Site-selective functionalization of Si6R6 siliconoids

Article abstract of DOI:10.1039/c8sc05591b

The recent progress in the synthesis of partially substituted neutral silicon clusters (siliconoids) revealed unique structures and electronic anisotropies that are reminiscent of bulk and nano surfaces of silicon. Here, we report the selective 2-lithiati

Full text of DOI:10.1039/c8sc05591b

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January 2016 Pages 1–812
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Site-Selective Functionalization of Si R Siliconoids
6
6
§
§
Yannic Heider, Nadine E. Poitiers, Philipp Willmes, Kinga Leszczyńska, Volker Huch,
Received 00th January 20xx,
Accepted 00th January 20xx
and David Scheschkewitz*
The recent progress in the synthesis of partially substituted neutral silicon clusters (siliconoids) revealed unique structures
and electronic anisotropies that are reminiscent of bulk and nano surfaces of silicon. Here, we report the selective 2-
DOI: 10.1039/x0xx00000x
lithiation of the global minimum Si
6
R
6
siliconoid at a different vertex than in the previously reported isomeric 4-lithiated
). In order to enable an intuitive distinction of the vertices of the global minimum Si scaffold
which can be considered the silicon analogue of benzene in terms of thermodynamic stability), we introduce a novel
nomenclature in analogy to the ortho-meta-para nomenclature of disubstituted benzenes. By treatment of the 2-lithiated
i
www.rsc.org/
derivative (R = 2,4,6- Pr
3
C
6
H
2
6 6
R
(
t
Si
6
cluster with Me
3 4 3 2 2 2
SiCl, SiCl H B·SMe , (Me N) PCl as well as with carboxylic acid chlorides RCOCl (R = Bu, Ph) various 2-
functionalized Si
6
clusters were obtained and characterized in solution and – in most cases – the solid state. The structural
and spectroscopic effect of the position of the newly introduced functional group is discussed by comparison to the
corresponding 4-functionalized derivatives.
29
substituted yet exhibit a similarly wide dispersion of Si NMR
Introduction
Partially substituted neutral silicon clusters (siliconoids)
14,16-18,22,24
1
-4
shifts.
Very recently, the protonation of silicon Zintl
are
anions to partially H-substituted anionic siliconoids was reported
fleeting intermediates during the production of silicon from
molecular precursors and can typically only be detected in the gas
30,31
independently by the groups of Fässler and Gschwind/Korber.
5-9
We had previously described the reductive functionalization of the
phase . The synthesis of stable derivatives has attracted
considerable interest as the unsubstituted vertices of siliconoids are
reminiscent of the free valences at bulk and nano surfaces of
dismutational Si R isomer 1 to the anionic siliconoid 3Li as well as
6
6
its reactivity with several representative electrophiles of Groups 13
32,33
1
0-13
to 15.
silicon, the so-called "dangling bonds”.
Since the report on the
first stable siliconoid Si5R6 with one "naked" vertex in a
1
4
hemispheroidal coordination environment by one of us,
numerous examples have been prepared by the groups of Wiberg,
15
1
6
17,18
19
20
14,21-23
Breher, Kyushin,
Iwamoto, Fässler and ourselves.
The
22
23
Si6R6 isomers 1 and 2 are lower energy isomers of the
hypothetical hexasilabenzene and as such prime examples of the
often drastic differences between carbon and silicon (Scheme 1).
While of the known C H isomers, benzene is by far the lowest in
6
6
energy, the tricyclic 2 corresponds to the global minimum isomer of
Si H and can therefore be considered as the silicon analogue of
6
6
23,24
benzene on grounds of thermodynamic stability.
The functionalization of such clusters is a prerequisite for the
further development of their chemistry and ultimately the
application of their fascinating electronic properties in extended
materials. Compared to the related Zintl Anions of the heavier
32
Scheme 1: Synthesis of 3Li
and 4Li (this work) from
2
5-31
Group 14 elements,
which are (poly)anionic, completely
dismutational hexasilabenzene isomer 1 and from the global
unsubstituted deltahedral clusters, siliconoids are partially minimum isomer 2 (Tip = 2,4,6-triisopropylphenyl).
Herein, we show that reduction of the global minimum isomer 2
a.
under similar conditions selectively affords the regiomeric anionic
Krupp-Chair of General and Inorganic Chemistry, Saarland University, 66123
Saarbrücken, Germany.E-Mail: scheschkewitz@mx.uni-saarland.de
Si siliconoid 4Li instead of 3Li by cleavage of an aryl substituent in
6
Electronic Supplementary Information (ESI) available: CCDC 1877380 (5a), 1877381
5b), 1877378 4Li), 1877379 6a), 1877382 6c), 1877383 6f See
DOI: 10.1039/x0xx00000x
the 2-position of the bridged propellane scaffold (Scheme 1). In
order to account for the rapidly increasing number of species with
(
(
(
(
( )
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the thermodynamically favored bridged-propellane scaffold and (“naked”) bridgehead silicon atoms in 1,3-position, thVeielwigAartticolepOrnelifniex
unequivocally distinguish between the different vertices, we to the mono-substituted vertices (4,6-posDitOioI:n1)0.1b0o3n9d/Ce8dSCt0o55o9n1Be
propose a novel terminology for this structural motif, inspired by substituent each, the remoto prefix to the remote bridge in 5-
the
well-established
ortho-meta-para
nomenclature for position and the privo prefix to the characteristically deshielded
3
4-37
disubstituted benzenes.
The functionalization of 3Li and 4Li with ("deprived" of electrons) atom in 2-position.
selected electrophiles is shown to result in several sets of
regiomeric derivatives allowing for the systematic comparison of
the structural and spectroscopic consequences of the functional
group's position.
Chart 1: Proposed prefixes for unique assignment of vertices in
bridged propellane-type ("benzpolarene") siliconoids.
Results and Discussion
Nomenclature of Si Siliconoids
6
Structures with [1.1.1.]propellane motif have intrigued
38,39
experimentalists and theorists alike ever since the early 1970s,
because of their non-classical structure containing bridgehead
atoms in an umbrella-type hemispheroidal coordination
environment. The bonding situation between the bridgehead atoms
of [1.1.1]propellanes can be described by biradical or ionic
Functionalization in ligato position
In addition to the previously reported persilabenzpolarenes,
investigated two further reactions of siliconoid 3Li with
electrophiles. The novel ligato functionalized siliconoids 5a,b were
thus obtained by treatment of 3Li in benzene at room temperature
3
2,33
we
4
0-43
contributions to the electronic ground state
and was discussed
4
4,45
by Shaik et al. as a “charge-shift-bond”.
Li show a closely related structure having two propeller blades
The Si siliconoids 2 and
6
3
29
with Me SiCl and benzoyl chloride, respectively (Scheme 2). The
bridged by one SiTip moiety. Strongly deshielded Si NMR signals
3
2
29
reactions proceed quantitatively according to
spectroscopy. The siliconoids 5a,b were isolated as single crystals
Si NMR
had been explained by a cluster-like delocalization of the two
2
3
electrons in question. Electron density determinations of the Si6
and fully characterized by x-ray analysis, NMR spectroscopy, UV/Vis
siliconoid 2 confirmed the absence of direct bonding between the
46
47,48
(
Table 1) and by IR spectroscopy in case of the CO containing
bridgehead silicon vertices. For nomenclature purposes,
the
species.
Si scaffold is nonetheless formally considered as tetracyclic system
6
with a direct connection between the hemispheroidally coordinated
vertices that are depicted in the schemes as a dashed line.
Scheme 2: Synthesis of ligato-functionalized siliconoids 5a-f.
The high thermodynamic stability of Si siliconoid 2 as the alleged
t
6
Reagents: a) Me SiCl, b) PhCOCl, c) BuCOCl, d) ClP(NMe ) , e)
3
2 2
2
3,24
global minimum of the Si H potential energy surface
suggests a
considerable prevalence of this structural motif. This received first
corroboration by the successful synthesis of mixed Group 14 5f: E = BH .
6
6
SiCl , f) BH ·SMe . 5a: E = Me Si, 5b: E = COPh and the previously
4
3
2
3
3
2
t
reported 5c: E = CO Bu, 5d: E = P(NMe ) , 5e: E = SiCl ,
2
2
3
−
3
49
50
systems and a Sn derivative recently. Saturated variations of
6
the six-atom scaffold occur in numerous other species throughout
51-54
main group chemistry.
In the past, the tetracyclic core structure
has been variously referred to as “edge-capped trigonal
5
5
56
bipyramid” “doubly edge-bridged tetrahedron” or “bridged
2
3
propellane”. As these terminologies do not seem to do justice to
the ubiquity of the structural motif, we propose a novel term that
29
echoes the relationship to the iconic benzene molecule and – at the The Si NMR spectra of 5a,b show the typical distribution of
same time – takes into account the extraordinary polarization of 2 chemical shifts for ligato functionalized persilabenzpolarenes as
3
9,57-64
32,33
and related species.
We thus suggest the term recently reported by our group for 5c-f:
the two unsubstituted
29
“benzpolarene”– in analogy to benzvalene – for the tetracyclic bridgehead silicon atoms give rise to two Si signals in a range of
arrangement of vertices in the cluster core of 2 and 3Li. In addition, −257 to −280 ppm (Table 1). The C2v symmetry of the benzpolarene
we feel that the availability of the first Si6 siliconoid regiomers scaffold of 2 is lowered to C in the substituted cases. As we had
s
described herein requires a descriptive nomenclature not unlike the shown by VT NMR studies for some of the ligato-functionalized
32
well-established ortho, meta and para prefixes used for species, hindered rotation further reduces the symmetry so that
disubstituted benzenes. The prefixes thus proposed in the following the two seemingly identical nudo atoms become diastereotopic.
reflect the characteristic bonding situation of each vertex (Chart 1).
The latin words for “naked” (lat. nudus), “bonded” (lat. ligatus), deshielded with signals at 169.9 (5a) and 174.7 ppm (5b). The
remote” (lat. remotus) and “deprived” (lat. privus) served as surprisingly downfield shifted signals (for tetracoordinate silicon
inspirations. The nudo prefix is assigned to the unsubstituted atoms) had been rationalized by invoking magnetically induced
The resonances of the SiTip groups in privo position are strongly
2
“
2
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Figure 1. Structures of 5a,b in the solid state (thermal ellipsoids at 50% probability). Hydrogen atoms and co-crystallized pentane
omitted. a: E = SiMe , b: E = COPh.
3
2
9
cluster currents or – in a complementary manner – by the strong anionic Si species as confirmed by Si NMR spectroscopy. The six
LUMO contribution at this atom.
6
49,65,66
29
The Si NMR chemical shifts resonances show a similar chemical shift distribution as in 3Li, but
of the remaining cluster vertices are located in the typical range for with distinctly different values suggesting the functionalization had
saturated silicon atoms and vary only slightly with the introduced taken place in another position of the benzpolarene scaffold
functionality. The longest wavelength absorption bands in the (Table 1). The reduction product 4Li was fully characterized by x-ray
UV/vis are observed at max = 459 nm (5a) and 477 nm (5b). The diffraction on single crystals, NMR spectroscopy and UV/Vis
-
1
characteristic CO stretching mode in the IR of 5b at  = 1605 cm
compares well with that of 5c.
spectroscopy.
32,33
29
Single crystals were obtained in A strongly deshielded Si NMR signal at 267.9 ppm is significantly
5 % (5a) and 66 % (5b) yield and the structures confirmed by x-ray broadened (presumably due to coupling to the quadrupolar Li-
The distances between the nucleus) and only shows a cross-peak to the aromatic H atoms of a
unsubstituted bridgehead silicon atoms Si1 and Si3 are slightly single Tip ligand in the 2D H/ Si correlation spectrum. These
shorter than in the global minimum isomer, in line with the observations led us to conclude that the anionic functionality of 4Li
observations for the previously reported ligato functionalized is located at the tetracoordinate silicon atom in the privo position
7
4
67
diffraction studies (Figure 1).
1
29
23
32,33
29
persilabenzpolarenes 3Li and 5c-f.
The distance between the Si2. The Si chemical shift of 267.9 ppm is particularly remarkable
ligato positions Si4 and Si6 of 5a-f increases with decreasing as saturated silyl anions typically show resonances at much higher
6
9
distance between the nudo positions Si1 and Si3, presumably in field often deep in the negative ppm region. According to our
66
order to minimize strain (Table 1). Apparently, the variation of the previous calculations, the magnetically induced cluster currents
ligato functionality of the benzpolarene structures 5a-f directly circumvent the privo position and thus cause its pronounced
influences the bonding between the unsubstituted silicon atoms Si1 deshielding even in case of the peraryl-substituted benzpolarene 2.
and Si3.
On the basis of a complementary explanation referring to the
49
topology of the LUMO, this phenomenon is probably due to the
pronounced silylene character of the privo atom. The presence of a
directly attached electron-releasing substituent could lead to an
increased localization of the vacant p orbital in the privo position
and thus to the observed even more pronounced deshielding. The
Synthesis of privo lithiated siliconoid 4Li
The reduction of the dismutational isomer of hexasilabenzene 1 had
yielded the lithiated siliconoid with a benzpolarene scaffold 3Li and
thus a functionalized derivative of the Si H global minimum isomer
6
6
32,33
signals for the remoto SiTip and the two ligato SiTip units appear at
2
2
.
In order to probe the possible intermediacy of 2, its reduction

= 15.3 and 100.2 / −43.8 ppm, respectively. The reason for the
with lithium/naphthalene was investigated (Scheme 3).
large difference between the chemical shifts is unclear although the
electronic environments of the ligato atoms are certainly
dominated by their relative position to the anionic functionality and
the lithium counter cation in privo position. The unsubstituted
bridgehead silicon atoms in the nudo positions are apparently not
compromised by the reduction and give rise to two signals at the
usual high field at  = −222.2 and −231.4 ppm, comparable to the
corresponding signals of the ligato lithiated 3Li. The constitution of
the reduction product of benzpolarene 2 was finally proven as the
privo functionalized 4Li by x-ray diffraction on single crystals (Figure
Scheme 3: Synthesis of privo-lithiohexasilabenzpolarene 4Li by
reduction with 2 equivalents of Li/C H .
10 8
67
Treatment of 2 with 2.2 equivalents of Li/C H in Et O and thf 2).
10
8
2
indeed results in the complete and uniform conversion into a novel
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Table 1: Comparison of NMR spectroscopic and structure data of ligato functionalized siliconoids 5a-f and privo functionalized
View Article Online
siliconoids 6a-f.
DOI: 10.1039/C8SC05591B
Position
of E
Functional
Group (E)
Si1,Si3
[ppm]
Si2
Si4
max
∆ Si1-Si3
∆ Si4-Si6
a,67
Hemispheroidality 
Comp.
m
b
[ppm] [ppm] [nm]
[Å]
[Å]
[Å]
2
3
-
2
Tip
−274.2 174.6
−7.5
−66.8
−43.8
−3.7
473
364
468
459
477
475
475
460
475
2.7076(8)
2.9037
0.08
-
1.3535
1.2805
1.3078
1.3283
1.3333
1.3458
1.3498
1.3409
1.3153
1.3308
-
32
ligato
privo
ligato
ligato
ligato
ligato
ligato
ligato
privo
privo
privo
privo
privo
privo
3Li
4Li
5a
5b
5c
5d
5e
5f
Li
−230.9 152.2
2.5506(9)
3.2243
−
232.6
−222.2 267.9
231.4
−257.8 169.9
266.6
−263.0 174.7
279.0
−264.7 171.8
271.1
−256.0 168.7
261.4
−252.3 175.4
264.2
−257.3 161.2
265.0
−242.0 193.6
253.3
−268.5 166.2
271.1
−263.1 173.1
265.8
−246.0 186.5
256.1
−251.6 161.7
258.9
−243.3 237.3
255.6
Li
2.5562(10) 2.9171(11)
-
−
TMS
2.6176(5)
2.6598(9)
2.6430(6)
2.6231(5)
2.635(1)
2.620(1)
2.6118(6)
-
2.9643(8)
2.8854(8)
2.9095
2.9508
2.8920
2.9988
2.9482(6)
-
−0.04
0.34
0.27
0.18
0.48
−0.48
−0.04
0.34
0.27
0.18
0.48
−0.48
−
COPh
−26.5
−27.4
−33.8
12.9
−
t
32
CO Bu
−
3
2
P(NMe2)2
−
3
2
SiCl3
−
3
2
BH3
−4.8
−
6a
6b
6c
6d
6e
6f
TMS
−15.9 469
−
COPh
−16.2
−14.5
−16.9
−6.4
-
−
t
CO Bu
473
2.6350(5)
-
2.9641(7)
-
1.3439
-
−
P(NMe2)2
SiCl3
-
-
−
-
-
-
−
−
BH3
−28.8 454
2.6024(8)
2.9431(7)
1.3155
−
a
−
For substituents BH
Figure 3: B(OH)
3
and Tip no Hammett parameters are available. The Hammett parameters of similar compounds were used for the correlation plot in
and (C
The hemispheroidality  is the distance of a naked cluster vertex from the plane spanned by its three substituents. Its value is taken as a measure for the
−
3
−
3
for BH
6
H
5
2
-4-CHMe ) for the Tip substituent (see reference 68).
b
degree of hemispheroidality of the vertex. For a detailed explanation see reference 4.
Tip-substituted siliconoid 2 and the ligato functionalized siliconoids
5a-b. This shortening is tentatively attributed to delocalization of
the lone-pair of the anionic silicon vertex into cluster bonding
orbitals. The formation of the two regioisomeric derivatives is
predominantly a consequence of the different topologies of the
LUMOs of both the dismutational Si6R6 isomer 1 and the
benzpolarene isomer 2. The initial reduction plausibly occurs at the
unsubstituted vertices of the starting materials (A, D), which
22,23
provide dominant contributions to the respective LUMOs.
Other
important LUMO contributions are located precisely at the silicon
vertices to which the preferentially eliminated aryl groups are
Figure 2: Structure of 4Li in the solid state (thermal ellipsoids at bonded. The subsequent isomerizations are likely driven by the very
5
0% probability). Hydrogen atoms and co-crystallized solvent low energy of the benzpolarene scaffold (Scheme 4). The lithiated
molecules omitted.
regioisomer 3Li is formed due to a syn TipLi elimination (B) followed
by a cyclobutene-bicyclobutane rearrangement (C). In case of the
reduction of the benzpolarene isomer 2, we suggest an orbital- and
The distance between the bridgehead silicon atoms (Si1-Si3 strain-controlled TipLi elimination followed by a 1,2- migration of
2
.5562(10) Å) is similar to that in 3Li, but shorter than in the fully the lithium counteraction (F) to yield 4Li.
4
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Scheme 4: Mechanistic explanation of the regioselective positions and a strongly deshielded signal for the privoVieswiliAcroticnleaOtonlmine,
formation of 3Li and 4Li.
which carries one Tip substituent and the fuDnOctI:io1n0.a1l0g3r9o/Cu8pSEC0in55t9h1iBs
case.
Scheme 5: Synthesis of privo-functionalized siliconoids 6a-d.
t
Reagents: a) Me SiCl, b) PhCOCl, c) BuCOCl, d) ClP(NMe ) , e)
3
2 2
t
SiCl , f) BH ·SMe . 6a: E = Me Si, 6b: E = COPh, 6c: E = CO Bu, 6d: E
4
3
2
3
−
=
P(NMe ) , 6e: E = SiCl , 6f: E = BH .
2 2 3 3
The UV/vis spectra of the isolated products 6a,c,f and 4Li show the
position of the longest wavelength absorption maximum to strongly
depend on the substituents of the Si scaffold (max:; 6a 469 nm; 6c
6
4
73 nm; 6f 454 nm; 4Li 468 nm). As in case of the ligato
functionalized species, it can presumably be assigned to the vertical
HOMO-LUMO singlet excitation.
The distances between the bridgehead silicon atoms Si1-Si3 in the
crystal structure of 6a,c,f (6a 2.6118(6), 6c 2.6350(5), 6f 2.6024(6)
3
2,33
Å) are longer than in the ligato lithiated siliconoid 3Li
and privo
lithiated siliconoid 4Li, but slightly shorter than in the ligato
functionalized siliconoids 5a,c,f. This is in line with a more effective

donation in the privo position. While for siliconoids 5a-f, a
reciprocal interdependency between the distances of Si1-Si3 and
Si4-Si6 is observed, no such relationship is present in case of the
privo functionalized siliconoids 6a-f.
29
The Si NMR resonances of the privo silicon atom are strongly
influenced by the nature of the pending functionality. The signal is
shifted to higher field with increasing electron-withdrawing power
t
of the substituent: Li > BH > TMS > P(NMe ) > Tip > CO Bu > COPh
3
2 2
>
SiCl . This sequence correlates nicely with the Hammett
3
70-73
Functionalization in privo position
parameter 
m
,
which is based on the relative reaction kinetics
In order to evaluate the suitability of privo lithiated siliconoid 4Li as of a second substitution in the meta position of benzene relative to
nucleophilic transfer reagent for the intact unsaturated Si scaffold, the functionality in question.
6
t
we treated it with several electrophiles (Me SiCl, PhCOCl, BuCOCl,
3
ClP(NMe ) , SiCl , BH ·SMe ). Indeed, the corresponding privo Correlation with Hammett parameters 
substituted siliconoids 6a-f are obtained by straightforward Figure 4 shows the two correlations between the Si NMR chemical
combination of the reagents in toluene at room temperature shift at the privo position of compounds 2, 5a-f and 6a-f and the
2
2
4
3
2
2
9
(
Scheme 5).
Hammett parameter  drawn separately for the two synthetically
m
2
9
According to Si NMR spectra, the reactions lead to full conversion accessible positions of the functional group. The correlations with
of 4Li to the privo functionalized siliconoids 6a-f. Crystallization of the  Hammett parameter are similar, but slightly less satisfactory
p
6
a,c,f from concentrated hexane solutions affords single crystals in (see Supporting Information).
moderate to good yields (6a: 66% ; 6c: 27% ; 6f: 78%), which were The plot for the ligato functionalized compounds 5a-f (Figure 3, top)
2
29
fully characterized by multinuclear NMR spectroscopy, UV/Vis shows a linear relationship (R = 0.912). The response of the Si
67
spectroscopy and x-ray diffraction (Figure 3). In case of 6b,d,e, the chemical shift, however, is moderate as indicated in its range (160
reactions were only performed on the NMR scale and characterized to 180 ppm) and the resulting slope (m = 14.346 ppm). Electron-
2
9
by multinuclear NMR spectroscopy. The Si NMR data of the privo withdrawing substituents in ligato position result in a stronger
2
9
substituted species display a similarly wide dispersion in chemical deshielding of the privo atom in the Si NMR (5b,c,d,e) while
29
shifts as the corresponding ligato isomers. The Si NMR spectra of
electron-donating groups exert the opposite effect (5a,f). The m
6a-f thus show two signals in the high-field region for the nudo
value for the Tip substituent (red triangle in Figure 4) had to be
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Figure 3. Structures of 6a, 6c and 6f in the solid state (thermal ellipsoids at 50% probability). Hydrogen atoms and co-crystallized solvent
t
−
molecules omitted. a: E = TMS, c: E = CO Bu , f: E = BH .
3
2
9
correlation of the Hammett parameters with the Si chemical shifts
of the nudo silicon atoms Si1 and Si3 (Table 1).
In case of the privo functionalized benzpolarenes (6a-f), the
2
1
1
1
1
1
00
80
60
40
20
00
correlation of the Hammett parameters  is even better with a
m
2
very good linear dependency (R = 0.978). This is due to a markedly
stronger response than in case of ligato functionalization with a
slope of m = −79.76 ppm and a consequently larger chemical shift
range (160 to 240 ppm). The stronger influence of the functional
group is readily explained by its direct attachment to the silicon
atom in question (Si2) vs. an additional distance of two Si-Si bonds
in case of ligato functionalization. Remarkably, the slope of the
linear fit is negative proving a reciprocal relationship between the
electron-withdrawing strength of the substituent and its
deshielding effect in the privo position. We had shown previously
that the formal substitution of the nudo silicon atoms by
germanium or tin results in a pronounced deshielding of the privo
positions as well, which we tentatively rationalized by the strong
influence of the LUMO shape on the paramagnetic contribution to
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
Hammett parameter meta
2
60
40
20
00
80
60
40
20
00
2
2
2
1
1
1
1
1
4
9
the chemical shift.
In contrast to our findings, in the case of mono-substituted carbon-
based benzenes, the correlation of the Hammett parameter  with
p
74
the chemical shift of the para-carbon atom is known, i.e. the ring
atom opposite to the one carrying the functional group.
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
CONCLUSION
Si siliconoids 1 and 2 can be selectively reduced to yield derivatives
6
Hammett parameter meta
of the global minimum isomer of the Si H potential energy surface
with an anionic functionality at distinct vertices. In order to
distinguish between the different positions of the tricyclic Si6
6
6
29
Figure 4. Plots of the Hammett parameters m vs. Si chemical
shifts of the privo silicon vertices of ligato (5a-f; top) and privo
functionalized hexasilabenzpolarenes (6a-f; bottom). The data pair
marked with red triangles corresponds to the unfunctionalized
hexaaryl derivative 2.
scaffold, we propose
a nomenclature that refers to the
characteristic environment of the four conceivable symmetry-
independent positions: nudo, privo, ligato and remoto. The
68
2
approximated by that of C H -4-CHMe
and was therefore anisotropic electronic structure of the global minimum Si
H
6 6
6
5
disregarded for the linear fit. Surprisingly, there is no apparent scaffold is accounted for by the introduction of "benzpolarene" as
6
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unique name for this ever more frequently occurring structural 19 T. Iwamoto, N. Akasaka, S. Ishida, Nat. Commun. 2014, 5,
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5
353 (7 pages).
motif. The privo lithiated hexasilabenzpolarene is accessible by
reductive cleavage of one of the Tip groups of the perarylated
derivative, while the ligato lithiated isomer had been obtained from
the dismutational isomer previously. The privo derivative is shown
to be an equally suitable nucleophilic reagent for the transfer of the
uncompromised benzpolarene framework. The electronic influence
DOI: 10.1039/C8SC05591B
2
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Conflicts of interest
There are no conflicts to declare
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