Twist it! the acid-dependent isomerization of homoleptic carbenic iridium(III) complexes

Article abstract of DOI:10.1039/c7cc00697g

The first successful meridional to facial isomerization of homoleptic carbenic iridium(iii) complexes is presented. The Br?nsted-acid-mediated procedure allows the conversion of large amounts of material and additionally provides an in situ purification b

Full text of DOI:10.1039/c7cc00697g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Twist it! The Acid-Dependent Isomerization of Homoleptic
Carbenic Iridium(III) Complexes
†
a
†b
c
d
e
Jaroslaw G. Osiak , Tobias Setzer , Peter G. Jones , Christian Lennartz , Andreas Dreuw ,
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
Wolfgang Kowalsky , Hans-Hermann Johannes*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first successful meridional to facial isomerization of homoleptic asymmetric chelate ligands in an octahedral complex leads to
carbenic iridium(III) complexes is presented. The Brønsted-acid- two possible geometrical isomers, namely the meridional (mer)
mediated procedure allows the conversion of large amounts of and the facial (fac) isomer (Scheme 1). These configurations
material and additionally provides an in situ purification because of often differ in physical and photophysical properties such as
2
precipitation of the target material during the reaction. The long term stability, quantum efficiency or radiative rates.
pronounced acid-dependency of the reaction yield observed for Unfortunately, most synthetic pathways yield a mixture of both
tris(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(III) and tris(N- isomers so that for certain applications additional purification
phenyl,N-benzyl-benzimidazol-2-yl)iridium(III) was investigated by or transformation of one isomer is necessary. The currently
labelling experiments and quantum chemical calculations. The available tools for these transformations can be divided into
3
,4
3,5
results reveal a subtle balance between the strength of the acid, the thermal and photoinduced isomerizations. So far, none of
coordinating power of the corresponding base and steric effects of these methods have been successfully applied to the compound
the ligand sphere. Based on these findings, general rules are given class of homoleptic carbenic Ir(III) complexes.
for a systematic and material-specific modification of the reaction The particular interest in those Ir(III) complexes featuring a
conditions for the mer-fac isomerization of homoleptic carbenic carbenic motif was sparked by the lack of sufficiently stable and
Ir(III) complexes.
highly efficient electrophosphorescent deep blue materials for
PhOLEDs.[ Especially the fac isomers, which show a specifically
narrow blue electroluminescence (EL) emission band width, are
of particular importance for device engineering. Unfortunately,
there is no robust route to synthesize the fac isomer selectively.
According to a recent publication by Thompson and Forrest, the
lack of conversion methods for these complexes can be ascribed
to the stronger metal-ligand bonds in carbenic Ir(III) compared
6]
Interest in iridium chemistry has markedly increased over the
last two decades. Cyclometallated Ir(III) complexes have spread
into the most diverse scientific fields in medical, catalytic,
1
sensing and optoelectronic applications. One major driving
force in this process has been their excellent electro-
phosphorescent properties, making them one of the most
commonly used class of emissive materials in commercial
phosphorescent organic light emitting diodes (PhOLEDs).
Prominent representatives of this compound class are
7
to the weaker Ir-N bonds in Ir(C^N) complexes.
Addressing the need for reliable mer-fac conversion
procedures, this work presents an acid-mediated isomerization
methodology at the example of the two deep blue
phosphorescent Ir(III) complexes tris(N-phenyl,N-methyl-
6
homoleptic tris-cyclometallated d -Ir(III) complexes. The use of
benzimidazol-2-yl)iridium(III) (
benzimidazol-2-yl)iridium(III) (
confirmed by X-ray analyses of the fac-
structures. The overall reaction yields depend on the employed
acid. Whereas hydrochloric acid (HCl), as a strong acid,
1
2
) and tris(N-phenyl,N-benzyl-
). The success of the method is
and mer-/fac- crystal
a.
Institut für Hochfrequenztechnik, Technische Universität Braunschweig,
Bienroder Weg 94, 38106 Braunschweig (Germany).
E-Mail: h2.johannes@ihf.tu-bs.de Address here.
BASF SE, ROM/CQ – B009, 67056 Ludwigshafen am Rhein (Germany).
Institut für Anorganische und Analytische Chemie, Technische Universität
Braunschweig (Germany)
trinamiX GmbH, Industriestraße 35, 67063 Ludwigshafen (Germany).
Interdisciplinary Center for Scientific Computing, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 205A, 69120 Heidelberg (Germany)
Email: dreuw@uni-heidelberg.de.
1
2
b.
c.
d.
instantaneously leads to degradation of both 1 and 2, malonic
e.
acid (MA) and trifluoroacetic acid (TFA) show a complex-
dependent selectivity. In order to gain a deeper understanding
1
†
These authors contributed equally to this publication
Electronic Supplementary Information (ESI) available: CCDC 1516674 (fac-1)
516675 (mer-2) and 1516676 (fac-2). For ESI and crystallographic data in CIF see
of this dependency, a H-NMR isotopic labeling experiment and
,
quantum chemical calculations were carried out.
1
DOI: 10.1039/x0xx00000x
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B
A
Ir
B
View Article Online
DOI: 10.1039/C7CC00697G
A
Ir
A
B
A
B
Brønsted acid
B
A
B
A
meridional
facial
N
N
N
Malonic Acid
EtOAc, 75 °C
1
Ir
Ir
N
3
3
N
N
N
CF3COOH
2
Ir
Ir
EtOAc, 75 °C
N
3
3
Scheme 1 Overview of the acid-mediated isomerization of tris-(N-phenyl,N-methyl-
benzimidazol-2-yl)iridium(III) (1) and tris-(N-phenyl,N-benzyl- benzimidazol-2-
yl)iridium(III) (2).
The procedure for the geometric isomerization of the Ir(III)
complexes can be directly carried out on the isolated isomeric
mixtures obtained from the non-selective preparation. The
complexes
1 and 2 were obtained as a mixture of the fac and
the mer isomers, whereby the mer isomer was formed in both
cases predominantly. According to HPLC analysis the mer : fac
ratios for the isomers were 6.1:1.0 for 1 and 7.2:1.0 for 2.
In agreement with literature, variation of the reaction
temperature, time or solvents had little influence on the
8
isomeric ratios. When the mixtures are dissolved in EtOAc with
the addition of a dilute Brønsted acid, the mer isomer is
converted to the fac isomer. An essential factor for the
successful conversion is the choice of an appropriate acid. While
a strong acid, such as HCl, leads to a rapid degradation of the
precious complex, a weak acid, as for instance acetic acid,
promotes at best a slow mer to fac conversion (if any) of the
respective material.
The optimized conditions for the conversion of the complexes
1
and are depicted in Scheme 1. MA was used for the conversion
2
Fig. 1. ORTEP plots of fac (1) and mer/fac (2). Thermal ellipsoids are drawn at 50%
of 1, but its acidity was insufficient for the geometric probability level. Hydrogen atoms and co-crystallized solvent molecules were omitted
for clarity.
isomerization of 2, for which a stronger acid, such as TFA, was
needed. The progress of the reaction is indicated under
optimized conditions by the precipitation of the desired fac system to study the key mechanistic features, conformational
isomer, while the mer isomer remains dissolved. This in situ issues caused by ligands with larger substituents than the
purification allows facile isolation of the fac isomer by filtration methyl group of
1 can be excluded. TFA was chosen as an
(see Supporting Information).
isomerization agent in the computations, because of its
From the four geometric isomers of the two complexes, three ambiguous behavior, it enables the geometric conversion of
single crystals suitable for X-ray structural analysis were while partially leading to its degradation.
1
obtained (fac of
displayed in Figure 1. Crystallographic information are given in isomerization, was studied by a systematic sampling of the
the ESI and via the CCDC.
protonation sites, which showed that a protic Ir-ligand bond
1, mer and fac of 2
). The structures are The initial protonation of I (mer-1), which triggers the
In order to study the practically inaccessible intermediates of cleavage most probably yields two energetically similar
the mer-fac isomerization complementary quantum chemical intermediates (G = -4.3 kJ/mol and G = 5.0 kJ/mol), both of
calculations have been conducted. By employing
1
as a model
which have a characteristic protonation at the phenyl moiety in
2
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common (see Supp. Info. SI-Fig. 6). A protonation at the carbene requiring G = 101.9 kJ/mol, marks the highest oveVireawllArrtieclaecOtniolinne
is unlikely as all resulting intermediates exhibit high energies barrier during the geometric conversioDnO. IT: 1h0e.10a3c9t/uCa7lCCm0e0r6-9f7aGc
(
G ≥ 106.0 kJ/mol).
conversion occurs between III (G = -45.2 kJ/mol) and IVa
/
IVb
‡
To justify the neglect of conformational influences caused by via TS-III-IVab (G = 43.3 kJ/mol). During this step the complex
different substituents of the ligand, a deuteration experiment loses its octahedral geometry and forms the trigonal
1
of mer-
2
was conducted using CF
3
COOD. The H-NMR results bipyramidal transition state TS-III-IVab. This step is initiated by
–
1
show a protonation of the phenylic and also of the benzylic replacement of
moieties (see Supp. Info. SI-Fig. 1). A comparison of the Ph . Similar transition states can be found in the literature for
computed energies of all possible constitutional isomers geometrically comparable systems.9–11 Returning to the
X with Cb , which moves in trans position to
2
relative to mer-2 (ΔG = 0.0 kJ/mol) rules out the benzyl moiety energetically favourable octahedral complex geometry, two
as a reasonable participant. Species with at least one benzyl- intermediates can be identified: IVa (G = 20.5 kJ/mol) and IVb
iridium(III) linkage are energetically disadvantageous (G = -20.0 kJ/mol). In IVa the vacant coordination site is
1
(
ΔG = 44.7 kJ/mol) compared to Ir(III)-phenyl linked isomers shielded by agostic interactions between Ph -H and Ir(III) while
–
12
–
(fac-
2
: ΔG = -3.8 kJ/mol). Therefore, substitution-dependent
X acts as a spectator. However, in IVb an X occupies the free
byproducts or intermediates can be excluded. Consequently, an Ir(III) coordination site yielding a thermodynamic sink. Finally,
initial phenyl-carbon-iridium(III) bond-breakage is a reasonable the thermodynamically favoured product (Δ-fac-
starting point to initiate the geometric transformation. (G = -0.2 kJ/mol) is reached by the energetically preferred
In accordance with existing literature, four different cyclometallation step from IVa via TS-IVa-V (G = 93.4 kJ/mol)
V
1)
‡
–
1
3
2
1
mechanistic scenarios featuring three different key transition where
X attacks from above the Cb -Cb -Ph -Ph plane and
9
,10
states were studied. The pathway having the lowest overall abstracts the proton as an external base.
energy barrier of all four mechanisms is shown in Scheme 2. On closer inspection of the quantum chemical results, the
Gibbs free energies are given in kJ/mol relative to
G = 0.0 kJ/mol).
The initial protonation of Δ-
I
+ TFA crucial role of the thermodynamic sinks (II
connected with the coordination strength of the corresponding
leads to II (G = -27.3 kJ/mol). base becomes apparent. If those sinks were too deep to be
, III and IVb),
(
I
Considering the preference of this system for electron-rich reversibly overcome during the reaction, the isomerization
species and a fast ligand exchange rate, the vacant coordination might stop or potential decomposition reactions might take
site is probably occupied by the electron-rich acid anion place.
–
-
CF
3
COO (
X
) yielding a significant energy sink. Subsequently, For the initial reaction step Δ-
I
+ HX → II the influence of three
HX = MA, TFA or HCl) was tested. The
corresponding energies obtained for II clearly depend on the
1
3
the dangling ligand of II rotates around the Cb -Cb axis in an different acids
anti-clockwise manner giving TS-II-III (G = 74.6 kJ/mol) while
(
‡
–
X
still occupies the vacant coordination site. This reaction step, coordinating power of the acid’s anion. Therefore, the use
TS-IVa-V
‡
G
= 93.4
Ph3
Ir
X–
+
TS-II-III
Ph3
‡
Cb1
Cb3
- HX
G
= 74.6
Ph3
N
N
Ph2
Ph1
Cb2
Cb1
N
N
N
Ph1
N
Cb3
N
Cb3
N
Cb1
Ir
N
Ir
TS-III-IVab
IVa
G = 20.5
N
Ph2
‡
G
= 43.3
Ph2
N
N
Ph1
N
N
Cb2
Cb1
N
X-
Cb2
+
Ph3
Cb3
Ph2
Cb2
N
Ph1
H
Ir

-I

-V
G = 0.0
G = –0.2
+
HX
IVb
G = –20.0
II
G = –27.3
Cb1
Ph1
III
Ph3
Ph3
N
Cb3
G = –45.2
N
X
Cb3
Ir
Cb1
Ir
X
Ph2
N
Ph2
Ph1
Ph3
N
Cb2
Cb2
Cb3
X
Cb1
Ir
N
Ph2
N
Ph1
Cb2
Scheme 2 Energetic isomerization sequence starting from I (Δ-mer-1) to finally produce V (Δ-fac-1).
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of two different MA conformers only yields very modest dependency of the reaction yields was found to bVeiewinAfrtliucleenOcnelinde
thermodynamic sinks
(ΔG(twisted) = -7.7 kJ/mol, by a subtle interplay of the acid strengthDOanI:d10t.1h0e39c/oCo7CrdCi0n0a6t9in7Gg
ΔG(planar) = -15.7 kJ/mol) (see SI-Fig. 3), while the sink for TFA power of its corresponding base. Based on the presented results
is almost twice as deep (ΔG = -27.3 kJ/mol). Finally, HCl leads to general guidelines for synthetic planning can be devised: (1) The
an
iridium-chloro-complex
that
is
stabilized
by acid strength generally determines whether or not the reaction
ΔG = -45.7 kJ/mol (see SI-Fig. 4). Therefore, the coordinating is kinetically inhibited, whereas very strong acids
power of the corresponding base directly correlates with the instantaneously result in material decomposition; (2) The
depth of the resulting energy sinks.
coordinating power of the corresponding base controls the
Besides the influence of different acids, a further comparison of thermodynamic barriers by means of energy sinks resulting in
IVa and IVb shows that the complex, depending on its structure, unwanted side reactions or material degradation; (3) Agostic
can intrinsically prevent these sinks by agostic interactions interactions between the ligand sphere and a vacant Ir(III) site
1
(
G = 20.5 kJ/mol vs. G = -20.0 kJ/mol). In this case the Ph -H help to prevent thermodynamic sinks by repelling free
group in IVa can weakly bind to Ir(III) by electrostatic coordinating agents.
-
interactions and prevent
X
from coordinating to the vacant We acknowledge the Federal Ministry of Education and
metal center. On the other hand, for III the vacant coordination Research (BMBF) and the BASF for financial support. We also
site is completely exposed, because the small methyl group want to thank Susanne Salzmann and Peter Deglmann for
–
points in the direction of
X
. In conclusion, it can be assumed fruitful discussions on DFT and the COSMO-RS model.
that thermodynamic sinks are highly unfavourable but
sometimes inevitable (see III). Deep energy sinks can cause the
mer-fac conversion to stop or even lead to further degradation
cascades. This finding is in excellent agreement with the
experimental observations for the geometric conversion of
Notes and references
1
2
(a) L. Lu, L.-J. Liu, W.-c. Chao, H.-J. Zhong, M. Wang, X.-P.
Chen, J.-J. Lu, R.-n. Li, D.-L. Ma, C.-H. Leung, Sci. Rep. 2015, 5,
1
4544; (b) T. Vaidya, A. C. Atesin, I. R. Herrick, A. J. Frontier,
mer-
leads to partial decomposition whereas HCl does not produce
any fac- ).
On the other hand, a successful mer- to fac-2 isomerization
1 (I): While MA gives the best reaction yield, TFA already
R. Eisenberg, Angew. Chem. Int. Ed. 2010, 49, 3363–3366; (c)
Y. You, S. Lee, T. Kim, K. Ohkubo, W.-S. Chae, S. Fukuzumi,
G.-J. Jhon, W. Nam, S. J. Lippard, J. Am. Chem. Soc. 2011,
133, 18328–18342; (d) H. Sasabe, J.-i. Takamatsu, T.
1 (V
2
was only achieved using TFA, while MA did not yield any
product. Both complexes essentially differ only in their
substitution pattern at the benzimidazole. The replacement of
all methyl groups by bulkier benzyl groups in III (Scheme 1)
reduces the available space for the coordination of acid anions
and in this way shields the vacant site of the Ir(III) complex.
Consequently, the crowded ligand sphere would hamper the
Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O. Molt,
E. Fuchs, C. Lennartz, J. Kido, Adv. Mater. 2010, 22, 5003–
5
007.
(a) T. Sajoto, P. I. Djurovich, A. Tamayo, M. Yousufuddin, R.
Bau, M. E. Thompson, R. J. Holmes, S. R. Forrest, Inorg.
Chem. 2005, 44, 7992–8003; (b) T. Karatsu, E. Ito, S. Yagai, A.
Kitamura, Chem. Phys. Lett. 2006, 424, 353–357; (c) R. J.
Holmes, S. R. Forrest, T. Sajoto, A. Tamayo, P. I. Djurovich, M.
E. Thompson, J. Brooks, Y.-J. Tung, B. W. D’Andrade, M. S.
Weaver et al., Appl. Phys. Lett. 2005, 87, 243507.
–
–
coordination of relatively large anions such as MA and TFA ,
–
while small anions such as Cl were still compact enough to
3
4
5
A. B. Tamayo, B. D. Alleyne, P. I. Djurovich, S. Lamansky, I.
Tsyba, N. N. Ho, R. Bau, M. E. Thompson, J. Am. Chem. Soc.
2003, 125, 7377–7387.
overcome this protection. Even with the crowded ligand sphere
lowering the influential impact of thermodynamic sinks in the
A. R. McDonald, M. Lutz, L. S. von Chrzanowski, G. P. M. van
Klink, A. L. Spek, G. van Koten, Inorg. Chem. 2008, 47, 6681–
mer-/fac-2 conversion, it is puzzling why the experimental
results prove that this conversion requires the use of TFA.
However, this necessity can be explained by a difference in the
6
691.
(a) K. Tsuchiya, E. Ito, S. Yagai, A. Kitamura, T. Karatsu, Eur. J.
Inorg. Chem. 2009, 14, 2104–2109; (b) T. Karatsu, T.
Nakamura, S. Yagai, A. Kitamura, K. Yamaguchi, Y.
rotational barrier for the mer-/fac-
Preliminary results indicate that it is significantly higher for mer-
fac- than for the mer-/fac- conversion. Therefore, the rate
of the mer-/fac- conversion is not only determined by
2 conversion compared to 1.
Matsushima, T. Iwata, Y. Hori, T. Hagiwara, Chem. Lett. 2003,
/
2
1
32, 886–887; (c) G. Treboux, J. Mizukami, M. Yabe, S.
2
Nakamura, Chem. Lett. 2007, 36, 1344–1345.
thermodynamic sinks but also by a kinetic energy barrier.
Employing the weaker and incompletely dissociated MA, only a
small portion of protonated educt molecules is generated, and
has to overcome the reaction energy barrier to finally recover
the acid. Thereby, the reaction rate approaches practically zero.
Consequently, not only the coordinating power of the
corresponding base but also the strength of the acid are key
criteria that determine the successful outcome of the mer-fac
isomerization.
6
7
O. Molt, K. Kahle, US7803948 B2, 2010.
(a) J. Lee, H.-F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich,
M. E. Thompson, S. R. Forrest, Nat. Mater. 2016, 15, 92–98;
(
b) K. Tsuchiya, S. Yagai, A. Kitamura, T. Karatsu, K. Endo, J.
Mizukami, S. Akiyama, M. Yabe, Eur. J. Inorg. Chem. 2010,
26–933.
C.-H. Chien, S. Fujita, S. Yamoto, T. Hara, T. Yamagata, M.
Watanabe, K. Mashima, Dalton Trans. 2008, , 916–923
M. Amati, F. Lelj, Chem. Phys. Lett. 2002, 363, 451–457.
0 J. G. Gordon, R. H. Holm, J. Am. Chem. Soc. 1970, 92, 5319–
332.
6,
9
8
7
9
1
5
In summary, a new and efficient acid-mediated mer-fac
isomerization, featuring an in situ purification, is presented for
homoleptic carbenic Ir(III) complexes and is confirmed by X-ray 12 N. M. Scott, V. Pons, E. D. Stevens, D. M. Heinekey, S. P.
11 I. Iwakura, H. Ebina, K. Komori-Orisaku, Y. Koide, Dalton
Trans. 2014, 43, 12824.
analysis of the single crystal structures. The pronounced acid-
Nolan, Angew. Chem. 2005, 117, 2568–2571.
4
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