Observation of Cationic Transition Metal-Alkane Complexes with Moderate Stability in Hydrofluorocarbon Solution.

ABSTRACT

In seeking to create more-stable transition metal-alkane complexes, we generated cationic alkane complexes of the type [(HEB)Re(CO)2(alkane)][Al(OR(f))4] (HEB = η(6)-hexaethylbenzene; alkane = cyclopentane (16) or pentane (17-19); OR(f) = perfluoro-tert-butoxy) via photolysis of the precursor complex [(HEB)Re(CO)3][Al(OR(f))4] (15) in the presence of the added alkane. The alkane complexes were generated in a hydrofluorocarbon (HFC) solvent, most often CF3CH2CF3, which is capable of simultaneously dissolving the ionic complex 15 and a small amount of alkane at low temperature (183 K). Use of the HFC solvent in tandem with the highly fluorinated, solubilizing, weakly coordinating [Al(OR(f))4](-) anion overcomes the technical difficulty of combining ionic species with alkanes in solution without the solvent molecules rapidly displacing the bound alkane ligand, as the alkanes bind in preference to the HFCs to the organometallic fragments employed in this study. The [(HEB)Re(CO)2(alkane)](+) complexes are more long-lived than the corresponding neutral alkane complexes [(HEB)W(CO)2(alkane)] and [CpRe(CO)2(alkane)] (Cp = η(5)-cyclopentadienyl), with samples of [CpRe(CO)2(cyclopentane)] decaying significantly more rapidly than [(HEB)Re(CO)2(alkane)](+) when present in the same solution. Intramolecular exchange of the methylene group bound to the metal within the cyclopentane ligand in 16 was observed at 212 K, with the 1,2 shifts appearing to be faster than 1,3 shifts.