An Al/P-based frustrated lewis pair as an efficient ambiphilic ligand: Coordination of boron trihalides, rearrangement, and formation of HBX 2 complexes (X = Br, I)

Article abstract of DOI:10.1021/ic5009126

The Al/P-based frustrated Lewis pair (FLP) Mes₂P-C(=CH-Ph)- Al(CMe₃)₂ (1) reacted with boron halides BX₃ (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF ₃ adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr ₃ and BI₃ complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes₂P-(μ-C=CH-Ph) (μ-HBX₂)-AlX(CMe₃) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes₂P-C(=CH-Ph)- AlX(CMe₃) (X = Br, I), with dihalogenboranes, HBX₂. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.

Full text of DOI:10.1021/ic5009126

Article
pubs.acs.org/IC
An Al/P-Based Frustrated Lewis Pair as an Efficient Ambiphilic
Ligand: Coordination of Boron Trihalides, Rearrangement, and
Formation of HBX2 Complexes (X = Br, I)
Werner Uhl,*,† Christian Appelt,† Agnes Wollschlager,† Alexander Hepp,† and Ernst-Ulrich Wurthwein‡
̈
̈
†Institut fur Anorganische und Analytische Chemie, Universitat Munster, Corrensstraße 30, D-48149 Munster, Germany
̈
̈
̈
̈
‡Organisch-Chemisches Institut, Universitat Munster, Corrensstraße 40, D-48149 Munster, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: The Al/P-based frustrated Lewis pair (FLP) Mes2P
C(CHPh)Al(CMe3)2 (1) reacted with boron halides BX3 (X =
F, Cl, Br, I) as an ambiphilic ligand to form complexes (2−5) in which
the boron atoms were coordinated to phosphorus and one of the
halogen atoms to aluminum. Nonplanar five-membered heterocycles
resulted that had five different ring atoms (AlCPBX). The distance of
the bridging halogen atoms to the AlCPB plane increased steadily with
the radius of the halogen atoms. Only the BF3 adduct showed a
dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in
other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for
each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective
rearrangement with the concomitant release of isobutene. The obtained products, Mes2P(μ-CCHPh)(μ-HBX2)
AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2PC(CHPh)
AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable
in the free form. Quantum−chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium
cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal
suprafacial 1,3-sigmatropic rearrangement.
phosphines.11 These reactions verify the strong donor−
INTRODUCTION
■
acceptor capability of these Al/P-based FLPs because the
HAlR2 adducts were formed in the presence of excess alkyne,
while the alkyne adducts were formed despite an excess of the
hydride. Most experiments of our group are based on the Al/P-
based FLP 1 (Scheme 1) that is easily available in a multigram
Frustrated Lewis pairs (FLPs) have found considerable interest
in recent research.1 They have Lewis acidic and basic centers in
single molecules or bimolecular systems and show a unique
chemical reactivity as evident from the efficient coordination
and activation of diverse substrates such as carbon dioxide or
dihydrogen. Only recently Al/P-based FLPs were shown to be
powerful alternatives to the usually applied B/P-based systems.
Because of the inherently high Lewis acidity of aluminum
atoms, these FLPs do not require an activation of the acceptor
atoms by electron-withdrawing fluorinated substituents. They
are excellent reagents for the coordination and activation of
inorganic and organic compounds such as carbon dioxide,2−4
terminal alkynes,2,5 alkenes,6 hydrogen,7 carbonyl compounds,8
or isocyanates.4 They are applicable as ion-pair receptors for the
effective coordination of strongly polar alkali metal hydrides,9 in
unprecedented hydride transfer reactions by phase-transfer
catalysis,9 and as efficient main-group-based catalysts for the
dehydrogenation of ammineboranes.10 Hydroalumination of
alkynylphosphines is a facile method for the generation of such
compounds,2 but steric shielding by bulky groups bonded to
phosphorus and aluminum is required to prevent secondary
reactions such as dimerization via intermolecular Al−P
interactions11 or the formation of persistent adducts with
sterically less shielded dialkylaluminum hydrides or alkynyl-
Scheme 1. Formation of Al/P FLP 1
scale by treatment of dimesityl-phenylethynylphosphine with
di(tert-butyl)aluminum hydride.2 The specific bifunctionality of
these compounds with donor and acceptor sites in single
molecules should allow their application as efficient ligands for
the coordination of polar or even ionic compounds as was
previously shown with alkali metal hydrides,9 dialkylaluminum
hydrides,11 or borane, BH3.10 In this Article, we report on the
reactions of 1 with boron trihalides. In addition to
Received: April 17, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
investigations in the coordination mode and the influence of
the size of the halogen atoms on the structure and stability of
the expected adducts, we were interested in possible secondary
reactions that in view of the well-known alkylation capability of
alkylaluminum groups could result in halogen−alkyl exchange
reactions.
RESULTS AND DISCUSSION
■
Formation of BX3 Adducts. The adducts 2−5 were
obtained by treatment of FLP 1 with BF3·THF or the halides
BCl3, BBr3, and BI3 in toluene below room temperature or −20
°C (Scheme 2). Solutions of 2 (X = F) and 3 (X = Cl) were
Scheme 2. Synthesis of BX3 Adducts 2−5
Figure 1. Molecular structure and numbering scheme of 2 (similar
scheme for 3); displacement ellipsoids are drawn at the 40% level.
Hydrogen atoms with the exception of H2 have been omitted. See
Table 1 for important bond lengths and angles.
thermally stable and could be stored at room temperature or
slightly elevated temperatures without decomposition. The
colorless crystalline products were isolated in yields of greater
than 87% after recrystallization from mixtures of n-hexane and
1,2-difluorobenzene or the pure fluorinated solvent. Com-
pounds 4 (X = Br) and 5 (X = I) in contrast were thermally
unstable and decomposed in solution at temperatures greater
than 0 °C. At −20 °C, they were formed in high purity.
Concentration and storage of the reaction mixtures at low
temperature afforded the colorless products in moderate yields
of 71% and 30%. Owing to the instability of these adducts, the
NMR data were recorded at 0 °C allowing the unambiguous
identification of the products despite the formation of some
minor impurities.
The molecular structures of compounds 2−4 confirmed the
formation of adducts between the bifunctional FLP 1 and BX3
via B−P bonding interactions and B−X−Al bridges (Figures 1
and 2). Five-membered heterocycles were formed that
contained five different ring atoms (BPCAlX). The Al−C
(204 pm), P−C (182 pm), and B−P distances (205 pm, Table
1) in the heterocycles are similar in all compounds. The B−P
bond lengths are relatively long compared to adducts of aryl- or
alkylphosphines with boron halides that usually are between
192 and 202 pm.12 The difference may be caused by ring strain,
steric interactions of BX3 with the bulky mesityl groups, and the
bifunctional coordination of the BX3 molecules via boron and
halogen atoms. All other structural parameters in the backbone
of the adducts correlate with the size of the halogen atoms. The
terminal B−X bond lengths are similar to values observed for
BX3−phosphine adducts,12 with the B−X distances to the
bridging halogen atoms expectedly being lengthened by 13 (X
= F) to 9 pm (X = Br). The Al−X distances are in the upper
range typically observed for Al−X−Al bridges of dialkyl- or
diarylaluminum halides.13 M−X−B bridges are rare in main-
group metal chemistry, possibly as a result of the weak donor
capability of boron-bound halogen atoms. Only a few examples
are reported in the literature, and those are limited exclusively
to the tetrafluorborate anion.14 A broader structural variety was
Figure 2. Molecular structure and numbering scheme of 4;
displacement ellipsoids are drawn at the 40% level. Hydrogen atoms
with the exception of H11 have been omitted. See Table 1 for
important bond lengths and angles.
observed for respective transition metal compounds.15
Interestingly, the angle P−C−Al of BF3 adduct 2 with relatively
short B−F and Al−F bonds is smaller (114.1(1)°) than that of
free FLP 1 (119.0(av)°),2 while larger values of 121.5(1) and
122.9(av)° were detected for 3 (X = Cl) and 4 (X = Br). The
intramolecular P···Al distances increase simultaneously from
322.5 to 339.8 pm (328.7 pm in 1). The unfavorable fit
between the bite of the bifunctional ligand (distance between
donor and acceptor centers of the FLP) and the steadily
increasing B−X and Al−X bond lengths results in an increasing
deviation of the bridging halogen atom from the average plane
of the four remaining ring atoms (BPCAl) and larger torsion
angles Al−C−P−B that may indicate an increasing steric strain
in these molecules. While the fluorine atom is almost in the
same plane as these atoms (only 23 pm above the plane), the
bromine atom deviates by 64 pm. The increasing displacement
of the halogen atoms results in a slight rotation of the
Al(CMe3)2 group in particular in comparison of the Cl and Br
B
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Selected Structural Parameters of BX3 Adducts 2−4 and HBI2 Adduct 7
b
2 (X = F)
3 (X = Cl)
4 (X = Br)
7 (HBI2)
P−B (pm)
B−Xt (pm)
B−Xbr (pm)
Al−Xbr (pm)
P−C−Al (deg)
B−X−Al (deg)
B···Al (pm)
205.3(3)
135.5 (av)
148.8(3)
191.3(2)
204.9(3)
182.0 (av)
193.2(4)
240.0(1)
121.5(1)
107.88(9)
351.3
205.1
199.2
207.9
255.8
122.9
103.1
364.4
339.8
63.8
199.3(3)
220.0(3)
229.7(3)
274.06(9)
121.7(1)
81.77(8)
331.4
c
a
114.1(1)
130.0(2)
308.8
a
P···Al (pm)
322.5
337.2
335.4
deviation of X from plane (pm)
torsion angle Al−C−P−B (deg)
activation barrier
23.2
59.4
152.9
9.3(2)
−11.6(2)
61.8 kJ/mol
15.7
−27.9(2)
−
c
48.3 kJ/mol
Dec.
a
b
For comparison: P−C−Al = 119.0° (av) and P···Al = 328.7 pm (av) in FLP 1. Average values from three independent molecules. Al−I(terminal)
= 256.10(9) pm.
compounds and leads to a relatively short contact between a
methyl group of a CMe3 substituent and the boron atom (B···C
distance is 424 pm in 3 and 413 pm in 4) despite an increasing
B···Al distance. These structural details are important for the
understanding of the dynamic behavior of these compounds in
solution and their reluctance to undergo secondary reactions
(see below). Regrettably, the instability of BI3 adduct 5 in
solution prevented the growing of single crystals and the
determination of its molecular structure.
barrier. The alternating coordination of two halogen atoms by
rotation of the BX3 groups about the P−B bonds may offer a
reasonable alternative explanation for the observed exchange
processes. Equivalent substituents were observed for the BCl3
adduct only upon heating a solution to about +60 °C.
Activation barriers for the rotation of the BX3 groups were
estimated to be 48.3 kJ/mol (X = F) and 61.8 kJ/mol (X =
Cl).16 The latter value should be treated with caution because
slow decomposition of 3 was observed at elevated temperature
that may influence the exchange process. Similar temperature-
dependent NMR experiments failed for BBr3 and BI3 adducts 4
and 5 because they readily rearranged in solution above room
temperature or even at 0 °C.
Formation of HBX2 Adducts 6 and 7. Storing solutions
of BBr3 adduct 4 in benzene at 2 °C for 5 days in a sealed NMR
tube yielded reproducibly in a selective and unprecedented
rearrangement compound 6 with only one tert-butyl group
bonded to aluminum and a hydridic hydrogen atom attached to
boron (Scheme 3). Isobutene was formed as a byproduct and
1
Only BF3 adduct 2 showed the expected simple H NMR
spectrum at room temperature with a singlet for both tert-butyl
groups and a single set of resonances for two chemically
equivalent mesityl groups. Cooling to −60 °C resulted in a
slight broadening of the tert-butyl singlet and a strong
broadening of the resonance of the ortho-methyl groups,
while the signal of the para-methyl groups remained
unchanged. The fluorine atoms showed a broad singlet in the
19F NMR spectrum at room temperature. A splitting into two
resonances with an intensity ratio of 2 to 1 was observed on
cooling a toluene solution to −15 °C. The second resonance
was assigned to the bridging fluorine atom and consists of a
doublet of triplets by coupling to the terminal fluorine atoms
and the phosphorus atom. This observation may be interpreted
in terms of a fast rotation of the BF3 group at room
temperature with an exchange of the fluorine atoms between
terminal and bridging positions. At low temperature, the
structure is fixed with the bridging fluorine atom being part of
an almost planar heterocycle and consequently equivalent tert-
butyl and mesityl substituents. In contrast, a splitting of the tert-
butyl and mesityl resonances was observed for solutions of
adducts 3−5 at room temperature that indicated a rigid
conformation with magnetically inequivalent substituents.
Hindered rotation across the P−C(mesityl) bonds resulted in
a further splitting of the ortho-Me groups into four resonances
in the 1H and 13C NMR spectra. This behavior correlates to the
radii of the heavier halogen atoms and the longer B−X and Al−
X bonds in the Al−X−B bridges that result in an increasing
displacement of the bridging halogen atoms from the plane of
the remaining four ring atoms (see molecular structures). With
these continuous structural changes, the oscillation of the
halogen atoms between both sides of the five-membered rings
becomes energetically unfavorable and requires a drastic
distortion of the bond parameters in the rings for a planar
transition state. The halogen atoms are preferably localized on
one side of the ring, and the equilibration of the substituents at
aluminum and phosphorus is prevented by a high activation
Scheme 3. Rearrangement of Adducts 4 and 5
was identified by its characteristic NMR shifts (δ(1H) = 4.74
and 1.59). All attempts to repeat this reaction on a preparative
scale failed. The reactions did not go to completion, and
inseparable mixtures of compounds were obtained. Compound
6 was identified by NMR spectroscopy as the main component.
A similar product (7) was obtained when FLP 1 was treated
with BI3 at 0 °C. The corresponding adduct 5 could only be
identified as an intermediate on the basis of its characteristic
NMR data. Compound 7 is insoluble in hydrocarbons and was
isolated directly from the reaction mixture as an amorphous
solid in 69% yield. Its NMR spectra had to be recorded in
CD2Cl2 solutions, although slow decomposition was observed
in this solvent at temperatures as low as 0 °C.
Only iodine compound 7 was obtained as a crystalline solid
and characterized by crystal structure determination (Figure 3).
The NMR spectra of 6 and 7 were almost identical, and it is a
C
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
C bonds2,9−11 and are similar in range to those of adducts 2−5.
The terminal B−H hydrogen atoms gave broad resonances at δ
= 5.07 and 4.50, and singlets were observed for the phosphorus
atoms in the 31P NMR spectra at δ = 6.3 and 5.1. It is
interesting to note that even the crude products showed only a
single 31P NMR resonance although there are two stereogenic
centers in these molecules (Al and B) that should give rise to
the formation of diastereomeric molecules. In order to exclude
a fast exchange between both possible isomers (R,R and R,S
plus their enantiomers) by ring opening and fast rotation, we
conducted low-temperature NMR experiments as low as −70
°C. No change was observed except the usual chemical shift
alterations and a lower half-width associated with the
resonances at lower temperature. The results of the NMR
spectroscopic characterization confirm a highly stereoselective
rearrangement and the formation of only a single stereo isomer
(and its enantiomer).
The reaction mechanism was elucidated by density functional
theory calculations. Preliminary geometry optimizations for the
gas phase were performed at the M062x/6-31G(D) level, and
final results were obtained by the M062x/6-311+G(d,p)+GD3
method, which includes the robust M062x functional of Zhao
and Truhlar19 as well as the GD3-dispersion correction as
developed by Grimme et al.20 In order to simulate unspecific
solvent effects, solvent reaction field calculations using the
CPCM polarizable conductor calculation model (solvent =
benzene) at the M062x/6-311+G(d,p)+GD3//M062x/6-
311+G(d,p) level were performed. For all calculations, the
GAUSSIAN09-D0121 package of programs was used. In the
following, we discuss structural properties and relative energies
(kcal/mol) based on the solvent model including zero point
corrections (M062x/6-31G(d)). Natural charges (natural bond
orbital (NBO) charges) were obtained from NBO calcu-
lations22 as implemented in Gaussian09.
We started our calculations with FLP 1 that is characterized
by a relatively long intramolecular nonbonding P−Al distance
of 332.5 pm (X-ray structure analysis: 328.7 pm). Positive NBO
charges were obtained for the Lewis-acidic Al (+1.998) and the
Lewis-basic P atoms (+0.876). Coordination of BBr3 afforded
compound 4 with a five-membered AlCPBBr heterocycle in an
exothermic reaction (−31.8 kcal/mol). The direct comparison
of the calculated total energies of 4 and 6 indicates that the
overall rearrangement reaction is exothermic by −14.5 kcal/
mol (−12.7 kcal/mol for the intermediate van der Waals
complex of 6 and isobutene, C). For the rearrangement of 4 to
6, we first considered a concerted process that at a first sight
seemed to offer an excellent explanation for the experimentally
observed high selectivity. However, the approach of tert-butyl
hydrogen atoms to the boron atom resulted in an almost
continuous increase in energy (up to about >68 kcal/mol) and
resulted in a high energy local minimum (Erel = 30.5 kcal/mol)
that is characterized by a B−H bond, an isolated bromide anion
(Al···Br 537.0 pm), and a substantially enlarged Al−C bond
(232.5 pm) to the leaving isobutene moiety. This not very
reasonable intermediate gives the observed product 6 over a
barrier of only a few kilocalories per mole in a strong
exothermic reaction. This calculated very endothermic reaction
pathway indicates that the negligibly low Lewis acidity of the
four-coordinate boron atom in 4 does not support a concerted
reaction pathway under the experimentally applied conditions.
In the next step, we resorted to mechanisms that were based
on ionic intermediates. BBr3 adduct 4 may be considered as an
internal Lewis acid−base pair (NBO charges: B, −0.250; Al,
Figure 3. Molecular structure and numbering scheme of 7;
displacement ellipsoids are drawn at the 40% level. Hydrogen atoms
with the exception of H1 and H2 have been omitted. See Table 1 for
important bond lengths and angles.
reasonable assumption that both compounds are isostructural.
Compound 7 may be described as an adduct of the unknown
halogenoborane HBI2 with the unique halogen-functionalized
FLP Mes2PC(CHPh)Al(CMe3)(I) in which com-
pared to the FLP 1 a tert-butyl group at aluminum has been
replaced by an iodine atom. Pure HBX2 compounds are
unstable in the condensed phase and have been obtained only
in matrix experiments, in the gas phase, or in equilibrium
mixtures with other boron species (with the exception of X =
I).17 In contrast, adducts with carbene, amine, or thioether
ligands are stable. They have been characterized by crystal
structure determination; some are commercially available and
have been used in hydroboration reactions.18 We did not find
any reference on HBI2. Hence, adduct 7 is a unique compound
in which unknown diiodoborane is stabilized by coordination to
our bifunctional FLP 1. Compounds 6 and 7 may be formed by
a halogen−tert-butyl exchange between boron and aluminum,
while the B−H bond is formed by β-hydride elimination. This
is the first time that the di(tert-butyl)aluminum group of FLP 1
reacted as an effective alkylating reagent to form new and
unexpected compounds. Five-membered heterocycles are
formed that feature Al−I−B bridges. The relatively long B−I
(229.7(3) pm) and Al−I distances (274.06(9) pm, Table 1) to
the bridging iodine atom cause its large displacement of 152.9
pm from the BPCAl plane. The angle P−C−Al (121.7(1)°) is
similar to that of BBr3 adduct 4. The remaining tert-butyl group
and the hydrogen atom bonded to boron are on the same side
of the central heterocycle. Boron and aluminum have both a
chiral coordination sphere with four chemically different
substituents. The R,S-configurated molecule (B: R; Al: S) is
depicted in Figure 3 with the enantiomeric form being
generated by the crystallographic center of symmetry.
The NMR spectra of both compounds are consistent with
the constitution of 7 in the solid state. Resonances of a tert-
butyl group and two independent mesityl substituents were
observed. The vinylic hydrogen atoms gave doublets by
coupling to the phosphorus atoms with 3JPH coupling constants
of 37.7 and 34.6 Hz that are characteristic of a cis arrangement
of hydrogen and four-coordinate phosphorus atoms across C
D
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+1.887). Cleavage of the endocyclic B−Br bond should result
+
in the formation of an open chain zwitterionic species, BBr2 
C(CHPh)AlBr(CMe3)2− (NBO charges: B, −0.033; Al
+1.777), featuring a four-coordinate aluminum atom and a
three-coordinate boron atom. This species is only 22.5 kcal/
mol higher in energy than adduct 4. It seemed to be an
excellent intermediate to activate a tert-butyl group by its Lewis-
acidic boron atom. However, ring closure to heterocyclic
compound 4 is strongly preferred over rearrangement.
A reasonable low-barrier mechanism was only obtained by
electrolytic dissociation and the complete removal of a terminal
bromine atom of 4 to yield the ionic species A (Figure 4). In
solution, this process may be supported by interactions with
solvent molecules and the presence of Lewis-acidic species such
as BBr3 or uncoordinated FLP 1 to yield the cation of A with a
counterion such as BBr4 (calculated Br− affinity of BBr3 in
−
benzene solution: −29.6 kcal/mol). The reaction energy for the
−
formation of A (sum of the cation of A and BBr4 ) from 4 and
BBr3 was calculated to be +36.3 kcal/mol considering the
particular treatment of the solvation process by the CPCM
approach. In comparison, the dissociation of 4 into the cation
of A and the Br− anion requires 65.8 kcal/mol according to the
same theoretical procedure. The intermediate formation of
cationic A is strongly supported by a recently obtained
compound in which a BBr2+ cation is stabilized by coordination
to a Lewis base. It was synthesized by treatment of sterically
highly shielding 2,6-dimesitylpyridine with two equivalents of
BBr3.23 The second equivalent of BBr3 captured the released
Br− anion to yield the [BBr4]− counterion. The cationic species
A with a three-coordinate boron atom has expectedly a
significantly higher Lewis acidity (and hydride affinity; NBO
charge at B: +0.065) at boron compared to 4. Indeed, the
calculated reaction pathway for the H···B approach starting
from A results now in smooth migration of one hydrogen atom
from a CMe3 group to the boron atom with an activation
barrier of only 21.6 kcal/mol with respect to A. The
corresponding transition state TS1 is characterized by C···H
and B···H distances of 141.6 and 131.6 pm and a significant
lengthening of the respective C−Al bond (210.0 pm).
Relaxation of the transition state results in C−H bond cleavage
with the formation of B and isobutene that is weakly
coordinated to aluminum with strongly differing Al−C
distances (Al···C(H2) 226.2 pm; Al···C(Me2) 274.1 pm; 13.7
kcal/mol lower in energy than A). Al−C distances of 230−264
pm were reported for few structurally characterized Al−alkene
complexes.24 The last step of this reaction sequence is the
−
nucleophilic addition of the bromide ion released from BBr4
to yield a van der Waals complex (C) with the isobutene
molecule placed between one of the two mesityl substituents at
phosphorus and the phenyl ring that is 12.7 kcal/mol lower in
energy compared to 4. Finally, experimentally observed
compound 6 is formed by the complete removal of isobutene
(Erel = −14.5 kcal/mol with respect to 4). Because of the
coordination of the alkene to aluminum in the cation of B, the
bromide ion is forced to approach selectively from the opposite
side of the heterocycle yielding in agreement with the
experimental observations exclusively the enantiomeric (R,S)-
or (S,R)-forms of compound 6.
On the basis of these results, the mechanism of the
stereoselective formation of BHX2 adducts 6 and 7 comprises
the cleavage of an Al−C bond with the fragmentation of a
CMe3 group into a hydride anion and isobutene, B−H bond
formation, and a sigmatropic 1,3-rearrangement of a bromide
Figure 4. Calculated structures and relative energies (kcal/mol)
(M062x/6-311+G(d,p)//M062x/6-311+G(d,p) + GD3+cpcm-
(benzene) + zpe (M062x/6-31G(d)) for the stereoselective rearrange-
ment of compound 4 (Erel = 0.0 kcal/mol) to 6; relative energies in
−
square brackets refer to the sum of the cation of A + BBr4 . Selected
bond lengths and interatomic distances (pm): 4: PB 206.1, BBrt
200.0 and 200.3, BBrbr 209.6, AlBrbr 261.5; A: PB 196.8, B
Brt 188.1, BBrbr 191.3, Al···Brbr 297,5; TS1: PB 199.8, BBrt
195.0, BBrbr 204.1, AlBrbr 254.1, AlC 210.0, CC 143.3, C···
H 141.6, B···H 131.6; B: PB 200.0, BBrt 198.6, BBrbr 212.7,
AlBrbr 249.5, AlC 226.2; C: PB 199.8, BBrt 199.9, BBrbr
209.8, AlBrbr 254.9; Al···C(H2)C 456.9, Al···CC(Me2) 543.8;
6: PB 200.7, BBrt 199.1, BBrbr 211.5, AlBrbr 253.7.
anion from boron to aluminum. According to the relevant bond
lengths, these rearrangement steps are characterized mechanis-
E
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
according to a literature procedure.2 The boron trihalides BX3 were
applied as purchased.
tically by significant pericyclic properties. The suggested route
with a cationic intermediate resembles a superfacial 1,3-
dyotropic rearrangement observed for an organosilicon
compound that has been published only recently and comprises
an exchange of a silicon bonded chlorine atom by a tert-butyl
group.25 In contrast to the mechanism reported in this article
(Figure 4), an intact tert-butyl group migrated without β-
elimination and formation of isobutene. A silyl cation was
calculated to be the key intermediate in this unique reaction.
Synthesis of the FLP/BF3 Adduct, 2. A solution of FLP 1 (0.541
g, 1.06 mmol) in 20 mL of toluene was treated with the
trifluoroborane−THF complex (F3B ← THF; 0.12 mL, 1.06 mmol)
at room temperature. The mixture was stirred at room temperature for
15 h. The solvent was removed in vacuum. The residue was dissolved
in a 1:1 mixture of n-hexane and 1,2-difluorobenzene (8 mL). Cooling
the solution to −20 °C afforded colorless crystals of 2 (0.582 g, 95%).
Anal. calcd (found) for C34H46AlPBF3 (%): C, 70.3 (70.4); H, 8.0
1
(7.5). mp (argon, sealed capillary): 149 °C (dec.). H NMR (C6D6,
400 MHz, 298 K): 1.21 (18 H, s, Al(CMe3)2), 1.94 (6 H, s, p-CH3),
CONCLUSION
■
2.29 (12 H, s, o-CH3), 6.59 (4 H, d, 4JPH = 3.2 Hz, m-HMes), 7.02 (1 H,
The bifunctionality of Al/P-based FLP 1 with donor and
acceptor sites in a single molecule allows its application as an
efficient ligand for the coordination of ambiphilic compounds
such as boron trihalides. The boron atoms of the products were
coordinated to the basic phosphorus atoms, and one of the
halogen atoms occupied a bridging position between Al and B.
The properties of these compounds depend considerably on
the size of the halogen atoms. The fluorine atom is almost in
the plane of the resulting AlCPBX heterocycle, while the
bromine atom deviates by 64 pm. The increasing deviation of
the halogen atoms from the average plane influences the
molecular dynamics and favors secondary rearrangement
reactions. The BBr3 adduct rearranges slowly at ambient
temperature, but the corresponding iodine compound is
difficult to handle in solution even at temperatures below 0
°C. Rearrangement results in the stereoselective formation of
HBX2 adducts. Interestingly the obvious concerted reaction
pathway has a very high activation barrier and can be ruled out
by the results of quantum−chemical calculations. They support
instead the formation of a cationic intermediate that results
from cleavage of a B−Br bond and contains a highly Lewis-
acidic borenium cation. This initially formed species activates a
tert-butyl group bonded to aluminum to give the migrating
hydride ion by β-hydride elimination. The isobutene leaving
group is coordinated to the aluminum atom and favors the
selective attack of the bromine atom from the backside of the
central heterocycle.
3
t, JHH = 7.3 Hz, p-HPh), 7.09 (2 H, pseudo-t, outer line spacing 14.6
3
3
Hz, m-HPh), 7.29 (2 H, d, JHH = 7.2 Hz, o-HPh), 7.96 (1 H, d, JPH
=
37.5 Hz, PCCH). 13C{1H} NMR (C6D6, 100 MHz, 298 K):
3
16.9 (s br, Al(CMe3)2), 20.8 (s, p-CH3), 24.8 (d, JPC = 5.6 Hz, o-
CH3), 31.5 (s, Al(CMe3)2), 122.1 (d, 1JPC = 54.8 Hz, ipso-CMes), 127.7
(s, o-CPh), 129.6 (s, m-CPh), 130.3 (s, p-CPh), 131.6 (d, 3JPC = 9.0 Hz,
3
m-CMes), 140.5 (s br, PCCH), 141.5 (d, JPC = 27.2 Hz, ipso-
CPh), 141.8 (d, 4JPC = 2.7 Hz, p-CMes), 143.5 (d, 2JPC = 8.5 Hz, o-CMes),
162.7 (d, JPC = 9.1 Hz, PCCH). 31P{1H} NMR (C6D6, 162
2
MHz, 298 K): −10.7 (br, not clearly resolved multiplet). 11B{1H}
NMR (C6D6, 128 MHz, 298 K): 4.3 (d, JPB = 160 Hz). 19F NMR
1
(C6D6, 377 MHz, 298 K): −127.1 (br). 19F NMR (C6D6, 377 MHz,
2
213 K): −125.0 (2 F, s br, terminal F), −131.9 (1 F, dt br, JFF = 78
Hz, JPF = 168 Hz, bridging F). IR (paraffin, CsI, cm−1): 1773 (vw),
2
1736 (vw), 1603 (m), 1585 (w), 1540 (w), 1522 (vw), 1506 (m)
ν(CC), phenyl; 1466 (vs), 1377 (vs) (paraffin); 1290 (w), 1269
(m) δCH3; 1211 (w), 1190 (w), 1173 (w), 1153 (vw), 1099 (w), 1074
(vw), 1059 (vw), 1028 (m), 961 (vw), 932 (m), 893 (w), 868 (m),
852 (m), 797 (s), 746 (s) νCC, νBF; 723 (s, paraffin); 692 (m)
δphenyl; 638 (m), 592 (m), 577 (m), 550 (m), 517 (vw), 490 (w),
463 (w), 424 (vw), 405 (w), 395 (w), 361 (m), 332 (vw) δCC, δBF,
νPB, νAlC. FT Raman (neat, cm−1): 3058 (w), 3032 (w), 2925 (m),
2871 (w), 2831 (m), 2761 (vw), 2695 (vw) νCH; 1600 (s), 1540 (vs)
νCC, phenyl; 1491 (w), 1461 (w), 1443 (w), 1381 (w), 1339 (vw),
1291 (w), 1214 (m), 1183 (w), 1052 (vw), 1026 (vw), 1002 (w), 964
(vw), 934 (vw), 896 (vw), 871 (vw), 812 (vw), 763 (vw) νCC, νBF;
622 (vw), 572 (w), 553 (w), 531 (vw), 494 (vw), 459 (vw), 426 (vw),
409 (vw), 359 (vw), 317 (vw), 296 (vw), 225 (vw), 197 (vw), 163
(vw), 134 (m), 108 (s), 72 (vs) δCC, δBF, νPB, νAlC. MS (EI, 20 eV,
363 K, m/z): 512 (6%) 1+, M+ − BF3; 456 (7%), 455 (22%) 1+ −
CMe3.
Uncoordinated HBX2 compounds are unstable, but they can
be isolated as adducts with different donor molecules and are
applicable in hydroboration reactions. HBI2 has previously not
been reported. In our case, these molecules are stabilized by the
specific donor−acceptor capability of the FLP. The unprece-
dented adducts are promising starting materials for secondary
reactions such as functionalization by salt elimination or
hydroboration. Reduction may yield low-valent, highly electron
deficient boron species that are stabilized by the donor−
acceptor functionality of 1. These investigations are in the focus
of our current research activities.
Synthesis of the FLP/BCl3 Adduct, 3. A solution of FLP 1
(0.406 g (0.79 mmol) in 20 mL of toluene was cooled to −20 °C and
treated with a solution of BCl3 in toluene (0.79 mL, 1.0 M, 0.093 g,
0.79 mmol). The mixture was warmed to 0 °C and stirred for 2 h at
this temperature. The solvent was removed in vacuum at 0 °C.
Residual volatiles were carefully removed in vacuum (10−3 Torr). The
residue was dissolved in 1,2-difluorobenzene and cooled to −20 °C to
yield colorless crystals of 3 (0.432 g, 87%). Anal. calcd (found) for
C34H46AlPBCl3 (%): C, 64.8 (65.2); H, 7.4 (7.4). mp (argon, sealed
1
capillary): >118 °C (dec.). H NMR (toluene-d8, 400 MHz, 280 K):
0.76 and 1.51 (each 9 H, s, Al(CMe3)2), 1.59, 2.20, 2.58, and 2.83
(each 3 H, s, o-CH3), 1.93 and 1.99 (each 3 H, s, p-CH3), 6.53 and
6.74 (each 1 H, s br, m-HMes), 6.55 (2 H, s, m-HMes), 7.01 (1 H, t, 3JHH
= 7.1 Hz, p-HPh), 7.07 (2 H, pseudo-t, outer line spacing 14.6 Hz, m-
HPh), 7.20 (2 H, d, 3JHH = 7.2 Hz, o-HPh), 7.96 (1 H, d, 3JPH = 36.5 Hz,
PCCH); similar shifts were observed at room temperature, but
the resonances became broad, and the signals of the para-Me groups
coincided; coalescence of the tert-butyl and ortho-Me resonances
occurred at about +60 °C, decomposition prevented a detailed
assignment of the spectra at elevated temperature in the range of a fast
exchange. 13C{1H} NMR (toluene-d8, 100 MHz, 280 K): 17.9 and
18.5 (each s br, Al(CMe3)2), 20.7 (s, p-CH3), 25.3, 25.4, 27.2, and 30.4
(each s br, o-CH3), 31.6 and 32.8 (each s, Al(CMe3)2), 125.2 and
125.3 (each s br, ipso-CMes), 128.0 (s, o-CPh), 130.0 (s, m-CPh), 130.4
EXPERIMENTAL SECTION
■
All procedures were carried out under an atmosphere of purified argon
in dried solvents (n-pentane and n-hexane with LiAlH4; toluene with
Na/benzophenone and 1,2-difluorobenzene with molecular sieves)
using standard Schlenks. NMR spectra were recorded in various
deuterated solvents using Bruker Avance I and III spectrometers (1H,
400; 13C, 100 MHz; 11B, 128 MHz; 31P, 162 MHz; 19F, 377 MHz) and
referenced internally to residual solvent resonances (chemical shift
data in δ). 13C, 31P, and 11B NMR spectra were all proton-decoupled.
The assignment of NMR spectra is based on HMBC, H,H-ROESY,
HSQC, and DEPT135 data. Elemental analyses were determined by
the microanalytic laboratory of the Westfalische Wilhelms-Universitat
Munster. IR spectra were recorded as Nujol mulls between CsI or KBr
plates on a Shimadzu Prestige 21 spectrometer. FLP 1 was obtained
̈
̈
(s, p-CPh), 131.5, 131.6, and 132.2 (each s br, m-CMes), 141.0 (s br,
̈
3
PCCH), 141.2 and 141.6 (each s br, p-CMes), 141.4 (d, JPC
=
F
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
27.5 Hz, ipso-CPh), 142.8, 143.5, 143.6, and 144.0 (each s br, o-CMes),
CH3), 29.3 (d, 3JPC = 3.1 Hz, o-CH3), 30.4 (d, 3JPC = 5.0 Hz, o-CH3),
2
1
162.4 (d, JPC = 6.5 Hz, PCCH). 31P{1H} NMR (toluene-d8,
31.7 (s, o-CH3), 32.4 and 33.6 (each s, Al(CMe3)2), 125.6 (d, JPC
=
=
=
=
=
=
4
1
53.7 Hz, ipso-CMes), 128.4 (d, JPC = 2.0 Hz, o-CPh), 129.2 (d, JPC
162 MHz, 280 K): 16.5 (poorly resolved quartet, br, outer line spacing
about 430 Hz). 11B{1H} NMR (toluene-d8, 128 MHz, 280 K): 9.9 (d,
1JPB = 165 Hz). IR (paraffin, CsI, cm−1): 1732 (vw), 1603 (m), 1533
(w) νCC, phenyl; 1456 (vs), 1375 (vs) (paraffin); 1292 (w), 1269
(w), 1246 (w) δCH3; 1209 (w), 1155 (w), 1101 (vw), 1076 (w), 1028
(m), 1005 (w), 961 (vw), 932 (s), 895 (w), 870 (s), 853 (s), 808 (s),
791 (m), 745 (s) νCC, νBCl; 723 (vs) (paraffin); 696 (m) δphenyl;
631 (s), 606 (w), 571 (s), 545 (m), 524 (w), 490 (m), 463 (m), 440
(w), 405 (w), 359 (w), 343 (m) δCC, νPB, νAlC, νBCl. MS (EI, 20
eV, 353 K, m/z): 512 (10%) 1+, M+ − BCl3; 456 (14%), 455 (41%) 1+
20.1 Hz, ipso-CMes), 130.0 (s, m-CPh), 130.3 (s, p-CPh), 131.5 (d, 3JPC
3
3
8.9 Hz, m-CMes), 131.6 (d, JPC = 10.8 Hz, m-CMes), 131.9 (d, JPC
3
3
11.1 Hz, m-CMes), 132.4 (d, JPC = 10.8 Hz, m-CMes), 141.0 (d, JPC
28.7 Hz, ipso-CPh), 141.66 (d, 4JPC = 3.0 Hz, p-CMes), 141.75 (d, 4JPC
2
2
2.9 Hz, p-CMes), 142.0 (d, JPC = 6.9 Hz, o-CMes), 144.2 (d, JPC = 4.0
Hz, o-CMes), 144.3 (d, 2JPC = 12.3 Hz, o-CMes), 144.9 (d, 2JPC = 9.3 Hz,
o-CMes), 144.9 (br, 1JPC = 14.0 Hz, PCCH), 161.2 (d, 2JPC = 4.4 Hz,
PCCH). 31P{1H} NMR (C6D6, 162 MHz, 280 K): 20.4 (poorly
resolved quartet, br., 1JPB = 123 Hz). 11B NMR (C6D6, 128 MHz, 280
K): −58.7 (d, 1JPB = 123 Hz). IR (paraffin, KBr, cm−1): 1603 (s), 1576
(m), 1553 (s), 1516 (s) νCC, phenyl; 1466 (vs), 1452 (vs), 1375
(vs) paraffin; 1337 (s), 1314 (s), 1292 (vs), 1265 (s), 1246 (s) δCH3;
1209 (s), 1173 (s), 1157 (m), 1109 (m), 1074 (s), 1026 (m), 1013
(s), 1001 (m), 968 (m), 957 (m), 932 (s), 895 (m), 870 (s), 851 (vs),
808 (vs), 777 (vs), 747 (vs) νCC; 723 (s) paraffin; 696 (s), 629 (s),
565 (vs), 529 (vs), 509 (s), 486 (vs), 461 (m), 430 (vs) δCC, νAlC,
νPC. MS (EI, 25 eV, 473 K, m/z, experimental intensities agree with
expected isotopic pattern; only the most intense peak of each fragment
is given): 764 (2%) M+ − Al(CMe3)2 + H, 653 (100%) M+ − BI −
CMe3 − butene, 525 (29%) M+ − 2CMe3 − BI2, 119 (31%) Mes+.
FLP/HBBr2 Adduct, 6. The selective rearrangement of 4 to 6 was
observed only in C6D6 solution in a sealed NMR tube at 275 K over 5
d. We were not able to reproduce this reaction in a preparative scale in
different solvents (toluene, benzene) and at different temperatures. In
all cases, mixtures of compounds were obtained with 6 as the main
component. Separation of the mixtures by recrystallization from
different solvents failed. The unambiguous characterization and
identification of 6 is based on the results of the NMR experiments
and a comparison with data from the corresponding iodine compound
+
− CMe3, 118 (1), 116 (1%) BCl3 .
Synthesis of the FLP/BBr3 Adduct, 4. A cooled (−20 °C)
solution of FLP 1 (0.31 g, 0.61 mmol) in toluene (10 mL) was treated
with BBr3 (0.06 mL, 0.15 g, 0.61 mmol). The pale yellow mixture was
stirred for 1 h at −20 °C, concentrated at 0 °C, and cooled to −45 °C
to afford colorless crystals of compound 4 (0.33 g, 71%). Compound 4
is stable as a solid material only at low temperatures (−32 °C); at
room temperature, it rearranges slowly in the solid state and in
solution to yield HBBr2 adduct 6. NMR data were recorded at low
temperature (275 K); rearrangement occurred already under this
condition, but the resonances of 4 could be assigned unambiguously.
Anal. calcd (found) for C34H46AlBBr3P (%): C, 53.5 (53.6); H, 6.1
(6.1). mp (argon, sealed capillary): 75 °C (dec.). 1H NMR (C6D6, 400
MHz, 275 K): 0.81 and 1.58 (each 9 H, s, Al(CMe3)2), 1.59, 2.34, 2.62,
and 2.94 (each 3 H, s, o-CH3), 1.90 and 1.98 (each 3 H, s, p-CH3),
6.53 (3 H, s, m-HMes), 6.75 (1 H, s, m-HMes), 6.98 (1 H, m, p-HPh),
7.05 (2 H, pseudo-t, outer line spacing 14.8 Hz, m-HPh), 7.20 (2 H, d,
3
3JHH = 7.4 Hz, o-HPh), 8.01 (1 H, d, JPH = 37.3 Hz, PCCH).
13C{1H} NMR (C6D6, 100 MHz, 275 K): 18.3 and 18.7 (each s br,
Al(CMe3)2), 20.7 (s, p-CH3), 25.6 (s, o-CH3), 26.9 (s, o-CH3), 28.5
1
7. H NMR (C6D6, 400 MHz, 300 K): 0.99 (9 H, s, Al−CMe3), 1.93
3
(d, JPC = 5.2 Hz, o-CH3), 31.1 (s br, o-CH3), 31.8 and 33.1 (each s,
and 1.97 (each 3 H, s, p-CH3), 2.19 and 2.55 (each 6 H, s, o-CH3),
5.07 (s br, HBBr), 6.57 (2 H, d, 4JPH = 2.8 Hz, m-HMes), 6.60 (2 H, d,
4JPH = 3.3 Hz, m-HMes), 6.96 (1 H, m, p-HPh), 7.07 (2 H, pseudo-t,
Al(CMe3)2), 125.1 (d, 1JPC = 54.0 Hz, ipso-CMes), 126.4 (d, 1JPC = 63.0
Hz, ipso-CMes), 128.2 (d, 4JPC = 1.0 Hz, o-CPh), 129.7 (s, m-CPh), 130.4
(s, p-CPh), 131.6 (d br, 3JPC = 10.0 Hz, m-CMes), 131.6 (s br, m-CMes),
3
outer line spacing 14.8 Hz, m-HPh), 7.56 (2 H, d, JHH = 7.4 Hz, o-
3
131.7 (s br, m-CMes), 132.2 (d br, JPC = 10.9 Hz, m-CMes), 141.3 (d,
3
HPh), 7.69 (1 H, d, JPH = 37.7 Hz, PCCH). 13C{1H} NMR
1
3JPC = 28.0 Hz, ipso-CPh), 141.7 (s, p-CMes), 142.2 (d, JPC = 35.0 Hz,
(C6D6, 100 MHz, 300 K): 17.7 (s br, AlCMe3), 20.7 (s, p-CH3), 24.2
PCCH), 142.6 (d, 2JPC = 8.0 Hz, o-CMes), 143.9 (s br, o-CMes), 144.4
3
3
(d, JPC = 4.1 Hz, o-CH3), 26.9 (d, JPC = 2.9 Hz, o-CH3), 30.2 (s,
2
2
(d br, JPC = 10.0 Hz, o-CMes), 162.0 (d, JPC = 5.4 Hz, PCCH).
31P{1H} NMR (C6D6, 162 MHz, 275 K): 20.1 (poorly resolved
quartet, br, 1JPB = 153 Hz). 11B NMR (C6D6, 128 MHz, 300 K): −5.7
(d, 1JPB = 153 Hz). IR (paraffin, CsI, cm−1): 1601 (m), 1580 (w), 1526
(w) νCC, phenyl; 1460 (vs), 1377 (vs) (paraffin); 1302 (vw), 1288
(w) δCH3; 1171 (w), 1157 (w), 1026 (m), 932 (m), 872 (m), 853
(m), 809 (m), 783 (m), 748 (s) νCC; 723 (vs) (paraffin); 694 (m),
671 (w) δphenyl; 646 (w), 625 (s), 569 (s), 490 (w), 461 (s) δCC,
νAlC, νPC, νBBr. MS (EI, 20 eV, 483 K, m/z, experimental intensities
agree with expected isotopic pattern; only the most intense peak of
each fragment is given): 764 (2.3%) and 762 (2.2%) M+, 372 (34%)
Mes2PC(H)CHPh+, 119 (17%) Mes+.
1
1
AlCMe3), 121.8 (d, JPC = 62.9 Hz, ipso-CMes), 123.8 (d, JPC = 54.8
Hz, ipso-CMes), 128.6 (s, o-CPh), 129.7 (s, m-CPh), 130.3 (s, p-CPh),
3
3
131.7 (d, JPC = 10.0 Hz, m-CMes), 131.8 (d, JPC = 10.0 Hz, m-CMes),
137.1 (s, PCCH), 140.6 (d, 3JPC = 26.7 Hz, ipso-CPh), 141.5 (d, 4JPC
= 2.8 Hz, p-CMes), 142.2 (d, 4JPC = 2.6 Hz, p-CMes), 143.0 (d, 2JPC = 6.8
Hz, o-CMes), 143.1 (d, 2JPC = 6.9 Hz, o-CMes), 161.6 (d, 2JPC = 4.1 Hz,
PCCH). 31P{1H} NMR (C6D6, 162 MHz, 300 K): 6.3 (s br). 11B
NMR (C6D6, 128 MHz, 300 K): −4.4 (s br).
Synthesis of the FLP/HBI2 Adduct, 7. A cooled (0 °C) solution
of FLP 1 (0.14 g, 0.27 mmol) in toluene (6 mL) was treated with a
solution of BI3 (0.11 g, 0.28 mmol) in toluene (4 mL). Stirring was
continued for 14 h at this temperature. All volatiles were removed in
vacuum, and the residue was treated with 1 mL of n-pentane. The
solvent was removed in vacuum, and the solid residue was washed with
2 mL of 1,2-difluorobenzene. The solvent was removed by a pipet, and
the solid was dried in vacuum to yield 7 as a colorless amorphous solid
in relatively high purity (0.16 g, 69%). Compound 7 is insoluble in
hydrocarbons and only sparingly soluble in 1,2-difluorobenzene. It
dissolves in CH2Cl2, but the solutions are unstable at room
temperature. The NMR spectra were recorded at 0 °C. Small
resonances indicate the formation of impurities even under these mild
conditions. Anal. calcd (found) for C30H38AlBPI3 (%): C, 42.5 (41.9);
Synthesis of the FLP/BI3 Adduct, 5. A cooled (−20 °C) solution
of FLP 1 (0.210 g, 0.41 mmol) in toluene (4 mL) was treated with a
solution of BI3 (0.160 g, 0.41 mmol) in 1 mL of the same solvent. The
yellow solution was stirred for 1 h at −20 °C. A colorless solid of 5 was
obtained without concentration upon storing of the solution at −20
°C (0.11 g, 30%). Adduct 5 is unstable and decomposes slowly at
room temperature in the solid state (in particular under vacuum) and
rearranges in solution to yield HBI2 adduct 7. Therefore, we were not
able to obtain a correct elemental analysis and to grow single crystals.
NMR spectra were recorded at 280 K. mp (argon, sealed capillary):
1
1
101 °C (dec.). H NMR (C6D6, 400 MHz, 280 K): 0.78 and 1.60
H, 4.5 (4.5). mp (argon, sealed capillary): >105 °C (dec.). H NMR
(each 9 H, s, Al−CMe3), 1.54, 2.46, 2.63, and 3.03 (each 3 H, s, o-
(CD2Cl2, 400 MHz, 275 K): 0.57 (9 H, s, Al−CMe3), 2.24 and 2.48
CH3), 1.60 (9 H, s, Al−CMe3), 1.90 and 1.98 (each 3 H, s, p-CH3),
(each 6 H, s, o-CH3), 2.30 and 2.32 (each 3 H, s, p-CH3), 4.50 (1 H, s
6.53, 6.54, 6.57, and 6.79 (each 1 H, s br, m-HMes), 6.98 (1 H, t, 3JHH
=
br, BH), 6.94 and 6.98 (each 2 H, d br, 4JPH = 3.0 Hz, m-HMes), 7.40 (3
3
7.4 Hz, p-HPh), 7.05 (2 H, pseudo-t, outer line spacing 14.8 Hz, m-
HPh), 7.22 (2 H, d, 3JHH = 7.3 Hz, o-HPh), 8.02 (1 H, d, 3JPH = 38.4 Hz,
PCCH). 13C{1H} NMR (C6D6, 100 MHz, 280 K): 18.7 and
19.2 (each s br, Al(CMe3)2), 20.7 (s, p-CH3), 26.0 (d, 3JPC = 1.6 Hz, o-
H, m, p-HPh and m-HPh), 7.60 (2 H, br, o-HPh), 7.74 (1 H, d, JPH
=
34.6 Hz, PCCH). 13C NMR (CD2Cl2, 100 MHz, 275 K): 17.7
(s br, AlCMe3), 21.0 and 21.1 (each s, p-CH3), 24.8 and 27.9 (each s
1
br, o-CH3), 29.6 (s, AlCMe3), 122.7 (d, JPC = 37.6 Hz, ipso-CMes),
G
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Crystal Data and Structure Refinement for Compounds 2−4 and 7
compd
2·C6H4F2
3
4
7·0.5C6H4F2
empirical formula
fw (M/(g mol−1))
cryst syst
space group
a (pm)
C40H50AlBF5P
694.56
C34H46AlBCl3P
629.82
C34H46AlBBr3P
763.20
C33H40AlBFI3P
905.11
triclinic
trigonal
R-3
monoclinic
P21/c
monoclinic
P21/n
P-1
940.52(5)
1211.37(6)
1754.0(1)
92.43(3)
103.07(3)
104.46(3)
1.8746(2)
2
4402.4(6)
4402.4(6)
966.5(2)
90
974.49(4)
3518.0(2)
3051.8(1)
90
1215.87(7)
1446.69(6)
2086.8(1)
90
b (pm)
c (pm)
α (deg)
β (deg)
90
94.685(1)
90
105.568(5)
90
γ (deg)
vol (nm3)
120
16.22(1)
18
10.4275(8)
12
3.5360(3)
4
Z
temp (K)
density (Mg m−3
153(2)
153(2)
153(2)
153(2)
)
1.230
1.160
1.458
1.700
abs coeff (mm−1
θ range (deg)
reflns collected
ind reflns
)
0.149 (Mo Kα)
1.75−27.96
14 175
0.344 (Mo Kα)
1.60−27.97
40 506
3.575 (Mo Kα)
1.86−28.01
106 291
2.748 (Mo Kα)
1.73−27.94
44 525
8920 (Rint = 0.0415)
475
8642 (Rint = 0.0758)
374
25043 (Rint = 0.0520)
1117
8428 (Rint = 0.0716)
398
parameters
a
R1
0.0591
0.0480
0.0379
0.0274
b
wR2 (all data)
0.1645
0.1002
0.0943
0.0618
largest diff. peak and hole (e nm−3
)
638, −570
386, −387
691, −453
1171, −1025
a
b
Observation criterion: I > 2σ(I). R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo − Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
1
123.3 (d, JPC = 47.0 Hz, ipso-CMes), 129.1 (s br, o-CPh), 129.5 (s, m-
contained half a molecule of 1,2-difluorobenzene per formula unit,
which was disordered across a crystallographic center of symmetry.
Further crystallographic data is summarized in Table 2. CCDC
numbers 997863 (2), 997864 (3), 997865 (4), and 997866 (7)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Database Centre via www.ccdc.cam.ac.uk/data_request/cif.
3
CPh), 130.2 (s, p-CPh,), 131.7 (d, JPC = 10.6 Hz, m-CMes), 131.9 (d,
3JPC = 10.1 Hz, m-CMes), 137.0 (s br, PCCH), 140.5 (d, 3JPC = 26.5
4
Hz, ipso-CPh), 141.9 and 142.6 (each d, JPC = 2.0 Hz, p-CMes), 142.9
2
(two resonances coincide, d, JPC = 9.0 Hz, o-CMes), 162.1 (s, PC
CH). 31P NMR (CD2Cl2, 162 MHz, 275 K): 5.1 (s br). 11B NMR
(CD2Cl2, 128 MHz, 275 K): −28.9 (br). IR (paraffin, CsI, cm−1):
2033 (w), 1912 (w), 1819 (w), 1736 (w), 1599 (m), 1570 (m), 1547
(w), 1533 (w) νCC, νBH, phenyl; 1434 (m), 1375 (m) paraffin;
1290 (w), 1267 (w), 1250 (w) δCH3; 1202 (w), 1172 (w), 1153 (w),
1098 (w), 1074 (w), 1026 (m), 932 (m), 878 (w), 849 (m), 808 (m),
789 (w), 747 (s) νCC; 723 (s) paraffin; 704 (m) δphenyl; 669 (w),
629 (m), 613 (sh), 583 (s), 559 (s), 515 (m), 478 (m), 469 (m), 442
(w), 397 (m), 380 (w), 353 (w), 330 (m) δCC, νAlC, νPC, νBI, νAlI.
MS (EI, 25 eV, 473 K, m/z, experimental intensities agree with
expected isotopic distribution; only the most intense peak of each
fragment is given): 791 (1%) M+ − CMe3, 652 (84%) M+ − CMe3 −
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray structure data (cif) and results of quantum−chemical
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: uhlw@uni-muenster.de; fax: +49 251 8336660.
■
+
HBI, 525 (8%) M+ − CMe3 − HBI2, 265 (8%) BI2 .
X-ray Crystallography. Crystals suitable for X-ray crystallography
were obtained by recrystallization from a mixture of n-hexane and 1,2-
difluorobenzene (2: −20 °C), 1,2-difluorobenzene (3: −20 °C; 7: +5
°C), or the reaction mixture (concentrated at 0 °C) at −32 °C (4).
Intensity data were collected on Bruker APEX II and QUAZAR
diffractometers with monochromated Mo Kα radiation. The collection
method involved ω scans. Data reduction was carried out using the
program SAINT+.26 The crystal structures were solved by direct
methods using SHELXTL.27 Non-hydrogen atoms were first refined
isotropically followed by anisotropic refinement by full matrix least-
squares calculation based on F2 using SHELXTL. Hydrogen atoms
were positioned geometrically and allowed to ride on their respective
parent atoms. Further details are summarized in Table 2. Compound 2
crystallized with a molecule of 1,2-difluorobenzene per formula unit;
the solvent molecule was disordered over two positions, and all atoms
were refined on split positions (0.66:0.34). The crystals of 3 contained
strongly disordered solvent molecules, which could not be localized
and refined. We applied the SQUEEZE program28 in PLATON and
found large voids with an electron count corresponding to five 1,2-
difluorobenzene molecules per unit cell. Compound 4 had three
independent molecules per asymmetric unit. The crystals of 7
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) (a) Frustrated Lewis Pairs I: Uncovering and Understanding; Erker,
G., Stephan, D. W., Eds.; Springer: Heidelberg, Germany, 2013; Vol.
332. (b) Frustrated Lewis Pairs II: Expanding the Scope; Erker, G.,
Stephan, D. W., Eds.; Springer: Heidelberg, Germany, 2013; Vol. 334.
(c) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science
2006, 314, 1124−1126. (d) Stephan, D. W.; Erker, G. Angew. Chem.
2010, 122, 50−81; Angew. Chem., Int. Ed. 2010, 49, 46−76.
(2) Appelt, C.; Westenberg, H.; Bertini, F.; Ehlers, A. W.; Slootweg, J.
C.; Lammertsma, K.; Uhl, W. Angew. Chem. 2011, 123, 4011−4014;
Angew. Chem., Int. Ed. 2011, 50, 3925−3928.
(3) (a) Men
́
ard, G.; Stephan, D. W. J. Am. Chem. Soc. 2010, 132,
ard, G.; Stephan, D. W. Angew. Chem. 2011, 123,
1796−1797. (b) Men
́
8546−8549; Angew. Chem., Int. Ed. 2010, 50, 8396−8399.
(4) (a) Roters, S.; Appelt, C.; Westenberg, H.; Hepp, A.; Slootweg, J.
C.; Lammertsma, K.; Uhl, W. Dalton Trans. 2012, 41, 9033−9045. See
H
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
also: (b) Boudreau, J.; Courtemanche, M. A.; Fontaine, F. G. Chem.
Commun. 2011, 11131−11133.
(17) (a) Moroz, A.; Sweany, R. L. Inorg. Chem. 1992, 31, 5236−5242.
(b) Bass, C. D.; Lynds, L.; Wolfram, T.; DeWames, R. E. J. Chem. Phys.
1964, 40, 3611−3618. (c) Frost, D. C.; Kirby, C.; McDowell, C. A.;
Westwood, N. P. C. J. Am. Chem. Soc. 1981, 103, 4428−4432.
(d) Westwood, N. P. C.; Lewis-Bevan, W.; Gerry, M. C. L. J. Mol.
Spectrosc. 1984, 106, 227−234. (e) Kasuya, T.; Lafferty, W. J.; Lide, D.
R. J. Chem. Phys. 1968, 48, 1−149. (f) Lee, K.-H.; Takeo, H.; Kondo,
S.; Matsumura, C. Bull. Chem. Soc. Jpn. 1985, 58, 1772−1777.
(g) Bouix, J.; Cueilleron, J. Bull. Soc. Chem. Fr. 1968, 3157−3161.
(h) Grant, D. J.; Dixon, D. A. chJ. Phys. Chem. A 2009, 113, 777−787.
(18) (a) Bissinger, P.; Braunschweig, H.; Kraft, K.; Kupfer, T. Angew.
Chem. 2011, 123, 4801−4804; Angew. Chem., Int. Ed. 2011, 50, 4704−
4707. (b) Vargas-Baca, I.; Findlater, M.; Powell, A.; Vasudevan, K. V.;
Cowley, A. H. Dalton Trans. 2008, 6421−6426. (c) Brown, H. C.;
Ravindran, N. J. Org. Chem. 1977, 42, 2533−2534. (d) Brown, H. C.;
Ravindran, N. Inorg. Chem. 1977, 16, 2938−2940. (e) Brown, H. C.;
Racherla, U. S. J. Org. Chem. 1986, 51, 895−897.
(5) (a) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131,
8396−8397. (b) Dureen, M. A.; Brown, C. C.; Stephan, D. W.
Organometallics 2010, 29, 6594−6607.
(6) Men
4488; Angew. Chem., Int. Ed. 2012, 51, 4409−4412.
(7) Menard, G.; Stephan, D. W. Angew. Chem. 2012, 124, 8397−
́
ard, G.; Stephan, D. W. Angew. Chem. 2012, 124, 4485−
́
8400; Angew. Chem., Int. Ed. 2012, 51, 8272−8275.
(8) Uhl, W.; Appelt, C. Organometallics 2013, 32, 5008−5014.
(9) Appelt, C.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew.
Chem. 2012, 124, 6013−6016; Angew. Chem., Int. Ed. 2012, 51, 5911−
5914.
(10) Appelt, C.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew.
Chem. 2013, 125, 4350−4353; Angew. Chem., Int. Ed. 2013, 52, 4256−
4259.
(11) Uhl, W.; Appelt, C.; Backs, J.; Westenberg, H.; Wollschlager, A.;
̈
(19) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241.
(20) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
2010, 132, 154104.
Tannert, J. Organometallics 2014, 33, 1212−1217.
(12) (a) Monkowius, U.; Nogai, S.; Schmidbaur, H. Dalton Trans.
2003, 987−991. (b) Weller, F.; Mohlen, M.; Dehnicke, K. Z.
̈
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Kristallogr. − New Cryst. Struct. 1997, 212, 159−160. (c) Evans, C.
E. B.; Lough, A. J.; Grondey, H.; Manners, I. New J. Chem. 2000, 24,
447−453. (d) Jones, T. L.; Willis, A. C.; Wild, S. B. Inorg. Chem. 1992,
31, 1411−1416. (e) Charmant, J. P. H.; Fan, C.; Norman, N. C.;
Pringle, P. G. Dalton Trans. 2007, 114−123. (f) Black, D. L.; Taylor, R.
C. Acta Crystallogr., Sect. B 1975, 31, 1116−1120. (g) Aubauer, C.;
Davidge, K.; Klapotke, T. M.; Mayer, P.; Piotrowski, H.; Schulz, A. Z.
̈
Anorg. Allg. Chem. 2000, 626, 2373−2378. (h) Braunschweig, H.;
Radacki, K.; Uttinger, K. Inorg. Chem. 2007, 46, 8796−8800.
(i) Dornhaus, F.; Bolte, M.; Lerner, H.-W.; Wagner, M. Eur. J. Inorg.
Chem. 2006, 1777−1785.
(13) Al−F−Al: (a) Atwood, J. L.; Newberry, W. R. J. Organomet.
Chem. 1974, 66, 15−21. (b) Rennekamp, C.; Stasch, A.; Muller, P.;
̈
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.; Uson, I. J. Fluorine
Chem. 2000, 102, 17−20. (c) Chen, M.-C.; Roberts, J. A. S.; Marks, T.
J. Organometallics 2004, 23, 932−935. (d) Chen, M.-C.; Roberts, J. A.
S.; Seyam, A. M.; Li, L.; Zuccaccia, C.; Stahl, N. G.; Marks, T. J.
Organometallics 2006, 25, 2833−28xx. Al−Cl−Al: (e) Graham, T. W.;
Ong, C.; Wei, P.; Stephan, D. W. J. Organomet. Chem. 2007, 692,
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.
(22) Reed, A. R.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736−1740.
(23) Mansaray, H. B.; Rowe, A. D. L.; Phillips, N.; Niemeyer, J.;
Kelly, M.; Addy, D. A.; Bates, J. I.; Aldridge, S. Chem. Commun. 2011,
47, 12295−12297.
4481−4485. (f) Uhl, W.; Rosener, C.; Stefaniak, C.; Choy, T.; Jasper-
̈
(24) (a) Hausen, H. D.; Todtmann, J.; Weidlein, J. Z. Naturforsch.
̈
Peter, B. Z. Naturforsch. 2012, 67b, 1081−1090. (g) Tian, X.; Frohlich,
̈
̈
1994, 49b, 430−433. (b) Uffing, C.; Ecker, A.; Koppe, R.; Merzweiler,
̈
R.; Mitzel, N. W. Z. Anorg. Allg. Chem. 2006, 632, 307−312.
(h) Albahily, K.; Gambarotta, S.; Duchateau, R. Organometallics 2011,
K.; Schnockel, H. Chem.Eur. J. 1998, 4, 2142−2147. (c) Schnockel,
̈
̈
H.; Leimkuhler, H.; Lotz, R.; Mattes, R. Angew. Chem. 1986, 98, 929−
̈
30, 4655−4664. Al−Br−Al: (i) Krossing, I.; Noth, H.; Staude, S. Z.
̈
930; Angew. Chem., Int. Ed. 1986, 25, 921−922. (d) Menard, G.;
Stephan, D. W. Angew. Chem. 2012, 124, 8397−8400; Angew. Chem.,
Int. Ed. 2012, 51, 8272−8275.
Naturforsch. 2008, 63b, 1045−1051. (j) Uhl, W.; Heller, D.; Rohling,
M.; Kosters, J. Inorg. Chim. Acta 2011, 374, 359−365. (k) Knabel, K.;
̈
Noth, H. Z. Naturforsch. 2005, 60b, 1027−1035. (l) Petrie, M. A.;
̈
(25) Uhl, W.; Bohnemann, J.; Layh, M.; Wurthwein, E.-U. Chem.
̈
Power, P. P.; Rasika Dias, H. V.; Ruhlandt-Senge, K.; Waggoner, K.
M.; Wehmschulte, R. J. Organometallics 1993, 12, 1086−1093.
(14) (a) Macdonald, C. L. B.; Gorden, J. D.; Voigt, A.; Cowley, A. H.
J. Am. Chem. Soc. 2000, 122, 11725−11726. (b) Macdonald, C. L. B.;
Gorden, J. D.; Voigt, A.; Filipponi, S.; Cowley, A. H. Dalton Trans.
Eur. J. 2014, 20, 8771−8781.
(26) (a) SAINT+, version 6.02 (includes XPREP and SADABS);
Bruker AXS INC.: Madison, WI, 1999. (b) Sheldrick, G. M. SADABS;
University of Gottingen: Gottingen, Germany, 1996.
̈
̈
(27) (a) SHELXTL-Plus, REL. 4.1; Siemens Analytical X-RAY
Instruments Inc.: Madison, WI, 1990. (b) Sheldrick, G. M. SHELXL-
2008, 1161−1176. (c) Kuzu, I.; Neumuller, B. Z. Anorg. Allg. Chem.
̈
2007, 633, 941−942. (d) Reger, D. L.; Little, C. A.; Smith, M. D.;
Rheingold, A. L.; Liable-Sands, L. M.; Yap, G. P. A; Guzei, I. A. Inorg.
Chem. 2002, 41, 19−27. (e) Bytheway, I.; Griffith, C. S.; Koutsantonis,
G. A.; Skelton, B. W.; White, A. H. Eur. J. Inorg. Chem. 2007, 3240−
3246. (f) Zhao, X.; Lough, A. J.; Stephan, D. W. Chem.Eur. J. 2011,
17, 6731−6743.
97, program for the refinement of structures; University of Gottingen:
̈
Gottingen, Germany, 1997. (c) Sheldrick, G. M. Acta Crystallogr. 2008,
A64, 112−122.
̈
(28) (a) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46,
194−201. (b) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13.
(15) Selected examples: (a) Bontemps, S.; Bouhadir, G.; Apperley, D.
C.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem.Asian J. 2009, 4,
428−435. (b) Emslie, D. J. H.; Cowie, B. E.; Oakley, S. R.; Huk, N. L.;
Jenkins, H. A.; Harrington, L. E.; Britten, J. F. Dalton Trans. 2012, 41,
3523−3535. (c) Cowie, B. E.; Emslie, D. J. H.; Jenkins, H. A.; Britten,
J. F. Inorg. Chem. 2010, 49, 4060−4072. (d) Braunschweig, H.; Chiu,
C.-W.; Wahler, J.; Radacki, K.; Kupfer, T. Chem.Eur. J. 2010, 16,
12229−12233.
(16) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-
copy, 3rd ed.; Wiley-VCH: Weinheim, Germany, 1998; p 307.
I
dx.doi.org/10.1021/ic5009126 | Inorg. Chem. XXXX, XXX, XXX−XXX