Silicon(II) coordination chemistry: N-heterocyclic carbene complexes of Si2+ and SiI+

Article abstract of DOI:10.1002/anie.201301363

Ligand swap: The exchange of N-heterocyclic carbene (NHC) ligands at Si^II centers is shown to provide access to a dicationic NHC complex of silicon(II), and an NHC adduct of the iodosilyliumylidene cation SiI ⁺, [SiI(IiPr₂Me₂)(IDipp)]⁺ (see picture). Characterization studies led to the discovery of an unprecedented C-H×××Si anagostic interaction for [SiI(IiPr ₂Me₂)(IDipp)]⁺. Copyright

Full text of DOI:10.1002/anie.201301363

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Angewandte
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DOI: 10.1002/anie.201301363
Ionic Silicon(II) Compounds
Silicon(II) Coordination Chemistry: N-Heterocyclic Carbene
Complexes of Si2+ and SiI+**
Alexander C. Filippou,* Yury N. Lebedev, Oleg Chernov, Martin Straßmann, and
Gregor Schnakenburg
Si2+ ions are highly reactive, two-valence-electron species that
have been generated in laser-induced plasmas and studied by
photoabsorption and microwave spectroscopy.[1] The ions
have also been employed as implants for waveguide fabrica-
tion in lithium niobate crystals, which are of great interest for
various photonic applications owing to their physical and
electro-optical properties.[2] A possible method of trapping
these highly electrophilic ions in the condensed phase, may
involve complexation by three strongly basic, neutral ligands
(L) to give dications of the general formula [SiL3]2+, in which
the silicon center attains a noble gas configuration. However,
dicationic silicon(II) complexes are presently unknown, in
marked contrast to the few germanium homologues, which
were reported some years ago by K. M. Baines et al.[3] This
difference can be explained by the anomalous low electro-
negativity of silicon versus germanium,[4] which makes the
isolation of silicon(II)-centered dications a highly challenging
goal. Even monocationic silicon(II) compounds are very rare,
and only the nido-type cluster cation [(C5Me5)Si]+,[5] the
b-diketiminato-substituted cation [HC(CMe)2(Ndipp)2Si]+
(dipp = 2,6-diisopropylphenyl)[6] and the SiII cation [{C10H6-
a,a-NP(nBu)3}SiCl]+ stabilized by a chelating bis(iminophos-
phorane) ligand[7] have been isolated thus far.
Since the first report of the silicon(0) compound [Si2-
(Idipp)2] (Idipp = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-
ylidene) by G. Robinson et al. in 2008,[8] N-heterocyclic
carbenes have been shown to be particularly effective ligands
for the stabilization of silicon compounds in unusually low
oxidation states. Remarkable examples include NHC adducts
of the dihalosilylenes SiX2 (X = Cl, Br),[9] which are very
valuable precursors in SiII chemistry,[9c,10] or NHC adducts of
the organohalosilylenes RSiCl (R = m-terphenyl, (2,6-diiso-
propylphenyl)(trimethylsilyl)amino),[11] which paved the way
for the preparation of the first complexes featuring metal–
silicon triple bonds.[12] Herein, the exchange of NHC ligands
at SiII centers is illustrated for the first time to provide access
Scheme 1. Multistep synthesis of the iodide salt of the dicationic SiII-
complex [Si(IMe4)3]2+ (3) starting from SiI4.
to unprecedented dicationic NHC complexes of silicon(II)
and NHC adducts of the iodosilyliumylidene cation SiI+.
The entry into this chemistry started with the triiodosilyl-
imidazolium salt 1 (Scheme 1), which was obtained from the
reaction of SiI4 with Idipp in toluene and isolated as a yellow,
thermally robust solid in 96% yield. Reduction of 1 with
potassium graphite (2.3 equiv) in benzene afforded the yellow
NHC-diiodosilylene adduct 2-I in 81% yield (Scheme 1).[13]
Under rigorous exclusion of air, the silicon(II) compound 2-I
is stable in benzene or toluene solution at ambient temper-
ature for several days, and decomposes in the solid state upon
heating above 1608C.
The solid-state structure of 1·3(CHCl3) was determined
by single-crystal X-ray crystallography.[13] It resembles that of
[SiBr3(Idipp)]Br·3(CH2Cl2)[9b] and reveals that the chloro-
form trissolvate of 1 is composed of well separated [SiI3-
(Idipp)]+ cations and iodide counter anions. The shortest Si···I
interionic contact of 5.94 ꢀ exceeds by far the sum of the van
der Waals radii of silicon and iodine (4.08 ꢀ),[14] thus
[*] Prof. Dr. A. C. Filippou, Dipl.-Chem. Y. N. Lebedev, Dr. O. Chernov,
Dipl.-Chem. M. Straßmann, Dr. G. Schnakenburg
Institut fꢀr Anorganische Chemie, Universitꢁt Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
E-mail: filippou@uni-bonn.de
ꢀ
excluding any bonding interaction. The mean Si I bond
length of the cation in 1·3(CHCl3) (2.420(8) ꢀ)[15] compares
well with that of SiI4 (2.432(5) ꢀ)[16] and the Si C1 bond
ꢀ
2
[**] We thank the Deutsche Forschungsgemeinschaft (SFB 813) for
generous financial support of this work. We also thank C. Schmidt,
K. Prochnicki, and H. Spitz for recording the solution NMR spectra,
Dr. S. Schwieger for collection of the data for one X-ray crystal
structure, Dr. W. Hoffbauer for recording the 29Si{1H} MAS-NMR
spectrum of 2-I and A. Martens for the elemental analyses.
ꢀ
(1.911(3) ꢀ) is only slightly longer than a covalent Si C(sp )
single bond (1.872 ꢀ),[17] as expected for an imidazolium
cation bearing
a SiI3 substituent at the C2 position
(Figure 1).[18]
1
The H, 13C{1H}, and 29Si{1H} NMR spectra confirm the
ionic nature of 1 in CDCl3 solution.[19] For example, the
29Si{1H} NMR spectrum of 1 displays a singlet signal at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301363.
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of pyramidalization (DP) of 2-I of 70% (sum of angles at Si =
297(1)8)[24] is slightly smaller than that of [SiBr2(Idipp)] (2-Br:
DP = 75%; sum of angles at Si = 292.78)[9b] and [SiCl2(Idipp)]
(2-Cl: DP = 78%; sum of angles at Si = 289.78);[9a] this trend
follows Bentꢁs rule.[25]
II
ꢀ
The Si I bond length of 2-I (2.575(3) ꢀ) compares well
with that of 4 (2.5591(6) ꢀ) (see below), but is considerably
longer than those of SiIV iodides (2.420(8) ꢀ (1); 2.432(5) ꢀ
(SiI4);[16] 2.453(4) ꢀ (MesSiI3)[26]) owing to the lower oxida-
tion state of silicon (+ II) in 2-I and 4.
ꢀ
The Si C bond length (1.984(7) ꢀ) of 2-I is nearly
identical with those of 2-Cl (1.989(3) ꢀ)[9a] and 2-Br
Figure 1. DIAMOND plot of the structure of the cation of 1·3CHCl3 in
the solid state. Thermal ellipsoids are set at 30% probability; hydrogen
atoms and solvent molecules were omitted for clarity. Selected bond
lengths [ꢂ] and angles [8]: Si–I1 2.439(1), Si–I2 2.410(1), Si–I3
2.412(1), Si–C1 1.911(3), C1–N1 1.371(4), C1–N2 1.352(4), N1–C2
1.370(4), N2–C3 1.373(4), C2–C3 1.352(5); I1-Si-I2 107.99(4), I1-Si-I3
108.16(4), I2-Si-I3 109.33(4), C1-Si-I1 107.5(1), C1-Si-I2 113.6(1), C1-Si-
I3 110.1(1), N1-C1-N2 105.4(3).
1.985(4) ꢀ[9b] and also the Si C bond dissociation energy of
ꢀ
2-I (121.4 kJmolꢀ1) compares well with those of 2-Cl
(121.4 kJmolꢀ1) and 2-Br (123.6 kJmolꢀ1), which suggests
ꢀ
the presence of a comparable strong C Si donor-acceptor
single bond in 2-X.[27] This is verified by the topological
ꢀ
analyses of the electron density distributions at the Si C bond
critical points.[27,28] These analyses further show that the Si C
ꢀ
dative bonds of 2-X have covalent character and are weaker
considerably lower field (d = ꢀ225.8 ppm) than that of SiI4
(d in C6D6 = ꢀ346.6 ppm). The same trend was observed for
[SiBr3(Idipp)]Br in CD2Cl2 (d = ꢀ63.9 ppm) vs. SiBr4 (d in
C6D6 = ꢀ90.8 ppm).[9b] In comparison, the 29Si NMR signal of
the neutral pentacoordinate complex [SiCl4(Idipp)][8] appears
in CD2Cl2 at considerably higher field (d = ꢀ108.9 ppm) than
that of SiCl4 (d in C6D6 = ꢀ18.5 ppm)[20] as a result of the
higher silicon coordination number.[21]
Compound 2-I is the first silicon(II) diiodide to be
characterized by X-ray crystallography (Figure 2).[13,22] Its
structure closely resembles those of the SiII halides [SiX2-
(Idipp)] (X = Cl, Br)[9a,b] and of the germanium analogue
[GeI2(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imida-
zol-2-ylidene),[23] and features a trigonal pyramidal coordi-
nated silicon center, which indicates the presence of a stereo-
chemically active lone pair of electrons at silicon. The degree
than those of SiMe4, which provides a rationale for the lower
ꢀ
Si C bond dissociation energies found for 2-X versus those of
alkyl silanes (ca. 370 kJmolꢀ1).[29] The electronic structure of
2-X compares well with those of the zwitterionic silenes
X2SiC(NH2)2 (X = H, Me, F, Cl, NH2, OH), which feature
a reversed Si C bond polarization,[30] that is best represented
ꢀ
by zwitterionic canonical formulas, as depicted for 2-I in
Scheme 2.[31] Compounds 2-X take an intermediate position
in the bonding continuum between the planar parent silene
=
=
H2Si CH2 at the one end, which features a short Si C bond
(1.704(2) ꢀ)[32] with a large Si C bond dissociation energy
ꢀ
(462 kJmolꢀ1),[31a] and the non-planar carbene–silylene
adduct [(NN)SiC(NN)] (NN = 1,2-(NCH2tBu)2C6H4) at the
ꢀ
other end, which features
a
ꢀ
long Si C single bond
(2.162(5) ꢀ) with a very low Si C bond dissociation energy
(13 kJmolꢀ1).[33] The position in this bonding continuum
depends on the sum of the singlet–triplet gaps SDEST of the
carbene and silylene fragments, which is tuned by the
substituents.[31,34]
The 29Si{1H} NMR spectrum of 2-I in C6D6 solution
displays a singlet at a similar position (d = ꢀ9.7 ppm) to
those observed in the solid-state 29Si{1H} NMR-MAS spec-
trum (d = ꢀ5.1 ppm (1Si), and ꢀ9.2 ppm (2Si).[35] This
suggests that compound 2-I has a similar structure in solution
and in the solid state. A comparison of the 29Si nucleus
chemical shift of 2-I with that of [SiBr2(Idipp)] (d =
10.9 ppm)[9b] and [SiCl2(Idipp)] (d = 19.06 ppm)[9a] reveals
the same 29Si nucleus deshielding upon halide substitution
(I!Br!Cl) as observed for SiIV halides.[36]
Figure 2. DIAMOND plot of the molecular structure of 2-I in the solid
state. Thermal ellipsoids are set at 30% probability; hydrogen atoms
are omitted for clarity. Selected bond lengths [ꢂ] and angles [8] (bond
lengths and bond angles of the other two independent molecules of
2-I are given in square brackets): Si1–I1 2.570(1) [2.585(1), 2.563(1)],
Si1–I2 2.576(1) [2.578(1), 2.577(1)], Si1–C1 1.997(4) [1.975(4),
1.980(4)], C1–N1 1.362(5) [1.371(5), 1.362(5)], C1–N2 1.363(5)
[1.369(5), 1.377(5)], N1–C2 1.388(5) [1.377(5), 1.383(5)], N2–C3
1.373(5) [1.378(5), 1.364(5)], C2–C3 1.339(6) [1.344(6), 1.338(6)];
I1-Si-I2 95.63(4) [96.72(4), 97.16(4)], C1-Si-I1 103.1(1) [104.9(1),
102.8(1)], C1-Si-I2 96.5(1) [97.5(1), 95.6(1)], N1-C1-N2 104.7(3)
[103.2(3), 103.7(3)].
Scheme 2. Ylidic canonical structures of 2-I, 32+ and 4+. Formal
charges are shown in circles. L=N-heterocyclic carbene.
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The silicon(II) iodide 2-I is a very promising starting
material for the preparation of cationic SiII complexes, since it
bears two iodide groups, which are good leaving groups and
should be easily displaced from silicon. In fact, addition of
1,3,4,5-tetramethyl-imidazol-2-ylidene (IMe4; 2.4 equiv) to
a solution of 2-I in fluorobenzene rapidly afforded, after
replacement of both iodide groups and the Idipp ligand, the
iodide salt of the dicationic SiII complex [Si(IMe4)]2+ (32+),
which immediately precipitated out of the reaction solution,
and was isolated as a light yellow powder in 79% yield
(Scheme 1). Complex salt 3 is only soluble in CH2Cl2, in which
it slowly decomposes after several hours at ambient temper-
ature. No evidence was found for any intermediates in the
reaction of 2-I with IMe4. This suggests that the first step,
which probably involves a Iꢀ/IMe4 exchange to give the
putative intermediate [SiI(IMe4)(Idipp)]I, is rate determining
in the overall reaction sequence ultimately leading to 3.[37]
The crystal structure of 3·2CD2Cl2 shows well separated
32+ ions and iodide counter anions, the closest Si···I contact
(5.88 ꢀ) being much longer than the sum of the van der Waals
radii of silicon and iodine (4.08 ꢀ).[14] The dication 32+ has an
almost C3 symmetric, propeller-like, pyramidal structure
(Figure 3), which resembles that of the Ge dication
[Ge(IiPr2Me2)3]2+.[3a] The degree of pyramidalization of 32+
(DP = 53%, sum of angles at Si = 3128) is lower than those of
[SiI(IiPr2Me2)(Idipp)]+ (4+) (DP = 65%, sum of angles at Si =
301.48; Figure 4) and 2-I (DP = 70%) and can be rationalized
with Bentꢁs rule, according to which silicon uses hybrid
orbitals of higher s character for bonding to the less electro-
negative NHC substituents in 32+ and 4+.[25] The double
positive charge of 32+ is strongly delocalized over the IMe4
groups, which suggests a strong contribution of the zwitter-
ionic canonical formula depicted in Scheme 2 to the bonding
in 32+.
Figure 4. DIAMOND plot of the molecular structure of the cation of
4·C6H5F in the solid state. Ellipsoids are set at 30% probability; the
2,6-bonded isopropyl substituents of the Idipp ligand, the solvent
molecule and the hydrogen atoms, except that (H36) involved in the
anagostic interaction with the silicon center, are omitted for clarity; the
anagostic interaction is visualized by a dashed line. Selected bond
lengths [ꢂ] and angles [8]: Si–I1 2.5591(6), Si–C1 1.947(2), C1–N1
1.358(3), C1–N2 1.374(3), N1–C2 1.386(3), N2–C3 1.377(3), C2–C3
1.341(3), Si–C28 1.967(2), C28–N3 1.362(3), C28–N4, 1.357(3), N3–
C29 1.384(3), N4–C30 1.389(3), C29–C30 1.357(3), N3–C36 1.489(3);
C1-Si-I1 103.49(6), C28-Si-I1 98.57(7), C1-Si-C28 99.32(9), N1-C1-N2
104.2(2), N3-C28-N4 105.8(2).
Evidence for this is provided by both the structural and
2+
spectroscopic features of 3 . Thus, the Si CNHC bonds of 32+
ꢀ
(1.915(3) ꢀ)[15] are even shorter than that of [SiCl(C6H3-2,6-
Trip2)(IMe4)] (1.963(2) ꢀ)[11a] and [Cp(CO)2Mo Si(C6H3-2,6-
=
Trip2)(IMe4)] (1.944(1) ꢀ),[12a] and approximate the Si Caryl
ꢀ
single bonds of [SiCl(C6H3-2,6-Trip2)(IMe4)] (1.937(2) ꢀ)[11a]
[12a]
=
and [Cp(CO)2Mo Si(C6H3-2,6-Trip2)(IMe4)] (1.920(2) ꢀ).
The 29Si{1H} NMR spectrum of 3 in CD2Cl2 shows
a strongly shielded 29Si NMR signal (d = ꢀ89.9 ppm), which
appears at considerably higher field than those of 2-I (d =
ꢀ9.7 ppm) and 4 (d = ꢀ55.3 ppm). The 13C{1H} NMR spec-
trum displays a distinctive signal for the Si-bonded CNHC atom
at d = 150.7 ppm, which is shifted considerably high-field
versus that of IMe4 (d = 212.7 ppm),[38] but appears at
a position close to that of the imidazolium salt (IMe4H)Cl
(d = 136.9 ppm).[11a] Finally, the 1H and 13C{1H} NMR spectra
of 3 display a single set of resonances, as expected for three
ꢀ
equivalent IMe4 groups rotating rapidly about the Si C bonds
on the NMR time scale.
Whereas reaction of 2-I with IMe4 directly afforded the
complex salt 3, addition of the sterically more demanding
N-heterocyclic carbene 1,3-Diisopropyl-4,5-dimethylimida-
zol-2-ylidene (IiPr2Me2) to a toluene solution of 2-I at
ambient temperature gave, after displacement of only one
iodide group, the SiII complex salt [SiI(IiPr2Me2)(Idipp)]I (4)
[Eq. (1)]. No further reaction of 4 with IiPr2Me2 was
observed, even at higher temperature (fluorobenzene,
608C). Compound 4 was isolated as a yellow, crystalline
Figure 3. DIAMOND plot of the molecular structure of the cation of
3·2(CD2Cl2) in the solid state. Ellipsoids are set at 30% probability;
hydrogen atoms and solvent molecules are omitted for clarity. The
smaller figure depicts the same structure in a side view that shows the
strong pyramidalization of silicon. Selected bond lengths [ꢂ] and
angles [8] (corresponding bond lengths and angles of the other two
NHC groups are included in squared brackets): Si–C1 1.918(3), Si–C8
1.909(3), Si–C15 1.917(3), C1–N1 1.359(3) [1.356(3), 1.355(3)], C1–N2
1.357(3) [1.352(3), 1.360(3)], N1–C2 1.389(3) [1.384(3), 1.386(3)], N2–
C3 1.402(3) [1.388(3), 1.385(3)], C2–C3 1.354(4) [1.360(4), 1.359(4)];
C1-Si-C8 103.9(1), C1-Si-C15 104.7(1), C8-Si-C15 103.4(1), N1-C1-N2
105.1(2) [104.8(2), 104.8(2)].
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C6H5F-monosolvate in 84% yield after crystallization from
fluorobenzene. Compound 2-I is moderately soluble in
fluorobenzene and THF, and well soluble in dichloromethane,
in which it slowly decomposes at ambient temperature.
The crystal structure of 4·PhF reveals the presence of well-
separated trigonal pyramidal SiII monocations [SiI(IiPr2Me2)-
(Idipp)]+ (4+) and iodide anions, the shortest interionic Si···I
contact being 7.42 ꢀ. The structure of the cation 4+ (Figure 4)
resembles those of 2-I and 32+.
the calculated results was given by the calculated Si–H
coupling constant of 9.0 Hz, which compares very well with
the experimental value of 10.4 Hz. Compound 4 is thus the
ꢀ
first example of an anagostic interaction of a C H bond with
an SiII center.
In conclusion, the isolation and full characterization of 3
and 4, which can be strikingly viewed as NHC complex salts of
+
Si2+ and [SiI] , highlights the synthetic potential of [NHC
ꢀ
SiX2] adducts (X = halogen) and opens a new avenue of
chemical exploration in SiII chemistry, one in which associa-
tive or dissociative substitution of the NHC ligands can
provide entry to a series of novel compounds of silicon in low-
In contrast to 3, the 1H and 13C NMR spectra of 4 in
CD2Cl2 show that rotation of both NHC ligands about their
ꢀ
respective Si CNHC bonds is frozen out on the NMR time-
ꢀ
scale. The stereochemical rigidity of 4 results from the
interference of the bulky 2,6-diisopropylphenyl and isopropyl
substituents, which locks cation 4+ in a specific conformation,
in which the five-membered rings of the NHC ligands are
perpendicularly disposed (dihedral angle of the least squares
planes defined by the NHC ring carbon atoms = 85.7(1)8).
This locked conformation renders all groups in the NHC
ligand backbone non-equivalent and leads to a large number
of 1H and 13C signals in the NMR spectra of 4, which were all
assigned by 2D NMR spectroscopy.[13] However, the most
intriguing spectroscopic feature of 4 is the doublet signal
observed for the 29Si NMR nucleus in the proton-coupled
29Si NMR spectrum at d = ꢀ55.3 ppm (J(Si,H) = 10.4 Hz).
Selective decoupling of one of two isopropyl methine protons
of the IiPr2Me2 ligand (septet signal at d = 4.96 ppm) trans-
formed the 29Si signal into a singlet, and 2D 29Si–1H correla-
tion spectroscopy revealed a cross-peak with the same
isopropyl methine proton.[13] The observation of 29Si spin
coupling to only one isopropyl methine proton of the IiPr2Me2
ligand, as well as the magnitude of the 29Si-1H coupling
constant (J = 10.4 Hz), which is even larger than those
oxidation states. Moreover, the unprecedented C H anagos-
tic interaction observed in 4 may serve as a valuable model for
ꢀ
intermediates in non-metal-mediated C H bond activation
reactions.
Received: February 15, 2013
Revised: April 14, 2013
Published online: May 27, 2013
Keywords: ionic compounds · low-valent compounds ·
.
N-heterocyclic carbenes · silicon · silyliumylidenes
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5683; b) A. C. Filippou, O. Chernov, G. Schnakenburg, Angew.
Chem. 2009, 121, 5797; Angew. Chem. Int. Ed. 2009, 48, 5687;
ꢀ
of 4, which reveals a close contact of one isopropyl C H
group of IiPr2Me2 to silicon leading to a much shorter Si···H
distance (2.57(1) ꢀ), than the sum of the van Waals radii of
silicon and hydrogen (3.30 ꢀ; Figure 4).[14] The nature of this
interaction was studied by a topological analysis of the
electron density.[28] A bond critical point (BCP) and a bond
path could be located between Si and H36, clearly indicating
ꢀ
a Si···H C interaction. The geometrical parameters of this
interaction (d(Si···H)calc. = 2.557 ꢀ; d(Si···H)exp. = 2.57(1) ꢀ;
](Si···H-C)calc. = 121.08; ](Si···H-C)exp. = 116.6(1)8), as well
as the substantial electron density (1BCP = 0.117 eꢀꢀ3), the
positive Laplacian (5 1BCP = 0.797 eꢀꢀ5), and the slightly
2
negative total energy density (HBCP = ꢀ0.0004 Hartreeꢀꢀ3) at
the bond critical point compare well with those found for
ꢀ
anagostic M···H C interactions in transition metal com-
plexes.[41] NBO second-order perturbation analysis shows
a small stabilization energy of 14 kJmolꢀ1 resulting from the
orbital interaction between the silicon lone pair and the
antibonding s* C H orbital.[27] Further proof of the validity of
ꢀ
Angew. Chem. Int. Ed. 2013, 52, 6974 –6978
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6977

.
Angewandte
Communications
c) A. C. Filippou, O. Chernov, G. Schnakenburg, Chem. Eur. J.
2011, 17, 13574.
b) R. S. Ghadwal, S. S. Sen, H. W. Roesky, G. Tavcar, S. Merkel,
D. Stalke, Organometallics 2009, 28, 6374.
[20] G. Engelhardt, R. Radeglia, H. Jancke, E. Lippmaa, M. Mꢃgi,
Org. Magn. Res. 1973, 5, 561.
[10] a) J. Li, S. Merkel, J. Henn, K. Meindl, A. Dçring, H. W. Roesky,
R. S. Ghadwal, D. Stalke, Inorg. Chem. 2010, 49, 775; b) R. S.
Ghadwal, S. S. Sen, H. W. Roesky, M. Granitzka, D. Kratzert, S.
Merkel, D. Stalke, Angew. Chem. 2010, 122, 4044; Angew. Chem.
Int. Ed. 2010, 49, 3952; c) R. S. Ghadwal, H. W. Roesky, M.
Granitzka, D. Stalke, J. Am. Chem. Soc. 2010, 132, 10018;
d) R. S. Ghadwal, H. W. Roesky, K. Prçpper, B. Dittrich, S.
Klein, G. Frenking, Angew. Chem. 2011, 123, 5486; Angew.
Chem. Int. Ed. 2011, 50, 5374; e) S. M. I. Al-Rafia, A. C.
Malcolm, R. McDonald, M. J. Ferguson, E. Rivard, Chem.
Commun. 2012, 48, 1308; f) O. Chernov, Dissertation, Novel
Molecular SiII Precursors for Synthesis of the First Compounds
with Metal-Silicon Triple Bonds, Universitꢃt Bonn, 2012.
[11] a) A. C. Filippou, O. Chernov, G. Schnakenburg, Chem. Eur. J.
2010, 16, 2866; b) H. Cui, C. Cui, Dalton Trans. 2011, 40, 11937.
[12] a) A. C. Filippou, O. Chernov, K. W. Stumpf, G. Schnakenburg,
Angew. Chem. 2010, 122, 3368; Angew. Chem. Int. Ed. 2010, 49,
3296; b) A. C. Filippou, O. Chernov, G. Schnakenburg, Angew.
Chem. 2011, 123, 1154; Angew. Chem. Int. Ed. 2011, 50, 1122.
[13] The Supporting Information contains the syntheses, analytical,
and spectroscopic data of compounds 1 – 4, as well as selected
NMR spectra for compounds 1 – 4 and crystallographic data for
1·3(CHCl3), 2-I, 3·2(CD2Cl2) and 4·C6H5F. CCDC 924156
(1·3(CHCl3)), 924157 (2-I), 924158 (3·2(CD2Cl2)) and 924159
(4·C6H5F) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
[21] E. A. Williams, Annu. Rep. NMR Spectrosc. 1983, 15, 235.
[22] Three independent molecules with very similar geometric
parameters were found in the unit cell of 2-I. The unweighted
mean (xu) values of the individual bonding parameters of the
three crystallographically distinct molecules of 2-I were used in
the discussion of the structure of 2-I. The standard deviations (s)
of xu (values given in parentheses) were calculated using the
equation s2 = S(xiꢀxu)2/n2ꢀn, where xi is the respective individ-
ual value and n is the total number of individual values.
[23] A. J. Arduengo III, H. V. R. Dias, J. C. Calabrese, F. Davidson,
Inorg. Chem. 1993, 32, 1541.
[24] A degree of pyramidalization of 100% corresponds to a sum of
angles of 2708, and a degree of pyramidalization of 0% indicates
a planar coordination of the central atom.
[25] H. Bent, Chem. Rev. 1961, 61, 275.
[26] N. Weidemann, G. Schnakenburg, A. C. Filippou, Z. Anorg. Allg.
Chem. 2009, 635, 253.
[27] Details of all quantum chemical studies, comparisons of selected
calculated and experimental bond parameters of 1+, its dissoci-
ation products, 2-X (X = Cl, Br, I) and 4+, and the cartesian
atomic coordinates of all calculated structures are given in the
Supporting Information.
[28] R. F. W. Bader, Chem. Rev. 1991, 91, 893.
[29] R. Walsh, Acc. Chem. Res. 1981, 14, 246.
[30] a) H. Ottosson, Chem. Eur. J. 2003, 9, 4144, and references
therein; b) H. Ottosson, A. M. Eklçf, Coord. Chem. Rev. 2008,
252, 1287.
[14] A. Bondi, J. Phys. Chem. 1964, 68, 441.
[15] The unweighted mean (xu) value of the Si I bond lengths of
ꢀ
[31] The reversed Si C polarization of 2-X is evidenced by the NBO
ꢀ
charges of the SiX2 fragments: 2-Cl, q(SiCl2) = ꢀ0.29e; 2-Br,
ꢀ
1·3(CHCl3) and the Si C bonds of 3·2CD2Cl2 is given. The
q(SiBr2) = ꢀ0.28e; 2-I, q(SiI2) = ꢀ0.27e.
standard deviation (s) of xu (value given in parentheses) was
calculated using the equation s2 = S(xiꢀxu)2/n2ꢀn, where xi is the
individual value and n = 3.
[32] S. Bailleux, M. Bogey, J. Demaison, H. Bꢂrger, M. Senzlober, J.
Breidung, W. Thiel, R. Fajgar, J. Pola, J. Chem. Phys. 1997, 106,
10016.
[16] M. Kolonits, M. Hargittai, Struct. Chem. 1998, 9, 349.
[33] W. M. Boesveld, B. Gehrhus, P. B. Hitchcock, M. F. Lappert,
P. v. R. Schleyer, Chem. Commun. 1999, 755.
[34] The DEST of the SiX2 and Idipp fragments were calculated in the
frozen geometries adopted by the fragments in 2-X, as well as in
the minimum (relaxed) geometries. DEST (in the frozen geo-
2
ꢀ
[17] The mean value of the length of a Si C(sp ) single bond was
derived from a CSD survey of tetravalent, four-coordinate
silicon compounds.
[18] Alternatively, [SiI3(Idipp)]+ can be regarded as a donor-acceptor
complex of Idipp with the triiodosilylium ion SiI3+. However, the
metries): SiCl2 246.9 kJmolꢀ1
,
Idipp 381.0 kJmolꢀ1
;
SiBr2
following findings are less supportive of this view, which implies
222.4 kJmolꢀ1, Idipp 379.7 kJmolꢀ1; SiI2 186.5 kJmolꢀ1, Idipp
+
ꢀ
the presence of a Si C dative bond in [SiI3(Idipp)] , and instead
378.4 kJmolꢀ1
; DEST (in the minimum geometries): SiCl2
ꢀ
suggest the presence of a covalent Si C bond (see the Support-
ing Information): The NBO charges of the SiI3 (q =+ 0.53e) and
222.1 kJmolꢀ1; SiBr2 199.4 kJmolꢀ1; SiI2 165.7 kJmolꢀ1; Idipp
358.3 kJmolꢀ1 (see the Supporting Information).
Idipp fragment (q =+ 0.47e) have similar values; also, the ZPVE
[35] The observation of two signals in a 1:2 ratio in the solid-state
29Si{1H} MAS-NMR spectrum of 2 suggests that two of three
independent molecules have the same 29Si shielding tensor.
[36] U. Niemann, Z. Naturforsch. B 1975, 30, 202.
[37] An exchange of the N-heterocyclic carbenes to give the neutral
intermediate [SiI2(IMe4)] is also conceivable in the first step, but
is less probable given the observation that reaction of 2-I with
the more bulky imidazol-2-ylidene IiPr2Me2 stops at the SiII salt
[SiI(IiPr2Me2)(Idipp)]I (4).
ꢀ
corrected energy required for the Si C bond homolysis to give
the fragments SiI3 and Idipp+ (BDEhom. = 315.4 kJmolꢀ1) is
ꢀ
similar to that required for the Si C bond heterolysis to give the
fragments SiI3 and Idipp (BDEhet. = 330.9 kJmolꢀ1); further-
+
+
ꢀ
more, the Si C bond dissociation energy of [SiI3(Idipp)]
compares well with those of typical Si C covalent bonds
ꢀ
[29]
(BDE ca. 370 kJmolꢀ1
)
and is considerably higher than
ꢀ
those of Si C dative bonds as found in 2-X (BDE ca.
120 kJmolꢀ1). For studies of donor–acceptor (dative) bonds,
see: a) A. Haaland, Angew. Chem. 1989, 101, 1017; Angew.
Chem. Int. Ed. Engl. 1989, 28, 992; b) V. Jonas, G. Frenking,
M. T. Reetz, J. Am. Chem. Soc. 1994, 116, 8741; c) H. Jiao,
P. v. R. Schleyer, J. Am. Chem. Soc. 1994, 116, 7429.
[38] N. Kuhn, T. Kratz, Synthesis 1993, 561.
ˇ
[39] a) E. Liepins, I. Birgele, P. Tomsons, E. Lukevics, Magn. Reson.
Chem. 1985, 23, 485; b) J. Ambati, S. E. Rankin, J. Phys. Chem.
A 2010, 114, 12613.
[40] For a recent review on through-space nuclear spin-spin cou-
plings, see: J.-C. Hierso, D. Armspach, D. Matt, C. R. Chim. 2009,
12, 1002.
[41] a) Y. Zhang, J. C. Lewis, R. G. Bergman, J. A. Ellman, E.
Oldfield, Organometallics 2006, 25, 3515; b) M. Brookhart,
M. L. H. Green, G. Parkin, Proc. Natl. Acad. Sci. USA 2007, 104,
6908.
[19] The structure of the Idipp adducts of SiX4 (X = Cl, Br, I)
depends on both the X group and the solvent. Thus, chlorinated
hydrocarbons such as CH2Cl2 and CHCl3 favor, in the case of
X = Br or I, the ionic form [SiX3(Idipp)]X, whereas [SiBr4-
(Idipp)] is composed of neutral trigonal bipyramidal molecules
in toluene or benzene solutions. In comparison, [SiCl4(Idipp)] is
nonionic in CH2Cl2 or benzene solutions: a) Refs. [8] and [9b];
6978 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 6974 –6978