Reactions of nitrogen donors with cycloheptatrienylidene complexes: Metal coordination versus nucleophilic attack on the carbene ligand

Article abstract of DOI:10.1021/om300867g

Cycloheptatrienylidene (CHT)-palladium complexes may react with N-donor molecules, showing two different pathways of reaction, either nucleophilic attack on the CHT ligand or coordination to the metal center. The first variant leads to a formation of wate

Full text of DOI:10.1021/om300867g

Article
pubs.acs.org/Organometallics
Reactions of Nitrogen Donors with Cycloheptatrienylidene
Complexes: Metal Coordination versus Nucleophilic Attack on the
Carbene Ligand
̈
̈
Kevser Mantas-Oktem, Karl Ofele, Alexander Pothig, Bettina Bechlars, Wolfgang A. Herrmann,
̈
and Fritz E. Kuhn*
̈
Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische Universitat Munchen,
̈
̈
Ernst-Otto-Fischer-Strasse 1, D-85747 Garching b. Munchen, Germany
̈
S
* Supporting Information
ABSTRACT: Cycloheptatrienylidene (CHT)−palladium complexes may react
with N-donor molecules, showing two different pathways of reaction, either
nucleophilic attack on the CHT ligand or coordination to the metal center. The
first variant leads to a formation of water-soluble η3-cycloheptatrienyl complexes, as
in the case of 3,5-lutidine or 3-chloropyridine. Reaction with 2,6-lutidine, on the
other hand, yields monomeric pyridine-substituted CHT−Pd compounds
comparable to NHC−PEPPSI complexes. Reaction with 1-methylimidazole yields
both a dimeric water-soluble cycloheptatrienyl palladium complex and a monomeric
CHT−Pd compound, depending on the conditions of the reaction. Furthermore, a
subsequent formation of a Pd3-sandwich type complex was observed, which has
been determined by single-crystal X-ray diffractometry. The nucleophilic attack of
morpholine on the CHT ligand reveals another possible reaction path: removal of
the CHT ligand from the metal under formation of a tropylidenimmonium cation.
However, as far as we know, no systematic investigation on the
reactivity of CHT complexes toward nucleophiles has been
conducted thus far. We, therefore, examined the reactions of A
and B with different secondary and tertiary amines and now
report the isolation and characterization of the reaction
products.
INTRODUCTION
■
Carbocyclic carbenes, such as cycloheptatrienylidene (CHT),
have already been applied as ligands for a variety of transition
metals.1 However, only few details are reported about their
reactivity toward donor molecules, especially under preserva-
tion of the metal−carbene bond. A well-established class of
precursors, the dinuclear halogene-bridged Pd complexes
[Pd(CHT)X2]2, can react with CH3CN or phosphines, forming
donor-substituted mononuclear CHT complexes (Scheme 1).
The latter are also known to be active catalysts for several C−C
and C−N cross-coupling reactions.2−4 The reaction with
DMSO, however, results in the substitution of the CHT ligand
and its conversion to tropone,5 which was observed earlier
when treating a pentacarbonyl tungsten CHT complex with
DMSO.6
Another rare example of a reaction with CHT ligands that
does not break the metal−carbene bond is the deprotonation of
the methylene bridge in a P-C(CHT)-P pincer iridium
complex. Its hydrogen atoms become acidic due to the
positively charged CHT ring, leading to an extended π system.7
The same iridium CHT complex is able to coordinate a
molybdenum tricarbonyl moiety by the seven-membered ring,
yielding a heterobimetallic carbene complex. It indicates that
the metal bound CHT ring and the tropylium ion are
comparable systems with respect to their reactivity.7
RESULTS AND DISCUSSION
■
We synthesized the precursors A and B according to the
procedures described in the literature.3 The formation of B
from 1,1′-dichlorocycloheptatriene and Pd0 proceeds through a
dinuclear neutral palladium complex H (Scheme 2).3 For
unsubstituted cycloheptatrienyl ligands, the general motif of a
η3-coordination has been proposed already in 1989,9 but was
confirmed by single-crystal diffraction experiments only
recently.4
Reaction of [Pd(CHT)X2]2 with Pyridine Derivatives.
The treatment of suspensions of complex A in THF with 3,5-
lutidine or 3-chloropyridine at 50 °C produces red crystalline
air-stable products in good yields (Scheme 3, 1a/2a). These
compounds are not soluble in THF, but, unlike the known
mononuclear complexes, such as C−G, they dissolve in water.
The subsequent addition of NH4PF6 to the aqueous
solutions resulted in almost quantitative formation of deep
purple precipitates, which are soluble in acetone and
In general, the reactivity of CHT complexes toward nitrogen
donor molecules is not well-examined. It was found, however,
that the compound 2a efficiently catalyzes α-ketoarylations.8
Received: September 7, 2012
Published: November 7, 2012
© 2012 American Chemical Society
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Scheme 1. Reaction of Donor Ligands with Dinuclear Pd−CHT Complexes2,3
Scheme 2. Synthesis of an η3-Cycloheptatrienyl Complex H and Its Conversion into a Carbene Complex B3
Scheme 3. Synthesis of Dinuclear Palladium Complexes with Pyridine-Substituted Cycloheptatrienyl Ligands
acetonitrile. Single-crystal X-ray diffraction analysis and 1H
NMR spectroscopy revealed the formation of compound 1b,
whose structure is similar to that of complex H, but, due to the
pyridine-substituted cycloheptatrienyl ligands, it is an ionic
compound (Scheme 3). Owing to the similarity of the proton
spectra, we assume that the structure of 2b is analogous. Unlike
phosphines and acetonitrile, the pyridine nucleophiles did not
react by coordination to the metal but instead attack the seven-
membered CHT ring. The analogous chloro-substituted
complexes were synthesized by reaction of compound B with
3,5-lutidine or 3-chloropyridine (Scheme 3, 3a/3b and 4a/4b).
The molecular structure of the C2-symmetric dinuclear
cationic complex in 1b is depicted in Figure 1. The Pd2(μ-Br)2
core is stabilized by η3-coordinated lutidine-substituted cyclo-
heptatrienyl ligands to bind to each palladium center. The
palladium atom Pd1 adopts a distorted trigonal-planar
coordination sphere formed by Cg1 and the bromine atom
Br1. The center of the triangle is formed by the palladium-
bound carbon atoms C2−C4. The equivalent positions for the
atoms Pd1a, Br1a, and C2a−C4a as well as for Cg1a are
generated by symmetry operations (1 − x, y, −z). The planes
formed by the described triangles, including Pd1 and Pd1a,
respectively, form an angle of 69.83°, while the Pd2X2 core is
almost planar in the neutral complexes H and B.4
Furthermore, the relatively simple pattern of the cyclo-
heptatrienyl signals points to a fluctuation of the palladium
center over the η3-coordinated tropylium-3,5-lutidine unit. This
behavior can be observed even at temperatures as low as −90
°C. The same phenomenon has been reported for the above-
mentioned chlorocycloheptatrienyl palladium complex H.4
In the cases of 1a and 2a, the conversion of the carbene
ligand into a cycloheptatrienyl moiety takes place during the
initial reaction (Scheme 3) of the N base with the CHT
complex A and not after dissolution of the primary products 1a
and 2a in water. This is confirmed by the 13C NMR solid-state
spectra of 1a and 2a, which show signals between 72 and 79
ppm, typical for allylic compounds.10 All attempts to stimulate a
rearrangement of the pyridine-substituted cycloheptatrienyl
complexes 1a and 2a to the corresponding mononuclear CHT
complexes C−G (Scheme 1) analogous to the rearrangement
of complex H to B (Scheme 2) failed.
The observed nucleophilic attack on the CHT ring can be
explained by considering that the Fischer-type carbene atom is
electrophilic and the tropylium resonance form of the
coordinated CHT ring is favored for low-valent metal
complexes,11 since comparable reactions of free tropylium
salts with amines are reported in the literature.12−14
Sterically hindered pyridines, such as 2,6-lutidine, react with
A or B under the same conditions as described above, yielding
different greenish yellow products. These easily prepared air-
and moisture-stable compounds were found to be the
mononuclear complexes 5a and 5b (Scheme 4), which are
the first CHT analogues to the Pd−NHC PEPPSI complexes.15
1
The H NMR spectrum of 1b shows three signals for 12
protons of two cycloheptatrienyl ligands in the region from 4.6
to 6.2 ppm, while the signals of the lutidine protons appear
downfield at 8.3 and 8.7 ppm. This indicates that the centers of
the positive charge are situated at the heterocyclic substituents.
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Figure 2. ORTEP view of 5a. Thermal ellipsoids are shown at a 50%
probability level. The hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Pd1−C1 1.960(4), Pd1−N1
2.155(3), Pd1−Br1 2.4369(5), Pd1−Br2 2.4546(5), C1−C2 1.397(4);
C1−Pd1−N1 178.39(14), C1−Pd1−Br1 88.22(11), N1−Pd1−Br1
90.16(9), C1−Pd1−Br2 89.36(11), N1−Pd1−Br2 92.26(9), Br1−
Pd1−Br2 177.582(19), C5−N1−Pd1 119.84(17). Symmetry oper-
ations for the equivalent atom positions a: x, 1/2 − y, z.
Figure 1. ORTEP view of the cation of 1b in the solid state. Thermal
ellipsoids are shown at the 50% probability level. The PF6− anions and
the hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Pd1−Br1 2.5665(4), Pd1−Br1a 2.5786(4), Pd1a−
Br1 2.5786(4), Pd1−C3 2.073(3), Pd1−C4 2.166(3), Pd1−C2
2.201(3), C1−N1 1.457(4); Pd1−Br1−Pd1a 71.393(11), Br1−Pd1−
Br1a 89.200(13), C3−Pd1−C4 39.26(13), C3−Pd1−C2 38.15(13),
C4−Pd1−C2 69.20(12), C3−Pd1−Br1 134.90(9), C4−Pd1−Br1
98.29(9), C2−Pd1−Br1 163.11(9), C3−Pd1−Br1a 134.22(10), C4−
Pd1−Br1a 172.11(9), C2−Pd1−Br1a 102.96(9), C7−C1−C2
130.3(3), C7−C1−N1 116.6(3), C2−C1−N1 113.1(3), C8−N1−
C1 119.5(3), C12−N1−C1 119.0(3). Symmetry operations for the
equivalent atompositions a: 1 − x, y, −z. Cg1 defines the mass center
of the allylic system C2, C3, and C4.
employing the bulky 2,6-lutidine, a nucleophilic attack on the
ligand is sterically hindered, only allowing the coordination of
2,6-lutidine to the Pd center. Accordingly, when treating 5a
with an excess of the less bulky 3,5-lutidine, the dinuclear
complex 1a was formed.
DFT calculations (B3LYP using 6-31G(d) for C, H, and N;
LANL2DZ for Pd; 6-311(s) for Br) for the different isomers
support this observation. We found that the observed
monocarbene complex 5a is slightly more stable by Δ(ΔG) =
0.5 kcal/mol than its (non-observed) isomer with a 3,5-lutidine
ligand. On the other hand, the observed cationic fragment of
dinuclear complex 1a or 1b is Δ(ΔG) = 6.9 kcal/mol lower in
energy than its (non-observed) isomer with 2,6-lutidinium
substituents, which corresponds to the increased steric demand
of the ortho-disubstituted pyridine.
Scheme 4. Synthesis of the 2,6-Lutidine-Substituted Carbene
Complexes 5a and 5b
Reaction of [Pd(CHT)X2]2 with 1-Methylimidazole.
Another tertiary amine with a pyridine-like nitrogen donor
atom, 1-methylimidazole, revealed a more complex reactivity
toward the dinuclear complex A, depending on the reaction
conditions. When a suspension of A in THF was treated with
an excess of 1-methylimidazole at 50 °C for 18 h, a red
microcrystalline water-soluble compound 6a was obtained.
After adding NH4PF6 to the aqueous solution, a purple
precipitate 6b soluble in acetone and acetonitrile was formed
(Scheme 5(I)).
The elemental analysis of 6a corresponds to the dinuclear
formula depicted in Scheme 5. The formation of an imidazole-
substituted cycloheptatrienyl complex 6b analogous to 1b and
2b was confirmed by the 1H NMR spectrum. Multiplets at 4.46,
4.99, and 6.03 ppm were observed for the cycloheptatrienyl
protons, almost identical to the corresponding signals of 1b and
2b. Compared to the free heterocycle, the imidazole signals are
shifted downfield to 7.19, 7.44, and 8.68 ppm, as in the case of
the pyridine signals in 1b and 2b.
Contrary to 1b and 2b, 5a and 5b are neutral complexes and
insoluble in water. Recrystallization of 5a in dichloromethane
afforded single crystals suitable for X-ray diffraction analysis
(Figure 2).
The palladium atom in 5a is coordinated by two trans-
oriented bromine atoms (Br1 and Br2), one carbon atom (C1)
of the CHT ligand, and the nitrogen atom (N1) of the 2,6-
lutidine ligand, adopting a slightly distorted square-planar
geometry. The molecule contains a mirror plane that includes
the atoms Pd1, Br1, Br2, C1, and N1, so that the CHT and the
lutidine ligands exhibit a coplanar orientation.
Compound 5b is stable against further addition of 2,6-
lutidine. When adding less sterically hindered 3,5-lutidine, it
reacts to form the dinuclear complex 1a. This transformation
supports our presumption that the reaction products occur
according to the ability of the pyridine derivatives to act as
nucleophiles, due to steric reasons.
When the reaction between A and 1-methylimidazole was
carried out without an excess of imidazole, a yellow water-
1
insoluble product 7 was isolated (Scheme 5(II)). Its H NMR
The reactivity of these nitrogen-donating pyridine com-
pounds toward A indicates that they preferably attack the
carbene ligand, rather than coordinating to the metal center.
Therefore, when treating A with 3-chloropyridine or 3,5-
lutidine, the dinuclear species 1a or 1b with substituted
cycloheptatrienyl ligands are obtained. On the contrary, when
spectrum is comparable to the spectrum of the 2,6-lutidine−
CHT complex 5a and corresponds to a mononuclear CHT
complex with 1-methylimidazole coordinated to the palladium
center. Additionally, in the 13C NMR solid-state spectrum of 7,
a typical signal for metal coordinated carbene-C atoms at 223.5
ppm is observed, similar to complex 5a. Subsequent treatment
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a
Scheme 5. Reactions of 1-Methylimidazole with [Pd(CHT)Br2]2
a
(I) Nucleophilic attack at the carbene ligand. (II) Coordination to the metal center. (III) Rearrangement of the metal-coordinated complex.
of a solution of complex 7 in THF with an excess of 1-
methylimidazole at 50 °C for 18 h led to complete conversion
into the water-soluble dinuclear cycloheptatrienyl complex 6a
(Scheme 5(III)).
Single crystals of 7 suitable for X-ray diffraction analysis were
grown from a solution in dichloromethane and n-hexane. As
shown in Figure 3, the palladium atom in 7 is coordinated by
Figure 4. ORTEP view of the cation of the complex 8a. Thermal
ellipsoids are shown at a 50% probability level. Hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Pd1−Pd2 2.7767(4), Pd2−Pd3 2.7825(4), Pd1−
Pd3 2.7319(5), Pd1−Br1 2.5648(6), Pd2−Br2 2.5556(5), Pd3−Br3
Figure 3. ORTEP view of 7. Thermal ellipsoids are shown at a 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
2.5570(6), Pd1−C6 2.244(4), Pd1−C7 2.202(4), Pd1−C15 2.180(4),
Pd1−C16 2.221(4), Pd2−C1 2.537(4), Pd2−C2 2.149(4), Pd2−C3
bond lengths (Å) and angles (deg): Pd1−C1 1.9460(16), Pd1−N1
2.447(4), Pd2−C17 2.238(4), Pd2−C18 2.166(4), Pd3−C4 2.158(4),
2.1225(15), Pd1−Br1 2.4413(3), Pd1−Br2 2.4602(2), C1−C2
Pd3−C5 2.251(4), Pd3−C13 2.185(4), Pd3−C14 2.392(4), N1−C1
1.403(3), C1−C7 1.407(3); N1−Pd1−C1 177.09(7), Br1−Pd1−C1
1.442(5), N3−C12 1.439(6); Pd2−Pd1−Pd3 60.67(1), Pd1−Pd2−
86.91(5), Br2−Pd1−C1 87.44(5), Br1−Pd1−N1 93.33(4), Br2−
Pd3 58.87(1), Pd1−Pd3−Pd2 60.46(1), Pd2−Pd1−Br1 148.17(2),
Pd1−C1 87.44(5), Br2−Pd1−N1 92.74(4), Br1−Pd1−Br2 169.93(1),
Pd1−Pd3−Br3 148.11(2), Pd3−Pd2−Br2 160.85(2), C1−N1−C8
C2−C1−C7 124.59(17), Pd1−C1−C7 118.78(14), Pd1−N1−C9
126.8(4), C12−N3−C19 124.9(3).
126.78(13).
bis(cycloheptatrienyl) tripalladium complex moiety in 8a is
cationic due to the positively charged imidazolium substituents
at the cycloheptatrienyl ligands.
two trans-oriented bromine atoms (Br1 and Br2), one carbon
atom (C1) of the CHT ligand, and the nitrogen atom (N1) of
the 1-methylimidazole ligand, also adopting a slightly distorted
square-planar geometry as was the case for 5a.
The Pd3 core of complex 8a is located between two planar
cycloheptatrienyl ligands as a sandwich compound. With bond
lengths between 2.782 and 2.731 Å, the Pd−Pd contacts are in
the range of typical Pd−Pd bonds.11 The two cycloheptatrienyl
rings are positioned staggered to each other with a distance of
2.100(1) and 2.095(1) Å, respectively, to the centroid of the
central Pd3 core. The C−C bond lengths of the cyclo-
heptatrienyl ligands (1.39−1.43 Å) correspond to an electronic
delocalization in the seven-membered carbocyclic ring systems.
A 5-fold hydrogen−halogen bonding to Br3 can be observed,
including two contacts to the acidic imidazolium protons at
C19 and C8′ of a second symmetry equivalent (Figure 5I), one
contact to the imidazolium backbone proton C20″ of a third
symmetry equivalent, and two hydrogen bonds to CHT
protons of C2′ and C18″ (not shown). Further hydrogen
The formation of a different reaction product was observed
after leaving a solution of 6a in D2O at room temperature
without being protected from air. Red crystals formed within 3
days, and single-crystal X-ray diffraction analysis revealed the
cycloheptatrienyl complex 8a based on a triangular tripalladium
center (Figure 4), which is the first example of this compound
class with substituted CHT rings. Only a few examples of such
tripalladium sandwich complexes are known. As Murahashi et
al. first reported in 2006,11 these complexes only possess
unsubstituted cycloheptatrienyl ligands like complex 9 (Scheme
6). Additionally, mechanistic investigations on the formation of
such compounds have been carried out, using different metals
in the cluster.16 In contrast to all of these complexes, the
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Scheme 6. Left: Conversion of the Dinuclear Complex 6a into Tripalladium Sandwich Complexes 8a and 8b. Right: Analogous
C7H7 Complex 911
Figure 5. (I) Hydrogen bonding in complex 8a between imidazolium
protons of C19 and C8′ and the bromo ligand (Br3) of the central
Pd3Br3 moiety. (II) Top view of complex 8a showing the distortion of
the Pd3−Br3−Br2 angle from ideal 120°.
Figure 6. Solid-state structure of compound 10. Thermal ellipsoids are
shown at a 50% probability level. Selected bond lengths (Å) and angles
(deg): Br1−Pd1 2.4296(4), Br2−Pd1 2.4403(4), Br3−Pd1 2.4414(4),
Br4−Pd1 2.4453(4), N1−C1 1.340(4), N1−C11 1.477(4), N1−C8
1.484(4), O1−C10 1.426(4), O1−C9 1.435(4), C8−C9 1.507(5),
C10−C11 1.500(5), C7−C1 1.427(4), C1−C2 1.443(4), C2−C3
1.357(5), C3−C4 1.409(5), C4−C5 1.362(5), C5−C6 1.416(5), C6−
C7 1.359(5); C1−N1−C11 125.5(2), C1−N1−C8 124.2(2), C11−
N1−C8 110.3(2), C10−O1−C9 109.6(2), N1−C8−C9 109.5(3),
O1−C9−C8 112.0(3), O1−C10−C11 111.1(3), N1−C11−C10
110.7(3), N1−C1−C7 119.5(3), N1−C1−C2 118.1(3), C7−C1−C2
122.4(3), C3−C2−C1 130.2(3), C2−C3−C4 131.3(3), C5−C4−C3
126.6(3), C4−C5−C6 127.1(3), C7−C6−C5 131.5(3), C6−C7−C1
129.7(3).
bonding can be found to Br1 and Br2, but only to less acidic
CHT protons of neighboring complexes. This is accompanied
by a distortion of the Pd3−Pd2−Br2 angle (Figure 5II).
The somewhat obscure generation of a formally Pd(I)
species 8a (Scheme 6(I)) from the Pd(II) complex 6a could be
reproduced by treatment of the latter with 1 equiv of Pd0(dba)2
in acetonitrile(Scheme 6(II)). This reaction led to an almost
quantitative conversion of the dinuclear complex into the
tripalladium sandwich, which could be further transformed into
complex 8b by exchange of the anion. Both compounds 8a and
8b are well characterized by elemental analysis and 1H and 13
C
NMR. Presumably, the observed conversion of the dinuclear
complex 6a into the tripalladium sandwich 8a by air and in
water is initiated by its partial decomposition, yielding Pd0
fragments that may react with remaining molecules of 6a,
forming the Pd3 core of 8a.
tigation into whether this behavior plays a role during the
catalytic application of CHT−Pd complexes in C−N coupling
reactions is warranted. Tropylidenimmonium cations similar to
compound 10 have been generated from addition products of
secondary amines to tropylium salts by bromination or hydride
abstraction.13
Catalytic Activity. All of the compounds described in this
work are catalytically active in C−C or C−N coupling
reactions. Our ongoing investigations are focused on improving
the catalytic application of these synthesized compounds.
Exemplarily, the results of 2a in the α-arylation of
propiophenone (Scheme 8) are described in Table 1.
Reaction of [Pd(CHT)X2]2 with Morpholine. The
efficient catalysis of the Hartwig−Buchwald coupling reaction
of aryl halides with morpholine is one of the few reported
catalytic applications of CHT−palladium complexes.2 To
obtain further information about the mechanism of this
process, A was treated with morpholine using the same
conditions as those in the experiments with pyridine
derivatives. It resulted in the formation of colloidal Pd and a
brown precipitate 10, which is insoluble in water. Single crystals
of 10 suitable for X-ray diffraction analysis were grown from a
solution in dichloromethane. It revealed the formation of a salt
consisting of a square-planar tetrabromopalladium(II) anion
and a planar tropylidenimmonium cation shown in Figure 6. A
reaction pathway is suggested in Scheme 7.
As observed in the cases of 3,5-lutidine and 3-chloropyridine,
morpholine could engage in a nucleophilic attack on the CHT
ligand to form the unstable dinuclear complex 10′ with
morpholine-substituted cycloheptatrienyl ligands as an inter-
mediate. It may decompose to 10 in a still obscure reaction
sequence, while generating dihydrogen and Pd(0). Inves-
CONCLUSION
■
We were able to show that, unlike other donor molecules, such
as phosphines and acetonitrile, the reactions of N-donor
molecules, such as pyridines, 1-methylimidazole, and morpho-
line, with CHT−Pd complexes exhibit two different reaction
pathways. One of them is the attack of sterically less hindered
N donors on the CHT ligand, followed by its rearrangement to
an η3-coordinated cycloheptatrienyl system. The second
possible variant is the coordination of sterically demanding
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Scheme 7. Reaction of CHT Complex B with Morpholine with Proposed Intermediate
Scheme 8. α-Arylation of Propiophenone Catalyzed by 2a
(CPMAS), and the rotation frequency of the probe head was 15
kHz. Elemental analyses were performed at the Microanalytical
Laboratory of TU Munich.
Table 1. Catalytic Runs of α-Arylation of Propiophenone
a
b
catalysis
T [°C]
yield [%]
Compounds 1a/1b. 3,5-Lutidine (0.7 mmol, 75 mg, 70.3 μL) was
added to an orange suspension of A (0.28 mmol, 199 mg) in THF (10
mL). The reaction mixture was stirred at room temperature overnight.
Within a few minutes, a color change was observed from orange to red.
The resulting red product was filtered off, washed with THF (1 × 5
mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and dried in vacuo.
The resulting red product 1a is soluble in water and less soluble in
D2O. Yield of 1a: 179 mg, 90%. Elemental analysis (%) calcd for
C28H30Br4N2Pd2: C, 36.28; H, 3.26; N, 3.02; Pd, 22.96. Found: C,
36.06; H, 3.50; N, 2.96; Pd, 22.82. 13C solid-state NMR of 1a (75.468
MHz): δ = 146.2, 144.3, 140.3, 138.9, 134.3, 126.3, 124.9, 79.5, 79.2,
21.05, 19.3.
1
2
rt
60
74
65
a
Reaction conditions: 1.0 mmol p-bromoanisole, 1.1 mmol
propiophenone, 1.5 mmol NaOtBu, 1 mol % 2a, 1 mL of THF, 65
°C, 18 h reaction time. Monitored by GC with benzophenone as
internal standard.
b
nucleophiles to the metal center under formation of
mononuclear substituted carbene complexes. Contrary to
pyridine derivatives, 1-methylimidazole can both perform the
coordination to the metal as well as attack the CHT ligand. The
observed inclusion of Pd0 in a dinuclear cycloheptatrienyl
palladium complex opens an easy way to Pd3-sandwich
compounds with N-donor-substituted cycloheptatrienyl ligands,
granting access to an interesting class of metal complexes. The
ability of CHT ligands to transform into cycloheptatrienyl
ligands through a nucleophilic attack presumably plays an
important role in the catalytic application of CHT−Pd
complexes involving N-donor molecules as solvents, supporting
ligands, or substrate components. During the reaction of
morpholine with a CHT−Pd complex, the carbene ligand is
transformed into a tropylidenimmonium cation by a
nucleophilic attack and removed from the metal. This reaction,
which is accompanied by the formation of colloidal palladium,
might have a crucial effect on the catalytic activity of CHT−Pd
complexes in C−N coupling reactions, currently being studied
in our laboratories.
After dissolution of 1a in water and addition of a saturated NH4PF6
solution, a brown precipitation was quantitatively formed. The brown
product 1b was filtered, washed several times with H2O, dried in
vacuo, and recrystallized from acetone by cooling in a freezer. 1H
NMR of 1b (500 MHz, (CD3)2CO): δ = 8.77 (s, 4H), 8.37 (s, 2H),
3
6.22 (d, J = 7 Hz, 4H), 5.14 (m, 4H), 4.62 (m, 4H), 2.48 (s, 12H).
13C {1H} NMR of 1b (500 MHz, (CD3)2CO): δ = 148.4, 142.5, 138.8,
79.6, 70.8, 68.3, 17.6.
Compounds 2a/2b. 3-Chloropyridine (0.6 mmol, 68 mg, 78 μL)
was added to an orange suspension of A (0.24 mmol, 171 mg) in THF
(10 mL). The reaction mixture was stirred at room temperature
overnight. Within a few minutes, a color change was observed from
orange to red. The resulting red product was filtered off, washed with
THF (1 × 5 mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and
dried in vacuo. The resulting red product 2a is soluble in water and
less soluble in D2O. Yield of 2a: 150 mg, 88%. Elemental analysis (%)
calcd for C24H20Br4Cl2N2Pd2: C, 30.61; H, 2.35; N, 2.97; Pd, 22.6.
Found: C, 30.43; H, 2.22; N, 2.86; Pd, 22.6. 13C solid-state NMR of 2a
(75.468 MHz): δ = 162.7, 145.9, 138.8, 135.3, 132.3, 128.7, 125.4,
79.1, 73.9.
EXPERIMENTAL SECTION
■
After dissolution of 2a in water and addition of a saturated NH4PF6
solution, a brown precipitation was quantitatively formed. The brown
product 2b was filtered, washed several times with H2O, and dried in
Materials. All experiments were carried out under dry argon, using
standard Schlenk or glovebox techniques if it is not otherwise noted.
Dry and oxygen-free solvents were used. Solvents were dried by
standard procedures, distilled, and kept under argon over molecular
sieves.
1
vacuo. H NMR of 2b (500 MHz, CD3CN): δ = 8.94 (m, 2H), 8.49
3
(m, 2H), 8.34 (m, 2H), 7.87 (m, 2H), 6.06 (d, J = 8 Hz, 4H), 5.15
Instruments. H and 13C spectra were recorded on a Bruker DPX
1
(m, 4H), 4.65 (m, 4H).
500 and referenced to the residual solvent signals.17 13C NMR spectra
in the solid state were measured at room temperature with a Bruker
Avance 300 and a 4 mm MAS probe head, and the samples were
packed in 4 mm ZrO2. 13C solid-state NMR chemical shifts are
reported relative to external adamantane (referenced to TMS). 13C
solid-state NMR spectra were measured with cross-polarization
Compound 5a. 2,6-Lutidine (2.5 mmol, 267 mg, 268 μL) was
added to a suspension of A (1.0 mmol, 712 mg) in THF (10 mL). The
reaction mixture was stirred at room temperature overnight. The
resulting suspension was filtered off, washed with THF (1 × 5 mL),
Et2O (2 × 10 mL), and n-pentane (2 × 10 mL), and dried in vacuo.
After recrystallization from dichloromethane, 5a was obtained as
8254
dx.doi.org/10.1021/om300867g | Organometallics 2012, 31, 8249−8256

Organometallics
Article
yellow needles. Yield: 655 mg, 92%. Elemental analysis (%) calcd for
C14H15Br2NPd: C, 36.28; H, 3.26; N, 3.02; Pd, 22.96. Found: C,
ASSOCIATED CONTENT
■
S
* Supporting Information
1
35.77; H, 3.66; N, 2.78; Pd, 21.8. H NMR (500 MHz, CD2Cl2): δ =
Supplementary crystallographic data of 1b, 5a, 7, 8a, and 10 in
CIF format and experimental details of all synthesized
compounds are described in the Supporting Information.
This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic data (excluding structure
factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication Nos. CCDC-873305, 873306,
873307, 873308, and 873309. These can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax: (+44)1223-336-033; e-
mail: deposit@ccdc.cam.ac.uk).
9.89 (d, 3J = 10 Hz, 2H), 8.21 (m, 2H), 7.97 (m, 2H), 7.56 (t, 3J = 8/7
3
Hz, 1H), 7.10 (d, J = 8 Hz, 2H), 3.31 (s, 6H). 13C {1H} NMR (500
MHz, CD2Cl2): δ = 228.5 (carben C), 164.4, 159.6, 145.6, 138.0,
123.3, 27.3. 13C solid-state NMR (75.468 MHz): δ = 220.5, 163.6,
160.7, 158.3, 147.8, 139.2, 123.6, 28.2.
Compounds 6a/6b. 1-Methylimidazole (1.22 mmol, 100.1 mg,
97.8 μL) was added to an orange suspension of A (0.49 mmol, 350
mg) in THF (10 mL). The reaction mixture was stirred at a
temperature of 50 °C overnight. Within a few minutes, a color change
was observed from orange to red. The resulting red product was
filtered off, washed with THF (1 × 5 mL), Et2O (1 × 5 mL), and n-
pentane (1 × 5 mL), and dried in vacuo. The resulting red compound
6a is soluble in water and less soluble in D2O. Yield of 6a: 386 mg,
90%. Elemental analysis (%) calcd for C22H24Pd2Br4N4: C, 30.13; H,
2.76; N, 6.39; Pd, 24.27. Found: C, 30.63; H, 2.76; N, 6.23; Pd, 25.46.
13C solid-state NMR of 6a (75.468 MHz): δ = 137.89, 132.88, 126.21,
120.57, 116.96, 91.27, 78.80, 74.82, 39.17.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +49 89 289 13081. Fax: +49 89 289 13473. E-mail: fritz.
kuehn@ch.tum.de.
After dissolution of 6a in water and addition of a saturated NH4PF6
solution, a brown precipitation was quantitatively formed. The brown
product 6b was filtered, washed several times with H2O, and dried in
vacuo. 1H NMR of 6b (500 MHz, CD3CN): δ = 8.68 (s, 2H), 7.44 (m,
Notes
The authors declare no competing financial interest.
3
2H), 7.19 (m, 2H), 6.03 (d, J = 8 Hz, 4H), 4.99 (m, 4H), 4.46 (s,
4H), 3.73 (s, 6H). 13C {1H} NMR of 6b (500 MHz, CD3CN): δ =
134.21, 124.39, 123.70, 96.30, 77.93, 71.10, 68.58, 36.99.
ACKNOWLEDGMENTS
■
The authors are grateful to Dr. Gabriele Raudaschl-Sieber for
the measurement of solid-state NMR and Dr. Markus Drees for
performing the DFT calculations. K.M.-O. thanks the TUM
Compound 7. 1-Methylimidazole (1.14 mmol, 93.6 mg, 90.8 μL)
was added to a suspension of A (0.57 mmol, 409 mg) in THF (10
mL). The reaction mixture was stirred at 50 °C overnight. The
resulting dark yellow suspension was filtered off, washed with THF (1
× 5 mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and dried in
vacuo. Yield: 204 mg, 82%. Elemental analysis (%) calcd for
C22H24Pd2Br4N4: C, 30.13; H, 2.76; N, 6.39; Pd, 24.27. Found: C,
̈
Graduate School for financial support.
REFERENCES
■
̈
(1) Ofele, K.; Tosh, E.; Taubmann, C.; Herrmann, W. A. Chem. Rev.
1
30.31; H, 2.98; N, 6.40; Pd, 23.2. H NMR (500 MHz, CD2Cl2): δ =
2009, 109, 3408.
3
̈
9.80 (d, J = 11 Hz, 2H), 8.17 (m, 2H), 8.06 (s, 1H), 7.90 (m, 2H),
(2) Taubmann, C.; Tosh, E.; Ofele, K.; Herdtweck, E.; Herrmann, W.
7.49 (m, 1H), 6.84 (m, 1H), 3.68 (s, 3H). 13C {1H} NMR (500 MHz,
CD2Cl2): δ = 227.0 (carben C), 164.0, 159.6, 145.5, 140.6, 137.8,
130.6, 120.4, 31.1. 13C solid-state NMR (75.468 MHz): δ = 223.5
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acetonitrile, and 0.340 mmol of Pd(dba)2 was added to the red
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25.70; H, 2.38; N, 4.91; Pd, 31.60. H NMR (500 MHz, D2O): δ =
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̈
8.43 (s, 2H), 7.34 (m, 4H), 6.04 (m, 4H), 5.04 (m, 4H), 4.66 (m, 4H),
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The precipitation of 8a was quantitatively carried out with NH4PF6
in water. 1H NMR of 8b (500 MHz, CD3CN): δ = 8.68 (s, 2H), 7.44
(m, 2H), 7.19 (m, 2H), 6.04 (d, 3J = 8 Hz, 4H), 4.99 (m, 4H), 4.47 (s,
4H), 3.73 (s, 6H). Elemental analysis (%) calcd for
C22H24Pd3Br3N4PF6: C, 25.20; H, 2.31; N, 5.34; Pd, 30.45. Found:
C, 24.90; H, 2.31; N, 5.01; Pd, 29.18.
Compound 10. Morpholine (0.263 mmol, 22.8 mg, 22.8 μL) was
added to a suspension of A (0.105 mmol, 75.3 mg) in THF (10 mL).
The reaction mixture was stirred at room temperature overnight. A
brown product was filtered off, washed with THF (1 × 5 mL), Et2O (2
× 10 mL), and (2 × 10 mL) n-pentane, and dried in vacuo. The
product 10 could be recrystallized from dichloromethane by addition
of diethyl ether. Yield: 0.15 g, 92%. Elemental analysis (%) calcd for
C22H28Br4N2O2Pd: C, 33.94; H, 3.63; N, 3.60; Pd, 13.67. Found: C,
32.14; H, 4.13; N, 3.54; Pd, 13.5.
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