Natural and synthetic metal oxalates – a topology approach

Abstract

Oxalates are found in minerals and in biology, are made in the laboratory, and are used on an industrial scale. They form coordination polymers and hydrogen bonded networks that often can be analysed using network topology. In this survey of known naturally occurring oxalates we note weddellite, [Ca(C₂O₄)]·xH₂O, that seems to be the first known naturally occurring metal–organic framework, forming the four-connected crb-net (zeolite CRB). The natural oxalates are typically 3D, 2D or 1D coordination polymers, with extensive hydrogen bonding in the latter cases. For example, humboldtine and lindbergite form the new 3- and 8-connected net hum by combining the 1D structure with strong hydrogen bonds. Tris-oxalates rarely occur in nature but stepanovite, [Mg(H₂O)₆][Na[Fe(ox)₃]]·3H₂O, is an exception and forms hcb-nets (honeycomb 2D layers) with the hexaaqua ions sealing any potential voids. Synthetic tris-oxalates on the other hand are well explored and normally form 2D hcb-nets or 3D chiral three-connected srs-nets. Theoretically a few other topologies should also be possible, and it was found that [Mn((R)-salmen)(CH₃OH)(CH₃CN)][MnCr(ox)₃]·0.5CH₃OH·1.25CH₃CN forms the achiral three-connected lig-net, [Fe(2,6-bis(pyrazol-3-yl)pyridine)₂][MnCr(ox)₃]·2,6-bis(pyrazol-3-yl)pyridine·CH₃OH forms the likewise achiral nod-net and [Cu(trans[14]dien)][KCr(ox)₃] the ths-net. A new binodal 3-connected net noa (with the derived 3c-,4c- new net mys) was found in [Fe^II(tren(imid)₃)]₂[Mn_2.5(CH₃OH)₃Cr₃(ox)₉]·(CH₃OH)_4.75·(H₂O)₄. The more complex [Fe(tren(imid)₃)]₂[Mn_2.5(CH₃OH)₃Cr₃(ox)₉] forms a new three-nodal 3-connected daz-net.