A series of chiral bis-N-heterocyclic carbene ligands H 2 [(S)-1a−d]X 2 (X = Br, I) on a spiro scaffold and their palladium complexes (S)-2a−d and (S)-3a,b were prepared and applied in the enantioselective oxidative kinetic resolution of secondary alcohols. The corresponding alcohols can be obtained in high yields with moderate to excellent ee values.

Novel chiral N,N′-bisaryl bis­(NHC) ligand precursors H2[(S)-2]­Cl2 on a spiro scaffold and H2[(S)-3b–g]­Cl2 with a binaphthyl linkage were rationally designed and their cyclometalated cis-chelated NHC palladium complexes (S)-5, (S)-6, and (S)-7b–g have been synthesized and fully characterized. Complexes 6 and 7b were further confirmed by X-ray single-crystal analysis. Both complexes adopted a slightly distorted square planar geometry around the Pd­(II) center. The structure of 6 consists of a rare dimeric arrangement incorporating two palladium­(II) centers bonded through a short metal–metal bond (2.853(2) Å), indicating a PdII–PdII intramolecular interaction (<3.00 Å). These N,N′-bisaryl-bis­(NHC)-Pd complexes together with N,N′-bisalkyl analogues {[(S)-1a–d]­PdX2} (X = I, (S)-4a; X = Br, (S)-4b–d) have been used in the asymmetric aryl–aryl cross-coupling reactions of arylboronic acids and aryl halides. The enantioselectivity of the biaryl products was greatly improved within 24 h (up to 74% ee) when complexes 7a–g were used as catalysts. The results show that for these types of bis­(NHC) palladium catalysts the structural characters of the chiral scaffolds play a decisive role in the enantioselectivities of cross-coupling reactions.

A multimetallic structure with a rigid linker can poise more than one catalytically active site in close proximity to control their reaction pathways in ethylene (co)­polymerization by intermetallic cooperativity. This elegant concept was mainly limited to bimetallic systems. We report the synthesis of a series of mono-, bi-, and trinuclear salicylaldiminato Ni­(II) catalysts based on a triptycene backbone. In ethylene (co)­polymerization, the bimetallic nickel catalyst not only showed higher activity than the mononickel catalyst but also achieved an enhanced incorporation rate of α-olefin and the polar amino olefin, while the trimetallic nickel catalysts were thermally robust up to 100 °C and achieved a unique long-chain branching level up to an 81% mole ratio of all. Trinuclear nickel catalysts were found to be very less active for ethylene copolymerization with 1-hexene due to the increased steric hindrance. Although the catalytic result is worse than expected, especially for trinickel complexes, this work is still meaningful for the development of multinuclear catalysts that can mediate important catalytic reactions with enhanced efficiency and cooperativity.

A series of new biaryl-bridged bis­(N-heterocylic carbene) palladium complexes has been synthesized and applied in asymmetric Suzuki cross-coupling of aryl halide and aryl boronic acid. X-ray crystallographic studies of square-planar palladium complexes of four of these bis­(NHC) ligands revealed that the dihedral angle of the bridging biaryl moiety depends on its identity and lies between 79.65° and 92.69°. A linear correlation between the dihedral angle in these Pd complexes and cross-coupling enantioselectivity is first reported. Moreover, larger Ccarbene–Pd–Ccarbene bite angles were found to lead to increased enantioselectivity in a first fast and then slow growing tendency.

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