Ulrich Koelle

(Tetramethylcyclobutadiene)cobalt Complexes − Syntheses of Tris(ligand) Derivatives and Structures of [(C4Me4)Co(CO)3]BF4 and [(C4Me4)Co(NCMe)3]PF6

Photolysis of [Cb*Co(C6H6)]PF6] (6) (Cb* = C4Me4) in acetonitrile solution produces the red solvento complex [Cb* Co(NCMe)3]PF6 (8) in quantitative yield. Alternatively, heating of the acetonitrile solution to reflux temperature may effect the displacement of the benzene ligand. Photolysis of 6 in liquid ammonia gives [Cb*Co(NH3)3]PF6 (9). Compound 8 is substitution-labile and reacts with a wide variety of Lewis bases to produce substitution products [Cb*CoL3]PF6 with L = P(OMe)3 (10), py (11), CNtBu (12), and the related compound (NEt4)2[Cb*Co(CN)3] (13). Thermal reaction of [Cb*Co(CO)3]BF4 (7) with PMe3 takes place spontaneously at ambient temperature and produces the monosubstitution product [Cb*Co(CO)2(PMe3)]BF4 (14), while irradiation combined with a purging stream of N2 effects an exhaustive decarbonylation and affords [Cb*Co(PMe3)3]BF4 (16). Irradiation of 7 in acetonitrile produces mixtures of the mono- and disubstitution products; if a purging stream of N2 is applied, [Cb*Co(CO)(NCMe)2]BF4 (18) is formed with a small admixture of [Cb*Co(NCMe)3]BF4. The iodide Cb*CoI(CO)2 reacts with 2,2′-bipyridine to produce the monocarbonyl complex [Cb*Co(CO)(bipy)]I (15). Single-crystal structure determinations of [Cb*Co(CO)3]BF4 (7) with Co−Cb(ring plane) 1.777(2) A (av.) and of [Cb*Co(NCMe)3]PF6 (8) with Co−Cb(ring plane) 1.68(1) A (av.) are reported. Synthetic procedures for the preparation of [Cb*Co(CO)3]BF4 (7), Cb*CoI(CO)2 (2), and [Cb*Co(C6H6)]PF6 (6) are also given. An electrochemical study of the compounds 8, 10−13, and 16 reveals chemically fully reversible oxidation reactions. Hard σ-donor ligands (CN−, py, NCMe) stabilize the corresponding 17e species much more so than soft donor ligands [PMe3, CNtBu, and P(OMe)3]. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)