Synthesis and x-ray crystal structure of [(THF)Zn(O2(OH)SiR)]4 (R = (2,6-i-Pr2C6H3)N(SiMe3)): Enroute to larger aggregates

Article abstract of DOI:10.1021/ic020514u

Reaction of aminosilanetriol RSi(OH)₃ (1) (R = (2,6-i-Pr₂C₆H₃)N(SiMe₃)) with diethyl zinc at room temperature in 1:1 stoichiometric ratio affords [(THF)Zn(O₂(OH)SiR)]₄ (2) (R = (2,6-i-Pr₂C₆H₃)N(SiMe₃)) in good yield. The single-crystal X-ray diffraction studies reveal that 2 is monoclinic, P2₁, with a = 17.117(3) A, b = 16.692(5) A, c = 17.399(4) A, α = y = 90°, β = 91.45(7)°, and Z = 2. The molecular structure of 2 contains two puckered eight-membered Zn₂Si₂O₄ rings, which are connected by the Zn-O bonds and form two planar four-membered Zn₂O₂ rings. Compound 2 contains an unreacted hydroxyl group on each silicon atom, and hence, we carried out the reactions of 2 with dimethylzinc and methyllithium to form [Zn₄(THF)₄(MeZn)₄ (O₃SiR)₄] (3) (R = (2,6-i-Pr₂C₆H₃)N- (SiMe₃)) and [(L)ZnLi(O₃SiR)]₄ (4) (L = 1,4-(Me₂N)₂C₆H₄, R = (2,6-i-Pr₂C₆H₃)N(SiMe₃)), respectively. This suggested that 2 could be an intermediate product formed during the synthesis of 3 and 4.

Full text of DOI:10.1021/ic020514u

Inorg. Chem. 2003, 42, 970−973
Synthesis and X-ray Crystal Structure of [(THF)Zn(O (OH)SiR)]4 (R )
2
(2,6-i-Pr2C6H3)N(SiMe3)): Enroute to Larger Aggregates†
,‡
Ganapathi Anantharaman,‡ Herbert W. Roesky,* Hans-Georg Schmidt,‡ Mathias Noltemeyer,‡ and
Jiri Pinkas§
Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Department of Inorganic Chemistry,
Masaryk UniVersity, CZ-61137 Brno, Czech Republic
Received August 14, 2002
Reaction of aminosilanetriol RSi(OH)3 (1) (R ) (2,6-i-Pr2C H )N(SiMe3)) with diethyl zinc at room temperature in
6
3
1:1 stoichiometric ratio affords [(THF)Zn(O (OH)SiR)]4 (2) (R) (2,6-i-Pr2C H )N(SiMe3)) in good yield. The single-
2
6 3
crystal X-ray diffraction studies reveal that 2 is monoclinic, P21, with a ) 17.117(3) Å, b ) 16.692(5) Å, c )
17.399(4) Å, R ) γ ) 90°, â ) 91.45(7)°, and Z ) 2. The molecular structure of 2 contains two puckered
eight-membered Zn2Si2O rings, which are connected by the Zn−O bonds and form two planar four-membered
4
Zn2O rings. Compound 2 contains an unreacted hydroxyl group on each silicon atom, and hence, we carried out
2
the reactions of 2 with dimethylzinc and methyllithium to form [Zn4(THF)4(MeZn)4(O SiR)4] (3) (R) (2,6-i-Pr2C H )N-
3
6 3
(SiMe3)) and [(L)ZnLi(O SiR)]4 (4) (L) 1,4-(Me2N)2C H , R) (2,6-i-Pr2C H )N(SiMe3)), respectively. This suggested
3
6
4
6 3
that 2 could be an intermediate product formed during the synthesis of 3 and 4.
Introduction
silica surfaces, used as catalysts, emerged as an important
topic of research in last two decades.2-5 Moreover, the
introduction of zinc species into H-ZSM-5 increases the
rate and selectivity of the aromatization reaction of alkanes.
Also, Zn/H-ZSM-5 is used as a catalyst due to the Lewis
acidic nature of the zinc atoms, for the conversion of methyl
halides to hydrocarbons and for the dehydrofluorination of
The Si-O-M linkages are the basic structural units
present in the naturally occurring silicate minerals in the
earth’s crust and in the synthetic zeolites. The type of the
Si-O linkages predominantly determines the structure of
these minerals and the nature of the metal atoms; they can
form cyclic, chain, and sheet type structures or three-
dimensional frameworks as in zeolites.1 The presence of a
metal in the siloxane framework increases the thermal
stability and improves the catalytic and conducting properties.
The studies on metal-incorporated zeolites and metal-doped
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Elsevier: Amsterdam, 1989; p 133.
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de. Fax: +49-551-393373.
† Dedicated to Professor Lev Yagupolskii on the occasion of his 80th
birthday.
‡ Universita¨t Go¨ttingen.
§ Masaryk University.
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970 Inorganic Chemistry, Vol. 42, No. 4, 2003
10.1021/ic020514u CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/22/2003

[(THF)Zn(O2(OH)SiR)]4
Scheme 1. Synthesis of Zinc Siloxane Starting from Aminosilanetriol 1
Freons. Although the catalytic properties of the Zn/H-ZSM-5
system are reported, no structural investigation of the
catalytic center has been described.5 Earlier, the studies on
metallasiloxanes have shown that these compounds are
equally important because of their ability to act as model
compounds for the complex zeolite systems.6
Currently, we have focused our attention on the formation
of larger aggregates from smaller molecules. In recent years
we have successfully synthesized a variety of organic soluble
metallasiloxanes6a,7 and metallaphosphonates,8 starting from
aminosilanetriol 1 and alkyl phosphonic acids with metal
alkyls. Some of the structures of these products are compa-
rable with the secondary building units (SBUs) of the
naturally occurring molecular sieves and zeolite systems.
Furthermore, these compounds can be utilized as precursors
for the corresponding metal-containing silicate systems.7
Consequently, we have been interested in synthesizing
aggregates of zinc siloxanes to mimic condensation pro-
cesses.
There are several methods reported for the synthesis of
zinc siloxanes starting from monosilanol with alkyl- or
amido-substituted zinc derivatives.9 However, the degree of
aggregation in these compounds is limited to a cubic
tetranuclear zinc siloxane. Earlier, we have reported an
oligomeric zinc siloxane compound [(L)ZnLi(O3SiR)]4 (4)
(L ) 1,4-(Me2N)2C6H4, R ) (2,6-i-Pr2C6H3)N(SiMe3))
prepared from dimethylzinc, methyllithium, and kinetically
stable aminosilanetriol 1 (Scheme 1).10 Instead of using
methyllithium in 4, the addition of an excess of dimethylzinc
to 1 (2:1 ratio) afforded the so far largest aggregate of zinc
siloxane [Zn4(THF)4(MeZn)4(O3SiR)4] (3) (R ) (2,6-i-
Pr2C6H3)N(SiMe3)). This result has been reported in a
communication.11 Subsequently, we were interested in the
product from the reaction between zinc alkyl and 1 in a 1:1
stoichiometric ratio. In this case we have obtained tetrameric
zinc siloxane [(THF)Zn(O2(OH)SiR)]4 (2) containing an
unreacted OH group on each silicon atom. Herein we wish
to report on the synthesis and single-crystal X-ray diffraction
studies of compound 2. Further, we have reported the
synthesis of 3 and 4 starting from 2. It is to be noted that all
three compounds (2-4) have the same stoichiometric ratio
of zinc to silicon with respect to the starting material.
(6) (a) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W.
Chem. ReV. 1996, 96, 2205. (b) Voronkov, M. G.; Maletina, E. A.;
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F. J.; Budzichowski, T. A. Polyhedron 1995, 14, 3239.
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Res. 1996, 29, 183. (b) Murugavel, R.; Bhattacharjee, M.; Roesky, H.
W. Appl. Organomet. Chem. 1999, 13, 227 and further references
therein.
(8) (a) Walawalkar, M. G.; Roesky, H. W.; Murugavel, R. Acc. Chem.
Res. 1999, 32, 117 and further references therein. (b) Chakraborthy,
D.; Horchler, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.
Inorg. Chem. 2000, 39, 3995. (c) Roesky, H. W. Solid State Sci. 2001,
3, 777.
(9) (a) Schindler, F.; Schmidbaur, H. Angew. Chem. 1967, 79, 697; Angew.
Chem., Int. Ed. Engl. 1967, 6, 683. (b) Schindler, F.; Schmidbaur, H.
Angew. Chem. 1965, 77, 865; Angew. Chem., Int. Ed. Engl. 1965, 4,
876. (c) Driess, M.; Merz, K.; Rell, S. Eur. J. Inorg. Chem. 2000,
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118, 3459.
(10) Anantharaman, G.; Reddy, N. D.; Roesky, H. W.; Magull, J.
Organometallics 2001, 20, 5777.
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114, 1274; Angew. Chem., Int. Ed. 2002, 41, 1226.
Inorganic Chemistry, Vol. 42, No. 4, 2003 971

Anantharaman et al.
Table 1. Crystal Data and Structure Refinement Details for
[(THF)Zn(O2(OH)SiR)]4 (2‚C7H8) (R ) (2,6-i-Pr2C6H3)N(SiMe3))
Experimental Section
General Procedure. All manipulations were performed on a
vacuum line or in the glovebox under a purified N2 atmosphere.
Solvents were distilled from Na/benzophenone ketyl prior to use.
Dimethylzinc (2 M solution in toluene), diethylzinc (1.1 M solution
in toluene), and N,N,N′,N′-tetramethyl-l,4-phenylenediamine (L)
were purchased from Aldrich and used as received. Alkylzinc
comounds were pyrophoric; hence, they were handled in a efficient
fumehood by wearing adequate protective clothing. Methyllithium
was purchased from Acros (1.6 M solution in toluene) and used as
received. The aminosilanetriol 1 was prepared as described in the
literature.12 Elemental analyses were performed by the Analytisches
Labor des Instituts fu¨r Anorganische Chemie, der Universita¨t
Go¨ttingen. NMR spectra were recorded on a AM200 and Bruker
Avance 500 instrument. Chemical shifts are reported in ppm with
reference to TMS. IR spectra were recorded on a Bio-Rad FTS-7
spectrometer in Nujol mull. Melting points were measured in a
sealed glass tube and were not corrected.
empirical formula
fw
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C83H148N4O16Si8Zn4
1944.25
P21
17.117(3)
16.692(5) Å
17.399(4)
90
91.45 (7)
90
4970(2)
200(2)
T (K)
λ (Å)
0.710 73
2
1.299
1.109
0.0569, 0.1386
0.1536, 0.1536
Z
D
calcd (Mg m-3)
µ (mm-1
)
R1 [I > 2σ(I)]a
wR2 (all data)a
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) [Σw(|Fo | - |Fc |)2/Σw|Fo| ]1/2
.
2
2
2
phenylenediamine (L) (0.64 g, 3.9 mmol), and the solution was
stirred for another 4 h. The solution was filtered and concentrated
to 1/3 of its volume. Colorless crystals of 4 were obtained at room
temperature (2.18 g, 45%).
X-ray Structure Determination and Refinement for Com-
pound 2. Data for the crystal structure of 2 were collected on a
Stoe-Siemens four-circle diffractometer using Mo KR radiation (λ
) 0.710 73 Å). The structure was solved by direct methods
(SHELXS-96)13 and refined against F2 using SHELXL-97.14 All
the heavy atoms were refined anisotropically. Hydrogen atoms were
included using the riding model with Uiso tied to the Uiso of the
parent atoms. A summary of cell parameters, data collection, and
structure solution is given in Table 1.
Synthesis of 2. A solution of RSi(OH)3 (1) (2.0 g, 6.1 mmol) in
THF/hexane (5 mL, 40 mL) was added to a solution of ZnEt2 (5.6
mL of a 1.1 M solution in toluene, 6.1 mmol) in hexane (10 mL)
at room temperature. After the evolution of ethane gas ceased, the
resulting solution was further stirred for 16 h at room temperature.
The volatile components were removed, and the residue was dried
for 6 h under vacuum. The remaining solid was washed with hexane
(5 mL). Colorless crystals of 2 were obtained in toluene (5 mL) at
room temperature (1.85 g, 62%). Mp: 162-165 °C. 1H NMR (500
MHz, CDCl3, TMS): δ 0.03 (s, 36H; SiMe3), 1.10 (d, 24H; J )
6.7 Hz, CH(CH3)(CH3)), 1.17 (d, 24H; J ) 6.7 Hz, CH(CH3)(CH3)),
1.68 (br, 16H; OCH2CH2), 2.35 (s, 3H; H3CC6H5), 3.62 (br, 16H;
OCH2), 3.70 (m, 8H; J ) 6.7 Hz, CH(CH3)2), 6.94 (m, 12H;
aromatic), 7.2 (m, 12H; H3CC6H5). 13C NMR (50.32 MHz, CDCl3,
TMS): δ 2.14 (Si(CH3)3), 21.45 (C6H5CH3), 24.96 (CH(CH3)-
(CH3)), 25.06 (CH(CH3)(CH3)), 25.75 (OCH2CH2), 27.42 (CH-
(CH3)2), 69.95 (OCH2), 123.03 (aromatic C-4), 123.36 (aromatic
C-3, C-5), 125.29 (p-C6H5CH3), 128.22 (m-C6H5CH3), 129.03 (o-
C6H5CH3), 137.86 (H3C-C6H5), 142.56 (aromatic C-2, C-6), 147.85
(aromatic C-1). 29Si NMR (99.36 MHz, CDCl3, TMS): δ 3.78
(Si(CH3)3), -62.16 (Si(OH)O2). IR (ν, cm-1): 3242, 1605, 1576,
1318, 1257, 1246, 1182, 1107, 1042, 1024, 967, 947, 908, 865,
823, 802, 753, 727, 693, 599, 549, 464. MS (EI, 70 eV) [m/e
(assignment, %)]: 162 (2,6-i-Pr2C6H3, 100%), 177 (2,6-i-Pr2C6H3-
NH2, 38%). Anal. Calcd for 2‚C7H8, C83H148N4O16Si8Zn4: C, 51.22;
H, 7.61; N, 2.88. Found: C, 50.75; H, 7.68; N, 2.87.
Synthesis of 3 from 2. A solution of ZnMe2 (1.6 mL of a 2 M
solution in toluene, 6.1 mmol) was added to a suspension of 2 (1.5
g, 0.77 mmol) in THF/hexane (15 mL, 30 mL) at room temperature.
After the evolution of methane gas ceased, the resulting clear
solution was further stirred for 16 h at room temperature. The
volatile components were removed, and the residue was dried for
6 h under vacuum. The remaining solid was washed with hexane
(5 mL). Colorless crystals of 3 are obtained in toluene (5 mL) at 0
°C (1.27 g, 67%).
Synthesis of 4 from 2. A solution of MeLi (5.2 mL of a 1.6 M
solution in diethyl ether, 6.1 mmol) was added to a suspension of
2 (4.0 g, 2.1 mmol) in THF/hexane (10 mL, 40 mL) at room
temperature. After the evolution of methane gas ceased, the resulting
clear solution was further stirred for 16 h at room temperature. To
the resulting solution was added solid N,N,N′,N′-tetramethyl-l,4-
Results and Discussion
The reaction of 1 with ZnEt2, in 1:1 molar ratio, produced
predominantly 2 by sequential formation of Si-O-Zn
linkages, whereas the reaction of 1 and dimethylzinc forms
not only 2 but also another product for which an additional
resonance was found in silicon NMR spectroscopy at 71.8
ppm. Compound 2 is highly soluble in common organic
solvents, such as benzene, toluene, diethyl ether, and THF.
We have fully characterized 2 by means of analytical and
spectroscopic techniques. Furthermore, the structure of 2,
the single crystals obtained in the former reaction, was
confirmed unambiguously by the single-crystal X-ray dif-
fraction studies.
The IR spectrum of 2 shows a broad band around 3300
cm-1 for the OH stretching frequencies of the silanol groups.
The 1H NMR spectrum of 2 displays the resonances of aryl,
isopropyl, and methyl groups and broad resonances assigned
to the coordinated THF molecules. The appearance of two
different resonances for isopropyl groups is consistent with
the crystal structure of 2 if the rotation of the aromatic groups
about the C-N bond is restricted. The absence of resonances
of the metal-bound ethyl groups in the region of near zero
suggests that both ethyl groups have reacted with the
silanetriol with the formation of 2. However, we were not
able to observe the resonances of free hydroxyls on each
(12) Winkhofer, N.; Voigt, A.; Dorn, H.; Roesky, H. W.; Steiner, A.; Stalke,
D.; Reller, A. Angew. Chem. 1994, 106, 1414; Angew. Chem., Int.
Ed. Engl. 1994, 33, 1352.
(13) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(14) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
972 Inorganic Chemistry, Vol. 42, No. 4, 2003

[(THF)Zn(O2(OH)SiR)]4
Table 2. Selected Bond Distance (Å) and Bond Angles (deg) of 2
Si(1)-O(1)
Si(1)-O(3)
Si(3)-O(5)
Zn(1)-O(9)
Zn(1)-O(12)
Zn(2)-O(6)
Zn(2)-O(3)
Zn(1)-Zn(2)
1.625(8)
1.585(9)
1.633(8)
1.846(7)
1.988(8)
1.882(7)
2.029(8)
Si(1)-O(2)
Si(3)-O(6)
Si(3)-O(4)
Zn(1)-O(3)
Zn(1)-O(13)
Zn(2)-O(12)
Zn(2)-O(14)
1.596(8)
1.603(7)
1.647(9)
1.950(7)
2.048(7)
1.938(7)
2.053(8)
2.850(3)
2.8396(19) Zn(3)-Zn(4)
O(3)-Si(1)-O(2)
O(2)-Si(1)-O(1)
O(6)-Si(3)-O(4)
O(3)-Zn(1)-O(12)
109.1(4)
O(3)-Si(1)-O(1)
O(6)-Si(3)-O(5)
O(5)-Si(3)-O(4)
O(9)-Zn(1)-O(13)
106.3(5)
113.5(4)
114.6(4)
88.0(3)
106.5(4)
107.9(4)
98.2(3)
O(3)-Zn(1)-O(13) 118.9(3)
O(6)-Zn(2)-O(12) 131.8(3)
O(12)-Zn(1)-O(13) 106.4(3)
O(6)-Zn(2)-O(14)
O(12)-Zn(2)-O(3)
98.4(3)
87.2(3)
O(6)-Zn(2)-O(3)
110.4(4)
distances observed in the Zn2O2 rings that are also responsible
for the eclipsed conformation. The bond lengths of the
σ-bonded Zn-µ3-O (Zn(1)-O(3) 1.950(7), Zn(2)-O(12)
1.938(7), Zn(3)-O(8) 1.939(8), and Zn(4)-O(5) 1.943(8))
are similar to the zinc siloxane compound reported values;9c
however, these values are shorter than the coordinated ones
(average 2.016(7) Å). The distances of coordinated Zn-µ3-O
bonds and the coordinated THF molecules are almost equal
(see Table 2). The average Zn-µ2-O (e.g. Zn(1)-O(9), Zn-
(2)-O(6), Zn(3)-O(11), and Zn(4)-O(2)) bond distance is
1.867(9) Å, which is shorter than the σ-bonded Zn-µ3-O
distance in the ring. The diagonal distances between the zinc
atoms in Zn2O2 (Zn(1)-Zn(2) 2.8396(19) and Zn(3)-Zn-
(4) 2.850(3)) suggest that there is a weak Zn‚‚‚Zn interac-
tion.9,11,16 The O-Zn-O bond angles within the Zn2O2 ring
are close to 90° showing that the four-membered rings are
almost perfect rectangles.
Figure 1. Core structure of 2 showing the drum polyhedron. The drawing
shows 50% probability thermal ellipsoids.
silicon atom. The resonances of the SiMe3 and Si(OH)O2
groups were observed in the 29Si NMR at 3.78 and -62.16
ppm, respectively. Moreover, the resonance due to Si(OH)-
O2 in 29Si NMR is slightly shifted toward downfield
compared to the observed value for 4 (-64.8 ppm) and it is
also different from those of 3 (-60.87, -61.34).10,11 By
comparing the analytical and spectroscopic data, we were
able to predict partially the structure of 2. However, the
formation of a drumlike structure was finally confirmed by
the single-crystal X-ray diffraction studies.
Conversion of 2 to 3 and to 4. The formation of eight-
(Zn2Si2O4) and four-(Zn2O2) membered rings and the ori-
entation of the OH groups around silicon atoms in 2 suggests
that it could be reacted further to construct 3 and 4 that have
been made previously from 1.10,11 Thus, 4 equiv of dimeth-
ylzinc reacts with 2 at room temperature to produce 3. The
fact that all of the spectroscopic and analytical data of this
product were identical to those of 311 strongly suggests that
2 could be an intermediate species in this reaction. Similarly,
4 was made from 2 with the addition of methyllithium in
the presence of N,N,N′,N′-tetramethyl-l,4-phenylenediamine
(L).
Conclusion
In this paper, we have shown that changing the stoichio-
metric ratio of zinc alkyls with respect to the aminosilanetriol
1 leads to the formation of the new zinc siloxane compounds
2-4. We have also shown that 2 can be an intermediate in
the formation of 3 and 4. The acidic protons in 2 are the
reactive centers for further reactions with different metal
alkyls.
X-ray Crystal Structure of 2. The molecular structure
of 2 is shown in Figure 1. Selected structural parameters
are given in Table 2. The compound crystallizes in the
monoclinic space group P21 with a molecule of toluene in
the asymmetric unit. The molecule of 2 possesses a pseudo-
4-fold (S4) symmetry and consists of two eight-membered
Zn2Si2O4 units connected by four Zn-O bonds forming a
drumlike structure. These two Zn2Si2O4 rings are puckered
and exist in a boat conformation. The four-membered Zn2O2
rings are almost planar, and the zinc atoms of one ring are
in register with the oxygens of the second one.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft, the Go¨ttinger Akademie
der Wissenschaften, and the VW foundation.
Supporting Information Available: Single-crystal X-ray struc-
tural data for compound 2 (CIF format). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC020514U
(15) (a) Murugavel, R.; Chandrasekhar, V.; Voigt, A.; Noltemeyer, M.;
Schmidt, H.-G.; Roesky, H. W. Organometallics 1995, 14, 5298. (b)
Klemp, A.; Haptop, H.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G. Inorg. Chem. 1999, 38, 5832.
(16) For compounds having Zn‚‚‚Zn interactions, see the following: (a)
Elias, A. J.; Schmidt, H.-G.; Noltemeyer, M.; Roesky, H. W.
Organometallics 1992, 11, 462. (b) Li, H.; Eddaoudi, M.; Groy, T.
L.; Yaghi, O. M. J. Am. Chem. Soc. 1998, 120, 8571.
The average Si-O bond length (1.61 Å) is slightly shorter
than the values of the same in silantriols, whereas the Si-
OH bond lengths (average 1.64 Å) are comparable with those
of silanetriols.15 There are two types of Zn-µ3-O bond
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