Imidazole phosphines: Synthesis, reaction chemistry, and their use in Suzuki C,C cross-coupling reactions

Article abstract of DOI:10.1021/om300145d

A straightforward consecutive synthesis methodology for the preparation of phosphino imidazoles 1-(4-PR₂-C₆H₄)-4,5-Me ₂-1H-C₃HN₂ (4a, R = C₆H₅; 4b, R = ^cC

Full text of DOI:10.1021/om300145d

Article
pubs.acs.org/Organometallics
Imidazole Phosphines: Synthesis, Reaction Chemistry, and Their Use
in Suzuki C,C Cross-Coupling Reactions
Bianca Milde, Rico Packheiser, Stefanie Hildebrandt, Dieter Schaarschmidt, Tobias Ruffer,
̈
and Heinrich Lang*
Department of Inorganic Chemistry, Institute of Chemistry, Faculty of Sciences, Chemnitz University of Technology, Straße der
Nationen 62, 09111 Chemnitz, Germany
S
* Supporting Information
ABSTRACT: A straightforward consecutive synthesis meth-
odology for the preparation of phosphino imidazoles 1-(4-PR2-
c
C6H4)-4,5-Me2-1H-C3HN2 (4a, R = C6H5; 4b, R = C6H11)
and 1-(4-PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2 (R = C6H5: 6a,
R′ = C6H5; 6b, R′ = cC6H11; 6c, R′ = cC4H3O; R = cC6H11: 6d,
c
c
R′ = C6H5; 6e, R′ = C6H11; 6f, R′ = C4H3O) is presented.
Phosphino imidazoles 6a−f were reacted with [PdCl2(SEt2)2]
(7), giving [Pd(1-(4-PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2)-
Cl2]2. Single crystals of [Pd(1-(4-P(C6H5)2-C6H4)-2-P-
(cC4H3O)2-4,5-Me2-1H-C3N2)Cl2]2 (8) suitable for single-
crystal X-ray structure analysis could be obtained by using the
synthesis-cum-diffusion strategy, confirming the formation of a
neutral 18-membered Pd2P4 cycle with two trans-configurated palladium centers. A Pt4P4 cyclic compound, accessible by
molecular recognition, was obtained via treatment of [Pt(dppf)(CC-C6H4-4-P(C6H5)2)2] (dppf = 1,1′-bis-
(diphenylphosphino)ferrocene) (11) with [PtCl2(SEt2)2] (12). The structure of 13 in the solid state was confirmed by
crystal structure determination, proving the formation of a neutral molecular square composed of Pt(dppf) and PtCl2 corner
units and 4-(C6H5)2P-C6H4-CC linkers. In addition, compounds 6a−f were applied in the palladium-promoted Suzuki cross-
coupling of 2-bromotoluene with phenylboronic acid using potassium carbonate as base. All in situ generated phosphino
imidazole palladium species showed high catalytic activity at which the diphosphino systems featuring phenyl and cyclohexyl
groups achieved the best results. Additionally, phosphine 6d was applied in the coupling of 4-chlorotoluene with phenylboronic
acid and in the synthesis of sterically hindered biaryls under mild reaction conditions, showing an excellent performance. In
comparison with other catalytically active species, equal or higher productivities were obtained using lower catalyst loadings and
lower temperatures.
successfully synthesized examples.6f,g Exo-bidentate metal−
INTRODUCTION
■
organic units including nitrogen donors and mainly palladium
and platinum metals have been employed as edges, while as
linking components generally organic units are applied.6
Examples of metallo-supramolecular squares are [{Pt(μ-4,4′-
Phosphino imidazoles represent an interesting family of
molecules due to their ambivalent donor character (P,N),
allowing their coordination to hard or soft transition metal
fragments.1 They can be used, for example, as easily tunable
ligands with good performance in palladium-catalyzed C,C
couplings,1a,d,2 as structural elements of ionic liquids in two-
phase catalytic reactions, allowing facile recycling,1b,d,e,2a,c,3 or
as model system for mimicking the catalytically active site in
bioinorganics.1c,4 In contrast to monophosphino imidazoles,
diphosphino imidazoles were investigated in combination with
chirality.2d,5
Discrete molecular architectures can be designed and
synthesized by molecular recognition using, for example,
organic and metal−organic or organometallic compounds as
building blocks containing the structural information to
construct the appropriate self-assembled 2D or 3D architec-
tures.6 Such structures include for instance molecular rods,
squares, hexagons, and boxes of which the appropriate squares
featuring transition metal groups were among the first
6f
7
bpy)(dppp)}4]8+ or [{Pt-μ-CC-CC)(dcpe)}4]4+ (dppp
= bis(diphenylphosphino)propane, dcpe = bis(dicyclohexyl-
phoshino)ethane). Applications of such metallamacrocycles
include homogeneous catalysis8 and host−guest chemistry.6i
This prompted us to combine the two topics phosphino
imidazoles and metallamacrocycles. In addition, the use of the
appropriate phosphines in Suzuki C,C cross-coupling reactions
is reported.
RESULTS AND DISCUSSION
Synthesis and Characterization of Phosphino Imida-
zoles and Metallamacrocycles. Diphosphino imidazoles 1-
■
Received: February 22, 2012
Published: May 2, 2012
© 2012 American Chemical Society
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(4-PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2 (R = C6H5: 6a, R′ =
2.6 ppm are characteristic, whereas the signal of the PR′2 unit
always appears at higher field. Nevertheless, the phosphino
groups do not show phosphorus−phosphorus couplings.
Phosphino imidazoles 6a−f can be applied in the Suzuki C,C
cross-coupling of 2-bromotoluene with phenylboronic acid
(vide infra) as well as in the synthesis of palladamacrocycles
(reaction 1). Molecules 6a−f react quantitatively and rapidly
c
c
C6H5; 6b, R′ = C6H11; 6c, R′ = C4H3O; R = cC6H11; 6d, R′ =
c
C6H5; 6e, R′ = cC6H11; 6f, R′ = C4H3O) were accessible in a
consecutive reaction sequence as shown in Scheme 1. Molecule
2 is remarkably sensitivity toward light, and hence rapid workup
is highly recommended.
a
Scheme 1. Synthesis of Diphosphino Imidazoles 6a−f
with [PdCl2(SEt2)2] (7) at ambient temperature to give air-
and moisture-stable yellow complexes of type [Pd(1-(4-PR2-
C6H4)-2-PR′2-4,5-Me2-1H-C3N2)Cl2]2. Within this reaction,
even using different reaction conditions, always nonsoluble
materials were obtained and hence characterization is limited.
However, with 6c as phosphino source single crystals of 8
suitable for single X-ray structure studies could be obtained,
which allowed determining the structure of this complex in the
solid state (Figure 1).
The very poor solubility of 8 even in common polar organic
solvents precluded any reasonable NMR data. However,
suitable crystals for single X-ray structure analysis were
obtained by the one-step synthesis-cum-diffusion strategy at
ambient temperature so that crystals could be formed in a slow
reaction (vide supra). These crystals were subjected to single-
crystal X-ray structure analysis. The molecular structure of 8 is
shown in Figure 1. Important bond lengths, bond angles, and
torsion angles are summarized in the caption of Figure 1. For
crystal and structure refinement data see the Experimental
Section.
Cyclic 8 crystallizes in the monoclinic space group P21/c. In
its solid state it forms a dimeric trans-palladium complex with
two bidentate imidazole ligands bridging two PdCl2 units,
forming an 18-membered metallacycle. Compound 8 possesses
a crystallographically imposed inversion symmetry with the
inversion center in the middle of Pd1 and Pd1A (Figure 1).
The coordination at Pd is distorted square-planar. The
phenylene groups are oriented parallel to each other with an
interplanar distance of ca. 6.2 Å. As expected, the imidazole unit
shows planarity (rms deviation 0.0094 Å), and the phenylene
moiety is rotated by 70.3(5)° out of the plane of the imidazole
entity. Therefore, the molecule is not planar but exhibits a
structure similar to cyclophanes.10a The Pd···Pd distance is
6.519 Å, alluding that no direct Pd−Pd contact exists. Similar
distances have been observed for [Pd(2,5-(P(C6H5)2)2-
C4H2S)Cl2]2.10b All other bond lengths and angles correspond
to related molecules, i.e., [Pd(1,3-i-Pr2-2-PPh2-4,5-Me2-C3N2)-
Cl3]11 and [Pd(2,5-(PPh2)2-C4H2S)Cl2]2.10b
a
LDA = LiN(iC3H7)2.
Palladium-promoted Stelzer coupling9 of 1-(4-I-C6H4)-4,5-
Me2-1H-cC3HN2 (2) with PR2H(BH3) (3a, R = C6H5; 3b, R =
cC6H11) gave 1-(4-PR2-C6H4)-4,5-Me2-1H-cC3HN2 (4a, R =
C6H5; 4b, R = cC6H11), which on deprotonation with
LiN(iC3H7)2 followed by subsequent treatment with PR′2Cl
(5a, R = C6H5; 5b, R = C6H11; 5c, R = C4H3O) produced
colorless diphosphines 6a−f (Table 1, Experimental Section).
Noteworthy is the high reactivity of phosphines 4 and 6 toward
oxygen. They are rapidly oxidized to the respective phosphine
oxides when exposed to air, solutions more quickly than solid
materials. This also is reflected by the somewhat low yields
characteristic for 4 and 6 (Table 1, Experimental Section).
Phosphines 4 and 6 were characterized by elemental analysis,
IR, and NMR (1H, 13C{1H}, 31P{1H}) spectroscopy and high-
resolution ESI TOF mass spectrometry.
c
c
The IR spectra of the mono- and diphosphino imidazoles are
less expressive, while NMR spectroscopy is more meaningful.
Very characteristic is the sp2-hybridized CH imidazole proton
signal at ca. 7.5 ppm for 4a and 4b, which upon substitution by
the PR′2 group (6a−f) disappears, allowing the monitoring of
the reaction progress. This also is possible using 13C{1H} NMR
spectroscopy because a shift of the imidazole carbon atom in
position 2 from 137 ppm in 4 (C−H) to 140−145 ppm in 6
1
(C−PR′2; JCP = 2−15 Hz) occurs (Experimental Section). A
further feature of 6a−f, as compared to 4, is the appearance of a
3
4
doublet of doublets (i.e., 6a: JCP = 3.1 Hz, JPC = 2.8 Hz) for
the meta-positioned phenylene C−H unit, due to its coupling
with both the PR2 and the PR′2 building blocks. In addition,
31P{1H} NMR spectroscopy is an efficient tool to describe the
introduction of a second phosphino moiety. While for 4 only
one resonance signal is observed at 2.7 (4a) and −5.9 (4b)
ppm, respectively, for 6a−f two resonances between −71.7 and
Table 1. Synthesis of Diphosphino Imidazoles 6a−f
a
a
compd
R
R′
yield/%
compd
R
R′
yield/%
6a
6b
6c
C6H5
C6H5
C6H5
C6H5
57
50
64
6d
6e
6f
cC6H11
cC6H11
cC6H11
C6H5
60
46
65
cC6H11
cC4H3O
cC6H11
cC4H3O
a
Based on 4a or 4b.
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Figure 1. ORTEP diagram (50% probability level) of the molecular structure of 8 crystallized from dichloromethane, with the atom-numbering
scheme. All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å), angles (deg), and torsion angles (deg): Pd1−P1 = 2.3127(12),
Pd1A−P2 = 2.3171(12), Pd1−Cl1 = 2.3008(11), Pd1−Cl2 = 2.3055(10), Pd1−P2A = 2.3171(12), C1−P1 = 1.805(4), C7−P1 = 1.814(4), C13−P1
= 1.823(4), C23−P2 = 1.796(4), C24−P2 = 1.780(5), C28−P2 = 1.799(4); Cl1−Pd1−Cl2 = 168.69(4), Cl1−Pd1−P1 = 90.80(4), Cl2−Pd1−P1 =
89.14(4), Cl1−Pd1−Cl2A = 93.61(4), Cl2, Pd1−P2A = 85.99(4), P1−Pd1−P2A = 174.79(4); C15−C16−N1−C23 = 70.3(5), C13−P1−Pd1−Cl1
= 45.28(16). (Symmetry generated atoms are indicated by the suffix A; symmetry code: −x+2, −y, −z+1.)
Supramolecular chemistry also allows the synthesis of a
platinum-molecular square, as given in the preparation of 13
(Scheme 2). Treatment of [Pt(dppf)Cl2] (dppf = 1,1′-
The organometallic bis(alkynyl) platinum complex 11 and
square 13 were characterized by elemental analysis, spectros-
1
copy (IR; H, 13C{1H}, 31P{1H} NMR), and single-crystal X-
ray structure analysis (Figure 2 (11), Figure 3 (13)).
Scheme 2. Synthesis of the Platinum-Molecular Square 13
from 9
Heterobimetallic 11 shows from the spectroscopic point of
view no peculiarities (Experimental Section). Nevertheless, the
4-(C6H5)2P-C6H4-CC groups are best suited to monitor the
progress of the reaction of 11 with 12 to form 13 since a shift
of the phosphorus signal from −6.6 ppm in 11 to 14.7 ppm in
13 (PtCl2 edge fragment) occurs in the 31P{1H} NMR
spectrum. Compounds 11 and 13 show a second phosphorus
signal at 13.6 ppm, which can be assigned to the Pt(dppf)
1
195Pt−31
P
building block with a characteristic J
coupling constant
of 2380 Hz. The phosphorus−platinum coupling constant
1J
= 3673 Hz (PtCl2) confirms the expected cis-
195Pt−31
P
1
arrangement at the platinum(II) center. The H NMR spectra
of 11 and 13 are easier to interpret due to their symmetrical
structure (Figures 2 and 3) exhibiting the expected signals of
the organic groups with remarkable AA′XX′ patterns for the
phenylene and the cyclopentadienyl groups. However, 13C{1H}
NMR spectroscopy delivered no usable information due to the
poor solubility of 13. The successful formation of 11 can be IR
spectroscopically controlled by the disappearance of the alkynyl
C−H stretching frequency through the course of the reaction of
9 with 10 (Experimental Section). The alkynyl units in 11 and
13 are characterized by strong CC vibrations at 2112 (10)
and 2114 cm−1 (11, 13), respectively.
Single crystals of 11 and 13 could be grown by slow diffusion
of n-hexane into a dichloromethane solution containing 11 at
ambient temperature or slowly cooling a saturated dichloro-
methane−13 solution from 50 °C to ambient temperature. The
molecular structure of 11 in the solid state is shown in Figure 2,
while that of 13 is depicted in Figure 3. Important bond lengths
(Å) and bond angles (deg) are summarized in the captions of
Figures 2 and 3, respectively. For crystal and structure
refinement data see the Experimental Section.
bis(diphenylphosphino)ferrocene) (9) and P(C6H5)2(C6H4-4-
CCH) (10) produced the bis(alkynyl)platinum complex
[Pt(dppf)(CC-C6H4-4-PPh2)2] (11) (Scheme 1). The
chelating ferrocenyl phosphine was used to enforce the
required cis-geometry at the platinum atom. Heterobimetallic
11 produced upon reaction with [Pt(SEt2)2Cl2] (12) via self-
assembly the neutral tetranuclear molecular square 13.
Appropriate workup gave 13 in an overall yield of 36%
(Experimental Section). Yellow 13 is poorly soluble in polar
and nonpolar solvents, and it is stable toward air and moisture.
The neutral tetraplatinum square 13 possesses alternating edges
composed of Pt(dppf) and PtCl2 moieties, which are connected
by linear alkynyl phosphine units 4-(C6H5)2P-C6H4-CC.
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Figure 2. ORTEP diagram (50% probability level) of the molecular structure of 11 with the atom-numbering scheme. All hydrogen atoms and one
molecule of chloroform have been omitted for clarity. The T-shaped π,π interactions between the aromatic C6 rings C23−C28 and C57−C62 are
indicated with a dashed line, whereby d refers to the geometrical centroid to geometrical centroid distance and ∠ to the interplanar angle. Further π
interactions are indicated by dashed lines; carbon−carbon distances are in the range 3.109 (C11−C35) to 3.495 Å (C16−C37). Selected bond
lengths (Å) and angles (deg): C35−C36 = 1.219(9), C55−C56 = 1.186(9), C35−Pt1 = 1.985(7), C55−Pt1 = 2.013(7), P1−Pt1 = 2.3010(18), P2−
Pt1 = 2.3287(17), D1−Fe1 = 1.643, D2−Fe1 = 1.642; C36−C35−Pt1 = 174.1(6), C56−C55−Pt1 = 177.5(6), C35−Pt1−C55 = 90.4(3), C35−
Pt1−P1 = 86.92(18), C55−Pt1−P1 = 177.06(19), C35−Pt1−P2 = 175.86(19), C55−Pt1−P2 = 85.60(19), P1−Pt1−P2 = 97.11(6), D1−Fe1−D2
= 177.5 (D1, D2 = denote the centroids of C5H4 at Fe1).
Complex 11 crystallizes in the monoclinic space group C2/c
and exhibits a square-planar geometry around Pt1 (rms
deviation 0.0062 Å, highest deviation from planarity observed
for Pt1 with −0.0156 Å) with a P1−Pt1−P2 bite angle of
97.11(6)° and a C35−Pt1−C55 angle of 90.4(3)°. The
ferrocene moiety exhibits a staggered conformation (−35.2°),
and the D1−Fe1 and D2−Fe1 separations at 1.642(3) and
1.640(3) Å (D1, D2 = centroids of C5H4) are similar to those
of related compounds.12a The cyclopentadienyl rings are
5.7(5)° deviated from parallelism. The Pt−CC−CPh units
are almost linear, with Pt1−C35−C36, C35−C36−C37, Pt1−
C55−C56, and C55−C56−C57 angles of 174.1(6)°, 173.8(7)°,
177.5(6)°, and 172.0(8)° (Figure 2). The Pt−CCC distances
of 1.985(7) and 2.013(7) Å and the CC bond lengths of
1.219(9) and 1.186(9) Å reflect the formal bond order of these
units.6b,12a Furthermore, complex 11 exhibits intramolecular T-
shaped and sandwich π,π interactions (Figure 2).12b The C35−
Pt1−C55 angle (see above) provides the best precondition for
the formation of molecular squares.
0.1890 Å). The Pt-CC−CPh units are essentially linear, with
Pt1−C35−C36, C35−C36−C37, Pt1−C74−C73, and C74−
C73−C70 angles of 177.4(17)°, 169(2)°, 173(2)°, and
174.1(18)° and CC distances of 1.23(2) and 1.14(2) Å,
respectively. The ferrocenyl moieties exhibit a staggered
conformation (−35.8°) with the cyclopentadienyl rings
oriented virtually parallel to each other (4.6°). In addition,
complex 13 exhibits intramolecular T-shaped, parallel-dis-
placed, and sandwich π,π interactions (Figure 3).12b The
Pt1···Pt1A and Pt2···Pt2A distances are 11.9350(4) and
16.2205(4) Å, respectively, allowing, for example, the place-
ment of small molecules or group 11 cations as already
described for [{Pt-μ-CC−CC)(dppe)}4] (dppe = bis-
(diphenylphosphine)ethane).6b Complex 13 crystallizes with 11
molecules of chloroform as packing solvent, of which four are
located in the cavity. However, the molecule is not planar due
to the geometry of the phosphorus atoms P3 and P4. All other
structural parameters are similar to those of related molecules,
e.g., [Pt(dppf)(CCPh)2]12a and [{Pt-μ-CC-CC)-
7
(dcpe)}4]4+ (dcpe = bis(dicyclohexylphoshino)ethane).
Complex 13 crystallizes in the monoclinic space group P2/n
and possesses a crystallographically imposed inversion
symmetry with the inversion center in the middle between
the platinum atoms Pt1 and Pt1A as well as Pt2 and Pt2A,
respectively (Figure 3). The 32-membered metallamacrocycle
consists of four platinum atoms at the corners, of which two are
coordinated by a chelating dppf and two monodentate PPh2
moieties. The geometry around Pt1 and Pt2 is distorted square
planar with C35−Pt1−C74, P1−Pt1−P2, P4−Pt2−P3, and
Cl1−Pt2−Cl2 angles of 85.7(7)°, 98.24(17)°, 96.44(18)°, and
86.20(18)° (Pt1: rms deviation 0.0250 Å, highest deviation
from planarity observed for Pt1 of 0.0484 Å; Pt2: rms deviation
0.1528 Å, highest deviation from planarity observed for P3 of
Suzuki C,C Cross-Coupling Reactions. Initially, mono-
(4a, 4b) and diphosphino imidazoles (6a−f) were applied in
the palladium-mediated Suzuki−Miyaura coupling of 2-
bromotoluene with phenylboronic acid, in situ generating the
catalytically active species by applying mixtures of [Pd(OAc)2]
and 4a or 4b (ratio 1:2) or 6a−f (ratio 1:1) (reaction 2) to test
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appropriate reaction solution to determine the rate of
conversion by 1H NMR spectroscopy. The obtained con-
1
versions equal H NMR spectroscopic yields and are based on
the respective aryl halides.
As it can be seen from Figure 4, all compounds are
catalytically active, whereas phosphines 6a−f are more
productive and active than 4a and 4b. Due to the more
electron-rich and bulky cyclohexyl groups, monophosphine 4b
is more active then 4a, featuring phenyl groups. While 4b
produces 2-methylbiphenyl in a yield of 59%, with 4a only a
conversion of 18% is achieved (Figure 4), which is comparable
with the performance of triphenylphosphino ligands in C,C
couplings.13
In general, the phenyl (6a,b)- and cyclohexyl (6d,e)-
functionalized diphosphino systems show significantly higher
activity and productivity than the furyl (6c,f) derivatives.
Furthermore, from Figure 4 it can be seen that the performance
of the catalysts 6c and 6f with furyl-substituted diphosphines is
similar to the catalytic behavior of the complexes carrying
monophosphines 4a and 4b. This result can be explained by the
weak σ-donating ability of the furyl phosphino group and hence
has a smaller influence on the active species. Also, the catalysts
based on diphosphines 6a, b, d, and e show a similar catalytic
behavior, reaching 86−100% conversion after 1 h (Figure 4).
Apparently, the second phosphino group featuring stronger σ
donors has a positive effect on the stabilization of the active
species visible by the higher activity. However, the difference of
the nature of the phosphino group (phenyl vs cyclohexyl) is
less decisive. Nevertheless, the catalytic systems featuring
phosphines 6d and 6e show a somewhat higher activity
compared to 6a and 6b.
We also investigated the catalytic behavior of our systems (i)
in the coupling of nonactivated 4-chlorotoluene with phenyl-
boronic acid using 0.01 mol % [Pd(OAc)2] and (ii) in the
synthesis of sterically hindered biaryls at 50 °C. To achieve
comparable results with literature-known systems, we changed
to non-aqueous reaction conditions including potassium
phosphate as base and toluene as solvent.
From Table 2 it can be seen that the Suzuki coupling of 4-
chlorotoluene with phenylboronic acid to give 4-methylbi-
phenyl using phosphines 4a and 6b in combination with
[Pd(OAc)2] gave 64% and 89% conversion, respectively (Table
2, entries 1, 2). Especially diphosphine 6d (TON: 8900) shows
similar results, when compared to the tert-butyl-substituted
monophosphino benzimidazole (TON: 8600) reported by
Beller et al.2b and the cyclohexyl phosphino imidazole (TON:
Figure 3. ORTEP diagram (50% probability level) of the molecular
structure of 13 with the atom-numbering scheme. All hydrogen atoms,
the packing solvent chloroform, and the phosphorus-bonded phenyl
groups, which are not involved in π,π interactions, have been omitted
for clarity. The T-shaped π,π interactions between the aromatic C6
rings C11−C16 and C37−C42 are indicated with a dashed line, at
which d refers to the geometrical centroid to geometrical centroid
distance and ∠ to the interplanar angle. Further parallel-displaced and
sandwich π,π interactions are indicated by dashed lines. Carbon−
carbon distances are in the range 3.024 (C23−C74) to 3.502 Å (C42−
C58). Selected bond lengths (Å) and angles (deg): C35−C36 =
1.23(2), C73−C74 = 1.14(2), C35−Pt1 = 1.996(17), C74−Pt1 =
2.030(18), P1−Pt1 = 2.290(5), P2−Pt1 = 2.311(4), P3−Pt2 =
2.264(5), P4−Pt2 = 2.239(6), Cl1−Pt2 = 2.352(5), Cl2−Pt2 =
2.361(5), D1−-Fe1 = 1.646, D2−Fe1 = 1.641; C36−C35−Pt1 =
177.4(17), C73−C74−Pt1 = 173(2), C35−Pt1−C74 = 85.7(7), C35−
Pt1−P1 = 87.0(5), C74−Pt1−P1 = 172.4(5), C35−Pt1−P2 =
173.2(6), C74−Pt1−P2 = 88.9(5), P1−Pt1−P2 = 98.24(17), P4−
Pt2−P3 = 96.44(18), P4−Pt2−Cl1 = 173.00(17), P3−Pt2−Cl1 =
88.51(19), P4−Pt2−Cl2 = 90.01(18), P3−Pt2−Cl2 = 166.45(18),
Cl1−Pt2−Cl2 = 86.20(18), D1−Fe1−D2 = 178.9. (D1, D2 = denote
the centroids of C5H4 at Fe1; symmetry -generated atoms are
indicated by the suffix A; symmetry codes: −x+1/2, y, −z+3/2; −x+3/
2, y, −z+3/2.)
the new phosphino imidazoles. The catalytic reactions were
carried out in 1,4-dioxane−water mixtures of ratio 2:1 in the
presence of potassium carbonate as base at 100 °C using 0.5
mol % of the monophosphine and 0.25 mol % of the
appropriate diphosphine, respectively, and 0.25 mol % of the
palladium source. Acetylferrocene as standard was added to the
Figure 4. Kinetic investigation of 4a and 6a−c (left) and 4b and 6d−f (right) in the Suzuki−Miyaura C,C cross-coupling of 2-bromotoluene with
phenylboronic acid to give 2-methylbiphenyl (0.5 mol % mono- or 0.25 mol % diphosphine, 0.25 mol % [Pd(OAc)2], conversion time 0−60 min);
reaction conditions based on ref 14.
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Table 2. Suzuki Coupling of 4-Chlorotoluene and Sterically Hindered Aryl Bromides
a
Reaction conditions:2b 4-chlorotoluene (3.0 mmol, 1.0 equiv), phenylboronic acid (4.5 mmol, 1.5 equiv), K3PO4 (6.0 mmol, 3.0 equiv), toluene (6
b
c
mL), [Pd(OAc)2]/phosphine (0.01 mol % [Pd], 0.1 mol % mono- or 0.05 mol % diphosphine), 100 °C, 20 h. Ligand from ref 2f. Ligand from ref
2b. Ligands from ref 15b. Reaction conditions:15b 4-chlorotoluene (3.0 mmol, 1.0 equiv), phenylboronic acid (4.5 mmol, 1.5 equiv), K3PO4 (6.0
d
e
mmol, 3.0 equiv), toluene (6 mL), [Pd]:P = 1:2, 100 °C, 20 h. Reaction conditions:16 aryl halide (1.0 mmol, 1.0 equiv), phenylboronic acid (1.5
f
mmol, 1.5 equiv), K3PO4 (3.0 mmol, 3.0 equiv), toluene (2 mL), [Pd2(dba)3]/phosphine (0.05 mol % [Pd], 0.1 mol % phosphine), 50 °C, 24 h.
g
Ligand from ref 15d, T = 100 °C, 16 h.
8800) reported earlier by our group2f (Table 2, entries 3, 4). In
summary, excellent results were obtained, when compared to
the cyclohexyl-functionalized phosphino imidazole (TON:
1500−6600) by Beller and co-workers2b (Table 2, entry 5).
substituted aryl bromides with phenylboronic acid in the
presence of potassium phosphate as base and [Pd2(dba)3] (dba
= dibenzylideneacetone) as palladium source at 50 °C. From
Table 2 it can be seen that the coupling of aryl bromides with
one or two ortho-substituents proves successful with a
palladium loading of only 0.05 mol % (Table 2, entries 6−9).
Even the coupling of the sterically demanding 1,3,5-tri-
isopropyl-2-bromobenzene achieves good results, with a
conversion of 78% (Table 2, entry 9). When compared to
other catalytic systems suitable for these reactions, e.g., Fe(η5-
(1-(4-t-Bu-C6H4)-2-P(2-CH3C6H4)2C5H3))(η5-C5H5)/
[Pd2(dba)3],15 only half the amount of palladium is required to
achieve quantitative conversion. Compared to the established
biphenyl-based catalyst by Buchwald15d (Table 2, entry 14),
Compared to the biphenyl ligand by Buchwald,15a,d
a
significantly higher conversion can be achieved when using
0.01 mol % palladium (Table 2, entry 6).15b However,
compared to trialkyl phosphino-based catalysts by Fu15c and
Beller15b (Table 2, entries 8, 9), catalyst system 6d/[Pd(OAc2]
showed under the equivalent reaction conditions a somewhat
lower conversion.15b
The preparation of sterically hindered biaryls still represents
a challenge in organic synthesis, especially under mild reaction
conditions. Therefore, we investigated the coupling of ortho-
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longer reaction times and higher catalyst loadings are required;
however, the catalytic reaction proceeds at considerably lower
temperature (50 °C vs 100 °C). Nevertheless, compared to
previously reported 1-(4-I-C6H4)-2-P(cC6H11)2-4,5-Me2-1H-
C3N2/[Pd2(dba)3],2f no enhanced catalytic activity, based on
the second phosphino group, could be observed.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under an
atmosphere of argon using standard Schlenk techniques. Tetrahy-
drofuran and dichloromethane were purified by distillation from
sodium/benzophenone and calcium hydride, respectively; di-isopro-
pylamine was purified by distillation from potassium hydroxide.
Column chromatography was carried out using silica with a particle
size of 40−60 μm (230−400 mesh (ASTM), Becker) or alumina with
a particle size of 90 μm (standard, Merck KGaA). For filtrations Celite
(purified and annealed, Erg. B.6, Riedel de Haen) was used.
CONCLUSIONS
It has been shown that mono- and bidentate phosphino
■
imidazoles of type 1-(4-PR2-C6H4)-4,5-Me2-1H-C3HN2 (R =
NMR spectra were recorded at 298 K with a Bruker Avance 250 or
a Bruker Avance III 500 spectrometer. The H NMR spectra were
c
1
C6H5, C6H11) and 1-(4-PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2
c
c
c
recorded at 250.13 or 500.3 MHz, the 13C{1H} at 125.7 MHz, and the
31P{1H} NMR spectra at 101.249 or 202.5 MHz, respectively.
Chemical shifts are reported in δ units (parts per million) downfield
from tetramethylsilane with the solvent as reference signal (1H NMR:
standard internal CDCl3, δ 7.26; 13C{1H} NMR: standard internal
CDCl3, δ 77.16; 31P{1H} NMR: standard external rel 85% H3PO4, δ
0.0; P(OMe)3, δ 139.0). High-resolution mass spectra were recorded
with a Bruker Daltonik micrOTOF-QII spectrometer (ESI-TOF). ESI-
TOF mass spectra of 11 were recorded on an Applied Biosystems
spectrometer. Elemental analyses were carried out with a Thermo
FlashAE 1112 series instrument. Melting points of analytically pure
samples were determined by a Gallenkamp MFB 595 010 M melting
point apparatus. FT IR spectra were recorded with a Thermo Nicolet
IR 200 spectrometer using KBr pellets or NaCl plates. All starting
materials were obtained from commercial suppliers and used without
further purification. 1-(4-Iodophenyl)-4,5-dimethyl-1H-imidazole
(2),2f borane-diphenylphosphine 3a,17 borane-dicyclohexylphosphine
3b,18 chlorophosphines 5b19 and 5c,19 [PdCl2(SEt2)2],20
(R = C6H5: R′ = C6H5, C6H11, C4H3O; R = C6H11: R′ =
c
c
C6H5, C6H11, C4H3O) can be prepared in straightforward
synthesis methodologies including Stelzer coupling9 and
selective metalation at position 2 of the imidazole unit. The
diphosphino imidazoles gave upon their reaction with
[PdCl2(SEt2)2] palladamacrocycles of composition [Pd(1-(4-
PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2)Cl2]2, which, however,
are insoluble in common organic solvents. However, it
c
appeared that the derivative with R = C6H5 and R′ = C4H3O
allowed the growth of single crystals suitable for X-ray structure
analysis using the synthesis-cum-diffusion strategy. The
structure of the latter molecule in the solid state shows the
formation of a neutral 18-membered metal−organic cycle with
two trans-configurated palladium centers. A further metal-
lamacrocycle was accessible by treatment of [Pt(dppf)(CC-
C6H4-4-PPh2)2] (dppf = 1,1′-bis(diphenylphosphino)-
ferrocene), to enforce the required cis-geometry at the platinum
atom, with [Pt(SEt2)2Cl2] to give by molecular recognition
[(Pt(dppf)(CC-C6H4-4-PPh2))PtCl2]2. X-ray structure de-
termination of this compound confirmed the molecular square
architecture composed of Pt(dppf) and PtCl2 corner units and
4-(C6H5)2P-C6H4-CC linkers. Similar compounds are known
and well described.6a,f,i,7 Furthermore, the mono- and bidentate
phosphino imidazoles 1-(4-PR2-C6H4)-4,5-Me2-1H-C3N2 and
1-(4-PR2-C6H4)-2-PR′2-4,5-Me2-1H-C3N2, respectively, were
used in the palladium-catalyzed Suzuki cross-coupling of, for
example, 2-bromotoluene with phenylboronic acid using
potassium carbonate as base under aqueous reaction con-
ditions.14 All in situ generated phosphino palladium species
showed moderate to high catalytic activity toward the
formation of 2-methylbiphenyl. It was found that the
monophosphine species were less active and productive than
the catalysts based on diphosphines. The diphosphino systems
featuring phenyl and cyclohexyl groups show a similar activity
and productivity with conversions of 86−100%; the ones
carrying weak σ-donating furyl groups show productivities
comparable to the appropriate monophosphines. However, the
best results were obtained with diphosphines carrying strong σ-
donating substituents. In addition, we investigated the catalytic
behavior of 1-(4-P(C6H5)2-C6H4)-4,5-Me2-1H-C3N2 and 1-(4-
P(cC6H11)2-C6H4)-2-P(C6H5)2-4,5-Me2-1H-C3N2 in the Suzuki
coupling of 4-chlorotoluene with phenylboronic acid under
non-aqueous conditions,2b in which especially the diphosphine
showed excellent productivities, with TONs up to 8900.
Furthermore, 1-(4-P(cC6H11)2-C6H4)-2-P(C6H5)2-4,5-Me2-1H-
C3N2 was applied in the synthesis of ortho-substituted biaryls
with a palladium loading of 0.05 mol % at 50 °C under non-
aqueous conditions,16 showing good to excellent conversions.
Compared to other catalytically active species2b,f,15,16 reported
by Fu, Beller, and Buchwald, our systems show at least the same
or higher productivities but at lower catalyst loadings and lower
temperatures.
[PtCl2(dppf)],21 and HCC-C6H4-4-P(C6H5)2 were prepared
22
according to published procedures.
Synthesis of 1-(4-(Diphenylphosphino)phenyl)-4,5-dimeth-
yl-1H-imidazole (4a). Compound 2 (0.50 g, 1.68 mmol), potassium
acetate (0.20 g, 2.04 mmol, 1.2 equiv), and [Pd(OAc)2] (3.8 mg, 1.0
mol %) were dissolved in dimethylacetamide (40 mL). Then borane-
diphenylphosphine 3a (0.34 g, 1.70 mmol) was added in a single
portion, and the reaction mixture was stirred for 3 h at 130 °C. After
cooling to ambient temperature, the reaction mixture was poured into
water (50 mL) and extracted twice with dichloromethane (20 mL).
The dichloromethane solution was then washed five times with water
(30 mL) and dried over magnesium sulfate. The solvent was removed
in a membrane-pump vacuum, and the crude product was purified by
column chromatography on silica (column size: 3.5 × 15 cm) using a
mixture of n-hexane−diethyl ether (ratio 1:1, v:v). Phosphine 4a could
be isolated as a colorless solid. Yield: 0.31 g (0.87 mmol, 52% based on
2). Anal. Calcd for C23H21N2P (356.40 g/mol): C, 77.51; H, 6.23; N,
7.86. Found: C, 77.13; H, 5.97; N, 7.80. Mp: 162 °C. IR (KBr, ν/
cm−1): 1432 (m, P−C), 1500 (s, NC), 1574/1594 (m, CC),
1
2912/2942/2998 (w, C−H), 3052 (w, C−H). H NMR (500.30
MHz, CDCl3, δ): 2.11 (s, 3 H, CH3), 2.22 (s, 3 H, CH3), 7.22 (dpt,
4
4
3JHH = 8.4 Hz, JHH = 1.9 Hz, JHP = 1.1 Hz, 2 H, Hm/C6H4), 7.33−
7.40 (m, 12 H, Ho,m,p/C6H5+Ho/C6H4), 7.50 (s, 1 H, H2/C3HN2).
13C{1H} NMR (125.81 MHz, CDCl3, δ): 8.9 (s, CH3), 12.1 (s, CH3),
3
122.6 (s, C4,5/C3HN2), 125.0 (d, JCP = 6.7 Hz, Cm/C6H4), 128.4 (d,
3JCP = 7.1 Hz, Cm/C6H5), 128.8 (s, Cp/C6H5), 133.5 (d, JCP = 19.8
2
Hz, Co/C6H5), 133.8 (s, C4,5/C3HN2), 134.3 (d, JCP = 19.6 Hz, Co/
2
C6H4), 134.7 (s, Cp/C6H4), 136.2 (d, 1JCP = 10.9 Hz, Ci/C6H5), 136.6
1
(s, C2/C3HN2), 137.8 (d, JCP = 13.7 Hz, Ci/C6H4). 31P{1H} NMR
(202.5 MHz, CDCl3, δ): −5.9 (s). HRMS (ESI-TOF) C23H21N2P [M
+ nH]+ m/z: calcd 357.1515, found 357.1516; [M + nNa]+ m/z: calcd
379.1335, found 379.1325; [2M + nNa]+ m/z: calcd 735.2777, found
735.2815.
Synthesis of 1-(4-(Dicyclohexylphosphino)phenyl)-4,5-di-
methyl-1H-imidazole (4b). To a dimethylacetamide solution (40
mL) containing 2 (0.50 g, 1.68 mmol), potassium acetate (0.20 g, 2.04
mmol, 1.2 equiv), and [Pd(OAc)2] (3.8 mg, 1.0 mol %) was added
borane-dicyclohexylphosphine 3b (0.36 g, 1.70 mmol) in a single
portion, and the reaction mixture was stirred for 3 h at 130 °C.
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Afterward, the reaction mixture was cooled to ambient temperature,
poured into water (50 mL), and extracted twice with dichloromethane
(20 mL). The dichloromethane solution was washed five times with
water (30 mL) and dried over magnesium sulfate. After removal of all
volatiles in a membrane-pump vacuum, the crude product was purified
by column chromatography on silica (column size: 3.5 × 15 cm) using
a mixture of n-hexane−diethyl ether (ratio 1:1, v:v). Phosphine 4b
could be isolated as a colorless solid. Yield: 0.30 g (0.81 mmol, 48%
based on 2). Anal. Calcd for C23H33N2P (368.50 g/mol): C, 74.97; H,
9.03; N, 7.60. Found: C, 74.46; H, 8.87; N, 7.52. Mp: 97 °C. IR (NaCl,
ν/cm−1): 1447 (m, P−C), 1499 (s, NC), 1595 (m, CC), 2849/
(KBr, ν/cm−1): 1433 (m, P−C), 1497 (m, NC), 1591 (w, CC),
1
2847/2918 (s, C−H), 3046/3068 (w, C−H). H NMR (500.30
MHz, CDCl3, δ): 0.94−1.22 (m, 10 H, C6H11), 1.52−1.65 (m, 10 H,
C6H11), 1.87 (s, 3 H, CH3), 2.01−2.06 (m, 2 H, H1/C6H11), 2.18 (s, 3
H, CH3), 7.02 (dpt, 3JHH = 8.2 Hz, 2 H, Hm/C6H4), 7.25−7.29 (m, 12
H, Ho/C6H4 + C6H5). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.7 (s,
CH3), 13.0 (s, CH3), 26.4 (s, C6H11), 26.8 (d, JCP = 8.3 Hz, C6H11),
26.9 (d, JCP = 11.8 Hz, C6H11), 29.4 (d, JCP = 7.8 Hz, C6H11), 30.4 (d,
1
JCP = 17.0 Hz, C6H11), 34.3 (d, JCP = 8.0 Hz, C1/C6H11), 125.3 (s,
3
3
C4,5/C3N2), 128.6 (d, JCP = 7.2 Hz, Cm/C6H5), 128.7 (dd, JCP = 3.1
Hz, JCP = 2.8 Hz, Cm/C6H4), 129.0 (s, Cp/C6H5), 133.7 (d, JCP
=
4
1
1
19.5 Hz, Co/C6H4), 133.9 (d, 1JCP = 19.9 Hz, Co/C6H5), 135.5 (s, C4,5/
2932 (s, C−H), 3032 (w, C−H). H NMR (500.30 MHz, CDCl3,
C3N2), 136.6 (d, JCP = 11.2 Hz, Ci/C6H5), 137.9 (d, JCP = 13.1 Hz,
Ci/C6H4), 138.0 (d, 3JCP = 1.8 Hz, Cp/C6H4), 144.7 (d, 1JCP = 13.9 Hz,
C2/C3N2). 31P{1H} NMR (202.5 MHz, CDCl3, δ): −23.1 (s,
P(C6H11)2), −5.6 (s, PPh2). HRMS (ESI-TOF) C35H42N2P2 [M +
nH]+ m/z: calcd 553.2825, found 553.2896.
1
1
δ): 0.99−1.07 (m, 2 H, C6H11), 1.11−1.39 (m, 8 H, C6H11), 1.63−
1.73 (m, 6 H, C6H11), 1.79−1.82 (m, 2 H, C6H11), 1.88−1.98 (m, 4 H,
3
C6H11), 2.14 (s, 3 H, CH3), 2.24 (s, 3 H, CH3), 7.25 (dpt, JHH = 8.2
Hz, 2 H, Hm/C6H4), 7.52 (s, 1 H, H2/C3HN2), 7.57−7.50 (m, 2 H,
Ho/C6H4). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.3 (s, CH3),
12.8 (s, CH3), 26.4 (s, C6H11), 26.9 (d, JCP = 7.4 Hz, C6H11), 27.2 (d,
Synthesis of 1-(4-(Diphenylphosphino)phenyl)-2-(di-2-furyl-
phosphino)-4,5-dimethyl-1H-imidazole (6c). Molecule 4a (0.30
g, 0.84 mmol) was reacted with lithium di-isopropylamide (0.42 mL,
0.84 mmol) and chlorodi-2-furylphosphine (5c) (0.17 g, 0.85 mmol)
as described above. The residue was purified by column chromatog-
raphy on silica (column size: 15 × 2.5 cm) using diethyl ether as
eluent. Molecule 6c was obtained as a colorless solid. Yield: 0.28 g
(0.54 mmol, 64% based on 4a). Anal. Calcd for C31H26N2O2P2
(520.50 g/mol): C, 71.53; H, 5.03; N, 5.38. Found: C, 71.87; H,
5.19; N, 5.18. Mp: 65 °C. IR (NaCl, ν/cm−1): 1006 (s, C−O), 1434
(m, P−C), 1497 (m, NC), 1592 (w, CC), 2919 (w, C−H), 3051
(w, C−H). 1H NMR (500.30 MHz, CDCl3, δ): 1.94 (s, 3 H, CH3),
JCP = 12.5 Hz, C6H11), 28.8 (d, JCP = 7.2 Hz, C6H11), 30.0 (d, JCP
=
16.2 Hz, C6H11), 32.6 (d, JCP = 12.1 Hz, C6H11), 122.7 (s, C4,5/
3
C3HN2), 124.6 (d, JCP = 7.3 Hz, Cm/C6H4), 134.7 (s, C4,5/C3HN2),
135.1 (d, 2JCP = 20.5 Hz, Co/C6H4), 135.1 (s, Cp/C6H4), 135.6 (d, 1JCP
= 19.6 Hz, Ci/C6H4), 137.3 (s, C2/C3HN2). 31P{1H} NMR (202.5
MHz, CDCl3, δ): 2.7 (s). HRMS (ESI-TOF) C23H33N2P [M]+ m/z:
calcd 369.2454, found 369.2447.
General Synthesis Procedure for Phosphines 6a−f. To 0.30 g
of 4a (0.84 mmol) or 4b (0.81 mmol) dissolved in dry diethyl ether
(40 mL) was added dropwise 1 equiv of a 2.0 M solution of lithium di-
isopropylamide at −30 °C. After warming the reaction mixture to
ambient temperature, it was again cooled to −30 °C, and 1 equiv of
the appropriate pure chlorophosphine (5a−c) was added dropwise.
The reaction mixture was stirred at ambient temperature for 2 h, and
the solvent was removed under vacuum. The crude product was
purified by column chromatography on silica and dried under vacuum.
Synthesis of 1-(4-(Diphenylphosphino)phenyl)-2-(diphenyl-
phosphino)-4,5-dimethyl-1H-imidazole (6a). Following the syn-
thesis procedure described above, 4a (0.30 g, 0.84 mmol) was reacted
with lithium di-isopropylamide (0.42 mL, 0.84 mmol) and the
chlorophosphine 5a (0.15 mL, 0.84 mmol). The resulting residue was
purified by column chromatography on silica (column size: 15 × 2.5
cm) using a mixture of n-hexane−diethyl ether (ratio 1:1, v:v) as
eluent. Phosphine 6a was obtained as a colorless solid. Yield: 0.26 g
(0.48 mmol, 57% based on 4a). Anal. Calcd for C35H30N2P2 (540.57
g/mol): C, 77.76; H, 5.59; N, 5.18. Found: C, 77.91; H, 5.76; N, 5.10.
Mp: 96 °C. IR (KBr, ν/cm−1): 1435 (s, P−C), 1479/1498 (m, N
C), 1593 (w, CC), 2863/2918/2966 (w, C−H), 3051 (w, C−H).
1H NMR (500.30 MHz, CDCl3, δ): 1.97 (s, 3 H, CH3), 2.25 (s, 3 H,
4
3
3
2.25 (s, 3 H, CH3), 6.31 (dt JHP = 1.6 Hz, JHH = 3.3 Hz, JHH = 1.8
Hz, 2 H, H4/C4H3O), 6.71 (m, 2 H, H3/C4H3O), 7.02 (m, 3JHH = 8.4
Hz, 2 H, Hm/C6H4), 7.30 (m, 2 H, Ho/C6H4), 7.35−7.40 (m, 10 H,
C6H5), 7.59 (m, 2 H, H5/C4H3O). 13C{1H} NMR (125.81 MHz,
3
CDCl3, δ): 9.5 (s, CH3), 13.2 (s, CH3), 110.9 (d, JCP = 6.6 Hz, C4/
2
C4H3O), 121.9 (d, JCP = 26.7 Hz, C3/C4H3O), 127.5 (s, C4/C3N2),
127.7 (dd, 3JCP = 2.5 Hz, 4JCP = 2.3 Hz, Cm/C6H4), 128.7 (d, 3JCP = 7.1
Hz, Cm/C6H5), 129.2 Hz, (s, Cp/C6H5), 133.9 (d, 2JCP = 19.9 Hz, Co/
2
4
C6H4), 134.1 (d, JCP = 19.8 Hz, Co/C6H5), 136.4 (d, JCP = 3.8 Hz,
C5/C3N2), 136.6 (d, 1JCP = 10.9 Hz, Ci/C6H5), 137.2 (d, 3JCP = 1.2 Hz,
Cp/C6H4), 138.7 (d, JCP = 13.6 Hz, Ci/C6H4), 140.4 (d, JCP = 12.5
1
1
Hz, C2/C3N2), 147.6 (d, JCP = 2.6 Hz, C5/C4H3O), 148.2 (d, JCP
=
4
1
4.3 Hz, C2/C4H3O). 31P{1H} NMR (202.5 MHz, CDCl3, δ): −71.7 (s,
P(cC4H3O)2), −5.6 (s, PPh2). HRMS (ESI-TOF) C31H26N2O2P2 [M +
nH]+ m/z: calcd 521.1542, found 521.1542.
Synthesis of 1-(4-(Dicyclohexylphosphino)phenyl)-2-(diphe-
nylphosphino)-4,5-dimethyl-1H-imidazole (6d). Based on the
general procedure described earlier, 4b (0.30 g, 0.81 mmol) was
reacted with lithium di-isopropylamide (0.41 mL, 0.82 mmol) and
chlorodiphenylphosphine (5a) (0.15 mL, 0.84 mmol). The residue
was purified by column chromatography on silica (column size: 15 ×
2.5 cm) using a mixture of n-hexane−diethyl ether (ratio 2:1, v:v) as
eluent. The product 6d was obtained as a colorless solid. Yield: 0.27 g
(0.49 mmol, 60% based on 4b). Anal. Calcd for C35H42N2P2 (552.67
g/mol): C, 76.06; H, 7.66; N, 5.07. Found: C, 75.86; H, 7.62; N, 4.94.
Mp: 155 °C. IR (NaCl, ν/cm−1): 1435 (m, P−C), 1497 (m, NC),
1592 (w, CC), 2849/2922 (s, C−H), 3051 (w, C−H). 1H NMR
(500.30 MHz, CDCl3, δ): 1.08−1.18 (m, 2 H, C6H11), 1.23−1.37 (m,
8 H, C6H11), 1.59−1.73 (m, 6 H, C6H11), 1.79−1.92 (m, 6 H, C6H11),
3
CH3), 7.00 (dpt, JHH = 8.4 Hz, 2 H, Hm/C6H4), 7.25−7.28 (m, 8 H,
C6H5), 7.32−7.38 (m, 10 H, Ho/C6H4 + C6H5), 7.40−7.45 (m, 4 H,
C6H5). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.7 (s, CH3), 13.2 (s,
3
4
CH3), 127.1 (s, C4,5/C3N2), 128.2 (dd, JCP = 3.1 Hz, JCP = 2.8 Hz,
Cm/C6H4), 128.4 (d, 3JCP = 7.4 Hz, Cm/C6H5), 128.7 (d, 3JCP = 4.2 Hz,
Cm/C6H5), 128.8 (s, Cp/C6H5), 129.2 (s, Cp/C6H5), 133.8 (d, JCP
=
2
20.7 Hz, Co/C6H5), 134.0 (d, 2JCP3 = 19.7 Hz, Co/C6H5), 134.1 (d, 2JCP
= 19.6 Hz, Co/C6H4), 136.3 (d, JCP = 6.2 Hz, C4,5/C3N2), 136.5 (d,
3JCP = 1.9 Hz, Cp/C6H4), 136.7 (d, 1JCP = 10.9 Hz, Ci/C6H5), 137.5 (d,
1JCP = 1.9 Hz, Ci/C6H5), 138.5 (d, 1JCP = 13.0 Hz, Ci/C6H4), 144.0 (d,
1JCP = 1.9 Hz, C2/C3N2). 31P{1H} NMR (202.5 MHz, CDCl3, δ):
3
1.97 (s, 3 H, CH3), 2.25 (s, 3 H, CH3), 6.98 (m, JHH = 7.9 Hz, 2 H,
−27.9 (s, {C3N2PPh2}), −5.5 (s, {C6H4PPh2}). HRMS (ESI-TOF)
C35H30N2P2 [M + nH]+ m/z: calcd 541.1957, found 541.1915.
Synthesis of 1-(4-(Diphenylphosphino)phenyl)-2-(dicyclo-
hexylphosphino)-4,5-dimethyl-1H-imidazole (6b). As described
earlier, 4a (0.30 g, 0.84 mmol) was reacted with lithium di-
isopropylamide (0.42 mL, 0.84 mmol) and chlorodicyclohexylphos-
phine (5b) (0.19 mL, 0.86 mmol). The crude product was purified by
column chromatography on silica (column size: 15 × 2.5 cm) using a
mixture of n-hexane−diethyl ether (ratio 2:1, v:v) as eluent. Phosphine
6b was obtained as a colorless solid. Yield: 0.23 g (0.42 mmol, 50%
based on 4a). Anal. Calcd for C35H42N2P2 (552.67 g/mol): C, 76.06;
H, 7.66; N, 5.07. Found: C, 75.79; H, 7.73; N, 4.93. Mp: 144 °C. IR
Hm/C6H4), 7.24−7.78 (m, 6 H, Hm,p/C6H5), 7.38−7.43 (m, 6 H, Ho/
C6H4 + Ho/C6H5). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.7 (s,
CH3), 13.2 (s, CH3), 26.6 (s, C6H11), 27.1 (d, JCP = 7.3 Hz, C6H11),
27.4 (d, JCP = 12.4 Hz, C6H11), 29.0 (d, JCP = 7.1 Hz, C6H11), 30.1 (d,
JCP = 16.1 Hz, C6H11), 32.7 (d, JCP = 12.2 Hz, C6H11), 127.2 (s, C4/
3
4
C3N2), 127.7 (dd, JCP = 2.8 Hz, JCP = 2.7 Hz, Cm/C6H4), 128.4 (d,
3JCP = 7.6 Hz, Cm/C6H5), 128.8 (s, Cp/C6H5), 134.0 (d, JCP = 20.7
2
Hz, Co/C6H5), 135.1 (d, JCP = 19.3 Hz, Ci/C6H5), 135.9 (d, JCP
20.1 Hz, Co/C6H4), 136.2 (d, 1JCP = 6.6 Hz, Ci/C6H4), 136.2 (d, 3JCP
2.0 Hz, C5/C3N2), 137.5 (d, 3JCP = 1.8 Hz, Cp/C6H4), 144.2 (d, 1JCP
=
=
=
1
2
2.8 Hz, C2/C3N2). 31P{1H} NMR (202.5 MHz, CDCl3, δ): −26.7 (s,
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PPh2), 2.6 (s, P(C6H11)2). HRMS (ESI-TOF) C35H42N2P2 [M + nH]+
m/z: calcd 553.2883, found 553.2896.
Synthesis of [Pt(dppf)(CC-C6H4-4-P(C6H5)2)2] (11).
[PtCl2(dppf)] (9, 0.7 g, 0.87 mmol) was dissolved in a dichloro-
methane−di-isopropylamine mixture (100 mL, ratio 7:3, v:v), followed
by the addition of [CuI] (5.7 mg, 0.1 mmol) and 2 equiv of 10 (0.5 g,
1.75 mmol). The reaction mixture was stirred for 6 h at ambient
temperature and filtered through a pad of alumina. Afterward, the
solvent was reduced under vacuum to 5 mL, and n-hexane (20 mL)
was added. The supernatant layer was removed, and the precipitate
was dissolved in dichloromethane (10 mL) and chromatographed on
alumina (column size: 5 × 1.5 cm) using dichloromethane as eluent.
After drying under vacuum, product 11 was obtained as a yellow solid.
Yield: 495 mg (0.45 mmol, 51% based on 9). Anal. Calcd for
C74H56FeP4Pt (1320.05 g/mol): C, 67.33; H, 4.28. Found: C, 67.40;
H, 4.50. Mp: 183 °C (dec). IR (KBr, ν/cm−1): 2114 (m, CC),
Synthesis of 1-(4-(Dicyclohexylphosphino)phenyl)-2-(dicy-
clohexylphosphino)-4,5-dimethyl-1H-imidazole (6e). Using the
general synthesis methodology described above, 4b (0.30 g, 0.81
mmol) was reacted with lithium di-isopropylamide (0.41 mL, 0.82
mmol) and chlorodicyclohexylphosphine (5b) (0.18 mL, 0.82 mmol).
The crude product was purified by column chromatography on silica
(column size: 15 × 2.5 cm) using a mixture of n-hexane−diethyl ether
(ratio 3:1, v:v) as eluent. Phosphine 6e was obtained as a colorless
solid. Yield: 0.21 g (0.37 mmol, 46% based on 4b). Anal. Calcd for
C35H54N2P2 (564.76 g/mol): C, 74.43; H, 9.64; N, 4.96. Found: C,
74.68; H, 9.37; N, 4.72. Mp: 66 °C. IR (NaCl, ν/cm−1): 1446 (m, P−
C), 1497 (m, NC), 1594 (w, CC), 2849/2923 (s, C−H), 3031
(w, C−H). 1H NMR (500.30 MHz, CDCl3, δ): 0.95−1.04 (m, 4 H,
C6H11), 1.08−1.35 (m, 16 H, C6H11), 1.60−1.70 (m, 16 H, C6H11),
1.77−1.79 (m, 2 H, C6H11), 1.83−1.91 (m, 4 H, C6H11), 1.94 (s, 3 H,
1
1432/1477 (s, P−C). H NMR (250.13 MHz, CDCl3, δ): 4.18 (dpt,
3JHH = 1.8 Hz, 3JHP = 1.7 Hz, 4 H, Hα/C5H4), 4.31 (pt, 3JHH = 1.8 Hz,
3
4 H, C5H4), 6.76 (dpt, JHH = 8.3 Hz, 4JHH = 1.7 Hz, 3JHP = 1.5 Hz, 4
CH3), 2.04−2.09 (m, 2 H, C6H11), 2.25 (s, 3 H, CH3), 7.09 (m, 3JHH
=
H, Hm/C6H4), 6.91−6.95 (m, 4 H, Hp/C6H5), 7.20−7.24 (m, 8 H,
3
3
8.0 Hz, 2 H, Hm/C6H4), 7.51 (m, JHH = 8.2 Hz, JHP = 1.9 Hz, 2 H,
Ho/C6H4). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.8 (s, CH3),
13.1 (s, CH3), 26.5 (s, C6H11), 26.5 (s, C6H11), 26.9 (d, JCP = 8.7 Hz,
C6H11), 27.0 (d, JCP = 12.1 Hz, C6H11), 27.1 (d, JCP = 6.8 Hz, C6H11),
27.3 (d, JCP = 12.4 Hz, C6H11), 29.0 (d, JCP = 7.4 Hz, C6H11), 29.7 (d,
H
o,m/C6H5), 7.28−7.30 (m, 20 H, Ho,m,p/C6H5), 7.35−7.38 (m, 4 H,
Ho/C6H4), 7.80−7.84 (m, 8 H, Ho,m/C6H5). 13C{1H} NMR (125.7
MHz, CDCl3, δ): 72.9 (pt, JPC = 3.3 Hz, Cβ/C5H4), 75.7 (pt, JPC
=
2
2
5.0 Hz, Cα/C5H4), 77.7 (Ci/C5H4*), 104.9 (d, 2JCP = 20.5 Hz, CC−
P), 110.1 (d, JCP = 34.8 Hz, CC-P), 128.0 (pt, JCP = 5.3 Hz, Cm/
1
3
C6H5(dppf)), 128.5 (d, 3JCP = 6.6 Hz, Cm/C6H5), 128.6 (s, Cp/C6H5),
JCP = 8.1 Hz, C6H11), 30.1 (d, JCP = 16.2 Hz, C6H11), 30.5 (d, JCP
=
128.9 (s, Cp/C6H4), 130.6 (s, Cp/C6H5(dppf)), 131.6 (d, JCP = 7.4
3
16.9 Hz, C6H11), 32.7 (d, JCP = 12.3 Hz, C6H11), 34.2 (d, JCP = 8.4 Hz,
3
4
C6H11), 125.8 (s, C4/C3N2), 128.2 (dd, JCP = 7.2 Hz, JCP1 = 4.3 Hz,
Hz, Cm/C6H4), 132.7 (d, JCP = 19.9 Hz, Co/C6H5(dppf)), 132.8 (pt,
2
Cm/C6H4), 135.1 (d, JCP = 19.3 Hz, Co/C6H4), 135.3 (d, JCP = 21.0
2
1JCP = 8.9 Hz, Ci/C6H5(dppf)), 133.4 (d, JCP = 5.1 Hz, Ci/C6H4),
1
Hz, Ci/C6H4), 135.4 (s, C5/C3N2), 138.1 (s, Cp/C6H4), 144.8 (d, 1JCP
= 15.4 Hz, C2/C3N2). 31P{1H} NMR (202.5 MHz, CDCl3, δ): −22.2
(s, {C3N2P(C6H11)2}), 2.6 (s, {C6H4P(C6H11)2}). HRMS (ESI-TOF)
C35H54N2P2 [M + nH]+ m/z: calcd 565.3871, found 565.3835.
Synthesis of 1-(4-(Dicyclohexylphosphino)phenyl)-2-(di-2-
furylphosphino)-4,5-dimethyl-1H-imidazole (6f). Molecule 4b
(0.30 g, 0.81 mmol) was reacted with lithium di-isopropylamide (0.41
mL, 0.82 mmol) and chlorodi-2-furylphosphine (5c) (0.17 g, 0.85
mmol) as described earlier. The crude product was purified by column
chromatography on silica (column size: 15 × 2.5 cm) using diethyl
ether as eluent. Phosphine 6f was obtained as a colorless solid. Yield:
0.28 g (0.53 mmol, 65% based on 4b). Anal. Calcd for C31H38N2O2P2
(532.59 g/mol): C, 69.91; H, 7.19; N, 5.26. Found: C, 70.15; H, 7.54;
N, 4.95. Mp: 115 °C. IR (KBr, ν/cm−1): 1007 (m, C−O), 1449 (m,
P−C), 1500/1507 (w, NC), 1598 (w, CC), 2854/2930 (s, C−
133.8 (d, JCP = 19.5 Hz, Co/C6H4), 135.0 (pt, JCP = 5.6 Hz, Co/
2
2
C6H5(dppf)), 137.9 (d, JCP = 10.8 Hz, Ci/C6H5). 31P{1H} NMR
1
(101.249 MHz, CDCl3, δ): 6.6 (P(C6H5)2), 13.5 (1J
Pt = 2374 Hz,
31P−195
dppf). MS (ESI-TOF) C74H56FeP4Pt [M + H]+ m/z: calcd 1320.24,
found 1320.6(45). *Signal concealed by CDCl3.
Synthesis of [Pt(dppf)(CC-C6H4-4-P(C6H5)2)2PtCl2)]2 (13).
To a solution of 11 (50 mg, 0.04 mmol) in dichloromethane (10
mL) was added [PtCl2(SEt2)2] (12, 14.2 mg, 0.04 mmol) in a single
portion, and the reaction mixture was stirred for 1 h at ambient
temperature. Afterward, the solvent was reduced under vacuum to 2
mL, and the formed complex was precipitated by addition of n-pentane
(10 mL). The supernatant layer was decanted, and the yellow residue
was washed twice with n-pentane (5 mL). After drying under vacuum,
13 was obtained as a yellow solid. Yield: 45 mg (0.014 mmol, 70%
based on 12). Anal. Calcd for C148H112Cl4Fe2P8Pt4 × 1/4 CH2Cl2
(3172.08 g/mol): C, 55.48; H, 3.54. Found: C, 55.20; H, 3.50. Mp:
>247 °C (dec). IR (KBr, ν/cm−1): 2114 (s, CC), 1432/1481 (s, P−
1
H), 3122/3143 (w, C−H). H NMR (500.30 MHz, CDCl3, δ):
0.93−1.05 (m, 2 H, C6H11), 1.07−1.17 (m, 4 H, C6H11), 1.20−1.37
(m, 6 H, C6H11), 1.57−1.72 (m, 6 H, C6H11), 1.78−1.87 (m, 4 H,
C6H11), 1.90 (s, 3 H, CH3), 2.21 (s, 3 H, CH3), 6.28 (dt, 4JHP = 1.6 Hz,
1
3
C). H NMR (250.13 MHz, CDCl3, δ): 4.19 (pt, JHH = 2.0 Hz, 8 H,
3
C5H4), 4.35 (pt, JHH = 2.0 Hz, 8 H, C5H4), 5.30 (s, CH2Cl2) 6.40−
3
3JHH = 3.3 Hz, JHH = 1.9 Hz, 2 H, H4/C4H3O), 6.63 (m, 2 H, H3/
6.57 (m, 8 H, Hm/C6H4), 6.90−7.01 (m, 8 H, Hp/C6H5), 7.09−7.40
(m, 64 H, C6H5 + Ho/C6H4), 7.80−7.84 (m, 16 H, Ho,m/C6H5).
C4H3O), 6.98 (m, 3JHH = 8.0 Hz, 2 H, Hm/C6H4), 7.42 (m, 3JHH = 8.2
3
Hz, JHP = 1.9 Hz, 2 H, Ho/C6H4), 7.58 (m, 2 H, H5/C4H3O).
31P{1H} NMR (202.5, CDCl3, δ): 13.6 (1J
= 3673.3 Hz,
31P−195Pt
13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.6 (s, CH3), 13.2 (CH3),
26.0 (s, C6H11), 27.1 (d, JCP = 7.5 Hz, C6H11), 27.3 (d, JCP = 12.2 Hz,
C6H11), 28.9 (d, JCP = 7.2 Hz, C6H11), 30.1 (d, JCP = 16.2 Hz, C6H11),
P(C6H5)2), 14.7 (1J
= 2380.1 Hz, dppf).
31P−195
General ProcedurePtfor the Suzuki Reaction (ref 14). 2-
Bromotoluene (500 mg, 2.92 mmol), phenylboronic acid (470 mg,
3.85 mmol, 1.3 equiv), potassium carbonate (1.21 g, 8.76 mmol, 3
equiv), and acetyl ferrocene (111 mg, 0.49 mmol) were dissolved in a
1,4-dioxane−water mixture (10 mL, ratio 2:1, v:v). After addition of
0.25 mol % of [Pd(OAc)2] and 0.5 mol % of the monophosphine (4)
or 0.25 mol % of the appropriate diphosphine (6), the reaction
mixture was stirred for 1 h at 100 °C. Samples of 1 mL were taken
after 2.5, 5, 10, 20, 30, and 60 min and filtered through a pad of silica
(column size: 6 × 2.5 cm) using diethyl ether as eluent. All volatiles
were evaporated under reduced pressure, and the conversions were
3
32.6 (d, JCP = 11.9 Hz, C6H11), 110.9 (d, JCP = 7.0 Hz, C4/C4H3O),
122.1 (d, 2JCP = 27.5 Hz, C3/C4H3O), 127.4 (s, C4/C3N2), 127.6 (dd,
3JCP = 2.5 Hz, 4JCP = 2.3 Hz, Cm/C6H4), 135.3 (d, 2JCP = 19.6 Hz, Co/
3
1
C6H4), 136.6 (d, JCP = 4.2 Hz, C5/C3N2), 137.2 (d, JCP = 14.9 Hz,
Ci/C6H4), 139.7 (d, 3JCP = 1.7 Hz, Cp/C6H4), 140.4 (d, 1JCP = 11.7 Hz,
C2/C3N2), 147.7 (d, JCP = 2.5 Hz, C5/C4H3O), 147.9 (d, JCP = 5.3
Hz, C2/C4H3O). 31P{1H} NMR (202.5 MHz, CDCl3, δ): −70.8 (s,
P(cC4H3O)2), 2.6 (s, P(C6H11)2). HRMS (ESI-TOF) C31H38N2O2P2
[M]+ m/z: calcd 549.2440, found 549.2430.
4
1
Synthesis of [Pd(1-(4-P(C6H5)2-C6H4)-2-P(cC4H3O)2-4,5-Me2-
1H-C3N2)Cl2]2 (8). Complex 8 was synthesized via the synthesis-
cum-diffusion strategy to ensure the formation of crystals suitable for
single-crystal X-ray structure analysis. Therefore, a solution of 6c (20
mg, 0.04 mmol) in dichloromethane (2 mL) was inserted into a test
tube and covered with a layer of dichloromethane (10 mL). Afterward,
a solution of [PdCl2(SEt2)2] (7, 13 mg, 0.04 mmol) in dichloro-
methane (2 mL) was added slowly. The resulting yellow crystals were
subjected to single-crystal X-ray structure analysis.
1
determined by H NMR spectroscopy.
General Procedure for the Suzuki Coupling of Aryl
Chlorides (ref 2b). 4-Chlorotoluene (379 mg, 3.0 mmol), phenyl-
boronic acid (550 mg, 4.5 mmol, 1.5 equiv), potassium phosphate
(1.27 g, 6.0 mmol, 3 equiv), and acetylferrocene (114 mg, 0.50 mmol)
were dissolved in toluene (6 mL). After addition of 0.01 mol %
[Pd(OAc)2] and 0.1 mol % of the appropriate monophosphine 4a or
0.05 mol % of the diphosphine 6d, the reaction mixture was stirred for
20 h at 100 °C. Afterward, a sample of 2 mL was taken and filtered
3669
dx.doi.org/10.1021/om300145d | Organometallics 2012, 31, 3661−3671

Organometallics
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through a pad of Celite. After evaporation of all volatiles under
reduced pressure, the conversions were determined by 1H NMR
spectroscopy.
Schaffner, B.; Zapf, A.; Spannenberg, A.; Borner, A.; Beller, M.
̈ ̈
Chem.Eur. J. 2009, 15, 1329. (f) Milde, B.; Schaarschmidt, D.;
Ruffer, T.; Lang, H. Dalton Trans. 2012, 41, 5377.
̈
General Procedure for the Synthesis of Sterically Hindered
Biaryls (ref 16). Phenylboronic acid (183 mg, 1.5 mmol, 1.5 equiv),
potassium phosphate (0.64 g, 3.0 mmol, 3.0 equiv), acetyl ferrocene
(114 mg, 0.50 mmol), 0.05 mol % [Pd2(dba)3], and 0.05 mol % of 6d
were dissolved in toluene (2 mL). Afterward, the appropriate aryl
bromide (1.0 mmol, 1.0 equiv) was added in a single portion, and the
reaction mixture was stirred for 24 h at 50 °C. Thereafter, the reaction
mixture was filtered through a pad of Celite, and the solvent was
removed in a membrane-pump vacuum. The conversions were
(3) (a) Saleh, S.; Fayad, E.; Azouri, M.; Hierso, J.-C.; Andrieu, J.;
Picquet, M. Adv. Synth. Catal. 2009, 351, 1621. (b) Consorti, C. S.;
Aydos, G. L. P.; Ebeling, G.; Dupont, J. Org. Lett. 2008, 10, 237.
(c) Andrieu, J.; Azouri, M. Inorg. Chim. Acta 2007, 360, 131.
(d) Dyson, P. J.; Geldbach, T. J. In Catalysis by Metal Complexes-Metal
Catalyzed Reactions in Ionic Liquids; James, B.; van Leeuwen, P. W. N.
M., Eds.; Springer: Dordrecht, 2005; Vol. 29. (e) Wasserscheid, P.;
Welton, T. Ionic Liquids in Synthesis, 2nd ed.; Wiley-VCH: Weinheim,
2008, Vol.1. (f) Zhang, J.; Dakovic, M.; Popovic, Z.; Wu, H.; Liu, Y.
Catal. Commun. 2012, 17, 160.
1
determined by H NMR spectroscopy.
Crystal Structure Determination. The crystal and intensity
collection data for 8, 11, and 13 are summarized in Table S1
(Supporting Information). All data were collected on an Oxford
Gemini S diffractometer with graphite-monochromatized Mo Kα
radiation (λ = 0.71073 Å) at 110 K (8) and graphite-monochrom-
atized Cu Kα radiation (λ = 1.54184 Å) at 100 K (11, 13). The
structures were solved by direct methods using SHELXS-9123 and
refined by full-matrix least-squares procedures on F2 using SHELXL-
97.24 All non-hydrogen atoms were refined anisotropically, and a
riding model was employed in the refinement of the hydrogen atom
positions.
(4) (a) Gilchrist, T. L. In Heterocyclenchemir; Neunhoeffer, H., Ed.;
Wiley-VCH: Weinheim, 1995; p 300. (b) Schulz, T.; Torborg, C.;
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̈
̈
Angew. Chem., Int. Ed. 2009, 48, 918. (c) Grimmett, M. R. In Science of
Synthesis-Five Membered Heteroarenes with Two Nitrogen or Phosphorous
Atoms; Neier, R., Vol. Ed.; Thieme: Stuttgart, 2002; Vol. 12, p 325.
(d) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. Synthesis
2003, 17, 2661.
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Chem.Eur. J. 2010, 16, 13095. (b) Artemova, N. V.; Chevykalova, M.
N.; Luzikov, Y. N.; Nifant’ev, I. E.; Nifant’ev, E. E. Tetrahedron 2004,
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Organomet. Chem. 1997, 529, 445. (d) Canac, Y.; Debono, N.;
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P. J.; Nicholson, B. K.; Skelton, B. W.; White, A. H. J. Chem. Soc.,
ASSOCIATED CONTENT
* Supporting Information
■
S
CCDC 867788, CCDC 867790, and CCDC 867789 contain
the supplementary crystallographic data for complexes 8, 11,
and 13. This material is available free of charge via the Internet
at http://pubs.acs.org. These data can also be obtained free of
charge from the Cambridge Crystallographic Database via
www.ccdc.cam.ac.uk/products/csd/request/.
Dalton Trans. 2002, 383. (c) Wurthner, F.; You, C.-C.; Saha-Moller, C.
̈
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R. Chem. Soc. Rev. 2004, 33, 133. (d) Ferrer, M.; Gutierrez, A.; Mounir,
M.; Rossell, O.; Ruiz, E.; Rang, A.; Engeser, M. Inorg. Chem. 2007, 46,
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116, 4981. (g) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem. Soc. 1990,
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J. A.; Stang, P. J.; Huang, S. D. Inorg. Chem. 1998, 37, 5595.
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Tessier, C. A.; Youngs, W. J. J. Am. Chem. Soc. 1998, 120, 12149.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: heinrich.lang@chemie.tu-chemnitz.de. Phone: +49(0)
371-531-21210. Fax: +49(0)371-531-21219.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for generous financial
support. Additionally, we would like to thank Sabrina Krauße
and Martina Steinert for assistance in the lab.
(9) Herd, O.; Heßler, A.; Hingst, M.; Tepper, M.; Stelzer, O. J.
Organomet. Chem. 1996, 522, 69.
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S. G.; Lee, S. J.; Park, J. K.; Park, K. M.; Shin, S. C. Bull. Korean Chem.
Soc. 2005, 26, 1390.
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