Luminescent complexes of iridium(III) containing N∧C∧N-coordinating terdentate ligands

Article abstract of DOI:10.1021/ic061172l

A family of bis-terdentate iridium(III) complexes is reported which contain a cyclometalated, N∧C∧N-coordinating 1,3-di(2-pyridyl)benzene derivative. This coordination mode is favored by blocking competitive cyclometalation at the C⁴ and C⁶ positions of the ligand. Thus, 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyxH) reacts with IrCl ₃·3H₂O to generate a dichlorobridged dimer [Ir(dpyx-N,C,N)Cl(μ-Cl)]₂, 1. This dimer is cleaved by DMSO to give [Ir(dpyx)-(DMSO)Cl₂], the X-ray crystal structure of which is reported here, confirming the N∧C∧N coordination mode of dpyx. The dimer 1 can also be cleaved by a variety of other ligands to generate novel classes of mononuclear complexes. These include charge-neutral bis-terdentate complexes of the form [Ir(N∧C∧N)(C∧N∧C)] and [Ir-(N∧C∧N) (C∧N∧O)], by reaction of 1 with C∧N∧C-coordinating ligands (e.g., 2,6-diphenylpyridine and derivatives) and C∧N∧O-coordinating ligands (based on 6-phenylpicolinate), respectively. Treatment of 1 with terpyridines leads to dicationic complexes of the type [Ir(N∧C∧N) (N∧N∧N)]²⁺, while 2-phenylpyridine gives [Ir(dpyx- N∧C∧N)-(ppy-C,N)Cl]. All of the charge-neutral complexes are luminescent in fluid solution at room temperature. Assignment of the emission to charge-transfer excited states with significant MLCT character is supported by DFT calculations. In the [Ir(N∧C∧N)(C∧N∧C)] class, fluorination of the C∧N∧C ligand afthe phenyl 2′ and 4′ positions leads to a blue-shift in the emission and to an increase in the quantum yield (λ_max = 547 nm, φ = 0.41 in degassed CH₃CN at 295 K) compared to the nonfluorinated parent complex (λ_max = 585 nm, φ = 0.21), as well as to a stabilization of the compound with respect to photodissociation through cleavage of mutually trans Ir-C bonds. [Ir(dpyx-N∧C∧N)(ppy-C,N)Cl] is an exceptionally bright emitter: φ= 0.76, λ_max = 508 nm, in CH₃CN at 295 K. In contrast, the [Ir(N∧C∧N)(C∧N∧O)] complexes are much less emissive, shown to be due to fast nonradiative decay of the excited state, probably involving reversible Ir-O bond cleavage. The [Ir(N∧C∧N) (N∧N∧N)]²⁺ complexes are very feeble emitters even at 77 K, probably due to the almost exclusively interligand charge-transfer nature of the lowest-energy excited state in these complexes.

Full text of DOI:10.1021/ic061172l

Inorg. Chem. 2006, 45, 8685−8699
Luminescent Complexes of Iridium(III) Containing N
Terdentate Ligands
∧C∧N-Coordinating
Andrew J. Wilkinson,† Horst Puschmann,‡ Judith A. K. Howard,‡ Clive E. Foster,§ and
J. A. Gareth Williams*,†
Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE U.K., and
Fujifilm Imaging Colorants Ltd, P.O. Box 42, Hexagon House, Blackley,
Manchester, M9 8ZS U.K.
Received June 27, 2006
A family of bis-terdentate iridium(III) complexes is reported which contain a cyclometalated, N
∧
C N-coordinating
∧
1,3-di(2-pyridyl)benzene derivative. This coordination mode is favored by blocking competitive cyclometalation at
the C4 and C6 positions of the ligand. Thus, 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyxH) reacts with IrCl3
3H2O
to generate a dichlorobridged dimer [Ir(dpyx-N,C,N)Cl( -Cl)]2, 1. This dimer is cleaved by DMSO to give [Ir(dpyx)-
(DMSO)Cl2], the X-ray crystal structure of which is reported here, confirming the N N coordination mode of
dpyx. The dimer 1 can also be cleaved by a variety of other ligands to generate novel classes of mononuclear
complexes. These include charge-neutral bis-terdentate complexes of the form [Ir(N N)(C C)] and [Ir-
(N N)(C O)], by reaction of 1 with C C-coordinating ligands (e.g., 2,6-diphenylpyridine and derivatives)
and C O-coordinating ligands (based on 6-phenylpicolinate), respectively. Treatment of 1 with terpyridines
leads to dicationic complexes of the type [Ir(N N)(N N)-
N)]2+, while 2-phenylpyridine gives [Ir(dpyx-N
‚
µ
∧C∧
∧
C
∧
∧ ∧
N
∧C
∧
∧N∧
∧ ∧
N
∧ ∧
N
∧
C∧
∧
N∧
∧C∧
(ppy-C,N)Cl]. All of the charge-neutral complexes are luminescent in fluid solution at room temperature. Assignment
of the emission to charge-transfer excited states with significant MLCT character is supported by DFT calculations.
In the [Ir(N
a blue-shift in the emission and to an increase in the quantum yield (λmax
CH3CN at 295 K) compared to the nonfluorinated parent complex (λmax
stabilization of the compound with respect to photodissociation through cleavage of mutually trans Ir
[Ir(dpyx-N N)(ppy-C,N)Cl] is an exceptionally bright emitter: φ ) 0.76, λmax 508 nm, in CH3CN at 295 K.
In contrast, the [Ir(N N)(C O)] complexes are much less emissive, shown to be due to fast nonradiative
decay of the excited state, probably involving reversible Ir O bond cleavage. The [Ir(N N)(N
N)]2+ complexes
∧
C∧
N)(C∧
N∧
C)] class, fluorination of the C
∧
N∧
C ligand at the phenyl 2
547 nm, φ ) 0.41 in degassed
585 nm, φ ) 0.21), as well as to a
C bonds.
′ and 4′ positions leads to
)
)
−
∧
C
∧
)
∧
C∧
∧N∧
−
∧
C∧
∧N∧
are very feeble emitters even at 77 K, probably due to the almost exclusively interligand charge-transfer nature of
the lowest-energy excited state in these complexes.
Introduction
and luminescent probes and sensors.3,4 For many years,
iridium played the part of Cinderella alongside its three sisters
Ru, Os, and Pt, probably reflecting the kinetic inertness of
the Ir(III) ion: harsh conditions and laborious purification
Interest in the photochemistry of second- and third-row
transition metal complexes with polypyridyl ligands grew
rapidly in the 1970s, as the potential of compounds such
as [Ru(bpy)3]2+ as excited-state electron-transfer agents
emerged.1,2 This interest has been maintained, indeed ac-
celerated, by new applications such as light-emitting devices
(1) Work in the field from the 1970s and 1980s has been extensively
reviewed; see for example Balzani, V.; Scandola, F. Supramolecular
Photochemistry; Ellis Horwood: New York, 1991.
(2) Hoffman, M. Z., Ed. Inorganic Photochemistry: State of the Art. J.
Chem. Educ. 1983, 60, 784-887.
(3) For a review of the science of OLEDs, see, for example: Hung, L.
S.; Chen, C. H. Mater. Sci. Eng. Rep. 2002, 39, 143.
(4) Schanze, K. S., Schmehl, R. H., Eds. Applications of Inorganic
Photochemistry in the Chemical and Biological SciencessContemporary
Developments. J. Chem. Educ. 1997, 74, 633-702.
* To whom correspondence should be addressed. E-mail:
j.a.g.williams@durham.ac.uk.
† Coordination Photochemistry Group, University of Durham.
‡ Chemical Crystallography Group, University of Durham.
§ Fujifilm Imaging Colorants Ltd.
10.1021/ic061172l CCC: $33.50
Published on Web 09/15/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 21, 2006 8685

Wilkinson et al.
are often required for the formation of its complexes, the
synthesis of [Ir(bpy)3]3+ being a good example.5 Thus,
notwithstanding some pioneering work in the 1980s, par-
ticularly by Watts and co-workers,6 it is principally in the
past decade that the photophysics and photochemistry of
iridium complexes have begun to enjoy widespread inves-
tigation.7
The large increases in device efficiency shown to be possible
have provided a major impetus for the recent synthesis and
investigation of a large range of derivatives of the Ir(N∧C)3,
Ir(N∧C)2(L∧X), and Ir(N∧N-)3 structural classes {N∧C )
bidentate cyclometalating ligands such as ppy; L∧X )
anionic ligands such as acac and 2-picolinate; N∧N- ) 5-(2-
pyridyl)pyrazoles and 5-(2-pyridyl)triazoles}.16-18 In parallel,
bis-cyclometalated iridium complexes, particularly those of
the form [Ir(N∧C)2(N∧N)]+ (N∧N ) bipyridine ligands),
have been studied as sensitizers for energy- and electron-
transfer reactions, for which they are particularly appropriate
given the long-lived nature of their excited states, the
tunability of the excited-state energies, and their strong
photoreducing properties.19 Applications in luminescent probe
and sensor molecules are also under investigation.20
N6-coordinated iridium complexes {e.g. [Ir(bpy)3]3+, [Ir-
(tpy)2]3+, and their derivatives} are typically characterized
3
by long-lived emissive excited states of primarily π-π*
character5,8,9 or, when strongly electron-donating substituents
are present in the ligands, intraligand charge transfer states
(ILCT).10,11 This contrasts with the unambiguous metal-to-
ligand charge transfer (MLCT) nature of Ru(II) analogues12
and arises from the higher oxidation state of the iridium metal
center (+3 rather than +2): this raises the energy of the
MLCT transitions in such Ir(III) complexes, normally to
beyond those of the intraligand excited states of the pyridyl
ligands. On the other hand, the introduction of anionic ligands
into the coordination sphere of the iridium ion (e.g., chloride
or a cyclometalating carbon) stabilizes the MLCT excited
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Almost all such work on cyclometalated iridium complexes
has focused hitherto on tris-bidentate complexes, primarily
of the generic types indicated above. In contrast, cyclom-
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have seen little investigation, despite the widely recognized
structural advantages of bis-terdentate complexes over tris-
bidentate analogues.21 For example, the latter are normally
chiral, so that a mixture of diasteromers is formed when two
or more such complexes are linked together, whereas the
former are achiral. Also, the axial symmetry of complexes
with two terdentate ligands allows three or more such
complexes to be connected linearly and without problems
of geometrical isomerism. Scandola and co-workers recently
described a series of cationic [Ir(N∧N∧N)(C∧N∧C)]+
complexes22 (N∧N∧N ) terpyridyl ligand; C∧N∧C ) 2,6-
diphenylpyridyl ligand), while Mamo and colleagues earlier
Tris-cyclometalated complexes such as fac-[Ir(ppy)3] have
attracted particular attention in the field of organic light-
emitting devices (OLEDs).13-15 The high efficiency of their
emission from triplet MLCT states, together with their good
chemical and photochemical stability and their charge
neutrality, offers potential as dopants for harvesting the
otherwise nonemissive triplet states formed in such devices.
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8686 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
Scheme 1. Schematic Structures of the Four Proposed Classes of
Charge-Neutral, Bis-terdentate Complexes of Iridium
Scheme 2. Synthetic Strategies Employed in the Preparation of the
Terdentate Ligands Used in This Studya
isolated cationic complexes containing either one or two
N∧N∧C-coordinated 2,6-bis(2′-quinolyl)pyridine ligands.23
We embarked upon a program to investigate the com-
plexation chemistry of iridium with terdentate, cyclometa-
lating ligands and to explore their excited-state and lumi-
nescence properties. Aside from the structural advantages
with respect to incorporation into multimetallic systems
discussed above, improved stabilities could, in principle, be
envisaged upon switching from bidentate to terdentate
ligands, and new approaches to tuning excited state energies
opened up. In particular, we sought to obtain charge-neutral
complexes since these are generally preferred over charged
compounds in applications such as OLEDs, the latter often
suffering from poorer device lifetimes and long turn-on times
due to the redistribution of ions under the applied electric
field.24
a The numbering system used in the assignment of the 1H NMR spectra
of the uncoordinated ligands is included.
[Ir(N∧C∧N)(N∧N∧N)]2+ complexes has been carried out.
Haga et al. have also been investigating complexes related
to those of Classes III and IV recently, but based on 1,3-
bis-(1-methyl-benzimidazol-2-yl)benzene as the N∧C∧N
ligand.26
2. Synthesis of the Required Ligands. 2.1. N∧C∧N
Ligands. The palladium-catalyzed Stille cross-coupling of
1,3-dibromobenzenes with 2-(trialkylstannyl)-pyridines has
previously been shown to offer a reliable route to 1,3-di(2-
pyridyl)-benzene (dpybH) and derivatives,27,28 and this
method was employed here. In the case of the xylyl-based
compound, 1,3-di(2-pyridyl)-4,6-dimethylbenzene (dpyxH),
the starting haloaromatic is 1,3-dibromo-4,6-dimethylben-
zene, which was readily prepared by dibromination of
m-xylene with bromine in the presence of a catalytic amount
of iodine (Scheme 2).29
2.2. C∧N∧O Ligands. The Hantzsch pyridine synthesis
or its variants are appropriate for the formation of 6-aryl-
2-picolinic acid derivatives. Thus, 4-p-tolyl-6-phenylpicolinic
acid (tppicH2) was prepared by a classical Kro¨hnke conden-
sation30 of 2-oxo-4-p-tolyl-but-3-enoic acid with 1-(2-oxo-
2-phenylethyl)pyridinium iodide and ammonium acetate
(Scheme 2).31
2.3. C∧N∧C Ligands. Since such ligands also feature a
central pyridine ring, a Hantzsch-type synthesis might again
be anticipated to be appropriate, as used by Kro¨hnke in the
Results and Discussion
1. Strategy. Our strategy has been to use a 2,6-dipyridyl-
benzene derivative as a formally monoanionic, cyclometa-
lating ligand offering terdentate N∧C∧N coordination in
combination with a second, dianionic terdentate ligand to
generate an Ir(III) complex with overall charge neutrality
(Scheme 1). Two distinct classes of dianionic ligand have
been investigated in this study, namely bis-cyclometalating
C∧N∧C-coordinating ligands such as 2,6-diphenylpyridine
(Class I in Scheme 1) and C∧N∧O- ligands based on
6-phenylpicolinate, where O- is a carboxylate oxygen (Class
II). We previously reported the first example of the first class
in a communication in this journal.25 Here, we describe in
detail the synthetic chemistry and photophysical properties
of the two classes, as well as a highly emissive complex
incorporating the combination of 2-phenylpyridine (N∧C)
and chloride (X-) in place of the terdentate dianionic ligand
(Class III). We also considered the principle of using an
N∧N∧N-coordinating terpyridyl ligand carrying two ionis-
able carboxylic acid groups as an alternative type of dianionic
ligand to furnish a charge-neutral complex (Class IV) for
which purpose an investigation of the properties of model
(26) (a) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno,
T.; Ishii, Y.; Sakai, K.; Haga, M. Inorg. Chem. 2005, 44, 4737. (b)
Haga, M.; Oral Communication, 16th International Symposium on the
Photochemistry and Photophysics of Coordination Compounds; Asi-
lomar, 2006.
(27) Beley, M.; Chodorowski, S.; Collin, J.-P.; Sauvage, J.-P. Tetrahedron
Lett. 1993, 34, 2933.
(28) Ca´rdenas, D. J.; Echavarren, A. M.; Ramirez de Arellano, M. C.
Organometallics 1999, 18, 3337.
(29) The method used was a modification of that described in Allinson,
G.; Bushby, R. J.; Jesudason, M. V.; Paillaud, J. L.; Taylor, N. J.
Chem. Soc., Perkin Trans. 2 1997, 147.
(23) Mamo, A.; Stefio, I.; Parisi, M. F.; Credi, A.; Venturi, M.; Di Pietro,
C.; Campagna, S. Inorg. Chem. 1997, 36, 5947.
(24) Slinker, J.; Bernards, D.; Houston, P. L.; Abrun˜a, H. D.; Bernhard,
S.; Malliaras, G. G. Chem. Commun. 2003, 2393.
(25) Wilkinson, A. J.; Goeta, A. E.; Foster, C. E.; Williams, J. A. G. Inorg.
Chem. 2004, 43, 6513.
(30) Kro¨hnke, F. Synthesis 1976, 1.
(31) Baba, A.; Wang, W.; Yong, K. W.; Strong, L.; Schmehl, R. H. Synth.
Comm. 1994, 24, 1029. Kipp, R. A.; Li, Y.; Simon, J. A.; Schmehl,
R. H. J. Photochem. Photobiol. A 1999, 121, 27.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8687

Wilkinson et al.
Scheme 3. (a) N∧C∧N and N∧C Binding Modes of
2,6-Dipyridylbenzene. (b) C∧N∧C and C∧N Binding Modes of 2,6-
Diphenylpyridine
tained following chromatographic separation (1% yield),
containing dpyb bound in the desired N∧C∧N-coordinating
manner. Other products isolated and identified by electro-
1
spray mass spectrometry and H NMR spectroscopy were
[Ir(dpyb-N,C4)(ttpy-N,N,N)Cl]+ and [Ir(dpyb-N,C4)2(ttpy-
N,N)]+ (Scheme 4), where cyclometalation had occurred
through the C4 position of the phenylene ring (Scheme 3a,
binding mode II). Thus, we may postulate that the orange
solid from the initial reaction of dpybH with IrCl3‚3H2O
contains at least the three species shown bracketed in the
center of Scheme 4. The predominance of the bidentate N,C4
coordination over N,C2,N may be related to the fact that the
C2 position is more sterically hindered by the pyridyl rings,
with metalation of the substitutionally inert iridium(III) ion
likely to be kinetically favored at the C4 position. A similar
coordination mode has been observed upon direct reaction
of dpybH with palladium acetate, leading to a dimeric species
dipalladated at C4 and C6.28
formation of 2,6-diphenylpyridine.30 However, this method
proved to be unsuitable for the formation of the fluorinated
derivative F4dppyH2. Competitive nucleophilic aromatic
substitution was observed during the formation of the
requisite enaminonesthe reaction of dimethylamine and
formaldehyde with 2,4-difluoroacetophenoneswith dimethy-
lamine replacing one or more fluorine atoms. Instead, this
ligand was prepared by Suzuki cross-coupling of 2,4-
difluorophenylboronic acid with 2,6-dibromopyridine (Scheme
2).
3. Synthesis of the Iridium Complexes. 3.1. Chloro-
Bridged Iridium Dimers and [Ir(N∧C∧N)(N∧N∧N)]2+
Complexes. 3.1.1. N∧C∧N ) 1,3-Di(2-pyridyl)benzene
(dpyb) and Its Associated Problems. Our initial investiga-
tions centered on the attempted complexation of 1,3-di(2-
pyridyl)benzene (dpybH) to iridium(III) as a terdentate,
N∧C∧N-coordinating ligand (Scheme 3a, binding mode I).
Binding of dpyb in such a manner has previously been
observed for the neighboring transition metals Ru(II),32 Os-
(II),32a,b and Pt(II).28,33 Cyclometalation of iridium(III) is
commonly performed in a mixture of 2-ethoxyethanol and
water, for example, in the preparation of chloro-bridged
dinuclear iridium complexes such as [Ir(ppy)2µ-Cl]2.6 Reac-
tion of dpybH with IrCl3‚3H2O at 130 °C in this solvent
system (3:1 by volume) resulted in an orange precipitate,
but 1H NMR spectroscopy in d6-DMSO revealed a complex
mixture of cyclometalated products, purification of which
was hindered by the low solubility in all common solvents.
The product mixture was therefore reacted directly with 4′-
(p-tolyl)-2,2′:6′,2′′-terpyridine (ttpy) in ethylene glycol at
reflux34 on the grounds that the resulting products could shed
light on its composition. A very small amount of the
anticipated product [Ir(dpyb)(ttpy)]2+ (Scheme 4) was ob-
3.1.2. N∧C∧N ) 1,3-di(2-pyridyl)-4,6-dimethylbenzene
(dpyx) and Its Major Advantages. To favor the desired
terdentate coordination mode, this competitive cyclometa-
lation at the C4 and C6 positions was blocked by the
introduction of methyl substituents into the ligand to give
dpyxH (Scheme 2). Reaction of dpyxH with IrCl3‚3H2O in
ethoxyethanol/water (3:1) at 80 °C again led to a yellow-
orange solid. Although the solid is insoluble in most common
solvents, a 1H NMR spectrum could be obtained in d6-DMSO
following heating and exhibited the same C2 symmetry as
the free ligand, but with the loss of the phenylene proton
signal between the two pyridyl rings (H2 in ligand, δ ) 7.43
ppm). As expected for cyclometalation at this carbon, a
significant upfield shift of the resonance of the proton para
to this position is also observed (from 7.26 to 6.98 ppm).
An X-ray diffraction study of a single crystal obtained from
DMSO solution revealed the structure to be [Ir(dpyx)-
(DMSO)Cl2] (Figure 1). The desired N∧C∧N coordination
is evident, with a sulfur-bound DMSO molecule trans to the
metalated carbon. The unambiguous identification of the
initially formed solid is apparently thus not possible by
crystallography or 1H NMR spectroscopy, but MALDI mass
spectrometry suggests that it is the chloride-bridged dimer
[Ir(dpyx)Cl(µ-Cl)]2 (m/z ) 1044) (structure 1 in Scheme 5),
which then undergoes cleavage of the bridging chlorides
upon solvation in DMSO. Such chloride bridge cleavage is
well-established for dinuclear cyclometalated complexes of
the type [Ir(ppy)2(µ-Cl)]2 and [Pt(ppy)(µ-Cl)]2.6,35,36
In the structure of [Ir(dpyx)(DMSO)Cl2] (Figure 1; hence-
forth denoted 1-DMSO), the iridium(III) center exhibits a
distorted octahedral geometry due to the geometric con-
straints of the terdentate binding of dpyx. The two Ir-Cl
bonds are approximately perpendicular to the plane of the
dpyx ligand, with the sulfur-bound DMSO molecule oc-
cupying the remaining coordination site of the iridium(III)
(32) (a) Barigelletti, F.; Flamigni, L.; Guardigli, M.; Juris, A.; Beley, M.;
Chodorowski-Kimmes, S.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem.
1996, 35, 136. (b) Barigelletti, F.; Flamigni, L.; Collin, J.-P.; Sauvage,
J.-P. Chem. Commun. 1997, 333. (c) Patoux, C.; Launay, J.-P.; Beley,
M.; Chodorowski-Kimmes, S.; Collin, J.-P.; James, S.; Sauvage, J.-
P. J. Am. Chem. Soc. 1998, 120, 3717.
(33) (a) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.;
Wilson, C. Inorg. Chem. 2003, 42, 8609. (b) Farley, S. J.; Rochester,
D. L.; Thompson, A. L.; Howard, J. A. K.; Williams, J. A. G. Inorg.
Chem. 2005, 44, 9690.
(35) Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem. 1999,
38, 2250.
(34) Such forcing conditions (ethylene glycol at 196 °C) have previously
been found necessary for the introduction of terpyridines into the
coordination sphere of iridium: see refs 8 and 9.
(36) For example: (a) Kvam, P.-I.; Songstad, J. Acta Chem. Scand. 1995,
49, 313. (b) Balashev, K. P.; Puzyk, M. V.; Kotlyar, V. S.; Kulikova,
M. V. Coord. Chem. ReV. 1997, 159, 109.
8688 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
Scheme 4. Iridium Complexes Isolated Containing N∧C or N∧C∧N-Bound 2,6-Dipyridylbenzene (dpyb)a
a In the third of the postulated intermediates, L is likely to be a monodentate neutral ligand, for example, monodentate N-bound dpybH.
center. Selected bond lengths and angles are listed in Table
1. The Ir-C bond {1.976(4) Å} is shortened with respect to
the analogous bonds in fac-[Ir(mppy)3] {2.024(6) Å; mppy
) 2-(4′-methylphenyl)pyridine},37 a result of the constraints
imposed by binding of two flanking pyridine rings. The Ir-N
distances of [Ir(dpyx)(DMSO)Cl2] {2.067(3) and 2.087(3)
Å} are also shortened with respect to those in fac-[Ir(mppy)3]
{2.132(5) Å}, attributable to the strong trans influence of
the cyclometalated carbons opposite the nitrogen atoms in
the latter.
Reaction of [Ir(dpyx)Cl(µ-Cl)]2 (1) with 4′-tolyl-terpyri-
dine (ttpy) in ethylene glycol at 196°C for 45 min gave [Ir-
(dpyx)(ttpy)]2+ cleanly in 87% yield (Scheme 5, compound
2b), a huge improvement over that achieved for [Ir(dpyb)-
(ttpy)]2+. Similarly, reaction with unsubstituted terpyridine
(tpy) readily gave [Ir(dpyx)(tpy)]2+ (2a), confirming the
generality of this new class of complex.38
3.2. [Ir(N∧C∧N)(C∧N∧C)] Complexes: [Ir(dpyx)-
(dppy)] (3) and [Ir(dpyx)(F4dppy)] (4). The application of
the N∧C∧N-bound iridium(III) precursor compound, 1, to
the synthesis of the charge-neutral complex [Ir(dpyx)(dppy)]
(3) was investigated. Reaction with 2,6-diphenylpyridine
(dppyH2) in refluxing ethylene glycol for up to 1 hs
conditions which led to the efficient formation of the [Ir-
(dpyx)(N∧N∧N)]2+ complexessgave none of the desired
1
product. Although H NMR spectroscopy of the fractions
containing the dpyx ligand isolated from chromatography
showed that it was still bound in an N∧C∧N manner, no
other aromatic proton resonances could be observed, and
electrospray ionization mass spectrometry suggested that the
remaining three coordination sites were occupied by weakly
bound solvent molecules. In contrast, reaction for 24 h under
the same conditions did allow the chromatographic isolation
(38) The X-ray crystal structure of a representative member, [Ir(dpyx)-
(ttpy)](PF6)2, was described in our preliminary communication (ref
25) and showed the anticipated pseudo-octahedral geometry at the
metal, with the two ligands occupying mutually orthogonal planes.
(37) Garces, F. O.; Dedeian, K.; Keder, N. L.; Watts, R. J. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1993, 49, 1117.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8689

Wilkinson et al.
complex as an orange solid in 37% yield. Similarly, reaction
with F4dppyH2 (mp ) 83-84 °C) under similar conditions
led to the fluorinated complex 4, a yellow solid, in 21% yield.
Unfortunately, neither complex is amenable to scale-up; in
the first case, because of the inherent instability of the product
in solution (see below), and in the second, because of the
need for large excess quantities of the F4dppyH2 ligand (if
the ratio of ligand to intermediate is reduced, the mixture
no longer melts below 110 °C and yields are greatly
decreased, while the use of higher temperatures leads to
decomposition). Preparation of the dithienyl analogue [Ir-
(dpyx)(dthpy)] was attempted in a similar manner, but no
evidence of complexation was observed in this case, nor
when the reaction was carried out in refluxing ethylene
glycol. The use of acetic acid as the solvent led to isolation
of [Ir(dpyx)(dthpyH)]+ (6), which contains the bidentate
C∧N-coordinated dthpyH ligand (Scheme 5). Again, the
identity of the ligand in the sixth site remians ambiguous.
3.3. [Ir(N∧C∧N)(C∧N)Cl] Complex: [Ir(dpyx)(ppy)-
Cl] (5). Heating 1 in 2-phenylpyridine (ppyH) in the presence
of AgOTf gave the complex [Ir(dpyx)(ppy)Cl] (5) in good
yield. There are two possible isomers of this complex, with
either a meridional or facial arrangement of the nitrogen
Figure 1. Structure of [Ir(dpyx)(DMSO)Cl2] (1-DMSO) crystallized from
DMSO; T ) 120 K; thermal ellipsoids shown at the 30% probability level.
1
of a dppyH-containing iridium(III) complex. However, H
NMR spectroscopy showed clearly that this ligand was only
bidentately bound (C∧N coordination), with the remaining
phenyl group unbound (mode B in Scheme 3b). The same
product could also be obtained at lower temperatures if the
reaction was carried out in the presence of Ag+ salts to
promote dechlorination, e.g., in ethanol at reflux for 6 days.
The identity of the ligand L in the sixth coordination site,
if there is one, remains ambiguous, but is most likely a
weakly bound solvent molecule. Thus, electrospray mass
spectrometry reveals only the mass of the species [Ir(dpyx)-
(dppyH)]+, even at the lowest ionization cone voltages. C2
symmetry is observed in the resonances of the unbound
1
atoms. The H-1H NOESY spectrum shows a cross-peak
between H6 of dpyx and H6 of ppy, indicative of the mer
complex. This is expected to be the thermodynamically
favored product, because the high trans influence of the
cyclometalated carbons will disfavor the alternative in which
the carbon atoms would be disposed trans to one another.
3.4. [Ir(N∧C∧N)(C∧N∧O)] Complexes: [Ir(dpyx)-
(tppic)] (7) and [Ir(dpyx)(hbqc)] (8). The picolinic acid
derivatives tppicH2 and hbqcH2 have high melting points,
rendering a solvent-free approach to complexation unsuitable.
Molten benzoic acid was selected as the reaction medium,
owing to its appropriate melting point (122-123 °C) and
potential for solubilizing pyridine-carboxylic acid function-
alized ligands. Thus, reaction of 1 with the respective ligand
in the presence of AgOTf in benzoic acid led to the
complexes [Ir(dpyx)(tppic)] (7) and [Ir(dpyx)(hbqc)] (8) in
good yield. Interestingly, all attempts to prepare a related
charge-neutral complex containing the O∧N∧O-coordinating
ligand, 2,6-dipyridinecarboxylate, in place of the C∧N∧O
ligand, were unsuccessful, leading to a complex mixture of
inseparable products.
1
1
phenyl ring in the H NMR spectrum, while the H-1H
NOESY reveals a through-space interaction between the
o-phenyl protons (H2′′, see Scheme 3b) and those adjacent
to the nitrogen of the pyridyl rings of dpyx (H6). These
observations indicate that the unbound phenyl is free to rotate
and to occupy a plane orthogonal to the rest of the dppyH
ligand. This contrasts with the work of Crabtree et al. on
cyclometalation of iridium phosphine species, where 2,6-
diphenylpyridine gave a mono-cyclometalated species [Ir-
(dppyH-N,C2)(PPh3)2H]+, in which the sixth site of the metal
was stabilized by an agostic bond to the noncyclometalated
phenyl ring.39 In a related system containing cyclometalated
2-isopropyl benzoquinolines, such an agostic interaction was
sterically disfavored, and the site opposite to metalation
remained empty.40 Such a possibility cannot be ruled out in
the present instance.
4. Photophysical Properties. 4.1. [Ir(N∧C∧N)(C∧N∧C)]
Complexes: [Ir(dpyx)(dppy)] (3) and [Ir(dpyx)(F4dppy)]
(4). 4.1.1. Ground-State Absorption. The absorption spectra
of 3 and 4 in acetonitrile solution at room temperature show
very intense bands in the near-UV region between 240 and
300 nm (Figure 2, Table 2). By comparison with the well-
established literature data for fac-[Ir(ppy)3], these are as-
The desired complex 3, containing the bis-cyclometalated
dppy ligand, was eventually obtained by using molten
dppyH2 itself as the reaction solvent (mp ) 74-76 °C).41
Thus, heating 1 with silver trifluoromethanesulfonate in
molten dppyH2 at 110 °C in the absence of any other solvent,
followed by chromatographic purification, led to the desired
1
signed to spin-allowed π-π* transitions of the ligands.6,16
A series of weaker, lower-energy features extending well
(41) Several other solution-based syntheses were also attempted using a
variety of solvents: ethoxyethanol/water, acetic acid, toluene, and
glycerol, in each case, both in the presence and absence of silver ions.
These reactions led either to intractable mixtures or, at best, to evidence
of products containing N∧C-bound diphenylpyridine, after chromato-
graphic purification.
(39) Albe´niz, A. C.; Schulte, G.; Crabtree, R. H. Organometallics 1992,
11, 242.
(40) Clot, E.; Eisenstein, O.; Dube´, T.; Faller, J. W.; Crabtree, R. H.
Organometallics 2002, 21, 575.
8690 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
Scheme 5. Structures of the Iridium Complexes Isolated, Containing the N∧C∧N-Bound Ligand dpyx, Obtained through the Intermediacy of
[Ir(dpyx)Cl(µ-Cl)]2 (1)a
a The numbering system used in the 1H NMR assignments of the complexes is included; the numbering of the dpyx ligand in [Ir(dpyx)(F4dppy)] is
representative of all the complexes.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
[Ir(dpyx)(DMSO)Cl2] (1-DMSO)
Ir-N(1)
Ir-N(3)
Ir-C(20)
Ir-Cl(1)
Ir-Cl(2)
Ir-S(1)
2.067(3)
2.087(3)
1.976(4)
2.352(2)
2.374(2)
2.438(2)
C(20)-Ir-S(1)
C(20)-Ir-N(1)
N(1)-Ir-N(3)
C(20)-Ir-Cl(1)
N(1)-Ir-Cl(2)
Cl(1)-Ir-Cl(2)
178.66(10)
80.00(13)
159.89(12)
90.94(11)
89.29(9)
176.20(3)
into the visible region are attributable to both allowed and
spin-forbidden metal-to-ligand charge transfer (1,3MLCT)
bands. It is clear that the bis-terdentate complexes show
significantly lower energy MLCT bands than fac- and mer-
[Ir(ppy)3] (which exhibit similar absorbance spectra to one
another), an effect which may be attributed to a lowering in
energy of the acceptor ligand π* orbitals (LUMO) upon
increased delocalization across the three aromatic rings of
the N∧C∧N ligand. The MLCT transitions of the fluorinated
complex 4 are blue-shifted with respect to 3, which may be
readily rationalized in terms of the stabilization of the HOMO
as a result of the electron-withdrawing nature of the fluorine
substituents (vide infra).
Figure 2. Absorption spectra of [Ir(dpyx)(dppy)] (3) (red) and [Ir(dpyx)-
(F4dppy)] (4) (green) in CH3CN at 295K, together with the spectrum of
fac-[Ir(ppy)3] (blue) for comparison. The low-energy portion of the excitation
spectrum of [Ir(dpyx)(dppy)], registered at 600 nm under the same
conditions, is also shown (red dotted line).
4.1.2. Emission. Both of the new complexes are strongly
luminescent in degassed solution, displaying a single, broad,
Inorganic Chemistry, Vol. 45, No. 21, 2006 8691

Wilkinson et al.
Table 2. Absorption Data for the Iridium Complexes in Acetonitrile
Solution at 295 K
complex
absorption λmax/nm (ꢀ/mol-1 dm3 cm-1
)
[Ir(dpyx)(DMSO)Cl2]
1-DMSO
286 (20 500), 388 (6100), 426sh (2020),
494 (455)
261 (38 500), 270 (35 300), 298br (26 000),
335sh (13 100), 377 (10 100), 435 (700),
465 (500)
264 (52 900), 283 (45 100), 301 (39 900),
361 (20 300), 377 (21 600), 435 (1500),
463 (1000)
[Ir(dpyx)(tpy)]2+
2a
[Ir(dpyx)(ttpy)]2+
2b
[Ir(dpyx)(dppy)]
3
244 (53 900), 269 (46 900), 349 (7700), 378
(6900), 406 (6500), 458 (5500), 481 (6800),
510 (7300)
[Ir(dpyx)(F4dppy)]
4
240 (49 200), 265 (48 000), 290sh (24 800),
304sh (21 200), 334 (8400), 360 (6000), 385
(5600), 447 (5600), 464 (6400), 487 (5200)
239 (44 700), 258 (39 700), 285 (37 000), 353
(6200), 369 (7800), 399 (10 000), 417
(11 300), 455 (3600), 492 (1300)
[Ir(dpyx)(ppy)Cl]
5
Figure 3. Emission spectra of [Ir(dpyx)(dppy)] (3) (red) and [Ir(dpyx)-
(F4dppy)] (4) (green) in CH3CN at 295 K, following excitation into the
lowest energy absorption band, together with the spectrum of fac-[Ir(ppy)3]
(blue) for comparison.
[Ir(dpyx)(tppic)]
7
[Ir(dpyx)(hbqc)]
8
242 (32 800), 273 (35 700), 337 (13 500), 393
(6900), 429 (6700), 459 (4300), 489 (1700)
245 (34 500), 290 (16 600), 315sh (10 400),
341 (6500), 363 (5900), 393 (4900), 428
(4600), 459 (2800), 492 (1100)
compared to [Ir(ppy)2(N∧N)]n+,18e19d,44 and [Pt(4,6-dfppy)-
(acac)] compared to [Pt(ppy)(acac)],45 traced back in each
case to the influence of the fluorine atoms on the HOMO
{4,6-dfppy ) 2-(4,6-difluorophenyl)pyridine; N∧N ) a 2,2′-
bipyridine ligand (n ) 1) or 2-triazolylpyridine (n ) 0)}.
The relatively long luminescence lifetimes, τ, of the
complexes, 3.7-3.8 µs in degassed solution at ambient
temperature (Table 3), are also suggestive of significant
ligand-centered character in the excited state, as opposed to
the rather shorter lifetimes typical of more purely MLCT
states.7,43,45 The emission is quenched strongly by dissolved
molecular oxygen, with bimolecular rate constants of quench-
ing kQ of the order of 6 × 109 M-1 s-1 (Table 3).
structureless emission band, typical of phosphorescence from
3
a MLCT state (Figure 3, Table 3). This assignment is
broadly supported by DFT calculations, which confirm that
the LUMO is localized almost exclusively on the N∧C∧N-
coordinating ligand, while the HOMO comprises a significant
contribution from the metal: contour plots of the frontier
orbitals are shown in Figure 4, and further details are
provided in the Supporting Information. The two ligands are
seen to make a significant contribution to the HOMO, as
well as the metal, and so the excited state is perhaps best
regarded as having mixed d(Ir)/π(dpyx)/π(dppy) f π*(dpyx)
character. Similar conclusions regarding the contribution of
ligand-centered (LC) and, for heteroleptic complexes, ligand-
to-ligand charge transfer (LLCT) character to the emissive
excited state have been reached in DFT studies of related
iridium complexes.22,42
The luminescence of 3 (λmax ) 585 nm) is substantially
red-shifted with respect to that of fac-[Ir(ppy)3] (λmax ) 528
nm). As in absorption, this is attributed to a lowering of the
N∧C∧N ligand π* orbital energy upon the increased
conjugation which accompanies the introduction of an
additional pyridyl ring compared to ppy. The blue-shift of
the emission band upon 4,6-difluorination (4, λmax ) 547
nm), which mirrors the effect in absorption, may be attributed
to a greater stabilization of the HOMO compared to the
LUMO upon introduction of the inductively electron-
withdrawing fluorine atoms. The molecular orbital plots from
the DFT calculations substantiate this interpretation since the
LUMO is seen to be almost entirely localized on the
unsubstituted dpyx ligandsand hence should not be signifi-
cantly affected by substituents in the dppy ligandswhile the
HOMO has a significant contribution from the dppy moiety.
Hypsochromic shifts have also been reported for fac-[Ir(4,6-
dfppy)3] compared to fac-[Ir(ppy)3],43 [Ir(4,6-dfppy)2(N∧N)]n+
The luminescence quantum yields (φ) of 3 and 4 in
degassed acetonitrile solution are 0.21 and 0.41, respectively.
Assuming that formation of the emissive state occurs with
unity efficiency, one may calculate an estimate of the
unimolecular radiative (kr) and nonradiative (∑knr) decay rate
constants by application of eqs 1 and 2:
τ ) 1/(k + k )
(1)
(2)
∑
r
nr
φ ) k /(k + k ) ) k . τ
∑
r
r
nr
r
Inspection of the parameters so obtained (Table 3) confirms
that the higher emission quantum yield of the fluorinated
complex compared to the parent is due both to a higher
radiative rate constant and to reduced nonradiative decay,
consistent with the predictions of the energy gap law: for a
homologous series of complexes, a decrease in the emissive
excited state energy is typically accompanied by a decrease
in φ.46
(44) Welter, S.; Lafolet, F.; Cecchetto, E.; Vergeer, F.; De Cola, L.
ChemPhysChem. 2005, 6, 2417.
(45) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. Inorg. Chem. 2002, 41, 3055.
(46) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. For an example of
the application of the law to metal-complex-doped light-emitting
polymers, see: Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R.
A.; Younus, M.; Khan, M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R.
H. J. Am. Chem. Soc. 2001, 123, 9412.
(42) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634.
(43) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba,
I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003,
125, 7377.
8692 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
Table 3. Luminescence Data for the Iridium Complexes in CH3CN Solution at 295K, and Corresponding Data for the fac- and mer-isomers of
[Ir(ppy)3] and [Ir(4,6-dfppy)3] in CH2Cl2
τ degassed
φ degassed
complex
λ
max/ nm
(aerated)/nsa
(aerated)b
kr/s-1 c
Σ knr/s-1 c
kQ/M-1 s-1 d
[Ir(dpyx)(dppy)] 3
fac-[Ir(ppy)3]e,f
585
510
512
547
450
460
508
603
562
502g
3800 (77)
1900
150
3700 (<100)
1600
0.21 (0.0013)
0.40
0.036
0.41 (0.0036)
0.43
0.053
0.76 (0.016)
0.053 (0.0092)
0.027 (0.0043)
5.5 × 104
2.1 × 105
2.4 × 105
1.1 × 105
2.7 × 105
2.5 × 105
4.8 × 105
4.8 × 105
1.6 × 105
2.1 × 105
3.2 × 105
6.5 × 106
1.6 × 105
3.6 × 105
4.5 × 106
1.5 × 105
8.6 × 106
5.7 × 106
-
6.7 × 109
-
mer-[Ir(ppy)3]e
-
[Ir(dpyx)(F4dppy)] 4
fac-[Ir(4,6-dfppy)3]e
mer-[Ir(4,6-dfppy)3]e
[Ir(dpyx)(ppy)Cl] 5
[Ir(dpyx)(tppic)] 7
[Ir(dpyx)(hbqc)] 8
[Ir(dpyx)(tpy)]2+ 2a
>5.1 × 109
-
210
-
1600 (<100)
110 (39)
170 (<100)
>4.9 × 109
8.7 × 109
>2.2 × 109
-
h
-
<0.001
-
a Lifetime of emission, λex ) 355 nm. b Luminescence quantum yield, measured using [Ru(bpy)3]Cl2(aq) as the standard (φ ) 0.02853). c Radiative (kr) and
nonradiative (Σknr) rate constants calculated from τ and φ values; estimated uncertainty (20%. d Bimolecular rate constant for quenching by O2, estimated
from τ values in degassed and aerated solutions. e In CH2Cl2, from ref 43. f The emission wavelength of fac-[Ir(ppy)3] in CH3CN is 528 nm (this work). g At
77 K. h A reliable lifetime could not be obtained using the available setup, owing to the very low intensity of the emission.
Figure 4. Density functional theory (DFT) calculation of the HOMO (left)
and LUMO (right) for [Ir(dpyx)(dppy)] (3).
A pertinent comparison to make is with the mer-[Ir(ppy)3]
derivatives, which incorporate the same meridional coordina-
tion geometry as the new terdentate complexes. The mer-
ppy complexes display much poorer luminescence efficien-
cies than their fac analogues due to a competing mer-to-fac
photoisomerization process.43 Thus, while mer-[Ir(ppy)3]
derivatives have similar radiative rate constants to the fac
isomers, their nonradiative rate constants are more than an
order of magnitude greater43 (Table 3) since the mer-to-fac
photoisomerization offers the excited state an additional
nonradiative decay pathway. For 3 and 4, the nonradiative
rate constants are of a comparable magnitude to those for
fac-[Ir(ppy)3] and are not elevated like those of mer-[Ir-
(ppy)3], suggesting that photodecomposition processes are
not a major pathway of nonradiative decay. Instead, lumi-
nescence quantum yields appear to be limited by the rather
lower radiative rate constants for these complexes, which
are 2-4 times smaller than those of the tris-bidentate
systems, possibly associated with the smaller contribution
of the metal to the frontier orbitals as discussed above.
4.1.3. Photodissociation. Given that the ∑knr values for
the new complexes are not abnormally high (e.g., they are
actually smaller than for the photostable fac-[Ir(ppy)3]), rates
of photodecomposition are likely to be low. Nevertheless, a
photodissociation process is observed for 3 under UV-vis
irradiation. 1H NMR spectroscopic analysis following irradia-
tion of an acetonitrile solution with a xenon lamp indicated
that the major compound formed is [Ir(dpyx)(dppyH)L]+
(Scheme 3b, dppyH binding mode B), the result of cleavage
Figure 5. Emission spectra of [Ir(dpyx)(dppy)] (3) in toluene (blue), THF
(green), CH3CN (orange) and CH2Cl2 (red), λex ) 480 nm, following intense
irradiation for 5 min. All spectra are normalized to a peak intensity of 1.0
prior to irradiation.
of one of the mutually trans Ir-C bonds. Just such a process
is likely to be the first step in the photoisomerization of mer-
Ir(ppy)3 to the fac form. However, the terdentate nature of
the ligands in 3 prohibits such an isomerization: a fac
arrangement is not possible, and the partially dissociated
species with bidentately bound dppyH is the result.
The solvent dependence of the rate of photodecomposition
was investigated in anhydrous solvents (Figure 5). The ob-
served trend, CH2Cl2 > CH3CN > THF ≈ toluene, may be
related to the ability of the solvent to solvate the sixth site
of the complex or to the acidity of the solvent. Given that a
species of the form [Ir(dpyx)(dppyH)]+ was a persistently
troublesome side-product in the preparation of 3, it is likely
that the difficulty in obtaining the latter complex in larger
quantities is associated with this instability during purification
by column chromatography. On the other hand, in the solid
state, the compound is stable indefinitely; the stability when
incorporated into an electroluminescent device such as an
OLED may, therefore, not necessarily be compromised.
Although the fluorinated derivative 4 also photodegrades
under prolonged irradiation in solution, the rate is signifi-
cantly decreased compared to the unsubstituted complex 3,
with a significant proportion of the emission intensity being
retained after irradiation for 1 h in acetonitrile solution.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8693

Wilkinson et al.
Figure 7. Absorption and emission spectra of [Ir(dpyx)(hbqc)] (8) (blue)
and [Ir(dpyx)(tppic)] (7) (red) in CH3CN at 295 K; λex ) 460 nm.
Figure 6. Absorption and emission (λex ) 456 nm) spectra of [Ir(dpyx)-
(ppy)Cl] (5) in CH3CN at 295 K (blue solid lines), together with the
corresponding spectra of [Ir(dpyx)(dppy)] (3) for comparison (red lines).
The low-energy portion of the excitation spectrum of [Ir(dpyx)(dppy)],
registered at 507 nm under the same conditions, is also shown (blue dotted
line).
tion (∑knr ) 1.5 × 105 s-1) (Table 3). Gradual photodisso-
ciation of Cl- is, however, observed in solution upon
prolonged irradiation. Interestingly, Haga and co-workers
have also observed very intense emission from a bis-benz-
imidazole-coordinated iridium complex, containing a com-
bination of ppy and Cl in the remaining coordination sites.26b
4.3. [Ir(N∧C∧N)(C∧N∧O)] Complexes: [Ir(dpyx)-
(tppic)] (7) and [Ir(dpyx)(hbqc)] (8). The ground-state
absorption spectra of 7 and 8 (Figure 7) show similar features
to that of 5. Again, the bands stretching into the visible region
(400-500 nm) are attributed to charge-transfer transitions,
blue-shifted with respect to 3, owing to the weaker ligand
field offered by the coordinating oxygen compared to the
cyclometalating carbon and consequent reduction in the
HOMO energy. In this respect, at least, the C∧N∧O ligand
is thus apparently similar to the combination of ppy and Cl
in 5.
4.2. [Ir(N∧C∧N)(C∧N)Cl] Complex: [Ir(dpyx)(ppy)-
Cl] (5). The ground-state absorption spectrum of 5 is shown
in Figure 6. As for [Ir(dpyx)(dppy)] (also shown in Figure
6 for comparison), the series of bands that extend into the
visible region may be assigned to charge-transfer transitions,
including the anticipated MLCT bands. They are significantly
blue-shifted compared to 3, which may be rationalized in
terms of a lowering of the HOMO energy upon substitution
of the second cyclometalating carbon atom of dppy by the
much weaker ligand-field chloride ligand. The LUMO is
again expected to be localized primarily on the dpyx ligand,
such that the introduction of the chloride would be expected
to have little effect on the LUMO energy.
5 is very strongly emissive in solution, exhibiting a
significantly blue-shifted spectrum compared to that of [Ir-
(dpyx)(dppy)] (Figure 6). A blue-shift in emission is again
consistent with a lowering of the HOMO energy upon
substitution of the strong-field cyclometalating carbon by a
chloride ligand, without significantly affecting the dpyx-
based LUMO. The observation of a more structured emission
profile, with a much smaller Stokes’ shift (600 cm-1
compared to 2400 cm-1 for [Ir(dpyx)(dppy)] in CH3CN at
295 K) is suggestive of an increase in LC character.
Therefore, the emission is assigned to a heavily mixed π-
(dpyx)/d(Ir) f π*(dpyx) (LC/MLCT) excited state, reflecting
the lowering of the predominantly metal-based HOMO in
[Ir(dpyx)(dppy)] to an energy similar to that of the dpyx
ligand π orbital.
The luminescence quantum yield in degassed acetonitrile
solution is outstandingly high, φ ) 0.76, and among the
highest hitherto reported for tris-cyclometalated iridium
complexes. The value is significantly higher not only than
that of [Ir(dpyx)(dppy)] (φ ) 0.21), which could be partly
expected on the basis of the energy gap law given the
difference in λmax, but also than that of fac-[Ir(ppy)3] (ΦPL
) 0.40 in toluene), which has a very similar emission energy.
Application of eqs 1 and 2 establishes that this improvement
is due to a combination of an increased radiative rate constant
(kr ) 4.8 × 105 s-1) and a decreased nonradiative contribu-
Both of the C∧N∧O complexes show a broad structureless
3
emission band, typical of phosphorescence from MLCT
states. However, DFT calculations suggest that there is a
significant amount of dpyx f tppic and dpyx f hbqc LLCT,
and LC character in both cases (Figure 8). The emission of
8 (λmax ) 562 nm) is blue-shifted with respect to 3 (585
nm), again attributable to a lower energy HOMO as a result
of the replacement of a cyclometalated carbon by the oxygen
atom. In contrast, the emission from 7 is significantly red-
shifted (λmax ) 603 nm), even though the absorption spectra
of both of the [Ir(N∧C∧N)(C∧N∧O)] complexes are very
similar to one another. This apparent anomaly is probably
related to the introduction of the p-tolyl substituent. Whereas
the HOMO-LUMO gap is likely to be similar in the ground
state of the two complexes, an additional stabilization will
be possible in the relaxed excited state of the tppic complex
by rotation of the p-tolyl group into the coplanar conforma-
tion required for maximal conjugation. We have observed a
similar effect of pendent aryl groups lowering the π-π*
emission energy, without significantly affecting the corre-
sponding absorption band, in aryl-substituted N∧C∧N-
coordinated platinum(II) complexes.33b
The luminescence quantum yields of the C∧N∧O com-
plexes are substantially lower than for the other complexes
described so far, and their lifetimes are very short (Table
8694 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
Figure 9. Absorption spectrum of [Ir(dpyx)(tpy)](PF6)2 (2a) in CH3CN
at 295 K (blue line, a), with the weak low-energy bands in the 400-500
nm region expanded for clarity (b). The emission spectrum (λex ) 370 nm,
red line, c) and low energy portion of the excitation spectrum (λem ) 501
nm, green line, d) displayed by this complex in an ethanol/methanol glass
(4:1 by volume) at 77 K are also shown.
Figure 8. DFT calculation of the HOMO (left) and LUMO (right) for
[Ir(dpyx)(tppic)] (7) (a) and [Ir(dpyx)(hbqc)] (8) (b).
3). Calculation of the radiative and nonradiative decay rate
constants reveals that the poor efficiency of emission is due
to rapid nonradiative processes: ∑knr is 1-2 orders of
magnitude larger than for the [Ir(N∧C∧N)(C∧N∧C)] com-
plexes (Table 3). Although we do not have conclusive
evidence, we suspect that the large nonradiative decay rate
constant is due to a photochemical cleavage of the Ir-O
bond in the excited state, analogous to that of the Ir-C bond
of 3. The Ir-O bond would be expected to be labilized by
the strong trans influence of the trans-disposed cyclometa-
lated carbon. Whereas photodissociation is irreversible for
[Ir(N∧C∧N)(C∧N∧C)], however, no net decomposition is
observed for the two C∧N∧O complexes, even upon intense
irradiation. Apparently, therefore, the Ir-O bond dissociation
is reversible, re-formation of this coordinate bond being
possible after nonradiative relaxation to the ground state, to
regenerate the starting complex.
4.4. [Ir(N∧C∧N)(N∧N∧N)]2+ Complexes: [Ir(dpyx)-
(tpy)]2+ (2a) and [Ir(dpyx)(ttpy)]2+ (2b). These complexes
are pale yellow in color, in contrast to the strong orange
colors of the other complexes discussed so far. The origin
of this difference is clear from the absorption data, which
reveal that the bands extending into the visible beyond 400
nm are very weak (Figure 9 and Table 2). These low-energy
bands are identified as almost purely dpyx f tpy LLCT
transitions on the basis of DFT calculations (Figure 10).
The complexes are scarcely emissive at room tempera-
ture: φ < 10-3 in degassed CH3CN. Even at 77 K, 2a is
only very weakly emissive, exhibiting a structured profile
with a maximum at 502 nm (Figure 9). The emission is
attributed to the dpyx f tpy LLCT transition. Given the
Figure 10. DFT calculation of the HOMO (left) and LUMO (right) for
[Ir(dpyx)(tpy)]2+ (2a).
small contribution of metal character to the excited state,
the oscillator strength and hence radiative rate constant, kr,
are expected to be particularly small.
Complexes of iridium(III) with an N5C donor set are rare.
Of the three instances in the literature, although no quantum
yield has been reported for the complex [Ir(bpy)2(ppy)]2+,6b,47
there was no suggestion that emission was particularly
weak,6b while [Ir(bpy-N,N′)2(Hbpy-C3,N′)]2+ was reported to
have a high quantum yield of 0.30.48 It is likely that in these
tris-bidentate complexes, there is greater MLCT character,
promoting the radiative transition. In contrast, the bis-
terdentate, mono-cyclometalated complex [Ir(bmpqpyH-
N∧N∧N)(bmpqpy-N∧N∧C)]2+ exhibits a luminescence quan-
tum yield of only 0.005 in deoxygenated acetonitrile
solution,23 behavior more consistent with that observed here
for [Ir(dpyx)(tpy)]2+ {bmpqpyH ) 2,6-bis(7′-methyl-4′-
phenyl-2′-quinolyl)-pyridine}.
Given the poor emission properties of this class of
complex, the strategy for obtaining emissive, charge-neutral
complexes represented by Class IV in Scheme 1 is clearly
not a viable one.
(47) Constable, E. C.; Leese, T. A. J. Organomet. Chem. 1987, 335, 293.
(48) (a) Watts, R. J.; Harrington, J. S.; van Houton, J. J. Am. Chem. Soc.
1977, 99, 2179. (b) Braterman, P. S.; Heath, G. A.; MacKenzie, A.
J.; Noble, B. C.; Peacock, R. D.; Yellowlees, L. J. Inorg. Chem. 1984,
23, 3425.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8695

Wilkinson et al.
Scheme 6. Schematic Summary of the Nature of the Frontier Orbitals
and of the Trends in Radiative Rate Constant (kr) and Emission
Quantum Yield (φ) for the Three Classes of Bis-terdentate Complexesa
Conclusions
This study demonstrates that a family of iridium(III)
complexes that incorporate an N∧C∧N-coordinating 1,3-di-
(2-pyridyl)benzene derivative are accessible. The introduction
of methyl substituents at C4 and C6 of the phenyl ring blocks
competitive cyclometalation at these positionssand hence
the alternative bidentate binding modesgreatly facilitating
the chemistry in terms of yields and purification. By
proceeding via the intermediacy of the dichlorobridged dimer,
1, complexes with overall charge neutrality can then be
obtained through the introduction of a second terdentate
ligand, C∧N∧C or C∧N∧O-coordinating, or through a
combination of a bidentate C∧N ligand with a monodentate
chloride anion in the sixth site.
The [Ir(N∧C∧N)(C∧N∧C)] complexes are strongly emis-
sive in fluid solution at room temperature, from states of
predominant MLCT character. For example, the quantum
yield of 4 in deoxygenated acetonitrile is comparable to that
of the widely studied electrophosphorescent material fac-
[Ir(ppy)3] (φ ) 0.443), with a similar emission wavelength
maximum, while 5 has an exceptionally high quantum yield
of 0.76 under the same conditions.
All of the complexes feature a meridional arrangement of
the nitrogen atoms. In the case of the [Ir(N∧C∧N)(C∧N∧C)]
complexes (3 and 4), this leads to two mutually trans-related
Ir-C bonds. Although the complexes show some tendency
to photodissociation in solution through cleavage of one of
these bonds, owing to the high trans influence of C-, they
are much more stable than mer-[Ir(ppy)3], as clearly evident
from the much smaller nonradiative decay rate constants
{e.g., ∑knr ) 1.6 × 105 s-1 for [Ir(dpyx)(F4dppy)] compared
to 6.5 × 106 s-1 for mer-Ir(ppy)3}. The [Ir(N∧C∧N)-
(C∧N∧O)] complexes exhibit comparatively low lumines-
cence quantum yields arising from rapid nonradiative decay,
possibly due to facile reversible photodissociation of the
Ir-O bond positioned trans to an Ir-C bond.
a The properties of the [Ir(N∧C∧N)(C∧N)Cl] complex are broadly
similar to those of the [Ir(N∧C∧N)(C∧N∧C)] class.
problems associated with diastereoisomer formation encoun-
tered with unresolved tris-bidentate complexes, while the ease
with which substituents can be incorporated into the central
five positions of the ligands will allow for the construction
of linear connections between units.49
Experimental Section
4-Tolyl-6-phenylpicolinic acid (tppicH2)31 and 2,6-di(2-thienyl)-
pyridine (dthpyH2)50 were prepared according to procedures reported
previously. 4-Hydroxybenzo[h]quinoline-2-carboxylic acid51 (hbqcH2)
was supplied by Avecia Ltd. as the ethyl ester, and hydrolyzed to
the acid using 5% NaOH(aq) at reflux. The synthesis and
characterization data for 3 and 2b were reported in the Supporting
Information of our previous communication.25
1. Synthesis of Ligands. 1.1. 1,3-Di(2-pyridyl)-4,6-dimethyl-
benzene (dpyxH). 1.1.1. Step 1: Preparation of 1,3-Dibromo-
4,6-dimethylbenzene (dibromo-m-xylene). The method was based
on a procedure described in the literature,29 but the yield was found
to be greatly improved by using >2 equiv of bromine rather the
∼1:1 ratio used previously. Bromine (43.8 g, 274 mmol) was added
over 5 min with stirring to m-xylene (13.0 g, 122 mmol), followed
by addition of iodine (0.5 g, 1.97 mmol) over a further 30 min.
The mixture was then stirred at room temperature for a further 3
h. After this period, NaOH solution (4 M, 100 mL) was added and
the reaction mixture was stirred for 15 min. The resulting precipitate
was isolated by filtration, dried under vacuum, and recrystallized
from ethanol to give the desired product as a white solid (15.7 g,
Finally, dicationic complexes of the form [Ir(N∧C∧N)-
(N∧N∧N)]2+ are also readily accessible from 1 through
reaction with a terpyridine ligand. These complexes are only
weakly emissive, probably associated with the high degree
of LLCT character of the excited state, a feature also
observed in [Ir(C∧N∧C)(N∧N∧N)]+ complexes.22
The trends in frontier orbital characteristics and their
influence on the emission properties (radiative rate constant,
kr, and quantum yield, φ) are illustrated schematically in
Scheme 6. The scheme serves to highlight the profound
influence of the ligands that complete the coordination sphere
of these iridium complexes containing one N∧C∧N-bound
ligand.
1
49%). H NMR (CDCl3, 200 MHz) δ 7.68 (1H, s, H2), 7.10 (1H,
s, H5), 2.31 (6H, s, Me). 13C{1H} NMR (CDCl3, 126 MHz) δ 137.0
(Cq), 135.0 (CH), 132.7 (CH), 122.1 (Cq), 22.4 (CH3). MS(EI) m/z
) 262/264/266 (M+), 183/185 (M+ - Br), 104 (M+ - 2Br), 103
(M+ - Br, HBr).
In summary, the compounds reported here represent a new
family of iridium complexes, in which emission energies and
intensities can be correlated with structural changes with the
aid of DFT analysis. Apart from potential applicability to
light-emitting device technology, the bis-terdentate binding
geometries will render such compounds particularly attractive
for incorporation into linear supramolecular multimetallic
architectures. The absence of chirality circumvents the
(49) For a recent example of the incorporation of a bis-terdentate complex
into a dimetallic assembly through cross-coupling, see: Arm, K. J.;
Williams, J. A. G. Dalton Trans. 2006, 2172.
(50) Constable, E. C.; Henney, R. P. G.; Tocher, D. A. J. Chem. Soc.,
Dalton Trans. 1992, 2467.
(51) Foster, R. E.; Lipscomb, R. D.; Thompson, T. J.; Hamilton, C. S. J.
Am. Chem. Soc. 1946, 68, 1327.
8696 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
1.1.2. Step 2: Formation of 1,3-di(2-pyridyl)-4,6-dimethyl-
Found: C, 66.91; H, 3.01; N, 4.64%. C17H9F4N requires: C, 67.33;
H, 2.99; N, 4.62%. Rf ) 0.6 on silica in hexane/diethyl ether, 90:
10.
benzene. A mixture of 2-tri-n-butylstannylpyridine (9.32 g, 81%
1
pure by mass by H NMR assay, 20.5 mmol), 1,5-dibromo-2,4-
dimethylbenzene (2.28 g, 8.64 mmol), bis(triphenylphosphine)-
palladium(II) chloride (225 mg, 0.32 mmol), and lithium chloride
(2.68 g, 63.2 mmol) in dry toluene (50 mL) was degassed by five
freeze-pump-thaw cycles then heated under reflux under a
nitrogen atmosphere for 24 h. The progress of the reaction was
followed by TLC (silica, hexane/diethyl ether, 75:25). After the
reaction mixture cooled to room temperature, saturated KF solution
(20 mL) was added and the solution stirred for 30 min. The
precipitated solid was removed by filtration and washed with water
(50 mL). NaHCO3 solution (10%, 100 mL) was then added to the
combined filtrates, extracted into dichloromethane (2 × 200 mL),
and dried over MgSO4. Removal of solvent under reduced pressure
and purification of the residue by column chromatography (silica,
hexane/diethyl ether, gradient elution from 100:0 to 10:90) gave
the desired product as a pale yellow solid (1.35 g, 61%), mp )
2. Synthesis of Complexes. 2.1. [Ir(dpyb)(ttpy)](PF6)2. A
suspension of 1,3-di(2-pyridyl)benzene (dpybH) (201 mg, 0.87
mmol) and iridium trichloride trihydrate (299 mg, 0.85 mmol) in a
mixture of 2-ethoxyethanol (15 mL) and water (5 mL) was heated
under reflux at 130 °C for 2.5 h. After the mixture cooled to room
temperature, the precipitated solid was collected by centrifuge,
washed with ethanol (2 × 20 mL), and dried under vacuum to give
an orange solid (342 mg). A suspension of this solid (crude [Ir-
(dpyb)Cl(µ-Cl)]2, see Discussion) (320 mg) and 4′-(p-tolyl)-2,2′:
6′,2′′-terpyridine (ttpy) (160 mg, 0.49 mmol), in ethylene glycol
(15 mL), was heated to 196 °C under a nitrogen atmosphere for 1
h. After cooling to room temperature, the reaction mixture was
added to water (20 mL) and filtered. Saturated KPF6 solution was
added, and the resulting red precipitate collected by centrifugation,
washed with water (3 × 20 mL), and dried under vacuum.
Purification by column chromatography (alumina, acetonitrile)
followed by a second column (silica, acetonitrile/water/saturated
KNO3 solution, gradient elution from 100:0:0 to 93.8:6.0:0.2) gave
the desired product as a red solid (8 mg, 1%). The complex was
not isolated in sufficient quantities for reliable elemental analysis
data to be obtained, but its identity was confirmed spectroscopi-
cally: 1H NMR (d6-acetone, 500 MHz) δ 9.43 (2H, s, H3′ ttpy),
9.03 (2H, d, J ) 8.2, H3 ttpy), 8.42 (2H, d, J ) 8.1, H3 dpyb), 8.33
(2H, d, J ) 7.9, H3′ dpyb), 8.23 (2H, d, J ) 8.2, Hb ttpy), 8.20
(2H, td, J ) 8.0, 1.3, H4 ttpy), 7.98 (2H, td, J ) 7.9, 1.3, H4 dpyb),
7.84 (2H, d, J ) 5.8, H6 dpyb), 7.73 (1H, t, J ) 7.9, H4′ dpyb),
7.57 (2H, dd, J ) 5.8, 1.2, H6 ttpy), 7.54 (2H, d, J ) 8.0, Ha ttpy),
7.45 (2H, ddd, J ) 7.8, 5.7, 1.2, H5 ttpy), 7.13 (2H, ddd, J ) 7.3,
5.9, 1.2, H5 dpyb), 2.48 (3H, s, Me ttpy). MS(ES+) m/z ) 373.5
(M2+), 892 (M(PF6)+).
1
83.8-85.6 °C. H NMR (CDCl3, 500 MHz) δ 8.67 (2H, d, J )
4.9, H6), 7.72 (2H, td, J ) 7.6, 1.8, H4), 7.47 (1H, s, H2′), 7.43
(2H, d, J ) 7.7, H3), 7.27 (1H, s, H5′), 7.23 (2H, ddd, J ) 7.5, 4.9,
1.2, H5), 2.40 (6H, s, Me). 13C{1H} NMR (CDCl3, 126 MHz) δ
159.8 (Cq), 149.3 (C6), 138.2 (Cq), 136.2 (C4), 135.9 (Cq), 133.4
(C3′), 131.2 (C6′), 124.3 (C3), 121.6 (C5), 20.1 (CH3). MS(EI) m/z
) 260 (M+), 259 (M+ - H), 245 (M+ - Me), 78 (C5NH4+•). MS-
(ES+) m/z ) 261 ([M + H]+), 283 ([M + Na]+). HRMS(ES+)
m/z ) 283.1198 ([M + Na]+); Calcd for C18H16N2Na, 283.1211.
Found: C, 80.85; H, 6.29; N, 10.28%. C18H16N2‚1/2H2O requires:
C, 80.27; H, 6.36; N, 10.40%. Rf ) 0.6 on silica in dichloromethane/
methanol, 90:10.
1.2. 2,6-Di(2,4-difluorophenyl)pyridine (F4dppyH2). The con-
ditions employed were similar to those used in our earlier work on
the application of the Suzuki reaction to the synthesis of terpy-
ridines.52 A combination of 2,6-dibromopyridine (820 mg, 3.46
mmol), 2,4-difluorophenylboronic acid (1.87 mg, 11.8 mmol), and
potassium carbonate (3.23 g, 23.4 mmol) in a mixture of 1,2-
dimethoxyethane (16 mL) and water (400 µL) was degassed by
three freeze-pump-thaw cycles. Addition of tetrakis(triphenylphos-
phine)palladium(0) (274 mg, 0.24 mmol) was followed by stirring
at room temperature under a nitrogen atmosphere for 30 min before
heating to 90 °C overnight. The progress of the reaction was
followed by TLC (alumina, hexane/ether, 50:50). After the mixture
cooled to room temperature, the precipitated solid was removed
by filtration and washed with 1,2-dimethoxyethane, followed by
removal of solvent from the combined filtrates under reduced
pressure. The residue was redissolved in dichloromethane (100 mL)
and washed with dilute NaHCO3 solution (3 × 100 mL). The
organic layer was dried over MgSO4, and the solvent removed under
reduced pressure before purification by column chromatography
(silica, hexane/diethyl ether, gradient elution from 100:0 to 98:2)
to give the desired product as a white solid (929 mg, 88%), mp )
2.2. [Ir(dpyx)(tpy)](PF6)2 (2a). A suspension of 1 (41 mg, 0.039
mmol) and 2,2′:6′,2′′-terpyridine (tpy) (22 mg, 0.094 mmol) in
ethylene glycol (5 mL) was heated to 196 °C under a nitrogen
atmosphere for 1 h. After cooling to room temperature, the reaction
mixture was added to water (10 mL) and filtered. Saturated KPF6
solution (20 mL) was added, and the resulting yellow precipitate
collected by centrifugation, washed with water (3 × 10 mL), and
dried under vacuum. Purification by column chromatography (silica,
acetonitrile/water/saturated KNO3 solution, gradient elution from
100:0:0 to 93.0:6.8:0.2) gave the desired product as a yellow solid
1
(44 mg, 58%), mp > 250 °C. H NMR (d6-acetone, 500 MHz) δ
9.17 (2H, d, J ) 8.3, H3′ tpy), 8.83-8.88 (3H, m, H4′ tpy, H3 tpy),
8.46 (2H, d, J ) 8.5, H3 dpyx), 8.22 (2H, td, J ) 8.0, 1.4, H4 tpy),
7.98 (2H, ddd, J ) 8.2, 7.6, 1.6, H4 dpyx), 7.78 (2H, dd, J ) 5.8,
0.9, H6 dpyx), 7.70 (2H, dd, J ) 5.8, 0.8, H6 tpy), 7.50 (2H, ddd,
J ) 7.6, 5.7, 1.2, H5 tpy), 7.43 (1H, s, H4′ dpyx), 7.11 (2H, ddd, J
) 7.3, 5.8, 1.1, H5 dpyx), 3.02 (6H, s, Me). 13C{1H} NMR (d6-
acetone, 126 MHz) δ 178.3 (Cq), 170.7 (Cq), 160.4 (Cq), 156.2 (C6
tpy), 154.5 (Cq), 152.8 (C6 dpyx), 142.7 (C4′ tpy), 142.1 (C4 tpy),
141.4 (C4 dpyx), 140.6 (Cq), 138.0 (Cq), 133.9 (C4′ dpyx), 130.1
(C5 tpy), 127.6 (C3 tpy), 126.6 (C3′ tpy), 125.5 (C3 dpyx), 124.6
(C5 dpyx), 22.6 (CH3). MS(ES+) m/z ) 342.5 (M2+), 830
1
83.6-84.4 °C. H NMR (CDCl3, 500 MHz) δ 8.14 (2H, td, J )
8.8, 6.8, H6), 7.83 (1H, t, J ) 7.8, H4′), 7.74 (2H, dd, J ) 7.5, 1.9,
H3′), 7.03 (2H, td, J ) 8.3, 2.1, H5), 6.93 (2H, ddd, J ) 8.8, 6.3,
2.4, H3). 13C{1H} NMR (CDCl3, 126 MHz) δ 163.2 (dd, J ) 309.4,
11.9, C2 or C4), 161.2 (dd, J ) 311.9, 12.1, C2 or C4), 152.4 (d, J
) 2.7, C2′), 137.3 (s, C4′), 132.4 (dd, J ) 9.7, 4.5, C6), 123.7 (dd,
J ) 11.7, 3.7, C1), 122.9 (d, J ) 10.2, C3′), 112.0 (dd, J ) 21.1,
3.5, C5), 104.5 (dd, J ) 26.8, 25.5, C3). 19F NMR (CDCl3, 470
MHz) δ -109.5 (1F, m), -112.8 (1F, m). MS(EI) m/z ) 303 (M+).
(M(PF6)+). HRMS(ES+) m/z ) 830.1460 (M(PF6)+); calcd for 193
-
IrC33H26F6N5P, 830.1459. Found: C, 40.08; H, 2.98; N, 6.82%.
C33H26F12IrN5P2 requires: C, 40.66; H, 2.69; N, 7.18%. One spot
by TLC(silica), Rf ) 0.4 in acetonitrile/water/saturated KNO3
solution, 90:9:1.
2.3. [Ir(dpyx)(F4dppy)] (4). 1 (41 mg, 0.039 mmol), 2,6-di-
(2,4-difluorophenyl)pyridine (F4dppyH2) (227 mg, 0.75 mmol) and
AgOTf (40 mg, 0.16 mmol) were ground together in a porcelain
(52) (a) Aspley, C. J.; Williams, J. A. G. New J. Chem. 2001, 25, 1136.
(b) Leslie, W.; Wild, K.; Arm, K. J.; Williams, J. A. G. J. Chem.
Soc., Perkin Trans. 2 2002, 1669.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8697

Wilkinson et al.
mortar. The resulting fine powder was heated to 110 °C and stirred
under a nitrogen atmosphere for 24 h. After the mixture cooled to
room temperature, the product was extracted into dichloromethane
(30 mL), and the remaining solid removed by filtration. Removal
of solvent under reduced pressure gave a brown residue. This was
purified by column chromatography (silica, hexane/diethyl ether,
gradient elution from 100:0 to 95:5; the unreacted excess F4dppyH2
elutes first, and can be recovered) followed by a second column
(silica, hexane/diethyl ether, gradient elution from 100:0 to 98:2)
to give the desired product as a yellow solid (12 mg, 21%), mp >
dpyx), 7.70 (1H, dd, J ) 7.5, 1.4, H3′ dthpyH), 7.34 (2H, ddd, J )
5.8, 1.6, 0.6, H6 dpyx), 7.17 (1H, dd, J ) 5.2, 3.8, H4′′ dthpyH),
7.07 (1H, s, H4′ dpyx), 7.04 (1H, dd, J ) 5.2, 0.7, H5′′ dthpyH),
7.03 (1H, d, J ) 4.9, H5 dthpyH), 6.99 (2H, ddd, J ) 7.5, 5.7, 1.3,
H5 dpyx), 5.69 (1H, d, J ) 4.9, H4 dthpyH), 2.86 (6H, s, CH3).
13C{1H} NMR (CDCl3, 101 MHz) δ 174.2 (Cq), 169.6 (Cq), 159.0
(Cq), 151.7 (CH), 149.2 (Cq), 148.7 (Cq), 140.8 (CH), 139.2 (Cq),
138.8 (C4 dpyx), 138.3 (Cq), 138.0 (Cq), 135.5 (Cq), 134.3 (C4′′
dthpyH), 132.9 (C3′′ dthpyH), 131.9 (C4′ dpyx), 130.2 (C4 dthpyH),
128.9 (C5 dthpyH or C5′′ dthpyH), 127.8 (CH), 123.4 (C3 dpyx),
123.2 (C5 dpyx), 117.4 (C3′ dthpyH), 117.1 (CH), 22.7 (CH3). MS-
(ES+) m/z ) 694 (M+). HRMS(ES+) m/z ) 694.0966 (M+); calcd
for 193IrC31H23N3S2, 694.0963.
1
250 °C. H NMR (CDCl3, 500 MHz) δ 8.16 (2H, d, J ) 8.2, H3′
F4dppy), 8.07 (2H, d, J ) 8.3, H3 dpyx), 7.85 (1H, t, J ) 8.1, H4′
F4dppy), 7.57 (2H, dd, J ) 5.7, 1.1, H6 dpyx), 7.49 (2H, ddd, J )
7.5, 6.0, 1.5, H4 dpyx), 6.97 (1H, s, H4′ dpyx), 6.63 (2H, ddd, J )
5.8, 4.9, 1.0, H5 dpyx), 6.20 (2H, ddd, J ) 12.8, 9.1, 2.4, H3 F4-
dppy), 5.65 (2H, dd, J ) 7.2, 2.4, H5 F4dppy), 2.94 (6H, s, Me
dpyx). 13C{1H} NMR (CDCl3, 126 MHz) δ 191.2 (Cq), 176.1 (Cq),
168.6 (Cq), 163.1 (dd, J ) 259.9, 10.3, CF), 162.2 (dd, J ) 260.8,
10.3, CF), 161.2 (d, J ) 7.21, Cq), 149.8 (CH), 138.4 (CH), 137.0
(Cq), 134.8 (Cq), 133.8 (CH6 dpyx), 131.0 (Cq), 127.2 (CH4′ dpyx),
122.6 (CH3 dpyx), 120.5 (CH), 118.0 (dd, J ) 13.5, 2.6, CH5 F4-
dppy), 117.6 (d, J ) 18.8, CH3′ F4dppy), 97.6 (t, J ) 26.2, CH3
F4dppy), 23.0 (CH3). 19F NMR (CDCl3, 282 MHz) δ -110.77 (2F,
q, J ) 8.8), -111.02 (2F, dd, J ) 13.4, 9.0). MS(ES+) m/z ) 754
([M + H]+). HRMS(ES+) m/z ) 754.1440 ([M + H]+); calcd for
193IrC35H23F4N3, 754.1452. Found: C, 55.45; H, 3.34; N, 5.21%.
C35H23F4IrN3 requires: C, 55.77; H, 3.08; N, 5.57%. Rf ) 0.2 on
silica in hexane/diethyl ether, 90:10.
2.6. [Ir(dpyx)(tppic)] (7). 1 (373 mg, 0.36 mmol), 4-p-tolyl-6-
phenylpicolinic acid (tppicH2) (1.03 g, 3.55 mmol), AgOTf (552
mg, 2.15 mmol), and benzoic acid (3.49 g, 28.6 mmol) were ground
together in a porcelain mortar. The resulting fine powder was heated
to 150 °C and stirred under a nitrogen atmosphere for 24 h. After
the mixture cooled to room temperature, the product was dissolved
in dichloromethane (50 mL) and washed with sodium hydrogen
carbonate solution (1 M, 3 × 50 mL). Drying over Na2CO3 was
followed by removal of solvent under reduced pressure. The brown
residue was purified by column chromatography (silica, dichlo-
romethane/methanol, gradient elution from 100:0 to 97:3) to give
the desired product as an orange solid (286 mg, 54%), mp > 250
°C. 1H NMR (CDCl3, 500 MHz) δ 8.48 (1H, d, J ) 1.3, H3 tppic),
8.31 (1H, d, J ) 1.4, H5 tppic), 8.04 (2H, d, J ) 8.1, H6 dpyx),
7.86 (2H, d, J ) 8.1, Hb tppic), 7.70 (1H, dd, J ) 7.9, 1.3, H6′
tppic), 7.59 (2H, ddd, J ) 8.1, 7.7, 1.5, H5 dpyx), 7.41-7.44 (4H,
m, Ha tppic, H3 dpyx), 6.94 (1H, s, H4′ dpyx), 6.79 (2H, ddd, J )
7.1, 6.0, 1.0, H4 dpyx), 6.78 (1H, ddd, J ) 8.7, 7.7, 1.3, H5′ tppic),
6.57 (1H, td, J ) 7.5, 1.2, H4′ tppic), 5.82 (1H, dd, J ) 7.8, 1.0,
H3′ tppic), 2.86 (6H, s, Me dpyx), 2.51 (3H, s, Me tppic). MS-
(ES+) m/z ) 740 ([M + H]+). MS(EI) m/z ) 739 (M+), 695 (M+
- CO2), 347.5 (M2+ - CO2). HRMS(ES+) m/z ) 740.1881 ([M
+ H]+); calcd for 193IrC37H29O2N3, 740.1884. Found: C, 59.74;
H, 4.28; N, 5.52%. C37H29IrN3O2 requires: C, 60.07; H, 3.95; N,
5.68%. Rf ) 0.5 in dichloromethane/methanol, 90:10.
2.4. [Ir(dpyx)(ppy)Cl] (5). 1 (35 mg, 0.033 mmol) and AgOTf
(44 mg, 0.17 mmol) in 2-phenylpyridine (ppyH) (250 µl, 1.75
mmol) were heated to 110 °C under a nitrogen atmosphere for 24
h. After the mixture cooled to room temperature, dichloromethane
(25 mL) was added and the remaining solid removed by filtration.
Washing of the filtrate with HCl (1 M, 3 × 25 mL), drying over
MgSO4, and removal of solvent under reduced pressure gave a
yellow residue. This was purified by column chromatography (silica,
dichloromethane/methanol, gradient elution from 100:0 to 99.75:
0.25) to give the desired product as a yellow solid (25 mg, 59%),
1
mp > 250 °C. H NMR (CDCl3, 500 MHz) δ 10.12 (1H, d, J )
2.7. [Ir(dpyx)(hbqc)] (8). 1 (40 mg, 0.038 mmol), 4-hydroxy-
benzo[h]quinoline-2-carboxylic acid (hbqcH2) (87 mg, 0.36 mmol),
AgOTf (59 mg, 0.23 mmol), and benzoic acid (314 mg, 2.57 mmol)
were ground together in an agar mortar. The resulting fine powder
was heated to 110 °C and stirred under a nitrogen atmosphere for
24 h. After the mixture cooled to room temperature, the solid was
extracted into acetonitrile and filtered through Celite, followed by
removal of solvent under reduced pressure. The residue was
dissolved in the minimum amount of acetonitrile and added to water
(50 mL). The resulting yellow precipitate was collected by
centrifugation, washed with water (3 × 10 mL), and dried under
vacuum. The solid, which may be solubilized in dichloromethane
by the addition of a few drops of trifluoroacetic acid, was purified
by column chromatography (silica, dichloromethane/methanol,
gradient elution from 100:0 to 97.5:2.5) to give the desired product
as an yellow solid (30 mg, 58%), mp > 250 °C. 1H NMR (CDCl3,
500 MHz) δ 8.12 (1H, d, J ) 9.0, H5 hbqc), 8.12 (1H, s, H3 hbqc),
8.06 (2H, d, J ) 8.4, H3 dpyx), 7.83 (1H, d, J ) 9.0, H6 hbqc),
7.63 (2H, td, J ) 7.9, 1.7, H4 dpyx), 7.28 (2H, d, J ) 5.8, H6
dpyx), 7.25 (1H, d, J ) 7.4, H7 hbqc), 7.00 (1H, br s, H4′ dpyx),
6.90 (1H, t, J ) 7.6, H8 hbqc), 6.78 (2H, ddd, J ) 7.0, 5.8, 1.0, H5
dpyx), 5.86 (1H, d, J ) 7.5, H9 hbqc), 2.87 (6H, s, Me dpyx).
13C{1H} NMR (CDCl3, 126 MHz) δ 183.8, 175.3, 171.0, 163.3,
151.5, 151.0, 144.5, 140.8, 138.1 (C4 dpyx), 138.1, 137.8, 134.8,
133.3, 130.7 (C6 hbqc), 130.4, 129.4 (C8 hbqc), 123.0 (C3 dpyx),
5.3, H6 ppy), 8.07 (1H, d, J ) 8.0, H3 ppy), 8.00 (2H, d, J ) 8.4,
H3 dpyx), 7.96 (1H, td, J ) 8.0, 1.4, H4 ppy), 7.65 (2H, d, J ) 5.6,
H6 dpyx), 7.59 (1H, d, J ) 7.7, H6′ ppy), 7.51-7.55 (3H, m, H4
dpyx, H5 ppy), 6.88 (1H, s, H4′ dpyx), 6.75 (2H, ddd, J ) 7.2, 5.9,
1.0, H5 dpyx), 6.71 (1H, td, J ) 7.4, 1.2, H5′ ppy), 6.54 (1H, td, J
) 7.5, 1.0, H4′ ppy), 6.00 (1H, d, J ) 7.7, H3′ ppy), 2.83 (6H, s,
Me). MS(ES+) m/z ) 606 ([M - Cl]+), 638 ([M - Cl + MeOH]+).
Found: C, 53.98; H, 3.81; N, 6.30%. C29H23ClIrN3 requires: C,
54.33; H, 3.62; N, 6.55%.
2.5. [Ir(dpyx)(dthpyH)](CH3CO2) (6). A suspension of 1 (41
mg, 0.039 mmol), 2,6-di(2-thienyl)pyridine (dthpyH2) (29 mg, 0.12
mmol), and AgOTf (86 mg, 0.33 mmol) in glacial acetic acid (2
mL) was heated to 110 °C under a nitrogen atmosphere overnight.
After the mixture cooled to room temperature, water (20 mL) was
added and the mixture extracted into dichloromethane (3 × 20 mL),
dried over MgSO4, and filtered. Removal of solvent under reduced
pressure followed by purification by column chromatography (silica,
dichloromethane/methanol, gradient elution from 100:0 to 95:5)
gave the product as a yellow solid (10 mg, 36%), mp > 250 °C.
Elemental analysis data was inconclusive due to the ambiguity of
1
coordination at the sixth site (L in Scheme 5). H NMR (CDCl3,
400 MHz) δ 8.22 (1H, dd, J ) 7.9, 1.4, H5′ dthpyH), 8.19 (1H, t,
J ) 7.8, H4′ dthpyH), 8.07 (2H, d, J ) 8.5, H3 dpyx), 7.95 (1H,
dd, J ) 3.8, 0.8, H3′′ dthpyH), 7.75 (2H, ddd, J ) 8.3, 7.6, 1.7, H4
8698 Inorganic Chemistry, Vol. 45, No. 21, 2006

Luminescent Complexes of Iridium(III)
122.2 (C5 dpyx), 120.9, 120.2, 119.5, 118.9, 108.1 (C5 hbqc or C3
hbqc), 22.6 (CH3 dpyx). MS(ES+) m/z ) 690 ([M + H]+). HRMS-
(ES+) m/z ) 690.1364 ([M + H]+); calcd for 193IrC32H23O3N3,
690.1363. Rf ) 0.3 in dichloromethane/methanol, 90:10. Found:
C, 55.26; H, 3.40; N, 5.74%. C32H22IrN3O3 requires: C, 55.80; H,
3.22.; N, 6.10%.
KR radiation (λ ) 0.71073 Å) on a Bruker SMART 1K area
detector diffractometer, equipped with a Oxford Cryosystems N2
open-flow cooling device. Series of narrow ω scans (0.3°) were
performed at several æ settings in such a way as to cover a sphere
of data to a maximum resolution between 0.70 and 0.77 Å. Cell
parameters were determined and refined using the Bruker SMART
software,55 and raw frame data were integrated using the Bruker
SAINT program.56 The structure was solved by direct methods and
refined by full-matrix least squares on F2 using SHELX.57 Reflec-
tion intensities were corrected by numerical integration using
SADABS.58 All non-hydrogen atoms were refined with anisotropic
displacement parameters, and the hydrogen atoms were positioned
geometrically and refined using a riding model. Crystal data and
structure refinement parameters: C20H21Cl2IrN2OS; Mr ) 600.55;
T ) 120(2) K; λ(Mo KR) ) 0.71073 Å; monoclinic, space group
P21/n a ) 14.379(11) Å, b ) 8.779(7) Å, c ) 15.889(12) Å, â )
97.811(7)°, V ) 1987(3) Å3; Dc ) 2.007 Mg m-3; Z ) 4; µ(Mo
KR) ) 7.107 mm-1; reflections collected/unique 21 215/4559
[Rint ) 0.0296]; final R indices [I > 2σ(I)]: R1 ) 0.0191 and wR2
) 0.0487 [6262]; R indices (all data): R1 ) 0.0237 and wR2 )
0.0496.
3. Photophysical Measurements. Absorption spectra were
measured 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;
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; final vapor pressure at
77 K was <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 air-equilibrated aqueous
solution (φ ) 0.02853) as the standard; estimated uncertainty in φ
is (20% or better.
Acknowledgment. We thank the EPSRC and Avecia Ltd
for a CASE studentship (to A.J.W.), Frontier Scientific Ltd
for key starting materials, the EPSRC National Mass
Spectrometry Service Centre for several mass spectra, and
Dr. A. Beeby for access to the laser system.
Samples for time-resolved measurements were excited at 355
nm using the third harmonic of a Q-switched Nd:YAG laser. The
luminescence was detected with a Hamamatsu R928 photomultiplier
tube and recorded using a digital storage oscilloscope, before
transfer to a PC for analysis. The estimated uncertainty in the quoted
lifetimes is (10% or better. Bimolecular rate constants for
quenching by molecular oxygen, kQ, were determined from the
lifetimes in degassed and air-equilibrated solution, taking the
concentration of oxygen in CH3CN at 0.21 atm O2 to be 1.9 mmol
Supporting Information Available: Crystallographic data in
CIF format; results of structural calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
dm-3 54
Where lifetimes in aerated solution were too short to be
.
IC061172L
measured accurately using the available instrumentation, kQ was
estimated from the corresponding integrated emission intensities.
4. Crystallography. The single-crystal X-ray diffraction experi-
ment was carried out at 120 K, using graphite-monochromated Mo
(55) SMART, Data Collection Software, version 5.625; Bruker Analytical
X-ray Instruments Inc.: Madison, WI, 2001.
(56) SAINT, Data Reduction Software, version 6.02A.; Bruker Analytical
X-ray Instruments Inc.: Madison, WI, 2001.
(53) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.
(54) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(57) Sheldrick, G. M. SHELX. Acta Crystallogr. 1990, A46, 467-473.
(58) Sheldrick, G. M. SADABS, Version 2.03; University of Go¨ttingen:
Go¨ttingen, Germany, 2002.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8699