Cationic Dialkyl Metal Compounds of Group 13 Elements (E = Al, Ga, In) Stabilized by the Weakly Coordinating Dianion [B12Cl12]2–

Abstract

The reactions of the trityl salt of the weakly coordinating dianion [B12Cl12]2– ([Ph3C]2[B12Cl12]) with Et3Al, Et3Ga·(OEt2) and Et3In in 1,2-difluorobenzene yielded (Et2Al)2B12Cl12, [Et2Ga(OEt2)2]2[B12Cl12], and (Et2In)2B12Cl12. The products were characterized by NMR (1H, 11B, 13C), IR, and Raman spectroscopy. Investigation of the symmetric carbon–metal stretching vibration (Raman) of the [Et2E]+ unit in (Et2E)2B12Cl12 (E = Al, In) compounds indicated a linear structure for E = In and a bent structure for E = Al. The latter was confirmed by a crystal structure determination of (Et2Al)2B12Cl12. While the reaction of the triethyl compounds Et3E (E = Al, In) and Et3Ga·(OEt2) proceeded via β-hydride abstraction and release of ethylene, Me3Al reacts with [Ph3C]2[B12Cl12] under methide transfer. The gaseous byproduct ethene was identified by IR spectroscopy, and solid byproducts (Ph3CH or Ph3CMe) were observed by NMR spectroscopy in solution. The formation of (Me2Al)2B12Cl12 was proven by X-ray diffraction and NMR spectroscopy. In the crystal structures of (Me2Al)2B12Cl12 and (Et2Al)2B12Cl12 the aluminum atoms are bound to two chlorine atoms, resulting in a distorted tetrahedral environment around aluminum. The aluminum–chlorine contacts are longer than a typical Al–Cl single bond but significantly shorter than the sum of the van der Waals radii. The bonding in both compounds can be described as ion-like. The underlying thermodynamics for β-hydride abstraction and methide transfer were investigated in the gas phase by DFT calculations, in 1,2-difluorobenzene solution by applying the COSMO solvation model, and in the solid state by Born–Haber–Fajans cycles using a volume-based approach to estimate lattice enthalpies. These estimations show that the reactions are unfavorable in the gas phase but become favorable when solvation and lattice energies are taken into account.