Cross-couplings in the elaboration of luminescent bis-terpyridyl iridium complexes: The effect of extended or inhibited conjugation on emission

Article abstract of DOI:10.1039/b313638h

The utility of Suzuki cross-coupling methodology for the in situ elaboration of bromo-functionalised bis-terpyridyl iridium(III) complexes has been explored. The complex [Ir(tpy)(tpy-φ-Br)]³⁺ {tpy-φ-Br = 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine} undergoes palladium-catalysed cross-coupling with aryl boronic acids to yield biaryl-substituted complexes directly. The biphenyl and 4-cyanobiphenyl-substituted products display relatively intense, long-lived (τ > 100 μs) yellow emission in degassed aqueous solution at room temperature, assigned to a ³π-π* state. A 4-aminobiphenyl-substituted analogue displays an additional low energy absorbance band, attributed to an intraligand charge-transfer (ILCT) excited state, and is scarcely emissive under the same conditions. The iridium(III) complex of 4′-mesityl-terpyridine is also reported. Its emission is much shorter-lived, with a spectral profile resembling that of unsubstituted [Ir(tpy)₂]^3-, confirming the need for the attainment of a roughly coplanar geometry for stabilisation of the ³π-π* excited state.

Full text of DOI:10.1039/b313638h

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Cross-couplings in the elaboration of luminescent bis-terpyridyl
iridium complexes: the effect of extended or inhibited conjugation
on emission
Wendy Leslie, Andrei S. Batsanov, Judith A. K. Howard and J. A. Gareth Williams*
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE.
E-mail: j.a.g.williams@durham.ac.uk
Received 28th October 2003, Accepted 7th January 2004
First published as an Advance Article on the web 26th January 2004
The utility of Suzuki cross-coupling methodology for the in situ elaboration of bromo-functionalised bis-terpyridyl
iridium() complexes has been explored. The complex [Ir(tpy)(tpy-ꢀ-Br)]3ϩ {tpy-ꢀ-Br = 4Ј-(4-bromophenyl)-
2,2Ј:6Ј,2Љ-terpyridine} undergoes palladium-catalysed cross-coupling with aryl boronic acids to yield biaryl-
substituted complexes directly. The biphenyl and 4-cyanobiphenyl-substituted products display relatively intense,
long-lived (τ>100 µs) yellow emission in degassed aqueous solution at room temperature, assigned to a 3π–π* state.
A 4-aminobiphenyl-substituted analogue displays an additional low energy absorbance band, attributed to an
intraligand charge-transfer (ILCT) excited state, and is scarcely emissive under the same conditions. The iridium()
complex of 4Ј-mesityl-terpyridine is also reported. Its emission is much shorter-lived, with a spectral profile
resembling that of unsubstituted [Ir(tpy)2]3ϩ, confirming the need for the attainment of a roughly coplanar geometry
for stabilisation of the 3π–π* excited state.
3
has been assigned to an excited state of predominantly π–π*
character,4–7 as opposed to the shorter-lived MLCT states
responsible for the luminescence of related cyclometallated
compounds.8 The complexes prepared during the present
exploration of the cross-coupling synthetic strategy incorporate
biphenyl-appended terpyridines, and hence offer an oppor-
tunity to investigate the effect of extended ligand conjugation
on the luminesence properties. Also discussed is the effect of
inhibiting conjugation, using a ligand carrying a 4Ј-mesityl
substituent, itself prepared by a cross-coupling procedure.
Introduction
The rich photophysical and electrochemical properties of
polypyridyl metal complexes render them of great interest in a
wide variety of applications, ranging from photocatalysts to
luminescent chemosensors, and from new electroluminescent
display materials to devices for the conversion of light to elec-
trical energy.1a–d Iridium() complexes with cyclometallating
ligands, for example, developed in the 1980s by Watts and
others,2 have recently enjoyed renewed interest, following
the demonstration that tris(2-phenylpyridine)iridium() is
able to act as a “triplet harvester” when incorporated into
electroluminescent devices, with accompanying large gains in
efficiency.1c,3 On the other hand, the longer lifetimes and appre-
ciable quantum yields found in iridium() complexes with an
N6 coordination sphere have sparked interest in their appli-
cation as luminescent sensors or labels amenable to time-
resolved detection procedures.4,5 With this latter application in
mind, we have been investigating the chemistry and lumines-
cence properties of bis-terpyridyl iridium() complexes.4 Their
preparation has proved challenging. Successful routes to date
involve harsh conditions and laborious purification, reflecting
the kinetic inertness of iridium() with respect to ligand
substitution processes: temperatures of 160 ЊC or higher are
required.4–7 Compared to the more widely-studied ruthenium
analogues (typically prepared at about 80 ЊC), these conditions
are very harsh, and represent a hurdle to be overcome in the
further development of this chemistry. This is especially
important if more delicate functionality is required in the ter-
pyridines, for example, in the case of responsive complexes as
chemosensors. In this paper, we report on the possibility of
using a simple, bromo-functionalised complex as a synthon in
the preparation of larger, potentially more elaborate complexes,
by palladium-catalysed cross-coupling with aryl boronic acids.
Since the cross-coupling can be achieved under much milder
conditions (80–85 ЊC) than those typically required for the
complexation of the terpyridines to the metal (160–200 ЊC), this
strategy could provide an attractive solution to the synthetic
problem.
Results and discussion
Synthesis and elaboration of complexes
The palladium-catalysed cross-coupling reaction of aryl
boronic acids with aryl halides has become an important tool
for C–C bond formation in organic chemistry, as it can tolerate
a very wide variety of functionality in both substrates, without
competitive homo-coupling of either of the reactants becoming
an issue.9 In the field of coordination chemistry, the procedure
has been applied to the synthesis of new ligands,10 but there are
few instances of the use of pre-formed metal complexes them-
selves as substrates in Suzuki couplings.11 The notable examples
are the formation of dinuclear homometallic complexes with
aryl spacers by cross-coupling of appropriate bromo-function-
alised complexes with 1,4-benzene-diboronic acid: originally
demonstrated for cyclometallated ruthenium complexes with
terdentate ligands,12a this approach has also been applied very
recently to bis-cyclometallated iridium() complexes.12b We
have reported recently on the use of bis-terpyridyl ruthenium()
complexes, that incorporate bromo- or boronic acid functional-
ity in the ligands, in Suzuki-type cross-coupling reactions.13
This strategy allows the in situ elaboration of both classes of
complex, upon Pd-catalysed reaction with aryl boronic acids or
aryl halides respectively. In the present work, we have sought to
extend this methodology to analogous iridium() complexes.
In order to investigate this possibility, the bromo-functional-
ised complex [Ir(tpy)(tpy-ꢀ-Br)](PF6)3 (tpy-ꢀ-Br = 4-bromo-
phenyl-terpyridine; Scheme 1) was prepared by reaction of
[Ir(tpy)Cl3] with tpy-ꢀ-Br in refluxing ethylene glycol. The
desired complex could be obtained in yields of 40–50% typi-
cally, following chromatography on silica. Cross-couplings with
Previous work has shown that the luminescence lifetime of
[Ir(tpy)2]3ϩ (tpy = 2,2Ј:6Ј,2Љ-terpyridine) is extended upon intro-
duction of simple aryl groups into the 4Ј-positions of the
terpyridine (e.g. 4-tolyl; 3,5-di-tert-butylphenyl7). The emission
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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Scheme 1
phenylboronic acid, 4-cyanobenzene boronic acid and 4-(di-
methylamino)benzene boronic acid were investigated, the latter
two being chosen in order to verify the applicability of the
method to systems with electron-withdrawing or -donating
groups. Our earlier work on ruthenium had revealed that the
widely-used catalyst Pd(PPh3)4, in combination with sodium
carbonate as a base, offered satisfactory results, provided that
DMSO was used as the solvent, as opposed to the less polar
solvents normally used in cross-coupling reactions of organic
substrates.13 This is probably a reflection primarily of the low
solubility of these rather highly-charged complexes in such sol-
vents. Applying similar conditions to [Ir(tpy)(tpy-ꢀ-Br)](PF6)3
in DMSO allowed the cross-coupling reactions to proceed,
leading to the desired complexes (Scheme 1). The reactions
proceeded rapidly at 80 ЊC and all starting complex had been
consumed within a few hours. This compares to considerably
slower reaction times observed for comparable cross-couplings
of uncomplexed tpy-ꢀ-Br with the same boronic acids, where
we found that, typically, reaction times of 24–48 h were
required.14 It is well-established that electron-withdrawing sub-
stituents in the aryl halide (especially those which are ortho or
para to the halogen) will increase the rate of cross-coupling
reactions (when the rate-determining step is the oxidative
addition to the palladium centre), owing to the increased polar-
isation and lability of the carbon–halogen bond.9 Thus, the
faster rate of reaction of the complexes, compared to the free
ligands, may be attributed to the presence of the electron-
withdrawing [Ir(tpy)23ϩ] core of the complex located para
to the halogen. On the other hand, it is also possible that
cross-couplings on the free ligands are somewhat retarded by
competitive complexation of the catalytic palladium species to
the terpyridine nitrogens, a possibility which clearly will not
arise for the pre-formed complexes.
Following the coupling reactions, anion exchange with aque-
ous KPF6 was carried out, to displace bromide anions and
remove water-soluble material, followed by column chromato-
graphy on silica. The overall yields were of the order 25–35%;
although rather low, it may be noted that these are the isolated
product yields, which generally tend to suffer from the relatively
inefficient nature of chromatographic purification of highly
charged salts. Thus, they do not compare unfavourably with
earlier work on such complexes,4,6,7 and higher yields could be
anticipated upon optimisation of reaction conditions. In the
case of the coupling with phenylboronic acid, the formation
of the desired heteroleptic complex, [Ir(tpy)L1]3ϩ, was accom-
panied by a significant amount of the homoleptic complex,
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[Ir(L1)2]3ϩ. Although these two complexes were readily separ-
ated by chromatography, the apparent scrambling that must
have taken place during the coupling reaction, in order to lead
to this side-product, is surprising, given the much higher tem-
peratures that are normally required to introduce the terpyr-
idines into the coordination sphere of iridium. For the other
cross-couplings, the amount of the equivalent side-products
formed was scarcely significant; however, in each case, thin layer
chromatography revealed the presence of the corresponding
“free” biphenyl ligand in solution after three hours (by com-
parison with the Rf values of samples of the pre-formed
ligands14), indicating some minor dissociation of the newly-
formed ligand. A control reaction, to examine whether the
coupling conditions {DMSO, aqueous Na2CO3 in the presence
of Pd(PPh3)4} favour ligand substitution processes around the
iridium centre, was carried out using [Ir(tpy)Cl3] and 4Ј-tolyl-
terpyridine (ttpy), but formation of [Ir(tpy)(ttpy)]3ϩ did not
occur at 85 ЊC, even after 72 h.
Fig. 1 The molecular structure of 4Ј-(2,4,6-trimethylphenyl)-terpyr-
idine (mesityl terpyridine, L4), showing thermal ellipsoids at the 50%
probability level. Atoms generated by the twofold axis are primed.
Given the success of this in situ coupling strategy in allowing
4Ј-bromophenylterpyridine to be elaborated whilst already
bound to the metal ion, it would be of interest to investigate
whether the analogous complex of 4Ј-bromoterpyridine,
[Ir(tpy)(tpy-Br)]3ϩ, (i.e. omitting the phenyl ring interposed
between the terpyridine unit and the bromo substituent), could
be functionalised similarly. Unfortunately, however, the pre-
pararation of this complex proved troublesome. Reaction of
the intermediate, [Ir(tpy)Cl3], with 4Ј-bromoterpyridine (tpy-
Br) under the usual conditions (ethylene glycol at reflux) gave a
mixture of products, amongst which there was no evidence of
the desired complex by 1H NMR analysis or mass spectrometry.
Bearing in mind the known instability of 4-bromopyridine with
respect to substitution reactions to give oligomeric products,
even under mild conditions,15 it is possible that tpy-Br cannot
itself tolerate these harsh, high temperature conditions. For the
reverse strategy of reacting [Ir(tpy-Br)Cl3] with terpyridine
(where tpy-Br is pre-coordinated to the metal under milder
conditions, and hence protected from such possible side-
reactions), the required product was detectable by electrospray
mass spectrometry, but exhaustive chromatography was un-
successful in separating it from the other components of the
mixture to give a pure sample.
units about the interannular bonds C(2)–C(12) and C(2Ј)–
C(12Ј). Such a configuration is commonly found in the solid
state structures of terpyridines, as it minimises unfavourable
electrostatic interactions between the nitrogen lone pairs.19 The
three pyridine rings which constitute the terpyridine moiety are
not quite coplanar, with an angle of 9.3Њ between the planes
of the lateral rings and that of the central pyridine ring. In this
respect too, the compound is similar to previously studied
systems, where an angle of 5–10Њ is typical. In contrast, the
dihedral angle between the mesityl group and the central pyr-
idine ring is large, 67.5Њ, compared to 11Њ between the central
pyridine and the aromatic pendant in 4Ј-phenylterpyridine.19
The mesityl ring is sandwiched between two lateral pyridyl rings
of adjacent molecules (interplanar angles 8.2Њ, mean inter-
planar separations ca. 3.5 Å), but no continuous stacks exist in
the structure.
The bis(4Ј-mesitylterpyridine)iridium() complex [Ir(L4)2]-
(PF6)2 was prepared readily by reaction of IrCl3ؒ3H2O with two
equivalents of the ligand (L4) in ethylene glycol at reflux
(Scheme 2), and isolated as the hexafluorophosphate salt after
anion exchange with aqueous KPF6. Purification was achieved
more readily than for related aryl-substituted complexes, by
repeated recrystallisation from acetone/toluene.
4Ј-Mesityl-terpyridine, L4 and its iridium(III) complex [Ir(L4)2]3؉
This complex was of interest for study of its photophysical
properties (see below) because, although it incorporates a 4Ј-
aryl substituent, conjugation of this ring with the terpyridine
will be inhibited due to the steric demand of the ortho methyl
groups. The use of “classical” terpyridine synthetic method-
ology, starting from 2-acetylpyridine and the pertinent aryl
aldehyde, in this case 2,4,6-trimethylbenzaldehyde, failed to
provide the desired ligand, possibly because of steric inhibition
of the Michael addition of the second equivalent of 2-acetyl-
pyridine to the initially formed α,β-unsaturated ketone.† On
the other hand, the ligand was readily prepared via a Suzuki
cross-coupling reaction of tpy-Br with the neopentyl glycol
ester of 2,4,6-trimethylbenzeneboronic acid in dimethoxy-
ethane, catalysed by Pd(PPh3)4 and in the presence of a strong
base, Ba(OH)2 (Scheme 2).14 A single crystal suitable for an
X-ray diffraction study was obtained from an ethanol solution.
The molecule of L4 in the crystal (Fig. 1) lies on a crystallo-
graphic twofold axis (passing through the N(1), C(4), C(5) and
C(8) atoms) and has a transoid arrangement of the pyridine
Photophysical properties
Absorption and emission data for the complexes prepared in
this study are collected in Table 1.
Absorption
With the exception of [Ir(tpy)L3]3ϩ, all of the complexes are
yellow or pale yellow in colour, with strong absorption bands in
the ultra-violet region tailing slightly into the visible (Fig. 2 and
Table 1). The large extinction coefficients of 104–105 MϪ1 cmϪ1
are typical of ligand-centred (1π–π*) transitions, as normally
observed in this region for iridium() complexes of bpy and
tpy ligands. The biphenyl and cyano-biphenyl substituted
complexes, [Ir(tpy)L1]3ϩ and [Ir(tpy)L2]3ϩ, display pronounced
absorption tails on the low-energy side, extending well beyond
400 nm, similar to those found previously for the tolyl-substi-
tuted complex [Ir(ttpy)2]3ϩ and related 4Ј-monoaryl-substituted
compounds.7 The tail is even more pronounced for the
homoleptic, bis-biphenyl-substituted complex [Ir(L1)2]3ϩ
(Fig. 2), and the extinction coefficients of this complex are also
significantly larger, as expected given the presence of two bi-
phenyl units. On the other hand, the profile of the bis-mesityl
complex [Ir(L4)2]3ϩ is quite different from those of the other
complexes at wavelengths > 330 nm, lacking the well-defined
low energy maxima around 380–390 nm, and tailing off at
† Our attempts were based on the widely used “one-pot” procedure,
which typically gives good results for simple 4Ј-aryl terpyridines (e.g.
aryl = phenyl, tolyl, 4-bromophenyl, 4-methoxyphenyl).16a An altern-
ative strategy involving initial deprotonation of 2-acetylpyridine with
KOBut, to pre-form the enolate, has been found to promote reaction
with unreactive α,β-unsaturated ketones formed from bulky aryl alde-
hydes,17,18 which may have proved more successful in this instance.
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Table 1 Photophysical parameters of the iridium() complexesa
[Ir(L4)2]3ϩ
[Ir(tpy)L1]3ϩ
[Ir(L1)2]3ϩ
[Ir(tpy)L2]3ϩ
Absorption (H2O) λmax/nm (ε)b
251 (34 300)
253 (40 700)
280 (45 600)
317 (23 700)
354 (18 000)
375 (16 100)
562
255 (70 900)
282 (85 400)
321 (49 300)
361 (41 700)
381 (44 300)
579
252 (43 200)
279 (49 400)
316 (31 000)
344 (25 500)
367 (21 300)
550, 582
514, 546 (sh), 604 (sh)
5.6
0.33
280 (35 600)
315 (16 800)
Emission (H2O) λmax/nm
Emission 77 Kc λmax/nm
ꢀ × 102 (H2O)d
458, 491, 526, 564 (sh)
461, 494, 532
0.73
517, 546 (sh), 604 (sh)
8.3
524, 557, 607 (sh)
2.2
ꢀ × 102 (CH3CN)d
0.17
0.44
0.58
τ/µs in H2O degassed (aerated)
τ/µs in CH3CN degassed (aerated)
τ/µs at 77 Kc
0.37 (0.32)
0.40 (0.38)
9.5, 0.60
107 (3.6f)
6.0 (0.63)
181
61 (3.6)e
17 (0.69)
163
144 (3.7)
6.2 (0.60)
230
a At 295 K except where stated otherwise. b The Beer–Lambert law was obeyed at concentrations up to at least 5 × 10Ϫ5 M. Absorption spectra in
CH3CN were essentially identical to those in water, with the same λmax values within the uncertainty of the measurement ( 1nm). c In EtOH/MeOH
d
(4:1 v/v), except for [Ir(L4)2]3ϩ, for which an EPA glass was used (EPA = ethanol/isopentane/diethyl ether, 2:5:5 v/v). In degassed solution,
measured using an excitation wavelength of 368 nm, excitation and emission band-passes of 2.5 nm, and using quinine sulfate as the standard
(ꢀ = 0.546 in 1M H2SO427); estimated uncertainty 15%. e A short-lived minor component to the emission was also observed; the quoted value is that
of the major component obtained by fitting the data to biexponential decay kinetics. f See also ref. 28.
Scheme 2
region). It seems likely, therefore, that the low energy maxima
and long wavelength tails are a consequence of additional con-
jugation at the 4Ј-position, which is only very limited in the
mesityl system owing to the steric encumbrance of the ortho
methyl groups.
In contrast to the other complexes, the amino-substituted
compound, [Ir(tpy)L3]3ϩ, is deep red in colour, owing to a
broad, relatively intense absorption band centred at 420 nm in
aqueous solution (Fig. 3). This is attributed to an intraligand
charge transfer (ILCT) transition, in which the NMe2 group
acts as the electron donor and the metal-bound terpyridyl
moiety as the electron acceptor. The free ligand L3 displays a
corresponding ILCT band around 350 nm (as reported in our
previous study of the ligands14). The red-shift in the complex
Fig. 2 UV-visible absorption spectra of [Ir(tpy)L1]3ϩ (thin solid line),
with respect to the ligand can be related to the increased
acceptor properties of the terpyridyl π* orbitals upon co-
[Ir(L1)2]3ϩ (dotted line), [Ir(tpy)L2]3ϩ (dashed line) and [Ir(L4)2]3ϩ
(alternating dashed and dotted line) in aqueous solution. The spectrum
ordination to the metal ion; shifts of comparable magnitudes
have been observed upon coordination of zinc to L3 and to
4Ј-(p-Me2N–C6H4)-terpyridine (L5),14,20 and also in the iridium
chloro complex [IrL6Cl3] compared to the free ligand, where
L6 = 4Ј-(p-Bu2N–C6H4–)-terpyridine.21 Further support for
the ILCT assignment comes from the substantial red-shift
(50 nm) observed in acetonitrile compared to water (Fig. 3).
of [Ir(ttpy)2]3ϩ (thick solid line) is shown for comparison. For each
complex, the spectra in acetonitrile are almost identical to those in
water.
shorter wavelengths. In this respect, it resembles much more
6,7
closely the unsubstituted, parent complex [Ir(tpy)2]3ϩ (albeit
with somewhat enhanced absorption in the 360–400 nm
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Scheme 3
which could be regarded as the simple model of the basic site.
This reduced basicity of the amine is to be expected, given
the presence of the electron-withdrawing metal–terpyridyl
fragment in direct conjugation with it, and the fact that proton-
ation will be inhibited by the ϩ3 charge on the complex (related
pyridyl-appended complexes display a comparable reduction in
pKa relative to pyridine4a).
Emission
Previous studies have attributed the luminescence from
[Ir(tpy)2]3ϩ and 4Ј-aryl-substituted analogues to a state of
primarily ligand-centred character.4–7 The former displays a
highly structured emission spectrum in solution at room
temperature, whilst the introduction of aryl rings into the 4Ј
positions of the ligands leads to a red-shift and a rather less
structured spectrum, which has been interpreted in terms of the
effect of enhanced conjugation on the 3π–π* excited state.7
A higher-lying MLCT state may also be implicated in the
luminescence behaviour.
Fig. 3 UV-visible absorption spectra of [Ir(tpy)L3]3ϩ in water and in
acetonitrile. The inset shows the change in absorption at 420 nm in
aqueous solution as a function of pH.
This negative solvatochromic behaviour is indicative of a polar
ground state and a less polar excited state, and is consistent with
a charge-transfer axis which lies colinear with the dipole axis.22
The behaviour is in marked contrast to that of the homoleptic
analogues [Ir(L3)2]3ϩ and [Ir(L5)2]3ϩ, which display only a very
weak (and positive) solvatochromism in the corresponding
band; in these complexes, there is no permanent dipole moment
in the ground state (D2d symmetry), and formation of the ILCT
excited state will be accompanied by, at most, a small increase
in the polarity of the system.23
Mesityl-substituted complex [Ir(L4)2]3؉
Despite possessing an aryl substituent at the 4Ј-position, the
bis-mesityl substituted complex [Ir(L4)2]3ϩ displays an emission
spectral profile (Fig. 4) which is almost identical to that of the
unsubstituted complex [Ir(tpy)2]3ϩ, and quite different from
those of all the previously reported aryl-substituted complexes,
such as [Ir(ttpy)2]3ϩ. From the position of the highest energy
(0–0) band, the luminescent level of this complex is higher in
energy by about 2000 cmϪ1 compared to other 4Ј-aryl substi-
tuted complexes. This result, which is in line with the absorp-
tion data, provides very strong evidence that it is indeed the
additional conjugation offered by the 4Ј-aryl group that nor-
mally leads to the significantly different spectra exhibited by the
aryl substituted systems compared to the parent complex: in the
specific case of the mesityl system, the steric barrier to attain-
ment of the necessary coplanar conformation will effectively
prevent the aryl group from augmenting the conjugation in the
excited state.
Upon acidification of a solution of [Ir(tpy)L3]3ϩ, this low
energy absorption band disappears, owing to protonation of
the -NMe2 group, which inhibits its ability to act as a donor in
the ILCT process (Scheme 3). This is very different from the
effect of acidification of the free ligand L3, where the band
undergoes a red-shift. In that case, the site of initial protonation
is the terpyridyl moiety, making it a better acceptor (Scheme
3).14,20 Clearly, in the complex, the terpyridyl nitrogens are not
available for protonation, leaving only the NMe2 unit accessible.
A pH titration in aqueous solution, obtained by monitoring the
absorbance at 420 nm, gives an inflexion point at 3.8 (Fig. 3),
significantly lower than the pKa of N,N-dimethylaniline (5.2),
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order 9 × 108
M
Ϫ1sϪ1, about twice as large as the value
for [Ir(ttpy)2]3ϩ in aqueous solution, and more in line with
values typically observed for polypyridine transition metal
complexes.24
In a frozen glass at 77 K (in EtOH/MeOH, 4:1 by volume),
these complexes display a significant hypsochromic shift in
emission of around 1500 cmϪ1 relative to room temperature
(Fig. 5). Such behaviour implies that the luminescent excited
state is coupled with nuclear rearrangements which are affected
by the state of the solvent. In contrast, the mesityl complex
shows almost no change in emission energy or spectral shape.
The differing behaviour is likely to be due to the torsional
rearrangement that may be expected upon excitation of the bi-
phenyl complexes, between the planes of the terpyridyl frag-
ment and the biphenyl moiety, and between the two constituent
rings of the latter. An approximately coplanar conformation of
the three planes will be required to maximise the conjugation in
the excited state whereas, in the ground state, the dihedral
angle of a biphenyl fragment is around 30Њ.25 The necesssary
rearrangement to the more favourable conformation will be
possible in fluid solution at room temperature, but not at 77 K,
accounting for the higher energy emission under the latter
conditions.
Fig. 4 Emission spectra of the complexes in aqueous solution at
295 K, λex =368 nm, excitation and emission band-passes set to 5 nm.
The same legend as in Fig. 2 is employed: [Ir(tpy)L1]3ϩ (thin solid line),
[Ir(L1)2]3ϩ (dotted line), [Ir(tpy)L2]3ϩ (dashed line), [Ir(L4)2]3ϩ
(alternating dashed and dotted line), [Ir(ttpy)2]3ϩ (thick solid line).
The measured luminescence lifetime (e.g. 400 ns in MeCN,
Table 1) is also more comparable to that of [Ir(tpy)2]3ϩ (τ =
1.2 µs under the same conditions7) than to the longer values
found for the aryl-substituted systems (τ ≥ 6 µs4,5,7). On the
other hand, it is intriguing that the introduction of the mesityl
group actually shortens the lifetime compared to the unsubsti-
tuted system. Since the quantum yield is similarly lowered,
it implies that the mesityl complex is subject to enhanced
non-radiative decay pathways. This could be associated simply
with the increased availability of proximate C–C stretching
vibrations able to facilitate the deactivation of the excited state,
but it could also reflect the inductively electron-donating nature
of the 2,4,6-trimethylphenyl substituent, which may serve
to raise the energy of the metal-centred orbital and thus lower
the energy of the higher-lying MLCT state, promoting it as a
pathway of decay.
Biphenyl-substituted complexes
Fig. 5 Emission spectra of the biphenyl complexes at 77 K, in an
ethanol/methanol (4:1 v/v) glass; λex =368 nm, band-passes of 5 nm.
Legend: [Ir(tpy)L1]3ϩ (thin solid line), [Ir(L1)2]3ϩ (dotted line),
[Ir(tpy)L2]3ϩ (dashed line). The room temperature spectrum of
[Ir(tpy)L2]3ϩ is also shown, to aid comparison (dashed line marked with
open circles).
The biphenyl, bis-biphenyl and cyano-biphenyl-substituted
complexes all display moderately intense room temperature
luminescence in degassed aqueous solution, with emission max-
ima in the range 550–590 nm (Fig. 4). This represents a signifi-
cant red-shift relative to related complexes which have just one
aryl ring at the 4Ј-position (as opposed to a biphenyl unit), such
as [Ir(ttpy)2]3ϩ, although the quantum yields of luminescence in
degassed solution are of a similar magnitude (0.02–0.08). The
cyano complex displays marginally the highest energy emission,
and also retains the limited but nevertheless distinctive
Interestingly, the complexes are substantially less emissive in
acetonitrile than in aqueous solution, (e.g. the quantum yield of
[Ir(tpy)L1]3ϩ is 18-times lower in acetonitrile than in water, in
the absence of oxygen), even though the emission profiles in the
two solvents are very similar. In air-equilibrated acetonitrile
solutions, the emission intensity is very weak indeed (ꢀ 5–7 ×
structure in the emission profile displayed by [Ir(ttpy)2]3ϩ
.
The increased conjugation offered by the additional phenyl
3
ring will lead to stabilisation not only of the π–π* states, but
10Ϫ4). The lifetimes are also shortened, e.g. for [Ir(tpy)L1]3ϩ
,
also of the MLCT excited states, since the ligand π* energy will
τ = 6.0 ( 0.5) µs in CH3CN compared to 107 ( 10) µs in H2O
(degassed solutions), an 18-fold decrease. That the quantum
yields and lifetimes are reduced by comparable factors (together
with the similar forms of the spectra), suggests that the reduc-
tion is due to increased rates of non-radiative deactivation in
acetonitrile, rather than to the radiative rate constant being
substantially different in the two solvents (which might be
expected if there were a switch to emission from a different type
of excited state).
A similar trend to lower quantum yields and shorter lifetimes
in acetonitrile has also been observed in the monoaryl series of
complexes; however, the difference in that case was less pro-
nounced (a factor typically of about 3).26 An explanation for
the more dramatic trend in the present instance possibly lies in
the extent to which higher-lying ILCT states may influence the
emission. As discussed earlier, in the amino-substituted com-
plex [Ir(tpy)L3]3ϩ, the presence of an ILCT state is revealed
experimentally by the low energy band in absorption. This
complex is scarcely emissive, suggesting that the charge-transfer
be lowered as the conjugation increases. However, on the basis
3
of the lifetime data, we again assign the emission to a π–π*
state; indeed, the very long lifetimes in aqueous solution, sub-
stantially longer than those of [Ir(ttpy)2]3ϩ, imply that any
MLCT contribution to the excited state must now be small.
Thus, the complexes [Ir(tpy)L1]3ϩ and [Ir(tpy)L2]3ϩ display
monoexponential decay, with observed lifetimes of 107 and 144
(
10) µs respectively in aqueous solution at room temperature,
compared to 27 ( 3) µs for [Ir(ttpy)2]3ϩ under the same condi-
tions. In line with their longer lifetimes and proposed greater
triplet character, the new complexes also display higher sensitiv-
ity to oxygen. In air-equilibrated solution, for example, lifetimes
and quantum yields are reduced by a factor of around 20,
compared to much smaller effects (ca. 4-fold) for [Ir(ttpy)2]3ϩ
,
with the result that the biphenyl complexes are significantly
poorer emitters (in terms of quantum yield) than the monaryl-
substituted systems, under these conditions. The bimolecular
rate constants of quenching by molecular oxygen are of the
D a l t o n T r a n s . , 2 0 0 4 , 6 2 3 – 6 3 1
628

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state is subject to efficient non-radiative deactivation. Analo-
gous ILCT excited states are expected in the other complexes,
albeit at higher energies (higher than the emitting 3π–π* states),
owing to the more weakly electron-donating nature of the bi-
phenyl and cyanobiphenyl pendants. However, such ILCT
states will be stabilised in acetonitrile compared to water (as
observed experimentally for the amino system – see above), and
4Ј-Mesityl-terpyridine L4
An oven-dried Schlenk tube, equipped with a reflux condenser,
was charged with 4Ј-bromoterpyridine (400 mg, 1.28 mmol),
the neopentyl glycol cyclic ester of 2,4,6-trimethyl-benzene-
boronic acid, (328 mg, 1.41 mmol, 1.1 equivalents), barium
hydroxide (513 mg, 3 mmol, 2.4 equiv.) and Pd(PPh3)4 (74 mg,
0.06 mol, 5 mol%), and flushed with nitrogen. Degassed di-
methoxyethane (13 mL) and water (2 mL) were added via
cannula and the mixture heated at reflux for 24 h. The solvent
was then evaporated under reduced pressure, and the residue
partitioned between dichloromethane and water (30 mL of
each). The organic layer was separated, washed with aqueous
sodium hydroxide solution (0.1 M, 2 × 30 mL), and dried over
anhydrous sodium carbonate. The solvent was removed to give
a light brown residue, which was recrystallised from ethanol to
give the desired compound as an off-white solid (275 mg, 61%).
3
may therefore mix with the emissive π–π* state to a greater
extent in the former solvent, possibly providing a pathway of
non-radiative decay to the ground-state. Configurational mix-
3
ing of 1,3ILCT excited states into emissive π–π* and MLCT
states has been reported by McMillin and co-workers in a series
of 4Ј-aryl substituted [Pt(tpy)Cl]ϩ complexes although, in
that case, it leads to an increase in lifetime and quantum yield
over the parent, unsubstituted system which, itself, is almost
non-emissive.18
1H NMR (500 MHz, (CDCl3): δ 8.64 (2H, d, J 7.5, H3), 8.62
3
(2H, d, 3J 5.0, H6), 8.26 (2H, s, H3Ј), 7.83 (2H, td, 3J 7.5, 4J 1.5,
H4), 7.28 (2H, dd, 3J 7.5, 5.0, H5), 6.90 (2H, s, Ha), 2.29 (3H, s,
para-CH3), 2.03 (6H, s, ortho-CH3). 13C{1H} NMR (CDCl3):
156.2, 155.5, 151.6 (quats), 149.1 (C6), 137.0 (quat), 136.7 (C4),
136.6 (quat), 134.9 (quat), 128.0 (Ca), 123.6 (C5), 121.9 (C3Ј),
121.1 (C3), 20.9 (para-CH3), 20.6 (ortho-CH3). MS (EI): 351
(Mϩ), 350 (Mϩ – H), 335 (Mϩ – CH3), 273 (Mϩ – py). HRMS
(ESϩ): 352.1812 (M ϩ Hϩ); calc. for C24H21N3ϩHϩ: 352.1813.
Found C, 78.95, H, 5.90, N, 11.74; calc. for C24H21N3ؒH2O: C,
78.00, H, 6.27, N, 11.37%.
Conclusion
Palladium-catalysed Suzuki-type cross-coupling methodology
is successful in allowing the bromo-phenyl-substituted iridium
bis-terpyridyl complex [Ir(tpy)(tpy-ꢀ-Br)]3ϩ to be elaborated
in situ, giving biphenyl-substituted complexes that are isolable
in modest yield. Notably, the conditions employed (e.g. reaction
temperature of 85 ЊC) are much milder than the aggressive con-
ditions and high temperatures (≈200 ЊC) that are normally
required to obtain such complexes. Thus, the approach offers a
pathway for accessing a more diverse range of complexes,
potentially including those appended with bioactive moieties
such as peptides and nucleic acids. The long luminescence life-
times of the biphenyl and cyano-biphenyl substituted com-
plexes in degassed aqueous solution are consistent with an
emissive state of primarily 3π–π* character, stabilised compared
to that of [Ir(ttpy)2]3ϩ by the additional conjugation of the bi-
phenyl appendage. The mesityl-substituted complex supports
these conclusions on the effect of conjugation, the inhibition of
which, in this complex, leads to behaviour similar to that of
[Ir(tpy)2]3ϩ. Finally, the introduction of an amino susbtituent
leads to a switch to a low-energy ILCT excited state. This new
information on the way in which absorption and emission ener-
gies, lifetimes and quantum yields are influenced by ligand 4Ј
substituents should facilitate further exploration of the bis-
terpyridyl class of iridium() complexes, the development of
which has, to date, been largely overshadowed by the much
more widely investigated cyclometallated systems.
[Ir(L4)2](PF6)3
A mixture of iridium trichloride trihydrate (102 mg, 0.29 mmol)
and 4Ј-(mesityl)-terpyridine (L4) (200 mg, 0.57 mmol) in ethyl-
ene glycol (25 mL) was heated with stirring at 100 ЊC for 2.5 h,
under an inert atmosphere of nitrogen. The temperature was
then raised to 198 ЊC and heating continued for a further 1.5 h.
After cooling to ambient temperature, the deep orange solution
was added to a saturated aqueous solution of KPF6, precipitat-
ing the hexafluorophosphate salt. Repeated recrystallisation
from acetone and toluene yielded the desired complex as a pale
yellow solid (150 mg, 40%). A sample for photophysical studies
was chromatographed on a short column of alumina, gradient
elution from CH3CN to 80% CH3CN/19% H2O/1% KNO3
(sat’d, aq). 1H NMR (500 MHz, (CD3)2CO): δ 9.16 (2H, s, H3Ј),
3
3
9.02 (2H, d, J 8.0, H3), 8.39 (2H, t, J 8.0, H4), 8.21 (2H, d,
3J 4.0, H6), 7.66 (2H, dd, J 8.0, 4.0, H5), 7.19 (2H, s, Ha),
3
2.36 (6H, s, ortho-CH3), 2.42 (3H, s, para-CH3). 13C NMR
((CD3)2CO): 143 (C4), 130 (C5), 129 (Ca), 128 (C3Ј), 128 (C6),
21.8 (para-CH3), 21.0 (ortho-CH3). ES-MS: m/z 298 (M3ϩ).
HRMS-ES(ϩ): m/z 1185.2384 (Mϩ2PF6)ϩ (calcd for C48H42-
N6IrP2F12: 1185.2378).
Experimental
Synthetic details
4Ј-(4-Bromophenyl)-2,2Ј:6Ј,2Љ-terpyridine
was
prepared
according to the one-pot procedure of Spahni and Calzaferri,16a
and separated from the isomeric by-product, 6Ј-(4-bromo-
phenyl)-2,2Ј:4Ј,2Љ-terpyridine, as previously reported.16b 4Ј-
Bromo-terpyridine was prepared from 4Ј-triflate-terpyridine in
almost quantitative yield upon treatment with a solution of
HBr in acetic acid at 110 ЊC for 4 h;14 the triflate was obtained
as described by Potts.29 [Ir(tpy)Cl3] was prepared as described
previously.7 Tetrakis(triphenylphosphine)-palladium() was
obtained by hydrazine reduction of PdCl2 in DMSO in the
presence of triphenylphosphine.30 Other reagents, including
2,2Ј:6Ј,2Љ-terpyridine and the boronic acids, were obtained from
commercial sources and were used as supplied.
[Ir(tpy)(tpy-ꢀ-Br)](PF6)3
A mixture of Ir(tpy)Cl3 (220 mg, 0.41 mmol) and 4Ј-bromo-
phenylterpyridine (tpy-ꢀ-Br) (159 mg, 0.41 mmol) in ethylene
glycol (12 mL) was heated strongly until a clear red solution was
obtained (15 min, oil temperature 215 ЊC). After cooling to
ambient temperature, the solution was diluted with water
(40 mL), and the small amount of red precipitate obtained
{unreacted Ir(tpy)Cl3} was separated off by centrifugation. The
remaining solution was then treated with saturated aqueous
KPF6 (10 mL), precipitating the crude product as the hexa-
fluorophosphate salt, which was collected and washed with
water. Purification was achieved by column chromatography on
silica, gradient elution from CH3CN to 53% CH3CN/45% H2O/
2% KNO3 (sat’d, aq). Fractions containing the product were
concentrated under reduced pressure and subject to a second
metathesis with KPF6, leading to the required complex (200
Proton and 13C NMR spectra, including NOESY and COSY,
were recorded on a Varian 500 MHz instrument, and referenced
to residual protio-solvent resonances. Coupling constants are in
Hertz. Electrospray ionisation mass spectra were acquired on a
time-of-flight Micromass LCT spectrometer; high resolution
spectra for accurate mass determinations were also carried out
on this instrument or at the EPSRC National Mass Spectro-
metry Service Centre, Swansea.
1
mg, 40%). H NMR (500 MHz, CD3CN): 9.05 (2H, s, H3Ј on
3
3
tpy-ꢀ-Br), 8.86 (2H, d, J 8.5, H3Ј on tpy), 8.77 (1H, t, J 8.5,
D a l t o n T r a n s . , 2 0 0 4 , 6 2 3 – 6 3 1
629

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H4Ј), 8.70 (2H, d, 3J 8.0, H3 on tpy-ꢀ-Br), 8.59 (2H, d, 3J 8.0, H3
1H NMR (500 MHz, CD3CN): 9.13 (2H, s, H3Ј on L2), 8.86
(2H, d, 3J 7.5, H3Ј on tpy), 8.78 (1H, t, 3J 7.5, H4Ј), 8.73 (2H, d,
3J 8.0, H3 on L2), 8.59 (2H, d, 3J 8.0, H3 on tpy), 8.35 (2H, d, 3J
8.5, Hb), 8.23 (4H, two overlapping t, H4 on both terpyridines),
on tpy), 8.21 (4H, two overlapping t, H4 on both terpyridines),
3
3
8.12 (2H, d, J 8.5, Hb), 7.98 (2H, d, J 8.5, Ha), 7.68 (2H, d,
3J 5.5, H6 on tpy or tpy-ꢀ-Br), 7.59 (2H, d, 3J 5.5, H6 on tpy-ꢀ-
Br or tpy), 7.48 (4H, m, H5 on both terpyridines). MS (ESϩ):
m/z 813 (Mϩ), 407 (M2ϩ), 479 (MϩPF6)2ϩ. HRMS (EI): m/z
813.0939 (Mϩ), 271.0331 (M3ϩ); calc. for C36H25BrN6Ir, Mϩ:
813.0955, M3ϩ: 271.0318.
3
3
8.14 (2H, d, J 8.5, Ha), 8.02 (2H, d, J 8.5, HbЈ), 7.95 (2H, d,
3
3J 8.5, HaЈ), 7.69 (2H, d, J 5.0, H6 on L2 or tpy), 7.59 (2H, d,
3J 4.5, H6 on L2 or tpy), 7.49 (4H, two overlapping dd, H5 on
both ligands). MS (ESϩ): m/z 278 (M3ϩ), 490 (MϩϩPF6)2ϩ
HRMS (ESϩ): 270.7350 (M3ϩ). calc. for C43H29N7Ir, M3ϩ
278.7373.
.
:
[Ir(tpy)L1](PF6)3
A Schlenk tube was charged with [Ir(tpy)(tpy-ꢀ-Br)](PF6)3
(84 mg, 0.067 mmol), phenylboronic acid (17 mg, 0.14 mmol),
sodium carbonate (22 mg, 0.20 mmol, dissolved in the mini-
mum volume of water), and DMSO (6 mL) and the system was
degassed via three freeze–pump–thaw cycles. Pd(PPh3)4 (5 mg,
4.2 µmol, 0.06 equiv.) was then added under a positive pressure
of nitrogen and the mixture heated at 85 ЊC for 4 h, after which
time, TLC indicated complete consumption of the starting
material. A small amount of the free ligand L1 was suspected to
be present in the crude mixture, (based on comparison of the
TLC with that for a previously prepared sample of the
ligand14); this was readily removed by diluting the reaction mix-
ture with water and washing with dichloromethane. Treatment
of the aqueous solution with KPF6 led to a precipitate, the
NMR and ESMS of which revealed it to be a mixture of the
desired product and a smaller amount of the homoleptic com-
plex [Ir(L1)2](PF6)3. Separation was achieved using column
chromatography on alumina; gradient elution from CH3CN to
86.5% CH3CN/12.0% H2O/1.5% KNO3 (sat’d, aq), eluting
[Ir(L1)2]3ϩ, and subsequently to 77% CH3CN/20% H2O/3%
KNO3 (sat’d, aq), for elution of [Ir(tpy)L1]3ϩ. Treatment with
KPF6 led to the complexes as their hexafluorophosphate salts:
[Ir(tpy)L1](PF6)3 (15 mg, 18%) and [Ir(L1)2](PF6)3 (8 mg, 9%).
[Ir(tpy)L2](PF6)3 –method 2
A mixture of L2 (prepared as described previously14) (55 mg,
0.13 mmol) and Ir(tpy)Cl3 (74 mg, 0.14 mmol) in ethylene glycol
(6 mL) was heated with stirring at 215 ЊC for 20 min. After
cooling to ambient temperature, the clear red solution was
diluted with water, and treated with KPF6(aq), to precipitate
the crude product as a yellow solid, which was collected and
washed with water. Purification was achieved by chromato-
graphy on silica, gradient elution from CH3CN to 68% CH3CN/
30% H2O/2% KNO3(aq); product-containing fractions were
treated with KPF6(aq), leading to the desired complex (76 mg,
1
43%). H NMR and ESMS data were consistent with those
given above.
[Ir(tpy)L3](PF6)3
The procedure was similar to that described above for [Ir-
(tpy)L1]3ϩ, in this case starting from 4-(N,N-dimethylamino)-
benzeneboronic acid, (26 mg, 0.16 mmol), [Ir(tpy)(tpy-ꢀ-
Br)](PF6)3 (100 mg, 0.080 mmol), sodium carbonate (25 mg,
0.24 mmol, dissolved in the minimum volume of H2O) and
Pd(PPh3)4 (5.4 mg, 4.8 µmol, 0.06 equiv.) in DMSO (12 mL).
The mixture was heated at 85 ЊC for 12 h, the progress of the
reaction being monitored by TLC. After anion exchange with
potassium hexafluorophosphate, the crude product was found
to contain the desired compound, together with small amounts
1
Data for [Ir(tpy)L1](PF6)3: H NMR (500 MHz, CD3CN):
9.13 (2H, s, H3Ј on L1), 8.86 (2H, d, 3J 8.0, H3Ј on tpy), 8.78 (1H,
t, 3J 8.0, H4Ј on tpy), 8.73 (2H, d, 3J 8.0, H3 on L1), 8.59 (2H, d,
3J 8.0, H3 on tpy), 8.33 (2H, d, 3J 8.5, Hb), 8.22 (4H, two over-
of the two homoleptic complexes [Ir(L3)2]3ϩ and [Ir(tpy)2]3ϩ
.
3
lapping t, H4 both terpyridines), 8.10 (2H, d, J 8.5, Ha), 7.87
The former was readily removed by chromatography on alu-
mina, gradient elution from CH3CN to 95% CH3CN/5% H2O;
however, a further column was required to separate all traces
of [Ir(tpy)]23ϩ, together with an unidentified, bright yellow
fluorescent impurity; gradient elution from 50% CH3CN/50%
(CH3)2CO to 41% CH3CN/41% (CH3)2CO/16% H2O/2%
KNO3. The fractions containing the product were concentrated
and treated with KPF6(aq), leading to the desired complex as the
3
3
(2H, d, J 7.5, HbЈ), 7.70 (2H, d, J 5.5, H6 on L1), 7.59 (4H,
overlapping m, H6 on tpy and HaЈ), 7.49 (5H, overlapping m, H5
on both terpyridines and HcЈ). MS (ESϩ): m/z 270 (M3ϩ), 478
(MϩPF6)2ϩ. HRMS (ESϩ): 270.4152 (M3ϩ). calc. for C42H30-
N6Ir, M3ϩ: 270.4055.
Data for [Ir(L1)2](PF6)3: 1H NMR (500 MHz, CD3CN):
9.14 (2H, s, H3Ј), 8.75 (2H, d, 3J 8.0, H3), 8.34 (2H, d, 3J 8.5, Hb),
3
3
8.25 (2H, t, J 8.0, H4), 8.10 (2H, d, J 8.5, Ha), 7.87 (2H, d,
3J 7.5, HbЈ), 7.73 (2H, d, 3J 5.0, H6), 7.59 (2H, t, 3J 7.5, HaЈ), 7.51
(3H, two overlapping m, H5 and HcЈ). MS (ESϩ): m/z 321
(M3ϩ).
1
hexafluorophosphate salt (17 mg, 26%). H NMR (500 MHz,
CD3CN): 9.11 (2H, s, H3Ј on L3), 8.86 (2H, d, 3J 8.5, H3Ј on tpy),
8.77 (1H, t, 3J 8.5, H4Ј), 8.73 (2H, d, 3J 8.0, H3 on L3), 8.59 (2H,
3
3
d, J 8.0, H3 on tpy), 8.27 (2H, d, J 8.5, Hb), 8.21 (4H, two
3
overlapping t, approx. J 8.0, H4 on both terpyridines), 8.04
[Ir(tpy)L2](PF6)3 –method 1
(2H, d, 3J 8.5, Ha), 7.76 (2H, d, 3J 8.5, HbЈ), 7.70 (2H, d, 3J 5.5,
H6 on L3 or tpy), 7.59 (2H, d, 3J 5.5, H6 on tpy or L3), 7.48 (4H,
The procedure was similar to that described above, in this
case using 4-cyanobenzene boronic acid (8.8 mg, 0.06 mmol),
[Ir(tpy)(tpy-ꢀ-Br)](PF6)3 (40 mg, 0.032 mmol), sodium carbon-
ate (9.5 mg, 0.09 mmol), Pd(PPh3)4 (2 mg, 1.8 µmol, 0.06
equiv.), in DMSO (5 mL). After 2 h at 80 ЊC, TLC indicated
complete consumption of the starting material. Following
anion exchange upon treatment with KPF6(aq), analysis of the
two overlapping dd, H5 on both terpyridines), 6.91 (2H, d, J
3
8.5, HaЈ), 3.06 (6H, s, CH3). MS (ESϩ): m/z 427 M2ϩ, 499
(MϩPF6)2ϩ
.
Photophysical measurements
1
crude product by H NMR indicated that the desired complex
Absorption spectra were measured on a Biotek Instruments
XL spectrometer, using quartz cuvettes of 1 cm path-length.
Steady-state luminescence spectra at room tempertaure were
measured using a Jobin Yvon FluoroMax-2 spectrofluorimeter
and those at 77 K with a Perkin-Elmer LS50B instrument. Both
instruments were fitted with red-sensitive Hamamatsu R928
photomultiplier tubes; the spectra shown are corrected for the
wavelength dependence of the detectors, and the quoted emis-
sion maxima refer to the values after correction. Samples for
emission measurements were contained within quartz cuvettes
of 1 cm pathlength modified with appropriate glassware to
was contaminated with the free ligand L2, as well as a trace of
[Ir(tpy)2]3ϩ. The former was removed by passage through a
small column of alumina (Brockman I), initially eluting with
hexane/ethyl acetate to remove L2, and then switching to
acetonitrile/water, to elute the mixture of complexes. These two
complexes were subsequently separated by chromatography on
alumina (Brockman II–III); gradient elution from CH3CN to
87% CH3CN/12% H2O/1% KNO3 (sat’d, aq). After treatment
with KPF6(aq), the desired complex was obtained as the hexa-
fluorophosphate salt (15 mg, 36%).
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630

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6 N. P. Ayala, C. M. Flynn, Jr., L. Sacksteder, J. N. Demas and
B. A. DeGraff, J. Am. Chem. Soc., 1990, 112, 3837.
allow connection to a high-vacuum line. Degassing was
achieved via a minimum of three freeze–pump–thaw cycles
whilst connected to the vacuum manifold; final vapour pressure
at 77 K was less than 10Ϫ3 mbar, as monitored using a Pirani
gauge.
Samples for time-resolved measurements were excited at
355 nm using the third harmonic of a Q-switched Nd:YAG
laser and the luminescence detected with a Hamamatsu R928
photomultiplier tube and recorded using a digital storage
oscilloscope, before transfer to a PC for analysis; estimated
uncertainty in quoted lifetimes is 10% or better. Low temper-
ature measurements were made using an Oxford Instruments
DN1704 cryostat, with helium as the inert atmosphere.
7 J.-P. Collin, I. M. Dixon, J.-P. Sauvage, J. A. G. Williams,
F. Barigelletti and L. Flamigni, J. Am. Chem. Soc., 1999, 121, 5009.
8 These trends and differences between the N6-coordinated complexes
and the cyclometallated systems have been reviewed: I. M. Dixon,
J.-P. Collin, J.-P. Sauvage, L. Flamigni, S. Encinas and F. Barigelletti,
Chem. Soc., Rev., 2000, 29, 385.
9 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
10 Examples include terpyridines, phenanthrolines and bipyridines;
representative references are, respectively: I. M. Dixon, J.-P. Collin,
J.-P. Sauvage, F. Barigelletti and L. Flamigni, Angew. Chem., Int.
Ed., 2000, 38, 1292; D. S. Tyson, J. Bialecki and F. N. Castellano,
Chem. Commun., 2000, 2355; E. C. Constable, C. E. Housecroft and
I. Poleschak, Inorg. Chem. Commun., 1999, 2, 565.
11 Bromo-substituted Ru(), Os() and Ir() complexes have been
used recently in other metal-catalysed coupling reactions; for
example, in Sonogashira couplings: P. J. Connors Jr., D. Tzalis, A. L.
Dunnick and Y. Tor, Inorg. Chem., 1998, 37, 1121; in Stille
couplings: G. R. Pabst, O. C. Pfüller and J. Sauer, Tetrahedron, 1999,
55, 8045; in Negishi couplings: Y. Q. Fang, M. I. J. Polson and G. S.
Hanan, Inorg. Chem., 2003, 42, 5; and in homo-coupling reactions
catalysed by nickel() to give dimeric products: S. Fanni, C. Di
Pietro, S. Serroni, S. Campagna and J. G. Vos, Inorg. Chem.
Commun., 2000, 3, 42; P. M. Griffiths, F. Loiseau, F. Puntoriero,
S. Serroni and S. Campagna, Chem. Commun., 2000, 2297.
12 (a) S. Chodorowski-Kimmes, M. Beley, J.-P. Collin and J.-P.
Sauvage, Tetrahedron Lett., 1996, 37, 2963; (b) E. A. Plummer, J. W.
Hofsraat and L. De Cola, J. Chem. Soc., Dalton Trans., 2003, 2080.
13 C. J. Aspley and J. A. G. Williams, New J. Chem., 2001, 25, 1136.
14 W. Goodall, K. Wild, K. J. Arm and J. A. G. Williams, J. Chem. Soc.,
Perkin Trans. 2, 2002, 1669.
15 J. P. Wibaut, J. Overfhoff and H. Geldof, Recl. Trav. Chim. Pays-Bas
Belg., 1935, 54, 807; J. P. Wibaut and F. W. Broekman, Recl. Trav.
Chim. Pays-Bas Belg., 1939, 58, 885.
16 See, for example: (a) W. Spahni and G. Calzaferri, Helv. Chim. Acta,
1984, 67, 450; (b) M. L. Turonek, P. Moore and W. Errington,
J. Chem. Soc., Dalton Trans., 2000, 441.
17 G. Albano, V. Balzani, E. C. Constable, M. Maestri and D. R.
Smith, Inorg. Chim. Acta, 1998, 277, 225.
18 J. F. Michalec, S. A. Bejune, D. G. Cuttell, G. C. Summerton,
J. A. Gertenbach, J. S. Field, R. J. Haines and D. R. McMillin,
Inorg. Chem., 2001, 40, 2193.
Crystallography
The X-ray diffraction experiment was carried out on a Bruker
3-circle diffractometer with a SMART 1K CCD area detector,
using graphite-monochromated Mo Kα radiation (λ = 0.71073
Å) and a Cryostream (Oxford Cryosystems) open-flow N2 cryo-
stat. Crystal data for L4: C24H21N3, M = 351.44, T = 120 K,
orthorhombic, space group Pbcn (no. 60), a = 8.217(1),
b = 13.359(1), c = 17.167(1) Å, U= 1884.4(3) Å3, Z = 4, Dc =
1.239 g cmϪ3, µ = 0.07 mmϪ1. A full sphere of reciprocal space
was covered by 5 sets of narrow (0.3Њ) ω scans, each set with
different ꢀ and/or 2θ angles, yielding 19602 reflections with 2θ ≤
55Њ, of which 2177 were independent (Rint = 0.050). The struc-
ture was solved by direct methods and refined by full-matrix
least squares against F 2 of all data, using SHELXTL software.
The refinement of 173 parameters converged at R = 0.046 [for
1728 reflections with F 2 ≥ σ(F 2)] and wR(F 2) = 0.119 [for all
data].
CCDC reference number 222965.
See http://www.rsc.org/suppdata/dt/b3/b313638h/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
19 For example, in 4Ј-phenylterpyridine: E. C. Constable, J. Lewis,
M. C. Liptrot and P. R. Raithby, Inorg. Chim. Acta, 1990, 178, 47;
and in related derivatives: E. C. Constable, F. K. Khan, P. R.
Raithby and V. E. Marquez, Acta Crystallogr., Sect. C, 1992, C48,
932.
20 W. Goodall and J. A. G. Williams, Chem. Commun., 2001,
2514.
We thank the University of Durham for a studentship (W.L.),
the Royal Society for a grant towards the purchase of a steady-
state fluorimeter, and the EPSRC for a Senior Research Fellow-
ship (J.A.K.H.). We are grateful to Dr. Andrew Beeby for kind
access to instrumentation for time-resolved luminescence
spectroscopy.
21 D. Roberto, F. Tessore, R. Ugo, S. Bruni, A. Manfredi and S. Quici,
Chem. Commun., 2002, 846.
22 See, for example: S. D. Cummings and R. Eisenberg, J. Am. Chem.
Soc., 1996, 118, 1949; unfortunately, the low solubility of the
complex in solvents less polar than acetonitrile, and the dearth of
appropriate solvents with polarities intermediate between those of
acetonitrile and water, prevents a thorough study of the solvato-
chromism.
23 In this and other respects, the properties of the homoleptic, amino-
substituted complexes are quite different from those of the
complexes reported here, and will be discussed, together with their
synthesis, elsewhere: W. Leslie, R. A. Poole and J. A. G. Williams,
manuscript in preparation.
24 D. M. Roundhill, Photochemistry and Photophysics of Metal
Complexes, Plenum, New York, 1994.
25 For example, Y. Kim and C. M. Lieber, Inorg. Chem., 1989, 28, 3990.
26 Based on data in ref. 4 and 7 and further unpublished results on
pyridyl-substituted complexes.
27 S. R. Meech and D. Phillips, J. Photochem., 1983, 23, 193.
28 [Ir(tpy)L1]3ϩ was reported very recently to display a lifetime of 1.2 µs
in water. This slightly smaller value compared to our measurement
of 3.6 µs, may be a result of the significantly higher concentration
employed in the earlier study (2.5 × 10Ϫ5 M) necessitated by the
nature of the experiment, namely the investigation of non-covalent
sensitisation of a cyclodextrin-appended [Ru(bpy)32ϩ] unit by the
iridium complex: J. M. Haider, R. M. Williams, L. De Cola and
Z. Pikramenou, Angew. Chem., Int. Ed., 2003, 42, 1830.
29 K. T. Potts and D. Konwar, J. Org. Chem., 1991, 56, 4815.
30 D. R. Coulson, Inorg. Synth., 1972, 13, 121.
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D a l t o n T r a n s . , 2 0 0 4 , 6 2 3 – 6 3 1
631