Tuning coordination in s-block carbazol-9-Yl complexes

1,3,6,8-Tetra-tert-butylcarbazol-9-yl and 1,8-diaryl-3,6-di(tert-butyl)carbazol-9-yl ligands have been utilized in the synthesis of potassium and magnesium complexes. The potassium complexes (1,3,6,8-tBu₄carb)K(THF)₄ (1; carb=C₁₂H₄N), [(1,8-Xyl₂-3,6-tBu₂carb)K(THF)]₂ (2; Xyl=3,5-Me₂C₆H₃) and (1,8-Mes₂-3,6-tBu₂carb)K(THF)₂ (3; Mes=2,4,6-Me₃C₆H₂) were reacted with MgI₂ to give the Hauser bases 1,3,6,8-tBu₄carbMgI(THF)₂ (4) and 1,8-Ar₂-3,6-tBu₂carbMgI(THF) (Ar=Xyl 5, Ar=Mes 6). Structural investigations of the potassium and magnesium derivatives highlight significant differences in the coordination motifs, which depend on the nature of the 1- and 8-substituents: 1,8-di(tert-butyl)-substituted ligands gave π-type compounds (1 and 4), in which the carbazolyl ligand acts as a multi-hapto donor, with the metal cations positioned below the coordination plane in a half-sandwich conformation, whereas the use of 1,8-diaryl substituted ligands gave σ-type complexes (2 and 6). Space-filling diagrams and percent buried volume calculations indicated that aryl-substituted carbazolyl ligands offer a steric cleft better suited to stabilization of low-coordinate magnesium complexes.