Laser photolysis studies of the reaction of chromium(III) octaethylporphyrin complex with triphenylphosphine and triphenylphosphine oxide

Article abstract of DOI:10.1021/ic0504487

The photoreaction of the chromium(III) octaethylpoprhyrin complex, [Cr(OEP)(Cl)(L)] (L = H₂O, Py, OPPh₃), in dichloromethane was studied using laser flash photolysis technique. Laser irradiation causes the generation of a coordinately unsaturated intermediate [Cr(OEP)(Cl)], which reacts with ligands in solution to give the parent complex, [Cr(OEP)(Cl)(L)], or a transient species, [Cr(OEP)(Cl)(H₂O)], when L = Py or OPPh ₃. Once produced [Cr(OEP)(Cl)(H₂O)] eventually exchanges the axial H₂O ligand with L to regenerate [Cr(OEP)(Cl)(L)]. The kinetics of the axial ligand substitution reaction was followed spectrophotometrically, and the ligand-concentration dependence of the ligand exchange rate revealed that the reaction occurs via a limited dissociative mechanism. The photoreaction of [Cr(OEP)(Cl)(OPPh₃)] containing excess PPh₃ in the bulk solution leads to the unfavorable coordination of the PPh₃ molecule to the chromium ion to give a transient complex, [Cr(OEP)(Cl)(PPh₃)]. The dynamic and thermodynamic properties of [Cr(OEP)(Cl)(PPh₃)] in solution are discussed on the basis of the kinetic parameters of the dissociation and association reactions of the PPh₃ ligand together with the steric aspect of the molecular structure of the related complexes.

Full text of DOI:10.1021/ic0504487

Inorg. Chem. 2005, 44, 6445−6455
Laser Photolysis Studies of the Reaction of Chromium(III)
Octaethylporphyrin Complex with Triphenylphosphine and
Triphenylphosphine Oxide
Masahiko Inamo,*,† Nao Matsubara,† Kiyohiko Nakajima,† Tsutomu S. Iwayama,‡ Hiroshi Okimi,§ and
Mikio Hoshino*,§
Departments of Chemistry and Physics, Aichi UniVersity of Education, Kariya, Aichi 448-8542,
Japan, and The Institute of Physical and Chemical Research, Wako, Saitama 351-0189, Japan
Received March 25, 2005
The photoreaction of the chromium(III) octaethylpoprhyrin complex, [Cr(OEP)(Cl)(L)] (L
) H2O, Py, OPPh3), in
dichloromethane was studied using laser flash photolysis technique. Laser irradiation causes the generation of a
coordinately unsaturated intermediate [Cr(OEP)(Cl)], which reacts with ligands in solution to give the parent complex,
[Cr(OEP)(Cl)(L)], or a transient species, [Cr(OEP)(Cl)(H2O)], when L
) Py or OPPh3. Once produced [Cr(OEP)-
(Cl)(H2O)] eventually exchanges the axial H2O ligand with L to regenerate [Cr(OEP)(Cl)(L)]. The kinetics of the
axial ligand substitution reaction was followed spectrophotometrically, and the ligand-concentration dependence of
the ligand exchange rate revealed that the reaction occurs via a limited dissociative mechanism. The photoreaction
of [Cr(OEP)(Cl)(OPPh3)] containing excess PPh3 in the bulk solution leads to the unfavorable coordination of the
PPh3 molecule to the chromium ion to give a transient complex, [Cr(OEP)(Cl)(PPh3)]. The dynamic and thermodynamic
properties of [Cr(OEP)(Cl)(PPh3)] in solution are discussed on the basis of the kinetic parameters of the dissociation
and association reactions of the PPh3 ligand together with the steric aspect of the molecular structure of the
related complexes.
Introduction
of the metalloporphyrins is labilized more than several orders
of magnitude in rate as compared with the complex having
nonmacrocyclic ligands, and this labilizing effect due to the
porphyrin ligation plays an important role in the catalytic
activity of the chromium(III) porphyrin complexes.5-17 To
elucidate the mechanism of this labilizing effect, the dynam-
Chromium porphyrins have been extensively studied as
catalysts for various organic syntheses such as the epoxide
carbonylation reaction,1 polycarbonate formation,2,3 and
polyene polymer epoxidation.4 In the former two cases, the
chromium(III) porphyrin complex acts as a Lewis acid in
the catalytic reactions, and the nature of the axial coordina-
tion site of the central chromium atom related to such
catalytic activity of the complex has been extensively
investigated to elucidate the mechanism of the catalytic
reactions. It has been well-recognized that the axial position
(5) Fleischer, E. B.; Krishnamurthy, M. J. Am. Chem. Soc. 1971, 93,
3784-3786.
(6) Fleischer, E. B.; Krishnamurthy, M. J. Coord. Chem. 1972, 2, 89-
100.
(7) Krishnamurthy, M. Inorg. Chim. Acta 1978, 26, 137-144.
(8) Ashley, K. R.; Leipoldt, J. G.; Joshi, V. K. Inorg. Chem. 1980, 19,
1608-1612.
* To whom correspondence should be addressed. E-mail: minamo@
auecc.aichi-edu.ac.jp.
(9) Leipoldt, J. G.; Basson, S. S.; Rabie, D. R. J. Inorg. Nucl. Chem.
1981, 43, 3239-3244.
† Department of Chemistry, Aichi University of Education.
‡ Department of Physics, Aichi University of Education.
§ The Institute of Physical and Chemical Research.
(10) O’Brien P.; Sweigart, D. A. Inorg. Chem. 1982, 21, 2094-2095.
(11) Leipoldt, J. G.; van Eldik, R.; Kelm, H. Inorg. Chem. 1983, 22, 4146-
4149.
(1) Schmidt, J. A. R.; Mahadevan, V.; Getzier, Y. D. Y. L.; Coates, G.
W. Org. Lett. 2004, 6, 373-376.
(12) Leipoldt, J. G.; Meyer, H. Polyhedron 1987, 6, 1361-1364.
(13) Ashley, K. R.; Kuo, J. Inorg. Chem. 1988, 27, 3556-3561.
(14) Ashley, K. R.; Trent, I. Inorg. Chim. Acta 1989, 163, 159-166.
(15) Inamo, M.; Sumi, T.; Nakagawa, N.; Funahashi, S.; Tanaka, M. Inorg.
Chem. 1989, 28, 2688-2691.
(2) Stamp, L. M.; Mang, S. A.; Holmes, A. B.; Knights, K. A.; de Miguel,
Y. R.; McConvey, I. F. Chem. Commun. 2001, 2502-2503.
(3) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.; Holmes, A.
B. Macromolecules 2000, 33, 303-308.
(16) Inamo, M.; Sugiura, S.; Fukuyama, H.; Funahashi, S. Bull. Chem. Soc.
Jpn. 1994, 67, 1848-1854.
(4) Davaras, E. M.; Diaper, R.; Dervissi, A.; Tornaritis, M. J.; Coutsolelos,
A. G. J. Porphyrins Phthalocyanines 1998, 2, 53-60.
(17) Inamo, M.; Nakajima, K. Bull. Chem. Soc. Jpn. 1998, 71, 883-891.
10.1021/ic0504487 CCC: $30.25
Published on Web 08/02/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 18, 2005 6445

Inamo et al.
Tesque) stabilized with 10 ppm 2-methyl-2-butene was used without
further purification. Anal. Calcd for 1 (C36H46ClCrN4O‚0.3H2O):
C, 67.18; H, 7.30; N, 8.70; Cl, 5.51. Found: 67.17; H, 7.32; N,
8.43; Cl, 5.67. Dark violet crystals of [Cr(TPP)(Cl)(OPPh3)] for
the X-ray crystallography were obtained by slow evaporation of
the solvent from a 1,2-dichloroethane-toluene solution. The
precipitated purple crystalline product was filtered and dried. Anal.
Calcd for C62H43N4OPClCr: C, 76.11; H, 4.43; N, 5.73. Found:
C, 76.09; H, 4.62; N, 5.79.
Absorption spectra were recorded on a Hitachi U-3000 spectro-
photometer. Laser photolysis studies were carried out with a Nd:
YAG laser (Surelite, Continuum) equipped with second (532 nm)
and third (355 nm) harmonic generators. The duration of the laser
pulse was 6 ns. The transient spectra were measured by an
intensified charge-coupled device detector (DH 520-18F-01, Andor
Technology). The decay of the transient absorption was monitored
by a detection system (TSP-601, Unisoku, Japan). The intensity of
the analyzing light beam from a xenon lamp (L2195, Hamamatsu
Photonics) passed through a sample cell was measured by a
photomultiplier (R2949, Hamamatsu Photonics) attached to the exit
of a monochromator. The kinetics of the axial substitution reaction
were studied using a stopped-flow spectrophotometer (Unisoku).
The temperature of the solutions was controlled to within (0.1 °C
using a thermostated circulating water bath. The concentration of
the chromium(III) porphyrin complex in dichloromethane was less
than 1.0 × 10-5 mol kg-1; and that of the axial ligand, more than
1.0 × 10-4 mol kg-1. The concentration of water in the dichloro-
methane solution was determined by a Karl Fischer titrator
(CA-06, Mitsubishi Chemicals).
The kinetics of the bleaching and those of the positive transients
were the same for all photoinduced reactions. The experimental
pseudo-first-order rate constant kobsd was obtained from the nonlinear
least-squares analysis of the absorbance-time traces observed for
the laser flash photolysis and stopped-flow experiments. The
reaction curves were averaged several times on the digital oscil-
loscope. The estimated standard deviation of kobsd was less than
(3%.
X-ray Crystallography. A prismatic single crystal (0.18 × 0.24
× 0.50 mm) of [Cr(TPP)(Cl)(OPPh3)] was glued onto a glass fiber
and coated with epoxy resin to avoid intensity changes during the
data collection. After removal from the mother liquor, the crystal
deteriorated gradually due to loss of the crystallization solvent.
Intensity data were collected on a Rigaku RAXIS-RAPID dif-
fractometer with graphite-monochromated Mo KR radiation (λ )
0.710 69 Å) at room temperature. The data were processed by the
PROCESS-AUTO program package. A total of 23 096 reflections
were collected to a maximum 2θ value of 55°, 12 999 of which
were independent (Rint ) 0.018). A set of 7522 reflections
(I > 2σ(I)) was used for the structure determination and refinement.
A numerical absorption correction using NUMABS26 was applied.
The structure was solved by direct methods (SIR92),27 expanded
using Fourier techniques (DIRDIF94),28 and refined by full-matrix
least-squares based on F with anisotropic displacement parameters
for non-hydrogen atoms except for those of toluene and 1,2-
dichloroethane. Hydrogen atoms were placed at idealized positions
and included in the refinement with fixed isotropic thermal
ics of the axial ligand substitution reaction of the chromium-
(III) porphyrin complexes has been studied using various
techniques, including conventional spectrophotometric method
and laser flash photolysis. It has been revealed that in many
cases the substitution reaction proceed via a limiting dis-
sociative mechanism in which a coordinately unsaturated
intermediate species is included. The high reactivity coupled
with the facile ligand substitution reaction at the axial
coordination site of the porphyrin complexes is of funda-
mental interest for the information they provide on the
chemistry of porphyrin complexes in solution.
We have been interested in the dynamics of the photoin-
duced reaction of the chromium(III) porphyrin complexes
as well as the axial ligand substitution, and laser flash
photolysis has been employed to investigate the mechanisms
of various photophysical and photochemical processes,
including photoinduced axial ligand dissociation and recom-
bination reactions.18-23 The reactivity of the metallopor-
phyrins in the ground and excited states are affected by
various factors, i.e., the molecular structure of the complex,
the electronic structure, and the axial ligation.24 In the present
study, in a continuing effort to develop our understanding
of the chromium porphyrin chemistry, we investigated the
reaction of the chromium(III) complex of 2,3,7,8,12,13,17,-
18-octaethylporphyrin with triphenylphosphine in dichloro-
methane with respect to the effect of the steric bulk of the
axial ligand on the dynamics of the ligand substitution
reaction using a nanosecond laser flash photolysis technique.
Studies on the dynamics of the reactions of the triphen-
ylphosphine complex in solution may provide some insight
into the effect of steric bulk around the coordinating
phosphorus atom on the reactivity of the porphyrin complex
toward the axial ligand. The mechanism of the steric effect
of the bulky ligand on the reaction dynamics will be
discussed on the basis of the kinetics of the photoinduced
reactions.
Experimental Section
General Information. Aquachloro(2,3,7,8,12,13,17,18-octaeth-
ylporphyrinato)chromium(III), [Cr(OEP)(Cl)(H2O)] (1), was pre-
pared and purified according to the synthetic procedure for the
chromium(III) 5,10,15,20-tetraphenylporphyrin complex, [Cr(TPP)-
(Cl)(H2O)].18,25 Pyridine (Py, Wako Pure Chemicals) was dried over
solid potassium hydroxide and then distilled. Triphenylphosphine
(PPh3, Wako) was purified by vacuum sublimation. Triphenylphos-
phine oxide (OPPh3, Wako) was recrystallized from a mixture of
ethanol and diethyl ether. Dichloromethane (spectral grade, Nakalai
(18) Inamo, M.; Hoshino, M.; Nakajima, K.; Aizawa, S.; Funahashi, S.
Bull. Chem. Soc. Jpn. 1995, 68, 2293-2303.
(19) Hoshino, M.; Tezuka, N.; Inamo, M. J. Phys. Chem. 1996, 100, 627-
632.
(20) Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Wakatsuki,
Y.; Inamo, M. J. Phys. Chem. A 1998, 102, 1297-1303.
(21) Inamo, M.; Hoshino, M. Photochem. Photobiol. 1999, 70, 596-601.
(22) Inamo, M.; Nakaba, H.; Nakajima, K.; Hoshino, M. Inorg. Chem. 2000,
39, 4417-4423.
(23) Inamo, M.; Eba, K.; Nakano, K.; Itoh, N.; Hoshino, M. Inorg. Chem.
2003, 42, 6095-6105.
(24) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. III, Chapter 1.
(26) Higashi, T. Program for Absorption Correction, Rigaku Corp., Tokyo,
Japan, 1999.
(27) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(28) Beurskens, P. T., Admiraal, G., Beurskens, G., Bosman, W. P., de
Gelder, R., Israel, R., Smits, J. M. M. The DIRDIF-94 Program
System, Technical Report of the Crystallography Laboratory, Uni-
versity of Nijmegen, The Netherlands, 1994.
(25) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am.
Chem. Soc. 1977, 99, 8195-8202.
6446 Inorganic Chemistry, Vol. 44, No. 18, 2005

[Cr(OEP)(Cl)(L)] Reaction with Phosphine Compounds
Figure 2. Transient spectra observed for the dichloromethane solution of
[Cr(OEP)(Cl)(H2O)] in the presence of 2.54 × 10-3 mol kg-1 H2O. The
spectra were taken at 20 ns after a 355 nm laser irradiation.
the reproducibility of the intensity of this weak band was
poor and that its intensity strongly depends on the purity of
PPh3. Spectrum d in Figure 1 is observed when carefully
purified PPh3 is used. However, this spectrum is almost
identical to that of [Cr(OEP)(Cl)(OPPh3)] observed with the
use of commercially available PPh3. These observations
suggest that a trace amount of OPPh3 may be contained in
the solid PPh3 as an impurity even if it is purified with
meticulous care. Probably, OPPh3 may preferentially coor-
dinate to the chromium atom due to the smaller steric
hindrance around the coordinating oxygen atom of OPPh3
than the phosphorus atom in PPh3. The small absorption band
observed in the lower energy region of the Soret band of
[Cr(OEP)(Cl)(OPPh3)], therefore, can be attributed to the
Soret band of [Cr(OEP)(Cl)(PPh3)], which will also be
indicated by the transient absorption spectrum as mentioned
later.
Figure 1. UV-visible absorption spectra of the dichloromethane solution
of the Cr(III)-OEP complex. [Cr(OEP)(Cl)(H2O)] (a), [Cr(OEP)(Cl)-
(OPPh3)] (b), [Cr(OEP)(Cl)(Py)] (c), and the solution containing the
Cr(III)-OEP complex and PPh3 (d).
parameters. All calculations were performed using the teXsan
crystallographic software package of Molecular Structure Corp.29
The relatively high R-factor value is due to the poor crystallinity.
The atoms of toluene and 1,2-dichloroethane were highly disor-
dered. The toluene molecule was found at two disordered positions.
The Cl(2), Cl(3), C(68), and C(69) atoms of 1,2-dichloroethane
were included in the final stages of the refinements with fixed
parameters.
Results
Photoreaction of [Cr(OEP)(Cl)(H2O)] in Dichloro-
methane. The transient absorption difference spectrum
observed after the laser irradiation of the dichloromethane
solution of [Cr(OEP)(Cl)(H2O)] is shown in Figure 2. The
transient spectrum does not show the typical porphyrin
triplet-state band around 500 nm.19 The transient spectrum
decays according to first-order kinetics without the formation
of any permanent photoproducts. Dependence of the pseudo-
first-order rate constants, kobsd, on the concentration of H2O
contained in the dichloromethane solution is shown in Figure
S1 (Supporting Information). We thus attribute the observed
transient spectrum (Figure 2) to that of [Cr(OEP)(Cl)]. This
is consistent with a similar finding for 1 in toluene.23
Therefore, the photoinduced reaction can be expressed by
UV-Visible Absorption Spectra of [Cr(OEP)(Cl)(L)]
(L ) H2O, Py, OPPh3, and PPh3). It has been demonstrated
that the chromium(III) porphyrin complex exists as
[Cr(porphyrin)(Cl)(H2O)] in a dichloromethane solution
which contains a small amount of water.22 The axial Cl ligand
does not dissociate in a solvent with low dielectric constant
such as dichloromethane.25 The axial H2O ligand, on the other
hand, can be easily replaced with an external ligand such as
Py and OPPh3, as evidenced by the UV-visible absorption
spectra shown in Figure 1. The change in the absorption
spectrum upon introducing the ligand, L (L ) OPPh3 or Py),
into the solution of [Cr(OEP)(Cl)(H2O)] can be explained
by the substitution reaction of the axial H2O ligand to give
[Cr(OEP)(Cl)(L)]. Meanwhile, the absorption spectrum of a
solution that contains the Cr(III)-OEP complex and PPh3
is very similar to that of [Cr(OEP)(Cl)(OPPh3)] with a slight
difference around the Soret band, as shown in Figure 1. The
intensity of the Soret band of the Cr(III)-OEP solution
containing PPh3 is slightly lower than that of [Cr(OEP)(Cl)-
(OPPh3)], and an additional weak absorption band appears
at the lower energy region of the Soret band. We found that
[Cr(OEP)(Cl)(H2O)] 9
hν8 [Cr(OEP)(Cl)] + H2O (1)
kH2
[Cr(OEP)(Cl)] + H2O y Oz [Cr(OEP)(Cl)(H2O)] (2)
k-H
O
2
Thus, kobsd is given by
kobsd ) kH O[H2O] + k-H O
(3)
2
2
(29) Crystal Structure Analysis Package, Molecular Structure Corp., 1985
and 1999.
where kH O and k-H O represent the second-order rate constant
2
2
Inorganic Chemistry, Vol. 44, No. 18, 2005 6447

Inamo et al.
Figure 3. Transient spectra observed for the dichloromethane solution of
[Cr(OEP)(Cl)(OPPh3)] in the presence of 1.90 × 10-3 mol kg-1 OPPh3
and 1.83 × 10-3 mol kg-1 H2O. The spectra were taken at 20 ns (a) and 1
µs (b) after a 355 nm laser irradiation.
for the recombination reaction of the H2O molecule with the
photoinduced five-coordinate complex, [Cr(OEP)(Cl)], and
the first-order rate constant for the dissociation of the axial
Figure 4. Plot of the pseudo-first-order rate constant kobsd of the decay of
[Cr(OEP)(Cl)(H2O)] as a function of the concentration of pyridine (A) and
of OPPh3 (B) for the laser photolysis of [Cr(OEP)(Cl)(L)] (L represents
H2O ligand, respectively. Since the rate constants kH O and
2
pyridine and OPPh3, respectively) in dichloromethane. [H2O]/mol kg-1
)
k-H O are also included in the rate law of the photoreaction
(2.2-2.9) × 10-3 (a), (5.3-5.9) × 10-3 (b), (1.3-1.4) × 10-2 (c), (2.3-
2.4) × 10-3 (d), (5.2-5.3) × 10-3 (e), (1.5-1.6) × 10-2 (f). T ) 25.0 °C.
2
of [Cr(OEP)(Cl)(L)], where L represents the axial ligand such
as Py, and the kinetic analysis of the data shown in Figure
S1 (Supporting Information) using eq 3 will be described in
a later section.
Observed transient spectra for the reaction of [Cr(OEP)(Cl)-
(Py)] are shown in Figure S3 (Supporting Information).
The fast process consists of the parallel reversible reactions
of 2 and 4:
Photoreaction of [Cr(OEP)(Cl)(OPPh3)] and [Cr(OEP)-
(Cl)(Py)] in Dichloromethane. The transient spectra ob-
served after laser irradiation of the dichloromethane solution
of [Cr(OEP)(Cl)(OPPh3)] containing an excess amount of
OPPh3 are shown in Figure 3. The decay of the transient
spectrum was found to be biphasic. The faster step of the
decay, which we call the “fast process”, is completed within
10-6 s after the laser pulse, and a much slower process (“slow
process”) follows it. As in the case of the photoreaction of
[Cr(OEP)(Cl)(H2O)], the transient spectrum observed just
after the laser irradiation for [Cr(OEP)(Cl)(OPPh3)] does not
have a broad absorption band around 500 nm that is typical
for the triplet excited state of the porphyrin complex, while
a new absorption band that can be attributed to the five-
coordinate [Cr(OEP)(Cl)] was observed around 430 nm. The
second transient has the same spectrum as [Cr(OEP)(Cl)-
(H2O)] as shown in Figure S2 (Supporting Information) and
is attributed to this species. The five-coordinate complex
should also react with OPPh3 to give the initial complex.
The decay of the first transient spectrum to the second one
can thus be ascribed to the parallel reactions of [Cr(OEP)-
(Cl)] with OPPh3 and H2O in the bulk solution to yield
[Cr(OEP)(Cl)(OPPh3)] or [Cr(OEP)(Cl)(H2O)], respectively.
One of the products of the fast process, [Cr(OEP)(Cl)(H2O)],
is a transient species, and it finally reverts to the initial
complex. The slow process can thus be attributed to the axial
substitution reaction of the H2O ligand in [Cr(OEP)(Cl)-
(H2O)] by the entering ligand OPPh3. A similar photoreaction
was observed for [Cr(OEP)(Cl)(Py)] in the present study.30
kOPPh
[Cr(OEP)(Cl)] + OPPh3 y
3z [Cr(OEP)(Cl)(OPPh3)] (4)
3
k-OPPh
The absorbance-time traces can be expressed as the sum
of two exponential functions. However, it is extremely
difficult to determine rate constants of these reactions from
reaction curves that seem to decay exponentially. Instead,
each rate constant involved in these equations can be obtained
from the kinetic analysis of the slow process.
In the slow process, the decay of the second transient
spectrum to the original spectrum corresponds to the axial
substitution reaction of H2O in [Cr(OEP)(Cl)(H2O)] by the
entering ligand L (L ) Py or OPPh3) and obeys the first-
order kinetics with respect to the porphyrin complex. kobsd
of the slow process was measured in the presence of an
excess amount of the axial ligand L. The dependence of kobsd
on the ligand concentration in the bulk solution for the slow
process of the photoreaction of [Cr(OEP)(Cl)(Py)] and
[Cr(OEP)(Cl)(OPPh3)] is shown in Figure 4. In the case of
[Cr(OEP)(Cl)(Py)], the reaction rate first increases with
increases in the concentration of Py and then decreases at
higher concentrations of Py. The rate constant also depends
on the H2O concentration in the bulk solution. These findings
can be interpreted by the dissociative mechanism with a dead-
(30) In the previous study we described the photoreaction of [Cr(OEP)-
(Cl)(Py)] in toluene.23 The solvent effect on the photoreaction
mechanisms was revealed to be small for these two solvents.
6448 Inorganic Chemistry, Vol. 44, No. 18, 2005

[Cr(OEP)(Cl)(L)] Reaction with Phosphine Compounds
Table 1. Kinetic and Thermodynamic Parametersa for the Axial
Substitution Reaction of [Cr(OEP)(Cl)(L)] at T ) 25.0 °C b
k-H O (s-1
)
4.17 × 105
57.2 ( 2.2
54.5 ( 7.2
2.45 × 109
7.2 ( 0.8
-40.9 ( 2.7
1.51 × 109
6.7 ( 1.5
-46.9 ( 5.0
5.91 × 102
-38.2 ( 1.9
-74.9 ( 6.4
2.33 × 107
17.1 ( 1.1
-46.4 ( 3.7
1.13 × 102
-24.8 ( 1.4
-43.8 ( 5.0
1.61 × 105
63.3 ( 1.6
66.9 ( 5.4
5.53 × 108
12.6 ( 1.7
-35.4 ( 5.7
2
q
∆H-H O
2
q
∆S-H O
2
k
H O (2mol-1 kg s-1
)
q
∆HH O
2
q
∆SH O
k
Py (2mol-1 kg s-1
)
q
∆HPy
q
∆SPy
K
(Py) (mol-1 kg)
∆H(Py)
°
°
∆S(Py)
kOPPh (mol-1 kg s-1
)
3
q
∆HOPPh
3
q
∆SOPPh
K
(OPPh )3 (mol-1 kg)
3
∆H(OPPh )
°
°
3
∆S(OPPh )
3
k-PPh (s-1
)
3
q
∆H-PPh
3
q
∆S-PPh
3
kPPh (mol-1 kg s-1
)
3
q
∆HPPh
3
q
∆SPPh
3
a ∆H, kJ mol-1; ∆S, J mol-1 K-1
.
b Values of the rate and equilibrium
Figure 5. Transient spectra observed for the dichloromethane solution of
[Cr(OEP)(Cl)(OPPh3)] in the presence of 1.90 × 10-3 mol kg-1 OPPh3,
9.80 × 10-3 mol kg-1 PPh3, and 1.45 × 10-3 mol kg-1 H2O (A) and in the
presence of 1.33 × 10-2 mol kg-1 OPPh3, 9.80 × 10-3 mol kg-1 PPh3,
and 1.62 × 10-3 mol kg-1 H2O (B). The spectra were taken at 20 ns (a),
500 ns (b), and 20 µs (c) after a 355 nm laser irradiation.
constants were calculated by using the determined ∆Hq, ∆Sq, ∆H°, and
∆S°.
end complex formation as a preequilibrium of the substitution
reaction (eqs 5-7).
lines in Figures S1 (Supporting Information) and 4 were
calculated using the kinetic parameters obtained.
K(Py)
[Cr(OEP)(Cl)(H2O)] + Py y z [Cr(OEP)(Cl)(HOH---Py)]
(5)
Although the bell-shaped ligand concentration dependence
of kobsd is hardly seen for the slow process of the photo-
reaction of [Cr(OEP)(Cl)(OPPh3)] as shown in Figure 4B,
it is probable that the axial substitution reaction of H2O in
[Cr(OEP)(Cl)(H2O)] by OPPh3 occurs by a mechanism
similar to the case of [Cr(OEP)(Cl)(Py)]. We have analyzed
the kinetic results for this slow process using an equation
k-H
[Cr(OEP)(Cl)(H2O)] y 2Oz [Cr(OEP)(Cl)] + H2O (6)
kH2
O
kPy
[Cr(OEP)(Cl)] + Py
9
8 [Cr(OEP)(Cl)(Py)]
(7)
The dead-end complex, [Cr(OEP)(Cl)(HOH‚‚‚Py)], repre-
sents the complex in which the Py molecule binds to the
hydrogen atom of the coordinating H2O ligand in [Cr(OEP)-
(Cl)(H2O)] by hydrogen bonding.22 Because the formation
of the hydrogen bond makes the O-H and Cr-O bonds
weaker and stronger, respectively, in [Cr(OEP)(Cl)-
(HOH‚‚‚Py)], the dissociation of the axial H2O ligand should
be suppressed. kobsd is expressed by eq 8 by applying the
steady-state approximation to the five-coordinate inter-
mediate, [Cr(OEP)(Cl)].
similar to eq 8 with a fixed value of k-H O determined from
2
the [Cr(OEP)(Cl)(Py)] and [Cr(OEP)(Cl)(H2O)] systems
mentioned above. The obtained kinetic and thermodynamic
parameters listed in Table 1 explain the observed kinetic
behavior shown by the solid lines in Figure 4B.
Photoreaction of [Cr(OEP)(Cl)(OPPh3)] in Dichloro-
methane Containing PPh3. The absorption spectrum of the
Cr(III)-OEP complex was measured in the presence of large
excesses of OPPh3 and PPh3 over the porphyrin complex,
and it was the same as that of [Cr(OEP)(Cl)(OPPh3)]. This
is probably due to the stronger coordinating ability of OPPh3
toward the chromium atom than PPh3. In Figure 5, the
transient absorbance difference spectra observed for the
solution of the Cr(III)-OEP complex containing excess
amounts of OPPh3 and PPh3 are shown. The transient
spectrum observed at 20 ns after a laser irradiation is quite
similar to that observed for the solution of [Cr(OEP)(Cl)-
(OPPh3)] just after laser irradiation (Figure 3a), indicating
the formation of the five-coordinate complex, [Cr(OEP)(Cl)].
On the other hand, the second transient spectrum observed
at 500 ns after a laser pulse is quite different from the second
transient spectrum for the solution that does not contain PPh3.
A new positive absorption band was observed around 470
kobsd ) k-H OkPy[Py] (kH O[H2O] +
2
2
kPy[Py])-1 (1 + K(Py)[Py])-1 (8)
The k-H O and kH O terms in this equation are the rate
2
2
constants for the water dissociation and recombination
reactions, respectively, and they are identical with those in
eq 3. The kinetic results of the slow process of the
photoreaction of [Cr(OEP)(Cl)(Py)] were simultaneously
analyzed together with those of the photoreaction of
[Cr(OEP)(Cl)(H2O)] using a weighted-least-squares fitting
calculation based on eqs 3 and 8. The kinetic and thermo-
dynamic parameters obtained are listed in Table 1. The solid
Inorganic Chemistry, Vol. 44, No. 18, 2005 6449

Inamo et al.
(Cl)(OPPh3)], is relatively slow, the medium process can be
ascribed to the process of equilibration between [Cr(OEP)-
(Cl)(PPh3)] and [Cr(OEP)(Cl)(H2O)] produced through the
fast process. In the time range of the medium process, the
reverse reactions of the formation of these transient com-
plexes are sufficiently fast, while the formation of the final
product [Cr(OEP)(Cl)(OPPh3)] is too slow to participate.
After establishing the equilibrium between [Cr(OEP)(Cl)-
(PPh3)] and [Cr(OEP)(Cl)(H2O)], a much slower reaction
back to [Cr(OEP)(Cl)(OPPh3)] occurs in the microsecond
to millisecond time frame (slow process). The reaction
mechanism is shown in Scheme 1. According to the proposed
mechanism, the ratio of [Cr(OEP)(Cl)(H2O)] over the sum
of [Cr(OEP)(Cl)(PPh3)] and [Cr(OEP)(Cl)(H2O)] after the
medium process can be expressed by
R2 )
k-PPh kH O[H2O](k-H OkPPh [PPh3](1 + K(OPPh )[OPPh3])-1
+
3
2
2
3
3
k-PPh kH O[H2O])-1 (10)
3
2
Figure 6. Absorbance-time traces of the photoreaction of the di-
chloromethane solution of [Cr(OEP)(Cl)(OPPh3)] in the presence of PPh3.
Values of R1 and R2 can be estimated by using rate constants
(vide infra) for the experimental conditions of the transient
spectrum measurements shown in Figure 5: R1 ) 0.40 and
R2 ) 0.24 for Figure 5A; R1 ) 0.43 and R2 ) 0.41 for Figure
5B. These values are consistent with the spectral change of
the medium process shown in Figure 5.
[OPPh3] ) 2.94 × 10-3 mol kg-1, [H2O] ) (2.0-2.2) × 10-3 mol kg-1
10-3 (d), and 1.11 × 10-2 (e). T ) 25.0 °C.
,
and [PPh3]/mol kg-1 ) 0 (a), 2.08 × 10-3 (b), 4.17 × 10-3 (c), 8.34 ×
nm as shown in Figure 5. Within 20 µs this second transient
spectrum gradually changed to the third one, the shape of
which is similar to that of the second transient spectrum.
This spectral change depends on the conditions: the lower
the concentration of OPPh3, the larger the absorbance change
for this process. Because the initial transient spectrum can
be attributed to [Cr(OEP)(Cl)], the second and third transient
spectra can be considered as an indication of the formation
of the transient species [Cr(OEP)(Cl)(PPh3)] and [Cr(OEP)-
(Cl)(H2O)], and the concentration of these two transient
species depends on the ligand concentrations. These species
should be given by the reaction of photoinduced [Cr(OEP)-
(Cl)] and the bulk PPh3 and H2O molecules, respectively.
The third transient spectrum decays back to the initial
absorption spectrum in the time range of microseconds to
milliseconds. The absorbance-time traces at the Soret band
of [Cr(OEP)(Cl)(OPPh3)] are shown in Figure 6, and the
three-step process of the decay of the transient spectrum,
which we call the fast process, medium process, and slow
process, is clearly shown.
As in the case of the fast process of the photoreaction of
the [Cr(OEP)(Cl)(OPPh3)] solution without PPh3, the fast
process of the present reaction system is the three parallel
reversible reactions shown in Scheme 1. The absorbance-
time traces are the sum of three exponential functions, and
we did not analyze the reaction curves to determine the rate
constants involved in these reactions. Instead, the kinetic
parameters can be obtained from the medium process and
slow process. The medium process corresponds to the
equilibration of the transient species, [Cr(OEP)(Cl)(H2O)]
and [Cr(OEP)(Cl)(PPh3)], produced through the fast process.
The reaction is first-order with respect to the complex, and
the dependence of kobsd on the concentration of PPh3 is shown
in Figure 7A.31 The mechanism of this process can be
expressed as
K(OPPh
[Cr(OEP)(Cl)(H2O)] + OPPh3 y
3)z
As mentioned above, the fast process gives a mixture of
the transient species [Cr(OEP)(Cl)(PPh3)] and [Cr(OEP)(Cl)-
(H2O)], and at the end of this process an equilibrium between
these two transient species is not established due to the
relatively slow rate of the reverse reactions. The amount of
these intermediate complexes is proportional to the forward
rate constant of the reaction of [Cr(OEP)(Cl)] with PPh3 or
H2O, and the ratio of [Cr(OEP)(Cl)(H2O)] over the total
amount of these two transient species can be given by
[Cr(OEP)(Cl)(HOH‚‚‚OPPh3)] (11)
k-H
[Cr(OEP)(Cl)(H2O)] y 2Oz [Cr(OEP)(Cl)] + H2O
(12)
kH2
O
kPPh
[Cr(OEP)(Cl)] + PPh3 y 3z [Cr(OEP)(Cl)(PPh3)] (13)
k-PPh
3
By applying the steady-state approximation with respect to
the five-coordinate intermediate [Cr(OEP)(Cl)], the pseudo-
first-order rate constant is expressed as
R1 ) kH O[H2O](kPPh [PPh3] + kH O[H2O])-1
(9)
kobsd ) (k-H OkPPh [PPh3](1 + K(OPPh )[OPPh3])-1
k-PPh kH O[H2O])(kPPh [PPh3] + kH O[H2O])-1 (14)
The least-squares analysis of the data shown in Figure 7A
+
2
3
2
2
3
3
The medium process and slow process can be ascribed to
the processes of equilibration. Since the reaction of [Cr-
(OEP)(Cl)] with OPPh3 to give the final product, [Cr(OEP)-
3
2
3
2
6450 Inorganic Chemistry, Vol. 44, No. 18, 2005

[Cr(OEP)(Cl)(L)] Reaction with Phosphine Compounds
Scheme 1
based on eq 14 was done together with the kinetic results
for the slow process because these two processes include
identical elementary processes of ligand-binding and ligand-
dissociation reactions (vide supra).
expressed by eq 15 by applying the steady-state approxima-
tion to the five-coordinate intermediate [Cr(OEP)(Cl)].
kobsd ) (k-H OkOPPh [OPPh3](1 +
2
3
kPPh [PPh3]kH O-1[H2O]-1))(kH O[H2O] + kPPh [PPh3] +
Once the equilibrium between [Cr(OEP)(Cl)(H2O)] and
[Cr(OEP)(Cl)(PPh3)] is established through the medium
process, then the slower, succeeding reaction (slow process)
occurs, as shown in Figure 6. kobsd depends on the concentra-
tion of H2O and OPPh3 of the bulk solution in the same
manner as the slow process of the photoreaction of [Cr(OEP)-
(Cl)(OPPh3)], and kobsd also depends on the concentration
of PPh3, as shown in Figure 7B. As the concentration of
PPh3 increases, kobsd decreases. Based on the mechanism
shown in Scheme 1, the pseudo-first-order rate constant is
3
2
2
3
kOPPh [OPPh3])-1(1 + K(OPPh )[OPPh3] +
3
3
-1
k-H OkPPh kH O
k
-1[PPh3][H2O]-1)-1 (15)
-PPh3
2
3
2
The least-squares analysis for the kinetic results of the
medium process and slow process, as shown in Figure 7,
was done simultaneously to determine the kinetic parameters
k
and k using values of other rate constants and K
-PPh3 (OPPh3)
PPh3
evaluated from the photoreaction of [Cr(OEP)(Cl)(H2O)] and
[Cr(OEP)(Cl)(OPPh3)]. Obtained values of the kinetic
parameters are listed in Table 1.
Quantum Yield Measurements. The quantum yield, Φdiss
,
for the photodissociation of the axial ligand L (L ) H2O,
Py, and OPPh3) from [Cr(OEP)(Cl)(L)] in dichloromethane
was determined by laser flash photolysis. In the present study,
the laser irradiation of [Cr(OEP)(Cl)(L)] gives solely the five-
coordinate species, [Cr(OEP)(Cl)], immediately after the laser
pulse, and the formation of the triplet excited states could
not be detected. Φdiss is expressed as
-1
Φdiss ) ∆Adiss∆ꢀ1
I
-1NA
(16)
abs
where ∆Adiss, ∆ꢀ1, Iabs, and NA represent the absorbance
change just after the laser excitation at a monitoring
wavelength, the difference in the molar absorption coef-
ficients between [Cr(OEP)(Cl)] and [Cr(OEP)(Cl)(L)], the
number of photons absorbed by [Cr(OEP)(Cl)(L)], and
Avogadro’s number, respectively. The molar absorption
coefficient of the five-coordinate intermediate [Cr(OEP)(Cl)]
was determined by the method previously described.18 Figure
Figure 7. Plot of the pseudo-first-order rate constant kobsd of the medium
process (A) and the slow process (B) of the photoreaction of the
dichloromethane solution of [Cr(OEP)(Cl)(OPPh3)] in the presence of PPh3
as a function of the concentration of PPh3. A: [OPPh3] ) 2.94 × 10-3 mol
kg-1, [H2O] ) (2.0-2.2) × 10-3 mol kg-1. T ) 15.0 (a), 25.0 (b), and
35.0 °C (c). B: [OPPh3]/mol kg-1 ) 2.94 × 10-3 (d) and 8.91 × 10-3 (e).
[H2O] ) (2.0-2.2) × 10-3 mol kg-1. T ) 25.0 °C.
(31) It is difficult to study the dependence of kobsd of the medium process
on the concentration of OPPh3 because the absorbance change becomes
smaller when the concentration of OPPh3 becomes larger. Judging
from the mechanism of the slow process of the corresponding reaction
system, it is probable that the preequilibrium (eq 11) may participate
in the medium process of the photoreaction.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6451

Inamo et al.
Table 2. Kinetic Parametersa for the Axial Substitution Reaction of
[Cr(OEP)(Cl)(OPPh3)] by Pyridine at T ) 25.0 °C b
S4 (Supporting Information) shows the absorption spectrum
of [Cr(OEP)(Cl)] as well as that of [Cr(OEP)(Cl)(L)] (L )
H2O and Py). To estimate the Iabs value, a benzene solution
of zinc(II) tetraphenylporphyrin, [Zn(TPP)], which has the
same absorbance at the laser excitation wavelength (532 nm)
as that of the dichloromethane solution of [Cr(OEP)(Cl)-
(L)], was used as a reference for the quantum yield
measurements. When the benzene solution of [Zn(TPP)] is
subjected to the laser pulse, the triplet excited state of
[Zn(TPP)], which has an absorption band around the 470
nm region, is produced. The absorbance change, ∆AT, at 470
nm after the laser pulse is expressed as
k
(s-1
)
2.48
-OPPh3
q
∆H-OPPh
77.8 ( 0.7
23.5 ( 2.3
6.56
89.1 ( 1.9
69.4 ( 6.4
1.57 × 10-2
8.4 ( 4.0
-6 ( 13
3
q
3
∆S-OPPh
k-Py (s-1
)
q
∆H-Py
q
∆S-Py
k
OPPh /kPy
3
∆HOPPh q - ∆HPy
q
3
∆SOPPh q - ∆SPy
q
3
a ∆H, kJ mol-1; ∆S, J mol-1 K-1
.
b Values of the rate and equilibrium
constants were calculated by using the determined ∆Hq, ∆Sq, ∆H°, and
∆S°.
-1
ligands in the bulk solution, as shown in Figure S5
(Supporting Information). At a constant concentration of
OPPh3, the rate constant first decreases with an increase in
the Py concentration and finally converges to a constant value
at a higher Py concentration, while the rate increases with
an increase in the concentration of OPPh3 at a constant Py
concentration. These results can be interpreted by a dissocia-
tive mechanism, as shown in eqs 20 and 21.
∆AT ) ΦTꢀTIabsNA
(17)
where ΦT (0.83) and ꢀT (7.3 × 104 M-1 cm-1) are the triplet
yield of [Zn(TPP)] and the molar absorption coefficient of
the triplet excited state of [Zn(TPP)] at 470 nm, respec-
tively.32 From eqs 16 and 17, eq 18 is derived:
Φdiss ) ∆Adiss∆AT-1ꢀT∆ꢀ1-1ΦT
(18)
k-OPPh
The values of Φdiss were determined as 0.64 ( 0.03 (L )
H2O), 0.76 ( 0.03 (L ) Py), and 0.64 ( 0.03 (L ) OPPh3).
The effect of dioxygen on Φdiss was investigated under the
conditions of PO2 ) 0-1 atm, and it was revealed that
dioxygen has no effect on the quantum yield of the
photodissociation of L (L ) H2O, Py, and OPPh3) from
[Cr(OEP)(Cl)(L)].
3
[Cr(OEP)(Cl)(OPPh3)] y
z [Cr(OEP)(Cl)] + OPPh3
kOPPh
3
(20)
[Cr(OEP)(Cl)] + Py yk z [Cr(OEP)(Cl)(Py)] (21)
Py
k-Py
By applying the steady-state approximation to [Cr(OEP)-
(Cl)], the pseudo-first-order rate constant can be given by
Axial Substitution Reaction of [Cr(OEP)(Cl)(OPPh3)]
by Py. Laser photolysis provides kinetic parameters for the
dissociation of L from [Cr(OEP)(Cl)(L)] and those for the
binding of L to the five-coordinate complex, [Cr(OEP)(Cl)],
as mentioned above (L ) H2O, PPh3). However, the kinetic
parameters for the axial OPPh3 dissociation reaction from
[Cr(OEP)(Cl)(OPPh3)] were not obtained from these experi-
ments since the rate of this reaction is too slow to participate
in the photoreaction dynamics. To determine these kinetic
parameters, we studied the kinetics of the axial substitution
reaction of [Cr(OEP)(Cl)(OPPh3)] by Py in dichloromethane
using a stopped-flow apparatus. A dichloromethane solution
of [Cr(OEP)(Cl)(OPPh3)] containing an excess amount of
OPPh3 was mixed with the Py solution, and the progress of
the substitution reaction was monitored spectrophotometri-
cally. The spectrum of the initial OPPh3 complex was
converted to that of the Py complex with well-defined
isosbestic points. The overall reaction can be expressed by
eq 19.
kobsd ) (k
[Py] +
-OPPh3
-1
k
OPPh kPy
k
[OPPh3])(kOPPh kPy-1[OPPh3] + [Py])-1 (22)
-Py
3
3
The values of k-OPPh , kOPPh kPy-1, and k-Py at each temperature
3
3
were determined by applying the least-squares fitting cal-
culation to the ligand concentration dependence of kobsd. Since
the Eyring’s plots of k-OPPh , kOPPh kPy-1, and k-Py proved linear
within experimental error, the enthalpy and entropy of
activation were determined by simultaneously fitting the
variable-temperature data to eq 22 and the Eyring equation.
Kinetic parameters thus obtained are summarized in Table
2.
Molecular Structure of [Cr(TPP)(Cl)(OPPh3)]. In the
present study, we investigated the photochemical reaction
of the Cr-OEP complexes in solution. To obtain supporting
evidence about the dynamic properties of the complexes
involved in the steric effect caused by the bulky PPh3 ligand,
we tried to prepare a single crystal of the Cr-OEP complex
having an axial PPh3 ligand and related complexes, but an
X-ray-quality single crystal was not obtained. Instead, we
succeeded in preparing a single crystal of [Cr(TPP)(Cl)-
(OPPh3)]. The molecular structure of [Cr(TPP)(Cl)(OPPh3)]
is shown in Figure 8. Crystallographic data are listed in Table
3. The selected bond lengths and angles are listed in Table
4. The four equatorial Cr-N lengths (2.027(4), 2.030(4),
2.028(4), and 2.033(4) Å) are close to each other. The
average Cr-N bond length of 2.030 Å is almost the same
as those of other Cr(III) porphyrins.17,18,22 The axial Cr-Cl
3
3
[Cr(OEP)(Cl)(OPPh3)] + Py h [Cr(OEP)(Cl)(Py)] +
OPPh3 (19)
The reaction rates were measured under various conditions
where entering and leaving ligands were present in large
excess over the porphyrin complex. The reaction was first-
order with respect to the porphyrin complex. kobsd was plotted
against the ratio of the concentration of entering and leaving
(32) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983,
38, 9-14.
6452 Inorganic Chemistry, Vol. 44, No. 18, 2005

[Cr(OEP)(Cl)(L)] Reaction with Phosphine Compounds
Figure 8. Molecular structure of [Cr(TPP)(Cl)(OPPh3)].
Table 3. Crystallographic Data and Experimental Details
Table 4. Selected Bond Lengths (Å) and Angles (deg) of the Porphyrin
Complex
compound
formula
M
cryst syst
space group
a (Å)
[Cr(TPP)(Cl)(OPPh3)]‚0.5C7H8‚0.5C2H4Cl2‚H2O
C
66.5H51N4O2PCl2Cr
1092.05
monoclinic
P21/n
24.8385(4)
17.9520(4)
13.3714(2)
90
89.699(2)
90
4
5962.2(2)
3.03
0.9083-0.9644
0.18 × 0.24 × 0.50
1.092
Cr(1)-Cl(1)
2.300(2)
2.033(4)
2.027(4)
2.030(4)
Cr(1)-N(3)
Cr(1)-N(4)
P(1)-O(1)
2.028(4)
2.033(4)
1.489(4)
Cr(1)-O(1)
Cr(1)-N(1)
Cr(1)-N(2)
Cl(1)-Cr(1)-O(1)
Cl(1)-Cr(1)-N(1)
Cl(1)-Cr(1)-N(2)
Cl(1)-Cr(1)-N(3)
Cl(1)-Cr(1)-N(4)
O(1)-Cr(1)-N(1)
O(1)-Cr(1)-N(2)
O(1)-Cr(1)-N(3)
179.1(1)
O(1)-Cr(1)-N(4)
N(1)-Cr(1)-N(2)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-N(4)
N(2)-Cr(1)-N(3)
N(2)-Cr(1)-N(4)
N(3)-Cr(1)-N(4)
Cr(1)-O(1)-P(1)
89.4(2)
90.5(2)
179.4(2)
89.6(2)
89.8(2)
177.5(2)
90.1(2)
174.0(2)
b (Å)
c (Å)
90.7(1)
91.1(1)
89.8(1)
91.4(1)
88.8(2)
88.1(1)
90.6(2)
R (deg)
â (deg)
γ (deg)
Z
V (Å3)
µ(Mo KR) (cm-1
transm factor
cryst size (mm3)
)
(rms) displacement for the entire core is 0.034 Å. The
chromium atom is located 0.043(1) Å from the mean plane
of the porphyrin core toward the chlorine atom. The Cl-
Cr-O angle of 179.1(1)° and the N-Cr-O angles which
span the range 89.8(1)-91.4(1)° are consistent with an in-
plane arrangement of the central chromium atom. The Cr-
O-P angle of 174.0(2)° is larger than the corresponding
value of 154.2(5)° for [Os(OEP)(OPPh3)2],33 but it is still
within the range of the metal-O-P angles in OPPh3
complexes of around 140-180°.
d
calcd (g cm-3
)
λ(Mo KR) (Å)
0.710 69
25
55
0.0838
0.1174
T (°C)
2θmax (deg)
R a
b
Rw
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b Rw ) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]1/2; w )
2
2
2
[σc (Fo) + (p2/4)|Fo| ]-1; p ) 0.0700.
bond length of 2.300(2) Å is close to that of the related
complexes, i.e., [Cr(TPP)(Cl)(Py)] (2.311(2) Å),18 [Cr(TPP)-
(Cl)(1-MeIm)] (2.317(2) Å, 1-MeIm ) 1-methylimidazole),17
[Cr(TPP)(Cl)(1,2-Me2Im)] (2.315(2) Å, 1,2-Me2Im ) 1,2-
dimethylimidazole),17 and [Cr(TPP)(Cl)(H2O)](2-MePy)2
(2.3114(7) Å, 2-MePy ) 2-methylpyridine).22 The axial
Cr-O bond length is 2.033(4) Å, which is also close to the
distance between Cr and the oxygen atom of the axial H2O
ligand in [Cr(TPP)(Cl)(H2O)](2-MePy)2 (2.057(2) Å).22
Figure S6 (Supporting Information) shows the perpendicular
displacements of the atoms from the mean plane of the
respective 24-atom porphyrin core. The root mean square
Discussion
It has been well-recognized that the excited singlet and
triplet states of the porphyrin π system of the chromium-
(III) porphyrin complex weakly interact with the central
chromium atom (S ) 3/2), giving singquartet (4S1), trip-
doublet (2T1), tripquartet (4T1), and tripsextet (6T1) excited
6
states.34,35 The lowest and second-lowest excited states, T1
(33) Che, C.-M.; Lai, T.-F.; Chung, W.-C.; Schaefer, W. P.; Gray, H. B.
Inorg. Chem. 1987, 26, 3907-3911.
Inorganic Chemistry, Vol. 44, No. 18, 2005 6453

Inamo et al.
Table 5. Kinetic and Thermodynamic Parameters for the Reaction of
[Cr(OEP)(Cl)] with L at T ) 25.0 °C
constant, kL, is extremely large, and the difference in these
activation parameters is not very large, indicating that the
reactivity of the five-coordinate intermediate toward the
ligand association reaction is so high that the discriminating
ability of the intermediate toward the entering ligand is poor.
These findings are in accord with a small activation enthalpy
and negative activation entropy for the axial ligand binding
reaction. The energy required for orientation of the reacting
molecules in a favorable configuration may be small for the
present reaction, like other metalloporphyrins with very large
rate constants (in the range of 108-109 mol-1 kg s-1) for
the axial ligand rebinding for Fe(II)36-38 and Co(III)39-41
porphyrins.
a
kL (mol-1
kg s-1
k-L
(s-1
KL
∆HL°
∆SL°
)
)
(mol-1 kg) (kJ mol-1
)
(J mol-1 K-1
)
H2O
Py
2.45 × 109 4.17 × 105 5.9 × 103
-50
-82
-61
-51
-95
-116
-70
1.51 × 109 6.56
2.3 × 108
9.4 × 106
OPPh3 2.33 × 107 2.48
PPh3
5.53 × 108 1.61 × 105 3.4 × 103
-102
a The value of the equilibrium constant KL was estimated using the
relationship KL ) kLk-L
-1
.
and 4T1, are in thermal equilibrium. We previously reported
on the photochemistry of the chromium(III) porphyrin
complexes with various axial ligands in order to clarify the
mechanism of photophysical and photochemical processes,
including the axial ligand dissociation and recombination
reactions.18-23 Laser photolysis studies have revealed that
photoinduced physical and chemical processes depend on
the nature of the porphyrin ligand and the chemical bond
between the chromium atom and the axial ligand, as well as
the reaction medium. Based on the quantum yield measure-
ments, it was found that the photoelimination of the axial
ligand occurs via two reaction paths, i.e., those through the
4S1 and 6T1 excited states, and that the yield of the
photoelimination largely depends on the donor ability of the
axial ligand. As for the tetraphenylporphyrin complex,
[Cr(TPP)(Cl)(Py)], the quantum yield of the axial Py
dissociation in the toluene solution is 0.65, in which the
quantum yield of 0.18 and 0.47 is through the singlet and
triplet excited states, respectively.21 The corresponding
quantum yield for the OEP complex, [Cr(OEP)(Cl)(Py)], in
toluene is 0.89, which was solely attributed to the singlet
excited state.23 The difference in the quantum yield of these
two complexes can be attributed to the nature of the
porphyrin ligand. For the OEP complex, the axial ligand
dissociation through the triplet excited state can only be
observed when the complex has a stronger chemical bond
at an axial site such as [Cr(OEP)(Cl)(1-MeIm)]. In the
present study, based on the independence of the quantum
yield on the dioxygen concentration, it was revealed that the
photodissociation reactions occur at the singlet excited state
for all complexes investigated here. Participation of the triplet
excited state in the photodissociation of the axial ligand
depends on the efficiency of the intersystem crossing from
4S1 to 4T1 competing with the dissociation of the axial ligand
Even though the kL value is extremely large for all ligands,
as mentioned above, its value is somewhat smaller for the
ligand that suffers from the steric hindrance caused by the
substituents around the donor atom of the ligand during the
complexation reaction with [Cr(OEP)(Cl)]: a large entering
ligand such as PPh3 may not favor the bimolecular reaction
due to the low probability of a relative geometrical arrange-
ment of the colliding molecules, and in addition, the
activation enthalpy for PPh3 is higher than that of H2O by
about 5 kJ mol-1, which implies that [Cr(OEP)(Cl)] prefers
H2O to PPh3 energetically. It is noteworthy that the most
crowded PPh3 ligand shows higher reactivity than the less
crowded OPPh3 ligand, probably due to the higher nucleo-
philicity of PPh3. A similar association reaction of PPh3 with
coordinately unsaturated metalloporphyrins has been inves-
tigated using laser flash photolysis, and the observed value
of kL was 6.9 × 107 M-1 s-1 (25 °C) for [Rh(OEP)(I)],42
which is slightly smaller than that of the present complex.
In contrast to the relatively small difference in the
reactivity of the ligand association reaction to [Cr(OEP)-
(Cl)], the rate constant of the dissociation of the ligand L
from [Cr(OEP)(Cl)(L)] extends over 5 orders of magnitude,
and the difference in the activation enthalpy amounts to more
than 30 kJ mol-1. Such features in activation parameters can
be considered an indication that the bond between the
chromium atom and the donor atom of the axial ligand L is
almost broken at the transition state of the axial ligand
dissociation reaction, reflecting the reaction mechanism of
the present ligand substitution process. The difference in the
rate constant of the axial ligand dissociation reaction between
the PPh3 and OPPh3 complexes, which amounts to a factor
of 6 × 104, should reflect the intrinsic properties of the
chemical bond at the axial coordination site of the complex.
The more labile character of PPh3 should reflect the
4
at the S1 state. Therefore, it can be concluded that the
chemical bond between the chromium atom and the axial
ligand L in [Cr(OEP)(Cl)(L)] is not so strong even for L )
4
4
OPPh3 that the intersystem crossing from S1 to T1 could
(36) Lavalette, D.; Tetreau, C.; Momenteau, M. J. Am. Chem. Soc. 1979,
101, 5395-5401.
not be observed due to the facile axial ligand dissociation at
the S1 state.
4
(37) Dixon, D. W.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1985, 107,
808-813.
In the present study, the rate constants of the dissociation
reaction of the axial ligand L from [Cr(OEP)(Cl)(L)] (L )
H2O, Py, PPh3, and OPPh3) at the electronic ground state
and those of the ligand association to [Cr(OEP)(Cl)] were
determined as listed in Table 5. The ligand association rate
(38) Maillard, P.; Schaeffer, C.; Te´treau, C.; Lavalette, D.; Lhoste, J.-M.;
Momenteau, M. J. Chem. Soc., Perkin Trans. 2 1989, 1437-1442.
(39) Tait, C. D.; Holten, D.; Gouterman, M. Chem. Phys. Lett. 1983, 100,
268-272.
(40) Tait, C. D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106,
6653-6659.
(41) Hoshino, M.; Kogure, M.; Asano, K.; Hinohara, T. J. Phys. Chem.
1989, 93, 6655-6659.
(34) Gouterman, M.; Hanson, L. K.; Khalil, G.-E.; Leenstra, W. R.; Buchler,
J. W. J. Chem. Phys. 1975, 62, 2343-2353.
(35) Harriman, A. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2727-2734.
(42) Suzuki, H.; Miyazaki, Y.; Hoshino, M. J. Phys. Chem. A 2003, 107,
1239-1245.
6454 Inorganic Chemistry, Vol. 44, No. 18, 2005

[Cr(OEP)(Cl)(L)] Reaction with Phosphine Compounds
intrinsically less attractive interaction of the phosphorus
ligand with the chromium(III) ion compared with the oxygen
ligand. In addition to this, the steric interaction caused by
the steric bulk of the axial ligand may affect the dynamics
of the axial ligand dissociation reaction. Although we were
unable to obtain an X-ray-quality single crystal of the PPh3
complexes in the present study, it is expected that the steric
bulk of the PPh3 ligand might cause the repulsive interaction
with the porphyrin ligand due to the smaller distance between
the phenyl group of PPh3 and the porphyrin π electron
system. Evidence of such interactions can be seen in an
umbrella-like conformation of the porphyrin core with the
pyrrole â-carbon atoms of OEP bowed toward the axial Cl
ligand trans to PPh3 observed in [Rh(OEP)(Cl)(PPh3)].43 This
type of deformation of the porphyrin core can also be
demonstrated in the tetraphenylpoprhyin complex having a
related phosphine ligand, diphenyl(phenylacetenyl)phosphine,
in [Rh(TPP)(CH3)(L)] (L ) diphenyl(phenylacetenyl)phos-
phine).44 The peripheral phenyl substituents of tetraphe-
nylporphyrin are also bowed toward the axial methyl group
to minimize steric interactions between the phosphine ligand
and the phenyl substituents of the porphyrin. Similar steric
interactions can be expected for [Cr(OEP)(Cl)(PPh3)] since
the bond length between the Cr and P atoms should be shorter
than that between the Rh and P atoms. On the other hand,
[Cr(TPP)(Cl)(OPPh3)] shows a relatively planar porphyrin
skeleton, which is documented in Figure S6 (Supporting
Information). The individual atomic displacements for the
porphyrin skeleton are all <0.08 Å, indicating that the steric
interaction between the porphyrin ligand and the axial Cl
and OPPh3 ligands can be negligible. Since the steric bulk
of the peripheral substituents is less pronounced for OEP
than TPP, repulsive interaction due to the OPPh3 ligand is
also negligible for [Cr(OEP)(Cl)(OPPh3)]. Therefore, the
expected repulsive interaction between the porphyrin skeleton
and the PPh3 ligand in [Cr(OEP)(Cl)(PPh3)] should cause
the weakening of the axial bond and be partly reflected in
the more labile nature of the axial PPh3 ligand compared
with the OPPh3 complex.
spectrum similar to that of [Cr(OEP)(Cl)(OPPh3)] as dem-
onstrated in Figure 1. Such a difference in the equilibrium
constant is caused by the much smaller k-L value of the
OPPh3 complex as compared with the PPh3 complex even
though the kL value of OPPh3 is 1/20th of that of PPh3.
Conclusion
Photoreactions of the chromium(III) octaethylpoprhyrin
complex, [Cr(OEP)(Cl)(L)] (L ) H2O, Py, OPPh3), in
dichloromethane were studied using a nanosecond laser flash
photolysis technique. The laser irradiation causes efficient
axial ligand dissociation at the electronic excited state of the
complex, probably at the 4S1 state, to produce a five-
coordinate intermediate, [Cr(OEP)(Cl)], and the generation
of the triplet excited states was not observed. The reactivity
of [Cr(OEP)(Cl)] toward external ligands in the solution is
extremely high, and the transient complex [Cr(OEP)(Cl)-
(H2O)] can easily be produced through the reaction with the
H2O molecule in the dichloromethane solution when L )
Py and OPPh3. Transient spectra show that the axial H2O
ligand is then replaced by a ligand such as Py or OPPh3 to
regenerate the parent complex. Unfavorable coordination of
the PPh3 ligand to [Cr(OEP)(Cl)] was observed during the
photoreaction of the [Cr(OEP)(Cl)(OPPh3)] solution contain-
ing an excess amount of PPh3 in the bulk solution. Although
the steric repulsion between the bulky PPh3 ligand and the
porphyrin skeleton is expected during the formation of
[Cr(OEP)(Cl)(PPh3)], the larger rate constant of the PPh3
binding reaction compared with that for OPPh3 results in
the formation of [Cr(OEP)(Cl)(PPh3)], which is eventually
transformed to the parent complex, [Cr(OEP)(Cl)(OPPh3)],
through the axial substitution reaction. Based on the rate law
and the kinetic parameters of the reaction, a limited dis-
sociative mechanism, including [Cr(OEP)(Cl)] as an inter-
mediate, is ascribed to the photoinduced axial ligand
substitution reactions of [Cr(OEP)(Cl)(L)] in solution.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (No. 16550053 and No.
17550056) from the Ministry of Education, Science, Sports,
and Culture of Japan.
As has been suggested from the UV-visible absorption
spectrum shown in Figure 1, OPPh3 should show a higher
affinity for coordination to the central chromium atom of
the OEP complex than PPh3, and this can be rationalized by
the equilibrium constant of the reaction of the five-coordinate
[Cr(OEP)(Cl)] with ligand L, as listed in Table 5. The
equilibrium constant for OPPh3 is 3000 times larger that that
of PPh3. When PPh3 is possibly contaminated with its
oxidation product, OPPh3, even in a quantity as small as
0.1%, the spectrum of the solution containing the Cr(III)-
OEP complex and PPh3 should exhibit an absorption
Supporting Information Available: Figures reporting the
kinetic results on the photoreaction of [Cr(OEP)(Cl)(H2O)], the
comparison of the transient spectrum of [Cr(OEP)(Cl)(OPPh3)] with
the difference spectrum between [Cr(OEP)(Cl)(H2O)] and
[Cr(OEP)(Cl)(OPPh3)], the transient spectrum of [Cr(OEP)(Cl)(Py)],
the UV-visible absorption spectrum of the five-coordinate
[Cr(OEP)(Cl)], the kinetic results on the substitution reaction of
[Cr(OEP)(Cl)(OPPh3)], a formal diagram of the porphyrin core,
top and side views of the complex, tables reporting bond distances,
angles, and positional and thermal parameters for the structurally
characterized porphyrin complex, and one X-ray crystallographic
file (in CIF format). This material is available free of charge via
the Internet at http://pubs.acs.org.
(43) Thackray, D. C.; Ariel, S.; Leung, T. W.; Menon, K.; James, B. R.;
Trotter, J. Can. J. Chem. 1986, 64, 2440-2446.
(44) Stulz, E.; Scott, S. M.; Bond, A. D.; Otto, S.; Sanders, J. K. M. Inorg.
Chem. 2003, 42, 3086-3096.
IC0504487
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