Cent. Eur. J. Chem. • 6(3) • 2008 • 400–403
DOI: 10.2478/s11532-008-0029-0
Central European Journal of Chemistry
A G2(MP2) theoretical study of substituent
effects on H3BNHnCl3-n (n= 3-0) donor-acceptor
complexes
Research Article
Hafid Anane1*, Soufiane El Houssame2, Abdelali El Guerraze2,
Abdeladim Guermoune2, Abderrahim Boutalib2, Abedellah Jarid2,
Ignacio Nebot-Gil3, Francisco Tomás3
1
Département Sciences de la matière, Faculté Polydisciplinaire de Safi,
Route Sidi bouzid B.P 4162, Université Cadi Ayyad, Morocco
2
3
Département de Chimie, Faculté des Sciences Semlalia,
B.P. 2390 Marrakech, Université Cadi Ayyad, Morocco
Institut de Ciencia Molecular, Departament de Quimica-Fisica,
Universitat de València, Dr. Moliner,
50 E-46100, Burjassot, València, Spain
Received 10 February 2008; Accepted 26 March 2008
Abstract: T he complexation energies of H3BNHnCl3-n (n= 3-0) complexes and the proton affinities of NHnCl3-n compounds have been computed
at the G2(MP2) level of theory. G2(MP2) results show that the successive chlorine substitution on the ammonia decreases both the
basicity of the NHnCl3-n ligands and the stability of H3BNHnCl3-n complexes. The findings are interpreted in terms of the rehybridisation
of the nitrogen lone-pair orbital. The NBO partitioning scheme shows that the variation of the N-H and N-Cl bond lengths, upon complexation, is due to variation of “s” character in these bonds.
Keywords: Ammonia-borane • Complex • Ab initio • G2(MP2) • Substituent effect
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1. Introduction
A typical property of electron deficient boranes is their
Lewis acidity which reflects the fact that boron possesses
less valence electrons than valence orbitals. As a result,
the simplest borane BH3 readily forms donor-acceptor
complexes with Lewis bases. The compounds containing
boron and nitrogen play an important role in synthetic
chemistry and have received extensive attention in
the chemical literature. Accurate knowledge of the
thermodynamics of complexation energies would serve
as a useful framework from which to build a detailed
and quantitative understanding of borane reactivity and
mechanism [1-3]. Many theoretical studies have been
devoted to boron donor-acceptor complexes concerning
their structural parameters, the nature of the bonding,
the factors affecting their stability and other physical
proprieties [4-21] where the methods used for analysis
differ.
In 1976, Morokuma and co-workers [4] studied
the effect of the methyl substitution of the hydrogen
atoms of ammonia. They reported that the N-methyl
substituent effect on the stability of H3BNH3 is very
small (1 kcal/mol). The H3BNH3, H3BNMe3, H3BPH3 and
H3BPMe3 complexes have been studied by Shibata et
al. [5]. They showed that there is a strong correlation
between the N-B and P-B distances and the formation
energy of the complexes. They reported also that the
structural changes occurring in the acceptors and
donors on complex formation are closely related to
the charge transfer. A study by Frenking et al., [6-7]
* E-mail: [email protected]
400
Unauthenticated
Download Date | 6/18/17 6:09 PM
H. Anane et al.
Table 1.
MP2(Full)/6-31G(d) calculated d(N-Y) bond lengths (in Å), ∠HBH bond angle (in °) and “2s” MP2-NBO contributions of nitrogen in the
N-H and N-Cl bonds (in %).
N-H
N-Cl
N-B
∠HBH
2s(N-H)
2s(N-Cl)
NH3
1.017
NH2Cl
1.022
1.754
25.76
25.15
14.40
NHCl2
1.026
1.759
26.19
14.19
1.773
NCl3
13.52
H3BNH3
1.020
1.657
113.88
H3BNH2Cl
1.024
1.753
1.660
114.31
22.73
15.58
H3BNHCl2
1.027
1.756
1.664
115.68
22.98
16.53
1.768
1.703
116.11
H3BNCl3
shows that there is no correlation between the charge
transfer from the donor to the acceptor, using the natural
bond orbitals partitioning scheme NBO [22], and bond
strength of H3BNH3, H3BNMe3, F3BNH3, F3BNMe3,
Cl3BNMe3, and Cl3AlNMe3 complexes. In contrast,
other works [8-9] have shown that there is a correlation
between the charge transfer from the NHnF3-n (n=0-3)
donors to the BH3 acceptor and the binding energies
of H3BNHnF3-n complexes using the density functional
theory and perturbation calculations. Furthermore, they
have found that the successive fluorine substitution on
ammonia decreases their stability. Recently, our group
have shown that the successive fluorine substitution
on phosphine favors complex formation (H3BPHnF3-n)
(n= 3-0), in spite of the reduction of the basicity of the
PHnF3-n ligands [21].
We expand here our study to the chlorine substitution
effect on ammonia borane complexes, investigating the
H3BNHnCl3-n (n= 3-0) compounds at the G2(MP2) [23]
level of theory, and discussing both their theoretically
predicted structures and their stabilities.
2. Computational Details
The proton affinities of NHnCl3-n (n= 3-0) Lewis bases
and their complexation energies with BH3 Lewis acid
have been computed at the G2(MP2) level of theory
[23]. Corrections for zero-point vibrational energies have
been taken into account from HF/6-31G(d) harmonic
frequencies scaled by 0.893 [24]. The investigation of
the electronic structure, using the natural bond orbitals
partitioning scheme NBO [22], was carried out at the
MP2(Full)/6-31G(d) level.
All calculations in this work were performed on IBM
RS/6000 workstations of the University of València using
the Gaussian 94 [25] series of computer programs.
21.62
17.28
3. Results and Discussion
Table 1 lists the MP2(Full)/6-31G(d) most important
geometrical parameters for NHnCl3-n (n= 0-3) moieties and
their complexes with the BH3 acid and 2s contributions of
nitrogen in the N-H and N-Cl bonds at MP2-NBO level.
In Table 2 we give the calculated complexation energies
for the H3BNHnCl3-n (n= 0-3) complexes, the proton
affinities of NHnCl3-n moieties and the transferred charge
from NHnCl3-n donors to BH3 acceptor. The theoretical
complexation energies (Ec) are calculated as the energy
difference between the complexes and the respective
sum of donor and acceptor compounds energies, while
the theoretical proton affinities (P.A) are taken as the
energy difference between the neutral and protonated
NHnCl3-n bases.
Table 1 shows that very small changes in the N-H
and N-Cl bond distances are predicted as a result of
chlorine substitution in NH3 isolated base. In fact, we
observed a lengthening of the N-H and N-Cl bonds. The
bond distances of N-H and N-Cl change from 1.017 to
1.026 Å and from 1.754 to 1.773 Å respectively. It has
been suggested [8,26] that the bond length changes are
caused by the rehybridisation of donor atom “N” upon
chlorination and also due to a reversion of polar effects
as the partial charge on the nitrogen change. It becomes
positive with increasing chlorine substitution on NH3.
The net charges on N, obtained at MP2-NBO level, are
-1.12, -0.85, -0.64 and -0.46 electron for NH3, NH2Cl,
NHCl2 and NCl3, respectively. With reference to our NBO
analysis, it may be claimed that the contribution rate of
the “s” character in the N-H and N-Cl bonds also plays
an important role in this context (see below).
Moreover, the B-N bonds in H3BNHnCl3-n (n= 2-0)
complexes are longer than the H3BNH3 one. This result
seems reasonable because NHnCl3-n (n= 0-2) bases are
weakly than NH3 one. Upon complexation, the MP2
calculations show that the formation of the complexes
by successive chlorination on nitrogen atom involves
401
Unauthenticated
Download Date | 6/18/17 6:09 PM
A G2(MP2) theoretical study of substituent effects
on H3BNHnCl3-n (n= 3-0) donor-acceptor complexes
Table 2.
G2(MP2) complexation energies Ec of H3BNH3-nCln (n = 0-3) complexes, proton affinities PA of NH3-nCln ligands (in kcal/mol) and the
MP2-NBO transferred charge Qt.
Complex
Ec
PA
Qt
H3BNH3
-25.97
202.5
0.35
H3BNH2Cl
-23.95
189.42
0.31
H3BNHCl2
-19.95
178.86
0.28
H3BNCl3
-15.76
170.43
0.25
-10
-10
NCl3
-15
-20
NHCl2
NH2Cl
-25
Ec(kcal/mol)
Ec(kcal/mol)
-15
NH3
-30
NCl3
NHCl2
-20
NH2Cl
-25
NH3
-35
-40
168
-30
173
178
183
188
193
198
0.2
203
Figure 1.
Linear correlation between G2(MP2) calculated proton
affinities of the NHnCl3-n Lewis bases and G2(MP2) calculated complexation energies of H3BNHnCl3-n (n= 0-3)
complexes.
a pyramidalisation of the BH3 molecule. One can see
from table 1 that the ∠HBH bond angle in the isolated
BH3 compound is 120° becomes 113.88°, 114.31°,
115.68 ° and 116.11° for H3BNH3, H3BNH2Cl, H3BNHCl2
and H3BNCl3 respectively. Then, we can observe that
this pyramidalisation decreases with the degree of
substitution (5.10%, 4.74%, 3.60% and 3.24% for
H3BNH3, H3BNH2Cl, H3BNHCl2 and H3BNCl3 complexes
respectively) in the same trend as the stability of
these donor-acceptor complexes. Geometries of the
H3BNHnCl3-n complexes show a tetrahedral arrangement
around the boron center. The ∠NBH bond angle is 101105°. The B-H bond is slightly longer in complexes than
in isolated BH3 (1.191 Å). These values are related
to the hybridization changes from sp2 in BH3 to sp3 in
complexes. Upon coordination, the MP2 calculation
shows a very small distortion of the N-Cl bond length
(0.08%, 0.17% and 0.29% for H3BNH2Cl, H3BNHCl2,
and H3BNCl3 respectively). Furthermore, this calculation
predicts both a lengthening of N-H and a shortening of
N-Cl bond lengths (Table 1). To explain this effect, we
have applied the NBO analysis. MP2-NBO calculations
show that in isolated NHnCl3-n (n=3-0) moieties the
lone pair on nitrogen has lower “s” character than in
complexes. Then, we can deduce from these results
that this change alone would imply a shortening of the
N-Cl bond lengths owing to an increased “s” character in
these bonds. Even, table 1 shows that the contribution of
the 2s AO of ammonia is more important in the isolated
moiety than in the H3BNHnCl3-n (n= 2-0) complexes, so
0.25
0.3
0.35
0.4
Qt(transferred charge)
PA (kcal/mol)
Figure 2.
Linear correlation between MP2-NBO transferred charge
Qt (in electron) from NHnCl3-n bases to BH3 acid and
G2(MP2) calculated complexation energies of
H3BNH‌n‌‌‌Cl3-n (n= 3-0) complexes.
we can understand why we had a lengthening of the
N-H bond length.
Table 2 shows that the H3BNH3 is more strongly
bound (-25.97 kcal/mol) than the H3BNHnCl3-n (n=2-0)
complexes (-23.95, -19.95, and -15.76 kcal/mol for
H3BNH2Cl, H3BNHCl2, and H3BNCl3, respectively). The
introduction of the first chlorine group on nitrogen atom
destabilizes the complex by about 2 kcal/mol, the second
by 6.02 kcal/mol, and the third by 10.21 kcal/mol. In short,
the successive chlorine substitution on nitrogen atom
reduces the stability of the corresponding complexes as
the basicity of NHnCl3-n (n= 3-0) bases. Moreover, the
stability of H3BNHnCl3-n (n= 3-0) complexes decreases
when the basicity of NHnCl3-n (n= 3-0) molecules
decreases (Figure 1). The good correlation between
the proton affinities of the bases and the complexation
energies computed at the G2(MP2) level of theory shows
that the stability of the complexes depends completely
on the type of base involved.
Before discussing calculated atomic partial charges,
we emphasize that dividing up the molecular electronic
charge into atomic regions is always based upon an
arbitrary partitioning scheme. The calculated charges
have no physical meaning by themselves; they should
only be used as a model to explain trends and properties
of molecules. In particular, the absolute values of the
partial charges should not be over interpreted [6-7].
Rather, the change in the partial charges upon complex
formation should be compared.
402
Unauthenticated
Download Date | 6/18/17 6:09 PM
H. Anane et al.
The MP2-NBO results (Table 2) show that the
transferred charge from NHnCl3-n (n= 3-0) to BH3
decreases when the degree of substitution increases
(0.35, 0.31, 0.28 and 0.25 electron for H3BNH3, H3BNH2Cl,
H3BNHCl2 and H3BNCl3 respectively). From the NBO
analysis, it follows that there is a correlation between
transferred charge and bond strength in H3BNHnCl3-n
(n= 3-0) donor-acceptor complexes (Figure 2).
4. Conclusion
stability of ammonia-borane compounds decreases with
the degree of the chlorine substitution at the nitrogen
atom. This study proves that the stability of H3BNHnCl3-n
(n= 3-0) complexes was related to the basicity of the
NHnCl3-n (n= 3-0) bases and the transferred charge
from donor to acceptor. Upon complexation, the MP2
structural parameters of H3BNHnCl3-n (n= 3-0) complexes
confirm the lengthening of N-H and a shortening of N-Cl
bonds. The analysis of the electronic structure using
the NBO partitioning scheme prove that this change
is related to the contribution rate of the “s” character in
these bonds.
The substitution effect on H3BNHnCl3-n (n= 3-0) donoracceptor complexes was investigated at the G2(MP2)
level of theory. The G2(MP2) results show that the
References
[1] E. L. Muetterties, Boron Hydride Chemistry, (Academic Press, New York, 1985)
[2] A. Pelter, K. Smith, H. C. Brown, Boron Reagents,
(Academic Press, New York, 1988)
[3] H. C. Brown, Organic Syntheses via Boranes, (Wiley,
New York, 1975)
[4] H. Umeyama, K. Morokuma, J. Am. Chem. Soc. 98,
7208 (1976)
[5] F. Hirota, K. Miyata, S. Shibata, J. Mol. Struct. (Theochem) 201,99 (1989)
[6] V. Jonas, G. Frenking, M. T. Reetz, J. Am. Chem.
Soc.116, 8741 (1994)
[7] A. Y Timoshkin, G. Frenking, J. Am. Chem. Soc. 124,
7240 (2002)
[8] A. Skancke, P. N. Skancke, J. Phys. Chem. 100,
15079 (1996)
[9] H. Anane et al., J. Mol. Struct. (Theochem) 709, 103
(2004)
[10] P. Jungwirth, R. Zahradník, J. Mol. Struct. (Theochem) 283, 317 (1993)
[11] A. El Guerraze et al., Chem. Phys. 313, 159 (2005)
[12] Ch. Aubauer, T. M. Klapötke, A. Schulz, J. Mol.
Struct. (Theochem) 543, 285 (2001)
[13] C. W. Bauschlicher Jr., A. Ricca, Chem. Phys. Lett.
237, 14 (1995)
[14] H. Anane, A. Boutalib, F. Tomás, J. Phys. Chem. A.
101, 7879 (1997)
[15] M. Bowden, T. Autrey, I. Brown, M. Ryan, Curr. Appl.
Phys. 498, 8 (2008)
[16] Y. Mo, J. Gao, J. Phys. Chem. A. 105, 6530 (2001)
[17] H. Anane, A. Jarid, A. Boutalib, I. Nebot Gil, F.
Tomás, J. Mol. Struct. (Theochem) 455, 51 (1998)
[18] H. Anane, A. Boutalib, I. Nebot Gil, F. Tomás, Chem.
Phys. Lett. 287, 575 (1998)
[19] H. Anane, A. Jarid, A. Boutalib, I. Nebot Gil, F.
Tomás, Chem. Phys. Lett. 296, 277 (1998)
[20] G. Rasul, G.K. Surya Prakash, G. A. Olah, J. Mol.
Struct. (Theochem). 818, 65 (2007)
[21] H. Anane, A. Jarid, A. Boutalib, I. Nebot Gil, F.
Tomás, Chem. Phys. Lett. 324, 156 (2000)
[22] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev.
88, 899 (1988)
[23] L. A. Curtiss, K. Raghavachari, J. A. Pople, J. Chem.
Phys. 98, 1293 (1993)
[24] W. J. Hehre, L. Radom, J. A. Pople, P. von R.
Schleyer, Ab Initio Molecular Orbital Theory, (Wiley,
New York, 1986)
[25] M. J. Frisch et al., Gaussian 94 (revision B.1),
Gaussian, Inc., Pittsburgh, PA, 1995.
[26] D. Christen, R. Minkwitz, R. Nass, J. Am. Chem.
Soc. 109, 7020 (1987)
403
Unauthenticated
Download Date | 6/18/17 6:09 PM