Compounds with Pentacoordinated Silicon Atoms

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. 

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. 

FIGURE 22. Histogram of Si—F bond lengths in compounds with a pentacoordinate silicon atom... 

Over the past few years, a series of zwitterionic spirocyclic bis[l,2-benzenediolato(2-)]-organosilicates, such as compounds 1 [Ic] and 2 [la] (Fig. 1), and related zwitterions with 2,3-n q)hthalene-diolato(2-) and symmetrically substituted l,2-benzenediolato(2-) ligands have been structurally charac-terized by X-ray diffraction [1]. The coordination polyhedra around the pentacoordinate silicon atoms of such compounds can be described using the idealized geometries of the trigonal bipyramid (TBP) and the square pyramid (SP). In the solid state, almost all transitions between a... 

Recently we reported an easy route obtaining enamine-functionalized silicon compounds with a pentacoordinate silicon atom from organotrichlorosilanes and ethyIene-iV,iV -bis(2-hydroxyaceto-phenoneimine) (salen H2) [1]. 

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

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]). 

O-Apical stereoisomers of pentacoordinated phosphoranes can be obtained by different routes, including the Kawashima s synthesis of the pentacoordinated phosphorus compound bearing a pentacoordinate silicon atom, both possessing Martin ligands in the structure or by the reaetion of 8-chloro-2-cyclohexyl-4-phenylbenzo[e]-1,2-oxaphosphinine-2-oxide with tetramethylenebis(magnesium bromide) as reported by Konovalov et al ... 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

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