One-Electron Oxidation of a Disilicon(0) Compound: An Experimental and Theoretical Study of [Si2]+ Trapped by N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/chem.201502199

One-electron oxidation of the disilicon(0) compound Si₂(Idipp)₂ (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with [Fe(C₅Me₅)₂][B(Ar^F)₄] (Ar^F=C₆H₃-3,5-(CF₃)₂) affords selectively the green radical salt [Si₂(Idipp)₂][B(Ar^F)₄] (1-[B(Ar^F)₄). Oxidation of the centrosymmetric1 occurs reversibly at a low redox potential (E_1/2=-1.250V vs. Fc⁺/Fc), and is accompanied by considerable structural changes as shown by single-crystal X-ray structural analysis of 1-B(Ar^F)₄. These include a shortening of the S-Si bond, a widening of the Si-Si-C_NHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1⁺. Comparative quantum chemical calculations of 1 and 1⁺ indicate that electron ejection occurs from the symmetric (n₊) combination of the Si lone pairs (HOMO). EPR studies of 1-B(Ar^F)₄ in frozen solution verified the inequivalency of the two Si sites observed in the solid-state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar ²⁹Si hyperfine coupling tensors in 1⁺. EPR studies of 1-B(Ar^F)₄ in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1⁺. The radical salt [Si₂(Idipp)₂][B(Ar^F)₄] was obtained upon one-electron oxidation of the disilicon(0) compound Si₂(Idipp)₂. The open-shell compound was characterized by experimental and quantum-chemical methods, providing a detailed insight into the unusual electronic structure and bonding situation of this σ-type radical.

Full text of DOI:10.1002/chem.201502199

DOI: 10.1002/chem.201502199
Full Paper
&
Disilicon Compounds
One-Electron Oxidation of a Disilicon(0) Compound: An
+
Experimental and Theoretical Study of [Si ] Trapped by N-
2
Heterocyclic Carbenes
[a]
[a]
[b]
[a]
Marius I. Arz, Martin Straßmann, Andreas Meyer, Gregor Schnakenburg,
[b]
[a]
Olav Schiemann,* and Alexander C. Filippou*
+
Abstract: One-electron oxidation of the disilicon(0) com-
pound Si (Idipp) (1, Idipp=1,3-bis(2,6-diisopropylphenyl)i-
Comparative quantum chemical calculations of 1 and 1 in-
dicate that electron ejection occurs from the symmetric (n+)
2
2
F
F
midazolin-2-ylidene) with [Fe(C Me ) ][B(Ar ) ] (Ar =C H -3,5-
combination of the Si lone pairs (HOMO). EPR studies of 1-
B(Ar ) in frozen solution verified the inequivalency of the
5
5 2
4
6
3
F
(
[
CF ) ) affords selectively the green radical salt [Si (Idipp) ]
3
2
F
2
2
4
F
B(Ar ) ] (1-[B(Ar ) ). Oxidation of the centrosymmetric 1
two Si sites observed in the solid-state, and point in agree-
ment with the theoretical results to an almost equal distribu-
tion of the spin density over the two Si atoms, leading to
4
4
occurs reversibly at a low redox potential (E =À1.250 V vs.
1
/2
+
Fc /Fc), and is accompanied by considerable structural
29
+
changes as shown by single-crystal X-ray structural analysis
quite similar Si hyperfine coupling tensors in 1 . EPR stud-
ies of 1-B(Ar ) in liquid solution unraveled a topomerization
with a low activation barrier that interconverts the two Si
F
F
of 1-B(Ar ) . These include a shortening of the SiÀSi bond,
4
4
a widening of the Si-Si-C
angles, and a lowering of the
NHC
+
symmetry, leading to a quite different conformation of the
sites in 1 .
+
NHC substituents at the two inequivalent Si sites in 1 .
Introduction
benes (NHCs) can be used to trap highly reactive, unsaturated
[
5]
main-group species. This provided access to a series of un-
[
6]
The structures and properties of small silicon clusters are of
fundamental importance for the understanding of the transi-
tion from molecular behavior to that of bulk silicon and have
significant implications for the chemical vapor deposition
usual zerovalent compounds, such as E (NHC) (E=SiÀSn,
2
2
[7] [8a] [8b]
B, P, As ) or E(bNHC) (E=Si, Ge; (bNHC)=chelating bis-N-
[
9]
heterocyclic carbene). Appealing examples in low-valent sili-
con chemistry for the complexation proclivity of N-heterocyclic
(
CVD) of amorphous and crystalline silicon in the semiconduc-
carbenes include the isolation of NHC adducts of SiX (X=Cl,
2
[
1]
[10]
[11]
+
[10d,12]
tor industry. In this context the electronic structure of the sili-
Br, I),
Si(Cl)R (R=aryl, amino),
[SiR] (R=I, aryl),
2
+ [10d]
[13]
[14]
0
con dimer (Si ) was investigated experimentally and by quan-
Si
,
SiR(Si(R)=SiR ) (R=aryl), R2Si=Ge, or the Si com-
2
2
tum theory, and the relevance of Si in cluster formation was
pound Si (Idipp)2 (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imi-
2
2
[2]
[6a]
pointed out in few studies. In comparison, studies on the
dazolin-2-ylidene), which has been described as a base-stabi-
+
[15]
[
Si2] cation are far less common and include mainly quantum
lized allotrope of silicon.
This description was fortified by
[3]
chemical calculations. The quartet ground state structure of
quantum chemical calculations suggesting that 1 can be
+
1
[
Si2] was established by electron spin-resonance (ESR) studies
viewed as a Si molecule trapped in its excited ( D ) electronic
2
g
[4]
[16]
in noble-gas matrices at 4 K.
state by two NHCs.
However, such a description can be
Main-group element chemistry has witnessed a renaissance
in recent years following the discovery that N-heterocyclic car-
questioned given the quite different lowest-energy configura-
3
À [3]
tion of Si (triplet, C S ) from that of 1 and the rather large
2
g
SiÀC
bond dissociation energies, which have to date pre-
NHC
[
17]
[
a] M. I. Arz, M. Straßmann, Dr. G. Schnakenburg, Prof. Dr. A. C. Filippou
Institut für Anorganische Chemie
Rheinische Friedrich-Wilhelms-Universität Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: filippou@uni-bonn.de
vented the use of 1 as a source of Si in reactions. Notably,
2
a recent experimental and theoretical study uncovered the iso-
lobal analogy of 1 with the diphosphene P Mes* (Mes*=
2,4,6-tBu -C H ) and the NHC-stabilized phosphasilenylidene
2
2
3
6
2
[
18]
[
b] A. Meyer, Prof. Dr. O. Schiemann
(Idipp)Si=PMes*.
In all three of these double-bond com-
Institut für Physikalische und Theoretische Chemie
Rheinische Friedrich-Wilhelms-Universität Bonn
Wegelerstrasse 12, 53115 Bonn (Germany)
E-mail: schiemann@pc.uni-bonn.de
pounds, the lowest-unoccupied molecular orbital (LUMO) is
the antibonding EÀE p* orbital, the highest-occupied molecu-
lar orbital (HOMO) the symmetric (n ) combination of the
+
[
19]
lone-pair orbitals and the HOMOÀ1 the EÀE p-bonding orbi-
tal, with the disilicon compound 1 revealing the smallest
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502199.
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energy gap of only 0.03 eV between the HOMO and HOMOÀ1
orbital. This electronic situation raised the question as to
whether ejection of one electron from 1 would occur from the
n+ level to give a s radical or from the p level to give a p radi-
cal. The outcome of the one-electron oxidation of 1 was ex-
pected to have a major influence on the structure and reactivi-
ing that 1 cannot be easily reduced. Furthermore two irreversi-
ble oxidation processes were observed in C H F at E =0.658
6
5
1/2
À1
and 1.001 V (scan rate: 50 mVs ) suggesting that a rapid
follow-up reaction occurs upon further oxidation of the radical
+
cation 1 to a putative dicationic analogue of a disilyne
2
+ [27]
(1 ). Remarkably, the electrochemical properties of 1 differ
+
+
ty of the resulting radical cation [Si (Idipp) ] C (1 )C given the
considerably from those of Si (CAAC) , the disilicon analogue
2
2
2
2
[
20,21]
known differences between organic s- and p-type radicals.
of 1 containing cyclic alkyl(amino)carbene (CAAC) substituents,
Studies addressing this issue in the case of the isolobal diphos-
which displays in THF a quasi-reversible one-electron reduction
+
[29]
phene radical cations [P R ] C were hampered by the high reac-
at E1/2 =À1.40 V. This difference can be rationalized with the
2
2
[
22]
[30]
tivity of these ions, which precluded so far their isolation.
much stronger p-acceptor properties of CAACs. The CVs of
+
EPR spectroscopy of [P Mes* ] C generated by g-irradiation of
1 also differ markedly from those of the diphosphenes P2R2
2
2
P Mes* in dilute Freon solution at 77 K suggested the pres-
(R=C(SiMe ) , Mes*, m-terphenyl), which show a reversible
2
2
3 3
[23]
ence of a s-type radical, and the lowest energy ionization
one-electron reduction at E1/2 =À2.23 to À1.82 V and an irre-
band in the photoelectron spectra of P R was assigned using
versible
oxidation
at
large
potentials
(E =0.90–
1/2
2
2
[24]
[18,22,31,32]
quantum chemical methods,
which have shown that the
1.44 V).
This marked difference reflects the large in-
order of the closely spaced n and p-orbital depends on the R
crease of both the LUMO and HOMO energies occurring upon
replacement of the two PR fragments in P R by the much less
+
[
24j]
substituent,
and in the case of aryl diphosphenes also on
2
2
[
25]
[18]
the aryl twist angle. Remarkably the same subtle change of
the n+ and p-orbital sequence was found recently for the iso-
lobal Si (NHC) compounds by quantum chemical calculation-
electronegative, isolobal Si(Idipp) fragments in 1.
This
energy shift reduces considerably the electron affinity of 1 and
[
33]
decreases its ionization potential, rendering 1 a strong one-
2
2
[6a,18,26]
s.
This prompted us to probe the one-electron oxidation
electron reducing agent that is comparable with cobaltocene
[
34]
[35]
of 1 and carry out a comprehensive experimental and theoreti-
(E1/2 =À0.85 V) or P (Idipp) (E =À0.97 V).
2
2
1/2
+
cal study of the structure, properties and dynamics of the 1
Following the results of cyclic voltammetry, we attempted
radical ion, which can be strikingly viewed as NHC-trapped
the chemical one-electron oxidation of 1 using [Fe(C Me ) ]
5
5 2
+
F
[36]
[
Si ] .
2
[B(Ar ) ] as oxidant (Scheme 1). In fact, vacuum transfer of
4
1
,3-C H F to an equimolar mixture of red-colored 1 and
6 4 2
F
green-colored [Fe(C Me ) ][B(Ar ) ] at À1968C followed by
5
5 2
4
Results and Discussion
Experimental studies
+
At first, the stability of the radical cation 1 was addressed
under electrochemical conditions. For this purpose, cyclic vol-
tammetric studies of 1 were carried out in fluorobenzene, tet-
rahydrofuran, and 1,2-difluorobenzene at room temperature.
The cyclic voltammograms (CVs) in all solvents used showed
a reversible one-electron oxidation of 1 at a low half-wave po-
tential (E ), which is slightly solvent-dependent, ranging from
F
4
Scheme 1. One-electron oxidation of 1 leading to 1-B(Ar ) . Compound 1
+
and the two resonance forms of 1 are depicted without formal charges;
the electron lone pairs at the silicon atoms are indicated by two dots and
the unpaired electron by one dot.
1
/2
À0.794 V (THF and 1,2-C H F ) to À0.730 V (C H F) vs. the
6
4
2
6
5
+
/0
[18,27,28]
[
Fe(C Me ) ]
redox couple (Figure 1).
No redox wave
5
5 2
was found at more negative potentials (until À2.0 V), suggest-
warming of the mixture to À408C was accompanied by a color
change to intense green. Addition of n-hexane to the dark-
green solution followed by crystallization at À608C afforded
F
the radical salt 1-B(Ar ) in 71% yield as a green extremely sen-
4
sitive solid, which turns instantaneous to a white solid upon
F
exposure to air. 1-B(Ar ) is reduced selectively back to 1 upon
4
[
27]
+
treatment with one equiv of C K in THF. Thus, 1 /1 is a rare
8
[
37]
example of a chemically reversible Si-based redox system.
F
1
-B(Ar ) can be stored at À308C for several months without
4
apparent sign of decomposition on visual inspection or using
EPR spectroscopy. However, storage of the solid at room tem-
perature for approximately one day afforded a red solid of un-
known composition. Similarly, greenish solutions of intact 1-
Figure 1. Single-wave cyclic voltammograms of 1 at different scan rates in
F
B(Ar ) in fluorobenzene or diethyl ether at À308C turned rap-
4
fluorobenzene at room temperature in the potential range À1.050 to
+
idly brown–red after warming to room temperature, indicating
À0.350 V; reference electrode: dmfc /dmfc/0.1m N(nBu)
4 6 6 5
PF /C H F
F
(
dmfc=decamethylferrocene).
an increased thermolability of 1-B(Ar ) in solution. Monitoring
4
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F
of the decomposition of 1-B(Ar ) in diethyl ether at ambient
pected to lead to an elongation of the SiÀSi bond as observed
4
[
40]
temperature by cw X-band EPR spectroscopy revealed a mono-
exponential decay with a half-life t1/2 of 14218 s, leading to
EPR-silent products in a brown-red solution. Several paramag-
netic intermediates of low concentration were detected during
in the one-electron oxidation of disilenes. Another distinct
structural change provoked by the one-electron oxidation is
a reduction of the symmetry from centrosymmetric in 1 (C) to
i
+
C in 1 , rendering the two Si sites (Si1 and Si2) inequivalent.
1
+
the decay and one of these could be identified as the radical
In fact, the Si2ÀC28 bond (1.899(7) Š) in 1 is shorter than the
+
+
cation [SiH (Idipp)] on the basis of the hyperfine splitting pat-
Si1ÀC1 bond (1.910(8) Š), and both SiÀC
bonds of 1 are
2
NHC
[
6a]
tern of its EPR signal (triplet of quintets of triplets) and the
slightly shorter than those of 1 (SiÀC =1.927(2) Š). Fur-
NHC
1
a
magnitude of the hyperfine coupling constants (A( H )=
thermore, the angle at Si2 (aC28-Si2-Si1=109.9(2)8) is larger
than that at Si1 (aC1-Si1-Si2=101.9(2)8), and both angles are
14
g
1
d
[27]
1
.2 mT, A( N )=0.2 mT, A( H )=0.05 mT). Analysis of the EPR
1
[6a]
silent products by H NMR spectroscopy indicated an unselec-
more obtuse than those of 1 (aCNHC-Si-Si=93.37(5)8). These
F
tive decomposition of 1-B(Ar ) to a mixture of Idipp-contain-
structural trends can be rationalized according to NBO analyses
4
F
ing products including 1 and (HIdipp)[B(Ar ) ].
with the increased s-character of the Si natural hybrid orbitals
4
F
+
The crystal structure of 1-B(Ar ) ·2(1,3-C H F ) is composed of
employed in the SiÀC
and the SiÀSi s-bonding of 1 , and
4
6
4
2
NHC
+
well-separated radical cations 1 and borate anions with the
closest Si···F contact (5.92(1) Š) being significantly longer than
the sum of the crystallographic van der Waals radii of silicon
are consistent with the slightly larger spin density at Si2 ob-
[
27]
tained by the DFT calculations. Finally, a change in the con-
formation of the NHC substituents is observed upon one-elec-
tron oxidation of 1. Thus, whereas in 1 the NHC groups adopt
[38]
+
and fluorine (3.6 Š). The radical cation 1 features as
a trans-bent planar CNHC-Si-Si-CNHC core with a torsion angle
[
6a]
1
an orthogonal conformation (aNNHC-CNHC-Si-Si#=90.88),
in
+
+
of 174.3(4)8 (Figure 2, left). The SiÀSi bond length of 1
2.178(3) Š) is 2.3% shorter than that of 1 (2.229(1) Š) and
1
the Si2-bonded NHC group is almost coplanar with the
[
6a]
(
CNHC-Si-Si-CNHC plane (aN3-C28-Si2-Si1=2.6(8)8), while the Si1-
bonded NHC group is twisted out of the CNHC-Si-Si-CNHC plane,
as shown by the torsion angle N2-C1-Si1-Si2 of 53.1(8)8
(
Figure 2, right). Remarkably, the same structural differences as
+
described above between 1 and 1 have been previously ob-
served between the one- and two-electron reduction product
[
41]
of the digermyne Ge R (R=C H -2,6-(C H -2,4,6-iPr ) ), indi-
2
2
6
2
+
6
3
3 2
À
cating the isolobal analogy of 1 with [Ge R ] and of 1 with
2
2
2À
[
Ge R ]
2 2
.
F
The electronic structure of 1-B(Ar ) was investigated by con-
tinuous-wave (cw) EPR spectroscopy. The EPR spectrum at X-
4
[
27]
F
band frequencies (9.4 GHz) of 1-B(Ar ) recorded in liquid dieth-
4
yl ether solution at 220 K showed an intense signal at a giso
+
value of 1.9979 (Figure 3a). This signal originates from the 1
isotopomers (90.9% relative abundance) containing the mag-
28
netically inactive nuclei Si (natural abundance: 92.23%) and
3
0
+
Si (natural abundance: 3.1%). Remarkably, the giso value of 1
compares well to that of the isolobal disilyne radical anion in
Figure 2. Left: DIAMOND plot of the molecular structure of the radical
+
F
[51]
[39]
4 6 4 2
cation 1 in the single crystal of 1-B(Ar ) ·2(1,3-C H F ). Thermal ellipsoids
[
K(dme) ][Si (SiiPrDsi ) ] (g =1.99962). It is however smaller
4 2 2 2 iso
are set at 30% probability at 100(2) K. Hydrogen atoms are omitted for clari-
than that of the free electron (g =2.0023), in contrast to the
e
ty. Selected bond lengths [Š], bond angles [8], and torsion angles [8]: Si1ÀSi2
commonly larger g-values of Si-centered radicals ranging from
2
1
.178(3), Si1ÀC1 1.910(8), Si2ÀC28 1.899(7); C1-Si1-Si2 101.9(2), C28-Si2-Si1
[
42,43]
F
09.9(2); C1-Si-Si2-C28 174.3(4), N2-C1-Si1-Si2 53.1(8), N3-C28-Si2-Si1 2.6(8).
2.0027–2.0077.
The central EPR signal of 1-B(Ar ) in the
4
Right: View of the radical cation along the CNHC-Si-Si-CNHC core showing the
different conformation of the NHC rings; the dipp substituents were not
drawn for better visibility.
liquid phase is flanked by a pair of inner and outer satellite sig-
+
nals, which originate from the 1 isotopomers bearing one or
29
two magnetically active Si nuclei (I=1/2, natural abundance:
.67%), respectively (Figure 3a). The observed multiplicity pat-
4
29
compares very well with the internal SiÀSi bond of the disilyne
radical anion in [K(dme) ][Si (SiiPrDsi ) ] (2.173(1) Š, Dsi=
tern of the Si satellite signals and their relative intensities
suggests unequivocally the presence of two equivalent Si sites,
4
2
2 2
28/30 29
CH(SiMe ) ) substantiating the isolobal relationship between
with the
Si Si isotopomers (relative abundance: 8.9%)
3
2
[
39]
+
the two ions. Contraction of the SiÀSi bond of 1 suggests
an increase in the SiÀSi bond order as shown by the increased
giving rise to a doublet signal (inner pair of satellites) and the
2
9
29
Si Si isotopomer (relative abundance 0.2%) giving rise to
a triplet signal (the central line of the triplet coincides with the
+
Wiberg bond index (WBI; WBI(SiÀSi) of 1 =2.046; WBI(SiÀSi)
[
27]
28/30 28/30
of 1=1.703). It originates according to natural bond orbital
intense signal of the
Si Si isotopomers and so only the
[
44]
(
NBO) analysis from a decreased lone-pair repulsion, and an in-
outer lines are visible). The signals are separated by half the
29
crease in bonding SiÀSi electron density occurring upon de-
value of the isotropic hyperfine coupling constant (A ( Si)=
iso
[27]
+
population of the n orbital. In contrast, removal of one
5.99 mT). Notably, the hyperfine coupling in 1 is considerably
+
electron from the Si=Si p-bonding orbital of 1 would be ex-
larger than in other Si radicals, in which the unpaired electron
Chem. Eur. J. 2015, 21, 12509 – 12516
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
a Q-band spectrum (34.1 GHz) of a solid state sample of 1-
F
B(Ar ) , and compares moreover favorably with the calculated
4
g-tensor at the TPSSh/aug-cc-pVTZ/SVP level of theory
F
(
Table 1). The X-band spectrum of 1-B(Ar ) in frozen solution
4
further displayed two broad satellites of weak intensity at both
sides of the intense major signal. Several cases were consid-
Table 1. Comparison of the calculated (TPSSh/aug-cc-pVTZ/SVP) g and
2
9
A( Si) tensor components of the optimized C
1
-symmetric minimum struc-
+
ture of 1 (B3LYP/6-311+G**/6-31G*) with the experimental values ob-
tained from the EPR spectrum of 1-B(Ar ) in frozen solution; hyperfine
4
couplings are given in mT.
F
[a,b]
g
11
g
22
g
33
g
iso
calculated
experimental
2.0036
2.0031
2.0022
2.0026
1.9888
1.9880
1.9982
1.9979
Si1
11
Si1
Si1
Si1
A
A
22
A
33
A
iso
calculated
experimental
À2.14
À8.29
À2.46
À4.30
3.11
8.64
3.82
5.19
Si2
Si2
Si2
Si2
A
11
A
22
A
33
A
iso
calculated
experimental
À2.70
À9.03
À3.09
À4.94
4.71
10.25
5.42
6.79
[
a] The isotropic values were calculated as the average of the g and
2
9
A( Si) tensor components: giso =(g11+g22+g33)/3 and Aiso =(A11+A22+A33)/
3
2
9
. [b] The sign of the A( Si) values cannot be determined in cw experi-
ments. However, the relative sign of the principal values A11, A22, and A33
of the hyperfine coupling tensor could be determined and only equal
signs led to a good fit of the experimental spectra. This was supported
by the DFT calculations, which gave negative values for the Si hyperfine
coupling constants due to the negative value of the magnetic moment
Figure 3. Experimental (c) and simulated (g) cw X-band EPR spectra of
F
1-B(Ar )
4
in diethyl ether at a) T=220 K (liquid state) and b) T=50 K (frozen
29
of the Si nucleus and the positive value of the spin density at both Si
atoms.
state). The marked signal (*) in a) originates from a decay product of 1-
F
4
B(Ar ) .
29
ered to assign all components of the A( Si) hyperfine coupling
tensors, which proved to be quite challenging owing to the su-
perposition of the satellite signals with the intense central
is delocalized over two Si atoms, such as the disilene radical
+
29
a
[40]
cation [Si (SiMetBu ) ] (A ( Si )=2.30 mT), or the disilyne
2
2 4
iso
À
29
a
[39]
[27]
radical anion [Si (SiiPrDsi ) ] (A ( Si )=3.92 mT) and is 3.5
signal and the large line width of the signals. These cases
2
2 2
iso
+
4
À
times larger than that in the [Si ] cation in its C S ground
were validated by quantification of the deviation between the
simulated and the experimental spectrum. The smallest devia-
tion was obtained assuming the presence of two magnetically
inequivalent Si atoms Si1 and Si2 differing in the isotropic but
2
g
29
[4]
state (A ( Si) =1.51–1.71 mT). This comparison suggests
iso
calc.
F
an increased s-character of the SOMO of 1-B(Ar ) supporting
4
+
the s-radical character of 1 , and reflects the quite different
+
+
[27]
electronic ground state of 1 (S=1/2) from that of [Si2] (S=
not in the dipolar part of the hyperfine coupling. The de-
[
3,45]
3
/2).
The equivalency of the two Si atoms of 1 in solution raised
the question as to whether the C -symmetric structure of 1 is
rived EPR parameters were backed up by quantum chemical
calculations, which revealed a very good agreement of the cal-
culated with the experimental g values and consistent trends
in the principal values of the hyperfine coupling tensors
+
+
1
a result of anisotropic interactions in the solid state or an in-
+
[27]
trinsic property of 1 , and which is the process in case of a C -
(Table 1).
1
F
symmetric minimum structure equilibrating the two Si sites in
The observation of a C -symmetric structure of 1-B(Ar ) with
1
4
solution. These issues were addressed by analyzing the EPR
two magnetically inequivalent Si atoms in the solid-state and
in frozen solution, but of a higher symmetric structure with
F
spectra of 1-B(Ar ) in frozen solution and in the solid state in
4
combination with quantum chemical calculations. The X-band
two magnetically equivalent Si atoms in solution, suggested
F
[46]
EPR-spectrum of 1-B(Ar ) in frozen diethyl ether solution at
that a topomerization
takes place in solution, leading to
4
5
0 K showed an intense signal between 334 mT and 338 mT,
a positional exchange of the two Si sites. Attempts to detect
this dynamic process by EPR spectroscopy in solution were not
which could be simulated best with a g-tensor of weak ortho-
rhombicity (g =2.0031, g =2.0026, g =1.9880) (Figure 3b).
successful, since even at 157 K, the melting point T of diethyl
1
1
22
33
M
The g-tensor could also be used for the simulation of the cor-
ether, the EPR spectrum showed only equivalent Si sites.
Taking into account the difference of the hyperfine coupling
responding X-band spectrum in the liquid phase as well as for
Chem. Eur. J. 2015, 21, 12509 – 12516
www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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2
9
constants of the two inequivalent Si nuclei in frozen solution
Dn=45 MHz), an upper limit for the Gibbs energy of activa-
(
°
À1
tion DG (<13.5 kJmol ) and a low limit for the exchange
[
27]
rate kex (>100 MHz) was calculated for the dynamic process.
Quantum-chemical calculations
To elucidate whether a topomerization process is operative in
+
solution, the potential energy hypersurface of 1 was analyzed
at the B3LYP/6-311 +G**/6-31G* level of theory. This analysis
+
ensued an overall C symmetric minimum structure of 1 as
1
found by X-ray crystallography and furthermore fully repro-
duced the experimentally observed structural differences be-
+
[27]
Figure 4. Schematic Gibbs energy profile (T=298 K) for the topomerization
tween 1 and 1 (Table 2). A comparison of the calculated
+
of 1 showing the B3LYP/6-311+G**/6-31G* optimized C
1
and C
. The 2,6-di-
isopropylphenyl groups and the hydrogen atoms are omitted for clarity.
i
symmetric
+
(
gas-phase) with the experimental bonding parameters of 1
minimum structures and the interconnecting transition state TS
1
F
derived from the X-ray diffraction analysis of 1-B(Ar ) revealed
4
Table 2. Comparison of selected calculated (B3LYP/6-311+G**/6-31G*) and experimental bond lengths [Š],
+
bond angles [8], and torsion angles [8] of 1 and 1.
space built from the DFT-derived
symmetric (n ) and antisymmet-
+
Si1ÀSi2 Si1ÀC1
2.223 1.944
2.178(3) 1.910(8) 1.899(7)
2.247 1.954 1.954
2.229(1) 1.927(2) 1.927(2)
Si2ÀC28 C1-Si1-Si2 C28-Si2-Si1 N2-C1-Si1-Si2 N3-C28-Si2-Si1
ric (n ) combination of the lone
À
calcd
exptl
calcd
1.942
106.6
101.9(2)
98.8
108.5
109.9(2)
98.8
39.4
53.1(8)
87.8
23.4
2.6(8)
À87.8
+
1
1
pair orbitals and the Si=Si p and
p* orbitals in order to elucidate,
whether a multiconfigurational
wave function is necessary for
[a]
exptl
93.37(5)
93.37(5)
90.8(1)
À90.8(1)
[
a] The experimental bonding parameters of 1 were taken from Ref. [6a].
+
the description of 1 . The ob-
tained CAS orbitals with their
energy values and occupancies
an overall good agreement, with the calculated SiÀSi and SiÀ
bond lengths being roughly 2% larger than the experi-
C
NHC
are depicted in Figure 5. Two determinants are necessary to
+
mental values (Table 2). The calculated and experimental struc-
describe the electronic structure of 1 . The major determinant
+
ture of 1 differ mostly in the conformation of the two N-het-
with a contribution of 94% to the overall wave function has
erocyclic carbenes as evidenced by the torsion angles N2-C1-
Si1-Si2 and N3-C28-Si2-Si1 (Table 2). This difference is not sur-
À1
prising given the minor energy of only few kJmol required
to change these torsion angles (Figure 4). In general, the struc-
tural differences between the Si1 and Si2 site in the C sym-
1
+
metric minimum structure of 1 were calculated to be smaller
in the gas-phase than found in the solid-state by X-ray crystal-
lography (Table 2).
Along with the global C -symmetric minimum structure also
1
+
Figure 5. Calculated CASSCF (5,4)/TZVP orbitals of 1 with their correspond-
a local minimum structure of C symmetry was found on the
À1
i
ing energy eigenvalues and occupancies; iso surface value: 0.04 ebohr
.
À1
energy hypersurface, lying only 3 kJmol higher in energy
[
47]
(
Figure 4). A quadratic synchronous transit (QST2) calculation
established that the two minimum structures are connected
the configuration [2-2-1-0], in which the n Si lone pair orbital
À
À1
via a transition state, which lies only 6 kJmol higher in
(HOMOÀ2) and Si=Si p-orbital (HOMOÀ1) are fully occupied,
energy than the C symmetric ground-state structure. The tran-
the n+ orbital (SOMO) is singly occupied and the Si=Si p*-orbi-
1
sition state shows an imaginary frequency at À4 cmÀ1 and in-
terconverts the two minimum structures along a mode involv-
ing a combined bending and torsion of the NHC substituents
tal (LUMO) is empty. This corroborates the description of 1 as
+
a s-type radical. The minor determinant with the configuration
[2-0-1-2] has only a small contribution (6%) to the overall wave
function. The energy gap between the n+ and Si=Si p-orbital
(
Figure 4). The calculated energy barrier is much lower than
the low limit for spectroscopic detection by EPR spectroscopy
(DE
=2.97 eV) is considerably larger than that in
SOMOÀHOMOÀ1
°
À1
(
DG <13.5 kJmol ) and is fully consistent with the observed
1 (DE
=0.03 eV), indicating that a significant ener-
HOMOÀHOMOÀ1
+
topomerization of 1 in diethyl ether solution occurring rapid-
ly even at the melting point of the solvent.
A complete active-space self-consistent field (CASSCF)
getic separation of the n+ orbital and the Si=Si p-orbital
occurs upon one-electron oxidation of 1.
A comparison of the frontier orbitals of the radical cation 1
[48]
[
49]
+
+
À
(
5,4)/TZVP calculation of 1 was carried out with an active
with those of the disilyne radical anions [Si R ] (R=singly
2
2
Chem. Eur. J. 2015, 21, 12509 – 12516
www.chemeurj.org
12513
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[39]
bonded silyl substituent) reveals the similar electronic struc-
ture (isolobal analogy) of the two opposite charged ions,
which can also be illustrated by the canonical formulae given
in Figure 6.
Table 3. Calculated ZPVE (zero point vibrational energy) corrected bond
0
dissociation enthalpies at 0 K (D (0)) for the stepwise dissociation of Idipp
+
[a]
from 1 and 1.
0
À1
+
D (0) [kJmol
]
The CASSCF(5,4) results indicate, that the radical cation 1
+
+
[
(
[
(
Si
(
Si
S=1/2)
Si (Idipp)]
S=1/2)
(Idipp)
S=0)
Si (Idipp)
S=0)
2
(Idipp)
2
]
!
[Si
(S=1/2)
2
(Idipp)]
+
+
+
+
Idipp
(S=0)
Idipp
(S=0)
Idipp
(S=0)
Idipp
(S=0)
243.1
431.3
126.5
185.0
(1)
(2)
(3)
(4)
can be approximately described by
a one-determinant
+
+
4
gÀ [b]
2
!
!
!
[Si
(S=3/2)
Si (Idipp)
(S=0)
2
]
(C S
)
2
2
2
3
gÀ [c]
2
Si
2
(C S
)
(
[
(S=1)
a] The total spin angular momentum S of each species is written in pa-
renthesis below the species and the term symbol of selected fragments is
given in parenthesis after the fragment. [b] The valence electronic config-
+
À
Figure 6. Canonical formulae of 1 (left) and [Si
2
R
2
]
(right). Formal charges
+
4
À
2
2
1
2
are encircled; two dots indicate the lone pairs of electrons and one dot the
unpaired electrons.
g g u g u
uration of [Si2] in the C S ground state is 4s 4s 5s 2p . [c] The va-
3
À
lence electronic configuration of Si2 in the C S
g
ground state is
2
2
2
2
g u g u
4s 4s 5s 2p .
+
method. This suggests that the EPR parameters of 1 can be
[27]
+
reasonably well calculated using DFT methods. Calculations
at three different levels of theory gave an excellent agreement
between the experimental and the calculated g tensors, and
consistent trends in the principal values of the experimental
sociation of the first Idipp has a doublet ground state as 1 ,
+
but in contrast to 1 is a p-type radical, in which the Si=Si p-
orbital is the SOMO followed by the n+ orbital (HOMOÀ1).
Similarly, in [Si (Idipp)] the order of two highest occupied mo-
2
[27]
and calculated hyperfine coupling tensors (Table 1). Notably,
lecular orbitals is reversed compared to 1 with the Si=Si p-orbi-
tal being the HOMO followed by the n+ orbital (HOMOÀ1).
2
9
the absolute value of A ( Si) is slightly larger at the Si2 site.
iso
This can be rationalized with the increased spin density at this
atom. In fact, a Mulliken spin density analysis revealed that the
This explains why upon one-electron oxidation of [Si (Idipp)] to
2
+
give [Si (Idipp)] , which involves a removal of one electron
2
+
overall spin density of 1 is mainly distributed over the Si1
from the Si=Si p-bonding orbital, the SiÀSi bond length is con-
and Si2 atoms and is slightly larger on the Si2 atom (Si1: 46%,
Si2: 48%). Furthermore, a slightly larger contribution of the Si2
atom orbitals to the SOMO was found (Si1: 34.3%, Si2: 34.8%).
siderably increased from 2.206 to 2.346 Š, whereas the oppo-
site is observed in the 1e-oxidation of 1 (d(SiÀSi) =2.247 Š)
calc.
+
[27]
to give 1 (d(SiÀSi) =2.223 Š) (Table 2).
calc.
+
The charge distribution in 1 was studied by natural popula-
tion analysis and compared with that in 1. One-electron oxida-
tion to give the radical cation 1 leads to an increase of the Si
Conclusion
+
partial charge from À0.10 in 1 to +0.24 (Si1) and +0.22 (Si2)
The present comprehensive structural, spectroscopic, and the-
+
F
in 1 . This shows that oxidation mainly occurs at the Si atoms
oretical study of 1-B(Ar ) suggests that the radical cation
4
À
+
leading to an overall removal of 0.66 e from these atoms.
[Si (Idipp) ] should be rather regarded as the unprecedented
2
2
À
Finally, the enthalpies of the stepwise dissociation of the
cationic counterpart of the disilyne radical anions [Si R ] (R=
2 2
+
+
Idipp groups from 1 were calculated at the B3LYP/6-311 +
singly bonded silyl group) than a bis-NHC adduct of [Si ] . Im-
2
G**/6-31G* level of theory and compared with those of 1 to
plications of this electronic analogy, which can be traced back
[27]
À
analyze the SiÀCNHC bond strengths (Table 3). Dissociation of
to the isolobal relationship between a Si(Idipp) and a [SiR]
+
the first Idipp group from 1 needs a considerably lower
fragment in low-valent silicon chemistry are currently under
study.
0
À1
energy (D (0) =243.1 kJmol ) than that of the second Idipp
1
+
0
À1
from [Si (Idipp)] (D (0) =431.3 kJmol ; Table 3). The same
2
2
trend is observed for the successive dissociation of Idipp from
Acknowledgements
+
+
1
. The overall energy required to dissociate 1 into [Si2]
4
À
0
0
À1
(
C S ) and two IDipp (D (0) +D (0) =674.4 kJmol ) is more
g 1 2
We thank the Deutsche Forschungsgemeinschaft (SFB813,
than twice the dissociation energy consumed to transform
“
Chemistry at Spin Centers”) for the financial support of this
3
À
0
0
1
3
into [Si ] (C S ) and two Idipp (D (0) +D (0) =
2
À1
g
3
4
work. We also thank Dr. B. Lewall for the cyclovoltammetric
studies and A. Martens for the elemental analyses.
11.5 kJmol ; Table 3). This can be explained with the in-
+
creased charge separation (Coulomb energy) in 1 . Remarka-
bly, the successive dissociation of both Idipp groups from 1
require a considerably higher energy (243.1 and 431.3 kJmol )
+
À1
Keywords: multiple bonds · N-heterocyclic carbenes · radical
ions · silicon
than those of typical CNHC!Si donor–acceptor bonds (for ex-
0
À1 [10b,d]
ample, D (0) of SiX (Idipp) (X=Cl, Br, I)=121–124 kJmol )
2
suggesting the presence of more covalent SiÀC
bonds in
NHC
+
[50]
+
1
.
Notably, the fragment ion [Si (Idipp)] resulting after dis-
2
Chem. Eur. J. 2015, 21, 12509 – 12516
www.chemeurj.org
12514
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[
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27] Details of the cyclic voltammetric studies are given in the Supporting
Information, which also contains the synthesis, the reduction, the EPR
spectroscopic and decay studies of 1-B(Ar )
quantum chemical studies and bonding analyses of 1 and 1 .
F
4
, as well as the comparative
+
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[
28] All potentials in the text are given vs. the [Fe(C
5
Me
5
)
2
]
redox couple,
1
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+
/0
owing to its favorable properties versus the [Fe(C
5
H
5
)
2
]
redox
[
10] a) R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn, D. Stalke, Angew.
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5
687; Angew. Chem. 2009, 121, 5797; c) A. C. Filippou, O. Chernov, G.
+
/0
son reasons, the half-wave potential of the [Fe(C
was also determined under the same conditions and found to be
5
H
5
2
) ]
redox couple
Schnakenburg, Chem. Eur. J. 2011, 17, 13574; d) A. C. Filippou, Y. N. Leb-
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6 5 6 4 2
0.440 V (in THF), 0.520 V (in C H F) and 0.562 V (in 1,2-C H F ) versus
+
/0
the [Fe(C
5
Me
5
)
2
]
redox couple.
[
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Chem. Eur. J. 2015, 21, 12509 – 12516
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Full Paper
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28/30 29
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Received: June 5, 2015
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www.chemeurj.org
12516
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim