Establishment of heterolytic and homolytic Y-NO2 bond dissociation energy scales of nitro-containing compounds in acetonitrile: Chemical origin of NO2 release and capture

Article abstract of DOI:10.1021/jo702364a

(Chemical Equation Presented) The first heterolytic and homolytic N(O)-NO₂ bond dissociation energy scales of three types Y-nitro (Y = N, O) compounds and corresponding radical anions in acetonitrile were established by using titration calorimetry combined with relevant electrochemical data through proper thermodynamic cycles.

Full text of DOI:10.1021/jo702364a

not until very recently was it found to be associated with many
destructive processes in living bodies. A wide spectrum of
diseases has been suggested to be related to the exogenously
and endogenously generated NO2.3c,d Conversely, it is also
Establishment of Heterolytic and Homolytic
Y-NO2 Bond Dissociation Energy Scales of
Nitro-Containing Compounds in Acetonitrile:
Chemical Origin of NO2 Release and Capture
-
reported that NO2 plays a crucial role in many NO-mediated
processes such as hypoxic vasodilation and cytoprotection from
cardiac and liver ischemia-reperfusion injury and in gene
expression.4 Nitrite is also suggested to be a potential therapeutic
in the treatment of various diseases.5
Xin Li, Xiao-Qing Zhu, Fan Zhang, Xiao-Xiao Wang, and
Jin-Pei Cheng*
State Key Laboratory of Elemento-Organic Chemistry,
Department of Chemistry, Nankai UniVersity,
Tianjin 300071, China
-
In order to understand the mechanistic details of NO2 or NO2
in chemical or biological processes, the basic information
concerning quantitative energetic changes in NO2-related bond
cleavage in solution should be of great value because it provides
the necessary information on the thermodynamic driving forces
of NO2-carrier molecules to release, capture, or transfer an NO2
(or NO2-) moiety. However, to our knowledge, no systematic
work was reported on experimental determinations of Y-NO2
bond energies in any solvent system where the NO2-related
chemical and biological activities are likely to take place. The
definite and immediate need of the Y-NO2 bond energy data
bank to deepen our understanding of the board range of NO2-
related activities has stimulated us to extend our current research
on NO-related bond energetics6 to cover the equally important
Y-NO2 bond energies. In this work, we report the first series
of experimentally determined Y-NO2 (Y ) N, O) heterolytic
and homolytic bond energy scales for three representative
families of synthetic or biochemical significance (Scheme 1).
This kind of bond energy scales should also be useful for the
study of stability and shock sensitivity of the Y-NO2-type
energetic materials7 and for estimating the nitrating ability of
chengjp@nankai.edu.cn; chengjp@most.cn
ReceiVed NoVember 14, 2007
The first heterolytic and homolytic N(O)-NO2 bond dis-
sociation energy scales of three types Y-nitro (Y ) N, O)
compounds and corresponding radical anions in acetonitrile
were established by using titration calorimetry combined with
relevant electrochemical data through proper thermodynamic
cycles.
(3) (a) Kirsch, M.; Korth, H.-G.; Sustmann, R.; de Groot, H. Biol. Chem.
2002, 383, 389. (b) Pfeiffer, S.; Mayer, B.; Hemmens, B. Angew. Chem.
1999, 111, 1824; Angew. Chem., Int. Ed. 1999, 38, 1714. (c) Espey, M. G.;
Xavier, S.; Thomas, D. D.; Miranda, K. M.; Wink, D. A. Proc. Natl. Acad.
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Hunter, C. J.; Schecheter, A. N.; Gladwin, M. T. Blood Cells Mol. Dis.
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Rodriguez, J.; Feelisch, M. Nat. Chem. Biol. 2005, 1, 290. (d) Webb, A.;
Bond, R.; Mclean, P.; Uppal, R.; Benjamin, N.; Ahluwalia, A. Proc. Natl.
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Xian, M.; Lu¨, J. -M.; Cheng, J.-P. J. Org. Chem. 2000, 65, 6729. (c) Xian,
M.; Zhu, X.-Q.; Lu¨, J.-M.; Wen, Z.; Cheng, J.-P. Org. Lett. 2000, 2, 265.
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2007, 46(2), 592.
Because of the recent discoveries on the significant roles of
nitrogen oxides (NxOy) in many important natural processes,
the chemistry and biochemistry of NxOy have regained tremen-
dous research attention in the last two decades.1 Among various
kinds of NxOy, nitric oxide (NO) is certainly the best known
“star molecule” for its very broad range of vital roles in human
physiological processes.2 However, the importance of nitrogen
dioxide (NO2) has largely been shadowed or even overlooked
during the same period. Nevertheless, the significant functions
of NO2 and nitrite ion (NO2-) in mammalian biology have
slowly been accumulated,3 and the research on the functions of
NO2 and NO2- has started to draw increasing attention recently.
Nitrogen dioxide has long been known in chemistry as a
strong oxidant and nitrating reagent for organic synthesis, but
(1) Augusto, O.; Bonini, M. G.; Amanso, A. M.; Linares, E.; Santos, C.
C. X.; Menezes, S. L. D. Free Radical Biol. Med. 2002, 32(9), 841.
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1991, 43(2), 109. (b) Culotta, E.; Koshland, D. E. Science 1992, 258, 1862.
(c) Feldman, P. L.; Griffith, O. W.; Stuehr, D. J. Chem. Eng. News 1993,
20, 26. (d) Butler, A. R.; Williams, D. L. H. Chem. Soc. ReV. 1993, 22,
233. (e) Averill, B. A. Chem. ReV. 1996, 96, 2951. (f) Ignarro, L. J. Biochem.
Pharmacol. 1991, 41, 485. (g) Ignarro, L. J. Annu. ReV. Pharmacol. Toxicol.
1990, 30, 535. (h) Gnewuch, C. T.; Sosnovsky, G. Chem. ReV. 1997, 97,
829. (i) Murad, F. Angew. Chem., Int. Ed. 1999, 38, 1857. (k) Ignarro, L.
J. Angew. Chem., Int. Ed. 1999, 38, 1883.
(7) (a) Melius, C. F.; Bulusu, S. N. Chemistry and Physics of Energetic
Materials; Kluwer: Dordrecht, 1990. (b) Owens, F. THEOCHEM 1996,
11, 370. (c) Rice, B. M.; Sahu, S.; Owens, F. THEOCHEM 2002, 69, 583.
10.1021/jo702364a CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/29/2008
2428
J. Org. Chem. 2008, 73, 2428-2431

SCHEME 1. Y-Nitro Compounds
acetonitrile were similarly estimated from eqs 3 and 4,
respectively, using the thermodynamic cycle constructed from
the Y-NO2 bond heterolytic and homolytic scission reactions
of the neutral Y-nitro compounds combined with certain
electrode potentials (Scheme 3).11 The enthalpy changes of the
+
reactions of Y- with NO2 (∆Hrxn) and the standard redox
potentials12 of the relevant species are listed in the Supporting
Information (Table S2, Figures S1-S3). The heterolytic and
homolytic N(O)-NO2 bond dissociation energies and the
(N(O)-NO2)•- bond dissociation energies of the corresponding
radical anion in acetonitrile are presented in Table 1.
SCHEME 3
nitro-releasing compounds in synthetic chemistry.8 Because a
rapid cleavage of Y-NO2 bond (releasing a nitrite) upon one-
electron reduction of a nitro compound has been observed for
years,9 in order to gain information concerning the role of
electron transfer in activating the N(O)-NO2 bond rupture, the
heterolytic (defined as nitrite ion-releasing) and homolytic
(defined as nitro radical-releasing) [N(O)-NO2]•- bond dis-
sociation energies of the (Y-NO2)•- radical anions were also
determined.
The data in Table 1 demonstrate that the ∆Hhet(N-NO2) and
∆Hhomo(N-NO2) bond energies of N-nitrosulfonamides (1)
range from 51.4 to 57.8 kcal/mol and from 36.0 to 38.2 kcal/
mol, respectively, while the ∆Hhet(N-NO2) and ∆Hhomo(N-
NO2) bond energies of N-nitroacylamides (2) range from 60.5
to 69.0 kcal/mol and from 31.9 to 34.9 kcal/mol, respectively.
The corresponding ∆Hhet(O-NO2)’s and ∆Hhomo(O-NO2)’s of
O-nitro alcohols (3) range from 61.2 to 73.7 kcal/mol and from
30.1 to 39.6 kcal/mol, respectively. It is clear that the N(O)-
NO2 heterolytic bond dissociation energies of these three types
of nitro compounds in acetonitrile are all greater than the
corresponding homolytic bond dissociation energies, which
SCHEME 2
(10) Arnett has shown that the temperature dependence of the redox
processes for resonance-delocalized anions and for the reversible cation-
to-radical conversions were experimentally found to be negligible. Then
the corresponding entropy changes for redox processes should be insig-
nificant. In this work, we have also examined the temperature effect on the
oxidation potentials of the N- or O- anions of interest, and found that
variation of temperature caused only minimal changes in anion oxidative
potentials (see Table S1 in the Supporting Information), suggesting that
neglecting the entropy effect for anion electro-oxidation in our thermo-
chemical cycle would not produce remarkable error to the derived bond
energy data. For previous validation of similar treatment, see: (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. (c) Troughton, E. B.; Molter, K. E.; Arnett, E. M. J.
Am. Chem. Soc. 1984, 106, 6726.
(11) Parker have published a series of papers in the early 1980s where
they showed that the entropies of electrode processes do not depend much
on solvent but do depend on structural factors. See: (a) Svaan, M.; Parker,
V. D. Acta Chem. Scand., Ser. B: Org. Chem. Biochem. 1984, B38(9),
759. (b) Svaan, M.; Parker, V. D. Acta Chem. Scand., Ser. B: Org. Chem.
Biochem. 1984, B38(9), 751. (c) Svaan, M.; Parker, V. D. Acta Chem.
Scand., Ser. B: Org. Chem. Biochem. 1982, B36(6), 365. (d) Svaan, M.;
Parker, V. D. Acta Chem. Scand., Ser. B: Org. Chem. Biochem. 1982,
B36(6), 357. (e) Svaan, M.; Parker, V. D. Acta Chem. Scand., Ser. B: Org.
Chem. Biochem. 1982, B36(6), 351. (f) Svaan, M.; Parker, V. D. Acta Chem.
Scand., Ser. B: Org. Chem. Biochem. 1981, B35(8), 559.
The desired Y-NO2 bond dissociation energies were derived
from the thermodynamic cycle (Scheme 2), which is analogous
to that in our earlier studies on NO affinity,6 where the difference
between the heterolytic (∆Hhet) and homolytic dissociation
energy (∆Hhomo) of the Y-NO bond is equal to the enthalpy of
electron transfer.10 As shown in Scheme 2, the ∆Hhet of Y-NO2
should be numerically equal to the reaction enthalpy change
+
(∆Hrxn) of the nitranion Y- with NO2 simply by switching
the sign of ∆Hrxn(Scheme 2, eq 1). ∆Hhomo can be derived
indirectly from the corresponding heterolytic Y-NO2 bond
dissociation energy with a suitable redox potential through the
thermodynamic cycle showed in Scheme 2 (eq 2). The hetero-
lytic and homolytic (Y-NO2)•- bond dissociation energies in
(8) (a) Park, Y. D.; Kim, H. K.; Kim, J. J.; Cho, S. D.; Kim, S. K.;
Shiro, M.; Yoon, Y. J. J. Org. Chem. 2003, 68, 9113. (b) Pedro, R.; Montse,
A.; Jordi, G.; Jaume, V. J. Org. Chem. 1991, 56, 7038. (c) Ariza, X.; Farras,
J.; Serra, C.; Vilarrasa, J. J. Org. Chem. 1997, 62, 1547. (d) Suri, S. C.;
Chapman, R. D. Synthesis 1988, 743.
(12) In this study, reversible potentials corrected from irreversible
potentials (CV) by the method in ref 13 were used in the thermochemical
cycles.
(13) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287
and references therein cited.
(14) Lee, K. Y.; Amatore, C.; Kochi, J. K. J. Phys. Chem. 1991, 95,
1285.
(15) Frangione, M.; Port, J.; Baldiwala, M.; Judd, A.; Galley, J.; DeVega,
N.; Linna, K.; Caron, L.; Anderson, E.; Goodwin, J. A. Inorg. Chem. 1997,
36, 1904.
(9) (a) Hoffmann, A. K.; Hodgson, W. G.; Maricle, D. L.; Jura, W. H.
J. Am. Chem. Soc. 1964, 86, 631. (b) Bowyer, W. J.; Evans, D. H. J. Org.
Chem. 1988, 53, 5234. (c) Ruhl, J. C.; Evans, D. H.; Hapiot, P.; Neta, P.
J. Am. Chem. Soc. 1991, 113, 5188.
Y - NO2 9
e8 Y - NO2•- f Y• + NO•2-
J. Org. Chem, Vol. 73, No. 6, 2008 2429

TABLE 1. Y-NO2 Bond Dissociation Energy ∆Hhet’s and
∆Hhomo’s of Y-Nitro Compounds and ∆Hhet•-’s and ∆Hhomo•-’s of
Corresponding Radical Anions in Acetonitrile (kcal/mol)a
insensitivity of the remote substitiuents on affecting homolytic
BDEs as seen from the ∆Hhomo data is also in consistence with
the literature observations.17
b
c
•-d
•-e
compd
∆Hhet
∆Hhomo
∆Hhet
∆Hhomo
A closer look at the ∆Hhomo data further indicates that the
pattern of the remote substituents to influence homolysis
energies of N-nitro compounds (1, 2) differs from that for
O-nitro compounds (3). While in series 3 the remote substituent
effect shows a normal O-pattern (i.e., EDG, bond weakening;
EWG, bond strengthening) as generally observed for heteroa-
tom-centered radicals in the literature,18 in series 1 and 2, the
direction is just reversed; i.e., the EDGs are bond-strengthening,
whereas EWGs generally bond-weakening. The latter phenom-
enon, i.e., the so-called counter-O type substituent effect, is
rarely observed,18 but it can be rationalized on the basis of
differential solvation of the nitro-containing compounds.
1
G ) 4-MeO
4-Me
4-H
4-Cl
56.7
56.0
55.2
53.9
54.2
51.4
51.6
57.0
57.8
57.5
37.3
36.8
36.6
36.3
36.4
36.0
36.1
37.7
37.1
38.2
-20.9
-21.3
-21.4
-20.2
-20.2
-18.3
-18.5
-20.7
-21.0
-20.4
-13.2
-13.8
-14.6
-14.4
-14.1
-14.6
-14.8
-13.1
-12.0
-12.9
4-Br
4-NO2
3-NO2
R1 ) Et
R2 ) CH3
CH3CH2
2
G ) 4-MeO
4-Me
4-H
4-Cl
67.6
67.1
66.5
65.3
65.7
64.0
68.1
69.0
60.5
68.9
33.6
33.4
33.3
32.9
32.7
32.6
34.9
33.6
31.9
33.2
-22.1
-22.1
-21.9
-19.8
-19.6
-17.6
-19.9
-21.8
-22.3
-24.4
0.1
-0.1
-0.5
0.8
1.6
2.1
1.6
1.9
-5.5
-0.5
The activation of the Y-NO2 bond upon one-electron
reduction of the nitro compounds to facilitate a rapid release of
nitrite is believed to be associated to the numerous physiological
and pharmacological functions of the NO2- ion.19 In the present
work, the role of electron transfer in activating N(O)-NO2 bond
rupture for species 1-3 can be estimated by comparison of the
Y-NO2 bond energies with the corresponding (Y-NO2)-• bond
cleavage energies as determined using the appropriate thermo-
dynamic cycle (Scheme 3).
4-Br
4-NO2
R1 ) Et
Pr
R2 ) ClCH2
ClCH2CH2
3
G ) 4-MeO
4-Me
4-H
4-Cl
4-Br
4-CF3
65.6
65.1
64.5
63.6
63.5
62.3
61.2
73.7
30.1
30.3
30.4
30.8
31.1
31.4
39.6
36.5
-32.2
-31.7
-30.8
-30.1
-29.6
-26.3
-16.0
-27.1
-8.5
-8.7
-8.5
-9.1
-9.0
-7.1
-6.2
-1.7
As seen from Table 1, the ∆Hhet(N-NO2)•-’s and ∆Hhomo
-
(N-NO2)•-’s of N-nitrosulfonamides (1•-) range from -18.3
to -21.4 kcal/mol and from -12.0 to -14.8 kcal/mol, respec-
tively; the ∆Hhet(N-NO2)•-’s and ∆Hhomo(N-NO2)•-’s of
N-nitroacylamides (2•-) range from -17.6 to -24.4 kcal/mol
4-NO2
R1 ) CH3CH2
a Relative uncertainties were estimated to be smaller than or close to 1
and from 2.1 to -5.5 kcal/mol, respectively, while the ∆Hhet
-
kcal/mol. b ∆Hhet(Y-NO2) obtained from eq 1. c ∆Hhomo(Y-NO2) derived
(O-NO2)•-’s and ∆Hhomo(O-NO2)•-’s of O-nitro alcohols (3•-)
range from -16.0 to -32.2 kcal/mol and from -1.7 to -9.1
kcal/mol, respectively. A simple comparison of these energetic
data with the corresponding bond energies of neutral molecules
immediately indicates that both the heterolytic and the homolytic
bond dissociation energies of the nitro compound radical anions
are substantially lower than those of the corresponding neutral
molecules. The main factor contributing to the remarkable bond-
weakening effect is that the reduction potential values of N-
(O)-nitro compounds are all very negative in acetonitrile (see
Table S1, Supporting Information). It is also obvious from the
data of Table 1 that the ∆Hhet(N(O)-NO2)•-’s are all much
lower than the corresponding ∆Hhomo(N(O)-NO2)•-’s because
the nitrite ion generated in the former bond-breaking process is
from eq 2, taking E1/2(NO2+/0) ) 0.91 V vs Fc+/0 14 d ∆Hhet(Y-NO2)•-
.
derived from eq 3, taking E1/2(NO20/-) ) 0.400 V vs Fc+/0 15
.
e ∆Hhomo(Y-
NO2)•- derived from eq 4, taking E1/2(NO2+/0) ) 0.91 V vs Fc+/0 14
.
indicates that homolysis of the Y-NO2 bond to generate the
NO2 radical (NO2 ) is energetically much more favorable than
•
the corresponding Y-NO2 bond heterolysis to generate a pair
of ions. Further inspection of the heterolytic bond dissociation
energies of these nitro compounds shows that the ∆Hhet
decreases gradually as the remote substituent changes from
electron-donating groups (EDG) to electron-withdrawing groups
(EWG). This results in a quite good linear correspondence
between the ∆Hhet values and the Hammett σ constants (see
Figures S4-S6, where the r value equals 0.998, 0.987, and 0.994
for series 1, 2, and 3, respectively). This feature can be
understood from the fact that the EDGs are anion-destabilizing
(causing an increase in ∆Hhet) and EWGs are anion-stabilizing
(causing a decrease in ∆Hhet).
Although solvents are known to have a great effect on
heterolysis energies, their effect on homolytic BDEs, where only
neutral species are formed in solution during the homolysis of
neutral molecules, is expected to be small and is indeed observed
to be true in the literature where the solution BDEs were found
to match the corresponding gas-phase values very closely for
many compounds.16 This feature gives one confidence to apply
the BDE scales established in this work to biochemical reactions,
since the Y-NO2 BDEs in water can be well approximated by
the data from the present BDE study in acetonitrile. The relative
•
better solvated than the NO2 radical and the heterolytic
cleavages of the (N(O)-NO2)•- bond are all exergonic. This
feature suggests that these three types of nitro compound radical
anions are very unstable at room temperature and able to
spontaneously release NO2- by heterolytic cleavage. Of these,
-
the O-nitro alcohols radical anions seem to be the best NO2
donors, since the corresponding ∆Hhet(Y-NO2)•- are in the
lowest ranges of those evaluated for the nitro radical anions.
The opposite influence of remote substituents in varying the
∆Hhet(N-NO2)•-’s and the ∆Hhomo(N-NO2)’s (note that both
types of bond rupture generate the same nitrogen radical) can
(17) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press: New York, 2003 and references cited.
(18) Wen, Z.; Shang, Z.; Cheng, J.-P. J. Org. Chem. 2001, 66, 1466.
(19) (a) Tamura, R.; Katayama, H.; Watabe, K.; Suzuki, H. Tetrahedron
1990, 46, 7557. (b) Chakrapani, H.; Gorczynski, M. J.; King, S. B. J. Am.
Chem. Soc. 2006, 128, 16332. (c) Gorczynski, M. J.; Huang, J.-M.; King,
S. B. Org. Lett. 2006, 8, 2305.
(16) Bordwell, F. G.; Cheng, J.-P.; Ji, G.; Satish, A. V.; Zhang, X. J.
Am. Chem. Soc. 1991, 113, 9790.
2430 J. Org. Chem., Vol. 73, No. 6, 2008

The heat of dilution of nitronium tetrafluoroborate was small enough
to be neglected for reaction heat measurements. The reported ∆Hrxn
is the average value of two or three independent runs. Spectroscopic
identification of the Y-nitro compounds, for example, N-methyl-
N-nitrobenzamide: 1H NMR (300 MHz, CDCl3) δ 3.66 (3H, s),
7.44-7.48 (2H, m), 7.56-7.61 (1H, m), 7.67-7.70 (2H, m); 13C
NMR (CDCl3, 75 MHz) δ 35.9, 128.3, 128.8, 133.2, 133.3, 169.6.
be understood in terms of the differential solvation of the starting
(N-NO2)•- radical anion and the nitrogen radical products.
In summary, we here outlined an approach to the bond
dissociation energies of the Y-NO2 compounds and their radical
anions and have experimentally established, for the first time,
all four types of the bond-breaking energy scales for three nitro
compound families in acetonitrile. This energetic study should
be important in helping chemists and biochemists to understand
-
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grant Nos. 20332020,
20472038, 20421202, 20672060, and 20072020) is gratefully
acknowledged. We thank Professor Vernon Parker for helpful
discussions. We also thank the reviewers for setting high
standards and for good suggestions on this work.
the mechanistic details of NO2/NO2 release, capture, and
transfer. A more inclusive construction of the Y-NO2 bond
energy databank is under our immediate research.
Experimental Section
General Procedure for Titration Calorimetry. Reaction of
+
NO2 (NO2+BF4-) with anions (K+ as counterion) in dry CH3CN
Supporting Information Available: Experimental procedure,
∆Hrxn’s, oxidative potentials of Y- and E°red(Y-NO2)’s of Y-nitro
compounds in acetonitrile, and correlation of the heats of heterolysis
of Y-NO2 bonds in Y-nitro compounds with σ values. This material
is available free of charge via the Internet at http://pubs.acs.org.
was rapid and gave the Y-NO2 coupling product quantitatively.
The titration calorimetry was performed in acetonitrile solution at
25 °C on a CSC 4200 isothermal titration calorimeter. Prior to use,
the instrument was calibrated against an internal heat pulse. The
reaction heat was determined following eight automatic injections
and obtained by area integration of each peak except the first one.
JO702364A
J. Org. Chem, Vol. 73, No. 6, 2008 2431