Competition between non-classical and classical hydrogen bonded sites in [BH3CN]-: Spectral, energetic, structural and electronic features

Article abstract of DOI:10.1016/j.molstruc.2005.12.029

Interaction of the salt (Ph₃P{double bond, short}N{double bond, short}PPh₃)BH₃CN with the various OH and NH proton donors in low polar media was studied by variable temperature (200-290 K) IR spectroscopy and theoretically

Full text of DOI:10.1016/j.molstruc.2005.12.029

Journal of Molecular Structure 790 (2006) 114–121
www.elsevier.com/locate/molstruc
Competition between non-classical and classical hydrogen bonded sites
in [BH3CN]K: Spectral, energetic, structural and electronic features
Oleg A. Filippov, Andrey M. Filin, Natalia V. Belkova, Viktoria N. Tsupreva,
*
Yulia V. Shmyrova, Igor B. Sivaev, Lina M. Epstein, Elena S. Shubina
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St 28, 119991 Moscow, Russian Federation
Received 31 October 2005; received in revised form 20 December 2005; accepted 20 December 2005
Available online 20 February 2006
Abstract
Interaction of the salt (Ph3PaNaPPh3)BH3CN with the various OH and NH proton donors in low polar media was studied by variable
temperature (200–290 K) IR spectroscopy and theoretically by DFT calculations. The formation of two types of complexes containing non-
classical dihydrogen bond to the hydride hydrogen (DHB) and classical hydrogen bond (HB) to nitrogen lone pair was shown in solution. The 1:1
complexes of both types (XH/H and XH/N) coexist in the presence of equimolar amount of proton donor. The addition of excess XH-acid leads
to the increase of the classical HB content and appearance of the 1:2 complexes, where two basic sites work simultaneously. The structure, spectral
characteristics, energy and electron redistribution were studied by DFT (B3LYP) method. The comparison DHB parameters of [BH3CN]K with
those of the unsubstituted analogue [BH4]K allowed analyzing the electronic effects of the CN group on the basic properties of boron hydride
moiety. The electronic influence of the BH3 group on CNK/HX hydrogen bond was also established by comparison with the corresponding
classical HB to the CNK anion.
q 2006 Elsevier B.V. All rights reserved.
Keywords: Cyanoborohydride; Hydrogen bond; IR spectroscopy; Quantum chemical calculations
1. Introduction
were analyzed. Our spectral study of closo-borate anions
[B10H10]2K and [B12H12]2K showed that they form rather weak
DHB, whose structure, energy and electron distributions have
been calculated [6]. Recently the spectral investigation of DHB
complexes to GaHꢀ4 in solution was combined with DFT
calculations of model DHB complexes of GaHꢀ4 ; BH4ꢀ, [7] and,
for the first time, the discussion about the mechanism of proton
transfer reaction to main group hydrides. However, the
problem of competition between DHB and classical HB was
practically not studied. Only in our recent paper the
coexistence of both types of hydrogen bonds was determined
at the example of the substituted polyhedral boron hydride
[B12H11SCN]2K [8].
The spectral, structural and energetic properties of the most
unusual unconventional hydrogen bonds (HB) to a hydride
hydrogen as a proton acceptor were actively investigated in the
recent years for the transition metal (XH/HM) or main group
(XH/HE) hydride complexes (see, for example reviews
[1–3]). This type of HB is usually named dihydrogen bond
(DHB). The one of the first aims of investigations was to prove
that these interactions are really HB’s. Significant similarity of
the nature, geometry and spectral features of both new and
classical HB was demonstrated for numerous hydride
complexes [1]. For main group hydrides the experimental
studies in the solid state [3] and in solution [4] as well as the
theoretical calculations were devoted initially to boron
hydrides [3,4]. Later the theoretical studies of the different
simple hydrides [5] appeared where DHB nature and structure
Continuing our work in this field we present in this paper the
spectral, energetic, structural and electronic features of HB’s of
the cyanoborohydride [BH3CN]K with different strength XH
proton donors (XZO, N). The competition between DHB and
HB to nitrogen lone pair and possibility of the classical
and DHB bonds coexistence in one hydride molecule
was studied theoretically and by IR spectroscopy, comparing
with DHB of the unsubstituted analogue [BH4]K and HB of
CNK anion.
*
Corresponding author. Tel.: C7 95 135 6448; fax: C7 95 135 5085.
E-mail address: shu@ineos.ac.ru (E.S. Shubina).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.12.029

O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
115
2. Experimental part
BH3CNK with CH3OH—classical hydrogen bonded
complex (1), DHB complex (2) and, in presence of two
proton donor molecules, 1:2 complex with both classical
and non classical H-bonds, (3) (Fig. 1). The same results
were obtained for BH3CNK$CF3CH2OH system
(complexes 5–7, correspondingly). Structural characteristics
of these complexes are collected in Table 1.
2.1. Computational details
Full geometry optimization for BH3CNK, BH4ꢀ and their
HB complexes with CH3OH and CF3CH2OH were carried out
with GAUSSIAN 98 [9] package at DFT level using the hybrid
B3LYP functional [10]. The 6-311CCG(d,p) basis set was
used for all the atoms. The nature of all the stationary points on
the potential energy surfaces was confirmed by a vibrational
analysis. Natural bond orbital (NBO) analysis was performed
on all optimized structures.
The linear arrangement of OH/L (LZH, N) moiety was
found for all the HB complexes, OH/L angles being 168.2–
˚
178.38. The L/H distances are rather short (1.625–1.839 A)
and substantially less than the sum of van der Waals radii
˚
˚
(2.4 A for H/H and 2.7 A for N/H) for both types of HB
complexes. As expected they shorten with the increase of the
proton donor strength. The elongation of the H/H distances in
the complexes 2 and 6 of BH3CNK anion in comparison with
analogous complexes of the unsubstituted BHK4 anion (4 and 8)
is induced by the electronic influence of CN group decreasing
proton accepting ability of the hydride hydrogen (the Ej factors
decrease will be discussed later). It is interesting that both
H/H and N/H distances in the 1:2 complexes with CH3OH
2.2. Spectroscopic studies
All manipulations were carried out under an argon
atmosphere by standard Schlenk techniques. [Ph3-
PaNaPPh3][BH3CN] was prepared by precipitation with
[Ph3PaNaPPh3]Cl from aqueous solution of Na[BH3CN]
(Merck) followed by drying of the precipitate formed over
phosphorus pentoxide. All solvents used were dried using
appropriate agents and freshly distilled prior to use. The
fluorinated alcohols were from P&M company, Moscow.
The IR spectra of dichloromethane or tetrahydrofuran
solutions (CaF2 cells, dZ0.012–0.120 cm) were measured on
Specord M-82 (Carl Zeiss Jena) spectrometer with 2–4 cmK1
resolution and Infralum FT-801 (Lumex) FTIR spectrometer.
Low temperature IR studies were carried out in the OH, BH
and CN stretching regions using Carl Zeiss Jena cryostat in the
temperature range 200–300 K with the accuracy of the temp-
erature setting G0.5 K. To examine spectra in the region of
nXH the concentration of BH3CNK was equal to 10K1–10K2 M,
and was 10K2–10K3 M in the region of nBH and nCN. The
˚
and CF3CH2OH (3 and 7) are 0.020–0.042 A longer, than in the
corresponding complexes 1:1 due to the electron density
redistribution upon HB formation to the second centre.
The hydrogen bond formation leads to the elongation of O–
H bonds from their values in the isolated molecules which is
larger in hydrogen bonded complexes on N atom lone pair
˚
(Dr(O–H)Z0.024–0.034 A) than in DHB complexes (Dr(O–
˚
H)Z0.014–0.018 A). The length of B–H bond implicated in
dihydrogen bonding increases, while the non-bonded B–H
bonds shorten in all DHB as well as classical HB complexes.
˚
The C–N bond shortens in the same extent (by 0.001–0.002 A)
in HB complexes on N atom lone pair and in DHB complexes.
The electrostatic attraction between the partial negative
charge of the hydrogen and/or nitrogen lone pair of BH3CNK
and the partial positive charge of the acidic hydrogen is the
driving force for both DHB (BHdK/HdCO) and classical HB
(NdK/HdCO). The HB formation leads to an increase of an
absolute values of both the positive charge on the acidic
hydrogen and the negative charge on the nitrogen or the hydride
atom (Table 2). The value of Dq(OH) increase is slightly larger
for the classical HB. In the same time the negative charge on the
non-bonded (B)H and (C)N atoms decreases in absolute value
(K0.008 for BH in the complex 1 and K0.023 for CN in the
complex 2). Notably, that polarization in the 1:2 complex 3
seems to be a superposition of these two changes (Dq(BH)Z
0.009 and Dq(CN)Z0.021) (Table 2).
concentration of XH-acids in the first region was equal to 10K2
10K3 M to avoid self-association and was taken in 1:1 ratio or in
excess (from 2 to 14 fold) in the region nBH and nCN
–
.
Both experimental and theoretical DnBH band shifts are
calculated from the position of A1 band as
free
BH
init
BH
init
BH
DnBH Zn Kn ðA1Þ or DnBH ZnbBoHnd Kn ðA1Þ.
3. Results and discussion
3.1. Theoretical study
The BH3CNK anion possesses two potential proton
accepting sites: the non-classical one is hydride hydrogen of
the BH3 group and classical one is the lone pair of CN group
nitrogen atom. These two sites could compete for the hydrogen
bonding with XH-acids leading to the DHB (BH/HX) and/or
classical HB (CN/HX) formation. To address this problem
the theoretical study of the interaction between BH3CN with
rather weak XH-acids was carried out, the structures of the
complexes formed as well as electronic redistributions and
vibrational frequencies were obtained.
Notably, all structural and electronic changes observed in
the HB complexes studied here are in agreement with general
trends established for hydrogen bonds to classical bases as well
as to the boron hydrides studied previously. The calculations
allow us to assume coexistence of both classical and non-
classical hydrogen bonds in one molecule.
3.2. Spectroscopic features of hydrogen bonds
The theoretical study show the real competition of the
two proton-accepting sites in BH3CNK—hydride ligand
and nitrogen lone pair. Three minima are found for
The theoretical approaches help us in IR assignments
and appear to able predicting different HB complexes

116
O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
Fig. 1. The optimized geometries (distances in A) of the model hydrogen bonded complexes between BH3CNK and CH3OH.
˚
Table 1
˚
The structural characteristics (angles in degrees, bond lengths and elongations in A) of HB complexes to boron hydride hydrogen or to nitrogen atom (in italics)
calculated by the DFT method
Complex
L/HO
r(L/H)
Dr(BH)
Dr(CN)
Dr(OH)
1
2
BH3CNK$HOCH3
NCBHK3 $HOCH3
178.3
169.7
171.3
168.2
172.8
176.4
177.4
175.7
174.4
176.4
1.839
1.751
1.859
1.788
1.654
1.727
1.625
1.761
1.668
1.553
K0.003
0.006
K0.001
K0.001
0.024
0.014
0.022
0.012
0.021
0.034
0.018
0.029
0.015
0.028
3
BH3CNK$2CH3OH
0.002
K0.003
4
5
6
BHK4 $HOCH3
BH3CNK$HOCH2CF3
NCBHK3 $HOCH2CF3
0.005
K0.004
0.009
K0.002
K0.002
7
8
BH3CNK$2HOCH2CF3
BHK4 $HOCH2CF3
0.004
0.007
K0.004
Table 2
The electronic characteristics (L/H bond orbital populations, o.p.; changes of the NBO charges, Dq) of HB complexes to boron hydride hydrogen or to nitrogen
atom (in italics) calculated by the DFT method
Complex
o.p. (L/H)
Dq(BH)
Dq(CN)
Dq(OH)
1
2
BH3CNK$HOCH3
NCBHK3 $HOCH3
0.092
0.078
0.088
0.062
0.174
K0.008
0.019
0.045
0.055
0.052
0.054
0.049
0.062
K0.023
3
4
BH3CNK$2CH3OH
BHK4 $HOCH3
0.009
0.021
0.021
DqZjqcomplexjKjqfreej.

O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
117
Table 3
Spectral features (in cmK1) in the nXH range of proton donors and their
hydrogen bonded complexes to BH3CNK
Table 5
The nBH bands in IR spectra of 1:1 and 1:2 hydrogen bonded complexes of
BH3CNK with HFIP
init
XH
free
BH
bond
BH
RXH
BH/HX
CN/HX
Complex
n
n
(DnBH
)
n
(DnBH)
a
a
BH3CNK$HFIP
BH3CNK$2HFIP
2330 (C12)
2330 (C12)
2348 (C30)
2278 (K38)
bond
XH
bond
XH
DnXH
DnXH
n
n
CH3OH
3630
3609
3600
3576
3472
3523
3456
3416
3383
3370
3355
3326
K107
K153
K184
K193
K102
K104
K132
3398
3344
3231
3128
3244
3227
3155
K232
K265
K369
K448
K228
K232
K303
FCH2CH2OH
CF3CH2OH
(CF3)2CHOH
Pyrrole
Band shifts are calculated as DnBH ZnfBreHe Kn ðA1Þ or DnBHZnBboHnd Kn ðA1Þ,
init
BH
init
BH
where nBH(A1)Z2318 cmK1
.
2-Phenyl-pyrrole 3459
Imidazole
3458
Table 6
Calculated frequency shifts (in cmK1) of the model hydrogen bonded
complexes of BH3CNK or BHK4 with CH3OH
a
Band shift DnXH ZnbXoHnd Kn
.
init
XH
formation. Interaction of the salt [Ph3PaNaPPh3][BH3CN]
with different strength proton donors: CH3OH, FCH2CH2OH
(MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), imidazole,
pyrrole, 2-phenyl-pyrrole, was investigated by IR spectroscopy
in low polar solvents (methylene chloride and tetrahydrofuran,
THF) in the temperature range 190–290 K. The two sites
competing for the hydrogen bonding with XH-acids in
agreement with computational results were found and
characterized by the studies in the ranges of the stretching
vibrations of the XH-acid bonds (nXHZ3700–3100 cmK1) as
well as BH and CN bonds of cyanoborohydride anion (2500–
2100 cmK1) were conducted according to our established
protocol [1a,4]. In this section we discuss the observed spectral
changes (Tables 3–5) in conjunction with results of frequency
calculations (Table 6) while the structural and electronic
features of the complexes calculated are discussed in the
separate section.
free
BH
bond
Complex
DnCN
Dn
Dn
BH
1
2
3
4
BH3CNK$CH3OH
NCBHK3 $CH3OH
25
31
24
22
38
49
17
37
11
13
24
K29
K6
K14
BH3CNK$2CH3OH 53
BHK4 $CH3OH
56
init bond
XH
DnXH Zn Kn
increase with the strength of the proton
donors (from 102 cmK1 for pyrrole to 193 cmK1 for HFIP).
XH
bond
XH
Notably, that the positions of the n
bands for the different
proton donors interacting with BH3CNK are higher and,
consequently, the band shifts DnXH are smaller than those for
the parent BHK4 anion [4a,b,7] (for example DnOH 184 cmK1 for
BH3CNK/TFE and 290 cmK1 for BHK4 /TFE). The difference
between DnOH for BH3CNK and BHK4 increases with the
increase of the proton donor strength. Thus, the presence of
electron accepting CN ligand weakens the proton accepting
strength of the hydride hydrogen, decreasing spectral shifts.
Similar effect of DnOH decrease was observed by us for DHB in
the system [B12H11SCN]2K/HOR in comparison with DHB’s
3.2.1. Range of nXH
The observed intensity decrease of the unbonded n bands
init
XH
of unsubstituted [B12H12]2K [8].
was accompanied by the appearance of two new broad low-
)
bond
XH
The second n
complexes is characterized by larger shift values (DnXH
band assigned to the BH3CN/HX
bond
XH
frequency bands of hydrogen bonded XH groups (n
(Fig. 2).
Z
232–448 cmK1) (Table 3) and greater intensity than those of
These two bonded bands could be assigned to hydrogen
bonding to two possible proton accepting sites (XH/HB and
XH/NC). Comparison of the data in Table 3 with the spectral
changes observed for the interaction of the same proton donors
with the unsubstituted hydride anion BHK4 [4b] and CNK anion
bond
XH
DHB. An overlap of two n
bands can mask the first band of
[11] allows to validate this suggestion.
The higher frequency n
bond
XH
bands of BH/HX complexes
appear in the range 3523–3326 cmK1. The frequency shifts
Table 4
Spectral features (in cmK1) of classical HB complexes BH3CN/HX in the
range of nCN
a
bond
CN
RXH
DnCN
n
CH3OH
2174
2176
2178
2182
2173
2174
2174
6
FCH2CH2OH
CF3CH2OH
(CF3)2CHOH
Imidazole
8
10
14
5
Pyrrole
6
Fig. 2. Variable temperature IR spectra in the nNH range of 2-phenyl-pyrrole
(0.023 M) in the presence of (Ph3PaNaPPh3)BH3CN (0.15 M). CH2Cl2, dZ
0.12 cm.
2-Phenyl-pyrrole
6
a
init
CN
Band shift DnCN ZnbCoNnd Kn
.

118
O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
Fig. 3. Variable temperature IR spectra in the nOH range of MFE (0.023 M) in
the presence of (Ph3PaNaPPh3)BH3CN (0.15 M). CH2Cl2, dZ0.12 cm.
Fig. 4. Variable temperature IR spectra in the nCN range of (Ph3PaNa
PPh3)BH3CN (0.01 M) in the presence of TFE (0.01 M) in THF, dZ0.04 cm.
OH/HB at low temperatures as indeed was observed in the
case of the BH3CNK/MFE system (Fig. 3).
But according to our calculations the proton donor coordi-
nation both to the nitrogen and to the hydride sites leads to the
high frequency shifts of nCN band of close values (Table 5).
This is due to the same shortening of CN bond in both HB
complexes (Table 1).
bond
OH
The Dn
value for BH3CN/HOCH3 hydrogen bonded
complex is less than that in the case of HB with CN anion [11]
(232 and 345 cmK1, respectively). The decrease of the CN
group proton accepting ability is due to the electron with-
drawing effect of the BH3 group analogous to the strong
electron accepting effect of the boron cage found by us
previously for [B12H11SCN]2K [8]. Significant decrease of
SCN group proton accepting ability was found in
[B12H11SCN]2K/HOCH3 systems in comparison with SCNK
anion hydrogen bonded to CH3OH [8,12].
The spectral picture in the range of nBH stretching vibrations
of the BH3CNK anion as well as its changes upon the
interaction with XH-acids are more complicated than these for
BHK4 [4a,b,7] according to the C3v symmetry of BH3CNK there
should be two nBH bands in the IR spectra belonging to A1 and
two degenerate E1 modes. However, the band decomposition
init
The frequency calculations for the system BH3CNK/CH3-
analysis for n band shows the presence of three overlapped
BH
bands at 2318 (s), 2284 (m) and 2268 (m) cmK1. This could be
due to removing of E1 mode degeneracy as the result of the
cation and solvent effects. In addition to three nBH bands two
weak bands of the overtones of BH deformation modes are
bond
XH
OH give two n
bands with low frequency shifts of 242
and 433 cmK1 for DHB (2) and classical HB (1) complexes,
respectively. Although the band shifts are overestimated in
comparison with experimental values for the complexes
with CH3OH (Table 3) due to carrying out the calculations
in the gas phase, they are in agreement with the
experimental trend.
observed at 2230 and 2215 cmK1
.
Analysis of all the spectra shows that the DHB and
CN/HO complexes coexist in the presence of one equivalent
of proton donor. Under these conditions the high frequency
shifted single band of complex shape is observed in the range
of nBH stretching vibrations with maximum at 2330 cmK1
3.2.2. Range of nBH and nCN
(DnBHZC12 cmK1). The band decomposition shows the
The data in the nXH region show the formation of HB
complexes of two types, but do not provide any information
about the centres of coordination. So, the study of concen-
tration and temperature dependences of nBH and nCN bands of
the BH3CN in the presence of different proton donors in THF
was performed in order to elucidate the types of coordination in
init
BH
decrease of the intensity of n band and appearance of new
low and high frequency shifted nBH bands (Fig. 5, Table 5).
free
BH
We assign the high frequency band to n modes of the both
free
complexes 1 and 2 on the basis of close values Dn
BH
calculated for these complexes (20–30 cmK1, Table 6). The
calculated low frequency shift of nBH stretching vibration of
B–H bond implicated in the dihydrogen bond
(DnbBoHnd ZK29 cmK1, 2) is in agreement with the band shift
(DnBboHnd ZK38 cmꢀ1) obtained by decomposition of the
observed nBH band (Fig. 5).
the different HB complexes.
The stretching vibration of n
of BH3CNK appears at
init
CN
2168 cmK1 in THF. The decrease of the intensity of this band
bond
CN
and the appearance of the high frequency broad intense n
band were observed in the presence of XH proton donors. The
band shift values DnCN increase with the proton donor strength
growing up from 6 to 14 cmK1 in the case of OH proton donors
(Table 4). The intensity of the broad bonded bands of
complicated shape increases on cooling from 290 to 210 K
(Fig. 4).
Twofold excess of proton donors leads to the intensity
decrease of the nbBoHnd Z2278 cmK1 and the intensity increase
bond
CN
of the nfBreHe Z2330 cmꢀ1, while the n
parallel with the intensity decrease of the n band (Figs. 6
band grows up in
init
CN
and 7).
This suggests that the hydrogen bonding equilibrium (Eq.
(1)) shifts to the right in the presence of excess proton donors
The high frequency shift of nCN band was assigned to the
coordination of proton donor on the nitrogen lone pair [11,13].

O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
119
Fig. 7. IR spectrum in the nBH range of (Ph3PaNaPPh3)BH3CN (0.015 M) in
the presence of excess of HFIP (0.03 M) and band decomposition. 290 K, THF,
dZ0.04 cm. Thin dashed—nBH bands of free BH3CNK; thin solid—nBH bands
Fig. 5. IR spectra in the nBH range of (a) (Ph3PaNaPPh3)BH3CN (0.015 M,
bold dashed) and (b) in the presence of HFIP (0.015, bold solid). Bands
decomposition of the last spectra (thin dashed lines are assigned to BH3CNK
anion bands and thin solid lines to HB complex). 290 K, THF, dZ0.04 cm.
of hydrogen bonded BH3CNK
.
results give additional support to this assignment: DnbCoNnd of the
HB complex 3 is equal to 24 cmK1, i.e. increases about twice in
comparison to complex 1:1 (Table 6). Calculated value of D
nbBoHnd ZK6 cmK1 in the complex 1:2 (3) is so small that its
and the content of the classical HB complexes increases.
K
RXH CBH3CNK%½RXH/H3BCNꢁ
K
C½BH3CN/HXRꢁ
(1)
experimental observation is impossible in the presence of much
init
stronger n band. On the other hand the new high frequency
BH
band at 2348 cmK1 appears in the n range at the BH3CNK/
free
BH
Upon cooling the equilibrium (1) shifts also to the right, that
init
init
becomes apparent from a decrease of the n and n bands as
CN
OH
HFIP concentration ratio being 1:2 and increases upon further
addition of proton donor (Fig. 7). We assign this new band to
well as an increase of the bonded bands (Figs. 4 and 3).
Temperature dependences indicate that the classical
free
BH
the n
vibration in the complex with two proton donor
(BH3CN/HX) complexes are thermodynamically more
molecules using calculation results indicating larger shift D
nfBreHe Z38–53 cmK1 for the complex 3. Thus this range is more
sensitive to the coordination of the second proton donor
molecule.
bond
XH
stable. Really, the intensity increase of n
of the classical
HB complexes (H3BCN/HXR) at low temperatures is larger
than this for DHB (for example, Figs. 2 and 3). Thus, the shift
of the equilibrium is mainly due to an increase of the classical
HB content.
Thus, we can suggest that in the case of proton donor excess
the complex 1:2 with two HB’s: classical and non-classical—
coexists in the equilibrium with HB complexes 1:1 (Eq. (2)).
It is interesting that the single high frequency shifted band
nCN is observed at the BH3CNK/HFIP ratios from 1:1 to 1:4.
A great excess of the proton donor (1:4, 1:8, 1:14) induces
appearance of the additional weak broad high frequency
shoulder in the nCN range (DnCNZ24 cmK1, Fig. 6), which we
assign to the complex of 1:2 composition. Computational
K
RXH CBH3CN%½RXH/H3BCNꢁ
K
K
C½BH3CN/HXRꢁ %½RXH/H3BCN/HXRꢁ
(2)
Table 7
The formation enthalpy values (KDH8, in kcal/mol) for hydrogen bonded
complexes of BH3CNK or BHK4 with RXH proton donors
RXH
Pia
BH3CNK$HXR
H3BH/
HXRb
BH/HX
CN/HX
CH3OH
0.63
0.74
0.89
1.05
0.65
0.67
0.78
2.3
3.2
3.6
3.8
2.2
2.3
2.8
4.4
4.8
6.1
6.9
4.3
4.4
5.3
4.1
FCH2CH2OH
CF3CH2OH
(CF3)2CHOH
Pyrrole
5.2
6.5
2-Phenyl-pyrrole
Imidazole
a
Fig. 6. IR spectra in the nCN range of (Ph3PaNaPPh3)BH3CN (0.015 M) (1)
and in the presence of HFIP. (Ph3PaNaPPh3)BH3CN/HFIP concentration
ratios: 1:1 (2), 1:2 (3), 1:8 (4), 1:14 (5). 290 K, THF, dZ0.04 cm.
The acidity factors of proton donors. Values from Ref. [14] and our own
measurements.
b
Data from Ref. [4b].

120
O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
Table 8
Energies of model HB complexes with ZPE correction and those enthalpies obtained from the Iogansen’s correlation
Complex
KDEcalc, kcal/mol
KDHcalc(Dn)a, kcal/mol
1
2
3
BH3CNK$HOCH3
NCBHK3 $HOCH3
BH3CNK$2CH3OH
12.09
8.23
6.76
4.53
19.47
4.12 (N/HO)
6.33 (BH/HO)
6.29
4
5
6
7
BHK4 $HOCH3
11.13
18.81
13.91
30.47
BH3CNK$HOCH2CF3
NCBHK3 $HOCH2CF3
BH3CNK$2HOCH2CF3
8.29
5.49
4.69 (N/HO)
7.51 (BH/HO)
7.64
8
BHK4 $HOCH2CF3
19.00
a
KDH8Z18DnOHcalc/(720CDnOHcalc).
3.3. Hydrogen bond formation enthalpy
established previously for different classes of organic bases as
well as for transition metal hydrides [1].
The formation enthalpies (KDH8, in kcal molK1) of DHB
and classical HB complexes for all the proton donors were
obtained from bands displacement DnXH using the Iogansen’s
correlation Eq. (3) proposed for organic systems [14]. Its
applicability for unconventional HB was proved previously by
us [1].
Using these dependences we calculated proton accepting
ability named as the basicity factor (Ej) of the both basic sites
from the Eq. (4), where KDH11 is the enthalpy of HB
formation for a ‘standard’ pair (phenol-diethylether) for which
PiZEjZ1.0, KDHij. is experimentally obtained enthalpy and
Pi is known acidity factor of proton donor [14a]. The Ej values
are independent of the partners and media and therefore are
more convenient for comparison of the basicity of different
proton accepting sites [1,4b,c].
18DnOH
KDH8 Z
(3)
DnOH C720
The classical HB’s to the CN group of BH3CNK are of
medium strength varying from 4.3 to 6.9 kcal molK1 (Table 7).
Thus they are stronger than the NCH2BH/HX hydrogen
bonds, which enthalpy values are in the range 2.2–
3.8 kcal molK1 (Table 7). The magnitudes of DHB complexes
of BH3CNK enthalpy are in all cases two thirds less than these
of DHBs of the unsubstituted BHK4 complexes with same proton
donors, quantitatively characterizing the strength of electron
accepting effect of CNKgroup.
DHij
Ej Z
(4)
DH11Pi
The Ej value of the hydride hydrogen of BH3CNK obtained
in this work is equal 0.80. Thus, the substitution of one hydride
by CN group (BHK4 /BH3CNK) induced the greater decrease of
Ej in comparison with the analogous changes produced by SCN
substitution in the hydride pair B12H21K2 =B12H11SCN2K. Intro-
duction of CN group decreases the basicity of BHK4 anion by a
factor of 1.56, while the basicity of B12H11SCN2K is 1.11 times
lower than that in B12H21ꢀ2 [8]. The scale of factors shows
position of Ej of hydride hydrogen for BH3CNK relative to the
Theoretically calculated KDE values were obtained as
difference of energies of HB complex and free reactants
(Table 8). The KDEcalc values are higher than experimentally
obtained because the calculations were carried out in gas phase
in the absence of counterion. Such overestimation of HB
energies was observed previously for other main group hydride
systems [4b,6,7]. Energy values of HB complexes depend on
proton donating ability (for example KDEZ8.23 for 2 and
13.91 for 6). Comparison of DHBs formed by cyanoborohy-
dride and tetraborohydride anions shows that introduction of
CN group greatly decreases KDE values as was obtained in
IR experiment. The hydrogen bond formation enthalpies,
KDHcalc(Dn), reported in Table 8 are calculated by means of
the Iogansen’s empirical rule (3) using the computed OH
frequency shifts. There is rather good agreement between the
computed hydrogen bonding enthalpies and those obtained
from the IR experiments.
The linear dependences between experimentally obtained
KDH8 of both BH/HX and CN/HX complexes and the
characteristics of the proton donating ability of XH-acids Pi
[14a] (Fig. 8) were found. Analogous dependences were
Fig. 8. Dependence of the formation enthalpy values (KDH8) for BH/HX
(circles) and CN/HX (triangles) bonds on the acidity factors (Pi) of the proton
donors.

O.A. Filippov et al. / Journal of Molecular Structure 790 (2006) 114–121
121
Acknowledgements
CN–
SCN–
BH3CN–
BH4–
This work was supported by the Russian Foundation for
Basic Research (No. 04-03-32456) and the Fundamental
Research Program of the Presidium of RAS.
B12H11SCN–
BH3CN–
CH3CN
2–
B12H11SCN–
B10H10
2–
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B12H12
Et3NBH3
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4. Conclusions
The cyanoborohydride BH3CNK, possessing two HB sites:
the hydride hydrogen and the nitrogen lone pair, appeared to be
very convenient model for the investigation of the competition
between non-classical and classical sites of hydrogen bonding.
The combination of the IR spectroscopy and DFT calculations
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The thermodynamic characteristics and proton accepting
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in the interacting molecules upon HB formation were studied
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