Synthesis and characterization of luminescent osmium(II) carbonyl complexes based on chelating dibenzoylmethanate and halide ligands

Article abstract of DOI:10.1039/b308340c

Octahedral Os(II) complexes 1-5 with formula [Os(CO)₃X(dbm)] are prepared through utilization of both solid-state pyrolysis and ligand exchange reactions. These complexes exhibit prominent ³π-π * phosphorescence with unusually long l

Full text of DOI:10.1039/b308340c

Synthesis and characterization of luminescent osmium(II) carbonyl
complexes based on chelating dibenzoylmethanate and halide ligands†
Yao-Lun Chen,a Chittaranjan Sinha,a I-Chia Chen,a Kuan-Lin Liu,a Yun Chi,*a Jen-Kan Yu,b Pi-Tai
Chou*b and Tian-Huey Luc
a Department of Chemistry, National Tsing Hua University, 300, Hsinchu, Taiwan R.O.C
b Department of Chemistry, National Taiwan University, 106, Taipei, Taiwan, R.O.C
c
Department of Physics, National Tsing Hua University, 300, Hsinchu, Taiwan, R.O.C.
E-mail: ychi@mx.nthu.edu.tw. E-mail: chop@ccms.ntu.edu.tw
Received (in Cambridge, UK) 27th July 2003, Accepted 13th October 2003
First published as an Advance Article on the web 31st October 2003
Octahedral Os(II
)
complexes 1–5 with formula
X = Cl (2), Br (3) and SCN (5)
[Os(CO)3X(dbm)] are prepared through utilization of both
solid-state pyrolysis and ligand exchange reactions. These
Based on the tfa complex 1, other related derivatives with X
= Cl (2), Br (3) and SCN (5) can be synthesized using simple
ligand substitution conducted in refluxing methanol. The only
exception is the iodide complex [Os(CO)3I(dbm)] (4), which
was alternatively prepared using the aforementioned solid state
pyrolysis, as the required starting material [Os2(CO)6(m-I)2]
was readily obtained from the reaction of Os3(CO)12 and I2.5
These Os metal complexes were fully characterized via
spectroscopic methods. Of particular interest is the IR n(CO)
spectrum of 1 in cyclohexane, which revealed three strong CO
stretching bands at 2125, 2047 and 2031 cm21 characteristic for
the mutually orthogonal [Os(CO)3] unit. The iodide complex 4
was then examined by a single crystal X-ray diffraction study.
The result showed an asymmetric unit containing two crystallo-
graphically distinct, but structurally identical, molecules. As
depicted in Fig. 1 the structure of one molecule reveals an
octahedral Os metal arrangement, which is surrounded by a
chelating dbm ligand, together with an iodide and three facial
carbonyl ligands. The gross structure of 4 can be referred to
those found in the closely related hexafluoroacetylacetonate
derivative [Os(CO)3(tfa)(hfac)]6 and the monometallic com-
plexes such as [Os(CO)3(tfa)2] and [Os(CO)3(pyS)2],7 pyS =
pyridine-2-thionate, of which one potentially chelating organic
ligand is converted to monodentate bonding mode in complet-
ing the six-coordinate geometry.
The absorption and luminescence spectra of representative
complexes 1, 2 and 4 are shown in Fig. 2, and the photophysical
as well as electrochemical data are summarized in Table 1.
Typically, these complexes exhibit the lowest energy absorp-
tion band in the visible region of ca. 367–380 nm with high
absorptivity of e > 1.3 3 104 M21cm21 at the peak maximum.
Due to their spectral similarity with the absorption of the
respective sodium salt (dbm)Na in acetonitrile solution ( ~ 352
nm), this absorption band can be reasonably assigned as an
intraligand p–p* transition of the deprotonated dbm. However,
3
complexes exhibit prominent p–p* phosphorescence with
unusually long lifetimes (29–64 ms) and high quantum yields
(0.08–0.13).
Photophysical studies of Re( ) tricarbonyl complexes with
I
structural formula fac-[Re(CO)3X(L)], X = halides and L =
bidentate heterocycles such as 2,2A-bipyridine, have received
considerable attention during the last decade.1 These metal
complexes exhibit either low-lying intraligand p–p* or metal-
to-ligand charge transfer (MLCT) absorptions, while the strong
emission originates from the excited states with considerable
triplet–singlet mixing as a result of enhanced spin–orbit
coupling. More recently, these Re( ) metal fragments have been
I
extensively used as the basic building blocks to construct
supramolecular transition-metal complexes that may serve as
model systems for photoresponsive molecular devices.2 Inter-
estingly, extension to the missing Os(CO)3X analogues has
never been realized possibly due to the lack of a suitable
molecular design, resulting in an abrupt discontinuation of this
class of highly luminescent metal carbonyl complexes. In this
paper, a new series of Os(II)(CO)3X are prepared, in which the
skeletal arrangement consists of one b-diketonate chromophore
such as (dbm)H = dibenzoylmethane to balance the +2 formal
charge on the metal, one anionic ligand X and three orthogonal
CO ligands located at the octahedral coordination site.
Accordingly, our first target complex [Os(CO)3(tfa)(dbm)]
(1) was prepared by heating the Os dimer complex
[Os2(CO)6(tfa)2]3 (tfa = CF3CO22) and a slight excess of
dibenzoylmethane (dbm)H in a Carius tube. This solid-state
pyrolysis allowed us to conduct the experiments at higher
temperatures and to avoid the usage of solvents that may
interfere with the reactions.4
[Os2(CO)6(m-X)2] + (dbm)H ? [Os(CO)3X(dbm)]
X = tfa (1) and I (4)
(1) + NaCl, NaBr or NH4SCN ? [Os(CO)3X(dbm)]
† Electronic supplementary information (ESI) available: experimental
details and the spectral data of all isolated Os complexes. See http://
www.rsc.org/suppdata/cc/b3/b308340c/
‡ CCDC 215957. See http://www.rsc.org/suppdata/cc/b3/b308340c/ for
crystallographic data in .cif format.
Fig. 1 The X-ray crystal structure of 4; selected distances: Os(1)–C(1) =
1.917(7), Os(1)–C(2) = 1.928(8), Os(1)–C(3) = 1.896(7), Os(1)–O(4) =
2.088(4), Os(1)–O(5) = 2.085(4) and Os(1)–I(1) = 2.721(1) Å.‡
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CHEM. COMMUN., 2003, 3046–3047
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Table 1 Photophysical properties and electrochemical data of complexes 1–5
labs/nm, CH2Cl2 at
Entry
Formula
RT (e/M21cm21
)
lem/nma
538 (541)f
557 (537)
563 (561)
574 (602)
539 (545)
530e
to/msa
Fem
0.13
kr/103 s21b
2.02
Epa/Vc
+1.75
+1.68
+1.64
+1.60
+1.74
—
Epc/Vc
1
2
3
4
5
[Os(CO)3(tfa)(dbm)]
[Os(CO)3Cl(dbm)]
[Os(CO)3Br(dbm)]
[Os(CO)3I(dbm)]
280 (21900)
367 (20000)
285 (19100)
372 (16200)
278 (19200)
375 (15600)
278 (23800)
380 (13500)
64 (3.0)f
46 (9.1)
29 (4.2)
0.72, 38 (2.7)
53 (3.4)
5 3 104
21.53
21.60
21.58
21.59
21.55
21.20
0.13
0.08
0.007
0.13
2.76
2.82
0.18
[Os(CO)3(SCN)(dbm)] 277 (26300)
372 (15300)
[(dbm)Na]
2.49
352 (22100)d
< 1023
< 0.05
a Solvent: CH2Cl2 and excitation wavelength: 380 nm. b kr = Fem/t. c In CH3CN with 0.1 M nBu4NPF6 (TBAH) supporting electrolyte and Ag/AgCl
reference electrode at RT; scan rate 100 mV s21, while Epa and Epc refer to the irreversible anodic and cathodic peak potentials, respectively. d Data recorded
in CH3CN. e The emission data were taken in a 77 K methanol glass. f Data in parentheses () were obtained in the solid state. Note that a faster decay
component ( < 0.2 ms) was observed for 1–5 in the solid state possibly due to the defective sites. Its integrated intensity is < 10% and is thus neglected.
manifesting the unusually strong spin–orbit coupling induced
by Os.
A unique, strong phosphorescence was also observed for 1–5
in the solid state, and the corresponding emission and lifetime
data are listed in Table 1. In comparison to that measured in the
solution phase the lifetime in solid state is relatively short,
possibly due to the crystal packing effect.
In summary, a new series of the Os(II) complexes with the
dbm chelate were synthesized. The results represent a rare class
of complexes possessing lowest excited states localized pre-
dominantly on the dbm ligand, resulting in unusually strong,
long-lived phosphorescence in room-temperature fluid solution
as well as in solid crystal. This class of Os(II) complexes with
profound photophysical and electrochemical data provided may
be advantageously exploited in a variety of photochemical
studies such as OLED or photovoltaic devices. Furthermore, the
Fig. 2 Absorption and emission spectra of complexes 1, 2 and 4 in CH2Cl2
current successes in assembling the Re-containing supramole-
solution.
cules may warrant a similar applicability of the dibenzoylme-
thanate Os(II) complexes. Work on the relevant subjects is in
progress and will be published in a forthcoming full paper.
1
because the corresponding MLCT signal of the related Os(II
)
polypyridyl complexes also occurs in the same region,8 the
possibility of a small contribution from the MLCT transition
cannot be ruled out.
Notes and references
Complexes 1–5 exhibit bright emission in deaerated CH2Cl2.
The hypsochromic shift of the emission peak maximum is in the
order of tfa ≈ SCN > Cl > Br > I, consistent with the trend
of electron withdrawing strength destabilizing the HOMO of
dbm and hence increasing the energy gap. The emission lifetime
is unusually long ( > 20 ms),9 while the quantum efficiencies,
except for 4, were measured to be as high as 0.08–0.13. In
addition, the emission was drastically quenched by O2 with a
1 (a) D. J. Stufkens, Comments Inorg. Chem., 1992, 13, 359; (b) A. P. Zipp,
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rate ~ 1.52 3 109 M21 21
for 2 in CH2Cl2. These observations
s
lead us to assign the emission to be phosphorescent in nature.
Further support is given by the power-dependent relaxation
dynamics. On increasing the excitation intensity the decay
becomes non-single exponential, indicating the prevalence of
triplet–triplet annihilation upon high triplet-state population.
Note that a shorter lifetime was also observed for the iodide
complex 4 (see Table 1), and is tentatively proposed to originate
from photodissociation due to the weak Os–I bond, as indicated
by the change of absorption spectral features as well as the
decrease of 574 nm phosphorescence upon a long period of
photolysis.
A comparative experiment was also performed for the dbm
anion (dbm)Na in basified (NaOH) methanol, and a phospho-
rescence maximized at ~ 530 nm was resolved in a 77 K
methanol glass, of which the spectral feature resembles that
observed in complexes 1–5. This result concludes that the room-
temperature phosphorescence in 1–5 originates predominantly
from the 3p–p* manifold of ligated dbm. The main difference in
phosphorescence properties lies in the much faster radiative
decay rate in the Os complexes (see Table 1 for comparison),
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9 Other examples are [MoCl12]
22, [Au(dppm)2]2+ and [Pt(dppe)(5,6-
Me2phen)]2+, see: K. T. Wan and C. M. Che, J. Chem. Soc., Chem.
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