N-Heterocyclic Carbene Adducts of Phenyldioxophosphorane and Its Heavier Sulfur and Selenium Analogues

Article abstract of DOI:10.1002/ejic.201700494

The reactions of the N-heterocyclic carbene–phenylphosphinidene adducts (NHC)PPh with dioxygen, elemental sulfur or selenium are reported. For NHC = 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe), the corresponding complexes (IMe)PE₂Ph of phenyl

Full text of DOI:10.1002/ejic.201700494

A Journal of
Accepted Article
Title: N-Heterocyclic Carbene Adducts of Phenyldioxophosphorane
and its Heavier Sulphur and Selenium Analogues
Authors: Dirk Bockfeld, Thomas Bannenberg, Peter G. Jones, and
Matthias Tamm
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
N-Heterocyclic Carbene Adducts of Phenyldioxophosphorane
and its Heavier Sulphur and Selenium Analogues
Dirk Bockfeld,[a] Thomas Bannenberg,[a] Peter G. Jones[a] and Matthias Tamm*[a]
Dedicated to Prof. Dr. Anthony J. Arduengo, III, on the occasion of his 65th birthday.
Abstract: The reactions of the N-heterocyclic carbene-phe-
nylphosphinidene adducts (NHC)PPh with dioxygen, elemental sul-
phur or selenium are reported. For NHC = 1,3,4,5-tetramethylimidaz-
olin-2-ylidene (IMe), the corresponding complexes (IMe)P(E)2Ph of
spectively.[19] Cleavage of the dimers can also be achieved by ad-
dition of Lewis bases, and for instance, pyridine-stabilised PhPE2
(E = S, Se) can be isolated in high yield by stirring the correspond-
ing dimers (PhPE2)2 in pyridine.[20,21] To the best of our knowledge,
NHC adduct formation with LR, WR or related phosphorus(V)
species has not been reported, while cleavage of the four-mem-
bered P2E2 ring was recently observed for NHC addition to related
dimeric “phosphinidene chalcogenides” (Ar*PE)2 (Ar* = 2,6-
bis(2,4,6-trimethylphenyl)phenyl, E = S, Se), which afforded the
phosphorus(III) adducts (NHC)P(E)Ar* (III) with NHC = 1,3-diiso-
propyl-4,5-dimethylimidazolin-2-ylidene (Scheme 1).[22]
phenyldioxophosphorane
(PhPO2),
phenyldithioxophosphorane
(PhPS2) and phenyldiselenoxophosphorane (PhPSe2) were isolated
and fully characterised, including determination of their molecular
structures by X-ray diffraction analyses. For the adducts of the steri-
cally more demanding carbenes 1,3-bis(2,4,6-trimethylphenyl)imidaz-
olin-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-
ylidene (IPr), the reactions with dioxygen afforded the phenyldioxo-
phosphorane complexes (IMes)PO2Ph and (IPr)PO2Ph, and their X-
ray crystal structures were established. In contrast, their reactions
with sulphur and selenium resulted in the cleavage of the NHC–phos-
phorus bond and the formation of 2,4-diphenyl-2,4-dithioxo-1,3,2,4-
dithiadiphosphetane, (PhPS2)2, and 2,4-diphenyl-2,4-diselenoxo-
1,3,2,4-diselenadiphosphetane, (PhPSe2)2 (Woollins’ reagent).
Introduction
Phenyldioxophosphorane (PhPO2, I), “the elusive phosphorus an-
alogue of nitrobenzene”, was recently prepared under matrix iso-
lation conditions by reaction of dioxygen with photochemically
generated triplet phenylphosphinidene (PPh).[1] Prior to this report,
the existence of dioxophosphoranes had only been inferred from
trapping experiments,[2–5] whereas monomeric dithioxo- and dise-
lenoxophosphoranes of the type ArPE2 (E = S, Se) were success-
fully isolated in the presence of very bulky (e.g. Ar = 2,4,6-tri-tert-
butylphenyl) or donor-functionalised aryl substituents.[6–10] In the
absence of donor ligands, phenyldioxophosphorane aggregates
to trimeric metaphosphonate (PhPO2)3 (II),[11–14] while the sulphur
and selenium analogues form the dimeric species 2,4-diphenyl-
2,4-dithioxo-1,3,2,4-dithiadiphosphetane, (PhPS2)2,[15] and 2,4-di-
phenyl-2,4-diselenoxo-1,3,2,4-diselenadiphosphetane,
Scheme 1. Selected phosphinidene chalcogenides, Ar* = 2,6-
bis(2,4,6-trimethylphenyl)phenyl, Dipp = 2,6-diisopropylphenyl.
(PhPSe2)2 (Woollins’ reagent, WR).[16,17] Monomeric species
ArPE2 are involved if these compounds or the corresponding sul-
phur derivative (ArPS2)2 (Ar = 4-methoxyphenyl, Lawesson’s rea-
gent, LR)[18] are used for thionation or selenation reactions, re-
N-Heterocyclic carbene-phenylphosphinidene adducts of the
type (NHC)PPh can be prepared directly from the free carbenes
such as 1,3,4,5-tetramethylimidazolin-2-ylidene (IMe) by reaction
with pentaphenylcyclopentaphosphane, (PPh)5,[23,24] or in two
steps by reaction with dichlorophenylphosphine, PPhCl2, followed
by reduction with KC8.[25] These species are best described as in-
versely polarised phosphaalkenes as illustrated by the mesomeric
structures A and B,[26] and the Lewis base behaviour and coordi-
nation chemistry of these species is consistent with the availability
of two lone pairs on the phosphorus atom.[27–29] Accordingly, we
reasoned that their reaction with chalcogens should lead to selec-
tive oxidation of the phosphinidene moiety to afford the corre-
sponding carbene-stabilised phenyldioxo-, phenyldithioxo- and
[a]
Institut für Anorganische und Analytische Chemie, Technische Uni-
versität Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
E-mail: m.tamm@tu-bs.de
http://www.tu-braunschweig.de/iaac
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
phenyldiselenoxophosphoranes (Scheme 1), potentially via inter-
mediate phosphorus(III) monochalcogenides such as III. With ox-
ygen, such reactivity has been observed for related phosphaal-
kenes of the type (Me2N)2C=PAr upon reaction with ozone (for Ar
= Ph)[30] or air (for Ar = 2,4,6-tri-tert-butylphenyl).[31] Similarly, “car-
bene-stabilised diphosphorus”[32] (IPr)2P2 (IPr = 1,3-bis(2,6-diiso-
propylphenyl)imidazolin-2-ylidene) reacts with dry O2 to form the
diphosphorus tetroxide (IPr)2P2O4 (IV),[33] and other NHC-phos-
phorus(I) compounds have also been shown to react slowly with
dioxygen.[34,35] In contrast, we are unaware of any study dealing
with the oxidation of NHC-phosphinidene adducts with sulphur or
selenium, whereas the reaction of cationic dicarbene-phospho-
rus(I) species afforded bromide salts of the type [(NHC)2PS2]Br
(V).[36] The reactivity of cyclic phosphanylidene-phosphoranes
with chalcogens was also studied,[37–39] and, for instance, the di-
thionated and diselenated compounds VI were structurally char-
acterised.[39] In contrast, reaction with dioxygen generally pro-
ceeds with cleavage of the P-P bond.[39,40]
1c fall in the range between –50 and –19 ppm (Table 1).[23–25] An
opposite trend is established by 13C NMR spectroscopy for the
carbene carbon atoms in 2a and 2b, which display doublets at
significantly higher field, viz. 145.5 ppm (1JP-C(NHC) = 99.4 Hz) and
153.8 ppm (1JP-C(NHC) = 80.8 Hz), in comparison with 1a and 1b.
For 2c, the corresponding Ccarbene resonance could not be found.
1
It appears noteworthy that the JP-C(NHC) couplings in 2a and 2b
are similar to or smaller than those in 1a and 1b, whereas an in-
1
crease of ca. 100 Hz is observed for the JP-C(Ph) coupling con-
stants upon oxidation of 1a–1c to 2a–2c (Table 1).
Scheme 2. Preparation of NHC-phosphinidene dioxides.
Herein, we demonstrate that the P-Ccarbene bonds in the car-
bene-phosphinidene adducts of the type (NHC)PPh (1a, NHC =
IMe, 1,3,4,5-tetramethylimidazolin-2-ylidene; 1b, NHC = IMes,
1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene; 1c, NHC = IPr,
1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) resist oxida-
tion with O2 to formally afford N-heterocyclic carbene adducts of
phenyldioxophosphorane (NHC)PO2Ph. Furthermore, their reac-
tivity towards S8 and grey selenium was studied and showed a
strong dependence on the nature of the carbene ligand.
Table 1. Chemical shifts of NHC-phenyldioxophosphoranes 2a, 2b and 2c
compared to the corresponding NHC-phosphinidenes 1a[24], 1b[23] and 1c[25]
.
a
b
c
31P
C
-53.5 ppm[a]
169.1 ppm[a]
-23.0 ppm[b]
170.0 ppm[a]
-18.9 ppm[b]
172.9 ppm[b]
13
NHC
(NHC)PPh
1JP-
98.0 Hz
102.8 Hz
104.7 Hz
(1)
C(NHC)
Results and Discussion
13
C
150.6 ppm[a]
49.7 Hz
140.1 ppm[a]
42.3 Hz
139.1 ppm[b]
43.2 Hz
Ph
We initially decided to study the reactivity of NHC-phosphinidene
adducts systematically, since, in a previous study,[41] we had un-
intentionally isolated single crystals of the dioxophosphorane
(IPr)PO2Mes from an NMR sample in C6D6 of the corresponding
mesitylphosphinidene species.[42] The molecular structure was
determined by X-ray diffraction analysis and is shown in the Sup-
porting Information (Figure S1). Exposure of THF (for 1a) or tolu-
ene (for 1b and 1c) solutions of the phenylphosphinidene adducts
1 to air under ambient conditions afforded the desired dioxo com-
pound 2 in satisfactory yield (81%) only in the case of 1c, whereas
product mixtures were obtained for 1a and 1b. In these cases, the
formation of significant amounts of imidazolium salts as side prod-
ucts indicates hydrolysis of the phenyldioxophosphoranes 2a and
2b with cleavage of the P–Ccarbene bonds. Performing the reactions
in air that was dried by passing through P4O10 afforded both com-
pounds in pure form as colorless solids in moderate yield (2a:
62%, 2b, 69%, Scheme 2). In the solid state, compounds 2 are
air-stable; they are soluble in polar solvents such as chloroform,
dichloromethane or DMSO to produce air-sensitive solutions. The
rate of decomposition seems to follow the order 2c < 2b < 2a as
indicated by NMR spectroscopy, which indicates formation of sub-
stantial amounts of imidazolium salt within a few hours for 2a and
2b or within several days for 2c.
1JP-C(Ph)
-1.0 ppm[c]
0.8 ppm[d]
31P
-1.2 ppm[e]
153.8 ppm[e]
80.8 Hz
0.3 ppm[d]
13
[f]
C
145.5 ppm[c]
99.4 Hz
–
NHC
(NHC)PO2Ph
1JP-
[f]
(2)
–
C(NHC)
13
C
139.8 ppm[c]
144.2 Hz
137.2 ppm[e]
150.0 Hz
136.6 ppm[d]
152.3 Hz
Ph
1JP-C(Ph)
[a] recorded in THF-d8. [b] recorded in C6D6. [c] recorded in DMSO-d6.
[d] recorded in CDCl3. [e] recorded in CD2Cl2. [f] not observed.
The molecular structures of 2a–2c were established by X-ray
diffraction analyses and are shown in Figures 1–3. 2c crystallizes
with imposed mirror symmetry. All compounds are monomeric
and display strongly distorted tetrahedral geometries around the
phosphorus atom with large O–P–O angles of ca. 122° and small
C–P–C angles of ca. 102°. The P–Ccarbene bond lengths are elon-
gated from 1.794(3) Å[24] to 1.8586(15) Å, from 1.763(6) Å[23] to
1.8711(15) Å and from 1.7658(10) Å[29] to 1.8753 (19) Å upon ox-
idation of 1a–1c to 2a–2c, respectively, whereas a marginal short-
ening of the P–Cphenyl bonds from 1.816(3) Å[24] to 1.8117(15) Å,
Upon oxidation, characteristic downfield shifts are observed
for the 31P NMR signals of 2a–2c, which are found close to 0 ppm,
whereas the chemical shifts reported for the starting materials 1a–
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
from 1.838(5) Å[23] to 1.8081(16) Å and from 1.8238(10) Å[29] to
1.805(2) Å is observed. The phosphorus-oxygen bond lengths are
between 1.4817(13) Å and 1.4841(11) Å, which falls in the range
of terminal P–O bonds in R2PO2 systems and related phos-
phinates.[31,43] The N–C–N angles (2a: 106.31(12)°, 2b:
105.77(13)°, 2c: 106.04(16)°) are slightly larger than those in the
C21
O1'
O1
P1
C1
corresponding NHC-phosphinidenes (1a: 104.7(2)°[24]
,
1b:
N1'
N1
104.0(5)°[23], 1c: 104.35(8)°[29]), which reveals a decrease of car-
bene character as indicated by the zwitterionic presentation of
compounds 2a–2c in Scheme 2.[44,45] The crystal structures of 2b
and 2c feature short O···H contacts between the oxygen atoms
on phosphorus and the NHC backbone, building up chains paral-
lel to the a axis (2b) or the b axis (2c). In addition, the crystal
structure of a solvate of 2c with one equivalent of chloroform and
half an equivalent of toluene was determined; values quoted here
are for the solvent-free form. The corresponding structural data,
together with selected packing diagrams of 2a, 2b and 2c can be
found in the Supporting Information.
Figure 3. Molecular structure of 2c with thermal displacement pa-
rameters drawn at the 50% probability level. The asymmetric unit
consists of half a molecule; N1’ and O1’ are generated via a mirror
plane. Hydrogen atoms were omitted for clarity. Selected bond
lengths [Å] and angles [°]: P–C1 1.8753(19), P–C21 1.805(2), P–
O1 1.4856(10), N1–C1–N1’ 106.04(16), C21–P–C1 102.41(9),
O1–P–O1’ 122.75(9).
O2
O1
P1
C11
The preparation of the heavier homologues of 2 proved to be
more challenging. Reaction of the sterically more demanding
NHC phosphinidenes 1b and 1c with elemental sulphur (S8) or
selenium (Segrey) did not lead to selective product formation.
Some of the reaction products that could be identified by NMR
spectroscopy are the corresponding imidazole-2-thione (IPr)S[46]
or imidazole-2-selenones (IMes)Se[47] and (IPr)Se,[48,49] which in-
N1
C1
N2
dicates lability of the NHC-phosphorus bond. Furthermore, the
Figure 1. Molecular structure of 2a with thermal displacement pa-
rameters drawn at the 50% probability level. Hydrogen atoms
were omitted for clarity. Selected bond lengths [Å] and angles [°]:
P–C1 1.8586(15), P–C11 1.8117(15), P–O1 1.4902(10), P–O2
1.4855(11), N1–C1–N2 106.31(12), C11–P–C1 101.07(6), O1–
P–O2 121.02(6).
[50–53]
formation of poly(phenylphosphinidenes) (PPh)n
was ob-
served by 31P NMR spectroscopy during the reaction of 1b with
selenium. The reaction of 1c with selenium in C6D6 afforded a
greenish solution with several 31P NMR signals between 83 and
120 ppm, indicating the formation of (PhP)3Se2 and (PhP)3Se3
(see the Supporting Information).[54,55] After prolonged storage of
this solution (up to ten days), small red crystals had formed which,
according to single crystal X-ray diffraction and elemental analy-
sis, proved to be Woollins’ reagent (WR, Scheme 1).
O1
C31
In contrast, the sterically less congested (IMe)PPh (1a) re-
acted readily with sulphur and selenium in THF to afford the cor-
responding NHC-phosphinidene disulfide 3 and diselenide 4 as
colourless solids in satisfactory yield, i.e. 75% (3) and 66% (4).
Both compounds are soluble in dichloromethane and chloroform,
but slowly decompose in these solvents with formation of imidaz-
olium salts, as shown by NMR spectroscopy. However, the stabil-
ity in dichloromethane was sufficient for their characterization. In
dimethyl sulfoxide, 2a was formed; in the case of 4, this conver-
sion was accompanied by the precipitation of elemental selenium.
P1
O2
C1
N1
N2
Figure 2. Molecular structure of 2b with thermal displacement pa-
rameters drawn at the 50% probability level. Hydrogen atoms
were omitted for clarity. Selected bond lengths [Å] and angles [°]:
P–C1 1.8711(15), P–C31 1.8081(16), P–O1 1.4841(11), P–O2
1.4857(12), N1–C1–N2 105.77(13), C31–P–C1 101.74(7), O1–
P–O2 122.68(7).
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
P=Se: 2.093 Å)[43] and lie in the range of comparable anionic
R2PE2 species.[56–62]
S2
P1
S1
C1
N2
C11
N1
Scheme 3. Preparation of an NHC-phosphinidene disulfide and
diselenide.
Figure 4. Molecular structure of 3 with thermal displacement pa-
rameters drawn at the 50% probability level. Hydrogen atoms
were omitted for clarity. Selected bond lengths [Å] and angles [°]:
P–C1 1.8659(12), P–C11 1.8164(12), P–S1 1.9780(4), P–S2
1.9688(4), N1–C1–N2 106.62(10), C11–P–C1 103.41(5), S1–P–
S2 120.007(19).
Table 2. Chemical shifts of NHC-phosphinidene dichalcogenides 3 and 4.
(IMe)P(S)2Ph (3)
52.9 ppm[a]
(IMe)P(Se)2Ph (4)
2.3 ppm[a]
31P
C
145.4 ppm[a] 1JP-C = 49.8 Hz
142.4 ppm[a] 1JP-C = 91.0 Hz
140.2 ppm[a] 1JP-C = 28.0 Hz
139.4 ppm[a] 1JP-C = 70.5 Hz
13
Se2
Se1
NHC
13
C
Ph
P1
C1
N1
[a] recorded in CD2Cl2.
C11
N2
As also established for the dioxo species 2, a pronounced
downfield shift of the 31P NMR resonance is observed upon oxi-
dation, viz. from –53.5 ppm in 1a[24] to 52.9 ppm in 3 (Table 2),
which is also at significantly lower field than found for 2a (0.8 ppm
in CDCl3, Table 1) and for the NHC-phosphinidene monosulfide
III (29 ppm, Scheme 1).[22] The diselenide 4 exhibits a 31P NMR
signal at 2.3 ppm, which shows coupling with the 77Se nuclei. Sim-
Figure 5. Molecular structure of 4 with thermal displacement pa-
rameters drawn at the 50% probability level. Hydrogen atoms
were omitted for clarity. Selected bond lengths [Å] and angles [°]:
P–C1 1.861(3), P–C11 1.827(3), P–Se1 2.1361(8), P–Se2
2.1299(8), N1–C1–N2 106.3(3), C11–P–C1 104.08(15), Se1–P–
Se2 119.37(4).
1
ilarly, a doublet at 71.3 ppm with JP-Se = 681 Hz is observed in
the 77Se NMR spectrum. In the 13C NMR spectra of 3 and 4, dou-
blets are also found for the carbene carbon atoms at 145.4 ppm
(1JP-C = 49.8 Hz) and 140.2 ppm (1JP-C = 28.0 Hz) and also for the
ipso-phenyl carbon atom at 142.4 ppm (1JP-C = 91.0 Hz) and 139.4
ppm (1JP-C = 70.5 Hz). These coupling constants are smaller than
those observed for dioxophosphorane 2a, but again, the 1JP-C(NHC)
1
are smaller than the JP-C(Ph) coupling constants, which reverses
the trend found for the phosphinidene adduct 1a (Table 1).[24]
The molecular structures of 3 and 4 were determined by X-ray
diffraction analyses and are presented in Figures 4 and 5. The
structures are not isotypic. Additionally, the structure of 4·C6D6
was determined (see Supporting Information) but was less accu-
rate, so that numerical values are quoted only for the solvent-free
structure. The distorted tetrahedral geometries around the phos-
phorus atoms in 3 and 4 resemble that in 2a, with large S–P–S
and Se–P–Se angles of ca. 120°. The P-C bond lengths are also
similar to those in 2a, with significantly longer P-Ccarbene than P-
Cphenyl bonds, i.e. 1.8659(12) Å vs. 1.8164(12) Å in 3 and 1.861(3)
Å vs. 1.827(3) Å in 4. This is consistent with the trend established
for the 31P-13C coupling constants (Table 2). The P–E bond
lengths (3: 1.9780(4) and 1.9688(4) Å; 4: 2.1361(8) and 2.1299(8)
Å) are longer than expected for P=E double bonds (P=S: 1.954 Å;
Scheme 3. Reaction of 1,3,4,5-tetramethylimidazolin-2-ylidene
(IMe) with Woollins’ reagent (WR); yields were determined by 1H
NMR spectroscopy of the crude reaction solution.
In view of the synthesis of the phosphinidene sulfide and sele-
nide of the type (NHC)P(E)Ar* (III) by NHC addition to dimeric
(Ar*PE)2,[22] we expected that disulfido and diselenido species
such as 3 and 4 might also be accessible by reaction of dimeric
phosphorus(V) compounds such Lawesson’s (LR) or Woollins’
(WR) reagent (Scheme 1) with two equivalents of an N-heterocy-
clic carbene. Indeed, addition of 1,3,4,5-tetramethylimidazolin-2-
ylidene (IMe) to WR in THF-d8 indicated formation of the
diselenide 4 in ca. 25% yield by 1H NMR spectroscopy (Scheme
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
3). The formation of this compound was further confirmed by ob- Conclusion
servation of the diagnostic doublet at 71.3 ppm in the 77Se NMR
spectrum (vide supra, see also the Supporting Information). The
main product, however, which was formed in ca. 75% yield, was
identified as the selenone (IMe)Se with (77Se) = 21.7 ppm,[63]
which reveals that WR mainly acts as a selenation reagent to-
wards the free carbene ligand.
We have studied the reactivity of the NHC-phosphinidene ad-
ducts (IMe)PPh (1a), (IMes)PPh (1b) and (IPr)PPh (1c) towards
dioxygen, elemental sulphur and selenium. The reaction with di-
oxygen furnished the corresponding phenyldioxophosphorane
adducts (NHC)PO2Ph 2a–2c. The reactions of 1a with sulphur
and selenium afforded the phenyldithi-
Table 3. NBO results (B97-D/6-311G**) for compounds 2a, 3 and 4.
oxophosphorane and phenyldiselenoxo-
phosphorane adducts (IMe)PS2Ph (3)
and (IMe)PSe2Ph (4), whereas decom-
position products such as polyphos-
phines and imidazole-2-thiones or imid-
Charges
WBI[a]
CNHC–P
Comp.
azole-2-selenones were formed with the
sterically more demanding species 1b
and 1c. Our spectroscopic, structural
and theoretical results suggest that the
classification of compounds 2–4 as N-
heterocyclic carbene adducts of phe-
nyldioxophosphorane (PhPO2), phenyl-
dithioxophosphorane (PhPS2) and phe-
nyldiselenoxophosphorane (PhPSe2) is
justified, at least to a larger extent than
q(CNHC
)
q(P)
q(CPh
)
q(E1) / q(E2)
P–E1
P–E2
P–CPh
1a
2a
3
0.11
0.19
–0.31
–0.38
–0.37
–0.37
-
1.25
0.64
0.71
0.75
-
-
1.00
0.15
0.15
0.15
2.08
1.06
0.93
–1.08 / –1.08
–0.59 / –0.60
–0.53 / –0.54
1.17
1.29
1.22
1.17
1.25
1.20
0.78
0.79
0.81
4
[a] WBI = Wiberg bond indices
regarding the starting materials, the inversely polarised phosphal-
kenes 1, as true “NHC-phosphinidene adducts”.
To analyse the bonding in the dichalcogenides 2a (E = O), 3
(E = S) and 4 (E = Se), their molecular structures were optimised
by applying the density functional theory (DFT) method B97-D
with the triple- 6-311G** basis set, which was followed by natural
bond orbital (NBO) analysis. The computational results are sum-
marised in Chapter S7 of the Supporting Information (Tables S3–
S6), and contour plots of selected NBOs are shown in Figures
S32–S34. The natural bond orbital (NBO) analysis revealed that
all compounds are best presented by ylidic mesomeric structures
as shown in Schemes 2 and 3 or, preferably, by the doubly zwit-
terionic structure in Table 3. While the NBO charges at the car-
bene and phenyl carbon atoms are very similar in all three com-
pounds (q(CNHC) = 0.15, q(CPh) = 0.38/0.37), the polarization of
the phosphorus-chalcogen bonds decreases significantly with de-
creasing electronegativity of the chalcogen atoms (O > S ≈ Se).
Accordingly, q(P) decreases from 2.08 (2a) to 1.06 (3) and 0.93
(4). In all cases, the polarization of the CNHC-P bond is significantly
higher than in the NHC-phosphinidene adduct (IMe)PPh (1a),
which exhibits q(P) = 0.19 (Table 3). This is also reflected by the
Wiberg bond indices (WBI) of the CNHC-P bond, which decrease
from 1.25 in 1a to 0.64, 0.71 and 0.75 in 2a, 3 and 4, respectively.
Notably, the WBI of the CPh-P bonds also decreases from 1.00 in
1a to ca. 0.80 upon oxidation, but the order of the CNHC-P and CPh-
P bond strengths is inverted compared to 1a, which agrees with
Experimental Section
Materials and Instruments: All manipulations were performed under a
strictly dry argon atmosphere using standard Schlenk-line techniques and
dry argon-filled gloveboxes. The solvents were dried with an MBraun sol-
vent-purification system. The starting materials, carbene-phosphinidene
adducts 1a[24], 1b[23] and 1c[25] were prepared according to previously pub-
lished procedures. To generate dry air for the syntheses of 2, compressed
air was passed through columns filled with phosphorus pentoxide. The 1H,
13C, 31P and 77Se NMR spectra were recorded with Bruker Avance DPX
200 (200 MHz), Bruker Avance II 300 (300 MHz), Bruker Avance III HD
300 (300 MHz), Bruker DRX 400 (400 MHz) and Bruker AV III 400
(400MHz) spectrometers. The chemical shifts are given in ppm relative to
residual solvent peaks (1H: δ = 7.26 (CDCl3), 5.32 (CD2Cl2), 2.50 (DMSO-
d6) ppm; 13C: δ = 77.16 (CDCl3), 53.84 (CD2Cl2), 39.52 (DMSO-d6)
ppm.)[65], H3PO4 or Me2Se. Coupling constants (J) are reported in Hz, and
splitting patterns are indicated as s (singlet), d (doublet), t (triplet), m (mul-
tiplet) and sept (septet). All spectra were recorded at room temperature.
Mass spectra were recorded with a linear iontrap coupled with an orbitrap
mass analyser (ThermoFisher Scientific, LTQ-Orbitrap Velos). Elemental
analyses were performed with a Vario Micro Cube system.
X-ray Structure Determinations: The crystallographic data are listed in
Tables S1 and S2. Suitable single crystals were mounted on a glass fiber
(2a, 2b, 2c, 2c·CHCl3·0.5(C7H8), 3, (IPr)PO2Mes·C6D6) or a human hair (4,
4·C6D6) in perfluorinated inert oil. The intensity measurements were per-
formed at 100 K with Oxford Diffraction diffractometers using monochro-
mated MoKα (for (IPr)PO2Mes·C6D6) or mirror-focussed CuKα radiation.
The diffractometer software CrysAlisPRO was employed.[66] Absorption
corrections were based on multi-scans or face indexation (4, 2c). The
structures were refined anisotropically on F2 with SHELXL14.[67] Hydrogen
atoms were included using a riding model or rigid methyl groups. Excep-
tions and special features: In 2c·CHCl3·0.5(C7H8) and 4·C6D6 disordered
1
the trend observed for the JPC coupling constants discussed
above (see Tables 1 and 2). For comparison, WBI values of 1.008
(CNHC-P) and 0.905 (CR-P) were calculated for the penta(car-
bonyl)tungsten of a related NHC-phosphinidene system (IMe)PR
(R = CH(SiMe3)2), which indicates a smaller degree of polarisation
upon metal coordination.[64]
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700494
European Journal of Inorganic Chemistry
FULL PAPER
solvent molecules were treated with the program SQUEEZE[68,69] to re-
move mathematically their residual electron density (further details see
Supporting Information).
(IMes)PO2Ph (2b): (IMes)PPh (1b) (0.147 g, 0.36 mmol) was dissolved in
toluene (20 mL). Dry air was passed through this solution for 15 min. The
product precipitated as a colourless solid. The solvent was removed with
a cannula and the remaining solid was washed with n-pentane. The prod-
uct was dried in vacuum to isolate 2b as a colourless solid. Yield: 0.110 g
(69 %). Single crystals were obtained from a concentrated solution in
DMSO layered with acetonitrile. 1H NMR (CD2Cl2, 300 MHz): δ = 7.31 -
7.15 (m, 3 H, o,p-CHPh), 7.10 (d, 2 H, C=CH-N, 4JH-P = 0.9 Hz), 7.09 - 6.97
(m, 2 H, m-CHPh), 6.93 (s, 4 H, m-CHMes), 2.37 (s, 6 H, p-CH3Mes), 1.98 (s,
CCDC 1542337 (2a), 1542338 (3), 1542339 (4), 1542340 (4·C6D6),
1542341 (2b), 1542342 (2c), 1542343 (2c·CHCl3·0.5(C7H8)) and 1542336
(IPrPMesO2·C6D6) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
12 H, o-CH3Mes) ppm. 13C NMR (CD2Cl2, 76 MHz): δ = 153.1 (d, 1 C, CNHC
-
=
1
1
P, JC-P = 80.8 Hz), 140.4 (s, 2 C, N-CMes), 137.2 (d, 1 C, P-CPh, JC-P
(IMe)PO2Ph (2a): (IMe4)PPh (1a) (0.548 g, 2.36 mmol) was dissolved in
THF (150 mL). Dry air was passed through this solution for 35 min. The
product precipitated as a pale yellow solid. The solvent was removed with
a cannula and the remaining solid was washed with n-pentane. The prod-
uct was dried in vacuum to isolate 2a as a pale yellow solid. Yield: 0.389
g (62 %). Single crystals were obtained from a concentrated solution in
CDCl3. 1H NMR (DMSO-d6, 300 MHz): δ = 7.73 - 7.65 (m, 2 H, m-CHPh),
150.0 Hz), 135.3 (s, 4 C, o-CMes), 133.7 (s, 2 C, p-CMes), 133.4 (d, 2 C, m-
CHPh, 4JC-P = 9.2 Hz), 130.8 (d, 1 C, p-CHPh, 5JC-P = 2.6 Hz), 129.3 (s, 4 C,
m-CHMes), 127.6 (d, 2 C, o-CHPh, 3JC-P = 13.1 Hz), 124.0 (d, 2 C, N-C=C-
N, 3JC-P = 2.6 Hz), 21.3 (s, 2 C, p-CH3Mes), 17.7 (s, 4 C, o-CH3Mes) ppm. 31
{1H} NMR (CD2Cl2, 122 MHz): δ = -1.21 (s, 1JP-C(NHC) = 80.8 Hz, 1JP-C(Ph)
P
=
150.0 Hz) ppm. C27H29N2PO2 (444.51 g/mol): calcd. C 72.96, H 6.58, N
6.30; found C 72.05, H 6.45, N 6.19.
4
7.43 - 7.35 (m, 3 H, o,p-CHPh), 4.05 (d, 6 H, N-CH3, JP-H = 0.6 Hz), 2.16
(s, 6 H, C-CH3) ppm. 13C NMR (DMSO-d6, 76 MHz): δ = 145.5 (d, 1 C,
CNHC-P, 1JC-P = 99.4 Hz), 139.8 (d, 1 C, P-CPh, 1JC-P = 144.2 Hz), 131.0 (d,
2 C, m-CHPh, 4JC-P = 9.4 Hz), 130.4 (d, 1 C, p-CHPh, 5JC-P = 2.7 Hz), 128.1
(d, 2 C, o-CHPh, 3JC-P = 12.4 Hz), 127.4 (d, 2 C, N-C=C-N, 3JC-P = 3.1 Hz),
32.8 (s, 2 C, N-CH3), 8.1 (s, 2 C, C-CH3) ppm. 31P {1H} NMR (DMSO-d6,
122 MHz): δ = -0.97 (s, JP-C(NHC) = 99.1 Hz, JP-C(Ph) = 144.2 Hz) ppm.
C13H17N2PO2 (264.26 g/mol): calcd. C 59.09, H 6.48, N 10.60; found C
58.75, H 6.55, N 10.96.
(IPr)P(O2)Ph (2c): (IPr)PPh (1c) (0.116 g, 0.23 mmol) was dissolved in
toluene (20 mL) and stirred for 1 h while exposed to air (This reaction was
also run using dry air, as described for the syntheses of 2a and 2b which
did not affect the yields). The product precipitated as a colourless solid.
The solvent was removed with a cannula and the remaining solid was
washed with n-pentane. The product was dried in vacuum to isolate 2c as
a colourless solid. Yield: 0.098 g (81 %). Single crystals were obtained
from a concentrated solution in CHCl3 layered with n-pentane. 1H NMR
(CDCl3, 200 MHz): δ = 7.48 (t, 2 H, p-CHDipp, 3JH-H = 7.8 Hz), 7.24 (d, 4 H,
1
1
(IMe)PS2Ph (3): Sulphur (0.016 g, 0.50 mmol) was added in one portion
to (IMe)PPh (1a) (0.047 g, 0.20 mmol) in THF (5 mL) and the yellow reac-
tion mixture was shaken until it became colourless (ca. 2 min). The cloudy
solution was quickly filtered through a syringe filter to remove unreacted
sulphur and the filter was washed with 1 mL of dichloromethane. The prod-
uct was dried in vacuum to isolate 3 as a colourless solid. Yield: 0.045 g
(75 %). Single crystals were obtained from a concentrated solution in
CDCl3 layered with n-hexane. 1H NMR (CD2Cl2, 300 MHz): δ = 8.19 - 8.10
(m, 2 H, m-CHPh), 7.49 - 7.43 (m, 3 H, o,p-CHPh), 3.74 (s, 6 H, N-CH3),
2.14 (s, 6 H, C-CH3) ppm. 13C NMR (CD2Cl2, 76 MHz): δ = 145.4 (d, 1 C,
CNHC-P, 1JC-P = 49.8 Hz), 142.4 (d, 1 C, P-CPh, 1JC-P = 91.0 Hz), 131.1 (d,
2 C, m-CHPh, 4JC-P = 12.1 Hz), 131.0 (d, 1 C, p-CHPh, 5JC-P = 3.3 Hz), 128.5
(d, 2 C, o-CHPh, 3JC-P = 13.6 Hz), 127.4 (d, 2 C, N-C=C-N, 3JC-P = 1.6 Hz),
34.6 (s, 2 C, N-CH3), 9.18 (s, 2 C, C-CH3) ppm. 31P {1H} NMR (DMSO-d6,
122 MHz): δ = 52.85 (s, 1JP-C(m-Ph) = 12.1 Hz, 1JP-C(NHC) = 49.8 Hz, 1JP-C(Ph)
= 91.0 Hz) ppm. HRMS (ESI): m/z calc. for C13H17N2PS2: 296.06545,
found: 296.06457. C13H17N2PS2 (296.38 g/mol): calcd. C 52.68, H 5.78, N
9.45; found C 52.50, H 5.87, N 8.96 (despite several attempts no reliable
sulphur content could be determined).
m-CHDipp 3JH-H = 7.8 Hz), 7.19 - 7.13 (m, 3 H, m,p-CHPh), 7.12 (d, 2 H,
,
4
C=CH-N, JH-P = 0.8 Hz), 7.01 - 6.96 (m, 2 H, o-CHPh), 2.57 (sept, 4 H,
CHMe2, 3JH-H = 6.8 Hz), 1.23 (d, 12 H, CH3, 3JH-H = 6.8 Hz), 1.10 (d, 12 H,
CH3, 3JH-H = 6.9 Hz) ppm. 13C NMR (CDCl3, 101 MHz): δ = 145.2 (s, 4 C,
o-CDipp), 136.6 (d, 1 C, P-CPh, 1JC-P = 152.3 Hz), 133.7 (d, 2 C, m-CPh, 3JC-
P = 9.4 Hz), 133.4 (s, 2 C, N-CDipp), 130.8 (s, 2 C, p-CDipp), 130.5 (d, 1 C,
p-CPh, 4JC-P = 2.6 Hz), 127.3 (d, 2 C, o-CPh, 2JC-P = 2.7 Hz), 124.3 (d, 2 C,
C=CH-N), 123.9 (s, 4 C, m-CDipp), 29.1 (s, 4 C, CHMe2), 25.6 (s, 4 C, CH3),
22.3 (s, 4 C, CH3) ppm. 31P {1H} NMR (CDCl3, 162 MHz): δ = 0.27 (s) ppm.
C33H41N2PO2 (528.68 g/mol): calcd. C 74.97, H 7.82, N 5.30; found C
75.33, H 7.83, N 5.53.
Acknowledgements
D.B. would like to thank Dr. Adinarayana Doddi for helpful discus-
sions and Lara Marie Schulze for preparative assistance. This
work was supported by the Deutsche Forschungsgemeinschaft
(DFG) through grant TA 189/16-1.
(IMe)PSe2Ph (4): Selenium (0.040 g, 0.50 mmol) was added in one portion
to (IMe)PPh (1a) (0.047 g, 0.20 mmol) in THF (5 mL) and the reaction
mixture was shaken until it became nearly colourless (ca. 2 min). The so-
lution was quickly filtered through a syringe filter to remove unreacted se-
lenium and the filter was washed with 1 mL of dichloromethane. The prod-
uct was dried in vacuum to isolate 4 as a colourless solid. Yield: 0.052 g
(66 %). Single crystals were obtained from a concentrated solution in THF
layered with n-hexane as well as from C6D6. 1H NMR (CD2Cl2, 400 MHz):
δ = 8.34 - 8.25 (m, 2 H, m-CHPh), 7.52 - 7.41 (m, 3 H, o,p-CHPh), 3.67 (s,
6 H, N-CH3), 2.14 (s, 6 H, C-CH3) ppm. 13C NMR (CD2Cl2, 101 MHz): δ =
140.2 (d, 1 C, CNHC-P, 1JC-P = 28.0 Hz), 139.4 (d, 1 C, P-CPh, 1JC-P = 70.5
Hz), 132.2 (d, 2 C, m-CHPh, 4JC-P = 12.6 Hz), 131.4 (d, 1 C, p-CHPh, 5JC-P
= 3.2 Hz), 128.6 (d, 2 C, o-CHPh, 3JC-P = 13.5 Hz), 127.5 (s, 2 C, N-C=C-
N), 35.0 (s, 2 C, N-CH3), 9.20 (s, 2 C, C-CH3) ppm. 31P {1H} NMR (CD2Cl2,
Keywords: N-Heterocyclic Carbenes • Phosphinidenes • Phos-
phaalkenes • Sulphur • Selenium
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This article is protected by copyright. All rights reserved.

10.1002/ejic.201700494
European Journal of Inorganic Chemistry
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10.1002/ejic.201700494
European Journal of Inorganic Chemistry
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The reactions of the N-heterocyclic
carbene-phenylphosphinidene ad-
ducts (NHC)PPh with dioxygen, ele-
mental sulphur or selenium afford
NHC-stabilised phenyldioxophospho-
rane, phenyldithioxophosphorane and
phenyldiselenoxophosphorane spe-
cies of the type (NHC)PE2Ph (E = O,
S, Se).
NHC-phosphinidene adducts
Dirk Bockfeld, Thomas Bannenberg, Pe-
ter G. Jones and Matthias Tamm*
Page No. – Page No.
N-Heterocyclic Carbene Adducts of
Phenyldioxophosphorane and its
Heavier Sulphur and Selenium Ana-
logues
This article is protected by copyright. All rights reserved.