Cyclometallated iridium(III) complexes with dicyanamide or tricyanomethanide

Article abstract of DOI:10.1016/j.inoche.2009.06.008

Reaction of cyclometallated iridium(III) complex Ir(ppy)₂(PPh₃)Cl (2, ppy = 2-phenylpyridine) with dicyanamide or tricyanomethanide gave neutral mononuclear complexes Ir(ppy)₂(PPh₃)N(CN)₂ (3a) or Ir(ppy)₂(PPh₃)C(CN)₃ (3b), and dicyanamide/tricyanomethanide-linked binuclear iridium(III) complexes [{Ir(ppy)₂(PPh₃)}₂N(CN)₂]⁺ (4a) or [{Ir(ppy)₂(PPh₃)}₂C(CN)₃]⁺ (4b). Substitution of coordinated chloride in the precursor 2 with dicyanamide or tricyanomethanide improved significantly the luminescence properties of 3a-4b. Compared with that in the precursor 2 (1.6%), 2.2 to 9.3-fold enhancement of emission quantum yields was detected in 3a-4b.

Full text of DOI:10.1016/j.inoche.2009.06.008

Inorganic Chemistry Communications 12 (2009) 758–760
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Cyclometallated iridium(III) complexes with dicyanamide or tricyanomethanide
*
Xu Zhang, Li-Yi Zhang, Lin-Xi Shi, Zhong-Ning Chen
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of cyclometallated iridium(III) complex Ir(ppy)2(PPh3)Cl (2, ppy = 2-phenylpyridine) with dicy-
anamide or tricyanomethanide gave neutral mononuclear complexes Ir(ppy)2(PPh3)N(CN)2 (3a) or
Ir(ppy)2(PPh3)C(CN)3 (3b), and dicyanamide/tricyanomethanide-linked binuclear iridium(III) complexes
[{Ir(ppy)2(PPh3)}2N(CN)2]+ (4a) or [{Ir(ppy)2(PPh3)}2C(CN)3]+ (4b). Substitution of coordinated chloride
in the precursor 2 with dicyanamide or tricyanomethanide improved significantly the luminescence
properties of 3a–4b. Compared with that in the precursor 2 (1.6%), 2.2 to 9.3-fold enhancement of emis-
sion quantum yields was detected in 3a–4b.
Received 8 May 2009
Accepted 8 June 2009
Available online 10 June 2009
Keywords:
Dicyanamide
Luminescence
Iridium
Ó 2009 Elsevier B.V. All rights reserved.
2-Phenylpyridine
Tricyanomethanide
In recent decades, organic light-emitting diodes (OLEDs) have
been under active investigations because of the enormous poten-
tial applications in flat panel display (FPD) and general lighting
[1]. The relevant studies are started from coordination complexes
of aluminium(III), iridium(III), and platinum(II) etc. [2], which have
been used as the light-emitting dopants in OLEDs with high quan-
tum efficiency. In particular, cyclometallated iridium(III) com-
plexes are intensively studied because they exhibit high emission
quantum efficiency in the visible spectral region [3,4]. The mixing
of the 1MLCT and 3MLCT states is formed due to strong spin-orbit
coupling of heavy metal ions in these complexes so that the 3MLCT
emission is enhanced by borrowing the intensity from the 1MLCT
state and leads to high phosphorescence efficiencies [5].
Reaction of sodium dicyanamide with 1 or 2 equiv. of 2 in metha-
nol-dichloromethane (v/v = 1:1) solution gave rise to isolation of
neutral mononuclear complex Ir(ppy)2(PPh3){N(CN)2} (3a) or dicy-
anamide-bridged binuclear iridium(III) complex [{Ir(ppy)2-
(PPh3)}2{l
-N(CN)2}]+ (4a). Complex 4a was also accessed by
reaction of 3a with equimolar 2 in a stepwise synthetic approach.
Similarly, tricyanomethanide-containing complexes Ir(ppy)2-
(PPh3){C(CN)3} (4b) and [{Ir(ppy)2(PPh3)}2{l
-C(CN)3}]+ (4b) were
accessed using potassium tricyanomethanide instead of sodium
dicyanamide.
Complexes 2–4b ware characterized by ESI-MS spectrometry,
1H NMR, and IR spectroscopy (Experimental section, ESI), and by
X-ray crystallography for 3a and 4b [9]. In the IR spectra of 3a–
4b, characteristic stretching vibration bands of dicyanamide or tri-
cyanomethanide were detected. Relative to v(N(CN)2) in the free
dicyanamide (2287, 2229, and 2181 cmꢀ1), those in mononuclear
complex 3a (2266, 2225, and 2161 cmꢀ1) showed a lower fre-
Metal complexes of dicyanamide [NðCNÞꢀ2 ] and tricyanometha-
nide [CðCNÞꢀ3 ] have attracted much attention because of their
intriguing optical, magnetic and electronic properties [6], where
the quasi-p-conjugated dicyanamide or tricyanomethanide serves
as an effective mediator to transmitting magnetic and electronic
interactions. We report herein the preparation and photophysical
properties of dicyanamide/tricyanomethanide-containing cyclo-
metallated iridium(III) complexes with ppy (ppy = 2-phenylpyri-
dine). Our research is aimed at exploring the influence on the
luminescence properties by substitution of the coordinated chlo-
ride in the precursor Ir(ppy)2(PPh3)Cl (2) with a dicyanamide or tri-
cyanomethanide ligand.
quency shift due to the r donation from dicyanamide to the metal
center upon coordination. The v(N(CN)2) in dicyanamide-bridged
binuclear complex 4a (2301, 2199 cmꢀ1), however, exhibited high-
er frequency shifts compared with those in terminal dicyanamide-
bound species 3a, arising probably from the integrated electronic
effects of
lic centers as well as
r
donation from the bridging ligand to the organometal-
back-donation from the organometallic cen-
p
ters to the ligand. Likewise, compared with the v(C(CN)3) in the
free tricyanomethanide (2179 cmꢀ1), a lower frequency shift oc-
curred in terminal tricyanomethanide-bound 3b (2176 cmꢀ1),
whereas a higher frequency shift was detected in tricyanometha-
nide-bridged binuclear species 4b (2192 cmꢀ1).
The synthetic routes to complexes 2–4b are depicted in Scheme
1. The precursor Ir(ppy)2(PPh3)Cl (2) [7] was prepared by reaction
of chloride-bridged iridium(III) dimer Ir2(ppy)4(l-Cl)2 (1) [8] with
triphenylphosphine (PPh3) according to the literature procedure.
The ORTEP drawings of 3a and 4b are depicted in Figs. 1 and 2,
respectively. The iridium(III) atom is located at a distorted octahe-
dral environment composed of C2N3P donors. The two ppy ligands
* Corresponding author. Fax: +86 591 8379 2346.
E-mail address: czn@fjirsm.ac.cn (Z.-N. Chen).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.06.008

X. Zhang et al. / Inorganic Chemistry Communications 12 (2009) 758–760
759
Scheme 1. Synthetic routes to complexes 2–4b.
are cis-arranged with the two C donors in cis-position and the two
N donors in trans-orientation. The Ir–Cppy and Ir–Nppy distances are
comparable to those in other cyclometallated iridium(III) com-
plexes of ppy [4]. For 3a, the dicyanamide is terminally bound to
the iridium(III) ion with Ir1–N3 distance of 2.128(7) Å. The Ir–
N„C–N array is quasi-linear with the Ir1–N3–C4 and N3–C4–N4
angles of 167.3(7) and 172.8(11)°, respectively. The C41–N4–C42
angle of dicyanamide is 122.8(11)° due to sp2 hybridized character
of N4 atom. For 4b, the binuclear iridium(III) centers are linked by
a bridging tricyanomethanide with Ir1ꢁ ꢁ ꢁIr2 separation of 8.1 Å. Of
the three C„N groups in CðCNÞꢀ3 , two C„N (1.151(5) and 1.155
(6) Å) bound to the iridium(III) centers are appreciably elongated
relative to that of the uncoordinated C„N (1.102(7) Å). The rele-
vant bonding distances and angles of dicyanamide or tricyano-
methanide in 3a or 4b are similar to those in other metal
complexes with dicyanamide or tricyanomethanide [6].
with wavelength longer than 330 nm due to d(Ir) ?
p
*(ppy) MLCT
(metal-to-ligand charge transfer) state [10]. Upon irradiation at
kex > 350 nm, 2–4b exhibit bright cyan emission in both solid states
and solutions with sub-microsecond ranges of lifetimes at ambient
temperature [11], arising most likely from 3[d(Ir) ? *(ppy)]
p
3MLCT triplet excited state. As depicted in Fig. 3, the emission of
3a–4b is obviously blue-shifted compared with that of 2 in both
solution and solid state (Table 1), suggesting that substitution of
the chloride in 2 with a stronger
r donating and p accepting ligand
such as dicyanamide or tricyanomethanide can raise the emission
energy in some extent. This can be rationally elucidated by the fact
that stronger ligand field would cause larger splitting of the d-orbi-
tal and consequently lower the energy of metal-centered HOMO
[12]. The emission quantum yields of 3a–4b are in the range
3.7–15% which are much higher than that of the precursor 2. This
reveals that strong-field ligand substitution of the coordinated
chloride results in significantly enhanced luminescence efficiency
because the stronger Ir–N bonding compared with that of Ir–Cl
favors suppressing the non-radiative process due to some
Ir-solvent interaction in solutions.
The UV–vis absorption spectra of 2–4b (Table 1) in dichloro-
methane solutions are featured by UV absorptions below 330 nm
from ligand centered characters together with low-energy bands
Fig. 2. ORTEP drawing of 4b with 30% thermal ellipsoids probability. The phenyl
groups in P atoms are omitted for clarity. Selected bond lengths (Å) and angles
(°):Ir1–C22 1.994(9), Ir1–C11 010(9), Ir–N1 2.050(8), Ir1–N2 2.065(7), Ir1–N5
2.079(7), Ir1–P1 2.419(3), N5–C81 1.160(10), N6–C83 1.134(11), N7–C84 1.118(15);
C22–Ir1–C11 89.6(3), C22–Ir1–N1 92.4(3), C11–Ir1–N1 80.0(4), C22–Ir1–N2
80.0(3), C11–Ir1–N2 91.2(3), N1–Ir1–N2 168.5(3), C22–Ir1–N5 173.1(3), C11–Ir1–
N5 87.8(3), N1–Ir1–N5 93.4(3), N2–Ir1–N5 93.7(3), C22–Ir1–P1 94.5(2), C11–Ir1–P1
175.6(2), N1–Ir1–P1 98.2(2), N2–Ir1–P1 90.99(19), N5–Ir1–P1 88.3(2).
Fig. 1. ORTEP drawing of 3a with 30% thermal ellipsoids probability. Selected bond
lengths (Å) and angles (°): Ir1–N1 2.044(5), Ir1–N2 2.087(6), Ir1–N3 2.128(7), Ir1–
C11 2.010(7), Ir1–C22 2.044(7), Ir1–P1 2.441(2), N3–C41 1.137(10), N4–C41
1.311(12), N5–C42 1.158(18) Å; N1–Ir–N2 171.5(2), N1–Ir–C11 80.3(3), N2–Ir–
C22 79.6(3), N3–C41–N4 172.8(11), N4–C42–N5 174.1(18), C11–Ir–C22 85.7(3),
C41–N3–Ir 167.3(7), C41–N4–C42 122.8(11)°.

760
X. Zhang et al. / Inorganic Chemistry Communications 12 (2009) 758–760
Table 1
Absorption and emission data of 2–4b.
Medium
kabs/nm (
e
/dm3 molꢀ1 cmꢀ1
)
298 K
77 K
kem/nma
s
/
ls
Uem
kem/nm
b
2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
Solid
CH2Cl2
512
480, 510sh
498
472, 496
497
466, 495
502
469, 496
502
469, 493
0.107
0.018
0.099
0.146
0.047
0.059
0.055.
0.057
0.024
0.026
500, 512
452, 481
490, 519
457, 483
481, 512
452, 481
509
475, 501
495
469, 496
258 (42,650) 347 (6210) 388 (3490)
250 (38,590) 302 (13,320) 378 (3010)
248 (50,390) 302 (15,260) 378 (4370)
249 (73,950) 302 (24,940) 378 (6580)
249 (88,600) 306 (26,010) 376 (6070)
0.016
0.150
0.061
0.110
0.037
3a
3b
4a
4b
a
Emission spectra are recorded in 10ꢀ5 M of dichloromethane solutions at 298 K.
The emission quantum yields are measured using fac-Ir(ppy)3 (Uem = 0.40 [10a]) as the standard.
b
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450
500
λem (nm)
550
600
Fig. 3. Emission spectra of 2–4b in dichloromethane solutions at 298 K.
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In summary, four mono- and binuclear iridium(III) complexes
were prepared by substitution of the chloride in the precursor Ir(p-
py)2(PPh3)Cl with dicyanamide or tricyanomethanide. Stronger
field ligand substitution gave rise to an obvious blue shift of the
emission and 2.2 to 9.3-fold enhancements of the luminescence
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Acknowledgements
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We thank financial supports from the NSFC, the 973 project
(2007CB815304) from MSTC, and NSF of Fujian Province
(2008I0027 and 2008F3117).
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group P21/n, a = 17.352(7) Å, b = 9.391(3) Å, c = 24.936(10) Å, b = 99.942(7)°,
V = 4003(3) Å3, Z = 4, T = 293(2) K,
l
(Mo K
k = 0.71073 Å, 2h = 55°. The structure, refined on F2, converged for 9188 unique
reflections (Rint = 0.050) and 7722 observed reflections with I > 2 (I) to give
a ,
) = 3.546 mmꢀ1, Dc = 1.516 g cmꢀ1
Appendix A. Supplementary material
r
R1 = 0.0552 and wR2 = 0.1527 and a goodness-of-fit = 1.140. Crystal data for
ꢀ
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2009.06.008.
4bꢁ5/2CH2Cl2: C86.5H67Cl5F6Ir2N7P3: M = 1973.03, triclinic, space group P1,
a = 15.693(5) Å,
b = 69.636(12)°,
b = 16.502(5) Å,
c = 19.029(5) Å,
a
= 72.358(13)°
c
= 71.034(11)°, V = 4268(2) Å3, Z = 2, T = 293(2) K,
l(Mo
Ka
) = 3.389 mmꢀ1, Dc = 1.535 g cmꢀ1, k = 0.71073 Å, 2h = 55°. The structure,
refined on F2, converged for 14,611 unique reflections (Rint = 0.0519) and 9916
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