Structure and photochemistry of dicyanocobalt(III) tetraphenylporphyrin. Photochromic reaction caused by photodissociation of axial ligand

Article abstract of DOI:10.1021/ic000408x

Chlorocobalt(III) tetraphenylporphyrin, (Cl)Co(III)TPP, reacts with potassium cyanide in dichlommethane or benzene containing 18-crown-6 to give a green solution of [crown-K⁺][(CN)₂Co(III)TPP^-]. The molecular structure of [crown-K⁺][(CN)₂Co(III)TPP^-] is identified by X-ray crystallography. In methanol, (Cl)Co(III)TPP plus KCN also gives a green solution of [(CN)₂Co(III)TPP^-]. The green methanol solution containing 1.4 x 10^-4 M KCN turns orange by continuous photolysis with a 250-W mercury lamp for 5 min. The orange solution returns to green when it is kept in the dark for 5 min. The kinetic study suggests that [(CN)₂Co(III)TPP^-] dissociates CN^- by continuous photolysis, giving rise to the formation of the orange species, (CH₃OH)(CN)Co(III)TPP. The photoproduct, (CH₃OH)(CN)-Co(III)TPP, regenerates the green species, [(CN)₂Co(III)TPP^-], by reaction with CN^-. The laser photolysis study of [(CN)₂Co(III)TPP^-] in methanol demonstrates that photodissociation of CN^- takes place within 20 ns after the 355-nm laser pulse, resulting in the formation of two transients, I (short-lived) and II (long-lived). The absorption spectra of both transients are similar to that of (CH₃OH)(CN)Co(III)TPP. These transients eventually return to [(CN)₂Co(III)TPP^-]. The decay of species I follows first-order kinetics with a rate constant k = 2 x 10⁶ s^-1, independent of the concentration of KCN. Species II is identified as (CH₃OH)(CN)Co(III)TPP, which is observed with the continuous photolysis of the solution. The laser photolysis of [crown-K⁺][(CN)₂Co(III)TPP^-] in dichloromethane gives the transient species, which goes back to the original complex according to first-order kinetics with a rate constant k = 5 x 10⁶ s^-1. [crown-K⁺][(CN)₂Co(III)TPP^-] is concluded to photodissociate the axial CN^- to form [crown-K⁺CN^-][(CN)Co(III)TPP] in which an oxygen atom of the crown moiety in [crown-K⁺CN^-] is coordinated to the cobalt(III) atom of [(CN)Co(III)TPP] at the axial position. The intracomplex reverse reaction of [crown-K⁺CN^-][(CN)Co(III)TPP] leads to the regeneration of [crown-K⁺][(CN)₂Co(III)TPP^-]. The structure and the reaction of the transient species I observed for [(CN)₂Co(III)TPP^-] in methanol are discussed on the basis of the laser photolysis studies of [crown-K⁺][(CN)₂Co(III)TPP^-] in dichloromethane.

Full text of DOI:10.1021/ic000408x

4850
Inorg. Chem. 2000, 39, 4850-4857
Structure and Photochemistry of Dicyanocobalt(III) Tetraphenylporphyrin. Photochromic
Reaction Caused by Photodissociation of Axial Ligand
Mikio Hoshino,*,†,‡ Hirotaka Sonoki,†,‡ Yoshio Miyazaki,§ Yasuhiro Iimura,† and
Kiyoko Yamamoto†
The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan, Interdisciplinary
Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuda-machi,
Midori-ku, Yokohama, Kanagawa 226-8502, Japan, and Department of Chemistry, Faculty of
Engineering, Toyo University, Kujirai, Kawagoe, Saitama 350-8585, Japan
ReceiVed April 10, 2000
Chlorocobalt(III) tetraphenylporphyrin, (Cl)CoIIITPP, reacts with potassium cyanide in dichloromethane or benzene
containing 18-crown-6 to give a green solution of [crown-K+][(CN)2CoIIITPP-]. The molecular structure of [crown-
K+][(CN)2CoIIITPP-] is identified by X-ray crystallography. In methanol, (Cl)CoIIITPP plus KCN also gives a
green solution of [(CN)2CoIIITPP-]. The green methanol solution containing 1.4 × 10-4 M KCN turns orange by
continuous photolysis with a 250-W mercury lamp for 5 min. The orange solution returns to green when it is kept
in the dark for 5 min. The kinetic study suggests that [(CN)2CoIIITPP-] dissociates CN- by continuous photolysis,
giving rise to the formation of the orange species, (CH3OH)(CN)CoIIITPP. The photoproduct, (CH3OH)(CN)-
CoIIITPP, regenerates the green species, [(CN)2CoIIITPP-], by reaction with CN-. The laser photolysis study of
[(CN)2CoIIITPP-] in methanol demonstrates that photodissociation of CN- takes place within 20 ns after the
355-nm laser pulse, resulting in the formation of two transients, I (short-lived) and II (long-lived). The absorption
spectra of both transients are similar to that of (CH3OH)(CN)CoIIITPP. These transients eventually return to
[(CN)2CoIIITPP-]. The decay of species I follows first-order kinetics with a rate constant k ) 2 × 106 s-1
,
independent of the concentration of KCN. Species II is identified as (CH3OH)(CN)CoIIITPP, which is observed
with the continuous photolysis of the solution. The laser photolysis of [crown-K+][(CN)2CoIIITPP-] in
dichloromethane gives the transient species, which goes back to the original complex according to first-order
kinetics with a rate constant k ) 5 × 106 s-1. [crown-K+][(CN)2CoIIITPP-] is concluded to photodissociate the
axial CN- to form [crown-K+CN-][(CN)CoIIITPP] in which an oxygen atom of the crown moiety in [crown-
K+CN-] is coordinated to the cobalt(III) atom of [(CN)CoIIITPP] at the axial position. The intracomplex reverse
reaction of [crown-K+CN-][(CN)CoIIITPP] leads to the regeneration of [crown-K+][(CN)2CoIIITPP-]. The structure
and the reaction of the transient species I observed for [(CN)2CoIIITPP-] in methanol are discussed on the basis
of the laser photolysis studies of [crown-K+][(CN)2CoIIITPP-] in dichloromethane.
Introduction
extensively studied by X-ray crystallography, NMR, electron
spin resonance, and Raman spectroscopy for an understanding
Synthetic metalloporphyrins with the central metals FeIII, CoIII,
and MnIII react with CN- to give dicyanide complexes.1-14 In
particular, the cyanide complexes of FeIII porphyrins have been
of the effects of the axial CN- on the electronic structures.1-9
The molecular structures of the cyano-metalloporphyrins
studied hitherto are limited to FeIII and MnIII porphyrins,1,12,14
which have two CN- anions at the axial positions. The crystal
structures of the cyanide complexes of cobalt porphyrins have
not been reported. This is partly because of the difficulty in
preparation of the crystals of the cyanide complex.
* To whom correspondence should be addressed: The Institute of
Physical and Chemical Research, Wako, Saitama 351-0198, Japan. Fax:
048-462-4668. Phone: 048-467-9426. E-mail: hoshino@postman.riken.go.jp.
† The Institute of Physical and Chemical Research.
‡ Tokyo Institute of Technology.
§ Toyo University.
Studies on the interaction between cyanide anions and CoIII
complexes are important not only in coordination chemistry but
also in toxicology. Because of the high affinity of CN- anions
for cobalt complexes, hydroxocobalamin15-17 and water-soluble
CoIII porphyrins18 have been investigated as an effective antidote
(1) Scheidt, W. R.; Haller, K. J.; Hatano, K. J. Am. Chem. Soc. 1980,
102, 3017-3021.
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T. N.; Zovinka, E. P. Inorg. Chem. 1997, 36, 3892-3897.
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Way, J. L. Fundam. Appl. Toxicol. 1994, 23, 76-80.
(11) Tait, C. D.; Holten, D.; Gouterman, M. J. Am. Chem. Soc. 1984, 106,
6653-6659.
(12) Galich, L. G.; Huckstadt, H.; Homborg, H. J. Porphyrins Phthalo-
cyanines 1998, 2, 79-87.
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1976, 98, 94.
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(6) Mclaughlin, V. B.; Farragi, M.; Leussing, D. L. Inorg. Chem. 1993,
32, 941-947.
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J. Phys. Chem. 1996, 100, 15274-15279.
(8) Sato, T.; Tanaka, N.; Moriyama, H.; Matsumoto, O.; Takenaka, A.;
Neya, S.; Funasaki, N. Bull. Chem. Soc. Jpn. 1992, 65, 739-745.
(9) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
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Hatano, K. Inorg. Chem. 1983, 22, 1516-1522.
10.1021/ic000408x CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/29/2000

Dicyanocobalt(III) Tetraphenylporphyrin
Inorganic Chemistry, Vol. 39, No. 21, 2000 4851
The intensity data were collected to a maximum 2θ value of 60.1°.
A total of 40 frames with 3.0° oscillation images were collected, each
being exposed for 30 min. A total of 16 728 reflections were collected.
The data were corrected for Lorentz and polarization effects but not
for absorption effects.
to the lethal effects of cyanide anions. The mechanism of the
detoxification clearly involves the formation of cyanide com-
plexes.
In previous papers, we have studied photochemical reactions
of CoIII porphyrins with neutral axial ligands, pyridine and
isonitriles.19,20 These complexes readily undergo photodisso-
ciation of the axial ligands to give CoIII porphyrins. However,
the CoIII porphyrins with anionic axial ligands such as Cl- and
The structure was solved by direct methods23 and expanded using
Fourier methods. The non-hydrogen atoms were refined isotropically.
Hydrogen atoms in the structure were included but not refined. The
final cycle of full-matrix least-squares refinement was based on 7387
observed reflections (I > 3.0σ(I)) and converged (largest parameter
shift was 0.03 times its standard deviation) with unweighted and
weighted agreement factors of R ) ∑||Fo| - |Fc||/∑|Fo| ) 0.137, Rw
NO2 were found to yield CoII porphyrins by photolysis.19,21
-
In the present study, we examined the photochemical reactions
of the cyanide complexes of CoIII tetraphenylporphyrins, which
have two CN- at the axial positions.
) {∑w(|Fo| - |Fc|)2/wFo }1/2 ) 0.210. The standard deviation for an
2
observation of unit weight was 1.68. The weighting scheme was based
on counting statistics and included a factor (p ) 0.20) to downweight
the intense reflections. The maximum and minimum peaks on the final
difference Fourier map corresponded to 1.85 and -1.14 e-/Å3,
respectively. Neutral-atom scattering factors were taken from Cromer
and Waber.24 Anomalous dispersion effects were included in Fc;25 the
values for ∆f ′ and ∆f ′′ were those of Creagh and McAuley.26 The
values for the mass attenuation coefficients are those of Creagh and
Hubbell.27 All calculations were performed using the TeXsan28 crystal-
lographic software package.
This paper reports (1) the crystal structure of the dicyanide
complex of cobalt(III) tetraphenylporphyrin, in which the
counterion is K+ trapped in 18-crown-6, (2) the mechanistic
studies for the formation of the dicyanide complex of cobalt-
(III) tetraphenylporphyrin, [(CN)2CoIIITPP-], in methanol, and
(3) the photochemical reactions of [(CN)2CoIIITPP-] studied
by continuous photolysis and nanosecond laser flash photolysis.
Experimental Section
Absorption spectra were recorded on a Hitachi 330 spectrophotom-
eter. Laser photolysis was carried out with a Nd:YAG laser (model
HY 500 from JK Lasers Ltd.) equipped with second (532 nm), third
(355 nm), and fourth (266 nm) harmonic generators. The transient
spectra were measured by the ICCD detector (DH520-18F-01 from
Andor Technology Ltd.). To monitor the decay of the transient
absorption, the output from the photomultiplier (R 758 from Hamamat-
su) was conducted to the digital storage oscilloscope (model Gould
630 from Gould Instrument System Ltd.). A 250-W mercury lamp (USH
250 D from Ushio Inc.) with cutoff filters was used as a light source
for continuous photolysis.
Reagent-grade potassium cyanide, 18-crown-6, toluene, benzene,
dichloromethane, and methanol were used without further purification.
Chlorocobalt(III) tetraphenylporphyrin, (Cl)CoIIITPP, was synthesized
and purified according to the procedure previously reported.22
The crown complex, [crown-K+][(CN)2CoIIITPP-], was synthesized
according to the method described below. To a 200-cm3 benzene
solution of (Cl)CoIIITPP (1.0 g) and 18-crown-6 (2.0 g), KCN (0.5 g)
was added with vigorous stirring for 2.0 h at room temperature. After
filtration of the solid residuals from the reaction mixture, hexane was
slowly added to the solution. The dark green crystals of [crown-
K+][(CN)2CoIIITPP-] thus obtained were recrystallized twice from
toluene solutions. 1NMR (500 MHz, CDCl3, the solvent peak at δ 7.24
was used as an internal standard): δ 8.75 (8H, s), δ 8.09 (8H, d), δ
7.62 (12H, m), δ 2.65 (24H, s). Anal. Calcd for [crown-K+][(CN)2-
CoIIITPP-]‚1/2CH3C6H5‚H2O: C, 67.70; H, 5.32; N, 7.71. Found: C,
67.53; H, 5.32; N, 7.63. For the structure determination by X-ray
crystallography, [crown-K+][(CN)2CoIIITPP-] was recrystallized again
from the toluene solution.
Diffraction data of a single crystal of [crown-K+][(CN)2CoIIITPP-]‚1/
2CH3C6H5‚H2O (0.47 × 0.09 × 0.06 mm) were collected on a Rigaku
Raxis-CS2 imaging plate diffractometer equipped with graphite-
monochromated Mo KR radiation and a cooling device operating at
115 K. Indexing was performed from three oscillation images, which
were exposed for 4.0 min. The camera radius was 143.5 mm. Readout
was performed in the 0.1-mm pixel mode. Cell constants and an
orientation matrix for data collections corresponded to a monoclinic
cell with the following dimensions: a ) 20.024(3) Å, b ) 25.386(6)
Å, c ) 22.64(1) Å, â ) 103.562(3)°, and V ) 11 186(5) Å3. The
calculated density is 1.30 g/cm3 for Z ) 4, and the formula weight is
2182.4. The space group is uniquely determined to be P21/n (No. 14)
by the systematic absences of h0l, h + 1 ) 2n + 1, and 0k0,
k ) 2n +1.
Results
Structure of [crown-K+][(CN)2CoIIITPP-]. Figure 1 shows
the molecular structure of [crown-K+][(CN)2CoIIITPP-] deter-
mined by X-ray crystallography. From the projection of the ac
plane, the dicyanocobalt(III) porphyrin in crystals was found
to have the structure {[crown-K+‚H2O]2[(CN)2CoIIITPP-]}[(CN)2-
CoIIITPP-]‚C7H8, in which one of the two [(CN)2CoIIITPP-]
takes two [crown-K+]. Two water molecules, O13 and O15,
were refined with a disorder model. The atomic coordinates
except for hydrogen atoms are listed in Table 1S. Bond distances
and angles are listed in Tables 2S and 3S, respectively.
Figure 2 shows the perspective view of the {[crown-K+‚
H2O]2[(CN)2CoIIITPP-]}+ moiety with the disordered H2O
molecules. Selected interatomic distances are listed in Table 1.
The Co-C(CN) separations are 1.98(2) and 1.94(2) Å. These
values are close to that of vitamin B12 (1.92 Å).29 The C-N
bond lengths of the cyanide ions, 1.09(2) and 1.13(2) Å, are
similar to those of [(CN)2MnIIITPP-] (1.15(2) and 1.16(2) Å)12
(15) Houeto, P.; Borron, S. W.; Sandouk, P.; Imbert, M.; Levillain, P.;
Baud, F. J. J. Toxicol., Clin. Toxicol. 1996, 34, 397-404.
(16) Houto, P.; Buneaux, F.; Galliot-Guilley, M.; Baud, F. J.; Levillain, P.
J. Anal. Toxicol. 1994, 18, 154-158.
(23) SIR92, see: Altomare, A. B.; Burla, M. C.; Camali, M.; Cascarano,
M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr.
1994, 27, 435-436.
(24) Cromer, D. T.; Waver, J. T. International tables for X-ray crystal-
lography; The Keynoch Press: Birmingham, 1974; Vol. IV, Table
2.2A.
(17) Forsyth, J. C.; Mueller, P. D.; Becker, C. E.; Osterloh, J.; Benowitz,
N. L.; Rumack, B. H.; Hall, A. H. J. Toxicol., Clin. Toxicol. 1993,
31, 277-294.
(25) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781-782.
(26) Creagh, D. C.; McAuley, W. J. International Tables for Crystal-
lography; Kluwer Academic Publishers: Boston, 1992; Vol. C, Table
4.2.6.8, pp 219-222.
(27) Creagh, D. C.; Hubbell, J. H. International Tables for Crystallography;
Kluwer Academic Publishers: Boston, 1992; Vol. C, Tables 4.2.4.3,
pp 200-206.
(28) TeXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation, 1985 and 1999.
(18) McGuinn, W. D.; Baxter, L.; Pei, L.; Petrikovics, I.; Cannon, E. P.;
Way, J. L. Fundam. Appl. Toxicol. 1994, 23, 76-80.
(19) Hoshino, M.; Kogure, M.; Amano, K.; Hinohara, T. J. Phys. Chem.
1989, 93, 6655-6659.
(20) Hoshino, M.; Nagamori, T.; Seki, H.; Chihara, T.; Tase, T.; Lillis, J.
P.; Wakatsuki, Y. J. Phys. Chem. A 1998, 102, 1297-1303.
(21) Seki, H.; Okada, K.; Iimura, Y.; Hoshino, M. J. Phys. Chem. A 1997,
101, 8174-8178.
(22) Sakurai, T.; Yamamoto, K.; Naito, H.; Nakamoto, N. Bull. Chem. Soc.
Jpn. 1976, 49, 3042-3046.
(29) Pratt, J. M. Inorganic Chemistry of Vitamine B12; Academic Press:
London, 1972.

4852 Inorganic Chemistry, Vol. 39, No. 21, 2000
Hoshino et al.
distance
Table 1. Selected Bond Distancesa
atoms
distance
atoms
Co(1)-N(1)
Co(1)-N(2)
Co(1)-N(3)
Co(1)-N(4)
Co(1)-C(45)
Co(1)-C(46)
Co(2)-N(7)
Co(2)-N(8)
Co(2)-N(9)
Co(2)-N(10)
Co(2)-C(103)
Co(2)-C(104)
K(1)-O(1)
1.94(1)
1.92(1)
1.94(2)
1.94(1)
1.94(2)
1.98(2)
1.96(2)
1.92(2)
1.94(2)
1.91(1)
1.93(2)
1.88(2)
2.78(1)
2.80(1)
2.92(2)
2.94(2)
2.77(2)
K(1)-O(6)
2.80(2)
2.74(2)
2.77(3)
2.81(2)
2.83(1)
2.82(2)
3.00(2)
2.71(2)
2.82(2)
2.88(2)
2.80(3)
2.78(3)
2.84(2)
1.13(2)
1.09(2)
1.22(2)
1.16(2)
K(1)-O(13)
K(1)-O(13′)
K(1)-N(5)
K(2)-O(7)
K(2)-O(8)
K(2)-O(9)
K(2)-O(10)
K(2)-O(11)
K(2)-O(12)
K(2)-O(15)
K(2)-O(15′)
K(2)-N(6)
K(1)-O(2)
N(5)-C(45)
N(6)-C(46)
N(11)-C(104)
N(12)-C(103)
K(1)-O(3)
K(1)-O(4)
K(1)-O(5)
a Distances are in angstroms. Estimated standard deviations in the
least significant figure are given in parentheses.
Figure 1. Stereoscopic view of {[crown-K+‚H2O]2[(CN)2CoIII-
TPP-]}[(CN)2CoIIITPP-]‚C7H8.
nm (ꢀ ) 1.36 × 104 M-1 cm-1). However, [crown-K+][(CN)2-
CoIIITPP-] in CH2Cl2 gives absorption peaks at 625 and 580
nm in the Q-band region and at 447 nm (ꢀ ) 1.02 × 105 M-1
cm-1) in the Soret band region. In comparison with (Cl)CoIII-
TPP, the two bands show a remarkable shift to red when the
dicyanide complex is formed.
As will be described later, the absorption spectrum of
[crown-K+][(CN)2CoIIITPP-] in CH2Cl2 is similar to that of
[(CN)2CoIIITPP-] in methanol. The absorption peaks of the Soret
and Q bands of the latter are slightly red-shifted in comparison
with those of the former because of solvent effects. The
photochemical product cannot be detected upon continuous
irradiation of [crown-K+][(CN)2CoIIITPP-] in dichloromethane
by the mercury lamp with a cutoff filter (λ < 360 nm) for 20
min.
Reaction of (Cl)CoIIITPP and KCN in Methanol. Accord-
ing to the previous conductometric studies, (Cl)CoIIITPP in
alcohols undergoes ionic dissociation:30
(Cl)CoIIITPP + 2CH3OH f [(CH3OH)2CoIIITPP+][Cl-]
(1)
[(CH3OH)2CoIIITPP+][Cl-] a (CH3OH)2CoIIITPP+ + Cl-
(2)
The major species (99.6%) of (Cl)CoIIITPP (ca. <10-4 M) in
methanol is the six-coordinate species (CH3OH)2CoIIITPP+. The
molar absorption coefficient, ꢀ, of (CH3OH)2CoIIITPP+ in
methanol has been obtained as 2.73 × 105 M-1 cm-1 at 425
nm.
Figure 3 shows the absorption spectra observed for the
methanol solution of 1.5 × 10-5 M (Cl)CoIIITPP at various
concentrations of KCN. At low concentrations of KCN ([KCN]
< 3.86 × 10-5 M), the absorption peak of (Cl)CoIIITPP located
at 426 nm shifts slightly to red by ca. 3 nm with an increase in
the concentration of KCN. The small change in the absorption
spectrum is likely to result from the formation of (CH3OH)-
(CN)CoIIITPP:
Figure 2. Molecular structure of the [crown-K+‚H2O]2[(CN)2CoIII-
TPP-] moiety with disordered H2O molecules.
and [(CN)2FeIIITPP-] (1.147(3) Å).1 Because the distance N11-
O15 (and O13) is 2.77-2.85 Å, a hydrogen bond is probably
formed between CN (N11) and H2O (O13 or O15).
The CCN-Co-CCN bond angle is obtained as 176.3(8)°. Thus,
the CCN-Co-CCN bond is almost linear as in the cases of
[(CN)2MnIIITPP-] and [(CN)2FeIIITPP-]. The N-C-Co angles
are 174(2)/178(2) and 175(2)/176(2)°, indicating that the CN
groups are linearly coordinated to the central cobalt atom.
The 1H NMR spectrum measured in CDCl3 is interpreted in
terms of the formation of [crown-K+][(CN)2CoIIITPP-] by
dissolving the crystals of {[crown-K+]2[(CN)2CoIIITPP-]}[(CN)2-
CoIIITPP-].
(CH3OH)2CoIIITPP+ + CN- a
(CH3OH)(CN)CoIIITPP + CH3OH (3)
The absorption spectrum of [crown-K+][(CN)2CoIIITPP-] in
a dichloromethane (CH2Cl2) solution differs markedly from that
of (Cl)CoIIITPP. The absorption peaks of (Cl)CoIIITPP in CH2-
Cl2 are located at 406 nm (ꢀ ) 1.27 × 105 M-1 cm-1) and 540
(30) Hoshino, M.; Katayama, S.; Yamamoto, K. Bull. Chem. Soc. Jpn. 1985,
58, 3360-3364.

Dicyanocobalt(III) Tetraphenylporphyrin
Inorganic Chemistry, Vol. 39, No. 21, 2000 4853
Figure 3. Absorption spectral changes observed for the methanol solution of (Cl)CoIIITPP at various concentrations of KCN: (1) 0.0 M, (2) 9.65
× 10-6 M, (3) 1.93 × 10-5 M, (4) 4.86 × 10-5 M, (5) 5.79 × 10-5 M, (6) 7.72 × 10-5 M, (7) 9.65 × 10-5 M, (8) 1.16 × 10-4 M, (9) 1.35 × 10-4
M, (10) 1.54 × 10-4 M, and (11) 7.7 × 10-4 M.
cobalt porphyrin has only two axial positions, and thus
coordination of three CN- to the central cobalt atom of (CH3-
OH)(CN)CoIIITPP is very unlikely. The assumption that KCN
is fully dissociated in methanol leads to [CN-] ) [K+]. Thus,
one CN- and two K+ are presumably responsible for n ) 3:
(CH3OH)(CN)CoIIITPP + CN- + 2K+ {K3
\}
{[K+]2[(CN)2CoIIITPP-]}+ + CH3OH (7)
Figure 4. Plot of log Y vs log [CN-] (see text).
The spectroscopic studies mentioned above suggest that the
green species produced by the reaction of (Cl)CoIIITPP and KCN
in methanol is a cationic species, {[K+]2[(CN)2CoIIITPP-]}+,
in which two CN- ions are coordinated to the central cobalt
atom. It is likely that two K+ are necessary to stabilize the green
anionic species, [(CN)2CoIIITPP-], in methanol.
The equilibrium reaction 3 mostly shifts to the right side at
[KCN] ) 3.86 × 10-5 M.
A remarkable change in the absorption spectrum is observed
when [KCN] > 3.86 × 10-5 M: new absorption bands appear
at 443, 572, and 612 nm. The spectral change at higher
concentrations of KCN is interpreted in terms of the further
association of CN- to (CH3OH)(CN)CoIIITPP:
The 1H NMR spectra (500 MHz, CD3OD, tetramethylsilane
is used as an internal standard) were measured for (Cl)CoIII-
TPP and the green species, [(CN)2CoIIITPP-]. The former gives
δ 9.34 (8H, s), δ 8.26 (8H, d), and δ 7.87 (12H, m), and the
latter gives δ 8.74 (8H, s), δ 8.14 (8H, d), and δ 7.72 (12H,
m). These NMR spectra indicate that both (CH3OH)2CoIIITPP+
and [(CN)2CoIIITPP-] have C4h symmetry; the ortho protons
of the phenyl group in these porphyrins are equivalent. The δ
value of the pyrrole proton in the cobalt(III) porphyrin markedly
shifts to the higher field by the effects of the CN- coordination.
Isolation and crystallization of both (CH3OH)(CN)CoIIITPP
and [(CN)2CoIIITPP-] from the methanol, ethanol, and acetone
solutions was not successful. When the solution is concentrated,
the cyanide complexes readily polymerize to give fine powders
that are insoluble in both organic and inorganic solvents.
Photochromic Reaction of [(CN)2CoIIITPP-] in Methanol.
When a methanol solution of 1.1 × 10-5 M (Cl)CoIIITPP
containing 1.4 × 10-4 M KCN was irradiated by a 250-W
mercury lamp with a cutoff filter (λ < 360 nm) for 5.0 min,
the color of the solution turned from green to orange. The
absorption spectrum observed at 5 s after 5.0 min of irradiation
was found to be almost identical with that of (CH3OH)(CN)-
CoIIITPP. The original absorption spectrum was restored when
(CH3OH)(CN)CoIIITPP + nCN- {Kn
\}
[(CN)n+1CoIIITPP]n- + CH3OH (4)
From eq 4, the absorbance at wavelength λ, Dλ, is formulated
as5
Y ) (Dλ - D0 )/(Dλ - D∞ ) ) Kn[CN-]n
(5)
λ
λ
λ
λ
where D0 and D∞ are the absorbances at [KCN] ) 3.86 ×
10-5 M and at an “infinite” concentration of CN-, respectively.
Equation 5 is rewritten as
log(Y) ) log(Kn) + n log[CN-]
(6)
Figure 4 shows the plot of log Y obtained at λ ) 440 nm vs
log[CN-]. The plot gives a straight line with a slope n ) 3.0
and the intercept log(Kn) ) 12.3 ((0.2) by least-squares fittings.
The former value, n ) 3, implies that three CN- ions are
necessary for the formation of the green species. However,

4854 Inorganic Chemistry, Vol. 39, No. 21, 2000
Hoshino et al.
Figure 5. Rate constants, k, for regeneration of [(CN)2CoIIITPP-] in
methanol, represented as a function of [CN]. The inset is the rise curve
of the absorbance change at 445 nm observed after the continuous
photolysis of [(CN)2CoIIITPP-] in methanol containing 1.88 × 10-4
M KCN.
Figure 6. Transient absorption spectra observed for the methanol
solution of 1.1 × 10-5 M [(CN)2CoIIITPP-] and 1.23 × 10-2 M KCN
at (a) 20 ns and (b) 1.0 µs after the 355-nm laser pulse. The inset is
the decay of the absorbance monitored at 450 nm.
the irradiated solution was preserved in the dark for 10 min.
On the basis of these findings, the photochemical reaction of
[(CN)2CoIIITPP-] is expressed by eq 8.
few milliseconds. The 20-ns spectrum has a positive peak at
435 nm and a negative one at 448 nm. The 1.0-µs spectrum
has positive and negative peaks at 430 and 445 nm, respectively.
No permanent products were observed after photolysis, indicat-
ing that the two transient species, I (short-lived) and II (long-
lived), are produced by the laser flash photolysis. Both transients,
I and II, eventually return to [(CN)2CoIIITPP-]. The rate
constant for the decay of the transient I is found to be
independent of the concentration of CN- in the concentration
range between 1.0 × 10-4 and 1.23 × 10-2 M.
{[K+]2[(CN)2CoIIITPP-]}+ + CH3OH {hν
\}
(CH3OH)(CN)CoIIITPP + CN- + 2K+ (8)
The orange product, (CH3OH)(CN)CoIIITPP, regenerates
{[K+]2[(CN)2CoIIITPP-]}+ according to eq 8. Hereafter, we
denote {[K+]2[(CN)2CoIIITPP-]}+ in methanol as [(CN)2-
CoIIITPP-].
As shown in the inset of Figure 5, the rate for the regeneration
of [(CN)2CoIIITPP-] was measured by monitoring the absor-
bance, D, of the irradiated solution at 445 nm. The increase in
D strictly follows first-order kinetics with the rate constant k.
The absorption spectra of the transients were obtained with
the method previously reported.31,32 When the parent molecule
gives the transient species after the laser pulse, the absorbance
change at λ, ∆D(λ), is given by
∆D(λ) ) R(Dtr(λ) - D0(λ))
(11)
(D∞ - D) ) (D∞ - D0) [1 - exp(-kt)]
(9)
Here R is a variable number, 0 < R < 1.0, dominated by the
energy of the laser pulse; Dtr(λ) and D0(λ) are the absorbances
of the transient species and the parent molecule, respectively.
Equation 11 is rewritten as
Here D0 and D∞ are the absorbances at 455 nm measured at t
) 0 and at an “infinite” time after 5.0 min irradiation by the
mercury lamp, respectively.
Figure 5 shows the plot of k represented as a function of the
concentration of CN-. The plot exhibits a concave upward curve
with an increase in the concentration of CN-. From eq 7, the
rate constant k for the regeneration of [(CN)2CoIIITPP-] is
formulated as
Dtr(λ) ) D0(λ) + ∆D(λ)/R
(12)
With the use of various R, Dtr(λ) is obtained as a function of λ.
When the R value is appropriate, the plot of Dtr(λ) versus λ
gives the absorption spectrum of the transient species. The
appropriate value of R is determined on the basis of the
assumption that the transient species has a smooth and undis-
torted absorption band in the Soret band region. From the
transient absorption spectra detected at 20 ns and 1 µs, the
absorption spectrum of the transient I was found to have a Soret
band peak at 439 ( 2 nm with ꢀ ) (2.5 ( 0.3) × 105 M-1
cm-1. Similarly, the transient II exhibits the Soret band peak
at 430 ( 2 nm with ꢀ ) (2.3 ( 0.3) × 105 M-1cm-1. The
absorption spectrum of the transient II is found to be almost
identical with that of (CH3OH)(CN)CoIIITPP in methanol.
The quantum yields, φ, for the formation of the two transients
were determined with the use of the laser photolysis method.
The absorbance change, ∆D(λ), observed for [(CN)2CoIIITPP-]
in methanol at 20 ns after the 355-nm pulse is represented by33
k ) kb + kf[CN-][K+]2 ) kb + kf[CN-]3
(10)
where kf is the forward rate constant of eq 7 and kb is the
backward rate constant: K3 ) kf/kb. The kb value is estimated
as 1.5 × 10-3 s-1 from Figure 5. The plot of log(k - kb) versus
log[CN-] gives a straight line with a slope of 3 and the intercept
log kf ) 9.7 ( 0.1 by least-squares fittings. From kf and kb
obtained here, log K3 is calculated as 12.5 ( 0.2. This value is
in good agreement with that (12.3 ( 0.2) obtained from the
spectroscopic studies described above.
Laser Photolysis of [(CN)2CoIIITPP-] in Methanol. The
355-nm laser photolysis studies were carried out for the green
methanol solution containing 1.1 × 10-5 M [(CN)2CoIIITPP-]
and 1.23 × 10-2 M KCN. Figure 6 shows the transient spectra
detected at 20 ns and 1 µs after a laser pulse. The transient
spectrum (a) was detected at 20-ns decays according to first-
order kinetics with a rate constant of 8.4 × 106 s-1, leaving the
transient spectrum (b) detected at 1.0 µs, which decays over a
(31) Seki, H.; Okada, K.; Iimura, Y.; Hoshino, M. J. Phys. Chem. A 1997,
101, 8174-8178.
(32) Inamo, M.; Hoshino, M. Photochem. Photobiol. 1999, 70, 596-601.
(33) Hoshino, M.; Nagamori, T.; Tase, T.; Chihara, T.; Lillis, J. P.;
Wakatsuki, Y. J. Phys. Chem. A 1999, 103, 3672-3677.

Dicyanocobalt(III) Tetraphenylporphyrin
Inorganic Chemistry, Vol. 39, No. 21, 2000 4855
-1
∆D(λ) ) φ ∆ꢀ(λ) Iabs NA
(13)
Here Iabs, ∆ꢀ(λ), and NA are the number of photons absorbed,
the difference in the molar absorption coefficient at λ between
[(CN)2CoIIITPP-] and the transient, and Avogadro’s number,
respectively. For determination of Iabs, a benzene solution of
benzophenone with the same absorbance at 355 nm as that for
the methanol solution of [(CN)2CoIIITPP-] was used as a
standard solution for monitoring the T-T absorption of the
triplet benzophenone after the pulse. The absorbance, ∆DT, of
the triplet benzophenone at 20 ns after the pulse is described as
-1
∆DT ) φTꢀTIabsNA
(14)
where φT is the triplet yield of benzophenone (φT ) 1.0) and
ꢀT is the molar absorption coefficient of the triplet benzophenone
(ꢀT ) 7.8 × 103 M-1 cm-1 at 530 nm) in benzene.34 From eqs
13 and 14, φ is expressed as
Figure 7. Transient absorption spectrum observed for the dichlo-
romethane solution of 1.0 × 10-5 M [crown-K+][(CN)2CoIIITPP-] at
20 ns after the 355-nm laser pulse. The inset is the decay of the
absorbance monitored at 440 nm.
φ ) φT(ꢀT/∆ꢀ)-1(∆D/∆DT)
(15)
Scheme 1
With the use of the molar absorption coefficients of the
transients I and II, the quantum yields for the formation of the
transients I and II are determined as 0.1 ((0.02) and 0.02
((0.005), respectively.
Laser Photolysis of [crown-K+][(CN)2CoIIITPP-]. The laser
photolysis studies of [crown-K+][(CN)2CoIIITPP-] in a dichlo-
romethane solution were carried out. Figure 7 shows the
transient absorption spectrum observed for 1.0 × 10-5
M
[crown-K+][(CN)2CoIIITPP-] after 355-nm laser pulsing. The
spectrum measured at 20 ns after the pulse has a positive band
at 440 nm and a negative one at 460 nm. From the transient
spectrum, the Co-CN bond in [crown-K+][(CN)2CoIIITPP-] is
concluded to be dissociated upon laser flash photolysis. The
decay of the transient strictly follows first-order kinetics with
the rate constant of 6.2 × 105 s-1. Thus, the structure of the
transient is suggested to be [crown-K+CN-][(CN)CoIIITPP].
Presumably, the oxygen atom in the [crown-K+CN-] moiety is
coordinated to the central cobalt atom. The photochemical
reaction of [crown-K+][(CN)2CoIIITPP-] is illustrated in
Scheme 1.
Table 2. First-Order Decay Rate Constants of the Transient Species
Produced by Laser Photolysis of [crown-K+][(CN)2CoIIITPP-] in
Several Solvents
rate constant
rate constant
solvent
toluene
dichloromethane
acetonitrile
× 10-6, s-1
solvent
× 10-6, s-1
0.73
0.62
0.59
acetone
ethanol
methanol
0.66
3.3
10.3
In methanol and ethanol solutions, [crown-K+][(CN)2CoIII-
TPP-] (ca. 1.0 × 10-5 M) thermally decomposes within 30 min
to give an orange solution. The product exhibits an absorption
spectrum identical with that of (CH3OH)(CN)CoIIITPP in
methanol. Irradiation of the solution by the mercury lamp
accelerated the rate for the decomposition. The laser photolysis
of the methanol solution revealed that the transient spectrum
detected at 20 ns after the pulse is similar to the difference
spectrum obtained by subtracting the spectrum of [crown-
K+][(CN)2CoIIITPP-] from that of (CH3OH)(CN)CoIIITPP. The
decay of the transient follows first-order kinetics with a rate
constant of 1.0 × 107 s-1, leaving the photoproduct, (CH3OH)-
(CN)CoIIITPP. In Table 2 are listed the decay rate constants of
the transient measured after laser photolysis of [crown-K+][(CN)2-
CoIIITPP-] in several solvents. The rate constants obtained with
methanol and ethanol solutions are much larger than those with
nonalcoholic solutions. It is noted that the thermal and photo-
accelerated decomposition of [crown-K+][(CN)2CoIIITPP-] is
limited to the alcoholic solutions studied.
the green species from the methanol solution could not be done,
the origin of the green solution is ascribed to the formation of
the dicyanide complex, [(CN)2CoIIITPP-]. A similar color
change is also observed when KCN and 18-crown-6 are added
to a benzene solution of (Cl)CoIIITPP. The X-ray structure
determination of [crown-K+][(CN)2CoIIITPP-] indicates that the
two axial cyanide anions are necessary for the observed color
change of CoIIITPP+.
A number of studies on the ligand exchange reaction of
iron(III) porphyrins by cyanide ions have been carried out.6,31,35-37
The formation of the dicyanide complexes of iron(III) porphyrins
has been interpreted in terms of the stepwise ligand exchange
mechanism.1 The monocyanide complex initially produced
further reacts with CN-, resulting in the formation of the
dicyanide complex of iron(III) porphyrins.
Discussion
(35) Arifuku, F.; Ujimoto, K.; Kurihara, H. Bull. Chem. Soc. Jpn. 1986,
A methanol solution of (Cl)CoIIITPP changes its color from
red to green with the addition of KCN. Although isolation of
59, 149-154.
(36) Wang, J. T.; Yeh, H. J. C.; Johnson, D. F. J. Am. Chem. Soc. 1978,
100, 2400-2405.
(37) Kajiro, K.; Uchimura, F.; Kikuchi, G. J. Biochem. 1956, 43, 539-
(34) Bensasson, R.; Land, E. J. Trans. Faraday Soc. 1971, 67, 1904-1915.
552.

4856 Inorganic Chemistry, Vol. 39, No. 21, 2000
Hoshino et al.
Scheme 2
A methanol solution of 1.1 × 10-5 M (Cl)CoIIITPP also
exhibits stepwise equilibria by the addition of KCN; the first
step equilibrium at [KCN] < 3.86 × 10-5 M is ascribed to the
formation of (CH3OH)(CN)CoIIITPP, and the second step is
for the formation of [(CN)2CoIIITPP-] from (CH3OH)(CN)CoIII-
TPP. The laser photolysis of [(CN)2CoIIITPP-] in methanol gives
transients I and II. The long-lived transient II is assigned to
(CH3OH)(CN)CoIIITPP, which is produced by full dissociation
of the axial CN-. The short-lived transient I decays according
to first-order kinetics. Thus, the transient I cannot be produced
from the full dissociation of CN-. It is likely that the dissociated
CN- is located nearby the central cobalt atom. Scheme 2 shows
the thermal and photochemical reactions of [(CN)2CoIIITPP-]
in methanol. Because CN- makes a strong hydrogen bond with
alcohols, the dissociated CN- is probably trapped by the axial
methanol of (CH3OH)(CN)CoIIITPP. The structure of the
transient I is assumed to be (CN-‚‚‚HOCH3)(CN)CoIIITPP, in
which the oxygen atom of the (CN-‚‚‚HOCH3) moiety is
coordinated to the central CoIII atom. The intracomplex ligand
exchange reaction of (CN-‚‚‚HOCH3)(CN)CoIIITPP leads to the
regeneration of [(CN)2CoIIITPP-] in methanol. Although there
is no experimental evidence, the interconversion from transient
I to transient II is one of the possible pathways to the
photochemically induced full dissociation of the axial CN- from
[(CN)2CoIIITPP-] in methanol.
1
ascribed to the formation of the green species. The H NMR
measurement of the green species in CD3OD reveals that the
chemical shifts of the pyrrole and phenyl protons are close to
those of [crown-K+][(CN)2CoIIITPP-]. Thus, the green species
in methanol is ascribed to [(CN)2CoIIITPP-].
The spectroscopic titration of (CH3OH)2CoIIITPP+ by KCN
in methanol indicates that two K+ and two CN- ions are
necessary for the formation of the stable green species,
[(CN)2CoIIITPP-]. The counterions, K+, are suggested to
stabilize [(CN)2CoIIITPP-] in methanol solutions.
A methanol solution of ca. 1.0 × 10-5 M [crown-K+][(CN)2-
CoIIITPP-] gradually decomposes to yield (CH3OH)(CN)CoIII-
TPP.
[crown-K+][(CN)2CoIIITPP-] + CH3OH a
[crown-K+] + CN- + (CH3OH)(CN)CoIIITPP (16)
Probably, [(CN)2CoIIITPP-] in methanol is not stable unless the
counterion [crown-K+] is located nearby the axial CN-.
The methanol solution of [(CN)2CoIIITPP-] displays photo-
chromism. The continuous photolysis of [(CN)2CoIIITPP-] in
methanol containing KCN gives (CH3OH)(CN)CoIIITPP as a
photoproduct, which thermally returns to [(CN)2CoIIITPP-]. The
kinetic studies on the regeneration of [(CN)2CoIIITPP-] clearly
indicate that, in agreement with the spectroscopic titration study
mentioned above, two K+ ions and one CN- ion are essential
Conclusion
In benzene or chloroform, (Cl)CoIIITPP reacts with KCN in
the presence of 18-crown-6, resulting in the formation of [crown-
K+][(CN)2CoIIITPP-]. The X-ray structure determination dem-
onstrates that both the CCN-Co-CCN and N-C-Co bonds are
almost linear. When KCN is added to the methanol solution of
(Cl)CoIIITPP, the dicyanocomplex, [(CN)2CoIIITPP-], is also
produced, and the structure is confirmed by NMR measure-

Dicyanocobalt(III) Tetraphenylporphyrin
Inorganic Chemistry, Vol. 39, No. 21, 2000 4857
ments. The methanol solution of [(CN)2CoIIITPP-] is found to
exhibit photochromism; the continuous photolysis of [(CN)2-
CoIIITPP-] yields (CH3OH)(CN)CoIIITPP, which thermally
returns to [(CN)2CoIIITPP-]. From the kinetic studies, it is
suggested that potassium ions are necessary to stabilize
[(CN)2CoIIITPP-] in methanol. The laser photolysis of [(CN)2-
CoIIITPP-] in methanol gives two transients: (CH3OH)(CN)-
CoIIITPP and (CN-‚‚‚HOCH3)CoIIITPP. The decay of (CN-‚‚‚H-
OCH3)CoIIITPP follows first-order kinetics with a rate constant
of 8.4 × 106 s-1. Because the rate constant is independent of
the concentration of CN-, (CN-‚‚‚HOCH3)CoIIITPP probably
returns to [(CN)2CoIIITPP-] via an intracomplex ligand ex-
change reaction.
Supporting Information Available: Tables listing bond lengths
and angles (except for hydrogen atoms), positional parameters, and
B(eq). This material is available free of charge via the Internet at
http://pubs.acs.org.
IC000408X