From unsymmetrically substituted benzamidinato and guanidinato dichlorohydridosilanes to novel hydrido n-heterocyclic silylene iron complexes

Article abstract of DOI:10.1021/om5005918

Starting from the unsymmetric N,N′-substituted thiourea compounds (R)N(H)C(=S)N(H)(^tBu) (1, R = Dipp: 2,6-ⁱPr₂-C₆H₃; 2, R = 1-adamantyl), the corresponding asymmetric carbodiimines (R)N=C=N(^t

Full text of DOI:10.1021/om5005918

Article
pubs.acs.org/Organometallics
From Unsymmetrically Substituted Benzamidinato and Guanidinato
Dichlorohydridosilanes to Novel Hydrido N‑Heterocyclic Silylene Iron
Complexes
Burgert Blom, Markus Pohl, Gengwen Tan, Daniel Gallego, and Matthias Driess*
Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universitat Berlin, Strasse des 17. Juni 135, Sekr. C2,
̈
D-10623 Berlin, Germany
S
* Supporting Information
ABSTRACT: Starting from the unsymmetric N,N′-substi-
tuted thiourea compounds (R)N(H)C(S)N(H)(tBu) (1, R
= Dipp: 2,6-iPr2-C6H3; 2, R = 1-adamantyl), the corresponding
asymmetric carbodiimines (R)NCN(tBu) (3, R = Dipp;
4, R = 1-adamantyl) are readily accessible in high yields upon
reduction with LiHMDS (Li[N(SiMe3)2]). The reaction of
compound 3 with PhLi followed by SiCl4 afforded, in a one-
pot reaction, the asymmetric benzamidinato-stabilized tri-
chlorosilane [PhC{(NtBu)(NDipp)}]SiCl3 (5). Similarly,
silanes [PhC{(NtBu)(NDipp)}]SiHCl2 (6), [(NMe2)C{(NtBu)(NDipp)}]SiHCl2 (7), and [PhC{(NtBu)(NAd)}]SiHCl2 (8)
could also be isolated. All novel trichloro- or dichlorohydridosilanes were fully spectroscopically characterized and studied by
single-crystal X-ray diffraction analyses, the latter revealing in all cases a distorted-trigonal bipyramidal five-coordinate silicon
center. The reactions of silanes 5−8 with K2[Fe(CO)4] were also explored: In the case of the reaction of silane 5 with
K2[Fe(CO)4], no reaction was observed even after prolonged heating. However, in the case of the silanes 6−8, the selective
formation of the corresponding hydrido SiII:→Fe0 complexes [[R1C{(NtBu)(NR2)}](H)Si:→Fe(CO)4] (9, R1 = Ph, R2 = Dipp;
10, R1 = NMe2, R2 = Dipp; 11, R1 = Ph, R2 = 1-adamantyl) could be achieved. Complexes 9−11 represent unprecedented
hydrido-N-heterocyclic silylene complexes, bearing asymmetric ligand backbones. Complexes 9−11 were fully spectroscopically
characterized, and in addition the single-crystal X-ray structure analysis of compound 10 is reported.
corresponding alcohols after workup.11 In the latter case, the
INTRODUCTION
■
SiII center appears to act in a cooperative way with the iron
One of our key research objectives in the preceding years has
center, thereby facilitating the catalytic process (Scheme 1).
been to investigate N-heterocyclic silylene (NHSi) transition
Although this is the first example whereby a hydrido-NHSi
metal complexes as possible precatalysts for a variety of
transformations, which, given the ready abundance of silicon in
complex successfully engages a catalytic transformation, the
prochiral ketones employed in this study did not in any
instance afford enantiomeric excesses (ee), due to the lack of
chiral induction. This would be highly desirable and represents
the earth’s crust, precipitated our early interest in this
direction.1 In general, NHSi complexes are accessible by
reactions of isolable “free” NHSis with a suitable transition
metal complex, which then affords the corresponding NHSi
a major advance in using low-valent silicon chemistry in
complex.2 One of the most useful NHSis for this purpose, used
homogeneous catalysis.
We therefore became interested in whether it might be
in our group and others in recent times, is Roesky’s
possible to prepare “asymmetric” benzamidinato-12 or related
guanidinato-stabilized NHSi complexes,13 which are not known
to date,14 since their isolation could enable asymmetric
catalysis.
chlorosilylene, PhC(NtBu)2SiCl.3 Taking advantage of the
facile coordination this chlorosilylene offers, a plethora of its
complexes (Chart 1) across the transition metals: titanium,4
vanadium,5 chromium, molybdenum or tungsten,6 rhenium or
manganese,7 iron,8 and even copper9 have been isolated and
studied to date.
Access to NHSi transition metal complexes relies almost
exclusively on the isolation of the “free” NHSi and then
coordination thereof to a transition metal center, usually by
ligand elimination.15 The isolation of the reactive free NHSi
can be experimentally difficult due to their intrinsic high
reactivity, particularly to moisture and oxygen.16 Accordingly,
access to hydrido-NHSi complexes involves isolation of the free
In preceding studies it was found that the robust nature of
this chlorosilylene, upon coordination to transition metals,
enables further functionalization of the SiII−Cl bond, and even
reactive SiII hydrides could be isolated in this manner.10
Previously it was also found that the hydrido SiII complex
[LHSi:→Fe(dmpe)2] (L = PhC(NtBu)2; dmpe = 1,2-bis-
(dimethylphosphino)ethane) is also catalytically active in the
hydrosilylation of ketones, affording high yields of the
Received: June 3, 2014
Published: September 19, 2014
© 2014 American Chemical Society
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In the present study we report our progress in this direction,
particularly a facile and new direct route to asymmetric
benzamidinato- or guanidinato-stabilized hydrido-NHSi com-
plexes of iron, which are readily accessible from a double
metathesis route using K2[Fe(CO)4] from the corresponding
dichlorohydridosilanes as SiIV precursors bearing asymmetric
ligands. This new route effectively saves two synthetic steps
(i.e., the isolation of the free NHSi and its coordination to a
transition metal) and, given the increasing interest these
complexes have attracted recently, might be of considerable
practical importance.
Chart 1. Some Examples of N-Heterocyclic Silylene
Complexes Based on the Chlorosilylene Ligand
PhC(NtBu)SiCl and the Related Derivative IV
RESULTS AND DISCUSSION
■
The seminal work of Roesky showed that the carbodiimine tBu-
NCN-tBu is the key building block to access benzamidi-
nato-stabilized trichloro- or dichlorohydridosilanes, upon its
reaction with PhLi, then the silane (SiCl4 or SiHCl3), in a one-
pot synthesis affording LSiCl3 or LSiHCl2 (L = PhC(NtBu)2),
respectively.3 Using a dehydrohalogenation reaction with the
latter precursor, the chlorosilylene LSiCl can be accessed in
higher yields than through reductive dechlorination of LSiCl3.
Tacke and co-workers12 have very recently employed a similar
strategy to access the first guanidinato-stabilized18 silanes, using
the symmetrical carbodiimine Dipp-NCN-Dipp (Dipp =
2,6-diisopropylphenyl). Its reaction with LiNMe2, followed by
trichlorosilane addition, affords selectively L′SiCl2H (L′ =
(NMe2)C(NDipp)2). The latter also then serves as a key
starting material to access the NHSi L′Si(Ntms2) (tms =
SiMe3). On the basis of these seminal efforts, we reasoned that
starting from an asymmetric carbodiimine, of the type R1-N
CN-R2 (R1 ≠ R2, R1, R2 = aliphatic or aromatic substituents),
a similar reaction sequence could afford asymmetric benzami-
dinato- or guanidinato-stabilized silanes of the type L*SiCl3 and
L*SiHCl2 (L* = PhC(NR1)(NR2) or (Me2N)C(NR1)(NR2)),
respectively. Hence the starting point in our investigation was
targeting the asymmetric carbodiimines Dipp-NCN-tBu
(3) and Ad-NCN-tBu (4) (Ad = 1-adamantyl), since they
bear substantially different substituents on the N atoms in both
cases. Both carbodiimines 3 and 4 are known, but their hitherto
reported syntheses are less convenient and require metal (Zr,
Fe, Ta, Sn)-mediated routes19 and/or workup procedures
involving distillation.20 In our hands, we found that
carbodiimine 3 or 4 can readily be accessed upon reduction
with LiHMDS (Li[N(SiMe3)2]) starting from the thiourea
precursors (Dipp)N(H)C(S)N(H)(tBu) (1)21 and (1-
adamantyl)N(H)C(S)N(H)(tBu) (2),22 respectively
(Scheme 3). This novel reductive route to carbodiimines 3
and 4 is clean and selective, and both can be isolated in high
yields (3, 72%; 4, 100%) without any additional purification
(distillation) steps necessary.
Scheme 1. Proposed Mechanism for the Hydrosilylation of
a
Ketones Mediated by [LHSi:→[Fe]], Showing Cooperative
Behavior between the NHSi and Iron Center11
a
L
= = Fe(dmpe)2, dmpe = bis-
PhC(NtBu)2; [Fe]
(dimethylphosphino)ethane.
halo-NHSi in a first synthetic step, subsequent coordination to
the metal center as a second step, followed by a halide−hydride
exchange reaction, typically by “superhydride” (HBEt3 anion)
or similar reagent (Scheme 2). All presently reported hydrido-
NHSi complexes, in themselves very rare, generally follow this
somewhat cumbersome and time-consuming three-step syn-
thetic route.17
Having succeeded in the facile isolation of the asymmetric
carbodiimines, we next turned our attention to the synthesis of
a
Scheme 2. Schematic Representation of the Hitherto Employed Synthetic Strategy for Access to Hydrido NHSi Complexes
a
X = halogen; L = PhC(NtBu)2; L* = ligand capable of elimination e.g., PMe3, CO, NHC.
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In the 29Si{1H} NMR spectra, the silanes 5−8 exhibit
strongly shielded singlet resonance signals, comparable to each
other within narrow limits, typical for SiIV complexes of this
type, in the range δ = −92.9 to −102.3 ppm (Table 1). The
Scheme 3. Novel Route to Asymmetric Carbodiimines 3 and
4 upon Reduction of Thioureas 1 and 2 with LiHMDS
Table 1. Summary of Some Key NMR Spectral Parameters
(in ppm, spectra recorded in C6D6 at 298 K) Observed for
Silanes 5−8
complex
29Si{1H}
1J (29Si,H)/Hz
13C{1H} of NCN
5
6
7
8
−93.8
−92.9
−102.3
−97.3
171.2
170.5
171.2
171.3
353
346
332
similarity in the chemical shifts suggests that the steric and/or
electronic changes in the ligand backbone do not affect the 29Si
NMR shift for this series of compounds. It is interesting to note
that silanes 5 and 6, both featuring the identical ligand
arrangement, reveal very similar chemical shifts, despite
compound 5 being a trichlorosilane and compound 6 a
dichlorohydridosilane. The most shielded in this series is silane
7, bearing the guanidinato ligand backbone, which exhibits a
chemical shift at δ = −102.3 ppm, ca. 10 ppm upfield shifted
from its benzamidinato analogue, compound 6, clearly due to
the increased electron-donating capacity of Me2N (in 7) vs Ph
(in 6).
The asymmetry of the bidentate benzamidinato or
guanidinato ligands should conceivably give rise to two
stereoisomers in the case of the silanes 6−8 (Figure 1). The
1H NMR spectra of all four silanes exhibit sharp lines and
reflect a symmetric pattern, consistent with only one
the corresponding SiIV compounds using them as initial
building blocks to construct the ligand backbone. By employing
a similar reaction strategy to that of Tacke and Roesky,12,13 it
was indeed possible to isolate several novel SiIV compounds23 in
a similar fashion (Scheme 4).
Reacting carbodiimine 3 or 4 with LiNMe2 or LiPh in an
initial step and subsequently with either tetrachlorosilane or
trichlorosilane afforded the desired asymmetric silanes (5−8) in
moderate to high yields (5, 84%; 6, 89%; 7, 83%; 8, 63%) as
colorless solids. This synthetic procedure seems to work in a
rather general fashion, as evidenced by the facile isolation of
these silanes, which represent only the second set of isolable
asymmetric silanes stabilized by benzamidinato or guanidinato
ligands.14 The silanes 5−8 are thermally robust, and all exhibit
melting points without decomposition (5, 175−178 °C; 6,
103−106 °C; 7, 123−128 °C; 8, 115−116 °C).
Scheme 4. Synthesis of Asymmetric Benzamidinato- Or Guanidinato-Stabilized Silanes from Asymmetric Carbodiimines 3 and
a
4
a
Reactions were carried out in diethyl ether under the following conditions: (a) −78 °C → rt, 3 h; (b) rt, 12 h; (c) −78 °C → rt, 12 h.
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chemical shift position. Also in the 13C NMR spectrum, a highly
symmetric pattern is observed for all the silanes.
IR spectroscopy proved to be a very useful tool in the
characterization of the silanes 6 and 7 in the solid state. In the
case of compound 6, comparison of its IR spectrum with
compound 5 showed a virtually identical spectrum with the
exception of two very strong absorption bands for compound 6
at ν = 2214 and 2193 cm−1, assignable to two ν(Si−H)
stretching vibrations (Figure 2). The existence of two bands is
expected if two stereoisomers are present. The stretching
vibration at ν = 2214 cm−1 is of lower intensity than that at
2193 cm−1. In close analogy with silane 7, two strong stretching
vibrations are also observed at ν = 2221 and 2141 cm−1,
respectively, again reflecting the coexistence of the two
stereoisomers in the solid state. In compound 7 the band at
ν = 2221 cm−1 is of higher intensity.
Figure 1. Stereoisomerism in the case of the silanes 6−8 derived from
t
the asymmetry of the ligands. R1, R2 = Dipp, Bu, Ad; R3 = Ph or
NMe2. (Ideal trigonal-bipyramidal geometry is shown for clarity.)
component present on the NMR time scale; this includes the
silanes 6−8, where stereoisomerism is expected. This
observation indicates that either there is rapid interconversion
between the two possible stereoisomeric forms for the silanes
6−8 on the time scale of the measurement, and the time-
averaged symmetric spectrum is observed, or one stereoisomer
is energetically preferred and the spectra reflect its presence
alone. The former scenario is more likely, since IR and single-
crystal X-ray diffraction data (vide infra) indeed provide
evidence for the coexistence of two stereoisomeric forms in
the solid state. The interconversion between the stereosiomers
in the case of compounds 6−8 most likely follows Berry
pseudorotation,24 typical for five-coordinate systems, which is
often very rapid on the NMR time scale.25 In fact, variable-
temperature NMR experiments (in THF-d8) were conducted
for compounds 6 and 7, but even at −80 °C, in both cases, this
exchange process could not be frozen out, and the spectra in
both compounds were analogous to those at room temperature.
Particularly diagnostic in silanes 6−8 is the presence of a Si-
Additionally, all four silanes were the subject of single-crystal
X-ray diffraction analyses, and crystals suitable for this purpose
were obtained from concentrated n-hexane (5−7) or toluene
(8) solutions upon slow cooling to 5 or −30 °C. Figure 3
1
H signal in the H NMR spectrum, which in all three cases
shows up as a sharp singlet resonance signal at δ = 6, 6.60; 7,
6.56; 8, 6.87 ppm. In all cases the presence of low-intensity 29Si
1
satellites derived from J(29Si,H) is detectable, with coupling
constants ranging from 332 Hz (8) to 353 Hz (6) (Table 1).
The most characteristic resonance signal in the 13C{1H} NMR
spectrum of compounds featuring benzamidinato or guanidi-
nato ligands is that of the NCN in the backbone of the four-
membered chelate ring. This signal is clearly visible in all of the
silanes 5−8 (Table 1) and exhibits a remarkably similar
Figure 3. ORTEP representation of the molecular structure of silane 5
in the solid state at the 50% probability level. H atoms are omitted for
clarity. Cl atoms are shaded green.
Figure 2. Comparative IR spectra of silanes 5 and 6 clearly showing two Si−H stretching vibrations for the latter in the range 1450−3400 cm−1.
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shows the solid-state structure of silane 5 (the structure of 7 is
presented in the SI). A comparison among silanes 5−7 is
presented in Table 2 since they have the same N-substitution
pattern, making their structural comparison possible.
reported by Tacke and co-workers. In the case of silanes 6
and 7, the H atom residing on the SiIV centers could be located.
In silanes 5−7 each SiIV center is five coordinate and exhibits a
distorted trigonal-bipyramidal geometry. The source of this
distortion is the ring strain associated with the benzamidinato
or guanidinato ligand system featuring an acute bite angle of
approximately 70° in all cases. The degree of distortion from
ideal trigonal-bipyramidal geometry can be computed by the τ
value26 and reveals that silane 7 deviates most notably from
idealized trigonal-bipyramidal geometry: 5, τ = 0.75; 6, τ =
0.76; τ = 7, 0.65. In the structure solution of silane 6,
fortunately, two molecules were found in the asymmetric unit,
which correspond to two stereoisomeric forms of compound 6
(Figure 4). This provides additional evidence for their existence
and also provides some rationale for the IR spectrum (vide
supra) where two Si−H stretching vibrations were observed,
corresponding to these two stereoisomers. Their mutual
presence in the asymmetric unit suggests they are energetically
very close on the potential energy hypersurface. In the case of
the silane 7, only one molecule is found in the asymmetric unit,
representing one of the possible stereoisomers, which might
suggest there is a greater energy difference between the two
stereoisomers in compound 7 and that only one preferentially
crystallized.
The silane 8, bearing the 1-adamantyl residue, is structurally
similar to the aforementioned silanes 5−7, and its structure is
presented in Figure 5 along with some key metric parameters.
The silicon-bound hydride could also be located in the Fourier
difference map and refined. Only one of the possible
stereoisomers is found in the asymmetric unit, akin to
compound 7. The computed τ value is comparable to those
of compounds 5 and 6 (τ = 0.73), suggesting that no major
geometric distortions around the silicon center arise from the
dramatic increase in steric bulk of the 1-adamantyl residue.
The availability of the silanes 5−8 initially prompted us to
explore their ability to undergo reduction/dehydrohalogenation
reactions to access the corresponding asymmetric mononuclear
NHSis. Therefore, we employed a range of reducing agents for
5 (K mirror, KC8, Na-naphthalenide) and dehydrohalogenation
Table 2. Summary of Selected Bond Lengths (Å) and Angles
[deg] of Silanes 5−7
b
metric parameter
5/Å or deg
6/Å or deg
7/Å or deg
Bond Lengths
N1−C1
N2−C1
N1−Si1
N2−Si1
Cl1−Si1
Cl2−Si1
Cl3−Si1
C1−Si1
1.370(2)
1.301(2)
1.771(1)
1.988(1)
2.1314(6)
2.0714(6)
2.0540(6)
2.345(2)
1.370(2)
1.297(2)
1.781(2)
2.018(2)
2.1427(7)
2.0636(7)
1.374(2)
1.342(2)
1.788(1)
1.921(1)
2.1922(5)
2.0922(5)
2.369(2)
2.335(1)
Bond Angles
N1−Si1−N2
C1−N1−Si1
69.2(6)
68.20(7)
96.7(1)
71.03(5)
94.24(8)
89.57(8)
105.2(1)
130.3(1)
124.6(1)
164.52(4)
125.30(4)
93.19(4)
92.73(2)
93.93(4)
a
95.7(1)
C1−N2−Si1
N1−C1−N2
N2−C1−C2
N1−C1−C2
N2−Si1−Cl2
N1−Si1−Cl1
N2−Si1−Cl1
Cl2−Si1−Cl1
Cl2−Si1−N1
88.4(1)
88.5(1)
106.6(1)
130.5(1)
122.9(1)
165.00(5)
120.92(5)
93.67(4)
95.08(3)
96.05(5)
106.6(2)
129.8(2)
123.7(2)
164.44(5)
118.68(6)
93.02(5)
96.68(3)
96.41(5)
a
C1 refers to the NCN atom in the backbone of the ligand in all cases.
Two molecules (stereoisomers) were found in the asymmetric unit,
b
and the metric parameters reported here reflect one of these.
The related silanes 5, 6, and 7 exhibit very similar metric
parameters, in terms of both bond lengths and angles, which are
comparable to each other within narrow margins (Table 2).
These parameters are also akin to those of the related but
symmetric silane L′SiHCl2 (L′ = (NMe2)C(NDipp)2)13
Figure 4. ORTEP representation of the molecular structure of one of the independent molecules (stereoisomers) in silane 6 in the solid state at the
50% probability level (left). All H atoms are omitted for clarity, except H1. ORTEP representation of the two stereoisomers of compound 6 in the
i
asymmetric unit (right). Thermal ellipsoids are set at 50% probability. All H atoms are omitted for clarity, except H1 and H2. Pr groups of Dipp
substituents and the Ph group in the backbone are omitted for clarity (right).
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Scheme 5. Direct Synthesis of the Asymmetric Hydrido
SiII:→Fe Complexes 9−11 from the Respective
Dichlorohydridosilanes, upon Reaction with K2[Fe(CO)4]
Figure 5. ORTEP representation of the molecular structure of silane 8
in the solid state. Thermal ellipsoids are set at the 50% probability
level. All H atoms, with the exception of H1, are omitted for clarity.
Selected bond lengths [Å]: Si1−N2 1.798(3), Si1−N1 1.941(3), Si1−
Cl1 2.084(1), Si1−Cl2 2.172(1), Si1−C1 2.325(3), N1−C1 1.312(5).
Selected bond angles [deg]: N2−Si1−N1 70.0(1), N2−Si1−Cl1
117.6(1), N1−Si1−Cl1 94.9(1), N2−Si1−Cl2 100.5(1), N1−Si1−Cl2
169.0(1), Cl1−Si1−Cl2 94.49(5), N2−Si1- C1 36.0(1), N1−Si1−C1
34.3(1), Cl1−Si1−C1 112.59(9), Cl2−Si1−C1 135.5(1), C1−N1−
C12 130.1(3), C1−N1−Si1 89.1(2).
Table 3. Summary of Some Key Chemical Shifts (in ppm,
spectra recorded in C6D6 at 298 K) Observed in Complexes
9−11
complex
29Si{1H}
1H NMR shift of Si−H
1J (29Si,H)/Hz
9
99.6
83.6
86.5
7.05
7.04
6.91
200
205
204
10
11
n
t
reagents (LiHMDS, NaHMDS, even BuLi, BuLi, etc.) for
silanes 6−8. Unfortunately, all experiments did not afford the
desired NHSi.
We then considered an alternative possibility: that the silanes
could, when reacted with a suitable metalate as reducing agent,
form the targeted NHSi metal complexes, directly by a salt
metathesis route. For this purpose we chose K2[Fe(CO)4]
given its ease of access and availability to probe this synthetic
idea.
Initially we reacted silane 5 with K2[Fe(CO)4], hoping to
form the corresponding NHSi complex [{PhC(NtBu)-
(NDipp)}(Cl)Si:→Fe(CO)4]. However, this reaction did not
proceed to any extent, even under reflux in THF after 72 h.
Strikingly, when the silanes 6−8 were reacted with K2[Fe-
(CO)4], the envisaged reaction did occur, and the correspond-
ing hydrido-NHSi iron(0) complexes [{R1C(NtBu)(NR2)}-
(H)Si:→Fe(CO)4] (9, R1 = Ph, R2 = Dipp; 10, R1 = NMe2, R2
= Dipp; 11, R1 = Ph, R2 = 1-adamantyl) could be isolated upon
workup (Scheme 5). This represents a facile new route to
access hydrido NHSi complexes, directly from the easily
accessible dichlorohydridosilanes, without the need to isolate
the “free” NHSi ligand.27
SiII group VIII metal complexes.28 Moreover, in all three
compounds, in the 1H NMR spectra, the silicon-bound hydride
atom is clearly detectable, flanked by 29Si satellites (Table 3).
In the case of complexes 9 and 10, IR spectroscopy in the
solid state was also recorded, and in both cases three sharp
bands are visible in the carbonyl region, in accordance with the
expectations for the iron centers exhibiting local C3v symmetry
(9, ν = 2013, 1984, 1887 cm−1; 10, ν = 2022, 1944, 1909
cm−1). This implies that the hydrido-NHsilylene ligands
necessarily occupy the apical position in the trigonal-
bipyramidal coordination sphere of the iron center in both
complexes, as equatorial coordination would afford a local C2v
symmetry at the iron center and four bands in the CO region
would be observed.29 In contrast to the silanes 6 and 7, only
one stretching vibration for the Si−H bond is observed in the
infrared spectra of the corresponding Fe0 complexes 9 and 10,
at ν = 2059 cm−1 (9) and ν = 2100 cm−1 (10), respectively.
Compared to the SiIV precursors, these stretching vibrations are
shifted to slightly lower wavenumbers in both cases, as would
be expected for a decrease in the oxidation state at Si.30 The
presence of only one stretching vibration, in both cases,
suggests a preference for formation of only one stereoisomer.
Efforts to obtain crystals suitable for single-crystal X-ray
diffraction analysis of complexes 9 and 11 remained
unsuccessful. In the case of complex 11, likely as a result of
the adamantyl residue, only amorphous wax-like blocks
resulted, despite attempting numerous solvents and crystal-
lization procedures. Fortunately, crystals suitable for X-ray
diffraction analysis were obtained in the case of complex 10,
from dilute hexane solutions with slow cooling to 5 °C. The
solid-state structure of complex 10 is depicted in Figure 6.
The hydrido SiII:→Fe0 complexes 9−11 were fully
characterized spectroscopically. In all three cases the [M +
H]+ signal could clearly be detected in the high-resolution ESI-
MS spectrum, with a fitting isotope distribution pattern
confirming their constitution. The 29Si{1H} NMR spectra of
all three complexes (in C6D6) reveal one sharp singlet,
downfield shifted from the corresponding silane precursors
6−8 (Table 3). This provides further evidence of the
coordination of the hydrido-NHSi to the iron center and
additionally is indicative that the oxidation state of the silicon
center is +2, since this is the typical spectral region for related
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observed after 24 h at 70 °C heating. The inactivity of the
catalysts contrasts with our previously reported electron-rich
hydrido-NHSi Fe(0) system11 and is likely due to the presence
of the electron-withdrawing CO ligands, which impedes the
reactivity of the system. In order to increase the electron
density on the iron atom, we are currently exploring the
possibility of inserting chelating electron-rich alkyl phosphanes
(for example dmpe) by CO elimination using complexes 9−11
as starting materials. This should yield access to heteroleptic
systems of the type [[R1C{(NtBu)(NR2)}](H)Si:→Fe(CO)n-
(η2-dmpe)x] (x = 1, n = 2 or x = 2, n = 0),32 which, due to an
increase in electron density at the iron center, might then act as
suitable catalysts for hydrosilylation reactions.
SUMMARY AND OUTLOOK
■
An improved route to asymmetric carbodiimines has been
developed, and these precursor molecules were employed for
the synthesis of corresponding asymmetric trichloro- and
dichlorohydridosilanes in high yields. Due to the asymmetry of
the carbodiimine, the resulting benzamidinato or guanidinato
ligands induce stereoisomer formation in the corresponding
silanes. Starting from the silanes, a new direct route to hydrido-
NHSi iron(0) complexes could be achieved upon salt
metathesis with K2[Fe(CO)4]. This represents a facile and
general new route to hitherto unprecedented asymmetric
hydrido-NHSi metal complexes, without the need to first
isolate any “free” NHSi ligand. Further experiments are devoted
to enabling entry to related iron(0) complexes capable of
asymmetric catalytic transformations (e.g., hydrosilylation).
Figure 6. ORTEP representation of the molecular structure of
complex 10 in the solid state. Thermal ellipsoids are set at the 50%
probability level. All H atoms, with the exception of H3, are omitted
for clarity. Selected bond lengths [Å]: Fe1−C20 1.779(6), Fe1−C21
1.780(6), Fe1−C22 1.787(5), Fe1−C23 1.770(5), O1−C22 1.139(7),
O2−C21 1.134(8), O3−C20 1.151(8), O4−C23 1.144(6), Fe1−Si1
2.234(1), Si1−N2 1.811(4), Si1−N1 1.829(4), Si1−C1 2.284(4).
Selected bond angles [deg]: N2−Si1−N1 72.7(2), N2−Si1−Fe1
124.1(1), N1−Si1−Fe1 126.0(1).
Complex 10 features an ideal trigonal-bipyramidal Fe0 site
and a distorted tetrahedral SiII center. The hydrido-NHSi ligand
occupies one of the apical positions of the coordination sphere
of the iron atom, and the other sites are occupied by CO
ligands, in accordance with the solid-state IR data (vide supra).
The apical location of the NHSi ligand is akin to the complex
[Fe(CO)4←:Si(OtBu)L] (L = PhC(NtBu)2) reported by
Roesky and co-workers,31 but contrasts our recently reported
hydrido-NHSi iron complex [LHSi:→Fe(dmpe)2] (L =
PhC(NtBu)2; dmpe = 1,2-bis(dimethylphosphino)ethane),11
in which the ligand is equatorially coordinated to iron. The
explanation for this is that, in the latter complex, the presence
of electron-rich alkyl phosphane ligands enhances the electron
density at the Fe center and thereby increases its propensity for
π-back-donation to the SiII center. Moreover, as was shown by
us previously, since the hydrido-NHSi ligand has the ability to
behave as a π-acidic ligand, under these conditions, it
preferentially adopts equatorial coordination. In complex 10,
the presence of π-acidic CO groups on the iron center
substantially diminishes its π-density, thereby weakening its
ability to form a π-back-donation and most likely accounts for
the apical coordination of the ligand. The Fe1−Si1 bond length
in complex 10 is 2.234(1) Å, which is comparable in magnitude
to that observed in [Fe(CO)4←:Si(OtBu)L] (2.237(7) Å), but
substantially longer than observed in [LHSi:→Fe(dmpe)2]:
2.184(2) Å. This is again due to the enhanced π-back-donation
facilitated by the electron-rich phosphane ligands in the latter
complex, increasing the electron density on the Fe(0) site.
Asymmetric iron complexes 9−11 were also screened for
catalytic hydrosilylation of pro-chiral ketones, in the hope of
observing enantiomeric excesses in the products. For all three
complexes, even at a high (10 mol %) catalyst loading,
employing Si(OEt)3H as silane, no catalytic activity was
EXPERIMENTAL SECTION
■
General Considerations. All experiments and manipulations were
carried out under a dry and oxygen-free atmosphere of nitrogen or
argon, using standard Schlenk techniques, or in an MBraun inert
atmosphere drybox containing an atmosphere of purified nitrogen.
Solvents were dried by standard methods and stored over activated
molecular sieves (4 Å). Prior to use, all solvents were degassed at least
once via a freeze−pump−thaw cycle. C6D6 was dried by stirring it over
KC8 for 48 h and vacuum-transfer over activated molecular sieves (4
Å), where it was stored for use. The NMR spectra were recorded on a
Bruker AV 200 or 400 spectrometer. Concentrated solutions of
samples in C6D6 were sealed off in a Young-type NMR tube for
1
measurements. The H and 13C{1H} were referenced to the residual
solvent signals as internal standards (δH = 7.15; δC = 128.00 ppm for
C6D6). The 29Si{1H} NMR spectra were referenced to TMS
(tetramethylsilane) as an external standard. Abbreviations: s = singlet;
m = multiplet; br = broad, quint = quintet. Unambiguous signal
assignments were made by employing a combination of DEPT-45 and
2D NMR H,C-COSY (HMQC/HMBC) experiments. Diastereotopic
i
groups on the Pr substituents are indicated with superscripts A, B, C,
or D. High-resolution mass spectra (ESI or APCI) were recorded on a
Thermo Fischer Scientific LTQ Orbitrap XL spectrometer. Electron
impact (EI) spectra were recorded on a Finnigan MAT 95S Sectorfield
spectrometer. The line of highest intensity in the isotope pattern is
reported. For the single-crystal X-ray structure determinations, crystals
were mounted on a glass capillary in perfluorinated oil and measured
in a cold N2 flow. The data for all compounds were collected on an
Oxford Diffraction Supernova, single source at offset, Atlas at 150 K
(Cu Kα radiation, λ = 1.5418 Å). The structures were solved by direct
methods and refined on F2 with the SHELX-97 software package. The
positions of the H atoms were calculated and considered isotropically
according to a riding model. K2[Fe(CO)4] was synthesized according
to a literature procedure,33 (Dipp)N(H)C(S)N(H)(tBu) (1)21 and
(1-Adamantyl)N(H)C(O)N(H)(tBu) (2)22 were prepared using
different routes, and for compound 2 the NMR data are included here.
For compounds 3 and 4 the NMR data are compared with literature.19
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Article
All other starting materials were obtained from commercial sources
and used as received, unless otherwise stated.
°C, ppm): δ 23.5 (s, 4 C, 2 × CH(CH3)2), 29.1 (s, 2 C, 2 ×
CH(CH3)2), 31.5 (s, 3 C, 1 × C(CH3)3), 55.8 (s, 1 C, 1 × C(CH3)3),
123.6 (s, 2 C, 2 × C3,5, Dipp), 125.0 (s, 1 C, 1 × C4, Dipp), 131.3 (s, 1
C, 1 × NCN), 135.1 (s, 1 C, 1 × C1, Dipp), 142.5 (s, 2 C, 1 ×
C2,6, Dipp).
N-(2,6-Diisopropylphenyl)-N′-(tert-butyl)thiourea (1). In a
250 mL Schlenk flask, 100 mL of distilled water was added.
Thiophosgene (7.50 g, 65.2 mmol) was added to the water, and
then diisopropylphenylamine (9.64 g, 54.4 mmol) over a period of 30
min. A two-phase reaction follows, after which the organic phase was
extracted and added to another 250 mL Schlenk flask together with
tert-butylamine (19.9 g, 272.0 mmol), and the mixture was stirred at
room temperature for 12 h. The reaction solution was then added to
200 mL of a dilute HCl solution, after which a precipitate was
separated from a supernatant solution. The precipitate was washed
with 5 mL of hexane, and the hexane washing discarded. The resulting
residue was taken up in diethyl ether, dried over magnesium sulfate,
and filtered. The clear filtrate was concentrated in vacuo until
incipience and cooled to 5 °C overnight, affording colorless crystals of
the desired product, which were separated from the supernatant
solution and dried in vacuo. Yield: 9.39 g, 32.1 mmol (59%). Mp: 131−
N-Adamantyl-N′-tert-butylcarbodiimine (4). In a 250 mL
Schlenk flask, compound 2 (1.90 g, 7.60 mmol) and LiHMDS (2.50
g, 15.10 mmol) were added. The two solids were mixed thoroughly
and cooled to −90 °C. Then 100 mL of precooled (−90 °C) THF was
added rapidly via cannula to the solid mixture. The reaction was
allowed to warm to room temperature over a period of several hours,
during which a white precipitate formed. The supernatant solution was
subsequently removed in vacuo, n-hexane (150 mL) was added to the
residue, and the solution was stirred at room temperature for 0.5 h.
The resulting suspension was filtered via a sintered glass frit, and the
solvent from the filtrate was removed in vacuo. This afforded a wax-like
1
solid in quantitative yields. H NMR (200.13 MHz, C6D6, 25 °C,
ppm): δ 1.22 (s, 9 H, 1 × C(CH3)3), 1.46 (m, 6 H, 3 × CH2, 1-
adamantyl), 1.86 (m, 9 H, 3 × CH2 + 3 × CH, 1-adamantyl). 13C{1H}
NMR (50.3 MHz, C6D6, 25 °C, ppm): δ 29.9 (s, 3 C, 3 × CH,
adamantyl), 31.2 (s, 3 C, 3 × CH3), 35.9 (s, 3 C, 3 × CH2, 1-
adamantyl), 45.0 (s, 3 C, 3 × CH2, 1-adamantyl), 54.5 (s, 1 C, 1 × C1,
1-adamantyl), 55.1 (s, 1 C, 1 × C(CH3)3), 138.9 (s, 1 C, 1 × NC
N). 13C shifts from literature19f in CDCl3: δ 29.9, 31.3, 36.1, 44.9, 54.9,
55.0, 139.0.
132 °C; lit.21 128−129 °C. H NMR (200.13 MHz, CDCl3, 25 °C,
1
ppm): δ 1.17 (d, 3JH−H = 7.0 Hz, 6 H, 2 × CHCHA ), 1.21 (d, 3JH−H
=
3
7.0 Hz, 6 H, 2 × CHCHB ), 1.41 (s, 9 H, 1 × C(CH3)3), 3.15 (sept,
3
3JH−H = 6.9 Hz, 2 H, 2 × CH(CH3)3), 5.19 (br s, 2 H, 2 × N-H), 7.20
4
(d, JH−H = 1.1 Hz, 1 H, C3 or 5-H, Dipp), 7.24 (br s, 1H, C3 or 5-H,
3
3
Dipp), 7.37 (dd, JH−H = 6.8 Hz, JH−H = 8.6 Hz, 1H, C4-H, Dipp).
Lit.21 (300 MHz, CDCl3, ppm): δ 1.22 and 1.26 (CHMe2), 1.42
(CMe3), 3.15 (CHMe2), 7.23 (Ar H-3 and -5), 7.37 (Ar H-4).
13C{1H} NMR (50.32 MHz, CDCl3, 25 °C, ppm): δ 23.5 (s, 2 C, 2 ×
CHCAH3), 24.3 (s, 2 C, 2 × CHCBH3), 28.5 (s, 2 C, 2 × CH(CH3)2),
28.9 (s, 3 C, 1 × C(CH3)3), 53.7 (s, 1 C, 1 × −C(CH3)3), 124.5 (s, 2
C, 1 × C3,5, Dipp), 130.0 (s, 1 C, 1 × C4, Dipp), 130.0 (s, 1 C, 1 × C1,
Dipp), 148.0 (s, 2 C, 1 × C2,6, Dipp), 180.0 (s, 1 C, 1 × −NH-C(S)-
NH). ESI-MS, m/z: calcd for C17H28N2S [M + H]+ 293.2041; found
293.2046. Anal. Calcd for C17H28N2S: C 69.81, H 9.65, N 9.58, S
10.96. Found: C 69.89, H 9.93, N 9.45, S 10.75.
(N-2,6-Diisopropylphenyl-N′-tert-butyl)benzamidinato-
trichlorosilane (5). A Schlenk tube was charged with 3 (0.664 g,
2.570 mmol), and 20 mL of diethyl ether was added to the solid. The
solution was cooled to −78 °C, and 1.45 mL of a 1.8 M solution of
phenyllithium (0.220 g, 2.621 mmol) in dibutyl ether added dropwise.
The yellow-colored reaction solution was stirred at −78 °C for 1 h,
slowly warmed to room temperature, and then stirred for a further 3 h.
Tetrachlorosilane (300 μL, 2.621 mmol) was added to the resulting
solution rapidly via syringe upon which the formation of a white
precipitate is observed. The reaction was stirred at room temperature
for 12 h, and the resulting orange suspension filtered via cannula. The
filtrate was concentrated in vacuo until the incipient point of
crystallization and cooled to 5 °C overnight, affording colorless
crystals of 5. Yield: 1.208 g, 2.159 mmol (84%). Mp: 175−178 °C. 1H
N-Adamantyl-N′-tert-butylthiourea (2). A Schlenk tube was
charged with 100 mL of hexane and 1-adamantylisothiocyanate (3.0 g,
15.5 mmol). Upon stirring at room temperature, 6 mL (54.4 mmol) of
tert-butyl amine was added, and the reaction solution stirred at room
temperature for 72 h. During the course of the reaction, a white
precipitate formed. This precipitate was separated from the super-
natant by cannula filtration, washed with a small amount of hexane,
and dried in vacuo at room temperature for 1 h. Yield: 3.3 g, 13.3
NMR (200.13 MHz, C6D6, 25 °C, ppm): δ 1.13 (d, 3JH−H = 6.9 Hz, 6
3
H, 2 × CHCHA ), 1.24 (s, 9 H, 1 × C(CH3)3), 1.51 (d, JH−H = 6.7
3
Hz, 6 H, 2 × CHCHB ), 3.42 (sept, JH−H = 6.8 Hz, 2 H, 2 ×
3
3
CH(CH3)2), 6.66−6.72 (m, 1 H, 1 × C4-H, Ph), 6.72−6.78 (m, 2 H, 1
× C3,5-H, Ph), 6.81 (d, 4JH−H = 1.5 Hz, 1 H, 1 × C3 or 5-H, Dipp), 6.83
(br s, 1 H, 1 × C3 or 5-H, Dipp), 6.89 (dd, 3JH−H = 6.5 Hz, 3JH−H = 7.7
Hz, 1 H, 1 × C4-H, Dipp), 7.07−7.11 (m, 2 H, 1 × C2,6-H, Ph).
13C{1H} NMR (50.3 MHz, C6D6, 25 °C, ppm): δ 23.8 (s, 2 C, 2 ×
1
mmol (86%). (NMR data not accessible in the literature.) H NMR
(200.13 MHz, CDCl3, 25 °C, ppm): δ 1.47 (s, 9 H, 1 × C(CH3)3),
1.68 (br m, 12 H, 6 × CH2, 1-adamantyl), 2.12 (m, 3 H, 3 × CH, 1-
adamantyl), 8.28 (s, 2 H, 2 × NH). 13C{1H} NMR (50.32 MHz, C6D6,
25 °C, ppm): δ 29.2 (s, 6 C, 6 × CH2, 1-adamantyl), 35.6 (s, 3 C, 1 ×
C(CH3)3), 43.8 (s, 6 C, 3 × CH, 1-adamantyl), 58.5 (s, 1 C, 1 ×
C(CH3)3), 115.0 (br s, 1 C, 1 × C1, 1-adamantyl), 130.0 (s, 1 C, 1 ×
CS).
CHCAH3), 25.7 (s, 2 C, 2 × CHCBH3), 29.2 (s, 2 C, 2 ×
−CH(CH3)2), 31.0 (s, 3 C, 1 × C(CH3)3), 55.8 (s, 1 C, 1 ×
C(CH3)3), 123.9 (s, 2 C, 1 × C3,5, Dipp), 127.1 (s, 2 C, 1 × C2,6, Ph),
127.8 (s, 2 C, 1 × C3,5, Ph (peak found by HMQC spectrum, hidden
under solvent signal), 128.6 (s, 1 C, 1 × C1, Ph), 129.0 (s, 1 C, 1 × C4,
Dipp), 130.7 (s, 1 C, 1 × C4, Ph), 131.8 (s, 1 C, 1 × C1, Dipp), 148.1
(s, 2 C, 1 × C2,6, Dipp), 171.2 (s, 1 C, NC(Ph)N). 29Si{1H} NMR
(50.3 MHz, C6D6, 25 °C, ppm): −93.8 (s, 1 Si, LSiCl3). EI-MS, m/z:
calcd for C23H31N2Cl3Si [M]+ 468 (100%); found 468. Anal. Calcd for
C23H31N2Cl3Si: C 58.78, H 6.65, N 5.96. Found: C 58.94, H 6.80, N
6.00.
(N-2,6-Diisopropylphenyl-N′-tert-butyl)benzamidinato-
dichlorosilane (6). A Schlenk tube was charged with 3 (0.961 g,
3.719 mmol), and 30 mL of diethyl ether was added to the solid. The
solution was cooled to −78 °C, and 2.10 mL of a 1.8 M solution of
phenyllithium (0.319 g, 3.793 mmol) in dibutyl ether was added
dropwise. The yellow-colored reaction solution was stirred at −78 °C
for 1 h, slowly warmed to room temperature, and then stirred for a
further 3 h. Trichlorosilane (400 μL, 3.793 mmol) was added to the
resulting solution rapidly via syringe, upon which the formation of a
white precipitate was observed. The reaction was then stirred at room
temperature for 12 h, and the resulting orange suspension filtered via
cannula. The filtrate was concentrated in vacuo until the incipient point
N-tert-Butyl-N′-2,6-diisopropylphenylcarbodiimine (3). In a
250 mL two-necked round-bottom flask, LiHMDS (3.962 g, 16.411
mmol) was added, along with 100 mL of THF, and the resulting
solution cooled to −78 °C. Dropwise, a solution of 1 (2.400 g, 8.205
mmol) in THF was added to the LiHMDS solution at −78 °C via
dropping funnel. The resulting reaction solution was slowly warmed to
room temperature over a period of 1 h and then refluxed for 2 h.
During the course of reflux, the formation of a white precipitate was
observed. The reaction was then allowed to cool to room temperature,
and the volatile components were removed in vacuo. The residue was
extracted with n-hexane and filtered via cannula, and the solvent from
the filtrate was removed in vacuo. This afforded a colorless oil as
product, which on the basis of NMR is completely pure, and no
further distillation or purification step was required. Yield: 1.53 g, 5.91
mmol (72%). 1H NMR (200.13 MHz, C6D6, 25 °C, ppm): δ 1.18 (s, 9
3
H, 1 × C(CH3)3), 1.23 (d, JH−H = 6.9 Hz, 12 H, 2 × CH(CH3)2),
3.63 (sept, 3JH−H = 6.9 Hz, 2 H, 2 × CH(CH3)2), 7.06 (s, 3 H, 3 × C-
H, Dipp). Literature (300 MHz, C6D6, ppm): 1.18 (s, 9 H, 1 ×
C(CH3)3), 1.24 (d, 12 H, 2 × CH(CH3)2), 3.64 (sept, 2 H, CHMe2),
7.07 (m, 3 H, m- and p-C6H3). 13C{1H} NMR (50.32 MHz, C6D6, 25
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Organometallics
Article
of crystallization and cooled to 5 °C overnight, affording colorless
crystals of 6. Yield: 1.446 g, 3.310 mmol (89%). Mp: 103−106 °C. 1H
NMR (200.13 MHz, C6D6, 25 °C, ppm): δ 1.12 (s, 9 H, 1 ×
MHz, C6D6, 25 °C, ppm): δ 1.15 (s, 9 H, 1 × C(CH3)3), 1.33 (m, 6 H,
3 × CH2, 1-adamantyl), 1.73 (m, 3 H, 3 × CH, 1-adamantyl), 1.90 (m,
6 H, 3 × CH2, 1-adamantyl), 6.80−6.92 (m, 6 H, 1JSi−H = 332 Hz 1 ×
SiH + 5 × Ar-H). 13C{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ
29.9 (s, 3 C, 3 × CH2, 1-adamantyl), 31.5 (s, 6 C, 6 × CH3), 36.1 (s, 3
C, 3 × CH2, 1-adamantyl), 43.5 (s, 3 C, 3 × CH, 1-adamantyl), 55.5 (s,
1 C, 1 × C(CH3)3), 57.2 (s, 3 C, C1, 1-adamantyl), 128.0 (signals
located by HMQC spectrum) (Ph-C), 128.2 (s, 1 C, 1 × Ph-C), 130.0
(s, 1 C, 1 × Ph-C), 133.2 (s, 1 C, 1 × C1, Ph) 171.3 (s, 1 C, 1 ×
NCN). 29Si{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ −97.3. ESI-
MS, m/z: calcd for C21H30N2ClSi [M − Cl]+ 373.1861; found
373.1868.
C(CH3)3), 1.12 (d, 3JH−H = 6.9 Hz, 6 H, 2 × CHCHA ), 1.50 (d, 3JH−H
3
3
= 6.8 Hz, 6 H, 2 × CHCHB ), 3.39 (sept, JH−H = 6.8 Hz, 2 H, 2 ×
3
CH(CH3)2), 6.60 (s, 1JSi−H = 353 Hz, 1 H, 1 × Si-H), 6.67−6.77 (m, 3
H, 1 × C3,4,5-H, Ph), 6.84 (d, JH−H = 1.6 Hz, 1 H, 1 × C3 or 5-H,
4
Dipp), 6.86 (br s, 1 H, 1 × C3 or 5-H, Dipp), 6.91 (dd, 3JH−H = 6.6 Hz,
3JH−H = 8.8 Hz, 1 H, 1 × C4-H, Dipp), 7.06−7.10 (m, 2 H, 1× C2,6-H,
Ph). 13C{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ 23.7 (s, 2 C, 2
× CHCAH3), 25.7 (s, 2 C, 2 × CHCBH3), 29.1 (s, 2 C, 2 ×
CH(CH3)2), 30.8 (s, 3 C, 1 × C(CH3)3), 55.1 (s, 1 C, 1 × C(CH3)3),
123.8 (s, 2 C, 1 × C3,5, Dipp), 127.3 (s, 2 C, 1 × C2,6, Ph), 127.7 (s, 1
C, 1 × C4, Ph (peak found by HMQC spectrum, hidden under solvent
signal), 128.7 (s, 1 C, 1 × C4, Dipp), 129.7 (s, 1 C, 1 × C1, Ph), 130.5
(s, 2 C, 1 × C3,5, Ph), 132.2 (s, 1 C, 1 × C1, Dipp), 147.9 (s, 2 C, 1 ×
C2,6, Dipp), 170.5 (s, 1 C, 1 × NC(Ph)N). 29Si{1H} NMR (50.32
MHz, C6D6, 25 °C, ppm): δ −92.8 (s, 1 Si, LSiHCl2). 29Si-INEPT
NMR (200.13 MHz, C6D6, 25 °C, ppm): δ −92.9 (s, 1 Si, LSiHCl2;
1JSi−H = 353 Hz). EI-MS, m/z: calcd for C23H32N2Cl2Si [M]+ 434
(N-(2,6-Diisopropylphenyl-N′-tert-butyl)benzamidinato)-
silylidene-irontetracarbonyl (9). A Schlenk tube was charged with
6 (0.546 g, 1.258 mmol) and K2[Fe(CO)4] (0.311 g, 1.258 mmol),
and the two solids were cooled to −78 °C. Precooled THF (−78 °C,
50 mL) was added under stirring to the solid mixture via cannula. After
completion of the addition of THF, the reaction mixture was slowly
allowed to warm to room temperature and stirred for a further 12 h.
The reaction solution turned light brown, and the formation of a white
precipitate was observed. The volatile components were subsequently
removed in vacuo, the remaining brown residue was extracted with
toluene (50 mL), and the toluene was removed in vacuo (in the
filtrate), affording a brown residue. This brown residue was then
extracted with n-hexane (50 mL), concentrated to 5 mL in vacuo, and
then stored at −30 °C. A light brown precipitate resulted, which was
separated carefully at −30 °C from the supernatant via syringe,
affording a light brown solid as product. Yield: 134 mg, 0.252 mmol
(100%); found 434. Anal. Calcd for C23H32N2Cl2Si: C 63.43, H 7.41,
N 6.43. Found: C 63.13, H 7.66, N 6.28. IR (KBr, [cm−1]): 2214 (s)
(Si−H), 2193 (s) (Si−H).
(N-2,6-Diisopropylphenyl-N′-tert-butyl)guanidinato-
dichlorosilane (7). A Schlenk tube was charged with compound 3
(1.726 g, 6.679 mmol), and 50 mL of THF was added to the solid.
The solution was cooled to −78 °C, and a solution of LiNMe2 (0.443
g, 8.683 mmol) in THF was added dropwise to it under rapid stirring.
The yellow-colored reaction solution was stirred at −78 °C for 1 h,
slowly warmed to room temperature, and then stirred for a further 2 h.
Trichlorosilane (300 μL, 2.621 mmol) was added to the resulting
solution rapidly via syringe (at −78 °C), upon which the formation of
a white precipitate was observed. The reaction was stirred at room
temperature for 12 h, and all volatiles were removed in vacuo. The
resulting residue was extracted with 100 mL of CH2Cl2 and filtered via
cannula, after which the CH2Cl2 was removed in vacuo. The residue
was washed with 5 mL of n-hexane, the washing discarded, and the
resulting solid dried in vacuo for 0.5 h, affording 7 as a white solid.
Yield: 2.221 g, 5.544 mmol (83%). Mp: 123−128 °C. 1H NMR
(200.13 MHz, C6D6, 25 °C, ppm): δ 1.09 (d, 3JH−H = 6.8 Hz, 6 H, 2 ×
(20%). 1H NMR (200.13 MHz, C6D6, 25 °C, ppm): δ 1.09 (d, 3JH−H
=
3
6.9 Hz, 6 H, 2 × CHCHA ), 1.28 (d, JH−H = 6.9 Hz, 6 H, 2 ×
3
3
CHCHB ), 1.46 (s, 9 H,1 × C(CH3)3), 3.29 (sept, JH−H = 6.9 Hz, 2
3
H, 2 × CH(CH3)2), 6.65−6.76 (m, 2 H, 2 × C-H, Ph), 6.80−6.93 (m,
3 H, 3 × C-H, Ph), 7.05 (s, 1JSi−H = 200 Hz, 1 H, 1 × Si-H), 6.94−7.11
(m, 3 H, 3 × C-H, Dipp). 13C{1H} NMR (50.32 MHz, C6D6, 25 °C,
ppm): δ 22.4 (s, 2 C, 2 × CHCAH3), 24.8 (s, 2 C, 2 × CHCBH3), 28.7
(s, 3 C, 1 × C(CH3)3), 28.9 (s, 2 C, 2 × CH(CH3)2), 51.5 (s, 1 C, 1 ×
C(CH3)3), 123.0 (s, 2 C, 1 × C3;5, Dipp), 124.7 (s, 1 C, 1 × C4, Ph),
128.2 (s, 2 C, Ph (peak found by HMQC spectrum, hidden under
solvent signal), 128.9 (s, 2 C, C, Ph), 136.7 (s, 1 C, 1 × C1, Dipp),
138.5 (s, 2 C, 1 × C2,6, Dipp), 146.4 (s, 1 C, 1 × C4, Dipp), 153.3 (s, 1
C, 1 × NC(Ph)N), 213.1 (s, 2 C, 2 × Fe(CO)A ), 214.9 (s, 2 C, 2 ×
CHCHA ), 1.26 (s, 9 H, 1 × C(CH3)3), 1.46 (d, 3JH−H = 6.8 Hz, 6 H, 2
4
3
Fe(CO)B ). 29Si{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ 99.6
× CHCHB ), 2.12 (s, 6 H, 1 × N(CH3)2), 3.34 (sept, 3JH−H = 6.8 Hz,
4
3
2 H, 2 × CH(CH3)2), 6.56 (s, 1 H, 1JSi−H = 346 Hz, 1 × Si-H), 6.99−
7.18 (m, 3 H, 1 × C3,4,5-H, Dipp). 13C{1H} NMR (50.32 MHz, C6D6,
25 °C, ppm): δ 23.7 (s, 2 C, 2 × CHCAH3), 25.6 (s, 2 C, 2 ×
CHCBH3), 28.6 (s, 2 C, 2 × CH(CH3)2), 30.5 (s, 3 C, 1 × C(CH3)3),
40.8 (s, 2 C, 1 × N(CH3)2), 54.1 (s, 1 C, 1 × −C(CH3)3), 124.3 (s, 2
C, 1 × C3,5, Dipp), 128.3 (s, 1 C, 1 × C4, Dipp), 134.4 (s, 1 C, 1 × C1,
Dipp), 147.5 (s, 2 C, 1 × C2,6, Dipp), 171.2 (s, 1 C, 1 × NC(Ph)N).
29Si-INEPT NMR (200.13 MHz, C6D6, 25 °C, ppm): δ −102.3 (s, 1
(s, 1 Si, LSi(H)-Fe(CO)4). 29Si-INEPT NMR (200.13 MHz, C6D6, 25
°C, ppm): δ 99.6 (s, 1 Si, LSi(H)-Fe(CO)4; 1JSi−H = 200 Hz). ESI-MS,
m/z: calcd for C27H32N2O4SiFe [M + H]+ 533.1509; found 533.1554.
IR (KBr, [cm−1]): 2059 (m) (Si−H), 2013 (s) (CO), 1984 (s) (CO),
1887 (s) (CO).
(N-(2,6-Diisopropylphenyl-N′-tert-butyl)guanidinato)-
silylidene-irontetracarbonyl (10). A Schlenk tube was charged with
7 (0.287 g, 0.711 mmol) and K2[Fe(CO)4] (0.175 g, 0.712 mmol),
and the two solids cooled to −78 °C. Precooled THF (−78 °C, 50
mL) was added under stirring to the solid mixture via cannula. After
completion of the addition of the THF, the reaction mixture was
slowly allowed to come to room temperature and stirred for a further
12 h. The reaction solution turned light brown, and the formation of a
white precipitate was observed. The volatile components were
subsequently removed in vacuo, the remaining brown residue was
extracted with toluene (50 mL), and the toluene was removed in vacuo
(in the filtrate), affording a brown residue. The brown residue was
carefully washed with n-hexane (20 mL) at 0 °C, and the n-hexane
washings were discarded. The remaining light brown residue was dried
in vacuo for 0.5 h, affording the product. Yield: 88 mg, 0.178 mmol
(25%). Mp: 60 °C (dec). 1H NMR (200.13 MHz, C6D6, 25 °C ppm):
δ 0.98 (d, 3JH−H = 6.8 Hz, 3 H, 1 × C6CHCHD3), 1.11 (d, 3JH−H = 6.9
1
Si, LSiHCl2; JSi−H = 346 Hz). EI-MS, m/z: calcd for C19H33N3Cl2Si
[M]+ 401 (100%); found 401. Anal. Calcd for C19H33N3Cl2Si: C
56.70, H 8.26, N 10.44. Found: C 56.81, H 8.70, N 10.46. IR (KBr,
[cm−1]): 2221 (s) (Si−H), 2141 (s) (Si−H).
(N-Adamantyl-N′-tert-butyl)benzamidinatodichlorosilane
(8). A Schlenk tube was charged with carbodiimine 4 (1.00 g, 4.40
mmol), and 100 mL of diethyl ether was added to the solid. The
solution was cooled to −78 °C, and 2.4 mL of a 1.8 M solution of
phenyllithium in dibutyl ether was added dropwise. The light orange
colored reaction solution was stirred at −78 °C for 1 h, slowly warmed
to room temperature, and then stirred for a further 2.5 h. The reaction
solution was then recooled to −50 °C, and trichlorosilane (0.44 mL,
4.40 mmol) was added to the resulting solution rapidly via syringe,
upon which the formation of a white precipitate was observed with a
yellow supernatant. The reaction was stirred at room temperature for
12 h, and the yellowish suspension was filtered via cannula. The filtrate
was concentrated in vacuo, where upon a large amount of the product
precipitated as a colorless solid. The remaining filtrate was decanted,
and the solid dried in vacuo for 0.5 h, affording a pure colorless solid, 8.
Hz, 3 H, 1 × C2CHCHB ), 1.25 (s, 9 H, 1 × C(CH3)3), 1.31 (d, 3JH−H
3
3
= 6.8 Hz, 3 H, 1 × C2CHCHA ), 1.49 (d, JH−H = 6.9 Hz, 3 H, 1 ×
3
C6CHCHC ), 2.04 (s, 6 H, 1 × −N(CH3)2), 2.91 (sept, JH−H = 6.7
3
3
Hz, 1 H, 1 × C6CH(CH3)2), 3.49 (sept, JH−H = 6.9 Hz, 1 H, 1 ×
3
C2CH(CH3)2), 6.96−7.02 (m, 2 H, 1 × C3,5-H, Dipp), 7.04 (s, 1 H, 1
1
1
Yield: 1.1 g, 2.7 mmol (61%). Mp: 115−116 °C. H NMR (200.13
× Si-H, Si-satellites: JSi−H = 205 Hz), 7.06−7.13 (m, 1 H, 1 × C4-H,
5280
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Article
Dipp). 13C{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ 22.8 (s, 1
C, 1 × C2CHCBH3), 24.6 (s, 1 C, 1 × C6CHCDH3), 25.5 (s, 1 C, 1 ×
C2CHCAH3), 26.0 (s, 1 C, 1 × C6CHCCH3), 28.2 (s, 1 C, 1 ×
C6CH(CH3)2), 28.9 (s, 1 C, 1 × C2CH(CH3)2), 30.1 (s, 3 C, 1 ×
C(CH3)3), 39.9 (s, 2 C, 1 × N(CH3)2), 54.4 (s, 1 C, 1 × −C(CH3)3),
124.4 (s, 1 C, 1 × C3, Dipp), 124.7 (s, 1 C, 1 × C5, Dipp), 128.3 (s, 1
C, 1 × C4, Dipp (peak found by HMQC spectrum, hidden under
solvent signal), 133.3 (s, 1 C, 1 × C1, Dipp), 145.5 (s, 1 C, 1 × C2,
Dipp), 146.0 (s, 1 C, 1 × C6, Dipp), 161.4 (s, 1 C, 1 × −N-C(NMe2)-
N−), 215.7 (s, 4 C, 1 × Fe(CO)4). 29Si-INEPT NMR (200.13 MHz,
2013, 19, 40. For a general review on silylenes see: (b) Blom, B.;
Driess, M. Stuct. Bonding 2014, 156, 85.
(3) (a) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Angew.
Chem. 2006, 118, 4052; Angew. Chem., Int. Ed. 2006, 45, 3948. (b) Sen,
S. S.; Roesky, H. W.; Stern, D.; Henn, J.; Stalke, D. J. Am. Chem. Soc.
2010, 132, 1123.
(4) Blom, B.; Inoue, S.; Gallego, D.; Driess, M. Chem.Eur. J. 2012,
18, 13355.
(5) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Hey, J.; Stalke, D.
Chem.−Asian J. 2012, 7, 528.
(6) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Wolf, H.; Stalke, D. J.
Am. Chem. Soc. 2012, 134, 2423.
(7) Ramachandran, A.; Sarish, S. P.; Roesky, H. W.; Hey, J.; Stalke, D.
Inorg. Chem. 2011, 50, 5039.
(8) (a) Wang, W.; Wang, H.; Chen, M.; Ding, Y.; Roesky, H. W.;
Jana, A. Inorg. Chem. 2009, 48, 5058. (b) Ref 11..
(9) Tan, G.; Blom, B.; Gallego, D.; Driess, M. Organometallics 2014,
33, 363.
(10) Si(II) hydrides are notoriously reactive species and somewhat
rare. As selected recent examples, see: (a) Jana, A.; Leusser, D.;
Objartel, I.; Roesky, H. W.; Stalke, D. Dalton Trans. 2011, 40, 5458.
(b) Rodriguez, R.; Gau, D.; Contie, Y.; Kato, T.; Saffon-Merceron, N.;
Baceiredo, A. Angew. Chem. 2011, 123, 11694; Angew. Chem., Int. Ed.
2011, 50, 11492. (c) Ref 4. (d) Ref 11. (e) Ref 17.
(11) Blom, B.; Enthaler, S.; Inoue, S.; Irran, E.; Driess, M. J. Am.
Chem. Soc. 2013, 135, 6703.
(12) For references concerning amidinato-stabilized Si(II) and Si(IV)
compounds see: (a) So, C.-W.; Roesky, H. W.; Gurubasavaraj, P. M.;
Oswald, R. B.; Gamer, M. T.; Jones, P. G.; Blaurock, S. J. Am. Chem.
Soc. 2007, 129, 12049. (b) Sen, S. S.; Hey, J.; Herbst-Irmer, R.;
Roesky, H. W.; Stalke, D. J. Am. Chem. Soc. 2011, 133, 12311.
(c) Azhakar, R.; Ghadwal, R. S.; Roesky, H. W.; Wolf, H.; Stalke, D.
Organometallics 2012, 31, 4588. (d) Junold, K.; Nutz, M.; Baus, J. A.;
Burschka, C.; Guerra, C. F.; Bickelhaupt, F. M.; Tacke, R. Chem.Eur.
J. 2014, 20, 1.
1
C6D6, 25 °C, ppm): δ 83.6 (s, 1 Si, LSi(H)-Fe(CO)4; JSi−H = 205
Hz). ESI-MS, m/z: calcd for C23H33N3O4SiFe [M + H]+ 500.1663;
found 500.1660. Anal. Calcd for C23H33N3O4SiFe: C 55.31, H 6.66, N
8.41. Found: C 55.46, H 6.84, N 8.62. IR (KBr, [cm−1]): ν = 2100
(br,w) (Si−H), 2022 (s) (CO), 1944 (s) (CO), 1909 (s) (CO).
(N-Adamantanyl-N′-tert-butyl)benzamidinato)silylidene-
irontetracarbonyl (11). A Schlenk tube was charged with 8 (0.25 g,
0.61 mmol) and K2[Fe(CO)4] (0.15 g, 0.61 mmol) and placed in a
−50 °C cold bath. Precooled THF (60 mL) was added under rapid
stirring to the solid mixture via cannula, and the reaction was left
stirring in the cold bath over 12 h, with gradual warming to room
temperature. During the course of the reaction, a white precipitate was
observed, with a light brown supernatant solution. The solvent was
removed in vacuo, and the remaining brown residue extracted with
toluene (20 mL) and filtered via cannula. The toluene of the filtrate
was removed in vacuo, affording a light brown solid, which was
scratched into a powder by a freeze−pump−thaw cycle and dried for
several hours at room temperature in vacuo. Yield: 0.23 g, 0.56 mmol
(92%). Mp: 137−140 °C (dec). 1H NMR (200.13 MHz, C6D6, 25 °C
ppm): δ 1.05 (s, 9 H, 3 × CH3), 1.31 (m, 9 H, 3 × CH2, 1-adamantyl
+3 × CH, 1-adamantyl), 1.77 (m, 6 H, 3 × CH2, 1-adamantyl), 6.91 (s,
1 H, 1 × Si-H, Si-satellites: 1JSi−H = 204 Hz, 1 × SiH), 6.81−6.94 (m, 5
H, 5 × Ar-H) . 13C{1H} NMR (50.32 MHz, C6D6, 25 °C, ppm): δ 9.7
(s, 3 C, 3 × CH, 1-adamantyl), 31.1 (s, 3 C, 3 × CH3), 35.7 (s, 3 C, 3
× CH2, 1-adamantyl), 44.0 (s, 3 C, 3 × CH2, 1-adamantyl), 54.9 (s, 1
C, 1 × C(CH3)3), 56.4 (s, 1 C, 1 × C1, 1-adamantyl), 127.8 (s, 2 C,
Ph), 128.0 (s, 2 C, Ph), 128.2 (s, 1 C, Ph), 130.7 (s, 1 C, Ph), 170.4 (s,
1 C, 1 × NC(Ph)-N), 217.1, (s, 4 C, 4 × CO). 29Si{1H} NMR
(50.32 MHz, C6D6, 25 °C, ppm): δ 86.5 ppm. ESI-MS, m/z: calcd for
C25H30FeN2O4Si [M + H]+ 507.1397; found 507.1398.
(13) Another “symmetrical” N-heterocyclic silylene (NHSi) has
recently been reported bearing the structurally similar guanidinato
ligand where the N atoms are substituted with 2,6-diisopropylphenyl
groups. Muck, F. M.; Junold, K.; Baus, J. A.; Burschka, C.; Tacke, R.
̈
Eur. J. Inorg. Chem. 2013, 5821.
(14) To date only one report from Karsch and co-workers exists of
an asymmetric amidinato silane. Substituted diazabutadienes R1N
C(R2)-(NC(CF3)2 a−c (a, R1 = tBu, R2 = Ph; b, R1 = Dipp, R2 = o-
Cl-Ph; c, R1 = Mes, R2 = Ph, Mes = 2,4,6-trimethylphenyl) were used
as starting materials, which when reacted with trichloro- or
dichlorofluorosilane afforded “asymmetric” benzamidinato-stabilized
silanes LSiCl3 or LSiCl2F (L = R1C(NR2)(NCH(CF3)2), respectively.
To our knowledge, subsequent to this report, no further attempts have
been made in accessing asymmetric benzamidinato or guanidinatosi-
lanes, or respectively the corresponding NHSi’s: Karsch, H. H.;
ASSOCIATED CONTENT
* Supporting Information
Selected NMR and MS spectra of the compounds reported
here as well as crystallographic details are available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: matthias.driess@tu-berlin.de.
Notes
■
Schluter, P. A.; Bienlein, F.; Herker, M.; Witt, E.; Sladek, A.; Heckel,
̈
M. Z. Anorg. Allg. Chem. 1998, 624, 295.
The authors declare no competing financial interest.
(15) See as selected examples: (a) Azhakar, R.; Ghadwal, R. S.;
Roesky, H. W.; Hey, J.; Stalke, D. Chem.−Asian J. 2012, 7, 528.
(b) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Millevolte, A. J.;
Powell, D. R.; West, R. J. Organomet. Chem. 2001, 636, 17.
(c) Ramachandran, A.; Sarish, S. P.; Roesky, H. W.; Hey, J.; Stalke,
ACKNOWLEDGMENTS
The authors thank the Cluster of Excellence UniCat (financed
by the Deutsche Forschungsgemeinschaft and administered by
■
̌
D. Inorg. Chem. 2011, 50, 5039. (d) Tavcar, G.; Sen, S. S.; Azhakar, R.;
the TU Berlin). Alma Jager is thanked for experimental
̈
Thorn, A.; Roesky, H. W. Inorg. Chem. 2010, 49, 10199. (e) Ref 6..
(f) Neumann, E.; Pfaltz, A. Organometallics 2005, 24, 2008.
(g) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H.
Organometallics 1988, 17, 5599. (h) Perti, S. H. A.; Eikenberg, D.;
Neumann, B.; Stammler, H.-G.; Jutzi, P. Organometallics 1999, 18,
2615. (i) Herrmann, W. A.; Haerter, P.; Gstoettmayr, C. W. K.;
Bielert, F.; Seeboth, N.; Sirsch, P. J. Organomet. Chem. 2002, 649, 141.
(j) Zeller, A.; Bielert, F.; Haerter, P.; Herrmann, A. A.; Strassner, T. J.
Organomet. Chem. 2005, 690, 3292.
assistance in the preparation of some of the compounds
reported here as part of an internship under the supervision of
B.B. Terry Brauer (University of South Africa) is gratefully
acknowledged for proofreading the manuscript.
REFERENCES
■
(1) Blom, B.; Gallego, D.; Driess, M. Inorg. Chem. Front. 2014, 1, 134
and references therein.
(2) For a recent and comprehensive review on the synthesis of NHSi
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(17) As a further example for Ni, where such a three-step synthesis
has been employed to access a hydrido SiII complex which
(29) Elschenbroich, C. Chapter 14. In Organometall-chemie, 6th ed.;
B. G. Teubner Verlag: Wiesbaden, 2008.
(30) See ref 17 for a discussion on how SiIV hydrides are expected to
exhibit stretching vibrations at higher wavenumbers than SiII hydrides.
(31) Wang, W.; Wang, H.; Chen, M.; Ding, Y.; Roesky, H. W.; Jana,
A. Inorg. Chem. 2009, 48, 5058.
stoichiometrically hydrosilylates alkynes, see: Stoelzel, M.; Prasang,
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(32) (a) Pohl, M. Masters Thesis, Technische Universitat Berlin,
̈
2014. (b) In fact, we have found that reaction of complex 10 with
dmpe at elevated temperatures does afford the desired complex
[[(NMe2)C{(NtBu)(NDipp)}](H)Si:→Fe(CO)2(η2-dmpe)] as a
major reaction product. The reaction is however not entirely selective,
and some other products are also formed during the course of the
reaction; isolation of this complex is ongoing. We do have some
spectroscopic evidence for its formation, which we include here:
31P{1H} NMR (C6D6) δ 68.8 (s), 65.3 (s) (cis and trans isomers); 29Si-
INEPT-NMR (200.13 MHz, C6D6, 25 °C, ppm) δ 69.0 (s, 1 Si,
LSi(H)-Fe(CO)2 (η2 -dmpe)); ESI-MS, m/z calcd for
C27H49FeN3O2SiP2 [M + H]+ 594.2941, found 594.2482. Attempts
at photolytic CO extrusion of [[(NMe2)C{(NtBu)(NDipp)}](H)Si:→
Fe(CO)4] in the presence of dmpe yielded even less satisfactory
results and a broad array of unidentified products.
(18) Guanidinato ligands have also been employed, particularly in the
Jones group, in stabilizing a large array of low-valent group 2, 13, 14, or
15 compounds. See: Jones, C. Coord. Chem. Rev. 2010, 254, 1273.
(19) (a) Cowley, R. E.; Golder, M. R.; Eckert, N. A.; Al-Afyouni, M.
H.; Holland, P. L. Organometallics 2013, 32, 5289. (b) Nguyen, A. I.;
Zarkesh, R. A.; Lacy, D. C.; Thorson, M. K.; Heyduk, A. F. Chem. Sci.
2011, 2, 166. (c) Saegusa, T.; Ito, Y.; Shimizu, T. J. Org. Chem. 1970,
35, 3995. (d) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.;
Warren, T. H. J. Am. Chem. Soc. 2005, 127, 11248. (e) Babcock, J. R.;
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N. A.; Elhaïk, J.; Holland, P. L. Chem. Commun. 2009, 1760.
(20) Typically carbodiimines are purified by distillation; see for
example: Schlack, P.; Keil, G. Liebigs Ann. Chem. 1963, 661, 164.
(21) (a) Knox, J. R.; Toia, R. F.; Casida, J. E. Agric. Food Chem. 1992,
40, 909. (b) Enders, E.; Hammann, I.; Stende, W. (Bayer
Aktiengesellschaft) N-aryl-N′-(cyclo)-alkyl-thioureas and their use as
agents for combinating animal pests and plant pests. U.S. Patent US
05/827, 1977, published 1980. (c) This compound has escaped
characterization by single-crystal X-ray diffraction analysis, and we
carried out such an investigation. The structure is presented in the SI..
(22) (a) Wolfgang, W.; Randau, G. Justus Liebigs Ann. Chem. 1969,
722, 52. (b) Arya, V. P.; Fernandes, F.; Ghate, S. P.; Costa-Pereira, F.
X. R.; Wasaiwalla, Y. H. Indian J. Chem. 1972, 10, 686.
(33) Baby, A.; Brunet, J.-J.; Kindela, F. B.; Neilbecker, D. Synth.
Commun. 1994, 24, 2827.
(23) For selected references on higher coordinate Si(IV) compounds
see: (a) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr.
Chem. 1986, 131, 99. (b) Cumit, C.; Corriu, R. J. P.; Reye, C.; Young,
J. C. Chem. Rev. 1993, 93, 1371.
(24) Berry, R. S. J. Chem. Phys. 1960, 32, 933.
(25) See as examples of similar complexes featuring similar
phenomena: (a) Hoffman, P. R.; Miller, J. S.; Ungermann, C. B.;
Caulton, K. G. J. Am. Chem. Soc. 1973, 95, 7902. (b) Ungermann, C.
B.; Caulton, K. G. J. Organomet. Chem. 1975, 94, C9. (c) Blom, B.
Reactivity of Ylenes at Late Transition Metal Complexes; Cuvillier Verlag,
2011; pp 77, 79, 94, 118. (d) Komiya, S.; Akita, M.; Yoza, A.; Kasuga,
N.; Fukuoka, A.; Kai, Y. J. Chem. Soc., Chem. Commun. 1993, 787.
(e) Hirano, M.; Akita, M.; Morikita, T.; Kubo, H.; Fukuoka, A.;
Komiya, S. Dalton Trans. 1997, 3453. (f) Blom, B.; Reactivity of Ylenes
at Late Transition Metal Centres; Cuvillier Verlag, 2011 (Doctoral
thesis University of Bonn).
(26) This index (τ) describes in five-coordinate systems, numerically,
the degree to which they are trigonal bipyramidal or square pyramidal
and can be considered a continuum. It is calculated by the formula τ =
(χ − δ)/60, where χ and δ are the two largest angles around the
central atom. A perfect TBP geometry yields τ = 1, while in a perfect
square-pyramidal geometry τ = 0. Distortions from either parent
geometry result in values in between these two extremes. See:
Anderson, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J. v.; Verschoor, G. C. J.
Chem. Soc., Dalton Trans. 1984, 1349.
(27) A similar strategy was reported to isolate donor-stabilized
silylidene complexes of the type [(CO)4FeSiR2←:D] (R = Cl,
tBuO, MesO, CH3, Cl), but to date no example exists where such a
double metathesis/reduction reaction can be employed to access a
hydridosilylidene complex. See: Leis, C.; Wilkinson, D. L.;
Handwerker, H.; Zybill, C.; Muller, G. Organometallics 1992, 11, 514.
̈
(28) Selected examples for SiII group VIII metal complexes:
(a) Schmedake, T. A.; Haaf, M.; Paradise, B. J.; Millevolte, A. J.;
Powell, D. R.; West, R. J. Organomet. Chem. 2001, 636, 17.
(b) Amoroso, D.; Haaf, M.; Yap, G. P. A.; West, R.; Fogg, D. E.
Organometallics 2002, 21, 534. (c) Yoo, H.; Carroll, P. J.; Berry, D. H.
J. Am. Chem. Soc. 2006, 128, 6038. (d) Cade, I. A.; Hill, A. F.; Kampfe,
̈
A.; Wagler, J. Organometallics 2010, 29, 4012. (e) Dysard, J. M.; Tilley,
T. D. Organometallics 2000, 19, 4726.
5282
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