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.

Reaction of trimethoxy(morpholinomethyl)silane (7) with lithium aluminum hydride yielded (morpholinomethyl)silane (8). Treatment of 8 with ethane-1,2-dithiol (molar ratio 1:2) in MeCN afforded bis[ethane-1,2-dithiolato(2−)](morpholiniomethyl)silicate (5; isolated as 5·MeCN). Reaction of 7 with ethane-1,2-diol (molar ratio 1:2) in MeCN yielded the oxygen analogue bis[ethane-1,2-diolato(2−)](morpholiniomethyl)silicate (6). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 5·MeCN and 6 are distorted trigonal bipyramids. Compounds 5·MeCN and 6 were additionally characterized by solid-state VACP/MAS NMR experiments (13C, 29Si). The structural investigations were complemented by quantum-chemical studies of the structure, bonding, and 29Si NMR chemical shift tensors of 5 (SiS4C skeleton) and 6 (SiO4C skeleton).

The hexacoordinate silicon(IV) complexes [NEt4]2[Si(NCS)6] (2), [K(18‐crown‐6)]2[Si(NCS)6]·2CH3CN (3·2CH3CN), and cis‐[(acac)2Si(NCS)2] (cis‐4; acac = acetylacetonato‐O, O) were synthesized, starting from Si(NCS)4 (1). Compounds 1, 2, 3·2CH3CN, and cis‐4 were structurally characterized in the solid state (13C, 15N (cis‐4 only), and 29Si MAS NMR; crystal structure analyses) and in solution (1H, 13C, and 29Si NMR; cis‐4 only). The experimental investigations were complemented by computational studies (RI‐MP2 geometry optimizations of 1, [Si(NCS)6]2—, cis‐4, and trans‐4; calculations of the 29Si NMR chemical shifts using the optimized structures).

A series of novel hexacoordinate silicon(IV) complexes with an SiO2N4 skeleton (compounds (OC-6-12)-3, (OC-6-12)-4, (OC-6-12)-5, (OC-6-12)-6, and (OC-6-2'2)-7) were synthesized, starting from Si(NCO)4 or Si(NCS)4. These compounds contain (i) two bidentate O,N-chelate ligands (or one tetradentate O,N,N,O-chelate ligand) derived from 4-aminopent-3-en-2-ones of the formula type Me-C(NRH)=CH-C(O)-Me (R = organyl) and (ii) two monodentate cyanato-N or thiocyanato-N ligands. Formally, the bidentate singly negatively charged O,N-chelate ligands (tetradentate 2-fold negatively charged O,N,N,O-chelate ligand) behave as ligands of the imino-enolato type. In addition, the adduct trans-8 was synthesized by reaction of Si(NCS)4 with 2 molar equiv of Me-C(Ni-PrH)=CH-C(O)-Me. This hexacoordinate silicon(IV) complex contains (i) four monodentate thiocyanato-N ligands and (ii) two neutral monodentate ligands of the iminio-enolato type. All compounds synthesized were structurally characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of these compounds, the experimental investigations were complemented by computational studies.

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.

The neutral hexacoordinate silicon(IV) complex cis‐[(acac)2Si(NCO)2] (cis‐8; acac = acetylacetonato‐O, O) was synthesized, starting from Si(NCO)4. Compound cis‐8 was structurally characterized in the solid state (13C, 15N, and 29Si MAS NMR; crystal structure analysis) and in solution (1H, 13C, and 29Si NMR). The experimental investigations were complemented by computational studies (RI‐MP2 geometry optimizations of cis‐8 and trans‐8 and of the corresponding cyanato‐O isomers cis‐[(acac)2Si (OCN)2] (cis‐9) and trans‐[(acac)2Si(OCN)2] (trans‐9); calculations of the 29Si NMR chemical shifts of cis‐8, trans‐8, cis‐9 and trans‐9 using the optimized structures).

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Reaction of tetramethoxysilane with three molar equivalents of oxalic acid and two molar equivalents of 1-(2-hydroxyethyl)-pyrrolidine or 1-(2-hydroxyethyl)piperidine in tetrahydrofuran yielded the λ 6 Si-silicates 1-(2-hydroxyethyl)pyrrolidinium tris[oxalato(2Ϫ)]silicate (4) and 1-(2-hydroxyethyl)piperidinium tris[oxalato(2Ϫ)]silicate (5). The related germanium compounds 1-(2-hydroxyethyl)piperidinium tris[oxalato(2Ϫ)]germanate (6) and triethylammonium tris[oxalato(2Ϫ)]germanate (7) were synthesized analogously, starting from tetramethoxygermane and using three molar equivalents of oxalic acid and two molar equivalents of 1-(2-hydroxyethyl)piperidine or triethylamine. Compounds 4Ϫ7 Dianionische Tris[oxalato(2؊)]silicat-und Tris[oxalato(2؊)]germanat-Komplexe: Synthese, Eigenschaften und strukturelle Charakterisierung im Festkörper und in LösungInhaltsübersicht. Die Umsetzung von Tetramethoxysilan mit drei Mol-Äquivalenten Oxalsäure und zwei Mol-Äquivalenten 1-(2-Hydroxyethyl)pyrrolidin oder 1-(2-Hydroxyethyl)piperidin in Tetrahydrofuran führte zu den λ 6 Si-Silicaten 1-(2-Hydroxyethyl)pyrrolidinium-tris[oxalato(2Ϫ)]silicat (4) und 1-(2-Hydroxyethyl)piperidinium-tris[oxalato(2Ϫ)]silicat (5). Die verwandten GermaniumVerbindungen 1-(2-Hydroxyethyl)piperidinium-tris[oxalato(2Ϫ)]-germanat (6) und Triethylammonium-tris[oxalato(2Ϫ)]germanat (7) wurden auf analoge Weise durch Umsetzung von Tetramethoxygerman mit drei Mol-Äquivalenten Oxalsäure und zwei Mol-Äqui-