Formation, structural characterization, and reactions of a unique cyclotrimeric vicinal Lewis pair containing (C6F5)2P-Lewis base and (C6F5)BH-Lewis acid components

Article abstract of DOI:10.1039/c4dt02081b

The synthesis of the new vicinal frustrated Lewis pair 5 containing (C₆F₅)₂P-Lewis base and (C₆F₅)BH-Lewis acid functionality is described. It forms a unique cyclotrimer (5)₃ which was structurally characterized by X-ray crystallography and high-resolution solid-state NMR spectroscopy. The relevant NMR Hamiltonian parameters (¹¹B and ³¹P chemical shielding tensors, ¹¹B quadrupolar coupling tensors, and ³¹P-¹¹B spin-spin coupling constants) indicate significant intramolecular covalent B...P interactions, consistent with results from density functional theory (DFT) calculations. In addition, the ¹¹B/³¹P and ³¹P/³¹P three-spin geometries are accurately reproduced by suitable high-resolution hetero- and homonuclear dipolar NMR experiments. As predicted from the bonding character portrayed by the solid-state NMR results, the cyclotrimer (5)₃ possesses only moderate catalytic activity. However, it undergoes an addition reaction with pyridine and hydroboration reactions with benzaldehyde and tert-butylacetylene. The products of the hydroboration reactions form stable adducts with pyridine. This journal is

Full text of DOI:10.1039/c4dt02081b

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Formation, structural characterization, and
reactions of a unique cyclotrimeric vicinal Lewis
pair containing (C6F5)2P-Lewis base and (C6F5)
BH-Lewis acid components†
Cite this: Dalton Trans., 2014, 43,
15159
Markus Erdmann,‡a Thomas Wiegand,‡b Jonas Blumenberg,a Hellmut Eckert,*b
Jinjun Ren,b Constantin G. Daniliuc,a Gerald Kehra and Gerhard Erker*a
The synthesis of the new vicinal frustrated Lewis pair 5 containing (C6F5)2P-Lewis base and (C6F5)-
BH-Lewis acid functionality is described. It forms a unique cyclotrimer (5)3 which was structurally charac-
terized by X-ray crystallography and high-resolution solid-state NMR spectroscopy. The relevant NMR
Hamiltonian parameters (11B and 31P chemical shielding tensors, 11B quadrupolar coupling tensors, and
31P–11B spin–spin coupling constants) indicate significant intramolecular covalent B⋯P interactions, con-
sistent with results from density functional theory (DFT) calculations. In addition, the 11B/31P and 31P/31
P
three-spin geometries are accurately reproduced by suitable high-resolution hetero- and homonuclear
dipolar NMR experiments. As predicted from the bonding character portrayed by the solid-state NMR
results, the cyclotrimer (5)3 possesses only moderate catalytic activity. However, it undergoes an addition
reaction with pyridine and hydroboration reactions with benzaldehyde and tert-butylacetylene. The pro-
ducts of the hydroboration reactions form stable adducts with pyridine.
Received 9th July 2014,
Accepted 8th August 2014
DOI: 10.1039/c4dt02081b
www.rsc.org/dalton
Introduction
Frustrated Lewis pairs are comprised of pairs of co-existing
Lewis acids and Lewis bases,1 which are prevented from
covalent adduct formation owing to either steric hindrance2–4
or specific electronic effects.5 The co-existent Lewis acid/base
pairs can undergo a variety of remarkable cooperative reactions
with added substrates. Dihydrogen activation is a most promi-
nent and often observed frustrated Lewis pair (FLP) feature2,6
and, consequently, protocols for metal-free FLP-catalyzed
hydrogenation reactions have been developed.7 FLPs were also
shown to add to a great variety of other small molecules,
including CO2,8 SO2,9 and nitrogen oxides.10 Intramolecular
FLPs play an important role in this chemistry.3 Recently we
introduced additional functionalities at vicinal P/B FLPs. This
can be done by modifying the C2-framework that is bridging
between the phosphane and borane sites,11 but also by direct
Scheme 1
modification of the Lewis acid and Lewis base components.
Previously, we introduced the much less Lewis basic (C6F5)2P-
building block5 and we converted the (C6F5)2B-group to (C6F5)-
BH by treatment with 9-BBN.12 This resulted in an effective
C6F5/H exchange with formation of the (C6F5)-9-BBN product
(3) and the new P/BH FLP 2 (see Scheme 1). We have now rea-
lized both features in one system containing the (C6F5)2P and
the (C6F5)BH functionalities in a saturated vicinal FLP. This
new system shows some rather unique features that will be
described and discussed in this article.
aOrganisch-Chemisches Institut, WWU Münster, Corrensstraße 40, D 48149 Münster,
Germany. E-mail: erker@uni-muenster.de; Fax: +49-251-83-36503
Results and discussion
Formation and structural characterization of the vicinal
bInstitut für Physikalische Chemie and Graduate School of Chemistry, WWU
Münster, Corrensstraße 30, D 48149 Münster, Germany.
(C6F5)2P/B(C6F5)H FLP
E-mail: eckerth@uni-muenster.de; Fax: +49-251-83-29159
†Electronic supplementary information (ESI) available: Experimental, analytical
We previously showed that the hydroboration of (C6F5)2P–
C(CH3)vCH2 with Piers’ borane [HB(C6F5)2]13 yields the vicinal
(C6F5)2P/(C6F5)2B-FLP 4.5 Compound 4 shows some typical FLP
details and crystallographic data. CCDC 1001587–1001589. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt02081b
‡These authors contributed equally.
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Scheme 2
reactions. However, it turns out to be less reactive than e.g. its
analogue 1 featuring the stronger basic Mes2P-Lewis base com-
ponent. Since the FLP 1 readily underwent the (C6F5) vs. H
exchange reaction (see Scheme 1)12 we also treated the
(C6F5)2P/(C6F5)2B-FLP 4 with the 9-BBN reagent (Scheme 2).
For that purpose compound 4 was generated in situ from the
(C6F5)2P–C(CH3)vCH2 phosphane and HB(C6F5)2 (toluene solu-
tion, 15 min, r.t.). Then 9-BBN was added and the mixture was
stirred overnight at room temperature. This resulted in the
anticipated C6F5/H exchange reaction at the borane site, and it
left the (C6F5)2P-unit untouched. The resulting product (5)3
forms during the reaction time as an insoluble precipitate that
was isolated in 45% yield. The product was characterized by C,
H-elemental analysis, X-ray diffraction and solid-state NMR
spectroscopy (see below).
The X-ray crystal structure analysis confirms that one C6F5
substituent at boron was replaced by hydride. However, the
resulting vicinal P/B FLP (C6F5)2P–C(CH3)–CH2–B(C6F5)H (5)
was not found as such but it had become a subunit of its cyclo-
trimer (5)3. The trimer is formed from three symmetry equi-
valent monomeric units. Each of them features a close to anti-
periplanar [P]-CHMe–CH2-[B] conformation [dihedral angle
P1–C1–C2–B1 169.2(2)°]. The three P/B FLP monomers are
each connected by P–B bonds (B1–P1A: 2.006(3) Å). The trimer
shows three symmetry equivalent stereogenic centers (at C1)
and three symmetry-equivalent hydridoborate centers; the
overall structure of (5)3 possesses the symmetry C3. Within the
cyclic structure the B1–B1A separation is 5.109 Å, and the
P1–P1A distance is found to be 5.758 Å. There are three boron–
phosphorus distances: the direct B1–P1A bond (see above), the
B1–P1 separation of 4.236 Å within the monomeric P/B FLP
unit and the transannular B1–P1B distance which amounts to
6.149 Å (see Fig. 1). The central core of compound (5)3 does
not deviate much from co-planarity. The P1⋯P1A⋯P1B and
the B1⋯B1A⋯B1B planes are only 0.421 Å apart. The planes of
the C6F5 groups at both boron and phosphorus are oriented
more or less normal to the central core plane of the cyclotrimer.
Fig. 1 A projection of the C3-symmetric P/BH cyclotrimer (5)3 (thermal
ellipsoids are shown with 15% probability).
structure and bonding. Many systems still possess weak
covalent interactions between the two Lewis centers moderat-
ing the FLP reactivity,11c,14 and various solid-state NMR para-
meters such as 11B quadrupolar coupling constants,
11B isotropic chemical shifts and 11B–31P spin–spin coupling
constants can serve to quantify the “degree of frustration” of
the investigated P/B systems. Based on these parameters, three
groups of P/B systems can be differentiated: (i) classical Lewis
adducts showing no FLP reactivity (e.g. the adduct between
PPh3 and B(C6F5)3),15,16 (ii) P/B systems with weak interactions
between the centers (such as vinylene-bridged FLPs)11a
showing moderate FLP reactivities and (iii) non-interacting
FLPs (e.g. the norbornylene-bridged intramolecular P/B
system),17 being extremely reactive. Solid-state NMR can serve
as a principal tool for understanding its reactivity in terms of
structure and bonding properties classifying the reactivity.
REDOR-type techniques11c,18 allow a very accurate determi-
nation of P⋯B distances, which are very important structural
parameters for understanding the potential of those systems
in FLP-type cooperative reactions. The recently proposed new
homonuclear re-coupling technique called DQ-DRENAR
(Double-Quantum based Dipolar Recoupling Effects Nuclear
Alignment Reduction)19,20 allows the determination of homo-
nuclear P–P distances, which opens up the possibility of ana-
lyzing the long-range 31P–31P dipolar interactions for proving
the presence of oligomeric structures. Due to the insolubility
of the cyclic trimer (5)3 in all common organic solvents, solid-
state NMR is a key technique for its structural characterization,
including the assessment of B/P intramolecular interaction
strengths. Fig. 2 shows the 11B{1H} MAS NMR spectrum of (5)3.
One single resonance is detected, confirming the crystallo-
graphic identity of the three boron atoms within the cyclo-
trimeric structure. The line shape is dominated by second order
quadrupolar perturbations. The corresponding parameters
(quadrupolar coupling constant CQ = 1.69 MHz and EFG asym-
Characterization of the cyclotrimer (5)3 by solid-state NMR
spectroscopy
Solid-state NMR techniques provide a good understanding of
the reactivity and catalytic activities of P/B FLPs in terms of
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Table 2 DFT-calculated 1J and 3J-coupling tensors for 1 (BP-86(PBE)/
TZ2P)a
Jiso(
31P–11B)/Hz Jxx/Hz Jyy/Hz Jzz/Hz Jxy/Hz ΔJ/Hz ηJ
1J 26.4
3J 31.9
−8.7
27.9
−0.03 87.9
28.8 38.9
−0,24 92.2
−0.04 10.6
0.14
0.13
a The following convention is used: ΔJ = Jzz − 1/2(Jxx + Jyy), ηJ = (Jyy
Jxx)/(Jzz − Jiso) and |Jzz − Jiso| > |Jxx − Jiso| > |Jyy − Jiso|.
−
Fig. 2 Left: 11B{1H} MAS NMR spectrum of (5)3 acquired at 9.4 T with a
spinning frequency of 13.0 kHz (a) and the corresponding line shape
simulation (b, for details see Table 1). Right: 31P{1H} CPMAS NMR spec-
trum of (5)3 measured at 9.4 T and a spinning frequency of 10.0 kHz.
Table 1 Experimentally and quantum chemically determined 11B and
31P solid-state NMR parameters
CQ
3%/
MHz
CS
CS
δ
(
11B)
ηQ
0.1
δ
(
31P)
Δσ(31P)
10/ppm
ησ(31P)
0.1
iso
iso
0.5/ppm
0.5/ppm
Fig. 3 (a) 11B{31P} REDOR curve of (5)3 measured at 7.05 T with a spin-
ning frequency of 14.0 kHz. The squares represent REDOR data and the
Exp. −14.6
Calc. −17.6a
1.69
0.35
17.4
99.3
0.57
1.67b
0.36b 26.4a
86.9c
0.62c
triangles compensated REDOR data (calibration factor
a = 1). (b)
a B3-LYP/def2-TZVP. b B97-D/def2-TZVP(mod.). c B3-LYP/TZVP. The
following convention is used: Δσ = σzz − 1/2(σxx + σyy), ησ = (σyy − σxx)/
(σzz − σiso) and |σzz − σiso| > |σxx − σiso| > |σyy – σiso|.
1H→31P{11B} CP-REAPDOR curve of (5)3 (for the conditions see (a)). The
curves in (a) and (b) represent numerical simulations: the dotted curves
belong to simulations based on the crystallographically determined B–P
distances (assuming a four-spin system) and the experimentally deter-
mined 31P CSA parameters, while the solid curves represent simulations
that additionally include a ΔJ-value of 100 Hz.
metry parameter ηQ = 0.35) as well as the isotropic chemical
shift δiso = −14.6 ppm are typical of a four-coordinated boron
atom with an unsymmetrical ligation pattern. These data are
in excellent agreement with the DFT calculated ones based on
the crystallographic data (see Table 1) and the CQ and δiso
values agree well with values determined for characteristic
intermolecular Lewis acid/Lewis base adducts (e.g. PPh3 and
B(C6F5)3).11c
The Wiberg bond order index21 close to one (0.85 in the
Cartesian Atomic Orbital, CAO, and 0.89 in the Natural Atomic
Orbital, NAO, basis) also suggests the presence of a covalent
boron–phosphorus single bond. The B–P internuclear distance
of the trimer (5)3 has been determined by 11B{31P} REDOR11c,18
and 31P{11B} REAPDOR14,22 experiments (see Fig. 3). The
numerical simulations (taking all the intramolecular B⋯P dis-
tances within the cyclotrimer, d = 2.01 Å, 4.24 Å, and 6.15 Å,
into consideration) leads in both experiments to very good
overall agreement with the prediction from the crystal struc-
ture. Upon close inspection of these results, one notices a
slight discrepancy, which could be remedied, in principle, by
assuming a slightly longer distance of 2.04 Å than the crystal-
lographically found distance of 2.006(0) Å.
The principal component of the 11B electric field gradient
tensor (Vzz) lies nearly parallel to the B–P bond vector indicat-
ing that the EFG is strongly dominated by the covalent inter-
actions among the two Lewis functionalities (the angle
between Vzz, B and P was determined by DFT calculations to be
14.5°, see ESI†). Altogether, the 11B MAS-NMR data indicate
rather strong interactions between the Lewis centers prevent-
ing any FLP-type reaction behavior.
The 31P{1H} CP-MAS NMR spectrum (see Fig. 2) reveals also
one single resonance at 17.4 ppm and the field-independent
linewidths indicate the influence of non-resolved 31P–11B spin–
spin interactions on the MAS-NMR line shape (FWHM (7.05 T)
= 188 Hz, FWHM (9.4 T) = 192 Hz). An attempt to measure
1J(31P–11B) by a heteronuclear J-resolved experiment was
thwarted by very short 11B and 31P spin–spin relaxation times.
Using GAUSSIAN09, this value was calculated to be 61.8 Hz
As previously discussed, such discrepancies between simu-
lated and experimental REDOR data can arise from the effect
of non-zero contributions of the J-anisotropy (ΔJ
∼
100 Hz).11c,14 Therefore, the numerically simulated REDOR
and REAPDOR curves assuming a ΔJ value of 100 Hz indicate
perfect agreement with the experimental data. This result is
also consistent with DFT calculations of the full J-coupling
tensor of (5)3 yielding a ΔJ value of 92 Hz and ηJ = 0.14, on the
BP-86(PBE)/TZ2P level of theory (see Table 2). In any event, our
results highlight the fact that strong intermolecular B–P bonds
are formed between the monomeric units, which are even
slightly shorter than in the classical Lewis adduct between
PPh3 and B(C6F5)3.
3
(1J = 32.2 Hz, J = 29.6 Hz) on the B3-LYP/TZVP level of theory
which in general gives results in close agreement with experi-
mental data. The data in Table 2, calculated via ADF (Amster-
dam Density Functional), reveal that the 1J and 3J coupling
contributions to the isotropic J(31P–11B) value are of compar-
able size.
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Reactions of the cyclotrimer (5)3
From a small series of chemical reactions we have obtained
evidence that the monomer might be available from (5)3 by
equilibration. We have isolated the corresponding trapping
products in a few cases. The reaction of (5)3 with pyridine is a
typical example. A suspension of the trimer was stirred with
3 molar equivalents of pyridine overnight. The resulting
product was then crystallized from pentane at −34 °C to give
compound 6 as a white solid. It was isolated in 43% yield. The
X-ray crystal structure analysis (see below) showed that we had
obtained and isolated the pyridine adduct to the boron atom
of the FLP monomer 5. This has resulted in the formation of a
stereogenic boron center. Together with the already present
persistent carbon chirality center in the α-position to phos-
phorus this may give rise to a pair of diastereomers. The NMR
analysis of the product in solution actually showed that both
diastereomers are present in a ratio of 6major : 6minor = 2 : 1, as
revealed by the observation of a pair of slightly different sets of
NMR signals (see Table 3). In the 19F NMR spectra we have e.g.
observed separate sets of o- and p-19F NMR signals of the
single C6F5-ligand at boron for the two isomers (the m-C6F5B
resonances are overlapping). The C6F5 substituents at phos-
phorus are diastereotopic and, consequently, give rise to a pair
of separate 19F NMR features for each diastereoisomer. Accord-
ingly, we also observe pairs of p-C6F5[P] 19F NMR resonances
for each diastereomer (in a ca. 2 : 1 intensity ratio). Actually
each of these signals appears as a triplet of triplets due to the
coupling of each p-C6F5[P] fluorine nucleus with a pair of adja-
cent m- and o-fluorine atoms (the spectra are depicted in the
Fig. 4 31P–31P DQ-DRENAR curve of 1 measured at 7.05 T with a spin-
ning frequency of 7.0 kHz and SIMPSON simulations based on a three
spin-system with and without 31P CSA effects.
The geometry of (5)3 can be further characterized on the
basis of the homonuclear dipole–dipole interactions between
the constituent 31P nuclei within the cyclotrimer. To this end,
a new homonuclear recoupling technique, denoted DQ-DRE-
NAR (Double-Quantum based Dipolar Recoupling Effects
Nuclear Alignment Reduction), was used. This method uses a
symmetry-based POST-C7 pulse sequence for the stimulation
of DQ-coherences, thereby leading to a “drainage” of z-magne-
tization. To distinguish this decrease from that caused by
other sources (relaxation, experimental imperfections),
a
1
corresponding reference experiment is carried out in which
the DQ-Hamiltonian is absent and the difference between
both signal functions is recorded. Fig. 4 compares the 31P–31P
DQ-DRENAR curve of (5)3 with the simulation based on a
three-spin system in a trigonal geometry with P–P distances of
5.758(0) Å. The simulation includes the effect of the chemical
shift anisotropy on the DRENAR curve, which is quite con-
siderable in the present case. For the initial data range (ΔS/S0
< 0.2) excellent agreement between simulation and experiment
can be noted. For longer mixing times a significant deviation
is observed due to the fact that the first order approximation
on which the simulation is based fails. Since the intermolecu-
lar P–P distances between different molecular units in mono-
meric P/B systems are significantly longer, the DRENAR results
are clearly consistent with the trigonal P–P–P geometry
suggested by the crystallographic data. Further work in our
laboratory is in progress for developing new DRENAR-based
techniques which allow an analysis of the oscillatory parts
usually observed for longer evolution times. This part, which
was not accessible in the present application, is quite sensitive
to the detailed spin geometry. Overall, the results of the
present study clearly show the utility of this solid-state NMR
approach for the study of weakly interacting spins at longer
intermolecular distances, which will be important in future
studies of other intermolecular P/B-based FLP assemblies and
polymeric systems.
ESI†). We also observe the broad H NMR [B]-H resonance of
both 6major,minor at δ 3.71 and 3.67.
The single crystals that we used for the X-ray crystal struc-
ture determination of compound 6 contained only a single dia-
stereomer (see Fig. 5). It shows a tricoordinate phosphorus
atom that has a pair of C6F5 substituents and the C1 carbon
atom of the bridge bound to it, resulting in a trigonal-pyramid-
al geometry (∑P1CCC = 308.6°). Carbon atom C1 represents a
stereogenic center. The boron atom is tetracoordinated. It has
a C6F5-group, the pyridine donor ligand, carbon atom C2 of
the bridge and the hydride bound to it in a pseudotetrahedral
coordination geometry (∑B1NCC = 330.0°).
Compound 5 contains an active [B]H borane functionality,
which reacts e.g. with benzaldehyde. Treatment of a suspen-
sion of (5)3 in dichloromethane overnight at room temperature
with a stoichiometric amount of PhCHO gave product 7, which
we isolated in 76% yield (see Scheme 3). The compound is
characterized by a 10B NMR signal at δ 48.0 indicating the pres-
ence of a planar-tricoordinate Lewis acidic boron center. Again
the C6F5 groups at phosphorus are diastereotopic due to the
adjacent carbon chirality center. The newly formed OCH2Ph
substituent at boron gives rise to an AB 1H NMR pattern of the
methylene group (δ 5.12/5.11 [13C: 71.4]). For additional data
see Table 3 and the ESI.†
Compound 7 adds pyridine to give the respective [B]-pyri-
dine adduct 8, which was isolated as an oil in 84% yield. The
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Table 3 Selected NMR parametersa of compounds (C6F5)2PCH(Me)CH2B(C6F5)R(L)
Compd.
6major
H
6minor
H
7
8
9
10major
CHvCHtBu
pyr
10minor
CHvCHtBu
pyr
R
L
OCH2Ph
OCH2Ph
pyr
CHvCHtBu
pyr
pyr
—
—
31P
10B
1H
−37.7
−5.1
−36.9
−5.1
1.00
1.06
2.93
26.8
25.9
−35.5
48.0
1.39
−36.1
10.5
1.29
1.00
2.86
25.9
24.5
−36.4
71.5
1.83
1.61
3.64
33.1
25.3
−37.7
0.4
1.03
−35.7
0.4
1.31
0.91
2.92
28.7
25.3
BCH2
1.08
0.81
3.00
25.6
25.9
1H
CH
BCH2
CH
3.62
27.9
23.6
3.10
28.7
25.3
13C
13C
a NMR spectra in CD2Cl2, chemical shifts in ppm (δ-scale).
pyridine unit (for details see the ESI†). In this case we have
apparently monitored the NMR signals of only a single dia-
stereoisomer. Consequently, the 19F NMR spectrum of 8 shows
three sets of C6F5 resonances in a 1 : 1 : 1 intensity ratio: two
originating from the pair of diastereotopic C6F5 groups at
phosphorus and one from the single C6F5 substituent at boron
(the spectra are depicted in the ESI†).
The [B]H functionality of compound 5 can be used for
hydroboration.23 As a typical example we treated a suspension
of the trimer (5)3 with tert-butylacetylene in CD2Cl2 overnight.
Compound 9 was obtained as a colorless oil and recovered in
90% yield. It features a typical 10B NMR signal of a planar-tri-
coordinate borane Lewis acid (δ 71.5) and a 1H NMR AX
pattern of the newly formed trans-[B]-CHvCH-tBu moiety
Fig. 5 A view of the molecular structure of one diastereomer of the
pyridine adduct 6 (thermal ellipsoids are shown with 15% probability).
3
(δ 6.82, 6.45, JHH = 17.7 Hz). It also shows the typical set of
19F NMR signals of the three C6F5-groups at boron and
phosphorus.
Compound 9 also added pyridine. After crystallization from
pentane at −34 °C we obtained the [B]-pyridine adduct 10 in
63% yield as a mixture of two diastereoisomers in a 10 : 6 ratio.
Both these isomers show 10B NMR signals typical of four-co-
ordinate pseudotetrahedral boron (see Table 3) and 31P NMR
signals typical of three-coordinate trigonal pyramidal phos-
1
phorus. Each isomer shows the typical H NMR features of a
trans-CHvCH-tBu substituent at boron (for details see the
ESI†).
We have obtained crystals of one diastereoisomer by
slow crystallization from a heptane–dichloromethane solution.
In the crystal this diastereoisomer of 10 shows an
extended antiperiplanar conformation of the central unit with
a dihedral angle P1–C1–C2–B1 of −177.1(3)°. The phosphorus
atom features
a
three-coordinate trigonal pyramidal
coordination geometry (∑P1CCC = 308.3°). The pseudotetra-
hedral coordinated boron atom has a single C6F5 substituent
in addition to carbon atom C2 of the bridge and the newly
added pyridine ligand (see Fig. 6). The fourth coordination
position is taken by the σ-CHvCH-tBu substituent. It features
Scheme 3
1H/13C NMR spectra show the typical signals of the a trans disubstituted CvC double bond (C4–C5: 1.303(5) Å,
[P]CHMeCH2[B] subunit (see Table 3), the signals of the θ B1–C4–C5–C6: −172.8(4)°); at its distal carbon atom C5 we
OCH2Ph substituent and the resonances of the coordinated find the bulky tBu substituent attached.
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do not show any appreciable P–B interaction any more as
revealed by their characteristic NMR parameters. This behavior
leaves room for the chemical functionalization of the system 5;
so we shall see if derivatives of 5 and related B(C6F5)H contain-
ing P/B systems will become useful in the development of
frustrated Lewis pair chemistry.
Experimental part
Solid-state NMR experiments were performed on a BRUKER
Avance 400 and a BRUKER Avance III 300 spectrometer equipped
with 4 mm double and triple resonance probes. 11B{1H} MAS
NMR studies were conducted at a Larmor frequency of
128.4 MHz. The single-pulse spectrum was acquired at a spin-
ning frequency of 13.0 kHz and a relaxation delay of 5 s, using
a 45° excitation pulse (2.0 μs in length) applying the TPPM-15
1H decoupling scheme during acquisition.24 31P{1H} CPMAS
NMR experiments were conducted at 7.05 T and 9.4 T with
rotation frequencies of 3.0–10.0 kHz using the following acqui-
Fig. 6 A view of the molecular structure of one diastereoisomer of 10
(thermal ellipsoids are shown with 15% probability).
1
sition parameters: H 90° pulses of 5.0–5.5 μs length, contact
Conclusions
time 5 ms, relaxation delays 2–5 s. {1H}→11B{31P} CP-REDOR
and {1H}→31P{11B} CP-REAPDOR experiments were conducted
at 7.05 T using radio frequency power levels corresponding to
180° pulses of 8.0 μs length for 11B and for 31P, respectively.
A spinning frequency of 14.0 kHz was used in both cases. For
creating a reproducible magnetization in each experiment, a
pre-saturation comb consisting of sixty 90° pulses was applied.
1H decoupling was achieved using the TPPM-15 scheme
during the evolution and data acquisition period with rf fields
corresponding to a 1H nutation frequency of 53 kHz. In the
case of the REAPDOR experiments the 11B resonance was
excited for one-third of the rotor period during the dipolar
evolution time. Simulations of REDOR and REAPDOR curves
were done with the SIMPSON code.25 DQ-DRENAR measure-
ments were conducted at 7.05 T applying a rotation frequency
of 7.0 kHz. All line shape simulations were performed using
the DMFIT software (version 2011).26
In this study we have prepared the vicinal intramolecular
Lewis acid/Lewis base pair 5 that is characterized by having a
P(C6F5)2 moiety of rather low Lewis basicity and a B(C6F5)H
Lewis acid component. Previous studies had shown that the
P(C6F5)2/B(C6F5)2 pairs do not undergo intra- or intermolecular
P/B adduct formation, probably because the Lewis basicity of
the former group is too low, although those systems showed a
limited set of FLP reactions.5 Our new vicinal P(C6F5)2/B(C6F5)
H Lewis pair 5 behaves differently: it features strong P–B inter-
actions that result in the formation of the observed unique
cyclotrimer (5)3. We think that two features may be responsible
for this behavior: first the presence of the small B(C6F5)H
Lewis acid moiety that will encourage adduct formation and,
second, the very low solubility of the cyclotrimerization
product that will remove the new vicinal FLP 5 from any equili-
brium situation by precipitation of (5)3.
The trimeric structure based on intermolecular P/B inter-
actions has been verified by modern solid-state NMR tech-
niques. Those studies reveal strong P⋯B covalent interactions
as reflected by 11B solid-state NMR parameters which are very
similar to those values observed for classical Lewis acid/-base
adducts. 11B{31P} REDOR and 31P{11B} REAPDOR measure-
ments allow an accurate determination of the corresponding
P⋯B distances, which agree well with the crystallographically
determined values if non-zero anisotropic contributions of the
J-coupling tensor are taken into account. The recently develo-
ped DQ-DRENAR sequence was applied for the first time in
FLP-related systems for measuring the weak long-range 31P–31P
homonuclear three-spin dipolar interaction within the cyclo-
trimer. Thus, our studies suggest that this technique will be
useful for exploring P⋯P distance geometries in oligomeric
and polymeric FLP-based systems.
DFT calculations
Most calculations were carried out using the program packages
TURBOMOLE (version 6.3)27,28 and GAUSSIAN (version
GAUSSIAN09).29 The geometry was taken from the crystal struc-
ture and the positions of the hydrogen atoms were optimized
on the DFT meta-GGA (TPSS)30 level of theory applying the
recently developed D3(BJ) dispersion correction31 and
Ahlrich’s def2-TZVP32 basis set. All geometry optimizations
were performed within the TURBOMOLE program suite. In all
TURBOMOLE SCF calculations an energy convergence
criterion of 10−7Eh and in all geometry optimizations an
energy convergence criterion of 5 × 10−7Eh were chosen. The
integration grid was set to m4 33 and the RI approximation34,35
was used.
The calculation of the 11B electric field gradient was per-
formed on a GGA DFT level (functional B97-D)36 using the
program package GAUSSIAN09. The def2-TZVP basis set
obtained from the EMSL database37,38 was modified in such a
Despite its apparent stability and its low solubility, the
system (5)3 undergoes typical hydroboration reactions of the
pendant B(C6F5)H moiety. The products are monomeric and
15164 | Dalton Trans., 2014, 43, 15159–15169
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Paper
way that tighter basis functions on the boron atom (extracted (ca. 5 mL) and the formed suspension was stored at −34 °C.
from the cc-pCVTZ basis set;39,40 for details see ref. 11c) were After two days the formed colourless precipitate was isolated
included for having a more accurate description of the region and dried in vacuo (24.1 mg, 43%). In the CD2Cl2 solution of
near the boron nucleus. The GAUSSIAN output files were ana- the obtained solid two diastereomers 6major and 6minor were
lysed using the program EFGShield,41 version 2.2, for determi- observed (major/minor ∼ 2/1). Crystals suitable for the X-ray
nation of CQ and η values and visualizing the orientation of single crystal structure analysis were obtained by slow crystalli-
the electric field gradient tensor in the molecular geometry sation from a dichloromethane solution of compound 6. Anal.
using the DIAMOND software.42 The magnetic shielding calcu- calc. for C26H12BF15NP (665.15 g mol−1): C 46.95, H 1.82,
lations were performed within the GIAO (gauge independent N 2.11; found: C 47.39, H 1.93, N 2.19. IR (KBr) ˜ν = 2419 (w,
1
atomic orbitals) framework.43,44 For 11B, magnetic shieldings B–H). 6major: H NMR (500 MHz, CD2Cl2, 299 K): δ = 8.55 (m,
were calculated on the B3-LYP45,46 level of theory with the 2H, o-Py), 8.05 (m, 1H, p-Py), 7.60 (m, 2H, m-Py), 3.71 (br d,
4
def2-TZVP basis set using the TURBOMOLE program package. 1JBH ∼ 90 Hz, 1H, BH), 3.00 (m, 1H, CH), 1.20 (dd, JPH
=
3
Chemical shifts are referenced to BF3*(OEt2) using B2H6 21.2 Hz, JHH = 6.7 Hz, 3H, CH3), 1.08, 0.81 (each m, each 1H,
(δ(B2H6) = 16.6 ppm vs. BF3/Et2O) as an external standard CH2B). 13C{1H} NMR (126 MHz, CD2Cl2, 299 K): δ = 147.0
(σB3-LYP(B2H6) = 84.23 ppm).47–50 31P chemical shifts were cal- (o-Py), 141.4 (p-Py), 126.3 (m-Py), 25.9 (m, CH), 25.6 (br, CH2B),
2
culated with the B3-LYP functional and the def2-TZVP basis 18.5 (d, JPC = 26.1 Hz, CH3), [C6F5 not listed]. 31P{1H} NMR
3
set. Chemical shifts were referenced to phosphoric acid (202 MHz, CD2Cl2, 299 K): δ = −37.7 (quin, JPF = 30.6 Hz). 10B
(σB3-LYP = 274.31 ppm). 31P chemical shift anisotropy (CSA) NMR (54 MHz, CD2Cl2, 299 K): δ = −5.1 (ν1/2 ∼ 150 Hz). 19F
parameters were calculated on the B3-LYP TZVP51 level of NMR (564 MHz, CD2Cl2, 299 K): δ = −129.4 (m, 2F, o-C6F5P),
3
theory using the program package GAUSSIAN09. 31P/11B in- −151.5 (tm, JFF = 20.4 Hz, 1F, p-C6F5P), −162.1 (m, 2F,
direct spin–spin coupling constants were calculated on the m-C6F5P), [Δδ19FpPm = 10.6]; −130.0 (m, 2F, o-C6F5P), −152.1 (tm,
B3-LYP TZVP level of theory using the program package 3JFF = 20.4 Hz, 1F, p-C6F5P), −161.9 (m, 2F, m-C6F5P), [Δδ19FPpm
=
GAUSSIAN09. Wiberg bond order indices52 in the Cartesian 9.8]; −134.4 (m, 2F, o-C6F5B), −159.5 (t, JFF = 19.9 Hz, 1F,
3
Atomic Orbital (CAO) basis were calculated from the TPSS p-C6F5B), −165.0 (m, 2F, m-C6F5B), [Δδ19FpBm = 5.5]. 6minor 1H
:
Kohn–Sham determinants using the TURBOMOLE program NMR (500 MHz, CD2Cl2, 299 K): δ = 8.55 (m, 2H, o-Py), 8.11
package and those in the Natural Atomic Orbital (NAO) basis (m, 1H, p-Py), 7.64 (m, 2H, m-Py), 3.67 (br, 1H, BH), 2.93 (m,
4
3
using the Natural Bonding Orbitals (NBO) program 1H, CH), 1.16 (dd, JPH = 21.2 Hz, JHH = 6.6 Hz, 3H, CH3),
(version 3.1)53 as included in GAUSSIAN09. 1.06, 1.00 (each m, each 1H, CH2B). 13C{1H} NMR (126 MHz,
J-Anisotropies ΔJ were calculated using the program CD2Cl2, 299 K): δ = 147.1 (o-Py), 141.6 (p-Py), 126.5 (m-Py), 26.8
2
package ADF and the CPL program included therein.54–56 For (br, CH2B), 25.9 (m, CH), 18.2 (d, JPC = 25.3 Hz, CH3), [C6F5
the J coupling tensors the Fermi contact (FC), paramagnetic not listed]. 31P{1H} NMR (202 MHz, CD2Cl2, 299 K): δ = −36.9
3
spin orbit (PSO), diamagnetic spin orbit (DSO), as well as spin (quin, JPF = 28.4 Hz). 10B NMR (54 MHz, CD2Cl2, 299 K): δ =
dipolar interaction (SD) contributions were considered. The −5.1 (ν1/2 ∼ 150 Hz). 19F NMR (564 MHz, CD2Cl2, 299 K): δ =
GGA functional BP-8657,58 was used for the SCF run together −129.7 (m, 2F, o-C6F5P), −151.6 (tm, JFF = 20.6 Hz, 1F,
3
with the PBE59,60 functional for determining the first-order p-C6F5P), −161.76 (m, 2F, m-C6F5P), [Δδ19FpPm = 10.2]; −129.8
perturbed MOs in the calculation of NMR properties. Those (m, 2F, o-C6F5P), −151.8 (tm, JFF = 20.3 Hz, 1F, p-C6F5P),
3
P
calculations were performed with the TZ2P basis set.61 The −161.80 (m, 2F, m-C6F5P), [Δδ19Fpm = 10.1]; −133.3 (m, 2F,
3
integration keyword was set to 6.0 and an SCF convergence cri- o-C6F5B), −159.7 (t, JFF = 20.1 Hz, 1F, p-C6F5B), −165.0 (m, 2F,
terion of 10−7Eh was used for the spin–spin coupling constant m-C6F5B), [Δδ19FpBm = 5.3].
calculations.
Preparation of compound 7. Benzaldehyde (8.6 μL,
Preparation of compound (5)3. (C6F5)2P–C(CH3)vCH2 0.09 mmol, 3.0 eq.) was added to a suspension of compound
(80.0 mg, 0.20 mmol, 1.0 eq.) and bis(pentafluorophenyl) (5)3 (50.0 mg, 0.03 mmol, 1.0 eq.) in CD2Cl2 (0.8 mL) and
borane (68.1 mg, 0.20 mmol, 1.0 eq.) were dissolved in toluene reacted overnight. Then all volatiles were removed and the
(10 mL). After stirring for 15 min 9-BBN (24.0 mg, 0.20 mmol, obtained solid was dried in vacuo to give compound 7
1.0 eq.) was added. The solution was stirred overnight and the (47.1 mg, 76%). Anal. calc. for C28H13BF15OP (692.17 g mol−1):
formed colourless precipitate was isolated and dried in vacuo C 48.59, H 1.89; found: C 48.63, H 1.82. 1H NMR (500 MHz,
(53.2 mg, 45%). Crystals suitable for the X-ray single crystal CD2Cl2, 299 K): δ = 7.38 (m, 2H, m-Ph), 7.32 (m, 1H, p-Ph), 7.31
2
structure analysis were obtained simultaneously with the iso- (m, 2H, o-Ph), 5.12, 5.11 (each d, JHH = 12.7 Hz, each 1H,
4
lated colourless precipitate. Mp: 192 °C (DSC). Anal. calc. CH2O), 3.62 (m, 1H, CH), 1.39 (m, 2H, CH2B), 1.17 (dd, JPH
=
3
for C63H21B3F45P3 (1758.12 g mol−1): C 43.04, H 1.20; found: 20.1 Hz, JHH = 6.9 Hz, 3H, CH3). 13C{1H} NMR (126 MHz,
C 42.99, H 1.39. IR (KBr) ˜ν = 2353 (m, B–H). CD2Cl2, 299 K): δ = 137.9 (i-Ph), 129.0 (m-Ph), 128.4 (p-Ph),
Preparation of compounds 6major and 6minor. Pyridine 127.1 (o-Ph), 71.4 (CH2O), 27.9 (br, CH2B), 23.6 (m, CH), 19.6
2
(7.0 μL, 0.09 mmol, 3.0 eq.) was added to a suspension of com- (d, JPC = 24.0 Hz, CH3), [C6F5 not listed]. 31P{1H} NMR
3
pound (5)3 (50.0 mg, 0.03 mmol, 1.0 eq.) in dichloromethane (202 MHz, CD2Cl2, 299 K): δ = −35.5 (quin, JPF = 30.4 Hz). 10B
(10 mL) and stirred overnight. Then all volatiles were removed NMR (54 MHz, CD2Cl2, 299 K): δ = 48.0 (ν1/2 ∼ 450 Hz). 19F
in vacuo and the remaining solid was dissolved in pentane NMR (470 MHz, CD2Cl2, 299 K): δ = −129.9 (m, 2F, o-C6F5P),
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3
−150.7 (tm, JFF = 20.4 Hz, 1F, p-C6F5P), −161.2 (m, 2F,
Preparation of compounds 10major and 10minor. tert-Butyl-
m-C6F5P), [Δδ19FpPm = 10.5]; −129.8 (m, 2F, o-C6F5P), −150.6 (br acetylene (10.5 μL, 0.09 mmol, 3.0 eq.) was added to a suspen-
3
t, JFF = 19.1 Hz, 1F, p-C6F5P), −161.2 (m, 2F, m-C6F5P), sion of compound (5)3 (50.0 mg, 0.03 mmol, 1.0 eq.) in
[Δδ19FPpm
=
10.6]; −133.3 m, 2F, o-C6F5B), −152.5 (tm, dichloromethane (10 mL) and stirred overnight. Then pyridine
3JFF = 19.6 Hz, 1F, p-C6F5B), −161.5 (m, 2F, m-C6F5B), [Δδ19FBpm
=
(7.0 μL, 0.09 mmol, 3.0 eq.) was added and the reaction
9.0].
mixture was stirred overnight again. All volatiles were removed
Preparation of compound 8. Benzaldehyde (8.6 μL, in vacuo and the remaining residue was dissolved in pentane
0.09 mmol, 3.0 eq.) was added to a suspension of compound (ca. 5 mL) and stored at −34 °C. After two days the formed col-
(5)3 (50.0 mg, 0.03 mmol, 1.0 eq.) in dichloromethane (10 mL) ourless precipitate was isolated and dried in vacuo (39.8 mg,
and stirred overnight. Then pyridine (7.0 μL, 0.09 mmol, 3.0 eq.) 63%). In the CD2Cl2 solution of the obtained solid two diaster-
was added and the reaction mixture was stirred overnight eomers 10major and 10minor were observed (major/minor ∼ 5/3).
again. All volatiles were removed in vacuo and the remaining Crystals suitable for the X-ray single crystal structure analysis
residue was dissolved in pentane (ca. 5 mL) and stored at were obtained by slow crystallisation from a heptane–dichloro-
−34 °C. After two days the formed oil was isolated and dried methane solution of 10. Mp: 259 °C. Anal. calc. for
in vacuo (55.4 mg, 84%). Anal. calc. for C3H18BF15NOP C32H22BF15NP (747.29 g mol−1): C 51.43, H 2.97, N, 1.87;
(771.27 g mol−1): C 51.39, H 2.35, N 1.82; found: C 51.65, H found: C 51.23, H 2.71, N, 1.81. 10major
:
1H NMR (600 MHz,
1
2.23, N 1.74. H NMR (500 MHz, CD2Cl2, 299 K): δ = 8.68 (br CD2Cl2, 299 K): δ = 8.47 (m, 2H, o-Py), 8.06 (m, 1H, p-Py), 7.60
3
4
m, 2H, o-Py), 8.05 (br m, 1H, p-Py), 7.60 (br m, 2H, m-Py), 7.33 (m, 2H, m-Py), 6.21 (dt, JHH = 17.7 Hz, JHH = 2.2 Hz, 1H,
3
(m, 2H, o-Ph), 7.30 (m, 2H, m-Ph), 7.21 (m, 1H, p-Ph), 4.52, BCH), 4.84 (d, JHH = 17.7 Hz, 1H, vCH), 3.10 (m, 1H, CH),
2
4.29 (each d, JHH = 13.0 Hz, each 1H, CH2O), 2.86 (m, 1H, 1.03 (m, 2H, CH2B), 1.00 (dd, 4JPH = 21.6 Hz, 3JHH = 6.5 Hz, 3H,
4
CH), 1.29, 1.00 (each m, each 1H, CH2B), 1.10 (dd, JPH
=
CH3), 0.92 (s, 9H, tBu). 13C{1H} NMR (151 MHz, CD2Cl2,
21.7 Hz, JHH = 6.2 Hz, 3H, CH3). 13C{1H} NMR (126 MHz, 299 K): δ = 146.8 (vCH), 146.2 (o-Py), 141.3 (p-Py), 133.8
3
CD2Cl2, 299 K): δ = 145.9 (o-Py), 142.0 (i-Ph), 140.7 (p-Py), 128.4 (br, BCH), 126.0 (m-Py), 34.1 (tBu), 29.7 (tBu), 28.7 (br, CH2B),
3
(m-Ph), 127.0 (p-Ph), 126.7 (o-Ph), 125.7 (m-Py), 65.9 (CH2O), 25.3 (br m, CH), 19.2 (d, JPC = 25.9 Hz, CH3), [C6F5 not
2
25.9 (br, CH2B), 24.5 (m, CH), 18.5 (d, JPH = 23.5 Hz, CH3), listed]. 31P{1H} NMR (243 MHz, CD2Cl2, 299 K): δ = −37.7
3
[C6F5 not listed]. 31P{1H} NMR (202 MHz, CD2Cl2, 299 K): δ = (quin, JPF = 29.8 Hz). 10B NMR (64 MHz, CD2Cl2, 299 K): δ =
−36.1 (quin, 3JPF = 27.5 Hz). 10B NMR (54 MHz, CD2Cl2, 299 K): 0.4 (br, ν1/2 ∼ 200 Hz). 19F NMR (564 MHz, CD2Cl2, 299 K): δ =
δ = 10.5 (ν1/2 ∼ 150 Hz). 19F NMR (470 MHz, CD2Cl2, 299 K): δ = −129.2 (m, 2F, o-C6F5P), −151.2 (tm, JFF = 20.3 Hz, 1F,
3
−129.2 (m, 2F, o-C6F5P), −151.0 (tm, JFF = 20.5 Hz, 1F, p-C6F5P), −161.9 (m, 2F, m-C6F5P), [Δδ19FpPm = 10.7]; −130.0 (m,
3
3
p-C6F5P), −161.5 (m, 2F, m-C6F5P), [Δδ19FpPm = 10.4]; −129.8 (m, 2F, o-C6F5P), −152.1 (tm, JFF = 20.3 Hz, 1F, p-C6F5P), −161.9
2F, o-C6F5P), −151.7 (tm, JFF = 19.8 Hz, 1F, p-C6F5P), −161.7 (m, 4F, m-C6F5P), [Δδ19FpPm = 9.8]; −132.4 (m, 2F, o-C6F5B),
3
(m, 2F, m-C6F5P), [Δδ19FPpm = 10.0]; −133.0 (m, 2F, o-C6F5B), −159.7 (t, JFF = 20.2 Hz, 1F, p-C6F5B), −164.9 (m, 2F, m-C6F5B),
3
3
−157.8 (t, JFF = 19.6 Hz, 1F, p-C6F5B), −164.2 (m, 2F, m-C6F5B), [Δδ19FBpm = 5.2]. 10minor: 1H NMR (600 MHz, CD2Cl2, 299 K): δ =
[Δδ19FBpm = 6.4].
8.47 (m, 2H, o-Py), 8.10 (m, 1H, p-Py), 7.60 (m, 2H, m-Py), 6.12
3
3
Preparation of compound 9. tert-Butylacetylene (5.3 μL, (d, JHH = 17.8 Hz, 1H, BCH), 5.05 (d, JHH = 17.8 Hz, 1H,
0.09 mmol, 3.0 eq.) was added to a suspension of compound vCH), 2.92 (m, 1H, CH), 1.31, 0.91 (each m, each 1H, CH2B),
4
3
(5)3 (25.0 mg, 0.03 mmol, 1.0 eq.) in CD2Cl2 (0.8 mL) and 1.00 (dd, JPH = 21.6 Hz, JHH = 6.5 Hz, 3H, CH3), 0.95 (s, 9H,
reacted overnight. All volatiles were removed in vacuo to obtain tBu). 13C{1H} NMR (151 MHz, CD2Cl2, 299 K): δ = 147.9
compound 9 as a colourless oil (25.7 mg, 90%). Anal. calc. for (vCH), 146.2 (o-Py), 141.3 (p-Py), 132.1 (br, BCH), 126.0
C27H17BF15P (668.19 g mol−1): C 48.53, H 2.56; found: C 47.09, (m-Py), 34.2 (tBu), 29.7 (tBu), 28.7 (br, CH2B), 25.3 (br m, CH),
1
3
3
H 2.32. H NMR (500 MHz, CD2Cl2, 299 K): δ = 6.82 (d, JHH
=
19.2 (d, JPC = 23.1 Hz, CH3), [C6F5 not listed]. 31P{1H} NMR
17.7 Hz, 1H, vCH), 6.45 (d, 17.7 Hz, 1H, BCH), 3.64 (m, 1H, (243 MHz, CD2Cl2, 299 K): δ = −35.7 (quin, JPF = 28.0 Hz). 10B
CH), 1.83 (dt, 2JHH = 16.2 Hz, 3JPH = 3JHH = 10.8 Hz, 1H, CH2B), NMR (64 MHz, CD2Cl2, 299 K): δ = 0.4 (ν1/2 ∼ 200 Hz). 19F NMR
1.61 (m, 1H, CH2B), 1.14 (dd, 3JPH = 20.2 Hz, 3JHH = 6.8 Hz, 3H, (564 MHz, CD2Cl2, 299 K): δ = −129.1 (m, 2F, o-C6F5P), −151.5
3
CH3), 1.08 (s, 9H, tBu). 13C{1H} NMR (126 MHz, CD2Cl2, (tm, JFF = 20.2 Hz, 1F, p-C6F5P), −161.4 (m, 2F, m-C6F5P),
3
299 K): δ = 176.3 (vCH), 128.2 (br, BCH), 36.3 (tBu), 33.1 (br d, [Δδ19FpPm = 9.9]; −129.6 (m, 2F, o-C6F5P), −151.8 (tm, 3JFF = 21.0
2JPH = 20.7 Hz, CH2B), 28.5 (tBu), 25.3 (m, CH), 20.1 (d, JPH
=
Hz, 1F, p-C6F5P), −161.8 (m, 2F, m-C6F5P), [Δδ19FpPm = 10.0];
2
25.6 Hz, CH3), [C6F5 not listed]. 31P{1H} NMR (202 MHz, −132.2 (m, 2F, o-C6F5B), −160.2 (t, JFF = 20.2 Hz, 1F, p-C6F5B),
3
CD2Cl2, 299 K): δ = −36.4 (quin, JPF = 31.0 Hz). 10B NMR −165.0 (m, 2F, m-C6F5B), [Δδ19FpBm = 4.8].
3
(54 MHz, CD2Cl2, 299 K): δ = 71.5 (ν1/2 ∼ 600 Hz). 19F NMR
(470 MHz, CD2Cl2, 299 K): δ = −129.5 (m, 2F, o-C6F5P), −150.3
(tm, JFF = 20.3 Hz, 1F, p-C6F5P), −161.1 (m, 2F, m-C6F5P),
3
Acknowledgements
[Δδ19FPpm = 10.8]; −129.9 (m, 2F, o-C6F5P), −150.8 (tm, JFF
=
3
20.3 Hz, 1F, p-C6F5P), −161.2 (m, 2F, m-C6F5P), [Δδ19FpPm = 10.5]; Financial support from the Deutsche Forschungsgemeinschaft
3
−132.5 (m, 2F, o-C6F5B), −153.6 (t, JFF = 20.3 Hz, 1F, p-C6F5B), SFB 858 – Cooperativity in Chemistry – (project A1) and by
−162.5 (m, 2F, m-C6F5B), [Δδ19FpBm = 8.9].
project EC168/10-1 (Dynamic Supramolecular Associations) is
15166 | Dalton Trans., 2014, 43, 15159–15169
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Dalton Transactions
Paper
gratefully acknowledged. T.W. thanks the Fond der
Chemischen Industrie and the NRW-Forschungsschule Mole-
cules and Materials for a doctoral stipend. We additionally
thank Prof. Stefan Grimme (Technische Universität Bonn) for
his support with the DFT calculations.
T. Jurca and D. W. Stephan, Chem. Commun., 2008, 1701;
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