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

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 (CH2=CHCN), acrylic acid (CH2=CHCO2H), methyl acrylate (CH2=CHCO2Me) and methyl vinyl ketone (CH2=CHC(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).