Laser photolysis of chromium(III) porphyrins with axial pyridines in dichloromethane and toluene solutions. Novel effects of a hydrogen bond in the ligand exchange reaction

Article abstract of DOI:10.1021/ic0002059

Laser photolysis studies were carded out for (chloro)(pyridine)(5,10,15,20-tetraphenylporphyrinato)chromium-(III), [Cr(TPP)(Cl)(Py)], in both dichloromethane and toluene containing water. The five-coordinate [Cr(TPP)(Cl)] produced by the photoinduced dissociation of pyridine from [Cr(TPP)(Cl)(Py)] initially reacts with H₂O to give [Cr(TPP)(Cl)(H₂O)], which eventually exchanges the axial H₂O with Py to regenerate [Cr(TPP)(Cl)(Py)]. The rate for the ligand exchange of [Cr(TPP)(Cl)(H₂O)] with exogenous Py is found to exhibit a bell-shaped pyridine-concentration dependence. Kinetic studies revealed that at a high Py concentration, the exogenous Py probably makes a hydrogen bond with the axial H₂O of [Cr(TPP)(Cl)(H₂O)] to yield [Cr(TPP)(Cl)(HO-H···Py)] as a dead-end complex. A similar structure of the Cr-TPP complex having 2-methylpyridine molecules bound to the coordinated H₂O ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange reaction of the axial HO-H···Py in [Cr(TPP)(Cl)(HO-H···Py)] by Py follows the dissociative mechanism: The first step is the dissociation of Py from [Cr(TPP)(Cl)(HO-H···Py)], and the second step is the dissociation of H₂O. The five-coordinate [Cr(TPP)(Cl)] thus produced reacts with Py to regenerate [Cr(TPP)(Cl)(Py)]. The direct ligand exchange reaction of the axial HO-H···Py in [Cr(TPP)(Cl)(HO-H···Py)] with exogenous Py does not occur. Probably, the hydrogen bond, HO-H···Py, increases the basicity of H₂O, and thus, the bond energy between Cr and O in [Cr(TPP)(Cl)(HO-H···Py)] becomes much stronger than that in [Cr(TPP)(Cl)(H₂O)]. The mechanism of the ligand substitution reaction of the chromium(III) porphyrins has been examined in detail on the basis of the laser photolysis studies of [Cr(TPP)(Cl)(L)] (L = pyridine, 3-cyanopyridine, and H₂O).

Full text of DOI:10.1021/ic0002059

Inorg. Chem. 2000, 39, 4417-4423
4417
Laser Photolysis of Chromium(III) Porphyrins with Axial Pyridines in Dichloromethane
and Toluene Solutions. Novel Effects of a Hydrogen Bond in the Ligand Exchange Reaction
Masahiko Inamo,* Hideyuki Nakaba, and Kiyohiko Nakajima
Department of Chemistry, Aichi University of Education, Kariya, Aichi 448-8542, Japan
Mikio Hoshino
The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan
ReceiVed February 24, 2000
Laser photolysis studies were carried out for (chloro)(pyridine)(5,10,15,20-tetraphenylporphyrinato)chromium-
(
(
III), [Cr(TPP)(Cl)(Py)], in both dichloromethane and toluene containing water. The five-coordinate [Cr(TPP)-
Cl)] produced by the photoinduced dissociation of pyridine from [Cr(TPP)(Cl)(Py)] initially reacts with H2O to
give [Cr(TPP)(Cl)(H2O)], which eventually exchanges the axial H2O with Py to regenerate [Cr(TPP)(Cl)(Py)].
The rate for the ligand exchange of [Cr(TPP)(Cl)(H2O)] with exogenous Py is found to exhibit a bell-shaped
pyridine-concentration dependence. Kinetic studies revealed that at a high Py concentration, the exogenous Py
probably makes a hydrogen bond with the axial H2O of [Cr(TPP)(Cl)(H2O)] to yield [Cr(TPP)(Cl)(HO-H‚‚‚Py)]
as a dead-end complex. A similar structure of the Cr-TPP complex having 2-methylpyridine molecules bound
to the coordinated H2O ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange
reaction of the axial HO-H‚‚‚Py in [Cr(TPP)(Cl)(HO-H‚‚‚Py)] by Py follows the dissociative mechanism: the
first step is the dissociation of Py from [Cr(TPP)(Cl)(HO-H‚‚‚Py)], and the second step is the dissociation of
H2O. The five-coordinate [Cr(TPP)(Cl)] thus produced reacts with Py to regenerate [Cr(TPP)(Cl)(Py)]. The direct
ligand exchange reaction of the axial HO-H‚‚‚Py in [Cr(TPP)(Cl)(HO-H‚‚‚Py)] with exogenous Py does not
occur. Probably, the hydrogen bond, HO-H‚‚‚Py, increases the basicity of H2O, and thus, the bond energy between
Cr and O in [Cr(TPP)(Cl)(HO-H‚‚‚Py)] becomes much stronger than that in [Cr(TPP)(Cl)(H2O)]. The mechanism
of the ligand substitution reaction of the chromium(III) porphyrins has been examined in detail on the basis of
the laser photolysis studies of [Cr(TPP)(Cl)(L)] (L ) pyridine, 3-cyanopyridine, and H2O).
Introduction
Studies on the photochemistry of the chromium(III) porphy-
rins have revealed that the nature of the excited states of the
complex can be interpreted in terms of weak coupling of the π
Although chromium(III) porphyrins are not naturally occur-
ring substrates in biological systems, their chemical and
photochemical properties have been extensively studied since
the ability of the various metalloporphyrins to characterize
hemoproteins was recognized. Among the various aspects of
their chemical features, interactions between metalloporphyrins
and axial ligands have been focused on in relation to the oxygen
binding ability of hemoproteins. It is commonly recognized that
the substitution reactions of the axial ligands of metallopor-
phyrins proceed much faster than those of other metal com-
plexes. These effects were well documented for the metallopor-
excited states of the porphyrin ligand and the d electrons of the
4
central chromium(III) ion, giving singquartet ( S1), tripdoublet
2
4
6
4
(
T1), tripquartet ( T1), and tripsextet ( T1) excited states. A
laser photolysis study of the chromium(III) porphyrins [Cr(TPP)-
Cl)(L)] (L is an axial ligand) has revealed that photoinduced
(
physical and chemical processes depend on both the nature of
the bond between chromium and the axial ligand L as well as
5
-10
the complex environment.
The mechanism of the photore-
action of chromium porphyrins has been discussed on the basis
of the laser photolysis studies, which revealed that the photo-
elimination of the axial ligand L occurs via two reaction paths,
1,2
phyrins with substitution-inert metal ions such as chromium(III),
which undergo the ligand exchange reaction based on the
dissociative mechanism. The dissociative mechanism of the
ligand exchange reactions of metalloporphyrins assumes the
coordinately unsaturated intermediate as a reactive species. Some
of the six-coordinate metalloporphyrins efficiently undergo
photochemical dissociation of the axial ligand to yield the
4
6
i.e., the path through the S1 and T1 excited states, and that the
yield of the photoelimination largely depends on the donor
10
ability of the axial ligand L. It was found that the coordinately
(
4) Gouterman, M.; Hanson, L. K.; Khalil, G.-E.; Leenstra, W. R.; Buchler,
J. W. J. Chem. Phys. 1975, 62, 2343.
3
coordinately unsaturated intermediate. Thus, the laser photolysis
(5) Yamaji, M.; Hama, Y.; Hoshino, M. Chem. Phys. Lett. 1990, 165,
technique has provided useful information on the ligand
exchange reaction of the metalloporphyrins via coordinately
unsaturated intermediates.
309.
(
6) Yamaji, M. Inorg. Chem. 1991, 30, 2949.
(7) Inamo, M.; Hoshino, M.; Nakajima, K.; Aizawa, S.; Funahashi, S.
Bull. Chem. Soc. Jpn. 1995, 68, 2293.
(
8) Hoshino, M.; Tezuka, N.; Inamo, M. J. Phys. Chem. 1996, 100, 627.
(1) Fleischer, E. B.; Krishnamurthy, M. J. Am. Chem. Soc. 1971, 93, 3784.
(2) O’Brien, P.; Sweigart, D. A. Inorg. Chem. 1982, 21, 2094.
(3) Sima, J. Struct. Bonding 1995, 84, 135.
(9) Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Wakatsuki,
Y.; Inamo, M. J. Phys. Chem. A 1998, 102, 1297.
(10) Inamo, M.; Hoshino, M. Photochem. Photobiol. 1999, 70, 596.
1
0.1021/ic0002059 CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/06/2000

4418 Inorganic Chemistry, Vol. 39, No. 20, 2000
Inamo et al.
unsaturated intermediate generated by the photoinduced axial
ligand dissociation reacts with a water molecule present in the
toluene solution to give [Cr(TPP)(Cl)(H2O)] as an intermediate
during the photoreaction of [Cr(TPP)(Cl)(3-CNPy)] (3-CNPy
)
3-cyanopyridine).7
In the present work, the photoreaction of [Cr(TPP)(Cl)(Py)]
in the dichloromethane and toluene solutions was studied using
a laser photolysis technique. The rate constant kobsd for the ligand
exchange reaction of H2O in the transient [Cr(TPP)(Cl)(H2O)]
with pyridine was found to exhibit a bell-shaped pyridine-
concentration dependence. The value of kobsd initially increases
with an increase in [Py]. After reaching the maximum value,
kobsd decreases smoothly with an increase in [Py]. These results
are interpreted in terms of the formation of a dead-end complex,
[Cr(TPP)(Cl)(HO-H‚‚‚Py)], at the high pyridine concentration.
Such a hydrogen-bonded complex was isolated as a stable
product when [Cr(TPP)(Cl)(H2O)] was recrystallized from a
solvent containing 2-methylpyridine, which does not directly
bind to the chromium atom due to the steric hindrance caused
by the methyl group of 2-methylpyridine, and its molecular
structure was investigated by X-ray crystallography. In this
paper, we describe the chemical properties of this newly found
complex together with the mechanism of the photochemical
reaction of the chromium(III) porphyrins in solution.
Figure 1. Transient spectrum observed for the dichloromethane
-
3
solution of [Cr(TPP)(Cl)(H
2
O)] in the presence of 2.02 × 10 mol
-
1
kg
2
H O at 25.0 °C. The spectrum was taken at 49 ns after a 532 nm
laser irradiation.
Experimental Section
maximum 2θ value of 55°, 9258 of which were independent (Rint
)
General Information. (Aqua)(chloro)(5,10,15,20-tetraphenylpor-
phyrinato)chromium(III), [Cr(TPP)(Cl)(H
0.035). A set of 7119 reflections (I > 2σ(I)) was used for the structure
2
O)], was prepared according
determination. A numerical absorption correction using the program
1
1
12
to the reported procedure. Pyridine and 2-methylpyridine (2-MePy)
dried over solid potassium hydroxide were fractionally distilled before
use. 3-Cyanopyridine was recrystallized from a mixture of o-xylene
and hexane. Toluene was refluxed on sodium, and then distilled.
Spectroscopic grade dichloromethane (Nacarai Tesque, Inc.) was used
without further purification. The preparation of the polystyrene film
doped with the chromium(III) porphyrins was previously described.8
The solution of [Cr(TPP)(Cl)(Py)] in dichloromethane or toluene was
prepared by adding Py to the corresponding solution of [Cr(TPP)(Cl)-
NUMABS was applied. The structure was solved by direct methods
13
14
(SIR92 ) and expanded using Fourier techniques (DIRDIF94 ). All
non-hydrogen atoms were anisotropically refined. Hydrogen atoms
except for those on O(1) were placed in idealized positions, which were
1
5
not refined. All calculations were performed using the teXsan
crystallographic software package from the Molecular Science Corp.
Results
Laser Photolysis of [Cr(TPP)(Cl)(H2O)] in Dichloro-
methane. The photoreaction of [Cr(TPP)(Cl)(H2O)] was studied
in dichloromethane. Figure 1 shows the transient spectrum
obtained at 49 ns after a 532 nm laser pulse at 25.0 °C. The
spectrum exhibits a positive peak at 435 nm and a negative
one at 450 nm. The transient spectrum decays according to
pseudo-first-order kinetics and eventually restores the initial
(
[
H
2
O)]. Dark violet crystals for the X-ray crystallography including
Cr(TPP)(Cl)(H O)] and 2-MePy were obtained by slow evaporation
-toluene solution. Anal. Calcd for C56 OClCr: C,
4.37; H, 4.90; N, 9.29. Found: C, 74.07; H, 4.95; N, 8.99.
UV-vis absorption spectra were recorded on a Hitachi U-3000
2
of a CHCl
7
3
44 6
H N
spectrophotometer. The concentration of water in the toluene and
dichloromethane was determined using a Karl Fischer titrator (CA-06,
Mitsubishi Chemical Co., Ltd.). Laser photolysis studies were carried
out with a Nd:YAG laser (Model Surelite 10, Continuum) equipped
with second (532 nm) and third (355 nm) harmonic generators. The
energy and duration of the laser pulse were, respectively, 100 mJ/pulse
and ca. 6 ns. The detection system of the transient was previously
spectrum of [Cr(TPP)(Cl)(H O)]. Figure 2 shows the plot of
2
the pseudo-first-order rate constant kobsd versus the concentration
of water present in the dichloromethane solution. The plot is a
straight line. These findings indicate that the transient is ascribed
to the five-coordinate species [Cr(TPP)(Cl)] produced by
photodissociation of the axial H2O and that the reaction observed
after the laser pulse is the rebinding of the H2O molecule to
the five-coordinate [Cr(TPP)(Cl)]. Thus, kobsd is given by eq 1.
7
described. The concentration of the chromium(III) porphyrins was ca.
-
5
-1
10
mol kg . The extraneous ligand concentrations were always more
-
4
-1
than 10 mol kg . The decay analysis of the transient species was
carried out using the nonlinear least-squares method. The standard
deviation for the mean value of the observed first-order rate constant
kobsd ) k [H O] + k
-H O
2
(1)
kobsd was less than (3%.
H O
2
2
X-ray Crystallography. A prismatic crystal having the approximate
dimensions of 0.20 × 0.25 × 0.30 mm was mounted on a glass fiber.
X-ray diffraction data were collected at -100 °C on a Rigaku RAXIS-
RAPID diffractometer with graphite-monochromated Mo KR radiation
The slope and the intercept of the line in Figure 2 afford the
bimolecular rate constant kH O for the H2O association of [Cr-
2
(λ ) 0.710 69 Å). A total of 44 images, corresponding to 220.0°
(12) Higashi T. Program for Absorption Correction, Rigaku Corp., Tokyo,
oscillation angles, were collected at two different goniometer settings.
The camera radius was 127.40 mm. The exposure time was 5 min/
degree. Readout was performed in the 0.100 mm pixel mode. Data
were processed by the PROCESS-AUTO program package. The
integration of the data yielded a total of 12 794 reflections to a
Japan, 1999.
(13) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
(
14) 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, University of
Nijmegen, The Netherlands, 1994.
(
11) Summerville, D. A.; Jones, R. D.; Hoffman, B. M.; Basolo, F. J. Am.
(15) Crystal Structure Analysis Package, Molecular Structure Corp., 1985,
1999.
Chem. Soc. 1977, 99, 8195.

Laser Photolysis of Chromium(III) Porphyrins
Inorganic Chemistry, Vol. 39, No. 20, 2000 4419
Cl)(Py)], which is in thermal equilibrium with the T1 state.8
4
(
The excited state uniformly decays according to first-order
6
-1
kinetics with a rate constant of 6.2 × 10 s at 25.0 °C. The
photochemical dissociation of Py does not occur in the
polystyrene film. In contrast to the case of a polystyrene film,
the decay of the transient in dichloromethane containing water
-
6
is biphasic; the fast step of the decay is completed within 10
s after the pulse, followed by a much slower step. The transient
spectrum in dichloromethane detected at 34 ns after a laser pulse
is slightly different from that in the polystyrene film. The
previous studies clearly indicated that the 34-ns spectrum is
6
composed of those of the T1 state and the five-coordinate
species [Cr(TPP)(Cl)] which is produced by the photodissocia-
1
0
tion of Py from [Cr(TPP)(Cl)(Py)]. The transient spectrum
Figure 3B) measured at 1.2 µs after the pulse is in good
agreement with the difference spectrum between [Cr(TPP)(Cl)-
H2O)] and [Cr(TPP)(Cl)(Py)] in dichloromethane, indicating
that the slower decay component is ascribed to the reaction of
Cr(TPP)(Cl)(H2O)], which is formed by the reaction between
(
(
[
16
the transient [Cr(TPP)(Cl)] and H2O in dichloromethane. The
intermediate [Cr(TPP)(Cl)(H2O)] eventually returns to [Cr(TPP)-
(Cl)(Py)] by the axial ligand substitution reaction with exog-
enous pyridine.
Figure 2. Dependence of the pseudo-first-order rate constant kobsd of
the decay of the transient spectrum for [Cr(TPP)(Cl)(H
concentration of H O in dichloromethane: T ) 15.0 °C (A), 25.0 °C
B), and 35.0 °C (C).
2
O)] on the
2
In a recent paper, we reported the quenching of the photo-
dissociation of L from [Cr(TPP)(Cl)(L)] by oxygen in the
(
1
0
toluene solution. The photochemical dissociation of the axial
4
ligand L was demonstrated to take place from both the S1 and
6
T1 excited states. The quantum yields for the photodissociation
of Py from [Cr(TPP)(Cl)(Py)] were determined to be 0.18 and
4
6
10
0
.47 in the S1 and T1 excited states, respectively. The
photochemical reaction of [Cr(TPP)(Cl)(Py)] can be represented
4
by eqs 2-10. The lifetime of the S1 state is predicted to be
hν
4
4
[
Cr(TPP)(Cl)(Py)] 98 [Cr(TPP)(Cl)(Py)]* ( S ) (2)
1
4
6
6
S f [Cr(TPP)(Cl)(Py)]* ( T )
(3)
(4)
(5)
(6)
(7)
1
1
4
S1 f [Cr(TPP)(Cl)] + Py
4
S1 f [Cr(TPP)(Cl)(Py)]
6
T1 f [Cr(TPP)(Cl)] + Py
6
T1 f [Cr(TPP)(Cl)(Py)]
Cr(TPP)(Cl)] + Py h [Cr(TPP)(Cl)(Py)]
[Cr(TPP)(Cl)] + H O h [Cr(TPP)(Cl)(H O)]
[
(8)
(9)
Figure 3. Transient spectra observed for the dichloromethane solution
2
2
-3
-1
of [Cr(TPP)(Cl)(Py)] in the presence of 7.99 × 10 mol kg pyridine
-
2
-1
and 1.84 × 10 mol kg
4 ns (A) and 1.2 µs (B) after a 532 nm laser irradiation. The transient
spectrum observed for the polystyrene film doped with [Cr(TPP)(Cl)-
Py)] taken at 59 ns is also shown (C).
H
2
O at 25.0 °C. The spectra were taken at
[Cr(TPP)(Cl)(H O)] + Py f [Cr(TPP)(Cl)(Py)] + H O
2 2
3
(10)
(
shorter than the laser pulse duration (6 ns). Thus, the transients
6
detected at 34 ns after the pulse are the T1 state of [Cr(TPP)-
(
TPP)(Cl)] and k-H O for the thermal dissociation reaction of
2
(
Cl)(Py)] and [Cr(TPP)(Cl)]. The rate constants for the decay
H2O from [Cr(TPP)(Cl)(H2O)], respectively. The values of kH O
2
6
6
9
-1
-1
of the T1 state at 25.0 °C are obtained as 9.1 × 10 and 1.1 ×
thus obtained are (1.59 ( 0.03) × 10 mol kg s at 15.0 °C,
7 -1
9
-1
-1
10 s in degassed dichloromethane and toluene, respectively,
by analyzing the reaction curve monitored around 490 nm. These
(
1.85 ( 0.04) × 10 mol kg s at 25.0 °C, and (2.16 (
9
-1
-1
0
.06) × 10 mol kg s at 35.0 °C. The k-H2O values were
6
-1
values are larger than that (6.2 × 10 s ) measured in the
determined with better accuracy in the laser photolysis experi-
ments of [Cr(TPP)(Cl)(Py)] as will be described later.
8
polystyrene film.
Axial Substitution Reaction. As mentioned above, the laser
photolysis of [Cr(TPP)(Cl)(Py)] in dichloromethane and toluene
containing water produces [Cr(TPP)(Cl)(H2O)] as an intermedi-
Laser Photolysis of [Cr(TPP)(Cl)(Py)]. Figure 3 shows the
transient spectra observed for [Cr(TPP)(Cl)(Py)] in dichloro-
methane (A and B) and in polystyrene film (C) after a 532 nm
laser pulsing. In a polystyrene film, the transient having a strong
negative absorption around 450-460 nm and a positive one
(
16) Existence of a water-bound complex, [Cr(TPP)(Cl)(H2O)], was
7
evidenced by the X-ray crystallography and laser photolysis of [Cr-
6
around 490 nm is ascribed to the T1 excited state of [Cr(TPP)-
(TPP)(Cl)(H2O)] described in this paper.

4420 Inorganic Chemistry, Vol. 39, No. 20, 2000
Inamo et al.
Chart 1
implies that the rate for the substitution reaction of H2O in [Cr-
TPP)(Cl)(H2O)] with Py slows at higher concentrations of Py.
(
Here, we consider that at the higher concentrations of Py, an
axial H2O in [Cr(TPP)(Cl)(H2O)] interacts with exogenous
pyridine to yield the hydrogen-bonded species I, [Cr(TPP)(Cl)-
(HOH- - -Py)] (Chart 1), as a dead-end complex:
KL
[
Cr(TPP)(Cl)(H O)] + Py y
\
z [Cr(TPP)(Cl)(HOH- - -Py)]
2
(14)
Figure 4. Pseudo-first-order rate constant kobsd of the decay of [Cr-
TPP)(Cl)(H O)] as a function of the concentration of pyridine for the
laser photolysis of [Cr(TPP)(Cl)(Py)] in toluene: CH O/mol kg ) 3.8
On the basis of the assumptions that (i) the dead-end complex
is inert against the direct substitution reaction of (HOH- - -Py)
with Py, (ii) the intramolecular ligand substitution reaction (eq
(
2
-1
2
-
3
-3
-3
-2
×
10 (A), 4.5 × 10 (B), 5.8 × 10 (C), and 2.2 × 10 (D). T )
1
5) does not occur, and (iii) the pyridine-substitution reaction
2
5.0 °C.
ate. The rates for the axial substitution reaction of the bound
H2O in [Cr(TPP)(Cl)(H2O)] by Py (eq 10) were measured under
the pseudo-first-order conditions where H2O and Py were present
in a large excess over the porphyrin complex. The decay of
[Cr(TPP)(Cl)(HOH- - -Py)] f [Cr(TPP)(Cl)(Py)] + H O
2
(
15)
takes place according to the combination of eqs 11, 12, and 14,
the rate constant kobsd is formulated as
[Cr(TPP)(Cl)(H2O)] strictly followed the pseudo-first-order
kinetics. The rate constants kobsd for the substitution reaction
were determined from the change of the absorbance in the Soret
band region. Figure 4 shows the plot of kobsd vs [Py] measured
at various concentrations of H2O in the toluene solution. At a
given [H2O], the plot of kobsd exhibits a bell-shaped [Py]
dependence; i.e., kobsd initially increases with an increase in [Py],
but it begins to decrease at a higher [Py]. The transient spectrum
of [Cr(TPP)(Cl)(H2O)] is almost invariant to the pyridine
concentrations studied.
-1
-1
kobsd ) k
kL[Py](k [H O] + k [Py]) (1 + K [Py])
-H O
H O
2
L
L
2
2
(
16)
Equation 16 explains the experimental values of kobsd represented
as functions of [H2O] and [Py] well. The rate constants k-H O,
the ratio kH O/kL, and the equilibrium constant KL were
determined by the curve fitting of kobsd using eq 16 and a least-
squares fitting program. We found that, without using assump-
tions i and ii, the curve fitting of kobsd does not go well. Figure
S1 in the Supporting Information shows the plots of kobsd vs
3-CNPy] at a constant H2O concentration for the reaction of
Cr(TPP)(Cl)(H2O)] with 3-CNPy obtained in the laser pho-
2
2
The substitution reaction of H2O in [Cr(TPP)(Cl)(H2O)] with
the ligand L has been interpreted in terms of a dissociative
mechanism.7
[
[
tolysis of [Cr(TPP)(Cl)(3-CNPy)]. A similar bell-shaped ligand-
concentration dependence of kobsd was clearly observed. The
values of k-H O, kH O/kL, and KL were determined in the
k
-
H2O
[
Cr(TPP)(Cl)(H O)] y z [Cr(TPP)(Cl)] + H O (11)
2
2
kH2
O
2
2
kL
temperature range 15.0-35.0 °C. From the van’t Hoff and
[
Cr(TPP)(Cl)] + L
98 [Cr(TPP)(Cl)(L)]
(12)
Eyring plots, the kinetic and thermodynamic parameters for each
q
process were evaluated. In Table 1 are listed ∆H-H O and
2
Here, [Cr(TPP)(Cl)] represents the reactive five-coordinate
intermediate. By applying the steady-state approximation to the
intermediate, the rate constant kobsd is simply expressed as
q
q
q
q
q
∆
S-H O for k-H O, ∆HH O - ∆HL and ∆SH2O - ∆SL for
2
2
2
0
2
0
kH O/kL, and ∆HL and ∆SL for KL obtained with L ) Py and
-CNPy in toluene. The laser photolysis studies of the pyridine
3
complex [Cr(TPP)(Cl)(Py)] in dichloromethane were also carried
out. The plots of kobsd vs [Py] are shown in Figures S2 and S3.
Obtained kinetic and thermodynamic parameters are listed in
Table 1.
-
1
kobsd ) k
kL[L](k [H O] + k [L])
(13)
-H O
H O
2
L
2
2
Equation 13 predicts that, at a given H2O concentration, kobsd
asymptotically increases with an increase in [L]. The experi-
mental values of kobsd obtained for L ) 3-CNPy were explained
As mentioned above, the rate constant kH O was directly
2
determined in the laser photolysis studies of [Cr(TPP)(Cl)(H2O)]
in dichloromethane. The corresponding rate constant for the
reaction in the toluene solution had been previously obtained.7
With the use of kH O and the kH O/kL ratio, the rate constant kL
-
2
-1 7
by eq 13 when [3-CNPy] < 1 × 10 mol kg . The rate
constant kobsd, obtained for L ) Py at low Py concentration is
also well expressed by eq 13.
2
2
In the present study, we measured the values of kobsd for L )
Py at much higher concentrations, and found that kobsd exhibits
a bell-shaped [Py] dependence as shown in Figure 4. This
for the association of L to [Cr(TPP)(Cl)] can be determined.
Table 2 lists the rate constants and the kinetic parameters thus
obtained. The second-order rate constants of these reactions are

Laser Photolysis of Chromium(III) Porphyrins
Inorganic Chemistry, Vol. 39, No. 20, 2000 4421
a
Table 1. Kinetic and Thermodynamic Parameters for the Water
Substitution Reaction of [Cr(TPP)(Cl)(H
L
2
O)] by the Pyridine Base
b
solvent
toluene
dichloromethane
k
k
K
k
K
∆
∆
∆
∆
∆
∆
∆
∆
∆
∆
-H O/s-
1
3.86 × 10
5
2
1.57 × 10
5
2
H2O/k
L
(L ) Py)
2.07
1.29
-
1
3
L
/mol kg (L ) Py)
5.48 × 10
1.97
1.25 × 10
H2O/k (L ) 3-CNPy)
L
-
1
L
/mol kg (L ) 3-CNPy)
6.89 × 10
51.3 ( 1.7
34.2 ( 5.5
-6.9 ( 1.4
-17.0 ( 4.7
-40.7 ( 1.2
-83.9 ( 4.0
q
H
-H O
56.3 ( 1.4
43.3 ( 4.5
1.6 ( 0.6
7.4 ( 1.9
-46.4 ( 1.4
-96.3 ( 4.7
2
q
S
-H O
2
q
q
H
S
H O - ∆H
L
(L ) Py)
2
q
q
H O - ∆S
L
(L ) Py)
2
H
L
° (L ) Py)
° (L ) Py)
S
H
S
L
q
q
H O - ∆H
L
q
(L ) 3-CNPy) -10.8 ( 2.0
2
q
H O - ∆S
L
(L ) 3-CNPy)
° (L ) 3-CNPy)
° (L ) 3-CNPy)
-30.7 ( 6.9
-37.8 ( 2.3
-91.6 ( 7.8
2
Figure 5. ORTEP diagram of the structure of the 2-MePy adduct of
H
L
[Cr(TPP)(Cl)(H
2
O)]. The thermal ellipsoids are drawn at the 30%
S
L
probability level.
a
H/kJ mol-1, ∆S/J mol-1 K-1. b Values of rate and equilibrium
∆
q q
constants were calculated by using the determined ∆H , ∆S , ∆H°,
and ∆S°.
Table 3. Crystallographic Data and Experimental Details
compd
[Cr(TPP)(Cl)(H2O)]
2-MePy)2
Z
2
(
V, Å3
2272.4(3)
3.59
0.9057-0.9389
0.20 × 0.25 × 0.30
1.322
0.710 69
-100
55
Table 2. Kinetic Parameters for the Reaction of the
Five-Coordinate Complex [Cr(TPP)(Cl)] with the Entering Ligand L
-
1
empirical
formula
M
C56H44N6ClCrO
µ(Mo KR), cm
transm factor
cryst size, mm
dcalcd, g cm
λ(Mo KR), Å
T, °C
2θmax, deg
R
/mol- kg s-1 ∆H
1
q
/kJ
∆S
q
/J
3
k
L
L
L
-1
904.45
-
1
-1
-3
solvent
L
(25.0 °C)
mol
mol
K
cryst system triclinic
space group P-1
9
9
9
8
8
dichloromethane H
2
O
1.85 × 10
1.43 × 10
1.17 × 10
5.65 × 10
5.94 × 10
8.9 ( 0.4 -37.6 ( 1.4
7.3 ( 0.7 -45.0 ( 2.4
8.6 ( 0.4 -42.4 ( 1.4
15.5 ( 1.5 -25.4 ( 4.9
19.4 ( 2.0 -11.7 ( 7.0
a, Å
11.7727(5)
Py
Oa
Py
-CNPy
b, Å
c, Å
R, deg
â, deg
γ, deg
19.277(2)
10.3870(3)
90.104(8)
105.424(5)
89.935(7)
toluene
H
2
a
0.058
0.089
b
Rw
3
a
Reference 7.
a
b
2
|2)1/2, w
R ) ∑||F
o
| - |F
c
||/∑|F
o
|.
R
w
) (∑w(|F
o
| - |F
c
|) /∑w|F
o
2
2
2
-1
very large but are, however, a little smaller than that for the
diffusion-controlled reaction, the value of which is estimated
) [σ
c
(F
o
)+(p /4)|F
o
| ] , p ) 0.0700.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the
Porphyrin Complex
to be on the order of 1010 mol kg s in these solvents. A
small energy barrier implying a small activation enthalpy and
a negative activation entropy indicates that the energetic cost
required for orienting the reacting partners in a favorable
configuration for bond formation would not be very large like
other metalloporphyrins with very large rate constants (in the
-1
-1
Cr(1)-Cl(1)
Cr(1)-O(1)
Cr(1)-N(1)
Cr(1)-N(2)
2.3114(7) Cr(1)-N(3)
2.057(2)
2.035(2)
2.750(3)
2.781(3)
2.057(2)
2.046(2)
2.035(2)
Cr(1)-N(4)
O(1)-N(5)
O(1)-N(6)
8
9
-1
-1
Cl(1)-Cr(1)-O(1) 178.75(6) N(1)-Cr(1)-N(2) 89.90(8)
range of 10 -10 mol kg s ) for the axial ligand rebinding
Cl(1)-Cr(1)-N(1) 90.94(6)
Cl(1)-Cr(1)-N(2) 91.23(6)
Cl(1)-Cr(1)-N(3) 92.46(6)
Cl(1)-Cr(1)-N(4) 92.27(7)
N(1)-Cr(1)-N(3) 176.60(8)
N(1)-Cr(1)-N(4) 90.43(8)
N(2)-Cr(1)-N(3) 90.14(8)
N(2)-Cr(1)-N(4) 176.48(9)
N(3)-Cr(1)-N(4) 89.33(8)
Cr(1)-O(1)-N(5) 118.3(1)
Cr(1)-O(1)-N(6) 121.2(1)
for iron(II)1
7-19
and cobalt(III)
20,21
studied by the laser photolysis
method.
Reaction of [Cr(TPP)(Cl)(H2O)] and 2-Methylpyridine in
Toluene. To obtain supporting evidence for the hydrogen-
bonded species I, we examined the reaction of [Cr(TPP)(Cl)-
H2O)] with a sterically hindered pyridine molecule. The
absorption spectrum of [Cr(TPP)(Cl)(H2O)] in toluene displays
O(1)-Cr(1)-N(1)
O(1)-Cr(1)-N(2)
O(1)-Cr(1)-N(3)
O(1)-Cr(1)-N(4)
88.00(8)
88.11(8)
88.60(8)
88.40(8)
(
-2
-1
The molecular structure of the complex including [Cr(TPP)-
Cl)(H2O)] and 2-MePy is shown in Figure 5. The crystal-
a slight change with the addition of 1 × 10 mol kg 2-MePy.
(
The absorption peak of the Soret band shifts to red only by ca.
lographic data are listed in Table 3. The angles R and γ are
close to 90°, so the crystal system was checked for a monoclinic
lattice. A 2-fold axis parallel to the b axis was found to be absent
because the agreement factor between the h,k,l and -h,k,-l
reflections was too large. On the basis of a statistical analysis
of the intensity distribution, and the successful solution and
refinement of the structure, the space group was determined to
be P-1. Selected bond lengths and bond angles are given in
Table 4, and complete listings of the structural data are given
in the Supporting Information.
As shown in Figure 5, 1 mol of [Cr(TPP)(Cl)(H2O)] ac-
companies 2 mol of 2-MePy via a hydrogen bond. The N(5)-
O(1) and N(6)-O(1) distances are 2.750(3) and 2.781(3) Å,
respectively. The angles of Cr(1)-O(1)-N(5) and Cr(1)-O(1)-
3
nm, suggesting that the H2O in [Cr(TPP)(Cl)(H2O)] hardly
undergoes the axial ligand exchange with the exogenous
2
2
-MePy. Presumably, the steric effect of the methyl group in
-MePy prohibits the axial ligation to the central chromium of
the complex.
(
(
(
(
(
17) Maillard, P.; Schaeffer, C.; T e´ treau, C.; Lavalette, D.; Lhoste, J.-M.;
Momenteau, M. J. Chem. Soc., Perkin Trans. 2 1989, 1437.
18) Dixon, D. W.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1985, 107,
8
08.
19) Lavalette, D.; Tetreau, C.; Momenteau, M. J. Am. Chem. Soc. 1979,
01, 5395.
20) Tait, C. D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106,
, 6653.
21) Hoshino, M.; Kogure, M.; Asano, K.; Hinohara, T. J. Phys. Chem.
989, 93, 6655.
1
6
1

4422 Inorganic Chemistry, Vol. 39, No. 20, 2000
Inamo et al.
N(6) are 118.3(1)° and 121.2(1)°, respectively. One of the two
to that obtained with [Cr(TPP)(Cl)] in the present study, i.e.,
4
-1
3
-1
2-MePy molecules is approximately positioned over the meso-C
1.2 × 10 mol kg in dichloromethane and 3.0 × 10 mol
atom of the porphyrin, and the other is over the pyrrole ring,
with the angle N(5)-O(1)-N(6) being 104.0(1)° as shown in
Figure S5. The latter 2-MePy molecule is so heavily disordered
that the exact distances and angles among the N(6) and C(51)-
C(56) atoms were not obtained. Although the crystal structure
was considered, it was not clear why one of the 2-MePy
molecules is significantly disordered in the crystal.
kg in toluene at 25.0 °C.
The axial water in [Cr(TPP)(Cl)(H O)] readily undergoes a
2
ligand exchange reaction in the presence of nitrogenous bases
to yield [Cr(TPP)(Cl)(L)] (L ) Py, 1-methylimidazole, 1,2-
dimethylimidazole). The molecular structures of these complexes
7,22
have been investigated by X-ray crystallography. The present
work has shown that 2-methylpyridine forms a hydrogen bond
with the axial H2O in [Cr(TPP)(Cl)(H2O)] to yield a stable
product in the crystal and that pyridine also forms a hydrogen
bond with [Cr(TPP)(Cl)(H2O)] in solution. However, the
hydrogen-bonded species [Cr(TPP)(Cl)(HO-H‚‚‚Py)] is not
stable and reverts to [Cr(TPP)(Cl)(Py)]. It is noteworthy that
the hydrogen bond formation between the axial water of the
metalloporphyrin and a nitrogenous base has been observed for
These hydrogen bonds affect the structure of the [Cr(TPP)-
(Cl)(H2O)] unit, especially for the axial bond lengths. The axial
Cr-O bond shortens from 2.239(3) to 2.057(2) Å, and the axial
Cr-Cl bond is elongated from 2.242(3) to 2.3114(7) Å due to
the hydrogen bond formation between the axial water molecule
and 2-MePy as compared with the structure of [Cr(TPP)(Cl)-
7
(
H2O)]. The Cr-Cl bond length of the present complex is close
7
23
to that for [Cr(TPP)(Cl)(Py)] (2.311(2) Å) and [Cr(TPP)(Cl)-
1-MeIm)] (2.317(2) Å, 1-MeIm ) 1-methylimidazole). On
metmyoglobin. The axial water coordinated to the central
2
2
(
iron(III) atom in metmyoglobin forms a hydrogen bond with
the distal histidine. Probably, the hydrogen bond formation
increases the basicity of the oxygen atom of the axial H2O,
leading to the stable Fe(III)-O bond in metmyoglobin.
The first-row transition-metal complexes of the porphyrins
the other hand, the hydrogen bond has little effect on the other
structural features. The average Cr-Np bond length of 2.043 Å
is almost the same as those of the other chromium(III)
porphyrins. The Cl-Cr-O angle of 178.75(6)° and the Np-
Cr-O angles, which span the range 88.00(8)-88.60(8)°, are
consistent with a modest off-plane displacement of the central
chromium atom toward the Cl atom. A formal diagram of the
porphyrin core is shown in Figure S4, in which the perpendicular
displacement of each atom from the 24-atom porphyrin mean
plane is displayed. The porphyrin core is approximately planar,
and the individual atomic displacements are all <0.14 Å. The
meso-phenyl groups are moderately tilted from the porphyrin
normal. Individual dihedral angles between the 24-atom por-
phyrin mean plane and the phenyl rings range from 66.15(8)°
to 79.89(10)°.
On the basis of these structural features of the complex, the
small shift in the absorption bands of [Cr(TPP)(Cl)(H2O)] in
toluene observed with the addition of 2-MePy is interpreted in
terms of the formation of the hydrogen bond between the N
atom in 2-MePy and the H2O ligand of [Cr(TPP)(Cl)(H2O)]. It
is likely that the formation of the hydrogen bond does not cause
a remarkable difference in the absorption spectra between
1
,3
1,3
have the excited electronic states (π,d)*, (d,π)*, and
1,3
(d,d)*, which lie below the porphyrin-localized S1 excited state
2
4
in energy. A number of studies on the photoelimination of
axial ligands from iron(II), cobalt(II), and cobalt(III) porphyrins
have shown that the dissociation of the ligand occurs from the
excited states involving an antibonding dz orbital of the central
metal. In contrast to the iron(II), cobalt(II), and cobalt(III)
2
3
porphyrins, the photodissociation of the axial ligand from
chromium(III) porphyrins has been demonstrated to occur in
4
6
10
both the S1 and T1 states. The dissociation yield of the axial
4
6
ligand in both the S1 and T1 states increases with a decrease
in the bond strengths between the central Cr atom and the axial
1
0
ligand L.
The laser photolysis studies of [Cr(TPP)(Cl)(Py)] in toluene
and dichloromethane containing water and pyridine have shown
that the initially produced [Cr(TPP)(Cl)] reacts with H2O to yield
[Cr(TPP)(Cl)(H2O)]. The axial H2O in [Cr(TPP)(Cl)(H2O)] is
then replaced by the exogenous Py, thus returning to [Cr(TPP)-
(Cl)(Py)]. Kinetic studies revealed that, at higher concentrations
of pyridine, [Cr(TPP)(Cl)(H2O)] interacts with Py to give the
hydrogen-bonded species I as illustrated in Chart 1. As
mentioned above, the absorption spectrum of the 2-MePy adduct
of [Cr(TPP)(Cl)(H2O)] is found to be very similar to that of
[Cr(TPP)(Cl)(H2O)] in the Soret band region. The interaction
between pyridine and the axial H2O in I is, therefore, postulated
to be too weak to cause the spectral change.
The rate for the ligand exchange reaction of I by Py is very
slow in comparison with that of [Cr(TPP)(Cl)(H2O)]. This
implies that the hydrogen bonding may increase the basicity of
the oxygen atom of the axial H2O, resulting in an increase in
the strength of the Cr-O bond. The kinetic studies for the
exchange reaction of the axial HO-H‚‚‚Py in I by exogenous
pyridine have shown that the sequential dissociation of Py and
then H2O from I gives a coordinately unsaturated [Cr(TPP)-
[Cr(TPP)(Cl)(H2O)] and its 2-MePy adduct.
The laser photolysis studies of [Cr(TPP)(Cl)(H2O)] in solution
-
2
-1
containing 1.0 × 10 mol kg 2-MePy were carried out to
elucidate the effects of the hydrogen bond on the photodisso-
ciation of the axial ligands. The detected transients are mostly
6
attributed to the T1 excited state. The photodissociation of the
axial ligand is markedly suppressed due to the formation of the
hydrogen bond between the axial water and 2-MePy. The
hydrogen bond formation increases the Cr-O bond strength,
which is reflected in the shortening of the Cr-O bond length,
resulting in the decrease of the yield for photodissociation of
the axial ligand.
Discussion
Reagent grade toluene and dichloromethane contain at least
-
3
-1
1
(
(
.0 × 10 mol kg water. Recrystallization of the chromium-
(Cl)]. Then, [Cr(TPP)(Cl)] reacts with exogenous Py, leading
III)-TPP complex from these solutions gives [Cr(TPP)(Cl)-
to the formation of [Cr(TPP)(Cl)(Py)].
The formation of the hydrogen-bonded species II, [Cr(TPP)-
H2O)], in which a water molecule is coordinated to the axial
16
position. Such axial coordination of water has also been
(
Cl)(HO-H‚‚‚3-CNPy)], is also observed when 3-CNPy is used
17
observed for iron(II) porphyrins in toluene. The equilibrium
constant for the H2O binding to the iron(II) porphyrins is on
the order of 10 -10 mol kg. These values are comparable
as an exogenous ligand. The equilibrium constant, KL, for the
3
4
-1
(23) Antonini, E.; Brunori, M. Hemoglobin and Myoglobin in Their
Reactions with Ligands; North-Holland: Amsterdam, 1971.
(24) Dolphine, D., Ed. The Porphyrins; Academic Press: New York, 1979.
(22) Inamo, M.; Nakajima, K. Bull. Chem. Soc. Jpn. 1998, 71, 883.

Laser Photolysis of Chromium(III) Porphyrins
Inorganic Chemistry, Vol. 39, No. 20, 2000 4423
q
-1
formation of II determined from the kinetic analysis is smaller
than that of I by a factor of ca. 8. This result is interpreted in
terms of the difference in the basicity between Py and 3-CNPy.
These complexes, I and II, are not equilibrium species, and
eventually return to [Cr(TPP)(Cl)(Py)] and [Cr(TPP)(Cl)-
[Cr(TPP)(Cl)(H2O)] in toluene, and ∆H ) 109 kJ mol , and
q
-1 -1
27
∆S ) 12 J mol
K
for the hydrated Cr(III) ions in water.
Although the solvent is different in the above two systems, it
was concluded that [Cr(TPP)(Cl)(H2O)] exchanges the axial H2O
more readily than the hydrated Cr(III) ions. These results are
well explained by the difference in the mechanism of the H2O
exchange reaction between [Cr(TPP)(Cl)(H2O)] and the Cr(III)
ions; the former proceeds via a dissociative mechanism, and
the latter via an associative mechanism. In general, the
intermediates of the ligand exchange reactions are the five-
coordinate species for the dissociative mechanism and the seven-
coordinate species for the associative mechanism. In the case
of [Cr(TPP)(Cl)(H2O)], the intermediate is the five-coordinate
species, indicating that the porphyrin ligand stabilizes the five-
coordinate species [Cr(TPP)(Cl)] rather than the seven-
coordinate species as an intermediate for the ligand exchange
reactions.
(3-CNPy)], respectively.
It has been recognized that the porphyrin ligand enhances
the water exchange rate of metal ions, and this effect has been
clearly demonstrated for the iron(III) complexes. Kinetic
parameters for the axial water exchange of the Fe(III)-TMPyP
complex (TMPyP ) 5,10,15,20-tetrakis(4-N-methylpyridyl)-
porphyrin) and the Fe(III)-TPPS complex (TPPS ) 5,10,15,-
2
0-tetrakis(4-sulfonatophenyl)porphyrin) have been determined
5
-1
q
-1
q
to be k ) 7.8 × 10 s (25 °C), ∆H ) 58 kJ mol , and ∆S
-
1
-1
7
-1
)
61 J mol
K
for the former, and k ) 1.4 × 10 s (25
q
-1
q
-1
-1
°
C), ∆H ) 57 kJ mol , and ∆S ) 84 J mol
K
for the
25
4
latter. The water exchange rate is enhanced by a factor of 10 -
1
5
2
-1
0 over that of the hydrated Fe(III) ion: k ) 1.6 × 10 s
q
-1
q
-1
-1 26
Supporting Information Available: Figures reporting the ligand
concentration dependence of kobsd for the reaction systems [Cr(TPP)-
Cl)(3-CNpy)] in toluene and [Cr(TPP)(Cl)(Py)] in dichloromethane,
a formal diagram of the porphyrin core, top and side views of the
complex, tables reporting bond distances and angles and positional and
thermal parameters for the structurally characterized porphyrin complex,
and on X-ray crystallographic file in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(25 °C), ∆H ) 64 kJ mol , and ∆S ) 12 J mol K .
Similar acceleration effects of the porphyrin ligand on the
exchange rate are observed in the present work; the dissociation
rate of H2O from [Cr(TPP)(Cl)(H2O)] in toluene is found to be
(
5
-1
3
(
.86 × 10 s (25 °C). This value is much greater than that
2.4 × 10 s-1 at 25 °C) from the hydrated Cr(III) ions in
-6
27
water. The kinetic parameters for the axial H2O exchange are
q
-1
q
-1
-1
∆
H ) 51.3 kJ mol and ∆S ) 34.2 J mol
K
for
IC0002059
(
25) Ostrich, I. J.; Liu, G.; Dodgen, H. W.; Hunt, J. P. Inorg. Chem. 1980,
9, 619.
26) Grant, M.; Jordan, R. B. Inorg. Chem. 1981, 20, 55.
1
(27) Xu, F.-C.; Krouse, H. R.; Swaddle, T. W. Inorg. Chem. 1985, 24,
(
267.