Substituent effects in pyridyl-functionalized pyrylium salts, pyridines and λ3,σ2-phosphinines: a fundamental and systematic study

Article abstract of DOI:10.1039/c8dt01441h

A series of substituted, pyridyl-functionalized 2,4,6-triarylpyrylium salts were prepared and investigated for their light absorption and emission properties. After reaction with P(SiMe₃)₃, the corresponding λ³-phosphinine

Full text of DOI:10.1039/c8dt01441h

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Substituent Effects in Pyridyl-Functionalized Pyrylium Salts,
Pyridines and λ3,σ2-Phosphinines: A Fundamental and Systematic
Study
Received 00th January 20xx,
Accepted 00th January 20xx
Antonia Loibl,a Wiebke Oschmann,a Maria Vogler,a Evgeny A. Pidko,b Manuela Weber,a Jelena
Wiecko,a and Christian Müller*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of substituted, pyridyl-functionalized 2,4,6-triarylpyrylium salts were prepared and investigated for their light
absorption and emission properties. After reaction with P(SiMe3)3 the corresponding λ3-phosphinines were obtained,
which carry on the 4- or 6-aryl ring +/-I and +/- M substituents. Supported by DFT calculations a systematic evaluation of
the σ-donor, π-donor as well as π-acceptor properties of these low-coordinate P,N-hybrid ligands was performed. The
modular synthetic approach allowed us at the same time to synthesize the structurally related bipyridine-derivatives for
comparison reasons. Reaction of the chelating ligands with [W(CO)6] in THF afforded the corresponding [(P^N)W(CO)4] and
[(N^N)W(CO)4] complexes. A crystallographic characterization of selected coordination compounds revealed significant
structural differences between the pyridyl-phosphinines- and the bipyridine-based compounds. Their characterization by
means of IR-spectroscopy gave experimental insight into the electronic properties of the respective ligands.
phosphorus variants of this compound class. Bipyridine-based
complexes found applications in homogeneous catalysis,
molecular devices or the development of new materials with
Introduction
λ3,σ2-Phosphinines differ in their electronic properties notably
from classical trivalent P(III) compounds and also from their
lower homologs, the pyridines.[1] The substantially reduced
basicity of the phosphorus atom in C5H5P (63.8% lone-pair s-
character) compared to the nitrogen atom in C5H5N (29.1%
lone-pair s-character) is caused both by an unfavourable 3s-3p
hybridization and oppositely polarized C-E bonds (Pauling
electronegativities: 2.5 for C, 2.1 for P and 3.1 for N).[2] These
peculiarities strongly determine the unique physical and
chemical properties of such low-coordinate phosphorus
heterocycles, which constitute an important object of research
fascinating photophysical properties.[5] When exchanging one
pyridine moiety of 2,2’-bipyridine by a phosphinine unit, the
semi-equivalent 2-(2’-pyridyl)phosphinine is obtained.
According to the HSAB concept of Pearson, these intriguing
chelates combine a “hard” σ-donating nitrogen with a “soft” π-
accepting phosphorus atom. Such bidentate hybrid ligands
have been reported for the first time by Mathey et al. with the
synthesis of 2-(2’-pyridyl)-4,5-dimethylphosphinine (NIPHOS).
Until today the unsubstituted parent compound remains
unknown.[6] However, we found that 2-(2’-pyridyl)-4,6-
diphenyl-phosphinine (2, Scheme 1) is readily available from
in our group. Our previous contributions have laid
a
the pyridyl-functionalized pyrylium salt
Interestingly, this route allows simultaneously the preparation
of the 2,2’-bipyridine derivative , starting from and NH3. In
1
and P(SiMe3)3.[7]
foundation for detailed investigations on the synthesis,
coordination chemistry and reactivity of functionalized 2,4,6-
triaryl-substituted phosphinines, which are generally
accessible via the modular pyrylium salt route.[3,4] Since 2,2’-
bipyridine and its derivatives are among the best studied
chelating ligands, we became interested in the exploration of
3
1
this way, heterocycles with an identical substitution pattern
are derived, which makes a direct comparison possible.
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tailor-made phosphinines where electronic and steric
properties can be tuned as necessary.
Scheme 1. Synthesis of 2 and 3 via the modular pyrylium salt route.
We report here now on the synthesis, coordination chemistry
During our investigations on the properties of pyridyl-
phosphinine it turned out that the corresponding
phosphinine-sulfide ( =S) reacts quantitatively with methanol
under formation of the unusual P(V)-species
(Figure 1).[8]
and evaluation of the stereo-electronic properties of 2, which
2,
contain additional substituents in 4- and 6-position of the
phosphinine-heterocycle.
2
A
Moreover, the synthetic access to a completely new set of
metal complexes based on phosphinines has been achieved by
us by making use of the chelate effect. Accordingly,
coordination compounds with metal centres in both low as
well as medium to high oxidation states could be prepared for
the first time, which was not possible with monodentate
phosphinines as ligands before (B
, Figure 1).[9] We also
demonstrated that Pt(II) and Ir(III) complexes containing
bidentate phosphinine-ligands can selectively activate small
molecules, such as CH3OH, H2O and NH3 as the result of the
rather reactive P=C double bond present in metal complexes
of type
the electronic properties of
substituents at the phenyl ring in 4-position of the heterocyclic
framework (
, Figure 1).[12]
B
and
C
(Figure 1).[10,11] More recently we proofed that
2
can be adjusted by introducing
D
Figure 2. Frontier molecular orbitals of phosphinine and pyridine.
Starting from the corresponding pyrylium salts, a systematic
modulation of both +/-I and +/-M effects has been achieved
depending on the type of substituent. For comparison reasons
we have extended our studies also to the structurally related
2,2’-bipyridine derivatives of 3 (see Scheme 1).
Results and discussion
Pyridyl-functionalized pyrylium-salts
2-(2-pyridyl)-4,6-diphenylpyrylium tetrafluoroborate 1a can be
synthesized starting from chalcone and acetylpyridine, which
are converted first to the corresponding diketone in the
presence of a base (Scheme 2, R = H).[7a,12]
Figure 1. Selected examples of the chemistry of pyridyl-phosphinine 2.
Compared to pyridine, the inversion of the π- and n-orbital
sequences in phosphinine leads to weak σ-donor
a
(energetically low-lying HOMO-2), but rather strong π-acceptor
properties (energetically low-lying LUMO), once coordinated
to
a
metal centre through the P-atom (Figure 2).[13,14]
Interestingly, it tunred out that the π-shaped HOMO has a
pronounced influence on the properties of phosphinine-metal
σ-complexes, which has not been considered for this ligand
class before.[12,
15]
Compelling results can be expected if the
unique characteristics of phosphinines could therefore be
applied, for example, in molecular materials or homogeneous
catalytic systems. Consequently, a prerequisite is the access to
Scheme 2. Synthesis of pyridyl-functionalized pyrylium salts. The substituent is
located on the 4-aryl-ring.
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Accordingly, the substitution pattern can easily be modified by and optical properties. Particularly their use as strongly
using differently substituted acetophenones and benzalde- fluorescent (laser) dyes and sensors is well documented.
hydes, respectively, during the preparation of the chalcone. Moreover, they have been used as photosensitizers for
With this modular procedure, pyrylium salts 1a-e, bearing electron-transfer reactions and in photocatalysis, especially as
either an H-, F-, -CF3-, -OCH3 or -SCH3-group as p-substituent in initiating systems for cationic polymerization, photoredox
the 4-aryl-ring of the 2,4,6-triarylpyrylium derivative were catalysis and photopolymerizations. Applications as charge
obtained by treatment of the diketone with excess BF3·Et2O transport materials and as photochromic materials have also
(Scheme 2). All compounds were obtained as yellow (R = H, F, been described in literature.[16]
CF3), orange (R = OCH3) and dark red (R = SCH3) solids in
reasonable yields (Figure 3). Likewise, the yellow-to dark red
coloured pyrylium salts 1f-h containing an additional F-, -OCH3
or -SCH3-group as p-substituent in the 6-aryl-ring were
prepared from the corresponding substituted chalcones and
acetylpyridine (Scheme 3).
Figure 3. Photograph of pyrylium salts 1a-h in the solid state.
Figure 4. Molecular structure of 1e in the crystal. a) top-view, b) front view.
Displacement ellipsoids are shown at the 50% probability level. Selected bond
lengths (Å) and angles (°): O(1)-C(1): 1.348(3); O(1)-C(5): 1.347(3); C(1)-C(2):
1.366(3); C(2)-C(3): 1.406(3); C(3)-C(4): 1.408(3); C(4)-C(5): 1.362(3). C(5)-O(1)-
C(1): 121.32(18).
As the optical effects of pyrylium salts strongly depend on the
Scheme 3. Synthesis of pyridyl-functionalized pyrylium salts. The substituent is
substitution pattern of the oxygen-containing pyrylium
located on the 6-aryl-ring.
heterocycle (Figure 3), we first decided to study the influence
of the substituents in 1b-e and 1f-h, respectively, on their
Red crystals suitable for X-ray diffraction were obtained by
absorption and emission properties, with 1a (R = H) as a
slow recrystallization of 1e from a hot solution in methanol. 1e
reference compound. Figure 5 shows the UVvis spectra of
crystallizes in the space-group P1̅ with one independent
pyrylium salts 1a-e in acetonitrile, while the UVvis spectra of
molecule in the asymmetric unit. The molecular structure is
depicted in Figure 4a/b along with selected bond lengths and
distances (For the crystallographic characterization of 1b and
1h see Supporting Information).
compounds 1a
,
f-h in acetonitrile are depicted in Figure 6.
From Figure 4b it is obvious that the central pyrylium-ring is
almost co-planar with the pyridyl-ring in 2- and the phenyl-ring
in 6-position, while the 4-methylthiophenyl-group is slightly
twisted with respect to the central heterocycle (torsion angles:
168° vs. 179° (pyridyl) and 176° (phenyl)). This observation
indicates delocalization of the π-systems over the aromatic
rings.
2,4,6-Triarylpyrylium salts and their derivatives have found
numerous applications due to their special chemical, physical
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(
1d). Analogous observations were made for substitution in
the x-chromophore with an even stronger bathochromic shift
of the x-band to λ = 445 nm and λ = 475 nm for 1g (R’ = OCH3)
and 1h (R’ = SCH3), respectively, while the y-band stays
unaffected at λ = 357 nm (1g) and λ = 360 nm (1h).
Figure 5. UVvis absorption spectra of 1a-e 1·10-5 M in acetonitrile at room
temperature.
On the other hand, the F-substituent in 1b (λ = 398, 361 nm)
and 1f (λ = 398, 358 nm) has almost no effect on the position
of the x- and y-bands compared to the parent pyrylium salt 1a
.
For 1c (R = CF3; λ = 404, 338 nm) the bands drift further apart
and a considerable decrease in intensity of both bands can be
detected.
Similarly, the influence of the different substituents in the
series 1b-e and 1f-h is also reflected in their emission
properties. Figure 8 shows a photograph of solutions of
pyrylium salts 1a-h in methanol under UV-light (λ = 365 nm).
Interestingly, changing the p-substituent in the 4-aryl-ring (1b-
1e) has an effect on the emission intensity, but influences the
colour of the emitted light only marginally. In contrast, the
colour of the emitted light changes drastically from blue (1f) to
yellow (1g) and finally to dark red (1h) upon changing the
substituent in the 6-aryl-ring of the pyrylium salt (1f-h).
Consequently, a striking difference exists especially for the
Figure 6. UVvis absorption spectra of 1a
temperature.
,
f-h 1·10-5 M in acetonitrile at room
Similar to triphenylmethane dyes 2,4,6-triarylpyrylium salts
can be viewed as two-dimensional chromophore systems,
allowing an in-depth interpretation of the photophysical
properties. The pyrylium ring together with the ortho-
substituents build the x-chromophore with a transition dipole
moment approximately through the 2- and 6-positions. The y-
chromophore composed of the pyrylium ring and the 4-aryl
group (Figure 7). Accordingly, the two absorption bands in the
visible area of light are assigned as x- and y-bands with the
longest-wavelength absorption corresponding to the x-
chromophore. The y-band is usually of higher intensity than
the x-band. Since the pyrylium ring is a strong electron
acceptor, a bathochromic shift of each band can be achieved
by increasing the donor strength of the substituents on the
respective chromophore.[17]
couples 1d
1b 1f (F).
/1g (OCH3) and 1e/1h (SCH3), while it is minimal for
/
Figure 8. Photograph of pyrylium salts 1a-h 1·10-2 M in methanol under UV-light
(λ = 365 nm).
Based on these qualitative observations, the emission spectra
of 1·10-4 M solutions of pyrylium salts 1a-h in methanol were
measured for an excitation wavelength of λ = 360 nm and the
results are illustrated in Figure 9.
Figure 7. 2,4,6-Triarylpyrylium salts as two-dimensional chromophore systems.
Accordingly, pyrylium salts 1a-h all display two bands between
350-550 nm. For reference compound 1a the x-band is located
at λ = 392 nm and the y-band at λ = 359 nm. Introduction of a
π-donating group in the y-chromophore causes a red shift of
the y-band to λ = 417 nm for 1d (R = OCH3) and even further to
λ = 461 nm for pyrylium salt 1e (R = SCH3) while the x-band
stays almost unchanged at λ = 392 nm (1e) and λ = 400 nm
Figure 9. Emission spectra of pyrylium salts 1a-h
temperature with excitation at λ = 360 nm.
1ꢀ
10-4 M in methanol at room
The emission spectra of 1a-h confirm that the most significant
change in the emission colour occurs when the substituents in
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the x-chromophore are modified. Substitution in the y- to obtain pyrylium salts with tuneable emission wavelengths of
chromophore, on the other hand, only lead to changes in strong intensity.
intensity of the emission.
To rationalise these results, the occurrence of non-radiative Pyridyl-functionalized pyridines and phosphinines
twisted intramolecular charge transfer (TICT) states in the x-
The 2,4,6-triarylpyrylium tetrafluoroborates 1a-h were further
and y-chromophores have to be taken into account. Rotation
converted with P(SiMe3)3 in acetonitrile to the corresponding
pyridyl-functionalized λ3-phosphinines 2a-h and with ammonia
of an aryl-ring out of the central pyrylium plane causes a
charge separation which decreases or quenches the emission.
in water/ethanol to the 2,2’-bipyridine derivatives 3a-h
,
Formation of such a TICT state is promoted by solvents of high
polarity and low viscosity as well as by a high acceptor strength
of the pyrylium ring. Since the pyrylium centre is stronger
accepting for substituents in 4-position, the y-chromophore is
more likely to twist than the x-chromophore. Additionally,
strong donating groups stimulate a charge separation and
thereby formation of a TICT state.[17]
respectively. The synthesis of phosphinines 2a-c,e and the
bipyridine 3a via the modular pyrylium salt route has already
recently been reported by us (Figure 10).[12]
In all pyrylium salts apart from 1d and 1e, the longest
wavelength absorption band and therefore the light emission
can be attributed to the x-chromophore. While reference
compound 1a emits blue light of λ = 464 nm, increasing the π-
donor strength of the substituent in the 6-aryl-ring shifts the
band to λ = 476 nm, λ = 566 nm and even λ = 637 nm for 1f (R’
= F), 1g (R’ = OCH3) and 1h (R’ = SCH3), respectively. The
intensity of the emission increases substantially by introducing
a fluorine substituent in 1f, but decreases strongly in the
presence of an OCH3- (1g) or an SCH3-group (1h). Supposedly,
twisting the x-chromophore becomes increasingly favourable
in the series 1a,f-h until the emission is essentially quenched
for 1h
.
Substitution in the y-chromophore, on the other hand, does
not have a significant impact on the emission wavelength. For
fluorine containing compounds 1b (R = F, λ = 462 nm) and 1c
(R = CF3, λ = 473 nm) the longest wavelength absorption is the
x-band, making the y-chromophore irrelevant for the
wavelength of the emitted light. For the stronger π-donating
Figure 10. Synthesised phosphinine- and bipyridine-derivatives.
substituents OCH3-
(1d) and SCH3- (1e) within the y-
chromophore, the y-band becomes the longest absorption
wavelength. The occurrence of non-radiative TICT states in the
y-chromophore can be considered very likely for these
substituting groups in a polar solvent of low viscosity, such as
methanol. The observed light emission for these pyrylium salts
at λ = 476 nm (1d, R = OCH3) and λ = 464 and λ = 476 nm (1e, R
= SCH3) can therefore evidently be attributed to the x-
chromophore, which is also supported by the absence of any
influence of the substituent on the emission wavelength.
The intensity of the light emission of one chromophore can be
increased by diminishing the acceptor strength of the pyrylium
All phosphinines were obtained as pure, yellow-orange solids
in up to 31% yield after column chromatography and
recrystallization from hot acetonitrile. The bipyridine-
derivatives were isolated in rather high yield as pure and white
solids.
Crystals of the bipyridine-derivative 3h, suitable for X-ray
diffraction, could be obtained by slow recrystallization from
hot ethanol. The molecular structure of 3h in the crystal is
depicted in Figure 11 along with selected bond lengths and
angles.
As expected, the graphical representation of 3h shows a
coplanar arrangement of the two pyridine-moieties with the
nitrogen atoms in trans-position, as the lowest energy
conformation of this molecule. In contrast, the central
pyridine-ring and the phenyl-group in 4-position are twisted
significantly with a torsion angle (C(2)-C(3)-C(17)-C(18)) of
140.2(3)°. Interestingly, this value is much higher compared to
the one observed for the pyrylium salt 1e (~168°), which
ring through encouraging
a TICT state in the other
chromophore.[17] This corresponds to pyrylium salts 1b and 1d
,
having a substituent that promotes twisting of the y-chromo-
phore and displaying emission of very high intensity from the
x-chromophore. Concluding from this, combining a H-, OCH3-
or SCH3-substituent in the x-chromophore with a F- or OCH3-
group in the y-chromophore should be an interesting strategy
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contains a methylthiophenyl-group in the 4-position of the phosphinines 2a-h and of the bipyridine derivatives 3a-h at the
central heterocycle (Figure 4). Moreover, the reported solid- B3LYP/6-311+G(d,p) level of theory.[12] Figure 13 shows a
state-structure of the bipyridine derivative 3e shows also an comparison of the shape and the relative energies of the
almost coplanar arrangement of the methylthiophenyl-group LUMO – HOMO-3 of the reference compounds 2a and 3a (R =
in 4-position and the central pyridine ring (torsion angle = H). For the graphical representation of the frontier orbitals of
171.9°).[18] Apparently, conjugative interactions by the strong all other compounds see supplementary material.
+M effect of the SCH3-substiutent override the non-bonding Qualitatively, both the sequence and the shape of the π- and
steric repulsions of the ortho-hydrogen atoms in 1e. These are n- orbitals of the pyridyl-functionalized phosphinine 2a
obviously much less pronounced in the neutral bipyridine resemble the ones depicted in Figure 2 for the parent
derivative 3h, which lacks the SCH3-group in the respective compound C5H5P. For the bipyridine-derivative 3a, however,
position (For the crystallographic characterization of 3c see the situation is rather different. The introduction of three
Supporting Information). Even though the difference of additional aryl-groups in 2-, 4-, and 6-position of the pyridine-
approximately 30° for these compounds is rather large, it ring seem to decrease the σ-donating ability of the ligand as
should be taken into account that these distortions could also the lone-pairs of the nitrogen atoms now appear in the
be induced also by packing effects.
HOMO-2, rather than in the HOMO. For 3a, π-accepting and π-
donor properties seem to dominate over σ-donation. From the
relative energies of the frontier orbitals of 2a in comparison to
3a it is obvious, that the pyridyl-functionalized phosphinine is
both the better π-acceptor and π-donor than the bipyridine-
derivative 3a, while their σ-donor capacities are very similar.
Figure 11. Molecular structure of 3h in the crystal. Displacement ellipsoids are
shown at the 50% probability level. Selected bond lengths (Å) and angles (°):
N(1)-C(1): 1.342(4); N(1)-C(5): 1.354(4); C(1)-C(2): 1.387(5); C(2)-C(3): 1.394(4);
C(3)-C(4): 1.389(5); C(4)-C(5): 1.383(5); C(5)-N(1)-C(1): 117.6(3); (C(2)-C(3)-C(17)-
C(18): 140.2(3).
Accordingly, the methylthiophenyl-substituent in 6-position of
bipyridine 3h is again almost coplanar to the central pyridine-
ring, with
a
torsion angle (C(2)-C(1)-C(11)-C(12)) of
approximately 166°. This observation is in line with
a
decreased non-bonding steric repulsion between only three
ortho-hydrogen atoms along with the strong +M effect of the
SCH3-substituent. An interesting observation can be made in
both crystallographic representations of 1e and 3h with
respect to the orientation of the methylthio-substituent. At
least in the solid state, the SCH3 groups are aligned in such a
way, that one of the two lone-pairs can efficiently overlap with
the π-system of the adjacent phenyl-group. This maximizes the
+M effect of the SCH3-substituent, although conjugative
interactions should be less pronounced in the neutral
bipyridine-derivative 3h compared to the charged pyrylium
salt 1e. The influence of a SCH3-group on any π-conjugation is
depicted in Figure 12 as mesomeric structures of compounds
Figure 12. Mesomeric structures of 2e/3e and 2h/3h.
2d/3d and 2h/3h. It is clear, that these resonance structures,
resulting from charge transfer, will have a drastic impact on
the electronic properties of the phosphinines and bipyridines,
respectively (vide infra).
As DFT-calculations are a powerful tool to investigate the
electronic properties of substituted phosphinines, we
calculated the frontier orbitals of the pyridyl-functionalized
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Figure 14. Energy levels of the LUMO-HOMO-3 for phosphinine-(orange) and
bipyridine-(blue) derivatives 2a-e and 3a-e (a) and 2a f-h and 3a f-h (b).
,
,
By introducing an F-substituent in the para-position of the 4-
aryl-ring (2b/3b), the frontier orbitals are slightly stabilized
with respect to the reference compounds 2a and 3a (R = H).
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 π-
accepting properties. The σ- and π-donating capacities, on the
other hand, should slightly decrease. This effect is visible but
seems to be marginal which can be attributed to the +M effect
of the fluorine substituent counterbalancing its -I effect, as is
also reflected in a Hammett constant of the F-group being very
close to zero (σP = 0.06).[19]
Figure 13. Energy levels of the LUMO-HOMO-3 for phosphinine- and bipyridine-
derivative 2a (left) and 3a (right).
The shape of the LUMO (π-accepting) and the HOMO (π-
donation) is very similar for all substituted phosphinines and
bipyridines (see Supporting Information), while the shapes of
the HOMO-1 – HOMO-3 are particularly influenced when
OCH3- and SCH3-groups are introduced in either the 6-aryl-ring
(2g, 2h, 3g, 3h) or in the 4-aryl-ring (2d, 2e, 3d, 3e). F- and CF3-
A much stronger effect is noticeable when a CF3-group, as a
pure -I substituent, is introduced. This leads to significant
π-accepting properties of both the phosphinine and the
bipyridine-derivative. Interestingly, Figure 14a suggests that
the CF3-substituted bipyridine derivative 3c has similar π-
substituents have a less significant effect. However, more
important is the change in the relative energies of the relevant
MOs, when substituents are introduced. This is illustrated in
Figure 14a for the series 2a-e and 3a-e (4-aryl-ring), and in
Figure 14b for the series 2f-h and 3f-h (6-aryl-ring).
accepting properties compared to 2,4,6-triarylphosphinine 2a
By introducing either an OCH3- (2d 3d) or an SCH3-substituent
2e, 3e), the frontier orbitals are drastically destabilized
.
,
(
compared to the reference compounds, while especially the
HOMOs are now much higher in energy than the HOMOs in 2a
and 3a. We believe that the rather strong +M effect
particularly of the OCH3- and SCH3-groups increases the
π-donor properties by conjugative interactions through the
HOMO, which is especially obvious from the shape of the
HOMOs of phosphinine 2d and 2e, respectively (Figure 15).
Figure 15. Visualized HOMO of 2d and 2e
.
Indeed, Figure 15 clearly illustrates the participation of the
OCH3- and SCH3-group in the respective molecular orbital, as it
is expected from a substituent with a strong +M effect. This
situation leads consequently to an energetically high-lying
HOMO and at the same time to stronger π-donor properties.
This would also be in line with the assumption of resonance
structures as shown for compound 2e in Figure 12. A similar
trend is observed when substituents are introduced in the
para-position of the 6-aryl-ring (Figure 14b). The F-group in 2f
and 3f again shows only a marginal change in the energy of the
frontier orbital compared to the reference compounds 2a/3a
.
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The strongest effects on the π-donor-capacities are caused complexes could be isolated as pure, dark red solids after
again by the OCH3- and SCH3-group, which significantly recrystallization (Figure 16).
destabilize the HOMO in 2g/3g as well as in 2h/3h. The Red crystals of the OCH3-substituted coordination compounds
stronger π-donor properties are again in line with anticipated 4g and 5g, suitable for X-ray diffraction, were obtained from a
resonance structures shown for compound 2h in Figure 12. hot acetonitrile solution and slow diffusion of diethyl ether
However, it is interesting to note that the substituents in the into a dichloromethane solution, respectively. The compounds
6-aryl-ring seem to have a stronger effect on the electronic crystallize in the space group P21/n and P21/c and the
properties of the corresponding heterocycles compared to the molecular structures are depicted in Figure 17a/b along with
influence of the substituents in the 4-aryl-ring. While the selected bond lengths and angles. The crystallographic
energies of the HOMOs are very different for the phosphinines characterization of both 4g and 5g makes a rather rare direct
and bipyridines 2d
similar in energy for 2g
Moreover, with respect to the same substituent, the energetic heteroatom (For the crystallographic characterization of 4f
difference between the HOMO of 2f-2h 3f-3h and the 5c 5d 5e 5f, and 5h see Supporting Information).
reference compound 2a 3a is slightly larger compared to the
difference calculated for 2b 3b and the unsubstituted
/
3d and 2e
/
3e (Figure 14a), they are very comparison of the structurally related coordination
3h (Figure 14b). compounds possible, which differ only in the nature of one
5b
/
3g as well as for 2h
/
,
,
/
,
,
/
,d
,
e
/
,d,e
heterocycles. A reason for this could be that the conjugative
interactions, especially by the OCH3- and SCH3-groups, are less
disturbed by the weaker non-bonding steric repulsions of the
three ortho-hydrogen atoms and the lone-pair in 2g
2h 3h (6-aryl-ring) compared to the situation in 2d 3d as well
as in 2e 3e (4-aryl-ring).
/3g and
/
/
/
In order to experimentally probe the electronic properties of
the series of phosphinines and bipyridines and to compare
them with the above-mentioned DFT calculations, we
prepared the corresponding [(P^N)W(CO)4] and [(N^N)W(CO)4]
complexes and analysed them by means of IR-spectroscopy.
The phosphinine-based coordination compounds 4a-h were
obtained quantitatively by reaction of equimolar amounts of
the ligand and [W(CO)6] in THF and under irradiation with UV-
light (Scheme 4).
Figure 16. Tungsten-carbonyl compounds of phosphinine- and bipyridine
derivatives.
Scheme 4. Synthesis of tungsten-carbonyl complexes.
The molecular structure of 4g in the crystal shows a slightly
distorted octahedral coordination geometry around the W-
centre with the P,N-hybrid ligand acting as a bidentate chelate.
Moreover, the difference between the pyridine moiety and the
aromatic phosphinine ring is obvious, which is best described
as a distorted hexagon due to the larger phosphorus atom and
the resulting longer P-Cα bonds in comparison to the nitrogen
case.[2b] The two heterocyclic rings in 4g are slightly twisted
with respect to one another (torsion angle N(1)-C(6)-C(5)-C(4)
= 171.8°), with an intercyclic C(5)-C(6) bond of 1.475(6) Å. The
P(1)-C(1) and P(1)-C(5) bond lengths are with 1.730(4) Å and
As the conversion of 2a-h to 4a-h can easily be monitored by
means of 31P{1H} NMR spectroscopy, it is worth mentioning
that an intermediate with W-satellites is observed during the
course of the reaction, while the complex formation is
complete after approximately 15 h. We assume, that the
observed intermediates are the mono-coordinated penta-
carbonyl species [N∩PW(CO)5]. A similar transient species has
been observed by Mathey et al. upon reaction of NIPHOS with
in situ generated [W(CO)5THF].[20,21] In case of the bipyridine-
based complexes 5a-h, the reactions of 3a-h with [W(CO)6] in
THF were monitored by means of 1H NMR spectroscopy and
1.725(4)
Å somewhat shorter than in free 2,4,6-triaryl-
phosphinines (1.74-1.76 Å), while the internal C(1)-P(1)-C(5)
quantitative formation was reached within
a day. All
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angle is with 104.8(2)° larger compared to free 2,4,6-triaryl- with 1.472(4) Å somewhat shorter than in free 2,2’-bipyridine
phosphinines (101.24-101.76°). The carbon-carbon bond (1.490(3) Å), while the N(2)-W(1) bond is shorter than the
lengths in the aromatic phosphinine subunit are in the usual N(1)-W(1) bond (2.221(3) Å vs. 2.284(3) Å). This differences is
range (1.389(6)-1.403(6) Å) observed for both free and most likely due to the fact that the aryl-substituent in α-
complexed phosphinine ligands.[22]
position of the N(1)-pyridine ring of 3g is rather close to the
As observed before in related transition metal complexes of coordination site, which creates steric repulsion between this
the P,N-ligand 2a, the metal centre is not located in the ideal group and the CO ligand trans to N(2). Due to the longer P(1)-
axis of the phosphorus lone-pair and clearly shifted towards C(1) and P(1)-W(1) bond lengths in 4g compared to the N(1)-
the nitrogen atom.[11,12] Obviously, this non-directional C(1) and N(1,2)-W(1) bond distance in 5g, this effect is much
coordination mode is necessary for an appropriate less pronounced in the phosphinine-based complex. Con-
complexation of the W(CO)4-fragment by the chelating P,N sequently, this situation leads to rather different torsion angles
ligand and enabled by the more diffuse and non-directional of P(1)-C(1)-C(11)-C(12) = 40.3(6)° and N(1)-C(1)-C(11)-C(12)
lone pair of the low-coordinate phosphorus atom compared to = -61.5(4)°. In contrast to the free ligand (and also the pyrylium
the formally sp2-hybridized nitrogen atom in pyridines. salt), the enforced orientation of the α-aryl-group in the
Consequently, this phenomenon is not observed for the N(1)- coordination compounds, however, should have direct
W(1) interaction.
consequences for a substituent effect in the 6-aryl-ring and
any resulting π-conjugation.
The two series of coordination compounds 4a-e/5a-e and 4f-
h/5f-h were further investigated by means of IR-spectroscopy
and the results are listed in Table 1 and Table 2. In agreement
with related bipyridine-W(CO)4 compounds, all complexes
show the characteristic wavenumbers ꢀȬ(CO) centred at around
1900 cm-1. First, we turned our attention to the series of
complexes having the substituent in the 4-aryl-ring of the
central heterocycle (4a-e/5a-e, Table 1).
Table 1. Solid state IR data (ꢀȬ(CO)) for complexes 4a-e and 5a-e.
4
5
(a) R = H
(b) R = F
2008, 1893, 1870, 1836
2012, 1893, 1877, 1840
1999, 1872, 1846, 1813
2000, 1872, 1853, 1802,
1783
(c) R = CF3
2014, 1920, 1857, n.d.
2002, 1997, 1887, 1877,
1867, 1859, 1848, 1805,
1817a
(d) R = OCH3
(e) R = SCH3
2015, 1973, 1891. 1857
2014, 1971, 1889, 1859
1996, 1873, 1846, 1806,
1777 (sh)
1999, 1934, 1865, 1823
Figure 17. Molecular structures of 4g (a) and 5g (b) in the crystal. Displacement
ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and
angles (°): 4g: P(1)-C(1): 1.730(4); P(1)-C(5): 1.725(4); C(1)-C(2): 1.392(6); C(2)-
C(3): 1.403(6); C(3)-C(4): 1.395(6); C(4)-C(5): 1.389(6); C(5)-C(6): 1.475(6); C(6)-
N(1): 1.364(5); P(1)-W(1): 2.4467(12); N(1)-W(1): 2.296(3); C(5)-P(1)-C(1):
a: two sets of values are observed, due to the presence of two independent
molecules in the asymmetric unit. See Supporting Information.
104.8(2); P(1)-C(1)-C(11)-C(12): 40.3(6). 5g
: N(1)-C(1): 1.356(4); N(1)-C(5):
1.364(4); C(1)-C(2): 1.393(4); C(2)-C(3): 1.389(4); C(3)-C(4): 1.391(4); C(4)-C(5):
1.392(4); C(5)-C(6): 1.472(4); C(6)-N(2): 1.351(4); N(1)-W(1): 2.284(3); N(2)-W(1):
2.221(3); C(5)-N(1)-C(1): 116.8(3); N(1)-C(1)-C(11)-C(12): -61.5(4).
From the IR-spectroscopic investigations it becomes obvious
that for the F-substituted compounds 4b and 5b the
wavenumbers ꢀȬ(CO) only slightly increase in comparison to the
reference compounds 4a and 5a. This is in line with both the
experimental and theoretical data described above. As
expected, this effect is much more pronounced for the
The molecular structure of the bipyridine-based complex 5g
reveals, that the two nitrogen-heterocycles are only slightly
twisted with respect to one another (torsion angle (N(2)-C(6)-
C(5)-N(2) = -4.9°). The intercyclic C(5)-C(6) bond distance is
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coordination compounds 4c and 5c, containing the CF3-group Finally we analysed the IR-spectroscopic data of the series of
as a pure -I substituent. metal complexes containing the substituent in the para-
For the OCH3- and SCH3-substituted phosphinine-based position of the 6-aryl ring (4f-h/5f-h, Table 2). Similar to the
complexes 4d and 4e we notice an even larger increase in the observation made for compounds 4a-e/5a-e, the F-substituted
wavenumbers ꢀȬ(CO) which is counterintuitive to the fact that complexes 4f and 5f show only a marginal increase of the
+M substituents should lead to a decrease of the stretching wavenumbers ꢀȬ(CO) in comparison to the reference compounds
frequencies. However, as described above, both the OCH3- and 4a and 5a. The SCH3-substituted pyridyl-phosphinine 4h shows
the SCH3-group increase drastically the π-donor properties of again a very significant increase of ꢀȬ(CO), while a slight decrease
the respective ligand by conjugative interactions through the of the wavenumbers ꢀȬ(CO) can be noticed for the OCH3-
HOMO, as shown in Figure 15. This situation leads functionalized compound 4g. Most strikingly, however, almost
consequently to a destabilization of the HOMO and at the no effect of the substituent is present for the substituted
same time to stronger π-donor properties, as evident from the bipyridines in the coordination compounds 5g and 5h
.
theoretical calculations. We anticipate that the particular As obvious from Figure 14b, these findings are in contradiction
electronic properties of phosphinines might deliver a plausible to the theoretical calculations, as the HOMO-energies of the
explanation: In contrast to common π-donor ligands, such as phosphinine- and pyridine-based ligands 2g/2h (OCH3) and
halides, S2- or SCN- (see spectrochemical series), the LUMO 3g
(π-accepting properties) and the HOMO (π-donor properties)
of 2d and 2e have particularly large coefficients at the
phosphorus donor-atom, which are similar in shape and which
point into the same direction (Figure 13). These molecular
orbitals would therefore interact with the same filled
tungsten-centred d-orbital in an octahedral complex, leading
to a repulsion and a net weakening of the P-M bond and,
consequently, to a shift of the IR stretching frequencies to
/3h (SCH3) are essentially identical.
Table 2. Solid state IR data ʚꢀȬ(CO)) for complexes 4a,f-h and 5a,f-h.
4
5
(a) R‘ = H
2008, 1893, 1870, 1836
2009, 1902, 1870, 1840
2007, 1882, 1854, 1827 (sh)
1999, 1872, 1846, 1813
2002, 1882, 1851, 1819
higher values, i.e. close to uncomplexed [W(CO)6] (ῦ = 1998
cm-1). Interestingly, the effect of the OCH3- and SCH3-
substituent is much less pronounced in the bipyridine-based
coordination compounds 5d and 5e. While a significant shift to
higher wavenumbers can still be detected for 5e (SCH3), it is
almost negligible for 5d (OCH3). A closer look to the HOMO
energies of the corresponding ligands 3d and 3e reveals,
however, that they are substantially stabilized compared to
the situation in the phosphinine-based compounds and that
especially the HOMO of 3d is much lower in energy than the
HOMO of 3e (Figure 14a). Moreover, the visualization of the
HOMOs of 3d and 3e reveals rather small coefficients at the
central nitrogen-atoms (Figure 18). Both observations lead to
the conclusion that the π-donor capacities of the substituted
bipyridines 3d and 3e are distinctly lower than in the pyridyl-
phosphinines.
(f) R‘ = F
(g) R‘ = OCH3
1989, 1867, 1853, 1817,
1810
(h) R‘ = SCH3
2016, 1975, 1893,1859
1991, 1873, 1850, 1816
We anticipate, that the orientation of the 6-aryl-group with
respect to the remaining ligand framework plays an
fundamental role in the interpretation of the IR-spectroscopic
data of the here presented coordination compounds. As
evident from the crystallographic characterization of 4g and 5g
(Figure 17), the 6-aryl-group is located close to the metal
centre, showing a rather large torsion angle. As a matter of
fact, we even found a hindered rotation of this group around
the C(1)-C(11)-bond for closely related bipyridine-based Re(I)-
complexes, which can be detected by means of 1H NMR
spectroscopy.[23] Consequently, any +M effect of a substituent
will most likely be diminished, as a result of a disturbed
conjugation between the two aromatic moieties through the
π-system. Thus, only inductive effects of the substituents
might play a role in the coordination compounds. In contrast,
the much longer P-C and P-W bond in the phosphinine-based
complexes compared to an N-C and N-W bond in the nitrogen
analogues results in the location of the 6-aryl-group much
further away from the metal fragment. Accordingly, this
should result in a reduced repulsion and a much lower
Figure 18. Visualized HOMO of 3d and 3e
.
rotational barrier around the C(1)-C(11)-bond in 4a-h, which
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allows that both inductive and mesomeric effects of the substituent effects and confirms that σ-bonded phosphinines
substituent can still be operative, at least to a certain extend.
can have pronounced π-donor properties. In contrast, the
experimental evidence for π-donor capacities was less distinct
for bipyridine-based complexes, which we attribute to a
smaller coefficient of the relevant orbitals on the nitrogen
atom compared to the phosphorus analogue. Most strikingly,
however, is the fact that almost no electronic effect caused by
the substituents in the 4-aryl-ring can be found in the
bipyridine-based coordination compounds, while the
phosphinine-based systems still show a considerable influence
of the substituents in the 4-aryl-ring on the IR-stretching
frequencies. We attribute these finding to a considerable steric
hindrance between the metal fragment and the 4-aryl-group,
leading to a rather large torsion angle and consequently to an
unfavourable conjugation between the two aromatic moieties
through the π-system.
Conclusions
We recently found that 2,4,6-triaryl-λ3-phosphinines can have
significant π-donor properties depending on the substituent in
para-position of the 4-aryl-ring. Inspired by these results, we
now made use of the modular pyrylium salt route in order to
prepare a series of pyridyl-functionalized phosphinines and,
for comparisons reasons, also of the structurally related
bipyridine-derivatives. We were able to introduce F-, CF3-, -
OCH3- and -SCH3-groups into the 4-aryl-ring, while we could
chose for 6-aryl-rings containing either F-, OCH3- or -SCH3-
substituents. These two series of compounds were compared
with one another in order to systematically study electronic
+/- I and +/- M substituent effects on the properties of the
corresponding heterocycles.
These results demonstrate a fundamental and systematic
investigation of substituent effects on the properties of
pyridyl-functionalized 2,4,6-triaryl-λ3-phosphinines and their
corresponding tungsten(0)-carbonyl complexes. These studies
are an important aspect for specific ligand design and the
application of such low-coordinate phosphorus heterocycles in
more applied research fields, such as homogeneous catalysis
or molecular materials science.
We found a drastic influence of the different substituents on
the optical properties of the pyrylium salts. Being two-
dimensional chromophore systems, these compounds exhibit
two bands in the visible area of light in their absorption
spectra. Each band can selectively be shifted using
substituents in the corresponding chromophore, thus changing
the colour in the solid state from bright yellow to orange and
even dark red. Likewise, the colour of their emission light can
be systematically tuned from blue to yellow and to red light.
Acknowledgements
This counts in particular for the substituents in the 6-aryl-ring, We thank the Freie Universität Berlin and the Deutsche
while substitution in the 4-aryl-ring mainly effects the emission Forschungsgemeinschaft (DFG) for financial support. E. A.
intensity.
Pidko thanks the Ministry of Education and Science of Russian
DFT calculations on each phosphinine- and bipyridine- Federation (GosZadanie no. 11.1706.2017PP).
derivative showed, that introducing F- and CF3-substituents
leads to a stabilisation of the frontier orbitals resulting in
Notes and references
molecules with better π-acceptor and weaker σ-donor
properties. Stronger π-donating substituents, such as OCH3-
and SCH3-groups, lead to significant donor abilities through the
energetically high-lying, π-shaped HOMO and HOMO-1.
Substituents in the 6-aryl-ring have a more pronounced effect
on the corresponding σ-donor, π-donor and π-acceptor
properties than the substituents in the 4-aryl-ring. We assume
that conjugative interactions especially by the OCH3- and -
SCH3-groups are less disturbed by the weaker non-bonding
steric repulsions of the three ortho-hydrogen atoms and the
lone-pair (6-aryl-ring) compared to the situation in the other
compounds (4-aryl-ring).
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