Pentacoordinate Silicon Compounds Research Articles

Templated Rearrangement of Silylated Benzoxazolin-2-ones: A Novel Tridentate (ONO)2- Chelating Ligand System

The reaction of phenyltrichlorosilane with benzoxazolin-2-one gives rise to phenyltris(benzoxazolin-2-on-1-yl)silane, which thermally rearranges to a pentacoordinate silicon compound bearing a novel (ONO) tridentate chelating ligand with interesting coordination properties.

A Stable, Highly Fluorescent Vinylogous β-Diketonate Silicon Complex

We report the synthesis of a novel, highly fluorescent pentacoordinate silicon compound which is apparently the first example of a vinylogous β-diketonate complex. The compound was generated by slow, solid-state oxidation of a methoxysilane derivative of tetraphenylcyclopentadiene.

Pentacoordinate Silicon Compounds: Structure and Bonding

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Theoretical study of the structure of (O→Si)-(acetoxymethyl)trifluorosilane dimers

According to the data of quantum chemical calculations (HF, DFT), the isolated sp,sp conformer of (acetoxymethyl)trifluorosilane — pentacoordinated silicon compound — forms dimers with two intermolecular bonds Si-F→Si. The =O→Si intramolecular coordination bonds become stronger, which leads to the general stabilizing effect. In polar media, dimer formation is energetically unfavorable relative to monomer formation.

Dimethyl-, diphenyl- and cyclotetramethylene silylchloride derivatives of guanidine, their synthesis and structure. Formation of pentacoordinated silicon compounds and an unexpected diionic disila-diguanidinium dichloride

The reaction of 1,3,4,6,7,8-hexahydro-2 H-pyrimido[1,2 a]pyrimidinyl potassium, [hpp] −K + ( 1), with dimethyldichlorosilane, diphenyldichlorosilane or cyclotetramethylenedichlorosilane (1,1-dichloro silolane) in THF resulted in the formation of the in solid-state unexpected diionic dimer λ 4 Si-[Me 2SiCl(hpp)] 2 ( 2) (76%), as well as λ 5 Si-Ph 2SiCl(hpp) ( 3) (87%) and λ 5 Si-(CH 2) 4SiCl(hpp) ( 4) (28%). Compounds 3 and 4 are the first synthesised examples of four ring (–λ 5 Si–N–C N–) guanidine structures. 1H and 13C NMR show the compounds to have a symmetric guanidine unit in solution. 29Si NMR analysis shows the structures, in selected solvents, to be of λ 5 Si-type for 3 and to the major part for 2. Compound 4, on the other hand, is of ionic λ 4 Si-type. The syntheses and molecular structures of the compounds are discussed.

A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

Remarkable hydrolytic stability in the range pH 7.0–8.0 is exhibited by the zwitterionic λ5Si-silicate 1, which can even be prepared in water and forms a stable hydrate 1⋅H2O. The synthetic route to 1 and the structure of the zwitterion in the crystal (O red; N blue; Si green; C gray) are shown in the picture.

A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

Ab initio and DFT study of the structure of pentacoordinated silicon compounds: Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes

The structure and thermodynamic stability of Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes were studied at the ab initio (HF, MP2 with different basis sets) and DFT (B3LYP/6-31G*) levels of theory. Performed calculations have demonstrated that these compounds exist explicitly in chelate forms with pentacoordinate silicon atom with energetic advantage of these forms over nonchelate structures equal to ∼3–6 kcal/mol (at T=0 and 298 K) depending on the nature of axial substituent at the silicon atom (X=OCOCH 3, OCOCF 3, F, Cl, Br). According to the Bader analysis of electron distribution, chelate cycles SiCNCO in the compounds studied are closed by weakly covalent Si–O bonds. A decrease in the Si–O bond distance upon variation of X is accompanied by increasing the stability of the (O–Si)chelate complexes and the extent of electron density transfer from the donor oxygen atom to the acceptor XSiC 3 fragment. The negative sign of the multi-center bond indices calculated for the X–Si–O fragment of Si-substituted (O–Si)dimethyl( N-acetylacetamidomethyl)silanes is suggestive of the (3c-4e) character of bonding in this fragment.

Novel pentacoordinate silicon compounds bearing [Si-N-C-N-C-N] chelate ring derived from biguanide ligands

The reactions between secondary carbosilanes 1– 3 and 1-propylbiguanide/1-phenylbiguanide, H 6bigR proceed via SiH/NH dehydrocoupling and afford 1,4-bis(silyl)-5-propyl/phenylbiguanides with the general formula (R 1R 2 2SiCH 2CH 2SiMeH) 2·H 4bigR, [R 1=Ph, R 2=Me, R= n Pr ( 4), R=Ph ( 5); R 1=Me, R 2=Ph, R= n Pr ( 6), R 1=R 2=Et, R=Ph ( 7)]. These compounds represent a new family of pentacoordinate silicon derivatives comprising of [Si-N-C-N-C-N] chelate ring and have been characterized by elemental analysis, FAB mass, IR and multinuclear ( 1H, 13C and 29Si) NMR spectral studies.

Modelling nucleophilic substitution at silicon using solution 19F-NMR chemical shift, 1JSiF and 2JCF coupling constant data of pentacoordinate silicon compounds. Correlation with other magnetic nuclei and X-ray structures

Solution 1 J SiF and 2 J CF NMR coupling constant and 19F-NMR chemical shift data have been analysed in a series of pentacoordinate silyl monofluoride complexes used to model nucleophilic substitution at silicon. Patterns in the data reveal strong correlations between both coupling constants and 19F-NMR chemical shifts and the degree of substitution displayed by each. Excellent correlation is obtained between the new data and our previous 13C- and 29Si-NMR studies to further confirm the validity of the NMR technique for structural correlation in solution. By pooling the X-ray crystal structures of a large number of compounds of this class from both our own results and the literature we are able to provide a possible explanation for some of the trends in the NMR data we observe in solution.

Theoretical investigation on pentacoordinated silicon compounds of H 3SiX (X=F, Cl, Br, I) with NH 3

The halosilane–ammonia adducts H 3SiX·NH 3 (X=F, Cl, Br) were studied with ab initio calculations at the G2(MP2) level. By following the G2(MP2) theory and using effective core potentials for F, Cl, Br, and I atoms, the adducts H 3SiX·NH 3 (X=F, Cl, Br, I) were also calculated. The calculations show that each halosilane can form two kinds of adducts with NH 3. One of them is an axial position adduct and the other is a Si–X bond side-adduct. The former is more stable and easy to be formed. The bonding properties of all adducts were analyzed and the influences of various halogen atoms on structures and stability of adducts were discussed in this paper. It is predicted that adducts of H 3SiX (X=Cl, Br, I) with ammonia or amines will give conducting solutions in solvents, which is well agreed with the experimental results.

Pentacoordinate spirosilicate anion, bis(2,2′-biphenyldiyl)methylsilicate, synthesized by the lithium cleavage of dimethoxyethane

The reaction of spirobisilafluorene (1) with lithium in dimethoxyethane produces lithium 1-methyl-spirobisilafluorenide (2), a stable pentacoordinate silicon compound with five carbon ligands, and lithium 2-methoxyethoxide, which was identified by trapping with Ph3SiCl to give Ph3Si-OCH2CH2OCH3 (4). The X-ray crystal structure of 2 shows that the geometry at silicon is an idealized trigonal bipyramid, slightly distorted toward a square pyramid. Methanolysis of 2 cleaves a Si-aryl bond producing a methyl biphenylsilafluorene, 3. Crystal structures are reported for 3 and 4.

Synthese des Cyclotetrasiloxans [(Me)(C 6H 4CH 2NMe 2-2)-SiO] 4 und dessen strukturanalytische Untersuchung

The cyclotetrasiloxane [(Me)(C 6H 4CH 2NMe 2-2)SiO] 4 ( 2) can be synthesized by hydrolysis of the pentacoordinated silicon compounds (C 6H 4CH 2NMe 2-2)MeSiCl 2 ( 1) or (C 6H 4CH 2NMe 2-2)MeSi(OR) 2 ( 3a, R=CH 2CHCH 2; 3b, R=CH 2CCH) in quantitative yield. The solid-state structure of 2 is reported. Compound 2 crystallizes in the triclinic space group P-1 with the following cell parameters: a=9.575(2), b=15.018(3), c=16.021(4) Å; α=94.72(12), β=90.96(2), γ=103.75(2)°; V=2228.5(9) Å 3 and Z=2. Due to sterical hindrance the silicon atoms in 2 are tetra-coordinated, which differs from the starting materials 1, 3a and 3b, where hypervalent silicon centers exist.

Reactions of silamorpholinones and acylsilamorpholines with electrophilic reagents. X-ray structure of products including a pentacoordinated silicon compound

Reactions of 2-sila-5-morpholinones, 4-acyl-2-silamorpholines and 4-acyl-2,6-disilamorpholines with electrophilic reagents generally lead to the opening of the sila- or disilacycle by cleavage of the Si–O bond with subsequent rearrangement to form five-membered chelate derivatives where the amide oxygen atoms coordinate with the silicon to form pentacoordinate silicon species. Multinuclear NMR spectroscopy and X-ray diffraction studies were used for structural investigation of the products. 4-Acyl-2,6-disilamorpholines initially form adducts with strong acids where the amide oxygen is protonated by the acid as demonstrated by X-ray crystallography.

Modelling nucleophilic substitution at silicon using hypervalent silicon compounds based on urea ligands

The X-ray crystal structure of a pentacoordinate silicon compound, 3, with a urea ligand shows partial Si–O bond formation and Si–Cl cleavage. The solution NMR parameters for the related series of compounds with different leaving groups provides structural information and this can be used to model substitution at silicon. The corresponding thioureas exhibit N–Si bond formation with the silicon remaining tetracoordinate.

Pentacoordinate Silicon Compounds with SiO5 Skeletons Containing SiOH or SiOSi Groups: Derivatives of the Pentahydroxosilicate(1−) Anion [Si(OH)5]- and Its Anhydride [(HO)4Si−O−Si(OH)4]2-

Three novel pentacoordinate silicon compounds with SiO5 skeletons containing SiOH or SiOSi groups were synthesized: triethylammonium bis[benzilato(2−)-O1,O2]hydroxosilicate (1; isolated as rac-1), anilinium bis[benzilato(2−)-O1,O2]hydroxosilicate [2; isolated as a conglomerate consisting of (Λ)-2·THF and (Δ)-2·THF], and anilinium tetrakis[benzilato(2−)-O1,O2]-μ-oxo-disilicate (3; isolated as rac-3·2CH3CN). The λ5Si-hydroxosilicate and λ5Si,λ5Si‘-μ-oxo-disilicate anions of these salts formally derive from the pentahydroxosilicate(1−) anion [Si(OH)5]- and its anhydride [(HO)4Si−O−Si(OH)4]2-, respectively. All compounds were characterized in solution (1H, 13C, and 29Si NMR) and in the solid state (29Si VACP/MAS NMR, crystal structure analyses).

A new relation between bond valence and bond distance.

A new empirical relationship s = s0(R0 - lambda)3/(R - lambda)3 between bond distances and bond valences is proposed, where s is the bond valence for the bond distance R, s0 is the reference bond valence for the reference system with the reference bond distance R0 and lambda is the sum of the cation radii of bonding atoms defined by Pauling. Since lambda is the size of the inner electron region, R - lambda represents the valence electron region between bonding atoms. The new relationship was derived based on the following three hypotheses. (i) The number of electrons (p) in the volumes of (R - lambda)3 in a coordination polyhedron are equal, even if the bond distances are not equal to each other. (ii) The average electron density p/(R - lambda)3 is a measure of covalent bond strength. (iii) The sum of the average electron densities around the central atom is conserved, even if the coordination number changes. The new relationship is applicable not only to polyhedra with one type of ligand atom, but also to polyhedra with two or more types of ligand atoms and explains why the Brown-Shannon formula [Brown & Shannon (1973). Acta Cryst. A29, 266-282] and the Brown-Altermatt formula [Brown & Altermatt (1985). Acta Cryst. B41, 244-247] work well. The new relationship was applied to a penta-coordinated silicon compound, strong hydrogen-bond systems and some organic compounds with carbon-carbon bonds.

Theoretical Study of Ultraviolet Absorption Spectra of Tetra- and Pentacoordinate Silicon Compounds

Single-excitation configuration interaction (CIS) and time-dependent density functional (TD-DFT) calculations have been carried out on the excited singlet states of tetracoordinate trisilane [(CH)3Si]2Si(Cl)CH3 (2) and pentacoordinate trisilane (5) to explain the remarkable red shift of the λmax (Δλmax = 10 nm) exhibited by 5. The calculated λmax of (H3Si)2Si(Cl)CH2NHCHO (3) was red-shifted by 11.2 nm with respect to that of (H3Si)2Si(Cl)CH3 (1), in agreement with experiment. In both compounds, the strongest absorption originates from the σSiSi → σ*SiSi transition. Compared with that of 1, the σSiSi orbital of 3 was substantially destabilized, whereas the σ*SiSi orbital was slightly changed. With the additional calculations on tetracoordinate model compounds having an Si−O bond, we concluded that the red shift of the UV spectrum of 5 is caused by the antibonding interaction of the pπ orbital on the oxygen atom and the σSiSi orbital of the same symmetry.

The Synthesis of Silicon-containing Polyester Directly from SiO2 and Characterization of Its Structure

ABSTRACTA new approach to the synthesis of a novel silicon-containing polyester by noncarbothermal method directly from SiO2 was studied. At first, a high reactive pentacoordinate organic silicon compound (KSi(OCH2CH2O)2(OCH2CH2OH)) was prepared by the reaction of the fumed silica with ethylene glycol (EG) and KOH. Then, a novel double hydroxy tetracoordinate silicon monomer (OCH2CH2O)Si(OCH2CH2OH)(OCH2CH2OCH2CH2OH) was synthesized by the reaction of ClCH2CH2OH with reactive pentacoordinate silicon compound. Finally, a silicon-containing polyester was synthesized by low temperature solution condensation between tetracoordinate silicon compound and p-phthaloyl chloride in dimethyl sulfoxide (DMSO). The product was characterized by FTIR, TG, GPC, etc. The IR spectra of polyester exhibited a characteristic strong absorption band for ester carbonyl in the 1722.2cm-1. IR spectra also showed absorption at 1124.6cm-1 due to the O-Si-O group. TG analysis showed its good thermal stability. GPC analysis showed that the number average molecular weight of this siliconcontaining polyester was about ninety thousand.

Inductive Effects in Neutral Pentacoordinated Silicon Compounds Containing a Si ← N Dative Bond. A Theoretical Study

Using ab initio MP2 and DFT calculations, we have studied the structure of a series of pentacoordinated silicon compounds containing a Si ← N dative bond and the influence of inductive effects on the Si ← N bond. The study includes two series of model systems, namely, Si(NH3)XnY4-n (X = OH, Y = H or CH3; n ranging from 0 to 4) and Si(NH3)(NH2)H2X (X = Cl or H), methyl(2,2‘,2‘ ‘-nitrilotriethoxy)silane, methyl(2,2‘,3-nitrilodiethoxypropyl)silane, 8-(trifluorosilylamino)quinoline, 8-(trimethylsilylamino)quinoline, a dimer of Me2NSiH2Cl, and a pentamer of Me2NSiH3. All these structures have dative Si ← N bonds. The nature of the bonds was analyzed using the Laplacian of the charge density distribution and the natural bond orbital (NBO) partitioning scheme. The calculations show that the dissociation energy of the Si ← N dative bond in the model systems is always lower than 9 kcal mol-1 and that the Si ← N bond length gets shorter as the number of electron-withdrawing groups linked to silicon increases. The different aggregation mode between the dimer of Me2NSiH2Cl and the pentamer of Me2NSiH3 is also analyzed.