Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction

Article abstract of DOI:10.1021/jacs.8b09214

The Lewis acid-base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C₆F₅)₃) to give the conventional B-Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid-base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs.

Full text of DOI:10.1021/jacs.8b09214

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Article
Evidence for Single Electron Shift in a Lewis Acid Base Reaction
Zhaowen Dong, Hanna H. Cramer, Marc Schmidtmann, Lucas A. Paul, Inke Siewert, and Thomas Müller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09214 • Publication Date (Web): 15 Oct 2018
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Evidence for Single Electron Shift in a Lewis Acid Base Reaction
Zhaowen Dong,† Hanna H. Cramer,† Marc Schmidtmann,† Lucas A. Paul,‡ Inke Siewert,‡ Thomas
Müller*,†
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†
Institute of Chemistry, Carl von Ossietzky University of Oldenburg, Carl von Ossietzky-Str. 9-11, D-26129 Oldenburg,
Federal Republic of Germany, European Union.
‡
Institute of Inorganic Chemistry, Göttingen University, Tammannstr. 4, D-37077 Göttingen, Federal Republic of
Germany, European Union.
ABSTRACT: The Lewis acid base reaction between
a nucleophilic hafnocene based germylene and tris-
pentafluorophenylborane (B(C6F5)3) to give the conventional B – Ge bonded species in almost quantitative yields is reported.
This reaction is surprisingly slow and during its course, radical intermediates are detected by EPR and UV-vis spectroscopy.
This suggests that the reaction is initiated by a single electron transfer step. The hereby-involved germanium radical cation
was independently synthesized by oxidation of the germylene by trityl cation or strong silyl-Lewis acids. A perfluorinated
tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the
results of DFT calculations indicate that the radical cation is a hafnium(III) centered radical that is formed by a redox
induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling
of the radicals to form the polar Lewis acid base product. The implications of this observation is shortly discussed in the
light of the recent finding that radical pairs are formed in frustrated Lewis pairs.
In this contribution, we will describe the Lewis acid base
1. INTRODUCTION
reaction between B(C6F5)3 (BCF), one of the most widely
used Lewis acids5 and a nucleophilic germylene 1, recently
The concept of Lewis acid base reactions is central to
chemistry. In general, the reaction between an electron
pair donor and an electron pair deficient acceptor results
in the formation of a ylide with a formal two-electron two-
center bond.1 The conventional textbook electron pushing
suggests the involvement of an electron pair in this process
(Figure 1a). Thirty-seven years ago, Pross and Shaik
recognized that the related SN2 reactions can be
understood in terms of single electron shift reactions,
which accounts for the observation of products derived
from both the typical concerted associative reaction
pathway as well as from single electron transfer.2 Using
this concept, the relationship between reactants and
product in a Lewis acid base reaction is a single electron
shift from the lone pair of the Lewis base (LB) to the empty
acceptor orbital of the Lewis acid (LA) and two spin-paired
electrons are generated in close proximity to each other.
Coupling of these two electrons into a single bond leads to
formation of the polar product LA--LB+ (Figure 1b, 1c). The
energy gain provided by this coupling after the single
electron shift favours the polar product over the products
of the single electron transfer (SET) reaction, where no
coupling occurs. Any factor that operates as to hinder the
coupling process will tend to favour the SET pathway over
the polar one.2b The recently detected SET reactions in
frustrated Lewis pairs in which the steric bulk of the
reaction partners inhibits the coupling process nicely
support this concept.3 A similar dichotomy between
concerted reaction pathway and SET reaction was recently
suggested in Diels Alder reactions.4
synthesized in our laboratory.6
A
fast valence
isomerization of the oxidized germylene prevents
immediate coupling after the electron shift and allows for
the experimental detection of the radical pair after SET.
E
a.
c.
SET
LA + :LB
LA LB
LA + LB
polar
b.
E (SET)
[LA] + [LB]
LA LB
LA LB
polar
LA + :LB
E (polar)
[LA] + [LB]
SET
reaction coordinate
Figure 1. Lewis Acid (LA) / Base (LB) reaction. a. Textbook
electron pushing description. b. Single electron shift which
opens two pathways, the conventional polar (see a.) and the
single electron transfer (SET) pathway. c. Schematic
configuration mixing diagram for competing polar (blue) and
SET (red) pathways.
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a.
B(C6F5)3
Ge
2. RESULTS AND DISCUSSION
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Ge
Syntheses. An equimolar ratio of a solution of BCF in
toluene was added to a yellow toluene solution of the
bicyclohexene-germylene (BCHGe) 1 at room temperature.
After 12 h the color of the reaction mixture had changed
from yellow to red-brown and the BCHGe/BCF Lewis
adduct 2 was formed (Scheme 1a). It was isolated as red-
brown oily product in nearly quantitative yield and was
fully characterized by NMR-, UV-vis spectroscopy, and
mass spectrometry. Orange crystals suitable for X-ray
diffraction (XRD) analysis were obtained from toluene
solution with slowly stirring the solution at room
temperature. BCHGe/borane adduct 2 is stable at room
temperature for more than half a year. The formation and
stability of the Lewis adduct 2 in solution is shown by NMR
spectroscopy. The chemical shifts of the 1H NMR
resonances of the two magnetically nonequivalent
cyclopentadienyl (Cp) substituents (δ1H = 5.00, 5.66) are
clearly different from those of BCHGe 1 (δ1H = 5.23, 5.55)
but are similar to those of related BCHGe-Fe(CO)4 (δ1H =
4 SiMe3
Hf IV
4 SiMe3
3
3
Me
Me
toluene
r.t.
2
2
Hf IV
1
+ B(C6F5)3
1
Me
Me
Me3Si
Me3Si
1
2
Ge
SiMe3
Me
Me
SET
Me3Si
Hf
+
[B(C6F5)3]
9
IV
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3'
IET
Ge
SiMe3
3
Me
4
2
1
Me
Me3Si
Hf
III
3
5.05, 5.67) or BCHGe-W(CO)5 (δ1H
= 5.08, 5.67)
b.
Ge
Ge
11
complexes6a. The relative sharp B NMR resonance at δ11B
= 1.6 (half width w(1/2) = 600 Hz) indicate tetra-
coordination for the boron atom in agreement with 19F
NMR data. In particular, the small NMR chemical shift
difference between the signals for the meta- and para-
fluorine atoms Δδ19Fm,p (Δδ19Fm,p = 8.5) is here informative.7
In agreement with the NMR data, the solid state structure
revealed the formation of the Lewis adduct 2 (Figure 2a).
The structural parameter of the germylene unit of 2 are
very close to that of the free BCHGe 1 (see Figure 2a and
Table 1). In particular, the close approach of the
germanium atom to the C2=C3 bond as shown by the small
flap angle (Ge) and the small Ge/C2/3 separations indicate
the stabilizing homoconjugative interaction.6a, 8 The boron
atom in compound 2 is tetra-coordinated and the Ge–B
bond is relative long (226.0 pm) compared to standard
values (Ge–B = 206 pm)9 and to related complexes between
germylenes and BCF (222.3 pm).10 In qualitative agreement
with the long Ge–B bond, its computed bond dissociation
energy (BDE) is relatively small (BDE(B–Ge) (2) = 75 kJ
mol-1, at M06-2X/def2-tzvp). Compound 2 is a typical
adduct between the Lewis basic BCHGe and Lewis acidic
BCF.
[Ph3C][B(C6F5)4]
benzene
SiMe3
4 SiMe3
Hf IV
3
3
Me
Me
4
-[Ph3C]
2
2
1
1
Me
Me
KC8
Me3Si
Me3Si
Hf
III
benzene
[B(C6F5)4]-
[B(C6F5)4]
3
1
Interestingly, in the course of 50 minutes after the addition
of BCF, the toluene solution of BCHGe 1 turned to deep
purple with a characteristic long wave absorption at max
=
544 nm. The intensity of this band decreases with time and
after 6 h the typical red-brown solution of the BCF adduct
2 was obtained (max = 394 nm, see Figures S2–S4). The
purple toluene solution was essentially NMR silent and
electron paramagnetic resonance (EPR) investigations
indicated the presence of two different radical species. One
intensive, sharp signal (giso = 1.9881) with satellite signals
due to hyperfine coupling to hafnium. The second broad
resonance was partly obscured by the sharp signal (Figure
S5). The intensity of the EPR signals decreased with time
and essentially vanished after 3.5 h (Figure S6). A 1H NMR
spectra recorded after 6 h indicated the quantitative
formation of the diamagnetic Lewis adduct 2. We suggest
that the intermediate radicals are formed in a single
electron transfer process prior to the formation of the
classical Lewis adduct (Scheme 1a).11 The detection of
hafnium satellites for the sharp EPR signal indicated that
the radical [BCHGe]
BCHGe 1. The resulting BCF radical anion, [BCF] , is the
source for the UV-vis absorption at max = 544 nm in
toluene (r.t.) and of the broad, ill-defined EPR resonance.
This is supported by the results of TD/DFT calculations
that predict for [BCF] an absorption at max = 514 nm
(Figure S30). [BCF] has been characterized by the Norton
group as a short living intermediate in THF (t ≈ 2 min)
Scheme 1. The reactions of BCHGe 1 with diverse Lewis
acids. a. Synthesis of the BCHGe/B(C6F5)3 adduct 2 via
radical complex 3. b. The reaction of BCHGe 1 with
[Ph3C][B(C6F5)4] to give the stable borate 3[B(C6F5)4] and
the chemical reduction of radical cation 3 with KC8 to
recover BCHGe 1.
• +
3 is derived from oxidation of
•-
• -
•-
½
and was suggested as reaction product resulting from
reactions using BCF as oxidant.11b A deep blue color with

max ≈ 600 nm is reported for its solution in THF at T = -50
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°C and the EPR spectrum consists of a 14-line signal
biphasic mixtures of different polarity that allow
separation of ionic from non-polar products.
centered at giso = 2.010.11b, 12
1
2
3
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These observations suggest that the apparently simple
Lewis acid / base reaction, shown in Scheme 1a, proceeds
via a single electron transfer steps followed by coupling of
the radical ions.
After addition of 1 equiv. of [Ph3C][B(C6F5)4] to a solution
of BCHGe 1 in benzene, the color immediately changed
from yellow to dark green. After 30 min reaction time, the
two phases were allowed to separate and the upper non-
polar light yellow benzene layer was investigated by EPR
spectroscopy which indicated the presence of trityl radical
(Ph3C•) (Scheme 1b and S15). From the lower polar phase
the residual solvent was removed and the deep-green oily
residue was dissolved in chlorobenzene-d5. As expected the
sample was NMR silent but we detected the strong EPR
signal of radical cation 3 (Figure 3) with clearly visible
satellite signals due to hyperfine coupling to hafnium.
Similar results were obtained when BCHGe 1 was treated
with equimolar amounts of the silyl arenium borate
[Et3Si(C6H6)][B(C6F5)4] or with silylium borate [(Me5C6)3Si]
[B(C6F5)4]. In these cases no silyl centered radicals were
detected, only secondary products such as (Me5C6)3SiH and
(Me5C6)3SiMe were identified by NMR spectroscopy (see SI
material).15 Under these conditions radical cation 3 is stable
for an extended period of time as the intensity of the EPR
singlet decreased only slightly during 14 d (Figure S14). UV-
vis spectroscopy in toluene showed two bands at  = 350
and 430 nm and two additional weak absorptions at  = 615
and 825 nm. Finally, radical cation borate 3[B(C6F5)4] was
characterized by the result of an XRD analysis (see below).
The chemical oxidation is fully reversible as reduction of a
freshly prepared solution of radical cation borate
3[B(C6F5)4] by an equimolar amount of KC8 in benzene-d6
resulted in the quantitative back-formation of BCHGe 1 as
shown by 1H NMR spectroscopy (Scheme 1b, Figure S22).
The cyclic voltammogram of BCHGe 1 showed an
irreversible oxidation at Epa = -0.51 V and a shoulder at
−0.36 V in chlorobenzene (vs. [Cp2Fe]+/[Cp2Fe], ν = 0.1 Vs−1,
0.1 M [nBu4N][B(C6F5)4], Figure S16). The shoulder vanishes
at high scan rates, which points to a chemical reaction after
initial oxidation forming a second redox-active product.
Consistent with this, the peak potential shifts with
increasing scan rates indicating fast chemical reaction after
initial oxidation and/or irreversible electron transfer
process (Figure S17).13 Irrespectively of the scan rate, the
shoulder is also present, when a mixture of 1 and BCF is
measured, which indicates the formation of the same
product after electrochemical oxidation and SET reaction
of 1 and BCF (Figure S18). In line with the experimentally
observed reversibility of reaction, the CV shows two
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reduction waves at −2.00
V
and −2.27
V
(vs.
[Cp2Fe]+/[Cp2Fe], ν = 0.1 Vs−1, Figure S19), which are only
present after initial oxidation of 1.
In order to gain more information about the intermediate
radical cation 3 and to reach its fast quantitative formation,
we switched to the stronger cationic oxidant trityl cation,
[Ph3C]+ (cf. E = -0.11 V in MeCN14). As counter anion we
choose the weakly coordinating [B(C6F5)4]- anion, which
provides additional kinetic stabilization of the radical
cation due to its low reactivity. In addition, solutions of the
salt [Ph3C][B(C6F5)4] in aromatic hydrocarbons form
a.
b.
Figure 2. a. Molecular structure of Lewis adduct 2 in the (thermal ellipsoids at 50% probability, all hydrogen atoms are omitted
for clarity). Selected atom distances [pm] and angles [°]: Ge1–B1 226.0(13), Ge1–C1 205.6(11), Ge1···C2 230.3(11), Ge1···C3 224.8(12),
Ge1–C4 201.4(12), Ge1···Hf1 297.7(6), C1–C2 148.9(17), C2–C3 142.2(17), C3–C4 149.4(17), C1–Hf1 225.9(12), C2···Hf1 286.4(12), C3···Hf1
289.5(13), C4–Hf1 225.9(12), (Ge) 100.5, (Hf) 127.5. b. Molecular structure of radical cation 3 in the solid state. Thermal ellipsoids
at 50% probability, all hydrogen atoms, solvent molecules and [B(C6F5)4]– are omitted for clarity. Selected atom distances [pm] and
angles [°], given as mean values from two independent molecules: Ge1–C1 203.1(4), Ge1···C2 260.7(36), Ge1···C3 262.4(37), Ge1–C4
203.5(4), Ge1···Hf1 289.9(6), C1–C2 142.6(5), C2–C3 144.7(5), C3–C4 142.9(5), C1–Hf1 233.4(4), C2–Hf1 269.5(37), C3–Hf1 268.5(36),
C4–Hf1 232.0(3), (Ge) 131.3, (Hf) 110.1. For the definition of the flap angles, see Table 1.
Structural characterization of borate 3[B(C6F5)4]. XRD
analysis of yellow-green single crystals revealed two
independent ion pairs of 3[B(C6F5)4] in the asymmetric unit
with well-separated radical cations 3 and borate anions
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(the shortest F / Hf or F / Ge atomic distances are 445.8 or
Page 4 of 7
[a] Experimental data from XRD, mean values from three
independent molecules, reference 10. [b] Experimental data
from XRD. [c] M06-2X/def2-tzvp. [d] UM06-2X/def2-tzvp.
[e] Flap angle (Hf) is defined as angle between the Hf
atom, the midpoint of C1/C4 and midpoint of C2/C3
distances. [f] Flap angle (Ge) is defined as angle between
the Ge atom, the midpoint of C1/C4 and the midpoint of
C2/C3 distances. [i] Sum of the covalent radii expected for
Hf−E single bonds: E = C: 227 pm, E = Ge: 273 pm. Sum of
the covalent radii expected for C−E single bonds: E = C: 150
pm, E = Ge: 196 pm.
1
2
3
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445.9 pm). The molecular structure of radical cation 3
differs significantly from that of BCHGe 1 and from the
germylene fragment of the Lewis adduct 2. (see Table 1, and
Figure 2b). Common to all three molecular structures is the
bent five-membered GeC4 ring with a regular envelope
conformation (Figure 2b, Table 1).6a The flap angle (Ge) of
131.3° in cation 3 is however significantly larger than the
flap angle (Ge) of 97.9° in BCHGe 1, which suggests that
the homoconjugative interaction between the Ge atom and
the remote C2C3 bond is canceled.6b This is further shown
by larger atomic distances between the Ge and the C2/3
atoms (260.7, 262.4 pm) compared with those in BCHGe 1
(226.7, 227.0 pm). The Ge–C1/4 bond lengths of cation 3
(Ge–C1/4 = 203.1, 203.5 pm) are typical for aryl- and alkyl-
substituted germylenes (Ge–C = 200–209 pm).16 The nearly
equidistant inner cyclic C–C bonds of the GeC4 ring (C–C =
142.6, 144.7, 142.9 pm) shows the delocalized structure of
the butadiene fragment. The flap angle (Hf) decreases
significantly upon oxidation (1: (Hf) = 124.8°; 3: (Hf) =
110.1°). This indicates the switch from the 2-coordination
of the hafnium atom to the C4Ge ring in BCHGe 1 to the 4
coordination in a 2, -mode in cation 3.6b In agreement,
the Hf–C1/4 bonds (Hf–C1/4 = 233.4, 232.0 pm) and Hf–C2/3
bonds (Hf–C2/3 = 269.5, 268.5 ppm) in cation 3 all are longer
than Hf–C1/4 σ-bonds in BCHGe 1 (Hf–C1/4 = 220.3, 220.1
pm)6a or the single bond covalent radii of Hf and C atoms
(Hf–C = 227 pm).9 The germanium and the hafnium atom
in cation 3 are separated by 289.9 pm which falls out of the
range of regular Hf – Ge single bonds (Ge–Hf = 273 pm)9
and indicate only small bonding interaction between both
centers. Summarized, these structural features indicate
that cation 3 is best described as a 4-butadiene hafnocene
complex with an independent germylene functionality.
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EPR analysis and electronic structure of radical cation
3. Radical cation 3 was characterized by X-band EPR
spectroscopy (Figure 3) at ambient temperature. The EPR
spectrum in chlorobenzene showed an intense singlet
accompanied by several satellite signals, which are
characteristic for a hafnium-centered radical. This singlet
showed the same spectroscopic characteristics as the
hafnium centered radical species obtained from the
reaction of BCF with germylene 1 (Figure S5). The
experimental EPR spectrum of radical cation 3 was
simulated based on the following parameter: giso = 1.9881,
Aiso(Hf) = 8.5 mT (Figure 3). These EPR data indicated a
localization of the unpaired electron at the hafnium atom.
To gain further insight into the electronic structure of 3,
density functional calculations were performed for 3 at the
UM06-2X/ADZP(Hf),def2-tzvppd(Ge,Si,C,H)//UM06-
2X/def2-tzvp level. The calculated structure for radical
cation 3 is very close to that determined by XRD analysis,
the largest deviation in atomic distances being less than 2
% (Table 1). In agreement with the ESR spectroscopic
results, the calculations indicated that the singly occupied
molecular orbital (SOMO) and the total spin density
distribution of 3 are mainly localised on the dz2 orbital of
the Hf atom with contributions from the * orbital of the
butadiene fragment and only minor contributions of the
lone pair of the germanium atom (Figures 4a-4c). The
highest doubly occupied molecular orbital is dominated by
the lone pair at the germanium atom (Figure 4d) and the
LUMO shows significant contributions from the 4p(Ge)
orbital (Figure 4e). This arrangement of frontier orbitals
point to the germylene nature of radical cation 3. Time
dependent (TD)-DFT calculations allowed the assignment
of the long wave UV-absorption of radical cation 3 in
toluene at max = 825 nm to the SOMO  LUMO transition
(calculated  = 816 nm, Figure S29).
Table 1. Comparison of structural parameter of
compounds 1-3 and calculated structure 2opt and 3opt
.
1[a]
2[b]
2opt[c]
148.9
142.0
149.1
3[b]
3opt[d]
141.6
145.3
141.2
203.2
262.3
261.3
203.4
–
C1–C2
C2–C3
C3–C4
C1–Ge
C2–Ge
C3–Ge
C4–Ge
B–Ge
148.3
142.5
148.2
210.0
226.7
227.0
208.8
–
148.9
142.2
149.4
205.6
230.3
224.8
201.4
226.0
225.9
286.4
289.5
225.9
297.7
86.6
142.6
144.7
142.9
203.1
260.7
262.4
203.5
–
202.8
223.6
220.8
203.8
224.9
225.0
281.8
284.5
224.3
302.7
86.6
C1–Hf
220.3
277.9
278.5
220.1
301.0
85.0
124.8
97.9
233.4
269.5
268.5
232.0
289.9
85.7
235.0
267.8
C2–Hf
C3–Hf
C4–Hf
Ge–Hf
C4–Ge–C1
 (Hf)[e]
 (Ge)[f]
268.0
235.8
295.2
85.5
127.5
100.5
124.8
97.2
110.1
107.5
132.4
131.3
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radical 3’ undergoes a valence isomerization in the form of
1
2
3
4
5
6
an intra-molecular electron transfer (IET) process, which
changed the formal oxidation state of hafnium from +IV to
+III. This process provides an additional example for a
redox induced electron transfer (RIET) process,17 in this
involving the hafnium and a non-innocent butadiene
ligand.
7
8
3. CONCLUSIONS
In conclusion, a new BCHGe/B(C6F5)3 Lewis adduct 2 was
obtained by the reaction of BCHGe 1 with B(C6F5)3 . This
Lewis acid base reaction was surprisingly slow but
proceeds with nearly quantitative yield. During this
reaction the radical pair 3[BCF] was identified by EPR and
UV-vis spectroscopy. Using [Ph3C][B(C6F5)4] or silylium
borates as oxidants the radical cation 3 was selectively
synthesized and isolated in the form of its tetraarylborate.
Its formation is chemically reversible as its reduction with
KC8 gave quantitatively BCHGe 1. Radical cation 3 is a
hafnocene (III) complex, which is formed from the
expected radical cation 3’ by an intramolecular electron
transfer process from the ligand to the hafnium center.
Scheme 1 summarizes these processes and suggests that
this Lewis acid/base reaction proceeds via a SET reaction.
The investigated example is unique as far as the described
valence isomerization 3’  3 is fast and it hampers the
formation of the Lewis adduct 2 as the electronic situation
in 3 does not allow for an immediate coupling of the radical
pair. Our results are in line with the idea that a Lewis acid
base reaction proceeds via a single electron shift in the
transition-state region and each factor that prevents
coupling to the polar product favors the SET process.2a We
are convinced that this work opens up new perspectives to
Lewis acid / base chemistry, particularly in view of the
recent discovery that frustrated Lewis pairs are prone to
SET reactions.3a, 3b
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Figure 3. Experimental (blue solid lines) and simulated (pink
solid lines) EPR spectrum for radical cation 3 in C6H5Cl
solution was measured at 298 K (microwave frequency = 9.403
GHz). Fit parameters: giso = 1.9881, Aiso(Hf) = 8.5 mT. The
simulated spectrum included an 18.6% 177Hf contribution and
13.6% 179Hf contribution.
a
b
c
Ge
e
d
4. EXPERIMENTAL SECTION
All the reactions were performed under dry argon atmosphere
using standard Schlenk techniques or in a nitrogen-filled
glovebox. BCHGe 1 was prepared according to a literature
procedure.6a Non-chlorinated Solvents were dried over
sodium/potassium alloy and distilled under nitrogen prior to
use. Details of experimental procedures, EPR analytical data,
UV-vis spectrum and X-ray structure determinations are given
in the Supplementary Information. All quantum chemical
calculations were carried out using the Gaussian09 package.
The NBO analyses were performed with the Version 6.0 of the
NBO program which was implemented in the G09 D.01
version of the Gaussian program.
Figure 4. Spin density map and key frontier molecular orbitals
of radical cation 3 calculated at the UM06-2X/ADZP(Hf), def2-
tzvppd(Ge,Si,C,H) // UM06-2X/def2-tzvp level. a. Spin
density map of 3opt (isovalue = 0.004). b. Singly occupied
molecular orbital (SOMO) of 3opt (E = -9.07 eV). c. Sketch of
the atomic orbital contributions of SOMO. d. Surface diagram
of SOMO-1(α) of 3opt (E = -10.88 eV). e. Surface diagram of
LUMO(α) of 3opt (E = -4.65 eV). (Color code: Yellow, silicon;
blue, germanium; orange, hafnium; gray, carbon; all hydrogen
atoms are omitted for clarity, isovalue for all MOs: 0.05).
Based on the computational investigations, EPR
spectroscopy and XRD analysis, we conclude that radical
cation 3 can be described as a hafnocene(III) complex
which is 4 coordinated in a 2, -mode to a 1-
Synthesis of germylene/borane adduct 2: A toluene (3 mL)
solution of B(C6F5)3 (84.48 mg, 0.165 mmol) was added to a
yellow toluene solution (3 mL) of BCHGe 1 (100 mg, 0.165
mmol) at room temperature. The reaction mixture was stirred
for overnight at room temperature. After that, the solvent was
removed under vacuum and the red-brown oily residue was
germacyclopenta-2,4-dienylidene.
Therefore,
the
oxidation of BCHGe 1 resulted in the formal reduction of
the hafnium atom. As the HOMO of BCHGe 1 is a
combination of the -C2C3 orbital and the 4p(Ge) atom,6a
the initial step of the one-electron oxidation is the
formation of the alkene radical cation 3’ (Scheme 1). Cation
identified as germylenes/borane adduct
spectroscopy in almost quantitative yield.
2
by NMR
Spectroscopic data of germylenes/borane adduct 2: 1H NMR
(499.87 MHz, 305.0 K, C6D6): δ = 5.66 (s, 5H, Cp), 5.00 (s, 5H,
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Cp), 1.62 (s, 6H, 2xCH3) 0.02 (s, 18H, Si(CH3)3). 13C{1H} NMR
(1) (a) Jensen, W. B. Chem. Rev. 1978, 78, 1-22. (b) Denmark, S. E.;
1
(125.71 MHz, 305.0 K, C6D6): δ = 148.1 (dm, o-CF, J(13C-19F) =
Beutner, G. L. Principles, Definitions, Terminology, and Orbital
Analysis of Lewis Base–Lewis Acid Interactions Leading to
Catalysis; In Lewis Base Catalysis in Organic Synthesis; Vedejs, E.;
Denmark, S. E.; Eds.; Wiley-VCH, Weinhein: 2016, pp 31-54.
(2) (a) Shaik, S. S. J. Am. Chem. Soc. 1981, 103, 3692-3701. (b) Pross, A.
Acc. Chem. Res. 1985, 18, 212-219. (c) Shaik, S. S. Acta Chem. Scand.
1990, 44, 205-221. (d) Eric, V.; Anslyn, D., Modern Physical Organic
Chemistry. University Science Books: Sausalito, CA, 2006.
(3) (a) Liu, L. L.; Cao, L. L.; Shao, Y.; Ménard, G.; Stephan, D. W. Chem
2017, 3, 259-267. (b) Liu, L. L.; Cao, L. L.; Zhu, D.; Zhou, J.; Stephan,
D. W. Chem. Commun. 2018, 54, 7431-7434. (c) Piers, W. E.;
Marwitz, A. J.; Mercier, L. G. Inorg. Chem. 2011, 50, 12252-12262.
(d) Merk, A.; Großekappenberg, H.; Schmidtmann, M.; Luecke,
M.-P.; Lorent, C.; Driess, M.; Oestreich, M.; Klare, H.; Müller, T.
Angew. Chem. Int. Ed., in press, DOI: 10.1002/anie.201808922.
(4) Cahill, K. J.; Johnson, R. P. J. Org. Chem. 2012, 78, 1864-1873.
(5) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345-354.
(6) (a) Dong, Z.; Reinhold, C. R. W.; Schmidtmann, M.; Müller, T.
Angew. Chem. Int. Ed. 2016, 55, 15899-15904. (b) Dong, Z.; Bedbur,
K.; Schmidtmann, M.; Müller, T. J. Am. Chem. Soc. 2018, 140, 3052-
3060.
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244 Hz), 141.3 (dm, p-CF, 1J(13C-19F) = 255 Hz), 137.7 (dm, m-CF,
1J(13C-19F) = 252 Hz), 136.4 (2xCCH3), 117.4 (br, ipso-C), 104.5
(Cp), 103.4 (Cp), 103.2 (2xCSi), 19.5 (CH3), 3.7 (2xSi(CH3)3).
29Si{1H} NMR (99.31 MHz, 305.0 K, C6D6): δ = -3.4. 11B{1H} NMR
(160.378 MHz, C6D6, 305 K): δ = 1.6. 19F{1H} NMR (470.348
MHz, C6D6, 305 K): δ = −162.5 (t, m-CF, 3J(19F-19F) = 20 Hz),
3
−154.0 (tm, p-CF, 3J(19F-19F) = 17 Hz), −127.1 (dm, o-CF, J(19F-
19F) = 19 Hz). HRMS (70 eV, CI) m/z [M+H]+ Calcd for
C40H35BF15GeHfSi2: 1121.0803 Found: 1121.0807. UV-vis (toluene
solution) [nm]: λ = 294, 394(sh).
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Synthesis of hafnium radical cation 3: A benzene (3 mL)
solution of [Ph3C][B(C6F5)4] (152.2 mg, 0.165 mmol) was slowly
added to a yellow benzene solution (3 mL) of BCHGe 1 (100
mg, 0.165 mmol) at room temperature. Then the reaction
mixture was stirred 30 min at this temperature and the
solution turned from a yellow to a deep green biphasic
mixture. After separation of the two phases, the upper
benzene phase was investigated by EPR analysis and showed
strong EPR signals, which suggested the formation of trityl
radical. The lower layer was evaporated under vacuum and the
residue was dissolved in C6D5Cl. The solution was NMR silent
but showed an intensive EPR signal. Yellow-green single
crystals suitable for X-ray diffraction analysis were obtained
(7) The 19F NMR of BCF can offer a reliable probe of the coordination
environment around the boron atom. The planar structure of BCF
allows efficient resonance between the empty 2p-orbital at the
boron atom and the -system of the perfluorinated aryl-
substituent. This -resonance leads to significant deshielding of
the p-fluorine atom. Coordination of a fourth substituent to the
boron atom results in loss of planarity and hampers the
resonance. As a consequence, the signal of the p-fluorine atom
moves to higher field. The fluorine atoms in the meta positions
°
by recrystallization from toluene solution at 6 C. (Yield: 167
mg (80%)).
are not influenced by the -resonance and their 19F NMR chemical
shifts do not change significantly by pyramidalization of the
boron center. Therefore, the separation between the meta and
para 19F chemical shifts (Δδm,p) is a convenient measure for the
pyramidalization of the boron atom. For neutral tri-coordinated
BCF it is large (Δδ19Fm,p (BCF) = 20.1), and it becomes smaller in
neutral tetra-coordinated boron compounds and in anionic
borates (i.e. Δδ19Fm,p (2) = 8.5; Δδ19Fm,p ([MeB(C6F5)3]-) ≈ 5.0). For
further details see Beringhelli, T.; Donghi, D.; Maggioni, D.;
D’Alfonso, G. Coord. Chem. Rev. 2008, 252, 2292-2313.
ASSOCIATED CONTENT
Supporting Information. Experimental and computational
details, NMR spectra and details of the structure solution of
compounds 2, 3 and (Me5C6)3SiMe. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
(8) Dong, Z.; Reinhold, C. R.; Schmidtmann, M.; Müller, T. J. Am.
Chem. Soc. 2017, 139, 7117-7123.
(9) Pyykkö, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 12770-12779.
(10) Del Rio, N.; Lopez‐Reyes, M.; Baceiredo, A.; Saffon‐Merceron, N.;
* thomas.mueller@uni-oldenburg.de
ORCID
Zhaowen Dong: 0000-0001-6163-470X
Thomas Müller: 0000-0002-4247-3776
Hanna H. Cramer: 0000-0001-7047-2511
Inke Siewert: 0000-0003-3121-3917
Lucas A. Paul: 0000-0001-9152-8668
Marc Schmidtmann: 0000-0001-8026-7271
Lutters, D.; Müller, T.; Kato, T. Angew. Chem. Int. Ed. 2017, 56,
1365-1370.
(11) (a) Harlan, C. J.; Hascall, T.; Fujita, E.; Norton, J. R. J. Am. Chem.
Soc. 1999, 121, 7274-7275. (b) Kwaan, R. J.; Harlan, C. J.; Norton, J.
R. Organometallics 2001, 20, 3818-3820. (c) Beddows, C. J.;
Burrows, A. D.; Connelly, N. G.; Green, M.; Lynam, J. M.; Paget, T.
J. Organometallics 2001, 20, 231-233. (d) Ishida, Y.; Sekiguchi, A.;
Kobayashi, K.; Nagase, S. Organometallics 2004, 23, 4891-4896. (e)
Zheng, X.; Wang, X.; Qiu, Y.; Li, Y.; Zhou, C.; Sui, Y.; Li, Y.; Ma, J.;
Wang, X. J. Am. Chem. Soc. 2013, 135, 14912-14915.
Notes
(12) Lawrence, E. J.; Oganesyan, V. S.; Wildgoose, G. G.; Ashley, A. E.
Dalton Trans. 2013, 42, 782-789.
The authors declare no competing financial interest.
(13) Zanello, P. Inorganic Electrochemistry: Theory, Practice and
Application; Royal Society of Chemistry: Cambridge, U.K., 2003.
(14) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.
(15) Schäfer, A.; Reißmann, M.; Schäfer, A.; Saak, W.; Haase, D.;
Müller, T. Angew. Chem. Int. Ed. 2011, 50, 12636-12638.
(16) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109,
3479-3511.
ACKNOWLEDGMENT
This work was supported by the Carl von Ossietzky University
Oldenburg, Göttingen University, by the Lower Saxony State
(Lichtenberg Fellowship to Z.D.) and by the DFG (SI 1577/2-1).
The simulations were performed at the HPC Cluster CARL,
located at the University of Oldenburg (Germany) and funded
by the DFG through its Major Research Instrumentation
Program (INST 184/108-1 FUGG) and the Ministry of Science
and Culture (MWK) of the Lower Saxony State.
(17) Miller, J. S.; Min, K. S. Angew. Chem. Int. Ed. 2009, 48, 262-272.
REFERENCES
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Insert Table of Contents artwork here
Lewis acid base reaction via SET process
LA
LB
LA
+
LB
LA
+
LB
EPR
SET
EPR
UV-vis
X-ray
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