Silatranes structure

A single hexacoordinated silatrane 46 possessing a very weak additional Si NMe2 bond (2.952 A) was obtained recently by Corriu and coworkers44. The hexacoordination in 46 is most likely due to the rigid geometry of its 8-dimethylaminonaphthyl group. The reluctance of silicon to become hexacoordinated in the silatrane structure is its special characteristic feature which, as will be shown below (Section II.C), predetermines the low reactivity of silafunctional silatranes toward nucleophiles. 

Another case in point is the silatrane structures, where, again, the relatively weak N-Si dative bond is much longer in the gas than in the crystal. The difference is 0.28 A for 1-fluorosilatrane [9-71], represented here (9-2) by the heavy-atom skeleton. 

Subsequent theoretical calculations confirmed the important role of the Si-<—N interaction in the formation of the silatrane structure. A significant Si- —N bond order was determined at the CNDO/2 level using the experimental solid state geometries of silatranes Thus, the Wiberg indexes of this bond in the crystalline a, and y... 

Carbasilatranes are another variation of the silatrane structure. The carba-silatranel44 has been used as a precursor to fix the optically active ligand system via the incorporated silicon atom on silica gel or mesoporous silicas (Scheme 39) and to prepare vanadium(V) complexes therefrom, which act as functiraial models for the sulfide-peroxidase [301]. Several 3,7,10-trimethylsilatranes and carbasilatranes have been synthesized and investigated by NMR spectroscopy [302]. 

Remarkable is the fact, that toxicity of silatranes varies within extremely wide limits, being determined mainly by the nature of the substituents R in structure 8 (Table 4). While 1-arylsilatranes are very toxic, 1-alkyl-, 1-vinyl-, and 1-ethinylsila-tranes are practically non-toxic. 1-Alkoxysilatranes have low toxicity and many of them can also be regarded as practically non-toxic. 

During the following years investigations in the field of organosilicon compounds were concentrated on the development of new methods of synthesis of silatranes and on the study of their steric and electronic structure, various physical properties and, especially, their biological activity2-15 ... 

The toxic effect and other biological properties of many silatranes are thus associated with their structural peculiarities and are mainly determined, in the author s opinion, by the following factors... 

Polyhedral structure of the silatrane skeleton and anomalous valency state of the silicon and nitrogen atoms involved. 

By extensive studies of the influence of a large number of silatrane derivatives of different structure on cultures of plant tissues several very active compounds were recognized and examined in their effects on seeds, seedlings of plants and various cultivary such as tomatoes, cucumbers, grapes, flax, etc. 

The possibilities considered of application of some silatrane derivatives as specific stimulators of the growth of plants suggest that these compounds influence, in some direct or indirect way, the components of the nucleic structures (such as nucleic acids, histones) constituting the genetic apparatus of the cell. 

Transannular interaction between the silicon and the nitrogen atoms in Si- and C-substituted silatranes is strongly proved by numerous X-ray studies5-7 13-15,18-22,29. Table 1 provides structural parameters for a large fraction of the approximately 60 crystal-lographically determined structures of 1-substituted silatranes which are mainly collected in a recent review29. 

The cage skeleton of most silatranes possesses an approximate threefold (C3) symmetry and in its five-membered (N—Si) chelate rings the carbon atoms in the a-position to the nitrogen deviate from the plane formed by the other atoms15,29 (Scheme 2). The partial disordered structure observed for some silatranes may be regarded as a frozen state of the dynamic flapping of the cr-C atoms in the crystals15. 

The energetic preference of the endo form was demonstrated by the first quantum chemical calculations of silatranes performed on 1-hydro- and 1-fluorosilatranes12,181. These calculations employed the CNDO/2 semiempirical method with spd basis set and geometries of the endo and exo forms corresponding to those obtained for 1-methylsilatrane by modified molecular mechanics method. The energy of the Si-e- N coordinative bonding was found to be about 20-25 kcalmol-1 and to be the main contributor to stabilization of the endo structure of silatranes. 

AMI and PM3 computation on 1-methyl-191, 1-fluoro-180, 1-chloro-192 and 1-isothiocyanatosilatranes68 using full geometry optimization confirmed the small energy cost for a shortening of the SN— N bond in silatranes. The potential profiles of these molecules were shown to be very flat in the 2.00-3.60 A range of the Si — N distance, with two distinct minima related to the endo and exo forms. Both parametrizations found the endo structure to be the more stable in the case of 1-isothiocyanato- and 1-halosilatranes, where the equilibrium Si —N distances are shorter than in 1-methylsilatrane. Both... 

Owing to the silicon hypervalency and the cage structure, silatranes are of considerable interest from the standpoint of molecular electrochemistry. However, little work has been devoted to electrochemistry of silatranes279-281. 

Dipole moment studies207,378,380 found a lower dipole moment of the Si — N bond in 3-homosilatranes (1-2 D) than in silatranes (1.5-3.1 D). The enthalpy of formation of this bond was estimated to be 3-11 kcalmol-1 depending on the substituent X and on the method of the bond dipole calculation207. The observation of an absorption band at 570-600 cm-1 in the IR spectra suggests a tricyclic structure of 3-homosilatranes in solution378. 

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