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-tBu(4)carb)K(THF)(4) (1; carb=C(12)H(4)N), [(1,8-Xyl(2)-3,6-tBu(2)carb)K(THF)](2) (2; Xyl=3,5-Me(2)C(6)H(3)) and (1,8-Mes(2)-3,6-tBu(2)carb)K(THF)(2) (3; Mes=2,4,6-Me(3)C(6)H(2)) were reacted with MgI(2) to give the Hauser bases 1,3,6,8-tBu(4)carbMgI(THF)(2) (4) and 1,8-Ar(2)-3,6-tBu(2)carbMgI(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.