Lanthanum(III) chloroaluminate and chlorogallate complexes with toluene and hexamethylbenzene: The effect of arene methylation on the structure

Two new lanthanum(III) chloroaluminate complexes with neutral arenes, [La(η6-C6H5Me)(AlCl4)3] (1) and [La(η6-C6Me6)(AlCl4)3]·1/2C6H6 (2), as well as the first lanthanide chlorogallate complex [La(η6-C6Me6)(GaCl4)3]·1/2C6H6 (3) have been synthesized and structurally characterized. The coordination polyhedron around the central lanthanum ion in 1–3 is a distorted pentagonal bipyramid formed by six Cl-atoms and an η6-bound arene. In contrast to the mononuclear complex 1 with toluene, 2 and 3 form dimers in the solid state via π–π stacking interactions of the coordinated hexamethylbenzene ligands with a sandwiched benzene molecule. Such dimerization has not been seen in the crystal structures of other known lanthanide chloroaluminate π-complexes, including those with C6Me6. The reasons for the dimer formation in the lanthanum(III)–hexamethylbenzene–benzene systems are discussed based on DFT calculations and 1H NMR experiments.