Synthesis, structure and π-delocalization of a phosphaalkenyl based neutral PNP-pincer

Article abstract of DOI:10.1016/j.ica.2011.02.078

Preparation, characterization and structural properties of a novel bis-phosphaalkenyl based PNP-pincer are reported. In this pincer the π-system is delocalized over all three donor sites, which was demonstrated with DFT calculations, UV-Vis measurements a

Full text of DOI:10.1016/j.ica.2011.02.078

Inorganica Chimica Acta 374 (2011) 211–215
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure and
PNP-pincer
p-delocalization of a phosphaalkenyl based neutral
⇑
Andreas Orthaber, Ferdinand Belaj, Rudolf Pietschnig
Karl-Franzens-University, Department of Chemistry, Schuberststraße 1, A-8010 Graz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 March 2011
Preparation, characterization and structural properties of a novel bis-phosphaalkenyl based PNP-pincer
are reported. In this pincer the
strated with DFT calculations, UV–Vis measurements and structural findings. As a consequence of this
extended delocalization the -system reveals near coplanarity which is evident from the first crystal
structure for an uncomplexed bisphosphaalkenyl PNP-pincer.
p-system is delocalized over all three donor sites, which was demon-
Dedicated to Prof. Dr. Wolfgang Kaim on the
occasion of his 60th birthday.
p
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Pincer
Phosphaalkene
DFT calculations
Crystal structure
1. Introduction
tuted lithiumsilylphosphanides [3,8]. For this sequence the specific
substitution pattern at the ketone and the phosphanide seem to be
essential and for substituent combinations other than in I and II
failure of this route has been reported [8]. In an alternative ap-
proach the phosphaalkene units are part of a phosphinine rings
and the pincer III is obtained using the Märkl route to phosphi-
nines via the corresponding bispyrylium salts [14].
Looking for a more general approach to such pincers, we fol-
lowed the Becker-route to phosphaalkenes – a strategy which
had been successful in our hands for the preparation of 1,10-
ferrocenylene bridged bisphosphaalkenes [16–18]. Accordingly,
the reaction of acid chloride 1 with bissilylphosphane 2 yields
the envisaged bisphosphaalkene 3 in a simple one pot reaction
(Scheme 2). After removal of the volatile byproducts, 3 can be iso-
lated in pure form by recrystallization.
Pincer ligands with a PNP-set of donor centers connected by a
central pyridine ring have become a common structural motif for
the synthesis of a variety of metal complexes [1–15]. Especially
for pincers of this type in which the phosphorus atoms are part
of a P@C double bond, conjugation of the pendant unsaturated
units with the central pyridine ring has to be considered [3,8,14].
In the course of our investigations dealing with the conjugation
of
p
-spacer-separated bis-r2k3-phosphanes [16–18], we became
interested in studying pyridine bridged bisphosphaalkenes for
which only a single structurally characterized example was re-
ported in the literature so far [8]. On the other hand, coordination
chemistry of low valent and unsaturated phosphanes was widely
studied over the last decades [19,20]. Here we describe synthesis,
crystal structure and DFT calculations of a 2,6-bisphosphaalkenyl
pyridine in which coplanarity of the ligand based p-systems is ob-
served and for which extended conjugation between the central
pyridine ring and the pendant units can be concluded.
In the 31P NMR spectra of 3 several signals in the expected re-
gion for this type of phosphaalkene can be observed (163–
172 ppm) owing to the presence of three possible diastereomers
EE-, ZZ- and EZ. A pentane solution of 3 was filtered over Celite
and concentrated to incipient crystallization. Storage at low tem-
perature (ꢀ70 °C) afforded yellow needle shaped crystals. The 31P
2. Results and discussion
NMR spectra of these crystals showed a single signal at
165.8 ppm which indicated that only one of the symmetric
diastereomers (ZZ) was separated from the isomeric mixture using
this procedure. The quality of the crystals obtained this way
was suitable for single crystal X-ray diffraction. Compound 3
crystallizes in the monoclinic space group P21/c (a = 18.007(2) Å,
The few phosphaalkenyl based PNP-pincers known so far
(Scheme 1) have been synthesized using very different approaches.
One way of preparing such compounds is an addition–elimination
sequence employing a pyridine based diketone and bulky substi-
b = 12.5768(17) Å,
c = 14.7045(19) Å,
b = 101.180(7)°,
V =
3267.0(7) Å3, Z = 4). Details of the structure solution are summa-
⇑
Corresponding author. Tel.: +43 316 380 5287; fax: +43 316 380 9835.
E-mail address: rudolf.pietschnig@uni-graz.at (R. Pietschnig).
rized in Table 1. The crystal structure analysis reveals that both
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.02.078

212
A. Orthaber et al. / Inorganica Chimica Acta 374 (2011) 211–215
t-Bu
t-Bu
H
H
Ph
Ph
C
P
N
C
P
N
C
P
N
C
P
P
P
Mes'
Mes'
Mes*
Mes*
Ph
Ph
Scheme 1. Survey of the literature known phosphaalkenyl based PNP-pincers I (Mes⁄ = 2,4,6-tri-tert-butylphenyl) [3], II (Mes0 = 2,4-di-tert-butyl-6-methylphenyl) [8] and III
[14].
SMT
TMS
O
Et2O
O
+ 2
N
O
O
N
-2 TMSCl
P
P
Cl
Cl
Mes
Mes
PTMS2
1
2
3
Scheme 2. Synthesis of pincer 3.
O
C
O
O
N
SiH3
H3Si
N
C
P
SiH3
P
4
5
P
C6H5
C6H5
C6H5
Scheme 4. Model compounds 4 and 5 used for ab initio calculations.
Fig. 1. ORTEP plot of 3 (ellipsoids at 50% probability). Hydrogen atoms are omitted
for clarity. Top: view along the (0 1 0)-axis with numbering. Bottom: View along the
(0 0 1) axis.
angle (N-C-C=P)
P@C units adopt the Z conformation in this isomer (Fig. 1). The dis-
tance between the two phosphorus atoms of ca. 4.6 Å should allow
concurrent coordination to a metal center as in related PNP pincer
systems.
Fig. 2. Energy diagram of the rigid PES scan around the dihedral angle N–C–C@P in
model compound 5. Relative energies are given in [kcal/mol].
The most noteworthy feature of the molecular structure of 3 is
the near coplanarity of the two phosphaalkene units with the cen-
tral pyridine ring (12.7(2)° and 13.3(3)°). This is in marked contrast
to the only other uncomplexed pyridylbisphosphaalkene which
has been structurally characterized, i.e. II (Scheme 1), in which
the P@C units are twisted by as much as 63° [8]. This significant
O
O
O
O
O
O
SMT
TMS
SMT
TMS
SMT
TMS
N
N
C
P
C
P
C
P
C
P
C
P
N
C
P
Mes
Mes
Mes
Mes
Mes
Mes
A
B
C
Scheme 3. Resonance structures of 3.

A. Orthaber et al. / Inorganica Chimica Acta 374 (2011) 211–215
213
twisting limits the electronic interaction between the P@C units
and the central aromatic ring so that no conjugation can be ex-
pected for II. By contrast in 3 substantial p-conjugation is obvious
which involves the electron rich OPC-allylic system and the pyri-
dine core as illustrated by the Lewis resonance structures in
Scheme 3. This interpretation is further supported by the increased
P@C bond distances in 3 (1.701(3) Å, 1.702(3) Å) compared to the
more isolated P@C bond in II (1.674(4) Å). By contrast the (ar-
yl)C–C(P-alkene) distances are very similar for 3 (1.484(3) and
1.485(4) Å) and III (1.48 Å).
In agreement with a fully delocalized
shows an intense yellow color while its twisted analog II was de-
scribed as colorless. The absorption can be attributed to a
p-system, compound 3
p–
p⁄
transition. To firmly assess the extent of conjugation in 3, quantum
mechanical calculations have been performed at the DFT and TD-
DFT B3LYP/6-311G⁄⁄ level of theory on two model systems 4 and
5 (Scheme 4). The CH3 groups have been omitted in the model
compounds and were replaced by hydrogen atoms to avoid rota-
tional degrees of freedom. Model compounds are fully optimized.
Furthermore a rigid scan on the PES for the dihedral angle around
the N–C–C@P bond for model compound 5 was performed, which
shows the influence of the conjugation of the two
p-systems
(Fig. 2). Two local minima are observed one with a dihedral angle
of ꢀ180° and around 34°, for the latter being ca. 2.1 kcal/mol high-
er in energy. These minima are separated by two barriers for which
N–C–C@P equals ꢀ90° (+7.5 kcal/mol) and +96° (+6.7 kcal/mol).
For these two orientations of the pyridine fragment no conjugation
Fig. 3. Highest occupied (HOMO, bottom) and lowest unoccupied (LUMO, top)
molecular orbital of model compound 4 from B3LYP/6-311G⁄⁄ calculations plotted
with the ChemCraft program [http://www.chemcraftprog.com] at an isolevel of
0.04 a.u.
of the two
of 6–8 kcal/mol which can be attributed to the effective conjuga-
tion of the extended systems. Hence the total stabilization en-
p systems should be possible, indicating a stabilization
p
ergy in ZZ-3 is expected to be in the range of 12–16 kcal/mol. In
agreement with this no isomerization of ZZ-3 has been observed
in solution over several days using 31P NMR.
Structural parameters for model compound 4 and the X-ray dif-
fraction data of 3 agree very well (i.e. P@C calc. 1.714 Å versus X-
ray 1.701(3) and 1.702(3) Å or Cpyr–C: 1.484 Å versus 1.484(3)
and 1.485(4) Å). The computed dihedral angles N–C–C@P in the
model compounds (11.5(3)° and ꢀ12.8(3)°) deviate less than 2°
from the X-ray data . Analysis of the molecular orbitals gives an in-
sight into the nature of the conjugated system (Fig. 3). The highest
molecular orbital (HOMO) shows essential contributions from the
p-orbitals of the P@C double bonds as well as from the p-system
of the central pyridine ring, whereas the LUMO is composed of
p⁄ orbitals of the P@C double bond and antibonding orbitals of
the pyridine fragment, which is line with the observed color of
compound 3. Furthermore the TD–DFT calculations confirm the
energetically lowest band in the UV–Vis to be a
of the extended conjugated system (Fig. 4). Mulliken and NBO
250
300
350
400
450
500
Wavelength [nm]
p–
p⁄ transition
Fig. 4. UV–Vis transitions in 4 based on TD–DFT calculations (B3LYP/6-311G⁄⁄).
O
O
O
O
N
C
P
SMT
Mes
N
TMS
Mes
SMT
P
Mes
TMS
Mes
P
P
(3)
CuBr∗Me2S
CuBr∗Me2S
indirect
direct
O
O
C
O
O
N
Cu
SMT
Mes
N
Cu
TMS
SMT
P
Mes
P
TMS
P
P
Mes
Mes
(6)
(7)
Scheme 5. Direct and indirect complexation of 3.

214
A. Orthaber et al. / Inorganica Chimica Acta 374 (2011) 211–215
charge analyses yield positively charged phosphorus centers (+0.43
and +0.63) as well as a negatively charged nitrogen atom (ꢀ0.36
and ꢀ0.45), which is in agreement with a significant contribution
of resonance structure C (Scheme 3) also underlined by the slightly
elongated P@C double bond.
4.1. Synthesis of 3
Mes-P(TMS)2 (2) (408 mg, 1.37 mmol) is added to a cooled
(ꢀ70 °C) suspension of 2,6-bis(chlorocarbonyl)pyridine (144 mg,
0.71 mmol) in toluene (20 ml). The resulting mixture is allowed
to warm to room temperature and stirred overnight. All volatiles
are removed in vacuum and the residue is extracted with pentane
and filtered over Celite. Upon concentration to ca. 5 ml the ZZ-
isomer of bisphosphaalkene 3 can be precipitated by prolonged
cooling to ꢀ70 °C as yellow crystalline material (172 mg, 41%).
m.p. (pentane) 282–284 °C.
In order to explore the coordination properties of 3 we investi-
gated its behavior towards Cu(I) halides as in related cases [14,15].
Direct addition of CuBrꢁMe2S to the isomeric mixture of 3 indicates
the formation of different phosphorus containing species as shown
by 31P NMR spectroscopy. Based on NMR also the direct complex-
ation of the ZZ isomer of 3 is feasible but has little synthetic use
owing to the tedious separation procedure via crystal picking. To
get around the likewise complexation of different E/Z isomers of
3 we considered attaching the metal to the corresponding acyl-
phosphane (Scheme 5), which is initially formed below ꢀ35 °C in
the synthesis of 3 but not isolated. The metal coordinated acyl-
phosphane 6 should then be prone to conversion into the corre-
sponding bisphosphaalkene 7 by twofold silyl migration. We
anticipated that via this indirect route the (ZZ) isomer of the bis-
phosphaalkene complex could form preferentially. Slow warming
to ambient temperature of the thus prepared reaction mixture is
accompanied with a significant color change. The dark red color
of the reaction mixture is in contrast to the yellow color of uncom-
plexed 3 and indicates significant interaction with the copper ion.
31P NMR spectroscopic investigations of the reaction mixture indi-
cate successful silyl migration and formation of the P@C moiety
associated with a coordination shift towards higher field of ca.
15 ppm. Interestingly, employing this indirect route the relative
amount of complexed Z,Z isomer 7 (d(31P) = + 148 ppm) is signifi-
cantly increased compared with the direct complexation method.
This indicates that the coordination sphere of a metal provides
some control over the E/Z ratio of the phosphaalkene via acylphos-
phane rearrangement. As our exploratory results show, direct com-
plexation of bis-phosphaalkene 3 seems to be inferior to indirect
complexation via its precursor, at least in the case of Cu(I).
1H NMR (CDCl3, 400 MHz): ꢀ0.09 (s, O-TMS, 18H), 2.29 (s, p-Me,
3
6H), 2.42 (s, o-Me 12H), 6.89 (s, Mes-H, 4H), 7.60 (t, JHH 7.8 Hz,
Pyr–H, 1H), 7.75 (dt, JHH 7.8 Hz, 1.8 Hz, Pyr–H 2H). 13C NMR
3
(CDCl3, 100 MHz): 0.23 (s, O-TMS), 21.05 (s, p-Me), 22.20 (d,
8.7 Hz, o-Me), 119.23 (dd, 17.5 Hz, 5.3 Hz), 128.19 (s, 30-Mes),
133.73 (d, 37.9 Hz, 10-Mes), 136.29 (s, Pyr), 138.20 (s, 40-Mes),
141.83 (d, 6.7 Hz, 20-Mes), 157.88 (d, 29.6 Hz, Pyr), 195.02 (d,
54.9 Hz, P@C). 31P NMR (CDCl3, 162 MHz): 165.8 (br. s.). UV–Vis
(pentane) , 370 nm, 385 nm (sh), 435 nm (br.), In the crude reac-
tion mixture further 31P NMR signals can be observed (172.3,
170.4, 165.8 (ZZ), 162.9, 162.7) which can be attributed to isomeric
phosphaalkene units.
Table 1
Summary for the acquisition and structure solution of 3.
Empirical formula
Formula weight
Crystal description
Crystal size
Crystal system, space group
Unit cell dimensions
a (Å)
b (Å)
C31H43NO2P2Si2
579.78
needle, yellow
0.35 ꢂ 0.25 ꢂ 0.19 mm
monoclinic, P21/c
18.007(2)
12.5768(17)
14.7045(19)
101.180(7)
3267.0(7)
4
1.179
1240
0.234
c (Å)
b (°)
V Å(3)
Z
Calculated density (Mg/m3)
F(0 0 0)
3. Conclusion
Linear absorption coefficient
Absorption correction
l
(mmꢀ1
)
semi-empirical from
equivalents
0.956 and 0.886
In summary, we report the preparation, characterization and
structural properties of a novel phosphaalkenyl based PNP-pincer
with a delocalized p-system. The delocalization over all three do-
nor sites was demonstrated based on DFT calculations, UV–Vis
measurements and structural findings. As a consequence of this ex-
Maximum and minimum transmission
Unit cell determination
2.31° <
H < 25.99°
9641 reflections used at 100 K
T (K)
100
Diffractometer
Radiation source
Radiation and wavelength
Monochromator
Scan type
Bruker APEX-II CCD
sealed tube
tended delocalization the p-system reveals near coplanarity which
is evident from the first crystal structure for an uncomplexed bis-
phosphaalkenyl PNP-pincer. This planarity is quite remarkable and
even for complexes of related phosphaalkenyl based PNP-pincer li-
Mo K
graphite
/ and
a, 0.71073 Å
x
scans
H
Range for data collection (°)
2.15–26.00
gands the coplanarity of the ligand based p-systems is not neces-
Index ranges
ꢀ22 6 h 6 22, ꢀ15 6 k 6 15,
sarily observed. In the Cu(I) complex of III for instance where the
P@C units are part of phosphinine rings the inter-planar angles
range between 25.5° and 31.4° [14]. The relevance of the herein
presented pincer system 3 may arise from its potential as an inter-
ꢀ16 6 l 6 18
Reflections collected/unique
Significant unique reflections
R(int), R(sigma)
28 091/6221
4954 with I > 2
0.0524, 0.0506
96.9%
r(I)
Completeness to
H = 26.0°
Refinement method
Full-matrix least-squares on F2
6221/369/0
face between extended
p-conjugated strands and metal centers
which we intend to explore in the future.
Data/parameters/restraints
Goodness-of-fit (GOF) on F2
1.049
Final R indices [I > 2
R indices (all data)
Extinction expression
Weighting scheme
r
(I)]
R1 = 0.0630, wR2 = 0.1685
R1 = 0.0780, wR2 = 0.1872
None
4. Experimental
2
w ¼ 1=½r2ðF2oÞ þ ðaPÞ þ bPꢃ
where P ¼ ðF2o þ 2Fc2Þ=3
0.1265, 1.5250
0.001
All reactions were carried out under dry argon atmosphere
with standard Schlenk techniques. Solvents were dried with a
PureSolve system and degassed prior to use. Mes-PTMS2 [21] and
2,6-bis(chlorocarbonyl)pyridine [22] were prepared according to
published procedures. All other chemicals were obtained from
Sigma–Aldrich or ABCR and used as received.
Weighting scheme parameters a, b
Largest
Dr in last cycle
Largest difference peak and hole (e/Å3)
Structure Solution Program
1.033 and ꢀ0.719
SHELXS-97 [23]
Structure Refinement Program
SHELXL-97 [23]

A. Orthaber et al. / Inorganica Chimica Acta 374 (2011) 211–215
215
4.2. Attempted synthesis of 6
Acknowledgment
Mes-P(TMS)2 (2) (154 mg, 0.52 mmol) is added to a cooled
(ꢀ70 °C) suspension of 2,6-bis(chlorocarbonyl)pyridine (54 mg,
0.26 mmol) in toluene (20 ml). The resulting mixture is allowed
to warm to ꢀ35 °C and an excess of CuBr⁄Me2S (82 mg, 0.40 mmol)
is added and stirred for 1 h at this temperature. Stirring is contin-
ued overnight warming the reaction mixture to ambient condi-
tions. 31P-Spectroscopic investigation of the crude reaction
mixture reveal the presence of broad signals around 150 ppm
(152.4, 148.7, 148.2 (presumably ZZ isomer)). All volatiles are re-
moved in vacuum and the residue is extracted with diethyl ether
leads to an inseparable mixture of isomeric compounds.
The authors A.O. and R.P. would like to thank the Austrian Sci-
ence Fund (FWF) for financial support (Grants P18591-B03 and
P20575-N19).
Appendix A. Supplementary material
CCDC 806606 contains the supplementary crystallographic data
for 3. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2011.02.078.
4.3. Computational details
References
Calculations were carried out with the Gaussian Inc. program
package (G03 Rev. B.04). Model systems have been investigated
at the B3LYP/6-311G⁄⁄ level of theory and were fully optimized
and were determined as true minima by inspection of their har-
monic frequencies having no imaginary frequency. The rigid PES
scan on model compound 5 was carried out on a fully optimized
system with fixed parameters (in z-matrix format) except for the
dihedral angle N–C–C@P.
[1] W.V. Dahlhoff, S.M. Nelson, J. Chem. Soc. A (1971) 2184.
[2] A. Sacco, G. Vasapollo, C.F. Nobile, A. Piergiovanni, M.A. Pellinghelli, M.
Lanfranchi, J. Organomet. Chem. 356 (1988) 397.
[3] A. Jouaiti, M. Geoffroy, G. Bernardinelli, Tetrahedron Lett. 33 (1992) 5071.
[4] M. Kawatsura, J.F. Hartwig, Organometallics 20 (2001) 1960.
[5] S. Ekici, D. Gudat, M. Nieger, L. Nyulaszi, E. Niecke, Angew. Chem., Int. Ed. 41
(2002) 3367.
[6] C. Hahn, M. Cucciolito, A. Vitagliano, J. Am. Chem. Soc. 124 (2002) 9038.
[7] D. Hermann, M. Gandelman, H. Rozenberg, L.J.W. Shimon, D. Milstein,
Organometallics 21 (2002) 812.
[8] A.S. Ionkin, W.J. Marshall, Heteroatom Chem. 13 (2002) 662.
[9] R.J. Trovitch, E. Lobkovsky, P.J. Chirik, Inorg. Chem. 45 (2006) 7252.
[10] M. Feller, E. Ben-Ari, T. Gupta, L.J.W. Shimon, G. Leitus, Y. Diskin-Posner, L.
Weiner, D. Milstein, Inorg. Chem. 46 (2007) 10479.
[11] A. Hayashi, M. Okazaki, F. Ozawa, R. Tanaka, Organometallics 26 (2007) 5246.
[12] S.M. Kloek, D.M. Heinekey, K.I. Goldberg, Angew. Chem., Int. Ed. 46 (2007)
4736.
[13] B.M. Cochran, F.E. Michael, J. Am. Chem. Soc. 130 (2008) 2786.
[14] C. Müller, E.A. Pidko, M. Lutz, A.L. Spek, D. Vogt, Chem. Eur. J. 14 (2008) 8803.
[15] J.I. van der Vlugt, E.A. Pidko, D. Vogt, M. Lutz, A.L. Spek, A. Meetsma, Inorg.
Chem. 47 (2008) 4442.
[16] C. Moser, M. Nieger, R. Pietschnig, Organometallics 25 (2006) 2667.
[17] C. Moser, A. Orthaber, M. Nieger, F. Belaj, R. Pietschnig, Dalton Trans. 32 (2006)
3879.
[18] C. Moser, F. Belaj, R. Pietschnig, Chem. Eur. J. 15 (2009) 12589.
[19] F. Mathey, Angew. Chem., Int. Ed. 42 (2003) 1578.
[20] P.L. Floch, Coord. Chem. Rev. 250 (2006) 627.
[21] G. Becker, O. Mundt, M. Roessler, E. Schneider, Z. Anorg. Allg. Chem. 443 (1978)
42.
[22] B.-L. An, J.-X. Shi, W.-K. Wong, K.-W. Cheah, R.-H. Li, Y.-S. Yang, M.-L. Gong, J.
Lumin. 99 (2002) 155.
[23] G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr. A64 (2008) 112.
4.4. Crystal structure solution of compound (3)
All measurements were performed using graphite-
monochromatized Mo K
solved by direct methods (SHELXS-97) and refined by full-matrix
least-squares techniques against F2
SHELXL-97). The non-hydrogen
a radiation at 100 K. The structure was
(
atoms were refined with anisotropic displacement parameters
without any constraints. The H atoms of the phenyl rings were
put at the external bisector of the C–C–C angle at a C–H distance
of 0.95 Å and a common isotropic displacement parameter was re-
fined for the H atoms of the central phenyl ring as well as for those
of the phenyl rings of the mesityl groups. The H atoms of the
methyl groups were refined with common isotropic displacement
parameters for the H atoms of the same group and idealized geom-
etry with tetrahedral angles, enabling rotation around the X–C
bond, and C–H distances of 0.98 Å. Further details are summarized
in Table 1.