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

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 <i>ion-like</i>. 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.