An experimental and theoretical evaluation of the reactions of silver hyponitrite with phosphorus halides. In search of the elusive phosphorus-containing hyponitrites

Article abstract of DOI:10.1021/jp045984j

In the reaction of F₂PBr, F₂P(O)Br, (C ₆F₅)₂PBr, (CH₃)₂P(S)Br, and (CH₃)₂P(O)Cl with silver hyponitrite (AgON=NOAg), nitrous oxide (N₂O) and μ-oxo phosphorus species were obtained in all cases rather than the plausible hyponitrite alternative. Theoretical calculations of the geometries and expected decomposition pathways of the phosphorus-containing hypothetical hyponitrites were carried out at the B3LYP/6-311+G-(2df)//B3LYP/6-31+G(d) level. The cis-hyponitrite, XON=NOX (X=PF₂, OPF₂), is predicted to concertedly decompose to N₂ plus phosphorus-containing radicals (OPF₂, O ₂PF₂) or to N₂O plus the μ-oxo phosphorus species, X-O-X, (X=PF₂, OPF₂) with the former pathway having a smaller activation barrier (4.6 kcal/mol, X=PF₂; 10.5 kcal/mol, X=OPF₂). On the other hand, trans-hyponitrite can only decompose to N₂ plus the phosphorus-containing radicals, because there is a very high barrier for rearrangement to cis-hyponitrite. These results are in disagreement with experiment, because only the μ-oxo phosphorus species are observed. Reconciliation between experiment and theory is made for X=OPF₂ when a silver cation is included in the calculations. In THF (as a model for neat F₂P(O)Br), the silver cation is predicted to reverse the order of the two transition states through stronger interactions with the oxygen atoms in the transition state of the N₂O-producing pathway. Thus, Ag(I) is predicted to be not only catalytic for X=OPF₂ but also product-specific toward the μ-oxo products.

Full text of DOI:10.1021/jp045984j

1420
J. Phys. Chem. A 2005, 109, 1420-1429
An Experimental and Theoretical Evaluation of the Reactions of Silver Hyponitrite with
Phosphorus Halides. In Search of the Elusive Phosphorus-Containing Hyponitrites
Hyun Joo, M. A. Salam Biswas,† William E. Hill,* and Michael L. McKee*
Department of Chemistry and Biochemistry, Auburn UniVersity, Auburn, Alabama 36849
ReceiVed: September 6, 2004; In Final Form: NoVember 15, 2004
In the reaction of F2PBr, F2P(O)Br, (C6F5)2PBr, (CH3)2P(S)Br, and (CH3)2P(O)Cl with silver hyponitrite
(AgONdNOAg), nitrous oxide (N2O) and µ-oxo phosphorus species were obtained in all cases rather than
the plausible hyponitrite alternative. Theoretical calculations of the geometries and expected decomposition
pathways of the phosphorus-containing hypothetical hyponitrites were carried out at the B3LYP/6-311+G-
(2df)//B3LYP/6-31+G(d) level. The cis-hyponitrite, XONdNOX (XdPF2, OPF2), is predicted to concertedly
decompose to N2 plus phosphorus-containing radicals (OPF2, O2PF2) or to N2O plus the µ-oxo phosphorus
species, XsOsX, (XdPF2, OPF2) with the former pathway having a smaller activation barrier (4.6 kcal/
mol, XdPF2; 10.5 kcal/mol, XdOPF2). On the other hand, trans-hyponitrite can only decompose to N2 plus
the phosphorus-containing radicals, because there is a very high barrier for rearrangement to cis-hyponitrite.
These results are in disagreement with experiment, because only the µ-oxo phosphorus species are observed.
Reconciliation between experiment and theory is made for XdOPF2 when a silver cation is included in the
calculations. In THF (as a model for neat F2P(O)Br), the silver cation is predicted to reverse the order of the
two transition states through stronger interactions with the oxygen atoms in the transition state of the N2O-
producing pathway. Thus, Ag(I) is predicted to be not only catalytic for XdOPF2 but also product-specific
toward the µ-oxo products.
Introduction
hyponitrite, Me3SiONdNOSiMe3, and reported that thermolysis
gave N2, trimethylsilanol, and hexamethyl disiloxane.
Phosphorus-containing hyponitrites are conspicuous by their
absence in the chemical literature. Furthermore, no attempts to
synthesize these potentially interesting molecules have been
reported. This is in spite the potential use of such hyponitrites
to generate extremely interesting radicals (e.g., F2PO, F2PO2,
F2P(S)O). The current work deals with the attempts to prepare
such hyponitrites by reaction with silver hyponitrite as shown
in the following equation (eq 1):
The existence of organic hyponitrites is well-documented in
the chemical literature. These hyponitrites have been synthesized
by reactions of appropriate organic halides and silver hyponi-
trite.1-3 The hyponitrites are a source of alkoxy or aryoxy
radicals. The geometries of alkyl hyponitrites are trans-based
on vibrational spectroscopy,4,5 dipole moment measurements,6
and low-temperature X-ray crystallography.7 Decomposition is
thought to occur by homolytic scission with the loss of N2.8-10
The presence of the NdN bond has been confirmed by its
ultraviolet spectrum.11 The trans isomer has been found to be
the product obtained by usual preparative methods as confirmed
by Raman12-14 and infrared15-17 spectroscopy. Cis-hyponitrous
acid has been postulated as an unstable intermediate in the
reaction of hydroxylamine and nitrous acid18 and in the reduction
of nitrous acid by europium(II).19
2R2P(Y)X + AgONdNOAg f
2AgX + R2P(Y)ONdNOP(Y)R2 (1)
(R ) CH3, F; Y ) O, S; X ) halide)
A number of alkali metal, alkaline earth metal, mercury,
silver, and lead hyponitrites have been synthesized. Mercury
and silver hyponitrites are known to have a trans configuration,4
while sodium hyponitrite has been synthesized in both the cis
and trans geometries.20,21 Several transition metal hyponitrites
have been prepared, including the very interesting [Ni(dppf)-
N2O2], [Pd(dppe)N2O2], and [Pt(dppf)N2O2] hyponitrites, all of
which have cis-hyponitrite geometries and decompose by
unimolecular kinetics.21,22
In the thermal decomposition of hyponitrites, the experimental
evidence suggests that trans-hyponitrites decompose to N2 plus
other products, while cis-hyponitrites may decompose to N2O
plus products.23 A related issue is whether the decomposition
is concerted (eq 2a) or stepwise (eq 2b).
RONdNOR f 2RO• + N2
(2a)
RONdNOR f RO• + •NdNOR f 2RO• + N2 (2b)
Beck et al.13 reported that the decomposition of triphenyltin
hyponitrite, Ph3SnONdNOSnPh3, yielded N2O and (Ph3Sn)2O.
On the other hand, Wiberg et al.14 prepared trimethylsilyl
Neuman and Bussey determined the activation volume of the
trans-di-tert-butyl hyponitrite decomposition and suggested the
reaction follows the concerted pathway.10 For trans-hyponitrites,
structural and mechanistic studies are well-established.20,24 On
the contrary, mechanistic studies on cis-hyponitrites are rare.22
* Corresponding authors.
† Current address: Chemistry Department, Tuskegee University, Tuske-
gee, AL 36088.
10.1021/jp045984j CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/22/2005

Reaction of Ag Hyponitrite with P Halides
J. Phys. Chem. A, Vol. 109, No. 7, 2005 1421
To understand the different chemical reactivities of trans-
and cis-hyponitrites, theoretical calculations were carried out.
No previous calculations have been reported on the structures,
stabilities, or decomposition reaction pathways of phosphorus-
containing hyponitrites. Hybrid density functional theory,
B3LYP, has been applied to study the decomposition pathways
of XONdNOY (X, Y ) PF2, OPF2) to establish the probable
reaction mechanisms leading to the observed final products.
went to completion at this temperature. In the second batch,
the reaction was monitored by 31P NMR on a Bruker AM 400
instrument at -30 °C, using 0.0981 g (0.47 mmol) of the
bromide. From the previous low-temperature reaction, it was
thought that the reaction might have started well below -23
°C. Therefore, the reactants were mixed at bromobenzene-
liquid nitrogen temperature (-30 °C). The amount of silver
hyponitrite taken was 0.079 g (0.03 mmol). After mixing the
reactants for 20 min, a 2 mL portion of the solution was
transferred by a syringe into an NMR tube, and the spectrum
was recorded at -30 °C. Two peaks were found at 66.0 and
91.3 ppm. These peaks were assigned to dimethylthiophosphinic
bromide and (CH3)2P(S)OP(S)(CH3)2, respectively.32 The reac-
tion was allowed to go for 6 h. Again, a 2 mL portion of the
liquid from the flask was withdrawn by syringe, and the 31P
NMR spectrum was taken at -30 °C. Only one peak was
observed at 91.3 ppm, which corresponds to the chemical shift
of phosphorus in (CH3)2P(S)OP(S)(CH3)2.
Experimental Section
Standard vacuum-line techniques were used to purify and
transfer all volatile substances. Nonvolatile compounds were
handled in a nitrogen-filled glovebag containing P4O10 as a
desiccant.
Infrared spectra were determined on a Perkin-Elmer 983
spectrophotometer. Liquids were examined as neat samples;
1
solids were studied as Nujol and fluorolube mulls. 31P and H
NMR spectra were obtained at 162 and 400 MHz on a Bruker
AM 400 spectrometer. 31P chemical shifts were measured
relative to H3PO4 (85%, external) and 1H relative to TMS
(internal). Mass spectra were recorded on a Finnigan 3300 mass
spectrometer equipped with a gas inlet system and a solid probe.
Gaseous reactants were obtained from Matheson. Toluene and
benzene (dried over sodium) and acetonitrile (purified by
distillation over phosphorus pentoxide and collected over
molecular sieves) were obtained from Fisher. Thiophosphoryl
chloride was obtained from Aldrich and distilled prior to use.
Bromopentafluorobenzene was obtained from Columbia Organ-
ics and dried over 4A molecular sieves prior to use. Phosphorus
trichloride was obtained from Matheson, Coleman & Bell and
distilled prior to use. Silver hyponitrite was prepared by
the literature method.25 (CH3)2P(S)Br,26,27 (CH3)2P(O)Cl,28
F2P(O)Br,29 (C6F5)2PBr,30 and F2PBr31 were prepared by
literature methods.
Reaction of Dimethylthiophosphinic Bromide with Silver
Hyponitrite in 2:1 Ratio. (CH3)2P(S)Br (0.462 g, 2.67 mmol)
was placed in a 100 mL three-necked flask containing a
magnetic stirring bar. Toluene (30 mL) was added to the flask
to dissolve the bromide. Silver hyponitrite (0.382 g, 1.38 mmol)
was placed in a thumb-shaped solid addition tube and added
slowly over a period of 15 min at room temperature. The system
was protected from light by wrapping the flask with aluminum
foil. The collected gas (1.33 mmol) was shown to be N2O by
its infrared and mass spectra. The yellowish liquid was filtered
off, and the solvent was removed in vacuo to give 0.269 g of
solid, identified as (CH3)2P(S)OP(S)(CH3)2 by its 31P NMR and
infrared spectrum.
Several low-temperature reactions were carried out to see if
any hyponitrite, (CH3)2P(S)ONdNO(S)P(CH3)2, could be de-
tected. In one of those reactions, 0.219 g (1.27 mmol) of
dimethylthiophosphinic bromide was taken in a 100 mL flask
as described in the previous reaction, using 25 mL of toluene
and 0.205 g (0.74 mmol) of silver hyponitrite. The reaction flask
was placed in a dry ice acetone slush bath to keep the reaction
mixture at -78 °C. The silver hyponitrite was then added slowly
over a 10 min period. After 20 min, no volatiles were observed.
The reaction flask was then quickly transferred to a chloroben-
zene-liquid nitrogen slush bath (-45 °C). Again, no gas was
detected after 20 min. The reaction flask was then transferred
to a carbon tetrachloride-liquid nitrogen slush bath (-23 °C).
After 20 min, 2 Torr (V ) 396 mL) of pressure was observed.
The color of the reaction mixture did not change. The reaction
flask was then transferred to an ice bath. After a while, the
pressure observed was 13 Torr (V ) 396 mL). The reaction
The mass spectrum of the solid product showed the following
peaks: M+ ) [(CH3)2P(S)OP(S)(CH3)2+] 202 (44%); (M -
CH3)+ 187 (3%); (M - HS)+ 169 (27%); (M - CH3-S)+ 155
(3%); (CH3)2P(S)OH+ 110 (7%); (CH3)2PS+ 93 (100%); H2Cd
PS+ 77 (58%); PS+ 63 (45%); PO+ 47 (77%); PCH2+ 45 (51%);
CH3+ 15 (41%). Mass spectrum of the gaseous product: N2O+
44 (100%); NO+ 30 (35%).
The infrared spectrum of the gaseous sample is given as
follows (all values in cm-1). N2O: 2234 (s), 2214 (s), 1299
(s), 1272 (s).
Reaction of Dimethylphosphinic Chloride with Silver
Hyponitrite in 2:1 Ratio. Dimethylphosphinic chloride (0.331
g, 2.96 mmol) was dissolved in 20 mL of toluene (Na dried) in
a 100 mL three-necked flask equipped with a magnetic stirring
bar. Silver hyponitrite (0.440 g, 1.60 mmol) was placed in a
thumb-shaped tube with ground joint and fitted to the flask.
The flask was then evacuated on the vacuum line. The entire
flask including the thumb-shaped tube was wrapped with
aluminum foil to protect the contents from light. Silver
hyponitrite was then added to the solution over a period of 20
min. The heterogeneous mixture was stirred for 2 h at room
temperature. The flask was then connected to the vacuum line,
and the gaseous product was collected (1.46 mmol) at -196
°C. The gas was shown to be N2O by infrared and mass
spectrometry. The mixture in the three-necked flask was filtered,
and the solvent was evaporated from the filtrate under high
vacuum. The solid recovered was 0.247 g (98% yield) based
on the amount of (CH3)2P(O)Cl reacted. The 31P NMR of the
solid run in toluene showed only one signal at 49.6 ppm. The
mass spectrum of the compound (CH3)2P(O)OP(O)(CH3)2
was: M+ peak not observed; (CH3)2PO2H+ 94 (41%); (CH3)2-
+
+
P(O)H2 79 (100%); HPO+ 48 (4%); H2PO+ 49 (7%); PO2
63 (3%); (CH3)2PdO+ 77 (5%). The mass spectrum and the
infrared spectrum of the gas collected at -196 °C are identical
with the results reported in the preceding section. The infrared
spectrum of the solid in Nujol showed the following peaks (all
values in cm-1): 1305 (s) (PdO), 972 (vs) (PsOsP, asym.),
871 (s) (PsOsP, sym.), 723 (m) (PsC).
Reaction of Difluorophosphinic Bromide with Silver
Hyponitrite in 2:1 Ratio. The reaction of difluorophosphinic
bromide was performed in the absence of light. F2P(O)Br (9.20
mmol) was condensed into a Fisher-Porter tube charged with
silver hyponitrite (1.701 g, 6.20 mmol). The mixture was
warmed to room temperature and left for 2 h. The product was
passed through four traps at -78, -130, -160, and -196 °C.
The -78 and -130 °C traps each contained 4.40 mmol of gas

1422 J. Phys. Chem. A, Vol. 109, No. 7, 2005
Joo et al.
33
with a liquid mixture of F2P(O)Br and F2P(O)OP(O)F2 (as
indicated by infrared spectroscopy). There were no products in
the -160 °C trap. The -196 °C trap contained 4.0 mmol of a
gas identified as N2O by its mass and infrared spectra. The data
are the same as those obtained for N2O in previous reactions.
The infrared spectrum of the samples from the -78 and -130
°C traps showed the following absorptions (all values in cm-1):
1425 (s) (PdO) F2P(O)OP(O)F2; 1375 (ms) F2P(O)Br; 1325
(s) F2P(O)Br; 1075 (vs) (POP asym. stretch) F2P(O)OP(O)F2;
993 (s) (POP sym. stretch) F2P(O)OP(O)F2; 976 (vs) F2P(O)-
OP(O)F2; 942 (vs) F2P(O)Br; 899 (s) F2P(O)OP(O)PF2; 885 (vs)
F2P(O)Br; 560 (vs) F2P(O)Br.
SCHEME 1
The mass spectra showed the following peaks for the gas in
the -130 °C trap: F2P(O)Br+ 164 (91%), 166 (91%); FP(O)Br+
+
145 (37%), 147 (37%); F3PO+ 104 (91%); PF3 88 (21%);
F2PO+ 85 (100%); PF+ 50 (35%); PO+ 47 (71%).
Reaction of Bis(perfluorophenyl)bromophosphine and
Silver Hyponitrite in 2:1 Ratio. (C6F5)2PBr (1.171 g, 2.63
mmol) was dissolved in 25 mL of benzene (Na dried) in a three-
necked flask. The arrangement and precautions were the same
as the previous reactions. Silver hyponitrite (0.376 g, 1.36 mmol)
was added slowly over a period of 10 min, and the mixture
was stirred for 2 h at room temperature and then filtered. The
amount of product, recovered from the filtrate by removing the
solvent in vacuo, was 0.956 g. The gas evolved (1.27 mmol)
was N2O, identified by mass spectral and infrared data. The
31P NMR spectrum run in benzene showed a multiplet (apparent
nonet) at +93.4 ppm assigned to (C6F5)2POP(C6F5)2. The mass
spectrum of the white solid, (C6F5)4P2O, was as follows:
(C6F5)4P2O+ 746 (30%); C24F19P2O+ 727 (3%); (C6F5)3P2O+
579 (11%); (C6F5)3P+ 532 (25%); (C6F5)2PO+ 381 (92%);
(C6F5)2P+ 365 (50%); C12F8+ 296 (100%); C12F7+ 277 (11%);
C6F5PF+ 217 (58%); C6F5P+ 198 (22%); C6F4P+ 179 (15%);
considered in our computational study (numbering system is
given for C). Hyponitrite B was included to examine the effects
of asymmetric oxygen substitution on phosphorus. The Gaussian
03 program package35 was used for all calculations. The density
functional method was used with the B3LYP choice exchange
and correlation functionals, which has been shown to yield good
results for a number of inorganic systems.36 For geometry
optimization, the 6-31+G(d) basis set was used for F, N, O,
and P atoms, and the LANL2DZ37 effective core potential (ECP)
and basis set was used for the Ag+ ion. For transition states
with strong biradical character, we applied the unrestricted
broken-symmetry UB3LYP method with the same basis set. At
every optimized geometry, frequency calculations were made
using the same level of theory to characterize the stationary
points as minima (0 imaginary frequencies) or transition states
(1 imaginary frequency). The transition vector of every transition
state (TS) structure was animated with the MOLDEN program.38
If necessary, intrinsic reaction coordinates (IRC) were calculated
to connect a TS to the corresponding minima. After the reaction
profile was constructed, single-point calculations were per-
formed on stationary points with the larger 6-311+G(2df) basis
set for F, N, O and P and the standard SDD39 effective core
potential and basis set for Ag. For convenience, “SBS” was
used to indicate B3LYP/6-31+G(d)+LANL2DZ and “LBS” to
indicate B3LYP/6-311+G(2df)+SDD. The natural population
analysis (NPA) charges were obtained by using the NBO40
program implemented in the Gaussian program.
+
+
+
+
C6F4 148 (20%); C6F3 129 (95%); C5F3 117 (35%); CF3
69 (100%); PO+ 47 (8%). The infrared spectrum showed two
intense peaks at 909 and 640 cm-1 (PsOsP asym. and sym.).
The absorptions at 1465 (s) and 1102 (s) cm-1 are PsC
stretching modes, while the bands at 1377 (s), 1303 (s), and
1232 (s) cm-1 are CsF stretching bands. The N2O gas was
identified as previously described.
Reaction of Bromodifluorophosphine and Silver Hyponi-
trite in 2:1 Ratio. Silver hyponitrite (1.701 g, 6.2 mmol) was
placed in a Fisher-Porter tube wrapped in aluminum foil. F2PBr
gas (12.4 mmol) was condensed into the tube. The mixture was
warmed to room temperature and kept for 2 h. The reactor
contents were passed through -90, -130, -160, and -196 °C
traps. Nothing was collected in the -90 °C trap. The -130 °C
trap contained 5.4 mmol of gas. The infrared spectrum showed
it to be a mixture of F2POPF2 34 and F2PBr. The -160 °C trap
stopped 4.1 mmol of gas, which was identified as N2O from its
infrared and mass spectrum. The mass spectrum of the products
The reactions of F2PBr and F2P(O)Br with AgONdNOAg
were carried out in neat liquid. To estimate the effect of the
medium, single-point energy calculations were carried out using
the conductor polarizable continuum model (CPCM)41 at the
B3LYP/6-311+G(2df) level with the dielectric constant of
tetrahydrofuran (THF). While the dielectric constants of F2PBr
and F2P(O)Br are unknown, the dielectric constant of Cl3PO is
known to be 14.0, which is reasonably close to that of THF
(7.58).42
+
collected in the -130 °C trap was as follows: F2POPF2 154
(60%); F2POPF+ 135 (23%); F3PO+ 104 (13%); F2PO+ 85
(33%); PF2 69 (29%); PF+ 50 (65%); PO+ 47 (49%); P+ 31
+
(4%); F2PBr+ 148 (96%); F2PBr+ 150 (100%); FPBr+ 129
The structures and geometrical parameters of F2PONdNOPF2
(A), F2P(O)ONdNOPF2 (B), and F2P(O)ONdNOP(O)F2 (C)
species are given in Figure 1. Relative energies, enthalpies, and
free energies (kcal/mol) at the B3LYP/LBS//B3LYP/SBS level
of theory are given for A in Table 1, for B in Table 2, and for
C in Table 3. A table of spin-squared values, absolute energies,
and free energies of solvation in THF is available as Supporting
Information (Table S1) as well as Cartesian coordinates of all
structures in Tables 1-3 and 5 (Table S2). Reaction profiles of
free energies (kcal/mol) at 298 K at the B3LYP/LBS//B3LYP/
(29%); FPBr+ 131 (23%). The mass spectrum of the -160 °C
+
trap gas was as follows: F2POPF2 154 (37%); F3PO+ 104
(33%); F2PO+ 85 (41%); PF2+ 69 (100%); PF+ 50 (31%); PO+
47 (46%); N2O+ 44 (85%).
Computational Methods
The decomposition of the three phosphorus-containing hy-
ponitrite species listed in Scheme 1 (A, B, C), where A and C
were the synthetic targets of our experimental study, were

Reaction of Ag Hyponitrite with P Halides
J. Phys. Chem. A, Vol. 109, No. 7, 2005 1423
SBS level of theory are provided for A (Figure 2), B (Figure
3), and C (Figure 4).
phosphinyl O2sP2 bond length (1.623 and 1.680 Å, respec-
tively), while the O1sN1 and N2sO2 bond lengths change in
the opposite direction (1.450 and 1.407 Å, respectively) which
suggests a resonance contribution from P1dO1‚‚‚N1dN2d
O2‚‚‚P2.
Results and Discussion
The reactions of silver hyponitrite with (CH3)2P(S)Br,
(CH3)2P(O)Cl, F2P(O)Br, and F2PBr have been evaluated as
potential synthetic paths to the hyponitrites R2P(Y)ONdNOP-
(Y)R2 (Y ) O, S; R ) CH3, F) and R2PONdNOPR2 (R )
C6F5, F). In all cases, the reactions were carried out in the
absence of moisture and light (because silver hyponitrite is light-
sensitive). The gaseous products were identified by infrared and
mass spectra; the solids were identified by NMR, mass, and
infrared spectra. The results obtained in this series of reactions
may be expressed by the following general equations (eq 3):
The elimination of N2O can occur after the phosphorus-
containing group (F2P or (F2P(O)) has rotated around the N-O
bond. This allows interaction between a phosphorus center and
a remote oxygen center (i.e., P2 with O1 in AC-1, BC-1, or
CC-1 or P1 with O2 in BC-2). In AC-1 and BC-1, the interacting
phosphorus center is an -OPF2 group which is more electro-
philic than an -OP(O)F2 group. As a consequence, the
nonbonded P2-O1 interactions in AC-1 and BC-1 cause the
O2-P2 bond lengths to increase relative to AC and BC (1.667
f 1.729 Å and 1.680 f 1.756 Å for AC f AC-1 and BC f
BC-1, respectively). In turn, the free energy of activation values
for the elimination of N2O are small for AC-1 f AC-TS2 and
BC-1 f BC-TS2 (7.9 and 1.1 kcal/mol, respectively). On the
other hand, in BC-2 and CC-1, the -OP(O)F2 group is the
electrophile, which causes a much smaller increase in the P-O
bond length relative to BC and CC (1.623 f 1.646 Å and 1.634
f 1.656 Å for BC f BC-2 and CC f CC-1, respectively)
and leads to a larger free energy of activation for N2O
elimination (21.4, 17.6 kcal/mol for BC-2 f BC-TS5, CC-1
f CC-TS2, respectively).
2R2P(Y)X + AgONdNOAg f
2AgX + N2O + R2P(Y)OP(Y)R2 (3a)
(R ) CH3, F; Y ) electron pair, O, S; X ) halide)
2R2PX + AgONdNOAg f
2AgX + N2O + R2POPR2 (3b)
(R ) C6F5, F; X ) Br)
The transition states for rotation around the NdN bond which
interconverts cis- and trans-hyponitrites (XCT-TS, X ) A, B,
C) were located. The transition states, which have strong
biradical character ( S2 ) 1.01-1.02), are reached after about
92° rotation around the NsN bond, which breaks the NdN
π-bond and causes the NsN bond to elongate by about 0.1
Å. The activation enthalpies (298 K) for the cis f trans
rearrangement are very similar for AC, BC, and CC (50.7-
51.9 kcal/mol, Tables 1-3).
In all three cases, the cis isomers are found to be global
minima and trans isomers are about 2 kcal/mol higher in free
energy. Their potential energy surfaces are very similar along
the free energy reaction coordinates (Figures 2-4). Rotation
about the N-O bond in the cis isomer leads to a transition state
(XC-TS1, X ) A, B, C) and then an intermediate (XC-1, X )
A, B, C) followed by another barrier (XC-TS2, X ) A, B, C)
to yield N2O plus product.
Thus, no hyponitrites were isolated from the synthetic
pathway even when reactions were carried out at low temper-
atures. In all cases, µ-oxo phosphorus-containing species were
obtained. The change in substituents from electron-releasing
methyl groups to the highly electronegative F (eq 3a, R ) CH3,
F) or from tricoordinate phosphorus to pentavalent phosphorus,
P ) O or P ) S (eq 3a, Y ) electron pair, O, S) did not change
the reaction pathway. We had hoped that the postulated
hyponitrites formed in these reactions might have stability
comparable to tert-butyl hyponitrite. Because the stability of
tert-butyl hyponitrite has been attributed to its lack of an
R-hydrogen,2 we reasoned that the proposed hyponitrites of
phosphorus might have comparable stability to tert-butyl
hyponitrite because they also lacked an R-hydrogen.
Cis-hyponitrites are known to be less stable than trans-
hyponitrites43,44 and decompose with the elimination of N2.
Therefore, the fact that we do not observe a phosphorus-
containing hyponitrite might be due to an initially formed cis-
hyponitrite (which decomposes) or to the rearrangement of the
trans-hyponitrite to cis-hyponitrite under experimental condi-
tions. It is interesting to note that the electronegative fluorine
or less electronegative perfluorophenyl groups on the trivalent
phosphorus did not result in an identifiable hyponitrite. To
determine whether this might have a thermodynamic or kinetic
origin, we turned to theoretical calculations.
Optimized geometries for the hypothetical hyponitrites are
shown in Figure 1, and NPA charges are given in Table 4. For
all three molecules, the cis isomer has slightly shorter NdN
bond lengths (about 0.007 Å) and longer NsO bond lengths,
which indicate less π-delocalization in the cis compared to the
trans isomer (Figure 1, AC/AT, BC/BT, CC/CT). As the
phosphinyl (F2P) groups in F2PONdNOPF2 are oxidized to
phosphoryl (F2P(O)) groups (AC f CC), the OsP bond lengths
are shortened from 1.667 to 1.634 Å. The oxidation to
phosphoryl groups makes these phosphorus atoms more posi-
tively charged (III f V) and leads to a strong bonding
interaction between P1 and O1 (See Scheme 1 for numbering).
Interestingly, in the unsymmetrical F2P(O)ONdNOPF2 (BC),
the phosphoryl P1sO1 bond length is much shorter than the
The cis isomer is predicted not to form the trans isomer due
to the high free energy barriers for breaking the NdN bond
((XCT-TS, X ) A, B, C), 51.2-52.5 kcal/mol). We would
like to emphasize that these predictions are for unimolecular
rearrangement of the cis isomer. It is possible that the coordina-
tion of an electrophile to the cis isomer could lower the barrier
for cis T trans isomerization. For example, coordination of Ag+
to nitrogen could lower the barrier for cis T trans isomerization
by stabilizing the transition state (Figure 5).
The trans isomers are predicted to undergo concerted
synchronous fragmentation (XT-TS, X ) A, B, C) of both N-O
bonds to form N2 plus two radicals. Thus, if a trans isomer is
initially formed, it can only fragment to produce N2, because
rearrangement to the cis isomer is blocked by a high barrier
(unassisted reaction). On the other hand, the cis isomer can
fragment to form either N2 (with radicals formed) or N2O (with
no radicals formed) with competitive activation free energies
(Figures 2-4). For A and C, the pathway for N2 production is
4.6 and 10.5 kcal/mol lower than the pathway for N2O
production, while for B, the N2O production pathway is lower
than the N2 production by 3.0 kcal/mol. The N2O production
pathway for C is much higher than for A or B, because only

1424 J. Phys. Chem. A, Vol. 109, No. 7, 2005
Joo et al.
Figure 1. Calculated geometries for species on the following PES: F2PONdNOPF2 (A), F2P(O)ONdNOPF2 (B), and F2P(O)ONdNOP(O)F2 (C)
at the B3LYP/SBS level.
the -OP(O)F2 group (not the -OPF2 group) can attack the
remote oxygen atom.
Because no radical products are observed for A and C, the
N2O production pathway must be the preferred pathway under
our experimental conditions. If the phosphorus-containing
hyponitrite (A, B, or C) is an intermediate in the reaction, this
suggests the following: (1) the cis-hyponitrite is formed in the
reaction, and (2) the cis-isomer decomposes via the N2O

Reaction of Ag Hyponitrite with P Halides
J. Phys. Chem. A, Vol. 109, No. 7, 2005 1425
TABLE 1: Relative Enthalpies and Free Energies (kcal/mol)
on the Potential Energy Surface of F2PONdNOPF2 at the
B3LYP/LBS//B3LYP/SBS Level
TABLE 3: Relative Enthalpies and Free Energies (kcal/mol)
on the Potential Energy Surface of F2(O)PONdNOP(O)F2 at
the B3LYP/LBS//B3LYP/SBS Level
a
a
∆Ee
0.0
6.0
1.9
∆H0K
∆H298K ∆G298K ∆GTHF
∆Ee
0.0
10.0
2.0
∆H0K ∆H298K ∆G298K ∆GTHF
AC
AC-1
0.0
5.7
1.9
0.0
5.8
0.0
5.7
1.9
0.0
6.0
2.2
CC
0.0
9.5
0.0
9.6
2.0
0.0
8.5
1.9
0.0
8.2
2.1
CC-1b
AT
1.9
CT
1.9
AC-TS1
AC-TS2
AC-TS3
AT-TS
ACT-TS
N2 + 2OPF2
6.8
6.6
13.3
9.1
13.1
50.7
6.1
13.4
9.7
13.9
50.7
-46.5
-47.7
7.0
13.2
7.8
12.1
50.5
-69.2
-58.3
7.5
13.9
9.3
13.1
51.2
-66.2
-55.8
CC-TS1b
CC-TS2
CC-TS3
CT-TS
CCT-TS
N2 + 2O2PF2
9.7
9.1
8.7
9.4
9.3
14.4
11.5
15.5
53.3
29.9
19.5
24.7
54.3
28.9
16.5
21.5
51.8
28.8
17.2
22.2
51.9
29.3
15.1
19.9
51.5
25.8
15.3
16.3
52.3
-44.3 -47.3
-9.7 -15.0 -13.9 -36.7 -30.7
N2O + F2P(O)OP(O)F2 -49.8 -50.5 -50.0 -61.7 -59.2
N2O + F2POPF2 -47.4 -48.2
a Including solvation free energy calculated by CPCM method in
THF.
a Including solvation free energy calculated by CPCM method in
THF. b Geometry optimized at the B3LYP/cc-pVDZ level.
TABLE 2: Relative Enthalpies and Free Energies (kcal/mol)
on the Potential Energy Surface of F2P(O)ONdNOPF2 at the
B3LYP/LBS//B3LYP/SBS Level
activations for A, the calculations clearly predict N2 production
for C, in conflict with experiment. Thus, the reaction profile of
C constructed by considering all the thermodynamic corrections
and solvation free energies fails to explain the reaction mech-
anism. The question is why.
a
∆Ee
0.0
6.1
8.5
∆H0K ∆H298K ∆G298K ∆GTHF
BC
0.0
5.8
0.0
5.8
8.3
0.0
6.0
8.3
0.0
6.6
7.5
BC-1
BC-2b
8.2
The possibility that the silver cation might have a catalytic
effect on the decomposition of the cis-isomer of C was
considered. Optimized geometries of silver cation complexes
with C are given in Figure 6. The most stable Ag(I)-C complex
is not with CC (the global minimum) but rather with CC-1
due to the much stronger Ag(I) interactions with the PdO
oxygens (2.366 and 2.336 Å, Figure 6). Structural parameters
of AgCC+ show slightly longer bond lengths for O1sN1 and
N2sO2 than those of CC-1 but shorter than P1sO1 and O2sP2
bond lengths. Interestingly, OdP1, OdP2, and N1sN2 bond
lengths remain essentially the same.
BT
2.2
7.4
9.6
14.5
8.3
30.4
19.2
54.0
2.2
7.1
8.5
11.9
8.0
29.5
16.5
51.5
2.2
6.6
8.6
12.5
7.5
29.2
17.1
51.5
2.2
7.4
8.8
10.5
8.8
30.8
15.3
51.2
-51.7
-60.9
2.2
7.9
7.7
10.9
7.8
28.9
15.7
49.9
-47.4
-58.6
BC-TS1
BC-TS2
BC-TS3
BC-TS4b
BC-TS5
BT-TS
BCT-TS
N2 + OPF2 + O2PF2 -26.3 -30.4 -29.4
N2O + F2POP(O)F2 -49.5 -50.2 -49.8
a Including solvation free energy calculated by CPCM method in
THF. b Geometry optimized at the B3LYP/cc-pVDZ level.
The effect of Ag(I) on the potential energy surface (PES) of
C is shown in Figure 7 and Table 5. The binding enthalpy (298
K) of Ag+ to both PdO oxygen atoms of the bidentate ligand
production pathway. While the two pathways (N2 production
and N2O production) have very similar free energies of
Figure 2. Reaction profiles of free energies for the reaction of A in THF solution at the B3LYP/LBS//B3LYP/SBS level.

1426 J. Phys. Chem. A, Vol. 109, No. 7, 2005
Joo et al.
Figure 3. Reaction profiles of free energies for the reaction of B in THF solution at the B3LYP/LBS//B3LYP/SBS level.
Figure 4. Reaction profiles of free energies for the reaction of C in THF solution at the B3LYP/LBS//B3LYP/SBS level.
is 37.8 kcal/mol (Table 5) relative to the lowest conformation
(CC) of the ligand and 47.4 kcal/mol relative to the less stable
CC-1 conformation. In comparison, the experimental gas-phase
binding enthalpy45 (298 K) of the first two waters to Ag+ is
58.7 (33.3 + 25.4) kcal/mol. Calculated Ag+ binding enthalpies
to oxygen range from 32 to 38 kcal/mol (water, methanol,
ethanol, diethyl ether, and acetone).46 In THF (which models
the F2P(O)Br neat reaction environment), the Ag+ cation is
predicted to be bound relative to Ag+ + CC by 6.0 kcal/mol in
free energy (Figure 7).

Reaction of Ag Hyponitrite with P Halides
J. Phys. Chem. A, Vol. 109, No. 7, 2005 1427
TABLE 4: Natural Population Analysis (NPA) Charges for
Stationary Points at the B3LYP/SBS Levela
P1 P2 N1 N2
O1
O2 OdP1 OdP2 Ag
AC
AC-1
AT
AC-TS1
AC-TS2
AC-TS3
BC
1.73 1.73 0.13 0.13 -0.70 -0.70
1.75 1.71 0.11 0.18 -0.78 -0.71
1.73 1.73 0.11 0.11 -0.68 -0.68
1.74 1.70 0.14 0.14 -0.72 -0.69
1.72 1.76 0.17 0.31 -1.12 -0.48
1.80 1.80 0.13 0.13 -0.78 -0.78
2.64 1.74 0.12 0.15 -0.70 -0.68 -1.02
2.65 1.73 0.11 0.20 -0.79 -0.63 -1.02
2.60 1.75 0.16 0.16 -0.70 -0.69 -1.02
2.53 1.66 0.10 0.13 -0.64 -0.64 -0.97
2.64 1.71 0.13 0.16 -0.72 -0.67 -1.01
2.65 1.79 0.19 0.30 -1.07 -0.50 -1.05
2.62 1.85 0.13 0.13 -0.72 -0.78 -0.99
2.60 1.74 0.14 0.16 -0.71 -0.67 -1.02
2.56 1.79 0.29 0.20 -0.49 -1.03 -1.04
2.63 2.63 0.14 0.14 -0.67 -0.67 -1.01 -0.67
2.62 2.60 0.15 0.17 -0.68 -0.69 -0.99 -1.11
2.63 2.63 0.11 0.11 -0.65 -0.65 -1.01 -1.01
2.62 2.60 0.15 0.16 -0.67 -0.69 -0.99 -1.02
2.66 2.56 0.23 0.31 -1.07 -0.48 -1.02 -1.00
2.61 2.61 0.14 0.14 -0.69 -0.69 -0.96 -0.96
2.70 2.69 0.18 0.22 -0.72 -0.70 -1.11 -1.14 0.96
BC-1
BC-2
BT
BC-TS1
BC-TS2
BC-TS3
BC-TS4
BC-TS5
CC
CC-1
CT
CS-TS1
CS-TS2
CS-TS3
AgCC+
AgCC-TS2+ 2.69 2.65 0.33 0.37 -1.19 -0.48 -1.20 -1.03 0.95
AgCC-TS3+ 2.67 2.67 0.20 0.20 -0.80 -0.68 -1.04 -1.09 0.95
a See Scheme 1 for numbering.
Figure 5. Illustration of possible catalytic effect of Ag+ on the cis f
trans isomerization of F2PONdNOPF2.
The bound Ag+ has a dramatic effect on the free energies of
the two reaction pathways, N2-producing and N2O-producing.
The bound Ag(I) reduces the free energy barrier of the N2O-
producing pathway from 25.8 to 8.0 kcal/mol. On the other hand,
the bound Ag(I) reduces the free energy barrier of the N2-
producing pathway by only 1.7 kcal/mol (15.3 f 13.6 kcal/
mol, Figure 7). Thus, the Ag(I) reverses the relative free energies
of the two pathways compared to the Ag(I)-free reaction and
may explain why the N2O-producing pathway is observed
experimentally.
Figure 6. Optimized geometries of F2P(O)ONdNOP(O)F2 with Ag-
(I) at the B3LYP/SBS level.
RSE is defined in this work as the enthalpy change of the
isodesmic reaction eq 4
HO-H + XO f XO-H + OH
X ) PF2, OPF2
(4)
The relative stabilities of possible radical products including
PF2, OPF2, and O2PF2 were also examined (Table 6). The PF2
radical has been observed and computed,47 but the OPF2 and
O2PF2 radicals have not yet been reported. Some experimental
evidence for the existence of O2PF2 has been given.48 From
the first four reaction enthalpies (298 K) in Table 6, it can be
seen that the O-P bond enthalpies vary from 70.8 to 96.8 kcal/
mol. This variation can be understood from differences in radical
stabilization energies (RSE), or, by the fact that the unpaired
electron density reorganizes from the nascent radical fragment
to the final radical product. From a comparison of the spin
densities and NPA charges (Table 7), it is clear that almost all
of the unpaired spin density is on phosphorus in PF2 and OPF2,
while it is distributed between the two oxygen atoms in O2PF2.
Thus, the initial radical formed by breaking the N-O bond in
N-OPF2 will undergo significant radical stabilization to form
a radical with unpaired spin on the phosphorus center, while
the initial radical formed by breaking the O-P or N-O bond
in O-PF2 or N-OP(O)F2 will undergo little radical stabilization.
where RSE ) ∆Hrxn ) ∆Hproducts - ∆Hreactants. From this
isodesmic reaction, the RSE of OPF2 was determined to be 24.2
kcal/mol, which is much larger than the value for O2PF2 (4.2
kcal/mol at the B3LYP/LBS//B3LYP/SBS level including ZPC
and Cp corrections to 298 K).
The much larger RSE of OPF2 compared to O2PF2 explains
the large change in reaction exothermicities in the N2-producing
pathways of AC, BC, and CC (Tables 2-4: -46.5, -29.4, and
-13.9 kcal/mol, respectively). The N2-producing pathway of
AC breaks two N-OPF2 bonds, compared to one N-OPF2 bond
for the BC reaction and none for the CC reaction. The difference
in reaction exothermicities (∆∆Hrxn) is 17.1 kcal/mol for the
BC/AC reactions and 15.1 kcal/mol for the CC/BC reactions,
which approximately equals the difference between the RSEs
of OPF2 and O2PF2 (20.0 kcal/mol).
The variation in the activation enthalpies for the N2-producing
pathway for XNdNX f 2X + N2, X ) OPF2, OP(O)F2, can
also be understood in terms of the RSE of the radicals
•
produced: •OPF2 and OP(O)F2 (Table 8). When two OPF2

1428 J. Phys. Chem. A, Vol. 109, No. 7, 2005
Joo et al.
Figure 7. Comparison of the free energy reaction profiles for C in THF with C catalyzed by Ag(I) in THF.
TABLE 5: Binding Energies, Enthalpies, and Free Energies
TABLE 8: Kinetic and Thermodynamic Data (kcal/mol) for
the Concerted N2-Producing Pathwaya
a
(kcal/mol) of Ag(I) with F2P(O)ONdNOP(O)F2
b
∆Ee
∆H0K
∆H298K
∆G298K
∆GTHF
trans-XNdNX
X )
∆Ha(298 K)
∆Hrxn
N2-Producing
(298 K)
RSEb
AgCC+
37.9
55.3
41.8
37.6
54.7
41.0
37.8
54.4
41.3
30.1
48.4
33.2
6.0
23.8
7.7
AgCC-TS2+
AgCC-TS3+
OPF2
O2PF2
OR
12.0
-48.4
-15.9
2‚24.2 ) 48.4
2‚4.2 ) 8.4
∼30d
20.2
25-30c
a Calculated at B3LYP/LBS+CPCM//B3LYP/SBS level. b Including
solvation free energy calculated by CPCM method in THF.
a Calculated with respect to the trans-hyponitrite isomer. See Tables
1-3. b From eq 4. c See ref 49 where RdCH3, CH3CH2, (CH3)2CH,
and (CH3)3C. d The RSE for OCH3 and OCMe3 are computed from eq
4 to be 15.8 and 14.0 kcal/mol, respectively. The N2-producing pathway
would produce two OR radicals which would give a total RSE of about
30 kcal/mol.
TABLE 6: Reaction Enthalpies and Free Energies (kcal/
mol) for Radical Recombinationsa
b
bond energy
∆Ee ∆H0K ∆H298K ∆G298K ∆GTHF
PF2 + OPF2 f F2P-OPF2
OPF2 + OPF2 f F2PO-P(O)F2
PF2 + O2PF2 f F2PO-P(O)F2
-72.2 -70.6 -70.8 -58.4 -58.8
-83.0 -80.7 -81.0 -68.7 -67.7
-92.3 -89.5 -89.8 -78.4 -80.5
hyponitrites RONdNOR, R ) CH3, CH3CH2, (CH3)2CH,
(CH3)3C, which are known to produce N2 upon decomposition47
with experimental barriers of 25-30 kcal/mol.49 The RSEs of
RO, RdCH3, and RdCMe3 are calculated to be 15.8 and 14.0
kcal/mol, respectively (Table 8). It is probable that the N2-
producing barrier of F2PONdNOPF2 (12.0 kcal/mol, calcd) is
lower than the H3CONdNOCH3 and Me3CONdNOCMe3
barriers (25-30 kcal/mol, exptl) due to the larger RSE of OPF2
compared to OCH3 and OCMe3 (Table 8).
OPF2 + O2PF2 f F2P(O)-OP(O)F2 -100.0 -96.5 -96.8 -84.5 -84.8
OPF2 + OPF2 f F2P(O)-P(O)F2
OPF2 + OPF2 f F2PO-OPF2
-63.5 -60.8 -61.1 -47.9 -46.0
7.2 8.0 8.0 20.6 21.2
a Calculated at B3LYP/LBS//B3LYP/SBS level. b Including solvation
free energy calculated by CPCM method in THF.
TABLE 7: Atomic Mulliken Spin Densities and NPA
Charges of Radical Species Calculated at the B3LYP/SBS
Level
PF2
OPF2
O2PF2
spin NPA
density charge density charge density charge
Conclusion
radical
atom
spin
NPA
spin
NPA
The reactions of neat F2PBr and F2P(O)Br with silver
hyponitrite (AgONdNOAg) did not produce the expected
phosphorus-containing hyponitrites, F2PONdNOPF2 and F2P-
(O)ONdNOP(O)F2, but rather N2O and µ-oxo phosphorus
species. A computational study of XONdNOY, X, Y ) PF2,
P(O)F2, using density functional theory was carried out to
explain the absence of any phosphorus-containing hyponitrites.
Single-point energies (B3LYP/6-311+G(2df)//B3LYP/6-31+G-
(d)) using the CPCM method with the dielectric constant of
P
F
O
1.02
-0.01
1.17
-0.58
0.77
-0.01
0.25
2.03
-0.55
-0.93
-0.09
0.01
0.53
2.57
-0.54
-0.74
radicals are formed by the N2-procducing pathway, the activation
barrier is smaller than when two O2PF2 radicals are produced
(12.0 compared to 20.2 kcal/mol, Table 8) due to the much
larger RSE of the OPF2 radicals. Support for this interpretation
comes from the experimental activation enthalpies of organic

Reaction of Ag Hyponitrite with P Halides
J. Phys. Chem. A, Vol. 109, No. 7, 2005 1429
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Supporting Information Available: Spin-squared values
at the B3LYP/SBS level, zero-point energies (kcal/mol), heat
capacity corrections to 298 K (kcal/mol), entropies (cal/mol‚
K), free energies of solvation in THF (kcal/mol), and total
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coordinates of all optimized structures in Tables 1-3 and 5 at
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