Synthesis, structure, photophysical and electrochemiluminescence properties of Re(i) tricarbonyl complexes incorporating pyrazolyl-pyridyl-based ligands

Article abstract of DOI:10.1039/c1dt00015b

Three rhenium carbonyl complexes 1-3 were synthesized by reaction of the appropriate bidentate pyrazolyl-pyridyl-based ligand L1, L2 (L1 = 2-[1-{4-(bromomethyl)benzyl}-1H-pyrazol-3-yl]pyridine; L2 = 1,4-bis(3-(2- pyridyl)pyrazol-1-ylmethyl)benzene) with [Re(CO)₅Cl] in toluene. They were characterized by elemental analyses, ESI-MS, ¹H spectroscopy, and X-ray crystallography for 1 and 2. Compounds 1-3 exhibit bright yellow-green luminescence in the solid state and in solution at 298 K with the lifetimes in the microsecond range. It is noteworthy that the luminescent quantum efficiencies of compounds 1-3 are between 0.040 and 0.051, which are much higher than that of the [Re(bpy)(CO)₃Cl] complex (= 0.019) (M. M. Richter et al., Anal. Chem., 1996, 68, 4370; J. Van Houten et al., J. Am. Chem. Soc., 1976, 98, 4853). Electrogenerated chemiluminescence (ECL) was observed in solutions of these complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) or 2-(dibutylamino)ethanol (DBAE) by stepping the potential of a Pt disk working electrode. The ECL spectra are identical to the photoluminescence spectra, indicating that the chemical reactions following electrochemical oxidation or reduction form the same ³MLCT excited states as that generated in the photoluminescence experiments. In most cases, the ECL quantum efficiencies of complexes 1-3 are comparable to that of the [Re(L)(CO)₃Cl] (L = bpy or phen) system. Oxygen tends to substantially decrease ECL intensities of the three rhenium complexes-TPrA system, which could allow them to be used as oxygen sensors.

Full text of DOI:10.1039/c1dt00015b

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PAPER
Synthesis, structure, photophysical and electrochemiluminescence properties
of Re(I) tricarbonyl complexes incorporating pyrazolyl–pyridyl-based ligands†
Qiao-Hua Wei,*a,b Fang-Nan Xiao,a Li-Jing Han,a Shen-Liang Zeng,a Ya-Nan Duana and Guo-Nan Chen*a
Received 5th January 2011, Accepted 2nd March 2011
DOI: 10.1039/c1dt00015b
Three rhenium carbonyl complexes 1–3 were synthesized by reaction of the appropriate bidentate
pyrazolyl–pyridyl-based ligand L1, L2 (L1 = 2-[1-{4-(bromomethyl)benzyl}-1H-pyrazol-3-yl]pyridine;
L2 = 1,4-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzene) with [Re(CO)5Cl] in toluene. They were
characterized by elemental analyses, ESI–MS, 1H spectroscopy, and X-ray crystallography for 1 and 2.
Compounds 1–3 exhibit bright yellow–green luminescence in the solid state and in solution at 298 K
with the lifetimes in the microsecond range. It is noteworthy that the luminescent quantum efficiencies
of compounds 1–3 are between 0.040 and 0.051, which are much higher than that of the
[Re(bpy)(CO)3Cl] complex(= 0.019) (M. M. Richter et al., Anal. Chem., 1996, 68, 4370; J. Van Houten
et al., J. Am. Chem. Soc., 1976, 98, 4853). Electrogenerated chemiluminescence (ECL) was observed in
solutions of these complexes in the absence or presence of coreactant tri-n-propylamine (TPrA) or
2-(dibutylamino)ethanol (DBAE) by stepping the potential of a Pt disk working electrode. The ECL
spectra are identical to the photoluminescence spectra, indicating that the chemical reactions following
electrochemical oxidation or reduction form the same 3MLCT excited states as that generated in the
photoluminescence experiments. In most cases, the ECL quantum efficiencies of complexes 1–3 are
comparable to that of the [Re(L)(CO)3Cl] (L = bpy or phen) system. Oxygen tends to substantially
decrease ECL intensities of the three rhenium complexes–TPrA system, which could allow them to be
used as oxygen sensors.
synthesized and their ECL properties have been investigated.2,3,7–9
At present, the mostly thoroughly studied on organometallic com-
plexes as ECL candidates are [Ru(bpy)3]2+ (bpy = 2,2¢-bipyridine)
Introduction
Electrogenerated chemiluminescence (ECL) is the process of
generating excited states in a photoactive molecule at an electrode
and its derivatives because of their excellent photochemical
surface, leading to luminescence upon return to the ground
and electrochemical properties. Such complexes are widely used
state. The design and synthesis of molecules that have reliable
in analytical applications, especially in clinical diagnostics and
ECL properties have been attracted considerable interest,1–5 since
Hercules and Bard et al. reported the first ECL system in the
mid-1960s.6 In the past several years, a number of new ECL-
emitting species, especially organometallic complexes, have been
bimolecular detection.10–16 Compared to other d6 transition metals
Ru(II) and Os(I)17 tris-diimine systems, the ECL properties of
rhenium tricarbonyl containing bidentate polypyridyl ligands
have been less studied, although many exhibit strong photolu-
minescence in solution. To the best of our knowledge, the only
reported ECL of a rhenium complex is [Re(L)(CO)3Cl] (where
L is bpy, 1,10-phenanthroline (phen), or a methyl-substituted
derivative).18,19
aMinistry of Education Key Lab of Analysis and Detection for Food Safety,
Fujian Provincial Key Lab of Analysis and Detection for Food Safety,
and Department of Chemistry, Fuzhou University, Fuzhou, 3500108, China.
E-mail: qhw76@fzu.edu.cn, gnchen@fzu.edu.cn; Fax: 86 59122866135
Since the ECL emission spectrum of [Re(L)(CO)3Cl] is similar
to that of the optical emission spectrum and is assigned to
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter and Graduate School of CAS, The Chinese
Academy of Sciences, Fuzhou, Fujian, 350002, China
3
emission from the lowest energy, MLCT state,18 subtle ligand
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic files in CIF format for the structure determination of compounds
1 and 2; unit cell pack structure of complex 1 and 2; CVs of 1 mmol L-1 1 in
acetonitrile solution at different scan rates and relationship curves of the
peak currents and scan rate; CVs of 1 mmol L-1 L1 and L2 in acetonitrile
solution at a scan rate of 100 mV s-1. CCDC reference numbers 806767
and 806768. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt00015b
modification can be sufficient to cause a significant change of
the ECL properties. We and other groups have described recently
a series of metal complexes which are bridged by a bidentate
pyrazolyl–pyridine ligand.20–22 The p* orbital (LUMO) energy
of the pyrazolyl–pyridine ligand is higher than that of the bpy
ligand, which may result in a negative shift of the reduction
5078 | Dalton Trans., 2011, 40, 5078–5085
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potential of its metal complexes. These considerations prompted
us to synthesize the rhenium tricarbonyl complexes containing
bidentate pyrazolyl–pyridine ligand and investigate where they
can be used for the production of ECL.
In this paper, three rhenium carbonyl complexes incorpo-
rating bidentate pyrazolyl–pyridyl-based ligands (as shown in
Scheme 1) were synthesized and characterized. Their photophys-
ical properties, electrochemical and ECL characteristics in the
absence or presence of coreactant tri-n-propylamine (TPrA) or 2-
(dibutylamino)ethanol (DBAE) were also extensively evaluated.
to 60 ◦C with vigorous stirring for 12 h. The red solution was
distilled off under vacuum, and the residue was dissolved in
2 mL dichloromethane. Diffusion of diethyl ether vapor onto the
dichloromethane solution gave yellow crystals of 2. Yield: 76%.
C27H20ClN6O3Re: C, 46.41; H, 2.89; N, 12.04. Found: C, 46.13;
H, 2.91; N, 12.34. ESI–MS [m/z (%)]: 699.5 (100) [M+H]+. IR
1
-1
(KBr, ncm ): 2019 (s, CO), 1920 (s, CO), 1887 (s, CO). H NMR
(CDCl3, ppm): 8.95 (1H, d, J = 4.0 Hz; coordinated pyridyl H6),
8.63 (1H, d, J = 4.0 Hz; pendant pyridyl H6), 7.96 (1H, ddd, J =
1.2, 1.6, 8.0 Hz; coordinated pyridyl H3), 7.92 (1H, d, J = 1.2 Hz;
coordinated pyridyl H4), 7.86 (1H, d, J = 0.8 Hz; pendant pyridyl
H4), 7.71 (1H, ddd, J = 1.2, 1.6, 8.0 Hz; pendant pyridyl H3), 7.47
(1H, d, J = 2.4 Hz; coordinated pyrazolyl H5), 7.42–7.40 (2H, m,
2 ¥ phenyl), 7.30–7.18 (5H, m; 2 ¥ phenyl, coordinated pyrazolyl
H4, and 2 ¥ pyridyl H5), 6.92 (1H, d, J = 2.4 Hz; pendant pyrazolyl
H5), 6.87 (1H, d, J = 2.8 Hz; pendant pyrazolyl H4), 5.63 (2H, s,
CH2-coordinated pyrazolyl), 5.41 (2H, s, CH2-pendant pyrazolyl).
[Re(CO)3Cl]2L2 (3). L2 (115.2 mg, 0.29 mmol) and
[Re(CO)5Cl] (236.1 mg, 0.65 mmol) in toluene (5 ml) were heated
to 85 ◦C with stirring for 12 h. The crude product was collected by
filtration and recrystallized from DMF– diethyl ether to give 3 as
yellow powders. Yields: 92%. Anal. calcd for C30H20Cl2N6O6Re2:
C, 35.89; H, 2.01; N, 8.37. Found: C, 35.81; H, 1.91; N, 7.89. ESI–
MS, m/z: 1039.9 (100) [M + Cl]-. IR (KBr, ncm ): 2024 (s, CO),
-1
1909 (s, CO), 1886 (s, CO). 1H NMR (D3CON(CD3)2, ppm): 8.90
(2H, d, J = 5.2, pyridyl H6), 8.37 (2H, d, J = 4.0, pyridyl H3), 8.32
(2H, t, J = 5.2, pyridyl H4), 8.27 (2H, td, J = 6.8,1.2 Hz, pyrazolyl
H5), 7.62 (2H, ddd, J = 6.4, 2.8, 1.6, pyridyl H5), 7.46 (2H, t, J =
2.8 Hz, pyrazolyl H4), 7.24 (4H, s, phenyl), 5.83 (2H, d, J = 16 Hz,
CH2), 5.58 (2H, d, J = 16.4 Hz, CH2).
Scheme 1 Synthesis of compounds 1–3.
Experimental
Materials
All synthetic operations were performed under dry nitrogen
atmosphere using Schlenk techniques and vacuum-line sys-
tems. [Re(CO)5Cl] and tetrabutylammonium hexafluorophos-
phate (Bu4NPF6) were purchased from Alfa Aesar. All reagents
were used as received and solvents were purified by standard
methods. 3-(2-pyridyl)pyrazole,23 2-[1-{4-(bromomethyl)benzyl}-
1H-pyrazol-3-yl]pyridine (L1) and 1,4-bis(3-(2-pyridyl)pyrazol-1-
ylmethyl)benzene (L2) were prepared according to the published
methods.24
Crystal structural determination
Crystal 1 coated with epoxy resin was measured on a BRUKER
SMART CCD diffractometer by w scan technique at 150 K with
˚
graphite-monochromated Mo-Ka (l = 0.71073 A) radiation, and
crystal 2 was measured on a BRUKER P4 diffractometer by w scan
technique at 293 K at Fujian Institute of Research on the Structure
of Matter, the Chinese Academy of Sciences. An absorption
correction by SADABS was applied to the intensity data. The
structures were solved by direct methods or by the Patterson
procedure and the heavy atoms were located from an E-map.
The remaining non-hydrogen atoms were determined from the
successive difference Fourier syntheses. All non-hydrogen atoms
were refined anisotropically except those mentioned otherwise.
The hydrogen atoms were generated geometrically with isotropic
thermal parameters. The structures were refined on F2 by full-
matrix least-squares methods using the SHELXTL-97 program
package.25 Crystallographic data of 1 and 2 are summarized in
Table 1. Full crystallographic data are provided in the CIF files
(ESI†).
Synthesis
[Re(L1)(CO)3Cl] (1). The ligand L1 (196.9 mg, 0.60 mmol)
and [Re(CO)5Cl] (180.8 mg, 0.50 mmol) in toluene were heated
to 60 ◦C with vigorous stirring for 5 h. The solvent was removed
under vacuum and the yellow residue was dissolved with 5 mL of
dichloromethane and 30 mL of diethyl ether. The yellow solution
was cooled in the refrigerator for several hours to give yellow prism
crystals. Yield: 60%. Anal. calcd for C19H14N3O3ReBrCl: C, 36.02;
H, 2.23; N, 6.64, found: C, 36.15; H, 2.23; N, 6.33. ESI–MS [m/z
-1
(%)]: 634 (30) [M], 248 (100) [M - Re(CO)3Cl]. IR (KBr, ncm ):
2025 (s, CO), 1910 (s, CO), 1893 (s, CO). 1H NMR (CDCl3, ppm):
8.90 (1H, d, J = 5.0 Hz; pyridyl H6), 8.11 (1H, m, pyridyl H3), 7.93
(1H, d, J = 2.75 Hz; pyridyl H4), 7.52–7.41 (2H, m, 2 ¥ phenyl),
7.26–7.16 (4H, m; 2 ¥ phenyl, pyrazolyl H5, and pyridyl H5), 6.92
(1H, d, J = 2.75 Hz; pyrazolyl H4), 5.69 (2H, d, J = 13.0 Hz;
CH2-pz), 4.60 (2H, d, J = 19.50 Hz; CH2Br).
Physical measurements
Elemental analyses (C, H, N) were carried out on a Perkin–Elmer
model 240 C automatic instrument. Electrospray mass spectra
(ESI–MS) were recorded on a Finnigan LCQ mass spectrometer
using dichloromethane–methanol or acetonitrile–methanol as the
mobile phase. UV-Vis absorption spectra in acetonitrile, DMF
[Re(L2)(CO)3Cl] (2). A mixture of L2 (239.2 mg, 0.61 mmol),
[Re(CO)5Cl] (180.8 mg, 0.50 mmol), toluene (5 mL) was heated
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◦
˚
Table 2 Selected bond distances (A) and angles ( ) for complex 1
Table 1 Crystallographic data for compounds 1·DMF and 2
Compound
1·DMF
2
Re(1)–C(3)
Re(1)–C(2)
Re(1)–C(1)
Re(1)–N(2)
Re(1)–N(1)
Re(1)–Cl(1)
O(1)–C(1)
O(2)–C(2)
O(3)–C(3)
1.904(9)
1.918(9)
1.925(9)
2.161(7)
2.201(7)
2.5428(16)
1.144(11)
1.156(11)
1.156(11)
Re(2)–C(32)
Re(2)–C(33)
Re(2)–C(31)
Re(2)–N(5)
Re(2)–N(4)
Re(2)–Cl(2)
O(4)–C(31)
O(5)–C(32)
O(6)–C(33)
1.904(11)
1.914(9)
1.917(10)
2.163(8)
Empirical formula
Formula weight
Space group
C41H35Br2Cl2N7O7Re2
1340.88
C27H20ClN6O3Re
698.14
P1
¯
¯
2.191(8)
P1
˚
2.5313(16)
1.145(12)
1.162(12)
1.158(11)
a/A
13.2187 (6)
13.2825 (5)
14.5639 (6)
96.440 (2)
114.512 (2)
105.043 (2)
2175.44 (16)
2
10.360 (2)
10.530 (2)
12.400 (3)
72.32 (3)
85.05 (3)
69.03 (3)
1203.1 (4)
2
1.927
5.206
0.71073
293(2)
0.0294
˚
b/A
˚
c/A
a/deg
b/deg
C(3)–Re(1)–C(2)
C(3)–Re(1)–C(1)
C(2)–Re(1)–C(1)
C(3)–Re(1)–N(2)
C(2)–Re(1)–N(2)
C(1)–Re(1)–N(2)
C(3)–Re(1)–N(1)
C(2)–Re(1)–N(1)
C(1)–Re(1)–N(1)
N(2)–Re(1)–N(1)
C(3)–Re(1)–Cl(1)
C(2)–Re(1)–Cl(1)
C(1)–Re(1)–Cl(1)
N(2)–Re(1)–Cl(1)
N(1)–Re(1)–Cl(1)
N(3)–C(12)–C(13)
C(16)–C(19)–Br(1)
O(1)–C(1)–Re(1)
O(2)–C(2)–Re(1)
O(3)–C(3)–Re(1)
88.9(4)
90.6(4)
89.7(4)
93.9(3)
101.3(3)
168.2(3)
95.9(3)
173.4(3)
94.8(3)
73.9(3)
178.9(3)
91.4(2)
C(32)–Re(2)–C(31)
C(32)–Re(2)–C(33)
C(33)–Re(2)–C(31)
C(33)–Re(2)–N(5)
C(32)–Re(2)–N(5)
C(31)–Re(2)–N(5)
C(33)–Re(2)–N(4)
C(32)–Re(2)–N(4)
C(31)–Re(2)–N(4)
N(5)–Re(2)–N(4)
C(33)–Re(2)–Cl(2)
C(32)–Re(2)–Cl(2)
C(31)–Re(2)–Cl(2)
N(5)–Re(2)–Cl(2)
N(4)–Re(2)–Cl(2)
N(6)–C(42)–C(43)
C(46)–C(49)–Br(2)
O(4)–C(31)–Re(2)
O(5)–C(32)–Re(2)
O(6)–C(33)–Re(2)
89.8(4)
90.4(4)
90.0(4)
94.2(3)
100.3(3)
168.9(4)
97.1(3)
170.7(4)
95.7(4)
73.6(3)
179.1(3)
89.7(3)
g /deg
3
˚
V/A
Z
r
calcd/g cm-3
2.047
7.577
0.71073
150 (2)
0.0513
0. 1433
1.034
m/mm-1
˚
radiation (l, A)
Temp/K
R1 (Fo)a
2
b
wR2(Fo
GOF
)
0.0641
1.120
a R1 = R|Fo - Fc|/RFo b wR2 = R[w(Fo - Fc2)2]/R[w(Fo2)]1/2
.
2
90.4(3)
91.0(3)
84.98(19)
83.70(18)
112.3(7)
110.0(6)
176.6(9)
179.1(8)
179.3(8)
84.86(19)
82.79(19)
111.7(7)
111.3(7)
177.7(9)
178.7(9)
178.0(9)
and dichloromethane solutions were measured on a Perkin–Elmer
Lambda 25 UV-Vis spectrometer. Infrared spectra were recorded
on a Magna750 FT–IR spectrophotometer with KBr pellet. H
1
NMR spectra were measured on a BRUKER AVANCE III
400 MHz spectrometer with SiMe4 as the internal reference. Emis-
sion and excitation spectra were recorded on a Perkin–Elmer LS
55 luminescence spectrometer with a red-sensitive photomultiplier
type R928. Emission lifetimes were determined on an Edinburgh
Analytical Instrument (F900 fluorescence spectrometer) using
LED laser at 340 nm excitation and the resulting emission
was detected by a thermoelectrically-cooled Hamamatsu R3809
photomultiplier tube. The instrument response function at the
excitation wavelength was deconvoluted from the luminescence
decay. The electrochemical and electrochemiluminescent (ECL)
measurements were performed at room temperature using a system
made in our lab,26 consisting of a BPCL Ultra-Weak Chemilumi-
nescence analyzer controlled by a personal computer (Institute of
Biophysics, Chinese Academy of Sciences) in conjunction with a
CHI model 1210 electrochemical analyzer (Shanghai Chenghua
Instrument Co., China). A three-electrode system was employed
with platinum wire as auxiliary electrode, SCE as reference
electrode, and platinum disk electrode (2 mm diameter) as working
electrode.
1 and 2 were soluble in most organic solvents, but compound
3 proved extremely insoluble in most organic solvent, and only
slightly soluble in DMF and MeCN, making recrystallization and
characterization more difficult.
Compounds 1–3 were satisfactorily characterized by elemental
analyses, ESI–MS, IR and 1H NMR spectroscopy, and by
X-ray crystallography for 1 and 2. The ESI–MS revealed that
the molecular ion fragments [M], [M + H]+ or [M + Cl]- as the
principal peaks with high abundance. Compounds 1–3 showed
1
C
O stretches in the frequency range 1886–2025 cm-1. The H
NMR spectra showed that in most cases the signals of the proton
of the coordinated pyridine and pyrazole ring were shifted to lower
field with respect to the signals of the free ligand due to metal ion
complexation, which agreed with the corresponding findings in
other pyrazolyl–pyridyl-based complexes.20–22
Crystal structures
The solid state structures of 1 and 2 have been accurately
determined by X-ray crystallography and ORTEP drawings of
1 and 2 are depicted in Fig. 1 and 2, respectively. Details of
the data collection and refinement are given in Table 1. Selected
bond lengths and angles for the molecules are listed in Table 2
and 3, respectively. Both complexes feature distorted octahedral
coordination with three CO groups being fac-oriented around the
Re(I) center. The relevant bonding lengths and angles around the
Re(I) center are in the normal ranges as observed in similar [Re(N–
N)(CO)3Cl] (N–N = diimine complexes).27
Results and discussion
Synthesis and characterization
The mononuclear complexes 1–2 (Scheme 1) were prepared in
good yields by reaction of the appropriate pyrazolyl–pyridyl-based
ligand (L1, L2) with [Re(CO)5Cl] (in 1.2 : 1 molar ratio) in toluene
solution at 60 ◦C under N2, and recrystallized from CH2Cl2–
diethyl ether, while the complex 3 was synthesized by reaction of L2
ligand with [Re(CO)5Cl] in 1 : 2.4 molar ratio in toluene solution
at 85 ◦C, and recrystallized from DMF–diethyl ether. The reaction
to prepare complex 2 in toluene solution requires a small excess of
the L2 ligand and requires the careful control of the temperature
to prevent further reaction to dinuclear complex 3. Compounds
As shown in Fig. 1, the pyridine ring and the pyrazole ring
are almost coplanar with a dihedral angle of 4.7◦. The bro-
momethyl substituted phenyl ring plane is almost orthogonal to
the pyrazolyl–pyridine ring plane with a dihedral angle of 85.8◦. In
5080 | Dalton Trans., 2011, 40, 5078–5085
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◦
˚
Table 3 Selected bond distances (A) and angles ( ) for complex 2
present between the oxygen atom of CO and the C atom of
˚
the methylene (C ◊ ◊ ◊ O = 3.419 A). In the case of complex 2
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(2)
Re(1)–N(1)
Re(1)–Cl(1)
C(3)–O(3)
C(1)–O(1)
1.868(4)
1.880(5)
1.891(5)
2.142(3)
2.145(3)
2.4073(12)
1.056(6)
1.122(5)
1.124(5)
86.76(18)
89.77(19
89.4(2)
C(3)–Re(1)–N(2)
C(1)–Re(1)–N(1)
C(2)–Re(1)–N(1)
C(3)–Re(1)–N(1)
N(2)–Re(1)–N(1)
C(1)–Re(1)–Cl(1)
C(2)–Re(1)–Cl(1)
C(3)–Re(1)–Cl(1)
N(2)–Re(1)–Cl(1)
N(1)–Re(1)–Cl(1)
O(1)–C(1)–Re(1)
O(2)–C(2)–Re(1)
O(3)–C(3)–Re(1)
N(6)–C(26)–C(23)
95.00(18)
173.95(15)
98.29(15)
93.59(16)
73.33(12)
95.30(14)
90.25(15)
174.88(13)
84.68(9)
81.42(9)
178.3(4)
179.2(4)
175.8(5)
(Fig. 2), the ligand is slightly twisted, with the result that the
angle between the coordinated and free pyrazolyl–pyridine rings
planes is 22.2◦. The coordinated pyrazolyl–pyridine ring plane is
essentially orthogonal to the phenyl ring with a dihedral angle of
95.9◦, while the dihedral angle of the free pyrazolyl–pyridine ring
plane and the phenyl ring is 73.7◦. Furthermore, the dihedral
angle between the coordinated pyrazole ring and the pyridine
ring is 1.5◦, that is smaller than that of the free pyrazole ring
and pyridine ring (5.9◦), which is due to the complexation of the
metal Re(I). Relatively weak intermolecular CH ◊ ◊ ◊ Cl hydrogen
C(2)–O(2)
C(1)–Re(1)–C(2)
C(1)–Re(1)–C(3))
C(2)–Re(1)–C(3)
C(1)–Re(1)–N(2)
C(2)–Re(1)–N(2)
101.38(15)
170.75(14)
112.1(3)
˚
bonds (C–H ◊ ◊ ◊ Cl = 3.746 A) and p ◊ ◊ ◊ p stacking interactions
involving the free pyrazolyl–pyridine rings can be observed. A
strong hydrogen bonding interaction is also present between the
oxygen atom of CO and the C atom of the free pyrazoline rings
˚
(C ◊ ◊ ◊ O = 3.220 A). This supermolecular association leads to
formation of a two-dimensional sandwich (Fig. S2†). In addition,
as shown in Fig. 1, two enantiomers are present in equal amounts
in a well defined arrangement within the lattice of compound 1,
while for compound 2 only one specific enantiomer (Fig. 2) is
selectively crystallized.
Photophysical properties
The absorption and emission data of complexes 1–3 are sum-
marized in Table 4. The electronic absorption spectra of 1–3
are characterized by an intense band at ca. 260–320 nm and a
low-energy band at ca. 320–400 nm, which are typical of ligand-
centered p → p* (pyrazolyl–pyridine) and MLCT [d(Re) → p*
(pyrazolyl–pyridine)] transitions,18,20–22,27,28 respectively (Fig. 3).
The electronic absorption spectra of 2 and 3 are similar, except
that the absorption bands at high energy and low energy all have
much higher intensity than those of the related complex 2.
Fig. 1 ORTEP drawing of 1 showing two enantiomers with atom labeling
scheme showing 30% thermal ellipsoids.
As listed in Table 4, compounds 1–3 exhibit bright yellow–
green luminescence in the solid state and in solution at
298 K with excitation at lex > 300 nm. The solid state lifetime at
298 K is in the range of microseconds, revealing that the emission is
most likely associated with a spin-forbidden triplet parentage.27,28
All of these complexes exhibit almost the same emission maxima,
centered at 506, 510 and 511 nm in the solid state. This indicates
that the 3MLCT excited states of the three complexes are almost
the same and have the same energy levels, no matter what kind of
Fig. 2 ORTEP drawing of 2 with atom labeling scheme showing 30%
thermal ellipsoids.
the complex 1 solid-state structure, there are several intermolecular
hydrogen bonds and p ◊ ◊ ◊ p stacking interactions involving the
phenyl ring and the pyrazolyl–pyridine ring, generating a unique
supermolecular structure (Fig. S1†). The Cl atom serves as
a hydrogen bond acceptor for three C atoms of the pyridine
ring, the pyrazole ring and the bromomethyl substituted phenyl
ring of three neighboring molecules (C–H ◊ ◊ ◊ Cl in the range of
Fig. 3 Electronic absorption spectra of 1 (solid), 2 (dash) in acetonitrile
solution and 3 (dot) in DMF solution at room temperature.
˚
3.485–3.588 A). A weak hydrogen bonding interaction is also
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Table 4 Photophysical data for compounds 1–3
b
Compound
Medium
l
abs /nm (e/dm3 mol-1 cm-1)
l
em/nm (tem/ms)a (298 K)
jem
1
2
3
Solid
MeCN
Solid
MeCN
Solid
DMF
—
506 (1.25)
538
510 (0.53)
524
511 (0.64)
528
—
0.040
—
0.042
—
0.051
282 (11343), 295 (11665), 338 (3813)
—
281 (17904), 303 (9586), 336 (3346)
—
285 (20472), 296 (21682), 337 (7075)
2+
a The excitation wavelength in the lifetime measurement is 340 nm. b Relative to Ru(bpy)3 (jem = 0.062).28
ligand is present in the molecules. The average value of ca. 510 nm
for the emission maxima is blue shifted by ca. 85 nm from that
of the similar complex [Re(bpy)(CO)3Cl],28 which is due to the
higher p* orbital (LUMO) energy of pyrazolyl–pyridine ligand
compared to that of the bpy ligand with the result of increasing
the energy gap between HOMO and LUMO. The emission in
MeCN at room temperature (Fig. 4), however, is red-shifted ca.
14–32 nm as compared to that in the solid state. This phenomenon
is typical of phosphorescence from the [d(Re) → p* (pyrazolyl–
pyridine)] 3MLCT excited state, which has been reported in many
rhenium(I) diimine tricarbonyl complexes in the literature.18,27–28
It is noteworthy that quantum efficiencies of luminescence of
compounds 1–3 are found to lie between 0.040 and 0.051, as
measured using [Ru(bpy)3]2+ (= 0.062) as a relative standard,
which are much higher than that of the [Re(bpy)(CO)3Cl] complex
(= 0.019).18,28
As shown the cyclic voltammogram in Fig. 5, complexes 1
and 3 display one irreversible oxidation wave (Ipa < Ipc), while
complex 2 shows one quasi-reversible oxidation wave (DE = 70 mV,
I
pa : Ipc ª 1). This oxidation has been assigned to the Re(I)/Re(II)
redox reaction. The more positive oxidation potential for 1
(E1/2 = 1.43 V) compared with 2 (E1/2 = 1.41 V) is due to the better
electron withdrawing ability of L1 than that of L2. The oxidation
potentials of 1–3 are more positive than that obtained for the
reference [Re(bpy)(CO)3Cl] (E1/2 = +1.32 V),29 which indicates
that complexes 1–3 are more difficult to oxidize. In the cathodic
region, one irreversible reduction wave is observed in the cyclic
voltammograms of complexes 1–3 (as shown in Fig. S4†), which
is assigned to the reduction of the pyrazolyl–pyridyl-based ligand
(L) → (L-). For complex 1 and 2, the reduction wave is not equal in
area to the oxidation wave, while no oxidation wave is found in the
cathodic region of complex 3. The irreversibility of the reduction
wave follows the order 1 < 2 < 3. In agreement with the electron
withdrawing ability of the free ligands L1, L2, the reduction
potential of 1 is less negative than that of 2. The reduction
potentials of the three present rhenium complexes are all more
negative than that obtained in the reference [Re(bpy)(CO)3Cl]
(E1/2 = -1.35 V),29 which is due to the higher p* orbital (LUMO)
energy of the pyrazolyl–pyridine ligand than that of the bpy
ligand.
Fig. 4 Emission spectra of 1 (solid), 2 (dot), 3 (dash dot dot) in acetonitrile
solution and 1 (dash), 2 (dash dot) and 3 (short dot) in the solid states at
room temperature.
Electrochemistry and electrochemiluminescence (ECL)
The redox behaviors of homo- or binuclear complexes 1–3 (1 ¥
10-3 mol L-1) and ligands L1, L2 (1 ¥ 10-3 mol L-1) were determined
by cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) using Bu4NPF6 (c = 0.1 mol L-1) as the supporting
electrolyte in deaerated acetonitrile solution. All electrochemical
data for the three rhenium complexes and the free ligands are
collected in Table 5. The free ligand L1 shows one irreversible
reduction wave at -1.09 V, while L2 exhibits two irreversible
reduction waves at -1.06 and -1.28 V, respectively (as shown in
Fig. S3†).
Fig. 5 CVs of 1 mmol L-1 1, 2 and 3 in deaerated acetonitrile solution at
scan rate of 100 mV s-1.
Under the experimental conditions and over the range of scan
rates from 10 to 400 mV s-1, the dependence of the anodic peak
current of the Re(I)/Re(II) redox pair of the complexes 1–3 was
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Table 5 Electrochemical data for complexes 1–3 and ligands L1, L2
Ep (V)a
Oxidation (V)
Reduction (V)
Compound
Epa
Epc
E1/2
Epa
Epc
E1/2
L1
L2
-1.09
-1.06, -1.28
-1.43
1
+1.39
+1.38
+1.36
+1.47
+1.45
+1.42
+1.43
+1.41
+1.39
1.32b
-1.38
-1.40
1.40
2
-1.45
3
[Re(bpy)(CO)3Cl]
-1.35b
a All potentials were determined at room temperature in deaerated MeCN solutions (0.1 M Bu4NPF6) vs. SCE at scan rate of 100 mv s-1. b From values
reported in ref. 29 in deaerated MeCN solutions.
Table 6 ECL quantum efficiencies of complexes 1–3
Annihilationa 1–3–coreactant systemb
investigated to identify the type of current. With the increase
of the scan rate, the oxidation and reduction peak currents of
the complexes 1–3 are increased markedly, and the ratio of them
(Ipa : Ipc) is also increased. The peak currents and scan rate show
good linear relationships, which indicates that the electrochemical
process is controlled by absorption (as shown in Fig. S5†).7
ECL of a 50 mmol L-1 solution of rhenium complexes 1–3,
containing 0.1 mol L-1 Bu4NPF6 was observed when the potential
of the Pt working electrode was pulsed between the oxidation
and reduction waves of the three rhenium complexes (~ +1.8 and
-2.0 V). The ECL spectra are similar to the photoluminescence
Compounds
(jECL
)
(jECL,TPA
)
(jECL,DBAE)
1
0.00176
0.00161
0.00852c
0.00029
0.00064
0.0602
0.0693
0.0255
0.52
0.0212
0.0227
0.0131
—
2
3
[Re(bpy)(CO)3Cl]d
[Re(phen)(CO)3Cl]d
0.087
—
a
2+
jECL were calculated with respect to the ECL efficiency of Ru(bpy)3 of
jECL = 0.0500 in MeCN.30 Reported values were averaged from at least
3
spectra, so the same MLCT states are probably formed in both
five scans with a relative standard deviation of ~10% in MeCN solutions.
b
ECL calculated with respect to jECL = 1.00 for Ru(bpy)32+. ECL solutions
experiments.18 It is noted that 3 displays the highest ECL intensity
owing to the existance of two [Re(CO)3Cl] groups in the structure.
No ECL emission was observed on scanning to potential less
positive or negative than those needed to form the anionic and
cationic species, respectively. The generation of luminescence upon
pulsing the potential could be explained by analogy to the well-
studied [Re(L)(CO)3Cl] (L = bpy, 1,10-phenanthroline (phen), or
a methyl-substituted derivative) system.18,19
j
contained 1 mM complex and 0.025 mol L-1 TPA or DBAE. c In DMF
solution. d From the reported values in ref. 18.
Coreactant system ECL also has been studied using 50 mM
solution of the rhenium complexes at the Pt working electrode with
tri-n-propylamine (TPrA) or 2-(dibutylamino)ethanol (DBAE) as
the coreactant. As shown in Fig. 6–7, the ECL intensity peaks
of three rhenium complexes 1–3 with TPrA as the coreactant
appear at potentials of 1.61 V, 1.55 V and 1.60 V, respectively,
whereas with DBAE as the coreactant (Fig. 7) at more negative
potentials of 1.53 V, 1.43 V and 1.50 V, respectively, which may
be attributed to the easier oxidation of DBAE than TPrA.31 It is
found that the ECL intensities of complexes 1–3 are all influenced
by the concentration of TPrA and DBAE. As shown in Fig. 8,
the ECL intensities of complexes 1–3 are increase noticeably with
the increasing concentration of DBAE. With the same DBAE
concentration, ECL intensity follows the order 1 > 2 > 3. When
TPrA is the coreactant, a maximum of the ECL intensities in
all the complexes 1–3 can be observed along with the increasing
concentration of TPrA. With the same TPrA concentration, ECL
intensity follows the order 2 > 1 > 3. When the concentrations
of TPrA and DBAE are lower than about 60 mM, the ECL
intensity of the complexes 1–3–TPrA system is higher than that of
the DBAE system. In most cases, the ECL quantum efficiencies
of complexes 1–3 are comparable to that of the [Re(L)(CO)3Cl]
(L = bpy or phen) system18 (Table 6). It is noted that oxygen
tended to substantially decrease the ECL intensities of the three
Fig. 6 ECL intensity–potential curve of 50 mmoL L-1 1 (solid), 2 (dash)
and 3 (dot) in acetonitrile solution containing 35 mmol L-1 TPrA.
rhenium complexes when the TPrA concentration is lower than
10 mM. Fig. 9 shows that the ECL intensity of 1 deaerated for
5 min is about six times higher than that in air-saturated solution,
then when oxygen is added into the solution the ECL intensity is
gradually decreased, which could enable it to be used as an oxygen
sensor.
Conclusions
Three rhenium carbonyl complexes 1–3 were prepared by reac-
tion of the appropriate bidentate pyrazolyl–pyridyl-based ligand
with [Re(CO)5Cl]. These complexes exhibit bright yellow–green
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reduction wave. The ECL of the acetonitrile solutions of complexes
1–3 in the absence or presence of co-reactant tri-n-propylamine
(TPrA) or 2-(dibutylamino)ethanol (DBAE) were observed by
stepping the potential of a Pt disk working electrode. The ECL
spectra were identical tothe photoluminescence spectra, indicating
that the chemical reactions following electrochemical oxidation
3
or reduction form the same MLCT excited states. It was found
that the concentration of TPrA and DBAE had a great effect
on the ECL intensities of complexes 1–3. The ECL quantum
efficiencies of the complex 1–3–TPrA systems are higher than that
of the analogous systems with DBAE. In most cases, the ECL
quantum efficiencies of complexes 1–3 are comparable to that of
[Re(L)(CO)3Cl] (L = bpy or phen) system. It is noted that oxygen
tends to substantially decrease the ECL intensities of the three
rhenium complexes when the TPrA concentration is lower than
10 mM, which could enable them to be used as oxygen sensors.
Fig. 7 ECL intensity–potential curve of 50 mmoL L-1 1 (solid), 2 (dash)
and 3 (dot) in acetonitrile solution containing 35 mmol L-1 DBAE.
Acknowledgements
This study was financially supported by the National Nature
Sciences foundation of China (20735002, 20801013, 20801014),
the Innovation Foundation for Young Technological Talents of
Fujian Province (2007F3050), and the project-sponsored by SRF
for ROCS, SEM (LXKQ0830) and Fuzhou University (XRC-
0723).
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