The Ever-surprising chemistry of boron: Enhanced acidity of phosphine·boranes

Article abstract of DOI:10.1002/chem.200802307

The gas-phase acidity of a series of phosphines and their corresponding phosphine·borane derivatives was measured by FT-ICR techniques. BH ₃ attachment leads to a substantial increase of the intrinsic acidity of the system (from 80 to 110 kJ mol^-1). This acidity-enhancing effect of BH₃ is enormous, between 13 and 18 orders of magnitude in terms of ionization constants. This indicates that the enhancement of the acidity of protic acids by Lewis acids usually observed in solution also occurs in the gas phase. High- level DFT calculations reveal that this acidity enhancement is essentially due to stronger stabilization of the anion with respect to the neutral species on BH₃ association, due to a stronger electron donor ability of P in the anion and better dispersion of the negative charge in the system when the BH₃ group is present. Our study also shows that deprotonation of ClCH₂PH₂ and ClCH ₂PH₂·BH₃ is followed by chloride departure. For the latter compound deprotonation at the BH₃ group is found to be more favorable than PH₂ deprotonation, and the subsequent loss of Cl^- is kinetically favored with respect to loss of Cl ^- in a typical S_N2 process. Hence, ClCH₂PH ₂·BH₃ is the only phosphine·borane adduct included in this study which behaves as a boron acid rather than as a phosphorus acid.

Full text of DOI:10.1002/chem.200802307

DOI: 10.1002/chem.200802307
The Ever-Surprising Chemistry of Boron: Enhanced Acidity of
Phosphine·Boranes
Marcela Hurtado,[a] Manuel Yꢀnez,*[a] Rebeca Herrero,[b] Andrꢁs Guerrero,[b]
Juan Z. Dꢀvalos,[b] Josꢁ-Luis M. Abboud,*[b] Brahim Khater,[c] and
Jean-Claude Guillemin*[c]
Abstract: The gas-phase acidity of a
series of phosphines and their corre-
sponding phosphine·borane derivatives
was measured by FT-ICR techniques.
BH3 attachment leads to a substantial
increase of the intrinsic acidity of the
system (from 80 to 110 kJmolꢀ1). This
acidity-enhancing effect of BH3 is enor-
mous, between 13 and 18 orders of
magnitude in terms of ionization con-
stants. This indicates that the enhance-
ment of the acidity of protic acids by
Lewis acids usually observed in solu-
tion also occurs in the gas phase. High-
level DFT calculations reveal that this
acidity enhancement is essentially due
to stronger stabilization of the anion
with respect to the neutral species on
BH3 association, due to a stronger elec-
tron donor ability of P in the anion and
better dispersion of the negative
charge in the system when the BH3
group is present. Our study also shows
that deprotonation of ClCH2PH2 and
ClCH2PH2·BH3 is followed by chloride
departure. For the latter compound de-
protonation at the BH3 group is found
to be more favorable than PH2 depro-
tonation, and the subsequent loss of
Clꢀ is kinetically favored with respect
to loss of Clꢀ in a typical SN2 process.
Hence, ClCH2PH2·BH3 is the only
phosphine·borane adduct included in
this study which behaves as a boron
acid rather than as a phosphorus acid.
Keywords: ab initio calculations ·
acidity · gas-phase reactions · ion
cyclotron resonance · phosphanes
Introduction
Thus, phosphine·boranes are much less volatile than the cor-
responding free compounds and not at all pyrophoric, and
many other of their physicochemical properties are also dif-
ferent.[1]
Phosphine·boranes are formed by the association of a phos-
phine and a borane, whereby the properties of both the free
phosphine and the free borane are completely modified.
The development of gas-phase ion chemistry in the last
decades of the 1900s led to a significant change in our view
of chemical reactivity. The absence of solute–solvent and
counterion interactions revealed the existence of reactivity
trends which were very different to those usually accepted
and obtained in condensed media. One point of interest was
whether a result observed in the condensed phase carries
over to the gas phase, that is, is an intrinsic effect or a sol-
vent effect. As a consequence, a great deal of effort was
concentrated on determining intrinsic reactivities, in particu-
lar intrinsic basicities and acidities.[2–5]
[a] Dr. M. Hurtado, Prof. Dr. M. Yꢀnez
Departamento de Quꢁmica, C-9
Universidad Autꢂnoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax : (+34)91-497-5238
E-mail: manuel.yanez@uam.es
[b] R. Herrero, A. Guerrero, Dr. J. Z. Dꢀvalos,
Prof. Dr. J.-L. M. Abboud
Instituto de Quꢁmica Fꢁsica Rocasolano, CSIC
C/Serrano, 119. 28006 Madrid (Spain)
E-mail: iqrla78@iqfr.csic.es
The gas-phase acidity of phosphines has been reported[6,7]
and the role played by the substituent on the acidity of the
molecule clearly evidenced. In particular the presence of an
a,b-unsaturated substituent[6] with or without an electron-
withdrawing group[5,7] led to a huge increase in acidity.
The gas-phase acidity of boranes has also attracted some
attention.[8,9] In recent studies[9] it was concluded that, al-
though carbon is more electronegative than boron, BH3 is a
[c] Dr. B. Khater, Dr. J.-C. Guillemin
Sciences Chimiques de Rennes, UMR 6226 CNRS-ENSCR
Avenue du Gꢃnꢃral Leclerc, 35708 Rennes Cedex 07 (France)
Fax : (+33)223-23-81-08
E-mail: jean-claude.guillemin@ensc-rennes.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802307.
4622
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Chem. Eur. J. 2009, 15, 4622 – 4629

FULL PAPER
Materials: Lithium aluminum hydride, aluminum trichloride, bromocyclo-
propane, dichlorophenylphosphine, diethyl methylphosphonate, diethyl
benzylphosphonate, chloromethylphosphonic acid dichloride, diethyl
chlorophosphate, borane·tetrahydrofuran complex solution, and tetra-
ethylene glycol dimethyl ether (tetraglyme) were purchased from Al-
drich. All experiments at atmospheric pressure were performed under ni-
trogen.
slightly stronger acid than methane, reflecting the fact that
the electron affinity of the BH2 radical is greater than that
of the methyl radical.[9] Quite unexpectedly, for boron a dra-
matic increase in acidity occurs on methyl substitution, and
methylborane is predicted to have an intrinsic acidity almost
200 kJmolꢀ1 larger than that of BH3, but also much larger
than that of methane, in spite of being a carbon acid. This
acidity enhancement reflects the large reinforcement of the
General: 1H (400 MHz), 13C (100 MHz), 31P (162 MHz), and 11B
(128.4 MHz) NMR spectra were recorded on a Bruker ARX400 spec-
trometer. Chemical shifts are given in parts per million (d) relative to tet-
ꢀ
C B bond, which on deprotonation becomes a double bond
ramethylsilane (1H), solvent (13C, d
ACTHNUTRGEN(UNG CDCl3)=77.0 ppm), external 85%
H3PO4 (31P NMR), and external BF3·Et2O (11B NMR). The NMR spectra
were recorded in CDCl3. High-resolution mass spectrometry (HRMS)
was performed on a Varian MAT 311 instrument.
through donation of the lone pair created on the carbon
atom into the empty p orbital on boron. For the same
reason, saturated and a,b-unsaturated boranes are much
stronger acids than the corresponding hydrocarbons, in spite
of also being carbon acids.[9]
Preparation of 1–10: Phosphine·boranes 6–10 were prepared starting
from free phosphines 1–5. The preparation of methylphosphine (1),[19]
phenylphosphine (3),[20] benzylphosphine (4),[20] chloromethylphosphine
(5),[21] methylphosphine·borane (6),[22] phenylphosphine·borane (8),[22]
and diborane[23] has already been reported, but several synthesis were
partially modified in this work. Cyclopropylphosphine (2)[24] was pre-
pared by reduction of the corresponding diethyl cyclopropylphosphonate
11. Experimental procedures for 1–6, 8, and 11 are given in the Support-
ing Information.
The formation of a complex between a primary phosphine
and borane will completely change the acidity of the hydro-
gen atoms on the phosphorus and boron atoms. Moreover
the role of the substituent could be completely changed by
the presence of the boron compound in the molecule. Thus,
ten years ago, the gas-phase negative-ion chemistry of a ter-
tiary phosphine·borane was investigated[10] and, in the ab-
sence of hydrogen atoms on the phosphorus atom,
Me3P·BH3 was found to be a stronger acid than BH3, by
712 kJmolꢀ1. In the gas phase, the Lewis acid behavior of
trimethylborane towards anions has long been known,[11]
and addition of alkoxide anions to borate esters has been re-
ported.[12]
The aim of this paper is to investigate the intrinsic acidity
of a suitable set of primary phosphine·boranes by means of
Fourier transform ion cyclotron resonance (FT-ICR) spec-
troscopy[13–17] and DFT calculations, to rationalize the role
played by complexation in the acidity of the complex. Five
phosphines 1–5 and the corresponding phosphine·boranes 6–
10 were selected for this study on the basis of molecular di-
versity. Methyl derivatives 1 and 6 are examples of alkyl sys-
tems, and phenyl derivatives 3 and 8 of aryl compounds. Cy-
clopropyl derivatives 2 and 7 and benzyl derivatives 4 and 9
were selected for the potential interaction between the rings
Cyclopropyl- (7) and chloromethylphosphine·borane (10): A solution of
BH3·thf or BH3·Me2S (5 mL, 1m, 5 mmol) was slowly added to a previ-
ously frozen (ꢀ1968C) solution of phosphine 2 or 5 (5 mmol) in dry di-
chloromethane (5 mL). The reaction mixture was allowed to warm to
room temperature and was stirred for 5 min at this temperature. The mix-
ture was then distilled off on a vacuum line and the corresponding phos-
phine·borane 7 or 10 was selectively condensed in a trap cooled to
ꢀ408C (0.1 mmHg). This cell was then disconnected from the vacuum
line by stopcocks and attached to the mass spectrometer. 7: Yield: 95%
(based on the free phosphine). 1H NMR (CDCl3, 258C): d=0.25 (qt, 1J-
ACHUTNGRENNUG CAHTUNGTERN(NUGN H,H)=7.5 Hz, 3H, BH3), 0.70–0.97 (m, 5H, cyclo-
(B,H)=98.8 Hz, 3J
propyl), 4.78 ppm (dq, 2J(H,P)=370 Hz, 3J
ACTHNUGTRENNUGN ACHTUNGTRENNNUG
31P NMR (CDCl3, 258C): d=ꢀ43.2 ppm (q, 1J
ACHTUNGTRENNUNG
(CDCl3, 258C): d=ꢀ5.9 (1J
(C,H)=170.1 Hz (t), 1J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
CH2), 3.2 ppm (1J(C,H)=167.6 Hz (d), 2J
ACTHNUGTRENNUGN ACHTUNGTRENNNUG
11B NMR (CDCl3, 258C): d=ꢀ45 ppm; IR (film, 77 K): n˜ =1045 (s), 1420
(s), 2348 (s, nBH), 2398 (s, nPH), 2959 (s), 3088 cmꢀ1 (w); HRMS calcd for
C3H10BP+: 88.0613; found: 88.062. 10: Yield: 93% (based on the free
phosphine). 1H NMR (CDCl3, 258C): d=0.20–1.10 (q, 1J
ACHTUNGTRENNUNG
3J
(H,H)=7.1 Hz, 3H, BH3), 3.76 (m, 3J
E
ACHTUNGTRENNUNG
2H, CH2), 5.01 ppm (dtq, 1JPH =380 Hz, 3J
ACHTUNGTRENNUNG
PH2); 31P NMR (CDCl3, 258C): d=ꢀ28.5 ppm (tq, 1J
ACHTUNGTRENNUNG
13C NMR (CDCl3, 258C): d=28.0 ppm (1J(C,P)=31.2 Hz (d), 1J
ACTHNUGTRENNUGN ACHTUNGTRENNUNG
ꢀ
and the C P bond. Chloromethylphosphine 5 and its borane
153.4 Hz (t), CH2); 11B NMR (CDCl3, 258C): d=ꢀ42.6 ppm; IR (film,
77 K): n˜ =871 (s, nCCl), 1470 (m), 2349 (m, nBH), 2396 (s, nPH), 2846 (w),
2915 (s), 2963 cmꢀ1 (w); HRMS calcd for CH711B35ClP+: 96.0067; found:
96.007.
complex 10 were chosen as compounds bearing a chlorine
atom, the former being a precursor of the simplest phos-
phaalkene (H2C=PH) under basic conditions.[18]
Benzylphosphine·borane (9): A solution of BH3·thf or BH3·Me2S (6 mL
of 1m sol., 6 mmol) was slowly added to a previously frozen (ꢀ1968C)
solution of phosphine 4 (5 mmol) in dry dichloromethane (5 mL). The re-
action mixture was allowed to warm to room temperature and stirred for
20 min at this temperature. The low-boiling compounds were then re-
moved in vacuo and the crude mixture was directly used without further
purification. Attempts to purify the crude compound by distillation on a
vacuum line (0.1 mmHg) led to a 2:1 mixture of compounds 4:9. Yield:
95% (crude and based on the free phosphine). 1H NMR (CDCl3, 258C):
d=0.60 (qt, 1J
(H,H)=6.6 Hz, 2J
N
ACHTUNGTRENNUNG
E
G
ACHTUNGTRENNUNG
31P NMR (CDCl3, 258C): d=ꢀ39.4 ppm (qt, J
ACHTUNGTRENNUNG
1
(CDCl3, 258C): d=23.5 (1J(C,H)=131.9 Hz (t), 1J
ACTHNUGTRENNUGN ACHTUNGTRENNNUG
Experimental Section
CH2), 126.7 (1J
(C,H)=159.2 Hz (d), 3J
(d), 4J
(C,H)=161.4 Hz (d), 5J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
T
E
ACHTUNGTRENNUNG
Caution: Phosphines and phosphine·boranes are malodorous and poten-
tially toxic compounds. All reactions and handling should be carried out
under a well-ventilated hood.
G
ACHTUNGTRENNUNG
Chem. Eur. J. 2009, 15, 4622 – 4629
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www.chemeurj.org
4623

J.-C. Guillemin, J.-L. M. Abboud, M. Yꢀnez et al.
Table 1. Experimental and calculated GA values[a] for compounds 1–10.
1600 (w, nC C), 2358 (s, nBH), 2398 (s, nPH), 3029 (m, nCH), 3231 cmꢀ1 (s);
=
GAACTHUNGTRENNUG ACTHUNGTRENNUNG
(RPH2)[b] GA(RPH2·BH3)[c] ꢀdGA[d]
ꢀDrG10[d] ꢀDrG04[d]
HRMS calcd for C7H1211BP+: 138.0770, found: 138.077.
R
FT-ICR spectrometry: A modified Bruker CMS 47 FT-ICR mass spec-
trometer was used. A detailed description of the original instrument is
given in reference [10]. It has already been used in a number of recent
studies.[25–27] Some salient features are as follows: the spectrometer is
linked to an Omega Data Station (IonSpec, CA), high vacuum is provid-
ed by a Varian TURBO V550 turbomolecular pump (550 Lsꢀ1), and the
magnetic field strength of the superconducting magnet is 4.7 T.
C6H5
1457.3ꢂ0.8
1375.0ꢂ2.5
82.3
76.0
113.4
115.1
101.1
105.8
118.1
120.5 (123.4)
52.1
50.8
49.3
56.0
55.7
128.8
162.8
163.2
176.2
1455.1
1379.1
C6H5CH2 1493.8ꢂ0.9
1380.4ꢂ2.5
1382.8
1495.3
c-C3H5
CH3
1510.0ꢂ3.0
1408.9ꢂ2.8
1507.8
1402.0
1530.0ꢂ2.5
1411.9ꢂ2.3
1530.7 (1528.2) 1410.2 (1404.8)
Determination of gas-phase acidities (GA): The GA value of a protic
acid AH, GA(AH), is the standard Gibbs energy change for reaction (1),
DrG0m(1).[28]
ClCH2
1364.3[e]
1312.2[e]
[a] All values in kJmolꢀ1. Calculated values, obtained at the B3LYP/6-311++
(3df,2p) level, are given in italics. Values in parentheses were calculated at
GACHTUNGTRENNUNG
HAðgÞ ! HþðgÞ þ AꢀðgÞ DrG0mð1Þ
ð1Þ
the G3X level. [b] Uncertainties taken as twice the absolute deviation.
[c] The reported uncertainties do not include those originating from dissocia-
tion of the adducts. [d] Defined in the text. [e] These values should be taken
as apparent acidities, because they do not correspond strictly speaking to the
process defined by Equation (1), since the deprotonation process triggers
chloride departure.
Gaseous mixtures of the phosphine RPH2 (or the adduct RPH2·BH3) and
a reference acid ArefH of known gas-phase acidity were introduced into
the high-vacuum section of the instrument. Typical partial pressures were
in the range 2ꢅ10ꢀ8 to 1ꢅ10ꢀ7 mbar. The average temperature of the cell
was 331 K. Isoamyl nitrite (iso-C5H11NO2) containing about 20% of
methanol was added (nominal pressures of (2–3)ꢅ10ꢀ8 mbar). Resonant
capture of electrons (nominal energies of 0.8 eV) by iso-C5H11NO2 and
CH3NO2 provided a mixture of the anions iso-C5H11Oꢀ and CH3Oꢀ. If
the total pressure was less than 2ꢅ10ꢀ7 mbar, argon was added up to a
total pressure on this order. After reaction times of 5–12 s, all iso-
C5H11Oꢀ and CH3Oꢀ were protonated by RPH2, RPH2·BH3, and/or
action (5) and assumed that the experimental readings of pressure corre-
spond to those of pure adducts RPH2·BH3(g). At least formally, this sug-
gests that the acidities thus determined are upper limits for the true GA
values.
Generally, when these problems are absent, the standard deviation for
the experimental GA values determined by averaging the results ob-
tained with at least three different references, ranges between 0.8 and
ꢀ
ArefH. In all cases, and whenever the couples RPHꢀ/Aref or
ꢀ
(RPH·BH3)ꢀ/Aref coexisted for periods ranging between 30 and 60 s, one
1.2 kJmolꢀ1. The values of GA
ACHTNUGTRENNUG(RPH2) and GAACHTUNGTREN(NUGN RPBH5), obtained as in-
of these ions was isolated by means of ion-selection techniques and al-
lowed to react with the neutral species, and the system was monitored
for up to 60 s in some cases. In all cases, it was established that reac-
tion (2) had reached a state of equilibrium during this time.
dicated above, are summarized in Table 1 and full details are given in Ta-
bles S1 and S2 in the Supporting Information. While the precision is satis-
factory, the accuracy is generally less so, and in the NIST database[28b] the
total uncertainty is generally taken as 8.4 kJmolꢀ1 to allow for anchoring
and other problems (full details are given in the Supporting Information).
We believe that this is a conservative estimate in our case.
RPH2 ꢁ BH3ðgÞ þ ArefꢀðgÞ ! ðRPH ꢁ BH3ÞꢀðgÞ þ ArefHðgÞ Kpð2Þ DrG0mð2Þ
ð2Þ
As discussed in detail below, deprotonation of 5 and 10 is followed by de-
parture of Clꢀ, and a true GA cannot be determined. Nevertheless, we
monitored the formation of chloride ion as a function of the gas-phase
acidity of the reference acids. As the acidity of the latter increases, this
becomes increasingly difficult because of the importance of the formation
of [(Aref)2H]ꢀ ions (our spectrometer is not fitted with an external ion
source). Therefore, although we report below some “limiting” values of
the acidities of reference acids leading to the formation of Clꢀ, it is clear
that these values must be considered with caution and only as reasonable
lower limits of the acidities of the corresponding ArefH.
Thus, from experiment to experiment, the limiting ratio of abundances of
the two ions remained constant within 5%, irrespective of whether ions
Arefꢀ, RPHꢀ, or (RPH·BH3)ꢀ were selected. In other experiments, no se-
lection was carried out, but the ratio was the same, within these limits.
Experimental GA values are determined from GA
through Equation (3).
ACHTUGNERTN(NUNG ArefH) and Kp(2)
GAðRPBH5Þ ¼ GAðArefHÞꢀRT ln Kpð2Þ
ð3Þ
Kp(2) involves the partial pressures P of the neutral and ionic species
taking part in reaction (2) and is given by the product of the two terms in
parentheses in Equation (4).
Computational Details
Kpð2Þ ¼ ðPA H=PRPHꢁBH ÞðPRPHꢁBH
ꢀ
=PA ꢀÞ
ð4Þ
Standard DFT and high-level ab initio calculations on the various systems
under study were performed with the Gaussian 03 suite of programs.[30]
These systems have three potential acidic sites, namely, CH, BH, and PH
bonds. In our survey we considered all possibilities and all possible con-
formers. In all cases, with the sole exception of ClCH2PH2·BH3, the most
stable anion was obtained by deprotonation of the PH2 group. For the
sake of simplicity, in the discussion which follows we consider exclusively
the most stable anion in its most stable conformation. All geometries
ref
3
3
ref
The pressures of the neutral species were measured with a Bayard–
Alpert ion gauge. Its readings were corrected according to the method of
Bartmess and Georgiadis;[29] full details are given in the Supporting Infor-
mation. The ratio of the pressures of the ionic species was taken as the
ratio of the relevant ion intensities.
It is unfortunate that both adducts partially dissociate in the gas phase,
and this phenomenon is more important in the case of the neutral species
[reaction (5)].
were optimized at the B3LYP/6-31+GACTHNUTRGNEUGN(d,p) level, which usually yields
good geometries with low computational demand. Harmonic vibrational
frequencies were obtained at the same level of accuracy to assess wheth-
er the structures found correspond to local minima of the potential-
energy surface and to evaluate the zero-point energy (ZPE) and other
thermal corrections. To obtain reliable energies, single-point calculations
RPH2 ꢁ BH3ðgÞ ! RPH2ðgÞ þ BH3ðgÞ Kpð5Þ
ð5Þ
at the B3LYP/6-311++GACTHNUTRGENUG(N 3df,2p) level were carried out. Although the
Hence, in the absence of a reliable experimental value for Kp(5) it is not
possible to accurately determine Kp(2). We thus ignored the effect of re-
level of theory used has been found to be reliable for the determination
of both intrinsic basicities and acidities, we assessed our model by com-
4624
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Chem. Eur. J. 2009, 15, 4622 – 4629

Acidity of Phosphine·Boranes
FULL PAPER
paring the calculated B3LYP/6-311++GACTHNUGTRNEUNG(3df,2p) acidities with those ob-
In all cases, the phosphines are significantly less acidic
than their BH3 adducts. This follows from the observation
that their corresponding anions RPHꢀ are fully re-protonat-
ed by the neutral adducts. The calculated GA values nicely
agree with the experimental results. The agreement between
experimental and calculated values is very good, the aver-
aged absolute deviation of 2.7 kJmolꢀ1 being on the same
order as the experimental error. The agreement between
G3X calculated values and the experimental data is slightly
worse than that obtained for DFT values.
tained with G3X theory.[31] For this assessment we chose compounds 1
and 6 as suitable model systems. As we discuss below, the differences be-
tween the two models are never larger than 4 kJmolꢀ1, but more impor-
tantly, G3X values are not superior to the B3LYP values in comparison
with experimental data.
To gain some insight into the electronic structure and bonding of the sys-
tems under investigation, we used the theory of atoms in molecules
(AIM)[32] and the electron localization function (ELF).[33] In the frame-
work of the former we evaluated the electron density at the different
bond critical points (BCP), which will help us to understand the changes
which occur on going from the neutral to the anionic compounds. The
ELF[33] is a function which becomes large in regions of space where elec-
tron pairs, either bonding or lone pairs, are localized. The function is con-
veniently scaled between [0,1], and thereby maps from the very low (0)
to very high (1) electron localization regimes. In this way it is possible to
locate electron localization basins defined by isosurfaces corresponding
to an ELF value around 0.87. The attraction basins of ELF have been
successfully related to key bonding concepts, such as core, valence, and
lone-pair regions, while their populations and synaptic orders have been
related to bond order. ELF grids and basin integrations were computed
with the TopMod package.[34]
Collision-induced decomposition (CID) of (R-PBH4)ꢀ
(using argon as the target gas) produced by on-resonance
(structural assignment) or slightly off resonance (SORI)[35]
(lowest energy pathway for decomposition) excitation of
these ions (in all cases using the lowest possible energy)
cleanly leads to the formation of RPHꢀ as the sole ionic
product. We take this as implying that 1) the acidic proton
ꢀ
belongs to the phosphine moiety and, 2) the P B bond is
most likely the weakest bond in these ions. As we discuss
below, computational results confirm that in this family of
compounds, deprotonation of the PH2 group is thermody-
namically favored over that of BH3, with the sole exception
of ClCH2PH2·BH3.
Results and Discussion
Syntheses of phosphines 1–5 by reduction of the correspond-
ing phosphonates or dichloro phosphines have already been
reported.[20–22] Phosphine·boranes 6–10 were prepared by ad-
dition of BH3 to free phosphines 1–5 (Scheme 1). Pure di-
In line with these observations, our calculations show that
in all cases, with the sole exception of ClCH2PH2·BH3,
which will be discussed below, all systems investigated
behave as phosphorus acids, and loss of the proton from the
BH3 group or a CH group is 180–250 or 50–150 kJmolꢀ1 less
favorable.
Deprotonation of ClCH2PH2 (5) does not lead to the ob-
servation of its corresponding anion (ClCH2PH)ꢀ. In all
cases, the product of the reaction between ClCH2PH2 and
the anion of the reference acid is chloride ion. This suggests
that the behavior in the gas phase is similar to that in solu-
tion.[18] Interestingly, this reaction is observed even in the
presence of the anion of p-hydroxybenzaldehyde, a very
strong acid endowed with a rather weak conjugate base in
the gas phase (GA=1364.4 kJmolꢀ1). This suggests that
chloride departure involves an elimination process.[36] This is
corroborated by our DFT calculations, which show that de-
protonation of the PH2 group leads to a complex in which a
Clꢀ ion is weakly bound to one of the H atoms of the meth-
ylidenephosphine (see Figure 1). Hence, under experimental
conditions, deprotonation of ClCH2PH2 is indeed followed
by chloride departure and the formation of methylidene-
phosphine. It is worth noting that the apparent acidity calcu-
Scheme 1.
borane was used with the volatile derivative (1!6) and a so-
lution of BH3·THF or BH3·Me2S with phosphines that
formed complexes easily separated from solvents by distilla-
tion (7–10). Three new compounds were prepared in this
study: cyclopropyl- (7), benzyl- (9) and chloromethylphos-
phine·borane (10). They were easily characterized by NMR
spectroscopy and mass spectrometry. In the 31P NMR spec-
trum, the downfield chemical shift [d=ꢀ28.5 (7), ꢀ39.4 (9),
1
ꢀ42.6 ppm (10)] and coupling constant JP, H around 380 Hz
are characteristic of these compounds.[22,23] The 11B NMR
chemical shift of about d=ꢀ42 ppm and observation of the
molecular ion by high-resolution mass spectrometry confirm
the structures. The optimized geometries of neutral and de-
protonated compounds as well as the calculated energies are
summarized in Tables S3 and S4 of the Supporting Informa-
tion, respectively.
The experimental values of GA
(RPH2·BH3), obtained as indicated above, are given in
Table 1, together with the B3LYP/6-311++G(3df,2p) calcu-
ACHTUNGRTENUN(NG RPH2) and GA-
Figure 1. Optimized structures of the deprotonated species of compounds
5 and 10. Structure (5-H)P is the P-deprotonated species of 5. Structures
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTERN(NUNG 10-H)B correspond to the P and B deprotonation of com-
lated values.
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4625

J.-C. Guillemin, J.-L. M. Abboud, M. Yꢀnez et al.
lated for compound 5 is equal to that of p-hydroxybenzalde-
hyde, which defines a lower limit for this magnitude. While
this extraordinary agreement is possibly fortuitous, it seems
to indicate that Clꢀ departure triggered by deprotonation of
5 involves a very low activation barrier. In fact our calcula-
Nevertheless, the observed loss of Clꢀ could be the result
of a simple SN2 reaction,[36] which in principle cannot be
ruled out. To investigate whether this possibility can com-
pete with the deprotonation process, we studied both mech-
anisms at the B3LYP/6-311+GACHTUGNTREN(NUNG 3df,2p)//B3LYP/6-31+G-
tions on the deprotonated species evolve to form
without an activation barrier.
(5-H)P
ACHTUNGTRENNUNG
model reactant.
We observed the same behavior for deprotonation of
ClCH2PH2·BH3 (10), that is, its anion was not detected
under any circumstances. Chloride ion is observed even in
the presence of the anion of o-toluic acid (GA=
1384.5 kJmolꢀ1). Again, this suggests the existence of two
reactive channels leading to formation of Clꢀ.[36] The theo-
retical survey of the possible deprotonation processes of this
compound shows that deprotonation of the PH2 group is en-
ergetically less favorable than deprotonation of the BH3
group. This theoretical finding agrees with experimental ob-
servations which suggest that also in this case deprotonation
of the system triggers chloride departure. In fact, as illustrat-
ed in Figure 1, in the less stable anion produced by deproto-
nation of the PH2 group, the Cl atom remains attached to
the carbon atom. Conversely, deprotonation of the BH3
group involved a drastic internal reorganization of the
Scheme 2.
Our results (see Table S5 of the Supporting Information)
indicate that reaction (6) is 80 kJmolꢀ1 more exothermic
than reaction (7). This means that the SN2 nucleophilic sub-
ꢀ
system, associated with cleavage of the P B bond and for-
ꢀ
mation of a rather strong C B bond with non-negligible
ꢀ
double-bond character, and cleavage of the C Cl bond. The
consequence of this drastic structural rearrangement is that
the most stable anion can be viewed as the interaction be-
tween Clꢀ and one of the H atoms of the PH2 group of the
neutral H2BCH2PH2 moiety (see Figure 1). In this case the
calculated apparent acidity of the system (1312.2 kJmolꢀ1) is
significantly different from that of o-toluic acid, a rough ex-
perimental estimate of the lower limit of the acidity of 10.
This energy difference is likely used to overcome the activa-
tion free-energy barrier associated with the geometrical re-
arrangement indicated above. In this respect, it is important
to note that the acidity of compound 10 when it behaves as
stitution is thermodynamically favored over the deprotona-
tion process. This difference becomes much smaller
(35 kJmolꢀ1) in terms of free energy, due to significant dif-
ferences between the entropy changes occurring in the two
reactions, which favor the deprotonation process. This is,
however, only part of the relevant information, since it is
necessary to know whether the SN2 reaction is also kinetical-
ly favored. To answer this question we also located the tran-
sition states of both reactions (see Figure 2).
a
P acid to yield anion CAHTUNTGRENUN(G 10-H)P (see Figure 1) of
1358.6 kJmolꢀ1 is much closer to this experimental lower
limit. Hence, one may perhaps infer that even though struc-
ture
tonation process takes place at the PH2 group, and this is
eventually followed by a rearrangement connecting (10-H)P
with (10-H)B. This kind of rearrangement, which would
ACHTUNGTRENNUNG(10-H)B in Figure 1 is the most stable anion, the depro-
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
imply an H shift from the BH3 to the PH group. It often
occurs via ion/neutral complexes and involves barriers
which are below the energy of the separated products, al-
though they are entropically disfavored. We have located
the corresponding transition state, which lies quite high in
Figure 2. Structures of the transition states involved in the deprotonation
(TSdeprot) and the SN2 reaction (TSSN2) between CNꢀ and ClCH2PH2·BH3.
energy (234 kJmolꢀ1 above
ACTHNUGTRENNG(U 10-H)P), that is, although ACHUTNGTREN(NUGN 10-
H)B is the most stable deprotonated form of 10, it is un-
reachable under the current experimental conditions.
Hence, we may in principle conclude that the ability of
these compounds to lose Clꢀ indicates that ClCH2PH2·BH3
is the only phosphine·borane complex included in this study
which should behave as a boron acid rather than as a phos-
phorus acid.
In the TS for the deprotonation process proton transfer
from the BH3 group towards the CNꢀ ion triggers departure
of a Clꢀ ion. Similarly, in the TS for the SN2 reaction, attach-
ment of CNꢀ leads to departure of Clꢀ. The important
result, however, is that this TS is estimated to be 45 kJmolꢀ1
higher in energy than that associated with the deprotonation
4626
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Acidity of Phosphine·Boranes
FULL PAPER
process. This difference becomes even higher (49 kJmolꢀ1)
in terms of free energy. Hence, although the SN2 reaction
leads to products which are more stable than those associat-
ed with the deprotonation process, the reaction is much less
favorable from a kinetic viewpoint, so in principle it cannot
be repudiated that the observed loss of Clꢀ is indeed that as-
sociated with the deprotonation process rather than with the
SN2 reaction.
same trend is observed in the electron densities at the B P
ꢀ
BCP, and accordingly there is a rather good linear correla-
tion between DrG04 and these densities (see Figure 3).
As mentioned above, for a given R, RPH2·BH3 is always a
stronger acid than RPH2. Although substantial, the effect is
not constant. It appears that while C6H5PH2 and its adduct
are the most acidic species, the acidity-enhancing effect of
BH3 as measured by dRGA is the smallest in the series. The
opposite holds in the case of methylphosphine and its
adduct. These facts can be analyzed as follows:
Figure 3. Linear correlation between DrG04 and the electron density at the
Consider thermodynamic cycle I; Equation (8) holds.
ꢀ
B P BCP (1BP).
DrG10 þ DrG20ꢀDrG03ꢀDrG40 ¼ 0
ð8Þ
Two factors seem to be responsible for the greater stabili-
zation of the anionic species on association with BH3. As il-
lustrated in Figure 4, taking 1 and 6 and their corresponding
deprotonated forms as suitable model systems, deprotona-
Since DrG02 =GA(RPH2·BH3) and DrG03 =GA
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
tion (9) follows
ꢀ
tion of 1, which changes a P H bond in the neutral species
dRGA ¼ DrG04ꢀDrG01
ð9Þ
to a lone pair in the anion, leaves an electron-rich P atom,
as reflected in a large population of the corresponding V(P)
basin. Accordingly the anion behaves as a better electron
donor towards the BH3 moiety. On the other hand, the pres-
ence of the BH3 group contributes to the dispersion of the
negative charge of the anionic system, because the hydrogen
atoms of this group can easily accumulate a larger net nega-
where dRGA is always negative and so are DrG01 and DrG40.
The difference between the latter DrG0 values measures the
difference between the “borane basicities” of RPH2 and
RPHꢀ. In other words, the acidity-enhancing effect of BH3
increases with increasing strength of the bond between
RPHꢀ and BH3 (increasing the absolute value of DrG04) and
is weakened by a strengthening of the bond between RPH2
and BH3 (increasing the absolute value of DrG01). The calcu-
lated values for DrG40 and DrG01 clearly show that the first
effect dominates, and the anion is always more stabilized by
complexation than the neutral species. This is in agreement
ꢀ
with the fact that the electron density at the B P BCP is
larger in the anion than in the neutral system. Our calculat-
ed values fulfill Equation (9) with an average deviation of
1.2 kJmolꢀ1. In the series R=Me, C6H5CH2, c-C3H5, C6H5,
the possibility of internal charge delocalization by resonance
increases in this order.[36–39] This charge delocalization can
reduce the availability of the lone pair(s) to yield both
RPH2·BH3 and (RPH·BH3)ꢀ, and thus reduce the absolute
values of DrG01 and DrG40. The results presented in Table 1
show that DrG01 is not very sensitive to substituent effects.
Consistently, the differences between the electron densities
ꢀ
at the corresponding B P BCPs are negligibly small. On the
other hand, the absolute values of DrG04 are larger and also
strongly depend on the resonance interaction between the
substituents and the reactive centers. As expected, the abso-
lute values of DrG40 decrease in the order Me>C6H5CH2 ꢃc-
C3H5 >C6H5. This is consistent with the antagonic effects of
resonance interactions indicated above. The methylene
group also efficiently reduces the resonance interaction be-
tween the phenyl group and the phosphine moiety. The
Figure 4. Three-dimensional representations of ELF isosurfaces with
ELF=0.80 for compounds 1 and 6 and their corresponding deprotonated
species. Lobes denoted as VACTHUNGTREN(NNUG C,H), VACHTUNGTREG(NNUN P,H), VACHTNUGTRENN(UGN B,H) correspond to basins
associated with CH, PH, and BH bonds. Lobes denoted VACTHNUTRGENUG(N B,P) corre-
ꢀ
spond to basins associated with B P bonds. Lobes denoted as V(P) corre-
spond to basins associated with P lone pairs. The populations of these
basins are also shown.
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4627

J.-C. Guillemin, J.-L. M. Abboud, M. Yꢀnez et al.
tive charge, which is apparent when comparing the ELFs of
CH3PH·BH3 and CH3PH2·BH3.
order of 1012. The computed DrG04 values (Table 1) lead to
Kp(10) values of at least 1020.
ꢀ
This ability of BH3 to better stabilize anionic species than
neutral species is also reflected in the dissociation energies
ꢀ
ꢀ
of the P B bonds. As we mentioned above, B P bond cleav-
age in neutral and anionic systems is measured by DrG01 and
DrG04, respectively. However, the values reported in Table 1
include relaxation of the two fragments. For instance, the
value calculated for CH3PH2·BH3 implies that the two frag-
ments CH3PH2 and BH3 are in their equilibrium conforma-
tion. Although the structural changes of the phosphine on
complexation are very small, it is clear that the BH3 group
within the complex is far from being in its equilibrium con-
formation, since it is strongly pyramidalized. This implies
that the dissociation energy, usually defined as the energy
difference between the complex and the two subunits at in-
finite separation, includes the relaxation energy of the BH3
group, which is significant. Hence, a more realistic estimate
Conclusions
Our experimental and theoretical study on a series of phos-
phines and their phosphine·borane adducts showed that BH3
attachment leads to a substantial increase (from 80 to
110 kJmolꢀ1) in the intrinsic acidity of the system. This acid-
ity-enhancing effect of BH3 is enormous, between 13 and 18
orders of magnitude in terms of ionization constants. This
indicates that the enhancement of the acidity of protic acids
by Lewis acids observed in solution[42] also occurs in the gas
phase. Density functional theory calculations reveal that this
acidity enhancement reflects the greater stabilization of the
anion with respect to the neutral species on association with
BH3, due to the greater electron-donor ability of P in the
anion and better dispersion of the negative charge within
the system when the BH3 group is present. Our study also
shows that deprotonation of ClCH2PH2 and ClCH2PH2·BH3
is followed by chloride departure. Although the loss of Clꢀ
could be also associated with a SN2 reaction, we have shown
that this process is much less favorable from the kinetic
viewpoint than the deprotonation reaction. Accordingly, it is
reasonable to expect the observed Clꢀ departure to be asso-
ciated with deprotonation of the system, which would imply
that ClCH2PH2·BH3 is the only phosphine·borane adduct in-
cluded in this study which should behave as a boron acid
rather than as a phosphorus acid. Although chloride depar-
ture impeded measurement of the acidity of these two com-
pounds, the experimental evidence is consistent with the
theoretical analysis.
ꢀ
of the B P bond strength would be obtained without allow-
ing relaxation of the fragments.[40] When this is done, the
dissociation energies increase by about 50 kJmolꢀ1 for the
neutral systems, and by about 85 kJmolꢀ1 for the anions.
More importantly, since, as we mentioned above, the geome-
try distortion undergone by the phosphine moiety is not sig-
nificant, more than 95% of this energy increase comes from
the relaxation of the BH3 moiety. In other words, the relaxa-
tion energy of the BH3 moiety, measured as the energy dif-
ference between the BH3 group in the complex and the BH3
group in its equilibrium geometry, is on the order of 80–
89 kJmolꢀ1 when the complex is an anion, and only
51 kJmolꢀ1 for the neutral complex.
Finally, we draw attention to the fact that, in all cases, the
acidity-enhancing effect of BH3 in the systems studied
herein, is enormous, between 13 and 18 orders of magnitude
in terms of ionization constants.[41] The enhancement of the
acidity of protic acids by Lewis acids in solution is known to
lead to extremely powerful acidic systems such as magic
acid.[42] Here we show that similar situations can be found in
the gas phase.
Acknowledgements
This work has been supported by Grants CTQ 2006-10178/BQU and
BQU2003-05827 from Ministerio de Educaciꢂn y Ciencia (CSIC) , the
CNES and PCMI (INSU-CNRS) (ENSCR), by the DGI Project No.
BQU2003-00894, Consolider on Molecular Nanoscience, CSD2007-00010,
and by the Project MADRISOLAR, Ref.: S-0505/PPQ/0225 of the Co-
munidad Autꢂnoma de Madrid. A generous allocation of computing time
at the CCC of the UAM is also acknowledged.
The fact that the calculated GA values for the adducts are
in reasonable agreement with the experimental data, implies
that the extent of dissociation of (RPH·BH3)ꢀ(g) is relative-
ly small. Consider now the ratio 1 of the pressures of RPHꢀ
and (RPH·BH3)ꢀ (1=PRPBH
/PRPHꢀ) It can be readily shown
ꢀ
4
that Equation (10) holds
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ð10Þ
3
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Acidity of Phosphine·Boranes
FULL PAPER
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Received: November 6, 2008
Published online: March 9, 2009
Chem. Eur. J. 2009, 15, 4622 – 4629
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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