Thermal and photochemical reactivity of Os(HNO)(CO)Cl2(PPh3)2: Evidence for photochemical HNO generation

Article abstract of DOI:10.1016/j.poly.2007.04.044

FTIR studies of the thermal and photochemical reactions of Os(N(O)H)(CO)Cl₂(PPh₃)₂ (1) are described. Though 1 is relatively stable, it readily reacts when irradiated to form multiple products, including a metal-carbonyl species and N₂O, the decomposition product of HNO. The relative yields of products varied depending on whether or not excess CO was present. A model is presented that includes initial photochemical release of HNO from 1 as a significant but not exclusive photoreaction.

Full text of DOI:10.1016/j.poly.2007.04.044

Polyhedron 26 (2007) 4638–4644
www.elsevier.com/locate/poly
Thermal and photochemical reactivity
of Os(HNO)(CO)Cl2(PPh3)2: Evidence for photochemical
HNO generation
a,b,
b
a
Jon Marhenke
*, Crisjoe A. Joseph , Marlena Z. Corliss ,
a
b,
*
Tim J. Dunn , Peter C. Ford
a
Biochemistry, California State University, Chico, CA 95929, USA
Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA
b
Received 15 January 2007; accepted 25 April 2007
Available online 17 May 2007
Abstract
FTIR studies of the thermal and photochemical reactions of Os(N(O)H)(CO)Cl2(PPh3)2 (1) are described. Though 1 is relatively sta-
ble, it readily reacts when irradiated to form multiple products, including a metal–carbonyl species and N2O, the decomposition product
of HNO. The relative yields of products varied depending on whether or not excess CO was present. A model is presented that includes
initial photochemical release of HNO from 1 as a significant but not exclusive photoreaction.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Carbonyl; Nitrous oxide; Nitroxyl; Osmium; Photoreaction
1. Introduction
solution is the focus of the research described here. There
have been several recent reports of compounds containing
coordinated HNO [4] including a complex with myoglo-
bin [5], and nitroxyl coordination chemistry has been
reviewed [6]. Complete structural characterization has
only been reported for a few metal–HNO complexes [7].
The first of these, Os(N(O)H)(CO)Cl2(PPh3)2 (1), was syn-
thesized by Grundy et al. in 1970 [8] and structurally
characterized by Wilson and Ibers in 1979 [7a]. Hetero-
lytic cleavage of metal–ligand bonds is a known photo-
Since the recognition of its importance in mammalian
biology in the mid-1980s, NO (nitric oxide) has been the
focus of considerable biological and chemical research
[1]. It has recently been suggested that NOꢀ (nitroxyl
anion) and its conjugate acid HNO (‘‘nitroxyl’’) may also
be important biologically [2]. For instance, it was shown
that HNO is produced in the physiologically relevant reac-
tion of S-nitrosothiols with thiols [3]. There has also been
some interest in the potential therapeutic value of HNO
and therefore in designing compounds that might be used
to deliver nitroxyl to specific biological sites.
chemical process in similar compounds [9], so
1
appeared to have potential as a photochemical HNO
donor. Presented here are results of studies of the thermal
and photochemical reactivity of 1.
Although nitroxyl has been characterized in argon and
dinitrogen matrices using infrared spectroscopy [10], study-
ing the photochemical formation of nitroxyl in solution is
challenging since HNO reacts with itself at near diffusion
controlled rates to form hyponitrous acid, which undergoes
dehydration to form N2O (Eq. (1)) [11]. Therefore, the
In this context, the possibility of photochemical gener-
ation of HNO from a transition metal–HNO complex in
*
Corresponding authors. Tel.: +1 805 893 2443; fax: +1 805 893 4120
(P.C. Ford).
E-mail addresses: jmarhenke@exchange.csuchico.edu (J. Marhenke),
ford@chem.ucsb.edu (P.C. Ford).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.04.044

J. Marhenke et al. / Polyhedron 26 (2007) 4638–4644
4639
detection of N2O is an indirect indicator for the formation
of HNO [3], although there are other sources of N2O in the
chemistry of nitric oxide, for example as a product of NO
disproportionation mediated by transition metal complexes
[12]
had settled to the bottom of the flask was isolated and used
without further purification.
In the second step, the crude Os(H)(CO)Cl(PPh3)3 was
used for the synthesis of Os(NO)(CO)Cl(PPh3)2, following
literature methods [5,6]. To a mixture of 300 mg Os(NO)(-
CO)Cl(PPh3)2 and 100 mg N-methyl-N-nitrosyl-p-toluene-
sulfonamide (Aldrich, ‘‘Diazald’’) was added 100 mL
ethanol. The mixture was allowed to reflux for 3 h, during
which time the color changed from yellow to brownish
orange. Upon cooling, a brownish orange powder settled
to the bottom. The ethanol was removed by cannula, and
the solid was washed repeatedly with pentane. The prod-
uct, Os(NO)(CO)Cl(PPh3)2, was characterized by its IR
spectrum, which was in good agreement with the literature
[5].
2HNO ! N2O + H2O
ð1Þ
2. Experimental
2.1. General
Infrared spectra were obtained using a Nicolet 4700
FTIR and a 0.5 mm-pathlength sealed IR cell. UV–Vis
spectra were collected on an HP 8453 diode array spectro-
photometer. GC–MS data were obtained using a Hewlett–
Packard 6890 Series GC System and a 5973 Mass Selective
Detector. Data manipulation was performed using Igor
Pro software. Irradiation of samples in IR cells was per-
formed using the 365 nm output of a Mineralight handheld
UV lamp. Irradiation in UV–Vis cells was performed using
Starna airtight cuvettes equipped with septum caps.
Os(N(O)H)(CO)Cl2(PPh3)2 (1) was formed by the reac-
tion of Os(NO)(CO)Cl(PPh3)2 with gaseous HCl. The syn-
thesis and isolation followed the general method of Wilson
and Ibers [5]. To produce gaseous HCl, H2SO4 was added
to NaCl. In the glovebox, crystals of Os(NO)(CO)-
Cl(PPh3)2 (ꢁ50 mg) were partially dissolved in dry, deaer-
ated benzene (4 mL) in a test tube with a stir bar. This test
tube was placed in a 500 mL three-neck round-bottom
flask. HCl gas was flushed through the flask while the ben-
zene solution was stirred. It was then allowed to stand for
ꢁ15 min, and then the whole system was flushed with N2.
The flask was then disconnected from the HCl set-up,
brought into the glovebox, and allowed to stand for an
hour to let the particles settle. The benzene solution was
transferred into a separate test tube, leaving behind a yel-
low/orange powder. Dry, degassed pentane (50 mL) was
allowed to slowly effuse into the benzene solution, produc-
ing red crystals. The product was characterized by its IR
Samples for spectroscopic studies were prepared under a
nitrogen atmosphere in the glovebox. Anhydrous chloro-
form (Aldrich, SureSeal) was transferred via cannula to a
Schlenk flask, deaerated, and brought into the glovebox.
Solutions of 1 were then prepared and, for studies under
nitrogen, loaded into IR cells and removed from the glove-
box. For studies performed under CO, samples were placed
in airtight flasks equipped with rubber septa and removed
from the glovebox, and then CO was bubbled through
the solution. These samples were then transferred to IR
cells via syringe. Molar concentrations of N2O in solution
were calculated from FTIR data using absorbance values
at 2220 cmꢀ1, an extinction coefficient of 910 Mꢀ1 cmꢀ1
[13], and a pathlength of 0.05 cm. Molar concentrations
of 1 in solution were estimated from FTIR data using
absorbance values at 1415 cmꢀ1 (a frequency at which
there appeared to be little overlap with any other species
spectrum (mCO = 1980 cmꢀ1 and
m
NO = 1415 cmꢀ1 in
CHCl3).
3. Results and discussion
The FTIR spectrum of Os(N(O)H)(CO)Cl2(PPh3)2 (1)
shows a strong mCO absorbance at 1980 cmꢀ1, as well as
an absorbance at 1415 cmꢀ1, assigned to the mNO of coordi-
nated HNO by comparison to the literature data [5]. The
peaks at 1435 cmꢀ1 and 1483 cmꢀ1 are assigned to vibra-
tional modes of the PPh3 ligands since these peaks are
observed for free PPh3 and other PPh3-containing osmium
species. The UV–Vis spectrum of 1 in CHCl3 shows an
absorption maximum at 364 nm (Fig. S1).
in solution), an extinction coefficient of 420 Mꢀ1 cmꢀ1
and a pathlength of 0.05 cm.
,
2.2. Synthesis of Os(N(O)H)(CO)Cl2(PPh3)2 (1)
Except where noted, all syntheses were carried out under
nitrogen. Reagents were used without additional purifica-
tion. Solvents were deaerated prior to use.
Os(N(O)H)(CO)Cl2(PPh3)2 (1) was synthesized in a
three-step process. First, Os(H)(CO)Cl(PPh3)3 was synthe-
sized from OsCl6 according to a literature method [14].
3.1. Photochemical reactivity of
Os(N(O)H)(CO)Cl2(PPh3)2 under nitrogen
2ꢀ
To a refluxing mixture of triphenylphosphine (1 g) and 2-
methoxyethanol (20 mL) was added a slurry of K2OsCl6
(330 mg) in 2-methoxyethanol. Then a 10 mL portion of
formaldehyde solution (37% in water/methanol) was cann-
ulated into this mixture. The color changed from brownish
red to yellowish green, and the mixture was allowed to
reflux for 30 min. After cooling, the off-white solid that
When a solution of 1 in CHCl3 under nitrogen was sub-
jected to continuous photolysis at 365 nm, the FTIR absor-
bance changes shown in Fig. 1 were observed. The peaks at
1415 cmꢀ1 and 1980 cmꢀ1 decreased in intensity, while a
new band grew in at 1925 cmꢀ1. The bands at 1435 cmꢀ1
and 1483 cmꢀ1 attributed to PPh3 were nearly unchanged

4640
J. Marhenke et al. / Polyhedron 26 (2007) 4638–4644
Fig. 1. Infrared spectral changes observed upon 365 nm continuous photolysis of a CHCl3 solution of 1 under nitrogen. The total time of photolysis
between the first and last spectrum was 30 min. The spectrum of 1 before photolysis is shown in black. The final spectrum is shown in red. Inset: an
enlargement of the spectral region 2200–2240 cmꢀ1 showing the growth of a peak indicating the formation of N2O. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
by photolysis. The spectral changes at other frequencies are
consistent with photochemical loss of HNO. The decrease
in intensity of the bands at 1415 cmꢀ1 and 1980 cmꢀ1 indi-
cates disappearance of 1, while the growth of a peak at
1925 cmꢀ1 suggests that the product of the photochemical
reaction contains a carbonyl ligand. A weak peak was also
observed to grow in at 2220 cmꢀ1 (inset in Fig. 1), consis-
tent with the appearance of N2O, which is known to form
rapidly from HNO [11]. For comparison, an FTIR spec-
trum of N2O-saturated CHCl3 was obtained, and it dis-
played an analogous peak at the same frequency
(2220 cmꢀ1) as the purported N2O peaks observed in these
photochemical studies (Fig. S2). To further confirm the
photochemical production of N2O, a solution of 1 in
CHCl3 was irradiated in an air-tight cuvette, and a small
sample of this solution was withdrawn and analyzed by
GC–MS. A peak with m/z = 44 (consistent with N2O)
was observed, overlapping with m/z = 32 (O2) and m/z =
28 (N2) peaks. (The sample had been exposed to air in
the transfer and injection process.) The elution time
matched well with an authentic sample of N2O in CHCl3,
and this peak was absent in analogous samples not sub-
jected to photolysis.
The disappearance of the HNO peak of 1, the observa-
tion of N2O formation and the appearance of a new peak
attributed to a metal–carbonyl stretch all point to the
possible operation of a primary photoreaction involving
dissociation of 1 to HNO plus Os(CO)Cl2(PPh3)2. A five-
coordinate species OsCl2(PPh3)3 similar to the proposed
osmium product has been reported [15]. The photochemi-
cally released HNO would form N2O through the known
dimerization pathway, although a competing pathway
might be the reaction of HNO with 1 considering that this
is present in greater concentration.
ing vibration of a linear nitrosyl, and weak peaks appeared
above 2000 cmꢀ1, which is an appropriate range for metal-
hydride or carbonyl stretches.
Longer term (2 h) photolysis demonstrated that the ini-
tial species formed are themselves subject to secondary
photolysis (Fig. 2). After 30 min (blue spectrum in
Fig. 2), 1 was nearly depleted, as indicated by the relatively
small absorbance at 1415 cmꢀ1, and the largest product
peak appeared at 1925 cmꢀ1. Upon further photolysis
(red spectrum in Fig. 2), the peak at 1925 cmꢀ1 decreased
in intensity, while peaks grew in at 1973 cmꢀ1 and
2040 cmꢀ1. The latter are consistent with the formation
of Os(CO)2Cl2(PPh3)2 (2), the infrared spectrum of which
has been reported [16]. After 60 min of photolysis, the peak
at 1415 cmꢀ1 is gone, and the peak at 2220 cmꢀ1 has
stopped growing. Thus appearance of N2O correlates well
with the disappearance of 1.
3.2. Thermal reactivity of 1 under nitrogen
A portion of the solution of 1 used for photolysis exper-
iments was stored under nitrogen in the dark at room tem-
perature for ꢁ3 h (the duration of the photolysis
experiments). During this time, no significant decomposi-
tion of 1 occurred, as indicated by the lack of spectral
changes (Fig. S3).
3.3. Photochemical reactivity of 1 under CO
When a solution of 1 in CHCl3 was equilibrated with
1 atm CO and subjected to 365 nm irradiation, the major
products were 2 and N2O, according to the changes in
the FTIR spectrum (Fig. 3). The evidence for ꢀth1e formation
of N2O is the growth of the peak at 2220 cm , coinciding
with the disappearance of the peak at 1415 cmꢀ1 character-
istic of 1. At the same time, peaks at 2043 cmꢀ1 and
ꢁ1975 cmꢀ1, corresponding to Os(CO)2Cl2(PPh3)2 [16],
In addition to the peaks attributed to N2O and the
osmium carbonyl product, a broad absorption centered
at ꢁ1840 cmꢀ1 appeared, possibly due to the mNO stretch-

J. Marhenke et al. / Polyhedron 26 (2007) 4638–4644
4641
Fig. 2. Spectral changes observed when a CHCl3 solution of 1 under nitrogen in an IR cell was subjected to 365 nm photolysis. Spectra were obtained
every 30 min, and the total time of photolysis between the first and last spectrum was 2 h. The spectrum before photolysis is shown in black. The spectrum
after 30 min of photolysis is shown in blue. The spectra after 60, 90, and 120 min of photolysis are shown in red. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Infrared spectral changes observed upon 365 nm continuous photolysis of a CHCl3 solution of 1 under ꢁ1 atm CO. The total time of photolysis
between the first and last spectrum was 60 min. The initial spectrum is shown in black. The final spectrum is shown in red. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
appeared. (The band at 1975 cmꢀ1 overlaps with the start-
ing material’s strong absorbance at 1980 cmꢀ1, but its pres-
ence can be inferred from the fact that the peak at
irradiation than in the dark. After 60 min of photolysis, 2
was the major product and 1 was almost completely gone,
but under analogous conditions in the dark, very little con-
version of 1 to 2 took place in a similar time frame.
The accelerated formation of N2O and Os(CO)2Cl2-
(PPh3)2 is further evidence that HNO is photochemically lib-
erated from 1. The dicarbonyl 2 should be formed more
rapidly by reaction of CO with the five-coordinate HNO-loss
photoreaction intermediate Os(CO)Cl2(PPh3)2 than by
direct reaction of CO with 1, the mechanism of which is
unknown. The likely pathway is irradiation-induced HNO
labilization followed by trapping of the resulting five-coordi-
nate osmium intermediate, as depicted in Scheme 1. The
additional FTIR absorbances that appear at 1840 cmꢀ1
and in the 2000–2100 cmꢀ1 range suggest that other products
are formed under photochemical conditions, very likely as
the result of CO photodissociation.
1980 cmꢀ1, which is stronger than the peak at 1415 cmꢀ1
,
does not decrease in intensity as quickly as the latter. Also,
the final spectrum obtained after exhaustive photolysis
clearly shows a peak at 1975 cmꢀ1.) In addition to peaks
corresponding to N2O and 2, weaker peaks appeared in
the 1800–1900 cmꢀ1 range and in the 2000–2150 cmꢀ1
range, as was observed in reactions under dinitrogen.
3.4. Thermal reactivity of 1 under CO
A portion of the same sample of 1, equilibrated under
1 atm CO, was allowed to sit in the dark for 80 min, the
result being the modest spectral changes shown in Fig. 4.
Consistent with the literature reports [8], a small amount
of 2 was formed, as indicated by the peak at 2040 cmꢀ1
.
The observation that the relative product yields varied
with reaction conditions is consistent with this model.
However, Os(CO)2Cl2(PPh3)2 is formed much faster under

4642
J. Marhenke et al. / Polyhedron 26 (2007) 4638–4644
Fig. 4. Infrared spectrum of a sample of 1 under ꢁ1 atm CO after 80 min reaction in the dark at room temperature.
O
nitrous oxide under these conditions. This analysis, how-
H
N
Os
Cl
CO
ever, does not account for other possible fates of nitroxyl.)
According to Scheme 1, the presence of excess CO in the
system would affect the net photochemistry in two ways.
First, CO could back-react with the five-coordinate inter-
mediate, Os(N(O)H)Cl2(PPh3)2, to re-form 1, thus reduc-
ing the net photochemistry along the CO-loss pathway.
This would increase the relative importance of the HNO-
loss pathway, thus increasing the yield of N2O relative to
disappearance of 1. The second impact of solution CO
would be to trap the HNO-loss photoreaction intermedi-
ate, Os(CO)Cl2(PPh3)2, to form 2. This would minimize
hν, -HNO
Cl
Ph3P
PPh3
CO
Cl
PPh3
CO
+CO, fast
Os(CO)(Cl)2(PPh3)2
Os
Cl
Ph3P
+HNO
(2)
(1)
hν, -CO
2HNO
H2O + N2O
+CO
Other products
Os(HNO)(Cl)2(PPh3)2
Scheme 1.
The yield of N2O relative to the amount of 1 consumed is
significantly greater under CO than under N2, as shown in
Fig. 5, where the concentration of N2O generated is plotted
against the concentration of 1. The yield of N2O relative to
disappearance of 1 is given by the negative of the slope of
this line. Under N2, the yield is 0.012, whereas under CO, it
is 0.14, more than an order of magnitude larger. (Note that
since two HNO are necessary to generate N2O, the latter
number implies that at least 28% of 1 decayed to give
H
O
N
H
O
O
N
N
Os
Cl
Cl
PPh3
H
-CO
PPh3
Cl
Ph3P
Cl
PPh3
CO
Os
Cl
Os
Ph3P
Ph3P
Cl
+CO
1840 cm-1
(1)
Scheme 2.
Fig. 5. Molar concentration of N2O vs. molar concentration of 1, calculated from the data shown in Figs. 1 and 3. Open squares represent data collected
when samples of 1 were irradiated under N2. Filled squares represent data collected when samples of 1 were irradiated under CO.

J. Marhenke et al. / Polyhedron 26 (2007) 4638–4644
4643
back reaction of Os(CO)Cl2(PPh3)2 with the HNO gener-
ated, thus allowing the latter to react along the N2O forma-
tion pathway. In either case, Scheme 2 predicts that the
presence of CO will increase the yield of N2O relative to
depletion of 1, as was observed. Notably, the reaction of
HNO with a metal center to form an HNO complex has
a precedent in the reported trapping of nitroxyl with deoxy-
myoglobin [5].
This conclusion is supported by the detection of N2O, the
product of HNO dimerization, as well as by the spectral
characterization of the organometallic products and com-
parison of reactivities under various conditions. A model
for the overall thermal and photochemical reactivity of 1
in CHCl3 is presented in Scheme 1.
Acknowledgements
Although net quantum yields were not measured for this
system, the first scenario would lead to a decrease in the
overall rate of conversion of 1 to the products, while the
second would lead to an increased rate if the experiments
with and without added CO were carried out under analo-
gous conditions of light intensity, etc. The qualitative
experiments illustrated in Fig. S4 show that there is no
great difference in the photoinduced decay of 1 regardless
of whether or not CO is present although that there was
a modest increase under CO. In the context of Scheme 1
and the data presented in Fig. 5, this would imply that
the major pathway(s) for the depletion of 1 is relatively
unaffected by CO but that the pathway generating N2O is
strongly enhanced. Thus, if CO photodissociation is fol-
lowed by an irreversible unimolecular reaction leading to
rapid depletion of the initially formed intermediate
Os(N(O)H)Cl2(PPh3)2, then the effect of CO on the CO-
loss pathway may be small while trapping of Os(CO)-
Cl2(PPh3)2 to form 2, would enhance the formation of
N2O by inhibiting the back reaction of HNO with that
unsaturated intermediate.
What might be a fast irreversible pathway depleting the
intermediate Os(N(O)H)Cl2(PPh3)2 of the CO-loss path-
way? Although the ‘‘other products’’ observed were not
conclusively elucidated, the appearance of an IR band at
1840 cmꢀ1 (Fig. 1) is consistent with the formation of line-
arly coordinated NO. Furthermore, the absence of a corre-
sponding carbonyl stretch for this species suggests that it
was formed via the CO-loss pathway. Hydride migration
from the N-coordinated HNO to the vacant coordination
site left by CO would give the nitrosyl hydride complex
Os(NO)(H)Cl2(PPh3)2, as proposed in Scheme 2. An anal-
ogous compound, Os(NO)(H)(O2CCF3)2(PPh3)2, has been
reported to have mNO = 1820 cmꢀ1 [17], a value similar to
that observed in this work (1840 cmꢀ1). While it might be
expected that the formation of the nitrosyl product would
be inhibited by excess CO owing to back reaction of the
intermediate to reform 1, the effect on the 1840 cmꢀ1 band
was relatively modest even when the photolysis is carried
out under a CO atmosphere. This would be consistent with
the unimolecular decay of the intermediate Os(N(O)H)-
Cl2(PPh3)2 (via hydride migration?) being sufficiently fast
to compete with the bimolecular back-reaction under the
conditions of this study.
This work was supported by grants from the National
Science Foundation. Carol Buckmann and Professor Ran-
dy M. Miller are thanked for their assistance with equip-
ment and instrumentation at CSU Chico.
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.2007.
04.044.
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