C–H Bond Activation Facilitated by Bis(phosphinoamide) Heterobimetallic Zr/Co Complexes

Abstract

The activation of C−H bonds using first-row transition metals poses a formidable challenge in the development of sustainable catalytic methods. Early/late heterobimetallic complexes provide a Lewis acidic binding site for directing groups, facilitating the activation of C−H bonds at an appended first-row transition metal center. Herein, the reactivity of the Zr IV /Co −I heterobimetallic complexes (THF)(I)Zr(XylNP i Pr 2 ) 2 Co(PR 3 ) (1-PR 3 ; Xyl = 3,5-dimethylphenyl; PR 3 = PMe 3 , PPh 2 Me) toward directed C−H bond activation is explored with pyridine and terminal alkyne derivatives. 1-PMe 3 reacts reversibly with 4methylpyridine to afford the C−H activated complex (4-Me-C 5 H 4 N)(I)Zr(XylNP i Pr 2 ) 2 (μ-4-Me-C 5 H 3 N)Co(PMe 3 )(H) (3-PMe 3 ). By using the more Lewis basic substrate 4-tertbutylpyridine, (I)Zr(XylNP i Pr 2 ) 2 (μ-4-t Bu-C 5 H 3 N)Co(PMe 3 )(H) (4-PMe 3 ) is formed irreversibly. In addition to pyridine derivatives, 1-PPh 2 Me can activate the C−H bond of terminal alkynes to form (THF)(I)Zr(XylNP i Pr 2 ) 2 (μ-R′CC)Co(PPh 2 Me)(H) (R′ = Ph (5-PPh 2 Me); R′ = SiMe 3 (6-PPh 2 Me)).

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

“…Their high reactivity also allowed for the observation of bimetallic C-H bond activation with pyridine and terminal alkynes (Scheme 16). 67 Another type of two-atom bridged late TM/main group metal complex is the Pt/Al complex 3.11 obtained by treatment of the organoaluminum precursor with [Pt(PPh 3 ) 2 (ethylene)]. It showed a signicant degree of ring strain due to the short Pt/ Al distance, 2.561(1) Å, which can be released in the CO 2 , H 2 or amide reaction products (Scheme 17).…”

“…Their high reactivity also allowed for the observation of bimetallic C–H bond activation with pyridine and terminal alkynes ( Scheme 16 ). 67 …”

One Bridge, Three Bonds: A Frontier in Multiple Bonding in Heterobimetallic Complexes

“…However, the ligand scaffold often inhibits substrate binding to the bimetallic core of the complexes, either limiting reactivity to the late metal site or necessitating ligand dissociation events to accommodate substrates. − Oxidative addition reactions of CO 2 , alkyl halides, and a wide variety of other substrates have been studied at the bimetallic core of A and A-N 2 , but in many cases, these reactions result in a phosphine ligand dissociation from the Co center to accommodate a bridging substrate fragment . A strategy to relieve this steric encumbrance about the metal–metal bond is to reduce the number of bridging ligands to two, breaking the C 3 symmetry of the systems and creating a “metallopincer”-type complex of the late transition metal. − Early/late heterobimetallic complexes with two bridging ligands retain bonding interactions similar to those of the C 3 -symmetric systems. ,,, Gratifyingly, these metallopincers are reactive toward σ-bond activation and catalysis, including directed C–H bond activation, hydrogenation of unsaturated hydrocarbons, and magnesiation of aryl fluorides. − , …”

“…25 As one example, the bis(phosphinoamide) complex (THF)-(I)Zr(XylNP i Pr 2 ) 2 CoPMe 3 (B; Xyl = 3,5-dimethylphenyl; Figure 1) was found to perform the oxidative addition of dihydrogen, the o-C−H bonds of pyridine derivatives, and the C−H bonds of terminal alkynes. 26,27 While the activated dihydride serves as a key intermediate in the semihydrogenation of internal alkynes, 27 the C−H activated products do not display any further reactivity. 26 This observation, along with the lack of catalytic reactivity of related first-row metallopincer and group 13 pincer complexes, 25,30,32 leads to the conclusion that these metallopincers may not have solved the aforementioned issue of coordinative saturation at the late transition metal site.…”

“…26,27 While the activated dihydride serves as a key intermediate in the semihydrogenation of internal alkynes, 27 the C−H activated products do not display any further reactivity. 26 This observation, along with the lack of catalytic reactivity of related first-row metallopincer and group 13 pincer complexes, 25,30,32 leads to the conclusion that these metallopincers may not have solved the aforementioned issue of coordinative saturation at the late transition metal site. 25,26 For this reason, we turned our efforts toward the synthesis of a Zr/Co mono(phosphinoamide) complex, a bimetallic complex with a single bridging ligand.…”