Indazolato Derivatives of Boron, Aluminum, and Gallium:  Characterization and Solvent-Dependent Regioisomeric Structures through π−π Interactions in the Solid State

The complexes [(η1:η1-ind)(μ-M)R2]2 (R = Me, M = Ga (1); R = Me, M = Al (2); R = Et, M = B (3)) were prepared by treatment of indazole with MR3 in toluene. These have dimeric molecular structures, with the indazolato ligands coordinated in a η1:η1 fashion. In solution, complexes 1 and 2 show a mixture of syn and anti isomers in a 3:5 ratio, respectively; however, in the solid state the anti isomers can be selectively crystallized in toluene and the syn isomers in benzene. These complexes constitute the first examples of organoaluminum and organogallium complexes for which both isomers observed in solution can be selectively crystallized. In contrast with 1 and 2, for boron complex 3 only the anti isomer was observed in solution and in the solid state. Results from theoretical calculations at the B3LYP/6-31G* level and NBO analyses were performed to gain insight into the nature of the metal−ligand bond and into the question of whether π−π interactions in the crystal packing play an important role in the molecular structure of the isomer crystallized.