Spontaneous carbon dioxide fixation: a µ4-carbonate bridged tetranuclear zinc(ii) complex of a heptadentate Schiff base

Abstract

The electrochemical reaction of zinc and 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine (H₃L¹) in acetonitrile in air yields the tetranuclear zinc complex [(Zn₂L)₂(CO₃)]·4H₂O, which crystallises with different solvent molecules in two different systems: [(Zn₂L)₂(CO₃)]·4H₂O·2CH₃CN 1a (monoclinic, Pn) and [(Zn₂L)₂(CO₃)]·4H₂O 1b (triclinic, P). The electrochemical reaction under a nitrogen stream leads to the isolation of crystals of [(Zn₂L)₂(CO₃)]·0·25H₂O·2CH₃CN 1c (monoclinic P2₁/c). All structures demonstrate the striking ability of this system to spontaneously fix carbon dioxide. The crystals solution reveal a µ₄-η²:η¹:η¹ binding mode for the carbonate group in all cases, leading to a tetranuclear complex by self-assembly of dinuclear units. Chemical reaction of Zn(CH₃COO)₂·2H₂O with H₃L yields [Zn₂L(CH₃COO)]·2H₂O. Its recrystallisation allows isolating [Zn₂L(CH₃COO)]·2H₂O·CH₃OH 2. Reaction of [Zn₂L(CH₃COO)]·2H₂O with (CH₃)₄NOH·5H₂O leads, again, to the tetranuclear carbonate compound.