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(sp2)–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 (κ2‐C,N‐R1R2C=CH)Co(PMe3)3 (1: R1 = C5H4N, R2 = H; 2: R1 = C5H4N, R2 = Ph; 3: R1 = C4H3N2,R2 = H). The oxidative addition of 1–3 with iodomethane afforded the mer‐trans trivalent cobalt complexes (κ2‐C,N‐R1R2C=CH)Co(CH3)I(PMe3)2 (4: R1 = C5H4N, R2 = H; 5: R1 = C5H4N, R2 = Ph; 6: R1 = C4H3N2, R2 = 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 η2‐bonded alkenyl moiety to provide (κ2‐C,N‐C5H4N–CH=CH)(κ3,η2‐C,C,N‐C5H4N–CH=CH2)Co(PMe3) (7). No cyclometalation was observed with 8‐vinylquinoline and Co(CH3)(PMe3)4, which afforded (κ3,η2‐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 η4‐fashion with the exocyclic vinyl group and the ortho‐carbon atoms to give (κ4,η4‐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 (κ2‐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 η2,σ1‐bound coordination motif (κ3,η2‐C,C,N‐C5H4N–CH=CH–C10H6)Co(PMe3)2 (11). The reaction of 11 with carbon monoxide provided the monocarbonyl complex (κ3,η2‐C,C,C‐C5H4N–CH=CH–C10H6)Co(CO)(PMe3)2 (12), accompanied by the release of the N‐coordination site. The carbonylation of (κ3‐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 (κ3‐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). The regioselective cyclometalation reactions of 2‐alk