Decomposition of fluorophosphoryl diazide: A joint experimental and theoretical study

Article abstract of DOI:10.1039/c4cp05694a

The photolytic and thermal decomposition of fluorophosphoryl diazide, FP(O)(N₃)₂, was studied using matrix isolation spectroscopy. Upon ArF laser photolysis (λ = 193 nm), FPO and a new geminal azido nitrene FP(O)(N₃)N were identified using matrix IR spectroscopy. The nitrene shows a triplet ground state with the zero-field parameters D/hc = 1.566 cm^-1 and E/hc = 0.005 cm^-1. Further decomposition of the nitrene into FPO was observed under an irradiation of λ > 335 nm. In contrast, no nitrene but only FPO was identified after flash vacuum pyrolysis of the diazide. To reveal the decomposition mechanism, quantum chemical calculations on the potential energy surface (PES) of the diazide using DFT methods were performed. On the singlet PES four conformers of the nitrene were predicted. The two conformers (syn and anti) showing intramolecular N_nitrene...N_α,azide interactions are much lower in energy (ca. 40 kJ mol^-1, B3LYP/6-311+G(3df)) than the other two exhibiting N_nitrene...O interactions. syn/anti refers to the relative orientation of the PO bond and the N₃ group. The interconversion of these species and the decomposition into FPO via a novel three-membered ring diazo intermediate cyclo-FP(O)N₂ were computationally explored. The calculated low dissociation barrier of 45 kJ mol^-1 (B3LYP/6-311+G(3df)) of this cyclic intermediate rationalizes why it could not be detected in our experiments. This journal is

Full text of DOI:10.1039/c4cp05694a

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Decomposition of fluorophosphoryl diazide: a
joint experimental and theoretical study†
Cite this:DOI: 10.1039/c4cp05694a
Dingqing Li,a Hongmin Li,a Bifeng Zhu,a Xiaoqing Zeng,*a Helge Willner,b
Helmut Beckers,*c Patrik Neuhaus,d Dirk Groted and Wolfram Sanderd
The photolytic and thermal decomposition of fluorophosphoryl diazide, FP(O)(N3)2, was studied using
matrix isolation spectroscopy. Upon ArF laser photolysis (l = 193 nm), FPO and a new geminal azido
nitrene FP(O)(N3)N were identified using matrix IR spectroscopy. The nitrene shows a triplet ground state
with the zero-field parameters |D/hc| = 1.566 cmÀ1 and |E/hc| = 0.005 cmÀ1. Further decomposition of
the nitrene into FPO was observed under an irradiation of l 4 335 nm. In contrast, no nitrene but only
FPO was identified after flash vacuum pyrolysis of the diazide. To reveal the decomposition mechanism,
quantum chemical calculations on the potential energy surface (PES) of the diazide using DFT methods
were performed. On the singlet PES four conformers of the nitrene were predicted. The two conformers
(syn and anti) showing intramolecular NnitreneÁÁÁNa,azide interactions are much lower in energy (ca. 40 kJ molÀ1
,
B3LYP/6-311+G(3df)) than the other two exhibiting NnitreneÁÁÁO interactions. syn/anti refers to the relative
orientation of the PQO bond and the N3 group. The interconversion of these species and the decomposition
into FPO via a novel three-membered ring diazo intermediate cyclo-FP(O)N2 were computationally explored.
The calculated low dissociation barrier of 45 kJ molÀ1 (B3LYP/6-311+G(3df)) of this cyclic intermediate
rationalizes why it could not be detected in our experiments.
Received 6th December 2014,
Accepted 21st January 2015
DOI: 10.1039/c4cp05694a
www.rsc.org/pccp
DEST values become relatively smaller, and in some cases a
singlet ground state has been suggested by quantum chemical
Introduction
Nitrene intermediates play an important role in organic synthesis calculations and proved by experimental observations.9 Such a
and biology.1,2 Photolytic or thermal decomposition of covalent significant electronic change was rationalized by an intramolecular
azides is a general method for producing highly reactive nitrenes by interaction between the nitrene center and the adjacent oxygen atom
releasing molecular nitrogen. The formation and reactions of (NnitreneÁÁÁO), which stabilizes the close-shell singlet by forming a
nitrenes have been attractive topics for extensive theoretical and cyclic oxazirine-like structure (Scheme 1).10–15
experimental studies in the past few decades.3–6
Such intramolecular interactions have also been proposed
The electronic properties of nitrenes largely depend on for the related singlet a-oxo nitrene intermediates R2P(O)N16–18
substitution. The parent nitrene HN and simple alkyl nitrenes and RS(O)2N.19–22 Even though a triplet ground state has recently
(RN) have triplet ground states, where two unpaired electrons been confirmed for the sulfonyl and phosphoryl nitrenes,
are localized on the hypovalent nitrene center. Large singlet– FS(O)2N19,20 and F2P(O)N,16 respectively, by matrix-isolation
triplet energy gaps (DEST) were estimated for these nitrenes by spectroscopy (IR, UV/Vis, and EPR), these oxo nitrenes are liable
calculations.7,8 However, in carbonyl nitrenes (RC(O)N) the to an exceptional photo-induced oxygen-shift isomerization from
XnE(O)N to XnENO (XnE = F(O)S and F2P). It has been concluded
that such a unique oxygen-shift Curtius-rearrangement will be
a College of Chemistry, Chemical Engineering and Materials Science,
facilitated by significant intramolecular NnitreneÁ Á ÁO interactions
in their lowest-lying singlet excited state (Scheme 1).16,19,20
Soochow University, 215123 Suzhou, China. E-mail: xqzeng@suda.edu.cn
b
¨
FB C-Anorganische Chemie, Bergische Universitat Wuppertal, 42119 Wuppertal,
Germany
c
¨
¨
Institut fur Chemie und Biochemie, Freie Universitat Berlin, 14195 Berlin,
Germany. E-mail: beckers@zedat.fu-berlin.de
d
¨
¨
Lehrstuhl fur Organische Chemie II, Ruhr-Universitat Bochum, 44780 Bochum,
Germany
† Electronic supplementary information (ESI) available: Observed matrix IR and
EPR spectra, calculated vibrational displacement vectors for the imaginary
frequencies of the transition states, and atomic coordinates for all species
discussed. See DOI: 10.1039/c4cp05694a
Scheme 1 Intramolecular O - N interactions in singlet a-oxo nitrenes.
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Matrix IR spectroscopy
Matrix-IR spectra were recorded on a FT-IR spectrometer in a
reflectance mode using a transfer optic. A KBr beam splitter
and an MCT detector were used in the spectral region of 4000–
550 cmÀ1. A Ge-coated 6 mm Mylar beam splitter combined with
a liquid helium-cooled Si bolometer and a CsI window at the
cryostat were used in the region of 700 to 200 cmÀ1. For each
spectrum, 200 scans at a resolution of 0.25 cmÀ1 were co-added.
Gaseous FP(O)(N3)2 was mixed with argon (1 : 1000) in a 1-L
stainless-steel container and then small amounts (ca. 1 mmol)
of the mixture were directed through an aluminium oxide
furnace and deposited within 30 minutes onto the cold matrix
support (16 K, Rh-plated Cu block) in a high vacuum. Details of
the matrix apparatus have been described elsewhere.29
27
Scheme 2 Possible decomposition pathways for OC(N3)2
FP(O)(N3)2.
and
Apart from nitrenes, decomposition of covalent polyazides was
utilized to produce some long-sought species.23,24 The presence of
an azide group in the initially generated nitrenes may alter their
electronic properties due to the lone-pair electrons on the nitrogen
atom at the a position of the azide group. This might provide access
to some synthetically difficult molecules. One recent example is the
synthesis of diazirinone (cyclo-N2CO) via pyrolysis of carbonyl diazide
(OC(N3)2),25,26 where the geminal azido nitrene (N3C(O)N) was
proposed to be involved as an intermediate (Scheme 2).27 Although
the intriguing reaction mechanism for the formation of cyclo-N2CO
from N3C(O)N has not yet been revealed, the possible formation of
novel small-ring systems from geminal diazides stimulates us to
extend our studies to the photolytic and thermal decomposition of
other main-group covalent azides.
In this work, we have studied the photolytic and thermal
decomposition of fluorophosphoryl diazide, FP(O)(N3)2, by matrix
isolation spectroscopy. This diazide is of particular interest in view
of the present study, since a stepwise N2 elimination might provide
access to two metastable intermediates, the geminal azido phos-
phorylnitrene FP(O)(N3)N, and the cyclic phosphadiazirine deriva-
tive FP(O)N2 (Scheme 2). Furthermore, the azido phosphorylnitrene
might be stabilized by two competing intramolecular interactions
to the hypovalent nitrene center, involving either the polar phos-
phoryl oxygen or the lone-pair electrons on the a-nitrogen atom of
the azido ligand.
Matrix EPR spectroscopy
For electron paramagnetic resonance (EPR) measurements, the
matrices were deposited on an oxygen-free high-conductivity
copper rod (75 mm in length, 3 mm in diameter) that was
cooled using a Sumitomo SHI-4-5 closed-cycle 4.2 K cryostat.
The vacuum system consisted of a vacuum shroud equipped
with a sample inlet valve and a half-closed quartz tube (75 mm
length, 10 mm diameter) at the bottom, and a vacuum-pump
system with a Pfeiffer Vacuum TMU071P turbo pump backed by
a Leybold two-stage, rotary-vane pump. During deposition, the
inlet port was positioned at the same height as the tip of the
copper rod, and for the measurement of EPR spectra the entire
apparatus was moved downwards so that the quartz tube and
copper rod were positioned inside the EPR cavity. X-band EPR
spectra were recorded using a Bruker Elexsys E500 ESR spectro-
meter equipped with an ER077R magnet (75 mm pole cap
distance), and an ER047 XG-T microwave bridge. Computer
simulation of EPR spectra was performed using the XSOPHE
computer simulation software suite (version 1.0.4),30 developed
by the Centre for Magnetic Resonance and Department of
Mathematics, University of Queensland, Brisbane (Australia)
and Bruker Analytik GmbH, Rheinstetten (Germany).
Flash vacuum pyrolysis and photolysis
Small amounts of the argon diluted (1 : 1000) sample were
passed through an aluminium oxide furnace (i.d. 1.0 mm,
o.d. 2.8 mm, length 25 mm), which was heated (voltage 5.6 V,
current 2.0 A) to about 1000 1C over a length of ca. 10 mm using
a tantalum wire (o.d. 0.25 mm, resistance 1.0 O). The residence
time in the hot zone is estimated to be of the order of a few
milliseconds. Laser photolysis was carried out using an ArF
excimer laser (Lambda-Physik, 2 mJ, 5 Hz). UV/Vis irradiation
was performed using a high-pressure mercury lamp (TQ 150,
Heraeus) by passing the light through water-cooled quartz
lenses combined with cutoff filters (Schott).
Experimental methods
Caution! Covalent azides are in general explosive. Although no
explosion was encountered with FP(O)(N3)2 during this work, it
should be handled with care in millimolar quantities (o5 mmol)
and appropriate safety precautions should be taken.
Sample preparation
Fluorophosphoryl diazide, FP(O)(N3)2, was prepared by the
reaction of F2P(O)Cl with NaN3 and purified according to the
literature.28 For the preparation of a 15N-labeled sample, 1-15N
sodium azide (98 atom% 15N, EURISO-TOP GmbH) was used
Computational details
as received. For the preparation of an 18O-labeled sample, Structural optimizations were performed using density functional
18O-enriched F2P(O)Cl (95 atom% 18O) was used.16 The purity theory (DFT) methods of B3LYP,31 BP86,32 MPW1PW91.33 The basis
of the samples was checked using IR spectroscopy.
set of 6-311+G(3df) was employed throughout. The complete basis
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set method (CBS-QB3)34 was also used for the calculation of bands of the diazide precursor were not affected by the irradiation.
relative energies. Local minima were confirmed by vibrational As can be seen in the spectrum shown in Fig. 1 (lower trace), the
frequency analysis, and transition states by additional intrinsic carrier of these bands exclusively produces FPO upon 4335 nm
reaction coordinate (IRC) calculations.35,36 All calculations were photolysis. Attempts to increase the yield of this intermediate by
performed using the Gaussian 03 software package.37
prolonged laser photolysis were unsuccessful due to its simulta-
neous decomposition into FPO. In addition to the band at
2187.5 cmÀ1, three weaker absorption bands at 1315.1, 1266.1,
and 923.6 cmÀ1 can be unambiguously identified. Interestingly,
these band positions are very close to those of the diazide precursor
(nas(N3): 2199.5 and 2178.3 cmÀ1, n(PO): 1338.7 cmÀ1, ns(N3):
1274.9 cmÀ1, and n(PF): 919.1 cmÀ1), which strongly suggests the
formation of the nitrene FP(O)(N3)N after the loss of one N2
molecule from the diazide.
Results and discussion
Photolysis of FP(O)(N3)2 in solid argon
As has been shown in our recent studies, short-pulsed (5 Hz) ArF
laser photolysis of matrix-isolated azides is a very useful method for
generating highly reactive nitrene intermediates in solid noble gas
matrices, e.g., F2P(O)N,16 FC(O)N,14 and FS(O)2N.19 Similarly,
FP(O)(N3)2 shows a strong UV absorption at about 200 nm, which
allows its photolysis using an ArF excimer laser (l = 193 nm). The IR
difference spectrum after photolysis, showing the depletion of the
precursor and formation of new species upon photolysis, is
depicted in Fig. 1 (upper trace).
Clearly, a few new bands appear immediately upon photolysis.
The intensities of the two strongest bands at 1292.5 and 812.5 cmÀ1
increase steadily with the time of photolysis, they are in good
agreement with the reported frequencies of n(PO) = 1292.2 cmÀ1
and v(PF) = 811.4 cmÀ1 for the triatomic species FPO,38 indicating a
complete decomposition of FP(O)(N3)2 upon laser photolysis. Inter-
estingly, a few new weak bands were also observed, especially a
strong one at 2187.5 cmÀ1, which occurred right between the two
bands for the N3 antisymmetric stretches of the diazide precursor
(2199.5 and 2178.3 cmÀ1).
Calculations on FP(O)(N3)N were performed for the lowest
energy singlet and triplet states (Table 1). Generally, the
observed band positions and relative intensities are in better
agreement with calculations for the triplet ground state (vide infra).
With the aid of calculations, weaker bands in the far-infrared
region recorded using a liquid He cooled Bolometer detector could
also be identified (Fig. S1, ESI†). Photolysis experiments were also
performed using 18O and 15N enriched FP(O)(N3)2 (IR difference
spectra are shown in Fig. S1–S2, ESI†). The 18O isotopic shifts for
n7 (Dnexptl = 1.1 cmÀ1, Dncalcd = 1.2 cmÀ1), n9 (Dnexptl = 6.1 cmÀ1
,
Dncalcd = 6.6 cmÀ1), n10 (Dnexptl = 6.7 cmÀ1, Dncalcd = 7.2 cmÀ1), and
n11 (Dnexptl = 1.8 cmÀ1, Dncalcd = 2.2 cmÀ1) strongly support the
assignment to the nitrene FP(O)(N3)N in its triplet state. As for the
15N enriched sample, the assignment of 15N isotopic shifts was
difficult due to the presence of several isotopomers in the matrix.
The 15N experiment confirms the assignment of the band at
1315.1 cmÀ1 to n(PO) since no 15N isotopic shift was observed for
this band.
To identify these weak bands, subsequent irradiation using
near-UV visible light (l 4 335 nm) was applied to the matrix-
isolated photolysis products. The resulting IR difference spectrum
is shown in Fig. 1 (lower trace). Some weak bands together with the
strongest one at 2187.5 cmÀ1 vanished completely, whereas the
To determine the nature of the electronic ground state of the
nitrene intermediate, a matrix EPR spectrum was recorded for
the products obtained by ArF excimer laser photolysis of
Table 1 Calculated and experimental vibrational frequencies (cmÀ1) and
intensities of triplet FP(O)(N3)N
Experimentala
Ar-matrix
Calculatedb
syn
anti
Assignmentc
2187.4 s
1315.1 ms
1266.1 ms
923.6 ms
813.9 w
739.6 vw
582.7 w
2305 (552)
1302 (124)
1352 (272)
901 (194)
796 (67)
726 (26)
592 (48)
579 (5)
427 (40)
400 (34)
352 (10)
268 (4)
2296 (551)
1321 (61)
1345 (379)
867 (150)
790 (67)
717 (16)
591 (41)
579 (14)
422 (40)
385 (31)
369 (14)
266 (3)
n1, nas(N3)
n2, n(PO)
n3, ns(N3)
n4, n(PF)
n5, nas(NPN)
n6, ns(NPN)
n7, di.p.(N3)
n8, do.o.p.(N3)
n9
455.5 w
404.1 w
363.8 vw
n10
n11
n12
243 (2)
133 (1)
57 (o1)
246 (3)
128 (1)
55 (o1)
n13
n14
n15
Fig. 1 IR difference spectra (transmission, T) of Ar-matrix isolated FP(O)(N3)2
at 16 K. Upper trace: after ArF laser irradiation (l = 193 nm). Lower trace: after
subsequent near-UV visible irradiation (l 4 335 nm). Bands of depleted species
point upward, while bands of newly formed species point downward. Bands
associated with FP(O)(N3)2 (a), FP(O)(N3)N (b), and FPO (c) are marked.
a
Experimentally observed frequencies and relative band intensities: s
b
strong, ms medium strong, w weak, vw very weak. Calculated IR
frequencies and intensities (km molÀ1) in parenthesis at the B3LYP/
c
6-311+G(3df) level of theory. Assignments are given for the syn conformer
(i.p. in plane, o.o.p. out of plane).
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FP(O)(N3)2 at 5 K. A weak but distinct signal showing character-
istic triplet nitrene nature was found (Fig. S3, ESI†). The derived
zero-field parameters at g = 2.0023, are |D/hc| = 1.566 cmÀ1 and
|E/hc| = 0.005 cmÀ1. These are fairly close to those of related
triplet phosphoryl nitrenes F2P(O)N (|D/hc| = 1.60 cmÀ1 and
|E/hc| = 0.0054 cm )
À1 16 and (PhO)2P(O)N (|D/hc| = 1.5408 cmÀ1
and |E/hc| = 0.00739 cmÀ1).39
Flash vacuum pyrolysis of FP(O)(N3)2
Flash vacuum pyrolysis of FP(O)(N3)2 diluted with argon
(1 : 1000) was applied at about 1000 1C and the products were
subsequently trapped at 16 K (Fig. 2, upper trace).
The strong IR bands at 1293.2 and 812.4 cmÀ1 assigned to
FPO clearly demonstrate the decomposition of FP(O)(N3)2. The
frequencies of these bands are slightly shifted compared to
those observed after photolysis of the diazide, due to the Fig. 3 Calculated conformers of FP(O)(N3)N in the triplet and singlet states at
the B3LYP/6-311+G(3df) level of theory. Selected bond lengths (Å) and selected
different local matrix environments. Traces of the precursor
still survived the pyrolysis, as evidenced by weak IR bands of
angles (1, red italics) are also shown. Nitrogen, oxygen, fluorine, and phos-
phorus atoms are shown in blue, red, cyan, and orange color, respectively.
the diazide (Fig. 2, lower trace). The byproducts HN3 and F3PO
were identified with the aid of 15N labeling. These side products
nitrene intermediates,9,17 it was also applied to FP(O)(N3)N.
are likely formed by traces of moisture. To identify weak IR
bands of photolabile intermediates, the matrix-isolated pyrolysis
products were irradiated with l 4 395 nm light. However, no
change was found in the IR spectra. Thus, flash vacuum pyrolysis
of FP(O)(N3)2 offers an efficient way of producing FPO in the gas
phase, which was formerly generated by either high-temperature
reaction of P(O)FBr2 with silver,38 electric discharge of a PF3/O2/
Ne,40 or flash vacuum pyrolysis of F2POPF2.41
Optimized structures are shown in Fig. 3. Their relative energies are
compiled in Table 2. Consistent with experimental EPR observa-
tions, a triplet ground state is conclusively predicted by all applied
methods. For the most stable conformers on the triplet and singlet
hypersurfaces DEST = 35.2 kJ molÀ1 (CBS-QB3). This is substantially
smaller than those of F2P(O)N (83.5 kJ molÀ1 16
(65.7 kJ mol
À1 17 at the same theoretical level.
)
and (CH3)2P(O)N
)
Two conformers, syn and anti, were predicted for both triplet
and singlet FP(O)(N3)N (Fig. 3), where syn/anti refers to the
relative orientation of the PQO and N3 moieties with respect to
the P–Na bridge. In both states, syn is slightly lower in energy
than anti (Table 2). A preference for the syn conformation has also
been found in F2P(O)N3, which was rationalized by anomeric
interaction from the nitrogen lone-pair (Na) to the s*(PQO).42
For the lowest singlet state two different syn and anti
conformers were found (conformer I and conformer II in
Fig. 3). The most striking structural differences between these
conformers are related to NnitreneÁ Á ÁO (conformer I) versus
NnitreneÁ Á ÁNa intramolecular interactions (conformer II): small
+OPN angles were observed in I (syn 78.7, anti 79.61), whereas
even smaller +NaPN angles were observed in II (syn 58.1, anti
57.11). Conformer II is lower in energy (Table 2), and the global
minimum on the singlet PES is syn–II, which corresponds to the
smallest DEST value of 35.2 kJ molÀ1 at the CBS-QB3 level.
Conformers of FP(O)(N3)N
Calculations on the structures and energies of the nitrene
FP(O)(N3)N in both its singlet and triplet states were performed
using various DFT methods (B3LYP, BP86, and MPW1PW91).
Since the complete basis set method CBS-QB3 was found to give
a better estimation of the singlet–triplet energy gaps (DEST) of
Table 2 Calculated relative energies (kJ molÀ1) of FP(O)(N3)N
Tripletb
Singletb
Methods
syn
anti
syn-I
syn-II
anti-I
anti-II
B3LYPa
0.0
0.0
0.0
0.0
5.6
4.4
5.9
6.6
91.4
64.4
102.7
69.6
55.9
22.3
58.3
35.2
100.7
72.3
113.1
82.0
56.7
23.9
59.2
37.0
BP86a
MPW1PW91a
CBS-QB3
Fig. 2 IR spectra (Absorption, A) of Ar-matrix isolated 15N-labeled
FP(O)(N3)2 before (lower trace) and after flash vacuum pyrolysis (upper
trace) at 16 K. Bands of FP(O)(N3)2 (a), FPO (c), F3PO (d), and HN3 (e) are
marked, bands of unknown impurities are marked by asterisks.
a
b
Basis set of 6-311+G(3df) was used. Structures of the conformers are
given in Fig. 3.
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Fig. 5 Calculated decomposition routes for two FP(O)(N3)N conformers
at the B3LYP/6-311+G(3df) level of theory. Relative energies are given in
kJ molÀ1, selected bond lengths (Å) and angles (1, red italics) are also shown.
Nitrogen, oxygen, fluorine, and phosphorus atoms are shown in blue, red,
cyan, and orange color, respectively.
Fig. 4 Inter-conversion of FP(O)(N3)N conformers on the singlet PES at the
B3LYP/6-311+G(3df) level of theory. Relative energies are given in kJ molÀ1
,
selected bond lengths (Å) and angles (1, red italics) are also shown. Nitrogen,
oxygen, fluorine, and phosphorus atoms are shown in blue, red, cyan, and
orange color, respectively.
and (CH3O)2P(O)N3 (174.6 kJ molÀ1).17 As the common decomposi-
tion product, a novel three-membered ring molecule cyclo-FP(O)N2
is predicted from FP(O)(N3)N (Fig. 5).
The rather strong NnitreneÁ Á ÁNa intramolecular interactions
in II are also evidenced by the Na–N bond lengths of 1.662 and
1.639 Å in syn-II and anti-II, respectively, which indicate three-
membered ring Na–P–N structures. As expected, the +OPN and
+NaPN angles of the conformers in the triplet ground state are
fairly close to a tetrahedral coordination at phosphorus, and
there are no indications for such intramolecular interactions in
the triplet ground state.
To gain insight into the photochemistry of Ar-matrix isolated
FP(O)(N3)N the inter-conversion of its four conformers on the
singlet PES was explored at the B3LYP/6-311+G(3df) level of theory
(Fig. 4). Transition states (TS) for the II to I rearrangements for both
the syn (TS1) and the anti (TS3) configurations were located at
activation energies of 63.0 and 70.4 kJ molÀ1, respectively. The
displacement vectors of the imaginary frequencies of TS1 and TS3
mainly correspond to the NPNa bending mode. The barrier for the
syn-I to anti-I conversion (TS2) is much lower and was calculated to
be 48.8 kJ molÀ1. The corresponding displacement vector of the
imaginary frequency mainly corresponds to a rotation of the azide
group around the P–Na bond. However, the analogous direct syn-II
The decomposition barrier of cyclo-FP(O)N2 was calculated
to be merely 44.9 kJ molÀ1, which is much lower than that of
cyclo-N2CO (103.1 kJ molÀ1) calculated at the same level of
theory. The latter compound has been very recently isolated as a
neat substance and found to have a half-life time of 30 h at room
temperature in a clean quartz container.26 The main difference
in the decomposition mechanisms of these two cyclic diazo
molecules is that the two P–N bonds in cyclo-FP(O)N2 break
simultaneously, while the two C–N bond undergo stepwise
cleavage in cyclo-N2CO to yield an open-chain isomer OCNN,
which undergoes further decomposition into CO and N2 without
a barrier on the singlet PES.44,45 The low decomposition barrier
of cyclo-FP(O)N2 probably accounts for its absence among the
flash vacuum pyrolysis products, where only its decomposition
product FPO was detected by IR spectroscopy.
Decomposition of FP(O)N4
to anti-II rearrangement is precluded by the strong NnitreneÁÁÁNa A Curtius-type rearrangement has recently experimentally been
interaction, which prevents the free rotation of the azide group.
observed for (N3)2P(O)N.23 The high energy compound N3P(O)N4
A singlet phosphoryl nitrene intermediate (Ph2P(O)N) has was formed under visible light irradiation of the nitrene.23 However,
been very recently detected by ultra-fast spectroscopy,43 and its all attempts to locate transition states for the N3-migration in singlet
life-time has been estimated to be about 480 ps. Given the low I to FP(O)(N3)N failed, and only the aforementioned TS1 and TS3 leading
II conversion barriers, and the expected rapid singlet–triplet inter- to the cyclic minima syn-II and anti-II (Fig. 4) were obtained. This
system crossing (ISC) in solid Ar matrices aided by the small DEST result is consistent with the experimental observation that no
(Table 2), the detection of the most stable triplet nitrene conformer rearrangement product but only FPO was found upon near-UV
in a solid argon matrix is reasonable. Nonetheless, the absence of visible light irradiation (l 4 335 nm).
FP(O)(N3)N in the flash pyrolysis products, and the almost exclusive
Nevertheless, molecular structures and decomposition path-
formation of FPO prompted us to also investigate competing ways of the possible rearrangement product FP(O)N4 were
secondary reactions of singlet FP(O)(N3)N such as rapid N2 elim- calculated. Two planar conformers, cis and trans, were found,
ination or its Curtius-type rearrangement.
which are close in energy and they differ mainly in the
configuration (cis or trans) of the PQO and the Na–Nb bonds
with respect to the P–Na bridge (Fig. 6). The transition states,
Formation and decomposition of cyclo-FP(O)N2
Rather low activation barriers (o10 kJ molÀ1) for the elimina- TS7 and TS8, leading to the concerted decomposition of cis and
tion of N2 from syn-II and anti-II were predicted on the singlet trans FP(O)N4 into FPO + 2 N2 were located at activation
PES (TS4 and TS5, Fig. 5 and Fig. S4, ESI†). These barriers are energies of 39.1 and 39.0 kJ molÀ1, respectively. With an energy
significantly lower than those predicted for the stepwise N2 release of more than 500 kJ molÀ1 these fragmentation reac-
elimination in the phosphoryl azides (CH3)2P(O)N3 (189.2 kJ molÀ1
)
tions of FP(O)N4 are highly exothermic.
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¨
3 S. Brase, C. Gil, K. Knepper and V. Zimmermann, Angew.
Chem., Int. Ed., 2005, 44, 5188.
4 S. Lang and J. A. Murphy, Chem. Soc. Rev., 2006, 35, 146.
5 N. P. Gritsan and M. S. Platz, Chem. Rev., 2006, 106, 3844.
6 H. Bock and R. Dammel, Angew. Chem., Int. Ed. Engl., 1987,
26, 504.
7 E. A. Pritchina, N. P. Gritsan, A. Maltsev, T. Bally, T. Autrey,
Y. Liu, Y. Wang and J. P. Toscano, Phys. Chem. Chem. Phys.,
2003, 5, 1010.
8 C. Buron and M. S. Platz, Org. Lett., 2003, 5, 3383.
9 J. Liu, J. L. Mandel, C. M. Hadad and M. S. Platz, J. Org.
Chem., 2004, 69, 8583.
10 J. Kubicki, Y. Zhang, S. Vyas, G. Burdzinski, H. L. Luk,
J. Wang, J. Xue, H.-L. Peng, E. A. Pritchina, M. Sliwa,
G. Buntinx, N. P. Gritsan, C. M. Hadad and M. S. Platz,
J. Am. Chem. Soc., 2011, 133, 9751.
Fig. 6 Calculated decomposition routes for two FP(O)N4 conformers at the
B3LYP/6-311+G(3df) level of theory. Relative energies are given in kJ molÀ1
,
selected bond lengths (Å) are also shown. Nitrogen, oxygen, fluorine, and
phosphorus atoms are shown in blue, red, cyan, and orange color, respectively.
11 E. A. Pritchina, N. P. Gritsan and T. Bally, Russ. Chem. Bull.,
Int. Ed., 2005, 54, 519.
12 S. Vyas, J. Kubicki, H. L. Luk, Y. Zhang, N. P. Gritsan,
C. M. Hadad and M. S. Platz, J. Phys. Org. Chem., 2012, 25, 693.
13 X. Q. Zeng, H. Beckers, H. Willner, D. Grote and W. Sander,
Chem. – Eur. J., 2011, 17, 3977.
14 J. Kubicki, Y. Zhang, J. Wang, H. L. Luk, H.-L. Peng, S. Vyas
and M. S. Platz, J. Am. Chem. Soc., 2009, 131, 4212.
15 M. P. Sherman and W. S. Jenks, J. Org. Chem., 2014, 79, 8977.
16 X. Q. Zeng, H. Beckers, H. Willner, P. Neuhaus, D. Grote and
W. Sander, Chem. – Eur. J., 2009, 15, 13466.
17 R. D. McCulla, G. A. Gohar, C. M. Hadad and M. S. Platz,
J. Org. Chem., 2007, 72, 9426.
18 G. A. Gohar and M. S. Platz, J. Phys. Chem. A, 2003,
107, 3704.
19 X. Q. Zeng, H. Beckers, P. Neuhaus, D. Grote and W. Sander,
Z. Anorg. Allg. Chem., 2012, 638, 526.
20 X. Q. Zeng, H. Beckers and H. Willner, J. Am. Chem. Soc.,
2013, 135, 2096.
21 J. Kubicki, H. L. Luk, Y. Zhang, S. Vyas, H.-L. Peng,
C. M. Hadad and M. S. Platz, J. Am. Chem. Soc., 2012,
134, 7036.
Conclusions
Both photolytic and thermal decomposition reactions of
FP(O)(N3)2 were studied by matrix isolation. Upon ArF laser
photolysis (l = 193 nm), a new triplet a-oxo nitrene intermediate,
FP(O)(N3)N, was produced and characterized by matrix IR and
EPR spectroscopy. Further decomposition of this nitrene into
FPO was observed under near-UV visible irradiation (l 4
335 nm). In contrast, flash vacuum pyrolysis of FP(O)(N3)2
exclusively yields FPO, and no intermediate species was found.
Different decomposition pathways of FP(O)(N3)N were theoreti-
cally explored on the singlet PES. The cyclic conformers syn-II
and anti-II (Fig. 3), stabilized by intramolecular NnitreneÁ Á ÁNa,azide
interactions, were found to be much lower in energy than syn-I
and anti-I (Fig. 3), stabilized by intramolecular NnitreneÁ Á ÁO
interactions. According to DFT calculations singlet FP(O)(N3)N
decomposes by stepwise N2 elimination to yield FPO via cyclo-
FP(O)N2. The low computed activation barrier of 44.9 kJ molÀ1
(B3LYP/6-311+G(3df)) for the decomposition of cyclo-FP(O)N2
into FPO and N2, however, rules out its experimental observation
in the flash pyrolysis products of FP(O)(N3)2.
22 V. Desikan, Y. Liu, J. P. Toscano and W. S. Jenks, J. Org.
Chem., 2008, 73, 4398.
Acknowledgements
23 X. Q. Zeng, H. Beckers and H. Willner, J. Am. Chem. Soc.,
2011, 133, 20696.
24 X. Q. Zeng, H. Beckers and H. Willner, Angew. Chem., Int.
Ed., 2011, 50, 482.
25 X. Q. Zeng, H. Beckers, H. Willner and J. F. Stanton, Angew.
Chem., Int. Ed., 2011, 50, 1720.
26 X. Q. Zeng, H. Beckers, H. Willner and J. F. Stanton, Eur.
J. Inorg. Chem., 2012, 3403.
This research was supported in part by the National Natural Science
Foundation of China (21372173, 21422304), the Project of Scientific
and Technologic Infrastructure of Suzhou (SZS201207), the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD), and the Deutsche Forschungsgemeinschaft
(WI 663/26-1). X.Q.Z. greatly appreciates Professor H. Willner for
a research stay in his lab. We are indebted to T. Benter and his
group for allowing us to make use of the ArF laser.
¨
27 C. J. Shaffer and D. Schroder, Angew. Chem., Int. Ed., 2011,
50, 2677.
28 S. R. O’Neill and J. M. Shreeve, Inorg. Chem., 1972, 11, 1629.
Notes and references
¨
29 H. G. Schnockel and H. Willner, Infrared and Raman Spectro-
scopy, Methods and Applications, VCH, Weinheim, 1994.
30 M. Griffin, A. Muys, C. Noble, D. Wang, C. Eldershaw,
K. E. Gates, K. Burrage and G. R. Hanson, Mol. Phys. Rep.,
1999, 26, 60.
1 M. S. Platz, Reactive Intermediate Chemistry, Wiley-
Interscience, New York, 2004.
2 W. Lwowski, Azides and Nitrenes, Academic Press, New York,
1984.
Phys. Chem. Chem. Phys.
This journal is ©the Owner Societies 2015

View Article Online
PCCP
Paper
31 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
32 J. P. Perdew, Phys. Rev. B: Condens. Matter Mater. Phys., 1986,
33, 8822.
33 C. Adamo and V. Barone, J. Chem. Phys., 1998, 108, 664.
34 J. A. Montgomery, M. J. Frisch, J. W. Ochterski and
G. A. Petersson, J. Chem. Phys., 2000, 112, 6532.
35 C. Gonzalez and H. B. Schlegel, J. Chem. Phys., 1989, 90, 2154.
36 C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1990,
94, 5523.
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03,
Revision D.01, Gaussian, Inc., Pittsburgh, PA, 2003.
38 R. Ahlrichs, R. Becherer, M. Binnewies, H. Borrmann,
¨
M. Lakenbrink, S. Schunck and H. Schnockel, J. Am. Chem.
Soc., 1986, 108, 7905.
39 M. Houser, S. Kelley, V. Maloney, M. Marlow, K. Steininger
and H. Zhou, J. Phys. Chem., 1995, 99, 7946.
37 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven, 40 B. Gatehouse, T. Brupbacher and M. C. L. Gerry, J. Phys.
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, Chem. A, 1999, 103, 560.
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, 41 H. Beckers, H. Bu¨rger, P. Paplewski, M. Bogey, J. Demaison,
´
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,
P. Drean, A. Walters, J. Breidung and W. Thiel, Phys. Chem.
Chem. Phys., 2001, 3, 4247.
Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, 42 X. Q. Zeng, M. Gerken, H. Beckers and H. Willner, Inorg.
H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, Chem., 2010, 49, 3002.
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, 43 S. Vyas, S. Muthukrishnan, J. Kubicki, R. D. McCulla,
R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
G. Burdzinski, M. Sliwa, M. S. Platz and C. M. Hadad,
J. Am. Chem. Soc., 2010, 132, 16796.
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, 44 A. A. Korkin, P. von R. Schleyer and R. J. Boyd, Chem. Phys.
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, Lett., 1994, 227, 312.
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, 45 C. J. Shaffer, B. J. Esselman, R. J. McMahon, J. F. Stanton
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
and R. C. Woods, J. Org. Chem., 2010, 75, 1815.
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