Crystalline hydrates: from two waters to none without stopping at one

Abstract

The citratobis(aqua)gadolinium compound with empírical formula [Gd(Hcitr)(H20)2] [Hcitr = citrate (3-, C6H5O7(3-)] has been reportad previously as a 1D polymer which in the solid is a stoichiometric monohydrate, [Gd(Hcitr)(H20)2]n·nH20, prepared hydrothermally at a temperatura of 100 ºC (10 days). (For present naming purposes the aqua ligand is treated as distinct from the unligated water, which we call water of hydration.) We describe here the preparation, by wet chemistry means, of the stoichiometric dihydrate, [Gd(Hcitr)(H20)2]n·2nH20, whose basic building block in the crystal is a two-dimensional polymer of the complex. The anhydrous form, also a two-dimensional polymer, is preparad from the dihydrate at 77 'C (several hours) in a single-crystal to single-crystal transformation. This transformation, in the course of which no evidence of a monohydrate is observad, is fully reversible, with the same single-crystal sample having been used for the characterization of the initial dihydrate, the form with no free water, and the restored dihydrate. The evolution of the SCSC transformation can be followed neatly by means of axial photographs. The initial citratobis(aqua)complex dihydrate and the dehydrated citratobis(aqua) complex possess the same two-dimensional layer structure, but with significant layer slippage occurring between the two forms.