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

Bis(triisopropylcyclopentadienyl)cobalt, [(i-Pr)(3)C5H2](2)Co, and bis(tetraisopropylcyclopentadienyl)cobalt, [(i-Pr)(4)C5H](2)Co, can be prepared from the reaction of K[(i-Pr)(3)C5H2] and K[(i-Pr)(4)C5H], respectively, with CoCl2 in THF. In contrast to [(i-Pr)(3)C5H2](2)Co, solid samples of the ''encapsulated'' [(i-Pr)(4)C5H](2)Co are indefinitely stable in air. Bis(triisopropylcyclopentadienyl) iron, [(i-Pr)(3)C5H2](2)Fe, and bis(tetraisopropylcyclopentadienyl)iron, [(i-Pr)(4)C5H](2)Fe, can be oxidized with NOBF4 to give the corresponding ferrocenium salts {[(i-Pr)(3)C5H2](2)Fe}-BF4 and {[(i-Pr)(4)C5H](2)Fe}BF4 in high yield. Oxidation of [(i-Pr)(3)C5H2](2)Co and [(i-Pr)(4)C5H](2)-Co with NH(4)X (X = [PF6](-), [BPh(4)](-)) yields the cobaltocenium complexes {[(i-Pr)(3)C5H2](2)Co}X and ([(i-Pr)(4)C5H](2)Co)X. The oxidation of [(i-Pr)(4)C5H](2)M (M = Fe or Co) is significantly slower than that for [(i-Pr)(3)C5H2](2)M. The single-crystal X-ray structures of the neutral [(i-Pr)(3)C5H2](2)M metallocenes and cationic {[(i-Pr)(3)C5H2](2)M}(+) and {[(i-Pr)(4)C5H](2)M}(+) 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)(3)C5H2](2)Fe), 2.08(2)-2.10(2) ({[(i-Pr)(3)C5H2](2)Fe}BF4), 2.122(7) ([(i-Pr)(3)C5H2](2)Co), and 2.05(2) Angstrom ({[(i-Pr)(3)C5H2](2)Co}BPh(4)). In contrast, {[(i-Pr)(4)C5H](2)Fe}BF4 and {[(i-Pr)(4)C5H](2)Co}-PF6 display sterically induced distortions in the M-C(ring) distances (lengthened to 2.14(2) and 2.09(2) Angstrom) and the inter-ring angles (ring normal-M-ring normal angles of 170.7 degrees and 171.5 degrees), making them the most distorted transition metal metallocenium complexes yet reported. The {[(i-Pr)(3)C5H2](2)Co}(0/+) and {[(i-Pr)(4)C5H](2)Co}(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)(4)C5H](2)Fe 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)(4)C5H](2)M}(0/+) complexes.