Synthesis and Excited State Raman Spectroscopy of Sterically Crowded Ruthenium(II) Octaethyltetraphenylporphyrin

Article abstract of DOI:10.1021/ic960773h

Pyridine and CO adducts of Ru^IIOETPP (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphinato) have been prepared and characterized by absorption and resonance Raman (RR) spectroscopy. Ru → porphyrin back-bonding is evident from the ground state RR spectra. Skeletal mode frequency downshifts in Ru^IIOETPP(Py)₂ relative to Ru^IIOETPP(CO)(Py) are comparable to those seen for the TPP (tetraphenylporphyrin) and OEP (octaethylporphyrin) analogs, indicating that distortion of the porphyrin ring from substituent crowding has little effect on the extent of back-bonding. Photoexcited Ru^IIOETPP(Py)₂ has absorption and RR spectra which are similar to those of Ru^IITPP(Py)₂ and are characteristic of a (dππ^*) charge transfer state. As for Ru^IITPP(Py)₂, the photoexcited RR spectrum has features indicating porphyrin anion formation and does not have pyridine anion features, as have been observed for photoexcited Ru^IIOEP(Py)₂. The ethyl substituents raise the porphyrin eg^* orbital above the lowest pyridine π^* orbital in Ru^IIOEP(Py)², but not in Ru^IIOETPP(Py)₂, for which the effect of the phenyl substituents is dominant.

Full text of DOI:10.1021/ic960773h

764
Inorg. Chem. 1997, 36, 764-769
Ar ticles
Synthesis and Excited State Raman Spectroscopy of Sterically Crowded Ruthenium(II)
Octaethyltetraphenylporphyrin
S. E. Vitols,† J. Scott Roman, Daniel E. Ryan,‡ Milton E. Blackwood, Jr., and
Thomas G. Spiro*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544-1009
ReceiVed June 27, 1996X
Pyridine and CO adducts of RuIIOETPP (OETPP ) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphinato)
have been prepared and characterized by absorption and resonance Raman (RR) spectroscopy. Ru f porphyrin
back-bonding is evident from the ground state RR spectra. Skeletal mode frequency downshifts in RuIIOETPP-
(Py)2 relative to RuIIOETPP(CO)(Py) are comparable to those seen for the TPP (tetraphenylporphyrin) and OEP
(octaethylporphyrin) analogs, indicating that distortion of the porphyrin ring from substituent crowding has little
effect on the extent of back-bonding. Photoexcited RuIIOETPP(Py)2 has absorption and RR spectra which are
similar to those of RuIITPP(Py)2 and are characteristic of a (dπ,π*) charge transfer state. As for RuIITPP(Py)2,
the photoexcited RR spectrum has features indicating porphyrin anion formation and does not have pyridine
anion features, as have been observed for photoexcited RuIIOEP(Py)2. The ethyl substituents raise the porphyrin
eg* orbital above the lowest pyridine π* orbital in RuIIOEP(Py)2, but not in RuIIOETPP(Py)2, for which the effect
of the phenyl substituents is dominant.
Introduction
The highly distorted dodecasubstituted porphyrins have
attracted much interest1-3 as model systems for studying the
effects of nonplanar conformational distortions on the biological
properties of the tetrapyrroles found in the photosynthetic
reaction center of Rhodopseudomanas Viridis4,5 and the antenna
system of Prosthecochloris Viridis.6 In the models, steric
crowding of the peripheral substituents takes the place of
environmental influences from the surrounding protein in the
photosynthetic systems7 and permits systematic exploration of
the electronic effects of out-of-plane distortions.
The steric clash of substituents at the meso and pyrrole â
positions (Figure 1) can produce ruffled or saddle-shaped
porphyrins.1,2 The lowered symmetry increases the complexity
* To whom correspondence should be addressed.
† Present address: CST-4 MS J586 TA-46, Spectroscopy and Biophysics
Group, Los Alamos National Laboratory, Los Alamos, NM 87545.
‡ Present address: Department of Biology, California Institute of
Technology, Pasadena, CA 91125.
X Abstract published in AdVance ACS Abstracts, February 1, 1997.
(1) (a) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner,
Figure 1. Structural diagram for RuIIOETPP.
M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851. (b) Senge,
M. O.; Medforth, C. J.; Sparks, L. D.; Shelnutt, J. A.; Smith, K. M.
of the vibrational spectrum by splitting degeneracies and
destroying the mutual exclusion of Raman and infrared modes.3,8,9
Optical properties are also affected. Relative to TPP and OEP
complexes, the Soret and Q bands of OETPP’s are red-shifted,
reflecting destabilization of the highest occupied molecular
orbital.1c The fluorescence spectrum is also red-shifted and
exhibits an anomalously large absorption/fluorescence Stokes
Inorg. Chem. 1993, 32, 1716. (c) Barkigia, K. M.; Renner, M. W.;
Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am. Chem.
Soc. 1993, 115, 3627. (d) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.;
Song, X.; Ema, T.; Nelson, N. Y.; Medforth, C. J.; Smith, K. M.;
Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon, J.-C.; Takeuchi,
T.; Goddard, W. A., III; Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117,
11085-11097.
(2) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1992, 114, 9859.
(3) Piffat, C.; Melamed, D.; Spiro, T. G. J. Phys. Chem. 1993, 97, 7441.
(4) Deisenhofer, J.; Michel, H. Science 1989, 245, 1463.
(5) Horning, T. L.; Fujita, E.; Fajer, J. J. Am. Chem. Soc. 1986, 108, 323.
(6) Tronrud, D. E.; Schmid, M. F.; Matthews, B. W. J. Mol. Biol. 1986,
188, 443.
(7) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am.
Chem. Soc. 1988, 110, 7566.
(8) Shelnutt, J. A.; Majumder, S. A.; Sparks, L. D.; Hobbs, J. D.; Medforth,
C. J.; Senge, M. O.; Smith, K. M.; Miura, M.; Luo, L.; Quirke, J. M.
E. J. Raman Spectrosc. 1992, 23, 525.
(9) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581.
S0020-1669(96)00773-2 CCC: $14.00 © 1997 American Chemical Society

Ruthenium(II) Octaethyltetraphenylporphyrin
Inorganic Chemistry, Vol. 36, No. 5, 1997 765
shift, as well as reduced quantum yield.10-12 Recent time-
resolved emission and picosecond transient absorption studies13
indicate that increased rates of both intersystem crossing (S1/
T1) and internal conversion (S1/S0) account for the reduced
reaction mixture was concentrated under reduced pressure and the
resulting white solid triphenylphosphine oxide was filtered off onto a
glass frit and washed with n-pentane. Most of pentane was removed
by vacuum distillation with a water aspirator. The product distilled as
a clear liquid at 129-130 °C to yield 5.593 g (72.6%) of 3-hexen-2-
one. 1H NMR (CDCl3): δ 6.79 (dt, 1H, J ) 6.1 Hz, H at C-4), 6.00
(dt, 1H, J ) 1.3 Hz, 16.1 Hz, H at C-3), 2.22-2.16 (m, 2H, H at C-5),
2.18 (s, 3H, H at C-1), 1.02 (t, 3H, J ) 7.4 Hz, H at C-6). NMR of
the isolated Ph3PdO (21.6 g, 98.9%) in CDCl3 was identical to that of
a reference sample.
1
quantum yield; a large structural reorganization in the (π,π*)
excited state may account for the large Stokes shift.13
Possible implications of the distortions for the mechanism
of photoinduced electron transfer have been addressed.7 How-
ever, to date, no distorted dodecasubstituted porphyrin has been
made which produces a charge-separated state upon photo-
excitation. To explore this dimension, we have prepared Ru(II)
adducts of OETPP (Figure 1), since ruthenium porphyrins form
photoinduced metal-to-ligand charge transfer states (MLCT)14-16
when the Ru(II) d orbital energies are tuned by appropriate
choice of axial ligand. The MLCT state is the lowest lying
excited state when pyridine is the axial ligand, but it is raised
3-Acetyl-4-ethylpyrrole. According to the literature procedure,19
in a dry flask under argon, 1.34 g of NaH (60% dispersion in mineral
oil, 33.6 mmol) was rinsed with hexane (3 × 7.5 mL), added via syringe
and removed via cannula. Diethyl ether (30 mL) was then syringed
into the flask. A solution of 3-hexen-2-one (1.50 g, 15.3 mmol) and
(p-tolylsulfonyl)methyl isocyanide (TOSMIC) (2.98 g, 15.3 mmol) in
90 mL of 2:1 ether/DMSO was prepared in another flask under argon
and was slowly added via cannula to the NaH slurry over a period of
50 min. Evolution of gas and formation of a tan solid were observed
during the addition. After another 40 min of stirring at room
temperature, the mixture was cooled in an ice bath and the reaction
slowly quenched with 60 mL of of H2O. The aqueous layer was
removed and extracted with ether (4 × 30 mL). The combined ether
extracts were washed with aqueous NaCl and dried over Na2SO4. After
removal of solvent, the residue was taken up in CH2Cl2 and and the
solution chromatographed on neutral alumina (activity I, CH2Cl2) to
yield a pale yellow oil which solidified upon cooling to -15 °C (1.73
g, 82.4%). 1H NMR (CDCl3): δ 8.98 (s (br), 1H, NH), 7.36 (t, 1H, J
) 2.4 Hz, H at C-5), 6.56 (s (br), 1H, H at C-2), 2.78 (q, 2H, J ) 7.4
Hz, CH2), 2.38 (s, 3H, COCH3), 1.17 (t, 3H, J ) 7.4 Hz, CH2CH3).
3,4-Diethylpyrrole. According to established procedures,17,20 to a
stirred mixture of lithium aluminum hydride (LAH) (2.67 g, 70.4 mmol)
in 75 mL of THF under argon at 25 °C was added a solution of 3-acetyl-
4-ethylpyrrole (1.73 g, 12.6 mmol) in 50 mL of THF via cannula over
a period of 2.5 h. The reaction mixture became a pale lime green color
and was allowed to stir for an additional 21.5 h at room temperature,
at which point it was heated to relux for 2 h. The solution was then
cooled to 0 °C and the reaction quenched by the sequential dropwise
addition of 2.7 mL of H2O, 2.7 mL of 15% aqueous NaOH, and 8.1
mL of H2O. Stirring was continued at 0 °C for 30 min, anhydrous
MgSO4 was then added, and stirring was resumed at room temperature
for another 30 min. The solid was removed by suction filtration and
washed with THF. The pale yellow filtrate was concentrated under
reduced pressure to ∼15 mL and then diluted with 50 mL of ether and
50 mL of saturated aqueous NaCl. The aqueous layer was extracted
with ether (3 × 50 mL), and the combined extracts were washed with
aqueous NaCl and dried over Na2SO4. Removal of solvent and brief
vacuum-drying yielded a viscous yellow oil which solidified upon
cooling to -15 °C (1.34 g, 86.5%). 1H NMR (CDCl3): δ 7.82 (s (br),
1H, NH), 6.52 (d, 2H, J ) 2.6 Hz, H at C-2 and C-5), 2.45 (q, 4H, J
) 7.6 Hz, CH2), 1.19 (t, 6H, J ) 7.6 Hz, CH3).
3
above the π-π* state when the axial ligand is CO, which
lowers the dπ orbital energy via back-bonding. It is of interest
to discover whether out-of-plane distortion of the porphyrin
affects the excited state ordering by perturbing the orbital
energies. In addition, the nature of the MLCT state is at issue.
Transient RR spectroscopy has established that the character
of the state depends on the porphyrin substituents.16 In
RuIITPP(Py)2 the acceptor orbital is a porphyrin π* orbital, but
in RuIIOEP(Py)2 the acceptor orbital is, surprisingly, on the axial
pyridine ligands. It is therefore of considerable interest to find
out where the electron goes when the porphyrin contains both
phenyl and ethyl substituents.
Experimental Section
General Information. Visible and FTIR spectra were obtained on
Hewlett-Packard 8452A and Nicolet 5DXB spectrometers, repectively.
1H NMR spectra were taken on a JEOL GSX-270 (270 MHz)
spectrometer. Chemical shifts are reported in ppm relative to residual
solvent resonance (δ 7.24 for CHCl3). FAB-MS analyses were
performed on a Kratos MS5ORFTC mass spectrometer, and 3-nitro-
benzyl alcohol was used to form the sample matrix.
Materials. Methylene chloride was distilled over calcium hydride
and passed through a basic alumina column immediately prior to use.
Benzene and diethyl ether were distilled over sodium/benzophenone.
Tetrahydrofuran was passed through basic alumina and then distilled
from sodium/benzophenone. Dimethyl sulfoxide was freshly distilled
and stored over molecular sieves (4 Å). Propionaldehyde and benzal-
dehyde were freshly distilled before use. Deuteriochloroform was
stored over K2CO3/molecular sieves and passed through a basic alumina
column before use. Ru3(CO)12 was purified by dissolution in benzene,
filtration, and removal of solvent under reduced pressure. All other
reagents were obtained from Aldrich and used without further purifica-
tion.
2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphyrin
[OETPP(H2)]. According to the referenced procedure with some
modification,1a a 2 L, two-necked flask was charged with 1 L of distilled
CH2Cl2, 3,4-diethylpyrrole (1.34 g, 10.9 mmol), and benzaldehyde (1.10
mL, 10.8 mmol). The flask was equipped with a reflux condenser and
was purged with argon for 15 min. The reaction vessel was then sealed
with a septum, and BF3‚OEt2 (0.134 mL, 1.09 mmol) was added via
syringe. The reaction mixture was shielded from ambient light and
was stirred continuously for 75 min at room temperature. The pale
orange solution was then concentrated under reduced pressure to a dark
purple, oily residue, which was dissolved in 75 mL of methanol and
stored overnight at -10 °C. The methanol solution was filtered onto
a glass frit, and the solid was washed with excess MeOH until the
filtrate was practically clear. The mauve solid was reprecipitated from
CH2Cl2/MeOH, filtered off, and washed with methanol to yield 571
mg (25.4%) of porphyrinogen.
3-Hexen-2-one. The synthesis followed established procedures,17,18
with slight modifications. To an argon-purged solution of 1-(tri-
phenylphosphoranylidene)-2-propanone (25.00 g, 78.5 mmol) in 100
mL of CH2Cl2 was added 6.25 mL (86.6 mmol) of propionaldehyde
via syringe. After 72.5 h of stirring at room temperature, the yellow
(10) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barigia, K. M.; Smith,
K. M. J. Am. Chem. Soc. 1991, 113, 4077.
(11) Medforth, C. J.; Smith, K. M. Tetrahedron Lett. 1990, 31, 5583.
(12) (a) Takeda, J.; Ohya, T.; Sato, M. Chem. Phys. Lett. 1991, 183, 384.
(b) Ravikanth, M.; Reddy, D.; Chandrashekar, T. K. J. Photochem.
Photobiol. A: Chem. 1993, 72, 61.
(13) Gentemann, S.; Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith,
K. M.; Fajer, J.; Holten, D. J. Am. Chem. Soc. 1994, 116, 7363.
(14) Levine, L. M. A.; Holten, D. J. Phys. Chem. 1988, 92, 714.
(15) (a) Rodriguez, J.; Kirmaier, C.; Holten, D. Chem. Phys. Lett. 1988,
147, 235. (b) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem.
Soc. 1988, 111, 6100.
To a solution of porphyrinogen (200 mg, 0.24 mmol) in 35 mL of
CH2Cl2 was added 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
(16) Vitols, S. E.; Kumble, R.; Blackwood, M. E., Jr.; Roman, J. S.; Spiro,
T. G. J. Phys. Chem. 1996, 100, 4180.
(17) Ryan, D. E. Ph.D. Dissertation, Princeton University, 1991.
(18) Ramirez, F.; Dershowitz, S. J. Org. Chem. 1957, 22, 41.
(19) Cheng, D. O.; LeGoff, E. Tetrahedron Lett. 1977, 12, 1469.
(20) Chamberlin, K. S.; LeGoff, E. Heterocycles 1979, 12, 1567.

766 Inorganic Chemistry, Vol. 36, No. 5, 1997
Vitols et al.
(216 mg, 0.96 mmol). The pink solution became dark green almost
immediately, and the reaction mixture was heated to reflux under argon
for 30 min. After concentration of the solution via reduced pressure,
the residue was chromatographed on neutral alumina (activity III, 2.5
8 cm) using 2% MeOH in CH2Cl2 as eluant. The major green fraction
was collected, while a small amount of insoluble purple material
remained at the top of the column. This purple material was dissolved
in a 0.4% KOH/ethanol solution, the mixture was filtered, and the filtrate
was combined with the green fraction. After removal of solvents, the
residue was dissolved in CH2Cl2 and methanol. Several drops of
triethylamine were added, and the green solution turned dark orange-
brown. The CH2Cl2 was removed under reduced pressure, and the
remainder was chilled and filtered to yield shimmering blue crystals.
The product was washed with a minimal amount of cold methanol and
vacuum-dried to afford 153 mg (76% based on porphyrinogen) of
OETPP(H)2. Mp f 300 °C. λmax (CH2Cl2), nm (log ꢀ): 350 (4.36),
456 (5.21), 556 (4.14), 602 (4.06), 632 (sh) (3.97), 708 (3.92). 1H
NMR (CDCl3): δ 8.31 (m, 8H, o-H) 7.68 (m 12H, m,p-H), 2.51 (m
(br), 8H, CH), 1.94 (m (br), 8H, CH2), 0.44 (t (br), 24H, CH3), -2.0
(s (br), 2H, NH). FTIR (KBr): 1596, 1442, 1371, 1113, 1054, 1016,
755 (vs), 736, 702 (vs) cm-1. FAB-MS gave a parent ion peak at m/z
840. The precise mass pattern matched very well with a computer-
simulated spectrum of OETPP(H)2 (C60H62N4). The wavelengths of
the electronic absorption bands differ somewhat from those cited in
ref 1a, and they did not change upon addition of triethylamine.
However, when trifluoroacetic acid was added to a solution of the free
base, the color changed from pale to bright green along with the
observed absorption maxima shifts (λmax ) 400, 468, 636, 652, 690
nm). We therefore believe that the free base rather than the dication
is prepared with our protocol.
Figure 2. Absorption spectra for the indicated complexes of
RuIIOETPP.
RuIIOETPP(CO). A 25 mL three-neck flask was charged with
OETPP(H)2 (20 mg, 2.38 × 10-5 mol), Ru3(CO)12 (77 mg, 1.20 ×
10-4), and 10 mL of Decalin. The mixture was refluxed under argon
for 20 h and the reaction monitored by visible spectroscopy. Upon
cooling, the entire mixture was chromatographed on neutral alumina.
After being eluted successively with hexanes, benzene, and di-
chloromethane, the olive-brown fraction was collected using 5% MeOH
in CH2Cl2. This collected band was chromatographed again on alumina
using C6H6, 50:50 C6H6/CH2Cl2 ,and 100% CH2Cl2. The broad olive-
colored band was eluted and the solvent removed under reduced
pressure. Attempts to further purify the product by TLC on silica
(CH2Cl2) resulted in partial decomposition. Therefore, the isolated
brown RuIIOETPP(CO) was vacuum-dried and stored in darkness at
-15 °C. λmax (CH2Cl2): 338, 418, 448 (sh), 558, 596 nm. 1H NMR
(CDCl3): δ 8.28 (d, 8H, J ) 6.7 Hz, o-H), 7.66 (m 12H, m,p-H), 2.19
(m (br), 16H, CH2), 0.46 (t (24H), J ) 7.4, 6.7 Hz, CH3). FTIR
(KBr): 2969, 2929, 2871, 1931 (vs), 1443, 1173, 1019, 760, 744, 702
cm-1. FAB-MS gave the expected base peak at m/z 968 amu (M + 2)
for C61H60N4ORu.
RuIIOETPP(Py)2. A solution of 10 mg of RuOETPP(CO) in 5 mL
of distilled pyridine was prepared under an inert atmosphere of nitrogen.
The olive-brown solution was gently heated to 30 °C while being purged
with argon. The solution was then irradiated by an Xe lamp for 30
min while being flushed with argon and monitored with visible
spectroscopy. The pyridine was removed under reduced pressure, and
the residue was eluted through a neutral alumina column with pure
CH2Cl2. The major brownish-olive fraction was collected, dried under
vacuum, and stored in darkness at -15 °C. λmax (pyridine): 432, 510,
540 nm.
Table 1. Electronic Absorption Maxima (nm) for Various
RuII(porphyrin)(L′)(L) Complexes
Soret
â
R
complex
NiOETPPa
RuOETPP(CO)
solvent [B(0,0)] [Q(1,0)] [Q(0,0)]
other
CH2Cl2
CH2Cl2
434
418
552
558
568
510
518
495
531
506
588
d
d
540
549
d
448 (sh), 596
460 (sh), 494
d
d
RuOETPP(CO)Py pyridine 426
RuOETPP(Py)2
pyridine 432
RuOEP(CO)(Py)b CH2Cl2
396
b
RuOEP(Py)2
benzene 395
521 450, 514
d
d
RuTPP(CO)(Py)c pyridine 412
d
c
RuTPP(Py)2
pyridine 412
426 (sh)
a Data obtained from ref 3. b Data obtained from ref 22. c Data
obtained from ref 14. d Unobserved.
the Soret band maximum.21 Solutions were placed in a 1 cm (TA) or
1 mm (TR3) path length cuvette modified with a Teflon stopcock
attached to the stem to prevent sample contamination with oxygen.
Sample integrity was monitored by comparison of probe-only spectra
taken before and after each time point as well as by comparison of
bulk absorption spectra taken before and after each experiment. TA
spectra were calibrated using HeNe, Ar+, and Kr+ emission lines. The
TR3 difference spectra were obtained by subtracting the “probe-only”
spectra from the “pump + probe” spectra. The intensity ratio for the
subtraction procedure was adjusted to avoid negative peaks.
Results
Ground State Optical Absorption Spectra. Absorption
spectra of the RuIIOETPP species (Figure 2 and Table 1) reveal
red-shifted π-π* electronic transitions, as has been observed
for other OETPP complexes.1a,3 Both the B and Q bands are
found at longer wavelengths than those for corresponding OEP
and TPP analogs. The red shift in the optical spectra of ruffled
porphyrins is attributed to a decrease in the energy separating
the highest occupied molecular π orbitals from the lowest
unoccupied π* molecular orbitals. The decrease in separation
Transient Absorption (TA) and Time-Resolved Resonance Ra-
man (TR3) Spectroscopy. The nanosecond time-resolved absorption
and RR apparatus have been described previously.16 The ground state
RR spectra of the RuIIOETPP complexes were recorded with a triple
polychromator equipped with an intensified diode array detector
(Princeton Instruments) utilizing a 135° backscattering geometry.
For the transient spectroscopy, sample solutions were prepared under
an inert atmosphere of nitrogen in a glovebox (Vacuum Atmospheres
D1-001-SD equipped with an He-493 Dri-Train) and diluted to a final
concentration of ∼1.5 mM (transient absorption) or ∼1 mM (transient
Raman) by assuming an extinction coefficient of 300 000 cm-1 M-1 at
(21) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: New York,
1975.

Ruthenium(II) Octaethyltetraphenylporphyrin
Inorganic Chemistry, Vol. 36, No. 5, 1997 767
Table 2. Resonance Raman Band Frequencies (cm-1
Ru- Ru-
)
modea
OETPP(CO) OETPP(Py)2 ∆νb ∆ν(TPP)c ∆ν(OEP)c
φ4 ν(CC)
1598
1555
1470
1447
1389
1349
1257
1234
1144
904
1596
1552
1470
1442
1385
1342
1257
1226
1141
899
-2
-3
0
-5
-4
-7
0
-8
-3
-5
0
0
d
-4
d
-5
d
-5
d
d
+4
d
-4
-3
-14
0
ν2 ν(CâCâ)
-CH2 scissors
ν3 ν(CRCm)
ν
29 ν(CRCm)
ν4 ν(CRN)sym
-CH2 twist
ν1 ν(CmCPh)
ν5 ν(CâC1)
ν6 pyr breathing
d
d
d
d
a Assignments from ref 3. b Frequency differences between the CO
adduct and the (Py)2 adduct. c (Py)2 minus CO frequency differences
for RuIITPP and RuIIOEP.26 d Unobserved or uncertain.
Figure 3. Resonance Raman spectra of the indicated complexes, with
B-band excitation (413.1 nm). Asterisks mark pyridine solvent bands.
is principally caused by the destabilization of the HOMOs by
the out-of-plane distortions, which raises their energies. In
contrast, the energies of the degenerate LUMOs remain
unchanged.1a,d,3
Relative to those of other metals, however, the Ru(II) B and
Q bands are blue-shifted. This well-known hypsochromic effect
of Ru(II)14,22 is attributed to the strong Ru(II)fporphyrin back-
bonding, which raises the energy of the porphyrin eg* orbitals
via interaction with the Ru(II) dπ orbitals. The effect is
especially pronounced when the axial ligands are pyridine but
is diminished when one or both pyridines are replaced by CO.
The Q-band red shift for RuIIOETPP(CO)(Py) relative to
RuIIOETPP(Py)2, also seen for the OEP and TPP analogs (Table
1), is due to the competition of the CO π* orbitals with the
porphyrin eg* orbitals for the Ru(II) dπ electrons. This
competition diminishes the dπ-eg* interaction and lowers the
eg* orbital energy. For the OEP and TPP analogs, the lowered
HOMO/LUMO gap results in a red shift of the Q bands, but
the B band is unaltered (Table 1). For OETPP, the B band
actually appears to blue shift, but this effect may result from
overlap with another, unidentified electronic transition, since
the 432 nm B band of RuIIOETPP(Py)2 is quite broad, while
RuIIOETPP(CO)(Py) has a 460 nm shoulder on its (narrow) 426
nm B band.
Figure 4. Photoinduced absorption spectra for the indicated complexes
with temporal overlap of pump (416 nm) and probe (broad band) pulses
(0 ns). The top trace, with the probe preceding the pump (-200 ns), is
the background.
transitions.23 In this respect, RuIIOETPP(Py)2 resembles
RuIITPP(Py)2 in showing no evidence for Ru f pyridine charge-
transfer transitions.16
Ground State Resonance Raman Spectra. Further evi-
dence of π-back-bonding effects is seen in the RR spectra of
RuIIOETPP(CO)(Py) and RuIIOETPP(Py)2 (Figure 3). The
bands are assigned with reference to NiOETPP,3 and it can be
seen that the frequencies of several skeletal modes (ν1, ν3, ν4,
ν6, ν29) downshift significantly when CO is replaced by a
pyridine ligand. This effect also occurs in OEP and TPP
adducts26 (Table 2) and is attributed to increased back-donation
to the porphyrin eg* orbitals when competition from the CO
π* orbitals is eliminated. By populating the antibonding eg*
orbital, the enhanced back-donation weakens bonds in the
This unidentified transition is not a Ru f pyridine charge-
transfer transition, since the RR spectra (see below) show no
enhancement of bound pyridine modes. Such modes are seen
in the spectrum of RuIIOEP(Py)2 when the excitation is in
resonance with absorption bands appearing between the B and
Q bands, which are assigned to Ru f pyridine charge-transfer
(23) (a) Schick, G. A.; Bocian, D. F. J. Am. Chem. Soc. 1984, 106, 1682.
(b) Barrow, W. L. Resonance Raman Studies of Charge-Transfer
Compounds. Dissertation Georgia Institute of Technology, 1982.
(24) Choi, S.; Spiro, T. G.; Langry, K. C.; Smith, K. M.; Budd, L. D.;
LaMar, G. N. J. Am. Chem. Soc. 1982, 104, 4345.
(25) Parthasarathi, N.; Hansen, C.; Yamaguchi, S.; Spiro, T. G. J. Am.
Chem. Soc. 1987, 105, 3865.
(26) Kim, D.; Su, O. Y.; Spiro, T. G. Inorg. Chem. 1986, 25, 3993.
(22) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. J. Am. Chem.
Soc. 1978, 100, 3015.

768 Inorganic Chemistry, Vol. 36, No. 5, 1997
Vitols et al.
Figure 5. RR spectra of the excited states of RuOETPP(Py)2 (left, bottom) and RuTPP(Py)2 (right, bottom) obtained by subtracting the ground
state spectra (450 nm pulsed excitation, top) from the spectra induced by pumping at 416 nm and probing at 450 nm, with temporal overlap of the
pulses. The RuOEPTPP(Py)2 ground state spectrum contains strong pyridine solvent bands (1219, 1481, and 1577 cm-1) because of sample dilution
(necessitated by degradation reactions encountered at higher concentrations).
porphyrin ring, leading to downshifts of skeletal modes.
Although the modes differ significantly in frequency and
composition among OEP, TPP, and OETPP,3 the pattern of
frequency shifts for the Ru(II) adducts is similar (Table 2). As
expected, modes associated with internal vibrations of the
substituents shift very little (e.g., the φ4 phenyl mode and the
CH2 scissors mode in the OETPP spectrasFigure 3). Excitation
of RuIIOETPP(CO)(Py) at other wavelengths (457.9 and 472.7
nm) produced spectra with somewhat altered intensity patterns,
but there was no evidence of bound-pyridine modes, as there is
for RuIIOEP(Py)2.23
The resemblance of the two excited state spectra is apparent.
Both are dominated by a strong band near 1200 cm-1, which is
assigned16 to the skeletal mode ν26. They both show a 15 cm-1
downshift of the ν4 mode and enhancement of a neighboring
band assigned to ν29. The enhancement of the non-totally
symmetric modes ν29, and especially ν26, is attributed to a Jahn-
Teller effect in the excited state, which splits the degeneracy
of the eg* acceptor orbitals and renders the vibrational modes
totally symmetric. In the RuIITPP(Py)2 excited state, enhance-
ment is also seen for ν2, downshifted from its ground state
position, and another non-totally symmetric mode, identified
as either ν10 or ν11. For RuIIOETPP(Py)2, these bands are too
weak to be identified reliably. Nevertheless, the similarity of
the two spectra, and their relationship to porphyrin anion RR
spectra,16 makes it apparent that they arise from the same kind
of excited state, namely a Ru(II) f porphyrin charge transfer
state. The downshifts in ν4 and ν2 reflect weakening of
porphyrin bonds when an electron is promoted to the eg*
orbitals.
Transient Absorption Spectra. When RuIIOETPP(Py)2 is
photoexcited, an induced absorption is observed at 685 nm
(Figure 4) in a region characteristic of charge-transfer states.15b
A similar absorption is seen for RuIITPP(Py)2 but with a
maximum at 670 nm. The lowest excited state of RuIITPP-
(Py)2 is known to be charge transfer in character,15b,16 and a
similar excited state is inferred for RuIIOETPP(Py)2. The red
shift in its absorption relative to that of RuIITPP(Py)2 is
consistent with the red-shifts of the π-π* absorptions in the
ground state.
Significantly, no bound pyridine modes are detected in the
excited state spectrum of RuIIOETPP(Py)2, in contrast to that
of RuIIOEP(Py)2. The charge transfer state involves acceptor
orbitals on the porphyrin, not on the pyridine.
TR3 Spectra. Figure 5 compares ground and excited state
RR spectra for RuIIOETPP(Py)2 (left) and RuIITPP(Py)2 (right).
The ground state spectra were obtained with 450 nm excitation
with a 10 Hz, 7 ns H2-Raman-shifted pulsed Nd:YAG laser,
while the excited state spectra were obtained by subtracting the
ground state spectra from the photoinduced spectra produced
by the same probe pulses when temporally overlapped with 416
nm pump pulses. (The appearance of strong peaks from the
pyridine solvent in the ground state spectrum of RuIIOETPP-
(Py)2 but not RuIITPP(Py)2 is due to a lower sample concentra-
tion for the former. The concentration of the RuOETPP(Py)2
sample had to be lowered in order to prevent degradation of
the sample from photochemistry occurring at higher concentra-
tions.)
Discussion
Strong back-bonding from Ru(II) to the porphyrin eg* LUMO
is evidenced by depressed porphyrin skeletal mode frequencies
in the RR spectrum of Ru(II) porphyrins and by the short-lived
dπ f π* character of the lowest electronic excited state.16 Back-
bonding to the porphyrin is weakened when CO is an axial
ligand, because the CO π* orbitals compete effectively for the
Ru(II) dπ electrons; as a result, the skeletal mode frequency
downshifts are relieved and the dπ f π* excited state is raised
in energy above the lowest π-π* state. It is evident that these

Ruthenium(II) Octaethyltetraphenylporphyrin
Inorganic Chemistry, Vol. 36, No. 5, 1997 769
consequences of back-bonding are also seen for RuIIOETPP-
(Py)2: the skeletal mode frequencies are depressed relative to
those of RuIIOETPP(CO), and the lowest excited state is dπ f
π* in character. This was not a foregone conclusion, since
porphyrin ruffling induced by the substituent crowding might
have been expected to diminish back-bonding because of orbital
misalignment. The out-of-plane distortion is probably less
pronounced in RuIIOETPP(Py)2 than in NiOETPP or CuOETPP,
since the distortion increases with decreasing metal-N (pyrrole)
distance, and Ru(II) is larger than Ni(II) and Cu(II); for
RuOEP(CO)(H2O), this distance is 2.051 Å,27 compared to 1.998
Å for CuOEP28 and 1.95829 or 1.929 Å30 for planar or ruffled
allotropes of NiOEP. The out-of-plane distortion in OETPP
remains substantial even for large metal ions, e.g. in ZnOETPP-
(MeOH) (Zn-N ) 2.063 Å.)31 However, it is possible that
RuIIOETPP(Py)2 resists distortion, even at the expense of steric
crowding, in order to preserve the back-bonding. Its crystal
structure would therefore be of considerable interest.
pyridine ligands in RuIIOEP(Py)2.16 The evidence for this
conclusion was that the TR3 spectrum of the ligands in
RuIIOEP(Py)2 showed upshifted porphyrin skeletal modes, as
would be expected for a Ru(III) porphyrin, together with
additional bands which were identified by isotope labeling as
pyridine anion modes. Evidently, the ethyl substituents, which
are electron donating, raise the porphyrin eg* orbitals above the
lowest pyridine π* orbital in RuIIOEP(Py)2. This inference is
supported by the detection of relatively low-energy Ru(II) f
Py charge-transfer transitions that selectively enhance pyridine
RR bands of RuIIOEP(Py)2.23 Such transitions were not found
in RuIITPP(Py)2;16 the phenyl substituents are electron with-
drawing and stabilize the eg* orbitals. Nor are Ru(II) f Py
charge-transfer transitions present in RuIIOETPP(Py)2, whose
TR3 spectrum likewise resembles that of RuIITPP(Py)2. Both
TR3 spectra are consistent with a Ru(III) porphyrin anion
formulation and show no evidence for pyridine anion modes.
Thus, when phenyl and ethyl substituents are both present, the
phenyl influence dominates the ethyl influence. This result is
consistent with the eg* orbital structure, in which the coefficients
are larger on the Cm atoms, where the phenyl substituents are
attached, than on the Câ atoms, where the ethyl substituents
are attached.32
Another point of interest is that the RuIIOETPP(Py)2 TR3
spectrum allows us to conclude that the direction of excited
state electron transfer is dominated by the phenyl rather than
by the ethyl substituents. We recently found that while the
electron is transferred from Ru(II) to the porphyrin in the lowest
excited state of RuIITPP(Py)2, it is transferred instead to the
Acknowledgment. This work was supported by NIH Grant
GM 33576.
(27) Kadish, K. M.; Hu, Y.; Mu, X. H. J. Heterocycl. Chem. 1991, 28,
1821.
(28) Pak, R.; Scheidt, W. R. Acta Crystallogr. 1991, C47, 431.
(29) Cullen, D. L.; Meyer, E. F. J. Am. Chem. Soc. 1974, 96, 2095.
(30) Meyer, E. F.; Acta Crystallogr. 1972, B28, 2162.
(31) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M.
W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851.
IC960773H
(32) Sekino, H.; Kobayashi, H. J. Chem. Phys. 1987, 86, 5045.