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

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.