Isomers of disulfur dinitride, S2N2

Article abstract of DOI:10.1002/anie.201407538

UV photolysis (λ=248 or 255 nm) of cyclic S₂N₂ isolated in solid argon matrices yields two open-shell S₂N₂ isomers, trans SNSN (³A″ ) and cis SNSN (³A″), as well as a closed-shell C_2v dimer (SN)₂ (¹A₁). These novel isomers have been characterized by their IR spectra and mutual photointerconversion reactions. Quantum chemical calculations support the experimental results and also provide insight into the complex potential energy surface of S₂N₂.

Full text of DOI:10.1002/anie.201407538

Angewandte
Chemie
DOI: 10.1002/anie.201407538
Elusive Isomers
Isomers of Disulfur Dinitride, S2N2**
Xiaoqing Zeng,* Andrea Flores Antognini, Helmut Beckers,* and Helge Willner
Abstract: UV photolysis (l = 248 or 255 nm) of cyclic S2N2
isolated in solid argon matrices yields two open-shell S2N2
isomers, trans SNSN (3A’’) and cis SNSN (3A’’), as well as
a closed-shell C2v dimer (SN)2 (1A1). These novel isomers have
been characterized by their IR spectra and mutual photo-
interconversion reactions. Quantum chemical calculations
support the experimental results and also provide insight into
the complex potential energy surface of S2N2.
even lower in energy than the SNSN D2h ring structure.[9a,c]
Trigonal planar (Y-shaped) isomers, planar chains, and
puckered four-membered rings represent alternative higher-
energy minima on both the singlet and the triplet PES of S2N2.
Some of these structures have already been established for
isoelectronic tetraatomic molecules with 22 valence electrons,
such as C2F2 (linear and planar Y-shaped),[10a,b] N2O2 (planar
cis chain),[7] CO3 (planar Y-shaped),[10c] and P4 (square
2ꢀ
planar).[10d] Of particular interest with regard to the polymer-
ization of S2N2 are proposed intermediate biradicals with cis-
bent SNSN structures.[9]
B
inary sulfur–nitrogen compounds are of broad interest in
structural and synthetic inorganic chemistry as well as in
material science.[1] The key compound of the rich sulfur–
nitrogen chemistry is tetrasulfur tetranitride, S4N4.[2,3] It was
discovered in 1835,[2] and has been the subject of extensive
studies.[3] By thermal cracking of gaseous S4N4 on silver wool,
the more challenging disulfur dinitride, S2N2, was obtained.[4]
Solid S2N2 was found to undergo thermally or photolytically
induced polymerization at low temperature to form poly-
sulfur nitride, (SN)x.[5] The outstanding properties of S2N2 are
of ongoing interest in inorganic chemistry.[6]
Although S2N2 (D2h) is metastable with respect to N2 and
S2 and even explodes violently when heated higher than
308C,[4,8a] its dissociation into two SN radicals was found to be
hindered by a barrier of about 200 kJmolꢀ1 [9]
On the other
.
hand, the lowest triplet state was predicted to be about
170 kJmolꢀ1 above the singlet ground state.[9b,c]
Despite the long history and numerous studies on S2N2, it
is surprising that only the D2h ring structure has been up to
now firmly established experimentally. In an early study,
Hassanzadeh and Andrews reported the matrix isolation of
binary sulfur–nitrogen species produced in a sulfur/nitrogen/
argon vapor discharge.[11] Although cyclic S2N2 (D2h) was not
observed under these conditions, formation of an acyclic
NSSN isomer assigned to an IR band at 1167.4 cmꢀ1 in solid
argon was claimed.[11] Searching for such elusive S2N2 isomers,
we describe herein the ring-opening UV photolysis of matrix-
isolated S2N2 (D2h) at which novel open-shell S2N2 isomers
have been established. The identity of our new S2N2 species
has been unambiguously proved by their vibrational spectra
in 15N-labeling experiments, in mutual photo-isomerization
reactions, and by quantum-chemical calculations.
Unlike the N N bonded N2O2,[7] the homologous disulfur
ꢀ
dinitride, S2N2, consists in a planar SNSN ring structure of D2h
symmetry (1,3,2,4-dithiadiazete).[6a,8] However, beyond this
planar ring structure, theoretical studies predict the existence
of several isomers on both the lowest singlet and triplet
potential energy surfaces (PES) of S2N2.[9] Among them,
linear SNNS (1Sg ) and planar cyclic 1,2,3,4-dithiadiazete
+
=
ꢀ
(C2v) with pairwise N N and S S bonds were found to be
[*] Prof. Dr. X. Q. Zeng
The Key Lab of Health Chemistry and Molecular Diagnosis of
Suzhou, College of Chemistry, Chemical Engineering and Materials
Science, Soochow University
Suzhou 215123 (China)
E-mail: xqzeng@suda.edu.cn
For the matrix-isolation experiments, S2N2 (ca. 5 mg) was
transferred in vacuo into a small U-trap and placed in front of
the matrix support. A gas stream of Ar (2 mmolhꢀ1) passed
over the cold sample (ꢀ158C), and the resulting mixture was
deposited onto the matrix support at 16 K in a high vacuum
(see the Supporting Information for details). UV irradiation
Dr. H. Beckers, Prof. Dr. H. Willner
FB C—Anorganische Chemie, Bergische Universitꢀt Wuppertal
42119 Wuppertal (Germany)
E-mail: beckers@uni-wuppertal.de
[12]
into the p–p* electronic band of S2N2 using two different
Dr. A. Flores Antognini
light sources of lmax = 255 and 248 nm strongly depleted its IR
INQUINOA (CONICET-UNT) Instituto de Quꢁmica Fꢁsica, Facultad
de Bioquꢁmica, Quꢁmica y Farmacia, Universidad Nacional de
Tucumꢂn
bands. Several new IR bands appeared in the region 1150–
1250 cmꢀ1 that are characteristic for terminal S N stretching
=
San Lorenzo 456, 4000 Tucumꢂn (Argentina)
vibrations (Supporting Information, Figures S1 and S2).
=
[**] This research was supported in part by the Deutsche Forschungs-
gemeinschaft (WI 663/26-1), the National Natural Science Foun-
dation of China (21372173), and the Project of Scientific and
Technologic Infrastructure of Suzhou (SZS201207). X.Q.Z. highly
appreciated Prof. H. Willner for a research stay in his lab, and thanks
Prof. X. G. Bao for helpful discussions. A.F.A. acknowledges
CONICET (Consejo Nacional de Investigaciones Cientꢁficas y
Tꢃcnicas) for financial support.
Several carriers bearing terminal S N bonds are consid-
ered and they are among those depicted in Figure 1. The
acyclic isomers may adopt either cis or trans planar config-
uration and might be trapped in their lowest singlet or triplet
states. Planar bent isomers with alternate SNSN structures
(b,c) are probably produced by photoinduced cleavage of one
S N bond of cyclic S2N2 (a). In contrast, formation of NSSN
structures (h,i) needs cleavage of two S N bonds, or
recombination of two SN radicals. In any case, all four
ꢀ
ꢀ
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407538.
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Figure 1. Planar cyclic and planar acyclic isomers of S2N2.
atoms of the precursor S2N2 (a) should stay within the matrix
cage under the photolysis. In accord with this expectation, the
absorption that is due to the thiazyl (SN) monomer in solid
Ar, previously attributed to an IR band at 1209.4 cmꢀ1,[11] was
not detected among the photolysis products. Searching for
two weakly interacting SN radicals at both B3LYP/6-311 + G
(3df) and CCSD/cc-pVTZ levels, we found a C2v dimer (d) as
Figure 2. IR difference spectra: Upper trace: conversion of (SN)2 (d,
3
C2v, 1A1) into cis (b, Cs, 3A’’) and trans SNSN (c, Cs, A’’) by irradiation
with UV light of lꢂ320 nm (5 min). Lower trace: reverse conversion
of 15N-labeled (14N:15N=1:1) cis (b) and trans SNSN (c) to (SN)2 (d)
under irradiation with green light (lmax ꢁ520 nm, 10 min). Primed
symbols refer to 15N enriched isotopologues. A band of an unknown
species is labeled by an asterisk.
=
another candidate that exhibits S N stretching frequencies.
The assignment of the isomers (b), (c), and (d) in the IR
spectra in Figure 2 and 3 was confirmed by using a 14/15N-
labeled precursor (1:1). The two bands at 1251.6 and
14/
1226.1 cmꢀ1 reveal distinct 1:1 doublet patterns, and their
15N isotope shifts of 28.8 and 27.9 cmꢀ1, respectively, are
similar to that reported for the SN monomer isolated in solid
Ar (Dn(14/15N) = 28.1 cmꢀ1).[11] Such isotopic patterns suggest
the formation of SNSN chain isomers (b,c) bearing a single
=
terminal S N bond. The photoinduced interconversion
between the initially formed S2N2 isomers (Figure 2) confirms
the formation of the bent SNSN species b and c and enables
the unambiguous assignment of several far-IR bands
(Figure 3) as well as two combination bands. The agreement
between experimental and calculated frequencies listed in
Table 1 is compelling (see also the Supporting Information,
Tables S1 and S2).
Both bent SNSN isomers (b) and (c) are depleted by
visible light irradiation of lmax ꢁ 520 nm, producing the
weakly bound SN dimer (d), (SN)2 (1A1, C2v; Figure 2,
bottom trace). Re-formation of species (b) and (c) at the
expense of (d) was achieved by irradiation with light of l ꢂ
320 nm (Figure 2, upper trace). From these spectra it is
Figure 3. Far-IR difference spectra: Upper trace: conversion from cis
1
(b, Cs, 3A’’) and trans SNSN (c, Cs, 3A’’) into (SN)2 (d, C2v, A1) by
irradiation with light of lmax ꢁ365 nm (20 min), Lower trace: reverse
conversion of 15N-labeled (14N:15N=1:1) (SN)2 (d) to cis (b) and trans
SNSN (c) under irradiation of UV light (lꢂ305 nm, 10 min). Primed
symbols refer to 15N enriched isotopologues.
Table 1: Calculated and observed IR frequencies and isotope shifts Dn(15N) [cmꢀ1] of S2N2 isomers.
3
1
cis-SNSN (b, Cs, A’’)
trans-SNSN (c, Cs, 3A’’)
(SN)2 (d, C2v, A1)
Calculated[a]
Observed[b]
Calculated[a]
Observed[b]
Calculated[a]
Observed[b]
ni
ni
Dn(15N)[b]
ni
ni
Dn(15N)[b]
ni
ni
Dn(15N)[b]
1654.2 (vw)[d]
1231 (53)
1048 (5)
416 (73)
338 (52)
183 (14)
156 (2)
30.1
1726.7 (vw)[e]
1261 (61)
1036 (3)
451 (108)
295 (24)
175 (1)
1226.1 (s)
1059.0 (w)
430.2 (s)
368.5 (m)
181.9 (m)
27.9
25.1
1.6
10.3
4.0
1251.6 (m)
28.8
1250 (6)
1200 (259)
368 (4)
234 (<1)
160 (0)
1215.8 (vw)
1194.5 (s)
351.1 (vw)
7.4, (27.9)[c]
21.1, 28.0
3.7, 8.1
473.2 (m)
312.7 (w)
7.4
4.2
102 (<1)
116 (4)
[a] Calculated IR frequencies (scaled by a factor of 0.9679) and intensities (in parentheses, in kmmolꢀ1) at the B3LYP/6-311+G(3df) level. [b] Solid Ar
matrix, 16 K, relative intensities: s strong, m medium, w weak, vw very weak, Dn(15N) relative to the natural isotopologue (for comparison with
calculated values, see the Supporting Information, Tables S1–S3). [c] Tentative assignment due to overlap with a matrix site of the natural isotopomer.
[d] Combination band (1226.1+430.2=1656.3 cmꢀ1). [e] Combination band (1251.6+473.2=1724.8 cmꢀ1).
2
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=
apparent that carrier (d) shows two S N bands at 1215.8 and
consistent with the observed photodegradation of the weak
1194.5 cmꢀ1 and the 14/15N mixed sample produced three
isotopologues of (d) (Supporting Information, Table S3).
band at 909.1 cmꢀ1. Thus, we are tempted to assign this
feature to trans SNNS (C2h, 3Au).
=
The presence of two coupled S N stretching modes
Calculated structures for the species of most interest in
this work are summarized in Figure 4. We note that DFT
supports the assignment of the SN dimer (d), and rules out
ꢀ
a trans S S bonded species (h) (C2h, Figure 1). Also the
vibrational splitting of 21.3 cmꢀ1 for the in- and out-of-phase
=
S N modes of (d) is much smaller than that predicted for the
ꢀ1
3
ꢀ
S S bonded species (i) (138 cm , cis NSSN, B1; Supporting
Information, Table S4). The averaged frequencies of the two
S N modes of (d) (1205.1 cmꢀ1) and their 14/15N isotopic shifts
=
are close to those of the SN monomer (1209.4 cmꢀ1
,
Dn(14/15N) = 28.1 cmꢀ1),[11] indicating only small perturbations
to the two SN moieties. Within the instrumental frequency
range available to this study (> 180 cmꢀ1) and in accord with
calculated vibrational data (Table 1), one additional weak
feature is attributed to this dimer in the far-IR spectrum at
351.1 cmꢀ1 (Figure 3).
The photochemistry of the new S2N2 isomers provides
insight into their electronic configuration. Time-dependent
(TD)-DFT calculations for the triplet isomers generally
reveal electronic transitions in the near UV/Vis region l >
350 nm, while the lowest-energy p–p* transitions of the
singlet species are predicted in the region l < 350 nm
(Supporting Information, Table S5). Thus, the photochemical
behavior of the bent SNSN photoproducts discloses their
open-shell nature, for example, triplet cis SNSN (3A’’, b), and
trans SNSN (3A’’, c). This assignment is consistent with
quantum chemical calculations,[9c] which revealed triplet
ground states for the planar bent SNSN isomers (b) and (c),
while their lowest singlet states were found to be energetically
close to the NS(2P) + NS(2P) dissociation limit.[9]
Figure 4. Calculated molecular structures of S2N2 isomers at the
B3LYP/6-311+G(3df) level of theory. Bond lengths [ꢄ], angles [8]
(italics), molecular symmetry, and electronic states are indicated.
Secondary products obtained after near-UV (280 nm ꢃ
B3LYP calculations put the closed-shell linear species SNNS
1Sg ) at the global minimum of S2N2 (Supporting
+
l ꢃ 405 nm) photolysis of matrix-isolated S2N2 are N2S (C1v
,
(D1h
,
[13]
2040.2 cmꢀ1, Dn(14/15N) = 31.1, 36.2, and 67.8 cmꢀ1
)
and
Information, Table S7), while inclusion of electron correla-
tion at the CCSD or CCSD(T) levels changes the relative
energies of the two lowest-energy isomers, and yields the
cyclic 1,2,3,4-dithiadiazete slightly lower in energy than linear
SNNS.
likely S atoms or N2 + S2 (IR-inactive). While IR bands of
bent NS2 (C2v),[11] previously observed in an Ar matrix at
1499.7 cmꢀ1 and 1225.2 cmꢀ1, were not found in our study, N2S
(C1v) could be formed through the photodecomposition of
ꢀ
ꢀ
hitherto unknown N N bonded S2N2 isomers. N N bonded
isomers bearing two equivalent N-atoms should provide
a readily recognizable 1:2:1 isotope pattern in experiments
using 15N-enriched samples (14N:15N = 1:1). Such an isotope
In summary, the experimental results provide insights into
the complex PES of the S2N2. Linear SNNS (j) and cyclic
1,2,3,4-dithiadiazete (g, C2v) were calculated to be lower in
energy than the familiar alternate SNSN ring (a, D2h), and
singlet trans NSSN (h) was predicted to be formed preferen-
tially from two SN (X2P) radicals.[9c] However, apart from the
known D2h ring structure none of these S2N2 isomers have as
yet been detected experimentally. Instead two new triplet
bent SNSN and a new cyclic (SN)2 species have been
established for the first time, and the formation of triplet
pattern has indeed been observed for a single absorption in
ꢀ1
the S N bond stretching region at 909.1 cm (Dn(14/15N) =
ꢀ
10.9 and 20.9 cmꢀ1) after irradiation of the bent SNSN species
using a green LED light (lmax ꢁ 520 nm; Supporting Informa-
tion, Figure S3). Near-UV LED (lmax ꢁ 405 nm) photolysis
caused depletion of this band and formation of N2S (C1v).
DFT calculations predict the formation of triplet trans SNNS
3
trans SNNS (C2h, Au) by photolysis of the matrix-isolated
3
(C2h, Au) from the bent SNSN species via a cyclic transition
SNSN species is suggested. The present study expands the
known chemistry of S2N2 and contributes to the ongoing work
to unravel the hitherto still unsettled mechanism of the solid-
state polymerization of cyclic S2N2 (a), in which open-shell
SNSN isomers have been suggested to be involved.[5]
state lower in energy (178 kJmolꢀ1) than the energy utilized
in the radiation (520 nm = 230 kJmolꢀ1; Figure S4 in the
ꢀ
Supporting Information). Calculated IR data for this N N
bonded isomer fits well to the appearance of a single
fundamental IR band at 889 cmꢀ1 (Dn(14/15N) = 10.8 and
20.8 cmꢀ1) within the spectral range available to this study
(Supporting Information, Table S6), and its predicted elec-
tronic transitions (Supporting Information, Table S5) are
Received: July 23, 2014
Published online: && &&, &&&&
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Keywords: disulfur dinitride · IR spectroscopy · matrix isolation ·
photochemistry · sulfur–nitrogen compounds
.
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Elusive Isomers
X. Q. Zeng,* A. Flores Antognini,
H. Beckers,* H. Willner
&&&& — &&&&
Isomers of Disulfur Dinitride, S2N2
Two new open-shell S2N2 isomers, trans
SNSN (3A’’) and cis SNSN (3A’’), as well
as a closed-shell (SN)2 dimer (C2v, 1A1) are
formed by UV photolysis (l=248 or
255 nm) of cyclic S2N2 (1Ag) in solid Ar at
16 K. These isomers were characterized
by their IR spectra and their mutual
photo-interconversion reactions.
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These are not the final page numbers!