Zwitterionic pentacoordinate silicate

The pentacoordinate silicon compounds 50,43 51,43 and 529 are spirocyclic zwitterionic A557-silicates with an Si04C skeleton. They contain two ethane-l,2-diolato(2-) ligands or two meso-oxolane-3,4-diolato(2-) ligands. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

The spirocyclic pentacoordinate silicon compounds 85,56 86,39 and 8739 are zwitterionic A5S7-silicates with an S702N2C skeleton. The chiral zwitterions... 

The spirocyclic pentacoordinate silicon compounds 90,45,58 91 45,58 and 9259 are zwitterionic A5Si-silicates with an Si05 skeleton. The chiral zwitter-ions contain two 2-methyllactato(2-) or two benzilato(2-) ligands. In these molecules, two five-membered SiC>2C2 rings are connected by the silicon spirocenter. Compounds 90 and 91 represent isoelectronic analogs of the zwitterionic A5Sz-silicates 58 and 59 (0/CH2 replacement see Section III,D). 

The pentacoordinate silicon compounds 94,23 95,23 96-98,60 99,60,61100,61,62 101-103,60 104,61,62 105,62 106,62 and 10761,63 are monocyclic zwitterionic A5S7-silicates with an Si02FC2 skeleton. The chiral zwitterions each contain one bidentate diolato(2-) ligand that formally derives from 1,2-dihydroxy-benzene, salicylic acid, glycolic acid, oxalic acid, benzohydroximic acid (tautomer of benzohydroxamic acid), 2-methyllactic acid, or (S)-mande-lic acid. 

Keywords Pentacoordinate Silicon / Zwitterionic 5/-Silicates / Crystal Structures / Acetohydroximato(2-) Ligand / Benzohydroximato(2-) Ligand... 

In continuation of our systematic studies on zwitterionic pentacoordinate silicon compounds, we have now succeeded in synthesizing the molecular A, 5/-silicates 5-10 (for reviews dealing with pentacoordinate silicon compounds, see [13-19]). These zwitterions belong to a new class of spirocyclic pentacoordinate silicon compounds with an SiO C skeleton. They contain diolato(2-) ligands derived from acetohydroximic acid and benzohydroximic acid, respectively (tautomers of acetohydroxamic acid and benzohydroxamic acid). Preliminary data concerning 5 have already been published elsewhere [8]. We report here on the syntheses and properties of compounds 5-10. In addition, the crystal structures of 6 and 8 are described. 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

Most of the zwitterionic compounds studied so far are chiral, with a chiral A5S/-silicate skeleton. Most of them have been isolated as racemic mixtures and in some cases as enantiomerically pure compounds, some of the optically active compounds being configurationally stable in solution. With these experimental investigations, in combination with computational studies, a new research area concerning the stereochemistry of molecular pentacoordinate silicon compounds has been developed. 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

A new class of pentacoordinate zwitterionic silicates (52-70) has been developed and reported by Tacke and his coworkers28-31 46-52 (cf Section II.A.3). These are generally high melting crystalline solids, which are almost insoluble in nonpolar organic solvents, and only slightly soluble in polar solvents. 

The zwitterionic X Si-silicate 28 is, to the best of our knowledge, the first pentacoordinate silicon species containing diolato(2-) ligands derived from a hydroximic acid (Scheme 8). 

Summary The syntheses and properties of derivatives of a new class of zwitterionic monocyclic 5i-silicates with an Si02C2F framework are reported. These molecular pentacoordinate silicon compounds contain bidentate diolato(2-) ligands that derive from benzohydroximic acid, oxalic acid, or glycolic acid. In addition, the crystal structures of these compounds are described. 

Over the past few years, we have synthesized and structurally characterized a series of zwitterionic acyclic and spirocyclic X S/-silicates, such as compounds 1 [1] and 2 [2] (Scheme 1). These zwitterions contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. We have now succeeded in preparing a series of related zwitterionic monocyclic A. S i-silicates. We report here on the syntheses, properties, and crystal structures of compounds 3a/3b-5a/5b (Scheme 1) [3] (for reviews dealing with pentacoordinate silicon compounds, see [4—10]). 

Keywords Pentacoordinate Silicon / Zwitterionic A, Si-Silicates / Monocyclic 5i-Silicates / Chirality / Isomerization / Epimerization / NMR Spectroscopy... 

Summary Derivatives of a new class of zwitterionic spirocyclic A, 5/-silicates with an SiO C framework have been prepared. The syntheses and the properties of these molecular pentacoordinate silicon compounds are reported. The crystal structures of two of the zwitterions are described. 

In contrast to the well-established chemistry of zwitterionic spirocyclic A, 5/-silicates with an S1O4C framework [1-11] (such as compounds 1 [1], 2 [9], and 3 [7]), only little is known about related zwitterions with an SiOs skeleton (such as compound 4 [11]). We report here on the syntheses, structures, and properties of the zwitterionic X Sr-silicates 5 and 6 (5/O4C skeletons) and their respective isoelectronic analogs 7 and 8 (SiOs skeletons). The studies presented here were carried out with a special emphasis on the comparison of the isoelectronic CH2/O pairs 5/7 and 6/8 (for reviews dealing with pentacoordinate silicon compounds, see [12-18]). 

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