Molecular hydrogen tweezers: Structure and mechanisms by neutron diffraction, NMR, and deuterium labeling studies in solid and solution

Article abstract of DOI:10.1021/ja206394w

The mechanism of reversible hydrogen activation by ansa-aminoboranes, 1-N-TMPH-CH₂-2-[HB(C₆F₅)₂]C ₆H₄ (NHHB), was studied by neutron diffraction and thermogravimetric mass-spectroscopic exp

Full text of DOI:10.1021/ja206394w

ARTICLE
pubs.acs.org/JACS
Molecular Hydrogen Tweezers: Structure and Mechanisms by Neutron
Diffraction, NMR, and Deuterium Labeling Studies in Solid and
Solution
Felix Schulz,† Victor Sumerin,‡ Sami Heikkinen,‡ Bj€orn Pedersen,§ Cong Wang,‡ Michiko Atsumi,||
Markku Leskel€a,‡ Timo Repo,*,‡ Pekka Pyykk€o,‡ Winfried Petry,§ and Bernhard Rieger*,†
†WACKER-Lehrstuhl f€ur Makromolekulare Chemie and §Forschungs-Neutronenquelle Heinz Maier-Leibnitz, Technische Universit€at
M€unchen, Lichtenbergstrasse 4, D-85747 Garching bei M€unchen, Germany
‡Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
Department of Physical and Analytical Chemistry, Quantum Chemistry, Uppsala University, P.O. Box 518, SE-75120 Uppsala, Sweden
S Supporting Information
b
ABSTRACT: The mechanism of reversible hydrogen activation by ansa-aminoboranes,
1-N-TMPH-CH2-2-[HB(C6F5)2]C6H4 (NHHB), was studied by neutron diffrac-
tion and thermogravimetric mass-spectroscopic experiments in the solid state as well
as with NMR and FT-IR spectroscopy in solution. The structure of the ansa-
ammonium borate NHHB was determined by neutron scattering, revealing a short
NꢀH HꢀB dihydrogen bond of 1.67 Å. Moreover, this intramolecular HꢀH
3 3 3
distance was determined in solution to be also 1.6ꢀ1.8 Å by 1H NMR spectroscopic
T1 relaxation and 1D NOE measurements. The X-ray BꢀH and NꢀH distances
deviated from the neutron and the calculated values. The dynamic nature of the
molecular tweezers in solution was additionally studied by multinuclear and variable-
temperature NMR spectroscopy. We synthesized stable, individual isotopic isomers
NDDB, NHDB, and NDHB. NMR measurements revealed a primary isotope effect
p
in the chemical shift difference Δ1H(D) = δ(NH) ꢀ δ(ND) (0.56 ppm), and
hence supported dihydrogen bonding. The NMR studies gave strong evidence that the structure of NHHB in solution is similar to
that in the solid state. This is corroborated by IR studies providing clear evidence for the dynamic nature of the intramolecular
dihydrogen bonding at room temperature. Interestingly, no kinetic isotope effect was detected for the activation of deuterium
hydride by the ansa-aminoborane NB. Theoretical calculations attribute this to an “early transition state”. Moreover, 2D NOESY
NMR measurements support fast intermolecular proton exchange in aprotic CD2Cl2 and C6D6.
’ INTRODUCTION
Therefore, the splitting of H2 has received intensive interest from
numerous researchers.4 The discoveries of nonclassical H2 com-
plexes by Kubas et al.,5 and of dihydrogen bonds by Crabtree,
Siegbahn, and co-workers,6 are important milestones in this area.
The recently developed class of nonmetal compounds, called
“frustrated Lewis pairs” (FLPs), have received a great amount of
interest. They have been shown to activate dihydrogen and other
small molecules.7 In most cases the release of hydrogen gas from
corresponding onium boron hydrides is not thermodynamically
favored. If the reaction is almost thermodynamically neutral, the
inverse reaction, a facile hydrogen liberation, can take place under
mild conditions. Thus, some of the reported FLP systems were
shown to reversibly activate hydrogen8ꢀ10 and, more recently, to
act as efficient metal-free hydrogenation catalysts.11ꢀ13 Since a
complete absence of transition metals is required in pharmaceu-
tical products, due to toxicity concerns, FLPs are a very attractive
Although, or maybe because, dihydrogen is the simplest exist-
ing nonpolar molecule, it is one of the most important ones in in-
dustry. Catalytic hydrogenation reactions are among the most
massive man-made chemical reactions in the world. The hydro-
cracking of crude oil, the Haber-Bosch process for ammonia syn-
thesis, and the FischerꢀTropsch process, which is gaining in
importance lately due to rising oil prices, are just a few examples.1
Moreover, hydrogen is also employed in the manufacturing of
polyolefins and a vast variety of base chemicals like methanol,
cyclohexane, and aniline and in the hardening of oils and fats, and
it is essential for the synthesis of many specialty chemicals and
pharmaceuticals, especially in the form of asymmetric hydro-
genations.2 In these processes the activation of the dihydrogen
molecule is most often achieved by a transition-metal center in
homogeneous, heterogeneous, or biological catalysts.3 Because of
this importance of hydrogenation reactions, even minor improve-
ments can lead to substantial monetary savings. In order to achieve
this, a good understanding of the mechanistic details is indispensable.
Received: July 10, 2011
Published: November 16, 2011
r
2011 American Chemical Society
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Chart 1. Molecular Structure of the ansa-Ammonium Bor-
ates NHHB (X = Y = H), NDDB (X = Y = D), NDHB (X = D,
Y = H), and NHDB (X = H, Y = D)
alternative to conventional hydrogenation catalysts currently
applied in the pharmaceutical industry.14 With the help of FLPs
even asymmetric hydrogenations are achievable.15,16
For further development of improved FLP-based catalysts,
knowledge of the mechanistic and structural details of these
systems is of utmost importance. Many research groups have
studied the mechanistic aspects of hydrogen activation by FLPs
theoretically before.17 In particular the initial step has been dis-
cussed.18,19 Some structural features, like the prearrangement
Figure 1. 300 MHz VT NMR spectra of NHHB in CD2Cl2: (a) NH
(around 7.8 ppm) and C6H4 (around 7.1 ppm) 1H NMR signals;
(b) CH2 1H NMR signals; (c) BH 1H NMR signals; (d) 11B NMR signals.
of fluorine-containing FLPs in solution due to NꢀH
F or
and quadrupolar relaxation effects of the neighboring 14N nucle-
us.26 The signal is slightly shifted downfield (0.22 ppm), com-
pared to the chemical shift observed in C6D6. This can be ascribed
to the different polarities of the solvents. Unfortunately, no 1JHH
coupling across the dihydrogen bond is visible in either NH or BH
resonances due to the broad line widths.27 The doublet at 4.45
ppm can be assigned to the benzylic CH2 bridging the phenyl ring
to the 2,2,6,6-tetramethylpiperidine (TMP). The observed cou-
pling, 3JHH = 5.5 Hz, originates from coupling to the NH. This was
readily confirmed by disappearance of the aforementioned splitting
from the benzylic CH2 signal with the NDHB sample (see below).
In Figure 1b the CH2 signal is shown to split into one large
signal approximately at 4.34 ppm and one small signal at 4.44
ppm, which could be caused by dynamic equilibria between two
tautomers or two conformers. In the case of a dynamic equilib-
rium within the dihydrogen bond, the proton can take up two
different positions between the nitrogen and the hydride (H8)28
within the molecule: one relatively close to N and far away from
H8, and the second position the other way around (far from N
and so close to H8 that a side-on-bound H2 complex forms). This
can be compared to the tautomerism found in systems containing
intramolecular hydrogen bonds (Scheme 1). For conformational
isomerism, two structures, which convert into each other through
ring-inversion of the piperidine moiety, are presented in
Scheme 2: NHHB(1) is identical with the solid-state structure.
It is not unreasonable to assume that it is also the most stable one
in solution. Hints to the existence of its conformational isomer
NHHB(2) were recently found by us when studying similar com-
pounds where TMP was replaced by a 2,2,4-trimethyl-1,2,3,
4-tetrahydroquinoline moiety.16 In a study performed by Berke
and co-workers, a similar conformational equilibrium was found
in the piperidinium borate [TMPH]+[HB(C6F5)3]ꢀ.29
3 3 3
CꢀH F bonds, are also suggested by experiments.20
3 3 3
In a recent theoretical study on hydrogen activation by various
previously reported FLPs, Rokob et al. concluded that the re-
action has to be slightly exergonic at standard conditions so that
the reverse reaction is also feasible.8 NHHB (Chart 1) is one of
the few nitrogen-based systems that can release the bound hydro-
gen in solution upon heating, but the computed Gibbs free energy
was too low, pointing at an at least partly incorrect description by
the applied theoretical methods.11 Despite the indication that it
also contains one of the shortest NꢀH
HꢀB dihydrogen
3 3 3
bonds reported so far in the literature, its exact length could not
be obtained by X-ray diffraction due to the poor visibility of
H-atoms by this method. In order to clarify this, we were moti-
vated to carry out a neutron diffraction study on NHHB.
To the best of our knowledge, the FLP systems based on the
ansa-ammonium borate concept have also the highest catalytic
activity among previously reported organocatalytic systems for a
wide range of nitrogen-containing substrates.13,16,21 We further-
more elucidated the solution structure and the hydrogen exchange
1
mechanisms by H NMR T1 relaxation measurements, selective
deuteriumꢀhydrogen exchange, and characterization of the hydron-
loaded22 compounds (Chart 1) by multinuclear NMR, selec-
tive 1D NOE measurements, 2D NOESY NMR, IR spectroscopy
and high-resolution ESI-TOF (HRESI-TOF),23 variable-temperature
1
(VT) H, 11B NMR and VT FIR/MIR solution-state experi-
ments, and TG-MS of the solids.
’ RESULTS AND DISCUSSION
NMR Studies. The new NMR spectra of NHHB recorded in
CD2Cl2 essentially do not differ from the previously reported
spectra measured from C6D6 solutions.11 The 1H NMR spectrum
of NHHB in CD2Cl2 shows the broad quartet at 3.76 ppm with a
coupling constant of 77 Hz (1JHB), characteristic for the BH hy-
dride and corresponding to the splitting observed in the doublet
at ꢀ20.84 ppm in 11B NMR data. The chemical shift difference
between the fluorine signals (Δδp,m = 3.40 ppm) in 19F NMR
provides further evidence for an anionic four-coordinate borate.24
In the 1H NMR data, the broad singlet25 (Δν1/2 = 165 Hz) at 7.77
ppm is assigned to the NH proton (Figure 1a). The broadness of
the signal can be attributed to a combination of proton exchange
In VT NMR studies the coupling of the CH2 signals cannot be
observed at lower temperatures, but this is most probably caused
by a slower rotation, resulting in line broadening. The ratio be-
tween the two CH2 signals was estimated to be on the order of
9:1, which would also be the ratio of the corresponding rate con-
stants. As pointed out before, conformer NHHB(1) is the more
stable one, and hence k1 > k2. At the same time, additional signals
appear for the hydrogen atoms attached to the piperidine ring
upon lowering of the temperature.23 It is therefore believed that
two conformational isomers exist, as shown in Scheme 2.
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Scheme 1. Possible Tautomerism of NHHB in Solution
Scheme 3. Reversible D2 Activation by the System NB/
NDDB
value would indicate the existence of a strong H-bond, as outlined
by Dziembowska and Rozwadowski in detail—herein a strong
dihydrogen bond—in NHHB.31 Equilibrium and intrinsic iso-
tope effects on chemical shifts have been discussed in the litera-
ture. The first arise from different zero-point energies (ZPEs) in
the different states of the equilibrium and the second from geo-
metric isotope effects arising from anharmonic ground isotopic
vibrations.32
Scheme 2. Schematic Equilibrium between Two Conforma-
tional Isomers of NHHB in Solution
At first we concluded that the large value of the isotope effect
might indicate the existence of a double-well potential and thus
tautomerism within the system NXYB (X, Y = H or D) in
solution (Scheme 1), but it has been shown that all the above-
mentioned effects contribute to the observed values.33 There-
fore, the reported effects do not indicate the presence of tautom-
erism. If such a tautomerism existed then the inter-hydron
distance in NB-H2 would be so small that a side-on-bound H2
complex would be likely.34 Such a structure would imply a rapid
exchange between the two hydrons caused by classic and quan-
tum mechanical exchange.35 This in turn would result in a co-
alescence of the NH and BH signals which in fact is not observed.
Therefore, a tautomerism as shown in Scheme 1 and hence also a
double-well potential can be excluded.
The BH signal in Figure 1c is not shifted, but the shape of the
quartet is not resolved any more at low temperature. This is also
an effect of line broadening which could be caused by less rotational
freedom and faster relaxation. The corresponding effect can be
observed in the 11B NMR spectrum, too (Figure 1d). The nicely re-
solved doublet becomes broader with decreasing temperature,
but does not change otherwise. In addition, this is the only signal
that can be detected. Therefore, the environment of 11B and H8
does not seem to change dramatically with the conversion of
NHHB(1) to NHHB(2).
Literature reports on such isotope effects regarding dihydro-
gen bonding were not found; only the more common hydrogen
bonds to O, N, and S have been previously studied in detail.36 We
therefore performed VT 1H NMR studies of NHHB in C6D5Br
between 253 and 356 K and VT 2H NMR studies of the products
of HD splitting by NB in CH2Cl2 between 213 and 293 K.23 It
can clearly be seen that the NH signal of NHHB shifts to higher
field upon increasing the temperature, even though the peak can-
not be clearly seen due to overlapping aromatic signals. At the
same time, the ND signal shifts to lower field upon decreasing the
temperature. The magnitudeof the observed chemical shiftchange
is in both cases around 0.5 ppm. This suggests two possible struc-
tures of the NH and the ND moiety, respectively: high- and
low-temperature structures. Due to different binding strengths
of H and D, the change between the structures happens at
different temperatures in both cases. The structure of NH
changes between 293 and 316 K, and the structure of ND
changes between 243 and 273 K. It is thought that the two
structures might be caused by weak NHꢀsolvent interactions.
The change of NH/ND chemical shifts in altered measurement
temperatures follows well-documented behavior for common
hydrogen-bonding groups (water, alcohols, amines, etc).37 The
shift of X to lower field suggests more pronounced bonding of X
with N, i.e., a shorter NꢀX distance at lower temperatures
which allows X to share its electrons with N more efficiently,
leading to a deshielding of X. The shift to higher field at elevated
temperatures is caused by weaker NꢀX bonding, possibly
The deuterated ansa-ammonium borate NDDB was synthe-
sized by analogy to NHHB: A yellow solution of NB in pentane
reacted rapidly with D2 at room temperature to give a white sus-
pension of NDDB in quantitative yield (Scheme 3). At 110 ꢀC a
solution of NDDB in toluene turned yellow again like the ansa-
ammonium borate NHHB, indicating that NB can reversibly
activate D2 just like H2.
By comparison of the spectroscopic data for NDDB and
NHHB the following observations can be made: the 19F NMR
spectra are identical (Δδp,m = 3.40 ppm); the 11B resonance shows
a singlet at ꢀ21.07 ppm instead of a doublet; the 1H spectra are to
a large extentidentical with theexceptions that the broad quartetat
3.76 ppm and the broad singlet at 7.77 ppm are absent, and the
CH2 signal at 4.44 ppm is a singlet instead of a doublet. These data
are perfectly in line with the formulation of NDDB as the only
product. In this case, the small couplings to deuterons were not
resolved because the couplings are smaller (JHD ≈ JHH/6.5), and
the quadrupolar D may have faster relaxation.30
The 2H NMR spectrum shows the ND and BD resonances as
broad singlets25 at 7.21 and 3.76 ppm, respectively. The latter of
the two should appear as a quartet due to the coupling to the 11B
nucleus, but the coupling constant seems to be too small with re-
spect to the line width to allow the detection of the fine structure.
The ND signal is shifted upfield by 0.56 ppm in comparison to the
corresponding signal of NHHB in the 1H NMR spectrum. The
p
difference in chemical shift Δ1H(D) = δ(NH) ꢀ δ(ND) be-
tween the two hydrons is called a primary isotope effect. It is now
positive and quite large. When comparing it to literature, such a
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arising from an increased NꢀX distance, and therefore increased
electron density on X.
HꢀD Exchange in Solution. When NHHB is dissolved in a
methanol-d1/benzene mixture (40/60) at room temperature, an
HꢀD exchange takes place immediately. By removal of the so-
lvent and addition of fresh CH3OD for a second time, NHHB is
converted completely into NDHB. Protic solvents lead in the
same way to the back-conversion of NDHB into NHHB.
The 1H NMR spectrum of NDHB in CD2Cl2 exhibits a broad
quartet25 at δ = 3.72 ppm with a coupling constant of 1JHB= 76
Hz showing the existence of the BH hydride, as documented
before for NHHB. The broad singlet of the protic NH is missing,
but all other signals stay put. The CH2 resonance at 4.44 ppm is a
singlet due to the small coupling constant of ND. The 11B NMR
signal at ꢀ20.86 ppm (doublet, 1JBH = 77 Hz) and the chemical
shift difference of F atoms (Δδp,m = 3.40 ppm) are consistent
with a four-coordinate anionic boron.24 It can be concluded that
the steric and electronic structures of the molecule do not change
dramatically. At the same time a broad singlet25 at 7.20 ppm can
be observed in a 2H NMR spectrum as the only signal which is as-
signed to ND. The primary isotope effect exists in this molecule,
too, and it is of the same size (0.57 ppm) as in NDDB.
Figure 2. Expansion of the NOESY spectrum (500 MHz, 600 ms
mixing time) recorded from a C6D6 sample of NB reacted with HD.
Negative peaks (diagonal) are given by two sparsely placed contours.
Positive signals are depicted by several densely spaced contours. The
rectangle centered at 4.00, 7.70 ppm (f2, f1) shows the limits for 2D
integration to determine the NHꢀHB correlation peak volume. Effec-
tively, only half of the cross-peak volume is captured, and the data have
to be multiplied by 2 to estimate the actual NHꢀHB NOESY cross-peak
volume.
Upon warming of a colorless solution of NDHB in toluene-d8
up to 60 ꢀC for several hours and subsequent cooling, the mol-
ecule stays intact. Formation of NHDB did not take place, as
evidenced by multiple-nucleus NMR spectra, implying that no
rotation of D H occurred. In a very recent publication, Piers
3 3 3
et al. observed the same for the ion pair [tBu3PH]+[DB(C6F5)3]ꢀ
dissolved in C6D5Br up to 100 ꢀC.19 Above 65 ꢀC the solution
started to turn slightly yellow and signals for NB could be de-
tected in NMR spectra. The exchangeability seems to be re-
stricted to the N-bound hydron, whereas the B-bound hydron
always remains in the same place until NB is formed. This is
another direct experimental proof that NB-H2 and hence the
tautomerism depicted in Scheme 1 cannot exist.
further details). The NHꢀHB cross-peak volumes of both experi-
ments gave evidence for the existence of all four isomers (Figure 2).38
Consequently, control experiments of separately synthesized
equimolar solutions of NHDB and NDHB were performed. After
an equilibration time of several hours in CD2Cl2, the recorded 2D
(H,H) NOESY spectrum showed an NHꢀHB cross-peak volume
that can be ascribed to an approximately 0.25:0.25:0.25:0.25
mixture of all four possible isotopomers NHHB, NHDB, NDHB,
and NDDB, demonstrating an intermolecular proton exchange.
The same measurement was performed, applying the dry aprotic
and apolar C6D6 as solvent, and again the spectrum gave evidence
for the existence of all four isotopomers with approximate ratios
of 0.15:0.35:0.35:0.15; i.e., the exchange is less complete than in
CD2Cl2.23
As discussed before, careful sample preparation ensured that
only traces of water can be expected in them, rendering significant
contributions of NH exchange between residual water impossible.
On the other hand, minuscule amounts of water (μg/mL range)
cannot be ruled out, and water molecules could act as proton
carriers, enabling a proton exchange between two molecules of
NXYB through H2O. Another possible mechanism could be
that protons migrate via the free-electron pairs of the fluorine
atoms (or chlorine in the case of CD2Cl2 as solvent). The
hydrons could not be directly exchanged between two ammo-
nium groups because of missing free-electron pairs.
In order to synthesize NHDB, a procedure similar to that used
for NDHB was applied. This time NDDB was used as the starting
compound, which was dissolved in a mixture of methanol/benzene
(40/60) at room temperature. After stirring, evaporation of sol-
vents, and repeating these steps once, NHDB was obtained as a
white solid in near-quantitative yield.
The NH resonance is detected at 7.70 ppm as a broad singlet25
in the proton NMR spectrum, and the CH2 group causes a doublet
at 4.44 ppm (3JHH = 6 Hz) as observed in the corresponding spec-
trum of NHHB. No broad quartet appears around 3.8 ppm, but
instead a broad singlet25 shows up at 3.80 ppm in the 2H NMR
spectrum which is in line with a BD fragment. Additionally, the 11B
spectrum shows a singlet at ꢀ21.15 ppm, and the chemical shift
difference of F atoms is Δδp,m = 3.39 ppm, indicating a four-
coordinate anionic borate.24 NHDB behaves in the same way as
NDHB: only the N-hydron can be exchanged easily in solution,
but no rotation of HꢀD can be observed at elevated temperatures
before NB is formed.
Reaction of NB with HD. Reacting HD gas at room tempera-
ture with a yellowish pentane NB solution, a white precipitate
forms within minutes. By multinuclear NMR, FT-IR, or HRESI-
TOF MS analysis methods, it could not be determined whether
only NHDB and NDHB, or all four possible isomers (NHHB,
NHDB, NDHB, NDDB), formed. Therefore, additional 2D
NOESY NMR studies of pure NHHB, used as a standard, and
this reaction mixture were performed. Spectra were recorded first
in CD2Cl2 and later in C6D6 (see the Experimental Section for
In conclusion, the fast intermolecular proton exchange pre-
vents the unambiguous proof of an intramolecular mechanism of
hydrogen splitting by NB. Nevertheless, additional experiments
show that the very strong Lewis acid B(C6F5)3 remains un-
changed during H2 activation by NB.23 This and theoretical calcu-
lations support intramolecular hydrogen activation.11 After split-
ting of the hydrogen, the proton exchange can take place with the
help of trace amounts of water, as suggested here.
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Scheme 4. Possible Transition States during Hydrogen
Activation by NB
Table 1. Reaction Energies of NB + HD f NHDB and
NDHB from CAM-B3LYP/6-31G** Calculations
(in kJ/mol)a
Transition States
ΔqE
ΔqE + ZPE
ΔqH
ΔqG
NHDB
NDHB
NHHB
NDDB
21.08
21.08
21.08
21.08
33.41
33.39
31.37
35.65
25.93
25.90
27.50
24.49
68.07
68.01
68.54
67.66
Products
ΔE
ΔE + ZPE
ΔH
ΔG
NHDB
NDHB
NHHB
NDDB
ꢀ89.52
ꢀ89.52
ꢀ89.52
ꢀ89.52
ꢀ56.24
ꢀ58.78
ꢀ52.29
ꢀ62.00
ꢀ64.40
ꢀ66.45
ꢀ60.27
ꢀ69.92
ꢀ22.66
ꢀ24.57
ꢀ20.25
ꢀ26.45
a The corresponding data of NHHB and NDDB are included for com-
parison. Δq and Δ denote the activation and reaction energies. E,
E + ZPE, H, and G correspond to electronic energy, electronic + zero
point energy, enthalpy, and Gibbs free energy at 298.15 K and 1 atm,
respectively. The computed rate constant ratio results in kNHDB
NDHB = 0.98.
/
1
2
At the same time, the H and H NMR data of the reaction
products of NB and HD in solution exhibited a ratio between
N-bound H (or D) and B-bound H (or D) atoms of 1:1. Thus, no
kinetic isotope effect (KIE) can be observed, which is not sur-
prising because in the theoretical calculation of the H2 splitting
by NB an early transition state (TS in Scheme 4) is found in a
concerted mechanism.11 In this structure the H2 bond is only
slightly elongated and the BꢀH and NꢀH bonds are formed
simultaneously.
k
The formation of an η2-bound adduct between NB and
dihydrogen (NB-H2 in Scheme 4) on the other hand could ex-
plain the absence of a KIE, too. H2 binding to the Lewis acidic
boron center would then be the rate-limiting step before the fast
intramolecular deprotonation of the adduct by the Lewis basic
nitrogen takes place to form NHHB. We did search for an η2 H2
transition state, but could not find any for NB in our calculations.
In addition we did not observe any rotation of HꢀD in heat-
ed solutions of NHDB and NDHB. Experimental evidence for
NB-H2 is therefore also missing.
Figure 3. Zero-point energies of starting compounds, transition states,
and products along the pathway of the reaction of NB with deuterium
hydride assuming an intramolecular mechanism.
Heitler-London” one.11 Thereit meant that the attraction between
the resulting counterions is comparable with the splitting energy
for H2. With more than one ion pair, a “collective Madelung ion-
ization” can result.17g
Neutron Diffraction. We previously reported the crystal
structure of NHHB determined by X-ray diffraction.11 In addi-
tion, we performed density functional theory (DFT) calculations
at the PBE/6-31G(d) level of theory for geometry optimizations,
and the reaction path, utilizing the PCM method to account for
solvation effects. Rokob et al. also optimized the geometry of this
ansa-ammonium borate, but only in the gas phase using DFT
at the M05-2X/6-31G(d) level.8 The geometries determined
by these different methods were in good agreement overall
but varied significantly in the value for the soft intramolecular
This is in contrast to the findings of Fan et al., who favored the
pathway of FLP-assisted side-on dihydrogen activation, similar
to transition metal-H2 σ bond complexes.18 Gao et al. reported
an intermediate on the proton-transfer pathway, consisting of
an H2 σ bond complex (CF3)3B(η2-H2) PH3.39 They also em-
3 3 3
phasized the dihydrogen bond aspect of the loaded BꢀH
3 3 3
HꢀP complex. The conceptually related mechanism of the
B(C6F5)3-catalyzed hydrosilylation of various functions, which
involves a boraneꢀsilane adduct during activation, is already
well-established.40
According to Table 1, the ZPE and Gibbs free energy isotopic
differences are quite small at TS, although they become larger for
the products. This supports the idea that the early transition state
does not produce an isotope effect (Figure 3). In their study of
phosphorusꢀboron-based FLP systems, Grimme et al.17h em-
phasized the role of ligandꢀligand attractions from dispersion
and found a barrier, or transition state, for one of their systems. It
was an “early” one, like ours. They also suggested replacing the
entire host molecule (here NB) by a strong electric field, an even
stronger theoretical simplification than the “Coulomb pays for
NꢀH HꢀB dihydrogen bond distance (Table 2). Unfortunately
3 3 3
this is the part of main interest within this molecule since these
two hydrogen atoms can either be released as dihydrogen gas
which can then be split up again by the resulting NB or
transferred toothermoleculeslikeimines, enamines, and nitrogen-
containing heterocycles. In order to determine precisely the
actual geometry of NHHB we conducted a neutron diffraction
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Table 2. Comparison of Structural Data of NHHB Obtained
by Different Methods: Single-Crystal Neutron Diffraction,
Single-Crystal X-ray Diffraction, ab Initio Geometry Optimi-
zation in Solution at the PBE/6-31G(d) Level of Theory
Utilizing the PCM Model, and ab Initio Geometry Optimi-
zation in the Gas Phase at the M05-2X/6-31G(d) Level (The
44.8 ms for BꢀH (T1(min)H1) and 92.1 ms for NꢀH (T1(min)H8)
at 213 K and 300 MHz. As can be seen from eq 3, the measured
relaxation rate 1/T1(H) is the sum of relaxation rates based on
dipoleꢀdipole interactions with all protons Hi and other NMR-
active nuclei X in close proximity to H. Because both H-atoms
(H1/H8) are additionally in close contact to other nuclei, which
contribute to a large extent to the fast relaxation (especiallyB, F, N,
and other H-atoms), the original T1 criterion as defined by
Crabtree and Hamilton is not valid.48 Instead, the enhanced
approach by Halpern and co-workers was utilized to determine
Presented Data Describe the NꢀH HꢀB Moiety)
3 3 3
PBE/
6-31G(d)a
M05-2X/
6-31G(d)b
property
d(HꢀH) [Å]
ND
XRDa
the H H distance in solution. Therefore, eqs 1 and 4 were
1.67
1.03
1.24
3.35
154.9
123.1
1.78
0.94
1.19
3.36
154.2
125.2
1.51
1.06
1.24
3.34
150.3
132.5
1.53
1.04
1.23
3.25
159.7
122.9
3 3 3
applied to determine the single contributions from dipoleꢀ
dipole relaxation to the observed relaxation rates 1/T1(min)H1 and
1/T1(min)H8. The distances (rHꢀH, rHꢀX) between H1/H8 and
other nuclei reported in the Supporting Information were taken
from the measured neutron diffraction structure.49 Additionally
the natural abundances of the different nuclei were also taken
into account.
d(NꢀH) [Å]
d(BꢀH) [Å]
d(NꢀB) [Å]
ψ(—NꢀH H) [deg]
3 3 3
θ(—BꢀH H) [deg]
3 3 3
a Data taken from ref 11. b Data taken from ref 8.
"
#
measurement on a single crystal grown out of a C6D6 solution by
slow cooling of a supersaturated solution.
1
3 γ4Hp2
10rH6 ꢀH 1 þ ω2Hτ2c
τc
4τc
1 þ 4ω2Hτ2c
¼
þ
ð1Þ
T1ðHꢀHÞ
The crystals were determined to be triclinic and could be
refined to yield the structure reported in Figure 8 unambiguously.
Good R values of 0.066 (R1) and 0.065 (wR2) could be reached,
and further refinement was impossible, mainly due to disordered
solvent molecules. It is well-known that NꢀH and BꢀH bond
lengths are underestimated when determined by X-ray diffraction.41,42
Both the calculated and the neutron values are clearly larger,
and mutually agree. The NꢀH bond length of NHHB (1.03 Å)
fits well the one in the stable Lewis acid/Lewis base adduct
0:6158
τc ¼
ð2Þ
ð3Þ
ω
1
1
1
¼
þ
∑
X
∑
T1ðHÞ
T1ðHꢀHiÞ
T1ðHꢀXÞ
i
1
2 γ2H γ2Xp2
15rH6 ꢀX
H3N BH3, determined previously by a neutron diffraction study
¼
IðI þ 1Þ
3
T1ðHꢀXÞ
to be 1.03 Å, which is a widely accepted standard value for NꢀH
"
#
bonds.6,43 The BꢀH bond of 1.17 Å in the H3N BH3 weak
3τc
6τc
τc
3
ꢁ
þ
þ
2
2
1 þ ω2Hτ2c
1 þ ðωH þ ωXÞ τ2c
1 þ ðωH ꢀ ωXÞ τ2c
complex is shorter. Further neutron values for BꢀH and BꢀD
bonds are 1.21 Å.6,43,44 Thus, the BꢀH bond in NHHB (1.24 Å)
is only very slightly elongated. The actual dihydrogen bond
length of only 1.67 Å is one of the smallest values reported in
ð4Þ
These values were then used to solve eq 3 to get 1/T1(min)H1ꢀH8
literature so far for NꢀH HꢀB compounds.45 For compar-
and 1/T1(min)H8ꢀH1. Therefore two independent values for
3 3 3
ison, the so-called van der Waals radii of both hydrogen atoms
sum up to 2.4 Å. The sum of the van der Waals radii is a measure
on the strength of dihydrogen bonds.46 It is therefore assumed
the H1 H8 distance resulted out of the calculation based
3 3 3
on the different T1(min) values: T1(min)H1 led to 1.72 Å and
T1(min)H8 to 1.69 Å.
that in the close H H contact within NHHB the dihydrogen
In addition, 300 MHz T1 data were also analyzed by fitting a
theoretical T1 curve to the measured T1 values of both nuclei H1
and H8. The fitted curve includes all relaxation contributions
from other nuclei (i.e., based on distances obtained from the
neutron diffraction structure). The full treatment of T1 curves of
3 3 3
bond is comparatively strong and partly covalent in nature.47
Additionally, the bonding angles ψ and θ (see Table 2) also agree
well with previously reported structures containing NꢀH
3 3 3
HꢀB dihydrogen bonds.6,45
H1 H8 Interproton Distance Determinations in Solu-
H1 and H8 resulted in H1 H8 distances of 1.77 and 1.68 Å,
3 3 3
3 3 3
tion Applying T1 Relaxation Measurements and Selective
1D NOE Experiments. Having determined the precise structure
of NHHB in the solid state, we were curious about its structure in
solution, since this is the surrounding where the system actually
activates, transfers, and liberates hydrogen. Therefore we con-
ducted T1 relaxation measurements by 1H NMR spectroscopy of
the activated dihydrogen within NHHB in CD2Cl2 solution.
According to eq 1, the relaxation rate 1/T1(HꢀH) of one proton
due to dipoleꢀdipole interaction with another proton depends
on the interatomic protonꢀproton distance rHꢀH. The symbols
γ, p, τc, and ω represent gyromagnetic ratio, Planck’s constant/
2π, correlation time, and Larmor frequency, respectively. Appli-
cation of minimum relaxation times T1(min) allows the use of eq 2
and hence a single correlation time τc. The determined T1(min)
respectively. The calculated correlation time at 213 K for H1 and
H8 were 4.6 ꢁ 10ꢀ10 and 3.4 ꢁ 10ꢀ10 s, respectively.23 This dif-
ference in correlation time values indicates a minor deviation
from isotropic molecular motion. However, the effect of aniso-
tropy on the distance results is not severe if distances are calculat-
ed from T1(min) according to Bakhmutov and Berke et al.50 This
is also observed when comparing the results from the T1(min)
approach to the values obtained from the full T1 curve treatment;
i.e., both methods retain similar distance values.
Obviously, both our T1(min) and full T1 curve-fitting ap-
proaches rely on several assumptions, i.e., interatom distances re-
trieved from the neutron diffraction study.51 Therefore, an
additional study was performed to confirm the above findings.
According to Bakhmutov, a convenient way to study internuclear
distances in hydrides is based on measurement of the NOE
values are rather short as expected for close H H contacts:
3 3 3
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buildup rate, i.e., cross-relaxation rate by selective 1D NOE ex-
periments.52 Selective 1D NOE measurements at 500 MHz were
performed to determine the H1 H8 distance at room tem-
3 3 3
perature. Two series of 1D NOE measurements with increasing
NOE mixing times were performed; an experiment with selective
pulse was applied to the H1 resonance and another one with
selective pulse was applied to the methyl resonance at 1.12 ppm.
In the first series, the NOE signal buildup at the H8 frequency
was monitored, whereas in the second one the NOE buildup of
the methyl signal at 1.51 ppm was monitored. The latter exper-
iment served as a reference, where the NOE buildup rate cor-
responds to an average distance of 3.16 Å between the protons of
the two methylꢀmethyl groups resonating at 1.12 and 1.51 ppm,
respectively. The reference distance was computed using the “rꢀ3
average” approach utilizing the methyl proton coordinates from
the neutron diffraction data.23,53
Figure 4. Comparison of solid-state DRAFT spectra of all four isotopic
isomers of NXYB in the NꢀX and BꢀY stretching frequency region.
In order to obtain comparable NOE buildup rates, NOE in-
tensities (integrals) were scaled according to nAnB, where nA and
nB are the number of chemically equivalent protons in the groups
contributing to the NOE signal (scaling factors 1 and 36 for H1ꢀ
H8 and methylꢀmethyl, respectively).53,54 In addition, NOE
intensities were corrected for different degree of selective inver-
sion and relaxation effects during excitation sculpting element in
the 1D DPFGSE NOE pulse sequence.55 This was achieved by
comparing the integrals of the inverted resonances in the spectra
recorded with the shortest mixing time of 12 ms, i.e., to return the
integral ratio H1:Me(1.12 ppm) back to 1:6. A correction factor
of 0.99 has to be applied to the data obtained from H1 selective
1D NOE.56 Assuming isotropic molecular motion, the distance
between protons H1 and H8 (rH1ꢀH8) can then be calculated
using eq 5, where rMeꢀMe is the reference distance 3.16 Å, while
σH1ꢀH8 and σMeꢀMe are the cross-relaxation rates (NOE buildup
rates) determined from the linear parts of the buildup curves.
Table 3. Frequency Regions of the NꢀX and BꢀY Stretching
Modes in the Solid State of NXYB (in cmꢀ1
)
mode
NHHB
NDDB
NHDB
NDHB
ν(NH)
ν(ND)
ν(BH)
ν(BD)
3110ꢀ2970a
ꢀ
3110ꢀ2970a
ꢀ
ꢀ
2400ꢀ2160
ꢀ
ꢀ
2440ꢀ1980b
2440ꢀ1980b
ꢀ
2440ꢀ2060
ꢀ
ꢀ
1780ꢀ1560
1780ꢀ1560
a The precise region cannot be determined because of overlapping CꢀH
stretching vibrations. b Due to a signal overlap it cannot be distinguished
between the NꢀD and BꢀD stretching modes.
NꢀXandBꢀY stretching modes can be easily identified (Table 3):
Due to the isotopic substitution of H by D the corresponding bands
shift to lower frequencies.36 When looking at the data it is obvious
that ν(NH) is considerably red-shifted when compared to a non-
perturbed NH group above 3500 cmꢀ1. This, and the broadening
of the vibration are proofs for the existence of hydrogen bonding—
in this case of dihydrogen bonding in the solid which further sup-
ports the findings of the neutron diffraction study.58,59 The shape of
the band cannot be discussed in detail because of the overlapping
CꢀH stretching vibrations.
This results in a value of 1.63 Å for rH1ꢀH8.
23,52 Additional measure-
ments at room temperature using 2D NOESY data resulted in
rH1ꢀH8 of 1.65 Å.23
rffiffiffiffiffiffiffiffiffiffiffiffiffiffi
σMeꢀMe
σH1ꢀH8
6
rH1ꢀH8 ¼ rMeꢀMe
ð5Þ
The H1 H8 interproton distances extracted from NMR
3 3 3
experiments agree reasonably well with each other and give
strong evidence that a structure similar to the one in the solid
state determined by neutron and X-ray scattering is maintained
in a CD2Cl2 solution. Conclusively, the collected NMR data sug-
gests a short dihydrogen bond length range of approximately
1.6ꢀ1.8 Å within NHHB in solution which is consistent with
1.67 Å determined in the solid state. At temperatures above room
temperature the dihydrogen bond distance of NHHB in solution
is thought to become even shorter, as evidenced above.
The unusual broadness of the BꢀH stretching mode and the
many maxima can be ascribed to anharmonicity leading to strong
coupling between different vibrations—one of them being pre-
sumably the NꢀH stretching mode. This is supported by the
different shapes and intensities of the BꢀD stretching vibrations
of NDDB and NHDB where the influence of the isotopic sub-
stitution of the protonic hydron is apparent. The specific shape of
the BꢀH stretching vibration is therefore another direct proof for
intramolecular dihydrogen bonding at room temperature as
observed in the solid-state structure of NHHB determined by
neutron diffraction at 10 K. Fermi coupling is also thought to
influence the band shape because of the complex pattern of vibra-
tions in the region below 1500 cmꢀ1 resulting in a large amount
of overtones and combination levels.36,57
FT-IR Spectra. Vibrational spectroscopy is a sensitive probe of
the potential energy surface of a molecule which determines the
dynamicsof its nuclearmotion. Based on the BornꢀOppenheimer
approximation, conclusions can then be drawn on structural and
dynamic properties of the matter.36 We measured FT-IR spectra in
the solid state and in solution to further study the dihydrogen
bonding in NHHB and its deuterated analogues. Due to the large
number of atoms in one molecule, the resulting IR spectra are
complex.57 Therefore, we focused on the regions of the NꢀX and
BꢀY stretching modes between 1500 and 3200 cmꢀ1. By
comparison of the solid-state spectrum of NHHB with the spectra
of its deuterated analogues (Figure 4) the bands representing the
Upon deuteration the value for ν(NH)/ν(ND) and ν(BH)/
√
ν(BD) should be close to 2 in the case of a harmonic oscillator
and the line width should also decrease by the inverse of this factor
√
1/ 2.60 Because of signal overlap and the broadness of the signals
of NXYB these values cannot be determined accurately, but the
ratio of the frequency shifts are in the region of 1.32ꢀ1.34, being
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Figure 6. TG curves of NHHB and NDDB at heating rates of 5 ꢀC/min.
Figure 5. Comparison of solution spectra of NB, NHHB and NDDB in
CH2Cl2 at 300 K in the NꢀX and BꢀY stretching frequency region. The
spectrum of the solvent has been subtracted in all cases.
quite close to the value mentioned above. In earlier publications it
was shown that this factor decreases with increasing strength of
hydrogen bonds and can even reach unity in the case of strong
H-bonds.36,58 This is not in line with our findings because NXYB
does exhibit a comparatively strong dihydrogen bond as shown
above. On the other hand several differences between the studied
systems are obvious. In contrast to NXYB the previously studied
systems were mainly intermolecular ones. Anomalies have been
reported before for intramolecular H-bonding, due to different
electronic configurations especially in rings.58 In addition, no system-
atic work in this field with regard to dihydrogen bonding is known.
High-level quantum mechanical calculations to determine the
potential energy surface would be needed for further interpre-
tations of the observed isotope effects.
Figure 7. Selected MS signals of TG-MS measurements of (a) NHHB
and (b) NDDB at a heating rate of 5 ꢀC/min.
In the case of solution spectra of NHHB, NDDB, and NB
(Figure 5), the NꢀX and BꢀY stretching vibrations can again be
identified easily, and do not differ apparently in their location
(see Table 3 for solid-state values). This finding coincides with
the result of the 1H NMR T1 relaxation study because it is a clear
proof for strong intramolecular dihydrogen bonding in solution.
The biggest difference seems to be in the NꢀH stretching mode,
but since the spectrum of the solvent had to be subtracted in this
region no detailed interpretation is possible. The equilibria be-
tween different structural isomers, as pointed out before, could
not be studied at variable temperatures because no significant dif-
ferences were detected in the spectra of solutions between 240
and 380 K.57 The changes that occur are presumably obscured by
the broadness of the bands. This is remarkable because above 338
K NHHB is destabilized and releases dihydrogen slowly. There-
fore it is assumed that the structure of the molecule—in particular
the dihydrogen bond and its intramolecular nature—is quite
stable in solution until the loss of H2 occurs and NB is formed
with the only change being the NꢀH and HꢀH distances as
mentioned above.
In addition, VT solution spectra of NHHB, NDDB, and NB
(only 300 K) were recorded in the low wavenumber region
(600ꢀ100 cmꢀ1) where the stretching vibration of the dihydro-
gen bond was assumed to be located.57 A rather large number of
bands was observed and a distinct assignment could not be made.
A normal coordinate analysis is mandatory for further investiga-
tions in this respect.61 An increase or decrease in intensity could
also not be observed upon temperature variations as reported
previously for dihydrogen bonds in other systems.62
TG-MS Measurements. The solid ionic pairs ammonium boro-
hydride and diammoniate of diborane which also consist of
NꢀH HꢀB dihydrogen bonds are known to release hydrogen
3 3 3
gas upon heating in the solid state starting at temperatures
above ꢀ40 and 85 ꢀC, respectively.62 Since NXYB can release XY
in solution forming NB the question was raised whether this is also
possible in the solid state. Therefore thermogravimetry (TG)
measurements of the pure solids NHHB and NDDB were con-
ducted under a flow of nitrogen at heating rates of 5 ꢀC/min. As
can be seen in Figure 6, NHHB starts to decompose at 165 ꢀC,
and at 185 ꢀC already 7.5 wt % is lost. Because NHHB contains
only 0.34 wt % of stored hydrogen, this significant mass loss re-
sults out of a degradation of the NB backbone. NDDB on the
other hand follows reproducibly different kinetics: decomposi-
tion starts at slightly higher temperatures around 170 ꢀC and pro-
ceeds very slowly. At 185 ꢀC only 1.5 wt % of the initial mass
is lost. At the same time it has to be considered that 0.69 wt %
of NDDB corresponds to the stored D2. Therefore, besides
D2 release, also partial decomposition of the NB structure starts.
At the same time the calculated transition temperatures, taken
as ΔG = 0 kJ/mol for the hydrogen-releasing reaction using the
data of Table 1, are at 170 and 200 ꢀC for NHHB and NDDB,
respectively.
To gain insight into the initial decomposition products upon
heating, scans of different masses were recorded on a coupled
EI-MS. The intensities of the largest m/z signals are depicted in
Figure 7. The signal with a value of 2 amu in Figure 7a can be
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+
1
p
assigned to H2+ and 168 amu most probably to C6HF5 . It can
clearly be seen that both fragments are detected simultaneously
in the beginning with increasing intensities. To analyze whether
the H2+ ion is the stored hydrogen or results from another part
of the molecule the same measurements were conducted on
by H NMR spectroscopy: the chemical shift difference Δ1H-
(D) = δ(NH) ꢀ δ(ND) amounts to 0.56 ppm. It could be shown
that this chemical shift difference is caused by different positions
1
of D and H in the hydrogen bond. H NMR spectroscopic T1
relaxation and selective 1D NOE measurements showed that the
+
NDDB. Figure 7b shows that D2 (m/z = 4 amu) is indeed
NꢀH HꢀB dihydrogen bond length is 1.6ꢀ1.8 Å—very
3 3 3
+
detected at temperatures around 170 ꢀC together with C6HF5
close to the value determined in the solid by neutron diffraction—
and even shorter at elevated temperatures. FT-IR spectra of NHHB,
NDDB, and NB support this finding because of very similar
frequencies and band shapes of the NꢀX and BꢀY stretching
vibrations in solid and solution spectra. In addition, no structural
changes could be detected in VT FT-IR studies (240ꢀ380 K)
upon cooling and heating of the solutions. Furthermore, only
the N-bound hydron in NXYB can easily be exchanged in
(m/z = 168 amu) and C6DF5+ (m/z = 169 amu). The first and
the latter signals are both missing in the TG-MS experiment with
NHHB. This—together with the observation that NDDB is
thermodynamically more stable than NHHB—leads to the fol-
lowing conclusions: (1) The structure NXYB is stabilized by XY.
(2) Once XY is released the residue decomposes quite fast. (3)
The perfluorinated phenyl rings are among the first parts of
NXYB that break up. (4) NDDB seems to experience a greater
solution, and even upon heating no rotation of X Y occurs
until XY is released.
3 3 3
stabilization by the stronger NꢀD DꢀB bond in comparison
3 3 3
to the NꢀH HꢀB bond in NHHB. The data in Table 1 sup-
Upon the activation of deuterium hydride by NB, no isotope
effect with regard to H/D distribution was detected.38 Theoret-
ical calculations attribute this to an early transition state. The
valuable insights into structures, reactivity, and mechanisms re-
vealed in this article will be useful for further developments of
FLP based hydrogenation catalysts.
3 3 3
port the smaller ZPE of NDDB compared to NHHB as a reason
for this stabilization. At the same time this difference is larger
than in D2/H2.23 About half of this isotopic effect arises from the
NꢀD and BꢀD stretching modes, and the remaining part has its
origin in the coupled modes between NꢀD, BꢀD and other
functional groups.23
Additionally a signal corresponding to HD+ (m/z = 3 amu)
could also be detected simultaneously to D2+ during heat-up of
NDDB.23 This also points to an uncontrolled degradation of
NXYB rather than a clean XY release. In summary, it was not pos-
sible to release H2 or D2 in the solid state leaving the intact com-
pound NB behind. A similar behavior could be observed upon
heating of NHHB under vacuum at 150 ꢀC. It seems that es-
pecially the BꢀC6F5 bondings are thermally too instable to allow
such an H2 release from the solid state. At isothermal conditions
the release of H2 (145 ꢀC) or D2 (165 ꢀC) was also always
accompanied by decomposition products of the NB backbone.
’ EXPERIMENTAL SECTION
General Methods and Materials. Whenever necessary, the experi-
ments were performed on double-manifold H2 (D2, HD, Ar)/vacuum
lines or in an argon glovebox (MBraun Labmaster 130). Dichloro-
methane, n-pentane, and toluene were dried by an MBraun solvent puri-
fication system (MB SPS-800). Deuterated solvents were received from
Deutero GmbH, dried by stirring over CaH2 overnight at room tempera-
ture, and distilled prior to use. Hydrogen gas (5.0) was purchased from
Linde and passed through a drying unit equipped with molecular sieves
(4 Å). D2 (99.96%) and HD (98%) lecture bottles were obtained from
Isotec and used without further purification. Methanol (extra dry) and
benzene (anhydrous) were purchased from Acros Organics and Sigma-
Aldrich and used as received. 1,1,2,2-Tetrachloroethane (Acros Organics)
was dried by stirring over CaH2 overnight at room temperature and
distilled prior to use. 1-N-TMP-CH2-2-[B(C6F5)2]C6H4 (NB)11 and
1-N-TMPH-CH2-2-[HB(C6F5)2]C6H4 (NHHB)11 were synthesized as
described previously.
’ CONCLUSIONS
Besides the previously reported NHHB, we could successfully
synthesize the individual, deuterated isomers NDDB, NHDB,
and NDHB. They were characterized by HRESI-TOF mass spec-
trometry, multinuclear NMR, and IR spectroscopy.
We have determined the crystal structure of NHHB by
neutron diffraction and could show that an extremely short intra-
NMR and VT-NMR experiments were performed on a Bruker ARX-
300 spectrometer (1H, 11B, 19F) or a Bruker DPX-400 (2H). 1H (2H)
NMR spectra are referenced to SiMe4 (SiMe4-d12) by referencing the
molecular dihydrogen bond exists with an H H contact of
3 3 3
residual solvent peak (also during low-temperature studies). 11B and 19
F
only 1.67 Å. TG-MS studies of NXYB showed that XY cannot be
released in the solid state without immediate degradation of NB.
The decomposition of NHHB begins at 145 ꢀC and around
165 ꢀC in the case of NDDB. The NꢀX and BꢀY stretching vi-
brations were identified in FT-IR spectra of the solids. A red shift
of ν(NX), the broadness of the ν(NX) and ν(BY) bands, and the
fact that several maxima are present in the latter case are explain-
ed by Fermi couplings and anharmonicity. This supports the
existence of dynamic intramolecular dihydrogen bonding in
the solid state at room temperature. The ratios of ν(NH)/
ν(ND) and ν(BH)/ν(BD) are in the region of 1.32ꢀ1.34.
By dilution, VT and 2D NOESY NMR spectroscopic experi-
ments the dynamic nature of NHHB in solution was studied, and
the part of the molecule in close proximity to the N atom was
identified to be in constant change. An equilibrium between at
least two conformational isomers (NHHB(1/2)) exists, with one
more stable and hence preferred isomer. Upon deuterium
labeling of NHHB a primary isotope effect could be identified
NMR spectra were referenced externally to BF3 Et2O at 0 ppm and
3
CF3CO2H at ꢀ78.5 ppm relative to CFCl3 at 0 ppm, respectively.
All 2D 1H,1H NOESY NMR measurements were performed at 27 ꢀC
using a Varian Unity Inova 500 spectrometer (500 MHz 1H-frequency)
equipped with a 5 mm 1H-{X} pulsed field gradient inverse detection
probe head. The standard sample preparation inside an argon glovebox
was to weigh 15 mg in total of pure or mixtures of crystalline NXYB in
5 mm NMR tubes and dissolve it in 0.5 mL of carefully dried CD2Cl2 or
C6D6. One-dimensional 1H NMR spectra from each sample were
recorded prior to NOESY measurements. Spectra covering a spectral
width of 5497.5 Hz were recorded using a 90ꢀ pulse with 64 transients
preceded by two steady-state scans. The relaxation delay was 10.0 s and
the number of acquired complex data points was 10 382 (acquisition
time 1.9 s). The FID was multiplied by an exponential weighting fun-
ction with a 5 Hz line-broadening factor and zero-filled up to 16 384
complex data points prior to Fourier transformation. Polynomial base-
line correction was applied (fifth order) and the receiver gain was kept
constant in every measurement. The pulse width was determined for
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each sample and variation in pulse lengths was below 5%, confirming
nonsignificant differences in dielectric loading of RF-coil and thus in the
sensitivity. Signal integrals were determined using the spectrometer
operating software VNMR 6.1C. To determine scaling factors in order to
normalize integration results in 1D as well as in NOESY with respect to
small differences in sample concentrations, signals of methylene protons
(4.5 ppm in CD2Cl2, 4.8 ppm in C6D6) were integrated. Note that in
C6D6 solution, methylene protons overlap with approximately half of
the boron-bound signals. In C6D6, the BH signal integral is evaluated by
integrating half of the signal (overlap-free) to avoid the CH2 signal and
the final result was multiplied by 2. Half of the BH integral is also
subtracted from the CH2 integral data to correct for the effect of overlap
before correction factor calculation. In the CD2Cl2 samples, part of the
NH signal (at 7.9 ppm) overlaps with aromatic proton signals. There-
fore, only the low-field half of the signal (from 7.9 to 8.6 ppm) is
integrated and the result is multiplied by 2.
Two-dimensional NOESY spectra were recorded to estimate the
portion of NHHB in the samples by comparison of cross-peak volumes
between NH and BH protons. NOESY spectra were recorded in phase
sensitive mode using a spectral width of 5497.53 Hz in both dimensions.
Spectra were recorded using 32 transients and 200 increments. The
number of acquired complex data points was 1100. NOESY mixing time
was 600 ms and a relaxation delay of 2.0 s was used. Receiver gain was
kept constant in all NOESY measurements. Time domain data was
apodized using Gaussian function and zero-filled to 2048 complex points
in both dimensions, prior to Fourier transformation. Polynomial base-
line correction (fifth order) was applied in f2-dimension before 2D
integration. Integration of the NOESY correlation peak between NH
and BH protons was accomplished using the VNMR 6.1C software.
Due to partial overlap of NHꢀHB correlation with NHꢀH2C and
H(aromatic)ꢀH2C correlations, only approximately half of the NHꢀHB
cross-peak (nonoverlapping part) was included in the integration area.
To obtain the estimate of total cross-peak volume, the integration result
was multiplied by 2. Before comparison of NHꢀHB cross-peak volumes,
the volume data was normalized using correction factors calculated from
1D data (see above).
100, 120, 150, and 200 ms. Spectra were recorded with 64 transients,
keeping receiver gain constant in every measurement. The spectral width
was 5497.5 Hz, the relaxation delay was 5.0 s, and the number of acquir-
ed complex data points was 10 995 (acquisition time 2.0 s). The FID was
multiplied by an exponential weighting function with a line-broadening
factor of 3 Hz and zero-filled up to 16 384 complex data points prior to
Fourier transformation. A baseline correction using a spline function was
applied prior to the integration of NOE peaks.
VT FT-IR studies (4000ꢀ100 cmꢀ1) of dissolved samples (concen-
trations: 0.01ꢀ0.5 mol/L) were conducted on a Bruker VERTEX 70
with a VT cell from Specac including a temperature controller. For the
low-temperature measurements liquid nitrogen was used for cooling.
The samples were dissolved inside an argon glovebox (MBraun Lab-
master 130) where the resulting solutions were transferred into the cell
in order to exclude any traces of water. The measurements in the low
frequency range (300ꢀ600 cmꢀ1) were performed with polyethylene as
window material (d = 1 mm). Unfortunately, it was impossible to pro-
vide the measurements in the range below 300 cmꢀ1 at temperatures
below room temperature due to the strong bands of the media CH2Cl2.
The use of other solvents was restricted because of the poor solubility of
the zwitterions in low polar solvents (toluene and linear hydrocarbons).
At room temperature and above C6H6 was used and hence spectra rang-
ing from 100 to 600 cmꢀ1 were obtained. The measurements in the me-
dium frequency range (800ꢀ4000 cmꢀ1) were performed using AgCl
windows (d = 100 μm). Unfortunately, it was impossible to provide the
measurements in the range below 800 cmꢀ1 at room temperature and
lower due to the strong bands of the media CH2Cl2. Above room tem-
perature 1,1,2,2-tetrachloroethane was used and hence spectra ranging
from 850 to 4000 cmꢀ1 were obtained. Diffuse reflectionꢀabsorption
FT-IR spectra (DRAFT) without solvent were recorded on a Bruker
VERTEX 70 by deposition of small amounts of pulverized substance
on the reflecting metal plate of a DRIFT unit (Spectra Tech: The
Collector).64
High-resolution mass spectra (HRESI-TOF) were recorded on a
Bruker micrOTOF-Q mass spectrometer utilizing acetonitrile as solvent.
TG-MS experiments were carried out on a Q5000 thermogravimetric
analyzer from TA Instruments connected to an HPR-20 from Hiden
Analytical.
NOESY spectra were recorded from the following samples: NHHB in
CD2Cl2, equimolar amounts of NHDB and NDHB in CD2Cl2, NHHB
in C6D6, equimolar amounts of NHDB and NDHB in C6D6, NB that
reacted with HD dissolved in CD2Cl2, NB that reacted with HD
dissolved in C6D6.
Crystals suitable for neutron diffraction were grown from a benzene-d6
solution. NHHB was dissolved in C6D6 at 60 ꢀC in a Schlenk tube
under an argon atmosphere to obtain a saturated solution. After addition
of a seed crystal, it was slowly cooled to 35 ꢀC where a large crystal
formed after several days. The temperature was finally lowered to room
temperature and a single crystal of the size 2 ꢁ 2 ꢁ 2 mm could be ob-
tained. The neutron diffraction study of NHHB was carried out on
thermal single crystal diffractometer RESI65 at FRM II, equipped with a
Huber Eulerian cradle and a SHI RDK-101 cryostat at 10.0(1) K using
the Cu-422 monochromator (λ = 1.03(1) Å). The X-ray structure was
used as a starting point for full-matrix least-squares refinement on F
(JANA2000/JANA200666). The H atoms were refined free, and the
disordered solvent molecules were treated as rigid bodies with ideal geo-
metry. All atoms were refined with isotropic thermal displacement para-
meters. Extinction was treated in an isotropic Becker-Coppens model as
implemented in JANA2006.67 Some bond distances are presented in
Table 4, and the structure is shown in Figure 8.
1H T1 relaxation measurements were carried out by the standard
inversionꢀrecovery (180ꢀꢀτꢀ90ꢀ) method with the use of a Bruker
ARX-300 spectrometer in deoxygenated CD2Cl2 solutions. Calculations
of the relaxation times were completed using the nonlinear three-
parameter fitting routine of the spectrometer. In order to determine
the minimum T1 times (T1(min)), the experiments were performed in
the temperature range of 183ꢀ293 K. Fitting of the T1 curves to the T1
values measured at different temperatures was accomplished via the MS
Excel Solver routine utilizing the GRG2 quasi-Newton nonlinear regres-
sion algorithm.63 The T1(min) values of all protons were determined
at 213 K.
Selective 1D NOE measurements were conducted with a Varian
Unity Inova 500 spectrometer at 27 ꢀC equipped with a 5 mm 1H-{X}
pulsed field gradient inverse detection probe head using the 1D double-
pulsed field-gradient-spin-echo nuclear Overhauser effect (DPFGSE-NOE)
pulse sequence.55 Selective inversion of H1 resonances was accom-
plished using a 5.4 ms 180ꢀ Gaussian pulse applied at an RF-power of
210 Hz, whereas a 15.5 ms 180ꢀ Gaussian pulse at an RF-power of 74 Hz
was used for the methyl resonance at 1.12 ppm. Hard rectangular pulses
were applied at an RF-power of 37 kHz. Selective experiments for H8
were not successful, since rapid transverse relaxation of H8 during
selective pulses as well as pulsed field gradient periods led to complete
disappearance of the signal. NOE mixing times were 12, 20, 30, 40, 60,
NHHB. Colorless crystal, C28H26BF10N 0.5C6D6, M = 619.38 g/mol,
3
crystal size 2 ꢁ 2 ꢁ 2 mm, T = 10 K, triclinic, space group P1 (No.2),
a = 10.587(10) Å, b = 12.183(10) Å, c = 12.520(10) Å, α = 86.77(5)ꢀ,
β = 70.73(5)ꢀ, γ = 64.83(5)ꢀ, V = 1372.95(2) Å3, Z = 2, F(calcd) = 1.498
Mg/m3, F(000) = 400, μ = 0.071 mmꢀ1, 9015 reflexes (2θmax = 110ꢀ),
5332 > 2.5σ [Rint = 7.0%], 295 parameters, R1 (I > 2.5σ(I)) = 0.066,
wR2 (all data) = 0.065, GooF = 1.42, largest diff. peak and hole 1.56
and ꢀ1.40 barns/Å3.
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Journal of the American Chemical Society
ARTICLE
Table 4. Selected Bond Distances As Determined by Neutron
Diffraction
the solvents in vacuo, this procedure was repeated once more. Subsequent
washing (three times) with n-pentane (6 mL) yielded 188 mg (0.33
mmol, 94%) of NDHB as a white solid. 1H NMR (CD2Cl2, 300 MHz): δ
7.15 (m, 4H, C6H4), δ 4.44 (s, 2H, NCH2), δ 3.72 (br q, 1H, 1JHB = 76
Hz, BH), δ 1.6ꢀ1.9 (m, 6H, CH2CH2CH2), δ 1.51 (s, 6H, CH3), δ 1.12
(s, 6H, CH3). 2H NMR (CH2Cl2, 400 MHz): δ 7.20 (br s). 11B NMR
(CD2Cl2, 96 MHz): δ ꢀ20.86 (d, 1JBH = 77 Hz). 19F NMR (CD2Cl2,
282 MHz): δ ꢀ133.67 (d, 6F, 3JFF = 22 Hz, o-C6F5), δ ꢀ163.66 (br s,
3F, p-C6F5), δ ꢀ167.06 (m, 6F, m-C6F5). HRMS ESI+-TOF calcd for
bond
distance [Å]
d(B1ꢀH1)
d(N8ꢀH8)
d(H1ꢀH8)
1.235(7)
1.033(6)
1.674(8)
C28H26BF10N Na+: 600.1891. Found: 600.1903. HRMS ESIꢀ-TOF
3
calcd for C28H25BF10Nꢀ: 576.1926. Found: 576.1930.
Synthesis of 1-N-TMPH-CH2-2-[DB(C6F5)2]C6H4 (NHDB).
In a glovebox, a 25-mL flame-dried Schlenk tube equipped with a stir bar,
a Teflon stopcock, and a glass stopper (Glindemann sealing rings were used
for conical joints instead of grease) was charged with 1-N-TMPD-CH2-2-
[DB(C6F5)2]C6H4 (NDDB, 0.35 mmol, 200 mg) and 3 mL of a
methanol/benzene (40/60) mixture. The reaction was stirred at
1000 rpm at room temperature overnight. After evaporation of the
solvents in vacuo, this procedure was repeated once more. Subsequent
washing (three times) with n-pentane (6 mL) yielded 191 mg (0.33
mmol, 95%) of NHDB as a white solid. 1H NMR (CD2Cl2, 300 MHz): δ
7.70 (br s, 1H, NH), δ 7.16 (m, 4H, C6H4), δ 4.44 (d, 2H, 3JHH = 5.5 Hz,
NCH2), δ 1.6ꢀ1.9 (m, 6H, CH2CH2CH2), δ 1.51 (s, 6H, CH3), δ 1.12
(s, 6H, CH3). 2H NMR (CH2Cl2, 400 MHz): δ 3.80 (br s). 11B NMR
(CD2Cl2, 96 MHz): δ ꢀ21.15 (s). 19F NMR (CD2Cl2, 282 MHz):
δ ꢀ133.66 (d, 6F, 3JFF = 22 Hz, o-C6F5), δ ꢀ163.63 (br s, 3F, p-C6F5),
δ ꢀ167.02 (m, 6F, m-C6F5). HRMS ESI+-TOF calcd for C28H25-
BDF10N Na+: 601.1954. Found: 601.2017. HRMS ESIꢀ-TOF calcd for
3
C28H24BDF10Nꢀ: 577.1989. Found: 577.2015.
Figure 8. ORTEP plot of the crystal structure of NHHB as determined
by neutron diffraction. Black circles, H; red circles, D.
Reaction of 1-N-TMP-CH2-2-[B(C6F5)2]C6H4 (NB) with Deute-
rium Hydride. In a glovebox, a 25-mL flame-dried Schlenk tube equipped
with a stir bar, a Teflon stopcock, and a glass stopper (Glindemann sealing
rings were used for conical joints instead of grease) was charged with
1-N-TMP-CH2-2-[B(C6F5)2]C6H4 (NB, 0.4 mmol, 230 mg) and 1.5 mL of
dry n-pentane. The reaction was degassed once with a freezeꢀpumpꢀthaw
cycle and refilled with D2 (1 bar). The reaction was stirred at 1000 rpm at
room temperature for 15 min and a white precipitate formed. After filtration
and washing (five times) with n-pentane (5 mL), 227 mg (0.4 mmol, 98%)
of a white solid was obtained. 1H NMR (CD2Cl2, 300 MHz): δ 7.76 (br s,
0.5H, NH), δ7.14 (m, 4H, C6H4), δ4.44 (overlap of s and d, 2H, NCH2), δ
3.72 (br q, 1H, 1JBH = 76 Hz, BH), δ 1.6ꢀ1.9 (m, 6H, CH2CH2CH2), δ
1.51 (s, 6H, CH3), δ 1.12 (s, 6H, CH3). 2H NMR (CH2Cl2, 400 MHz):
δ 7.23 (br s, 1D, ND), δ 3.78 (br s, 1D, BD). 11B NMR (CD2Cl2, 96 MHz):
δ ꢀ20.81 (overlap of s and d). 19F NMR (CD2Cl2, 282 MHz): δ ꢀ133.67
(d, 6F, 3JFF = 22 Hz, o-C6F5), δ ꢀ163.66 (br s, 3F, p-C6F5), δ ꢀ167.05
NMR Measurements of 1-N-TMPH-CH2-2-[HB(C6F5)2]C6H4
(NHHB). 1H NMR (CD2Cl2, 300 MHz): δ 7.77 (br s, 1H, NH), δ 7.16
(m, 4H, C6H4), δ 4.45 (d, 2H, 3JHH = 5.5 Hz, NCH2), δ 3.76 (br q, 1H,
1JHB = 77 Hz, BH), δ 1.6ꢀ1.9 (m, 6H, CH2CH2CH2), δ 1.51 (s, 6H,
CH3), δ 1.13 (s, 6H, CH3). 11B NMR (CD2Cl2, 96 MHz): δ ꢀ20.84
(d, 1JBH =78Hz). 19FNMR(CD2Cl2, 282 MHz): δ ꢀ133.44 (d, 6F, 3JFF
=
22 Hz, o-C6F5), δ ꢀ163.45 (br s, 3F, p-C6F5), δ ꢀ166.85 (m, 6F, m-C6F5).
Synthesis of 1-N-TMPD-CH2-2-[DB(C6F5)2]C6H4 (NDDB).
In a glovebox, a 25-mL flame-dried Schlenk tube equipped with a stir bar,
a Teflon stopcock and a glass stopper (Glindemann sealing rings were used
for conical joints instead of grease) was charged with 1-N-TMP-CH2-2-
[B(C6F5)2]C6H4 (NB, 1.0 mmol, 575 mg), and 3 mL of dry n-pentane.
The reaction was degassed once with a freezeꢀpumpꢀthaw cycle and
refilled with D2 (1 bar). The reaction was stirred at 1000 rpm at room
temperature for 15 min and a white precipitate formed. After filtration
and washing (five times) with n-pentane (15 mL), 568 mg (1.0 mmol,
(m, 6F, m-C6F5). HRMS ESI+-TOF calcd for C28H25BDF10N Na+:
3
601.1954. Found: 601.1929. HRMS ESI+-TOF calcd for C28H26BF10N
3
Na+: 600.1891. Found: 600.1911. HRMS ESIꢀ-TOF calcd for C28H24-
BDF10Nꢀ: 577.1989. Found: 577.1994. HRMS ESIꢀ-TOF calcd for
C28H25BF10Nꢀ: 576.1926. Found: 576.1977.
1
98%) of NDDB was obtained as a white solid. H NMR (CD2Cl2,
300 MHz): δ 7.15 (m, 4H, C6H4), δ 4.44 (s, 2H, NCH2), δ 1.6ꢀ1.9
(m, 6H, CH2CH2CH2), δ 1.51 (s, 6H, CH3), δ 1.12 (s, 6H, CH3).
2H NMR (CH2Cl2, 400 MHz): δ 7.21 (br s, 1D, ND), δ 3.76 (br s, 1D,
BD). 11B NMR (CD2Cl2, 96 MHz): δ ꢀ21.07 (s). 19F NMR (CD2Cl2,
282 MHz): δ ꢀ133.67 (d, 6F, 3JFF = 22 Hz, o-C6F5), δ ꢀ163.66 (br s, 3F,
p-C6F5), δ ꢀ167.03 (m, 6F, m-C6F5). HRMS ESI+-TOF calcd for
Theoretical Analysis. The calculations were carried out using
DFT with the PBE68 and CAM-B3LYP69 functionals and a 6-31G** basis
set70 for all atoms. The computed structures were presented in our
earlier paper.11 The effects of the hydrogenꢀdeuterium isotopic sub-
stitution are new. Gaussian 0971 was employed for the DFT calculations.
In addition, a potential energy distribution analysis was performed by the
VEDA program.72
C28H25BDF10N Na+: 601.1954. Found: 601.1931. HRMS ESIꢀ-TOF
3
calcd for C28H24BDF10Nꢀ: 577.1989. Found: 577.1992.
Synthesis of 1-N-TMPD-CH2-2-[HB(C6F5)2]C6H4 (NDHB).
In a glovebox, a 25-mL flame-dried Schlenk tube equipped with a stir
bar, a Teflon stopcock, and a glass stopper (Glindemann sealing
rings were used for conical joints instead of grease) was charged with
1-N-TMPH-CH2-2-[HB(C6F5)2]C6H4 (NHHB, 0.35 mmol, 200 mg)
and 3 mL of a methanol-d1/benzene (40/60) mixture. The reaction was
stirred at 1000 rpm at room temperature overnight. After evaporation of
’ ASSOCIATED CONTENT
S
Supporting Information. Characterization of compounds
b
by multinuclear NMR spectroscopy, packing diagrams, and geo-
metric parameters from single-crystal neutron diffraction analysis;
crystallographic data and CIF file; FTIR DRAFT and VT-FTIR
20255
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Journal of the American Chemical Society
ARTICLE
solution spectra; 1H NMR T1 relaxation data, 2D NOESY spectra,
VT-NMR spectra and MS signals of TG-MS measurements;
complete ref 71. This material is available free of charge via the
Internet at http://pubs.acs.org.
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V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem.—Eur. J. 2010,
16, 4895–4902. (r) Jiang, C. F.; Blacque, O.; Berke, H. Organometallics
2010, 29, 125–133. (s) Neu, R. C.; Ouyang, E. Y.; Geier, S. J.; Zhao,
X. X.; Ramos, A.; Stephan, D. W. Dalton Trans. 2010, 39, 4285–4294.
(t) Alcarazo, M.; Gomez, C.; Holle, S.; Goddard, R. Angew. Chem., Int.
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’ AUTHOR INFORMATION
Corresponding Author
timo.repo@helsinki.fi; rieger@tum.de
’ ACKNOWLEDGMENT
(8) Rokob, T. A.; Hamza, A.; Pꢀapai, I. J. Am. Chem. Soc. 2009, 131,
10701–10710.
(9) Chapman, A. M.; Haddow, M. F.; Orton, J. P. H.; Wass, D. F.
We gratefully acknowledge the beam time obtained at FRM II,
the neutron source at the Technische Universit€at M€unchen. We
thank G. Gemmecker, K. Ruhland, S. Vagin, K. Chernichenko,
J. Dengler, A. Jonovic, and P. Heinz for help with the data col-
lections and S. Vagin for helpful discussions. The work was
funded by the Academy of Finland (123248, 139550) and DAAD
(D/05/51658). P.P. belongs to the Finnish Centre of Excellence
in Computational Molecular Science. M.A. thanks the Swedish
Institute.
Dalton Trans. 2010, 39, 6184–6186.
(10) (a) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc.
2009, 131, 52–53. (b) Ullrich, M.; Lough, A. J.; Stephan, D. W.
Organometallics 2010, 29, 3647–3654.
(11) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskel€a,
M.; Repo, T.; Pyykk€o,P.;Rieger,B.J. Am. Chem. Soc. 2008,130, 14117–14119.
(12) (a) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fr€ohlich,
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P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701–1703.
(c) Wang, H. D.; Fr€ohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
2008, 5966–5968. (d) Axenov, K. V.; Kehr, G.; Fr€ohlich, R.; Erker, G.
J. Am. Chem. Soc. 2009, 131, 3454–3455. (e) Jiang, C.; Blacque, O.;
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S.; Tumay, T. A.; Axenov, K. V.; Peuser, I.; Kehr, G.; Fr€ohlich, R.;
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Lubbe, G.; Wibbeling, B.; Fr€ohlich, R.; Kehr, G.; Erker, G. Organo-
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