Synthesis, structures, and properties of encapsulated iron and cobalt metallocenes with highly isopropylated cyclopentadienyl rings

Article abstract of DOI:10.1021/om960740+

Bis(triisopropylcyclopentadienyl)cobalt, [(i-Pr)₃C₅H₂]₂Co, and bis(tetraisopropylcyclopentadienyl)cobalt, [(i-Pr)₄C₅H]₂Co, can be prepared from the reaction of K[(i-Pr)₃C₅H₂] and K[(i-Pr)₄C₅H], respectively, with CoCl₂ in THF. In contrast to [(i-Pr)₃C₅H₂]₂Co, solid samples of the encapsulated [(i-Pr)₄C₅H]₂Co are indefinitely stable in air. Bis(triisopropylcyclopentadienyl)iron, [(i-Pr)₃C₅H₂]₂Fe, and bis(tetraisopropylcyclopentadienyl)iron, [(i-Pr)₄C₅H]₂Fe, can be oxidized with NOBF₄ to give the corresponding ferrocenium salts [(i-Pr)₃C₅H₂]₂Fe}-BF₄ and {[(i-Pr)₄C₅H]₂Fe}BF₄ in high yield. Oxidation of [(i-Pr)₃C₅H₂]₂Co and [(i-Pr)₄C₅H]₂-Co with NH₄X (X = [PF₆]^-, [BPh₄]^-) yields the cobaltocenium complexes {[(i-Pr)₃C₅H₂]₂Co}X and {[(i-Pr)₄C₅H]₂Co}X. The oxidation of [(i-Pr)₄C₅H]₂M (M = Fe or Co) is significantly slower than that for [(i-Pr)₃C₅H₂]₂M. The single-crystal X-ray structures of the neutral [(i-Pr)₃C₅H₂]₂M metallocenes and cationic [(i-Pr)₃C₅H₂]₂M}⁺ and {[(i-Pr)₄C₅H]₂M}⁺ metallocenes are reported. The structures of the bis(triisopropylcyclopentadienyl)metallocenes are undistorted, with nearly parallel rings and average M-C(ring) distances of 2.05(1) ([(i-Pr)₃C₅H²]₂Fe), 2.08(2)-2.10(2) ({[(i-Pr)₃C₅H₂]₂Fe}BF₄), 2.122(7) ([(i-Pr)₃C₅H₂]₂Co), and 2.05(2) ? ({(i-Pr)₃C₅H₂]₂Co}BPh₄). In contrast, {[(i-Pr)₄C₅H]₂Fe}BF₄ and {[(i-Pr)₄C₅H]₂Co}-PF₆ display sterically induced distortions in the M-C(ring) distances (lengthened to 2.14(2) and 2.09(2) ?) and the inter-ring angles (ring normal-M-ring normal angles of 170.7° and 171.5°), making them the most distorted transition metal metallocenium complexes yet reported. The {[(i-Pr)₃C₅H₂]₂Co}^0/+ and {[(i-Pr)₄C₅H]₂Co)^0/+ redox couples have essentially identical electrochemical potentials, despite the two additional electron-donating isopropyl substituents in the latter complexes. Additionally, oxidation of [(i-Pr)₄C₅H]₂Fe is more favorable than expected for a ferrocene with eight alkyl groups. The anomalous electrochemical data can be attributed to the severe steric crowding that is present in the {[(i-Pr)₄C₅H]₂M)^0/+ complexes.