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 (Cp-ttt) ligand, the rhenium(V) dioxo species (Cp-ttt)ReO2 could now be observed, in equilibrium with the dimeric form [(Cp-ttt)Re(O)mu-O](2), and characterized by NMR, IR, and UV-vis spectroscopies, as well as electrospray ionization mass spectrometry. (Cp-ttt)ReO2 is shown to be the primary product of reduction of the rhenium(VII) complex (Cp-ttt)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.