Spectroscopic Characterization of a Monomeric, Cyclopentadienyl-Based Rhenium(V) Dioxo Complex

Mononuclear, coordinatively unsaturated rhenium­(V) dioxo species of the type XReO2 (X = Me, substituted cyclopentadienyl) have long been postulated as intermediates in rhenium-catalyzed deoxydehydration, but their characterization was precluded because of aggregation into dimeric or oligomeric structures. Using the bulky 1,2,4-tri-tert-butylcyclopentadienyl (Cpttt) ligand, the rhenium­(V) dioxo species (Cpttt)­ReO2 could now be observed, in equilibrium with the dimeric form [(Cpttt)­Re­(O)­μ-O]2, and characterized by NMR, IR, and UV–vis spectroscopies, as well as electrospray ionization mass spectrometry. (Cpttt)­ReO2 is shown to be the primary product of reduction of the rhenium­(VII) complex (Cpttt)­ReO3 with PPh3 and demonstrated to react with ethylene glycol significantly faster than its dimeric counterpart, supporting its role as an intermediate in rhenium-catalyzed deoxydehydration reactions.