Mechanism of generation of: Closo -decaborato amidrazones. Intramolecular non-covalent B-H?π(Ph) interaction determines stabilization of the configuration around the amidrazone CN bond

Article abstract of DOI:10.1039/c8nj01018h

Three types of N(H)-nucleophiles, viz. hydrazine, acetyl hydrazide, and a set of hydrazones, were used to study the nucleophilic addition to the CN group of the 2-propanenitrilium closo-decaborate cluster (Ph₃PCH₂Ph)[B₁₀H₉NCEt], giving N-closo-decaborato amidrazones. A systematic mechanistic study of the nucleophilic addition is provided and included detailed synthetic, crystallographic, computational and kinetic work. As a result, two possible mechanisms have been proposed, which consist of firstly a consecutive incorporation of two Nu(H) nucleophiles, with the second responsible for a subsequent rapid proton exchange. The second possible mechanism assumes a pre-formation of a dinuclear [Nu(H)]₂ species which subsequently proceeds with the nucleophilic attack on the boron cluster. The activation parameters for hydrazones indicate a small dependence on bond formation [ΔH^? = 6.8-15 kJ mol^-1], but significantly negative entropies of activation [ΔS^? ranges from -139 to -164 J K^-1 mol^-1] with the latter contributing some 70-80% of the total Gibbs free energy of activation, ΔG^?. In the X-ray structure of (Z)-(Ph₃PCH₂Ph)[B₁₀H₉N(H)C(Et)NHNCPh₂], very rare intramolecular non-covalent B-H?π(Ph) interactions were detected and studied by DFT calculations (M06-2x/6-311++G?? level of theory) and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method). The estimated strength of these non-covalent interactions is 0.8-1.4 kcal mol^-1.

Full text of DOI:10.1039/c8nj01018h

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S. Bolotin, A. S. Mikherdov, A. S. Novikov, P. Mokolokolo, A. Roodt, V. P. Boyarskiy, D. Dar'in, M. Krasavin,
V. V. Suslonov, A. P. Zhdanov, K. Yu. Zhizhin and N. T. Kuznetsov, New J. Chem., 2018, DOI:
10.1039/C8NJ01018H.
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Mechanism of Generation of closoꢀDecaborato Amidrazones. Intramolecular
Nonꢀcovalent B–H•••π(Ph) Interaction Determines Stabilization of the
Configuration around the Amidrazone C=N Bond
Valeria K. Burianova,1 Dmitrii S. Bolotin,*1 Alexander S. Mikherdov,1 Alexander S. Novikov,1
Pennie Petrus Mokolokolo,2 Andreas Roodt,*2 Vadim P. Boyarskiy,1 Dmitry Dar'in,1
Mikhail Krasavin,1 Vitalii V. Suslonov,3 Andrey P. Zhdanov,4 Konstantin Yu. Zhizhin,4
and Nikolay T. Kuznetsov4
1Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab., 7/9, Saint Petersburg 199034, Russian
Federation
2Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa
3Center for Xꢀray Diffraction Studies, Saint Petersburg State University, Universitetskii Pr., 26, Saint Petersburg
198504, Russian Federation
4N. S. Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky Pr., 31,
Moscow 119991, Russian Federation
Abstract
Three types of N(H)ꢀnucleophiles, viz. hydrazine, acetyl hydrazide, and a set of hydrazones, were
used to study the nucleophilic addition to the C≡N group of the 2ꢀpropanenitrilium closoꢀdecaborate
cluster (Ph3PCH2Ph)[B10H9NCEt], giving Nꢀclosoꢀdecaborato amidrazones. A systematic
mechanistic study of the nucleophilic addition is provided and included detailed synthetic,
crystallographic, computational and kinetic work. As a result, two possible mechanisms have been
proposed, which consist of firstly a consecutive incorporation of two Nu(H) nucleophiles, with the
second responsible for a subsequent rapid proton exchange. The second possible mechanism
assumes a preꢀformation of a dinuclear [Nu(H)]2 species which subsequently proceeds with the

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nucleophilic attack on the boron cluster. The activation parameters for hydrazones indicate a small
dependence on bond formation [ꢀH≠ = 6.8–15 kJ⋅mol–1], but significantly negative entropies of
activation [ꢀS≠ ranges from –139 to –164 J⋅K–1⋅mol–1] with the latter contributing some 70–80% of
the total Gibbs free energy of activation, ꢀG≠. In the Xꢀray structure of (Z)ꢀ
(Ph3PCH2Ph)[B10H9N(H)=C(Et)NHN=CPh2], very rare intramolecular nonꢀcovalent interactions B–
H•••π(Ph) were detected and studied by DFT calculations (M06ꢀ2X/6ꢀ311++G** level of theory)
and topological analysis of the electron density distribution within the framework of Bader’s theory
(QTAIM method). Estimated strength of these nonꢀcovalent interactions is 0.8–1.4 kcal/mol.
Introduction
Polyhedral borane clusters have been a subject of significant attention for the past 15 years due to
their broad utility in inorganic1 and coordination chemistry,2 and also in pharmacology, where they
are generally applied as active species for potential boron neutronꢀcapture cancer therapy3 and as
inhibitors of platelet aggregation,3g antiviral agents,4 and modulators of important hormone
receptors.3g Although closoꢀdodecaborate and relevant carboranes have been extensively studied,
the chemistry of a lighter congener, viz. closoꢀdecaborate, has received significantly less attention.
The closoꢀdecaborate cluster is generally involved in two types of reactions, for example as
(i) redox transformations of the cluster (reduction to the nidoꢀdecaborane B10H14 or 6ꢀsubstituted
nidoꢀboranes B10H13X (where X is OH, F, Cl, Br, or I)5 or oxidative dimerization leading to
[B20H18]4–)6 and (ii) sideꢀchain reactions, viz. substitution of one or two hydrides by a variety of Cꢀ
,7 Oꢀ8 or N7, 9ꢀnucleophilic species, including nitriles RC
very easily functionalizable species by nucleophilic addition and 1,3ꢀdipolar cycloaddition
respectively, due to the availability of a highly activated C N moiety. This nitrile easily reacts with
≡
N.9 The nitrilium closoꢀdecaborates are
≡
a spectrum of Cꢀ,10 Nꢀ11 and Oꢀnucleophiles,12 and also with nitrones13 and azides.14
2

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The nucleophilic addition of simple amines RNH2 (R = Alk, Ar) and Alk2NH to the C≡N
group is known for closoꢀdecaborateꢀ,11 nidoꢀdecaboraneꢀ,15 and cobalt(III) bis(1,2ꢀdicarbollide)16ꢀ
bound nitriles, however, the reactions of nitrilium boron clusters with hydrazine and its derivatives
have not been studied previously. Based on this, we chose three types of N(H)ꢀnucleophiles, viz.
hydrazine, acetyl hydrazide, and a set of hydrazones to study their nucleophilic addition to the 2ꢀ
propanenitrilium closoꢀdecaborate cluster giving Nꢀclosoꢀdecaborato amidrazones. Driven by the
success of our synthetic experiments and high selectivity of these reactions, we undertook a
systematic mechanistic study of the addition. This included detailed kinetic work, and as a result,
we obtained a range of nucleophilicity for the N(H)ꢀnucleophiles that includes a wideꢀranging
variation of their structural types. Diverse analogous reactions involving nucleophilic addition of
amines to unsaturated species have been reported earlier.17 These reactions serve as reference for
our mechanistic interpretation of the process, which we conclude from the detailed kinetic study
reported herein.
Results and Discussion
Addition of N(H)ꢀnucleophiles to 2ꢀPropanenitrilium closoꢀDecaborate. As the starting
materials for the study of the addition of N(H)ꢀnucleophiles to 2ꢀpropanenitrilium closoꢀdecaborate
(Ph3PCH2Ph)[1] (Scheme 2), we addressed three types of Nꢀdonors, viz. hydrazine (2a; Scheme 1),
acetyl hydrazide (2b), and hydrazones (2c–g).
NH2
R2
NH2
NH2
NH2
NH2 CF3
Ph
R3
MeO
H
NOS
2c
2d
2e
2f
2g
N
NH2
O
HN
NH2
R2
Br
H2N
Me
H
R3
2a
2b
2c–g
Scheme 1. Structures of three types of the nucleophiles used in this study.
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The reaction of any one of 2a–g with the nitrilium salt (Ph3PCH2Ph)[1] proceeds in MeCN or THF
at RT over a period of 30 min giving 2ꢀiminium closoꢀdecaborates (Ph3PCH2Ph)[3a–g] in isolated
yields of 59–92% (Scheme 2; for synthesis, characterization and discussion of spectral data see
Supporting Information) as mixtures of the (E/Z)ꢀisomers (for (Ph3PCH2Ph)[3a–f]), which varies
for different products and ranges from 45/55 to 85/15. The benzophenone hydrazone derivative
(Ph3PCH2Ph)[3g] exists exclusively in the (Z)ꢀform, because of the availability of attractive nonꢀ
covalent interactions B–H•••Ph (see later). The reaction of 2a with (Ph3PCH2Ph)[1] proceeds in
THF resulting in the formation of a precipitate of (Ph3PCH2Ph)[3a], whereas the reactions with 2b–
g do not result in spontaneous formation of precipitates.
Et
NH2
HN
(Ph3PCH2Ph)
R1
N
H
E/Z-ratio
45/55
85/15
Yield (%)
92
75
NOS
(Ph3PCH2Ph)[3a]
(Ph3PCH2Ph)[3b]
R1
H
Ac
NH
HN
2a–b
a
R1
(Ph3PCH2Ph)
Et
N
Et
(Ph3PCH2Ph)
(Ph3PCH2Ph)[1]
N
H
NH
N
- BH
- B
b
R2
R3
MeO
H
Br
CF3
H
E/Z-ratio
80/20
75/25
70/30
70/30
0/100
NOS
Yield (%)
R2
(Ph3PCH2Ph)[3c]
(Ph3PCH2Ph)[3d]
(Ph3PCH2Ph)[3e]
(Ph3PCH2Ph)[3f]
(Ph3PCH2Ph)[3g]
NH2
NH2
NH2
NH2
Ph
84
58
59
68
78
NH2
R2
N
R3
R3
2c–g
Scheme 2. Addition of the N(H)ꢀnucleophiles to (Ph3PCH2Ph)[1].
Iminium salts (Ph3PCH2Ph)[3a–g] are stable at RT in air in the solid state for at least
3 months. In MeCN and toluene solutions, these compounds are stable for at least 1 week at RT and
for 8 h at the boiling points of these solvents. These species decompose upon reflux in EtOH in the
presence of NaOH (10 mol% respectively to (Ph3PCH2Ph)[3a–g], ca. 0.02 M solutions) giving
–
(Ph3PCH2Ph)[B10H9NH3], EtCO2 , and starting 2c–g, which were detected by HRESI–ꢀMS (2a and
2b are unobservable in the spectra due to m/z < 80 Da). Noticeably that in the cases of
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(Ph3PCH2Ph)[3b–f] no formation of free or boronꢀbound 1,2,4ꢀtriazoles was detected by HRESI–ꢀ
MS, in spite of the fact that these heterocycles are typically generated from acyl amidrazones.18
Theoretical Study of the Intramolecular NonꢀCovalent Interactions B–H•••π(Ph) in
(Z)ꢀ(Ph3PCH2Ph)[3g]. Inspection of the crystallographic data reveals the presence of
intramolecular nonꢀcovalent interactions B–H•••π(Ph) in crystal structure of (Z)ꢀ(Ph3PCH2Ph)[3g]
(Figure 1). Indeed, the shortest distance H•••C in corresponding fragment of molecule is 2.99 Å,
which is slightly larger than the sum of Bondi’s (the shortest)19 van der Waals radii of H and C
atoms (2.90 Å), comparable with the sum of Alvarez’s20 van der Waals radii (2.97 Å), but lesser
than sum of Rowland’s21 van der Waals radii (3.02 Å). Note that the van der Waals radius of the
boron hydride hydrogen atom was supposed to be 1.39 Å as negatively charged thereby having
increased atomic volume (instead of “classical” Bondi’s van der Waals radius for H atom = 1.20
Å).22 This type of nonꢀcovalent interactions is very rare and only few systems demonstrating such
contacts were reported, viz. Xꢀray diffraction studies of [Cp*Ir(PR3)S2C2B10H10] complexes reveal
that the nonꢀcovalent interaction between the carborane B–H bonds and the phosphine aryl
substituents involves a B–H•••π contacts (H•••π distances in these systems are 2.40−2.76 Å, which
is significantly shorter than those in (Z)ꢀ(Ph3PCH2Ph)[3g]) (Figure 1).23 Thus, in addition to the
structural study, detailed computational analysis of the intramolecular nonꢀcovalent interactions B–
H•••π(Ph) in crystal structure of (Z)ꢀ(Ph3PCH2Ph)[3g] is desirable.
Figure 1. Molecular structure of (Z)ꢀ(Ph3PCH2Ph)[3g]. Thermal ellipsoids are given at the 50%
probability level. Dotted line indicates the nonꢀcovalent intramolecular interactions B–H•••π(Ph).
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In order to confirm or disprove the hypothesis on the existence of these nonꢀcovalent
intramolecular interactions B–H•••π(Ph) in (Z)ꢀ(Ph3PCH2Ph)[3g] and to quantify their energy from
a theoretical viewpoint, we carried out single point DFT calculations based on the experimental Xꢀ
ray geometry of (Z)ꢀ(Ph3PCH2Ph)[3g] at the M06ꢀ2X/6ꢀ311++G** level of theory. We also
performed topological analysis of the electron density distribution within the framework of Bader’s
theory (QTAIM method)24 (Table 1). This approach has already been successfully used by us in
studies of different nonꢀcovalent interactions and properties of coordination bonds in various
transition metal complexes.25 It is wellꢀknown that Xꢀray diffraction experiments cannot indicate
the precise location of H atoms, and hydrogens are practically always placed at idealized positions
even if a suitable electron density maximum could be found from a difference Fourier map. To
obtain the theoretically (more correct) positions of H atoms and to exclude possible crystal packing
effects, we have also carried out the geometry optimization procedure in the gas phase for (Z)ꢀ
(Ph3PCH2Ph)[3g] and used the resulting equilibrium geometry for topological analysis of the
electron density distribution. The contour line diagrams of the Laplacian distribution ∇2ρ(r), bond
paths, and selected zeroꢀflux surfaces for nonꢀcovalent interactions B–H•••π(Ph) in (Z)ꢀ
(Ph3PCH2Ph)[3g] are shown in Figure 2. To visualize the studied nonꢀcovalent interactions,
reduced density gradient (RDG) analysis26 was carried out, and RDG isoꢀsurfaces for
(Ph3PCH2Ph)[3g] were plotted (Figure 2).
Table 1. Values of the density of all electrons – ρ
(r), Laplacian of electron density – ∇2ρ(r), energy
density – Hb, potential energy density – V(r), and Lagrangian kinetic energy – G(r) (Hartree) at the
bond critical points (3, –1), corresponding to nonꢀcovalent interactions B–H•••π(Ph) in(Z)ꢀ
(Ph3PCH2Ph)[3g], bond lengths – l (Å), as well as energies of these interactions Eint (kcal mol–1),
defined by two approaches.*
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a
b
∇2ρ(r)
0.016
0.022
0.025
DFT calculation based on the
Experimental Xꢀray geometry 0.006
Hb
V(r)
G(r) Eint Eint
l
ρ(r)
0.000 −0.003 0.003 0.9 0.8 2.99
0.001 −0.004 0.005 1.3 1.4 2.93
0.001 −0.004 0.005 1.3 1.4 2.80
Equilibrium optimized
geometry in gas phase
0.008
0.009
* The Poincare–Hopf relationship is satisfied, thus all critical points have been found.
a Eint = –V(r)/227
b Eint = 0.429G(r)28
Figure 2. Contour line diagrams of the Laplacian distribution ∇2ρ(r), bond paths and selected zeroꢀ
flux surfaces (left) and RDG isoꢀsurfaces (right) referring to the nonꢀcovalent interactions B–
H•••π(Ph) in (Z)ꢀ(Ph3PCH2Ph)[3g] (M06ꢀ2X/6ꢀ311++G** level of theory). Bond critical points (3,
–1) are shown in blue, nuclear critical points (3, –3) – in pale brown, ring critical points (3, +1) – in
orange, and cage critical points (3, +3) – in light green. Length units – Å, RDG isoꢀsurface values
are given in Hartree.
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The QTAIM analysis demonstrates the presence of appropriate bond critical points (BCPs)
for nonꢀcovalent interactions B–H•••π(Ph) in (Z)ꢀ(Ph3PCH2Ph)[3g] both in solid state and in gas
phase. The geometry optimization of (Z)ꢀ(Ph3PCH2Ph)[3g] led to shortening of B–H•••π(Ph)
distances compared to the solid state geometry and to the formation of bifurcated contacts B–
H•••C•••H–B. The low magnitude of the electron density (0.006–0.009 Hartree), positive values of
the Laplacian (0.016–0.025 Hartree), and zero or close to zero (0.001 Hartree) positive energy
density in these BCPs are typical for nonꢀcovalent interactions. We have defined energies for these
contacts according to the procedures proposed by Espinosa et al.27 and Vener et al.28 (Table 1) and
found that estimated strength of these nonꢀcovalent interactions is 0.8–1.4 kcal mol–1. This is typical
for weak hydrogen bonding following the classification of Jeffrey (“strong” Hꢀbonds: 40–15 kcal
mol–1, “moderate” Hꢀbonds: 15–4 kcal mol–1, “weak” Hꢀbonds: < 4 kcal mol–1).29 The balance
between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the BCPs reveals
the nature of these interactions: if the ratio –G(r)/V(r) > 1 is satisfied, then the nature of the
appropriate interaction is purely nonꢀcovalent, while in case where –G(r)/V(r) < 1 some covalent
interaction also takes place.30 Based on this criterion one can state that covalent contribution is
absent in the B–H•••π(Ph) contacts in (Z)ꢀ(Ph3PCH2Ph)[3g], and as can be inferred from inspection
of calculated NBO atomic charges, all C atoms in the Ph moiety involved in the nonꢀcovalent
interactions B–H•••π(Ph) in (Z)ꢀ(Ph3PCH2Ph)[3g] are negatively charged (from –0.14 to –0.20),
whereas the appropriate H centers are slightly positively (0.03) charged. However, it is of note that
recently Hobza et al.31 considered the nature of the nonꢀcovalent interactions in different B–
H•••π(Ph) motifs and shown that it is not only electrostatically attractive nonꢀclassical hydrogen
bonds, but rather nonꢀspecific weak dispersionꢀdriven contacts.
The nonꢀcovalent interactions B–H•••π(Ph) determine the configuration of (Z)ꢀ
(Ph3PCH2Ph)[3g]. Indeed, we performed geometry optimization procedure in the gas phase for
other isomer of (Z)ꢀ(Ph3PCH2Ph)[3g] without nonꢀcovalent interactions B–H•••π(Ph) and found that
this isomer is less stable (by 8.6 kcal mol–1 in terms of Gibbs free energies) (Table 4S, Supporting
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Information). It may thus be concluded that the contribution of these weak interactions to the
stabilization of (Z)ꢀ(Ph3PCH2Ph)[3g] compared to isomerization is only 30–33%. The discussion
about the theoretical calculations of relative energies for different isomers of this and other reported
compounds in acetonitrile solution are given in the Supporting Information (Tables 1S–3S).
Determination of Plausible Mechanism of Generation of (Ph3PCH2Ph)[3a–g].
(i) Kinetics. The kinetics of the nucleophilic addition of a range of nucleophiles (2a–c and 2e–g;
Scheme 1) to the nitrile moiety of the boron cluster was investigated next. Amidrazone 2d
gradually decomposed under the reaction conditions to give 3,6ꢀdiphenylꢀ1,2,4,5ꢀtetrazine and no
satisfactory kinetic data could thus be obtained for this species.32 Figure 3 shows a typical profile
of the reaction progress as monitored by UV/Vis timeꢀresolved spectrophotometry. The absorbance
vs. time data was fitted to a single exponential as described previously,33 yielding clean reactions.
Figure 3. UV/Vis spectral changes of absorbance vs. time for the addition of nucleophile 2e to
(Ph3PCH2Ph)[1] (MeCN; 25 °C). Insert indicates fit of Abs.vs time data to a firstꢀorder exponential
at 360 nm.33 ꢁt = 0.75 min and 2 min, ttotal = 30 min. [2e] = 2.5×10–4 M,
(Ph3PCH2Ph)[1] = 5×10–5 M.
To our surprise, a clear secondꢀorder [Nu(H)] dependence of the observed pseudo firstꢀorder rate
constants was obtained, which at first glance might seem questionable considering the final
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products (1:1 ratio of Nu(H) to the boron cluster) as isolated (see Experimental Section and
Figure 1). However, as illustrated in Figures 4, 5 and 7, the experimental kinetic data clearly
confirms the secondꢀorder dependence for all six the entering nucleophiles 2a–c and 2e–g. Visually
this is manifested best by the logarithmic plots shown in Figure 4, which all yielded slopes equal to
2 within experimental error, see also Table 2.
Figure 4. Plot of kobs vs. entering log [Nu(H)] for the addition reaction of 2a–c and 2e–g to
(Ph3PCH2Ph)[1] at 25 °C in MeCN, yielding linear plots with slopes ca. 2;
(Ph3PCH2Ph)[1] = 5×10–5 M (λ = 330 nm).
Since similar behaviour has been reported in literature in distant related systems,17d, e we pursued
the arguments further in more detail. Thus, where Figure 4 clearly supports the secondꢀorder
logarithmic dependence, Figure 5 even more so, wherein the pseudo firstꢀorder kinetic data for all
the entering nucleophiles were subjected to nonꢀlinear leastꢀsquares fits of the complete appropriate
rate equation(s) as defined in Scheme 3 (a and b) below, and clearly describe the exponential
secondꢀorder increase in kobs with increasing [2].
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Figure 5. Plots of kobs vs. [Nu(H)] for the nucleophilic addition of 2a–c and 2e–g to
(Ph3PCH2Ph)[1] at 25 °C in MeCN. Insert indicates slower nucleophiles (2b and 2g; also both
exhibiting clear 2nd order behavior with respect to [Nu(H)]). Data are given in Tables 5S and 6S.
(ii) Reaction Mechanism. We identified two likely scenarios for the kinetic behaviour and
the results obtained are therefore interpreted in cognisance of secondꢀorder [Nu(H)] behaviour as
presented in Scheme 3 (a and b) below, respectively.
(a)
(1a)
(2a)
where RDS denotes the rateꢀdetermining step.
This yields the following Rate Law for the formation of 3 (with BNu(H) a steady state):
d[3]/dt=k2[BNu(H)][Nu*(H*)] – kꢀ2[BNu(H*)][Nu*(H)] (3a)
(b)
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(1b)
(2b)
(3b)
This yields the following Rate Law for the formation of 2′ (with [Nu(H)]2 a steady state):
d[2’]/dt=k2K1[B][Nu(H)]2 – kr[B[Nu(H)]2]
(4b)
Scheme 3. Reaction schemes for nucleophilic addition of 2a–c and 2e–g to (Ph3PCH2Ph)[1],
denoted by ‘Nu(H)’ and ‘B’, respectively. (a) Assuming a direct B–Nu attack (Eq. 1a) as a first
rateꢀdetermining step followed by a fast proton exchange (Eq. 2a). To distinguish between the first
Nu(H) involved in Eq. 1a, and the second nucleophile in Eq. 2a, asterisked nucleophiles are used.
However, note that in all cases Nu(H) is identical to Nu*(H*). (b) Assuming a fast, bimolecular
equilibriumꢀformation of a dinuclear [Nu(H)]2 entity in Eq. 1b, which reacts in a rateꢀdetermining
step with B (Eq. 2b), followed by a rapid loss of the extra Nu(H) to yield the final product BNu(H)
(3) (Eq. 3b). In both Scheme 3 (a) and (b) the reverse rate constant kr is indicated but is not
interpreted further.
As indicated in Scheme 3, two potential mechanisms were identified for the kinetics
observed. The observed pseudo firstꢀorder rate constants for (a) and (b) are given by Eq. 4a and
Eq. 5b, respectively, derived from basic kinetic principles and assuming [Nu(H)] >> [B].
Thus, firstly, as described in Scheme 3 (a), a B–Nu(H) interaction (Eq. 1a) as a first rateꢀ
determining step is assumed, followed by a subsequent fast proton exchange (Eq. 2a). The pseudo
firstꢀorder rate constant for this process is given by Eq. 4a. Motivation for this mechanism stems
from analogous behavior of the oximes,12a as well as the assumed principle that the nucleophiles are
expected to proceed via a direct attack at the boron cluster.
Under conditions where [Nu(H)] >> [B], the pseudo firstꢀorder rate constant is given by
Eq. 4a after incorporating [BNu(H)] as a steady state:
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kaobs = (k1KD[Nu(H)]2)/(1 + KD[Nu(H)]) + kr
(4a)
In Eq. 4a, KD =k2/kꢀ1 = discrimination factor, i.e., the ability of the boron cluster to discriminate
between different entering nucleophiles, and kr = reverse rate constant for the overall process.
Secondly, in Scheme 3 (b) we consider a fast, bimolecular equilibriumꢀformation of a
dinuclear [Nu(H)]2 entity in Eq. 1b, which reacts in a subsequent rateꢀdetermining step with B
(Eq. 2b), followed by a rapid loss of the extra Nu(H) to yield the final product BNu(H) (Eq. 3b).
The pseudo firstꢀorder rate constant for this process is given by Eq. 5b. Although one may envisage
that the formal attack of [Nu(H)]2 should be potentially sterically inhibited in the dinuclear form,
some evidence to its existence comes from the infrared spectra as presented in the Supporting
Information, Figure 54S.
Under conditions where [Nu(H)] >> [B]
d[B]/[B]dt = kbobs = kf[Nu(H)]2 + kr
(5b)
In Eq. 5b, kf =k2K1 = forward thirdꢀorder rate constant, and kr = reverse rate constant for the overall
process.
As illustrated in Figures 4 and 5, both rate laws as simplified in Eqs. 4a and 5b describe the
kinetics observed very well. In particular the logarithmic plots of both kaobs and kbobs vs. log[Nu(H)]
in Figure 4 which yield clear linear relationships with slopes close to 2 for all six nucleophiles 2a–
c, 2e–f, see Table 2, convincingly support a 2nd order in [Nu(H)]. Similarly, Figure 5 illustrates the
plots of kaobs and kbobs vs. [Nu(H)] and the nonꢀlinear leastꢀsquares fits of the data to Eqs. 4a and 5b,
wherein good secondꢀorder fits for all six nucleophiles 2a–c, 2e–f, were obtained.
It is important to note that in the current study, the lack of solubility of the entering
nucleophiles did not allow KD to be separated from k1 [i.e., in Eq. 4a KD[Nu(H)] << 1], effectively
yielding Eq. 5b shown above. An attempt to estimate a value for KD is nevertheless illustrated in
Figure 56S (see Supporting Information) by visually modelling the shape34 and thus acceptable
curves where values of KD of 1–50 M–1 gave reasonable fits for nucleophile 2e. However, for the
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purpose of this study, and more importantly, to allow direct comparison between the two
mechanisms as illustrated in Scheme 3 (a and b) a KD = 1 M–1 was utilised in all the fits involving
Eq. 4a.
By performing experiments wherein the concentration of (Ph3PCH2Ph)[1] is in excess with
respect to the entering nucleophile, we obtained the predicted firstꢀorder dependence on [B], see
Figure 6 (a and b). This was already illustrated in (and required from) both Eq. 3a and 4b, which
yields Eq. 6ab following leastꢀsquares analysis and incorporation of the secondꢀorder dependence
of [Nu(H)]. Thus, under conditions where [B] >> [Nu(H)], both mechanisms in Scheme 3 (a and b)
and their rate laws in both Eq. 3a and 4b simplifies to
d[Nu(H)]/[Nu(H)]dt = kabobs = kf’[B] + kr
(6ab)
where kf’ = kfX = k1KDX (Eq. 4a) or kf = k2K1X (Eq. 5b), and X = [Nu], to yield the
corresponding thirdꢀorder rate constants.
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Figure 6. Plots of kobs vs. [B] in MeCN for the nucleophilic addition of (a) 2c, 2e and 2f to
(Ph3PCH2Ph)[1] at 25 °C; (b) 2e to (Ph3PCH2Ph)[1] at different temperatures.
[Nu(H)] = 2.5×10ꢀ4 M.
In the case where [B] >> [Nu(H)], the pseudo firstꢀorder rate constants described by
Eq. 6ab yield reasonably similar forward thirdꢀorder rate constants to those obtained when
monitoring the reactions with [Nu(H)] >> [B] (and utilising Eqs. 4a and 5b), see Table 2,
considering the differences in experiments and solubility problems experience. This leads to the
conclusion that both reaction schemes [Scheme 3 (a and b)] and the corresponding rate laws
appropriately describe the overall reaction. We take this as strong supporting evidence that we
selected appropriate reaction mechanisms in Scheme 3.
A clear nonꢀzero small parallel/reverse reaction (intercept of linear dependence kabobs vs. [B],
see Figure 6 (a and b)) was obtained from this study. Although not as conclusive, the exponential
data wherein [Nu(H)] >> [B], as illustrated in Figure 5, could not verify this, but also not exclude
this since the standard deviations obtained do not allow accurate interpretation, see Table 7S. This
aspect is not considered further in this report since it does not really influence any conclusions
made.
We consider Scheme 3 (a) the more likely mechanism since (i) the nucleophilic attack
seems the more likely reaction to be critically dependent on the electron density at the N atom and
(ii) intermolecular proton transfer and proton transfer reactions in general, are usually very fast.
For this latter process, Eq. 4a may be derived and clearly describes the kinetics observed,
yielding the appropriate rate constants as presented in Tables 2 and 3. This leaves us with a
mechanistic interpretation typical of many squareꢀplanar substitution reactions wherein a
‘discriminating factor’, i.e., the preference of the proton transfer (k2) over the dissociation of the
entering nucleophile (kꢀ1) has been considered.35 As indicated above, however, KD (=k2/kꢀ1) is
directly associated with k1 and could not be separated therefrom (kf = k1KD) with all the data on
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hand. Although one expects a variation in KD, all relative trends therein are interpreted in
combination with k1, and therefore with respect to kf.
(iii) Variation of Entering Nucleophiles. The nucleophiles 2a–c and 2e–g as varied exhibited
a significant range of almost five ordersꢀofꢀmagnitude in forward rate constants as manifested by kf,
with 2c being the more reactive nucleophile and 2g the least reactive (see Figures 4 and 5, and
Table 2). Nucleophiles 2g and 2b appear to have low reactivity, with hydrazine intermediate, and
the three electronꢀrich systems, 2e, 2f and, in particular, 2c the most reactive.
Table 2. Rate constants for the nucleophilic addition of 2a–c and 2e–g to (Ph3PCH2Ph)[1] in MeCN
at 25 °C; determined from leastꢀsquares fits of the observed pseudo firstꢀorder rate constants to
Eqs. 4a and 5b, and Eq. 6ab, which follows from (i.e., Fig. 5) as well as the logarithmic
dependences as illustrated in Fig. 4
d)
kf
kr
(sꢀ1)
Nu(H)
Slopes from
log plots a)
Eq. 4a
(k1KD)b)
(Mꢀ2sꢀ1)
Eq. 5b
Eq. 6ab
(=k2K1) d)
(Mꢀ2sꢀ1)
ꢀꢀ
(=k2K1)c)
(Mꢀ2sꢀ1)
52±4
1.8±0.1
1.8±0.1
2.1±0.1
2.0±0.2
2.0±0.1
1.9±0.1
52±4
ꢀꢀ
ꢀꢀ
2a
2b
2c
2e
2f
0.21±0.04
4794±81
1394±18
1285±35
0.06±0.03
0.19±0.02
4785±81
1388±18
1281±36
0.06±0.02
ꢀꢀ
2200±300 0.0023± 0.0003
800±80
440±80
ꢀꢀ
0.0006±0.0001
0.0005±0.0001
ꢀꢀ
2g
a) From slopes in Fig. 4.b) Eq. 4a; Data in Fig. 5. c) Eq. 5b; Data in Fig. 5. d) Eq. 6ab; Data in Fig. 6
However, it must again be noted that the forward rate constants kf as reported in Tables 2–4
represent potential combinations of (i) rate (k1) and discrimination constant (KD) for the proposed
mechanism in Scheme 3 (a), [kf=k1KD] or on the other hand (ii) rate k2 and equilibrium constant for
dinuclear [Nu(H)]2 in the mechanism in Scheme 3 (b) [kf=k2K1]. Relative positions in the order are
thus dependent on both these contributions and may not necessary be clearly defined. Nevertheless,
it is noted that the forward rate constants kf for the nucleophilic attack are in general agreement with
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the nucleophiles’ expected coordinating and accepted electron donating capabilities, and indeed,
spanning a significant five ordersꢀofꢀmagnitude range of reactivities.
(iv) Activation parameters. Figure 7 illustrates the temperature dependence of the observed
pseudo firstꢀorder rate constants for the nucleophilic addition of 2e to (Ph3PCH2Ph)[1]. The lines
represent the leastꢀsquares fitted values for the experimental data points to a global model of the
exponential form of the Eyring equation, as reported previously.35 This yields a very low
ꢀH≠ = (6.9±0.8) kJ⋅mol–1and large negative ꢀS≠ = (–195±3) J⋅K–1⋅mol–1, see Table 3.
Values for the activation parameters for 2c and 2f as entering nucleophiles were also
obtained and are given in Table 3 (for experimental fits, see Supporting information; Table 8S).
Figure 7. Global fit of the observed pseudo firstꢀorder rate constants (at different temperatures) to
the exponential form of the Eyring equation for the addition of 2e to (Ph3PCH2Ph)[1] in MeCN.
Table 3. Data from leastꢀsquares fit of the observed pseudo firstꢀorder rate constants for the
nucleophilic addition of 2c, 2e, and 2f to (Ph3PCH2Ph)[1] in MeCN.
Temp
(°C)
2c
Fig. 6S
Eq. 4aa)
ꢀꢀ
2e
2f
Constant
Fig. 5 Eq. Fig. 5 Eq. Fig. 6S Eq.
4aa)
5bb)
4aa)
1037±28
ꢀꢀ
865±33
621±33
ꢀꢀ
35
30
25
15
5
1698±29 1654±52
1543±33 1512±53
ꢀꢀ
kf (M–2 s–1)
4785±81 1417±29 1388±18
ꢀꢀ
ꢀꢀ
1300±29 1255±52
1138±29 1119±52
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ꢀH≠(kJ mol–1)
ꢀꢀ
ꢀꢀ
14±2 a)
6.9±0.8a) 6.1±0.7b)
15±1 a)
–162±6a) –195±3a) –164±3b) –139±4 a)
ꢀS≠ (J K–1 mol–1)
a) Data from global fits of kobs vs. [2c], [2e] and [2f] to Eq. 4a at different temperatures. b) Data from global fits of kobs
vs. [2e] to Eq. 5b at different temperatures.
The activation parameters for the nucleophilic addition of 2e to (Ph3PCH2Ph)[1] yield as
indicated the very low ꢀH≠ = (6.9±0.6) kJ mol–1 and large negative ꢀS≠ = (–195±3) J K–1 mol–1, and
indicate that the reaction is primarily entropy controlled, with ꢀS≠ contributing 87% to the Gibbs
free energy of activation at 25 °C, see Table 4. The values for the activation parameters for 2c and
2f as entering nucleophiles are comparable to that of 2e, with ꢀS≠ contributing some 73% of the
Gibbs free energy of activation at 25 °C in both. This is in general agreement with the literature
values on related reactions,33 and indicate smaller contributions of bond formation to the total
Gibbs’ energy of activation in these systems.
Table 4. Data from leastꢀsquares fits of the observed pseudo firstꢀorder rate constants [via a global
fit33] for the nucleophilic addition of 2c, 2e and 2f to (Ph3PCH2Ph)[1] in MeCN at 25°C.
Nucleophile
2c
2e
2f
865±33
15±1 b)
–139±4 b)
56±2
kf (M–2 s–1)
4785±81
1388±18
14±2 a,b)
6.9±0.8 b)
ꢀH≠ (kJ mol–1)
ꢀS≠ (J K–1 mol–1)
ꢀG≠ 298 (kJ mol–1)
ꢀS≠ fraction of
ꢀG≠ (%)
–162±6 a,b) –195±3 b)
52±4
73
65±1
89
73
a)ꢀH≠ = 21±3 kJ⋅mol–1, ꢀS≠ = (ꢀ121±11 J⋅K–1⋅mol–1) from Fig. 5(b); b) Data from individual fits of kobs vs. [2e] to Eq. 3
at different temperatures; see Table 3.
Conclusions
The nucleophilic addition to the C≡N group of the 2ꢀpropanenitrilium closoꢀdecaborate
cluster of three types of N(H)ꢀnucleophiles, viz. hydrazine, acetyl hydrazide, and a set of
hydrazones giving Nꢀclosoꢀdecaborato amidrazones has been studied. In the Xꢀray structure of (Z)ꢀ
(Ph3PCH2Ph)[3g], very rare intramolecular nonꢀcovalent interactions B–H•••π(Ph) were detected
and studied by DFT calculations (M06ꢀ2X/6ꢀ311++G** level of theory) and topological analysis of
the electron density distribution within the framework of Bader’s theory (QTAIM method).
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Estimated strength of these nonꢀcovalent interactions is 0.8–1.4 kcal/mol. A systematic mechanistic
study of the nucleophilic addition was undertaken and included detailed synthetic, crystallographic,
computational and kinetic work. As a result, a nucleophilicity series for the N(H)ꢀnucleophiles, with
a range of structural types, was obtained. A clear secondꢀorder dependence on the entering
nucleophiles has been obtained for all six the nucleophiles, yielding overall thirdꢀorder kinetics and
the reactivities ranged over five ordersꢀofꢀmagnitude.
Based upon the experimental data, two possible mechanisms have been proposed. The first
possibility includes a consecutive incorporation of two Nu(H) nucleophiles, with the second step
responsible for a subsequent rapid proton exchange. The second possible mechanism assumes a preꢀ
formation of a dinuclear [Nu(H)]2 species, which subsequently proceeds with the nucleophilic
attack on the boron cluster. We consider that the first mechanism is more plausible since (i) the
nucleophilic attack seems the more likely reaction to be critically dependent on the electron density
at the N atom and (ii) intermolecular proton transfer, and proton transfer reactions in general, are
usually very fast. The activation parameters for amidrazones employed as N(H)ꢀnucleophiles
indicate a small dependence on bond formation, but significantly negative entropies of activation
with the latter contributing ca. 70–80% of the total Gibbs free energy of activation, ꢀG≠.
Thus, this work provides a broad series of nucleophilicity of Nu(H)ꢀnucleophiles with wideꢀ
ranging variation of their structural types (Scheme 1). Although the obtained series follow general
principles of physical organic chemistry, the quantitative data are certainly helpful for planning
physical chemistry experiments. Moreover, metalꢀfree and metalꢀinvolving nucleophilic additions
of N(H)ꢀnucleophiles comprise a field of intensive studies, and we believe that our nucleophilicity
series will be useful to control various nucleophilic additions of Nꢀdonors. Further development of
kinetics and reaction mechanisms of addition of other nucleophiles to 2ꢀnitrilium closoꢀdecaborates
is underway in our group.
Experimental Section
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i
Materials and Instrumentation. Solvents MeCN, Et2O, CH2Cl2, PrOH and 1 M hydrazine
solutions in MeCN and in THF were obtained from commercial sources and used as received.
Benzyltriphenylphosphonium 2ꢀnitriliumꢀclosoꢀdecaborate, (Ph3PCH2Ph)[1],36 acetyl hydrazide
2b,37 and benzophenone hydrazone 2g38 were synthesized according to the literature methods.
Melting points were measured on a Stuart SMP30 apparatus in capillaries and are not corrected.
Elemental analysis for boron was performed by the FSUE IREA 291 Center (Moscow) on an aiCAP
6300 Duo ICP spectrometer using H3BO3 as an internal standard. Electrospray ionization mass
spectra were obtained on a Bruker micrOTOF spectrometer equipped with an electrospray
ionization (ESI) source. The instrument was operated both in negative and in positive ion modes
using an m/z range of 50–3000. The nebulizer gas flow was 0.4 bar, and the drying gas flow was
4.0 L/min. For ESI, the clusters were dissolved in MeOH. In the isotopic pattern, the most intensive
peak is reported. Molar conductivities of 7×10–4 M solutions in MeCN were measured on a Mettler
Toledo FE30 conductometer using an Inlab710 sensor. Infrared spectra (3600–500 cm–1) were
1
recorded on a Shimadzu IR Prestigeꢀ21 instrument in KBr pellets. H{11B} and 11B{1H} NMR
spectra were measured on a Bruker Avance 400 spectrometer in CD3CN at ambient temperature;
residual solvent signals were used as the internal standard for 1H{11B} and 13C{1H} NMR, whereas
BF3•Et2O was used as the external standard for 11B{1H} NMR.
Xꢀray Structure Determination. A singleꢀcrystal Xꢀray diffraction experiment was carried
out
using
Agilent
Technologies
«Xcalibur»
((Z)ꢀ(Ph3PCH2Ph)[3b]•MeCN,
(Ph3PCH2Ph)[3d]•MeCN, and (Z)ꢀ(Ph3PCH2Ph)[3g]•MeCN) and «SuperNova» ((E)ꢀ
(Ph3PCH2Ph)[3a]•MeCN, (Z)ꢀ(Ph3PCH2Ph)[3c], (E)ꢀ(Ph3PCH2Ph)[3e], and (E)ꢀ(Ph3PCH2Ph)[3f])
diffractometers with monochromated MoKα or CuKα radiation, respectively. The studied crystals
were kept at 100(2) K during data collection. The structures had been solved by the Superflip39
structure solution program using Charge Flipping and refined by means of the ShelXL40 program,
incorporated in the OLEX2 program package.41 Empirical absorption correction was applied in
CrysAlisPro42 program complex using spherical harmonics, implemented in SCALE3 ABSPACK
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scaling algorithm. CCDC numbers 1563793–1563799 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.
Kinetic Equipment and Procedures. Exploratory UV/visible measurements were
performed on a Varian Cary 50 Conc UV–visible spectrophotometer with thermostated automated
multicell changers (10 cells virtually simultaneously monitored) equipped with a JulaboF12ꢀmV
temperature cell regulator (accurate within 0.1 °C) in 1.000±0.001 cm quartz tandem cuvette cells.
Figure 3 shows a typical timeꢀresolved reaction progress monitored by UV/vis. Reactions were also
1
monitored by H NMR where appropriate and the rates and products obtained where in agreement
with the UV/vis measurements.
Kinetic Data Treatment. Stability tests were conducted over several days on a UV–vis
spectrometer for (Ph3PCH2Ph)[1] and nucleophiles 2a–c; 2e–g to ensure no decomposition
occurred. However, amidrazone 2d gradually decomposed to give 3,6ꢀdiphenylꢀ1,2,4,5ꢀtetrazine.32
Because of the good solubility of (Ph3PCH2Ph)[1] only in CH2Cl2 and MeCN among most common
organic solvents, all the reactions were performed in MeCN, which is a more convenient solvent for
variable temperature experiments.
Computational Details. The full geometry optimization and single point calculations based
on the experimental Xꢀray geometries have been carried out at the DFT level of theory using the
M06ꢀ2X functional (this functional parameterized for mainꢀgroup elements and specifically
developed to describe weak dispersion forces and nonꢀcovalent interactions)43 with the help of
Gaussianꢀ09 program package.44 The standard 6ꢀ311++G** basis sets were used for all atoms. No
symmetry restrictions have been applied during the geometry optimization. The Hessian matrix was
calculated analytically for optimized structures in order to prove the location of correct minima (no
imaginary frequencies), and to estimate the thermodynamic parameters, the latter being calculated
at 25 °C. The solvent effects were taken into account using the SMD continuum solvation model by
Truhlar and coworkers45 with acetonitrile as solvent. The topological analysis of the electron
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density distribution with the help of the atoms in molecules (QTAIM) method developed by
Bader24has been performed by using the Multiwfn program (version 3.3.7).46 The atomic charges
were computed by using the natural bond orbital (NBO) partitioning scheme.47 The Cartesian
atomic coordinates for all model species are presented in Table 4S (Supporting Information).
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the…
AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail: d.s.bolotin@spbu.ru (D.S.B.)
*Eꢀmail: roodta@ufs.ac.za (A.R.)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
D.S.B. is much obliged to the Russian Foundation for Basic Research for financial support for the
synthetic part of the work (project 16ꢀ03ꢀ00573). The kinetic part of this work was conducted under
Saint Petersburg State University Project 12.37.214.2016. A.S.N. is grateful to the Russian
Foundation for Basic Research for supporting of the theoretical part of the work (project 16ꢀ33ꢀ
60063) and thankful to the Saint Petersburg State University and Santander Bank for the
opportunity to conduct a scientific internship at the Instituto Superior Técnico, Universidade de
Lisboa (Lisbon, Portugal). A.R. and P.P.M. acknowledge the University of the Free State, SASOL,
and the South African National Research Foundation for financial support. Physicochemical studies
were performed at the Center for Magnetic Resonance, Center for Xꢀray Diffraction Studies, and
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Center for Chemical Analysis and Materials Research (all belong to Saint Petersburg State
University).
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TABLE OF CONTENTS
Three types of N(H)ꢀnucleophiles were used to study the nucleophilic addition to the C≡N group of
the 2ꢀpropanenitrilium closoꢀdecaborate cluster giving Nꢀclosoꢀdecaborato amidrazones. A
nucleophilicity series for the N(H)ꢀnucleophiles, with a range of structural types, has been obtained.
The activation parameters for hydrazones indicate a small dependence on bond formation, but
significantly negative entropies of activation with the latter contributing some 70–80% of the total
Gibbs free energy of activation, ꢀG≠. In the Xꢀray structure of (Z)ꢀ(Ph3PCH2Ph)[3g], very rare
intramolecular nonꢀcovalent interactions B–H•••π(Ph) were detected and studied theoretically.
28