Six-coordinate Silicon Research Articles

A Six-Coordinate Silicon Dihydride Embedded in a Porphyrin: Enhanced Hydride-Donor Properties and the Catalyst-Free Hydrosilylation of CO2.

The catalyst-free hydrosilylation of CO2 under mild conditions remains limited. Herein, we report the synthesis, characterization, and reactivity of 5,10,15,20-tetraphenylporphyrinato(dihydrido)silicon(IV) (1) as a six-coordinate silicon dihydride. The Si-H moiety of 1 reacts with polar double bonds and CO2 in the absence of a catalyst to afford hydrosilylated products. Combining the hydrosilylation with subsequent transformation furnishes formic acid from CO2 . Computational studies indicate that the hydride-donor properties of 1 are exceptionally high for a neutral silicon hydride, and that the direct hydride transfer from silicon to carbon is a pivotal step in the hydrosilylation of CO2 with 1.

Neutral six-coordinate bis(dithiocarbamato)silicon(iv) complexes with an SiCl2S4 skeleton.

Treatment of SiCl4 with lithium dithiocarbamates of the formula type Li[R2NCS2] (R = Ph, iPr) in a molar ratio of 1 : 2 afforded the respective six-coordinate silicon(iv) complexes [Ph2NCS2]2SiCl2 (3) and [iPr2NCS2]2SiCl2 (4), which were isolated as the solvates 3·MeCN and 4·MeCN. Compounds 3·MeCN and 4·MeCN were structurally characterised by single-crystal X-ray diffraction and multinuclear NMR spectroscopic studies in the solid state and in solution. In this study, dithiocarbamato ligands were implemented in silicon coordination chemistry for the first time. Compounds 3 and 4 represent the first six-coordinate silicon(iv) complexes with an SiCl2S4 skeleton.

Bis(amidinato)- and bis(guanidinato)silylenes and silylenes with one sterically demanding amidinato or guanidinato ligand: synthesis and reactivity.

This article deals with the synthesis and reactivity of the bis(amidinato)silylene [iPrNC(Ph)NiPr]2Si (5), the bis(guanidinato)silylene [iPrNC(NiPr2)NiPr]2Si (6), the mono(amidinato)silylene [DippNC(Ph)NDipp]SiNMe2 (Dipp = 2,6-diisopropylphenyl) (8) and the mono(guanidinato)silylene [DippNC(NMe2)NDipp]SiN(SiMe3)2 (9). The reactions studied include (i) nucleophilic substitutions, (ii) Lewis acid/base reactions, (iii) Brønsted acid/base reactions, (iv) oxidative additions (bond activations, small-molecule activations), (v) cycloaddition reactions with unsaturated organic substrates, (vi) reductions and (vii) radical formation by muonium addition. Silylenes 5, 6, 8 and 9 have been demonstrated to display unique reactivity profiles and to be versatile precursors for the synthesis of novel four- or five-coordinate silicon(ii) and four-, five- or six-coordinate silicon(iv) complexes with unprecedented structural motifs. This article also deals with the structural characterisation of all the new silicon(ii) and silicon(iv) compounds synthesised. Crystal structure analyses and 29Si NMR spectroscopic studies in the solid state and in solution were used to characterise the silicon coordination polyhedra of these compounds.

Reactivity of the Donor-Stabilized Silylenes [iPrNC(Ph)NiPr]2 Si and [iPrNC(NiPr2 )NiPr]2 Si: Activation of CO2 and CS2.

Activation of CO2 by the bis(amidinato)silylene 1 and the analogous bis(guanidinato)silylene 2 leads to the structurally analogous six-coordinate silicon(IV) complexes 4 (previous work) and 8, respectively, the first silicon compounds with a chelating carbonato ligand. Likewise, CS2 activation by silylene 1 affords the analogous six-coordinate silicon(IV) complex 10, the first silicon compound with a chelating trithiocarbonato ligand. CS2 activation by silylene 2, however, yields the five-coordinate silicon(IV) complex 13 with a carbon-bound CS2 (2-) ligand, which also represents an unprecedented coordination mode in silicon coordination chemistry. Treatment of the dinuclear silicon(IV) complexes 5 and 6 with CO2 also affords the six-coordinate carbonatosilicon(IV) complexes 4 and 8, respectively.

Reactions of the donor-stabilized silylene bis[N,N'-diisopropyl-benzamidinato(-)]silicon(II) with Brønsted acids.

Reaction of the donor-stabilized silylene 1 (which is three-coordinate in the solid state and four-coordinate in solution) with [HMCp(CO)3 ] (M=Mo, W; Cp=cyclopentadienyl) leads to the cationic five-coordinate silicon(IV) complexes 2 and 3, respectively, and reaction of 1 with CH3 COOH yields the neutral six-coordinate silicon(IV) complex 4. Compounds 2-4 were structurally characterized by crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution. The formation of 2-4 can be formally described in terms of a Brønsted acid/base reaction, coupled with a redox process (Si(II) →Si(IV) , H(+) →H(-) ).

Activation of sulfur dioxide by bis[N,N'-diisopropylbenzamidinato(-)]silicon(II): synthesis of neutral six-coordinate silicon(IV) complexes with chelating O,O'-Sulfito or O,O'-dithionito ligands.

The neutral six-coordinate silicon(IV) complexes 2 and 3 (mixture of cis-3 and trans-3) were synthesized by reaction of the donor-stabilized silylene bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) (1) with SO2 . Compounds 2 and 3 are the first silicon(IV) complexes with chelating sulfito or dithionito ligands, and 3 is even the first molecular compound with a chelating dithionito ligand. Compounds 2 and 3 were structurally characterized by crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution.

The Donor‐Stabilized Silylene Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Synthesis, Electronic Structure, and Reactivity

A convenient and robust synthesis of bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) (1), a donor-stabilized silylene, has been developed (35 g scale). To get further information about the reactivity profile of 1, a series of oxidative addition reactions were studied. Treatment of 1 with PhSe-SePh (Se-Se bond activation), C6F6 (C-F activation), and CO2 (C=O activation/cycloaddition) yielded the neutral six-coordinate silicon(IV) complexes 10, 11, and 13, respectively. Treatment of 1 with N2O resulted in the formation of the dinuclear five-coordinate silicon(IV) complex 12 (oxidative addition/dimerization), which contains a four-membered Si2O2 ring. Compounds 10-13 were characterized by NMR spectroscopic studies in the solid state and in solution and by crystal structure analyses. Silylene 1 is three-coordinate in the solid state (from crystal structure analysis) and exists as the four-coordinate isomer 1' in benzene solution (from computational studies). Based on state-of-the-art relativistic DFT analyses, the four-coordinate species 1' was demonstrated to be the thermodynamically favored isomer in benzene solution (favored by ΔG = 6.6 kcal mol(-1) over the three-coordinate species 1). The reason for this was studied by bonding analyses of 1 and 1'. Furthermore, the (29)Si NMR chemical shifts of 1 and 1' were computed, and in the case of 1' it was analyzed how this NMR spectroscopic parameter is affected by solvation. These studies further supported the assumption that the silylene is four-coordinate in solution.

Neutral Six-Coordinate and Cationic Five-Coordinate Silicon(IV) Complexes with Two Bidentate Monoanionic N,S-Pyridine-2-thiolato(−) Ligands

A series of neutral six-coordinate silicon(IV) complexes (4-11) with two bidentate monoanionic N,S-pyridine-2-thiolato ligands and two monodentate ligands R(1) and R(2) was synthesized (4, R(1) = R(2) = Cl; 5, R(1) = Ph, R(2) = Cl; 6, R(1) = Ph, R(2) = F; 7, R(1) = Ph, R(2) = Br; 8, R(1) = Ph, R(2) = N3; 9, R(1) = Ph, R(2) = NCO; 10, R(1) = Ph, R(2) = NCS; 11, R(1) = Me, R(2) = Cl). In addition, the related ionic compound 12 was synthesized, which contains a cationic five-coordinate silicon(IV) complex with two bidentate monoanionic N,S-pyridine-2-thiolato ligands and one phenyl group (counterion: I(-)). Compounds 4-12 were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses (except 7). These structural investigations were performed with a special emphasis on the sophisticated stereochemistry of these compounds. These experimental investigations were complemented by computational studies, including bonding analyses based on relativistic density functional theory.

Phosphine and Diphosphine Complexes of Silicon(IV) Halides

The reaction of SiX4 (X = Cl or Br) with PMe3 in anhydrous CH2Cl2 forms trans-[SiX4(PMe3)2], while the diphosphines, Me2P(CH2)2PMe2, Et2P(CH2)2PEt2, and o-C6H4(PMe2)2 form cis-[SiX4(diphosphine)], all containing six-coordinate silicon centers. With Me2PCH2PMe2 the product was trans-[SiCl4(κ(1)-Me2PCH2PMe2)2]. The complexes have been characterized by X-ray crystallography, microanalysis, IR, and multinuclear ((1)H, (13)C{(1)H}, and (31)P{(1)H}) NMR spectroscopies. The complexes are stable solids and not significantly dissociated in nondonor solvents, although they are very moisture and oxygen sensitive. This stability conflicts with the predictions of recent density functional theory (DFT) calculations (Wilson et al. Inorg. Chem. 2012, 51, 7657-7668) which suggested six-coordinate silicon phosphines would be unstable, and also contrasts with the failure to isolate complexes with SiF4 (George et al. Dalton Trans. 2011, 40, 1584-1593). No reaction occurred between phosphines and SiI4, or with SiX4 and arsine ligands including AsMe3 and o-C6H4(AsMe2)2. Attempts to make five-coordinate [SiX4(PR3)] using the sterically bulky phosphines, P(t)Bu3, P(i)Pr3, or PCy3 failed, with no apparent reaction occurring, consistent with predictions (Wilson et al. Inorg. Chem. 2012, 51, 7657-7668) that such compounds would be very endothermic, while the large cone angles of the phosphines presumably preclude formation of six-coordination at the small silicon center. The reaction of Si2Cl6 with PMe3 or the diphosphines in CH2Cl2 results in instant disproportionation to the SiCl4 adducts and polychlorosilanes, but from hexane solution very unstable white [Si2Cl6(PMe3)2] and [Si2Cl6(diphosphine)] (diphosphine = Me2P(CH2)2PMe2 or o-C6H4(PMe2)2) precipitate. The reactions of SiHCl3 with PMe3 and Me2P(CH2)2PMe2 also produce the SiCl4 adducts, but using Et2P(CH2)2PEt2, colorless [SiHCl3{Et2P(CH2)2PEt2}] was isolated, which was characterized by an X-ray structure which showed a pseudo-octahedral complex with the Si-H trans to P. Attempts to reduce the silicon(IV) phosphine complexes to silicon(II) were unsuccessful, contrasting with the isolation of stable N-heterocyclic carbene adducts of Si(II).

Synthesis and structure of (C=O→Si←O′=C′)bis(2-methyl-4-oxopyran-3-yloxy)difluoro-(λ6)siliconium

A new six-coordinate silicon compound, (C=O→Si←O′=C′)bis(2-methyl-4-oxopyran-3-yloxy)-difluoro(λ6)siliconium, containing two five-membered rings closed by C=O→Si coordination bonds, forms o protodesilylation trifluoro(phenyl)silane with 3-hydroxy-2-methylpyran-4-one (maltol). According to multinuclear NMR and IR spectral data and quantum-chemical calculations, the silicon atom in this compound has an octahedral environment with two cis-arranged C=O→Si bonds

Synthesis and specific features of the structure of the mixed anionic six-coordinate silicon complexes with the (O,O)-dianionic and (C,O)-monoanionic chelate ligands

Dicyclohexylammonium salts of bis(1,2-catecholato-O,O′)-[(N-methylacetamido)methyl-C,O]silicate and bis(1,2-catecholato-O,O′)-[(2-oxohexahydroazepin-1-yl)methyl-C,O]silicate were synthesized by the transesterification of the corresponding trimethoxysilyl derivatives by pyrocatechol in the presence of dicyclohexylamine. According to the X-ray diffraction data, the synthesized salts are the anionic silicon complexes, in which the Si atoms are six-coordinate due to two (O,O′)-dianionic and one (C,O)-monoanionic chelate ligands. In the latter, chelating occurs due to the formation of the intramolecular (intraionic) coordination bond O→Si.

Five and six-coordinate silicon complexes with an O,N,O′-chelating ligand derived from o-hydroxyacetophenone–N-(2-hydroxyethyl)imine

The reaction of o-hydroxyacetophenone–N-(2-hydroxyethyl)imine, 1, with suitable chlorosilanes in the presence of triethylamine gives access to five and six-coordinate silicon complexes. The molecular structures of TBPY-5–34-[2-oxy-κO-acetophenone–N-(2-oxy-κO-ethyl)iminato-κN]diphenylsilane, C22H21NO2Si, 2, and OC-6–22′-bis[2-oxy-κO-acetophenone–N-(2-oxy-κO-ethyl)iminato-κN]silicon·chloroform, C20H22N2O4Si·CHCl3, 3, have been determined as representative examples.

Synthesis and Characterization of a Six-Coordinate Silicon (IV) Complex with a Tripodal Phenalenyl-Based Hexadentate Ligand

We report the preparation and solid state characterization of a silicon (IV) salt based on a new redox active tripodal hexadentate ligand [H 3L = l,l,l-Tris(l′-oxophenalenyl-9′-N-methyl) ethane]. X-ray analysis of the complex reveals a short Si-N bond distance (av 1.878(16)). The compound is shown to undergo three reversible one-electron reduction steps (without decomposition) due to the formation of radical, anion and dianion of the phenalenyl moiety.

Optical Resolution of a Six-Coordinate Silicon(IV) Complex with a Tripodal Hexadentate Ligand

Abstract The reaction of silicon tetraacetate with a tripodal hexadentate ligand (H3L = 1,1,1-tris(N-salicylideneaminomethyl)ethane) afforded the colorless [Si(L)]+ complex. The X-ray analysis of the complex revealed that the Si–N bond distance, av. 1.926(2) Å, is much longer than the Si–O bond distance, av. 1.723(2) Å. The complex is stable toward hydrolysis and could be optically resolved by fractional crystallization of the diastereomeric salt with (−)-dibenzoyl-l-tartaric acid.

31P and 29Si NMR study of sol–gel-synthesized phosphate ceramics

31 P and 29Si solid-state magic angle spinning (MAS) and liquid-state nuclear magnetic resonance (NMR) techniques have been applied to the study of sol–gel-synthesized phosphate ceramic composites. This study emphasizes the chemistry and structural properties for both sols and gels prepared by directly reacting P2O5 with tetraethoxysilane (TEOS). This new method of formation of both the sols and gels is a drastic improvement in the retention of phosphorus during the formation of the phosphosilicate gels, and it provides a lower temperature route to six-coordinate silicon. Both31P and 29Si liquid-state NMR spectra for sols prepared without water showed the presence of P–O–Si bonds. The 31P NMR spectra for sols prepared with water resembled those for the PO(OH)x(OR)3–xprecursors, while29Si NMR spectra indicated that TEOS had undergone partial hydrolysis and condensation. Hexacoordinated silicon was observed for the first time in gels prepared at low temperature (70 °C). P–Si gels prepared directly by using P2O5 were compared with those using other molecular precursors.

Stereodynamics of neutral six-coordinate silicon chelates: evidence for two non-dissociative rate processes

A DNMR spectroscopic study of the Stereodynamics of novel neutral six-coordinate silicon chelates reveals two distinct ligand site exchange processes, both of which are intramolecular non-dissociative.

Molecular and chemical dynamics of a biradical with six-coordinate silicon atom

The intramolecular dynamics of a silicon biradical complex witho-semiquinone ligands was studied in a frozen toluene solution. Analysis of the broadening and shifts of the canonical components in the ESR spectrum of this biradical allows one to detect the abrupt reorientation of the magnetic axes due to intramolecular single bond-unpaired electron exchange. The frequencies of chemical exchange at low temperatures were estimated.

Hypervalent Siliconate Materials. Synthesis and Characterization of Novel Ladder and Network Ionomers

ABSTRACTThe first representatives of novel materials that contain five- and six-coordinate anionie silicon are reported. Linear (or cyclic) ionomers incorporating pentacovalent siliconate were synthesized by condensation of 1,2,4,5-tetrahydroxybenzene (THB) with alkyl and aryl trialkoxysilylanes. Network ionomers that incorporate pentacovalent siliconates were formed from THB and alkyl and aryl bistrialkoxysilanes. In addition, network ionomers with hexacoordinate silicon result from condensation of TEOS and THB. Spectroscopie evidence is presented to verify the identity of the novel structural features of these materials.

Nuclear magnetic resonance studies of silicon(IV) complexes in aqueous solution—II. Tris-tropolonato and tris-3-hydroxypyridin-4-onato complexes

Quantitative data for the reaction: 3LH+Si(OH) 4+H + →2 [SiL 3] ++4H 2O in aqueous solution have been obtained by 1H NMR spectroscopy, where LH  tropolone, 4-methyltropolone, 3-hydroxy-2-methylpyridin-4-one, 1,2-dimethyl-3-hydroxypyridine-4-one and 1-ethyl-3-hydroxy-2-methylpyridin-4-one. Equilibrium studies were carried out in D 2O solutions for both types of complex. The measurements for the complexes derived from 3-hydroxypyridin-4-ones were repeated in H 2O and the results are compared with the D 2O data. We present the first air-stable organic substrate capable of etching glass at a significant rate in non-fluorinated acidic solution. The dissolution of silica was confirmed by the detection of the 29Si NMR resonance characteristic of the six-coordinate silicon(IV) complex formed. Information has been obtained on the rate of dissolution of glass by 1-ethyl-3-hydroxy-2-methylpyridin-4-one in acid solution.

Six-coordinate silicon: new high-pressure silicates with the benitoite and wadeite structures

Six-coordinate silicon: new high-pressure silicates with the benitoite and wadeite structures