Oxygen compounds intramolecular 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. 

The second structural motif, B (Figure 72), involves also a dimeric structure in which two alkoxy oxygen atoms bridge between two zinc atoms, but now an additional intramolecular coordinating substituent is present, resulting in the formation of four-coordinate zinc atoms. Most of these strucmral investigations are aimed at the elucidation of the mechanism of the enantioselective addition of organozinc compounds to aldehydes, a topic that will be discussed in Chapter 13. 

Compounds of the type Cp2TiR (R = alkyl or aryl) have been shown to be unstable unless complexation by a donor molecule containing nitrogen or oxygen occurs. However, the stability is enhanced significantly when the active site is blocked sterically or by intramolecular coordination. 

That the intramolecular O Sn coordination in compounds 320a-c is relatively weak was also shown by replacing the intramolecularly coordinating oxygen by the external strong Lewis base BU3PO to yield 1 1 or 1 2 adducts. In the latter, the tin atom remains pentacoordinated and the added Lewis base displaces a halide group. The tendency to form the 1 2 adduct decreases in the sequence I > Br > Cl. The Cl derivative forms no adduct. 

The highest coordination state of Mg is hexacoordinate, even when further intramolecular coordination is possible. The Mg of the crown ether compound, 2-bromomagnesio-l,3-xylyl-18-crown-5 [Compound 16] [17], is coordinated to only four of the five oxygen atoms as well as the Br and the aromatic carbon. 

Also in compounds 86-89, intramolecular Sn—O coordination is relatively weak as was shown by replacement of the intramolecular coordinating oxygen atom by external ligands like pyridine 139,140). 

An intramolecular coordination of a Lewis base atom affording pentacoordi-nate silicon similar to the situation in silatranes has been established for cyclo-bis(benzamidodimethyl)silane 192 (318) and, more recently, for a new class of organosilicon compounds (aroyloxymethyl)trifluorosilanes 193 (319) (Figure 26). The pentacoordinate character of silicon in compounds 192 and 193 has been confirmed by X-ray diffraction. In the latter case, the Si <- 0 bond length is 1.94 A which is much less than the sum of van der Waals radii (3.4 A) for silicon and oxygen. Similarly, the product of the reaction of bis-(trimethylsilyl)-... 

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