2-acylpyrroles as mono-anionic O,N-chelating ligands in silicon coordination chemistry

Article abstract of DOI:10.1002/chem.201402803

Kryptopyrrole (2,4-dimethyl-3-ethylpyrrole) was acylated with, for example, benzoyl chloride to afford 2-benzoyl-3,5-dimethyl-4-ethylpyrrole (L ¹H). With SiCl₄ this ligand reacts under liberation of HCl and formation of the complex L¹₂SiCl₂. In related reactions with HSiCl₃ or H₂SiCl₂, the same chlorosilicon complex is formed under liberation of HCl and H₂ or liberation of H₂, respectively. The chlorine atoms of L ¹₂SiCl₂ can be replaced by fluoride and triflate using ZnF₂ and Me₃Si-OTf, respectively. The use of a supporting base (triethylamine) is required for the complexation of phenyltrichlorosilane and diphenyldichlorosilane. The complexes L ¹₂SiCl₂, L¹₂SiF ₂, L¹₂Si(OTf)₂, L¹ ₂SiPhCl, and L¹₂SiPh₂ exhibit various configurations of the octahedral silicon coordination spheres (i.e. cis or trans configuration of the monodentate substituents, different orientations of the bidentate chelating ligands relative to each other). Furthermore, cationic silicon complexes L¹₃Si⁺ and L ¹₂SiPh⁺ were synthesized by chloride abstraction with GaCl₃. In contrast, reaction of L¹ ₂SiCl₂ with a third equivalent of L¹H in the presence of excess triethylamine produced a charge-neutral hexacoordinate Si complex with a new tetradentate chelating ligand which formed by Si-templated C-C coupling of two ligands L¹. Si surround systems: 2-Benzoyl-3,5-dimethyl-4-ethyl-1H-pyrrole (LH) is an interesting O,N-chelator for silicon. The complexes L₂SiCl₂, L₂SiF ₂, L₂Si(OTf)₂, L₂SiPhCl, and L ₂SiPh₂ exhibit various configurations of their Si coordination spheres. Cationic silicon complexes L₃Si⁺ and L₂SiPh⁺ were synthesized by chloride abstraction with GaCl₃. Reaction of L₂SiCl₂ with a third equivalent of LH in the presence of excess base leads to Si-templated C-C coupling of two ligands L.

Full text of DOI:10.1002/chem.201402803

DOI: 10.1002/chem.201402803
Full Paper
&
Silicon Coordination Chemistry
2-Acylpyrroles as Mono-anionic O,N-Chelating Ligands in Silicon
Coordination Chemistry
Alexander Kꢀmpfe,[a] Erica Brendler,[b] Edwin Kroke,[a] and Jçrg Wagler*[a]
In memory of Prof. Daniel Kost
Abstract: Kryptopyrrole (2,4-dimethyl-3-ethylpyrrole) was
acylated with, for example, benzoyl chloride to afford 2-ben-
zoyl-3,5-dimethyl-4-ethylpyrrole (L1H). With SiCl4 this ligand
reacts under liberation of HCl and formation of the complex
L1 SiCl2, L1 SiF2, L1 Si(OTf)2, L1 SiPhCl, and L1 SiPh2 exhibit
2 2 2 2 2
various configurations of the octahedral silicon coordination
spheres (i.e. cis or trans configuration of the monodentate
substituents, different orientations of the bidentate chelating
ligands relative to each other). Furthermore, cationic silicon
complexes L1 Si+ and L1 SiPh+ were synthesized by chloride
L1 SiCl2. In related reactions with HSiCl3 or H2SiCl2, the same
2
chlorosilicon complex is formed under liberation of HCl and
H2 or liberation of H2, respectively. The chlorine atoms of
3
2
abstraction with GaCl3. In contrast, reaction of L1 SiCl2 with
2
L1 SiCl2 can be replaced by fluoride and triflate using ZnF2
a third equivalent of L1H in the presence of excess triethyl-
amine produced a charge-neutral hexacoordinate Si complex
with a new tetradentate chelating ligand which formed by
Si-templated CÀC coupling of two ligands L1.
2
and Me3Si-OTf, respectively. The use of a supporting base
(triethylamine) is required for the complexation of phenyltri-
chlorosilane and diphenyldichlorosilane. The complexes
Introduction
Among the great variety of silicon compounds with a hyper-
coordinate Si atom,[1,2] a large group of complexes have been
synthesized with the aid of mono-anionic bidentate O,N-chelat-
ing ligands[3] (such as I, II, III and IV). Furthermore, silicon com-
plexes with other ligand systems (such as in V, VI and VII) con-
tain the pyrrolide moiety as an N-donor function of the
ligand.[4] Surprisingly, silicon complexes with pyrrole-functional-
ized mono-anionic bidentate O,N-chelators have not been re-
ported to date, even though a simple class of that kind of li-
gands, that is, 2-acylpyrroles, is accessible via convenient syn-
theses and has been investigated as the ligand moiety in vari-
ous other coordination compounds of main group and transi-
tion metals.[5]
In the course of our investigations, we have studied silicon
complexes with bi-, tri-, and tetradentate O,N-donor li-
gands,[3b,c,f,4a,b,6] silicon complexes with five-membered N-heter-
ocyclic donor moieties,[2g,h,7] and other silicon compounds of
[a] Dipl.-Chem. A. Kꢀmpfe, Prof. Dr. E. Kroke, Dr. J. Wagler
Institut fꢁr Anorganische Chemie
pyrrole-derived ligand systems.[8] In the work reported herein,
we now address silicon complexes with pyrrole-functionalized
bidentate O,N-chelating ligands.
Technische Universitꢀt Bergakademie Freiberg
09596 Freiberg (Germany)
Fax: (+49)3731394058
E-mail: joerg.wagler@chemie.tu-freiberg.de
[b] Dr. E. Brendler
Results and Discussion
Institut fꢁr Analytische Chemie
Technische Universitꢀt Bergakademie Freiberg
09596 Freiberg (Germany)
Base-supported acylation of “kryptopyrrole” (2,4-dimethyl-3-
ethylpyrrole) with acylchlorides provides easy access to various
2-acyl-3,5-dimethyl-4-ethyl-1H-pyrroles (e.g., ligands L1–4H,
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402803.
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Scheme 1. Syntheses of hexacoordinate dichlorosilicon complexes with two
chelating 2-acylpyrrolide ligands.
Scheme 1) as crystalline solids. The molecular structures of L1H,
L2H, L3H, and L4H in the crystal are displayed in the Supporting
Information. These compounds react with SiCl4 under libera-
tion of HCl, even in the absence of a supporting base
(Scheme 1), to afford the hexacoordinate dichlorosilicon bis-
chelates L1–42SiCl2, which precipitated from the reaction mix-
ture as yellow powders that are poorly soluble in various or-
ganic solvents. Hexacoordination of their Si atoms was con-
firmed by 29Si CP/MAS NMR spectroscopy (diso =À171 ppm in
all cases). Crystals suitable for X-ray diffraction analyses were
grown by slow reaction in undisturbed dilute solutions.
Thus, the molecular structures of compounds L1 SiCl2,
2
L2 SiCl2, and L4 SiCl2 were determined by single-crystal X-ray
2
2
diffraction analysis (Figure 1). In general, these complexes ex-
hibit an all-trans configuration of their hexacoordinate Si coor-
dination spheres and, due to the location of the Si atom on
a center of inversion, 1808 trans angles. Comparison of some
selected bond features of these compounds reveals the subtle
influence of the remote acyl substituent (R) on the Si coordina-
tion characteristics of the individual ligands. Along the series
L4 SiCl2, L1 SiCl2, L2 SiCl2 (R=thienyl, phenyl, p-nitrophenyl) we
2
2
2
observe a shortening of the SiÀCl bonds and a lengthening of
the SiÀO and SiÀN bonds. These features of the Si coordination
spheres and the shortening of the N1ÀC4 and O1ÀC9 bonds
observed along the same series are indicators for the lowering
of the ligand donor strength from R=thienyl via phenyl to p-
nitrophenyl. As the ligand variation revealed only a subtle in-
fluence on the silicon coordination chemistry, we limited our
further studies to complexes of the benzoyl-substituted ligand
L1H.
Figure 1. Molecular structures of (from top) L12SiCl2, L22SiCl2, and L42SiCl2 in
the crystal. Selected atoms are labelled and hydrogen atoms are omitted for
clarity. In all cases the Si atom is located on a crystallographic center of
inversion. Selected bond lengths [ꢂ]: L12SiCl2: Si1ÀCl1 2.228(1), Si1ÀO1
1.766(1), Si1ÀN1 1.875(2), N1ÀC4 1.409(3), C4ÀC9 1.381(3), O1ÀC9 1.325(2);
L22SiCl2 Si1ÀCl1 2.219(1), Si1ÀO1 1.781(1), Si1ÀN1 1.887(2), N1ÀC4 1.400(2),
C4ÀC9 1.388(3), O1ÀC9 1.311(2); L42SiCl2 Si1ÀCl1 2.241(1), Si1ÀO1 1.765(1),
Si1ÀN1 1.863(2), N1ÀC4 1.411(2), C4ÀC9 1.385(3), O1ÀC9 1.323(2).
In analogy to the reaction with SiCl4 (see Scheme 1; yield
92%), ligand L1H was treated with HSiCl3 and H2SiCl2 in tolu-
ene, again in a 2:1 stoichiometric ratio (Scheme 2). Surprisingly,
the same dichlorosilicon complex L1 SiCl2 formed in both
2
cases and in similar yield (92% and 87%), as confirmed by 29Si
CP/MAS NMR spectroscopy and elemental analyses. Thus, de-
hydrogenative coupling must have taken place, as a combina-
tion of dismutation (into SiCl4 and SiH4) and chelation under
HCl elimination would have caused yields below 75% or below
50% for the reactions with HSiCl3 and H2SiCl2, respectively.
Even though silicon complexation under H2 elimination proved
to be an elegant method for the preparation of hypercoordi-
nate Si complexes,[9] this dehydrogenative coupling is particu-
larly interesting because of the unsaturated ligand moieties in-
Scheme 2. Alternative synthesis routes of complex L12SiCl2.
volved, that is, the carbonyl function. As shown in previous
studies, addition of SiÀH bonds to unsaturated ligand moieties
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(e.g., 1,3-H-transfer to imine C atoms[10] and 1,5-H-transfer to
phenanthroline[11]) would provide an alternative reaction path-
way, which has not been observed in the current case though.
Halide exchange in L1 SiCl2 with ZnF2 (Scheme 3) afforded
2
the difluorosilicon complex L1 SiF2, which exhibited very good
2
solubility in solvents such as THF and benzene, whereas the di-
Scheme 3. Substitution of chloride in L12SiCl2.
chlorosilicon precursor was hardly soluble in various organic
solvents. The 29Si NMR spectra of this compound confirm sili-
con hexacoordination: In solution, a triplet (1JSi,F =181 Hz) ap-
pears at d=À170.9 ppm, and in the CP/MAS spectrum a com-
plex signal was observed at d=À170 ppm (see the Supporting
Information), which resembles the signal shape of another hex-
acoordinate difluorosilicon-bis-chelate reported by Tacke
et al.[12] Its signal characteristics can be attributed to dipolar
coupling effects that arise from the cis-disposed SiÀF bonds.[13]
Single-crystal X-ray structure analysis of the benzene solvate of
Figure 2. Molecular structures of (from top) L12SiF2 and L12Si(OTf)2 in the
crystal. Selected atoms are labelled, hydrogen atoms and the solvent mole-
cule of L12SiF2 · C6H6 are omitted for clarity. The Si atom is located on a crys-
tallographic twofold axis (in L12SiF2) or on a center of inversion (in
L12Si(OTf)2). Selected bond lengths [ꢂ] and angles [8]: L12SiF2: Si1ÀF1
1.632(1), Si1ÀO1 1.842(1), Si1ÀN1 1.883(2), N1ÀC4 1.403(3), C4ÀC9 1.386(3),
O1ÀC9 1.309(2); F1-Si1-F1* 93.6(1), F1-Si1-O1 176.9(1), N1-Si1-N1* 165.4(1);
L12Si(OTf)2: Si1ÀO1 1.746(1), Si1ÀO2 1.870(1), Si1ÀN1 1.842(1), N1ÀC4
1.411(1), C4ÀC9 1.383(2), O1ÀC9 1.331(1).
L1 SiF2 (Figure 2) revealed the configurational change of the
2
silicon coordination sphere upon halide exchange, that is,
transformation into a complex with cis-arranged halides. Simi-
lar behavior (cis SiF2 vs. trans SiCl2 moieties in related hexa-
coordinate silicon O,N-bis-chelates) has previously been report-
ed for other ligands and has been attributed to the size differ-
ence of atoms in the Si coordination sphere Cl>O>F.[14] In the
gands (L1 SiCl2 1.766(1), 1.875(2) vs. L1 Si(OTf)2 1.746(1),
1.842(1) ꢂ, respectively). As expected, SiÀO bond shortening
causes a lengthening of the C9ÀO1 bond (to 1.331(1) ꢂ).
2
2
Whereas complexes of the type L2SiCl2 were prepared with-
out the aid of a sacrificial base (and further complexes could
be derived by anion exchange reactions), phenylchlorosilanes
exhibit significantly lower reactivity towards the 2-benzoylpyr-
role ligand L1H, that is, upon heating PhSiCl3 and L1H in tolu-
ene under reflux for 3 h the unreacted ligand was recovered as
the sole solid, and PhSiCl3 was the only silane detected by 29Si
NMR spectroscopy of the crude “reaction” solution. Base-sup-
ported substitution, using triethylamine as the base, was ap-
plied to successfully synthesize the O,N-bis-chelate complexes
L1 SiPhCl and L1 SiPh2 from the corresponding chlorosilanes
particular case of L1 SiF2, the C1-bound methyl group thus
2
causes less repulsion with the Si-bound F atom, whereas in an
all-trans configuration related to L1 SiCl2 this methyl group
2
would face the slightly larger O atom O1 of the opposite bi-
dentate ligand, which would result in enhanced steric repul-
sion. The ligand arrangement of L1 SiF2 results in the O-donor
2
atom being trans-disposed to the SiÀF bond. Therefore, the
stronger SiÀF bond causes significant lengthening of the SiÀO
bond with respect to that in the precursor compound with
a
trans O-Si-O orientation (L1 SiF2 1.842(1) vs. L1 SiCl2
2
2
2
2
1.766(1) ꢂ), whereas the still trans-oriented SiÀN bonds show
less response to the configurational change of the complex
(L1 SiF2 1.883(2) vs. L1 SiCl2 1.875(2) ꢂ). SiÀO bond lengthening
(Scheme 4).
In these hexacoordinate Si complexes we found the all-trans
configuration for L1 SiPhCl in analogy to the dichlorosilicon
2
2
2
furthermore results in a shortening of the intra-ligand bond
complex, whereas L1 SiPh2 reveals an alternative configuration
2
C9ÀO1 (L1 SiCl2 1.325(2) vs. L1 SiF2 1.309(2) ꢂ).
with cis-arranged monodentate groups and this time trans-dis-
posed SiÀO bonds (Figure 3). This particular configuration can
be rationalized in a manner similar to that for the related con-
figuration of L1 SiF2, that is, the methyl groups in the pyrrole-
2-position need to face the sterically least demanding substitu-
ent, which is the O atom of the chelating ligand. Whereas this
2
2
A different effect on the silicon coordination sphere is ob-
served upon replacing the Si-bound chlorine by the more ionic
triflate (Scheme 3, Figure 2), which proceeds under retention
of the all-trans configuration and which causes a similar short-
ening of both the SiÀO and SiÀN bonds to the bidentate li-
2
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forced toward the sterically less demanding substituent (Cl),
and again the Si-bound phenyl group gains sufficient spatial
freedom (sum of cis angles C31-Si1-X: 379.28).
Whereas compounds L1 SiR2 (R=Cl, F, OTf, Ph) reveal silicon
2
hexacoordination in the solid state and do not exhibit suffi-
cient solubility for solution-state 29Si NMR spectroscopic inves-
tigation (R=Cl, OTf, Ph) or prove to contain hexacoordinate sil-
icon in the solution state (R=F), compound L1 SiPhCl shows
2
good solubility and ionic dissociation of the SiÀCl bond in
CDCl3 solution (Scheme 5), as indicated by a 29Si NMR shift
characteristic of pentacoordinate silicon (d=À109.3 ppm). The
Scheme 4. Base-supported syntheses of hexacoordinate phenylsilicon com-
plexes with two chelating 2-benzoylpyrrolide ligands.
Scheme 5. Ionic dissociation of the SiÀCl bond in L12SiPhCl.
progress of the initial stages of this dissociation, however, is re-
flected by the solid-state structures of three different solvates
of L1 SiPhCl (THF, toluene, MeCN), which contain the mole-
2
cules of L1 SiPhCl in similar conformation but with different
2
SiÀCl bond lengths (THF solvate: 2.311(1) and 2.327(1) ꢂ, tolu-
ene solvate 2.340(2) ꢂ, MeCN solvate 2.384(1) ꢂ; for the molec-
ular structures see the Supporting Information). For the solu-
tion state we have investigated the temperature dependence
of the ionic dissociation of L1 SiPhCl by variable-temperature
2
29Si NMR spectroscopy in the temperature range 223–316 K. At
low temperatures (306 K and below) the 29Si NMR signal ap-
pears at d=À105.1 ppm, thus representing the shift of the
pentacoordinate Si atom in the cation L1 SiPh+. At higher tem-
2
Figure 3. Molecular structures of (from top) L12SiPhCl (in the solvate
L12SiPhCl · MeCN) and L12SiPh2 in the crystal. Selected atoms are labelled
and hydrogen atoms (and solvent molecule from L12SiPhCl · MeCN) are
omitted for clarity. Selected bond lengths [ꢂ] and angles [8]: L12SiPhCl:
Si1ÀCl1 2.384(1), Si1ÀO1 1.781(1), Si1ÀO2 1.791(1), Si1ÀN1 1.889(1), Si1ÀN2
1.885(1), Si1ÀC31 1.926(2), N1ÀC4 1.411(2), C4ÀC9 1.388(2), O1ÀC9 1.320(2);
Cl1-Si1-C31 178.7(1), O1-Si1-O2 172.4(1), N1-Si1-N2 168.4(1); L12SiPh2:
Si1ÀO1 1.805(1), Si1ÀO2 1.800(1), Si1ÀN1 1.980(1), Si1ÀN2 1.993(1), Si1ÀC31
1.942(1), Si1ÀC37 1.937(1); O1-Si1-O2 171.4(1), N1-Si1-C31 171.8(1),
N2-Si1-C37 171.9(1), C31-Si1-C37 97.3(1).
peratures the signal is shifted to higher field (e.g., d=
À111.2 ppm at 316 K). This counter-intuitive temperature de-
pendence (ionic dissociation upon cooling) has already been
reported by Kost et al.[3b] for complexes of the type
(O,N)2SiPhCl (where (O,N) is a mono-anionic hydrazide-derived
chelating ligand), and can be attributed to better anion solva-
tion at lower temperatures. For the dissociation of L1 SiPhCl
2
into L1 SiPh+ and ClÀ we have derived DH=À39.2 kJmolÀ1
2
and DS=À122.4 JmolÀ1 KÀ1 from a linearized LN(K) versus 1/T
plot (see the Supporting Information). The use of GaCl3 as an
efficient chloride scavenger gave rise to the preparation of the
requirement would be fulfilled in both the all-trans and this
particular cis configuration, the sterically demanding, and in
this compound sterically competing, two Si-bound phenyl
groups can gain additional freedom in this cis-configuration by
forcing the bidentate ligands into the same opposite hemi-
sphere of the coordination sphere. Thus, the sum of the four
C-Si-X cis angles of each phenyl group can amount to 372.7
and 371.78 for C31 and C37, respectively, whereas the sum of
angles would have to be close to 3608 in the highly symmetric
all-trans configuration. In the case of just one Si-bound phenyl
salt [L1 SiPh][GaCl4], which contains the cationic pentacoordi-
2
nate silicon complex [L1 SiPh]+ both in CDCl3 solution (d29Si=
2
À104.6 ppm) and in the solid state (Figure 4, CP/MAS diso29Si=
À105.4 ppm).
The silicon coordination sphere in [L1 SiPh]+ is intermediate
2
between trigonal bipyramidal and square pyramidal, which is
reflected by the structural parameter t=0.53[15] (derived from
the two widest angles of the Si coordination sphere, i.e., the
O-Si-O and N-Si-N angles). Lowering of the silicon coordination
number results in significant shortening of the SiÀN (by
group, that is, in L1 SiPhCl, the two bidentate ligands can be
2
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Figure 5. Molecular structure of [L13Si]+ in the crystal structure of
[L13Si][GaCl4] · MeCN. Selected atoms are labelled and hydrogen atoms,
anion and solvent molecule are omitted for clarity. Selected bond lengths
[ꢂ] and angles [8]: Si1ÀO1 1.796(1), Si1ÀO2 1.777(2), Si1ÀO3 1.780(2), Si1ÀN1
1.890(2), Si1ÀN2 1.884(2), Si1ÀN3 1.863(2); O2-Si1-O3 179.0(1), O1-Si1-N3
173.2(1), N1-Si1-N2 170.0(1).
Figure 4. Molecular structure of [L12SiPh][GaCl4] in the crystal. Selected
atoms are labelled and hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢂ] and angles [8]: Si1ÀO1 1.733(2), Si1ÀO2 1.730(2), Si1ÀN1
1.874(2), Si1ÀN2 1.860(2), Si1ÀC31 1.874(2), N1ÀC4 1.405(3), C4ÀC9 1.369(3),
O1ÀC9 1.351(3); O1-Si1-O2 129.9(1), N1-Si1-N2 161.8(1).
0.02 ꢂ), SiÀC (by 0.05 ꢂ), and SiÀO bonds (by 0.05 ꢂ) with re-
This mer-configuration of the cation could be confirmed crys-
spect to the precursor L1 SiPhCl. The predominant shortening
tallographically in the solid-state structure of [L1 Si][GaCl4] ·
2
3
of the SiÀC and SiÀO bonds results from their equatorial posi-
MeCN. So far, only two kinds of hexacoordinate silicon tris-che-
lates with three identical (O,N)-bidentate ligands have been
characterized crystallographically (III[3g] and IV[3h]). Whereas the
urea-derived compound III comprises four-membered chelate
rings and fac-configuration of the Si coordination sphere, the
phenalenyl-derived compound IV features six-membered che-
late rings and mer-configuration of the Si coordination sphere.
tions in [L1 SiPh]+ (with respect to a trigonal-bipyramidal Si
2
coordination sphere). The shortening of the SiÀO bonds has
particular influence on the CÀO bonds, which are 0.03 ꢂ
longer than in the precursor L1 SiPhCl.
2
Substitution of the remaining SiÀCl bonds in L1 SiCl2 by an-
2
other L1 ligand equivalent (also in this case the use of a sacrifi-
cial base is required, and the exact stoichiometry of the base
applied is essential, vide infra) affords a cationic silicon (O,N)-
Thus, the herein reported compound [L1 Si][GaCl4] · MeCN is
3
the first crystallographically characterized silicon tris-chelate of
that kind which comprises five-membered chelate rings. For
the cationic tris-oxinato silicon complex,[16] which also compris-
es five-membered chelate rings, we also found mer-configura-
tion of the Si coordination sphere, but crystallographic evi-
dence for this configuration in the solid state could not be ob-
tained.
tris-chelate complex with a hexacoordinate Si atom [L1 Si]+
3
(Scheme 6), which could be characterized in the crystal struc-
ture of the salt [L1 Si][GaCl4] (Figure 5).
3
In a CDCl3 solution of the salt [L1 Si]Cl, the ligands give rise
3
1
to three sets of H and 13C NMR signals of equal intensity, and
in the 29Si NMR spectrum a sole signal appears at d=
À172.6 ppm. These properties are in accord with the octahe-
dral complex in mer-configuration, whereas the fac-isomer
would comprise three chemically equivalent chelating ligands.
As mentioned earlier, the correct stoichiometry of the sup-
porting base applied is crucial for the successful synthesis of
the cationic silicon compound [L1 Si]+. Excess base leads to
3
a deprotonation and CÀC cou-
pling reaction of two of the che-
lating ligands (Scheme 6), and
the solid-state structure of the
cation [L1 Si]+ (Figure 5) already
3
exhibits the prerequisite for this
reaction. The pyrrole-bound
methyl group C35 is in close
proximity to the carbonyl
C
atom C24 of a neighboring che-
late ligand (separation C35···C24
3.22 ꢂ), thus deprotonation at
C35 and nucleophilic addition to
C24 occurs upon treatment with
excess base, furnishing a novel
(O,N,O’,N’) tri-anionic tetraden-
tate ligand as confirmed crystal-
lographically in compound L1L’Si
(Figure 6).
Scheme 6. Synthesis of the cationic hexacoordinate silicon-tris-chelate L13Si+ and its interligand CÀC coupling
upon deprotonation.
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Scheme 8. Inter-ligand CÀC coupling under formation of a novel tetraden-
tate ligand from two bidentate (O,N)-chelators.
establishes a shorter bond Si1ÀN2. The SiÀO and SiÀN bond
lengths within the remaining bidentate ligand moiety (O1N1 in
Figure 6. Molecular structure of L1L’Si in the crystal structure of L1L’Si · 3
MeCN. Selected atoms are labelled, phenyl groups are shown as stick
models, and hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: Si1ÀO1 1.717(1), Si1ÀO2 1.787(1),
Si1ÀO3 1.800(1), Si1ÀN1 1.826(1), Si1ÀN2 1.846(1), Si1ÀN3 1.901(1);
O1-Si1-O2 170.2(1), O3-Si1-N2 168.3(1), N1-Si1-N3 171.6(1).
[L1 Si]+, O3N3 in L1L’Si) are hardly altered by this transforma-
3
tion of the Si coordination sphere and by the different net
charges of the two complexes.
This interligand CÀC coupling reaction with formation of
L1L’Si from [L1 Si]+ is related to the fusion of two (O,N)-biden-
3
tate ligands observed by Kost et al. (Scheme 8),[17] which also
proceeds along the route of deprotonation of an alkyl group
in the b-position to an Si-bound N atom and nucleophilic addi-
tion of the carbanion to a carbonyl-analogous electrophilic
center of a second ligand (i.e., to an imine C atom in this case).
Since three of the five possible octahedral complex configu-
rations (i.e., all-trans, NN-trans, and OO-trans in Scheme 9) have
been encountered with our bis-chelate complexes L1 SiX2, we
2
analyzed the relative energies of the five isomers for com-
pounds L1 SiCl2, L1 SiF2, L1 SiPh2, and L1 SiPhCl by quantum
2
2
2
2
chemical analyses with Gaussian09,[18] that is, optimization of
the molecular geometries for the gas phase (at the DFT
MPW1PW91/6-311G(d,p) level) and single-point energy analy-
ses of the local minima (at the MP2/6-311G(d,p) level) (Table 1).
Scheme 7. Schematic representation of corresponding bond lengths [ꢂ] of
the Si coordination spheres of the compounds shown in Figure 5 and 6
(atomic labels correspond to Figure 5 and 6 as well). The dashed line indi-
cates the CÀC coupling between the two bidentate ligand moieties.
(Note: For L1 SiPhCl, six isomers were considered due to the
2
chemically non-equivalent positions X and X’ in the all-cis
isomer.)
Comparison of corresponding bond lengths in the com-
pounds [L1 Si]+ and L1 L’Si (Scheme 7) reveals some interesting
For the four compounds under investigation we found an in
general pronounced instability of the XX-trans isomer. This can
be attributed to the two methyl groups in pyrrole-2-position
which face each other in this isomer and cause pronounced
interligand repulsion. For L1 SiCl2 and L1 SiF2 the computation-
3
insights into the coordination behavior of 2-acylpyrrole anions
as ligands in general. Whereas in L1L’Si the rather short SiÀO
bond to the alkoxy O atom O1 is expected upon ligand cou-
pling (formal transition from a dative to a covalent SiÀO bond,
SiÀO bond length shortening by 0.06 ꢂ upon transition from
2
2
al analyses confirm the experimentally found configurations as
energetic minima that display reasonable energetic difference
[L1 Si]+ to L1L’Si), basically the same shortening is observed for
3
the SiÀN1 bond to the same bidentate ligand moiety. Hence,
to the alternative isomers. Interestingly, for L1 SiPh2 the all-cis
2
the originally mono-anionic charge of the bidentate ligand
isomer is predicted to be slightly more stable than the experi-
moiety O2N2 in [L1 Si]+ (Scheme 7 left) must have been highly
mentally encountered configuration, and the all-trans isomer is
3
delocalized, and upon nucleo-
philic addition to the carbonyl C
atom of this ligand moiety both
donor atoms (now O1 and N1 in
L1L’Si) have gained from the ad-
ditional anionic charge to a simi-
lar extent. The shorter bond Si1À
O1 in L1L’Si causes a lengthening
of the trans-disposed Si1ÀO2
bond and therefore a shift of the
delocalized negative charge of
Scheme 9. Different configurational isomers of the hexacoordinate silicon-bis-chelates L12SiX2 analyzed by quan-
tum-chemical calculations (pyrrole-bound alkyl substituents are omitted here for clarity but have been included in
this bidentate ligand moiety to
the donor atom N2, which thus the computational analyses).
Chem. Eur. J. 2014, 20, 9409 – 9418
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ed molecules (gas phase) using Gaussian 09.[18] The Cartesian coor-
dinates of the optimized molecular structures are available in the
Supporting Information.
Table 1. Relative energies [kJmolÀ1] of the different octahedral isomers
for each of the compounds L12SiCl2, L12SiF2, L12SiPh2, and L12SiPhCl at
the MPW1PW91/6-311G(d,p) level. Relative single-point energies at the
MP2/6-311G(d,p) level are given in parentheses. Data corresponding to
the experimentally (X-ray) found configuration are given in italics.
Syntheses and Characterization
General considerations: Most of the starting materials commer-
cially available were used as received without further purification,
benzoyl chloride was distilled prior to use. 2,4-Dimethyl-3-ethylpyr-
role[19] (kryptopyrrole) was prepared according to a literature pro-
cedure. Solvents were distilled from sodium benzophenone (THF
and diethyl ether) or sodium (hexane and toluene) or CaH2 (di-
chloromethane) and were stored over sodium wire (diethyl ether,
hexane, toluene) or activated molecular sieves 3 ꢂ under an argon
atmosphere (THF, dichloromethane). Amylene-stabilized chloroform
and acetonitrile were received as spectroscopic grade and were
stored over activated molecular sieves 3 ꢂ. All reactions were car-
ried out under an atmosphere of dry argon utilizing standard
Schlenk techniques. Solution-state 1H, 13C and 29Si NMR spectra
were recorded on a Bruker Avance III 500 MHz spectrometer or on
a Bruker DPX 400 spectrometer (Me4Si as internal standard), solid-
state 29Si NMR spectra were recorded on a Bruker AvanceTM
400 MHz WB spectrometer in 4 mm ZrO2 rotors or in 7 mm ZrO2
rotors with KelF inserts. Elemental analyses were performed by
using an Elementar vario MICRO cube. Single-crystal X-ray diffrac-
tion data were collected on a STOE IPDS 2/2T single-crystal diffrac-
tometer using Mo Ka-radiation. The structures were solved by
direct methods using SHELXS-97 and refined by applying the full-
matrix least-squares method of F2 against all reflections with
SHELXL-97[20] in WinGX.[21] Graphics of the molecular structures
were generated with ORTEP32[22] and POV-RAY.[23] (Color versions of
the molecular structures shown in this paper are available in the
Supporting Information.) All non-hydrogen atoms were anisotropi-
cally refined. C-bound hydrogen atoms were isotropically refined
in idealized positions (riding model), N-bound H atoms were locat-
ed as residual electron density peaks and were refined isotropically.
CCDC-993365 (L1H), CCDC-993366 (L2H), CCDC-993377 (L3H),
Compound
L1 SiCl2
L1 SiF2
L1 SiPh2
L1 SiPhCl
2
2
2
2
all-trans
XX-trans
NN-trans
OO-trans
all-cis
0.0 (0.0)
28.9 (30.1)
68.2 (63.6)
0.0 (0.0)
30.6 (14.7)
17.6 (7.5)
9.2 (10.5)
55.7 (55.3)
32.2 (38.5)
8.8 (8.4)
3.8 (9.2)
55.3 (56.9)
0.0 (7.1)
51.9 (47.3)
10.9 (8.8)
16.8 (1.3)
17.6 (2.5)
5.9 (0.0)
0.0 (0.0)
8.8 (6.3)[a]
6.7 (5.0)[b]
[a] Cl-trans-O, Ph-trans-N. [b] Cl-trans-N, Ph-trans-O.
energetically equivalent to the experimentally found arrange-
ment. Hence, the initial interpretation of the origin of the mo-
lecular configuration of L1 SiPh2 has only limited significance,
2
and further contributions such as intermolecular crystal pack-
ing effects may play the more important role in this case. For
L1 SiPhCl the five isomers all-trans, NN-trans, OO-trans, and all-
2
cis exhibit similar relative energies. In this context it is surpris-
ing that in the three different crystal structures (three different
solvates) of this complex we have found the all-trans isomer
exclusively. Unfortunately, in both cases, investigation of the
coexistence of various isomers in solution (e.g., by variable-
temperature solution-state NMR spectroscopy) was hampered
by poor solubility (of L1 SiPh2) or by ionic dissociation (of
2
L1 SiPhCl).
2
Conclusion and Outlook
In this study we have shown that 2-acylpyrroles can be utilized
as mono-anionic O,N-chelators in silicon coordination chemis-
try. Our first examples of silicon complexes synthesized there-
from already indicate both flexibility of those ligands in the Si
coordination sphere (arrangement of bis-chelates such as
L1 SiCl2, L1 SiF2 and L1 SiPh2 in various configurations) and re-
CCDC-993372 (L4H), CCDC-993367 (L1 SiCl2), CCDC-993375
2
(L2 SiCl2), CCDC-993373 (L4 SiCl2), CCDC-993374 (L1 SiF2·benzene),
2
2
2
CCDC-993369 (L1 Si(OTf)2), CCDC-993378 (L1 SiPh2), CCDC-993371
2
2
(L1 SiPhCl·toluene), CCDC-993370 (L1 SiPhCl·MeCN), CCDC-993368
2
2
(L1 SiPhCl·THF), CCDC-993381 ([L1 SiPh][GaCl4]), CCDC-993380
2
2
([L1 Si][GaCl4]·MeCN) and CCDC-993379 (L1(L’)Si·3 MeCN) contain
3
2
2
2
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Se-
lected parameters of data collection and structure refinement are
listed in the Supporting Information.
activities of the ligands which exceed the pure coordination
chemistry (Si-templated CÀC coupling, high yield of L1 SiCl2
2
along various syntheses routes such as dehydrogenative cou-
pling of L1H with H2SiCl2, and syntheses of cationic silicon com-
plexes by chloride abstraction with GaCl3).
Synthesis of 2-benzoyl-3–5-dimethyl-4-ethyl-1H-pyrrole L1H: To
a solution of kryptopyrrole (6.15 g, 50.0 mmol) and Et3N (7.07 g,
50.30 mmol, excess) in THF (50 mL) at room temperature was
added benzoyl chloride (5.19 g, 51.3 mmol, excess) over few mi-
nutes. Thereafter the mixture was heated under reflux for 6 h and
then cooled to room temperature. The triethylamine hydrochloride
precipitate was removed by filtration and was washed with THF
(4ꢃ5 mL). From the combined filtrate and washings the volatiles
were removed in vacuo (trap condensation) and the solid residue
was recrystallized from methanol. Yield: 7.82 g (34.40 mmol, 69%),
colorless solid. M.p. 1378C; 1H NMR (500 MHz, CDCl3): d=1.05 (t,
3J(H,H)=7.56 Hz, 3H, CH2CH3), 1.90 (s, 3H, CH3), 2.25 (s, 3 H CH3),
In principle, complexes of the type L2SiX2 with L=anion of
a 2-acylpyrrole offer various sites for electronic modifications,
that is, substituents at the pyrrole backbone, various acyl
groups and different monodentate Si-bound substituents X.
Further exploration of this interesting system of Si complexes
is currently under way.
Experimental Section
Computational Analyses
3
2.38 (q, J(H,H)=7.56 Hz, 2H, CH2CH3), 7.41–7.52 (m, 3H, ar), 7.61–
All quantum-chemical calculations for the geometry optimization
(at DFT MPW1PW91 6–311G (d,p) level) and the single-point
energy analyses (at MP2 6–31G (d) level) were performed on isolat-
7.63 (m, 2H, ar), 9.09 ppm (s (br.), 1H, NH); 13C NMR (125 MHz,
CDCl3) d=11.6 (pyrrole 4 CH3), 11.7 (pyrrole 2 CH3), 15.1 (CH2CH3),
17.2 (CH2CH3), 125.3, 126.9, 128.3, 132.7 (pyrrole ring), 128.2, 128.2,
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130.7, 140.5 (phenyl), 185.4 ppm (C=O); elemental analysis (%)
calcd for C15H17NO: C 79.26, H 7.54, N 6.16; found: C 79.38, H 7.55,
N 6.13.
Ligands L2H, L3H, and L4H were synthesized in an analogous
tomaceous earth, washed with dichloromethane (2ꢃ1 mL), and the
volatiles were evaporated in vacuo from the combined filtrate and
washings. Recrystallization of the residue from benzene and de-
cantation of the supernatant afforded L1 SiF2 · C6H6 as a colorless
2
crystalline product (composition according to single-crystal X-ray
structure analysis). Yield 274 mg (0.46 mmol, 62%). Upon storage
at room temperature the crystalline product loses solvent of crys-
tallization. 1H NMR (400 MHz, CDCl3): d=1.07 (t, 3J(H,H)=7.7 Hz,
3H, CH2CH3); 2.01 (s, 3H, CH3), 2.41 (q, 3J(H,H)=7.7 Hz, 2H,
CH2CH3), 2.56 (s, 3H, CH3), 7.38–7.49, 3 H (m.p,), 7.63 ppm (d,
7.9 Hz, 2H, oPh); 13C NMR (100.1 MHz, CDCl3): d=12.2 (pyrrole
CH3); 13.2 (pyrrole CH3), 14.5 (CH2CH3), 17.7 (CH2CH3), 128.0 (Ph o/
m); 129.3 (Ph o/m), 131.1 (Ph p), 132.1 (Ph i), 133.0, 133.6, 135.3,
155.2 (pyrrole), 173.6 ppm (C=O); 19F NMR (376.5 MHz, CDCl3): d=
manner, for yield and characterization see Supporting Information.
Synthesis of L1 SiCl2 from SiCl4: To a stirred solution of 2-benzoyl-
2
3,5-dimethyl-4-ethyl-1H-pyrrole L1H (1.06 g, 4.66 mmol) in toluene
(100 mL) was added SiCl4 (440 mg, 2.56 mmol, 10% excess) drop-
wise at room temperature, whereupon precipitation of a bright
yellow solid commenced. After complete addition of SiCl4, stirring
was continued for 1 h. Subsequently, the yellow product was fil-
tered off, washed with toluene (3ꢃ5 mL), and dried in vacuo. Yield
1.18 g (2.14 mmol, 92%). Crystals suitable for X-ray analysis were
obtained by addition of SiCl4 (70 mg, 0.41 mmol) to a solution of
L1H (170 mg, 0.75 mmol) in toluene (75 mL) at 508C and leaving
the solution undisturbed at this temperature for 7 h. M.p. (sealed
capillary, uncorrected) 3058C (decomp); 29Si CP/MAS NMR
(79.5 MHz, nspin =4 kHz): diso =À171.0 ppm; elemental analysis (%)
calcd for C30H32N2O2SiCl2: C 65.33, H 5.85, N 5.08; found: C 65.35, H
6.02, N 4.91. The compound has very poor solubility in solvents
such as chloroform, THF, benzene, toluene, and acetonitrile. In
DMSO it undergoes dissociation and formation of a DMSO-solvated
1
À125.3 ppm (s, 29Si satellites J(Si-F)=181 Hz); 29Si NMR (79.5 MHz,
1
CDCl3): d=À170.9 ppm (t, J(Si,F)=181 Hz); 29Si CP/MAS (79.5 MHz,
nspin =5 kHz): diso =À170.0 ppm (for signal shape see Supporting
Information); elemental analysis (%) calcd for C30H32N2O2SiF2
·
0.25C6H6: C 70.30, H 6.27, N 5.21; found: C 70.20, H 6.55, N 5.50.
Synthesis of L1 Si(OTf)2: To a suspension of L1 SiCl2 (180 mg,
2
2
0.326 mmol) in MeCN (10 mL) was added Me3SiOTf (220 mg,
0.99 mmol, 3 equiv) in one portion at 808C. The resulting clear so-
lution was stored undisturbed at room temperature whereupon
a yellow solid began to crystallize after 1 h. After leaving the mix-
ture undisturbed overnight the supernatant liquid was removed by
decantation and the remaining yellow solid was washed with two
portions of MeCN (2ꢃ1 mL). Yield 131 mg (0.168 mmol, 52%). M.p.
(sealed capillary, uncorrected) 2358C (decomp); 29Si CP/MAS
(79.5 MHz, nspin =10 kHz): diso =À175.1 ppm; elemental analysis (%)
calcd for C32H32N2O8SiS2F2: C 49.35, H 4.14, N 3.60, S 8.23; found: C
49.37, H 4.15, N 3.68, S 8.44. Solution NMR spectroscopy (1 H, 13C,
29Si) was possible only in [D6]DMSO and produced the same signals
silicon complex,[6s] that is [L1 Si(DMSO)2]2+, thus it is soluble in
2
[D6]DMSO and produces the same set of NMR signals as L1 Si(OTf)2
2
does, which thus forms the same complex in DMSO solution.
1H NMR (400 MHz, [D6]DMSO): d=0.98 (t, 3J(H,H)=7.7 Hz, 6H,
3
CH2CH3), 2.18 (s, 6H, CH3), 2.22 (s, 6H, CH3), 2.39 (q, J(H,H)=7.7 Hz,
4H, CH2CH3), 7.22 (d, 7.1 Hz, 2H, Si(oPh)), 7.39 (m, 2H, Si(mPh)),
7.49 (m, 1H, Si(pPh)), 7.73 (m, 4H, mPhL), 7.83 (m, 2H, pPhL),
8.07 ppm (d, 7.4 Hz, 4H, oPhL); 13C NMR (100.1 MHz, [D6]DMSO):
d=12.4, 13.3, 13.9, 16.9 (alkyl), 34 (broad multiplet, Si-coordinated
[D6]DMSO), 129.1, 129.9, 130.1, 133.8, 133.9, 137.5, 139.0, 158.9,
168.5 ppm (C=O); 29Si NMR (79,5 MHz, [D6]DMSO): d=À175.5 ppm.
as L1 SiCl2.
2
Synthesis of L1 SiPhCl: To a solution of L1H (1.15 g, 5.00 mmol)
Synthesis of L1 SiCl2 from HSiCl3: To a stirred solution of L1H
2
2
and Et3N (550 mg, 5.50 mmol) in MeCN (75 mL) was added PhSiCl3
(525 mg, 2.50 mmol) at room temperature in one portion, where-
upon a yellow-colored solution resulted from which, after initial
stirring was discontinued, a yellow crystalline solid precipitated
upon storage at 58C for 6 h. The supernatant liquid was decanted
off and the yellow solid was washed with three portions of MeCN
and, subsequently, dried in vacuo. Yield 783 mg (1.23 mmol, 50%)
(1.00 g, 4.40 mmol) in toluene (100 mL) at 08C was added HSiCl3
(300 mg, 2.20 mmol) in one portion. Upon evolution of gas,
a bright yellow solid precipitated. The mixture was stirred at 08C
for 30 min and was then stored at room temperature overnight.
Thereupon, the yellow product was filtered off, washed with dieth-
yl ether (3ꢃ5 mL) and dried in vacuo. Yield 1.11 g (2.01 mmol,
92%). 29Si CP/MAS NMR (79.5 MHz, nspin =4 kHz): diso =À171.0 ppm;
elemental analysis (%) calcd for C30H32N2O2SiCl2: C 65.33, H 5.85, N
5.08; found: C 65.09, H 5.82, N 4.96.
1
3
of L1 SiPhCl · MeCN. H NMR (400 MHz, CDCl3): d=1.13 (t, J(H,H)=
2
7.7 Hz, 6H, CH2CH3), 2.33 (s, 6H, CH3), 2.39 (s, 6H, CH3), 2.51 (q,
3
3J(H,H)=7.7 Hz, 4H, CH2CH3), 7.22 (d, J(H,H)=7.1 Hz, 2H, Si(oPh)),
Synthesis of L1 SiCl2 from H2SiCl2: To a stirred solution of L1H
7.39 (m, 2H, Si(mPh)), 7.49 (m, 1H, Si(pPh)), 7.66 (m, 4H, mPhL),
7.74 (m, 2H, pPhL), 7.89 ppm (d, 3J(H,H)=7.5 Hz, 4H, oPhL);
13C NMR (100.1 MHz, CDCl3): d=13.1, 14.1, 14.9, 17.9 (alkyl), 128.9,
129.3 (o/mPhL), 129.5, 130.1 (o/mPhL), 131.5, 134.0, 134.4, 135.0,
139.7, 141.5, 162.8 (PhL, PhSi, pyrrole), 168.9 ppm (C=O); 29Si NMR
2
(1.00 g, 4.40 mmol) in toluene (100 mL) at 08C was added 555 mg
of a H2SiCl2 solution in toluene (40% w/w H2SiCl2, 2.20 mmol).
Soon after the addition, gas evolution commenced and a bright
yellow solid precipitated. The mixture was stirred at 08C for 2 h
and was then stored at room temperature overnight. Thereupon,
the yellow product was filtered off, washed with diethyl ether (3ꢃ
5 mL) and dried in vacuo. Yield 1.06 g (1.91 mmol, 87%). 29Si CP/
MAS NMR (79.5 MHz, nspin =5 kHz): diso =À171.0 ppm; elemental
analysis (%) calcd for C30H32N2O2SiCl2: C 65.33, H 5.85, N 5.08;
found: C 65.61, H 5.84, N 5.04.
(79,5 MHz, CDCl3): d=À109.3 ppm; 29Si CP/MAS (79.5 MHz, nspin
=
5 kHz): diso =À156.9; elemental analysis calcd (%) for C36H37N2O2SiCl
· MeCN: C 71.96, H 6.36, N 6.62; found: C 72.05, H 6.34, N 6.37.
Synthesis of [L1 SiPh][GaCl4]: To a solution of GaCl3 (55 mg,
2
0.31 mmol) in CHCl3 (1 mL) was given L1 SiPhCl·MeCN (200 mg,
2
0.31 mmol) as a solid. Upon diffusion of pentane into the resulting
greenish yellow solution, a crystalline solid was obtained within
three days. The solvent was decanted off, the remaining yellow
solid was washed with CHCl3/hexane 2:1 (v/v) (1.5 mL), and dried
in vacuo. Yield 210 mg (0.27 mmol, 87%). M.p. (sealed capillary, un-
Compounds L2 SiCl2, L3 SiCl2 and L4 SiCl2 were synthesized from
2
2
2
SiCl4 as described for L1 SiCl2. For yield and characterization see
2
Supporting Information.
Synthesis of L1 SiF2: To
a
suspension of L1 SiCl2 (410 mg,
2
2
1
0.743 mmol) in dichloromethane (25 mL) was added ZnF2 (155 mg,
1.50 mmol). Upon stirring at room temperature the initially colour-
less liquid turned yellow within 5 min. Stirring was continued for
1 h. After standing overnight the mixture was filtered through dia-
corrected) 2608C (decomp.); H NMR (400 MHz, CDCl3): d=1.13 (t,
3J(H,H)=7.7 Hz, 6H, CH2CH3), 2.33 (s, 6H, CH3), 2.37 (s, 6H, CH3),
3
2.51 (q, J(H,H)=7.7 Hz, 4H, CH2CH3), 7.25 (m, 2H, Si(oPh)), 7.41 (t,
3J(H,H)=7.2 Hz, 2H, Si(mPh)), 7.50 (t, 3J(H,H)=7.2 Hz, 1H,
Chem. Eur. J. 2014, 20, 9409 – 9418
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3
3
Si(pPh)),7.65 (t, J(H,H)=7.4 Hz, 4H, mPhL), 7.72 (t, J(H,H)=7.4 Hz,
2H, pPhL), 7.19 ppm (m, 4H, oPhL); 13C NMR (100.1 MHz, CDCl3): d=
13.1, 14.1, 15.1, 17.9 (alkyl), 129.0, 129.2 (o/mPhL), 129.3, 130.2 (o/
mPhL), 131.7, 134.2, 134.4, 135.1, 140.1, 141.8, 163.4 (PhL, PhSi, pyr-
role), 168.8 ppm (C=O); 29Si NMR (79,5 MHz, CDCl3): d=
CH2CO(pyr’)(Ph)), 76.0 ((pyr-CH2CO(pyr’)(Ph)), 108.3, 121.5, 122.3,
126.5, 127.2, 127.7, 128.0, 128.2, 130.0, 130.1, 130.5, 131.3, 131.7,
132.2, 133.0, 133.2, 133.3, 134.4, 135.2, 135.8, 135.9, 147.0, 153.2,
154.2 (24 C pyrrol, phenyl) 172.7, 174.9 ppm (C=O); 29Si NMR
(79.5 MHz, CDCl3): d=À167.6 ppm; elemental analysis calcd (%) for
C45H47N3O3Si·0.5 MeCN: C 76.05, H 6.73, N 6.75; found: C 76.02, H
6.55, N 6.50.
À104.6 ppm; 29Si CP/MAS (79.5 MHz, nspin =5 kHz): diso
=
À105.4 ppm; elemental analysis calcd (%) for C36H37N2O2SiGaCl4: C
56.20, H 4.85, N 3.64; found: C 56.28, H 4.74, N 3.48.
Synthesis of L1 SiPh2: This was carried out analogously to that of
2
Acknowledgements
L1 SiPhCl starting from L1H (170 mg, 0.750 mmol), Et3N (80 mg,
2
0.80 mmol), and Ph2SiCl2 (100 mg, 0.400 mmol) using MeCN
(25 mL) as solvent. Crystallization of the product commenced after
2 h of undisturbed storage at room temperature. After one day the
supernatant solvent was decanted off, the remaining yellow solid
was washed with MeCN (3ꢃ1 mL), and it was dried in vacuo. Yield
173 mg (0.27 mmol, 72%). M.p. (sealed capillary, uncorrected)
This work was performed within the Cluster of Excellence
“Structure Design of Novel High-Performance Materials via
Atomic Design and Defect Engineering (ADDE)” that is finan-
cially supported by the European Union (European regional de-
velopment fund) and by the Ministry of Science and Art of
Saxony (SMWK).
2468C (decomp); 29Si CP/MAS (79.5 MHz, nspin =5 kHz): diso
=
À159.9 ppm; elemental analysis calcd (%) for C42H42N2O2Si: C 79.46,
H 6.67, N 4.41; found: C 79.75, H 6.60, N 4.32. This compound has
very poor solubility in solvents such as chloroform, DMSO, THF,
benzene, toluene, and acetonitrile. Therefore, only solid-state char-
acterization methods were applied.
Keywords: chelating ligands
· coordination chemistry ·
hexacoordination · hypercoordination · isomerism · silicon
Synthesis of [L1 Si][GaCl4]: To a suspension of L1 SiCl2 (220 mg,
[1] For reviews on hypercoordinate silicon compounds see, for example:
a) J. Wagler, U. Bçhme, E. Kroke in Structure and Bonding, Vol. 155 (Ed.:
D. Scheschkewitz), Springer, 2014, pp. 29–105; b) W. Levason, G. Reid,
W. Zhang, Coord. Chem. Rev. 2011, 255, 1319–1341; c) D. Kost, I. Kalikh-
man, Acc. Chem. Res. 2009, 42, 303–314; d) R. Tacke, O. Seiler in Silicon
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3
2
0.40 mmol) in CHCl3 (3 mL) was added a solution of L1H (90 mg,
0.40 mmol) and Et3N (40 mg, 0.40 mmol) in CHCl3 (2 mL). Upon stir-
ring for 5 h an almost clear solution resulted, which was filtered.
After washing the filter residue with CHCl3 (2 mL) the combined fil-
trate and washings were evaporated to dryness and the residue
was dissolved in MeCN (1.5 mL). This solution was added to a solu-
tion of GaCl3 (70 mg, 0.40 mmol) in THF (0.5 mL) (layering of the
MeCN phase over the THF solution was intended, but layering
failed). The resulting solution was stored at À208C for three days
to initialize crystallization, which then proceeded at room tempera-
ture and afforded a yellow crystalline solid, which was isolated by
decantation and washed with THF (3ꢃ0.1 mL) and dried in vacuo.
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Yield of [L1 Si][GaCl4]·0.5 MeCN: 195 mg, 0.21 mmol (53%).
3
1H NMR (400 MHz, CDCl3): d=1.00–1.08 (m, 9H, CH2CH3), 1.56 (s,
3H, CH3), 2.01 (s, 3H, CH3); 2.12 (s, 3H, CH3), 2.16 (s, 3H, CH3), 2.27
(s, 3H, CH3), 2.33 (s, 3H, CH3), 2.35–2.50 (m, 6H, CH2CH3), 7.51–
7.78 ppm (m, 15H, aromatic); 13C NMR (100.1 MHz, CDCl3): d=11.0,
12.7, 12.9, 13.1, 14.0, 14.2, 14.2, 14.4, 14.4, 17.7, 17.8, 17.9 (alkyl),
128.5, 128.8, 129.0, 129.6, 129,9, 129,9, 131.0, 131.2, 132.8, 133.3,
133.8, 134.3, 134.4, 135.0, 135.3, 136.1, 136.5, 137.3, 137.6, 139.0,
140.0, 156.0, 157.9, 158.3 (3ꢃ PhL,3x pyrrole), 170.4, 172.8,
173.6 ppm (C=O); 29Si NMR (79.5 MHz, CDCl3): d=À172.6 ppm; ele-
mental analysis calcd (%) for C45H48N3O3SiGaCl4 ·0.5 MeCN: C 58.84,
H 5.31, N 5.22; found: C 59,03, H 5.53, N 5.51.
Synthesis of L1(L’)Si: To
a
suspension of L1 SiCl2 (89 mg,
2
0.16 mmol) in MeCN (2 mL) was given a solution of L1H (37 mg,
0.16 mmol) and Et3N (40 mg, 0.4 mmol, 2.5 equiv) in MeCN (3 mL).
Upon stirring at 808C for 1 h the resulting solution turned brown-
ish red. Upon standing at room temperature for one week an
orange crystalline solid had formed which was isolated from the
mixture by decantation and washing with MeCN (2ꢃ0.1 mL), and
dried in vacuo. Yield of L1(L’)Si ·0.5 MeCN : 35 mg, 0.048 mmol
1
3
(30%). H NMR (400 MHz, CDCl3): d=0.85 (t, J(H,H)=7.48 Hz, 3H,
CH2CH3), 0.89 (t, 3J(H,H)=7.53 Hz, 3H, CH2CH3), 1.04 (t, 3J(H,H)=
7.53 Hz, 3H, CH2CH3), 1.36 (s, 3H, CH3), 1.70 (s, 3H, CH3), 1.97 (s,
3H, CH3), 2.13 (s, 3H, CH3), 2.15–2.23 (m, 4H, CH2CH3), 2.30 (s, 3H,
CH3), 2.40 (q, 3J(H,H)=7.48 Hz, 2H, CH2CH3), 3.50 (d, 2J(H,H)=
16.38 Hz, 1H, CHH), 3.61 (d, 2J(H,H)=16.38 Hz, 1H, CHH), 7.14–
7.90 ppm (m, 15H, ar); 13C NMR (100.1 MHz, CDCl3) d=9.2, 11.1,
12.4, 12.5, 12.8, 14.4, 15.0, 15.9, 17.6, 17.7, 18.3 (alkyl), 35.4 (pyr-
Chem. Eur. J. 2014, 20, 9409 – 9418
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9417
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Chem. Eur. J. 2014, 20, 9409 – 9418
www.chemeurj.org
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim