USE OF THE 2,6-BIS[(DIMETHYLAMINO)METHYL]PHENYL LIGAND FOR THE STUDY OF NUCLEOPHILIC-SUBSTITUTION AT HEXACOORDINATE SILICON CENTERS - EVIDENCE SUGGESTIVE OF A HEPTACOORDINATE SILICON TRANSITION-STATE

Two silicates bearing the potentially bis chelating ligand Ar = (C6H3(CH(2)NMe(2))(2)-2,6) have been made with the objective of studying nucleophilic substitution reactions at hexacoordinate siIicon. The X-ray crystal structure of the silicate [ArSi(O2C6H4-1,2)(2)](PPN+)-P--, 3b, has revealed that the anion exhibits slightly distorted octahedral geometry, with only one NMe(2) being coordinated to the silicon. Furthermore, the results of solution NMR studies of 3b and [ArSiF4](-) [K, 18-crown-6](+), 4, are consistent with dynamic coordination, in which the NMe(2) groups are displacing each other through a heptacoordinate transition state and the central silicon atom is essentially hexacoordinated.