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Ba[Si(OCH2CH2O)3] a Hexaalkoxysilicate Synthesized from SiO2.

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171 R W. Ratcliffe. T. N. Salzmann. B. G. Christensen. Terraliedron Lerr.
1980. 31-34.
[XI M. Rudolph. D;.mvrurioii. Technische Universitiit Miinchen. 1991.
[Y] C . Yu. G. C. Levy, J. Am. Cliem. Soc. 1984, 106. 6533-6537.
[lo] Review' R. M. Williams. Sjntliesi.7 of'Opricullj Acltiv a-Amino Acid$,Self
Rc,proi6tctroii o/ Chtruli~y.Pcrgamon, New York. 1989. pp. 62 - 84.
[l I] Review: M. Regitz. G. Maas, W. Illgcr, Diusoulkanc., Eigenscliufren rind
S i . i i r / t i w . Thieme. Stuttgart. 1977.
[12] The van der W a d s volume of the trifluoromethyl group (42.6 A') is more
than double that of the methyl group; its van der W a d s radius (2.7 A) is
betseen that of the methyl group (2.0 A) and the rerr-hutyl group(3.5 A):
D Seehach, Angrn. Client. 1990. 102, 1363 . 1409, Angrit-. Clivm. Inr. Ed.
En'?/. 1990. .?Y. 1320 1367. and references therein.
1131 K Burger. M. Rudolph. H. Neuhauser, Lii4ig.s Ann. Clirm. 1991. 1365136X.
[14] H. Leuchs, K . Bormann. Ber. Drsch. Clieiii. G m 1919, 52, 2086-2097.
115) E M. L. Tan. L. Ryhdnen. J. Uitto. J. f n i w r . Dermurol. 1983, HO, 261 267;
C % m i . Ahstr. 1983. 98. 137603s.
(161 W D. Klohs. R. W. Steinkampf. M. S. Wicha. A. E. Mertus. J. B Tunac.
W, R. Leopold. J. Null. Cunwr Inst. 1985. 75. 353-359; Chen?.Ahstr. 1985,
IOi. I 1 5 868 b.
[17] Melting points (uncorrected): Tottoli apparatus (Biichi). IR spectra:
Perkin-Elmer spectrometers 157G and 257. 'H N M R : Bruker AM360
(360.1 MHr), tetramethylsilane as internal standard; " C N M R : Bruker
AM 360 (90.6 MHz), tetramethylsilane as internal standard; I9F N M R :
Jcol C 6 0 H L (56.5 MHz) or Bruker AM360 (338.8 MHz), trifluoroacetic
acid as external standard. Elemental analyses: C,H.N analyzer EA 41510.
Monar system (Herdeus). Correct elemental analyses were obtained for all
the coiiipounds described: CkO.17. H+0.31. Nk0.12.
Ba[Si(OCH,CH,O)J, a Hexaalkoxysilicate
Synthesized from SiO, **
(1,2-O,C2Me,),] (M = K, Li; 1,2-O,C2Me, = dianion of
pinacole) have been synthesized directly from GeO,; however, silicon analogues are still unknown.[241Polyhedral germanates with four, five, and six oxygen donors at G e have
been prepared, but it has been suggested[25]that the ability
of G e (but rarely Si) to exhibit sixfold 0-coordination is
because it is a post transition metal element. Materials with
sixfold 0-coordinated Si atoms are limited to the rare mineral stishovite formed from a meteor impact.
Given that no examples of hexaalkoxysilicates have been
described in the literature since the synthesis of the tris(benzenediyldioxido) complexes in 1931, it can be inferred that
they d o not form readily. However, theoretical calculations
suggest that simple silicate dianions with six 0 ligands without supporting aromatic groups should be stable.[2h1We
report here the first such example.
We recently reported the synthesis of the pentacoordinate
silicates M[Si(OCH,CH,O),(OCH,cH,OH)] and M,[Si,(OCH,CH,O),] (M = Li, Na, K. Cs) by direct reaction of
one equivalent of MOH with one equivalent of SiO-,in
. excess
ethylene glycol.'271Efforts to delineate the scope of this reaction led to studies of analogues with alkali metals. for example M[Si,(OCH,CH,O),] (M = Mg, Ca, Ba), whose stoichiometry would be ideal for the preparation of aluminosilicates such as CaO 2 SiO, A1,0, (garnet). These studies
provided rather unique and unexpected results. We report
here the synthesis of the first example of the heretofore unknown dianionic hexaalkoxysilicate 1 directly from silica,
ethylene glycol, and barium oxide.
By Mur-tin L. Hoppe, Richard M . Luine,* Jejfrey Kumpf,
Mark S. Gordon, and Larry W Burggraf
Anionic (pentacoordinate) and dianionic (hexacoordinate) organosilicates were first reported more than 60 years
Pentacoordinate Si complexes have recently received considerable attention because of their role in sol-gel
processing of silicate glasses,[4-61their utility in organic synthesis and polymerization. [' - 91 and their use as precursors to
organosilicon compounds.["Pentacoordinate silicates
are easily prepared from compounds with tetrasubstituted Si
centers.["- "I Although polyhalogenated anionic silicates
are relatively common, only perfluorinated derivatives are
found to form hexacoordinate silicate dianions, for example
[ S i F J - .[231 Salts of the dianion [Si(l ,2-0,C,H,),]2-, first
prepared in the early 30s, are the most common nonfluorinated hexacoordinate silicates.i2,3, l o ]
Surprisingly, these salts are thermally and hydrolytically
more stable than pentacoordinate alkoxysilicates, possibly
because the aromatic rings provide charge delocalization.r'"l
Hexacoordinate dianionic germanium complexes M,[Ge-
When mixtures of BaO (85.2 g, 0.56 mol), SiO, (30.0 g,
0.5 mol), and glycol (500 mL) were heated such that excess
glycol and the H,O formed in the reaction distilled off, the
SiO, dissolved completely in 2-4 h. When the clear solution
was cooled, the white microcrystalline methanol-soluble
product 1 precipitated (z90% yield).[*'] CaO and MgO can
be used in place of BaO; the yields are low with MgO because
of the low reactivity of MgO with ethylene glycol. X-ray
quality single crystals of the Ba complex were grown by
vapor diffusion of acetonitrile into a solution of the Ba complex in glycol.
The structure determined by single-crystal X-ray diffraction (Fig. I ) ["I reveals two crystallographically independent
BaZ+[Si(OCH,CH,O),]*- formula units per asymmetric
unit. "Si MAS solid-state NMR data support these finding~.'~''Each Si atom has a slightly distorted octahedral
coordination environment with three bidentate ethanediolato hgands. Each unit cell contains 13 glycol molecules
in the crystal lattice. Coulombic Ba . . ' 0 interactions and
strong hydrogen bonding with the glycol molecules in the
lattice I 3 l 1 likely account for the stability of the complex and
also modify the stereochemistry about each silicon atom.
One Si atom has a A configuration; its three bidentate
ethanediolato hgands adopt a (j.66) configuration. The configuration about the other Si atom is A(666). The crystallographic inversion center also generates Si centers with A(62.i)
and A(222.) stereochemistry.
There are no chemically significant differences in any of
the Si-0 o r C-C bond lengths or angles for the two molecules except for those imposed by stereochemistry. However,
the Ba atoms occupy quite different environments in the
crystal lattice. Each Ba atom has nine contacts with 0 atoms
< 3.2 k. For Bal, two of these contacts are with the Sil-tris-
Prof'. R M . Laine. M . L. Hoppe
Dcp;irtinent of Materials Science and Engineering
H. H. Dow Building. 2300 Hayward St.
Univercity of Michigan
Ann Arbor. M I 48109-2136 (USA)
J K:impf
Dcpartment of Chemistry, University of Michigan (USA)
M. S Gordon
Dcpartmcnt of Chemistry.
North Dakota State University. Fargo. N D (USA)
L W. Burggraf
Air Force Office of Scientific Research. Washington DC (USA)
Thi\ work was supported by the Air Force Office or Scientific Research.
We would like to thank Prof. Linda Nazar. Waterloo, and Prof. Florence
Bahonneau, Paris. for continuing coliaborations, especially for running
soliii-state Kh4R studies: and Prof. R. J. P. Corriu for numerous helpful
discussions that have made this publication possible.
silicates;1361our work suggests that Mg[Si(OR),] may be an
intermediate. The Mg analogue of 1 can be used as a tractable
cordierite precursor.r371The isolation of hexaalkoxysilicates
also provides support for the formation of hexacoordinate
silicate anion intermediates in the nucleophilic reactions of
pentacoordinated silicates.
Received: September 5, 1992 [Z 5557 IE]
German version: Angrw. CIirni. 1993. 105. 283
Fig. I . ORTEP plot of the crystallographically independent formula units of
Ba[Si(OCH,CH,O),] (1) (atoms represented as 50% thermal ellipsoids). Selected interatomic distances [A]: Si(2)-0(7) 1.835(2). SI(Z)-O(X)1.762(2),Si(Z)-O(Y)
1.753(2). Si(Z)-O(lO) 1.781(2), Si(2)-0(11) 1.809(2). Si(2)-0(12) 1.770(1).
Ba(2)..-0(7) 2.772(2), Ba(2)...0(10) 2.956(2), Ba(2)...0(12) 2.682(1),
Ba(2)...Si(2) 3.481(1). Mean bond lengths have typical values: Si-0 1.785(1).
C-01.419(3). C-C1.516(4).
(ethanedio1ato)complex (mean distance) (2.904(2)), two are
with the Si2 complex (2.739(2)), and five are with glycol
molecules in the lattice (2.832(2)A). For Ba2, two contacts
are with the Sil complex (2.739(2)), three are with the Si2
complex (2.803(2)),and four are with glycol molecules in the
lattice (2.827(2)A). The Ba . . . Si distances are also quite dissimilar. The Bal -Si2 and Bal -Si2 distances are 3.874(1)
and 3.768(1) A, respectively, while the Ba2-Sil and Ba2Si2 distances are 3.684(1) and 3.481(1)A. The closest
Ba ' . Ba distance is quite long, 4.750(1) A.
Thermogravimetric analyses of 1 reveal the expected
weight loss calculated from the chemical analysis,rzs1and at
temperatures greater than 800°C lead to the formation of
phase-pure BaSiO, (JCPDS File No. 26-1402).[321
Because no chemically significant differences in the Si-0
bonds are observed in the crystal structure, despite the
coulombic interactions with Ba2+ ions, we reexamined the
synthesis of the analogous hexacoordinate alkali metal silicates with two equivalents of LiOH o r K O H ; the reactions
provided the pentacoordinate complexes in lower yields.[271
The presence of a dication may be required to stabilize dianionic hexaalkoxysilicates; however, this conclusion is not
supported by preliminary theoretical studies.[2hJ
Preliminary a b initio calculations with a minimal basis set
(STO-2G) r331 were conducted on the free, isolated
[Si(OCH,CH,0)3]2- dianion to determine if it has any inherent asymmetry. Geometry optimization using analytical
gradient routines (GAMESS 1341) was conducted with C ,
symmetry. The resulting structure (which has a positive definite hessian) predicts Si-0 bond lengths that are within an
0.002 A range (1.805-1.807 A). The semiempirical AM1
method[351predicts all six bond lengths to be 1.794 A. Both
of these results are remarkably close to the average experimental Si-0 distance of 1.785 A in 1. (EXAFS studies of 1
indicate that the Si-0 bond lengths, which range from 1.753
to 1.809 A, are essentially identical to those in stishovite,
1.757 and 1.809 A.r391).Thus, at these levels of theory the
isolated dianion exhibits no bond-length distortion. The distortions seen in the X-ray structure of 4 probably result from
crystal packing effects, H-bonding, and Ba-0 coulombic
The monomers M[Si(OCH,CH,O),] (M = Mg, Ca, Ba)
and tractable polymeric derivatives may offer access to novel
silicon-containing materials, including glasses and ceramics.
Mg(OR),/Si(OR), mixtures have been used to form novel
[ I ] a) H. Meerwein, Jirsrus Liehig.r Ann. C/iwii. 1929. 476, 113: b) R. Muiler.
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S. Li. Y. Lu. G . Yang. Clirm. M a r r r . 1992, 4. 808- 812.
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(Murrr. Rrs. Suc. Srnip. Proc. 1986. 73. 529 -542).
R. M . Laine. K. Y. Blohowiak, T. R. Robinson. M. L. Hoppe, P. Nardi. J.
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Simple filtration followed by washing with anhydrous ethanol and acetonitrile and then drying at 100 C under vacuum 2-6 h provided analytically pure samples. Correct C.H,Ba.Si analysis for 1 . 3.25 HOCH,CH,OH [29].
[29] Crystal data for I . 3.25 HOCH,CH,OH: triclinic Pi, u =10.151(8),
h =13.865(5).~=15.709(6)A.x =102.90(3).p = 91.04(3).:. =109.75(3) .
V = ZOiX(1) A'.
Z = 4, c ) = 1.70
~ gcm
~ '. ~p(Mok,)
= 2.08 mm I .
h'(O00)= 1080. colorless rectangular plate, 0.2 x 0.5 x 0.44 mm3, 13038 reflections collected with 5 < 20 < 50 . 9298 unique reflections.
K,,,,, = 0.0312: 8999 reflections with 2 6 a ( F ) used in the refinement.
Data were collected on a Siemens R3;v diffrdctometer equipped with a
LT-2 low-tcmperiiture device d t 175K and corrected for absorption ( i p
scans) and secondary extinction. Thc structure was solved by direct methods a n d refined using Siemens SHELXTL PLUS. All non-H atoms were
refined anisotropically. H atoms were located by difference Fourier synthesis and refined isotropically. The 0 atom of one glycol of solvation is
disordered over two positions at refined occupancies of 0.700(1) and
0.300(1). The related H atom was not placed. 745 parameters were refined
with a wjeighting scheme [ii, ' = n * ( F )+ 0.000382(F;,)2]to give a final
convergence of R = 0 0282. R, = 0.0374. and a final difference electron
density max. of +OX0 e
', min. -0.82 e '. Further details of the
cryhtal structure investigation are available on request from the Director
of the Cambridge Crystallographic Centre, 12 Union Road, GB-Cambridge CB2lEZ (UK). on quoting the full journal citation.
[30] Solid-state "Si MAS NMR: 4 = -143.3. - 145.5; M. Hoppe, L. Nazar.
R. M. Laine. unpublished results.
[31] Significant H-bonding interactions for 1 : Distances [A] between the hydroxyl proton of a glycol molecule in the lattice and the numbered oxygen
of ii coordinated glycol. Approximate errors are 0.04 A. a) O(11) 1.84,
O(4) 1.97; b ) 0 ( 3 ) 1.60. O(6) 1.94: c ) 0 ( 7 ) 1.52, O(4) 1.91, O(13) 1.97;
d ) O ( l l ) 1.70. O(10) 1.87. Symmetry transformations a ) . x . ~:;, b) 1 .x,
1 -,I'.--1.
) 1 --I. 1 -,v. -:;d)2-.x,
[32] Iktiiils o f the pyrolysis experiments with the Mg. Ca. and Ba complcxes
will be described elsewhere. P. Kansal. K. W. Chew. M. L. Hoppe. R. M.
Liiine. unpublished results.
1.331 W. .I.Hehre. R. F. Stewart. J. A. Pople. J. Clievi7. P h ~ s 1969,
51. 2657.
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[37] L.-F. Zhang. M. L. Hoppe. J. A. Rahn, S.-M. Koo, R. M. Laine in Srnlhr\i.\
ond Pi-oi~cssinjio/ Coramii..s Scieni(fir l.ssue\ (Murcr. Res. Soc. S ~ ~ i i p .
Proi.. 1992. 249. 81 86).
[38] R. J. P. Corriu, C. Guerin, B. J. L. Henner. Q. Wang. Orjynmnrtu//ic.$
1991. 10. 3200.
1391 J. S Tse. D. D. Klug, B. X. Yang, X. H. Feng, R. M. Laine. Phw. Chmn.
. M i n c ~ submitted.
Previous attempts at modeling the active site in oxyhemerythrin proceeded logically from the synthesis of coordinatively unsaturated Fe: complexes, which were treated
with 0, a t low temperatures in aprotic solvents in the hope
of stabilizing the oxyhemerythrin form.''] Despite some preliminary success this approach is problematic because of the
extreme lability of Fe" complexes. Therefore, we decided to
try another approach in which more robust, coordinatively
unsaturated Fe!' complexes are treated with H202.[31We
report here on the synthesis and reactivity of a complex of
this type.
The dinuclear complex I, which is asymmetric with respect
to the two Fe"' ions with different coordination, was synthesized by the reaction of [(bpy),Fe,O,(ac),](PF,)"
(bpy = 2,2'-bipyridine; ac = acetate) with LFeC1,'4b1
(L = 1,4,7-trimethyl-l,4,7-triazacyclononane) and 2.2'bipyridine in acetonitrile. As the X-ray structure analysis[51
shows (Fig. 1 top), one Fe"' ion is bound to the tridentate
amine L and the other to 2,2'-bipyridine and a terminal
choride ion; the two metal ions are connected by an 0x0 and
A Hemerythrin Model Complex
with Catalase Activity**
By Bert Mauerer, Jonathan Crane, Jiirgen Schuler,
Karl Wieghardr," and Bernhard Nuber
In the past few years the structural and electronic properties of the active site of the metalloprotein hemerythrin have
been modeled successfully with low-molecular-weight systems."] Current efforts are concentrated on the preparation
of ,jicnctionul model complexes. The active form of the enzyme, deoxyhemerythrin, has one penta- and a hexacoordinate, high-spin iron(rr) ion bridged by a hydroxy or aqua
ligand, whereas in the 0, containing form, oxyhemerythrin,
both Fe ions are in the oxidation state + 111 and one Fe"' ion
binds a hydroperoxo ligand (Scheme 1).
Asp 106
Giu 58
Asp 106
Fig. 1. Crystal structures of the cations in 1 (top) and 2 (bottom). Selected
distances [A] and angles
1: Fe2-CIl 2.376(2); Fel-01 1.786(5). Fe2-01
1.797(5), Fel ' . Fe2 3.097(2). Fe2-N4 2.206(6), Fe2-N5 2.146(7). Fel-Nl
2.184(7). Fel-N2 2.252(7), Fel-N3 2.211(7); Fel-01-Fe2 119.6(3). 2 : Fel-01
1.806(8), F e l - 0 2 1.995(9), F e l - 0 4 2.017(9), Fel-N4 2.173(12). Fel-N5
2.242(15). Fel-N6 2.190(12). Fe2-01 1.785(7). Fe2-03 2.048(9). Fe2-05
2.017(8). Fe2-Nl 2.218(10), Fe2-N2 2.148(10). Fe2-06 2.153(9), Fel . . . Fe2
3.102(3); Fel-01-Fe2 1 1 9 3 4 ) .
Glu 58
Scheme 1
[*] Prof. Dr. K . Wieghardt. Dip[.-Chem. B. Mauerer, Dr. J. Crane,
Dip].-Chem. J. Schuler
Lehrstuhl fur Anorganische Chemie I der Universitdt
Postfach 102148, D-W-4630 Bochuni (FRG)
Dr 8. Nuber
Anorganisch-chemisches Institut der UniversitHt
Heidelberg ( F R G )
[**I This research was supported by the Fonds der Chemischen Industrie. J. C.
thanks the Alexander-von-Humboldt-Stiftung for a stipend.
two symmetric acetate bridges. Thus, this is an asymmetric
methemerythrin model complex. The chloro ligand in 1 can
by replaced by H,O in a reaction with AgCIO;H,O
CH,CI, giving complex 2, whose X-ray structure was also
ILFe(,i-O)(~-ac),Fe(bpy)CI]X la, X = PF,, Ib, X
[LFe(~c-O)(p-ac),Fe(bpy)(H,O)](CIO,), . CH,CI,
determined (Fig. 1 bottom).151The structural and electronic
properties of 1 and 2 are similar to those of their sym-
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