CoCH3(PMe3)4 is known for a long time for its potential to activate less reactive C–H Bonds under extremely mild conditions by methane elimination. A spontaneous second C–H bond activation at the ...same center of cobalt complex is not expected. In contrast to previous investigations with certain aryl‐iminophosphane chelating ligands, in this work two consecutive metalation steps at the same complex center lead to a rare example of a cobalt metallabicycle. In the first step a C–H activation results in a five‐membered chelate ring, whereas the second ring is formed by an incomplete C–H activation which stops at a stable, rarely observed δ‐C–H agostic interaction. The results are supported by NMR spectroscopy, X ray structure analysis and density functional theory (DFT) calculations. Knowledge of such agostic interactions are essential in order to understand catalytic reaction paths and studies of such molecular structures of their ring sizes, bite angles and electronic properties have shown that these ligands allow sensitive control within the metal coordination sphere.
“Snapshot”: A double metalation through the coordination of an ortho‐iminophosphane anchoring group can initiate a new bicyclometalation reaction affording the mononuclear cobalt complex (16 valence electrons) (1, 2). The first ring closing step involves a C–H activation, while the second one stops at the level of a rarely observed δ‐C–H agostic interaction supported by quantum chemical calculations.
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‐alkenylpyridine/‐pyrazine derivatives with Co(CH3)(PMe3)4 provide cobalt complexes by vinylic C(sp2)–H activation. These cobalt complexes smoothly add iodomethane but readily demetalate with carbon monoxide. Conversely, reductive elimination through the release of ethane is the dominant reaction with the same ligands and Fe(CH3)2(PMe3)4.
Steric control dictates the cyclometalation of aromatic substrates containing imine anchoring groups upon reaction with basic iron or cobalt complexes. As structural isomers, the azadiene systems 1 ...and 2 undergo regiospecific activation of the (vinylic or aromatic) CH bond of the CH group that has switched places with the N atom.
The novel tetranuclear mixed valent iron(II)/iron(0) complex (1) was prepared by ortho-metalation of benzophenone with zerovalent Fe(PMe3)4 (d8) followed by a subsequent insertion of an additional ...equivalent of benzophenone under ring expansion (seven-membered metallacycle). Dimerization and spontaneous dihydrogen loss, constitutes a four membered Fe2O2 core structure in which both incorporated chelating phenylrings are additionally attached by low-valent Fe(PMe3)3-fragments in η4-mode. In a single electron transfer reaction, by C–C-coupling of two molecules benzophenone with zerovalent Co(PMe3)4 (d9), cyclometalated mononuclear five-membered cobalt(II) alcoholate complex (2) is obtained. The crystal structures of 1 and 2 were determined and supported by gas phase DFT calculations and the formation mechanisms of both complexes are discussed.
Reaction cascade: Ortho-metalation of benzophenone with zerovalent iron and cobalt is observed followed by a subsequent addition/insertion of an additional equivalent benzophenone. Display omitted
•Reaction cascade of benzophenone includes C–H activation and C–C coupling steps.•Subsequent insertion/addition sequence with low valent Fe(PMe3)4 and Co(PMe3)4.•Dimerization constitutes a four membered Fe2O2 core structure.•Single electron transfer reaction afforded cyclometalated mononuclear cobalt complex.•Formation mechanisms of both complexes explain the key steps of the proposed sequence.
Four thought: The first nickel(IV) complex that contains only alkyl ligands (see picture) was serendipitously obtained in a reaction between a nickel(0) species and a strained diolefin. The twisted ...conformation of the ligand provides the necessary steric shielding to stabilize the complex. DFT calculations show this conformation to be the ground state and NMR spectra indicate the diamagnetic nature and high oxidation state of the nickel center.
The title compound, Co2{P(C6H5)2}2(C3H9P)4, was obtained by the addition of di-phenyl-phosphane to a solution of Co(CH3)(C3H9P)4. The dinuclear complex mol-ecule exhibits inversion symmetry with the ...inversion centre located between the two Co(I) atoms. The short Co-Co distance of 2.3670 (8) Å lies within the range of metal-metal double bonds. As a result of inversion symmetry, the four-membered Co2P2 core is rigorously planar, and the two bridging P(C6H5)2-ligands and the terminal C3H9P ligands are arranged in a pseudo-tetra-hedral fashion about the Co(I) atom.
We have studied the reactivity of a series of terminal alkenyldiphenylphosphines {Ph2P(CH2) n -CHCH2), n = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co(I). Methane and ...ethane elimination occurs when different olefinic P,C-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co(CH3)(PMe3)4 with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic P,C,P-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe3)2Co{κ3-(P,C,P)-(Ph2)CH2CHCH-CH(PPh2)} (1). X-ray study of 1 revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C(sp2)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co(CH3)(PMe3)4 afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe3)2Co{κ2(P,C)-HCCHCH2PPh2} (2); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co(CH3)(PMe3)4 afforded a η3-coordinated π-allyl complex of composition (PMe3)2Co{η3-κ4-(P,C,C,C)-H2CCH-CH(CH2)2PPh2} (3). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed.
Single crystals of octahedral mer‐cis‐CoIIII(CH3)2(PMe3)3 (1) and square planar trans‐NiIICl(CH3)(PMe3)2 (2), were obtained from solvent mixtures (methylcylohexane / pentane 1:1) and have been ...analyzed by X‐ray crystallography for the first time.
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