Rhenium(V) Oxo Complexes with N-Heterocyclic Carbenes

Air-stable rhenium(V) oxo complexes are formed when [ReOCl3(PPh3)2] is treated with N-heterocyclic carbenes
of the 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene type, LR (R = Me, Et, i-Pr). Complexes of the compositions
[ReO2(LR)4]+, [ReOCl(LR)4]2+, or [ReO(OMe)(LR)4]2+ can be isolated depending on the alkyl substituents at the
nitrogen atoms of the ligands and the reaction conditions applied. Despite the steric overcrowding of the equatorial coordination spheres of the metal atoms by each of the four carbene ligands, stable complexes with six-coordinate rhenium atoms are obtained. Steric demands of the alkyl groups allow control of the stability of the
mono-oxo intermediates. Air-stable cationic complexes of the compositions [ReOCl(LMe)4]2+, [ReOCl(LEt)4]2+, and
[ReO(OMe)(LMe)4]2+ have been isolated, whereas reactions of [ReOCl3(PPh3)2] or other rhenium(V) precursors with
the more bulky 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Li-Pr) directly yield the dioxo complex [ReO2(Li-Pr)4]+.
X-ray structures of [ReO2(Li-Pr)4][ReO4], [ReO2(Li-Pr)4][PF6], [ReO2(LMe)4][ReO4]0.45[PF6]0.55, [ReO(MeOH)(LMe)4][PF6]2,
and [ReOCl(LEt)4][PF6]2 show that the equatorial coordination spheres of the rhenium atoms are essentially planar
irrespective of the steric demands of the individual carbene ligands.