Dopant-controlled crystallization in metal-organic frameworks: The role of copper(II) in zinc 1,4-benzenedicarboxylate

A series of Zn-Cu 1,4-benzenedicarboxylate(bdc) metal organic frameworks(MOF) have been synthesized by introducing N,N-dimethylformamide(DMF) solutions of 1,4-benzenedicarboxylic acid to metal salt solutions with a range of concentrations of Cu(NO ₃) ₂ and Zn(NO ₃) ₂ across the entire composition range. At the Zn-rich side of the range, the presence of Cu(II) had the effect of changing the polymorph of the solid from the predominantly P2 ₁/n structure, associated with MOF-2, to the C2/m structure, shared with Cu(bdc). There was an associated change in crystal morphology that accompanied the addition of a Cu salt impurity. BET surface area measurements of the thermally desolvated product showed that the surface area of the Zn(bdc) increased with the addition of Cu. At the Cu-rich side, magnetic susceptibility measurements indicated that the presence of the Zn impurity increased the fraction of paramagnetic Cu centers, suggesting the formation of a large fraction of heterometallic secondary building units(hSBU) in the framework.