Chelating ionic versus bridged molecular structures of group 13 metal complexes with bidentate ligands

Abstract

Complexes of aluminum and gallium trihalides with ethylenediamine (en) and N,N,N′,N′-tetramethylethylenediamine (tmen) of 2:1 composition have been synthesized and structurally characterized by single crystal X-ray diffraction analysis. In contrast to known molecular complexes of hydrido and methyl-substituted analogs, these solid complexes adopt ionic structures of the general type [M1X2LL]+[M2X4]− (X = Br, I; M1, M2 = Al or Ga; LL = en, tmen).

Quantum chemical computations at B3LYP/LANL2DZ(d,p) level of theory showed that molecular complexes M1X3LLM2X3 are more stable in the gas phase compared to the [M1X2LL]+[M2X4]− ion pairs. Ionic complexes of metal halides with en and tmen are predicted to exist in the solid state due to strong X− affinity of MX3 and favorable crystallization energy. Considerably lower H− and CH3− affinity of MH3 and M(CH3)3 favors molecular structures for complexes of hydrides and methyl derivatives both in the gaseous and condensed state.

Comparative case study between complexes of AlCl3 with ammonia and en points out that AlCl3NH3 adduct exist in molecular form due to strong intermolecular Hcdots, three dots, centeredCl interaction. Molecular structure of GaBr3NH3 adduct was experimentally confirmed by single crystal X-ray diffraction analysis.