Post-synthetic modification of zirconium metal–organic frameworks by catalyst-free aza-Michael additions

Abstract

The reactions of the zirconium MOF [Zr₆O₄(OH)₄(bdc-NH₂)₆] (UiO-66-NH₂, bdc-NH₂ = 2-amino-1,4-benzenedicarboxylate) with the Michael acceptors acrylonitrile (CH₂CHCN), acrylic acid (CH₂CHCO₂H), methyl acrylate (CH₂CHCO₂Me) and methyl vinyl ketone (CH₂CHC(O)Me) led to post-synthetic modification of the MOF through C–N bond formation without loss of crystallinity. The reactions with acrylonitrile and acrylic acid go to completion, yielding [Zr₆O₄(OH)₄(bdc-NHCH₂CH₂CN)₆] (UiO-66-AN, 1) and [Zr₆O₄(OH)₄(bdc-NHCH₂CH₂CO₂H)₆] (UiO-66-AA, 2) respectively, whereas those with methyl acrylate and methyl vinyl ketone are incomplete, yielding [Zr₆O₄(OH)₄(bdc-NH₂)_0.66(bdc-NHCH₂CH₂CO₂Me)_5.34] (UiO-66-MA, 3) and [Zr₆O₄(OH)₄(bdc-NH₂)_2.76(bdc-NHCH₂CH₂C(O)Me)_3.24] (UiO-66-MVK, 4), respectively. The acrylonitrile-modified MOF UiO-66-AN undergoes further reaction with sodium azide in the presence of zinc(II) chloride in n-butanol to form the tetrazolate-modified MOF [Zr₆O₄(OH)₄(bdc-NHCH₂CH₂CN)_4.74(bdc-NHCH₂CH₂CN₄H)_1.26] (UiO-66-TZ, 5).