Phosphino imidazoles and imidazolium salts for Suzuki C-C coupling reactions

Article abstract of DOI:10.1039/c2dt12322c

The consecutive syntheses of imidazoles 1-(4-X-C₆H ₄)-4,5-R₂-^cC₃HN₂ (3a, X = Br, R = H; 3b, X = I, R = Me; 3c, X = H, R = Me; 5, X = Fc, R = H; 7, X = CCFc, R = H; 9, X = C₆H

Full text of DOI:10.1039/c2dt12322c

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PAPER
Phosphino imidazoles and imidazolium salts for Suzuki C–C coupling
reactions†
Bianca Milde, Dieter Schaarschmidt, Tobias Rüffer and Heinrich Lang*
Received 2nd December 2011, Accepted 27th February 2012
DOI: 10.1039/c2dt12322c
The consecutive syntheses of imidazoles 1-(4-X-C6H4)-4,5-R2-cC3HN2 (3a, X = Br, R = H; 3b, X = I,
R = Me; 3c, X = H, R = Me; 5, X = Fc, R = H; 7, X = CuCFc, R = H; 9, X = C6H5, R = Me; Fc =
Fe(η5-C5H4)(η5-C5H5)), phosphino imidazoles 1-(4-X-C6H4)-2-PR′2-4,5-R2-cC3N2 (11a–k; X = Br, I, Fc,
FcCuC, Ph; R = H, Me; R′ = Ph, cC6H11, cC4H3O), imidazolium salts [1-(4-X-C6H4)-3-R′′-4,5-
R2-cC3HN2]I (16a; X = Br, R = H, R′′ = n-Bu; 16b, X = Br, R = H, R′′ = n-C8H17; 16c, X = I, R = Me,
R′′ = n-C8H17, 16d, X = H, R = Me, R′′ = n-C8H17) and phosphino imidazolium salts [1-C6H5-2-PR′2-3-
n-C8H17-4,5-Me2-cC3N2]PF6 (17a, R′ = C6H5; 17b, R′ = cC6H11) or [1-(4-P(C6H5)2-C6H4)-3-n-C8H17-
4,5-Me2-cC3HN2]PF6, (20) and their selenium derivatives 1-(4-X-C6H4)-2-P(vSe)R′2-4,5-R2-cC3N2
(11a-Se–f-Se; X = Br, I; R = H, Me; R′ = C6H5, cC6H11, cC4H3O) are reported. The structures of 11a-Se
and [(1-(4-Br-C6H4)-cC3H2N2-3-n-Bu)2PdI2] (19) in the solid state were determined. Cyclovoltammetric
measurements were performed with the ferrocenyl-containing molecules 5 and 7 showing reversible redox
events at E0 = 0.108 V (ΔEp = 0.114 V) (5) and E0 = 0.183 V (ΔEp = 0.102 V) (7) indicating that 7 is
more difficult to oxidise. Imidazole oxidation does not occur up to 1.3 V in dichloromethane using
[(n-Bu)4N][B(C6F5)4] as supporting electrolyte, whereas an irreversible reduction is observed between
−1.2–−1.5 V. The phosphino imidazoles 11a–k and the imidazolium salts 17a,b and 20, respectively,
were applied in the Suzuki C–C cross-coupling of 2-bromo toluene with phenylboronic acid applying
[Pd(OAc)2] as palladium source. Depending on the electronic character of 11a–k, 17a,b and 20 the
catalytic performance of the in situ generated catalytic active species can be predicted. As assumed, more
electron-rich phosphines with their higher donor capability show higher activity and productivity.
Additionally, 11e was applied in the coupling of 4-chloro toluene with phenylboronic acid showing an
excellent catalytic performance when compared to catalysts used by Fu, Beller and Buchwald.
Furthermore, 11e is eligible for the synthesis of sterically hindered biaryls under mild reaction conditions.
C–C Coupling reactions with the phosphino imidazolium salts 17b and 20 in ionic liquids [BMIM][PF6]
and [BDMIM][BF4] were performed, showing less activity than in common organic solvents.
example, as model systems imitating the catalytically active site
Introduction
in bio-inorganic molecules.1c,2 In addition, they can be used as
Imidazole- and imidazolium-functionalised phosphines represent
an interesting family of molecules because they can act as
ambivalent P,N donors which allow the coordination of either
hard or soft transition metals.1 These species can be applied, for
easily tuneable ligands with good performance in palladium-cat-
alysed C–C coupling reactions.1a,d,3 Imidazolium phosphines
contain structural elements of an ionic liquid and have been
obtained by selective N-protonation or N-alkylation of the appro-
priate neutral molecules. Their application in 2-phase catalytic
reactions is advantageous since they allow facile recycling and
easy separation of the catalyst from the products.1b,d,e,3a,c,4,6
During the last years, this field of chemistry developed rapidly,
however, only little is known about aryl-substituted imidazo-
les2b,d,3b,e,5a,e and imidazolium salts,3a,d,5 which is attributed to
their elaborate multistep preparation procedures. Compared to
classical monodentate ligands such as phosphines, phosphino
imidazoles and imidazolium salts are advantageous due to their
flexible coordination behavior and their ability to reduce metal
leaching in homogeneous multiphase catalysis.1e,6 Recently, it
was reported that, for example, dialkylphosphino imidazoles can
Chemnitz University of Technology, Faculty of Science, Institute of
Chemistry, Department of Inorganic Chemistry, Straße der Nationen
6209111 Chemnitz, Germany. E-mail: heinrich.lang@chemie.tu-chemnitz.de;
Fax: +49 (0)371-531-21210; Tel: +49 (0)371-531-21210
†Electronic supplementary information (ESI) available: Details of the
synthesis and experimental data of 11a–k, 11a-Se–f-Se and 16a–d are
given in ESI-Experimental section. Reaction profiles for the Suzuki
cross-couplings in the presence of phosphines 11, 17, 20 and
[Pd(OAc)2] are given in ESI-General. In the file ESI–X-ray the restraints
used for the refinement of cis-19 are given, together with an indication
why they were needed. CCDC 11a-Se (848041), trans-19 (848043) and
cis-19 (848042). For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt12322c
This journal is © The Royal Society of Chemistry 2012
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be used in the Sonogashira3e and Suzuki3b C–C as well as Buch-
wald–Hartwig3b C–N coupling.
alkyl iodides R′′I (R′′ = n-Bu, n-C8H17) did result in the for-
mation of a mixture of compounds 13–15, from which desired
13 could not be isolated in pure form, neither by chromatography
nor crystallisation (Scheme 2, Experimental section).
This prompted us to develop straightforward consecutive syn-
thesis methodologies for the preparation of organic- and organo-
metallic-functionalised aryl phosphino imidazoles which were
converted into the appropriate imidazolium salts to study the
electronic influence of the N-heterocyclic component on the
phosphino group. The use of appropriate phosphines in carbon–
carbon cross-couplings is reported both in organic solvents and
ionic liquids.
Since no separation of molecules 13–15 from each other was
possible, another synthesis procedure had to be developed. The
crucial step was the quaternisation of 3a and 3b with 12 yielding
the imidazolium salts [1-(4-X-C6H4)-3-R′′-4,5-R2-cC3HN2]I
(16a; X = Br, R = H, R′′ = n-Bu; 16b, X = Br, R = H, R′′ =
n-C8H17; 16c, X = I, R = Me, R′′ = n-C8H17; 16d, X = H, R =
Me, R′′ = n-C8H17) (Scheme 3). Decreasing the melting point of
the imidazolium salts by longer alkyl chains,6d results in the for-
mation of ionic liquids. For this reason, we chose 16c as a start-
ing compound for the synthesis of ionic phosphines. Therefore,
16c was deprotonated with n-BuLi followed by the reaction of
16c·Li with PR′2Cl to give after anion exchange with K[PF6] a
mixture of [1-(4-I-C6H4)-2-PR′2-3-R′′-4,5-Me2-cC3N2]PF6 (13a:
R′ = C6H5, R′′ = n-C8H17; 13b: R′ = cC6H11, R′′ = n-C8H17) and
[1-(C6H5)-2-PR′2-3-R′′-4,5-Me2-cC3N2]PF6 (17a: R′ = C6H5,
Results and discussion
Synthesis
As a starting molecule 1-(4-X-C6H4)-4,5-R2-cC3HN2 (3a, X =
Br, R = H; 3b, X = I, R = Me, 3c,19 X = H, R = Me) was chosen
which was synthesised by a modified reaction protocol as
described by Zhang and co-workers2d (Scheme 1). These species
can be further functionalised by applying C–C cross-couplings
including the palladium-promoted Negishi, Sonogashira and
Suzuki reactions. Compound 1-(4-Fc-C6H4)-cC3H3N2 (5) (Fc =
Fe(η5-C5H4)(η5-C5H5)) was accessible by Negishi ferrocenyla-
tion of the appropriate bromo-substituted imidazole 3a with
FcZnCl (4), available by mono-lithiation of ferrocene according
to Mueller-Westerhoff7 followed by treatment with dry
[ZnCl2(thf)2], in presence of catalytic amounts of [Pd(PPh3)4]
(Scheme 1). Imidazole 1-(4-FcCuC-C6H4)-cC3H3N2 (7) was
obtained by the reaction of 3a with ethynyl ferrocene (6) in pres-
ence of catalytic amounts of [CuI] and [PdCl2(PPh3)2] in NEt3
under Sonogashira cross-coupling conditions. Suzuki C–C coup-
ling of 3b with phenylboronic acid, potassium carbonate as base
and [Pd(dppf)Cl2] (dppf = 1,1′-bis(diphenylphosphino) ferro-
cene) as catalyst afforded 1-(4-C6H5–C6H4)-4,5-Me2-cC3HN2 (9)
(Scheme 1, Experimental section).
c
R′′ = n-C8H17; 17b: R′ = C6H11, R′′ = n-C8H17), respectively
(Scheme 3). However, these mixtures could not be separated like
13–15 (vide supra). Nevertheless, a straightforward synthesis
procedure to produce pure 17a and 17b starts from halide-free 1-
(C6H5)-4,5-Me2-cC3HN2 (3c) applying the same reaction
sequence as just described (Scheme 3). Quaternisation of 3c with
12b gave 16d, which on subsequent reaction with LDA, PR′2Cl
and K[PF6] produced 17a and 17b (Scheme 3). After appropriate
work-up, 17a and 17b could be isolated as colourless solids in
an overall yield of 66 and 71%, respectively (Experimental
section).
On the other hand, a diphenyl phosphino group could be suc-
cessfully introduced at the phenyl imidazolium building block
using a synthesis protocol firstly described by Stelzer et al.8 as
demonstrated in reaction (1). After appropriate work-up, the cor-
responding imidazolium salt [1-(4-PPh2-C6H4)-3-n-C8H17-4,5-
Me2-cC3HN2]PF6 (20) could be isolated as a colourless solid in
63% yield (Experimental section).
Imidazoles 3, 5, 7 and 9 could be converted to the appropriate
phosphino imidazoles 1-(4-X-C6H4)-2-PR′2-4,5-R2-cC3N2 (11a–
c
To verify the σ-donor properties of the newly synthesized
phosphines 11a–f we converted them into the appropriate seleno
phosphines 11a-Se–f-Se by addition of a 2-fold excess of
elemental selenium in toluene at 100 °C (reaction (2)). In
general, the σ-donor ability of phosphines towards selenium
k; X = Br; I, Fc, FcCuC, Ph; R = H, Me; R′ = C6H5, C6H11,
cC4H3O) (Scheme 2, Table 1) by the consecutive reaction of
either lithium di-i-propylamide or n-butyl lithium followed by
addition of phosphines PR′2Cl (10) in tetrahydrofuran (LDA) or
diethyl ether solutions (n-BuLi). The reaction of 11a–k with
Scheme 1 Synthesis of imidazoles 3a–c, 5, 7 and 9 (Fc = (η5-C5H4)(η5-C5H5), dppf = 1,1′-bis(diphenylphosphino)ferrocene).
5378 | Dalton Trans., 2012, 41, 5377–5390 This journal is © The Royal Society of Chemistry 2012

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Scheme 2 Synthesis of phosphino imidazoles and imidazolium salts 11a–k and 13–15.
ð1Þ
acceptors can be quantified by the phosphorus–selenium coup-
Table 1 Synthesis of phosphino imidazoles 11a–k
ling constant as indicated by 31P{1H} NMR spectroscopy.9 Elec-
Yielda [%]
1
Compd.
X
R
R′
tron-withdrawing groups increase JPSe, due to the increased s
character of the phosphorus orbital involved in the PvSe
bonding. This results in shorter bond distances between the
phosphorus and the acceptor carbon atoms, which directly
affects the steric demand around the P atom resulting in marked
changes in the behaviour of the respective transition metal
complexes.
11a
11b
11c
11d
11e
11f
11g
11h
11i
Br
Br
Br
I
H
H
H
Me
Me
Me
H
C6H5
60
49
52
65
55
55
63
44
67
61
72
cC6H11
cC4H3O
C6H5
I
cC6H11
cC4H3O
C6H5
I
Fcb
FcCuCb
C6H5
C6H5
C6H5
H
C6H5
Me
Me
Me
C6H5
11j
11k
cC6H11
cC4H3O
a Based on 3a and 3b, respectively. b Fc = Fe(η5-C5H4)(η5-C5H5).
ð2Þ
phosphino imidazolium salts are hygroscopic. Therefore, rapid
work-up and storage under argon are mandatory.
The chemistry of N-heterocyclic carbenes developed rapidly
during the last years due to their outstanding potential as sup-
porting ligands in homogeneous catalysis.10 For this reason, we
did react the imidazolium salts 16a–c with [PdCl2(cod)] (cod =
cycloocta-1,5-diene) as palladium source and KOt-Bu as depro-
tonating agent (reaction (3)). Although the reactants were readily
consumed only in the case of 16a we succeeded in isolating a
pure palladium carbene complex. By halide exchange the mono-
nuclear palladium carbene complex 19 could be isolated in
minor yield (Experimental section). The obtained few crystals
were characterised by X-ray diffraction studies indicating that 19
was formed both in the cis and trans configuration (Fig. 2). Due
to the very low yield of 19 no further analytical characterisations
could be performed.
Characterisation
The identities of all newly synthesised imidazoles 3b, 5, 7 and
9, phosphino imidazoles 11a–k and 11a-Se–f-Se and imidazo-
lium salts 16a–d, 17a, 17b and 20 have been confirmed by
elemental analysis, IR and NMR spectroscopy (1H, 13C{1H},
31P{1H}) as well as high-resolution mass spectrometry (Exper-
imental section). Cyclovoltammetric measurements of 5, 7 and
the halide derivatives 3a and 3b were additionally carried out.
The structures of 11a-Se and 19 in the solid state were deter-
mined by single crystal X-ray structure analysis.
It is important to note that all compounds are sensitive
towards light and quickly turn yellow. Furthermore, the phos-
phine-carrying imidazoles are sensitive towards air and the
The IR spectra of the newly prepared molecules are less
expressive with exception of the imidazole and imidazolium
CvN and CvC vibrations (Experimental section). The
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ð3Þ
Scheme 3 Synthesis of 13, 16 and 17.
introduction of phosphino groups resulted in the visibility of
characteristic P–C stretching frequencies and for the appropriate
seleno derivatives additional ν˜PvSe absorptions are observed.
Apparently there is nearly no influence on the CuC vibration
when the imidazole ring is functionalised by a PPh2 group as
given in 7 (ν˜CuC = 2206 cm−1) and 11h (ν˜CuC = 2204 cm−1),
respectively.
the heterocyclic C-2 carbon atom at ca. 135 ppm (3, 16) which
is shifted to 140–147 ppm in the phosphino-containing species
11 and 17. Conspicuous in 11a, 11c, 11d and 11f and the corre-
sponding selenium derivatives 11a-Se–e-Se is that C-5 shows a
3JCP coupling, while C-4 appears as a singlet. All other mol-
ecules of 11 solely show a singlet. Also very characteristic is the
meta-carbon C6H4 atom in 11a–k, 17 and 20 emerging as a
doublet with a 4JCP coupling of 3–6 Hz at ca. 130 ppm.
Contrastingly, the NMR spectra of the respective imidazole
and imidazolium species are in some extend more meaningful. A
striking feature in the 1H NMR spectra is the shift of the cC3HN2
core proton in position 2 to a lower field, when changing from
the imidazoles to the appropriate imidazolium salts, i.e. 3b,
7.45 ppm and 16c, 9.82 ppm (Experimental section). For the
phenylene C6H4 units, as expected, a characteristic AA′XX′
coupling pattern with 3JHH = 8.7 Hz is found, which splits into a
more complex pattern, when a phosphorus atom is attached to
the heterocyclic core as given in 11a–k and 11a-Se–f-Se (Exper-
imental section). For the ferrocenyl-containing molecules 5, 7
and 11g and 11h a singlet as well as two pseudo-triplets are
observed between 4.0 (C5H5) and 4.7 ppm (C5H4), respectively,
resulting from the cyclopentadienyl ring protons. For the two
methyl groups in 3b, 9, 11d–f, 11d-Se–f-Se, 16c, 16d, 17a, 17b
and 20 two individual singlets are characteristic due to the asym-
metry of the cC3N2 core unit.
31P{1H} NMR spectroscopy can be used to monitor the pro-
gress of the reaction of lithiated 3, 5, 7, 9 and 16 with the
respective chloro phosphines PR′2Cl (10a–c) since a singlet of
the phosphorus atom appears between −73–5 ppm depending on
the nature of the organic groups R′ (Experimental section). In
addition, the donor properties of the phosphino imidazoles 11a–f
1
can be quantified by the coupling constant JPSe of the appropri-
ate seleno phosphines 11a-Se–f-Se. An electron-withdrawing
1
group at P increases JPSe as a result of the increased s character
of the phosphorus orbital involved in the phosphorus–selenium
bonding. As a consequence thereof, a shorter bond length
between the phosphorus and the acceptor R′ units is observed.9
This electronic impact has direct influence on the P donor ability.
1
The 31P{1H} NMR data together with the JPSe coupling con-
stant for the seleno phosphines 11a-Se–f-Se are summarised in
Table 2.
The interpreting of the 13C{1H} NMR spectra of all imida-
zoles and imidazolium salts is somewhat more complex due to
the presence of ipso-carbon atoms. However, when phosphino
From Table 2 it can be seen that the seleno phosphines 11a-
Se–c-Se show higher coupling constants than the respective 11d-
Se–f-Se derivatives featuring methyl groups in positions 4 and 5
indicating that the latter three molecules are better σ-donors. The
most electron-donating systems are the aliphatic cyclohexyl
c
groups are attached either to the C3N2 or C6H4 unit the spectra
became more simplified, since characteristic P–C couplings
appear (Experimental section). Very distinctive is the signal of
1
seleno phosphines 11b-Se and 11e-Se with JPSe of 718 and
5380 | Dalton Trans., 2012, 41, 5377–5390
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Table 2 Chemical shifts and 1JPSe values of 11a-Se–f-Se
Compd.
δ/ppm
1JPSe/Hz
Compd.
δ/ppm
1JPSe/Hz
11a-Se
11b-Se
11c-Se
19.1
41.6
−20.7
753
725
790
11d-Se
11e-Se
11f-Se
17.6
41.2
−21.4
745
718
781
Table 3 Cyclovoltammetric data (potentials vs. FcH/FcH+), scan rate
100 mV s−1 at a glassy-carbon electrode of 1.0 mmol L−1 solutions
of 3a, 3b, 5, and 7 in dry dichloromethane containing 0.1 mol L−1 of
[(n-Bu)4N][B(C6F5)4] as supporting electrolyte at 25 °C
Compd.
Ered–irrev/V
Compd.
E0 (ΔEp)/V
Ered–irrev/V
Fig. 1 ORTEP diagram (50% probability level) of the molecular struc-
ture of 11a-Se with the atom numbering scheme. (Hydrogen atoms are
omitted for clarity.) Standard uncertainties of the last significant digit(s)
are shown in parentheses. Selected bond distances (Å) and angles (°):
P1–Se1 = 2.1032(6), P1–C1 = 1.813(2), P1–C10 = 1.813(2), P1–C16 =
1.820(2), N1–C1 = 1.373(3), N1–C3 = 1.371(3), N2–C1 = 1.321(3),
N2–C2 = 1.372(3), C2–C3 = 1.368(3); C1–P1–Se1 = 113.56(8), C10–
P1–Se1 = 113.18(8), C16–P1–Se1 = 114.43(8), C1–P1–C10 = 103.02
(10), C1–P1–C16 = 105.16(11), C16–P1–C10 = 106.48(11).
3a
3b
−1.22
−1.23
5
7
0.108 (0.114)
0.183 (0.102)
−1.48
−1.52
E0 = redox potential, ΔEp = difference between oxidation and reduction
potential, Ered–irrev = irreversible reduction potential.
725 Hz, respectively (Table 2). In summary, the σ-donor ability
decreases in the order 11e > 11b > 11d > 11a > 11f > 11c. The
data summarised in Table 2 are indicative of classifying the suit-
ability of the phosphino imidazoles as suitable ligands in palla-
dium-promoted Suzuki C–C cross-couplings (vide infra).
Complexes 5 and 7 contain a redox-active ferrocenyl group
and hence they have been subjected to cyclovoltammetric
measurements in dry dichloromethane solutions utilising [(n-
Bu)4N][B(C6F5)4] as supporting electrolyte.11 The cyclovoltam-
metric studies were carried out at scan rates of 100 mV s−1. All
potentials are referenced to the FcH/FcH+ redox couple as
internal standard as recommended by IUPAC.12 The cyclovol-
tammetric data are summarised in Table 3 and the cyclovoltam-
mograms (= CV) of 5 and 7 are shown in Fig. S1 in the
ESI-General.†
One ferrocenyl-related oxidation half-reaction in the anodic
CV sweep and reduction half-reaction in the cathodic CV sweep
could be observed. The ferrocenyl substituents showed reversible
electrochemical behaviour (ΔEp = 0.114 (5) and 0.102 V (7)),
while for the imidazole unit one irreversible reduction event is
observed at −1.48 V (5) and −1.52 V (7), respectively. From
Table 3 it is obvious that molecule 7 with its alkynyl unit is
more difficult to oxidise (5, E0 = 0.108 V; 7, E0 = 0.183 V). The
imidazole starting compounds 3a and 3b show only irreversible
reduction events at −1.22 (3a) and −1.23 V (3b) for the hetero-
cyclic core (ESI-General, Fig. S1,† Table 3).13 This indicates
that, as expected, organometallics 5 and 7 are more electron rich
due to the electron donating ferrocenyl groups present.
Seleno phosphine 11a-Se crystallises in the monoclinic space
group P21/n. The molecular structure of 11a-Se is set-up by two
phenyl groups, the imidazole unit and one selenium atom which
are bound to the phosphorus atom resulting in a tetrahedrally dis-
torted geometry (Fig. 1). The phosphorus–carbon bond separ-
ations with 1.813(2) (P1–C1/P1–C10) and 1.820(2) Å (P1–C16)
are representative of P–Caryl bond distances.9c,g,h Very character-
istic is the P–Se bond of 2.1032(6) Å, which is similar to other
seleno phosphines, i.e. 1-tert-butyl-2-(diphenylphosphino sele-
nide)-1H-imidzole1c and triphenyl seleno phosphine.14 The
C–P–C angles at P1 (Fig. 1) are from 103.02(10)–106.48(11)° in
the typical range of tertiary seleno phosphines.1c,9c,g,h,14 The imi-
dazole core entity with atoms N1, N2 and C1–C3 is planar
(r.m.s. deviation 0.0006 Å, highest deviation from planarity
observed for C3 with 0.0010 Å). The C6H4Br moiety of the imi-
dazole unit is rotated by 58.1° with respect to the imidazole core
entity.
As outlined earlier, from the palladium complex 19 single
crystals could be separated in a very low yield by slow evapor-
ation of a chloroform solution below 0 °C. Complex trans-19
Table 4 Reaction of 2-bromo toluene with phenylboronic acid to give
2-methyl biphenyla
Entry Compd. Conversion/% Entry Compd. Conversion/%
1
2
3
4
5
6
7
11a
11b
11c
11d
11e
11f
7
80
0
20
100
5
8
9
10
11
12
13
14
11h
11i
11j
11k
17a
17b
20
60
24
100
5
55
97
66
An advantage of the seleno phosphines is their tendency to
crystallise easier than the appropriate phosphino imidazoles or
imidazolium salts. Exemplary, the molecular structure of 11a-Se
in the solid state was established by single crystal X-ray structure
analysis. Suitable crystals were obtained by slow evaporation
of a saturated dichloromethane solution containing 11a-Se at
ambient temperature. The ORTEP diagram, selected bond dis-
tances (Å) and angles (°) are shown in Fig. 1. The crystal and
structure refinement data are presented in the Experimental
section.
11g
32
a Reaction conditions:16 2-bromo toluene (1.0 eq), phenylboronic
acid (1.5 eq), K2CO3 (3.0 eq), 1,4-dioxane–water (ratio 2.1, v/v) (10 mL
mmol–1), [Pd(OAc)2]/phosphine (0.25 mol% [Pd], 0.5 mol% phosphine,
100 °C, 1 h. The reaction times were not minimized.
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Catalysis
crystallises in the form of pale yellow plates, whereas cis-19
crystallises as colourless needles. For trans-19 the structure
determination by X-ray crystallography proceeds unproblemati-
cally with the use of Mo Kα radiation. In case of cis-19 the
needles were extremely tiny and Cu Kα radiation had to be used
with comparatively long exposure times. Due to this, an “icing”
of the crystal occurred at higher diffraction angles and the data
set obtained so far did not reach full completeness (Table 7,
Experimental section). As no further suitable crystals of cis-19
could be selected the incomplete data set has been used for
refining. Thus, Fig. 2 displays the molecular structure of cis-19
and selected bond ranges, without further discussion (Exper-
imental section). Fig. 2 presents further the molecular structure
of trans-19 (right). Important bond distances (Å) and bond
angles (°) for trans-19 are summarised in the caption of this
figure. Crystal and structure refinement data are presented in the
Experimental section.
The application of phosphino imidazoles 11a–k and phosphino
imidazolium salts 17 and 20 in the homogeneous palladium-cat-
alysed Suzuki C–C cross-coupling of 2-bromo toluene with phe-
nylboronic acid was studied as model systems. The catalytic
active species was in situ generated by applying mixtures of
[Pd(OAc)2] and the respective phosphines 11a–k, 17 and 20 in
the ratio of 1 : 2 (reaction (4)). The catalytic reactions were
carried out in 1,4-dioxane–water mixtures of ratio 2 : 1 (v:v) in
presence of potassium carbonate at 100 °C using 0.5 mol% of
the appropriate phosphine and 0.25 mol% of the palladium
source. Acetyl ferrocene as standard was added to the appropri-
ate reaction solution to determine the rate of conversion using 1H
NMR spectroscopy.16 The obtained conversions equal H NMR
spectroscopic yields and are based on the respective aryl halides.
1
Both isomers of complex 19 crystallise in the triclinic space
ˉ
group P1. The Pd1 atom of trans-19 possesses a planar PdC2I2
coordination set-up (r.m.s. deviation 0.0169 Å, highest deviation
from planarity observed for I2 with −0.0213 Å) with trans-
oriented I1–Pd1–I2 and C1–Pd1–C14 angles of 178.963(13) and
178.49(12)°, respectively (Fig. 2). As expected, the imidazole
units show planarity (r.m.s. deviation 0.0025 and 0.0020 Å) and
are twisted to the C6H4Br moieties with angles of 47.1 and
47.3°, respectively. The main structural feature of the imidazole
ligands is the slightly decreased N1–C1–N2 angle (104.8(3)°)
which is attributed to the extent of electronic delocalisation. A
similar behaviour was found for other palladium imidazole
species.15 The Pd–I, Pd–Csp2 and the bond distances of the imi-
dazole and phenyl units correspond to the bond lengths of the
related molecules, i.e. [(1,3-Me2-cC3H2N2)2PdI2].15a The for-
mation of both the kinetic and thermodynamic product might be
attributed to the isolation method of crystals of 19 (Experimental
section).
ð4Þ
As can be seen from Table 4 and Fig. S2–S5 (ESI-General†)
all in situ generated palladium phosphines are catalytically
active, of which the cyclohexyl-substituted phosphines 11b, 11e,
11j and 17b are significantly more productive and active than the
others (Table 4; entries 2, 5, 9 and 13), which originates from
the electron-richness and bulkiness of these compounds. Com-
paring the alkyl- and aryl-functionalised phosphines among each
other, the following trend concerning the activity and pro-
ductivity of their palladium complexes can be seen: 11b > 11a >
11c > and 11e > 11d > 11f. In general, the dimethyl functiona-
lised phosphino imidazole systems (Table 4; entries 4–6), which
are also more easily and in better yields accessible (vide supra),
are by far more productive than the non-substituted derivatives
Fig. 2 ORTEP diagram (50% probability level) of the molecular structure of cis-19 (left), showing only one molecule, and trans-19 (right) with the
atom-numbering scheme. For cis-19 and trans-19 all hydrogen atoms and one molecule of the packing solvent chloroform (cis-19) have been omitted
for clarity. Selected bond distances (Å), angles (°) and torsion angles (°) for trans-19: Pd1–I1 = 2.6152(4), Pd1–I2 = 2.6092(4), Pd1–C1 = 2.031(3),
Pd1–C14 = 2.032(3), N1–C1 = 1.341(4), N1–C3 = 1.390(4), N1–C4 = 1.463(4), N2–C1 = 1.367(4), N2–C2 = 1.395(4), N2–C8 = 1.422(4), N3–C14
= 1.335(4), N3–C15 = 1.393(4), N3–C18 = 1.463(4), N4–C14 = 1.376(4), N4–C16 = 1.386(4), N4–C22 = 1.422(4), C2–C3 = 1.343(5), C15–C16 =
1.341(5); C1–Pd1–C14 = 178.49(12), I1–Pd1–I2 = 178.963(13), I1–Pd1–C1 = 89.48(9), I1–Pd–C14 = 91.66(9), I2–Pd1–C1 = 90.34(9), I2–Pd1–C14
= 88.54(9), N1–C1–N2 = 104.8(3), N3–C14–N4 = 104.8(3). Range of bond distances for cis-19: Pd–I = 2.601(4)–2.68(4), Pd–Csp2 = 1.91(2)–
2.07(2).
5382 | Dalton Trans., 2012, 41, 5377–5390
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Table 5 Suzuki coupling of 4-chloro toluene and sterically hindered aryl bromides
Entry
1
Aryl halide
Ligand
Catalyst/mol%
0.01
Conversion/%
28a
TON
2800
11d
2
3
11e
0.01
0.01
88a
8800
8600
86a,b
4
5
0.01
0.05
15–66a,b
1500–6600
99c,d
1860
6
7
8
9
0.01
0.01
0.01
0.05
47c,d
92c,d
94c,d
100e
4700
9200
9400
2000
P(t-Bu)3
P(n-Bu)Ad2
11e
10
11
11e
11e
0.05
0.05
43e
860
100e
2000
12
13
11e
0.05
0.01
76e
1520
9700
97e,f
a Reaction conditions:3b 4-chloro toluene (3.0 mmol, 1.0 eq), phenylboronic acid (4.5 mmol, 1.5 eq), K3PO4 (6.0 mmol, 3.0 eq), toluene (6 mL),
[Pd(OAc)2]/phosphine (0.01 mol% [Pd], 0.1 mol% phosphine, 100 °C, 20 h. b Ligand from ref. 3b. c Ligands from ref. 18b. d Reaction conditions:18b
4-chloro toluene (3.0 mmol, 1.0 eq), phenylboronic acid (4.5 mmol, 1.5 eq), K3PO4 (6.0 mmol, 3.0 eq), toluene (6 mL), [Pd]:P = 1 : 2, 100 °C, 20 h.
e Reaction conditions:19 aryl halide (1.0 mmol, 1.0 eq), phenylboronic acid (1.5 mmol, 1.5 eq), K3PO4 (3.0 mmol, 3.0 eq), toluene (2 mL),
[Pd2(dba)3]/phosphine (0.05 mol% [Pd], 0.1 mol% phosphine, 50 °C, 24 h. f Ligand from ref. 18d, T = 100 °C, 16 h.
11a–c (Table 4; entries 1–3), explainable by the more electron-
donating capability of the imidazole rings. Since these com-
pounds possess with the bromo and iodo substituents reactive
functionalities, we decided to introduce organic and organome-
tallic catalytic inert groups at the phenylene building block. This
led to the derivatives 11i–k. It was found that the productivities
of these species are similar to their halide counter parts (Table 4;
entries 4–6 and 9–11). However, their activities are considerably
higher (Fig. S3, ESI-General;† Table 4) indicating that the halide
carrying phosphines 11a–f are involved in the catalytic reactions.
In addition, the reaction profiles (Fig. S3, ESI-General†) indicate
that the influence of the halide is less prominent when electron-
deficient phosphino groups are present, explainable by the
slower oxidative addition. Comparing the biphenyl- and ferroce-
nyl-functionalised molecules with each other, it is obvious that
the organometallic compounds 11g and 11h are superior ligands
in the catalysis, however, less active than the respective cyclo-
hexyl phosphines.
From Table 4 it further becomes clear that the neutral as well
as the ionic cyclohexyl phosphino systems show the same pro-
ductivity with a slightly higher activity for 11e. In the case of the
molecules featuring phenyl groups it is obvious that the imidazo-
lium derivative 17a gives a more active and productive catalyst
than 11d (Table 4; entries 4 and 12; Fig. S5, ESI-General†) most
likely attributed to the better solubility of ionic 17a. The palla-
dium complex carrying the imidazolium salt 20 with the phos-
phino donating group located at the phenylene unit is, when
compared with ligand 17a, more active and productive, which
can be explained with the better σ-donor ability of the triphenyl-
phosphine derivative. Compared with the respective neutral
phosphino imidazole molecule,17 the imidazolium salt 20 is a
better ligand for C–C couplings.
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We also studied the ability of our systems (i) to couple non-
activated 4-chloro toluene using low palladium loadings, and (ii)
to convert sterically hindered biaryls at only 50 °C (Table 5). As
can be seen from Table 5, the reaction of 4-chloro toluene with
phenylboronic acid in the presence of phosphines 11d and 11e
gave 28 and 88% conversion, respectively, using only 0.01 mol
% [Pd(OAc)2] (Table 5; entries 1 and 2). Especially the cyclo-
hexyl derivative 11e can compete under these reaction conditions
with TONs up to 8800 with the di-t-butylphosphino-substituted
benzimidazole (TON: 8600) and shows excellent results com-
pared to the dicyclohexylphosphino imidazole (TON:
1500–6600) reported by Beller and co-workers3b (Table 5;
entries 3 and 4). In comparison with the biphenyl ligand first
described by Buchwald,18a,d significantly higher conversions can
be achieved using 0.01 mol% palladium (Table 5, entry 6).18b
However, comparison with the trialkyl phosphino ligands by Fu
et al.18c and Beller et al.18b showed a slightly lower conversion
under the applied reaction conditions.18b
Another challenge in organic synthesis is the preparation of
sterically hindered biaryls especially under mild reaction con-
ditions. As can be seen from Table 5 the coupling of aryl bro-
mides with one or two ortho-substituents proves the successful
use of only 0.05 mol% palladium at 50 °C (Table 5; entries
9–12). Even highly congested 1,3,5-tri-i-propyl-2-bromo
benzene can be coupled with good conversions of 76% (Table 5;
entry 12). Compared to previously reported catalyst systems suit-
able for these couplings, i.e. Fe(η5-(1-(4-t-Bu-C6H4)-2-P(2-
CH3C6H4)2C5H3))(η5-C5H5)/[Pd2(dba)3]19a (dba = dibenzylide-
neacetone)19 only half the amount of palladium is required to
achieve good to excellent results. In comparison with the estab-
lished biphenyl catalyst system by Buchwald et al.18d (Table 5,
entry 13) higher catalyst loadings and longer reaction times are
necessary, yet, the catalytic reaction can be performed at con-
siderably lower temperature (50 °C vs. 100 °C).
from the catalytic active species.6 The results thereof are shown
in Table 6. To our surprise, the conversions in the first run are
lower when compared to the reaction in 1,4-dioxane (Table 6;
entries 1 and 4). Furthermore, recycling proved unsuccessful and
almost no conversion could be detected in further runs (Table 6;
entries 2, 3, 5 and 6). These inactivity of our systems might be
caused by the formation of 1,3-dialkylimidazole-2-ylidene palla-
dium complexes, which can lead, due to their stability regarding
dissociation, to inactive polycarbene palladium complexes.20 To
rule out the possibility of the formation of inactive complexes,
we chose 1-butyl-2,3-dimethylimidazolium tetrafluoroborate
([BDMIM][BF4]) as ionic liquid. However, the Suzuki reaction
in [BDMIM][BF4] resulted in even lower conversions after 1 h
when compared to the couplings in [BMIM][PF6] or 1,4-dioxane
(Table 6; entries 7 and 8). Furthermore, precipitation of metallic
palladium occurred during the catalytic reaction, which could
not be suppressed by carrying out the reaction at 75 °C but with
0.5 mol% of palladium (Table 6; entries 9 and 10). These
obtained results show that the catalytic species is only active for
a short time leading to rapid decomposition, most probably
attributed to the lower σ-donor ability of the ionic phosphines
and hence insufficient stabilisation of the active species.
Conclusions
Within this study the synthesis and characterisation (IR, NMR,
MS, electrochemistry, elemental analysis) of a series of imida-
zoles 1-(4-X-C6H4)-4,5-R2-cC3HN2 (X = Br, I, Fc, FcCuC, Ph;
R = H; Me; Fc = Fe(η5-C5H4)(η5-C5H5)), phosphino imidazoles
1-(4-X-C6H4)-2-PR′2-4,5-R2-cC3N2 (X = Br, I, Fc, FcCuC, Ph;
R = H; Me; R′ = Ph, cC6H11, cC4H3O), imidazolium [1-(4-
X-C6H4)-3-R′′-4,5-R2-cC3HN2]I (X = Br, I; R = H, Me; R′′ =
n
nBu, C8H17) and phosphino imidazolium salts [1-C6H5-2-PR′2-
c
3-nC8H17-4,5-Me2-cC3N2]PF6 (R′ = Ph, C6H11) or [1-(4-PPh2-
C6H4)-3-nC8H17-4,5-Me2-cC3HN2]PF6, and their selenium
Ionic phosphines 17b and 20 were additionally tested under
similar catalytic conditions (vide supra) in the coupling of 2-
bromo toluene with phenylboronic acid but exchanging the
organic solvent (1,4-dioxane) with the ionic liquid [BMIM][PF6]
(BMIM = 1-butyl-3-methylimidazolium), which should provide
excellent solubility due to its structural similarity. It is well-
stated that ionic liquids in homogeneous catalysis show some
benefits including better solubility, stability under catalytic con-
ditions and easier separation of the organic coupling products
derivatives 1-(4-X-C6H4)-2-P(= Se)R′2-4,5-R2-cC3N2 (X = Br, I;
c
c
R = H, Me; R′ = Ph, C6H11, C4H3O) are reported. They have
been prepared by straightforward synthesis methodologies
including alkylation, metallation, P–C coupling (Stelzer) and
C–C cross-coupling reactions (Suzuki, Negishi, Sonogashira).
To verify the σ-donor ability of the phosphino imidazoles the
appropriate selenium derivatives have been prepared by addition
of selenium in its elemental form to the respective phosphine.
Table 6 Reaction of 2-bromo toluene with phenylboronic acid in ionic liquid
Entry
Run
Compd.
Conversion/%
Entry
Run
Compd.
Conversion/%
1
2
3
1
2
3
17ba
65
5
1
7
8
1
2
17bb
26
3
9
10
1
2
17bc
36
3
4
5
6
1
2
3
20a
55
1
0
a Reaction conditions: 2-bromo toluene (1.0 eq), phenylboronic acid (1.5 eq), K2CO3 (3.0 eq), water (10 mL), [BMIM][PF6] (7 g, 24.6 mmol),
[Pd(OAc)2]/phosphine (0.25 mol% [Pd], 0.5 mol% phosphine, 100 °C, 1 h. b Reaction conditions: 2-bromo toluene (1.0 eq), phenylboronic acid (1.5
eq), K2CO3 (3.0 eq), water (10 mL), [BDMIM][BF4] (6 g, 25.0 mmol), [Pd(OAc)2]/phosphine (0.25 mol% [Pd], 0.5 mol% phosphine, 100 °C, 1 h.
c Reaction conditions: 2-bromo toluene (1.0 eq), phenylboronic acid (1.5 eq), K2CO3 (3.0 eq), water (10 mL), [BDMIM][BF4] (6 g, 25.0 mmol),
[Pd(OAc)2]/phosphine (0.5 mol% [Pd], 1.0 mol% phosphine, 75 °C, 1 h.
5384 | Dalton Trans., 2012, 41, 5377–5390
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1
High JPSe values indicate electron-poor phosphines and hence
filtrations Celite (purified and annealed, Erg. B.6, Riedel de
Haen) was used. Column chromatography was carried out using
either alumina with a particle size of 90 μm (standard, Merck
less electron donating σ-donors.9 The structure of 1-(4-Br-
C6H4)-2-P(= Se)Ph2-cC3H2N2 in the solid state was confirmed
by single crystal X-ray diffraction studies showing the pseudo-
tetrahedral environment about the phosphorus atom. The reaction
of [1-(4-Br-C6H4)-3-n-Bu-cC3H3N2]I towards [PdCl2(cod)]
(cod = cyclo-1,5-octadiene) gave the respective cis- and trans-
palladium carbene complexes [Pd(1-(4-Br-C6H4)-3-n-Bu-
cC3H2N2)2I2], which were characterised by single crystal X-ray
structure measurements confirming the molecular square-planar
coordination about Pd and additionally shows the structural
characteristics typical of palladium–carbene complexes.15 Fur-
thermore, the phosphino imidazoles and imidazolium salts were
applied in the palladium-promoted Suzuki C–C cross-coupling.
As a model reaction the conversion of 2-bromo toluene and phe-
nylboronic acid as organic substrates and potassium carbonate as
base were chosen. All in situ generated phosphino palladium
species showed catalytic activity towards the formation of 2-
methyl biphenyl. It was found that the more electron-rich the
phosphines are, the more active and productive the catalytic
system is. This was impressively demonstrated by the cyclo-
hexyl-functionalised derivatives. Additionally, the phosphino
imidazolium species showed higher activity and productivity
than their neutral derivatives. Also it is obvious that the halide
ligands at the phenylene units should be replaced by innocent
organic or organometallic moieties since otherwise the C–Br or
C–I bonds are involved in the catalytic reactions. Compared
with, for example, dialkylphosphino-functionalised imidazoles
and benzimidazoles applying [Pd(OAc)2]3b as palladium source,
the compounds described within this study show excellent pro-
ductivities with TONs up to 8800. Furthermore, the cyclohexyl-
functionalised phosphino imidazole 1-(4-I-C6H4)-2-P(cC6H11)2-
4,5-Me2-cC3N2 was used in the synthesis of sterically hindered
biphenyls showing good to excellent results at 50 °C and catalyst
loadings of only 0.05 mol%. In further studies C–C coupling
reactions of the appropriate imidazolium molecules in presence
of the ionic liquid [BMIM][PF6] were carried out. Surprisingly,
the conversions were lower when compared to the reaction in
1,4-dioxane, and recycling was not possible. These observations
are most likely attributed to deactivation of the catalyst due to
the formation of stable, inactive polycarbene palladium com-
plexes.20 To avoid this type of reaction we carried out the Suzuki
catalysis in [BDMIM][BF4]. Nevertheless, the observed conver-
sions were even lower when compared to [BMIM][PF6] or 1,4-
dioxane. In addition, the formation of “palladium-black” was
observed, leading to the assumption that the phosphino imidazo-
lium salts are not suited to stabilise the active species due to
their lower σ-donor ability.
KGaA) or silica with
(230–400 mesh (ASTM), Becker).
a particle size of 40–60 μm
NMR spectra were recorded with a Bruker Avance III 500
spectrometer. The 1H NMR spectra were recorded at 500.3 MHz,
the 13C{1H} and 31P{1H} NMR spectra at 125.7 MHz and
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, respectively). High resolution mass spectra were recorded
with a Bruker Daltonik micrOTOF-QII spectrometer (ESI-TOF).
Elemental analyses were carried out with a Thermo FlashAE
1112 series instrument. The melting points of the analytical pure
samples were determined using a Gallenkamp MFB 595 010 M
melting point apparatus. FT IR spectra were recorded with a
Thermo Nicolet IR 200 spectrometer using either KBr pellets or
NaCl plates. Cyclovoltammetric measurements of dry degassed
dichloromethane solutions (1.0 mmol L−1) of 3a, 3b, 5 and 7
containing [(n-Bu)4N][B(C6F5)4] (0.1 mol L−1) as supporting
electrolyte were conducted under a blanket of purified argon at
25 °C utilising a Radiometer Voltalab PGZ 100 electrochemical
workstation interfaced with a personal computer. A three elec-
trode cell, which utilised a Pt auxiliary electrode, a glassy carbon
working electrode (surface area 0.031 cm2) and an Ag/Ag+
(0.01 mol L−1 [AgNO3]) reference electrode mounted on a
luggin capillary was used. The working electrode was pre-treated
by polishing on a Buehler microcloth first with 1 μ and then
1
4
with a μ diamond paste. The reference electrode was con-
structed from a silver wire and inserted into an acetonitrile sol-
ution containing [AgNO3] (0.01 mol L−1) and [(n-Bu)4N]-
[B(C6F5)4] (0.1 mol L−1), in a luggin capillary with a vycor tip.
This luggin capillary was inserted into a second luggin capillary
with a vycor tip filled with a 0.1 mol L−1 [(n-Bu)4N][B(C6F5)4]
solution in dichloromethane. Experimental potentials were refer-
enced against an Ag/Ag+ reference electrode but results are pre-
sented referenced against ferrocene as an internal standard as
required by IUPAC.12 Data were manipulated on a Microsoft
Excel worksheet to set the formal reduction potentials of the
FcH/FcH+ couple to 0.0 V (FcH = Fe(η5-C5H5)2). Under
our conditions the FcH/FcH+ redox couple was at 320 mV vs.
Ag/Ag+.
All starting materials were obtained from commercial
suppliers and used without further purification. 1-(4-Bromophe-
nyl)-1H-imidazole (3a),5a 1-phenyl-1H-imidazole (3c),21
[PdCl2(PPh3)2],22 [PdCl2(cod)],23 ethynylferrocene (6),24 diphe-
nylphosphine25 and the chlorophosphines 10b26 and 10c27 were
prepared according to published procedures.
Experimental
General procedures
Synthesis of 1-(4-iodophenyl)-4,5-dimethyl-1H-imidazole (3b)
All reactions were carried out under argon atmosphere using
standard Schlenk techniques. Toluene and tetrahydrofuran were
purified by distillation from sodium–benzophenone; triethyl-
amine was purified by distillation from calcium hydride. Diethyl
ether and dichloromethane were received from a MBRAUN
(MB-SPS 800) solvent drying and purification system. For
Following the synthesis procedure described by Zhang et al.,2d
4-iodoaniline (10.95 g, 0.05 mol) dissolved in methanol
(300 mL) were reacted with 2,3-butanedione (4.4 mL,
0.05 mol). After stirring for 16 h at ambient temperature,
ammonium chloride (5.35 g, 0.1 mol) and 30% aq.
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Synthesis of 1-(4-(ethynylferrocenyl)phenyl)-1H-imidazole (7)
formaldehyde (10 mL, 0.1 mol) were added and the mixture was
refluxed for 2 h. Afterwards, phosphoric acid (7 mL) was added
in a single portion and the reaction mixture was refluxed for
additional 6 h. After cooling down to ambient temperature all
volatiles were removed in vacuum and the dark brown residue
was poured onto ice and neutralised with potassium hydroxide
until pH ≈ 9 was reached. The resulting mixture was extracted
with dichloromethane (300 mL) and dried over magnesium
sulfate. The solvent was removed in vacuum and the residue was
purified on silica (column size: 15 × 5 cm) using diethyl ether as
eluent. The product 3b was obtained as pale yellow solid. Yield:
4.3 g (14.4 mmol, 29% based on 4-iodoaniline). Anal. Calcd for
C11H11IN2 (298.12 g mol−1): C, 44.32; H, 3.72; N, 9.40. Found:
C, 45.34; H, 3.90; N, 8.75. Mp.: 86 °C. IR (KBr, ν˜/cm−1): 1495
(m, NvC), 1588 (w, CvC), 2854/2917/2959 (w, C–H), 3060/
Ethynylferrocene (6, 0.5 g, 2.38 mmol), 3a (0.53 g, 2.38 mmol),
[CuI] (45 mg, 10 mol%) and [PdCl2(PPh3)2] (83 mg, 5 mol%)
were dissolved in dry triethylamine (60 mL) and stirred at 60 °C
for 16 h. After removal of all volatiles in vacuum, the residue
was purified by column chromatography on silica (column size:
15 × 3.5 cm) using diethyl ether and then ethyl acetate as
eluents. Product 7 was obtained as yellow solid. Yield: 0.37 g
(1.05 mmol, 44% based on 3a). Anal. Calcd for C21H16FeN2
(352.21 g mol−1): C, 71.61; H, 4.58; N, 7.95. Found: C, 71.45;
H, 4.60; N, 7.95. Mp.: 160 °C (dec.). IR (KBr, ν˜/cm−1): 1490
(w, NvC), 1524 (m, CvC), 2206 (w, CuC), 2851/2921 (w,
C–H), 3108 (w,vC–H). 1H NMR (500.30 MHz, CDCl3, δ):
3
4.26 (s, 5 H, C5H5), 4.27 (pt, JHH = 1.8 Hz, 2 H, C5H4),
3
1
4.52 (pt, JHH = 1.9 Hz, 2 H, C5H4), 7.24 (m, 1 H, C3H3N2),
3105 (w, vC–H). H NMR (500.30 MHz, CDCl3, δ): 2.08 (s, 3
3
3
7.32 (m, 1 H, C3H3N2), 7.35 (dpt, JHH = 8.5 Hz, 2 H, C6H4),
H, CH3), 2.21 (s, 3 H, CH3), 7.01 (dpt, JHH = 8.7 Hz, 2 H, Ho/
3
7.59 (dpt, JHH = 8.4 Hz, 2 H, C6H4), 7.89 (m, 1 H,
3
C6H4), 7.45 (s, 1 H, H2/C3HN2), 7.80 (dpt, JHH = 8.7 Hz, 2 H,
H2/C3H3N2). 13C{1H} NMR (125.81 MHz, CD2Cl2, δ): 64.8 (s,
Ci/C5H4), 69.2 (s. C5H4), 70.1 (s, C5H5), 71.6 (s, C5H4), 84.6
(s, CuC), 90.2 (s, CuC), 121.1 (s, C4/C3H2N2), 121.3 (s,
Co/C6H4), 123.2 (s, Cp/C6H4), 123.5 (s, C5/C3H2N2), 133.0 (s,
Cm/C6H4), 133.2 (s, Ci/C6H4), 136.5 (s, C2/C3H3N2). HRMS
(ESI-TOF) C21H16FeN2 [M + nH]+ m/z: calcd: 353.0719, found:
353.0736.
Hm/C6H4). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.2 (s,
CH3), 12.9 (s, CH3), 93.2 (Cp/C6H4), 122.8 (s, C4/C3HN2),
127.4 (s, Co/C6H4), 135.0 (s, C2/C3HN2), 135.1 (s, C5/C3HN2),
136.8 (s, Ci/C6H4), 138.8 (s, Cm/C6H4). HRMS (ESI-TOF)
C11H11IN2 [M + nH]+ m/z: calcd: 299.0050, found: 299.0040.
Please, notice that the results of the elemental analysis deviate
from the calculated values due to decomposition of 3b because
this compound is highly light sensitive.
Synthesis of 1-(4-(1,1′-biphenyl))-4,5-dimethyl-1H-imidazole (9)
Imidazole 3b (1.29 g, 6.5 mmol), phenylboronic acid (8, 0.95 g,
7.81 mmol, 1.2 eq), potassium carbonate (2.69 g, 19.5 mmol,
3 eq) and [PdCl2(dppf)] (23 mg, 0.5 mol%) were dissolved in a
1,4-dioxane–water mixture (50 mL, ratio 2 : 1, v:v) and stirred at
100 °C for 16 h. After cooling to ambient temperature the
organic phase was separated, dried over magnesium sulfate and
purified by column chromatography on silica (column size: 18 ×
3.5 cm) using diethyl ether as eluent. Biphenyl 9 was obtained
as a pale yellow solid. Yield: 1.31 g (5.3 mmol, 82% based on
3b). Anal. Calcd for C17H16N2 (248.32 g mol−1): C, 82.22; H,
6.49; N, 11.28. Found: C, 81.61; H, 6.60; N, 11.13. Mp.:
103 °C. IR (KBr, ν˜/cm−1): 1450/1490 (s, NvC), 1595/1605 (w,
Synthesis of 1-(4-ferrocenylphenyl)-1H-imidazole (5)
Ferrocene (840 mg, 4.52 mmol) was dissolved in dry tetrahydro-
furan (60 mL) and cooled to −78 °C. Then potassium tert-butox-
ide (64 mg, 0.57 mmol, 0.125 eq) was added in a single portion
followed by dropwise addition of t-BuLi (5.6 mL, 8.96 mmol, 2
eq). After stirring the reaction mixture for 45 min at −78 °C,
[ZnCl2(thf)2] (1.4 g, 4.99 mmol, 1.1 eq) and 3a (1.0 g,
4.48 mmol) was added in a single portion at 0 °C. The mixture
was stirred for 30 min at this temperature, followed by addition
of [Pd(PPh3)4] (26 mg, 0.5 mol%). After heating the reaction
mixture to 50 °C for 24 h, all volatiles were removed and the
residue was purified by column chromatography on silica
(column size: 15 × 3.5 cm) using diethyl ether as eluent and
then a mixture of diethyl ether–ethyl acetate (ratio 1 : 1, v:v).
The product was obtained as an orange solid. Yield: 0.94 g
(2.86 mmol, 63% based on 3a). Anal. Calcd for C19H16FeN2
(328.19 g mol−1): C, 69.53; H, 4.91; N, 8.54. Found: C, 69.06;
H, 5.06; N, 8.61. Mp.: 132 °C. IR (KBr, ν˜/cm−1): 1509/1532 (s,
NvC), 1560 (w, CvC), 2868/2931 (w, C–H), 3124 (w,vC–H).
1H NMR (500.30 MHz, CD2Cl2, δ): 4.04 (s, 5 H, C5H5), 4.38
1
CvC), 2860/2919 (w, C–H), 3033/3102 (w,vC–H). H NMR
(500.30 MHz, CDCl3, δ): 2.12 (s, 3 H, CH3), 2.24 (s, 3 H, CH3),
3
7.30 (dpt, JHH = 8.4 Hz, 2 H, H3/C6H5–C6H4), 7.36 (m, 1 H,
H4′/C6H5–C6H4), 7.45 (m, 2 H, H3′/C6H5–C6H4), 7.51 (s, 1 H,
3
H2/C3HN2), 7.59 (m, 2 H, H2′/C6H5–C6H4), 7.66 (dpt, JHH
=
8.4 Hz, 2 H, H2/C6H5–C6H4). 13C{1H} NMR (125.81 MHz,
CDCl3, δ): 9.2 (s, CH3), 12.8 (s, CH3), 122.9 (s, Ci), 125.7 (s,
C3/C6H5–C6H4), 127.1 (s, C2′/C6H5–C6H4), 127.8 (s, C4′/C6H5–
C6H4), 128.1 (s, C2/C6H5–C6H4), 128.9 (s, C3′/C6H5–C6H4),
134.6 (s, Ci), 135.1 (s, C2/C3HN2), 136.1 (s, Ci), 139.8 (s, Ci),
141.0 (s, Ci). HRMS (ESI-TOF) C17H16N2 [M + nH]+ m/z:
calcd: 249.1386, found: 249.1381; [M + nNa]+ m/z: calcd:
271.1206, found: 271.1206.
3
3
(pt, JHH = 1.8 Hz, 2 H, C5H4), 4.69 (pt, JHH = 1.8 Hz, 2 H,
C5H4), 7.35 (dpt, JHH = 8.6 Hz, 2 H, Ho/C6H4), 7.37 (pt, JHH
3
3
= 1.5 Hz, 1 H, H4/C3H3N2), 7.43 (pt, JHH = 1.5 Hz, 1 H, H5/
3
C3H3N2), 7.60 (dpt, JHH = 8.6 Hz, 2 H, Hm/C6H4), 8.33 (m, 1
3
H, H2/C3H3N2). 13C{1H} NMR (125.81 MHz, CD2Cl2, δ): 66.8
(s, C5H4), 69.8 (s, C5H4), 69.9 (s, C5H5), 83.2 (s, Ci/C5H4),
120.0 (s, C4/C3H3N2), 122.2 (s, Co/C6H4), 127.3 (s, Cm/C6H4),
127.9 (s, C5/C3H3N2), 133.6 (s, Ci/C6H4), 136.8 (s, C2/C3H3N2),
141.2 (s, Cp/C6H4). HRMS (ESI-TOF) C19H16FeN2 [M + nH]+
m/z: calcd: 329.0715, found: 329.0736.
General synthesis procedure for phosphines 11a–f
To 0.5 g of 3a (2.24 mmol), 3b (1.68 mmol), 5 (1.52 mmol), 7
(1.42 mmol) or 9 (2.01 mmol) dissolved in dry diethyl ether
(3a, 5, 7, 9, 40 mL) or tetrahydrofuran (3b, 40 mL) one eq of a
5386 | Dalton Trans., 2012, 41, 5377–5390
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2.5 M solution of n-BuLi (3a, 5, 7, 9) or a 2.0 M solution of
lithium di-i-propylamide (3b) was added dropwise at −30 °C.
After warming up the solution to ambient temperature, it was
again cooled to −30 °C and one eq of the chlorophosphines
10a–c was added dropwise. The reaction mixture was stirred at
ambient temperature for 2 h and the solvent was removed in
vacuum. The crude product was purified by column chromato-
graphy on silica or alumina and dried in vacuum.
6 H, Hm,p/C6H5–P), 7.33–7.37 (m, 7 H, Ho/C6H5–P + Hm,p
/
C6H5). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 9.5 (s, CH3),
9.7 (s, CH3), 14.2 (s, CH2(CH2)6CH3), 22.7 (s, CH2(CH2)6CH3),
26.6 (s, CH2(CH2)6CH3), 28.9 (s, CH2(CH2)6CH3), 29.0
(s, CH2(CH2)6CH3), 29.7 (s, CH2(CH2)6CH3), 31.7 (s,
3
CH2(CH2)6CH3), 47.9 (d, JCP = 10.6 Hz, CH2(CH2)6CH3),
127.7 (s, C4/C3N2), 128.8 (d, 4JCP = 6.6 Hz, Co/C6H5), 129.6 (d,
3JCP = 7.3 Hz, Cm/C6H5–P), 129.9 (s, Cp/C6H5), 130.4 (s, Cm/
2
C6H5), 130.5 (s, Cp/C6H5–P), 131.0 (s, C5/C3N2), 132.9 (d, JCP
3
= 20.2 Hz, Co/C6H5–P), 133.0 (m, Ci/C6H5–P), 133.9 (d, JCP
=
1.2 Hz, Ci/C6H5), 141.1 (d, JCP = 54.0 Hz, C2/C3N2). 31P{1H}
NMR (202.5 MHz, CDCl3, δ): −144.6 (sept, JPF = 712.5 Hz,
PF6), −23.5 (s, P(C6H5)2). HRMS (ESI-TOF) [C31H38N2P]PF6
1
General procedure for the synthesis of seleno phosphines
11a-Se–f-Se
1
To a toluene solution of 11a–f (100 mg), 2 eq of elemental sel-
enium was added in a single portion and stirred for 2 h at
100 °C. After cooling to ambient temperature, the solvent was
removed in membrane-pump vacuum and the respective seleno
phosphines were purified using column chromatography on
silica (column size: 2.5 × 8 cm) and dried in membrane-pump
vacuum.
[M]+ m/z: calcd: 469.2767, found: 469.2767.
Synthesis of 1-phenyl-2-(dicyclohexylphosphino)-3-n-octyl-4,5-
dimethyl-1H-imidazolium hexafluorophosphate (17b)
Compound 16d (0.50 g, 1.21 mmol) was dissolved in dry
dichloromethane (40 mL) and cooled to −78 °C. n-BuLi
(0.48 mL, 1.20 mmol) was added dropwise and the reaction
mixture was stirred for 45 min at this temperature. After drop-
wise addition of chlorodicyclohexylphosphine (0.27 mL,
1.22 mmol) the reaction mixture was warmed up to ambient
temperature. Then all volatiles were removed in vacuum and the
residue was dissolved in acetone (20 mL). A solution of potass-
ium hexafluorophosphate (0.22 g, 1.20 mmol) in water (20 mL)
was added dropwise and the mixture was stirred at ambient
temperature for 1 h. The solvent was removed in membrane-
pump vacuum and the crude product was purified by column
chromatography (column size: 2.5 × 12 cm) on silica using
diethyl ether as eluent. Phosphine 17b was obtained as a colour-
less solid. Yield: 0.54 g (0.86 mmol, 72% based on 16d). Anal.
Calcd for C31H50F6N2P (626.68 g mol−1): C, 59.41; H, 8.04; N,
4.47. Found: C, 59.58; H, 8.21; N, 4.32. Mp.: 58 °C. IR (NaCl,
ν˜/cm−1): 1450 (m, P–C), 1500 (m, NvC), 1595/1635 (w,
CvC), 2853/2927 (s, C–H), 3067 (w,vC–H). 1H NMR
General procedure for the synthesis of imidazolium salts
16a–16d
To 3a–c (0.5 g) dissolved in acetonitrile (50 mL) one eq of
n-BuI (12a) or n-C8H17I (12b) was added in a single portion and
the reaction mixture was stirred at 70 °C for 5 (12a) or 14 (12b)
days. The progress of the reaction was monitored using 1H NMR
spectroscopy. After completion of the reaction the solvent was
removed in membrane-pump vacuum. The crude product was
washed five times with diethyl ether (10 mL portions) and dried
in membrane-pump vacuum.
Synthesis of 1-phenyl-2-(diphenylphosphino)-3-n-octyl-4,5-
dimethyl-1H-imidazolium hexafluorophosphate (17a)
To 16d (0.30 g, 0.73 mmol) in dry dichloromethane (30 mL),
n-BuLi (0.30 mL, 0.75 mmol) was added dropwise at −78 °C.
The reaction mixture was stirred for 45 min at this temperature,
followed by dropwise addition of chlorodiphenylphosphine
(0.14 mL, 0.78 mmol). After warming the mixture to ambient
temperature, all volatile materials were removed in membrane-
pump vacuum and the residue was dissolved in acetone (20 mL).
Then a solution of potassium hexafluorophosphate (0.14 g,
0.76 mmol) in water (20 mL) was added in a single portion and
the mixture was stirred at ambient temperature for an additional
hour. After removal of all volatiles, the residue was purified by
column chromatography (column size: 2.5 × 12 cm) on silica
using diethyl ether as eluent. Phosphine 17a was obtained as a
colourless solid. Yield: 0.30 g (0.49 mmol, 67% based on 16d).
Anal. Calcd for C31H38F6N2P (614.58 g mol−1): C, 60.58; H,
6.23; N, 4.56. Found: C, 60.50; H, 6.53; N, 4.25. Mp.: 92 °C. IR
(KBr, ν˜/cm−1): 1436 (m, P–C), 1499 (m, NvC), 1595/1632 (w,
3
(500.30 MHz, CDCl3, δ): 0.86 (t, JHH = 7.1 Hz, 3 H,
CH2(CH2)6CH3), 1.05–1.19 (m, 10 H, CH2(CH2)6CH3
+
C6H11), 1.27–1.44 (m, 12 H, CH2(CH2)6CH3 + C6H11),
1.60–1.65 (m, 8 H, CH2(CH2)6CH3 + C6H11), 1.74–1.78 (m, 4
H, CH2(CH2)6CH3 + C6H11), 1.92 (s, 3 H, C3N2), 2.34 (s, 3 H,
C3N2), 4.31 (m, 2 H, CH2(CH2)6CH3), 7.29 (d, 3JHH = 7.4 Hz, 2
H, Ho/C6H5), 7.60–7.66 (m, 3 H, Hm,p/C6H5). 13C{1H} NMR
(125.81 MHz, CDCl3, δ): 9.4 (s, CH3), 9.4 (s, CH3), 14.1 (s,
CH2(CH2)6CH3), 22.6 (s, CH2(CH2)6CH3), 25.6 (s, C6H11), 26.2
(d, JCP = 1.3 Hz, C6H11), 26.3 (d, JCP = 6.9 Hz, C6H11), 26.5
(s, CH2(CH2)6CH3), 28.9 (s, CH2(CH2)6CH3), 29.0 (s,
CH2(CH2)6CH3), 30.2 (s, CH2(CH2)6CH3), 31.1 (d, JCP = 9.1
Hz, C6H11), 31.7 (d, JCP = 23.5 Hz, C6H11), 31.7 (s,
1
CH2(CH2)6CH3), 35.0 (d, JCP = 11.2 Hz, C1/ C6H11), 47.4 (d,
3JCP = 17.1 Hz, CH2(CH2)6CH3), 127.4 (m, Co/C6H5), 129.9 (s,
1
CvC), 2855/2927/2953 (m, C–H), 3058 (w,vC–H). H NMR
Cp/C6H5), 130.3 (s, Cm/C6H5), 131.6 (m, C4/C3N2), 132.3 (d,
3
1
(500.30 MHz, CDCl3, δ): 0.86 (t, JHH = 7.2 Hz, 3 H,
3JCP = 1.7 Hz, C5/C3N2), 134.1 (m, Ci/C6H5), 143.3 (d, JCP
=
CH2(CH2)6CH3), 1.07–1.25 (m, 10 H, CH2(CH2)6CH3),
1.35–1.41 (m, 2 H, CH2(CH2)6CH3), 1.97 (s, 3 H, CH3), 2.41 (s,
3 H, CH3), 4.15 (m, 2 H, CH2(CH2)6CH3), 6.99 (dpt, 3JHH = 8.5
Hz, 4JHH = 1.9 Hz, 5JHP = 1.2 Hz, 2 H, Ho/C6H4), 7.18–7.23 (m,
61.7 Hz, C2/C3N2). 31P{1H} NMR (202.5 MHz, CDCl3, δ):
1
−144.6 (sept, JPF = 712.5 Hz, PF6), −13.8 (s, P(C6H5)2).
HRMS (ESI-TOF) [C31H50N2P]PF6 [M]+ m/z: calcd: 481.3706,
found: 481.3679.
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Synthesis of [(1-(4-Br-C6H4)-cC3H2N2-3-n-Bu)2PdI2] (19)
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
appropriate phosphine (11, 17, 20), 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 chromatographed on silica gel
(column size: 6 × 2.5 cm) using diethyl ether as eluent. All vola-
tiles were evaporated under reduced pressure and the conversions
were determined by 1H NMR spectroscopy.
To 16a (106 mg, 0.26 mmol) dissolved in dry tetrahydrofuran
(10 mL) a solution of potassium tert-butoxide (58 mg,
0.53 mmol, 2 eq) in dry tetrahydrofuran (5 mL) was added drop-
wise at ambient temperature. After stirring the solution for 2 h,
[PdCl2(cod)] (37 mg, 0.13 mmol) was added in a single portion
and the mixture was stirred for additional 16 h. All volatiles were
removed in membrane-pump vacuum and the residue was dis-
solved in chloroform and filtered through a pad of Celite. By
slow evaporation in membrane-pump vacuum at 0 °C two differ-
ent types of crystals (pale yellow plates and colourless needles)
suitable for X-ray diffraction analysis could be isolated.
General procedure for the Suzuki reaction in ionic liquids
Phenylboronic acid (470 mg, 3.85 mmol), potassium carbonate
(1.21 g, 8.76 mmol), acetyl ferrocene (111 mg, 0.49 mmol),
0.25 mol% of [Pd(OAc)2] and 0.5 mol% of the appropriate phos-
phine (17b, 20) were dissolved in [BMIM][PF6] (7.0 g,
24.6 mmol) or [BDMIM][BF4] (6.0 g, 25.0 mmol), followed by
the addition of degassed water (5 mL). After stirring of the reac-
tion mixture for 5 min at ambient temperature, 2-bromo toluene
(500 mg, 2.92 mmol) was added in a single portion and the
mixture was stirred for 60 min at 100 °C. After cooling down to
ambient temperature the reaction mixture was continuously
extracted with n-pentane until no more colouring of the solution
was observable. In the case of [BMIM][PF6], the water layer was
removed and the ionic liquid was washed once with water
(10 mL) and then was used in the next run. The [BDMIM][BF4]
solution was used without further purification in the 2nd run.
Finally, the n-pentane solution was evaporated in membrane-
Synthesis of 1-(4-(diphenylphosphino)phenyl)-3-n-octyl-4,5-
dimethyl-1H-imidazolium hexafluorophosphate (20)
Diphenylphosphine (0.18 mL, 1.03 mmol) was added in a single
portion to a dimethyl acetamide solution (30 mL) containing 16c
(0.5 g, 0.93 mmol), potassium acetate (109 mg, 1.11 mmol, 1.2
eq) and [Pd(OAc)2] (2.1 mg, 1.0 mol%) and stirred for 2 h at
130 °C. After cooling the reaction mixture to ambient tempera-
ture, all volatile materials were removed in vacuum and the
residue was dissolved in acetone (10 mL). Then a solution of
potassium hexafluorophosphate (172 mg, 0.93 mmol) in water
(10 mL) was added in a single portion and the mixture was
stirred at ambient temperature for 1 h. The solvent was removed
in membrane-pump vacuum and the residue was purified by
column chromatography (column size: 2.5 × 12 cm) on silica
using a mixture of acetone–diethyl ether (ratio 1 : 1, v:v). Ionic
phosphine 20 could be isolated as colourless solid. Yield: 0.36 g
(0.59 mmol, 63% based on 16c). Anal. Calcd for C31H38F6N2P
(614.58 g mol−1): C, 60.58; H, 6.23; N, 4.56. Found: C, 60.60;
H, 6.24; N, 4.31. Mp.: 138 °C. IR (KBr, ν˜/cm−1): 1434 (m,
P–C), 1560 (s, NvC), 1595/1634 (w, CvC), 2855/2925/2951
1
pump vacuum and the conversion was determined by H NMR
spectroscopy.
General procedure for the Suzuki coupling of aryl chlorides
4-Chloro toluene (379 mg, 3.0 mmol), phenylboronic acid
(550 mg, 4.5 mmol, 1.5 eq), potassium phosphate (1.27 g,
6.0 mmol, 3 eq) and acetyl ferrocene (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 phosphine
(11d, 11e), the reaction mixture was stirred for 20 h at 100 °C.
Afterwards, a sample of 2 mL was taken and filtered through
a pad of Celite. After evaporation of all volatiles under
1
(s, C–H), 3056/3070/3160 (w,vC–H). H NMR (500.30 MHz,
3
CDCl3, δ): 0.86 (t, JHH = 7.2 Hz, 3 H, CH2(CH2)6CH3),
3
1.20–1.38 (m, 10 H, CH2(CH2)6CH3), 1.84 (pent, JHH = 7.3
Hz, 2 H, CH2(CH2)6CH3), 2.13 (s, 3 H, CH3), 2.29 (s, 3 H,
3
CH3), 4.10 (t, JHH = 7.8 Hz, 2 H, CH2(CH2)6CH3), 7.31–7.38
(m, 12 H, Ho,m,p/C6H5+Hm/C6H4), 7.40–7.43 (m, 2 H, Ho/
C6H4). 13C{1H} NMR (125.81 MHz, CDCl3, δ): 8.5 (s, CH3),
9.1 (s, CH3), 14.1 (s, CH2(CH2)6CH3), 22.7 (s, CH2(CH2)6CH3),
26.4 (s, CH2(CH2)6CH3), 29.0 (s, CH2(CH2)6CH3), 29.1
(s, CH2(CH2)6CH3), 29.5 (s, CH2(CH2)6CH3), 31.8 (s,
1
reduced pressure, the conversions were determined by H NMR
spectroscopy.
3
CH2(CH2)6CH3), 47.8 (s, CH2(CH2)6CH3), 125.9 (d, JCP = 6.3
Hz, Co/C6H4), 127.7 (s, C3N2), 128.0 (s, C3N2), 129.0 (d, 3JCP
=
General procedure for the synthesis of sterically hindered
biaryls
7.3 Hz, Cm/C6H5), 129.5 (s, Cp/C6H5), 133.2 (s, Ci/C6H4), 133.3
(s, C2/C3N2), 134.1 (d, 2JCP = 20.1 Hz, Co/C6H5), 134.9 (d, 2JCP
1
= 19.2 Hz/Cm/C6H4), 135.8 (d, JCP = 10.6 Hz, Ci/C6H5), 142.3
Phenylboronic acid (183 mg, 1.5 mmol, 1.5 eq), potassium
phosphate (0.64 g, 3.0 mmol, 3.0 eq), acetyl ferrocene (114 mg,
0.50 mmol) 0.05 mol% [Pd2(dba)3] and 0.1 mol% of 11e
were dissolved in toluene (2 mL). Afterwards, the appropriate
aryl bromide (1.0 mmol, 1.0 eq) was added in a single
portion and the reaction mixture was stirred for 24 h at 50 °C.
Afterwards, the reaction mixture was filtered through a pad
of Celite and the solvent was removed in membrane-pump
vacuum. The conversions were determined by 1H NMR
spectroscopy.
1
(d, JCP = 16.5 Hz, Cp/C6H4). 31P{1H} NMR (202.5 MHz,
1
CDCl3, δ): −144.5 (sept, JPF = 712.6 Hz, PF6), −5.4 (s,
P(C6H5)2). HRMS (ESI-TOF) [C31H38N2P]+ [M]+ m/z: calcd:
469.2767, found: 469.2739.
General procedure for the Suzuki reaction
2-Bromo toluene (500 mg, 2.92 mmol), phenylboronic acid
(470 mg, 3.85 mmol), potassium carbonate (1.21 g, 8.76 mmol)
5388 | Dalton Trans., 2012, 41, 5377–5390
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Table 7 Crystal and intensity collection data for 11a-Se, trans-19 and cis-19
11a-Se
trans-19
cis-19
Formula weight
Chemical formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
486.20
918.56
1037.93
C21H16BrN2PSe
Monoclinic
P21/n
14.6515(2)
6.86120(10)
19.3482(3)
C26H30Br2I2N4Pd
Triclinic
ˉ
P1
C27H31Br2Cl3I2N4Pd
Triclinic
ˉ
P1
16.3428(11)
16.6465(12)
19.6587(15)
88.435(6)
79.579(6)
79.799(6)
8.7047(5)
12.7269(6)
14.2494(7)
78.787(4)
82.971(4)
74.495(4)
1487.97(13)
2.050
96.639(2)
V (ų)
1931.97(5)
1.672
960
5176.7(6)
1.998
2964
ρcalc (mg m−3
F(000)
)
872
Crystal dimensions (mm)
Z
Max. and min. transmission
Absorption coefficient (λ, mm−1
Scan range (°)
0.4 × 0.2 × 0.2
4
1.00000, 0.37259
5.885
4.60–62.00
−16 ≤ h ≤ 16
−7 ≤ k ≤ 7
−21 ≤ l ≤ 22
7108
0.35 × 0.35 × 0.08
2
1.00000, 0.55328
5.405
3.20–26.12
−9 ≤ h ≤ 10
−15 ≤ k ≤ 14
−11 ≤ l ≤ 17
9837
0.10 × 0.03 × 0.02
6
1.00000, 0.25423
23.419
3.28–62.00
−11 ≤ h ≤ 18
−12 ≤ k ≤ 19
−22 ≤ l ≤ 22
23957
)
Index ranges
Total reflections
Unique reflections
Rint
2999
0.0191
5836
0.0237
14 330
0.0603
Data/restraints/para-meters
Goodness-of-fit on F2
R1 a, wR2 a [I 2σ(I)]
R1 a, wR2 a (all data)
2999/0/235
1.060
0.0253, 0.0659
0.0283, 0.0671
0.669, −0.620
5836/0/316
0.927
0.0248, 0.0519
0.0344, 0.0531
0.757, −0.668
14 330/670/1112
0.893
0.0997, 0.2499
0.1714, 0.2796
3.955, −1.841
Largest differences in peak and hole peak in final Fourier map (e Å−3
)
2 2
4
2 2
a R1 = [Σ(||Fo| − |Fc|)/Σ|Fo|]; wR2 = [Σ(w(Fo2 − Fc ) )/Σ(wFo )]1/2; S = [Σw(Fo2 − Fc ) ]/(n − p)1/2. n = number of reflections, p = parameters used.
Crystal structure determination
Acknowledgements
The crystal and intensity collection data for 11a-Se, trans-19
and cis-19 are summarised in Table 7. All data were collected
on an Oxford Gemini S diffractometer with graphite monochro-
matised Mo Kα radiation (λ = 0.71073 Å) at 105 K (trans-19)
and graphite monochromatised Cu Kα radiation (λ = 1.54184 Å)
at 100 K (11a-Se, cis-19). The structures were solved by direct
methods using SHELXS-9128 and refined by full-matrix least-
square procedures on F2 using SHELXL-97.29 All non-hydro-
gen atoms were refined anisotropically and a riding model was
employed in the refinement of the hydrogen atom positions. For
cis-19 the crystal available for measuring was comparatively
tiny and needed the use of Cu Kα radiation in order to get
reasonable results. At higher diffraction angles large exposure
times have been used, leading to an ‘icing’ of the crystal. The
measurement was therefore stopped and hence, the ratio
between unique to observed reflections is almost poor. As no
further crystals were available no re-measurement could be per-
formed. Despite this, all non-hydrogen atoms could be refined
anisotropically and a number of fragments could be refined dis-
ordered over two positions. Occupation factors: 0.17/0.83 for
Pd3, I5, I6/Pd3′, I5′, I6′. 0.49/0.51 for C71–C73/C71′–C73′.
0.75/0.25 for C33, C34/C33′/C34′. 0.48/0.52 for C58–C60/
C58′–C60′. 0.39/0.61 for C80, Cl1–Cl3/C80′, Cl1′–Cl3′. Trials
to introduce further disordered fragments did not give reliable
results, which is attributed to the already poor data-to-parameter
ratio.
We are grateful to Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for generous financial support.
Furthermore, we would like to thank Sabrina Krauße and
Martina Steinert for assistance in the lab.
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