Experimental observation of the 16-electron molecules SPN, SNP, and cyclic PSN

Article abstract of DOI:10.1002/anie.201108636

16-Electron triatomic ring: Novel thiazylidynephosphane (SNP) was produced by either flash vacuum pyrolysis (ca. 1000 °C) or laser photolysis (193 nm) of SP(N₃)₃. Its photointerconversion to cyclic thiazaphosphirine (cyc-PSN) and thiophosphoryl nitride (SPN) was found in Ar matrix at 16 K. Cyc-PSN is the first experimentally observed 16-electron cyclic triatomic molecule. Copyright

Full text of DOI:10.1002/anie.201108636

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DOI: 10.1002/anie.201108636
Small-Ring Systems
Experimental Observation of the 16-Electron Molecules
SPN, SNP, and Cyclic PSN**
Xiaoqing Zeng,* Helmut Beckers, Helge Willner, and Joseph S. Francisco*
Dedicated to Professor Dianxun Wang on the occasion of his 70th birthday
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Small molecules of second-row elements and their third-row
analogues have been the focus of extensive theoretical and
experimental studies due to their novel structure and bonding
properties.[1] In particular, low-coordinated phosphorus mol-
ecules have been discovered in circumstellar and dense
interstellar clouds,[2] and some phosphorus–nitrogen triply
bonded species have been attracting much attention due to
their applications as building blocks in organophosphorus
chemistry.[3] The recent direct observation of diatomic PN in
supergiant star VY CMa[4] suggests the existence of more
phosphorus molecules in the presence of oxygen or sulfur.
Moreover, elusive species such as N2S,[5,6] P2O/P2S,[7,8] and
OPN/SPN[9,10] have been of great interest. Dinitrogen sulfide
(N2S), first produced by the thermolysis of thiatriazoles, was
confirmed to be a linear molecule with sulfur at the terminal
position.[5,6] The diphosphorus compounds P2O[7] and P2S[8]
SPN to cyc-PSN is 138 kJmolꢀ1, and cyc-PSN is separated
from SNP by a barrier of 95 kJmolꢀ1. CCSD(T)/cc-pVTZ
calculations were consistent with the B3LYP results, and cyc-
PSN and SPN are 93 and 73 kJmolꢀ1 higher in energy than
SNP, respectively. The results, given in Figure 1, are consistent
with previous calculations at the configuration interaction
(CI) level.[13] The collective calculations and barriers suggest
that all three isomers should be experimentally detectable at
low temperatures.
=
have first been identified in solid argon matrices by its P O
stretching vibration (1270.4 cmꢀ1) and the isotope pattern of
32
ꢀ1
the P S mode ( S: 891.4 cm , 34S: 887.8 cmꢀ1), respectively.
=
ꢁ
Both adopt a linear structure featuring a terminal P P bond.
To date, cyclic isomers of P2O and P2S have not been detected.
After more than two decades since the discovery of
ONP,[9] the OPN isomer was obtained by photolytic decom-
position of OP(N3)3 in a solid Ar matrix.[10] However, their
cyclic isomer was not found. As a rule, second-row triatomic
molecules having 16 valence electrons adopt a linear struc-
ture.[11] This is because of the favorable involvement of these
atoms in strong p-type bonds, and due to their unfavorable
bonding in small rings.[12] Thus, their cyclic isomers, if they
exist at all, are weakly bonded high-energy species.[13–15] On
the other side, for the third and higher row analogues, which
avoid using their s valence electrons in multiple bonding,
cyclic molecules may become experimentally observable
species.[13,14,16] However, to the best of our knowledge, cyclic
third-row triatomic molecules having 16 valence electrons
have not been observed so far.
Figure 1. Calculated reaction coordinate of SPN isomers along the P-
N-S angle at the B3LYP/6-311+G(3df) level of theory. Bond lengths
[ꢀ] of the minima calculated at the CCSD(T)/cc-pVTZ are given in
italics.
The SPN molecule was generated experimentally by
thermal or photolytic decomposition of SP(N3)3, which
extrudes four moles of nitrogen molecules [Eq. (1)]. The
highly explosive triazide precursor has been very recently
obtained as a neat substance.[17]
SPðN3Þ3 ! SPN þ 4 N2
ð1Þ
Here, we report the first neutral cyclic third-row triatomic
species, cyc-PSN, and its photointerconversion with two linear
isomers SNP and SPN in solid noble gas matrices.
Preliminary exploration of the potential energy surface
for the SPN system by using density functional calculations
with the B3LYP/6-311 + G(3df) method combined with
intrinsic reaction coordinate (IRC) analysis shows that the
linear SNP isomer is the global minimum. The barrier from
The triazide, diluted in Ar (approximately 1:1000) was
heated to about 10008C, and the mixture was then deposited
onto the cold matrix support at 16 K. The IR spectrum
(Figure S1 in the Supporting Information) shows that only
traces of triazide were left. Among the products, the nitrogen-
containing species of PN (1327.4 cmꢀ1, Dn(14/15N) =
30.3 cmꢀ1),[18] SN (1208.6 cmꢀ1, Dn(14/15N) = 28.0 cmꢀ1),[19]
SN2 (2039.9 cmꢀ1, Dn(14/15N) = 36.2 cmꢀ1),[20] and HN3
(2135.3 cmꢀ1, 1146.1 cmꢀ1)[21] were identified by using 15N
labeling and by comparing to reference spectra. Traces of HN3
might be formed from the hydrolysis of the triazide. Diatomic
PS was also formed, as evidenced by the occurrence of a very
weak band at 729.3 cmꢀ1, consistent with the reported value of
729.5 cmꢀ1 in Ar matrix.[8]
[*] Dr. X. Q. Zeng, Dr. H. Beckers, Prof. Dr. H. Willner
Bergische Universitꢁt Wuppertal, FB C, Mathematik und Natur-
wissenschaften, Fachgruppe Chemie
Gaußstr. 20, 42119 Wuppertal (Germany)
E-mail: zeng@uni-wuppertal.de
Prof. Dr. J. S. Francisco
Among the unknown IR bands, the strongest one at
1374.1 cmꢀ1 was found to be accompanied by two weak bands
at 641.2 cmꢀ1 and 358.1/354.4 cmꢀ1 (exhibiting matrix site
splitting). Their band positions and relative intensities
(Table 1) are in good agreement with the prediction for
SNP at 1407, 661, and 358 cmꢀ1 from the CCSD(T)/aug-cc-
pV5Z level of theory. Their assignment to SNP is further
Purdue University, Department of Chemistry
West Lafayette, IN 47907-2084 (USA)
E-mail: francisc@purdue.edu
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) and the Fonds der Chemischen Industrie. The Frontispiece
shows a photo of Interstellar Plant taken from http://www.centauri-
dreams.org/wp-content/uploads/2011/08/pulsar planet.jpg.
supported by the isotopic shifts of the naturally abundant 34
S
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108636.
and the 15N labeled samples summarized in Table 1. Accord-
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Table 1: Observed and calculated IR frequencies [cmꢀ1] of SPN isomers.
Species
Mode
number
Mode
description
Observed[a]
Ar matrix
CCSD(T)
cc-pVTZ[c]
15N[b]
obsd
34S[b]
obsd
aug-cc-pVQZ
aug-cc-pV5Z
calcd[d]
calcd[d]
SNP
1
2
3
PN stretch
NS stretch
SNP bend
1374.1 (100)
641.2 (1)
358.1/354.4 (4)
1403 (384)
652 (4)
353 (11)
1401
657
353
1407
661
358
33.0
2.1
9.8
38.4
0.2
9.7
2.5
9.2
0.8
2.4
10.2
0.9
cyc-PSN
1
2
3
PN stretch
Sym-ring
Asym-ring
1049 (1)
543 (2)
330 (27)
1062
548
345
1069
552
351
24.0
4.1
5.9
540.0 (34)
346.1/341.9 (100)
4.2
5.9
SPN
1
2
3
PN stretch
PS stretch
SPN bend
1314.4 (55)
637.1 (100)
196.8/195.2 (24)
1313 (0.5)
630 (33)
191 (5)
1321
639
193
1331
644
194
24.0
4.9
2.3
28.0
4.7
2.6
[a] Observed band positions in Ar matrix and relative integrated intensities in parenthesis. [b] Isotopic shifts relative to the natural 14N and 32S sample.
[c] The IR intensities [km molꢀ1] in parentheses were calculated at the MP2/cc-pVTZ level of theory based on the CCSD(T)/cc-pVTZ optimized
structures. [d] Calculations at the CCSD(T)/cc-pVTZ level of theory.
ing to the very different 15N isotopic shifts of the two
stretching vibrations of n1 (33.0 cmꢀ1) and n2 (2.1 cmꢀ1), they
are more appropriately described as out-of-phase and in-
phase S-N-P stretches, respectively. Due to a Fermi reso-
nance, the first band of S15NP splits into two bands at 1341.4
(n1) and 1329.1 cmꢀ1 (n2 + 2n3), in which the sum of their
intensities is almost equal to that of S14NP (Figure S2). No
indication for the presence of the less stable isomers was
obtained in the pyrolysis of the triazide at about 10008C.
Thus, SNP seems to be the most robust isomer against further
decomposition.
When UV irradiation (l = 255 nm, 20 min) was applied to
the matrix-isolated pyrolysis products of the triazide, the IR
Figure 3. IR spectral changes in the region of 560–190 cmꢀ1 obtained
spectrum clearly showed the selective depletion of the bands
a) after UV light irradiation (l=255 nm), b) subsequent UV/vis irradi-
ation (l> 395 nm), and c) further UV/vis irradiation (l> 280 nm) of
an Ar matrix which contains the pyrolysis products of SP(N3)3. IR
associated with SNP. Two weak bands at 1314.4 and
637.1 cmꢀ1 in the mid-infrared region (MIR, Figure 2a), and
three even weaker ones at 540.0, 346.1/341.9, and 196.8/
195.2 cmꢀ1 in the far-infrared region (FIR, Figure 3a),
appeared simultaneously. The two new bands in the MIR
are assigned to the SPN isomer, which was calculated to have
three weak IR absorptions at 1331, 644, and 194 cmꢀ1 at the
bands are assigned to either SPN, SNP, or cyc-PSN, and the bands of
depleted species point downward and those of formed species
upward. The feature marked by an asterisk is an artificial spike caused
by the spectrometer.
CCSD(T)/aug-cc-pV5Z level of theory (Table 1). The
observed rather weak band at 196.8/195.2 cmꢀ1 in the FIR is
reasonably assigned to the bending mode of SPN. This
assignment is supported by the 14/15N isotope shifts in Table 1,
which agree quite well with the predictions. A 32/34S isotopic
shift of 10.8 cmꢀ1 for the strongest band (637.1 cmꢀ1) was also
observed, which is in line with the prediction of 10.9 cmꢀ1.
The observed band positions for the remaining two weak
bands in the FIR at 540.0 and 346.1/341.9 cmꢀ1 are very close
to the CCSD(T)/aug-cc-pV5Z predicted frequencies for the
cyc-PSN isomer at 552 and 351 cmꢀ1, for the in-phase and out-
of-phase P-S-N stretches, respectively. This assignment is
corroborated by their 14/15N isotope shifts (Table 1). Features
due to the naturally abundant cyc-P34SN were not observed.
Due to the low abundance of cyc-PSN in the matrix and also
Figure 2. IR spectral changes in the region of 1450–600 cmꢀ1 obtained
a) after UV light irradiation (l=255 nm), and b) subsequent UV
irradiation (l=330 nm) of an Ar matrix which contains the pyrolysis
products of SP(N3)3. IR bands are assigned to either SPN or SNP, and
the bands of depleted species point downward and those of formed
species upward.
ꢀ
the low IR intensity, the P N stretch was not observed in our
experiments. The lack of intensity for the breathing mode
makes such cyclic triatomic molecules generally difficult to
observe in the MIR region, especially for third and higher row
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analogues, for which the stronger out-of-phase stretches are
expected to appear in the FIR region.[14] This probably
explains why similar cyclic species have not yet been
experimentally detected. The cyc-PSN isomer represents the
first observed 16-electron cyclic third-row triatomic molecule.
Another experimentally known cyclic third-row triatomic
molecule is SiC2 (12-electron).[22]
In subsequent experiments, visible light irradiation (l >
395 nm, 20 min) was applied to the matrix, this resulted in the
conversion of cyc-PSN to SNP (Figure 3b) and also traces of
SPN. Further UV/vis irradiation (l > 280 nm) caused
isomerization from SPN to SNP (Figure 3c). Such
Scheme 1. Formation of SPN isomers.
Table 2: Molecular structure and rotational constants for SPN isomers.
conversion was also accomplished by a more selec-
tive UV irradiation of l = 330 nm (Figure 2b). A
similar photolytic interconversion was observed for
the two linear PNO and NPO isomers.[10]
Structure[a]
Rotational constants[b]
Species, ABC Basis set
R(AB) R(BC) q(ABC)
A
B
C
SNP
cc-pVTZ
1.573 1.537 180.0
3321 3321
3314 3314
3348 3348
3365 3365
In addition to the thermal decomposition of
SP(N3)3, its photolysis in solid Ar was studied.
Notably, a stepwise dissociation of the triazide was
observed. First, the azido nitrene ((N3)2P(S)N) was
formed upon photolysis (l = 193 nm) with an ArF
excimer laser, which then undergoes Curtius rear-
rangement to form N3P(S)N4 under visible light
irradiation (l > 420 nm). The N3P(S)N4 further
decomposed into SNP using UV/vis irradiation of
l > 320 nm (Figure S3). Since the IR bands associ-
ated with the two azide intermediates were found to
be rather weak and complicated by matrix site
splittings, their identification was mainly aided by
DFT B3LYP/6-311 + G(3df) calculations (Figure S4
and Table S1). Based on the above observations, the
aug-cc-pVTZ 1.575 1.538 180.0
aug-cc-pVQZ 1.567 1.503 180.0
aug-cc-pV5Z 1.563 1.527 180.0
cyc-PSN
SPN
cc-pVTZ
2.104 1.934 46.3
23043 7098 5427
22950 7109 5428
23122 7232 5509
23224 7294 5550
aug-cc-pVTZ 2.103 1.938 46.3
aug-cc-pVQZ 2.084 1.930 46.4
aug-cc-pV5Z 2.074 1.924 46.4
cc-pVTZ
1.912 1.513 180.0
3937 3937
3933 3933
3983 3983
4009 4009
aug-cc-pVTZ 1.914 1.514 180.0
aug-cc-pVQZ 1.902 1.506 180.0
aug-cc-pV5Z 1.895 1.502 180.0
[a] Bond lengths in ꢀ and angles in 8, respectively. [b] In units of MHz.
thermal and photolytic decomposition pathways of SP(N3)3
are summarized in Scheme 1. In both cases, the final decom-
position product is the most stable SNP isomer, which forms
SPN and cyc-PSN isomers by selective irradiations.
The structural and bonding properties of the three SPN
isomers were explored by using coupled-cluster calculations
combined with natural bond orbital (NBO) analysis. A
complete set of optimized structural parameters is given in
Table 2 along with rotational constants for all isomers. For the
two linear isomers, the bonding situations, elucidated in
Scheme 2, are similar to the corresponding OPN isomers. The
structural features of the SNP and SPN molecules can be
explained by two Lewis resonance structures. A phosphorus–
Scheme 2. Bonding properties of three SPN isomers.
suggest that this cyclic isomer is very flexible, and may easily
undergo ring-opening or eliminate sulfur at high temper-
atures. This probably prevents its observation under the flash
ꢀ
nitrogen triple bond in SPN is evidenced by a P N stretching
frequency of 1314.4 cmꢀ1, which is close to that of diatomic
pyrolysis conditions. Indeed, the formation of PN[24] and S2
[25]
PN (1327.4 cmꢀ1). The calculated P N bond lengths in SPN
fragments by the flash vacuum pyrolysis (FVP) of SP(N3)3 has
been confirmed by their characteristic UV spectra (Figures S5
and S6).
Frontier molecular orbitals (FMOs) for the three SPN
isomers were also examined. Their highest occupied
(HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) are depicted in Figure 4. The FMOs of the two
linear isomers are similar to those of the corresponding PNO
isomers.[26] Replacement of O by S alters the HOMOs by
a large contribution from the sulfur p orbital. For the cyclic
ꢀ
(1.502 ꢀ) and SNP (1.527 ꢀ) at the CCSD(T)/aug-cc-pV5Z
level of theory are slightly larger than the sum of the triple
bond covalent radii for P (0.94 ꢀ) and N (0.54 ꢀ).[23] Thus,
SPN is regarded to be one of the very few examples[7,8,10] with
phosphorus in a s2l5 coordination. For the cyclic isomer, the
ꢀ
P N bond length is calculated to be 1.582 ꢀ, within the sum of
the double bond covalent radii for P (1.02 ꢀ) and N
(0.60 ꢀ).[23] The CCSD(T)/aug-cc-pV5Z calculated S P and
ꢀ
ꢀ
S N bond lengths are 2.074 and 1.924 ꢀ, respectively. The
ꢀ
NBO analysis shows significant positive charge on P (+ 1.02e)
and negative charge on N (ꢀ0.90e). The calculated very long
isomer, the HOMO is a combination of the out-of-plane P N
bonding p-orbital with nonbonding to the sulfur p orbital, and
ꢀ
ꢀ
S N bond and the small negative charge on S (ꢀ0.11e)
the LUMO is simply the p*(P N) orbital.
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transition states were confirmed by intrinsic reaction coordinate
(IRC) calculations.[31] Natural bond order (NBO) analysis was
performed using NBO 3.1 implemented in Gaussian 03 with the
B3LYP method.[32] All calculations were performed using the
Gaussian 03 software package.[33]
Received: December 7, 2011
Published online: January 20, 2012
Figure 4. Molecular orbital plots (isovalue=0.04) of the HOMO and
LUMO for SNP, SPN, and cyc-PSN at the CCSD(T)/cc-pVTZ level of
theory.
Keywords: ab initio calculations · azides · flash pyrolysis ·
IR spectroscopy · small-ring systems
.
Experimental Section
Caution! Covalent azides are hazardous and explosive, and hence
SP(N3)3 should be handled in submillimolar quantities only. Although
we have not experienced any explosions during this work, safety
precautions of wearing face shields, leather gloves, and protective
leather clothes are strongly recommended when handling liquid and
solid SP(N3)3.
Thiophosphoryl triazide, SP(N3)3, was prepared and purified
according to published protocols.[17] 1-15N sodium azide (98 atom%
15N, EURISO-TOP GmbH) was used for preparing the 15N labeled
sample.
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Matrix IR spectra were recorded on a FT-IR spectrometer (IFS
66v/S Bruker) in a reflectance mode using a transfer optic. A KBr
beam splitter and MCT detector were used in the region of 4000–
500 cmꢀ1 and a Ge-coated 6-m Mylar beam splitter with a He(l) cooled
Si bolometer in the region of 700–180 cmꢀ1 (CsI window). For each
spectrum, 200 scans at a resolution of 0.25 cmꢀ1 were co-added.
Matrix UV/vis spectra were measured with the PerkinElmer Lambda
900 UV spectrometer in the range of 200 to 700 nm with a data point
distance of 0.5 nm and an integration time of 2 s. The radiation of the
spectrometer was directed into a 150 cm long optical quartz fiber,
through a quartz lens inside the cryostat and twice passed through the
matrix deposit on the cold Rh mirror. A second quartz lens and fiber
collected the reflected radiation and returned it to the spectrometer.
Details of the matrix apparatus have been described elsewhere.[27]
The gaseous sample was mixed by passing a flow of argon gas
through a U-trap containing ca. 10 mg of solid SP(N3)3, which was
kept in a cold ethanol bath at ꢀ278C. By adjusting the flow rate of
argon gas (2 mmolhꢀ1), a small amount of the resulting mixture
(azide/Arꢂ 1:1000 estimated) was deposited at a high vacuum within
30 min onto the matrix support (16 K for Ar matrix, 6 K for Ne
matrix, Rh-plated Cu block), and used for photolysis experiments. For
the pyrolysis experiments, the gas mixture was passed through an
aluminium oxide tube furnace (i.d. 1.0 mm, o.d. 2.8 mm, length
25 mm), which was heated (voltage 5.3 V, current 2.0 A) to about
10008C over a length of ca. 10 mm with a tantalum wire (o.d. 0.25 mm,
resistance 0.8 W), prior to deposition on the cold matrix support in
high vacuum.
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Photolysis experiments were carried out with an ArF excimer
laser (Lambda-Physik, 193 nm, 5 Hz), or with
a high-pressure
mercury arc lamp (TQ 150, Heraeus) by conducting the light through
water-cooled quartz lenses combined with different Schott cut off
filters of ꢃ 280, ꢃ 360, ꢃ 395, ꢃ 420, ꢃ 455, and ꢃ 570 nm, or Schott
interference filter of 255 nm (Schott) and band-pass filter of 330FS10–
50 (Lot-Oriel).
Computational details: The structures and IR frequencies were
calculated using coupled cluster theory with single and double
excitation, and including a perturbative triples correction (CCSD(T))
method[28] and the following basis sets: cc-pVTZ, augmented aug-cc-
pVQZ and aug-cc-pV5Z.[29] Their IR intensities were estimated at the
CCSD(T)/cc-pVTZ optimized structures using the second-order
Møller-Plesset (MP2) approximation at the same basis set.[30] Local
minima were confirmed by vibrational frequency analysis, and
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