New 2,3-disubstituted-5-hydroxyquinoxaline ligands and their coordination chemistry with cyclometallated iridium(iii): Syntheses, structures and tunable electronic properties

Article abstract of DOI:10.1039/c1dt10707k

A new series of para-substituted 2,3-diphenyl-5-hydroxyquinoxaline ligands (LHⁿ) were synthesised and characterised. These ligands were prepared in high yield via a two-step synthetic method. Four novel heteroleptic iridium(iii) complexes were

Full text of DOI:10.1039/c1dt10707k

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PAPER
New 2,3-disubstituted-5-hydroxyquinoxaline ligands and their coordination
chemistry with cyclometallated iridium(III): syntheses, structures and tunable
electronic properties†
Andrew J. Hallett,* Benson M. Kariuki and Simon J. A. Pope*
Received 19th April 2011, Accepted 5th July 2011
DOI: 10.1039/c1dt10707k
A new series of para-substituted 2,3-diphenyl-5-hydroxyquinoxaline ligands (LHn) were synthesised and
characterised. These ligands were prepared in high yield via a two-step synthetic method. Four novel
heteroleptic iridium(III) complexes were correspondingly prepared in high yield giving [Ir(ppy)2(Ln)].
Two X-ray crystallographic studies were undertaken on LH3 and [Ir(ppy)2(L2)] with each confirming the
proposed formulations, with the complex showing the O,N-coordination mode of the quinoxalinato
ligand. Density functional theoretical calculations were performed, firstly to compare the coordinated
quinoxalinato system with the related quinolinato analogue, and secondly to probe the influence of the
variation in para-substitution on the ancillary ligand. The calculations suggest that for either the
quinoline or quinoxaline systems ligand-centred character appears to dominate the HOMO and
LUMOs. Experimental electrochemical and spectroscopic characterisation showed that the subtle
variations in absorption and emission wavelengths are probably due to ligand-dominated transitions
that are influenced by the electronic nature of the para-substituted phenyl units in coordinated Ln.
Introduction
Whilst there have been numerous reports on metal ion complexes
(for example Al(III), B(III) and Pd(II))1 containing commercially
available 8-hydroxyquinoline derivatives, only a few describe
the incorporation of substituted hydroxyquinoline ligands into
iridium(III) complexes.2 Given the recent and rapid development
of Ir(III) complexes towards a variety of opto-electronic related
applications such as OLEDs,3 electrochemical cells (LECs),4
photovoltaic5 and luminescent imaging applications,6 hydrox-
Fig. 1 The structures of [AlQ3] and [Ir(py)2Q].
yquinoline ligands and related systems may offer useful avenues for
future applications. The ability to easily tune the optical properties
of Ir(III) complexes via the use of ancillary ligands is therefore of
nature of the ancillary hydroxyquinoline ligand determines the
extent of the LUMO localisation on that ligand.9
great general value. In the specific case of hydroxyquinoline species
In light of this, we have developed a range of related 5-
the ligand-centred excited state character can be tuned through a
hydroxyquinoxaline ligands that are easily substituted with various
detailed appreciation of the IL(phenol-to-pyridine)CT transition.7
aryl units. Our theoretical studies suggest that the LUMO of
The accepting ability (i.e. the LUMO energy) of the pyridine ring
these ligands can be more effectively localised upon the hydrox-
is particularly sensitive to substitution at the 5-position.8 In this
yquinoxaline unit. As a consequence, the corresponding Ir(III)
manner, modulation of the electronic character at the 5-position
complexes demonstrate some tunable behaviour with respect to
can conveniently tune the absorption and emission profiles of the
their luminescent properties.
Here we report a new, convenient procedure for the synthesis
of functionalised 5-hydroxyquinoxaline species, yielding three
new ligands LH2, LH3 and LH4 with substituents of varied
electronic character in the 2,3-positions of the pyrazine ring.
Together with the known ligand LH1, we describe their coor-
dination chemistry with a bis-cyclometallated Ir(III) precursor,
giving the first examples of neutrally charged, heteroleptic Ir(III)
complexes containing functionalised quinoxalinato-based ligands.
ligand and resultant complexes based on Al(III) and Ir(III) (Fig. 1).
In the case of the bis-cyclometalled Ir(III) complexes the electronic
School of Chemistry, Main Building, Cardiff University, Cardiff, CF10
3AT. E-mail: popesj@cardiff.ac.uk, hallettaj@cardiff.ac.uk; Fax: (+44)
029-20874030; Tel: (+44) 029-20879316
† CCDC reference numbers 822159 and 822160. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt10707k
9474 | Dalton Trans., 2011, 40, 9474–9481
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Table 1 Parameters associated with the single crystal diffraction data
collection for LH3 and [Ir(ppy)2(L2)]
The syntheses, selected structures and spectroscopic properties of
the ligands and complexes are reported herein.
LH3
[Ir(ppy)2(L2)]·0.85Et2O
Results and discussion
Formula
C22H18N2O
326.38
150(2)
0.71073
Monoclinic
P21/n
13.9500(4)
16.8880(5)
15.3970(4)
90.00
102.071(2)
90.00
3547.14(17)
8
1376
0.076
C44.4H33.8Br2IrN4O2
1007.37
150(2)
0.71073
Monoclinic
P21/n
18.3163(6)
9.7199(4)
23.3380(5)
90.00
110.101(2)
90.00
3901.84(20)
4
1965
5.513
16143
Formula weight
T/K
Wavelength/A
Synthesis and characterisation
˚
Four para-substituted 2,3-diphenyl-5-hydroxyquinoxaline ligands,
LH1–4 (1 = p-H, 2 = p-Br, 3 = p-Me, 4 = p-OMe), were prepared
in a simple two-step reaction procedure (Scheme 1). First, 2-
amino-3-nitrophenol was reduced by heating to reflux in acidic
EtOH for 14 h in the presence of zinc dust, giving the corre-
sponding 2,3-diamino species. Subsequent condensation with a
range of substituted diones in refluxing EtOH for 16 h gave the
desired ligands, and for LH1, LH3 and LH4, without recourse
to further purification. Column chromatography (silica, CH2Cl2)
was, however, required for the purification of LH2: all of the
ligands were isolated in high yields (82–100%). Although LH1 has
been previously reported, these did not include full spectroscopic
characterisation, which are therefore included here for clarity and
comparison.10
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
F000
m/mm-1
Reflections collected
14775
8077 (0.0642)
Independent reflections (Rint
Final R1 [I > 2s(I)]: R1, wR2 0.0705, 0.1390 0.0692, 0.1217
)
8941 (0.0782)
◦
2
˚
Table 2 Selected bond lengths (A) and angles ( ) for [Ir(ppy)2(L )] and
the values calculated from DFT studies
◦
[Ir(ppy)2(L2)]
Calculated values
˚
Bond length (A)/angle ( )
Ir(1)–N(1)
Ir(1)–N(3)
Ir(1)–N(4)
Ir(1)–O(1)
Ir(1)–C(1)
Ir(1)–C(2)
O(1)–Ir(1)–N(1)
N(1)–Ir(1)–N(3)
N(3)–Ir(1)–N(4)
2.232(7)
2.036(7)
2.040(7)
2.132(5)
1.996(8)
1.991(8)
78.1(2)
2.310
2.052
2.039
2.150
1.996
2.008
76.6
95.2(3)
173.7(3)
93.0
174.6
mixture composition of 2 : 1 ligand to [(ppy)2Ir(m-Cl2)Ir(ppy)2] in
2-methoxyethanol, in excellent yields of 88–96 %. The complexes
were characterised in the solution state using H and 13C{ H}
NMR, UV-vis. and luminescence spectroscopies. Due to the
unsymmetrical nature of the quinoxaline ligands, rendering the
1
1
1
1
phenylpyridine ligands inequivalent, the H and 13C{ H} NMR
Scheme 1 Synthetic route to ligands LHn and complexes [Ir(ppy)2Ln] (n =
1–4).
spectra were very complex. For example, [Ir(ppy)2L1] showed 38
1
unique resonances in the aromatic region of the 13C{ H} NMR
1
The ligands were each characterised using 1H and 13C{ H}
spectrum. ES mass spectrometry (including HR) revealed the
protonated parent ions [M+H]+ for each complex.
NMR spectroscopy, mass spectrometry and electronic spec-
troscopy. The distinguishing features of the 1H NMR studies
revealed characteristic resonances associated with the quinoxaline-
appended para-substituted phenyl groups. For example, LH2 (the
para-Br derivative) gave two signature multiplet resonances at 7.44
and 7.30 ppm, attributed to the phenyl groups, observed upfield
from the starting dione precursor (cf. 7.77 and 7.60 ppm in CDCl3).
In all four ligands the proton resonances of the quinoxaline unit
are perturbed by the subtle electronic changes imposed by the
substituted phenyl groups, suggesting an element of electronic
communication throughout the ligand and quinoxaline unit. ES+
mass spectrometry revealed the protonated parent ions [M+H]+
for LH1, LH2 and LH4, whereas EI showed the parent ion
[M]+ for LH3. The ligand LH3 was also characterised by X-ray
crystallography (see X-ray crystallography section).
X-ray crystallography
Single crystals suitable for X-ray diffraction studies were isolated
using vapour diffusion of Et2O in to CHCl3 solutions of the
ligand or complex over a period of 24 h at room temperature.
The parameters associated with the data collection are presented
in Table 1 with selected bond lengths and angles (principally
involving the coordination sphere of [Ir(ppy)2(L2)]) shown in
Table 2.
The solid-state structural determination of LH3 confirmed the
formation of the quinoxaline ligand (Fig. 2, Table 1) showing
that both tolyl groups are twisted out of the plane of the
quinoxaline unit, presumably to minimise steric repulsions.
The structural analysis of [Ir(ppy)2(L2)] also supports the pro-
posed formulation determined from solution state spectroscopic
The neutrally charged hetero-ligand complexes [Ir(ppy)2Ln]
(n = 1–4) were isolated following addition of base to a reaction
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Fig. 2 The structure of LH3. All hydrogen atoms other than H(1) are
omitted for clarity.
analyses (Fig. 3, Tables 1 and 2). The complex possesses pseudo-
octahedral coordination geometry, with a chelated quinoxalinato
derivative bound through the oxygen and nitrogen donor atoms.
As with numerous related examples of this type, the relative
coordination geometry of the cyclometallated phenylpyridine
ligands is retained from that of the precursor (i.e. cis C,C and trans
N,N).11 From a steric perspective, it is noted that the 2-substituted
para-BrC6H4 group is twisted close to orthogonality in an effort
to minimise clashes with the coordinated phenylpyridine ligand.
Fig. 4 Graphical representation of the frontier orbitals of [Ir(ppy)2Q] and
[Ir(ppy)2(quinox)].
HOMO is distributed over the whole of the quinoline ligand, with
no electron density located on the cyclometallated phenylpyridine
ligands. The LUMO entails distribution over the metal centre
and ancillary quinoline ligand, with additional coverage on the
cyclometallated phenylpyridine ligands.
Replacing the quinolinato moiety with quinoxalinato gives
the reference compound [Ir(ppy)2(quinox)], wherein the HOMO
remains essentially unchanged. Importantly, the LUMO is pre-
dicted on the metal centre and ancillary ligand, with little or
no contribution from the cyclometallated ligands (Fig. 4). These
results therefore suggest that appropriate functionalisation of 5-
hydroxyquinoxaline ligands could feasibly yield tunable optical
characteristics within the corresponding Ir(III) complexes.
Fig. 3 The structure of [Ir(ppy)2(L2)]. Hydrogen atoms and solvent
molecules are omitted for clarity.
For the quinoxalinato complexes described here the energy
levels of both the HOMO and LUMO are sufficiently different
(DE > 0.2 eV) from the other MOs to be considered independent.
As with the unsubstituted [Ir(ppy)2(quinox)], both the HOMO and
LUMO are located primarily on the quinoxaline ligand and the
metal 5d(Ir) centre (Fig. 5). Population analyses (Table 3) revealed
that the distribution of the frontier orbitals over the various ligands
and metal is very similar in each case for [Ir(ppy)2Ln] (HOMO:
15.2–16.3% Ir; 77.6–78.9% Ln; 2.1–2.2% ppy trans to N; 3.7–4.2%
ppy trans to O. LUMO: 1.8–1.9% Ir; 94.0–95.2% Ln; 1.5–2.2%
ppy trans to N; 1.3–2.0% ppy trans to O). Although there is
no direct contribution from the substituted phenyl rings to the
HOMO coverage, the nature of the para-substituent does appear
to impart an influence on the overall energy level. In contrast,
the LUMO is partially delocalised over the substituted phenyl
rings. Of course, due to relativistic effects, absolute energy levels
cannot be reliably calculated for complexes of heavy metal ions,
but relative energy levels can still be informative. For example, the
complex of the electron withdrawing para-Br ligand [Ir(ppy2)L2]
has the lowest HOMO energy level (E = -5.03 eV) whereas the
complex of the electron donating para-OMe has the highest (E =
-4.73 eV). Similarly, the LUMO energies also reflect this variation
(para-Br ELUMO = -2.12 eV; para-OMe ELUMO = -1.74 eV). Again,
The bond lengths and angles of [Ir(ppy)2(L2)] were compared
with the optimised values calculated from density functional
theory (DFT) (Table 2). In general, a reasonable agreement is
obtained between the theoretical and experimentally observed
bond lengths, although some small differences are found. The
optimised Ir–C bond lengths are slightly different with Ir–C(1)
˚
˚
(1.996 A) shorter than Ir–C(2) (2.008 A) conflicting with the
experimental data. In the case of the Ir–O bonds, the calculated
˚
˚
value is 2.150 A, which is longer by 0.018 A than the value obtained
by X-ray crystallography; similarly the Ir–Nquinoxaline bond, where
˚
the calculated value is 0.078 A longer.
Density functional theory (DFT) studies
In an effort to elucidate the nature of the electronic transitions
within this class of complex, DFT calculations (computed using
the B3PW91 hybrid orbital) were undertaken. In these examples,
an assessment of the frontier orbitals provided a qualitative insight
into the HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital) energy levels.
Firstly, it is noteworthy that here, as in previous studies,12 the
HOMO of the related 8-hydroxyquinoline complex [Ir(ppy)2Q] is
located primarily on the metal and ancillary ligand (Fig. 4). The
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n
Table 3 Percentage distribution of HOMO and LUMO in [Ir(ppy)2L ] (n = 1–4)
[Ir(ppy)2L1]
HOMO
[Ir(ppy)2L2]
HOMO
[Ir(ppy)2L3]
HOMO
[Ir(ppy)2L4]
LUMO
LUMO
LUMO
HOMO
LUMO
Ir
16.0
78.1
2.2
1.9
94.7
1.8
1.7
16.3
77.6
2.2
1.9
95.2
1.5
1.3
15.3
78.4
2.1
1.8
94.2
2.1
1.9
15.2
78.9
2.1
1.8
94.0
2.2
2.0
Ln
ppy 1
ppy 2
3.7
3.8
4.2
3.8
Fig. 5 Graphical representation of the frontier orbitals of [Ir(ppy)2Ln] (n = 1–4 left to right). Bottom, HOMO; top, LUMO.
this calculated data suggests that variation of the remote para
substituent can lead to a degree of tunable optical properties within
this series of complexes. Interestingly, the calculated HOMO–
LUMO bandgaps do not vary a great deal (2.91–2.99 eV).
were not fully reversible (Table 4). Each complex also showed an
irreversible reduction wave close to the edge of the solvent window.
The first oxidation potential varies very little across the series of
complexes (0.87–0.90 V): the difference in oxidation potentials is
presumably due to the subtle donor variations of the substituted
quinoxalinato ligands. The EHOMO values were calculated using the
reported equations13 and the resultant values, ranging from -5.20
to -5.23 eV, are detailed in Table 4.
Electrochemical studies
An investigation into the electrochemical behaviour of the four
complexes was undertaken in de-aerated CH2Cl2. Electrochemical
studies were performed in order to approximate the HOMO
energy levels for each complex, since the ionisation potential
Spectroscopic properties of the ligands and complexes
The UV-vis absorption spectra of the ligands and complexes were
measured as aerated MeCN solutions. The absorption properties
of the ligands are dominated by a broad visible absorption
around 320–410 nm attributed to IL transitions, possibly including
phenol-to-pyrazine charge transfer. The electronic nature of the
para-substituted groups subtly influences the positioning of this
absorption band. Indeed, the relatively electron poor Ph and
for the first oxidation can be used to establish EHOMO assuming
0/1+
that the absolute level of the FeCp2
redox couple is 4.8 eV
below the vacuum level. The cyclic voltammograms, measured at a
platinum disc electrode (scan rate 200 mV s-1, 1 ¥ 10-3 M solutions,
0.1 M NBu4PF6 as a supporting electrolyte) of the complexes
[Ir(ppy)(Ln)] (n = 1–4) each showed one or two oxidations, which
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Table 4 Absorption, emission and electrochemical properties of the free ligands and corresponding iridium complexes
f,h
Ligand/Complex
lmax (e/M-1cm-1)/nma
Eox/Vb
HOMO/eVc
E
bandgap/eVd
LUMO/eVe
l
em/nmf
t (ns) aeratedf
Uem
LH1
289(19600), 350(6200)
294(16950), 349(5100)
292(16450), 355(7050)
297(20000), 364(9200)
302(18050)
302(16650)
294(14450)
—
—
—
—
0.87
0.89
0.88
0.90
—
—
—
—
-5.20
-5.22
-5.21
-5.23
—
—
—
—
1.83
1.83
1.83
1.83
—
—
—
—
-3.37
-3.39
-3.38
-3.40
459
473
474
438
462
436
456
503
5.5
7.8
3.0
5.3
5.0
6.7g
4.4
4.9
—
—
—
—
0.010
0.008
0.013
0.019
LH2
LH3
LH4
[Ir(ppy)2(L1)]
[Ir(ppy)2(L2)]
[Ir(ppy)2(L3)]
[Ir(ppy)2(L4)]
286(14450)
a Absorption spectra measured as MeCN solutions (6.4 ¥ 10-5 mol dm-3). b Oxidation potentials measured as CH2Cl2 solutions at 200 mVs-1 with 0.1 M
[NBu4][PF6] as supporting electrolyte calibrated with Fc/Fc+. c The HOMO energy level was calculated using the equation HOMO (eV) = Eox - EFc/Fc+
+ 4.8. d Ebandgap was determined from the absorption edge of the iridium complexes. e The LUMO energy level was calculated using the equation LUMO
(eV) = HOMO + Ebandgap. f Excitation at 372 nm. g Excitation at 295 nm. h Estimated errors of 15%.16
para-Br substituted ligands, LH1 and LH2, respectively, possess
similar absorption maxima (~350 nm), whereas the electron-
donating para-OMe substituted ligand, LH4, showed a significant
bathochromic shift (364 nm) together with an increase in molar
absorption coefficient (Table 4). In addition, the bands at 289–
297 nm are assigned to purely intra-ligand p–p* transitions for
each example.
the related hydroxyquinoline chromophores, it is reasonable to
assume that this is dominated by an IL p–p* transition. The
subtleties associated with the emission wavelengths of the ligands
can, in part, be attributed to the electronic influence of substitution
in the para position of the aryl units. For example, the methoxy
derivative, LH4, possesses the highest energy emission wavelength.
1
The complexes all absorb in the visible region (Fig. 6, Table 4):
the features are in the range 250–600 nm and are typically
broad in appearance. The complexes each exhibit a relatively
weak absorption band that tails into the visible region, assigned
to spin allowed (singlet) and possibly spin forbidden (triplet)
metal-to-ligand charge transfer (MLCT) transitions, together
with superimposed perturbed ligand-centred bands. Significantly
stronger absorption bands resulting from ligand-centred p–p*
transitions are observed in the higher energy region around
300 nm.
Fig. 7 Excitation (dashed) and emission (solid) spectra of [Ir(ppy)2(L1)]
(blue), [Ir(ppy)2(L2)] (green), [Ir(ppy)2(L3)] (orange) and [Ir(ppy)2(L4)]
(pink). lex = 372 nm.
The photophysical data for the complexes is also presented
in Table 4 and shows that the nature of the emission from
the complexes is probably dominated by ligand(quinoxalinato)-
centred character (see Fig.7). When compared to the free ligands,
the emission wavelengths and lifetimes of the complexes suggest
a metal-perturbed, ligand-dominated emissive state. The low
quantum yields are also consistent with such an assignment since
in aerated solvent the iridium centre would be expected to assist
non-radiative deactivation of the ligand-centred excited states.
This contrasts with related reports on quinolinato complexes of
iridium: such complexes are not uniformly emissive (attributed
to unfavourable mixing of singlet and triplet excited states), but
those that are show some weak phosphorescence in degassed
Fig.
6
UV-vis spectra of [Ir(ppy)2L1] (blue), [Ir(ppy)2L2] (green),
[Ir(ppy)2L3] (orange) and [Ir(ppy)2L4] (pink) as MeCN solutions (6.4 ¥
10-5 M).
Luminescence measurements were conducted on MeCN solu-
tions using a 372 nm excitation (Table 4). Firstly, the free ligands
display a short-lived (t = < 10 ns) fluorescence, which appeared
as a broad band in the visible region (ca. 440–480 nm). As with
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solvent.14 Related quinolinato complexes of Al(III) have shown
that there is also significant CT character that originates from
intraligand phenolate-to-pyridine transitions.8b Our DFT studies
suggest that a similar electronic character is inherent within the
Ir complexes described here. The calculations propose that the
HOMO in each case comprises significant phenolate character,
together with some Ir-centred orbital contributions; in each case
the LUMO has significant pyrazine-localised electron density.
Experimentally, a clear trend is evident in the complexes
whereby an electron-withdrawing group (L2) blue-shifts the emis-
sion peak, but an electron donating group red-shifts (L4). The
wavelength variations within this series, demonstrates an ability to
tune the emission character of the complexes as a function of the
remote para-substituent of the ancillary ligand.
out using a Parstat 2273 potentiostat in conjunction with a three-
electrode cell. The auxiliary electrode was a platinum wire and
the working electrode a platinum (1.0 mm diameter) disc. The
reference was a silver wire separated from the test solution by a
fine porosity frit and an agar bridge saturated with KCl. Solutions
(10 ml CH2Cl2) were 1.0 ¥ 10-3 mol dm-3 in the test compound
and 0.1 mol dm-3 in [NBun ][PF6] as the supporting electrolyte.
4
Under these conditions, E0¢ for the one-electron oxidation of
[Fe(h-C5H5)2], added to the test solutions as an internal calibrant,
is 0.47 V. Unless specified, all electrochemical values are at n = 200
mVs-1.
Data collection and processing
Diffraction data for LH3 and [Ir(ppy)2(L2)] were collected on
a Nonius KappaCCD using graphite-monochromated Mo-Ka
Conclusions
˚
radiation (l = 0.71073 A) at 150 K. Software package Apex 2
(v2.1) was used for the data integration, scaling and absorption
correction.
This paper describes the high-yielding, two-step synthesis of a
range of new 5-hydroxyquinoxaline ligands, conveniently utilising
a commercially available 2-amino-3-nitrophenol starting material.
Variation of the para-phenyl substituents provides the means for
moderately tuning the electronic characteristics of the ligands.
The resultant coordination chemistry yields the first examples
of cyclometallated Ir(III) complexes containing functionalised
quinoxalinato-based ancillary ligands.
Supporting DFT calculations suggest that both the HOMO
and LUMOs are almost completely localised on the coordinated
quinoxalinato ligands. Spectroscopically, this facilitates a degree
of tuning in the electronic properties of the complexes, whilst the
photophysical properties of the complexes appear to be dominated
by ligand-centred character at room temperature under ambient
conditions.
Structure analysis and refinement
The structure was solved by direct methods using SHELXS-97
and was completed by iterative cycles of DF-syntheses and full-
matrix least squares refinement. All non-H atoms were refined
anisotropically and difference Fourier syntheses were employed in
positioning idealised hydrogen atoms and were allowed to ride on
their parent C-atoms. All refinements were against F2 and used
SHELX-97.17
DFT studies
All calculations were performed on the Gaussian 03 program.18
Geometry optimisations were carried out without constraints
using the B3PW91 functional. The LANL2DZ basis set was used
for the Ir centres, and was invoked with pseudo-potentials for the
core electrons, a 6-31G(d,p) basis set for all coordinating atoms
with a 6-31G basis set for all remaining atoms. All optimisations
were followed by frequency calculations to ascertain the nature of
the stationary point (minimum or saddle point).
Experimental
All reactions were performed with the use of vacuum line and
Schlenk techniques. Reagents were commercial grade and were
used without further purification. [(ppy)2Ir(m-Cl)2Ir(ppy)2] was
prepared according to the literature procedure.15 1H and 13C-
1
{ H} NMR spectra were run on NMR-FT Bruker 400 or 250
1
1
spectrometers and recorded in CDCl3. H and 13C-{ H} NMR
chemical shifts (d) were determined relative to internal TMS and
are given in ppm. Low-resolution mass spectra were obtained
by the staff at Cardiff University. High-resolution mass spectra
were carried out at the EPSRC National Mass Spectrometry
Service at Swansea University. UV-Vis studies were performed
on a Jasco V-570 spectrophotometer as MeCN solutions (6.4 ¥
10-5 M-1). Photophysical data were obtained on a JobinYvon–
Horiba Fluorolog spectrometer fitted with a JY TBX picosecond
photodetection module as MeCN solutions. Emission spectra were
uncorrected and excitation spectra were instrument corrected.
The pulsed source was a Nano-LED configured for 372 nm
output operating at 500 kHz. Luminescence lifetime profiles were
obtained using the JobinYvon–Horiba FluoroHub single photon
counting module and the data fits yielded the lifetime values
using the provided DAS6 deconvolution software. Quantum yield
measurements were obtained on aerated acetonitrile solutions of
the complexes, using [Ru(bpy)3](PF6)2 in aerated acetonitrile as
a standard (Uem = 0.016).16 Electrochemical studies were carried
Syntheses of the ligands
Ligand LH1. Zn dust (~1 g) was heated in conc. HCl (15 ml)
for 10 min and then added to a solution of 2-amino-3-nitrophenol
(0.315 g, 2.04 mmol) in EtOH : HCl (1 M) (1 : 1, 30 ml). The
mixture was heated at reflux for 14 h during which time the
solution turned from red to pale yellow. The solvents were then
removed in vacuo and the crude mixture dissolved in EtOH (10
ml). After filtering, benzil (0.340 g, 1.62 mmol) was added and the
mixture heated at reflux for 16 h. The solvent was removed in vacuo
and the black product dissolved in CHCl3 (35 ml). The organic
solution was washed with water (2 ¥ 40 ml) and brine (40 ml) and
then heated with activated charcoal and MgSO4 for 30 min. The
solution was filtered and the solvent removed in vacuo to give a
1
yellow crystalline powder. Yield = 0.397 g (1.33 mmol) 82%. H
NMR (400 MHz, CDCl3) dH = 7.82 (1H, br s), 7.58–7.64 (2H,
m), 7.41–7.43 (4H, m), 7.24–7.31 (6H, m), 7.16 (1H, dd, JHH
=
=
6.9 and 1.8 Hz) ppm. 13C-{ H} NMR (101 MHz, CDCl3) dC
1
154.7, 152.4, 151.5, 141.9, 139.3, 138.9, 131.6, 131.5, 130.3, 130.2,
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Dalton Trans., 2011, 40, 9474–9481 | 9479
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129.4, 129.3, 128.7, 128.6, 120.0, 111.3 ppm. UV-vis (MeCN): lmax
(e dm3 mol-1 cm-1) = 289 (19600), 350 (6200) nm. AP MS found
m/z 299.1, calculated m/z 299.3 for [M+H]+.
128.0, 127.3, 127.0, 126.5, 126.1, 126.0, 122.8, 122.7, 120.8, 120.5,
120.1, 118.1, 118.0, 117.4, 117.1, 111.0 ppm. UV-vis (MeCN): lmax
(e dm3 mol-1 cm-1) = 302 (18 050) nm. ES MS found m/z 799.2,
calculated m/z 798.9 for [M+H]+. HR MS found m/z 797.2024,
calculated m/z 797.2020 for [C42H30O1N4191Ir1]+.
Ligand LH2. Prepared similarly from 2-amino-3-nitrophenol
(0.300 g, 1.95 mmol) and 4,4¢-dibromobenzil (0.422 g, 1.15 mmol).
The product was further purified by column chromatography
(silica, CH2Cl2) and eluted as the first fraction with CH2Cl2. The
solvent was removed in vacuo to give the product as a yellow
crystalline solid. Yield = 0.482 g (1.05 mmol) 93%. 1H NMR
(400 MHz, CDCl3) dH = 7.62 (2H, m), 7.44 (4H, d, JHH = 8.4
[Ir(ppy)2(L2)]. Prepared
similarly
from
[(ppy)2Ir(m-
Cl)2Ir(ppy)2] (0.045 g, 0.042 mmol), LH2 (0.038 g, 0.083
mmol) and Na2CO3 (0.010 g, 0.094 mmol). Yield = 0.073 g (0.076
mmol) 91%. 1H NMR (400 MHz, CDCl3) dH = 8.61 (1H, d,
JHH = 5.6 Hz), 7.54–7.77 (6H, m), 7.36 (1H, d, JHH = 7.7 Hz),
7.17–7.20 (4H, m), 7.10 (1H, d, JHH = 8.1 Hz), 6.95–7.02 (2H, m),
6.89 (2H, 2 ¥ d, JHH = 8.2 Hz), 6.81 (1H app. t, JHH = 5.9 Hz),
6.32–6.71 (6H, m), 6.25 (1H, app. t, JHH = 7.6 Hz), 5.80 (1H, d,
1
Hz), 7.30 (4H, m), 7.17 (1H, d, JHH = 6.4 Hz) ppm. 13C-{ H}
NMR (101 MHz, CDCl3) dC = 153.2, 152.3, 149.9, 142.0 137.9,
137.5, 132.9, 132.2, 132.1, 131.8, 131.7, 131.5, 124.3, 124.2, 120.0,
111.8 ppm. UV-vis (MeCN): lmax (e dm3 mol-1 cm-1) = 294 (16950),
349 (5100) nm. ES MS found m/z 456.9, calculated m/z 457.1 for
[M+H]+.
1
JHH = 7.6 Hz), 5.49 (1H, d, JHH = 7.6 Hz) ppm. 13C-{ H} NMR
(101 MHz, CDCl3) dC = 171.1, 169.2, 168.7, 154.4, 151.5, 150.7,
150.0, 149.4, 148.9, 146.4, 145.8, 143.5, 143.2, 137.9, 137.4, 137.3,
136.6, 134.6, 134.3, 132.7, 131.5, 131.4, 131.2, 130.9, 130.3, 130.2,
124.5, 124.4, 123.8, 123.4, 122.3, 122.1, 121.7, 119.7, 119.6, 119.3,
118.7, 112.5 ppm. UV-vis (MeCN): lmax (e dm3 mol-1 cm-1) = 302
(16 650) nm. ES MS found m/z 957.0, calculated m/z 956.7 for
[M+H]+. HR MS found m/z 953.0235, calculated m/z 953.0230
for [C42H28O1N4Br2191Ir1]+.
Ligand LH3. Prepared similarly to LH1 from 2-amino-3-
nitrophenol (0.540 g, 3.50 mmol) and 4,4¢-dimethylbenzil (0.679 g,
2.85 mmol) to give an orange powder. Yield = 0.930 g (2.85 mmol)
1
100%. H NMR (400 MHz, CDCl3) dH = 7.65 (1H, d, JHH = 8.4
Hz), 7.58 (1H, br), 7.35 (4H, d, JHH = 8.0 Hz), 7.07 (5H, m), 2.31
1
(3H, s), 2.28 (3H, s) ppm. 13C-{ H} NMR (101 MHz, CDCl3) dC =
154.3, 152.0, 151.2, 141.4 139.1, 139.0, 136.0, 135.7, 131.0, 130.9,
129.8, 129.7, 129.1, 129.0, 119.4, 110.8, 21.4, 21.3 ppm. UV-vis
(MeCN): lmax (e dm3 mol-1 cm-1) = 292 (16450), 355 (7050) nm. EI
MS found m/z 326.1, calculated m/z 326.4 for [M]+.
[Ir(ppy)2(L3)]. Prepared
similarly
from
[(ppy)2Ir(m-
Cl)2Ir(ppy)2] (0.048 g, 0.045 mmol), LH3 (0.031 g, 0.094
mmol) and Na2CO3 (0.010 g, 0.094 mmol). Yield = 0.065 g (0.079
mmol) 93%. 1H NMR (400 MHz, CDCl3) dH = 8.62 (1H, d, JHH
=
Ligand LH4. Prepared similarly from 2-amino-3-nitrophenol
5.6 Hz), 7.84 (1H, d, JHH = 5.7 Hz), 7.47–7.68 (6H, m), 7.33
(1H, d, JHH = 7.6 Hz), 7.11 (2H, 2 ¥ d, JHH = 8.0 Hz), 6.90–6.99
(5H, m), 6.75–6.83 (3H, m), 6.67 (1H, app. t, JHH = 7.5 Hz), 6.56
(1H app. t, JHH = 7.5 Hz), 6.47 (1H, app. t, JHH = 7.6 Hz), 6.24
(0.300 g, 1.95 mmol) and anisil (0.300 g, 1.41 mmol) to give a
1
yellow crystalline solid. Yield = 0.488 g (1.36 mmol) 96%. H
NMR (400 MHz, CDCl3) dH = 7.80 (1H, br), 7.60 (2H, app. quin
{coincident dd and d}, JHH = 8.2 Hz), 7.42 (4H, m), 7.13 (1H, d,
JHH = 8.0 Hz), 6.81 (4H, d, JHH = 8.2 Hz), 3.78 (3H, s), 3.77 (3H,
(2H, br), 6.11 (1H, app. t, JHH = 7.6 Hz), 5.83 (1H, d, JHH
=
7.5 Hz), 5.45 (1H, d, JHH = 7.6 Hz), 2.14 (3H, s), 1.96 (3H, s)
1
ppm. 13C-{ H} NMR (101 MHz, CDCl3) d = 169.5, 168.0, 167.2,
1
s) ppm. 13C-{ H} NMR (101 MHz, CDCl3) dC = 160.8, 160.7,
159.3, 154.4, 152.0, 149.7, 147.9, 147.7, 147.6, 145.3, 144.3, 142.1,
141.9, 136.9, 136.5, 135,5, 135.1, 134.9, 132.0, 131.9, 131.3, 129.8,
128.6, 128.3, 127.9, 127.8, 127.3, 127.0, 122.7, 122.6, 120.8, 120.4,
120.0, 118.0, 117.3, 117.1, 111.0, 20.3, 20.1 ppm. UV-vis (MeCN):
lmax (e dm3 mol-1 cm-1) = 294 (14 450) nm. ES MS found m/z
827.3 and 890.3, calculated m/z 827.0 and 890.0 for [M+H]+
and [M+MeCN+Na]+ respectively. HR MS found m/z 825.2329,
calculated m/z 825.2333 for [C44H34O1N4191Ir1]+.
154.6, 152.2, 142.8, 142.0, 131.9, 131.7, 131.6, 131.5, 131.3, 131.1,
119.8, 114.3, 114.1, 110.9, 55.8, 55.7 ppm. UV-vis (MeCN): lmax
(e dm3 mol-1 cm-1) = 297 (20 000), 364 (9200) nm. ES MS found
m/z 359.1, calculated m/z 359.4 for [M+H]+.
Syntheses of the complexes
[Ir(ppy)2(L1)]. [(ppy)2Ir(m-Cl)2Ir(ppy)2] (0.040 g, 0.037 mmol),
LH1 (0.023 g, 0.077 mmol) and Na2CO3 (0.010 g, 0.094 mmol)
were heated at 120 ◦C in 2-methoxyethanol (10 ml) for 16 h. The
solvent was removed in vacuo and the crude product dissolved in
CH2Cl2 (25 ml). The organic solution was washed with water (2
¥ 25 ml) and brine (25 ml), dried over MgSO4 and filtered. The
solvent was lowered in volume (ca. 2 ml) in vacuo and the product
precipitated by the slow addition of Et2O (20 ml) to give a red
solid, which was dried in vacuo. Yield = 0.048 g (0.061 mmol) 88%.
1H NMR (400 MHz, CDCl3) dH = 8.64 (1H, d, JHH = 5.6 Hz), 7.85
(1H, d, JHH = 5.1 Hz), 7.50–7.69 (6H, m), 7.34 (1H, d, JHH = 7.7
Hz), 6.94–7.14 (9H, m), 6.81 (1H, app. t, JHH = 5.9 Hz){app. =
apparent (coincident dd)}, 6.64–6.69 (2H, m), 6.56 (1H, app. t,
JHH = 7.6 Hz), 6.39 (1H app. t, JHH = 7.1 Hz), 6.10 (1H, app. t,
JHH = 7.3 Hz), 5.82 (1H, d, JHH = 7.1 Hz), 5.49 (1H, d, JHH = 7.6
[Ir(ppy)2(L4)]. Prepared
similarly
from
[(ppy)2Ir(m-
Cl)2Ir(ppy)2] (0.045 g, 0.042 mmol), LH4 (0.032 g, 0.089
mmol) and Na2CO3 (0.010 g, 0.094 mmol). Yield = 0.069 g (0.080
mmol) 96%. 1H NMR (400 MHz, CDCl3) dH = 8.63 (1H, d, JHH
=
5.1 Hz), 7.82 (1H, d, JHH = 5.1 Hz), 7.48–7.67 (5H, m), 7.32 (1H,
d, JHH = 6.9 Hz), 7.15 (1H, d, JHH = 7.8 Hz), 7.10 (1H, d, JHH
=
8.0 Hz), 6.93–6.99 (4H, m), 6.77 (1H, app. t, JHH = 7.2 Hz), 6.66
(1H, app. t, JHH = 7.5 Hz), 6.53–6.58 (3H m), 6.44 (1H, app. t,
JHH = 8.0 Hz), 6.17 (1H, app. t, JHH = 7.6 Hz), 6.02 (2H, br), 5.82
(1H, d, JHH = 7.6 Hz), 5.53 (1H, d, JHH = 6.9 Hz), 3.62 (3H, s),
3.56 (3H, s) ppm. 13C-{ H} NMR (101 MHz, CDCl3) dC = 168.9,
1
167.3, 166.5, 157.7, 156.9, 153.4, 151.1, 149.5, 147.9, 147.3, 146.9,
144.8, 143.6, 141.5, 135.0, 134.3, 131.5, 130.8, 129.8, 129.3, 129.2,
128.6, 128.0, 127.2, 126.9, 122.1, 121.9, 120.2, 119.8, 119.5, 117.4,
117.2, 116.4, 116.3, 111.4, 111.1, 110.3, 53.5, 53.0 ppm. UV-vis
(MeCN): lmax (e dm3 mol-1 cm-1) = 286 (14 450) nm. ES MS found
1
Hz) ppm. 13C-{ H} NMR (101 MHz, CDCl3) dC = 169.5, 168.3,
167.2, 154.6, 151.9, 150.3, 147.9, 147.7, 145.4, 144.2, 142.1, 141.9,
137.9, 135.7, 135.0, 134.1, 132.3, 131.2, 129.9, 128.7, 128.3, 128.2,
9480 | Dalton Trans., 2011, 40, 9474–9481
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©

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m/z 859.2, calculated m/z 859.0 for [M+H]+. HR MS found m/z
857.2234, calculated m/z 857.2231 for [C44H34O3N4191Ir1]+.
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Acknowledgements
The EPSRC are thanked for funding. Dr Robert Jenkins, Mr Robin
Hicks and Mr David Walker are thanked for running low resolu-
tion mass spectra. The staff of the EPSRC MS National Service
at the University of Swansea are also gratefully acknowledged.
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This journal is
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Dalton Trans., 2011, 40, 9474–9481 | 9481
©