Mechanistic Aspects of the Thermal and Photochemical Interconversion of Permethylscandocene Tellurolates and Tellurides. X-ray Structures of (C₅Me₅)₂ScTeCH₂C₆H₅, [(C₅Me₅)₂Sc]₂(µ-Te), and [(C₅Me₅)₂Sc]₂(µ-Se)

Treatment of Cp*₂ScR (Cp* = η⁵-C₅Me₅; R = CH₂SiMe₃, CH₂C₆H₅, erythro-CH(D)CH(D)-t-C₄H₉, CH₂CH(D)CH₂CH₂CH=CMe₂, C₆H₅, CH₂(c-C₅H₉)) with 1 equiv of elemental tellurium or Te=P-n-Bu₃ yields the tellurolates Cp*₂ScTeR (1a-e,g) in moderate to good yields (31–71%). The tellurolates are monomeric, as inferred from the X-ray structure of 1b (Sc-Te = 2.8337(14) Å; Sc‒Te‒C = 121.61(21)°). Insertion of elemental selenium into the Sc‒C bond of Cp*₂ScCH₂SiMe₃ produces selenolate 3 (55%). Reaction of 0.5 equiv of tellurium or selenium with the deuteride Cp*₂ScD results in elimination of D₂ and production of chalcogenide dimers [Cp*₂Sc]₂(μ-E) (E = Te (2), Se (4)). Evidence that the production of 2 proceeds via Cp*₂ScTeD is presented. The X-ray structures of 2 (Sc‒Te = 2.7528(12) Å Sc-Te-Sca = 172.07(5)°) and 4 (Se‒Sc = 2.5425(16) Å Sc‒Se‒Sca = 173.74(10)°) were performed and are reported for comparison. Telluride dimer 2 was also produced via the thermal (70–110 °C) or photochemical (λ > 290 nm) extrusion of TeR₂ from tellurolates 1a-d. Further thermal reactions between 2 and TeR₂ were observed at higher temperatures, leading to a new scandium telluride of empirical formula Cp*ScTe. Experiments utilizing tellurolates 1c,d indicate that both the thermal and photochemical eliminations of TeR₂ proceed without involving Te‒C bond cleavage, leading to R• intermediates. Photochemical reactions between 2 and TeR₂ substrates showed that the elimination process is photochemically reversible, leading to varying mixtures of compounds 1, 2, and TeR₂, depending on the nature of R. Use of Te[(CH₂)₄CH=CH₂]₂, which contains an alkyl group sensitive to the presence of R•, shows cyclization in the 5-hexenyl group leading to 1g, indicating the involvement of radicals in the back-photoreaction. The presence of a photostationary equilibrium in these systems is further evidenced in the slow conversion of Cp*₂ScTe(CH₂)₄CH=CH₂ (1f) to 1g over prolonged irradiation periods. Comparison of the UV‒visible spectra and the photochemistry associated with these tellurium compounds with their selenium congeners indicates that the primary photoevents in these reactions involve Te → Sc ligand to metal charge-transfer absorptions. Crystallographic data for 1b, 2, and 4 are as follows: 1b, monoclinic, space group P2₁/c, a = 14.932(5) Å, b = 10.8122(8) Å, c = 17.496(8) Å, β = 113.64°, V = 2587.5(15) ų, Z = 4, R = 0.041, R_w = 0.038; 2, tetragonal, space group P42₁c, a = 15.0324(7) Å, c = 18.8884(14) Å, V = 4268.3(4) Å3, Z = 4, R = 0.040, R_w = 0.034; 4, tetragonal, space group P42₁c, α = 14.7663(12) Å, c = 19.101(4) Å, Z = = 4, V = 4164.9(9) ų, R = 0.050, R_w = 0.043.