A new class of iridium complexes suitable for stepwise incorporation into linear assemblies: Synthesis, electrochemistry, and luminescence

Article abstract of DOI:10.1021/ic701788d

A new family of cationic iridium(III) complexes is reported that contain two cyclometalating terdentate ligands. The complex [Ir(N∧C∧N-dpyx) (N∧N∧C-phbpy)]⁺ (1) contains one N∧C∧N-coordinating ligand, cyclometalating through the central phenyl ring, and one N∧N∧C-coordinated ligand, cyclometalated at the peripheral phenyl ring [dpyxH = 1,3-di(2-pyridyl)-4,6-dimethylbenzene; phbpyH = 6-phenyl-2,2′- bipyridine]. This binding mode dictates a mutually eis arrangement of the cyclometalated carbon atoms: the complexes are thus bis-terdentate analogues of the well-known [Ir(N∧C-ppy)₂(N∧N-bpy)]⁺ family of complexes, which similarly contain a cis-C₂N₄ coordination environment. The dpyx ligand can be brominated regioselectively at the carbon atom para to the metal under mild conditions. Starting from a modified complex, [Ir(N∧C∧N-dpyx)(N∧N∧C-mtbpy-φ-Br)]⁺ (2), which incorporates a pendent bromophenyl group, a sequential cross-coupling- bromination-cross-coupling strategy can be applied for the stepwise introduction of aryl groups into the ligands, using in situ palladium-catalyzed Suzuki reactions with arylboronic acids [mtbpyH-φ-Br = 4-(p-bromophenyl)-6-(m- tolyl)bipyridine]. Dimetallic complexes 6 and 7 have similarly been prepared by a palladium-catalyzed reaction of complex 2 with 1,4-benzenediboronic acid and 4,4′-biphenyldiboronic acid, respectively. All five monometallic complexes and both dimetallic systems are luminescent in solution, emitting around 630 nm in MeCN at 298 K, with quantum yields in the range of 0.02-0.06, superior to [Ir(ppy)₂(bpy)]⁺. The luminescence, electrochemistry, and singlet-oxygen-sensitizing abilities of the new family of complexes are discussed in the context of the tris-bidentate analogues and related bis-terdentate compounds that contain a trans arrangement of cyclometalated carbon atoms.

Full text of DOI:10.1021/ic701788d

Inorg. Chem. 2008, 47, 6596-6607
A New Class of Iridium Complexes Suitable for Stepwise Incorporation
into Linear Assemblies: Synthesis, Electrochemistry, and Luminescence
Victoria L. Whittle and J. A. Gareth Williams*
Department of Chemistry, UniVersity of Durham, Durham DH1 3LE, U.K.
Received September 11, 2007
A new family of cationic iridium(III) complexes is reported that contain two cyclometalating terdentate ligands. The
complex [Ir(N∧C∧N-dpyx)(N∧N∧C-phbpy)]+ (1) contains one N∧C∧N-coordinating ligand, cyclometalating through
the central phenyl ring, and one N∧N∧C-coordinated ligand, cyclometalated at the peripheral phenyl ring [dpyxH )
1,3-di(2-pyridyl)-4,6-dimethylbenzene; phbpyH ) 6-phenyl-2,2′-bipyridine]. This binding mode dictates a mutually
cis arrangement of the cyclometalated carbon atoms: the complexes are thus bis-terdentate analogues of the
well-known [Ir(N∧C-ppy)2(N∧N-bpy)]+ family of complexes, which similarly contain a cis-C2N4 coordination environment.
The dpyx ligand can be brominated regioselectively at the carbon atom para to the metal under mild conditions.
Starting from a modified complex, [Ir(N∧C∧N-dpyx)(N∧N∧C-mtbpy-φ-Br)]+ (2), which incorporates a pendent
bromophenyl group, a sequential cross-coupling-bromination-cross-coupling strategy can be applied for the stepwise
introduction of aryl groups into the ligands, using in situ palladium-catalyzed Suzuki reactions with arylboronic
acids [mtbpyH-φ-Br ) 4-(p-bromophenyl)-6-(m-tolyl)bipyridine]. Dimetallic complexes 6 and 7 have similarly been
prepared by a palladium-catalyzed reaction of complex 2 with 1,4-benzenediboronic acid and 4,4′-biphenyldiboronic
acid, respectively. All five monometallic complexes and both dimetallic systems are luminescent in solution, emitting
around 630 nm in MeCN at 298 K, with quantum yields in the range of 0.02–0.06, superior to [Ir(ppy)2(bpy)]+. The
luminescence, electrochemistry, and singlet-oxygen-sensitizing abilities of the new family of complexes are discussed
in the context of the tris-bidentate analogues and related bis-terdentate compounds that contain a trans arrangement
of cyclometalated carbon atoms.
Introduction
those of [Ru(bpy)3]2+ and have been receiving increased
attention over the past decade.6–9 They too have been studied
as sensitizers of energy- and electron-transfer reactions.10
Although generally rather less brightly emissive than triscy-
clometalated complexes such as [Ir(ppy)3],11 their lumines-
The intense and sustained interest in [Ru(bpy)3]2+, and its
widespread application in studies of energy and electron
transfer over the past 30 years, has been due to a combination
of attractive properties.1–3 The lowest excited state has
unambiguous triplet metal-to-ligand charge-transfer (3MLCT)
character, with a lifetime sufficiently long for energy- and
electron-transfer processes to compete with radiative and
other nonradiative decay processes. Thermodynamically, the
3MLCT state is both strongly oxidizing and reducing,
favoring light-induced electron-transfer processes. Biscyclo-
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4,5
metalated iridium(III) complexes such as [Ir(ppy)2(bpy)]+
(Scheme 1) have excited-state properties quite similar to
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* To whom correspondence should be addressed. E-mail: j.a.g.williams@
durham.ac.uk.
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6596 Inorganic Chemistry, Vol. 47, No. 15, 2008
10.1021/ic701788d CCC: $40.75  2008 American Chemical Society
Published on Web 01/11/2008

New Class of Iridium Complexes
Scheme 1. Structures of Cationic Iridium Complexes Incorporating
Two Metalated Carbon Atoms in the Coordination Sphere (N4C2
Coordination)a
assemblies. These isomers are usually difficult to separate
yet may display subtly different photophysical properties.
Complexes containing two terdentate ligands, instead of three
bidentate ones, are advantageous from this point of view, in
that they are achiral with D2d symmetry or lower; moreover,
substitution at the central 4′ position of the ligands allows
linear rodlike extension.16 However, in ruthenium chemistry,
complexes such as [Ru(tpy)2]2+ are subject to severe non-
radiative decay via low-lying, thermally populated metal-
centered states, leading to very short excited-state lifetimes.17
Strategies attempting to overcome this problem have been
reviewed recently by Medlycott and Hanan18 and include
the use of modified terdentate ligands (e.g., cyclometa-
lates,19,20 triazines,21 and six-membered chelating rings22)
all of which, however, tend to lower the MLCT energies.
In the case of iridium(III), the bis-terdentate complexes
perhaps most closely analogous to [Ir(ppy)2(bpy)]+ are those
of the form [Ir(C∧N∧C-Ar′-dppy)(N∧N∧N-Ar′′-tpy)]+ re-
cently reported by Scandola et al. (Scheme 1; Ar′-dppy )
4-substituted 2,6-diphenylpyridine, Ar′′-tpy ) 4′-substituted
terpyridine).23,24 However, these compounds have a mutually
trans arrangement of the cyclometalated carbon atoms, as
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D. C. M.; Zhu, N. Y.; Cheung, K. K. Coord. Chem. ReV. 2005, 249,
1434. (b) Lo, K. K. W.; Hui, W. K.; Chung, C. K.; Tsang, K. H. K.;
Lee, T. K. M.; Li, C. K.; Lau, J. S. Y.; Ng, D. C. M. Coord. Chem.
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a
(a) The well-established tris-bidentate complexes [Ir(N∧C)2(N∧N)]+,
containing mutually cis-cyclometalated carbon atoms (top). (b) Scandola’s
bis-terdentate complexes containing dppy, with trans-disposed carbons
(middle). (c) The generic structure of [Ir(C∧N∧C)(N∧N∧C)]+-coordinated
complexes with cis-disposed carbons, the subject of the present work
(bottom).
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Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712.
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cence quantum yields are comparable to that of [Ru(bpy)3]2+,
and recent results have revealed their potential as emitters
in electroluminescent devices.12,13 Other applications include
the novel probes and sensors for biomolecules being
pioneered by Lo and co-workers14 and emerging potential
as photocatalysts for “water splitting” to generate hydrogen,
recently demonstratedbyBernhard andco-workers.15
The intrinsic chirality of such D3 and C2 complexes means
that they are normally isolated as racemic mixtures, resulting
in the formation of stereoisomers when they are linked to
form dyads or incorporated into donor-chromophore-acceptor
(18) Medlycott, E. A.; Hanan, G. S. Chem. Soc. ReV. 2006, 250, 1763.
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L.; Taylor, N. J.; Watanabe, M.; Tanaka, Y.; Loiseau, F.; Passalacqua,
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F.; Balzani, V. Inorg. Chem. 1992, 31, 3518. (c) Serroni, S.; Juris,
A.; Campagna, S.; Venturi, M.; Denti, G.; Balzani, V. J. Am. Chem.
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Arm, K. J.; Williams, J. A. G. Photochem. Photobiol. Sci. 2007, 6,
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Kritikos, M.; Eriksson, L.; Norrby, P.-O.; Bergquist, J.; Sun, L.;
Åkermark, B.; Hammarström, L. Inorg. Chem. 2005, 44, 3215. (b)
Abrahamsson, M.; Jäger, M.; Österman, T.; Eriksson, L.; Persson, P.;
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2006, 128, 12616.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6597

Whittle and Williams
Scheme 2. Synthesis of 1 via the Chloro-Bridged Dimer Ir2
opposed to the cis disposition normally observed in the
bidentate systems; density functional theory (DFT) calcula-
tions reveal clear-cut localization of the highest occupied
molecular orbital (HOMO) on the dppy and the lowest
unoccupied molecular orbital (LUMO) on the tpy ligand, and
hence an excited state of predominant ligand-to-ligand CT
(LLCT) nature, characterized by a low radiative rate constant.
Earlier, using more conjugated quinolyl ligands, Mamo et
al. isolated an interesting homoleptic iridium complex
containing two N∧N∧C-coordinating 2,6-(2′-quinolinyl)py-
ridine ligands, which emits from a 3MLCT state.25,26
Complexes containing one terdentate cyclometalating ligand
and one bidentate cyclometalating ligand, some of which are
strongly emissive, have also been recently reported by Haga
and co-workers.27 In these cases, the sixth site is occupied
by a monodentate ligand such as chloride.
In this contribution, we describe the synthesis and proper-
ties of a simple bis-terdentate iridium complex containing
cis-disposed cyclometalated carbon atoms, of the generic type
represented in Scheme 1c and analogous to the popular
[Ir(ppy)2(bpy)]+ systems. We show that complexes based on
this core structure are amenable to facile, in situ elaboration
along the principal axis of the molecule, in a stepwise
manner, using palladium-catalyzed cross-coupling reactions,
making them potentially ideal as versatile units for incor-
poration into linear photoactive assemblies.
Results and Discussion
1. Strategy. Previously, we showed that 1,3-di(2-pyridyl)-
4,6-dimethylbenzene (dpyxH) can bind symmetrically to
iridium in an N∧C∧N manner.28 Substituents at the C4 and
C6 positions were required to prevent competitive C4
cyclometalation leading to bidentate N∧C coordination. Thus,
the reaction of dpyxH with iridium trichloride generates the
chloro-bridged dimeric product [Ir(N∧C∧N-dpyx)Cl(µ-Cl)]2
(denoted Ir2; Scheme 2), from which the complex [Ir(N∧C∧N-
dpyx)(C∧N∧C-dppy)] could be prepared, containing three
cyclometalated carbon atoms in the coordination sphere of
the metal.29 Despite displaying attractive luminescence
properties (λmax ) 585 nm, φ ) 0.21, and τ ) 3.9 µs in
MeCN), this complex proved to be unstable in solution with
respect to photodissociation of one of the two mutually trans-
cyclometalated carbons, effectively ruling out further de-
rivatization. In the present work, we sought to use the same
intermediate, Ir2, to obtain a cationic complex containing
two cis-cyclometalating carbon atoms, and to explore the
possibility of elaborating it further in situ, using palladium-
catalyzed cross-coupling reactions.
2. Synthesis of Parent Complex 1. The reaction of the
dimer Ir2 with 6-phenyl-2,2′-bipyridine (phbpyH) in ethylene
glycol at 196 °C, followed by ion exchange with KPF6 and
chromatographic purification on silica, gave [Ir(N∧C∧N-
dpyx)(N∧N∧C-phbpy)](PF6) (1) as a yellow/orange solid
(Scheme 2). Complex 1 is the first example of a new family
of heteroleptic iridium complexes comprising two different
terdentate, cyclometalating ligands. Evidence for cyclo-
metalation of the 6-phenylbipyridine ligand is provided by
(23) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.; Scandola, F.
Inorg. Chem. 2004, 43, 1950.
(24) Polson, M.; Ravaglia, M.; Fracasso, S.; Garavelli, M.; Scandola, F.
Inorg. Chem. 2005, 44, 1282.
(25) Mamo, A.; Stefio, I.; Parisi, M. F.; Credi, A.; Venturi, M.; Di Pietro,
C.; Campagna, S. Inorg. Chem. 1997, 36, 5947.
1
the H NMR spectrum, which shows a very low-frequency
(26) Di Marco, G.; Lanza, M.; Mamo, A.; Stefio, I.; Di Pietro, C.; Romeo,
G.; Campagna, S. Anal. Chem. 1998, 70, 5019.
(27) (a) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno,
T.; Ishii, Y.; Sakai, K.; Haga, M.-A. Inorg. Chem. 2005, 44, 4737.
(b) Obara, S.; Itabashi, M.; Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii,
Y.; Nozaki, K.; Haga, M. A. Inorg. Chem. 2006, 45, 8907.
(28) Wilkinson, A. J.; Goeta, A. E.; Foster, C. E.; Williams, J. A. G. Inorg.
Chem. 2004, 43, 6513.
(29) Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.; Foster, C. E.;
Williams, J. A. G. Inorg. Chem. 2006, 45, 8685.
6598 Inorganic Chemistry, Vol. 47, No. 15, 2008

New Class of Iridium Complexes
Scheme 3. Treatment of the Parent Complex 1 with NBS in MeCN at
Room Temperature Leading to Bromination of Both of the
Cyclometalated Rings at the Position Para to the Metal, 1-Br2
doublet at 5.94 ppm coupled to a second doublet at 6.62
ppm, each with an integral of one. These signals are assigned
to H6′′ and H5′′, respectively (see Scheme 2 for the atom-
numbering system employed). In cyclometalated complexes
of arylpyridine ligands, low-frequency/high-field shifts are
typically characteristic of protons adjacent to the site of
cyclometalation. In the present case, the effect is augmented
by the protons being positioned above the plane of the central
aryl ring of the N∧C∧N-coordinating ligand, an environment
in which they will experience an upfield shift, owing to the
diamagnetic ring current associated with the latter. The
isolation of the cyclometalated N∧N∧C-coordination mode
of phbpy with iridium(III) contrasts with the result obtained
upon the reaction of phbpyH with [Ir(N∧N∧N-tpy)Cl3],
which, under similar reaction conditions, led only to bidentate
N∧N coordination to give [Ir(N∧N∧N-tpy)(N∧N-phbpy-
H)Cl]2+.30
3. Bromination of 1. We previously demonstrated that
biscyclometalated, tris-bidentate complexes of the type
[Ir(ppy)2(bpy)]+ are susceptible to facile electrophilic bro-
mination of the cyclometalating phenyl rings, which occurs
selectively at the positions para to the metal ion.31,32 This
allows such complexes to be activated to subsequent cross-
coupling reactions, offering a versatile, stepwise route to
multimetallic complexes. This remarkable reactivity, similar
to that observed for [Ru(bpy)2(ppy)]+ by Coudret et al.,33
can be attributed to the increase in the electron density in
the phenyl rings that accompanies cyclometalation.
conditions employed because radical benzylic bromination
normally requires elevated temperatures.
Interestingly, the ligand 1-bromo-3,5-di(2-pyridyl)-2,6-
dimethylbenzene, which is formed in situ in the above
bromination reaction, appears to be an elusive compound as
the “free” ligand. In an attempt to synthesize it, 1,3,5-
tribromo-2,6-dimethylbenzene was prepared by bromination
of 3,5-dimethylaniline, followed by deamination via the
diazonium salt, and then subjected to palladium-catalyzed
Stille cross-coupling with 2-tri-n-butylstannnylpyridine. Us-
ing 2 equiv of the stannane, the only product formed in
significant amounts was the singly reacted compound 1,3-
dibromo-5-pyridyl-2,6-dimethylbenzene. Further reaction
with more stannane proceeds more slowly and, surprisingly,
the rate of cross-coupling at the 1 position proves to be
comparable, and indeed slightly faster, than that at the 3
position. The two dipyridyl isomers that form cannot be
readily separated by chromatography or crystallization. The
tripyridyl compound, on the other hand, could be obtained
in good yield (see the Supporting Information).
With a view to developing a pathway for linear
elaboration of the new [Ir(N∧C∧N)(N∧N∧C)]+ complexes,
the reactivity of 1 with respect to bromination was
investigated. A solution of 1 in an acetonitrile solution
was treated with 1 equiv of N-bromosuccinimide (NBS)
1
at room temperature, and the evolution of the H NMR
spectrum was followed. The characteristic singlet reso-
nances of the H4′ proton of dpyx at 7.21 ppm and of the
two degenerate methyl groups at 2.97 ppm declined over
a period of several hours, accompanied by the growth of
a new methyl singlet at 3.19 ppm (data in acetone-d6).
These changes are consistent with bromination at C4′ of
dpyx (Scheme 3). The resonances of the protons of the
phbpy ligand also underwent substantial changes, sug-
gesting that bromination was occurring simultaneously at
C4′′ of phbpy. Indeed, the dibrominated complex (1-Br2;
Scheme 3) could be obtained in essentially quantitative
yield upon the addition of a second 1 equiv of NBS. While
the rate of bromination appeared to be a little faster for
the dpyx ligand, the relative rates proved to be too similar
for selective bromination of this ligand to be achieved
using just 1 equiv of NBS. Despite the widespread use of
NBS in organic chemistry for benzylic bromination, it
should be noted that no bromination of the methyl groups
of dpyx was observed. This selectivity stems from the mild
4. Synthesis of Modified Complex 2. In order to achieve
the goal of stepwise elaboration of the [Ir(N∧C∧N)(N∧N∧C)]+
system specifically along the principal axis [i.e., the
C4′dpyx-Ir-C4′
axis, which is the equivalent of the C2
phbpy
axis of bis(terpyridyl) complexes], the competitive bromi-
nation of the cyclometalating ring of phbpy must be blocked,
while a means for functionalization of this ligand along the
axis must be introduced. Complex 2 (Scheme 2) was
identified as an appropriate target for this purpose. The new
ligand 4-(p-bromophenyl)-6-(m-tolyl)bipyridine (mtbpyH-φ-
Br) was synthesized in adequate yield by a modified Krönhke
procedure, in which the initial aldol reaction of m-methyl-
acetophenone with p-bromobenzaldehyde and the subsequent
Michael reaction with 2-acetylpyridine were carried out under
solvent-free conditions (reaction scheme and experimental
(30) Bexon, A. J. S.; Williams, J. A. G. C. R. Chimie 2005, 8, 1326.
(31) Arm, K. J.; Williams, J. A. G. Chem. Commun. 2005, 230.
(32) Arm, K. J.; Williams, J. A. G. Dalton Trans. 2006, 2172.
(33) Coudret, C.; Fraysse, S.; Launay, J.-P. Chem. Commun. 1998, 663.
Fraysse, S.; Coudret, C.; Launay, J.-P. J. Am. Chem. Soc. 2003, 125,
5880.
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Whittle and Williams
Scheme 4. Sequential Cross-Coupling-Bromination-Cross-Coupling Reactions on the Modified Complex 2, Leading to Complexes 3–5, Respectively
details are provided in the Supporting Information).34 This
ligand reacts with the iridium dimer Ir2 under the same
conditions as phbpyH to generate [Ir(N∧C∧N-dpyx)(N∧N∧C-
mtbpy-φ-Br)](PF6) (2; Scheme 2), in which cyclometalation
of mtbpy-φ-Br occurs exclusively at the position para to the
methyl group. The isolated yield is high (84%), and there is
no evidence for the formation of the alternative isomer
comprising metalation ortho to the methyl group, which is
presumably disfavored on both steric and electronic grounds.
5. Sequential Cross-Coupling Reactions of Complex
2. Previously, we showed that bromo-substituted bis(terpy-
ridyl)ruthenium(II) and -iridium(III) complexes can undergo
palladium-catalyzed Suzuki cross-coupling reactions with
arylboronic acids to generate compounds with more elaborate
ligands in situ.32,35,36 When similar conditions are applied
in the present instance, complex 2 was found to undergo
smooth cross-coupling with phenylboronic acid in a dimethyl
sulfoxide solution, in the presence of sodium carbonate as
the base and using the conventional catalyst Pd(PPh3)4, to
generate 3 (Scheme 4), which incorporates a biphenyl
pendent on the N∧N∧C ligand. Complex 3 was isolated in
good yield after precipitation of the hexafluorophosphate salt
and chromatographic purification, with the main side product
being a trace of the debrominated complex.
disappearance of the H4′ resonance in the 1H NMR spectrum
and an accompanying shift of the dpyx methyl singlet from
2.98 to 3.22 ppm (in acetone-d6). The new bromo-function-
alized complex 4 (Scheme 4) is now primed for further cross-
coupling reactions but, in this case, on the N∧C∧N ligand.
We selected 3,5-dimethylbenzeneboronic acid to explore this
possibility, which readily underwent cross-coupling under
the same conditions as those used in the conversion of 2 f
3 {Na2CO3, Pd(PPh3)4, dimethyl sulfoxide, 80 °C}. Appar-
ently, the steric hindrance expected to be associated with
the two methyl groups flanking the C-Br bond of 4 does
not significantly impede the cross-coupling reaction nor
necessitate the use of stronger bases as is sometimes observed
for coupling of sterically hindered partners in purely organic
systems.38 In the resulting product 5, the main change in
1
the H NMR spectrum compared to 4 is the appearance of
three new singlets in the aromatic region. The inequivalence
of the two protons ortho to the aryl-aryl bond (i.e., Hb and
Hd in structure 5, Scheme 4) arises from the lack of rotation
about this bond due to the flanking methyl groups on the
dpyx unit. This leads to the pendent ring being positioned
in a plane that is essentially orthogonal to the plane of dpyx.
Protons Hb and Hd of dpyx are then clearly in different
environments with respect to the asymmetric N∧N∧C-
coordinating phbpy ligand: as represented in Scheme 4, Hb
is in line with the pyridyl ring of phbpy, whereas Hd is on
Treatment of a solution of complex 3 in an acetonitrile
solution with NBS at room temperature led to bromination
specifically at the C4′ position of dpyx, as revealed by the
1
the side of the phenyl ring. Variable-temperature H NMR
(34) Cave, G. W. V.; Raston, C. L. Chem. Commun. 2000, 2199.
(35) Aspley, C. J.; Williams, J. A. G. New J. Chem. 2001, 25, 1136.
(36) Leslie, W.; Batsanov, A.; Howard, J. A. K.; Williams, J. A. G. Dalton
Trans. 2004, 623.
experiments in dimethyl-d6 sulfoxide revealed no evidence
of the onset of rotation about the aryl-aryl bond, even at
363 K. In fact, the barrier to rotation in 2,6-dimethylbiphenyl
(37) Ren and co-workers have also employed in situ Suzuki and Sono-
gashira reactions recently to build up metal complexes, in their case,
on diarylformamidinate-bridged diruthenium compounds: (a) Xu, G.-
L.; Ren, T. Inorg. Chem. 2006, 45, 10449. (b) Chen, W. Z.; Ren, T.
Inorg. Chem. 2006, 45, 9175.
(38) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207.
6600 Inorganic Chemistry, Vol. 47, No. 15, 2008

New Class of Iridium Complexes
Scheme 5. Synthesis of the Dinuclear Complexes 6 and 7 by Cross-Coupling of 2 with Aryldiboronic Acids
has been estimated to exceed 100 kJ mol-1,39 which
compares to a value of only 3 kJ mol-1 for RT at 363 K.
6. Dimetallic Assemblies. The potential of the complexes
for incorporation into multimetallic assemblies has been
assessed in a preliminary study. Complex 2 was cross-
coupled with 1,4-benzenediboronic acid, using a molar ratio
of approximately 2:1 (complex-diboronic acid). The reaction
proceeded readily, under the same conditions as those
described in the preceding section for elaboration of 2 with
arylboronic acids, to give the dimetallic assembly [{Ir(d-
pyx)2}2µ-(mtbpy-φ3-mtbpy)]2+ (6), in which the constituent
iridium complexes are effectively connected by a bridge of
three phenyl rings (Scheme 5). An analogous reaction with
4,4′-biphenyldiboronic acid led to the corresponding as-
sembly [{Ir(dpyx)2}2µ-(mtbpy-φ4-mtbpy)]2+ (7), with a four-
ring bridge. Homometallic dimers (dyads) of the elements
ruthenium(II), osmium(II), and iridium(III) bound to poly-
pyridyl ligands and bridged by poly(phenylene) linkers have
been the subject of much research interest over the past 20
years.40,41 Those based on tris-bidentate systems, such as
the [{Ir(ppy)2}2µ-(bpy-φ-bpy)]2+ analogues of 6 and 7,41 are
necessarily formed, as noted in the introduction, as diaster-
eomeric mixtures of two pairs of enantiomers, whereas single
products are isolated in the present instance, which makes
use of bis-terdentate metal centers. Somewhat related dimers
of the form { [Ir(dpyx)}2µ-(tpy-φn-tpy)]4+ (n ) 0–3) have
been reported recently by Collin et al., prepared using
preformed bridging terpyridine ligands.42
electrochemical processes within the range of -2.0 to +1.5
V in an acetonitrile solution. The cyclic voltammogram of
complex 1 (Figure 1) is representative in the profile of all
of the complexes. A one-electron-reduction process, which
occurs at -1.39 V vs SCE in the case of 1 (Table 1), is
assigned to the reduction of the N∧N∧C ligand on several
grounds. First, the value is close to the reduction potential
of -1.35 V observed for the analogous tris-bidentate
complex, [Ir(ppy)2(bpy)]+, where reduction is unambiguously
centered on the bipyridine ligand. In complex 1, the reduction
is likely to be centered on the bipyridyl fragment of the
N∧N∧C ligand, with the phenyl ring in the 6 position having
only a small influence. Second, the value is mildly displaced
to more negative potentials compared to that of Scandola’s
[Ir(dppy)(tpy)]+ complexes (E° of ca. -1.27 V vs SCE),
7. Electrochemistry. All five of the new monometallic
complexes display at least two reversible or quasi-reversible
(39) Grumadas, A. J.; Poshkus, D. P.; Kiselev, A. V. J. Chem. Soc., Faraday
Trans. 1982, 78, 2013.
(40) Welter, S.; Salluce, N.; Belser, P.; Groeneveld, M.; De Cola, L. Coord.
Chem. ReV. 2005, 249, 1360.
Figure 1. Cyclic voltammogram of 1 in an acetonitrile solution at 298 K,
in the presence of 0.1 M Bu4NBF4 as the supporting electrolyte, at a scan
rate of 300 mV s-1. The insets show the first oxidation and first reduction
at a series of scan rates (50, 100, 200, 300, 400, and 500 mV s-1); the
current varies linearly with the square root of the scan rate.
(41) Lafolet, F.; Welter, S.; Popovic, Z.; De Cola, L. J. Mater. Chem. 2005,
15, 2820.
(42) Auffrant, A.; Barbieri, A.; Barigelletti, F.; Collin, J.-P.; Flamigni, L.;
Sabatini, C.; Sauvage, J.-P. Inorg. Chem. 2006, 45, 10990.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6601

Whittle and Williams
Table 1. Ground-State UV-Visible Absorption and Electrochemical Data of the Iridium Complexes in CH3CN at 298 K
complexa absorbance λmax/nm (ꢀ/M-1 cm-1 Eo1x/2/V (∆E/mV)b E1re/2d/V (∆E/mV)b
[Ir(ppy)2(bpy)]+
)
255 (46 600), 303 (22 400), 334 (9210), 370 (6160), 405 (3710), 465
(720)
283 (86 300), 323 sh (53 700), 437 (15 000), 482 sh (10 900), 514
(9200)
240 (24 500), 265 (22 100), 294 sh (16 600), 367 (5360), 411 (5490),
479 (640)
232 (61 700), 242 (61 600), 284 (65 150), 372 (17 870), 411 sh
(12 100), 479 (1800)
1.28c
-1.35c
d
[Ir(tdppy)(tpy-φ-Br)]+
[Ir(dpyx)(phbpy)]+ (1)
1.08 (90)
1.21 (83)
1.22 (80)
-1.24 (95)
-1.39 (86)
-1.15 (100)
[Ir(dpyx)(mtbpy-φ-Br)]+ (2)
[Ir(dpyx)(mtbpy-φ-Ph)]+ (3)
[Ir(Brdpyx)(mtbpy-φ-Ph)]+ (4)
232 (65 300), 293 (60 900), 375 (23 200), 412 sh (13 200), 479 (1940)
235 (58 000), 286 (50 700), 376 (16 360), 425 (8160), 483 (1600)
0.96 (90)
0.94 (100)
0.89 (90)
-1.54 (80)
-1.82 (100)
-1.66 (90)
[Ir(Me2-C6H3-dpyx)(mtbpy-φ-Ph)]+ (5) 229 (149 000), 273 sh (122 000), 293 (136 000), 375 (47 500), 420
(23 400), 481 (4160)
[{Ir(dpyx)2}2µ-(mtbpy-φ3-mtbpy)]2+ (6) 233 (86 300), 275 (63 500), 292 (67 400), 320 (64 600), 379 (51 800),
479 (32 500)
0.84 (100)
-1.85e (100)
[{Ir(dpyx)2}2µ-(mtbpy-φ4-mtbpy)]2+ (7) 232 (71 100), 287 (74 100), 320 (77 000), 377 (61 000), 479 (33 380)
1.01 (140)
-1.72e (130)
a Hexafluorophosphate salts in each case. b Using Bu4NPF6 (0.1 M) as the supporting electrolyte; values are at scan rate 300 mV s-1 (except for 5, at 50
mV s-1) and are reported relative to SCE, measured using ferrocene as the standard (Eo1/x2 ) 0.42 V vs SCE). c In DMF solution, data from ref 5a. d Data
from refs 23 and 24. e Reduction waves were poorly defined for the dimetallic systems, and a more detailed study was hampered by their poor solubility in
MeCN.
where a clear-cut reduction on the terpyridine ligand is
assigned. That a change from bipyridine to terpyridine has
a small stabilizing influence on the reduction is well
established, for example, from the reduction potentials of
[Ir(bpy)3]3+ and [Ir(tpy)2]3+ (E° ) -1.0 and -0.77 V vs
SCE, respectively43,44). Finally, DFT studies carried out using
the methods described previously29 reveal that the LUMO
is almost exclusively localized on the bipyridyl fragment of
the N∧N∧C ligand (see the Supporting Information for plots
of the frontier orbitals).
At positive potentials, there is a reversible one-electron
oxidation, occurring at 1.21 V for 1, which is similar to the
value of 1.28 V observed in the tris-bidentate analogue
[Ir(ppy)2(bpy)]+. As has been noted elsewhere,6,23,24,45 the
strong covalent character of the Ir-C bonds in the cyclo-
metalated complexes means that the oxidation wave cannot
be assigned simply to metal-centered oxidation, and indeed
the DFT calculations confirm that the HOMO is delocalized
over the metal and the cyclometalating ring of the N∧C∧N
ligand. The values of 1.21 and 1.28 V for these complexes
Figure 2. Absorption spectra of the new [Ir(N∧C∧N)(N∧N∧C)]+ iridium
containing cis-disposed carbon atoms are somewhat higher
than the values observed for Scandola’s series of [Ir(dp-
py)(tpy)]+ complexes in which there is a trans arrangement
of the two carbon atoms. This is in line with studies of the
fac and mer isomers of [Ir(ppy)3], which show that the mer
isomers, having a trans disposition of two of the carbon
atoms, are more readily oxidized than the fac isomers by
about 0.1 V.
The introduction of the pendent p-bromophenyl group onto
the N∧N∧C ligand is seen to facilitate reduction by 240 mV,
which can be attributed to a net electron-withdrawing effect
and more extended conjugation introduced into the N∧N∧C
complexes 1–5 in an acetonitrile solution at 298 K, normalized at 411 nm
to aid comparison.
work. This is also manifest in the decreased oxidation
ox
potential of this complex, with E being shifted by 250 mV
1/2
to less positive potential compared to 1. Meanwhile, sub-
stituents on the N∧C∧N ligand also stabilize slightly the
oxidation while destabilizing the reduction (Table 1).
8. Ground-State UV-Visible Absorption. The absorp-
tion spectra of all of the complexes in an acetonitrile solution
at room temperature show very intense broad bands in the
region between 220 and 330 nm (Figure 2 and Table 1). By
comparison with the literature data for biscyclometalated
cationic iridium complexes, these are assigned to spin-
allowed 1π–π* transitions of the ligands. A noteable differ-
ence between the derivative 2 and parent complex 1 is the
appearance of a relatively sharp and well-defined band
centered at 285 nm, which must be associated with the
introduction of the pendent aryl unit. This feature persists
in complexes 3–5. A similar band centered around 280 nm
was previously observed in [Ir(tpy)2]3+ complexes bearing
ligand, upon which the LUMO is localized. On the other
red
hand, further extension to a biphenyl pendent shifts E to
1/2
more negative potentials, so the net effect in this case must
be one of electron donation through the π-bonding frame-
(43) Kahl, J. A.; Hanck, K. W.; DeArmond, K. J. Phys. Chem. 1978, 82,
540.
(44) Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.;
Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009.
(45) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
6602 Inorganic Chemistry, Vol. 47, No. 15, 2008

New Class of Iridium Complexes
biphenyl pendents.36 A series of bands in the region 330–450
nm are attributable to spin-allowed CT transitions, the lowest
of which is well-resolved in 1 (411 nm) but less so in 2–5
because of increased absorption around 375 nm. In each
complex, there is also a weak, low-energy band ca. 480 nm,
likely to be due to direct absorption to the lowest-energy
triplet state facilitated by the high spin-orbit coupling
constant of the iridium center (ꢀ ) 3909 cm-1 46). Both of
the latter bands (i.e., those at ca. 411 and 480 nm) are slightly
red-shifted in complexes 4 and 5 but not in 2 and 3,
suggesting that their energy is influenced primarily by the
N∧C∧N ligand substitution.
While the positions of the bands are roughly comparable
to those displayed by [Ir(ppy)2(bpy)]+ (Table 1), the spectra
are quite different from those of the trans-cyclometalated
complexes [Ir(C∧N∧C-Ar′-dppy)(N∧N∧N-Ar′′-tpy)]+, which
show intense bands extending to much lower energy23,24
(longest wavelength band around 514 nm with ꢀ ∼ 9000;
Table 1). In that case, DFT calculations revealed a HOMO
comprising both metal and dppy character and a tpy-localized
LUMO as in the present instance, where the LUMO is
localized on the bpy moiety of the N∧N∧C ligand as
described above. The lower energy of the excited state in
the dppy-tpy complexes can be attributed to the cumulative
effect of the lowering in energy of the acceptor ligand π*
orbitals upon increased delocalization across all three rings
of the tpy ligand, compared to just two in 1, and of the
destabilized HOMO in the trans-configured complexes as
discussed in the preceding section.
The absorption spectra of the dinuclear complexes are quite
different from those of the monomers, with greatly enhanced
absorption in the region 330–400 nm, which swamps the
previously discernible band around 400 nm. This is no doubt
due to the intense spin-allowed 1π–π* transitions associated
with the oligo(phenylene) units that are anticipated in this
region.47
9. Emission. The new complexes are luminescent in
solution at room temperature, λmax ∼ 630 nm, displaying a
single, structureless emission band in each case, typical of
phosphorescence from excited states of primarily CT char-
acter. The excitation and emission spectra (at 298 and 77
K) of the parent complex 1 are shown in Figure 3. On the
basis of the earlier discussion of the frontier orbitals, the
emissive state is probably best regarded as having mixed
3d(Ir)/π(N∧C∧N) f π*(N∧N∧C) character (3MLCT/LLCT).
The large blue shift in the emission maximum (2600 cm-1)
on cooling to 77 K in a rigid glass is typical of CT transitions,
in which a considerable degree of molecular reorganization
accompanies the formation of the excited state. The emission
energy of 1 is similar to that of [Ir(ppy)2(bpy)]+ (Table 2), which
also displays a similarly large thermally induced Stokes shift
(2260 cm-1).48 On the other hand, the emission energy is
substantially higher than that for the trans-C-configured complex
Figure 3. Emission and excitation spectra (solid lines) and absorption
spectrum (dotted line, offset for clarity) of 1 in MeCN at 298 K (λex ) 410
nm and λem ) 600 nm, respectively; bandpasses ) 3 nm). The emission
spectrum at 77 K in an EPA glass is also shown (dashed line).
[Ir(tdppy)(tpy-φ-Br)]+ (λmax ) 690 nm23), mirroring the obser-
vations made for the absorption spectra.
The aryl substituents have a modest influence on the
emission maximum, with a total variation in energy across
the series of five complexes of 470 cm-1 (Figure 4 and Table
2). In principle, for an excited state of purely CT character,
a linear correlation is anticipated between the energy of the
CT excited state and the difference between the ground-state
first-oxidation and first-reduction potentials (∆E1/2) because
the formation of the excited state formally involves oxidation
of the HOMO and reduction of the LUMO.1,49 Using the
trend in energy of the emission maxima as a guide to the
trend in 0–0 energy of the excited state (an approximation
that holds if the degree of reorganization is similar for each
ox
red
complex), this quantity can be plotted against E - E , as
1/2
1/2
shown in the inset to Figure 4. It can be seen that there is a
close linear correlation for complexes 1–4, supporting the
CT assignment, but that complex 5 is anomalous in that it
displays an emission energy lower than expected on the basis
of the redox potentials. This is most probably due to the
fact that the E1/2 values, which relate to the ground state,
underestimate the influence of the N∧C∧N aryl substituent
in the excited state. In the ground state, the pendent aryl
group will be essentially orthogonal to the N∧C∧N ligand
owing to the flanking methyl groups, whereas the barrier to
planarization is likely to be at least partially overcome upon
formation of the excited state.
The luminescence lifetimes, τ, are in the range of 100–200
ns, which are reduced by a factor of 2–3 in an air-equilibrated
solution. The luminescence quantum yield of the parent
complex 1 is 0.023, similar to the value of 0.018 found for
[Ir(ppy)2(bpy)]+.48 The quantum yield increases slightly upon
introduction of the pendent p-bromophenyl ring into the
N∧N∧C ligand and further to 0.063 for the biphenyl-appended
(46) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006.
(47) Grimme, J.; Kreyenschmidt, M.; Uckert, F.; Mullen, K.; Scherf, U.
AdV. Mater. 1995, 7, 292.
(48) Glusac, K. D.; Jiang, S.; Schanze, K. S. Chem. Commun. 2002, 2504.
(49) Cummings, S.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1040.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6603

Whittle and Williams
Table 2. Luminescence Data for the Iridium Complexes, in CH3CN Solution at 298 K except Where Stated Otherwise
emission
at 77 Kf
b
Φlum × 102
degassed
(aerated)
τ/nsc
degassed
(aerated)
emission
kOQ /10
kr/105
∑knr/106
e
9
2
complexa
λ
max/nm
M-1 s-1
s-1
s-1
λ
max/nm
τ/µsc
d
e
[Ir(ppy)2(bpy)]+
610g
690
632
639
636
625
644
632
633
1.8h
3.5
180h
1.0h
0.19
1.9
2.2
4.2
2.5
2.9
1.9
1.9
5.4h
0.58
8.1
7.5
6.3
5.0
9.7
7.0
6.8
520i
j
[Ir(tdppy)(tpy-φ-Br)]+
[Ir(dpyx)(phbpy)]+ (1)
1680 (198)
120 (49)
130 (53)
150 (57)
190 (63)
100 (47)
140 (58)
144 (58)
2.3
6.4
5.9
5.7
5.6
5.9
5.3
5.4
662k
2.3 (0.81)
2.8 (1.1)
6.3 (2.5)
4.8 (1.6)
2.9 (1.2)
2.7
544, 576
556, 592
553, 590
544, 582
556, 593
565, 596
577, 613
3.6
3.9
4.0
4.3
[Ir(dpyx)(mtbpy-φ-Br)]+ (2)
[Ir(dpyx)(mtbpy-φ-Ph)]+ (3)
[Ir(Brdpyx)(mtbpy-φ-Ph)]+ (4)
[Ir(Me2-C6H3-dpyx)(mtbpy-φ-Ph)]+ (5)
[{Ir(dpyx)2}2µ-(mtbpy-φ3-mtbpy)]2+ (6)
[{Ir(dpyx)2}2µ-(mtbpy-φ4-mtbpy)]2+ (7)
3.5
3.1, 68l
2.7
2.9, 72l
a Hexafluorophosphate salts in each case. b Luminescence quantum yield measured using [Ru(bpy)3]Cl2(aq) as the standard. c Lifetime registered at the
emission maximum following excitation at 374 nm. d Bimolecular rate constant for quenching by O2, estimated from τ values in degassed and aerated
solutions. e Radiative (kr) and nonradiative (∑knr) rate constants calculated from τ and φ values at 298 K. f In EPA glass. g In MeOH.5b h In degassed THF.48
i In 2-MeTHF.43 j Data from refs 23 and 24. k In 4:1 MeOH/EtOH. l Values estimated by fitting separately the short and long components of the decay.
Figure 5. Absorption and room-temperature emission spectra of the
dimetallic complex 6 (solid lines) in MeCN (λex ) 380 nm). The emission
Figure 4. Emission spectra of iridium complexes 1–5 in a degassed
spectra of 6 and 7 in an EPA glass at 77 K are also shown (dashed and
acetonitrile solution at 298 K (λex ) 410 nm; excitation and emission
dotted lines, respectively).
bandpasses ) 3 nm). Spectra shown are corrected and normalized to the
same intensity; corresponding quantum yields are given in Table 2. Inset:
plot of the emission energy (estimated from λmax) as a function of the
difference between the first oxidation and first reduction potentials.
contribution of the biscyclometalated dppy to the HOMO.
In the present case, there is probably less LLCT character
but rather a larger contribution of the metal to the HOMO,
which is a key factor in determining the extent to which the
formally forbidden triplet-to-singlet emissive transition is
promoted.
The dinuclear complexes 6 and 7 are similarly luminescent
in solution, displaying emission profiles, lifetimes, and
quantum yields that are very similar to those of the parent
complex 1 (Figure 5 and Table 2). At 77 K, however, some
differences emerge. The emission is displaced to lower
complex 3. Inspection of the radiative (kr) and nonradiative
(∑knr) decay rate constants (estimates of which are listed in
Table 2, calculated from the experimentally determined τ
and Φ values) reveals that the increase in the luminescence
efficiency in going from complex 1 to 3 is due primarily to
an increase in the radiative rate constant. In contrast, the
introduction of an aryl substituent into the N∧C∧N ligand
(complex 5) has a mildly detrimental effect owing to a small
decrease in kr and an increase in ∑knr.
Comparison with the trans-cyclometalated [Ir(Ar′-dpp-
y)(Ar′′-tpy)]+ complexes is instructive. Scandola et al.
reported that they emit at substantially lower energy, λmax
∼ 690 nm, and were found to have unusually low radiative
rate constants, ∼2 × 104 s-1,23,24 1 order of magnitude lower
than the values commonly found for cyclometalated iridium
complexes, including those of the present work (Table 2).
The low kr was interpreted in terms of a high degree of LLCT
character in the excited state, arising from the major
energy, with the effect being larger for the tetraphenyl bridge
em
than the triphenyl: λ ) 544, 553, 565, and 577 nm for 1,
max
3, 6, and 7, respectively. Moreover, the emission decay
becomes biexponential for the dinuclear complexes (Figure
6). While the shorter component has a lifetime of around 3
µs, similar to that for all of the monometallic complexes and
hence attributable to an excited state of similar CT character,
the longer component (around 70 µs) is much longer than
anticipated for an excited state with significant metal
character. We suspect that this emission emanates from the
6604 Inorganic Chemistry, Vol. 47, No. 15, 2008

New Class of Iridium Complexes
with the previously reported instability of tricationic irid-
ium(III) complexes, such as [Ir(bpy)3]3+, to oxidative pho-
tochemical processes.52
1
Previously, Thompson et al. have recorded O2 quantum
yields for a series of charge-neutral [Ir(N∧C)2(L∧X)] com-
plexes, which were on the order of 0.6–1.0 in benzene.53
Because cationic [Ir(N∧C)2(N∧N)]+ complexes have appar-
1
ently not been investigated, we measured the O2 quantum
yield for [Ir(ppy)2(bpy)]+, which gave a value of 0.50, and
for the trans-disposed carbon complex [Ir(dppy)(tpy-φ-
CO2Et)]+, for which a value of 0.60 was recorded. Thus,
the values of the cationic complexes (0.4–0.6) are apparently
consistently a little lower than those for the charge-neutral
complexes.
Concluding Remarks
The new [Ir(N∧C∧N)(N∧N∧C)]+ complexes are readily
synthesized and display electrochemical and photophysical
properties that are closely comparable to those of the well-
established class of tris-bidentate complexes of which
[Ir(N∧C-ppy)2(N∧N-bpy)]+ is the archetypal example. This
contrasts with the chemistry of ruthenium(II), where the
attractive excited-state properties of[Ru(bpy)3]2+ are largely
lost upon going to the bis-terdentate analogue [Ru(tpy)2]2+,
owing to increased nonradiative decay associated with low-
lying metal-centered states. The achiral nature of the bis-
terdentate iridium(III) systems avoids the problems associated
with diastereoisomer formation that are encountered for the
tris-bidentate complexes. Moreover, the present work reveals
that the new complexes are particularly amenable to linear
stepwise elaboration along the principal axis of the molecule,
thanks to the ease with which the N∧C∧N-coordinated ligand
can be brominated, activating the molecule to cross-coupling
reactions. By using appropriately substituted partners (e.g.,
boronic acids, stannanes, acetylides), incorporation of this
attractive iridium core into larger conjugated assemblies will
clearly be feasible.
Figure 6. Decay kinetics of the dimetallic complex 7 in an EPA glass at
77 K registered at 600 nm (λex ) 374 nm). The estimated lifetime of the
short component is 2.9 µs (blue fitted line to experimental data), while a
value of 72 µs is obtained for the long component (red line and
corresponding data, inset).
triplet π–π* excited state associated with the bridging ligand.
CT states are destabilized in a rigid matrix at 77 K, and so
it is plausible that the CT and π–π* states have similar energy
at 77 K, leading to the two components observed, even
though the former is unequivocally the lower at room
temperature, which then ensures exclusively CT emission
and monoexponential decay kinetics. A somewhat similar
observation has been made by De Cola et al. for [{Ir(F2-
ppy)2}2µ-(bpy-φn-bpy)]2+ dinuclear complexes (φ ) 3 and
4).41 In that case, biexponential decay was observed even at
room temperature. The difference probably arises from the
fact that the CT states associated with the Ir(F2ppy)(bpy)+
units are higher in energy than those of the corresponding
monometallic units in the present study.
10. Sensitization of Singlet Oxygen. The triplet states of
many transition-metal complexes can act as sensitizers of
Experimental Section
singlet oxygen, O2 ∆g, by energy transfer to the 3Σg- ground
1H and 13C NMR spectra, including NOESY and COSY, were
recorded on a Varian 500 MHz instrument. Chemical shifts (δ)
are in parts per million, referenced to residual protiosolvent
resonances, and coupling constants are in hertz. Electrospray
ionization mass spectra were acquired on a time-of-flight Micromass
LCT spectrometer. All solvents used in preparative work were at
least Analar grade, and water was purified using the Purite system.
Solvents used for optical spectroscopy were high-performance liquid
chromatography grade. 1,3-Di(2-pyridyl)-4,6-dimethylbenzene
(dpyxH),29 6-phenyl-2,2′-bipyridine (phbpyH),54 and bis(µ-chlo-
ro)bis[1,3-di(2-pyridyl)-4,6-dimethylbenzene-N,C2′,N-iridium chlo-
ride] (Ir2)28 were prepared as described previously. The synthesis
and characterization of mtbpyH-φ-Br are described in the Support-
ing Information.
1
state of molecular oxygen. The quenching of the lumines-
cence of 1 by dissolved oxygen was accompanied by the
appearance of emission in the near-IR attributable to the
formation and subsequent radiative decay of 1O2, which emits
∼1270 nm. The quantum yield of singlet oxygen formation
by complex 1 under irradiation at 355 nm was determined
using perinaphthenone as a standard,50 giving a value of 0.4
( 0.05. No significant quenching of singlet oxygen by the
complex was observed: the rate of decay of the 1O2 emission
in the near-IR was 1.67 × 104 s-1, essentially identical with
the literature value of 1.64 × 104 s-1 in MeCN following
production via standard sensitizers.51 This observation is also
consistent with the observed photostability of the complex
under irradiation in air-equilibrated solution and contrasts
[Ir(N∧C∧N-dpyx)(N∧N∧C-phbpy)](PF6) (1). A suspension of
Ir2 (50 mg, 0.048 mmol), phbpyH (25 mg, 0.11 mmol), and silver(I)
(52) Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am. Chem. Soc. 1977,
99, 3547.
(50) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. J. Photochem.
Photobiol. A 1994, 79, 11.
(53) Gao, R.; Ho, D. G.; Hernandez, B.; Selke, M.; Murphy, D.; Djurovich,
P. I.; Thompson, M. E. J. Am. Chem. Soc. 2002, 124, 14828.
(54) Kröhnke, F. Synthesis 1976, 1.
(51) Clennan, E. L.; Noe, L. J.; Wen, T.; Szneler, E. J. Org. Chem. 1989,
54, 3581.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6605

Whittle and Williams
trifluoromethanesulfonate (58 mg, 0.23 mmol) in ethylene glycol
(4 mL) was stirred at room temperature under N2 for 1 h and then
heated to 196 °C for a further 3 h. After cooling to room
temperature, the precipitated AgCl was collected by centrifuge and
washed with acetonitrile (2 × 2 mL). The acetonitrile and ethylene
glycol solutions were combined and added to a saturated aqueous
KPF6 solution (20 mL). The resulting yellow/orange precipitate was
collected by centrifuge, washed with water (3 × 5 mL), and dried
under vacuum. Purification by flash column chromatography (silica,
CH2Cl2/MeOH, gradient elution from 100/0 to 99/1) gave the
three freeze-pump-thaw cycles. Tetrakis(triphenylphosphine)-
palladium(0) (4.4 mg, 0.0038 mmol) was added under a positive
pressure of N2. The resultant orange suspension was stirred at 80
°C under a N2 atmosphere for 18 h. The mixture was cooled to
room temperature, diluted with acetonitrile (1 mL), and filtered to
remove the insoluble palladium black that had formed. The filtrate
was added to a saturated aqueous KPF6 solution (25 mL); the
resultant orange precipitate was collected by a centrifuge, washed
with water (10 × 3 mL), and dried under vacuum. Purification by
flash column chromatography (silica, CH2Cl2/MeOH, gradient
elution from 100:0 to 99:1) gave the desired product as an orange
solid (49 mg, 72%). 1H NMR (acetone-d6, 500 MHz): δ 9.10 (1H,
1
desired product as a yellow/orange solid (61 mg, 78%). H NMR
3
(acetone-d6, 500 MHz): δ 8.72 (1H, d, J ) 8.0, H3-NNC), 8.70
(1H, d, 3J ) 8.0, H3′-NNC), 8.54 (1H, d, 3J ) 8.5, H5′-NNC), 8.38
(1H, t, 3J ) 8.0, H4′-NNC), 8.31 (2H, d, 3J ) 8.5, H3-NCN), 8.10
d, J ) 1.0, H3′-NNC), 9.00 (1H, dd, J ) 13.5, J ) 7.5, H3′′-
4
3
3
4
NNC), 8.93 (1H, d, J ) 1.0, H5′-NNC), 8.35 (4H, m, Hb- and
3
4
3
(1H, td, J ) 8.0, J ) 1.5, H4-NNC), 7.86 (1H, d, J ) 7.5, H3′′-
H3-NCN), 8.14 (1H, td, 3J ) 7.5, 4J ) 1.5, H4-NNC), 8.02 (2H, d,
3J ) 8.0, Ha), 7.97 (1H, s, H3-NNC), 7.84 (4H, m, H4-NCN, Hb′),
7.79 (2H, d, 3J ) 6.0, H6-NCN), 7.68 (1H, d, 3J ) 4.5, H6-NNC),
7.56 (2H, t, 3J ) 7.5, Ha′), 7.46 (1H, t, 3J ) 7.5, Hc′), 7.42 (1H, dd,
3J ) 7.0, 3J ) 5.5, H5-NNC), 7.22 (1H, s, H4′-NCN), 6.96 (2H, td,
NNC), 7.83 (2H, td, 3J ) 8.0, 4J ) 1.5, H4-NCN), 7.68 (2H, d, 3J
) 5.5, H6-NCN), 7.65 (1H, d, J ) 5.0, H6-NNC), 7.39 (1H, t, J
3
3
) 7.0, H5-NNC), 7.21 (1H, s, H4′-NCN), 6.95 (2H, t, J ) 7.0,
3
H5-NCN), 6.85 (1H, t, 3J ) 7.5, H4′′-NNC), 6.62 (1H, td, 3J ) 7.5,
3J ) 1.0, H5′′-NNC), 5.94 (1H, d, J ) 7.5, H6′′-NNC), 2.97 (6H,
3J ) 6.5, J ) 1.0, H5′′-NCN), 6.50 (1H, d, J ) 7.5, H5-NNC),
5.84 (1H, d, 3J ) 7.5, H6′′-NNC), 2.98 (6H, s, Me-NCN), 2.17 (3H,
s, Me-NNC). MS (MALDI, DCTB matrix): m/z 849.4 [M]+. HRMS
(ES+). Calcd for 191IrC47H36N4: m/z 847.25405. Found: m/z
847.25409 [M]+. Elem anal. Calcd for C47H36F6IrN4P: C, 56.9; H,
3.7; N, 5.6. Found: C, 56.6; H, 3.7; N, 5.4. One spot by TLC (silica)
Rf ) 0.51 in dichloromethane/methanol (90:10).
3
4
3
s, Me). 13C NMR (acetone-d6, 500 MHz): δ 150.9 (C6-NCN), 138.4
(C4-NCN), 130.0 (C4′-NCN), 123.7 (C5-NCN), 122.6 (C3-NCN),
120.4 (C3′). MS (ES+): m/z 681.3 [M]+. HRMS (ES+). Found: m/z
681.17581 for [M]+. Calcd for 191IrC34H26N4: m/z 681.17580. Elem
anal. Calcd for C34H26F6IrN4P: C, 49.4; H, 3.2; N, 6.8. Found: C,
48.8; H, 3.4; N, 6.7. One spot by thin-layer chromatography (TLC;
silica): Rf ) 0.43 in dichloromethane/methanol (90:10).
[Ir(N∧C∧N-Brdpyx)(N∧N∧C-mtbpy-O-Ph)](PF6) (4). A mixture
of iridium complex 3 (29 mg, 0.029 mmol) and N-bromosuccin-
imide (NBS; 5.2 g, 0.029 mmol) was stirred in acetonitrile (2 mL)
for 20 h, during which some precipiation of an orange solid was
observed. Additional acetonitrile (2 mL) was added to fully dissolve
the product. The solution was then added to a saturated aqueous
KPF6 solution (20 mL), giving a precipitate, which was collected
by centrifuge, washed with water (3 × 5 mL), and dried under
vacuum to give the product as an orange solid (31 mg, 100%). 1H
NMR (acetone-d6, 500 MHz): δ 9.13 (1H, s, H3′-NNC), 9.04 (1H,
1-Br2. 1H NMR and mass spectrometric data for the dibrominated
complex 1-Br2 are recorded in the Supporting Information.
[Ir(N∧C∧N-dpyx)(N∧N∧C-mtbpy-O-Br)](PF6) (2). A suspension
of Ir2 (52 mg, 0.050 mmol), mtbpyH-φ-Br (41 mg, 0.10 mmol),
and silver(I) trifluoromethanesulfonate (80 mg, 0.31 mmol) in
ethylene glycol (3 mL) was stirred at room temperature for 1 h
under a N2 atmosphere. The suspension was then stirred and heated
to 196 °C under N2 for a further 2 h. After cooling to room
temperature, an orange precipitate formed. The solids were collected
by centrifuge, and an orange product was extracted into acetonitrile
(5 mL), leaving a gray residue of AgCl. The acetonitrile and
ethylene glycol solutions were combined and reduced in volume,
before being added to a saturated aqueous KPF6 solution (25 mL).
The resulting orange precipitate was collected by centrifuge, washed
with water (3 × 5 mL), and dried under vacuum. Purification by
flash column chromatography (silica, CH2Cl2/MeOH, gradient
elution from 100:0 to 98.75:1.25) gave the desired product as an
3
3
t, J ) 8.5, H3′′-NNC), 8.96 (1H, s, H5′-NNC), 8.46 (2H, d, J )
8.5, H3-NCN), 8.38 (2H, d, J ) 8.5, Hb), 8.17 (1H, t, J ) 7.0,
3
3
H4-NNC), 8.05 (2H, d, J ) 8.0, Ha), 8.00 (1H, s, H3-NNC), 7.92
3
(2H, t, J ) 6.5, H4-NCN), 7.87 (4H, m, Hb′- and H6-NCN), 7.77
3
(1H, d, J ) 5.5, H6-NNC), 7.58 (2H, t, J ) 7.5, Ha′), 7.49 (1H,
3
3
3
3
3
t, J ) 7.5, Hc′), 7.43 (1H, t, J ) 7.5, H5′′-NNC), 7.05 (2H, t, J
) 7.5, H5-NCN), 6.53 (1H, d, 3J ) 7.0, H5-NNC), 5.86 (1H, d, 3J
) 8.0, H6′′-NNC), 3.22 (6H, s, Me-NCN), 2.20 (3H, s, Me-NNC).
MS (MALDI, DCTB matrix): m/z ) 927.3 [M]+. HRMS (ES+).
Calcd for 191IrC47H35N479Br: m/z 925.16456. Found: m/z 925.16344.
Elem anal. Calcd for C47H35BrF6IrN4P: C, 52.8; H, 3.3; N, 5.2.
Found: C, 52.3; H, 3.6; N, 4.8. One spot by TLC (silica) Rf ) 0.53
in dichloromethane/methanol (90:10).
1
orange solid (84 mg, 84%). H NMR (acetone-d6, 500 MHz): δ
4
3
9.05 (1H, d, J ) 2.0, H3′-NNC), 8.95 (1H, d, J ) 8.0, H-NNC),
8.88 (1H, d, 4J ) 1.0, H5′-NNC), 8.32 (2H, d, 3J ) 8.0, H3-NCN),
3
3
4
8.21 (2H, d, J ) 8.5, Hb), 8.13 (1H, td, J ) 8.0, J ) 1.5, H4-
NNC), 7.90 (3H, m, H3′′-NNC, Ha), 7.83 (2H, td, J ) 8.0, J )
3
4
1.5, H4-NCN), 7.76 (2H, dd, J ) 6.0, J ) 1.0, H6-NCN), 7.68
[Ir(N∧C∧N-Me2C6H3-dpyx)(N∧N∧C-mtbpy-O-Ph)](PF6) (5). A
mixture of the brominated iridium complex 4 (35 mg, 0.035 mmol),
3,5-dimethylphenylboronic acid (10 mg, 0.070 mmol), and sodium
carbonate (12 mg, 0.11 mmol in 100 µL water) in dimethyl
sulfoxide (5 mL) was degassed by three freeze-pump-thaw cycles.
Tetrakis(triphenylphosphine)palladium(0) (4.9 mg, 0.0042 mmol)
was added under a positive pressure of N2. The resultant orange
suspension was stirred at 80 °C under N2 for 18 h. The dimethyl
sulfoxide solution was cooled to room temperature, diluted with
acetonitrile (2 mL), and filtered to remove the insoluble palladium
black that had formed. The filtrate was added to a saturated aqueous
KPF6 solution (25 mL); the resultant orange precipitate was
collected by centrifuge, washed with water (3 × 5 mL), and dried
under vacuum. Purification by flash column chromatography (silica,
3
4
(1H, d, J ) 5.0, H6-NNC), 7.41 (1H, ddd, J ) 8.5, J ) 7.0, J
3
3
3
4
) 1.0, H5′′-NNC), 7.21 (1H, s, H4′-NCN), 6.94 (2H, td, J ) 7.5,
3
4J ) 1.0, H5-NCN), 6.49 (1H, d, 3J ) 7.5, H5-NNC), 5.83 (1H, d,
3J ) 8.0, H6′′-NNC), 2.97 (6H, s, Me-NCN), 2.16 (3H, s, Me-NNC).
MS (MALDI, DCTB matrix): m/z 851.3 [M]+. HRMS (ES+). Calcd
for 191IrC41H31N479Br: m/z 849.13326. Found: m/z 849.13533 [M]+.
Elem anal. Calcd for C41H31BrF6IrN4P: C, 49.5; H, 3.1; N, 5.6.
Found: C, 49.0; H, 3.5; N, 6.3. One spot by TLC (silica) Rf ) 0.65
in dichloromethane/methanol (90:10).
[Ir(N∧C∧N-dpyx)(N∧N∧C-mtbpy-O-Ph)](PF6) (3). A mixture
of iridium complex 2 (32 mg, 0.032 mmol), phenylboronic acid
(7.8 mg, 0.064 mmol), and sodium carbonate (10 mg, 0.096 mmol
in 100 µL water) in dimethyl sulfoxide (5 mL) was degassed by
6606 Inorganic Chemistry, Vol. 47, No. 15, 2008

New Class of Iridium Complexes
CH2Cl2/MeOH, gradient elution from 100:0 to 99.1:0.9) gave the
desired product as an orange solid (20 mg, 53%). 1H NMR (acetone-
d6, 500 MHz): δ 9.12 (1H, d, 4J ) 1.0, H3′-NNC), 9.02 (1H, d, 3J
isotope patterns match the theoretical simulation (see the Supporting
Information). One spot by TLC (silica) Rf ) 0.57 in an acetonitrile/
water/saturated KNO3 solution (90:9.8:0.2).
4
) 8.5, H3′′-NNC), 8.94 (1H, d, J ) 1.0, H5′-NNC), 8.37 (4H, m,
Photophysical Measurements. Absorption spectra were mea-
sured on a Biotek Instruments XS spectrometer, using quartz
cuvettes of 1 cm path length. Steady-state luminescence spectra
were measured using a Jobin Yvon FluoroMax-2 spectrofluorimeter,
fitted with a red-sensitive Hamamatsu R928 photomultiplier tube
(PMT); the spectra shown are corrected for the wavelength
dependence of the detector, and the quoted emission maxima refer
to the values after correction. Samples for emission measurements
were contained within quartz cuvettes of 1 cm path length modified
with appropriate glassware to allow connection to a high-vacuum
line. Degassing was achieved via a minimum of three freeze-pump-
thaw cycles while connected to the vacuum manifold; the final vapor
pressure at 77 K was <5 × 10-2 mbar, as monitored using a Pirani
gauge. Luminescence quantum yields were determined by the
method of continuous dilution, using [Ru(bpy)3]Cl2 in an air-
equilibrated aqueous solution as the standard (φ ) 0.02855); the
estimated uncertainty in φ is (20% or better.
The luminescence lifetimes of the complexes were measured by
time-correlated single-photon counting (TCSPC), following excita-
tion at 374.0 nm with an EPL-375 pulsed-diode laser. The emitted
light was detected at 90° using a Peltier-cooled R928 PMT after
passage through a monochromator. The estimated uncertainty in
the quoted lifetimes is (10% or better. For the dimetallic complexes
6 and 7, an estimate of the short component of the decay was made
in this way using TCSPC, while the longer component was analyzed
using multichannel scaling following excitation by a pulsed xenon
lamp. Bimolecular rate constants for quenching by molecular
oxygen, kQ, were determined from the lifetimes in a degassed and
Hb-NNC and H3-NCN), 8.16 (1H, td, 3J ) 8.0, 4J ) 1.5, H4-NNC),
3
8.03 (2H, d, J ) 8.5, Ha), 7.99 (1H, s, H3-NNC), 7.83 (6H, m,
Hb′, H4-NCN, and H6-NCN), 7.76 (1H, d, 3J ) 5.0, H6-NNC), 7.56
(2H, t, 3J ) 7.5, Ha′), 7.45 (2H, m, Hc′ and H5′′-NNC), 7.16 (1H, s,
Hc), 7.07 (1H, s, Hb-NCN or Hd), 7.02 (1H, s, Hb-NCN or Hd),
3
4
3
6.97 (2H, td, J ) 6.0, J ) 1.5, H5-NCN), 6.55 (1H, d, J ) 8.5,
H5-NNC), 5.95 (1H, d, 3J ) 8.0, H6′′-NNC), 2.66 (6H, s, Me-NCN),
2.46 (6H, d, 4J ) 3.5, Me-NCN-pendent), 2.19 (3H, s, Me-NNC).
MS (MALDI, DCTB matrix): m/z 953.4 [M]+. HRMS (ES+). Calcd
for 191IrC55H44N4: m/z 951.31665. Found: m/z 951.31694 [M]+.
Elem anal. Calcd for C55H44F6IrN4P: C, 60.3; H, 4.1; N, 5.1. Found:
C, 59.9; H, 3.9; N, 5.7. One spot by TLC (silica) Rf ) 0.48 in
dichloromethane/methanol (90:10).
[{Ir(N∧C∧N-dpyx)2}2µ-(N∧N∧C-mtbpy-(O)3-N∧N∧C-mtbpy)]-
(PF6)2 (6). A mixture of complex 2 (60 mg, 0.060 mmol), 1,4-
benzenediboronic acid (6 mg, 0.036 mmol), and sodium carbonate
(9.5 mg, 0.09 mmol in 100 µL water) in dimethyl sulfoxide (5 mL)
was degassed by three freeze-pump-thaw cycles. The catalyst
Pd(PPh3)4 (4.2 mg, 0.0036 mmol) was added under a positive
pressure of N2. The resulting orange suspension was stirred at 80
°C under a N2 atmosphere for 18 h. Upon cooling to room
temperature, the mixture was diluted with acetonitrile (2 mL) and
filtered to remove the insoluble palladium black that had formed.
The filtrate was added to a saturated KPF6 solution (25 mL), and
the orange precipitate that formed was collected by centrifuge,
washed with water (3 × 5 mL), and dried under vacuum.
Purification by flash column chromatography (silica, acetonitrile/
water/saturated KNO3 solution, gradient elution from 100:0:0 to
95:4.95:0.05) gave the desired product as an orange solid (23 mg,
30%). 1H NMR (CD3CN, 500 MHz): δ 8.79 (2H, s, H3′), 8.71 (2H,
s, H5′), 8.67 (2H, d, 3J ) 8.0, H3-NNC), 8.32 (4H, d, 3J ) 8.0, Hb),
8.26 (4H, d, 3J ) 8.0, H3-NCN), 8.16 (4H, d, 3J ) 8.0, Ha-NNC),
air-equilibrated solution, taking the concentration of oxygen in
46
CH3CN at 0.21 atm of O2 to be 1.9 mmol dm-3
.
The quantum yields of singlet oxygen production were obtained
1
by measuring the intensity of the O2 near-IR luminescence in an
air-saturated MeCN solution and comparing it with that measured
for a solution of perinaphthenone {1-H-phenanlen-1-one, Φ(1O2)
) 0.9550} of the same optical density (<0.2). The samples were
excited at 355 nm using the third harmonic of a Nd:YAG laser,
and the emission in the near-IR was detected using a liquid-N2-
cooled germanium photodiode detector. A filter was used to
eliminate light of <1100 nm. Measurements at a range of incident
light powers were made and the quantum yields determined from
the ratio of the gradients of intensity versus power for the sample
and standard. Representative plots are shown in the Supporting
Information.
3
8.07 (4H, s, Hb′), 8.03 (2H, t, J ) 7.0, H4-NNC), 7.90 (2H, s,
3
3
H3′′), 7.78 (4H, t, J ) 7.5, H4-NCN), 7.61 (4H, d, J ) 6.0, H6-
NCN), 7.56 (2H, d, 3J ) 5.5, H6-NNC), 7.28 (2H, t, 3J ) 6.5, H5),
7.21 (2H, s, H4′-NCN), 6.89 (4H, t, 3J ) 6.5, H5-NCN), 6.57 (2H,
d, J ) 9.0, H5′′), 5.82 (2H, d, J ) 7.5, H6′′), 2.96 (12H, s, Me-
NCN), 2.46 (6H, s, Me-NNC). MS (MALDI, DCTB matrix): m/z
1765.4 [M + PF6]+, 1620.4 [M + e-]+. Experimental isotope
patterns match the theoretical simulation (see the Supporting
Information). One spot by TLC (silica) Rf ) 0.51 in a acetonitrile/
water/saturated KNO3 solution (90:9.8:0.2).
3
3
[{Ir(N∧C∧N-dpyx)2}2µ-(N∧N∧C-mtbpy-(O)4-N∧N∧C-mtbpy)]-
(PF6)2 (7). This dimeric complex was prepared in the same way
and on the same molar scale as 6, but using 4,4′-biphenyldiboronic
acid (8.7 mg, 0.036 mmol) in place of benzenediboronic acid.
Purification of the hexafluorophosphate salt by flash column
chromatography (silica, acetonitrile/water/saturated KNO3 solution,
gradient elution from 100:0:0 to 95:4.95:0.05) gave the desired
product as an orange solid (0.021 g, 35%). 1H NMR (CD3CN, 700
MHz): δ 8.78 (2H, s, H3′), 8.70 (2H, s, H5′), 8.66 (2H, d, 3J ) 7.0,
Acknowledgment. We thank EPSRC and the University
of Durham for the award of a DTA studentship to V.L.W.
and Frontier Scientific Ltd. for their valued help in supplying
key reagents. We are indebted to Dr. Supiah Navaratnam
and Dr. Ruth Edge for their assistance in measuring O2
quantum yields, carried out with support from CCLRC
Daresbury Laboratory.
1
3
3
H3-NNC), 8.30 (4H, d, J ) 7.7, Hb), 8.26 (4H, d, J ) 7.7, H3-
1
Supporting Information Available: Syntheses, H NMR and
3
NCN), 8.14 (4H, d, J ) 7.7, Ha), 8.04–8.00 (10H, m, H4-NNC,
MS data, DFT calculations, frontier orbitals, singlet oxygen quantum
yields, and a complete reference for Gaussian. This material is
available free of charge via the Internet at http://pubs.acs.org.
3
Hb′, Ha′), 7.89 (2H, s, H3′′-NNC), 7.78 (4H, t, J ) 7.0, H4-NCN),
7.61 (4H, d, 3J ) 6.3, H6-NCN), 7.55 (2H, d, 3J ) 6.3, H6-NNC),
7.27 (2H, t, 3J ) 7.0, H5-NNC), 7.21 (2H, s, H4′-NCN), 6.89 (4H,
3
3
3
t, J ) 6.3, H5-NCN), 6.57 (2H, d, J ) 8.4, H5′′), 5.82 (2H, d, J
) 8.4, H6′′), 2.96 (12H, s, Me-NCN), 2.25 (6H, s, Me-NNC). MS
(MALDI, DCTB matrix): m/z 1841.4 [M + PF6]+. Experimental
IC701788D
(55) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6607