Reversible ON-OFF switching of single-molecule-magnetism associated with single-crystal-to-single-crystal structural transformation of a decanuclear dysprosium phosphonate

{Dy (EDDC) (μ -AcO) (μ -C H PO )(μ -C H PO )(μ -AcO) (AcO) (H O)(CH OH) } (μ -C O )· H O ( ), where H EDDC is ', '', , ', '', ', ''-(ethane-1,2-diylidene)dipyrazine-2-carbohydrazide and C H PO H is 9-anthrylmethylphosphonic acid, is found to undergo two consecutive single-crystal-to-single-crystal transformations. The first is under UV irradiation ( = 365 nm for 3 d in air) to {Dy (EDDC) (μ -AcO) (μ -C H PO ) (μ -AcO) (AcO) (H O) } (μ -C O )· H O ( ) where the two CH OH are replaced by two H O and the second by annealing under N at 100 °C on a diffractometer or under Ar in a glovebox to {Dy (EDDC) (μ -AcO) (μ -C H PO ) (μ -AcO) (H O)} (μ -C O ) ( or ) where it has lost two H O molecules. The second transformation is reversible by exposure to air at room temperature ( ). While the overall structures are the same (retaining the space group 2 / ), there is a considerable expansion of the unit cell from (8171 Å ) to (8609 Å ) and (8610 Å ) and the coordinations of the Dy atoms undergo major reconstructions. This is associated with switching the single-molecule-magnetism (SMM) from OFF for to ON for and to OFF again for in air. Such a switching mechanism associated with the retention of crystallinity is unique in the chemistry of dysprosium. The structure of the molecule is formed from two symmetry related pentamers joined by an oxalate. A related compound containing two isolated neutral pentamers {Dy (EDDC) (μ -AcO) (μ -C H PO ) (μ -AcO) (AcO) (H O) }{Dy (EDDC) (μ -AcO) (μ -C H PO )(μ -C H PO )(μ -AcO) (AcO) (CH OH) }·2CHCl ( ) has also been isolated with closely related Dy coordination and it exhibits similar SMM behaviour in zero field.