Tuning the electronic effects of aromatic phosphorus heterocycles: An unprecedented phosphinine with significant P(π)-donor properties

Article abstract of DOI:10.1039/c4cc03469d

A hitherto unprecedented electronic situation has been observed for a substituted, pyridyl-functionalized phosphinine. In contrast to previous studies, this compound shows considerable π-donor properties as the result of the rather strong +M effect of the

Full text of DOI:10.1039/c4cc03469d

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Tuning the electronic effects of aromatic phosphorus
heterocycles: an unprecedented phosphinine with
significant P(π)-donor properties
The novel methylthiol-substituted 2,4,6-triarylphosphinine
derivative shows considerable π-donor properties as a result
of the rather strong +M effect of the CH3S-group. This
unprecedented situation opens up the possibility to tune the
electronic properties of such aromatic phosphorus heterocycles
effectively with respect to potential applications in homogeneous
catalysis and molecular material sciences.
See Christian Müller et al.,
Chem. Commun., 2014, 50, 8842.
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Tuning the electronic effects of aromatic
phosphorus heterocycles: an unprecedented
phosphinine with significant P(p)-donor
properties†
Cite this: Chem. Commun., 2014,
50, 8842
Received 8th May 2014,
Accepted 6th June 2014
Antonia Loibl,a Iris de Krom,b Evgeny A. Pidko,c Manuela Weber,a Jelena Wieckoa
DOI: 10.1039/c4cc03469d
and Christian Muller*ab
¨
www.rsc.org/chemcomm
A hitherto unprecedented electronic situation has been observed
for a substituted, pyridyl-functionalized phosphinine. In contrast to
previous studies, this compound shows considerable p-donor properties
as the result of the rather strong +M effect of the CH3S-substituent,
changing the electronic properties of this low-coordinate and aromatic
phosphorus heterocycle substantially.
Fig. 1 Low-coordinated l3s2-phosphorus compounds.
Since the first successful preparation of 2,4,6-triphenylphosphinine
of phosphinines indicate that these aromatic heterocycles act
indeed as electron withdrawing ligands with properties similar to
phosphites. Interestingly, their p-accepting behaviour has led to the
development of very active Rh-catalysts for the hydroformylation of
(internal) alkenes.5 It has further been suggested that the HOMO
and HOMOꢀ1 p-orbitals can participate in Z6-coordination
towards a transition metal center via the aromatic ring, which
accounts for the range of coordination modes observed in various
transition metal complexes based on phosphinines.6
With respect to potential applications, it has long been discussed
whether or not only the steric but also the electronic properties of
phosphinines can be modified successively. From the MO diagram
shown in Fig. 2 it is, for example, not obvious why potentially
p-donor properties of s-coordinated phosphinines have neither
been considered, nor discussed in the literature. Especially as the
HOMO is rather high in energy, this situation should have a
¨
(A, Fig. 1) by Markl in 1966, this low-coordinate phosphorus
compound has been regarded as an aromatic heterocycle with
significantly different electronic properties compared to its
nitrogen counterpart (pyridine), as well as classical trivalent
phosphorus(III) species.1
Both the reluctance of phosphorus to undergo 3s–3p hybridiza-
tion and the differences in electronegativities of N (3.1), P (2.1) and
C (2.5) lead to a substantial decrease in lone-pair basicity in C5H5P
(63.8% lone-pair s-character) compared to C5H5N (29.1% lone-pair
s-character). This special situation causes an inversion of the p-and
n-orbital sequences between phosphinine and pyridine (Fig. 2).2
When coordinated to a metal center through the P-atom,
phosphinines have been described to act as weak s-donor
(energetically low-lying HOMOꢀ2), but rather strong p-acceptor
(energetically low-lying LUMO) ligands, which can stabilize
efficiently metal centers in low oxidation states.3 In contrast, the
p-excess aromatic l3s2-benzazaphospholes (B, Fig. 1), investigated
by Heinicke et al., show considerable P(p)-donor properties due to
a Nꢀ–CQP 2 N+QC–Pꢀ conjugation.4 As a matter of fact,
IR-stretching frequencies of transition metal carbonyl complexes
a
¨
¨
Institut fur Chemie und Biochemie, Freie Universitat Berlin, Fabeckstr. 34/36,
14195 Berlin, Germany. E-mail: c.mueller@fu-berlin.de; Fax: +49 30 838 454004;
Tel: +49 30 838 54004
b Department of Chemical Engineering and Chemistry, Eindhoven University of
Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands
c Institute of Complex Molecular Systems, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
† Electronic supplementary information (ESI) available: X-ray data and electro-
chemical data. CCDC 988905. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc03469d
Fig. 2 Frontier orbitals of pyridine (left) and phosphinine (right).
8842 | Chem. Commun., 2014, 50, 8842--8844
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Scheme 2 Synthesis of phosphinine-based tungsten(0) complexes.
Scheme 1 Synthetic route towards substituted, pyridyl-functionalized
phosphinines.
Fig. 3 Time-dependent 31P{1H} spectra for the conversion of phosphinine
3b with W(CO)6 towards 5b. * Unidentified species.
significant effect on the nature of the metal–phosphorus bond.
On the other hand, a systematic investigation into the influence of
substituents on the electronic properties of phosphinines has so far
remained elusive, mainly due to synthetic reasons. During the
course of our investigations into chelating phosphinines, we
recently reported on the preparation and coordination chemistry
of 2-(20-pyridyl)-4,6-diphenylphosphinine, which was obtained via
the classical pyrylium-salt route (3a, Scheme 1).7 This P,N-hybrid
ligand enforced s-coordination to a metal center and is therefore an
ideal bidentate ligand to probe influences of substituents on the
electronic properties of the aromatic phosphorus heterocycle,
because p-coordination can be excluded. We anticipated that the
modular synthetic route towards 3a would offer the possibility of
introducing defined substituents into the phenyl group in the
4-position of the heterocyclic framework. This would allow for the
first time a systematic modulation of the electronic properties of 3a
not only via +/ꢀI-effects of the substituents, but also through the
participation of the PQC double bond in conjugative interactions
via +/ꢀM-effects. Starting from p-substituted benzaldehydes, the
corresponding diketones 1a–d were synthesized by condensation reac-
tion of the substituted chalcones with acetylpyridine. Subsequently,
the pyrylium salts 2a–d, bearing either a H, F, CF3, or CH3S-group
could be obtained as yellow-red solids in good yields. We were able to
convert all pyrylium-salts into the corresponding phosphinines 3a–d
by reaction with P(SiMe3)3 in CH3CN (Scheme 1), although the
isolated yields of the products were rather low.8
while the final complex 5b has been formed almost quantitatively
1
after 10 h (d(ppm) = 201.0, JP–W = 278 Hz). We assume that the
observed intermediate is the mono-coordinated pentacarbonyl-
species [W(k-P,N)(CO)5] 4b, depicted in Scheme 2. A similar
transient species has been observed by Mathey et al. upon
reaction of NIPHOS with in situ generated [W(CO)5THF].9,10
All complexes could be isolated as red solids after recrystallization
and were analyzed by means of NMR spectroscopic techniques. Red
crystals of the fluorine-substituted compound 5b suitable for X-ray
diffraction were obtained from a hot THF solution. The compound
crystallizes in the orthorhombic space group Pna21 and the mole-
cular structure is depicted in Fig. 4 along with selected bond lengths
and angles. The molecular structure of 5b in the crystal shows a
distorted octahedral coordination geometry around the W-center
with the P,N-hybrid ligand acting as a bidentate chelate. As observed
before in related transition metal complexes of the unsubstituted
P,N-ligand 3a, the metal center is not located in the ideal axis of the
phosphorus lone-pair and clearly shifted towards the nitrogen atom.
Coordination compounds 5a–d were further investigated by
means of IR-spectroscopy (Table 1). In agreement with related
bypridine–W(CO)4 compounds, all complexes show the char-
acteristic wavenumbers n~(CO) centered at around 1900 cmꢀ1
.
For the fluorine-substituted derivative 5b we noticed only a
Compounds 3a–d show the characteristic downfield-shift of
the phosphorus-signal in the 31P{1H} NMR spectrum at d(ppm) =
186.7 (3a), 186.6 (3b), 192.2 (3c) and 185.8 (3d). In order to probe
the electronic properties of 3a–d we decided to prepare the
corresponding [W(k-P,k-N)(CO)4] complexes and to analyze them
by means of IR-spectroscopy. According to 31P NMR spectroscopy,
coordination compounds 5a–d were obtained quantitatively by
reaction of equimolar amounts of the ligand and W(CO)6 in THF
and under irradiation with UV-light (Scheme 2).
The reactions could be monitored by means of 31P{1H} NMR
spectroscopy and the course of the reaction is depicted for the
fluorine-substituted P,N-ligand 3b in Fig. 3.
Fig. 4 Molecular structure of 5b in the crystal. Displacement ellipsoids are
shown at the 50% probability level. Selected bond lengths (Å) and angles (1):
W(1)–P(1): 2.464(4); W(1)–N(1): 2.31(2); P(1)–C(1): 1.77(2); P(1)–C(5): 1.72(2);
Interestingly, an intermediate with W-satellites (1JP–W = 275 Hz)
is observed at d(ppm) = 160.8 upon consumption of the ligand, C(5)–C(6): 1.49(2); C(1)–P(1)–C(5): 104.1(8).
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Table 1 IR-wavenumbers n~(CO) of 5a–d [in cmꢀ1
]
Remarkably, upon introducing a CH3- or CH3S-substituent (3d), the
frontier orbitals are destabilized compared to the reference com-
pound 3a, while especially the HOMO is now much higher in energy
than the HOMO in 3a. We believe that the rather strong +M effect of
especially the CH3S-group increases the p-donor properties of 3d by
conjugative interactions through the HOMO. This situation leads
consequently to an energetically high-lying HOMO and at the same
time to stronger p-donor properties. However, this does not clarify
why the IR-stretching frequencies in 5d are shifted to higher
wavenumbers compared to those in 5a. We anticipate that the
particular electronic properties of phosphinines might deliver a
plausible explanation: in contrast to common p-donor ligands, such
as halides, S2ꢀ or SCNꢀ (see the spectrochemical series), the LUMO
(p-accepting properties) and the HOMO (p-donating properties) in
3d have particularly large coefficients at the phosphorus donor-
atom, which are similar in shape and which point to the same
direction (Fig. 5). These molecular orbitals would therefore interact
with the same filled metal-centered d-orbital in an octahedral
complex, leading to repulsion and net weakening of the P–M bond
and, consequently, to a shift of the IR stretching modes to higher
wavenumbers, i.e. close to uncomplexed W(CO)6 (n~ = 1998 cmꢀ1).
We have demonstrated that the electronic properties of 2,4,6-
triarylphosphinine derivatives can be modulated systematically
and rather efficiently by introducing substituents into specific
positions of the heterocyclic framework. Furthermore, we found
evidence that the HOMO and HOMOꢀ1 of p-symmetry play a
significant role in the properties of metal complexes containing
s-coordinated phosphinine ligands, which has so far been
neglected for this class of compounds. This particular electro-
nic situation is caused by the special shape and orientation of
the LUMO and the HOMO of the phosphinine ligand and is
unique in comparison with other p-donors.
5a (–H)
5b (–F)
5c (–CF3)
5d (–SCH3)
n~1
n~2
n~3
n~4
2008
1893
1870
1836
2012
1893
1877
1840
2014
1920
n.d.
2014
1971
1889
1858
1857
marginal shift to higher wavenumbers compared to the reference
compound 5a (R = H). We attribute this minor increase in the
p-acceptor capacity of the phosphinine 3b to a +M effect of the
fluorine atom, which counterbalances the –I effect of this electron-
withdrawing substituent. Indeed, the IR-data of compound 5c,
which bears a –CF3 as a pure –I substituent in the para-position,
proved the higher p-accepting properties of ligand 3c. In the
presence of an electron-donating group in the same position of
the heterocycle, one would expect a shift to lower wavenumbers
due to an increased electron density in the antibonding p*(C–O)
orbitals. Much to our surprise, however, we clearly observed a
significant opposite effect for coordination compound 5d, which
contains a CH3S-substituent in the para-position. As a matter of
fact, we detected a considerable shift of the IR bands to higher
wavenumbers, rather than to lower ones. In order to explain this
unusual observation we carried out DFT-calculations at the B3LYP/
6-311+G(d,p) level on the pyridyl-functionalized phosphinines
3a–d and compared the relative energies of the frontier orbitals.
The results are depicted in Fig. 5. For comparison, we included
the H3C-substituted coordination compound in the calculations,
although the corresponding P,N-ligand could not be synthesized.8
Fig. 5 (right side) shows the shape of the frontier orbitals of the
CH3S-substituted phosphinine 3d. Qualitatively, both the sequence
and the shape of the p- and n-orbitals resemble the ones depicted
in Fig. 2 for the parent compound C5H5P with the exception that
the HOMOꢀ1 in C5H5P is now represented by the HOMOꢀ1 and
HOMOꢀ2 in 3d. This is also the case for phosphinines 3a–c. Upon
introducing a fluorine substituent (3b), the frontier orbitals are
slightly stabilized. This is in line with our chemical intuition that
the electron withdrawing fluorine substituent should lower the
energy of the LUMO which would consequently lead to stronger
p-accepting properties as observed also in the IR stretching
frequencies in 5b. This effect, however, seems to be only marginal
and might be attributed to the +M effect of the fluorine substituent.
The European Initial Training Network SusPhos (317404) is
gratefully acknowledged for financial support.
Notes and references
¨
¨
1 (a) G. Markl, Angew. Chem., 1966, 78, 907; (b) G. Markl, in Multiple
Bonds and Low Coordination in Phosphorus Chemistry, ed. M. Regitz
and O. J. Scherer, Thieme Verlag, Stuttgart, 1990, pp. 220–257.
2 (a) C. Batich, E. Heilbronner, V. Hornung, A. J. Ashe III, D. T. Clark,
U. T. Gobley, D. Kilcast and I. Scanlan, J. Am. Chem. Soc., 1973,
´
95, 929; (b) N. Mezailles, F. Mathey and P. Le Floch, Prog. Inorg.
Chem., 2001, 455.
3 (a) C. Mu¨ller, L. E. E. Broeckx, I. de Krom and J. J. M. Weemers, Eur.
J. Inorg. Chem., 2013, 187; (b) P. Le Floch, Coord. Chem. Rev., 2006,
250, 627; (c) P. Le Floch and F. Mathey, Coord. Chem. Rev., 1998,
179–180, 771; (d) C. Mu¨ller and D. Vogt, Dalton Trans., 2007, 5505.
¨
¨
´
4 (a) M. Ghalib, L. Konczol, L. Nyulaszi, P. G. Jones, G. J. Palm and
J. W. Heinicke, Dalton Trans., 2014, 43, 51; (b) M. Ghalib, P. G. Jones,
G. J. Palm and J. W. Heinicke, RSC Adv., 2013, 3, 17726.
5 (a) B. Breit, R. Winde, T. Mackewitz, R. Paciello and K. Harms,
´
Chem. – Eur. J., 2001, 7, 3106; (b) L. Kollar and G. Keglevich, Chem.
Rev., 2010, 110, 4257.
6 E. F. DiMauro and M. C. Kozlowski, J. Chem. Soc., Perkin Trans. 1,
2002, 439.
7 I. de Krom, E. A. Pidko, M. Lutz and C. Mu¨ller, Chem. – Eur. J., 2013,
19, 7523.
8 Unfortunately, the pyrylium salt of the –CH3-substituted derivative
could not be obtained in a pure form.
`
9 A. Breque, C. C. Santini, F. Mathey, J. Fischer and A. Mitschler, Inorg.
Chem., 1984, 23, 3463.
¨
Fig. 5 Frontier orbitals of 3a–d and the CH3-derivative.
8844 | Chem. Commun., 2014, 50, 8842--8844
10 J. Deberitz and H. Noth, J. Organomet. Chem., 1973, 49, 453.
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