Mehrfachbindungen zwischen hauptgruppenelementen und übergangsmetallen: VII. Oxidative decarbonylierungsprozessein der photochemie von tricarbonyl(η5-pentamethylcyclopentadienyl)rhenium: Neue oxo—rhenium-komplexe

Short-wavelength photolysis of tricarbonyl(η⁵-pentamethylcyclopentadienyl)rhenium (1) (λ < 300 nm, quartz-glass) in tetrahydrofuran yields under partial or complete decarbonylation three novel organorhenium compounds, with the relative yields depending upon the irradiation time. Formed by exhaustive oxidative decarbonylation, the complex trioxo(η⁵-pentamethylcyclopentadienyl)rhenium(VII) (5) represents the first example of the new class of oxo half-sandwich complexes. The derivatives (μ-O)[(η⁵-C₅Me₅)Re(CO)₂]₂ (2; Me = CH₃) and (η⁵-C₅Me₅)₂Re₂(CO)₂O₂ (3; Me = CH₃) containing both oxo and carbonyl ligands are formed from 1 at shorter irradiation times. Both compounds are isolable intermediates along the mechanistically not yet fully established sequence 1 → → 5 as they are degraded to 5 under CO₂ elimination in the presence of air and/or light. The mixed oxo carbonyl complex 2 has been characterized by means of single-crystal X-ray diffraction techniques (triclinic, space group P$\text{1}$-C_i¹; a 907.4(2), b 1040.2(3), c 1414.3(4) pm; α 79.99(2), β 88.42(2), γ 66.18(2)°; R_iso = 0.068, R_aniso = 0.037, R_w = 0.031). The molecular structure centres around an isosceles Re₂O triangle whose metal centres exhibit a strongly distorted square-pyramidal geometry; the metal-metal distance recorded at 281.7(1) pm is in agreement with a single bond by the EAN rule. The centrically coordinated, planar five-membered ring ligands are parallel to each other (interplanar angle 2°) and occupy trans-positions with respect to the central Re₂O geometry.