Nitrogen, pentacoordinated

The literature on these structures is sparse, and generally the articles report preparations of several of them in similar ways. These compounds are constituted by oxygen, sulfur, or nitrogen adducts at the silicon atom, thus providing pentacoordinated silicon compounds or structural analogs (Scheme 33). 

Photoswitching has allowed Kawashima et al. to control the configuration of the N-N double bond and the coordination number of potassium 18-crown-6 2-(phenylazo)phenyltetrafluorosilicates 836 and 837, which are accessible by the reaction of 2-(phenylazo)phenyltrifluorosilane 838 with KF/18-crown-6 (Scheme 116).830 In the (Z)-form, the silicon atom of 837 is pentacoordinated, whereas in the (E)-form, one nitrogen atom of the azo group of 836 intramoleculary coordinates to the silicon atom, thus extending its coordination sphere to six ligands. Almost... 

The traditional limitation of the stereoisomer concept to isomeric compounds with different configurations stems from the idea that configurations are not interconvertible under the observations conditions. ) A definition of stereoisomers on this basis is questionable as its application is then dependent upon observation conditions. It also fails to account for the fact that many mobile interconversions of configurations (e.g. configurations involving tricoordinate Nitrogen or pentacoordinate Phosphorus) are known, as well as thermally stable conformations (e.g. the atropisomers). 

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. 

Chirality was also achieved by using cyclic (chiral) substituents on the imino nitrogen. Complexes 79 and 80 (the latter obtained by reduction of the C=N double bond) were tested for the ATH of butyrophenone in isopropanol at 60 °C with NaOH. The activity (26% versus 68% conversion for 79a,b) and stereoselectivity (64% versus 3% for 79a,b) were sharply reduced when a methyl group was introduced into the 6-position of the pyridine ring. Tetracoordinated cationic complexes and pentacoordinated neutral species displayed similar efficiencies (64 versus 66% ee for 79a and 80a, respectively). 

Si-N bonds in compounds with pentacoordinate silicon and tricoordinate nitrogen atoms... 

Pentacoordinate silicon forms two types of bonds with tricoordinate nitrogen atoms, a pure covalent bond and a N - Si dative bond. The first is significantly shorter than the second. The average covalent Si—N bond length in compounds where pentacoordinate silicon atom is bonded to tricoordinate nitrogen atom was calculated from 48 XRD experimental values to be 1.761 A (s.d. 0.06 A, s.m. 0.009 A). An example of the difference in bond length is shown in 127141 where the covalent Si—N bond lengths are 1.766 and 1.770 A and the dative bond is 2.333 A. 

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