2-Imino-2,3-dihydrobenzoxazole—a useful platform for designing rare- and alkaline earth complexes with variable di- and trianionic O,N,N, ligands

Abstract

The reactions of 2-imino-2,3-dihydrobenzoxazole LH with M[N(SiMe₃)₂]₂(THF)₂ (M = Yb and Ca) and Y(CH₂SiMe₃)₃(THF)₂ proceed with the opening of the dihydrobenzoxazole ring and the elimination of HN(SiMe₃)₂ or SiMe₄. Besides, in the case of Yb[N(SiMe₃)₂]₂(THF)₂, an electron transfer from Yb(II) to L takes place and Yb(III) complex 1 is coordinated by a dianionic phenolate ligand containing a pendant radical-anionic diazabutadiene fragment form. When LH is reacted with Ca[N(SiMe₃)₂]₂(THF)₂, C–H bond activation of a methyl fragment by imino nitrogen occurs and affords a dimeric calcium complex 2. In 2, the phenolate ligand is dianionic due to the presence of the amido-imino fragment [R–NC(CH₃)–C(CH₂)–NR′]⁻ (R = 2,4-tBu₂-C₆H₂O; R′ = 2,6-iPr₂C₆H₃). In situ generated ate-complex {Na(Et₂O)_n}{Ca[N(SiMe₃)₂]₃} also enables C–H bond activation, however the dianionic phenolate ligand in the resulting complex 3 contains an amido-imino fragment [R–N–C(CH₂)–C(CH₃)NR′]⁻ featuring the sequence of N–C and NC bonds opposite to that in 2. The reaction of Y(CH₂SiMe₃)₃(THF)₂ with LH affords mono(alkyl) yttrium complex 4. 4 contains a dianionic amido-imino phenolate ligand resulting from the migration of one alkyl group to the CN bond [R–N–C(Me)(CH₂SiMe₃)–C(Me)NR′]. 4 undergoes slow intramolecular C–H bond activation of the residual CH₃ group to afford a yttrium complex coordinated by a trianionic diamido-phenolate ligand [R–N–C(Me)(CH₂SiMe₃)–C(CH₂)NR′].