Lewis acidity of organosilicon acetates

Abstract

Thermodynamic data, obtained calorimetrically, are reported for the formation of adducts of organosilicon acetates, R n Si(OCOME) 4- n (0 ⩽ n ⩽ 3) with a wide variety of Lewis bases in acetonitrile solution. Only 1 1 adducts are formed, in which the silicon is 5-coordinate and there is evidence that the coordination number cannot exceed five in these compounds. Enthalpies of adduct formation (−Δ H 0) increase regularly as R is replaced by OCOMe and become very large with Si(OCOMe) 4, exceeding 150 kJ mol −1 for addition of amines to this compound. Despite the large enthalpies of formation, adduct stabilities are not high and adduct formation constants, K, rarely exceed 200 l mol −1; stabilites are similar for 0 ⩽ n ⩽ 2 but a little greater for n = 3 and are thus little affected by variations in the enthalpy.