Tetra(o-phenyldiazo-p-methylphenoxy)silane - A silicon complex with a fourfold capped-tetrahedral [4+4] coordination sphere

Article abstract of DOI:10.1016/j.inoche.2007.05.005

2-Hydroxy-5-methylazobenzene (HO-C₆H₃-p-Me-o-N{double bond, long}N-C₆H₅, 1) reacts with SiCl₄ in the presence of triethylamine to yield the [4+4]-coordinate silicon complex 2 [Si(O-C₆H₃-p-Me-o-N{double bond, long}N-C₆H₅)₄]. Its molecular structure has been determined by X-ray crystallography, and 2 represents the first structurally characterized Si-complex bearing o-diazophenoxy ligands. ²⁹Si NMR data suggest that the capped tetrahedral coordination sphere of the Si-atom is retained in solution. Even the difluorocompound 3 [F₂Si(O-C₆H₃-p-Me-o-N{double bond, long}N-C₆H₅)₂] still exhibits a tetracoordinate Si-atom, whereas 4 [F₂B(O-C₆H₃-p-Me-o-N{double bond, long}N-C₆H₅)], the byproduct of the formation of 3 from 2, bears a bidentate chelating o-phenyldiazo-p-methylphenoxy ligand.

Full text of DOI:10.1016/j.inoche.2007.05.005

Inorganic Chemistry Communications 10 (2007) 952–955
www.elsevier.com/locate/inoche
Tetra(o-phenyldiazo-p-methylphenoxy)silane – A silicon complex with
a fourfold capped-tetrahedral [4+4] coordination sphere
*
Daniela Gerlach, Jo¨rg Wagler
Institut fu¨r Anorganische Chemie, Technische Universita¨t Bergakademie Freiberg, Leipziger Str. 29, D-09596 Freiberg, Germany
Received 22 March 2007; accepted 6 May 2007
Available online 18 May 2007
Abstract
2-Hydroxy-5-methylazobenzene (HO–C6H3-p-Me-o-N@N–C6H5, 1) reacts with SiCl4 in the presence of triethylamine to yield the
[4+4]-coordinate silicon complex 2 [Si(O–C6H3-p-Me-o-N@N–C6H5)4]. Its molecular structure has been determined by X-ray crystallog-
raphy, and 2 represents the first structurally characterized Si-complex bearing o-diazophenoxy ligands. 29Si NMR data suggest that the
capped tetrahedral coordination sphere of the Si-atom is retained in solution. Even the difluorocompound 3 [F2Si(O–C6H3-p-Me-o-
N@N–C6H5)2] still exhibits a tetracoordinate Si-atom, whereas 4 [F2B(O–C6H3-p-Me-o-N@N–C6H5)], the byproduct of the formation
of 3 from 2, bears a bidentate chelating o-phenyldiazo-p-methylphenoxy ligand.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Azobenzene; Boron; Chelate; Hypercoordination; Silicon
Various silicon complexes of bidentate (ON) chelating
ligands have been investigated during the past decades
[1,2] and the class of bidentate Schiff base ligands have
proved to be suitable ligands for the syntheses of penta-
and hexacoordinate silicon compounds [2].
Schiff base ligands are easily accessible via condensation
of o-hydroxyaryl carbonyl compounds or acetylacetone
with primary amines and this is a major reason for their
widespread use in complex chemistry. Azobenzene-derived
(ON) bidentate ligands are easily accessible, and their
structural similarities to bidentate Schiff base ligands are
obvious (Scheme 1).
Surprisingly, with respect to silicon complexes of such
ligands, there is no structural information within the Cam-
bridge Crystallographic Data Base and literature is limited
to patents concerning dyes and electrographic toners which
are based on compounds bearing the Si(O–C6Hn-o-N@N-R)
moiety [3].
As shown by Kano et al., the o-diazophenyl group
exhibits the ability to coordinate silicon atoms in a biden-
tate fashion with formation of five-membered chelates.
Penta- and hexacoordinate silicon complexes bearing an
o-diazophenyl ligand have been reported (Scheme 2) [4].
o-Diazophenolates, however, should be able to realize five-
and/or six-membered chelates (Scheme 1). Therefore, we
prepared the (ON) bidentate ligand 1 and investigated its
reaction with SiCl4 to yield 2 (Scheme 3) [5].
The molecular structures of both 1 and 2 have been
determined by X-ray crystallography (Figs. 1 and 2, respec-
tively) [6]. The molecular conformation (E) of 1 is similar
to that found in the crystal structure of the Schiff base N-
phenylsalicylaldimine [7]. Fig. 1 clearly demonstrates that
a hydrogen bridge is established between O1 and N2. The
diazo moiety N1@N2 is in plane with the benzene ring
C1–C6 (torsion C1–C6–N1–N2: 0.1(3)°) and, contrary to
the structure of N-phenylsalicylaldimine, the phenyl group
at N2 is only slightly out of plane (torsion N1–N2–C8–C9:
3.4(3)°), whereas this torsion is ca 48° in the Schiff base
mentioned above. The presence of N1 with its lone pair
instead of an azomethine C–H can explain this structural
difference.
*
Corresponding author. Tel.: +49 3731 39 3556; fax: +49 3731 39 4058.
E-mail address: joerg.wagler@chemie.tu-freiberg.de (J. Wagler).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.05.005

D. Gerlach, J. Wagler / Inorganic Chemistry Communications 10 (2007) 952–955
953
Ph
C10
N N
C9
R
OH
C11
C12
C5
N1
C8
C7
C4
N2
H vs. SiR3
Ph
C6
C1
C13
Ph
?
N
C3
N N
O
or
N
SiR3
SiR3
C2
R
O1
R
O
Fig. 1. Molecular structure of 1 in the crystal (ORTEP plot with 50%
˚
Scheme 1.
probability displacement ellipsoids). Selected bond lengths [A]: C1–O1
1.349(3), C1–C2 1.391(3), C2–C3 1.375(3), C3–C4 1.401(3), C4–C5
1.380(3), C5–C6 1.395(3), C1–C6 1.410(3), C6–N1 1.401(2), N1–N2
1.265(2), N2–C8 1.427(2).
Ph
Ph
N
N
N N
F
F
Ph
N2
N1
Si
F
Si
F
F
F
N6
N5
Scheme 2.
O1
O3
Si1
NH2
O2
N7
OH
O4
N8
N3
N4
i
ii
N
Fig. 2. Molecular structure of 2 in the crystal (ORTEP plot with 30%
probability displacement ellipsoids and selected atomic labels, hydrogen
atoms and phenyl groups omitted for clarity). Selected interatomic
N N
OH
iii
N
1
Si
O
˚
distances [A]: Si1–O1 1.618(1), Si1–O2 1.625(1), Si1–O3 1.622(1), Si1–
4
O4 1.621(1), Si1–N1 3.067(1), Si1–N3 3.115(1), Si1–N5 3.105(1), Si1–N7
3.097(1).
iv
2
N
2
molecular configuration of the ligand 1, the original chela-
tor N2 (and its analogues N4, N6, N8) point outside the
molecule of 2 and nitrogen atoms N1 (and its analogues
N3, N5, N7) exert weak donor interactions towards the
Si-atom. N1, N5 and N3, N7 point towards the widened
angles O1–Si1–O3 and O2–Si1–O4, respectively. The
angles between the capping N-atoms N1, N3, N5, N7
and the trans-situated bonds Si1–O4, Si1–O1, Si1–O2 and
Si1–O3, respectively, are close to 170°, i.e., 170.1(1),
168.4(1), 169.9(1) and 170.3(1)°, respectively.
This capping of the tetrahedral faces of this tetraphen-
oxysilane derivative causes a slight upfield shift of the
29Si NMR signal (d = À108.4 ppm in the solid state) with
respect to the 29Si NMR shift of tetraphenoxysilane
(d = À101 ppm) [9]. Even in solution (d = À105.6 ppm)
the 29Si nucleus of 2 is still noticeably more shielded than
that of tetraphenoxysilane. The slightly diminished shield-
ing of the 29Si nucleus of 2 in solution indicates configura-
tional differences to the arrangement found in the solid
state. The difference Dd29Si between Si(OPh)4 and 2 in
N N
N
BF2
SiF2
O
O
4
2
3
Scheme 3. Reagents: (i) HCl, NaNO2, (ii) NaOH, (iii) Et3N, SiCl4, THF
and (iv) BF3 Æ Et2O, THF.
Compound 2 exhibits a fourfold capped tetrahedral
coordination sphere at the Si-atom with Si–N separations
to the capping atoms N1, N3, N5 and N7 being signifi-
cantly shorter than the sum of the van der Waals radii
˚
(3.65 A) [8]. The Si–O bond lengths are in the expected
range for tetracoordinate silicon compounds. Their relative
orientation, however, indicates a distortion of the tetrahe-
dral coordination sphere. Two O–Si–O angles (O1–Si1–
O3 114.58(6)°, O4–Si1–O2 113.47(6)°) are significantly
wider than 109.5°, whereas the other four are smaller, rang-
ing from 106.94(6)° to 107.71(6)°. With respect to the

954
D. Gerlach, J. Wagler / Inorganic Chemistry Communications 10 (2007) 952–955
solution, however, may arise from through-bond substitu-
ent effects and does not necessarily indicate any hypercoor-
dination of the Si-atom of 2 in solution.
benzene to act as a bidentate ligand by formation of
six-membered chelates, a chelating pattern typical for Schiff
base ligands. In silicon complexes, however, this ligand
system exhibits noticeable differences and gives rise to com-
pounds with capped-tetrahedral coordination spheres
under formation of five-membered pseudo-chelates.
As known from studies of Kano et al. [4], phenyldiazo-
benzene ligands can undergo cis–trans-isomerization upon
UV-irradiation. Irradiation of 2 with UV revealed that
the phenyldiazophenolate ligand is also susceptible to
photo-induced isomerization [10]. However, due to the
presence of four phenyldiazophenolate ligands a mixture
of various compounds has been detected by 29Si NMR
spectroscopy after irradiation.
As the hypercoordinate Si-compounds of Kano et al. [4]
bear Si-bound fluorine atoms as electron withdrawing
substituents, we examined the effect of Si-bound fluorine
atoms on our ligand system by partial replacement of dia-
zophenolate ligands by F-atoms to yield 3 (Scheme 3). So
far, we have not been able to isolate compound 3 as a solid,
but its 29Si NMR data in solution (d À110.2, triplet,
1JSi–F = 174.1 Hz) indicates both the successful twofold
fluorination and the presence of a still four-coordinate,
potentially [4+2]-coordinate, Si nucleus.
Appendix A. Supplementary material
CCDC 641409, 641407 and 641408 contain the supple-
mentary crystallographic data for 1, 2 and 4. These data
can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html, or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.inoche.2007.05.005.
References
[1] D. Kost, I. Kalikhman, Adv. Organomet. Chem. 50 (2004) 1;
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Z. Naturforsch. B 60 (2005) 1054;
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(2006) 1826.
[2] M. Schley, J. Wagler, G. Roewer, Z. Anorg. Allg. Chem. 631 (2005)
2914;
The byproduct of this transformation, boron complex 4,
crystallized from the reaction mixture. This compound has
already been described by Hohaus and Wessendorf [11].
However, a CSD search indicated that there are currently
no structural data for BF2-complexes with 2-phenyldiazo-
phenolate ligands available. Therefore, the structure of 4
is briefly described here (Fig. 3).
J. Wagler, U. Bo¨hme, E. Brendler, G. Roewer, Organometallics 24
(2005) 1348;
O. Seiler, C. Burschka, M. Fischer, M. Penka, R. Tacke, Inorg.
Chem. 44 (2005) 2337.
The two crystallographically independent molecules of 4
have similar conformations, therefore only one is discussed
here. As a result of the coordination of the B-atom, the
bond N1@N2 is significantly stretched with respect to the
corresponding bond length in compound 1. The boron
[3] K. Maeda, Y. Nakamura, N. Suzuki, Jpn. Kokai Tokkyo Koho
(2002), JP 2002088664 A 20020327;
K. Maeda, Y. Nakamura, N. Suzuki, Jpn. Kokai Tokkyo Koho
(2001), JP 2001316600 A 20011116;
˚
atom is located 0.387(3) A out-of-plane from the least-
squares-plane (O1, C1, C6, N1, N2). In contrast to 1, there
is a notable tilt of the phenyl group at N2 out of the plane
of the chelate in 4 (torsion N1–N2–C8–C9: 25.9(3)°).
The structures of the free ligand acid 1 and the boron
complex 4 demonstrate the ability of 2-oxy-5-methylazo-
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(1975) 808.
[4] N. Kano, F. Komatsu, M. Yamamura, T. Kawashima, J. Am. Chem.
Soc. 128 (2006) 7097;
M. Yamamura, N. Kano, T. Kawashima, J. Organomet. Chem. 692
(2007) 313.
C10
[5] Ligand 1 has been prepared according to a general literature method
(e.g., J. Song, C. Guo, L. Zhu, Z. Sun, Huaxue Shiji, 26 (2004) 89.)
from a solution of p-cresol and Na2CO3 and a solution of aniline,
HCl and NaNO2. The product has been recrystallized from ethanol.
Preparations of 2, 3 and 4 have been carried out under an atmosphere
of dry argon in dry solvents. Compound 2: A solution of 1 (5.00 g,
23.6 mmol) in 25 ml THF was added dropwise to a À78 °C cold
solution of SiCl4 (1.00 g, 5.90 mmol) and Et3N (2.86 g, 28.3 mmol) in
5 ml THF, then stirred at room temperature. The hydrochloride
precipitate was filtered after 1 d and washed with 25 ml THF. The
solvent was removed from the filtrate under vacuum and the red solid
was recrystallized from toluene (5 ml). Red crystals of 2 were filtered
off, washed with toluene : hexane (1:1) and dried in vacuum. Yield:
3.90 g (4.47 mmol, 76%). Mp: 125 °C. 1H NMR (CDCl3, 400 MHz):
C9
C8
C11
C12
C7
C5
C4
N1
C6
N2
C3
C13
B1
C1
C2
O1
F2
F1
Fig. 3. Molecular structure of one of the two crystallographically
independent molecules of 4 in the crystal (ORTEP plot with 50%
probability displacement ellipsoids, hydrogen atoms omitted). Selected
3
d = 2.22 (s, 12H, CH3), 6.89 (d, 4H, ar, JHH = 8.2 Hz), 7,10 (d, 4H,
3
ar, JHH = 8.2 Hz), 7.25–7.32 (m, 12H, ar), 7.42 (s, 4H, ar), 7.76 (d,
3
8H, ar, JHH = 7.2 Hz). 13C NMR (CDCl3, 101 MHz): d = 20.6
˚
bond lengths [A]: B1–O1 1.449(3), B1–N2 1.616(3), B1–F1 1.373(3), B1–
(CH3), 116.3, 121.4, 123.2, 128.5, 130.3, 132.1, 132.9, 142.3, 149.8,
F2 1.372(3), N1–N2 1.281(3), C1–O1 1.334(3).

D. Gerlach, J. Wagler / Inorganic Chemistry Communications 10 (2007) 952–955
955
152.6 (ar). 29Si NMR (CDCl3, 79 MHz): d = À105.6. 29Si CP/MAS
NMR (79 MHz): diso = À108.4. Anal. Calc. for C52H44N8O4Si: C,
71.54; H, 5.08; N, 12.83. Found: C, 71.33; H, 5.33; N, 13.11%.
Compounds 3 and 4: A solution of BF3 Æ Et2O (0.68 g, 4.81 mmol) in
10 ml THF was added dropwise to a solution of 2 (2.10 g, 2.41 mmol)
in 20 ml THF. After 14 days storage at ambient temperature,
compound 3 was detected in the reaction mixture as the main
silicon-containing product by 29Si NMR. Compound 3: 29Si NMR
Z = 4, Dcalc = 1.274 Mg/m3, T = 296(2) K, hmax = 28.5°, 8206 reflec-
tions (Rint = 0.0219), 1487 unique reflections, 1265 with I > 2r(I),
148 parameters refined. Final R indices [I > 2r(I)] R1 = 0.0391,
wR2 = 0.1159, all data: R1 = 0.0504, wR2 = 0.1347. Data collection
and structure refinement of 2 (C52H44N8O4Si): Mw = 873.04, Mono-
˚
clinic, P21, a = 11.6139(4), b = 16.1977(7), c = 11.8699(4) A,
b = 94.732(2)°, V = 2225.34(14) A , Z = 2, Dcalc = 1.303 Mg/m3,
3
˚
T = 93(2) K,
hmax = 30°, 28571 reflections (Rint = 0.0215), 12787
1
(THF, 79 MHz): d = À110.2 (t, JSi–F = 174.1 Hz). The solvent was
unique reflections, 11548 with I > 2r(I), 587 parameters refined. Final
R indices [I > 2r(I)] R1 = 0.0431, wR2 = 0.1143, all data: R1 = 0.0504,
then removed under vacuum, the residue dissolved in 1 ml toluene
and 1.2 ml hexane was added. Some red crystals of 4 had formed
which were filtered and dried in vacuum. Yield: 0.47 g (1.81 mmol, 38
%), m.p.: 120 °C. 3a: 1H NMR (CDCl3, 101 MHz): d = 2.40 (s, 3H,
wR2 = 0.1228. Data collection and structure refinement of
4
(C13H11BF2N2O): Mw = 260.05, Orthorhombic, Pca21, a =
3
˚
˚
30.0987(11), b = 7.2396(2), c = 10.9512(5) A, V = 2386.29(15) A ,
Z = 8, Dcalc = 1.448 Mg/m3, T = 93(2) K, hmax = 30°, 30633 reflec-
tions (Rint = 0.0750), 3656 unique reflections, 2897 with I > 2r(I),
343 parameters refined. Final R indices [I > 2r(I)] R1 = 0.0415,
wR2 = 0.0942, all data: R1 = 0.0630, wR2 = 0.1023.
3
CH3), 7.07 (d, 1H, ar, JHH = 8.8 Hz), 7.50–8.07 (mm, 7H, ar). 13C
NMR (CDCl3, 101 MHz): d = 119.5, 123.4, 129.4, 131.7, 131.8, 132.7,
136.5, 142.6, 145.0, 148.4 (ar). 11B NMR (CDCl3, 128 MHz):
1
d = À1.6 (t, JB-F = 15.2 Hz). Anal. Calc. for C13H11N2OBF2: C,
60.04; H, 4.27; N, 10.77. Found: C, 59.73; H, 4.47; N, 10.34%.
[6] Crystallographic data have been recorded on a Bruker-Nonius X8
APEX2-CCD diffractometer with graphite monochromatized Mo Ka
[7] F. Arod, M. Gardon, P. Pattison, G. Chapuis, Acta Crystallogr. C 61
(2005) o317.
[8] A. Bondi, J. Phys. Chem. 68 (1964) 441.
˚
radiation (0.71073 A), structures were solved with direct methods
[9] J. Pickies, W. Wojnowski, Z. Anorg. Allg. Chem. 511 (1984) 0219.
[10] A solution of 2 (1.85 g, 2.12 mmol) in 130 ml THF was irradiated
with a medium pressure Hg lamp (kmax = 365–436 nm) in a 150 mL
reactor at 15 °C for 4 h. The solvent has then been removed in
vacuum and the reaction mixture was dissolved in CDCl3 for 29Si
NMR analysis. In the 29Si NMR spectrum peaks were found at
À100.1, À100.4, À100.6, À105.1, À105.3 and À107.5 ppm in
approximate ratio 2:1:2:1:5:1. The presence of further less intense
peaks can not be excluded. The peak at À105.3 ppm may arise
from 2 and four of the other peaks may be due to the four
alternative isomers. The origin of further peaks, however, is not
clear yet.
(SHELXS) and refined with full-matrix least-squares methods against
F2 (SHELXL). All non-hydrogen atoms were anisotropically refined,
hydrogen atoms were placed in idealized positions and refined
isotropically (riding model), except the hydroxy H atom in 1, which
was localized from residual electron density and isotropically refined
without bond length restraints. Friedel data have been merged for the
refinement of the structures of 1 and 4 as the absolute configura-
tion could not be determined due to poor anomalous scattering.
The Flack-Parameter for
twinning model with 2:1 domain ratio. Data collection and structure
refinement of 1 (C13H12N2O): Mw = 212.25, Orthorhombic, Pca21,
a = 11.6101(4), b = 12.7255(4), c = 7.4884(2) A, V = 1106.37(6) A ,
2 refined to 0.0(2) using a racemic
3
˚
˚
[11] E. Hohaus, K. Wessendorf, Z. Naturforsch. B 35 (1980) 319.