Sterically crowded triarylphosphines bearing cyano groups

Article abstract of DOI:10.1016/j.tetlet.2018.04.065

Sterically crowded triarylphosphines bearing one, two, and three cyano groups at 4-positions of aromatic substituents, namely, (4-cyano-2,6-diisopropylphenyl)_n(2,4,6-triisopropylphenyl)_3–nP (n = 1, 2, 3), were synthesized. Influence

Full text of DOI:10.1016/j.tetlet.2018.04.065

Tetrahedron Letters xxx (2018) xxx–xxx
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Tetrahedron Letters
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Sterically crowded triarylphosphines bearing cyano groups
Shigeru Sasaki
Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Sterically crowded triarylphosphines bearing one, two, and three cyano groups at 4-positions of aromatic
substituents, namely, (4-cyano-2,6-diisopropylphenyl)n(2,4,6-triisopropylphenyl)3–nP (n = 1, 2, 3), were
synthesized. Influence of the introduction of cyano groups is clearly reflected to the oxidation potentials
and the fluorescence wavelengths. As the number of cyano groups increases, the oxidation potentials are
raised and the fluorescence wavelengths are blue-shifted. Synthetic utility of the (cyanoaryl)phosphine is
also demonstrated.
Received 27 February 2018
Revised 20 April 2018
Accepted 24 April 2018
Available online xxxx
InChIKeys:
Ó 2018 Elsevier Ltd. All rights reserved.
LMIVDGLRYYXTLO-UHFFFAOYSA-N
VKTNOELLZIRSFI-UHFFFAOYSA-N
BTLUGJZUEKEUFY-UHFFFAOYSA-N
AQSLMGLXUWSEFZ-UHFFFAOYSA-N
A cyano group is a strong electron withdrawing substituent
and is employed as a key substituent of typical electron acceptors
such as DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone),
TCNQ (7,7,8,8-tetracyanoquinodimethane), and TCNE (tetracya-
noethylene). In addition, a cyano group can coordinate to transition
metals, and nitriles can serve as key synthetic intermediates to
high p-character and the steric protection. Triarylphosphines simi-
lar to 1 have been employed as key functional sites in various
electron systems and demonstrated to work as reversible redox
sites.9 Among various
-electronic systems studied, sterically
p-
p
crowded triarylphosphines connected to acceptors such as carbonyl
(2)9g and quinone (3)9e,9h moieties have been found to show typical
donor-acceptor interaction such as intramolecular charge transfer.
Therefore, the sterically crowded triarylphosphines bearing cyano
groups are also expected to show unique properties taking
characteristics of the triarylphosphines and the similarity to
triarylamines into consideration. In this letter, synthesis,
structure, and properties of (cyanoaryl)phosphines 4a, 4b, and 4c
are described. Influence of the introduction of cyano groups was
evaluated spectroscopically and electrochemically. Synthetic
utility of the (cyanoaryl)phosphine is also presented.
We have synthesized various sterically crowded triarylphosphi-
nes bearing functional sites by using the (bromoaryl)phosphines as
key intermediates.9c,9e,9g,9h Triarylphosphines 4a, 4b, and 4c bearing
cyano groups were synthesized by the reaction of (bromoaryl)
phosphines 5a,9c 5b, and 5c9e with copper(I) cyanide in refluxing
N,N-dimethylformamide, respectively (Scheme 1). Phosphines 4a,
4b, and 4c were purified by column chromatography on aluminum
and recrystallization, and obtained as yellow air-stable solids.
1H (500 MHz, CDCl3, 300 K) and 13C (126 MHz, CDCl3, 300 K)
NMR spectra of 4a, 4b, and 4c exhibit inversion averaged signal
patterns typical of the sterically crowded triarylphosphines such
as 1.8a,8c,8d For example, symmetrical 4c gives two methyl, one
methyne, and one aromatic 1H signals. The introduction of cyano
groups leads to downfield shifts of 31P NMR resonances (dP –50.1
(4a), –47.7 (4b), –45.6 (4c), –51.9 (1)), upfield shifts of ipso-
construct larger
p
-conjugated systems as shown in conversion to
ketones,1 cyclic
p
-conjugated systems,2 and phthalocyanines.3 Aro-
matic amines bearing cyano groups have been reported to exhibit
unique photophysical properties (Chart 1). 4-Dimethylaminoben-
zonitrile is a fluorescent molecule and has been studied as a typical
TICT molecule,4 and can coordinate to transition metals.5 Tris(4-
cyanophenyl)amine emits fluorescence in solution and solid with
high quantum yield,6a also works as a ligand for MOFs,6b and has
been studied as a candidate for SHG materials.6c,6d On the other
hand, studies on 4-cyanophenylphosphines have been limited to
the synthesis. Cyanophenylphosphines have been synthesized by
Vilsmeier-Haack reaction of the corresponding amides,7a,7b
reaction of cyanophenylzinc reagents with chlorophosphines,7c,7f
nucleophilic substitution of phosphide anions with cyanophenyl
fluorides,7d palladium catalyzed phosphination of cyanophenyl
bromides and triflates.7e Sterically crowded triarylphosphines
such as tris(2,4,6-triisopropylphenyl)phosphine (1)8 have pla-
narized phosphorus atoms substituted by bulky substituents and
can be reversibly oxidized to the corresponding radical cations at
low potentials, because of the synergic effect of the high HOMO
resulting from the phosphorus lone pair residing in the orbital of
E-mail address: ssasaki@m.tohoku.ac.jp
https://doi.org/10.1016/j.tetlet.2018.04.065
0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sasaki S. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.04.065

2
S. Sasaki / Tetrahedron Letters xxx (2018) xxx–xxx
Chart 1. Aromatic amines and phosphines bearing cyano groups and sterically crowded triarylphosphines.
118.8 (4c), 118.82 (benzonitrile);10
m
(CN)/cm–1 2226 (4a), 2225
(4b), 2228 (4c), 2230 (benzonitrile)10). The structure of 4c was
investigated by X-ray crystallography (Fig. 1). The bond angles
and lengths around the phosphorus atom (average 111.7°, 1.847
Å) are similar to those of 1 (111.5°, 1.845 Å).8a The bond lengths
around the cyano groups (CN 1.146(3), 1.144(3) 1.142(3); C–CN
1.442(3), 1.439(3), 1.449(3) Å) are within the typical values of
aromatic nitriles (CN 1.138(1); C–CN 1.442(1))11 such as benzoni-
trile (CN 1.137 (14); C–CN 1.401(14))12 and tris(4-cyanophenyl)
amine (CN 1.137(2), 1.137(2), 1.129(3); C–CN 1.429(2), 1.429(2),
1.434(3)).6c The nitrogen atoms of 4c in the crystal have contacts
less than sum of the van der Waals radii with the aromatic
(2.642, 2.658 Å) and methyl protons (2.679, 2.697 Å) to
contribute tight packing (Fig. S27). Although the phosphorus
atom still takes pyramidalized geometry, the whole shape of 4c
looks rather flat because the bent angles between the P–C bond
and the aromatic ring calculated from the angle of the P–C bond
and the C(ipso)–C(para) are deviated from 180° (P1–C1–C4
162.85°, P1–C14–C17 168.83°, P1–C27–C30 158.50°). Thus, 4c is
Scheme 1. Synthesis of (cyanoaryl)phosphines.
carbons of 4-cyanoaryl groups with smaller 1JPC (dC 143.5 (32.8 Hz)
(4a), 141.2 (30.5 Hz) (4b), 139.1 (28.4 Hz) (4c)), and downfield
shifts of p-carbons of cyanoaryl groups (dC 111.8 (4a), 113.0 (4b),
114.0 (4c), 112.41 (benzonitrile)10). Similar trend was observed
in naphthoquinonyl substituted triarylphosphines 3,9e but the
effect is larger for cyano groups. On the other hand, influences on
the 13C NMR chemical shifts and infrared absorptions (
the cyano groups are very limited (dC 119.8 (4a), 119.3 (4b),
m(CN)) of
Figure 1. An ORTEP drawing of 4c (top view) obtained by X-ray crystallography with 50% thermal ellipsoids and a space filling model (side view). Selected bond angles (°) and
lengths (Å): C1–P1–C14 114.57(9), C1–P1–C27 113.37(9), C14–P1–C27 107.03(9), P1–C1 1.846(2), P1–C14 1.844(2), P1–C27 1.851(2).
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S. Sasaki / Tetrahedron Letters xxx (2018) xxx–xxx
3
expected to work as a ligand for two-dimensional coordination
polymers.
Redox properties of 4a, 4b, and 4c were studied by cyclic
voltammetry (Fig. 2, Table 1). The introduction of cyano groups
raises the first oxidation potentials in an additive manner and
destabilizes the redox systems (oxE1/2 = 0.57 (4a), 0.76 (4b), 1.00
(4c), 0.38 (1)9g V vs Ag/Ag+). The cyanoaryl group effectively
lowers HOMO level of the sterically crowded triarylphosphines.
The redox waves corresponding to the reduction of the cyano
group were not observed up to –2.7 V.
Triarylphosphines 4a, 4b, and 4c exhibit a lemon-yellow color
and the corresponding absorption and emission were studied by
UV-Vis and fluorescence spectroscopy in n-hexane and dichloro-
methane (Fig. 3, Fig. S25, Table 1). The UV-Vis spectrum of 4a in
dichloromethane shows two peaks in 300–450 nm region.
Whereas that of 4b shows a single broad peak and that of 4c shows
a single narrow peak. Replacement of an isopropyl group of 1 by a
cyano group apparently leads to red shift of the absorption, but the
effect of the second and the third cyano groups looks limited. On
the other hand, the fluorescence wavelengths show a blue shift
as the number of cyano groups increases. The measurements in
Figure 3. UV-Vis (solid lines) and fluorescence (dotted lines) spectra of 4a, 4b, and
4c in dichloromethane at 293 K. Excitation wavelengths: 375 nm for 4a, 420 nm for
4b and 4c.
n-hexane show a similar trend, and the absorptions and emissions
are blue-shifted relative to those in dichloromethane (Fig. S25,
Table 1). Stokes shifts of 4a, 4b, 4c, and 1 calculated from the
difference between abskmax and emkmax are similar and suggest sim-
ilar photochemical processes. The fluorescence intensities of the
cyano substituted triarylphosphines are weak, and the phosphines
suffered from degradation probably due to P–C bond cleavage typ-
ical of triarylphosphines13 during the repeated fluorescence mea-
surements with excitation by visible light. The TD-DFT
calculation (RB3LYP TD-FC 6–311+G(2d,p)) of 4a, 4b, and 4c qual-
itatively agrees with the experimental results (for details, see Sup-
plementary Data). The calculation predicts two transitions, HOMO
to LUMO and HOMO to LUMO+1, for the absorption in this region.
The HOMOs of 4a, 4b, and 4c mainly reside on the phosphorus lone
pair, the LUMOs are
LUMO+1s are
p
⁄-orbitals on the 4-cyanoaryl group, and the
p
⁄-orbitals on the 2,4,6-triisopropylphenyl (4a) or
the 4-cyanoaryl (4b, 4c) groups (Fig. S28). The calculated
wavelengths, oscillator strengths, and spectra (4a: k= 393.65 nm,
f = 0.1466, and k= 331.66 nm, f = 0.1360, 4b: k= 398.00 nm, f =
0.1597, and k= 361.60 nm, f = 0.1244, 4c: k= 384.14 nm, f =
0.1585, and k= 384.06 nm, f = 0.1591, Fig. S29) qualitatively agree
with the trend observed in the experiment, although the absorp-
Figure 2. Cyclic voltammograms of 4a, 4b, and 4c. Conditions: in tetrahydrofuran
with 0.1 mol L–1 tetra-n-butylammonium perchlorate, scan rate 30 mV sec–1
E1/2(ferrocene/ferrocenium) = 0.20 V, 293 K.
,
Table 1
Oxidation potentials, UV-Vis absorption and fluorescence wavelengths of sterically crowded triarylphosphines bearing cyano groups and the related compounds.
4a
4b
4c
1a
2ab
/Vc
0.57
0.76
1.00
0.38
0.52
ox
E
1/2
in hexanes
abs
k
k
(
e
)/nm
361 (6800), 325 (7100)
598
10,978
352 (16700)
584
11,286
357 (16700), 248 (54700)
562
10,218
326 (14300)
502
10,755
388 (7900)
635
10,025
max
em
/nmd
max
Stokes Shift/cm–1e
in dichloromethane
abs
k
k
(
e
)/nm
375 (5550), 322 (5900)
646
11,187
360 (18000)
624f
11,752
363 (17100)
599f
10,854
327 (13500)
516
11,201
407 (7800)
712
10,525
max
em
/nmd
max
Stokes Shift/cm–1e
a
b
c
d
e
f
Refs. 8a and 8c.
Ref. 9g.
Conditions: see Fig. 2. E1/2(ferrocene/ferrocenium) = 0.20 V.
abs
Excited at
k
unless otherwise stated.
max
abs
em
k
–
k
/cm–1
.
max
max
Excited at 420 nm.
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4
S. Sasaki / Tetrahedron Letters xxx (2018) xxx–xxx
tion spectrum of 4b is an unresolved overlap of the two transitions.
The calculation shows that the introduction of cyano groups lowers
the LUMO levels as well as the HOMO levels, which causes the
trend observed in the UV-Vis spectra. The properties of sterically
crowded triarylphosphines bearing cyano groups revealed in this
study allow comparison with the related compounds. The sub-
stituent and the solvent effects on the electronic spectra of nitrile
4a are smaller than those of aldehyde 2a,9g which has slightly
higher HOMO and considerably lower LUMO judging from the
oxidation and reduction potentials. Comparison of tris(4-
The structures of 6 and 7 were determined by NMR and mass
spectroscopies. The properties of 7 clarify the difference of behav-
iors of the phosphorus and the nitrogen donor moieties connected
by a carbonyl group and are worth mentioning. The crowded tri-
arylphosphine and carbonyl moieties of 7 give characteristic 31P
(dP –51.0) and 13C NMR (dC 195.5) chemical shifts, respectively.
The phosphorus chemical shift lies in a range typical of sterically
crowded triarylphosphines such as parent 1 (dP –51.9). The similar
chemical shifts of carbonyl carbons of 7 and benzophenone (dC
196.50)10 as well as 2b (dC 197.04)9 g suggest limited influence of
cyanoaryl)phosphine 4c and tris(4-cyanophenyl)amine (abskmax
the phosphine moiety. The cyclic voltammogram of 7 in
em
338,
k
(/) 397 (0.26) nm in acetonitrile)6a shows that the
tetrahydrofuran shows two-step reversible oxidation (E1/2 = 0.47,
0.83 V vs Ag/Ag+) and the irreversible reduction of the
benzophenone moiety (Ep = –2.33 V) (Fig. 4). The first and the
second oxidations are assignable to those of the triarylphosphine
and triarylamine moieties from the comparison with the
previously reported 2b (E1/2 = 0.52 V). The UV-Vis spectrum of an
max
phosphine exhibits an absorption in longer wavelength than the
amine reflecting the higher HOMO characteristic of the sterically
crowded triarylphosphines. The fluorescence of the phosphines
exhibits larger Stokes shifts and lower quantum yields compared
to the amines reflecting the fluorescent properties generally
observed in conventional triarylphosphines.
From synthetic viewpoint, nitriles can be transformed to vari-
ous functional groups and are generally accepted as useful syn-
thetic intermediates. The hydrolysis of 4a under basic condition
gave the corresponding amide 6 (Scheme 2). The addition of the
aryllithium derived from (4-bromophenyl)bis(4-methylphenyl)
amine to the cyano group of 4a followed by hydrolysis afforded
benzophenone derivative 7. Nitrile 4a proved to be a key synthetic
intermediate for the introduction of the sterically crowded tri-
arylphosphine moieties.
orange solution of
7 in dichloromethane exhibits a peak
maximum in visible region (abskmax
(e) 380 (28900) nm). The
wavelength and intensity, which are similar to those of 4,40-bis
abs
(diphenylamino)benzophenone
(
k
(e
)
380 (33100), 288
) 401 (7200), 318 (8700)
nm),9 g suggest dominant contribution of the (4-carbonylaryl)
amine moiety. Triarylphosphine emits weak fluorescence
535 nm) in wavelength similar to that of 4-bis(4-
max
(25700) nm)14 rather than 2b (abskmax
(e
7
em
(
k
max
methylphenyl)aminobenzaldehyde (abskmax 357,
k
500 nm
em
max
in tetrahydrofuran)15 rather than 2b (emkmax 710 nm in dichloro-
methane), suggesting emission from the (4-carbonylaryl)amine
moiety. Interestingly, the fluorescence intensity increased upon
repeated fluorescence measurements of triarylphosphine
7
(Fig. S26). The spectral change can be explained by the P–C bond
cleavage,13 which leads to formation of the aminobenzophenone
derivative of higher fluorescence quantum yield (Scheme S1).
In conclusion, a series of sterically crowded triarylphosphines
bearing one, two, and three cyano groups have been synthesized
and systematically characterized. The cyano groups lower the
HOMO and LUMO levels of the phosphines, and the influence is
reflected to the oxidation potentials and the electronic spectra.
The (cyanoaryl)phosphines presented in this work should allow
access to coordination polymers as well as extended
p-electron
systems bearing the sterically crowded triarylphosphine moieties
as functional sites. This work also shows that the sterically
crowded triarylphosphines connected to
p-electron systems
behave in a similar manner to triarylamines as donor sites, but
they still take over properties typical of triarylphosphines such as
a photolytic cleavage of a phosphorus–carbon bond.
Scheme 2. Reactions of (cyanoaryl)phosphine 4a.
Figure 4. (a) Cyclic voltammograms of 7 in tetrahydrofuran with 0.1 mol L–1 tetra-n-butylammonium perchlorate as a supporting electrolyte. Working electrode; grassy
carbon, counter electrode; Pt wire, reference electrode; Ag/AgNO3 in acetonitrile with 0.1 mol L–1 tetra-n-butylammonium perchlorate. E1/2 (ferrocene/ferrocenium = 0.20 V.
Temperature; 293 K, scan rate 30 mV sec–1. (b) UV-Vis (solid lines) and fluorescence (dotted lines) spectra of 7 in dichloromethane at 293 K. Excitation wavelength: 380 nm.
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S. Sasaki / Tetrahedron Letters xxx (2018) xxx–xxx
5
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Acknowledgements
This work was supported by JSPS KAKENHI Grant numbers
22605001, 25600020. We thank Research and Analytical Center
for Giant Molecules, Graduate School of Science, Tohoku Univer-
sity, for taking mass spectra and elemental analysis.
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These data include MOL files and InChiKeys of the most important
compounds described in this article.
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