Chapter 10: Open Framework and Microporous Metal Phosphonate MOFs with Piperazine-based Bisphosphonate Linkers Check Access

The aluminium methylphosphonates of Maeda opened up the field of fully ordered, microporous organic–inorganic hybrids. They possess remarkable adsorption properties, but subsequent efforts to prepare other microporous methylphosphonates met with relatively little success, and porous metal carboxylates have since dominated this field. More recently, though, there has been an upsurge in interest in metal phosphonate MOFs, where bisphosphonate or phosphonate-carboxylate linkers enable extended 3D frameworks to be prepared, some with permanent porosity. In particular, the N,N′-piperazinebis(methylenephosphonic acid) is a versatile framework-forming linker, in large part because of its ability to display variable coordinative behaviour through different numbers of O and N atoms, and of the methylenephosphonate groups to adopt either axial or equatorial configurations. The first frameworks prepared with this linker, with divalent cations in tetrahedral coordination, were small pore solids that showed reversible dehydration but no permanent porosity. Our further studies in this area have found a rich structural chemistry with both divalent and trivalent metal cations. For divalent cations that display octahedral coordination, the structure type STA-12 (M₂L·nH₂O, M = Mn, Fe, Co, Ni, n = 7 or 8) is found to be of particular importance, displaying large pores, a rich structural chemistry and catalytic activity. For trivalent cations, there is a particularly diverse range of metal phosphonate MOFs, where the phosphonate MOF to form depends on cation size, pH of crystallising medium and the presence or absence of methyl groups on the piperazine ring. The products include the MIL-91 (Al, Fe); lanthanide phosphonates La–Nd with the lanthanide in 7 or 8-fold coordination and lanthanide phosphonates Gd–Yb with the metal octahedral. These exhibit reversible dehydration, cation exchange, ‘breathing’ and permanent porosity.