Multiple Bonds between Transition Metals and Main-Group Elements. 124. Structures and Reactivity of Acylperrhenates

Two structural types of perrhenic anhydrides result from treatment of Re₂O₇ with carboxylic anhydrides (RCO)₂O in acetonitrile or tetrahydrofuran solution. Strongly electron-withdrawing groups, cf. R = CF₃, thus lead to complexes of type RC(O)-O-ReO₃·2L (L = CH₃CN, thf) exhibiting η¹-coordinated carboxylato groups and octahedral coordination at rhenium(VII), both in solution (IR, ¹⁷O NMR) and in the solid state (X-ray crystal structure of CF₃C(O)−O−ReO₃·2CH₃CN (crystal data: space group P2₁/n (No. 14), a = 10.522(3) Å, b= 8.193(1) Å, c = 13.094(4) Å, β = 90.59(1)°, Z = 4)). Analogous coordination isobserved in the X-ray crystal structure of Re₂O₇·2CH₃-CN (space group P2₁/m (No. 11), a = 5.363(1) Å, b = 11.763(2) Å, c = 8.273(2) Å, β = 94.395(9)°, Z = 2)). By way of contrast, anhydrides of weak carboxylic acids, cf. R = CH₃, yield mixed anhydrides of general formula RC(O)-OReO₃·L (L = CH₃CN, thf), the structures of which compounds display octahedral rhenium(VII) and η₂-coordinated carboxylato groups. These structures are again observed in solution and in the solid state (X-ray crystal structure of CH₃C(O)-OReO₃·CH₃CN (space group C2/m (No. 12), a = 14.818(8) Å, b = 6.976(2) Å, c = 7.390(4) Å, β = 94.64(2)°, Z = 4)). As a result of these structural features, only mixed anhydrides containing strongly electron-withdrawing substituents react cleanly with alkyltin compounds to yield the alkylrhenium(VII) oxides of the type alkyl-ReO₃.