Synthesis of ligands based on 4H-imidazoles and pyridine subunits: Selective complexation and bathochromically absorbing complexes

Article abstract of DOI:10.1002/ejic.200800914

New multifunctional ligands based on the combination of 4H-imidazoles with pyridine subunits have been synthe-sised. The long wavelength absorptions originating from the merocyanine-chromophore can be shifted towards the near infrared (MR) region by introduction of additional auxochromic groups. Due to different complexation spheres these hybrid-ligands form the requirements for the construction of heterobimetallic complexes. In addition, the hybrid ligands are redox active compounds showing two separated fully reversible reductions.

Full text of DOI:10.1002/ejic.200800914

FULL PAPER
DOI: 10.1002/ejic.200800914
Synthesis of Ligands Based on 4H-Imidazoles and Pyridine Subunits: Selective
Complexation and Bathochromically Absorbing Complexes
Jörg Blumhoff,[a] Rainer Beckert,*[a] Sven Rau,[b] Sebastian Losse,[b] Martin Matschke,[a]
Wolfgang Günther,[a] and Helmar Görls[c]
Keywords: N ligands / Imidazole / Bipyridine / Palladium / Iridium
New multifunctional ligands based on the combination of
4H-imidazoles with pyridine subunits have been synthe-
sised. The long wavelength absorptions originating from
the merocyanine-chromophore can be shifted towards the
near infrared (NIR) region by introduction of additional
auxochromic groups. Due to different complexation spheres
these hybrid-ligands form the requirements for the construc-
tion of heterobimetallic complexes. In addition, the hybrid
ligands are redox active compounds showing two separated
fully reversible reductions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
The dual function of the ligand is clearly evident in this
example. However, these azaheterocycles only absorb in the
ultraviolet region which seriously impedes application of
the so formed photochemical molecular device since this
energy rich radiation damages biological matter and leads
to different photochemical reactions such as bond cleavage.
It is therefore highly desirable to retain the well documented
ligand properties of 1,4-diazadienes and introduce new
chromophoric properties into this ligand system. Recently,
details of optimised ligands for the construction of dye-sen-
sitised solar cells were published,[11] the latter showing in-
tense absorptions in a broad range of the spectra.
Introduction
Coordination chemistry plays a fundamental role in sup-
ramolecular chemistry.[1] Among other chelating agents, oli-
gopyridines and other 1,4-diazadienes in the coordination
of metal-containing building blocks have attracted special
interest.
With respect to the conversion of solar energy, dyads,
triads and other arrangements in which an antenna mole-
cule is connected with an electron-accepting part have be-
come increasingly important as they allow for directional
energy transfer.[2] For example, bi- and terpyridine com-
plexes of several metals are often part of such supramolec-
ular architectures.[3] Besides oligopyridines, metal com-
plexes of other azaheterocycles[4–9] have become of increas-
ing importance. The construction of architectures aimed at
converting light energy into chemical energy or information
requires the use of multifunctional well understood building
blocks. An illustrative example of the implementation of
these design principles is the EuIII cryptate in which the
hexapyridodiaza cryptate serves not only as a ligand for the
europium ion but also as a UV-antenna, absorbing light
energy and transferring it to the EuIII centre. The sensitised
transition at the europium results in emission of light from
the metal centre.[10]
To the best of our knowledge, oligopyridines which pos-
sess an additional functional dye substructure are quite rare
to date.[12] There is still a need to integrate functional chro-
mophoric systems into pyridines/oligopyridines. Moreover,
especially for the application of light driven energy conver-
sion on a molecular level, storage and triggered release of
redox equivalents within a molecular subunit are extremely
important properties as they facilitate the two electron re-
duction of protons to molecular hydrogen even in systems
containing only one photo-redox active one-electron trans-
2+
fer agent such as a Ru(bpy)3 unit.[5] The 4H-imidazoles 1
(Figure 1) for which different syntheses[13] were developed
[a] Institut für Organische und Makromolekulare Chemie der
Friedrich-Schiller-Universität,
Humboldtstr. 10, 07743 Jena, Germany
Fax: +49-3641-948212
E-mail: c6bera@uni-jena.de
[b] Institut für Anorganische und Allgemeine Chemie der
Friedrich-Alexander-Universität, Erlangen Nürnberg
Egerlandstraße 1, 91058 Erlangen, Germany
[c] Institut für Anorganische und Analytische Chemie der
Friedrich-Schiller-Universität,
Figure 1. Principle of a UV-antenna and structure of “chromo-
phore-ligands” of type of 4H-imidazoles 1.
Lessingstr. 8, 07743 Jena, Germany
2162
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 2162–2169

Ligands Based on 4H-Imidazoles and Pyridine Subunits
in our group are unique substances which fulfil these cri- ogous Zn complex,[17b] this Pd complex has good solubility
teria:
in most common solvents and could, therefore, be charac-
• The protonation/deprotonation is connected with a terised completely.
drastic alteration of colour due to the change of the chro-
mophore [cyanine-merocyanine-(aza)oxonole].[14]
• Absorption of visible light (λmax up to 580 nm) with
high extinction coefficients (ε up to 4.5).
•
Reversible two-electron redox systems,[15a,15b] in
cyclised form (boratetraazapentalenes), the corresponding
radical anion (SEM-form) shows unusual high KSEM values
(up to 1015).[16]
• The peripheral 1,4-diamino/imino substructures pres-
ent good prerequisites for the construction of metal chelate
complexes; the recently reported Pd complexes proved to be
electrochemically active.[17c]
Due to the last fact, the combination of oligopyridines
with 4H-imidazoles might result in two different coordina-
tion spheres. Such types of ligands offer some advantages
for the construction of heterobimetallic complexes and for
supramolecular structures. Recently, the value of such types
of complexes in generating hydrogen has been clearly dem-
onstrated.[5] Obviously, the bridging ligand can be regarded
as key-part in such compounds. In the following part, the
syntheses and the properties of some new multifunctional
ligands are presented.
Scheme 1. Synthesis of the (4H-imidazol)2-Pd complex 1-Pd.
For the synthesis of novel multifunctional ligand systems,
two different methods were employed (Scheme 2). Method
A[13] starts from the appropriate nitrile/dinitrile 2 whereas
in method B[18] carboxylic acid chlorides 6 serve as building
blocks. Reactions of the nitriles 2 with lithium hexamethyl-
disilazide (LiHMDS) yielded the corresponding amidinates
which were converted into the persilylated amidines by
heating with chlorotrimethylsilane under reflux. Finally, the
4H-imidazoles of type 1 were obtained by a deprotection-
cyclisation sequence of the persilylated amidines using bis-
(imidoyl chlorides) 4 in the presence of KF/18-crown-6. Al-
Results and Discussion
In addition to the Pd complexes mentioned above[17c] we ternatively, in method B,[18] the cyclisation reaction between
also succeeded in synthesising an appropriate palladium oxalamidines 5 and carboxylic acid chlorides 6 formed the
complex consisting of two chelating 4H-imidazoles. Com- desired products.
pared with the analogous PdII-allyl complex,[17c] 1-Pd
Recently, we reported the synthesis of the pyridyl-substi-
shows a bathochromic shift of 30 nm to 630 nm. The syn- tuted 4H-imidazole 1a (R = 2-pyridyl, Ar = 4-tolyl)[13c]
thetic route is depicted in Scheme 1. In contrast to an anal- starting from the commercially available pyridyl-2-carb-
Scheme 2. Two synthetic pathways to 4H-imidazoles 1.
Eur. J. Inorg. Chem. 2009, 2162–2169
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2163

R. Beckert et al.
FULL PAPER
amidine. Its synthesis has now been improved using cheaper
2-cyanopyridine as the starting material. The conversion
into the corresponding imidoester derivative (NaOMe/
methanol) and subsequent aminolysis reaction with
ammonium chloride gave the amidine in high yield (93%)
which was finally cyclised according to method A to yield
1a. In an adapted synthesis starting from nitriles 2b and 2d,
derivatives 1b and 1d can be isolated in medium yields.
For the preparation of 1c and 1e, the commercially avail-
able carboxylic/dicarboxylic acids formed the basis. Upon
treatment with thionyl chloride in the presence of small
amounts of DMF they were converted into the correspond-
ing acid chlorides 6c and 6e. The 4H-imidazoles 1c and 1e
were formed by slow addition of 6c/e to a THF solution
containing the deprotonated (nBuLi) oxalamidine
5
(method B). The immediate formation of the products (as
anions) can be followed visually by the abrupt colour
change from yellow to deep purple. After work-up and pu-
rification by column chromatography both 4H-imidazoles
were obtained in yields of about 50%. All new synthesised
derivatives 1a–e are air-stable red microcrystalline com-
pounds. The spectroscopic data are in agreement with their
structures. Besides the characteristic patterns of fragmenta-
tion, the M+ peak could clearly be detected in the mass
spectra of all compounds. Due to the high symmetry in the
molecules of 1b, 1d, 1e and 1f, the 1H NMR spectra feature
single sets of signals. The aromatic protons of the tolyl
groups form the characteristic AB-system and can be easily
detected as two separated doublets. In derivatives 1b and 1f
the protons of the terpyridine subunits are characteristically
shifted towards low field.
The easy protocol according to method B gave rise to an
improvement in the synthesis of derivative 1a making this
way the method of choice. Starting from picolinic acid chlo-
ride, derivative 1a can be obtained in a yield of 50%.
Figure 2 shows the absorption and emission spectra of
derivatives 1b und 1e in their neutral, protonated as well
Figure 2. UV/Vis-spectra of 1e and 1b.
as deprotonated forms. As depicted in this figure, all new about 100 nm (Figure 3). This successful transformation
synthesised derivatives display, in solution, strong absorp- may open the way for obtaining novel ligands showing
tions in the region between 300 and 550 nm. Protonation/ NIR-absorptions.[15b]
deprotonation causes an alteration of their chromophoric
According to previous findings, 4H-imidazoles proved to
systems and consequently of their long wavelength absorp- be reversible two-electron redox systems.[15a,15b,16] In ad-
tion in their UV/Vis spectra. Compared with the neutral dition to the pH dependence of their UV/Vis spectra, their
species, both the protonated (cyanine) and the deprotonated electrochemical behaviour was studied. Exemplified by
4H-imidazole (azaoxonole) display a bathochromic shift in compound 1b for mono-4H-imidazoles, electrochemical
their UV/Vis spectra.
measurements were performed. The reduction of 1b oc-
1/2
1/2
In order to achieve a stronger bathochromic shift for the curred in two steps (U1 = –0.815 V, U2 = –1.465 V;
neutral state, the introduction of auxochromic substituents KSEM = 1.04ϫ1011).
was tested next. This objective could be realised by an ex-
Recently, new palladium complexes of simple 4H-imid-
change of peripheral arylic substituents in the merocyanine azols were synthesised and fully characterised.[17c] Based on
system of compounds 1.[13b,14] Thus, the novel 4H-imid- these findings, some of the ligands described here were
azole 1f (formula 1b, Ar = 4-Me2N-C6H4) was obtained tested with respect to their selective coordination behaviour.
by replacement of both tolyl residues by a transamination
The model reaction between ligand 1a and allyl-PdII
reaction of 1b with 4-(dimethylamino)aniline (Scheme 3). chloride dimer in THF in the presence of triethylamine gave
Derivative 1f was isolated in the form of a black-bluish only the Pd complex 1a-Pd. In this very fast complexation
powder which is soluble in THF giving a strong blue colour. reaction, the metal coordinated to both peripheral N-atoms
As expected, in the UV/Vis-spectrum of 1f, the introduction of the 4H-imidazole as well as at the 1,4-diazadiene re-
of auxochromic groups caused a bathochromic shift of sulting from the combination of pyridine-4H-imidazole.
2164
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 2162–2169

Ligands Based on 4H-Imidazoles and Pyridine Subunits
Scheme 3. Synthesis of ligand 1f by means of a transamination reaction.
trum of 1a-Pd is very similar to those of other palladium
complexes of 4H-imidazoles featuring a strong absorption
maximum at 560 nm (Figure 4).
Figure 3. Bathochromic shift of derivative 1f compared with the
parent compound 1b.
Consequently, the positively charged homobimetallic com-
plex with a chloride as the counterion could be isolated
(Scheme 4).
Figure 4. UV/Vis-spectrum of complex 1a-Pd.
Ligand 1b in which the terpyridine subunit is in competi-
tion with the chelating sphere of the 4H-imidazole was the
next promising ligand for the construction of heterobimet-
allic species. In this case we succeeded in differentiating be-
tween both chelating spheres. Besides the expected homobi-
metallic complex, the neutral monopalladium complex 1b-
Pd was isolated as by-product in small amounts (Scheme 5).
The structure of this complex was mainly elucidated from
its MS and NMR spectroscopic data. The protons of the
allyl ligand absorb characteristically in the regions of 5.2–
5.8 ppm (CH) and 3.0–4.0 ppm (CH2). The coordination of
the metal at the 4H-imidazole unit could be clearly high-
lighted by NOESY experiments in which the coupling of
the ortho protons of the aryl substituents with the protons
of the CH2 group of the allyl ligand was detected. No coup-
ling to protons of the terpyridine subunit was observed.
Scheme 4. Synthesis of the complexes 1a-Pd.
All attempts to differentiate between both complexation These findings demonstrate that the combination of pyr-
spheres, however, failed. In the mass spectrum of 1a-Pd, the idines with 4H-imidazoles should, in principle, allow a new
base peak (m/z = 646) is consistent with the homobimetallic entry to heterobimetallic complexes with long wavelength
complex. Its further fragmentation by gradual elimination absorptions.
of both allyl groups and palladium is characteristic for such
In contrast to palladium, iridium allowed the very selec-
complexes and can easily be seen in the spectrum. Further- tive formation of a monometallic complex 1b-Ir in which
more, the existence of these two different allyl ligands was only the 4H-imidazole core acted as a chelating ligand
confirmed by NMR experiments in the course of which an sphere (Scheme 5). This complex was obtained in good
unambiguous assignment was possible. The UV/Vis spec- yield as a brown powder. As expected, the iridium complex
Eur. J. Inorg. Chem. 2009, 2162–2169
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2165

R. Beckert et al.
FULL PAPER
Scheme 5. Synthesis of the complexes 1b-Pd and 1b-Ir.
Scheme 6. Assignment of H-atoms 1–18 (shifts in ppm) of 1b-Ir.
1b-Ir showed a broad absorption in the region between 500 determination (Scheme 6). Theoretically, three different iso-
and 650 nm. The 1H NMR spectrum and, in addition, mers can be formed during the reaction i.e. the N,N-trans-,
TOCSY-, ROESY-, NOESY- and HSQC-experiments as the C,N-trans- and the C,C-trans isomers (with respect to
well as the 13C-NMR spectra, allowed a complete structural the coordinating atoms of the two phenylpyridine ligands)
of 1b-Ir. Due to the presence of double sets of signals in
some cases, it is most likely that two geometrical isomers of
1b-Ir were formed during the reaction in a ratio of approxi-
mately 9:1. The analytical data reported refer to the N,N-
trans-isomer of 1b-Ir being the main product (Figure 5).
Final and unambiguous proof for the selective complex-
ation reaction was obtained by X-ray structural analysis on
a single crystal of complex 1b-Ir (Figure 5).
Conclusions
Novel multifunctional hybrid-ligands have been prepared
based from the combination of pyridines with 4H-imid-
Figure 5. Molecular structure of 1b-Ir (the H atoms have been
azoles. They are characterised by broad absorptions in their
omitted for clarity). Selected bond lengths [Å]: Ir–N1 2.173(8), Ir–
UV/Vis spectra spanning 400–750 nm. The synthesis of
complex 1b-Ir clearly demonstrated that ligands consisting
N2 2.191(9), Ir–N8 2.045(8), Ir–C49 2.000(10), Ir–C60 2.015(11),
Ir–N9 2.034(9).
2166
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 2162–2169

Ligands Based on 4H-Imidazoles and Pyridine Subunits
8.95 (s, 2 H), 8.73 (m, 6 H), 8.12 (d, 2 H), 8.02 (d, 4 H), 7.93 (t, 2
H), 7.40 (t, 2 H), 7.26 (d, 4 H), 2.39 (s, 6 H) ppm. 13C NMR
(63 MHz, [D8]THF): δ = 188.4, 157.2, 156.9, 150.1, 144.3, 137.5,
136.9, 134.2, 133.7, 131.8, 130.2, 128.9, 128.1, 124.8, 121.7, 119.5,
21.2 ppm. MS (DEI): m/z = 584 [M + H]+, 583 [M]+, 568 [M –
CH3]+. UV/Vis (THF): λmax (lg ε) = 497 nm (4.16), 526 nm (4.04).
C38H29N7 (583.66): calcd. C 78.19, H 5.01, N 16.80; found C 77.91,
H 5.46, N 16.63.
of a terpyridyl substructure and a 4H-imidazole are capable
of acting as selective and multifunctional ligands. In this
case, only the N-ligand atoms of the 4H-imidazole were in-
tegrated into the complex formation with a catalytically
active metal such as iridium. Depending on the number of
4H-imidazoles, ligands were synthesised which are capable
of binding two or three metal centres. The syntheses and
properties of further metal complexes, applicable as an-
tenna parts in light-harvesting systems will be the subject
of a forthcoming paper.
Hybrid Ligand 1d: The reaction mixture was evaporated to dryness
and the residue was suspended in water. The precipitate was filtered
off and washed with water. The residue was then washed with
methanol until the filtrate was almost colourless and finally washed
1
with n-pentane and dried in vacuo; yield 289 mg (41%). H NMR
(250 MHz, CD2Cl2): δ = 9.37 (s, 2 H), 9.00 (d, 2 H), 8.41 (d, 2 H),
7.97 (d, 8 H), 7.35 (d, 8 H), 2.44 (s, 12 H) ppm. 13C NMR
(100 MHz, CD2Cl2): δ = 187.5, 157.5, 156.8, 150.0, 140.7, 137.5,
135.3, 129.9, 123.8, 120.4, 119.7, 21.0 ppm. MS (DEI): m/z = 704
[M]+. UV/Vis (THF): λmax (lg ε) = 502 (4.43), 530 nm (4.32).
C44H36N10 (704.80): calcd. C 74.98, H 5.15, N 19.87; found C
75.09, H 5.30, N 19.61.
Experimental Section
General: All reactions were monitored by TLC, carried out on
0.25 mm Merck TLC aluminium sheets (aluminium oxide 150 F254
neutral or silica 60 F254) and visualised using UV light. 1H and 13
C
NMR spectra were recorded with a Bruker DRX 400 (400 MHz)
or a Bruker AC 250 (250 MHz) spectrometer. UV/Vis spectra were
recorded on a Perkin–Elmer Lambda 19 spectrophotometer. MS
spectra were taken from measurements on a Finnigan MAT SAQ
710 mass spectrometer. Elemental analyses were carried out in-
house with an automatic LECO CHNS 932 analyser.
General Procedure for the Synthesis of 1c, 1e: In a 100 mL one-
necked round-bottom flask, equipped with a magnetic stirrer, the
corresponding carboxylic acid (1 mmol) was dissolved or sus-
pended in thionyl chloride (50 mL). A catalytic amount of DMF
was added and the reaction mixture was heated under reflux for
2 h. The resultant yellow solution was cooled to room temperature,
the solvent was evaporated and the remaining residue dried in
vacuo. The acyl chlorides thus formed 6a, 6c, 6e and 6f were then
added to a THF solution containing the deprotonated oxalamidine
5, yielded by deprotonation of 5 with nBuLi (0.8 mL of a 2.5 
solution in n-hexane). For the completion of the reaction the mix-
ture was stirred for 20 h at room temperature.
Synthesis of Hybrid Ligand 1a: Pyridine-2-carbonitrile (0.1 mol,
10.4 g) was dissolved in methanol (100 mL) and NaOMe (0.02 mol,
1.08 g) was added and the solution stirred for 12 h at room tem-
perature. After adding ammonium chloride (0.1 mol, 5.76 g) the
reaction mixture was heated under reflux for 3 h. The solution was
then evaporated to dryness and the residue suspended in ethanol
(100 mL) and heated under reflux for 1 h. The mixture was filtered
off and the filtrate was concentrated to 25 mL. The resultant pre-
cipitate was recrystallised from 2-propanol/n-pentane to yield
11.26 g (93%) of pyridine-2-carbamidine.
Hybrid Ligand 1c: The reaction mixture was evaporated to dryness
and the red compound was purified by column chromatography
1
Pyrdidine-2-carbamidine (1 mmol, 121 mg) and bis(imidoyl chlo-
ride) 4 (1 mmol) were dissolved in THF (100 mL) in the presence
of triethylamine (2 mmol, 0.28 mL) and the reaction mixture was
heated under reflux for 6 h. The reaction mixture was evaporated
to dryness and the residue suspended in CH2Cl2. The filtrate was
again evaporated to dryness and purified by column chromatog-
raphy (alumina, activity V, toluene at the beginning, then toluene/
ethanol, 1:1); yield 190 mg (54%). Analytical data were in agree-
ment with those reported.[13c]
(alumina, activity V, toluene/acetone, 5:1); yield 196 mg (44%). H
NMR (250 MHz, [D8]THF): δ = 9.41 (s, 1 H), 8.82 (d, 1 H), 8.55
(d, 1 H), 8.41 (s, 1 H), 8.30 (d, 1 H), 8.00 (d, 4 H), 7.27 (d, 4 H),
7.18 (d, 1 H), 2.39 (s, 6 H) ppm. 13C NMR (100 MHz, [D8]THF):
δ = 187.0, 157.2, 155.6, 149.4, 148.9, 147.5, 140.6, 131.4, 129.4,
124.5, 121.6, 121.2, 120.0, 20.1 ppm. MS (DEI): m/z = 444 [M]+,
429 [M – CH3]+. UV/Vis (THF): λmax (lg ε) = 502 (4.0), 532 nm
(3.89). C28H24N6 (444.51): calcd. C 75.65, H 5.44, N 18.91; found
C 75.21, H 6.02, N 18.77.
General Procedure for the Synthesis of Hybrid Ligands 1a, 1b, 1d:
To a suspension of the corresponding nitrile 2a, 2b, 2d (1 mmol) in
THF (5 mL) in a Schlenk tube 2a, 2b (1 mL) or 2d (2 mL) as a 1 
lithium hexamethyldisilazide solution (in THF) was added drop-
wise. The reaction mixture was then stirred for 4 d at room tem-
perature. The solution was evaporated to dryness and the residue
was suspended in toluene. Chlorotrimethylsilane (1 mmol, 0.13 mL
for 2a, 2b or 1 mmol, 0.13 mL for 2a, 2b) was added and the reac-
tion mixture was heated under reflux for 12 h. The mixture was
again evaporated to dryness and the remaining residue was treated
with bis(imidoyl chloride) 4 (1 mmol, 305 mg for 2a, 2b or 2 mmol,
610 mg for 2d) in the presence of KF/18-crown-6 (3 mmol, 174 mg/
793 mg for 2a, 2b or 6 mmol, 349 mg/1586 mg for 2d) in THF. For
the completion of the reaction the mixture was stirred for 24 h at
room temperature.
Hybrid Ligand 1e: The reaction mixture was evaporated to dryness
and the residue suspended in warm methanol. The precipitate was
filtered and washed two times with methanol (50 mL). The pure
compound was obtained by recrystallisation from THF; yield
1
411 mg (58%). H NMR (250 MHz, [D8]THF ): δ = 9.76 (s, 2 H),
8.89 (d, 2 H), 8.81 (d, 2 H), 8.02 (d, 8 H), 7.28 (d, 8 H), 2.39 (s, 12
H) ppm. MS (DEI): m/z = 704 [M]+, 703 [M – H]+, 689 [M –
CH3]+. UV/Vis (THF): λmax (lg ε) = 504 (3.60), 535 nm (3.48).
C44H36N10 (704.80): calcd. C 74.98, H 5.15, N 19.87; found C
74.91, H 5.25, N 19.84.
Synthesis of Hybrid Ligand 1f: Compound 1b (1 mmol, 584 mg)
was suspended in toluene (50 mL) in a Schlenk tube. Freshly dis-
tilled N,N-dimethyl-p-phenylenediamine (4 mmol, 545 mg) and a
catalytic amount of p-toluenesulfonic acid were then added. The
reaction mixture was heated to 85 °C and kept at this temperature
for 8 h during which time the colour turned a deep-blue. The reac-
tion mixture was cooled and the formed precipitate was filtered off
and dried in vacuo; yield 546 mg (85%). 1H NMR (400 MHz,
CDCl3): δ = 8.83 (s, 2 H), 8.78 (d, 2 H), 8.70 (m, 4 H), 8.07 (d, 2
Hybrid Ligand 1b: The reaction mixture was evaporated to dryness
and the residue was suspended in warm methanol. The precipitate
was filtered off, washed three times with warm methanol and dried
1
in vacuo; yield 262 mg (45%). H NMR (250 MHz, [D8]THF): δ =
Eur. J. Inorg. Chem. 2009, 2162–2169
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2167

R. Beckert et al.
H), 7.91 (t, 2 H), 7.39 (t, 2 H), 7.26 (d, 4 H), 7.20 (d, 4 H), 3.07 (s, 148.5, 143.5, 140.4, 136.8, 130.8, 128.9, 128.3, 127.0, 124.8, 123.8,
FULL PAPER
12 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 188.3, 156.2, 156.1,
151.5, 149.2, 137.9, 136.9, 135.8, 129.0, 128.3, 125.6, 125.5, 125.3,
123.9, 121.4, 119.0, 112.25, 30.3 ppm. MS (DEI): m/z = 641 [M]+.
UV/Vis (THF): λmax (lg ε) = 596 (4.16), 836 (4.05) nm. C40H35N9
(641.74): calcd. C 74.86, H 5.50, N 19.64; found C 75.02, H 5.61,
N 19.30.
121.1, 118.7, 115.3, 61.7, 20.8 ppm. MS (FAB in Nba): m/z = 730
[M]+, 689 [M – allyl]+, 584 [M – allyl – Pd]+.
Synthesis of Complex 1b-Ir: In a 100 mL one-necked round-flask,
equipped with a magnetic stirrer, 1b (0.5 mmol, 292 mg) and bis(2-
phenylpyridine)iridium(I) chloride dimer (0.25 mmol, 268 mg) were
suspended in DMF (50 mL). After addition of triethylamine
(0.5 mmol, 0.07 mL) the reaction mixture was heated to 115 °C and
kept on this temperature for 7 h. The resultant deep red solution
was cooled and, after addition of water, the brown precipitate was
filtered off, washed with water, diethyl ether, n-pentane and dried
in vacuo; yield 412 mg (76%). 1H NMR (400 MHz, CDCl3): δ =
8.76 (m, 6 H), 8.51 (d, 2 H), 8.05 (d, 1 H), 7.94 (d, 2 H), 7.87 (t, 2
H), 7.72 (d+d, 2H+ 2 H, isochronic signals), 7.35 (m, 4 H), 7.14
(t, 2 H), 6.88 (d, 4 H), 6.72 (m, 8 H), 6.20 (d, 2 H), 2.17 (s, 6
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 194.6, 178.9, 168.4,
156.3, 156.0, 150.6, 149.2, 143.4, 143.0, 136.9, 136.7, 134.7, 134.6,
131.8, 130.8, 130.7, 129.6, 128.1, 126.8, 126.5, 125.0, 123.8, 122.3,
121.4, 121.1, 119.0, 118.5, 118.4, 117.3, 21.0 ppm. MS (FAB in
Nba): m/z = 1085 [M + 2]+. UV/Vis (CHCl3): λmax (lg ε) = 518
(3.9), 625 (3.6) nm.
Synthesis of Complex 1-Pd: In a 100 mL one-necked round-bot-
tomed flask equipped with a magnetic stirrer was dissolved ligand
1 (1 mmol, 352 mg, Ar1 = phenyl, Ar2 = Ar3 = 4-tolyl) in THF
(35 mL). After addition of triethylamine (1 mmol, 0.14 mL) and a
solution of PdII acetate (0.5 mmol, 112 mg) the reaction mixture
was stirred at room temperature for 24 h. The resultant precipitate
was filtered off and washed with THF. Afterwards, the precipitate
was purified by chromatography on a short column (alumina, ac-
tivity V, toluene/acetone, 12:1). The residue was extracted with
methylene chloride and the resultant solution was evaporated to a
small volume. The precipitated solid was filtered off, washed with
n-pentane and dried in vacuo yielding the pure Pd complex; yield
120 mg (43%). 1H NMR (250 MHz, CD2Cl2): δ = 8.17 (d, 4 H),
7.55 (t, 2 H), 7.40 (t, 4 H), 7.05 (d, 8 H), 6.76 (d, 8 H), 2.25 (s, 12
H) ppm. 13C NMR (100 MHz, CD2Cl2): δ = 199.3, 181.0, 140.1,
136.5, 134.1, 132.7, 130.4, 128.7, 128.2, 125.1, 20.9 ppm. MS
(DEI): m/z = 807 [M – H]+. UV/Vis (THF): λmax (lg ε) = 630
(3.9) nm. C46H38N8Pd (809.24): calcd. C 68.27, H 4.73, N 13.85;
found C 67.95, H 4.30, N 13.11.
Crystal Structure Determination: The intensity data for the com-
pound were collected on a Nonius KappaCCD diffractometer,
using graphite-monochromated Mo-Kα radiation. Data were cor-
rected for Lorentz and polarisation effects but not for absorption
effects.[19,20] The structure was solved by direct methods
(SHELXS[21]) and refined by full-matrix least-squares techniques
against Fo2 (SHELXL-97[22]). All hydrogen atoms were included in
calculated positions with fixed thermal parameters. All non-hydro-
gen atoms were refined anisotropically.[22] XP (SIEMENS Analyti-
cal X-ray Instruments, Inc.) was used for structure representations.
Synthesis of Complex 1a-Pd: In a 100 mL one-necked round-bot-
tom flask, equipped with a magnetic stirrer was dissolved ligand 1a
(1 mmol, 353 mg) in THF (60 mL). After addition of triethylamine
(1 mmol, 0.14 mL) and a solution of (allyl)palladium(II) chloride
dimer (1 mmol, 366 mg) in THF (20 mL), the reaction mixture was
stirred at room temperature for 20 h. The resultant precipitate was
filtered off and dried in vacuo yielding the pure Pd complex; yield
415 mg (64%). 1H NMR (400 MHz, CDCl3): δ = 9.18 (d, 1 H),
8.42 (d, 1 H), 8.16 (t, 1 H), 8.08 (m, 1 H), 7.38 (d, 4 H), 7.25 (d, 4
H), 5.75 (m, 1 H), 5.34 (m, 1 H), 3.99 (d, 1 H), 3.67 (m, 2 H,
broad), 3.34 (m, broad, 3 H), 2.91 (d, 1 H), 2.74 (m, broad, 1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 190.4, 171.1, 155.5,
144.1, 139.7, 137.9, 129.6, 126.8, 124.8, 122.8, 121.9, 117.0, 109.7,
63.9, 61.7, 61.0, 21.2 ppm. MS (FAB in Nba): m/z = 648 [M +
H]+, 607 [M – allyl]+, 500 [M – allyl – Pd]+, 459 [M – 2 allyl –
Pd]+, 352 [M – H – 2 allyl-2 Pd]+. UV/Vis (THF): λmax (lg ε) = 557
(3.8), 590 (3.7) nm. C28H28ClN5Pd2 (682.83): calcd. C 49.25, H
4.13, N 10.26; found C 49.49.02, H 4.31, N 10.07.
Crystal Data for 1b-Ir:[23] C60H44IrN9·2.5CHCl3·2H2O,
M =
1453.73 gmol–1, brown prism, size 0.06ϫ0.06ϫ0.02 mm, triclinic,
¯
space group P1, a = 13.4240(9), b = 13.4260(8), c = 18.4943(13) Å,
α = 82.233(4), β = 82.453(4), γ = 83.768(3)°, V = 3260.0(4) Å3, T
= –90 °C, Z = 2, ρcalcd. = 1.481 gcm–3, µ(Mo-Kα) = 24.08 cm–1,
F(000) = 1458, 21717 reflections in h(–17/17), k(–17/16), l(–23/23),
measured in the range 1.79°ՅθՅ27.52°, completeness θmax
=
96%, 14423 independent reflections, Rint = 0.067, 8165 reflections
with Fo Ͼ 4σ(Fo), 771 parameters, 0 restraints, R1obs = 0.0876,
wR2obs = 0.2000, R1all = 0.1731, wR2all = 0.2418, GOOF = 1.035,
largest difference peak and hole: 2.610/–0.943 e·Å–3.
Synthesis of Complex 1b-Pd: In a 100 mL one-necked round flask,
equipped with a magnetic stirrer, 1b (1 mmol, 584 mg) was dis-
solved in THF (50 mL). Triethylamine (1 mmol, 0.14 mL) and (al-
lyl)palladium(II) chloride dimer (0.5 mmol, 183 mg) dissolved in
THF (10 mL) were added dropwise at room temperature. The reac-
tion mixture was heated to reflux for 2 h until completion. The
mixture was evaporated to dryness and the remaining residue was
suspended in methanol. The filtrate was again evaporated to dry-
ness and purified by column chromatography (alumina, activity V,
first toluene/ethyl acetate, 1:1, then pure THF). Although the
homobimetallic complex is the main product, complex 1b-Pd could
be purified and isolated as a by-product by column chromatog-
raphy since it is the only neutral complex formed in this reaction;
yield 37 mg (5%). 1H NMR (250 MHz, CD2Cl2): δ = 8.84 (s, 2 H),
8.74 (m, 4 H), 8.52 (d, 2 H), 8.01 (d, 2 H), 7.94 (t, 2 H), 7.60 (d, 4
H), 7.41 (t, 2 H), 7.26 (d, 4 H), 5.67 (m, 1 H), 3.67 (d, 2J = 6.8 Hz,
2 H), 3.20 (d, 2J = 12.4 Hz, 2 H), 2.43 (s, 6 H) ppm. 13C NMR
(100 MHz, CD2Cl2): δ = 196.0, 177.8, 156.2, 156.0, 151.5, 149.2,
Acknowledgments
We thank Deutsche Forschungsgemeinschaft (DFG) (BE1366/8-1)
for financial support. Roche Diagnostics GmbH, Syngenta and
Clariant are greatfully acknowledged for support with chemicals.
[1] J. M. Lehn, Supramolecular Chemistry Concepts and Perspec-
tives, VCH 1995.
[2] V. Balzani (Ed.), Electron Transfer in Chemistry, Wiley-VCH,
2001, vol. 1–5.
[3] V. Balzani, M. Venturi, A. Credi, Molecular Devices and Ma-
chines, Wiley-VCH, Weinheim (Germany), 2003; Artificial Pho-
tosynthesis (Eds.: A. F. Collings, C. Critchley), Wiley-VCH,
Weinheim (Germany), 2005.
[4] R. Konduri, H. Ye, F. M. MacDonell, S. Serroni, S. Campagna,
K. Rajeshwar, Angew. Chem. Int. Ed. 2002, 41, 3185–3187; R.
Konduri, N. R. de Tacconi, K. Rajeshwar, F. M. MacDonell,
J. Am. Chem. Soc. 2004, 126, 11621–11629.
2168
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 2162–2169

Ligands Based on 4H-Imidazoles and Pyridine Subunits
[5] S. Rau, B. Schäfer, D. Gleich, E. Anders, M. Rudolph, M.
Friedrich, H. Görls, W. Henry, J. G. Vos, Angew. Chem. 2006,
118, 6361–6364.
[6] P. J. Steel, Acc. Chem. Res. 2005, 38, 243–250.
[7] a) E. A. Medlycott, G. S. Hanan, Chem. Soc. Rev. 2005, 34,
133–142; b) S. Kohlmann, S. Ernst, W. Kaim, Angew. Chem.
1985, 97, 698–699.
[8] D. P. Rillema, R. Sahai, P. Matthews, A. K. Edwards, R. J.
Shaver, Inorg. Chem. 1990, 29, 167–175.
[9] L. Zhang, P. Carroll, E. Meggers, Org. Lett. 2004, 6, 521–523.
[10] a) B. Alpha, J. M. Lehn, G. Mathis, Angew. Chem. 1987, 99,
259–261; b) B. Alpha, J. M. Lehn, G. Mathis, Angew. Chem.
Int. Ed. Engl. 1987, 26, 266–267.
Raabe, W. Günther, H. Görls, Z. Naturforsch., Teil B 2006, 61,
437–447.
J. Fabian, H. Görls, R. Beckert, J. Atzrodt, J. Prakt. Chem./
Chemiker-Ztg. 1997, 339, 735–741.
a) M. Matschke, C. Käpplinger, R. Beckert, Tetrahedron 2006,
62, 8586–8590; b) M. Matschke, R. Beckert, Molecules 2007,
12, 723–734.
T. Gebauer, R. Beckert, D. Weiß, K. Knop, C. Käpplinger, H.
Görls, Chem. Commun. 2004, 1860–1861.
a) J. Atzrodt, R. Beckert, K. Nordhoff, M. Bräuer, E. Anders,
H. Görls, Eur. J. Org. Chem. 1998, 2557–2563; b) J. Atzrodt,
R. Beckert, H. Görls, Heterocycles 1999, 51, 763–783; c) J.
Blumhoff, R. Beckert, D. Walther, S. Rau, M. Rudolph, H.
Görls, W. Plass, Eur. J. Inorg. Chem. 2007, 481–486.
[14]
[15]
[16]
[17]
[11] C.-Y. Chen, S.-J. Wu, C.-G. Wu, J.-G. Chen, K.-C. Ho, Angew.
Chem. 2006, 118, 5954–5957.
[18]
D. Müller, R. Beckert, H. Görls, Synthesis 2001, 601–606.
[12] a) L. Flamigni, F. Barigelletti, N. Armaroli, J.-P. Collin, J.-P.
Sauvage, J. A. G. Williams, Chem. Eur. J. 1998, 4, 1744–1754;
b) L. Flamigni, F. Barigelletti, N. Armaroli, B. Ventura, J.-P.
Collin, J.-P. Sauvage, J. A. G. Williams, Inorg. Chem. 1999, 38,
661–667; c) L. Flamigni, F. Barigelletti, N. Armaroli, J.-P. Col-
lin, I. M. Dixon, J.-P. Sauvage, J. A. G. Williams, Coord. Chem.
Rev. 1999, 190–192, 671–682; d) T. V. Duncan, I. V. Rubtson,
[19] COLLECT, Data Collection Software, Nonius B. V., The Ne-
therlands, 1998.
[20] Z. Otwinowski, W. Minor, Processing of X-ray Diffraction Data
Collected in Oscillation Mode, in: Methods in Enzymology, vol.
276, Macromolecular Crystallography, part A (Eds: C. W. Car-
ter, R. M. Sweet), p. 307–326, Academic Press, 1997.
H. T. Uyela, M. J. Therien, J. Am. Chem. Soc. 2004, 126, 9474– [21] G. M. Sheldrick, Acta Crystallogr., Sect. A 1990, 46, 467–473.
9475; e) T. Akasaka, J. Otsuki, K. Araki, Chem. Eur. J. 2002,
8, 130–136; f) T. Akasaka, T. Mutai, J. Otsuki, K. Araki, Dal-
ton Trans. 2003, 1537–1544.
[22] G. M. Sheldrick, SHELXL-97 (release 97-2), University of
Göttingen, Germany, 1997.
[23] CCDC-719100 (for 1b-Ir) contains the supplementary crystal-
lographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
[13] a) J. Atzrodt, J. Brandenburg, C. Käpplinger, R. Beckert, W.
Günther, H. Görls, J. Fabian, J. Prakt. Chem./Chemiker-Ztg.
1997, 339, 729–734; b) J. Atzrodt, R. Beckert, W. Günther, H.
Görls, Eur. J. Org. Chem. 2000, 1661–1668; c) R. Beckert, C.
Hippius, T. Gebauer, F. Stöckner, C. Lüdigk, D. Weiß, D.
Received: September 12, 2008
Published Online: April 1, 2009
Eur. J. Inorg. Chem. 2009, 2162–2169
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2169