Facile CH aryl and OH bonds activation at dirhenium site of [Re2(CO)8(THF)2] complex

Abstract

The activation of small molecules is a topic of great interest and previously we reported on the easy activation of different types of EH bonds at the dinuclear complex [Re₂(CO)₈(THF)₂] (1) where labile THF molecules coordinate to adjacent rhenium(0) atoms. Here we extend the reactivity of 1 reporting on the oxidative addition of benzene and toluene at room temperature to give [Re₂(μ-H)(μ-κC-Ar)(CO)₈], Ar = −C₆H₅ (2) and −C₆H₄Me (3). Compound 3 is a new example of μ-κC-Ar dinuclear rhenium complex and has been obtained as para and meta isomers (3a,b). It is known from the literature that 2 can activate arenes and heteroarenes via reductive elimination of benzene and oxidative addition of CH bonds to the dinuclear fragment. Here we have studied the reaction of 2 with C₆D₆ and H₂CC(H)Ph and determined the kinetic constants by ¹H NMR (1.4 × 10⁻⁵ s⁻¹ at 308 K and 1.1 × 10⁻⁵ s⁻¹ at 298 K, respectively). The results indicate that the rate-determining step of the reaction is the reductive elimination of benzene, while the oxidative addition is fast. Water and methanol react with 1 in toluene at room temperature to give the hydroxo and methoxo hydrido complexes [Re₂(μ-H)(μ-OR)(CO)₈], RH (5) and CH₃ (6). On reacting 1 with water in deuterated toluene, and monitoring by ¹H/²H NMR, a preferential deuteration of the hydride site to give [Re₂(μ-D)(μ-OH)(CO)₈] is evidenced. This finding excludes the oxidative addition of water on the dinuclear “Re₂(CO)₈” fragment while supporting a heterolytic addition of water via protonation at the µ-κC-tolyl group, elimination of toluene and addition of OH⁻. Single crystal X-ray diffraction analyses have been performed for complexes 3a, 5 and 6 and their solid state structures have been determined. In particular, the crystal structure of 5 results in a new polymorphic form (5b) and it is discussed in comparison with the already known one (5a).