80
F. Sagan and M. P. Mitoraj
tion which can reach the regime of typical covalently bonded species [59]. Echeverria
has studied a series of homopolar dihydrogen interactions and also concluded on the
importance of dispersion forces [24, 26, 27]. Furthermore, we have confirmed the
stability of the M cryst -Hex adduct bonded through CH•••HC by ab initio Born–Oppenheimer molecular dynamics simulations (DFT/BLYP-D3/TZP)—hexane was found
to dynamically glide in the proximity of iPr ligands, and no spontaneous drifting
apart was observed [52]. Furthermore, the calculations have shown that the sterically
crowded cis-isomer exhibiting quasi-tetrahedral geometry is more stable than the corresponding planar trans-conformation at both energy and free energy levels, Fig. 6a.
It is very striking result in the light of intuitively expectable steric repulsion between
closely located iPr units in the case of cis-NiL 2 , Fig. 6a. Substitution of bulky iPr
units by smaller Me groups leads not only to the planar conformation of cis-NiL 2 ,
but also the trans-isomer is now thermodynamically preferred [52], Fig. 6c. Therefore, the tetrahedral geometry of cis-NiL 2 originates from the existence of numerous
cooperative intramolecular non-covalent interactions: CH•••HC, CH•••S CH•••Ni,
Fig. 6b. In turn, it is related to the bulky iPr units that are close to each other in the
cis-NiL 2 . It is very beautiful example showing how two quite bulky groups being
close to each other, classically considered as the source of steric repulsion, lead not
only to overall stabilization, but also to the determination of the complex geometry.
These data are perfectly in accord with recent topical findings on the importance of
CH•••HC interactions and London dispersion forces in various branches of chemistry
[6, 7, 10, 25, 52, 57–64]. It must be emphasized that dispersion contribution has been
also recently recognized as a crucial factor (in addition to well-established charge
transfer term) for agostic interactions (C–H•••metal) due to elegant and accurate
energy decomposition DLPNO-CCSD(T) implemented in the Orca program [64].
Ammonia borane, named also as borazane, is considered nowadays as one of the
most promising hydrogen storage materials predominantly due to significant hydrogen content (19.6%) as well as high melting point (104 °C). The latter property
is attributed in the literature to the existence of polar (proton–hydride) dihydrogen
bonds N–H
δ+ •••
−δ H–B between AB monomers. These types of interactions are crucial for hydrogen storage materials [28, 29, 31]. Very recently, McGrady and coworkers have published a series of high-quality papers which demonstrate thepreparation
of various hydrogen storage materials including LiN(CH 3 ) 2 BH 3 and KN(CH 3 ) 2 BH 3
in which untypical hydride–hydride interactions B–H
δ− •••
−δ H–B are observed from
the QTAIM results [22, 66, 67]. These are very interesting suggestions since hydrogen
atoms involved in such homopolar contacts B–H
δ− •••
−δ H–B carry negative partial
charges, what intuitively shall lead to overall repulsion due to destabilizing electrostatic contribution. In order to shed some light on the role of B–H
δ− •••
−δ H–B and
other types of chemical bonds in LiN(CH 3 ) 2 BH 3 and KN(CH 3 ) 2 BH 3, we have performed a comprehensive in-depth study of bonding situation based on ETS-NOCV,
IQA, NCI methods, and molecular electrostatic potentials [23].
ETS-NOCV method allowed to determine that the major inter-molecular bonding
in LiN(CH 3 ) 2 BH 3 stems from B–H•••Li contacts—it is dominated by the electrostatics which covers 55% (E elstat –35.56 kcal/mol) of the total stabilization, followed
by the orbital interaction (E orb –18.17 kcal/mol) and dispersion (17%, E disp
F. Sagan and M. P. Mitoraj
tion which can reach the regime of typical covalently bonded species [59]. Echeverria
has studied a series of homopolar dihydrogen interactions and also concluded on the
importance of dispersion forces [24, 26, 27]. Furthermore, we have confirmed the
stability of the M cryst -Hex adduct bonded through CH•••HC by ab initio Born–Oppenheimer molecular dynamics simulations (DFT/BLYP-D3/TZP)—hexane was found
to dynamically glide in the proximity of iPr ligands, and no spontaneous drifting
apart was observed [52]. Furthermore, the calculations have shown that the sterically
crowded cis-isomer exhibiting quasi-tetrahedral geometry is more stable than the corresponding planar trans-conformation at both energy and free energy levels, Fig. 6a.
It is very striking result in the light of intuitively expectable steric repulsion between
closely located iPr units in the case of cis-NiL 2 , Fig. 6a. Substitution of bulky iPr
units by smaller Me groups leads not only to the planar conformation of cis-NiL 2 ,
but also the trans-isomer is now thermodynamically preferred [52], Fig. 6c. Therefore, the tetrahedral geometry of cis-NiL 2 originates from the existence of numerous
cooperative intramolecular non-covalent interactions: CH•••HC, CH•••S CH•••Ni,
Fig. 6b. In turn, it is related to the bulky iPr units that are close to each other in the
cis-NiL 2 . It is very beautiful example showing how two quite bulky groups being
close to each other, classically considered as the source of steric repulsion, lead not
only to overall stabilization, but also to the determination of the complex geometry.
These data are perfectly in accord with recent topical findings on the importance of
CH•••HC interactions and London dispersion forces in various branches of chemistry
[6, 7, 10, 25, 52, 57–64]. It must be emphasized that dispersion contribution has been
also recently recognized as a crucial factor (in addition to well-established charge
transfer term) for agostic interactions (C–H•••metal) due to elegant and accurate
energy decomposition DLPNO-CCSD(T) implemented in the Orca program [64].
Ammonia borane, named also as borazane, is considered nowadays as one of the
most promising hydrogen storage materials predominantly due to significant hydrogen content (19.6%) as well as high melting point (104 °C). The latter property
is attributed in the literature to the existence of polar (proton–hydride) dihydrogen
bonds N–H
δ+ •••
−δ H–B between AB monomers. These types of interactions are crucial for hydrogen storage materials [28, 29, 31]. Very recently, McGrady and coworkers have published a series of high-quality papers which demonstrate thepreparation
of various hydrogen storage materials including LiN(CH 3 ) 2 BH 3 and KN(CH 3 ) 2 BH 3
in which untypical hydride–hydride interactions B–H
δ− •••
−δ H–B are observed from
the QTAIM results [22, 66, 67]. These are very interesting suggestions since hydrogen
atoms involved in such homopolar contacts B–H
δ− •••
−δ H–B carry negative partial
charges, what intuitively shall lead to overall repulsion due to destabilizing electrostatic contribution. In order to shed some light on the role of B–H
δ− •••
−δ H–B and
other types of chemical bonds in LiN(CH 3 ) 2 BH 3 and KN(CH 3 ) 2 BH 3, we have performed a comprehensive in-depth study of bonding situation based on ETS-NOCV,
IQA, NCI methods, and molecular electrostatic potentials [23].
ETS-NOCV method allowed to determine that the major inter-molecular bonding
in LiN(CH 3 ) 2 BH 3 stems from B–H•••Li contacts—it is dominated by the electrostatics which covers 55% (E elstat –35.56 kcal/mol) of the total stabilization, followed
by the orbital interaction (E orb –18.17 kcal/mol) and dispersion (17%, E disp