Synthesis and Crystal Structures of an Unexpected Tetranuclear Zinc(II) Complex and a Benzoquinone Compound Derived from Zn^II- and Cd^II-Promoted Reactivity of Schiff Base Ligands

Abstract

An unexpected polyhydroxyl-bridged tetranuclear ZnII complex and a benzoquinone compound derived from metal-ion promoted reactivity of Schiff base ligands were synthesized and characterized. The reaction of zinc(II) acetate dihydrate with oxime-type Schiff base ligand HL1 [HL1 = 1-(3-((3,5-dibromosalicylaldehyde)amino)phenyl)ethan-1-one O-benzyl oxime] in methanol, acetone, and acetonitrile resulted in the chemoselective cleavage of the C=N bond of the Schiff base HL1, and then the further addition of acetone to two salicylaldehyde molecules derived from cleavage of the C=N bond in situ α,α double aldol reaction promoted by ZnII ions. The newly formed ligands H4L2 coordinate to four ZnII ions forming a defect-dicubane core structure [ZnII4(H2L2)2(μ3-OCH3)2(μ-OCH3)2(CH3OH)2] (1) bridged exclusively by oxygen-based ligands. The similar ligand HL3 [HL3 = 1-(3-((3,5-dichlorosalicylaldehyde)amino)phenyl)ethan-1-one O-benzyl oxime)] was employed to react with CdII acetate dihydrate under the same reaction conditions. No aldol addition occurred but a unexpected benzoquinone compound 2,5-bis(((3-(1-((benzyloxy)imino)ethyl)phenyl)imino)methyl)-1,4-benzoquinone (2) formed. The results provided interesting insights into one-pot routes involving in situ reactions act as a strategy for obtaining a variety of polymeric/polynuclear complexes which are inconvenient to obtain from directly presynthesizing the ligands.