Luminescent Iridium(III) Complexes Supported by N-Heterocyclic Carbene-based C^C^C-Pincer Ligands and Aromatic Diimines

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Iridium(III) hydrido complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-butylimidazolin-2-ylidene)phenyl anion (C(1)^C^C(1)) or 1,3-bis(3-butylbenzimidazolin-2-ylidene)phenyl anion (C(2)^C^C(2)) and aromatic diimine (2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), or dipyrido-[3,2-f:2',3'-h]-quinoxaline (dpq)) in the form of [Ir(C^C^C)(N^N)(H)](+) have been prepared. Crystal structures for these complexes show that the Ir-CNHC distances are 2.043(5)-2.056(5) Å. The hydride chemical shifts for complexes bearing C(1)^C^C(1) (-20.6 to -20.3 ppm) are more upfield than those with C(2)^C^C(2) (-19.5 and -19.2 ppm), revealing that C(1)^C^C(1) is a better electron donor than C(2)^C^C(2). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ = 340-530 nm (ε ≤ 10(3) dm(3) mol(-1) cm(-1))) originate from a dπ(Ir(III)) → π*(N^N) metal-to-ligand charge transfer transition, where the dπ(Ir(III)) level contain significant contribution from the C^C^C ligands. All these complexes are emissive in the yellow-spectral region (553-604 nm in CH3CN and CH2Cl2) upon photo-excitation with quantum yields of 10(-3)-10(-1).

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## OPEN

received: 22 June 2015

accepted: 16 September 2015

Published: 21 October 2015

# Luminescent Iridium(III) Complexes Supported by N-Heterocyclic Carbene-based C^C^C-Pincer Ligands and Aromatic Diimines

Lai-Hon Chung 1 , Hoi-Shing Lo 1,2 , Sze-Wing Ng 1 , Dik-Lung Ma 3 , Chung-Hang Leung 4 &
Chun-Yuen Wong 1,2

Iridium(III) hydrido complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1butylimidazolin-2-ylidene)phenyl anion (C 1 ^C^C 1 ) or 1,3-bis(3-butylbenzimidazolin-2-ylidene)
phenyl anion (C 2 ^C^C 2 ) and aromatic diimine (2,2′-bipyridine (bpy), 1,10-phenanthroline (phen),
4,4′-dimethyl-2,2′-bipyridine (Me2bpy), or dipyrido-[3,2-f:2′,3′-h]-quinoxaline (dpq)) in the form
of [Ir(C^C^C)(N^N)(H)] + have been prepared. Crystal structures for these complexes show that
the Ir–CNHC distances are 2.043(5)–2.056(5) Å. The hydride chemical shifts for complexes bearing
C 1 ^C^C 1 (−20.6 to −20.3 ppm) are more upfield than those with C 2 ^C^C 2 (−19.5 and −19.2 ppm),
revealing that C 1 ^C^C 1 is a better electron donor than C 2 ^C^C 2 . Spectroscopic comparisons and timedependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic
transition associated with these complexes (λ = 340–530 nm (ε ≤ 10 3 dm 3 mol −1 cm −1 )) originate
from a dπ(Ir III ) → π*(N^N) metal-to-ligand charge transfer transition, where the dπ(Ir III ) level contain
significant contribution from the C^C^C ligands. All these complexes are emissive in the yellowspectral region (553–604 nm in CH3CN and CH2Cl2) upon photo-excitation with quantum yields of
10 −3 –10 −1 .

Polypyridyl ruthenium(II) or other d 6 -transition metal complexes represent an important class of
emissive molecular material 1–14 . Their triplet [dπ(M) → π *(polypyridyl)] metal-to-ligand charge transfer ( 3 MLCT) excited-states are known to derive rich photophysical and photochemical properties, and
their applications in solar energy conversion 11,15, organic light emitting devices (OLEDs) 16, photochemistry 11,12, and bio-labelling reagents 17 have received considerable interest. Regarding the design of transition metal-containing luminophores, iridium(III) center has received great attention on the basis that it
is a heavier analogue of ruthenium(II) center 18,19 .

After the isolation of stable NHCs by Arduengo and co-workers in 1991 20, intensive investigations
on NHCs and the derived metal complexes have been centralized on the development of catalytic reagents for organic transformations 21–25 . On the other hand, employment of N-heterocyclic carbenes
(NHCs)-derived ligands as an alternative of polypyridines in the design of transition metal-based luminophores is growing to be an important research topic recently. For example, emissive Ru(II) 26, Ir(III) 18,19,
and Pt(II) 27 complexes supported by NHCs have been reported. Several emissive NHC-containing

1Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
SAR. 2 State Key Laboratory of Millimeter Waves, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong
Kong SAR. 3 Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR. 4 State Key
Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau,
[Macao, China. Correspondence and requests for materials should be addressed to C.-Y.W. (email: acywong@cityu.](mailto:acywong@cityu.edu.hk)
[edu.hk)](mailto:acywong@cityu.edu.hk)

Figure 1. Synthetic route of 1–2

multinuclear Cu, Ag, and Au complexes have also been prepared, in which the NHCs facilitate the
metal-metal interaction-induced emissions 28,29 .

We have initiated a program to develop organometallic Ru(II)/Os(II)–diimine and related
luminophores 30–33, and very recently we have reported emissive osmium(II) carbonyl complexes bearing 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion ( Me C 1 ^C^C 1Me ) or 1,3-bis(3-m
ethylbenzimidazolin-2-ylidene)phenyl anion ( Me C 2 ^C^C 2Me ) and aromatic diimine in the form of

[Os(C^C^C)(N^N)(CO)] + 30 . Spectroscopic and theoretical investigations on [Os(C^C^C)(N^N)(CO)] +
have revealed that the emissive state for [Os(C^C^C)(N^N)(CO)] + originates from a dπ(Os II ) → π *(N^N)
MLCT transition, where the C^C^C ligands contribute significantly to both the dπ(Os II ) and π *(N^N)
levels. This suggests that the NHC-derived ligands would not only act as point charge/spectator ligands, but can also be involved in the emissive excited-state to modify the photophysical properties of a
metal–diimine luminophore. As an extension to scrutinize the effect of C^C^C pincer ligands on the
photophysical properties of a [M(N^N)] moiety, we now present the preparation, spectroelectrochemical, photophysical, and theoretical investigations of a class of emissive hydrido iridium(III) complexes
bearing C^C^C pincer ligands and aromatic diimines, [Ir(C^C^C)(N^N)(H)] + .

Results
Synthesis. Emissive Ir(III) complexes [Ir(C^C^C)(N^N)(H)] + ( 1 - 2 ) were prepared by refluxing

[Ir(C^C^C)(CH3CN)(Br)(H)] with N^N in ethylene glycol (Figure 1). [Ir(C^C^C)(CH3CN)(Br)(H)]
were synthesized analogously to the corresponding known complexes [Ir( Ar C^C^C Ar )(CH3CN)(Cl)
(H)] and [Ir( Me C 1 ^C^C 1Me )(CH3CN)(I)(H)] ( Ar C^C^C Ar = 1,3-bis(1-arylimidazolin-2-ylidene)phenyl
anion) 23,24 . The presence of the hydride ligands in 1 - 2 was confirmed by the 1 H NMR signals at -20.6
to -19.2 ppm and νIr–H at 2126 to 2189 cm −1 . Both the 1 H and 13 C NMR spectra signify that 1 - 2 possess
a pseudo-plane of symmetry in solution on the NMR time scale at room temperature. For instances,
there are 17 and 19 sets of aromatic 13 C signal for 1a and 1b, respectively. The 13 C NMR signals at 167.8–
180.6 ppm for 1–2 are typical for metalated NHC. It is noted that the hydride chemical shifts for 1a - 1d
(-20.55 to -20.27 ppm) are nearly 1 ppm more upfield than those for 2a - 2d (-19.50 to -19.21 ppm).
Since the hydride chemical shifts indicate that the electronic shielding effect of the hydrido group results
from the metal core’s electron cloud, they can be used as probes for the donating ability of the C^C^C
ligands 34 . Therefore, the more upfield hydride chemical shifts for 1a - 1d when compared with 2a - 2d
reveals that C 1 ^C^C 1 is a stronger electron donor than C 2 ^C^C 2 . The same conclusion has recently been
made in the comparison of the νCO between [Os( Me C 1 ^C^C 1Me )(N^N)(CO)] + and [Os( Me C 2 ^C^C 2Me )
(N^N)(CO)] + 31 . These findings are also consistent with the NHC donor strengths determined by Huynh
_et al._, where benzimidazolin-2-ylidene is suggested to have a weaker donor strength compared with
imidazolin-2-ylidene 35 . Complex [Ir(C 1 ^C^C 1 )2] + has also been synthesized according to the method
reported in literature 23 for spectroscopic comparisons.

The molecular structures of 1a (ClO4), 2a (ClO4), and [ 2b (ClO4)]3·CH3CN have been determined by
X-ray crystallography. Perspective views of the cations 1a and 2b are depicted in Figure 2; selected bond
distances and angles are summarized in Table 1. In each case, the Ir atom adopts a distorted octahedral
geometry, with the C^C^C-pincer coordinating in a meridional mode. The ring systems on C^C^C are
not perfectly co-planar: the NHC moieties (i.e. imidazolin-2-ylidene or benzimidazolin-2-ylidene units)
are tilted towards the hydride ligands, and the angles between the NHC planes are 12.36–22.54°. These
angles are larger than those found in [Ir( Me C 1 ^C^C 1Me )(CH3CN)(I)(H)], [Ir(C^ Me C Me ^C)(CH3CN)(I)2]
(C^ Me C Me ^C = 1,3-bis(1-butylimidazolium)-4,6-dimethylbenzene) and [Ir( Ar C^C^C Ar )(CH3CN)(Cl)
(H)] in which the angles between the NHC planes are 2.73°, 3.63° and 5.60–15.58° respectively 23,24 . As
a comparison, the ring systems on C^C^C for [Os( Me C 1 ^C^C 1Me )(N^N)(CO)] + and [Os( Me C 2 ^C^C 2Me )
(N^N)(CO)] + (N^N = bpy or phen) are more close to a co-planar configuration (angles between the
NHC planes are 2.30–13.00°) 31 . The CNHC–Ir–CPh angles for these complexes are 77.22(11)–78.74(15)°,
which are only slightly larger than the CNHC–Os–CPh angles in [Os( Me C 1 ^C^C 1Me )(N^N)(CO)] + and

[Os( Me C 2 ^C^C 2Me )(N^N)(CO)] + (75.6(3)–76.8(3)°) 31 . The Ir–CNHC distances (2.043(5)–2.056(5) Å) are
notably longer than the Ir–CPh distances (1.959(4)–1.986(5) Å). Similar findings have been observed

Figure 2. Perspective views of 1a (top) and 2b (bottom); thermal ellipsoids are at the 50% and 30%
probability level, respectively. Hydrogen atoms are omitted for clarity.

complex 1a 2a 2b

[2.052(5), 2.056(5)]; [2.045(5), 2.049(5)];
Ir–CNHC 2.049(3), 2.055(3) 2.044(4), 2.046(4)

[2.043(5), 2.044(5)]

Ir–CPh 1.975(3) 1.959(4) 1.986(5); 1.982(5); 1.980(5)

Ir–NN^N (trans to Ph) 2.130(2) 2.134(3) 2.140(4); 2.134(4); 2.149(4)

Ir–NN^N (trans to H) 2.144(2) 2.154(3) 2.135(5); 2.148(4); 2.141(5),

[77.7(2), 77.8(2)]; [77.8(2), 77.9(2)];
CNHC–Ir–CPh 77.22(11), 77.86(11) 78.30(15), 78.74(15)

[77.7(2), 78.7(2)]

∠Ph/NHC 7.90, 16.96 5.06, 7.35 [4.86, 10.71]; [8.21, 8.35]; [3.35, 10.50]

∠NHC/NHC 22.54 12.36 14.83; 16.41; 13.85

Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1a, 2a, and 2b. The angle between the rings
(∠Ph/NHC or ∠NHC/NHC) are calculated from all non-hydrogen atoms on the ring moiety). For 2b, the
crystal contains three crystallographically independent Ir complexes in an asymmetric unit; structural data
from each complex are grouped in brackets and listed in the order of Ir(1), Ir(2), and Ir(3).

Complex

_E_ 1/2/V vs Cp2Fe +/0

reduction oxidation

1a - 1.93 _E_ pa = 0.7

1b - 1.95 _E_ pa = 0.71

1c - 2.00 _E_ pa = 0.69

1d - 1.83 _E_ pa = 0.70

2a - 1.87 _E_ pa = 0.76

2b - 1.87 _E_ pa = 0.76

2c - 1.95 _E_ pa = 0.73

2d - 1.73 _E_ pa = 0.77

Ir(C 1 ^C^C 1 )2] + — 0.71

Table 2. Electrochemical Data. Supporting electrolyte: 0.1 M [Bu4N]PF6 in CH3CN. _E_ 1/2 = ( _E_ pc + _E_ pa)/2
at 298 K for reversible couples. Anodic peak potential ( _E_ pa) at scan rate of 100 mV s −1 are recorded for
irreversible oxidation wave.

in [Ir( Me C 1 ^C^C 1Me )(CH3CN)(I)2] 23, [Ir( Me C 1 ^C^C 1Me )(CH3CN)(I)(H)] 24, [Ir(C^ Me C Me ^C)(CH3CN)
(I)2] (C^ Me C Me ^C = 1,3-bis(1-butylimidazolium)-4,6-dimethylbenzene) 23, [Ir( Ar C^C^C Ar )(CH3CN)(Cl)
(H)] 24, and Zr, Rh, and Os complexes bearing similar C^C^C-pincer ligands 31,36 . Since the Ir–CPh distances in _fac_ -[Ir(C^C)3] and _mer_ -[Ir(C^C)3] (C^C = 1-phenyl-3-methylbenzimidazolin-2-ylidene- _C_, _C_ 2′ )
are in the range of 2.071(7)− 2.099(4) Å 18, the significantly shorter Ir–CPh distances in this work most
likely arise from the strain intrinsic to the metal–C^C^C moieties.

Electrochemistry. Cyclic voltammetry has been used to examine the electrochemistry of the complexes (Table 2; all values vs Cp2Fe +/0 ). 1 - 2 show irreversible first oxidation waves at _E_ pa = 0.69 to 0.77 V
(scan rate = 100 mV s −1 ), and reversible first reduction couples at _E_ 1/2 = -2.00 to -1.73 V. It is noted that
both the first oxidation waves and the first reduction couples are sensitive to the change of C^C^C and
N^N. For example, the first reduction potentials for 1a - 1d (-2.00 to -1.83 V) are slightly more negative
than those for 2a - 2d (-1.95 to -1.73 V), and the ease of reduction follows the order: d - b ≈ a - c . These
findings suggest that both the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied
molecule orbitals (LUMOs) for 1 - 2 contain contributions from the C^C^C and N^N, in agreement with
our DFT calculations (see discussion below). Moreover, the contribution of N^N to the LUMOs of 1 - 2
is apparent as [Ir(C 1 ^C^C 1 )2] + does not feature any reduction wave within the solvent window.

UV–Visible Absorption and Spectroelectrochemistry. The UV–visible spectral data for 1, 2,
and [Ir(C 1 ^C^C 1 )2] + are summarized in Table 3, and their absorption spectra are depicted in Figure 3.
1–2 exhibit intense, high-energy absorptions at λ ≤ 340 nm (ε ≥ 10 4 dm 3 mol −1 cm −1 ), and moderately
intense bands at λ - 340 nm (ε ≈ 10 3 dm 3 mol −1 cm −1 ) with tailing up to 530 nm. In the literature, Ir(III)
complexes bearing aromatic diimine ligands such as [Ir(bpy)3] 3+ and [Ir(phen)3] 3+ feature highly intense
absorptions at λ ≤ 320 nm (ε ≥ 10 4 dm 3 mol −1 cm −1 ), and these are ascribed to π → π *(N^N) intraligand
(IL) transitions 37–39 . In addition, [Ir(C 1 ^C^C 1 )2] + exhibits intense absorptions at λ ≤ 330 nm (ε ≥ 10 4 dm 3
mol −1 cm −1 ), which are expected to be a mixture of dπ(Ir III ) → π *(C^C^C) metal-to-ligand charge transfer (MLCT) and π → π *(C^C^C) IL transition. With the origin of absorptions for [Ir(N^N)3] 3+ and

[Ir(C 1 ^C^C 1 )2] + as references, the high-energy absorptions at λ ≤ 340 nm for complexes 1 - 2 are assigned
to be a mixing of π → π *(C^C^C) IL, π → π *(N^N) IL, and dπ(Ir III ) → π *(C^C^C) MLCT transitions.

On the other hand, the electronic transitions at λ = 340–530 nm (ε ≤ 10 3 dm 3 mol −1 cm −1 ) for 1 2 should contain some dπ(Ir III ) → π *(N^N) MLCT character, reasons are as follows: (1) [Ir(bpy)3] 3+,

[Ir(phen)3] 3+, and [Ir(ppy)2(bpy)] + (ppy = 2-phenylpyridine) feature dπ(Ir III ) → π *(N^N) MLCT transitions in similar energy region (λmax = 370–520 nm, ε ≤ 10 3 dm 3 mol −1 cm −1 ); 38–43 (2) a red-shift in
absorption energy is observed when N^N is changed from Me2bpy to bpy, and from phen to dpq; (3)
1 - 2 display solvatochromic effect in the spectral region concerned. For example, the λmax for 1a within
this spectral region is 374 nm in CH3CN, and is 384 nm in CH2Cl2; (4) there are no corresponding
absorption bands for [Ir(C 1 ^C^C 1 )2] + . This assignment is consistent with the TD-DFT calculations on
complexes 1a and 2a, which suggest that the nature of electronic transitions in the spectral region concerned to be mainly attributed to the HOMO–1 → LUMO and HOMO–2 → LUMO transitions, where
the HOMO–1 and HOMO–2 have higher Ir contribution (27–59%) than that in LUMO (3–4%), and
LUMO has higher N^N contribution (93%) than those in HOMO–1 and HOMO–2 (3–15%) (see discussion below). The contribution of N^N to the LUMOs for 1 and 2 is further confirmed by spectroelectrochemistry. Thin-layer UV–visible spectroelectrochemistry has been employed to acquire the absorption
spectra for 1a − and 2a −, the reduced forms of 1a and 2a respectively (Figure 4). The isosbestic spectral

complex λmax/nm (εmax/dm 3 mol −1 cm −1 )

Solvent = CH3CN

1a 241 (sh, 36670), 283 (26380), 308 (sh, 14980), 319 (sh, 11720), 374 (4240), 439 (sh, 1620)

1b 246 (33910), 260 (30980), 281 (sh, 21780), 308 (10680), 320 (11250), 370 (5460), 455 (sh, 1720)

1c 242 (sh, 37920), 281 (27830), 304 (sh, 16760), 321 (sh, 13090), 371 (4640), 430 (sh, 1950)

1d 250 (sh, 57990), 256 (58720), 288 (37050), 319 (14630), 374 (7180), 459 (sh, 2210)

2a 247 (63480), 271 (44590), 285 (sh, 35650), 309 (25000), 321 (22070), 370 (sh, 5240), 422 (sh, 1710)

2b 248 (57050), 269 (46670), 286 (sh, 30100), 312 (sh, 17530), 321 (19930), 376 (sh, 5090), 430 (sh, 1720)

2c 247 (64180), 270 (48540), 307 (26390), 322 (23220), 369 (sh, 5520), 425 (sh, 1650)

2d 248 (77070), 256 (74100), 285 (47240), 320 (23660), 365 (sh, 7060), 453 (sh, 1660)

[Ir(C 1 ^C^C 1 ) 2 ] + 279 (18810), 305 (15410), 318 (15270)

Solvent = CH2Cl2

1a 240 (sh, 40380), 285 (30500), 308 (sh, 15420), 318 (sh, 12550), 351 (4400), 384 (4510), 447 (sh, 1770)

1b 247 (sh, 34120), 262 (33960), 280 (sh, 22810), 309 (10810), 321 (11075), 375 (5690), 472 (sh, 1750)

1c 242 (sh, 40650), 283 (30940), 306 (sh, 16480), 322 (13600), 378 (4670), 422 (sh, 1980)

1d 252 (sh, 60080), 257 (61150), 289 (36820), 320 (14480), 381 (7310), 467 (sh, 2280)

2a 248 (67690), 272 (48350), 282 (sh, 45580), 309 (25340), 321 (23470), 375 (sh, 5460), 441 (sh, 1560)

2b 249 (65520), 269 (56000), 282 (sh, 40080), 311 (sh, 19100), 322 (22270), 385 (sh, 5890), 448 (sh, 1680)

2c 249 (63520), 272 (49670), 308 (25150), 323 (23440), 372 (sh, 5490), 431 (sh, 1820)

2d 249 (80990), 257 (76520), 286 (49670), 321 (23940), 379 (7330), 465 (sh, 1940)

[Ir(C 1 ^C^C 1 ) 2 ] + 281 (19600), 305 (15680), 319 (15680)

Table 3. UV–Visible Absorption Data.

changes suggest that the electrochemical reductions of 1a and 2a are clean conversions. Notably, reductions of 1a and 2a result in enhancement of absorption at ~380 nm and new absorption doublet band
near 500 nm. These absorption features were observed in the reduction of [Ir(bpy)3] 3+ and are characteristic absorptions for anionic bpy radical (bpy •− ) 44 .

Emission Spectroscopy. The emission properties of the complexes in fluid solution (CH3CN and
CH2Cl2) at 298 K have been investigated (Table 4). Figure 5 depicts the emission spectra for 2a, 2c,
2d, and [Ir(C 1 ^C^C 1 )2] + in CH3CN at 298 K. Emission maxima of 1 - 2 range from 553 to 604 nm in
CH3CN and CH2Cl2 which are significantly blue shifted when compared with [Os(C^C^C)(N^N)(CO)] +
(λem = 676–731 nm, solvents = CH3CN and CH2Cl2) 30 . Quantum yields (Φ ) and emission lifetimes (τ ) of
1 - 2 are around 10 −3 –10 −1 and 10 2 –10 1 ns respectively, while those parameters for [Os(C^C^C)(N^N)
(CO)] + are around 10 −4 –10 −2 and 1–6 μ s respectively 30 . Similar to Os(C^C^C)(N^N)(CO)] +, these photophysical parameters for 1 - 2 are sensitive to the change of C^C^C and N^N, revealing that the emissive
state involve both the C^C^C and N^N moieties. For example, in both 1 - 2 and [Os(C^C^C)(N^N)
(CO)] +, blue-shift on emission maxima, higher emission quantum yield, and longer excited state lifetime
are observed when changing the N^N from 2,2-bipyridine to 1,10-phenanthroline 30 . The resemblance
of the excitation profiles to the absorption spectra signifies that the emissions originate from the energy
dissipation of the dπ(Ir III ) → π *(N^N) MLCT transitions. Interestingly, similar conclusion has been made
on the nature of the emissive excited states in [Os(C^C^C)(N^N)(CO)] + 30 . The emission profile for

[Ir(C 1 ^C^C 1 )2] + is highly structured and the emission maxima (378 and 398 nm) are not sensitive to the
change of solvent, therefore these emissions are assigned as π → π * (C^C^C) 3 IL emissions.

Theoretical Calculations. Time-dependent density functional theory (TD-DFT) calculations have
been performed on modeling complexes [Ir( Me C 1 ^C^C 1Me )(bpy)(H)] + ( 1a’ ) and [Ir( Me C 2 ^C^C 2Me )(bpy)
(H)] + ( 2a’ ), in which their metal cores are the same as 1a and 2a but the butyl chains on the C^C^C
are replaced by methyl groups to reduce computational cost. The ground-state structures of 1a’ and
2a’ have been optimized at the DFT level (functional = PBE0) 45,46 without symmetry constrain. The
conductor-like screening model (COSMO) 47 has been applied to account for solvent effects upon the
electronic transition. All the optimized geometries are in satisfactory agreement with their crystal structures. For example, the Ir–CNHC and Ir–CPh bond distances calculated for 1a’ (2.05–2.06 and 1.97 Å
respectively) are similar to those for 1a determined by X-ray crystallography (Ir–CNHC: 2.049(3) and
2.055(3) Å; Ir–CPh: 1.975(3) Å).
The excitation energies and oscillator strengths for the calculated vertical transitions with λ - 360 nm
are summarized in Table 5. Table 6 summarized the compositions of the molecular orbitals (MOs) which

Figure 3. UV–visible absorption spectra of selected complexes in CH3CN at 298 K.

are involved in the lowest-energy electronic transitions in these complexes. Figure 6 depicts the simulated
absorption spectra. It is noted that the calculated lowest-energy dipole allowed transitions (λ - 360 nm)
mainly originate from the HOMO–1 → LUMO and HOMO–2 → LUMO transitions. The HOMOs–1 and
HOMOs–2 have higher Ir contribution (27–59%) than that in LUMOs (3–4%), whereas the LUMOs have
higher N^N contribution (93%) than those in HOMOs–1 and HOMOs–2 (3–15%), therefore the transitions contain some Ir → π *(N^N) MLCT character. This finding is consistent with the spectroscopic
observation that a red-shift in absorption energy is observed when N^N is changed from Me2bpy to bpy,
and from phen to dpq. Besides, the contribution of C^C^C to both the HOMOs–1 and HOMOs–2 are
not low (27–69%), suggesting that the C^C^C ligands contribute significantly to the dπ(Ir III ) levels. The
electronic difference density plots for 1a’ and 2a’ in their lowest-energy excited state (Figure 6, generated
by taking the difference in the excited-state electron density and ground-state electron density) clearly
show that electronic charge is depleted from the Ir center and accumulated at the N^N moiety. The
emissions from complexes 1 - 2 are thus believed to be originated from the triplet dπ(Ir III ) → π *(N^N)
MLCT states.

Conclusion
In this work a series of emissive Ir(III) hydrido complexes bearing the NHC-derived tridentate
C^C^C pincer ligands and aromatic diimines have been prepared. This joint experimental and theoretical study reveals that the lowest-energy absorptions associated with these complexes arise from a
dπ(Ir III ) → π *(N^N) MLCT transition, where the C^C^C ligands contribute significantly to the dπ(Ir III )
level. It is therefore evident that the C^C^C ligands can modulate the photophysical properties via the
formation of the hybrid [Ir + C^C^C] molecular orbitals, and this work highlights the opportunities of
using NHC-derived ligands to modulate the photophysics of a [M(N^N)] core.

Figure 4. UV–visible absorption spectra for 1a (top) and 2a (bottom) in CH3CN at 298 K during
electrochemical reduction at - 1.95 V vs Cp2Fe +/0 (10 s traces; initial trace is shown in red).

Methods
General Procedure. All reactions were performed under an argon atmosphere using standard Schlenk
techniques unless otherwise stated. All reagents and solvents were used as received. The C^C^C ligand
precursors, i.e. benzene-bridged bisimidazolium bromide 48, and [Ir(1,5-cod)Cl]2 (cod = 1,5-cyclooctadiene) 49, were prepared according to literature methods. [Ir(C^C^C)(CH3CN)(Br)(H)] were synthesized analogously to the corresponding known complexes [Ir( Ar C^C^C Ar )(CH3CN)(Cl)(H)] and

[Ir( Me C 1 ^C^C 1Me )(CH3CN)(I)(H)] 23,24 . 1 H, 13 C{ 1 H}, DEPT-135, 1 H– 1 H COSY, and 1 H– 13 C HSQC NMR
spectra were recorded on Bruker 400 DRX FT-NMR spectrometer. Figure 7 depicts the labeling scheme
for the H and C atoms. Peak positions were calibrated with solvent residue peaks as internal standard.
Electrospray mass spectrometry was performed on a PE-SCIEX API 3000 triple quadrupole mass spectrometer. Infrared spectra were recorded as KBr plates on an Avatar 360 FTIR spectrometer. UV–visible
spectra were recorded on a Shimadzu UV-1700 spectrophotometer. Elemental analyses were done on an
Elementar Vario Micro Cube carbon–hydrogen–nitrogen elemental micro-analyzer. Cyclic voltammetry
was performed with a CH Instrument model 600C series electrochemical analyzer/workstation. All the
electrochemical measurements were performed in CH3CN solution with [ _n_ -Bu4N]PF6 (0.1 M) as supporting electrolyte at room temperature. The glassy-carbon working electrode was polished with 0.05
_μ_ m alumina on a microcloth, sonicated for 5 min in deionized water, and rinsed with CH3CN before
use. An Ag/AgNO3 (0.1 M in CH3CN) electrode was used as reference electrode, with a platinum wire
as the counter electrode. All solutions were degassed with nitrogen before experiments. The _E_ 1/2 value
of the ferrocenium/ferrocene couple (Cp2Fe +/0 ) measured in the same solution was used as an internal
reference. Steady-state emission spectra were obtained on a Jobin Yvon Fluorolog-3-TCSPC spectrophotometer. Sample and standard solutions were degassed with at least three freeze-pump-thaw cycles. The
emission quantum yields for complexes 1 - 2 were measured by the method of Demas and Crosby 50 with

[Ru(bpy)3](PF6)2 in degassed CH3CN as standard (Φr = 0.062), whereas that for [Ir(C 1 ^C^C 1 )2] + was
measured with quinine sulphate in 0.1M H2SO4 as standard (Φr = 0.58) 51 . The emission quantum yields
were calculated by Φs = Φr( _B_ r/ _B_ s)( _n_ s/ _n_ r) 2 ( _D_ s/ _D_ r), where the subscripts s and r refer to sample and reference standard solution, respectively, _n_ is the refractive index of the solvents, _D_ is the integrated intensity,
and Φ is the luminescence quantum yield. The quantity _B_ is calculated by _B_ = 1 - 10 _−AL_, where _A_ is the
absorbance at the excitation wavelength and _L_ is the optical path length 52 .

[Ir(C^C^C)(N^N)H](ClO4), 1–2(ClO4). A mixture of [Ir(1,5-cod)Cl]2 (0.10 mmol), benzene bridged
bisimidazolium or bisbenzimidazolium bromide (0.20 mmol), and Cs2CO3 (0.43 mmol) was refluxed in
CH3CN (30 ml) for 16 h. Upon cooling to room temperature, the solvent was removed by reduced pressure and the residue was extracted with CH2Cl2. The [Ir(C^C^C)(CH3CN)(Br)(H)] obtained from this

complex λem/nm Quantum yield (Φ) Lifetime (τ)/ns

Solvent = CH3CN

1a 577 4.53 × 10 −3 21

1b 563 3.50 × 10 −2 244

1c 565 1.09 × 10 −2 38

1d 580 7.97 × 10 −3 30

2a 588 2.00 × 10 −3 10

2b 575 8.99 × 10 −3 51

2c 575 4.18 × 10 −3 15

2d 604 2.45 × 10 −3 11

[Ir(C 1 ^C^C 1 ) 2 ] + 378 4.22 × 10 −1 _b_ 4663

398 4802

sh, 416 5001

Solvent = CH2Cl2

1a 568 1.34 × 10 −2 47

1b 553 1.19 × 10 −1 790

1c 555 3.31 × 10 −2 91

1d 567 3.82 × 10 −2 121

2a 578 6.03 × 10 −3 26

2b 560 4.92 × 10 −2 181

2c 566 1.35 × 10 −2 43

2d 584 1.16 × 10 −2 51

[Ir(C 1 ^C^C 1 ) 2 ] + 378 6.07 × 10 −1 _b_ 4906

398 5094

sh, 416 5298

Table 4. Emission Data for Complexes 1, 2, and [Ir(C 1 ^C^C 1 )2] + in solution at 298 K. Measurement
conditions: Concentration = 3.0 × 10 −5 M; λex = 420 nm for 1 - 2, 340 nm for [Ir(C 1 ^C^C 1 )2] + . Quantum
yields for complexes 1 - 2 and [Ir(C 1 ^C^C 1 )2] + were determined using [Ru(bpy)3] 2+ and quinine sulphate as
references, respectively.

Figure 5. Emission spectra for 2a, 2c, 2d, and [Ir(C 1 ^C^C 1 )2] + (λex = 420 nm for 1–2, 340 nm for

[Ir(C 1 ^C^C 1 )2] + ).

extract was used for the synthesis of 1 - 2 without further purification. A mixture of [Ir(C^C^C)(CH3CN)
(Br)(H)] (0.15 mmol) and diimine (0.5 mmol) was refluxed in ethylene glycol for 3 h. Upon cooling to
room temperature, the resultant solution was added to a saturated NaClO4 solution to give brown solids.
The crude product was eluted by column chromatography (neutral alumina, 9:1 (v/v) CH2Cl2/CH3CN as

complex experimental λmax/cm −1 (εmax/dm 3 mol −1 cm −1 )

TD-DFT calculations

excitation energy/cm −1

(oscillator strength) contribution

1a’ 22780 (sh, 1620) 21540 (0.0086) 88.8% HOMO–1 → LUMO

7.41% HOMO–2 → LUMO

1.30% HOMO → LUMO

26740 (4240) 25100 (0.1077) 87% HOMO–2 → LUMO

7.07% HOMO–1 → LUMO

2a’ 23700 (sh, 1710) 23260 (0.0121) 89.6% HOMO–1 → LUMO

5.12% HOMO–2 → LUMO

3.02% HOMO–3 → LUMO

1.13% HOMO–8 → LUMO

27030 (sh, 5240) 25920 (0.1109) 86.6% HOMO–2 → LUMO

6.11% HOMO–1 → LUMO

2.09% HOMO–3 → LUMO

Table 5. Calculated Vertical Transition Energies (λ > 360 nm) for 1a’ and 2a’ at the TD–DFT/COSMO
level (Solvent = CH3CN). Excitations with oscillator strength < 5 × 10 −3 are omitted; solvent = CH3CN.

% composition

complex MO

Ir N^N C^C^C Hhydride

1a’

2a’

HOMO–2 58.93 14.51 26.56 0.00

HOMO–1 34.22 3.77 61.99 0.02

HOMO 33.74 4.45 61.53 0.28

LUMO 3.50 92.93 3.57 0.00

HOMO–3 10.46 1.13 88.41 0.00

HOMO–2 56.27 13.54 30.19 0.00

HOMO–1 27.45 3.35 69.20 0.01

HOMO 27.81 3.80 68.07 0.32

LUMO 3.41 92.87 3.72 0.00

Table 6. Selected Molecular Orbital Compositions (%) for 1a’ and 2a’.

eluent) as a yellow band. After removal of solvent, the yellow solid was recrystallized by slow diffusion
of Et2O into CH3CN solution to give yellow crystals.

Complex 1a. Yield: 0.06 g, 50%. Anal. Calcd for C30H34N6Ir(ClO4): C, 46.78; H, 4.45; N, 10.91. Found:
C, 46.70; H, 4.51; N, 10.88. 1 H NMR (400 MHz, CD3CN): _δ_ –20.46 (s, 1H, Ir− H), 0.58–0.99, 1.18–1.32
(m, 14H, C3H7 of _n_ -Bu); 3.17–3.22 (m, 4H, CH2 of _n_ -Bu); 7.05 (d, 2H, _J_ = 2.1 Hz, Hl); 7.16–7.16 (m,
1H, Hg); 7.22–7.29 (m, 3H, Hi + Hj); 7.41 (d, 1H, _J_ = 5.2 Hz, Hh); 7.65–7.69 (m, 1H, Hb); 7.74 (d, 2H,
_J_ = 2.1 Hz, Hk); 7.88 (td, 1H, _J_ = 8.0, 1.6 Hz, Hf); 8.21 (td, 1H, _J_ = 8.0, 1.6 Hz, Hc); 8.38 (d, 1H, _J_ = 8.0
Hz, He); 8.55 (d, 1H, _J_ = 8.0 Hz, Hd); 9.67 (d, 1H, _J_ = 5.2 Hz, Ha). 13 C NMR (100 MHz, CD3CN): _δ_ 13.7,
20.2, 34.5, 50.1 ( _n_ -Bu); 108.3 (Cj); 117.0 (Ck); 121.6 (Cl); 122.9 (Ci); 124.6 (Ce); 125.3 (Cd); 127.7 (Cg);
129.0 (Cb); 138.2 (Cc); 138.3 (Cf); 142.9 (Ir− CPh); 146.1 (Quaternary C in C 1 ^C^C 1 ); 151.7 (Ch); 156.9,
157.0 (Quaternary C in bpy); 157.2 (Ca); 167.9 (Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2189, νCl–O = 1086.
ESI-MS: _m/z_ 670 [M + ].

Complex 1b. Yield: 0.06 g, 50%. Anal. Calcd for C32H34N6Ir(ClO4): C, 48.39; H, 4.31; N, 10.58. Found:
C, 48.08; H, 4.36; N, 10.40. 1 H NMR (400 MHz, CD3CN): _δ_ –20.27 (s, 1H, Ir− H), 0.14–0.51, 0.89–1.06
(m, 14H, C3H7 of _n_ -Bu); 2.92–3.15 (m, 4H, CH2 of _n_ -Bu); 6.98 (d, 2H, _J_ = 2.0 Hz, Hl); 7.21–7.37 (m,
3H, Hi + Hj); 7.51 (dd, 1H, _J_ = 8.0, 5.0 Hz, Hg); 7.74 (d, 2H, _J_ = 2.0 Hz, Hk); 7.77 (d, 1H, _J_ = 5.0 Hz,
Hh); 8.04 (dd, 1H, _J_ = 8.0, 5.0 Hz, Hb); 8.12 (d, 1H, _J_ = 8.8 Hz, He); 8.22 (d, 1H, _J_ = 8.8 Hz, Hd); 8.44
(d, 1H, _J_ = 8.0 Hz, Hf); 8.78 (d, 1H, _J_ = 8.0 Hz, Hc); 9.97 (d, 1H, _J_ = 5.0 Hz, Ha). 13 C NMR (100 MHz,
CD3CN): _δ_ 13.6, 20.0, 34.4, 50.1 ( _n_ -Bu); 108.4 (Cj); 117.1 (Ck); 121.6 (Cl); 123.2 (Ci); 126.4 (Cg); 127.8
(Cb); 128.8 (Cd); 128.9 (Ce); 132.1, 132.6 (Quaternary C in phen); 137.4 (Cc); 137.6 (Cf); 142.6 (Ir− CPh);

Figure 6. TD-DFT calculated absorption spectrum for model complexes 1a’ and 2a’ in CH3CN.
Excitation energies and oscillator strengths are shown by the blue vertical lines; spectrum (in black) is
convoluted with a Gaussian function having a full width at half-maximum of 3000 cm −1 . Inserts show
the electronic difference density plots for 1a’ and 2a’ at the vertical transitions marked with * (isodensity
value = 0.002 au; charge accumulation and depletion are represented in red and blue respectively).

Figure 7. Labeling scheme for H and C atoms in 1–2.

146.4 (Quaternary C in C 1 ^C^C 1 ); 148.4, 148.8 (Quaternary C in phen); 152.6 (Ch); 157.3 (Ca); 168.1
(Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2179, νCl–O = 1094. ESI-MS: _m/z_ 695 [M + ].

Complex 1c. Yield: 0.05 g, 40%. Anal. Calcd for C32H38N6Ir(ClO4): C, 48.14; H, 4.80; N, 10.53. Found:
C, 48.44; H, 5.08; N, 10.45. 1 H NMR (400 MHz, CD3CN): _δ_ –20.55 (s, 1H, Ir− H), 0.58–0.98, 1.19–1.38
(m, 14H, C3H7 of _n_ -Bu); 2.39 (s, 3H, CH3 of Me2bpy); 2.61 (s, 3H, CH3 of Me2bpy); 3.26 (t, 4H, _J_ = 7.8
Hz, CH2 of _n_ -Bu); 6.96 (d, 1H, _J_ = 5.6 Hz, Hg); 7.05 (d, 2H, _J_ = 2.1 Hz, Hl); 7.20 (d, 1H, _J_ = 5.6 Hz, Hh);
7.21–7.29 (m, 3H, Hi + Hj); 7.50 (d, 1H, _J_ = 5.6 Hz, Hb); 7.73 (d, 2H, _J_ = 2.1 Hz, Hk); 8.23 (s, 1H, He);
8.39 (s, 1H, Hd); 9.46 (d, 1H, _J_ = 5.6 Hz, Ha). 13 C NMR (100 MHz, CD3CN): _δ_ 13.7, 20.3, 34.5, 50.1
( _n_ -Bu); 21.1, 21.3 (CH3 of Me2bpy); 108.2 (Cj); 117.0 (Ck); 121.5 (Cl); 122.7 (Ci); 125.2 (Ce); 125.8 (Cd);
128.4 (Cg); 129.6 (Cb); 143.4 (Ir− CPh); 146.1 (Quaternary C in C 1 ^C^C 1 ); 150.5, 150.7 (Quaternary C
in Me2bpy); 150.9 (Ch); 156.6 (Ca); 156.8, 156.9 (Quaternary C in Me2bpy); 168.4 (Ir− Ccarbene). IR (KBr,
cm −1 ): νIr−H = 2159, νCl–O = 1107. ESI-MS: _m/z_ 699 [M + ].

Complex 1d. Yield: 0.06 g, 45%. Anal. Calcd for C34H34N8Ir(ClO4): C, 48.25; H, 4.05; N, 13.24. Found:
C, 48.47; H, 4.28; N, 13.06. 1 H NMR (400 MHz, CD3CN): _δ_ –20.29 (s, 1H, Ir− H), 0.27–0.75, 1.08–1.21
(m, 14H, C3H7 of _n_ -Bu); 2.91–3.22 (m, 4H, CH2 of _n_ -Bu); 7.00 (d, 2H, _J_ = 2.1 Hz, Hl); 7.24–7.37 (m, 3H,
Hi + Hj); 7.65 (dd, 1H, _J_ = 8.2, 5.2 Hz, Hg); 7.76 (d, 2H, _J_ = 2.0 Hz, Hk); 7.87–7.88 (m, 1H, Hh); 8.18 (dd,
1H, _J_ = 8.2, 5.2 Hz, Hb); 9.18 (d, 1H, _J_ = 2.1 Hz, He); 9.22 (d, 1H, _J_ = 2.1 Hz, Hd); 9.41 (dd, 1H, _J_ = 8.2,
1.2 Hz, Hf); 9.75 (dd, 1H, _J_ = 8.2, 1.2 Hz, Hc); 10.09 (dd, 1H, _J_ = 5.2, 1.2 Hz, Ha). 13 C NMR (100 MHz,
CD3CN): _δ_ 13.5, 20.1, 34.3, 50.2 ( _n_ -Bu); 108.5 (Cj); 117.2 (Ck); 121.7 (Cl); 123.3 (Ci); 127.6 (Cg); 128.9

(Cb); 130.8, 131.5 (Quaternary C in dpq); 134.0 (Cc); 134.2 (Cf); 140.5, 140.5 (Quaternary C in dpq);
142.2 (Ir− CPh); 146.3 (Quaternary C in C 1 ^C^C 1 ); 147.7, 147.8 (Cd + Ce); 149.7, 150.0 (Quaternary C
in dpq); 153.9 (Ch); 158.5 (Ca); 167.8 (Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2131, νCl–O = 1108. ESI-MS:
_m/z_ 747 [M + ].

Complex 2a. Yield: 0.07 g, 50%. Anal. Calcd for C38H38N6Ir(ClO4): C, 52.44; H, 4.40; N, 9.66. Found:
C, 52.45; H, 4.38; N, 9.46. 1 H NMR (400 MHz, CD3CN): _δ_ –19.50 (s, 1H, Ir− H), 0.40–1.07, 1.23–1.47
(m, 14H, C3H7 of _n_ -Bu); 3.30–3.59 (m, 4H, CH2 of _n_ -Bu); 7.07 (t, 1H, _J_ = 6.4 Hz, Hg); 7.28–7.35 (m, 2H,
Hm); 7.37–7.50 (m, 6H, Hh + Hi + Hl + Hn); 7.71–7.74 (m, 1H, Hb); 7.77–7.87 (m, 3H, Hj + Hf); 8.21 (d,
2H, _J_ = 8.0 Hz, Hk); 8.27–8.31 (m, 1H, Hc); 8.38 (d, 1H, _J_ = 8.2 Hz, He); 8.60 (d, 1H, _J_ = 8.2 Hz, Hd); 9.51
(d, 1H, _J_ = 5.3 Hz, Ha). 13 C NMR (100 MHz, CD3CN): _δ_ 13.8, 20.4, 33.3, 47.5 ( _n_ -Bu); 109.7 (Cj); 111.9
(Cl/Cn); 112.2 (Ck); 123.6 (Ci); 123.9 (Cm); 124.6 (Ce); 124.7 (Cl/Cn); 125.5 (Cd); 127.9 (Cg); 129.3 (Cb);
133.0, 135.6 (Quaternary C in C 2 ^C^C 2 ); 138.7 (Cf); 138.8 (Cc); 142.9 (Ir− CPh); 146.6 (Quaternary C
in C 2 ^C^C 2 ); 152.2 (Ch); 156.6 (Quaternary C in bpy); 157.2 (Ca); 157.3 (Quaternary C in bpy); 180.1
(Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2126, νCl–O = 1075 ESI-MS: _m/z_ 771 [M + ].

Complex 2b. Yield: 0.05 g, 40%. Anal. Calcd for C40H38N6Ir(ClO4): C, 53.71; H, 4.28; N, 9.40. Found:
C, 53.75; H, 4.30; N, 9.45. 1 H NMR (400 MHz, CD3CN): _δ_ –19.21 (s, 1H, Ir− H), 0.19–0.35, 0.38–0.45,
0.52–0.67 (m, 14H, C3H7 of _n_ -Bu); 3.26–3.46 (m, 4H, CH2 of _n_ -Bu); 7.26–7.39 (m, 4H, Hl/Hm + Hn);
7.39–7.48 (m, 3H, Hg + Hm/Hl); 7.52 (t, 1H, _J_ = 8.0 Hz, Hi); 7.81 (d, 2H, _J_ = 5.1 Hz, Hf); 7.87 (d, 2H,
_J_ = 8.0 Hz, Hj); 8.08–8.18 (m, 2H, Hb + He); 8.21–8.29 (m, 3H, Hd + Hk); 8.41 (d, 1H, _J_ = 8.2 Hz, Hh); 8.88
(d, 1H, _J_ = 8.2 Hz, Hc); 9.96 (d, 1H, _J_ = 8.2 Hz, Ha). 13 C NMR (100 MHz, CD3CN): _δ_ 13.8, 20.3, 33.3, 47.6
( _n_ -Bu); 109.8 (Cj); 111.9 (Cl/Cm/Cn); 112.3 (Ck); 123.9 (Ci); 124.0 (Cl/Cm/Cn); 124.8 (Cl/Cm/Cn); 126.7
(Cg); 128.1 (Cb); 128.8 (Cd); 129.1 (Ce); 132.2, 132.7 (Quaternary C in phen); 133.1, 135.7 (Quaternary
C in C 2 ^C^C 2 ); 137.9 (Ch); 138.1 (Cc); 142.6 (Ir− CPh); 146.9 (Quaternary C in C 2 ^C^C 2 ); 148.3,
148.9 (Quaternary C in phen); 153.2 (Cf); 157.4 (Ca); 180.4 (Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2129,
νCl–O = 1090. ESI-MS: _m/z_ 795 [M + ].

Complex 2c. Yield: 0.05 g, 40%. Anal. Calcd for C40H42N6Ir(ClO4): C, 53.47; H, 4.71; N, 9.35. Found:
C, 53.53; H, 4.77; N, 9.57. 1 H NMR (400 MHz, CD3CN): _δ_ –19.48 (s, 1H, Ir− H), 0.60–0.97, 1.34–1.47 (m,
14H, C3H7 of _n_ -Bu); 2.33 (s, 3H, CH3 of Me2bpy); 2.67 (s, 3H, CH3 of Me2bpy); 3.39–3.67 (m, 4H, CH2 of
_n_ -Bu); 6.90 (d, 1H, _J_ = 5.7 Hz, Hg); 7.25 (d, 1H, _J_ = 5.7 Hz, Hh); 7.28–7.38 (m, 2H, Hm); 7.40–7.50 (m, 5H,
Hi + Hl + Hn); 7.54–7.65 (m, 1H, Hb); 7.81 (d, 2H, _J_ = 8.0 Hz, Hj); 8.17–8.35 (m, 3H, He + Hk); 8.46 (s,
1H, Hd); 9.42 (d, 1H, _J_ = 5.6 Hz, Ha). 13 C NMR (100 MHz, CD3CN): _δ_ 13.8, 20.5, 33.4, 47.6 ( _n_ -Bu); 21.1,
21.3 (CH3 of Me2bpy); 109.6 (Cj); 111.9 (Cl/Cn); 112.2 (Ck); 123.5 (Ci); 123.9 (Cm); 124.7 (Cl/Cn); 125.3
(Ce); 126.0 (Cd); 128.6 (Cg); 130.0 (Cb); 133.0, 135.7 (Quaternary C in C 2 ^C^C 2 ); 143.4 (Ir− CPh); 146.7
(Quaternary C in C 2 ^C^C 2 ); 151.1, 151.3 (Quaternary C in Me2bpy); 151.3 (Ch); 156.6 (Quaternary C in
Me2bpy); 156.7 (Ca); 157.1 (Quaternary C in Me2bpy); 180.6 (Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2133,
νCl–O = 1094. ESI-MS: _m/z_ 799 [M + ].

Complex 2d. Yield: 0.06 g, 40%. Anal. Calcd for C42H38N8Ir(ClO4): C, 53.30; H, 4.05; N, 11.84. Found:
C, 53.41; H, 4.25; N, 11.94. 1 H NMR (400 MHz, CD3CN): _δ_ –19.22 (s, 1H, Ir− H), 0.23–0.79, 1.18–1.48
(m, 14H, C3H7 of _n_ -Bu); 3.17–3.61 (m, 4H, CH2 of _n_ -Bu); 7.28–7.46 (m, 6H, Hl + Hm + Hn); 7.54 (t,
1H, _J_ = 8.0 Hz, Hi); 7.59 (dd, 1H, _J_ = 8.2, 5.2 Hz, Hg); 7.89 (d, 2H, _J_ = 8.0 Hz, Hj); 7.91–7.93 (m, 1H,
Hh); 8.16–8.35 (m, 3H, Hb + Hk); 9.18 (d, 1H, _J_ = 2.1 Hz, He); 9.24 (d, 1H, _J_ = 2.1 Hz, Hd); 9.37 (dd, 1H,
_J_ = 8.2, 1.4 Hz, Hf); 9.85 (dd, 1H, _J_ = 8.2, 1.3 Hz, Hc); 10.11 (dd, 1H, _J_ = 5.2, 1.3 Hz, Ha). 13 C NMR (100
MHz, CD3CN): _δ_ 13.7, 20.4, 33.3, 47.7 ( _n_ -Bu); 109.9 (Cj), 112.0 (Cl/Cm/Cn); 112.3 (Ck); 124.1 (Ci); 124.1,
124.9 (Cl/Cm/Cn); 127.8 (Cg); 129.2 (Cb); 130.9, 131.6 (Quaternary C in dpq); 133.1 (Quaternary C in
C 2 ^C^C 2 ); 134.5 (Cc), 134.6 (Cf), 135.7 (Quaternary C in C 2 ^C^C 2 ); 140.4, 140.5 (Quaternary C in dpq);
142.3 (Ir− CPh), 146.9 (Quaternary C in C 2 ^C^C 2 ), 147.8 (Ce); 147.9 (Cd); 149.5, 150.2 (Quaternary C
in dpq); 154.5 (Ch); 158.6 (Ca); 180.1 (Ir− Ccarbene). IR (KBr, cm −1 ): νIr−H = 2130, νCl–O = 1097. ESI-MS:
_m/z_ 847 [M + ].

X-ray Crystallography. X-ray diffraction data for 1a (ClO4), 2a (ClO4), and [ 2b (ClO4)]3·CH3CN were
collected on an Oxford Diffraction Gemini S Ultra X-ray single crystal diffractometer with Cu K _α_ radiation (λ = 1.54178 Å) at 133 K. The data were processed using CrysAlis 53 . The structures were solved
by Patterson and Fourier methods and refined by full-matrix least-squares based on _F_ 2 with program
SHELXS-97 and SHELXL-97 54 within WinGX 55 . All non-hydrogen atoms were refined anisotropically
in the final stage of least-squares refinement. The positions of H atoms were calculated based on riding
mode with thermal parameters equal to 1.2 times that of the associated C atoms. CCDC 1416088–
1416090 contain the supplementary crystallographic data for this paper, which can be obtained free
[of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.](http://www.ccdc.cam.ac.uk/data_request/cif)

Computational Methodology. DFT calculations were performed on model complexes

[Ir( Me C 1 ^C^C 1Me )(bpy)(H)] + ( 1a’ ) and [Ir( Me C 2 ^C^C 2Me )(bpy)(H)] + ( 2a’ ). Their electronic ground states

were optimized without symmetry constrain using the density functional PBE0 45,46 . The def2-SVP basis
sets were used for the H, C, and N atoms, while the def2-TZVP(-f) basis sets were used for the Ir
atoms 56 . Zero-order regular approximation (ZORA) was employed to account for relativistic effects.
Tight SCF convergence (10 −8 au) was used for all calculations. The vertical transition energies for these
model complexes in CH3CN were computed at their respective gas-phase optimized ground-state geometries using time-dependent-DFT (TD-DFT) method with the same density functional and basis sets in
the geometry optimizations. The combination of the resolution of the identity and the “chain of spheres
exchange” algorithms (RIJCOSX) 57 was used to accelerate all DFT and TD-DFT calculations with the use
of appropriate auxiliary basis sets. The conductor-like screening model (COSMO) 47 was used to account
for solvent effects upon the electronic transition. All the calculations were performed using the ORCA
software package (version 3.0.2) 58 .

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Acknowledgements
The work described in this paper was supported by the Hong Kong Research Grants Council (Project
No. CityU 103911), and the Special Equipment Grant from the Hong Kong University Grants Committee
(SEG_CityU02). We are grateful to Dr. Shek-Man Yiu for X-ray diffraction data collection and Mr. PakKei Pat for technical support.

Author Contributions
L.H.C, H.S.L. and S.W.N. carried out all the experiments and performed the data analysis; D.L.M.,
C.H.L. and C.Y.W. designed the experiments, analyzed the results and wrote the manuscript. All authors
reviewed the manuscript.

Additional Information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Chung, L.-H. _et al._ Luminescent Iridium(III) Complexes Supported by
N-Heterocyclic Carbene-based C^C^C-Pincer Ligands and Aromatic Diimines. _Sci. Rep._ 5, 15394; doi:
10.1038/srep15394 (2015).

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