A singlet biradicaloid zinc compound and its nonradical counterpart

Article abstract of DOI:10.1021/ja402351x

Metal ions with radical centers in their coordination sphere are key participants in biological and catalytic processes. In the present study, we describe the synthesis of the cAAC:ZnCl₂ adduct (1) using a cyclic alkylaminocarbene (cAAC) as donor ligand. Compound 1 was treated with 2 equiv of KC₈ and LiB(sec-Bu)₃H to yield a deep blue-colored dicarbene zinc compound (cAAC)₂Zn (2) and the colorless hydrogenated zinc compound (cAACH)₂Zn (3), respectively. Compounds 2 and 3 were well characterized by spectroscopic methods and single-crystal X-ray structural analysis. Density functional theory calculations were performed for 2 which indicate that this molecule possesses a singlet biradicaloid character. Moreover, we show the application of 2 in CO₂ activation, which yields a zwitterionic cAAC·CO₂ adduct.

Full text of DOI:10.1021/ja402351x

Article
pubs.acs.org/JACS
A Singlet Biradicaloid Zinc Compound and Its Nonradical
Counterpart
Amit Pratap Singh,† Prinson P. Samuel,† Herbert W. Roesky,*,† Martin C. Schwarzer,‡
Gernot Frenking,*,‡ Navdeep S. Sidhu,† and Birger Dittrich*,§
†Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
̈
‡Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerweinstraße, 35032 Marburg, Germany
̈
§Institut fur Anorganische und Angewandte Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, 20146-Hamburg, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Metal ions with radical centers in their coordination sphere are key
participants in biological and catalytic processes. In the present study, we describe the
synthesis of the cAAC:ZnCl2 adduct (1) using a cyclic alkylaminocarbene (cAAC) as
donor ligand. Compound 1 was treated with 2 equiv of KC8 and LiB(sec-Bu)3H to yield a
deep blue-colored dicarbene zinc compound (cAAC)2Zn (2) and the colorless
hydrogenated zinc compound (cAACH)2Zn (3), respectively. Compounds 2 and 3 were
well characterized by spectroscopic methods and single-crystal X-ray structural analysis.
Density functional theory calculations were performed for 2 which indicate that this
molecule possesses a singlet biradicaloid character. Moreover, we show the application of 2
in CO2 activation, which yields a zwitterionic cAAC·CO2 adduct.
Furthermore, we show the application of 2 in CO2 activation.
INTRODUCTION
■
The coordination of CO2, its activation, and its conversion into
organic compounds are rapidly growing domains of coordina-
tion chemistry, organometallic chemistry, and catalysis.7 Most
of the literature reports indicate that activation of CO2 requires
high temperature, or at least room temperature, and often the
presence of an activator or catalyst.8 In this respect, CO2
activation at lower temperature merits attention. Compound 2
activates CO2 at −30 °C within a few minutes to give a
zwitterionic cAAC·CO2 (4) adduct. Furthermore, 4 acts as a
stable cAAC source which eliminates CO2 in the presence of
zinc dichloride. Such a reversible fixation of CO2, which has so
far been observed only in polymers bearing N-bases,9 offers an
exciting challenge for further investigations in the context of
environmental chemistry.
Radicals and biradicals are very reactive chemical species that
play important roles in combustion, atmospheric chemistry,
plasma and polymerization chemistry, biochemistry, and a
number of other chemical processes.1 Metal ions with radical
centers in their coordination sphere are key participants in
biological and catalytic radical processes.2 There are a few
examples reported in the literature on the synthesis of metal
ions with a radical center in their coordination sphere by using
an organic ligand with a radical center or incorporating an in
situ-generated radical species.3 In 2007, Fedushkin and co-
workers4 reported on the synthesis and characterization of a
biradical dizinc compound supported by a radical anionic
ligand, 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene
(dpp-Bian). The dpp-Bian is bound to zinc in a bidentate
mode, resulting in a five-membered chelate ring. A similar type
of bonding was observed previously by Raston and co-workers5
for the 1,4-di-tert-butyl-1,4-diazabutadiene ligand with magne-
sium and zinc. These compounds were found to be triplets in
their ground state. However, a singlet biradical of two-
coordinated zinc is still elusive. In this context, we have
established a completely different approach for the synthesis of
a singlet biradical two-coordinated zinc compound, (cAAC)2Zn
(2), using a cyclic alkylaminocarbene (cAAC).6 So far no
physical methods are available to characterize unambiguously
the biradicaloid status of such compounds, and this prompted
us to synthesize also a nonradical counterpart, (cAACH)2Zn
(3), of the biradicaloid zinc compound 2 (Scheme 1).
RESULTS AND DISCUSSION
■
Synthesis. Compounds 2 and 3 were each synthesized in
two steps. In the first step, 2 equiv of cAAC were treated with 1
equiv of ZnCl2 in THF to give a 1:1 adduct of cAAC:ZnCl2
(1); 1 equiv of carbene remained unreactive. In the second
step, the reaction mixture was treated with 2 equiv of KC8 at
−78 °C. A deep blue color appeared in the solution at −50 °C.
After 1 h of stirring, the reaction mixture was filtered, and the
filtrate was stored for 1 day at −4 °C to give deep blue crystals
of 2. However, the reaction mixture of 1 and cAAC with
Received: March 6, 2013
Published: April 21, 2013
© 2013 American Chemical Society
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1
showed quite sharp resonances in its H NMR spectrum with
slightly different positions in comparison with those of the free
carbene cAAC and 3. In addition, 3 showed a singlet resonance
Scheme 1. Synthesis of Compounds 1−4
in its 1H NMR spectrum at 3.34 ppm for CH−Zn protons. 13
C
NMR spectra of 2 and 3 exhibited resonances for the “carbene”
carbon at 151 ppm (C•−Zn) and 78 ppm (CH−Zn),
respectively. In the FT-IR spectrum of 4, the ν(CO2) vibrations
were observed at 1666 and 1610 cm−1.
Crystal Structure Analysis. In order to establish
unambiguously the structural features of 2−4, single-crystal
X-ray structural analysis were carried out. The crystal and
structure refinement parameters for 2−4 are given in Table S1
(Supporting Information). The molecular structures of 2 and 3
show that these compounds are analogues of the R2Zn
compounds (e.g., dimethylzinc and diethylzinc). The latter
are very important reagents in various organic and inorganic
transformations.11 Under standard conditions both dimethyl-
zinc and diethylzinc exist as volatile, pyrophoric liquids
(Me2Zn: mp −42 °C, bp 46 °C; Et2Zn: mp −28 °C, bp 118
°C). Recently Steiner and co-workers12 reported the crystal
structures of Me2Zn and Et2Zn which exhibit a rich solid-phase
behavior. The solid-state studies of diarylzinc and bulky
dialkylzinc derivatives revealed that these molecules exhibit
monomeric structures.13 The crystal structures of 2 and 3 show
that the zinc atom is covalently bound to the two cAAC
molecules, resulting in a linear geometry as observed in the case
of Me2Zn and Et2Zn. The C1−Zn1 bond length in 2 is
1.8850(17) Å, which is shorter than the corresponding bond
lengths found in 3 (2.041(4) Å) and Me2Zn (1.927(6) Å).12
The C1−Zn1−C1′ bond angles in both 2 and 3 are 180.0°; the
zinc atom is found in the inversion center (Figures 1 and 2). In
LiB(sec-Bu)3H yielded a colorless clear solution at room
temperature. After removal of the solvent, a colorless solid of 3
was obtained. The crude compound was purified by
crystallization. Single crystals suitable for X-ray diffraction
analysis of 3 were obtained from a concentrated toluene
solution at −32 °C. In an alternative route for the synthesis of
2, we reacted cAAC and ZnCl2 in a 1:1 ratio to prepare the
cAAC:ZnCl2 adduct 1, and this adduct was then treated with
cAAC and KC8 in a ratio of 1:2.1 to give 2. In another
experiment, when CO2 was bubbled through the THF solution
of 2 at −30 °C for 2−3 min, an immediate color change from
blue to light yellow was observed. The concentrated THF
solution gave colorless crystals of 4 at −4 °C. In the literature it
is reported that R2Zn compounds do not react with CO2 at
room temperature, especially in the absence of a catalyst.10 In
contrast, 2 reacts rapidly at −30 °C. We assume that, due to the
presence of radical centers at the “carbene” carbon atoms in 2,
CO2 is easily activated to give the stable cAAC·CO2 adduct 4 as
a colorless compound. We observed that cAAC does not react
with CO2 at −30 °C, but reacts very slowly at 25−30 °C to give
4. Furthermore, 3 does not react with CO2 even at higher
temperatures up to 50 °C. These experiments clearly indicate
that the biradicaloid nature of 2 promotes CO2 activation even
at low temperatures. Compounds 2−4 have been fully
characterized by elemental analysis, various spectroscopic
methods, and single-crystal X-ray structural analysis. The
UV−visible absorption maximum for the intensely blue-colored
THF solution of 2 appeared at 630 nm. However, 2 was found
to be electron paramagnetic resonance (EPR) silent, and it
Figure 1. Molecular structure of 2. H atoms are omitted for clarity.
Anisotropic displacement parameters are depicted at the 50%
probability level. Selected experimental [calculated at RI-BP86/def2-
SVP singlet; triplet; singlet biradical] bond lengths [Å] and angles [°]:
Zn1−C1 1.8850(17) [1.896; 1.924; 1.902], C1− N1 1.376(2) [1.388;
1.398; 1.392], C1−C1′3.770 [3.791; 3.851; 3.805], C1−Zn1−C1′
180.0 [180.0; 177.6; 180.0].
2 and 3 the geometry around the “carbene” carbon atom is
trigonal planar and distorted tetrahedral, respectively. This
geometric difference between 2 and 3 unambiguously shows
the presence of the hydrogen atom at the “carbene” carbon of
the latter. The shorter C−Zn bond length of 2 is further
explained by the density functional theory (DFT) calculations.
The crystal structure of the reaction product 4, shown in Figure
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DFT Calculations for Compounds 2−4. Compound 2
exhibits no EPR resonance, suggesting a closed-shell ground
state of 2. In order to gain more insight into the electronic
situation of 2, DFT calculations were performed for the
geometries of the singlet and triplet states at the RI-BP86/def2-
SVP level of theory.14,23,24 The bond lengths and angles of 2
depicted in the legend of Figure 1 illustrate that the calculated
values for the singlet biradical are in better agreement with the
experimental values when compared with those of the
superposition of the X-ray structure with the calculated singlet
and triplet forms (Supporting Information). The experimental
structure shows a nearly perfect alignment with the singlet
form, while the calculated triplet exhibits a significantly greater
deviation. Calculations at the RI-BP86/def2-TZVPP//def2-
SVP level predict that the singlet is 6.9 kcal/mol lower in
energy than the triplet, whereas the singlet biradical is 0.1 kcal/
mol below the closed-shell singlet. The DFT calculations using
the functionals M05-2X, B3LYP, and PBE0 suggest that the
triplet state of 2 is slightly lower in energy than the closed-shell
singlet but the singlet biradical form is always more stable than
the triplet state (Table S2, Supporting Information). Thus, the
calculations agree with the conclusion reached from the EPR
analysis that 2 is not a triplet biradical. At the same time the
DFT calculations also suggest that the molecule possesses
biradicaloid character. This is supported by a CASSCF[2,2]/
def2-SVP calculation of 2 which gives coefficients of 0.81 and
−0.59 for the ground and first excited configurations. Figure
4a,b shows the highest lying degenerate singly occupied
Figure 2. Molecular structure of 3. H atoms (except CH−Zn) and
solvent molecules are omitted for clarity. Anisotropic displacement
parameters are depicted at the 50% probability level. Selected
experimental [calculated at RI-BP86/def2-SVP singlet] bond lengths
[Å] and angles [°]: C1−Zn1 2.041(4) [2.014], C1−N1 1.457(6)
[1.472], C1−Zn−C1′ 180.0 [179.9].
3, exhibits a zwitterionic bonding state. The nearly equivalent
C−O bond distances (1.2395(19) and 1.2352(19) Å) indicate
that the negative charge is delocalized equally among the
central carbon (C21) and the two oxygen (O1 and O2) atoms.
In contrast, the C1−N1 bond distance (1.2920(18) Å) is
substantially shorter than 1.376(2) Å as found in the parent
compound 2. This indicates a double bond character between
C1 and N1 atoms resulting in a formal positive charge at the
nitrogen atom in 4. The C1−C21 bond length (1.521(2) Å) is
longer than those reported for analogous zwitterionic CO2
adducts,8c which indicates that CO2 is only loosely bound to
cAAC. Therefore, treatment of 3 with even 0.5 equiv of ZnCl2
results in the elimination of CO2 to give a 1:1 mixture of 1 and
cAAC, which can be further used to obtain 2, thus completing
the cycle (Scheme 1). Moreover, 4 is quite stable for long
period of time under an inert atmosphere and does not release
CO2.
Figure 4. (a,b) Plots of the highest lying degenerate SOMOs of the
singlet biradical state of 2. (c) Plots of the HOMO of the closed-shell
singlet state of 2. (d) Plot of the spin density of the singlet biradical
state 2. All calculations at the RI-BP86/def2-TZVPP//def2-SVP level.
molecular orbitals (SOMOs) of the singlet biradical state of
2. The π-type orbitals are polarized toward either carbon atom.
The highest occupied molecular orbital (HOMO) of the
closed-shell singlet state of 2 is symmetric with respect to the
two ligands (Figure 4c). The calculated spin density of the
singlet biradical state of 2 (Figure 4d) indicates that the
unpaired electrons are mainly located at the carbon donor
atoms of the ligands and to a minor extent at the nitrogen
atoms.
Figure 3. Molecular structure of 4. H atoms are omitted for clarity.
Anisotropic displacement parameters are depicted at the 50%
probability level. Selected experimental [calculated at RI-BP86/def2-
SVP singlet] bond lengths [Å] and angles [°]: C1−C21 1.521(2)
[1.520], C1−N1 1.2920(18) [1.323], C21−O1 1.2395(19) [1.262],
C21−O2 1.2352(19) [1.241], O1−C21−O2 131.41(14) [131.4].
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The bonding situation in 2 and the preference for a singlet
biradicaloid structure can thus be explained as follows. The C−
Zn σ bonds come from electron-sharing interactions between
the unpaired electrons in the σ orbitals of the carbene carbon
atom of the cAAC ligands in the triplet state and Zn (Figure 5).
even more reactive than the longer one when both react with
CO2. The difference in the reaction temperature is more than
80 °C.10 This result leads us to assume that a singlet
biradicaloid system could be an important reagent for various
other chemical processes. Such investigations will be reported
in due course from our laboratory.
EXPERIMENTAL SECTION
■
All reactions and handling of reagents were performed under an
atmosphere of dry nitrogen or argon using standard Schlenk
techniques or a glovebox where the O2 and H2O levels were usually
kept below 1 ppm. The cyclic alkylaminocarbene (cAAC) was
synthesized following a reported procedure.6 Solvents were purified
with the M-Braun solvent drying system. Solution NMR spectra were
recorded on Bruker Avance 200, Bruker Avance 300, and Bruker
Avance 500 MHz NMR spectrometers. Elemental analyses were
Figure 5. Schematic representation of the chemical bonding between
Zn and the carbon atoms in the singlet biradical state of 2. The σ
orbital at Zn sketches the two lobes of the sp hybrids which point
toward the carbon atoms.
performed by the Analytisches Labor des Instituts fur Anorganische
̈
Chemie der Universitat Gottingen. Melting points were measured in
̈
̈
sealed glass tubes on a Buchi B-540 melting point apparatus.
̈
The use of the excited triplet state of the ligands as reference
state is supported by the finding that the HOMO of 2 is a π
orbital. Electron-sharing bonds are much stronger than donor−
acceptor bonds, which compensate for the singlet−triplet
excitation energy of the ligands. A similar situation was recently
found for the Si−C bonding in (cAAC)2SiCl2.18 The unpaired
π electrons at the carbene carbon atom couple via the vacant
p(π) atomic orbital of Zn which receives an equal amount of α
and β electrons from the ligands. Therefore, the spin density
rests on the carbene carbon atom with small contributions at
the nitrogen lone pair orbital which couple with the unpaired
electrons.
We also optimized the geometries of 3 and 4. Theoretical
values for bond lengths and angles are in very good agreement
with the experiment (Figures 2 and 3 and Supporting
Information). It is noteworthy that the calculated C1−N1
bond length increases from 4 (1.323 Å) to 2 (1.392 Å) and 3
(1.472 Å) which is in accord with the experimental values. It
indicates that the N1→C1 π interaction which is quite strong in
4, but negligible in 3, plays an intermediate role in the π
conjugation and thus in the stabilization of the biradicaloid
species. This is supported by the shape of the SOMO (which
are N1−C1 antibonding) and the energetically lower lying
doubly occupied π orbitals (which are N1−C1 bonding) that
have quite large coefficients at N1. The Wiberg bond order for
the N1−C1 bond for 4 is 1.48, for 2 is 1.15, and for 3 is 0.97,
giving numerical evidence for the strength of the partial π
bonding in the molecules. According to the RI-BP86/def2-
TZVPP//RI-BP86/def2-SVP calculations, the hydrogenation of
2 yielding 3 is exergonic by −36.7 kcal/mol. The bond strength
of the cAAC−CO2 complex is calculated at the same level of
theory to be 15.8 kcal/mol.
Synthesis of cAAC:ZnCl2 (1). Tetrahydrofuran (30 mL) was
cooled to −50 °C and added to a 1:1 mixture of cAAC (3.50 mmol,
1.00 g) and ZnCl2 (3.50 mmol, 478 mg). The reaction mixture was
slowly warmed to room temperature and stirred for 3 h. A white
precipitate appeared which was then filtered over a frit and dried under
1
vacuum to obtain pure 1: yield 1.18 g, 80%; mp 194−195 °C; H
NMR (CD3CN, 298 K, 500 MHz, δ ppm) 7.48−7.45 (m, 1Har, p-H),
7.38−7.37 (d, 2Har, m-H), 2.85−2.80 (m, 2H, CHMe2), 2.14 (s, 2H,
CH2), 1.51 (s, 6H, CH3), 1.41 (s, 6H, CH3), 1.31−1.29 (m, 12H,
CHCH3); 13C NMR (CD3CN, 298 K, 126 MHz, δ ppm) 146.7, 135.2,
133.o, 131.1, 126.4, 56.8, 50.1, 30.2, 29.8, 29.4, 28.8, 26.2, 22.1.
Elemental analysis, found (calcd) for C20H31Cl2NZn: C, 56.31
(56.95); H, 7.21 (7.41); N, 3.16 (3.32).
Synthesis of (cAAC)2Zn (2). Tetrahydrofuran (50 mL) was
cooled to −78 °C and added to a 1:1:2.1 mixture of 1 (2.37 mmol,
1.00 g), cAAC (2.37 mmol, 676 mg), and KC8 (4.97 mmol, 672 mg)
while stirring. The reaction mixture was slowly warmed to room
temperature and stirred for an additional hour to give a deep blue
solution. The solution was then filtered, and the volume of the filtrate
was reduced to half and stored at −4 °C to give blue crystals suitable
for single-crystal X-ray analysis of 2: yield 1.17 g, 78%; mp 175−180
1
°C; UV/vis λab = 630 nm; H NMR (THF-d8, 298 K, 500 MHz, δ
ppm) 7.19−7.18 (m, 2Har, p-H), 7.12−7.11 (d, 4Har, m-H), 3.26−3.23
(m, 4H, CHMe2), 1.88 (s, 4H, CH2), 1.38 (d, 12H, CHCH3), 1.29 (s,
12H, CH3), 1.19 (d, 12H, CHCH3), 0.84 (s, 12H, CH3); 13C NMR
(THF-d8, 298 K, 126 MHz, δ ppm) 151.2, 139.4, 130.2, 128.1, 126.2,
69.7, 56.9, 47.5, 34.0, 30.6, 30.4, 30.1, 29.9, 28.9. Elemental analysis,
found (calcd) for C40H62N2Zn: C, 74.15 (75.50); H, 10.16 (9.82); N,
4.29 (4.40).
Synthesis of (cAACH)2Zn (3). Tetrahydrofuran (50 mL) was
added to a 1:1 mixture of 1 (2.37 mmol, 1.00 g) and cAAC (2.37
mmol, 676 mg). The reaction mixture was cooled to −78 °C, and
LiB(sec-Bu)3H in THF (1 M, 4.74 mmol, 4.74 mL) was added slowly
with stirring. The reaction mixture was warmed to room temperature
and stirred for an additional 10 min to give a colorless solution. The
volatiles were removed under reduced pressure to obtain a colorless
solid. The crude solid was then dissolved in toluene and stored at −32
°C to give colorless crystals of 3 suitable for single-crystal X-ray
CONCLUSION
■
1
We have developed novel methodologies for the synthesis of
singlet biradicaloid dicarbene zinc compound 2. The
biradicaloid nature of the compound was proven by the
synthesis of a nonradical counterpart 3. The biradicaloid zinc
compound has shorter C−Zn bond lengths than the nonradical
counterpart due to the strong back-bonding. DFT calculations
for 2 indicate that this molecule possesses a singlet biradicaloid
character. We have also shown that such a zinc centered singlet
biradicaloid activates CO2 at low temperature. A comparison of
the C−Zn bond lengths of 1.927 Å for Me2Zn and 1.885 Å for
(cAAC)2Zn indicates that a shorter C−Zn bond in the latter is
analysis: yield 1.21 g, 80%; mp 169−173 °C; H NMR (THF-d8, 298
K, 500 MHz, δ ppm) 7.17−7.14 (m, 2Har, p-H), 7.12−6.90 (m, 4Har,
m-H), 4.26−4.23 (m, 4H, CHMe2), 3.34 (s, 2H, CH-Zn), 1.50 (s, 4H,
CH2), 1.25 (d, 12H, CHCH3), 1.06 (d, 12H, CHCH3), 0.90 (s, 12H,
CH3), 0.50 (s, 12H, CH3); 13C NMR (THF-d8, 298 K, 126 MHz, δ
ppm) 140.9, 129.6, 127.2, 125.6, 78.2, 65.4, 52.9, 45.3, 33.1, 32.6, 29.4,
28.1, 27.9, 24.5. Elemental analysis, found (calcd) for C54H80N2Zn: C,
78.48 (78.84); H, 9.56 (9.80); N, 3.53 (3.41).
Synthesis of cAAC·CO2 (4). CO2 was passed over a tetrahy-
drofuran solution (50 mL) of 2 (0.78 mmol, 500 mg) at −30 °C for
2−3 min. An immediate color change of the solution was observed
from blue to light yellow with little turbidity. The latter was
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characterized as Zn metal by elemental analysis. The reaction mixture
was then filtered through a frit, and the filtrate was stored at −4 °C to
give colorless needle-shaped crystals of 4: yield 450 mg, 87%; mp
182−185 °C; 1H NMR (THF-d8, 298 K, 500 MHz, δ ppm) 7.39−7.27
(m, 3Har), 2.89−2.76 (m, 2H, CHMe2), 2.30 (s, 2H, CH2), 1.59 (s,
6H, CH3), 1.46 (s, 6H, CH3), 1.33−1.27 (m, 12H, CHCH3); 13C
NMR (THF-d8, 298 K, 126 MHz, δ ppm) 195.1, 157.7, 147.1, 135.1,
130.9, 126.1, 50.3, 48.2, 30.6, 30.4, 29.2, 28.9; FT-IR (KBr) ν(CO2)
1666 and 1610 cm−1. Elemental analysis, found (calcd) for
C21H31NO2: C, 75.84 (76.55); H, 9.62 (9.48); N, 4.08 (4.25).
Crystal Structure Determination. Molecular structures of 2−4
were established by single-crystal X-ray crystallographic studies, and
the corresponding ORTEP representations are shown in Figures 1−3.
Crystals were measured on a Bruker three-circle diffractometer
equipped with a SMART 6000 CCD area detector and a Cu Kα
rotating anode. Integrations were performed with SAINT.19 Intensity
data for all compounds were corrected for absorption and scaled with
SADABS.20 Structures were solved by direct methods and initially
refined by full-matrix least-squares methods on F2 with the program
SHELXL-97,21 utilizing anisotropic displacement parameters for non-
hydrogen atoms. Visualization and modeling were done using
SHELXLE.22
Computational Details. Geometry optimizations were carried out
using the DFT functionals BP86,14 (for 2−4) M05-2X,15 B3LYP,16
and PBE017 (all for 2) with def2-SVP basis sets.23 The RI
approximation was applied whenever possible.24 The optimized
geometries were verified as minima on the potential energy surface
by calculation of the vibrational frequencies analytically (AO-
FORCE).25 Improved energies were calculated with the larger basis
sets def2-TZVPP26 using geometries obtained with the SVP basis set.
Calculations were carried out with the program packages Gaussian0927
and Turbomole 6.3.28
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ASSOCIATED CONTENT
* Supporting Information
Crystallographic data (CIF), details of DFT calculations, and
complete ref 27. This material is available free of charge via the
Internet at http://pubs.acs.org.
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S
AUTHOR INFORMATION
Corresponding Author
hroesky@gwdg.de; frenking@chemie.uni-marburg.de; bdittri@
gwdg.de and birger.dittrich@chemie.uni-hamburg.de
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
H.W.R. thanks the Deutsche Forschungsgemeinschaft (DFG
RO 224/60-1) for financial support.We are thankful to Dr. A.
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(18) Mondal, K. C.; Roesky, H. W.; Schwarzer, M. C.; Frenking, G.;
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Claudia Stuckl for EPR measurements. N.S.S. thanks Prof.
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