Study of the planarization of the tricordinate phosphorus in phospholes; Photoelectron spectra and structure of partially planarized phospholes

Article abstract of DOI:10.1016/S0022-328X(98)00685-8

The gradual flattening of the tricoordinate phosphorus in phosphole by the increasing steric bulk of the substituent group is shown on the HF/6-31G* optimized geometries of alkylarylphospholes. By the decreasing pyramidality, aromaticity indices show incr

Full text of DOI:10.1016/S0022-328X(98)00685-8

Journal of Organometallic Chemistry 566 (1998) 29–35
Study of the planarization of the tricordinate phosphorus in
phospholes; photoelectron spectra and structure of partially planarized
phospholes
La´szlo´ Nyula´szi a,*, La´szlo´ Soo´s a, Gyo¨rgy Keglevich b
a Department of Inorganic Chemistry, Technical Uni6ersity of Budapest, H-1521 Budapest Gelle´rt te´r 4, Hungary
b Department of Organic Chemical Technology, Technical Uni6ersity of Budapest, H-1521 Budapest Mu¨egyetem rakpart 3, Hungary
Received 27 November 1997
Abstract
The gradual flattening of the tricoordinate phosphorus in phosphole by the increasing steric bulk of the substituent group is
shown on the HF/6-31G* optimized geometries of alkylarylphospholes. By the decreasing pyramidality, aromaticity indices show
increase, as a result of the increased conjugation. The aromaticity of 1-(2,4,6-tri-tertiarybutyl)-phosphole is similar to that of furan
according to the geometrical indices. In the photoelectron spectra of the alkylaryl substituted phospholes the phosphorus lone pair
ionization energy also decreases along the decreasing pyramidality. 1-(2,4,6-tri-Tertiarybutyl-phenyl)-3-methylphosphole has the
lowest ionization energy value ever reported for a phosphole. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Phospholes; Aromaticity; Photoelectron spectra; Ab initio calculations
highly aromatic according to geometric and magnetic or
energetic measures of aromaticity [6–11].
1. Introduction
Planarization of phosphole itself, however, is only
partially successful up to now. To achieve some flattening
Phosphole (1) belongs to the simplest heterocyclic
species. It is not aromatic in contrast to its congeners
of the phosphorus pyramid Quin and Keglevich used
pyrrole and thiophene [1]. As it has been suggested by
bulky substituents on phosphorus [12–15]. The flatten-
Mislow [2], the lone pair of the inherently nonplanar
ing—as measured conveniently by the bond angle sum
tricoordinate phosphorus [3] is unable to take part in the
about the tricoordinate phosphorus—in 1-(2,4,6-tri-ter-
conjugative interaction with the y-system. Planar phos-
phole, however, which is not a minimum but a first order
saddle point on the potential energy hypersurface, would
tiarybutyl)-3-methyl-phosphole (2) is 331.7° [12]. This
value is considerably larger than in 1-benzyl-phosphole
[16] (302.7°). According to Schmidpeter’s classification
be among the most aromatic five-membered heterocycles
[17], with a bond angle sum of 330–340° a significant
according to computational investigations [4–6]. Al-
planarization is achieved and the tricoordinate phospho-
though not yet synthesized, pentaphosphole [6–8], and
rus can be considered as an intermediate state between
1-(2,4,6-tri-tertiarybutyl)-1,2,4-triphosphole [9] are pla-
phosphane and bis-methylene-phosphorane [18].
nar according to ab initio calculations. 1-(bis-trimethylsi-
Schmidpeter achieved flattening of the phosphorus pyra-
mid by using y-electron acceptor groups (PPh3+). Quan-
tum chemical calculations also indicate some flattening
of the tricoordinate phosphorus, when the phosphole
ring is substituted by y-electron acceptor groups (e.g.
BH2) [5].
lyl-methyl)-3,5-bis-trimethylsilyl-1,2,4-triphosphole has
recently been synthesized and characterized structurally
as a planar compound [10]. All these planar systems are
* Corresponding author.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00685-8

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L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
Scheme 1.
Recent theoretical studies [6] have demonstrated that
lated the MNDO and ab initio values to test the
reliability of the method of our choice. For the inver-
sion barrier about the tricoordinate phosphorus atom
in phosphole 20.3 kcal mol−1 was calculated by using
the MNDO method. At the HF/6-31G*, MP2/6-31G*
and B3LYP/6-311+G** levels, 26.4 [5], 17.19 [5] and
18.0 [6] kcal mol−1 were obtained, respectively. The
inversion barrier of a substituted (1-isopropyl-2-methyl-
5-phenyl)phosphole was 17 kcal mol−1 as determined
by NMR spectroscopy [2]. The comparisons indicate
that preliminary MNDO calculations will provide a
reasonable estimate for the pyramidality problem of the
tricoordinate phosphorus in phospholes. Results of fur-
ther calculations on the out of plane (OOP) angle about
phosphorus (see Fig. 1) with bulky P-substituents are
summarized in Table 1.
The MNDO and the HF/6-31G* OOP angles are in
reasonable agreement with each other, however,
MNDO structures are somewhat more flattened
(smaller OOPs) than those obtained by the ab initio
calculation. The methyl substituent in the i position of
the phosphole ring has only a small effect. Similarly,
the para substituent on the phenyl ring also has a
negligible influence on the OOP, in accordance with the
expectations. Therefore—to save computer time—ab
initio geometry optimizations of 2, 3, 4 and 5 were
carried out only for model compounds (2%, 3%, 4% and 5%,
respectively), lacking the para substituent in the ben-
zene ring, and the 3-methyl group on the phosphole
ring. The OOP angles obtained are also in reasonably
good agreement with the available X-ray structural
data (see below).
the reduction of the pyramidality of the tricoordinate
phosphorus in phosphole is correlated with the increase
of the aromatic character. The use of bulky substituents
on phosphorus (as in 2–4), results indeed in an en-
hancement of the aromatic character. 1-(2,4,6-tri-Ter-
tiarybutyl)-3-methylphosphole (2) is the first phosphole
showing electrophilic substitution reactivity [12]. Also
the Bird aromaticity index (BI) [19] calculated from the
X-ray structural data of 2 (56.5) [12] is clearly higher
than for 1-benzyl-phosphole (35.5) [20]. Bond length
equalization [6,11,17], and complex formation charac-
teristic for aromatic compounds (the first p5-complex of
a phosphole [21]) is also observed for partially pla-
narized 1,2,4-triphosphole (Scheme 1). The decrease of
pyramidality of phosphorus, and the increasing aro-
maticity could be shown by a combined photoelectron
spectroscopic quantum chemical investigation of the
phosphorus lone pair ionization energy in case of 1-(di-
tert-butyl-tolyl)-3-methyl-phosphole (4) [22]. In the
present work our aim is to extend the previous investi-
gations to a series of phospholes substituted on the
phosphorus atom with aryl groups of different steric
bulk. Photoelectron spectroscopy is used to study the
interaction of the phosphorus lone pair with the y-sys-
tem at differently pyramidalized phosphorus.
While 2 [12], 3 [15] and 4 [13,14] have been synthe-
sized earlier, 1-mesityl-phosphole (5) has been prepared
for the photoelectron spectroscopic investigations in the
present study. Instead of the photoelectron spectrum of
3-methyl-1-phenyl phosphole (6) that of 1-phenyl-phos-
phole (6% published earlier [23]) will be discussed.
The HF/6-31G* optimized structure of 2% is shown in
Fig. 2. All the other structures (3%–5%), are similar, with
the phosphole and the phenyl rings being perpendicular
to each other. Selected structural parameters of the
2. Results and discussion
To get initial structures for the ab initio calculations
and to obtain a general overview about the effect of the
substituents, first semiempirical calculations were car-
ried out by the MNDO method. Since one important
parameter from the point of view of the present study is
the inversion barrier about phosphorus, first we calcu-
Fig. 1. Graphical definition of the OOP angle in phosphole.

L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
31
Table 1
Effects of substituents (as shown in the drawing) on the OOP angle (in degree) on phosphorus, as calculated by the MNDO and ab initio methods
R1
R2
R3
R4
MNDO
HF/6-31G*
B3LYP/6-31G*
Exp.
—
H
—
H
—
H
—
H
H
Me
H
H
Me
H
H
Me
H
1
73.6
62.0
55.5
55.5
53.7
54.0
54.1
52.6
52.2
52.7
49.1
74.3
68.3
59.5
59.4
57.6
73.6
67.1
6%
5%
5
Me
Me
t-Bu
Me
Me
i-Pr
i-Pr
i-Pr
t-Bu
Me
Me
Me
t-Bu
t-Bu
i-Pr
i-Pr
i-Pr
t-Bu
Me
Me
Me
tBu
tBu
H
i-Pr
i-Pr
H
4%a
55.7
56.4
4
3%
58.0
49.0
3
2%
58.0b
45.9c
a
With Me group instead of H at the p-position of the phenyl ring.
b X-ray structure of 3 from [15].
c X-ray structure of 2 from [12].
HF/6-31G* optimized phospholes 2%–5% are collected in
Table 2. In accordance with the expectations, the bond
angle sum at the tricoordinate phosphorus atom is
increasing with the steric bulk of the substituent, as
shown by the bond angle sum (SUMBA). Simulta-
neously, the C2–P–C5 angle in the ring is opening up.
(For a planar tricoordinate phosphorus with sp2 hybrid
120° bond angle would be expected. For the nonplanar
tricoordinate phosphorus the bonding angles are some-
what larger than 90°.) With the increasing bulk of the
substituent at phosphorus the PC bond shows shorten-
ing and the CC bond lengths approach each other in
agreement with the expected increase of the conjugation
in the ring. BI [19], and bond shortening index (BD-
SHRT) [25]—as shown in Table 2—are increasing
simultaneously with the increasing steric bulk of the
substituent. The geometrical parameters of phospholes
2% and 3% are in reasonably good agreement with the
published X-ray structures, although for 2% both the
OOP angle (Table 1) and the SUMBA (Table 2) are
somewhat underestimated at the HF/6-31G* level. The
BI calculated from the experimental data was also
somewhat larger than that obtained from the HF/6-
31G* geometry [26]. To make a fair comparison, the BI
and the BDSHRT of furan, thiophene and pyrrole, all
calculated using data at the HF/6-31G* level [26], are
also given in Table 2. According to the geometric
aromaticity indices, 2% has a similar aromaticity to
Fig. 2. HF/6-31G* optimized structure of 1-(2,6-ditertiary-butyl-
phenyl)-phosphole as visualized by the MOLDEN program [24].

32
L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
Table 2
Selected HF/6-31G* structural parameters and calculated geometrical aromaticity indices of phospholes 1-6 (in angstrom and degrees).
a
b
c
CPC
SUMBA
BI
BDSHRT
1
6%
5%
4%
3%
2%
1.821
1.817
1.807
1.802
1.803
1.790
1.362
1.725
1.362
1.332
1.333
1.337
1.339
1.340
1.342
1.361
1.345
1.358
1.471
1.473
1.466
1.463
1.464
1.459
1.431
1.437
1.427
89.7
89.6
90.3
90.6
90.5
91.3
106.6
91.3
109.5
291.3
300.1
312.3
314.9
314.3a
325.5b
—
33
34
40
42
42
46
46
58
67
49
49
50
50
50
51
49
56
55
furan
thiophene
pyrrole
—
360.0
a
From the X-ray structure of 3 [15] 314.4° was obtained as bond angle sum.
b From the X-ray structure of 2 [12] 331.7° was obtained as bond angle sum.
furan. It has been shown before [22] that the phospho-
rus lone pair has considerable participation in the y-
system at OOP angles between 40 and 50°. Since
phosphorus (due to its lower electronegativity) is a
much better electron pair donor than oxygen, it takes
part in the conjugation effectively, even if the overlap is
unfavorable as a result of the nonplanarity.
longs to the unperturbed butadienic HOMO, and the
fourth one to the phosphorus lone pair orbital. The two
rings in 2%–5% are perpendicular to each other according
to the calculations (cf. Fig. 2). The same structure was
suggested by crystallographic investigations [12,15] and
The bulky substituent on the tricoordinate phos-
phorus atom decreases the inversion barrier signifi-
cantly, in accordance with the expectations. For 2% with
the phosphorus atom under planarity constraint (but
allowing all other parameters to relax) an inversion
barrier of 10.98 kcal mol−1 was calculated at the
HF/6-31G* level, that is smaller than that of 1 (26.5
kcal mol−1 at the same level [5]). For 1-(2,6,ditertbutyl-
phenyl)phospholane the inversion barrier was 27.46
kcal mol−1, that is clearly larger than for 2%, but
smaller than that for 1-methyl-phospholane (48.0 kcal
mol−1). Thus, the steric effect of the bulkiest sub-
stituent considered here is to reduce the inversion bar-
rier by 15–18 kcal mol−1
.
The photoelectron spectra of the compounds under
investigation (2–5) can be seen in Fig. 3, while the
spectral data are collected in Table 3. The spectra are
characterized by a band at low ionization energies and
a broad bump. The distinct band has a shoulder at the
low energy side for compounds 2–5. According to the
HF/6-31G* orbital energies, four ionizations should be
expected in the region of low ionization energies (Table
3). Two of them are to be attributed to the y-ioniza-
tions of the substituting aryl ring. One ionization be-
Fig. 3. Photoelectron spectra (intensities in cps) of the alkylaryl
substituted phospholes 2–5.

L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
33
Table 3
HF/6-31G* (Koopmans) and measured ionisation energies of phospholes 2%–6%a and 2–5 and 6%, respectively, in eV
6%
meas.
6%
calc.
5
5%
4
4%
3
3%
2
2%
meas.
calc.a
meas.
calc.a
meas.
calc.a
meas.
calc.a
nP
8.45
9.04
8.50
9.14
9.38
8.1a
8.89
8.41
8.59
8.64
7.9a
8.38
8.40
8.62
8.72
7.9a
8.52
8.54
8.61
9.17
7.5a
8.09
8.47
8.70
9.03
yCCCC 8.45
yAr 9.25
8.58
8.35
8.21
8.34
8.58 (8.42)b
8.35 (8.35)b
8.21 (8.26)b
8.34 (8.21)b
a The figures in italics indicate shoulders.
b The data in brackets are the measured yAr ionization energies of the aromatic moiety RH, corresponding to the alkylaryl substituent (R) on
phosphorus. The ionization energies measured for mesitylene and 1,3,5-tri-tert-butylbenzene are in good agreement with those published in [27].
by the 13C-NMR studies [14] earlier. Thus, no interac-
tion can be expected between the phosphorus lone pair
and the y-system of the aryl ring in 2–5. Indeed, the
maximum of the bands appears at the ionization ener-
gies, close to those measured for RH, where R is the
substituting alkylaryl group on the phosphorus atom
(see data in brackets in Table 3). For 2%, 3% and 4% the
HOMO is the phosphorus lone pair orbital, while for 5
and 6 it is the butadienic y-MO. Nevertheless, from the
spectrum published [23] it was clear even for 6, that the
butadienic y and the phosphorus lone pair ionization
energies contribute to one broad band, thus their en-
ergy difference is small. The shoulder at low ionization
energies of phospholes 2–5 shows increasing separation
from the lowest ionization energy band as the steric
bulk of the substituent (and thus the OOP angle)
increases. The phosphorus lone pair orbital is getting
also destabilized—alongside with the flattening of the
phosphorus pyramid—according to the calculations.
This is in agreement with our earlier results [22], where
methylphospholes were calculated with constrained
OOP angle about the phosphorus atom. Also, between
the first ionization energies of 2–5 and the calculated
SUMBA of 2%–5%, a linear correlation (cc. 0.959) was
found. In accordance with the flattening of the phos-
phorus pyramid the lowest ionization energy (7.5 eV) is
exhibited by the supermesityl-phosphole 2. This 7.5 eV
ionization energy of 2 is the lowest value ever reported
for a phosphole. The decrease of the ionization energy
is due to the increase of the p character of the lone pair
orbital of phosphorus, which allows better overlap with
the y-system, than in the case of 6 having a rather
pyramidal phosphorus atom (see data for 6% in Table 1)
and consequently a large s character.
lowest ionization energy value ever reported for a phos-
phole. The decrease of the ionization energy is in
accordance with the increase in the p character of the
corresponding orbital, as concluded from the calculated
MOs. This allows for a better overlap with the butadi-
enic y-system, resulting in bigger aromaticity with the
increasing bulk of the substituent, as shown by the
different geometric aromaticity indices, as the BI [19],
or the BDSHRT [24]. Also the inversion barrier at the
tricoordinate phosphorus decreases with the increasing
steric bulk of the P-substituent.
Considering the twisting of the phenyl ring from the
symmetrical position in 2% [26], it seems apparent that
additional increase of the steric bulk of a two dimen-
sional phosphorus substituent results in only moderate
further flattening of the phosphorus pyramid. More-
over, any attempted synthesis would be rather difficult
due to the steric hindrance. Thus, the increase of the
bond angle sum above 340° can only be conceived,
when applying other substituents (e.g. y-electron accep-
tors, such as trimethylsilyl, or boryl which were shown
to have effective before computationally [5]) on the
carbon atoms of the phosphole ring.
4. Experimental section
Photoelectron spectra have been recorded at the He I
resonance line on an instrument described earlier [27].
The resolution during the measurement was 40 meV at
2
the P1/2 Ar line. Quantum chemical calculations were
carried out by using the Gaussian 94 package [28].
Preliminary calculations were carried out by the
semiempirical MNDO method [29]. As a result of these
calculations, model compounds (see below) were se-
lected to account for the steric bulk of the substituents
with a possibly minimum cost of computer time. Ge-
ometries of model compounds were fully optimized at
the HF/6-31G* level.
3. Conclusions
With the gradual flattening of the phosphorus pyra-
mid due to the increasing space requirement of the
P-substituent, the phosphorus lone pair ionization en-
ergy decreases in the phospholes investigated. 1-(2,4,6-
tri-tertbutylphenyl)-3-Methylphosphole (2) has the
The NMR spectra were taken on a Bruker DRX 500
spectrometer. The couplings are given in Hz. Mass
spectra were recorded on a MS 25 RFA instrument at
75 eV. Aryl-phospholes 2, 3 and 4 were prepared, as
described earlier [12–15].

34
L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
C2,–CH3), 2.25 (s, C4,–CH3) total int. 12H, 6.42 (d,
J=38.9, 1H, C2–H), 6.83–6.91 (m, 2H, C4–H, C5–H);
MS m/z, (rel. int.) 216 (M+, 100), 201 (27), 161 (26),
119 (16) (Scheme 2).
Acknowledgements
Financial support from OTKA Grants No T014955
and T014917 is gratefully acknowledged. We are grate-
ful to K. Ujsza´szy and T. Ka´rpa´ti for the recording of
the mass spectra and some of the photoelectron spectra,
respectively. We also thank for Professor L.D. Quin for
his continuous interest and advice.
Scheme 2.
References
4.1. 3-Methyl-1-(2,4,6-trimethylphenyl)phosphole (5)
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than for other five membered rings;
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The solution of 2,4,6-trimethylphenylmagnesium bro-
mide prepared from 18.3 g (0.093 mol) of 1-bromo-
2,4,6-trimethylbenzene and 2.2 g (0.092 gatom) of
magnesium in 70 ml of dry THF was added dropwise to
the solution of 11.0 g (0.082 mol) of chlorophospholene
7 [30] in 70 ml of THF with stirring, at 0°C, in nitrogen
atmosphere. After complete addition, contents of the
flask were stirred at room temperature for 1 h. The
solvent was removed by distillation in vacuum and the
residue so obtained extracted with 4×100 ml of n-hex-
ane to give 17.7 g (99%) of phospholene 8 after evapo-
rating the solvent. MS, m/z (rel. int.) 218 (M+, 100),
203 (26), 190 (12), 175 (15), 150 (22), 135 (34), 119 (10).
To 17.7 g (ca. 0.082 mol) 8 from the previous reac-
tion dissolved in 400 ml of n-hexane was added the 25
ml dichloromethane solution of 4.2 ml (0.082 mol) of
bromine with stirring at 0°C, over a period of 20 min.
After addition was complete, the mixture was stirred at
room temperature for 2 h. Phospholium bromide 9
appeared as a yellow precipitated material.
The n-hexane solution of 9 obtained in the previous
reaction was treated with 16.2 ml (0.164 mol) of 2-pico-
line dissolved in 50 ml of dichloromethane. After stir-
ring at room temperature for 20 h and at the boiling
point for 24 h, the mixture consisted of two phases. The
upper layer was decanted and concentrated in vacuum.
The crude product so obtained was purified by column
chromatography (silica gel, 3% methanol in chloro-
form) to give 8.2 g (44%) of phosphole 2 in a purity of
95%. 31P-NMR (CDCl3) l −0.81; 13C-NMR (CDCl3)
l 18.9 (J=3.9, C3,–CH3), 21.0 (C4,–CH3), 21.4 (J=
15.5, C2,–CH3), 125.7 (J=2.1, C2), 128.9 (J=5.4, C,3),
129.0 (J=13.8, C,1), 133.1 (C5), 137.5 (J=13.0, C4),
140.1 (C4), 144.6 (J=15.6, C3), 146.0 (J=15.6, C,2);
1H-NMR (CDCl3) l 2.20 (d, Jꢀ1, C3–CH3), 2.21 (s,
[20] C.W. Bird, Tetrahedron 46 (1990) 5697.

L. Nyula´szi et al. / Journal of Organometallic Chemistry 566 (1998) 29–35
35
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7808.
6% the phenyl ring is somewhat distorted from the symmetry plane
of the phosphole ring, while in the X-ray structure of 6 this
distortion is negligibly small. The distorted phenyl ring has a
smaller steric repulsion than the symmetric one.
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