Zinc Complexes with an Ethylene-Bridged Bis(β-diketiminate) Ligand: Syntheses, Structures, and Applications as Catalysts in the Borylation of Aryl Iodides

A dinucleating bis­(β-diketiminate) ligand with a flexible bridge has been employed to synthesize zinc complexes. The ligand, abbreviated H₂L (H₂L = N-(4-((2-((4-((2,6-diisopropylphenyl)­imino)­pent-2-en-2-yl)­amino)­ethyl)­imino)­pent-2-en-2-yl)-2,6-diisopropylaniline), was deprotonated with ZnEt₂ to afford [LZn₂Et₂] (<b>1</b>). Reactions of <b>1</b> with 2 equiv of BnOH and ⁿBuOH, respectively, gave access to [LZn₂(OBn)₂]·C₆H₁₄ (<b>2</b>·C₆H₁₄) and [LZn₂(OⁿBu)₂] (<b>3</b>). Treatment of <b>1</b> with 2 equiv of I₂ in THF produced [LZn₂I₂(THF)₂]·2THF (<b>4</b>·2THF). X-ray single-crystal diffraction analyses revealed that they are all heteroleptic bimetallic compounds with two metal centers being chelated by one ligand set. The structurally similar compounds <b>1</b> and <b>4·</b>2THF possess approximate <i>C</i>₂ symmetry, with two β-diketiminate units being arrayed in head-to-tail antiparallel mode. Thus, the molecular structures of <b>1</b> and <b>4</b>·2THF exhibit a seesaw-like topology. The structures of <b>2</b>·C₆H₁₄ and <b>3</b> are almost identical, in which two zinc atoms are shared by two ZnN₂C₃ six-membered rings, two Zn₂ON₂C₂ seven-membered rings, and one Zn₂O₂ four-membered ring. Therefore, the metal cores of <b>2</b>·C₆H₁₄ and <b>3</b> display a crownlike topology. All complexes are catalytically active for the borylation of aryl iodides with B₂Pin₂ (B₂Pin₂ = 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bis­(1,3,2-dioxaborolane). Complex <b>1</b> shows higher activity in comparison to <b>2</b>, <b>3</b>, and <b>4·</b>2THF. The borylation reactions catalyzed by <b>1</b> could proceed under mild conditions and can be applied to a series of substrates with high functional group generality. This methodology thus represents a novel use of β-diketiminate zinc complexes for C–I borylation.