Dissertation/Thesis Abstract

Synthese, Struktur und Eigenschaften neuer Silicium(II)- und Silicium(IV)-Komplexe mit Guanidinato-Liganden
by Mück, Felix Maximilian, Ph.D., Bayerische Julius-Maximilians-Universitaet Wuerzburg (Germany), 2016, 378; 27766770
Abstract (Summary)

This thesis is a contribution to the chemistry of donor-stabilized silylenes with guanidinato ligands. The main focus of this work was the synthesis, characterization, and reactivity studies of the two novel silicon(II) complexes 23 and 24 with different sterically demanding ligand systems. A second focus was the characterization of the resulting four-, five-, and six-coordinate silicon(II) or silicon(IV) complexes.

In the course of these studies, the donor-stabilized three-coordinate silylenes 23 and 24, the neutral four-coordinate silicon(II) complexes 25·C4H8O and 26, the neutral four-coordinate silicon(IV) complexes 27–36, 38, 47–49, and 51, the neutral five-coordinate silicon(II) complexes 39·0.5C6H5CH3, 40–42 and 46, the neutral five-coordinate silicon(IV) complexes 18, 19, 37, and 56, the cationic five-coordinate silicon(IV) complexes 52 and 53, and the neutral six-coordinate silicon(IV) complexes 20, 55·0.5C6H5CH3, 57, and 58 were prepared for the first time.

These compounds were characterized by elemental analyses (except 33), NMR spectroscopic studies in the solid state (15N, 29Si, 31P (27 only), and 77Se VACP/MAS NMR (32, 35, 50, and 53 only) as well as 11B (39·0.5C6H5CH3 only), 27Al (40 and 41 only), and 125Te HPDec/MAS NMR (33, 36, and 51 only)) and in solution (except 39, 40, 52, and 53; 1H, 13C, 27Al (41 only), 29Si, 31P (27 only), 77Se (32, 35, and 50 only), and 125Te NMR (33, 36, and 51)), and single-crystal X-ray diffraction.

Synthesis and characterization of two novel donor-stabilized mono- and bis(guanidinato)-silylenes

The donor-stabilized silylenes 23 and 24 were synthesized by treatment of the five-coordinate dichlorohydrido(guanidinato)silicon(IV) complex 18 and six-coordinate chlorohydrido-bis(guanidinato)silicon(IV) complex 20, respectively, with potassium bis(trimethylsilyl)amide (reductive hydrogen chloride elimination). Compound 18 was prepared by treatment of trichlorosilane with one molar equivalent of lithium N,N´´-bis(2,6-diisopropylphenyl)-N´N´-dimethylguanidinate, and 19 was obtained by treatment of trichlorosilane with two molar equivalents of lithium N,N´,N´,N´´-tetraisopropylguanidinate. All attempts to synthesize silylene 22 by reduction of the corresponding five-coordinate trichloro(guanidinato)silicon(IV) complex 19 with alkali metals failed.

The silicon coordination polyhedra of the five-coordinate silicon(IV) complexes 18 and 19 are strongly distorted trigonal bipyramids, with a chlorine and nitrogen atom in the axial positions. The silicon coordination polyhedron of 20 is a strongly distorted octahedron, with the chloro and hydrido ligands in cis positions.

The silicon atoms of silylenes 23 and 24 are coordinated in a pseudo-tetrahedral fashion by three nitrogen atoms and the lone electron pair as the fourth “ligand”. Both silylenes are three-coordinate both in the solid state and in solution (one bidentate guanidinato and one monodentate amido/guanidinato ligand). The three-coordination of 24 in solution was also confirmed by quantum chemical calculations. This is in contrast to the analogous bis(amidinato)silylene 1, which is three-coordinate only in the solid state and four-coordinate in solution.

Reactivity studies of the donor-stabilized mono(guanidinato)silylene 23

Starting from silylene 23, the four-coordinate silicon(II) complexes 25 and 26, the four-coordinate silicon(IV) complexes 27–36 and 38, and the five-coordinate silicon(IV) complex 37 were synthesized. The formation of these products is based on Lewis acid/base (25, 26) or oxidative addition reactions (27–38). Except for the formation of 25, 27, and 34–36, the typical silylene reactivity of 23 is coupled with additional reactivity facets, such as (i) a switch of the coordination mode of the guanidinato ligand from bidentate binding to only one coordination center to bidentate binding to two different coordination centers (→ 26), (ii) a 1,3-SiMe3 shift of one of the two SiMe3 groups of the amido ligand (→ 28–33), or (iii) a nucleophilic reaction of one of the two nitrogen ligand atoms of the guanidinato ligand as part of a rearrangement reaction (→ 38).

Silylene 23 reacts with zinc chloride and zinc diethyl to give the neutral four-coordinate silicon(II) complexes 25 (isolated as 25·C4H8O) and 26, respectively, with a silicon–zinc bond. In these transformations silylene 23 reacts as a Lewis base to furnish the Lewis acid/base adducts 25 and (upon an additional rearrangement) compound 26.

The silicon coordination polyhedra of 25·C4H8O and 26 are (strongly) distorted tetrahedra. In the case of 25, the guanidinato ligand binds in a bidentate and in 26 in a monodentate fashion to the silicon atom.

The four-coordinate silicon(IV) complexes 27–36 and 38 and the five-coordinate silicon(IV) complex 37 were formed in an oxidative addition reaction by treatment of 23 with diphenylphosphoryl azide (→ 27), 2,4-hexadiyne (→ 28), 1,4-diphenylbutadiyne (→ 29), dinitrogen monoxide (→ 30), diphenyl disulfide (→ 31), diphenyl diselenide (→ 32), diphenyl ditelluride (→ 33), sulfur (→ 34), selenium (→ 35), tellurium (→ 36), carbon dioxide (→ 37), and carbon disulfide (→ 38) respectively. Additionally, compound 37 could also be synthesized by treatment of 30 with carbon dioxide.

The reaction of 23 with diphenylphosphoryl azide proceeds with a nitrogen elimination and formation of 27 with a silicon–nitrogen double bond. Compound 27 and can be formally described as an intramolecularly donor-stabilized silaimine.

Compounds 28 and 29 can be formally described as donor-stabilized silaimines with a silicon-bound trisubstituted vinyl group. The reaction mechanism is postulated to be a [1+2] cycloaddition of 23 with one of two C–C triple bonds of the diynes to form the corresponding silacyclopropenes, which then undergo a rearrangement with a 1,3-shift of one of the two SiMe3 groups from the nitrogen atom of the amido ligand to the carbon atom of the silacyclopropene ring.

Compounds 30–33 represent the first thermally stable donor-stabilized silaimines with an SiN3El skeleton (El = O, S, Se, Te). The formation of 30 can be rationalized in terms of an oxidation of 23 with dinitrogen monoxide to give a four-coordinate silicon(IV) complex with an silicon–oxygen double bond, which then undergoes a 1,3-shift of one of the two SiMe3 groups from the nitrogen to the oxygen atom to give 30 (including elimination of nitrogen). The formation of 31–33 can be rationalized in terms of a homolytic El–El bond activation (El = S, Se, Te) of the corresponding diphenyl dichalcogenides (formation of two Si–ElPh groups), followed by a 1,3-shift of one of the two SiMe3 groups to one of the two Si–ElPh moieties and elimination of Me3SiElPh.

Reaction of 23 with the elemental chalcogens sulfur, selenium, and tellurium proceeds also in terms of an oxidative addition to form compounds 34–36 with a silicon–chalcogen double bond.

For the formation of 37, a three-step process is proposed. In a first two-stage step, silylene 23 reacts with one molecule of carbon dioxide to give the stable four-coordinate silicon(IV) complex 30 as an intermediate (elimination of carbon monoxide). Addition of a second carbon dioxide molecule to the silicon–nitrogen double bond of 30 finally afforded the five-coordinate silicon(IV) complex 37 with an N,O-chelating carbamato ligand. This mechanism is strongly supported by the finding that treatment of 30 with an excess of CO2 also afforded compound 37.

Reaction of 23 with carbon disulfide leads to the cyclic silicon(IV) complex 38.

The silicon coordination polyhedra of 27–36 in the crystal are strongly distorted tetrahedra, with a bidentate guanidinato ligand, an amido ligand (27 and 34–36 only), and an imino ligand (28–33), respectively, and with an Si–El single bond (28, 29: El = C; 30: El = O; 31: El = S; 32: El = Se; 33: El = Te) and an Si–El double bond (27: El = N, 34: El = S; 35: El = Se; 36: El = Te), respectively.

The silicon coordination polyhedron of 37 is a strongly distorted trigonal bipyramid, with the oxygen atom of the carbamato ligand and a nitrogen atom of the guanidinato ligand in the axial positions.

The silicon coordination polyhedron of 38 is a distorted tetrahedron.

Reactivity of the donor-stabilized silylene 24

Silylene 24 reacts with the Lewis acids triphenylborane, triphenylalane, and zinc chloride to give the respective five-coordinate silicon(II) complexes 39, 40, and 42, which contain an

Si–B, Si–Al, and Si–Zn bond, respectively. In these transformations, silylene 24 reacts as a Lewis base to afford Lewis acid/base adducts.

The silicon coordination polyhedra of 39, 40, and 42 in the crystal are strongly distorted trigonal bipyramids, with the boron, aluminum, and zinc atom in an equatorial position. NMR spectroscopic studies demonstrated that the silicon–zinc compound 42 is also stable in solution, whereas the silicon–boron and silicon–aluminum compounds 39 and 40, respectively, are unstable in solution. Both complexes dissociate quantitatively to form 24 and ElPh3 (El = B, Al).

The bis(guanidinato)silicon(II) complexes 39 and 40 and the analogous bis(amidinato)silicon(II) complexes 3 and 41 are characterized by similar structures each. However, the respective amidinato/guanidinato analogues 3/39 and 41/40 differ significantly in their chemical stability in solution. As 39 and 40 even dissociate at lower temperature (T = –20 °C) and the corresponding amidinato analogues 3 and 41 are stable at higher temperatures (T = 70 °C), the bis(amidinato)silylene 1 is suggested to be a better σ-donor and thus a stronger Lewis base compared to the bis(guanidinato)silylene 24.

Furthermore, silylene 24 reacts as a nucleophile with the transition-metal carbonyl complexes [M(CO)6] (M = Cr, Mo, W) and [Fe(CO)5] to form the corresponding four-coordinate silicon(II) complexes 43–45 and the five-coordinate silicon(II) complex 46.

The silicon coordination polyhedra of 43–45 are strongly distorted tetrahedra, with one silicon-bound bidentate guanidinato ligand and a second guanidinato ligand that bridges the silicon and the transition-metal atom. The two silicon coordination polyhedra of 46 are strongly distorted trigonal bipyramids, with the iron atom in an equatorial site.

Comparison of the bis(guanidinato)silicon(II) complexes 43–46 with the respective amidinato analogues 4–7 reveals that only the iron complexes 7 and 46 have analogous structures. In contrast, the bis(amidinato)silylene 1 reacts with [M(CO)6] (M = Cr, Mo, W) in terms of a monosubstitution (replacement of one of the six carbonyl ligands) to give the five-coordinate silicon(II) complexes 4–6, whereas treatment of [M(CO)6] with the bis(guanidinato)silylene 24 leads to a disubstitution (replacement of two carbonyl ligands) to afford the four-coordinate silicon(II) complexes 43–45.

The four-coordinate silicon(IV) complexes 47–51 were synthesized in terms of an oxidative addition reaction by treatment of 24 with trimethylsilyl azide (→ 47), dinitrogen monoxide

(→ 48), sulfur (→ 49), selenium (→ 50), and tellurium (→ 51), respectively. The formation of 47 and 48 proceeds with the elimination of nitrogen.

The silicon coordination polyhedra of 47–51 in the crystal are strongly distorted tetrahedra. The dinuclear complex 48 contains two monodentate guanidinato ligands each and an Si2O2 ring. Compounds 47 and 49–51 represent the first four-coordinate bis(guanidinato)silicon(IV) complexes with a silicon–nitrogen or silicon–chalcogen double bond (S, Se, Te), respectively.

The formation of compounds 47–51 once again emphasizes the different reactivities of the amidinato/guanidinato-analogous silylenes 1 (three-coordinate in the solid-state and four-coordinate in solution) and 24 (three-coordinate both in the solid state and in solution). It is interesting to note that the oxidative addition reactions of the amidinato/guanidinato analogues 1 and 24 with trimethylsilyl azide, dinitrogen monoxide, sulfur, selenium and tellurium lead to products with different silicon coordination numbers. Compounds 8 and 10–12 represent five-coordinate silicon(IV) complexes with two bidentate amidinato ligands, whereas the corresponding analogues 47 and 49–51 are four-coordinate silicon(IV) complexes that contain one bidentate and one monodentate guanidinato ligand. Likewise, compound 9 is a dinuclear five-coordinate silicon(IV) complex with one bidentate and one monodentate amidinato ligand, whereas the dinuclear four-coordinate complex 48 contains two monodentate guanidinato ligands each.

The cationic five-coordinate silicon(IV) complexes 52 and 53 were also synthesized in terms of an oxidative addition reaction by treatment of 24 with diphenyl disulfide (→ 52) and diphenyl diselenide (→ 53), respectively.

The silicon coordination polyhedra of 52 and 53 are strongly distorted bipyramids, with the sulfur or the selenium atom in an equatorial position.

The formation of 52 and 53 is formally based on a heterolytic chalcogen–chalcogen bond activation of diphenyl disulfide and diphenyl diselenide by the bis(guanidinato)silylene 24. In contrast, a homolytic Se–Se bond activation was observed for the reaction of diphenyl diselenide with the analogous bis(amidinato)silylene 1 (formation of the six-coordinate silicon(IV) complex 13).

Furthermore, the reactivity of silylene 24 towards small molecules was investigated. The six-coordinate silicon(IV) complexes 55, 57, and 58 and the five-coordinate silicon(IV) complex 56 were prepared in terms of an oxidative addition reaction by treatment of 24 with an excess of carbon dioxide (→ 55), with an equimolar amount of carbon disulfide (→ 56), with a stoichiometric amount of sulfur dioxide (→ 57), and with a vast excess of liquid sulfur dioxide (which served also as the solvent; → 58), respectively.

Compound 58 was isolated as a co-crystallizate of the isomers cis-58 and trans-58, which differ in their relative orientation of the two exocyclic oxygen atoms. The silicon coordination polyhedra of 55·C6H5CH3, 57, and 58 are strongly distorted octahedra. The oxygen ligand atoms of the bidentate O,O´-chelating carbonato (55), sulfito (56), and dithionito (57) ligands are found in cis positions each.

Compound 58 is the second structurally characterized silicon compound with a bidentate O,O´-chelating dithionito ligand, and 55, 57, and 58 represent very rare examples of main-group element compounds with an O,O´-chelating carbonato, sulfito, or dithionito ligand. Complex 57 and its amidinato analogue 16 represent two of three main-group element compounds with an O,O´-chelating sulfito ligand, and complexes 55 and 58 (together with their amidinato analogues 14 and 17) are even the only known molecular compounds that contain an O,O´-chelating carbonato and non-bridging dithionito ligand, respectively.

The formation of 55, 57, and 58 is one of the rare examples of reactions of the amidinato/guanidinato-analogous silylenes 1 and 24 that lead to structurally analogous products (amidinato/guanidinato analogues 14/55, 16/57, and 17/58), whereas in most cases different reactivity profiles were observed.

The silicon coordination polyhedron of 56 is a strongly distorted trigonal bipyramid, with the carbon atom in an equatorial position. The five-coordinate silicon(IV) complex 56 with its carbon-bound CS22– ligand represents an unprecedented coordination mode in silicon chemistry, and the formation of 56 is a further example of the different reactivity profiles of the amidinato/guanidinato-analogous silylenes 1 and 24. The bis(amidinato)silylene 1 reacts with carbon disulfide to give the six-coordinate silicon(IV) complex 15 with an S,S´-chelating trithiocarbonato ligand and thereby differs from its guanidinato analogue 56 by both the silicon-coordination number and the coordination mode.

Indexing (document details)
Advisor: Tacke , Reinhold , Finze , Maik
Commitee:
School: Bayerische Julius-Maximilians-Universitaet Wuerzburg (Germany)
School Location: Germany
Source: DAI-C 81/7(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Chemistry
Keywords: Silicon
Publication Number: 27766770
ISBN: 9781392545027
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