Alkaline Earth Metal Organoamide Complexes Displaying Close Ae–F(C) (Ae = Ca, Mg) Interactions

Abstract

Protolysis of [Ca{N(SiMe3)2(thf)2] (thf = tetrahydrofuran) with the N,N-dialkyl-N′-2,3,5,6-tetrafluorophenylethane-1,2-diamines, p-HC6F4NH(CH2)2NR2 (LRH; R = Me or Et), yields [Ca(p-HC6F4N(CH2)2NR2)2(thf)2], whilst protolysis of di-n-butylmagnesium by the same amines yields [Mg(p-HC6F4N(CH2)2NR2)2] complexes. X-Ray crystal structures show that the calcium derivatives contain eight-coordinate calcium atoms, with tridentate (N,N′,o-F) coordination of the organoamide ligands and cisoid thf donors. It is noteworthy that the Ca–F(C) bond lengths are shorter than the Ca–NR2 bond lengths. By contrast, magnesium forms centrosymmetric, six-coordinate, homoleptic complexes, in which the Mg–NR2 bond lengths are shorter than the Mg–F(C) bond lengths. 19F{1H} NMR spectra of the R = Me complexes support retention of Ae–F(C) linkages in solution at low temperatures. Heating the complexes for a prolonged period in C6D6 at 80°C showed no evidence of C–F activation reactions.