Bichromophoric rhodamine-rhenium(I) and -iridium(III) sensory system: Synthesis, characterizations, photophysical and selective metal ions binding studies Dedicated to Prof. Claude Lapinte.

Article abstract of DOI:10.1016/j.poly.2014.07.010

A new class of rhenium(I) tricarbonyl diimine complexes containing rhodamine derivative as a sensory moiety, have been synthesized and characterized. Their photophysical and selective ion-binding properties have also been investigated, with the comparison

Full text of DOI:10.1016/j.poly.2014.07.010

Polyhedron xxx (2014) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Bichromophoric rhodamine-rhenium(I) and -iridium(III) sensory
system: Synthesis, characterizations, photophysical and selective metal
ions binding studies
Keith Man-Chung Wong a,b, , Chunyan Wang a,b, Ho-Chuen Lam b, Nianyong Zhu b
⇑
a Department of Chemistry, South University of Science and Technology of China, No. 1088, Tangchang Boulevard, Nanshan District, Shenzhen, Guangdong, China
b Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of rhenium(I) tricarbonyl diimine complexes containing rhodamine derivative as a sensory
moiety, have been synthesized and characterized. Their photophysical and selective ion-binding proper-
ties have also been investigated, with the comparison of the previously reported cyclometalated
iridium(III) system. One of the complex was found to exhibit selective binding toward Hg(II) ion with
electronic absorption and emission spectral changes. On the other hand, the iridium(III) complexes were
found to give similar electronic absorption response but different emission spectral changes upon
addition of Hg(II) ion. Interestingly, efficient intramolecular energy transfer from rhodamine 6G to the
cyclometalated iridium(III) moiety was suggested in one of the complexes, which could be modulated
by selective ion-binding process upon ring-opening of rhodamine 6G. This report shows that the modi-
fication of rhodamine derivatives and the choice of transition metal ion could be important to influence
the selective binding properties for development of colorimetric and/or luminescent probe.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 28 February 2014
Accepted 6 July 2014
Available online xxxx
Dedicated to Prof. Claude Lapinte.
Keywords:
Bichromophoric
Rhodamine
Rhenium(I)
Iridium(III)
Ion-binding
1. Introduction
co-worker [8], the modification of rhodamine derivatives as
chemosensors with high selectivity and sensitivity has been
There has been growing research interest in modification of
luminescent probes for different metal ions since the pioneering
work of crown ether by Pedersen in 1967 [1]. Host–guest chemis-
try has subsequently come to maturity by the work of macromol-
ecules [1–4], such as cryptands, spherands and crown ethers,
contributed by Lehn and Cram later on. After their endeavour to
sensing chemistry, scientists have started to pay a great deal of
attention to the development of luminescent sensors. Noelting
et al. firstly documented the preparation of rhodamine derivatives
[5]. As fluorophore and chromophore probes, rhodamine-based
dyes have been widely used in the past several decades [6,7],
owing to their great photo-stability and excellent spectroscopic
properties, such as long excitation and emission wavelengths, large
molar extinction coefficient and high fluorescence quantum yield,
as well as the favorable solubility in water. Since the innovative
work on the ring-opening behaviour of a rhodamine B derivative
as a fluorescent chemosensor for copper(II) ion by Czarnik and
actively explored [9–19].
In view of the general limitations commonly encountered in
organic fluorophores, such as high photobleaching rates, short
fluorescence lifetimes and small Stokes’ shifts, the search for new
chromophores and luminophores to act as luminescent chemosen-
sors continues. Transition-metal complexes were found to have a
series of advantages for chemosensing, including longer lumines-
cent lifetimes in the process of detection; kinetic inertness and
photochemical stability; relatively high triplet quantum yields
arising from the heavy metal effect; lower excitation energy
required. Therefore, the design and utilization of transition
metal-based luminescent chemosensors which commonly display
phosphorescence lead to attractive alternatives that could fulfill
roles that are complementary to those offered by organic fluoro-
phores. Although there are a few reports on the combination of
the rhodamine unit with another organic chromophore or lumino-
phore in a single molecule [20,21], it is surprising to find that the
related construction of bichromophoric array by the incorporation
of a luminescent transition metal complex into a rhodamine deriv-
ative with sensory functionality is relatively unexplored [22,23]. Of
particular interest to us is the possibility that incorporation of
rhodamine derivative into a luminescent transition metal complex
⇑
Corresponding author at: Department of Chemistry, South University of Science
and Technology of China, No. 1088, Tangchang Boulevard, Nanshan District,
Shenzhen, Guangdong, China.
E-mail address: keithwongmc@sustc.edu.cn (K.M.-C. Wong).
http://dx.doi.org/10.1016/j.poly.2014.07.010
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: K.-M.C. Wong et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.07.010

2
K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
may allow access to a variety of new bichromophoric sensory
systems. Therefore a program was initiated to design and synthe-
size different rhodamine-containing ligands and to coordinate
them to the bichromophoric systems with the combination of var-
ious transition metal complexes and rhodamine-derivatives.
Herein, we report to design and synthesize rhodamine-appended
rhenium(I) tricarbonyl diimine complexes and their photophysical
and selective ion-binding properties, with the comparison of the
previously reported cyclometalated iridium(III) system [22].
(dd, J = 5.0, 0.8 Hz, 1H, bipyridyl proton), 8.66 (dd, J = 5.0, 0.8 Hz,
1H, bipyridyl proton); positive EI-MS, m/z: 665 [M+H]+.
L3: The procedure was similar to that described for the synthe-
sis of L1 except rhodamine 6G hydrazide was used instead of rho-
damine B hydrazide. Yield: 0.59 g (98%). 1H NMR (400 MHz, CDCl3,
298 K, relative to Me4Si)/ppm: d = 1.32 (t, J = 7.1 Hz, 6H, N–CH2–
CH3), 1.87 (s, 6H, CH3 on xanthene ring), 2.42 (s, 3H, CH3 on bipyr-
idyl ring), 3.20–3.25 (m, 4H, N–CH2–CH3), 3.47–3.50 (m, 2H, NH),
6.35 (s, 2H, xanthene protons), 6.43 (s, 2H, xanthene protons),
7.06 (dd, J = 1.5 and 6.0 Hz, 1H, phenyl proton on rhodamine),
7.12 (dd, J = 0.8 and 4.9 Hz, 1H, bipyridyl proton), 7.45–7.52 (m,
2H, phenyl protons on rhodamine), 7.74 (dd, J = 1.4 and 5.1 Hz,
1H, bipyridyl proton), 8.04 (dd, J = 1.7 and 5.8 Hz, 1H, phenyl pro-
ton on rhodamine), 8.15 (s, 2H, CH@N and bipyridyl proton), 8.25
(s, 1H, bipyridyl proton), 8.50 (d, J = 5.1 Hz, 1H, bipyridyl proton),
8.57 (d, J = 5.1 Hz, 1H, bipyridyl proton); positive EI-MS: m/z: 609
[M]+.
L4: The procedure was similar to that described for the synthe-
sis of L3 except rhodamine 6G ethylamine was used instead of
rhodamine 6G hydrazide. Yield: 0.52 g (90%). 1H NMR (400 MHz,
CDCl3, 298 K, relative to Me4Si)/ppm: d = 1.31 (t, J = 7.1 Hz, 6H,
N–CH2–CH3), 1.84 (s, 6H, CH3 on xanthene ring), 2.44 (s, 3H, CH3
on bipyridyl ring), 3.19 (q, J = 7.1 Hz, 4H, N–CH2–CH3), 3.46–3.63
(m, 6H, NH and N–CH2CH2–N), 6.21 (s, 2H, xanthene protons),
6.35 (s, 2H, xanthene protons), 7.04 (dd, J = 2.1 and 5.1 Hz, 1H, phe-
nyl proton on rhodamine), 7.14 (d, J = 0.6 and 4.9 Hz, 1H, bipyridyl
proton), 7.41–7.48 (m, 2H, phenyl protons on rhodamine), 7.56 (dd,
J = 1.4 and 5.0 Hz, 1H, bipyridyl proton), 7.92–7.96 (m, 2H, CH@N
and phenyl proton on rhodamine), 8.21 (s, 1H, bipyridyl proton),
8.39 (s, 1H, bipyridyl proton), 8.54 (d, J = 5.0 Hz, 1H, bipyridyl pro-
ton), 8.65 (d, J = 5.0 Hz, 1H, bipyridyl proton); positive EI–MS: m/z:
637 [M]+.
2. Experimental
2.1. Materials
All the solvents for synthesis were of analytical grade. Methanol
for analysis was of HPLC grade. Rhodamine 6G and rhodamine B
base were purchased from Acros Organics Company. 4,40-
Dimethyl-2,20-bipyridine, iridium(III) chloride hydrate, rhenium(I)
pentacarbonyl bromide and ammonium hexafluorophosphate
were purchased from Aldrich Chemical Company. Mercury(II)
perchlorate, lead(II) perchlorate, copper(II) perchlorate, zinc(II)
perchlorate, cadmium(II) perchlorate, barium(II) perchlorate,
manganese(II) perchlorate, silver(I) perchlorate, potassium(I)
perchlorate, sodium(I) perchlorate and lithium(I) perchlorate were
purchased from Aldrich Chemical Company with purity over 99.0%
and were used as received. Hydrazine monohydrate and ethylene-
diamine were purchased from Alfa Aesar Company. The precursors
4-methyl-40-carbonyl-2,20-bipyridine [24], rhodamine 6G hydra-
zide [25], rhodamine 6G ethylamine [25], rhodamine B hydrazide
[26], rhodamine B ethylamine [26], and Ir2(dpqx)4Cl2 (dpqx = 2,3-
diphenylquinoxaline) [27] were synthesized according to the mod-
ification of literature methods.
[Re(CO)3(L1)Br] (1): This was synthesized by modification of a
literature procedure [28].
A mixture of Re(CO)5Br (0.07 g,
2.2. Synthesis
0.17 mmol) and L1 (0.09 g, 0.17 mmol) in toluene (20 ml) was
heated to reflux under N2 in the dark for 4 h. The solvent was
removed under reduced pressure. Subsequent recrystallization by
vapour diffusion of diethyl ether into dichloromethane solution
of products gave 1 as red-orange crystals. Yield: 121 mg, 72%. 1H
NMR (300 MHz, CDCl3, 298 K, relative to Me4Si)/ppm: d 1.16 (td,
J = 7.1, 1.4 Hz, 12H, N–CH2–CH3), 2.64 (s, 3H, CH3 on bipyridyl ring),
3.41–3.25 (m, 8H, N–CH2–CH3), 6.26 (m, 2H, xanthene protons),
6.48 (m, 4H, xanthene protons), 7.34 (m, 2H, bipyridyl proton
and phenyl proton on rhodamine), 7.58 (m, 2H, phenyl proton on
rhodamine), 8.04 (m, 1H, phenyl proton on rhodamine), 8.10
(s, 1H, bipyridyl proton), 8.43 (s, 1H, bipyridyl proton), 8.73 (s,
1H, CH@N), 8.87 (m, 2H, bipyridyl protons); positive FAB-MS:
m/z: 987.1 [M–H]+, 907.1 [MꢀBr]+; elemental Anal. Calc. for com-
plex 1: C, 52.17; H, 4.34; N, 8.49. Found: C, 52.22; H, 4.27; N, 8.30%.
[Re(CO)3(L2)Br] (2): The procedure was similar to that described
for the synthesis of 1 except L2 was used instead of L1. Yield:
118 mg, 68%. 1H NMR (300 MHz, CDCl3, 298 K, relative to Me4Si)/
ppm: d 1.17 (dt, J = 6.0 Hz, J = 3.0 Hz, 12H, N–CH2–CH3), 2.62 (s,
3H, CH3 on bipyridyl ring), 3.29–3.36 (dq, J = 6.0 Hz, J = 3.0 Hz,
8H, N–CH2–CH3), 3.56 (m, 4H, CH2CH2), 6.25 (m, 2H, xanthene pro-
tons), 6.41 (m, 4H, xanthene protons), 7.09 (m, 1H, phenyl proton
on rhodamine), 7.34 (m, 1H, bipyridyl proton), 7.45 (m, 3H, phenyl
protons on rhodamine and bipyridyl proton), 7.90 (m, 1H, phenyl
protons on rhodamine), 8.01 (s, 1H, N@CH), 8.14 (s, 1H, bipyridyl
proton), 8.51 (m, 1H, bipyridyl proton), 8.88 (d, J = 5.7 Hz, 1H,
bipyridyl proton), 9.03 (d, J = 5.6 Hz, 1H, bipyridyl proton), positive
FAB-MS: m/z 1017.4 [M]+; elemental Anal. Calc. for complex 2ꢁH2O:
C, 52.27; H, 4.45; N, 8.13. Found: C, 52.49; H, 4.10; N, 7.85%.
[Ir(dpqx)2L3]PF6 (3): Complex 3 was obtained by the reaction of
L3 (38 mg, 0.06 mmol) with 0.5 equiv of dinuclear iridium(III)
precursor complex, Ir2(dpqx)4Cl2 (50 mg, 0,03 mmol), in a solvent
L1: The mixture of rhodamine B hydrazide (0.456 g, 1 mmol)
and 4-methyl-40-carbonyl-2,20-bipyridine (0.198 g, 1 mmol) in
dichloromethane (15 ml) was heated to reflux for 12 h. The mix-
ture was cooled to room temperature and the solvent was removed
in vacuum to afford brown oil. Methanol was added to precipitate a
white solid which was then filtered and washed with methanol.
Yield: 0.56 g, 88%. 1H NMR (300 MHz, CDCl3, 298 K, relative to Me4-
Si)/ppm: d = 1.16 (t, J = 7.0 Hz, 12H, N–CH2–CH3), 2.41 (s, 3H, CH3
on bipyridyl ring), 3.32 (q, J = 7.0 Hz, 8H, N–CH2–CH3), 6.24 (dd,
J = 8.9, 2.6 Hz, 2H, xanthene protons), 6.47 (d, J = 2.5 Hz, 2H, xan-
thene protons), 6.54 (d, J = 8.8 Hz, 2H, xanthene protons), 7.12
(m, 2H, bipyridyl proton and phenyl proton on rhodamine), 7.48
(m, 2H, phenyl protons on rhodamine), 7.76 (dd, J = 5.2, 1.5 Hz,
1H, phenyl proton on rhodamine), 8.01 (dd, J = 6.2, 1.2 Hz, 1H,
bipyridyl proton), 8.17 (m, 2H, bipyridyl proton and CH@N), 8.41
(s, 1H, bipyridyl proton), 8.51 (d, 1H, J = 5.0 Hz, bipyridyl proton),
8.58 (d, 1H, J = 5.1 Hz, bipyridyl proton); positive EI-MS, m/z: 637
[M+H]+.
L2: The procedure was similar to that described for the synthe-
sis of L1 except rhodamine B ethylamine was used instead of rho-
damine B hydrazide. Yield: 0.58 g, 87%. 1H NMR (300 MHz, CDCl3,
298 K, relative to Me4Si)/ppm: d 1.16 (t, J = 7.0 Hz, 12H, N–CH2–
CH3), 2.43 (s, 3H, CH3 on bipyridyl ring), 3.32 (q, J = 7.0 Hz, 8H,
N–CH2–CH3), 3.50 (s, J = 2.8 Hz, 4H, N–CH2CH2–N), 6.25 (dd,
J = 8.9, 2.6 Hz, 2H, xanthene protons), 6.47 (d, J = 2.5 Hz, 2H, xan-
thene protons), 6.54 (d, J = 8.8 Hz, 2H, xanthene protons), 7.11
(m, 2H, bipyridyl proton and phenyl proton on rhodamine), 7.43
(m, 2H, phenyl protons on rhodamine), 7.57 (m, 1H, phenyl proton
on rhodamine), 7.91 (m, 1H, bipyridyl proton), 8.08 (s, 1H, CH@N),
8.21 (m, 1H, bipyridyl proton), 8.46 (m, 1H, bipyridyl proton), 8.53
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K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
3
mixture (30 ml) of dichloromethane and methanol (1:1 v/v) under
reflux condition for 4 h. After removal of solvent in vacuum, the red
solid residue was then dissolved in hot methanol. A saturated
methanolic solution of ammonium hexafluorophosphate was
added to precipitate the red solid. Subsequent recrystallization of
the complexes by diffusion of diethyl ether vapour into a solution
of the complex in dichloromethane afforded complex 3 as an
orange solid: Yield: 57 mg, 60%. 1H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: d = 1.24–1.29 (m, 6H, N–CH2–CH3), 1.83
(s, 6H, CH3 on xanthene ring), 2.62 (s, 3H, CH3 on bipyridyl ring),
3.08–3.21 (m, 4H, N–CH2–CH3), 3.44–3.55 (m, 2H, NH), 6.20 (s,
1H, xanthene protons), 6.21 (s, 1H, xanthene protons), 6.37–6.43
(m, 4H, xanthene and 2,3-diphenylquinoxaline protons), 6.64–
6.70 (m, 2H, 2,3-diphenylquinoxaline protons), 6.72–6.78 (m, 2H,
2,3-diphenylquinoxaline protons), 7.08–7.19 (m, 4H, 2,3-diphenyl-
quinoxaline protons), 7.28–7.34 (m, 3H, phenyl protons on rhoda-
mine and 2,3-diphenylquinoxaline proton), 7.51–7.62 (m, 4H,
phenyl proton on rhodamine and 2,3-diphenylquinoxaline pro-
tons), 7.67–7.71 (m, 6H, 2,3-diphenylquinoxaline protons), 7.75–
7.78 (m, 4H, 2,3-diphenylquinoxaline and bipyridyl protons), 7.84
(d, J = 6.4 Hz, 2H, 2,3-diphenylquinoxaline protons), 7.91 (s, 1H,
bipyridyl proton), 7.97–8.03 (m, 3H, phenyl proton on rhodamine
and 2,3-diphenylquinoxaline protons), 8.07 (s, 1H, bipyridyl pro-
ton), 8.51–8.54 (m, 2H, CH@N and bipyridyl proton), 8.59 (d,
J = 5.7 Hz, 1H, bipyridyl proton); positive ESI-MS: m/z: 1364.5
[M–PF6]+; elemental Anal. Calc. for C78H62F6IrN10O2Pꢁ4H2O: C,
59.26; H, 4.46; N, 8.86. Found: C, 59.08; H, 4.20; N, 8.79%.
[Ir(dpqx)2L4]PF6 (4): The procedure was similar to that described
for the synthesis of 3 except L4 (40 mg, 0.06 mmol) was used
instead of L3. Yield: 72 mg, 74%. 1H NMR (400 MHz, CDCl3, 298 K,
relative to Me4Si)/ppm: d = 1.26–1.31(m, 6H, N–CH2–CH3), 1.82
(s, 6H, CH3 on xanthene ring), 2.59 (s, 3H, CH3 on bipyridyl ring),
3.14–3.20 (m, 4H, N–CH2–CH3), 3.29 (t, J = 6.4 Hz, N–CH2), 3.49–
3.62 (m, 4H, N–CH2 and NH), 6.09 (s, 1H, xanthene protons), 6.12
(s, 1H, xanthene protons), 6.31 (s, 1H, xanthene protons), 6.33 (s,
1H, xanthene protons), 6.45 (t, J = 6.2 Hz, 2H, 2,3-diphenylquinox-
aline protons), 6.68–6.72 (m, 2H, 2,3-diphenylquinoxaline pro-
tons), 6.75–6.80 (m, 2H, 2,3-diphenylquinoxaline protons), 7.04
(t, J = 4.2 Hz, 1H, 2,3-diphenylquinoxaline protons), 7.14–7.20 (m,
4H, 2,3-diphenylquinoxaline protons), 7.31–7.36 (m, 2H, phenyl
protons on rhodamine), 7.42 (t, J = 4.2 Hz, 2H, phenyl protons on
rhodamine), 7.54–7.60 (m, 3H, 2,3-diphenylquinoxaline protons),
7.67–7.69 (m, 6H, 2,3-diphenylquinoxaline protons), 7.82–7.87
(m, 5H, 2,3-diphenylquinoxaline and bipyridyl protons), 7.90 (s,
1H, CH@N), 7.98–8.02 (m, 2H, 2,3-diphenylquinoxaline protons),
8.05 (d, J = 5.7 Hz, 1H, bipyridyl proton), 8.25 (s, 1H, bipyridyl pro-
ton), 8.41 (s, 1H, bipyridyl proton), 8.52 (d, J = 5.6 Hz, 1H, bipyridyl
proton), 8.62 (d, J = 5.6 Hz, 1H, bipyridyl proton); positive ESI-MS:
m/z: 1390 [M–PF6]+; elemental Anal. Calc. for C80H66F6IrN10O2P: C,
62.53; H, 4.33; N, 9.11. Found: C, 62.49; H, 4.10; N, 8.85%.
recorded on a Spex Fluorolog-3 Model FL3-211 fluorescence spec-
trofluorometer equipped with a R2658P PMT detector. The emis-
sion spectroscopic investigations of all solution required
a
degassing procedure on a high-vacuum line in a degassing cell,
which contains 10-ml Pyrex bulb and a 1-cm path length quartz
cuvette and is sealed from the atmosphere by a Bibby Rotaflo
HP6 Telflon stopper. At least four successive freeze-pump-thaw
cycles were performed to degas the solutions prior to the
measurement.
2.4. Crystal structure determination
Single crystals suitable for X-ray diffraction studies were grown
by slow diffusion of diethyl ether vapor into a dichloromethane
and acetone solutions of the complexes, respectively, for 1 and 4.
After the structural solution and refinement, one ethyl substituent
of amino group in 4 was found to be converted into hydrogen on
the rhodamine moiety, 40 was accordingly denoted for such com-
plex. The X-ray diffraction data were collected on a Bruker Smart
CCD 1000 diffractometer using graphite monochromatized Mo
Ka
radiation (k = 0.71073 Å). Raw frame data of 40 were integrated
with SAINT [29] program. Semi-empirical absorption correction with
SADABS [30] was applied. The crystal data and refinement details are
listed in Table 1. The structure was solved by direct methods
employing SHELXS-97 program [31]. Re, Br, Ir, P and many non-H
atoms were located according to the direct methods. The positions
of the other non-hydrogen atoms were found after successful
refinement by full-matrix least-squares using program SHELXL-97
[32]. For 1, there was one formula unit in the asymmetric unit. Half
of diethylether was located with O at the special position. For 40,
there was one formula unit in the asymmetric unit. Two halves
of PF6 anions were located, in which P atoms locate at the special
positions. Two acetone solvent molecules and one water O were
located. Restrains were applied to the acetones, assuming O–C
and C–C bond lengths of around 1.25(2) and 1.53(2) Å, respec-
tively. Other solvent molecules could not be located reliably, so
Platon-squeeze method was employed to abstract the influence
of the unlocated solvent molecules. This gave rise to the potential
solvent accessible void volume of 1094 Å3 per unit cell, which cor-
responds to 137 electron counts per unit cell.
2.5. Ion-binding studies
Binding constants for 1:1 complexation were determined by
nonlinear least-squares fits to Eq. (1), in which the derivations
were described previously [33].
Xlim ꢀ X0
X ¼ X0 þ
f½MꢂT þ ½Hg2þꢂ þ 1=KS
2½MꢂT
2
1=2
ꢀ ½ð½MꢂT þ ½Hg2þꢂ þ 1=KSÞ ꢀ 4½Mꢂ ½Hg2þꢂꢂ
g
ð1Þ
T
2.3. Physical measurements
where X0 and X are the absorbance (or luminescence intensity) of
the rhenium(I) or iridium(III) complex at a selected wavelength in
the absence and presence of the Hg(II) ion, respectively, [M]T is
the total concentration of the rhenium(I) or iridium(III) complex,
[Hg2+] is the concentration of the Hg(II) ion, Xlim is the limiting
value of absorbance (or luminescence intensity) in the presence of
excess Hg(II) ion and Ks is the stability constant.
All 1H NMR spectra were obtained from a Bruker DPX 300
(300 MHz) or Bruker DPX 400 (400 MHz) Fourier-transform NMR
spectrometers with chemical shifts, which were reported relative
to tetramethylsilane, (CH3)4Si. Fast atom bombardment (FAB) and
electron impact (EI) ionization spectra were obtained and Finnigan
MAT95 mass spectrometer. All positive-ion electrospray ionization
(ESI) spectra were performed on a Thermo Scientific DFS high-
resolution magnetic sector mass spectrometer. Elemental analyses
of the complexes were carried out on a Carlo Erba 1106 elemental
analyzer at the institute of Chemistry of the Chinese Academy of
Sciences, Beijing, China. The UV–Vis absorption spectra were
obtained by utilizing a Hewlett-Packard 8452A diode. Steady state
excitation and emission spectra at room temperature were
3. Results and discussion
3.1. Synthesis and characterization
The preparation of L3, L4, 3 and 4 has been communicated
recently [22]. The synthetic pathways of the rhodamine deriva-
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4
K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
Table 1
Crystal and structure determination data of 1 and 40.
Complex
1
40
Empirical formula
Formula weight
T (K)
C43H40BrN6O5Reꢁ½(C4H10O)
1023.98
301(2)
C82H72F6IrN10O5P
1614.67
301(2)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.71073
monoclinic
C2/c
25.488(6)
12.781(3)
28.862(6)
90
0.71073
triclinic
ꢀ
P
17.274(2)
18.135(2)
18.443(2)
115.72(1)
114.62(1)
94.54(1)
a
(°)
b, (°)
98.14(2)
90
c
(°)
V
9307(4)
8
1.462
4487.1(9)
2
1.195
Z (Å3)
Dcalc (g cmꢀ3
)
Crystal size, mm ꢃ mm ꢃ mm
Theta range for data collection (°)
Index ranges
0.300.25 ꢃ 0.15
1.79–25.68
–31 6 h 6 31
–14 6 k 6 15
–34 6 l 6 35
26856/8786
99.4
0.48 ꢃ 0.13 ꢃ 0.11
1.70–25.68
–21 6 h 6 21
–22 6 k 6 22
–22 6 l 6 22
46816/16984
99.6
Reflections collected/unique
Completeness to theta = 25.68° (%)
Goodness-of-fit (GOF) on F2
1.039
1.073
Final R indicesa [I > 2
r
(I)]
R1 = 0.0448
wR2 = 0.1260
1.378 and –1.020
R1 = 0.0408
wR2 = 0.1236
1.467 and –0.825
Largest different peak and hole (e Å–3
)
a
Rint
=
R
jF2o – F2o(mean)j/
R
[F2o], R1
=
R
||Fo| – |Fc||/R .
||Fo| and wR2 = {[w(F2o – F2c)2]/[w(F2o)2]}1/2
O
O
N
NH2
N
OHC
OHC
N
N
N
n
n
+
N
N
N
O
N
N
N
O
n = 0 (L1)
1 (L2)
O
O
N
N
NH2
N
N
N
n
n
+
n = 0 (L3)
1 (L4)
N
N
N
H
N
H
N
O
N
H
O
H
Scheme 1. Synthesis of L1–L4.
tive-containing bipyridine ligands, L1–L4, are summarized in
Scheme 1. Ligands L1–L4 were similarly prepared from the con-
densation reaction of 4-carboxaldehyde-40-methyl-2,20-bipyridine
with the corresponding rhodamine derivative hydrazide and ethyl-
amine intermediates in methanol in reflux condition under nitro-
gen atmosphere for 5 h. The syntheses of complexes 1–4 are
outlined in Scheme 2. Rhenium(I) tricarbonyl diimine complexes
1 and 2 were synthesized in good yield through the incorporation
of rhodamine-appended bipyridine ligands, L1 and L2, into the
precursor complex, [Re(CO)5Br], in reflux condition under an inert
atmosphere of nitrogen in toluene. Reaction of L3 and L4 with 0.5
equivalent of dinuclear iridium(III) precursor complex, Ir2(dpqx)4
Cl2 (dpqx = 2,3-diphenylquinoxaline) [27], followed by metathesis
with NH4PF6 afforded the rhodamine derivative-containing
iridium(III) complexes, 3 and 4 in reasonable yield. The identities
of complexes 1–4 were confirmed by 1H NMR spectroscopy, satis-
factory elemental analyses and positive-ion FAB or ESI mass spec-
trometric methods. For 1H NMR measurement, the complexes
appended with rhodamine moieties (rhodamine 6G or rhodamine
B) show triplet or multiplet signals in the range of d 1.14–1.41
and quartet or multiplet signals in the range of d 3.13–3.54, which
can be assigned as N–CH2–CH3 and N–CH2–CH3 of the rhodamine
moieties, respectively. Rhodamine 6G-containing complexes 3
and 4 have unique singlet signals at about d 1.82, which can be
referred to CH3 on the xanthene ring of the rhodamine 6G. How-
ever, the absence of these signals was noted in the 1H NMR spectra
of complexes 1 and 2, which were appended with rhodamine B
derivative.
3.2. X-ray crystal structure
Single crystals of 1 suitable for X-ray diffraction studies were
grown by slow diffusion of diethyl ether vapor into a dichloro-
methane solution. Although single crystals were obtained by slow
diffusion of diethyl ether vapor into an acetone solution of 4, one
ethyl substituent of amino group in 4 was found to be converted
into hydrogen on the rhodamine moiety after the structural
solution and refinement and 40 was accordingly denoted for such
complex. Bulk crystals were collected for the record of 1H NMR
spectrum which supported the identity of 40 with one ethyl
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K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
5
O
O
N
CO
Re CO
CO
N
[Re(CO)5Br]
N
N
N
N
N
N
n
n
Br
n = 0 ( )
L1
L2
n = 0 (
)
)
N
N
N
N
O
O
O
1
1 (
2
1 ( )
+
O
−
N
PF6
N
O
N
N
N
Ir
n
Ir2(dpqx)4Cl2
NH4PF6
N
N
N
n
N
N
N
O
H
H
L3
L4
N
2
n = 0 (
)
)
N
H
N
1 (
H
3
n = 0 ( )
4
1 ( )
Scheme 2. Synthesis of 1–4.
substituent of amino group converted into hydrogen, indicative of
high degree of completeness for such conversion during recrystal-
lization. It is noteworthy that oxidative N-dealkylation reactions
have been observed in the presence of catalyst. Attempts have
been made to verify the presence of possible catalyst in the form
of impurity during the course of recrystallization, however no
detectable candidate has been observed. The perspective drawings
of complexes 1 and 40 are depicted in Figs. 1 and 2, respectively.
The crystal structure determination data are listed in Table 1 and
the selected bond lengths and angles are given in Table 2. To the
best of my knowledge, they are the first examples showing the
crystal structures of transition metal complex tethered with rhoda-
mine derivative. For 1, the rhenium(I) metal centre coordinates to
one rhodamine derivative tethered bipyridine chelate, three car-
bonyl ligands with facial arrangement, and one bromide groups
to give a distorted octahedral geometry, characteristic of other rhe-
nium(I) tricarbonyl diimine system [34–36]. The N(1)–Re(1)–N(2)
bond angle of 74.5° about the rhenium(I) metal centre, deviated
from the idealized values of 90°, is attributed to the bite distance
exerted by the steric demand of the chelating bipyridine ligand.
For 40, the coordination geometry at the iridium(III) metal centre
was distorted octahedral with two cyclometalated dpqx and one
rhodamine derivative-containing bipyridine (L1) lignads. The coor-
dinated carbon atoms of two cyclometalated dpqx ligands are in a
cis arrangement to each other and trans to the nitrogen atom of the
bipyridine chelate. The Ir–C and Ir–N bond distances are normal
and comparable to those in other related cyclometalated
iridium(II) diimine system [37,38]. The Ir–N bond distances in
the bipyridine chelate (2.17 and 2.19 Å) are longer than those in
Fig. 2. Perspective drawing of complex cation of complex 40 with atomic number-
ing scheme. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn
at the 15% probability level.
the cyclometalated ligand (2.06 and 2.09 Å) because of the trans
position of the cyclometalated carbon atom having stronger trans
effect. It is interesting to note that the quinoxaline moiety and
the cyclometalated phenyl ring are not co-planar with the inter-
planar angles of (25.8° and 19.9°), probably for the relief of the
mutual repulsion. Similarly, the C–Ir–N (79.0° and 79.4°) and
N–Ir–N (74.9°) bond angles about the iridium(III) metal centre
were found to be less than 90°, as required by the bite distance
exerted by the steric demand of the chelating cyclometalated and
bipyridine ligands. In both 1 and 40, the molecular structure clearly
indicates that the rhodamine derivatives are in ring-closed spiro-
lactam form with the C–C–N bond angles (99.1° in 1 and 99.0° in
40) and N–C bond distances (1.51 Å in 1 and 1.53 Å in 40). The
C(15)–N(3)–N(4) bond angles (121.2° in 1 and 121.6° in 40) and
the C–N bond distances (1.29 Å in 1 and 1.27 Å in 40) about the
imine group are typical of sp2 C@N bond character. The co-planar-
ity of the bipyridine ligand is extended through the C@N conjuga-
tion to the spirolactam group with the torsion angle of 178.3° and
179.3° in 1 [C(6)–C(15)–N(3)–N(4)] and 40 [C(48)–C(52)–N(7)–
N(8)], respectively.
Fig. 1. Perspective drawing of 1 with atomic numbering scheme. Hydrogen atoms
are omitted for clarity. Thermal ellipsoids are drawn at the 15% probability level.
Please cite this article in press as: K.-M.C. Wong et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.07.010

6
K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
Table 2
Selected bond lengths (Å) and bond angles (deg) of 1 and 40.
1.0
3
4
1
40
0.8
Bond lengths (Å)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–Br(1)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
C(15)–N(3)
N(3)–N(4)
N(4)–C(23)
C(22)–C(23)
1.93
1.90
2.00
2.15
2.17
3
1.13
1.18
0.95
1.29
1.35
1.51
1.53
Ir(1)–C(1)
Ir(1)–C(21)
Ir(1)–N(1)
Ir(1)–N(3)
Ir(1)–N(5)
Ir(1)–N(6)
C(52)–N(7)
N(7)–N(8)
N(8)–C(60)
C(59)–C(60)
1.99
2.00
2.06
2.09
2.19
2.17
1.27
1.34
1.53
0.6
0.4
0.2
1.51
0.0
500
550
600
650
700
750
800
Wavelength/nm
Bond angles (°)
Re(1)–C(1)–O(1)
Re(1)–C(2)–O(2)
Re(1)–C(3)–O(3)
N(1)–Re(1)–N(2)
C(15)–N(3)–N(4)
C(22)–C(23)–N(4)
178.5
177.2
170.9
74.5
121.2
99.1
C(1)–Ir(1)–N(1)
C(21)–Ir(1)–N(3)
N(5)–Ir(1)–N(6)
C(52)–N(7)–N(8)
N(8)–C(60)–C(59)
79.4
79.0
74.9
121.6
99.0
Fig. 4. Normalized emission spectra of 3 and 4 in methanol at 298 K.
properties of complexes 1–4 have also been investigated and all of
them were found to exhibit emission upon excitation at k > 350 nm
in methanol at 298 K. Both complexes 1 and 2 are found to exhibit
similar green emission at 565 nm. Such emission is probably
attributed to the rhodamine B emission due to the presence of
trace amount of the open form of rhodamine. Although the lumi-
3.3. Photophysical behaviours
nescence lifetime is of <0.1
l
p
s, the assignment of emission origin
The electronic absorption behaviour of 1–4 has been studied in
methanol at 298 K and the electronic absorption spectra are
depicted in Fig. 3. The rhenium(I) complexes, 1 and 2, showed
intense high-energy absorption bands at 275 and 318 nm with
extinction coefficients of the order of 104 dm3 mol–1 cm–1, and a
low-energy absorption band at about 380–400 nm. With reference
to the spectroscopic features of other related rhenium(I) tricarbonyl
diimine system [39,40], the high-energy absorption bands are
derived from the 3MLCT [d
(Re) ? p⁄(bipy)] excited state could
not be excluded. On the other hand, complexes 3 and 4 showed
an intense emission band at 675 nm (Fig. 4). The large Stokes shifts
and the observed luminescence lifetime of about 0.6 ls suggest
that emission is from a triplet parentage. In view of the fact that
a similar emission band at about 675 nm was observed in the rho-
damine derivative-free model complex, [Ir(dpqx)2(4-carboxalde-
hyde-40-methyl-2,20-bipyridine)]PF6, such lower energy emission
assigned as
ponents, while the low-energy absorption band is ascribed to the
[d
(Re) ? p⁄(bipy)] metal-to-ligand charge transfer (MLCT) transi-
p ?
p⁄ transition of the bipyridine and rhodamine com-
is ascribed to the triplet excited state of MLCT [dp
(Ir) ? p⁄(dpqx)]
origin. Related iridium(III) 2,3-quinoxalinato complexes [27,37]
p
have also been reported to show similar emission at about 622–
tion. For the iridium(III) complexes, 3 and 4, a high-energy absorp-
tion band at 298 and 366 nm and a less intense low-energy
absorption band at 472 nm were observed. The high-energy absorp-
670 nm with the assignment of 3MLCT [d (Ir) ? p⁄(2,3-quinoxali-
p
nato)] excited state. It is interesting to note that a rather weak
emission at 550 nm characteristic of rhodamine 6G in 3 was
observed, probably due to the existence of trace amount of the
open form of rhodamine 6G in methanol. No such rhodamine 6G
emission was found in 4, suggesting that the introduction of
ethylene group separating two nitrogen atoms may render the
ring-closed form more stable in the same solvent system.
tion bands are ascribed to
p ?
p⁄ transition of the 2,3-quinoxali-
nate, bipyridine and rhodamine moieties, whereas the low-energy
absorption band is assigned as a spin-allowed metal-to-ligand
charge transfer (MLCT) [dp
(Ir) ? p⁄(dpqx)] transition. Similar to
the other related iridium(III) cyclometalated complexes
[27,37,38], the observation of weak shoulder beyond 500 nm is
probably due to the spin-forbidden 3MLCT transition. The emission
3.4. Selective metal ion binding studies
Selective binding properties of complexes 1–4 have been inves-
tigated by electronic absorption and emission spectroscopic meth-
ods for various alkali, alkaline-earth and transition-metal cations,
8
1
7
such as K+, Na+, Li+, Ca2+, Mg2+, Ba2+, Pb2+, Cd2+, Zn2+, Co2+, Ni2+
,
2
Ag+, Fe2+, Cu2+ and Hg2+. Solutions of 1, 3 and 4 in methanol exhibit
drastic colour changes from pale yellow to magenta selectively in
the presence of Hg(II) ion, no observable colour was observed in
2 upon the addition of the same concentration of Hg(II) ion. Upon
addition of Hg(II) ion, into the methanolic solution of 1, a new
absorption band at 558 nm was observed in the electronic absorp-
tion spectra with increasing concentration of Hg(II) ion. The
corresponding electronic absorption spectral changes are shown
in Fig. 5(a). On the basis of the absorption band of rhodamine B
occurred at similar energy, the emergence of such new absorption
band is ascribed to the conversion of the spirolactam form to
the corresponding ring-opened amide form upon Hg(II) ion binding
in 1. The logKs values for Hg(II) ion binding to 1 was found to be 4.76
( 0.12), from the result of theoretical fit of the 1:1 stoichiometric
6
3
4
5
4
3
2
ε
1
0
300
400
500
600
Wavelength/nm
Fig. 3. Electronic absorption spectra of 1–4 in methanol at 298 K.
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K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
7
(a)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.2
0.8
0.4
0.0
0
100
200
300
[Hg2+]x106M
550
600
650
700
750
800
Wavelength /nm
300
400
500
600
Wavelength/nm
(b)
(b)
0
20 40 60 80 100
[Hg2+] x106M
500 550 600 650 700 750 800
Wavelength /nm
Fig. 6. (a) The emission spectral changes of 3 (a) and 4 (b) in methanol at 298 K
upon addition of various concentrations of Hg(II) ion.
500 550 600 650 700 750 800 850
Wavelength/nm
tory result could be obtained to find the binding constants of 1
for Hg(II) ion by the emission titration. The deviation from the the-
oretical binding fit probably due to the contribution of 3MLCT
emission from the rhenium(I) moiety at similar energy region. Sim-
ilar to the electronic absorption study, no significant emission
response was found in 2, showing the weak binding affinity and
ring opening behaviour toward target Hg(II) ion. As reported in
the previous communication [22], the corresponding electronic
absorption and emission titrations of 3 and 4 have been studied.
Both of them were found to exhibit ring opening process of the
rhodamine derivative upon addition of Hg(II) ion into the metha-
nolic solution. Their electronic absorption spectral changes are
similar that there is a new absorption band at 530 nm and the
absorbance was found to increase with the concentration of Hg(II)
ion. The log Ks values for Hg(II) ion binding to 3 and 4 were found
to be 4.55 ( 0.09) and 3.63 ( 0.04), respectively, from the result of
theoretical fit of the 1:1 stoichiometric binding mode. The binding
constants of complexes 1 and 3, which contain the almost same
rhodamine appended diimine ligand, are comparable and in the
same order of magnitude. It is interesting to note that 3 and 4 were
found to show essentially different emission spectral changes upon
Hg(II) ion binding. The iridium(III)-based MLCT emission intensity
at 675 nm was found to increase greatly, with a slight increase in
the rhodamine 6G emission at 555 nm, in 3, while only the rhoda-
mine 6G emission was substantially enhanced with no observable
change in the iridium(III) emission in 4 (Fig. 6). In the system of 3
as a bichromophoric array, the emission enhancement of irid-
ium(III) luminophore is ascribed to an efficient intramolecular
energy transfer process from rhodamine 6G to the cyclometalated
iridium (III) moiety, upon the ring-opening of the rhodamine deriv-
ative. However, in the case of 4 with an ethyl group as non-conju-
gated linker between rhodamine 6G and the iridium(III)
luminophore, such energy transfer is inefficient.
Fig. 5. (a) The electronic absorption spectra of
tion = 17.1
M) at 298 K upon addition of various concentrations of Hg2+. Inset
shows the plots of the absorbance at 558 nm as a function of the concentration of
Hg2+. (b) The emission spectra of 1 in MeOH (concentration = 17.1
M) at 298 K
upon addition of various concentrations of Hg2+. Inset shows the plots of the
emission intensity at 582 nm as a function of the concentration of Hg2+
1 in MeOH (concentra-
l
l
.
binding mode. On the other hand, no significant electronic absorp-
tion spectral changes were observed in 2, indicative of its insensi-
tivity for Hg(II) ion binding. Together with the amide group, the
nitrogen atom on the imine unit is anticipated to involve in the
coordination of Hg(II) ion. The poor sensitivity in 2, as a result of
the poor coordinating ability, is rendered by the separation of ethyl
linker between amide moiety and imine nitrogen atom. is ascribed
to the difference in the separation distance of the amide group
from the nitrogen atom on the imine unit, which is anticipated
to participate in the binding process of Hg(II) ion.
Similar emission titration experiments have also been per-
formed to study the cation-sensing processes of complex 1. Upon
excitation at the isosbestic wavelength of 340 nm, the solution of
1
was found to exhibit a drastic emission enhancement at
588 nm with increasing the concentration of Hg(II) ion, attributed
to the rhodamine B emission as a result of ring-opening process.
The changes in emission intensities of 1 toward Hg(II) ion are fea-
tured in Fig. 5(b) and there is a red shift of the emission from 580 to
588 nm upon increasing the concentration of Hg(II) ion. Since the
emission energies of the rhenium(I) tricarbonyl diimine system
as well as the rhodamine B are very close, the mixing of the rhe-
nium(I)-emission into such emission enhancement at 588 nm, as
well as energy transfer between rhenium(I) complex and rhoda-
mine B, is possible. Unlike to the absorption titration, no satisfac-
Please cite this article in press as: K.-M.C. Wong et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.07.010

8
K.M.-C. Wong et al. / Polyhedron xxx (2014) xxx–xxx
from the Cambridge Crystallographic Data Centre, 12 Union Road,
4. Conclusion
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk.
A new class of transition metal complexes including rhenium(I)
tricarbonyl diimine and cyclometalated iridium(III) diimine sys-
tems, which contain rhodamine derivative as a sensory moiety,
have been synthesized and characterized. The X-ray crystal
structures of 1 and 40 (4 with one ethyl substituent of amino group
converted into hydrogen on the rhodamine derivative) have been
determined. Their photophysical and selective ion-binding proper-
ties have also been investigated. 1 was found to exhibit selective
binding toward Hg(II) ion with electronic absorption and emission
spectral changes, while another rhenium(I) analogue, 2, showed no
such binding behaviour. Iridium(III) complexes, 3 and 4, were
found to give similar electronic absorption response but different
emission spectral changes upon addition of Hg(II) ion. Efficient
intramolecular energy transfer from rhodamine 6G to the cyclo-
metalated iridium (III) moiety was suggested in 3, which could
be modulated by selective ion-binding process upon ring-opening
of rhodamine 6G. Although comparable binding constants towards
Hg(II) ion are obtained in 1 and 3 with almost the same rhodamine
tethered diimine ligand, the variation of coordinating sphere of
rhenium(I) or iridium(III) metal centre may also influence the
binding affinities, probably due to the differences in the electronic
effect and steric bulkiness, in particular to the cases of 2 and 4. This
report shows that the modification of rhodamine derivatives and
the choice of transition metal ion could be important to influence
the selective binding properties for development of colorimetric
and/or luminescent probe. Through a judicious design and choice
of ligand systems with different functionalized bridging spacers,
the selectivity of the sensing ability and the efficiency of energy-
transfer are anticipated to be varied.
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Appendix A. Supplementary data
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CCDC 986107 and 986108 contains the supplementary crystal-
lographic data for 1 and 40. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
Please cite this article in press as: K.-M.C. Wong et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.07.010