Investigation of Chromium complexes with a series of Tripodal Ligands

Article abstract of DOI:10.1002/zaac.201300170

A series of chromium(II) complexes containing the tripodal ligands tmpa, Me₂uns-penp, Me₄apme and Me₆tren were prepared and investigated with regard to their reactivity towards dioxygen. It was possible to structurally cha

Full text of DOI:10.1002/zaac.201300170

ARTICLE
DOI: 10.1002/zaac.201300170
Investigation of Chromium Complexes with a Series of Tripodal Ligands
Sabrina Schäfer,[a] Jonathan Becker,[a] Alexander Beitat,[a] and Christian Würtele*[a]
Keywords: Chromium; Tripodal ligands; Toluene oxidation; X-ray diffraction; Catalysis
Abstract.
A series of chromium(II) complexes containing the and [Cr(Me4ampe)Cl3]. The complexes show reactions with dioxygen,
tripodal ligands tmpa, Me2uns-penp, Me4apme and Me6tren were but it was not possible to characterize intermediates of these reactions.
prepared and investigated with regard to their reactivity towards di-
oxygen. It was possible to structurally characterize the complexes
[Cr(Me2uns-penp)Cl]2(BPh4)2, [Cr(Me6tren)Cl]Cl, [Cr(Me4apme)Cl]BPh4
The oxidation behavior of the complexes towards organic substrates
like toluene was investigated and oxidation of toluene to benzaldehyde
was observed.
Introduction
Oxidation reactions play an important role in biochemistry,
industrial processes, and organic syntheses.[1] Therefore, there
is high interest in the direct oxidation of organic substrates
using a metal catalyst and air as the oxidant.
The activation of dioxygen leads to different “dioxygen ad-
duct” transition metal complexes such as, for example, su-
peroxido, peroxido or oxido species. The formation of “di-
oxygen adduct” complexes can cause the transfer of oxygen to
a substrate. In that regard Würtele et al. as well as Karlin and
co-workers recently demonstrated that a copper peroxido com-
plex could be used to oxidize toluene to benzaldehyde and
benzyl alcohol.[2] The copper peroxido complexes used in
these investigations were based on the tripodal ligands shown
in Figure 1. However, oxidations in organic syntheses are still
often performed using chromium(VI) compounds such as
K2Cr2O7. These compounds are problematic with regard to se-
lective oxidation reactions and their toxicity.[3] With this back-
ground it seemed interesting to investigate the possibility to
perform selective oxidation reactions with low valent chro-
mium complexes and dioxygen using the tripodal ligands
shown in Figure 1.
Especially tris(2-pyridylmethyl)amine (tmpa, Figure 1) has
turned out to be a versatile ligand in bioinorganic chemistry to
model the reactivity of different metal enzymes based on iron,
copper, manganese or zinc in the active centre as well as in
other applications.[4] Most importantly, tmpa was used in cop-
per chemistry where Karlin and co-workers succeeded in struc-
turally characterizing a dinuclear copper peroxido complex for
Figure 1. Tripodal ligands investigated in this work.
the first time. They were able to model the reversible uptake
of dioxygen of copper enzymes.[5]
Systematic variation of the ligand tmpa was performed in
the past in an effort to gain better understanding of the reactiv-
ity of copper(I) complexes towards dioxygen and the potential
of the formed intermediates to oxidize organic substrates.[6] In
the ligands in Figure 1 the pyridine N-donor atoms of tmpa
were systematically substituted with aliphatic amine donor
atoms leading from tmpa (1) to (2-dimethyl-aminoethyl)bis(2-
pyridylmethyl)amine (Me2uns-penp (2)), bis(2-dimethyl-
aminoethyl)(2-pyridylmethyl)amine (Me4apme (3)) and tris(2-
dimethyl-aminoethyl)amine (Me6tren (4)). Copper(I) com-
plexes with all of these four ligands react with dioxygen to
form a mononuclear end-on superoxido copper complex that
further reacts to a dinuclear copper peroxido complex accord-
ing to the mechanism shown in Figure 2.[7]
* Dr. C. Würtele
Fax: +49-641-99-34149
The peroxido complex [(Me6tren)CuO2(Me6tren)]2+ was
structurally characterized.[2a] As described above it could be
demonstrated that these complexes can be used to catalytically
oxidize toluene to benzaldehyde.[2a] However, one of the prob-
lems to investigate these compounds in solution is the neces-
sity to perform all these studies at low temperatures. Therefore,
E-Mail: Christian.E.Wuertele@anorg.chemie.uni-giessen.de
[a] Institut für Anorganische und Analytische Chemie
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 58
35392 Gießen, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300170 or from the au-
thor.
Z. Anorg. Allg. Chem. 2013, 639, (12-13), 2269–2275
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Schäfer, J. Becker, A. Beitat, C. Würtele
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Figure 2. Mechanism for the reaction of copper(I) complexes (L =
tripodal ligand) with dioxygen (Charges are omitted for clarity).
low temperature stopped-flow measurements were performed.
Unfortunately, even at –80 °C detection, many times the reac-
tion intermediates could not be spectroscopically identified fast
enough.[6b,7]
Due to the fact that stable chromium peroxido complexes
are well known, e.g. [Cr(dien)(O2)2]·H2O (dien = diethylenetri-
amine),[8] we assumed that we might overcome some of these
problems using chromium complexes with the tripodal ligands
shown in Figure 1. Further support for this assumption came
from Nam and co-workers who recently succeeded to structur-
ally characterize a superoxido chromium(III) complex using
TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)
as a ligand.[9] Furthermore, they obtained the according chro-
mium(IV) or chromium(V) oxido complexes.[10] Using the
smaller macrocycle cyclen as a ligand a side-on peroxido chro-
mium(IV) complex was prepared and structurally charac-
terized by the same group.[11] Reactivity studies demonstrated
the interesting potential of these complexes.
Figure 3. UV/Vis spectrum of 1b (solid line) and product complex
(dashed line) after passing dioxygen through the solution (CH3CN,
25.0 °C, complex concentration = 0.8 mmol·L–1).
formation. The same reaction has been previously observed by
Gafford et al. when [Cr(tmpa)](ClO4)2 reacted with dioxygen
in ethanol.[13a] This bis(μ-hydroxido) chromium(III) dimer has
also been reported by Hodgson et al.[14] and Gafford et al.[15]
with perchlorate or bromide as anions. All these bis(μ-hy-
droxido) complexes showed a similar UV/Vis spectrum with
absorption maxima at 386 nm and 540 nm.
Using the synthetic protocol of Robertson et al. for the tmpa
complexes we prepared the according complexes with the li-
gands Me2uns-penp, Me4apme and Me6tren in a similar way
without any problems. Crystals suitable for structural analysis
Results and Discussion
Chromium(II) complexes with the ligand tmpa were could be obtained at approx. –40 °C for the complexes de-
previously reported by Robertson et al.[12] They struc- scribed below. The crystallographic data are summarized in
turally characterized complexes [Cr(tmpa)Cl2] (1a) and Table 1.
[Cr(tmpa)Cl]2(BPh4)2 (1b). We were able to reproduce their
The crystallographic data of [Cr(Me2uns-penp)Cl2] (2a) are
syntheses of both compounds. Furthermore, they described that reported in the Supporting Information. Its molecular structure
from the reaction of 1b in acetonitrile (with traces of water or is very similar to 1a. In both complexes the two chloride
air in the solvent) the formation of a μ-oxido chromium(III) anions are coordinated to the chromium ion. The molecular
complex, {[Cr(tmpa)Cl]2(μ-O)}(BPh4)2, was observed as an structure of the cation of [Cr(Me2uns-penp)Cl]2(BPh4)2 (2b) is
impurity. This compound was structurally characterized as presented in Figure 4. Selected bond lengths and angles are
well, however, no spectroscopic data were reported. We thus summarized in Table 2. 2b crystallized as a dimer. One unit of
reacted 1b with dioxygen in acetonitrile to enforce this reac- this complex has a distorted square pyramidal coordination
tion and observed the spectral changes in the UV/Vis spectrum (τ = 0.31).[16] Two of these units are connected through two
as shown in Figure 3.
chloride bridges. The bond length Cr–Cl(1)* is longer than the
The spectrum of 1b shows two absorption maxima at bond length between chromium and the coordinated chloride.
306 nm and at 376 nm as described in literature.[12] After reac- Cr(1)–Cl(1)* = 3.278 Å vs. Cr(1)–Cl(1) = 2.3358(9) Å. This
tion with dioxygen the spectrum changed and new absorption observation is in accordance to the observations for the tmpa
bands at 349 nm with a shoulder at 363 nm as well as at 393, complex as described previously by Robertson et al.[12]
420 and 446 nm were observed. The infrared spectrum showed
Crystals of 3a suitable for crystal structure analysis could be
a Cr–O–Cr stretching vibration at 844 cm–1. The spectroscopic obtained, but refinement was not possible. However, complex
data compare well with other related μ-oxido chromium com- [Cr(Me4apme)Cl]BPh4 (3b) could be crystallized. Figure 5
plexes described previously.[13]
shows the molecular structure of the cation of 3b (selected
Avoiding chloride as an additional ligand we performed the bond lengths and angles are summarized in Table 2). In con-
reaction of Cr(OTf)2·4H2O with tmpa in acetonitrile. From this trast to 1b and 2b it crystallizes as a monomer. The arrange-
reaction we obtained crystals suitable for crystallographic ment is best described as an intermediate between a square
analysis that showed that a bis(μ-hydroxido) chromium(III) pyramidal and trigonal bipyramidal coordination (τ = 0.57).[16]
complex was obtained. The molecular structure of the cation
Furthermore, a chromium(III) complex with the ligand
of [Cr(tmpa)OH]2(OTf)4 (1c) is reported in the Supporting In- Me4apme, [Cr(Me4apme)Cl3] (3d), could be crystallized as a
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Investigation of Chromium Complexes with a Series of Tripodal Ligands
Table 1. Crystallographic data.
2b
3b
4a
3d
Formula
Formula weight
Temperature
C40H42BClCrN4
677.04 g·mol–1
193(2) K
C38H46BClCrN4
657.05 g·mol–1
193(2) K
C12H30Cl2CrN4
353.30 g·mol–1
193(2) K
C14H26Cl3CrN4
408.74 g·mol–1
190(2) K
Wavelength
Crystal system
Space group
0.71073 A
monoclinic
P21/n
0.71073 A
orthorhombic
Pbca
0.71073 A
cubic
P213
0.71073 A
triclinic
P1
¯
Unit cell dimensions
a = 14.9600(3) Å
b = 16.2770(3) Å
c = 15.9060(3) Å
α = 90°
a = 16.2700(3) Å
b = 16.6890(3) Å
c = 25.4950(5) Å
α = 90°
a = 11.9655(14) Å
b = 11.9655(14) Å
c = 11.9655(14) Å
α = 90°
a = 7.1750(14) Å
b = 8.7870(18) Å
c = 16.2250(3) Å
α = 97.88(3)°
β = 93.92(3)°
γ = 112.27(3)°
929.6(3) Å3
β = 117.35(3)°
γ = 90°
β = 90°
γ = 90°
β = 90°
γ = 90°
Cell volume
3440.2(12) Å3
4, 1.307 Mg·m–3
0.445 mm–1
1424
6923(2) Å3
8, 1.261 Mg·m–3
0.440 mm–1
2784
1713.1(3) Å3
4, 1.370 Mg·m–3
0.974 mm–1
752
Z/Calculated density
Absorption coefficient
F(000)
2, 1.460 Mg·m–3
1.048 mm–1
426
θ range
Limiting indices
2.83 to 28.12°
–19 Յ h Յ 19,
–20 Յ k Յ 20,
–20 Յ l Յ 21
30871 / 8273
[R(int) = 0.0924]
98.4%
1.92 to 24.22°
–18 Յ h Յ 18,
–19 Յ k Յ 19,
–29 Յ l Յ 29
40311 / 5555
[R(int) = 0.1426]
99.7%
3.81 to 28.07°
–14 Յ h Յ 15,
–15 Յ k Յ 15,
–14 Յ l Յ 15
15301 / 1404
[R(int) = 0.1200]
99.6%
2.54 to 27.46°
–9 Յ h Յ 9,
–11 Յ k Յ 11,
–21 Յ l Յ 20
15992 / 4250
[R(int) = 0.0973]
99.7%
Reflections collected / unique
Completeness to θ
Absorption correction
Refinement method
none
none
none
none
Full-matrix least-squares on F2
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
8273 / 0 / 426
0.839
5555 / 0 / 410
0.827
1404 / 0 / 76
0.977
4250 / 0 / 303
1.029
R1 = 0.0471,
wR2 = 0.1027
R1 = 0.1002,
wR2 = 0.1153
0.515 and
R1 = 0.0510,
wR2 = 0.1143
R1 = 0.1140,
wR2 = 0.1299
0.596 and
R1 = 0.0427,
wR2 = 0.0896
R1 = 0.0647,
wR2 = 0.0974
0.407 and
R1 = 0.0523,
wR2 = 0.1297
R1 = 0.0777,
wR2 = 0.1425
1.015 and
R indices (all data)
Largest diff. peak and hole
–0.385 e·Å–3
–0.269 e·Å–3
–0.411 e·Å–3
–0.809 e·Å–3
Table 2. Selected bond length /Å and bond angles /°.
2b 3b 4a
2.3358(9) 2.3180(13) 2.3346(18) 2.3406(11)
3d
Cl(1)–Cr(1)
Cl(1)*–Cr(1)
Cl(2)–Cr(1)
Cl(3)–Cr(1)
3.278
–
–
–
–
–
–
–
–
–
2.3150(13)
2.3161(11)
2.153(2)
2.168(3)
2.096(3)
–
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(1)
Cr(1)–N(4)
2.076(2) 2.107(4)
2.090(2) 2.142(4)
2.110(2) 2.110(4)
2.402(2) 2.337(4)
2.219(3)
2.219(3)
2.114(5)
2.219(3)
N(2)–Cr(1)–N(3)
N(2)–Cr(1)–N(1)
N(3)–Cr(1)–N(1)
N(2)–Cr(1)–N(4)
N(3)–Cr(1)–N(4)
N(1)–Cr(1)–N(4)
N(2)–Cr(1)–Cl(1)
N(3)–Cr(1)–Cl(1)
N(1)–Cr(1)–Cl(1)
N(4)–Cr(1)–Cl(1)
158.71(9) 83.83(17) 118.46(3) 83.88(10)
79.83(9) 79.70(14) 82.82(8)
81.41(9) 143.15(15) 82.82(8)
97.94(9) 80.28(15) 118.46(3)
88.86(9) 111.30(15) 118.46(3)
80.71(8) 98.13(15) 82.82(8)
98.00(7) 177.52(12) 97.18(8)
100.46(7) 98.52(12) 97.18(7)
78.56(10)
92.27(11)
–
–
–
91.05(8)
174.92(7)
177.42(7) 97.90(11) 180.00(7) 87.10(8)
101.05(6) 99.54(11) 97.18(8)
–
–
Figure 4. Molecular dimer structure of the cation of 2b shown as an
ORTEP plot with thermal ellipsoids set at 50% probability.
Cl(1)–Cr(1)–Cl(1)* 95.18
–
–
by-product of the synthesis of complex 3a. Upon crystalli- in Figure 6 (selected bond lengths and angles can be found in
zation we obtained a mixture of 3a and 3d that allowed picking Table 2). This complex is sixfold coordinated with three chlor-
of crystals of both compounds suitable for crystal structure ide ligands and three N-donor atoms of the ligand Me4apme.
analysis. Obviously, some part of the chromium(II) complex One aliphatic arm of the ligand is not coordinated, because it
has been oxidized during the synthesis leading to this mixture. has been replaced by a chloride ion. This behavior is well
The molecular structure of [Cr(Me4ampe)Cl3] (3d) is shown known from related copper chemistry.[17]
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S. Schäfer, J. Becker, A. Beitat, C. Würtele
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Figure 5. Molecular structure of the cation of 3b shown as an ORTEP
plot with thermal ellipsoids set at 50% probability.
Figure 7. Molecular structure of the cation of 4a shown as an ORTEP
plot with thermal ellipsoids set at 50% probability.
towards toluene. GC–MS measurements showed no oxidation
products of toluene. In contrast, complexes of the type
[Cr(L)Cl]BPh4 demonstrated oxidation potential towards tolu-
ene. GC–MS measurements showed that similar to the copper
complexes benzaldehyde was formed with a maximum yield
for complex 3b of 25%. Thus the chromium complexes unfor-
tunately were not better than analogous copper compounds:
maximum yield of benzaldehyde with copper complexes was
approx. 40% with L = tmpa.[2] The yields of all chromium
complexes are summarized in Figure 8. Due to the quite low
yields no further detailed studies were performed on these oxi-
dation reactions.
Figure 6. Molecular structure of 3d shown as an ORTEP plot with
thermal ellipsoids set at 50% probability.
Furthermore, complexes with Me6tren as ligand were struc-
turally characterized. Figure 7 shows the molecular structure
of the cation of [Cr(Me6tren)Cl]Cl (4a). In contrast to the com-
plexes of 1a and 2a complex 4a is coordinated only by one
chloride ligand. The complex is fivefold coordinated and has
trigonal bipyramidal coordination (τ = 1).[16] Therefore com-
plex 4a is quite similar to [Cr(Me6tren)Cl]BPh4 (4b). The mo-
lecular structure and crystallographic data of 4b are reported
in the Supporting Information. 4b again crystallizes as a mono-
meric unit that has perfect trigonal bipyramidal coordination
(τ = 1).[16]
Figure 8. Oxidation of toluene and yield of benzaldehyde in%.
The low yields of benzaldehyde are a consequence of sev-
eral side reactions that cause formation of e.g. phenylbenzene
or 4-phenyl-3-buten-2-one (reaction of benzaldehyde with
acetone) in combination with simple oxidation reactions of the
chromium(II) complex to a chromium(III) compound.
Whereas it is principally not correct to assume that solid-
state structures are retained in solution, one could at least
Oxidation of Toluene
All synthesized chromium complexes were tested for their speculate if this is the case for the chromium complexes dis-
potential as catalysts for the oxidation of toluene in compari- cussed above. This would give a possible explanation for the
son with the analogous reaction of the according copper com- different yields of benzaldehyde obtained by oxidation with
plexes reported previously.[2a] For this investigation the com- the four complexes discussed in this study. The nearly perfect
plexes were dissolved in a minimum of a mixture of acetone trigonal bipyramidal coordination of 4b seems to suppress re-
and toluene and then dioxygen was passed through the solu- activity towards a substrate whereas 3b with an arrangement
tion. The resulting product solutions were analyzed by GC–MS between square pyramidal and trigonal bipyramidal supports
spectrometry. 1a, 2a, 3a and 4a showed no oxidation potential the oxidation reaction of toluene to benzaldehyde. With com-
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Investigation of Chromium Complexes with a Series of Tripodal Ligands
plexes 1b and 2b lower yields were most likely observed due substrates with dioxygen as an oxidant. Furthermore, it was
to equilibria of dimers and monomers. Two different mechan- shown that this reactivity strongly depends on the tripodal li-
istic pathways were suggested by Lucas et al. for the oxidation gand used. Unfortunately, our hope that using chromium com-
of toluene by copper complexes.[2b] However, these proposed plexes as an alternative to copper or iron complexes would
mechanisms so far are not based on solid kinetic studies and simplify our understanding of these reaction mechanisms
might be different for the chromium complexes described in turned out to be too naive. Similar to related copper chemistry,
here anyway.
the reaction behavior of chromium complexes with dioxygen
is quite complicated to analyze. Furthermore, we were not able
to structurally characterize some of the postulated reactive in-
termediate chromium “dioxygen adduct” complexes so far.
However, the reactivity of the chromium(II) complexes in prin-
ciple is promising. Optimizations of these reactions using dif-
ferent “better” ligands might lead to an alternative way for
oxidation reactions in organic chemistry.
Low Temperature Stopped-Flow Spectroscopy
In regard to the finding that some toluene oxidation was
observed with complex 3b, its reaction with dioxygen was
furthermore investigated by low temperature stopped-flow
measurements. When 3b (the spectrum is presented as bold
line in Figure 9b) reacted with dioxygen, a new complex
rapidly formed (thin line in Figure 9b).
Experimental Section
Materials
Reagents and solvents used were commercially available. Chromi-
um(II) triflate tetrahydrate was prepared according to literature.[18]
Furthermore, all tripodal ligands were synthesized according to pub-
lished procedures.[19]
Equipment
UV/Vis absorption spectra were recorded using an Agilent 8453-spec-
trometer. The measurements were performed using quartz cuvettes.
GC-MS data were obtained using a Quadrupol-MS HP MSD 5971 (EI)
with a J&W quartz glass GC column (30 m ϫ 0.25 mm, 0.25 μm DB-
5 MS). Stopped-flow experiments at ambient pressures were per-
formed using a commercially available Hi Tech Limited SF-61SX2
(Salisbury, UK) instrument. IR measurements were performed using a
JASCO FT/IR-4100 spectrometer. Elemental analyses were obtained
Figure 9. a) Time resolved spectra for the reaction of a 0.5 mmol·L–1 using a vario EL III element analyzer from Elementar.
solution of 3b in acetonitrile with dioxygen [0.01 mol·L–1] at –40.0 °C
(overall reaction time = 450 s, Δ = 1.5 ms). b) UV/Vis spectrum of
3b (0.5 mmol·L–1 in acetonitrile) and selected single spectra from the
stopped-flow experiment.
Synthesis of Complexes with the Ligands L = tmpa,
Me2uns-penp, Me4apme and Me6tren
The spectral features of this new compound (320, 381, 528
and 631 nm) are similar to the reported data for the end-on
The synthesis of these complexes is based on the preparation of the
complex [Cr(tmpa)Cl2] (1a) and [Cr(tmpa)Cl]2(BPh4)2 (1b) described
superoxido chromium(III) complex by Nam and co-workers.[9]
Unfortunately it was not possible to further characterize this
intermediate because in a slower consecutive reaction a dif-
ferent complex formed (dashed line in Figure 9b, absorbance
maxima at 319 (shoulder), 378 and 574 nm). So far, the final
product of this reaction is unknown. Efforts to isolate and char-
acterize this product complex failed. Furthermore, a detailed
kinetic analysis was not possible due to a more complex reac-
tion behavior (parallel reactions – no isosbestic points were
observed). In regard to these problems together with low yields
on the oxidation of toluene this reaction further was not inves-
tigated.
in literature.[12] Complex [Cr(tmpa)OH]2(OTf)4 (1c) was also synthe-
sized according to literature.[13a]
[Cr(Me2uns-penp)Cl2] (2a): Anhydrous CrCl2 (0.05 g, 0.37 mmol)
and
Me2uns-penp
(2-dimethyl-aminoethyl)bis(2-pyridylmethyl)-
amine) (0.10 g, 0.37 mmol) were stirred in tetrahydrofuran under inert
atmosphere. The solution was stirred and a brown precipitate occurred.
The solid was collected, dissolved in a minimum of acetonitrile and
filtered. Brown crystals could be obtained by ether diffusion at –40 °C.
Yield: 0.08 g (55%). Anal.: C16H22N4Cl2Cr (393.27); C 48.46 (calcd.
48.86); H 5.65 (5.64); N 14.73 (14.25)%. UV/Vis (CH3CN): λmax (lg
ε): 257 nm (3.7), 346 nm (3.1), IR (KBr): ν = 3020, 2985, 2927, 1661,
1604, 1472, 1437, 1288, 1156, 1028, 818, 765, 650 cm–1.
[Cr(Me2uns-penp)Cl]2(BPh4)2 (2b): Anhydrous CrCl2 (0.05 g,
0.37 mmol) and NaBPh4 (0.13 g, 0.37 mmol) were stirred with
Me2uns-penp (0.10 g, 0.37 mmol) in tetrahydrofuran under inert atmo-
sphere. The resulting precipitate was collected, dried and dissolved in
Conclusions
It was demonstrated that chromium(II) complexes with
tripodal ligands in principle can be used to oxidize organic acetonitrile at 70 °C. The solution was filtered hot and brown crystals
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S. Schäfer, J. Becker, A. Beitat, C. Würtele
ARTICLE
could be obtained by ether diffusion at –40 °C. Yield: 0.06 g (22%).
signals were calculated on the basis of comparisons to a calibration
Anal.: C80H84N8B2Cl2Cr2 (1354.10); C 69.90 (calcd. 70.96); H 6.31 curve according to the internal standard xylene. Also control reactions
(6.25); N 8.97 (8.28)%. UV/Vis (CH3CN): λmax (lg ε): 350 nm (3.1),
426 nm (2.8). IR (KBr): ν = 3052, 2994, 2983, 2833, 2370, 2309,
with chromium(II) chloride and potassium dichromate were carried out
and showed no reaction. And the blank only with acetone and toluene
1945, 1881, 1816, 1677, 1607, 1579, 1478, 1439, 1424, 1288, 1266, yielded no benzaldehyde.
1160, 1097, 1054, 1030, 985, 845, 732, 706, 612 cm–1.
[Cr(Me4apme)Cl]Cl (3a)/[Cr(Me4ampe)Cl3] (3d): Anhydrous CrCl2
(0.05 g, 0.40 mmol) and Me4apme (bis(2-dimethyl-aminoethyl)(2-pyr-
idylmethyl)amine) (0.10 g, 0.40 mmol) were stirred in tetrahydrofuran
under inert atmosphere. The solution was stirred and a green precipi-
tate occurred. The solid was collected, dissolved in a minimum of
acetonitrile and filtered. During crystallization two species were ob-
tained. Green crystals of 3d were suitable for structural analysis (Fig-
ure 6). Brown crystals of 3a were measurable, but refinement was not
possible. Anal.: C14H26N4Cl3Cr (408.74); C 41.00 (calcd. 41.14); H
6.24 (6.41); N 13.50 (13.71)%. UV/Vis (CH3CN): λmax (lg ε): 366 nm
(2.8), IR (KBr): ν = 3062, 3024, 2966, 2919, 1606, (1566), 1474,
(1438), 1293, 1156, 1099, 1027, 934, 809, 773 cm–1.
X-ray Diffraction
The X-ray crystallographic data of 2b, 3b and 4a were collected on a
STOE IPDS-diffractometer equipped with a low temperature system
(Karlsruher Glastechnisches Werk). Mo-Kα radiation (λ = 0.71073 Å)
and a graphite monochromator. No absorption corrections were ap-
plied. The structures were solved by direct methods in SHELXS97
and refined by using full-matrix least-squares in SHELXL97.[20] All
hydrogen atoms were positioned geometrically and all non-hydrogen
atoms were refined anisotropically.
The X-ray crystallographic data of 3d was collected with a BRUKER
NONIUS KappaCCD system using a BRUKER NONIUS. FR591 rot-
ating anode as radiation source and an OXFORD CRYOSYSTEMS
low temperature system. Mo-Kα radiation with wavelength 0.71073 Å
and a graphite monochromator was used. The PLATON MULABS
semi-empirical absorption correction using multiple scanned reflection
was applied. The structure was solved by direct methods in
SHELXS97 and refined with SHELXL97 using full-matrix least-
squares.[20] All non-hydrogen atoms were refined anisotropically. Only
the 50% occupied hydrogen atoms were positioned geometrically.
[Cr(Me4apme)Cl]BPh4 (3b): Anhydrous CrCl2 (0.05 g, 0.40 mmol)
and NaBPh4 (0.14 g, 0.40 mmol) were stirred with Me4apme (0.10 g,
0.40 mmol) in tetrahydrofuran under inert atmosphere. The resulting
precipitate was collected, dried and dissolved in a minimum of acetoni-
trile at 70 °C. The solution was filtered hot and green crystals could
be obtained by ether diffusion at –40 °C. Yield: 0.12 g (45%). Anal.:
C38H46N4BClCr (657.05); C 69.00 (calcd. 69.46); H 7.24 (7.06); N
8.58 (8.53)%. UV/Vis (CH3CN): λmax (lg ε): 365 nm (2.8). IR (KBr):
ν = 3052, 2998, 2982, 2965, 2366, 2309, 1941, 1892, 1817, 1606,
1577, 1475, 1437, 1427, 1300, 1265, 1155, 1090, 1052, 1022, 985,
930, 806, 770, 752, 734, 704, 609 cm–1.
Crystallographic data for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-924632 for 1c, CCDC-924633
for 2b, CCDC-924634 for 3b and CCDC-924635 for 4a and
CCDC-924636 for 3d. Copies of the data can be obtained, free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The crystallographic data for 2a
and 4b are not deposited (see Supporting Information).
[Cr(Me6tren)Cl]Cl (4a): Anhydrous CrCl2 (0.05 g, 0.43 mmol) and
Me6tren (tris(2-dimethylaminoethyl)amine) (0.10 g, 0.43 mmol) were
stirred in tetrahydrofuran under inert atmosphere. The resulting pre-
cipitate was collected, dried and dissolved in a minimum of acetoni-
trile. Light blue crystals were obtained by ether diffusion at –40 °C.
Yield: 0.11 g (72%). Anal.: C12H30N4Cl2Cr (353.30); C 40.21 (calcd.
40.80); H 8.56 (8.56); N 15.39 (15.86)%. UV/Vis (CH3CN): λmax (lg
ε): 900 nm (2.3), IR (KBr): ν = 2970, 2895, 2427, 1637, 1560, 1469,
1289, 1174, 1100, 1018, 966, 933, 802, 772, 594, 492 cm–1.
Supporting Information (see footnote on the first page of this article)
Crystallographic data and selected bond lengths and angles of com-
pounds of 1c, 2a and 4b.
[Cr(Me6tren)Cl]BPh4 (4b): Anhydrous CrCl2 (0.05 g, 0.43 mmol)
and NaBPh4 (0.15 g, 0.43 mmol) were stirred with Me6tren (0.10 g,
0.43 mmol) in tetrahydrofuran under inert atmosphere. The resulting
precipitate was collected, dried and dissolved in acetonitrile at 70 °C.
The solution was filtered hot and green crystals could be obtained by
ether diffusion at –40 °C. Yield: 0.09 g (33%). Anal.: C36H50N4BClCr
(637.07); C 66.22 (calcd. 67.87); H 7.68 (7.91); N 8.68 (8.79)%. UV/
Vis (CH3CN): λmax (lg ε): 900 nm (2.3), IR (KBr): ν = 3054, 2999,
2981, 2892, 2839, 1958, 1891, 1822, 1579, 1474, 1466, 1428, 1348,
1286, 1266, 1244, 1173, 1133, 1098, 1038, 1017, 932, 897, 845, 802,
768, 734, 705, 611 cm–1.
Acknowledgments
Christian Würtele is grateful to the postdoctoral program Just’us for
financial support. Furthermore we thank Prof. Dr. Siegfried Schindler
for support of this work and Günter Koch for his assistance in the
handling of the X-ray crystallographic measurements.
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Received: March 24, 2013
Published Online: August 15, 2013
Z. Anorg. Allg. Chem. 2013, 2269–2275
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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