Osidation of a water-soluble phosphine and some spectroscopic probes with nitric oxide and nitrous acid in aqueous solutions

Article abstract of DOI:10.1021/ic900688g

In acidic aqueous solutions, nitrogen monoxide oxidizes monosulfonated triphenylphosphine, TPPMS^-, to the corresponding phosphine oxide. The NO-derived product Is N₂O. This chemistry parallels that reported for the reaction of NO wit

Full text of DOI:10.1021/ic900688g

Inorg. Chem. 2009, 48, 6979–6985 6979
DOI: 10.1021/ic900688g
Oxidation of a Water-Soluble Phosphine and Some Spectroscopic Probes with
†
Nitric Oxide and Nitrous Acid in Aqueous Solutions
Andreja Bakac,* Margaret Schouten, Alicia Johnson, Wenjing Song, Oleg Pestovsky, and Ewa Szajna-Fuller
Iowa State University, Ames, Iowa 50011
Received April 8, 2009
-
In acidic aqueous solutions, nitrogen monoxide oxidizes monosulfonated triphenylphosphine, TPPMS , to the
corresponding phosphine oxide. The NO-derived product is N O. This chemistry parallels that reported for the reaction
ofNOwiththeunsubstitutedtriphenylphosphinein nonpolarorganic solvents, but therateconstant measuredin this work,
2
6
-2 -1
-
5
aqueous solution. The increased rate constant in the present work appears to be a medium effect, and unrelated to the
.14ꢀ 10 M s , isgreaterbyseveral ordersofmagnitude. This makes TPPMS a useful analytical reagent for NO in
-
introduction of a single sulfonate group in the phosphine. The reaction between nitrous acid and TPPMS has a 2:1
[
identifies the second-order self-reaction of HNO as the rate-limiting step that generates the active oxidant(s) for the fast
-
2
TPPMS ]/[HNO ] stoichiometry and generates NH OH quantitatively. The rate law, rate = 4k [HNO ] [TPPMS ],
-
2 2 d 2
2
-
subsequent reaction with TPPMS . It appears that the active oxidant is N O , although the oxides NO and NO derived
3
from it may be also involved. Bimolecular self-reaction of HNO also precedes the oxidations of ABTS and TMPD.
2
Competing with this path are the acid-catalyzed oxidations of both reagents via NO .
2
2
2-
þ
Introduction
Nitrite and nitrogen oxides play important roles in various
For a number of reactive substrates the step(s) in eq 2 is
fast relative to both forward and reverse steps in eq 1.
Under those circumstances, the kinetics become second order
in [HNO ] and zero-order in [substrate], eq 3, where k =
1
2,3
biological processes in mammals and plants, in nitrogen
4
5
2
d
dynamics in soil, atmospheric (photo)chemistry, and in the
laboratory. The first example of nitrite acting as electron
donor for anoxygenic photosynthesis has also been discov-
þ
-1 -1
(
2
10.9 þ 175 [H ]) M
s
in acidic aqueous solutions at
13
5 °C, and n is the stoichiometric factor, i.e., the number
6
of equivalents of substrate consumed per rate-determining
step.
ered recently.
In view oftheir widespread involvement in nature, nitrogen
oxides and nitrite/nitrous acid have been a subject of numer-
d
2
k , -H O
7-11
2HNO
2
N
2
O
3
a NO þ NO
2
ð1Þ
ous mechanistic studies.
One of the mechanisms by which
nitrous acid reacts with substrates involves the disproportio-
N
2
O
3
and=or NO and=or NO
2
þ substrate f products ð2Þ
nation of HNO followed by the reaction of one or both
2
12,13
nitrogen oxideswith the substrate, as shown in eq 1 and 2.
2
-
d½substrateꢁ=dt ¼ nk ½HNO ꢁ
ð3Þ
d
2
†
Reported in part at the 235th ACS National Meeting in New Orleans
To whom correspondence should be addressed. E-mail: bakac@ameslab.
Another common mechanism for oxidations with nitrous
*
þ
þ
)
2
acid involves acid-catalyzed formation of NO (or H NO
2
gov.
þ
followed by the oxidation of substrates with NO , eqs 4 and
(
1) Nitric Oxide. Biology and Pathobiology; Ignarro, L. J., Ed.; Academic:
þ
San Diego, 2000.
5. The rate law, eq 6, is first-order in each [H ], [HNO ], and
2
(
(
(
(
2) Neill, S. J.; Desikan, R.; Hancock, J. T. New Phytol. 2003, 159, 11–35.
3) Crawford, N. M. J. Exp. Bot. 2006, 57, 471–478.
4) Van Cleemput, O.; Samater, A. H. Fert. Res. 1996, 45, 81–89.
5) Vione, D.; Maurino, V.; Minero, C.; Pelizzetti, E.; Harrison, M. A. J.;
[substrate], which makes this mechanism kinetically distin-
guishable from the disproportionation mechanism above.
þ
þ
HNO
2
þ H a NO þ H
2
O
K
4
ð4Þ
ð5Þ
ð6Þ
Olariu, R.-I.; Arsene, C. Chem. Soc. Rev. 2006, 35, 441–453.
(6) Griffin, B. M.; Schott, J.; Schink, B. Science 2007, 316, 1870.
(7) Ford, P. C.; Laverman, L. E. Coord. Chem. Rev. 2005, 249, 391–403.
(8) Goldstein, S.; Czapski, G. Inorg. Chem. 1995, 34, 4041–4048.
(9) Roncaroli, F.; Videla, M.; Slep, L. D.; Olabe, J. A. Coord. Chem. Rev.
þ
NO þ substrate f products k5
þ
2
007, 251, 1903–1930.
-
d½substrateꢁ=dt ¼ k K ½HNO ꢁ½H ꢁ½substrateꢁ
5
4
2
(
(
(
(
10) Wolak, M.; van Eldik, R. Coord. Chem. Rev. 2002, 230, 263–282.
11) Bakac, A. Adv. Inorg. Chem. 2004, 55, 1–59.
12) Stedman, G. Adv. Inorg. Chem. Radiochem. 1979, 22, 113–170.
13) Pestovsky, O.; Bakac, A. Inorg. Chem. 2002, 41, 901–905.
Nitrogen dioxide, generated in eq 1, might be expected to
act as an oxygen atom donor toward convenient acceptors,
r 2009 American Chemical Society
Published on Web 06/22/2009
pubs.acs.org/IC

6
980 Inorganic Chemistry, Vol. 48, No. 14, 2009
Bakac et al.
but the available data do not support that notion.
Even though a number of cases of photochemically
induced oxygen atom transfer (OAT) from NO have
2
1
4-16
been published,
the data on thermal OAT to typical
oxygen acceptors are quite limited. Even when net
1
7
OAT does occur, as in the oxidation of thioethers or
1
8
olefins, the reaction requires initiation by electron
transfer. In the former reaction, the nitrosonium
þ
-
donor-acceptor complex [R S, NO ] NO is a critical
2
3
intermediate generated by thioether-induced dispropor-
tionation of NO2. The oxidation of olefins requires
electron transfer to generate olefin cation radicals,
1
7
18
followed by OAT from NO . We are not aware of any
2
kinetic data even for reactions with “standard” oxygen
atom acceptors, such as PPh3.
Experimental Section
,2’-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) dia-
The NO /PPh reaction to generate NO and OPPh is
2
3
3
2
thermodynamically favorable. For a gas-phase reaction,
0 0
mmonium salt ((NH ) ABTS, 98% pure), N,N,N ,N -tetra-
4 2
0
a standard enthalpy change ΔHr is about -50 kcal/mol,
methyl-para-phenylenediamine (TMPD, 99% pure), and
ninhydrinwerepurchasedfromSigmaAldrich. Diphenylpho-
sphinobenzene-3-sulfonic acid sodium salt, Na[P(C (me-
ta-SO -C H ] (NaTPPMS, >90% pure), was obtained from
as calculated from the standard thermochemical data
and assuming that the enthalpies of sublimation of PPh3
6 5 2
H )
and PPh O are not greatly different. Oxidation of PPh3
3
3
6
4
to the corresponding oxide is among the most common
and convenient reactions that allow one to gauge the
ability of an oxidant to act as an oxygen atom donor.
One-electron oxidation of PPh3, which will ultimately
TCI America and used as received. In-house distilled water
was further purified by passage through a Barnstead EASY-
pure III setup. The ionic strength was maintained at 0.10 or
1.0 M with HClO
Solutions of nitrous acid were generated in situ by addition
of the appropriate amount of 0.10 M HClO into a spectro-
and LiClO .
4
4
also yield the oxide OPPh , is limited to strong 1-e
3
4
oxidants because of the rather high reduction potential
•
þ
19-21
photometric cell containing sodium nitrite (99.999%, Fisher).
Gaseous NO (Matheson) was purified by passage through
Ascarite, sodium hydroxide and water. Stock solutions of
for the PPh3 /PPh couple.
3
The reaction of nitrogen monoxide with PPh , on the
24
3
other hand, has been studied, although not in aqueous
NO were prepared by bubbling the purified gas through
argon-saturated solutions of desired acidity (HClO ) for
30 min. Such a solution typically contained 1.7 mM NO
2
2,23
solutions.
The products, N O and OPPh , were gener-
2 3
4
2
ated in a kinetically third-order process, rate=k[PPh ][NO] ,
3
2
5
that exhibited strong dependence on solvent polarity and
phosphine basicity in a series of para-substituted triarylpho-
sphines.
The complete lack of data on the reactivity of NO2
toward oxygen atom acceptors, and the availability of
and 0.2-0.3 mM total nitrite. Hydroxylamine was detected
and quantified with ninhydrin, which reacts with primary
26-28
amines to yield the intensely blue product, λmax 580 nm.
The molar absorptivity of the product, determined in calibra-
tion experiments with known amounts of hydroxylamine, was
-1
-1
found to be 4910 M cm . The results were confirmed with
. The
NO/PPh data only in nonaqueous solutions prompted
3þ
3
another test that involved the reduction of Fe(phen)
3
us to examine the reaction of nitrous acid as a convenient
reaction solution was mixed with aqueous Fe(III) and phenan-
throline (1:3 ratio) in aqueous acetate buffer (pH 4.2), and the
absorbance increase was monitored at the 510 nm maximum
source of both NO and NO with a phosphine. Because
2
the biological chemistry of nitrite and nitrogen oxides
typically takes place in an aqueous milieu, the phosphine
that we selected for this study is the water-soluble
2
þ
4
-1
-1
of Fe(phen)3 (ε=1.14ꢀ10 M cm ). The rate constant
matched that measured independently with genuine NH OH
under the same conditions. The concentration of NH OH was
calculated from the absorbance increase using the indepen-
dently established 1:1 [Fe(III)]:[NH OH] stoichiometry.
Kinetic measurements were initiated byinjecting the desired
2
-
-
P(C H ) (3-SO -C H ) (hereafter TPPMS ). The re-
2
6
5 2
3
6
4
actions of nitrous acid with two one-electron donors,
tetramethylphenylene diamine (TMPD) and 2,2 -azino-
bis(3-ethylbenzothiazoline-6-sulfonate (ABTS ), were
also examined. The structures of various reagents used in
this work are shown below.
0
2
2
-
2-
-
substrate (ABTS , TMPD, or TPPMS ) into a spectropho-
tometric cell containing all the other reagents. The absorbance
change was monitored by conventional spectrophotometry
(Shimadzu 3101 PC) at a wavelength of maximum absorbance
of ABTS , TMPD , or TPPMS . The kinetics of the NO/
•-
•þ
-
-
TPPMS reaction were toofast for thismethodand required a
stopped flow (Applied Photophysics DX 17 MV). Ionic
strength was adjusted with HClO and LiClO . All of the
4 4
kinetic data were obtained at 25.0 ( 0.2 °C under air-free
conditions (argon atmosphere). Kinetic analyses were per-
formed with KaleidaGraph 3.6 PC software.
The solid NaTPPMS used in this work was found to
contain ∼9% (by molar ratio) unreactive impurities, most
(
14) Tanaka, N.; Oike, J.; Shibuya, K.; Kudoh, S.; Nakata, M. Res.
Chem. Int. 1998, 24, 893–903.
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(17) Bosch, E.; Kochi, J. K. J. Org. Chem. 1995, 60, 3172–3183.
(18) Bosch, E.; Kochi, J. K. J. Am. Chem. Soc. 1996, 118, 1319–1329.
(19) Yasui, S.; Kenji, I.; Ohno, A. Heteroatom Chem. 2001, 12, 217–222.
(20) Nakamura, M.; Miki, M.; Majima, T. J. Chem. Soc., Perkin Trans. 2
2
1
1
7
000, 1447–1452.
21) Shinro, Y.; Tsujimoto, M.; Kosei, S.; A., O. Chem. Ber./Recueil 1997,
30, 1699–1707.
22) Lim, M. D.; Lorkovic, I. M.; Ford, P. C. Inorg. Chem. 2002, 41,
026–1028.
23) Longhi, R.; Ragsdale, R. O.; Drago, R. S. Inorg. Chem. 1962, 1, 768–
70.
(
(
(
24) Pestovsky, O.; Bakac, A. J. Am. Chem. Soc. 2002, 124, 1698–1703.
25) Song, W.; Bakac, A. Chem.;Eur. J. 2008, 14, 4906–4912.
(
(26) Moore, S.; Stein, W. J. Biol. Chem. 1948, 176, 367.
(27) Moore, S.; Stein, W. J. Biol. Chem. 1954, 211, 907.
(28) Moore, S. J. Biol. Chem. 1968, 243, 6281.
(

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6981
a
Table 1. Spectral Data for Some Species in this Work
3
-1
-1
þ
ε (ꢀ10 M cm ) ([H ]/M)
species
λ=259 nm
λ=266 nm
λ=369 nm
-
a
TPPMS
9.69 (0.10)
11.2 (0.005)
1.59 (0.10)
-
TPP(O)MS
2.08 (0.10)
1.70 (0.005)
HNO
2
0.050 (0.10)
a
-
Mixture of TPPMS and TPP(H)MS.
likely moisture and water of crystallization. Such solutions
were only moderately stable toward autoxidation (∼5%
degradation in 24 h by UV-vis). Most experiments were
carried out with freshly prepared solutions and/or under
anaerobic conditions.
þ
Figure 1. Plot of Absobs/Abs
P
against [H ] for the protonation of
-
Molar absorptivities of TPPMS (λmax 259 nm) and its
oxidized form, TPP(O)MS , were obtained by oxidizing
-
-
TPPMS . Conditions: λ=259 nm, 100 μMTPPMS , 0.02- 0.10 M HClO ,
0.10 M ionic strength, argon atmosphere. The solid line is a fit to eq 7.
4
-
-
TPPMS (∼160 μM) with a limiting amount of hydrogen
•
þ
29
To avoid interference from the secondary TMPD /
HNO reaction, we studied the kinetics by the initial rate
method. Concentrations were varied over a wide range
and included runs with either one or the other reagent in
excess. The ionic strength was kept at 1.0 M. In the acidity
range used, 5 mM g [H ] g 2 mM, TMPD exists as a
mixture of singly and doubly protonated forms (Ka1 =
.3ꢀ10 M), and nitrous acid as a mixture of HNO
- -4
2 a
peroxide (33.5 μM, standardized with TiOSO ) under anae-
4
robic conditions in 0.10 M HClO . This experiment provided
2
4
3
-1
-1
the values of Δε259 =8.10 ꢀ 10 M cm (0.10 M HClO
4
)
3
-1
-1
and 9.5 ꢀ 10 M cm (5.0mMHClO
4
). Anothersample of
TPPMS was oxidized with a slight excess of H O . The
-
2
2
absorbance change at 259 nm was used to calculate the
absolute concentration of TPPMS in the sample, which
led to the molar absorptivities in Table 1. All of the TPPMS
þ
-
-
-3
34
6
2
solutions in this work were standardized by UV-vis spectro-
photometry at 259 nm.
35
and NO (K = 6.3 ꢀ10 M).
The initial rates were independent of the concentration
of TMPD, and linearly dependent on [HNO ] . By ex-
2
pressing the concentration of HNO in terms of total
2
The acidity constant KaP of TPP(H)MS was determined
from the effect of [H ] (0.01-0.10 M HClO ) on the absor-
þ
2
4
bance of the phosphine (100 μM) at 259 nm under anaerobic
-
conditions and 0.10 M ionic strength (HClO
4
þ NaClO
4
). The
concentration ([HNO2]tot = [HNO ] þ [NO ]), eq 8, one
2
2
data were fitted to eq 7, where Absobs represents the measured
absorbance in the presence of added H , Abs is the absor-
obtains the relation in eq 9, which suggests a plot de-
scribed by eq 10, where R stands for initial rate.
þ
P
þ
i
bance inthe absence of added H , andεHP and ε
absorptivities of TPP(H)MS and TPPMS , respectively.
P
are themolar
-
þ
½
H ꢁ
½
HNO2ꢁ ¼
½HNO2ꢁtot
ð8Þ
ð9Þ
þ
Kaþ½H ꢁ
Absobs
1
þ
¼
ꢀ ðKaP þ εHP=ε
P
ꢀ½H ꢁÞ
ð7Þ
þ
Abs
P
½H ꢁþKaP
!
Figure 1 shows the fit of the experimental data to eq 7 which
yielded KaP =0.49(0.05 M and εHP/ε =0.30(0.06. Similar,
but much less precise data were obtained in an earlier study at
2
þ
½H ꢁ
P
2
Ri ¼ kobs½HNO2ꢁ ¼ kobs
½HNO2ꢁtot
þ
ðK þ½H ꢁÞ
a
þ
30
.
variable ionic strength which resulted in H -dependent KaP
Results
þ
2
TMPD/HNO2 Reaction. Upon mixing of the two
reagents under air-free conditions, the solution turned
dark blue, indicative of the formation of the radical cation
R ðK þ½H ꢁÞ
i
a
2
tot
¼
kobs½HNO2ꢁ
ð10Þ
þ 2
½
H ꢁ
•þ
The fit, shown in Figure 2 yielded kobs = 12.3 (0.3 M-1 s-1.
TMPD . The kinetics were monitored at 610 nm where
the molar absorptivity of the radical cation is 8600 M
-
1
At the acid concentrations used, the values of k calculated
for the mechanism in eqs 1-3 from the parameters estab-
d
-
1 31
cm
.
The reaction stoichiometry, expressed as
•
þ
1
3
-1 -1
Δ[TMPD ]/Δ[HNO ], was cleanly 1.0 when TMPD
2
lished in our earlier work range from 11.2 M
s
(2 mM
þ
-1 -1
þ
(5 mM H ). The experimental value
derived above agrees with this calculation to within less than
was used in large excess. When TMPD was limiting, the
initial build-up of the blue radical cation was followed by
its disappearance. We interpret these data to mean that
H ) to 11.8 M
s
1
0% and strongly supports the mechanism in eqs 1-3.
Because NO does not react with TMPD, see below, the
observed 1:1 [TMPD]/[HNO ] stoichiometry requires that
each molecule of NO oxidizes two molecules of TMPD.
2
ABTS /HNO Reaction. Kinetic traces were exponen-
2
tial when [ABTS ] (0.2-0.8 mM) was present in large
the radical cation also reacted with HNO to generate a
2
2
þ 32
much less absorbing species, TMPD
with the known reaction between NO and TMPD , k =
, in agreement
2
•
þ
2
4
-1 -1
33
2
-
1
.3 ꢀ 10 M
s
at pH 5.5, 40% CH CN/H O.
3 2
2-
(
(
(
(
(
29) Pestovsky, O.; Bakac, A. J. Am. Chem. Soc. 2004, 126, 13757–13764.
30) Wright, G.; Bjerum, J. Acta Chem. Scand. 1962, 16, 1262–1270.
31) Rao, P. S.; Hayon, E. J. Phys. Chem. 1975, 79, 1063–1066.
32) Chaka, G.; Bakac, A. Dalton Trans. 2009, 318–321.
(34) Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783–1787.
(35) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Oxford University Press: New York, 1997.
33) Pestovsky, O.; Bakac, A. Inorg. Chem. 2003, 42, 1744–1750.

6
982 Inorganic Chemistry, Vol. 48, No. 14, 2009
Bakac et al.
The kinetic behavior changed at higher concentrations
of HNO when ABTS became the limiting reagent. The
2
-
2
appearance of the kinetic plots was close to second-order,
especially at low [H ] (<0.05 M). Initial rates were used
þ
to calculate the second-order rate constants kHNO2 under
the assumption that the mechanism in eqs 1-3 domi-
nates. Data analysis, analogous to that outlined above for
TMPD, yielded the second-order rate constants of 16.5
-
1
-1
þ
-1 -1
þ
at 0.05 M H ,
M
s
at 0.010 M H and 19.8 M
close to the expected values, 12.7 and 19.6 M
respectively. These calculations assumed an overall 1:1
s
-
1
-1
s
,
2
-
[HNO ]/[ABTS ] stoichiometry based on the lack of
2
reactivity of ABTS toward NO and the 2:1 [ABTS ]/
2
-
2-
Figure 2. Plot of the initial rate of formation of TMPD•þ, adjusted for the
proportion of [HNO ] present as nitrite, against the square of the total con-
2
centration of HNO ] =
[NO ] stoichiometry.
2
-
TPPMS /HNO Reaction. The stoichiometry was de-
2
2
-5
2
. Conditions: [TMPD]=(1-30)ꢀ10 M, [HNO
termined with either reagent in excess. In experiments
with excess HNO , the absorbance change corresponding
to the oxidation of TPPMS to TPP(O)MS was mea-
-5
þ
-3
(5 - 30) ꢀ 10 M, [H ] = (2.0 -5.0) ꢀ 10 M, ionic strength 1.0 M.
2
-
-
3
-1
-1
sured at 259 nm (Δε =8.10ꢀ10 M cm ), 290 nm
2
59
3
3
(
Δε290=3.48 ꢀ 10 ) or 300 nm (Δε =1.43 ꢀ 10 ). When
300
-
excess TPPMS was employed, the stoichiometry was
determined from the absorbance change at either 369 nm
(
-
1
-1
εHNO2=50 M cm ) or a convenient wavelength below
-
300 nm where TPPMS absorbs. Throughout the range of
concentrations used, i.e., 1.0 mM g [HNO ] g 0.02 mM
2
-
and 1.0 mM g [TPPMS ] g 0.1 mM, and regardless of
which reagent was in excess, the overall stoichiometry,
-
[
TPP(O)MS ] /Δ[HNO ], was in the range 1.9 ( 0.1.
inf
2
Consistent with this stoichiometry, the HNO -derived
product was hydroxylamine. In an experiment starting
with 1.0 mM TPPMS and 1.0 mM HNO in 5.0 mM
2
-
2
-
2
-
þ
]tot[H ] for the reaction of HNO
2
2-
HClO , all of the TPPMS was oxidized, and the final
4
Figure 3. Plot of kobs vs [ABTS
(
and 1.0 M ionic strength.
þ
concentration of NH OH was 0.51 ( 0.03 mM (average
0.01-0.1 mM) with excess ABTS (0.2-0.8 mM) at 0.20-0.80 M H
2
of two determinations) by ninhydrin method, and 0.41 (
0.01 by iron phenanthroline method. The reaction is thus
described by eq 11.
excess over [HNO ] (0.01-0.10 mM), and yielded pseu-
2
do-first-order rate constants that exhibited first-order
dependence on each [ABTS ] and [H ]. A plot of k
2
-
þ
þ
-
-
obs
HNO þ 2TPPMS þ H O f NH OH þ 2TPPðOÞMS
2
2
2
2
-
against the product [ABTS ]tot ꢀ [H ] at 1.0 M ionic
ð11Þ
strength, shown in Figure 3, yielded the value of the third-
order rate constant, k =695(16 M
-
2 -1
s . The concen-
tration of H was varied in these experiments over the
A
þ
The kinetic data were obtained by the method of initial
rates at 2.0-5.0 mM HClO . The rate of the formation of
range 0.20-0.80 M. Under these conditions, most of
ABTS exists as a singly protonated species, HABTS ,
4
2
-
-
-
TPP(O)MS was second order in [HNO ] and indepen-
2
3
pK =2.08. Surprisingly, when a correction was made
6
-
a
dent of [TPPMS ] as shown in Figure 4 which yielded k =
s , close to the calculated value of 4k
-
1 -1
for the protonated fraction, and the corrected value used
in a plot analogous to that in Figure 3, i.e., assuming that
only the dianion ABTS was reactive, the data strongly
39.0 ( 1.6 M
(defined in eq 1) in this acidity range (i.e., 45 M
d
-
1 -1
s
at 5 mM H ). The rate law is
at
2
-
þ
-1 -1
þ
2 mM H , and 47 M
given in eq 12.
s
2
-
deviated from linearity. Clearly, ABTS is not much
more reactive than HABTS .
The data are consistent with the mechanism shown in
-
-
-
d½TPPMS ꢁ=dt ¼ -2d½HNO2ꢁ=dt
2
-
-
eqs 4-6, with both ABTS and HABTS reacting with
þ
-7
-1 37
2
NO , and k =K k . For K =3ꢀ10
M
s . In view of the rate
, one obtains
A
4
5
4
¼ 4k ½HNO ꢁ
ð12Þ
d
2
9
-1 -1
k = (2.32 ( 0.05) ꢀ 10 M
5
constant being close to diffusion-controlled value, it is not
surprising that the reactivity is not greatly affected by
converting ABTS (E = 0.68 V) to a somewhat less
At higher acid concentrations, the appearance of the
traces was still second-order, but the rate constants
increased more than expected for the mechanism in eqs
2
-
0
-
0
36
2-
reducing HABTS (E =0.81 V).
1-3. As in the ABTS reaction, it appears that the
þ
mechanism in eqs 4-6 begins to contribute as [H ]
increases. These conditions were not explored further.
(
36) Scott, S. L.; Chen, W.-J.; Bakac, A.; Espenson, J. H. J. Phys. Chem.
993, 97, 6710–6714.
37) Bayliss, N. S.; Dingle, R.; Watts, D. W.; Wilkie, R. J. Aust. J. Chem.
963, 16, 933–942.
-
TPPMS /NO Reaction. The 2:1 stoichiometry was
determined from the absorbance changes at 259 nm
caused by the addition of limiting amounts of NO to an
1
1
(

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6983
-
2
-
Figure 4. Dependenceofinitialrate for the TPPMS /HNO
[
2
reaction on
]=(2-25)ꢀ
Figure 5. Plot of kobs versus [NO] for the reaction with TPPMS in
-
-5
-
-6
HNO
2
]. Conditions: [TPPMS ]=(2-30) ꢀ 10 M, [HNO
2
2.0 mM HClO
4
. [TPPMS ] = 5 ꢀ 10 M.
-
5
þ
1
0
M, [H ] = 2-5 mM, ionicstrength 0.10M. The line is afittosecond-
-
1 -1
order kinetics with k = 39.0 ( 1.6 M
s
.
satisfactory in view of the slowness of the step that
generated the NO. Despite all the precautions, some
loss of NO to traces of air over such long times is un-
avoidable.
-
excess of TPPMS . The formation of N O as reaction
2
product was confirmed by GC-MS. The overall reaction
is shown in eq 13.
-
-
Discussion
2
NO þ TPPMS f N2O þ TPPðOÞMS
The kinetics were studied under pseudo-first-order con-
ð13Þ
-
The NO/TPPMS reaction in water closely parallels the
22
NO/PPh reaction in organic solvents. In both cases the
3
-
ditions using a large excess of NO over TPPMS . A plot
of the observed pseudo-first-order rate constants against
overall stoichiometry is [NO]/[Phosphine] = 2, kinetics are
second order in NO, and N O is produced, as in eq 13,14. The
rates are, however, quite different; the oxidation of TPPMS
2
2
NO] is linear, Figure 5, confirming the second-order
-
[
dependence on [NO], eq 14, and yielding k = (5.14 (
in H O is orders of magnitude faster than the NO/PPh3
reaction in toluene and chloromethanes, Table 2. Even
though electron-donating substituents in the para-position
1
3
2
6
-2 -1
0
.20) ꢀ 10 M
s
.
-
-
2
have an accelerating effect on the PPh reaction (F = -1.5,
-
d½TPPMS ꢁ=dt ¼ k ½TPPMS ꢁ½NOꢁ
ð14Þ
TPPMS as Analytical Reagent for NO. The large rate
3
13
recalculated from data in ref 22 which reported F = -4.5),
-
even the most reactive phosphine studied, P(4-OCH -
3
3
-2 -1
constant and substantial associated absorbance changes
in the UV, see Table 1, are attractive features for an
analytical method for quantitative determination of small
concentrations of NO. This idea was tested by using
C H ) , has a rate constant of only 1.6 ꢀ 10 M
s
in
6
4 3
toluene.
-
The much greater reactivity of TPPMS appears to be
mostly solvent effect and not the electronic effect of a single
sulfonate group on one of the phenyls. We base this conclu-
sion on the magnitude of the effect, which is disproportio-
nately large when compared to that observed in P(4-X-
C H ) , see above. In those cases, the replacement of the
-
TPPMS to quantify NO generated in the TMPD/
HNO reaction.
2
On the basis of the observed 1:1 TMPD/HNO stoi-
2
chiometry, and the value of the rate constant for the
disappearance of TMPD approaching k , we concluded
6
4 3
d
most electron donating substituent (X = OCH ) with the
3
that only one of the nitrogen oxides generated in eq 1
reacted with TMPD. If both NO and NO were reactive,
most electron-withdrawing one (Cl) in all three phenyls
caused the rate constant in toluene to decrease 260-fold.
2
then both the stoichiometry and the kinetics would be
twice larger than observed. Moreover, TMPD does not
have the potential to reduce NO, so that one should
Upon introducing a sulfonate in a single phenyl in PPh and
3
changing the solvent to water, the rate constant increased
4
5
.6 ꢀ 10 -fold. Even though the sulfonate renders the phos-
expect NO to accumulate in TMPD-HNO solutions.
2
phine anionic, the electron density at phosphorus does not
-
4
The reaction between TMPD (1ꢀ 10 M) and HNO2
appear to be dramatically greater than in PPh , as shown by
3
-
5
(
5ꢀ 10 M) in 5.0 mM HClO at 0.10 M ionic strength
4
the similarity of acidity constants for the protonated forms of
3
8
was allowed to go to completion (9600 s), as established
by monitoring the absorbance increase at the 610-nm
maximum of TMPD . At that point, TPPMS was
PPh (ca. 1 M, estimated from the data in H O/CH CN)
3
2
3
-
and TPPMS (0.49 M, see above). The difference between
the acidity constants for (4-X-C H ) P (X=Cl, H) are much
þ
-
6
4 3
-
5
38
injected (final concentration 7.86 ꢀ 10 M) and the
greater, at least in the mixed H O/CH CN solvent.
2
3
reaction with NO was monitored at 259 nm. From the
The strong solvent effect as well as the electronic effects in
phenyl-substituted phosphines are both consistent with the
-
5
absorbance decrease, we obtained [NO] = 3.81 ꢀ 10 M,
6
-2
22
and from the initial rate we calculated k = 5.33 ꢀ 10 M
two mechanisms already considered in the literature, i.e.,
the reaction of the phosphine with NO dimer, eq 15, or the
formation of a phosphine/NO adduct followed by the reac-
tion with another molecule of NO, eq 16. This step generates
-
1
s
constant and that determined independently for the
. The excellent agreement between the measured rate
-
TPPMS /NO reaction confirms that the species reacting
with TPPMS was indeed NO. The concentration of NO
-
-
5
was 76% of that calculated (5ꢀ 10 M) from the 1:1
(38) Pestovsky, O.; Shuff, A.; Bakac, A. Organometallics 2006, 25, 2894–
TMPD/HNO stoichiometry. We consider the agreement
2
2898.

6
984 Inorganic Chemistry, Vol. 48, No. 14, 2009
Bakac et al.
a
Table 2. Summary of Kinetic Data Obtained in This Work
b
reaction
rate law
stoich.
1:2
conditions
products
-
c
-
2
þ
-
TPPMS þ NO
k[TPPMS ][NO]
k = (5.14 ( 0.20) ꢀ 10 M
2-5 mM H
CH Cl
CHCl
PhCH
2 - 5 mM H
TPP(O)MS þ N
2
O
6
-2 -1
s
d
2
PPh
PPh
PPh
3
3
3
þ NO
þ NO
þ NO
k[PPh
3
][NO]
1:2
1:2
1:2
2
2
OPPh
OPPh
OPPh
3
3
3
þ N
þ N
2
2
2
O
O
O
3
-2 -1
k = (1.57 ( 0.02) ꢀ 10 M
s
d
d
2
k[PPh
3
][NO]
3
3
-2 -1
k = (1.05 ( 0.01) ꢀ 10 M
s
2
k[PPh
3
][NO]
3
þ N
-
2 -1
k = 91 ( 1 M es
2
þ
•þ
TMPD þ HNO
2
k
k
k
d
[HNO
[HNO
2
]
]
1:1
1:1
1:1
TMPD þ NO
2
-
2 e
þ
þ
•-
ABTS þ HNO
2
d
2
2 - 10 mM H
0.2-0.8 M H
ABTS þ NO
2
-
þ
•-
ABTS þ HNO
2
A
[ABTS]tot[HNO
2
][H ]
ABTS þ NO
-
2 -1
k
k
A
= 695 ( 16 M
s
9
-1 -1 f
5
= (2.32 ( 0.05) ꢀ 10 M
s
-
2 e
-
TPPMS þ HNO
2
4k [HNO
d
2
]
2:1
2-5 mM
TPP(O)MS þ NH
2
OH
a
b
) aqueous solutions at 298 K, unless stated otherwise. For disappearance/formation of absorbing species (TPPMS , PPh
-
In acidic (HClO
4
3
,
ABTS , TMPD ). The rate ofdisappearance ofcoreactantsis related tothe given rate lawby the stoichiometricfactorgiven in third column. TPPMS =
•-
•þ
c
-
-
d
). 294 K, see ref 22. k
e
þ
-1 -1
f
, ref 13, see eq 1. For reaction of NO with ABTS /HABTS .
þ
2-
-
P(C
6
5 2
H ) (3-SO
3
-C
6
H
4
d
= (10.9 þ 175 [H ]) M
s
-
TPP(N O )MS intermediate, which eliminates N O and
pH 7, because neither reactant exhibits any acid/base chem-
istry in this pH range.
Regardless of the details, the fast NO/TPPMS reaction in
2
2
2
-
forms TPP(O)MS .
-
aqueous solutions does provide a convenient method for the
determination of NO, as demonstrated in the HNO /TMPD
2
K
dim
þTPPMS-,k15
-
2
NO a ðNOÞ2
f TPPðOÞMS þ N
2
O
ð15Þ
reaction which generates 1 equiv. of NO. 2
-
In its reactions with TMPD and ABTS , HNO utilizes
2
K
NO
þNO,k16
-
-
both mechanisms outlined in the Introduction. As expected,
NO þ TPPMS a TPPðNOÞMS
f
the mechanism in eqs 1-3 is prevalent at high [HNO ] and
2
þ
low [H ], i.e., conditions that favor second-order kinetics,
whereas high [H ] activates the mechanism in eqs 4-6.
Both ABTS and TMPD are known to be oxidized by
-
-
þ
-
½
TPPðN O ÞMSꢁ f TPPðOÞMS þ N O
ð16Þ
2
2
2
2
7
-1 -1
41
NO with rate constants 2.2 ꢀ 10 M
s
(pH 7) and 1.8 ꢀ
22
2
The preference for the second mechanism was expressed on
the basis of the small estimated equilibrium constant for the
formation of (NO) from NO in the gas phase (K < 3 ꢀ
8
-1 -1
33
1
0 M
s
(pH 5.5, 40% CH CN/H O), respectively. This
3 2
fast one-electron oxidation draws the disproportionation
equilibrium of eq 1 to the right and generates the observed
products. As one would expect, the NO that is also generated
2
dim
-4
-1 39
1
0
M ), and the assumption that Kdim has a similar
value in toluene. In the meantime, the binding energy for
NO) in the gas phase has been reported as only 696 cm ,
again suggesting that the equilibrium concentration of N O2
in NO is extremely small and would require an exceptionally
2-
in eq 1 does not react with either TMPD or ABTS , as shown
-1 40
(
2
by the overall 1:1 stoichiometry for the reaction of each with
2
HNO , and by finding close to one equivalent of NO in
2
solution after completion of the TMPD/HNO reaction.
5
-1 -1
2
large rate constant, ∼1 ꢀ 10 M
s
, for the (NO) /PPh3
þ
2-
2
The oxidation by NO of both ABTS and TMPD is
extremely rapid. The rate constant for ABTS /HABTS
reaction to explain the experimental data. On this basis, the
2-
-
(
NO) route would seem to be disfavored.
The much greater rate constant for the NO/TPPMS
reaction observed in this work would seem to strengthen
the antidimer case even more, provided that Kdim does not
increase greatly in water. If this is the case, then the rate
constant for the (NO) /TPPMS reaction would have to be
9
-1 -1
2
obtained here is 2.32 ꢀ 10 M
s
, and that for TMPD is
-
expected to also be close to diffusion controlled, given that
2-
TMPD is a much better reductant than ABTS . These rapid
þ
reactions with NO make the mechanism in eqs 4-6 func-
tional under the conditions that promote the formation of
NO , i.e., at high [H ].
Oxidation of TPPMS with HNO . In view of the
-
þ
þ
2
10
-1 -1
unreasonablyhigh, about 1 ꢀ 10 M
s
, whichwouldrule
is
-
2
out the (NO) route. Unfortunately, the value of K
2
dim
second-order dependence on HNO , some variant of the
2
probably not constant across various solvents, and may well
be much greater in water, leaving the (NO) mechanism a
weak) possibility.
We prefer the mechanism in eq 16, not only by disfavoring
eq 15, but also because the highly polar intermediate,
TPP (NO)2 MS] , would be stabilized by polar solvents,
thus explaining the strong positive effect of solvent polarity
on the rate.
The rate constant for the NO/TPPMS reaction was
mechanism in eqs 1-3 must be operative. In a temptingly
2
simple scenario, the disproportionation of HNO would
be followed by the reactions of TPPMS with the
2
(
-
nitrogen oxides formed. One might expect NO2 to
react by oxygen atom transfer, and NO as in eq 13. This
mechanism, summarized in Scheme 1, predicts a 1:1
δþ
δ-
-
[
stoichiometry and N O as product, clearly contradicting
2
experimental facts, i.e., 2:1 stoichiometry and formation
of NH OH with a rate constant approaching 4k .
-
2
d
determined in this work in 2-5 mM HClO , but the value
obtained should remain unchanged at higher pH, including
4
Undoubtedly, the second-order self-reaction of HNO2
is still involved, but subsequent chemistry deviates from
(
(
39) Forte, E.; Van den Bergh, H. Chem. Phys. 1978, 30, 325–331.
40) Wade, E. A.; Cline, J. I.; Lorenz, K. T.; Hayden, C.; Chandler, D. W.
(41) Forni, L. G.; Mora-Arellano, V. O.; Packer, J. E.; Willson, R. L. J.
Chem. Soc., Perkin Trans 2 1986, 1–6.
J. Chem. Phys. 2002, 116, 4755–4757.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6985
Scheme 1
Scheme 2
reduction of nitrite, where successive addition of elec-
trons and protons to coordinated nitrogen species
generates ammonia, hydroxylamine, and N O. In con-
2
the simple picture in Scheme 1. The reaction of the
phosphine with NO is known, and that with NO might
be reasonably assumed to involve intermediates. In the
NO reaction, the evidence comes from the kinetics.
Regardless of the exact nature of the intermediate, either
trast to those reactions, which are believed to be
initiated by coordination of nitrite to the metal fol-
lowed by coupled proton-electron transfers, the initi-
al interaction with the phosphine does not involve
2
-
HNO /NO but a product(s) of its bimolecular self-
2
2
(
NO) or phosphine-NO adduct, the reaction would be
2
reaction, as established by the second order rate law of
eq 12.
impeded at low, steady-state concentrations of NO, i.e.,
when the sole source of NO is eq 1. Under such condi-
tions, the NO-phosphine reaction may take a different
course than that shown in eq 13. In particular, the
intermediate may react with additional phosphine when
the supply of NO is extremely low. A similar situation
Conclusions
2
-
In the reactions with TMPD and ABTS , HNO2
þ
utilizes two mechanisms, disproportionation and NO -
mediated oxidation, either one of which can dominate
depending on the conditions. The disproportionation
would apply to the NO -phosphine adduct.
2
It is also feasible, and perhaps more likely, that N O ,
2
3
route generates NO and NO followed by the fast NO /
formed in the first step in Scheme 1, reacts with the
phosphine directly, without prior dissociation to NO
and NO . Several fast subsequent steps would follow to
2
2
substrate reaction. Nitrogen monoxide does not react with
either of these substrates, but instead accumulates in
2
-
solution, as confirmed with TPPMS which we found to
generate the observed products, Scheme 2. As pointed out
by a reviewer, the first two-electron step in N O /
TPPMS reaction may generate HNO followed by the
be a good analytical reagent for quantitative determina-
tion of NO in aqueous solutions. The reaction of nitrous
2
3
-
-
-
acid with TPPMS also involves bimolecular self-reaction
reaction of HNO with additional TPPMS to yield
NH OH, similar to the reaction of HNO with thiolates.
4
2
of HNO , but produces NH OH, suggesting rapid follow-
2
2
2
up steps between the phosphine and N O . No evidence
To some extent, our results also bear similarity to
metal-mediated enzymatic
2
3
4
3
44
was found for direct OAT from NO2.
and electrocatalytic
(
42) Wong, P. S.-Y.; Hyun, J.; Fukuto, J. M.; Shirota, F. N.; DeMaster,
E. G.; Shoeman, D. W.; Nagasawa, H. T. Biochemistry 1998, 37, 5362–5371.
43) Einsle, O.; Messerschmidt, A.; Huber, R.; Kroneck, P. M. H.; Neese,
F. J. Am. Chem. Soc. 2002, 124, 11737–11745.
44) Barley, M. H.; Takeuchi, K. J.; Meyer, T. J. J. Am. Chem. Soc. 1986,
08, 5876–5885.
Acknowledgment. We are grateful to the reviewers for
useful and constructive comments. This work was sup-
ported by a grant from National Science Foundation,
CHE 0602183. Some of the work was conducted with the
use of facilities at the Ames Laboratory.
(
(
1