Cationic dialkyl metal compounds of group 13 elements (E = Al, Ga, In) stabilized by the weakly coordinating dianion [B_(12)Cl _(12)]~(2-)

摘要

The reactions of the trityl salt of the weakly coordinating dianion [B _(12)Cl_(12)]~(2-) ([Ph_3C]_2[B _(12)Cl_(12)]) with Et_3Al, Et_3G? (OEt_2) and Et_3In in 1,2-difluorobenzene yielded (Et _2Al)_2B_(12)Cl_(12), [Et _2Ga(OEt_2)_2]_2[B_(12)Cl _(12)], and (Et_2In)_2B_(12)Cl_(12). The products were characterized by NMR (~1H, ~(11)B, ~(13)C), IR, and Raman spectroscopy. Investigation of the symmetric carbon-metal stretching vibration (Raman) of the [Et_2E]~+ unit in (Et_2E)_2B_(12)Cl_(12) (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 (Et _2Al)_2B_(12)Cl_(12). While the reaction of the triethyl compounds Et_3E (E = Al, In) and Et_3G? (OEt_2) proceeded via β-hydride abstraction and release of ethylene, Me_3Al reacts with [Ph_3C]_2[B _(12)Cl_(12)] under methide transfer. The gaseous byproduct ethene was identified by IR spectroscopy, and solid byproducts (Ph_3CH or Ph_3CMe) were observed by NMR spectroscopy in solution. The formation of (Me_2Al)_2B_(12)Cl_(12) was proven by X-ray diffraction and NMR spectroscopy. In the crystal structures of (Me_2Al)_2B_(12)Cl_(12) and (Et _2Al)_2B_(12)Cl_(12) 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.