Carboxy derivatised Ir(iii) complexes: synthesis, electrochemistry, photophysical properties and photocatalytic hydrogen generation

Article abstract of DOI:10.1039/c5dt00324e

In this contribution the synthesis and characterisation of a series of novel mixed ligand iridium(iii) complexes, functionalised with a carboxy ester or phosphonate groups are reported. These groupings are introduced on the 4-position of either the phenyl pyridine or the 2,2′-bipyridyl ligands. A low temperature high yield synthesis for the precursor [Ir(ppy-COOEt)₂(μ-Cl)]₂ was developed. The photophysical and electrochemical properties of these compounds are also described, together with their behaviour as photosensitisers for the generation of hydrogen from water.

Full text of DOI:10.1039/c5dt00324e

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Carboxy derivatised Ir(III) complexes: synthesis,
electrochemistry, photophysical properties and
photocatalytic hydrogen generation†
Cite this: Dalton Trans., 2015, 44,
10423
Avishek Paul, Nivedita Das, Yvonne Halpin, Johannes G. Vos* and Mary T. Pryce*
In this contribution the synthesis and characterisation of a series of novel mixed ligand iridium(III) com-
plexes, functionalised with a carboxy ester or phosphonate groups are reported. These groupings are
introduced on the 4-position of either the phenyl pyridine or the 2,2’-bipyridyl ligands. A low temperature
high yield synthesis for the precursor [Ir(ppy-COOEt)2(μ-Cl)]2 was developed. The photophysical and
electrochemical properties of these compounds are also described, together with their behaviour as
photosensitisers for the generation of hydrogen from water.
Received 23rd January 2015,
Accepted 29th April 2015
DOI: 10.1039/c5dt00324e
www.rsc.org/dalton
their stability, as is shown by Ru-based dyes used in the Grätzel
cell.6 Recently, we reported novel preparation methods for NiO
Introduction
Recently there has been considerable interest in the excited surfaces which would be highly suitable for this propose.7 The
state properties of Ir(III) complexes based on cyclometallated ultimate photocatalytic device is likely to be based on hetero-
ligands of the type [Ir(ppy)2(L–L)]n+, where ppy is 4(2-pyridyl) geneous surface bound photocatalytic systems where semi-
benzene and L–L a chelating ligand. This interest was instigated conductor electrodes such as NiOx act as an electron donor and
by the work of Thompson1 and others2 on the application of this would eliminate the need for sacrificial electron donors. At
cyclometallated iridium(III) compounds in the development of present carboxy and phosphonate moieties are considered the
OLEDs and more recently have been investigated as dyes in most suitable linkers and these have been used for the immo-
solar cells.3 While photovoltaic cells generate electricity from bilisation of iridium dyes in sensitised solar cells.8
sunlight, this energy cannot easily be stored and as a result the
In this contribution we report the synthesis and characteris-
development of alternative systems capable of storing solar ation of a new series of iridium mononuclear complexes
energy as a fuel or as stock chemicals using molecular photo- functionalised with carboxy ester and phosphonate groups for
catalysts is under consideration. Promising options include the future immobilisation on p-type semiconductor surfaces such
development of catalytic systems for the production of hydrogen as NiOx. (see Fig. 1). The photophysical and electrochemical
from water or CO2 reduction using visible light.4 Recently,
cyclometallated iridium(III) complexes have been studied as
photosensitisers for CO2 reduction, and analogues such as
[Ir(ppy)2(bpy)]+, have shown excellent catalytic efficiency in solar
driven hydrogen production in solution.5 Importantly their
photophysical properties can be fine-tuned by ligand modifi-
cation and as a result a wide range of Ir(III) based complexes
have been prepared for various applications.
However, homogeneous photocatalytic systems contain sacri-
ficial agents such as TEA, or TEOA which can damage the cata-
lytic activity of the photocatalyst. Immobilisation of the
photocatalysts on NiO surfaces would furthermore increase
School of Chemical Sciences, Dublin City University, Dublin 9, Ireland.
E-mail: mary.pryce@dcu.ie, han.vos@dcu.ie; Fax: (+353) 17005503;
Tel: (+353) 17008005
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
Fig. 1 Molecular structures of the synthesised Ir(III) complexes and
c5dt00324e
photocatalytic hydrogen generation.
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properties of the compounds were investigated and photo-
catalytic studies on the hydrogen generation capacity of the
compounds were carried out. Turnover numbers (TON defined
as number of hydrogen molecules formed per molecule of
catalyst) of up to 162 for photocatalytic hydrogen generation
were obtained. The results obtained indicate that the excited
state properties of the cyclometallated Ir(III) complexes are
maintained with variation in the R group on the bipyridyl
ligand, and in addition the compounds show well defined
reversible electrochemistry.
Scheme 2 Various reaction conditions for the synthesis of [Ir(ppy-
COOEt)2(µ-Cl)]2, (1).
In our case, the ppy ligand was functionalised with thermally
unstable carboxy ester or phosphonate groups, thus limiting
the possibility of high temperature reactions. When a 3 : 1
ethoxy ethanol/water solvent system was used (see Scheme 2)
for the preparation of carboxy ester containing iridium com-
plexes, relatively low yields were obtained (70–6%) and the
crude reaction mixture contained a mixture of products.
As a result more benign reaction conditions were con-
sidered and the solvent system was altered from ethoxy
ethanol/water to ethanol/water. The reaction mixture was
heated at reflux temperature for 24 hours in 3 : 1 ethanol/water
(Scheme 2). This reaction provided a better yield of about 70%
compared to the ethoxyethanol literature procedure which gave
a yield of 20%. Surprisingly, the product was soluble in a 3 : 1
ethanol/water mixture but not in water. Therefore, the product
was precipitated from the reaction mixture with the addition
of excess water. Further optimisation of the yield was obtained
by changing from ethanol/water to ethanol. Carrying out the
reaction without water provided a 93% yield. The use of an
inert (N2) atmosphere did not increase the yield. In summary,
absolute ethanol was found to be the best solvent for synthe-
sising [Ir(ppy-COOEt)2(µ-Cl)]2, (1) with excellent yields and
without the need for inert conditions.
Results and discussion
Synthesis
Synthetic procedure for ethyl 4-(2-pyridyl)benzoate (ppy-
COOEt). The 4 position of the phenyl ring in phenylpyridine
was functionalised with an ethyl carboxy ester group by using
4-(2-pyridyl)benzaldehyde as the starting material. There are
many reported procedures for the synthesis of carboxy ester
containing phenylpyridines9,10 such as the Negishi11,12 and
other cross-coupling reactions.13 In the approach reported in
this manuscript, esterification was carried out using a two-step
procedure, which is inexpensive and less time consuming than
previous reports. The first step involves oxidation of the alde-
hyde group and the second step is an acid catalysed esterifica-
tion (see Scheme 1). The oxidation of the aldehyde group can
be carried out using various literature procedures.14 Here the
oxidation was carried out using silver oxide and aqueous
NaOH solution, providing the products with yields of 90%.
The reaction was performed at room temperature and was
complete within 30 minutes. 10 cm3 of THF was added to the
reaction mixture to fully dissolve the 4-(2-pyridyl)benz-
aldehyde. THF was removed from the filtrate by rotary evapor-
ation after filtration of the reaction mixture. The presence of
THF in the filtrate can cause the formation of a soluble acidi-
fied product which will not precipitate from the aqueous solu-
tion and reduce the yield of 4-(2-pyridyl)benzoic acid. 4-(2-
Pyridyl)benzoic acid was esterified using a well-studied acid
catalysed reaction.9
NMR spectroscopy
1H NMR and 2D NMR spectra of the ligands and the Ir(III)
complexes are provided in the ESI (see ESI Fig. S1 to S14†).
Interestingly, there are considerable differences in the chemical
shifts of the ester groups present in [Ir(ppy-COOEt)2(dpb)](PF6)
(1c) and [Ir(ppy-COOEt)2(dceb)](PF6) (1e) as shown in Fig. 2.
The chemical shifts of the protons in ppy-COOEt ligands do
not vary significantly with the addition of a bpy-based ligand.
(See also ESI, Fig. S9, S11 and S13†).
Synthesis of [Ir(ppy-COOEt)2(μ-Cl)]2, (1). Several procedures
have been reported to synthesise Ir(III) compounds, containing
ppy or functionalised ppy ligands.15 Many of these synthetic
procedures require high boiling point solvents such as ethoxy
ethanol, methoxy ethanol and glycerol with 20–50% of water.
NMR data also suggests identical chemical environments
for the two-ester groups on the two different ppy-COOEt
ligands. Based on this observation, two possible geometries,
(I) and (II) (Fig. 3) are possible but crystallographic data are
required to identify the correct geometry. NMR data rules out
the possibility of geometry (III).
Electrochemistry
Electrochemical data for all Ir(III) complexes are tabulated in
Table 1. A reversible oxidation wave, representing the Ir(III)/Ir(IV)
redox couple, is observed in the cyclic voltammogram of
Scheme 1 Reaction scheme for synthesis of ethyl 4-(2-pyridyl)
benzoate.
10424 | Dalton Trans., 2015, 44, 10423–10430
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Table 1 Redox properties of compounds (1a–1e), vs. SCE,. The electro-
lyte employed was 0.1 M TBAPF6 in acetonitrile. Redox data from the lit-
erature are included for comparison. * Not reported
E1/2, oxidation (V)
[ΔEp (mV)]
E1/2, reduction [V]
[ΔEp (mV)]
Complex
1
+1.17 [70]
+1.52 [70]
+1.54 [70]
−1.79 (Epc
−1.90 [80]
−2.08 [90]
−2.23 [50]
−2.44 [135]
−1.27 [70]
−1.72 [70]
−1.93 [75]
−2.22 [100]
−2.35 [120]
)
1a
1b
−1.15 (Epc
−1.27 [60]
−1.70 (Epc
−1.75 (Epc
−1.95 [75]
−2.17 (Epc
−1.29 [60]
−1.43 [60]
−1.80 [90]
)
Fig. 2 A comparison of the ester CH2 groups signals (q) in (A) [Ir(ppy-
COOEt)2(dpb)](PF6), 1c and (B) [Ir(ppy-COOEt)2(dceb)](PF6), 1e.
)
)
)
1c
+1.47 [70]
+1.47 [70]
+1.50 [70]
−2.00 [110]
−1.19 [70]
−1.71 [75]
−1.89 [70]
1d
−2.13 (Epc
−0.97 [80]
−1.50 [70]
−1.90 [80]
−2.14 (Epc
)
1e
Fig. 3 Three possible geometries of [Ir(C^N)2(N^N)]+ type complexes.
)
[(Ir(ppy)2Cl)2]
Ir(ppy)3
+1.01
+0.73 [*]
Not observed
−2.28 [*]
17
each complex. This is a one-electron reversible process as indi-
cated by a peak to peak separation (ΔEp) of between 60 and
100 mV and an anodic to cathodic current ratio close to unity.
The oxidation of the dinuclear species, 1 occurs at a potential
at least 300 mV less positive than the corresponding process in
1a through to 1e as a result of the replacement of the bridging
chlorine atoms with bipy type ligands.
−2.53 [*]
−1.42 [70]
[Ir(ppy)2(bpy)]+ 18
+1.25 [65]
Compared to the parent compound [(Ir(ppy)2Cl]2, the oxi-
dation potential of compound 1 is 160 mV more positive, illus-
trating the electron withdrawing capacity of the ester groups.
The bipyridine ligand’s strong π-acceptor character is evident
when comparing the oxidation potential of 1a with that of 1
with the latter being 350 mV less positive than the former. The
addition of carboxy ester groups on the ppy ligand clearly
alters the electrochemical properties of the complexes. Bern-
hard et al. reported an oxidation potential for [Ir(ppy)2(bpy)]+
of 1.25 V vs. SCE18 at least 220 mV less positive than the corres-
ponding process in compounds 1a to 1e. The data show that
as expected the carboxy ester substituents act as strong accep-
tor groups capable of lowering the energy gap between the
HOMO and LUMO of the complex by strongly stabilising the
LUMO and only slightly stabilising the HOMO.16
Fig. 4 Cyclic voltammetry of 1a ([Ir(ppy-COOEt)2(bpy)]+), using 0.1 M
TBAPF6 in acetonitrile as the supporting electrolyte. Scan rate: 100 mV s−1
.
Several reductive processes are observed in the cyclic vol- becomes quasi-reversible. The first (least negative) reductive
tammograms of each complex. Fig. 4 reveals the cyclic voltam- process in 1a to 1e is observed at considerably more positive
mogram of 1a where multiple quasi-reversible waves are potentials when compared to [Ir(ppy)2(bpy)]+ species for which
observed. The first (least negative) reduction of both 1 and 1b the first reduction is found at −1.42 V vs. SCE (Table 1). This is
is irreversible at slow scan rates (0.01 V s−1 to <10 V s−1). again associated with electron withdrawing capacity of the
However, at scan rates of 10 V s−1 and higher the feature ester moiety.
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Photophysical properties
Absorption spectra. The absorption data for all the iridium
complexes are summarised in Table 2, with spectra for repre-
sentative complexes displayed in Fig. 5. All spectra obtained in
this study are similar to that of other iridium complexes
reported in the literature. The absorption bands for the
iridium complexes in the UV region (λ < 300 nm) are attributed
to the ligand-based transition (π–π*) and the less intense
absorption bands in the region greater than 300 nm are
assigned to metal → ligand charge transfer (1MLCT and
3MLCT) transitions.16 In the case of 1e, the absorption band at
the lowest energy (λmax = 489 nm, is more pronounced than
any of the other complexes studied. This is at slightly higher
energy than of a similar compound, [Ir(ppy)2(dceb)]+ where a
λmax was reported at 530 nm.19
Fig. 5 Absorption spectra of Ir(III) complexes in acetonitrile. (Concen-
trations of all the complexes are ∼10−5 M).
Photophysical data. The lifetimes and emission quantum
yields obtained for complexes 1a–1e are summarised in
Table 2, with the emission spectra displayed in Fig. 6 All
complexes have similar emission profiles, with complex 1e,
which contains four carboxy units, substantially red shifted
by 80 nm (2363 cm−1) compared to 1a, which has two
carboxy units present. Thus the λmax can be tuned in these
iridium(III) complexes with functionalisation of the ligands.
Surprisingly, the dpb ancillary ligand (emission occurs at
554 nm) has the minimum electronic effect on the emission
wavelength compared to the bipy ligand (emission occurs at
540 nm). This difference has been attributed to the better
mesomeric and inductive ability of the carboxy ester groups
compared to the other functional groups. The lifetime data
obtained for complexes 1a–1e are all within the range
215–342 ns, and increase to between 320–457 ns when
de-aerated. These lifetimes and also the emission quantum
yields in this manuscript are somewhat higher to that pre-
viously reported in the literature for the parent complex
[Ir(ppy)2(bpy)](PF6) (338 ns).21 The excited state lifetimes do
not vary significantly with the location or number of carboxy
units on the cyclometallated ppy or the bipy based ancillary
ligands. The absence of strong vibrational structure in the
emission spectra suggests that the emission is mostly 3MLCT
in character.22
Table 2 Photophysical and photocatalytic data for hydrogen production for the Ir(III) complexesa
TONs (H2)
355 nm
λabs
(nm)
ε × 103
λem
(nm)
λem
(nm) 77 K
τa
(ns)
τb
(ns)
τ (ns)
77 K
(M−1 cm−1
)
ϕem
ϕem
0.06
0.21
470 nm
6320
a
b
Complex
[Ir(ppy)2(bpy)]+
1a
337
477
487
427
402
361
487
422
402
355
485
422
402
348
495
428
404
356
489
420
353
0.6
583
540
520
—
—
338
457
—
0.72
4.29
4.94
7.67
0.98
4.38
4.99
9.56
1.13
4.38
4.99
9.07
1.14
3.45
4.57
7.12
1.24
8.02
10.74
0.035
0.033
0.02
256
720
76
109
22
23
570
585
1b
1c
1d
1e
564
554
566
619
565
0.28
0.19
0.20
0.18
292
215
230
342
424
325
320
384
719
724
740
560
162
70
560
0.032
0.03
147
34
121
58
540
585
620
a All absorption spectra were obtained at room temperature. Emission spectra were obtained using 355 nm as the excitation wavelength with an
absorbance of ∼0.15, in nitrogen purged dichloromethane, lifetime measurements were performed at 355 nm in aerateda and deaeratedb CH2Cl2,
and the emission quantum yields in aerateda and deaeratedb CH3CN.
10426 | Dalton Trans., 2015, 44, 10423–10430
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do show that the introduction of ester or phosphonate groups
do not prevent the compounds from acting as a photosensiti-
ser. In agreement with prior studies reported by Bernhard
et al.,20 on similar complexes the water reduction cascade is
thought to involve reductive quenching of the excited iridium
photosensitiser by TEA, followed by electron transfer to the
platinum catalytic centre.
It is difficult to compare TONs reported here to that of
the parent compound [Ir(ppy)2(bpy)]+, as the reaction con-
ditions are very different. However, when our results are
compared with the iridium complex [Ir(ppy)2(bpy)]+, where a
TON = 63 was reported, both complexes 1b and 1d show
improved TONs.20
Fig. 6 Emission spectra of the Ir(III) complexes recorded in CH2Cl2. All
samples had an absorbance of ∼0.15 at the excitation wavelength
(355 nm).
Conclusions
This study introduces ligands containing the carboxy ester
group in heteroleptic monomeric iridium complexes. Modified
synthetic procedures for the ligand ppy-COOEt and the associ-
ated chloro-bridged iridium precursor complex were develo-
ped, with high yields. All the complexes show electrochemical
reversibility and they produced hydrogen photocatalytically at
the two wavelengths employed. The introduction of the ester
or phosphonate groups does not seem to fundamentally
change the photophysical behaviour of the compounds. At
this stage it is not possible to explain the differences in
the efficiencies for photocatalytic hydrogen generation until
the nature of the excited states involved are identified.
However, it is expected that the nature of the lowest energy
triplet excited state may be affected and this is at present
under investigation.
Photocatalytic hydrogen generation
The iridium complexes, 1a–e were assessed for their ability to
act as a photosensitiser for hydrogen production. The reac-
tions were carried out in mixtures containing acetonitrile/TEA/
water (8 : 1 : 1) and using K2[PtCl4] as the catalyst. Two
irradiation wavelengths were employed; 350 and 470 nm, and
the TONs obtained are given in Fig. 1 and Table 2, together
with plots for hydrogen production over a 24 hour period in
Fig. 7. Control vials with no catalyst added produced negligible
amounts of hydrogen. For the iridium complexes investigated,
TONs at λexc = 355 nm ranged from 22 to 147, while at λexc
=
470 nm TONs ranged from 23 to 162, where the amount of
hydrogen varied with the substituent, and also with the wave-
length employed, however no obvious trend was notable.
Although these values are not among the highest reported they
Experimental
Physical measurements
1H NMR (400 MHz) and 2D COSY were recorded in deuteriated
solvents (d3-acetonitrile, CDCl3) on a Bruker AC400 NMR and
AC600 NMR Spectrometer with TMS or residual solvent peaks
as reference. XWIN-NMR processor and ACDLABS 12.0 NMR
processor software were employed to process the free induction
decay (FID) profiles. The H–H 2-D COSY NMR involved the
accumulation of 128 FIDs of 16 scans. Elemental analyses
(CHN) were carried out using Exador Analytical CE440 by the
Microanalytical Department, University College Dublin,
Ireland. UV–Vis absorption spectra were recorded on
a
Shimadzu 3100 UV-Vis/NIR spectrophotometer with interfaced
to an Elonex PC575 desktop computer. Emission spectra were
recorded on a Perkin-Elmer LS0B luminescence spectro-
photometer, and the excitation wavelength was set at 350 nm.
All emission spectra were initially generated by Perkin-Elmer
FL Winlab custom built software. The optical densities of all
the sample solution for the emission spectra and lifetime
studies were approximately 0.1–0.2 A.U. at the excitation wave-
length 350 nm.
Fig. 7 Amount of hydrogen produced is expressed in TONs at various
time intervals from 0 to 24 h using 470 nm as irradiation source. [PS] =
[K2PtCl4] = 1.2 × 10−4 M.
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Electrochemical experiments were carried out using a CH cooled in an ice bath. 2 g (10.91 mmol) of 4-(2-pyridyl)benz-
Instruments Version 8.15 software controlled electrochemical aldehyde was added to the cooled reaction mixture with stir-
bipotentiostat (CHI750C) with typical analyte concentrations ring. The procedure was modified by adding 10 cm3 of THF to
of 1 mM. The working electrode was a glassy carbon (GC) the reaction mixture for complete dissolution of 4-(2-pyridyl)
macro electrode (3 mm in diameter) with a Pt wire as the benzaldehyde. After 30 minutes, no more reactant particle was
counter electrode. All electrochemical experiments were refer- left to dissolve and a silver mirror appeared on the side of the
enced against a Hg/HgSO4 reference electrode. All potential conical flask. The reaction was completed in 30 minutes as fol-
values were then calibrated against the saturated calomel elec- lowed by TLC. The black silver suspension was removed by
trode (SCE) by adding ferrocene as an internal reference (E1/2 suction filtration. The black residue was further washed with
(Fc/Fc+) = +0.43 V vs. SCE). The GC electrode was prepared by hot water to obtain maximum amount of product. The com-
manually polishing with 0.05 µm alumina slurry (CH Instru- bined filtrate was acidified with aqueous HCl solution to pre-
ments, Inc.) followed by sonicating in Millipore water and cipitate out the product from the aqueous solution. The
finally in acetonitrile. Tetrabutylammonium hexafluoro- product was then collected by filtration. Yield: 90% (1.95 g,
1
phosphate (TBAPF6, Fluka, electrochemical grade ≥99.0%) in 9.81 mmol). H NMR (400 MHz, DMSO-d6) δ ppm 7.43 (dd, J =
acetonitrile (Aldrich anhydrous, 99.8%) was used as electrolyte 7.52, 4.74 Hz, 1 H), 7.94 (dd, J = 7.77, 7.77 Hz, 1 H), 8.01–8.10
and solvent respectively. The electrochemical cell was degassed (m, 3 H), 8.17–8.27 (m, 2 H), 8.72 (dd, J = 6.57, 1.01 Hz, 1 H).
using argon for at least 15 minutes prior to each experiment
and a blanket of argon was maintained throughout.
Ethyl 4-(2-pyridyl)benzoate (ppy-COOEt). Esterification of
4-(2-pyridyl)benzoic acid was performed using a literature
procedure.28 1 g (5.02 mmol) of 4-(2-pyridyl)benzoic acid was
Photocatalytic hydrogen generation experiments
converted to ethyl 4-(2-pyridyl)benzoate. Yield: 93% (1.06 g,
1
Photocatalytic hydrogen production experiments were carried 4.66 mmol). H NMR (400 MHz, DMSO-d6) δ ppm 1.35 (t, J =
out using an air-cooling apparatus for maintaining the solu- 7.07 Hz, 3 H), 4.35 (q, J = 7.07 Hz, 2 H), 7.43 (dd, J = 7.45, 4.80,
tions at room temperature at 22 °C, during irradiation using 0.88 Hz, 1 H), 7.94 (td, J = 7.77, 1.89 Hz, 1 H), 8.04–8.10
LEDs (355 and 470 nm). Acetonitrile used was dried over (m, 3 H), 8.21–8.28 (m, 2 H), 8.72 (dd, J = 4.80, 0.76 Hz, 1 H).
calcium hydride and the triethylamine dried over sodium
[Ir(ppy-COOEt)(μ-Cl)]2·2H2O, 1. 79 mg (2.64 mmol) of
before being freshly distilled under nitrogen. The samples IrCl3·3H2O and 1.2 g (5.28 mmol) of ppy-COOEt were added to
were prepared in GC vials (4.9 ml volume, VWR) with a known 30 cm3 EtOH and refluxed for 24 hours. The reaction mixture
headspace of 2.9 ml. Photosensitiser (Iridium complexes) and turned from dark brown to orange. The reaction solution was
catalyst K2[PtCl4] were taken in 1 : 1 ratio and dissolved in 4 : 1 reduced to 2–5 cm3 by rotary evaporation and 50 cm3 water
Acetonitrile/H2O solution containing 1.2 × 10−4 M of PS. was added. A yellow precipitate was collected by filtration. The
1.9 ml of the Ir/Pt solution and 90 µL of triethylamine were product was washed with water and diethyl ether, and further
added to the GC vials in 2 ml sample under argon atmosphere. recrystallised from DCM/ethanol solution. Yield: 1.67
g
Subsequently, the GC vials were irradiated with 355(22 × 10−3 (1.24 mmol, 93%). H NMR (400 MHz, DMSO-d6) δ ppm 1.16
W m−2) and 470 nm (0.4 W m−2) wavelength LEDs for (td, J = 7.14, 1.64 Hz, 12 H), 4.03–4.15 (m, 8 H), 6.28 (s, 2 H),
18 hours. After irradiation, 100 μL samples were drawn from 6.85 (s, 2 H), 7.42 (dd, J = 8.08, 1.77 Hz, 2 H), 7.47 (dd, J = 8.08,
the headspace and injected into the GC. The experiments were 1.77 Hz, 2 H), 7.62 (dd, J = 7.33, 7.33 Hz, 2 H), 7.72 (t, J =
1
repeated two times.
7.20 Hz, 2 H), 7.89 (d, J = 8.34 Hz, 2 H), 7.94 (d, J = 8.08 Hz,
2 H), 8.13 (t, J = 8.46 Hz, 2 H), 8.22 (td, J = 7.77, 1.39 Hz, 2 H),
8.33 (d, J = 7.83 Hz, 2 H), 8.42 (d, J = 8.34 Hz, 2 H), 9.57 (d, J =
Synthesis
All the solvents used for spectroscopy were of spectroscopic 4.55 Hz, 2 H), 9.87 (d, J = 6.06 Hz, 2 H). Elemental analysis of
grade. IrCl3·3H2O and 4-(2-pyridyl) benzaldehyde used for the C56H48Cl2Ir2N4O8·2H2O; M.W. = 1400.40; Calc: C 48.17, H 3.75,
reactions below were obtained from Sigma-Aldrich and used N 4.01. Found: C 48.33, H 3.61 and N 3.78%.
without further purification. All other solvents and reagents
[Ir(ppy-COOEt)2(bpy)](PF6), 1a. [Ir(ppy-COOEt)2(bpy)](PF6).
used were reagent grade. 4,4′-Dicarboxyethyl-2,2′-bipyridine 100 mg (0.07 mmol) of [Ir(ppy-COOEt)(μ-Cl)]2 and 25.23 mg
(dceb) was synthesised according to literature procedure.23 (0.16 mmol) of bpy were refluxed in 30 cm3 4 : 1 ethanol/
Synthetic procedure details for 4,4′-bis(diethylmethylphos- dichloromethane mixture for 6 hours in the dark. The solvent
phonato)-2,2′-bipyridine (dpb),24 2,2′:5′,2″-terpyridine (bpp)25 was completely removed by rotary evaporation. 30 cm3 of water
and 5-bromo-2,2′-bipyridine (5Brbpy)26 can be found in ESI.†
was added and sonicated for 10 minutes. The aqueous solu-
4-(2-Pyridyl)benzoic acid (ppy-COOH). 4-(2-Pyridyl)benzoic tion was filtered twice. A saturated aqueous solution of KPF6
acid (ppy-COOH) was synthesised following a modified litera- was added to the filtrate and stirred for 30 minutes. The
ture procedure.27 2.94 g (73.5 mmol) of NaOH was dissolved in aqueous layer was extracted with DCM. The product was
50 cm3 of water and the brown suspension of silver oxide was further washed with diethyl ether. The crude product was
prepared by adding a solution of 6.32 g (37.22 mmol) of further purified by recrystallisation from DCM/ethanol solu-
AgNO3 solution in 50 cm3 of water. The solution was stirred tion. Yield: 53% (66.34 mg, 0.07 mmol). 1H NMR (400 MHz,
continuously during the addition of silver nitrate solution to Acetonitrile-d3) δ ppm 1.23 (t, J = 7.20 Hz, 6 H), 4.18 (q, J =
complete the reaction. The brown semisolid mixture was then 7.07 Hz, 4 H), 6.87 (d, J = 1.26 Hz, 2 H), 7.18 (dd, J = 7.45, 5.94,
10428 | Dalton Trans., 2015, 44, 10423–10430
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1.52 Hz, 2 H), 7.52 (dd, J = 7.71, 5.43, 1.01 Hz, 2 H), 7.68 (dd, Elemental analysis for C44H40F6IrN4O8P, M.W.: 1092. Calc:
J = 8.21, 1.64 Hz, 2 H), 7.72 (dd, J = 5.81, 0.76 Hz, 2 H), 7.93 (d, C 48.48, H 3.70, N 5.14. Found: C 48.03, H 3.48, and N 4.93%.
J = 8.08 Hz, 2 H), 7.95–8.03 (m, 4 H), 8.13–8.23 (m, 4 H), 8.56
(d, J = 8.08 Hz, 2 H). Elemental analysis for C38H32F6IrN4O4P,
M.W.: 947.88. Calc: C 48.25, H 3.34, N 5.92. Found: C 48.22,
H 3.34 and N 5.89%.
Acknowledgements
[Ir(ppy-COOEt)2(5Br bpy)](PF6)·H2O, 1b. [Ir(ppy-COOEt)2- The authors thank the Environmental Protection Agency (EPA
(5Brbpy)](PF6), 1b was synthesised using synthetic procedure Grant 2008-ET-MS-3-S2) and SFI/TIDA/E2763, SFI/TIDA/2435
for 1a. 610.23 mg (0.44 mmol) of [Ir(ppy-COOEt)(μ-Cl)]2 and for financial support.
232 mg (0.98 mmol) of 5Brbpy were reacted. Yield: 41%
(370 mg, 0.36 mmol). 1H NMR (400 MHz, Acetonitrile-d3)
δ ppm 1.23 (t, J = 7.20 Hz, 6 H), 4.18 (qd, J = 7.07, 4.29 Hz,
8 H), 6.82 (s, 2 H), 6.88 (s, 2 H), 7.16–7.23 (m, 4 H), 7.54 (ddd,
Notes and references
J = 7.64, 5.49, 1.26 Hz, 2 H), 7.64–7.71 (m, 6 H), 7.81 (dd, J =
6.57, 1.52 Hz, 2 H), 7.87–8.02 (m, 12 H), 8.13–8.24 (m, 6 H),
8.34 (dd, J = 8.84, 2.27 Hz, 2 H), 8.47 (d, J = 8.34 Hz, 2 H), 8.54
(d, J = 8.08 Hz, 2 H). Elemental analysis for C38H31BrF6IrN4O4P·
H2O. M.W.: 1044.79. Calc: C 44.68, H 3.38, N 5.36. Found:
C 43.79, H 2.78 and N 5.30%.
1 (a) S. Lamansky, D. Murphy, F. A. Razzaq, H. E. Lee,
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[Ir(ppy-COOEt)2(dpb)](PF6), 1c. [Ir(ppy-COOEt)2(dpb)](PF6)
was synthesised using the synthetic procedure for 1a. 100 mg
(0.07 mmol) of [Ir(ppy-COOEt)(μ-Cl)]2 and 73 mg (0.16 mmol)
1
of dpb were reacted. Yield: 6% (10 mg, 0.008 mmol). H NMR
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(400 MHz, Acetonitrile-d3) δ ppm 1.08–1.25 (m, 18 H),
3.42–3.62 (m, 4 H), 3.95–4.12 (m, 8 H), 4.16 (q, J = 7.07 Hz,
4 H), 6.87 (d, J = 1.52 Hz, 2 H), 7.19 (t, J = 6.32 Hz, 2 H), 7.46
(d, J = 5.31 Hz, 2 H), 7.63 (dd, J = 8.08, 1.77 Hz, 2 H), 7.75 (d, J =
5.56 Hz, 2 H), 7.86–7.92 (m, 4 H), 7.96 (t, J = 7.33 Hz, 2 H), 8.19
(d, J = 8.08 Hz, 2 H), 8.76 (br. s., 2 H). Elemental analysis for
C48H56F6IrN4O10P3·2H2O. M.W.: 1284.14. Calc: C 43.68, H 4.71,
N 4.36. Found: C 43.84, H 4.36 and N 4.29%.
[Ir(ppy-COOEt)2(bpp)](PF6), 1d. [Ir(ppy-COOEt)2(dpp)](PF6)
was synthesised using the synthetic procedure for 1a. 200 mg
(0.15 mmol) of [Ir(ppy-COOEt)2(μ-Cl)]2 and 75.35 mg
(0.32 mmol) 2,5-di(2-pyridyl)pyridine (bpp) were reacted.
1
Yield: 184.5 mg (0.18 mmol, 60%). H NMR (400 MHz, Aceto-
nitrile-d3) δ ppm 1.23 (td, J = 7.14, 1.14 Hz, 7 H), 4.15–4.22 (m,
4 H), 6.88 (d, J = 1.26 Hz, 1 H), 6.90 (d, J = 1.26 Hz, 1 H), 7.18
(m, 2 H), 7.38 (dd, J = 7.58, 4.80 Hz, 1 H), 7.53 (dd, J = 7.64,
5.49 Hz, 1 H), 7.66 (dt, J = 8.08, 1.01 Hz, 1 H), 7.70 (td, J = 8.59,
1.77 Hz, 2 H), 7.75 (dd, J = 6.44, 1.39 Hz, 1 H), 7.81–7.86 (m,
2 H), 7.90–8.03 (m, 5 H), 8.15–8.23 (m, 3 H), 8.55 (dd, J = 5.68,
1.89 Hz, 1 H), 8.60 (d, J = 8.34 Hz, 1 H), 8.62–8.67 (m, 2 H),
8.70–8.75 (m, 1 H). Elemental analysis for C34H35F6IrN5O4P;
M.W.: 1024.96; Calc: C 50.49, H 3.45, N 6.85. Found: C 50.37,
H 3.33 and N 6.56%.
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1
0.07 mmol). H NMR (400 MHz, Acetonitrile-d3) δ ppm 1.23 (t,
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4 H), 4.49 (q, J = 7.07 Hz, 4 H), 6.85 (d, J = 1.77 Hz, 2 H), 7.17
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Hz, 2 H), 7.95–8.03 (m, 4 H), 8.19 (dd, J = 5.56, 0.76 Hz, 2 H),
8.21 (d, J = 7.83 Hz, 2 H), 9.10 (dd, J = 1.64, 0.63 Hz, 2 H).
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