Organotin compounds five-coordination

Few X-ray diffraction data are available for the organotin alkoxides. Trimethyltin methoxide in the crystal is a five-coordinate linear polymer 47,380 and both tin atoms in the methanol solvate, CH2(SnPh2OMe)2-MeOH, are five coordinate, one by intramolecular coordination, and the other by solvation 48.381 Dimethyltin dimethoxide 49,382 dimethyltin diphenoxide 50,383 and isopropyltin triisopropoxide 51384 are five-coordinate oxygen-bridged dimers. All these compounds are monomers in solution. 

Chemical interactions with solvents Various workers have observed (1, 4, 7) that five- and six-coordinate organotin compounds show U9Sn signals which occur at much lower frequencies than those of the four-coordinate derivatives. For this reason, the choice of solvent in preparing samples for NMR measurements may affect the value of the observed u9Sn chemical shift. 

Reported changes in chemical shift (50) with temperature and concentration of trimethyltin chloride in methanol or ethanol indicate a more complicated process than a simple monomer complex equilibrium. It has been suggested that ionization of the organotin compound occurs in these solvents, probably forming a solvated five-coordinate (RjSnLz) cationic species. 

In the condensed phase Me3SiF molecules show no intermolecular interactions, while the same germanium derivatives are associated as a dimer due to intermolecular F Ge coordination. According to the tendency for tetrahedral main-group 14 elements to expand the coordination sphere, organotin fluorides show a strong tendency to associate in the solid state and even in triorganotin fluorides the tin atom is five-coordinate A common feature of this class of compounds in the solid state is coordination expansion of the tin atom due to hypervalent interaction, which in turn often results in formation of polymeric materials. 

The five-coordinate complexes of type 421 possess a rigid structure, which is retained in solution as indicated by comparing the Sn NMR in solution and in the solid state . For 421a-c, isotope-induced chemical shifts A ( C( Sn) were determined for the first time for five-coordinate tin nuclei at natural abundance of A positive sign of 1 Ai2/i3c(ii Sn) was found, in contrast with most tetracoordinate organotin compounds. 

Baba and coworkers reported the unusual reactivity of highly coordinated tin compounds such as organotin(IV) enolates or organotin(IV) hydrides. NMR studies disclose the existence of five-coordinated tin enolate arising from the addition of certain ligands . In particular, the coordination to tin enolates by a bromide anion (from Bu4NBr) shows novel types of selective reactions. ... 

Little has been reported yet on the coordination chemistry of mono-organotin compounds. Coordination numbers of five to eight for the tin atom have been reported in such compounds (39,50,51). It has been suggested, however, that in solution a trigonal bipyramidal arrangement at tin is the most favorable one (49). 

The ability of organotin compounds to self-assemble, with an increase of the coordination number of tin from four to five and six, has been recognized quite early, and the resulting compounds were termed autocomplexes [345]. 

A mixture of ethyl bromosuccinate and tinfoil in the presence of catalysts was stirred for 4 h, for example, and the tinfoil soon disappeared from the reaction mixture in the process. A product was easily isolated as a solid form from the reaction mixture, and two crystalline diastereomeric species were obtained by fractional crystallization of the above product. Based on the IR spectra of the products, the author first proposed a five-membered ring structure for organotin compounds in which lone electron pair of carbonyl oxygen coordinates to a tin atom, as shown in Eq. (2.4). 

The most substantial study of this sort, analysing structures of 186 four-, five- and six-coordinated tin compounds, was published by Britton and Dunitz (1981) under the memorable title Pathways for SN2 and SN3 Substitution at Sn(IV) . Organotin halides readily increase their coordination number to 5 or 6 (Davies and Smith, 1982), and a wide range of structures are stable in the solid state. 

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