Synthesis, photoelectron spectroscopy and quantum chemical study of kinetically unstabilized phosphines complexed by borane

Article abstract of DOI:10.1039/b823186a

Ethynyl- and allenylphosphine-boranes have been prepared by addition at low temperature of borane on the free phosphine. Purification was performed by selective trapping in vacuo and the complexes were characterized by NMR and infrared spectroscopy and ma

Full text of DOI:10.1039/b823186a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, photoelectron spectroscopy and quantum chemical study of
kinetically unstabilized phosphines complexed by borane†
Bal a´ zs N e´ meth,a,b Brahim Khater, Tam a´ s Veszpr e´ mi* and Jean-Claude Guillemin*
a
b
a
Received 24th December 2008, Accepted 17th February 2009
First published as an Advance Article on the web 17th March 2009
DOI: 10.1039/b823186a
Ethynyl- and allenylphosphine-boranes have been prepared by addition at low temperature of borane
on the free phosphine. Purification was performed by selective trapping in vacuo and the complexes
were characterized by NMR and infrared spectroscopy and mass spectrometry. A kinetic stability lower
than that of the corresponding free systems was observed. A series composed of these compounds and
methyl-, vinyl-, allyl- and propargylphosphine-boranes was investigated by photoelectron spectroscopy
and B3LYP/aug-cc-pVTZ quantum chemical study in order to define the variation in the electronic
effects between the free systems and the corresponding complexes. Although the complexation only led
to minor changes in the unsaturated moiety, the P–C bond shortens in all cases because of the charge
transfer from phosphorus to boron. Similar rotamers can be found in the complexes and the free
systems, and the order of the relative stability is reversed only in the case of the allenyl derivative. The
-
1
calculated complexation energies are between 80–100 kJ mol in agreement with flash vacuum
thermolysis experiments. The photoelectron spectra can be easily described in the case of
a,b-unsaturated compounds by the change of the direct conjugation between the lone electron pair and
the p-bond in the free phosphines to hyperconjugation of the sP–B bond with the unsaturated moiety in
the corresponding complexed derivatives. In the case of b,g-unsaturated derivatives the observed
hyperconjugation in phosphines disappears on complexation.
Introduction
environment around the phosphorus atom was found to change
significantly upon complexation with borane, with the P–C bond
length shortening and the bond angles widening.
Primary alkyl- and arylphosphine-boranes have been well known
for a long time and have been used in the last few decades for
the preparation of functionalized or chiral phosphines. However,
only tertiary phosphines or alkyl- and aryl primary or secondary
derivatives have been extensively studied. As recently reported,
Twenty years ago, a pioneering work of Norman et al.
described the synthesis of some functionalized derivatives, the
1
9
primary allylic and propargylic phosphine-boranes. A low kinetic
1
stability was observed for these unsaturated species. Recently,
phosphine-boranes (as well as amine-boranes) have potential as
10
the vinylphosphine-borane has been synthesized. The gas phase
2–7
H
2
storage materials.
The gas electron diffraction (GED) study of methylphosphine-
infrared spectrum of the vinylphosphine-borane complex was
-1
recorded in the 3500–600 cm range and analyzed with the aid of
borane, as one of the simplest examples of a primary phosphine-
borane, has been recently reported and provides a starting point
for the investigation of larger functionalized primary phosphines.
A range of ab initio methods was used, including the CCSD(T)
method, with correlation-consistent basis sets. The structural
quantum mechanical DFT calculations at the B3LYP/6–31+G**
level of theory. The low kinetic stability of this compound made
the recording and the accurate analysis of the spectra a challenge.
In the gas phase, only the syn form of the complex was observed
8
10
and assigned. Except for the compounds mentioned above, only
a few functionalized phosphine-boranes have been prepared.
The complexation of primary phosphines by borane has nu-
merous consequences: the previously lone pair on the phosphorus
atom is now involved in the P–B bond, the compound formed is
less volatile than the corresponding free system and less sensitive
to oxidation. The acidity of the hydrogens on the phosphorus atom
a
Sciences Chimiques de Rennes, Ecole Nationale Sup e´ rieure de Chimie de
Rennes-CNRS, 35708 Rennes, France. E-mail: jean-claude.guillemin@ensc-
rennes.fr
b
Budapest University of Technology and Economics, Department of In-
organic and Analytical Chemistry, H-1521 Budapest, Hungary. E-mail:
Tveszpremi@mail.bme.hu
†
borane and flash vacuum thermolysis of phosphine-boranes 7 and 8, NMR
spectra of compounds 10, 12, NPA charges of heavy atoms in the investi-
gated structures (Table S1), calculated Wiberg indices (Table S2), calculated
P–C rotational barriers (Table S3), calculated structural parameters of
some related compounds (Table S4), calculated IEs for allylphosphine
and allylphosphine-borane (Table S5), calculated complexation energies
Electronic supplementary information (ESI) available: Preparation of di-
11
is dramatically modified.
On the other hand, the strong conjugations or hyperconju-
gations observed for free unsaturated phosphines between the
substituent and the phosphorus lone pair or the C–P bond
12–14
respectively,
would be completely different after the for-
of PH
3
–BH
3
(Table S6). Energy changes, Wiberg index changes and NPA
mation of complexes. Our aim is to study the variation in
the electronic effects during the complexation of the simplest
unsaturated derivatives. We report here the synthesis of ethynyl-
and allenylphosphineborane, the first a,b-unsaturated primary
charge changes of C, P and B atoms in methylphosphine complexation
with borane (Fig. S1–S3) and in ethenylphosphine complexation with
borane respectively (Fig. S4–S6), Photoelectron spectra (S7–S13). See
DOI: 10.1039/b823186a
3
526 | Dalton Trans., 2009, 3526–3535
This journal is © The Royal Society of Chemistry 2009

View Article Online
1
phosphine-boranes bearing a 1-alkynyl or allenyl substituent,
respectively. To efficiently evaluate the role of the substituent,
methyl, vinyl, allyl and propargyl derivatives were added to the
series. A complete study by photoelectron spectroscopy assisted
by quantum chemical calculations is described and the obtained
data have been compared with those of the free systems.
with the signals of the corresponding free systems. The
J
PH
coupling constant dramatically increased from about 200 Hz for
phosphines 4 and 6 to about 370 Hz for complexes 10 and 12.
NMR signals are observed at about 100 ppm, down-field of those
of the free systems. B NMR chemical shifts around -40 ppm are
3
1
P
1
1
9,10
typical of primary phosphine-boranes.
The kinetic stability of compounds 10 and 12 is lower than
that of the corresponding free systems and pure compounds
Results and discussion
◦
10,12 should be kept at low temperature (-78 C). At room
Synthesis of phosphine-boranes
temperature, a half-life of seven days was observed for 12 diluted
in deuterochloroform and in the presence of small amounts of
duroquinone (a radical inhibitor) but, under the same conditions,
the ethynylphosphine-borane 10 is quickly decomposed and NMR
spectra were recorded at low temperature. After a cautious
purification of the complexes 7–12 by selective condensation
in a cold trap and after dilution in deuterochloroform or
dideuterodichloromethane, the presence of the corresponding
low boiling free phosphine (2–5%) was easily detected by room
The six phosphine-boranes 7–12 (Chart 1) have been synthesized in
good yields by reaction at low temperature of the free phosphines
1–6 with diborane or borane complexed with THF or dimethyl-
sulfide in low boiling solvents (Scheme 1). The purification was
performed in a vacuum line by selective condensation of the
complexes at low temperature. To easily separate 1- or 2-carbon
phosphine-boranes (7,8,10) from the solvent, diborane in pentane
was used as reagent. Borane complexed with THF or Me S or
2
1
temperature H NMR spectroscopy. This proves the presence in
diborane can be used to prepare the 3-carbon derivatives 9,11,12.
The first primary 1-alkynyl- (10) and allenylphosphine-borane (12)
have been thus isolated.
the solvent of an equilibrium between the complexed and the free
systems. The B–P bond is quite weak and easily broken by flash
◦
vacuum thermolysis (FVT) at 300 C, a very low temperature for
FVT experiments: the FVT of phosphine-boranes 7,8 gave the
corresponding free phosphines 1,2 in a very good yield.
Structures
The calculated structural parameters of the investigated com-
pounds are compiled in Table 1. The NPA (Natural Population
15
Analysis) atomic charges are given in Table S1 of the ESI.† All
the stable phosphine-borane conformers 7–12 found can be seen
in Fig. 1.
The molecular properties during the complexation were in-
vestigated for the methyl 1,7 and ethenyl derivatives 2,8. It was
found that the Wiberg bond index and the charge transfer from
phosphorus to boron changes almost linearly within the P–B
˚
˚
distance between 1.5 A and 2.8 A (Fig. 2). In all the investigated
stable conformers the Wiberg indices are close to 1, which suggests
a single bond formation (Table S2 and Figures S1–S6 of the ESI†).
On the other hand the charge of the methyl and vinyl groups does
not change during the complexation. The complexation energies
Chart 1
-
1
-1
are between 80 kJ mol and 100 kJ mol , about 30–50% less than
16
those in amine-boranes. No direct correlation could be found
between the P–B bond lengths and the complexation energies.
The most conspicuous difference in the geometries of phos-
phines 1–6 and phosphine-boranes 7–12 is the P–C bond shorten-
ing in the complexes which can be observed in all the investigated
molecules (Table 1). This is the obvious consequence of the
enhanced positive charge on the phosphorus atom during the
complexation. The rotation around the P–C bond in the complexes
and in the free phosphines was studied and found to be similar.
Astonishingly, in all cases the complexes exhibit very similar con-
formers to the free phosphines (Fig. 1). Our calculations also prove
that the relative stability of the different conformers and the energy
barriers between them are well comparable (Table 2 and S3 of the
ESI†). Although the accuracy of the B3LYP method is normally
lower than the calculated energy differences between the studied
conformers, the calculated relative stabilities of phosphines 3, 5
and 6 fit the measurements well. Therefore the calculated order
Scheme 1
Compounds 10 and 12 were easily characterized by infrared and
H, C, B and P NMR spectroscopy and mass spectrometry.
A pure sample of compounds 10,12 was condensed on a KBr
window cooled at 77 K to record the infrared spectra. The nB–H
1
13
11
31
-
1
bond was observed around 2350 cm and the nP–H bond is blue
-
1 10
shifted at around 2400 cm . The expected NMR chemical shifts
1
and coupling constants were observed for 10 and 12: on the H
NMR spectrum, the signals corresponding to the two hydrogens
on the phosphorus atom are shifted down-field by comparison
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3526–3535 | 3527

View Article Online
Table 1 Calculated bond distances of the investigated phosphines and phosphine-boranes (in A˚ )a
No.
P–B
a
b
c
1
c
2
R-
R-PH
2
R-PH
2
-BH
3
R-PH
2
-BH
3
R-PH
2
R-PH
1.843
2
-BH
3
R-PH
2
R-PH
2
-BH
3
R-PH
2
R-PH
2
-BH
3
R-PH
2
2 3
R-PH -BH
1
7
1.947
1.869
2
2
I
II
8I
8II
1.935
1.936
1.840
1.830
1.814
1.810
1.328
1.329
1.327
1.327
3
3
3
3
3
I
9I
1.932
1.933
1.932
1.934
1.937
1.884
1.889
1.892
1.871
1.879
1.849
1.856
1.856
1.838
1.847
1.492
1.495
1.494
1.502
1.501
1.495
1.498
1.498
1.502
1.503
1.329
1.328
1.328
1.326
1.327
1.327
1.327
1.326
1.326
1.326
II
III
IV
V
9II
9III
9IV
9V
4
10
1.940
1.776
1.760
1.204
1.201
5
5
I
II
11I
11II
1.929
1.930
1.891
1.896
1.855
1.861
1.447
1.450
1.449
1.450
1.200
1.200
1.199
1.198
6
6
I
II
12I
12II
1.933
1.933
1.846
1.843
1.817
1.816
1.301
1.299
1.303
1.301
1.300
1.302
1.296
1.299
a
B3LYP/aug-cc-pVTZ level of the theory.
Table 2 Complexation energies and relative energies of the different R-
a
PH
2 3
-BH conformers
obs
b
calc
calc
calc
R-
DE PH2
DE PH2
DE complex DE complexation
H
CH
3
C-
2
99.57
=CH-
I
0.00
1.11
0.00
2.39
3.67
8.04
9.17
0.00
2.96
0.00
2.52
5.61
7.77
12.56
92.94
91.09
96.49
96.36
94.55
96.77
93.10
80.44
94.14
89.20
89.80
91.91
II
I
0.00
1.6(3)
II
CH
2
=CH-CH
2
-
III 1.4(3)
IV n. o.
V
n. o.
HC∫C-
HC∫C-CH
2
-
I
0.00
1.5 ± 2
0.00
0.00
2.02
0.00
1.33
0.00
6.96
0.00
II
I
CH
2
=C=CH-
II
2.1(4)
-0.78
a
-1
Energy values are in kJ mol , calculation performed at the B3LYP/aug-
18,19
cc-pVTZ + ZPE level, counter poise correction
complexation energies. Microwave measurements are in References 20–
was used for calculating
b
2
2. In the microwave experiment of 2 both conformers were observed, but
23,24
the energy difference could not be determined.
n. o.= not observed.
of the relative stabilities reflects the energy differences in the gas
phase well. The relative energies of the conformers are not changed
dramatically by the complexation, the calculated stability order of
the studied phosphine and phosphine-borane rotamers is reversed
only in the case of the allenyl derivative.
Two stable conformations of the ethenyl derivatives 2 and 8 were
found, the planar C
forms. The energy difference between them is only 1.1 kJ mol for
the phosphine 2, and about 3 kJ mol for the phosphine-borane 8,
s
symmetry syn and the non-planar gauche
-
1
-
1
-
1
-1
the barrier heights are 7.6 kJ mol and 4.2 kJ mol , respectively
Table 2 and S3 of the ESI†). This indicates that the rotation of
the PH or BH group is free only at moderate temperature.
(
17
Fig. 1 Stable conformers of 7–12 as shown by the MOLDEN Program.
2
3
3
528 | Dalton Trans., 2009, 3526–3535
This journal is © The Royal Society of Chemistry 2009

View Article Online
Fig. 2 Complexation of ethenylphosphine (2II) with borane. The variation of a) relative energy, b) Wiberg bond index and c) NPA charges with the P–B
distance (in A˚ ). The vertical line indicates the equilibrium bond distance.
The structure of the different conformations of ethenylphos-
Five minima were found on the potential energy surfaces of both
the allyl derivative 3 and its complex 9, which can be divided into
12
13
phine 2 has been briefly studied before and it was suggested
that there is no remarkable interaction between the double bond
and the lone electron pair of phosphorus in the slightly more
stable planar conformer, while some conjugation is expected in the
two groups, the more stable out-of-plane structures (3I, 3II, 3III
and 9I, 9II, 9III) and the less stable planar orientations (3IV, 3V
and 9IV, 9V). These conformers differ only in the direction of the
P–B bond or the phosphorus lone pair. Between the two groups
˚
higher energy non-planar structure. The P–C bonds of 2 (1.830 A)
-
1
˚
and 8 (1.810 A) are definitely shorter than in the parent methyl
the energy difference is 5–8 kJ mol . The total energy differences
-
1
˚
˚
derivatives 1 (1.869 A) and 7 (1.843 A). The data indeed suggest a
considerable interaction between the p-system and the phosphorus
lone pair. It is important to mention that as the total electron
charge on the C=C bond does not change during the complex
formation, the C=C bond length is also unchanged (Table 1 and
between the lowest and highest energy conformers are 9.2 kJ mol
-
1
and 12.6 kJ mol for the phosphine and the complex, respectively.
The rotation around the C–C and the C–P axes requires rather
high energy; the maximum energy during a full rotation is about
20 kJ mol for the free phosphine and about 27 kJ mol for the
complex (Table S3 of the ESI†).
-
1
-1
S1 of the ESI†).
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3526–3535 | 3529

View Article Online
In our earlier work it was found that the most stable confor-
mations of allylphosphine, 3I–III, are unprecedentedly stabilized
by the phosphorus lone pair and much less by the possible
Photoelectron spectroscopy
The UV photoelectron spectra of methylphosphine 1,
methylphosphine-borane 7, ethenylphosphine 2, allylphosphine 3,
ethynylphosphine 4, and allenylphosphine 6 have been published
13
hyperconjugation between the p-orbital and the C–P bond.
The C–P bond clearly illustrates the effect, as it is elongated
compared to that in 1) and the neighboring C–C single bond
12–14,27–29
(
previously.
The spectra of propargylphosphine 5 as well
shortened (compared to that in propene, Table S4 of the ESI†).
The bond length differences are more pronounced in the I–III
conformations than in IV–V. Since the complex formation does
not affect the net charge of the C=C carbon atoms, the double
bond lengths are almost unchanged (Table 1 and S1 of the
ESI†).
as those of borane complexes 7–12 are collected in the ESI
(Fig. S7–S13†).
The first photoelectron band of methylphosphine 1 is found at
9.70 eV and ordered to the lone electron pair of the phosphorus.
The complexation shifts the bands related to the phosphine moiety
to higher ionization energies and those related to the borane group
to lower ionization energies because of the strong charge shift from
the phosphine to the borane. The lone pair sets up the sP–B bond,
which gives the first PE band of the complex at 10.4 eV. According
Only one conformer was found in the triple bonded compounds
˚
4
and 10. The P–C bond in the free phosphine is 0.093 A shorter
than in methylphosphine 1. This suggests a very strong effect
between the p-system and the PH -group, which is confirmed by
30
to our OVGF (Outer Valence Green’s Function) calculations the
next two bands (11.0 eV and 11.6 eV) belong to the sB–H orbitals
(Table 3).
2
14
an earlier UPS study. The drastic bond shortening is in line with
the observation of similar unsaturated compounds containing
e.g. Si, Ge or Sn, where the C–E (E = Si, Ge, Sn) bond length
In ethenylphosphine 2 the two energetically close conformers
(2I, 2II) suggest different interactions and MO structures. Some
conjugation can be assumed in the nonplanar (gauche) 2II between
25,26
decreases in the order ethyl > vinyl > ethynyl.
This shortening
is mainly associated with an increase in the polarization of the C–E
bond caused by the increasing electronegativity of the unsaturated
group. Indeed, the positive charge on the phosphorus atom in
ethyl-, vinyl- and ethynylphosphine is 0.295, 0.318 and 0.357,
respectively. In addition to this effect, the conjugative interaction
between the lone electron pair and the appropriate symmetry
p-component may also shorten the C–P bond. The same C–P bond
shortening can be observed in the complex. In this case, however,
the lone pair effect is substituted by the hyperconjugation between
the p-system and the P–B bond.
the lone pair of phosphorus (n ) and the p-system while no
P
interaction is expected in the planar (syn) 2I. Although the
effect of conjugation is clearly seen when comparing the PE
spectra of methylphosphine 1, propene and ethenylphosphine 2
(Fig. 3), because of the very small energy difference between
2I and 2II, probably both conformers are present together
12,23,24
(Table 2).
The complexation modifies the type of interac-
tion as the hyperconjugation of the sP–B orbital substitutes the
direct conjugation of the lone pair. The first band at 10.5 eV
originates from the sP–B - p combination. The next intense band
belongs to the sB–H orbitals (two ionizations) while at the high
energy side of the band the sP–B + p combination is expected
(Fig. 3). This assignment is supported by the width of the
PE band. Nevertheless, the small energy difference suggests a
similar mixture of the conformers in the complex 8 and the free
phosphine 2.
In the case of the propargyl derivatives, 5 and 11, two
energetically close conformers (I and II) were found. The energy
-
1
-1
difference between them is 2 kJ mol (about 7 kJ mol in the
complex), which is in excellent agreement with the experimentally
-
1 21
found 1.5 ± 2 kJ mol . The maximum barrier for the rotation of
-
1
-1
the -PH
2
group (-PH
2
-BH group) is 19.4 kJ mol (9.6 kJ mol
3
for the complex). Because of the electronic attraction of the
unsaturated group, the neighboring C–C single bond is very short
13
The PE spectrum of allylphosphine 3 as it was clarified earlier,
˚
clearly demonstrates the interaction between the p and the nP
(
1.44–1.45 A), the shortest in all the investigated compounds. The
C–P bond is, on the other hand, the longest of all the studied
molecules because of the possible lone pair or hyperconjugation
effect.
bands. On the other hand, according to the calculations, the
possible hyperconjugation with the P–C bond is negligibly small.
In the complex 9, the double bond and the P–B bond are so
far from each other that no interaction is expected. Indeed, the
first broad band of the spectrum includes four peaks and can be
assigned to the p, sP–B and the two sB–H orbitals (Table 3 and
S5 of the ESI†). All of them are almost at the same energies as
the respective ionizations found in propene and methylphosphine-
borane.
An early MNDO calculation for allenylphosphine 6, suggests
14
only one stable conformation. Later, a microwave study indicated
the two coexisting conformers, a C symmetry syn and a non-
symmetric gauche form (Fig. 1, Table 2), which were also
S
22
found in our calculations. The syn conformer was found to
be 2.1 kJ mol- more stable than the gauche conformer (the
1
-
1
calculated result is 1.33 kJ mol ). The same orientations were
observed in the complex (the direction of the B–P bond is similar
to that of the phosphorus lone pair), however, according to
our calculations, the gauche conformer is slightly more stable
The only ethynylphosphine conformer 4 gives relatively sharp
14
bands in the PE spectrum. When an acetylene hydrogen is
substituted by a PH group, two different effects are expected.
2
First, due to the charge shift from the phosphorus toward the
C∫C bond, the p-orbitals are destabilized. Second, the phosphorus
-
1
(
by 0.78 kJ mol ).
Since both lone pair interaction and hyperconjugation can
lone pair can interact with one of the p components causing
a considerable energy splitting of the originally degenerated p
orbitals. All these effects are clearly observable in the PE spectrum
and can be seen in the respective correlation diagram (Fig. 4). The
appear in the free phosphine 6 which indicates the opposite effect
for the P–C bond length, the resultant C–P bond is rather similar
to the vinyl derivative. In the complex a small hyperconjugation
can be assumed according to the P–C distances and the charges of
the P and B atoms (Tables 1 and S1 of the ESI†).
n -p interaction in the complex disappears and is substituted by a
P
hyperconjugation between the p-component and the sP–B orbital.
3
530 | Dalton Trans., 2009, 3526–3535
This journal is © The Royal Society of Chemistry 2009

View Article Online
a
Table 3 Experimental and calculated ionization energies (in eV)
Phosphine
Calculated
Phosphine-borane
Calculated
R-
I
II
III
Measured
Assign.
I
II
III
Measured
Assign.
H
3
C-
1
2
9.71
2.52
2.95
4.86
5.04
9.70
12.40
12.70
14.45
n
s
s
p
7
8
10.94
11.05
11.09
13.83
14.18
10.4
11.0
11.6
13.5
s
s
s
s
s
P-B
B-H
B-H
1
1
1
1
P-C
PH2, sC-H
Methyl group
P-C
PH2, sC-H
Methyl group
1
5.49
15.1
10.5
CH
CH
2
=CH-
9.85
0.46
2.32
3.71
4.35
6.09
8.61
9.31
9.60
10.85
12.60
n
n
s
s
s
s
p
p
- p
10.95
11.12
11.12
11.14
13.26
15.05
15.08
10.55
11.06
11.09
11.71
13.33
14.76
15.52
sP-B - p
1
1
1
1
1
1
11.04
12.40
13.45
14.60
15.85
19.79
+ p
P-C, sC-H
PH2
C-C
C-C
11.0
11.6–11.8
13.0
14.8
16.0
s
s
s
s
s
B-H
P-B + p
14.30
15.65
P-C, sC-H
PH2
C-C
b
b
2
=CH-CH
2
-
3
9.24
0.41
2.58
2.79
2.87
4.21
5.01
6.23
8.88
9.55
10.00
12.32
13.15
12.90
14.26
15.00
16.25
20.89
9.46
9.32
10.25
n
n
p
p
- p
+ p
9
10.14
11.03
11.06
11.20
13.26
13.62
14.00
14.89
15.59
10.19
10.93
11.01
11.06
12.94
13.88
13.93
15.02
15.48
16.59
10.15
10.97
10.98
11.08
12.95
13.52
14.40
14.99
15.69
16.38
10.1
10.5
11.0
11.6
12.8
13.4
p
s
s
s
s
s
s
s
s
s
1
1
1
1
1
1
1
1
10.10
12.32
13.36
12.62
14.26
15.07
16.16
20.65
P-B
B-H
B-H
C-C
P-C, sP-H
P-C, sP-H
C-C, sC-C
C-C, sC-C
C-C, sC-C
14.7
15.3
16.3
1
6.79
HC∫C-
4
5
9.76
0.88
1.85
4.25
4.84
7.05
8.08
9.77
10.81
11.83
13.83
14.79
17.55
n
p
n
s
s
s
- p
10
11
10.83
11.40
11.47
11.49
12.39
15.51
15.99
10.87
11.5
s
P-B + p
p
2
p
1
1
1
1
1
1
1
1
+ p
PH2
1
p
2
, sB-H
P-C, sC-C
C-C, sC-H
11.8–12.0
14.8
s
s
s
P-B - p
PH2
1
P-C, sC-C
CH∫CCH
-
9.55
0.49
0.86
3.19
3.24
5.69
5.82
8.01
9.80
10.38
10.60
13.49
13.24
15.58
15.82
17.98
9.67
10.36
10.61
12.4
13.0
15.3
n
p
n
- p
10.52
11.05
11.22
11.26
11.63
14.34
14.50
16.28
10.57
10.89
11.15
11.18
11.33
14.37
14.72
16.00
16.55
10.4
10.9
p
p
2
p
1
1
1
1
1
1
1
1
1
2
2
p
+ p
PH2
1
s
s
s
s
11.4 - 11.8
14.0
sP-B, sB-H
P-C
C-H
s
s
s
s
P-C
C-C, sC∫C
PH2
15.8
C-H
1
6.52
C-C, sC∫C
CH
2
=C=CH-
6
9.69
9.16
9.81
9.55
10.02
11.17
13.33
n
p
n
s
s
s
s
s
p
- p
1
12
10.30
10.59
11.03
11.06
11.11
14.80
14.95
15.83
10.14
10.54
11.09
11.14
11.74
14.94
14.52
15.95
10.12
10.56
11.0–11.2
s
2
P-B - p
1
9
.98
2
p
1
1
1
1
1
1
0.27
3.68
3.73
5.40
5.66
7.33
11.16
13.33
13.76
15.31
15.72
17.48
p
+ p
PH2
1
s
s
s
s
s
s
B-H
B-H
P-C
11.6
P-B + p
PH2
P-C
C-H
1
15.23
16.87
C-H
C=C, sC=C
C=C, sC=H
14.7–15.6
a
b
Calculations are performed by the OVGF/aug-cc-pVTZ method. Calculated ionization energies for conformers IV and V are in the ESI.
Because of the strong charge transfer toward the BH
P–B and p bands are strongly stabilized.
No considerable charge difference is observable between the
triple bonds of propyne and propargylphosphine 5, therefore the
position of the non-interactive p component does not change.
Comparing the respective PE-bands to those of the parent
compounds, only a slight conjugation can be found between the
lone pair and the p-bond which causes a 0.24 eV shift in the
3
group, the
position of the p
is faded and the separation between the two p-components is
decreased. The p -band is at the same position as in propyne, the
band can be found at about 10.9 eV, while the broad shoulder
between 11.4–11.8 eV contains the sP–B, and the sB–H components
(Fig. 4, Table 3).
1
band. In the complex even the weak interaction
s
1
p
2
2
Since the PH group in allenylphosphine 6 is in the vinyl
2
position to the neighboring double bond and in the allyl position
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3526–3535 | 3531

View Article Online
Fig. 3 Correlation diagram of ethenyl- 2,8 and allyl-derivatives 3,9.
Fig. 4 Correlation diagram of ethynyl- 4,10 and propargyl derivatives 5,11.
to the second double bond, theoretically both lone pair and
hyperconjugation interactions are expected in 6. Indeed, compared
to the spectra of allene and methylphosphine 1, a strong splitting
of the originally degenerated p-orbitals can be observed which is
similar in size to the p-orbital splitting of vinylphosphine 2 (Fig. 5).
The “only” difference is the appearance of the second p-band in
structure to that in 8 (plus a non-interactive p-bond). The lowest
energy component of the band at 10.12 eV can be assigned to the
p
1
orbital, which is followed by the sP–B-p
2
combination at 10.56 eV
components are
(Table 3 and Fig. 5). The sB–H and the sP–B + p
2
hidden in a broad shoulder between 11.0–11.2 eV and at about
11.6 eV, respectively. Because of the very small energy difference
between 12I and 12II, a similar mixture of the two conformers to
that in 8 can also be assumed.
6
. The position of the p band is unchanged (10.02 eV) which may
1
suggest that the interaction with the further double bond is small.
Similarly to the other derivatives, the dative bond formation in
the complex strongly stabilizes the PE bands originating from the
Conclusion
allenylphosphine moiety and destabilizes the bands of the BH
3
group. The interaction between the nearest p-bond and the lone-
pair is substituted by the hyperconjugation of the P–B bond, which
causes almost the same splitting as the former effect. On the other
hand, no interaction can be expected with the farthest p-bond. As
a result, the first broad band of the spectrum exhibits a similar
The variation in the electronic effects during the complexation of
the simplest unsaturated phosphine-boranes has been studied. To
evaluate the role of the substituent, methyl, vinyl, allyl and propar-
gyl derivatives were prepared and, ethynyl- and allenylphosphine-
boranes have been synthesized for the first time. A complete study
3
532 | Dalton Trans., 2009, 3526–3535
This journal is © The Royal Society of Chemistry 2009

View Article Online
Materials
Lithium aluminum hydride, aluminum chloride and tetra-
ethyleneglycol dimethyl ether (tetraglyme) were purchased from
Aldrich. All the experiments at atmospheric pressure were per-
formed under nitrogen.
General
1
13
31
11
H, C, P and B NMR spectra were recorded on a Bruker
ARX400 spectrometer. Chemical shifts are given in ppm (d) rela-
1
13
tive to tetramethylsilane ( H), solvent ( C, CDCl
external H
(
3
, d 77.0 ppm),
(85%) ( P NMR) and external BF -Et
B NMR). The NMR spectra were recorded using CDCl
3
1
3
PO
4
3
2
O
1
1
3
as
solvent. HRMS (high-resolution mass spectrometry) experiments
were performed on a Varian MAT 311 instrument. The yields
given for the synthesis of phosphine-boranes 7–12 are determined
starting from 15 mmol of free phosphine and 5 mmol of diborane
(
0.67 equiv.). Distillation of larger amounts led to lower yields.
HeI photoelectron spectra were recorded on an instrument
Fig. 5 Correlation diagram of allenyl derivatives 6,12.
31
2
described earlier. The resolution at the Ar P1/2 line was 40 meV
during the measurements. For internal calibration N
He peaks were used.
2
, MeI and
by photoelectron spectroscopy assisted by quantum chemical
calculations has been performed and the obtained data have
been compared with those of the free systems. Methylphosphine-
borane provided a starting point for the investigation of the
larger functionalized primary phosphines. The observed low
temperature of gas phase thermolysis to form the free phosphine
+
Calculations
Preliminary DFT (Density Functional Theory) and MP2 (Møller–
Plesset second order perturbation) calculations for the molecular
14
agrees with the calculated complexation energies, which are 80–
geometry of some unsaturated phosphines and similar S, Se,
Te compounds proved that the bond length difference between
1
1
00 kJ mol- for the investigated compounds. Surprisingly, the
32–34
complexation has no remarkable influence on the a,b-unsaturated
compounds since similar structures have been found for the
conformers of the complexes and the corresponding free systems.
The only significant difference in the geometries is the P–C bond
shortening in the complexes, which is the obvious consequence of
the charge transfer during complexation. Except for the allenyl
derivative the order of the relative stability remained the same
in phosphines and phosphine-boranes and the rotational barriers
are also comparable. The photoelectron spectra of these species
and of methyl, allyl and propargyl derivatives were recorded and
analyzed with the help of quantum chemical calculations
and compared to those of the free systems. Interestingly, the
spectra can be easily described in the case of the a,b-unsaturated
compounds since the direct conjugation between the lone electron
pair and the p-bond has to be exchanged to a hyperconjugation
between the sP-B bond and the unsaturated moiety. In the case
of b,g-unsaturated derivatives, the observed hyperconjugation in
phosphines disappears with the complexation and no interaction
with the phosphorus atom could be observed. Other spectroscopic
studies on the a,b-unsaturated phosphine-boranes and the use of
these reactive compounds in the synthesis of polyfunctionalized
phosphorus derivatives are currently in progress in our laboratory.
the two methods is generally less than 0.02 angstrom.
The
MP2 method provides a somewhat longer multiple bond and a
somewhat shorter C–M (M = P, S, Se, Te) bond than the DFT
method. Since our main purpose was the comparison of the
geometry of related molecules, our calculations were performed
with the DFT method using the hybrid functional B3LYP
(Becke’s three parameter functional employing the Lee, Yang
35,36
and Parr correlational functional)
and aug-cc-pVTZ basis set.
The stationary points were characterized by second derivative
calculations. Zero-point vibrational energy (ZPE) correction for
18,19
relative energies and counter poise correction
for complexation
energies were calculated using the same level of theory. To
confirm the reliability of the DFT method used, MP2/aug-cc-
pVTZ//MP2-aug-cc-pVTZ and CBS-QB3 calculations for the
geometry and the complexation energy were also carried out for
the PH -BH system. It was found that the difference between the
3
3
B3LYP and CBS-QB3 methods was less than 2 kJ mol- (Table S6
1
-
1
in the ESI†). The MP2 method gives around 10 kJ mol higher
15
result than all the other methods. NPA calculations were carried
out at the B3LYP/aug-cc-pVTZ level to determine atomic charges
in the studied molecules. HF/aug-cc-pVTZ molecular orbitals
were calculated to support the assignment of the ionization bands.
Prediction of ionization energies were achieved by the OVGF
30
method with the aug-cc-pVTZ basis set. All calculations were
Experimental section
37
carried out using the Gaussian 03 program package.
Caution
Preparation of compounds (7)–(12)
Phosphines and phosphine-boranes are malodorous and poten-
tially toxic compounds. All reactions and handling should be
carried out in a well-ventilated hood.
Methylphosphine-borane 7 has been prepared as previously
reported starting from the corresponding free phosphine.
38
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3526–3535 | 3533

View Article Online
The phosphine-boranes 8–12 have been synthesized starting
2 F. H. Stephens, R. T. Baker, M. H. Matus, D. J. Grant and D. A. Dixon,
39
14
D. A., Angew. Chem., Int. Ed., 2007, 46, 746–749.
from the vinylphosphine 2, allylphosphine 3, ethynylphosphine
3
4
5
6
7
M. E. Bluhm, M. G. Bradley, R. Butterick III, U. Kusari and L. G.
Sneddon, J. Am. Chem. Soc., 2006, 128, 7748–7749.
41
40
41
4
,
propargylphosphine 5 and propadienylphosphine 6.
C. W. Yoon and L. G. Sneddon, J. Am. Chem. Soc., 2006, 128, 13992–
Preparation of phosphine-boranes (8)–(12): general procedure.
1
3993.
42
In a Schlenk flask, diborane (about 0.28 g, 10 mmol) was slowly
M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and K. I.
Goldberg, J. Am. Chem. Soc., 2006, 128, 12048–12049.
◦
added to a frozen solution (-196 C) of primary phosphine 2–6
M. H. Matus, M. T. Nguyen and D. A. Dixon, J. Phys. Chem. A, 2007,
(
about 15 mmol) and pentane (5 mL). The reaction mixture was
1
11, 1726–1736.
allowed to warm to room temperature and was stirred for 5 min at
this temperature. The mixture was then distilled off in a vacuum
line and the phosphine-borane 8–12 was selectively condensed in
D. J. Grant and D. A. Dixon, J. Phys. Chem. A, 2005, 109, 10138–
10147.
8 R. Noble-Eddy, S. L. Masters (n e´ e Hinchley), D. W. H. Rankin, D. A.
Wann, B. Khater and J.-C. Guillemin, Dalton Trans., 2008, 5041–
◦
a trap cooled at -60 C. This cell was then disconnected from the
5
047.
vacuum line by stopcocks and adapted to the PE spectrometer.
9
R. H. Shay, B. N. Diel, D. M. Schubert and A. D. Norman, Inorg.
Chem, 1988, 27, 2378–2382.
Instead of diborane, BH
3
-THF in THF and BH
3
-SMe in diethyl
2
1
1
0 B. Khater, J.-C. Guillemin, A. Benidar, D. B e´ gu e´ and C. Pouchan,
J. Chem. Phys, 2008, 129, 2243081–11.
1 M. Hurtado, M. Y a´ n˜ ez, R. Herrero, A. Guerrero, J. Z. D a´ valos,
J.-L. M. Abboud, B. Khater and J.-C. Guillemin, Chem.–Eur. J., 2009,
DOI: 10.1002/chem.200802307.
2 D. Gonbeau, S. Lacombe, M.-C. Lasnes, J.-L. Ripoll and G. Pfister-
Guillouzo, J. Am. Chem. Soc., 1988, 110, 2730–2735.
3 S. Le Serre, J.-C. Guillemin, T. K a´ rp a´ ti, L. Soos, L. Nyul a´ szi and T.
Veszpr e´ mi, J. Org. Chem., 1998, 63, 59–68.
4 S. Lacombe, W. Dong, G. Pfister-Guillouzo, J.-C. Guillemin and J.-M.
Denis, Inorg Chem., 1992, 31, 4425–4427.
ether can be added to the free phosphine 2–6. Purification was
easily performed in a vacuum line (0.1 mbar) by selective trapping
◦
◦
of the phosphine-borane in a trap cooled at -35 C for the 3-
carbon derivatives 9,11,12. A temperature of -50 C was used
1
1
1
with the 2-carbon derivatives 8 and 10 but the purification was
more difficult in this case. Spectroscopic data have already been
10
9
reported for phosphines 8, 9 and 11.
Ethynylphosphine-borane (10). Kinetically very unstable at
◦
room temperature, the pure product, stable at -60 C, was collected
15 A. E. Reed, R. B. Weinstock and F. Weinhold, J. Chem. Phys., 1985,
3, 735–746.
8
diluted in a solvent in an NMR tube and kept at low temperature
1
1
6 A. Haaland, Angew. Chem., Int. Ed. Engl., 1989, 28, 992–1007.
7 G. Schaftenaar and J. H. Noordik, Molden: a pre- and post-processing
program for molecular and electronic structures, J. Comput.-Aided Mol.
Design, 2000, 14, 123–134.
◦
1
(
<-50 C) before analysis. Yield: 43% (based on borane). H NMR
1
◦
(
d, 400 MHz, CDCl
3
, -40 C): 0.80 (qm, 3H, JBH = 97.2 Hz,
3
3
4
J
HH = 8.0 Hz, BH
3
); 3.07 (dt, 1H, JHP = 9.0 Hz, JHH = 2.2 Hz,
1
3
4
18 S. Simon, M. Duran and J. J. Dannenberg, J. Chem. Phys., 1996, 105,
1024–11031.
9 S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553–566.
CH); 5.45 (dqd, JHP = 399.6 Hz, JHH = 8.0 Hz, JHH = 2.2 Hz,
1
3
1
◦
1
PH
2
). P NMR (d, 162 MHz, CDCl
3
, -40 C): -80.2 ( JPB
, -40 C): 65.6 ( JCP
=
=
1
1
3
◦
1
2
1
1.0 Hz). C NMR (d, 100 MHz, CDCl
3
20 H. Møllendal, J. Demaison and J.-C. Guillemin, J. Phys. Chem. A, 2002,
1
2
106, 11481–11487.
00.6 Hz (d), CP); 97.1 ( JCH = 255.4 Hz (d), JCP = 14.9 Hz (d),
2
1 J. Demaison, J.-C. Guillemin and H. Møllendal, Inorg. Chem., 2001,
1
1
◦
CH). B NMR (d, 128.4 MHz, CDCl
3
, -40 C): -40.6. IR (film,
4
0, 3719–3724.
-
1
7
(
7K): n/cm 2069 s (C∫C), 2359 m (BH), 2400 s (PH), 3250 m
22 H. Møllendal, J. Demaison, D. Petitprez, G. Wlodarczak and J.-C.
HC∫C). Attempts to record the mass spectrum were unsuccessful.
Guillemin, J. Phys. Chem. A., 2005, 109, 115–121.
3 P. Dr e´ an, J.-M. Colmont, A. Lesarri and J. C. L o´ pez, J. Mol. Spectrosc.,
1996, 176, 180–184.
24 P. Dr e´ an, M. Le Guennec, J. C. L o´ pez, J. L. Alonso, J. M. Denis, M.
Kreglewski and J. Demaison, J. Mol. Spectrosc., 1994, 166, 210–223.
5 O. M o´ , M. Y a´ n˜ ez, M. Decouzon, J. F. Gal, P.-C. Maria and J.-C.
Guillemin, J. Am. Chem. Soc., 1999, 121, 4656–4663.
26 J.-F. Gal, M. Decouzon, P.-C. Maria, A. I. Gonz a´ lez, O. M o´ , M. Y a´ n˜ ez,
2
Allenylphosphine-borane (12). Yield: 75% (based on borane).
1
t
4
7
1/2 (5% in CDCl
00 MHz, CDCl
.3 Hz, BH
); 5.05 (dtd, 2H, JHH = 6.8 Hz, JHH = 3.1 Hz, JHP
.9 Hz, CH
3
at room temperature) ª 7 days. H NMR (d,
◦
1
3
3
, 25 C): 0.70 (qt, 3H, JBH = 100.2 Hz, JHH
=
2
4
5
4
3
=
1
3
4
2
); 5.11 (dm, 2H, JHP = 374.9 Hz, JHH = 7.3 Hz,
3
5
4
S. El Chaouch and J.-C. Guillemin, J. Am. Chem. Soc., 2001, 123, 6353–
J
J
HH = 4.2 Hz, JHH = 3.1 Hz, PH
2
31
); 5.30 (ttd, 1H, JHH = 6.8 Hz,
6
359.
3
2
HH = 4.2 Hz, JHP = 6.8 Hz, CH). P NMR (d, 162 MHz, CDCl
3
,
,
2
7 S. Cradock, E. A. V. Ebsworth, W. J. Savage and R. A. Whiteford,
J. Chem. Soc., Faraday Trans. 2, 1972, 68, 934–939.
28 M. F. Lappert, J. B. Pedley, B. T. Wilkins, O. Steizer and E. Unger,
J. Chem. Soc., Dalton Trans., 1975, 1207–1214.
9 A. H. Cowley, R. A. Kemp, M. Lattman and M. L. McKee, Inorg.
Chem., 1982, 21, 85–88.
◦
◦
1
13
2
2
(
5 C): -55.3 ( JPB = 37.0 Hz (q)). C NMR (d, 100 MHz, CDCl
3
1
1
5 C): 70.1 ( JCH = 175.1 Hz (d), JCP = 59.2 Hz (d), CH); 75.9
1
3
J
CH = 170.3 Hz (t), JCP = 12.4 Hz (d), CH
2
); 215.0 (s, C=C=C).
2
1
1
◦
B NMR (d, 128.4 MHz, CDCl
3
, 25 C): -42.1. IR (film, 77K):
-
1
3
0 J. V. Ortiz, V. G. Zakrzewski, and O. Dolgounircheva, in Conceptual
Perspectives in Quantum Chemistry, Ed. J.-L. Calais, and E. Kryachko,
n/cm 850 (m), 1949 (w, nC=C=C), 2349 (m, nBH), 2396 (s, nPH).
1
1
+
+
HRMS: calcd for C
3
7
H BP [M - H] : 85.03784, found: 85.0382.
(
Kluwer Academic), 1997, 465–517.
3
3
1 T. Veszpr e´ mi and Gy. Zsombok, Magy. Kem. Foly., 1986, 92, 39–40.
2 B. Khater, J.-C. Guillemin, G. Bajor and T. Veszpr e´ mi, Inorg. Chem.,
Acknowledgements
2
008, 47, 1502–1507.
3
3
3 J.-C. Guillemin, G. Bajor, E.-H. Riague, B. Khater and T. Veszpr e´ mi,
Organometallics, 2007, 26, 2507–2518.
4 G. Bajor, T. Veszpr e´ mi, El H. Riague and J.-C. Guillemin, Chem.–
Eur. J., 2004, 10, 3649–3656.
5 A. D. Becke, J. Chem. Phys, 1993, 98, 5648–5652.
6 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785–789.
7 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G.; Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
The authors thank the Hungarian Scientific Research Foundation
(
OTKA T048796) for financial support. This study was supported
◦
by a Balaton project (N K-76806) and N. B. receives a fellowship
from the French government for a joint PhD.
3
3
3
References
1
A. C. Gaumont and B. Carboni, Science of Synthesis, 2004, 6, 485–
12.
5
3
534 | Dalton Trans., 2009, 3526–3535
This journal is © The Royal Society of Chemistry 2009

View Article Online
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian 03 (Revision
C.02) Gaussian, Inc., Wallingford CT, 2004.
38 K. Bourumeau, A.-C. Gaumont and J.-M. Denis, J. Organomet. Chem.,
1997, 529, 205–213.
39 J.-L. Cabioch and J.-M. Denis, J. Organomet. Chem., 1989, 377, 227–
233.
40 J.-C. Guillemin and K. Malagu, Organometallics, 1999, 18, 5259–5263.
41 J.-C. Guillemin, P. Savignac and J.-M. Denis, Inorg. Chem., 30, 2170–
2173.
42 W. L. Jolly, Inorg. Synth., 1968, 11, 124–126.
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3526–3535 | 3535