Reactivity Diversification - Synthesis and Exchange Reactions of Cobalt and Iron 2-Alkenylpyridine/-pyrazine Complexes Obtained by Vinylic C(sp2)-H Activation

The reactivity of 2-alkenylpyridine derivatives with trimethylphosphane-supported iron- and cobalt-methyl adducts were investigated and provided a series of C,N-cyclometalated complexes through smooth vinyl C(sp(2))-H activation. The reactions of Co(CH3)(PMe3)(4) with 2-vinylpyridine, 2-(1-phenylvinyl)pyridine, and 2-vinylpyrazine provided dark green crystals of the five-membered metallacycles ((2)-C,N-R1R2C=CH)Co(PMe3)(3) (1: R-1 = C5H4N, R-2 = H; 2: R-1 = C5H4N, R-2 = Ph; 3: R-1 = C4H3N2,R-2 = H). The oxidative addition of 1-3 with iodomethane afforded the mer-trans trivalent cobalt complexes ((2)-C,N-R1R2C=CH)Co(CH3)I(PMe3)(2) (4: R-1 = C5H4N, R-2 = H; 5: R-1 = C5H4N, R-2 = Ph; 6: R-1 = C4H3N2, R-2 = H) in modest yields. The reaction of 2 with an additional equivalent of 2-(1-phenylvinyl)pyridine incorporated a second C,N metallacycle with an (2)-bonded alkenyl moiety to provide ((2)-C,N-C5H4N-CH=CH)((3),(2)-C,C,N-C5H4N-CH=CH2)Co(PMe3) (7). No cyclometalation was observed with 8-vinylquinoline and Co(CH3)(PMe3)(4), which afforded ((3),(2)-C,C,N-C9H6N-CH=CH2)Co(CH3)(PMe3)(2) (8) with retention of the Co-CH3 group; therefore, a suitable bite angle is required for C-H activation. No N coordination was observed with Fe(CH3)(2)(PMe3)(4) and 8-vinylquinoline (reductive elimination of C2H6), which afforded a low-valent Fe(PMe3)(3) moiety bound in an (4)-fashion with the exocyclic vinyl group and the ortho-carbon atoms to give ((4),(4)-C,C,C,C-C9H6N-CH=CH2)Fe(PMe3)(3) (9). In polar solvents, an equilibrium of cyclometalated 7,8-benzoquinoline exists between the mer-trans (90%) and mer-cis (10%) configurations of ((2)-C,N-C13H6N)Fe(CH3)(PMe3)(3) (10). At variance with the previously reported bicyclometalation of 2-(2-naphthalene-1-ylvinyl)pyridine with iron adducts, similar reactions with methylcobalt species provided the monocyclometalated (2),sigma(1)-bound coordination motif ((3),(2)-C,C,N-C5H4N-CH=CH-C10H6)Co(PMe3)(2) (11). The reaction of 11 with carbon monoxide provided the monocarbonyl complex ((3),(2)-C,C,C-C5H4N-CH=CH-C10H6)Co(CO)(PMe3)(2) (12), accompanied by the release of the N-coordination site. The carbonylation of ((3)-C,C,N-C5H4N-CH=C-C10H6)Fe(PMe3)(3) resulted in the exchange of one PMe3 ligand with the retention of the coordination sites to generate ((3)-C,C,N-C5H4N-CH=C-C10H6)Fe(CO)(PMe3)(2) (13). All new compounds were identified by NMR spectroscopy and structurally characterized by single-crystal X-ray crystallography (with the exception of 10 and 13).