A novel luminescent iridium(I)-cadmium(II) binuclear complex displaying a long-lived metal-to-ligand charge-transfer excited state. Synthesis and structural characterisation of I(CO)2Ir(μ-Ph2Ppy)2CdI2 [Ph2Ppy = 2-(diphenylphosphino)pyridine]

Article abstract of DOI:10.1039/a705431i

The first binuclear Ir^I-Cd^II complex to be structurally characterised, I(CO)₂Ir(μ-Ph₂Ppy)₂CdI₂ [Ph₂Ppy = 2-(diphenylphosphino)pyridine], has a donor-acceptor metal-metal bond of length 2.784(1) A; at room temperature, excitation of a solid sample of the complex with UV light led to a red emission at 739 nm with a lifetime of 6.48 μs that presumably originates from a ³MLCT excited state.

Full text of DOI:10.1039/a705431i

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A novel luminescent iridium(I)᎐cadmium(II) binuclear complex
displaying a long-lived metal-to-ligand charge-transfer excited state.
Synthesis and structural characterisation of I(CO)2Ir(ì-Ph2Ppy)2CdI2
[Ph2Ppy ؍ 2-(diphenylphosphino)pyridine]
Shan-Ming Kuang,a Feng Xue,a Zheng-Zhi Zhang,*,b Wen-Mei Xue,c Chi-Ming Che*,c and
Thomas C. W. Mak *,†,a
a
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong
b Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, China
c
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
iridium and cadmium atoms are supported by a pair of biden-
The first binuclear IrI᎐CdII complex to be structurally
tate µ-Ph2Ppy ligands. The co-ordination geometry at the Ir(1)
atom is distorted octahedral, which is seldom found for irid-
ium() although it is quite common for iridium() and higher
oxidation states. The co-ordination geometry at Cd(1) is tri-
gonal bipyrimidal. The measured Ir(1)᎐Cd(1) distance of
2.784(1) Å is much longer than the related IrI᎐HgII distances of
2.618(3) Å in Cl2Hg(µ-Cl)2HgIr(CO)Cl(dpm)(µ-dpmAuCl)4b
and 2.587(1) Å in [(η5-C5Me5)(CO)2IrHgCl][HgCl3].4d The
Cd(1)᎐N bond distances, both 2.51(1) Å are slightly longer
than those [2.397(5) and 2.412(5) Å] found in (CO)3Fe-
(µ-EtPhPpy)2Cd(NCS)2 [EtPhPpy = 2-(ethylphenylphosphino)-
pyridine].11 The P᎐Ir᎐P and N᎐Cd᎐N fragments are each
characterised, I(CO)2Ir(µ-Ph2Ppy)2CdI2 [Ph2Ppy = 2-(diphenyl-
phosphino)pyridine], has a donor–acceptor metal–metal bond
of length 2.784(1) Å; at room temperature, excitation of a solid
sample of the complex with UV light led to a red emission at
739 nm with a lifetime of 6.48 µs that presumably originates
3
from a MLCT excited state.
There has been considerable interest in the metal–metal inter-
action, electronic absorption spectra, and luminescence proper-
ties of binuclear complexes with d8–d8,1 d10–d10 2 and d8–d10 3
electronic configurations. Several years ago, Balch and co-
workers reported a number of luminescent binuclear iridium–
Group 11 complexes such as [AuIr(CO)Cl(µ-dpm)2][PF6],3a
‡ Preparation of complex 3. Carbon monoxide was passed through a
CH2Cl2 solution containing [Ir(cod)Cl]2 (0.10 g, 0.15 mmol) and
Ph2Ppy (0.16 g, 0.60 mmol) for 1 h.6 Solid CdI2 (0.28 g, 0.60 mmol) was
then added. The mixture was stirred for 24 h at room temperature, after
which the solvent was reduced to about 5 ml and diethyl ether (15 ml)
added to the solution to give orange microcrystals. Yield: 0.12 g (57%)
(Found: C, 34.62; H, 2.34; N, 2.20. Calc. for C36H28CdI3IrN2O2P2: C,
3c
[AuIr(CNMe)2(µ-dpm)2][PF6]2 and [AuIr(CNMe)3(µ-dpm)2]-
3c
[PF6]2
[dpm = bis(diphenylphosphino)methane]. However,
related studies on iridium–Group 12 binuclear complexes which
exhibit a significant donor–acceptor metal–metal interaction
are very scarce.4 To our knowledge, only one IrI᎐CdII complex,
namely CdIr(CO)2Cl2(η5-C5Me5), has been reported in the liter-
ature, but its crystal structure is unknown.4d Herein we describe
the synthesis, spectroscopic properties and crystal structure
of a new binuclear IrI᎐CdII complex, I(CO)2Ir(µ-Ph2Ppy)2CdI2
[Ph2Ppy = 2-(diphenylphosphino)pyridine], which features an
unusual distorted octahedral co-ordination geometry about the
IrI centre.
34.10; H, 2.23; N, 2.21%). IR (CH2Cl2): 2001.7, 2066.0 cmϪ1 31P-{1H}
.
NMR (CDCl3, 300 MHz, 298 K): δ 10.45.
Infrared spectra were recorded on a Shimadzu 435 spectrometer, and
UV/VIS spectra were obtained on a Milton Roy Spectronic 3000 diode-
array spectrophotometer. The 31P-{1H} NMR data were measured on a
AC-P200 spectrometer. Steady-state emission spectra were recorded on
a Spex Fluorolog-2111 spectrofluorimeter equipped with a Hamamatsu
R928 photomultiplier, and transient difference absorption spectroscopy
was performed by using the 355 nm output of a Quanta-Ray GCR-150
pulsed Nd-YAG laser as the excitation source of the flash-photolysis
set-up.
§ Crystal data for I(CO)2Ir(µ-Ph2Ppy)2CdI2 3. C36H28CdI3IrN2O2P2,
M = 1267.8, monoclinic, space group P21/c (no. 14), a = 15.333(3),
b = 13.573(3), c = 18.197(4) Å, β = 95.85(3)Њ, U = 3767(2) Å3, Z = 4,
Dc = 2.235 Mg mϪ3, F(000) = 2352, orange prism with dimensions
0.15 × 0.20 × 0.18 mm, µ(Mo-Kα) = 6.67 mmϪ1. Intensity data were
collected on a MSC/Rigaku RAXIS IIC imaging-plate diffractometer
In the presence of CO, reaction of CdI2 with Ir(CO)-
Cl(Ph2Ppy-P)2 1, prepared from [Ir(cod)Cl]2 (cod = cycloocta-
1,5-diene) and Ph2Ppy, afforded I(CO)2Ir(µ-Ph2Ppy)2CdI2 3,‡ in
which a halide exchange has occurred (Scheme 1).
The equilibrium between complex 1 and intermediate 2 is
substantiated by IR spectral data recorded in dichloromethane
solution at room temperature, which show that complex 1 has
only one ν(CO) absorption band at 1969.7 cmϪ1, whereas 2
exhibits two bands at 1970.3 and 1935.1 cmϪ1. Complex 2 is not
stable enough to be isolated in the solid state. Compared with 2,
the ν(CO) bands of 3 shift to higher values (2001.7, 2066.0
cmϪ1), indicating that the IrI᎐CdII bond is donor–acceptor in
nature with a reduction of electron density at the iridium centre.
This observation is in accord with previous studies in the
formation of hetero-binuclear Fe0᎐M complexes bearing the
N,P-bridging ligand Ph2Ppy.7
at 294
K
using graphite-monochromatized Mo-Kα radiation
(λ = 0.710 73 Å) from a rotating-anode X-ray generator operating at
50 kV and 90 mA (crystal to plate distance 69.5 mm, 2θmax = 55.2Њ,
36 5Њ oscillation frames in the range 0–180Њ, exposure 8 min per
frame).8 A self-consistent semiempirical absorption correction based
on Fourier coefficient fitting of symmetry-equivalent reflections was
applied using ABSCOR.9 The structure was determined by direct
methods and refined by full-matrix least squares using the
SHELXTL-PC program package.10 Hydrogen atoms were placed in
their calculated positions with C᎐H = 0.96 Å, assigned fixed isotropic
thermal parameters, and allowed to ride on their respective parent C
atoms. Refinement of 425 parameters for 47 non-hydrogen atoms and
The molecular structure of I(CO)2Ir(µ-Ph2Ppy)2CdI2 3 has
been determined by single-crystal X-ray analysis (Fig. 1).§ The
5382 observed data [|Fo| > 6σ(Fo) out of 7102 unique reflections]
2
᎐
᎐
converged to RF ᎐᎐ Σ(|Fo| Ϫ |Fc|)/Σ|Fo| = 0.062 and RwF ᎐ [{Σw(|Fo| Ϫ
᎐
¹
|Fc|)2}/{Σw|Fo|2}]² = 0.073 with wϪ1 = σ2(|Fo|) ϩ 0.000 05 |Fo|2. CCDC
† E-Mail: tcwmak@cuhk.edu.hk
reference number 186/687.
J. Chem. Soc., Dalton Trans., 1997, Pages 3409–3410
3409

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εmax > 2000, they are assigned to metal-to-ligand charge-
transfer (MLCT) bands of the type IrI → phosphine or CO.
Complex 1 is weakly emissive and shows an emission at 650
nm with a lifetime of 87 µs measured in a frozen CH2Cl2 solu-
tion at 77 K. Under the same conditions, complex 3 shows an
emission at 666 nm with a lifetime of 249 µs. The emission from
a CH2Cl2 solution of 3 is hardly observable at room temper-
ature, but a solid sample shows a red emission at 739 nm with a
lifetime of 6.48 µs at 298 K. We tentatively assign this lumin-
Ph
Ph
Ph
Ph
P
P
N
N
N
+CO
–CO
Ph2Ppy
CO
OC
OC
Cl
[Ir(cod)2Cl]2
Ir
P
Ir
Cl
N
OC
Ph
P
Ph
Ph
Ph
3
escence to emission from a MLCT excited state.
1
2
The long excited life-time of complex 3 suggests that it may
possess rich photochemical properties. Upon flashing
a
CdI2
degassed acetonitrile solution of 3 and N,NЈ-dimethyl-4,4Ј-
bipyridinium dihexafluorophosphate (Mv[PF6]2), the excited-
state electron-transfer reaction shown in equation (1) was
observed.
Ph
Ph
P
N
Cd
N
ϩ
IrI᎐CdII ϩ Mv2ϩ → IrII᎐CdII ϩ Mv
(1)
CO
᝽
I
I
Ir
P
ϩ
I
Formation of the Mv radical cation was substantiated by
OC
᝽
recording the different absorption spectrum 10 µs after the 355
nm laser flash. The bands at 400 and 630 nm are characteristic
Ph
Ph
of the Mv ϩ radical cation.
᝽
3
Scheme 1
Acknowledgements
This work is supported by Hong Kong Research Grants Coun-
cil Earmarked Grant CUHK 311/94P.
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Fig. 1 An ORTEP 5 drawing (35% thermal ellipsoids) showing the
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lengths (Å) and angles (Њ): Ir(1)᎐Cd(1) 2.784(1), Ir(1)᎐P(1) 2.357(4),
Ir(1)᎐P(2) 2.359(4), Cd(1)᎐N(1) 2.51(1), Cd(1)᎐N(2) 2.51(1);
Cd(1)᎐Ir(1)᎐P(2) 89.0(1), P(1)᎐Ir(1)᎐P(2) 177.5(1), Ir(1)᎐Cd(1)᎐N(1)
85.5(3), Ir(1)᎐Cd(1)᎐N(2) 86.2(3), N(1)᎐Cd(1)᎐N(2) 171.0(4). Torsion
angles (Њ): P(1)᎐Ir(1)᎐Cd(1)᎐N(1) 26.0, P(2)᎐Ir(1)᎐Cd(1)᎐N(2) 22.8
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almost linear with bond angles P(1)᎐Ir(1)᎐P(2) 177.5(1)Њ and
N(1)᎐Cd(1)᎐N(2) 171.0(4)Њ, respectively. The Ir(µ-Ph2Ppy)2Cd
core is distorted from planarity, the relevant torsion angles
being P(1)᎐Ir(1)᎐Cd(1)᎐N(1) 26.0Њ and P(2)᎐Ir(1)᎐Cd(1)᎐N(2)
22.8Њ. This twisting of the chelate rings allows the iridium and
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The UV/VIS spectrum of complex 1 in dichloromethane
solution shows absorption bands at 338, 386 and 440 nm with
the last having the lowest εmax value. On the other hand, com-
plex 3 shows broad absorptions ranging from 300 to 390 nm.
Since the absorptions of 1 and 3 at 300᎐400 nm have
11 S.-M. Kuang, Z.-Z. Zhang, B.-M. Wu and T. C. W. Mak,
J. Organomet. Chem., 1997, 540, 55.
Received 28th July 1997; Communication 7/05431I
3410
J. Chem. Soc., Dalton Trans., 1997, Pages 3409–3410