Disproportionation reaction of NO2/N2O4 with a Ru(II) porphyrin

Article abstract of DOI:10.1039/b307959g

A Ru(II) porphyrin rapidly reacts with NO₂/N₂O ₄ with the formation of a stable Ru(II) nitrosyl nitrato complex.

Full text of DOI:10.1039/b307959g

Disproportionation reaction of NO2/N2O4 with a Ru(II) porphyrin
Yanlong Kang, Grigory V. Zyryanov and Dmitry M. Rudkevich*
Department of Chemistry & Biochemistry, University of Texas at Arlington, TX 76019-0065, USA;
Tel: + 817 272 5245; Fax: + 817 272 3808. E-mail: rudkevich@uta.edu
Received (in Columbia, MO, USA) 11th July 2003, Accepted 27th August 2003
First published as an Advance Article on the web 4th September 2003
A Ru(II) porphyrin rapidly reacts with NO2/N2O4 with the
formation of a stable Ru(II) nitrosyl nitrato complex.
of the product, which possesses identical spectroscopic features
with 2 (for example, Fig. 1D). Ru(TTP)(NO)(ONO) porphyrin
3 can be obtained analogously from 4 and AgNO2 (Scheme
1).6
Nitrogen dioxide (NO2) belongs to the family of NOX gases.1
NOX is the term describing the sum of nitric oxide (NO), NO2,
N2O3 (NO·NO2), N2O4 (NO2·NO2), and N2O5. Most of these
are toxic atmospheric pollutants, originating in large quantities
from fuel combustion, power plants, and large-scale industrial
processes. NOX gases are widely involved in various nitrosation
processes in biological tissues.2 Of particular importance is the
interaction of NOX with metalloporphyrins.3 A large number of
iron heme complexes with NO have been obtained and
characterized.3,4 Stable nitrosyl porphyrin complexes can be
prepared by replacing iron with other transition metals, and
ruthenium nitrosyl derivatives hold a special place in mimicking
biorelevant NO–metal interactions.5–7 Compared to NO, chem-
istry between NO2 and metalloporphyrins is much less
developed. We report here a previously unnoticed reaction
between NO2/N2O4 and a Ru(II) porphyrin. It causes dis-
proportionation of N2O4 and leads to a stable nitrosyl nitrato
complex. Our findings offer a new insight into the mechanism
of sensing and fixation of NO2/N2O4.8
NO2 is a paramagnetic gas of an intense brown–orange color.
Reacting with itself, it forms the colorless dimer, dinitrogen
tetroxide (N2O4).10 As the temperature raises N2O4 rapidly
dissociates back to NO2. The position of the equilibrium
between the two gases and the color of the system vary with
temperature. Below 221 °C, only pure, solid N2O4 is present.
Above 140 °C the system is 100% NO2. At normal/standard
conditions, the dynamic interconversion between NO2 and
N2O4 makes it impossible to study either of these species alone.
N2O4 is known to exist in several isomeric forms, including
symmetrical O2N–NO2 and two cis–trans ONONO2.11 It may
2 12,13
also disproportionate to ionic NO+ONO2
.
Metals react
with N2O4 with the formation of ionic nitrosonium nitrates,13
and we propose, that the N2O4 disproportionation takes place
We found that reaction of 2 eq. NO2 with Ru(II) carbonyl
tetra-p-tolylporphyrin Ru(TTP)(CO)9 1 in CH2Cl2 results in a
rapid formation of nitrosyl nitrato Ru(TTP)(NO)(ONO2) 2 in
~ 95% yield (Scheme 1).†
Orange solution of 1 turned dark-green upon addition of NO2.
In the IR spectrum of 2 (in KBr), the carbonyl nCO band at 1950
cm21 is replaced by a new, very strong nitrosyl band nNO at
1852 cm21, characteristic for the linear Ru–NNO geometry (Fig.
1A). In addition, nitrate bands appeared at 1515, 1269, and 950
cm21. Reaction with NO2 caused obvious changes in the UV-
vis spectra of the porphyrin. Upon mixing, the Soret band at
lmax = 412 nm decreased in its intensity, while the Q-band at
lmax = 528 nm disappeared and a new band at lmax = 564 nm
emerged (Fig. 1B). When performed in the NMR tube in CDCl3,
the above experiment resulted in a visible transformation of the
spectra (Fig. 1C). In particular, prior to the NO2 exposure, the
pyrrole b-CH and the meso-tolyl CH protons of 1 were seen as
a singlet at 8.70 ppm and two pair of doublets (J ~ 7.5 Hz) at
8.08, 8.02, 7.52, and 7.50 ppm, respectively. After the reaction,
these were transformed into a singlet at 9.08 ppm and three
apparent doublets (J = 8 Hz) at 8.16, 8.14, and 7.61 ppm, 1 : 1
: 2 ratio, respectively.
For a spectroscopic comparison, Ru(TTP)(CO) 1 was treated
with NO gas in CH2Cl2. As expected,5,6 nitrosyl nitrito
Ru(TTP)(NO)(ONO) porphyrin 3 was obtained, whose features
are different from those of 2. Specifically, in the IR spectrum of
3 (in KBr), a strong nitrosyl band nNO at 1842 cm21 and two
1
nitrite bands were detected at 1512 and 928 cm21. In the H
NMR spectrum of 3 in CDCl3, the pyrrole b-CH are seen as a
singlet at 9.01 ppm, and the meso-tolyl CH protons appear as
two pair of doublets at 8.16, 8.08, 7.59, and 7.57 ppm.
An independent structural evidence for complex 2 was
obtained from the reaction of Ru(TTP)(NO)Cl porphyrin 4 with
AgNO3 (Scheme 1), following the procedure of Bohle and co-
workers.6 Treatment of a benzene–MeCN solution of 4 with
AgNO3 at room temperature resulted in a quantitative formation
Scheme 1 Reactions of NOX with Ru(II) porphyrins (L = solvent).
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This journal is © The Royal Society of Chemistry 2003

for the described here experiments. Nitrosonium cation then
replaces the carbonyl, and nitrate, most probably, coordinates at
the other side of the porphyrin.
the gas sensing. We will report on the progress due to the
course.
We acknowledge Professor G. B. Richter-Addo and Dr J. Lee
for their most valuable insights and discussions, and experi-
mental assistance. We thank Professor T. S. Kurtikyan for the
reprints of his important work. Financial support was kindly
provided by the American Cancer Society Institutional Re-
search Grant to the University of North Texas Health Science
Center, and the Petroleum Research Fund, administered by the
American Chemical Society. DMR is an A. P. Sloan Research
Fellow.
Chemically, Ru(TTP)(NO)(ONO2) 2 is quite stable and can
be stored for weeks without protection at room temperatures.
This is in contrast to recently described Fe(III)(TPP-
)(NO)(ONO2) complex,14 which exists only at very low
temperatures. At the same time, 2 essentially decomposes on
silica gel and neutral and basic aluminium oxide columns,
which indicates that the nitrate anion is rather weakly
coordinated and can be lost or replaced upon chromatog-
raphy.
Notes and references
† 1H NMR spectra were obtained on
Introducing higher concentrations of NO2/N2O4 leads to
further transformations of 2. In the UV-vis spectra, second set of
isosbestic points appears, with the broad shoulder growing at
a JEOL Eclipse, 500 MHz
spectrometer; IR and UV-vis spectra were recorded on Bruker Vector 22
FTIR and Varian Cary 50 spectrometers, respectively. HRMS MALDI data
were obtained on a IonSpec Ultima FTMS. Ru(TTP)(NO)(ONO2) (2).
Procedure 1. NO2 was generated from copper and conc. HNO3 at rt. The
NO2 stock solution in CH2Cl2 (3 mL, 0.46 mmol) was added dropwise to the
solution of porphyrin 1 (100 mg, 0.23 mmol) in CH2Cl2 (10 mL), and the
reaction mixture was further stirred for 30 min. The solvent was evaporated
in vacuum to yield 2 as a dark-green solid ( > 95%): mp > 320 °C; 1H NMR
(CDCl3, d) 9.08 (s, 8 H), 8.16 (2 3 d), J = 8 Hz, 8 H), 7.61 (d, J = 8 Hz,
8 H), 2.73 (s, 12 H). FTIR (KBr, n, cm21) 1852 s, 1515 m, 1269 s, 950 m.
UV-vis (lmax, nm) 412 (e 270000), 564. HRMS (MALDI-FTMS, m/z):
800.1969, 770.1996. Calc for C48H36N6O4Ru: 800.2000 (M 2 NO32),
1
lmax ~ 440 nm. In the H NMR spectra, the signals of 2
disappear and new sets emerge, featuring multiplets between
9.4 and 9.2 ppm. Most probably, nitration of the pyrrole rings
occurs, as N2O4 has previously been used to nitrate metal-
loporphyrins into the b-position.15
In summary, a new reaction can now be added to the broad
spectrum of processes between NOX gases and metalloporphyr-
ins. NO2/N2O4 rapidly disproportionates over a Ru(II) por-
phyrin with the formation of a Ru(II) nitrosyl nitrato complex.
This and recently described Fe(III)(TPP)(NO)(ONO2) are the
only known nitrosyl nitrato metalloporphyrins,6,14 and in
general, metalloporphyrin-nitrato adducts are rare.3,4 We are
currently looking at other metalloporphyrins to broaden the
reaction scope. While previously overlooked, these complexes
may be very important intermediates in biological and industrial
processes involving NOX. For instance, while metallated
porphyrins and phthalocyanines have been known for years as
effective sensors for NO2,16 chemical reactions and structures
responsible for this are largely unknown.
2
770.2000 (M 2 NO3 2 NO+). Procedure 2.6 To the solution of
Ru(TTP)(NO)Cl 4 (20 mg, 0.024 mmol) in CH3CN–benzene, 1 : 1 (20 mL)
AgNO3 (6 mg, 0.036 mmol) was added with stirring. The solution was
stirred for additional 30 min, after which the resulted AgCl was filtered
through Celite. The solvent was evaporated in vacuum to afford 2 in > 95%
yield.
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Fig. 1 Spectra of Ru(TTP)(NO)(ONO2) 2. A: FTIR spectrum of 2 in KBr.
B: UV-vis spectral changes of Ru(TTP)(CO) 1 upon reaction with NO2/
N2O4 in CH2Cl2. C: 1H NMR spectrum (500 MHz, CDCl3, 295 K) of 2
obtained from Ru(TTP)(CO) 1 and NO2/N2O4. D: 1H NMR spectrum of 2
obtained from Ru(TTP)(NO)Cl 4 and AgNO3. The residual CHCl3 signal is
marked “·”.
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