(Tetramethylcyclobutadiene)cobalt Complexes with Five-Electron Carbo- and Heterocyclic Ligands

Tetramethylcyclobutadiene(cyclopentadienyl)cobalt complexes Cb*Co(C5H4R) (Cb* = η4-C4Me4; R = H (5a), Me (5b), SiMe3 (5d), C(O)H (5f), and C(O)Me (5g)) were obtained by reaction of cyclopentadienide anions either with the (carbonyl)iodide complex Cb*Co(CO)2I (1) (method A) or with the more reactive acetonitrile complex [Cb*Co(MeCN)3]+ (2) (method B). Analogous compounds Cb*CoCp* (5c), Cb*Co(1,3-C5H3(SiMe3)2) (5e), and Cb*Co(η5-indenyl) (6) can be prepared only by method B. Treatment of 5f,g with NaBH4/AlCl3 or LiAlH4 affords the alkyl derivatives 5b and 5h (R = Et) or the alcohols 5i (R = CH2OH) and 5j (R = CH(OH)Me), respectively. The reaction of 1 with fluorene/AlCl3 yields complex [Cb*Co(η6-fluorene)]+ (8), which was deprotonated by KOBut to give Cb*Co(η6-fluorenyl) (9). Visible light irradiation of 9 induces η6→η5 haptotropic rearrangement to afford Cb*Co(η5-fluorenyl) (7). The pyrrolyl and phospholyl complexes Cb*Co(C4R4N) (R = H (10a), Me (10c)) and Cb*Co(C4R4P) (R = H (11a), Me (11c); R4 = H2Me2 (11b)) were obtained by reaction of 2 with the corresponding pyrrolide or phospholide anions. Improved procedures for the preparation of the starting materials 1 and 2 were developed. Using a one-pot procedure, the iodide 1 was obtained in high yield from 2-butyne and Co2(CO)8. Complex 2 was prepared by heating or irradiation of the toluene complex [Cb*Co(C6H5Me)]+ (4b) in acetonitrile. Structures of 5g, 6, and 11c were investigated by X-ray diffraction. Electrochemistry and joint UV−visible and EPR spectroelectrochemistry of complexes prepared were studied.