Synthesis, characterisation and photophysical studies of leucotriarylmethanes-containing ligands and their rhenium(i) tricarbonyl diimine complexes

Article abstract of DOI:10.1039/b927167h

A series of pyridine- and bipyridine-containing leucotriarylmethane ligands has been successfully synthesised and incorporated into tricarbonyl rhenium(i) diimine complexes. The X-ray crystal structures of two of the complexes have also been determined. T

Full text of DOI:10.1039/b927167h

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www.rsc.org/dalton | Dalton Transactions
Synthesis, characterisation and photophysical studies of
leucotriarylmethanes-containing ligands and their rhenium(I) tricarbonyl
diimine complexes†
Chi-Chiu Ko,* Chi-On Ng, Hua Feng and Wing-Kin Chu
Received 4th January 2010, Accepted 7th May 2010
First published as an Advance Article on the web 15th June 2010
DOI: 10.1039/b927167h
A series of pyridine- and bipyridine-containing leucotriarylmethane ligands has been successfully
synthesised and incorporated into tricarbonyl rhenium(I) diimine complexes. The X-ray crystal
structures of two of the complexes have also been determined. The photoreactivity, photophysical and
electrochemical properties of these ligands and their rhenium complexes were investigated. The
photo-ionisation of the leucotriarylmethanes in the free ligands and their metal complexes and the
subsequent change in absorption properties were also studied. Additionally, the electrochemistry of
these ligands and complexes were investigated.
bipyridine-4-carboxylic acid were prepared by a slight modifica-
Introduction
tion of a reported procedure.4 [Re(CO)3(N–N)(MeCN)](CF3SO3)
t
Leucotriarylmethanes are well known to exhibit photochromism
(N–N = Bu2bpy, phen, Br2phen, NO2phen) were synthesized
according to literature method.5 nBu4NPF6 was obtained from
Aldrich Chemical Company and recrystallised three times from
ethanol and dried in vacuo at 100 ◦C for 24 h before use for
electrochemical measurements. Acetonitrile was distilled over
CaH2 under argon. THF was distilled over sodium prior to use.
All other reagents and solvents were of analytical grade and were
used as received.
due to their reversible photo-ionisation reactions, which give the
coloured triphenylmethyl cation and the counter anion upon UV-
light excitation.1 Owing to these properties, they are widely used in
many applications such as colourless copying papers, photoimag-
ing systems, photo-responsive polymers, reversible photomechan-
ical transfer materials and photocontrollable vesicle formation.1b,2
In recent years, the design and study of ligands with pho-
tochromic moieties, such as stilbenes, azo compounds, spiroox-
azines, spiropyrans and diarylethenes, and their coordination
compounds have received considerable attention, as these metal
complexes have been demonstrated to show novel and perturbed
photochromic behaviour with their characteristic functionality
being switchable by the photochromic reactivity.3 To extend the
design of photochromic diarylethene and spirooxazine-containing
ligands and their transition metal complexes,3d-3i we have designed
the first series of leucotriarylmethanes-containing ligands and
their transition metal complexes. In this study, we describe the
syntheses, characterisation, photophysics, photochemistry and
electrochemistry of two leucotriarylmethane-containing ligands
and their rhenium(I) tricarbonyl diimine complexes.
Synthesis
All reactions were performed under strictly anaerobic and anhy-
drous conditions in an inert atmosphere of argon using standard
Schlenk techniques.
Bis(4-(dimethylamino)phenyl)(pyridin-4-yl) methanol (L1OH).
To a THF solution of 4-bromo-N,N-dimethylaniline (1 g, 5.0
mmol) at -78 ◦C, n-BuLi (3.125 mL, 1.6 M, 5.0 mmol) was added
in a dropwise manner. The resulting mixture was allowed to stir at
-78 ◦C for 1 h. Thereafter, methyl isonicotinate (0.343 g, 2.5 mmol)
in THF (5 mL) was added dropwise. The resulting solution was
allowed to slowly warm to room temperature over 3 h, after which
the reaction was quenched by the addition of water (30 mL) and
extracted with CHCl3 (3 ¥ 30 mL). The chloroform extract was
then washed with H2O and dried over MgSO4. After removal of the
solvent under reduced pressure, the residue was further purified
by column chromatography on alumina using CHCl3 as an eluent
to afford analytically pure L1OH as a white powdery solid. Yield:
Experimental
Reagent and materials
[Re(CO)5Br] was obtained from Strem Chemicals, Inc. Methyl
isonicotinate, 4-bromo-N,N-dimethylaniline, 4,4¢-dimethyl-2,2¢-
bipyridine (Me2bpy), 4,4¢-di-tert-butyl-2,2¢-bipyridine (tBu2bpy),
5-nitro-1,10-phenanthroline (NO2phen) and 1,10-phenanthroline
(phen) were obtained from Aldrich Chemical Company.
5,6-Dibromo-1,10-phenanthroline (Br2phen) and 4¢-methyl-2,2¢-
1
450 mg, 1.30 mmol; 52%. H NMR (400 MHz, CDCl3, 298 K,
TMS): d 2.66 (s, 1H, –OH), 2.95 (s, 12H, NCH3), 6.65 (d, 4H,
J = 8.9 Hz, phenyl protons at 2,2¢,6,6¢ positions), 7.09 (d, 4H,
J = 8.9 Hz, phenyl protons at 3,3¢,5,5¢ positions), 7.30 (d, 2H, J =
4.6 Hz, pyridyl protons at 3,5 positions), 8.51 (d, 2H, J = 4.6 Hz,
pyridyl protons at 2,6 positions); positive-ion ESI-MS: m/z: 348.5
[M + H]+; IR (KBr disc): n/cm-1: 3125 (O–H); elemental analyses
calcd (%) for C22H25N3O·0.25H2O (351.9): C 75.08, H 7.30, N
11.96; found: C 75.30, H 7.02, N 11.91.
Department of Biology and Chemistry, City University of Hong Kong, Tat
Chee Avenue, Kowloon, Hong Kong, China. E-mail: vinccko@ctyu.edu.hk;
Fax: 852-2788-7406; Tel: 852-3442-6958
† CCDC reference numbers 760665 and 760666. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b927167h
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Bis(4-(dimethylamino)phenyl)(pyridin-4-yl) acetonitrile (L1).
Concentrated hydrochloric acid (0.267 mL) was added dropwise to
a solution of L1OH (1 g, 2.89 mmol) in DMSO (30 mL) at 65 ◦C,
after which the solution changed from pale yellow to greenish
blue. KCN (1.03 g, 16 mmol) in H2O (5 mL) was then added
dropwise into the resulting greenish blue mixture and stirred at
this temperature for an additional 30 min. It was then poured
over deionised water (900 mL) to commence precipitation. The
crude product was filtered and air dried. Further purification by
column chromatography on alumina using CH2Cl2 as the eluent
resulted in analytically pure L1 as a white powder. Yield: 291 mg,
n/cm-1: 3395 (O–H); elemental analyses calcd (%) for C28H30N4O
(438.56): C 76.68, H 6.89, N 12.78; found: C 76.42, H 6.81, N
12.57.
[Re(CO)3(tBu2bpy)(L1)](PF6) (1). The complex was prepared
by modification of a literature method for the related Re(I) diimine
complexes.5 [Re(CO)3(tBu2bpy)(CH3CN)](CF3SO3) (100 mg, 0.14
mmol) and L1 (52.6 mg, 0.15 mmol) were dissolved in THF
(30 mL). The resulting solution was refluxed overnight under an
inert atmosphere of nitrogen. After removal of the solvent under
reduced pressure, the residue was redissolved in methanol (0.5
mL). Subsequent metathesis reaction with a saturated methanolic
ammonium hexafluorophosphate solution gave the target complex
1
0.81 mmol; 28%. H NMR (400 MHz, CDCl3, 298 K, TMS): d
2.96 (s, 12H, NCH3), 6.63 (d, 4H, J = 8.9 Hz, phenyl protons
at 2,2¢,6,6¢ positions), 7.02 (d, 4H, J = 8.9 Hz, phenyl protons
at 3,3¢,5,5¢ positions), 7.22 (d, 2H, J = 4.6 Hz, pyridyl protons
at 3,5 positions), 8.59 (d, 2H, J = 4.6 Hz, pyridyl protons at
-
as a PF6 salt. After column chromatography on silica gel using
CH2Cl2 as the eluent, the complex was further purified by
recrystallisation by the slow diffusion of diethyl ether vapour
into a concentrated dichloromethane solution of the complexes
to yield 1 as an analytically pure yellow crystalline solid. Yield:
2,6 positions); positive-ion ESI-MS: m/z: 357 [M + H]+; IR
-1
≡
(KBr disc): n/cm : 2230 (C N); elemental analyses calcd (%)
1
71 mg, 0.068 mmol; 50%. H NMR (400 MHz, CDCl3, 298 K,
for C23H24N4 (356.46): C 77.50, H 6.79, N 15.72; found: C 77.40,
t
TMS): d 1.47 (s, 18H; Bu), 2.91 (s, 12H, NCH3), 6.61 (d, 4H,
H 6.78, N 15.55.
J = 8.9 Hz, phenyl protons at 2,2¢,6,6¢ positions), 6.95 (d, 4H,
J = 8.9 Hz, phenyl protons at 3,3¢,5,5¢ positions), 7.33 (dd, 2H,
J = 5.3, 1.5 Hz, pyridyl protons at 3,5 positions), 7.66 (dd, 2H,
J = 5.9, 1.8 Hz, bipyridyl protons at 5,5¢ positions), 8.15 (dd, 2H,
J = 5.3, 1.5 Hz, pyridyl protons at 2,6 positions), 8.37 (d, 2H, J =
1.8 Hz, bipyridyl protons at 3,3¢ positions), 8.89 (d, 2H, J = 5.9 Hz,
bipyridyl protons at 6,6¢ positions); positive-ion ESI-MS: m/z: 895
[M–PF6] ; IR (KBr disc): n/cm : 2035, 1940, 1925 (C O); 835 (P–
F); elemental analyses calcd (%) for C44H48N6O3RePF6 (1040.06):
C 50.81 H 4.65 N 8.08; found: C 50.89 H 4.51 N 8.08.
Methyl 4¢-methyl-2,2¢-bipyridine-4-carboxylate. Concentrated
sulfuric acid (0.96 mL) was added dropwise to 4¢-methyl-2,2¢-
bipyridine-4-carboxylic acid (500 mg, 2.33 mmol) in methanol
(20 mL) at 0 ◦C. The resulting solution was refluxed overnight and
poured into H2O (100 mL). After neutralisation with aqueous
sodium hydroxide solution (25%) to pH 8, the mixture was
extracted with CHCl3 (3 ¥ 50 mL). The combined extracts was
washed with H2O and dried over MgSO4. After removal of the
solvent under reduced pressure, the white solid was further purified
by column chromatography on silica gel using CHCl3 as the
eluent to afford analytically pure methyl 4¢-methyl-2,2¢-bipyridine-
4-carboxylate as a white powdery solid. Yield: 419 mg, 1.84 mmol;
+
-1
≡
[Re(CO)3(phen)(L1)](PF6) (2). The title complex was synthe-
sised according to a procedure similar to that of 1, except
that [Re(CO)3(phen)(CH3CN)](CF3SO3) (88 mg, 0.14 mmol) was
used in place of [Re(CO)3(tBu2bpy)(CH3CN)](CF3SO3) in the
1
79%. H NMR (400 MHz, CDCl3, 298 K, TMS): d 2.46 (s, 3H,
CH3), 3.99 (s, 3H, OCH3), 7.18 (d, 1H, J = 4.9 Hz, proton at 5¢
position), 7.87 (d, 1H, J = 5.0 Hz, proton at 5 position), 8.25 (s,
1H, proton at 3¢ position), 8.58 (d, 1H, J = 4.9 Hz, proton at 6¢
position), 8.82 (d, 1H, J = 5.0 Hz, proton at 6 position), 8.93 (s,
1H, proton at 3 position); positive-ion ESI-MS: m/z: 229 [M +
1
substitution reaction. Yield: 53 mg, 0.063 mmol; 40%. H NMR
(400 MHz, CDCl3, 298 K, TMS): d 2.90 (s, 12H, NCH3), 6.57 (d,
4H, J = 8.9 Hz, phenyl protons at 3,3¢,5,5¢ positions), 6.86 (d, 4H,
J = 8.9 Hz, phenyl protons at 2,2¢,6,6¢ positions), 7.22 (dd, 2H,
J = 5.3, 1.5 Hz, pyridyl protons at 3,5 positions), 8.11 (dd, 2H,
J = 5.3, 1.5 Hz, pyridyl protons at 2,6 positions), 8.15 (dd, 2H,
J = 8.3, 5.2 Hz, phenanthrolinyl protons at 3,8 positions), 8.21
(s, 2H, phenanthrolinyl protons at 5,6 positions), 8.83 (dd, 2H,
J = 8.3, 1.3 Hz, phenanthrolinyl protons at 4,7 positions), 9.50
(dd, 2H, J = 5.2, 1.3 Hz, phenanthrolinyl protons at 2,9 positions);
positive-ion ESI-MS: m/z: 806.9 [M–PF6]+; IR (KBr disc): n/cm-1:
+
-1
=
H] ; IR (KBr disc): n/cm : 1735 (C O); elemental analyses calcd
(%) for C13H12N2O2 (228.3): C 68.41, H 5.30, N 12.27; found: C
68.12, H 5.71, N 12.05.
Bis(4-(dimethylamino)phenyl)(4¢-methyl-2,2¢-bipyridin-4-yl)
methanol (L2). The title compound was synthesised according
to a procedure similar to that of L1OH, except that methyl 4¢-
methyl-2,2¢-bipyridine-4-carboxylate (300 mg, 1.31 mmol) was
used in place of methyl isonicotinate. After purification by
column chromatography on alumina using CH2Cl2 as the eluent,
analytically pure L2 was obtained as a pale yellow powder. Yield:
≡
2035, 1930, 1920 (C O); 840 (P–F); elemental analyses calcd (%)
for C38H32N6O3RePF6 (951.87): C 47.95 H 3.39 N 8.83; found: C
47.67 H 3.63 N 8.71.
1
141 mg, 0.32 mmol; 25%. H NMR (400 MHz, CDCl3, 298 K,
[Re(CO)3(Br2phen)(L1)](CF3SO3) (3). The title complex was
synthesised according to a procedure similar to that of 1, except
that [Re(CO)3(Br2phen)(CH3CN)](CF3SO3) (110 mg, 0.14 mmol)
was used in place of [Re(CO)3(tBu2bpy)(CH3CN)](CF3SO3) in the
TMS): d 2.68 (s, 3H, bipyridyl CH3), 2.94 (s, 12H, NCH3), 6.66
(d, 4H, J = 8.9 Hz, phenyl protons at 2,2¢,6,6¢ positions), 7.09
(d, 1H, J = 4.9, bipyridyl proton at 5¢ position) 7.16 (d, 4H, J =
8.9 Hz, phenyl protons at 3,3¢,5,5¢ positions), 7.25 (dd, 1H, J =
5.3, 1.8 Hz, bipyridyl proton at 5 position), 8.18 (s, 1H, bipyridyl
proton at 3¢ position), 8.49 (d, 1H, J = 4.9 Hz, bipyridyl proton
at 6¢ position), 8.51 (dd, 1H, J = 1.8 Hz, 0.7 Hz, bipyridyl proton
at 3 position), 8.57 (dd, 1H, J = 5.3, 0.7 Hz, bipyridyl proton at 6
position); positive-ion ESI-MS: m/z: 439 [M + H]+; IR (KBr disc):
-
substitution reaction, and the complex was obtained as a CF3SO3
salt. Yield: 91 mg, 0.082 mmol; 60%. 1H NMR (400 MHz, CDCl3,
298 K, TMS): d 2.91 (s, 12H, NCH3), 6.50 (d, 4H, J = 9.0 Hz,
phenyl protons at 2,2¢,6,6¢ positions), 6.87 (d, 4H, J = 9.0 Hz,
phenyl protons at 3,3¢,5,5¢ positions), 7.23 (dd, 2H, J = 5.2,
1.7 Hz, pyridyl protons at 3,5 positions), 8.32 (dd, 2H, J = 5.2,
6476 | Dalton Trans., 2010, 39, 6475–6482
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1.7 Hz, pyridyl protons at 2,6 positions), 8.35 (dd, 2H, J = 8.7,
5.1 Hz, phenanthrolinyl protons at 3,8 positions), 9.17 (dd, 2H, J =
8.7, 1.3 Hz, phenanthrolinyl protons at 4,7 positions), 9.68 (dd,
2H, J = 5.1, 1.3 Hz, phenanthrolinyl protons at 2,9 positions);
Electronic absorption spectra were recorded on a Hewlett-
Packard 8452A diode array spectrophotometer or a Shimadzu
UV-1700 spectrophotometer. Photoexcitation was carried out
using a 500 W ozone-free Orieal Hg(Xe) lamp, and monochro-
matic light was obtained by passing the light through an Oriel
77200 1/4m monochromator. Steady-state emission and excitation
spectra at room temperature and at 77 K were recorded on a
Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. Solutions
were rigorously degassed on a high-vacuum line in a two-
compartment cell with no less than four successive freeze pump-
thaw cycles. Measurements of the EtOH–MeOH (4 : 1, v/v) glass
samples at 77 K were carried out with the diluted EtOH–MeOH
sample solutions contained in a quartz tube inside a liquid nitrogen
filled quartz optical Dewar flask. Luminescence quantum yields
were determined using the method described by Demas and
Crosby6 with aqueous solution of [Ru(bpy)3]Cl2 (fem = 0.0427
with 436-nm excitation) as reference. Luminescence lifetimes of
the samples were measured using time-correlated single photon
counting (TCSPC) technique on the TCSPC spectrofluorometer
in a Fast MCS mode with a NanoLED-375LH excitation source,
which has its excitation peak wavelength at 375 nm and pulse
width shorter than 750 ps.
positive-ion ESI-MS: m/z: 964.7 [M–CF3SO3]+; IR (KBr disc):
-1
≡
n/cm : 2035, 1940, 1920 (C O); elemental analyses calcd (%) for
C38H30Br2N6O3ReCF3SO3·0.5CH2Cl2 (1156.23): C 41.03 H 2.70 N
7.27; found: C 41.28 H 2.74 N 7.46.
[Re(CO)3(NO2phen)(L1)](PF6) (4). The title complex was syn-
thesised according to a procedure similar to that of 1, except
that [Re(CO)3(NO2phen)(CH3CN)](CF3SO3) (94 mg, 0.14 mmol)
was used in place of [Re(CO)3(tBu2bpy)(CH3CN)](CF3SO3) in the
1
substitution reaction. Yield: 60 mg, 0.060 mmol; 44%. H NMR
(400 MHz, CDCl3, 298 K, TMS): d 2.88 (d, 12H, J = 4.5 Hz,
NCH3), 6.56 (d, 4H, J = 6.8 Hz, phenyl protons 2,2¢,6,6¢ positions),
6.88 (d, 4H, J = 6.8 Hz, phenyl protons at 3,3¢,5,5¢ positions), 7.28
(dd, 2H, J = 5.3, 1.5 Hz, pyridyl protons at 3,5 positions), 8.13 (dd,
2H J = 5.3, 1.5 Hz, pyridyl protons at 2,6 positions), 8.26 (dd, 1H,
J = 8.0, 5.1 Hz, phenanthrolinyl proton at 8 position), 8.28 (dd,
1H, J = 8.0, 5.2 Hz, phenanthrolinyl proton at 3 position), 8.98
(dd, 1H, J = 8.0, 1.3 Hz, phenanthrolinyl proton at 7 position),
9.08 (s, 1H, phenanthrolinyl proton at 6 position), 9.55 (dd, 1H,
J = 8.0, 1.4 Hz, phenanthrolinyl proton at 4 position), 9.59 (dd,
1H, J = 5.1, 1.3 Hz, phenanthrolinyl proton at 9 position), 9.65
(dd, 1H, J = 5.2, 1.4 Hz, phenanthrolinyl proton at 2 position);
positive-ion ESI-MS: m/z: 851.9 [M–PF6]+; IR (KBr disc): n/cm-1:
Cyclic voltammetric measurements were performed by using a
CH Instruments, Inc. Model CHI 620 Electrochemical Analyzer.
Electrochemical measurements were performed in acetonitrile
n
solutions with 0.1 M Bu4NPF6 as the supporting electrolyte at
room temperature. The reference electrode was a Ag/AgNO3
(0.1 M in acetonitrile) electrode, and the working electrode was
a glassy carbon electrode (CH Instruments, Inc.) with a platinum
wire as the auxiliary electrode. The working electrode surface was
polished with a 1 mm a-alumina slurry (Linde) and then a 0.3 mm
a-alumina slurry (Linde) on a microcloth (Buehler Co.). The
ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal
reference. All solutions for electrochemical studies were deaerated
with pre-purified argon gas prior to measurements.
≡
2035, 1935, 1920 (C O); 840 (P–F); elemental analyses calcd (%)
for C38H31N7O5RePF6·0.5CH2Cl2 (1039.33): C 44.49 H 3.10 N
9.43; found: C 44.42 H 3.30 N 9.71.
[Re(CO)3(L2)Br] (5). Complex 5 was synthesised by refluxing
a suspension of [Re(CO)5Br] (100 mg, 0.25 mmol) and L2
(129.6 mg, 0.3 mmol) in benzene (40 mL) for 5 h under an inert
atmosphere of argon. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on
silica gel using dichloromethane–acetone (9 : 1 v/v) as the eluent.
Slow diffusion of diethyl ether vapour into a concentrated CH2Cl2
solution of the complex gave a yellowish orange crystalline solid
of 5. Yield: 98 mg, 0.12 mmol; 50%. 1H NMR (400 MHz, CDCl3,
298 K, TMS): d 2.55 (s, 1H, bipyridyl CH3), 2.97 (d, 12H, J =
1.3 Hz, NCH3), 6.68 (m, 4H, phenyl protons at 2,2¢,6,6¢ positions),
7.13 (m, 4H, phenyl protons at 3,3¢,5,5¢ positions), 7.29 (d, 1H,
J = 5.4 Hz, bipyridyl proton at 5¢ position), 7.35 (dd, 1H, J = 5.8,
1.7 Hz, bipyridyl H at 5 position), 7.96 (s, 1H, bipyridyl proton
at 3¢ position), 8.44 (d, 1H, J = 1.7 Hz, bipyridyl proton at 3
position), 8.76 (d, 1H, J = 5.4 Hz, bipyridyl H at 6¢ position);
8.88 (d, 1H, J = 5.8 Hz, bipyridyl H at 6 position); positive-ion
ESI-MS: m/z: 708.8 [M–Br]+; IR (KBr disc): n/cm-1: 2015, 1910,
Crystal structure determination†
Experimental details for the crystal structure determinations are
summarised in Table 1. Crystals of 2 and 3 suitable for X-ray
diffraction studies were obtained by slow diffusion of diethyl
ether vapour into a dichloromethane solution of the complexes.
Crystals of 2 and 3 mounted in glass capillaries were used for data
collection on an Oxford Diffraction Gemini S Ultra X-ray single
crystal diffractometer using graphite monochromatised Mo-Ka
˚
radiation (l = 0.71073 A). The structure was solved by direct
methods employing the SHELXL-97 program8 on a PC. Re and
many non-H atoms were located according to the direct methods.
The positions of other non-H atoms were found after successful
refinement by full-matrix least-squares using the SHELXL-97
program8 on a PC. In the final stage of least-squares refinement, all
non-H atoms were refined anisotropically. H atoms were generated
by the SHELXL-97 program.8 The positions of H atoms were
calculated based on riding mode with thermal parameters equal
to 1.2 times that of the associated C atoms and participated in the
calculation of the final R-indices. A conventional index R1 based
on observed F values larger than 4s(Fo) is given (corresponding to
≡
1890 (C O); elemental analyses calcd (%) for C31H30N4O4ReBr
(788.71): C 47.21, H 3.83, N 7.10; found: C 47.37, H 4.12, N 6.99.
Physical measurements and instrumentation
1H NMR spectra were recorded on a Bruker AV400 (400 MHz)
FT-NMR spectrometer. Chemical shifts (d, ppm) were reported
relative to tetramethylsilane (Me4Si). All positive-ion ESI mass
spectra were recorded on a PE-SCIEX API 300 triple quadrupole
mass spectrometer. Elemental analyses of all the compounds were
performed on an Elementar Vario EL III elemental analyser.
ꢀ
ꢀ
intensity ≥ 2s(I)). wR2 = { [w(Fo - Fc )2]/ [w(Fo2)2]}1/2, R1 =
2
2
ꢀ
ꢀ
ꢀFo| - |Fcꢀ/ |Fo|, The Goodness of Fit is always based on
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Table 1 Crystal and structure determination data for complexes 2 and 3
2
3
Formula
Mr
C38H32N6O3RePF6 C39H30Br2F3N6O6ReS
951.88
293 (2)
1113.77
298 (2)
T/K
˚
A/A
16.7916 (4)
11.0807 (3)
26.4430 (7)
90.00
128.189 (2)
90.00
17.4444 (3)
11.1975 (2)
20.8885 (4)
90.00
91.2079 (15)
90.00
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
3867.04 (17)
Orange
Monoclinic
P2/c
4079.32 (13)
Red
Monoclinic
P2/n
4
Crystal colour
Crystal system
Space group
Z
4
Scheme 1 Synthetic routes for L1, L2 and their rhenium(I) complexes.
F(000)
1880
1.635
2168
1.813
Dc/g cm-3
conditions afforded complex 5. The identities of the ligands L1, L2
and their rhenium complexes 1–5 were confirmed by satisfactory
elemental analyses, 1H NMR, IR spectroscopy and ESI mass
spectrometry, and for complexes 2 and 3, by X-ray crystallography.
Crystal dimensions/mm
0.20 ¥ 0.20 ¥ 0.20 0.20 ¥ 0.20 ¥ 0.20
˚
l/A (graphite
0.71073
0.71073
monochromated, Mo-Ka)
m/mm-1
3.257
5.056
Collection range/◦
3.15 ≤ q ≤ 25.00 3.07 ≤ q ≤ 25.00
h: -19 to 19
k: -8 to 13
l: -21 to 31
96.7
h: -20 to 20
k: -13 to 13
l: -24 to 24
99.5
X-Ray crystal structures
Completeness to theta (%)
No. of data collected
No. of unique data
No. of data used in
refinement, m
Fig. 1 depicts the perspective drawings of the complex cations
of 2 and 3. The experimental details for the crystal structure
determinations and selected bond distances and bond angles
13 037
62 975
6579
3976
7169
4632
No. of parameters refined, p 498
599
Ra
0.0371
0.0776
0.846
0.0390
0.0796
0.917
wRa
Goodness-of-fit, S
Maximum shift, (D/s)max
Residual extrema in final
0.002
+1.479, -0.895
0.001
+1.505, -0.984
-3
˚
difference map/e A
2
2
a w = 1/[s (Fo2) + (ap)2 + bP], where P = [2Fc + max(Fo2,0)]/3.
ꢀ
F2: GooF = S = { [w(Fo - Fc )2]/(n - p)}1/2, where n is the
2
2
number of reflections and p is the total number of parameters
2
refined. The weighting scheme is: w = 1/[s (Fo2) + (aP)2 + bP],
where P is [2Fc + max(Fo2,0)]/3.
2
Results and discussion
Synthesis and characterisation
The synthetic routes for the formation of the leucotriarylmethane-
containing ligands are summarised in Scheme 1. Reaction of lithi-
ated 4-dimethylaminobenzene with methyl isonicotinate or methyl
4¢-methyl-2,2¢-bipyridine-4-carboxylate, which was prepared by
the esterification of 4-4-carboxyl-4¢-methyl-2,2¢-bipyridine4b with
methanol using H2SO4 as catalyst, gave L1OH and L2, respec-
tively. Acidification of L1OH with hydrochloric acid, which gener-
ates the triarylmethane cation, followed by cyanation with potas-
sium cyanide afforded pyridine-containing leucotriarylmethane
nitrile (L1). Substitution of [Re(CO)3(N–N)(MeCN)](CF3SO3)
with L1 gave complexes 1–4 that, after purification by column
chromatography using silica gel and subsequent recrystallisation
from CH2Cl2–Et2O, were isolated as yellow to orange crystalline
solids, depending on the nature of the diimine ligands. Substitution
reactions of Re(CO)5Br with L2 in anhydrous benzene under reflux
Fig. 1 Perspective drawing of complex cations of (a) 2 and (b) 3 with
atomic numbering. Hydrogen atoms have been omitted for clarity. Thermal
ellipsoids are shown at the 30% probability level.
6478 | Dalton Trans., 2010, 39, 6475–6482
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◦
˚
Except for complex 4, all complexes in CH2Cl2 solution
displayed luminescence (Fig. 2) with lem in the range of 540–
591 nm and emission lifetime in the range of 0.2–0.7 ms, which is
in the typical range for room temperature MLCT phosphorescence
of the rhenium complexes,5,9 upon excitation at l = 355 nm. These
emissions showed a similar energy dependence on the p-accepting
ability of the diimine ligands as the lowest energy absorption
shoulder. With reference to the previous spectroscopic work on
related complexes5,9,10 and the observed emission energy depen-
dence on the diimine ligands, these emissions were also assigned
as being derived from MLCT phosphorescence. The luminescence
quantum yields of these emissions were also determined to be in
the range of 1.5–5.7 ¥ 10-3 with 436-nm excitation. The absence
Table 2 Selected bond distances (A) and angles ( ) with estimated
standard deviations (e.s.d.s) in parentheses for 2 and 3
2
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
1.922(9)
1.888(8)
1.913(9)
2.168(5)
2.177(5)
75.5(2)
179.0(6)
88.8(3)
86.1(3)
Re(1)–N(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
2.202(5)
1.144(8)
1.166(8)
1.151(8)
1.130(7)
177.3(7)
177.6(7)
88.3(3)
179.4(9)
2.224(4)
1.139(8)
1.143(8)
1.126(7)
1.121(7)
178.1(6)
176.9(7)
88.4(3)
Re(1)–N(2)
C(38)–N(6)
N(1)–Re(1)–N(2)
Re(1)–C(2)–O(2)
C(1)–Re(1)–C(2)
C(2)–Re(1)–C(3)
Re(1)–C(1)
Re(1)–C(2)
Re(1)–C(3)
Re(1)–N(1)
Re(1)–N(2)
N(1)–Re(1)–N(2)
Re(1)–C(2)–O(2)
C(1)–Re(1)–C(2)
C(2)–Re(1)–C(3)
Re(1)–C(1)—O(1)
Re(1)–C(3)—O(3)
C(1)–Re(1)—C(3)
C(37)–C(38)–N(6)
Re(1)–N(3)
3
1.932(8)
1.933(9)
1.931(7)
2.182(5)
2.174(5)
75.13(18)
177.2(7)
87.1(3)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
C(38)–N(6)
3
of the MLCT phosphorescence of 4 can be explained by the
Re(1)–C(1)—O(1)
Re(1)–C(3)—O(3)
C(1)–Re(1)—C(3)
C(21)–C(38)–N(6)
3
non-radiative deactivation pathway to the lower-lying IL state
associated with the NO2phen ligands.10d
87.7(3)
178.1(7)
are summarised in Tables 1 and 2, respectively. The rhenium
metal centres of both structures adopted a distorted octahedral
geometry, with angles subtended by the nitrogen atoms of
phenanthroline at the rhenium centre of complexes 2 and 3,
N(1)–Re(1)–N(2), of 75.5◦ and 75.13◦, respectively, which are
much smaller than the ideal angle of 90◦ in a regular octahedral
geometry. The deviation from the ideal angle of 90◦ is due to the
steric requirements of the chelating phenanthroline ligands, which
is commonly observed in other related systems.5,9 The Re–C, C O,
≡
Re–N (pyridine) bond lengths in these complexes are in the range
˚
˚
˚
Fig. 2 Overlaid emission spectra of complexes 1, 2 and 5 in degassed
dichloromethane solution at 298 K.
of 1.888–1.933 A, 1.126–1.166 A and 2.202–2.224 A, respectively.
These bonding parameters are similar to those reported for other
related rhenium(I) tricarbonyl diimine pyridine complexes.5,9 In
these structures, the phenyl and pyridyl rings of the L1 moiety
are all connected through an sp3 carbon, which is in a tetrahedral
geometry with bond angles in the range of 106–116◦. The non-
coplanarity of these rings suggested that there is no p-conjugation
between these rings in this form.
All complexes displayed strong luminescence in EtOH–MeOH
4 : 1 glass at 77 K. These emissions followed the same emission
energy trends as the solution emissions, but they were blue shifted.
3
They were also assigned as being derived from MLCT origin.
The blue shift in emission energy in the glass medium relative
to the solution state is attributed to the increased rigidity in
the glass medium, which is commonly observed in other related
systems.11 The sub-microsecond excited state lifetime observed in
glass medium at 77 K is also supportive of the phosphorescence
assignment.
Upon prolonged irradiation at 313 nm, all ligands and com-
plexes in CH2Cl2 and DMF solution displayed photochromic
behaviour with the formation of new absorption bands peaking
ca. 420 and 640 nm (Fig. 3), corresponding to the generation
of the triarylmethane cation (Scheme 2).1 The slight red shift of
the absorption bands of the triarylmethane cation in 5 (420 and
668 nm) versus that in free ligand L2 (410 and 637 nm) is attributed
to perturbation of the transitions by the metal centre in the metal
complex. In CH2Cl2 solution, the backward reaction rate is very
slow, as reflected by the new absorptions remaining unchanged
on prolonged standing. The observation of the very well-defined
Photophysical and photochromic properties
The electronic absorption data of the ligands and their rhenium
complexes in CH2Cl2 solution at 298 K are collected in Table 3.
The absorption spectra of the ligands in CH2Cl2 showed very
intense absorption bands from 250–320 nm, with molar extinction
coefficients on the order of 104 dm3 mol-1 cm-1, which was
tentatively assigned to the p → p* and n → p* transitions of the
bipyridine, pyridine and N,N-dimethylamino benzene moieties.
Complexes 1–5, in addition to the very intense IL p → p* and
n → p* absorptions at 250–360 nm, had a moderately intense
shoulder ca. 400–450 nm in their electronic absorption spectra.
This absorption shoulder with molar extinction coefficients in the
order of 103 dm3 mol-1 cm-1 was ascribed to the metal-to-ligand
charge transfer (MLCT) [dp(Re) → p*(diimine)] transition that
is typical of rhenium(I) tricarbonyl diimine systems,5,9 probably
with some mixing of a ligand-centred character. With the same
leucotriarylmethane-containing pyridine ancillary ligand, an ab-
sorption energy dependence of this band of 1 > 2 > 3 > 4,
which is inversely related to the p-accepting ability of the diimine
ligands (tBu2bpy < phen < Br2phen < NO2phen), was observed,
in agreement with the MLCT assignments.
Scheme 2 Photochromism of L1.
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Table 3 Photophysical data for the ligands and their rhenium(I) complexes
Medium
(T/K)
Emission
em/nm (to/ms)
Absorptiona
abs /nm (e/dm3 mol-1 cm-1)
Compound
l
Uem/10-3
l
L1
CH2Cl2(298)
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
460 (< 0.1)
420 (< 0.1)
—
62.3
266(44 830), 274(49 600), 300(12 640), 341(770)
L1cc
L2
312, 418, 640
d
515 (< 0.1)
459 (< 0.1)
146
252(27 485), 270(38 550), 284(26 630), 312(7580), 345(8105)
L2cc
1
—
300, 410, 637
254(34 755), 274(51 180), 303(23 085), 317(19 745),
345(8105), 405(1725)
d
540 (0.5)
1.47
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
479 (5.2)
1cc
2
—
318, 422, 647
d
558 (0.7)
1.70
1.89
229(41 075), 275(60 240), 310(16 940), 355(7070), 414(1735)
490 (13.4)
2cc
3
—
420, 647
d
567 (0.5)
520 (9.3)
250(42 920), 275(53 950), 286(46 475), 313(17 355),
348(8305), 438(1340)
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
Glassb(77)
CH2Cl2(298)
CH2Cl2(298)
3cc
4
—
—
416, 646
d
d
272(77 465), 315(19 660), 358(9760), 442(1540)
530 (0.2)
4cc
5
—
418, 644
d
591 (0.6)
5.72
257(39 000), 269(44 340), 293(27 305), 313(16 515),
386(5195), 450(1275)
Glassb(77)
CH2Cl2(298)
530 (5.7)
5cc
—
420, 668
d
a In CH2Cl2 at 298 K. b EtOH–MeOH (4 : 1 v/v). c Triarylmethane cation form. d Non-emissive.
The identities of the triarylmethane cationic form were also further
confirmed by the ESI-mass spectrometry. The ESI-mass spectra of
5 in CH2Cl2 after photo-irradiation are depicted in Fig. 4. Except
for the free ligand L1, in which the half-life of the first order decay
of triarylmethane cation in DMF solution was determined to be
360 s at 16 ◦C (Fig. 5), the half-lives of the triarylmethane cation
form of L2 and all rhenium complexes in DMF solutions were too
short to be determined with certainty by monitoring the UV-vis
absorption changes. The much shorter lifetimes for these cations in
the metal complexes can be attributed to the electron-withdrawing
effect of the metal centres, as well as the overall positive charge
on the metal complexes, both of which render nucleophilic attack
on the triarylmethane cation more likely.1,13 These photochromic
processes can be recycled a number of times without significant
decomposition, as exemplified in the time dependence of the
Fig. 3 UV-vis absorption spectral changes of (a) L1 and (b) 1 in CH2Cl2
solution at room temperature upon excitation at 313 nm.
isosbestic points in these UV-vis absorption spectral changes
(Fig. 3), and the close resemblance of the absorption spectral
changes for all complexes and their free ligands are suggestive of a
clean conversion process for the generation of the triarylmethane
cations. In DMF solution, these new absorptions decreased
in intensity, indicative of the reversibility of the photochromic
reaction. The significant difference in the backward reaction
rate in CH2Cl2 solution is attributed to the formation of the
relatively stable chloride salt of the triarylmethane cation.12 This
has been commonly observed in other leucotriarylmethanes.1,12
Fig. 4 ESI-mass spectrum of a CH2Cl2 solution of 5 after photoirradia-
tion. The inserts show the simulated isotope pattern for the triarylmethane
cation form of 5 (top) and the expanded ion cluster from the parent peak
of the ESI-mass spectrum (bottom).
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Fig. 5 UV-vis absorption spectral changes of L1 in DMF solution at
16 ◦C after excitation at 313 nm. The insert shows the UV-vis absorption
changes of L1 at 645 nm with 313-nm excitation and the subsequent decay
trace in the dark over 3 cycles.
absorption spectral changes at 645 nm of L1 in DMF solution
with alternating UV excitation and thermal decolouration cycles
(Fig. 5, insert).
Fig. 6 Cyclic voltammograms of (a) the oxidative scan and (b) reductive
scan of 1 in MeCN (0.1 mol dm-3 nBu4NPF6).
Apart from the change in the absorption properties, the emission
intensity of all complexes was found to decrease upon excitation
into the isosbestic absorption point during the conversion to the
triarylmethane cation form in CH2Cl2 solution. This is probably
attributed to the quenching of the emissive excited state by the
lowly lying excited state associated with the triarylmethane cation
as reflected by the characteristic low-energy absorption at ca.
647 nm.
be assigned to oxidation of the L1 moiety, probably arising from
the removal of a lone pair electron on the nitrogen atom of the
dimethylamino moiety.
The close resemblance of the first two oxidations of complex
5 and the free ligand L2 are also suggestive of ligand-centred
oxidations of the L2 moiety.
The second quasi-reversible oxidation couple in the rhenium
complexes at ca. +1.72 to +1.81 V was found to be dependent on
the nature of the diimine ligand and was assigned to the metal-
centred oxidation of Re(I) to Re(II). The potential for the oxidation
was found to follow the order: 4 (+1.81 V) ª 3 (+1.80 V) > 2
(+1.75 V) > 1 (+1.72 V), in line with the p-accepting ability of
Electrochemical studies
Complexes 1–5 displayed two quasi-reversible oxidation couples
at +0.67 to +0.88 V and +1.72 to +1.81 V in the oxidative
scan of the cyclic voltammograms in acetonitrile (0.1 mol dm-3
nBu4NPF6), while one quasi-reversible reduction couple and one
to two irreversible reduction waves at -0.58 to -1.78 V vs. SCE
were observed in the reductive scan. Complex 5 also showed an
irreversible oxidation wave at ca. +0.96 V. The electrochemical data
for the free ligands and their rhenium(I) complexes are summarised
in Table 4, and the representative cyclic voltammograms of 1 are
shown in Fig. 6.
t
the diimine ligands: NO2phen ≥ Br2phen > phen > Bu2bpy. In
general, the better the p-accepting ability of the diimine ligand,
the more stable the dp(Re) orbital is, and hence, the more difficult
the oxidation process becomes. Similar oxidation couples ascribed
to a Re(I)/Re(II) oxidation have been reported in other related
systems, such as [Re(CO)3(N–N)(py)].14
The first two reductions were tentatively assigned to diimine
ligand-centred reductions, as they are commonly observed in
related systems.14 The trends of these reduction potentials are
also in line with the electron-richness and p-accepting ability of
the diimine ligands as discussed above, but with a much stronger
dependence when compared to the metal-centred oxidation.
The insensitivity of the change of the diimine ligands of the first
oxidative couple in complexes 1–4, as well as the close resemblance
of the potential of the first irreversible oxidation wave with that
of the free ligands, suggested that the first oxidative couple of 1–4
Table 4 Electrochemical data for ligands and their complexesa in ace-
tonitrile solution (0.1 mol dm-3 nBu4NPF6) at 298 K
b
b
1
1
Oxidation E /V vs.
Reduction E /V vs.
Conclusion
2
2
Compound
SCE (Epa/V vs. SCE)
SCE (Epc/V vs. SCE)
A series of photochromic leucotriarylmethane-containing ligands
and their rhenium(I) tricarbonyl diimine complexes were suc-
cessfully synthesised, and their photophysical and photochromic
properties were studied. Except for 4, all complexes displayed
MLCT phosphorescence in CH2Cl2 solution and in a 77 K EtOH–
MeOH glass medium. Upon UV excitation at 313 nm, all of
the ligands and complexes underwent photo-ionisation in CH2Cl2
solution. In DMF solution, the triarylmethane cations of all of the
ligands and complexes underwent the thermal backward reaction.
The half-life of the first order decay of the triarylmethane cation
L1
L2
1
(+0.99)
(-2.49)
(+0.76), (+0.97)
+0.88, +1.72
+0.88, +1.75
+0.88, +1.80
+0.88, +1.81
+0.67, (+0.96), +1.81
(-2.29)
-1.26, (-1.60)
-1.15, (-1.52), (-1.78)
-0.97, (-1.57)
-0.58, -1.13
-0.97, (-1.57)
2
3
4
5
a Working electrode, glassy carbon; scan rate, 100 mVs-1. b
from (Epa + Epc)/2; Epa and Epc are peak anodic and peak cathodic
potentials, respectively.
E
is estimated
1
2
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of L1 in DMF solution was found to be 360 s at 16 ◦C. The
electrochemistry of these ligands and complexes was also studied.
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The work described in this paper was supported by a RGC GRF
grant from the Research Grants Council of the Hong Kong
(Project No. CityU 101008), the Special Equipment Grant from
the University Grants Committee of the Hong Kong (SEG_City
U02) and a grant from City University of Hong Kong (Project
No. 7002307). H. Feng acknowledges the receipt of a Postgraduate
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6482 | Dalton Trans., 2010, 39, 6475–6482
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