Isolation and X-ray crystal structures of triarylphosphine radical cations

Article abstract of DOI:10.1021/ja4012113

Salts containing triarylphosphine radical cations 1^?+ and 2^?+ have been isolated and characterized by electron paramagnetic resonance (EPR) and UV-vis absorption spectroscopy as well as single-crystal X-ray diffraction. Radical 1^?+ exhibits a relaxed pyramidal geometry, while radical 2^?+ becomes fully planar. EPR studies and theoretical calculations showed that the introduction of bulky aryl groups leads to enhanced p character of the singly occupied molecular orbital, and the radicals become less pyramidalized or fully flattened.

Full text of DOI:10.1021/ja4012113

Communication
pubs.acs.org/JACS
Isolation and X‑ray Crystal Structures of Triarylphosphine Radical
Cations
Xiaobo Pan, Xiaoyu Chen, Tao Li, Yizhi Li, and Xinping Wang*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, China
S
* Supporting Information
the planar geometry for the nitrogen analogues (Ar3N•+).9
ABSTRACT: Salts containing triarylphosphine radical
cations 1•+ and 2•+ have been isolated and characterized
by electron paramagnetic resonance (EPR) and UV−vis
absorption spectroscopy as well as single-crystal X-ray
diffraction. Radical 1•+ exhibits a relaxed pyramidal
geometry, while radical 2•+ becomes fully planar. EPR
studies and theoretical calculations showed that the
introduction of bulky aryl groups leads to enhanced p
character of the singly occupied molecular orbital, and the
radicals become less pyramidalized or fully flattened.
Although extremely crowded aryl ligands cause significant
flattening,8 whether Ar3P•+ radicals have a planar geometry
remains ambiguous. Conspicuously lacking is an X-ray
diffraction structure.
By using weakly coordinating anions,10 we recently
succeeded in stabilizing aniline, benzidine, and anthracene
radical cations.11 In this paper, we report the isolation,
characterization, and structures of two triarylphosphine radical
cations, one of which displays a perfectly planar geometry.
Phosphine 1 was synthesized by the reaction of MesCu(I)
(Mes = 2,4,6-trimethylphenyl) with Trip2PCl (Trip = 2,4,6-
triisopropylphenyl) (Scheme 1). Previously reported phosphine
he isolation and characterization of stable radicals of
T
heavier main-group elements is an area of high current
interest.1 A number of phosphorus radicals in solution have
been observed by electron paramagnetic resonance (EPR)
spectroscopy,2 but only a few species have been structurally
characterized in the gas phase (Figure 1A)3 or the solid state
Scheme 1
2 was synthesized by the reaction of TripCu(I) with
phosphorus trichloride.8d Upon one-electron oxidation with
AgX (X = SbF6; [Al(ORF)4], ORF = OC(CF3)3; [Al(ORMe)4],
ORMe = OC(CF3)2Me) in CH2Cl2,10,12 1 and 2 were converted
to deep-red radical cations 1•+ and 2•+, respectively, in high
yields.13 Their absorption spectra (Figure 2 ; for those of 2•+
with other anions, see Figure S2 in the Supporting
Information)13 are similar to those of previously reported
crowded triarylphosphine radical cations in solution.8 The
Figure 1. Structurally characterized phosphorus radicals.
(Figure 1B−K).4 On the other hand, triarylphosphines (Ar3P)
not only have a wide variety of catalytic applications5,6 but are
potential candidates for redox centers in functional materials.7
Although the structures of hundreds of Ar3P species are known,
no X-ray structural data have appeared on the corresponding
oxidized radical species Ar3P•+. In fact, triarylphosphine radical
cations have been the subject of numerous solution EPR studies
and were shown to adopt a pyramidal geometry,8 in contrast to
solution EPR spectra of 1•+[Al(ORF)4]− and 2•+SbF6 (Figure
−
3)14 at 273 and 77 K show typical signals of triarylphosphine
radical cations.8 The EPR signals of 2•+[Al(ORF)4]− and
2•+[Al(ORMe)4]− show high-resolution anisotropic hyper-
Received: February 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4012113 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 4. Thermal ellipsoid (50%) drawings of (a) 1•+ (in
−
1•+[Al(ORF)4]−) and (b) 2•+ (in 2•+SbF6 ). H atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg) in 1•+:
C31−P1, 1.798(3); P1−C16, 1.820(3); P1−C1, 1.820(3); C1−P1−
C16, 111.76(3); C16−P1−C31, 119.16(4); C1−P1−C31, 118.23(4).
In 2•+: C1−P1, 1.817(4); C1−P1−C1′, 119.997(4); C1−P1−C1″,
119.995(4); C1′−P1− C1″, 120.001(4).
Figure 2. Absorption spectra of 1 × 10−4 M 1, 2, 1•+[Al(ORF)4]−, and
−
2•+SbF6 in dichloromethane at 25 °C.
is fully flattened with a plane through the central triangle of the
three ipso carbon atoms of the Trip rings.
The above data show that the removal of one electron from
triarylphosphines have considerable effects on their ground-
state structures. To rationalize the experimental results, we
carried out some calculations for the model radical species
Ph3P•+ (3•+), Dmp3P•+ (4•+; Dmp = 2,6-dimethylphenyl), and
Dipp3P•+ (5•+; Dipp =2,6-diisopropylphenyl). Full geometry
optimizations were performed at the UB3LYP/6-31G(d) level,
and the obtained stationary points were characterized by
frequency calculations.17 As shown in Table 2, consistent with
the experimental data, the addition of bulky substituents causes
significant flattening of triarylphosphine radical cations that is
induced by the steric crowding and electron donation of the
alkyl groups.8f The flattening is accompanied by an increase in
the Mulliken spin density on the phosphorus atom. The spin
density of 0.89 on the phosphorus atom of 5•+ calculated at the
UM05-2X/def2-SVP level is consistent with the considerably
high spin localization on the phosphorus atom observed by
EPR spectroscopy.18 This value is close to that of
dialkylphosphinyl radical K (0.90)4i and significantly higher
than that of carbene-stabilized phospheniumyl radical H
(0.67).4g
Figure 3. EPR spectra of 1 × 10−3 M solutions of (a, b)
1•+[Al(ORF)4]− in CH2Cl2 at (a) 273 and (b) 77 K and (c, d)
14
−
2•+SbF6 in (c) CH2Cl2 at 273 K and (d) frozen CH3CN at 77 K.
couplings in the frozen solutions at 160 K but become broad at
77 K (Figures S4 and S5). The ratios of isotropic and
anisotropic hyperfine constants between the radicals and
phosphorus atoms suggest that ∼64 and ∼5% of the unpaired
electron are localized on the P 3p and P 3s orbitals,
respectively, in 1•+[Al(ORF)4]− (∼77 and ∼4%, respectively,
The singly occupied molecular orbital (SOMO) of 5•+
calculated at the UB3LYP/6-31G(d) level has almost pure P
3p character with small contributions from carbon atoms
(Figure 5). In contrast, a mixing of P 3s and P 3p orbitals for
the SOMO of 3•+ is observed, and that of 4•+ is in the middle.
In the hybridization term, the mixing of P 3s and P 3p character
affords pyramidalization at phosphorus (Figure S6).8f The
introduction of bulky aryl group leads to enhanced P 3p
character of the SOMO, and the radical becomes less
pyramidalized.8f In 5•+ (and hence experimental 2•+), the
repulsive interactions between the Dipp/Trip groups are so
strong that the mixing of the P 3s and P 3p orbitals is effectively
resisted, and the radicals become fully planar, leading to an
ideal trigonal planar sp2 hybridization with one P 3p orbital
(the SOMO) perpendicular to the plane.
We herein have described the stabilization and structural
characterization of the triarylphosphine radical cations 1•+ and
2•+. The former shows a relaxed pyramidal geometry while the
latter has a perfectly planar structure. These radical species are
thermally stable under anaerobic conditions at room temper-
ature. Preliminary investigations of the reactivities of 1•+ and
2•+ showed that both species readily react with O2. With excess
nBu3SnH in CH2Cl2, the 1•+[Al(ORF)4]− solution rapidly turns
pale yellow, but the deep-red solutions of 2•+X− (X = SbF6,
in 2•+SbF6 ).8,15 The sums of C−P−C angles in solution (349°
−
in 1•+[Al(ORF)4]− and 352° in 2•+SbF6 ) were consequently
−
estimated from Coulson’s equation.15
Crystals suitable for X-ray crystallographic studies were
obtained by cooling solutions of 1•+[Al(ORF)4]− and 2•+X− (X
= SbF6, [Al(ORF)4], [Al(ORMe)4]) in various solvents.13,16 The
structures of 1•+ (in 1•+[Al(ORF)4]−) and 2•+ (in 2•+SbF6 )
−
are shown in Figure 4, and their important structural
parameters, as well as those of the parent molecules 1 and 2,
are given in Table 1. The structural parameters of radical cation
2•+ with the three different anions are similar, except that the
C−P bonds in 2•+[Al(ORF)4]− and 2•+[Al(ORMe)4]− are
−
slightly shorter than those in 2•+SbF6 , which is probably
associated with the weaker coordinating property of larger
anions. In the structures of 1•+ and 2•+, the Mes and Trip
ligands are aligned in a propeller shape. The C−P bond lengths
are shorter and the C−P−C angles are larger than those in the
corresponding neutral phosphines 1 and 2. It is worth noting
that radical 1•+ becomes less pyramidalized and that radical 2•+
B
dx.doi.org/10.1021/ja4012113 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. X-Ray Structural Parameters for 1, 1•+, 2, and 2•+
a
1•+[Al(ORF)4]−
2
2•+SbF6
2•+[Al(ORF)4]−
2•+[Al(ORMe)4]−
−
1
C−P (Å)
1.835(2)
1.841(2)
1.854(2)
111.35(10)
110.28(10)
110.78(10)
332.41
1.798(3)
1.820(3)
1.820(3)
118.23(4)
119.16(4)
111.76(3)
349.15
1.843(2)
1.839(2)
1.851(2)
111.61(3)
110.88(1)
111.93(3)
334.42
1.817(4)
1.817(4)
1.817(4)
1.783(4)
1.806(5)
1.772(4)
119.5(2)
119.0(2)
121.3(2)
359.8
1.789(3)
1.786(3)
1.783(3)
118.82(4)
119.82(5)
121.00(4)
359.64
∠C−P−C (deg)
119.997(4)
120.001(4)
119.995(4)
359.993
∑(∠C−P−C) (deg)
a
Data taken from ref 8d.
Table 2. Calculated Bond Lengths (Å), Bond Angles (deg),
and Spin Densities on P in 3•+, 4•+, and 5•+
Foundation of Jiangsu Province (Grant BK2011549) for
financial support.
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[Al(ORF)4], [Al(ORMe)4]) remain unchanged. The higher
stability of 2•+ radicals is very possibly due to their higher
degree of steric crowding. Further studies of oxidized
triarylphosphines with various ligands and their chemical
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section, crystallographic data (CIF), EPR and
UV−vis spectra, results of DFT calculations, and complete ref
17a. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
xpwang@nju.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(Grants 91122019 and 21171087) and the Natural Science
■
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(13) 1•+SbF6− is unstable in solution. 1•+[Al(ORMe)4]− did not afford
crystals suitable for X-ray crystallography, and no detailed structure
was obtained.
−
(14) The EPR spectrum of 2•+SbF6 has a sharp peak in the frozen
CH2Cl2 solution at 77 K, and no hyperfine coupling was observed
(Figure S3).
(15) (a) Coulson, C. A. In Contributions a
Moleculaire: Volume Commemoratif Victor Henri; Maison Desoer:
Liege, Belgium, 1948; p 15. (b) Coulson’s equation is rough
́
l’Etude de la Structure
́
́
̀
approximation and not sensitive to subtle differences in the structure
close to ∠C−P−C = 120°.8b Whether the radical cations have a planar
geometry in solution remains elusive.
(16) X-ray data for 1•+[Al(ORF)4]−, 2•+X−, and neutral 1 are listed in
Table S1 in the SI. CCDC 921459−921463 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(17) (a) All of the calculations were performed using the Gaussian 09
program suite: Frisch, M. J.; et al. Gaussian 09, revision B.01; Gaussian,
Inc.: Wallingford, CT, 2010. See the SI for geometries and coordinates.
(b) A highly relaxed pyramidal geometry for 5•+ [∠C−P−C = 118.9°,
∑(∠C−P−C) = 356.7°] was obtained upon optimization at the
UHF/6-31G(d) level by Boere
́
and co-workers.8a
(18) Calculated spin densities are usually overestimated in
comparison to those generated from experimental EPR spectra.4f−i
D
dx.doi.org/10.1021/ja4012113 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX