Pentacoordinate Silicon Compounds Research Articles

N-Silylmethyl-2-(1-Naphthyl)Acetamides: Synthesis, Structure and Computational Screening

Abstract: Synthesis of new hybrid organosilicon compounds based on the amides 1- naphthylacetic acid was described. N-Organyl-2-(1-naphthyl)-N-[(triethoxysilyl)methyl]- acetamides were obtained by the reaction of 1-naphthylacetyl chloride with α-silylamines RNHCH2Si(OEt)3 (R = Me, i-Pr and Ph). Their subsequent interaction with N(CH2CH2OH)3 led to the formation of N-organyl-2-(1-naphthyl)-N-(silatranylmethyl)acetamides. The structure of these hybrid compounds was characterized by 1H, 13C, and 29Si NMR spectroscopy. The structure of N-methyl- and N-isopropyl-2-(1-naphthyl)-N-(silatranylmethy)acetamides was confirmed by X-ray diffraction analysis. Results of computational screening showed that these silatranes are bioavailable and have drug-likeness.

Stereomutations in silicate oligomerization: the role of steric hindrance and intramolecular hydrogen bonding.

Stereomutation has previously been explored in the literature with regard to the different mechanisms and activation energy barriers between different forms. However, the forces which govern stereomutations within the intermediate steps in chemical reactions have not been explored previously, a topic addressed in this article. The process of silicate oligomerization has been chosen in this study and it is demonstrated that steric hindrance of molecules and intramolecular hydrogen bonding govern the stereomutation of intermediate pentacoordinate silicon compounds in the silicate oligomerization process. It could be observed that the combined effect of intramolecular hydrogen bonding and steric hindrance in the silicate oligomerization process facilitates the conventional berry pseudorotation mechanism rather than other pseudorotation mechanisms reported in the literature.

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives

AbstractStereogenic silicon centres in functionalised tetracoordinated organosilanes generally exhibit very high configurational stability under neutral conditions. This stability drops completely when higher coordination states of the silicon centre are reached due to rapid substituent exchange. Herein we describe the synthesis of chiral and neutral pentacoordinate silicon derivatives with high configurational stability. The zwitterionic nature of these air‐ and water‐tolerant species allows for the first time their direct and efficient optical resolution using chiral HPLC techniques. By means of this method, pentacoordinate silicon compounds exhibiting high Si‐inversion have been obtained as single enantiomers. A rationalisation of the enantiomerisation pathways has been also carried out using DFT calculations.

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives.

Stereogenic silicon centres in functionalised tetracoordinated organosilanes generally exhibit very high configurational stability under neutral conditions. This stability drops completely when higher coordination states of the silicon centre are reached due to rapid substituent exchange. Herein we describe the synthesis of chiral and neutral pentacoordinate silicon derivatives with high configurational stability. The zwitterionic nature of these air- and water-tolerant species allows for the first time their direct and efficient optical resolution using chiral HPLC techniques. By means of this method, pentacoordinate silicon compounds exhibiting high Si-inversion have been obtained as single enantiomers. A rationalisation of the enantiomerisation pathways has been also carried out using DFT calculations.

Pentacoordinate silicon compounds based on 2,2′-dihydroxyazobenzene ligand

The preparation and structures of 6,6-diorganylbenzo[d,h][1,3,6,7,2]dioxadiazasilonines containing the N→Si intramolecular dative bond are described. The complexes were characterized by NMR (1H, 13C, 29Si), IR and UV spectra. The crystal structures of 6-methyl-6-phenyl- and 6-(chloromethyl)-6-methylbenzo[d,h][1,3,6,7,2]dioxadiazasilonines were determined by X-ray diffraction.

Complementary bonding analysis of the N-Si interaction in pentacoordinated silicon compounds using quantum crystallography.

The N-Si interaction in two pentacoordinated silicon compounds is investigated based on a complementary bonding analysis, which consists of bonding descriptors from real space and orbital space. These are derived from X-ray wavefunction refinements of high-resolution X-ray diffraction data of single crystals and from isolated-molecule theoretical wavefunctions. The two pentacoordinated compounds only differ in one methylene group, so that the amino substituent is more flexible in one of the structures, hence probing the attractive or repulsive character of the N-Si interaction. All studies suggest weak dative interactions, which do, however, greatly influence the character of the Si-F bond: A strong N-Si interaction results in a weakened Si-F bond, which is quantified in this study experimentally and theoretically.

Metal free mild and selective aldehyde cyanosilylation by a neutral penta-coordinate silicon compound.

This study demonstrates the preparation and structural characterization of a Si(iv) hydride (PhC(NtBu)2SiH(CH3)Cl) (1) and its use as a catalyst for the cyanosilylation of a variety of aldehydes. Compound 1 represents the first neutral penta-coordinate silicon(iv) species that catalyzes cyanosilylation of aldehydes under mild conditions.

Saturated Heavier Group 14 Compounds as σ -Electron-Acceptor (Z-Type) Ligands.

This review article describes the chemistry of transition-metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ-electron-acceptor (Z-type) ligands and discusses the characteristics of bonds between the transition metal and Z-type ligand. Moreover, we review the iridium hydride mediated cleavage of E-X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.

The hydrolysis of (O[sbnd]Si)-chelate [N-(acetamido)methyl]dimethylchlorosilanes. DFT and MP2 study, QTAIM and NBO analysis

The hydrolysis reaction of (amido)dimethylchlorosilanes 1, 5, containing pentacoordinate silicon atom involved in the intramolecular coordination bond CO→Si, and the solvate complexes of 1 with water molecules 2–4 leading to silanol hydrochlorides was studied computationally at the B3LYP/6-311+G∗∗ and MP2/6-311G∗∗ levels of theory. Transition states of the hydrolysis of reactants 1–5 have a similar structure and contain a hexacoordinate silicon atom. The energy barrier of hydrolysis of pentacoordinate silicon compounds is much lower than that of their tetracoordinate analogs and is decreased with the increase of the O→Si or decrease of the SiCl bond strength in the complexes of reactants with water, and depends on the number and location of the H-bonded water molecules. On going from gas to solution the activation energy is decreased. The strength and the nature of the coordination bond CO→Si as well as of the NH⋯OH, CO⋯HO, Cl⋯HO and HO⋯HCl hydrogen bonds in the reactants and complexes of reactants and products with water is analyzed using the QTAIM and NBO methods.

Analysis of the hypersensitivity of the (29)Si NMR chemical shift of the pentacoordinate silicon compounds to the temperature effect. N-(Silylmethyl)acetamides.

Theoretical investigation of the phenomenon of hypersensitivity of the (29)Si NMR chemical shift, δ, in the pentacoordinate silicon compounds to the temperature effect has been performed by the example of N-(silylmethyl)acetamides MeC(O)NMeCH2SiX3 (X = Me, 1; OMe, 2; F, 3) and MeC(O)NMeCH2SiMe2F (4) with the use of experimental dynamic NMR (DNMR) (29)Si data. It is based on the following: (i) the analysis of the potential energy surface of molecules 1-4 in polar solvents and the energetics of interconversion between their possible isomeric forms; (ii) the calculations of δ at different temperatures taking into account the dependence of the dielectric constant (ε) of the medium on T, and (iii) the isolation of dynamic, geometrical, and polar contributions to the temperature drift of δ. The results obtained allowed us to give a consistent explanation of the DNMR (29)Si spectra of acetamides 1-4 and to elucidate the nature of an unusual effect of T on δ.

Correlation among the Gas-Phase, Solution, and Solid-Phase Geometrical and NMR Parameters of Dative Bonds in the Pentacoordinate Silicon Compounds. 1-Substituted Silatranes

Silatranes XSi(OCH2CH2)3N exhibit a good linear relationship between their experimental and calculated (IGLO and GIAO) values of the NMR chemical shifts of (15)N, δN, and the lengths of dative bonds Si←N, dSiN, determined in the gas phase (ED, CCSD), solutions (COSMO PBE0, B3PW91), and crystals (X-ray). An aggregate of the obtained data provides strong evidence that the gas-phase value of dSiN in MeSi(OCH2CH2)3N should be greater by ∼0.05 Å than that determined in the electron diffraction (ED) experiment (2.45 Å). Given this condition, a long-standing contradiction between the data of the structural (X-ray, ED) and NMR (15)N experiments for the molecules of 1-methyl- and 1-fluorosilatrane regarding the sensitivity of their coordination contact Si←N to the medium effect is resolved.

A new type of chiral pentacoordinated silicon compounds with azomethine ligands made from acetylacetone and amino acids

Azomethines of acetylacetone with amino acids have been utilized to prepare chiral pentacoordinated silicon compounds. The sodium salt of the ligand 6 has been synthesized from acetylacetone and L-phenylalanine in presence of sodium hydroxide. Reaction of 6 with equimolar amounts of dichlorodiorganosilanes in presence of triethylamine gave the chiral silicon complexes 7a–c. These contain a R2SiONO’ skeleton, wherein the pentacoordination is reached by coordination of the acetylacetonate oxygen, the imine nitrogen, and one carboxylate oxygen atom of the azomethine ligand to the SiR2 unit. 7a–c were characterized by elemental analyses, single crystal X-ray diffraction, UV–Vis–spectroscopy, value of optical rotation, and NMR-spectroscopy.

Migrations of Alkyl and Aryl Groups from Silicon to Nitrogen in Silylated Aryloxyiminoquinones

Silylation of the oxidized ligand in lead(II) bis(3,5-tert-butyl-1,2-quinone-(3,5-di-tert-butyl-2-hydroxy-1-phenyl)imine), Pb(ONOQ)2, with chlorosilanes RSiX2Cl (R = Me, Ph; X = Me, Ph, Cl) results in the tetracyclic, pentacoordinate silicon compounds (ON[R]O)SiX2 with a reduced, dianionic amine-bisphenolate ligand in which the methyl or phenyl group has migrated to nitrogen. Methyl migration is observed exclusively over phenyl migration, though phenyl migration is observed if no alkyl groups are present on silicon. Chlorotriphenylstannane reacts similarly, with migration of the phenyl group from tin to nitrogen. The five-coordinate products adopt a trigonal-bipyramidal geometry with long bonds to the apical amine donors (d(Si–N) = 2.3–2.9 Å). Chlorine occupies the other axial position in strong preference to methyl or phenyl, while methyl has a modest preference for the axial position in comparison to phenyl (8.7:1).

Study of Binuclear Silicon Complexes of Diketopiperazine at SN2 Reaction Profile

A series of binuclear pentacoordinate silicon complexes of diketopiperazine have been synthesized and substituent (leaving group) effects on the Si-O bond coordination have been studied by comparison of the five differently substituted analogous structures (X = F, Cl, OTf, Br, and I). Variable-temperature NMR spectroscopy supported by X-ray crystallography shows, for the first time in binuclear pentacoordinate silicon complexes, a complex equilibrium with both nonionic (O-Si) and ionic (Si-X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution in a binuclear pentacoordinate silicon compound at the silicon atom.

Reactions of Hydridochlorosilanes with 2,2′-Bipyridine and 1,10-Phenanthroline: Complexation versus Dismutation and Metal-Catalyst-Free 1,4-Hydrosilylation

Stable in the solid state and isolable in high yields are adducts of H(2)SiCl(2), HSiCl(3), and RSiCl(3) (R = Me, Ph) with the N,N'-chelating ligands 1,10-phenanthroline (phen; 1c), 2,2'-bipyridine (bipy; 1b), and (to a limited extent) N,N,N',N'-tetramethylethylenediamine (tmeda; 1a). The products were comprehensively characterized via multinuclear solution and solid-state NMR spectroscopy, including analysis of the (29)Si NMR chemical shift anisotropy tensors, Raman spectroscopy, elemental analyses, and, for SiCl(4)(phen) (2c), HSiCl(3)(bipy) (3b), H(2)SiCl(2)(bipy) (4b), MeSiCl(3)(phen) (5c), and PhSiCl(3)(phen) (6c), single-crystal X-ray structure analyses. The latter revealed that the nonchlorine substituents (i.e., H, Me, and Ph) are exclusively trans-disposed to the N-donor atoms of the chelating ligands. A dismutation of the complexes HXSiCl(2)(bipy) and HXSiCl(2)(tmeda) (X = H or Cl) was observed in polar solvents at elevated temperatures. This reaction is more pronounced when phen is used instead of bipy or tmeda. For MeHSiCl(2)(phen), in addition to undergoing H-Cl redistribution accompanied by the formation of 5c, an unexpected 1,4-hydrosilylation was observed. The latter was proven NMR-spectroscopically and by a single-crystal X-ray structure analysis of the product MeSiCl(2)(4H-phen) (7), a pentacoordinated silicon compound with a trigonal-bipyramidal arrangement of the subsituents and the methyl group located in an equatorial position.

Neutral Pentacoordinate Silicon(IV) Complexes with Silicon–Chalcogen (S, Se, Te) Bonds

The neutral pentacoordinate silicon(IV) complexes 1 (SiS2ONC skeleton), 2 (SiSeSONC), 3 (SiTeSONC), 6/9 (SiSe2O2C), 7 (SiSe2S2C), and 8/10 (SiSe4C) were synthesized and structurally characterized by using single-crystal X-ray diffraction and multinuclear solid-state and solution-state (except for 6-9) NMR spectroscopy. With the synthesis of compounds 1-3 and 6-10, it has been demonstrated that pentacoordinate silicon compounds with soft chalcogen ligand atoms (S, Se, Te) can be stable in the solid state and in solution.

A Kinetic and Mechanistic Study of Nucleophilic Substitution at a Pentacoordinated Silicon Atom

AbstractA model system for the exploration of nucleophilic substitution at a pentacoordinate silicon centre is reported. The degenerate nucleophilic exchange of free N‐methylimidazole (NMI) with coordinated NMI in a chiral pentacoordinate silicon compound, 4, has been studied by dynamic NMR spectroscopy. In 4 the axial ligands are N‐methylimidazole and an intramolecularly‐coordinated and chelating oxygen atom of an amido group. The equatorial ligands are N‐CH2, Ph and Me. Two competing mechanisms for the nucleophilic substitution at silicon have been distinguished from the data. One mechanism takes place with retention of configuration at silicon and is zeroth order in added nucleophile. This is interpreted as involving rate limiting dissociation of the N‐methylimidazole followed by rapid re‐association of another NMI molecule. The other mechanism takes place with both inversion and retention of configuration at silicon but is also of zeroth order in added nucleophile. A mechanism for this reaction in which the O‐Si coordination is lost in the rate‐determining step is consistent with the data. In this particular example there is no evidence for a mechanism in which pentacoordinate silicon is attacked to give a hexacoordinate intermediate or transition state.

N-(o-Aminophenyl)-2-oxy-4-methoxybenzophenoneimine – Si-chelation by a tridentate ONN ligand system versus benzimidazoline formation

Reaction of 2-hydroxy-4-methoxybenzophenone (o-HO-p-MeO–C6H3–C(Ph)=O) with o-phenylenediamine (o-H2N–C6H4–NH2) yielded the tridentate chelating ligand 1a (N-(o-aminophenyl)-2-oxy-4-methoxybenzophenoneimine, o-HO-p-MeO–C6H3–C(Ph)=N-(o-C6H4)–NH2). Ligand 1a reacts with diphenyldichlorosilane in the presence of triethylamine to yield the pentacoordinate silicon compound 2a (Ph2Si[o-O-p-MeO–C6H3–C(Ph)=N-(o-C6H4)–NH]). N-acetylation of 1a yields 1b, the reaction of which with Ph2SiCl2 surprisingly yields a silylated benzimidazoline 3.

Addition Reaction of Pentacoordinated Silicon Compounds by R<SUB>3</SUB>SiX with NR’<SUB>3</SUB>

对R3SiX（R=H、CH3；X=F、Cl、Br、I）与NR＇3（R’=H、CH3）的加成物用量子化学密度泛函方法在B3LYP／631g（d,p）基组下（X原子采用cep-121g基组）进行了两种加成方式的研究．一种是NR＇3沿Si-X键轴向位置的加成，另一种是NR＇3沿Si-X键侧向接近的加成．计算结果表明，前者更稳定且更容易形成加成物；Si上斥电子基团不利于Si-N键的形成，而N上斥电子基团则有利于Si-N键的形成；NH3-H3SiX系列和N（CH3）3-H3SiX系列均能以两种方式进行加成，NH3-H2（CH3）SiX系列仅能沿Si-X键轴向进行加成，而NH3-H（CH3）2SiX和NH3-（CH3）3SiX系列两种方式都不能进行加成；在同系列加成产物中，以X=Cl时所得加成物最稳定．讨论了所有加成物中各键的性能、NBO电荷变化、取代基对加成物结构和稳定性的影响，并对H3SiX（X=F、Cl、Br、I）与NH3及N（CH3）3加成物在有机溶剂中导电的可能性进行了讨论．

Complexation of tris(pentafluorophenyl)silanes with neutral Lewis bases

A detailed analysis of monodentate and bidentate complexation of tris(pentafluorophenyl)silyl (TPFS) derivatives with neutral Lewis bases was performed. The NMR spectroscopy and X-ray diffraction analysis (11 structures) were the key methods to characterize tetra- or pentacoordinate silicon compounds, whereas the peculiarities of crystal packing were analyzed by means of DFT calculations. The interaction of TPFS-X (X = F, Cl, OTf) with strong Lewis bases (HMPA, N-methylpyrrolidinone) may afford three different species: neutral pentacoordinate TPFS(X)-L, cationic tetracoordinate TPFS-L + X −, and cationic pentacoordinate TPFS-(L) + 2X −, representatives of each type were characterized by X-ray diffraction. A variety of complexes with bidentate complexation, featuring the trigonal bipyramidal geometry with apical C 6F 5-group was prepared and structurally characterized. The extent of Si–C apical bond elongation depends on the donating ability of the coordinating ligand, with the longest Si–C bond of 1.981(1) Å observed for six-membered complex of TPFS-ether of N-(2-hydroxybenzoyl)pyrrolidine.