Azobenzene-functionalized iridium(III) triscyclometalated complexes

Article abstract of DOI:10.1039/c4dt02018a

Twelve Ir(III) triscyclometalated compounds containing up to three azobenzene fragments on their structure have been synthesized based on photochromic 2-phenylpyridyl type ligands 1-4. These complexes are intended to study the possibility of transferring

Full text of DOI:10.1039/c4dt02018a

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Azobenzene-functionalized iridium(III)
triscyclometalated complexes†
Cite this: DOI: 10.1039/c4dt02018a
J. Pérez-Miqueo,a A. Telleria,a M. Muñoz-Olasagasti,a A. Altube,b E. García-Lecina,b
A. de Cózarc,d and Z. Freixa*a,d
Twelve Ir(III) triscyclometalated compounds containing up to three azobenzene fragments on their struc-
ture have been synthesized based on photochromic 2-phenylpyridyl type ligands 1–4. These complexes
are intended to study the possibility of transferring the photochromicity of the azobenzene fragment
to the organometallic compound, and the effect of the substitution pattern, relative distance of the
azobenzene to the metal centre, and number of azobenzenes on their properties.
Received 3rd July 2014,
Accepted 15th August 2014
DOI: 10.1039/c4dt02018a
www.rsc.org/dalton
photoisomerization which induces not only structural changes
but also important electronic modifications in the ligands.8 In
Introduction
In the last few decades, the development of photo-responsive the early examples of photochromic metallocomplexes, efforts
materials has become an intensive area of research. These concentrated on how metal coordination could affect the
substances are intended for the production of “smart chemical photochromism of the organic switch located at the organic
systems”, whose properties—and eventually functionality— ligand.9 More recently, the focus shifted toward the modi-
are controlled by changes of the environment (light irradi- fication of the properties and eventually the functionality of
ation). These systems have been already implemented in a the metal complex upon isomerization of the photochromic
wide range of modern materials and devices for daily appli- fragment present in the ligands.
cations such as sunglass lenses, memory devices, photo-
There is a considerable amount of examples of azobenzene-
containing coordination compounds, mainly based on
chromic inks, etc.1,2
In spite of the importance and versatility of transition- neutral nitrogen or phosphorus based ligands: phosphine,10–13
metal complexes, smart photo-responsive examples remain pyridine,5,14 2,2′-bipyridine,15–24 terpyridine,6,24–27 or bridging
rather unexplored in comparison with the large number of azobis(4-pyridine)28–34 ligands. Despite the fact that organo-
well known light-triggered organic switches. In principle, metallic complexes (containing a C–metal bond) are used in a
photo-responsive metal complexes can be obtained by in- wide number of important applications—setting aside ferro-
corporation of organic photochromic units in the structure of cene derivatives5,35–37—only a handful of examples of azo-
their ligands.3–6 These photo-sensitive ligands, rather than benzene based organometallics are known. From these, just
acting as conventional spectators that tune the properties of the most recent ones are intended for the construction of
their complexes, transform them into dynamic smart entities photo-sensitive organometallics.38–43
able to offer a functional response to an external stimulus.7
Cyclometalated phenylpyridine ligands are among the most
Azobenzene is the photochromic fragment most frequently popular ligands for the construction of organometallic com-
used for this purpose. It does experience a reversible trans-to-cis plexes for many applications. Nevertheless, to the best of our
knowledge, there are no examples of photochromic phenylpyri-
dine azobenzenes.
We report here the synthesis of a series of phenylpyridine
aDepartment of Applied Chemistry, Faculty of Chemistry, University of the Basque
ligands that incorporate a photochromic azobenzene frag-
Country (UPV-EHU), San Sebastián, Spain. E-mail: Zoraida_freixa@ehu.es
bSurfaces Division, IK4-CIDETEC, San Sebastián, Spain
ment at different positions of the phenyl ring. The synthesis,
characterization and photochromism of their heteroleptic
iridium(III) complexes formed by cyclometalation of the
ligand are also presented. A posteriori modification of the
complexes by Suzuki cross-coupling was assayed to generate
complexes containing more than one azobenzene fragment
per complex.
cDepartment of Organic Chemistry II, Faculty of Chemistry, University of the Basque
Country (UPV-EHU), San Sebastián, Spain
dIKERBASQUE, Basque Foundation for Science, Bilbao, Spain
†Electronic supplementary information (ESI) available: For detailed synthetic
procedures, NMR, UV-vis, and voltammetry data of all the new compounds, and
computational details. CCDC 1011970–1011974. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4dt02018a
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3-iodoazobenzene sequentially with n-BuLi and trimethyl
borate at −112 °C, and diluted sulfuric acid at 0 °C, as pre-
Results and discussion
We designed a series of 2-phenylpyridyl type ligands that viously reported for compound d.44 4-Azophenylboronic acid
incorporate an azobenzene fragment in their structure (1–3 pinacol ester f was synthesized via condensation of aniline
Scheme 1). In these ligands, the phenyl moiety intended for boronic acid pinacol ester with nitrosobenzene, as reported
cyclometalation is also part of the azobenzene. They respond elsewhere.45
to the three possible substitution patterns (ortho, meta and
Ligands 2-(o-phenylazobenzene)pyridine (1), 2-(m-phenyl-
para). Ligand (2-phenyl(4-azophenyl)pyridine) was con- azobenzene)pyridine (2), and 2-(p-phenylazobenzene)pyridine
4
structed a posteriori by palladium-catalyzed cross-coupling on (3) were obtained by palladium-catalyzed cross-coupling
(5-bromo-2-pyridyl)phenyl iridium(III) cyclometalated com- between 2-bromopyridine and the corresponding (E)-2-azophe-
plexes with 4-azophenylboronic acid pinacol ester (f). It was nylboronic acid, (E)-3-azophenylboronic acid or (E)-4-azo-
designed to introduce more than one azobenzene photochro- phenylboronic acid pinacol ester respectively (yields 70–80%).
mic unit per complex. Altogether this set of azobenzene-con- Alternatively, ligand 2 was also obtained by Suzuki cross-coup-
taining phenylpyridyl ligands was used to study the photo- ling between 2-bromopyridine and 3-aminophenylboronic
chromism of a series of azobenzene-containing iridium(III) acid, and subsequent condensation of the formed 3-amino-
cyclometalated complexes. The goal was to analyze the phenyl-2-pyridine with nitrosobenzene in acetic acid at 90 °C
effect that the substitution pattern, the distance from the azo- for 3.5 h (overall yield 32%).
benzene to the metal centre, and the number of azobenzenes
per complex has on the photochromism of the corresponding
iridium derivatives.
Iridium(III) complexes
Heteroleptic meridional iridium(III) complexes of general
formula [Ir(ppy)2(L)] (A) and [Ir(Fppy)2(L)] (B) (ppy = 2-phenyl-
pyridyl, Fppy = 2-(2,4-difluorophenyl)pyridyl, L = cyclometa-
lated ligands 1–3) were synthesized by cleavage of the
corresponding chloro-bridged dimers [Ir(Fppy)2Cl]2 or [Ir-
(ppy)2Cl]2 in acetone using a chloride abstractor (AgOTf),
excess of ligand and NEt3 as base, as shown in Scheme 3.
Ligands synthesis
Synthetic routes towards ligands 1–3 are shown in Scheme 2.
Azophenylboronic acids d and e, were obtained by reacting
Et2O solutions of the corresponding 2-iodoazobenzene or
Scheme 3 (i) AgOTf, acetone (55 °C, 2 h). (ii) 1–3 (4 equiv.), NEt3 (7.5
Scheme 1 Azobenzene-containing ligands used in this work.
equiv.), acetone (55 °C, 15 h).
Scheme 2 (i) n-BuLi (1.1 equiv.), Et2O, −112 °C; B(OMe3) (1.1 equiv.), Et2O, −112 °C; dil. H2SO4, 0 °C. (ii) Nitrosobenzene (1.5 equiv.), CH3CO2H,
90 °C, 3.5 h. (iii) 2-Bromopyridine (1.1 equiv.), Pd(PPh3)4 (2 mol%), THF, Na2CO3 (1 M aq., 1.6 equiv.), 80 °C, overnight.
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Following the aforementioned synthetic protocol, hetero-
leptic complexes derived from ligands 2 and 3, containing two
additional cyclometalated ligands: either 2-phenylpyridyl (A2
and A3) or 2-(2,4-difluorophenyl)pyridyl (B2, B3) were obtained
as pure compounds after purification by column chromato-
graphy (Rf = 0.8, CH2Cl2). Complexes A3 and B3 were obtained
with considerably better yields (56% and 70% respectively)
compared to the analogous compounds derived from ligand 2
(yields 15–20%). These meridional complexes are chiral, but
were obtained as racemates. The lack of any element of sym-
metry in their structure is evidenced by the complexity of their
1H-NMR and 13C-NMR spectra. Complexes derived from [Ir-
(ppy)2Cl]2 (A2 and A3) present 39 different aromatic carbon
signals in their 13C-NMR, and 28 aromatic resonances in the
corresponding 1H-NMR spectra. The 1H-NMR spectroscopic
patterns of compounds B2 and B3, derived from the fluori-
nated dimer [Ir(Fppy)2Cl]2, are slightly more simple; “only” 24
aromatic signals are expected, but 13C-NMR spectra are rather
complicated due to the splitting of some of the 39 aromatic
Fig. 1 ORTEP representation of the molecular structure of the cation
[Ir(ppy)2(1)]+ obtained by X-ray diffraction. Ellipsoids are drawn at the
50% probability level. Relevant distances (Å): N4–N5 = 1.250(5), N1–Ir =
2.050(3), N2–Ir = 2.070(3), N3–Ir = 2.209(3), N4–Ir = 2.166(3).
signals expected due to F–C coupling. A comparative analysis a metal centre has been a recurring topic for many years. From
of the different H-NMR and 13C-NMR spectra, combined with the early reports that proposed such a coordination mode,46,47
1
COSY and HSQC experiments, allowed us to identify the many X-ray structures have been obtained for cyclometalated
number of signals expected in all the cases. Although an azobenzenes,48–54
2-(arylazo)pyridines,55–60
2,2′-azobis-
unambiguous assignment of all the signals could not be (pyridine),61–63 and 2-(arylazo)phenol64 ligands in which a five-
accomplished, the spectra are consistent with the expected or six-membered chelate ring was formed. All of them are well
compounds (see ESI†). Their purity was also confirmed by known redox-active ligands. The N–N distance is indicative of
elemental analysis and HR-MS spectroscopy.
the charge on the azo function, being longer for one-electron
When the same procedure as that described before for the (i.e., anion radical) and two-electron (hydrazido) reduced
synthesis of complexes of type A and B was conducted using ligands compared to unreduced NvN bonds. The N4–N5 dis-
2-(o-phenylazobenzene)pyridine (ligand 1), the properties of tance measured in the molecular structure of [Ir(ppy)2(1)]+
the obtained compounds (labelled A1′ and B1′) were rather (1.250(5) Å) corresponds to an unreduced coordinated azoaro-
different compared to the ones of the complexes derived from matic ligand. In order to gain more insight into the properties
ligands 2 and 3. The residues of these reactions were dark crys- of this molecule, DFT geometry optimizations were performed.
talline solids that were purified by column chromatography The computed N–N distance in compound [Ir(ppy)2(1)]+ is
using ethyl acetate–acetone as eluents. A first fraction (eluted 1.268 Å with a Wiberg bond index of 1.71. Moreover, analysis
with ethyl acetate) was identified as unreacted ligand (Rf 1 = of the orbital population by CASSCF(4,4) single point calcu-
0.8; Rf A1′ethyl acetate = Rf B1′ethyl acetae = 0), the eluent polarity lations on the optimized structure showed that the molecule
was gradually increased with acetone to collect a second does not present radical character (see Fig. S7 in ESI†).
fraction containing the metallic complexes (Rf A1′
=
As observed frequently in other complexes containing che-
acetone
Rf B1′acetone = 0.7). 1H-NMR spectra of these compounds are con- lating azoaromatics, in the molecular structure of complex [Ir-
centration dependent, indicative of some aggregation process (ppy)2(1)]+ the “free” phenyl ring is twisted with respect to the
in solution (see ESI†). Complexes A1′ and B1′ were fully charac- phenyl involved in the chelate ring (the angle between the
terized by NMR spectroscopy (CDCl3, 0.075 M and 0.1 M, phenyl planes defined as the torsion angle C33–N4–N5–C37 is
respectively). Elemental analysis and HR-MS spectroscopy were 167.3(3)°). This twist produces a rupture in the conjugation
in agreement with the cationic structures [Ir(ppy)2(1)]OTf (A1′) within the azobenzene fragment. In this case, there is also an
and [Ir(Fppy)2(1)]OTf (B1′) for both complexes. This molecular aromaticity loss between the phenyl and pyridyl fragments that
structure, in which the ligand acts as a neutral N,N-bidentate form the chelate to permit the formation of the six-membered
through the pyridine and one of the azobenzene nitrogen chelate ring (the angle between these planes defined as the
atoms, could be confirmed in the case of complex A1′ by X-ray torsion angle N3–C27–C28–C33 is 40.9(7)°).
diffraction analysis of a crystalline sample (Fig. 1). Crystals
were obtained by slow diffusion of diethyl ether into a satu- zene unit per complex, we explored the possibility to synthesize
rated CH2Cl2 solution of the complex.
chloride-bridged iridium dimers using the azobenzene-contain-
In an effort to include more than one photochromic azoben-
In the case of ligand 1, the coordination of the ligand as a ing ligand 3, [Ir(3)2Cl]2. Such a complex was intended as the
neutral chelate is favoured over cyclometalation, probably due starting material for the synthesis of triscyclometalated com-
to the close disposition of the azo group to the metal centre. pounds incorporating at least two azobenzene units. We explore
Coordination of the nitrogen atom of an azoaromatic ligand to this strategy using 2-(p-phenylazobenzene)pyridine (3) because
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this ligand is the one that rendered better yields in the cyclo- cyclometalated complexes. Initially, to confirm the viability of
metalation reaction described above for the synthesis of this methodology, we synthesized meridional [Ir(ppy)2((5-
derivatives A and B. IrCl3·3H2O was refluxed in a mixture of bromo-2-pyridyl)phenyl)] C and [Ir(Fppy)2((5-bromo-2-pyridyl)-
3 : 1 2-ethoxyethanol–H2O overnight in the presence of 2.2 phenyl)] D, both containing a bromine atom in one of the
equivalents of ligand 3 (as described for the synthesis of the cyclometalated ligands. Complexes C and D were synthesized
known precursors [Ir(ppy)2Cl]2 and [Ir(Fppy)2Cl]2).65 At the by cleavage of the corresponding iridium chloride-dimers in
end of the reaction period, unreacted starting compounds acetone with an excess of 2-(4-bromophenyl)pyridine, follow-
were recovered as the only reaction product. As an alternative, ing a procedure analogous to the one described for complexes
we considered the possibility of synthesizing directly the tris- A and B. The monobrominated compounds C and D were
cyclometalated complex fac-[Ir(3)3]. A known methodology for obtained with yields of 40% and 30%, respectively (Scheme 4).
the synthesis of facial homoleptic triscyclometalated Ir(III) These complexes were assayed in the Suzuki cross-coupling
complexes (i.e. fac-[Ir(ppy)3]) consists of introducing Ir(acac)3 with (E)-4-azophenylboronic acid pinacol ester (f). We chose
(acac
= acetylacetonate) and the corresponding ligand (E)-4-azophenylboronic acid pinacol ester (f) as the azoben-
(3 equiv.) in a closed digestion bomb for 24 h at 240 °C.66 This zene source to avoid conformational mixtures of products that
synthetic methodology was previously tested for the synthesis could be obtained with meta and ortho boronic acid derivatives
of fac-[Ir(ppy)3]. In our hands it rendered a pure compound d and e. Iridium complexes C and D were refluxed with 1.1
after column chromatography (silica/CH2Cl2) with 50% yield. equiv. of (E)-4-azophenylboronic acid pinacol ester (f) in a
When this synthetic procedure was tested with ligand 3, in an mixture of 2 : 1 THF–H2O using 2 mol% of Pd(PPh3)4 and
attempt to synthesize fac-[Ir(3)3], an unidentified insoluble Na2CO3 as a base. This synthetic procedure rendered azoben-
black material was obtained at the end of the reaction.
zene containing complexes A4 and B4 with yields of 85% and
In view of the lack of reactivity of ligand 3 for the synthesis 88% respectively after purification by column chromatography
of bis or triscyclometalated iridium(III) compounds when com- (Scheme 4).
monly applied methodologies were used, we considered the
The high yields obtained in the coupling reaction for the
possibility to introduce the photochromic azobenzene units synthesis of compounds A4 and B4 prompted us to assay the
a posteriori on preformed cyclometalated complexes. To make same reaction conditions on iridium complexes containing
use of the formerly described azobenzene boronic acids, we two bromine-containing cyclometalated ligands. For this
designed a synthetic route based on a Suzuki cross-coupling of purpose, a biscyclometalated iridium dimer based on 2-(4-bromo-
azobenzene boronic acids with bromo-containing iridium phenyl)pyridine E was synthesized according to a published
Scheme 4 (i) 2-(4-Bromophenyl)pyridine (2.5 equiv.), 3 : 1 mixture of 2-ethoxyethanol–H2O, 120 °C, 24 h. (ii) AgOTf, acetone (55 °C, 2 h). (iii) 2-(4-
Bromophenyl)pyridine, 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine or acetylacetone (4 equiv.), NEt3 (7.5 equiv.), acetone (55 °C, 15 h). (iv)
(E)-4-Azophenylboronic acid pinacol ester (C, D: 1.1 equiv. and F: 2.1 equiv.), solvent 2 : 1 THF–Na2CO3aq. (1 M), Pd(PPh3)4 (2 mol%), 80 °C, overnight.
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methodology.67 Dimeric complex E was then cleaved by using
AgOTf, an excess of ligand, and NEt3 in refluxing acetone to
form mononuclear heteroleptic bromine containing complexes
F. The ligands used for this reaction were commercially avail-
able 2-phenylpyridine (F-ppy), 2-(2,4-difluorophenyl)pyridine
(F-Fppy) and acetylacetone (F-acac). These complexes were
obtained as pure compounds after column chromatography
with 60–80% yield.
Catalytic cross-coupling of iridium complexes
F with
boronic acid azobenzenes should render compounds G con-
taining two photochromic units per complex (Scheme 4).
Iridium complexes F were refluxed with 2.1 equiv. of (E)-4-azo-
phenylboronic acid pinacol ester (f) using the same conditions
as those described above for compounds A4 and B4. Complex
G-acac was obtained with a yield of 80% after purification by
column chromatography (alumina) using dichloromethane as
the eluent. Due to the low solubility of compounds G-ppy and
G-Fppy in all the solvents assayed, only a small quantity of the
full sample was purified by column chromatography
(alumina). Their low solubility hampered a full characteriz-
ation of these compounds by 13C-NMR, COSY or HSQC spectro-
scopy. Their 1H-NMR spectra, EA and HRMS are consistent
with the proposed formulation. The molecular structure of
complex G-acac, containing two azobenzene fragments on
different phenylpyridyl ligands, was obtained by X-ray diffrac-
tion of a crystalline sample obtained by evaporation of satu-
rated CH3CN solutions. The obtained molecular structure is
shown in Fig. 2.
The molecular structure of G-acac confirms the expected
octahedral coordination environment of the iridium centre,
with the two phenylpyridyl ligands coordinating in a transoid
manner (N1 and N4 occupy relative trans positions). The mole-
cule is C2-symmetric in solution, as observed by NMR spectro-
scopy. This symmetry is broken in the solid state due to
slight distortions and the co-crystallization of acetonitrile
molecules. The four aromatic rings of each phenylpyridyl
ligand are not completely coplanar, but a slight torsion angle
is observed, being more pronounced in one ligand than in the
other (twist angles of 8(2)° and 40.3(19)° respectively).
Scheme 5 (i) AgOTf, acetone (55 °C, 2 h). (ii) 2-(4-Bromophenyl)pyri-
dine (4 equiv.), NEt3 (7.5 equiv.), acetone (55 °C, 15 h). (iii) Solvent 2 : 1
THF–Na2CO3aq. (1 M), 2 mol% of Pd(PPh3)4, 80 °C, 15 h.
In view of the good yields obtained in the Suzuki couplings
of brominated complexes with (E)-4-azophenylboronic acid
pinacol ester (f), we decided to explore the possibility to intro-
duce three azobenzene units per metal centre (Scheme 5). For
that purpose, we synthesized the triscyclometalated complex H
[Ir(5-bromo-2-pyridyl)phenyl)3]. Dimeric iridium complex E
was cleaved using an excess of 2-(4-bromophenyl)pyridine, to
form tris-brominated homoleptic meridional complex H. Crys-
tals suitable for X-ray diffraction were obtained for mono-, bis-
and tris-brominated complexes C, F-Fppy and H. C was
obtained by slow diffusion of hexane into saturated CH2Cl2–
CDCl3 solutions of the complex, F-Fppy was obtained by evap-
oration of saturated CDCl3–CH2Cl2 solutions and H was
obtained by evaporation of CDCl3–acetone solutions (see ESI†).
Complex H was subjected to Suzuki cross-coupling with 3.2
equiv. of (E)-4-azophenylboronic acid pinacol ester (f) follow-
ing the procedure described above for derivatives A4, B4 and
G, rendering meridional homoleptic compound I which con-
tains three photochromic azobenzene units. The lack of sym-
metry of this compound hampers full assignment of its NMR
spectra. Nevertheless, the 1H-NMR spectra showed aromatic
signals which could fit 48 protons integration. More revealing
is the 13C-APT-NMR spectra. It presented, as expected, 69
carbon signals, 21 of which are quaternary.
UV-vis absorption data
UV-vis spectra of ligands 1–4‡ and their complexes A–I were
measured in order to evaluate the effect of the ligand coordi-
nation on the electronic structure of the azobenzene frag-
ments. Their band maxima and molar extinction coefficient
are presented in Table 1.
Ligands 1–4 present two absorption bands in the region
300–500 nm (λmax and λ2) characteristic of azobenzene deriva-
tives.8 The more intense and energetic absorption (λmax) is
attributed to a π→π* transition, and the less intense red-
shifted λ2 to the symmetry-forbidden n→π* transition. Cationic
complexes A1′ and B1′, in which azobenzene is coordinated to
Fig. 2 Molecular structure of the complex G-acac obtained by X-ray
diffraction. ORTEP ellipsoids at the 50% probability level. Solvent
molecules (CH3CN) and hydrogen atoms have been omitted for clarity.
Relevant distances (Å): N2–N3 = 1.252(18), N5–N6 = 1.259(15) N1–Ir =
2.016(10), N4–Ir = 2.043(10), C11–Ir = 2.012(14), C12–Ir = 1.965(14),
O1–Ir = 2.1682(10), O2–Ir = 2.141(11).
‡Ligand
4 was synthesized a posteriori by Suzuki cross-coupling of f and
2-bromopyridine and was used only for comparative purposes.
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the complexes analyzed, as expected, it is proportional to the
number of azobenzenes in the complex. This trend has already
been observed for cobalt complexes containing bipyridine–azo-
benzene ligands.18
Table 1 UV/Vis spectroscopic data of ligands 1–4 and complexes A–I
measured in CH3CN
b
λmax [nm]
Δλmax
λ2 [nm]
Compound
(ε [M−1 cm−1])
[nm] ([cm−1])
(ε [M−1 cm−1])
Cyclic voltammetry
1
2
3
4
A1′
B1′
A2
B2
A3
323 (1.6 × 104)
319 (1.8 × 104)
339 (2.8 × 104)
348 (3.8 × 104)
304 (2.2 × 104)
310 (2.5 × 104)
398 (1.9 × 104)
387 (1.8 × 104)
346 (2.9 × 104)
348 (2.8 × 104)
356 (3.7 × 104)
352 (3.8 × 104)
354 (6.5 × 104)
350 (6.4 × 104)
353 (6.8 × 104)
355 (8.9 × 104)
—
—
—
—
454 (0.4 × 103)
429 (0.4 × 103)
439 (0.9 × 103)
441 (1.4 × 103)
—
The electrochemical properties of azobenzene-containing
iridium complexes were studied in dimethylformamide (DMF)
solutions. The corresponding half-wave potentials are sum-
marized in Table 2. The HOMO and LUMO levels of all com-
plexes have been deduced by the equation69–71 EHOMO/ELUMO
(eV) = −(4.8 + Eonset) and ΔE has been obtained as the differ-
ence LUMO–HOMO.
The obtained voltammograms (Fig. S1–S6 in ESI†) and
redox potential values were compared to the well-known mer-Ir-
(ppy)3.69–71 According to the literature, the voltammogram of
mer-Ir(ppy)3 exhibits two reversible reduction peaks assigned
to the reduction of the phenylpyridine ligands.72 It also pre-
sents a reversible oxidation wave attributed to the IrIV/IrIII
redox couple.73,74
−19 (1935)
−13 (1928)
79 (−6222)
68 (−5508)
7 (−597)
9 (−763)
8 (−646)
4 (−327)
6 (−487)
2 (−164)
5 (−407)
7 (−567)
—
466 (9.5 × 103)
436 (9.0 × 103)
454 (6.0 × 103)
433 (5.5 × 103)
464 (5.4 × 103)
449 (5.6 × 103)
481 (7.1 × 103)
469 (8.7 × 103)
470 (8.4 × 103)
476 (9.2 × 103)
B3
A4
B4
G-ppya
G-Fppya
G-acac
I
a Measured in CH2Cl2 due to their low solubility in CH3CN. b Δλmax
λmax(complex) − λmax(ligand)
=
.
As has been reported before, the values of the redox poten-
tials of iridium complexes containing ppy derivatives depend
on the electron-donating or electron-withdrawing nature of
their substituents.75 The azobenzene-containing complexes
analyzed in this work exhibit their first oxidation peak at
potentials between 0.53 and 1.27 V vs. Fc/Fc+ and their first
reduction peak potentials between −1.81 and −1.60 V. These
values are all anodically shifted with respect to those corres-
ponding to Ir(ppy)3. The lower reduction and higher oxidation
potentials observed, compared to those of Ir(ppy)3, could be
attributed to the presence of the low-lying π* orbital of the azo-
benzene.76 This trend is more pronounced for derivatives of
ligand 1 (within A and B series of compounds), probably due
to the cationic nature of complexes A1′ and B1′, and the loss of
aromaticity (and thus thermodynamic stability towards the
reduction process) observed in the ligands upon chelation.
Comparing the values of the oxidation potentials measured
for molecules of series A (ppy derivatives) to B (Fppy deriva-
tives), it can be observed that the presence of 2 electron-with-
drawing F atoms on each ligand leads to an anodic shift of the
corresponding oxidation peaks. The reduction wave is affected
to a lesser extent. Consequently, fluorine-containing com-
plexes show larger ΔE values than their fluorine-free counter-
parts (see Fig. 4 for a representative example). This observation
is in agreement with the published data for other iridium and
ruthenium polypyridyl complexes.77,78
the metal through one of the nitrogen atoms, do not present
any intense band in the region assigned to a π→π* transition,
but only a shoulder at shorter light wavelengths compared to
the parent ligand. In the case of derivatives of ligands 2–4,
metal coordination produces a bathochromic shift in the band
attributed to the π→π* transition. This shift is larger in the
case of complexes A2 and B2 (79 and 68 nm respectively) com-
pared to the ones derived from ligands 3 or 4 (Δλmax < 10 nm).
See Fig. 3 for representative absorption spectra of ligand 2 and
complex A2. This observation suggests that a para relative posi-
tion of the metal coordination and the azobenzene group
establishes stronger electronic communication between them,
compared to the meta substitution. Extending the aromaticity
on the phenylpyridyl ligand (see A3, B3 vs. A4, B4) produces a
slight increase in the molar absorptivity and in λmax, as
observed for fluorenyl tethered Ir(ppy) complexes.68 The pres-
ence of fluorine substituents in derivatives B does not produce
any systematic change in the UV-vis absorption spectra of their
complexes. By comparing the molar absorptivity at λmax in all
The reduction and oxidation peak potentials found for
complex A4 compared to A3 indicate that the incorporation of
an additional benzene ring in the structure of the phenylpyri-
dine ligand has a slight effect on the redox behaviour of the
complex.
The influence of the replacement of a second ppy ligand by
an azobenzene-containing ppy on complex A4 (to form
complex G-ppy) is almost negligible. However, a similar substi-
tution on Fppy derivative B3 rendering complex G-Fppy pro-
duces a notable cathodic shift on the oxidation potential. This
Fig. 3 Normalized UV-vis spectra of ligand 2 and complexes A2 and
B2, CH3CN.
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Table 2 Electrochemical properties of the Ir complexes
Ereda [V]
Eoxa [V]
Eonsetreda [V]
LUMOb [eV]
Eonsetoxa [V]
HOMOc [eV]
ΔE [eV]
Ir(ppy)3
A1′
A2
A3
B1′
B2
B3
A4
B4
G-ppy
G-Fppy
G-acac
I
−2.64
−1.63
−1.81
−1.68
−1.62
−1.77
−1.62
−1.61
−1.65
−1.64
−1.60
−1.63
−1.71
0.49
0.86
0.55
0.55
1.27
0.91
0.90
0.53
0.85
0.56
0.67
0.77
0.53
−2.41
−1.47
−1.64
−1.50
−1.47
−1.61
−1.43
−1.44
−1.46
−1.44
−1.47
−1.43
−1.45
−2.39
−3.33
−3.16
−3.30
−3.33
−3.19
−3.37
−3.36
−3.34
−3.36
−3.33
−3.37
−3.35
0.33
0.72
0.43
0.39
1.02
0.76
0.71
0.38
0.72
0.39
0.55
0.60
0.40
−5.13
−5.52
−5.23
−5.19
−5.82
−5.56
−5.51
−5.18
−5.52
−5.19
−5.35
−5.40
−5.20
2.75
2.19
2.07
1.89
2.49
2.37
2.14
1.82
2.18
1.83
2.02
2.03
1.85
a Potential values are reported versus Fc/Fc+. b Determined from the onset reduction potential. c Determined from the onset oxidation potential.
Table 3 Main transitions computed at TD-CAM-B3LYP(pcm)/6-31G* &
LANL2DZ level from geometries optimized at B3LYP(pcm)/6-31G* &
LANL2DZ level
a
λmax [nm]
(E [eV])
ΔEHOMO–LUMO
[eV]
f
Transition
2
3
4
317.9 (3.90) 0.97 HOMO → LUMO (65%)
340.6 (3.64) 1.39 HOMO → LUMO (68%)
346.1 (3.58) 1.77 HOMO → LUMO (64%)
—
—
—
A2 368.7 (3.36) 1.25 HOMO−1 → LUMO (64%) 2.88
A3 372.6 (3.29) 0.47 HOMO−1 → LUMO (65%) 2.69
B2 351.6 (3.52) 1.28 HOMO−1 → LUMO (62%) 3.56
B3 367.2 (3.37) 0.66 HOMO−1 → LUMO (65%) 3.28
A4 358.9 (3.45) 1.67 HOMO−2 → LUMO (36%) 2.62
HOMO−1 → LUMO (50%)
B4 356.8 (3.47) 1.74 HOMO−2 → LUMO (32%) 3.25
HOMO−1 → LUMO (54%)
Fig. 4 Cyclo-voltammograms (10−3 M, dry DMF) of A3 and B3 contain-
ing 0.1 M TBAPF6 as the supporting electrolyte, scan rate of 100 mV s−1
a Computed at B3LYP(pcm)/6-31G* & LANL2DZ level.
.
computed by means of time dependent DFT calculations at
the CAM-B3LYP(pcm)/6-31G* & LANL2DZ level of theory. The
data associated with the main transitions computed for
selected compounds are presented in Table 3, and the shape
of the involved frontier molecular orbitals is depicted in Fig. 5.
As is shown in Table 3, the absorption bands are mainly
associated with the HOMO → LUMO transition in the isolated
ligands (2, 3 and 4) and to the HOMO−1 → LUMO transition
in compounds A2,3 or B2,3. Moreover, a second transition
is probably due to the substitution of one of the F-containing
ppy ligands for a less electron-withdrawing ligand 4.
The oxidation potentials of complexes G are affected by the
chemical nature of the ancillary L–X ligand. This is revealed by
the results concerning the substitution of the ppy ligand of
complex G-ppy by the more electron-withdrawing Fppy and
acac ligands. The presence of those ligands markedly increases
the oxidation potential from 0.56 (G-ppy) to 0.67 (G-Fppy) and
0.77 V (G-acac). It should be noted that minor changes in the
reduction potential are also observed.
(HOMO−2
→ LUMO) appears to be relevant when an
additional phenyl group is included in the azobenzene ligands
Concerning the homoleptic complex I, the oxidation poten-
tial of I is anodically shifted from 0.49 to 0.53 V, compared to
the reference compound Ir(ppy)3. According to this value, it
seems that the presence of the azobenzene on the phenylpyri-
dine ligands stabilizes the iridium(III) state towards oxi-
dation.76 As can be expected, this complex is also easily
reduced. It shows a low ΔE value (1.85 eV).
(compounds A4 and B4).
In the complexes, these bands are assigned to the spin
allowed π→π* transition of molecular orbitals mainly located
in the azobenzene moiety and the iridium atom (Fig. 5). Never-
theless, there is a small but non-negligible contribution of the
phenylpyridyl ligands on the occupied orbitals. Therefore, an
effect in the absorption bands will be expected when electron-
withdrawing groups or electron-donating groups are included
in the phenylpyridyl ligands. Our results showed that the
TD-DFT calculations
Electronic energies and oscillator strengths of ligands 2, 3, effect would be higher on the compounds that incorporate
and 4 and compounds A2, A3, A4, B2, B3 and B4 were the azobenzene in the para position (with respect to the metal)
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position has almost no effect on these computed gaps (A3, B3
vs. A4 and B4).
The fluorination effect was also observed on the computed
HOMO–LUMO gaps. In all cases there was found a widening of
the gap due to fluorination (A2, A3 and A4 vs. B2, B3 and B4
respectively, see ESI†). These results are in good agreement
with the experimental values obtained via cyclic voltammetry
(vide infra).
Photoisomerization
It is well known that azobenzenes experience a E → Z isomeriza-
tion by irradiation at a light wavelength close to their absorp-
tion maxima.8 As described by Monkowius et al. this process is
more effective when the irradiation wavelength is individually
optimized for every compound.42 This process and the reverse
thermal cis-to-trans isomerization can be easily followed by
UV-vis absorption spectroscopy due to the smaller extinction
coefficient of the band associated with the absorptivity of the
π→π* transition in the cis complex compared to the trans form.
Following Monkowius’ procedure, to maximize the population
of the cis form in the PSS, kinetic data (first order rate con-
stants and half-life times) on the reverse Z → E thermal iso-
merization of ligands 1–3 and their complexes A–I were
obtained (see Table 4). As this process is temperature-depen-
dent,8 the measurements were performed in CH3CN at 55 °C
to reduce the analysis time which otherwise was taking several
days.
Complexes derived from ligands 1 and 2 showed no spectral
changes upon irradiation. In the case of A1′ and B1′ the lack of
photoisomerization is probably caused by a strong coordi-
nation of the nitrogen atom of the azobenzene to the metal
centre, as weakly coordinating azobenzenes or azopyridines
showed an active photoisomerization process.54,63,79 In the
case of A2 and B2 the low degree of isomerization could be
attributed to steric reasons or to a stabilizing effect of the trans
form upon metal coordination. Complexes derived from
ligands 3 and 4 presented thermally induced back isomeriza-
tion processes with similar reaction rates (see Fig. 6 for a
representative example of the isomerization process monitored
by UV-vis absorption spectroscopy). Compared with other
Fig. 5 Most relevant MO associated with the vertical excitations shown
in Table 3. Occupied and unoccupied orbitals are represented by red &
blue or yellow & green surfaces respectively.
(A2 or B2), compared to the meta substituted ones (A3, B3,
A4 or B4) due to the different participation of these ligands
on the involved molecular orbitals (see ESI† for further details
of the expansion of molecular coefficients). Our results also
show a hypsochromic shift in the absorption wavelength
maxima of A2 compared to its fluorinated counterpart B2 of
ΔΛmax(A2–B2) = 17.1 nm (in good agreement with the experi-
mental shift of 11 nm, Table 1). In the case of ligand 3 deriva-
tives these calculated shifts are smaller (ΔΛmax(A3–B3) = 5.4 nm
and ΔΛmax(A4–B4) = 2 nm). Comparative values of −2 nm and
4 nm were found experimentally, respectively. The anoma-
lous behaviour of the pair A3–B3 (small bathochromic effect
due to fluorination) cannot be explained by means of TD-DFT
calculations.
Table 4 Kinetic data of the Z → E isomerization in CH3CN at 55 °C
Compound
λirrada [nm]
k [s−1
]
τ1/2 [s]
1
2
3
4
327
321
343
354
351
355
367
359
366
358
0.7 × 10−4
0.2 × 10−4
0.9 × 10−4
1.3 × 10−4
1.1 × 10−4
1.0 × 10−4
1.2 × 10−4
1.3 × 10−4
1.2 × 10−4
1.3 × 10−4
9500
33 000
8100
5500
6600
6700
6000
5500
5900
5200
A3
B3
A4
B4
G-acac
I
The substitution pattern has also an important effect on
the computed HOMO–LUMO gaps. Our results show that the
para substituted compounds (A2 or B2) present a gap of about
0.2 eV higher than its meta counterpart compounds (A3 or B3).
Moreover, inclusion of an additional phenyl spacer in the para
a Optimized wavelength for the E → Z photoisomerization. Complexes
G-Fppy and G-Fppy could not be analysed due to their poor solubility
in CH3CN.
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measurements were performed using a Bio-Logic VMP3 poten-
tiostat–galvanostat.
UV-vis absorption measurements were performed with an
Agilent 8453 diode-array spectrophotometer utilizing 10 mm
cell-path quartz cuvettes (110 QS). Measurements of thermal
cis to trans isomerization rates were performed using a 25 μM
solution of A–F and H in acetonitrile and a 9 μM solution of
G-acac, G-Fppy, G-ppy and I in acetonitrile. To maximize the
initial population of Z derivatives on the PSS, we followed the
procedure described by Monkowius:42 using a Shimadzu
RF-540 fluorimeter, a 3 mL portion of each sample was irra-
diated at the corresponding λmax (associated with its π–π* tran-
sition band) until no further change in the UV-vis absorption
spectra was observed. The λ of the maximum observed after
subtracting the first and last spectra of the series was con-
sidered as the optimal light wave-length to promote the Z–E
photoisomerization (λopt). Fresh samples were irradiated at
(λopt) for 60 min, and then placed in a UV-vis spectropho-
tometer. Their absorbance spectral changes were measured as
a function of time for 10 hours. The temperature was main-
tained at 55 °C controlled with a HP 89090A Peltier tempera-
ture control accessory.
Fig. 6 UV-vis spectral changes before (black line) and after photo-
irradiation at 366 nm (blue line), and thermal cis to trans isomerization
of complex G-acac, 9 µM solution in CH3CN (55 °C).
azobenzene-containing organometallic compounds, the
kinetic data obtained locate the stability of the cis form of
these complexes between the one of NHC–azobenzene contain-
ing complexes of gold (k40 °C ∼ 8 × 10−4 s−1 42
and the one of
)
organoplatinum azobenzenes described by Puddephatt (k40 °C
= 4.5 × 10−5 s−1).41 An attempt to measure the degree of photo-
1
isomerization by H-NMR was unsuccessful due to the overlap
Synthetic procedures
of signals.
3-Azophenylboronic acid e. The title compound was
obtained following a procedure analogous to the one reported
for 2-azophenylboronic acid d.44 3-Iodoazobenzene (3.08 g,
10.0 mmol) was dissolved in 80 mL of distilled Et2O, n-BuLi
(1.60 M in hexane, 6.9 mL, 11.0 mmol) was added at −112 °C,
and the reaction mixture was stirred for 30 minutes keeping
the solution at −112 °C. This solution containing lithiated
3-azobenzene was added to an Et2O solution (10 mL) of tri-
methyl borate (1.25 mL, 11.2 mmol) at −112 °C. The reaction
temperature was gradually raised overnight to r.t. The mixture
was quenched with water at 0 °C, and treated with dilute sulfu-
ric acid (100 mL, 1 M). The reaction mixture was extracted with
Et2O (3 × 40 mL). The organic layer was treated with aq. NaOH
(100 mL, 1 M), and the aqueous layer was separated and
washed with Et2O (3 × 40 mL) to remove organic side-products.
The aqueous layer was neutralized with diluted sulfuric acid.
The product precipitated as an insoluble yellow solid. Spectro-
scopic data are in agreement with the ones previously pub-
lished for this compound.86
Experimental
General considerations
IrCl3·3H2O, Pd(PPh3)4, 2-phenylpyridine, 2-(2,4-difluorophenyl)-
pyridine and other general chemicals were obtained from
commercial sources and used without further purification.
2-Azophenylboronic acid d,44 4-azophenylboronic acid pinacol
ester f,45 [Ir(ppy)2Cl]2,65,80,81 [Ir(Fppy)2Cl]2,65,82 4-bromophenyl-
boronic acid pinacol ester,83 2-(4-bromophenyl)pyridine,84
and [Ir((5-bromo-2-pyridyl)phenyl)2Cl]2 E67,85 were synthesized
following published methodologies.
Solvents were dried and purified by known procedures and
freshly distilled under nitrogen from appropriate drying agents
prior to use. All manipulations and reactions involving air
and/or moisture-sensitive organometallic compounds were
performed under an atmosphere of dry nitrogen using stan-
dard Schlenk techniques.
General procedure for the synthesis of compounds 1, 2 and 3
Characterization methods
4.424 mmol of the corresponding azobenzene boronic acid
All electrochemical measurements were carried out in a sealed (d, e or f) were dissolved in 15 mL of degassed THF. To this
glass cell under a N2 atmosphere on 10−3 M solutions of A–I in mixture, 2-bromopyridine (0.762 g, 0.462 mL, 4.822 mmol), Pd-
anhydrous DMF (containing 0.1 M TBAPF6 as the supporting (PPh3)4 (2 mol%, 0.102 g, 0.088 mmol), and Na2CO3 (1 M aq.,
electrolyte) at a scan rate of 100 mV s−1. The working electrode 7.5 mL) were added successively under nitrogen. The reaction
was a glassy-carbon rod (5 mm diameter) and a Pt wire encap- mixture was heated overnight at 80 °C with continuous stir-
sulated on a porous glass tube was used as the counter elec- ring. The reaction was cooled to room temperature and 50 mL
trode. The potentials were controlled using a Metrohm Ag/ of water was added. The resulting mixture was extracted with
AgCl reference electrode. On the other hand a ferrocene/ferro- ethyl acetate (5 × 10 mL), dried (MgSO4), filtered and evapor-
cenium couple (+0.352 V vs. Ag/AgCl) was used as the internal ated in vacuo. The resulting solid was purified on a silica gel
standard (1 × 10−3 M) and all potentials are related to it. The column.
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2-(o-Phenylazobenzene)pyridine, 1. Purification by column
1H NMR (300 MHz, CDCl3): δ 8.77 (ddd, J = 4.8, 1.3, 1.3,
chromatography: silica/eluent CH2Cl2 (Rf 1 = 0.2). It was 1H), 8.16 (d, J = 8.5, 2H), 8.07 (d, J = 8.6, 2H), 8.00 (dd, J = 8.2,
obtained as a red oil. 0.917 g, 80% yield. 1.5, 2H), 7.88–7.81 (m, 6H), 7.61–7.50 (m, 3H), 7.33–7.28 (ddd,
1H NMR (500 MHz, acetone-D6): δ 8.73 (d, J = 4.0, 1H), 8.36 J = 5.0, 5.0, 3.2, 1H). 13C NMR (75 MHz, CDCl3): δ 156.85 (1C,
(d, J = 8.6, 2H), 8.07–8.04 (m, 3H), 7.98 (d, J = 7.2, 2H), 7.92 (td, quat.), 152.76 (1C, Cquat.), 151.91 (1C, Cquat.), 149.72 (1C, CH),
C
J = 7.8, 1.7, 1H), 7.64–7.55 (m, 3H), 7.38 (ddd, J = 7.4, 4.8, 0.7, 143.03 (1C, Cquat.), 140.64 (1C, Cquat.), 138.80 (1C, Cquat.),
1H). 13C NMR (126 MHz, acetone-D6): δ 156.50 (1C, Cquat.), 136.86 (1C, CH), 130.99 (1C, CH), 129.09 (2C, CH), 127.71 (2C,
153.60 (1C, Cquat.), 153.56 (1C, Cquat.), 150.71 (1C, CH), 142.68 CH), 127.51 (2C, CH), 127.43 (2C, CH), 123.43 (2C, CH), 122.87
(1C, Cquat.), 137.90 (1C, CH), 132.23 (1C, CH), 130.17 (2C, CH), (2C, CH), 122.28 (1C, CH), 120.58 (1C, CH). Elemental analysis:
128.42 (2C, CH), 123.90 (2C, CH), 123.75 (1C, CH), 123.61 (2C, calculated for C23H17N3: C, 82.36; H, 5.11; N, 12.53. Found:
CH), 121.38 (1C, CH). Elemental analysis: calculated for C, 82.17; H, 5.03; N, 12.53. Exact mass (MALDI) – m/z:
C17H13N3·0.1CH2Cl2: C, 76.47; H, 4.96; N, 15.64. Found: C, 336.1485 for [M + H]+.
77.00; H, 4.80; N, 15.06. Exact mass (MALDI) – m/z: 260.1178
General procedure for the synthesis of compounds B and C
for [M + H]+.
2-(m-Phenylazobenzene)pyridine, 2. Purification by column [Ir(ppy)2Cl]2 (0.150 g, 0.140 mmol) and AgOTf (0.108 g,
chromatography: silica/eluent CH2Cl2 (Rf 2 = 0.2). It was 0.420 mmol) were dissolved in degassed acetone (8 mL) and
obtained as a dark-red oil. 0.906 g, 79% yield.
refluxed under nitrogen for 2 h. The solution was cooled to
1H NMR (500 MHz, acetone-D6): δ 8.74 (d, J = 4.1, 1H), 8.71 room temperature and gravity-filtered to remove AgCl. The fil-
(t, J = 1.6, 1H), 8.31 (d, J = 7.7, 1H), 8.07 (d, J = 8.0, 1H), trate was refluxed under nitrogen for 1 h and added to a 1 h
8.01–7.98 (m, 3H), 7.93 (td, J = 7.7, 1.8, 1H), 7.72 (t, J = 7.8, refluxed solution of the corresponding ligand 1–3 or 2-(4-
1H), 7.64–7.58 (m, 3H), 7.39 (ddd, J = 7.6, 4.7, 0.9, 1H). bromophenyl)pyridine (0.560 mmol) and NEt3 (0.147 mL,
13C NMR (126 MHz, acetone-D6): δ 156.20 (1C, Cquat.), 153.45 1.054 mmol) in degassed acetone (4 mL). The resulting solu-
(1C, Cquat.), 152.99 (1C, Cquat.), 150.16 (1C, CH), 140.96 (1C, tion was refluxed overnight under nitrogen. After removing the
Cquat.), 137.46 (1C, CH), 131.74 (1C, CH), 129.99 (1C, CH), solvent, the residue was purified by column chromatography.
129.67 (2C, CH), 129.59 (1C, CH), 123.28 (1C, CH), 123.21 (1C, [Ir(ppy)21][OTf], A1′. Purification by column chromato-
CH), 123.12 (2C, CH), 121.43 (1C, CH), 120.67 (1C, CH). graphy: silica/eluent EtOAc (Rf 1 = 0.8; Rf A1′ = 0), when the
Elemental analysis: calculated for C17H13N3 0.7CH2Cl2: C, ligand is eluted the polarity is gradually increased to pure
67.16; H, 4.57; N, 13.30. Found: C, 67.41; H, 4.15; N, 13.79. acetone (Rfacetone A1′ = 0.7). It was obtained as a dark-red crys-
Exact mass (MALDI) – m/z: 260.1175 for [M + H]+.
2-(p-Phenylazobenzene)pyridine, 3. Purification by column
talline solid. 0.178 g, 70% yield.
1H NMR (300 MHz, CDCl3, 0.075 M) δ 8.37 (d, J = 8.0, 1H),
chromatography: silica/eluent CH2Cl2 (Rf 3 = 0.2). It was 8.25–8.20 (m, 2H), 8.16 (d, J = 8.3, 1H), 8.03 (d, J = 8.0, 1H),
obtained as an orange powder. 0.803 g, 70% yield. 8.00–7.97 (td, J = 7.5, 1.4, 1H), 7.93 (d, J = 5.7, 1H), 7.85 (d, J =
1H NMR (500 MHz, acetone-D6): δ 8.73 (d, J = 4.0, 1H), 8.36 7.8, 1H), 7.71–7.66 (m, 2H), 7.59 (dd, J = 5.6, 1.1, 1H), 7.50 (dd,
(d, J = 8.6, 2H), 8.07–8.04 (m, 3H), 7.98 (d, J = 7.2, 2H), 7.92 (td, J = 7.6, 1.7, 1H), 7.40 (td, J = 7.6, 1.1 1H), 7.33–7.25 (m, 3H),
J = 7.8, 1.7, 1H), 7.64–7.55 (m, 3H), 7.38 (ddd, J = 7.4, 4.8, 0.7, 7.20–7.10 (m, 2H), 7.04 (td, J = 7.4, 1.1, 1H), 6.95–6.78 (m, 7H),
1H). 13C NMR (126 MHz, acetone-D6): δ 156.50 (1C, Cquat.), 6.23 (d, J = 7.6, 1H), 6.14 (dd, J = 7.3, 1.3, 1H), 5.77 (dd, J = 8.0,
153.60 (1C, Cquat.), 153.56 (1C, Cquat.), 150.71 (1C, CH), 142.68 0.9, 1H). 13C NMR (75 MHz, CDCl3, 0.075 M): δ 166.85 (1C,
(1C, Cquat.), 137.90 (1C, CH), 132.23 (1C, CH), 130.17 (2C,
CH), 128.42 (2C, CH), 123.90 (2C, CH), 123.75 (1C, CH), 123.61
C
C
quat.), 166.37 (1C, Cquat.), 153.40 (1C, Cquat.), 152.15 (1C,
quat.), 149.96 (1C, CH), 149.69 (1C, CH), 148.85 (1C, Cquat.),
(2C, CH), 121.38 (1C, CH). Elemental analysis: calculated for 148.59 (1C, Cquat.), 147.82 (1C, CH), 143.66 (1C, Cquat.), 143.31
C17H13N3: C, 78.74; H, 5.05; N, 16.20. Found: C, 78.78; (1C, Cquat.), 143.25 (1C, Cquat.), 140.33 (1C, CH), 138.26 (1C,
H, 5.04; N, 15.94. Exact mass (MALDI) – m/z: 260.1178 for CH), 137.85 (1C, CH), 132.83 (1C, CH), 132.67 (1C, Cquat.),
[M + H]+.
132.22 (1C, CH), 132.00 (1C, CH), 130.44 (1C, CH), 129.87 (1C,
2-Phenyl(4-azophenyl)pyridine, 4‡. 2-(4-Bromophenyl)pyri- CH), 129.84 (1C, CH), 129.64 (1C, CH), 129.52 (1C, CH), 128.99
dine (0.300 g, 0.128 mmol) was dissolved in 4 mL of degassed (2C, CH), 126.63 (1C, CH), 125.35 (1C, CH), 124.43 (1C, CH),
THF. To this mixture, 4-azophenylboronic acid pinacol ester, 123.26 (1C, CH), 123.09 (1C, CH), 122.87 (1C, CH), 122.27 (2C,
f (0.434 mg, 0.141 mmol), Pd(PPh3)4 (2 mol%, 2.958 mg, CH), 120.88 (2C, CH), 119.86 (1C, CH), 118.22 (1C, CH), 117.63
0.003 mmol), and Na2CO3 (1 M aq., 2 mL) were added succes- (1C, CH). Elemental analysis: calculated for C40H29F3IrN5O3S:
sively under nitrogen, and heated overnight at 80 °C with con- C, 52.85; H, 3.22; N, 7.70; S, 3.53. Found: C, 52.47;
tinuous stirring. The reaction was cooled to room temperature H, 3.62; N, 7.69; S, 3.54. Exact mass (EI) – m/z: 760.2070
and 50 mL of water was added. The resulting mixture was for [M]+ with (193Ir).
extracted with ethyl acetate (5 × 10 mL). The organic phase was
[Ir(ppy)22], A2. Purification by column chromatography:
dried (MgSO4), and filtered. After evaporation of the solvent silica/CH2Cl2 (Rf A2 = 0.8). It was obtained as a light-orange
from the filtrate, the resulting solid was purified by column solid. 0.042 g, 20% yield.
chromatography eluting with dichloromethane (Rf = 0.2). The
product was obtained as a light orange powder (0.260 g, 60%).
1H NMR (500 MHz, CDCl3): δ 8.31 (s, 1H), 8.12 (m, 2H),
7.96 (d, J = 4.8, 1H), 7.86 (d, J = 7.5, 2H), 7.83 (d, J = 8.1, 2H),
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7.72–7.67 (m, 3H), 7.60 (d, J = 5.4, 1H), 7.56 (t, J = 7.9, 1H), 124.36 (1C, CH), 124.12 (2C, CH), 122.54 (1C, CH), 122.08 (1C,
7.53–7.47 (m, 4H), 7.43 (d, J = 7.1, 1H), 7.14 (d, J = 7.7, 1H), CH), 121.15 (1C, CH), 121.00 (1C, CH), 119.23 (1C, CH), 119.13
6.99–6.95 (m, 3H), 6.92 (t, J = 7.5, 1H), 6.87 (t, J = 7.0, 1H), 6.74 (1C, CH), 118.62 (1C, CH), 118.52 (1C, CH). Elemental analysis:
(t, J = 6.2, 2H), 6.62 (d, J = 5.9, 1H), 6.48 (d, J = 7.4, 1H). calculated for C33H23BrIrN3·C6H14: C, 57.13; H, 4.55; N, 5.13.
13C NMR (126 MHz, CDCl3): δ 187.58 (1C, Cquat.), 174.60 (1C, Found: C, 57.02; H, 4.50; N, 5.20. Exact mass (MALDI) – m/z:
C
C
quat.), 170.51 (1C, Cquat.), 167.98 (1C, Cquat.), 167.78 (1C, 733.0684 for [M]+ with (193Ir)(79Br).
quat.), 158.62 (1C, Cquat.), 153.41 (1C, Cquat.), 153.31 (1C, CH),
General procedure for the synthesis of compounds A and D
151.35 (1C, CH), 148.80 (1C, Cquat.), 147.88 (1C, CH), 146.57
(1C, Cquat.), 144.71 (1C, Cquat.), 142.40 (1C, Cquat.), 138.57 (1C, [Ir(Fppy)2Cl]2 (0.150 g, 0.123 mmol) and AgOTf (0.095 g,
CH), 136.86 (1C, CH), 135.85 (1C, CH), 134.48 (1C, CH), 132.81 0.370 mmol) were dissolved in degassed acetone (8 mL) and
(1C, CH), 130.70 (1C, CH), 130.14 (1C, CH), 129.82 (1C, CH), refluxed under nitrogen for 2 h. The solution was cooled to
129.80 (1C, CH), 129.11 (2C, CH), 124.73 (1C, CH), 124.26 (1C, room temperature and gravity-filtered to remove AgCl. The fil-
CH), 124.11 (1C, CH), 122.94 (1C, CH), 122.34 (2C, CH), 122.20 trate was refluxed under nitrogen for 1 h and added to a 1 h
(1C, CH), 121.38 (1C, CH), 121.18 (1C, CH), 119.65 (1C, CH), refluxed solution of the corresponding ligand 1–3 or 2-(4-
119.23 (1C, CH), 118.80 (1C, CH), 118.59 (1C, CH), 117.84 (1C, bromophenyl)pyridine (0.492 mmol) and NEt3 (0.130 mL,
CH). Elemental analysis: calculated for C39H28IrN5: C, 61.72; 0.932 mmol) in degassed acetone (4 mL). The resulting solu-
H, 3.72; N, 9.23. Found: C, 61.57; H, 3.59; N, 8.94. Exact mass tion was refluxed overnight under nitrogen. After removing the
(MALDI) – m/z: 759.1978 for [M]+ with (193Ir).
solvent, the residue was purified by column chromatography.
[Ir(ppy)23], A3. Purification by column chromatography:
[Ir(Fppy)21][OTf], B1′. Purification by column chromato-
silica/CH2Cl2 (Rf A3 = 0.8). It was obtained as a dark-orange graphy: silica/eluent EtOAc (Rf 1 = 0.8; Rf B1′ = 0) and when the
solid. 0.119 g, 56% yield.
ligand is eluted the polarity is gradually increased to pure
1H NMR (500 MHz, CDCl3): δ 8.22 (d, J = 5.7, 1H), 8.00 (d, acetone (Rfacetone B1′ = 0.7). It was obtained as a red crystalline
J = 8.1, 1H), 7.97 (d, J = 5.3, 1H), 7.87 (d, J = 8.4, 1H), 7.81 (d, J solid. 0.190 g, 79% yield.
= 7.9, 4H), 7.70 (d, J = 6.9, 1H), 7.65 (t, J = 7.2, 2H), 7.60 (d, J =
1H NMR (300 MHz, CDCl3, 0.1 M): δ 8.52 (dd, J = 7.8, 2.2,
5.9, 1H), 7.57 (d, J = 1.8, 1H), 7.53 (t, J = 7.2, 1H), 7.49–7.37 (m, 1H), 8.45 (d, J = 7.9, 1H), 8.25 (td, J = 7.8, 1.6, 1H), 8.15–7.98
5H), 6.98–6.89 (m, 4H), 6.85 (t, J = 7.3, 1H), 6.73 (t, J = 6.1, 1H), (m, 5H), 7.71 (t, J = 7.7, 1H), 7.56 (dd, J = 5.6, 1.3, 1H), 7.45 (td,
6.72 (t, J = 6.1, 1H), 6.63 (d, J = 6.2, 1H), 6.51 (d, J = 7.5, 1H). J = 7.7, 0.9, 1H), 7.36–7.31 (m, 3H), 7.26–7.19 (m, 2H),
13C NMR (126 MHz, CDCl3): δ 179.60 (1C, Cquat.), 174.90 (1C, 6.93–6.86 (m, 4H), 6.60–6.44 (m, 2H), 5.83 (d, J = 8.0, 1H), 5.65
C
C
quat.), 170.78 (1C, Cquat.), 168.36 (1C, Cquat.), 167.73 (1C, (dd, J = 8.5, 2.3, 1H), 5.56 (dd, J = 8.4, 2.3, 1H). 13C NMR
quat.), 158.73 (1C, Cquat.), 153.75 (1C, CH), 153.48 (1C, Cquat.), (75 MHz, CDCl3, 0.1 M): δ 163.25 (d, J = 7.2, 1C, Cquat.), 162.89
153.26 (1C, Cquat.), 151.71 (1C, CH), 148.73 (1C, Cquat.), 147.99 (d, J = 6.6, 1C, Cquat.), 162.22 (dd, J = 257, 12.4, 2C, Cquat.),
(1C, CH), 145.00 (1C, Cquat.), 142.44 (1C, Cquat.), 136.79 (1C, 160.70 (d, J = 260, 12.4, 1C, Cquat.), 159.51 (d, J = 259, 12.5, 1C,
CH), 136.33 (1C, CH), 135.82 (1C, CH), 134.40 (1C, CH), 132.96
(1C, CH), 130.90 (1C, CH), 130.59 (1C, CH), 130.26 (1C, CH), (1C, Cquat.), 149.96 (2C, CH), 148.23 (1C, CH), 147.97 (1C,
130.05 (1C, CH), 129.08 (2C, CH), 125.12 (1C, CH), 124.39 (2C, quat.), 147.60 (d, J = 7.3, 1C, Cquat.), 140.97 (1C, CH), 139.16
Cquat.), 153.30 (1C, Cquat), 152.38 (d, J = 6.5, 1C, Cquat.), 151.87
C
CH), 123.03 (1C, CH), 122.99 (2C, CH), 122.28 (1C, CH), 121.32 (1C, CH), 138.74 (1C, CH), 133.42 (1C, CH), 132.63 (1C, Cquat.),
(1C, CH), 121.21 (1C, CH), 120.02 (1C, CH), 119.35 (1C, CH), 131.00 (1C, CH), 130.69 (1C, CH), 129.96 (1C, CH), 129.12 (2C,
118.85 (1C, CH), 118.73 (1C, CH), 112.40 (1C, CH). Elemental CH.), 127.32 (1C, CH), 127.19 (1C, Cquat.), 127.16 (1C, Cquat.),
analysis: calculated for C39H28IrN5: C, 61.72; H, 3.72; N, 9.23. 125.66 (1C, CH), 123.77 (d, J = 21.1, 1C, CH), 122.73 (1C, CH),
Found: C, 61.43; H, 3.33; N, 9.61. Exact mass (MALDI) – m/z: 122.66 (1C, CH), 122.14 (d, J = 19.6, 1C, CH), 121.45 (2C, CH),
759.1971 for [M]+ with (193Ir).
117.53 (1C, CH), 114.45 (d, J = 18.4, 2C, CH), 99.58 (pst, J =
[Ir(ppy)2((5-bromo-2-pyridyl)phenyl)], C. Purification by 27.0, 1C, CH), 99.23 (pst, J = 27.0, 1C, CH). Elemental analysis:
column chromatography: silica/CH2Cl2–hexane 5 : 1 (Rf C = calculated for C40H25F7IrN5O3S: C, 48.98; H, 2.57; N, 7.14; S,
0.7). It was obtained as a yellow powder. 0.082 g, 40% yield.
3.27. Found: C, 49.40; H, 2.62; N, 6.99; S, 3.41. Exact mass (EI)
1H NMR (300 MHz, CDCl3): δ 8.14 (dd, J = 5.8, 0.9, 1H), – m/z: 832.1658 for [M]+ with (193Ir).
7.95–7.89 (m, 2H), 7.86–7.82 (m, 2H), 7.73–7.51 (m, 7H), 7.14
[Ir(Fppy)22], B2. Purification by column chromatography:
(dd, J = 8.3, 2.2, 1H), 7.04 (d, J = 2.1, 1H), 7.02–6.83 (m, 5H), silica/CH2Cl2 (Rf B2 = 0.8). It was obtained as a light-orange
6.80 (ddd, J = 7.3, 5.9, 1.4, 2H), 6.76 (ddd, J = 7.3, 5.9, 1.4, 2H), solid. 0.031 g, 15% yield.
6.62–6.58 (m, 1H), 6.45 (dd, J = 7.4, 1.1, 1H). 13C NMR
1H NMR (500 MHz, CDCl3): δ 8.31 (s, 1H), 8.24 (t, J = 8.6,
(75 MHz, CDCl3): δ 181.42 (1C, Cquat.), 173.94 (1C, Cquat.), 2H), 8.15 (d, J = 8.0, 1H), 8.09 (d, J = 5.3, 1H), 7.92 (d, J = 5.0,
170.39 (1C, Cquat.), 167.99 (1C, Cquat.), 167.50 (1C, Cquat.), 1H), 7.88 (d, J = 7.5, 2H), 7.76 (t, J = 7.4, 1H), 7.62 (t, J = 7.7,
158.09 (1C, Cquat.), 153.26 (1C, CH), 151.23 (1C, CH), 147.72 1H), 7.58–7.54 (m, 3H), 7.52–7.49 (m, 2H), 7.45 (d, J = 7.1, 1H),
(1C, CH), 144.65 (1C, Cquat.), 144.09 (1C, Cquat.), 142.12 (1C, 7.10 (d, J = 7.7, 1H), 7.04 (t, J = 6.1, 1H), 6.78 (t, J = 6.3, 2H),
C
quat.), 140.03 (1C, CH), 136.55 (1C, CH), 135.65 (1C, CH), 6.47 (t, J = 9.4, 1H), 6.45 (t, J = 9.4, 1H), 6.03 (d, J = 5.6, 1H),
134.29 (1C, CH), 132.61 (1C, CH), 130.48 (1C, CH), 130.01 (1C, 5.84 (d, J = 8.1, 1H). 13C NMR (126 MHz, CDCl3): δ 183.67 (1C,
CH), 129.81 (1C, CH), 126.71 (1C, Cquat.), 125.80 (1C, CH),
Cquat.), 180.10 (1C, Cquat.), 167.50 (1C, Cquat.), 166.90 (d, J = 8.1,
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1C, Cquat.), 164.68 (d, J = 6.9, 1C, Cquat.), 164.62 (dd, J = 10.1, J = J = 261, 1C, Cquat.), 161.87 (d, J = 6.4, 1C, Cquat), 161.69 (dd, J =
259, 1C, Cquat.), 163.93 (dd, J = 12.5, J = 254, 1C, Cquat.), 162.38 13.1, J = 257, 1C, Cquat.), 153.14 (1C, CH), 150.91 (1C, CH),
(dd, J = 10.9, J = 263, 1C, Cquat.), 162.36 (d, J = 6.4, 1C, Cquat.), 147.64 (1C, CH), 143.71 (1C, Cquat), 139.61 (1C, CH), 137.21
161.78 (dd, J = 13.4, J = 259, 1C, Cquat.), 153.25 (1C, Cquat.), (1C, CH), 136.73 (1C, CH), 135.49 (1C, CH), 127.84 (1C, Cquat),
153.20 (1C, CH), 151.04 (1C, CH), 149.17 (1C, Cquat.), 147.78 126.84 (1C, Cquat.), 126.37 (1C, Cquat.), 126.04 (1C, CH), 125.08
(1C, CH), 146.13 (1C, Cquat.), 138.13 (1C, CH), 137.49 (1C, CH) (1C, CH), 122.89 (1C, CH), 122.87 (d, J = 20.3, 1C, CH), 122.8
136.91 (1C, CH), 135.64 (1C, CH), 130.13 (1C, CH), 129.16 (2C, (d, J = 21, 1C, CH), 122.38 (1C, CH), 121.50 (1C, CH), 119.51
CH), 127.90 (1C, Cquat.), 126.58 (1C, Cquat.), 125.02 (1C, CH), (1C, CH), 113.65 (d, J = 14.1, 1.9, 1C, CH), 111.99 (d, J =
123.29 (1C, CH), 123.04 (d, J = 20.8, 1C, CH), 122.78 (d, J = 16.5, 2.1, 1C, CH), 97.56 (pst, J = 27.2, 1C, CH), 95.80 (pst,
19.7, 1C, CH), 122.49 (1C, CH), 122.44 (2C, CH), 121.69 (1C,
J = 27.1, 1C, CH). Elemental analysis: calculated for
CH), 120.02 (1C, CH), 118.12 (1C, CH), 113.85 (d, J = 13.0, 1C, C33H19BrF4IrN3·3C3H6O: C, 51.48; H, 3.81; N, 4.29. Found: C,
CH), 112.14 (d, J = 16.8, 1C, CH), 97.72 (pst, J = 27.2, 1C, CH), 51.57; H, 3.55; N, 4.52. Exact mass (EI) – 806.0393 for [M + H]+
95.79 (pst, J = 27.2, 1C, CH). Elemental analysis: calculated for with (193Ir)(79Br).
C39H24F4IrN5·C3H6O: C, 56.75; H, 3.40; N, 7.88. Found: C,
56.46; H, 3.43; N, 7.43. Exact mass (EI) – 832.1677 for [M + H]+ ppy); Fppy (F-Fppy); (5-bromo-2-pyridyl)phenyl (J). [Ir((5-
with (193Ir).
bromo-2-pyridyl)phenyl)2Cl]2, E (0.150 g, 0.108 mmol) and
[Ir((5-bromo-2-pyridyl)phenyl)2(L)] L = acac (F-acac); ppy (F-
[Ir(Fppy)23], B3. Purification by column chromatography: AgOTf (0.084 g, 0.324 mmol) were dissolved in degassed
silica/CH2Cl2 (Rf B3 = 0.8). It was obtained as a dark-orange acetone (8 mL) and refluxed under nitrogen for 2 h. The solu-
solid. 0.143 g, 70% yield.
tion was cooled to room temperature and gravity-filtered to
1H NMR (500 MHz, CD2Cl2): δ 8.27 (t, J = 7.1, 2H), 8.21 (d, remove AgCl. The filtrate was refluxed under nitrogen for 1 h
J = 5.0, 1H), 8.08 (d, J = 8.2, 1H), 7.96–7.94 (m, 2H), 7.83 (d, J = and added to a 1 h refluxed solution of the corresponding
7.3, 2H), 7.78 (td, J = 8.1, J = 1.4, 1H), 7.68–7.61 (m, 3H), ligand acetylacetone, 2-phenylpyridine, 2-(2,4-difluorophenyl)-
7.53–7.45 (m, 5H), 7.06 (t, J = 6.0, 1H), 6.86 (ddd, J = 7.4, 6.0, pyridine or 2-(4-bromophenyl)pyridine (0.432 mmol) and tri-
1.4, 1H), 6.83 (ddd, J = 7.3, 6.0, 1.3, 1H), 6.53–6.49 (m, 2H), ethylamine (0.113 mL, 0.810 mmol) dissolved in degassed
6.08 (dd, J = 7.4, 2.3, 1H), 5.92 (dd, J = 9.1, 2.2, 1H). 13C NMR acetone (4 mL). The resulting solution was refluxed overnight
(126 MHz, CD2Cl2): δ 180.09 (1C, Cquat.), 175.72 (1C, Cquat.), under nitrogen. After removing the solvent, the residue was
166.92 (1C, Cquat.), 166.74 (d, J = 7.9, 1C, Cquat.), 164.54 (dd, J = purified by column chromatography.
10.0, J = 260, 1C, Cquat.), 164.48 (d, J = 7.2, 1C, Cquat.), 163.90
[Ir((5-bromo-2-pyridyl)phenyl)2(acac)] F-acac. Purification by
(dd, J = 12.7, J = 254, 1C, Cquat.), 162.86 (d, J = 7.6, 1C, Cquat.), column chromatography: alumina/CH2Cl2 (Rf F-acac = 0.6). It
162.39 (dd, J = 10.9, J = 263, 1C, Cquat), 161.80 (dd, J = 12.9, J = was obtained as a yellow powder. 0.131 g, 80% yield.
260, 1C, Cquat.), 153.35 (1C, CH), 153.16 (1C, Cquat.), 152.93
Spectroscopic data are in agreement with the ones pre-
(1C, Cquat.), 151.25 (1C, CH), 148.38 (1C, Cquat.), 148.01 (1C, viously published for this compound.67,85
CH), 137.52 (1C, CH), 136.99 (1C, CH), 135.73 (1C, CH), 134.71
[Ir((5-bromo-2-pyridyl)phenyl)2(ppy)], F-ppy. Purification by
(1C, CH), 130.76 (1C, CH), 129.07 (2C, CH), 128.15 (1C, Cquat.), column chromatography: silica/CH2Cl2–hexane 5 : 1 (Rf F-ppy =
126.73 (1C, Cquat.), 125.32 (1C, CH), 123.47 (1C, CH), 123.04 (d, 0.7). It was obtained as a yellow powder. 0.105 g, 60% yield.
J = 21.4, 1C, CH), 122.74 (d, J = 20.2, 1C, CH), 122.70 (2C, CH),
1H NMR (300 MHz, CDCl3): δ 8.08 (dd, J = 5.8, 0.9, 1H),
122.66 (1C, CH) 121.90 (1C, CH), 120.41 (1C, CH), 113.74 (dd, 7.98–7.92 (m, 2H), 7.78 (m, 3H), 7.71–7.50 (m, 6H), 7.17 (dd,
J = 13.8, 2.0 1C, CH), 113.49 (1C, CH), 112.11 (dd, J = 16.2, 2.0, J = 8.3, 2.1, 1H), 7.10 (dd, J = 8.3, 2.0, 1H), 7.04–6.92 (m, 4H),
1C, CH), 97.43 (pst, J = 27.2, 1C, CH), 95.48 (pst, J = 27.2, 1C, 6.81 (ddd, J = 7.4, 1.4, 2H), 6.79 (ddd, J = 7.4, 1.4, 2H), 6.69 (d,
CH). Elemental analysis: calculated for C39H24F4IrN5: C, 56.38; J = 2.0, 1H), 6.52 (d, J = 2.0, 1H). 13C NMR (75 MHz, CDCl3):
H, 2.91; N, 8.43. Found: C, 56.84; H, 2.64; N, 8.25. Exact mass δ 178.34 (1C, Cquat.), 175.75 (1C, Cquat.), 169.74 (1C, Cquat.),
(EI) – m/z: 832.1656 for [M + H]+ with (193Ir).
[Ir(Fppy)2((5-bromo-2-pyridyl)phenyl)], D. Purification by 153.57 (1C, CH), 151.58 (1C, CH), 148.24 (1C, CH), 145.50 (1C,
168.76 (1C, Cquat.), 167.23 (1C, Cquat.), 161.56 (1C, Cquat.),
column chromatography: silica/CH2Cl2 (Rf D = 0.9). It was
Cquat.), 144.00 (1C, Cquat.), 141.89 (1C, Cquat.), 138.23 (1C, CH),
obtained as a light-orange solid. 0.060 g, 30% yield.
137.26 (1C, CH), 136.43 (1C, CH), 135.57 (1C, CH), 135.05 (1C,
1H NMR (300 MHz, CDCl3): δ 8.29–8.23 (d, J = 8.3, 2H), 8.11 CH), 133.28 (1C, CH), 130.58 (1C, CH), 127.20 (1C, Cquat),
(dd, J = 5.9, 1.1, 1H), 7.92 (d, J = 8.22, 1H), 7.88 (dd, J = 5.6, 0.9, 126.26 (1C, CH), 125.56 (1C, Cquat.), 125.85 (1C, CH), 124.72
1H), 7.72 (t, J = 7.4, 1.7, 1H C5H5N), 7.66–7.58 (m, 3H), 7.51 (1C, CH), 124.69 (1C, CH), 122.99 (1C, CH), 122.86 (1C, CH),
(dd, J = 5.9, 0.9, 1H, C5H5N), 7.19 (dd, J = 8.3, 2.1, 1H), 7.02 122.66 (1C, CH), 122.24 (1C, CH), 122.07 (1C, CH), 119.58 (1C,
(ddd, J = 7.2, 5.6, 1.3, 1H), 7.01 (d, J = 2.19, 1H), 6.84 (ddd, J = CH), 119.40 (1C, CH), 119.17 (1C, CH). Elemental analysis: cal-
7.3, 5.9, 1.4, 1H), 6.82 (ddd, J = 7.3, 5.9, 1.4, 1H), 6.53–6.41 (m, culated for C33H22Br2IrN3·C6H14: C, 57.19; H, 6.02; N, 3.92.
2H), 5.99 (dd, J = 7.5, 2.4, 1H), 5.81 (dd, J = 9.1, 2.4, 1H). Found: C, 57.20; H, 6.11; N, 3.77. Exact mass (MALDI) – m/z:
13C NMR (75 MHz, CDCl3): δ 179.38 (1C, Cquat.), 178.16 (1C, 810.9785 for [M]+ with (193Ir)(79Br).
Cquat), 167.20 (1C, Cquat), 166.77 (d, J = 7.9, 1C, Cquat), 164.65
[Ir((5-bromo-2-pyridyl)phenyl)2(Fppy)], F-Fppy. Purification
(d, J = 7.1, 1C, Cquat), 164.49 (dd, J = 10.0, J = 257, 1C, Cquat), by column chromatography: silica/CH2Cl2 (Rf F-Fppy = 0.9). It
163.85 (dd, J = 12.5, J = 252, 1C, Cquat.), 162.23 (dd, J = 10.8, was obtained as a yellow powder. 0.110 g, 60% yield.
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5
1H NMR (300 MHz, CDCl3): δ 8.36 (ddd, JF–H = 1.7, J = 8.3, 8.22 (dd, J = 5.7, 0.8, 1H), 8.01–7.83 (m, 9H), 7.76–7.67 (m,
1.0, 1H), 8.04 (brdd, J = 5.8, 1.4, 1H), 7.98 (brdd, J = 5.5, 1.6, 4H), 7.62–7.49 (m, 7H), 7.36 (dd, J = 8.1, 2.0, 1H), 7.31 (d, J =
1H), 7.82 (d, J = 8.1, 2H), 7.74–7.68 (brddd, J = 8.3, 7.3, 1.6, 1.9, 1H), 7.00–6.99 (m, 2H), 6.97–6.86 (m, 3H), 6.80–6.74 (m,
1H), 7.67–7.51 (m, 5H), 7.18 (dd, J = 8.3, 2.1, 1H), 7.11 (dd, J = 2H), 6.71–6.69 (m, 1H), 6.53 (dd, J = 7.1, 1.4 1H). 13C NMR
8.3, 2.0, 1H), 6.97 (ddd, J = 7.1, 5.6, 1.2, 1H), 6.88 (ddd, J = 7.3, (75 MHz, CD2Cl2): δ 177.29 (1C, Cquat.), 173.91 (1C, Cquat.),
5.9, 1.4, 1H), 6.84 (ddd, J = 7.3, 5.9, 1.4, 1H), 6.63 (d, J = 2.0, 169.67 (1C, Cquat.), 167.09 (1C, Cquat.), 167.04 (1C, Cquat.),
1H), 6.49–6.37 (m, 3H). 13C NMR (126 MHz, CDCl3): δ 182.15 158.34 (1C, Cquat.), 152.45 (1C, CH), 152.03 (1C, Cquat.), 150.65
(1C, Cquat.), 176.10 (1C, Cquat.), 169.15 (1C, Cquat.), 166.89 (1C, (1C, Cquat.), 150.53 (1C, CH), 147.15 (1C, CH), 145.15 (1C,
C
quat.), 164.94 (d, J = 7.6, 1C, Cquat.), 164.16 (dd, J = 9.1, J = 258,
C
C
quat.), 144.19 (1C, Cquat.), 144.05 (1C, Cquat.), 141.67 (1C,
quat.), 139.37 (1C, Cquat.), 135.99 (1C, CH), 135.15 (1C, CH)
1C, Cquat.), 162.48 (dd, J = 10.6, J = 262, 1C, Cquat.), 160.19
(1C, Cquat.), 153.04 (1C, CH), 151.33 (1C, CH), 147.85 (1C, CH), 135.02 (1C, CH), 133.62 (1C, CH), 131.89 (1C, CH), 130.07 (1C,
143.43 (1C, Cquat.), 141.52 (1C, Cquat.), 137.42 (1C, CH), 136.42 CH), 129.73 (1C, CH), 129.11 (1C, CH), 128.90 (1C, CH), 128.32
(1C, CH), 135.24 (1C, CH), 135.06 (1C, CH), 132.95 (1C, CH), (2C, CH), 126.75 (2C, CH), 123.84 (1C, CH), 123.47 (1C, CH),
128.42 (1C, Cquat.), 126.80 (1C, Cquat.), 126.00 (1C, CH), 125.67 123.33 (1C, CH), 122.28 (2C, CH), 121.94 (2C, CH), 121.83 (1C,
(1C, CH), 125.40 (1C,
Cquat.), 124.62 (1C, CH), 123.38 CH), 121.44 (1C, CH), 120.68 (1C, CH), 120.25 (1C, CH), 119.71
(d, J = 21.7, 1C, CH), 122.93 (1C, CH), 122.85 (1C, CH), 122.48 (1C, CH), 118.69 (1C, CH), 118.31 (1C, CH), 118.01 (1C, CH),
(1C, CH), 122.02 (1C, CH), 119.27 (1C, CH), 119.04 (1C, 117.79 (1C, CH). Elemental analysis: calculated for
CH), 118.63 (d, J = 13.7, 2.0, 1C, CH), 97.85 (pst, J = C45H32IrN5·1.5C3H6O: C, 64.48; H, 4.48; N, 7.59. Found: C,
27.5, 1C, CH, C6H2F2). Elemental analysis: calculated for 63.97; H, 4.08; N, 7.72 Exact mass (MALDI) – m/z: 835.2299 for
C33H20Br2F2IrN3·CH2Cl2: C, 43.75; H, 2.38; N, 4.50. Found: [M]+ with (193Ir).
C, 43.92; H, 1.98; N, 4.28. Exact mass (MALDI) – m/z: 846.9603
[Ir(Fppy)24], B4. [Ir(Fppy)2((5-bromo-2-pyridyl)phenyl)],
(0.100 g, 0.124 mmol) was dissolved in 4 mL of degassed THF.
D
for [M]+ with (193Ir)(79Br).
[Ir((5-bromo-2-pyridyl)phenyl)3], H. Purification by column To this mixture, (E)-4-azophenylboronic acid pinacol ester (f)
chromatography: silica/CH2Cl2 (Rf H = 0.9). It was obtained as (0.042 mg, 0.137 mmol), Pd(PPh3)4 (2 mol%, 2.867 mg,
a yellow powder. 0.116 g, 60% yield.
0.002 mmol), and Na2CO3 (1 M aq., 2 mL) were added succes-
1H NMR (300 MHz, CDCl3): δ 8.09 (dd, J = 5.8, 0.9, 1H), 7.91 sively under nitrogen. The reaction mixture was heated over-
(m, 2H), 7.81 (m, 2H), 7.71–7.50 (m, 7H), 7.16 (dd, J = 8.3, 2.0, night at 80 °C with continuous stirring. The reaction was
2H), 7.10 (dd, J = 8.3, 1.9, 1H), 7.00 (d, J = 2.0, 1H), 6.98 (ddd, cooled to room temperature and 50 mL of water were added to
J = 7.1, J = 5.4, J = 1.4, 1H), 6.86 (ddd, J = 7.3, 5.9, 1.4, 1H), 6.80 precipitate the formed compound. The solution was gravity fil-
(ddd, J = 7.3, 5.9, 1.4, 1H), 6.64 (d, J = 2.0, 1H), 6.46 (d, J = 1.9, tered and the solid was washed with hexane (20 mL). The
1H).13C NMR (75 MHz, CDCl3): δ 178.82 (1C, Cquat.), 176.43 resulting solid was purified by column chromatography (silica)
(1C, Cquat.), 169.09 (1C, Cquat.), 167.31 (1C, Cquat.), 166.82 (1C, eluting with dichloromethane (Rf = 0.8), obtaining 0.099 g
C
quat.), 159.86 (1C, Cquat.), 153.22 (1C, CH), 151.16 (1C, CH), (yield 88%) as an orange powder.
147.72 (1C, CH), 143.87 (1C, Cquat.), 143.47 (1C, Cquat.), 141.33
1H NMR (300 MHz, CDCl3): δ 8.30–8.24 (m, 2H), 8.21 (dd,
(1C, Cquat.), 139.79 (1C, CH), 136.98 (1C, CH), 136.22 (1C, CH), J = 5.8, 1.1, 1H), 8.03 (d, J = 8.1, 1H), 7.98–7.92 (m, 5H), 7.87 (d,
135.02 (1C, CH), 134.96 (1C, CH), 132.79 (1C, CH), 126.73 (1C, J = 8.2, 1H), 7.74 (td, J = 7.6, 1.7, 1H), 7.65–7.52 (m, 8H), 7.39
Cquat.), 126.69 (1C, Cquat.), 125.27 (1C, Cquat.), 125.92 (1C, CH), (dd, J = 8.1, 2.0, 1H), 7.26 (d, J = 1.9, 1H), 7.02 (ddd, J = 7.1, 5.6,
125.87 (1C, CH), 125.62 (1C, CH), 125.28 (1C, 1Cquat.), 124.87 1.1, 1H), 6.83 (ddd, J = 7.2, 5.7, 1.4, 1H), 6.81 (ddd, J = 7.2, 5.7,
(1C, CH), 124.46 (1C, CH), 122.75 (2C, CH), 122.71 (1C, CH), 1.4, 1H), 6.53–6.43 (m, 2H), 6.07 (dd, J = 7.5, 2.3, 1H), 5.90 (dd,
121.83 (1C, CH), 119.33 (1C, CH), 119.08 (1C, CH), 118.96 (1C, J = 9.2, 2.3, 1H). 13C NMR (75 MHz, CDCl3): δ 180.48 (1C, J =
CH). Elemental analysis: calculated for C33H21Br3IrN3·3C3H6O: 3.1, Cquat.), 175.49 (1C, Cquat.), 167.73 (1C, Cquat.), 166.99 (d, J =
C, 47.33; H, 3.69; N, 3.94. Found: C, 47.26; H, 3.36; N, 4.04. 8.0, 1C, Cquat.), 164.74 (d, J = 7.2, 1C, Cquat.), 164.57 (dd, J = 9.6,
Exact mass (EI) – m/z: 889.8982 for [M + H]+ with (193Ir)(79Br).
[Ir(ppy)24], A4. [Ir(ppy)2((5-bromo-2-pyridyl)phenyl)],
J = 257, 1C, Cquat.), 163.93 (dd, J = 12.6, J = 253, 1C, Cquat.),
162.82 (d, J = 5.9, 1C, Cquat.), 162.67 (dd, J = 10.8, J = 262, 1C,
C
(0.100 g, 0.136 mmol) was dissolved in 4 mL of degassed THF.
Cquat.), 161.75 (dd, J = 13.1, J = 257, 1C, Cquat.), 153.23 (1C, CH),
To this mixture, (E)-4-azophenylboronic acid pinacol ester (f) 152.84 (1C, Cquat.), 151.61 (1C, Cquat.), 150.99 (1C, CH), 147.76
(0.046 mg, 0.150 mmol), Pd(PPh3)4 (2 mol%, 3.143 mg, (1C, CH), 145.10 (1C, Cquat.), 144.40 (1C, Cquat.), 140.85 (1C,
0.003 mmol), and Na2CO3 (1 M aq., 2 mL) were added succes-
sively under nitrogen. The reaction mixture was heated over- 135.30 (1C, CH), 130.78 (1C, CH), 129.05 (2C, CH), 127.93 (1C,
night at 80 °C with continuous stirring. The reaction was quat.), 127.65 (2C, CH), 126.46 (1C, Cquat.), 124.72 (1C, CH),
Cquat.), 137.12 (1C, CH), 136.60 (1C, CH) 135.72 (1C, CH),
C
cooled to room temperature and 50 mL of water were added to 123.21 (2C, CH.), 122.84 (d, J = 20.3, 1C, CH), 122.81 (2C, CH),
precipitate the formed compound. The solution was gravity fil- 122.68 (1C, CH), 122.65 (d, J = 21, 1C, CH), 122.30 (1C, CH),
tered and the solid was washed with hexane (20 mL). The 121.53 (1C, CH), 121.11 (1C, CH), 119.56 (1C, CH), 113.74 (d,
resulting solid was purified by column chromatography (silica) J = 14.0, 1.6, 1C, CH), 111.98 (d, J = 16.0, 1.7, 1C, CH), 97.46
eluting with dichloromethane (Rf = 0.8), obtaining 0.097 g (pst, J = 27.4, 1C, CH), 95.52 (pst, J = 27.2, 1C, CH). Elemental
(yield 85%) as an orange powder. 1H NMR (300 MHz, CDCl3): δ analysis: calculated for C45H28F4IrN5·2CH2Cl2: C, 52.42; H,
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3.00; N, 6.50. Found: C, 52.18; H, 2.49; N, 6.43. Exact mass 3.051 mg, 0.003 mmol), and Na2CO3 (1 M aq., 2 mL) were
(MALDI) – m/z: 907.1906 for [M]+ with (193Ir).
added successively under nitrogen. The reaction mixture was
[Ir(4)2(ppy)], G-ppy. [Ir((5-bromo-2-pyridyl)phenyl)2(ppy)], heated overnight at 80 °C with continuous stirring. The reac-
F-ppy (0.100 g, 0.123 mmol) was dissolved in 4 mL of degassed tion was cooled to room temperature and 50 mL of water were
THF. To this mixture, (E)-4-azophenylboronic acid pinacol added to precipitate the formed compound. The solution was
ester (f) (0.080 mg, 0.258 mmol), Pd(PPh3)4 (2 mol%, gravity filtered and the solid was washed with hexane (20 mL).
2.843 mg, 0.002 mmol), and Na2CO3 (1 M aq., 2 mL) were The resulting solid was purified by column chromatography
added successively under nitrogen. The reaction mixture was (alumina) eluting with dichloromethane (Rf = 0.4), obtaining
heated overnight at 80 °C with continuous stirring. The reac- 0.101 g (yield 80%) as an orange powder.
tion was cooled to room temperature and 50 mL of water were
1H NMR (300 MHz, CDCl3): δ 8.64 (dd, J = 5.7, 0.8, 2H),
added to precipitate the formed compound. The solution was 7.97–7.79 (m, 12H), 7.67 (d, J = 8.1, 2H), 7.58–7.49 (m, 10H),
gravity filtered and the solid was washed with hexane (20 mL). 7.24 (ddd, J = 7.2, 5.8, 1.4, 2H), 7.18 (dd, J = 8.1, 1.7, 2H), 6.60
The resulting dark-orange powder (0.110 g, 80%) is highly (d, J = 1.7, 2H), 5.31 (s, 1H), 1.87 (s, 6H). 13C NMR (75 MHz,
insoluble in all the solvents assayed (i.e. solubility is less than CDCl3): δ 184.76 (2C, Cquat.), 168.16 (2C, Cquat.), 152.80 (2C,
3 mg in 10 mL of CDCl3). A small sample (5 mg) was purified
Cquat.), 151.45 (2C, Cquat.), 148.32 (2C, CH), 147.76 (2C, Cquat.),
by column chromatography (alumina) eluting with dichloro- 144.99 (2C, Cquat.), 144.46 (2C, Cquat.), 140.00 (2C, Cquat.),
methane (Rf = 0.8). The 1H-NMR of the eluted compound is vir- 137.02 (2C, CH), 131.43 (2C, CH), 130.75 (2C, CH), 129.04 (4C,
tually identical to the one of unpurified sample, the colour CH), 127.74 (4C, CH), 124.10 (2C, CH), 122.94 (4C, CH), 122.75
being much brighter tough. The low solubility hampered com- (4C, CH), 121.62 (2C, CH), 120.27 (2C, CH), 118.71 (2C, CH),
plete NMR characterization. 1H NMR (300 MHz, CDCl3): δ 8.24 100.54 (1C, CH, H), 28.77 (2C, CH3, H). Elemental analysis: cal-
(dd, J = 5.8, 1.1, 1H), 8.06 (dd, J = 5.7, 1.1, 1H), 8.00 (dd, J = 8.3, culated for C51H39IrN6O2: C, 63.80; H, 4.09; N, 8.75. Found: C,
1.2, 1H), 7.96–7.89 (m, 7H), 7.85–7.78 (m, 3H), 7.75–7.48 (m, 63.67; H, 4.11; N, 8.23. Exact mass (EI) – m/z: 960.2753 (M)+
16H), 7.36 (dd, J = 8.1, 2.0, 1H), 7.27 (dd, J = 8.1, 1.9, 1H), with (193Ir).
7.09–6.94 (m, 6H), 6.86–6.79 (m, 3H). Elemental analysis: cal-
[Ir(4)3], I. [Ir((5-bromo-2-pyridyl)phenyl)3], H (0.100 g,
culated for C57H40IrN7·2C3H6O: C, 66.88; H, 4.63; N, 8.67. 0.112 mmol) was dissolved in 4 mL of degassed THF. To this
Found: C, 66.20; H, 4.06; N, 8.28. Exact mass (MALDI) – m/z: mixture, (E)-4-azophenylboronic acid pinacol ester (f)
1015.2990 for [M]+ with (193Ir).
(0.110 mg, 0.359 mmol), Pd(PPh3)4 (2 mol%, 2.588 mg,
[Ir(4)2(Fppy)], G-Fppy. [Ir((5-bromo-2-pyridyl)phenyl)2(Fppy)], 0.002 mmol), and Na2CO3 (1 M aq., 2 mL) were added succes-
F-Fppy (0.100 g, 0.118 mmol) was dissolved in 4 mL of sively under nitrogen. The reaction mixture was heated over-
degassed THF. To this mixture, (E)-4-azophenylboronic acid night at 80 °C with continuous stirring. The reaction was
pinacol ester (f) (0.076 mg, 0.247 mmol), Pd(PPh3)4 (2 mol%, cooled to room temperature and 50 mL of water were added to
2.727 mg, 0.002 mmol), and Na2CO3 (1 M aq., 2 mL) were precipitate the formed compound. The solution was gravity fil-
added successively under nitrogen. The reaction mixture tered and the solid was washed with hexane (20 mL). The
was heated overnight at 80 °C with continuous stirring. The resulting solid was purified by column chromatography (silica)
reaction was cooled to room temperature and 50 mL of water eluting with dichloromethane (Rf = 0.8), obtaining 0.123 g
were added to precipitate the formed compound. The solution (yield 82%) as an orange powder.
was gravity filtered and the solid was washed with hexane
1H NMR (300 MHz, CDCl3): δ 8.31 (dd, J = 5.8, 0.9, 1H), 8.10
(20 mL). The resulting orange powder (0.111 g, 90%) is highly (dd, J = 5.4, 0.9, 1H), 8.03 (d, J = 8.3, 1H), 7.97–7.86 (m, 15H),
insoluble in all the solvents assayed (i.e. solubility is less than 7.83 (d, J = 5.4, 1H), 7.79–7.77 (m, 2H), 7.70 (td, J = 7.32, 1.56,
1.5 mg in 10 mL of CDCl3). A small sample (5 mg) was purified 1H), 7.65–7.49 (m, 17H), 7.41–7.35 (m, 3H), 7.30 (dd, J = 8.0,
by column chromatography (alumina) eluting with dichloro- 1.8, 1H), 7.05 (d, J = 1.8, 1H), 6.99 (ddd, J = 6.9, 5.6, 0.9, 1H),
methane (Rf = 0.8). The 1H-NMR of the eluted compound is vir- 6.89 (d, J = 1.8, 1H), 6.88–6.81 (m, 2H). 13C NMR (75 MHz,
tually identical to the one of impure sample, the colour being CDCl3): δ 177.69 (1C, Cquat.), 175.33 (1C, Cquat.), 170.09 (1C,
much brighter tough. Its low solubility hampered 13C-NMR
C
C
quat.), 167.98 (1C, Cquat.), 167.58 (1C, Cquat.), 159.12 (1C,
quat.), 153.43 (1C, CH), 152.81 (3C, Cquat.), 151.42 (3C, Cquat.),
characterization. 1H NMR (300 MHz, CDCl3): δ 8.42–8.38 (m,
1H), 8.19 (dd, J = 5.6, 0.7, 1H), 8.12–8.09 (m, 1H), 7.95–7.89 151.42 (1C, CH), 148.00 (1C, CH), 145.62 (1C, Cquat.), 145.28
(m, 7H), 7.83 (d, J = 8.1, 1H), 7.78 (d, J = 8.8, 1H), 7.74–7.46 (m, (1C, Cquat.), 145.18 (1C, Cquat.), 145.02 (1C, Cquat.), 144.59 (1C,
14H), 7.36 (dd, J = 8.2, 2.0, 1H), 7.27 (dd, J = 8.1, 1.9, 1H, 1H),
6.99–6.83 (m, 5H), 6.75 (d, J = 1.7, 1H), 6.55–6.41 (m, 4H).
C
C
quat.), 142.73 (1C, Cquat.), 140.79 (1C, Cquat.), 140.70 (1C,
quat.), 140.23 (1C, Cquat.), 136.65 (1C, CH), 136.16 (1C, CH),
Elemental analysis: calculated for C57H38F2IrN7: C, 65.13; H, 135.79 (1C, CH), 134.40 (1C, CH), 131.10 (1C, CH), 130.76 (1C,
3.64; N, 9.33. Found: C, 64.75; H, 3.456; N, 8.816. Exact mass CH), 130.69 (2C, CH), 129.05 (1C, CH), 129.01 (6C, CH), 127.80
(MALDI) – m/z: 1051.2782 for [M]+ with (193Ir).
(2C, CH), 127.72 (2C, CH), 127.53 (2C, CH), 124.53 (1C, CH),
[Ir(4)2(acac)], G-acac. [Ir((5-bromo-2-pyridyl)phenyl)2(acac)], 124.48 (1C, CH), 124.41 (1C, CH), 123.13 (2C, CH), 123.07 (2C,
F-acac (0.100 g, 0.132 mmol) was dissolved in 4 mL of CH), 123.02 (2C, CH), 122.76 (6C, CH), 122.51 (1C, CH), 122.30
degassed THF. To this mixture, (E)-4-azophenylboronic acid (1C, CH), 121.53 (1C, CH), 120.53 (1C, CH), 120.50 (1C, CH),
pinacol ester (f) (0.085 mg, 0.277 mmol), Pd(PPh3)4 (2 mol%, 119.32 (1C, CH), 118.96 (1C, CH), 118.80 (2C, CH). Elemental
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analysis: calculated for C69H48IrN9·2C3H6O: C, 68.68; H, 4.61; 10 M. D. Segarra-Maset, P. W. N. M. van Leeuwen and
N, 9.61. Found: C, 68.88; H, 4.16; N, 9.68. Exact mass (MALDI)
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11 N. Kawamura, R. Kiyotake and K. Kudo, Chirality, 2002, 14,
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12 H. Bricout, E. Banaszak, C. Len, F. Hapiota and
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Conclusions
Twelve new triscyclometalated Ir(III) organometallic complexes
containing up to three photoswitchable azobenzene units have
been synthesized. In complexes derived from ligand 1, the
ligand acts as a neutral N–N donor coordinating through the
pyridinic nitrogen and one of the nitrogen atoms of the azo-
benzene. Consequently, derivatives B1′ and A1′ are cationic
compounds, containing twisted azobenzene units, in which
the photoisomerization process of the azobenzene is inhibited.
Analysis of the spectroscopic and electronic properties of com-
plexes derived from ligands 2, 3 and 4 compared to the well
known [Ir(ppy)3] showed that in derivatives of ligand 2, a
strong electronic communication between the metal centre
and the azobenzene unit is established. Probably this produces
an additional stabilization of the trans form of the azobenzene
responsible of the nearly undetectable photoisomerization
process for derivatives B2 and A2. Complexes incorporating
ligands 3 or 4 behave as photochromic compounds with iso-
merization rates comparable to those reported for other azo-
benzene-containing organometallic complexes.41,42 Currently,
other photochromic derivatives of these ligands are being syn-
thesized in our laboratory.
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Acknowledgements
This work was supported by MICINN (CTQ2011-23333),
SAIOTEK (S-PE13UN020), UPV-EHU (GIU13/06) and Basque
Government grants to J.P-M (BFI-2012-147MEC) and M.M-O
(COL_2013_1_351). UPV-EHU Research Support Unit (SGIker)
is acknowledged for technical assistance. We are indebted to
Prof. V.V. Grushin whose comments inspired this work.
26 T. Yutaka, M. Kurihara, K. Kubo and H. Nishihara, Inorg.
Chem., 2000, 39, 3438–3439.
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