Bis(diethylamino)(pentafluorophenyl)phosphane - A push-pull phosphane available for coordination

Article abstract of DOI:10.1002/ejic.201100138

A facile large-scale synthesis of the "push-pull"-substituted ligand bis(diethylamino)(pentafluorophenyl)phosphane is reported. A selenophosphorane as well as complexes with CuI and PdCl₂, can be formed almost quantitatively from suitable starting materials. The Pd ^II complex shows a square-planar coordination with significant distortions of the Cl-Pd-Cl moiety in the solid state. In contrast, the phosphane ligand exhibits a large flexibility in the trigonal-planar coordination of the Cu salt, as proven by X-ray crystallography. C-C cross-coupling reactions and 1,3-dipolar cycloadditions have been tested for the Pd^II and Cu^I complexes, respectively. Whereas the reactivity of the Pd^II complex is good at low temperature, the Cu^I complex reveals remarkable reaction rates at temperatures up to 130 °C. Furthermore, the Cu^I-catalyzed azide/alkyne 1,3-dipolar cycloaddition was successfully adapted for flow conditions. Unlike their previously reported congeners, "push-pull"-substituted (amino)(fluoroaryl)phosphanes have been made available for reactions at phosphorus includingcoordination to metal centers, which was employed for catalyzing C-C coupling and "click" reactions.

Full text of DOI:10.1002/ejic.201100138

FULL PAPER
DOI: 10.1002/ejic.201100138
Bis(diethylamino)(pentafluorophenyl)phosphane – a Push–Pull Phosphane
Available for Coordination
Andreas Orthaber,[a] Michael Fuchs,[b] Ferdinand Belaj,[a] Gerald N. Rechberger,[c]
C. Oliver Kappe,[b] and Rudolf Pietschnig*[a]
Keywords: Phosphorus / Fluorinated ligands / Homogeneous catalysis / Click chemistry
A facile large-scale synthesis of the “push–pull”-substituted
ligand bis(diethylamino)(pentafluorophenyl)phosphane is re-
ported. A selenophosphorane as well as complexes with CuI
and PdCl2, can be formed almost quantitatively from suitable
starting materials. The PdII complex shows a square-planar
coordination with significant distortions of the Cl–Pd–Cl moi-
ety in the solid state. In contrast, the phosphane ligand exhi-
bits a large flexibility in the trigonal-planar coordination of
the Cu salt, as proven by X-ray crystallography. C–C cross-
coupling reactions and 1,3-dipolar cycloadditions have been
tested for the PdII and CuI complexes, respectively. Whereas
the reactivity of the PdII complex is good at low temperature,
the CuI complex reveals remarkable reaction rates at tem-
peratures up to 130 °C. Furthermore, the CuI-catalyzed az-
ide/alkyne 1,3-dipolar cycloaddition was successfully
adapted for flow conditions.
hydrocarbonylation, hydrocarboxylation, hydroformylation,
and Suzuki coupling reactions yielding reasonable activi-
ties.[12–14]
Introduction
Over the last decades palladium has achieved a promi-
nent role in catalysis and organic synthesis. Different types
of reaction protocols have been developed for the formation
of C–C bonds by the use of palladium catalysts.[1] In many
cases optimized phosphane ligands play a crucial role in
palladium catalysis, be it for electronic or steric reasons.[2–4]
Mono-, bis- and tris(amino)-substituted phosphanes repre-
sent electron-rich ligand systems,[5] which have been investi-
gated for instance in relation to hydrovinylation reactions.[5]
Especially (diethylamino)phosphanes have been widely
studied with respect to their coordination behavior[6] and
their potential in different catalytic reactions.[7,8]
The opposite approach, that is the design of electron-
deficient phosphane ligands also had a profound impact on
catalyst design. Since the first preparation of (C6F5)3P in
1960[9] and its structural characterization,[10] a wealth of co-
ordination compounds based on this ligand have been pre-
pared.[11] Thus, complexes with electron-withdrawing sub-
stituted phosphane ligands have been successfully tested in
It is generally accepted that in palladium-catalyzed cross-
coupling reactions cis-phosphane-stabilized Pd0 species un-
dergo oxidative addition with an aryl halide.[3] In recent
years, other more abundant metals with the same valence
electron configuration (d10), for example CuI, have also at-
tracted significant attention in catalysis. In particular, CuI-
catalyzed Huisgen azide/alkyne 1,3-dipolar cycloaddition
(CuAAC)[15] has become a very popular variety of “click
chemistry.”[15–19] Nitrogen-based ligands have been used ex-
tensively to trigger these cycloaddition reactions.[20–22] In
the past few years also aminophosphane ligands have been
used for this purpose, showing good stabilization of the CuI
species involved.[23,24]
C6F5P(NEt2)2 was reported already in the 1960s[25,26] and
since then became a subject of occasional physicochemical
studies,[27–29] which contrasts with the rich chemistry of its
nonfluorinated analogue, PhP(NEt2)2. Here we report a
straightforward and practical alternative to the original
synthetic procedure and further investigations into the co-
ordination and catalytic chemistry of C6F5P(NEt2)2 in stan-
dard model reactions. Our findings are corroborated by X-
ray crystallography, spectroscopic methods, and ab initio
calculations.
[a] Institute of Chemistry, University of Graz,
Schubertstr. 1, 8010 Graz, Austria
Fax: +43-316-380-9835
E-mail: rudolf.pietschnig@uni-graz.at
[b] Christian Doppler Laboratory for Microwave Chemistry and
Institute of Chemistry, University of Graz,
Heinrichstrasse 28, 8010 Graz, Austria
[c] Institute of Molecular Biosciences, Department of Mass
Spectrometry, University of Graz,
Results and Discussion
Heinrichstrasse 31, 8010 Graz, Austria
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100138.
Re-use of this article is permitted in accordance with the Terms
and Conditions set out at http://onlinelibrary.wiley.com/journal/
10.1002/(ISSN)1099–0682c/homepage/2005_onlineopen.html
Bis(diethylamino)(pentafluorophenyl)phosphane
(1)
combines electron-releasing amino groups with an electron-
withdrawing fluoroaryl unit at the same phosphorus atom.
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Push–Pull Phosphanes
Therefore, we were interested in the properties of this the fluoroaryl substituent is in fact not decisive. This means
“push–pull”-substituted phosphane and in further explor- in turn that steric factors are most likely responsible for the
ing the reactivity of this class of compounds. Compound quite drastic difference in reactivity on going from R = Et
1 was synthesized by the straightforward and cost-efficient to R = iPr in C6F5P(NR2)2. The selenophosphorane 2 could
reaction of LiC6F5 with ClP(NEt2)2 at low temperature also be characterized by means of X-ray diffraction and by
yielding the pure product after distillation (Scheme 1). EI-MS showing the characteristic pattern for the selenium-
Spectroscopic data for 1 are in agreement with the available containing species. Geometric parameters are similar as for
literature data and have been further amended.[28]
HC6F4P(=Se)(NEt2)2.[30] A remarkable feature of these
compounds is the almost coplanar orientation of one P–N
bond relative to the aromatic moiety, resulting in short
F···N contacts [2.767(7) Å and 2.844(7) Å] for both mole-
cules in the asymmetric unit. Furthermore, weak π–π and
π–Se interactions can be concluded from the solid-state
packing. A graphical representation of 2 is depicted in Fig-
ure 1. Our observation that the sterically less shielding di-
ethylamino groups significantly enhance the reactivity in 1
should open the way to the coordination chemistry of this
class of phosphane ligands.
Scheme 1. Synthesis of phosphane 1. (a) nBuLi, –50 °C, Et2O; (b)
–80 °C to room temp., ClP(NEt2)2.
Reactivity at the Phosphorus Atom
The closely related analog of 1 in which ethyl groups
were replaced by isopropyl groups was reported in 2003.[6]
Interestingly, in case of the isopropyl derivative
C6F5P(NiPr2)2 neither formation of a selenophosphorane Scheme 2. Formation of the selenophosphorane 2. (a) CHCl3,
room temp., 15 min, Segray (1.2 equiv).
nor coordination to (carbonyl)Rh complexes could be
achieved, which was attributed to steric and electronic fac-
tors.[6] By contrast, the facile selenation of a further related
phosphane, HC6F4P(NEt2)2, in which the amino groups are
identical but the fluoroaryl substituent differs by one fluor-
ine atom, was published very recently.[30] In the light of this
discrepancy, we wondered whether 1 would be unavailable
for reactions at the lone pair, just like its isopropyl-substi-
tuted analog, or prone to oxidation and coordination like
its partially fluorinated congener. In an initial attempt to
explore the reactivity of the phosphorus lone pair in this
compound we investigated the oxidation of 1 with gray sele-
nium (Scheme 2). The envisaged selenophosphorane 2 is
easily obtained at room temperature in quantitative yield.
In the 31P NMR spectra a single resonance at δ =
+49.4 ppm is observed showing the expected 77Se satellites
(1JPSe = 800 Hz), which is shifted to high field compared
1
with 1. In general, the JPSe coupling constant in seleno-
phosphoranes is a rough estimate for the overall electron-
withdrawing property of the substituents at the phosphorus
Figure 1. ORTEP drawing of the selenophosphorane 2 at a prob-
atom featuring larger values for more electron-withdrawing ability level of 50%. Hydrogen atoms and the second unit are omit-
substituents.[31] The coupling constant of 800 Hz in 2 is in
ted for clarity. Intra- and intermolecular contacts are indicated by
dashed lines. Selected distances [Å] and angles [°]: P1–Se1 2.102(2),
P2–Se2 2.087(2), P1–C21 1.845(7), P2–C41 1.839(7), F26–N12
2.844(7), F46–N32 2.767(7), Se2–C43 3.346(7), C25–C45 3.329(10);
the typical range for tris(alkylamino)selenophos-
phoranes,[32] whereas the 77Se NMR signal of 2 resonates
at lower field compared to (Me2N)3PSe, while the 31P NMR
C26–C21–P1–N12 –5.3(7), C46–C41–P2–N32 –10.7(7).
signal is observed at higher field {δSe = –136.9 ppm, δP
=
49.4 ppm (2) vs. δSe = –366 ppm, δP = 81.8 ppm [(Me2N)3-
PSe]}. Selenophosphorane 2 shows similar spectroscopic
data as its partially fluorinated analog HC6F4P(=Se)-
(NEt2)2.[30] The facile reaction of 1 with elemental selenium
Coordination Behavior
In order to study the coordination behavior of phos-
is in marked contrast to the reactivity of the analogous phane 1 we investigated its reactivity towards late transition
iPr2N-substituted phosphane reported by Dyer et al.[6] Ac- metals like Pd and Cu. The corresponding Pd complex (3)
tually, our findings suggest that electronic deactivation by can be synthesized from a suitable precursor, that is
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Cl2Pd(MeCN)2, and ligand 1 in nearly quantitative yield bonded to the Pd atoms in trans orientation. The pentafluo-
within a short time (Scheme 3). Coordination of the free rophenyl rings adopt orientations where an N atom of a
ligand 1 to the metal center is accompanied by a color diethylamino group is coplanar to the ring [N–P–C–C
1
change to intense orange. In the H NMR spectra of 3 the smaller than 1.8(2)°]. The most remarkable structural fea-
multiplet of the CH2 group splits into two distinctive mul- ture of 2 is the significant distortion from planarity around
tiplets. This diastereotopic effect for the protons of the the central Pd atom. Whereas the P–Pd–P angle is almost
methylene group upon coordination of the metal atom to linear [178.27(2)°], the Cl–Pd–Cl angle deviates strongly
phosphane 1 is comparable to that observed for benzyl- from linearity with 164.96(2)°.
phosphanes.[33]
There are only a few examples in the literature with a
bent linear arrangement of the phosphane ligands across
the metal atom with a Cl–Pd–Cl angle significantly smaller
than 180°. In these cases, such bending was imposed by the
use of rigid bidentate ligands,[36–39] which in one case is also
supported by a chlorine atom bridging two different metal
centers (Pd–Cl–Pt).[40] In the crystal structure of 3 the least-
squares (l.s.) plane through P1, Pd1, P3, and Cl1 encloses
an angle with the Pd1–Cl3 bond of 14.86(3)°. This motif is
best described as a distorted planar geometry, because the
Cl–Pd–P angles are close to a right angle [87.87(2)° to
91.81(2)°]. The Pd–Cl distances [2.309(1) Å and 2.320(1) Å]
are rather long for dichloridobis(phosphane)palladium
complexes, with the exception of bridging chlorine atoms,
Scheme 3. Synthesis of complexes 3 and 4 (ArF = C6F5). (a)
Cl2Pd(CH3CN)2, CH3CN, room temp., 2 h; (b) CuI, CHCl3, room
that is Pd–Cl–Pd, which can have Pd–Cl distances up to
temp., 0.5 h.
2.407(5) Å.[41] The Pd–Cl bond observed in 3 is 0.11 Å
Coordination shifts (Δδ = δ31Pcomplex – δ31Pfree ligand) have longer than for example in PdCl2(PPh3)2 (= M2). The Pd–
been studied by Trzeciak et al. showing a correlation be- P contacts are 2.322(1) Å and 2.355(1) Å and thus in the
tween Δδ, the phosphorus cone angle[34] and the Pd–P bond upper range of Pd–P distances for this class of com-
strength.[35] The coordination shift (Δδ) in the 31P NMR pounds.[42] Compared with C6F5P(NiPr2)2 [1.683(3) Å/
spectra of this phosphane is quite small (Δδ = –3.6 ppm; 1: 1.669(4) Å][6] and p-C6F4[P(NEt2)2]2 [1.675(2) Å/1689(2) Å]
[43]
82.0 ppm, 3: 78.4 ppm). A single crystal structure analysis
the P–N bonds in 3 are shortened upon coordination to
of Pd complex 3 confirms that in the solid state the ligand the metal center [1.646(2) Å–1.662(2) Å], whereas the P–C
coordinates through the phosphorus atom to the metal cen- bond is essentially unaffected. In contrast, the crystal struc-
ter. Two independent molecules of complex 3 with very sim- ture of tris(pentafluorophenyl)phosphane derivative
ilar structural parameters are present in the unit cell. One [(C6F5)3P]2PdCl2 shows much shorter Pd–P and Pd–Cl dis-
is located near the center of inversion and shows disorder tances [2.291(1) Å and 2.305(1) Å, respectively], and the
over two orientations, whereas the second molecule is not palladium center shows an almost planar coordination ge-
disordered. Consequently, the geometric parameters are dis- ometry.[44] The molecular structure of 3 is depicted in Fig-
cussed only for the molecule without disorder. The bis(di- ure 2, and further crystallographic data are given in the
ethylamino)(pentafluorophenyl)phosphane ligands in 3 are Supporting Information. In solution the spectrometric data
Figure 2. ORTEP drawing of Pd complex 3 at a probability level of 50% and schematic inset illustrating the trans configuration. Hydrogen
atoms and the second unit are omitted for clarity. Selected distances [Å] and angles [°]: Pd1–Cl3 2.3090(5), Pd1–Cl1 2.3199(5), Pd1–P3
2.3221(5), Pd1–P1 2.3554(5), P1–N11 1.646(2), P1–N12 1.662(2), P1–C21 1.855(2), P3–N32 1.651(2), P3–N31 1.655(2), P3–C41 1.861(2);
Cl3–Pd1–Cl1 164.96(2), P3–Pd1–P1 178.27(2).
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Push–Pull Phosphanes
are in good agreement with the solid-state structure. Dif- Cl–Pd–Cl unit can be reasonably reproduced suggesting
ferent mass spectrometric methods [i.e., DI-EI (direct inlet that secondary interactions of the ligand with the Pd–Cl
electron ionization) and the mild DI-ESI] lead to fragmen- moiety are responsible for this distorted geometry. Further-
tation of the complex, and only the ligand and its fragments more, it can be excluded that the significant distortion from
could be observed (i.e., ArFH+, ArF+–NEt2, C6F5PH+). In planarity results from crystal packing effects.
order to test the thermal stability of 3, we performed 31P
variable-temperature (VT) NMR experiments. Measure-
ments of 3 from room temperature (room temp.) up to
70 °C in C6D6 solution showed that no decomposition oc-
curs, and the complex is stable at these temperatures for at
Coordination to CuI
To check how general the unusual geometric features in
least 30 min. The integrity of the complex is further corrob- Pd complex 3 are, we set out to explore the coordination
orated by the persistence of the two distinctive multiplets behavior of phosphane ligand 1 towards a d10 metal ion
1
for the methylenic protons in the H NMR spectra at ele- where little backbonding can be expected. We have chosen
vated temperatures (70 °C). Formation of a second isomer CuI to extend our study owing to its relevance in “click”
(i.e., the cis isomer) could not be detected within the limits chemistry. The CuI complex 4 of phosphane 1 was obtained
of resolution and quantification in 31P NMR spectroscopy. by direct reaction of the ligand with CuI in a 2:1 ratio in
chloroform or dichloromethane (Scheme 3). As previously
observed for Pd complex 3, the coordination shift of Cu
complex 4 in the 31P NMR spectra is quite small (Δδ =
–8 ppm). Similar to complex 3, the methylenic protons are
Computational Study of the Coordination Geometry at PdII
For a better understanding of the unusual geometry diastereotopic. Slow evaporation of chloroform or dichloro-
around the palladium center in complex 3 we investigated methane under ambient conditions gave colorless hexago-
two model compounds by means of DFT calculations. In nal crystals of 4a. Crystal structure analysis of the complex
our model compounds M1 {[C6F5(Me2N)2P]2PdCl2} the shows a trigonal-planar surrounding of the central copper
ethyl groups of 3 were replaced by methyl groups to avoid atom with almost ideal parameters. The P–Cu–P angle
nonessential rotations around the CH2–CH3 bond. The [129.60(1)°] is distinctly larger than the I–Cu–P angles
DFT studies (B3LYP/6-31G*/LANL2DZ) were performed [113.62(1)° and 116.77(1)°]. The C6F5 groups are almost co-
on two isomers of M1, namely cis- and trans-M1, which are planar [7.41(7)°] suggesting that π-stacking may govern the
compared to computational results for the structurally well- relative orientation of the two phosphane ligands. Thus, the
characterized (Ph3P)2PdCl2 (= M2). A detailed analysis of inter-aryl distance is as short as 3.074(2) Å, which is 0.10 Å
the molecular orbitals was carried out in order to under- shorter than the sum of their van der Waals (vdW) radii.
stand the bonding situation of these complexes. A compari- The pentafluorophenyl rings adopt orientations in which
son of measured and calculated geometric parameters as the N atom of one diethylamino group is coplanar relative
well as a natural bond orbital (NBO) analysis should reveal to the aryl ring [torsion angles N–P–C–C smaller than
the differences and commonalities of both complexes. The 4.86(15)°]. The N atoms are consequently below/above the
P–Pd–P and Cl–Pd–Cl angles in trans-M1 are 178.9° and least-squares plane spanned by I1, Cu1, P1, P2 [N11
168.8°, respectively, which differs only slightly from the ex- –1.331(4) Å, N12 +1.396(4) Å, N21 –1.444(4) Å, N22
perimental values obtained by X-ray analysis.[45] Calculated +1.298(4) Å]. The P–N bonds are slightly longer
bond lengths are generally slightly longer than the mea- [1.651(1) Å–1.668(1) Å] than in complex 3. The observed
sured ones, nevertheless the experimental geometry is nicely Cu–I distance [2.5476(2) Å] is in the normal range for this
reproduced by the calculations. Differences in the natural class of compounds. On the basis of the structure analysis,
charges of M1 with the well-known M2 should reveal the we conclude that the nitrogen atoms do not contribute to
effect of the electron-withdrawing group of the phosphane the stabilization of the copper atom.
ligand. An NBO charge analysis on the C6F5-substituted
Interestingly, compound 4 is completely air-stable and
phosphane ligand in M1 results in a net charge of 1.46/ shows no formation of CuII species over months under am-
1.48 a.u. on P and 0.39 a.u. on Pd. In complex M2 a quite bient conditions. Its stability is, however, limited to non-
similar charge for Pd (0.40 a.u.) is found but a less positive acidic media owing to the lability of the P–N bonds toward
P-center (1.09 a.u.), which is in line with the electron-with- acid-mediated bond cleavage. The EI mass spectra of com-
drawing properties of the C6F5 group of ligand 1. The natu- plex 4 exclusively show the ligand and its fragments,
ral charge of the chlorine atoms seems to be unaffected by whereas in the ESI spectra also some fragments (L+ – NEt2)
the phosphane ligands (–0.55 and –0.56 for M1 and M2, and adducts of the complex (4+ – I and 4+ + Cu) could be
respectively). A comparison of the absolute gas phase ener- observed.
gies of cis-/trans-M1 shows that the trans isomer is just
In order to investigate the redox properties of this com-
15.3 kcalmol–1 lower in energy (ΔG298). Thus, formation of plex we performed cyclic voltammetry (CV) measurements
the cis isomer may occur easily with suitable reaction part- and compared the results of 4 with those obtained for the
ners. Owing to the distorted geometry in trans-M1 and 3, free ligand (Figure 3). Ligand 1 exhibits a single oxidation
the isomerization barrier is expected to be rather low. From wave at +0.83 V, whereas a single irreversible oxidation
our calculations the experimentally observed bending of the wave at +0.30 V dominates the cyclovoltammogram of
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complex 4. The irreversible nature of the latter process 4a both C6F5 groups of the ligands are in trans orientation
again suggests a ligand oxidation, however now at lower to the iodide [Cu–I–P–C –172.02(5)° and –177.59(5)°] in
potential owing to the vicinity of the metal cation. Further compound 4b (crystallization from MeCN) only one of the
oxidation processes occur at potentials of +1.22 V and C6F5 rings adopts this trans orientation [Cu–I–P–C
+1.57 V, which give a quasireversible single reduction wave 175.00(4)°], and the other one is in a cis orientation [Cu–I–
at +1.05 V. The reversible nature of this process in combina- P–C –16.67(4)°]. The P–Cu–P angle is also slightly larger
tion with the potential range suggests that the couple CuI/ than in 4a, but the sum of the angles around the Cu atom
CuII may be involved probably bonded to a second ligand is for all complexes ca. 360° [4a 359.98°, 4b 360.00°, 4c
unit. In essence these data suggest that 1 and 4 should be 359.77°]. The Cu–I bond is also slightly longer in 4b. The
stable against oxidation in air in the absence of water (Fig- structure obtained by recrystallization from THF (4c)
ure 4).
shows very similar structural parameters. Details of the
structure solution for 4a,b,c are given in the Supporting In-
formation. Phosphane ligand 1 entails a high spatial flexi-
bility in these copper complexes, which therefore should be
predestined to adapt to a large variety of substrates in cata-
lytic reactions.
Catalytic Studies
In order to test the catalytic scope of our push–pull phos-
phane complexes, we carried out two types of reactions. The
catalytic performance of Pd complex 3 was studied with a
Suzuki–Miyaura-type
C–C
cross-coupling
reaction
(Scheme 4) and compared to the activity of PdCl2(PPh3)2.
Reactions were carried out under ambient conditions in 1,4-
dioxane or toluene. The reaction conditions and achieved
conversions are summarized in the Supporting Information.
Neither solvents nor other reaction conditions (additional
base, salts) were optimized for these catalytic studies. The
stability of the complex against oxidation to PV under am-
bient conditions is sufficient, and short exposure of 3 to
air does not reduce its reactivity. At elevated temperatures
(110 °C) under microwave heating palladium catalyst 3
shows an activity comparable to PdCl2(PPh3)2 and reac-
tions without addition of ligand to PdCl2. Conducting these
cross-couplings at ambient temperature shows superior
conversion rates using 3 (i.e., time 48 h/74% conv.) com-
pared to the ones of the control experiments [PdCl2 (48 h/
34% conv.) and (PPh3)2PdCl2 (48 h/43% conv.)]. The higher
activity of 3 compared with PdCl2 and (PPh3)2PdCl2 is
probably a result of the structural distortion discussed be-
fore associated with longer and consequently weaker and
more reactive Pd–P and Pd–Cl bonds.
Figure 3. ORTEP drawing of complex 4a at a probability level of
50%. Hydrogen atoms are omitted for clarity. Selected distances
[Å] and angles [°]: I1–Cu1 2.5476(2), Cu1–P1 2.2373(4), Cu1–P
22.2609(4); P1–Cu1–P2 129.60(1), P1–Cu1–I1 116.77(1), P2–Cu1–
I1 113.62(1), I1–Cu1–P1–C1 –172.02(5), I1–Cu1–P2–C21
–177.59(5).
As outlined before, CuI species are often used as catalysts
to perform CuAAC “click” reactions. Suitable catalysts are
(amine-)stabilized CuI halides[46] or can be generated in situ
from CuII precursors and a reducing agent, that is CuSO4
and sodium ascorbate,[47] or by synproportionation of Cu0
Figure 4. Cyclovoltammograms of 1 (dashed) and 4 (solid) in
–
CH3CN (0.06 m Bu4N+BF4 ) vs. Ag0/Ag+ (referenced externally to
FcH/FcH+ at 0.96 mV) at a scan rate of 100 mVs–1.
Since CuAAC (“click”) cycloaddition reactions are often and CuII. Elemental copper exhibits much better perform-
carried out in donating/coordinating solvents, we wanted to ance upon pretreatment with H2O2, suggesting Cu2O as the
explore the behavior of the complex towards solvents hav- active species.[48] Thus, we wanted to explore a homogen-
ing oxygen (THF) and nitrogen (MeCN) donor sites. In eous 1,3-dipolar cycloaddition reaction with 4 as catalyst.
solution no significant differences in the 31P NMR spectra Initial studies in a microwave reactor on a model reaction
can be observed. We were able to grow single crystals from (Scheme 4, II) demonstrated that 4 nicely triggers the
both solvents. Although the copper atom still has a trigo- CuAAC process (Table 1), whereas conversions without li-
nal-planar coordination environment, the two phosphorus gand (5 mol-% CuI) are below the detection limits (En-
ligands show significantly different orientations. Whereas in tries 1, 3, 5, 7). The catalytically active species may be
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Push–Pull Phosphanes
Scheme 4. Reactions for catalytic studies. I: [Pd] 1.0 mol-% 3, K2CO3, 1,4-dioxane, room temp. II [Cu] 2.5 mol-%, 1,4-dioxane, ΔT.
formed in situ from phosphane 1 with CuI showing excel- at higher temperatures, we explored the possibility to adapt
lent conversions (Entries 2, 4, 6, 8). Full conversion is this catalyst system for continuous-flow conditions. Pre-
reached at 60 °C after 90 min (Entry 6). Catalyst 4 or in situ vious studies on CuAAC chemistry performed in flow ex-
generated catalyst give comparable results for conventional clusively employed immobilized CuI species.[48,50] The reac-
heating at 65 °C (Entries 9, 10). Full conversion at higher tion was carried out by using a high-temperature capillary
temperatures is already reached after 15 min (entry 11). Pre- reactor (X-Cube Flash, ThalesNano).[51] Best results were
liminary attempts to detect adducts of the copper catalyst obtained at 130 °C and 4 mLmin–1 flow rate, in which case
4 and either the alkyne or the azide by means of NMR a conversion of 80% and a similar isolated yield was ob-
spectroscopy have been unsuccessful. In the absence of az- tained (75%). Hence, CuI complex 4 seems to be an excel-
ide, Glaser-type coupling reactions have been observed at lent catalyst for CuAAC reactions over a wide range of tem-
ambient conditions with formation of the corresponding bis- peratures suitable for continuous-flow applications in high-
(alkyne), which could be proven by X-ray analysis. This throughput or combinatorial chemistry. As an additional
suggests a competitive reactivity between Glaser-type and advantage, simple removal of the catalyst can be envisaged
“click” reactions, which seems much faster for the latter, as by acidic cleavage of the amino groups and subsequent
evidenced by the complete lack of homocoupling products aqueous extraction,[48] or by fluorous extractions.[52]
in the presence of azide.
The good activity of the complex is mainly attributed to
the excellent stabilization of the CuI species. This is also
underlined by the fact that solid complex 4 can be stored
at ambient conditions in air over months, and neither de-
composition nor disproportionation reactions are observed.
Table 1. CuAAC of benzyl azide and phenyl acetylene. Reaction
conditions: benzyl azide (0.5 mmol), phenyl acetylene (1.1 mmol);
o.b.: conventional oil-bath heating; MW: microwave heating.
Entry
Additive
Temp.
[°C]
Time
[min]
Conv.
[%][a]
Yield
[%][b]
1[c]
2[c]
3[c]
4[c]
5[c]
6[c]
7[c]
8[c]
9[d]
10[e]
11[e]
MW
MW
MW
MW
MW
MW
MW
MW
o.b.
–
1
–
1
–
1
–
1
4
1
4
60
60
60
60
60
60
60
60
65
65
100
5
5
Ͻ1
21
Ͻ1
70
Ͻ1
98
Ͻ1
Ͼ99
–
Ͻ1
16
Conclusions
45
45
90
90
120
120
90
90
15
Ͻ1
76[f]
Ͻ1
Ͼ99[f]
Ͻ1
Ͼ99
98
We have demonstrated the cost-efficient synthesis of the
pentafluorophenyl-substituted aminophosphane ligand 1,
which is available for reactions at the phosphorus lone pair.
Besides the selenation also the coordination of 1 towards
PdII and CuI centers has been accomplished, which is in
marked contrast to its isopropyl analogue as described in
the literature. On the basis of the distorted geometry of the
PdII complex, which was confirmed by X-ray and theoreti-
cal studies, we concluded good reactivity in C–C coupling
reactions. This was confirmed for Suzuki–Miyaura-type
cross-coupling reactions at ambient conditions. Therefore,
this catalyst is a promising candidate for C–C coupling re-
actions of temperature-sensitive compounds. The analogous
o.b.
o.b.
–
–
97
98
[a] Conversions determined by a calibrated HPLC-UV analysis. [b]
Isolated yields with NMR purity Ͼ95%. [c] Carried out in dioxane
(2 mL), MW, 60 °C; compound 1 (5 mol-%) and CuI (5.0 mol-%)
were added. [d] Carried out in dioxane (2 mL), o.b., 2.5 mol-% 4
was added. [e] Carried out in dioxane (2 mL), o.b., compound 1
(5 mol-%) and CuI (5.0 mol-%) were added. [f] Contains impurities
of 1.
Owing to the stability of the catalyst at temperatures up CuI complex shows a trigonal-planar conformation for the
to 100 °C, we envisaged to adapt the CuAAC process central atom, whereas orientation of the ligands strongly
(Scheme 4, II) for continuous-flow conditions. A particu- depends on the coordination properties of the solvent. The
larly attractive feature of the continuous-flow technology is catalytically active species was either added directly or pre-
the ease of scaling reaction conditions – without the need pared in situ showing equally good performance in CuAAC
for reoptimization – through the operation of multiple sys- reactions, conducted under batch or continuous-flow condi-
tems in parallel (numbering-up, scaling-out), thereby tions. Further studies must be carried out to investigate the
achieving production-scale capabilities.[49] Owing to the scope and the limitations on substrates and reaction condi-
good performance of Cu complex 4 for this model reaction tions of this push–pull phosphane ligand system.
Eur. J. Inorg. Chem. 2011, 2588–2596
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2593

R. Pietschnig et al.
FULL PAPER
3
NMR (CDCl3, 400 MHz, 298 K): δ = 1.18 (t, JHH = 7.1 Hz, 12
Experimental Section
H), 3.28 (m, 8 H). 13C NMR (CDCl3, 75.5 MHz, 298 K): δ = 14.08
3
2
1
General: NMR spectroscopic data were collected either with a Var-
ian INOVA 400, or a Bruker Avance 300 operating at a proton
frequency of 400 or 300 MHz, respectively; 1H and 13C NMR spec-
tra were referenced internally to residual solvent signals; 31P and
19F NMR spectra were referenced externally to H3PO4 (85%) and
C6H5CF3 (δ = –63.7 ppm), respectively. Compound 1 was prepared
according to a modified literature procedure.[6,25] All reactions were
carried out under an inert gas by using modified Schlenk tech-
niques or a glove box. Solvents were dried with Al2O3 columns
by the PureSolv solvent purification system. All chemicals were
purchased from Aldrich or ABCR (C6F5H) and used without fur-
ther purification. Owing to the evolution of HF, the determination
of conventional combustion analyses was not possible with the
equipment available to us. Therefore, the purity of products was
determined with NMR, HPLC, or GC, and relevant spectra are
depicted in the Supporting Information.
(d, JPC = 2.7 Hz), 44.69 (d, JPC = 5.1 Hz), 113.32 (dt, JPC
=
3
1
91.6 Hz, JCF = 18.4 Hz), 137.84 (dm, JCF = 252.7 Hz), 142.85
(dm, JCF = 254.1 Hz), 146.09 (dm, JCF = 251.3 Hz) ppm. 19F
NMR (C6D6, 282.4 MHz, 298 K): δ = –133.72 (m, o-F), –150.52
(m, p-F), –160.65 (m, m-F) ppm. 31P NMR (CDCl3, 145.1 MHz,
1
1
298 K): δ = 48.4 (s, JPSe = 800 Hz) ppm. 77Se NMR (C6D6,
1
1
57.2 MHz, 298 K): δ = –136.9 (d, JPSe = 800 Hz) ppm.
Preparation of trans-Dichloridobis[bis(diethylamino)(pentafluoro-
phenyl)phosphane]palladium(II), trans-Pd[P(C6F5)(NEt2)2]2Cl2 (3):
A mixture of PdCl2 (29 mg, 0.16 mmol) and acetonitrile (MeCN,
5 mL) was refluxed, until the brown residue was completely dis-
solved and the (MeCN)2PdCl2 complex formed. The solution was
allowed to reach room temperature and subsequently added to a
solution of 1 (112 mg, 0.33 mmol) in acetonitrile (3 mL); the mix-
ture was stirred at room temperature for 2 h. The solvent was re-
moved and the residue extracted with dichloromethane to yield 3
as an oily material. Recrystallization from cold MeCN (–28 °C)
yielded 95% (134 mg, 0.16 mmol) spectroscopically pure 3. 1H
Electrochemical Studies: CV spectra were recorded with a Gamry
Reference 600 device in CH3CN solution (Bu4NBF4 0.06 m) of the
respective substance under argon. A standard three-electrode sys-
tem [Ag+/Ag reference electrode, Pt auxiliary wire, Pt working elec-
trode (diameter 3 mm)].
3
NMR (C6D6, 300 MHz, 298 K): δ = 1.04 (t, JHH = 7.0 Hz, 24 H),
3.19 (m, 8 H), 3.30 (m, 8 H) ppm. 13C NMR (C6D6, 75.5 MHz,
3
298 K): δ = 14.11 (br. s, CH3), 43.23 (br. s, CH2), 111.13 (tm, JCF
1
1
= 21.1 Hz), 137.54 (dm, JCF = 253.8 Hz), 142.22 (dm, JCF
=
MS Measurements: DI-EI measurements were performed with an
Agilent 5975C quadrupole mass spectrometer equipped with a di-
rect injection unit. HRMS measurements: Samples were dissolved
in CH3CN and directly infused into the ESI ion source of a Synapt
HDMS Q-TOF mass spectrometer (Waters, Manchester, UK).
Samples were analyzed in positive-ion mode with a capillary volt-
age of 2.50 kV and a source temperature of 100 °C. The [MH]+
peak at m/z = 556.2771 of Leu-Enk (Sigma, 100 pgμL–1 in H2O/
ACN, 1:1, v/v, 0.1% HCOOH) was used as the reference signal. A
table containing selected HRMS data is given in the Supporting
Information.
242.5 Hz) 146.80 (dm, JCF = 249.8 Hz) ppm. 19F NMR (C6D6,
282.4 MHz, 298 K): δ = –132.28 (br. d, J = 20.5 Hz), –151.46 (br.
tt, J = 21.9, 4.0 Hz), –162.43 (br. m) ppm. 31P NMR (C6D6,
121.5 MHz, 298 K): δ = 78.4 (br. s) ppm.
1
Preparation of Bis[bis(diethylamino)(perfluorophenyl)phosphane]-
iodidocopper(I), Cu[P(C6F5)(NEt2)2]2I (4): To
1
(349 mg,
0.70 mmol) in CDCl3 (700 μL) was added CuI (68 mg, 0.36 mmol)
and the mixture stirred for 30 min. After filtration and removal of
the solvent, pure product was obtained in quantitative yield (99%,
1
3
302 mg). H NMR (CDCl3, 300 MHz, 298 K): δ = 1.04 (t, JHH
=
7.0 Hz, 24 H, CH3), 3.10 (m, 8 H, CH2), 3.20 (m, 8 H, CH2) ppm.
Preparation of Bis(diethylamino)(pentafluorophenyl)phosphane (1):
CAUTION! The lithio intermediate in the preparation of compound
1 is highly reactive and prone to LiF elimination at temperatures
above –50 °C under the formation of very reactive aryne species! To
a cooled (–50 °C) solution of C6F5H (7.58 g, 45.0 mmol) in THF
(120 mL) was added nBuLi (28.3 mL, 45.3 mmol) over a period of
15 min, and stirring was continued at this temperature for 1 h. The
suspension was cooled to –80 °C, and ClP(NEt2)2 (9.50 g,
45.1 mmol) was added slowly. The mixture was allowed to reach
room temp. and stirred overnight. After removal of the solvent, the
residue was extracted with pentane yielding crude 1. Distillation
under reduced pressure (89 °C, 0.026 mbar) yielded 80% pure prod-
13C (100 MHz, CDCl3, 298 K): δ = 14.23 (s,CH3), 43.30 (br. s,
1
CH2), 113.88 (br. s), 137.32 (dm, JCF = 194.1 Hz, m-Aryl), 141.14
1
(dm,1JCF = 255.3 Hz, p-Aryl), 145.62 (dm, JCF = 246.0 Hz, o-
Aryl) ppm. 19F NMR (CDCl3, 282.4 MHz, 298 K): δ = –135.01
(m), –152.70 (m), –161.28 (m) ppm. 31P NMR (C6D6, 161.9,
298 K): δ = 74.0 ppm.
Catalytic Studies
Example Procedure for Suzuki–Miyaura-Type Cross Coupling:
Scheme 4. 4-Iodobenzonitrile (229 mg, 1 mmol), phenylboronic
acid (145 mg, 1.2 mmol), potassium carbonate (207 mg, 1.5 mmol),
and compound 2 (8.5 mg, 0.01 mmol) were weighed into a 10-mL
Biotage microwave vial. Dry toluene (2.0 mL) was added, and the
vial was capped with a Teflon-coated rubber septum and crimped.
The reaction mixture was stirred for the time indicated in the Sup-
porting Information, and samples (10 μL) were taken by syringe
and cannula, diluted in MeOH (1 mL), filtered through a syringe
filter, and subjected to GC-FID analysis. Further details (condi-
tions and yields) are provided in the Supporting Information.
1
uct (12.30 g, 35.9 mmol). H NMR (C6D6, 300 MHz, 298 K): δ =
3
3
4
0.97 (t, JHH = 7.1 Hz, 12 H), 2.93 (tt, JHH = 7.1 Hz, JHF
=
10.4 Hz) ppm. 13C NMR (C6D6, 75.5 MHz, 298 K): δ = 14.94 (d,
3JPC = 3.5 Hz), 44.69 (d, JPC = 19.2 Hz), 117.11 (dt, JPC
=
2
1
3
4
5
52.6 Hz, JCF = 23.4 Hz; pseudo q, JFC = JCF = 3.7 Hz), 137.63
1
1
(dm, JCF = 232.8 Hz), 140.95 (dm, JCF = 229.5 Hz), 146.21 (dm,
1JCF = 243.8 Hz) ppm. 19F NMR (C6D6, 282.4 MHz, 298 K): δ
= –136.96, –156.48, –163.02 ppm. 31P NMR (C6D6, 121.5 MHz,
298 K): δ = 82.0 ppm.
Isolation of 4-Cyanobiphenyl: The reaction mixture was stirred for
168 h achieving a GC-FID conversion of 90%. The reaction mix-
ture was filtered through a syringe filter, the solvent was removed
under reduced pressure, the residue was dissolved in dichlorometh-
Preparation of Bis(diethylamino)(pentafluorophenyl)(seleno)phos-
phorane (2): To a solution of 1 (920 mg, 2.7 mmol) in chloroform
(2 mL) a slight excess of gray selenium (255 mg, 3.2 mmol, ane (1.5 mL) and adsorbed on a silica samplet of the Biotage SP1
1.2 equiv.) was added. The suspension was stirred at ambient tem-
perature for 15 min. After filtration and removal of all volatiles,
pure product was obtained in almost quantitative yield as a color-
less solid (92%, 1.04 g, 2.5 mmol). M.p. 56–58 °C (CHCl3). 1H
system. After drying the samplet at 50 °C for 2 h, automated
chromatography using a petroleum ether/EtOAc gradient (0–40%)
gave 4-cyanobiphenyl as a colorless solid (149 mg, 83%) with the
following physical properties identical to a reference sample: M.p.
2594
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2588–2596

Push–Pull Phosphanes
1
84 °C (ref.[54] 84–85 °C). H NMR (300 MHz, CDCl3, 298 K): δ = sure to yield 1-benzyl-4-phenyl-1,2,3-triazole with slight ligand
7.76–7.68 (m, 4 H), 7.62–7.60 (m, 2 H), 7.53–7.44 (m, 3 H) ppm. contaminations (122 mg, 2.5% ligand contamination purity was
13C NMR (75 MHz, CDCl3, 298 K): δ = 145.7, 139.2, 132.6, 129.1, checked by NMR, 97.5% pure product according to NMR analysis
128.7, 127.8, 127.3, 119.0, 110.9 ppm.
with identical physical properties as reported above).
General Procedure for the Copper-Catalyzed Cycloaddition (MW
General Procedure for the Copper-Catalyzed Cycloaddition Under
Conditions): Scheme 4. Compound 3 (11 mg, 0.013 mmol) was Flow Conditions: An X-Cube flash flow reactor, equipped with a
weighed into
a
10-mL Biotage microwave vial. 1,4-Dioxane
16 mL coil, was adjusted to the following settings: T = 130 °C,
4.0 mLmin–1 flow, p = 80 bar. Compound 3 (11 mg, 0.013 mmol),
benzyl azide (64 μL, 68 mg, 0.5 mmol), phenyl acetylene (60 μL,
56 mg, 0.55 mmol), and 1,4-dioxane (2.0 mL) were combined in a
5-mL vial. As the reactor reached the reaction conditions, the mix-
ture was introduced into the flow system. After the introduction,
the pumps were switched back to the solvent reservoir, and 64 mL
of solvent was collected in a 100-mL round-bottomed flask. The
experiment was stopped, the solvent was removed under reduced
pressure, and a sample of the remaining solid was subjected to
NMR analysis to determine the conversion (80%). The residual
reaction mixture was taken up in acetone (2 mL), the product was
precipitated by the addition of water (6 mL), and the mixture was
filtered through a small frit (D3) to give 1-benzyl-4-phenyl-1,2,3-
triazole (9) as a pale yellow solid (89 mg, 75% isolated yield) with
(2.0 mL), benzyl azide (64 μL, 68 mg, 0.5 mmol), and phenyl
acetylene (60 μL, 56 mg, 0.55 mmol) were added, and the vial was
capped with a Teflon-coated rubber septum and crimped. The vial
was placed into a microwave reactor (Biotage Initiator 2.5), and
the reaction mixture was subjected to microwave irradiation by
using the following settings: T = 60 °C, t = 1.5 h, prestirring = 10 s,
sample absorption = normal. After the irradiation was finished, a
sample (50 μL) was collected and subjected to HPLC analysis to
determine the conversion. The remaining solution was concen-
trated under reduced pressure to yield 1-benzyl-4-phenyl-1,2,3-tri-
azole (9) with slight ligand contaminations [122 mg, 2.5% ligand
contamination (NMR impurity), 98%] with the following physical
properties: M.p. 132 °C (ref.[55] 132–133 °C). H NMR (300 MHz,
1
DMSO, 298 K): δ = 8.67 (s, 1 H), 7.86 (m, 2 H), 7.52 (m, 8 H),
5.65 (s, 2 H) ppm. 13C NMR (75 MHz, DMSO, 298 K): δ = 147.1, identical physical properties as reported above.
136.5, 131.1, 129.4, 129.3, 128.6, 128.4, 125.6, 122.0, 53.5 ppm.
Crystal Structure Determination: The crystal structures were deter-
General Procedure for the Copper-Catalyzed Cycloaddition (Batch
Conditions, Conventional Heating): Scheme 4. Compound 3 (11 mg,
0.013 mmol) was weighed into a 10-mL Biotage vial. 1,4-Dioxane
(2.0 mL), benzyl azide (64 μL, 68 mg, 0.5 mmol), and phenyl
acetylene (60 μL, 56 mg, 0.55 mmol) were added, and the vial was
capped with a Teflon-coated rubber septum and crimped. The solu-
tion was stirred for the indicated time at the respective temperature
(see Table 1). The solution was concentrated under reduced pres-
mined with a Bruker APEX-II CCD diffractometer by using graph-
ite-monochromatized Mo-Kα radiation (0.71073 Å). Structures
were solved by direct methods and refined by using the SHELXL
and SHELXS suite of programs.[53] Further details are given in the
Supporting Information. Structural and refinement data for 2, 3,
4a, 4b, and 4c are given in Table 2. CCDC-795562 (for 2), and
-771838 (for 3), -771839 (for 4a), -771840 (for 4b), and -771841 (for
4c) contain the supplementary crystallographic data for this paper.
Table 2. Summary of structural and refinement data of 2, 3, 4a, 4b, and 4c.
4a
C14H20F5N2PSe C28H40Cl2F10N4P2Pd C28H40CuF10IN4P2 C28H40CuF10IN4P2 2·(C28H40CuF10IN4P2)·C4H8O
2
3
4b
4c
Empirical formula
Formula mass
Crystal description
Crystal size [mm]
Crystal system
Space group
Unit cell dimensions
a [Å]
421.25
needle, colorless
0.62ϫ0.10ϫ0.09 0.298ϫ0.192ϫ0.068 0.34ϫ0.12 ϫ0.10
orthorhombic
861.88
plate, orange
875.02
needle, colorless
875.02
1822.14
plate, colorless
0.35ϫ0.30ϫ0.16
monoclinic
Ia
plate, colorless
0.38ϫ0.34ϫ0.13
monoclinic
P2/c
monoclinic
I2/a
monoclinic
P21/c
Pca21
17.5973(5)
7.0871(2)
27.5772(7)
90
3439.26(16)
8
1.627
1696
2.324
24.1455(10)
9.0003(4)
48.655(2)
92.825(2)
10560.8(8)
12
1.626
5232
0.850
16.8291(7)
11.6826(5)
18.7734(7)
104.779(2)
3568.9(3)
4
1.629
1752
1.644
18.8155(11)
10.0910(4)
20.5851(8)
115.0890(10)
3539.7(3)
4
1.642
1752
1.658
25.4888(9)
18.3825(6)
16.3908(6)
100.3930(10)
7553.9(5)
4
1.602
3664
1.558
b [Å]
c [Å]
β [°]
Volume [Å3]
Z
Calculated density [Mgm–3
F(000)
]
μ [mm–1]
Absorption correction
Temperature [K]
Θ range for data collection [°]
Index ranges
semiempirical from equivalents
100
100
100
100
100
2.31–26.00
–21 Յ h Յ 21
–8 Յ k Յ 8
–33 Յ l Յ 34
46642/6691
6392
1.85–30.00
–33 Յ h Յ 32
–12 Յ k Յ 12
–68 Յ l Յ 67
113786/15338
12368
2.07–30.00
23 Յ h Յ 23
–16 Յ k Յ 16
–26 Յ l Յ 26
105420/10398
9069
0.0350, 0.0185
10398/480/0
1.038
2.18–35.00
–30 Յ h Յ 30
–16 Յ k Յ 16
–33 Յ l Յ 33
31479/13206
12829
0.0199, 0.0335
13206/520/16
1.007
1.96–30.00
–35 Յ h Յ 35
–24 Յ k Յ 25
–21 Յ l Յ 23
135615/21718
18627
0.0279, 0.0198
21718/1122/368
1.273
Reflections collected/unique
Reflections observed [I Ͼ 2σ(I)]
R(int), R(sigma)
Data/restraints/parameters
Goodness-of-fit on F2
R1, wR2 [I Ͼ 2σ(I)]
0.0410, 0.0316
6691/428/14
1.187
0.0403, 0.0284
15338/737/36
1.034
0.0559, 0.1378
0.0588, 0.1390
0.0344, 0.0845
0.0490, 0.0935
1.419/–1.089
0.0208, 0.0496
0.0277, 0.0530
0.805/–0.549
0.0162, 0.0382
0.0170, 0.0385
0.725/–0.390
0.0357, 0.0720
0.0447, 0.0753
1.047/–1.534
R1, wR2 (all data)
Largest difference peak/hole [eÅ–3] 2.330/–0.998
Eur. J. Inorg. Chem. 2011, 2588–2596
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2595

R. Pietschnig et al.
FULL PAPER
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Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
Financial support by the Austrian Science Fund (FWF) (Grants
P18591-B03 and P20575-N19) and the EU-COST Action (CM0802
“PhoSciNet”) is gratefully acknowledged. This work was supported
in part by the Christian Doppler Research Society (CDG). A. O.
and R. P. would like to thank Dr. Manuel Volpe for helpful dis-
cussions and Dr. Jörg H. Albering for aquiring the diffraction data
of compound 2.
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Received: February 9, 2011
Published Online: April 27, 2011
2596
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