Addition of Small Electrophiles to N-Heterocyclic-Carbene-Stabilized Disilicon(0): A Revisit of the Isolobal Concept in Low-Valent Silicon Chemistry

Article abstract of DOI:10.1021/jacs.6b01018

Protonation and alkylation of (Idipp)S=Si(Idipp) (1) afforded the mixed-valent disilicon(I)-borates [(Idipp)(R)Si^II=Si⁰(Idipp)][B(Ar^F)₄] (1R[B(Ar^F)₄]; R = H, Me, Et; Ar^F = C₆H₃-3,5-(CF₃)₂; Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂) as red to orange colored, highly air-sensitive solids, which were characterized by single-crystal X-ray diffraction, IR spectroscopy and multinuclear NMR spectroscopy. Dynamic NMR studies in solution revealed a degenerate isomerization (topomerization) of the "σ-bonded" tautomers of 1H[B(Ar^F)₄], which proceeds according to quantum chemical calculations via a NHC-stabilized (NHC = N-heterocyclic carbene) disilahydronium ion ("π-bonded" isomer) and is reminiscent of the degenerate rearrangement of carbenium ions formed upon protonation of olefins. The topomerization of 1H[B(Ar^F)₄] provides the first example of a reversible 1,2-H migration along a Si=Si bond observed in a molecular system. In contrast, 1Me[B(Ar^F)₄] adopts a "rigid" structure in solution due to the higher energy required for the interconversion of the "σ-bonded" isomer into a putative NHC-stabilized disilamethonium ion. Addition of alkali metal borates to 1 afforded the alkali metal disilicon(0) borates 1M[BAr₄] (M = Li, Ar = C₆F₅; M = Na, Ar = Ar^F) as brown, air-sensitive solids. Single-crystal X-ray diffraction analyses and NMR spectroscopic studies of 1M[BAr₄] suggest in concert with quantum chemical calculations that encapsulation of the alkali metal cations in the cavity of 1 predominantly occurs via electrostatic cation-π interactions with the Si=Si π-bond and the peripheral NHC aryl rings. Displacement of the [Si(NHC)] fragments by the isolobal fragments [PR] and [SiR]^- interrelates the cations [(NHC)(R)Si=Si(NHC)]⁺ to a series of familiar, multiply bonded Si and P compounds as verified by analyses of their electronic structures.

Full text of DOI:10.1021/jacs.6b01018

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Article
Addition of Small Electrophiles to NHC-stabilized Disilicon(0): A
Revisit of the Isolobal Concept in Low-Valent Silicon Chemistry
Marius I. Arz, Martin Straßmann, Daniel Geiß, Gregor Schnakenburg, and Alexander C. Filippou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01018 • Publication Date (Web): 15 Mar 2016
Downloaded from http://pubs.acs.org on March 16, 2016
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Addition of Small Electrophiles to NHC-stabilized Disilicon(0): A Revisit
of the Isolobal Concept in Low-Valent Silicon Chemistry
Marius I. Arz, Martin Straßmann, Daniel Geiß, Gregor Schnakenburg, and Alexander C.
Filippou*
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Institute of Inorganic Chemistry, University of Bonn, GerhardꢀDomagkꢀStraße 1, Dꢀ53121 Bonn, Germany
Abstract: Protonation and alkylation of (Idipp)Si=Si(Idipp) (1) afforded the mixedꢀvalent
II
0
F
F
F
disilicon(I)ꢀborates [(Idipp)(R)Si =Si (Idipp)][B(Ar )
4 4 6 3
] (1R[B(Ar ) ]; R = H, Me, Et; Ar = C H ꢀ
3
,5ꢀ(CF ; Idipp = C[N(C ꢀ2,6ꢀiPr )CH] ) as red to orange colored, highly airꢀsensitive
3
)
2
6
H
3
2
2
solids, which were characterized by singleꢀcrystal Xꢀray diffraction, IR spectroscopy and
multinuclear NMR spectroscopy. Dynamic NMR studies in solution revealed a degenerate
F
isomerisation (topomerisation) of the “σꢀbonded” tautomers of 1H[B(Ar )
4
], which proceeds
according to quantum chemical calculations via a NHCꢀstabilized (NHC = Nꢀheterocyclic
carbene) disilahydronium ion (“πꢀbonded” isomer) and is reminiscent of the degenerate
rearrangement of carbenium ions formed upon protonation of olefins. The topomerisation of
F
1H[B(Ar )
4
] provides the first example of a reversible 1,2ꢀH migration along a Si=Si bond
F
observed in a molecular system. In contrast, 1Me[B(Ar )
4
] adopts a “rigid” structure in
solution due to the higher energy required for the interconversion of the “σꢀbonded” isomer
into a putative NHCꢀstabilized disilamethonium ion. Addition of alkali metal borates to 1
F
afforded the alkali metal disilicon(0) borates 1M[BAr ] (M = Li, Ar = C F ; M = Na, Ar = Ar ) as
4
6 5
brown, airꢀsensitive solids. Singleꢀcrystal Xꢀray diffraction analyses and NMR spectroscopic
studies of 1M[BAr ] suggest in concert with quantum chemical calculations that
4
encapsulation of the alkali metal cations in the cavity of 1 predominantly occurs via
electrostatic cation πꢀinteractions with the Si=Si πꢀbond and the peripheral NHC aryl rings.
−
Displacement of the [Si(NHC)] fragments by the isolobal fragments [PR] and [SiR]
+
interrelates the cations [(NHC)(R)Si=Si(NHC)] to a series of familiar, multiply bonded Si and
P compounds as verified by analyses of their electronic structures.
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1. Introduction
Stabilization of highly reactive lowꢀvalent mainꢀgroup element species by utilizing Nꢀ
heterocyclic carbenes (NHCs) as particularly strong σꢀdonors led in recent years to a
plethora of novel compounds with intriguing synthetic potential and to a flourishing, rapidly
0
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developing field in chemistry. Appealing examples in lowꢀvalent silicon chemistry include the
0
2
Si compounds (Idipp)Si=Si(Idipp) (1, Idipp = C[N(dipp)CH]
2
, dipp = C
6
H
3
ꢀ2,6ꢀiPr
2
) and
3
I
2
Si(bNHC) (bNHC = bidentate Nꢀheterocyclic carbene), the Si halides Si
2
X
2
(Idipp)
2
(X = Cl,
4
4
II
5
2
Br, I ), the NHCꢀadducts of the Si halides SiX (X = Cl – I), Si(X)R (X = Cl, H; R = aryl,
6
7
+ 8
amino), and of the disilenylsilylene SiR(SiR=SiR ), or the NHCꢀstabilized cations [Si ] ,
2
2
+
2+ 5d,6e,9
10
[SiR] and Si .
The same strategy also enabled to trap silagermenylidenes (R
2
Si=Ge),
11
12
phosphasilenylidenes (RP=Si) and disilavinylidenes (R
2
Si=Si).
Analysis of the bonding between mainꢀgroup elements and NHCs raised a controversial
1
3
debate in the literature. G. Frenking et al. viewed 1 as a Si molecule that is trapped in its
2
1
excited ( ꢀ ) electronic state by two NHCs, and calculated the donorꢀacceptor Si–C
g
NHC
1
4
interactions in 1 using the EDAꢀNOCV method. Although this donorꢀacceptor model
provides a simplified interpretation of the bonding in 1, which has been also used to
14,15
rationalize the bonding in other NHCꢀstabilized diatomic molecules,
it is not adequate to
) electronic state of
1
forecast the reactivity of 1 given the considerably higher energy of the ( ꢀ
g
3
− 16
Si
2
than that of its triplet groundꢀstate configuration (Χ Σ
g
)
and the rather large Si–CNHC
in reactions.
1
7
bond dissociation energies, which prevented so far to use 1 as a source of Si
2
An alternative view of 1 was recently presented in an experimental and theoretical study
by our group, which uncovered the similarity of the electronic structure of 1 with that of the
diphosphene (E)ꢀMes*P=PMes* (Mes*
=
C
6
H
2
ꢀ2,4,6ꢀtBu
3
)
and the NHCꢀstabilized
phosphasilenylidene (Idipp)Si=PMes*, with all three compounds showing a similar sequence
18
of the frontier orbitals with almost isoenergetic n (E–E) lone pair (E = Si or P) and π(E=E)
+
1
1
orbitals as the HOMO and the HOMO−1, respectively. This relationship, which can be
19
−
traced back to the isolobal analogy of the fragments [Si(NHC)], [PR] and [SiR] , allowed
also to rationalize the experimentally determined structure of the 1eꢀoxidation product of 1
+
(
[Si
2
(Idipp)
2
] ), and to interrelate its electronic structure with that of the disilyne radical anions
−
8,20
[Si
2
R
2
] .
Additional support for this view can be found by comparing the reactivity of the isolobal
1
2
1
1
compounds. Remarkably, diphosphenes have been shown to adopt either η , η or ꢁ(η ;η )
2
1
coordination modes in complexes with coinageꢀmetals,
and similar coinage metal
1
2
complexes of the disilicon(0) compound 1 of the general formulas [MCl(η ꢀ1)], [M(η ꢀ
1
2
1
)][B(C
6
F
5
)
4
] and [M
2
Cl
2
ꢀꢁꢀ(1κSi :2κSi ꢀ1)] (M = coinage metal atom) have been reported
22
recently by G. Robinson et al. and our group.
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In comparison, reactivity studies of diphosphenes towards simple electrophiles such as H
+
or R are very limited. Methylation of (E)ꢀMes*P=PMes* has been reported to give the
+
23a
isolable phosphanylphosphenium cation [Mes*(Me)P=PMes*] , whereas attempts to detect
+
21b,23b
the corresponding protonation product [Mes*(H)P=PMes*] failed.
This prompted us to study the hitherto unexplored reactivity of 1 towards the electrophiles
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+
+
H and R and alkali metal cations. Herein, we report on the protonation and electrophilic
alkylation of 1 leading to the disilicon(I) compounds [(Idipp)(R)Si=Si(Idipp)]A (R = H, Me, Et;
A = borate anion) as well as the reactions of 1 with alkali metal borates, yielding the alkali
2 2
metal disilicon(0) borates [Si (M)(Idipp) ]A (M = Li, Na). All products were thoroughly studied
by a combination of experimental and theoretical methods. In addition, we report on a
detailed quantum chemical study, which corroborates the isolobal analogy of the cations
+
[
(Idipp)(R)Si=Si(Idipp)] to a series of familiar, multiply bonded Si and P compounds.
2. Results and discussion
2.1
Protonation
of
(Idipp)Si=Si(Idipp).
The
hydridodisilicon(I)ꢀborate
F
F
[(Idipp)(H)Si=Si(Idipp)][B(Ar ) ] (1H[B(Ar ) ]) was obtained selectively upon protonation of 1
4
4
F
F
24
with Brookhart’s acid ([H(Et
2
O)
2
][B(Ar )
4
], Ar = C
6
H
3
ꢀ3,5ꢀ(CF
3
)
2
)
in fluorobenzene and
isolated after crystallization from a fluorobenzene / nꢀhexane mixture at −30 °C as a dark red,
highly airꢀsensitive solid in 80 % yield (Scheme 1) (see supporting information (SI), chapter
F
2
.1). Compound 1H[B(Ar )
4
] is a thermally robust solid, which decomposes upon melting in a
F
vacuumꢀsealed glass capillary at 221 °C. Slow decomposition of 1H[B(Ar )
4
]
to
F
1
[
4
IdippH][B(Ar ) ] and other unidentified products was however observed by H NMR
spectroscopy in THFꢀd
8
solution at room temperature within 24 h.
+
Idipp
Si
Idipp
F
+
ꢀ
[H(Et O) ][B(Ar ) ]
2
2
4
2 Et O
−
2
[B(ArF)
]
4
Si
Idipp
Si
Si
Idipp
C H F, RT
6
5
H
F
1
1H[B(Ar ) ]
4
F
4
Scheme 1. Synthesis of compound 1H[B(Ar ) ]. Formal charges are encircled.
F
The solidꢀstate structure of 1H[B(Ar )
4
] was determined by singleꢀcrystal Xꢀray diffraction,
+
and confirmed that the cations 1H (Figure 1) are well separated from the borate anions. The
closest SiꢁꢁꢁF contacts of 4.99(1) Å are significantly longer than the sum of the van der Waals
25
radii of silicon and fluorine (3.6 Å). The H85 atom was localized in the difference Fourier
map and isotropically refined. It is σꢀbonded to the Si1 atom, which is trigonal planar bonded
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according to the sum of bond angles of 360(1)°. The torsion angles C1ꢀSi1ꢀSi2ꢀC28 and
H85ꢀSi1ꢀSi2ꢀC28 of 177.61(8)° and −6(1)° indicate a planar CNHCꢀSi(H)ꢀSiꢀCNHC core with a
transꢀarrangement of the NHCꢀsubstituents. The Si1ꢀbonded central ring of the NHCꢀ
substituent adopts a coplanar conformation (
ϕNHC1 = 8.60(6)°) with respect to the CNHCꢀSiꢀSiꢀ
CNHC plane, whereas the Si2ꢀbonded NHCꢀsubstituent is perpendicularly oriented (
ϕ
NHC2
=
0
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71.06(6)°) (Figure 1, right).
N4
N3
C28
N4
H85
Si2
Si1
C28
Si2
N1
N2
C1
H85 Si1
N3
N1
C1
N2
+
F
Figure 1. Left: DIAMOND plot of the molecular structure of the cation 1H in the crystal lattice of 1H[B(Ar )
4
] at
123(2) K. Thermal ellipsoids are set at 30 % probability. Hydrogen atoms, except the Si1ꢀbonded H85 atom, were
omitted for clarity. Selected bond lengths [Å], bond angles [°] and torsion angles [°]: Si1–C1 1.882(2), Si1–Si2
2.1873(8), Si2–C28 1.940(2), Si1–H85 1.32(2); C1ꢀSi1ꢀSi2 116.73(7), C1ꢀSi1ꢀH1 106(1), Si2ꢀSi1ꢀH85 138(1),
+
C28ꢀSi2ꢀSi1 95.34(6); C1ꢀSi1ꢀSi2ꢀC28 177.61(8), H85ꢀSi1ꢀSi2ꢀC28 −6(1). Right: Different view of 1H illustrating
the different orientation of the Siꢀbonded NHCꢀrings; hydrogen atoms (except the H85 atom) and the Nꢀbonded
dipp substituents were omitted for clarity.
+
The Si=Si bond (2.1873(8) Å) of 1H compares well with those of the isolobal compounds
(
Z)ꢀTbb(Br)Si=Si(SIdipp) (2.167(2) Å, Tbb
=
C
6
H
2
ꢀ2,6ꢀ[CH(SiMe
3
)
2
]
2
ꢀ4ꢀtBu, SIdipp
=
1
2
Si
Si
Si
C[N(dipp)CH ] ),
(IMe )(R )Si=SiR
(2.1989(6) Å, IMe4
=
C[N(Me)CMe]2,
R
=
2
2
4
27
Si
Si
28
SiiPr[CH(SiMe
3
)
2
]
2
), [Li(dme)
3
][R (H)Si=SiR ] (2.2034(9) Å, dme = 1,2ꢀdimethoxyethane)
4
and [(Idipp)(I)Si=Si(Idipp)][B(C F ) ] (2.1739(9) Å), but is shorter (1.9 %) than that of 1
6
5 4
2
29
(
2 2
2.229(1) Å) and Si (SIdipp) (2.2323(8) Å). Shortening of the Si=Si bond of 1 upon
protonation can be rationalized according to comparative natural bonding orbital (NBO)
+
analyses of 1calc and 1H calc (see SI, Tables S7 and S8) with the increased sꢀcharacter of the
+
hybrid orbital on the Si1 atom employed in the Si–Si σꢀbond. Consistently, 1H displays a
shortened Si1–C1 bond (1.882(2) Å) and a widened C1ꢀSi1ꢀSi2 angle (116.73(7)°) compared
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to 1 (Si–CNHC: 1.927(2) Å; ∠CNHCꢀSiꢀSi# = 93.37(5)°). Another salient feature of the structure
+
of 1H is the narrow C28ꢀSi2ꢀSi1 angle of 95.34(6)°, which suggests the presence of a lone
pair of high sꢀcharacter at the Si2 atom, in full agreement with the results of the NBO
+
calc
analysis of 1H
(see SI, Table S8).
F
The ATR FTIR spectrum of a solid state sample of 1H[B(Ar )
4
] displays a weak absorption
(SiꢀH) stretching vibration (see SI, Figure S1).
This band does not appear in the IR spectrum of the deuterated derivative
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−
1
band at 2142 cm , which is assigned to the
ν
F
F
[
[
(Idipp)(D)Si=Si(Idipp)][B(Ar )
4
] (1D[B(Ar )
4
]), which was analogously obtained from 1 and
F
F
D(Et
2
O)
2
][B(Ar )
4
] (see SI, chapter 2.2). However, the
ν
(SiꢀD) absorption band of 1D[B(Ar )
4
],
−
1
which is expected to appear at 1541 cm , was not detected due to its weak intensity and
superimposition with other absorption bands in this region. The ν(SiꢀH) absorption band of
F
1
H[B(Ar )
4
] emerges at a similar wavenumber as the
ν
(SiꢀH) absorptions of the 1,2ꢀ
−1
dihydridodisilenes (E)ꢀSi
2
H
2
R
2
(ν
(SiꢀH) = 2160 cm , R = Bbp = C
6
H
3
3 2 2
ꢀ2,6ꢀ[CH(SiMe ) ] ; ν(Siꢀ
−1
30
H) = 2151 cm , R = Bbt = C H ꢀ2,6ꢀ[CH(SiMe ) ] ꢀ4ꢀC(SiMe ) , both in KBr), but appears at
6
2
3 2 2
3 3
n
lower wavenumbers than those of hydridosilanes (e.g. RSiH Cl3−n (n = 1 – 3): ν(SiꢀH) = 2171
−1
31
–
2240 cm ; R = C
6
H
3
ꢀ2,6ꢀ(C
6
H
2
ꢀ2,4,6ꢀiPr
3 2
) ).
F
Notably, 1H[B(Ar ) ] is fluxional on the NMR timescale. The dynamic process was studied
4
1
13
1
29
by variable temperature H, C{ H} and Si NMR spectroscopy in THFꢀd
8
and involves a
3
2
+
degenerate isomerization (topomerization) between the “σꢀbonded” tautomers of 1H ,
which leads to an exchange of the two heterotopic silicon sites in solution (Scheme 2). Thus,
29
two well separated Si doublet signals at
δ
= 69.4 ppm and 125.4 ppm are observed in the
2
9
F
slow exchange limit Si NMR spectrum of 1H[B(Ar )
4
] in THFꢀd
8
at 213 K for the threeꢀ
coordinated (Si1) and twoꢀcoordinated (Si2) silicon atom, respectively (Figure 2, left). In
29
29
F
4
comparison, no Si NMR signal was detected in the Si NMR spectrum of 1H[B(Ar ) ] in
THFꢀd at 333 K, even after long accumulation time, due to rapid site exchange.
8
+
+
+
H
Idipp
H
Idipp
Idipp
THFꢀd8
THFꢀd8
Si
Si
Idipp
Si
Si
Si
Si
Idipp
H
Idipp
F
4 8
Scheme 2. Topomerization of 1H[B(Ar ) ] in THFꢀd solution. The tentative intermediate, a NHCꢀstabilized
F
−
disilahydronium ion, is depicted in curly brackets. The [B(Ar )
4
] counteranion and formal charges were omitted for
simplicity.
29
F
4
Both Si NMR signals of 1H[B(Ar ) ] appear in the slow exchange limit spectrum at similar
+
+
frequencies as those of the structurally related cations 1Me and 1Et (see Table 1 and
2
chapter 2.2), but at a considerably higher field than that of 1 (
δ
= 224.5 ppm in C D ). The
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4
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
4
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4
4
4
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1
29
1
2
29
1
J( Si, H) (204.2 Hz) and J( Si, H) (14.8 Hz) coupling constants compare well with those of
1
29
1
the 1,2ꢀdihydridodisilenes (E)ꢀSi
2
H
2
R
2
(R = Bbp, Bbt; J( Si, H) = 216 Hz and 210 Hz,
2
29
1
30
respectively; J( Si, H) = 16 Hz in both compounds), indicating an increased sꢀcharacter of
the Si hybrid orbital involved in the Si–H bond in full agreement with the NBO analysis results
+
of 1H (see SI, Table S8).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
29
F
1
29
1
4
Figure 2. Left: Si NMR spectrum of 1H[B(Ar ) ] at 213 K displaying two doublets with characteristic J( Si, H)
2
29
1
1
F
and J( Si, H) coupling constants. Right: Excerpt of the H NMR spectrum of 1H[B(Ar )
4
] at 25 °C displaying the
= 5.62 ppm with a mean J( Si, H) coupling constant due to the positional exchange of the Si
sites; the signal marked with an asterisk originates from a small amount of an unknown impurity.
29
1
SiꢀH resonance at
δ
1
Additional evidence for the dynamic process was provided by the slow exchange limit H
13
1
F
4 8
and C{ H} spectra of 1H[B(Ar ) ] (THFꢀd , 213 K), which display a double set of signals for
1
13
1
the heterotopic Idipp substituents, whereas the fast exchange limit H and C{ H} spectra of
F
1
H[B(Ar )
4
] (THFꢀd
8
, 333 K) reveal only one set of signals (see SI, Figures S2 – S7). The
1
13
1
number and relative intensities of the Idipp signals in the slow exchange H and C{ H} NMR
spectra clearly indicate that both NHC groups rotate fast about the Si–CNHC bonds on the
13
1
NMR time scale. Most distinctive are the signals of the CNHC nuclei in the C{ H} NMR
spectra (Table 1). The signal of the Si1ꢀbonded CNHC atom appears at lower frequency (
61.8 ppm) than that of the Si2ꢀbonded CNHC atom ( = 174.5 ppm), and both CNHC atoms
δ
=
1
δ
3
3
have a chemical shift in between that of Idipp (δ = 220.6 ppm in C D ) or 1 (δ = 196.3 in
6 6
2
5b
C D ) and the imidazolium salt (IdippH)Cl (
δ
= 139.3 ppm in DMSOꢀd6). The same trend is
6
6
F
F
observed for 1Me[B(Ar )
4
] and 1Et[B(Ar )
4
] and suggests a higher “imidazolium character” of
F
the Si1ꢀbonded NHC ring than that of the Si2ꢀbonded NHC in 1R[B(Ar )
4
] (R = H, Me, Et)
(
Table 1).
Topomerisation of 1H[B(Ar )
F
4
] involves an intramolecular 1,2ꢀH migration and proceeds
according to quantum chemical calculations (chapter 2.3) via an NHCꢀstabilized
disilahydronium intermediate (Scheme 2). Evidence for the intramolecularity of the process
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was provided by the Si satellite signals of the SiꢀH resonance in the H NMR spectrum of
F
29
1
1
4 8
H[B(Ar ) ] in THFꢀd at 298 K (δ = 5.62 ppm), which displays an averaged J( Si, H)
3
4
coupling constant of 94.8 Hz (Figure 2, right). A similar topomerisation process has been
1
proposed by G. Robinson et al. for the copper complex [CuCl(
η
ꢀ1)], however without
22a
unambiguous experimental verification.
Furthermore, recent studies in our group have
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+
1
shown that the cation [(Idipp)(I)Si=Si(Idipp)] and the coinage metal complexes [MCl(
η
ꢀ1)]
1
F
and [M(
topomerisation process in solution as observed for 1H[B(Ar )
The thermodynamic parameters of the topomerisation were obtained by full lineꢀshape
η ꢀ1)(PMe )][B(Ar ) ] (M = Cu, Ag, Ag) undergo the same intramolecular
3
4
F
4,22bꢀd
4
].
4
,5
analysis of the NHC backbone C ꢀH ring proton signals in the temperature range of 213 –
≠
−1
≠
−1
−1
2
73 K and were found to be
see SI, chapter 3). Remarkably, the Gibbs energy of activation
= 241 K, T = coalescence temperature) compares well with that calculated for the
isomerization of the transꢀbent disilene Si
ꢀH = 38.5(±2.4) kJ mol and ꢀS = −54.3(±7.7) J K mol
≠
−1
(
ꢀG (51.6(±0.6) kJ mol at
T
c
c
2
H
4
into the monoꢀhydrogenꢀbridged minimum
−
1
35
F
structure (59 kJ mol ), and the entropy of activation of the topomerisation of 1H[B(Ar )
4
] is
similar to that found for the intramolecular 1,2ꢀhydrogen migration of the 1,2ꢀdihydridodisilene
−
1
−1
30
(E)ꢀSi
2
H
2
Bbt
2
(−63(8) J K mol ), which leads via a silylsilylene intermediate to a silane.
F
The topomerization of 1H[B(Ar )
4
] is the first example of a reversible 1,2ꢀhydrogen
migration observed along a Si=Si bond in a molecular system and provides a molecular
model for the migration of hydrogen atoms on Si(001) surfaces via “dangling bonds” (i.e.
3
6
coordinatively unsaturated silicon centers), which is suggested to account for the decrease
in efficiency of photovoltaic cells based on hydrogenated amorphous silicon (Staeblerꢀ
3
7
F
Wronski effect). Topomerization of 1H[B(Ar )
4
] is reminiscent of that of diazenium ions in
acidic solutions, which has been suggested to proceed either via an intraꢀ or intermolecular
+
38
H
transfer, and of the degenerate rearrangement of carbenium ions formed upon
3
9
protonation of olefins.
+
F
Table 1: Comparison of selected NMR data of the cations 1R in 1R[BAr
4
] (R = H, Me, Et, Na: Ar = Ar ; R = I, Li:
Ar = C ) and 1.
6 5
F
29
δ
( Si) [ppm]
δ(CNHC) [ppm]
Si1
Si2
C
NHCꢀSi1ꢀR
C
NHCꢀSi2
+
[c]
[c]
[d]
[e]
[c]
[c]
1
H
69.4
125.4
115.2
117.1
161.8
174.5
177.1
176.1
[d]
[e]
[d]
[f]
+
102.8
03.0
[d]
[d]
1Me
162.9
1
+
[d]
[d]
[d]
[f]
1
Et
111.6
87.2
75.3
164.2
+[a]
[f]
[f]
1I
−26.4
153.6
172.2
+
[g]
[g]
[g]
[h]
[g]
[g]
1
Li
301.1
288.8
224.5
301.1
288.8
224.5
188.8
188.8
189.1
196.3
+
[g]
[h]
[g]
[g]
[h]
1
Na
189.1
[b]
[h]
1
196.3
[
a]: Data taken from ref. [4]. [b]: Data taken from ref. [2]. [c]: THFꢀd
03 K. [g]: C Cl, 298 K. [h]: C , 298 K.
8 8 6 5 8
, 213 K. [d]: THFꢀd , 298 K. [e]: C H F, 298 K [f]: THFꢀd ,
2
6
D
5
6 6
D
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2.2 Alkylation of (Idipp)Si=Si(Idipp). Electrophilic alkylation of 1 was achieved upon
F
treatment of 1 with the corresponding iodoalkane in the presence of Na[B(Ar )
4
] to afford the
II
0
F
F
mixedꢀvalent alkyldisilicon(I) borates [(Idipp)(R)Si =Si (Idipp)][B(Ar )
4
] (1R[B(Ar )
4
], R = Me,
F
Et) (Scheme 3). Remarkably, addition of Na[B(Ar )
4
] to a solution of 1 in fluorobenzene led to
a color change from dark red to brown to afford selectively the alkalimetal disilicon(0) borate
0
1
2
3
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9
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
8
9
0
F
F
[
Si
2
(Na)(Idipp)
2
][B(Ar )
4
] (1Na[B(Ar )
4
]) as discussed in chapter 2.4 (vide infra). Subsequent
F
addition of the iodoalkane to the solution of 1Na[B(Ar )
4
] in fluorobenzene was accompanied
by a color change from brown to bright orange and precipitation of NaI leading selectively to
F
the alkyldisilicon(I) borate salts 1R[B(Ar )
4
] (Scheme 3). Remarkably, preactivation of 1 via
F
complexation of Na[B(Ar )
4
] is not indispensable for the formation of the alkyldisilicon(I)
F, which affords the
+
cations 1R , as evidenced by the reaction of 1 with iodomethane in C
6
H
5
29
analogous iodide salt [(Idipp)(Me)Si=Si(Idipp)]I according to Si NMR spectroscopy. The
F
salts 1R[B(Ar )
4
] were isolated after crystallization from fluorobenzene / nꢀhexane mixtures at
−
30 °C as highly airꢀsensitive, orange solids in moderate yields (R = Me: 36 %, R = Et: 50 %)
(
see SI, chapters 2.3 and 2.4). Notably, 1 did not react with 2ꢀiodopropane indicating that
2
bulky electrophiles cannot access the sterically protected Si core of 1. The compounds
F
1
R[B(Ar )
4
] are thermally robust and decompose upon melting in vacuumꢀsealed glass
capillaries at elevated temperatures (R = Me: 213 °C, R = Et: 209 °C). As in the case of
F
F
1
H[B(Ar )
4
], the salts 1R[B(Ar )
4 8
] decompose slowly in THFꢀd solution at ambient
F
temperature, the decomposition leading to [IdippH][B(Ar )
4
] and other unidentified products.
+
Idipp
Si
F
Idipp
R
1
2
) + Na[B(Ar ) ]
4
) + RꢀI /
ꢀ
NaI
F
ꢀ
[
B(Ar ) ]
4
Si
Idipp
Si
Si
Idipp
C H F,
ꢀ
30 °C
6
5
F
1
1
R[B(Ar ) ]
4
F
Scheme 3. Electrophilic alkylation of 1 to give the compounds 1R[B(Ar )
4
] (R = Me, Et). Formal charges are
encircled.
F
F
Singleꢀcrystal Xꢀray diffraction analyses of 1Me[B(Ar )
4
] and 1Et[B(Ar )
4
] revealed that
F
+
these compounds are isotypic with 1H[B(Ar ) ]. The cations 1R are well separated from the
4
+
borate anions as evidenced by the shortest SiꢁꢁꢁF distances of 5.416(5) Å (1Me ) and
+
25
+
5.030(9) Å (1Et ) (cf.
Σr(SiꢁꢁꢁF)vdW = 3.6 Å), and reveal similar structural features as 1H , i.e.
a transꢀbent planar CNHCꢀSiꢀSiꢀCNHC core, in which a trigonalꢀplanar coordinated Si1 atom
+
+
(sum of angles at Si1: 359.6(1)° (1Me ), 359.9(2)° (1Et )) is connected via a Si=Si bond to a
Vꢀshaped coordinated Si2 atom with a narrow angle at the Si2 atom (C28ꢀSi2ꢀSi1: 95.13(6)°
+
+
(1Me ), 100.08(7)° (1Et )) (Figure 3, Figure S34 (SI), Table 2). The Si–R (Si1–C56) bond
+
+
lengths (1Me : 1.885(2) Å, 1Et : 1.893(2) Å) are typical of Si–C single bonds and only slightly
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shorter than the Si1–CNHC bond lengths (1Me : 1.901(2) Å, 1Et : 1.912(2) Å). Whereas the
Si2ꢀbonded central ring of the NHC group adopts in all cases an almost orthogonal
orientation with respect to the planar CNHCꢀSiꢀSiꢀCNHC core of the cations (ϕNHC2 = 71.06(6)°
+
+
+
26
(
1H ), 79.11(7)° (1Me ), 88.03(7)° (1Et )), the increasing bulkiness of the Si1ꢀbonded
+
+
+
substituent R in the series 1H → 1Me → 1Et provokes a conformational change of the Si1ꢀ
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56
57
58
59
60
+
bonded NHC substituent from an almost coplanar orientation in 1H (
ϕ
NHC1 = 8.60(6)°) and
+
+
1Me
(ϕNHC1 = 13.23(8)°) to a synclinal conformation in 1Et (ϕNHC1 = 52.52(8)°). The
increased steric bulk of the R group can also be invoked to rationalize the gradual elongation
+
+
+
of the Si1–C1 bonds in the series 1H (1.882(2) Å) → 1Me (1.901(2) Å) → 1Et (1.912(2) Å)
+
+
and the decrease of the C1ꢀSi1ꢀSi2 angles in the same series (1H (116.73(7)°) → 1Me
+
(
114.62(7)°) → 1Et (108.36(7)°)) (Table 2).
N2
C1
N1
Si1
Si2
C56
C28
N4
N3
+
F
Figure 3. DIAMOND plot of the molecular structure of the cation 1Me in the crystal lattice of 1Me[B(Ar )
4
] at
1
23(2) K. Thermal ellipsoids are set at 30 % probability. Hydrogen atoms were omitted for clarity. Selected bond
lengths [Å], bond angles [°] and torsion angles [°]: Si1–C1 1.901(2), Si1–Si2 2.1909(8), Si2–C28 1.947(2), Si1–
C56 1.885(2); C1ꢀSi1ꢀSi2 114.62(7), C1ꢀSi1ꢀC56 111.3(1), Si2ꢀSi1ꢀC56 133.71(8), C28ꢀSi2ꢀSi1 95.13(6); C1ꢀSi1ꢀ
Si2ꢀC28 −177.4(1), C28ꢀSi2ꢀSi1ꢀC56 10.7(1).
F
F
In contrast to 1H[B(Ar ) ], multinuclear NMR spectroscopic studies of 1R[B(Ar ) ] (R = Me,
4
4
Et) in THFꢀd indicate that no degenerate isomerization (topomerization) of the “σꢀbonded”
8
40
1
13
1
isomers occurs in solution up to 333 K. Thus, the H and C{ H} NMR spectra display a
double set of signals for the heterotopic Idipp substituents (see SI, Figures S12 – S15 and
1
13
1
S17 – S21), which could be unequivocally assigned by a combination of Hꢀ C HMQC, Hꢀ
1
3
1
29
C HMBC and Hꢀ Si HMBC correlation spectroscopies using the correlation between the
4
,5
29
C ꢀH proton signals of the NHCs and the Si NMR signals. The number and relative
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2
2
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intensity of the signals of each Idipp group indicates that both NHCs rotate fast about the
29
respective Si–CNHC bonds on the NMR time scale. Similarly, the Si NMR spectra of
F
1R[B(Ar )
4
] (R = Me, Et) in THFꢀd
8
show two separate resonances at 298 K (Table 1), which
1
29
29
were unequivocally assigned by twoꢀdimensional Hꢀ Si HMBC spectroscopy. The Si NMR
F
spectrum of 1Me[B(Ar )
4
] was also recorded in C
), i.e. at almost the same position as those observed in
(102.8 and 115.2 ppm, respectively) (Table 1). This observation excludes a
6 5
H F and displayed two signals at 103.0 and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
17.1 ppm (calibrated against SiMe
4
THFꢀd
8
coordination of THF to the potentially electrophilic Si2ꢀcenter of the cations
+
29
[(Idipp)(Me)Si=Si(Idipp)] in solution. Moreover, the Si NMR chemical shifts of the
6 5
corresponding iodide salt 1Me[I] in C H F (103.5 and 112.6 ppm) are quite similar to those of
F
−
+
1
Me[B(Ar )
4
] suggesting a negligible interaction of the stronger nucleophile I with 1Me .
2
9
+
A comparison of the Si NMR chemical shifts of 1R reveals, that the threeꢀcoordinated Si
F
nuclei (SiꢀR) are progressively deshielded in the series 1H[B(Ar ) ] (
δ = 69.4 ppm) →
4
F
F
1Me[B(Ar )
4 4
] (δ = 102.8 ppm) → 1Et[B(Ar ) ] (δ = 111.6 ppm) (Table 1). Notably, a similar,
however less pronounced trend is observed in the series of the alkyl silanes SiMe R upon
3
replacement of the R substituent (H (
δ
δ δ =
= −18.5 ppm) → Me ( = 0.0 ppm) → Et (
4
1
29
1
.6 ppm)). In comparison, the twoꢀcoordinated Si NMR nuclei are progressively shielded
F
F
4 4
in the same series 1H[B(Ar ) ] (δ ] (δ
= 125.4 ppm) → 1Me[B(Ar ) = 115.2 ppm) →
F
1Et[B(Ar )
4
] (
δ
= 87.2 ppm) (Table 1). Remarkably, a similar influence of the substituents R
13
α
β
was observed on the C NMR chemical shifts of the C and C atoms in the alkenes Rꢀ
β
α
42
2
C H=C H (R = H, Me, Et).
+
Table 2: Comparison of selected bond lenghts [Å], bond angles [°] and dihedral angles [°] of the cations 1R in
F
1R
[BAr
4
] (R = H, Me, Et, Na: Ar = Ar ; R = I, Li: Ar = C
6
F
5
) and
1.
[
a]
[a]
Si–Si
[Å]
Si1–CNHC
[Å]
Si2–CNHC
[Å]
C
NHCꢀSi1ꢀSi2
C
NHCꢀSi2ꢀSi1
[°]
ϕ
NHC1
ϕ
NHC2
[°]
[°]
[°]
+
1H
2.1873(8)
2.1909(8)
2.1726(9)
2.1739(9)
2.234(1)
2.248(2)
2.229(1)
1.882(2)
1.901(2)
1.912(2)
1.901(2)
1.941(3)
1.948(4)
1.927(2)
1.940(2)
1.947(2)
1.941(2)
1.931(2)
1.938(3)
1.948(4)
1.927(2)
116.73(7)
114.62(7)
108.36(7)
112.83(7)
94.9(1)
95.34(6)
95.13(6)
100.08(7)
96.61(7)
98.7(1)
8.60(6)
71.06(6)
+
1Me
13.23(8) 79.11(7)
52.52(8) 88.03(7)
96.69(7) 95.78(7)
+
1Et
+
[b]
1I
+
1Li
79.0(1)
87.7(1)
108.9(1)
87.7(1)
+
1Na
95.4(1)
95.4(1)
[
c]
1
93.37(5)
93.37(5)
87.11(5) 87.11(5)
[
a]: ϕNHC1 and ϕNHC2 are the dihedral angles between the leastꢀsquare plane of the CNHCꢀSiꢀSiꢀCNHC atoms and the Si1ꢀ and Si2ꢀ
bonded NHCs rings, respectively. [b]: Data taken from ref. [4]. [c]: Data taken from ref. [2].
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2.3 Quantum chemical studies of 1H and 1Me . In order to get a closer insight into the
+
+
topomerisation of 1H , the energy hypersurface of 1H was analyzed and compared with that
+
43aꢀd
of 1Me by quantum chemical calculations at the B97ꢀD3 level of theory,
including the
43e
43f,g
COSMO solvation model and RIꢀJCOSX approximations (see SI, chapter 5).
The def2ꢀ
TZVP basis sets were used for the Si, N, heterocyclic C atoms and the Si1ꢀattached H atom
and methyl group, and the def2ꢀSVP basis sets for all peripherical carbon and all other
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54
55
56
57
58
59
60
4
3h,i
hydrogen atoms.
The level of theory is abbreviated in the following as B97ꢀD3/I.
+
+
Geometry optimization of 1H and 1Me afforded minimum structures with a Si1–H and
4
4
+
Si1–Me σꢀbond (“σꢀbonded” isomers), which are designated in the following as 1H calc and
+
1
Me calc. A comparison of the structural parameters revealed an excellent agreement
between the calculated and the experimental bond lengths and angles obtained for
F
F
1
4 4
H[B(Ar ) ] and 1Me[B(Ar ) ] by Xꢀray crystallography. Remarkably, also the quite different
conformation of the Si1ꢀbonded NHC compared to the Si2ꢀbonded NHC could be well
reproduced by the quantum chemical calculations (see SI, Tables S4 and S5).
+
A second minimum structure could be localized on the energy hypersurface of 1H and
+
+
+
−1
1Me , which is however less stable than that of 1H calc and 1Me calc by 30.5 kJ mol and
−1
+
95.4 kJ mol
(ꢀG values at 298 K), respectively. (1H )’calc features a “symmetric” Hꢀbridged
structure, in which the H atom is located over the Si–Si bond at almost the same distance
4
4
+
from the two Si atoms (“
π
ꢀbonded” isomer). Notably, the Si–Si bond of (1H )’calc (2.301 Å) is
+
longer than the Si=Si bond of 1H
(2.198 Å), and its length approaches the Si–Si single
calc
+
4
5
bond length in
α
ꢀSi (2.352 Å). (1H )’
features a 3cꢀ2e SiꢀHꢀSi interaction, which leads to
calc
+
longer Si–H bonds (1.716 and 1.710 Å) than the 2cꢀ2e Si–H bond (1.486 Å) of 1H
(“σꢀ
calc
+
TS
bonded” isomer). The two minimum structures are connected via a transition state (1H ) calc
,
−
1
+
which lies 42.9 kJ mol higher in energy than 1H calc (Figure 4). The calculated barrier
agrees acceptably well with the experimentally derived Gibbs energy of activation
≠
−1
+
+
(ꢀG (298 K) = 54.7±0.2 kJ mol ). The conversion of 1H calc to (1H )’calc involves a Hꢀ
migration from the endꢀon to a bridging position and a concomitant rotation of the Si1ꢀbonded
+
heterocyclic ring from an almost coplanar orientation in 1H calc
(
ϕNHC1 = 3.63°) to an almost
+
26
orthogonal orientation in (1H )’calc
(
ϕNHC1 = 76.17°) (see SI, Table S4).
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9
0
+
Figure 4. Schematic Gibbs profiles (T = 298 K) for the degenerate isomerizations of [Si
2
2
(H)(Idipp) ] (left) and
+
+
+
+
[
2
Si (Me)(Idipp)
2
]
(right) which includes the optimized minimum structures 1H calc
,
1Me calc
,
(1H )’calc and
+
+ TS
(1Me )’calc and the transition state (1H ) calc with selected bonding parameters. The Nꢀbonded dipp substituents
and H atoms, except the Siꢀbonded H atom, were omitted for clarity and only one half of the symmetric energy
profiles is depicted.
+
+
In contrast to (1H )’calc, the second minimum structure ((1Me )’calc) located on the energy
+
hypersurface of 1Me reveals an asymmetric bridging of the Si–Si single bond (2.336 Å) by
the methyl group, as evidenced by the markedly different Si1–C56 (2.029 Å) and Si2–C56
(2.307 Å) bonds (Figure 4). Both Si–CMe bonds are longer than the 2cꢀ2e Si–CMe bond of the
+
+
“
σꢀbonded” isomer 1Me
(1.891 Å). A relaxed potential energy surface scan of 1Me was
calc
+
performed starting from 1Me calc, which involved a progressive decrease of the Si2–CMe
+
distance from 377 pm (the Si2–CMe distance in 1Me calc) to 137 pm in 12 steps. This scan
−
1
afforded a maximum, which displayed one imaginary frequency of −177 cm and a Gibbs
−
1
energy of 97 kJ mol . Further optimization of this maximum structure did not furnish the
+ TS
+
+
transition state (1Me ) calc, but instead led to the minimum structures 1Me calc and (1Me )’calc
indicating a flat progression of the potential energy surface during interconversion of
+
+
(
1Me )’calc into the overall minimum structure 1Me calc (Figure 4). The significantly higher
+
+
+
energy barrier for the transformation of 1Me calc into (1Me )’calc compared to that of 1H
+
confirms the observed rigidity of 1Me on the NMR timescale.
+
+
NBO (natural bond orbital) analyses of the “σꢀbonded” isomers 1H calc and 1Me calc
revealed a localized Si1–R (R = H, Me) σꢀbond NBO, a Si–Si σꢀ and πꢀbond NBO, which are
both polarized towards the Si1 atom, and a lone pair of electrons at Si2 in a natural hybrid
43j
orbital (NHO) with high sꢀcharacter (ca. 77 %) (see SI, Tables S8 and S10). The rather
+
+
high Wiberg bond Index (WBI) of the Si=Si bond of 1.61 (1H calc) and 1.62 (1Me calc) and the
high covalent contribution to the overall NRT (natural resonance theory) Si–Si bond order of
+
+
1
.85 (1H calc) and 1.84 (1Me calc) provide additional support for the presence of a slightly
polar, covalent Si=Si bond.
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+
+
Natural population analyses of 1H calc and 1Me calc revealed a considerable electron
+
+
transfer from 1 to the electrophile (H , Me ), leading to a negative partial charge at the H
atom (q = −0.14e) and the methyl group (q = −0.38e). The charge flow occurs mainly from
+
the Si atoms, as evidenced by a comparison of the Si partial charges in 1H calc (q(Si1) =
+
0.31e, q(Si2) = 0.26e) and 1Me calc (q(Si1) = 0.65e, q(Si2) = 0.20e) with those in 1calc
10
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55
56
57
58
59
60
(
q(Si1,Si2) = −0.10e) (see SI, Tables S7, S8 and S10).
+
+
In comparison, the “πꢀbonded” isomers (1H )’calc and (1Me )’calc display only one nonꢀ
polarized NBO corresponding to the Si–Si σꢀbond, with each Si atom carrying a lone pair of
electrons in a NHO with high sꢀcharacter (see SI, Tables S9 and S11). This leads to a
+
+
+
considerably lower Si–Si WBI ((1H )’calc: 1.01; (1Me )’calc: 0.90) than those of 1H calc and
+
+
+
1
Me
and to a Si–Si NRT bond order ((1H )’ : 0.96; (1Me )’calc: 0.90), which lies at half
calc calc
+
+
the value of that in the “σꢀbonded” isomers 1H calc and 1Me calc. All these results suggest the
+
+
presence of a Si–Si single bond in (1H )’calc and (1Me )’calc
.
+
+
The largest difference between (1H )’calc and (1Me )’
exists with respect to the SiꢀRꢀSi
calc
+
interaction. In (1H )’calc, a highly localized NBO is found corresponding to the 3cꢀ2e SiꢀHꢀSi
interaction, which is formed of an almost pure p NHO of each Si atom and the hydrogen 1s
orbital and is polarized towards the H atom (Figure 5, left). In comparison, only a NBO for the
+
σ(Si1–CMe) bond, but no NBO for the Si2–CMe bond was found in (1Me )’calc (see SI, Table
S11). Instead, the Si2 atom bears a lone vacancy of pure pꢀcharacter (with an occupancy of
0
.26 electrons), which attains electron density from the σ(Si1–C ) and one σ(C–H) orbital of
Me
the methyl group according to second order pertubation theory. The corresponding second
(2)
−1
−1
order pertubation energies E amount to 196.6 kJ mol and 156.9 kJ mol , respectively.
This “agosticꢀtype” interaction locates one of the methyl hydrogen atoms (H1) in the Si2ꢀSi1ꢀ
C56 plane and leads to an elongation of one CMe–H1 (C56–H1) bond reducing the electron
deficiency at the Si2 atom (Figure 5, right).
H2
H1
dC56–H1 = 1.125 Å
dC56–H2 = 1.088 Å
C56–H3 = 1.083 Å
Si2–H1 = 2.030 Å
dSi1–Si2 = 2.336 Å
H3
Idipp
H
C56
Si1
d
Si1–H = 1.716 Å
Si2–H = 1.710 Å
Si1–Si2 = 2.301 Å
d
d
Idipp d
Si2 Si1
d
∠
Si2
Si1ꢀHꢀSi2 = 84.39°
∠
∠
Si1ꢀC56ꢀSi2 = 51.81°
Si2ꢀSi1ꢀC56ꢀH1 = 11.51°
Idipp
Idipp
2e3c(SiꢀHꢀSi) bond
"agosticꢀtype " interactions
+
Figure 5. Left: Schematic presentation of the SiꢀRꢀSi interaction in the “
π
ꢀcoordinated” isomers of (1H ) (left) and
+
1
Me (right) according to NBO analyses with selected bonding parameters.
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2.4 Addition of alkali metal borates to (Idipp)Si=Si(Idipp). During the syntheses of
F
F
1
R[B(Ar )
4
] (R = Me, Et) from 1, Na[B(Ar )]
4
and RI, a color change of the dark red solution of
F
1 in C
6
H
5
F to brown was observed upon treatment with Na[B(Ar )
4
]. This stimulated us to
study the interaction of 1 with alkali metal cations. In fact, compound 1 readily reacted with
4
6
F
24b,47
the alkali metal borates [Li(Et
2
O)2.5][B(C
6
F
5 4
) ]
and Na[B(Ar )
4
]
to afford after
0
1
2
3
4
5
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7
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0
1
2
3
4
5
6
7
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9
0
1
2
3
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5
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1
2
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0
1
2
3
4
5
6
7
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9
0
crystallization from fluorobenzene / nꢀhexane mixtures the alkali metal disilicon(0)ꢀborates
F
[Si
2
(M)(Idipp)
2
][BAr
4
] (1M[BAr
4
]; M = Li, Ar = C
6
F
5
; M = Na, Ar = Ar ) as brown solids in 43 %
(
M = Li) and 70 % yield (M = Na), respectively (Scheme 4) (see SI, chapters 2.5 and 2.6).
The compounds rapidly decompose upon contact with air, but are thermally quite robust
F
solids (decomposition at 202 °C (1Li[B(C
6
F
5
)
4
) and 211 – 212 °C (1Na[B(Ar )
4
])).
+
Idipp
Idipp
M
+
[M(Et O) ][BAr ]
2 n 4
[BAr ]ꢀ
Si
Si
Idipp
Si
Si
4
C H F, RT
6
5
Idipp
1M[BAr4]
1
Scheme 4. Synthesis of the alkali metal disilicon(0)ꢀborates 1M[BAr
4
] (M = Li, Ar = C
6 5
F , n = 2.5; M = Na, Ar =
F
6
Ar , n = 0). Formal charges and the
η ꢀcoordination of the two Nꢀbonded dipp substituents are not depicted.
Dissolution of 1M[BAr
led to an instant decomplexation of the alkali metal ions to give back 1 and the alkali metal
borates. However, solutions of 1M[BAr ] in nonꢀcoordinating polar solvents (fluorobenzene,
chlorobenzene) were found to be stable for several days at room temperature.
The solid state structures of 1Li[B(C ]ꢁ(nꢀC 14) (Figure 6) and 1Na[B(Ar )
Figure S35) were determined by singleꢀcrystal Xꢀray diffraction studies. The C ꢀsymmetric
4
] (M = Li, Na) in coordinating solvents, such as diethyl ether or THF,
4
F
6
F
5
)
4
6
H
4
] (see SI,
1
+
+
cations 1Li and C
2
ꢀsymmetric cations 1Na are well separated from the borate
counteranions, as apparent from the shortest SiꢁꢁꢁF distances of 6.155(3) Å and 6.809(6) Å,
2
5
respectively (cf. Σr(SiꢁꢁꢁF)vdW = 3.6 Å). The alkali metal cations are encapsulated in the
+
+
2
cavity of the (Idipp)Si=Si(Idipp) core of 1Li and 1Na via η ꢀcoordination to the Si=Si πꢀbond
6
and η ꢀcoordination to two peripheral 2,6ꢀdiisopropylphenyl (dipp) rings (Figure 6 and Figure
+
S35 (SI)). The Si–Li bond distances in 1Li (Si1–Li: 2.822(6) Å, Si2–Li: 2.925(7) Å) are quite
2 2
long and comparable to that of the contact ion pair R Si=Si(R)Li(dme) (2.853(3) Å, R =
4
8
C
6
H
2
ꢀ2,4,6ꢀiPr
3
). They suggest a weak SiꢁꢁꢁLi interaction, which is corroborated by the only
2
slightly elongated Si=Si bond (2.234(1) Å, 0.2 %) compared to 1 (2.229(1) Å). The Si–Na
+
bond lenghts in 1Na (3.096(3) Å) are similar to the Si–Na bond lengths in the dimer
4
9
+
(
NaSitBu
3
)
2
(3.060(4) and 3.073(4) Å), and the Si=Si bond (2.248(2) Å) of 1Na is slightly
+
more elongated (0.9 %) than in 1Li .
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Li
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60
N1
N4
Si2
C1
C28
Si1
N2
N3
+
Figure 6. DIAMOND plot of the molecular structure of 1Li in the crystal lattice of 1Li[B(C
6
F
5
)
4
]ꢁ(nꢀC
6
H
14) at
123(2) K. Thermal ellipsoids are set at 30 % probability. Hydrogen atoms were omitted for clarity. Selected bond
lengths [Å], bond angles [°] and torsion angles [°]: C1–Si1 1.941(3), C28–Si2 1.938(3), Li–Si1 2.822(6), Li–Si2
2
.925(7), Si1–Si2 2.234(1); Si1ꢀLiꢀSi2 45.7(1), C1ꢀSi1ꢀSi2 94.9(1), C1ꢀSi1ꢀLi 95.9(2), Si2ꢀSi1ꢀLi 69.6(1), C28ꢀSi2ꢀ
Si1 98.7(1), C28ꢀSi2ꢀLi 93.7(2), Si1ꢀSi2ꢀLi 64.7(1); C1ꢀSi1ꢀSi2ꢀC28 175.5(1).
+
+
Encapsulation of the Li and Na ions in the cavity of 1 leads only to minor structural
changes of the (Idipp)Si=Si(Idipp) core (Table 1). This suggests a mainly electrostatic
bonding of the alkali metal ions to the Si=Si
π bond and the πꢀelectron systems of the two
5
0
dipp substituents, which is typical for alkali metal cationꢀπ interactions. Notably the M–
+
+
arenecentroid distances (2.292 Å and 2.451 Å in 1Li , 2.557 Å in 1Na ), compare well with
6
those of the alkali metal cationꢀπ complexes [Li(η ꢀtoluene)][CB11Me12] (2.39, 2.45 Å) and
6
51
[
Na(η ꢀbenzene)
2
][CB11Me12] (2.69 and 2.71 Å). Remarkably, a similar reaction of the
diboron(0) compound (Idipp)B≡B(Idipp) with alkali metal borates was reported recently to
+
give the cations [B
structurally characterized.
Multinuclear NMR spectroscopic analyses of 1M[BAr
encapsulation of the alkali metal cations is retained in solution. Thus, the Si{ H} NMR
2
(M)(Idipp)
2
] (M = Li, Na), which however could not be isolated and
5
2
4
6 5
] in C D Cl indicate clearly that the
29
1
6 5 4
spectrum of 1Li[B(C F ) ] displays one broadened signal at 301.1 ppm and that of
F
29
1Na[B(Ar )
4
] one sharp Si NMR signal at 288.8 ppm (see SI, Figures S26 and S30). Both
2
9
2
Si NMR resonances appear at considerably lower field compared to 1 (
δ
= 224.5 ppm).
29
2
Remarkably, the opposite trend was observed for the Si NMR resonances of η ꢀdisilene
complexes of transition metals, which can be explained by the considerable metalꢀligand π
53
29
backꢀbonding in the latter compounds. No coupling between the Si (I = 1/2) and the
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1
quadrupolar Li (I = 3/2) nucleus could be resolved in the Si{ H} NMR spectrum of
29
1
6 5 4
Li[B(C F ) ] at 298 K and 243 K, due to signal broadening. However, the width of the Si
2
9
7
NMR signal of ꢀν ≈ 56 Hz (298 K) suggests, that the upper limit of the scalar Siꢀ Li coupling
1
29
7
would be 14 Hz. This value is considerably smaller than those in lithiosilanes ( J( Si, Li) =
5
4
32.8 – 65.3 Hz), providing additional evidence for the mainly electrostatic interaction of the
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
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7
8
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0
1
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1
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0
7
alkali metal ions with the Si=Si πꢀbond in 1M[BAr
Li[B(C ] (
at considerably higher field than that of [Li(Et
4
]. Notably, the Li NMR signal of
+
55
1
6
5
F )
4
δ
= −6.7 ppm) appears in the common range of πꢀcomplexated Li ions, but
2
O)2.5][B(C
6
5
F )
4
] (
δ
= −1.1 ppm in C
6
H
5
F). This
+
observation suggests a tight encapsulation of the Li ion by 1 in solution.
1
13
The H and C NMR spectra of 1M[BAr
4
] display a single set of signals for the two Idipp
6
substituents indicating that a rapid site exchange of the η ꢀcoordinated dipp groups with
those which are not bonded to the alkali metal ions occurs on the NMR time scale (see SI,
1
Figures S23 – S25 and S27 – S29). No splitting of the H NMR signals was observed at
−
6 5
40 °C just above the freezing point of C D Cl (−45 °C). It is presently unclear whether this
site exchange is an intramolecular process involving a successive or concomitant rotation of
+
the NHC groups about the respective Si–CNHC bonds. An intermolecular Li exchange seems
29
less probable given the broadening of the Si NMR resonance induced by the unresolved
spinꢀspin coupling.
+
Quantum chemical calculations of the cation 1Li were carried out on the B97ꢀD3/I level of
+
43
theory to analyze the interaction of Li with the (Idipp)Si=Si(Idipp)
2
core of 1. The structural
+
parameters of the optimized minimum structure 1Li calc agree well with the experimental
values (see SI, Table S6).
+
A look at the KohnꢀSham orbitals of 1Li calc revealed the same sequence of frontier
orbitals as in 1, in which the n
+
(Si–Si) orbital is the HOMO, the Si=Si πꢀorbital is the HOMO−1
8
,18
and the n (Si–Si) orbital is the HOMO−2 (see SI, Figure S43).
−
No orbitals pointing to an
+
2
6
interaction between Li and the η ꢀcoordinated Si=Si bond or the η ꢀcoordinated aryl
+
substituents were found by the calculations due to the mainly electrostatic bonding of Li ,
+
which was further confirmed by an NBO analysis of 1Li calc (see SI, Table S12). The Li atom
displays a lone vacancy with a small occupancy of 0.13. Second order pertubation theory
analysis of the Fock matrix in the NBO basis revealed only small interaction energies
between the Siꢀbased or arylꢀbased donor NBOs and the lone vacancy on the Li atom (see
5
6
SI, Table S13), which is characteristic of electrostatic cationꢀπ interactions.
+
Consequently, the encapsulation of the Li cation in the cavity of 1 is accompanied by only
+
+
a minor charge transfer to the Li ion, which still carries a high positive charge in 1Li
calc
according to an NPA charge analysis (q(Li) = 0.85e). This charge transfer occurs exclusively
+
from the NHCs, as evidenced by the essentially identical NPA charges of the Si atoms in 1Li
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(q(Si1) = −0.12e, q(Si2) = −0.11e) and in 1 (q(Si1,Si2) = −0.10e) (see SI, Tables S7 and
S12).
2.5 Quantum chemical calculations of a series of isolobal compounds with
II
0
+
[(Idipp)(R)Si =Si (Idipp)] . Formal replacement of the [Si(Idipp)] fragments in
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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II
0
+
+
−
19,20
[
(Idipp)(R)Si =Si (Idipp)] (1R , R = H, Me, Et) by the isolobal fragments [SiR] and [PR]
leads to a series of Si and P multipleꢀbonded compounds B – F, which are depicted in
1
2
27
23a
Scheme 5. Class B,
C
and F
compounds are very rare, whereas disilenide anions
28,48,57
58
(D)
and phosphasilenes (E) are familiar classes of compounds, the synthetic potential
of which has been exploited. A comparison of A with B – F unraveled many common
structural features, which stimulated us to study their electronic structures.
+
−
+
NHC
Si
R
R
R
R
R
Si
NHC
Si
NHC
Si
Si
Si
NHC
Si
Si
Si
P
P
P
R
R
R
R
R
R
R
R
R
A
B
C
D
E
F
II
0
+
+
Scheme 5. Isolobal derivatives of the cations [(NHC)(R)Si =Si (NHC)] (
A;
A
=
1R for NHC = Idipp and R = H,
Me, Et).
Me
For this purpose quantum chemical calculations of the model systems with NHC
C[N(Me)CH]
=
2
and R = Me were carried out on the B97ꢀD3/RIꢀJCOSX/def2ꢀTZVP level of
4
3
theory. The optimized minimum structures are depicted in Figure 7 and selected bonding
parameters are given in Table 3. The structures of compounds A, C, D, E and F are quite
similar as evidenced by the similar Si–Si, Si–CNHC and Si–Cmethyl bond lengths, the similar C1ꢀ
E1ꢀE2, C2ꢀE2ꢀE1, C1ꢀE1ꢀC3 and C3ꢀE1ꢀE2 bond angles (E = Si or P), the planar geometry
around the E1 atoms and the orthogonal orientation of the Si2ꢀbonded NHC substituent in
compounds A and C with respect to the C1ꢀSi1ꢀSi2ꢀC2 plane. The structural parameters of
compound B differ slightly from those of compounds A and C, which becomes apparent from
the slight pyramidalization of the Si1 atom and the synclinal orientation of the Si1ꢀbonded
NHC substituent towards the C1ꢀSi1ꢀSi2ꢀC2 plane.
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0
1
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8
9
0
A
D
B
C
E
F
2
(NHC = C[N(Me)CH] , R = Me)
Me
Figure 7. Calculated minimum structures of the model compounds
A
–
F
depicted in Scheme 5. Hydrogen atoms were omitted for simplicity reasons.
Table 3: Selected calculated bond lengths, bond angles and dihedral angles of compounds A – F depicted in
Figure 7; E = Si or P.
E1–E2
Å]
E1–C1
[Å]
E2–C2
[Å]
E1–C3
[Å]
C1ꢀE1ꢀE2
[°]
C2ꢀE2ꢀE1
[°]
[
A
B
C
D
E
F
2.213
2.240
2.192
2.210
1.890
1.882
1.910
1.935
1.946
1.962
1.931
1.997
1.900
1.911
1.900
1.927
118.45
121.59
129.55
132.00
89.50
101.14
88.91
99.53
[c]
[c]
1.885
[c]
[c]
[c]
[c]
2.083
2.010
1.889
1.881
129.14
102.08
1.812
C3ꢀE1ꢀE2
[°]
1.850
1.807
130.06
104.02
[a]
[b]
[b]
C1ꢀE1ꢀC3
Σ(E1)
ϕ
NHC1
ϕ
NHC2
[
°]
[°]
[°]
[°]
A
B
C
D
E
F
111.66
109.06
108.23
104.95
129.85
124.43
122.18
123.05
359.96
355.08
359.96
360.00
12.79
91.96
53.23
–
–
–
–
–
96.26
–
–
–
[
c]
[c]
[c]
111.76
112.80
119.76
360.00
360.00
117.14
[
a]: Σ(E1) is the sum of angles around the E1 atom. [b]: ϕNHC1 and ϕNHC2 are the dihedral angles between the
leastꢀsquare planes of the C1ꢀE1ꢀE2ꢀC2 atoms and the heterocyclic ring atoms of the NHC substituents bonded
to E1 and E2, respectively. [c]: E1 = Si, E2 = P.
A look at the KohnꢀSham orbitals of compounds A – F reveals the same sequence of
frontier orbitals as expected for isolobal compounds (Table 4). Thus, the HOMOs are in all
cases the E=E πꢀorbitals, the HOMOs−1 the lone pair orbitals at the twoꢀcoordinated E2
atoms (E2 = Si or P), and the LUMOs of compounds A, B, D, E and F the E=E π*ꢀorbitals. In
the NHCꢀsubstituted compounds A and B a significant stabilization of the Si=Si π*ꢀorbital
Me
occurs via a bonding interaction with the CNHCꢀ and NNHCꢀcentered π*ꢀorbital of the NHC ꢀ
substituent. In compound C the Si=Si π*ꢀorbital is the LUMO+1, whereas the LUMO is mainly
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the CNHCꢀ and NNHCꢀcentered π* orbital of the Si2ꢀbonded NHCꢀsubstituent. It is noteworthy
+
+
that the KohnꢀSham orbitals of 1H calc, 1Me calc (see SI, Figures S39 and S41),
+
4
12
[(Idipp)(I)Si=Si(Idipp)] , Tbb(Br)Si=Si(SIdipp) and the model compound (IMe
4
)(R)Si=SiR (R
2
7
=
SiMe ) compare well those of the respective model systems depicted in Table 4.
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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31
32
33
34
35
36
37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4: Selected KohnꢀSham orbitals and their energy eigenvalues of compounds
A
–
F
depicted in Figure 7;
LUMO
Me
−3
NHC = C[N(Me)CH]
2
, R = Me. Isosurface value: 0.05 e Bohr .
HOMO−1
HOMO
−7.23 eV
−3.50 eV
Me
Me +
[
(NHC )(R)Si=Si(NHC )] (A)
−7.89 eV
−5.26 eV
Me
(
NHC )(R)Si=SiR (B)
−
3.95 eV
−1.73 eV
Me
R
2
Si=Si(NHC ) (C)
−4.34 eV
−3.42 eV
−1.06 eV
−
[
R
2
Si=SiR] (
D)
0.30 eV
0.94 eV
3.54 eV
R
2
Si=PR (
E)
−5.76 eV
−4.87 eV
−1.70 eV
+
[R
2
P=PR] (
F)
−
11.82 eV
−11.30 eV
−8.07 eV
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Whereas compounds A – F reveal the same number, symmetry properties and the same
occupation of the frontier orbitals with electrons, the absolute energies of the frontier orbitals
of A – F differ significantly (Table 4). This is not surprising given the different charges of the
compounds and the considerable higher electronegativity of the [PR] than that of the
−
[Si(NHC)] or [SiR] fragment, and implies a quite different reactivity of A – F, which remains
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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0
1
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9
0
to be exploited for the silicon compounds A – C. The interrelationship of A – F provides a
different view of the bonding between NHCs and mainꢀgroup elements from that developed
1
4
by G. Frenking, and enables one to conceive many presently unknown Si compounds that
are related to known phosphorus congeners.
3
. Summary
F
4
The mixedꢀvalent disilicon(I) borates 1R[B(Ar ) ] (R = H, Me, Et) were obtained upon
+
electrophilic addition of R to 1 and were comprehensively analyzed by a combination of
structural, spectroscopic and quantum chemical methods. The unprecedented 1,2ꢀhydrogen
F
migration observed along the Si=Si bond of 1H[B(Ar )]
4
is reminiscent of the degenerate
isomerization of carbenium ions, and can be considered as a molecular model for the Hꢀ
migration in hydrogenated amorphous silicon that is of relevance in photovoltaics. The
+
cations 1R have considerable synthetic potential in silicon chemistry given their many
potential reactivity sites, and can be strikingly viewed as NHCꢀtrapped examples of the
+
cations [Si
2
R] , providing another example for the exceptional ability of NHCs to stabilize lowꢀ
valent silicon centers in unusual bonding environments. The interrelation of the electronic
+
structure of the cations 1R to those of a series of Si and P multipleꢀbonded compounds
−
which are obtained upon replacement of the [Si(NHC)] by the isolobal [PR] and [SiR]
fragments, provides a different view on the electronic structure of NHCꢀstabilized mainꢀgroup
0
element compounds. The Si compound 1 bears an optimal cavity for the complexation of
4
alkali metal cations as demonstrated by the isolation and full characterization of 1M[BAr ] (M
= Li, Na), which may become valuable reagents for the functionalization of 1 in reactions with
mainꢀgroup element or transitionꢀmetal halides.
Associated Content
Supporting Information
The supporting information (SI) contains the syntheses, analytical data and illustrations of the
1
13
1
29
1
F
F
F
H, C{ H} and Si{ H} NMR spectra of 1H[B(Ar )
4
], 1Me[B(Ar )
4
], 1Et[B(Ar )
4
], 1Li[B(C
6
F
5
)
4
],
F
F
and 1Na[B(Ar )
4
]. The SI also contains the analysis of the dynamic process of 1H[B(Ar )
4
] in
solution as well as details of the quantum chemical calculations. This material is available
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F
free of charge via the Internet at http://pubs.acs.org. Crystallographic data for 1H[B(Ar )
4
],
] have been deposited
with the Cambridge Structural Database under the deposition numbers CCDC 1448827 –
448831.
F
F
F
1
4 4 6 5 4 6 4
Me[B(Ar ) ], 1Et[B(Ar ) ], 1Li[B(C F ) ]ꢁ(nꢀC H14) and 1Na[B(Ar )
1
10
11
12
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14
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Author Information
Corresponding Author
filippou@uniꢀbonn.de
Notes
The authors declare no competing financial interest.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (SFB813, “Chemistry at Spin Centers”) for
financial support of this study. We also thank Dr. Ujjal Das and Dr. Senada Nozinovic for
helpful discussions.
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2
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3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
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0
1
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(9)
(
,
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2
(
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(
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(
17) The B3LYP/6ꢀ311+G**/6ꢀ31G* computed ZPVE corrected (ZPVE = zero point vibrational energy)
dissociation energies for the stepwise dissociation of the carbene substituents from Si
2
(Idipp)
2
at 0 K
−1
−1
amount to 126.5 kJ mol (Si (Idipp) → Si (Idipp) (S = 0) + Idipp (S = 0)) and 185.0 kJ mol (Si (Idipp) →
2
2
2
2
3
−
g
Si
2
(S = 1, Χ Σ
) + Idipp (S = 0)) (S is the total spin angular momentum of the electronically and
geometrically relaxed fragments), see ref. [8].
(
+ −
18) For a quantum chemical description of throughꢀbond and throughꢀspace interactions leading to n and n
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nd
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(
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(
20) In an approach for the formal classification of covalent compounds of transition metals, a neutral 2eꢀdonor
−
as Idipp has been also considered as equivalent to R (R = alkyl, aryl): Green, M. L. H. J. Organomet.
Chem. 1995, 500, 127.
(
21) (a) Cowley, A. H.; Norman, N. C.; Pakulski, M. J. Chem. Soc., Chem. Commun. 1984, 1054. (b) Cowley, A.
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(
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10
11
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14
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16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
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53
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55
56
57
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59
60
(
23) (a) Loss, S.; Widauer, C.; Grützmacher, H. Angew. Chem. Int. Ed. 1999, 38, 3329; Angew. Chem. 1999,
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Organomet. Chem. 1992, 428, 431.
(
25) Batsanov, S. S. Inorg. Mat. 2001, 37, 871.
26) NHC2 denotes the dihedral angle between the CNHCꢀSiꢀSiꢀCNHC leastꢀsquare plane and the leastꢀ
square plane of the Nꢀheterocyclic ring bonded to the Si1 and Si2 atom, respectively.
(
ϕ
NHC1 and ϕ
(
(
(
27) Yamaguchi, T.; Sekiguchi, A.; Driess, M. J. Am. Chem. Soc. 2010, 132, 14061.
28) Kinjo, R.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2007, 129, 26.
29) Chernov, O. Novel Molecular Si(II) Precursors for Synthesis of the First Compounds with MetalꢀSilicon
Triple Bonds. Ph.D. Thesis, University of Bonn, 2012.
(
(
(
(
30) Agou, T.; Sugiyama, Y.; Sasamori, T.; Sakai, H.; Furukawa, Y.; Takagi, N.; Guo, J.ꢀD.; Nagase, S.;
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≠
(40) Taking the equation
ꢀ
G = 0.01914ꢁTꢁ[9.972+lg(T/ꢀν)], the smallest separation (ꢀν) of two signals of the
1
heterotopic Idipp substituents in the H NMR spectrum at 298 K (ꢀν = 7.6 Hz) and the boiling point of THFꢀ
≠
−1
d
8
(T = 338 K), a low limit of the Gibbs energy of activation can be estimated to be
ꢀG = 75 kJ mol
(
Sandström, D. Dynamic NMR Spectroscopy; Academic Press: London, 1992).
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0
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(
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Wisconsin, Madison, 2013
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7
.
(
44) The term “σꢀbonded” isomer was used to indicate that this isomer features a Si–R σꢀbond (R = H, Me, Et)
+
resulting from the interaction of the n
+
lone pair orbital of
1
with the LUMO of the electrophile R (R = H,
Me, Et). In comparison, the “πꢀbonded” isomer features a 3cꢀ2e bond resulting from the interaction of the
Si=Si π bond of with the LUMO of the electrophile.
1
(
45) Holleman, A. F.; Wiberg, E. Inorganic Chemistry, Academic Press: San Diego/London, 2001; Lehrbuch der
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(
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(
(
50) For a review on cationꢀπ interactions and their importance in biological systems and material science, see:
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55) The Li NMR resonances of πꢀcomplexated Li atoms exhibit diagnostic upfield shifts, which result from the
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