Electron transfer in complexes of BII cations with organic π-acceptors: A combined experimental and quantum-chemical study

Article abstract of DOI:10.1039/c9dt03151k

Due to their combined Lewis acidity and electron-donor capability, B^II cations exhibit an interesting reactivity, which is almost unexplored so far. In this work, we compare the reduction in a dicationic diborane of a series of vicinal diones with different redox potentials, namely 3,5-di-tert-butylbenzoquinone, 3,4,5,6-tetrachlorobenzoquinone, 1,2-naphthalene-dione, 9,10-phenanthrene-dione, 2,2′-dichlorobenzil, benzil and 1,2-acenaphthylene-dione. The experimental work is complemented by quantum-chemical calculations, illuminating the electron-transfer step in the reactions.

Full text of DOI:10.1039/c9dt03151k

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Volume 46
Number 10
14 March 2017
Pages 3073-3412
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Electron transfer in complexes of BII cations with organic -
DOI: 10.1039/C9DT03151K
acceptors: A combined experimental and quantum-chemical study
Daniel Vogler, Nina Wolf, Elisabeth Kaifer, Hans-Jörg Himmel *
Abstract. Due to their combined Lewis acidity and electron-donor capability, BII
cations exhibit an interesting reactivity, which is almost unexplored up to date. In this
work, we compare the reduction at a dicationic diborane of a series of vicinal diones
with different redox potentials, namely 3,5-di-tert-butylbenzoquinone, 3,4,5,6-
tetrachlorobenzoquinone, 1,2-naphthalene-dione, 9,10-phenanthrene-dione, 2,2’-
dichlorobenzil, benzil and 1,2-acenaphthylene-dione. The experimental work is
complemented by quantum-chemical calculations, illuminating the electron-transfer
step in the reactions.
Introduction
Diborane(4) compounds are versatile reagents, that are widely applied in borylation
and diboration reactions, providing convenient access to valuable intermediates in
various functionalization and coupling reactions.[1] Diborane(4) compounds with two
sp2-hybridized boron atoms (sp2-sp2 hybridized diboranes(4)) are generally
electrophilic despite of their low boron oxidation number. In the last years several
strategies were developed to modify the electrophilicity and also to turn diborane(4)
compounds into nucleophiles.[2] For example, Yamashita et al. reported the synthesis
and reactivity of highly-electrophilic tetra(o-tolyl)diborane(4) [3] and the unsymmetrical
diborane (4) pinB-BMes2 (pin = pinacolato, Mes = 2,4,6-Me3C6H2).[4] Due to the high
electrophilicity, tetra(o-tolyl)diborane(4) could even be reduced with metallic lithium or
magnesium to the dianion, with double-bonded boron atoms.[5] A few dianionic sp3-
sp3-hybridized diboranes with boron-boron single bonds are also known.[6-11]
Braunschweig et al. recently reported an optimized synthetic access route to the
*
D. Vogler, N. Wolf, Dr. E. Kaifer, Prof. H.-J. Himmel
Anorganisch-Chemisches Institut, Ruprecht-Karls-University of Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
1

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tetrahalodiboranes(4) B2X4 (X = halogen) with graded electrophilicityD.[O1I2:]10.A103s9/Ct9oDT03151K
electrophilic diboranes(4), preactivated, nucleophilic neutral and anionic sp2-sp3-
hybridized diboranes were obtained by addition of a carbene, an alkoxide or fluoride
to a diborane(4) such as B2(pin)2.[13-15] Mahon et al. used the diboranate, resulting
from reaction between a ß-diketiminato magnesium n-butyl derivative with B2(pin)2,
as a source of the nucleophilic [B(pin)] in a number of reactions,[16] including the
synthesis of unsymmetrical diboranes by diborane metathesis.[17]
Our group has synthesized an especially electron-rich neutral diborane(4) derivative
[HB(hpp)]2 (1), featuring two bridging 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-
a]pyrimidinate (hpp) guanidinate substituents.[18] The nucleophilicity of this compound
enables catalyst-free hydroboration of CO2 and diboration of nitriles.[19] Moreover it
allows the preparation of several unprecedented cationic oligoboranes.[20,21] Reaction
of 1 with two equivalents of methyltriflate (MeOTf) yields the diborane [TfOB(hpp)]2
(2),[22] that was shown to be a source for {[B(hpp)]2}2+ with two directly connected BII
cations. Double triflate abstraction from 2 with AlCl3 or GaCl3 gave salts of the
unprecedented, fluorescent tetracation {[B4(hpp)4}4+,[23] exhibiting a (4c,4e) bond
between the four boron atoms in the rhomboid core.
N
4+
N
B
N
B
1/2
abstraction of the triflato
substituents by a Lewis
acid
N
N
N
N
B
N
B
N
N
N
> 2eq. MCl3
- 2 OTf
TfO
OTf
B
B
N
N
N
N
N
N
L
L
- 2 OTf
2+
N
L
L
2
B
B
substitution of the triflato
substituents by a Lewis
base
N
N
N
N
N
N
Scheme 1. Different reactions of diborane 2 with Lewis acids and Lewis bases.
2

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Reaction of 2 with neutral -Lewis bases (that are -donors or mild -accepVtoiewrsAr)ticle Online
DOI: 10.1039/C9DT03151K
affords a variety of dicationic diboranes and also a tetracation with two diborane units
(Figure 1).[24] In these complexes, the B-B bond length slightly decreases with
increasing -acceptor character.
-donor substrates

4+
2+
N
N
N
N
N
N
N
N
N
N
N
N
Me2N
Me2N
NMe2
N
N
N
N
N
N
B
B
B
B
N
N
N
N
NMe2
B
B
N
N
N
N
N
N
N
N
N
N
-acceptor substrates

decrease of the B-B bond length
N
N
N
2+
2+
2+
N
N
N
N
N
B
B
B
B
B
B
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Figure 1. Fully characterized cationic boron compounds (all with triflate couterions)
from previous work.
A continuative reactivity is observed if the -acceptor character is further increased.
Hence reaction of 2 with the substrate 3,5-di-tert-butyl-o-quinone (S1), being a -
Lewis base and a relatively strong -electron acceptor, leads not only to
complexation, but also to double electron-transfer from the B-B bond to S1 to give the
corresponding catecholate complex (Scheme 2).[24] The cleavage of the B-B bond
due to BII → BIII oxidation initiates a redistribution process, restoring the electron
octet at each boron atom to give the final product 3(OTf) (Scheme 2).
3

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2 OTf
2+
2 OTf View Article Online
1) complexation
DOI: 10.1039/C9DT03151K
t-Bu
t-Bu
2+
t-Bu
2) electron
transfer
t-Bu
t-Bu
O
+
O
B
O
O
B
O
t-Bu
O
2
B
B
N
N
N
N
N
N
N
N
N
N
N
N
t-Bu
+
t-Bu
O
B
O
3) redistribution
OTf
TfO
B
N
N
N
N
N
N
3
(OTf)
Scheme 2. Reaction sequence leading to 3(OTf), comprising 1) complexation of a -
donor/-acceptor, 2) electron transfer from the diborane to the -acceptor and 3)
redistribution to restore the coordination number of four at each boron atom.
Herein we systematically vary the -acceptor strength (the redox-potential) of the
dione substrate to evaluate the scope of this reactivity. The analysis of the electron-
transfer step requires the application of substrates from one class of compounds,
which bind in a similar way to the diboron reagent but differ in their redox-potential.
To this end, the reactions of the ditriflato-diborane 2 with seven different dione
substrates S1 – S7 (see Lewis structures in Figure 2) were compared. The redox
potentials of these substrates differ considerably. Hence the E1/2 values (vs. Fc+/Fc)
increase in the row benzil (S6, -1.51 V) < 1,2-acenaphthylene-dione (S7, -1.26 V) <
9,10-phenanthrene-quinone (S4, -1.04 V) < 1,2-naphthalene-dione (S3, -0.96) < 3,5-
di-tert-butyl-o-benzoquinone (S1, -0.92 V) < 3,4,5,6-tetrachloro-o-benzoquinone (S2, -
0.29 V).[25] The experimental work is complemented by detailed quantum-chemical
calculations, providing useful information about the mechanism of the electron-
transfer step (concerted two-electron transfer versus two consecutive one-electron
transfer steps).
4

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DOI: 10.1039/C9DT03151K
Cl
Cl
O
Cl
Cl
O
O
O
O
O
S1
S2
S3
O
Cl
Cl
O
O
O
O
S4
S5
O
O
O
S7
S6
Figure 2. Lewis structures of the seven diones relevant for this study: 3,5-Ditertbutyl-
benzoquinone (S1), 2,3,4,5-tetrachloro-benzoquinone (S2), 1,2-naphthalene-dione
(S3), 9,10-phenanthrene-dione (S4), 2,2’-dichlorobenzil (S5), benzil (S6) and 1,2-
acenaphthylene-dione (S7).
Results and Discussion
Experimental work. First we reacted diborane 2 with 1,2-naphthalene-dione (S3).
The two reactants were mixed at low temperature (-40 °C) in CH2Cl2 solution and the
reaction mixture then allowed to warm up to room temperature and stirred for 1 h.
The product 4(OTf) crystallized at -20 °C in a yield of 51% after partial solvent
removal (Scheme 3). The experimental solid state structure is illustrated in Figure 3a.
A triflato group is bound to one of the boron atoms. Due to the bridging position of
one of the oxygen atoms of reduced S3, each boron atom is bound to four atoms. The
boron atoms are separated by 2.443 Å, indicating that the B-B bond (c.f. 1.708(4) Å
[
in 2 22] and 1.772(3) Å in 1) [26] is completely cleaved. The inlet in Figure 3a
highlights the large tilt of the naphthalene plane with respect to the O1-B1-O2 plane,
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leading to a B1-O1-O2-C16 torsion angle of 150.26°. This conforDmOI:a10ti.1o0n39/Ci9sDT03151K
presumably important to maintain a low barrier for electron transfer between the B-B
bond and the organic -system in the complex. The structure is preserved in solution.
Hence the 11B NMR spectrum shows two boron signals at  = 6.5 and 2.4 ppm.
Using simple DFT (BP86/def2-SV(P)) calculations, the chemical shifts of the two
boron atoms were calculated to be  = 6.1 (OBO) and 1.4 (OBOSO2CF3) ppm, in
pleasing agreement with the experiment. Two signals, at  = -77.0 ppm for bound
19
triflate and -78.94 ppm for free triflate, are also present in the F NMR spectrum. In
the UV-Vis spectra, the broad band around 400 nm due to S3 disappeared and new
bands in the UV region appeared. Hence the results clearly show that S3 is
complexed and reduced by the diborane.
+
OTf
O
O
S
3, CH2Cl2
OTf
B
B
-40 °C - r.t., 1 h
N
N
N
N
51%
N
N
TfO
OTf
B
N
B
N
N
N
N
4
(OTf)
N
+
OTf
2
S
4, CH2Cl2
O
O
OTf
-40 °C - r.t., 1 h
B
B
52%
N
N
N
N
N
N
5
(OTf)
Scheme 3. Reaction of the diborane(4) [(TfO)B(hpp)]2 (2) with the substrates S3 and
S4 to give the new compounds 4(OTf) and 5(OTf).
Next, the reaction was repeated with 9,10-phenanthrene-quinone (S4) in place for S3.
Reaction in CH2Cl2 led quantitatively to 5(OTf) (Scheme 3), isolated by crystallization
from a concentrated CH2Cl2 solution in a yield of 52%. The structure of 5(OTf) is
depicted in Figure 3b. The tilt of the phenanthrene plane leads to a B1-O1-O2-C16
torsion angle of 146.75°. Again, the structure is preserved in solution. Hence in the
6

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11B NMR spectrum in CD2Cl2 solution, two signals of equal intensity appeared aVtiew A=rticle Online
DOI: 10.1039/C9DT03151K
6.6 and 3.1 ppm, fitting to the predicted values of  = 6.5 (OBO) and 2.3
(OBOSO2CF3) ppm from DFT (BP86/def2-SV(P)) calculations. Moreover, two signals
( = -77.03 and -79.00 ppm) show in the 19F NMR spectrum in line with the structure
in the solid state. Finally, the comparison between the UV-Vis spectra of 5(OTf) and
9,10-phenanthrene-dione (S4) confirms the reduction to the catechol. Hence the
broad band around 350 nm in the spectrum of S4 vanished and a new band at 307
nm appeared (see SI).
a)
b)
Figure 3. Illustration of the structures of the monocations 4 (a) and 5 (b) in the salts
4(OTf)·CH2Cl2 and 5(OTf)·CH2Cl2. Hydrogen atoms omitted, displacement ellipsoids
drawn at the 50% probability level. Selected bond distances (in Å) for 4: B1···B2
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2.443, B1-O1 1.453(1), B1-O2 1.555(2), B2-O2 1.512(2), B2-O3 1.493(2), O1-C15
1.367(2), O2-C16 1.416(2). Selected bond distances (in Å) for 5: B1···B2 D2O.I4: 130.410,39B/C19D-T03151K
O1 1.442(4), B1-O2 1.572(3), B2-O2 1.531(4), B2-O3 1.495(3), O1-C15 1.367(3),
O2-C16 1.447(3). The inlets show the structures along the B···B axis to highlight the
tilt of the naphthalene/phenanthrene plane with respect to the O1-B1-O2 plane.
As expected from its high redox potential, reaction between 2 and 3,4,5,6-tetrachloro-
11
o-benzoquinone (S2) also led to substrate reduction. In the B NMR spectrum two
signals at  = 7.2 and 2.8 ppm appeared, close to the signals at  = 5.6 and 2.1 ppm
reported for 3(OTf),[24]  = 6.5 and 2.4 ppm for 4(OTf) and  = 6.6 and 3.1 ppm for
5(OTf). Using DFT (BP86/def2-SV(P)) calculations, the chemical shifts of the two
boron atoms were calculated to be  = 6.3 (OBO) and 1.9 (OBOSO2CF3) ppm. The
19F NMR spectrum showed two signals at  = -77.10 (bound triflate) and -78.97 ppm
(free triflate). Moreover, the broad band at  = 456 nm in the UV/Vis spectrum of
3,4,5,6-tetrachloro-o-benzoquinone was replaced by a strong absorption in the UV
region at 298 nm, indicating reduction to the catechol (see SI). Hence all results
confirm a reaction similar to those with S1, S3 and S4, to give the product 6(OTf)
(Scheme 4). However, in difference to the other products, 6(OTf) slowly decomposed
in CH2Cl2 solutions (see SI). In an attempt to crystallize the decomposition product,
the solid was washed with n-pentane, dissolved in CH2Cl2, filtrated and stored for
several days at a temperature -20 °C. A few crystals precipitated in this period of time,
consisting of the decomposition product (hppH2)[B(3,4,5,6-tetrachloro-o-catecholato)2]
(Scheme 4, illustration of the crystal structure in the SI). The boranate ion of this salt
11
gives rise to a sharp intense signal at  = 14.1 ppm [27] in the B NMR spectra of
solutions of 6(OTf) stored for 4 weeks at -20 °C, indicating its formation in larger
quantities. Hence the strongly coordinating 3,4,5,6-tetrachloro-o-catecholato
substituents replaced the hpp units, that captured two protons from the solvent or
from traces of water. The presence of this decomposition product is fully consistent
with oxidation of the boron atoms and reduction of the 3,4,5,6-tetrachloro-o-
benzoquinone in the course of the reaction.
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
+
OTf
Cl
Cl
O
DOI: 10.1039/C9DT03151K
Cl
Cl
Cl
Cl
Cl
Cl
Cl
CH2Cl2
-40 °C - r.t.,
1 h
CH2Cl2
+
H
N
H
N
O
4 weeks,
-20 °C
O
O
O
TfO
B
B
B
2
S2
+
O
N
N
N
N
48%
N
N
Cl
N
Cl
+ other products
Cl
6
(OTf)
Scheme 4. Synthesis of 6(OTf), slowly decomposing in CH2Cl2 solution to give
(among other unidentified products) the salt [hppH2][B(S2)2] with reduced substrate
units.
Next we reacted compound 2 with 2,2'-dichlorobenzil (S5) in CH2Cl2 solution. The 11B
NMR spectrum of the reaction mixture showed two signals of equal intensity at  =
5.4 and 2.2 ppm. Moreover, in the 19F NMR spectrum two signals at  = -76.96
(bound triflate) and -78.97 ppm (free triflate) appeared. These results resemble those
obtained for the reactions of compound 2 with S1-S4. Assuming an analogue
structure for the product (denoted 7(OTf)), the chemical shifts of the two boron atoms
were calculated (BP86/def2-SV(P)) to be  = 5.1 and 0.7 ppm (see SI for more
information), in good agreement with the experimental results. Hence all results
indicate that 7(OTf) exhibits a structure similar to 4(OTf), 5(OTf) and 6(OTf).
11
Unfortunately, it was not possible to isolate a clean reaction product. The B NMR
spectra of all products are compared in Figure 4, showing that in all cases similar
products are formed.
In further experiments, we tested the possibility of eliminating the diborane unit from
the reduced substrate. The three isolated products 3(OTf), 4(OTf) and 5(OTf) were
dissolved in methanol and stirred for three days at 50 °C. Both NMR (see SI) and
mass spectrometric data indicate for each reaction the formation of the free catechol
(S1+2H, S3+2H and S4+2H). However, the NMR spectra also indicate that elimination
of the diborane unit is accompanied by its decomposition.
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DOI: 10.1039/C9DT03151K
Figure 4. 11B NMR spectra (128 MHz, CD2Cl2) for the diborane reactant 2 (black) and
the products 3(OTf) (red), 4(OTf) (blue), 5(OTf) (green), 6(OTf) (magenta) and the
product of the reaction of 2 with S5 (brown), 7(OTf).
Reaction of 2 with S6 gave no clear results. Although the 11B NMR spectrum in
CD2Cl2 solution displayed a new signal at  = 2.2 ppm, the signal of 2 in the 19F NMR
spectrum remained unchanged. Unfortunately it was not possible to identify the boron
11
species responsible for the new signal in the B NMR spectrum. After a few days a
small amount of crystals were isolated from the reaction mixture, that turned out to be
+
the condensation product of 1,2-diphenyl-1,2-ethenediol with hppH2 , crystallizing
together with triflate as counter-ion (see SI for a structural characterization). Although
the isolated crystals do not contain the direct product of addition of compound 2 to S6,
the structure of the crystallized salt still indicates reduction of the benzil during the
reaction. Finally, reaction of 2 with S7 also led to no clear results. Although the
11
change of the color of the solution from orange to red indicates a reaction, the B
NMR spectrum shows no specific signals. On the other hand, several signals
19
appeared in the F NMR spectrum, indicating degradation of the triflate anions (see
SI). This result points to the formation of reactive radical intermediates (see
discussion under consideration of the theoretical results below). Unfortunately an
identification of the products was not possible.
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Quantum-chemical calculations. The experiments indicate that electrDoOnI:-1t0r.a10n39s/fCe9DrT03151K
takes place for a variety of vicinal dione substrates with different redox-potentials,
demonstrating the wide scope of this reactivity. Quantum-chemical calculations were
carried out to gain further information on the electron-transfer step. In these
calculations we concentrated on the reaction sequence depicted in Scheme 5. The
B3LYP functional in combination with the def2-TZVP basis set was chosen for these
calculations, since this method has shown previously to provide reliable results for
guanidinate-bridged diborane molecules.[18c] First the structures of the products of
complexation and reduction of the substrates S1-S7 by the dication [B2(hpp)2]2+
(denoted Red(S1)-Red(S7) in the following) were calculated. Illustrations of the
structures of Red(S1)-Red(S4) can be found in the SI, and pictures of the structures
of Red(S5)-Red(S7) are included in Figures 5-7 (vide infra). Selected structural
parameters for all products are compiled in a Table in the SI. The large separations
of the two boron atoms (2.357 – 2.417 Å) indicate in all cases cleavage of the B-B
bond. Moreover, the other parameters signal reduction of the vicinal diones to the
corresponding catecholato forms.
2+
2+
2+
N
O
B
O
B
N
N
O
B
O
B
S1-S4
B
N
B
N
N
N
N
N
N
N
N
N
N
N
N
N
N
[B2(hpp)2]2+
-
Ox(S ) Ox(S )
Red(S ) Red(S )
-
1
7
1
7
Scheme 5. Reaction sequence analysed in the quantum-chemical calculations.
In Table 1, the E, H (0 K) and G (298 K) values calculated for the reactions
between [B2(hpp)2]2+ and Sn (n = 1 - 7) to give the products Red(Sn) are compared.
All reactions are exothermic and exergonic. Interestingly, the reaction with S1 (G = -
258 kJ mol1) is more exergonic than that with S2 (G = -230 kJ mol1), although S2
clearly exhibits the higher redox potential (E1/2 = -0.29 V vs. Fc+/Fc for S2 and -0.92 V
for S1). Furthermore, similar G values were calculated for the reactions with S6 (-
132 kJ mol1) and S7 (-138 kJ mol1), although S6 exhibits a significantly lower redox-
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potential (E1/2 = -1.51 V vs. Fc+/Fc for S6 and -1.26 V for S7). ObviouslyD,OtIh: 1e0.10lo39w/Ce9DrT03151K
Lewis basicity of the reduced catecholates diminishes the energy of complexation to
the boron atoms, leading in the sum to a less exergonic reaction. Formation of
Red(S6) is associated with the smallest, but still considerable negative Gibbs free
energy change (G = -132 kJ mol1).
Table 1. Reaction energies E, enthalpies H at 0 K and Gibbs free energies G at
298 K (all in kJ mol1) calculated with B3LYP/def2-TZVP for the reactions between
[B2(hpp)2]2+ and the seven dione substrates S1-S7 to give the complexed and reduced
compounds Red(S1) - Red(S7).
Reaction
E
H (0 K) G (298 K)
[B2(hpp)2]2+ + S1 → Red(S1) -331 -322
[B2(hpp)2]2+ + S2 → Red(S2) -303 -295
[B2(hpp)2]2+ + S3 → Red(S3) -296 -288
[B2(hpp)2]2+ + S4 → Red(S4) -274 -267
[B2(hpp)2]2+ + S5 → Red(S5) -245 -235
[B2(hpp)2]2+ + S6 → Red(S6) -203 -196
[B2(hpp)2]2+ + S7 → Red(S7) -208 -200
-258
-230
-224
-201
-178
-132
-138
In the case of the three substrates S5 – S7 (but not for S1 – S4), the calculations found
a second minimum on the potential energy hypersurface, corresponding to the
intermediates Ox(S5) - Ox(S7) prior to electron transfer. These intermediates exhibit a
higher energy than the products of two-electron transfer Red(S5) - Red(S7). The
energy difference decreases in the order E = 108 kJ mol1 for S5, 67 kJ mol1 for S6
and only 42 kJ mol1 for S7. For these three substrates, we also tried to calculate the
products of one-electron transfer, being biradicals with open-shell singlet or triplet
states. Broken-symmetry calculations were not successful, as they converged to the
structures Red(S5) - Red(S7). On the other hand, the lowest energy triplet states,
denoted T(S5) - T(S7) in the following, were successfully calculated. The calculated
structures are illustrated in Figures 5 - 7. The trends in the calculated bond distances
(see the numbers in the Tables integrated in Figures 5 – 7) are fully consistent with
the interpretation of the triplet structures as being the products of one-electron
transfer, with one unpaired electron in the B-B bond orbital (1e2c bond) and another
unpaired electron on the vicinal dione unit. For example, the distance between the
12

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two boron atoms increases from 1.656 Å in Ox(S5) to 1.865 Å in T(S5) and finally to
DOI: 10.1039/C9DT03151K
2.360 Å in Red(S5). For comparison, Wagner et al. reported an increase of the B-B
distance from the dianion of bis(9-borafluorenyl)methane (1.906(3) Å for a 2e2c bond)
to the monoanion (2.166(4) Å for a 1e2c bond) and finally to the neutral compound
(2.534(2) Å, no direct bond).[7] Interestingly, the energies of the triplet states T(Sn) are
for all three substrates close to those of the Ox(Sn) states.
bond
Ox(S5)
1.658
1.643
1.643
1.249
1.249
1.553
Red(S5)
2.360
1.346
1.346
1.401
1.400
1.342
T(S5)
1.865
1.455
1.455
1.320
1.320
1.431
B1-B2
B1-O1
B2-O2
O1-C1
O2-C2
C1-C2
Figure 5. Structures and relative energies of the three minima Red(S5), Ox(S5) and
T(S5). Selected structural parameters (in Å) for the minima on the closed-shell singlet
and triplet potential energy hypersurfaces are compiled in the inserted table.
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DOI: 10.1039/C9DT03151K
bond
Ox(S6)
1.660
1.624
1.627
1.256
1.256
1.544
Red(S6)
2.357
1.342
1.342
1.410
1.410
1.353
T(S6)
B1-B2
B1-O1
B2-O2
O1-C1
O2-C2
C1-C2
1.865
1.446
1.446
1.329
1.329
1.431
Figure 6. Structures and relative energies of the three minima Red(S6), Ox(S6) and
T(S6). Selected structural parameters (in Å) for the minima on the closed-shell singlet
and triplet potential energy hypersurfaces are compiled in the inserted table.
bond
Ox(S7)
1.669
1.637
1.637
1.240
1.240
1.578
Red(S7)
2.417
1.354
1.354
1.385
1.385
1.374
T(S7)
1.892
1.462
1.462
1.311
1.311
1.452
B1-B2
B1-O1
B2-O2
O1-C1
O2-C2
C1-C2
Figure 7. Structures and relative energies of the three minima Red(S7), Ox(S7) and
T(S7). Selected structural parameters (in Å) for the minima on the closed-shell singlet
and triplet potential energy hypersurfaces are compiled in the inserted table.
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The results indicate that the B-B distance could serve as adequatDeOI:r1e0.1a0c39t/iCo9nDT03151K
coordinate for a further evaluation of the electron-transfer step. We therefore
optimized the energies of the compounds with fixed B-B distances for several B-B
distances in the range 1.5 – 2.6 Å. In the calculations carried out for S4 (see SI), the
energy decreased until it reached a single minimum at a distance of 2.340 Å. The
calculated structure at the minimum complies with the fully-optimized structure
Red(S4), proving complete barrier-less two-electron transfer. There existed no further
local energy minimum corresponding to the intermediate Ox(S4), but the calculations
found a shallow potential energy curve in the area around a B-B distance of 1.7 Å
(see SI). In the case of the three substrates S5 – S7, the potential energy scan found
the two minima that correspond to Red(S4) and Ox(S4), and also provided estimates
for the barriers for two-electron transfer along the singlet potential energy curves. As
example, the energy relative to the Red(S7) global-energy minimum is plotted in
Figure 8a as a function of the B-B distance for the complex with S7. One can see that
a barrier of ca. 29 kJ mol1 separates the two minima Ox(S7) and Red(S7) on the
singlet potential-energy curve. At the barrier, the B-B bond is ca. 1.95 Å long, being
close to the equilibrium distance of 1.892 Å calculated for the triplet state, T(S7).
Similar calculations for the substrates S5 and S6 lead to lower barriers of ca. 22 kJ
mol1 for Ox(S6) → Red(S6) and ca. 9 kJ mol1 for Ox(S5) → Red(S5). Hence the
barrier height decreases with increasing absolute energy difference between the
isomers with oxidized (Ox(Sn)) and reduced (Red(Sn)) substrate, and also with
increasing G values for the electron transfer step. Such a relationship is expected
from the Marcus theory, although this theory is based on the assumption of an outer-
sphere mechanism, that clearly is not fulfilled here. To further elaborate on this point,
we plotted the square-root of the activation energy Ea as a function of the reaction
energy. Although the significance of such a plot is reduced due to the small number
of data points, the almost perfect linear fit (see Figure 8b) indicates that the two-
electron transfer step in these compounds complies with the behavior expected from
Marcus theory. Obviously, it is not meaningful to derive a parameter  from this plot.
For the substrates S6 and S7, the barrier for simultaneous two-electron transfer along
the singlet potential energy curve is already quite high. In such cases a stepwise one-
electron transfer process should be preferred, since the triplet state is significantly
lower in energy in the region of this barrier. Presumably, an open-shell singlet state
15

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lies close to this triplet state and allows the transfer of an electron withoutDcOIh: 1a0n.10g3e9/Co9DfT03151K
spin multiplicity. This process produces radical intermediates, provoking side
reactions. This might explain the different results of the experiments in which S6 and
S7 were applied. Especially the degradation of the triflate counterions in the reaction
with S7 indicates the formation of reactive radical intermediates in solution.
Conclusions
In this work, the ditriflato-substituted diborane [TfOB(hpp)]2 with bridging guanidinate
(hpp) substituents (compound 2) was applied as a source for the dicationic diborane
[B2(hpp)2]2+, being both a Lewis acid and an electron donor. The reactions with a
series of vicinal diones with different redox potentials demonstrate the ability of this
dicationic diborane to complex organic substrates in the first step and reduce them in
a second electron-transfer step. The two-electron reduction of the organic substrate
is accompanied by loss of the boron-boron bond (BII → BIII). Quantum-chemical
calculations were then applied to gain more insight into the electron-transfer step. For
three substrates it was possible to locate two minima on the singlet potential-energy
hypersurface corresponding to the complexed substrate before and after two-electron
transfer. Moreover, the calculations show that the decrease of the energy difference
between these two minima leads to an increase of the activation barrier for
simultaneous two-electron transfer. The relationship between thermodynamics and
kinetics of this simultaneous two-electron transfer complies with the expectations
from Marcus theory (although not obeying an outer-sphere mechanism). Additional
calculations show that the lowest-energy triplet state, corresponding to the product of
one-electron transfer, exhibits an energy similar to that of the diborane-substrate
complex before electron-transfer. This indicates that the presence of a significant
barrier for the simultaneous two-electron transfer along the singlet potential energy
curve leads to a change of the electron-transfer process to a pathway with stepwise
transfer of the two electrons, explaining the experimental findings. Ongoing work in
our laboratory aims at the isolation of redox-isomers and the synthesis of open-shell
biradicals in which one unpaired electron is in the -system of the organic substrate
and the other in the B-B bond orbital. The results of this work build a solid fundament
for research in these directions.
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DOI: 10.1039/C9DT03151K
a)
b)
Ea (Ea)0.5
Reaction
E
Ox(S5)→Red(S5) -107.86 9.02 3.00
Ox(S6)→Red(S6) -67.45 21.64 4.65
Ox(S7)→Red(S7) -42.46 29.34 5.42
Figure 8. a) Calculated relative energy as a function of the B-B distance for the
diborane complex with substrate S7. b) Marcus-type plot of the square-root of the
activation energy, (Ea)0.5, as a function of the energy difference E. The inserted
table summarizes the estimated energy differences and activation energies (Ea). All
values are given in kJ mol1.
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DOI: 10.1039/C9DT03151K
Experimental Details
All reactions were carried out under a dry Ar atmosphere by using standard Schlenk
technique. The reagents 1,2-naphthalene-dione 9,10-phenanthrene-dione and
3,4,5,6-tetrachlorobenzoquinone were purchased from Sigma–Aldrich. The reagents
benzil, 2,2’-dichlorobenzil and 1,2-acenaphthylene-dione were purchased from Fluka,
Alfa Aesar and abcr. All reagents were used without further purification. The
compound [(TfO)B(hpp)]2 was synthesized according to the literature.[22] All solvents
were rigorously dried by the solvent purification system MB SPS-800 MBRAUN and
stored over molecular sieves (4 Å) after being degassed by the freeze-pump-thaw
method. Infrared spectra were recorded as KBr discs or as powder with a BIORAD
Excalibur FTS 3000 and a Bruker Alpha Platinum-ATR. BRUKER Avance DPX 200,
BRUKER Avance II 400 and BRUKER Avance III 600 devices were used for NMR
spectroscopy. NMR spectra were recorded at 298 K and 11B NMR chemical shifts are
given in ppm relative to BF3·Et2O. Elemental analyses were performed at the
Microanalytical Laboratory of the University of Heidelberg using the vario EL and
vario MICRO cube devices from Elementar Analysensysteme GmbH. Mass spectra
were obtained with JOEL JMS-700 magnetic sector and BRUKER ApexQe hybrid 9.4
T FT-ICR spectrometer at the MS laboratory of the University of Heidelberg. UV-Vis
spectra were recorded at 298 K using a Varian Cary 5000 UV/Vis NIR spectrometer.
4(OTf)
In a dry argon-flushed Schlenk flask [(TfO)B(hpp)]2 (2, 50 mg, 0.08 mmol) was
dissolved in 1.0 mL of absolute dichloromethane. After cooling to ‒40 °C with a
cooling bath a solution of 1,2-naphthalene-dione (S3, 13 mg, 0.08 mmol) in 1.0 mL of
absolute dichloromethane was added. The cooling bath was removed, and the
reaction mixture stirred for 1 h at room temperature. The volume of the reaction
mixture was reduced to an eighth of its original value and the solution stored at ‒20
°C. Colorless crystals precipitated after a few days. The mother liquor was removed
with a syringe and the crystalline solid dried in vacuo to yield the product in a yield of
32 mg (0.04 mmol, 51%). C,H,N analysis (%) for C26H30B2F6N6O8S2·CH2Cl2 (754.29
1
g mol-1): calcd. C 38.64, H 3.84, N: 10.01; found C 38.05, H 3.93, N 10.06. H NMR
(400 MHz, CD2Cl2): δ = 8.07 (d, 1 H, 3J = 8 Hz, Harom), 7.88 (d, 1 H, 3J = 8 Hz, Harom),
18

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7.58–7.49 (m, 3 H, Harom), 7.40 (d, 1 H, 3J = 8 Hz, Harom), 3.57–3.10 (m, 16DHO,I: 1N0.−10C39H/C92D),T03151K
2.13–1.96 (m, 6 H, CH2), 1.79–1.70 (m, 2 H, CH2) ppm. 11B NMR (128 MHz, CD2Cl2):
13
δ = 6.52 (s, 1 B), 2.35 (s, 1 B, BOSO2CF3) ppm. C NMR (100 MHz, CD2Cl2): δ =
154.3 (Cq, hpp), 144.8 (Carom−O), 137.9 (Carom−O), 133.0 (Carom), 128.7 (Carom), 127.2
(Carom), 127.0 (Carom), 123.5 (Carom), 121.4 (Carom), 121.4 ( SO3CF3), 117.4 (Carom),
112.6 ( Carom), 48.4 (NCH2), 48.2 (NCH2), 40.5 (NCH2), 40.0 (NCH2), 21.5 (CH2) ppm.
-
19F NMR (376 MHz, CD2Cl2): δ = −77.00 (s, BOSO2CF3), −78.94 (s, free SO3CF3 )
ppm. MS (HR-ESI+): m/z (%) = 429.19 ({[hppH]2[SO3CF3]}+, 100%). UV-Vis (CH2Cl2,
c = 7.34·10-6 mol L-1): λ (ε in L mol-1 cm-1) = 230 (8.45·104), 285 (7.97·103), 328
(4.73·103) nm. IR (powder): ν
̃ = 2961(w), 2879(w), 1630(m), 1595(m), 1542(m),
1460(w), 1446(w) 1388(m), 1322(m), 1219(m), 1150(s), 1114(s), 1071(m), 1042(m),
1026(s), 1005(m), 953(s), 875(w), 799(m), 774(w), 760(w), 740(w), 698(w), 675(w),
635(s), 623(s), 601(m), 573(m), 514(m) cm-1. Crystal data for 4(OTf)·CH2Cl2,
C27H32B2Cl2F6N6O8S2, Mr = 839.27, 0.541 x 0.466 x 0.336 mm, triclinic, space group
P‒1, a = 12.366(4), b = 13.352(5), c = 13.473(3) Å, α = 101.35°, β = 108.56°, γ =
117.56°, V = 1705.8(12) Å3, Z = 2, ρcald = 1.634 Mg·m-3, Mo-Kα radiation (λ = 0.71073
Å), T = 120 K, θrange = 2.410 - 29.990°. Reflections collected 9838, independent
reflections 8515, Rint = 0.0408. Final R indices [I > 2σ(I)]: R = 0.0471, wR = 0.1180.
5(OTf)
In a dry argon-flushed Schlenk flask [(TfO)B(hpp)]2 (2, 50 mg, 0.08 mmol) was
dissolved in 1.0 mL of absolute dichloromethane. After cooling to ‒40 °C with a
cooling bath a solution of 9,10-phenanthrene-dione (S4, 17 mg, 0.08 mmol) in 1.0 mL
of absolute dichloromethane was added. The cooling bath was removed, and the
reaction mixture stirred for 1 h at room temperature. The volume of the reaction
mixture was reduced to an eighth of its original value and the solution stored at
‒20 °C. Colorless crystals precipitated after a few days. The mother liquor was
removed with a syringe and the crystalline solid dried in vacuo to yield the product in
a
yield of 35 mg (0.04 mmol, 52%). C,H,N analysis (%) for
C30H32B2F6N6O8S2·1.5 CH2Cl2 (804.35 g mol-1): calcd. C 40.61, H 3.79, N: 9.02;
1
found C 40.44, H 3.86, N 9.44. H NMR (400 MHz, CD2Cl2): δ = 8.81−8.72 (m, 2 H,
Harom), 8.26−8.24 (m, 2 H, Harom), 7.79−7.60 (m, 4 H, Harom), 3.54–3.20 (m, 16 H,
N−CH2), 2.06–1.97 (m, 6 H, CH2), 1.78–1.66 (m, 2 H, CH2) ppm. 11B NMR (128 MHz,
CD2Cl2): δ = 6.58 (s, 1 B), 3.07 (s, 1 B, BOSO2CF3) ppm. 13C NMR (100 MHz,
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CD2Cl2): δ = 151.5 (Cq, hpp), 139.7 (Carom−O), 128.3 (Carom−O), 128.0 (Carom), 127.9
DOI: 10.1039/C9DT03151K
(Carom), 127.8 (Carom), 127.3 (Carom), 127.6 (Carom), 126.1 (Carom), 125.2 (Carom), 124.1
(Carom), 123.9 (Carom), 123.4 (Carom), 122.9 (Carom), 122.3 (Carom), 120.6 ( SO3CF3),
47.3 (NCH2), 38.5 (NCH2), 21.0 (CH2) ppm. 19F NMR (376 MHz, CD2Cl2): δ = −77.03
-
(s, BOSO2CF3), −79.00 (s, free SO3CF3 ) ppm. MS (HR-ESI+): m/z (%) = 429.19
({[hppH]2[SO3CF3]}+, 2%). UV-Vis (CH2Cl2, c = 2.09·10-5 mol L-1): λ (ε in L mol-1 cm-1)
= 257 (3.53·104), 307 (7.36·103), 345 (9.91·102), 361 (1.01·103) nm. IR (KBr disk): ν
̃
= 2957(w), 2882(w), 1633(m), 1601(m), 1552(m), 1449(m), 1393(m), 1357(m),
1325(s), 1271(s), 1224(s), 1152(s), 1118(m), 1078(m), 1062(m), 1030(s), 977(m),
947(m), 803(w), 757(m), 724(m), 669(w), 638(s), 600(w), 573(w), 517(m) cm-1.
Crystal data for 5(OTf)·CH2Cl2, C31H34B2Cl2F6N6O8S2, Mr = 889.28, 0.40 x 0.40 x
0.40 mm, triclinic, space group P ‒1, a = 11.882(2), b = 12.192(2), c = 14.297(3) Å, α
= 83.12°, β = 77.39°, γ = 75.38°, V = 1951.2(8) Å3, Z = 2, ρcald = 1.514 Mg·m-3, Mo-Kα
radiation (λ = 0.71073 Å), T = 120 K, θrange = 1.730 - 28.000°. Reflections collected
9387, independent reflections 6947, Rint = 0.0420. Final R indices [I > 2σ(I)]: R =
0.0807, wR = 0.1906.
6(OTf)
In a dry argon-flushed Schlenk flask [(TfO)B(hpp)]2 (2, 50 mg, 0.08 mmol) was
dissolved in 1.0 mL of absolute dichloromethane. After cooling to ‒40 °C with a
cooling bath a solution of 3,4,5,6-tetrachloro-o-benzoquinone (S2, 19 mg, 0.08 mmol)
in 0.5 mL of absolute dichloromethane was added. The cooling bath was removed,
and the reaction mixture stirred for 1 h at room temperature. The solvent was
removed and the solid was dried in vacuo to yield 36 mg of the product (0.04 mmol,
48%). C,H,N analysis (%) for C22H24B2Cl4F6N6O8S2 (842.00 g mol-1): calcd. C 31.38,
1
H 2.87, N: 9.98; found C 30.53, H 3.62, N 9.83. H NMR (400 MHz, CD2Cl2): δ =
3.44–3.28 (m, 16 H, N−CH2), 2.09–1.88 (m, 8 H, CH2) ppm. 11B NMR (128 MHz,
CD2Cl2): δ = 7.22 (s, 1 B), 2.86 (s, 1 B, BOSO2CF3) ppm. 13C NMR (100 MHz,
CD2Cl2): δ = 154.0 (Cq, hpp), 141.4 (Carom−O), 139.4 (Carom−O), 137.1 (Carom), 129.1
(Carom), 125.7 (Carom), 122.0 (SO3CF3), 118.5 (Carom), 48.2 (NCH2), 48.0 (NCH2), 40.6
19
(NCH2), 39.9 (NCH2), 21.4 (CH2), 20.9 (CH2) ppm. F NMR (376 MHz, CD2Cl2): δ =
−
−77.10 (s, BOSO2CF3), −78.97 (s, free SO3CF3 ) ppm. MS (HR-ESI+): m/z (%) =
429.19 ({[hppH]2[OTf]}+, 100%). UV-Vis (CH2Cl2, c = 7.36·10-5 mol L-1): λ (ε in L mol-1
cm-1) = 228 (7.85·103), 298 (1.37·103) nm. IR (KBr disk): ν
̃ = 2954(w), 2885(w),
20

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1629(s), 1604(m), 1548(m), 1457(m), 1408(m), 1388(m), 1325(s), 1265(s), 1224(s),
DOI: 10.1039/C9DT03151K
1153(m), 1114(m), 1070(m), 1031(s), 1005(m), 952(m), 810(w), 797(w), 678(w),
638(s), 605(w), 574(w), 517(m) cm-1.
Details of the XRD studies. Suitable crystals for single-crystal structure
determination were taken directly from the mother liquor, taken up in perfluorinated
polyether oil and fixed on a cryo loop. Full shells of intensity data were collected at
low temperature with a Nonius Kappa CCD diffractometer (MoK radiation, sealed X-
ray tube, graphite monochromator) and a Bruker D8 Venture, dual source (Mo- or
Cu-K radiation, microfocus X-ray tube, Photon III Detector). Data were processed
[28]
[29]
with the standard Nonius
and
APEX3
software. Multiscan absorption
correction was applied using the SADABS program.[30] The structures were solved by
intrinsic phasing and refined using the SHELXTL software package (Version 2014/6
and 2018/3).[31] Graphical handling of the structural data during solution and
refinement was performed with XPMA and OLEX2.[32] All nonhydrogen atoms were
given anisotropic displacement parameters. Hydrogen atoms were generally set at
calculated positions and refined with a riding model. CCDC 1944030 (4(OTf)),
1944027 (5(OTf)), 1944028 (C20H16BCl10N3O4), and 1944029 (C22H24F3N3O4S)
contain the supplementary crystallographic data for this paper.
Details of the quantum-chemical calculations. DFT calculations were performed
[33,34,35]
with the TURBOMOLE
or ORCA 4.4.1 [36] program packages. The B3LYP
the def2-TZVP basis set.[39] For
functional [37,38] was used in combination with
the calculation of the 11B NMR shifts the BP86 functional [40] was used in combination
with the def2-SV(P) basis set.[41] The structures were visualized with the Diamond 3.0
software.[42]
Conflicts of interest
There are no conflicts of interest to declare.
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DOI: 10.1039/C9DT03151K
Acknowledgements
The authors gratefully acknowledge financial support by the Deutsche
Forschungsgemeinschaft (DFG).
[1] E. C. Neeve, S. J. Geier, I. A. I. Mkhalid, S. A. Westcott, T. B. Marder, Chem.
Rev., 2016, 116, 9091–9161.
[2] H.-J. Himmel, Eur. J. Inorg. Chem., 2018, 2139–2154.
[3] a) N. Tsukahara, H. Asakawa, K.-H. Lee, Z. Lin, M. Yamashita, J. Am. Chem.
Soc., 2017, 139, 2593–2596; b) Y. Katsuma, N. Tsukahara, L. Wu, Z. Lin, M.
Yamashita, Angew. Chem., 2018, 130, 6217–6222; Angew. Chem. Int. Ed.,
2018, 57, 6109–6114; c) Y. Katsuma, L. Wu, Z. Lin, S. Akiyama, M. Yamashita,
Angew. Chem., 2019, 131, 323-327; Angew. Chem. Int. Ed., 2019, 58, 317–321.
[4] a) H. Asakawa, K.-H. Lee, Z. Lin, M. Yamashita, Nat. Commun., 2014, 5, 4245–
4253; b) H. Asakawa, K.-H. Lee, K. Furukawa, Z. Lin, M. Yamashita, Chem. Eur.
J., 2015, 21, 4267–4271; c) Y. Katsuma, H. Asakawa, K.-H. Lee, Z. Lin, M.
Yamashita, Organometallics, 2016, 35, 2563–2566; d) Y. Katsuma, H. Asakawa,
M. Yamashita, Chem. Sci., 2018, 9, 1301–1310.
[5] S. Akiyama, K. Yamada, M. Yamashita, Angew. Chem. Int. Ed., 2019, 58, in
press (DOI: 10.1002/anie.201907400).
[6] J. Landmann, J. A. P. Sprenger, M. Hailmann, V. Bernhardt-Pitchougina, H.
Willner, N. Ignat’ev, E. Bernhardt, M. Finze, Angew. Chem., 2015, 127, 11411–
11416; Angew. Chem. Int. Ed., 2015, 54, 11259–11264.
[7] A. Hübner, T. Kaese, M. Diefenbach, B. Endeward, M. Bolte, H.-W. Lerner, M. C.
Holthausen, M. Wagner, J. Am. Chem. Soc., 2015, 137, 3705–3714.
[8] W. J. Grigsby, P. P. Power, J. Am. Chem. Soc., 1996, 118, 7981–7988.
[9] Y. Shoji, T. Matsuo, D. Hashizume, M. J. Gutmann, H. Fueno, K. Tanaka, K.
Tamao, J. Am. Chem. Soc., 2011, 133, 11058–11061.
[10] W. Preetz, B. Steuer, Z. Naturforsch. B: J. Chem. Sci., 1996, 51, 551–556.
[11] G. Bélanger-Chabot, H. Braunschweig, Angew. Chem. Int. Ed. 2019, in press
(DOI: 10.1002/anie.201906666).
[12] M. Arrowsmith, J. Böhnke, H. Braunschweig, A. Deißenberger, R. D. Dewhurst,
W. C. Ewing, C. Hörl, J. Mies, J. H. Muessig, Chem. Commun., 2017, 53, 8265–
8267.
22

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[13] R. D. Dewhurst, E. C. Neeve, H. Braunschweig, T. B. Marder, Chem.DCOIo: 1m0.1m039u/Cn9.D,T03151K
2015, 51, 9594–9607.
[14] a) W. Clegg, C. Dai, F. J. Lawlor, T. B. Marder, P. Nguyen, N. C. Norman, N. L.
Pickett, W. P. Power, A. J. Scott, J. Chem. Soc., Dalton Trans., 1997, 839–846; b)
S. Pietsch, U. Paul, I. A Cade, M. J. Ingleson, U. Radius, T. B. Marder, Chem. Eur.
J., 2015, 21, 9018–9021; c) M. Eck, S. Wuertemberger-Pietsch, A. Eichhorn, J. H.
J. Berthel, R. Bertermann, U. S. D. Paul, H. Schneider, A. Friedrich, C. Kleeberg,
U. Radius, T. B. Marder, Dalton Trans., 2017, 46, 3661–3680.
[15] S. Pietsch, E. C. Neeve, D. C. Apperley, R. Bertermann, F. Mo, D. Qiu, M. S.
Cheung, L. Dang, J. Wang, U. Radius, Z. Lin, C. Kleeberg, T. B. Marder, Chem.
Eur. J., 2015, 21, 7082–7099.
[16] a) A.-F. Pécharman, A. L. Colebatch, M. S. Hill, C. L. McMullin, M. F. Mahon, C.
Weetman, Nat. Commun., 2017, 8, 15022–15028; b) A.-F. Pécharman, M. S.
Hill, M. F. Mahon, Dalton Trans., 2018, 47, 7300–7305; c) A.-F. Pécharman, M.
S. Hill, C. L. McMullin, M. F. Mahon, Organometallics, 2018, 37, 4457–4464; d)
A.-F. Pécharman, M. S. Hill, G. McMullon, C. L. McMullin, M. F. Mahon, Chem.
Sci., 2019, 10, 6672–6682.
[17] A.-F. Pécharman, M. S. Hill, M. F. Mahon, Angew. Chem., 2018, 130, 10848–
10851; Angew. Chem. Int. Ed., 2018, 57, 10688–10691.
[18] a) O. Ciobanu, E. Kaifer, M. Enders, H.-J. Himmel, Angew. Chem., 2009, 121,
5646-5649; Angew. Chem. Int. Ed., 2009, 48, 5538–5541; b) N. Schulenberg, O.
Ciobanu, E. Kaifer, H. Wadepohl, H.-J. Himmel, Eur. J. Inorg. Chem., 2010,
5201–5210; c) A. Wagner, S. Litters, J. Elias, E. Kaifer, H.-J. Himmel, Chem.
Eur. J., 2014, 20, 12514–12527.
[19] a) M. Frick, E. Kaifer, H.-J. Himmel, Angew. Chem., 2017, 129, 11804–11807;
Angew. Chem. Int. Ed., 2017, 56, 11645–11648; b) M. Frick, J. Horn, H.
Wadepohl, E. Kaifer, H.-J. Himmel, Chem. Eur. J., 2018, 24, 16983–16986.
[20] a) S. Litters, M. Enders, E. Kaifer, H.-J. Himmel, Nat. Chem., 2013, 5, 1029–
1034; b) S. Litters, E. Kaifer, H.-J. Himmel, Angew. Chem., 2016, 128, 4417–
4420; Angew. Chem. Int. Ed., 2016, 55, 4345–4347; c) S. Litters, E. Kaifer, H.-J.
Himmel, Eur. J. Inorg. Chem., 2016, 4090–4098.
[21] H.-J. Himmel, Angew. Chem., 2019, 131, 11724–11742; Angew. Chem. Int. Ed.,
2019, 58, 11600–11617.
23

Dalton Transactions
Page 24 of 25
View Article Online
DOI: 10.1039/C9DT03151K
[22] J. Horn, A. Widera, S. Litters, E. Kaifer, H.-J. Himmel, Dalton Trans., 2018, 47,
2009–2017.
[23] A. Widera, H. Wadepohl, H.-J. Himmel, Angew. Chem., 2019, 131, 5957–5961;
Angew. Chem. Int. Ed., 2019, 58, 5897–5901.
[24] A. Widera, D. Vogler, H. Wadepohl, E. Kaifer, H.-J. Himmel, Angew. Chem.,
2018, 130, 11627–11630; Angew. Chem. Int. Ed., 2018, 57, 11456–11459.
[25] a) M. T. Huynh, C. W. Anson, A. C. Cavell, S. S. Stahl, S. Hammes-Schiffer, J.
Am. Chem. Soc., 2016, 138, 15903–15910; b) M. Hanif, L. Ping, G. Cheng, W.
Zhi-ming, Y. Shi-min, Y. Bing, W. Chun-Lei, M. Yu-guang, Chem. Res. Chinese
Univ., 2009, 25, 950–956; c) I. P. Romanova, A. V. Bogdanov, V. F. Mironov, O.
A. Larionova, S. K. Latypov, A. A. Balandina, D. G. Yakhvarov, O. G. Sinyashin,
Mendeleev Commun., 2009, 19, 306–308.
[26] O. Ciobanu, P. Roquette, S. Leingang, H. Wadepohl, J. Mautz, H.-J. Himmel,
Eur. J. Inorg. Chem., 2007, 4530–4534.
[27] C. Feuvrie, I. Ledoux, J. Zyss, H. Le Bozec, O. Maury, C. R. Chimie, 2005, 8,
1243-1248.
[28] DENZO-SMN: Z. Otwinowski & W. Minor, Processing of X-ray Diffraction Data
Collected in Oscillation Mode, Methods Enzymol., 1997, 276, Eds C. W. Carter
R. M. Sweet, Academic Press.
[29] APEX3 software package, Bruker AXS, 2016.
[30] a) G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction,
University of Göttingen, Germany; b) G. M. Sheldrick, SADABS, Bruker AXS
GmbH, Karlsruhe, Germany, 2004-2014; c) L. Krause, R. Herbst-Irmer, G. M.
Sheldrick, D. Stalke, J. Appl. Cryst., 2015, 48, 3.
[31] a) G. M. Sheldrick, SHELXT, Program for Crystal Structure Solution, University
of Göttingen, Germany, 2014–2018; b) G. M. Sheldrick, Acta Crystallogr. Sect. A,
2015, 71, 3–8.
[32] a) O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Puschmann,
OLEX2: A complete structure solution, refinement and analysis program, J.
Appl. Cryst. 2009, 42, 339–341; b) P. v. d. Sluis, A. L. Spek, Acta Cryst. A, 1990,
46, 194; c) A. L. Spek, Acta Cryst. C, 2015, 71, 9.
[33] TURBOMOLE V7.2 2017, a development of University of Karlsruhe and
Forschungszentrum Karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since
2007; available from http://www.turbomole.com.
24

Page 25 of 25
Dalton Transactions
View Article Online
[34] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett.,D1O9I:8109.1,03196/C29D,T03151K
165–169.
[35] O. Treutler, R. Ahlrichs, J. Chem. Phys., 1995, 102, 346–354.
[36] ORCA V4.4.1, Mühlheim a. d. Ruhr, Germany, 2017.
[37] A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
[38] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem.,
1994, 98, 11623–11627.
[39] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–3305.
[40] a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098–3100; b) J. P. Perdew, Phys. Rev.
B, 1986, 33, 8822–8824.
[41] D. Andrae, U. Haeussermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim. Acta,
1990, 77, 123–141.
[42] Diamond V3.0, Bonn, Germany, 1997‒2004.
TOC entry
The complexation and reduction of a series of
diones with different redox-potentials at a
dicationic diborane are analysed.
2+
Electron transfer in complexes
of BII cations with organic -
acceptors: A combined
experimental and quantum-
chemical study
O
B
O
B
2+
2e- transfer
O
B
O
B
D. Vogler, N. Wolf, E. Kaifer, H.-J.
Himmel*
N
N
N
N
N
N
N
N
N
N
N
N
Keywords: boron, diborane, reduction, electron-transfer, boron-boron bond
25