A unique six-membered chelated iridium complex

Article abstract of DOI:10.1039/b807735p

The reaction of 2-(3-methylbiphenyl-2-yl)pyridine with IrCl ₃?nH₂O in the presence of water incorporates a tightly bonded CO at the iridium center; in the subsequent reaction with picolinic acid a complex revealing an unusual CH activation of a methyl group is formed.

Full text of DOI:10.1039/b807735p

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A unique six-membered chelated iridium complex†
Sven Ammermann,a Constantin Daniliuc,b Peter G. Jones,b Wolf-Walther du Mont,b Wolfgang Kowalskya and
Hans-Hermann Johannes*a
Received 6th May 2008, Accepted 6th June 2008
First published as an Advance Article on the web 16th June 2008
DOI: 10.1039/b807735p
The reaction of 2-(3-methylbiphenyl-2-yl)pyridine with
IrCl3·nH2O in the presence of water incorporates a tightly
bonded CO at the iridium center; in the subsequent reaction
with picolinic acid a complex revealing an unusual CH
activation of a methyl group is formed.
4 and 1 as a two step reaction (Scheme 1). The first step was
carried out following procedures from Nonoyama6 leading to a l-
chloro-bridged dimer whereas the second step followed mild room
temperature procedures from Coppo et al.7 preventing scrambling
of the ligands.
We characterized our isolated product 2 from the second
step (14% yield) and found a strong band at 2021 cm−1 in
the IR spectrum, indicating a CO moiety in the molecule. The
NMR spectra displayed a large number of signals indicating an
unsymmetric molecule.‡ Based on our experience with heteroleptic
complexes, such as the proposed complex 1, we expected two
independent sets of signals originating from the ligand 3. Instead
we found only one set of signals and also the methyl group
disappeared. We concluded that the obtained product was not
consistent with the proposed structure 1, and subjected it to
X-ray analysis, revealing an unexpected and unique structure 2
(Fig. 2) consisting of a six-membered chelated iridium complex.§
The results were finally confirmed by repetition of the reactions.
Late transition-metals such as Rh, Pt and Ir exhibit interesting
properties. Among those properties, catalytic activity such as
selective CH activation is most attractive to the synthetic chemist.
Up to today there has been no general solution available for a
catalytic functionalization process of unactivated sp3 CH bonds.1
In addition, various complexes of these metals can also be
employed in OLEDs (organic light-emitting diodes) as high
efficient triplet emitting materials.2 Due to their significant spin–
orbit coupling second and third row transition metals are most
effective in harvesting the efficiency limiting triplet states.
We were searching for new materials that are applicable in
OLEDs and were stimulated by a patent containing an interesting
structural motif of a tweezer-like metal complex (an example is
shown in Fig. 1).3 The depicted idea of the ligand embracing the
metal center for use as an emitter in OLEDs was related to some
of our work.4
Fig. 1 Structures of a proposed tweezer-like metal complex 1 from a
patent3 and of isolated complex 2.
We focused on complex 1 from the literature3 and since no
reaction details were given, we conducted the synthesis according
to the following procedures. The ligand 2-(3-methylbiphenyl-2-
yl)pyridine 3 was prepared according to literature methods5 and
obtained in an overall yield of 23% after HPLC purification. The
cyclometalation was planned with regard to the expected products
Fig. 2 ORTEP representation of the complex 2 with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms and solv◦ent molecules
˚
are omitted for clarity. Selected bond lengths (A) and angles ( ): Ir1–C25 =
1.835(7), C25–O3 = 1.134(8), Ir1–Cl1 = 2.3392(15), Ir1–C12 = 2.070(6);
N1–C1–C6–C11 = −45.7(8); O1–Ir1–N2 = 78.33(18).
In order to understand the reaction we returned to the precursor
4 or 5 from the first step. We were not able to crystallize the
compound, but IR investigations showed evidence that the CO is
already inserted in this step. A fairly strong band, weaker than
the CO band of 2, was observable at 2025 cm−1, suggesting that
the CO bonding modes are closely related to each other (see also
ref. 8 for a review on vibrational frequencies in binary unsaturated
transition-metal carbonyls).
aLabor fu¨r Elektrooptik am Institut fu¨r Hochfrequenztechnik, Technische
Universita¨t Braunschweig, Postfach 3329, 38023, Braunschweig, Germany.
E-mail: h2.johannes@ihf.tu-bs.de; Fax: +49-531-391-2006; Tel: +49-531-
391-8004
bInstitut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t
Braunschweig, Postfach 3329, 38023, Braunschweig, Germany
† Electronic supplementary information (ESI) available: Experimental
procedures for preparation of compounds 2, 3 and 5a, detailed spectro-
scopic data. CCDC reference number 675630. For ESI or crystallographic
data in CIF or other electronic format see DOI: 10.1039/b807735p
Given that our reactions were carried out in a N2 atmosphere,
insertion of gaseous CO as often described in the literature9 is
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Scheme 1 Expected (upper path) and observed (lower path) reaction together with proposed intermediate products 4, 5a and 5b. Compound 2 was only
synthesized from intermediate product 5a.
excluded. Initially we focused on the starting material IrCl3·nH2O.
We examined the employed portion by IR spectroscopy and found
only a weak signal at 2100 cm−1, eliminating this material as the
source of CO. Since CO was inserted in the first step, we focused
on water as the source of oxygen in the CO. Several groups have
monitored and investigated the incorporation of CO into iridium
complexes with the aid of 18O-labeled water, demonstrating that
the oxygen originates from water.10 We thus performed the first
reaction step by replacing H216O (leading to compound 5a) with
H218O (leading to compound 5b) and subsequently identified a
shift of the CO band from 2025 to 1985 cm−1 for the first step (IR
results are shown in Fig. 3). This result is in accordance with the
theoretically calculated shift to 1977 cm−1.8
ligand 3 and 13C-labeled solvent. Since both compounds are
impractical we decided to perform a series of experiments to
indirectly narrow down the source of the carbon. At first we
examined the aqueous filtrate from step 1 to see if we could
find evidence for a demethylation of 3. According to GC-MS
analysis only 3, which has been used in excess, can be found.
Then we changed the solvent to phenol, di-n-butyl ether and
diethylene glycol dimethyl ether (diglyme) to examine the influence
thereof. Generally the yield of precipitable products decreased
1
in all cases. We compared the H NMR spectra (especially the
methylene protons) and the IR spectra (especially the CO band)
of all reactions performed and found, that with the exception of
2-ethoxyethanol none of the other solvents led to the occurrence
of a methylene bridge and a CO moiety. In addition the fingerprint
region indicates a closely related cyclometalation of all reactions
(a comparison of ligand 3 and two performed reactions is shown
in Fig. 4). In another reaction we changed the ratio of 3 and
Fig. 3 IR spectra of compounds 5a and 5b demonstrating shift of the CO
band around 2000 cm−1 for reaction with H218O (upper graph) and H216O
(lower graph).
Fig. 4 IR spectra of compound 3 and products of reaction of 3 in
2-ethoxyethanol and in diethylene glycol dimethyl ether (diglyme) as
solvents. The middle and lower graph show a similar change of the
fingerprint region compared to starting material 3 implying a closely
related cyclometalation. The lower graph exhibits an unambiguous lack of
a CO band at 2025 cm−1 compared to the middle graph.
According to ref. 10 the source of carbon is an alkyne which is
apparently not present in our case. We therefore considered two
options for the source of the carbon: the solvent 2-ethoxyethanol
and the methyl group of the ligand 3 itself. Comparable to the
experiments with H218O a direct proof would ask for 13C-labeled
4096 | Dalton Trans., 2008, 4095–4098
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IrCl3·nH2O from 2 : 1 to 1 : 1. We found a decrease in the yield of
more than 50% and no evidence for a decrease of the CO content
in 5a. To rule out hard to detect impurities in the solvent as the
source of carbon we also performed a reaction with a portion
of the solvent from a different manufacturer. The reaction led
to comparable results to those received with the solvent from the
initial manufacturer. These results indirectly prove, that the solvent
2-ethoxyethanol is the source of carbon and in addition we learn,
that the formation of CO and the activation of the methyl group
are closely related to each other.
Furthermore we examined and compared our NMR and IR
results of compounds 2, 3 and 5a. The NMR spectra of 5a revealed
two types of ligand or types of bonding to the metal center with
a ratio of 1 : 2, whereas only one ligand is cyclometalated via a
methylene bridge (shown by the characteristic and isolated signals
of the magnetically inequivalent methylene protons around 4.3
and 3.9 ppm). We therefore propose the intermediate products
5a and 5b to be unsymmetric (Scheme 1). A splitting of 5a as
described by Gill et al.11 and used by Mirabelli and Sneddon12
would lead to solvent-stabilized intermediates 6 and 7 (Scheme 2).
Since only 6 can react to produce 2 we find an explanation for
the low yield of the reaction. What happens to the other half of
the l-chloro-bridged dimer 5a remains unclear. Due to the tensed
situation of these tweezer-like ligands in a complex a purely dative
bonding mode of the pyridine nitrogen atom towards the center
is conceivable. This weak interaction would in the end lead to an
insoluble iridium salt and unreacted ligand 3. This speculation is
certainly based on the CH activation as the rate limiting step.
the nitrogen of the pyridine ring and the alkyl group of the toluene
ring. To the best of our knowledge only two complexes, one with a
manganese center and another with a palladium center, share this
rare motif with the present complex 2.13 Even six-membered rings
without participation of alkyl groups are scarce. Only one series
of complexes with a six-membered ring containing an iridium
center is known to us.14 The above mentioned manganese complex
described by Djukic et al.13a was an unexpected formation obtained
in 5% yield, whereas Stoccoro et al.13b intended to synthesize a
complex coordinating via a methylene group. In order to optimize
the creation of such bonds they used a palladium based precursor
with a methyl group as one of the ligands and added a designed
bidentate ligand with a free tolyl or a xylyl group to obtain a fixed
system. Presumably a hydrogen from the methyl group of the tolyl
or xylyl group is then transferred to the methyl ligand, whereas
methane will be released and the bond formed.
The other feature is the Ir(CO)Cl moiety which can often be
found in various metal complexes. CO can on the one hand
stabilize a metal complex and on the other hand easily be replaced
by other ligands or leave a vacant position for catalytic processes.
In addition CO can be used as a structural probe. The group of
Yersin15 recently compared the commonly used OLED emitting
material Ir(ppy)3 with Ir(ppy)2(CO)Cl, where one of the three
bidentate phenylpyridine rings (ppy) is replaced by CO and Cl.
They were able to investigate the photophysics using the motif of
Ir(CO)Cl, which is of great importance to materials research in
the field of OLEDs.
In summary, we have presented a new iridium complex with a
unique structural motif of a six-membered ring coordinating via
a methylene group and a pyridine nitrogen atom, and provided
reasonable explanations of the nature of the intermediate product
and the source of CO insertion. Although possible applications
of complex 2 are yet unknown, catalytic activity or data for
understanding energy transfer in OLEDs are feasible.
Acknowledgements
We would like to thank the Federal Ministry of Education and
Research (BMBF) and the BASF AG for financial support. Also
we would like to thank Dr K. Kahle and Dr P. Erk, BASF AG,
Ludwigshafen, Germany for helpful discussions.
Notes and references
‡ 1H NMR (400 MHz, CDCl3, TMS) for 2: d(ppm) = 8.77 (d, J = 5.1 Hz,
1H), 8.39 (d, J = 7.8 Hz, 1H), 8.10 (dt, J = 7.8, 7.7, 1.3 Hz, 1H), 7.81 (d,
J = 5.9 Hz, 1H), 7.66 (ddd, J = 7.7, 5.1, 1.4 Hz, 1H), 7.53 (d, J = 7.6 Hz,
1H), 7.37 (t, J = 7.6, 7.6 Hz, 1H), 7.32 (ddd, J = 8.1, 7.5, 1.5 Hz, 1H),
7.23–6.79 (m, br, 6H), 7.08 (ddd, J = 8.1, 1.6, 0.5 Hz, 1H), 6.90 (ddd, J =
7.5, 5.9, 1.6 Hz, 1H), 4.21 (d, J = 10.8 Hz, 1H), 3.76 (d, J = 10.8 Hz, 1H).
13C NMR (100 MHz, CDCl3) for 2: d(ppm) = 173.2 (s), 157.9 (s), 156.5
(s), 151.4 (s), 150.7 (s), 150.5 (d), 149.7 (d), 142.3 (s), 141.3 (s), 139.8 (d),
137.7 (d), 136.0 (s), 130.4 (d), 129.5 (d, br), 129.2 (d), 129.2 (d), 128.8 (d),
128.6 (d, br), 128.5 (d), 127.3 (d), 124.9 (d), 123.7 (d), 8.8 (t).
Scheme 2 Solvent-induced splitting of the l-chloro-bridged dimer 5a
explaining the low yield.
§ Crystal data for 2: C26H19Cl4IrN2O3, M = 741.43, triclinic, a =
◦
˚
˚
˚
11.4605(11) A, b = 14.1259(1◦4) A, c = 16.6417(16) A, a = 89.693(2) ,
Complex 2 exhibits some interesting structural motifs. The most
striking feature is the six-membered ring containing the iridium
and a methylene group. The necessary CH activation as well as
the synthesis and incorporation of CO are attributable to the
catalytic activity of iridium. The cyclometalating ligand 3 can be
reduced to a bidentate o-tolyl-pyridine ligand that coordinates via
◦
3
˚
b = 83.124(2) , c = 85.740(2) , V = 2667.3(4) A , T = 133(2) K, space
¯
˚
group P1, Z = 4, k = 0.71073 A, 17162 reflections measured, 17162 unique
[R(int) = 0.0000] which were used in all calculations, R1 = 0.0425, wR2 =
0.1083 (I>2r(I) for R1 and all data for wR2).
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