Diverse structures and adsorption properties of quasi-Werner-type copper(II) complexes with flexible and polar axial bonds.

The quasi-Werner-type copper(II) complex, [Cu(PF6)2(4-mepy)4] (1), in which 4-mepy is the 4-methylpyridine ligand, has flexible and polar axial bonds of Cu–PF6. Flexibility of the Cu–PF6 bonds induces diverse and unprecedented guest-inclusion structures, such as {[Cu(PF6)2(4-mepy)4][Cu(PF6)(4-mepy)4(acetone)]·PF6·4acetone} (γ-1⊃2.5acetone), {[Cu(PF6)2(4-mepy)4][Cu(PF6)(4-mepy)4(2-butanone)]·PF6·3.5(2-butanone)} (γ-1⊃2.25(2-butanone)), {[Cu(PF6)2(4-mepy)4][Cu(PF6)(4-mepy)4(H2O)]·PF6·4benzene} (γ-1⊃0.5H2O·2benzene), and {[Cu(PF6)2(4-mepy)4]·2benzene} (γ-1⊃2benzene). Exposure of the dense form, α-1, to benzene vapor affords the benzene-inclusion complex {[Cu(PF6)2(4-mepy)4]·2benzene} (γ-1⊃2benzene), all benzene guests of which are easily removed by vacuum drying, reforming guest-free, dense α-1′ with smaller sized crystals than α-1. In contrast to α-1, which shows almost no CO2 adsorption, α-1′ adsorbs CO2 gas with structural transformations, this being the first example that exhibits adsorption of gas in a dense Werner-type complex and a drastic change in adsorption properties depending on the size of the crystals.