New synthetic routes to biscarbonylbipyridinerhenium(I) complexes cis, trans-[Re(X2bpy)(CO)2(PR3)(Y)](n+) (X2bpy = 4,4'-X2-2,2'-bipyridine) via photochemical ligand substitution reactions, and their photophysical and electrochemical properties

Article abstract of DOI:10.1021/ic991190l

Photochemical ligand substitution of fac-[Re(X₂bpy)(CO)₃(PR₃)]⁺ (X₂bpy = 4,4'-X₂-2,2'-bipyridine; X = Me, H, CF₃; R = OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans-[Re(X₂bpy)(CO)₂(PR₃)(MeCN)]⁺, coordinated with four different kinds of ligands. Similarly, other biscarbonylrhenium complexes, cis,trans-[Re(X₂bpy)(CO)₂(PR₃)(Y)](n+) (n = 0, Y = Cl^-; n = 1, Y = pyridine, PR'₃), were synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me₂-bpy)(CO)₂{P(OEt)₃}(PPh₃)](PF₆) was determined by X-ray analysis. Crystal data: C₃₈H₄₂N₂O₅F₆P₃Re, monoclinic, P2₁/a, a = 11.592(1) A, b = 30.953(4) A, c = 11.799(2) A, V = 4221.6(1) A³, Z = 4, 7813 reflections, R = 0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their ³MLCT state with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed in the cases Y = Cl^-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y = Cl^- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-[Re(X₂bpy)(CO)₂(PR₃)(MeCN)]⁺.

Full text of DOI:10.1021/ic991190l

Inorg. Chem. 2000, 39, 2777-2783
2777
New Synthetic Routes to Biscarbonylbipyridinerhenium(I) Complexes
cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (X2bpy ) 4,4′-X2-2,2′-bipyridine) via Photochemical
Ligand Substitution Reactions, and Their Photophysical and Electrochemical Properties
Kazuhide Koike,† Junji Tanabe, Shigeki Toyama, Hideaki Tsubaki, Kazuhiko Sakamoto,
Jeremy R. Westwell,†,‡ Frank P. A. Johnson,§ Hisao Hori,† Hideki Saitoh, and
Osamu Ishitani*
Graduate School of Science and Engineering and Faculty of Science, Saitama University,
255 Shimo-Okubo, Urawa, Saitama 338-8570, Japan
ReceiVed October 8, 1999
Photochemical ligand substitution of fac-[Re(X2bpy)(CO)3(PR3)]+ (X2bpy ) 4,4′-X2-2,2′-bipyridine; X ) Me,
H, CF3; R ) OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans-
[Re(X2bpy)(CO)2(PR3)(MeCN)]+, coordinated with four different kinds of ligands. Similarly, other biscarbon-
ylrhenium complexes, cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (n ) 0, Y ) Cl-; n ) 1, Y ) pyridine, PR′3), were
synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me2-
bpy)(CO)2{P(OEt)3}(PPh3)](PF6) was determined by X-ray analysis. Crystal data: C38H42N2O5F6P3Re, monoclinic,
P21/a, a ) 11.592(1) Å, b ) 30.953(4) Å, c ) 11.799(2) Å, V ) 4221.6(1) Å3, Z ) 4, 7813 reflections, R )
0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their 3MLCT state
with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed
in the cases Y ) Cl-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y )
Cl- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-
[Re(X2bpy)(CO)2(PR3)(MeCN)]+.
Introduction
transfer reactions,4-6 and macrocyclic tetrametallic assemblies,
“molecular squares”, have been constructed using the rhenium
complexes as corners.7
On the other hand, only a few biscarbonyl diimine complexes
[Re(LL)(CO)2(L′)(Y)]n+ have been reported due to the lack of
a general synthetic method.8-11 From the few examples reported
of the biscarbonyl complexes, several intriguing features are
apparent: (1) relatively long-lived emissions in solution even
at room temperature, (2) photocatalytic functions of some
complexes for the reduction of CO2, (3) thermal and photo-
Many kinds of tricarbonyldiiminerhenium complexes fac-
[Re(LL)(CO)3(L′)]n+ (LL ) diimine ligand, L′ ) monodentate
ligand, n ) 0 or 1) have been synthesized, and their photo-
physical and photochemical properties have received a great
deal of attention in the last two decades.1-3 Recently, they have
been frequently used as building blocks for multifunctional
bimetallic complexes and supramolecular systems because the
photochemical and electrochemical properties of rhenium
complexes can be widely changed by the relatively straightfor-
ward introduction of various kinds of mono- and bidentate
ligands (L′ and LL).4 For example, covalently linked donor-
acceptor pairs containing the rhenium complex(es) have been
utilized in many studies for intramolecular electron- and energy-
(5) Schanze, K. S.; Walters, K. A. In Photoinduced electron transfer in
metal-organic dyads; Ramamurthy, V., Schanze, K. S., Eds.; Marcel
Dekker: New York, 1998; Vol. 2, pp 75-127.
(6) Schanze, K. S.; MacQueen, D. B.; Perkins, T. A.; Cabana, L. A.
Coord. Chem. ReV. 1993, 122, 63-89.
(7) (a) Slone, R. V.; Hupp, J. T. Inorg. Chem. 1997, 36, 5422. (b) Slone,
R. V.; Yoon, D. I.; Calhoun, R. M.; Hupp, J. T. J. Am. Chem. Soc.
1995, 117, 7, 11813-11814.
* Corresponding author. E-mail: ishitani@apc.saitama-u.ac.jp.
† National Institute for Resources and Environment, 16-3 Onogawa,
Tsukuba, Ibaraki 305, Japan.
(8) Although the biscarbonyl complexes [Re(LL)(CO)2(PR3)2]+ were
spectroscopically detected following the electrochemical reduction of
[Re(LL)(CO)3(PR3)]+ in the presence of excess PR3, the products have
not been isolated, and consequently, the use of this electrochemical
reaction as a synthetic method has not been established. See refs 36a,b
in this paper.
‡ Present address: Unilever Research, Port Sunlight Laboratory, Quarry
Road East, Bebington, The Wirral CH63 3JW, U.K.
§ Department of Chemistry, University of Nottingham, Nottingham NG7
2RD, U.K.
(1) Stufkens, D. J. Comments Inorg. Chem. 1992, 13, 359-385.
(2) (a) Koike, K.; Hori, H.; Westwell, J. R.; Ishizuka, M.; Takeuchi, K.;
Ibusuki, T.; Enjouji, K.; Konno, H.; Sakamoto, K.; Ishitani, O.
Organometallics 1997, 16, 5724-5729. (b) Hori, H.; Johnson, F. P.
A.; Koike, K.; Ishitani, O.; Ibusuki, T. J. Photochem. Photobiol., A
1996, 96, 171-174. (c) Hori, H.; Johnson, F. P. A.; Koike, K.;
Takeuchi, K.; Ibusuki, T.; Ishitani, O. J. Chem. Soc., Dalton Trans.
1997, 1019-1023.
(9) (a) It has been reported that this substitution reaction is initiated by
electron transfer from an electron donor to the excited state of Re-
(bpy)(CO)3Cl and, consequently, proceeds via a reaction mechanism
different from the photochemical ligand substitution reactions reported
in this paper. (b) Pac, C.; Kaseda, S.; Ishii, K.; Yanagida, S. J. Chem.
Soc., Chem. Commun. 1991, 787.
(10) Ishitani, O.; George, M. W.; Ibusuki, T.; Johnson, F. P. A.; Koike,
K.; Nozaki, K.; Pac, C.; Turner, J. J.; Westwell, J. R. Inorg. Chem.
1994, 33, 4712-4717.
(11) (a) Casper, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984,
23, 2104-2109. (b) Schutte, E.; Helms, J. B.; Woessner, S. M.; Bowen,
J.; Sullivan, B. P. Inorg. Chem. 1998, 37, 2618-2619.
(3) Kalyanasundaram, K. Photochemistry of polypyridine and porphyrin
complexes; Academic Press Ltd.: London, 1992.
(4) Balzani, V.; Juris, A.; Ventyri, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759-833.
10.1021/ic991190l CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/27/2000

2778 Inorganic Chemistry, Vol. 39, No. 13, 2000
Koike et al.
Materials. Acetonitrile and CH2Cl2 were dried over CaH2. Tetra-
hydrofuran (THF) was distilled from Na/benzophenone just before use.
Ligands bpy and Me2bpy (Tokyo Kasei Kogyo Co.) were used as
received. 2-Chloro-4-trifluoromethylpyridine was kindly supplied from
Ishihara Sangyo Co. and used for the preparation of (CF3)2bpy.13 ClRe-
(bpy)(CO)314 was synthesized according to the literature, and a similar
method was used for the synthesis of ClRe(Me2bpy)(CO)3 and ClRe-
{(CF3)2bpy}(CO)3 using Me2bpy and (CF3)2bpy as the chelating ligands,
respectively.
chemical stability, (4) the appearance of the MLCT (metal-to-
ligand charge-transfer) absorption band at longer wavelength
by ∼40 nm compared with those of the corresponding tricar-
bonyl complexes, and (5) C2V symmetry, which simplifies the
interpretation of the ν(CO) region of the IR spectra compared
to those of the tricarbonyl complexes.10,11b In addition, the
biscarbonyl complexes may potentially work as building blocks
for “linear-shaped” supramolecules such as photoactive molec-
ular-scale wires. In contrast, tricarbonyl complex(es) [fac-Re-
(LL)(CO)3(L′)]n+ could be used only as building blocks for “L-
shaped” supramolecules because the substitutable ligands, i.e.,
LL and L′, are in positions cis to one another.
Only two practical synthetic methods for the biscarbonyl
diimine complexes have been reported:8 (1) thermal substitution
of the tricarbonyl complex ClRe(bpy)(CO)3 with a phosphine
ligand such as PPh3 in solution at high temperature to give
cis,trans- or cis,cis-[Re(bpy)(CO)2(PR3)2]+ 11 and (2) the photo-
chemical substitution of ClRe(bpy)(CO)3 by P(OEt)3 in the
presence of an electron donor to yield cis,trans-[Re(bpy)(CO)2-
{P(OEt)3}2]+.9 Unfortunately, these methods have serious
limitations as general synthesis routes for biscarbonyl diimine
complexes because the effective substitution can occur only with
two identical phosphorus ligands.
Synthetic Procedures. While the synthesis of the SbF6- salt of [Re-
(bpy)(CO)3(PR3)]+ (R ) OEt or Ph) was reported as a two-step
synthesis,15 the PF6- salt of [Re(X2bpy)(CO)3{P(OEt)3}]+ (X ) H, Me,
CF3) can be prepared in a single step in reasonable yields as follows.
[Re(bpy)(CO)3{P(OEt)3}]+PF6 (1a). A THF solution (100 mL)
-
containing 104 mg (0.23 mmol) of ClRe(bpy)(CO)3 and 65 mg (0.25
mmol) of silver trifluoromethanesulfonate was refluxed under an argon
atmosphere for 3 h. After removal of the AgCl precipitate by filtration,
P(OEt)3 (1.2 mL) was added to the filtrate. The solution was refluxed
overnight under an Ar atmosphere. Evaporation under reduced pressure
left a yellow solid, which was washed with ether and then recrystallized
from CH2Cl2-ether to yield 132 mg of solid. To the solution of the
solid in 5 mL of methanol was added dropwise a concentrated
-
-
NH4+PF6 methanolic solution. The precipitated PF6 salt of 1a was
collected by filtration, washed with water, and then dried in vacuo.
-
1
Yield: 133 mg, 80%. For the PF6 salt, it was confirmed that the H
NMR spectrum and the IR bands for CO stretching are essentially
We report here the first example of the photochemical
substitution of the axial CO ligand in a tricarbonylbipyridine-
rhenium(I) complex, which may open up a novel general syn-
thetic route for a new class of biscarbonyl complexes, cis,trans-
[Re(X2bpy)(CO)2(PR3)(Y)]+, coordinated with four different
kinds of ligands (eq 1). Their structures, spectroscopy, and
electrochemistry are also reported.
identical to those reported for the SbF6 salt.15
-
[Re(bpy)(CO)3(PPh3)]+PF6- (1d). A similar procedure for 1a was
applied to the synthesis of 1d. Yield: 88%. The spectroscopic data of
1d have been reported elsewhere.15
[Re(Me2bpy)(CO)3{P(OEt)3}]+PF6- (1i) and [Re{(CF3)2bpy}(CO)3-
{P(OEt)3}]+PF6- (1j) were synthesized in a manner similar to that of
1a using ClRe(Me2bpy)(CO)3 and ClRe{(CF3)2bpy}(CO)3 instead of
ClRe(bpy)(CO)3.
Data for 1i. Yield: 93%. Anal. Calcd for C22H27N2O9F3PReS: C,
34.33; H, 3.54; N, 3.64. Found: C, 34.11; H, 3.32; N, 3.65. 1H NMR
(δ, 300 MHz, CDCl3): 8.84 (s, bpy-3,3′), 8.65 (d, J ) 5.7, bpy-6,6′),
7.38 (d, J ) 5.7 Hz, bpy-5,5′), 3.78 (quintet, JH,P ) J ) 7.0 Hz, POCH2),
2.68 (s, CH3-bpy), 1.04 (t, J ) 7.0 Hz, CH3-[CH2OP]). 13C NMR
(δ, 75.5 MHz, CDCl3): 193.6, 193.4 (CdO), 156.0 (bpy-2,2′), 154.1
(bpy-4,4′), 152.1 (bpy-6,6′), 128.4, 126.6 (bpy-3,3′, bpy-5,5′), 62.4 (d,
JC,P ) 7.3 Hz, CH2OP), 21.4 (bpy-CH3), 15.8 (d, JC,P ) 5.8 Hz, CH3-
[CH2OP]). IR (CH3CN): ν(CO)/cm-1 ) 2045, 1958, 1926. UV/vis
(CH3CN): λmax/nm (ꢀ/103 M-1 cm-1) ) 330 (sh, 5.9), 315 (18.6), 304
(15.8).
Data for 1j. Yield: 85%. Anal. Calcd for C21H21N2O6F12P2Re: C,
28.87; H, 2.42; N, 3.21. Found: C, 28.85; H, 2.24; N, 2.99. 1H NMR
(δ, 300 MHz, CDCl3): 9.15 (d, J ) 5.8, bpy-6,6′), 8.78 (s, bpy-3,3′),
7.88 (d, J ) 4.7 Hz, bpy-5,5′), 3.78 (quintet, JH,P ) J ) 7.0 Hz, CH2-
OP), 0.98 (t, J ) 7.0 Hz, CH3). 13C NMR (δ, 75.5 MHz, CDCl3): 192.2,
192.0 (CdO), 156.3 (bpy-2,2′), 155.0 (bpy-6,6′), ∼141 (m, bpy-4,4′),
124.2, 121.3 (bpy-3,3′, bpy-5,5′), 62.8 (d, JC,P ) 7.5 Hz, CH2OP), 15.4
(d, JC,P ) 6.1 Hz, CH3-[CH2OP]). IR (CH3CN): ν(CO)/cm-1 ) 2053,
1971, 1935. UV/vis (CH3CN): λmax/nm (ꢀ/103 M-1 cm-1) ) 363 (3.7),
322 (9.5), 309 (9.6), 274 (15.8).
The complexes 2 were synthesized by two methods: a photochemical
one-step method and/or a two-step method via thermal ligand substitu-
tion of the acetonitrile complexes 2a and 2f. The yields and elemental
analysis, and IR data are summarized in Tables 1 and 2, respectively.
Proton NMR data are given in the Supporting Information.
One-Step Method. [Re(bpy)(CO)2{P(OEt)3}(MeCN)]+PF6- (2a).
A vessel containing an acetonitrile solution (80 mL) of 200 mg (0.27
mmol) of 1a was immersed in a water bath maintained at ∼25 °C and
then irradiated under an argon atmosphere using a high-pressure
mercury lamp with a uranyl glass filter (>330 nm) for 12 h. The solvent
Experimental Section
General Procedures. UV/vis spectra were recorded using a Hitachi
330 spectrophotometer or an Otsuka-Denshi Photal-2000 multichannel
spectrophotometer with a D2 (25 W)/I2 (25 W) mixed lamp. Emission
spectra at 25 °C were measured with a Hitachi F-3000 fluorescence
spectrophotometer. To determine the quantum yields for emission, a
solution (0.5 M H2SO4) of quinine bisulfate was used as the standard
(φem ) 0.546),12 after applying a correction for differing refractive
indices of the solvents. IR spectra were recorded in acetonitrile with a
JEOL JIR-6500 FTIR spectrophotometer using 1 cm-1 resolution.
Proton NMR was measured on a Bruker AC300P system (300 MHz)
or a JEOL JNM-LA 500FT system (500 MHz). Emission lifetime
measurements were carried out as described elsewhere.11 The redox
potentials of the complexes were measured in an acetonitrile solution
containing tetra-n-buthylammonium tetrafluoroborate (0.1 M) as the
supporting electrolyte by cyclic voltammetric techniques using an ALS/
CHI CHI-620 electrochemical analyzer, with a glassy-carbon disk
working electrode (3 mm diameter), a Ag/AgNO3 (0.1 M) reference
electrode, and a Pt counter electrode. The supporting electrolyte was
dried in vacuo at 100 °C for 1 day prior to use.
(13) Furue, M.; Maruyama, K.; Oguni, T.; Naiki, M.; Kamachi, M. Inorg.
Chem. 1992, 31, 3792-3795.
(14) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.
(15) Hori, H.; Koike, K.; Ishizuka, M.; Takeuchi, K.; Ibusuki, T.; Ishitani,
O. J. Organomet. Chem. 1997, 530, 169-176.
(12) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.

Synthesis of Biscarbonylrhenium(I) Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2779
Table 1. Yields and Elemental Analysis Data of 2
yield/%a
[Re(X2bpy)(CO)2(PR3)(Y)]+
elemental anal./%b [found (calcd)]
one-step
method
two-step
methodc
X
PR3
Y
C
N
H
2a
2b
2c
2c
2df
2e
2f
2g
2h
2i
H
H
H
H
H
H
H
H
P(OEt)3
P(OEt)3
P(OEt)3
PPh3
P(OEt)3
P(OEt)3
PPh3
MeCN
py
PPh3
P(OEt)3
P(OEt)3
Cl-
MeCN
PPh3
87
68
75d
50e
76
65g
65
84
90
96
55
32.31 (32.00)
35.37 (35.03)
44.62 (44.49)
5.52 (5.60)
5.21 (5.33)
2.80 (2.88)
3.40 (3.49)
3.56 (3.58)
3.86 (3.94)
84
90
70
36.02 (36.03)
45.51 (45.39)
53.73 (53.98)
45.97 (45.65)
35.61 (35.68)
30.07 (30.83)
4.60 (4.67)
4.84 (4.96)
2.54 (2.62)
2.71 (2.80)
2.83 (3.08)
2.68 (2.77)
3.90 (3.86)
3.07 (3.10)
3.50 (3.59)
4.30 (4.23)
4.51 (4.66)
3.44 (3.58)
PPh3
Me
Me
CF3
P(OEt)3
P(OEt)3
P(OEt)3
PPh3
P(OEt)3
P(OEt)3
2j
a Isolated yields of the PF6- salts except for the neutral complex 2e. b For the PF6- salts except for 2e (neutral complex) and 2i (CF3SO3- salt).
c Based on 2a or 2f used. d The triethyl phosphite complex 1a was used as the starting material, and 2g was produced in 4% yield. e The
triphenylphosphine complex 1d was used as the starting material, and 2d was produced in 25% yield. f The reported compound.9 g Accompanied
by 6% recovery of the starting complex even after 34 h of irradiation.
Table 2. ν(CO) Frequencies (cm-1) and EFFF Force Constants (N
m-1) for 2 in CH2Cl2 Solution
of NH4PF6 (1 mL) and then a small amount of water. The solution
was kept in a refrigerator overnight. The resulting orange-red solid was
washed with water. Alternatively, 2j was successfully isolated by
column chromatography on silica gel using CH2Cl2 eluent.
a
b
complex
ν(CO)
kCO
kCO,CO
2a
2b
2c
2d
2e
2f
2g
2h
2hc
2i
1954, 1878
1945, 1871
1948, 1876
1956, 1881
1936, 1857
1944, 1872
1938, 1867
1946, 1873
1944, 1865
1954, 1879
1963, 1891
1483
1471
1477
1487
1453
1471
1463
1473
1466
1484
1501
58.8
57.0
55.6
58.1
60.5
55.5
54.6
56.3
60.8
58.1
56.0
-
Two-Step Method. [Re(bpy)(CO)2{P(OEt)3}(PPh3)]+PF6 (2c).
The acetonitrile complex 2a was photochemically synthesized using
190 mg of 1a as described above. A solution of 2a and 2 g of PPh3 in
150 mL of acetone was refluxed for 12 h under an argon atmosphere,
and then the solution was evaporated to dryness under reduced pressure.
Excess PPh3 was removed from the crude mixture by column
chromatography on silica gel using CH2Cl2-ethyl acetate (90:10, v/v)
as eluent. The yellow product 2c was recrystallized using ether-
CH2Cl2 and dried in vacuo.
2j
[Re(bpy)(CO)2{P(OEt)3}Cl] (2e). A methylene chloride solution
(80 mL) containing 2a (190 mg) and NEt4+Cl- (1.69 g) was refluxed
under an Ar atmosphere for 8 h. After evaporation of the solution, the
residue was dissolved in 100 mL of CH2Cl2, and then the solution was
washed three times with water. The organic layer was dried on MgSO4
and evaporated to dryness under reduced pressure to leave a yellow
solid. Recrystallization from ether-CH2Cl2 gave 2e.
a Principal force constants. b Interaction force constants. c In KBr
disk.
was then evaporated under reduced pressure. The resulting dark yellow
solid 2a was washed with ether, recrystallyzed from ether-CH2Cl2,
and dried in vacuo.
Procedures similar to those described above were adopted for the
one-step preparation of 2b,d,f,g as shown below.
[Re(bpy)(CO)2{P(OEt)3}(py)]+PF6- (2b) was obtained by irradia-
tion of a THF solution (50 mL) containing 200 mg of 1a and 1 g of
pyridine under an argon atmosphere for 12 h. The subsequent
procedures are analogous to those used for 2a.
[Re(bpy)(CO)2{P(OEt)3}2]+PF6- (2d) was obtained by irradiation
of an acetonitrile solution (150 mL) containing 500 mg of 1a and 10
mL of P(OEt)3 under an argon atmosphere for 12 h.
[Re(bpy)(CO)2(PPh3)2]+PF6 (2g). An acetone solution (60 mL)
-
containing 2f (100 mg) and PPh3 (1.0 g) was refluxed under an Ar
atmosphere for 12 h. After the solution was evaporated to dryness under
reduced pressure, the residue was washed with 100 mL of ether and
separated with column chromatography on silica gel using CH2Cl2-
ethyl acetate as eluent. The yellow layer was collected and evaporated
under reduced pressure, giving the product 2g as a yellow solid, which
was recrystallized using ether-CH2Cl2 and dried in vacuo.
Crystal Structure Determination of [Re(Me2bpy)(CO)2{P(OEt)3}-
(PPh3)]+PF6 (2h). All X-ray data were collected on a Mac Science
-
[Re(bpy)(CO)2(PPh3)(MeCN)]+PF6- (2f) was prepared by irradia-
tion of an acetonitrile solution (50 mL) containing 200 mg of 1d under
an argon atmosphere for 4 h. Longer irradiation should be avoided,
because decomposition of 2f occurred to give green byproducts.
MXC18K four-circle diffractometer using graphite-monochromated Mo
KR (λ ) 0.710 73 Å) radiation. The unit cell dimensions were
determined from 22 reflections in the range 31° < 2θ < 35°. The
crystallographic data are given in Table 3. A total of 9546 diffraction-
intensity data were collected using the 2θ-θ scan over all the 2θ range
(2.0-62.0°). Three standard reflections were measured at intervals of
100 reflections. The intensity data were corrected for Lorentz and
polarization effects. The absorption correction was not applied. The
crystal structure was solved by the direct method using the SIR92
program.16 All computations were carried out using the CRYSTAN-
GM program system.17 Of the 7813 unique reflections (Rint ) 0.060),
over the index ranges 14 > h > 0, 43 > k > 0, and 15 > l > -15, the
4349 reflections with criterion I0 > 1.5σ(I0) were used in the full-matrix
least-squares refinement based on minimization of the function ∑w(|Fo|
-
[Re(bpy)(CO)2(PPh3)2]+PF6 (2g) was obtained by irradiation of
an acetone solution (50 mL) containing 52 mg of 1a and 0.31 g of
PPh3 under an argon atmosphere for 20 h.
[Re(Me2bpy)(CO)2{P(OEt)3}2]+PF6- (2i). An acetonitrile solution
-
(150 mL) containing the CF3SO3 salt of 1i (513 mg) and 10 mL of
P(OEt)3 was irradiated under an argon atmosphere for 1 h. The solution
was then evaporated to dryness under reduced pressure, and the resulting
dark yellow solid was washed with ether. The product was recrystal-
-
lyzed using ether-CH2Cl2 and dried in vacuo. The PF6 salt of the
product 2i was obtained using a procedure analogous to that for 1a.
-
Elemental analysis of the CF3SO3 salt was carried out.
2
- |Fc|)2. The weighting scheme was w-1 ) σ2(Fo) + 0.0002|Fo| . The
[Re{(CF3)2bpy}(CO)2{P(OEt)3}2]+PF6- (2j). An acetonitrile solu-
tion (150 mL) containing 500 mg of 1j and 10 mL of P(OEt)3 was
irradiated under an argon atmosphere for 8 h. The solution was
evaporated under reduced pressure to give a brown oil, which contained
mainly 2j with a small amount of the starting complex 1j. To a solution
of the oil in 10 mL of methanol was added a saturated methanol solution
atomic and anomalous scattering factors were taken from the literature.18
(16) Altomare, A.; Cascarano, G.; Ciacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(17) Edwards, C.; Gilmore, C. J.; Mackey, S.; Stewart, N. CRYSTAN-
GM, Ver. 6.3.3; Mac Science: Yokohama, Japan, 1996.

2780 Inorganic Chemistry, Vol. 39, No. 13, 2000
Table 3. Crystallographic Data for 2h
Koike et al.
empirical formula
fw
cryst syst
space group
a/Å
C38H42N2O5F6P3Re
999.90
monoclinic
P21/a (no. 14)
11.592 (1)
â/deg
V/Å3
Z
94.312(7)
4221.6(1)
4
295
1.57
Ra
Rw
Sc
0.066
0.058
2.813
0.0005
1.01
b
T/K
∆/σ
D
calcd/g cm-3
∆Fmax/e Å-3
∆Fmin/e Å-3
b/Å
c/Å
30.953 (4)
11.799 (2)
µ/cm-1
F(000)
30.951
1992
-1.01
2
a R ) ∑(||Fo| - |Fc||)/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
c S ) [∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 2h
Bond Lengths
Re(1)-P(1)
Re(1)-N(1)
Re(1)-C(1)
O(1)-C(1)
2.448(5)
2.179(15) Re(1)-N(2)
1.84(3)
1.18(4)
Re(1)-P(2)
2.349(7)
2.145(17)
1.88(3)
Re(1)-C(2)
O(2)-C(2)
1.17(3)
Bond Angles
P(1)-Re(1)-N(1)
P(1)-Re(1)-C(1)
P(2)-Re(1)-C(1)
P(2)-Re(1)-N(1)
90.9(5)
90.1(8)
90.3(8)
86.7(5)
P(1)-Re(1)-N(2)
P(1)-Re(1)-C(2)
P(2)-Re(1)-C(2)
P(2)-Re(1)-N(2)
89.3(5)
91.2(7)
91.2(7)
89.9(5)
N(1)-Re(1)-N(2) 74.1(7)
N(1)-Re(1)-C(1) 95.9(8)
N(1)-Re(1)-C(2)
N(2)-Re(1)-C(2)
P(1)-Re(1)-P(2)
173.6(9)
99.9(9)
177.6(2)
N(2)-Re(1)-C(1) 170.0(8)
C(1)-Re(1)-C(2)
Figure 1. In situ FT-IR absorption spectra of an Ar-saturated
acetonitrile solution containing 1a (0.1 M) under irradiation with a 30
s monitoring interval. The cell path length was 2 mm.
90.1(10)
Table 5. Electronic Absorption and Emission Data for 2 at Room
Temperature
Therefore, the principal and interaction force constants can be
calculated using the following equations:
absorption λmax/nmb
emissionb
π-π*
π-π*
λmax
nm
/
τem/
ns
complexa
MLCT
(bpy)
(PPh3)
Φem
kCO ) K(µ-1){(νsym)2 + (νasym)2}
kCO,CO ) K(µ-1){(νsym)2 - (νasym)2}
2a
2b
2c
2d
2e
2f
386 (4000) 294 (19 000)
397 (4100) 295 (23 000)
387 (3800) 291 (18 000) 269 (20 000) 620
369 (4000) 289 (17 000)
423 (2100) 300 (13 000)
401 (3800) 301 (20 000) 270 (17 000) 650 <0.002
639 <0.002
651 <0.002
d
12
0.031 350
0.017c 250c
618c
nde
Results and Discussion
Photochemical Ligand Substitution Reaction of 1a. An Ar-
purged acetonitrile solution containing 1a was irradiated to give
biscarbonyl complex cis,trans-[Re(bpy)(CO)2{P(OEt)3}(MeCN)]+
(2a) in an 88% isolated yield (eq 1). The progress of this ligand
substitution reaction was followed by in situ IR measurements
in solution using a gastight optical cell of calcium fluoride, into
which an Ar-purged acetonitrile solution had been introduced.
This cell was placed in the optical chamber of an FT-IR
spectrometer and then irradiated using an optical fiber through
which 365 nm light was introduced from a Xe lamp with a band-
pass filter placed outside the spectrometer. Figure 1 shows the
IR spectral changes in the CO-stretching region following the
irradiation; two new bands attributable to 2a appear with
isosbestic points as the three bands of 1a decrease. This spectral
change clearly demonstrates the quantitative photochemical
conversion of 1a to 2a. The quantum yield of the photochemical
reaction was 0.089 at 25 °C. The triphenylphosphine complex
1d was also photochemically converted to the corresponding
biscarbonyl complex 2f in quantitative yield.
d
2g
2h
2i
410 (2800) 295 (20 000) 270 (22 000) 619
0.037 640
0.040 560
0.026c 340c
0.005c 20c
381 (3700) 290 (18 000) 267 (21 000) 612
364 (5100) 283 (22 000)
404 (4900) 297 (23 000)
609c
668c
2j
-
a All complexes are PF6 salts except for the neutral complex 2e.
b Data were obtained from DMF solutions except for the data denoted
by the superscript c. c Data were obtained from MeCN solutions. d Not
measured. e Not detected.
Most hydrogen atoms were assigned by calculation and their parameters
restrained for C-H length 0.96 Å and fixed thermal parameters Uiso
0.1-0.3. No secondary extinction corrections were applied. Table 4
shows selective bond lengths and angles of 2h.
Energy-Factored Force Field Calculations. Energy-factored force
field (EFFF) theory, when applied to transition metal carbonyls, has
been well demonstrated to determine both the principal force constant(s)
kCO and the interaction force constant(s) kCO,CO
complexes 2 have two equivalent CO groups and hence only two force
constants: kCO and kCO,CO. With only two observed ν(CO) frequencies,
and using the EFFF assumptions, it is possible to solve the force field
for this compound exactly (Table 5). In theory, CO stretching
frequencies (ν, cm-1) are described by the following secular equations:
≈
37-39
.
The biscarbonyl
It is well-known that the complexes fac-[Re(LL)(CO)3(Z)]
(Z ) alkyl or metal carbonyl moieties such as Me, Et, and
M(CO)5 (M ) Re, Mn, etc.)) are photolabile enough to undergo
3
facile homolytic fission of the Re-Z bond via a σπ* (and/or
λ ) K(νsym)2 ) µ(kCO + kCO,CO
λ ) K(νasym)2 ) µ(kCO - kCO,CO
)
3σσ*) excited state to give [Re0(LL)(CO)3] and (Z•).1,19-22 In
contrast to this well-known reaction type, the present photo-
)
(19) Rossenaar, B. D.; George, M. W.; Johnson, F. P. A.; Stufkens, D. J.;
Turner, J. J.; JrVlcek, A. J. Am. Chem. Soc. 1995, 117, 11582-11583.
(20) Rossenaar, B. D.; Kleverlaan, C. J.; Vandeven, M. C. E.; Stufkens,
D. J.; Vlcek, A. Chem. Eur. J. 1996, 2, 228-237.
(21) Stufkens, D. J. Coord. Chem. ReV. 1990, 104, 39-112.
(22) Meyer, T. J.; Casper, J. V. Chem. ReV. 1985, 85, 187-218.
where µ is the reduced mass (0.145 83) and K ) 4.04 × 10-4
.
(18) Ibers, J. A.; Hamilton, W. C. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, 1974; Vol. IV.

Synthesis of Biscarbonylrhenium(I) Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2781
chemical ligand substitution is entirely different, providing the
first example for the exclusive loss of the axial CO ligand from
tricarbonyl complexes fac-[Re(LL)(CO)3(L′)]n+. A notable
structural feature of the reactions is that L′ must be a phosphorus
ligand.23 This type of photochemical ligand substitution reactions
was not observed in the case of L′ ) halides and pyridine
derivatives.
New Synthesis Methods of 2. We have developed two
synthetic routes for 2, i.e., the one-step and two-step methods,
by taking advantage of the photochemical ligand substitution
reactions of 1 described above.
(1) Photochemical one-step synthesis: This procedure is
straightforward and involves irradiation of a solution containing
[fac-Re(X2bpy)(CO)3(PR3)]+ and ligand L′, e.g., pyridine and
phosphorus compounds at >330 nm for 4-48 h, followed by
the usual workup.24 Isolated yields of 2 are synthetically
reasonable (55-96%), as shown in Table 1. Although there are
two possible structural isomers of 2, the cis,cis and cis,trans
forms, only the cis,trans isomer is produced by this method.
The method is certainly suitable for high-yield synthesis of
the biscarbonyl complexes which have the same phosphorus
ligands or one phosphorus and another neutral ligand, i.e., 2a,
2b, 2d, 2f, 2g, 2i, and 2j. The one-step synthesis of 2c having
two different phosphorus ligands can be achieved in reasonable
yield but is accompanied by the formation of the complex 2d
or 2g having two identical ligands (P(OEt)3 or PPh3) in 5-30%
yields depending on the irradiation time. The two-step method
described below is more suitable for the synthesis of this type
of compound, because the isolation of 2c and 2h from the
byproducts is not straightforward. In the case of the chloro
complex 2e, the two-step method is more suitable because the
one-step reaction is very slow; even after 34 h of irradiation,
6% of the starting complex still remained in the solution.
(2) Two-step method: This method utilizes the acetonitrile
complexes 2a and 2f as the precursor compound which can be
easily prepared in good isolated yields (eq 1 and Table 1) using
the photochemical one-step method described above. An
important point of this method is that the acetonitrile ligand is
much more thermally labile than the other ligands (eq 2). This
Figure 2. ORTEP drawing and labeling scheme for the atoms of the
cation of 2h. Thermal ellipsoids are at the 50% probability level.
phosphorus ligands. The phosphorus ligands are not thermally
substituted even under reflux in the presence of an excess of
the ligand. The chloride complex 2e was also obtained by this
method in a good yield.
Characterization of 2 and the X-ray Crystal Structure of
2h. All the complexes have two intense IR bands in the ν(CO)
region, which are similar in intensity (Table 2). In addition,
elemental analysis (Table 1) and electrospray-ionization mass
spectroscopic data support the presence of two carbonyl ligands
in 2, i.e., [Re(X2bpy)(CO)2(PR3)(Y)]n+ (n ) 0, Y ) Cl-, n ) 1,
1
Y ) neutral ligands). In H NMR, only three or four sets of
peaks appear for X2bpy or bpy, clearly indicating that all the
complexes 2 have C2V symmetry, i.e., the trans configuration
of PR3 and Y coordinating to the central rhenium atom (eqs
1 and 2).
X-ray crystallographic data confirmed the identity of 2h. The
ORTEP drawing of the [Re(Me2bpy)(CO)2{P(OEt)3}(PPh3)]+
unit is shown in Figure 2. The two CO ligands are in cis
positions; P(OEt)3 and PPh3 are, actually, in trans positions. This
is the first example of X-ray crystallographic data for biscar-
bonyldiiminerhenium complexes, though more than 10 sets of
crystal data have been reported for tricarbonyldiiminerhenium
complexes.25-36a The complex 2h has a steric structure similar
reaction efficiently proceeds just upon refluxing a solution
containing the acetonitrile complex (2a or 2f) and ligand Y for
several hours. The progress of the reaction can be easily
followed by monitoring the CO-stretching bands using FT-IR
spectroscopy so that the end point of the reaction may be
detected. Removal of the solvent and the excess ligand from
the reaction mixture gave cis,trans-[Re(bpy)(CO)2(PR3)(Y)]n+
(25) Horn, E.; Show, M. R. Aust. J. Chem. 1980, 33, 2369-2376.
(26) Yam, V. W. W.; Wong, K. M. C.; Cheung, K. K. Chem. Commun.
1998, 135-136.
(27) Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V.; Hursthouse, M.
B.; Coles, S. J. J. Chem. Soc., Dalton Trans. 1996, 203-209.
(28) Guilhem, J.; Pascard, C.; Lehn, J.-M.; Ziessel, R. J. Chem. Soc., Chem.
Commun. 1989, 1449-1454.
(Y ) the neutral ligand, n ) 1, PF6 salt; Y ) Cl-, n ) 0) in
-
good yields (Table 1). The cis,cis isomer was not detected during
the reaction at all.
This method is especially suitable for the synthesis of the
biscarbonyl complexes 2c and 2h, which have two different
(29) Winslow, L. N.; Rillema, D. P.; Welch, J. H.; Singh, P. Inorg. Chem.
1989, 28, 1596-1599.
(30) Yam, V. W. W.; Lau, V. C. Y.; Cheung, K. K. Organometallics 1996,
15, 1740-1744.
(23) Recently, Rillema and co-workers reported photochemical ligand
substitution reactions of tetracarbonyldiiminerhenium(I) complexes
[Re(LL)(CO)4]+(CF3SO3-), giving the corresponding tricarbonyl
complexes [Re(LL)(CO)3(CF3SO3)]. Shaver, R. J.; Rillema, D. P.
Inorg. Chem. 1992, 31, 4101-4107.
(24) Irradiation using 313 nm light instead of >330 nm light caused a
decrease in the yield of 2 accompanied by the formation of green
complexes. The photochemistry of 2 is under investigation in our
laboratory.
(31) Civitello, E. R.; Dragovich, P. S.; Karpishin, T. B.; Novick, S. G.;
Bierach, G.; Oconnell, J. F.; Westmoreland, T. D. Inorg. Chem. 1993,
32, 237-241.
(32) Moya, S. A.; Schmidt, R.; Pastene, R.; Sartori, R.; Muller, U.; Frenzen,
G. Organometallics 1996, 15, 3463-3465.
(33) Wallace, L.; Woods, C.; Rillema, D. P. Inorg. Chem. 1995, 34, 2875-
2882.
(34) Shaver, R. J.; Perkovic, M. W.; Rillema, D. P.; Woods, C. Inorg.
Chem. 1995, 34, 5446-5454.

2782 Inorganic Chemistry, Vol. 39, No. 13, 2000
Koike et al.
Table 6. Electrochemical Data for 2a
to that of the reported tricarbonyl bipyridine complexes; i.e.,
the octahedral coordination on the rhenium center is rather
distorted due to the chelating effect of the bpy ligand (Table
4). Some interesting features can be seen for the X-ray structure
of 2h compared with those of tricarbonyl complexes: shorter
Re-CO bond distances (1.84(3) and 1.88 (3) Å) but longer CO
bond distances for the carbonyl ligands (1.18(4) and 1.17(3)
Å) vs 1.90-1.94 Å for Re-C; 1.10-1.16 Å for equatorial CO
in most tricarbonyl complexes. The axis line P-Re-P is bent
(2.4°) toward the space between the carbonyl ligands.
complex
E1/2red/Vb
Epred/Vc
E1/2ox/Vb
Epox/Vd
2a
2b
2c
2d
2e
2f
2g
2h
2i
-1.69 (75)
-1.87
-1.64
-2.26
-2.32
-1.87
-1.91
-2.26
-2.33
-2.37
-1.96
0.92 (68)
0.89 (61)
-1.71 (65)
-1.71 (65)
1.09
1.13
1.10 (77)
0.47 (67)
0.89 (69)
-1.70 (64)
-1.71 (64)
-1.80 (65)
-1.80 (69)
-1.26 (67)
1.10
1.05
1.06 (82)
1.23 (86)
EFFF theory, when applied to transition metal carbonyls, has
2j
been well demonstrated to determine both the principal force
a The cyclic voltammograms of 2 were taken in MeCN solutions
containing n-Bu4NBF4 (0.1 M) at a 200 mV/s scan rate using a glassy-
carbon working electrode, a Pt counter electrode, and a Ag/AgNO3
(0.1 M, an MeCN solution) reference electrode under an Ar atmosphere.
All potentials are reported against a Ag/AgNO3 reference. b Redox
potential for the reversible or quasi-reversible process and peak potential
difference (mV) (in parentheses). c Peak potential for irreversible
reduction. d Peak potential for irreversible oxidation.
37-39
constant(s) kCO and the interaction force constant(s) kCO,CO
.
The biscarbonyl complexes 2 have only two equivalent CO
groups, thus allowing the complete analysis of the force fields.40
The calculated force constants for 2 are shown in Table 2. In
principle, the force constants of the CO groups in metal
carbonyls should be affected by the electron density on the
central metal because of the π-back-donation from the d-orbital
of the metal ion to the antibonding orbitals of the CO groups.
Therefore, the values of kCO depend on the electron-withdrawing
strength of the ligands Y and the substituents X, which are
in the order P(OEt)3 > MeCN > PPh3 > py > Cl- for Y and
CF3 > H >Me for X.
It has been reported that the C-O bond length rCO41 and the
bond angle θ between the CO groups in metal carbonyls38 can
be estimated by the following equations:
rCO ) 1.647 - 0.184 ln kCO (rCO, Å; kCO, mdyn/Å)
tan2(θ/2) ) I(antisym)/I(sym)
Figure 3. UV/vis absorption (solid line) and emission (dotted line)
spectra of 2h in a DMF solution at room temperature.
where I is the intensity of relevant vibration modes. The
calculated rCO and θ using the above equations and the observed
IR frequencies in a KBr disk (Table 2) are 1.153 Å and 99.8°.
dependence of the band on the property of X in the X2bpy
ligands. A red shift of the band occurs by 10-15 nm upon
changing the solvent from DMF to CH2Cl2. A further shift to
the red occurs with an increase in the electron-withdrawing
strength of X in the order CF3 > H > Me. The shift is also
dependent upon Y, shifting to the red in the order Cl- > py >
PPh3 > MeCN > P(OEt)3 > CO. This change is attributable to
changes in the electron density on the central metal, which is
affected by the σ- and π-donating/accepting abilities of L and
PR3. This order for the electron-donating properties of the
ligands is also supported by the IR spectroscopic results (vide
supra) and electrochemical data (vide infra).
The values obtained from the crystallographic data are rCO
)
1.18(4) and 1.17(3) Å and θ ) 91.2° (Table 6). The differences
between the calculated and observed values are less than 3%
for rCO but are relatively large (8.6%) for θ.
Spectroscopic and Electrochemical Properties of 2. Table
5 summarizes the UV/vis absorption and emission data for 2.
Figure 3 shows the UV/vis absorption and emission spectra for
2h as a typical example. For all the complexes, the lowest energy
absorption band is attributed to the metal-to-ligand (X2bpy)
charge-transfer transition. This assignment is based on a
considerable solvatochromic shift of the band and substantial
The strong absorption band around 290 nm can be assigned
to the π-π* transition localized on the bpy ligand, which shows
relatively low sensitivity to both solvent polarity and electronic
properties of the ligand L.42 The π-π* transition band localized
on the phenyl groups of the PPh3 ligand appears around 268
nm.42
The complexes which have two phosphorus ligands, except
for 2j, emit around 620 nm with emission lifetimes of 250-
640 ns at room temperature. The emission was efficiently
quenched upon addition of O2. We recently reported that the
emissive state of 2d is triplet MLCT, which is the lowest excited
state.10 Since the emission properties of 2c, 2g, 2h, and 2i
are similar to those of 2d, their emissive states should also be
3MLCT. On the other hand, 2b and 2j emit with lower emission
quantum yields and shorter lifetimes compared with the
complexes described above (Table 5). In contrast, the complexes
(35) Anderson, P. A.; Keene, F. R.; Horn, E.; Tiekink, E. R. T. Appl.
Organomet. Chem. 1990, 4, 523-533.
(36) (a) Stor, G. J.; Hartl, F.; Vanoutersterp, J. W. M.; Stufkens, D. J.
Organometallics 1995, 14, 1115-1131. (b) Johnson, F. P. A.; George,
M. W.; Hartle, F.; Turner, J. J. Organometallics 1996, 15, 3374-
3387. (c) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A.
J. Chem. Soc., Dalton Trans. 1992, 1455-1463. (d) Paolucci, F.;
Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C. A.; Amatore, C.
J. Phys. Chem. B 1998, 102, 4759-4769. (e) Klein, A.; Vogler, C.;
Kaim, W. Organometallics 1996, 15, 236-244.
(37) Cotton, F. A.; Kraihazel, C. S. J. Am. Chem. Soc. 1962, 84, 4432.
(38) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London,
1975.
(39) Burdett, J. K.; Poliakoff, M.; Timmey, J. A.; Turner, J. J. Inorg. Chem.
1978, 17, 948.
(40) For tricarbonyl complexes, analysis of the force fields can be done
via tedious experimentation, e.g., the use of 13CO isotope labeling, or
using arbitrary approximations. This is necessary as these complexes
exhibit only three ν(CO) frequencies, but possess four force constants.
Gamelin, D. R.; George, M. W.; Glyn, P.; Grevels, F. W.; Johnson,
F. P. A.; Klotzbucher, W.; Morrison, S. L.; Russell, G.; Schaffner,
K.; Turner, J. J. Inorg. Chem. 1994, 33, 3246-3250.
(42) The absorption maxima of free bpy and PPh3 were observed at 280
and 236 nm and 260 and 194 nm in MeCN solution, respectively.
(41) Morrison, S. L.; Turner, J. J. J. Mol. Struct. 1994, 317, 39.

Synthesis of Biscarbonylrhenium(I) Complexes
Inorganic Chemistry, Vol. 39, No. 13, 2000 2783
or Cl- complexes (2b, 2e) were irreversibly reduced as shown
in Figure 4b, giving the solvento complex 2a, the E1/2red of which
is -1.69 V, as the product (Table 6). It has been reported that
the one-electron reduction of the tricarbonyl complexes with
L′ ) Cl- or pyridine causes dissociation of the L′ ligand, while
the one-electron-reduced species of [Re(bpy)(CO)3{P(OEt)3}]+
is much more stable because of the stronger π-acidic character
of the P(OEt)3 ligand.2a,b,36,43 The one-electron-reduced species
of 2b and 2e should be unstable enough to undergo a facile
ligand substitution reaction as shown in eqs 3 and 4.43,44
2b•- + MeCN f 2a•- + py
2c•- + MeCN f 2a•- + Cl-
(3)
(4)
Table 6 summarizes the electrochemical properties of 2. It is
noteworthy that the first oxidation waves for the complexes with
acetonitrile, pyridine, or Cl- ligand (2a, 2b, 2e, 2f) are reversible
on a time scale of the cyclic voltammetric measurements (Figure
4b). This is in contrast to the irreversible anodic waves for most
of the reported tricarbonyl diimine complexes.36c,d The oxidized
complexes of 2a, 2b, 2e, and 2f are stabilized to a greater degree
than the oxidized tricarbonyl complexes, probably arising from
much weaker π-acidities and the stronger σ-basicities of the
acetonitrile, pyridine, and Cl- ligands, compared to the axial
CO ligand of the latter. On the other hand, only quasi-reversible
or irreversible oxidation waves were observed for the complexes
with two phosphorus ligands, which are relatively strongly
π-acidic (Figure 4a). Inspection of the data in Table 6 clearly
indicates that the oxidation potentials are more strongly affected
by the electronic properties of the ligand L than the reduction
potentials (for example, 2a-g), while the substituents X on the
bpy ligand give important effects on the reduction potentials
(2d, 2i, 2j). These results suggest that the anodic waves of 2
are attributable to a metal-based oxidation, e.g., ReI/ReII. This
is analogous to the behavior of Re(R-diimine)(CO)3X (X )
halide).44
Figure 4. Cyclic voltammograms of 0.5 mM (a) 2c and (b) 2b in
acetonitrile solutions containing 0.1 M n-Bu4NBF4 taken using a glassy-
carbon working electrode, a Pt wire counter electrode, and a Ag/AgNO3
reference electrode. The scan rate was 200 mV/s. The cathodic curves
were obtained by scanning 1.5 times.
containing the Cl- or MeCN ligand (2a, 2e, and 2f) reveal no
or only weak emission at room temperature. Most of 2 show
3
the shorter lifetime of the MLCT excited state and the lower
emission quantum yield than the corresponding tricarbonyl
complexes, such as [Re(bpy)(CO)3{P(OEt)3}]+ (τem ) 1034 ns,
Φem ) 0.088, λmax ) 522 nm in MeCN), [Re(bpy)(CO)3(py)]+
(τem ) 669 ns, Φem ) 0.16, λmax ) 558 nm in MeCN), and
[Re(bpy)(CO)3(MeCN)]+ (τem ) 1201 ns, Φem ) 0.41, λmax
)
530 nm in MeCN).15,45 One of the reasons should be the smaller
3
energy gap of 2 between the MLCT excited state and the
ground state than those of the tricarbonyl complexes. The energy
gap low, however, does not interpret the “abnormally” lower
emission quantum yields of 2a, 2b, and 2f. Although, in this
point, we do not know the reason, it is noteworthy that 2 h of
irradiation to an acetonitrile solution containing 2b and P(OEt)3
did not cause decomposition of the complex at all, and
consequently, photolability of the complex is not the reason.
Figure 4a illustrates the cyclic voltammograms of 2c mea-
sured for an acetonitrile solution. In the cathodic scan, one
reversible wave and another irreversible wave were observed,
and similar cathodic waves were observed for the complexes
which have two phosphorus ligands, i.e., 2c, 2d, and 2g-j. The
electrochemical properties of the tricarbonyl bipyridine com-
plexes [Re(X2bpy)(CO)3(L′)]n+ (L′ ) halide, pyridine, MeCN,
PR3, etc.) have been well investigated, and it has been
established that the one-electron reduction of these complexes
is attributable to a bpy-based reduction.2,15,36,43 In a previous
paper,10 we have reported that the first reversible reduction of
2d yields [ReI(bpy•-)(CO)2{P(OEt)3}2] on the basis of spec-
troscopic data obtained by flow electrolysis. In an analogy,
therefore, the reversible wave of the other complexes might also
be attributable to a bpy-based reduction. In contrast, the pyridine
Conclusion
The present investigation has provided the first examples for
the photochemical substitution of the axial CO ligand in
tricarbonylbipyridinerhenium complexes with a phosphorus
ligand. The photochemical ligand substitution reaction opens
up synthetic routes for a new class of the biscarbonylbipyridine-
rhenium complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+. It
has been found that they are relatively stable for the most part
and, in some cases, are strongly emissive even in a fluid solution
at room temperature. This may suggest their suitability for use
as photocatalysts. There is potential for the synthetic methods
to be applied to make a building block for “linear-shaped” (one-
dimensional) supramolecules because the substitutable ligands,
i.e., PR3 and Y, are in positions trans to each other. These studies
are underway in our laboratory.
Acknowledgment. We thank Dr. C. Pac (Kawamura Institute
for Chemical Research) for his comments.
Supporting Information Available: An X-ray crystallographic file
for 2h, in CIF format, and H NMR chemical shift data for 2. This
material is available free of charge via the Internet at http://pubs.acs.org.
1
IC991190L
(43) (a) Hori, H.; Ishihara, J.; Koike, K.; Takeuchi, K.; Ibusuki, T.; Ishitani,
O. J. Photochem. Photobiol., A 1999, 5159, 1-6. (b) Hori, H.; Johnson,
F. P. A.; Koike, K.; Takeuchi, K.; Ibusuki, T.; Ishitani, O. J. Chem.
Soc., Dalton Trans. 1997, 1019-1023.
(44) Breikss, A. I.; Abrun˜a, H. D. J. Electroanal. Chem. 1986, 347-358.
(45) Casper, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 13, 359.