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

Abstract

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).

Solvent-Free Hydrosilylation of Alkenes Catalyzed by Well-Defined Low-Valent Cobalt Catalysts

“…A rapid color change from orange to green was observed when styrene was mixed with Co­(PMe 3 ) 4 , but the expected alkene-coordinated cobalt complex could not be confirmed (eq ). The similar alkene-coordinated cobalt complex was reported by Klein . When Ph 2 SiH 2 was added to the n -pentane solution of Co­(PMe 3 ) 4 , the reaction solution gradually changed from orange to red, accompanied by the precipitation of red crystals of dinuclear cobalt­(I) hydride 4 (eq 3).…”

Spontaneous Bicyclometalation of a Single Cobalt(I) Complex Stabilized by a δ‐C–H Agostic Interaction

“…While the distance between the cobalt atom and the metalated vinylic carbon atom C(7) of 1.909(1) Å is in the usual range of literature known complexes, the distance between the metalated aromatic carbon atom C(1) and the cobalt center is extremely long [2.161(5) Å]. [9b], Similar, relatively long Co–C bonds are detected in η 6 ‐cyclopentadienyl‐ or η 6 ‐aryl cobalt complexes, or recently from a pincer‐type cobalt complex reported …”

“…An important feature in previously reported reactions with Co(CH 3 )(PMe 3 ) 4 is the irreversible release of methane in cooperation with nitrogen or phosphorus donor atoms (D) . A spontaneous second C–H bond activation at the same cobalt center of the complex is not expected (Scheme ) …”

New molecular architectures containing low-valent cluster centres with di- and trimetalated 2-vinylpyrazine ligands: synthesis and molecular structures of Ru5(CO)15(μ5-C4H2N2CHCH)(μ-H)2 and Ru8(CO)24(μ7-C4H2N2CHC)(μ-H)3

“…[13][14][15][16][17][18][19] The chemistry of the closely related 2-vinylpyrazine, which contains an additional nitrogen atom within the heterocyclic ring, has been far less studied. [20][21][22] Usually, reactions of these ligands with unsaturated appendages proceed through C-H activation of the vinyl group with concomitant coordination of the adjacent ring nitrogen. However, 2-vinylpyrazine can also coordinate to trimetallic centres through metalation of its aromatic ring utilizing the second ring nitrogen (F, Chart 1), especially if the dominant C-H bond activation pathway does not furnish a kinetically stable product.…”

“…However, 2-vinylpyrazine can also coordinate to trimetallic centres through metalation of its aromatic ring utilizing the second ring nitrogen (F, Chart 1), especially if the dominant C-H bond activation pathway does not furnish a kinetically stable product. 22 Herein we detail the reaction of Ru 3 (CO) 12 with 2vinylpyrazine, the aim of which was to synthesize clusters in which all of the donor atoms of the 2-vinylpyrazine ligand participate in bonding to the cluster core. Two new polyruthenium clusters have been isolated in which the 2-vinylpyrazine shows novel and unprecedented coordination modes by utilizing all the available donor atoms for bonding (G and H, Chart 1).…”