Si Dative Bond Research Articles

Methacrylate Silatrane: New Building Block for Development of Functional Polymeric Coatings through Controlled Silanization.

Organosilicons are prevalent for the development of nanostructures, adhesives, fillers, and surface functionalization due to their ease of operation, availability, and effective modification on various surfaces. 3-(Trimethoxysilyl) propyl methacrylate (TMSPMA) is a widely used commercial silane for designing hybrid polymers via radical polymerization for a wide spectrum of applications. However, the chemical stability and processibility of TMSPMA encounter burdens due to its susceptibility to hydrolysis and aggregation, resulting in limiting its functionality and implementations. In this work, methylacrylate silatrane (MAST) was newly developed to bear a chemically stable tricyclic caged silatranyl ring and a transannular N → Si dative bond for excellent stability, processability, and progressive deposition. A complete, uniform, and thin assembled adlayer of MAST on a silicon wafer was verified by a contact angle goniometer, ellipsometry, atomic force microscopy, X-ray photoelectron microscopy, and Fourier transform infrared spectroscopy. The good homogeneity and molecular orientation of the MAST coatings are attributed to controlled silanization on oxide surfaces and strong intermolecular hydrogen bonds between internal urea groups. Moreover, the zwitterionic monomer of 2-methacryloyloxyethyl phosphorylcholine (MPC) was employed to copolymerize with MAST or TMSPMA to afford macromolecular modifiers of p(MPC9-co-MAST1) and p(MPC9-co-TMSPMA1), respectively, for surface modification and antifouling properties on silicon substrates. p(MPC9-co-MAST1) well preserved the reactivity of the silatrane groups after the polymerization process, whereas the hydrolysis of silane groups of p(MPC9-co-TMSPMA1) obviously occurred, giving rise to aggregation of polymer chains. Therefore, the p(MPC9-co-MAST1) film on surfaces exhibited superior wettability, grafting density, and antifouling properties compared to p(MPC9-co-TMSPMA1). Accordingly, we envision the great potential of the MAST building block for the development of functional hybrid polymers, well-defined polymeric thin films, and nanomaterials.

Versatile Thiol- and Amino-Functionalized Silatranes for in-situ polymerization and Immobilization of Gold Nanoparticles

BackgroundOrganosilanes have been massively implemented as building blocks in the fields of sol–gel material synthesis, surface functionalization, and nanotechnology. However, organosilane has been humbled by limitations, such as poor reproducibility, fast hydrolysis and heterogeneous molecular orientation. An emerging silatrane is gaining tremendous interest due to the unique tricyclic caged structure and transannular N→Si dative bond to provide chemical stability in aqueous solution and controllable deposition on surfaces. MethodsAminopropyl silatrane (APS), Mercaptopropyl silatrane (MPS) and 3-Mercaptopropyl trimethoxysilane (MPTMS) were applied to modification of silicon oxide substrates to in-situ graft hydrophilic polymers and immobilize gold nanoparticles (AuNPs). Thin and uniform silatrane coatings have been proved by atomic force spectroscopy, ellipsometry and X-ray photoelectron spectroscopy. Significant FindingsAntifouling poly(ethylene glycol) methacrylate (PEGMA) was grafted separately on MPS and MPTMS via surface-initiated thiol-ene photopolymerization. We found that MPS-PEGMA films were 30% superior to MPTMS-PEGMA films in protein repellency. Moreover, APS and MPS were co-deposited to attract and conjugate gold nanoparticles. AuNPs cluster size on APS films was found to increase 3.43 times along with broader AuNPs absorbance peaks after acid treatment, whereas cluster size increased only 4% on mixed silatrane-modified films. These discoveries are pertinent to robust and versatile properties that silatrane offers.

N-[(Trifluorosilyl)methyl]carboxanilides: Synthesis and structural features

The first representatives of (O-Si) chelated N-[(trifluorosilyl)methyl]anilides were synthesized. N-(Trimethylsilyl)acetanilide reacts with bifunctional silane ClCH2SiF3 with the formation of N-[(trifluorosilyl)methyl]acetanilide as the only product. N-[(Trifluorosilyl)methyl]benzanilide was obtained by similar alkylation of benzanilide with ClCH2SiF3 in the presence of DBU as a base. The structure of the products was proved by NMR and X-ray analysis. The energy of the C=O→Si dative bond in the obtained anilides was estimated by the QTAIM analysis. The calculated values lie near 40 kcal/mol, which is larger than in previously studied N-[difluoro(methyl)silyl]methylcarboxamides (14–30 kcal/mol).

Controlled Silanization: High Molecular Regularity of Functional Thiol Groups on Siloxane Coatings.

A comparative study on deposition and molecular regularity of two organosilanes, i.e., commercially available (3-mercaptopropyl)trimethoxysilane (MPTMS) and newly developed mercaptopropylsilatrane (MPS), was conducted in this work. MPTMS and MPS were applied to modify silicon surfaces to characterize their deposition kinetics, surface morphology, thickness, and elemental composition and the reactivity of thiol end groups based on gold-thiol and thiol-ene chemistries. MPS possesses a tricyclic caged structure and a transannular N → Si dative bond, making it chemically stable and controllable to avoid fast hydrolysis and aggregation in solution. The results indicate that MPS allows faster deposition and better formation of thin and homogeneous films than MPTMS. More importantly, the functional thiol groups on MPS coatings enable immobilization of a large amount of gold nanoparticles and effective thiol-ene photopolymerization with zwitterionic sulfobetaine acrylamide. Postmodification on silanized surfaces with MPS endows excellent plasmonic and antifouling properties, potentially leading to valuable applications to biosensing and biomaterials. The work demonstrated the feasibility and applicability of the functional silatrane molecule for surface silanization in a controlled manner.

Controlled Silanization Using Functional Silatrane for Thin and Homogeneous Antifouling Coatings.

Organosilicons for surface modification are gaining prominence because of their easy and rapid preparation, high availability, and effective modification for varying interfacial properties. However, their implementation has been humbled by poor control of the packing density, thickness, and molecular structures due to the uncontrollable hydrolysis and condensation. This study reports for the first time new functional silatrane chemistry for the precision deposition of a thin and homogeneous zwitterionic coating. Sulfobetaine silatrane (SBSiT) has a tricyclic caged structure and a transannular N → Si dative bond, which shows excellent chemical stability in the presence of water and an acid-modulated hydrolysis characteristic. Results from X-ray photoelectron spectroscopy indicate the progressive deposition of SBSiT on a silicon surface. Characterization using atomic force microscopy and ellipsometry shows the uniform and thin SBSiT films on silicon surfaces. The superior antifouling properties of SBSiT coatings were demonstrated by resisting bacterial and protein adsorption. More importantly, the stable and complete formation of the SBSiT coatings allows an accurate interpretation of the interfacial phenomena for sensing and nanomaterial applications.

Electrochemical Oxidation and Radical Cations of Structurally Non-rigid Hypervalent Silatranes: Theoretical and Experimental Studies.

Using 18 silatranes XSi(OCH2 CH2 )3 N (1) as examples, the potentials of electrochemical oxidation E0 of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials Ep (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E0 (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (dSiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1+.(s) , whereas the second one, 1+.(l) , has a longer (l) Si⋅⋅⋅N distance (dSiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom Ns . The vertical model of adiabatic ionization (1→1+.(s) or 1→1+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E0 (1) with the strength characteristics of the dative contact N→Si were revealed.

Ab initio computational study of 1-methyl-4-silatranone and attempts at its conventional synthesis

1-Methyl-4-silatranone could exhibit the structural aspects of a typical silatrane including a short N–Si bond distance reflecting a dative bond. But given the significant amide resonance in a [3.3.3] bridgehead bicyclic lactam, the lone pair could be shared with the carbonyl group leading to a very long N–Si bond, essentially a “non-silatrane.” Ab initio calculations (MP2/6-311 + G*) predict that ground state conformations of this molecule are best regarded as lactams rather than silatranes, the most stable having a calculated N–Si bond length of 2.902 A and an N–CO bond length of 1.387 A. The calculated transition state for inversion of the amide ring retains very little amide resonance (N–CO, 1.440 A). Some of this loss is compensated through tightening of the N–Si bond (2.422 A), leading to a net energy of activation of ca 8 kcal/mol. Attempts to synthesize 1-methyl-4-silatranone using conventional pathways successful for 1-methylsilatrane [condensations employing N,N-bis(2-hydroxyethyl)glycolamide in place of tris(2-hydroxyethyl)amine] were unsuccessful. This is due to the net loss in resonance energy of the amide reactant relative to that in the [3.3.3] system, the essential absence of the N–Si dative bond, and the rigidity introduced by the planar amide linkage in the starting material. A more likely pathway to successful synthesis should be formation of the amide linkage in the final step.

Silane-Functionalized N-Heterocyclic Carbene–Cobalt Complexes Containing a Five-Coordinate Silicon with a Covalent Co–Si Bond

Novel cobalt complexes featuring five-coordinate silicon centers with a covalent Co–Si bond, i.e., [(SiN2C)Co(OPy)] complexes (SiN2C = tetradentate silane-anchored bis(pyridine)-N-heterocyclic carbene (NHC) ligand, OPy = 2-pyridonato), were prepared by reaction of bidentate hydrosilyl-NHC-supported cobalt(II) complexes with 2-pyridone and characterized by 1H, 13C, and 29Si NMR, single-crystal X-ray diffraction, and elemental analyses. Theoretical calculations support a covalent nature of the Co–Si interaction, in contrast to the metal→Si dative bonds in metallasilatranes.

Synthesis and Properties of Pentacoordinated Phenoxysilane and Carboxysilanes with Intramolecular Nitrogen–Silicon Coordination

Reactions of a neutral pentacoordinated monohydrosilane bearing a dative N‒Si bond with phenol and benzoic acid derivatives gave the corresponding pentacoordinated phenoxysilane and carboxysilanes, respectively. X-ray crystallographic analyses of these silanes revealed that the distance of the N‒Si dative bond is shortened and the pentacoordination character of the silicon center becomes greater with the increase in the electron-withdrawing character of the apical substituent on silicon.

Cage Silaphosphanes with a P→Si Dative Bond

The design of the cage silaphosphanes XSi(−L−)3P (X = Me (a), F (b); L = YCH2Z (4−19), YC6H4 (20−23), C6H4Z (24−27), C10H6 (28), YCH2CH2Z (29-37), YC6H4Z (38−46); Y, Z = O, NH, CH2, S) has been carried out by MP2 and B3LYP methods using 6-31G(d) and 6-311+G(d) basis sets. These species can exist in two forms, namely, with the phosphorus lone electron pair (LEP) directed inside the cage (endo), enabling P→Si coordination, or outside the cage (exo), excluding it. The relative stabilities of the two isomers depend on the properties of the Si and P bridgehead atom surroundings as well as on the size and nature of the side chains (L). Among the 50 studied cage species XSi(−L−)3P possessing the bridgehead Si and P atoms in the 1- and 5-positions, only 3 structures (L = SC6H4 (23a,b), C10H6 (28b)) exist exclusively in the endo form with the pentacoordinate silicon atom. On going from the systems XSi(−L−)3P with the 1,5-bridgehead Si and P atoms to the similar 1,6-bridgehead systems, the number of stable endo iso...

Chemisorption of Tetrakis(dimethylamido)titanium on Si(100)-2 × 1: C−H and C−N Bond Reactivity Leading to Low-Temperature Decomposition Pathways

Tetrakis(dimethylamido)titanium, Ti[N(CH3)2]4 or TDMAT, is a metal alkylamide precursor often used in the deposition of titanium nitride films. The mechanisms of decomposition of TDMAT upon its adsorption on the Si(100)-2 × 1 surface are investigated by infrared (IR) spectroscopy, temperature-programmed desorption (TPD), and density functional methods. Two experimental observations, the formation of Si−H bonds at temperatures as low as 220 K and methane desorption at temperatures below 400 K, indicate the scission of C−H and C−N bonds. The unusual low-temperature reactivity of these bonds is explained in terms of bond activation due to the zwitterionic character of the silicon−silicon dimers of the Si(100)-2 × 1 surface. Silicon surface atoms with predominantly nucleophilic character are able to attract hydrogen atoms, forming Si−H bonds. Silicon surface atoms with electrophilic character are able to form N−Si dative bonds that weaken the C−N bond, favoring its scission. Density functional calculations support this model, finding that the C−H scission is favored thermodynamically and kinetically provided that the TDMAT molecule has already undergone dissociative adsorption via N−Ti bond scission. Several decomposition pathways are considered, and in all cases the surface products are stabilized upon the formation of Si−C bonds, which agrees with the formation of an interface with high carbon content when metalorganic precursors are employed. The repercussion of these facile mechanisms of decomposition on the first stages of film deposition is discussed.

Synthesis and structure of silicon-containing N-methyland N-benzoylamides of diisopropylphosphoric acid

Diisopropyl N-benzoyl-N-(trimethylsilyl)phosphoramidate reacts with ClCH2SiMe2Cl under mild conditions to form diisopropyl N-benzoyl-N-[(chlorodimethylsilyl)methyl]phosphoramidate (III). Diisopropyl N-methyl-N-(trimethylsilyl)phosphoramidate with ClCH2SiMe2Cl affords an N-transsilylation product which does not rearrange into diisopropyl N-[(chlorodimethylsilyl)methyl]-N-methylphosphoramidate (XV) even under severe conditions (4 h, 130°C). Compound XV was prepared by the reaction of diisopropyl phosphorochloridate with N-[(methoxydimethylsilyl)methyl]-N-methylamine followed by treatment of diisopropyl N-[(methoxydimethylsilyl)methyl]-N-methylphosphoramidate with boron trichloride. Analysis of experimental and calculated 29Si chemical shifts points to a five-coordinate silicon atom in compound III and a fourcoordinate silicon atom in compound XV. According to B3LYP calculations with due regard to solvent effects, compound III is an isomer with a C=O→Si bond. By variation of substituents at silicon, phosphorus, and carbonyl carbon atoms, chelate structures with either C=O→Si or P=O→Si dative bonds can be obtained.

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.

Donor-Stabilized Silyl Cations. 10. Pentacoordinate Siliconium-Ion Salts with a Triphenylphosphinimino-N Ligand Group: Two-Bond P−N−Si Coupling as a Measure for Coordination Strength1

A new donor group, triphenylphosphinimino, is described as a ligand in pentacoordinate siliconium halide dichelates, [XSiL2]+hal-, where L is the bidentate ligand −OC(CH2Ph)NN(PPh3)−, X = Me, C6H11, i-Bu, CH2Cl, CHCl2, Cl, Br, and hal = Cl, Br. The complex 16 (X = Me, hal = Cl) was characterized by crystal structure analysis, and the others were characterized by NMR spectroscopy and elemental analysis. The phosphinimino ligand is a stronger donor than previously studied dimethylamino and isopropylidenimino ligands, causing all of the complexes to be pentacoordinate siliconium-halide salts in the crystal as well as in solution. Two-bond 31P−N−29Si coupling constants across the N→Si dative bond increase with increasing monodentate-ligand electron-withdrawing power and may serve as a criterion for coordination strength.

Octahedral and Bicapped-Tetrahedral Silicon Configurations in the Solid State and Their Dynamic Coexistence in Solution

Hexacoordinate silicon bis chelates with the SiC2O2N2 ligand framework have been prepared, with different nitrogen-donor ligands, NMe2 and NCMe2. Crystal diffraction analyses revealed that the former type had a bicapped-tetrahedral geometry and the latter an octahedral molecular geometry in the solid state. The two N→Si dative bonds in the isopropylideneimino-coordinated compound are sufficiently strong to change silicon from its tetrahedral ground-state configuration to the higher energy octahedral configuration. In contrast, the weaker NMe2 donor groups do not form strong enough bonds to cause this change, and as a result silicon remains tetrahedral and forms only substantially longer and weaker N→Si dative bonds (2.7−2.8 A). Remote electron-releasing NMe2 substituents on the chelate rings render the ligand NMe2 groups stronger donors, resulting in reversal of the solid-state geometry back to octahedral. 29Si, 1H, and 13C NMR spectra at various temperatures provide evidence for the coexistence in equili...

Surprising Insights in the Various Molecular Structures of Hypercoordinate Bis(oxinato)silicon Complexes

The syntheses of two cyclic diorganosilicon enamines =CH2) [R = Ph (2a), Me (2b)] are described. These compounds react with 8-oxyquinoline leading to bis(oxinato)silicon complexes RPhSi(oxinate)2 [R = Ph (5a), Me (5b)]. Their X-ray structures reveal hexacoordination of the Si atom with the monodentate substituents in cis-positions and N atoms as well as O atoms in trans-positions. In crystalline dimethylbis(oxinato)silicon, Me2Si(oxinate)2 (7), the silicon atom is only bicapped tetrahedrally coordinated, while for dichlorobis(oxinato)silicon, Cl2Si(oxinate)2 (8), there is an octahedral coordination of the Si atom with chlorine atoms in trans-positions. This conclusion is based on the results of spectroscopic analysis (IR, 29Si CP/MAS NMR) as well as quantum chemical calculations. The first example of a silicon-bis-oxinate with the N→Si dative bonds in a trans-arrangement has been detected in the hexacoordinate silicon tris-chelate (oxinate)2Si(PhN-CH2CH2-NPh) (11). Its configuration was proven by X-ray structure analysis. Thus, for hexacoordinate bis(oxinato)silicon compounds three new architectures were found which complement the previously established building pattern of the N,N’-cis-O,O’-trans-bis(oxinato)silicon complexes. The mer-tris(oxinato)siliconium cation (9+) (its configuration being proven by 1H and 13C NMR spectroscopy) features at least three coordination patterns with (O,O;N,N)-cis,cis-, -cis,trans- as well as -trans,cis-arrangements of two oxinate ligands.

Spin Coupling through the N→Si Dative Bond in Penta- and Hexacoordinate Hydrido- and Fluorosilicon Complexes; Coupling through a Rapidly Dissociating−Recombining N→Si Bond

Penta- and hexacoordinate silicon complexes with spin ½ nuclei ( 1 H and 1 9 F) directly attached to silicon have been prepared and used to study NMR spin-spin interactions across the N→Si dative bond. In both hydrido and fluoro complexes, two-bond coupling constants were found to drop sharply as the geminal bond angle deviated from 90°. Three-bond coupling constants showed a Karplus-type dependence upon the corresponding dihedral angles for both 1 H and 1 9 F complexes. Four-bond coupling constants 4 J( 1 9 F-Si-N-C- 1 H) across the dative bond were observed. The correlations can be used as tools for the interpolation of bond and dihedral angles from NMR data, in cases where crystal structures are unavailable. They also demonstrate that the N→Si dative bond in hypercoordinate silicon complexes behaves essentially in the manner expected from normal covalent bonds. Rapid dissociation and recombination of the N-Si dative bond was established in several pentacoordinate complexes; however, one-, two-, and three-bond coupling constants could nevertheless be measured across this rapidly dissociating bond.

Selective functionalization of the Si(1 0 0) surface by switching the adsorption linkage of a bifunctional organic molecule

Selective functionalization by switching the adsorption linkage of a bi-functional organic molecule, 1-dimethylamino-2-propyne (DMAP), on Si(1 0 0) has been studied using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). The HREELS and STM results clearly indicate that DMAP adsorbs at the down dimer atom of Si(1 0 0)c(4 × 2) through an N → Si dative bond leaving the –CCH unit of DMAP free at low temperature (65–90 K). The adsorption linkage is switching from the N → Si dative bond to the Si–C di-σ bonds by annealing the adsorbed surface to 300 K, and thus the tertiary amino group becomes free.

Diastereomeric hexacoordinate silicon complexes: preparation, structure and epimerization

Abstract The synthesis of a neutral hexacoordinate silicon complex ( 6 ) with a chiral silicon and an adjacent chiral carbon center, via an intramolecular nucleophilic displacement, is described. 6 exists in a mixture of two diastereomers. One of the diastereomers has been characterized by an X-ray crystallographic analysis, and is found to have a nearly octahedral geometry about the silicon. 6 undergoes two intramolecular ligand-site exchhange processes, observable by NMR spectroscopy: reversible dissociation of the N–Si dative bond, leading to interchange of the N -methyl groups ( Δ G ∗ =16.7±0.2 kcal mol −1 at 365 K), and inversion of configuration at the silicon center, measured by selective inversion recovery NMR spectroscopy (21.0 kcal mol −1 at 350 K).

Pentacoordinate Silyl Cations Stabilized by Coordination with Oxygen Donors: Crystal Structure, Charge Distribution, and Stereodynamics

Attempted preparation of neutral hexacoordinate silicon chelates with two O→Si dative bonds (4), in analogy to previously reported bis-N→Si isomers (2), resulted in unexpected formation of pentacoordinate siliconium complexes (5 and 6), characterized by a crystal structure. The barrier for inversion of configuration at silicon was determined by selective inversion recovery NMR measurements: ΔG* = 20.5 ± 0.2 kcal mol-1.