Generation of nitric oxide by photolysis of silver hyponitrite in suspension induced by LMCT excitation

Article abstract of DOI:10.1016/j.inoche.2007.07.017

The yellow colour of silver hyponitrite is attributed to an LMCT band at λ_max = 419 nm. Solid Ag₂N₂O₂ and its suspensions in water or acetonitrile are light sensitive. LMCT excitation leads to a photolysis according to the equation Ag₂N₂O₂ → 2Ag + 2NO. In the presence of air and water, NO is finally converted to HNO₂.

Full text of DOI:10.1016/j.inoche.2007.07.017

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Inorganic Chemistry Communications 10 (2007) 1294–1296
www.elsevier.com/locate/inoche
Generation of nitric oxide by photolysis of silver hyponitrite
in suspension induced by LMCT excitation
*
Horst Kunkely, Arnd Vogler
Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany
Received 29 May 2007; accepted 28 July 2007
Available online 3 August 2007
Abstract
The yellow colour of silver hyponitrite is attributed to a LMCT band at kmax = 419 nm. Solid Ag2N2O2 and its suspensions in water
or acetonitrile are light sensitive. LMCT excitation leads to a photolysis according to the equation Ag2N2O2 ! 2Ag + 2NO. In the pres-
ence of air and water, NO is finally converted to HNO2.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Electronic spectra; LMCT; Hyponitrite complexes; Silver complexes; Photochemistry; Nitric oxide
Owing to the biological significance of nitric oxide (NO)
[1–5], its generation from a variety of different precursors
has been studied quite extensively [6,7]. The photochemical
formation of NO is of special value because it is controlled
by light. In addition to organic precursors [8–12], light-sen-
sitive coordination compounds [13–18] play an important
role. In particular, nitrosyl complexes have been investi-
gated in quite some detail. As a non-innocent ligand, coor-
dinated NO may be present as NO+, NO or NOꢀ. In this
context the relationship between NOꢀ and its formal
The compound Ag2N2O2 has been known since the 19th
century [20,21]. It is easily accessible [22] and precipitates,
for example, from an aqueous solution of Na2N2O2
(Aldrich) upon the addition of silver salts [23]. Unfortu-
nately, Ag2N2O2 is completely insoluble in all conventional
solvents. However, it dissolves in acids with the release of
H2N2O2 as indicated by the loss of the yellow colour of
Ag2N2O2 and the appearance of an absorption at
248 nm, which shows the presence of free hyponitrite [23].
Owing to the insolubility of Ag2N2O2 all further exper-
iments were carried out with the solid compound. Its
absorption spectrum was obtained as a scattered-transmis-
sion spectrum of an aqueous suspension (Fig. 1). Bands
appear at kmax = 207 (sh), 255 (sh) and 419 nm. The solid
does not show any photoluminescence at r.t. or 77 K.
The light sensitivity of Ag2N2O2 was originally noticed in
connection with its first preparation [20,21]. Dry Ag2N2O2
is slightly light sensitive and slowly turns dark upon irradi-
ation. As an aqueous suspension the light sensitivity of
Ag2N2O2 is increased considerably. The yellow suspension
is rapidly converted to a black one which consists of ele-
mental silver. In contrast to Ag2O, elemental silver does
not dissolve in aqueous ammonia, but only in nitric acid.
The aqueous solution, which is obtained upon removal of
the solid constituents from the photolysis suspension, does
not display prominent absorptions in the near UV when
dimer, N2O2ꢀ (hyponitrite), is apparent. Accordingly, it is
2
conceivable to generate NO by the photolysis of a suitable
hyponitrite complex. Indeed, such an example has been
reported quite recently [19]. Unfortunately, in this case
the hyponitrite complex is not easily accessible and its com-
plicated composition, [Ru2(CO)4(l-H)(l-PtBu2)(l-dppm)
(l-g2N2O2)], and structure interfere with the facile identifi-
cation of the reactive excited state. Generally, since only a
few hyponitrite complexes are known at all [19], the choice
of a suitable complex is rather restricted. Nevertheless, we
explored this possibility and selected Ag2N2O2 for the pres-
ent study.
*
Corresponding author. Tel.: +49 941 943 4716.
E-mail address: arnd.vogler@chemie.uni-regensburg.de (A. Vogler).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.07.017

H. Kunkely, A. Vogler / Inorganic Chemistry Communications 10 (2007) 1294–1296
1295
hyponitrite radical is a reductant as well as an oxidant
[26,27]. While its oxidizing nature favours the reversal of
the primary photochemical step, its oxidatꢀion by a second
Ag+ ion leads to the formation of NO (N2O2 ꢀ eꢀ ! 2NO).
It follows that the photolysis of Ag2N2O2 induced by
LMCT oxidation should finally proceed according to the
simple equation
Ag2N2O2 ! 2 Ag0 + 2NO
ð1Þ
The presence of NO in the photolyzed solution was in-
ferred from its subsequent reaction with oxygen and water
to produce nitrous acid
2NO + 1/2O2 + H2O ! 2HNO2
ð2Þ
Fig. 1. Scattered-transmission spectrum of an aqueous suspension of
finely powdered Ag2N2O2 at room temperature, 1-cm cell.
The water is not only necessary for the formation of ni-
trous acid but also facilitates the photolysis of solid
Ag2N2O2. It could favour product formation by readily
removing NO from the surface of the Ag2N2O2 particles.
In conclusion, suspensions of silver hyponitrite undergo
an efficient photolysis with the generation of elemental sil-
ver and nitric oxide. In the photolyzed suspension NO is
apparently the only product which accumulates in the
liquid phase. Since neither Ag2N2O2 nor elemental silver
is soluble, this photoreaction represents a simple method
for obtaining NO in solution without by-products. In the
context of biological considerations [1–5] it is also of inter-
est that silver and silver compounds are antibacterial
agents [30].
kept under argon. However, upon exposure to air an
absorption spectrum (Fig. 2) develops which unambigously
indicates the presence of HNO2 [24,25]. The photolysis is
still very efficient when the irradiation is restricted to kirr>
425 nm. A suspension of Ag2N2O2 in acetonitrile is also
light sensitive although less in water. Addition of water
to acetonitrile increases the photosensitivity of the solid.
Hyponitrite can be reduced but is essentially known as a
strong reductant [26,27]. Accordingly, it should also act as
a CT donor. Since Ag+ can function as a CT donor and an
acceptor, we suggest that the yellow colour of Ag2N2O2 is
caused by an LMCT band (kmax = 419 nm). This assign-
ment is in agreement with the photochemical and lumines-
cence properties of Ag2N2O2. Silver(I) complexes are well
known to emit from IL states [28–30] and also from CT
states provided Ag+ is the CT donor [31,32]. In contrast,
Ag2N2O2 is not luminescent under any conditions. This is
a strong indication that the lowest excited state is indeed
of the LMCT type [30,31]. Moreover, LMCT states of sil-
ver(I) complexes are not only luminescent but also gener-
ally reactive. LMCT excitation is well known to lead to
the reduction to elemental silver and oxidation of the
ligand [30,33]. As a one-electron transition the LMCT exci-
tation of Ag2N2O2 will yield a silver atom and a hyponitrite
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