Mechanism and driving force of NO transfer from S-nitrosothiol to cobalt(II) porphyrin: A detailed thermodynamic and kinetic study

Article abstract of DOI:10.1021/ic061427v

The thermodynamics and kinetics of NO transfer from S- nitrosotriphenylmethanethiol (Ph₃CSNO) to a series of α,β,γ,δ-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCo^II], generating the nitrosyl cobalt atom center adducts [T(G)PPCo^IINO], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectrophotometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer pathways suggests that the most likely route is a concerted process of the homolytic S-NO bond dissociation and the formation of the Co-NO bond. The kinetic investigation on the NO transfer shows that the second-order rate constants at room temperature cover the range from 0.76 × 10⁴ to 4.58 × 10⁴ M^-1 s^-1, and the reaction rate was mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in Ph₃CSNO is a Lewis acid and the T(G)PPCo^II is a Lewis base; the main driving force for the NO transfer is electrostatic charge attraction rather than the spin-spin coupling interaction. The effective charge distribution on the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCo^II], the transition-state, and the product [T(G)PPCo^IINO], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCo^II], 2.350 in the transition state, and 2.503 in the product [T(G)PPCo^IINO], which indicates that the concerted NO transfer from Ph₃CSNO to T(G)PPCo^II with the release of the Ph₃CS^? radical was actually performed by the initial negative charge (-0.350) transfer from T(G)PPCo^II to Ph₃CSNO to form the transition state and was followed by homolytic S-NO bond dissociation of Ph₃CSNO with a further negative charge (-0.153) transfer from T(G)PPCo^II to the NO group to form the final product T(G)PPCo^IINO. It is evident that these important thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO and metal porphyrins in both chemical and biochemical systems.

Full text of DOI:10.1021/ic061427v

Inorg. Chem. 2007, 46, 592−600
Mechanism and Driving Force of NO Transfer from S-Nitrosothiol to
Cobalt(II) Porphyrin: A Detailed Thermodynamic and Kinetic Study
Xiao-Qing Zhu,* Jian-Yu Zhang, and Jin-Pei Cheng*
Department of Chemistry and the State Key Laboratory of Elemento-Organic Chemistry, Nankai
UniVersity, Tianjin 300071, China
Received July 29, 2006
The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph
3
CSNO) to a series of
II
R
,
â
,
γ
,
δ
-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCo ], generating the nitrosyl cobalt atom center adducts
II
[
T(G)PPCo NO], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectropho-
tometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer
pathways suggests that the most likely route is a concerted process of the homolytic S NO bond dissociation and
−
the formation of the Co
constants at room temperature cover the range from 0.76
−
NO bond. The kinetic investigation on the NO transfer shows that the second-order rate
4
10 M s-1, and the reaction rate was
4
-1
×
10 to 4.58
×
mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in
Ph
CSNO is a Lewis acid and the T(G)PPCoII is a Lewis base; the main driving force for the NO transfer is
electrostatic charge attraction rather than the spin
3
−
spin coupling interaction. The effective charge distribution on
II
the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCo ], the transition-state, and
the product [T(G)PPCo NO], was estimated to show that the cobalt atom carries relative effective positive charges
of 2.000 in the reactant [T(G)PPCo ], 2.350 in the transition state, and 2.503 in the product [T(G)PPCo NO], which
II
II
II
II
•
indicates that the concerted NO transfer from Ph
actually performed by the initial negative charge (
3
CSNO to T(G)PPCo with the release of the Ph
0.350) transfer from T(G)PPCo to Ph
3
CS radical was
II
−
3
CSNO to form the transition
state and was followed by homolytic S
−
NO bond dissociation of Ph
3
CSNO with a further negative charge (
−
0.153)
II
II
transfer from T(G)PPCo to the NO group to form the final product T(G)PPCo NO. It is evident that these important
thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO
and metal porphyrins in both chemical and biochemical systems.
2
Introduction
directly react with it. Since the metal centers are generally
regarded as the principal targets for NO in mammalian
biology, the interaction of free NO with metal complexes
Extensive research into nitric oxide (NO) has been
stimulated since the discovery that NO plays essential
regulatory roles in processes such as smooth-muscle relax-
ation, regulation of blood pressure, modulation of neu-
such as metal porphyrins is of special interest, and this area
has been studied extensively.
3-11
However, a great deal of
1
rotransmission, and platellet adhesion in vivo. A number
(
2) (a) Palmer, R. M. J.; Ferrige, A. G.; Moncada, S. Nature 1987, 327,
524-526. (b) Hibbs, J. B., Jr.; Taintor, R. R.; Vavrin, Z. Science 1987,
35, 473-476. (c) Moncada, S.; Palmer, R. M. J.; Higgs, E. A.
of disease states involving NO imbalances were proposed,
and much attention has been given to the chemistry, biology,
and pharmacology of NO and the compounds which can
2
Pharmacol. ReV. 1991, 43, 109-142. (d) Wink, D. A.; Hanbauer, I.;
Grisham, M. B.; Laval, F.; Nims, R. W.; Laval, J.; Cook, J.; Pacelli,
R.; Liebmann, J.; Krishna, M.; Ford, P. C.; Mitchell, J. B. Curr. Top.
Cell. Regul. 1996, 34, 159-187.
*
To whom correspondence should be addressed. E-mail: xqzhu@
nankai.edu.cn (X.-Q.Z.).
1) (a) Feldman, P. L.; Griffith, O. W.; Stuehr, D. J. Chem. Eng. News
993, 71, 26. (b) Karupiah, G.; Xie, Q.; Buller, R. M. L.; Nathan, C.;
(3) (a) Coppens, P.; Novozhilova, I.; Kovalevsky, A. Chem. ReV. 2002,
102, 861-883. (b) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002,
102, 993-1017. (c) Butler, A. R.; Megson, I. L. Chem. ReV. 2002,
102, 1155-1165.(d) Wasser, I. M.; de Vries, S.; Moenne-Loccoz, P.;
Schroder, I.; Karlin, K. D. Chem. ReV. 2002, 102, 1201-1234.
(4) Richter-Addo, G. B. Acc. Chem. Res. 1999, 32, 529-536 and
references therein.
(
1
Duarte, C.; MacMicking, J. D. Science 1993, 261, 1445. (c) Lipton,
S. L. Nature 1993, 364, 626. (d) Edelman, G. M.; Gally, J. A. Proc.
Natl. Acad. Sci. U.S.A. 1992, 89, 11651.
592 Inorganic Chemistry, Vol. 46, No. 2, 2007
10.1021/ic061427v CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/16/2006

NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin
evidence has shown that the concentration of free NO in
Table 1. Enthalpy Changes of the Reactions of Ph3CSNO with
II
T(G)PPCo in Benzonitrile (kcal/mol) along with Redox Potentials of
mammalian tissue is very low (∼4 nM); the species most
II
II
T(G)PPCo and T(G)PPCo NO in Benzonitrile (V vs Fc)
related to NO found in mammalian tissue are nitrate, nitrite,
II
and NO-donors,1
2,13
substituents (G)
∆Hra
E1/2[T(G)PPCo NO] b E1/2[T(G)PPCoII] b
which means that the interaction of metal
p-OMe
p-Me
p-i-pr
m-Me
p-H
m-OMe
p-Cl
p-Br
m-Cl
m-Br
m-NO2
p-NO2
-31.6
-31.1
-31.0
-30.6
-30.2
-29.9
-29.4
-29.5
-29.6
-29.3
-27.8
-27.6
0.887
0.923
0.925
0.945
0.962
0.967
1.014
1.015
1.055
1.058
1.105
1.131
0.581
0.621
0.608
0.646
0.676
0.684
0.731
0.733
0.772
0.775
0.844
0.872
center complexes such as metal porphyrin with NO-donors
may be more common and important than that with free NO
in vivo. Since S-nitrosothiols (RSNOs) show many impor-
14
tant biological properties similar to those of NO itself and
generally have been regarded as the best candidates for the
endogenous storage and transports of NO1
3,15-17
and metal
porphyrins have been regarded as the most likely target to
accept NO in vivo, the interaction of RSNOs with metal
porphyrins should be not only very important but also
ubiquitous in vivo.3
,6,13
However, bibliographic research
a
Measured in benzonitrile at 298 K in kcal/mol by titration calorimetry.
The data given were the average values of at least two independent runs,
each of which was again an average value of at least 9 consecutive titrations.
The reproducibility was e1.0 kcal/mol. b Measured in benzonitrile at 298
K in V, taken as first redox potentials by CV method vs the ferrocenium/
ferrocene redox couple. Reproducible to 5 mV or better.
shows that compared to the reports on the interaction of free
NO with metal porphyrin, the publication with respect to
the interaction of RSNOs with metal porphyrin is quite poor,
and the many important chemical questions for this aspect
are still not clear.1
3,18
What are the final products of the
changes of each elementary step in the pathway of the NO
transfer from RSNOs to metal porphyrin? What are the
kinetic characters of the NO transfer from RSNOs to metal
porphyrin in the rate-determining step? What is the nature
of the driving force which pushes NO transfer from RSNOs
to metal porphyrin? Is it the electrostatic dipole charge
attraction or the spin-spin coupling interaction? It is evident
that all these interesting questions should be the keys to open
the door of comprehensively understanding the interaction
mechanism of metal porphyrins with RSNOs. In the present
reaction of RSNOs with metal porphyrin? What is the form
of NO in the initial transfer step from RSNOs to metal
porphyrin? Is it NO , NO ,or NO ? What are the energy
+
-
(
5) (a) Suzuki, N.; Higuchi, T.; Urano, Y.; Kikuchi, K.; Uchida, T.; Mukai,
M.; Kitagawa, T.; Nagano, T. J. Am. Chem. Soc. 2000, 122, 12059-
12060. (c) Linder, D. P.; Rodgers, K. R.; Banister, J.; Wyllie, G. R.
A.; Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 2004, 126,
4136-14148.
6) (a) Hoshino, M.; Laverman, L.; Ford, P. C. Coord. Chem. ReV. 1999,
1
(
187, 75-102 and references therein. (b) Laverman, L.; Ford, P. C.
Coord. Chem. ReV. 2005, 249, 391-403 and references therein.
7) Lim, M. D.; Lorkovic, I. M.; Ford, P. C. J. Inorg. Biochem. 2005, 99,
(
1
51-165 and references therein.
3
article, S-nitrosotriphenylmethanethiol (Ph CSNO) was cho-
(
8) (a) Hoshino, M.; Kogure, M. J. Phys. Chem. 1989, 93, 5478. (b)
Morlino, E. A.; Rodgers, M. A. J. J. Am. Chem. Soc. 1996, 118,
sen as representative of S-nitrosothiols, and tetraphenylpor-
II
phinatocobalt(II) derivatives [T(G)PPCo ] were chosen as
1
1798-11804. (c) Hoshino, M.; Nagashima, Y.; Seki, H. Inorg. Chem.
1
9
1998, 37, 2464-2469.
the model of metal porphyrins. Detailed thermodynamics
and kinetics of the NO transfer from Ph CSNO to a series
(
9) (a) Wolak, M.; Stochel, G.; Hamza, M.; van Eldik, R. Inorg. Chem.
3
2000, 39, 2018. (b) Zheng, D.; Birke, R. L. J. Am. Chem. Soc. 2001,
II
1
23, 4637.
of T(G)PPCo in benzonitrile were investigated using
titration calorimetry and stopped-flow spectrophotometry,
respectively. The experimental results can be used to answer
the above-mentioned questions.
(
10) (a) Thamae, M. A.; Nyokong, T. J. Porphyrins Phthalocyanines 2001,
5
, 839-845. (b) Wolak, M.; Zahl, A.; Schneppensieper, T.; Stochel,
G.; van Eldik, R. J. Am. Chem. Soc. 2001, 123, 9780-9791. (c) Zheng,
D.; Birke, R. L. J. Am. Chem. Soc. 2002, 124, 9066-9067. (d) Sharma,
V. S.; Pilz, R. B.; Boss, G. R.; Magde, D. Biochemistry 2003, 42,
8
900-8908.
11) (a) Hoshino, M.; Ozawa, K.; Seki, H.; Ford, P. C. J. Am. Chem. Soc.
993, 115, 9568-9575. (b) Laverman, L. E.; Hoshino, M.; Ford, P.
Results
(
1
Twelve para- and meta-substituted R,â,γ,δ-tetraphenylpor-
C. J. Am. Chem. Soc. 1997, 119, 12663-12664. (c) Laverman, L. E.;
Ford, P. C. J. Am. Chem. Soc. 2001, 123, 11614-11622. (d) Kurtikyan,
T. S.; Martirosyan, G. G.; Lorkovic, I. M.; Ford, P. C. J. Am. Chem.
Soc. 2002, 124, 10124-10129. (e) Fernandez, B. O.; Lorkovic, I. M.;
Ford, P. C. Inorg. Chem. 2004, 43, 5393-5402. (f) Kurtikyan, T. S.;
Gulyan, G. M.; Martirosyan, G. G.; Lim, M. D.; Ford, P. C. J. Am.
Chem. Soc. 2005, 127, 6216-6224.
II
phinatocobalts(II) [T(G)PPCo ] were synthesized according
19a,20
to the literature method
and were treated with S-
CSNO) in benzonitrile at
nitrosotriphenylmethanethiol (Ph
3
room temperature to give the corresponding cobalt-nitrosyl
product para- or meta-substituted nitrosyl-R,â,γ,δ-tetraphe-
(
12) Bellamy, T. C.; Griffiths, C.; Garthwaite, J. J. Biol. Chem. 2002, 277,
II
31801-31807.
nyl-porphinatocobalt(II) [T(G)PPCo NO] and the dimer
(
13) Wang, P. G.; Xian, M.; Tang, X.-P.; Wu, X.-J.; Wen, Z.; Cai, T.-W.;
Janczuk, A. J. Chem. ReV. 2002, 102, 1091-1134.
trityldisulfide (Ph
coordination of RSNO to the Co center porphyrin (eq 1).
The products were identified by MS, H NMR, and UV-
3 2
CS) as the final products without any
21
(
(
14) Zhang, Y.; Hogg, N. Free Radical Biol. Med. 2004, 36, 947.
15) Williams, D. L. H. Acc. Chem. Res. 1999, 32, 869-876 and references
therein.
1
vis spectra. The stoichiometry of reaction 1 was obtained
(
16) Malinski, T.; Bailey, F.; Zhang, Z. G.; Chopp, M. J. Cereb. Blood
Flow Metab. 1993, 13, 355-358.
(
17) (a) Andreasen, L. V.; Lorkovic, I. M.; Richter-Addo, G. B.; Ford, P.
C. Nitric Oxide: Biol. Chem. 2002, 6, 228-235. (b) Lee, J.; Chen,
L.; West, A. H.; Richter-Addo, G. B. Chem. ReV. 2002, 102, 1019-
(19) (a) Zhu, X.-Q.; Li, Q.; Hao, W.-F.; Cheng, J.-P. J. Am. Chem. Soc.
2002, 124, 9887-9893 and references therein. (b) Zhu, X.-Q.; Hao,
W.-F.; Tang, H.; Wang, C.-H.; Cheng, J.-P. J. Am. Chem. Soc. 2005,
127, 2696-2708.
1065.
(
18) (a) de Oliveira, M. G.; Shishido, S. M.; Seabra, A. B.; Morgon, N. H.
J. Phys. Chem. A, 2002, 106, 8963-8970 and references therein. (b)
Anthony, R. B.; Peter, R. Anal. Biochem. 1997, 249, 1-9. (c) Zhu,
X.-Q.; Zhang, J.-Y.; Mei, L.-R.; Cheng, J.-P. Org. Lett. 2006, 8, 3065-
(20) Alder, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
II
(21) The combination of Ph3CSNO and T(G)PPCo may occurred before
the NO initial transfer, while the RSNO-Co porphyrin coordinated
product wasn’t obtained.
3067.
Inorganic Chemistry, Vol. 46, No. 2, 2007 593

Zhu et al.
Figure 1. Isothermal titration calorimetry (ITC) for the reaction heat of
II
Ph3CSNO with T(H)PPCo in benzonitrile at 298 K. Titration was conducted
II
by adding 10 µL of T(H)PPCo (1.8 mM) every 300 s into the benzonitrile
Figure 2. UV-vis spectra obtained from the reaction of Ph CSNO ([Ph -
3
3
-
4
II
II
containing Ph3CSNO (∼1.2 mM).
CSNO] ) 5.63 × 10 M) with (m-NO )Co TPP ([(m-NO )Co TPP] )
2
2
-5
6
.33 × 10 M) in benzonitrile at 25 °C at time intervals of 125, 375, 625,
from the product analyses via the reaction of various molar
CSNO and T(G)PPCo . The results suggest that
3
mol of T(G)PPCo is required to consume 1 mol of Ph -
CSNO thoroughly to form 1 mol of T(G)PPCo NO and 0.5
mol of of trityldisulfide (Figure S1 in Supporting Informa-
tion). The reaction heats were determined with titration
calorimetry (Figure 1), and the redox potentials of the relative
species were measured using the CV method.19a Detailed
875, 1250, and 1875 ms (going down). The inset shows the (a) decay of
II
II
the kinetic trace for T(m-NO2)PPCo monitored at 420 nm and the (b)
growth of the kinetic trace for T(m-NO2)PPCo -NO monitored at 440 nm.
ratios of Ph
1
3
II
II
II
Table 2. Second-Order Rate Constants, k2, for the Reactions of
II
Ph3CSNO with T(G)PPCo in Benzonitrile at Different Temperatures
between 298 and 318 Ka
k2 (×10- M-1 s-1
3
)
G
298 K
303 K
308 K
313 K
318 K
experimental results are summarized in Table 1.
p-OCH3
p-CH3
p-i-pr
m-CH3
p-H
m-OCH3
p-Cl
p-Br
m-Cl
m-Br
m-NO2
p-NO2
45.80
40.90
34.57
32.84
30.88
24.98
19.18
18.78
16.30
15.72
7.74
61.95
53.18
46.68
43.44
41.40
32.20
25.12
24.85
22.48
21.34
10.57
10.36
77.05
67.27
60.83
56.14
53.74
42.30
35.10
34.42
28.58
28.18
14.24
14.20
100.45
89.21
77.58
74.80
69.34
56.52
46.76
45.41
39.12
37.58
19.47
19.58
125.10
114.25
96.03
91.52
84.00
70.28
58.14
57.25
48.58
48.68
26.45
25.96
7.55
a
Standard deviation of <5%.
q
and activation entropy (∆S ) for the reaction of Ph
3
CSNO
with T(G)PPCo in benzonitrile are summarized in Table 3;
they were derived from Arrhenius plots of ln k and Eyring
plots of ln(k /T) versus the reciprocal of the absolute
II
2
2
temperature (1/T), respectively (Figures S8-S13 in Sup-
porting Information). The heterolytic S-NO bond dissocia-
3
tion energy of Ph CSNO in benzonitrile [∆Hhet(S-NO)] was
obtained from the reaction heat of the triphenylmethanethiol
anion (Ph CS ) with NO cation (NO ClO ) in dry
3 4
The kinetics of reaction 1 were monitored at different
wavelengths (from 415 to 420 nm) depending on the
substituents of T(G)PPCo using stopped-flow spectrometry
-
+
+
-
anaerobic benzonitrile solution (see experimental details in
II
22-23
Supporting Information); the result is given in Table 4
.
3
under pseudo-first-order conditions with Ph CSNO in more
The homolytic S-NO bond dissociation energy of Ph
3
CSNO
∆Hhomo(S-NO)], as well as the heterolytic and homolytic
II
than 15-fold excess (Figure 2). The decays of T(G)PPCo
absorbance followed single exponentials from which pseudo-
first-order rate constants were derived. The second-order rate
[
(
-•
S-NO) bond dissociation energies for the radical anion
constants, k
2
, at different temperatures between 298 and 318
3 2 8
(22) The S-S BDE of (Ph CS) (60.4 kcal/mol) is smaller than that of S
(
63.6 kcal/mol) by ∼3.2 kcal/mol, the reason could be the steric
K are given in Table 2, which were derived from the line
slope of the plots of the pseudo-first-order rate constants
versus the concentrations of Ph
porting Information). The Arrhenius activation energy (∆E
and Eyring activation parameters: activation enthalpy (∆H )
hindrance of the former being larger than that of the latter. (cf., Brown,
T. L.; LeMay, H. E. Jr.; Bursten, B. E. Chemistry the Central Science;
Pearson Education North Asia Limited and China Machine Press,
Prentice Hall Inc.: Shanghai, China, 2002; p 289).
23) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-
294.
3
CSNO (Figure S7 in Sup-
a
q
)
(
594 Inorganic Chemistry, Vol. 46, No. 2, 2007

NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin
Table 3. Activation Parameters for the Reactions of Ph3CSNO with
Scheme 1. Possible Pathways of NO Transfer from Ph3CSNO to
II
T(G)PPCoII
T(G)PPCo in Benzonitrile
G
∆Ea a
∆Hqb
∆Sqc
-T∆Sqd
∆Gqe
p-OCH3
p-CH3
p-i-pr
m-CH3
p-H
m-OCH3
p-Cl
p-Br
m-Cl
m-Br
m-NO2
p-NO2
9.4
9.7
9.6
9.8
9.5
8.8
9.1
9.0
9.2
8.9
-7.75
-7.05
-7.55
-7.16
-8.20
-7.26
-5.13
-5.26
-6.71
-5.69
-4.08
-3.63
2.3
2.1
2.3
2.1
2.4
2.2
1.5
1.6
2.0
1.7
1.2
1.1
11.1
11.2
11.3
11.3
11.3
11.5
11.6
11.7
11.7
11.7
12.2
12.2
9.9
9.3
10.7
10.7
10.3
10.7
11.6
11.7
10.1
10.1
9.7
10.0
11.0
11.1
a
From the slope of the Arrhenius plots; the units are kcal/mol. b From
a
∆
H(a) ) ∆Hhet(S-NO)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
c
the slope of the Eyring plots; the units are kcal/mol. From the intercept
of the Eyring plots; the units are cal mol
and T ) 298 K. Obtained from the equation ∆G ) ∆H - T∆S ; the
units are kcal/mol.
-
1
-1 d
K . The units are kcal/mol
e
q
q
q
b
c
d
e
f
∆H(b) ) ∆Hhet(S-NO) - ∆Hhet(CoIII-NO)
II
∆G(c) ) -F[E°red(Ph
3
CSNO) - E1/2(TPPCo )]
Table 4. Enthalpy Change of the Reaction of Ph3CS- with the NO+
Cation, Dissociation Energies of the Heterolytic and Homolytic S-NO
Bond of Ph3CSNO and Its Radical Anion, Dissociation Energy of the
Homolytic S-S Bond of Ph3CS-SCPh3, and Redox Potentials of the
Relative Species in Benzonitrile
II
∆
H(d) ) ∆Hhomo(S-NO) - ∆Hhomo(Co -NO)
∆H(e) ) ∆Hhomo(S-NO)
III
a
52.1d
∆H(f) ) -∆Hhet(Co -NO)
∆Hr
-52.1
∆Hhet(Ph3CS-NO)
∆Hhomo(Ph3CS-NO)
+
/0
b
e
E1/2(NO
)
0.8265
23.4
E1/2(NO0/-)
-0.287
b
c
∆Hhet(S-NO)
-‚
3.4
7.4
f
g
h
i
∆H(g) ) ∆Hhomo(S-NO-•) - ∆Hhomo(Co -NO)
III
0
/-
-‚
g
E°red(Ph3CSNO
)
-1.156
∆Hhomo(S-NO)
E°ox(Ph3CSNO+/0)
1.704
c
∆Hhomo(S-S)
60.4
h,22
∆
H(h) ) ∆Hhet(S-NO-•) - ∆Hhomo(Co -NO) - F[E1/2(TPPCo ) - E1/2(NO0/-)] (9)
II
II
0
/-
c
E°ox(Ph3CS
)
-0.418
a
Hr is the enthalpy change of the reaction of NO with anion Ph3CS-,
+
∆
II
∆
H(i) ) -∆Hhomo(Co -NO)
(11)
(11)
(12)
+
-
derived from the reaction heat of NO with Ph3CS in benzonitrile by
switching the sign of the heat value, which was determined by using
isothermal titration calorimettry (ITC); the reproducibility was within 1.0
kcal/mol, and the units are kcal/mol. Measured at 298 K in V vs Fc by
CV. Reproducible to 10 mV or better. The standard redox potemtials were
j
G(j) ) -F[E1/2(TPPCo NO) - E°ox(Ph
3
CS-)]
II
∆
b
c
k
3 3
∆H(k) ) -1/2 ∆Hhomo(Ph CS-SCPh )
derived from the corresponding irreversible redox potentials calibrated by
2
3
using Wayner and Parker’s method, respectively; the units are V vs Fc.
the formation of dimer trityldisulfide (Ph
3
CS)
2
, which
d
+
-
Derived from the enthalpy change of the reaction of NO with Ph3CS
e
releases a great amount of heat (∼60.4 kcal/mol). Because
of the importance of NO transfer from S-nitrosothiols
to metal center complexes in vivo, it should be necessary to
in benzonitrile; the units are kcal/mol. Obtained from the equation S3;
f
the units are kcal/mol. Obtained from the equation S4; the uncertainty is
g
estimated to be e1 kcal/mol, and the units are kcal/mol. Obtained from
equation S5; the uncertainty is estimated to be e1 kcal/mol, and the units
h
3
know the pathway of the NO transfer from Ph CSNO to
are kcal/mol. Derived from the reaction heat of Ph3CSNO with T(H)P-
II
II
PCo , according to the equation ∆Hhomo(S-S) ) 2∆Hhomo(S-NO) -
T(G)PPCo , since this reaction would be a good mimic
II
2
∆Hhomo(Co -NO) - 2∆Hrxn; the uncertainty is estimated to be e2 kcal/
3,17
system of NO transfer in vivo. According to the equation
mol, and the units are kcal/mol.
of reaction 1 and the chemical properties of S-nitrosothiols
and tetraphenylporphinatocobalt(II), the possible pathways
3
of Ph CSNO, was also estimated according to the equations
II
of NO transfer from Ph
3
CSNO to T(G)PPCo can be
S3-S5, which were derived from appropriate thermodynamic
cycles (Scheme S1 in Supporting Information). The results
and some redox potentials of relative species are also
summarized in Table 4.
proposed as shown in Scheme 1. From Scheme 1, it is clear
that there are at least six possible pathways involved in the
NO transfer. Since the ability of S-nitrosothiols to act as NO,
+
-
NO , and NO donors under physiological conditions may
be responsible for certain biological activites,24 the six
proposed pathways shown in Scheme 1 would all be
Discussion
Mechanism for the Reactions of Ph
3
CSNO with T(G)-
PPCo in Benzonitrile. From eq 1, it is evident that the
nitrosylation of cobalt in T(G)PPCoII by Ph
CSNO in
benzonitrile solution was completed by NO transfer from
2
5
possible. But, which one is the actual pathway of the NO
II
II
3
transfer from Ph CSNO to T(G)PPCo ? To thoroughly
3
elucidate the most likely mechanisms of the NO transfer and
to provide the information necessary to postulate the intrinsi-
II
Ph
3
CSNO to the cobalt atom in T(G)PPCo . According to
the reaction heats listed in Table 1, the reactions of Ph
3
-
(
24) (a) Zhu, X.-Q.; Zhang, J.-Y,; Cheng, J.-P. Org. Lett. 2006, 8 (14),
II
CSNO with T(G)PPCo are all highly exothermic (27.6-
1.6 kcal/mol), which indicates that the NO transfer from
3
065-3067. (b) Stamler, J. S.; Singel, D. J.; Loscalzo, J. Science 1992,
3
Ph
258, 1898-1902 and references therein.
(25) (a) Arnelle, D. R.; Stamler, J. S. Arch. Biochem. Biophys. 1995, 318,
II
3
CSNO to T(G)PPCo in benzonitrile should be easy in
279. (b) Wang, K.; Zhang, W.; Xian, M.; Hou, Y.-C.; Che, X.-C.;
the view of the thermodynamics. The main reason could be
Cheng, J.-P.; Wang, P. G. Curr. Med. Chem. 2000, 7, 821.
Inorganic Chemistry, Vol. 46, No. 2, 2007 595

Zhu et al.
Table 5. Energetics of Each Mechanistic Step for the Reactions of Ph3CSNO with T(G)PPCoII in Benzonitrile Shown in Scheme 1 (kcal/mol)a
H (or 4G)
fg
4
G
ab
bc
cd
de
ef
gh
hi
ij
jk
kl
p-OCH3
p-CH3
p-i-pr
m-CH3
p-H
m-OCH3
p-Cl
p-Br
m-Cl
m-Br
m-NO2
p-NO2
52.1
52.1
52.1
52.1
52.1
52.1
52.1
52.1
52.1
52.1
52.1
52.1
28.9
30.2
30.2
31.3
31.8
32.0
33.6
33.2
34.0
34.6
37.4
38.0
40.1
41.0
40.7
41.6
42.2
42.4
43.5
43.6
44.5
44.5
46.1
46.8
-1.2
-0.7
-0.8
-0.1
0.0
0.1
0.6
0.1
0.0
23.4
23.4
23.4
23.4
23.4
23.4
23.4
23.4
23.4
23.4
23.4
23.4
-23.2
-21.9
-21.9
-20.8
-20.3
-20.1
-18.5
-18.9
-18.1
-17.5
-14.7
-14.1
-10.1
-9.8
-9.5
-9.2
-9.4
-9.4
-8.9
-9.3
-9.4
-8.9
-7.7
-7.8
-41.2
-41.6
-41.4
-41.6
-42.2
-42.3
-42.9
-43.4
-44.4
-43.9
-43.5
-44.4
-24.6
-24.1
-24.2
-23.5
-23.4
-23.3
-22.8
-23.3
-23.4
-22.8
-20.8
-21.1
-30.1
-30.9
-31.0
-31.5
-31.8
-32.0
-33.0
-33.1
-34.0
-34.1
-35.1
-35.7
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
-30.2
0.6
2.6
2.3
a
b -l
All values were derived in benzonitrile solution at 298 K in kcal/mol.
From eqs 2-12, respectively. The relative heterolytic and homolytic Co-NO
19a
II
III
bond dissociation energies of T(G)TPPCo NO and its cation T(G)TPPCo NO were obtained from the previous paper. The relative heterolytic and
homolytic S-NO bond dissociation energies of Ph3CSNO and its radical anion were obtained from the present work (Table 4 and Supporting
Information).
cal reaction mechanism, it is very useful and necessary to
Table 3, it is found that the activation free-energetic scales
II
make detailed thermodynamic analysis on each elementary
for the reactions of Ph
3
CSNO with T(G)TPPCo in ben-
to
II
step for the NO transfer from Ph
3
CSNO to T(G)PPCo NO
zonitrile range from 11.1 kcal/mol when G is p-OCH
3
in benzonitrile. On the basis of Scheme 1, eleven equations
12.2 kcal/mol when G is p-NO . A comparison of the
determined activation free energies of the reactions with the
2
-
1
(
2-12) (where the Faraday constant, F, ) 23.06 kcal mol
-
1
26
V ) were developed, from which the change of the
standard state enthalpy or free energy for each elementary
step shown in Scheme 1 can be derived according to the
heterolytic and homolytic S-NO, (S-NO) , Co -NO, and
Co -NO bond-dissociation energies of the relative NO-
corresponding standard-state free-energy changes in the five
likely initial reactions (a, b, c, d, and e) clearly shows that
the activation free energies (11.1-12.2 kcal/mol) are much
smaller than the corresponding standard-state free energy of
the five initial reactions a (52.1-52.1 kcal/mol), b (28.9-
38.0 kcal/mol), c (40.1-46.8 kcal/mol), and e (23.4-23.4
kcal/mol) but are larger than the corresponding standard state
free energy change of the initial reaction d (-1.2-2.3 kcal/
mol). On the basis of a general reaction law which states
that the activation free-energy change is always larger than
or at least equal to the corresponding standard-state free-
energy change for any elemental reaction, it is evident that
the initial reactions a, b, c, and e should be all ruled out as
3
-•
II
III
carried compounds and the redox potentials of the relative
species, respectively. Detailed thermodynamic values are
summarized in Table 5.
The most striking feature of the data in Table 5 is that the
energy changes of the initial steps (a, b, c, d, and e) in the
six possible pathways of the NO transfer are all larger than
that of the following steps (f, g, h, i, j, and k), which indicates
that the initial step of the NO transfer is more difficult in
thermodynamics, regardless of the actual pathway of the NO
transfer. By comparison of the energy changes of the six
initial steps (see columns a, b, c, d, and e in Table 5), it is
clear that the most favorable initial step of the NO transfer
is step d because, among the six initial steps, only step d
took place with a decrease or slight increase of the state free
energy (-1.2 to 2.3 kcal/mol), which indicates that the
pathway of d-k should be most likely mechanism for
reaction 1. Since the free-energy change of step k (-30.2
kcal/mol) is much more negative than that of step d (-1.2
to 2.3 kcal/mol), it is conceivable that the step d could be
the initial reaction in the reactions of Ph CSNO with T(G)-
II
PPCo ; the only remaining reaction, d, is suitable for the
reaction law, which means that the mechanism of the
II
reactions of Ph CSNO with T(G)PPCo is unambiguously
3
the d-k pathway. Since the state energy change of the initial
step d is close to zero, it is easy to conceive that NO transfer
II
II
from Ph CSNO to T(G)PPCo to form T(G)PPCo NO and
3
•
radical Ph CS could be reversible. In the reaction of Ph -
CSNO with T(G)PPCo , the main reason for the rapidly
complete transfer of NO from Ph
be that the formed (triphenylmethyl)sulfanyl radical Ph
3
3
II
II
3
CSNO to T(G)PPCo could
CS•
3
the rate-determining step. To further verify the mechanism
in the initial step can rapidly dimerize to generate the
disulfide and this releases a great amount of heat, which
makes the reaction equilibrium to move to the side of
products. The concerned NO transfer mechanism present here
would be similar to the direct NO transfer form S-nitrosothi-
ols to iron center complex reported by Butler and co-
II
of the reactions of Ph
3
CSNO with T(G)TPPCo (pathway
d-k), the kinetics of the reaction was determined. From
(
26) It should be pointed out herein that we used the term free-energy
change, ∆Get, to replace the enthalpy change, ∆Het, in eqs 4-6 for
the electron-transfer processes. This treatment is valid because the
entropies associated with electron transfer are negligible; this has been
verified by Arnett’s and Bordwell’s work. See the following refer-
ences: (a) Arnett, E. M.; Amarnath, K.; Harvey, N. G.; Cheng, J.-P.
Science 1990, 247, 423. (b) Arnett, E. M.; Amarnath, K.; Harvey, N.
G.; Cheng, J.-P. J. Am. Chem. Soc. 1990, 112, 344-355. (c) Bordwell,
F. G.; Cheng, J.-P.; Harrelson, J. A., Jr. J. Am. Chem. Soc. 1988, 110,
27
workers, suggesting that the NO transfer from the S-nitro-
sothiols to metal center complex act in a direct NO transfer
way.
1
229. (d) Bordwell, F. G.; Bausch, M. J. J. Am. Chem. Soc. 1986,
(27) Bulter, A. R.; Elkins-Daukes, S.; Parkin, D.; Williams, D. L. H. Chem.
Commun. 2001, 1732.
108, 1979.
596 Inorganic Chemistry, Vol. 46, No. 2, 2007

NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin
According to the values of the second-order rate constants
in Table 2, it is clear that the range of the rate constants of
II
NO transfer from Ph
1
3
CSNO to T(G)PPCo is from 0.76 ×
4
4
-1 -1
0 to 4.58 × 10 M s , which is much smaller than the
encounter-controlled kinetic criterion, indicating the NO
transfer should be reaction-controlled, which is supported
*
by the measured the activation free-energy change (∆G )
11.1-12.2 kcal/mol). From an examination of activation
enthalpy and activation entropy of the NO transfer, it is clear
q
that the ∆G is mainly derived from the contribution of
q
activation enthalpy, ∆H , because the activation entropy
change of the NO transfer is very small (-3.63 to -7.75
-
1
-1
cal mol K ). One key reason for the small activation
entropy could be that the length of S-NO bond is long
2
8
(
1.793 Å), much longer than that of N-NO bond about
2
9
Figure 3. Correlations of log k (at 298 K) vs Hammett parameter 4σ for
by 0.4 Å, which results in a larger flexibility of the related
2
II
the reaction of Ph3CSNO with T(G)PPCo in benzonitrile.
groups bound by S-NO bond in the transition state. Since
Ph CSNO has larger steric hindrance, in general, than the
3
other familiar small molecule S-nitrosothiols, it is possible
that for the NO transfers from the general smaller molecule
S-nitrosothiols to metal porphyrins, the activation entropies
all could be very small, the kinetics of the NO transfer should
be mainly controlled by activation enthalpy. According to
the definition of enthalpy, it is understandable that the
magnitude of activation enthalpy in the NO transfer
II
from Ph
3
CSNO to T(G)PPCo is dependent on the breaking
of the old S-NO bond and the formation of the new Co-
NO bond in the transition state. Generally, the weaker the
II
S-NO bond is or the stronger the Co -NO bond is, the
faster the rate of the NO transfer, which can be well
II
supported by the plot of the log k
2
against the Co -NO
II
BDEs of T(G)PPCo NO (Figure S17 in Supporting Informa-
tion). From this case, a very important prediction can be
made that, in the living body, the nitrosylation of metal
porphyrin by free NO should be much faster than that by
S-nitrosothiols, since for the former reaction, no S-NO bond
needs to be broken. In fact, this prediction has been supported
q
Figure 4. Hammett plots of the ∆H for the reaction of Ph3CSNO with
II
II
T(G)PPCo (O), ∆Hhomo(Co -NO) (9), and ∆GeT for the electron transfer
from T(G)PPCoII to NO+ (() vs 4σ.
II
31
and T(G)PPCo also is a neutral radical, it is easy to believe
that the spin-spin coupling affinity could be the dominant
II
driving force pushing NO from Ph
3
CSNO to T(G)PPCo . Is
2
7,30
by recent paper.
that true? To elucidate the nature of the driving force, the
spin-delocalization effect and polar effect of the substituents
G in T(G)PPCo on the reaction rate constants were
Nature of the Driving Force of NO Transfer from
II
Ph
3
CSNO to T(G)PPCo . According to the examination
CSNO with
II
above on the mechanism of the reaction of Ph
T(G)PPCo , it is evident that the reaction of Ph3CSNO with
T(G)PPCo was initiated by NO transfer. Since NO is a
3
examined. The results show that no good linear relationship
II
with the spin-delocalization parameter, 4σ•jj, was
32
of log k
2
II
observed for the porphyrin substituents (Figure S14 in
neutral radical and the oxidation number of cobalt atom from
Supporting Information) meaning that the rate of NO transfer
II
II
T(G)PPCo to T(G)PPCo NO has no change, an interesting
II
from Ph
3
CSNO to T(G)PPCo is insensitive to the spin-
question can be produced about what force made NO transfer
delocalization effect of the substituents G, that is, the spin-
spin coupling effect could not be the main driving force of
the NO transfer. For the plot of log k with Hammett polar
2
parameter 4σ (Figure 3), an excellent straight line with line
slope of -0.19 was observed, indicating that the electrostatic
II
from Ph
3
CSNO to T(G)PPCo in the rate-determining step.
•
Is it spin-spin coupling affinity like the force making H
•
and H form H
2
or electrostatic charge attraction like the force
+
-
making H and H form H
2
? Since NO is a neutral radical
dipole charge attraction would be the main driving force to
II
(
28) Arulsamy, N.; Bohle, D. S.; Butt, J. A.; Irvine, G. J.; Jordan, P. A.;
make NO transfer from Ph
3
CSNO to T(G)PPCo . In fact,
Sagan, E. J. Am. Chem. Soc. 1999, 121, 7115-7123.
from the good binary-correlation plot of log k
2
against 0.76σ
(
29) (a) Ohwada, T.; Miura, M.; Tanaka, H.; Sakamoto, S.; Yamaguchi,
K.; Ikeda, H.; Inagaki, S. J. Am. Chem. Soc. 2001, 123, 10164. (b)
Bartberger, M. D.; Houk, K. N.; Powell, S. C.; Mannion, J. D.; Lo,
K. Y.; Stamler, J. S.; Toone, E. J. J. Am. Chem. Soc. 2000, 122, 5889.
30) Franke, A.; Stochel, G.; Suzuki, N.; Higuchi, T.; Okuzono, K.; Eldik,
R. J. Am. Chem. Soc. 2005, 127, 5360-5375.
(31) Kamachi, M.; Shibasaka, M.; Kajiwara, A.; Mori, W.; Kishita, M.
Bull. Chem. Soc. Jpn. 1989, 62, 2465-2473.
(32) (a) Jiang, X.-K.; Ji, G.-Z. J. Org. Chem. 1992, 57, 6051. (b) Jiang,
X.-K. Acc. Chem. Res. 1997, 30, 283.
(
Inorganic Chemistry, Vol. 46, No. 2, 2007 597

Zhu et al.
Scheme 2. Charge-Distribution Map on the Rate-Determining Transition State for the Reaction of T(G)PPCoII with Ph3CSNO in Benzonitrilea
a
The porphyrin ring is shown as a square.
•
+
0.07σ jj (Figure S15 in Supporting Information), it is clear
to the nature of the Hammett substituent effect, it is possible
that the sign of the line slope values reflects increase or
decrease of the effective charge on the cobalt atom and that
the magnitude of the line slope values is a measure of the
effective charge change at the cobalt atom during the
corresponding reaction processes. Since the positive line
slope of 1.573 ( 0.047 for the plot of ∆GeT versus 4σ is
that the effect of spin-spin interaction on the driving force
is very small. To further support the nature of the driving
force, the dependence of the reaction rate on the oxidation
potentials of the NO acceptors [T(G)PPCo ] was examined
Figure S16 in Supporting Information), the results exhibit
an excellent straight line with a slope of -2.75 (r ) 0.98),
which means that the driving force of the NO transfer should
be significantly dependent on the electron-donating ability
Lewis basicity) of T(G)PPCo . Since the value of the line
slope is negative (-2.75), it is possible that T(G)PPCo is a
Lewis bases, NO in Ph CSNO is a Lewis acid, and the
combination of them in reaction d was essentially driven by
the interaction of Lewis acid and Lewis bases. In fact, from
Table 1, it is immediately evident that the oxidation potential
of T(G)PPCo NO is larger than that of the corresponding
T(G)PPCo by 0.259-0.306 V, indicating that the electro-
II
(
19a
equivalent to the oxidation number increase of the cobalt
II
III
atom from T(G)PPCo to T(G)PPCo , it is clear that the
II
(
q
positive line slope of 0.550 ( 0.057 for ∆H would also be
II
equivalent to the oxidation number increase of the cobalt
3
II
atom from T(G)PPCo to the transition state, but the negative
II
line slope of -0.792 ( 0.093 for ∆Hhomo(Co -NO) would
be equivalent to the oxidation number decrease of the cobalt
II
II
II
atom from T(G)PPCo NO to T(G)PPCo . If we define the
II
II
oxidation numbers of the cobalt atom in T(G)PPCo and
T(G)PPCo as +2 and +3, respectively, it can be deduced
III
II
chemical cobalt center in T(G)PPCo NO is a poorer electron
donor than that in the corresponding T(G)PPCo .
II
that the effective charge on the cobalt atom would be 2.350
Evaluation of the Effective Charge Distribution on the
( 0.057 and 2.503 ( 0.047 units in the transition state and
in T(G)PPCo NO, respectively. The details of the effective
II
Co Atom in NO Transfer Process. To elucidate the Lewis
II
acid nature of NO group in T(G)PPCo NO and examine the
charge change on the cobalt atom during NO transfer from
II
charge transfer during the NO transfer from Ph
3
CSNO to
Ph CSNO to T(G)PPCo is shown in Scheme 2. From
3
II
T(G)PPCo , the effective charge distribution on the cobalt
Scheme 2, it is clear that although the NO went from Ph -
3
II
atom in the reactant T(G)PPCo , the transition-state, and the
II
CSNO to T(G)PPCo without heterolytic S-NO bond
II
product T(G)PPCo NO needs to be estimated. Since Ham-
3
dissociation in Ph CSNO, the effective positive charge on
mett linear free-energy relationship analysis can provide a
very efficient access to estimate the effective charge distribu-
the cobalt atom always kept increasing from the reactant
II
T(G)PPCo (+2.000) to the transition state (+2.350) and
tion,3
3,34
the plots of the activation enthalpy (∆H ) for the
q
II
finally to the product T(G)PPCo NO (+2.503). The con-
II
II
reaction of Ph
3
II
CSNO with T(G)PPCo , the Co -NO BDEs
II
certed NO transfer from Ph
3
CSNO to T(G)PPCo with the
in T(G)PPCo NO, and the free-energy changes (∆GeT) for
•
release of Ph
3
CS radical was actually performed by the initial
negative charge (-0.350) transfer from T(G)PPCo to Ph
II
+
electron transfer from T(G)PPCo to NO against the
Hammett substituent constant 4σ were made, and three
excellent straight lines with slopes of 0.550 ( 0.057, -0.792
II
3
-
CSNO to form the transition state, followed by homolytic
0.093, and 1.573 ( 0.04735 were obtained, respectively
S-NO bond dissociation of Ph
charge (-0.153) transfer from T(G)PPCo to the NO group
to form the final product T(G)PPCo NO. Since the negative
3
CSNO with a further negative
(
(
II
Figure 4), which means that Hammett linear free-energy
relationship holds in the three reaction processes. According
II
charge on nitrosyl group are increased as the increase of the
positive charge on the cobalt atom from the reactant T(G)-
(
33) (a) Page M.; Williams A. Organic and Bio-organic Mechanism;
Addison Wesley Longman: Boston, 1997; Chapter 3, pp. 52-79. (b)
Williams A. Acc. Chem. Res. 1984, 17, 425. (c) Williams A. Acc.
Chem. Res. 1989, 22, 387. (d) Williams A. AdV. Phys. Org. Chem.
II
PPCo (+2.000) to the transition state (+2.350) finally to
the product, it is evident that the nature of the driving force
of the NO transfer in reaction step d is indeed electrostatic
attraction of positive charge and negative charge. In addition,
if one compares the charge distribution in the transition state
1991, 27, 1. (e) Williams A. J. Am. Chem. Soc. 1985, 107, 6335.
(
34) (a) Zhu, X.-Q.; Cao, L.; Liu, Y.; Yang, Y.; Lu, J.-Y.; Wang, J.-S.;
Cheng, J.-P. Chem.sEur. J. 2003, 9, 3937. (b) Zhu, X.-Q.; Li, H.-R.;
Li, Q.; Ai, T.; Lu, J.-Y.; Yang, Y.; Cheng, J.-P. Chem.sEur. J. 2003,
9, 871.
II
(
35) The line slope of the plot of ∆GeT against 4σ is 1.573 ( 0.047 rather
and in the product T(G)PPCo NO, one finds that the charge
1
8a
than the 1.618 ( 0.07 value in the previous literature. The value of
.618 in the previous paper is wrong because of our careless mistake.
II
change on the cobalt atom from the reactants T(G)PPCo to
1
Herein, a special apology is given to the readers.
the transition state is 0.350 unit, which is larger than the
598 Inorganic Chemistry, Vol. 46, No. 2, 2007

NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin
half of the change from the reactants to the product T(G)-
were treated according to standard procedure, and the benzonitrile
solution was carefully degassed before each experiment.
II
PPCo NO (0.503/2 ) 0.252 unit), suggesting that the
II
Spectrophotometric Kinetic Measurements. The kinetic de-
transition state of the nitrosylation of T(G)PPCo by Ph
3
-
II
termination of the reactions of T(G)PPCo with Ph
3
CSNO were
CSNO would be productlike (Scheme 2), similar to the case
for the nitrosations of alanine, glycine, valine, and anilines
performed on an Applied Photophysics SX.18MV-R stopped-flow
spectrophotometer under anaerobic operation, which was thermo-
3
6
by nitrosating agents such as ONCl, ONBr, ONSCN, etc.
stated ((0.1 °C) by circulating water, by rapid mixing of the T(G)-
II
PPCo with an excess of Ph
3
CSNO(15 ∼70 fold) using single
Conclusions
mixing mode. The kinetic traces were recorded on an Acorn
computer and analyzed by Pro-K Global analysis/simulation
software or translated to PC for further analysis. The pseudo-first-
order rate constants (kobs) can be calculated by fitting the data
In this work, the thermodynamics and kinetics of NO
II
3
transfer from Ph CSNO to T(G)PPCo in benzonitrile were
investigated, and the reaction mechanism of the NO transfer
was elucidated according to the related thermodynamic and
kinetic parameters, for which the determination of the
heterolytic and homolytic S-NO bond dissociation energies
with the equation of abs ) P
1
2
exp(-kobst) + P , which was
integrated in the Pro-K Global analysis/simulation software or
Origin Software (Figure S6 in the Supporting Information). The
application of the Guggenheim method gives the same value of
the rate constants kobs. The second-order-rate constants were derived
3
of Ph CSNO and its radical anion in benzonitrile were also
made. The following conclusions can be made. (1) The
3
from the slopes of the plots of versus the concentration of Ph -
II
II
CSNO (Figure S7 in the Supporting Information). In each case,
it was confirmed that the rate constants derived from five to
seven independent measurements agreed within an experimental
error of (5%.
reaction of Ph
NO and the dimer of (Ph
3
CSNO with T(G)PPCo to yield T(G)PPCo -
CS) is highly exothermic, which
3
2
indicates that the reactions are much favorable in thermo-
dynamics; the main reason is that the formed (triphenylm-
Isothermal Titration Calorimeter Experiments. Isothermal
titration calorimeter experiments were performed in benzonitrile
solution at 298 K on a CSC 4200 isothermal titration calorimeter.
Prior to use, the instrument was calibrated against an internal heat
pulse. Data points were collected every 2 s. For the reaction of
•
ethyl)sulfanyl radical Ph
3
CS can dimerize to release large
3
amounts of heat. (2) NO transfer from Ph CSNO to T(G)-
II
PPCo is a concerted process of the homolytic S-NO bond
dissociation and the formation of the Co-NO bond; the rate
of the NO transfer was controlled by activation enthalpy,
which is obviously different from the previous thought that
the NO transfer from S-nitrosothiol to metal porphyrin could
-
+
Ph CS with NO , the reaction heat was determined following nine
3
automatic injections from a 250 µL injection syringe containing
+
+
-
0.85 mM NO (NO ClO
4
) into the reaction cell (1.00 mL)
-
+
24
(containing 0.47 mM Ph CS ). Injection volumes (10 µL) were
be initiated by NO transfer. (3) In the reaction of Ph
3
-
3
II
delivered at 0.5 s intervals with 400 s between each set of injections.
CSNO with T(G)PPCo , NO in the S-nitrosothiol is a typical
Lewis acid and T(G)PPCo is a typical Lewis base; the main
II
The reaction heat was obtained by area integration of each peak
II
except the first one. For the reaction of Ph
3
CSNO with T(G)PPCo ,
driving force of the NO transfer is electrostatic dipole charge
attraction rather than a spin-spin coupling interaction. (4)
The effective charge distribution on the cobalt atom in the
porphyrin ring at the various stages, the reactant [T(G)-
the reaction heat was determined following eight automatic injec-
tions from a 250 µL injection syringe containing 1.37 mM T(G)-
II
PPCo into the reaction cell (1.00 mL) (containing 1.54 mM
3
Ph CSNO). Injection volumes (10 µL) were delivered at 0.5 s
II
II
PPCo ], the transition-state, and the product [T(G)PPCo -
NO], was estimated to show that the cobalt atom carries
intervals with 400 s between every two injections. The reaction
heat was obtained by area integration of each peak except the
first one.
relative effective positive charges of 2.000 in the reactant
II
[
T(G)PPCo ], 2.350 in the transition state, and 2.503 in the
Measurement of Redox Potentials. All electrochemical experi-
ments were carried out by CV (sweep rate, 100 mV/s) using a BAS-
100B electrochemical apparatus in dry benzonitrile solution under
II
product [T(G)PPCo NO], respectively, which indicates that
the concerted NO transfer from Ph
II
3
CSNO to T(G)PPCo was
1
9a
an argon atmosphere at 298 K as described previously. n-Bu
4
-
practically performed by the initial negative charge transfer
NPF (0.1 M) was employed as the supporting electrolyte. A
6
II
of -0.350 from T(G)PPCo to Ph
3
CSNO to form the
standard three-electrode cell consists of a glassy carbon disk as
working electrode, a platinum wire as counter electrode, and 0.1
M AgNO /Ag (in 0.1 M Bu NPF -PhCN) as the reference electrode.
3 4 6
transition state and an additional negative charge transfer
II
(
-0.153) from T(G)PPCo to NO.
All sample solutions were 1.5 mM. The ferrocenium/ferrocene redox
couple (Fc ) was taken as the internal standard. The reproducibility
Experimental Section
+/0
of the potentials was usually e5 mV for ionic species and e10
mV for neutral species.
II
Materials. Compounds T(G)PPCo (G ) p-OCH
Pr, m-CH , H, m-OCH , p-Cl, p-Br, m-Cl, m-Br, m-NO
CSNO37 and
were synthesized as previously described.
3
3
, p-CH
3
, p-i-
3
3
2
, p-NO )
2
19a,20
Acknowledgment. Financial support from the Ministry
of Science and Technology of China (Grant 2004CB719905),
the National Natural Science Foundation of China (Grants
Ph
were prepared according to the published procedures
and used as fresh reagents (see Supporting Information). All solvents
+
-38
NO ClO
4
2
0272027, 20332020, 20472038, and 20672060), Ministry
(
36) (a) da Silva, G.; Kennedy, E. M.; Dlugogorski, B. Z. J. Am. Chem.
Soc. 2005, 127, 3664-3665. (b) da Silva, G.; Kennedy, E. M.;
Dlugogorski, B. Z. Ind. Eng. Chem. Res. 2004, 43, 2296-2301.
37) Arulsamy, N.; Bohle, D. S.; Butt, J. A.; Irvine, G. J.; Jordan, P. A.;
Sagan, E. J. Am. Chem. Soc. 1999, 121, 7115-7123.
of Education of China (Grant 20020055004), and Natural
Science Fund of Tianjin (Grant 033804311) is gratefully
acknowledged.
(
(
Supporting Information Available: Experimental details,
estimation of the heterolytic and homolytic S-NO bond-dissociation
38) Markowitz, M. M.; Ricci, J. E.; Goldman, R. J.; Winternitz, P. F. J.
Am. Chem. Soc. 1957, 99, 3659.
Inorganic Chemistry, Vol. 46, No. 2, 2007 599

Zhu et al.
II
II
energies of Ph
for the reactants and products, the data fitting of kobs and the
derivation of k , the Arrhenius plot of ln k versus 1/T, as well as
the Eyring plot of ln (k /T) versus 1/T for the NO transfer reaction,
(at 298 K) versus (i) spin-delocalization parameter
3
CSNO, the corresponding IR and UV-vis spectra
(iv) Co -NO BDEs of T(G)PPCo NO for the reactions of T(G)-
II
PPCo with Ph CSNO. This material is available free of charge
via the Internet at http://pubs.acs.org.
3
2
2
2
plots of log k
2
•
JJ, (ii) σ and σ•JJ, (iii) oxidation potentials of T(G)PPCo , and
II
4
σ
IC061427V
600 Inorganic Chemistry, Vol. 46, No. 2, 2007