Hexacoordinate Silicon Complexes Research Articles

Ylenes in the MII→SiIV (M=Si, Ge, Sn) Coordination Mode

The reaction of the methimazolyl (mt, i.e., 2-mercapto-1-methylimidazolide) substituted silane Si(mt)(4) with SnCl(2) and GeCl(2) in dioxane affords the paddlewheel-shaped complexes [ClSi(μ-mt)(4)MCl] (M=Sn (1) and Ge (2), respectively). These compounds represent the first crystallographically characterized hexacoordinate silicon complexes comprising a Sn or Ge atom in the Si coordination sphere. An attempt to synthesize the related silicon compound 3 [ClSi(μ-mt)(4)SiCl] instead afforded the trisilane [ClSi(μ-mt)(4)Si-SiCl(3)] (3a), which provides the first crystallographic evidence for the feasibility of oligosilanes with adjacent hexacoordinate Si atoms. One of the hexacoordinate Si atoms of 3a features the unprecedented (Si(2)S(4))Si skeleton. Natural bonding orbital (NBO) analyses of compounds 1, 2, 3a (and the target compound 3) revealed characteristics of M(II)→Si(IV) (for 2 and 3) or M(I)→Si(IV) (for 3a) dative bonding in the systems with M=Si and Ge, whereas compound 1 exhibits a covalent Sn(III)-Si(III) bond.

Neutral Penta‐ and Hexacoordinate Silicon(IV) Complexes Containing Two Bidentate Ligands Derived from the α‐Amino Acids (S)‐Alanine, (S)‐Phenylalanine, and (S)‐tert‐Leucine

The neutral hexacoordinate silicon(IV) complex 6 (SiO(2)N(4) skeleton) and the neutral pentacoordinate silicon(IV) complexes 7-11 (SiO(2)N(2)C skeletons) were synthesized from Si(NCO)(4) and RSi(NCO)(3) (R = Me, Ph), respectively. The compounds were structurally characterized by solid-state NMR spectroscopy (6-11), solution NMR spectroscopy (6 and 10), and single-crystal X-ray diffraction (8 and 11 were studied as the solvates 8 x CH(3)CN and 11 x C(5)H(12) x 0.5 CH(3)CN, respectively). The silicon(IV) complexes 6 (octahedral Si-coordination polyhedron) and 7-11 (trigonal-bipyramidal Si-coordination polyhedra) each contain two bidentate ligands derived from an alpha-amino acid: (S)-alanine, (S)-phenylalanine, or (S)-tert-leucine. The deprotonated amino acids act as monoanionic (6) or as mono- and dianionic ligands (7-11). The experimental investigations were complemented by computational studies of the stereoisomers of 6 and 7.

ChemInform Abstract: Synthesis and Structural Characterization of Novel Neutral Hexacoordinate Silicon(IV) Complexes with SiO2N4 Skeletons

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Novel neutral hexacoordinate silicon(iv) complexes with two bidentate monoanionic benzamidinato ligands

The novel neutral hexacoordinate bis(benzamidinato)silicon(iv) complexes 1-10 (SiN(4)X(2) skeletons; X = F, Cl, Br, C, N, S, Se) were synthesised and characterised by elemental analyses, single-crystal X-ray diffraction (except for 2) and NMR spectroscopy in the solid state and in solution. The dynamic behavior of 1 (SiN(4)Cl(2) skeleton) and 3 (SiN(4)F(2)) was additionally studied by variable-temperature NMR experiments. Compounds 1 and 2 (SiN(4)Br(2)) were obtained by reaction of SiCl(4) and SiBr(4), respectively, with two molar equivalents of the corresponding lithium amidinate. Compound 1 served as the starting material in the syntheses of 3-10, in which the two chloro ligands of 1 were substituted by two (pseudo)halogeno or one bidentate dianionic S,S, S,Se or Se,Se ligand. Compound 4 contains an SiN(4)C(2) skeleton and 5-7 contain an SiN(6) skeleton. With the preparation of 8 (SiN(4)S(2) skeleton), 9 (SiN(4)SSe) and 10 (SiN(4)Se(2)) it could be demonstrated that syntheses of hexacoordinate silicon(iv) complexes with soft chalcogeno ligand atoms are indeed feasible. Compounds 9 and 10 are the first hexacoordinate silicon(iv) complexes with Si-Se bonds.

A Distorted Trigonal Antiprismatic Cationic Silicon Complex with Ureato Ligands: Syntheses, Crystal Structures and Solid State 29Si NMR Properties

AbstractInsertion of phenyl isocyanate into the Si–N bond of N‐(trimethylsilyl)diethylamine yields the N′‐silylated N,N‐diethyl‐N′‐phenylurea 1, which undergoes transsilylation with SiCl4 to yield the C3‐symmetric cationic hexacoordinate silicon complex 3s+ [tris‐κ‐O,N′‐(N,N‐diethyl‐N′‐phenylureato)siliconium] as chloride salt, which was characterized crystallographically. The cationic complex 3s+ exhibits a distorted trigonal antiprismatic coordination sphere about the silicon atom with fac arrangement of the three N‐atoms (and the three O‐atoms) relative to one another. This C3‐symmetric complex undergoes isomerization into its asymmetric isomer 3a+ (mer arrangement of NNN or OOO relative to one another) in CDCl3 solution. Hence, two 29Si NMR signals appear and four sets of signals emerge in the 1H and 13C NMR spectra. Despite its pronounced axial symmetry, the 29Si NMR shielding tensor of the cation 3s+ in its chloride salt exhibits an unusually small span (less than 20 ppm), which was analyzed CP/MAS NMR spectroscopically and by computational methods.

Novel hexacoordinate organosilicon(IV) complexes of diethylenetriamine Schiff base with SiO2N3 skeleton

Some novel hexacoordinate silicon(IV) complexes with a SiO2N3 skeleton were synthesized by using potentially pentadentate ligand N,N'-diethylenetriamine-bis(salicylideneimine). The aforesaid ligand, obtained by the condensation of diethylenetriamine and salicylaldehyde, contains two N of azomethine C=N group, two O of phenolic -OH group and one N of -NH- group capable of coordination. The ligand reacts with RSiCl3 (R = CH3, C6H5, C2H5, CH=CH2) in the presence of triethylamine to yield hexacoordinate silicon complexes 1-4. The resulting complexes were characterized by IR spectroscopy, 1 H, 13 C, 29 Si NMR spectroscopy and elemental analysis. The geometry and hexacoordination was confirmed by comparing the experimental results with computational studies by using GAUSSIAN 03 series of programs. The IR frequencies and 29 Si NMR shifts calculated for isomer B with Density Functional Theory (B3LYP-3-21+G* basis set) correlate with experimental values and suggested the geometry of synthesized complexes. Total energies, dipole moment and bond lengths were calculated from the geometries optimized with semi-empirical methods.

Stability and Structure of Carbene-Derived Neutral Penta- and Hexacoordinate Silicon Complexes

The stability and structure of the penta- and hexavalent complexes of SiF4 and SiCl4 with different carbenes have been investigated computationally. Apart from the known carbene complexes containing a pentavalent silicon center, stable neutral bis-carbene complexes with hexavalent silicon are also predicted to be stable compounds. While the largest binding energies of the pentavalent structures are somewhat higher in the SiF4 derivatives (21−30 kcal mol−1) than those for the analogous SiCl4 compounds (17−19 kcal mol−1), the hexacoordinated compounds exhibit almost the same stabilities (40−60 kcal mol−1) for both the chlorine and the fluorine derivatives. These stabilities are significantly higher than those for the hitherto synthesized amino and phosphino complexes. Not only the imidazol-2-ylidene-type carbene complexes but also the hexavalent bis(diisopropylamino)cyclopropylen-3-ylidene complex of SiCl4 or SiF4 are suggested to be an appropriate synthetic target.

Synthesis and Structural Characterization of Neutral Hexacoordinate Silicon(IV) Complexes with SiO2N4 Skeletons

A series of novel neutral hexacoordinate silicon(IV) complexes with SiO(2)N(4) skeletons (compounds 4-8) was synthesized and structurally characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy ((13)C, (15)N, (29)Si). The silicon(IV) complexes each contain two bidentate monoanionic O,N Schiff base ligands and two cyanato-N or thiocyanato-N ligands. Compounds 4-8 were prepared from Si(NCO)(4) or Si(NCS)(4), whereby the complex formation involved some unexpected chemical transformations of the ligands.

Hexacoordinated Oligosilanes from a Hexacoordinated Silicon(IV) Complex Containing an O,N,N,O Salen-type and Thiocyanato-N Ligands

Two new neutral hexacoordinated silicon complexes with SiN(4)O(2) (6) and SiN(3)O(2)C (7) coordinating frameworks were synthesized by reaction of the O,N,N,O-donor salen-type ligand 1,2-bis[[(2-hydroxy-4-methoxyphenyl)(phenyl)methylene]amino]ethane (H(2)salen*) with Si(NCS)(4) and HMeSi(NCS)(2), respectively. The complexes 6 [Si(salen*)(NCS)(2)] and 7 [Si(salen*)Me(NCS)] were studied in the solid-state by (29)Si and (15)N CP/MAS NMR and in solution by (1)H, (13)C, and (29)Si insensitive nuclei enhanced by polarization transfer (INEPT) NMR, UV/vis and FT-IR spectroscopy. Elemental analysis and single-crystal X-ray diffraction analysis were used to confirm the composition and structure for compounds 6 and 7. Both complexes contain the dianionic salen-type ligand coordinated in an equatorial fashion to the silicon center, while the axial positions are occupied by two thiocyanato-N ligands for 6 and one thiocyanato-N and one methyl ligand for 7. Complex 6, which contains two Si-NCS functional groups, was used as monomer to produce a mixture of linear oligosilanes with a hexacoordinated silicon backbone (formulated SCN-[Si(salen*)](n)-NCS, n = 2-8) 8, via a Wurtz-type coupling reaction. Oligomers 8 were identified by solid-state (29)Si cross polarization-magic angle spinning NMR and solution (1)H and (29)Si NMR spectroscopy, matrix assisted laser desorption ionization-time of flight (MALDI-TOF), gel-permeation chromatography (GPC), FT-IR, UV/vis spectroscopy and thermogravimetric analysis (TGA). Conclusive evidence of the oligomeric nature of 8 was provided by MALDI-TOF spectrometry and was supported by quantitative solution (29)Si NMR and GPC studies.

Hypercoordinate Silicon Complexes Based on Hydrazide Ligands. A Remarkably Flexible Molecular System

Though only one row apart on the periodic table, silicon greatly differs from carbon in its ability to readily form five- and six-coordinate complexes, termed "hypercoordinate silicon compounds". The assorted chemistry of these compounds is varied in both structures and reactivity and has generated a flurry of innovative research endeavors in recent years. This Account summarizes the latest work done on a specific class of hypercoordinate silicon compounds, specifically those with two hydrazide-derived chelate rings. This family is especially interesting due to the ability to form multiple penta- and hexacoordinate complexes, the chemical reactivity of pentacoordinate complexes, and the observation of intermolecular ligand crossovers in hexacoordinate complexes. Pentacoordinate complexes in this family exhibit marked structural flexibility, as demonstrated by the construction of a complete hypothetical Berry-pseudorotation reaction coordinate generated from individual crystallographic molecular structures. Although hexacoordinate complexes generally crystallize as octahedra, with a decrease in the ligand donor strength the complexes can crystallize as bicapped tetrahedra. Hexacoordinate complexes bearing a halogen ligand undergo a solvent-driven equilibrium ionic dissociation, which is controlled by solvent, temperature, counterion, and chelate structure and has been directly demonstrated by conductivity measurements and temperature-dependent (29)Si NMR. Hexacoordinate silicon complexes can also undergo reversible neutral nonionic dissociation of the N-Si dative bond. Ionic pentacoordinate siliconium salts react readily via methyl halide elimination, initiated by their own counterion acting as a base. Pentacoordinate complexes can also undergo intramolecular aldol condensations of imines, which may find potential as a template for organic synthesis. In addition, these complexes are capable of performing an uncatalyzed intramolecular hydrosilylation of imine double bonds. Perhaps the most striking manifestations of flexibility are the facile and complete intermolecular ligand crossovers. Crossovers have been observed between different hexacoordinate complexes, and between complex molecules and their differently substituted precursors, and take place within minutes. Although the precise mechanisms of these transformations remain elusive, NMR and single-crystal X-ray diffraction measurements have shed light on these interesting phenomena. A profusion of crystallographic data and careful NMR experimentation has led to an improved understanding of penta- and hexacoordinate hydrazide-based silicon dichelates. The diverse chemical reactivity of these complexes demonstrates both the scope and complexity of silicon chemistry. Future exploration into the structures and chemistry of hypercoordinate silicon will continue to enhance our understanding and appreciation of this unique element.

Synthesis and Structural Characterization of Novel Neutral Hexacoordinate Silicon(IV) Complexes with SiS2O2N2 Skeletons

AbstractA series of novel hexacoordinate silicon(IV) complexes with two tridentate dianionic S,N,O ligands was synthesized, starting from Si(NCO)4. All the compounds synthesized were structurally characterized by single‐crystal X‐ray diffraction and/or solid‐state NMR spectroscopy. In addition, most of the compounds were characterized by solution NMR spectroscopy.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Competing reactions of hypercoordinate silicon dichelates

AbstractNeutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino‐donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol‐type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four‐membered ring. In the absence of α‐protons necessary for the condensation, the four‐membered ring residue adds directly to one of the imino‐carbon atoms forming a new CC bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 α‐protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t‐butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and Structural Characterization of Novel Neutral Higher‐Coordinate Silicon(IV) Complexes with SiON3C and SiON4C Skeletons

AbstractThe neutral pentacoordinate silicon(IV) complex 10 (SiON3C skeleton) and the neutral hexacoordinate silicon(IV) complex 11 (SiON4C skeleton) were synthesized, starting from methyldi(thiocyanato‐N)silane (7). In addition to their monodentate thiocyanato‐N and methyl ligands, these compounds contain a tridentate dianionic O,N,N ligand (10) or a tridentate monoanionic O,N,N ligand (11). Compounds 10 and 11 were characterized by single‐crystal X‐ray diffraction and solid‐state and solution NMR spectroscopy. According to these studies, compounds 10 and 11 exist in solution as well.

Neutral Hexa- and Pentacoordinate Silicon(IV) Complexes withSiO6andSiO4N Skeletons

The neutral heteroleptic hexacoordinate silicon(IV) complexes 4 and 5 (SiO(6) skeletons) and the neutral pentacoordinate silicon(IV) complexes 7-9 (SiO(4)N skeletons) were synthesized, starting from the hexacoordinate precursor 2 and the pentacoordinate precursor 6, respectively. In these reactions, two monoanionic cyanato-N ligands are replaced by one dianionic bidentate O,O-chelate ligand. Compounds 4, 5, and 7-9 were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. The chiral silicon(IV) complexes 4, 5, 7, and 8 were obtained as racemic mixtures, whereas 9 was isolated as a cocrystallizate consisting of the two diastereomers, (C,S)-9 and (A,S)-9 (ratio 1:1). The stereodynamics of 5 and 8 were studied by variable-temperature (1)H NMR experiments.

Competitive Molecular Rearrangements in Hexacoordinate Cyano-Silicon Dichelates

Pentacoordinate siliconium chloride or neutral hexacoordinate silicon complexes with imino-nitrogen donor groups react with cyanotrimethylsilane in two competing reactions, leading either to addition of the cyano group to the imino carbon or to hexacoordinate cyano-silicon complexes. The latter may further transform to a rearranged tricyclic pentacoordinate complex. The common driving force for these reactions seems to be the conversion of one of the two initial N→Si dative bonds, present in the starting complexes, to a shorter formal covalent bond.

Syntheses of Allyl- and 3-Silylpropyl-substituted Salen-like Tetradentate Ligands via Hypercoordinate Silicon Complexes

The reaction of allylchlorosilanes (AllSiCl3, AllSiPhCl2) with the tetradentate salen-like ligand 1, o-HO-p-MeO-C6H3-C(Ph)=N-(CH2)2-N=C(Ph)-C6H3-p-OMe-o-OH, yields pentacoordinate silicon complexes of an allyl-substituted ligand system (o-O-p-MeO-C6H3-C(Ph)=N-(CH2)2- N-C(Ph,All)-C6H3-p-OMe-o-O)SiR (2a: R = Ph, 3: R = Cl). The allyl shift step involved in the formation of 2a and 3 probably occurs via an intermediate hexacoordinate silicon complex. This reaction is diastereoselective. The missing diastereomer of 2 (2b) was prepared using an alternative synthesis route, which starts from the trimethylsilyl derivative of ligand 1 o-Me3SiO-p-MeO-C6H3-C(Ph)=N- (CH2)2-N=C(Ph)-C6H3-p-OMe-o-OSiMe3. The diastereomers of 2 obtained from modified reaction pathways give rise to suggestions about the mechanism of formation of these complexes. Further functionalization of the allyl-substituted ligand system of 3 was carried out by hydrosilylation with HSiCl3 to yield complex 4 (o-O-p-MeO-C6H3-C(Ph)=N-(CH2)2-N-C(Ph)[(CH2)3-SiCl3]- C6H3-p-OMe-o-O)SiCl.

Intramolecular interligand charge transfer and a red hexacoordinate Si-complex with salen-type ligand vs. colorless tetracoordinate salen-Si-complexes with similar substituents

The reactions between the tetradentate ligand o-HO- p-OMe–C 6H 3–C(Ph) N–(CH 2) 2–N C(Ph)–C 6H 3- o-OH- p-OMe ( 1) and the dichlorosilacycles [PhN-(CH 2) 2–NPh]SiCl 2 ( 2) and [Me 3SiN-( o-C 6H 4)-NSiMe 3]SiCl 2 ( 4) afforded the red colored complexes [ o-O- p-OMe–C 6H 3–C(Ph) N–(CH 2) 2–N C(Ph)–C 6H 3- o-O- p-OMe]Si{2N}, {2N} = [PhN–(CH 2) 2–NPh] ( 3), {2N} = [Me 3SiN-( o-C 6H 4)-NSiMe 3] ( 5) with hexacoordinate central silicon atom, respectively. The intense red color of these compounds originates from interligand charge transfer [(amide) → Si → (salen)]. Compound 3a, a cyclic dimer of 3 with tetracoordinate Si atoms, was isolated from the reaction mixture. It is colorless, thus demonstrating that the interligand charge transfer from the amide ligand to the salen-type ligand requires the connection of the imine N atoms to the Si atom and is not possible via an Si–O-connection to the salen-type ligand only. The reaction between ligand 1 and the dichlorosilacycle 7 [Ph 2MeSiN–(CH 2) 2–NSiMePh 2]SiCl 2 with bulky substituents at the amide N atoms gave rise to the monomeric salen-Si-compound 8 [ o-O- p-OMe–C 6H 3–C(Ph) N–(CH 2) 2–N C(Ph)–C 6H 3- o-O- p-OMe]Si[Ph 2MeSiN–(CH 2) 2–NSiMePh 2] bearing only tetracoordinate Si atoms. Compound 8 is also colorless. The formation of 8 from 1 and 7 was proven to occur via a hexacoordinate silicon complex 8a, which, however, could not be isolated so far.

Easy access to chiral penta- and hexacoordinate silicon compounds

Chiral penta- and hexacoordinate silicon complexes are accessible with O,N,O-ligands, which contain exclusively one stereoisomer prepared from amino acids of the chiral pool.

Synthesis and Structural Characterization of Neutral Higher‐Coordinate Silicon(iv) Complexes with Tridentate Dianionic Chelate Ligands

The neutral pentacoordinate silicon(IV) complexes 8 and 9 with an SiO2N3 skeleton and the neutral hexacoordinate silicon(IV) complex 10.1/2 CH3CN with an SiO4N2 skeleton were synthesized, starting from tetra(cyanato-N)silane or tetra(thiocyanato-N)silane. Compounds 8 and 9 contain one tridentate dianionic ligand derived from 4-[(2-hydroxyphenyl)amino]pent-3-en-2-one and two monodentate singly charged cyanato-N or thiocyanato-N ligands bound to the silicon(IV) coordination center, whereas the silicon(IV) center of 10 is coordinated by two of these tridentate dianionic ligands. All compounds were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of the compounds studied, the experimental investigations were complemented by computational studies.

Donor-Stabilized Silyl Cations. 9. Two Dissociation Patterns of Hexacoordinate Silicon Complexes: A Model Nucleophilic Substitution at Pentacoordinate Silicon1

Hexacoordinate silicon bischelates with bulky monodentate alkyl ligands tend to ionize, while those with electron-withdrawing CF 3 substituents on the chelate rings resist ionization. A hexacoordinate silicon complex withboth of these structural features was prepared and was found, by its temperature-dependent 2 9 Si NMR spectroscopy, to undergo a reversible neutral dissociation of the N-Si dative bond, which satisfies both tendencies. The results are supported by crystallographic structure analyses and by nonempirical MO calculations of isodesmic reaction energies.