Calix[4]arenes of Aluminum and Gallium with Benzimidazolyl Ligands: Steric Control of the Conformation via Substitution on the Ligand

Complexes [bzimAlR2]4 [bzim = benzimidazolate; R = Et (2), iBu (3)], [mbzimAlR2]4 [mbzim = 2-methylbenzimidazolate; R = Et (6), iBu (7)], [dmbzimAlR2]4 [dmbzim = 5,6-dimethylbenzimidazolate; R = Me (9), Et (10), iBu (11)], and [tmbzimAlR2]4 [tmbzim = 2,5,6-trimethylbenzimidazolate; R = Me (12), Et (13), iBu (14)] have been prepared via alkane elimination and coordinative self-assembly upon the reaction of benzimidazole ligands with aluminum alkyls in benzene, toluene, or xylene. Characterization of the complexes was achieved by spectroscopic methods, microanalysis, and X-ray crystallography of 2, 7, 10, 11, 13, and 14. The complexes reported herein and the aluminum and gallium analogues 1, 4, 5, and 8 reported in a previous paper are predominantly tetranuclear aggregates related to calix[4]­arenes in which the benzimidazolyl ligands bind two metal atoms in a η1:η1 fashion. X-ray crystallography demonstrates that modulation of the conformation adopted by these metallacalix[4]­arenes is achieved by proper substitution on the C atom at the 2 position of the benzimidazolyl ligand. An H substituent for 1, 2, 4, 10, and 11 favors a chair conformation with a small cavity and approximate C2h symmetry, while a CH3 substituent for 5, 7, 8, 13, and 14 introduces enough repulsion to switch the conformation to a 1,3-alternate or double cone with a concomitant larger cavity and approximate C2v symmetry.