Dihydrogen bond intermediated alcoholysis of dimethylamine-borane in nonaqueous media

Article abstract of DOI:10.1021/acs.jpca.5b01921

Dimethylamine-borane (DMAB) acid/base properties, its dihydrogen-bonded (DHB) complexes and proton transfer reaction in nonaqueous media were investigated both experimentally (IR, UV/vis, NMR, and X-ray) and theoretically (DFT, NBO, QTAIM, and NCI). The e

Full text of DOI:10.1021/acs.jpca.5b01921

Subscriber access provided by SUNY DOWNSTATE
Article
Dihydrogen Bond Intermediated Alcoholysis of
Dimethylamine-Borane in Non-Aqueous Media
Igor E. Golub, Ekaterina S. Gulyaeva, Oleg Andreevich Filippov, Viktor P. Dyadchenko,
Natalia V. Belkova, Lina M. Epstein, Dmitry E. Arkhipov, and Elena S. Shubina
J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 30 Mar 2015
Downloaded from http://pubs.acs.org on March 31, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry A is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Dihydrogen Bond Intermediated Alcoholysis of
DimethylamineꢀBorane in NonꢀAqueous Media
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
a,b
a,b
a
b
Igor E. Golub, Ekaterina S. Gulyaeva, Oleg A. Filippov, Victor P. Dyadchenko, Natalia V.
a
a
a
a*
Belkova, Lina M. Epstein, Dmitry E. Arkhipov, Elena S. Shubina
a A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilova 28, 119991 Moscow, Russia
b Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1/3,
1
19234 Moscow, Russia
AUTHOR EMAIL ADDRESS: shu@ineos.ac.ru (Shubina E. S.)
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 2 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT
Dimethylamineꢀborane (DMAB) acid/base properties, its dihydrogenꢀbonded (DHB) complexes
and proton transfer reaction in nonꢀaqueous media were investigated both experimentally (IR,
UV/vis, NMR and Xꢀray) and theoretically (DFT, NBO, QTAIM and NCI). The effects of
DMAB concentration, solvents polarity and temperature on the degree of DMAB selfꢀ
association are shown and the enthalpy of association is determined experimentally for the first
time (−ꢁH°assoc = 1.5–2.3 kcal/mol). The first case of “improper” (blueꢀshifting) NHꢂꢂꢂF
hydrogen bonds was observed in fluorobenzene and perfluorobenzene solutions. It was shown,
that hydrogenꢀbonded complexes are the intermediates of proton transfer from alcohols and
phenols to DMAB. The reaction mechanism was examined computationally taking into account
the coordinating properties of the reaction media. The values of the rate constants of proton
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−4
transfer from HFIP to DMAB in acetone were determined experimentally [(7.9 ± 0.1)×10 to
−3
−1 −1
(
1.6 ± 0.1)×10 mol ·s ] at 270−310 K. Computed activation barrier of this reaction
‡
theor
ꢁ
G
298K(acetone) = 23.8 kcal/mol is in good agreement with the experimental value of the
‡exp
activation free energy ꢁG 270K = 21.1 kcal/mol.
KEYWORDS: amineꢀboranes; boron hydrides; DFT ; IR spectroscopy
INTRODUCTION
1
2
The amineꢀboranes (R
x y
NH(3ꢀx)BH(3ꢀy)R , where x,y ≤ 3) are well known as hydroborating
agents and selective reducing agents, (for example, chiral amineꢀborane complexes can be used
1
ꢀ4
5ꢀ8
in enantioselective syntheses). These compounds find many interesting applications and are
considered as prospective materials for reversible hydrogen storage systems due to their high H2
9ꢀ12
volumetric and gravimetric density. Amineꢀboranes as well as related bifunctional compounds
13
14ꢀ17
(metal borohydride ammonia borane complexes, metal tetrahydroborate ammoniates,
metal
18ꢀ19
20
21ꢀ22
amidoborane borohydrides
and amidoborane ammoniates , metal hydrazineꢀboranes
2
ACS Paragon Plus Environment

Page 3 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
etc.) containing both acidic and basic groups are of particular interest for these purposes, as these
2
3
compounds are capable to evolve hydrogen at heating. Definitely, this reactivity relies on the
24ꢀ26
existence dihydrogen bonds between acidic and basic functional groups.
27
Widely recognized nowadays, dihydrogen bond to borohydrides was suggested almost 50
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
4ꢀ25, 28
δ− +δ
years ago, but proved only in late 1990’s
after reports on MH ··· HX hydrogen bond
2
9ꢀ33
formed by hydrides of transition metals.
Thus, in 1964 Burg observed by IRꢀspectroscopy
that the stretching vibration of NHꢀgroup of dimethylamineꢀborane (DMAB) appears in CS
2
and
NH. On the basis of perturbation
analysis of stretching vibrations it was suggested that NH···H B interaction similar to hydrogen
34
4 3 2
CCl solutions at lower frequency than νNH of gaseous (CH )
3
bond exists in DMAB solution. In 1967 Rudolph and Parry, analyzing the high lattice energy of
solid PH BH , suggested that “possible source of this lattice energy is the interaction of the
3
3
acidic phosphine hydrogens and the hydridic borane hydrogens in an unusual type of hydrogen
δ+ −δ
35
bond” in PH ··· HB manner. This novel idea was not appreciated and unfairly forgotten until
the 1990’s. Then in 1968 Titov et al. explained the increased chemical reactivity of amineꢀ
boranes toward the H
2
loss by "close spatial arrangement of oppositely charged hydrogen
36
atoms". In 1968−1974 Brown et al. investigated the IR spectra of L·BH (L = Py, Me NH
3
2
Me
3
N, Et
3
N, Et
3
P) and Me
3
NBH
2
X (X = Cl, Br, I) in CCl
4
solutions and interactions of these
t
37ꢀ39
compounds with weak acids (MeOH, BuOH, PhOH, pꢀFC
6
H
4
OH).
The lowꢀfrequency shift
4
of νNH band was observed for DMAB in CCl . It was found that the IR spectra of DMAB are
“both concentrationꢀ and temperatureꢀdependent in a manner characteristic of compounds
undergoing intermolecular hydrogen bonding”. The preliminary results of Xꢀray analysis of solid
37
DMAB have shown the molecular association with short “B···HN” contacts. The lowꢀ
frequency shift of OH stretching vibration was observed upon the interaction of amineꢀ and
38ꢀ39
phosphineꢀboranes with acids.
3 2
On the basis of these data it was suggested that BH and BH
3
8
groups can act as proton acceptors despite the lack of lone pairs or π electrons. Even the
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 4 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
enthalpy of the interaction was estimated (1.7–3.5 kcal/mol), but no conclusion about the hydride
ligand participation in hydrogen bonding was made.
Much later, in 1995–1999, the analysis of amineꢀborane dimers in Cambridge Structural
40
24ꢀ25
Database by Crabtree et al.
revealed that in many cases the H···H distances (1.7–2.2 Å) are
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
less than the sum of van der Waals radii of hydrogen atoms (2.4 Å) and the B–H···H(N) angles
range from 95 to 175°. In the same time some of us investigated the ability of various main
n
n
group hydrides (anionic Bu
Me NAlH ) to form BH···HX dihydrogen bond upon the interaction with proton donors in
solution. It has been found these hydrides give the weak to medium strength DHBs adducts (1.1–
4 4 4 3 3 3 3
NBH and Bu NGaH4, neutral (EtO) PBH , Et NBH and
3
3
28, 41ꢀ42
6.5 kcal/mol).
14, 43ꢀ44
Recent publications
show still high interest to dihydrogen bonds in chemistry of BNH
compounds. However there is no real conception of the role of DHB in the activation processes
and the exact mechanisms of BH/NH activation (both offꢀmetal and onꢀmetal).
In this paper we present the results of the spectral and theoretical exploration of
dimethylamineꢀborane (DMAB) reactivity aiming to establish its properties in intermolecular
interactions and proton transfer reactions with organic acids and bases, to determine the structure
of intermediates and the mechanism of the proton transfer processes.
Experimental section
General Considerations. All manipulations were performed under a dry argon atmosphere
using standard Schlenk technique. Commercially available argon (99.9%) was additionally
purified from traces of oxygen and moisture by sequential passage through Ni/Cr catalyst
column and 4 Å molecular sieves.
The HPLC grade solvents (Acros Organics) were used for sample preparation after additional
purification by standard procedures. Dichloromethane (DCM), fluorobenzene (FBz) and
acetonitrile (MeCN) were dehydrated over CaH
2
,
tetrahydrofuran (THF) – over
Na/benzophenone, toluene – over Na. All solvents were freshly distilled under argon prior to use.
4
ACS Paragon Plus Environment

Page 5 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Fluorinated alcohols provided by P&M (Moscow, Russia) and Fluka Analytical were used.
Dimethylamineꢀborane (DMAB) was provided by Aviabor (Dzerzhinsk, Russia) as received.
Other reagents were from Sigma Aldrich.
Variableꢀtemperature IR measurements. IR spectra were recorded on FTIR Nicolet 6700
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
and FTIR Shimadzu IR Prestigeꢀ21 spectrometers using 0.04–0.22 cm CaF cells. Low
temperature IR studies were carried out in the 190−300 K temperature range using the homeꢀ
modified cryostat (Carl Zeiss Jena). The cryostat modification allows transfer of the reagents
(premixed at either low or room temperature) under an inert atmosphere directly into the cells.
−2
−3
For measurements in the νOH range, the acid concentrations were 10 −10 M to avoid selfꢀ
association, whereas DMAB was taken in 10ꢀfold excess. For measurements in the νBH range, the
equimolar ratio or 10ꢀfold excess acids were used.
NMR experiments. NMR spectra were recorded on a Bruker Avance II 400 MHz
1
spectrometer. H chemical shifts are reported in parts per million (ppm) relative to
1
1
tetramethylsilane (TMS) and were calibrated against the residual solvent resonance, while
was referenced to BF ⋅Et O.
A screwedꢀcap NMR tube was loaded with 1.8 mg of DMAB (0.06 mmol) under an inert
atmosphere and then 500 ꢃL (final concentration ca. 0.06 M) of dry and degassed CD Cl or
acetoneꢀd was transferred into the tube via syringe, under inert atmosphere. From 1 to 5 eq. of
B
3
2
2
2
6
proton donors (TFE, HFIP, TFA, PNP in solution) then was syringed into this solution at room
temperature.
Products characterization. XꢀRay diffraction studies.
Single crystals suitable for Xꢀray diffraction analysis were isolated directly from the reaction
mixtures. Xꢀray diffraction measurements were carried out using Smart APEX II diffractometer.
4
5
The frames were integrated and corrected for absorption by the APEX2 program package. The
details of crystallographic data and experimental conditions are given in Supplementary
materials (Table S1).
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 6 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The structures were solved by the direct method and refined by fullꢀmatrix leastꢀsquares
2
technique against F in the anisotropicꢀisotropic approximation. Hydrogen atoms were located
from the difference Fourier maps and refined in rigid body model. All calculations were
46
performed using the APEX2 program package.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
−
Crystallographic data for the structural analysis of B(OH)
3
ꢂC
9 7 9 7
H NꢂC H NH Cl and
+
−
Me NH [B(OCH CF ) ] has been deposited with the Cambridge Crystallographic Data
2
2
2
3 4
40
Centre , CCDC NOS. 1000135 – 1000136.
+
−
In IR spectra of isolated Me
2
NH
2
[B(OCH
2
CF
3
)
4
] product the following bands were observed
−1
−1
−1
−1
−1
(
in nujol): νNH = 3114 cm , νBO/νCO = 1290 cm , 1165 cm , 1107 cm , 1040 cm , νCF = 975
−1
−1
cm and 963 cm .
Computational details. Full geometry optimizations were carried out with the Gaussian09
47
48
(
Revision C.01) package at the density functional theory (DFT) level using the M06,
49
50ꢀ51
52ꢀ56
B3LYP, BP86
and MP2
levels of theory.
+
+
57ꢀ58
The 6ꢀ311 G(d,p)
basis set was used for all atoms. Frequency calculations were
performed for all optimized complexes in the gas phase and are reported without the use of
scaling factors. The nature of all the stationary points on the potential energy surfaces was
confirmed by a vibrational analysis. Transition state (TS) structures showed only one negative
eigenvalue in their diagonalized force constant matrices, and their associated eigenvectors were
confirmed to correspond to the motion along the reaction coordinate under consideration using
5
9ꢀ60
the Intrinsic Reaction Coordinate (IRC) method.
The complex formation energy was calculated in the gas phase taking into account the basis
61
sets superposition error (by the Bernardi and Boys method, BSSE), ZPVE correction was
62ꢀ63
determined from the unscaled harmonic frequencies.
Inclusion of nonspecific solvent effects in the calculations was performed by using the SMD
6
4
2 2
method. The interaction energy was calculated in THF (ε = 7.4), CH Cl (ε = 8.9),
Me CO (ε = 20.5), MeCN (ε = 35.6) and H O (ε = 78.4) for the gas phase optimized geometries.
2
2
6
ACS Paragon Plus Environment

Page 7 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Changes in Gibbs energies and enthalpies in the solvent were determined using corresponding
65
corrections obtained for the gas phase :
corr
ꢁ
ꢁ
H
DCM = ꢁEDCM + ꢁH gas
corr
G
DCM = ꢁEDCM + ꢁG gas
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The magnetic shielding calculations were performed within the gaugeꢀindependent atomic
66
11
orbitals (GIAO) framework. The B magnetic shielding were calculated on M06, B3LYP,
6
7
++
BP86 and MP2 levels of theory with the IGLOꢀIII basis set using M06/6ꢀ311G (d,p)
geometries. Chemical shifts were referenced to BF ⋅Et O as an external standard:
3
2
1
1
11
ꢁ
δ( B)= δ(BF
3 2
·Et O) − δ( B)
Results and discussion
In this paper we present the results of combined variable temperature IR and NMR
spectroscopic study of the DMAB interaction with organic acids and bases backedꢀup by
theoretical calculations. The computational results are discussed in the end of the paper; some of
the optimized structures are used as illustrations.
Selfꢀassociation of DMAB
The coexistence of acidic and basic groups within the DMAB molecule allows the interaction
between the two molecules, where the NHꢀgroup of one molecule acts as a proton donor and
BH
3
ꢀgroup of another molecule plays the role of a proton acceptor. The existence of DMAB as a
2
4ꢀ25, 68ꢀ69
dimer (Figure 1) in the solid state is well recognized nowadays
but there are only old
37
4
data on its selfꢀassociation in solution limited to CCl .
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 8 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 1. M06ꢀoptimized structures of DMAB and its selfꢀassociates. Formation SMD
theor
enthalpies calculated in DCM relative to DMAB monomer: ꢁHf
(dimer) = –6.5 kcal/mol;
DCM
theor
ꢁ
H
f
DCM (tetramer) = –11.6 kcal/mol.
Our IR measurements show DMAB selfꢀassociation in nonꢀpolar solvents. Three BH
as
−1
−1
s
stretching vibrations of DMAB were observed in CCl
4
: νBH at 2370 cm , 2301 cm and νBH at
−1
70
−1
2
265 cm (i.e., close to the literature values in nujol at 2379, 2295 and 2264 cm ). However,
this range is rather complicated for the analysis due to the overlap with the δBH bending vibration
71
10
72
11
overtones and B–H stretching vibrations modes, that appear at higher frequency than B–H
and lead to bands broadening. Therefore, the quantitative data were obtained using the NH
free
−1
stretching vibrations. The DMAB monomer has the band of νNH at 3299 cm (absorbance at
–1
band maximum A = 0.22; full width at half maximum FWHM = 10.8 cm ) in CCl
4
(ε = 2.2).
bond
−1
free
The νNH
appears at 3207 cm and its intensity is significantly greater than that of the νNH
band even at low DMAB concentration (c = 0.04 M) (Figure 2). Thus, in this solvent the major
bond
form is the associated one. The intensity of νNH
band increases with the temperature lowering
bond
(295–265 K, Figure S1) and the concentration growth (0.04 M–0.11 M, Figure S3). The νNH
−1
−1
70
position is close to that of DMAB in the solid state (3203 cm in nujol, 3211 cm in KBr).
bond
The νNH
band is nonꢀsymmetric and after the band deconvolution (Figures S2–S3) we
−
1
−1
–1
obtained the three bands at 3233 cm (∆νNH = −66 cm ; A = 0.12; FWHM = 31.9 cm ), 3207
−1
−1
–1
−1
cm (the major form, ∆νNH = −92 cm ; A = 0.85; FWHM = 28.8 cm ) and 3184 cm (∆νNH
=
−1
–1
−
115 cm ; A = 0.04; FWHM = 21.4 cm ).
8
ACS Paragon Plus Environment

Page 9 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
0
.5
νNHfree
2 2
CH2Cl2
CH Cl
νNHbond
3202 cm-1
3
288 cm-1
CCl4
CCl
4
A
hex
Hexane
1
.5
0
207 cm-1
3
3299 cm-1
305 cm-1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
224 cm-1
3
3
350
3300
3250
3200
3150
ν, cmꢀ
1
Figure 2. IR spectra of DMAB (0.04 M) in the νNH region in CH
2 2 4
Cl (black solid line), CCl (red
dashed line) and in hexane (blue dashꢀdot line) at 290 K, l =1.2 mm.
The quantity of the associated DMAB molecules in nonpolar hexane (ε = 1.9) is even higher
free
NH
−1
than in CCl : the band deconvolution (Figure S4) gives ν
at 3305 cm (A = 0.05; FWHM =
4
–1
bond
−1
−1
–
1
2.2 cm ) and four νNH
bands at 3229 cm (∆νNH = −76 cm ; A = 0.25; FWHM = 47.8 cm
1
−1
−1
–1
−1
)
, 3213 cm (∆νNH = −92 cm ; A = 0.14; FWHM = 11.9 cm ), 3202 cm (the major form,
−1
–1
−1
−1
∆νNH = −103 cm ; A = 0.91; FWHM = 13.2 cm ) and 3186 cm (∆νNH = −119 cm ; A =
–
1
bond
0
.22; FWHM = 38.3 cm ). The presence of several νNH
bands allows suggesting the
presence of other hydrogenꢀbonded forms of DMAB besides the dimer. These could be linear
6
9
68
chains linked by dihydrogen bond (Figure 1) or other forms of DMAB (e.g., trimers,
6
8, 73
tetramers
).
In the solvent of medium polarity – CH
2
Cl
2
(ε = 8.9) DMAB is predominantly nonꢀassociated
free
NH
−1
–1
(ν
at 3288 cm ; A = 1.26 ; FWHM = 14.8 cm ), (Figures S5–S6) and the growth of band
bond
−1
−1
–1
intensity νNH
increase of DMAB concentration or temperature is significantly smaller than in CCl
Blue-shifted H-bond. When DMAB is dissolved in fluorobenzene (FBz, ε = 5.4) a new blueꢀ
at 3224 cm (∆νNH = –64 cm ; A = 0.05; FWHM = 63.8 cm ) with the
4
(Figure 2).
bꢀs
−1
−1
–1
shifted NH band νNH (3312 cm , ∆νNH = +30 cm ; A = 0.07; FWHM = 20.9 cm , Figure S7–
S9) appears in the range of NH group stretching vibrations besides the bands of DMAB
−
1
–1
−1
monomer (3282 cm ; A = 0.17; FWHM = 19.2 cm ) and associated form (3215 cm , ∆ν
=
9
NH
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 10 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
−
1
–1
−
66 cm ; A = 0.01; FWHM = 24.5 cm ). The blueꢀshift of ν band can be explained by the
NH
74ꢀ76
formation of “improper” NH···F hydrogen bond that was predicted theoretically.
Indeed, in
−1
perfluorobenzene (PFBz, ε = 2.0) DMAB exists only in two forms: dimeric (νNH = 3216 cm ,
bond
free
–1
∆
ν
NH = νNH
(in PFbz)− νNH (in Fbz) = −66; A = 0.23; FWHM = 31.5 cm ) and NH···F
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−1
bond
free
−1
hydrogen bonded one (νNH = 3321 cm , ꢁνNH = νNH (in PFbz)− νNH (in Fbz) = +39 cm ; A
–1
= 0.06; FWHM = 19.2 cm , Figure 3, Figure S10). In contrast only nonꢀassociated DMAB and
free
−1
its dimer are observed in toluene (ε = 2.4) exhibiting two bands νNH = 3269 cm (A = 0.36;
–1
bond
−1
−1
–1
FWHM = 16.3 cm ) and νNH
= 3213 cm (∆νNH = −56 cm ; A = 0.08; FWHM = 29.9 cm ,
Figure 3 and Figure S11–S13). Absence of a high frequency νNH band in this aromatic solvent
argues that NH···π interaction does not lead to the highꢀfrequency shift of νNH band in
fluorobenzenes. So, this is the first ever spectroscopic evidence of such “improper” NH···F
interaction, previously predicted by theory.
0
.4
269 cm-1
3
t
T
o
o
l
luene
bz
bz
A
νNHfree
FB
F
z
0.3
νNHbond
3221 cm-1
PF
P
B
F
z
282 cm-1
3
0
0
.2
.1
0
νNHbꢀs
3213 cm-1
3215 cm-1
3
312 cm-1
316 cm-1
3
3350
3300
3250
ν, cmꢀ
3200
3150
1
Figure 3. IR spectra of DMAB in the νNH region in toluene (black solid line), FBz (red dashed
line) and in PFBz (blue dashꢀdot line); 290 K, l =0.4 mm.
The calculations reveal two types of “improper” hydrogen bonded complexes of DMAB with
−1
one and two molecules of FBz (Figure 4a, b) that give blue shifts of νNH band by +4 cm and +8
−1
cm at the MP2 level. “Improper” Hꢀbonded complex was also obtained in the case of DMAB
−1
interacting with PFBz (Figure 4c; blueꢀshift of νNH by +18 cm at the MP2 level). Note, the
calculations at M06, B3LYP and BP86 theory level give redꢀshifted νNH frequencies (Table 1).
1
0
ACS Paragon Plus Environment

Page 11 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
The MP2 method, which was typically used in theoretical calculations of blue shifted
74ꢀ75, 77
complexes
, is the only one that gives the νNH blueꢀshift for HB complexes of DMAB with
fluorobenzenes (Table 1). Note that the MP2ꢀmethod provides better agreement between the
experimental and calculated frequency shifts for all types of complexes investigated in this work
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
see part Theoretical investigation of DHB complexes Frequency analysis).
a)
b)
c)
Figure 4. DFT MP2ꢀoptimized geometries of “improper” HB complexes DMAB–FBz (a),
DMAB–2FBz (b) and DMAB–PFBz (c).
Table 1. The calculated frequencies of NH stretching vibrations of DMAB interacting with FBz
or PFBz.
DMAB–FBz
DMAB–2FBz
DMAB–PFBz
Method
ꢁ
r
NH
ꢁνNH
+30
+4
ꢁrNH
ꢁνNH
+30
+8
ꢁrNH
ꢁνNH
+39
+18
−7
Experimental
MP2
0.001
0.001
0.001
0.003
0.001
0.003
0.002
0.003
0.000
0.001
0.001
0.001
M06
−4
+3
B3LYP
BP86
−10
−32
−14
−32
0
−6
The enthalpy of DMAB selfꢀassociation (ꢁH°assoc) was determined using the Iogansen’s
7
8ꢀ80
empirical correlation (Eq. 1)
which correlates the hydrogen bond formation enthalpy (−ꢁH°,
kcal/mol) with the experimental or theoretical values of the νXH frequency shifts. Since the
enthalpy depends on the solvent polarity, the ꢁH°assoc values decrease with the increase of the
solvent dielectric permittivity ε (Table 2).
1
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 12 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
8⋅ ꢀν XH
o
−
ꢀH =
(1)
7
20 + ꢀν XH
−1
Table 2. The frequencies of NH stretching vibrations (in cm ) of DMAB measured in different
solvents and DMAB association enthalpies (in kcal/mol).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a
free
NH
bond
NH
o
Solvent
ε
ν
ν
ꢁνNH
ꢁH assoc
Hexane
PFBz
1.9
2.0
2.2
5.4
9.1
3305
–
3202
3215
3207
3215
3218
–103
−2.3
−1.5
−2.0
−1.5
b
−56
CCl
FBz
CH Cl
4
3299
3281
3297
–92
–66
–79
2
2
−1.8
a
81
b
bond
free
dielectric permittivity values taken from ref. ; ꢁν = νNH (in PFbz)− νNH (in Fbz).
NH
Interaction of DMAB with organic bases
In order to assess the protonꢀdonating properties of NHꢀgroup we studied DMAB interactions
with a number of organic bases (Y) of different strength – quinoline (Qu), pyridine (Py),
СH (СH PO (TOPO), [(CH N] PO (HMPA), Et N (Figures S15–S16). The IR
measurements were done in CCl solution using excess of bases. This solvent allowed to avoid
[
3
2
)
7
]
3
3
)
2
3
3
4
the overlap of bands in strong NH···Y complexes with C–H stretching vibrations of common
organic solvents.
In the presence of bases the νNH bands of DMAB monomer and dimer decrease or disappear
simultaneously. The new broad νNH bands corresponding to NH···Y bond (Y = N or O atom of
base) appear at lower frequencies (Figure 5). Formation of Hꢀbonded complexes with Nꢀbases
(
3
Py, Qu, Et N) becomes evident at high base excess (20–50 equiv). The Hꢀbonded complexes
with HMPA and TOPO have much higher formation constants so in this case no DMAB
monomer is visible.
1
2
ACS Paragon Plus Environment

Page 13 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
DMAB 0.03M 265 K
1
A
.0
.8
.6
.4
.2
.0
νNHbond
3205 cm-1
2.0
DMAB+HMPA= 1:10 265 K
DMAB+HMPA= 1:10 270 K
DMAB+HMPA= 1:10 275 K
DMAB+HMPA= 1:10 280 K
DMAB+HMPA= 1:10 285 K
A
ν
NH•••O=P
3119 cm-1
DMAB 0.03M 270 K
0
0
0
0
0
DMAB + NEt3 = 1:10 270 K
DMAB + NEt3 = 1:20 270 K
DMAB + NEt3 = 1:30 270 K
DMAB + NEt3 = 1:40 270 K
DMAB + NEt3 = 1:50 270 K
1.5
bond
νNH
νNH•••N
088 cm-1
-1
DMAB+HMPA= 1:10 290 K 3210 cm
3
DMAB+HMPA= 1:10 295 K
1
.0
free
ν
3
NH
297 cm
-1
0.5
0.0
free
ν
3
NH
296 cm-1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
340
3240
ν, cmꢀ1
3140
3040
3350
3250
ν, cmꢀ1
3150
3050
Figure 5. IR spectra of DMAB (0.03 M) in the νNH region in CCl
4
solution: in the presence of
1
0÷50 eq. NEt at 270 K (left) and 10 eq. HMPA at 295–265K (right), l =2.2 mm.
3
The value of ꢁνNH increases with the increase of proton accepting ability of base (E
S4). The enthalpies of hydrogen bond formation determined by Eq. 1 (−ꢁH°, Table 3) vary
depending on the strength of the organic base from 1.2 to 4.1 kcal /mol in solvents (ꢁH11(CCl
5.3) and from 1.5 to 6.0 kcal /mol for calculated HB complexes in gaseous phase (ꢁH11(gas) =
.4). The acidity factor P which characterizes the proton donating ability independent of proton
j
, Table
4
)
=
6
i
acceptor and solvent, was determined for the NHꢀgroup of DMAB from the linear dependence
o
ꢁH (
vs E (Figure S20) (P = 0.45 ± 0.01). It is three times higher than P (Et NH) = 0.15 and
j i i 2
CCl4)
78
slightly lower than P
i
(Ph
2
NH) = 0.50. Thus, the coordination of the electronꢀdeficient BH
NH from 0.11 (calculated from the
by Eq. 1 and Eq. 3) to 0.45 in agreement with the increase of the
3
ꢀ
group increases the proton donating ability of Me
2
8
2ꢀ83
experimental data in ref.
gasꢀphase acidity from free Me
H° = ꢀH ⋅ P ⋅ E
84
2
3 2
NH to BH NHMe .
ꢀ
(3)
1
1
i
j
1
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 14 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 3. Formation enthalpies (in kcal/mol) for Hꢀbond of organic bases with DMAB.
a
d
e
Base
MeCN 0.75 MeCN
Me CO 0.93 Acetone
THF 1.04 THF
TOPO 1.65 CH Cl
E
j
Solvent −ꢁH°exp −ꢁHtheor(ꢁν)
1.2
2.2
2.5
1.5
2.4
3.7
–
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
c
2.8
2
2
HMPA 1.49
CCl4
3.6
4.8
4.6
4.7
–
Py
1.53
CCl
CCl
CCl
CCl
4
4
4
4
3.6
b
Qu
1.60
3.8
TOPO 1.65
3.8
Et
a
3
N
1.70
4.1
6.0
78, 85ꢀ86
b
The basicity factors E
j
are taken from
; calculated using the experimental data from
87
c
d
ref.
;
calculated from the temperature dependence of formation constants. ꢁH°exp(ꢁv)
e
calculated by Eq. 1 using experimental; ꢁHtheor(ꢁν)– and calculated by Eq. 1 using MP2 ꢁνXH
values.
IR spectra of DMAB in polar aprotic solvents (MeCN, Me CO, THF) feature only one band of
2
free
NH stretching vibrations at frequencies lower than νNH in the low polar nonꢀcoordinating
solvents described above. We suggest that these bands belong to DMAB associates with the
solvents through the NHꢂꢂꢂY hydrogen bond where Y is the heteroatom of MeCN, Me
2
CO or
free
−1
2 2
THF. The band shifts ꢁνNH estimated relative to νNH in CH Cl change in the order –52 cm
−1
−1
(
MeCN) < –99 cm (Me CO) < –113 cm (THF) (Figure S34–S36). This sequence correlates
2
7
8
MeCN
j
with the order of the proton accepting properties (E
j
) of these solvent molecules E
(0.75) <
Me
2
CO
THF
j
E
j
(0.93) < E
DMAB interaction with acids
We have also studied the hydrogen bonding between the BH
acids: CH OH, CH CH OH, FCH CH OH (MFE), CF CH OH (TFE), (CF ) CHOH (HFIP),
(1.04).
3
ꢀgroup of DMAB and weak
3
3
2
2
2
3
2
3 2
(CF
3
)
3
COH (PFTB), phenol (PhOH), pꢀNO
2 6 4
C H OH (PNP) and indole. The IR measurements
were performed in CH
2
Cl (under conditions that exclude selfꢀassociation of DMAB); the
2
changes in the regions of OH, NH and BH stretching vibrations were considered.
1
4
ACS Paragon Plus Environment

Page 15 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
The IR spectra of proton donors in the presence of DMAB show the intensity decrease of the
free
proton donor band νXH and appearance of new broad lowꢀfrequency band corresponding to
bond
XH
ν
(Figure 6, Table 4). Such spectral picture evidences the involvement of XHꢀgroups in Hꢀ
bonding (XHꢂꢂꢂHB). The band shift, ꢁνXH, depends on the proton donating ability of acids (Table
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
bond
4
). The lowꢀfrequency shifted νNH
bands of DMAB are of low intensity and can correspond
bond
not only to the NH stretching vibrations in DMAB dimer. For DMAB dimer the νNH
band
−
1
−1
was observed at 3218 cm , ꢁνNH = 63 cm , with the intensity A= 0.05 (0.06 M DMAB in
CH Cl at 190 K) while this band drifts to lower frequencies and increases in intensity: e.g,. it
2
2
−1
−1
appears at 3216 cm , ꢁνNH = 65 cm , A = 0.16 for 0.06 M DMAB in presence of 0.15 M PNP
dimer
at 190K in CH
2
Cl
2
(Figure 6, right). This can result from the superposition of the νNH
and
νNH···O band of Hꢀbonded complex where NH interacts with the oxygen atom of a proton donor
see below “Theoretical investigation of DHB complexes”).
(
νNHfree 3281 cm-1
νOHfree
free
1
.8
.6
.4
.2
0
DMAB + HFIP 190
DMAB + HFIP 210
DMAB + HFIP 230
DMAB + HFIP 250
DMAB + HFIP 270
HFIP 190
ν
DMAB + PNP 210
556 cm-1
NH
3
3281 cm-1 DMAB + PNP 230
DMAB + PNP 250
0.5
A
0
0
0
0
DMAB + PNP 260
0.4
DMAB + PNP 270
288 cm-1
3542 cm-1
3
bond
νOH
3
0.3
366 cm-1
3284 cm-1
νOHfree
3573 cm-1
νOHassoc
νNHbond
3216 cm
0.2
0.1
-1
νNHbond
3224 cm-1
571 cm-1
3392 cm
-1
3
bond
νOH
3396 cm-1
290 cm-1
3222 cm-1
3
0.0
3650
3550
3450
3350
3250
3150
3
500
ν, cmꢀ1
3 300
3 100
2 2
Figure 6. IR spectra in the νXH region of DMABꢀHOR mixture in CH Cl : 0.02 M HFIP in the
presence of 0.06 M DMAB at 190–270 K (left) and 0.15 M PNP in the presence of 0.06 M
DMAB at 210–270 K (right), l = 0.4 mm.
−1
Table 4. Spectral characteristics (in cm ) and formation enthalpies (in kcal/mol) for hydrogen
bonds of the proton donors with DMAB in CH Cl .
2
2
a
free
bond
XH
b
c
XH
P
i
XH : DMAB νXH
ν
ꢁνXH −ꢁH°exp(ꢁν)
−ꢁHtheor(ꢁν)
Indole 0.50
EtOH 0.63
1 : 10
1 : 10
3491 3426
3615 3531
−65
−84
1.5
1.7
–
c
2.5
1
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 16 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
MFE 0.78
TFE 0.89
1 : 3.5
1 : 5
3606 3512
3600 3479
3569 3414
3576 3412
3547 3372
3525 3366
−94
2.1
.6
3.0
3.5
2
−121
−155
−164
−175
−179
PhOH 1.00
HFIP 1.05
1 : 3.5
1 : 5
3.2
3.3
3.5
2.7
4.7
3.2
4.9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
PNP
1.27
5 : 1
PFTB 1.33
1 : 5
3.6
a
86,96 b
The acidity factors for proton donors are taken from;
ꢁH°exp(ꢁv) calculated by Eq. 1
c
c
using experimental; ꢁHtheor(ꢁν) – calculated by Eq. 1 using calculated MP2 ꢁνXH values;
Calculated MP2 value for MeOH.
as
s
The intensity of νBH and νBH stretching vibration bands decreases upon DHB bond formation
bond
and new band νBH
appears at lower frequency. The position of this band depends on the
−1
proton donor shifting to lower frequencies with the increase of the XH strength from 2338 cm
−
1
−1
for DMAB : PNP =1:1 in CH
2
Cl
2
(ꢁνBH = 29 cm , Figure 7, left) to 2319 cm for DMAB in
−1
MeOH (ꢁνBH = 48 cm , Figure 7, right). Such changes indicate that BH
3
ligand is a proton
8
8
bond
−1
accepting site in hydrogen bonding. The νBH
2 2
band is very narrow (ꢁν1/2 = 5 cm ) in CH Cl
−1
in comparison to ꢁν1/2 = 36 cm in MeOH (Figure 7).
bond
DMAB 0.06 M 190 K
DMAB 0.06 M 190 K
DMAB 0.06 M 210 K
DMAB 0.06 M 230 K
DMAB 0.06 M 250 K
DMAB 0.06 M 270 K
DMAB 0.06 M 290 K
DMAB 0.06 M 190 K DCM
1
.0
νBH
as
BH
DMAB:PNP 0.06 M 1:1 210 K
DMAB:PNP 0.06 M 1:1 230 K
DMAB:PNP 0.06 M 1:1 250 K
DMAB:PNP 0.06 M 1:1 270 K
DMAB:PNP 0.06 M 1:1 290 K 0.6
DMAB:PNP 0.06 M 1:1 303 K
ν
2338 cm-1
A
0.8
A
2319 cm-1
2367 cm-1
367 cm-1
2
0
.8
.6
0
DMAB:PNP 0.06 M 1:1 310 K
2377 cm-1
2272 cm-1
2268 cm-1
0
.4
0.2
.0
s
ν
2
0.4
BH
368 cm-
1
PNP overtone
0
0
.2
.0
449 cm-1
2
0
2500
2400
2300
ν, cm
2200
2100
2450
2350
2250
2150
ν, cmꢀ1
ꢀ1
Figure 7. IR spectra in the νBH region of DMAB: 0.06 M DMAB (black line), 0.06 M DMAB in
the presence of 1 eq. PNP at 210–310 K in CH Cl (left); 0.06 M DMAB in MeOH at 190–290 K
2
2
(right), l = 0.4 mm.
The hydrogen bond formation enthalpy (−ꢁH°; Table 4) determined by Eq. 1 shows linear
7
8
dependence on the proton donor strength (acidity factors P
i
, Figure S19). Its slope gives (Eq. 3)
the basicity factor E = 0.62 ± 0.02 characterizing the proton accepting ability of boron hydride
j
1
6
ACS Paragon Plus Environment

Page 17 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
independent of proton donor and solvent. This value is slightly higher than that of neutral borane
28
3 3 j 3 3 j
complexes (EtO) PBH (E = 0.42) and Et NBH (E = 0.53), but smaller than that of anionic
−
BH
4
(E
j
= 1.25).
Theoretical investigation of DHB complexes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Structural analysis. Geometry optimizations were performed for Hꢀbonded complexes of
DMAB with MeOH, TFE and HFIP at DFT/M06, B3LYP, BP86 and MP2 theory levels, giving
similar geometry of Hꢀbonded complexes (Table S5–S7). All the minima were confirmed by all
these approaches, except the complexes of IIBHꢀNH type, which do not possess local minima and
converge into IBHꢀNH complex during B3LYP and BP86 optimizations, indicating the instability
of complexes of this type. The calculations revealed four possible types of complexes (Scheme
1): those with Hꢀbond to one center of DMAB (INH and II ) and complexes where the alcohol
BH
acts simultaneously as a proton donor and a proton acceptor (IBHꢀNH and IIBHꢀNH). The minimum
corresponding to cyclic bifurcate DHB complex (IIBHꢀNH) was not found for MeOH. Three of the
Hꢀbonded complexes (INH, IIBH and IBHꢀNH) are similar to complexes of DMAB interacting with
8
9ꢀ90
phenol and aniline described previously
by ab initio simulation. However, the influence of
the alcohol strength on HBꢀcomplexes properties and proton transfer reaction pathway as well as
the electronꢀdensity analysis were not studied at that time.
Scheme 1. Atom numbering scheme and possible types of DHB complexes of proton donors
with DMAB.
Table 5. Structural parameters of M06ꢀoptimized Hꢀbonded complexes of DMAB with alcohols.
MeOH
contact
TFE
contact
HFIP
∠
contact
r
∠
r
∠
r
1
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 18 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
I
NH
1.916 159
NH···O(H) 1.962 157 NH···O(H) 2.015 151 NH···O(H)
1
1
1
1.969 156
2.118 133
1.842 148
2.002 140
OH···H (B) 1.933 156 OH···H (B) 1.804 164 OH···H (B)
IIBH
2
2
2
OH···H (B) 1.932 140 OH···H (B) 2.014 130 OH···H (B)
3
3
2
OH···H (B) 1.683 160 OH···H (B) 1.645 160 OH···H (B)
I
BHꢀNH
NH···O(H) 2.277 130 NH···O(H) 2.269 129 NH···O(H)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
3
1
.813 148 OH···H (B) 1.731 154 OH···H (B)
2
2
IIBHꢀNH
2.283 121 OH···H (B) 2.218 123 OH···H (B)
−
2
.142 136 NH···O(H) 2.307 131 NH···O(H)
The structural parameters of these complexes are in the range typical for DHB involving boron
88
hydrides. The DHB distances are in the range 1.645–1.969 Å for primary (shorter) contact and
.932–2.283 Å for secondary (longer) interaction, both are less than the sum of van der Waals
radii of two hydrogen atoms (2.4 Å). The O–H···H angles for primary contact vary from 148 to
1
1
64°.
Formation of Hꢀbonded complexes entails shortening of the B–N distance by 0.009–0.018 Å
(
Table S8) for all types of complexes, the largest changes being found for complexes of type
IBHꢀNH. In complexes with NHꢂꢂꢂO coordination (I , IBH–NH and IIBH–NH) the N–H bond
NH
interacting with the HOR oxygen atom elongates by 0.004–0.009 Å. The B–H bond primarily
involved in DHB formation elongates by 0.002–0.011 Å. These changes are typical for Hꢀ
88, 91
bonded complexes formed by boron hydrides and amineꢀboranes.
Interaction Energies. The formation energies of complexes differ by less than 4 kcal/mol in
gas phase with the preference of IBHꢀNH complex (Table 6). Taking into account basis set
superposition error (BSSE) lowers the formation energies by ca. 10% only and does not change
2 2
the relative order. Taking into account the nonspecific solvation by SMD method (CH Cl ,
acetone) leads to a significant lowering of formation energy and complex IIBH becomes slightly
more stable than IBHꢀNH and IIBHꢀNH. The enthalpies of Hꢀbond complexes formation in
dichloromethane (Table 6, −∆HDCM = 2.0 – 5.5 kcal/mol) are comparable with the experimental
1
8
ACS Paragon Plus Environment

Page 19 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
enthalpy values (Table 4, −ꢁH°exp = 1.5 – 3.6 kcal/mol). But by other criteria, including the
QTAIM energy of HB formation (EH···X) derived from (3,−1) critical point characteristics, the
most preferable are the cyclic complexes (IBHꢀNH and IIBHꢀNH). That agrees with previous
89
reports.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 6. Formation energies (in kcal/mol) of M06ꢀoptimized Hꢀbonded complexes of DMAB
with HFIP.
Complex
a
a
b
c
ꢁ
E
ꢁEZPVE
ꢁEBSSE ꢁHDCM ꢁHMe
2
CO ꢁHtheor(ꢁνOH
)
E
H···X
type
I
NH
−10.7
−11.7
−14.0
−12.2
−9.0
−9.9
−7.8
−8.8
−3.2
−5.4
−5.5
−4.3
−2.4
−5.2
−4.9
−3.9
–
−6.6
IIBH
−3.7
−4.7
−4.8
−6.5
d
e
I
BHꢀNH
−12.2
−10.3
−11.2
−9.0
−8.5 /−4.5
c
d
IIBHꢀNH
−7.5 /−3.9
a
ꢁH
DCM and ꢁHMe
2
CO – energies with SMD solvent correction in CH
2
Cl
2
and acetone,
b
respectively; ꢁHtheor(ꢁν) – calculated by Eq. 1 using MP2ꢀcomputed νNH frequencies for INH and
OH frequencies for complexes of other types; EH···X – energy of HB formation derived from
QTAIM by equation EH···X = 0.5 × V(r). Energy of OH···HB bond; Energy of NH···O(H)–R
bond.
c
ν
d
e
Frequency analysis. Frequency calculations were performed for the gas phase optimized
geometries. The MP2ꢀmethod provides better agreement between the experimental and
calculated frequency shifts, which are presented on the example of complexes with TFE in Table
8. Formation of hydrogen bond where OH group acts as the proton donor (complexes IBH⋅NH, II)
−1
leads to the expected large frequency shift ꢁνOH (ꢁνOH = −132 ÷ −180 cm ). The exceptionally
−1
low frequency shift in complex INH (ꢁνOH = −17 cm ) is explained by the participation of OH
group only as proton acceptor in NHꢂꢂꢂOH bonding.
1
9
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 20 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ꢀ
1
Table 7. Frequency shifts (ꢁν, cm ) and intensity changes (ꢁA, Km/mol) calculated for MP2ꢀ
optimized Hꢀbonded complexes of DMAB with TFE.
Complex
type
as
as
s
ꢁ
ν
OH ꢁA ꢁνNH ꢁA ꢁνBH1
ꢁA ꢁνBH2
ꢁA ꢁνBH ꢁA ꢁνBN ꢁA
INH
−13
5
−21 209
−11
18
30
−16
−4
−22
−28
−9
1
9
20
20
29
23
−2
−3
−3
−3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
IIBH
−134 378 24
3
−15
−12
−32
32
INHꢀBH −177 371
7
69
14
−28
−19
−75 −35 95
−36 −27 88
IINHꢀBH −168 269 −21 90
13
Cyclic complexes (IBHꢀNH and IIBHꢀNH) exhibit the largest frequency shifts of νBH bands, which
as
are the most similar to the experimentally observed band shifts. In these complexes, the νBH2
s
−1
as
and νBH modes shift to lower frequencies by 20–33 cm , while νBH1 mode undergoes a highꢀ
frequency shift. Such pattern is consistent with the experimental spectra, where the new band
−
1
appears at lower frequency (ꢁνBH = −26 ÷ −48 cm ) (Figure 7). The decrease of the B–N
distance upon Hꢀbond formation results in the increase of νBN stretching vibration frequency
regardless of the complex type.
The computed frequencies can be used to estimate the hydrogen bond formation enthalpy
(ꢁHtheor(ꢁν), Table 6) and the basicity factor of BHꢀligand. This value calculated by Eq. 1 and
79
Eq. 3 and ꢁH =6.4 kcal/mol in gas phase is in perfect agreement with the experimental value
1
1
of the proton donating ability (E
.05 what is significantly lower than the experimental value. The calculated value of P
complexes INH (Table S10) is 0.48 ± 0.02, that is close to the experimental data (P
.01).
So, the complexes of type IBHꢀNH conform to the experimental observations by a number of
j j
= 0.62 ± 0.02). The E calculated for INH–BH complexes is 0.54 ±
0
i
for NH in
= 0.45 ±
i
0
criteria (energetic, spectral).
Electron-density analysis. Electron density redistribution, which occurs upon DHB formation,
9
2
was analyzed using different approaches, namely, natural population analysis (NPA), Wiberg
93
94ꢀ96
bond indexes (WBIs), Bader’s Quantum Theory of “Atoms in Molecules” (QTAIM)
and
2
0
ACS Paragon Plus Environment

Page 21 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
9
7ꢀ98
analysis of nonꢀcovalent interaction indexes (NCIs).
All these methods provide a powerful
99ꢀ101
tool to recognizing hydrogen bond from other types of nonꢀcovalent interaction.
As expected, upon DHB formation the charge on the proton of HOR becomes more positive,
whereas the charge on the interacting hydridic hydrogen(s) becomes more negative (Table S14).
The charge of NH group proton becomes more positive upon bonding with O atom in complexes
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
I , I
and IIBHꢀNH.
NH BHꢀNH
Within the framework of the QTAIM theory, a hydrogen bond is characterized by the presence
102
of (3, −1) critical point that allows one to distinguish it from other types of interaction. The
NH···O(H) interaction in complexes INH, IBHꢀNH and IIBHꢀNH always possess a (3, −1) critical
c
point (ρ = 0.01–0.03 au, εNHꢂꢂꢂO = 0.09–0.31, Table S17). In bifurcate complexes (IIBH and IIBHꢀ
NH), the (3, −1) critical point was found only for the closest contact with the most linear
O−H···H(B) arrangement (Figure 8a) despite the presence of secondary OH···HB contact. The
values of the electron density at the (3, −1) bond critical point (ρ
, Table S17) depend slightly on
c
the proton donor strength being in the range 0.01–0.03 au. The presence of additional
interactions causes deviation of the OH···H moiety from linearity and reflected in the values of
the H···H bond ellipticity (εHꢂꢂꢂH= 0.19–0.26 in IBHꢀNH and IIBHꢀNH) (Figure 12a). Extremely high
εHꢂꢂꢂH ellipticity values (0.92–3.54) are observed for bifurcate DHB complexes of type IIBH
.
9
7ꢀ98
From the analysis of nonꢀcovalent interactions (NCI)
we obtained NCI isosurfaces (Figure
8b, Figures S24, S26 and S28) which are similar to the QTAIM molecular graphs (Figures S24,
S26 and S28) except for the IIBH complexes. The NCI analysis gives in this case two negative
values of sign of (λ )ρ(r) (−0.015 and −0.014, Table S18) indicating attractive (dihydrogen
2
bonding) interaction between the proton and both BH ligands, which are spotted in blue color on
2
gradient isosurfaces (Figure 8b). One positive value of sign (λ )ρ(r) (+0.014) evidences the
nonbonding overlap and is depicted in red color (Figure 12b). Thus NCI analysis allows
assigning the IIBH complexes as bifurcate dihydrogen bond despite only one Hꢀbond path in
terms of QTAIM theory. In case of IINHꢀBH complexes, which geometrical parameters are typical
2
1
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 22 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
for bifurcate Hꢀbond, the analysis of electron density (NBO, QTAIM, NCI) shows only one
XHꢂꢂꢂHB interaction.
2.0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
0
0
.5
.0
.5
.0
-0.2
-0.1
0.0
0.1
0.2
sign( 2
λ )ρ a.u.
a)
b)
c)
Figure 8. Topological analysis of electron density for DMAB·MeOH complex of type IIBH
:
a) QTAIM molecular graph (ε = 1.54); b) NCI gradient isosurface for s = 0.3 au and blueꢀgreenꢀ
red color scale from −0.01< sign(λ )ρ(r) < +0.01 au; c) plot of the reduced density gradient
2
2
(RDG) versus sign(λ )ρ(r).
DMAB protonation in nonꢀaqueous media.
IR study. IR study ascertains that proton transfer proceeds via the Hꢀbonded complex: the
bond
presence of excess proton donors leads to the appearance of new strong band of νBH
stretching vibrations at low frequency in CH Cl (Figure 9) as well as in coordinating solvents
2
2
like MeCN, Me
2) begins, taking several days at room temperature in low polar media (CH
reaction of DMAB with weak acid TFE (10 eqiv.) in lowꢀpolar FBz was completed in 14 days at
2
CO and THF (Figures S38–S40). Above 250 K the hydrogen evolution (Scheme
2
Cl , FBz). Thus, the
2
+
−
room temperature. The monocrystals of [Me NH ] [B(OCH CF ) ] were isolated from the
2
2
2
3 4
reaction mixture by slow solvent evaporation and characterized by Xꢀray (see below), IR and
NMR spectroscopy. The use of coordinating solvent (MeCN, Me CO or THF) and stronger
2
proton donors reduces the reaction time to several hours. The protonation of DMAB by 2 eqiv.
HFIP in acetone has the reaction halfꢀtime of ca. 4 h at 290–310 K.
2
2
ACS Paragon Plus Environment

Page 23 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
Scheme 2. Mechanism of the proton transfer to DMAB.
In the course of the reactions the gradual fall of all bands in the region of νBH stretching
prod
vibrations is observed (Figure 9). Simultaneously two new bands νBH
appear at higher
−1
+
−
frequencies (2402 and 2430 cm ), corresponding to the reaction products ([Me
2
NH
2
] [X
2
BH
2
] ,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
103ꢀ104
bond
where X could be OR or Solv).
In the same time the decrease of the νNH
band, which
belongs to N–H group hydrogen bonded to acetone, is observed (Figure S30).
1.0
νBHas
2364 cm-1
A
0.8
0.6
0.4
2335cm-1
2
295 cm-1
2305 cm-1
νBHs
2266 cm-1
prod
BH
2402 cm-1
ν
2
430 cm-1
0.2
0.0
ν, cm−1
2500
2400
2300
2200
Figure 9. IR monitoring (νBH region) of the reaction between DMAB (black line, 0.06 M) and 2
eqiv. HFIP (0.12 M) in acetone (310 K, l = 0.4 mm) over ca. 280 min.
The experimental kinetic data obtained for DMAB reaction with 2 eqiv. HFIP in acetone obey
a second order reaction law (Eq. 4). The values of the rate constants determined vary from (4.2 ±
−5
−3
−1 −1
‡
0
.4)×10 to (1.6 ± 0.1)×10 mol ·s in the 190−310 K temperature range with ꢁG = 14.8–
22.2 kcal/mol (Table S18).
d[DMAB]
−
= k [DMAB][HOR]
(4)
obs
dt
UV-Vis. The interaction of DMAB with pꢀNO
2
C H
6 4
N=NC
6
H
4
2 2
OH (PNAP) in CH Cl could be
conveniently investigated by UVꢀVis spectroscopy. At low temperatures the spectra show strong
105ꢀ106
blue shift of the PNAP band (λ = 423 nm, ꢁλ = 45 nm) indicative of DHB formation
2
3
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 24 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
Figure 10) and above 270 K the band of PNAP anion appears at 530 nm evidencing the
beginning of the proton transfer reaction.
PNDP 270K
DMAB + 3PNDP 190K
DMAB + 3PNDP 210K
DMAB + 3PNDP 230K
DMAB + 3PNDP 250K
DMAB + 3PNDP 270K
DMAB + 3PNDP 290K
A
1.2
423 nm
3
78 nm
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0.8
0.4
0.0
530 nm
3
00
400
500
600
λ, nm
Figure 10. UVꢀVis spectra of PNAP (black line, 0.003 M) and PNAP in the presence of 5 eq.
DMAB in CH Cl , l = 0.4 mm.
2
2
Characterization of the reaction products. The product of DMAB protonation by TFE in FBz
+
−
– the compound [Me
2 2 2 3 4
NH ] [B(OCH CF ) ] was isolated as individual monocrystals and
11
characterized by Xꢀray diffraction (Figure 11). The
B
singlet signals of
+
−
[
Me
2 2 2 3 4 2 2
NH ] [B(OCH CF ) ] dissolved in CD Cl appear at 2.7 and 2.4 ppm, these values are
11
+
−
consistent with previously reported B chemical shifts for Na [B(OCH
2
CF
3 4
) ] (2.9 ppm) and
+
−
107
11
Li [B(OCH CF ) ] (3.2 ppm). Appearance of two B singlets could be due to the formation
2
3 4
1
08
of contact and solvent separated ion pairs in solution.
In the presence of even small amount of
water this compound transforms into boric acid with a broad signal at 19.5–20.3 ppm.
+
−
Figure 11. General view of [Me NH ] [B(OCH CF ) ] structure according to XRD analysis
2
2
2
3 4
(50% probability ellipsoids) (top) and M06 optimization (bottom).
2
4
ACS Paragon Plus Environment

Page 25 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
−
109
Similar [B(OR) ] products were obtained previously in NH BH alcoholysis by MeOH and
4
3
3
1
10
111
TFE and in case of NaBH
4
methanolysis. Another products were isolated by us as the result
+
−
of DMAB interaction with quinoline and NEt
3
in CCl
4 3 9 7 9 7
: B(OH) ꢂC H NꢂC H NH Cl (Figure 12)
+
−
and [Me
2 2
NH ] Cl . XRD study of dimethylammonium chloride revealed the same unit cell
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
12
dimensions as published CSDꢀDMEACL05.
+
−
3 9 7 9 7
Figure 12. General view of B(OH) ꢂC H NꢂC H NH Cl (50% probability ellipsoids).
NMR-monitoring of the reaction.
11
The B NMR signal of initial DMAB in acetoneꢀd
7.0 Hz). Upon the DMAB reaction with 2 eqiv. HFIP (Figure S35) a new triplet signal appears
at +3.1 ppm (t, JBH = 113.6 Hz) in ca. 30 min after mixing, corresponding to
6
appears as a quartet at −13.6 ppm (q, JBH
=
9
113
[
Me
2
NH
2
][BH
2
(OR)
2
]
.
2 2 2
In low polar media (CH Cl ) the reaction apparently yields Me NH–
1
04, 114
BH OR (+4.0 ppm, t, 111.2 Hz) and Me NH–BH(OR) (+2.4 ppm, d, 78.1 Hz) species.
2
2
2
3 3
The hydrolysis products B(OH) and BX were observed at 20.3 ppm. When initial compound is
11
1
almost consumed the new singlet signal is observed in B{ H} spectra at 2.2 ppm (Figure S32)
+
−
assigned to [Me
In the presence of 1 eqiv. pꢀnitrophenol (PNP) in acetoneꢀd
of BH X species is observed at −0.6 ppm (t, J = 111.2 Hz) (Figure 13). The exact position of
2 2 4
NH ] [B(OR) ]
6
solution of DMAB the new triplet
2
2
BH
−
this signal is highly dependent on the OR nature (Table 8), supporting its asignment to
2
5
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 26 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
Me NH ][BH (OR) ] species. Similar spectral picture was observed in time for this and other
2 2 2 2
proton donors (see Table 8 and Figures S33–S36).
11
It should be noted that B signals in the +3 ÷ −1 ppm region could be assigned to the neutral,
anionic or even cationic tetracoordinated boron in [BH
2
(X)
2
] species depending on the electron
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
–
115
−110
donating ability of X. In our case when RO residue (Me
2
NHBH
2
OR and [BH
2
2
(OR) ]
) is
formed as the result of proton transfer and H evolution, the formation of cationic species like
2
+
116
2
117ꢀ118
[BH
2
X
2
] (X = NHMe
or Solv
) is quite unlikely.
11
Figure 13. The B NMR spectra of DMAB (0.06 M) in the presence of PNP in acetoneꢀd
6
at
2
97 K: a) DMAB:PNP (1:2) (black line) after 1.5 h of reaction, DMAB:PNP (1:3) (blue line)
after 2 h of reaction, DMAB:PNP (1:4) (red line) after 2.5 h of reaction; b) DMAB:PNP (1:5)
after 10.5 h of reaction; c) DMAB:PNP (1:5) after 3 days.
2
6
ACS Paragon Plus Environment

Page 27 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
1
1
Table 8. Experimental values of B NMR chemical shifts for the reaction products and
calculated by GIAO-MP2/IGLO III method.
−
−
−
−
ROH
[BH
2
(OR)
2
]
[BH(OR)
3
]
[B(OH)
4
]
[B(OR)
4
]
B(OH)
+18.0
+19.5
3
+
4.0
+6.3
Calculated
for MeOH
+
1.3
−
+1.6
+
−
+
−
[
Me
2
NH
2
] [BH
2
(OMe)
2
]
[Me
2
NH
2
] [B(OMe)
3
H]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
2.4 (s)
2.7 (s)
TFE
HFIP
PNP
TFA
−
−
+
(broad s)
+20.3
+
3.1 (t)
BH = 113.6 Hz
0.6 (t)
BH = 111.2 Hz
1.1 (t)
BH = 118.3 Hz
−
−
−
+2.2(s)
J
J
J
(broad s)
+19.3
−
+2.5 (s)
+2.6 (s)
+1.1 (s)
−
(broad s)
+20.2
−
+0.7(d)
BH = 151.4 Hz
−
J
(broad s)
1
In H NMR spectra of DMAB in acetoneꢀd
6
the signal of NHꢀgroup is observed at 5.2 ppm (s),
of CH ꢀgroup ꢀ at 2.4 (d) and BH ꢀgroup ꢀ at 1.5 (q). In the presence of 5 equiv. PNP the new
3
3
+
signals appear at 5.7(s) and 2.6 (d) ppm that correspond to NH
respectively. The BHꢀsignals intensity decreases and a new broad line appears at 1.4 ppm (s) that
can be assigned to X BH species.
2 3 2 2
ꢀ and CH ꢀgroups of [Me NH ] ,
2
2
11
To predict the magnetic shielding of B nuclei several approaches were used (GIAOꢀ
MP2/M06/B3LYP/BP86 with IGLO III basis set). The GIAOꢀMP2/IGLO III//M06/6ꢀ311++G
11
(
d,p) and GIAOꢀM06/IGLO III//M06/6ꢀ311++G(d,p) methods gave reliable results for B NMR
11
chemical shift calculations (Table S19) and therefore were used to calculate the B magnetic
+
−
shielding in [Me
2
NH
2
] [BH(4ꢀx)(OR)
x
] and B(OR)
3
alcoholysis products assisting the signals
11
assignment in the experimental B NMR spectra (Table 8,Table S19–S20).
2
7
ACS Paragon Plus Environment

The Journal of Physical Chemistry
Page 28 of 43
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Mechanism of proton transfer
Several mechanisms for amineꢀboranes (AB) hydrolysis were proposed in the literature.
1
19ꢀ120
The first pathway implies the proton transfer to BH
3
ꢀgroup with hydrogen evolution and
119
subsequent fast hydrolysis by the water present in the system. The second pathway involves
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
electrophilic displacement of BH
3
through the proton attack to nitrogen, accompanied by the
1
19ꢀ121
rupture of B–N bond and fast hydrolysis of solvated BH ꢀgroup.
Another pathway implies
3
120
the dissociative activation of B–N bond.
However all these schemes were proposed for acid hydrolysis by strong inorganic acids (HCl,
SO ) where the proton transfer reaction is fast making the intermediates observation
H
2
4
impossible. We investigate the DMAB reaction with weak organic acids (fluorinated alcohols,
substituted phenols) in the argon atmosphere in anhydrous solvents that allows observing the
proton transfer reaction intermediates.
As it follows from the lowꢀtemperature IR and UVꢀstudy and DFT calculations in the presence
of weak OH acids DMAB prefers to form cyclic HB complexes of type IBHꢀNH rather than
complexes of type IIBH with bifurcate BH⋅⋅⋅HX bonds. Having small difference in formation
energy (less 4 kcal/mol) both these HB complexes can be envisaged as intermediates of the
2
reaction leading to H evolution. So, several pathways of proton transfer were considered in our
quantum chemical calculations with or without direct participation of a solvent molecule. The
calculations were performed taking into account the bulk solvent effect by means of SMD
approach.
Proton transfer in non-coordinating low polar solvents. Three possible mechanisms for this
reaction were considered (Scheme 3): the proton transfer to BH
transfer from the alcohol to BH ꢀgroup assisted by proton transfer from NH group to the
alcohol’s oxygen (TSNH BH) and the insertion of proton donor into B–N bond (TSB–N). Figure 14
3
ꢀgroup (TSBH1), the proton
3
ꢀ
presents the corresponding transition states obtained at the M06 theory level, the energy profiles
are given on Figure 15.
2
8
ACS Paragon Plus Environment

Page 29 of 43
The Journal of Physical Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Possible pathways for DMAB protonation.
The direct protonation of BHꢀsite (TSBH1) proceeding via the DHB intermediate IIBH and
‡
yielding Me
2
NH–BH
2
OR has the highest activation barrier (ꢁG DCM = G(TS) – G(Hꢀbond) =
50.3 kcal/mol for HFIP, Figure 15). This pathway is similar to the first pathway suggested for
AB hydrolysis but is not feasible without additional BH activation or B–N bond dissociation due
to low BH basicity and hence high activation energy. The activation energy for subsequent
proton transfer to second and third hydride ligands (TSBH2 and TSBH3, Figure S48) is lower (37.2
122
and 38.4 kcal/mol, respectively). This trend is common for boronhydrides protonation.
The next possible mechanism is the double proton transfer (TSNHꢀBH), which proceeds via
‡
cyclic HB complex of type IBHꢀNH yielding Me
2
N=BH
2
with lower activation barrier (ꢁG DCM
=
28.4 kcal/mol for HFIP and 24.4 kcal/mol for two HFIP molecules). The product of this process
1
23ꢀ124
Me N=BH is stable only at low temperatures
and easily forms cyclic dimmer (Me N–
2
2
2
1
25ꢀ127
BH
2
2
) .
However in the presence of alcohol excess it can react with HOR by Ad
E
‡
mechanism (TSAdE, Scheme 3; ꢁG DCM = 28.2 kcal/mol for HFIP and 21.2 kcal/mol for two
2 2
HFIP molecules) yielding Me NH–BH OR species as in case of direct protonation of BHꢀsite.
2
9
ACS Paragon Plus Environment