Conformation and Structure of Dichlorophosphoryl Isocyanate in the Gaseous and Solid Phases

Article abstract of DOI:10.1002/zaac.201800191

The conformational properties and the molecular structure of dichlorophosphoryl isocyanate, Cl₂P(O)NCO, were studied by combining vibrational spectroscopy (IR and Raman), gas-phase electron diffraction (GED), X-ray crystallography (XRD), and quantum-chemical calculations. Computationally, two conformers of C_s symmetry with the P=O bond being in syn or anti configuration relative to the NCO group are predicted to be close in energy (ca. 2 kJ mol^–1). Experimentally, both gas-phase and matrix-isolation IR spectra of Cl₂P(O)NCO suggest the presence of a single conformer, which was determined to be the energetically more favorable syn-conformer. This was also found to exist in the solid state by low-temperature XRD. However, the molecule in the solid state is significantly distorted from ideal C_s symmetry with an O–P–N–C dihedral angle of 38.1(1)° due to intermolecular C···O contacts [2.881(4) ?]. In the gas phase, the GED analysis suggests that the molecule exists predominantly as syn-conformer but with dynamic behavior about the two minimum structures (syn and anti).

Full text of DOI:10.1002/zaac.201800191

Title: Conformation and structure of dichlorophosphoryl isocyanate in
the gaseous and solid phase
Authors: Norbert Werner Mitzel, Jan Schwabedissen, Dingqing Li,
Christian Reuter, Hans-Georg Stammler, Eduard Bernhardt,
and Xiaoqing Zeng
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.201800191
Link to VoR: http://dx.doi.org/10.1002/zaac.201800191

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800191
Conformation and structure of dichlorophosphoryl
isocyanate in the gaseous and solid phase
a
b
a
a
a,
Jan Schwabedissen, Dingqing Li, Christian G. Reuter, Hans-Georg Stammler, Norbert W. Mitzel, *
Eduard Bernhardt, and Xiaoqing Zengb
c
th
Dedicated to Professor Bernt Krebs on the Occasion of his 80 Birthday
aUniversität Bielefeld, Fakultät für Chemie, Lehrstuhl für Anorganische Chemie und Strukturchemie,
Centrum für molekulare Materialien CM2, Universitätsstraße 25, 33615 Bielefeld, Germany; Email:
mitzel@uni-bielefeld.de
bCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123
Suzhou, China; E-mail: xqzeng@suda.edu.cn
cBergische Universität Wuppertal, FB C – Anorganische Chemie, Gaußstraße 20, 42097 Wuppertal,
Germany
KEYWORDS
Isocyanate – phosphorus – gas electron diffraction – vibrational spectroscopy – X-ray diffraction –
molecular structure - conformation
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Abstract
The conformational properties and the molecular structure of dichlorophosphoryl isocyanate, Cl
been studied by combining vibrational spectroscopy (IR and Raman), gas-phase electron diffraction (GED), X-ray
crystallography (XRD), and quantum-chemical calculations. Computationally, two conformers of C symmetry
with the P=O bond being in syn- or anti-configuration relative to the NCO group are predicted to be close in energy
2
P(O)NCO, have
s
–
1
(
ca. 2 kJ mol ). Experimentally, both gas-phase and matrix-isolation IR spectra of Cl P(O)NCO suggest the
2
presence of a single conformer, which was determined to be the energetically more favorable syn-conformer. This
was also found to exist in the solid state by low-temperature XRD. However, the molecule in the solid state is
significantly distorted from ideal C symmetry with an O–P–N–C dihedral angle of 38.1(1)° due to intermolecular
s
C∙∙∙O contacts (2.881(4) Å). In the gas phase, the GED analysis suggests that the molecule exists predominantly as
syn-conformer but with dynamic behaviour about the two minimum structures (syn and anti).
Introduction
1
Covalent isocyanates R–NCO are versatile reagents in synthetic and materials chemistry. For example,
toluene diisocyanate and methylene diphenyl isocyanate have been widely used in industry for
manufacturing of flexible and rigid polyurethane products as thermoplastic elastomers and thermoset
2
resins. Chemically, isocyanates are not only reactive towards alcohols and amines but also unstable due to
self-additions and trans-condensations. This is particularly the case for simple α-oxo-isocyanates such as
3
4
acyl isocyanates (RC(O)–NCO), sulfinyl isocyanates (RS(O)–NCO), and sulfonyl isocyanates (RS(O) –
2
5
NCO), most of which undergo facile oligomerization or polymerization reactions in the condensed phase
at room temperature. Therefore, the fundamentally important conformational and structural properties of
these compounds remained attracting theoretical and experimental interest in the recent few decades.
As another class of α-oxo-isocyanates, phosphoryl isocyanates (R
2
P(O)-NCO) have been used for the
synthesis of phosphorylated urethanes and ureas, and some physiologically active ones find applications as
6
7
insecticides and drugs. Similar to the synthesis of phosphorus(III) isocyanates, most phosphoryl
isocyanates can be easily prepared from phosphoryl halides and silver isocyanate. However, in contrast to
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Zeitschrift für anorganische und allgemeine Chemie
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other α-oxo-isocyanates, only very few simple phosphoryl isocyanates have been isolated and structurally
characterized. Recently, the vibrational spectroscopy and solid-state structure of the simplest phosphoryl
8
isocyanate F
2
P(O)NCO have been reported. According to the quantum-chemical calculations, two
−
1
energetically similar conformers of F
2
P(O)NCO (ΔEcal = 4.4 kJ mol , CBS-QB3) are expected to coexist,
but instead, only the lower-energy conformer with the P=O bond being in syn-configuration to the NCO
group was ascertained in the solid state by low-temperature X-ray crystallography. The preference of a
9
PNCO,7
single conformation has also been observed in other phosphorus(III) isocyanates F
2
PNCO and Cl
2
1
0
but instead, two conformers have been observed for the isoelectronic azides F PN
2
3
. Very recently, the
complex conformational landscape of triallyl phosphate has been explored by combining matrix-isolation
IR spectroscopy and density functional theory (DFT) calculations, and more than 50 conformers have been
identified.11
Continuing our interest in the synthesis and characterization of simple isocyanates7,9 and azides,7,12 herein,
we report a first-time full characterization of dichlorophosphoryl isocyanate, Cl P(O)NCO, by combining
2
vibrational spectroscopy, gas-phase electron diffraction (GED), X-ray crystallography (XRD), and
quantum-chemical calculations. Prior to this study, this isocyanate has been only characterized by IR
spectroscopy in the gas phase.13
Experimental Section
Sample preparation
Dichlorophosphoryl isocyanate, Cl
2
P(O)NCO, was synthesized by the reaction of PCl with urethane
5
1
3
(
EtOC(O)NH ) according to literature. The volatile part of the reaction mixture was distilled by passing
2
through three cold traps held at −20, −100 and −196 °C. Pure Cl
2
P(O)NCO was retained in the first trap.
The quality of Cl
2
P(O)NCO was checked by gas-phase IR spectroscopy and 31P NMR spectroscopy
(singlet, −11.9 ppm, reference to external standard H
3
PO ).
4
Vibrational and NMR spectroscopy
Gas-phase IR spectra were measured in an IR gas cell (optical path length 20 cm, Si windows, 0.5 mm
thick), which was fitted into the sample compartment of the FT-IR instrument (Bruker, Tensor 27).
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Zeitschrift für anorganische und allgemeine Chemie
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Raman spectra of the liquid were recorded on a Bruker-Equinox 55 FRA 106/S FT-Raman spectrometer
−
1
using a 1064 nm Nd:YAG laser (200 mW) with 200 scans at a resolution of 2 cm . Matrix IR spectra were
recorded on an FT-IR spectrometer (Bruker 70v) in a reflectance mode by using a transfer optic. A KBr
beam splitter and a liquid-nitrogen-cooled MCT detector were used in the spectral region of 4000–600
−
1
−1
cm . For each spectrum, 200 scans were collected at a resolution of 0.5 cm . The gaseous Cl
was mixed with neon (1:1000) in a 1-L stainless-steel container, the Cl P(O)NCO/noble gas mixture was
deposited (2 mmol h , 1.5 h) onto the cold matrix support (2.8 K, Rh-plated Cu block) in a high vacuum
2
P(O)NCO
2
−
1
−
8
(
~10 mbar), using a closed-cycle helium cryostat (Sumitomo Heavy Industries, SRDK-408D2-F40H)
inside the vacuum chamber. For the IR spectra measurement, the cryostat was horizontally rotated (±180°)
to irradiate the matrix-isolated sample with the IR beam.
Quantum-chemical calculations
A potential energy surface scan was performed on the MP2 ab initio level of theory as well as based on
several density functional theories with varying Dunning-type or Pople basis set by rotating both
isocyanate groups around the P–N bond. Subsequent optimisations of the minimum structures were
performed on the respective combination of method and basis set including the calculation of vibrational
frequencies to assure true minima. All calculations were performed using the GAUSSIAN09 quantum-
chemical program at the version D.01.14
Gas-phase electron diffraction experiment
The electron diffraction patterns were recorded on the heavily improved Balzers Eldigraph KD-G2 gas-
phase electron diffractometer at Bielefeld University. Experimental details are found in the Supporting
1
5
Information (Table S1). Instrumental details are reported elsewhere. Electron diffraction patterns were
measured on the Fuji BAS-IP MP 2025 imaging plates, which were scanned by using calibrated Fuji
BAS.1800II scanner. Intensity curves (Figure 1) were obtained by applying the method described earlier.16
Sector function and electron wavelength were refined using carbon tetrachloride diffraction patterns,17
recorded in the same experiment as the substances.
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Figure 1. Experimental (○) and model (–) molecular scattering intensities and differences for long (upper curves) and short
lower curves) nozzle-to-detector distances for Cl P(O)NCO.
(
2
Gas-phase electron diffraction structural analysis
1
8
The structural analysis was performed using the UNEX program. All refinements were performed using
two intensity curves simultaneously (Figure 1), one from the short and another from the long nozzle-to-
detector distance. These were obtained by averaging intensity curves measured in independent experiments
obtained at the same camera distance. The anti- and syn-conformers were assumed to be of C
s
symmetry
while for all the other pseudo-conformers C symmetry was used. The differences between values of
1
parameters refined in the same group were kept fixed at the values taken from the MP2/aug-cc-pVTZ
calculations. In order to calculate start values for amplitudes of vibrations and curvilinear corrections used
in the gas-phase electron-diffraction refinements, analytical quadratic and numerical cubic force fields
were calculated for all pseudo-conformers employing the MP2/aug-cc-pVTZ approximation. The bond
lengths, angles and dihedrals as well as amplitudes of vibration were refined in the same groups among the
conformers. The input potential was initially taken form MP2/aug-cc-pVTZ calculations and then refined
without any restrictions during the refinement steps. Between each pair of the ten pseudo-conformers used
explicitly by setting up individual z-matrices, ten pseudo-conformers were taken in to the refinement,
whose geometries were interpolated between the respective pairs of pseudo conformers. The mean square
amplitudes and vibrational corrections to the equilibrium structure were calculated with the SHRINK
program.19
X-Ray crystallography
A single crystal of dichlorophosphoryl isocyanate was grown in situ at 235 K and cooled to 218 K at a rate
−
1
of 1 K h . The single crystal was measured on a Rigaku SuperNova diffractometer using Mo-Kα radiation
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(
λ = 0.71073 Å) at 100.0(1) K. Using Olex2,20 the structure was solved with the Superflip21 structure
2
2
solution program using charge flipping and refined with the ShelXL refinement package using least-
−
1
squares refinements. All atoms were refined anisotropically. Crystal Data: (M = 159.89 g mol ):
monoclinic, space group P2 /c (no. 14), a = 7.18096(14) Å, b = 11.17462(18) Å, c = 6.58354(11) Å, β =
02.0804(18)°, V = 516.593(16) Å , Z = 4, μ(Mo Kα) = 1.442 mm , ρcalc = 2.056 g cm , 28035 reflections
measured (5.802° ≤ 2Θ ≤ 59.972°), 1493 unique (Rint = 0.0547, Rsigma = 0.0140) which were used in all
calculations. The final R was 0.0208 for 1475 reflections with I > 2σ(I) and wR was 0.0583 for all data.
1
3
−1
−3
1
1
2
CCDC 1579328 contains the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Discussion
Quantum chemical calculations
For the determination of possible conformers of Cl P(O)NCO existing, several energy profiles for the
2
rotation of the isocyanate group around the P–N bond were calculated. The results of the energy curves are
shown in Figure 2. The used combinations of methods and basis sets for determining the potential shoed
trustworthiness in previous examinations of the rotational barrier of isocyanantes and azides bound to
phosphorus (vide infra).
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Figure 2. Potential energy scan for the rotation of the isocyanate group around the P–N bond on different levels of theory: a
PBE0/aug-cc-pVTZ, b – MP2/aug-cc-pVTZ, c – MP2/6-311G(2df), d – B3PW91/aug-cc-pVTZ.
–
All four energy curves confirm the existence of two minima. The first and lowest in energy with a dihedral
angle ϕ(OP–NC) of 0° is the syn-conformer whereas the second occupies a dihedral angle of 180° and is
therefore called anti. Both conformers are depicted in Figure 3. Although all four methods show the same
tendency in the conformers, some differences occur among the calculations. For example the combination
2
3
24
of the ab initio perturbation theory MP2 and the Pople-type basis set 6-311G(2df) show a very shallow
progression around the minimum of the anti-conformer and the two maxima in the energy curve (ΔE(max–
−
1
25
minanti) ≈ 0.3 kJ mol ). However, the combination of the Dunning-type basis set aug-cc-pVTZ and MP2
−
1
results in a relative low minimum of the anti-conformer (ΔE(max–minanti) ≈ 0.8 kJ mol ). Furthermore, a
−
1
high-energy barrier for the transition from the syn to the anti-conformer with about 3 kJ mol is found in
2
6
the B3PW91 /aug-cc-pVTZ calculation.
Figure 3. Optimized minimum structures of dichlorophosphoryl isocyanate.
−
1
−1
Table 1. Calculated relative differences in total energy (ΔE, kJ mol ), Gibbs free energy (ΔG, kJ mol ) between the
anti- and syn-conformers and the relative abundance of the anti-conformer in the gas-phase at 298 K.
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Zeitschrift für anorganische und allgemeine Chemie
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method
ΔE ΔG anti
HF/cc-pVTZ
4.6 2.69 26%
B3PW96/cc-pVTZ
PBE0/cc-pVTZ
MP2/cc-pVTZ
HF/6-311G(2df)
B3PW96/6-311G(2df) 2.5 1.6
PBE0/6-311G(2df)
MP2/6-311G(2df)
2.6 1.8
1.9 1.3
2.0 2.3
4.4 1.7
32%
37%
28%
33%
35%
36%
30%
1.9 1.4
1.8 2.1
In Table 1 some calculated differences in the total energy (ΔE) as well as the thermodynamically corrected
Gibbs Free Energy (ΔG) are given. The abundancies of the anti-conformer were calculated based on ΔG
values for free molecules. It becomes clear that in the row of the cc-pVTZ calculations the Hartree-Fock27
and the MP2 calculation set the same trend of about 26% and 28% of the anti-conformer, respectively,
while the density-functional calculations predict the anti-conformer to be energetically preferred (32% and
3
7%). Regarding the calculations using the 6-311G(2df) basis set the calculation show lower energy
differences between the two conformers.
Table 2. Calculated geometrical parameters on different levels of theory.
HF/cc-pVTZ MP2/cc-pVTZ PBE0/cc-pVTZ
parametersa syn
anti syn anti syn anti
r(P=O)
r(P–N)
r(P–Cl)
r(N=C)
r(C=O)
1.436 1.431 1.467 1.463 1.479 1.475
1.640 1.632 1.664 1.659 1.678 1.673
1.999 2.007 2.006 2.015 2.036 2.045
1.202 1.195 1.225 1.223 1.224 1.222
1.128 1.130 1.165 1.165 1.168 1.168
∡
∡
∡
∡
(Cl–P–Cl) 103.5 103.3 103.0 102.8 103.1 103.0
(O=P–N) 116.5 114.8 117.4 114.6 117.5 114.0
(P–N–C)
132.1 141.1 129.5 133.7 133.1 137.5
(N=C=O) 175.7 176.4 174.2 175.7 174.4 174.8
a
Distances r in Å, angles and dihedral angles in deg. b
7
In comparison to the analogous phosphorus(III) compound Cl PNCO the determination of possible
2
conformers in this case is more convincing and unambiguous. In the previous case of dichlorophosphanyl
isocyanate different levels of theory suggested different conformers to be stable and it was not even clear
which conformer was the global minimum. This indicates an interaction of the phosphoryl moiety P=O
8
with the isocyanate group. The fluorinated analogue F
2
P(O)NCO only showed a very shallow minimum
for the anti-conformer on an energy curve calculated at the B3LYP/6-311+G(3df) level of theory. This
implies that the fluorine atoms also affect the conformational properties, which can be attributed to steric
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Zeitschrift für anorganische und allgemeine Chemie
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reasons because the shorter P–F bond brings the fluorine atoms closer to the isocyanate group. The higher
−
1
barrier of rotation of about 5 kJ mol also adds to this fact. Moreover, the more electronegative fluorine
atom can diminish the conjugation effects among the phosphoryl and the isocyanate groups by
withdrawing electron density. Calculated structural parameters at different levels of theory are listed in
Table 2. Along the series HF, MP2 and PBE0 in general all the bond lengths increase, whereas most of the
angles remain more or less unchanged. However, the angle
∡
(P–N–C) and in particular its variance
(N=C=O) is in all cases
between both conformers are larger in the HF calculation (Δ = 9°). The angle
∡
predicted to be about 175°. Regarding the effects of the conformation on the isocyanate group, the
differences among the conformers are negligible, as they get smaller by performing calculations at DFT or
ab initio level. The parameters varying most among the conformers are the angles
∡
(O=P–N) and
∡(P–N–
C). Whereas the angle (O=P–N) is by two or three degrees smaller in the anti-conformer, the
∡
∡
(P–N–C)
is widened by up to nine degrees in the anti-conformer (HF) and by about four degrees in the other
calculations.
Vibrational Spectra
The IR (gas-phase and Ne-matrix) and Raman (liquid) spectra of Cl
2
P(O)NCO are shown in Figure 4. To
aid the assignment, calculations of the IR spectra for the two conformers (syn and anti) B3LYP/6-
3
11+G(3df) were performed and the results are collected in Table 3.
In the IR spectra, the observed strongest bands at 2290 cm−1 (gas phase) and 2288.6 cm−1 (Ne-matrix),
belong to the antisymmetric stretching vibration of the NCO group (as(NCO)), which appears as a very
weak band at 2283 cm−1 in the Raman spectrum. The band position is red-shifted comparing to that of
−
1
−1
−1 8
F P(O)NCO (gas-phase IR: 2309 cm , Ar-matrix IR: 2306.9 cm , liquid Raman: 2307 cm ), but it is
2
−
1 28
blue-shifted compared to that of OP(NCO)
3
(liquid IR: 2265 cm ). These shifts coincide with the
(NCO)) and P=O ( PO) stretching vibrations
in the Ne-matrix locate at 1417.5 and 1312.5 cm , respectively. It is noteworthy that in the Raman
electronegativity order F > Cl > NCO. The NCO symmetric (

s

−
1
−
1
spectrum of the liquid the P=O stretching vibration shifts to 1292 cm , indicating the presence of
intermolecular interactions including the phosphoryl oxygen atom in the condensed phase. The P–N
−
1
stretching vibration (

PN) in Cl
2
P(O)NCO (747.1 cm , Ne-matrix) is slightly lower in frequency than that
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800191
−
1
8
in F
2
P(O)NCO (751.8 cm , Ar-matrix). The in-plane and out-of-plane deformation modes of the NCO
−
1
group in the Ne matrix are observed at 629.7 and 611.5 cm , and the asymmetric and symmetric stretching
−
1
vibrations of the ClPCl units in the liquid state occur at 597 and 517 cm , respectively.
1
.8
.6
1
A
*
IR1.4
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
TIR
IRam
0
2
400 2100 1800 1500 1200 900
600
300
1
−
1
Figure 4. Upper trace: IR spectrum of Cl
the frequency of CO is marked by asterisk (*). Middle trace: IR spectrum of gaseous Cl
transmission T, resolution: 2 cm ). Lower trace: Raman spectrum of liquid Cl
2
P(O)NCO isolated in a Ne-matrix at 2.6 K (absorbance A, resolution: 0.5 cm ),

/ cm
2
2
P(O)NCO at 300 K
2
P(O)NCO at 300 K (Raman intensity I,
−
1
(
−
1
resolution: 2 cm ).
Solid-state structure
As the isocyanate Cl
manually a crystal seed at 235 K and subsequently slowly chilling to 100 K. The crystallographic details can be
2
P(O)NCO is a liquid at room temperature, a suitable crystal was grown in situ by generating
found in the Experimental Section. Figure 5 shows the molecular structure of Cl
structural parameters are listed in Table 4.
2
P(O)NCO in the solid state and
−
1
2
Table 3. Calculated and Experimental Vibrational Frequencies (cm ) and Intensities of Cl P(O)NCO.
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Zeitschrift für anorganische und allgemeine Chemie
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Observedb
IR
Calculated (IIR)[IRa]a
Modec
Raman
syn
anti
Gas-phase Ne-Matrix Liquid
2
1
1
362 (1576) [5] 2367 (1449) [5] 2290 vs
469 (129) [34] 1488 (136) [27] 1421 w
313 (146) [14] 1329 (166) [16] 1319 m
2288.6 vs 2283 vw  (NCO)
as
1417.5 m
1312.5 s
747.1 s
1425 w
1292 m
740 vw
s
 (NCO)
(PO)
(PN)
7
6
6
5
5
3
3
2
2
1
8
2
28 (163) [2]
32 (139) [1]
16 (45) [<1]
80 (236) [2]
06 (61) [21]
72 (18) [6]
50 (6) [3]
713 (213) [2]
621 (31) [<1]
620 (44) [1]
569 (254) [2]
489 (39) [24]
348 (3) [5]
744 m
618 m
629.7 m
611.5 s
δ
δ
i.p.(NCO)
o.o.p.(NCO)
597 m
517 w
603.5 s
590 vw
520 vs
381 m
362 m
299 vw
247 vw
205 m
as(ClPCl)
s
 (ClPCl)
δ(OPN)
347 (6) [4]
82 (2) [3]
34 (1) [1]
92 (<1) [2]
5 (<1) [2]
321 (21) [1]
238 (1) [<1]
196 (<1 [2])
72 (2) [3]
4 (<1) [3]
23 (<1) [3]
a
B3LYP/6-311+G(3df) calculated IR intensities (km mol ) in parentheses and Raman intensities (Å amu−1) in square
brackets. Band positions and intensities: vs very strong, s strong, m medium strong, w weak, vw very weak and br
broad. Tentative assignments were made according to the vibrational displacement vectors of the syn Cl P(O)NCO.
2
−1
4
b
c
The most remarkable difference to the calculations is the deviation from ideal C
isocyanate group occupies a gauche-conformation regarding the P=O bond with a dihedral angle of
8.1(2)°, defined by the angle of torsion for P1-N1-C1-O1. This deviation has nearly no effect on the non-
s
symmetry. The
3
symmetrical P–Cl bonds, which are within experimental error of the same length (1.987(1) and 1.985(1)
Å), but slightly different O2-P1-Cl angles of 112.4(1) and 114.3(1) for Cl1 resp. Cl2.
a
Table 4. Structural parameters of Cl
2
P(O)NCO and related molecules in the solid state.
r(P=O)
r(P–N)
r(N=C)b
r(C=O)b
r(P–X)
∡(OPN)
∡(PNC) /
∡(NCO) /
∡(X–P–X)
ϕ(OP–NC) /
ϕ(XP–NC) /
ϕ(XP–NC)
∡(PNN)
∡(NNN)
ϕ(OP–NN)
−38.1(2)
Cl
2
P(O)NCO
PNCO
1.455(1)
1.623(1)
1.688(2)
1.615(2)
1.639(2)
1.638(2)
1.213(2)
1.202(2)
1.216(3)
1.225(3)
1.247(3)
1.149(2)
1.160(2)
1.146(3)
1.113(3)
1.117(3)
1.987(1)/1.985(1)
2.054(1)/2.049(1)
1.519(2)/1.517(2)
1.525(2)/1.527(2)
1.523(2)/1.526(2)
118.4(1)
135.2(1)
136.1(2)
132.5(2)
117.3(2)
118.6(2)
174.1(1)
174.6(2)
174.2(3)
173.4(3)
172.4(3)
103.6(1)
99.9(1)
99.4(1)
98.6(1)
99.0(1)
161.7(1) (Cl1)
121.3(2)
Cl
2
F
2
P(O)NCO
P(O)N b
P(O)N b
1.438(2)
1.442(2)
1.447(2)
119.9(1)
120.1(1)
120.1(1)
−18.9(5)
−0.2(3)
−0.7(3)
F
2
3
F
2
3
a
b
Distances r in Å, angles and dihedral angles in deg. In this case two crystallographically independent molecules were found
The asymmetric molecular structure is a consequence of the formation of dimers in the solid state (Figure
). In these dimers the two monomers are linked by P=O···C contacts at 2.789(1) Å which is below the
6
sum of their van der Waals radii (Σ(C,O) = 3.22 Å). The related angle is 128.1(1)° (∡P=O···C). These
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800191
dimers are linked by contacts of at 2.929(1) Å building layers perpendicular to the a axis as shown in
Figure 6, the related angle is 137.7(1)° ( P=O···C) This interaction between the electron-rich oxygen atom
∡
and the most electropositive carbon atom is also found in the solid state structure of the fluorinated
8
analogue . Hence, in F
2
P(O)NCO a trimeric species is formed with longer P=O···C contacts (2.881(4) and
7
3
.087(4) Å). Solid-state contacts in the phosphanyl isocyanate are established via the isocyanate oxygen
and the carbon atom of the adjacent molecule with 2.887(2) Å forming polymeric chains. In the extended
solid-state structure the layers are packed to a three-dimensional network by σ hole contacts of Cl2 to an
oxygen atom from an isocyanate group of the next sheet, the P1–Cl(2)···O(1) contact is 3.306(1) ) Å, the
angle at Cl2 is 160.0(1)°, see Figure 7.
Parameters of comparable substances in the solid state are listed in Table 3. A well-established indication
2
9
of the electronegativities of the atoms or groups connected to the phosphoryl moiety is the P–O bond. As
the electronegativity increases in the row F, Cl, NCO, N the shortest P–O bond is found in
difluoroisocyanate followed by difluoroazide and the longest bond is found in the substance under
investigation. On the contrary, the P–N bond elongates along the following series: F P(O)NCO,
Cl P(O)NCO, F P(O)N , Cl PNCO. This might indicate that the azide group favours less conjugation to
3
2
2
2
3
2
the phosphoryl moiety. Between both phosphoryl isocyanates the bonds N=C and C=O do not vary much
but in the phosphanyl isocyanate the N=C bond is by 0.01 Å slightly shorter whereas the C=O bond is
slightly longer by the same magnitude. The P–Cl bond is shorter in the phosphoryl isocyanate than that in
the phosphanyl isocyanate as the lone pair at phosphorus in the later one repells the chlorine atoms. For the
same reason the Cl–P–Cl angle is tighter in the phosphorus(III) compound. The angle at the nitrogen atom
P–N–C or P–N–N is larger in the isocyanate compounds.
Figure 5. Molecular structure of dichlorophosphoryl isocyanate in the solid state. a shows the numbering, b the view
along the P–N bond. Displacement ellipsoids are depicted with 50% probability.
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800191
2
Figure 6. Aggregation to layers of Cl P(O)NCO by O···C contacts in the solid state, view approximately along the
crystallographic a-axis. Thick stippled lines for the contacts with 2.789(1) Å, thin ones for 2.929(1) Å.
Among these three isocyanates the largest angle is found in the phosphanyl isocyanate and the shortest in
F
2
P(O)NCO. The N–C–O angle is in all three isocyanates about 174.3° within the error limits, whereas the
corresponding N–N–N angle in the azide is slightly smaller.
2
Table 5. Structural parameters of the gas-phase structure of Cl P(O)NCO and related molecules.
r(P=O)
r(P–N)
r(N=C)
r(C=O)
r(P–X)
∡(OPN) ∡(PNC) ∡(NCO) ∡(X–P–X)
118.3(2) 135.8(2) 175.3(2) 102.8(1)
137.2(6) 171.5(6) 99.4(2)
Cl
Cl
2
P(O)NCO, r
g
1.442(1) 1.646(1) 1.212(1) 1.157(1) 1.995(1)
1.667(3) 1.203(1) 1.159(1) 2.045(1)
2
PNCO, r
g
F
2
F
2
F
2
P(O)N
PN
PNCO, r
3
, rh1
, rh1
1.437(4) 1.657(2) 1.251(3) 1.130(2) 1.5316(6) 118.9(3) 117.8(5) 172(2)
98.8(3)
96.4(2)
97.9(8)
3
1.716(3) 1.247(3) 1.127(3) 1.572(2)
1.683(6) 1.256(6) 1.168(5) 1.563(3)
117.7(5) 177(1)
130.6(8)
a
a
b
Distances r in Å, angles and dihedral angles in deg. For the azides the analogous parameters are given.
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Concerning the conformation in the solid state, the azide compound with a dihedral angle
ϕ(OP–NN) of 0.2(3)° and 0.7(3)° is closest to an ideal syn-conformation in the solid state
while the isocyanates deviate more from C symmetry. The fluorine analogue adopts a dihedral
−
−
s
angle ϕ(OP–NC) of 18.9(5)° and the dichloride a larger one of 38.1(1)°. This reflects the
higher flexibility of the isocyanate group and the conformational stability caused by the
fluorine atoms
.
2
Figure 7. Packing of Cl P(O)NCO in the solid state. View approximately along the crystallographic c-axis.
Gas-phase structure
The structure of free molecules of Cl
2
P(O)NCO was determined by gas-phase electron
diffraction. A dynamic model7,30 was used to describe the molecular structure, because neither
a static one- nor a two-conformer model resulted in a satisfactory agreement with the
experimental data. Especially in the region between 4 and 5 Å in the radial distribution curve
where mainly the distances between the isocyanate oxygen atom and the atoms bound to
phosphorus are mapped. The dynamic model was set up using ten different pseudo-conformers
with a dihedral angle ϕ(OP–NC) being varied over a range starting at ϕ(OP–NC) = 0° (syn) to
ϕ(OP–NC) = 180° (anti) with steps of 20°. For all pseudo-conformers, distinct cubic force
fields were calculated at the MP2/aug-cc-pVTZ. The starting potential was taken from the
MP2/aug-cc-pVTZ calculation. For modelling the potential during the refinement, three
parameters were used and refined. As the potential was refined without any restrictions, it is
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Zeitschrift für anorganische und allgemeine Chemie
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possible that the model can result in static one or two conformer models or in models with less
dynamic character than the one set up at the start of the refinement. The refined model resulted
in overall a disagreement factor R of 2.47%. The final radial distribution curve is depicted in
f
Figure 8 also containing some labelled interatomic distances of the syn-conformer, which is
lowest in energy.
Figure 8. Experimental (circles ○) and model (continuous line –) radial distribution functions of
Cl P(O)NCO. The difference curve is shown below. Vertical bars indicate interatomic distances of the syn-
2
conformer (lowest in energy), selected ones are labelled.
The potential described by these three parameters deviates from quantum-chemically predicted
potential. Both are depicted in Figure 9. By comparison of the both dynamic models for the
Cl
2
PNCO and the here examined Cl P(O)NCO it is evident that in the phosphorus(V)
2
compound a more local dynamic model, is describing the composition in the gas phase. The
term local dynamic means that not the full range of the dynamic parameter (here ϕ(OPNC)) is
necessary to describe the gas-phase composition but pseudo conformers close to the minimum
structures are used for modelling the gas-phase structure. The usage of the local dynamic
model implies that the phosphoryl moiety in Cl P(O)NCO is stabilizing both conformers
2
observed in the gas phase and that the isocyanate group is less flexible that in the
phosphorus(III) analogue. Thus, the gas-phase structure was described by using the two
minimum conformers and additionally pseudo conformers whose dihedral angle ϕ(OPNC) vary
slightly from the respective ones of the minimum conformers. Because of the local dynamic
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.201800191
description of the gas phase structures in here, Cl
2
P(O)NCO seems to be more related to the
azides F
2
PN
3
and F
2
P(O)N , as these compounds show a local dynamic behaviour as well in
3
the gas phase.
Figure 9. Potential a) calculated at the (MP2/aug-cc-pVTZ) level of theory and b) refined against
experimental scattering intensities.
In Table 5 gas-phase structural parameters of related molecules are listed. The effect of the
introduction of an oxygen atom and the resulting higher oxidation state of the phosphorus atom
is a decreasing bond length to the other atoms at phosphorus. In the isocyanates, the P–N bond
is shortened by 0.02 Å to 1.646(1) Å and the P–Cl bond is 0.05 Å shorter in the phosphoryl
compound. The same tendency is seen for the azides. However, only minor changes are
observed in the pseudo-halide moieties. The bond lengths are the same within experimental
error, whereas the N=C bond is slightly longer in phosphoryl isocyanate. Interestingly the
deviation from linearity in the pseudo halides cannot be correlated to the oxidation state of the
central phosphorus atom. In general it is to be mentioned that the angle
∡(NCO) was often
described as a parameter hard to determine, thus, in some cases it is not even reported31 or
treated as a fixed parameter.32 Anyhow, in the here examined case contributions from
interatomic scattering of the isocyanate oxygen and the chlorine made it possible to determine
this angle with reasonable accuracy.
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10.1002/zaac.201800191
Conclusions
Dichlorophosphoryl isocyanate, Cl
2
P(O)NCO, was synthesized and fully characterized. The
molecular conformation and structure was examined by means pf gas-phase electron
diffraction, vibrational spectroscopy in different phases and in the solid state by X-ray
diffraction. The applied theoretical methods all suggest the syn-conformer to be the most stable
one besides an anti-conformer, as the barrier for interconversion was calculated to be between
−
1
2
and 3 kJ mol . A dynamic model was used in the gas-phase structure refinement. The
resulting abundances of the different pseudo conformers show that the isocyanate group is
vibrating close to the computationally determined minima. In the solid state dimers are formed
via C···O contacts, which build layers in the crystal phase.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Deutsche Forschungsgemeinschaft (DFG, core facility GED@BI,
grant MI477/35-1) and the National Natural Science Foundation of China (21372173,
2
1422304), the Beijing National Laboratory for Molecular Sciences (20140128), the Priority
Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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