Luminescent iridium complexes for detection of molybdate

Article abstract of DOI:10.1039/c1dt11360g

Reactions of [Ir(CN)₂Cl]₂ [HCN = 2-(3-R-phenyl) pyridine, 2-(3-R-phenylpyrazole) R = H, Me] with Me₂-phencat give luminescent complexes [Ir(CN)₂(Me₂-phencat)][PF ₆] (Me₂-2a, b, c

Full text of DOI:10.1039/c1dt11360g

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PAPER
Luminescent iridium complexes for detection of molybdate†
Carmen E. Castillo,b,c David L. Davies,*a Anne-K. Duhme Klair,*b Kuldip Singha and Shalini Singha
Received 19th July 2011, Accepted 22nd September 2011
DOI: 10.1039/c1dt11360g
Reactions of [Ir(CŸN)2Cl]2 [HCŸN = 2-(3-R-phenyl)pyridine, 2-(3-R-phenylpyrazole) R = H, Me] with
Me2-phencat give luminescent complexes [Ir(CŸN)2(Me2-phencat)][PF6] (Me2-2a, b, c)[PF6].
Deprotection of the methoxy groups with BBr3 is problematic as simultaneous bromination of the
cyclometallated phenyl groups occurs. However, deprotection of Me2-phencat with BBr3 followed by
complexation with [Ir(CŸN)2Cl]2 gives luminescent complexes [Ir(CŸN)2(H2-phencat)][PF6] (H2-3a,
c)[PF6], which are luminescent sensors for molybdate.
Introduction
Kinetically inert polypyridine complexes of metal ions with d6 low-
spin configuration, such as Ru(II), Re(I) and Ir(III), are increasingly
used as signalling units in sensors and probes for biological
targets, based on their high quantum yields, long excitation and
emission wavelengths and reasonably long lifetimes.1,2 In the field
of anion recognition, metal-based luminophores of this type have
shown promise in the development of sensors3 for biologically
Fig. 1 The reaction of [MoO4]- with catecholamides (R = signalling unit).
important oxoanions, such as nitrate, sulfate, phosphate and
phosphate derivatives.2 Similarly, the detection of oxometalates
of biological4 and environmental5 relevance, such as molybdate
(MoO42-), tungstate (WO42-) and vanadate (HVO42-), has recently
attracted significant attention.6
with the weaker donor in 2-position trans to the oxo-ligands to
avoid competition for the same d-orbitals.10
Ru(II) and Re(I)-based signalling units respond to depro-
tonation or molybdate-binding to the receptor unit with a
drastic decrease in emission intensity.11 We are now interested
to test whether catecholamide-linked Ir(III)-luminophores would
respond similarly with a pH- and oxomatalate-dependent change
in emission intensity.
In our previous work,7 we have linked Ru(II)-and Re(I)-based
luminophores to catecholamide-based receptor units that are able
2-
to distinguish biologically relevant oxometalates, such as MoO4
or HVO42-, from structurally related oxoanions, such as SO4 or
2-
HPO42-, as well as potentially competing cations, such as Cu2+
and Fe3+. Upon binding to two catecholamide receptor units, the
molybdenum centre increases its coordination number from four
to six, as shown in Fig. 1. In the resulting distorted octahedral com-
plex, the two strong oxo donors are positioned cis to each other, in
order to maximise p-bonding.8 As observed for 2, 3-dihydroxy
benzoic acid,9 a recently obtained crystal structure of the cis-
dioxoMo(VI) complex of a catecholamides-based luminescent
sensor revealed that the receptor unit, coordinates preferentially
Following the report by Thompson et al. in 199912 of an OLED,
containing cyclometalated iridium complex [Ir(ppy)3] (Hppy =
2-phenylpyridine) as a dopant there has been a huge upsurge
of interest in complexes [Ir(CŸN)3] and [Ir(CŸN)2(XY)]. These
complexes have high quantum yields for emission due to spin–
orbit coupling and large Stokes shifts and as a result have been
applied in luminescent sensors.1,2 Early examples involved their use
as oxygen sensors based on the quenching of emission by molec-
ular oxygen.13 Subsequently complexes with specific recognition
sites appended to the ligands have been synthesised. Complexes
[Ir(CŸN)2(XY)] can be modified on the cyclometallating ligand or
on the ancillary (XY) ligand. Huang et al. showed an aldehyde on a
cyclometallated phenyl could react selectively with homocysteine
causing a change in emission wavelength from 615 nm (red) to
525 nm (green), with a large enhancement in emission intensity.14
Zhao et al. reported a complex [Ir(CŸN)2(bipy)]+ containing
bismesitylboryl groups on the cyclometallated phenyls, which is
a highly selective chemosensor for fluoride ions detectable by the
naked eye.15 Alternatively, attaching the recognition site to the
aDepartment of Chemistry, University of Leicester, Leicester, UK, LE1
7RH. E-mail: dld3@le.ac.uk; Fax: +44-116-2523789; Tel: +44-116-2092
bDepartment of Chemistry, University of York, York, UK, YO10 5DD.
E-mail: anne.duhme-klair@york.ac.uk; Fax: +44-1904-322516; Tel: +44-
1904-322587
cDept. de Ciencia de los Materiales e Ingeniena Metalurgica y Quimica
Inorganica, Facultad de Ciencias, Universidad de Cadiz, Cadiz, Spain
† Electronic supplementary information (ESI) available. CCDC reference
number 836003 for [Me2-2b]. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11360g
628 | Dalton Trans., 2012, 41, 628–635
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XY ligand has also been successful. Phenathroline and bipyridyl
ligands have been functionalised with thioureas to sense anions16
or with crown ethers or other ligands for metal ion sensing.17
Lo et al. attached a biotin,18 the complexes formed were non-
emissive in aqueous buffer but enhanced emission intensities
and extended lifetimes were observed upon binding to avidin.
Given this range of sensing applications based on [Ir(CŸN)2(XY)]
complexes we decided to investigate their appplication for sensing
molybdate based on a phenanthroline modified ligand we have
designed previously.7
N—H and the adjacent O atom of Me2-phancat, as observed in the
corresponding Ru(II)-and Re(I)-complexes.7 The two OMe groups
give rise to singlets at d 3.98 and d 4.11 assigned to MeA and
MeB respectively due to NOEs to H8 and H5 respectively. H4 is
easily identified as the only other singlet at d 9.02. The NOESY
spectrum then allows identification of H3 and H3¢ (NOE to H5
and H4 respectively) and the COSY spectrum assignment of H1,1¢
and H2,2¢. In the free ligand (Me2-phencat), the signals for H1,1¢ are
found at d 9.16 and 9.04 respectively, but on co-ordination they
shift to higher field (d 8.53 and ca. 8.3, respectively) due to ring
current effects from the neighbouring cyclometallated phenyls.
H1 shows an NOE to phenyl and pyrazole protons Ha and Hg¢
respectively, similarly Ha¢ and Hg both show NOEs to H1¢ which
then allows assignment of all the other protons of the phenyl
(Ha,a¢–d,d¢) and pyrazole (He,e¢–g,g¢) rings using the COSY spectrum.
The protons Ha,a¢ are observed as overlapping doublet of doublets
at high field (d 6.42 and d 6.41 respectively) characteristic of the
[Ir(CŸN)2] fragment.19 The phenyl protons give rise to only four
signals integrating to two protons each, similarly the pyrazole
groups are difficult to resolve. Presumably, the asymmetry of the
Me2-phencat ligand is too far away to make the phenylpyrazole
ligands sufficiently different to resolve.
Results and discussion
The initial synthetic strategy was analogous to that used for
the corresponding Ru(II) and Re(I) complexes7 i.e. complexation
of the protected ligand Me2-phencat and then deprotection of
methoxy groups using BBr3 (Scheme 1). The ligand Me2-phencat
was prepared as reported earlier.7 The dimers 1a,b,c react with
Me2-phencat and KPF6 at 60 ◦C under microwave irradiation for
20 min to form compounds [Me2-2a, b, c](PF6) as yellow solids
with yields of greater than 90% (Scheme 1).
1
The H and 13C NMR spectra of [Me2-2a, b, c](PF6) are very
The 1H NMR spectrum of [Me2-2b]+ is similar to that of [Me2-
2a]+ except in [Me2-2b]+ one proton on each phenyl has been
replaced by a methyl (MeC,C¢), which are observed as coincident
singlets at d 2.34. The most downfield singlet signals, at d 10.82
and d 9.03 are assigned to H5 and H4 respectively. The orientation
of the amide is the same as [Me2-2a]+ as evidenced by the NOE
between the NH, and one of the OMe groups (MeB) at d 4.11 and
the short N—H ◊ ◊ ◊ O distance observed in the crystal structure
(see below). The 1H NMR spectrum of [Me2-2c]+ is also similar to
[Me2-2a]+ with the amide proton H5 being observed at d 10.81. The
complex due to the lack of C2-symmetry and there is overlap
of some signals in the aromatic region. For example, [Me2-2a]+,
1
has in principle, 31 inequivalent protons, however, the H NMR
spectrum shows only seventeen different signals suggesting that
there is substantial overlap. Nevertheless through the use of COSY,
NOESY, TOCSY and HMQC measurements we have been able
to assign the spectra. The most downfield signal in the 1H NMR
spectrum is a singlet at d 10.84 assigned to the amide proton H5
1
(confirmed by no cross peak in the HSQC H–13C) and is con-
sistent with an intramolecular hydrogen bond between the amide
Scheme 1 Synthesis of complexes [Me2-2a, b, c] and [H2-3a, c, d] with labelling for NMR assignments.
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pyridine protons Hh,h¢ observed at 9.25 in the starting dimer20 are
shifted to d 8.06 due to the ring current effect of the coordinated
phencat-OMe ligand. The FAB mass spectra show molecular ions
for the cations at m/z 838, 866, 860 for [Me2-2a, b, c]+ respectively.
All three complexes gave satisfactory microanalyses.
deprotection as evidenced by peaks ca 80 Daltons higher showing
a correct isotope pattern for substitution of one hydrogen by
bromine. Unfortunately, it was not possible to separate out a pure
component from these mixtures. If the reaction was left for a long
time, and additional BBr3 added as necessary, conversion to the
dibrominated product [H2-3d]+ was possible. Iodotrimethylsilane
was also tried instead of BBr3 as a deprotecting reagent for [Me2-
2c](PF6), however this only gave the mono deprotected product as
judged by ES-MS. Corresponding reactions of BBr3 with [Me2-
2a](PF6) and [Me2-2b](PF6) also gave inseparable mixtures of
products with bromination being evident in the ES-MS for both
of the complexes. For [Me2-2b](PF6) bromination cannot occur
on the position para to the metal; however, the actual site of
Single crystals of [Me2-2b](PF6) were obtained by slow diffusion
of hexane into a concentrated DCM solution of the salt. The
˚
crystal structure is shown in Fig. 2, with selected bond lengths (A)
and angles (◦). The Ir(III) has a distorted octahedral coordination
geometry [N(1)–Ir(1)–N(3) is 171.32◦], with cis metallated carbons
and trans nitrogen atoms, as expected for such systems.21 The
catechol unit is held planar by an intramolecular hydrogen bond
∑∑∑
˚
N–H O (d(N–O) = 2.680 A) as discussed above and as observed
1
˚
in similar Re(I) and Ru(II) complexes (d(N–O) = 2.649 A and 2.641
A respectively).
bromination could not be identified, as the H NMR spectrum
showed very broad peaks.
7
˚
Since deprotection of the complexed ligand was complicated
by simultaneous bromination of the cyclometallated phenyl(s),
the alternative approach of deprotecting the ligand and then
complexation to the metal was attempted. The ligand Me2-
phencat was deprotected using BBr3 however, the product is
insoluble in organic solvents and in water, hence it was purified
by washing successively with MeOH, DCM and diethylether. A
1H NMR spectrum could be obtained in D2O in the presence of
NaOD, however the spectrum showed some evidence of exchange
processes occurring and the solution decomposed over time hence
the spectrum was not fully assigned. However, even though the
ligand is not soluble it is able to react with the iridium dimers 1a
and 1c under microwave irradiation. to give the expected products
[H2-3a](PF6) and [H2-3c](PF6) respectively in high yields (~ 80%)
(Scheme 1).
Fig. 2 X-ray crystal structure of the cation of [Me2-2b] with selected
bond lengths (A) and bond angles ( ): Ir(1)–N(1), 2.007(5); Ir(1)–N(3),
1.997(5); Ir(1)–N(5), 2.128(4); Ir(1)–N(6), 2.124(4); Ir(1)–C(9), 2.005(6);
Ir(1)–C(18), 2.009(6); N(1)–Ir(1)–N(3), 171.32(19); N(1)–Ir(1)–C(9),
80.4(2); N(3)–Ir(1)–C(18), 80.1(2); N(5)–Ir(1)–N(6), 77.07(18).
◦
˚
1
The H and 13C NMR spectra of [H2-3a]+ and [H2-3c]+ are
similar to those of [Me2-2a]+ and [Me2-2c]+ respectively, except the
signals due to the phencat-ligands. The amide proton H5 is not
observed for either complex, probably due to exchange with the
solvent (MeOD). The only singlet is therefore assigned to H4 (d
8.81 and 8.87 for [H2-3a]+ and [H2-3c]+ respectively) which shows
an NOE to H3¢. The catechol protons H6–8 are identified using
the HMBC spectra as proton H6 shows a cross peak to C10 and
proton H7 shows a cross peak to C9 respectively in each case. The
other assignments are made on the same basis as for [Me2-2a]+,
in some cases the protons of the cyclometallated ligands (Ha–g/h
and Ha¢–g¢/h¢) are accidentally equivalent. Protons Ha,a¢ are again at
high field and show NOEs to H1 and H1¢ respectively. The 13C–
In order to bind molybdate the methoxy groups need to be
deprotected to provide the catechol. This was attempted using
BBr3 following the literature method.7 In the first attempt this led
cleanly to a new complex which showed a mass approximately
160 units higher than expected. The 1H NMR spectra of the
product showed no signals for OMe groups showing that the
deprotection had been successful. In addition there were only three
signals for each cyclometallated phenyl, doublets for Ha/a’ and Hd/d’
and doublet of doublets for Hb/b’ consistent with bromination
on both phenyl rings para to the metal. The other signals were
similar to [Me2-2c]+ hence the product was identified as [H2-3d]+.
Note, a direct bromination of the phenyl ring at the para positon
with respect to the metal in [Ir(ppy)2Cl]2 has been reported using
1
{ H} NMR spectra show the expected signals and the FAB mass
spectra show peaks for ions at m/z 810 and 832 for [H2-3a]+ and
[H2-3c]+, respectively.
1
pyridinium tribromide.22 The 13C–{ H} NMR spectra of [H2-3d]+
show the expected signals though there is some overlap between
signals of related groups. The FAB mass spectrum shows an ion at
m/z 990 with the appropriate isotope pattern due to [H2-3d]+ and
the microanalysis is satisfactory, confirming the dibromination.
In an attempt to prevent bromination of the phenyl complex,
[Me2-2c](PF6) was reacted with a 10-fold molar excess of BBr3 in
DCM at -78 ◦C. Monitoring by ES-MS showed that deprotection
of the first methyl occurred within one hour, but deprotection
of the second methyl was much slower requiring several days
at room temperature (RT) and a large excess of BBr3 to go to
completion, which suggests one OMe is perhaps less basic than
the other as found previously.7 Unfortunately, bromination of
one of the cyclometallated phenyls is competitive with the second
Photophysical measurements
Selected spectroscopic characteristics for complexes [Me2-2]+ and
3 are shown in Table 1. Due to the limited water solubility of the
complexes, to examine the effect of pH on absorption a mixed
solvent system consisting of acetonitrile and water (20 : 1) was
used. The absorption spectra of the protected complexes [Me2-
2a]+ and [Me2-2c]+ are pH independent (between pH 0.1 and 11),
but those of the deprotected ones [H2-3a, c, d]+ show an increase in
intensity with increase in pH due to deprotonation and formation
of [H-3a, c, d], as shown for [H2-3a]+ in Fig. 3. and [H2-3c]+ and
[H2-3d]+ in the supporting information.
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Table 1 Selected spectroscopic data and protonation constants for [Me2-2a]+ and [Me2-2c]+ and the protonated and mono-deprotonated forms of 3a,
3c and 3d (aerated solutions at room temperature in aqueous acetonitrile (5% water))
[Me2-2a]+
[Me2-2c]+
[H2-3a]+
[H-3a]
[H2-3c]+
[H-3c]
[H2-3d]+
[H-3d]
Absorption lmax/nm
325
596
(325)
325
(600)
—
378
609
(330)
340
(609)
—
319
596
(326)
280, 326
(596)
5.6
326
—
(326)
—
329, 375 sh
610
(330)
285, 330
(610)
5.6
334 sh, 374 sh
—
(330)
314
(610)
375 sh, 420 sh
400 sh
—
(400)
—
Emission lem/nm (lex/nm)
588
(400)
375
(588)
5.7
Excitation lex/nm (lem/nm)
(596)
(588)
pKa
pHia
—
—
6.0
6.0
5.6
sh = shouldera pHi is the pH at the inflection point of the fluorescence titration curve.
Fig. 3 Absorption spectra recorded between pH 1.0 and 10.6 during
Fig. 4 Emission spectra recorded between pH 1.0 and 10.6 during
the titration of an acidic solution (0.015 mM) of [H2-3a]+ in aqueous
acetonitrile (5% water) with [Me4N]OH.
the titration of an acidic solution (0.015 mM) of [H2-3a]+ in aqueous
acetonitrile (5% water) with [Me4N]OH.
The absorption spectra of 2, 3-dihydroxybenzamides generally
show an increase in the intensity of the lowest energy absorbance
band upon deprotonation of the ortho OH group of the cate-
cholamide unit10,23 hence, the increase in absorbance of [H2-3a,
c, d]+ with pH is attributed to the deprotonation of the ortho-OH
group on the catechol unit of the phencat ligand. This is consistent
with the Ru(II) and Re(I) complexes of the same ligand.7 From
the pH profiles obtained, pKa-values of 5.6, 5.6 and 5.7 can be
estimated for [H2-3a]+, [H2-3c]+ and [H2-3d]+, respectively (Table
1). Consequently, the change of the ancillary ligand from 1a to 1c
and 1d has no significant effect on the pKa value of the phencat
receptor unit.
pH for each complex (as shown for [H2-3a]+ in Fig. 4 and for [H2-
3c]+[H2-3d]+ in the supporting information†) which is attributed
to the deprotonation of the catechol unit, giving [H-3a, c, d].
This assertion is further supported by the observation that the
emission intensity of the protected complexes [Me2-2a, c]+ is pH in-
dependent. In addition, emission quenching upon deprotonation
of phenolic OH groups has been reported for similar systems.25
From the inflection points of the pH-profiles obtained for [H2-3a]+,
[H2-3c]+ and [H2-3d]+, pHi values of 6.0, 6.0 and 5.6, respectively,
were estimated (Table 1), again indicating that the ancillary ligand
has only little influence on the donor strength of the phencat
ligand.
Upon excitation both the protected and deprotected complexes
2 and 3, respectively, show emission in acetonitrile and the data
are tabulated in Tables 1. The emission of complex [H2-2a]+ is
solvent sensitive, a red shift (10 nm) is observed upon increasing
the polarity of the solvent from DCM to acetonitrile/MeOH,
which is consistent with a charge transfer component in the
emissive state. Each of the deprotected complexes [H2-3a, c, d]+
also show a red shift (10–20 nm) in emission upon changing
the solvent from acetonitrile to a mixture of acetonitrile:water
(20 : 1). In aqueous acetonitrile, complexes [H2-3a, c, d]+ show
long wavelength emission bands with maxima at 596, 610 and
588 nm, respectively. For complex [H2-3d]+ the emission is higher
energy than for [H2-3c]+ consistent with an electron withdrawing
substituent (Br) on the cyclometalated phenyl para to the metal.24
The intensity of the emission decreases sigmoidally with increasing
The addition of 0.5 equivalents of molybdate to solutions of the
deprotected complexes [H2-3a, c, d]+ results in a decrease of the
emission intensity in the acidic pH range. The emission intensity at
596 for [H2-3a]+ at various pH is shown in Fig. 5 (for [H2-3c, d]+ see
supporting information†). As expected from previous studies with
Re(I) and Ru(II) complexes the decrease in emission intensity of
the complexes [H2-3a, c, d]+ is proportional to the concentration of
molybdate, due to deprotonation of the catechol units upon metal-
ion coordination. The observation that the emission intensity of
the methyl protected complexes [Me2-2a]+ and [Me2-2c]+ is not
influenced by the presence of molybdate supports this assertion
and demonstrates that the decrease in emission intensity is not due
to intermolecular quenching processes.
To determine the composition of the Mo complexes formed,
the solutions of [H2-3a, c, d]+ were titrated with aqueous solutions
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Scheme 2 Alternate binding mode for monodeprotonated ligand.
pH. A shift to the salicylate mode of binding is conceivable
since cis-dioxo Mo(VI) salicylate complexes, such as [MoO2(sal)2]2-
(salH2 = salicylic acid), are documented in the literature.28 Since
protonation and shift to the salicylate mode of binding decreases
the stability of the Mo-complex, partial dissociation takes place
below pH 4 and the presence of the released [H2-3a]+ leads to an
increase in emission intensity.
Fig. 5 Emission intensity at 596 nm as a function of pH recorded for
[Me2-2a] (triangles), [H2-3a]+ (squares) and [H2-3a]+ + 0.5 equiv. molybdate
(circles) upon titration of acidic 0.015 mM solutions with [Me4N]OH
(aqueous acetonitrile, 5% water).
Conclusions
We have shown that bis-cyclometallated iridium complexes can
be used as luminescent reporters of changes in pH or molyb-
date concentration using our previously described catecholamide
receptor for molybdate. Further fine tuning of the ligand environ-
ment around iridium is needed to improve the response beyond
ruthenium analogues. Notably the synthesis of these complexes,
differs from the previous ruthenium and rhenium complexes. In
the case of iridium the deprotection of the catechol has to be done
before complexation to the metal due to competing bromination
of the cyclometallated phenyl groups by BBr3. The reactivity of
the cyclometallated phenyls towards electrophilic reagents may be
a general problem for other ligand modifications carried out after
complexation.
Fig. 6 Emission intensity at 596 nm and absorbance at 355 nm of a 0.02
mM solution of [H2-3a]+ in aqueous acetonitrile at pH 4.1 as a function of
molar MoO4 fractions.
2-
Experimental
of molybdate, as shown for [H2-3a]+ in Fig. 6 (for [H2-3c, d]+ see
supporting information†). During the titrations, the solutions were
buffered at pH values 4.1, 4.7 and 4.1 for [H2-3a, c, d]+, respectively.
Upon the addition of molybdate the emission intensity of all the
three sensors decreases almost linearly until a ratio of sensor
to molybdate of approximately 2 : 1 is reached. This ratio is
consistent with the predominant formation of cis-dioxo-Mo(VI)-
dicatecholate complexes at these pH values. Complexes of this
composition are well known in the literature.9,10,26
Below pH 4, protonation of the Mo-complex is observed, which
leads to an increase in emission intensity. The most likely first
protonation site is the basic OH-group in meta-position of the
catecholamide unit. Such protonation can give rise to a change
in the co-ordination mode of the catecholamide unit, which can
facilitate dissociation. For Fe(III)-catecholamide complexes, it is
known that coordination to the carbonyl oxygen rather than the
basic phenolate in meta-position becomes more favourable as the
pH is lowered (salicylate mode of binding, Scheme 2). We have
previously investigated the Mo-complex of a Ru-based sensor that
contains the same catecholamide receptor unit as [H3-3a,c,d]+ in
more detail at pH 4.0.27 For the Ru-based sensor, we were able
to confirm that a change in binding mode rather than complete
catecholamide dissociation is the predominant mechanism at this
Unless stated otherwise all reactions were carried out under an
inert atmosphere of nitrogen and under microwave irradiation.
After work up all the complexes were stable in air. Microwave
reactions were carried out in a CEM-Discover commercial mi-
crowave reactor. H, and 13C–{ H} NMR spectra were obtained
using a DRX 400 MHz spectrometer. Chemical shifts were
recorded in ppm (on d scale with tetramethylsilane as internal
reference), and coupling constants are reported in Hz. FAB mass
spectra were obtained on a Kratos concept mass spectrometer
using NOBA as matrix. The electrospray (ES) mass spectra were
recorded using a micromass Quattra LC mass spectrometer in
HPLC grade acetonitrile. Microanalyses were performed by the
Elemental Analysis Service (London Metropolitan University).
Emission and excitation spectra were recorded on a Hitachi F-4500
fluorimeter, equipped with a red-sensitive R928F photomultiplier
tube. All starting materials were obtained from Aldrich or Alfa
Aesar with the exception of dimers 1a–c24 and Me2-phencat7 ligand
which were prepared according to literature methods.
1
1
Preparation of [Me2-2a](PF6)
Dimer 1a (70 mg, 0.068 mmol), Me2-phencat (59 mg, 0.164 mmol)
and KPF6 (25 mg, 0.136 mmol) were placed in a microwave vial
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and methanol (3 ml) was added. Nitrogen was bubbled through the
solution for 2 min and the vial was then sealed with a septum cap.
The reaction mixture was then heated under microwave irradiation
for 20 min at 60 ◦C. After this time the solvent was removed in
vacuo leaving behind a solid which was dissolved in DCM (15
ml) and passed through celite. The volume of the filtrate was
reduced and hexane was added slowly to induce precipitation. The
precipitate was filtered, washed with hexane and dried in vacuo to
yield [Me2-2a](PF6) as a yellow solid (121 mg, 91%). Anal. Calcd
for C39H31N7O3IrPF6: C, 47.66, H, 3.18, N, 9.98. Found: C, 47.76,
H, 3.24, N, 9.92%. 1H NMR (CD3CN): d 10.84 (1H, s, H5), 9.02
(1H, s, H4), 8.77 (1H, dd, J = 8.6, 1.2, H3), 8.69 (1H, dd, J = 8.6,
1.6, H3¢), 8.53 (1H, dd, J = 5.1, 1.2, H1), 8.38–8.37 (3H, m, H1¢, e, e¢),
7.96 (1H, dd, J = 8.2, 5.1, H2), 7.82 (1H, dd, J = 8.2, 5.1, H2¢), 7.73
(1H, dd, J = 7.1, 2.7, H6), 7.53 (2H, d, J = 8.2, Hd, d¢), 7.36–7.30
(2H, m, H7, 8), 7.13 (2H, tt, J = 7.4, 1.2, Hc, c¢), 6.97–6.92 (4H, m,
H3¢), 8.37 (1H, dd, J = 5.2, 1.4, H1), 8.22 (1H, dd, J = 5.2, 1.4,
H1¢), 8.06 (2H, m, Hh, h¢), 7.94 (1H, dd, J = 8.6, 5.2, H2), 7.84 (2H,
dd, J = 7.7, 1.0, Hd, d¢), 7.82–7.76 (3H, m, H2¢, g, g¢), 7.71 (1H, dd,
J = 6.9, 2.6, H6), 7.45 (2H, ddd, J = 5.8, 2.1, 1.4, He, e¢), 7.33–7.27
(2H, m, H7, 8), 7.09, 7.08 (2H, 2 ¥ td, J = 7.6, 1.4, Hc, c¢), 6.97, 6.96
(2H, 2 ¥ td, J = 7.4, 1.4, Hb, b¢), 6.88, 6.87 (2H, 2 ¥ td, J = 7.2,
1.2, Hf, f¢), 6.40, 6.39 (2H, 2 ¥ dd, J = 7.6, 0.8, Ha, a¢) 4.07 (1H, s,
MeB) 3.95 (1H, s, MeA). 13C NMR: 167.52, 167.47 (Ck, k¢), 164.44
(C13), 152.95 (C9), 151.44 (C1), 150.16 (C1¢), 150.02 (Ci, i¢), 149.68
(C14), 149.41 (Ce, e¢), 147.59 (C10), 147.33 (C12), 144.53 (C11¢), 144.33
(C12), 144.27 (Cj, j¢), 138.49 (Cg, g¢), 138.07 (C3¢), 134.16 (C11), 132.82
(C3), 131.75, 131.70 (Ca, a¢), 130.37 (Cb, b¢), 127.00 (C2¢) 126.79 (C2),
124.87 (C7, d, d¢), 123.40, 123.36 (Cf, f¢), 122.66 (Cc, c¢), 122.12 (C6),
119.82 (Ch, h¢), 118.12 (C4) 116.88 (C8), 61.59 (MeB), 55.93 (MeA).
MS (FAB): m/z 860 [M]+.
H
b, b¢, g, g¢), 6.51, 6.50 (2H, 2 ¥ t, J = 2.7, Hf, f¢), 6.42, 6.41 (2H, 2 ¥
dd, J = 7.4, 0.8, Ha, a¢), 4.11 (3H, s, MeB), 3.98 (3H, s, MeA). 13C
NMR: 164.48 (C13), 152.94 (C9), 151.75 (C1), 150.46 (C1¢), 147.97
(C12), 147.56 (C10), 145.22 (C12¢), 143.45, 143.38 (Ch, h¢), 138.85
(Cg, g¢), 138.13 (C3¢), 134.04 (C11), 133.24, 133.19 (Ca, a¢), 132.98
(C3), 131.85, 131.52, 131.30 (C11¢, 14, i, i¢), 127.91 (Ce, e¢), 126.78 (C2¢),
126.53 (Cb, b¢), 126.32 (C2), 124.82 (C7), 123.36 (Cc, c¢) 122.08 (C6),
117.99 (C4), 116.80 (C8), 111.96 (Cd, d¢), 108.05 (Cf, f¢), 61.57 (MeB),
55.91 (MeA). MS (FAB): m/z 838 [M]+.
Attempted deprotection of [Me2-2c](PF6)
Under an inert atmosphere, [Me2-2c](PF6) (100 mg, 0.099 mmol)
was dissolved in dry DCM (8 ml). The solution was cooled to
-78 ◦C and a 10-fold molar excess (per methoxy) of 1.0 M BBr3 in
DCM was added slowly. The reaction was stirred at -78 ◦C for 1 h
and then allowed to warm to room temperature. The reaction was
then stirred at room temperature for a further 14 days and a total
of 28 equiv (per methoxy) of 1.0 M BBr3 in DCM was added at
different time intervals. The reaction was monitored via 1H NMR
spectroscopy and ES mass spectrometry. Molecular ions were
observed at m/z 846, 832, corresponding to the monodeprotected
complex, and di-deprotected cation [H2-3c]+, along with ions at
m/z 924, 910, 1004 and 990 (i.e. approximately 80 and 160 mass
units higher) the isotope patterns of which were consistent with
containing one or two bromine atoms respectively. After 14 days
only m/z 990 was observed so the reaction was worked up by
slow addition of water until no HBr was evolved. The reaction
mixture was evaporated to dryness and the residues were taken up
in methanol. KPF6 (32.2 mg, 0.175 mmol) was added the mixture
was stirred for 30 min. and was then evaporated to dryness and the
solid was dissolved in DCM (20 ml) and passed through celite. The
volume of the filtrate was reduced and hexane was added slowly
to induce precipitation. The precipitate was isolated, washed with
hexane and dried in vacuo to yield [H2-3d](PF6) as a yellow solid
(79 mg, 75%). Anal. Calcd for C41H27N5O3Br2IrPF6: C, 43.40,
Preparation of [Me2-2b](PF6)
The procedure was the same as for [Me2-2a](PF6) using dimer
1b (100 mg, 0.092 mmol), Me2-phencat (79.3 mg, 0.221 mmol)
and KPF6 (40.7 mg, 0.221 mmol), and after work up gave
[Me2-2b](PF6) as a yellow solid (154 mg, 83%). Anal. Calcd for
C41H35N7O3IrPF6: C, 48.71, H, 3.49, N, 9.70. Found: C, 48.80, H,
1
3.57, N, 9.65%. H NMR (400 MHz, CD3CN): d 10.82 (1H, s,
H5), 9.03 (1H, s, H4), 8.77 (1H, dd, J = 8.6, 1.2, H3), 8.69 (1H, dd,
J = 8.6, 1.6, H3¢), 8.56 (1H, dd, J = 5.1, 1.2, H1), 8.40 (1H, dd, J =
5.1, 1.2, H1¢), 8.34, 8.33 (2H, 2 ¥ d, J = 2.7, He, e¢), 7.97 (1H, dd,
J = 8.6, 5.1, H2), 7.83 (1H, dd, J = 8.2, 5.1, H2¢), 7.74 (1H, dd, J =
7.0, 2.7, H6), 7.39 (2H, s, Hd, d¢), 7.36–7.30 (2H, m, H7, 8), 6.89, 6.88
(2H, 2 ¥ d, J = 2.7, Hg, g¢), 6.79 (2H, bd, J = 7.4, Hb, b¢), 6.49, 6.48
(2H, 2 ¥ t, J = 2.7, Hf, f¢), 6.26, 6.25 (2H, 2 ¥ d, J = 7.4, Ha, a¢), 4.11
(3H, s, MeB), 3.98 (3H, s, MeA), 2.34 (6H, s, MeC, C¢). 13C NMR:
165.50 (C13), 154.02 (C9), 152.82 (C1), 151.53 (C1¢), 149.10 (C12),
148.65 (C10), 146.36 (C12¢), 144.52, 144.45 (Ch, h¢), 139.76 (Cg, g¢),
139.12 (C3¢), 135.05 (Cc, c¢), 134.02 (Ca, a¢), 133.97 (C14), 133.89 (C3),
132.35 (C11¢), 128.66 (Cb, b¢), 128.42 (Ce, e¢), 128.34 (Ci, i¢), 127.81
(C11), 127.61 (C2¢) 127.39 (C2), 125.93 (C7), 123.18 (C6), 119.03
(C4), 117.92 (C8), 113.76 (Cd, d¢), 109.02 (Cf, f¢) 62.66 (MeB), 56.99
(MeA), 21.11 (MeC, C¢). MS (FAB): m/z 866 [M]+.
1
H, 2.40, N, 6.17. Found: C, 43.32, H, 2.39, N, 6.14%. H NMR
(400 MHz, CD3CN): d 10.59 (1H, s, H5), 8.84 (1H, dd, J = 8.6,
1.6, H3), 8.67 (1H, dd, J = 8.6, 1.6, H3¢), 8.67 (1H, s, H4), 8.37
(1H, dd, J = 5.1, 1.6, H1), 8.27 (1H, dd, J = 5.1, 1.2, H1¢), 8.08,
(2H, bd, J = 8.2, He, e¢), 8.01 (2H, d, J = 1.9, Hd, d¢), 7.87 (1H, dd,
J = 8.6, 5.1, H2), 7.84–7.78 (4H, m, H2¢, 6, f, f¢), 7.46 (2H, m, Hh, h¢),
7.17 (1H, dd, J = 7.8, 1.6, H8), 7.11, 7.10 (2H, 2 ¥ dd, J = 8.2,
1.9, Hb, b¢), 6.95–6.91 (3H, m, H7, g, g¢), 6.27, 6.26 (2H, 2 ¥ d, J =
8.2, Ha, a¢). 13C NMR: 169.67 (C13), 166.89, 166.81 (Ck, k¢), 152.77
(C1), 151.85 (C1¢), 150.84, 150.76 (Ch, h¢), 149.02 (Ci, i¢), 148.73 (C10),
148.20 (C12), 147.91, 147.85 (Cj, j¢), 146.87 (C9), 146.13 (C12¢), 139.89
(Cf, f¢), 139.50 (C3¢), 135.81 (C3), 134.91 (C12), 134.59, 134.56 (Ca, a¢),
Preparation of [Me2-2c](PF6)
The procedure was of the same as for [Me2-2a](PF6) using dimer
1c (70 mg, 0.065 mmol), Me2-phencat (56.1 mg, 0.156 mmol)
and KPF6 (26.4 mg, 0.144 mmol), and after work up gave
[Me2-2c](PF6) as a yellow solid (119 mg, 91%). Anal. Calcd for
C43H33N5O3IrPF6: C, 51.39, H, 3.31, N, 6.97. Found: C, 51.41, H,
3.26, N, 6.94%. 1H NMR (CD3CN): d 10.81 (1H, s, H5), 9.03 (1H,
s, H4), 8.74 (1H, dd, J = 8.6, 1.4, H3), 8.65 (1H, dd, J = 8.4, 1.4,
133.76 (Cb, b¢), 132.29 (C11¢), 129.23 (C11), 128.59, 128.56 (Cd, d¢
)
128.10 (C2¢), 127.58 (C2), 125.32 (Cg, g¢), 122.82 (C4), 121.52, 121.47
(Ce, e¢), 120.99 (C7), 120.91 (C8) 120.56 (C6), 117.40 (C14), 117.05
(Cc, c¢). MS (FAB): m/z 990 [M]+.
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Preparation of H2-phencat
(Ci, i¢), 148.71 (Ch, h¢), 147.31 (C12), 147.11, (C10), 146.03 (C9), 144.84
(C12¢), 144.06, 144.01 (Cj, j¢), 138.23 (Cf, f¢), 138.08 (C3¢), 133.54 (C3),
131.59, 131.52 (Ca, a¢), 131.37 (C11¢), 130.18, 130.14 (Cb, b¢), 127.67
(C11), 126.72 (C2¢) 126.32 (C2), 124.63 (Cd, d¢), 123.04, 123.01 (Cg, g¢),
122.43 (Cc, c¢), 120.27 (C4), 119.74 (C6), 119.61 (Ce, e¢), 119.24 (C7),
118.91 (C8), 116.90 (C14). MS (FAB): m/z 832 [M]+.
An excess of BBr3 (8.4 mmol of 1.0 M solution in CH2Cl2) was
added to Me2-phencat (150 mg, 0.43 mmol) in CH2Cl2 (5 ml) at
-78 C and left to stir overnight. After 24 h H2O was added slowly
until no more HBr evolved. Volatile impurities were removed by
addition of methanol (3 ¥ 10 ml) followed by rotary evaporation.
The residual solid was washed with methanol CH2Cl2 and ether.
The resulting solid was insoluble in common organic solvents and
water however an 1H NMR spectrum could be run in D2O/NaOD:
d 8.85 (1H, d, J = 3.5, phen), 8.73 (1H, br s, phen), 8.57 (1H, d,
J = 8.5, phen), 8.07 (2H, s, phen), 7.67 (1H, dd, J = 4, 8.5, phen),
7.48 (1H, br s, phen), 7.25 (1H, dd, J = 2, 8, cat), 6.85 (1H, dd, J =
1.5, 7.5, cat), 6.57 (1H, t, J = 7.5, cat). ES-MS m/z 332 [M+H]+.
General titration procedure
Unless otherwise stated, the following general conditions apply.
All titrations were carried out in air at room temperature in 10 mm
quartz cuvettes. The solvent system used consisted of a mixture of
acetonitrile and water (20 : 1). Adjustments to the pH were carried
out with 0.6 M and 0.1 M solutions of HCl in this solvent system
and 0.6 M, 0.1 M and 0.05 M solutions of tetramethyl ammonium
hydroxide [(Me4N)OH] in water. pH values were determined using
a WTW Profilab pH 597 pH meter with a Mettler Toledo Inlab
422 electrode and are given as measured in the solvent system.
The concentrations of [H2-3a, 3c and 3d] were made to 0.015 mM,
0.041 mM and 0.083 mM, respectively. These concentrations gave
an absorbance within the Beer–Lambert range. The excitation
wavelength was set to 326 nm, 330 nm and 400 nm for [H2-3a,
3c and 3d], respectively. Excitation and emission spectra were
corrected for the photomultiplier response and are smoothed.
Preparation of [H2-3a](PF6)
A mixture of dimer 1a (70 mg, 0.068 mmol) and H2-phencat
(72.8 mg, 0.176 mmol) in methanol (2 ml) was degassed and heated
under microwave irradiation for 2 h. at 70 ◦C. The orange-yellow
solution was then cooled to room temperature and KPF6 (32.3 mg,
0.176 mmol) was added to the solution and stirred for 30 min. The
mixture was evaporated to dryness and the solid was dissolved in
DCM (15 ml) and passed through celite. The volume of the filtrate
was reduced and hexane was added slowly to induce precipitation.
The precipitate was filtered, washed with hexane and dried in vacuo
to yield [H2-3a](PF6) as a yellow solid (101 mg, 78%). Anal. Calcd
for C37H27N7O3IrPF6: C, 46.54, H, 2.85, N, 10.27. Found: C, 46.63,
H, 2.76, N, 10.27%. 1H NMR (400 MHz, MeOD): d 8.87 (1H, dd,
J = 8.6, 1.2, H3), 8.81 (1H, s, H4), 8.69 (1H, dd, J = 8.6, 1.2, H3¢),
8.55–8.53 (3H, m, H1, e, e¢), 8.43 (1H, dd, J = 5.1, 1.6, H1¢), 7.94 (1H,
dd, J = 8.6, 5.1, H2), 7.86 (1H, dd, J = 8.6, 5.1, H2¢), 7.60 (1H, dd,
J = 8.2, 1.2, H6), 7.55 (2H, bd, J = 7.8, Hd, d¢), 7.12–7.05 (3H, m,
Determination of pH profiles
The solution of the sensors was adjusted to the starting pH value
in the acidic range using the standard acid (mentioned above)
and a spectrum was recorded. Small aliquots of base were added
to the sample. The pH of the solution was allowed to stabilise
before each spectrum was recorded. The spectra were recorded
at intervals of approximately 0.5 pH units, across the pH range
of ca. 1–10. Analogous titrations were conducted in the presence
of stoichiometric quantities of molybdate. An aqueous standard
solution of Na2MoO4 was used for this purpose.
H
8, c, c¢), 6.96, 6.95 (2H, 2 ¥ d, J = 2.3, Hg, g¢), 6.91, 6.90 (2H, 2 ¥ td,
J = 7.4, 0.8, Hb, b¢), 6.87 (1H, t, J = 8.2, H7), 6.53, 6.52 (2H, 2 ¥ d,
J = 2.3, Hf, f¢), 6.42, 6.41 (2H, 2 ¥ dd, J = 7.4, 1.2, Ha, a¢). 13C NMR:
167.78 (C13), 151.32 (C1), 150.38 (C1¢), 148.00 (C12), 147.06 (C10),
146.01 (C9), 145.58 (C12¢), 143.35, 143.31 (Ch, h¢), 138.26, 138.23
(Cg, g¢), 138.15 (C3¢), 133.87 (C11), 133.62 (C3), 133.05, 133.00 (Ca, a¢),
131.49 (C11¢), 131.13 (Ci, i¢), 127.52 (Ce, e¢), 126.32 (C2¢, b, b¢), 125.93
(C2), 123.16 (Cc, c¢) 120.21 (C4), 119.72 (C6), 119.29 (C7), 118.95
(C8), 117.14 (C14), 111.56 (Cd, d¢), 107.94, 107.92 (Cf, f¢). MS (FAB):
m/z 810 [M]+.
Metal-to-Sensor titrations
Titrations for the determination of the composition of the
complexes were conducted using the following procedure. The
standard sample solutions (0.02 mM, 0.042 mM and 0.083 mM
for [H2-3a, 3c and 3d](PF6), respectively) of the sensors were
buffered with 10 mL of 2, 4-lutidine, and the pH was adjusted
to the required value (4.08 for [H2-3a, and 3d], and 4.67 for [H2-
3c]) with standard acid and base solutions. To the above solutions,
5 mL aliquots of the standard solution of Na2MoO4 (0.6 mM,
1.26 mM and 2.49 mM for [H2-3a, 3c and 3d] respectively) were
added. After each addition, the sample was stirred for ca. 3 min to
allow the solution to equilibrate before the emission was recorded.
Aliquots of Na2MoO4 were added until an approximate 1 : 1 ratio
was reached.
Preparation of [H2-3c](PF6)
The procedure was of the same as for [H2-3a](PF6) using dimer
1c (60 mg, 0.056 mmol), H2-phencat (60 mg, 0.146 mmol) and
KPF6 (25.7 mg, 0.139 mmol) and after work up gave [H2-3c](PF6)
as a yellow solid (87 mg, 80%). Anal. Calcd for C41H29N5O3IrPF6:
C, 50.41, H, 2.99, N, 7.17. Found: C, 50.32, H, 2.93, N, 7.11%.
1H NMR (400 MHz, MeOD): d 8.90 (1H, dd, J = 8.6, 1.2, H3),
8.87 (1H, s, H4), 8.69 (1H, dd, J = 8.6, 0.8, H3¢), 8.41 (1H, dd,
J = 5.1, 1.2, H1), 8.29 (1H, dd, J = 5.1, 1.6, H1¢), 8.13 (2H, bd,
J = 8.2, He, e¢), 7.96 (1H, dd, J = 8.2, 5.1, H2), 7.88–7.84 (3H, m,
X-ray crystal structure determination of [Me2-2b](PF6)
H
2¢, d, d¢), 7.82, 7.80 (2H, 2 ¥ td, J = 7.4, 1.6, Hf, f¢), 7.60 (1H, dd,
Data were collected on a Bruker Apex 2000 CCD diffractometer
J = 8.2, 1.6, H6), 7.49 (2H, bd, J = 5.8, Hh, h¢), 7.11–7.03 (3H,
m, H8, c, c¢), 6.97–6.90 (4H, m, Hb, b¢, g, g¢), 6.84, (1H, t, J = 7.8, H7),
6.42, 6.40 (2H, 2 ¥ dd, J = 7.4, 0.8, Ha, a¢). 13C NMR: 167.99,
167.95 (Ck, k¢), 167.73 (C13), 151.04 (C1), 150.08 (C1¢), 149.74, 149.40
using graphite monochromated Mo-Ka radiation, l = 0.7107
A. The data were corrected for Lorentz and polarisation effects
and empirical absorption corrections were applied. The structure
was solved by direct methods and with structure refinement on
˚
634 | Dalton Trans., 2012, 41, 628–635
This journal is
The Royal Society of Chemistry 2012
©

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F2 employed SHELXTL version 6.1029 Hydrogen atoms were
Sci., 1996, 12, 767–771; B. K. Pal, K. A. Singh and K. Dutta, Talanta,
1992, 39, 971–975.
7 H. D. Batey, A. C. Whitwood and A. K. Duhme-Klair, Inorg. Chem.,
2007, 46, 6516–6528.
˚
included in calculated positions (C–H = 0.93–1.00 A, O–H =
˚
0.84 A) riding on the bonded atom with isotropic displacement
parameters set to 1.5Ueq (O) for hydroxyl H atoms, 1.5Ueq (C) for
methyl hydrogen atoms and 1.2Ueq (C) for all other H atoms. All
non-hydrogen atoms were refined with anisotropic displacement
parameters without positional restraints. Disordered solvent was
removed the Squeeze option in PLATON.30 Figures were drawn
using the program ORTEP.31 Crystal data for [Me2-2b](PF6):
8 W. P. Griffith and T. D. Wickins, J. Chem. Soc. A, 1968, 400–404; D.
C. Brower, J. L. Templeton and D. M. P. Mingos, J. Am. Chem. Soc.,
1987, 109, 5203–5208; M. Kaupp, Angew. Chem., Int. Ed., 2001, 40,
3534–3565.
9 W. P. Griffith, H. I. S. Nogueira, B. C. Parkin, R. N. Sheppard, A.
J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans., 1995,
1775–1781.
10 V. A. Corden, A. K. Duhme-Klair, S. Hostachy, R. N. Perutz and N.
Reddig, Inorg. Chem., 2011, 50, 1105–1115.
11 A. K. Duhme-Klair, Eur. J. Inorg. Chem., 2009, 3689–3701.
12 M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R.
Forrest, Appl. Phys. Lett., 1999, 75, 4–6.
13 G. Di Marco, M. Lanza, M. Pieruccini and S. Campagna, Adv. Mater.,
1996, 8, 576–580; M. C. DeRosa, D. J. Hodgson, G. D. Enright, B.
Dawson, C. E. B. Evans and R. J. Crutchley, J. Am. Chem. Soc., 2004,
126, 7619–7626; G. Di Marco, M. Lanza, A. Mamo, I. Stefio, C. Di
Pietro, G. Romeo and S. Campagna, Anal. Chem., 1998, 70, 5019–
5023; R. M. Gao, D. G. Ho, B. Hernandez, M. Selke, D. Murphy,
P. I. Djurovich and M. E. Thompson, J. Am. Chem. Soc., 2002, 124,
14828–14829.
˚
C43H38Cl5F6IrN7O3P, M = 1215.22, monoclinic, a = 24◦.571(6) A,
◦
◦
˚
˚
˚
b = 12.865(2) A, c = 29.199(6) A, a = 90 , b = 99.084(5) , g = 90 ,
3
V = 9114(3) A , T = 150(2)K, space group C2/c, Z = 8, m(Mo-
Ka) = 0.519 mm-1, 8945 reflections measured, 8945 independent
reflections (Rint = 0.0000). reflections measured, 8945 independent
reflections (Rint = 0.0000). The final R1 values were 0.0433 (I >
2s(I)) and 0.0653 (all data). The final wR(F2) values were 0.0846
(I > 2s(I)) and 0.0888 (all data). The goodness of fit on F2 was
0.856.
14 H. L. Chen, Q. Zhao, Y. B. Wu, F. Y. Li, H. Yang, T. Yi and C. H.
Huang, Inorg. Chem., 2007, 46, 11075–11081.
15 Q. Zhao, F. Li, S. Liu, M. Yu, Z. Liu, T. Yi and C. Huang, Inorg. Chem.,
2008, 47, 9256–9264.
16 K. K.-W. Lo, J. S.-Y. Lau, D. K.-K. Lo and L. T.-L. Lo, Eur. J. Inorg.
Chem., 2006, 4054–4062.
Acknowledgements
We thank the University of Leicester for funding, Johnson
Matthey for a loan of iridium trichloride and Vincent A. Corden
and. J. Morgan for experimental assistance.
17 M. L. Ho, F. M. Hwang, P. N. Chen, Y. H. Hu, Y. M. Cheng, K. S.
Chen, G. H. Lee, Y. Chi and P. T. Chou, Org. Biomol. Chem., 2006,
4, 98–103; M. Schmittel and H. W. Lin, Inorg. Chem., 2007, 46, 9139–
9145; N. Zhao, Y.-H. Wu, H.-M. Wen, X. Zhang and Z.-N. Chen,
Organometallics, 2009, 28, 5603–5611.
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