2D Corrugated Magnesium Carboxyphosphonate Materials: Topotactic Transformations and Interlayer “Decoration” with Ammonia

In this paper we report the synthesis and structural
characterization
of the 2D layered coordination polymer Mg­(BPMGLY)­(H2O)2 (BPMGLY = bis-phosphonomethylglycine, (HO3PCH2)2N­(H)­COO2–). The Mg ion is found
in a slightly distorted octahedral environment formed by four phosphonate
oxygens and two water molecules. The carboxylate group is deprotonated
but noncoordinated. This compound is a useful starting material for
a number of topotactic transformations. Upon heating at 140 °C
one (of the two) Mg-coordinated water molecule is lost, with the archetype
2D structure maintaining itself. However, the octahedral Mg in Mg­(BPMGLY)­(H2O)2 is now converted to trigonal bipyramidal in
Mg­(BPMGLY)­(H2O). Upon exposure of the monohydrate Mg­(BPMGLY)­(H2O) compound to ammonia, one molecule of ammonia is inserted
into the interlayer space and stabilized by hydrogen bonding. The
2D layered structure of the product Mg­(BPMGLY)­(H2O)­(NH3) is still maintained, with Mg now acquiring a pseudo-octahedral
environment. All of these topotactic transformations are also accompanied
by changes in hydrogen bonding between the layers.