Hypercoordinate diorganosilanes featuring an (ONO) tridentate ligand. A surprising equilibrium between penta- And tetracoordination

Article abstract of DOI:10.1515/znb-2007-0214

The reaction of the tridentate ligand-precursor molecule 1, o-HO-p-MeO-C₆H₃-C(Ph)=N-(o-C₆H₄)-OH, with diorganodichlorosilanes Me₂SiCl₂, Ph ₂SiCl₂ and PhAllSiCl₂

Full text of DOI:10.1515/znb-2007-0214

Hypercoordinate Diorganosilanes Featuring an ꢀONOꢁ Tridentate
Ligand. A Surprising Equilibrium Between Penta- and Tetracoordination
Jo¨rg Waglera and Erica Brendlerb
a Institut fu¨r Anorganische Chemie, Technische Universita¨t Bergakademie Freiberg,
Leipziger Str. 29, D-09596 Freiberg, Germany
b Institut fu¨r Analytische Chemie, Technische Universita¨t Bergakademie Freiberg,
Leipziger Str. 29, D-09596 Freiberg, Germany
Reprint requests to Dr. Jo¨rg Wagler. Fax: (+49) 3731 39 4058.
E-mail: joerg.wagler@chemie.tu-freiberg.de
Z. Naturforsch. 2007, 62b, 225 – 234; received June 23, 2006
The reaction of the tridentate ligand-precursor molecule 1, o-HO-p-MeO-C6H3-C(Ph)=N-(o-
C6H4)-OH, with diorganodichlorosilanes Me2SiCl2, Ph2SiCl2 and PhAllSiCl2 yields pentacoordi-
nate silicon complexes R,RꢂSi[o-O-p-MeO-C6H3-C(Ph)=N-(o-C6H4)-O] (2aꢂ: R, Rꢂ = Me; 2b: R,
Rꢂ = Ph; 2c: R = Ph, Rꢂ = All). 2b and 2c have pentacoordinate silicon atoms in the solid state as
well as in chloroform solution. Surprisingly, for 2a an isomer 2aꢂ with Si-tetracoordination is pre-
ferred in the solid state, while the Si-pentacoordinated isomer 2a may dominate in solution. The
hypercoordinate allylsilane 2c with a C=X → Si moiety is stable in solution and does not undergo a
1,3-allyl-shift-reaction.
Key words: Allyl, Chelate, Equilibrium, Hypercoordination, Silicon
Introduction
The chemistry of complexes with hypercoordinate
silicon atoms is interesting from many points of view.
Silanes with electron-withdrawing substituents (e. g.
Cl, F, CF3) in particular are likely to form hyperco-
ordinate complexes [1]. With chelating ligands coor-
dination numbers higher than 4 at the silicon atom
are favoured even if it is bearing only less electron-
withdrawing substituents. Although pentacoordinate
Si complexes with up to five carbon donor atoms have
already been synthesized by Kolomeitsev et al. [2] and
Lammertsma et al. [3], there is still a high interest
in the enhanced chemical reactivity and its relation-
ship to the structure of the hypercoordinate organosi-
lanes. Due to the increased coordination number of
the Si atom, the Si-C bonds may be significantly ac-
tivated and the complexes may be useful reagents in
organic synthesis. Palladium-catalyzed cross-coupling
reactions with difluorotriphenylsilicate [4], allyl shift
reactions with hypercoordinate allylsilanes as interme-
diates [5] and photochemically induced 1,3-organyl-
shift reactions [6] were reported (Scheme 1).
Scheme 1.
of research [7]. Recently, we reported the syntheses of
various hexacoordinate diorganosilanes with tetraden-
tate ꢀONNOꢁ-chelating salen-type ligands and Si-C
bond cleavage reactions [6, 8, 9]. The switch to an
ꢀONOꢁ-chelating tridentate ligand system led to new
insights into both the C=N → Si coordination and the
activation of the Si-C bond by hypercoordination.
Results and Discussion
The control of the Si atom’s coordination behav-
ior by subtle differences in electronic effects of the
Si-bonded organyl groups is also a challenging field me 2, top) was prepared from 2-hydroxy-4-methoxy-
The tridentate ligand-precursor molecule 1 (Sche-
0932–0776 / 07 / 0200–0225 $ 06.00 © 2007 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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226
J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
˚
Scheme 3. Bond lengths in A: a) 1.445(2), b) 1.422(2),
c) 1.373(2), d) 1.417(2), e) 1.367(2), f) 1.424(2), g) 1.433(2),
h) 1.294(1), i) 1.362(1), k) 1.323(1).
Scheme 2.
Scheme 4.
Fig. 1. Molecular structure of 1 (ORTEP plot with 50 % prob-
ability ellipsoids, carbon-bound hydrogen atoms omitted).
benzophenone and o-aminophenol. Its crystal structure
is presented in Fig. 1.
The acidic hydrogenatoms H1a and H2a were found
on residual electron density maps. H1a is situated in
a hydrogen bridge position between O1 and N1 in
closer proximity of the nitrogen atom. Since the donor
site of N1 is thus occupied, the O2-H2a bond is di-
rected away from it. In comparison with the bond
C15-O2 [1.357(1) A], the bond C1-O1 [1.294(1) A] is
quite short. The location of H1a close to N1, the short
bond C1-O1 as well as the alternating bond lenghts
from C1 to C7 (depicted in Scheme 3, left) suggest
a significant contribution of the o-quinoid structure
drawn in Scheme 3.
Fig. 2. Molecular structure of 2a (ORTEP plot with 30 %
probability ellipsoids, hydrogen atoms omitted).
icon atom, as well as its isomer 2aꢂ, in which the sili-
con atom is pentacoordinated (Scheme 4). Complex 2a
was isolated as an almost colorless crystalline solid
which proved to be the pure isomer with a tetracoor-
dinate Si-atom (29Si CP/MAS NMR: δ = −6.3 ppm,
single crystal X-ray diffraction: see Fig. 2). However,
this tetracoordination is not maintained in chloroform
solution, where an equilibrium between the two coor-
dination modes is established (Scheme 4).
˚
˚
Compound 1 was reacted with Me2SiCl2, Ph2SiCl2
and PhAllSiCl2 in the presence of triethylamine as a
supporting base to yield 2a / 2aꢂ, 2b and 2c, respec-
tively (Scheme 2, bottom).
In the crystal the molecules 2a are disordered in
1 : 1 ratio (see X-ray structure analyses section). Al-
though the quality of the structure 2a is notably lower
than those of 2b and 2c, many parts of the disor-
The reaction with dimethyldichlorosilane in thf dered molecules are well separated and some inter-
yields complex 2a, which exhibits a tetracoordinate sil- esting structural features can be discussed. Due to
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J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
Table 1. Data of crystal structure determination and refinement of 1, 2a, 2b and 2c.
227
Compound
1
2a
2b
2c
Empirical formula
T (K)
C20H17NO3
93(2)
monoclinic
P21/c
13.2801(8)
8.5246(5)
14.1406(8)
90
101.312(1)
90
C22H21NO3Si
93(2)
C32H25NO3Si
90(2)
monoclinic
P21/n
11.0142(3)
14.4717(4)
15.6984(5)
90
101.217(1)
90
C29H25NO3Si
296(2)
Crystal system
Space group
triclinic
triclinic
¯
¯
P1
P1
˚
a (A)
6.7311(6)
8.6034(3)
10.0242(3)
14.8781(5)
99.509(1)
106.328(1)
93.228(1)
2, 1207.27(7)
1.275, 0.129
488
˚
b (A)
11.6420(11)
12.6761(12)
90.062(5)
90.029(5)
98.585(5)
2, 982.21(16)
1.270, 0.141
396
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
Z, V (A )
4, 1569.72(16)
1.351, 0.091
672
4, 2454.43(12)
1.352, 0.132
1048
ρcalc (g cm−3), µ (mm−1
F(000)
)
Crystal size (mm3)
0.40×0.22×0.05
0.30×0.20×0.15
0.60×0.45×0.20
0.58×0.30×0.18
2θmax (◦)
56
51
80
55
Index ranges
−17 ≤ h ≤ 17,
−11 ≤ k ≤ 11,
−18 ≤ l ≤ 18
17573, 0.0298
3762
−8 ≤ h ≤ 8,
−14 ≤ k ≤ 13,
−15 ≤ l ≤ 12
8488, 0.0257
3609
−17 ≤ h ≤ 19,
−26 ≤ k ≤ 25,
−28 ≤ l ≤ 28
92085, 0.0255
15181
−11 ≤ h ≤ 11,
−12 ≤ k ≤ 8,
−19 ≤ l ≤ 19
14338, 0.0192
5500
Reflections collected, Rint
Independent reflections
Parameters
223
439
335
314
R Indices [I ≥ 2σ(I)]
R1 = 0.0369,
wR2 = 0.0942
R1 = 0.0481,
wR2 = 0.0993
0.328, −0.186
R1 = 0.0633,
wR2 = 0.1316
R1 = 0.1040,
wR2 = 0.1430
0.286, −0.261
R1 = 0.0361,
wR2 = 0.1098
R1 = 0.0445,
wR2 = 0.1154
1.033, −0.550
R1 = 0.0402,
wR2 = 0.1130
R1 = 0.0560,
wR2 = 0.1208
0.269, −199
R Indices (all data)
−3
˚
Largest diff. peak and hole (e A
)
the tetracoordination of the Si atom, the Si-O bonds nate silicon atom produces two 13C as well as 1H NMR
˚
˚
(Si1-O1 1.651(5) A, Si1-O3 1.658(6) A) are signifi- signals for the Si-bonded methyl groups while there is
cantly shorter than in 2b and 2c (compare Table 1). only one Si-CH3 signal for the species with the penta-
˚
The C=N bond (C7=N1 1.285(7) A) is slightly shorter coordinate Si atom. (In CDCl3 coincidence of the two
than in 2b and 2c. This is to be expected as in the ab- diastereotopic 1H (Si-CH3) NMR signals is achieved at
sence of Si-N coordination the C=N bond is strength- r. t., see experimental section.) In both the isomers with
ened. The lone pair of N1 appears to point out of the tetra- and some isomers with pentacoordinate Si atoms
molecule. Thus, the interaction of this lone pair with the Si-bonded methyl groups are diastereotopic. In
hydrogen bridge donors (e. g. a chloroform molecule) case of TBP coordination with an O-Si-O axis and
may support the Si-N dissociation with formation of equatorially situated Si-C bonds, the Si-CH3 groups
the tetracoordinate isomer 2a. The tetracoordination of are chemically equivalent. Such coordination has been
the Si atom in 2a in the solid state may also be due observed in a silicon complex of ligand 1 bearing Si-N,
to the lower solubility of this isomer. In order to prove Si-CH3 and Si-Si bonds in equatorial positions [11]
the hypothesis that chloroform supports the configura- and may therefore be taken into account as one rea-
1
tion with a tetracoordinate Si atom as it has been found son for the emerging of only one H and 13C NMR
in another case of (imine)N→ Si coordination equi- Si-CH3 signal for the component 2aꢂ. The presence
librium [10], the 29Si NMR spectrum of 2a was also of other arrangements of the five donor atoms to-
recorded in toluene-d8 and dmso-d6 at ambient temper- gether with rapid configurational exchange, however,
ature. The concentration ratio tetra- : pentacoordinate can also be taken into account. The configurational ex-
isomer was found to be solvent dependent: 1.7 in change of each of the isomers with the tetracoordi-
CDCl3, 0.9 in toluene-d8, 3.0 in dmso-d6. The high nate silicon atom seems to be slow on the NMR time
content of the tetracoordinate isomer in dmso-solution scale.
indicates the domination of this isomer in polar sol-
vents even if they are not hydrogen bond donors. Sur-
prisingly, in each case the isomer with the tetracoordi-
For a closer insight into the equilibrium between the
isomers 2a and 2aꢂ, 1H and 29Si(INEPT) NMR spectra
of a solution of 2a in toluene-d8 were recorded at vari-
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228
J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
Fig. 5. Linearized plot of ln(K) vs. 1/T for the equilibrium
between 2aꢂ with penta- and its isomer 2a with tetracoordi-
nate Si atom.
Fig. 3. Stack plot of the 1H NMR spectra (methyl group sig-
nals) of 2a / 2aꢂ in toluene-d8 at various temperatures (from
front to back [◦C]: −30, −20, −10, 5, 20, 30, 40, 50, 60).
lnK vs. 1/T plot (y = −764.8x+2.569,Fig. 5). A strik-
ing kink in the lnK vs. 1/T plot at about 5 ◦C suggests
that below this temperature level the equilibration pro-
cess is disturbed by a ◦kinetic barrier. (Thus, only the
◦
data from 5 C to 60 C were taken into account for
thermodynamic analysis of the equilibration processes
between the isomers with tetra- and pentacoordinate
Si-atom.)
The thermodynamic data obtained (2aꢂ → 2a disso-
ciation: ∆H = 6.36 kJ/mol, ∆S = 21.4 J/molK) indi-
cate a small enthalpy gain (−6.36 kJ/mol) by N → Si
bond formation. This value is much smaller than the
−31.5 kJ/mol which were found for complexation of
the Si atom with a bidentate 2-iminomethylphenolate
ligand and an ꢀOOꢂNC(sp2)ꢁ-donor situation at the
Si-atom [10]. The N → Si bond dissociation in 2a
seems to be facilitated by the special bonding pat-
tern of ligand 1. The phenyl group of the 2-
oxybenzophenoneimine provides a π-electron system,
which may interact with the imine double bond’s
π-electron system only in the tetracoordinate silicon
complex (see Fig. 2, top, C7=N1 in conjugation with
phenyl C8 - C13) but not in a pentacoordinate Si-com-
plex (compare with Figs. 6 and 7). Finally, the lower
Lewis acidity of the Si centre with its two electron rich
methyl substituents weakens any additional N → Si
donor action. The entropy change (21.4 J/molK for
the Si-N dissociation reaction) appears to be associ-
ated with the relatively rigid chelate systems in both
isomers.
Fig. 4. Stack plot of 29Si INEPT NMR spectra of 2a / 2aꢂ
in toluene-d8 at various temperatures. Section between −14
and −46 ppm omitted for clarity, from front to back [◦C]:
−30, −20, −10, 5, 20, 30, 40, 50, 60.
ous temperatures (Figs. 3 and 4). The 1H NMR signals
of the Si-bonded methyl groups of the tetra- and penta-
coordinate isomers are well separated in the spectrum
and can be used to determine the concentration ratio
of the isomers. One of the two singlets of the tetraco-
1
ordinate isomer changes its H NMR shift in such a
way that coincidence of the singlets is reached at 5 ◦C.
At higher temperatures, however, these signals sepa-
rate again, indicating that coalescence has not been
achieved.
◦
Beside the kink in the lnK vs. 1/T plot at 5 C,
Assuming an equilibrium as given in Scheme 4, two other notable features of this equilibrium are the
lnK vs. 1/T were plotted and ∆H and ∆S for the coincidence of the Si-CH3 ◦1H NMR signals of the
dissociation were estimated from the equation lnK = tetracoordinate isomer at 5 C as well as the differ-
−(∆H/R · 1/T) + (∆S/R) and the linearization of the ent shift rates ∆δ29Si of the signals which represent
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J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
229
nal for all species featuring this coordination number.
The rapid exchange processes of pentacoordinate sili-
con complexes have already been mentioned. The ex-
change between tetra- and pentacoordination,however,
is slow and gives rise to two distinct signals, one for
each coordination number.
The rapid exchange between 2a(E) and 2a(Z)
(with 2a(E) favored at the higher temperature level)
can account for the shift of their 29Si NMR signals
to higher field at higher temperatures: In 2a(E) the
tetrahedral coordination sphere of the Si-atom may be
N-capped with the 29Si nucleus better shielded.
The isomers with tetracoordinate Si-atom represent
macrocycles (9-membered ring systems). Both of them
bear diastereotopic Si-CH3 groups, which may equi-
librate by ring inversion processes to achieve chem-
ical equivalence on the NMR time scale. These ex-
change processes seem to be NMR-slow, thus lead-
ing to two 1H and 13C NMR resonances, respec-
tively, for the isomers with tetracoordinate Si-atom.
Temperature dependent changes in the molar frac-
tions of rapidly exchanging isomers 2a(E) and 2a(Z)
Scheme 5.
tetracoordinate (∆δ29Si = −0.030 to −0.018 ppmK−1
)
and pentacoordinate silicon complexes (∆δ29Si =
0.048 ppmK−1, steady shift). The latter two features
indicate the superposition of more than one configura-
tional exchange processes. Both 29Si NMR signals ex-
hibit temperature dependent shifts. For slow exchange
processes between two species only a change in in-
tensities is to be expected [7b, c]. An explanation of
the observations can be that both coordination modes
include more than one isomer which undergo rapid
interconversion and lead to one representative signal
for each coordination number. The chemical shift of
each of these signals is then defined by the ratio of the
contributions from the rapidly interconverting diastere-
omers [10, 12]. A comparison between the molecular
structures of 2a and 2b, 2c (Figs. 6 and 7) reveals that
the C=N moiety of the tridentate ligand system has
Z configuration in 2a and E configuration in the penta-
coordinate Si-complexes 2b and 2c. Therefore, one can
discuss an isomer 2a(E) (featuring a tetracoordinate
Si-atom and E configuration of the imine system) as an
intermediate between the tetracoordinate silicon com-
plex 2a(Z) and the pentacoordinate species 2aꢂ. The
temperature dependent 1H and 29Si NMR properties of
the solution of 2a/2aꢂ can be explained assuming the
equilibria which are depicted in Scheme 5 as a simple
model (superposition of additional exchange processes
can not be excluded).
1
could cause shifts of the weighted avarage H reso-
1
nances. The assumption that the H resonance of one
of the diastereotopic Si-CH3 groups is much more in-
fluenced by the E/Z isomerization than that of the other
Si-CH3 group provides an explanation for the striking
change of one of the 1H NMR signals beyond coinci-
dence.
Taking into account that all isomerization processes
among the isomers with tetracooordinate Si-atom are
rapid on the NMR time scale (1 29Si signal only!), slow
ring inversion, which leads to identical species with di-
astereotopic Si-CH3 groups, is the only process which
1
can cause the emerging of two H and 13C NMR sig-
nals for the Si-CH3 groups of these tetracoordinate sil-
icon compounds. According to the slow and rapid iso-
merization processes in our model (Scheme 5), this
ring inversion may occur via the pentacoordinate sil-
icon complex 2aꢂ, meaning that the rapid transitions
from 2a(Z) to 2a(E) and back must be accompanied
by the return of the diastereotopic Si-CH3 groups into
their initial positions exclusively.
The 29Si NMR signal of the pentacoordinate species
also depends on the temperature (Fig. 4). Increasing
temperature causes a shift of the 29Si NMR signals to-
Interconversions between the potential isomers with wards each other (as◦well as a discernible broadening
tetracoordinate Si-atom (E/Z-isomerization at the C=N of the signals at 60 C, however far away from coa-
moiety) must be rapid on the NMR time scale, thus lescence). This observation as well as the shift of the
leading to only one representative 29Si NMR sig- 1H NMR signal of the Si-CH3 protons in the pentaco-
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230
J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
˚
Scheme 6. Bond lengths in A: a) 1.408(1), b) 1.401(1),
c) 1.394(1), d) 1.406(1), e) 1.374(1), f) 1.413(1), g) 1.447(1),
h) 1.349(1), i) 1.353(1), k) 1.307(1).
electronegativity of the carbon atom gives rise to a less
Lewis-acidic σ*-MO which is expected to be essen-
tial for the interaction with the trans-situated donor
Fig. 6. Molecular structure of 2b (ORTEP plot with 50 %
probability ellipsoids, hydrogen atoms omitted).
˚
atom. The Si-C bonds in 2b (Si1-C21 1.878(1) A,
ordinate isomer(s) to lower field at lower temperatures
can be explained with a rapid interconversion of two
or more pentacoordinate diastereomers (as depicted in
Scheme 5) or increasing N → Si coordination strength
at lower temperature levels. Both, the favored forma-
tion of TBP shaped silicon complexes bearing an ax-
ially situated Si-CH3 group as well as the shortening
of the N → Si bond distance, which are accompanied
with a lengthening of the average Si-C bond distance,
could result in a better shielding of the 29Si nucleus as
well as decreased shielding of the Si-CH3 protons at
lower temperatures. Thus, one can not exclude one of
these possibilities.
˚
Si1-C27 1.906(1) A), however, are significantly shorter
than those of the hexacoordinate diorganosilanes men-
˚
tioned above (ca. 1.93 – 1.97 A). A slight but signifi-
cant difference in the distances Si1-C21 and Si1-C27
is based on their different location in the Si coordina-
tion sphere.
Surprisingly, the Si-O bonds (Si1-O1 and
˚
Si1-O2 1.700(1) A) have the same length, although O1
is part of a six- and O2 is part of a five-membered
chelate.
Referring to the interesting bonding situation in the
free ligand acid 1 (see Scheme 3), the analogous C-C
bonds are less alternating in the silicon complex 2b
Different coordination behavior was found with 2b
and 2c. 2b, recrystallized from thf, forms light
yellowish-orange colored crystals suitable for X-ray
analysis (Fig. 6).
˚
(Scheme 6, left) and the bonds C1-O1 (1.349(1) A)
˚
as well as C7-N1 (1.307(1) A) are also lengthened
and shortened, respectively. This result allows the
conclusion that the oxophilic silicon atom reduces
the o-quinoid character of the ligand illustrated in
Scheme 3. In chloroform solution the pentacoordi-
nation of the Si atom of 2b is retained (δ29Si =
−84.6 ppm).
The silicon atom is situated in a coordination geom-
etry which is almost midway between trigonal bipyra-
mid (TBP) and square pyramid (SQP): 47 % TBP
with N1 and C27 in axial positions and the oxygen
atoms O1 and O2 in equatorial positions. C21 can
be referred to either as part of the equator of a TBP
A solution of 2b in THF was irradiated with UV
light for 6 h (reaction conditions see ref. [6]) but
no indication for Si-C bond cleavage and 1,3-shift
of a phenyl group was found. The Si-C bond
˚
or as the top of the SQP. The Si-N bond 2.107(1) A
is quite long in comparison with Si-N(sp2) bonds in
hexacoordinate diorganosilanes with salen-like ligands
˚
˚
Si1-C27 (1.906(1) A) is significantly longer than the
(ca. 1.98– 2.00 A, see [8]). The decrease in coordi-
Si-C bonds in some previously published pentacoordi-
nate Si complexes with salen-like ligands. This slight
plus in Si-C bond activation appears to be not enough
to allow any UV-stimulated organylation of the imine
carbon atom.
nation number should be accompanied by a decrease
in bond lengths to the same sort of atoms. The rea-
son for the contrasting behavior in case of 2b can be
found if the relative position of the Si-N bond is taken
into account. While in hexacoordinate diorganosi-
lanes with salen-type ligands the Si-N bonds are
Compound 2c was prepared in a similar manner.
trans to Si-O bonds, the Si-N bond in 2b is trans Suitable crystals for X-ray analysis were obtained from
(N1-Si1-C27 165.6(1)◦) to a Si-C bond. The lower diethylether/pentane (Fig. 7).
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J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
231
◦
˚
Table 2. Selected bond lengths [A] and angles [ ] of 2b
and 2c.
2b
2c
Si1-O1
Si1-O2
Si1-N1
Si1-C21
Si1-C27
C7-N1
1.700(1)
1.700(1)
2.107(1)
1.878(1)
1.906(1)
1.307(1)
1.349(1)
165.6(1)
137.2(1)
111.2(1)
108.9(1)
1.711(1)
1.713(1)
2.058(1)
1.875(2)
1.907(2)
1.312(2)
1.341(2)
160.1(1)
142.7(1)
107.3(1)
108.3(1)
C1-O1
N1-Si1-C27
O1-Si1-O2
O1-Si1-C21
O2-Si1-C21
Scheme 7.
the crystal may be due to reasons of molecular pack-
ing. To the best of our knowledge, this is the only
structurally confirmed example of a hypercoordinate
Si-complex with N → Si donor action in trans-position
to an sp3-hybridized monodentate carbon substituent
in the presence of a monodentate sp2-hybridized car-
bon substituent so far. Due to the position of the donor
atoms N1 and C27 in a SQP geometry, the bond length
˚
Si1-N1 (2.058(2) A) is significantly shorter than the
˚
Si-N bond in 2b (2.107(1)A). The opposite Si-C bonds
(Si1-C27 in both cases) are not influenced by this mod-
ified donor situation. The other features of the molecu-
lar structure of 2c are similar to those of 2b (e. g. sim-
ilar bond lenghts Si1-O1 and Si1-O2 and throughout
the tridentate ligand system as depicted in Scheme 6).
1H, 13C and 29Si NMR spectra of 2c were recorded.
Fig. 7. Molecular structure of 2c (ORTEP plot with 30 %
probability ellipsoids, hydrogen atoms omitted). The allyl
group is disordered over two sites (ratio 9 : 1). Only the dom-
inating part is depicted.
Analogous to the situation in 2b, the Si atom of 2c
is also situated in a coordination geometry on the The intense peak at −76.2 ppm in the 29Si NMR spec-
way from TBP to SQP, but closer to a square pyra- trum indicates the presence of the pentacoordinate sil-
midal arrangement of the donor atoms (O1, O2, N1 icon complex also in solution. In comparison with the
and C27 in the base and C21 on top of the pyra- spectrum of 2b, the significant low field shift mainly
mid; 29 % TBP, compare Table 2). This transition to- originates from the substitution of an sp2-hybridized
wards an SQP coordination geometry is mainly caused for an sp3-hybridized Si-bound carbon atom. This find-
by changes in the angles O1-Si1-O2, O1-Si1-C21 ing allows the conclusion that the Si-C bond in penta-
and N1-Si1-C27 (see Table 2). Surprisingly, the sp3- coordinate allylsilanes with ꢀONOꢁ-chelating ligands
hybridized carbon atom C27 is located opposite to the is not activated enough for a rearrangement to an imine
imine N atom N1, which causes a slightly longer Si-C- carbon atom as reported for tetradentate ꢀONNOꢁ-
2
bond Si1-C27 (1.907(2) A) than to the sp -hybridized chelating ligands (Scheme 7) [9]. The 13C NMR spec-
˚
˚
phenyl carbon atom (Si1-C21: 1.875(2) A). Gener- trum of 2c consists of only 25 signals. As previ-
ally a carbon atom of a phenyl moiety has a higher ously reported [6], the phenyl rotation in oxybenzophe-
electronegativity than an sp3-hybridized allyl carbon noneimine ligands is slow on the NMR time scale.
atom. Therefore, an interaction between the lone pair Therefore, diastereotopic o- and m-carbon atoms of
of imine nitrogen atom N1 and the σ* orbital of the this group are expected in 2c. They should give rise to a
bond Si1-C21 should be preferred. The arrangement in sum of 27 13C NMR signals. The chemical identity of
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232
J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
these o- and m-carbon atoms is conditioned by either and stored over sodium wire. Solution NMR spectra were
recorded on a BRUKER DPX 400 instrument (CDCl3 so-
lutions with TMS as internal standard), solid state spectra
on a BRUKER Avance 400WB spectrometer using a 7 mm
CP/MAS probehead (νspin = 5 kHz). Elemental analyses
were carried out on a Foss Heraeus CHN-O-Rapid instru-
ment.
a rapid configurational inversion around the Si atom
in 2c or by coincidence of signals.
So far, only few examples of hypercoordinate sil-
icon complexes with ꢀONOꢁ-chelating ligands of the
2-iminomethylenolate type were reported. Pentacoor-
dinate Si complexes derived from the N-o-hydroxy-
phenylimine of acetylacetone, the structures of which
were characterized by Tacke et al., exhibit TBP config-
urations with the N atom of the tridentate ligand in ax-
ial position [13]. Another kind of ꢀONOꢁ-chelating lig-
ands, pyridine-2,6-dimethanols and -diethanols, have
also been reported to form pentacoordinate diorganosi-
lanes with more or less intense N → Si donor action
and trigonal bipyramidal coordination [14].
1: Procedure following a literature method [15]: In
dry ethanol (90 mL) 2-hydroxy-4-methoxybenzophenone
(22.6 g, 99.3 mmol) and 2-aminophenol (10.8 g, 99.3 mmol)
were dissolved under reflux and piperidine (8.76 g,
103 mmol) as well as triethylorthoformate (11.7 g,
79.1 mmol) were added. The resulting mixture was re-
fluxed for 24 h, then the solvent was distilled off and the
residue was recrystallized from isopropanol (100 mL) to
yield an orange crystalline powder which◦was filtered off,
washed with isopropanol and dr◦ied at 70 C. Yield: 12.5 g
(39.4 %, 39.1 mmol), m. p. 190 C. – 1H NMR (400 MHz,
CDCl3): δ = 3.82 (s, 3H, O-CH3), 5.51 (s, 1H, ar OH),
6.30 – 7.40 (12H, ar), 14.26 (s, 1H, ar OH). – 13C NMR
(101 MHz, CDCl3): δ = 55.5 (O-CH3), 101.4, 106.7, 113.6,
115.3, 119.9, 122.6, 126.2, 128.3, 128.6, 129.4, 133.6, 133.9,
134.1 (ar), 149.0, 164.4, 165.9, 175.5 (ar C-O, C=N). –
C20H17NO3 (319.36): calcd. C 75.22, H 5.37, N 4.39; found
C 75.21, H 5.12, N 4.45.
Conclusions
The complexation of various diorganodichlorosi-
lanes with the tridentate ꢀONOꢁ-ligand 1 allows new
insights into the reactivity of hypercoordinate silicon
compounds. Pentacoordinate Si complexes exhibit in-
creased reactivities due to both the higher coordina-
tion number of the Si atom as well as its vacancy for
additional donor atoms [1]. Regarding intramolecular
rearrangements such as 1,3-shifts of organyl groups,
a line could be drawn between highly reactive hexa-
coordinate Si complexes with a tetradentate ꢀONNOꢁ-
ligand system, and their inert counterparts with a tri-
dentate ꢀONOꢁ-ligand and a pentacoordinate silicon
2a / (2aꢂ): In thf (30 mL) ligand 1 (2.00 g, 6.25 mmol) and
triethylamine (1.90 g, 18.8 mmol) were stirred at −10 ◦C and
a solution of dimethyldichlorosilane (0.85 g, 6.59 mmol) in
thf (20 mL) was added dropwise within 15 min. The resulting
orange mixture was stirred at −10 ◦C for further 15 min, then
the triethylamine hydrochloride was filtered off and washed
atom. The characterization of Si complexes with lig- with thf (20 mL). The volatiles were removed from the fil-
trate under reduced pressure. Then the oily residue was dis-
and 1 illustrates the coordination behavior of this kind
of chelating agents which leads to a coordination equi-
librium between penta- and tetracoordinate Si-com-
plexes in solution. An isomer with the lower Si coor-
dination number is found in the solid state. Coordina-
tion motifs in the pentacoordinate silicon complexes
are midway between TBP and SQP. A correlation be-
tween increasing TBP character of the Si coordina-
tion with increasing electronegativity of the “axially”
situated monodentate substituent was found for com-
plexes 2b and 2c as well as in structurally related com-
pounds [11].
solved in chloroform (1.5 mL) and n-hexane (4 mL) was
◦
added. This solution was stored at 8 C for 10 d, then the
colorless crystalline product 2a was filtered off and washed
with hexane (5◦mL) and dried in a vacuum. The filtrate was
stored at −18 C to yield a second crop of colorless crys-
◦
tals of 2a. Yield: 1.05 g (2.80 mmol, 45 %), m. p. 144 C.
–
1H NMR (400 MHz, CDCl3) selected data of the tetra-
coordinate isomer 2a: δ = 0.33 (s, 6H, Si-CH3), 3.72 (s,
3H, O-CH3), selected data of the pentacoordinate isomer 2aꢂ:
δ = 0.27 (s, 6H, Si-CH3), 3.85 (s, 3H, O-CH3). – 13C NMR
(101 MHz, CDCl3) selected data of the tetracoordinate iso-
mer 2a: δ = −3.6, −2.5 (Si-CH3), 55.2 (O-CH3), selected
data of the pentacoordinate isomer 2aꢂ: δ = 2.8 (Si-CH3),
55.6 (O-CH3). – 29Si NMR (79 MHz, CDCl3): δ = −7.3
(2a), −52.4 (2aꢂ) (79 MHz, CP/MAS): δ = −6.3 (2a). –
C22H21NO3Si (375.50): calcd. C 70.37, H 5.64, N 3.73;
found C 70.67, H 5.91, N 4.05.
Experimental Section
Commercially available reagents were used. All manip-
ulations were carried out under an inert atmosphere of dry
argon. Triethylamine was distilled from calcium hydride and
˚
stored over molecular sieve 3 A. Chloroform (stabilized with
2b: In thf (100 mL) ligand 1 (4.00 g, 12.5 mmol) and
amylene) was dried over molecular sieve 3 A. Ether, thf, hex- triethylamine (3.80 g, 37.6 mmol) were stirred at r. t. and
˚
ane and pentane were distilled from sodium/benzophenone diphenyldichlorosilane (3.25 g, 12.85 mmol) was added
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J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
233
◦
dropwise. The resulting orange mixture was stored at 8 C ment for all reflections against F2 with SHELXL-97). All
overnight. Then the precipitated hydrochloride was filtered non-hydrogen atoms were refined anisotropically. Hydrogen
off and washed with thf (30 mL). 100 mL of the volatiles atoms were placed in idealized positions and refined isotrop-
were removed from the filtrate under reduced pressure and ically (riding model). Hydrogen atoms of OH and NH groups
the remaining solution was allowed to stand overnight to were found by residual electron density and refined isotropi-
yield orange crystals of 2b which were filtered off, washed
cally.
In the structure of 2a the space groups P21/m (Z = 2, vir-
with thf (7 mL) and dried in a vacuum. Yield: 3.55 g
◦
(7.11 mmol, 57 %), m. p. 234 C. – 1H NMR (400 MHz,
tual bisecting plane in the two-molecules) and P21 (Z = 2,
twofold screw axis, the same kind of disorder) were simu-
lated. P21/m together with Z = 2 is impossible due to the
absence of a bisecting plane within molecule 2a. In order
to account for Z = 2 and the missing anomalous diffraction,
the structure was solved and refined in the triclinic space
3
CDCl3): δ = 3.87 (s, 3H, O-CH3), 5.77 (d, 1H, ar, JHH
=
8.0 Hz), 6.35 – 6.45 (m, 2H, ar), 6.74 (d, 1H, 3JHH = 9.2 Hz),
4
6.81 (d, 1H, ar, JHH = 2.4 Hz), 6.95 – 7.50 (mm, 17H,
ar). – 13C NMR (101 MHz, CDCl3): δ = 55.7 (O-CH3),
104.8, 109.2, 116.3, 116.5, 119.6, 120.3, 127.2, 127.3, 128.1,
129.0, 129.5, 129.9, 132.6, 133.5, 134.0, 135.6, 142.4 (ar),
154.1, 161.0, 165.5, 166.1 (ar C-O, C=N). – 29Si NMR
(79 MHz, CDCl3): δ = −84.6. – C32H25NO3Si (499.64):
calcd. C 76.93, H 5.04, N 2.80; found C 76.29, H 5.25,
N 2.91.
¯
group P1. The diffraction pattern exhibited weak reflections
with non-integer h-indices. Therefore, structure solution and
refinement was also tried using a cell with doubled a axis.
Attempts to refine the structure in other space groups than
¯
P1 (e. g. using the initial unit cell in space group P21 or us-
2c: In thf (40 mL) ligand 1 (1.0 g, 3.1 mmol) and tri-
ethylamine (0.75 g, 7.4 mmol) were stirred at r. t. and al-
lylphenyldichlorosilane (0.70 g, 3.2 mmol) was added drop-
wise. The resulting orange mixture was stored at 8 ◦C for 2 h.
Then the precipitated hydrochloride was filtered off and
washed with thf (5 mL). The volatiles were removed from
the filtrate under reduced pressure and the remaining orange
solid was dissolved in diethyl ether (5 mL). This solution
was filtered and pentane (3 mL) was added to the clear fil-
trate. Within 1 week at 8 ◦C orange crystals of 2c had formed.
The mother liquor was removed with a syringe and the crys-
tals were dried in a vacuum. Yield: 0.58 g (1.3 mmol, 42 %),
m. p. 124 C. – H NMR (400 MHz, CDCl3): δ = 1.94 (m,
2H, Si-CH2), 3.88 (s, 3H, O-CH3), 4.70 (m, 2H, -CH=CH2),
5.79 (d, 1H, ar, 3JHH = 8.4 Hz), 5.92 (m, 1H, CH2-CH=CH2),
6.30-7.60 (mm, 16H, ar). – 13C NMR (101 MHz, CDCl3):
δ = 28.6 (Si-CH2), 55.7 (O-CH3), 104.8, 109.0, 112.1, 116.1,
116.7, 119.3, 120.2, 127.2, 127.4, 128.2, 129.0, 129.5, 129.9,
132.5, 133.4 (2×), 135.7, 137.2, 141.2 (ar, CH2-CH=CH2),
154.2, 161.2, 165.5, 166.0 (ar C-O, C=N). – 29Si NMR
(79 MHz, CDCl3): δ = −76.2. – C29H25NO3Si (463.61):
calcd. C 75.13, H 5.44, N 3.02; found C 74.99, H 5.43,
N 2.99.
ing a doubled unit cell and space groups Pc or P21/c) led
to the same result of molecular disorder which could not be
resolved by inclusion of racemic twinning in case of the non-
centrosymmetric space groups. The diffraction pattern exhib-
ited only very broad reflections which underline the pres-
ence of disturbances such as heavy disorder in the crystal,
which make determination of the correct space group impos-
sible. The disorder in the structure of 2a was resolved us-
ing numerous restraints [16]: SAME was applied for frag-
ments [O1, C1-C6], [O3, C15-C20], [N1, C7-C13] to fit
the corresponding fragments of the disordered counterpart;
bond lengths Si1-C21, Si1-C22 were restrained to the same
lengths as in the corresponding counterpart and the aromatic
bond lengths in [C8-C13] were restrained to be identical us-
ing SADI. Identical anisotropic displacement parameters for
atoms in similar positions were applied.
Selected data of structure determination and refinement
are presented in Table 1. Crystallographic data (excluding
structure factors) for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-611921
(1), CCDC-611918 (2a), CCDC-611919 (2b), and CCDC-
611920 (2c). Copies of the data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
◦
1
X-Ray structure analyses
X-Ray structure data were recorded on a Bruker-Nonius-
X8-APEX2-CCD diffractometer with MoKα radiation (λ =
0.71073 nm) and semi-empirical correction (SADABS). The
structures were solved with Direct Methods (SHELXS-97)
and refined by full-matrix least-squares methods (refine-
Acknowledgements
This work was financially supported by the German Sci-
ence Foundation (DFG) and the German Chemical Industry
Fund.
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J. Wagler – E. Brendler · A Surprising Equilibrium Between Penta- and Tetracoordination
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