Exohedral functionalization: Vs. core expansion of siliconoids with Group 9 metals: Catalytic activity in alkene isomerization

Article abstract of DOI:10.1039/d0sc02861d

Taking advantage of pendant tetrylene side-arms, stable unsaturated Si6 silicon clusters (siliconoids) with the benzpolarene motif (the energetic counterpart of benzene in silicon chemistry) are successfully employed as ligands towards Group 9 metals. The pronounced σ-donating properties of the tetrylene moieties allow for sequential oxidative addition and reductive elimination events without complete dissociation of the ligand at any stage. In this manner, either covalently linked or core-expanded metallasiliconoids are obtained. [Rh(CO)2Cl]2 inserts into an endohedral Si-Si bond of the silylene-functionalized hexasilabenzpolarene leading to an unprecedented coordination sphere of the Rh centre with five silicon atoms in the initial product, which is subsequentially converted to a simpler derivative under reconstruction of the Si6 benzpolarene motif. In the case of [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) a similar Si-Si insertion leads to the contraction of the Si6 cluster core with concomitant transfer of a chlorine atom to a silicon vertex generating an exohedral chlorosilyl group. Metallasiliconoids are employed in the isomerization of terminal alkenes to 2-alkenes as a catalytic benchmark reaction, which proceeds with competitive selectivities and reaction rates in the case of iridium complexes.

Full text of DOI:10.1039/d0sc02861d

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DOI: 10.1039/D0SC02861D
ARTICLE
Exohedral functionalization vs. core expansion of siliconoids with
Group 9 metals: catalytic activity in alkene isomerization
Received 00th January 20xx,
Accepted 00th January 20xx
Nadine E. Poitiers, Luisa Giarrana, Volker Huch, Michael Zimmer, David Scheschkewitz*
DOI: 10.1039/x0xx00000x
6
Taking advantage of a pending tetrylene side-arms, stable unsaturated Si silicon clusters (siliconoids) with the
benzpolarene motif (the energetic counterpart of benzene in silicon chemistry) are succesfully employed as ligands
towards Group 9 metals. The pronounced -donating properties of the tetrylene moieties allow for sequential oxidative
addition and reductive elimination events without complete dissociation of the ligand at any stage. In this manner, either
2 2
covalently linked or core-expanded metallasiliconoids are obtained. [Rh(CO) Cl] inserts into an endohedral Si-Si bond of
the silylene-functionalized hexasilabenzpolarene leading to an unprecedented coordination sphere of the Rh centre with
five silicon atoms in the initial product, which is subsequentially converted to a simpler derivative under reconstruction of
the Si
6
benzpolarene motif. In case of [Ir(cod)Cl]
2
(cod = 1,5-cyclooctadiene) a similar Si-Si insertion leads to the
contraction of the Si
6
cluster core with concomitant transfer of a chlorine atom to a silicon vertex generating an exohedral
chlorosilyl group. The metallasiliconoids are employed in the isomerization of terminal alkenes to 2-alkenes as a catalytic
benchmark reaction, which proceeds with competitive selectivities and reaction rates in case of the iridium complexes.
extraordinarily electron-rich ligands towards transition metal
1
1
INTRODUCTION
centres, and Zintl anions of Group 14 elements heavier than
silicon can be converted to transition metal-centred
derivatives M@E
The control of the reactivity of transition metal centres is a
pivotal aspect of homogenous catalysis and therefore the
development of novel ligands for transition metals one of the
priorities of organometallic chemistry. Unsaturated silicon
x− 12
n
.
In all reported cases, the negative
charges of the polyanionic precursors are (at least partially)
retained in the transition metal-containing products with often
adverse consequences for their solubility and stability, limiting
their application, e.g. in homogeneous catalysis. Siliconoids
with their stabilizing shell of organic ligands and high solubility
due to their charge-neutrality appear to be the logical choice
to overcome both limitations.
1
compounds with the two major sub-categories of silylenes
and disilenes1 are typically characterized by a surplus of
electrons and are therefore inherently stronger -donors than
the corresponding carbon species, while retaining -acceptor
a,2
3
properties in some cases due to their unsaturated nature.
The first transition metal-substituted siliconoids, ligato-
hexasilabenzpolarenes Si
First applications in homogeneous catalysis include C-H
6 2 6 2
-Zr(Cp) Cl and Si -Hf(Cp) Cl with
4
5
borylation , reduction of organic amides , Sonogashira and
covalently attached metallocene moiety were disclosed just
6
7
Heck cross-coupling reactions and hydrosilylation of ketones .
Recently, a third widely occurring sub-category was introduced
to the class of stable unsaturated silicon species, the so-called
siliconoids, partially unsubstituted neutral silicon clusters.
Despite their unsaturated nature, applications of siliconoids as
1
3
recently. In contrast, the application of siliconoids as direct
charge-neutral ligands towards transition metals remained
unsuccessful so far although the introduction of a silylene side-
8
4
arm allowed for the coordination to Fe(CO) moieties in the
1
3
periphery of the hexasilabenzpolarene motif. Now we show
that by using the Group 9 metals rhodium and iridium a much
larger variety of unprecedented coordination modes to
siliconoids can be realized. We demonstrate that covalent
bonding modes between the metal and the uncompromised
hexasilabenzpolarene scaffold are related to the endohedral
incorporation of the metal centre into the siliconoid cluster in
a reversible manner. Finally, with the isomerization of alkenes
we provide a first proof-of-principle for the application of the
thus prepared soluble transition metal/silicon hybrid clusters
as homogenous catalysts.
ligands towards transition metals are virtually unexplored.
With our report on anionically functionalized Si derivatives,
6
9
however, a conceptual link towards (poly)anionic, completely
1
0
unsubstituted deltahedral Zintl anions of silicon
was
established, which in fact exhibit a rich chemistry towards
transition metals: silicides and their heavier congeners of
germanium and tin have frequently been employed as
a. Krupp Chair of General and Inorganic Chemistry, Saarland University, D-66123
Saarbrücken, Germany
Electronic
Supplementary
Information
(ESI)
available:
See
DOI:
1
0.1039/x0xx00000x
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covalent bonding situation. The endohedral bond distance
between the iridium-bonded vertex and that bearing the
RESULTS AND DISCUSSION
View Article Online
DOI: 10.1039/D0SC02861D
Synthesis of iridium complexes
tetrylene moiety (Si3–Si4 2a: 2.549(2) Å, 2b: 2.565(1) Å) is
considerably elongated in comparison to those in the afore-
Treatment of tetrylene-functionalized siliconoids 1a-c with 0.5
equivalents of bis[(1,5-cyclooctadiene)iridium(I) chloride]
2
,5
14
mentioned chlorosilyltricyclo[2.1.0.0 ]pentasilane (2.356 Å)
and a dianionic Si cluster with the same tricyclic scaffold
affords the tetrylene-Si
spectroscopically quantitative manner (Scheme 1). Complexes
a-c were fully characterized by x-ray diffraction on single
6
iridium complexes 2a-c in an NMR
5
1
5
(2.3822 Å).
2
crystals, elemental analysis, as well as multinuclear NMR
spectroscopy.
The 29Si NMR spectrum of 2a-c shows six sharp resonances in a
much narrower range than usual for hexasilabenzpolarenes
6
m,p,t
(+175.4 to −279.0 ppm),
which provides a first hint at the
rearrangement of the cluster scaffold and the ensuing loss of
the spherical aromaticity and the associated magnetically
2
9
1
induced cluster current. On the basis of the 2D Si/ H
correlation NMR spectrum, the signals at 56.7 ppm (2a), 52.6
ppm (2b) and 50.2 ppm (2c) are assigned to the endohedral
SiTip
ppm (2c) are very close in chemical shift to that of the
exocyclic silicon atom in
chlorosilyltricyclo[2.1.0.0 ]pentasilane (5.8 ppm) and were
therefore tentatively attributed to the extrusion of one SiTip
unit from the cluster core of 2a-c. The resonances at −38.4
2a), −41.5 (2b) and −42.1 ppm (2c) are assigned to the SiTip
2
moieties. The signals at 13.3 (2a), 12.8 (2b) and 12.2
2
,5
14
2
(
2
9
vertices. Most remaining Si signals are observed at higher
field (see SI) with the exception of one distinct resonance of 2a Fig. 1 Representative molecular structure of silylene-functionalized siliconoid
iridium complex 2a in the solid state. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are set at 50% probability. For structures of 2b,c see
Supporting Information. Selected bond lengths [Å] and angles [°]: 2a: Ir–Si7
at 33.4 ppm, which is apparently due to the pending silylene
centre.
2
.320(1), Ir–Si3 2.334(1), Si1–Si3 2.305(2), Si1–Si4 2.313(2), Si1–Si6 2.364(2),
6
Scheme 1 Synthesis of tetrylene-Si iridium complexes 2a-c from tetrylene-
Si1–Si2 2.369 (2), Si3–Si6 2.353(1), Si3–Si4 2.548(2), Si3–Si7 2.764(2), Si4–Si5
2
9
Si7 53.33(3), Si7–Si4–Si3 67.81(4); 2b: Ir–Si3 2.3517(7), Ir–Ge 2.4113(3), Ge–Si4
2.4444(8), Ge–Si3 2.8481(8), Si1–Si3 2.296(1), Si1–Si4 2.302(1), Si1–Si2 2.356(1),
Si1–Si6 2.375(1), Si3–Si6 2.339(1), Si3–Si4 2.565(1), Si4–Si5 2.389(1), Si5–Si6
functionalized Si
6
Siliconoids 1a-c (2a: E = Si, 2b: E = Ge, 2c: E = Sn).
.391(2), Si4–Si7 2.402(2), Si7–N2 1.857(3), Si7–N1 1.884(3), Cl–Si2–Si1
9.03(6), Si1–Si3–Ir 124.02(5), Ir–Si3–Si6 174.93(6), Ir–Si3–Si4 105.99(5), Ir–Si3–
Cl
Tip
Tip
Tip
Si
Tip
Si
Si
Tip
Tip
Si
Si
Tip
Si
[Ir(cod)Cl]2
benzene
Si
Tip
Tip
Si
Si
Si
Si
Si
Tip
N
2
.368(1), Ge–N2 1.995(2), Ge–N1 2.005(2); Cl–Si2–Si1 100.91(4), Ir–Si3–Ge
54.243(17), Ir–Si3–Si4 106.90(3), Si6–Si3–Ir 173.61(4), Si1–Si3–Ir 124.75(4),
E
E
tBu
Ir
tBu
tBu
N
N
tBu
N
Ir–Ge–Si3 52.322(16), Ir–Ge–Si4 108.95(2), Si3–Ir–Ge 73.436(19), Ge–Si4-Si3
Ph
Ph
69.25(3).
1a-c
2a-c
This is probably a consequence of the back and forth electron
Single crystals of 2a-c were obtained by crystallization from transfer between iridium and cluster orbitals, but may also
hexane in 68 % (2a), 61 % (2b) and 63 % (2c) yield and the indicate
propellane-like bonding situation as the
tricyclic structures of the siliconoid-iridium complexes hemispheroidality of Si4 suggests although Si3 does not fulfill
a
8
a
confirmed by x-ray diffraction in the solid state (Fig. 1). As the criterion of hemispheroidality and is very near planar-
anticipated on the basis of NMR data, the privo-vertex has tetracoordination instead (2a: (Si3)= −0.0154 Å, (Si4)=
been extruded from the cluster core due to the additional +0.7559 Å; 2b: (Si3)= −0.0060 Å, (Si4)= +0.7276 Å) a.
bond to the chlorine atom transferred from the iridium centre, Conversely, the bond lengths between the bridgehead silicon
which in turn is not only coordinated by the pending tetrylene atoms Si1 and Si3 in 2a,b (2a: 2.305(2) Å, 2b: 2.2962(2) Å) are
moiety but also by the former nudo-vertex of the now at the short end of the usual range for silicon single bonds
2
,5
benzpolarene starting material. The structural model of 2c as also observed in the chlorosilyltricyclo[2.1.0.0 ]pentasilane
1
4
29
could not be refined in a satisfactory manner; the following (2.312 Å). The Si CP/MAS spectra of 2a-c show very similar
discussion thus focuses on 2a,b. The Ir–E bond lengths (E = Si, signals as in in C D solution thus confirming the integrity of
6
6
Ge, Sn; 2a: Si3–Ir: 2.320(1) Å, Si7–Ir: 2.334(1) Å; 2b: Si3–Ir: the coordination modes upon solvation (the stannylene-Si6
1
2e,16
2
.3517(7) Å, Ge–Ir: 2.4113(3) Å) are those of single bonds
.
iridium complex 2c shows a double set of signals due to two
The slightly longer distance between the pending tetrylene crystallographically independent molecules in the asymmetric
and the iridium centre may be explained by the dative vs. unit; see SI). The longest wavelength absorption bands in the
2
| J. Name., 2012, 00, 1-3
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UV/vis spectra at max = 576 nm (2a), 580 nm (2b), 592 nm (2c) 45.489(4)°, centre(Si1–Si4)–Rh–Si2 79.149(4)°, centre(Si1–
View Article Online
DOI: 10.1039/D0SC02861D
are strongly red-shifted compared to previously reported Si4)–Rh–Si3 78.869(4)°, centre(Si1–Si4)–Rh–Si7 75.997(4)°,
9
,13
ligato-substituted siliconoids (max = 364 to 521 nm).
C16–Rh–centre(Si1–Si4) 175.493(6)°, Si7–Rh–Si3 132.095(5)°).
The geometric parameter  = (−)/60 is commonly used for
pentacoordinate complexes as an index of the degree of the
trigonality in trigonal-bipyramidal and square-planar pyramidal
Synthesis of rhodium complexes
The
reaction
with
1
equivalent
of
bis[(1,5-
1
7
structural motifs. With the two largest angles  and  of 3 (:
C16–Rh–centre(Si1–Si4) 175.493(6)°; : Si7–Rh–Si3
32.095(5)°) the angular parameter is  = 0.72 in line with a
cyclooctadiene)rhodium(I) chloride] with 1a-c led to a
complicated mixture of products in all cases, presumably due
to competing oxidative addition and reductive elimination
reactions. In one crystallization attempt of the product mixture
from 1b (E = Ge) a few red-brownish crystals were collected
and then investigated by x-ray diffraction showing the same
motif as observed in 2a-c (see SI). In the anticipation that it
might react in a similar manner despite the differing ligand set,
we considered the rhodium(I) dicarbonyl chloride dimer
as alternative. The reactions of 1a-c with 1
Cl] , however, led to uniform conversion
only in the case of 1a and inseparable mixtures of products for
b,c according to NMR spectra. In addition, H NMR
monitoring of the reaction mixture revealed the
1
distorted trigonal-bipyramidal coordination sphere of Rh. The
chlorine atom is shifted to one of the former nudo-vertices so
that the extrusion of the privo-SiTip moiety as in 2a-c is
2
avoided in this case. Intriguingly, the chloro-substituted silicon
vertex (Si3) is conferred a significant silylene character: the
corresponding bond distance to rhodium (Si3–Rh 2.2455(7) Å)
is considerably shorter than that of the amidinato silylene
moiety, which binds to rhodium at a distance (Si7–Rh
[
Rh(CO) Cl]
2 2
equivalent of [Rh(CO)
2
2
2
.3104(7) Å) similar to those reported for other complexes
5
,16e
1
with this motif
. The former privo-vertex binds to the Rh
1
centre at a distance (Si2–Rh 2.3872(8) Å) in line with the
covalent radii of silicon and rhodium. Si2, Si3 and Si7 bind in
equatorial positions and thus form the base of the trigonal-
bipyramidal coordination environment at Rh.
rearrangement of the initial product overnight in case of 1a
(Scheme 2).
Scheme 2 Synthesis of Si
siliconoid 1.
7
rhodium complexes 3 and 4 from silylene-functionalized Si
6
Tip
Si
Si
Tip
Tip
Si
Tip
Tip
Si
Si
Si
[
Rh(CO)2Cl]2
[Rh(CO) Cl]2
2
Si
toluene
-
benzene
rt, over night
tBu
N
N
tBu
26°C, 5 min
Ph
1
Tip
Si
Tip
Tip
Cl
Si
Si
Si
Tip
Tip
Si
Tip
Tip
Tip
Tip
Si
10 mol% of
Si
Si
Si
Si
1
Si
Si
Tip
Rh
0.4 eq [Rh(CO)2Cl]2
OC Rh CO
OC
Si
Cl
Si
tBu
N
N
tBu
3
4
tBu
N
N tBu
Ph
Ph
2 2
Treatment of 1a with 1 equivalent of [Rh(CO) Cl] in toluene
followed by cooling to −26 °C after three minutes of stirring at
ambient temperature yields dark red crystals of the primary Fig. 2 Molecular structure of silylene-functionalized siliconoid rhodium complex 3 in
the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at
product 3 after storage for 2 to 3 h in 56 % crystalline yield.
Conversely, the secondary product 4 is obtained as red-
brownish crystals in 63 % yield by crystallization from hexane
5
2
0% probability. Selected bond lengths [Å] and angles [°]: Rh–Si3 2.2455(7), Rh–Si7
.3104(7), Rh–Si5 2.3872(8), Rh–Si2 2.5936(7), Rh–Si1 2.5965(7), Si3–Cl 2.102(1), Si1–
Si2 2.279(1), Si1–Si5 2.373(1), Si1–Si4 2.499(1), Si2–Si7 2.254(1), Si2–Si6 2.333(1), Si3–
after stirring the reaction mixture overnight. Both, 3 and 4, Si4 2.308(1), Si4–Si6 2.401(1), Si7–N1 1.837(2), Si7–N2 1.822(2), Si3–Rh–Si7 132.05(3),
Si3–Rh–Si5 97.85(3), Si7–Rh–Si5 116.303, Si3–Rh–Si2 87.62(2), Si7–Rh–Si2 54.36(2),
Si5–Rh–Si2 102.19(2), Si3–Rh–Si1 72.23(2), Si7–Rh–Si1 98.48(2), Si5–Rh–Si1 56.69(2),
Si4–Si1–Rh 94.98(3), Si7–Si2–Rh 56.40(2), Si1–Si2–Rh 64.02(3), Si6–Si2–Rh 115.58(3).
were fully characterized by multinuclear NMR spectroscopy
and x-ray diffraction on single crystals (Fig.s 2 and 3).
In contrast to the iridium complexes 2a-c, the rhodium centre
of 3 is fully incorporated into the core structure under
expansion to a 7-vertex motif. Only one of the CO ligands is
retained in 3 completing the distorted trigonal-pyramidal
coordination sphere at rhodium in an apical position (C16–Rh–
Si2 102.586(4)°, C16–Rh–Si3 104.881(5)°, C16–Rh–Si7
The Si1–Si4 bond is unusually short (2.279(1) Å) and occupies
the remaining apical positions at Rh at relatively long distances
(Si4–Rh 2.5936(7) Å and Si1–Rh 2.5965(7) Å). Both Si1 and Si4
exhibit a hemispheroidal coordination environment with
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hemispheroidalities of (Si1) = +0.8003 Å and (Si4) = +0.4764 rhodium was confirmed by x-ray diffraction in the solid state
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8
a
DOI: 10.1039/D0SC02861D
Å. The degree of metallacyclopropane character of this (Fig. 3).
coordinating interaction according to the Dewar-Chatt- The rhodium centre of 4 exhibits a typical square-planar
Duncanson model18 is difficult to estimate due to the coordination sphere, with the hexasilabenzpolarene moiety
complexity of the bonding situation as the Si1–Si4 distance, indeed connected through the ligato-position Si4.
6
albeit shorter than a usual single bond, is heavily involved in Astonishingly, not only the Si moiety has been reinstated
cluster bonding.
The 29Si NMR spectrum in C
during the isomerization from 3, but the amidinato silylene –
D is consistent with the bonding now disconnected from the siliconoid and coordinated to the
6 6
situation as discussed on the basis of the solid state structure. rhodium centre in trans-fashion – has reacquired its chloro-
Four of the seven resonances are split into doublets by the substituent as well. The coordination sphere of rhodium is
coupling to the 1 Rh nucleus suggesting the coordination of completed by two CO ligands, which requires the "come-back"
rhodium being uncompromised by solvation. The signals are, of the initially dissociated CO molecule. The Si7–Rh bond
03
however, not as broadly dispersed as typically observed for Si
6
length of 2.305(1) Å is in line with reported donor-acceptor
The former privo-vertex Si2 gives rise to a low- bond length of Si-Rh complexes.5,16e Interestingly, it is
field Si NMR signal, albeit split into a doublet at 158.8 ppm significantly shorter than the covalent Si4–Rh bond length of
9
,13
siliconoids.
2
9
2
9
103
with a coupling constant of J Si, Rh) = 41.0 Hz. The silylene 2.398(1) Å in the same molecule. The distance between the
character of the former nudo-vertex Si3, now bearing the bridgehead silicon atoms (Si1–Si3 2.6188(4) Å) is similar as in
9
,13
chloro substituent, results in the significant deshielding of the previously reported ligato-substituted Si
corresponding 29Si NMR signal at 165.7 ppm. The interaction longest wavelength absorption bands are observed at max
with the rhodium centre is reflected in the coupling constant 461 nm (3) and 466 nm (4) and thus slightly blue-shifted
6
siliconoids.
The
=
2
9
103
103
of J Si, Rh = 53.4 Hz. The
Rh coupling of the two compared to the ligato-metalated siliconoids Zr and Hf (Zr: 521
1
3
hemispheroidally coordinated vertices Si1 and Si4 is too small nm, Hf: 497 nm). The Rh complexes 3 and 4 exhibit IR
to be resolved resulting in singlets at −140.2 and −122.1 ppm. characteristics of rhodium carbonyl complexes with CO
Although a discussion of the magnitude of experimental stretching modes at  = 1978 cm (3) and 1951, 1949 cm (4).
coupling constants is next to impossible in polycyclic systems
1
9
−1
−1
such as 3 the absence of detectable coupling is in line with a
3
a
predominant -character of the coordination to rhodium.
The assignment is backed by the absence of cross-peaks to Tip
2
9
1
groups in the 2D Si/ H correlation. The doublet at 108.7 ppm
(
J Si,103Rh = 59.6 Hz) is assigned to the N-heterocyclic silylene
2
9
moiety (Si7) based on the observation of a cross-peak to the t-
butyl groups. In notable contrast, the signal of the not directly
Rh-bonded Si6 at 58.3 ppm (assigned on the basis of a cross-
peak to one Tip substituent) is split into a doublet with
2
9
103
J Si, Rh = 14.3 Hz.
Surprisingly, after the rearrangement of Rh(I) complex 3 to 4,
2
9
the diagnostic wide dispersion of Si NMR shifts is again
observed, which suggested the re-establishment of an
9
,13
uncompromised benzpolarene scaffold. Besides the
characteristic highfield resonances for the nudo-vertices Si1
2
9
and Si3 at −256.1 and −258.3 ppm, the Si NMR signal of the
tetracoordinate privo-vertex Si2 appears at the typical low
2
9
field at 162.6 ppm. The doublet in the Si NMR at 48.2 ppm
with J Si,103Rh = 84.5 Hz is attributed to the N-heterocyclic
silylene moiety on the basis of a cross-peak to the t-butyl
groups in the 2D 29Si/ H correlation and the large coupling
indicative of pronounced s-orbital contributions. In contrast,
29
Fig. 3 Molecular structure of silylene-functionalized Siliconoid rhodium complex
in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids
are set at 50% probability. Selected bond lengths [Å] and angles [°]:Rh–Si7 2.305(1),
Rh–Si4 2.398(1), Si3–Si6 2.342(1), Si2–Si3 2.352(1), Si3–Si4 2.386(1), Si1–Si2 2.619(1),
4
1
2
9
103
the doublet at −9.0 ppm with J Si, Rh = 31.5 Hz suggests a Si1–Si4 2.336(1), Si1–Si6 2.351(1), Si1–Si2 2.392(1), Si1-Si3 2.619(1), Si4–Si5 2.384(1),
Si5–Si6 2.371(1), Si7–N1 1.826(3), Si7–N2 1.831(3), Si7-Cl 2.077(2), Si7–Rh–Si4
72.63(4), Si3–Si4–-Rh 131.22(5), Si5–Si4-Rh 132.67(5), Si1–Si4–Rh 124.01(5), Si4–
1
03
covalent bond to the Rh nucleus. Due to the absence of a
1
2
9
1
cross-peak to a Tip group in the 2D Si/ H correlation NMR
spectrum, it can be attributed to the ligato-vertex Si4. The
remaining 29Si NMR chemical shifts are located in the usual
range of saturated silicon atoms; only one of the signals
showing a small coupling to the Rh centre (21.8 ppm;
Si1–Si2 97.15(5).
Mechanistic considerations
2
9
103
J Si, Rh = 8.5 Hz). The anticipated structure of 4 as an Due to the flexibility of the coordination environments,
uncompromised benzpolarene scaffold covalently attached to reactions involving transition metals generally proceed
4
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Table 1 Selected 29Si NMR data of 2a-c, 3, 4
View Article Online
DOI: 10.1039/D0SC02861D
29
solid
Si2
 Si6
solid
29
 Si7
solid
[ppm]
32.9
-
29
2
9
29
29
29
29
29
Siliconoid
 Si2
[
 Si5
 Si6
 Si3
 Si4
 Si7
ppm]
[ppm]
[ppm]
[ppm]
[ppm]
[ppm]
[
ppm]
11.5
[ppm]
−41.6
−41.5
−42.2
54.1
2
2
a
b
13.3
12.8
12.2
158.8
162.6
55.3
52.6
50.6
−58.3
17.6
−38.4
−41.6
−42.1
58.3
−128.2
−90.8
−103.8
165.7
-
−126.8
−121.7
−120.2
-
32.9
-
-
108.7
48.2
12.3
10.2
157.3
161.8
2
c
-
3
4
107.7
42.7
21.8
−9.0
14.5
through multiple steps and, consequently, the mechanisms are to the Rh centre. The question of the re-establishment of the
often complicated, especially when backbone structures are intact hexasilabenzpolarene scaffold from intermediate 3 to
reconstructed such as in the present case through a sequence yield the final product 4 is even more daunting as it requires
of cleavage and formation of Si–Si bonds. Although, a the return of the previously eliminated CO ligand into the
computational treatment of such mechanisms is well out of coordination sphere of rhodium. Notably, a solution of
reach due to the anticipated complexity of the potential crystallized 3 turned out to be inert towards exposure to
energy surfaces and the large size of the involved molecules, monoxide CO atmosphere as well as the addition of excess
we propose a plausible mechanism for the Ir and Rh structures [(CO)
based on the structurally characterized products (Chart 1).
2
RhCl]
can only be attained by a reactive species formed in situ during
the reaction of 1a with [(CO) RhCl] . Indeed, treatment of a
2
. We therefore assumed that the formation of 4
2
2
Chart 1 Proposed mechanism of formation of 2a (top) and 3 (bottom).
solution of isolated crystals of 3 with 10 mol% of silylene-
substituted siliconoid 1a and 0.4 equivalents of additional
[(CO) RhCl] results in the uniform conversion of isolated 3 to 4
2 2
in the course of 24h. We speculate that either an extremely
short-lived intermediate of monomeric [(CO)
R
R
R
R
R
R
R
R
(1)
R
R
R
R
Cl
R
R
Cl
R
Cl
Si
Si
Ir
Si
20
Ir
Ir
3
RhCl] or a
L
L
L
heterodimer not unlike the one reported by Braunschweig et
2
1
al. might be responsible for the CO delivery.
R
R
R
R
R
L
R
R
R
R
R
R
(1)
(2)
R
Alkene isomerization catalysis
R
R
R
Despite the plethora of synthetic methods available for the
introduction of C–C double bonds, the regioisomerization of
an existing C–C double bond is a viable alternative. The so-
called alkene isomerization, however, results in mixtures of (E)
and (Z)-alkenes in many cases or further migration along a
saturated carbon chain. The selective transformation of
terminal alkenes to 2-alkenes has thus attracted considerable
interest.24 We anticipated that electron-rich siliconoid ligands
might fulfill two functions in a homogenous catalyst for alkene
isomerization: (a) act as an electron reservoir and thus
facilitate oxidative addition reactions and (b) provide sufficient
steric bulk to improve the selectivity regarding the number of
positions the C-C double bond migrates. We therefore probed
the isomerization of terminal alkenes to 2-alkenes in the
presence of catalytic quantities of complexes 2a-c, 3 and 4.
As first preliminary tests rapidly indicated, no solvent is
required for catalytic activity of 2a-c, 3 and 4 and therefore all
runs were carried out with neat substrate, an attractive
feature from both ecologic and economic perspectives.
Allyltrimethylsilane and 1-hexene were used as neat substrates
Cl
Rh
OC Cl
Rh
Si
R
OC
Si
Rh
OC
OC
OC
Cl
Si
3)
L
CO
L
22
(
23
R
R
Cl
R
R
R
R
R
R
(4)
R
R
Si
Si
R
Si
Cl Si
Si
Si
R
R
R
-CO
OC Rh
OC Cl
Rh
OC
Rh
OC
L
L
L
All reactions are likely initiated by the straightforward
coordination of the pending silylene ligand to the metal. For
the subsequent rearrangements, we suggest the oxidative
addition to the Si2–Si3 single bond as the common first step
(step 1 in Chart 1). From the second step onwards, however,
the isomerizations proceed through distinct pathways for the
rhodium and iridium species. In case of 2a, the oxidative
addition is directly followed by reductive elimination of the
2
chloro group and the SiTip moiety from the Ir centre resulting
in the formation of the exohedral chlorosilyl group in the final
product 2a (and by extension 2b,c). In contrast, in case of the
primary rhodium product 3 the chlorine migrates to Si3 (step
on an NMR scale using a C
were calculated from H NMR integrations (Table 2). Fig. 4
shows the spectroscopic conversion to 2-trans-hexene using
D
6 6
capillary as locking signal; yields
1
2
). In step 3, the formation of the chlorosilylene Si3 is
0
.05 mol % (2a), 0.1 mol% (2b, 2c) of the catalysts at room
temperature as a function of time. The side chain migration to
-trans-hexene is disregarded in the plots due to the overlap
suggested, while a formal double bond between Si1-Si4 is
formed as a consequence. In the last step, the final product 3
is formed by elimination of a CO unit enforced by the
coordination of the Si1–Si4 bond and the silylenes (Si3 and Si7)
3
of the chemical shifts, and estimated to be <10 % from the
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Journal Name
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View Article Online
DOI: 10.1039/D0SC02861D
Table 2 Reaction conditions, conversion determined by H NMR spectroscopy
Catalyst
Substrate
Cat. loading
Time
[h]
Temperature
[°C]
 conversion
E-Isomer
[%]
Z-Isomer
[%]
TON
TOF
[h ]
−
1
[mol %]
[%]
2
2
2
a
b
c
1-hexene
1-hexene
1-hexene
0.05
0.1
0.1
0.8
0.8
0.8
16
72
100
26
30
30
25
25
25
60
60
60
95
97
75
94
89
78
-
-
-
77
73
63
-
-
-
17
16
15
1902.7
968.8
748.4
117.4
111.6
97.4
118.9
13.5
7.5
4.5
3.7
2
2
a
b
allyl-SiMe
allyl-SiMe
3
3
2
c
allyl-SiMe3
3.2
spectroscopic data. Blind tests without catalyst or in the intramolecular hydrogen migration is supported through the
presence of [(cod)IrCl] or [(CO) RhCl] led to no detectable extremely electron-rich hexasilabenzpolarene scaffold.
2
2
2
conversion under identical conditions. The reaction can
therefore easily be quenched after the formation of 2-trans-
hexene is complete by simple addition of water leading to the
hydrolysis of the catalyst. The catalytic performance is best in
0
0
0
.8 mol % Si-Ir 2a, Conversion to vinyl-E-SiMe3
100
80
.8 mol % Ge-Ir 2b, Conversion to vinyl-E-SiMe3
−1
case of 2a (E = Si, TOF = 119 h ) with catalyst loadings as low
as 0.05 mol% and strongly decreases from 2b (E = Ge, TOF =
.8 mol % Sn-Ir 2c, Conversion to vinyl-E-SiMe3
−1
−1
1
3.5 h ) to 2c (E = Sn, TOF = 7.5 h ) both requiring double
catalyst loads. Preliminary results show a much lower catalytic
activity of the rhodium complexes 3 and 4 which was therefore
not investigated in detail (see SI). The isomerization of
allyltrimethylsilane to 2-E/Z-vinyltrimethylsilane proceeds
significantly slower even with the more active 2a-c and
requires higher temperatures as well as one order of
magnitude larger amounts of catalyst (0.8 mol %, 60 °C, Fig. 5,
SI).
60
4
0
0
0
2
0
5
10
15
20
25
30
35
time [h]
0.05 mol % Si-Ir 2a, Conversion to 2-trans-hexene
0.1 mol % Ge-Ir 2b, Conversion to 2-trans-hexene
0.1 mol % Sn-Ir 2c, Conversion to 2-trans-hexene
Fig. 5 Plot of the spectroscopically determined conversion to E-
vinyltrimethylsilane 60 °C using 0.8 mol% catalyst. black = Si-Ir 2a, red = Ge-Ir 2b,
blue = Sn-Ir 2c.
100
8
6
4
2
0
0
0
0
0
CONCLUSION
In conclusion, with the Group 9 metal complexes 2a-c and 3
we reported the first siliconoids with endohedral incorporation
of transition metals. As the isomerization of 3 to the 4 with
complete reconstitution of the uncompromised benzpolarene
scaffold demonstrates, a temporary change in coordination
mode of these ligands is possible in principle. All isolated
complexes show catalytic activity in the isomerization of
alkenes with the best (2a) reaching competitive selectivity at
satisfying conversion rates in case of 1-hexene.
0
10 20 30 40 50 60 70 80 90 100 110 120
time [h]
Fig. 4 Plot of the spectroscopically determined conversion to 2-trans-hexene.
black = Si-Ir 2a (0.05 mol%), red = Ge-Ir 2b (0.1 mol%), blue = Sn-Ir 2c (0.1 mol%).
Conflicts of interest
There are no conflicts to declare.
It should be noted, however, that the reaction at room
temperature is probably slowed down even further due to the
moderate solubility of the crystalline samples of the catalysts
2
a-c neat allyltrimethylsilane. This phenomenon is manifest in
Acknowledgements
Funding by the Deutsche Forschungsgemeinschaft (DFG
SCHE906/4-1 and 4-2) is gratefully acknowledged.
an extended induction period of approximately 5 h (Fig. 5)
after which the spectroscopic yield increases much faster due
to rapid dissolution in the mixture of allyltrimethylsilane and
the isomerization products.
The driving force of the alkene isomerization is the higher
2
5
thermodynamic stability of internal alkenes. As the negative
blind tests with the siliconoid-free precursors suggest, the
6
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3
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The unprecedented metallasiliconoids are readily accessible
from the silylene-substituted Si siliconoid and Group 9 metal
fragments. With the isomerization of terminal alkenes to 2-
alkenes, catalytic benchmark reaction of the
metallasiliconoids was investigated ( = silicon).
6
a
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