Five-co-ordinate magnesium complexes: synthesis and structure of quadridentate Schiff-base derivatives

Abstract

Reaction of dibenzylmagnesium in tetrahydrofuran (thf) with the quadridentate Schiff-base ligands NN′-ethylenebis(acetylacetoneimine)(H₂acen), NN′-ethylenebis(salicylideneimine)(H₂salen), NN′-o-phenylenebis(salicylideneimine)(H₂salphen), or 7,16-2H-6,8,15,17-tetramethyldibenzo[b,i][1,4,8,11]tetra-azacyclotetradecine (H₂tmdbtd) afforded in a good yield the corresponding magnesium complexes [{Mg(acen)}₂](1), [{Mg(salen)}₂](2), [{Mg(salphen)}₂](3), and [Mg(tmdbtd)(thf)](4). The dimeric nature of the Schiff-base complexes (1)–(3), which was proven by an X-ray analysis on complex (1), allows magnesium five-co-ordination. The acidity of a four-co-ordinate magnesium in complex (4) is shown by the formation of a very stable adduct with thf. The structure of (4) has also been determined by X-ray analysis. Complex (1): space group P2₁/c(monoclinic), a= 10.825(2), b= 11.845(3), c= 10.734(3)Å, β= 92.80(2)°, Z= 2, R= 0.050 for 957 observed reflections. Complex (4): space group Cmcm(orthorhombic), a= 14.212(2), b= 8.306(1), c= 19.350(4)Å, Z= 4, R= 0.042 for 900 observed reflections.