Synthesis, photophysical, photochemical and electrochemical properties of rhenium(I) diimine complexes with photoisomerizable pyridyl-azo, -ethenyl or -ethyl ligands

Article abstract of DOI:10.1039/a801575i

A series of rhenium(I) diimine complexes with pyridyl-azo, -ethenyl or -ethyl type ligands, [Re(CO)₃(N-N)L]ClO₄ and [{Re(CO)₃(N-N)}₂L′][ClO₄]₂ {N-N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen); L = 4-phenylazopyridine (PHAZO), 4-styrylpyridine (STYPY), [2-(4-pyridyl)ethyl]benzene (PEB), 4-(4-nitrostyryl)pyridine (NSP); L′ = 4,4′-azopyridine (AZO), 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPENB), 1,4-bis[2-(4-pyridyl)ethyl]benzene (BPEB)} have been synthesized and their photophysical properties studied. The quantum yields for the photoinduced isomerization of the complexes [Re(CO)₃(bpy)(NSP)]⁺ and [Re(CO)₃(phen)(NSP)]⁺ have been determined.

Full text of DOI:10.1039/a801575i

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Synthesis, photophysical, photochemical and electrochemical
properties of rhenium(I) diimine complexes with photoisomerizable
pyridyl-azo, -ethenyl or -ethyl ligands
Vivian Wing-Wah Yam,* Victor Chor-Yue Lau and Li-Xin Wu
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
A series of rhenium() diimine complexes with pyridyl-azo, -ethenyl or -ethyl type ligands, [Re(CO)3(N᎐N)L]ClO4
and [{Re(CO)3(N᎐N)}2LЈ][ClO4]2 {N᎐N = 2,2Ј-bipyridine (bpy), 1,10-phenanthroline (phen); L = 4-phenylazo-
pyridine (PHAZO), 4-styrylpyridine (STYPY), [2-(4-pyridyl)ethyl]benzene (PEB), 4-(4-nitrostyryl)pyridine (NSP);
LЈ = 4,4Ј-azopyridine (AZO), 1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPENB), 1,4-bis[2-(4-pyridyl)ethyl]benzene
(BPEB)} have been synthesized and their photophysical properties studied. The quantum yields for the
photoinduced isomerization of the complexes [Re(CO)3(bpy)(NSP)]ϩ and [Re(CO)3(phen)(NSP)]ϩ have
been determined.
N
N
N
N
N
N
Photochromic molecules and their photoisomerization reac-
tions have been the subject of extensive investigation in the
field of molecular photochemistry.1,2 Incorporation of a photo-
responsive component into a supramolecular structure can lead
to artificial photoresponsive species that may be quite valuable
as model systems for theoretical studies and as photochemical
molecular devices.1,2 The basic requirements for the design of
artificial photoresponsive supramolecular systems are as
follows: (a) a component of the supramolecular system must be
able to absorb light; (b) as a consequence of light excitation, the
chromophoric component (or another component) must
undergo a structural change; (c) such a molecular structural
change must cause a ‘functional’ change (that is a change in
some properties relevant to a function) in another component
or in the whole supramolecular structure.1,2 With the photo-
isomerizable properties of pyridyl-azo or -ethenyl type ligands
and our recent interests in the design of luminescent metal
complexes,3 it would be extremely interesting to extend our
studies to inorganic systems and supramolecular systems of
photoswitchable materials. In this paper, the synthesis, photo-
physics, photochemistry and electrochemistry of [Re(CO)3-
(N᎐N)L]ClO4 and [{Re(CO)3(N᎐N)}2LЈ][ClO4]2 {N᎐N =
2,2Ј-bipyridine (bpy), 1,10-phenanthroline (phen); L = 4-
phenylazopyridine (PHAZO), 4-styrylpyridine (STYPY), [2-(4-
pyridyl)ethyl]benzene (PEB), 4-(4-nitrostyryl)pyridine (NSP);
LЈ = 4,4Ј-azopyridine (AZO), 1,4-bis[2-(4-pyridyl)ethenyl]-
benzene (BPENB), 1,4-bis[2-(4-pyridyl)ethyl]benzene (BPEB)}
are described; some of which have been previously commun-
icated.3a,b The structures of these ligands are shown in Scheme 1.
The quantum yields for the photoinduced isomerization of the
complexes [Re(CO)3(bpy)(NSP)]ϩ and [Re(CO)3(phen)(NSP)]ϩ
have been determined.
N
N
N
N
NO2
AZO
PHAZO
N
STYPY
NSP
PEB
N
N
N
BPENB
BPEB
Scheme 1 The schematic drawings of the pyridyl-azo, -ethenyl and
-ethyl ligands
Physical measurements
The UV/VIS absorption spectra were obtained on a Hewlett
Packard 8452A diode array spectrophotometer, IR spectra as
Nujol mulls on a Bio-Rad FTS-7 Fourier-transform infrared
spectrophotometer (4000–400 cmϪ1), and steady-state excit-
ation and emission spectra on a Spex Fluorolog 111 spectro-
fluorometer equipped with a Hamamatsu R-928 photomultiplier
tube. Low-temperature (77 K) spectra were recorded by using
an optical Dewar sample holder. A narrow slit width was
selected for all emission measurements, in particular those of
complexes 1–9 which contain a photoisomerizable moiety. The
complexes were found to be relatively stable and showed
Experimental
Materials
The compound [Re(CO)5Cl] was purchased from Strem
Chemicals Inc. and used as received. 2,2Ј-Bipyridine, 1,10-
phenanthroline, silver trifluoromethanesulfonate Ag(OTf) and
substituted pyridines which were used for subsequent ligand
synthesis were obtained from Aldrich Chemical Co. Ltd. All
solvents for synthesis were of analytical grade and were used
without further purification.
J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468
1461

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no significant UV/VIS spectral change before and after the
measurements were taken. Proton NMR spectra were recorded
either on a JEOL JNM-GSX270 (270 MHz) or a Bruker
DPX300 (300 MHz) Fourier-transform NMR spectrometer.
Electron-impact (EI) and positive-ion FAB mass spectra were
recorded on a Finnigan MAT95 mass spectrometer. Elemental
analyses of the newly synthesized complexes were performed
either by Butterworth Laboratories Ltd. or the Institute of
Chemistry, Chinese Academy of Sciences.
Emission-lifetime measurements were performed using a
conventional laser system. The excitation source was the 355
nm output (third harmonic) of a Quanta-Ray Q-switched
GCR-150-10 pulsed Nd-YAG laser. Luminescence decay
signals were recorded on a Tektronix model TDS-620A (500
MHz) digital oscilloscope, and analyzed using a program for
exponential fits. All solutions for photophysical studies were
prepared under vacuum in a round-bottomed flask (10 ml)
equipped with a side-arm fluorescence cuvette (1 cm) and sealed
from the atmosphere by a Kontes quick-release Teflon stopper.
Solutions were rigorously degassed with no fewer than four
freeze–pump–thaw cycles.
benzene in degassed tetrahydrofuran was added at 0 ЊC under
nitrogen, and allowed to reflux overnight. Any unreacted 4-
methylpyridine was distilled off. The reaction mixture was
then poured into an ice–water mixture, and extracted with
diethyl ether (3 × 10 ml). The ethereal layer was dried over
anhydrous MgSO4, filtered and evaporated under vacuum.
The solid residue was then purified by column chromato-
graphy on silica gel with diethyl ether as eluent. The first
band was the unreacted 1,4-bis(bromomethyl)benzene, while
the second band was the desired product. Yield 1.53 g, 45%. 1H
NMR [270 MHz, (CD3)2CO, 298 K, relative to SiMe4]: δ 2.91
(8 H, s, CH2CH2 of BPEB), 7.05–7.27 (8 H, br, phenyl H and
pyridyl H meta to N), 8.44–8.49 (4 H, d, pyridyl H ortho to N).
Positive-ion EI-MS: ion clusters at m/z 287 {M}ϩ (Found: C,
83.05; H, 7.01; N, 9.39. Calc. for C20H20N2: C, 83.33; H, 6.94;
N, 9.72%).
[2-(4-Pyridyl)ethyl]benzene PEB. The procedure was similar
to that of BPEB except 1 equivalent of benzyl bromide was used
instead of 1,4-bis(bromomethyl)benzene. Yield 1.51 g, 70%. 1H
NMR (270 MHz, CDCl3, 298 K, relative to SiMe4): δ 2.92 (4 H,
s, CH2CH2 of PEB), 7.04–7.10 (2 H, dd, pyridyl H meta to N),
7.13–7.27 (5 H, m, phenyl H), 8.44–8.51 (2 H, dd, pyridyl H
ortho to N). Positive-ion EI-MS: ion clusters at m/z 183 {M}ϩ
(Found: C, 85.23; H, 7.23; N, 7.45. Calc. for C13H13N: C, 85.25;
H, 7.10; N, 7.65%).
Cyclic voltammetric measurements were performed by using
a CH Instrument Inc., model CHI 620 Electroanalytical
Analyzer, interfaced to an IBM-compatible personal computer.
The ferrocenium–ferrocene couple was used as the internal
reference in the electrochemical measurements in acetonitrile
(0.1  Bun NPF6). The working electrode was a glassy carbon
4
(Atomergic Chemetals V25) electrode, with a Ag–AgNO3 (0.1 
in MeCN) electrode as the reference electrode and a platinum
foil acting as the counter electrode.
Chemical actinometry was employed for the photochemical
quantum yield determination.4 Incident light intensities were
taken from the average values measured just before and after
each photolysis experiment using ferrioxalate actinometry.4
The time of irradiation was chosen to monitor 10% or less of
the photolysis reaction in order to minimize any effects arising
from secondary photolysis.
Synthesis of rhenium complexes
The rhenium complexes [Re(CO)3(N᎐N)Cl] and [Re(CO)3-
(N᎐N)(MeCN)]OTf (N᎐N = bpy, phen) were prepared by
literature methods.9,10
General synthesis of [Re(CO)3(N᎐N)L]ClO4. This was syn-
thesized by modification of a reported procedure.9m,10j–l A mix-
ture of [Re(CO)3(N᎐N)(MeCN)]OTf (1 equivalent) and ligand
L (1.5 equivalents) in tetrahydrofuran was heated to reflux
under an inert atmosphere of nitrogen for 12 h. The resulting
solution was allowed to cool, and evaporated to dryness under
vacuum to give a bright yellow-orange residue, which was
Synthesis of pyridyl-azo, -ethenyl or -ethyl type ligands
The ligands AZO,5 PHAZO,5 STYPY6 and BPENB6 were
synthesized according to literature procedures.
Ϫ
then dissolved in methanol and metathesized to its ClO4 salt
upon addition of a saturated methanolic solution of LiClO4.
The yellow-orange solid obtained was recrystallized by vapor
diffusion of diethyl ether into an acetonitrile solution of the
product. Yields of the product ranged from 75 to 95%.
4-(4-Nitrostyryl)pyridine NSP. This was prepared by modifi-
cation of a procedure reported by McCleverty and co-workers.7
A mixture of 4-ethenylpyridine (1.97 g, 18.8 mmol), 4-
bromonitrobenzene (3.03 g, 15 mmol), palladium() acetate
(34 mg, 0.15 mmol), triethylamine (1.90 g, 18.8 mmol) and PPh3
(79 mg, 0.3 mmol) was sealed in a Schlenk tube under N2, and
heated to 100 ЊC for 3 d. The resulting solid was partitioned
between dichloromethane and water. The organic layer was
separated, dried over anhydrous MgSO4, filtered and evapor-
ated to dryness, and the crude product was recrystallized from
General synthesis of [{Re(CO)3(N᎐N)}2LЈ][ClO4]2. This was
similar to the procedure for the synthesis of [Re(CO)3(N᎐N)-
L]ClO4 except 0.5 equivalent of LЈ was used in place of 1.5
equivalents of L.9m,10j–l
[{Re(CO)3(bpy)}2(AZO)][ClO4]2 1. Orange solid, yield 80%.
1H NMR [270 MHz, (CD3)2CO, 298 K, relative to SiMe4]:
δ 7.80–7.83 (2 H, dd, pyridyl H meta to N), 7.95–8.00 (2 H, td,
5,5Ј-H of bpy), 8.05–8.10 (2 H, td, 4,4Ј-H of bpy), 8.50–8.53
(2 H, d, 3,3Ј-H of bpy), 8.84–8.87 (2 H, d, pyridyl H ortho to
N), 9.21–9.25 (2 H, td, 6,6Ј-H of bpy). IR (Nujol mull, cmϪ1):
ν(CO) 1922, 1938, 2027. Positive-ion FAB-MS: ion clusters at
m/z 1138 {MؒClO4}ϩ, 1038 {M}ϩ, 611 {M Ϫ Re(CO)3(bpy)}ϩ,
427 {Re(CO)3(bpy)}ϩ (Found: C, 33.01; H, 2.01; N, 8.18.
Calc. for C36H24Cl2N8O14Re2ؒ1.5CH2Cl2: C, 33.02; H, 1.98; N,
8.22%).
1
acetone. Yield 2.37 g, 70%. H NMR (300 MHz, CDCl3, 298
K, relative to SiMe4): δ 7.08–7.10, 7.20–7.26 (2 H, d, vinyl H,
JHH(trans) = 16 Hz), 7.40–7.41 (2 H, d, pyridyl H meta to the N),
7.70–7.77, 8.23–8.35 (4 H, dd, phenyl H), 8.64–8.66 (2 H, dd,
pyridyl H ortho to N). Positive-ion EI-MS: ion clusters at m/z
226 {M}ϩ (Found: C, 68.73; H, 4.16; N, 12.31. Calc. for
C13H10N2O2: C, 69.03; H, 4.42; N, 12.39%).
1,4-Bis[2-(4-pyridyl)ethyl]benzene BPEB. The ligand was syn-
thesized by modification of a literature method reported by
Osuch and Levine.8 4-Methylpyridine (0.93 g, 1 ml, 11.8 mmol)
was dissolved in degassed tetrahydrofuran, and methyllithium
(7.3 ml of 1.4  solution in hexane) was added to the solution
mixture at 0 ЊC under an inert atmosphere of nitrogen, during
which the colorless solution turned orange-red. The solution
was then allowed to reflux for 2 h. The reaction mixture was
allowed to cool down, and a solution of 1,4-bis(bromomethyl)-
[Re(CO)3(bpy)(PHAZO)]ClO4 2. Orange crystalline solid,
yield 82%. H NMR [270 MHz, (CD3)2CO, 298 K, relative to
1
SiMe4]: δ 7.60–7.70 (5 H, m, 2,4,6-H of phenyl ring and pyridyl
H meta to N), 7.78–7.80 (2 H, dd, pyridyl H ortho to N), 7.90–
8.00 (2 H, dd, 3,5-H of phenyl ring), 8.05–8.10 (2 H, td, 5,5Ј-H
of bpy), 8.45–8.55 (2 H, 4,4Ј-H of bpy), 8.78–8.84 (4 H, td, 3,3Ј-
H of bpy), 9.50–9.55 (2 H, dd, 6,6Ј-H of bpy). IR (Nujol mull,
cmϪ1): ν(CO) 1911, 1941, 2031. Positive-ion FAB-MS: ion
clusters at m/z 610 {M}ϩ, 427 {M Ϫ PHAZO}ϩ (Found: C,
1462
J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468

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39.58; H, 2.49; N, 9.44. Calc. for C24H17ClN5O7ReؒH2O: C,
39.63; H, 2.61; N, 9.63%).
solid, yield 85%. 1H NMR (270 MHz, CD3CN, 298 K, relative
to SiMe4): δ 7.09–7.15 (2 H, d, vinyl-H of BPENB), 7.43–7.48
(6 H, 2d, d vinyl-H of BPENB and d pyridyl-H meta to N),
7.55 (4 H, s, phenyl H of BPENB), 7.97–8.37 (8 H, m, 3,8- and
5,6-H of phen), 8.45–8.48 (4 H, d, pyridyl H ortho to N),
9.06–9.10 (4 H, dd, 4,7-H of phen), 9.88–9.91 (4 H, dd, 2,9-H
of phen). IR (Nujol mull, cmϪ1): ν(CO) 1909, 1933, 2032.
Positive-ion FAB-MS: ion clusters at m/z 1285 {MؒClO4}ϩ,
735 {M Ϫ Re(CO)3(phen)}ϩ, 427 {M Ϫ Re(CO)3(phen) Ϫ
BPENB}ϩ (Found: C, 42.20; H, 2.32; N, 5.79. Calc. for
C48H32Cl2N6O14Re2ؒ0.5CH2Cl2: C, 42.44; H, 2.38; N, 5.88%).
[Re(CO)3(bpy)(PEB)]ClO4 10. Yellow crystalline solid, yield
90%. 1H NMR [270 MHz, (CD3)2CO, 298 K, relative to SiMe4]:
δ 2.66–2.81 (4 H, m, CH2CH2 of PEB), 6.93–7.00 (4 H, m,
pyridyl H meta to N and 3-H of phenyl ring of PEB), 7.10–7.14
(3 H, m, 2,4-H of phenyl ring of PEB), 7.75–7.80 (2 H, td, 5,5Ј-
H of bpy), 8.02–8.05 (2 H, dd, pyridyl H ortho to N), 8.23–8.29
(2 H, td, 4,4Ј-H of bpy), 8.30–8.40 (2 H, d, 3,3Ј-H of bpy),
9.18–9.21 (2 H, dd, 6,6Ј-H of bpy). IR (Nujol mull, cmϪ1):
ν(CO) 1923, 1940, 2027. Positive-ion FAB-MS: ion clusters at
m/z 610 {M}ϩ, 427 {M Ϫ PEB}ϩ (Found: C, 43.42; H, 2.90; N,
5.83. Calc. for C26H21ClN3O7Reؒ0.5H2O: C, 43.45; H, 3.06;
N, 5.85%).
[Re(CO)3(phen)(PHAZO)]ClO4 3. Orange crystalline solid,
1
yield 88%. H NMR [270 MHz, (CD3)2CO, 298 K, relative to
SiMe4]: δ 7.81–7.85 (2 H, d, pyridyl H meta to N), 7.40–7.50
(5 H, m, phenyl H of PHAZO), 8.35–8.40 (4 H, dd ϩ s, dd
3,8-H of phen, s 5,6-H of phen), 8.55–8.60 (2 H, dd, 4,7-H of
phen), 9.08–9.11 (2 H, dd, 2,9-H of phen), 9.12–9.15 (2 H, dd,
pyridyl H ortho to N). IR (Nujol mull, cmϪ1): ν(CO) 1907, 1945,
2030. Positive-ion FAB-MS: ion clusters at m/z 634 {M}ϩ, 451
{M Ϫ PHAZO}ϩ (Found: C, 40.76; H, 2.19; N, 9.25. Calc. for
C26H17ClN5O7Reؒ0.5CH2Cl2: C, 40.98; H, 2.19; N, 9.02%).
[Re(CO)3(bpy)(STYPY)]ClO4 4. Yellow crystalline solid,
1
yield 88%. H NMR (270 MHz, CD3CN, 298 K, relative to
SiMe4): δ 7.03–7.04 (1 H, d, vinyl H), 7.40–7.44 (3 H, m, aryl H
and pyridyl H meta to N), 7.49–7.56 (1 H, d, vinyl H), 7.58–7.60
(2 H, m, aryl H), 7.75–7.82 (2 H, td, 5,5Ј-H of bpy), 8.12–8.15
(2 H, d, pyridyl H ortho to N), 8.27–8.30 (2 H, td, 4,4Ј-H of
bpy), 8.41–8.44 (2 H, d, 3,3Ј-H of bpy), 9.22–9.24 (2 H, d,
6,6Ј-H of bpy). IR (Nujol mull, cmϪ1): ν(CO) 1920, 1943, 2027.
Positive-ion FAB-MS: ion clusters at m/z 608 {M}ϩ, 427
{M Ϫ STYPY}ϩ (Found: C, 43.29; H, 2.56; N, 5.89. Calc. for
C26H19ClN3O7Reؒ0.5H2O: C, 43.57; H, 2.79; N, 5.86%).
[Re(CO)3(phen)(STYPY)]ClO4 5. Yellow crystalline solid,
yield 90%. H NMR [270 MHz, (CD3)2CO, 298 K, relative to
[Re(CO)3(phen)(PEB)]ClO4 11. Yellow crystalline solid, yield
92%. 1H NMR [270 MHz, (CD3)2CO, 298 K, relative to SiMe4]:
δ 2.66–2.81 (4 H, m, CH2CH2 of PEB), 6.93–7.00 (4 H, m,
pyridyl H meta to N and 3-H of phenyl ring of PEB), 7.10–7.14
(3 H, m, 2,4-H of phenyl ring of PEB), 8.07–8.10 (4 H, d ϩ dd,
d pyridyl H ortho to N, dd 3,8-H of phen), 8.18 (2 H, s, 5,6-H of
phen), 8.82–8.86 (2 H, dd, 4,7-H of phen), 9.58–9.60 (2 H, dd,
2,9-H of phen). IR (Nujol mull, cmϪ1): ν(CO) 1922, 1942, 2027.
Positive-ion FAB-MS: ion clusters at m/z 634 {M}ϩ, 451
{M Ϫ PEB}ϩ (Found: C, 45.59; H, 2.76; N, 5.68. Calc. for
C28H21ClN3O7Re: C, 45.86; H, 2.87; N, 5.73%).
1
SiMe4]: δ 6.94–6.97 (1 H, d, vinyl-H of STYPY), 7.25–7.27
(2 H, dd, pyridyl H meta to N), 7.33–7.58 (5 H, m, phenyl H of
STYPY), 8.01–8.04 (2 H, dd, pyridyl H ortho to N), 8.10–8.24
(4 H, m, 3,8- and 5,6-H of phen), 8.83–8.86 (2 H, dd, 4,7-H of
phen), 9.61–9.63 (2 H, dd, 2,9-H of phen). IR (Nujol mull,
cmϪ1): ν(CO) 1900, 1939, 2030. Positive-ion FAB-MS: ion
clusters at m/z 632 {M}ϩ, 451 {M Ϫ STYPY}ϩ (Found: C,
45.52; H, 2.54; N, 5.51. Calc. for C28H19ClN3O7Reؒ0.5H2O: C,
45.40; H, 2.70; N, 5.68%).
[Re(CO)3(bpy)(NSP)]ClO4 6. Yellow solid, yield 88%. 1H
NMR (300 MHz, CD3CN, 298 K, relative to SiMe4): δ 7.21–7.26
(1 H, d, vinyl H of NSP), 7.40–7.42 (2 H, d, pyridyl H meta to
N), 7.49–7.55 (1 H, d, vinyl H ortho to N), 7.74–7.79 (4 H,
d ϩ t, d phenyl H of NSP and t 5,5Ј-H of bpy), 8.18–8.29 (6 H,
2d ϩ t, one d phenyl H of NSP, one d pyridyl H ortho to N and
t 4,4Ј-H of bpy), 8.36–8.39 (2 H, d, 3,3Ј-H of bpy), 9.22–9.24
(2 H, d, 6,6Ј-H of bpy). IR (Nujol mull, cmϪ1): ν(CO) 1907,
1922, 2027. Positive-ion FAB-MS: ion clusters at m/z 659
{M}ϩ, 427 {M Ϫ NSP}ϩ (Found: C, 40.48; H, 2.23; N, 7.38.
Calc. for C26H18ClN4O9Reؒ0.5H2O: C, 40.98; H, 2.49; N,
7.36%).
[{Re(CO)3(bpy)}2(BPEB)][ClO4]2 12. Yellow crystalline solid,
yield 95%. H NMR [270 MHz, (CD3)2CO, 298 K, relative to
1
SiMe4]: δ 2.50–2.82 (8 H, m, CH2CH2 of BPEB), 6.82 (4 H, s,
phenyl H of BPEB), 6.96–6.99 (4 H, d, pyridyl H meta to N),
7.92–7.98 (8 H, td, 5,5Ј-H of bpy), 8.30–8.39 (4 H, d, pyridyl H
ortho to N), 8.43–8.45 (4 H, td, 4,4Ј-H of bpy), 8.65–8.68 (4 H,
d, 3,3Ј-H of bpy), 9.40–9.42 (4 H, dd, 6,6Ј-H of bpy). IR (Nujol
mull, cmϪ1): ν(CO) 1922, 1935, 2027. Positive-ion FAB-
MS: ion clusters at m/z 1283 {MؒClO4ؒCH3CN}ϩ, 714
{M Ϫ Re(CO)3(bpy)}ϩ, 427 {M Ϫ Re(CO)3(bpy) Ϫ BPEB}ϩ
(Found: C, 41.00; H, 2.57; N, 6.23. Calc. for C46H34Cl2-
N6O14Re2: C, 41.27; H, 2.54; N, 6.28%).
[Re(CO)3(phen)(NSP)]ClO4 7. Yellow crystalline solid, yield
90%. 1H NMR [270 MHz, (CD3)2CO, 298 K, relative to SiMe4]:
δ 7.30–7.40, 7.60–7.70 (2 H, d, vinyl H of NSP), 7.51–7.55 (2 H,
d, pyridyl H meta to N), 7.80–7.88, 8.20–8.26 (4 H, d, phenyl H
of NSP), 8.35–8.40 (4 H, dd ϩ s, dd 3,8-H of phen, s 5,6-H of
phen), 8.55–8.60 (2 H, dd, pyridyl H ortho to N), 9.08–9.11
(2 H, dd, 4,7-H of phen), 9.92–9.95 (2 H, dd, 2,9-H of phen).
IR (Nujol mull, cmϪ1): ν(CO) 1902, 1934, 2026. Positive-ion
FAB-MS: ion clusters at m/z 677 {M}ϩ, 451 {M Ϫ NSP}ϩ
(Found: C, 38.92; H, 2.11; N, 6.62. Calc. for C28H18ClN4-
O9Reؒ1.5CH2Cl2: C, 39.16; H, 2.32; N, 6.20%).
[{Re(CO)3(bpy)}2(BPENB)][ClO4]2 8. Yellow crystalline
solid, yield 77%. 1H NMR (270 MHz, CD3CN, 298 K, relative
to SiMe4): δ 7.09–7.15 (2 H, d, vinyl-H of BPENB), 7.43–7.48
(6 H, 2d, d vinyl-H of BPENB and d pyridyl-H meta to N), 7.55
(4 H, s, phenyl H of BPENB), 8.05–8.10 (2 H, td, 5,5Ј-H of
bpy), 8.45–8.55 (2 H, 4,4Ј-H of bpy), 8.78–8.84 (4 H, td, 3,3Ј-H
of bpy), 9.50–9.55 (2 H, dd, 6,6Ј-H of bpy). IR (Nujol mull,
cmϪ1): ν(CO) 1922, 1950, 2027. Positive-ion FAB-MS: ion
clusters at m/z 1237 {MؒClO4}ϩ, 711 {M Ϫ Re(CO)3(bpy)}ϩ,
427 {M Ϫ Re(CO)3(bpy) Ϫ BPENB}ϩ (Found: C, 40.65; H,
2.38; N, 6.16. Calc. for C46H32Cl2N6O14Re2ؒH2O: C, 40.76; H,
2.51; N, 6.20%).
[{Re(CO)3(phen)}2(BPEB)][ClO4]2 13. Yellow crystalline
solid, yield 90%. H NMR [270 MHz, (CD3)2CO, 298 K, rel-
1
ative to SiMe4]: δ 2.50–2.82 (8 H, m, CH2CH2 of BPEB), 6.82
(4 H, s, phenyl H of BPEB), 6.96–6.99 (4 H, dd, pyridyl H meta
to N), 8.02–8.14 (8 H, 2dd, dd pyridyl H ortho to N and dd
3,8-H of phen), 8.16 (4 H, s, 5,6-H of phen), 8.81–8.85 (4 H, dd,
4,7-H of phen), 9.58–9.60 (4 H, dd, 2,9-H of phen). IR (Nujol
mull, cmϪ1): ν(CO) 1916, 1945, 2030. Positive-ion FAB-MS: ion
clusters at m/z 1287 {MؒClO4}ϩ, 738 {M Ϫ Re(CO)3(phen)}ϩ,
451 [M Ϫ Re(CO)3(phen) Ϫ BPEB}ϩ (Found: C, 42.43; H,
2.84; N, 5.72. Calc. for C48H34Cl2N6O14Re2ؒ0.5CH2Cl2: C, 42.38;
H, 2.59; N, 5.87%).
Results and Discussion
The complexes [Re(CO)3(N᎐N)L]ClO4 and [{Re(CO)3(N᎐N)}2-
LЈ][ClO4]2 were prepared by refluxing [Re(CO)3(N᎐N)(Me-
CN)]OTf (1 equivalent) and the corresponding ligand L (1.5
equivalents) or LЈ (0.5 equivalent) in dry THF under an inert
atmosphere of nitrogen, respectively. After allowing the reac-
tion mixture to heat to reflux overnight, the solvent was
removed under vacuum, and the solid residue was dissolved in
methanol, filtered and the filtrate was reduced in volume, and
metathesized to the perchlorate salt. Subsequent recrystalliz-
[{Re(CO)3(phen)}2(BPENB)][ClO4]2 9. Yellow crystalline
J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468
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Table 1 Photophysical data of rhenium() diimine complexes
Complex
Solvent (T/K)
Absorption λmax/nm (ε/Ϫ1 cmϪ1
)
Emission λem/nm(τ0/µs)
1
2
[{Re(CO)3(bpy)}2(AZO)][ClO4]2
[Re(CO)3(bpy)(PHAZO)]ClO4
CH2Cl2 (298)
CH2Cl2 (298)
250 (10 100), 272 (9970), 321 (6540), 366 (4090)
300 (sh) (25 400), 315 (40 000), 321 (sh) (9400), 344
(3350)
232 (16 100), 260 (11 000), 279 (13 000), 300 (9400),
343 (11 800), 450 (sh) (5000)
288 (sh) (6000), 324 (9550), 340 (9330)
240 (20 200), 266 (sh) (17 100), 312 (28 400), 320 (sh)
(33 700), 334 (29 100)
535 (1.04)
550 (0.50)
3
4
[Re(CO)3(phen)(PHAZO)]ClO4
[Re(CO)3(bpy)(STYPY)]ClO4
CH2Cl2 (298)
563 (0.2, 1.5)a
CH2Cl2 (298)
CH3CN (298)
530 (0.63)
540 (0.46)
5
6
[Re(CO)3(phen)(STYPY)]ClO4
[Re(CO)3(bpy)(NSP)]ClO4
CH2Cl2 (298)
CH3CN (298)
276 (24 200), 338 (24 500), 400 (sh) (2910)
242 (25 700), 282 (sh) (26 700), 312 (sh) (32 400), 320
(38 200), 346 (36 800)
258 (sh) (21 500), 278 (26 100), 294 (sh) (20 500), 348
(34 000)
230 (sh) (32 000), 254 (sh) (19 000), 278 (26 100), 346
(33 000)
244 (54 200), 276 (41 700), 310 (43 300), 320 (49 300),
360 (43 300)
548 (2.3)
540b
7
[Re(CO)3(phen)(NSP)]ClO4
CH2Cl2 (298)
CH3CN (298)
CH3CN (298)
550b
550b
8
9
[{Re(CO)3(bpy)}2(BPENB)][ClO4]2
415 (<0.1), 556 (0.24)
423 (<0.1), 540 (2.0)
[{Re(CO)3(phen)}2(BPENB)][ClO4]2 CH3CN (298)
224 (96 200), 252 (64 200), 276 (67 100), 300 (sh)
(35 900), 350 (sh) (59 800), 384 (94 300), 406 (sh)
(66 000)
10
11
[Re(CO)3(bpy)(PEB)]ClO4
[Re(CO)3(phen)(PEB)]ClO4
CH3CN (298)
249 (19 200), 263 (18 900), 305 (11 200), 319 (12 000),
348 (3530)
540 (0.23)
Solid (298)
Solid (77)
CH2Cl2 (298)
510 (1.5)
500
550 (2.0)
259 (27 500), 276 (30 000), 294 (sh) (16 300), 334 (sh)
(5340), 368 (sh) (4310)
CH3CN (298)
230 (sh) (11 800), 276 (9600), 308 (sh) (7400), 322
(8290), 363 (sh) (3550)
571 (1.4)
Solid (298)
Solid (77)
CH3CN (298)
515 (1.9)
500
586 (0.2)
12
13
[{Re(CO)3(bpy)}2(BPEB)][ClO4]2
[{Re(CO)3(phen)}2(BPEB)][ClO4]2
249 (38 300), 263 (37 400), 305 (21 700), 319 (22 900),
347 (6710)
Solid (298)
Solid (77)
CH2Cl2 (298)
508 (1.2)
508
550 (2.5)
259 (43 200), 275 (47 700), 295 (sh) (24 600), 329 (sh)
(8490), 383 (sh) (5830)
CH3CN (298)
258 (sh) (38 100), 276 (45 000), 327 (sh) (9350), 368
(sh) (6130)
570 (1.5)
Solid (298)
Solid (77)
557 (1.1)
540
a It is a biexponential decay. b As the complexes were rather light-sensitive as reflected by a change in the UV/VIS absorption spectra after laser-flash
excitation, no lifetime measurements were made.
Table 2 Electronic absorption spectral data of the pyridyl ligands in
ation from acetonitrile–diethyl ether or dichloromethane–
diethyl ether gave the analytically pure complexes. All the newly
synthesized complexes were characterized by 1H NMR and
IR spectroscopy, FAB-MS and gave satisfactory elemental
analyses. All complexes showed three ν(CO) stretches in the IR
spectra, typical of Re() diimine tricarbonyl complexes with the
carbonyls arranged in a facial configuration.9a–j
MeCN at 298 K
Compound
Absorption λmax/nm (ε/Ϫ1 cmϪ1
)
AZO
PHAZO
STYPY
252 (3830), 312 (7160), 460 (295)
228 (9470), 310 (23 360), 450 (265)
228 (21 160), 236 (sh) (16 340), 292 (31 300),
320 (sh) (15 435)
NSP
218 (sh) (14 345), 234 (10 865), 276 (17 755),
332 (30 875)
Electronic absorption spectroscopy
BPENB
238 (15 655), 246 (12 925), 334 (47 070), 350
(57 120), 370 (sh) (38 000)
The electronic absorption spectral data of the rhenium() com-
plexes are shown in Table 1 and those of the ligands are summar-
ized in Table 2. The electronic absorption spectra of complexes
1–5 and 13 in dichloromethane and complexes 6–12 in
acetonitrile all show an absorption band/shoulder at ca. 320–
400 nm at 298 K. With reference to previous spectroscopic work
on related rhenium() diimine systems,9,10 this absorption band
is most likely assigned as the dπ(Re) → π*(bpy/phen) metal-
to-ligand charge-transfer (MLCT) transition. However, with
complexes 1–9, closer scrutiny on their absorption spectra
showed that the band in this region has an absorption coefficient
of the order of 104, which is much larger than an order of 103
Ϫ1 cmϪ1 commonly observed for rhenium() diimine MLCT
transitions. Further examination of the free ligands associated
with complexes 1–9, that is the AZO, PHAZO, NSP, STYPY
and BPENB ligands, showed that they absorbed strongly in the
same region with absorption coefficients of the order of 104 Ϫ1
cmϪ1. Therefore it is likely that the lowest energy absorption
PEB
BPEB
258 (2145), 288 (sh) (230)
222 (14 920), 266 (14 270), 280 (13 050)
band in complexes 1–9 may consist of an admixture of intra-
ligand (IL) π → π* (L or LЈ) and MLCT character. On the
other hand, the electronic absorption spectra of complexes 10–
13 which do not contain the conjugated pyridyl ligands showed
low energy absorption bands with absorption coefficients of 103
Ϫ1 cmϪ1, typical of MLCT transitions of rhenium() diimine
systems. Moreover, for complexes 12 and 13, where the two
rhenium() centers were bridged by a non-conjugated BPEB
ligand which does not show any appreciable absorption beyond
300 nm, the absorption coefficients for the lowest energy
absorption band/shoulder are about twice that observed for the
respective mononuclear complexes 10 and 11. These findings
suggest that the lowest energy absorption band/shoulders in
1464
J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468

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complexes 10–13 are pure MLCT transitions. Similar observ-
ations have been reported in other related binuclear and
supramolecular Re() and Ru() polypyridine systems.9,10,11a,b
Emission spectroscopy
Excitation of the complexes at λ у 350 nm in fluid solution at
room temperature results in a yellowish-green luminescence
centered at ca. 510–580 nm. The detailed photophysical data
are summarized in Table 1. The large Stokes shift together with
the relatively long radiative lifetime observed are suggestive
of an emissive origin of triplet parentage, possibly derived
from the 3MLCT [dπ(Re) → π*(bpy/phen)] state. The
emission energies of the complexes are comparable to those
of other rhenium() tricarbonyl diimine systems, such as [Re-
10j
(CO)3(bpy)(4-Etpy)]ClO4 and [Re(CO)3(phen)(py)]ClO4,9n,o
Fig. 1 The UV/VIS spectral trace of complex 2 upon steady-state
suggesting the relative insensitivity of the emission energy to
the nature of the pyridyl ligand.11
photolysis at λ > 350 nm at 0, 125, 190, 250, 370, 490 and 790 min in
degassed CH2Cl2
The almost identical excited-state lifetimes and emission
maxima of the binuclear complexes 8, 9, 12 and 13 with their
respective mononuclear complexes 4, 5, 10 and 11 are suggest-
ive of weak to no interaction between the two rhenium centers,
typical of complexes containing weakly conjugated/non-
conjugated bridging ligands. It is interesting to note that for the
complexes which contain photoisomerizable bridging C᎐C
᎐
or N᎐N units, in particular, complexes 1–3, 6 and 7, their
᎐
radiative lifetimes are found to be shorter than those of com-
plexes 10–13. This may be rationalized by the presence of an
intramolecular excited-state energy transfer to the photo-
isomerizable unit to effect a trans–cis isomerization process, i.e.
the presence of an additional non-radiative decay pathway in
1–3, 6 and 7 as compared to complexes 10–13.
Photoisomerization studies
Spectroscopic measurements. With complexes 1–9 which con-
Fig. 2 The emission spectra of complex 2 before (––––––) and after
(———) photolysis
tained a photoisomerizable C᎐C or N᎐N unit, a photoinduced
᎐
᎐
trans–cis isomerization was observed upon visible light excit-
ation. Upon irradiation of a degassed dichloromethane solu-
tion of [Re(N᎐N)(CO)3L]ϩ or [{Re(N᎐N)(CO)3}2LЈ]2ϩ, where L
and LЈ contain an isomerizable pyridyl ligand, at λ > 350 nm,
UV/VIS spectral changes with a clean and well defined isos-
bestic point were observed. For example, a solution of com-
plex 2 gives a clean UV/VIS spectral trace with an isosbestic
point at ca. 310 nm (Fig. 1). The spectral changes were sug-
gested to result from the trans–cis isomerization of the azo unit
of the PHAZO ligand. To our surprise, the emission intensity
was shown to be greatly enhanced after steady-state photolysis
for 12 h, using a conventional 200 W xenon lamp as the light
source. The luminescence quantum yield (φlum) measured after
photolysis was ca. 40 times greater than that before photolysis,
and its emission maximum is red-shifted by ca. 20 nm from 550
nm to 570 nm [≈0.08 eV, eV ≈ 1.602 × 10Ϫ19 J] (Fig. 2). It is
well known that for the free azo chromophore the triplet-state
energy for the trans isomer is generally lower than that of the cis
counterpart as a result of the steric constraints in the cis isomer
which reduces the π electron delocalization.4b Thus it is likely
that in the 3MLCT excited state some of its excited-state energy
is intramolecularly transferred to the trans-azo moiety, provid-
ing an additional non-radiative deactivation pathway. On the
contrary, with the cis-azo ligand formed upon photolysis, such
intramolecular energy-transfer processes are highly disfavored
or blocked owing to the higher triplet-state energy of the cis
isomer, and hence resulting in a higher luminescence quantum
Fig. 3 The emission spectral traces of complex 7 upon irradiation at
λ = 330 nm in degassed CH2Cl2
compared to the azo analog,4b the steady-state photolysis
reaction of complex 4 was found to be extremely slow under
similar experimental conditions. However, upon replacement
of styrylpyridine with NSP, an enhancement in the efficiency of
the isomerization process was obtained.
Steady-state photolysis of complex 7 under similar experi-
mental conditions required ca. 4 h to give no further UV/VIS
spectral changes with an isosbestic point at ca. 300 nm and a
greater than 35 times increase in the luminescence quantum
yield. The emission traces resulting from the photoisomeriz-
ation of a degassed dichloromethane solution of complex 7
upon irradiation at λ = 366 nm are shown in Fig. 3. It is likely
that irradiation of the lowest energy absorption band which
yield [for complex 2, φlum(trans) = 7.2 × 10Ϫ4
; φlum(cis) =
0.027].3a,b
In order to further our understanding on the photoinduced
trans–cis isomerization processes in these complexes, a com-
parison of their photoreactivities with the bridging C᎐C ana-
᎐
log, i.e. the styrylpyridine ligand was made. As expected, due
to the relatively higher triplet-state energy of the ethylene moiety
J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468
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Table 3 Quantum yields for the photoisomerization processes of
compounds 6, 7 and NSP
Compound
Quantum yield, φtrans–cis
cis Isomer formed (%)
6
0.66
0.47
0.81
74
74
80
7
NSP
Fig. 4 Plots of the cis isomer of complexes 6, 7 and the NSP ligand
formed (%) against the time of irradiation at 366 nm
was assigned as an admixture of MLCT–IL transitions resulted
in an efficient trans–cis isomerization process.
Quantum yield measurements. In order to obtain a quanti-
tative measure of the photoinduced isomerization processes of
the rhenium() diimine complexes, the photochemical quantum
yield for the trans–cis isomerization process of the complexes
was determined using chemical actinometry. In the present
study, complexes with the styrylpyridine moiety were chosen
instead of those with 4-phenylazopyridine, as it contains a
Fig.
5
Transient absorption difference spectrum for the intra-
molecular energy-transfer process of complex 3 (25 m) recorded 10 µs
after laser flash
difference spectrum of 3 showed a ground-state bleaching at ca.
350–425 nm, while for both complexes 2 and 3 a strong absorp-
tion at ca. 450–700 nm was observed. The broad, featureless
transient absorption band of both complexes at ca. 600 nm was
tentatively assigned as the triplet-state absorption of the trans
PHAZO moiety. Similar assignments have been suggested by
Shaw and co-workers.11a,b The triplet-state absorption of trans-
4,4Ј-azopyridine has also been reported to occur at similar
regions.12 The observation of the triplet-state absorption of the
PHAZO moiety is indicative of an intramolecular energy-
transfer quenching mechanism.
CH᎐CH group which shows a characteristic 1H NMR coupling
᎐
constant in the trans (16 Hz) and cis (12 Hz) forms, and could
be conveniently used for monitoring the photoreaction. How-
ever, the photoreactions of the complexes with STYPY ligands
were extremely slow. In this context, the complexes with the
NSP ligand where an electron withdrawing NO2 group was
introduced to the 4-position of the phenyl ring, i.e. complexes 6
and 7, were employed in the studies, which were extremely
sensitive to light irradiation and showed effective photo-
isomerization. The photoreactions were monitored by 1H NMR
Intermolecular energy-transfer quenching. In order to provide
further support on the intramolecular energy-transfer process
occurring in the complexes such as complex 3, an inter-
molecular energy-transfer quenching study of the related com-
plex [Re(CO)3(phen)(py)]ClO4 by 4-phenylazopyridine and 4-
(4-nitrostyryl)pyridine, respectively, in acetonitrile was per-
formed. From the Stern–Volmer quenching plot, it was found
that the excited-state luminescence properties of [Re(CO)3-
(phen)(py)]ClO4 were quenched effectively by both PHAZO
and NSP. The diffusion-corrected quenching rate constants, kqЈ,
spectroscopy. Upon irradiation, the CH᎐CH signals for the
᎐
trans isomers which occurred at δ 7.29–7.35 and 7.58–7.64 (2 H,
JHH 16 Hz) for 6 and 7 respectively in CD3CN were found to
decrease gradually while at the same time the signals due to the
cis isomers which occurred at δ 6.47–6.51 and 6.80–6.90 (2 H,
JHH 12 Hz) (6 and 7 respectively) gradually built up in intensity.
1
From the H NMR results, 74% of both [Re(CO)3(bpy)(cis-
NSP)]ϩ and [Re(CO)3(phen)(cis-NSP)]ϩ were formed upon
irradiation while in the case of the pure ligand, NSP, 80% of
the cis isomer was formed. The plots of the percentage of cis
isomer formed or the percentage of trans isomer disappeared
against the time of irradiation for complexes 6, 7 and the ligand
NSP are depicted in Fig. 4. The quantum yields of the photo-
chemical reactions are summarized in Table 3 where an order
of NSP > 6 > 7 is observed. A control experiment to correct
for the presence of any thermal trans–cis isomerization reac-
tion was performed in each case, in which an equal amount
of the complex/compound was dissolved in the same amount
of degassed deuteriated acetonitrile and placed in the dark. A
real-time monitoring of the thermal control by 1H NMR spec-
troscopy was performed in each case, from which no observable
cis isomer was formed in the absence of light.
were found to be in the order of 8.5 × 109 and 4.7 × 109 Ϫ1 sϪ1
,
respectively. Based on the intermolecular quenching data, it
is envisaged that the corresponding intramolecular energy-
transfer quenching in complex 3 would be very facile, with rate
constants greater than that of 109 sϪ1
.
Electrochemical properties
Cyclic voltammetric data for the rhenium() complexes and
their related compounds9d,10c,h,j,n,o,13 are collected in Table 4.
All the complexes showed a characteristic quasireversible to
irreversible oxidation wave in the range of ϩ1.72 to ϩ1.78 V vs.
SCE. This oxidation wave is assigned as the oxidation of the
rhenium center, i.e. ReI → ReII, as similar findings have been
reported in related systems.13 The assignment of the oxidation
as ligand-centered oxidation is unlikely since both the diimine
ligand as well as the free pyridyl-azo, -ethenyl or -ethyl ligands
do not show any oxidation wave in this region.
Quenching studies
Excited-state absorption properties: transient absorption spec-
troscopic studies for intramolecular energy-transfer quenching.
The transient absorption spectrum of complex 3 in CH3CN at
room temperature is shown in Fig. 5. The transient absorption
However, for the reductive scan, a more complicated picture
is obtained. For the 2,2Ј-bipyridine systems, the cyclic voltam-
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J. Chem. Soc., Dalton Trans., 1998, Pages 1461–1468

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Acknowledgements
Table 4 Electrochemical data for rhenium() diimine complexes
V. W.-W. Y. acknowledges financial support from the Research
Grants Council and The University of Hong Kong. V. C.-Y. L.
acknowledges the receipt of both the Croucher Foundation
Studentship (1993–1995) and Scholarship (1995–1996), admin-
istered by the Croucher Foundation and a Sir Edward Youde
Memorial Fund Fellowship, administered by the Sir Edward
Youde Memorial Fund Council.
Complex/
compound
Reduction E or Epc a/V
Oxidation E a,b/V
₁
₁
₂
₂
vs. SCE (∆E/mV)
vs. SCE (∆E/mV)
1
2
3
4
6
Ϫ1.27 (90), Ϫ1.52,c Ϫ1.63,c
ϩ1.77 (91)
Ϫ1.73,c Ϫ2.14c
Ϫ0.75 (79), Ϫ1.11 (57),
Ϫ1.28 (67),d Ϫ1.83,c Ϫ2.05d
Ϫ0.75 (70), Ϫ1.07 (60),d
Ϫ1.27 (70),d Ϫ1.86,c Ϫ2.02c
Ϫ1.18 (53), Ϫ1.39 (98),d
Ϫ1.74,c Ϫ2.00,c Ϫ2.18c
Ϫ0.98 (67), Ϫ1.14 (70),
Ϫ1.29 (63),d Ϫ1.65,c Ϫ1.90,c
Ϫ2.02,c Ϫ2.16,c Ϫ2.24c
Ϫ0.96 (65), Ϫ1.13 (47),d
Ϫ1.27 (87),d Ϫ1.82,c Ϫ2.12,c
Ϫ2.20c
ϩ1.78 (60)
ϩ1.78 (60)
ϩ1.76 (113)
ϩ1.75 (96)
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7
ϩ1.72 (63)
8
9
Ϫ1.19 (30), Ϫ1.39 (94),d
Ϫ1.81,c Ϫ2.11,c Ϫ2.31c
Ϫ1.07 (197),d Ϫ1.44 (90),d
Ϫ1.83,c Ϫ2.06,c Ϫ2.30c
Ϫ1.18 (54), Ϫ1.40 (175)d
Ϫ1.19 (90), Ϫ1.51 (37)d
Ϫ1.21 (70), Ϫ1.81 (110)
Ϫ1.47 (90), Ϫ1.89 (180)
Ϫ1.58 (73), Ϫ1.85,c Ϫ2.02
(107)d
ϩ1.77 (84)
ϩ1.78 (54)
10
12
ϩ1.74 (88)
ϩ1.74 (62)
—
—
—
AZOe
PHAZOe
STYPY
NSP
BPENB
Ϫ1.05 (104),d Ϫ1.70c
—
—
Ϫ1.69 (120),d Ϫ1.94 (62),d
Ϫ2.13 (85)d
a All the electrochemical measurements were done in MeCN (0.1 
Bu NPF6); E = (Epa ϩ Epc)/2; Epa = anodic peak potential and Epc
cathodic peak potential; working electrode, glassy carbon; scan rate,
100 mV sϪ1; ∆E = |Epa Ϫ Epc| in mV. b All the oxidation waves are found
to be dependent on the scan rate, and are quasireversible. c Irreversible
reduction wave, the potential reported is Epc. d The reductive wave is
found to be dependent on the scan rate, and is quasireversible. e Taken
from ref. 14.
n
₁
=
4
₂
4 (a) C. G. Hatchard and C. A. Parker, Proc. R. Soc. London, Ser. A,
1956, 235, 518; (b) S. L. Murov, Handbook of Photochemistry,
Marcel Dekker, New York, 1973.
5 N. Campbell, A. W. Henderson and D. Taylor, J. Chem. Soc., 1953,
1281; A. Das, J. P. McCleverty, J. A. N. Badiola and M. D. Ward,
J. Chem. Soc., Dalton Trans., 1993, 681.
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mograms generally consist of a reversible–quasireversible
reduction wave at ca. Ϫ1.15 V vs. SCE with ∆E ≈ 60 mV and a
quasireversible to irreversible wave at ca. Ϫ1.35 V vs. SCE.
Based on previous work on related systems,9d,10c,h, j,n,o,13 the re-
duction couple at ca. Ϫ1.15 V is assigned as the reduction of
the 2,2Ј-bipyridine ligand, bpy0/Ϫ1. The reduction at ca. Ϫ1.35
V is assigned as the second reduction of the 2,2Ј-bipyridine,
i.e. bpyϪ1/Ϫ2, and this assignment has generally been supported
by the findings of Demas and co-workers9c and Meyer and
co-workers.9h, j However, according to the works by Moya and
co-workers13c,d and Guarr and co-workers,13e the reduction
wave at ca. Ϫ1.35 V is assigned as a metal-localized reduction,
i.e. ReI/0. Although we cannot completely exclude the possibility
of this assignment, we favor its assignment as the bipyridine-
centered reduction. For the 1,10-phenanthroline systems, the
reversible–quasireversible wave at ca. Ϫ1.10 V vs. SCE is
assigned as the reduction of the 1,10-phenanthroline ligand, i.e.
phen0/Ϫ1 as similar assignments have been reported in related
rhenium()–phen systems.9d,10c,h, j,n,o,13 Besides the above assigned
diimine-based reduction waves, other reduction waves were
probably due to the reduction of the pyridyl-azo, -ethenyl or
-ethyl ligands. However, no attempts were made to unambig-
uously assign all the reduction waves, as the reduction of these
Re diimine complexes is often complicated by coupled chemical
processes. It is interesting to note that for complex 12 which
contains the bridging ligand BPEB, only one metal-centered
oxidation wave and two reduction waves for the bpy ligand are
observed, which are suggestive of the non-interacting nature
of the two rhenium diimine units, leading to the observation
that the oxidation and reduction waves of the two units are
indistinguishable.15
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Received 24th February 1998; Paper 8/01575I
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