B=N bonds and BCN rings - Reactivity and charge density studies

Article abstract of DOI:10.1002/zaac.201200483

(F₅C₆)₂BNMe₂ (1) was prepared and (F₃C)₂BNMe₂ (2) was resynthesized. The crystal structures of both compounds were determined. In contrast to 2, compound 1 does not react with diazomethanes under carbine insertion into the BN bond. In this way the known (F₃C)₂BCH(SiMe₃)NMe ₂ (3) was resynthesized from 2 and its solid-state structure was determined. The electron densities of compounds 1, 2, and 3 and those of the simpler model compound (F₃C)₂BCH₂NMe ₂ (4) as well as that of the BN/CC isoelectronic compound (F ₃C)₂CCH₂CMe₂ (5) for comparison were calculated and their topologies analysed in terms of the Quantum Theory of Atoms in Molecules (QTAIM). Despite 2 has a shorter B-N bond in calculations and in the crystal than 1, the atomic charges and electron density parameters to QTAIM are very similar for both compounds and cannot explain the different reactivity. The three-membered rings 3 and 4 do not show bond-critical points for their B-N bonds. The nature of the B-N bond region was also analysed in terms of the source function contributions in QTAIM, which demonstrated the importance of the close proximity of a third atom in three-membered rings for the charge density topologies in such systems. Copyright

Full text of DOI:10.1002/zaac.201200483

Job/Unit: Z12483
/KAP1
Date: 14-12-12 11:08:36
Pages: 11
ARTICLE
DOI: 10.1002/zaac.201200483
B=N Bonds and BCN Rings – Reactivity and Charge Density Studies
Daniel Winkelhaus,[a] Yuri V. Vishnevskiy,[a] Raphael J. F. Berger,[b]
Hans-Georg Stammler,[a] Beate Neumann,[a] and Norbert W. Mitzel*[a]
In Memoriam Richard F. W. Bader
Keywords: Nitrogen; Boron; Three-membered rings; Electron density; Chemical bonding
Abstract. (F5C6)2BNMe2 (1) was prepared and (F3C)2BNMe2 (2) was lecules (QTAIM). Despite 2 has a shorter B–N bond in calculations
resynthesized. The crystal structures of both compounds were deter-
mined. In contrast to 2, compound 1 does not react with diazomethanes
under carbine insertion into the BN bond. In this way the known
and in the crystal than 1, the atomic charges and electron density pa-
rameters to QTAIM are very similar for both compounds and cannot
explain the different reactivity. The three-membered rings 3 and 4 do
(F3C)2BCH(SiMe3)NMe2 (3) was resynthesized from 2 and its solid- not show bond-critical points for their B–N bonds. The nature of the
state structure was determined. The electron densities of compounds B–N bond region was also analysed in terms of the source function
1, 2, and
3
and those of the simpler model compound contributions in QTAIM, which demonstrated the importance of the
close proximity of a third atom in three-membered rings for the charge
(F3C)2BCH2NMe2 (4) as well as that of the BN/CC isoelectronic com-
pound (F3C)2CCH2CMe2 (5) for comparison were calculated and their density topologies in such systems.
topologies analysed in terms of the Quantum Theory of Atoms in Mo-
form of an atom within a molecule and assign the electrons to
an atom and thus to determine partial charges.
Introduction
Description of the nature of chemical bonds is a fundamen-
tal concern in chemistry.[1] Within the range of possible de-
scriptions and definitions of chemical bonds the valence bond
approach and the molecular orbital description including the
projection of MO’s onto localized orbitals or natural bond or-
bitals (NBO) are the most popular.[2] In contrast to these
models, Bader’s quantum theory of atoms in molecules
(QTAIM)[3] works with an experimentally observable quan-
tity – the electron density distribution. QTAIM describes
chemical bonding by analyzing topology features of the charge
density in molecules. These features include “bond-paths”
(BPs), “bond-critical” (BCPs), and “ring-critical points”
(RCPs) and a range of other descriptors like the ellipticity
(along bond paths).[3] Local charge accumulations and de-
pletions are localized using the Laplacian, the second deriva-
tive of charge density. QTAIM enables to define the size and
The concept was useful for the description of many cases of
chemical bonding and was applied to complicated problems,
e.g. hydrogen–hydrogen bonding[4] and agostic interactions.[5]
However, there are also several reports, which questioned the
definition of the chemical bond in terms of QTAIM (e.g. H–H
bonding in planar biphenyl[6] and the bonding in the hypotheti-
cal molecule He@adamantane[7]). There remains still some
confusion, which arises from the ambiguous use of terms of
classical chemistry within QTAIM reports and the empirical
correlation of well-defined QTAIM parameters with traditional
pictures of the classification of chemical bonding.[8]
In this contribution we will focus on systems with three-
membered rings that include one dative bond. Such systems
are, for instance, important in explaining the so called α-effect,
an old postulate from the 1960ies, which was used to rational-
ize the increased reactivity of the silicon due to a geminal do-
nor function forming a three-membered ring with a dative
bond. Interestingly, no significant donor-acceptor interaction is
found for the ground states of compounds of the type
* Prof. Dr. N. W. Mitzel
Fax: +49-521-106-6026
E-Mail: mitzel@uni-bielefeld.de
[a] Universität Bielefeld
[9]
X3Si–CH2–NR2 and X3Si-C(RЈ2)–OR[10] (X = electron with-
Fakultät für Chemie
Lehrstuhl für Anorganische Chemie und Strukturchemie
Universitätsstr. 25
drawing group, R = alkyl) with SiCN and SiCO units. In con-
trast to these, compounds with SiON unit (X3Si–O–NR2)[11]
and SiNN units (X3Si–N(RЈ)–NR2)[12] show short Si–N dis-
tances and small Si–O–N and Si–N–N angles, respectively –
consistent with the above mentioned α-effect. Despite the veri-
fication of a structurally pronounced silicon nitrogen interac-
tion in the model compounds F3CSiF2SiONMe2 and
F3SiONMe2 the electron density topology according to the
33615 Bielefeld, Germany
[b] Paris-Lodron Universität Salzburg
FB Materialwissenschaften und Physik
Abteilung Materialchemie
Hellbrunner Str. 34
5020 Salzburg, Austria
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200483 or from the au-
thor.
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N. W. Mitzel et al.
ARTICLE
QTAIM theory represents neither a bond-path nor a bond- and
ring-critical point for the Si···N interactions. Doubtless the
dative Si–N bonds exhibit very low electron densities, how-
ever, for the similar electronic situation at the silicon and nitro-
gen atoms in the adduct (F3C)(MeO)F2Si–NMe3 a bond path
is found. By contrast this not involved in a three-membered
ring.
Scheme 2. Reaction of of Cl2BNMe2 with F3CBr/P(NEt2)3.
Molecular Structures of 1 and 2
Herein we report on a related absence of a bond path,
bond-, and ring-critical points for the typical three-membered
BCN systems with dative B–N bonds (F3C)2BC(R)HNMe2
(R = H, SiMe3). These compounds have earlier been prepared
and characterized by Bürger and co-workers.[13,14] In addition
to the (F3C)2B group containing systems, which require a com-
paratively high preparative effort, we also started to investigate
(F5C6)2B and (p-HF4C6)2B containing systems.[15] These fluo-
roaryl groups are less electron-withdrawing but more sterically
demanding than the CF3 group. However, we learned that the
reactivity of the starting materials (C6F5)2BNMe2 (1) and
(F3C)2BNMe2 (2) is very different and thus we included these
systems also into our charge density topology studies.
Single crystals of the pentafluorophenyl-substituted amino-
borane 1 were obtained from a concentrated hexane solution
at –35 °C. The structure of 1 is displayed in Figure 1; selected
structural parameters are listed in the caption. A single crystal
of 2 suitable for X-ray diffraction was generated in a sealed
glass capillary by slowly cooling the melt after establishing a
solid-liquid equilibrium. The structure of 2 is depicted in Fig-
ure 2 with selected structural parameters.
Results and Discussion
Synthesis of the Aminoboranes (C6F5)2BNMe2 (1) and
(F3C)2BNMe2 (2)
The reaction of the chloroborane [(C6F5)2BCl][16] with lith-
ium dimethylamide (LiNMe2)[17] at –30 °C in hexane afforded
the pentafluorophenyl-substituted aminoborane (C6F5)2BNMe2
(1) as a yellowish solid in 74% yield (Scheme 1).
Figure 1. Molecular structure of 1 in the crystal. Hydrogen atoms are
omitted for clarity. Selected bond lengths /Å and angles /°: N(1)–B(1)
1.372(2), N(1)–C(1) 1.464(2), N(1)–C(2) 1.467(2), B(1)–C(9)
1.590(2), B(1)–C(3) 1.591(2), B(1)–N(1)–C(1) 123.8(1), B(1)–N(1)–
C(2) 124.5(1), C(1)–N(1)–C(2) 111.7(1), N(1)–B(1)–C(9) 122.1(1),
N(1)–B(1)–C(3) 121.4(1), C(9)–B(1)–C(3) 116.6(1).
Scheme 1. Reaction of (C6F5)2BCl with lithium dimethylamide.
The compound was characterized by elemental analysis and
1
NMR spectroscopy (1H, 11B, 13C, and 19F). In the H and 13C
NMR spectra of compound 1 the methyl groups are detected as
signals at δ = 2.25 and 40.7 ppm, respectively. The 11B NMR
spectrum shows a single resonance at δ = 33.7 ppm (ν1/2
=
209 Hz) indicating a tri-coordinated boron atom. 19F NMR
spectroscopy shows the signals of the pentafluorophenyl
groups at δ = –161.3 (m-C6F5), –152.2 (p-C6F5), and
–132.5 ppm (o-C6F5).
The trifluoromethyl-substituted aminoborane (F3C)2BNMe2
(2) was synthesized according a procedure published by
Bürger, Pawelke et al.[14,18] In a Ruppert reaction[19] the chloro
substituted aminoborane (Cl2BNMe2)[20] was reacted with the
CF3 transfer reagent F3CBr/P(NEt2)3 to obtain the aminobor-
ane 2 in 74% yield as a colorless liquid (Scheme 2).
Figure 2. Molecular structure of 2 in the crystal. Hydrogen atoms are
omitted for clarity. Selected bond lengths /Å and angles /°: N(1)–B(1)
1.356(3), N(1)–C(1) 1.471(3), N(1)–C(2) 1.474(3), B(1)–C(4)
1.607(3), B(1)–C(3) 1.616(3), B(1)–N(1)–C(1) 124.7(2), B(1)–N(1)–
C(2) 125.1(2), C(1)–N(1)–C(2) 110.3(2), N(1)–B(1)–C(4) 124.5(2),
N(1)–B(1)–C(3) 122.0(2), C(4)–B(1)–C(3) 113.5(2).
2
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B=N Bonds and BCN Rings – Reactivity and Charge Density
In the pentafluorophenyl-substituted compound
1 the
B(1)–N(1) bond length is 1.372(2) Å. This is in the range ex-
pected for a C6F5 substituted monomeric aminoborane with
partial B–N multiple bond [1.376(4) Å in (C6F5)2BNiPr2[21]].
The B–N distance in the CF3 substituted aminoborane 2 has a
value of 1.356(3) Å and is thus 0.016 Å shorter than 1 [cf.
1.365(5) Å in (F3C)2BNiPr2[22]]. Due to the stronger electron
withdrawing character of the trifluoromethyl group compared
to the pentafluorophenyl group the increased electron de-
ficiency of the boron atom in 2 strengthens the (BN)-π-interac-
tion, which explains the shorter B–N bond in 2 compared to
1.
Both aminoboranes 1 and 2 exhibit a planar C2BNC2 core.
In 2 the core is perfectly planar, while in 1 the values for the
dihedral angles C(3)–B(1)–N(1)–C(2) and C(9)–B(1)–N(1)–
C(1) are 174.2 and 176.4°, respectively, showing a small tor- Figure 3. Molecular structure of 3 in the crystal. Hydrogen atoms are
omitted for clarity. Selected bond lengths /Å and angles /°: N(1)–C(5)
sion. The sum of angles about the boron atoms B(1) and the
1.525(2), N(1)–B(1) 1.567(2), B(1)–C(5) 1.574(2), B(1)–C(4)
1.617(2), B(1)–C(3) 1.621(2), N(1)–C(1) 1.482(2), N(1)–C(2)
1.486(2), C(1)–N(1)–C(2) 109.7(1), C(1)–N(1)–C(5) 117.7(1), C(2)–
nitrogen atoms N(1) in both compounds are 360° which under-
lines the double bond character of the B–N bond in 1 and 2.
Both, the C–B–C angle in 1 [116.6(1)° for C(9)–B(1)–C(3)]
and in 2 [113.5(2)° for C(3)–B(1)–C(4)], and the C–N–C angle
in 1 [111.7(1)° for C(1)–N(1)–C(2)] and in 2 [110.3(2)° for
C(1)–N(1)–C(2)], are compressed with respect to the ideal
value for a trigonal planar arrangement (120°). This decrease
is a consequence of the repulsive interaction (BN)-π- and the
N–C- and B–C-σ-bond, respectively.
The N–C bond lengths in 1 have values of 1.464(2)
[N(1)–C(1)] and 1.467(2) Å [N(1)–C(2)]. This is longer than
the same bond in 2 with values of 1.471(3) [N(1)–C(1)] and
1.474(3) Å [N(1)–C(2)].
N(1)–C(5) 116.1(1), C(1)–N(1)–B(1) 123.2(1), C(2)–N(1)–B(1)
120.8(1), C(5)–N(1)–B(1) 61.2(1), N(1)–B(1)–C(5) 58.1(1), N(1)–
B(1)–C(4) 121.3(1), C(5)–B(1)–C(4) 123.4(1), N(1)–B(1)–C(3)
117.2(1), C(5)–B(1)–C(3) 119.0(1), C(4)–B(1)–C(3) 109.6(1), N(1)–
C(5)–B(1) 60.8(1), N(1)–C(5)–Si(1) 127.2(1), B(1)–C(5)–Si(1)
134.5(1).
bond C(5)–N(1) [1.525(2) Å] is significantly shorter than the
corresponding one in 4 [1.567(4) Å] and 5 [1.556(5) Å]. The
exocylic C–N bond lengths to the nitrogen methyl bonds
[N(1)–C(1), N(1)–C(2)] have values of 1.482(2) and
1.486(2) Å. This is remarkably longer than the endocyclic
C–N bond length in 5 and marginally longer than the C–N
bond length in the aminoborane 2 [cf. 1.471(3) and 1.474(3) Å
for N(1)–C(1) and N(1)–C(2), respectively]. The B(1)–C(5)
bond in 3 at 1.574(2) Å is the shortest in comparison to the
other ring systems with values of 1.595(5) in 4 and 1.586 Å in
5, respectively.
The bond angles of the exocyclic substituents at the tetraco-
ordinate boron and nitrogen atoms have values of 109.7(1) and
109.6(1)° for C(1)–N(1)–C(2) and C(4)–B(1)–C(3), respec-
tively, which is slightly larger than the average values of
108.0(3) in 4 and 109.1(8)° in 5.
Syntheses of Three-membered BCN Ring Systems
According the work of Bürger and co-workers the reaction
of trifluoromethyl-substituted aminoborane (2) with trimethyl-
silyldiazomethane results in the formation of three-membered
BCN ring system (F3C)2BCH(SiMe3)NMe2 (3) under ex-
trusion of dinitrogen (Scheme 3).[12]
The angles at the N(1)–B(1)–C(5) and B(1)–C(5)–N(1) are
not bisected by the plane defined by the exocylic substituents
as it would be in the idealized case. In fact, both planes are
twisted in the direction of the NMe2 group by 2.9° [plane at
B(1), C(3), C(4)] and 5.1° [plane at C(5), H, Si(1)]. By con-
Scheme 3. Reaction of aminoborane 2 with diazomethane.
Single crystals of 3 were obtained from a hexane/dichloro- trast, this distortion is more pronounced in the analogous three-
methane mixture at –30 °C after several days. The structure of membered rings 4 (9.5 and 8.3°) and 5 (8.8 and 9.7°), most
3 is displayed in Figure 3; selected structural parameters are likely due to the weaker steric repulsion of the substituents in
listed in the caption.
3.
The structure of 3 exhibits a three-membered BNC ring sys-
Analogous to the synthesis of 3, the pentafluorophenyl-sub-
tem with endocyclic bond angles of 60.8(1) [N(1)–C(5)–B(1)], stituted aminoborane 1 was treated with trimethylsilyldiazome-
61.2(1) [C(5)–N(1)–B(1)], and 58.1(1)° [N(1)–B(1)–C(5)]. thane (Scheme 4). However, no reaction occurred even after
The B(1)–N(1) bond length has a value of 1.567(2) Å, which 24 h at room temperature. At this point it appeared likely, that
is in the range of analogous three-membered ring systems the less electron-withdrawing pentafluorophenyl group leads
[1.560(5) Å in (F3C)2BC(SiMe3)(CH2C6H5)NMe2 (4),[13] to a less reactive B–N double bond compared to the CF3 sub-
1.568(6) Å in (F3C)2BC(C6H5)2NMe2 (5)[23]]. By contrast, the stituted aminoborane (2). However, one could also quote a ste-
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N. W. Mitzel et al.
ARTICLE
ric argument: the ortho-fluorine substituent at the C6F5 groups this picture of a formal or partial double bond. The main struc-
in 1, placed almost perpendicular to the C2BN plane, shields tural parameters of 1 and 2 obtained by XRD were reproduced
the boron atom more efficiently that the CF3 groups in 2, and by the calculations in a satisfactory way concerning the fact
thus prevent reagents from easily accessing the boron atom.
that calculations describe free molecules and XRD the solid
state, which has to be expected to be significantly different
due to the facts that very polar molecules are interacting. The
calculated and XRD values for the B–N distance are 1.387 and
1.372(2) Å for 1 and 1.379 and 1.356(3) Å for 2, respectively.
This demonstrates that, consistent with expectation, the bonds
become shorter when free molecules are incorporated into the
polar solid state surrounding.[27]
Despite the differences in structure and charges, the charge
density topology and topological parameters describing the
B–N bonds of 1 and 2 within the QTAIM picture are remarka-
bly similar. Figure 4 and Figure 5 represent the charge density
distribution and the Laplacian for 2, whereas Figure 6 and Fig-
ure 7 display the according properties for 1. Both, charge den-
sity distribution and Laplacian plots are very similar in the
bonding regions about the B–N bond.
Scheme 4. Treatment of aminoborane 1 with diazomethane.
Different reactivity of (C6F5)2BNMe2 (1) and (F3C)2BNMe2
(2) was also observed for other reagents like hydrazines.
Whereas (C6F5)2BNMe2 (1) reacts with MeHNNMe2 to give
the expected substitution with liberation of HNMe2 and forma-
tion of (C6F5)2BN(Me)NMe2, (CF3)2BNMe2 (2) reacts with
the same reagent to afford (CF3)2B(NHMe2)[N(Me)NMe2].
Details of this will be reported elsewhere in a larger con-
text,[24] but we mention it herein, because it demonstrates a
clearly different electronic or steric behavior of 1 and 2. The
more Lewis-acidic and at the same time less bulkily substituted
boron atom in 2 prevents the leaving group HNMe2 to be ab-
stracted.
In order to understand such differences more clearly we car-
ried out charge density studies of 1 and 2, as well as those of
the three-membered ring compound 3 and related molecules.
Charge Density Studies of the Aminoboranes 1 and 2
Charge densities of the molecules described within this sec-
tion were calculated at the RI-MP2/TZVPP level of theory
(implemented in Turbomole version 6.2). Many attempts to
obtain suitable crystalline material of the compounds for an
experimental charge density study remained unsuccessful. The
analyses of the atomic properties and charge density
topology features were carried out using the program
AIMALL(11.12.19).[26]
Figure 4. Electron density contour map of 2 in the C2BNC2 plane
showing the atomic interaction lines and bond critical points.
Due to the above described differences in reactivity of 1 and
2 we first examined atomic charges. The higher electronegativ-
ity of the ligands at the boron atom in (CF3)2BNMe2 (2) as
compared to that in (C6F5)2BNMe2 (1) does not lead to a
higher positive charge on boron (1: qB = 2.14 e, 2: qB
2.11 e) – in fact the values are nearly identical – but also to a
less negative charge on nitrogen (1: qN = –1.51 e, 2: qn
=
=
–1.52 e). The charge difference between boron and nitrogen
atoms, i.e. the polarity of the B–N bond is therefore virtually
the same in 1 and in 2 (1: Δq = 3.65 e, 2: Δq = 3.63 e).
It seems surprising, that 1 and 2 have such similar charges
of boron and nitrogen atoms, despite the different substituent
and different structural parameters (observed in the solids).
However, the chemical shifts of the boron atoms in the 11B
NMR spectra are also very similar: δ(1) = 33.7 ppm and
δ(2) = 26.5 ppm.[18]
Figure 5. Contour map of the Laplacian of the electron density of 2
in the C2BNC2 plane. Dashed and solid lines correspond to the nega-
tive and positive Laplacian, respectively.
The B–N bonds in 1 and 2 can in the picture of Lewis-
The position of the bond critical point is also very similar:
formulae be described as combination of single and dative the lengths of the B–BCP vectors are 32.8 and 32.9% of the
B–N bond, and the planarity of the C2BNC2 skeleton adds to B–N vector for 1 and 2. Table 1 and Table 2 list some repre-
4
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B=N Bonds and BCN Rings – Reactivity and Charge Density
Table 2. Electron density topology parameters for selected bonds in
(C6F5)2BNMe2 (1) in the calculated ground state as obtained at the RI-
MP2/TZVPP level of theorya).
Atoms
ρ(BCP)
–ٌ2ρ(BCP)
ε
B(1)–N(1)
B(1)–C(3)
B(1)–C(9)
N(1)–C(1)
N(1)–C(2)
1.44
1.16
1.16
1.79
1.79
–18.41
1.49
1.50
17.45
17.44
0.14
0.12
0.12
0.04
0.04
a) The values given are the distances of the bond critical points (BCP)
to the nuclear positions along the bond paths (in Å), the electron densi-
ties ρ(BCP) (in e Å–3), and the values of the Laplacians –ٌ2ρ(BCP) in
e Å–5 and the ellipticities of the interaction at the BCPs.
ever, are strongly negative, which is normally regarded to be
typical of closed shell type or ionically minted. By contrast
and as expected, the values for the B–C bonds are slightly and
those of the C–N bonds strongly positive.
Figure 6. Electron density contour map of 1 in the C2BNC2 plane
showing the atomic interaction lines and bond critical points.
The double bond character of the B–N bond is also repre-
sented by the bond ellipticities ε, which are 0.15 and 0.14 for
1 and 2. Bond ellipticity is defined within QTAIM theory[3a]
as ε = λ1/λ2 – 1 (in which λ1 and λ2 are the negative curvatures
of the electron density at the bond critical point) and provides
a direct measure for the deviation of the charge distribution
from cylindrical symmetry; it serves as an indicator for pres-
ence of significant orbital interactions in addition to a σ bond.
Typical values for bond ellipticities are those of C–C bonds in
ethane (0.00), benzene (0.23) and ethylene (0.45),[30] indicat-
ing the different degree of π bonding in the latter two cases.
However, there is a large degree of variability in this param-
eter, and for instance diimine,[29] with a typical N=N double
bond, has ε = 0.16. So, despite the B=N unit is isoelectronic
to the C=C bond, care is suggested when comparing these elas-
ticity parameters. Interesting is also that the ellipticities in the
B–C bonds are almost of the same quantity (ε = 0.12) as those
of the B–N bonds.
Figure 7. Contour map of the Laplacian of the electron density of 1
in the C2BNC2 plane. Dashed and solid lines correspond to the nega-
tive and positive Laplacian, respectively.
sentative parameters to describe the bonding within the
QTAIM picture. The electron densities at the bond critical
points are very similar and 1.46 and 1.44 e Å–3 for 1 and 2.
This is large compared to dative B–N bonds (compare the
dative bonds in H3NǞBF3 1.04(1) e Å–3),[28] and in the range
between strong (ethane, 1.6 e Å–3) and weak (hydrazine,
1.0 e Å–3) single bonds, but much smaller than for typical
double bonds in ethene (2.4 e Å–3) and diimine (3.1 e Å–3).[29]
This effect was noticed in other boranes and is ascribed to
hyperconjugation in the orbital interaction picture of the NBO
formalism. For instance in BMe3 the ellipticities for the B–C
bonds are ε = 0.28 and a comparable value of ε = 0.26 has
been calculated for 3-methyl-1-boraadamantane, which has a
boron atom with pyramidal coordination sphere.[31]
Electron Density Studies of the BCN Compound 3 and
Related Systems
Table 1. Electron density topology parameters for selected bonds in
(CF3)2BNMe2 (2) in the calculated ground state as obtained at the RI-
MP2/TZVPP level of theorya).
Having studied the nature of the B–N partial double bond
in 1 and 2, the situation in the ring compound 3 seemed to be
an interesting contrasting case. As described in the introduc-
tion, we have earlier found that weak bonds involved in three-
membered rings constitute unusual situations concerning the
charge density topology. As above, electron densities were cal-
culated at the RI-MP2/TZVPP level of theory, while experi-
mental charge density studies were not feasible due to the lack
of suitable quality crystalline specimens.
Atoms
ρ(BCP)
–ٌ2ρ(BCP)
ε
B(1)–N(1)
B(1)–C(3)
B(1)–C(2)
N(1)–C(1)
N(1)–C(2)
1.46
1.19
1.18
1.75
1.75
–19.18
3.03
3.01
16.72
16.77
0.15
0.12
0.15
0.03
0.03
a) The values given are the electron densities ρ(BCP) (in e Å–3), and
the values of the negative Laplacians –ٌ2ρ(BCP) in e Å–5 and the
ellipticities of the interaction at the BCPs.
The electron density maps of (F3C)2BCH(SiMe3)NMe2 (3),
The negative Laplacians of the electron density at the bond as well as those of the simpler non-silylated (F3C)2BCH2NMe2
critical points (Table 1 and Table 2) of the B–N bonds, how- (4) and the BN/CC isoelectronic compound (F3C)2CCH2CMe2
Z. Anorg. Allg. Chem. 0000, 0–0
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ARTICLE
(5) are shown in Figure 8, Figure 9, and Figure 10, the corre-
sponding Laplacian plots in Figure 11, Figure 12, and Fig-
ure 13, respectively.
Figure 10. Electron density contour map of (F3C)2CCH2CMe2 (5) in
the C3 plane showing the atomic interaction lines and corresponding
bond critical points. The orientation is analogous to that of 4 in Fig-
ure 9: C(4) bears the two CF3 groups, C(6) the two Me groups.
Figure 8. Electron density contour map of (F3C)2BCH(SiMe3)NMe2
(3) in the BCN plane showing the atomic interaction lines and corre-
sponding bond critical points. The point between the B and N atoms
is not a bond critical point, but represents the minimum of electron
density on the line connecting the B and N nuclei.
Table 3. Electron density topology parameters for selected bonds in 3,
4, and 5 in the calculated ground state as obtained at the RI-MP2/
TZVPP level of theory. Electron densities ρ(BCP) are given in e Å–3,
Laplacians –ٌ2ρ(BCP) in e Å–5.
(CF3)2BCH(SiMe3)NMe2 (3)
Atoms
ρ(BCP)
–ٌ2ρ(BCP)
ε
B(1)–C(5)
N(1)–C(5)
B(1)–C(3)
B(1)–C(4)
N(1)–C(1)
N(1)–C(2)
Si(1)–C(5)
1.09
1.51
1.15
1.16
1.73
1.72
0.76
0.26
7.76
0.93
0.90
16.16
16.05
–6.09
0.27
0.57
0.10
0.10
0.01
0.02
0.02
(CF3)2BCH2NMe2 (4)
Atoms
ρ(BCP)
–ٌ2ρ(BCP)
ε
B(4)–C(5)
N(6)–C(5)
B(4)–C(1)
B(4)–C(9)
N(6)–C(7)
N(6)–C(13)
1.12
1.56
1.17
1.17
1.72
1.72
–0.30
9.13
1.03
1.04
16.12
16.12
0.22
0.61
0.10
0.10
0.01
0.01
Figure 9. Electron density contour map of (F3C)2BCH2NMe2 (4) in
the BCN plane showing the atomic interaction lines and corresponding
bond critical points. The point between the B and N atoms is not a
bond critical point, but represents the minimum of electron density on
the line connecting the B and N nuclei.
(F3C)2CH2CMe2 (5)
Atoms
ρ(BCP)
–ٌ2ρ(BCP)
ε
C(4)–C(5)
C(5)–C(6)
C(4)–C(6)
C(4)–C(1)
C(4)–C(9)
C(6)–C(7)
C(6)–C(13)
1.60
1.68
1.55
1.83
1.80
1.70
1.70
11.01
12.81
9.94
18.29
17.69
15.56
15.56
0.52
0.37
0.57
0.05
0.05
0.02
0.01
Some selected topology parameters of these compounds are
listed in Table 3.
The most intriguing feature of the three-membered BCN
rings in 3 and 4 is the absence of atomic interaction lines (bond
paths) between the boron and nitrogen atoms and the absence
of ring critical points. This would mean, that – on first sight – For 3 this is in slight variance to those parameters determined
QTAIM theory describes these systems as open chain. This is by XRD, but this is expected as dative bonds are generally
in obvious contradiction to the structural parameters because found to be shorter in the solid state than in the free molecules
in both cases all bond angles within the three-membered rings or in the gas phase.[27] So the distances and structural behavior
are close to 60° and the B–N distances (dative bonds) are upon phase change clearly indicate the presence of a strong
shorter than the B–C or C–N bonds (covalent bonds). Selected attractive B–N interaction.
structural parameters from the calculations are (3 / 4): B–N
For comparison, the electron density topology of the cyclo-
1.576 / 1.584, B–C 1.578 / 1.565, C–N 1.514 / 1.503 Å, and propane (F3C)2CCH2CMe2 (5) (Figure 10) has an atomic inter-
B–C–N 61.3 / 62.1, C–N–B 61.4 / 60.8, C–B–N 57.4 / 57.0°. action line between C(4) and (C6), the corresponding isoelec-
6
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B=N Bonds and BCN Rings – Reactivity and Charge Density
tronic part to the B–N bond in 4. Expectedly, atomic interac-
tion lines in the three-membered rings are no straight connec-
tions between the corresponding nuclei, but are slightly deviat-
ing to the outside.
The Laplacians for 3, 4, and 5 are shown in Figure 11, Fig-
ure 12 and Figure 13. They show the regions of charge ac-
cumulation and depletion in a way we expected for such
bonds, with some deviations which can be rationalized by tak-
ing into consideration the bent bond paths. Even the Laplacian
topology in the regions of the B–N bonds in 3 and 4 are as
expected for typical and strong dative B–N bonds.[28]
Figure 13. Contour map of the Laplacian of the electron density of 5
in the C3 plane. Dashed and solid lines correspond to the negative and
positive Laplacian, respectively.
approximation, where a bond critical point for these bonds
might be expected. These points are also drawn in Figure 8
and Figure 9 and also in Figure 11 and Figure 12. They have
electron densities of 0.91 and 0.90 e Å–3, which is relatively
large, even if compared to the BCPs of typical aminoborane
adducts like H3NBF3 [1.04(1) e Å–3,[28] an experimental value
from a shortened bond in the solid state]. The position of these
points is also close to the phase change of the Laplacian func-
tion, similar to the situation in 1 and 2.
Figure 11. Contour map of the Laplacian of the electron density of
(F3C)2BCH(SiMe3)NMe2 (3) in the BCN plane. Dashed and solid lines
correspond to the negative and positive Laplacian, respectively. Also
shown are the atomic interaction lines and corresponding bond critical
points. The point between the B and N atoms is not a bond critical
point, but represents the minimum of electron density on the line con-
necting the B and N nuclei.
However in the cases of 3 and 4 there is no reason to assume
lacking attractive interaction between boron and nitrogen
atoms on the basis of structural parameters or phase-dependent
behavior. So instead of assuming the B–N interaction to be-
come too weak, one should consider the influence of the third
partner within the ring onto the region under consideration.
This can be approached by another part QTAIM theory: the
source function. This allows describing the contribution of the
various regions of a molecule to the electron density at a cer-
tain point. One way in which it can be expressed is the contri-
bution of atomic basins to a selected point in space. Table 4
lists the source function contributions at the point of minimal
density on the lines between boron and nitrogen in 3 and 4, as
well as those describing the contributions to the BCP of the
C(4)–C(6) bond in 5. Untypical for “normal bonds”, there is a
high degree of contribution of an atom that is not involved
directly in this bond in the normal understanding in terms of a
Lewis structural formula. For 3 and 4 the contribution of the
carbon atoms (ca. 15%) is even higher than that of the boron
atoms. The situation is less pronounced in 5, where the atom
C(5) still contributes ca. 8% to the BCP of the bond C(4)–
C(6) in terms of the source function. The existence of a BCP
in the case of 5 is thus the result of a relatively high electron
density of the covalent C–C bond and a significant but low
source function contribution of the neighboring carbon atom,
whereas in 3 and 4 the source function contribution to the re-
gion between boron and nitrogen atoms describes the overlay
Figure 12. Contour map of the Laplacian of the electron density of
(F3C)2BCH2NMe2 (5) in the BCN plane. Dashed and solid lines corre-
spond to the negative and positive Laplacian, respectively. Also shown
are the atomic interaction lines and corresponding bond critical points.
The point between the B and N atoms is not a bond critical point, but
represents the minimum of electron density on the line connecting the
B and N nuclei.
In order to describe this region in more detail we have se- of electron density originating from the carbon atom, which is
arched the points of minimum electron density on the straight large enough to cover the topological features of a low-elec-
lines between the boron and nitrogen atoms in 3 and 4 as an tron-density dative B–N bond.
Z. Anorg. Allg. Chem. 0000, 0–0
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N. W. Mitzel et al.
ARTICLE
Table 4. Source contributions of atomic basins at the point of minimal region. This close proximity of atoms represents a state, where
density on the line between B and N for 3 and 4 and to the BCP of
the C(4)–C(6) bond in 5.
the analyses of the electron density topology concerning the
concept of atomic interaction line (bond path) in the QTAIM
(F3C)2BCH(SiMe3)NMe2 (3)
and the classical concept of a chemical bond defined by chemi-
cal and physical properties and structural features can show a
discrepancy, when interactions with low-electron density be-
tween the nuclei (dative, coordinative) are involved. This ob-
servation needs to be taken into consideration also for other
ring systems like four-membered rings, in particular in situa-
tions, where short but low-electron-density bonds are involved.
Atomic basin
ρ
% ρ
B(1)
N(1)
C(5)
0.14
0.29
0.14
15.23
31.69
15.63
(F3C)2BCH2NMe2 (4)
Atomic basin
ρ
% ρ
B(4)
N(6)
C(5)
0.13
0.28
0.14
14.39
30.78
15.29
Experimental Section
(F3C)2CCH2CMe2 (5)
General Methods: All manipulations were performed in a rigorously
dry inert atmosphere of argon using standard Schlenk and glove-box
techniques. Hexane was dried with LiAlH4 before being employed in
reactions. C6D6 was dried with Na/K alloy and degassed.
(C6F5)2BCl,[16] lithium dimethylamide,[17] (F3C)2BN(CH3)2 (2),[18,19]
and (F3C)2BCH(SiMe3)NMe2 (3)[13] were synthesized according to lit-
erature procedures. NMR measurements were undertaken with Bruker
DRX 500 and Bruker Avance 500 spectrometers. NMR chemical shifts
were referenced to the residual peaks of the protons of the used sol-
vents (1H, 13C) or externally (11B, BF3·OEt2; 19F, CFCl3). Elemental
Atomic basin
ρ
% ρ
C(4)
C(6)
C(5)
0.54
0.53
0.13
34.54
34.45
8.32
Conclusions
(F5C6)2BNMe2 (1) was prepared and (F3C)2BNMe2 (2) was
resynthesized. The crystal structures of both compounds were analyses were performed with a Leco CHNS 932 instrument.
determined. Compounds 1 and 2 show a very different reactiv-
(C6F5)2BNMe2 (1): To a suspension of lithium dimethylamide (0.13 g,
ity pattern. 2 is known to react with diazomethanes to afford
2.6 mmol) in hexane (5 mL) was added a solution of (C6F5)2BCl
BCN compounds like (F3C)2BCH(SiMe3)NMe2 (3), but 1 does
(1.00 g, 2.63 mmol) in hexane (5 mL) at –30 °C. The reaction mixture
was warmed to room temperature and stirred for additional 2 h. After
filtration of the mixture the solvent was removed in vacuo, resulting
not. The solid-state structure of 3 was also determined.
The electron densities of these compounds and those of
(F3C)2BCH2NMe2 (4) and the BN/CC isoelectronic compound
(F3C)2CCH2CMe2 (5) were calculated. Despite structural dif-
in a light yellow solid. Single crystals suitable for single-crystal X-ray
diffraction were obtained from a concentrated hexane solution at
ferences between 1 and 2 (2 having a shorter B–N bond in –35 °C after several days. Yield: 0.75 g (74%). C19H16BF10N
(459.14): calcd. C 43.23, H 1.55, N 3.60%; found C 43.03, H 1.82, N
3.59%. H NMR (500.1 MHz, C6D6, 298 K): δ = 2.25 ppm (s, 6 H,
calculations and crystal), the atomic charges and electron den-
sity parameters according to the Theory of Atoms in Molecules
are very similar for both compounds. It remains to interpret
the different reactivity patterns in terms of steric shielding of
the boron atom in (F5C6)2BNMe2 (1).
The charge density topology of the three-membered rings
systems 3–5 demonstrates that reported statements[32] like the
“link between bonding models and topological properties was
fully achieved for main group metals, while the correspon-
dence rules derived from this area of chemistry cannot be fully
extended in a straightforward manner to (transition metal) or-
ganometallic compounds” should be questioned with regard to
the part concerning main group compounds and modified with
respect to the too optimistic view concerning the understanding
of the bonding in main group chemistry. Despite extremely
short B–N bonds in 3 and 4, there are no bond-critical points
for the B–N bonds, despite relatively high electron densities in
1
CH3). 11B{1H} NMR (160.4 MHz, C6D6, 298 K): δ = 33.7 ppm (ν1/2
= 209 Hz). 13C{1H} NMR (125.7 MHz, C6D6, 298 K): δ = 146.4 (dm,
1JCF = 238.9 Hz, C6F5), 142.0 (dm, 1JCF = 254.7 Hz, C6F5), 137.6 (dm,
1JCF = 250.3 Hz, C6F5), 111.4 (br., i-C6F5), 40.7 ppm (CH3). 19F NMR
2
3
(470.5 MHz, C6D6, 298 K): δ = –161.3 (td, JFF = 23.1 Hz, JFF
=
3
8.5 Hz, 4F, m-C6F5), –152.2 (t, JFF
=
20.5 Hz, 2F, p-C6F5),
–132.5 ppm (dd, JFF = 20.7 Hz, JFF = 5.3 Hz, 4F, o-C6F5).
2
3
Crystallographic Structure Determinations: Single crystals of 1 and
3 suitable for X-ray diffraction measurement were picked inside a
glove-box, suspended in a paratone-N/paraffin oil mixture, mounted
on a glass fibre and transferred onto the goniometer of the dif-
fractometer. A single crystal of 2 was grown in situ. The measurements
were carried out with Mo-Kα radiation (λ = 0.71073 Å). The structures
were solved by direct methods and refined by full-matrix least-squares
cycles (program SHELX-97[34]). The positions of the CF3 groups in
(F3C)2BN(CH3)2 (2) were disordered and were refined using a “split”
this region. By contrast the BN/CC isolelectronic system 5 model. Crystallographic data and structure refinement results for com-
pounds 1–3 are summarized in Table 5.
shows this topology feature. The source contribution of the
carbon atom, bridging the boron and nitrogen atoms, to the
B–N region in 3 and 4 was evaluated and found to be un-
usually large.
Similar situations have been documented for SiON[11] and
SiNN[12] rings and also been observed in related situations.
Thus one may state that small ring systems feature a situation
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-907295 (1), CCDC-907296 (2), and CCDC-907297
(3) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk,
of more than two atoms contributing electron density to a small http://www.ccdc.cam.ac.uk).
8
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B=N Bonds and BCN Rings – Reactivity and Charge Density
Table 5. Crystallographic data for compounds 1, 2, and 3.
1
2
3
Formula
Mr
C14H6BF10N
389.01
C4H6BF6N
192.91
C8H16BF6NSi
279.12
T /K
100(2)
100(2)
100(2)
Crystal size /mm
Crystal system
Space group
a /Å
b /Å
c /Å
0.30ϫ0.30ϫ0.25
monoclinic
P21/c
11.1839(3)
10.1422(3)
13.6619(4)
90
110.4680(16)
90
1451.83(7)
4
0.40ϫ0.24ϫ0.24
orthorhombic
Pbca
12.6993(3)
8.1182(2)
7.5930(2)
90
90
90
778.50(3)
4
1.646
0.30ϫ0.30ϫ0.20
triclinic
P1
¯
6.8311(4)
6.9908(4)
15.2901(9)
80.857(3)
86.981(3)
66.365(4)
661.23(7)
2
α /°
β /°
γ /°
V /Å3
Z
ρcalcd. /g cm–3
μ /mm–1
F(000)
1.780
0.195
768
1.402
0.226
288
0.198
384
Θ range /°
Index range
3.1 to 27.5
–14 Յ h Յ 14
–12 Յ k Յ 13
–17 Յ l Յ 17
19557
3.1 to 30.0
–17 Յ h Յ 17
–11 Յ k Յ 11
–10 Յ l Յ 10
21777
3.2 to 27.5
–8 Յ h Յ 8
–9 Յ k Յ 9
–19 Յ l Յ 19
14682
Reflns collected
Unique reflns
3323
1206
2963
Observd refl (2σ)
Rint
2692
0.028
987
0.041
2613
0.033
Data / restr. / param
3323 / 0 / 237
1.037
1206 / 0 / 92
1.066
2963 / 0 / 159
1.030
GoF (F2)
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
Δρ(max/min) /e·Å–3
0.0335, 0.0869
0.0449, 0.0941
0.30 / –0.26
0.0605, 0.1601
0.0709, 0.1682
0.57/ –0.28
0.0330, 0.0841
0.0386, 0.0888
0.38 / –0.33
[8] a) R. F. W. Bader, J. Phys. Chem. A 2009, 113, 10391–10396; b)
W. H. E. Schwarz, H. Schmidbaur, Chem. Eur. J. 2012, 18, 4470–
4479.
Supporting Information (see footnote on the first page of this article):
Tables with details on the charge density topology analysis.
[9] N. W. Mitzel, K. Vojinovic´, T. Foerster, H. E. Robertson, K. B.
Borisenko, D. W. H. Rankin, Chem. Eur. J. 2005, 11, 5114–5125.
[10] N. W. Mitzel, Z. Naturforsch. B 2003, 58, 759–763.
[11] a) N. W. Mitzel, K. Vojinovic´, R. Fröhlich, T. Foerster, D. W. H.
Rankin, J. Am. Chem. Soc. 2005, 127, 13705–13713; b) N. W.
Mitzel, U. Losehand, A. Wu, D. Cremer, D. W. H. Rankin, J. Am.
Chem. Soc. 2000, 122, 4471–4482; c) N. W. Mitzel, U. Losehand,
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Losehand, Angew. Chem. 1997, 109, 2897–2899; Angew. Chem.
Int. Ed. Engl. 1997, 36, 2807–2809.
[12] a) N. W. Mitzel, Chem. Eur. J. 1998, 4, 692–698; b) K. Vojinovic´,
L. J. McLachlan, S. L. Hinchley, D. W. H. Rankin, N. W. Mitzel,
Chem. Eur. J. 2004, 10, 3033–3042.
[13] A. Ansorge, D. J. Brauer, H. Bürger, T. Hagen, G. Pawelke, An-
gew. Chem. 1993, 105, 429–430; Angew. Chem. Int. Ed. Engl.
1993, 32, 384–385.
Acknowledgements
This work was supported by Deutsche Forschungsgemeinschaft
(SPP1178, Experimental Electron Density). We are grateful to Fonds
der Chemischen Industrie for support and a stipend to D.W.
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ARTICLE
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Received: November 3, 2012
Published Online: ᭿
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Date: 14-12-12 11:08:36
Pages: 11
B=N Bonds and BCN Rings – Reactivity and Charge Density
D. Winkelhaus, Y. V. Vishnevskiy, R. J. F. Berger,
H.-G. Stammler, B. Neumann, N. W. Mitzel* ................. 1–11
B=N Bonds and BCN Rings – Reactivity and Charge Density
Studies
Z. Anorg. Allg. Chem. 0000, 0–0
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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