Silicon amidinates

Keywords silicon amidinates, germanium amidinates, hypervalency, low-valency... 

Keywords Silicon Amidinates / Gennanium Amidinates / Bidentate Coordination / Pentacoordination / Hexacoordination... 

Reactions of different stoichiometric amounts of the lithium amidinates (A) with SiCLt result in mono-, bis- and trisubstituted silicon amidinate derivates (Eq. 1). 

Table 1. Si-NMR data for the mono- and disubstituted silicon amidinates.

By variation of the substitution pattern of the amidinate ligands and of the silicon or germanium coordination centers, we obtained a number of compounds with a variable degree of Si/Ge-N hypervalent interaction, i.e. compounds with essentially no bones and ones with very strong element-nitrogen bonds ( double bonds ) are formed, providing tetra-, penta- or hexa-coordinated species [4], New or newly structurally characterized examples of silicon amidinate complexes with pentacoordinate silicon are obtained (Eq. 1 Table 1). 

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

The characteristic 1540 nm emission of Er3"1" was observed via a silicon exciton-mediated energy transfer for Si Er3+ nanocrystals (12 nm) derived from an erbium amidinate precursor (Ji et al., 2004). Similarly, the PL spectrum of Er3"1" implanted Si nanocrystals showed two lines at 1535 and 1546 nm, having a much higher intensity when compared to Er3"1" implanted Si02 nanocrystals or Er implanted Si nanocrystals (Priolo et al., 2001). 

As shown by X-ray structure determination (Fig. 3), 7 does not contain a hexacoordinated silicon atom stabilized by amidinate ligands but rather may be regarded as an amidine derivative with a tetracoordinated silicon atom. 

The system HSiC /DBU may be used as a SiClj synthon in the presence of electrophilic heterobutadienes. The resulting heterocycles are not only precursors for fluorine substituted imidazoles [13], but also may be transformed to amidinate complexes with silicon in a pentacoordinated state. Monophosphorous analogues to amidinates are synthons for silyl-substituted phosphaalkenes. 

In addition, an analogous species (3) containing a silicon-hydrogen IxMid was obtained by treating HSiCU with 1 equiv. of a lithium amidinate ligand (Fig. 3, Table 2). As may be expected, the hydrogen atom adopts an equatorial position at the pentacoordinate silicon atom. 

This study demonstrates the utility of amidinate ligands for the synthesis of hypervalent silicon and germanium compounds, and also, for the synthesis of low-valent germanium compounds. In addition, surprising results with dianionic amidinate ligands [N-C(R)—NR ] at silicon centers are documented. 

By using modified colloidal particles as templates, silicon oxide macroporous materials with uniform submicrometer-sized pores can be synthesized.[14] Modified polystyrene emulsion microspheres (200 1000 nm) can be electronegative (sulfates) or electropositive (amidines). After these microspheres are packed in an orderly fashion, they can interact with surfactants and silicon oxides to form macroporous solid composites, and further to form macroporous materials after the removal of the templates by calcination. The sizes of the macropores in the products range from 150 to 1000 nm. Macroporous Ti02 can also be prepared in a similar way. 

Suitable starting materials for the synthesis of silicon or germanium amidinate complexes are lithium amidinates, which are easily accessible by the reaction of lithium amides with nitriles (A) or by addition of organolithium compounds to carbodiimides (B) (Scheme 1). 

In the case of the monosubstituted amidinate complexes, a crystallographic study of 2c (Fig. I) confirms a monodentate coordination of the amidinate ligand to the silicon center. The C-N bonds are nearly equal. The Si-N bond (1.625(3) A) is remarkably short and indicates some double bond character [6]. The analogous germanium structure (Fig. 2), apart Irom the longer Ge-N bond (1.776(2) A), is quite similar. Though the N-H hydrogen atom has not been located in either case, it seems plausible that an N(2)-H functionality is present. 

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