changed to green, and a solution of 4 (156.2 mg, 1 mmol) and a further
5
mL of lithium naphthalenide in THF were added. The solution turned
Experimental Section
blue-violet. After evaporation, addition of 20 mL of toluene and filtra-
tion, 15 crystallized on adding n-hexane as needle-shaped crystals with an
intense metallic luster. After decanting the mother liquor and washing
with n-hexane, 15 was isolated in 21% yield. M.p. 202–2038C; IR (KBr):
n˜ =2960 (s), 2901 (m), 2866 (w), 1602 (s), 1488 (w), 1460 (w), 1407 (w),
1361 (w), 1279 (w), 1261 (w), 1205 (s), 1160 (m), 1049 (w), 1017(m), 963
(m), 860 (w), 803 (s), 777 (m), 735 cm (w); elemental analysis (%)
calcd for C100
N 6.98.
General: All titanium compounds were synthesized and handled in an
inert gas atmosphere (Schlenk techniques). The solvents were thoroughly
dried and saturated with nitrogen prior to use. Compound 1 was pre-
[
42]
pared according to a literature procedure and 2 by following the same
procedure starting with [(tBuCp) TiCl ]. 2: yield: 78%; m.p. 65–678C,
, 303 K): d=À0.14 (s, 18H; SiCH ), 0.66 (s,
), 6.48 (m, 4H; C ), 6.73 ppm (m, 4H; C ); C NMR
À1
2
2
1
H NMR (300 MHz, C
8H; C(CH
6
D
6
3
128 8 4
H N Ti : C 73.52, H 7.90, N 6.86; found: C 73.26, H 7.76,
1
3
1
3
)
3
5
H
4
5 4
H
976
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 969 – 978

Titanium-Based Molecular Squares and Rectangles
FULL PAPER
Table 3. Crystal structure data for complexes 6, 7, 8, 9, 14, and 15.
6
a
7
8
9
14
15
empirical formula
formula weight
diffractometer
crystal dimen-
sions [mm]
color
C
80
H
72
N
8
Ti
4
·4C10
H
12
C
56
H
56
N
16Ti
4
C
88
H
120
N
8
Ti
4
·2C
6
H
14
C
112 136 8 4 4 8 68 64 8 4 4 8 100 128 8 4 6 6
H N Ti ·11C H O C H N Ti ·8C H O C H N Ti ·7C H
1865.84
STOE IPDS
0.5ꢄ0.39ꢄ0.25
1040.62
STOE IPDS
0.10ꢄ0.10ꢄ0.09 0.55ꢄ0.38ꢄ0.14
1653.86
STOE IPDS
2579.03
STOE IPDS
0.55ꢄ0.36ꢄ0.12
1761.70
STOE IPDS
2180.46
STOE IPDS
1.00ꢄ0.18ꢄ0.18
0.45ꢄ0.18ꢄ0.07
black
black
black
black
black
black
crystal system
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
tetragonal
18.8020(5)
18.8020(5)
13.3756(4)
90
tetragonal
12.3928(8)
12.3928(8)
14.8824(11)
90
monoclinic
21.9682(6)
13.6003(4)
31.9150(9)
90
90.630(3)
90
9534.8(5)
triclinic
trigonal
23.3597(6)
23.3597(6)
17.5773(3)
90
90
120
8306.5(3)
triclinic
15.8890(4)
19.2117(7)
25.4357(10)
110.921(4)8
91.649(4)8
91.223(4)8
7245.3(4)
18.4806(14)
19.6711(14)
20.1828(10)
110.746(7)8
110.169(7)8
95.414(9)8
6241.7(7)
b [8]
90
90
90
90
g [8]
V [ꢃ ]
3
4728.5(2)
2285.7(3)
¯
¯
space group
P4
2
/n
P42
1
c
P2
1
/n
P1
P3
1
21
P1
Z
1
2
2
4
2
3
2
À3
[Mgm
]
1.310
0.383
1968
1.512
0.729
1072
1.152
0.371
3568
1.182
0.273
2784
1.057
0.328
2808
1.160
0.299
2332
À1
m [mm
]
F
000
l(MoKa, graphite) [ꢃ] 0.71073
0.71073
193(2)
2.14–26.13
0.71073
193(2)
2.12–26.01
0.71073
193(2)
2.10–25.99
0.71073
193(2)
2.09–26.03
0.71073
193(2)
1.95–26.00
T [K]
193(2)
q range for collec-
tion [8]
2.16–25.99
no. of reflns colld
45897
18505
1012
89745
12979
90165
14227
72831
6503
77141
8407
no. of observed reflns 3063
I>2s(I)]
no. of indep reflns
absorption correction numerical
[
4626
2265
none
0.9373/0.9306
18643
numerical
0.9499/0.8220
26487
numerical
0.9680/0.8644
10228
none
0.9433/0.7349
22746
numerical
0.9794/0.8771
max./min. transmis-
0.9104/0.8316
sion
data/restraints/param- 4626/0/306
eters
2265/0/136
18643/36/977
26487/130/1398
10228/40/425
22746/0/1295
R indices (all data)
R
1
0.0704
0.1113
0.1444
0.0742
0.0896
0.1772
0.1394
0.2551
0.1015
0.1799
0.2002
0.2060
wR
final R indices
I>2s(I)]
2
[
R
1
0.0411
0.0488
0.0616
0.0838
0.0649
0.0803
wR
GOF on F
2
0.1007
0.906
0.0566
0.715
0.1663
1.023
0.2240
0.920
0.1612
0.934
0.1662
0.823
2
largest diff. peak/
0.393/À0.260
0.280/À0.246
0.771/À1.170
1.143/À0.829
0.680/À0.305
1.451/0.428
À3
hole [eꢃ
]
Crystal structure determinations: Data for 6, 7, 8, 9, 14, and 15 were col-
lected on a STOE-IPDS diffractometer with graphite-monochromated
MoKa radiation (l=0.71073 ꢃ). Crystal data and intensity collection and
refinement parameters are reported in Table 3. Intensities were measured
[1] S. Leininger, B. Olenyuk, P. J. Stang, Chem. Rev. 2000, 100, 853–908.
[2] G. F. Swiegers, T. J. Malefetse, Chem. Rev. 2000, 100, 3483–3537.
[3] B. J. Holliday, C. A. Mirkin, Angew. Chem. 2001, 113, 2076–2097;
Angew. Chem. Int. Ed. 2001, 40, 2022–2043.
[4] S. Schmatloch, U. S. Schubert, Chem. Unserer Zeit 2003, 37, 180–
187.
[5] W. R. McWhinnie, J. D. Miller, Adv. Inorg. Chem. Radiochem. 1969,
11, 135–215.
[6] D. J. Berg, J. M. Boncella, R. A. Andersen, Organometallics 2002,
21, 4622–4631 .
at 193(2) K. All structures were solved by direct phase determination
2
(
SHELXL 97) and refined on F (SHELXL 97) with anisotropic thermal
parameters for all non-hydrogen atoms. CCDC-250439 (4), CCDC-
47973 (6a), CCDC-247976 (7), CCDC-247979 (8), CCDC-247978 (9),
2
CCDC-247977 (14), CCDC-247980 (15), and CCDC-250440 (16) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.
cam.uk).
[7] W. Kaim, Coord. Chem. Rev. 2002, 230, 127–139.
[
8] W. Kaim, Angew. Chem. 1983, 95, 201–221; Angew. Chem. Int. Ed.
Engl. 1983, 22, 171–191.
[
9] S. Kitagawa, R. Kitaura, S.-I. Noro, Angew. Chem., 2004, 116, 2388–
2
430; Angew. Chem. Int. Ed. 2004, 43, 2334–2375.
[
10] M. P. Coles, D. C. Swenson, R. F. Jordan, V. G. Young, Jr., Organo-
metallics 1997, 16, 5183–5194.
Acknowledgement
[11] R. Robson, J. Chem. Soc. Dalton Trans. 2000, 3735–3744 .
[
12] M. Fujita, N. Fujita, K. Ogura, K. Yamaguchi, Nature 1999, 400, 52–
This work was supported by the Fonds der Chemischen Industrie.
55.
Chem. Eur. J. 2005, 11, 969 – 978
www.chemeurj.org
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
977

R. Beckhaus et al.
[
[
[
13] P. N. W. Baxter, J.-M. Lehn, G. Baum, D. Fenske, Chem. Eur. J.
[34] L. J. Radonovich, M. W. Eyring, T. J. Groshens, K. J Klabunde, J.
Am. Chem. Soc. 1982, 104, 2816–2819.
[35] Crystallographic data: Stoe-IPDS diffractometer (MoKa radiation);
4: Number of exposures 194, Df per measurement 1.68, T=193 K,
1
999, 5, 102–112.
14] B. Olenyuk, A. Fechtenkçtter, P. J. Stang, J. Chem. Soc. Dalton
Trans. 1998, 1707–1728.
15] S. R. Batten, R. Robson, Angew. Chem., 1998, 110, 1558–1595;
Angew. Chem. Int. Ed. 1998, 37, 1460–1494.
8 2 r
crystal dimensions 0.26ꢄ0.22ꢄ0.16 mm, C10H N , M =156.18, tri-
¯
clinic, space group P1, a=8.7644(6), b=8.7793(7), c=11.0108(9) ꢃ,
3
[
[
[
16] P. J. Stang, B. Olenyuk, Acc. Chem. Res. 1997, 30, 502–518.
17] M. Fujita, Chem. Soc. Rev. 1998, 27, 417–425.
V=826.23(11) ꢃ , a=85.264(9), b=85.399(9), g=78.714(8)8, Z=4,
À1
À1
1
calcd=1.256 gcm
,
m(MoKa)=0.077 mm
,
F(000)=328,
max
V =
18] Multinuclear metal complexes based on polypyridine building
blocks attracted broad attention during the last decade. Two re-
views: a) J.-P. Launay, Chem. Soc. Rev. 2001, 30, 386–397; b) S. Ser-
roni, S. Campagna, F. Puntoriero, C. Di Pietro, N. McClenaghan, F.
Loiseau, Chem. Soc. Rev. 2001, 30, 367–375.
26.078, 10171 measured reflections, 3021 independent reflections
(Rint =0.0404), 2087 observed reflections (I>2s(I)), 281 parameters,
2
GOF (F )=0.946, final R indices: R
1
=0.0353, wR
2
=0.0832, max./
À3
min. residual electron density 0.191/À0.142 eꢃ . All non-hydrogen
atoms were refined freely. The H atom positions were calculated.
16: Number of exposures 225, Df per measurement 1.28, T=193 K,
[
19] L. L. Schafer, J. R. Nitschke, S. S. H. Mao, F.-Q. Liu, G. Harder, M.
Haufe, T. D. Tilley, Chem. Eur. J. 2002, 8, 74–83.
crystal dimensions 0.90ꢄ0.33ꢄ0.30 mm,
20 34 2 2 r
C H N OSi ·THF, M =
[
[
20] L. L. Schafer, T. D. Tilley, J. Am. Chem. Soc. 2001, 123, 2683–2684.
21] S. Kraft, R. Beckhaus, D. Haase, W. Saak, Angew. Chem. 2004, 116,
374.67, monoclinic, space group P2 /c, a=6.6406(10), b=18.184(2),
1
3
c=18.489(5) ꢃ, V=2228.1(7) ꢃ , a=90, b=93.64(3), g=908, Z=4,
À1
À1
1
609–1614; Angew. Chem. Int. Ed. 2004, 43, 1583–1587.
1calcd =1.117 gcm
,
m(MoKa)=0.169 mm
,
F(000)=816, Vmax =
[
[
[
22] P. J. Stang, J. A. Whiteford, Res. Chem. Intermed. 1996, 22, 659–665.
23] P. Schinnerling, U. Thewalt, J. Organomet. Chem. 1992 431, 41–45.
24] U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannen-
berg, Organometallics 2003, 22, 884–900.
25.968, 23592 measured reflections, 4193 independent reflections
(Rint =0.0473), 3329 observed reflections (I>2s(I)), 362 parameters,
2
GOF (F )=0.997, final R indices: R =0.0338, wR =0.0843, max/
1
2
À3
min residual electron density 0.223/À0.184 eꢃ . All non-hydrogen
atoms were refined freely. The H atom positions were calculated.
[36] T. Eicher, S. Hauptmann The Chemistry of Heterocycles, Thieme,
Stuttgart, 1995.
[
[
[
[
[
[
[
[
[
25] a) S. Kraft, PhD Thesis, University of Oldenburg, 2004; b) M. L. H.
Green, C. R. Lucas, J. Chem. Soc. Dalton Trans. 1972, 1000–1003.
26] Formation of 10 was proved by comparison with powder diffraction
data of pure 10; data of 14 were absent.
[37] R. Gyepes, P. T. Witte, M. Horacek, I. Cisarova, K. Mach, J. Orga-
nomet. Chem. 1998, 551, 207–213.
27] T. Rajendran, B. Manimaran, F.-Y. Lee, G.-H. Lee, S.-M. Peng,
C. M. Wang, K.-L. Lu, Inorg. Chem. 2000, 39, 2016–2017.
28] I. M. Piglosiewicz, S. Kraft, R. Beckhaus, W. Haase, W. Saak, Eur. J.
Inorg. Chem. 2004, in press.
29] A. M. McPherson, B. F. Fieselmann, D. L. Lichtenberger, G. L.
McPherson, G. D. Stucky, J. Am. Chem. Soc. 1979, 101, 3425–3430.
30] P. T. Witte, R. Klein, H. Kooijman, A. L. Spek, M. Polasek, V.
Varga, K. Mach, J. Organomet. Chem. 1996, 519, 195–204.
31] M. Schwach, H.-D. Hausen, W. Kaim, Inorg. Chem. 1999, 38, 2242–
[38] We thank Prof. Dr. M. Jansen, Max-Planck-Institut fꢀr Festkçrper-
forschung, Stuttgart, for the magnetic measurements.
[39] J. W. Lauher, R. Hoffmann, J. Am. Chem. Soc. 1976, 98, 1729–1742.
[40] J. C. Green, Chem. Soc. Rev. 1998, 27, 263–272.
[41] W. W. Lukens, Jr., M. R. Smith, III, R. A. Andersen, J. Am. Chem.
Soc. 1996, 118, 1719–1728.
[42] V. V. Burlakov, A. V. Polyakov, A. I. Yanovsky, Y. T. Struchkov,
V. B. Shur, M. E. Vol’pin, U. Rosenthal, H. Gçrls, J. Organomet.
Chem. 1994, 476, 197–206.
2
243.
32] M. Glockle, K. Hubler, H. J. Kummerer, G. Denninger, W. Kaim,
Inorg. Chem. 2001, 40, 2263–2269.
33] M. H. Chisholm, J. C. Huffman, I. P. Rothwell, P. G. Bradley, N.
Kress, W. H. Woodruff, J. Am. Chem. Soc. 1981, 103, 4945–4947.
Received: August 26, 2004
Published online: December 20, 2004
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