Synthesis and coordination properties of 6,6′-dimesityl-2,2′-bipyridine

Article abstract of DOI:10.1515/znb-1999-0421

A synthetic route to 6,6′-dimesityl-2,2′-bipyridine is presented that involves a Suzuki coupling of 6,6′-dibromo-2,2′-bipyridine with mesityl boronic acid. The new sterically crowded ligand is investigated by X-ray analysis and its coordination behavior in the presence of copper(I) is examined.

Full text of DOI:10.1515/znb-1999-0421

Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
Michael Schmittela+*, Andrea Ganz3, Wolfdieter A. Schenkb, Michael Hagelb
a Institut für Organische Chemie der Universität Würzburg,
b Institut für Anorganische Chem ie der Universität Würzburg, Am Hubland,
D-97074 Würzburg, Germany
Z. Naturforsch. 54b, 5 59-564 (1999); received January 11, 1999
Suzuki Coupling, Copper(I) Com plex, X-Ray Data
A synthetic route to 6,6'-dimesityl-2,2'-bipyridine is presented that involves a Suzuki cou-
pling of 6,6'-dibrom o-2,2'-bipyridine with mesityl boronic acid. The new sterically crowded
ligand is investigated by X-ray analysis and its coordination behavior in the presence of
copper(I) is examined.
During the last years, the preparation of large su-
pramolecular structures has become an attractive
goal. Many research groups have explored the
chances of coordination chemistry since the well-
defined geometric situation at individual metal
sites facilitates the design of complex architectures
[1], Among others, tetrahedrally coordinating met-
als, such as copper(I), and rigid chelating ligands,
2,2'-bipyridine 2. Bipyridine ligands contain an
such as phenanthrolines, have been used for the
preparation of fascinating structures like helices
[lb, 2], catenanes [3], rotaxanes [4] and many
others.
analogous chelating bisimine unit, however display
a higher flexibility due to the lacking aromatic
bridge [7]. Because of the higher flexibility we ex-
pected ligand 2 to exhibit a lower kinetic barrier
in the complexation step which could be of impor-
tance for the self-repair of supramolecular struc-
tures. Herein, we now describe the preparation of
2, its solid state structure and some investigations
concerning its coordination chemistry with cop-
per(I) ions.
While so far most studies concentrated on bis-
homoleptic copper(I) complexes as building
blocks, we have recently developed a strategy to
prepare defined heteroleptic copper(I) bisphen-
anthroline complexes [5] that allow us to construct
molecular boxes in an elegant self-assembly pro-
cess [6]. The key constituent of our modus ope-
randi relies on 2,9-diaryl phenanthrolines, such as
1, whose sterically crowded aryl groups prevent
the formation of the thermodynamically most sta-
ble bishomoleptic complex [Cu(l)2]+, thus render-
ing the heteroleptic species the most stable one
[5].
To further understand the physical organic
background of this concept we wished to investi-
gate the properties of the accordingly substituted
Results and Discussion
Reprint requests to Prof. Dr. M. Schmittel.
It has been reported for other bipyridine ligands
that substitution in 6,6'-positions can be achieved
by addition of the appropriate aryllithium com-
pound to the unsubstituted bipyridine 3 followed
by oxidation to rearomatize the bipyridine core, in
N ew address after 01.04.1999: FB 8-OC 1 (C h em ie-
B iologie), Universität GH Siegen, A dolf-R eichwein-
Str., D-57068 Siegen, Germany.
Fax: Int. +492717403270
E-mail: schmittel@ chemie.uni-siegen.de
0932-0776/99/0400-0559 $06.00 © 1999 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
D
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560
M. Schmittei et al. • Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
analogy to the preparation of the corresponding
phenanthroline ligands [8]. Therefore, we applied
similar reaction conditions as in the preparation of
ligand 1 [9]. The first substitution step afforded the
monosubstituted 6-mesityl-2,2'-bipyridine (4) in a
yield of 60%, which is comparable to the yield for
the preparation of the phenanthroline ligand. In
the second step, however, no reaction could be ob-
served under the usual conditions (i.e. room tem-
perature). When heating the mixture to 50-60 °C
a trisubstituted product 5 was isolated in 9% yield
accompanied by a large amount of unreacted start-
ing material (4).
1. Bu-Li
2. CuCI2l 0 2
Et20, -78 °C
N
Br
39%
Me
Pd(PPh3)4,Na2C03
toluene, MeOH, H20,
80-90 °C, 18 h
(H O bB-d^KM e
Me
Mes
,Mes
1. Mes-Li, Et20,
RT, 12-16 h
2. H20
3. Mn02
60%
Table I. Selected bond lengths and angles of 2.
Bond lengths [A]
Dihedral angles [°]
Me
1.346
N (l)-C (2 1 )
N (l)-C (2 5 )
N (2)-C (31)
N (2)-C (35)
C (21)-C (35)
N (l)-C (2 5 )-C (ll)-C (1 6 ) -116.77
1. Mes-Li, Et20,
benzene, reflux,
2. H20
1.365 N (2 )-C (3 1 )-C (4 1 )-C (4 6 )
-64.48
4
h
Mes =
-Me
1.324
1.335
1.490
3. Mn02
Me
Mes
C (11) —C(25) 1.478
C (31)-C (41) 1.505
+
4(81%)
the bipyridine skeleton by only 25-30° [14]. Thus,
the steric hindrance of the methyl groups in 2
causes the aryl group to be heavily tilted, thereby
reducing the conjugation with the bipyridine unit.
To investigate the coordination behavior of 2 we
mixed [Cu(MeCN)4]BF4 with an equimolar
5 (9%)
Therefore, to prepare the desired ligand 2 a dif-
ferent synthetic route had to be used. As we could
recently prepare sterically crowded biaryl com-
pounds [9] using the Suzuki coupling method [10],
we decided to apply this method as well. Hence, amount as well as with an excess of ligand 2 in
6,6'-dibromo-2,2'-bipyridine (7) was prepared by chloroform or TCE (1,1,2,2-tetrachloroethane).
oxidative coupling of 2,6-dibromopyridine (6) [11]. Even after heating these mixtures to 150 °C only
Subsequent Suzuki coupling of 7 with mesityl bo- the monocoordinated product [Cu(2)(MeCN)]+
ronic acid (8) [12] afforded the new ligand 2 in a was detected (see p. 561).
yield of 72%.
By crystallization from deuterochloroform at
room temperature, suitable crystals of 2 for X-ray
Thus, even the more flexible bipyridine skeleton
can not overcome the steric shielding of the bis-
imine coordination site exerted by the mesityl
analysis were obtained. The X-ray structure shows groups, therefore preventing the formation of the
a transoid, almost planar geometry at the bipyri- bishomoleptic complex in an effective manner.
dine moiety as observed for many other bipyridine
These experiments show that 2 matches the profile
ligands [13]. The sterically crowded mesityl groups of steric coordination control [5, 9] which is funda-
are rotated with respect to the bipyridine skeleton mental to our concept of preparing stable hetero-
by a dihedral angle of 63° and 64° respectively leptic copper(I) complexes. To our knowledge,
(Fig. 1, Table 1).
For comparison, the phenyl groups of 6,6'-di- anthroline-bipyridine copper(I) complexes, except
phenyl-2,2'-bipyridine are rotated with respect to
there has been no report on stable mixed phen-
in cases where the geometrical constraint of one
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561
M. Schmittei et al. ■Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
Fig. 1. X-Ray structure (ball stick representation) of 2.
[Cu(MeCN)4]BF4
+
2
TCE,
140-150 °C
Cu(MeCN)*
+
2
[Cu(2)(MeCN)]
ligand prevented the formation of a bishomoleptic
compound [15]. As a consequence, we can now
present the first preparation of an unconstrained of [Cu(MeCN)4]PF6 and the 4,7-disubstituted phe-
mixed phenanthroline-bipyridine copper(I) com- nanthroline 9 [16] furnished the new heteroleptic
plex. Reacting ligand 2 with an equimolar amount copper(I) complex [Cu(2)(9)]PF6in a yield of 96%
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M. Schmittei et al. • Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
562
7.9 Hz, 4J = 0.9 Hz, 1H), 8.44 (dt, V = 8.2 Hz, 4J =
0.9 Hz, 1H), 8.68 (m. 1H); 13C NMR (50 MHz,
CDCI3 ): (3 = 20.6, 21.2, 108.3, 119.3, 121.9, 124.3,
125.3, 128.9, 136.4, 137.1, 137.5, 137.9, 149.5, 156.4,
156.9, 159.1.
after purification. It was unambiguously identified
on the basis of its spectral and analytical data.
The straightforward preparation of the uncon-
strained mixed phenanthroline bipyridine cop-
per(I) complex supports the generality of our con-
cept. Future investigations shall explore whether
the kinetic barrier of complex dissociation is lower
for [Cu(2)(9)]PF6 than for [Cu(l)(9)]PF6 which
could play a major role for the repair of metallo-
supramolecular architectures spanning several of
such coordination sites to yield the thermodynami-
cally most stable structure.
Experiment to prepare 2 via 4. Bromomesitylene
(4.46 g, 22.4 mmol) was dissolved in diethylether
(100 ml) and a 1.6 M solution of «-butyllithium in
«-hexane (22.4 mmol) was added at room temper-
ature. After 1 h stirring at room temperature,
ligand 4 (700 mg, 4.49 mmol) dissolved in benzene
(12 ml) was added and the mixture heated to re-
flux for 4 h. After work-up according to that of 4
the brown oily residue was separated by column
chromatography (silica gel, cyclohexane:diethyl-
ether = 100:1), furnishing the starting material 4
(997 mg, 3.64 mmol, 81%) and the trisubstituted
product 5 (193 mg, 380//mol). The latter was iden-
tified by its characteristic 'H NMR spectrum. 'H
NMR (200 MHz, CDCI3 ): <5 = 2.11 (s, 18H, Me),
2.36 (m, 9H, Me) , 6.92 (m, 6H, Mes) , 7.20 (d, J =
8.0 Hz, 1H), 7.30 (m, 1H), 7.69 (d, J = 8.0 Hz, 1H),
7.76 (m, 1H), 8.65 (m, 1H).
Experimental Section
'H and 13C NMR spectra were recorded on
Bruker AC-200 or AM-250 instruments and cal-
ibrated with tetramethylsilane as an internal refer-
ence (TMS, d = 0.0 ppm). IR spectra were re-
corded on a Perkin-Elmer 1605 series FT-IR-
spectrometer. Elemental analyses were measured
on a Carlo Erba Elemental Analyzer 1106. Melt-
ing points were determined by using a Mettler
FP5.0. All reactions were carried out under an
inert atmosphere. Solvents were dried using stand-
ard methods. Chemicals were purchased and used
without further purification.
6,6'-Bis(2,4,6-trimethylphenyl)-2,2'-bipyridin (2) by
Suzuki coupling
7
(200 mg, 637 //mol) [11] and tetrakis(triphen-
ylphosphan)palladium(O) (6.00 mg, 5.20 //mol)
were dissolved in boiling toluene (40 ml). A solu-
tion of 8 (260 mg, 1.59 mmol) [12] in methanol
(5 ml) and a 2 M sodium carbonate solution (8 ml)
were added and the mixture was heated to reflux
for 4 h. After further addition of 8 (100 mg) heat-
ing was continued for 3 days. After cooling, the
layers were separated and the organic layer
washed with a solution of sodium carbonate con-
taining a small amount of ammonia. The aqueous
layer was extracted with dichloromethane
(2x50 ml) and the organic layers equally washed
with the sodium carbonate solution. The combined
organic layers were dried (M gS04) and concen-
trated. The residue was purified by column chro-
matography (silica gel, dichlormethane : cyclohex-
ane = 3:1; Rf = 0.52) to furnish 2 (182 mg, 465
//mol, 72%) as a colorless solid. M.p. 237-238 °C;
IR (KBr) v = 2946, 2916, 2851, 1612 (C=C), 1571
(C=C), 1441, 1375, 1150, 1080, 1034, 989, 851, 809,
758 cm -1; ‘H NMR (200 MHz, CDC13): (3 - 2.16
(s, 12H, Me), 2.39 (s, 6H, Me), 7.02 (s, 4H, Mes),
7.25 (dd, 3J = 7.6 Hz, 4J = 1.2 Hz, 2H), 7.84 (t, J =
7.6 Hz, 2H), 8.37 (dd, V = 7.6 Hz, 4J = 1.2 Hz,
2H); 13C NMR (50 MHz, CDCI3 ): d = 20.5, 21.2,
119.2, 124.7, 128.5, 136.0, 136.9, 137.5, 138.1,
156.3, 159.1.
Failure to prepare 2 via 4
6-(2,4,6-Trimethylphenyl)-2,2'-bipyridine
(4).
Bromomesitylene (3.18 g, 16.0 mmol) was dis-
solved in diethylether (70 ml) and a 1.6 M solution
of «-butyllithium in «-hexane (16.0 mmol) added
at room temperature. After 1 h stirring at room
temperature, 2,2'-bipyridine (3) (1.00 g, 6.40
mmol) was added and the mixture stirred over
night. After hydrolysis with water, the deep or-
ange organic layer was separated and the aqueous
layer extracted with dichloromethane (3x50 ml).
Manganese dioxide (10.0 g) was added in portions
to the combined organic layers and the mixture
stirred for 90 min. After addition of magnesium
sulfate, the mixture was filtered, concentrated and
the brown oily residue purified by column chro-
matography (silica gel, dichloromethane m eth a-
nol
= 100:1, Rf = 0.53) affording 2 (1.04 g,
3.80 mmol, 60%) as a yellow solid. IR (KBr) v =
3057, 2954, 2857, 1614 (C=C), 1574 (C=C), 1470,
1434, 1378, 783, 754 cm“1; ’H NMR (250 MHz.
CDC13): Ö = 2.10 (s, 6H, Me), 2.35 (s, 3H, Me),
6.98 (s, 2H, Mes), 7.26 (dd, V - 7.3 Hz. 4J = 0.93
Hz, 1H), 7.29 (m, 1H), 7.75 (td, V = 7.9 Hz, 4J =
1.8 Hz, 1H), 7.87 (t, J = 7.9 Hz, 1H), 8.36 (dd, V =
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M. Schmittei et al. • Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
563
C28H28N2 (392.5)
Coordination properties o f 2
Calcd
C 85.67 H7.19 N 7.14%,
Reaction o f 2 with copper(I): [Cu(MeCN)4]PF6
(5.0 mg, 13 //mol) was dissolved in CDC13 (0.7 ml)
and 2 (10.46 mg, 26 //mol) was added. The yellow
solution was examined by NMR spectroscopy and
the complex [Cu(2)(MeCN)]+ identified by char-
acteristic chemical shifts, besides uncoordinated 2.
!H NMR (200 MHz, CDC13): <5= 1.98 (s, 15H, Me,
CH3-CN ), 2.32 (s, 6H, Me), 6.95 (s, 4H), 7.54 (d,
J = 7.5 Hz, 2H), 8.22 (t, J = 7.8 Hz, 2H), 8.37 (d,
J = 7.8 Hz, 2H). When the above reactants were
heated in TCE (140-150 °C) for 3 h the subse-
quent NMR analysis still showed a mixture of
[Cu(2)(MeCN)]+: 2 in a ratio of 1:1 and no traces
of [Cu(2)2]+.
Found C 85.36 H 7.21 N7.13%.
Crystal data o f 2: The structure of 2 was estab-
lished by an X-ray structural analysis. Crystal data
and the details of the procedure are compiled in
Table II. The structure was solved by direct meth-
ods (SHELXS 96 program) [17] and refined by the
block-matrix least-squares method (SHELXL 96
program) [17].
Crystallographic data for the structure reported
in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supple-
mentary material (CSD 410686 a).
Preparation o f a heteroleptic complex
[Cu(2)(9)]PF6. [Cu(MeCN)4]PF6 (19.5 mg, 52
//mol) was dissolved in dichloromethane (10 ml)
and a solution of 2 (20.0 mg, 52 //mol) and 9
(20.0 mg, 52//mol) [16] in dichloromethane
(10 ml) was added. After stirring the deep violet
mixture for 30 min, the solvent was evaporated
and the residue purified by column chromatogra-
phy (silica gel, dichloromethane:methanol = 10:1)
to afford [Cu(2)(9)]PF6 (49.0 mg, 50 //mol, 96%)
as a deep violet solid. M.p. >250 °C; IR (KBr) v -
2919, 2828, 2211 (C=C), 1614 (C=C), 1591
(C=C), 1563 (C=C), 1509, 1463, 1444, 1423, 1377,
821 (P-F), 805, 758, 732, 690 cm -1; !H NMR (250
MHz, CDC13): <5 = 1.52 (s, 6H, [2]), 1.72 (s, 12H,
[2]), 5.89 (s, 4H, [2]), 7.47 (d, J = 7.3 Hz, 2H, [2]),
7.50 (m, 6H, [9]), 7.77 (m, 4H, [9]), 7.87 (d, J = 4.9
Hz, 2H, [9]), 8.25 (t, J = 7.7 Hz, 2H, [2]), 8.42 (s,
2H, [9]), 8.48 (d, J = 4.9 Hz, 2H, [9]), 8.54 (d, J =
8.2 Hz, 2H, [2]); 13C NMR (63 MHz, CDC13): <5 =
20.6, 20.8, 78.4, 84.3, 101.9, 120.7, 124.9, 126.7,
126.8, 126.9, 128.2, 128.9, 129.9, 130.3, 132.2, 134.8,
137.4, 137.5, 139.1, 142.8, 146.9, 152.2, 158.3.
Table II. Crystallographic data.
Formula (M F)
CP228H 28N 2 (392.52)
Space group
Lattice constants [A]
a = 6.8785(12)
b = 15.228(2)
c = 10.974(2)
a = 90°
ß = 95.701(8)°
y = 90°
Z
2
F(000)
420
Cell volum e [A 3]
Temperature
D ensity [g em -1] calc.
Diffractom eter
Radiation
1143.9(3)
293(2) K
1.140
Enraf-Nonius-CA D4
M oK a, Graphite m on o-o
chromator, A = 0.71073 A
Index range
0 < h < 8
- 4
<
k < 18
- 1 1
<
1
<
11
0-Range
2.29°-25.95°
3149
Total reflections
Symmetry independent
reflections (N )
Parameters (P)
N/P
C56H44N4CuPF6•2H20 (1017.53)
2925
278
10.52
Calcd
C 66.10 H 4.76 N5.51%,
Found C 66.56 H 4.89 N5.46%.
^-value
R,„-value
R 1 = 0.0762, wR2 = 0.1139
R \ = 0.0408, wR2 = 0.0928
Acknowledgements
We gratefully acknowledge financial support
from the Deutsche Forschungsgemeinschaft
(Schm 647/5-2, SFB 347) and the Fonds der Che-
mischen Industrie.
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564
M. Schmittei et ai ■Synthesis and Coordination Properties of 6,6'-Dimesityl-2,2'-bipyridine
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