Photophysical and photochemical properties of W(0) and Re(I) carbonyl complexes incorporating ferrocenyl-substituted pyridine ligands

Article abstract of DOI:10.1021/ic010713y

The photophysical and photochemical properties of a series of W(O) and Re(I) carbonyl complexes incorporating ferrocenyl-substituted pyridine ligands were investigated.

Full text of DOI:10.1021/ic010713y

Inorg. Chem. 2002, 41, 132−135
Photophysical and Photochemical Properties of W(0) and Re(I) Carbonyl
Complexes Incorporating Ferrocenyl-Substituted Pyridine Ligands
Shih-Sheng Sun, Dat T. Tran, Onduru S. Odongo, and Alistair J. Lees*
Department of Chemistry, State UniVersity of New York at Binghamton,
Binghamton, New York 13902-6016
Received July 5, 2001
Chart 1
Introduction
Complexes of polypyridyl d6-metal (W0 and ReI) carbonyl
complexes have received a great deal of attention because
of their relatively straightforward synthetic procedures and
their intriguing photophysical, photochemical, and electro-
chemical properties.1 Ferrocene and its derivatives have been
studied for their mixed-valence,2 charge-transfer,3 and elec-
trochemical properties;3a,4 their special redox-active features
have also been used to develop electrochemical sensors.5
However, while most of these studies are focused on
homonuclear ferrocenes bridged by different organic spacers,6
the incorporation of a ferrocene moiety within metal carbonyl
complexes has been investigated much less.7 A few studies
have appeared which have primarily focused on the synthesis,
electrochemistry, and potential application as nonlinear
optical materials of such species,7 but investigations of their
photophysical and photochemical properties are lacking.8
In this paper, we report the photophysical and photo-
chemical properties of several polypyridyl d6-metal (W0 and
ReI) carbonyl complexes incorporating ferrocene end-capped
pyridine ligands. The structures of these complexes and
Experimental Section
ligands are illustrated in Chart 1.
Materials. Complexes 1-4 (see Chart 1) were prepared accord-
ing to published procedures.6,7a Other chemicals were commercially
available and used as received.
* Author to whom correspondence should be addressed. E-mail: alees@
binghamton.edu.
(1) (a) Lees, A. J. Chem. ReV. 1987, 87, 711. (b) Geoffroy, G. L.;
Wrighton, M. S. Organometallic Photochemistry; Academic Press:
New York, 1979.
(2) (a) Liu, T.-Y.; Chen, Y. J.; Tai, C.-C.; Kwan, K. S. Inorg. Chem.
1999, 38, 674. (b) D´ıaz, C.; Arancibia, A. Inorg. Chim. Acta 1998,
269, 246.
(3) (a) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121. (b)
Mata, J. A.; Uriel, S.; Llusar, R.; Peris, E. Organometallics 2000, 19,
3797. (c) Oldenburg, K.; Vogler, A. J. Organomet. Chem. 1997, 544,
101.
(4) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 1998, 37, 894.
(5) Beer, P. D.; Gale, P. A.; Chen, G. Z. J. Chem. Soc., Dalton Trans.
1999, 1897.
(6) Lin, J. T.; Wu, J. J.; Li, C.-S.; Wen, Y. S.; Lin, K.-J. Organometallics
1996, 15, 5028.
(7) (a) Lin, J. T.; Sun, S.-S.; Wu, J. J.; Liaw, Y. C.; Lin, K.-J. J.
Organomet. Chem. 1996, 517, 217. (b) Lin, J. T.; Sun, S.-S.; Wu, J.
R.; Lee, L.; Lin, K.-J.; Huang, Y. F. Inorg. Chem. 1995, 34, 2323. (c)
Yeung, L. K.; Kim, J. E.; Chung, Y. K.; Rieger, P. H.; Sweigart, D.
A. Organometallics 1996, 15, 3891. (d) Kotz, J. C.; Nivert, C. L.;
lieber, J. M.; Reed, R. C. J. Organomet. Chem. 1975, 91, 87.
(8) (a) Miller, T. M.; Ahmed, K. J.; Wrighton, M. S. Inorg. Chem. 1989,
28, 2347. (b) Thomas, K. R. J.; Lin, J. T.; Lin, H.-M.; Chang, C.-P.;
Chuen, C.-H. Organometallics 2001, 20, 557.
General Procedures and Methods. UV-vis spectra were
obtained using a HP 8450A diode array spectrophotometer.
Emission spectra were recorded in deoxygenated solvent solution
at 293 K with an SLM 48000S lifetime fluorescence spectropho-
tometer equipped with a red sensitive Hamamatsu R928 photo-
multiplier tube. The emission lifetimes were collected on a PRA
system 300 time-correlated pulsed single-photon apparatus or using
a Laser Photonics nitrogen laser as the excitation source (337 nm)
and a Tektronix TDS 544A digitizer for decay data acquisition.
The obtained lifetimes were fitted to a single-exponential decay in
each case (PRA system software version 3.0 or Sigma plot version
4.0) and found to be reproducible to within 5%. The average ø2
was less than 1. Detailed procedures for luminescence and lifetime
experiments have been described in previous papers.9 Irradiations
at 405 and 428 nm were carried out with light from an Oriel
(9) (a) Sun, S.-S.; Lees, A. J. J. Am. Chem. Soc. 2000, 122, 8956. (b)
Wang, Z.; Lees, A. J. Inorg. Chem. 1993, 32, 1493.
132 Inorganic Chemistry, Vol. 41, No. 1, 2002
10.1021/ic010713y CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/14/2001

NOTE
Table 1. Absorption and Emission Data
emission
absorptiona
compd
λ
max, nm (ꢀ × 10-3, M-1 cm-1
)
λ
em, nm
472
Φem
<10-4
τ, ns
kr, s-1
1b
229 (74.9), 243 (75.8), 284 (31.0), 328 (14.6),
361 (16.6), 397 (18.6), 502 (sh, 5.1)
255 (64.8), 289 (sh, 37.8), 347 (14.5), 464 (12.7)
230 (68.8), 326 (35.1), 390 (23.3)
250 (27.8), 262 (27.9), 314 (30.0), 319 (31.8),
336 (sh, 23.4), 390 (5.7), 470 (2.7)
32
<3.1 × 103
2b
3b
4c
560
494
562
<10-4
27
35
132
<3.7 × 103
<2.9 × 103
1.4 × 104
<10-4
1.8 × 10-3
a Deoxygenated CH3CN solution at 293 K. b The emission was measured in EPA at 77 K. λex ) 400 nm. c The emission was measured in deoxygenated
CH3CN solution at 293 K. λex ) 370 nm.
Figure 2. Electronic absorption (s) and emission (‚‚‚) spectra of complex
4 at 293 K.
Figure 1. Electronic absorption spectra at 293 K of 2.5 × 10-5
complexes 1 (bold line, s), 2 (light line, s), and 3 (- - -) in CH3CN.
M
Complexes 1-3 do not exhibit room-temperature emission
Instruments 450-W mercury arc lamp using interference filters
(Ealing Corp., 10-nm band-pass) to isolate the excitation wave-
length. Typical light intensity was 5 × 10-8 to 1 × 10-7 einstein/
min. A Lexel Corp. model 95-4 argon ion laser (4 W all lines) was
used to perform the photolysis at 458, 488, and 514 nm with typical
laser powers of between 20 and 30 mW. The photochemical
quantum efficiencies were calculated on the basis of the first 40%
conversion of the reaction, and the errors were found to be within
10% (with quantum efficiencies above 10-6). Detailed procedures
for the photolysis experiments and the calculation of the photo-
chemical quantum yield have been described in previous papers.10
and only show very weak emission in 77 K glasses in the
470-560 nm region with quantum yields less than 10-4. The
emission maxima and lifetimes of complexes 1-3 at 77 K
are also listed in Table 1. The broad, structureless emission
is assigned to the 3MLCT transition, and the emission
maxima from the complexes are in the energy order of 2
(560 nm) < 3 (494 nm) < 1 (472 nm). The lower energy
emission band position of 3 compared to 1 is rationalized
on the basis that the benzene ring in ligand 6 of the complex
is distorted in the excited state in a way that maximizes the
π electron conjugation, thereby lowering the energy of the
π* orbital.12 Alternatively, the reordering of the excited-state
energy may arise from the contribution of reorganization
energy.13 It is noticeable that the lifetimes of complexes 1-3
do not vary much as they are each in the range 27-35 ns.
However, the relatively slow radiative decay process (kr)
indicates that the intramolecular energy transfer from the
3MLCT state to the excited state of the ferrocene moiety is
highly efficient in each of complexes 1-3, even at 77 K.
Figure 2 shows the absorption and emission spectra of 4
in deoxygenated CH3CN solution at 293 K. Excitation of
complex 4 at 337 nm generates an emission maximum at
562 nm with a single-exponential decay lifetime of 132 ns.
In addition, the reduction potentials of complex 4 show that
the bpy-based first reduction potential is significantly lower
than the ligand 7 based reduction potentials.7a On the basis
of the structureless emission spectral profile, long lifetime,
and the reduction potentials, we tentatively assign this
Results and Discussion
Electronic absorption spectral data obtained for 1-4 are
summarized in Table 1. Figure 1 illustrates the absorption
spectra of 1-3 in CH3CN at 293 K. The absorption spectra
of 1-3 display a series of high-energy absorption bands
below 360 nm, which are assigned to the π-π* transitions
localized on the pyridine ligands.6 The lowest energy absorp-
tion bands in 1-3 are assigned to W (dπ) to pyridine ligand
(π*) charge-transfer transitions (MLCT).6,11 They are in the
energy order of 2 (464 nm) < 1 (397 nm) < 3 (390 nm),
which is consistent with the weaker π acceptor nature of
PPh3 compared to CO and the lower amount of conjugation
in ligand 6 compared to ligand 5. The less intense ligand
field (LF) bands from the W(0) center are significantly con-
voluted with the intense π-π* and MLCT transitions, and
their exact energy positions are not specifically identi-
fied.1,11,12
(10) (a) Dunwoody, N.; Sun, S.-S.; Lees, A. J. Inorg. Chem. 2000, 39,
4442. (b) Lees, A. J. Anal. Chem. 1996, 68, 226.
(11) Lees, A. J.; Fobare, J. M.; Mattimore, E. F. Inorg. Chem. 1984, 23,
2709.
(12) (a) Zulu, M. M.; Lees, A. J. Inorg. Chem. 1988, 27, 1139. (b) Manuta,
D. M.; Lees, A. J. Inorg. Chem. 1986, 25, 1354.
(13) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439.
Inorganic Chemistry, Vol. 41, No. 1, 2002 133

NOTE
Table 2. Wavelength Dependence of Photosubstitution Quantum Yields
in CH2Cl2 at 293 Ka
405 nm
428 nm
458 nm
488 nm
514 nm
complex
103Φcr
103Φcr
105Φcr
105Φcr
105Φcr
1
2
3
17
2.5
27
5.1
1.6
6.5
13
3.1
31
4.0
1.1
0.82
0.81
0.53
b
a Photolysis performed in deoxygenated solution of ∼ 4 × 10-5
M
complex containing 0.01 M PPh3 as entering ligand. b The complex absorbs
insufficiently at 514 nm to obtain an accurate quantum yield.
emission band to the triplet dπ (Re) to π* (bpy) CT
transition. Although a single-exponential decay of the
emissive excited state was observed in complex 4, the
3
possibility of a fast equilibrium between the bpy-based -
MLCT and ligand 7 based MLCT states cannot be com-
Figure 3. Spectrophotometric titration curves of complex 4 by Ce(IV) in
CH3CN solution.
3
pletely ruled out. The relatively low emission quantum yield
and shorter lifetime of 4 compared to (bpy)Re(CO)3(4-Etpy)-
(PF6) (Φem ) 0.027 and τ ) 218 ns)14 is attributed to the
presence of low-lying nonemissive LF excited states localized
on ferrocene with the possible involvement of a ligand-to-
ligand (ferrocene to bpy) charge-transfer (LLCT) excited
state.15 Some recent studies on rhenium(I) carbonyl systems
have indicated that low-lying intraligand (IL) states16 or
ligand-to-ligand charge transfer (LLCT) states17 are competi-
tive in excited-state energy relaxation processes with the
lowest energy MLCT state. The participation of these excited
states would largely account for the decreasing emission
quantum yields and shorter lifetimes of the emissive 3MLCT
excited states.
Photosubstitution quantum efficiencies of complexes 1-3
upon photolysis in the presence of excess PPh3 as entering
ligand are collected in Table 2. It was observed that the
MLCT band slowly decreased with an increasing irradiation
period. By comparing the IR and UV-vis absorption spectra
of the photolysis products to literature values,6,18 we can
conclude that the irradiation of complexes 1-3 in the
presence of excess scavenging ligand in solution is the con-
sequence of the dissociation of the pyridine ligands. Scheme
1 illustrates the reactions that occur upon photoexcitation
of (CO)4W(PPh3)2 instead of (OC)3W(PPh3)2(L) for complex
2 indicate the labilization of the weak-field pyridine ligand
1
2
1
2
1
following A1(e4b2 ) f E(e3b2 a1 ) excitation.1,12,18a The
observed wavelength-dependent photochemical quantum
efficiencies, which decrease with lower energy excitation,
3
are consistent with the model of a reactive LF state lying
above the unreactive MLCT state.12 Complex 4 shows no
3
photochemical reactivity under the same experimental condi-
tions. The lack of spectroscopically accessible LF bands in
complex 4 is associated with this photochemical inertness.
Notably, Wrighton and co-workers have reported the
observation of a ligand-to-metal charge transfer (LMCT)
band from the oxidized ferrocenium ion when they performed
controlled-potential electrolysis of a series of ferrocenylphos-
phine-coordinated rhenium(I) tricarbonyl complexes.8a In
addition, Oldenburg and Vogler have observed this LMCT
band from the ferrocenium moiety in complex (dppf)Re-
(CO)3Cl where dppf is 1,1′-bis(diphenylphosphino)ferrocene.3c
UV-vis spectral traces of complexes 1-4 were recorded
during their titration with oxidizing reagent ammonium
cerium(IV) nitrate. In each of complexes 1-3, the addition
of Ce(IV) bleaches the MLCT band and it also causes a slow
decomposition reaction. However, in contrast, there is no
sign of decomposition for complex 4 during the titration.
The spectrophotometric titration curves obtained from com-
plex 4 in CH3CN solution are displayed in Figure 3. It is
clear that there are a series of isosbestic points (311, 351,
393, 426, and 584 nm) in the spectral traces from 4. In
particular, a low-energy band at 722 nm appears which has
an extinction coefficient of 552 M-1 cm-1; this is charac-
teristic of the ferrocenium ion.19
a
Scheme 1
(15) Extended Hu¨ckel calculation derived from CAChe 3.8 shows that the
HOMO is a Fe-centered MO and LUMO is a bpy-centered MO. In
addition, the very low oxidation potential of Fe2+ in ligand 77a also
provides the possible existence of a low-energy emission-silent LLCT
state in complex 4.
(16) (a) Ferraudi, G.; Feliz, M.; Wolcan, E.; Hsu, I.; Moya, S. A.; Guerrero,
J. J. Phys. Chem. 1995, 99, 4929. (b) Guerrero, J.; Piro, O. E.; Feliz,
M. R.; Ferraudi, G.; Moya, S. A. Organometallics 2001, 20, 2842.
(17) (a) Schanze, K. S.; MacQueen, D. B.; Perkins, T. A.; Cabana, L. A.
Coord. Chem. ReV. 1993, 122, 63. (b) Liard, D. J.; Vlcek, A. Inorg.
Chem. 2000, 39, 485.
(18) (a) Dahlgren, R. M.; Zink, J. I. Inorg. Chem. 1977, 16, 3154. (b)
Mathieu, R.; Poilblanc, R. Inorg. Chem. 1972, 11, 1858.
(19) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971,
93, 3603.
a
λ ) 405, 428, 458, 488, and 514 nm.
for complexes 1-3. The formation of (CO)5W(PPh3) instead
of (CO)4W(PPh3)(L) for complexes 1 and 3 and the formation
(14) Tapolsky, G.; Duesing, R.; Meyer, T. J. Inorg. Chem. 1990, 29, 2285.
134 Inorganic Chemistry, Vol. 41, No. 1, 2002

NOTE
Accompanying these changes in the absorption spectra,
the infrared CO stretching frequencies are also shifted to
higher frequencies although to somewhat different extents.
This observation is expected as the oxidized ferrocenium
moiety is more electron-withdrawing than a neutral ferrocene
moiety and, thus, this reduces the amount of π back-bonding
from the metal to the CO ligands.4,7d,8a The order of the
average CO frequency shift on oxidation is 3 (1 cm-1) ∼ 4
(1 cm-1) < 1 (5 cm-1) < 2 (11 cm-1). While complexes 3
and 4 exhibit no significant shift in CO frequencies,
complexes 1 and 2 do show appreciable frequency shifts,
which reflects the stronger interaction between the ferrocene
moiety and the metal center. In the case of complex 4, the
positively charged Re(I) atom seems to neutralize the effect
from the oxidation of ferrocene as this complex has the
shortest separation between the Re(I) and ferrocene moieties.
In the case of complex 3, the lack of planarity between the
phenyl ring and pyridine ring, as evidenced in its solid-state
structure6 as well as the highest energy MLCT transition
among complexes 1-3, is attributed to the small shift in CO
frequencies.
Acknowledgment. We are grateful to the Division of
Chemical Sciences, Office of Basic Energy Sciences, Office
of Science, U.S. Department of Energy (Grant DE-FG02-
89ER14039), for support of this research.
Supporting Information Available: A figure illustrating a
representative spectral sequence obtained upon photolysis at 428
nm of complex 3 in deoxygenated CH2Cl2 solution containing an
excess amount of entering ligand PPh3. A table listing the IR data
of photolysis products of complexes 1-3 and a table summarizing
the CO stretching frequencies of complexes 1-4 and their oxidized
forms. This material is available free of the charge via the Internet
at http://pubs.acs.org.
IC010713Y
Inorganic Chemistry, Vol. 41, No. 1, 2002 135