Formation and Deprotonation Kinetics of the Sitting-Atop Complex of Copper(II) Ion with 5,10,15,20-Tetraphenylporphyrin Relevant to the Porphyrin Metalation Mechanism. Structure of Copper(II)-Pyridine Complexes in Acetonitrile As Determined by EXAFS Spectroscopy

Article abstract of DOI:10.1021/ic980420d

The formation of a sitting-atop (SAT) complex of Cu(II) ion with 5,10,15,20-tetraphenylporphyrin (H₂tpp) in acetonitrile has been observed, and the kinetic parameters for the formation were determined as follows; k_so = (3.6 ±0.1) × 10⁵ mol^-1dm³ at 25.0 °C, ΔH^so? = 56 ±5 kJ mol^-1, and ΔS_SO? = 46 ±19 J mol^-1 K^-1. The ¹H NMR spectrum of the SAT complex (Cu(H₂tpp)²⁺) indicated that two pyrrolenine nitrogens coordinate to the Cu(II) ion and that two protons bound to the pyrrole nitrogens remain. The protons were abstracted by the addition of pyridine (py) as the Br?nsted base to give the Cu(tpp) metalloporphyrin. In the presence of py, the product for the reaction of the Cu(II) ion with H₂tpp was Cu(tpp) instead of the SAT complex. The observed conditional rates for the formation of Cu(H₂tpp)²⁺ and Cu(tpp) were interpreted by the contribution of Cu²⁺, Cu(py)²⁺, and Cu(py)₂²⁺ species, and the second-order rate constants of the SAT complex formation were k_S1 = (3.5 ± 0.3) × 10⁴ mo^-1 dm³ s^-1 for Cu(py)²⁺ and k_S2 = 90 ± 2 mol^-1 dm³ s^-1 for Cu(py)₂²⁺. Deprotonation rates were measured by following the reaction between the SAT complex and py as a function of the py concentration, and the second-order rate constant was determined to be (2.3 ± 0.1) × 10² mol^-1 dm³ s^-1. The present kinetic results have indicated that the SAT complex exists during the course of the metalation process and that the SAT complex formation is a rate-determining step.

Full text of DOI:10.1021/ic980420d

Inorg. Chem. 1998, 37, 5519-5526
5519
Formation and Deprotonation Kinetics of the Sitting-Atop Complex of Copper(II) Ion with
5,10,15,20-Tetraphenylporphyrin Relevant to the Porphyrin Metalation Mechanism.
Structure of Copper(II)-Pyridine Complexes in Acetonitrile As Determined by EXAFS
Spectroscopy
Yasuhiro Inada, Yumi Sugimoto, Yuko Nakano, Yuki Itoh, and Shigenobu Funahashi*
Laboratory of Analytical Chemistry, Faculty of Science, Nagoya University, Nagoya 464-8602, Japan
ReceiVed April 13, 1998
The formation of a sitting-atop (SAT) complex of Cu(II) ion with 5,10,15,20-tetraphenylporphyrin (H2tpp) in
acetonitrile has been observed, and the kinetic parameters for the formation were determined as follows: kS0
)
(3.6 ( 0.1) × 105 mol-1 dm3 s-1 at 25.0 °C, ∆HS0q ) 56 ( 5 kJ mol-1, and ∆SS0q ) 46 ( 19 J mol-1 K-1. The
1H NMR spectrum of the SAT complex (Cu(H2tpp)2+) indicated that two pyrrolenine nitrogens coordinate to the
Cu(II) ion and that two protons bound to the pyrrole nitrogens remain. The protons were abstracted by the
addition of pyridine (py) as the Brønsted base to give the Cu(tpp) metalloporphyrin. In the presence of py, the
product for the reaction of the Cu(II) ion with H2tpp was Cu(tpp) instead of the SAT complex. The observed
conditional rates for the formation of Cu(H2tpp)2+ and Cu(tpp) were interpreted by the contribution of Cu2+
Cu(py)2+, and Cu(py)22+ species, and the second-order rate constants of the SAT complex formation were kS1
,
)
(3.5 ( 0.3) × 104 mol-1 dm3 s-1 for Cu(py)2+ and kS2 ) 90 ( 2 mol-1 dm3 s-1 for Cu(py)22+. Deprotonation
rates were measured by following the reaction between the SAT complex and py as a function of the py
concentration, and the second-order rate constant was determined to be (2.3 ( 0.1) × 102 mol-1 dm3 s-1. The
present kinetic results have indicated that the SAT complex exists during the course of the metalation process
and that the SAT complex formation is a rate-determining step.
Introduction
and this intermediate was called a sitting-atop (SAT) complex,
in which two pyrrolenine nitrogens coordinate to the metal ion
Many metalloporphyrins and analogues, such as hemoglobin,
myoglobin, and chlorophyll, play significant roles in vivo.1-3
The function of these metalloporphyrins occurs at the axial site
of an incorporated metal ion, and the majority of studies on the
metalloporphyrin focus on the axial site.4 On the other hand,
the metalloporphyrin formation reaction is also one of the most
important processes in biological systems, and the importance
has been pointed out in relation to the biosynthesis of heme.3-9
The metalloporphyrin formation mechanism is not, however,
well established, not only in biological systems but also in
simple systems in vitro.10 In 1960, Fleisher and Wang proposed
that an intermediate should exist in the metalation process,11
and two protons on the pyrrole nitrogens still remain. The
existence of the SAT complex could explain the experimental
kinetics of the metalation for several porphyrins in N,N-
dimethylformamide (DMF),12-15 acetic acid,16 dimethyl sulfox-
ide (DMSO),17 and H2O.18,19 However, whereas the formation
constant of the SAT complex was reported as being on the order
of 102-104 mol-1 dm3 for Cu(II) and Zn(II) ions in DMF,12,14
the fast reaction corresponding to the formation of the SAT
complex was not observed. Furthermore, the reports on the
detection of the SAT complex have been very limited, only in
peculiar systems,20-23 and kinetic investigations of the SAT
complex formation have never been examined.
The metalloporphyrin formation (reaction 1) between a metal
ion (Mn+) and a general porphyrin (H2p) certainly proceeds by
multiple steps. At least two steps, i.e., the coordination of the
(1) da Silva, J. J. R. F.; Williams, R. J. P. The Biological Chemistry of
the Elements; Clarendon Press: Oxford, 1991.
(2) Fenton, D. E. Biocoordination Chemistry; Oxford Science Publica-
tions: Oxford, 1995.
(3) Lavallee, D. K. Mol. Struct. Energ. 1988, 9, 279.
(4) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978;
Vols. VI and VII.
(5) Blackwood, M. E., Jr.; Rush, T. S., III; Medlock, A.; Dailey, H. A.;
Spiro, T. G. J. Am. Chem. Soc. 1997, 119, 12170.
(6) Lloyd, S. G.; Franco, R.; Moura, J. J. G.; Moura, I.; Ferreira, G. C.;
Huynh, B. H. J. Am. Chem. Soc. 1996, 118, 9892.
(7) Conn, M. M.; Prudent, J. R.; Schultz, P. G. J. Am. Chem. Soc. 1996,
118, 7012.
(8) Taketani, S. In Regulation of Heme Protein Synthesis; Fujita, H., Ed.;
AlphaMed Press: OH, 1994.
(9) Dailey, H. A. Biosynthesis of Heme and Chlorophylls; McGraw-Hill:
New York, 1990.
(12) Funahashi, S.; Yamaguchi, Y.; Tanaka, M. Bull. Chem. Soc. Jpn. 1984,
57, 204.
(13) Funahashi, S.; Yamaguchi, Y.; Tanaka, M. Inorg. Chem. 1984, 23,
2249.
(14) Robinson, L. R.; Hambright, P. Inorg. Chim. Acta 1991, 185, 17.
(15) B.-Ackerman, M. J.; Lavallee, D. K. Inorg. Chem. 1979, 18, 3358.
(16) Funahashi, S.; Saito, K.; Tanaka, M. Bull. Chem. Soc. Jpn. 1981, 54,
2695.
(17) Pasternack, R. F.; Vogel, G. C.; Skowronek, C. A.; Harris, R. K.;
Miller, J. G. Inorg. Chem. 1981, 20, 3763.
(18) Hambright, P.; Chock, P. B. J. Am. Chem. Soc. 1974, 96, 3123.
(19) Turay, J.; Hambright, P. Inorg. Chem. 1980, 19, 562.
(20) Fleischer, E. B.; Dixon, F. Bioinorg. Chem. 1977, 7, 129.
(21) Burnham, B. F.; Zuckerman, J. J. J. Am. Chem. Soc. 1970, 92, 1547.
(22) Letts, K.; Mackay, R. A. Inorg. Chem. 1975, 14, 2993.
(23) Macquet, J. P.; Theophanides, T. Can. J. Chem. 1973, 51, 219.
(10) Lavallee, D. K. The Chemistry and Biochemistry of N-Substituted
Porphyrins; VCH: New York, 1987.
(11) Fleischer, E. B.; Wang, J. H. J. Am. Chem. Soc. 1960, 82, 3498.
10.1021/ic980420d CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/18/1998

5520 Inorganic Chemistry, Vol. 37, No. 21, 1998
n+ + H2p h M(p)(n-2)+ + 2H+
Inada et al.
under a nitrogen atmosphere, we confirmed that water less than 10-2
mol dm-3 did not affect any results.
M
(1)
Spectrophotometric Titration. To determine the formation con-
stants of Cu(II)-py complexes in AN, spectrophotometric titrations
were performed. The measurement apparatus consisted of an automatic
buret (E665, Metrohm), a glass titration vessel, a flow cell (light path
) 1 cm), a Teflon coated diaphragm pump (E-30, Kyoritsu), and a
UV-vis spectrophotometer (UV-265FW, Shimadzu). The Cu(II) ion
solution was titrated with a py solution, and the absorption spectra for
each titration point were recorded after achieving equilibrium. The
obtained absorbance data were analyzed by the least-squares calculation
using the program MQSPEC over the wavelength range 500-850 nm.31
EXAFS Measurements. The sample solutions for the extended
X-ray absorption fine structure (EXAFS) measurements were absorbed
in porous glass disks, which were sealed in a polyethylene bag in order
to prevent moisture ingress and evaporation of AN. X-ray absorption
spectra were measured in the vicinity of the Cu K-edge using the BL-
10B station at the Photon Factory of the National Laboratory for High
Energy Physics.32,33 The white synchrotron radiation was monochro-
matized by a Si(311) channel-cut crystal. The incident and transmitted
X-ray intensities were simultaneously measured by an ionization
chamber with a length of 17 and 31 cm, filled with N2 gas and a 3:17
mixture of Ar and N2 gas, respectively.
pyrrolenine nitrogens to Mn+ and the deprotonation of the
pyrrole proton should be involved in the overall process of
reaction 1. According to previous kinetic experiments,12-19
however, the apparent reaction proceeded in a single step. To
elucidate the metalation mechanism, kinetic observation for each
step is needed. Prevention of the deprotonation of the pyrrole
protons will be achieved by the appropriate selection of solvents
with low Brønsted basicity, because released protons cannot
be stabilized in such a solvent. In general, we must use
noncharged metal species such as MXn (X ) halogenide) as a
source of metal ions, because a solvent with low basicity cannot
solvate the free metal ion. The use of such metal species,
however, leads to complexity in the mechanistic analysis of the
metalation reaction. The conflict concerning the selection of
solvent and metal was overcome by the use of acetonitrile (AN)
in this work.
The basicity of AN is much lower than that of other aprotic
solvents, such as DMF, DMSO, and pyridine (py).24-26 On the
other hand, it is well-known that AN is a good solvent for the
metal salts, such as M(ClO4)n and M(CF3SO3)n, though its
donating ability is not so great.27 Thus, the use of AN is the
best selection for the mechanistic investigation of the metalation
reaction.28 In this work, we performed kinetic measurements
between the Cu(II) ion and H2tpp in AN and first succeeded in
detecting directly the SAT complex and evaluating independ-
ently its formation and deprotonation kinetics. First of all,
equilibria and structures of Cu(II) complexes in the presence
of py in AN were characterized in order to determine the initial
state for the reaction systems subsequently investigated.
The details of EXAFS data analysis were previously reported.34,35
The model function of EXAFS oscillation øcalc(k) is given as36-41
ø
calc(k) )
Nj
2rj
Fj(π,k) sin{2krj - Rj(k)} exp -2σj2k2 -
(2)
∑
2
{ }
(
)
λ
j
krj
where Fj(π,k) is the backscattering amplitude from each of Nj scatterers
j at a distance rj from the Cu center. σj is the Debye-Waller factor,
λ is the mean free path of an ejected photoelectron, and Rj(k) is the
total phase shift. The values of Fj(π,k) and Rj(k) reported by McKale
et al. were used.42 The parameters E0 and λ were determined by an
EXAFS spectrum of an aqueous solution of Cu(II) ion as a standard
sample on the basis of the structure parameters determined by the X-ray
diffraction method.43,44 The obtained values of E0 and λ were kept
constant during the course of the structural analysis of the other sample
solutions, and the values of rj, σj, and Nj, were optimized as variables.
The least-squares calculations for refinement of the structure parameters
were applied to the Fourier filtered k3ø(k) values so as to minimize the
error-squares sum, ∑{k3øobsd(k) - k3øcalc(k)}2. Calculations were
performed using the program REX (Rigaku).45
Experimental Section
Materials. A small excess of CF3SO3H was dropped into suspended
CuO (Wako, 99.99%) in water. The solution was stirred for a day,
residues were filtered, and the resultant solution was then concentrated
to obtain blue crystals of Cu(H2O)6(CF3SO3)2. The containing water
was completely expelled by heating at 300 °C, and a white powder of
Cu(CF3SO3)2 was obtained. Anal. Found: Cu, 17.6%. Calcd for
Cu(CF3SO3)2: Cu, 17.6%.
Pyridine (Wako, Pr. G.) was distilled twice under nitrogen atmo-
sphere after dehydration using 4A molecular sieves. H2tpp (Dojin) was
used without further purification, because it was spectrophotometrically
confirmed that the existence of impurities, such as 5,10,15,20-tetra-
phenylchlorine, was negligible. 5,10,15,20-Tetraphenylporphyrinato-
copper(II) (Cu(tpp)) was synthesized according to a reported proce-
dure.29,30 Acetonitrile was dried over 4A molecular sieves and distilled
under nitrogen. Sample solutions were prepared by dissolving each
reagent in AN under a nitrogen atmosphere in order to prevent
contamination by water in air. The content of water in the sample
solutions was confirmed to be less than 5 × 10-3 mol kg-1 by the
Karl-Fisher method. Although all the measurements were carried out
NMR Measurements. 1H NMR spectra for sample solutions were
measured using an AMX400 FT-NMR spectrometer (Bruker) at 400.13
MHz and 21 ( 1 °C for the following sample solutions: (1) 4.0 ×
10-2 mol dm-3 of H2tpp in CDCl3, (2) 1.0 × 10-4 mol dm-3 of H2tpp
(31) Suzuki, H.; Ishiguro, S. Inorg. Chem. 1992, 31, 4178.
(32) Nomura, M. KEK Report 85-7; National Laboratory for High Energy
Physics: Tsukuba, 1985.
(33) Nomura, M.; Koyama, A. KEK Report 89-16; National Laboratory
for High Energy Physics: Tsukuba, 1989.
(34) Inada, Y.; Sugimoto, K.; Ozutsumi, K.; Funahashi, S. Inorg. Chem.
1994, 33, 1875.
(35) Inada, Y.; Ozutsumi, K.; Funahashi, S.; Soyama, S.; Kawashima, T.;
Tanaka, M. Inorg. Chem. 1993, 32, 3010.
(24) Barrette, W. C., Jr.; Johnson, H. W., Jr.; Sawyer, D. T. Anal. Chem.
1984, 56, 1890.
(25) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publication: London, 1990.
(26) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data
1984, 13, 695.
(36) Sayers, D. E.; Stern, E. A.; Lytle, F. W. Phys. ReV. Lett. 1971, 27,
1204.
(37) Stern, E. A. Phys. ReV. B 1974, 10, 3027.
(38) Lytle, F. W.; Sayers, D. E.; Stern, E. A. Phys. ReV. B 1975, 11, 4825.
(39) Stern, E. A.; Sayers, D. E.; Lytle, F. W. Phys. ReV. B 1975, 11, 4836.
(40) Lengeler, B.; Eisenberger, P. Phys. ReV. B 1980, 21, 4507.
(41) Lee, P. A.; Citrin, P. H.; Eisenberger, P.; Kincaid, B. M. ReV. Mod.
Phys. 1981, 53, 769.
(27) Gutmann, V. The Donor-Acceptor Approach to Molecular Interac-
tions; Plenum Press: New York, 1978.
(28) Although the solubility of H2tpp is quite low in AN, the high molar
absorptivity of H2tpp in the visible region permitted runs of metalation
reactions.
(42) McKale, A. G.; Veal, B. W.; Paulikas, A. P.; Chan, S. K.; Knapp, G.
S. J. Am. Chem. Soc. 1988, 110, 3763.
(29) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. J. Am. Chem. Soc.
1951, 73, 4315.
(30) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443.
(43) Ohtaki, H.; Radnai, T. Chem. ReV. 1993, 93, 115.
(44) Marcus, Y. Chem. ReV. 1988, 88, 1475.
(45) Teranishi, T.; Harada, M.; Asakura, K.; Asanuma, H.; Saito, Y.;
Toshima, N. J. Chem. Phys. 1994, 98, 7967.

Kinetics of the Sitting-Atop Complex of Copper(II)
Inorganic Chemistry, Vol. 37, No. 21, 1998 5521
Table 1. Experimental Conditions for EXAFS Measurements
mole fraction
CCu
Cpy
2+
2+
2+
sample
solvent
mol dm-3
mol dm-3
Cu2+
Cu(py)2+
Cu(py)2
Cu(py)3
Cu(py)4
W
H2O
AN
AN
AN
AN
AN
AN
0.482
0.498
0.498
0.498
0.498
0.498
0.498
1.00
1.00
0.46
0.01
0
0
0
0
0
0.48
0.21
0.02
0
0
0
0.06
0.72
0.55
0.04
0
0
0
0
0.06
0.41
0.53
0.01
0
0
0
0
0.02
0.43
0.99
P0
P1
P2
P3
P4
P5
0
0.303
0.919
1.21
1.69
2.21
0
in CD3CN, and (3) a CD3CN solution containing 7.0 × 10-3 mol dm-3
Table 2. Structure Parameters of the Cu(II)-py Complexes in
of H2tpp and 3.5 × 10-2 mol dm-3 of Cu2+
.
Acetonitrilea
Kinetic Measurements. Absorption spectra were recorded on a
UV-vis spectrophotometer (UV-265FW), and the spectral changes for
the fast reactions were followed by a stopped-flow rapid-detection
system (RSP-600-06, Unisoku). The absorbance changes for the
reaction between the Cu(II) ion and H2tpp were monitored at 416 nm.
A stopped-flow three-sample mixer (newly developed and now com-
mercially available RS-3, Unisoku) with three sample syringes and two
mixing chambers was used for the kinetic measurements of the
deprotonation reaction; after a first mixing of AN solutions of the Cu(II)
ion and H2tpp, the absorbance changes at 414 nm for the deprotonation
were followed by a subsequent mixing of the first mixed solution and
a py solution.
b
c
d
Cu-N(an)eq
Cu-N(py)eq
Cu-N(an)ax
N
r/pme
N
r/pme
N
r/pme
2+
Cu(an)6
Cu(py)(an)5
4
3
2
1
199
202
202
205
2
2
2
2
2
218
230
230
232
237
2+
1
2
3
4
199
202
202
204
2+
2+
2+
Cu(py)2(an)4
Cu(py)3(an)3
Cu(py)4(an)2
a E0 ) 9002.6 ( 0.3 eV, λ ) 594 ( 19 pm. b The σ value of 5.9
pm, which was obtained from the data of sample P0, was used for all
samples. c The σ value of 5.8 pm, which was obtained from the data
of sample P5, was used for all samples. d The σ values were 16.7, 15.8,
13.8, 13.2, and 11.1 pm for Cu(an)62+, Cu(py)(an)52+, Cu(py)2(an)4
,
2+
Results and Discussion
Cu(py)3(an)32+, and Cu(py)4(an)22+, respectively. e Error of r is estimated
Equilibria and Structures of Pyridine Complexes of Cu(II)
Ion in Acetonitrile. The Cu(II) ion solutions (the initial
concentration ) 2.45 × 10-2 and 4.51 × 10-3 mol dm-3) were
to be ca. 1 pm.
The values of CCu and Cpy of the sample solutions for the
EXAFS measurements and the calculated mole fraction of each
Cu(II)-py complex are summarized in Table 1. The observed
EXAFS oscillations øobs(k) weighted by k3 and the Fourier
transforms are shown in Figures S2 and S3, respectively. The
main peaks (ca. 160 pm) in Figure S3 were attributed to the
nearest Cu-N or Cu-O interactions. In AN, the longer Cu‚‚‚C
nonbonding interactions appeared in the r range 200-300 pm,
where the multiple scatterings would also be included. In the
case of the sample solutions containing py, the peaks originating
from the nonbonding Cu‚‚‚C(py) interactions and the multiple
scatterings were also observed around 300-400 pm. The
intensities of these peaks gradually increased with respect to
an increase in Cpy. In this work, the main peaks (85 < r/pm <
205 in Figure S3) were extracted and used for the structural
analysis. The Fourier filtered and the calculated EXAFS
oscillations are consistent with each other, as shown in Figure
S4, and the obtained structure parameters are summarized in
Table 2.
titrated with the py solutions of 0.544 and 0.551 mol dm-3
,
respectively. An example of the titration curve at 700 nm is
shown in Figure S1A (S: Supporting Information). The
observed spectral data were analyzed by the least-squares
calculation on the basis of eq 3
4
2+
A )
ꢀ ,i[Cu(py)n
2+
Cu(py)n
]
(3)
∑
i
n)0
2+
where Ai is the absorbance at wavelength i and ꢀCu(py) ,i is the
n
molar absorption coefficient of Cu(py)n2+ at wavelength i. The
total concentrations of the Cu(II) ion (CCu) and py (Cpy) are
described by eq 4 using the formation constant ân of the Cu(II)-
py complexes defined as ân ) [Cu(py)n2+][Cu2+ -1 [py]-n
.
]
4
CCu ) [Cu2+] +
â [Cu2+][py]n,
∑
n
n)1
4
nâ [Cu2+][py]n (4)
For the structural analysis of the Cu(II)-py complexes, the
Cpy ) [py] +
∑
n
2+
structure parameters around the Cu center of Cu2+ and Cu(py)4
n)1
were first determined from the data of samples P0 and P5 (see
Table 1), respectively. Because sample P1 contained both Cu2+
and Cu(py)2+, the contribution of Cu(py)2+ to the k3øobs(k)
function was extracted using the Fourier filtered k3ø(k) values
of sample P0 on the basis of mole fractions. The structure
The obtained absorption spectra of the Cu(py)n2+ species are
shown in Figure S1B, and the values of ân at 25.0 ( 0.1 °C are
as follows: log(â1/mol-1 dm3) ) 6.4 ( 0.5, log(â2/mol-2 dm6)
) 11.9 ( 0.6, log(â3/mol-3 dm9) ) 15.8 ( 0.6, and log(â4/
mol-4 dm12) ) 18.5 ( 0.6. In water, the corresponding values
were reported as 2.60, 4.54, 5.80, and 6.70, respectively.46 It is
reasonable that the formation constants in AN are much larger
than those in water due to the weaker solvation of AN for the
Cu(II) ion. This is consistent with the negative value (-55.3
kJ mol-1) of the transfer free energy of Cu2+ from AN to
water.47 We have used the ân values in order to calculate the
distribution of the species in the sample solutions containing
py.
2+
parameters of Cu(py)3 were similarly determined from the
data of samples P4 and P5. In the case of samples P2 and P3,
the r values for three kinds of interaction, i.e., Cu-N(an)eq (the
nitrogen of AN in the equatorial site), Cu-N(py)eq (the nitrogen
of py in the equatorial site), and Cu-N(an)ax (the nitrogen of
AN in the axial site), and the σ value for the Cu-N(an)ax
interaction were optimized by fixing the N values for all
interactions and the σ values for Cu-N(an)eq and Cu-N(py)eq.
Because the obtained r values of samples P2 and P3 were
accepted as the mean values of some Cu(II)-py species
(46) Sun, M. S.; Brewer, D. G. Can. J. Chem. 1967, 45, 2429.
(47) Cox, B. G.; Parker, A. J.; Waghome, W. E. J. Phys. Chem. 1974, 78,
1731.
2+
2+
(Cu(py)2+ and Cu(py)2 for sample P2; Cu(py)2 and Cu-
2+
2+
(py)3 for sample P3), the structure parameters of Cu(py)2

5522 Inorganic Chemistry, Vol. 37, No. 21, 1998
Inada et al.
Cu(tpp) (λmax ) 414 nm) and has a characteristic broader Soret
band compared with those of H2tpp and Cu(tpp). This fact
suggests that the porphyrin ring in the product should be
1
distorted. According to the H NMR results described just
below, in addition to the above results, we have concluded that
the product of the reaction between Cu2+ and H2tpp is the SAT
complex (Cu(H2tpp)2+), which is stable at least for a few hours
in AN.
Figure 1B shows 1H NMR peaks assigned to N-H and
â-pyrrole protons for H2tpp in CDCl3 and the SAT complex in
CD3CN, and the chemical shift values are summarized in Table
3 together with the reported values for H2tpp.52,53 Although
the SAT complex contains the paramagnetic Cu(II) ion having
S )1/2, the â-pyrrole protons have been clearly observed as
seen in some low-spin Fe(III)-porphyrin complexes.54,55 The
1H NMR spectra of H2tpp in CDCl3 and CD3CN were almost
identical, and the signals of the N-H and â-pyrrole protons
were observed at a ratio of 1:4 in intensity. The finding that
the signal of the N-H protons in the SAT complex was clearly
observed at -2.05 ppm relative to TMS strongly indicated that
the N-H protons remain in the SAT complex (Cu(H2tpp)2+).
In contrast to the case of H2tpp, two kinds of â-protons for the
SAT complex were observed with a ratio of 1:1, which had
twice the area of the peak for N-H protons. The peaks, one a
singlet and the other a doublet, were attributed to the protons
in the pyrrolenine group coordinated and in the pyrrole group
not coordinated to the Cu(II) ion, respectively. A similar
splitting was reported for H2tpp at the low temperature of -80
°C, because the N-H tautomerism at such a low temperature
was frozen,52,53 where the signal of the â-pyrrole protons of
the pyrrole groups with N-H protons was observed at a lower
field relative to that of the pyrrolenine groups without the N-H
protons (see Table 3). The opposite trend in chemical shift was,
however, observed for the SAT complex, i.e., the doublet peak
assigned to the â-pyrrole protons appears at a higher field (see
Figure 1B). This indicates that the two pyrrolenine nitrogens
without an N-H proton bind to the paramagnetic Cu(II) ion,
because the dipole-dipole and scalar coupling interactions with
the paramagnetic ion lead to their downfield shift.56,57 Fur-
thermore, the peak for N-H protons was shifted downfield
relative to that of H2tpp. This is ascribed to both the distortion
of the porphyrin ring and the dipole-dipole interaction with
the paramagnetic ion. Because the N-H protons in H2tpp are
highly shifted to the upper field due to the ring current of the
planar porphyrin ring, the distortion of the porphyrin ring in
the SAT complex then leads to the downfield shift. Two types
of porphine skeletons (1 and 2 in Chart 1) are possible for the
SAT complex with pyrrolenine nitrogens coordinating to the
Figure 1. Spectral change for the formation of the SAT complex (A)
and H NMR spectra for H2tpp and the SAT complex (B): (A) the
1
spectra at 10.4, 20.4, 30.4, 40.4, and 60.4 ms after the start of the
reaction, [H2tpp]0 ) 1.5 × 10-6 mol dm-3, [Cu2+] ) 5.47 × 10-4 mol
dm-3, and 25.0 ( 0.1 °C; (B) the peaks of the N-H (higher field) and
â-pyrrole protons (lower field) for H2tpp (upper, 4.0 × 10-2 mol dm-3
of H2tpp in CDCl3) and the SAT complex (lower, CD3CN solution
containing 7.0 × 10-3 mol dm-3 of H2tpp and 3.5 × 10-2 mol dm-3 of
Cu2+).
were determined on the basis of mole fractions, and they were
in agreement with each other within the experimental uncertain-
ties.
The r value of the Cu-N(py)eq interaction in Cu(py)42+ was
consistent with the mean value (204.6 pm) of the Cu-N bond
lengths of Cu(py)4 (OCOCF3)2 in a single crystal48 and was by
5 pm longer than that of the Cu-N(an)eq of Cu2+ in AN. The
longer M-N bond lengths in pyridine derivatives compared to
those in AN were also reported for Mn(II) and Ni(II) ions.49-51
The fact that a gradual increase in r was observed with respect
to an increase in the number of py molecules around the Cu
center for both the Cu-N(an)eq and Cu-N(py)eq interactions
reflects the stronger σ-donation of py compared to that of AN.
Characterization of the SAT Complex in Acetonitrile.
Figure 1A shows the spectral change for the reaction between
Cu2+ and H2tpp in AN. This process was completed within
ca. 100 ms even in an excess Cu2+ concentration of the order
of 10-4 mol dm-3 and was extremely fast in comparison with
the usual metalloporphyrin formation of the Cu(II) ion.12,15,17,19
The absorption spectrum of the product (λmax ) 406 nm) is
apparently different from those of H2tpp (λmax ) 417 nm) and
1
Cu(II) ion. The observed H NMR spectrum determines the
symmetrical structure of 1, because in the case of 2 the two
â-protons of the pyrrolenine ring are not identical. Furthermore,
the clear splitting of two kinds of â-protons indicates that the
N-H tautomerism in the SAT complex is not so fast with
respect to the time scale of the 1H NMR measurements at 21 (
1 °C.
(52) Storm, C. B.; Teklu, Y. J. Am. Chem. Soc. 1972, 94, 1745.
(53) Storm, C. B.; Teklu, Y.; Sokoloski, A. Ann. N.Y. Acad. Sci. 1973,
206, 631.
(48) Pradilla, S. J.; Chen, H. W.; Koknat, F. W.; Fackler, J. P., Jr. Inorg.
Chem. 1979, 18, 3523.
(49) Kurihara, M.; Ozutsumi, K.; Kawashima, T. J. Sol. Chem. 1995, 24,
(54) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 9207.
(55) Tan, H.; Simonis, U.; Shokhirev, N. V.; Walker, F. A. J. Am. Chem.
Soc. 1994, 116, 5784.
719.
(56) Berliner, L. J.; Reuben, J. Biological Magnetic Resonance; Plenum
Press: New York, 1993.
(57) Bertini, I.; Luchinat, C. NMR of Paramagnetic Molecules in Biological
Systems; Benjamin: California, 1986.
(50) Inada, Y.; Funahashi, S. Anal. Sci. 1997, 13, 373.
(51) Inada, Y.; Sugata, T.; Ozutsumi, K.; Funahashi, S. Inorg. Chem. 1998,
37, 1886.

Kinetics of the Sitting-Atop Complex of Copper(II)
Inorganic Chemistry, Vol. 37, No. 21, 1998 5523
δ (relative to TMS)/ppm
Table 3. Values of Chemical Shift for H2tpp and the SAT Complex
solute
H2tpp
solvent
CDCl3
CD3CN
CDCl3/CS2
CDCl3/CS2
CD3CN
N-H
phenyl(o)
phenyl(m,p)
pyrrole(â)a
-2.77(s)
-2.87(s)
-3.33(s)
7.71-7.78(m)
7.80-7.87(m)
7.8(m)
8.20-8.23(m)
8.22-8.25(m)
8.3(m)
8.84(s)
8.85(s)
8.75(s)
8.90(d), 8.61(s)
8.66(d), 8.77(s)
H2tpp
H2tppb
H2tppb,c
Cu(H2tpp)2+
-2.05(s)
7.31-7.50(m)
8.10-8.12(m)
a When two values are represented, the former is the value of the â-H on the pyrrole groups with N-H proton and the latter is that of the other.
b Reference 52. c At -80 °C.
Chart 1
value of 56 kJ mol-1 for the SAT complex formation in
comparison with that for the solvent exchange is attributable to
the distortion pre-equilibrium of the porphyrin ring. To distort
the highly conjugated π-system of the porphyrin ring must
require a positive enthalpy change as calculated by Scheidt et
al. using molecular mechanics.63 The distortion pre-equilibrium
should also lead to a large positive value of ∆Sq due to the
increase in intramolecular freedom of the stretching and bending
modes in the porphyrin ring. Thus, we have concluded that
the SAT complex formation is followed by the reaction scheme
as follows:
Table 4. Rate Constantsa for Formation and Deprotonation of the
SAT Complex
H2tpp {K
\
D} H2tpp†
(5)
(6)
(7)
Formation
kS0/mol-1 dm3 s-1
∆HS0q/kJ mol-1
(3.6 ( 0.1) × 105
56 ( 5
Cu2+ + H2tpp† {KOS} Cu2+‚‚‚H2tpp†
∆SS0q/J mol-1 K-1
kS1/mol-1 dm3 s-1
kS2/mol-1 dm3 s-1
46 ( 19
(3.6 ( 0.3) × 104
90 ( 2
kex
Cu2+‚‚‚H2tpp† 98 Cu(H2tpp)2+
Deprotonation
k-H1/mol-1 dm3 s-1
(2.3 ( 0.1) × 102
(5.1 ( 0.5) × 103
where KD and KOS represent the distortion pre-equilibrium
constant and the outer-sphere association constant between the
distorted H2tpp (H2tpp†) and Cu2+, respectively. The rate-
determining step (reaction 7) is thought to be the exchange with
a dissociative-interchange mechanism between a bound AN
molecule at the axial site and a pyrrolenine nitrogen of H2tpp†.
The succeeding chelate ring closure step to form the SAT
complex, in which H2tpp coordinates to the Cu(II) ion as a
bidentate ligand, is generally faster than the coordination of a
first donor atom.
k-H2KSP/mol-2 dm6 s-1
a At 25.0 ( 0.1 °C.
Formation Kinetics of the SAT Complex in Acetonitrile.
The conditional first-order rate constants (kobs) for the SAT
complex formation were determined under the pseudo-first-order
conditions with excess Cu(II) concentration (CCu) (Table S1)
and evaluated to be proportional to CCu as shown in Figure S5.
Thus, kobs values were given as kobs ) kS0CCu. The second-
order rate constant kS0, the activation enthalpy ∆HS0q, and the
activation entropy ∆SS0q for the SAT complex formation were
determined to be (3.6 ( 0.1) × 105 mol-1 dm3 s-1 at 25.0 (
0.1 °C, 56 ( 5 kJ mol-1, and 46 ( 19 J mol-1 K-1, respectively
(Table 4).
Usually, the rates of both solvent exchange and complexation
reactions of the Cu(II) ion are extremely fast due to the lability
at elongated axial sites. The values of ∆Hq and ∆Sq for solvent
exchange were respectively reported as 11.5 kJ mol-1 and -21.8
J mol-1 K-1 in H2O,58 24.3 kJ mol-1 and 8.1 J mol-1 K-1 in
DMF,59 and 17.2 kJ mol-1 and -44.0 J mol-1 K-1 in
methanol,60,61 and it has been interpreted that the reaction
proceeds via a dissociative-interchange mechanism at a labilized
axial site. Although unfortunately the ∆Hq value in AN has
not been available, it is estimated to be 10-20 kJ mol-1 by
analogy to the trend of ∆Hq values for the first-row transition
metal(II) ions in H2O, DMF, and methanol.62 The larger ∆Hq
Zero intercept of the plot of kobs values as a function of CCu,
as shown in Figure S5, suggests the quantitative formation of
the SAT complex, and the overall formation constant can be
estimated to be larger than 107 mol-1 dm3. As seen in the
formation constants of the Cu(II)-py complexes, the larger
formation constant of the SAT complex in AN rather than in
the other solvents12,17 is due to the weaker solvation ability of
AN for the Cu(II) ion. Furthermore, in comparison with the
formation constant of Cu(py)2+ (â1 ) 106.4 mol-1 dm3), the
larger value for the SAT complex formation supports that H2tpp
coordinates to the Cu(II) ion as a bidentate ligand, because the
basicity of H2tpp is smaller than that of py.64-66 This is
1
consistent with the determination of structure 1 by H NMR.
Formation Kinetics of SAT and Cu(tpp) Complexes in the
Presence of Pyridine. The spectral changes for the reaction
of H2tpp with the Cu(II)-py complexes in the presence of
various amounts of py showed that the reaction product was
(58) Powell, D. H.; Helm, L.; Merbach, A. E. J. Chem. Phys. 1991, 95,
9258.
(59) Powell, D. H.; Furrer, P.; Pittet, P.-A.; Merbach, A. E. J. Phys. Chem.
1995, 99, 16622.
(60) Poupko, R.; Luz, Z. J. Chem. Phys. 1972, 57, 3311.
(61) Helm, L.; Lincoln, S. F.; Merbach, A. E. Inorg. Chem. 1986, 25, 2550.
(62) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes; VCH: New York, 1991.
(63) Cheng, B.; Munro, O. Q.; Marques, H. M.; Scheidt, W. R. J. Am.
Chem. Soc. 1997, 119, 10732.
(64) The reported pKa value of H3tpp+ (4.4 in water)65 is smaller than that
of Hpy+ (5.3 in water).66 The trend of basicity in AN is usually the
same as in water.25
(65) Aronoff, S. J. Phys. Chem. 1958, 62, 428.
(66) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1975.

5524 Inorganic Chemistry, Vol. 37, No. 21, 1998
Inada et al.
Figure 2. Plots of logarithmic values of conditional first-order rate
constants for formation of the SAT complex (circles) and Cu(tpp)
(triangles) as a function of Cpy, where (a) CCu ) (1.41-1.50) × 10-4
Figure 3. Plot of logarithmic values of kobs for Cu(tpp) formation
shown in Figure 2 as a function of log([py]/mol dm-3). Circles, triangles,
and squares represent the values at CCu ) (1.41-1.50) × 10-4, (2.98-
3.01) × 10-4, and (7.67-7.79) × 10-4 mol dm-3, respectively.
mol dm-3, (b) CCu ) (2.98-3.01) × 10-4 mol dm-3, and (c) CCu
)
(7.67-7.79) × 10-4 mol dm-3
.
the slope of the log-log plot of kobs/[Cu2+] against [py] is varied
from 1 to 2. Thus, it can be deduced that the reacting Cu(II)
species are Cu(py)2+ and Cu(py)22+. The reactions to form
Cu(tpp) are then expressed as reactions 12 and 13, and kobs
values were analyzed using eq 14.
changed from the SAT complex (Figure S6A) to Cu(tpp) (Figure
S6B) with increasing Cpy. It was indicated that Cu(tpp) was
not formed until the concentration of free py ([py]) exceeded
twice the total concentration of H2tpp ([H2tpp]0). This clearly
means that a proton acceptor, py in this case, is necessary to
form Cu(tpp) in AN. Unobservable formation of the SAT
complex in the sample solutions with [py] . 2[H2tpp]0 suggests
kS1
′
Cu(py)2+ + H2tpp py 8 Cu(tpp) + 2Hpy+
(12)
2+
that the formation constant of the SAT complex with Cu(py)n
will be decreased with an increasing number of n. The
conditional first-order rate constants (kobs) determined as a
function of Cpy at 25.0 ( 0.1 °C (Table S2) are plotted against
Cpy in the case of three different CCu in Figure 2.
kS2
′
Cu(py)22+ + H2tpp py 8 Cu(tpp) + 2Hpy+
(13)
(14)
2+
kobs ) kS1′[Cu(py)2+] + kS2′[Cu(py)2
]
The change in kobs for the SAT complex formation (circles
in Figure 2) was perfectly reproduced by taking into consider-
ation the contribution of Cu(py)2+ and Cu(py)2 species and
2+
Under the experimental conditions where the Cu(tpp) formation
occurs, the contribution of Cu2+ is negligible because of its
extremely small concentration. It is reasonable that the protons
released by reactions 12 and 13 will be stabilized by the
coexisting py. The rate constants were determined as follows:
kS1′ ) (2.6 ( 0.8) × 104 mol-1 dm3 s-1 and kS2′ ) (1.3 ( 0.6)
× 102 mol-1 dm3 s-1. The finding that the values of kS1 and
kS2 are almost identical to those of kS1′ and kS2′, respectively,
indicates that the SAT complexes should be formed in the
process of the Cu(tpp) formation and that the SAT complex
formation should be a rate-determining step for the Cu(tpp)
formation, because kS1′ and kS2′ might be much smaller than
kS1 and kS2, respectively, if the SAT complex formation is a
pre-equilibrium state prior to a rate-determining deprotonation.
This conclusion was also supported by the results of the
deprotonation rates of the SAT complex (vide infra). Using
all points in Figure 2 according to eq 11, the values of kS1 and
kS2 were finally determined to be (3.6 ( 0.3) × 104 mol-1 dm3
s-1 and 90 ( 2 mol-1 dm3 s-1 at 25.0 ( 0.1 °C, respectively
(Table 4). The calculated curves in Figure 2 reproduce very
well the experimental kobs values over the whole range of Cpy.
2+
2+
no contribution of Cu(py)3 and Cu(py)4 species, as shown
in eqs 8-10.
kS0
9
Cu2+ + H2tpp
8 Cu(H2tpp)2+
(8)
kS1
9
8 Cu(H2tpp)(py)m2+ + (1 - m)py,
Cu(py)2+ + H2tpp
m ) 0, 1 (9)
kS2
9
Cu(py)22+ + H2tpp
8 Cu(H2tpp)(py)n2+ + (2 - n)py,
n ) 0-2 (10)
Under the present experimental conditions ([H2tpp]0 ) 1.5 ×
10-6 mol dm-3, CCu ) 1.46, 2.98, and 7.72 × 10-4 mol dm-3
,
and Cpy ) (1.75-14.3) × 10-4 mol dm-3), kobs is given by eq
11, because it is acceptable that the formation and dissociation
rates of the Cu(II)-py complexes are more rapid than the
formation rate of the SAT complex.
kobs ) kS0[Cu2+] + kS1[Cu(py)2+] + kS2[Cu(py)2
]
(11)
2+
It should be noted that the rate constant of the SAT complex
formation gradually decreases with an increasing number of py
molecules coordinating to the Cu(II) ion, kS0 > kS1 > kS2, though
the Cu-N(an)eq bond length becomes longer with respect to
the number of py molecules around the Cu(II) ion (vide supra).
It has been known that the exchange of a bound solvent is
labilized by the coordination of an electron donor to the metal
By a least-squares analysis fixing the ân (n ) 1-4) and kS0
values independently determined, the second-order rate constants
were obtained as follows: kS1 ) (2.7 ( 1.1) × 104 mol-1 dm3
s-1 and kS2 ) (7 ( 2) × 103 mol-1 dm3 s-1
.
The kobs values of the Cu(tpp) formation (triangles in Figure
2) also decreased with increasing Cpy. As shown in Figure 3,

Kinetics of the Sitting-Atop Complex of Copper(II)
Inorganic Chemistry, Vol. 37, No. 21, 1998 5525
The final spectrum was perfectly identical to that of the
independently prepared Cu(tpp), which was characterized by a
specific absorption peak of Cu(tpp) at 535 nm. The values of
log(kobs/s-1) for the deprotonation of the SAT complex (Table
S3) are then plotted against log([py]/mol dm-3) in Figure 4B.
The slope of the plot is changed from 1 to 2. The kobs values
have been analyzed using eq 15 derived according to the reaction
scheme of eqs 16-19,
kobs ) k-H1[py] + k-H2KSP[py]2
(15)
(16)
(17)
Cu(H2tpp)2+ + py {KSP} Cu(py)(H2tpp)2+
k-H1
Cu(H2tpp)2+ + py
8 Cu(Htpp)+ + Hpy+
k-H2
Cu(py)(H2tpp)2+ + py
8 Cu(py)m(Htpp)+ + Hpy+ +
(1 - m) py, m ) 0, 1 (18)
fast
Cu(py)m(Htpp)+ + py 8 Cu(tpp) + Hpy+ + mpy,
m ) 0, 1 (19)
where KSP is the equilibrium constant for binding of py to the
Cu(II) ion in the SAT complex, and k-H1 and k-H2 are the
second-order rate constants for the deprotonation of pyrrole
protons in Cu(H2tpp)2+ and Cu(py)(H2tpp)2+, respectively. The
contribution of the other Cu(II)-py complexes can be ruled
out by the independence of kobs from CCu. Because the plot of
kobs against [py] was through the origin, we could also ignore
a reaction pathway of the self-deprotonation of the SAT
complex. This finding was consistent with the results obtained
in pure AN. The kinetic parameters at 25.0 ( 0.1 °C were
then obtained as follows: k-H1 ) (2.3 ( 0.1) × 102 mol-1 dm3
s-1 and k-H2KSP ) (5.1 ( 0.5) × 103 mol-2 dm6 s-1 (Table 4).
In effect, the conditional rate evaluated using these parameters
exceeds the conditional rate for the Cu(tpp) formation between
H2tpp and the Cu(II)-py complexes in the presence of py. This
fact strongly supports that the SAT complex formation is the
rate-determining step in the overall metalation process. The
report that there is no isotope effect on the rates for the
metalation of H2tpp and D2tpp with the Zn(II) ion in DMF71 is
consistent with the present conclusion.
Figure 4. Absorption spectral change during the deprotonation of the
SAT complex by pyridine in acetonitrile (A) and the log-log plot of
conditional first-order rate constants of the deprotonation against [py]
(B). In (B), circles and triangles respresent kobs values at CCu ) 1.94 ×
10-4 and 4.41 × 10-4 mol dm-3, respectively.
center.67-70 The trend in rates of SAT complex formation with
2+
Cu2+, Cu(py)2+, and Cu(py)2 is opposite to the expectation
from the change in the Cu-N(an)eq bond length and the electron
donation by the coordinating py. The solvent exchange on the
Jahn-Teller distorted Cu(II) ion proceeds with the fast intra-
molecular axial-equatorial (ax/eq) interconversion, and at that
time, all three N(an)-Cu-N(an) axes can interconvert.58-61 For
Cu(py)2+ with an unsymmetrical coordination sphere, the ax/
eq interconversion is limited only for two N(an)-Cu-N(an)
axes. In the case of Cu(py)22+, in which the py molecules are
reasonably coordinated in cis geometry, the exchange of AN
molecules in equatorial sites is thought to proceed by direct
exchange with the AN molecules in the second coordination
sphere, because interconversion is impossible, and thus the rate
should be much slower. The decrease in the rate constants of
the SAT complex formation can be interpreted by a decrease
in the frequency of the ax/eq interconversion. This is the first
report of the systematic determination of the rates for the
complexation of a series of Cu(II) species.
The overall deprotonation of the SAT complex may be
separated into two steps corresponding to the stepwise proton
dissociations (eqs 20 and 21).
Cu(H2tpp)2+ + py a Cu(Htpp)+ + Hpy+
Cu(Htpp)+ + py a Cu(tpp) + Hpy+
(20)
(21)
Deprotonation Kinetics of the SAT Complex. For the
overall metalation process of H2tpp, the release of the N-H
protons is required for its completion. Figure 4A shows the
spectral change in the reaction between the SAT complex and
py. Because the regeneration of free H2tpp by the addition of
py was not observed during the course of the reaction, py
directly reacted with the SAT complex. There were clear
isosbestic points at 405 and 425 nm, and the absorbance change
at 414 nm was first order with respect to the SAT complex.
The deprotonation reaction of the anticipated intermediate,
Cu(Htpp)+, is considered to be faster than that of Cu(H2tpp)2+
,
because the distance between the central copper and a remaining
pyrrole nitrogen with a proton in Cu(Htpp)+ must be shorter
than those in Cu(H2tpp)2+. If the first deprotonation (reaction
20) is a pre-equilibrium state prior to a rate-determining second
deprotonation (reaction 21), the apparent absorbance change for
deprotonation must not show first-order kinetic behavior. The
present results then indicate that the first deprotonation is a rate-
determining step for the overall deprotonation.
(67) Hioki, A.; Funahashi, S.; Tanaka, M. Inorg. Chem. 1986, 25, 2904.
(68) Ishii, M.; Funahashi, S.; Tanaka, M. Inorg. Chem. 1988, 27, 3192.
(69) Mizuno, M.; Funahashi, S.; Nakasuka, N.; Tanaka, M. Inorg. Chem.
1991, 30, 1550.
The deprotonation can also be observed in the presence of
other bases, such as 4-phenylpyridine (pKa of its conjugate acid
(70) Cusanelli, A.; Frey, U.; Richens, D. T.; Merbach, A. E. J. Am. Chem.
Soc. 1996, 118, 5265 and references therein.
(71) Lavallee, D. K.; Onady, G. M. Inorg. Chem. 1981, 20, 907.

5526 Inorganic Chemistry, Vol. 37, No. 21, 1998
Inada et al.
(28)
Rf ) KDKOSkSAT[MS62+][H2p]
in water ) 5.49), 3-hydroxypyridine (pKa ) 5.23), and
isoquinoline (pKa ) 5.40) but cannot be observed in the case
of a weaker base such as pyrazine (pKa ) 0.97).72 It should be
noted that, because water can act as a base, proton abstraction
was observed in the presence of a large amount of water (pKa
) -1.74) over 0.1 mol dm-3. According to our preliminary
experiments, the deprotonation rates corresponding to k-H1 in
the case of py were dependent on the Brønsted basicity of the
bases, i.e., the rate was on the order of 4-phenylpyridine ≈
isoquinoline > py > 3-hydroxypyridine . pyrazine > water.
Metalloporphyrin Formation Mechanism. We can gener-
ally describe the mechanisms of metalloporphyrin formation on
the basis of the present crucial results obtained in AN, together
with the previously reported results for a variety of metal ions
and porphyrins. The resolvable elementary reactions during the
metalation processes of a general porphyrin (H2p) with a
hexasolvated divalent metal ion in a usual donating solvent (S)
are summarily expressed by the distortion of the porphyrin ring
(eq 22), the outer-sphere encounter formation (eq 23), the
exchange of a bound solvent molecule with a first pyrrolenine
nitrogen (eq 24), the chelate ring closure to form the SAT
complex (eq 25), the first deprotonation of the pyrrole nitrogen
in the SAT complex (eq 26), and the second deprotonation to
form a metalloporphyrin (eq 27).
-1
-1
-H1
-1
RC
Rb ) K-H2
K
K
k
-SAT
[HS+]2[M(p)] (29)
These expressions for the metalation and demetalation rates can
explain some previously observed trends as follows: (1) the
parallel dependence of the metalation rate on the solvent
exchange rate and the lability of a bound solvent due to the
kSAT step;13,15,18,19,74-77 (2) the acceleration of the metalation
rate for the nonplanar porphyrins due to the KD step;10,12-15,78-81
(3) the dependence of the metalation rate on the Brønsted
basicity of porphyrins due to the kSAT step;14,82,83 (4) the [H+]2-
dependent rate law for the demetalation due to the KH1 and KH2
steps;84-86 (5) the dependence of the metalation rate of the
charged porphyrins on the ionic strength due to the KOS step.87
However, the pathway such as the second order with respect to
metal ion concentration found in DMF12 and acetic acid16 for
metalation of H2tpp is not involved. The formation and
deprotonation kinetics of the SAT complex, obtained by the
strategic use of AN as a solvent with very weak basicity, have
been independently first demonstrated in this work, and we have
unified the reaction scheme for the overall metalation of the
porphyrin.
kD
kD
H2p { } H2p†, KD )
(22)
Acknowledgment. This work was supported by the Grants-
in-Aid for Scientific Research (Nos. 09874131, 10440221,
10740305, and 10874081) from the Ministry of Education,
Science, Sports and Culture of Japan and the Kurata Research
Grant from the Kurata Foundation. The EXAFS measurements
were performed under the approval of the Photon Factory
Program Advisory Committee (Proposal Nos. 96G004 and
97G054).
k-D
k-D
kOS
kOS
MS62+ + H2p† { } MS62+‚‚‚H2p†, KOS
)
(23)
(24)
(25)
k-OS
k-OS
MS62+‚‚‚H2p† { kSAT } MS5(H2p)2+ + S
k-SAT
Supporting Information Available: Conditional first-order rate
constants for the SAT complex formation in AN (Table S1), for the
formation of the SAT complex and Cu(tpp) in the presence of py (Table
S2), and for the deprotonation of the SAT complex (Table S3); the
titration curves and absorption spectra (Figure S1) for the complexation
reactions of the Cu(II) ion with py in AN, the observed EXAFS
oscillations (Figure S2), the Fourier transform magnitude (Figure S3),
the Fourier-filtered and calculated EXAFS oscillations (Figure S4), the
kRC
kRC
MS5(H2p)2+
{
} MS4(H2p)2+ + S, KRC
)
k-RC
k-RC
k-H1
MS4(H2p)2+ + S* { } MS3(Hp)+ + HS*+ + S,
kH1
k-H1
kH1
kobs values for the SAT complex formation as a function of CCu (Figure
K-H1
)
(26)
S5), and the changes in absorption spectra corresponding to formation
of the SAT complex and Cu(tpp) in the presence of py (Figure S6) (8
pages). Ordering information is given on any current masthead page.
k-H2
k-H2
MS3(Hp)+ + S* { } M(p) + HS*+ + 3S, K-H2
)
IC980420D
kH2
(27)
kH2
(74) Choi, E. I.; Fleischer, E. B. Inorg. Chem. 1963, 2, 94.
(75) Fleischer, E. B.; Choi, E. I.; Hambright, P.; Stone, A. Inorg. Chem.
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(76) Kingham, D. J.; Brisbin, D. A. Inorg. Chem. 1970, 9, 2034.
(77) Tabata, M.; Tanaka, M. Inorg. Chem. 1988, 27, 203.
(78) Shah, B.; Shears, B.; Hambright, P. Inorg. Chem. 1971, 10, 1828.
(79) Funahashi, S.; Ito, Y.; Kakito, H.; Inamo, M.; Hamada, Y.; Tanaka,
M. Mikrochim. Acta, Wien I 1986, 33.
(80) Shimizu, Y.; Taniguchi, K.; Inada, Y.; Funahashi, S.; Tsuda, Y.; Ito,
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1993, 32, 1119.
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The value of kD is reported to be 4.6 × 107 s-1 at 25 °C for
5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin in water,73 and
the pathway of kOS is accepted to be diffusion-controlled.
Furthermore, the kRC step is generally considered to be faster
than the coordination of a first donor atom (kSAT). The present
results indicate that the kSAT path is a rate-determining step for
the metalation process in the presence of py as a proton acceptor
in AN. According to reactions 22-27, the forward and
backward rates for reaction 1 (Rf and Rb) are described by eqs
28 and 29, respectively.
(72) Pettit, L. D.; Powell, K. J. IUPAC Stability Constants Database SC-
Database for Windows, release 3; IUPAC and Academic Software:
U.K., 1997.
(73) Pasternack, R. F.; Sutin, N.; Turner, D. H. J. Am. Chem. Soc. 1976,
98, 1908.
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397.
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