Precursors for assembly of supramolecules containing quadruply bonded Cr24+ units: Systematic preparation of Cr 2(formamidinate)n(acetate)4-n

Article abstract of DOI:10.1002/ejic.200700414

Four quadruply bonded dichromium complexes with mixed-ligand sets, Cr ₂(formamidinate)_n(acetate)_4-n (n = 2-4), were synthesized from reactions of anhydrous Cr₂(O₂CCH ₃)₄ and formamidinate anions. The Cr-Cr bond lengths fall in the range for supershort Cr-Cr bonds as they vary from 1.8897(5) A to 2.012(1) A. The distance variation depends on the presence or absence of weak axial interactions. Because formamidinate ligands are less labile than acetate groups, these compounds may be useful building blocks for the construction of neutral supramolecules having dichromium units linked by polydentate dianions. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700414

FULL PAPER
DOI: 10.1002/ejic.200700414
Precursors for Assembly of Supramolecules Containing Quadruply Bonded
4+
Cr2 Units: Systematic Preparation of Cr2(formamidinate)n(acetate)4–n
(n = 2–4)
F. Albert Cotton,[a][†] Zhong Li,[a] and Carlos A. Murillo*[a]
Keywords: Metal–metal bonds / Supramolecular chemistry / Chromium / Corner piece precursors
Four quadruply bonded dichromium complexes with mixed-
ligand sets, Cr2(formamidinate)n(acetate)4–n (n = 2–4), were
synthesized from reactions of anhydrous Cr2(O2CCH3)4 and
formamidinate anions. The Cr–Cr bond lengths fall in the
range for “supershort” Cr–Cr bonds as they vary from
1.8897(5) Å to 2.012(1) Å. The distance variation depends on
the presence or absence of weak axial interactions. Because
formamidinate ligands are less labile than acetate groups,
these compounds may be useful building blocks for the con-
struction of neutral supramolecules having dichromium units
linked by polydentate dianions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
and linkers, three basic assembly schemes have been gen-
erally employed: (1) Two metal–metal bonded units sup-
ported by three formamidinate groups are linked to give
dimers of dimers as shown in Scheme 1. Examples are the
dimers of dimolybdenum compounds that have been ob-
Introduction
There is emerging interest in the use of metal atoms (or
ions) as key elements in the self-assembly of metal-organic
supramolecular arrays.[1] Most of these studies have focused
on systems with single-metal building blocks of the type
ML2, where the L ligands are usually diamine, diphosphane
or dicarboxylate and the metal atoms are Pt, Pd, Zn or
Cd.[2] These species usually have ions with large charges.
The introduction of metal–metal bonded dinuclear units
into this field have enriched the diversity of the geometry
and properties of the supramolecular systems and allowed
the syntheses of neutral molecules.[3] Appropriate selection
of the metal containing units and suitable linkers have led
to the formation of a variety architectures, including dimers
of dimers, loops, triangles, squares, complex polygons and
extended 2D and 3D materials.[4]
One of the best strategies for construction of large mole-
cules containing metal–metal bonds has been the use of pre-
cursors having subunits containing paddlewheel species
with mixed ligands in which some of the paddles are nonla-
bile formamidinate groups, while the remaining coordina-
tion sites are occupied by easily displaceable ligands, such
as acetonitrile molecules or carboxylate groups. The labile
ligands are then replaced by polydentate linkers to assemble
the target supramolecules whose structures depend on the
dimensions of the linkers and shape of the corner piece pre-
cursors. On the basis of the appropriate choice of subunits
+
tained by reactions of Mo2(DAniF)3(CH3CNeq)2 species
(DAniF = N,NЈ-di-p-anisylformamidinate) with tetraalk-
ylammonium salts of various dicarboxylates.[5] When Mo2-
(DAniF)3(O2CCH3) was used as starting material, even the
more basic diamidate or the all-nitrogen-donor linker fluo-
flavinate may be used as linkers and these ligands enable
very high intramolecular electronic communication.[6] Com-
pounds having two Ru2(DAniF)3 units linked by
terephthalate anions have also been made by reaction of
(NBu4)2O2CC6H4CO2 with the building block Ru2(DAniF)3-
(O2CCH3)Cl[7] and analogous Ru2 compounds with di-
phenylformamidinate have also been made.[8] (2) Dimetal
units with two cisoid formamidinate groups with rigid di-
anions allow the formation of squares, triangles (Scheme 2)
and, under some circumstances, loops. Examples are the
series of molecular squares assembled by reactions of [cis-
M2(DAniF)2(CH3CNeq)4]2+ (M = Mo, Rh) and dicarboxyl-
ate linkers.[9] Sometimes equilibria between triangles and
the corresponding squares exist in solution because of en-
tropy-dominated processes.[10] (3) Precursors with struc-
tures having transoid nonlabile formamidinate groups are
expected to give one-dimensional polymers (Scheme 3).
Current work in our laboratory involves the synthesis of
ladder structures from trans-M2(DAniF)2(O2CCH3)2 com-
pounds. In addition, by utilizing the axial coordination and
a combination of these linkage modes, more complex archi-
tectures can be made.[11] What is clear from this discussion
is that building blocks with mixed-ligand sets of different
[a] Department of Chemistry and Laboratory for Molecular Struc-
ture and Bonding, Texas A&M University,
P. O. Box 3012, College Station, Texas 77842-3012, USA
E-mail: murillo@tamu.edu
[†] Deceased February 20, 2007.
Eur. J. Inorg. Chem. 2007, 3509–3513
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3509

F. A. Cotton, Z. Li, C. A. Murillo
FULL PAPER
labilities allow selective substitution of the more labile li-
gands, and preparation of such species is critical for the
successful assembly of large molecules.
Results and Discussion
Syntheses
Reaction of 2 equiv. of LiDPhClF with 1 equiv. of anhy-
drous Cr2(O2CCH3)4 in THF at –70 °C affords 1 and 2 as
shown by the isolation of two types of crystals when a ben-
zene solution of the isomer mixture was layered with iso-
meric hexanes: red acicular crystals of cis-Cr2(DPhClF)2-
(O2CCH3)2 (1) and yellow-orange block crystals of trans-
Cr2(DPhClF)2(O2CCH3)2 (2). Because of their distinctive
shape and color, these isomers can be manually separated
in a relatively easy manner and isolated in reasonable yields.
It should be mentioned that the appropriate selection of the
formamidinate ligands is important as earlier studies had
shown that an analogous reaction of Cr2(O2CCH3)4 with
2 equiv. of LiDPhOMeF [DPhOMeF = (o-CH3OC6H4N)2CH]
gave only the cis product,[16] while reaction of the heavier
congener Mo2(O2CCH3)4 and 2 equiv. of HDAniF in the
presence of NaOCH3 exclusively produced the trans species,
namely trans-Mo2(DAniF)2(O2CCH3).[17] In the latter
study, the cis isomer, cis-Mo2(DAniF)2(O2CCH3)2, was ob-
tained from a two-step reaction in which first Mo2-
Scheme 1.
Scheme 2.
An additional advantage of using M–M bonded units in (DAniF)4 and (Et3O)BF4 in CH3CN reacted to produce cis-
building supramolecules is that a wide selection of transi- Mo2(DAniF)2(CH3CNeq)42+. This reaction was followed by
tion metal atoms are available and their electronic struc- addition of acetate anions which replaced the acetonitrile
tures are generally well understood, which allow the intro- molecules in the intermediate.
duction of internal spectroscopic probes. Thus far, most of
Once 1 and 2 are formed and isolated, these isomers are
1
the studies have been limited to dimolybdenum compounds stable as the H NMR spectra of these isomers in CDCl3,
and a handful of molecules containing W2, Ru2 and Rh2 which are significantly different from each other, do not
units. Even though the first-row transition metal, chro- change over a period of many hours. The spectra are thus
mium, is prone to support metal–metal bonds, and hun- consistent with retention of the geometry in solution.
4+
dreds of compounds with Cr2 core have been studied no
An analogous reaction between Cr2(O2CCH3)4 but with
efforts have been devoted to the methodical construction of 3 equiv. of LiDAniF gave complex 3 which has three DAniF
appropriate corner pieces.[12] It should be noted that gen- groups and one acetate ligand. It should be noted that even
erally chromium compounds are useful in a variety of appli- when only 2 equiv. of LiDAniF were used, the isolated
cations such as catalysts in organic synthetic applications[13] product was Cr2(DAniF)3(O2CCH3) and under such condi-
and are also found to be of importance in biosystems.[14] tions no species having the formula Cr2(DAniF)2(O2-
Recently an interesting compound with a Cr-to-Cr bond of CCH3)2 was observed. Furthermore, when traces of water
order 5 has been reported, in a work that should encourage were present in the reaction mixture or whenever an excess
further exploration of the chemistry of this element.[15]
of LiDAniF was used, compound 4 was the only isolated
As a first step in the syntheses of large molecules con- species.[18] This compound can also be prepared conve-
4+
taining Cr2 units, here we report the systematic syntheses niently and in better yield by reaction of LiDAniF and
of dichromium compounds of the type Cr2(formamidinate)n- CrCl2. Compounds 1–4 are soluble in common solvents
(acetate)4–n (n = 2–4). The compounds are a pair of isomers, such as THF, dichloromethane, toluene and benzene.
namely cis-Cr2(DPhClF)2(O2CCH3)2, 1, [DPhClF = (o-
The CVs of 1–4 were recorded in CH2Cl2 using scan rates
ClC6H4N)2CH], and trans-Cr2(DPhClF)2(O2CCH3)2 (2), as of 100 mV/s and 0.1  Bu4NPF6 as electrolyte. The oxi-
well as Cr2(DAniF)3(O2CCH3) (3) and the parent com- dation processes are irreversible and the potentials ranged
4+
pound Cr2(DAniF)4 (4). All of these compounds have been from 0.62 V to 1.00 V. This is not unusual for Cr2 species
characterized structurally and by various spectroscopic which usually decompose upon oxidation. The only Cr2
5+
techniques.
product known so far was produced by oxidation of
Scheme 3.
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3509–3513

Preparation of Cr2(formamidinate)n(acetate)4–n (n = 2–4)
FULL PAPER
Cr2(DPPC)4 (DPPC is the guanidinate-type species N,NЈ- tion. The Cr–Cr bond length of 2.0010(6) Å (Table 1) falls
diphenyl-1-pyrrolidine carbodiimidate). For Cr2(DPPC)4 in the range of 1.83–2.60 Å found for Cr–Cr distances in
two quasi-reversible waves were observed in the CV having paddlewheel species.[12] For such compounds, the metal–
E1/2 values of 0.02 and 1.10 V.
metal distances are affected not only by the identity of the
supporting ligand but are very sensitive to the presence or
absence of axial interactions. For example, the Cr–Cr dis-
tance for the Cr2(O2CCH3)4 molecule in gas phase is
1.97 Å, but this distance increases to 2.369(2) Å when two
pyridine molecules coordinate to the axial sites.[19] In 1,
there is no direct axial coordination, however, there are
weak axial interactions because the two cis formamidinate
ligands provide one chlorine atom each at distances of 2.817
and 2.864 Å for Cr(1)···Cl(1) and Cr(2)···Cl(4), respectively.
For comparison, in the paddlewheel compound of
Cr2(DPhClF)4, [Cr–Cr 2.208(2) Å] the distances between the
chlorine and chromium atoms (2.766 Å) are shorter than
those in 1.[20] As a consequence, the Cr–Cr bond length in
Cr2(DPhClF)4 [2.208(2) Å] is considerably longer than that
in 1 [2.0010(6) Å]. The effect of weak axial interactions on
dichromium units has also been documented in compounds
with ligands such as 2,6-bis(phenylimino)piperidine, 2,2Ј-di-
pyridylamine.[21]
Structural Results
Compounds 1–4 have been structurally characterized by
single crystal X-ray diffraction and their structures are
shown in Figure 1. Compound 1 crystallized in the space
group P21/n with the molecule residing on a general posi-
Table 1. Selected bond lengths [Å] for 1–4.
1
2·C6H6
3
4
Cr(1)–Cr(2)
Cr(1)–Cr(1a)
Cr(1)–N(1)
Cr(1)–O(1)
Cr(1)–N(3)
Cr(1)–O(3)
Cr(1)–N(5)
Cr(1)–O(5)
Cr(1)–N(7)
Cr(1)–O(7)
Cr(2)–N(2)
Cr(2)–O(2)
Cr(2)–N(4)
Cr(2)–N(6)
Cr(2)–O(6)
Cr(2)–O(8)
2.0010(6)
1.8897(5)
2.012(1)
2.068(3)
2.004(3)
1.924(1)
2.067(2)
2.051(2)
2.051(2)
2.049(2)
2.019(2)
2.044(2)
2.049(2)
2.050(2)
2.054(2)
2.018(2)
2.001(2)
2.055(2)
2.027(2)
2.027(2)
2.049(3)
2.017(3)
2.043(2)
2.048(2)
2.033(2)
1.992(2)
2.015(2)
2.025(2)
Compound 2, an isomer of 1, crystallized in the space
¯
group P1 with the molecule residing on an inversion center.
The Cr–Cr bond length of 2.012(1) Å is very similar to that
in 1. Again the two trans formamidinate ligands provide
one chlorine donor atom capable of interacting with a chro-
mium atom at a distance of 2.793 Å.
For 3, which crystallized in the space group P21/c with
Z = 4, the Cr–Cr bond length of 1.8897(5) Å is within the
range found in super-short quadruply bonded compounds.
This is due to the lack of axial coordination by either sol-
vent or other donor atoms in the formamidinate ligand.
The bulky formamidinate groups also prevent intermo-
lecular interactions between Cr and O atoms which tend
to lengthen the Cr–Cr distances such in the structure of
anhydrous Cr2(O2CR)4, in which weak intermolecular
Cr···O interactions allow the association of Cr2(O2CR)4
molecules to form a polymer.[12]
Figure 1. Structures of 1–4 with displacement ellipsoids drawn at
the 40% probability level. Note the presence of weak Cr···Cl axial
interactions in 1 and 2 and the absence of axial interactions in 3
and 4. The Cr–Cr distances [2.0010(6) and 2.012(1) Å] for 1 and 2,
respectively, are longer than for 3 and 4 [1.8897(5) and 1.924(1) Å,
respectively].
Eur. J. Inorg. Chem. 2007, 3509–3513
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3511

F. A. Cotton, Z. Li, C. A. Murillo
FULL PAPER
with Pt working and auxiliary electrodes, Ag/AgCl reference elec-
trode, scan rate of 100 mV/s, and 0.10  Bu4NPF6 (in CH2Cl2) as
electrolyte.
In compound 4, which crystallized in the space group
C2/c with Z = 4, the molecule is located on a general posi-
tion. The Cr–Cr bond length of 1.924(1) Å resembles that
of 1.9178(11) Å in the Cr2(DPhm–OMeF)4 analogue[22] and
it is consistent for a species without axial interactions. It
should be noted that the Cr–Cr distance in the
Cr2(DPho–OMeF)4 isomer, which has o-methoxy groups pro-
viding axial interactions, is 2.140(2) Å.[20] This distance is
about 0.22 Å longer than that in 4.
Preparation of cis-Cr2(DPhClF)2(O2CCH3)2 (1) and trans-
Cr2(DPhClF)2(O2CCH3)2 (2): To a solution of HDPhClF (265 mg,
1.00 mmol) in THF (20 mL) at –70 °C, methyllithium in diethyl
ether (0.32 mL, 1.6 ) was added slowly with stirring. The yellow
solution was warmed to 0 °C and then transferred to a suspension
of anhydrous chromium acetate (170 mg, 0.500 mmol) in THF
(20 mL). The color of the brown suspension turned gradually to
orange. After 4 h, the solvent was removed under reduced pressure
and the solid residue was extracted with benzene (15 mL). The ben-
zene solution was filtered through a Celite-packed frit, and the fil-
trate was layered with isomeric hexanes. Two types of crystals
formed within 7 d: red acicular crystals of cis-Cr2(DPhClF)2-
(O2CCH3)2 (1) and yellow-orange block crystals of trans-
Cr2(DPhClF)2(O2CCH3)2 (2). These were manually separated.
Concluding Remarks
Systematic syntheses and characterization are reported
for a series of dichromium compounds with mixed ligand
sets of formamidinate and acetate groups, Cr2(formamidin-
ate)n(acetate)4–n (n = 2–4). The Cr–Cr bond lengths are
strongly influenced by the presence or absence of axial in-
teractions. These compounds are expected to serve as pre-
cursors for the preparation of supramolecules with different
architectures utilizing selective ligand substitution based on
the fact that acetate groups are generally more labile than
formamidinate groups.
For 1: Yield: 138 mg (37%). 1H NMR (C6D6): 8.62 (s, 2 H,
–NCHN–), 7.18 (d, 4 H, aromatic C–H), 6.80 (m, 8 H, aromatic
C–H), 6.48 (t, 4 H, aromatic C–H), 2.33 (s, 6 H, –CH3). UV-vis:
λmax (ε, M–1 mol–1)
=
400 (1.5ϫ103) nm. 1·2H2O:
C30H26Cl4Cr2N4O5 (786): calcd. C 45.82, H 3.58, N 7.12; found C
46.02, H 3.96, N 6.92.
For 2: Yield: 104 mg (28%). 1H NMR (C6D6): 8.67 (s, 2 H,
–NCHN–), 7.03 (d, 4 H, aromatic C–H), 6.80 (m, 8 H, aromatic
C–H), 6.55 (t, 4 H, aromatic C–H), 2.25 (s, 6 H, –CH3). UV-vis:
λmax (ε, M–1 mol–1)
=
450 (1.1ϫ103) nm. 2·H2O:
Experimental Section
C30H24Cl4Cr2N4O4 (768): calcd. C 46.89, H 3.43, N 7.26; found C
47.08, H 4.07, N 6.87.
Materials and Methods: All reactions and manipulations were per-
formed under nitrogen, using either a dry box or standard Schlenk
line techniques. Solvents were purified under argon using a Glass
Preparation of Cr2(DAniF)3(O2CCH3) (3): A procedure analogous
to that described above for 1 and 2 but using HDAniF (383 mg,
1.50 mmol) and Cr2(O2CCH3)4 (170 mg, 0.500 mmol) in THF gave
an orange product. Crystals were obtained by layering a benzene
solution of 3 with hexanes. Yield: 315 mg (68%). 1H NMR (C6D6):
8.94 (s, 2 H, –NCHN–), 8.76 (s, 1 H, –NCHN–), 6.68 (d, 16 H,
aromatic C–H), 6.40 (d, 8 H, aromatic C–H), 6.24 (d, 8 H, aromatic
C–H), 3.22 (s, 12 H, –OCH3), 3.10 (s, 6 H, –CH3 in DAniF). UV-
vis: λmax (ε, M–1 mol–1) = 410 (1.3ϫ103) nm. Cr2C47H48N6O8 (929):
calcd. C 60.77, H 4.62, N 9.05; found C 60.88, H 4.36, N 9.14.
Contour solvent purification system or distilled under nitrogen
[23]
over appropriate drying agents. Anhydrous Cr2(O2CCH3)4
and
the formamidinate precursors[24] were synthesized following re-
ported procedures; other commercially available chemicals were
used as received.
Analytical and Physical Measurements: Elemental analyses were
performed by Robertson Microlit Laboratories, Madison, New Jer-
sey. Electronic spectra were measured on a Shimadzu UV–2501PC
1
spectrometer in CH2Cl2 solution. H NMR spectra were recorded
on a Inova–300 NMR spectrometer with chemical shifts (δ, ppm)
Preparation of Cr2(DAniF)4 (4): To a suspension of anhydrous
referenced to residual CHCl3 in CDCl3. Cyclic voltammograms CrCl2 (130 mg, 1.00 mmol) and HDAniF (510 mg, 2.00 mmol) in
(CVs) were collected on a CH Instruments electrochemical analyzer
THF (30 mL) at –78 °C, a solution of methyllithium in diethyl ether
Table 2. X-ray crystallographic data for 1–4.
1
2·C6H6
3
4·2C6H6
Empirical formula
Fw
Space group
a [Å]
b [Å]
c [Å]
C30H24Cl4Cr2N4O4
750.33
P21/n
19.062(3)
7.662(1)
22.258(3)
90
102.687(2)
90
C36H30Cl4Cr2N4O4
828.44
P1
C47H48Cr2N6O8
928.91
P21/c
21.648(5)
10.750(3)
18.945(5)
90
92.235(4)
90
C72H72Cr2N8O8
1281.38
C2/c
33.293(10)
10.002(3)
20.914(6)
90
111.888(6)
90
6462(3)
4
¯
8.342(4)
10.701(5)
10.967(5)
102.929(8)
91.142(8)
111.594(7)
881.6(7)
1
α [°]
β [°]
γ [°]
Volume [Å3]
Z
3171.6(7)
4
4405.(4)
4
T [K]
213
0.71073
1.571
1.064
0.0392, 0.0874
213
0.71073
1.560
0.966
0.0569, 0.1453
213
0.71073
1.401
0.555
0.0431, 0.1045
213
0.71073
1.317
0.399
0.0831, 0.1087
λ [Å]
dcalcd. [g/cm3]
µ [mm–1]
[b]
R1,[a] wR2
2
2 2
[a] R1 = Σ|Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2]]1/2
.
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3509–3513

Preparation of Cr2(formamidinate)n(acetate)4–n (n = 2–4)
FULL PAPER
[4] See for example: a) R. H. Cayton, M. H. Chisholm, J. C. Huff-
man, E. B. Lobkovsky, J. Am. Chem. Soc. 1991, 113, 8709–
8724; b) F. A. Cotton, J. P. Donahue, C. Lin, C. A. Murillo,
Inorg. Chem. 2001, 40, 1234–1244; c) F. A. Cotton, C. Lin,
C. A. Murillo, Inorg. Chem. 2001, 40, 472–477; d) F. A. Cotton,
C. A. Murillo, S.-E. Stiriba, X. Wang, R. Yu, Inorg. Chem.
2005, 44, 8223–8233; e) F. A. Cotton, L. M. Daniels, C. Lin,
C. A. Murillo, Chem. Commun. 1999, 841–842; f) Y. Ke, D. J.
Collins, H. C. Zhou, Inorg. Chem. 2005, 44, 4154–4156.
[5] a) F. A. Cotton, P. J. Donahue, C. A. Murillo, J. Am. Chem.
Soc. 2003, 125, 5436–5450; b) F. A. Cotton, P. J. Donahue,
C. A. Murillo, L. M. Pérez, J. Am. Chem. Soc. 2003, 125, 5486–
5492.
(0.65 mL, 1.6 ) was added slowly with stirring. The gray suspen-
sion changed gradually to a turbid orange dispersion. After 3 h,
the solvent was removed under reduced pressure and the residue
washed with hexanes (3ϫ15 mL). The dry solid was dissolved in
benzene (15 mL) and filtered through a Celite-packed frit, and then
the filtrate was layered with hexanes. Yield: 420 mg (75%). 1H
NMR (δ, ppm in C6D6): 8.69 (s, 4 H, –NCHN–), 6.62 (d, 16 H,
aromatic C–H), 6.38 (d, 16 H, aromatic C–H), 3.19 (s, 24 H,
–CH3 in DAniF). UV-vis: λmax (ε, M–1 mol–1) = 502 (1.2ϫ103) nm.
Cr2C60H60N8O8 (1125.17): calcd. C 64.05, H 5.37, N 9.96; found
C 63.89, H 4.86, N 9.64.
[6] a) F. A. Cotton, C. Y. Liu, C. A. Murillo, D. Villagrán, X.
Wang, J. Am. Chem. Soc. 2003, 125, 13564–13575; b) F. A.
Cotton, C. Y. Liu, C. A. Murillo, D. Villagrán, X. Wang, J.
Am. Chem. Soc. 2004, 126, 14822–14831; c) F. A. Cotton, Z.
Li, C. Y. Liu, C. A. Murillo, D. Villagrán, Inorg. Chem. 2006,
45, 767–778.
X-ray Structural Determinations: Single crystals suitable for X-ray
analysis were mounted on the tips of cryoloops attached to a goni-
ometer head. Data for 1, 2·C6H6, 3 and 4 were collected at –60 °C
with a Bruker SMART 1000 CCD area detector system. Cell pa-
rameters were determined using the program SMART.[25] Data re-
duction and integration were performed with the software
SAINT,[26] while the absorption corrections were applied by using
the program SADABS.[27] The structures were solved by direct
methods and refined using the SHELXS–97 program.[28] Non-hy-
drogen atoms, except for those of disordered solvent molecules,
were refined with anisotropic displacement parameters. Hydrogen
atoms were added in calculated positions. Crystallographic data are
given in Table 2 and selected bond lengths in Table 1.
[7] P. Angaridis, J. F. Berry, F. A. Cotton, P. Lei, C. Lin, C. A.
Murillo, D. Villagrán, Inorg. Chem. Commun. 2004, 7, 9–13.
[8] M. C. Barral, S. Herrero, R. Jiménez-Aparicio, M. R. Torres,
F. A. Urbanos, Inorg. Chem. Commun. 2004, 7, 42–46.
[9] a) F. A. Cotton, C. Lin, C. A. Murillo, Inorg. Chem. 2001, 40,
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Acknowledgments
We thank the Robert A. Welch Foundation and Texas A&M Uni-
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Received: April 16, 2007
Published Online: June 18, 2007
Eur. J. Inorg. Chem. 2007, 3509–3513
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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