4-(p-methylthio)phenyl-2,6-diphenyl phosphinine: The first X-ray crystal structure of a λ3-triaryl-phosphabenzene

Article abstract of DOI:10.1007/s10870-006-9147-8

The new donor functionalized methylthio-phosphabenzene 3 was synthesized by reaction of the corresponding pyrylium salt with P(SiMe₃) ₃. This compound was characterized crystallographically, showing eight independent molecules in the

Full text of DOI:10.1007/s10870-006-9147-8

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Journal of Chemical Crystallography, Vol. 36, No. 12, December 2006 ( 2006)
DOI: 10.1007/s10870-006-9147-8
4-(p-Methylthio)phenyl-2,6-diphenyl phosphinine: The first
X-ray crystal structure of a λ3-triaryl-phosphabenzene
Christian Mu¨ller,(1)∗ Martin Lutz,(2) Anthony L. Spek,(2) and Dieter Vogt(1)
Received May 15, 2006; accepted September 5, 2006
Published Online November 5, 2006
The new donor functionalized methylthio-phosphabenzene 3 was synthesized by reaction of
the corresponding pyrylium salt with P(SiMe3)3. This compound was characterized crystal-
lographically, showing eight independent molecules in the asymmetric unit. 3 crystallized
˚
in the orthorhombic space group Pna21 (no. 33) with cell parameters a = 48.3565(8) A,
3
˚
˚
˚
b = 7.64784(7) A, c = 40.8432(4) A, V = 15104.7(3) A . This result represents the first
crystal structure analysis of a λ3-triaryl-phosphabenzene.
KEY WORDS: Phosphinine; phosphabenzene; crystal structure; phosphorus-heterocycle.
Introduction
We recently reported on the preparation of
hydroxy-substituted phosphinines, which were
λ3-Phosphabenzenes (phosphinines), the
converted into chiral bidentate phosphabenzene-
phosphites and applied as ligands in the
rhodium-catalyzed hydrogenation of prochiral
substrates.5,6 During the course of our investiga-
tions on functionalized phosphabenzenes, a novel
methylthio substituted phosphinine has been pre-
pared. Single crystals of this compound were ob-
tained, which were subject to an X-ray crystal
structure analysis. We report here on the synthe-
sis and the first X-ray crystallographic character-
ization of a λ3-triaryl-phosphabenzene, which re-
sembles the original compound reported by Ma¨rkl
many decades ago.
higher homologues of pyridines, are known for
many decades, due to the pioneering work of
Ma¨rkl and Ashe III in the late 1960s.1,2 These het-
erocycles are planar, aromatic systems in which
–
–
one CH group of the aryl moiety is sub-
stituted by a phosphorus atom. Up to now, a
few λ3-phosphabenzenes and their corresponding
transition-metal complexes have been character-
ized crystallographically.3 However, an X-ray
crystal structure analysis of the original com-
pound reported by Ma¨rkl, the 2,4,6-triphenyl-
phosphabenzene, has still remained elusive, even
though attempts have been made before.4
(1) Department of Chemical Engineering and Chemistry, Schuit In-
stitute of Catalysis, Laboratory of Homogeneous Catalysis, Eind-
hoven University of Technology, 5600 MB Eindhoven, The
Netherlands.
Experimental
General considerations
(2) Bijvoet Center for Biomolecular Research, Crystal and Structural
Chemistry, Utrecht University, Utrecht, The Netherlands.
All reactions concerning the synthesis of
∗
To whom correspondence should be addressed; e-mail:
c.mueller@tue.nl
compound 3 were carried out under an argon
869
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1074-1542/06/1200-0869/0 2006 Springer Science+Business Media, LLC

870
Mu¨ller, Lutz, Spek, and Vogt
atmosphere using standard Schlenk techniques.
The solvents were dried and distilled prior to
use. Elemental analyses were obtained from
H. Kolbe, Mikroanalytisches Laboratorium,
Mu¨lheim (Germany). p-methylhiobenzaldehyde,
acetophenone and HBF4·Et2O (54 Gew.-%
in Et2O) were obtained from Aldrich and
used without further purification. P(SiMe3)3
was prepared according to the literature.7 1H,
13C{1H} and 31P{1H} NMR spectra were
recorded on a Varian Gemini spectrometer at
200 and 400 MHz. Chemical shifts are reported
relative to tetramethylsilane for 1H and 13C{1H}
as internal reference and to H3PO4 85% for
31P{1H}.
Synthesis of
4-(p-Methylthio)phenyl-2,6-Diphenyl pyrylium
tetrafluoroborate (2)
A solution of 1 (20.0 g, 78.6 mmol) and
acetophenone (4.72 g, 39.3 mmol, 4.6 mL) in
dichloroethane (20 mL) was heated to T = 75◦C
and a solution of HBF4 in diethylether (52%)
(13.34 g, 78.6 mmol) was added drop wise. The
solution turned dark red and stirring was con-
tinued for 2.5 h. After cooling the solution to
room temperature, red crystals were forming.
The solution was decanted off and the dark red
solid was recrystallized from methanol. Yield:
10 g (22.6 mmol, 58%). 1H NMR (DMSO-
–
d6): δ (ppm) = 2.62 (s, 3H, SCH3), 7.52 (d,
JH−H = 7.4 Hz, 2H), 7.77 (m, 6H), 8.52 (m, 6H),
9.00 (s, 2H, heteroarom.-H). 13C NMR (DMSO-
–
d6): δ (ppm) = 14.34 ( SCH3), 113.95, 126.09,
Synthesis of
p-methylthio-benzylidene-acetophenone (1)
128.14, 129.01, 129.59, 130.26, 130.85, 135.26,
151.03, 163.94, 169.54. 19F NMR (DMSO-d6):
δ (ppm) = −148.19, −148, 25 (3:1). Anal Calcd.
for C24H19BF4OS (442.28): C, 65.18; H, 4.33.
Found: C, 65.12; H, 4.33.
The chalcone 1 was prepared according to a
modified patent procedure:8 A solution of ace-
tophenone (22.1 mL, 22.8 g, 189.5 mmol) in
ethanol (32 mL) was added slowly to an ice-
cold solution of NaOH (9.62 g, 240.5 mmol)
in ethanol (64 mL) and water (64 mL) un-
der vigorous stirring. Subsequently, a solution
of 4-methylthiobenzaldehyde (25 mL, 28.6 g,
187.9 mmol) in ethanol (32 mL) was added and
the solution was stirred for two hours at 0◦C
and for overnight at room temperature. Water
(150 mL) was added and stirring was continued
for another 12 h, during which a yellow precip-
itate was formed. The solid was filtered off and
dried in vacuo, resulting in a yellow, fine pow-
Synthesis of
4-(p-Methylthio)phenyl-2,6-Diphenyl
phosphabenzene (3)
3.0 g (11.97 mmol, 2.04 equiv.) of P(SiMe3)3
was added drop wise at room temperature to a
stirred solution of 2 (2.6 g, 5.9 mmol) in 20 mL
of acetonitrile. The resulting dark reaction mix-
ture was heated to T = 85◦C and subsequently
refluxed for 5 h. After cooling to room tem-
perature, the volatiles were removed under vac-
uum. The residue was dissolved in CH2Cl2 and
added to an appropriate amount of silica gel
(ca. 3 g). Evaporation of the solvent was fol-
lowed by flash chromatography with petroleum
ether/ethyl acetate (19:1). Yield: 1.2 g (3.2 mmol,
1
der. Yield: 35.0 g (137.6 mmol, 73%). H NMR
–
(CDCl3): δ (ppm) = 2.48 (s, 3H, SCH3), 7.22
(d, JH−H = 8.2 Hz, 2H), 7.51 (m, 6H), 7.75 (d,
JH−H = 13.6 Hz, 1H), 8.00 (d, JH−P = 7.0 Hz,
2H). 13C NMR (CDCl3): δ (ppm) = 15.06
–
( SCH3), 120.91, 125.89, 128.46, 128.62, 128.85,
1
131.29 (Cq), 132.75, 138.26 (Cq), 142.44 (Cq),
144.37, 190.50 (CO). Anal Calcd. for C16H14OS
(254.35): C, 75.56; H, 5.55. Found: C, 75.85;
H, 5.80.
55%) of a yellow solid. H NMR (C6D6): δ
–
(ppm) = 1.99 (s, 3H, SCH3), 7.15 (m, 8H, m-
, p-phenyl-H), 7.21, 7.26 (2 × d, JH−H = 8.4 Hz,
7.2 Hz, 2 × 2H, o-, m-methylthiophenyl-H), 7.66

4-(p-Methylthio)phenyl-2,6-diphenyl phosphinine
871
Table 1. Crystal Data for 3
Crystal data Compound 3
(d, JH−H = 8.0 Hz, 4H, o-phenyl-H), 8.06 (d,
JH−P = 6.0 Hz, 2H, heteroarom.-H). 13C NMR
(CDCl3): δ (ppm) = 15.68 (-SCH3), 127.20 (3C),
127.60, 128.00 (3C), 128.09, 128.13, 128.39,
128.50, 128.59, 128.71, 128.93, 143.38 (2 × d,
2JC−P = 16.5, 21.8 Hz, Cβ), 138.78, 131.40 (d,
3JC−P = 11.9 Hz, Cγ ), 172.30 (d, 1JC−P = 51.8 Hz,
Cα). 31P NMR (C6D6): δ(ppm) = 183.5. Anal
Calcd. for C24H19PS (370.45): C, 77.81; H, 5.17.
Found: C, 77.73; H, 5.11.
CCDC deposit no.
Empirical formula
Molecular weight
Crystal colour
604687
24H19PS
370.42
C
yellow
Crystal size (mm3)
Temperature (K)
Crystal system
0.66 × 0.12 × 0.09
150(2)
orthorhombic
Pna21 (no. 33)
48.3565(8)
7.64784(7)
40.8432(4)
15104.7(3)
32
Space group
˚
a (A)
˚
b (A)
˚
c (A)
3
˚
V (A )
Crystal structure determination of (3)
Z
Calculated density (mg/m3)
1.303
µ (mm−1
)
0.261
Crystals of 3 suitable for X-ray diffraction
were obtained by slow recrystallization from ace-
tonitrile under an argon atmosphere.
Abs. correction method
Abs. correction−r1ange
multi-scan10
0.76–0.98
0.53
˚
(sin θ/λ)max (A
)
Reflections were measured on a Nonius Kap-
paCCD diffractometer with rotating anode and
graphite monochromator (λ = 0.71073 A). The
Reflections (measured/unique)
Rint
111884/18443
0.0789
1882/3057
0.1011/0.2338
0.1461/0.2731
1.061
Parameters/restraints
R1/wR2 [I > 2σ (I)]
R1/wR2 (all refl.)
S
˚
intensities were integrated with Eval149 using
an accurate description of the crystal form and
the diffraction geometry. Based on the internal
R-value and the systematic extinctions, space
group Pna21 (non-centrosymmetric) and Pnam
(centrosymmetric) were considered. The structure
was solved successfully with Direct Methods11 in
Pna21. Detailed inspection of the atomic coor-
dinates with PLATON/ADDSYM12 and the con-
formational differences between the eight crys-
tallographically independent molecules rule out a
transformation to an ordered structure in the cen-
trosymmetric space group Pnam. Structure refine-
ment was performed with SHELXL-9713 on F2
of all reflections (Table 1). Non-hydrogen atoms
were refined freely with anisotropic displacement
parameters. All hydrogen atoms were introduced
in geometrically optimized positions and refined
with a riding model. The non-centrosymmetric
structure was refined as an inversion twin result-
ing in a twin fraction of 0.54(12). Positive resid-
3
˚
ρmin/max (e/A )
− 0.45/2.14
contained in the integrated Bragg peaks but also
in additional diffuse scattering. Indeed, a close
inspection of the raw CCD images shows, that all
reflections hkl with h = 2n + 1 are diffuse residing
on streaks in the l direction. This was visualized in
the form of simulated “precission” images.14 The
stacking disorder for h = 2n + 1 is also reflected
in an analysis of the normalized structure factors
(E2 values) using the SIR-97 software, which in-
dicates a 30% pseudo-translational symmetry for
h = 2n.15 In order to obtain a stable least-squares
refinement distance/angle (SHELXL-97/SAME)
and displacement (SHELXL-97/ISOR) restraints
were introduced.
Geometry calculations, drawings and check-
ing for higher symmetry were performed with the
PLATON package.12
ual density peaks in the difference Fourier map
3
˚
up to 2.14 e/A are an indication that the refine-
ment model is only an incomplete approximation
and that there must be significant disorder. Due
to the disorder, not all intensity information is
CCDC 604687 contains the supplemen-
tary crystallographic data for this paper. These
data can be obtained free of charge from The

872
Mu¨ller, Lutz, Spek, and Vogt
O
O
O
NaOH
H
+
S
S
1
Scheme 1. Synthesis of Chalcone 1.
Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
Results and discussion
Fig. 1. Molecular Structure of 3 in the crystal (displacement
ellipsoid plot12 at 50% probability level). Only one of eight
independent molecules is shown.
The methylthio-functionalized chalcone (1)
was prepared in high yields by condensation
reaction of acetophenone and p-methylthio ben-
zaldehyde, according to a modified patent litera-
ture (Scheme 1).8
Yellow crystals of 3, suitable for X-ray
diffraction, were obtained by slow recrystalliza-
tion from acetonitrile. 3 crystallized in space
group Pna21 (orthorhombic) with eight indepen-
dent molecules in the asymmetric unit. The molec-
ular structure of one molecule is illustrated in
Fig. 1.
The P-heterocycle is essentially planar due
to the aromatic character of the phosphabenzene
moiety. The maximal torsion angle within this
ring is 7.3(13)◦. The phenyl substituent in para
position with respect to the phosphorous atom
is slightly rotated out of this plane with magni-
Compound 1 was further converted into
the corresponding dark-red pyrylium salt 216
by reaction with acetophenone and HBF4·Et2O.
7
Subsequently, treatment with P(SiMe3)3 gave
the methylthio-functionalized phosphabenzene 3
(Scheme 2).
Phosphabenzene 3 was fully characterized
1
by H, 31P and 13C NMR spectroscopy as well
as elemental analysis. The 31P NMR spectrum of
3 shows a single resonance at δ = 183.5 ppm. In
the 1H NMR spectrum a doublet at δ = 8.06 ppm
(JH−P = 6.0 Hz) is characteristic for the two het-
eroaromatic protons, which couple to the phos-
phorus atom.
–
–
tudes of the C14–13 C16 C111 torsion angles
of 28.3(1)-40.2(10)◦. The variation within these
torsion angles is thus relatively small and the
magnitudes closely resemble the expectation
S
S
O
C H CCH
6
5
3
P(SiMe3)3
CH3CN
BF4
1
HBF Et O
4
2
O
2
P
3
Scheme 2.
Synthesis of 4-(p-methylthio)phenyl-2,6-
diphenyl phosphinine 3.
Fig. 2. Quaternion fit of the eight independent molecules of
3. Hydrogen atoms are omitted for clarity.

4-(p-Methylthio)phenyl-2,6-diphenyl phosphinine
873
the a axis. For clarity reasons the eight indepen-
dent molecules in Fig. 3 have been visualized in
eight different colours (colour online).
In summary, we have synthesized the new
donor functionalized methylthio-phosphabenzene
3 by reaction of the corresponding pyrylium salt
with P(SiMe3)3. This compound was character-
ized crystallographically, showing eight indepen-
dent molecules in the asymmetric unit. This result
represents the first X-ray structure analysis of a
λ3-triaryl-phosphabenzene.
Acknowledgment
Fig. 3. Packing of the molecules of 3 in the unit cell as
a space filling plot. View along the crystallographic-b axis.
The eight independent molecules are shown in eight different
colours (colour online).
This work was supported in part (ML, ALS)
by the Council for the Chemical Sciences of the
Netherlands Organization for Scientific Research
–
(CW NWO).
values for biphenyl-type compounds.17 A much
bigger variation is found in the conformations in
the ortho phenyl rings and S-Me group (Fig. 2).
Bond lengths and angles of the eight
independent molecules were strongly restrained
with respect to each other in the least-squares
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–
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˚
–
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19
˚
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–
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˚
–
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874
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