SYNTHESIS, STRUCTURES, AND PROPERTIES OF ENCAPSULATED IRON AND COBALTMETALLOCENES 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 pr epared from the reaction of K[(i-Pr)(3)C5H2] and K[(i-Pr)(4)C5H], resp ectively, with CoCl2 in THF. In contrast to [(i-Pr)(3)C5H2](2)Co, soli d 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 s alts {[(i-Pr)(3)C5H2](2)Fe}-BF4 and {[(i-Pr)(4)C5H](2)Fe}BF4 in high y ield. 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 neutr al [(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 structu res of the bis(triisopropylcyclopentadienyl)metallocenes are undistort ed, 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)C5 H2](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 17 1.5 degrees), making them the most distorted transition metal metalloc enium complexes yet reported. The {[(i-Pr)(3)C5H2](2)Co}(0/+) and {[(i -Pr)(4)C5H](2)Co}(0/+) redox couples have essentially identical electr ochemical potentials, despite the two additional electron-donating iso propyl substituents in the latter complexes. Additionally, oxidation o f [(i-Pr)(4)C5H](2)Fe is more favorable than expected for a ferrocene with eight alkyl groups. The anomalous electrochemical data can be att ributed to the severe steric crowding that is present in the {[(i-Pr)( 4)C5H](2)M}(0/+) complexes.