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

{Dy5 (EDDC)2 (μ3 -AcO)2 (μ5 -C15 H11 PO3 )(μ4 -C15 H11 PO3 )(μ2 -AcO)2 (AcO)2 (H2 O)(CH3 OH)2 }2 (μ4 -C2 O4 )· x H2 O ( I ), where H2 EDDC is N ', N '', E , N ', N '', E-N ', N ''-(ethane-1,2-diylidene)dipyrazine-2-carbohydrazide and C15 H11 PO3 H2 is 9-anthrylmethylphosphonic acid, is found to undergo two consecutive single-crystal-to-single-crystal transformations. The first is under UV irradiation ( λ = 365 nm for 3 d in air) to {Dy5 (EDDC)2 (μ3 -AcO)2 (μ5 -C15 H11 PO3 )2 (μ2 -AcO)2 (AcO)2 (H2 O)3 }2 (μ4 -C2 O4 )· x H2 O ( I-UV ) where the two CH3 OH are replaced by two H2 O and the second by annealing under N2 at 100 °C on a diffractometer or under Ar in a glovebox to {Dy5 (EDDC)2 (μ3 -AcO)2 (μ5 -C15 H11 PO3 )2 (μ2 -AcO)4 (H2 O)}2 (μ4 -C2 O4 ) ( I-A-N2 or I-A-Ar ) where it has lost two H2 O molecules. The second transformation is reversible by exposure to air at room temperature ( I-A-N2 -cool ). While the overall structures are the same (retaining the space group P 21 / c ), there is a considerable expansion of the unit cell from I (8171 Å3 ) to I-UV (8609 Å3 ) and I-A-N2 (8610 Å3 ) and the coordinations of the Dy atoms undergo major reconstructions. This is associated with switching the single-molecule-magnetism (SMM) from OFF for I to ON for I-UV and to OFF again for I-A-Ar 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 {Dy5 (EDDC)2 (μ3 -AcO)2 (μ5 -C15 H11 PO3 )2 (μ2 -AcO)3 (AcO)2 (H2 O)2 }{Dy5 (EDDC)2 (μ3 -AcO)2 (μ5 -C15 H11 PO3 )(μ4 -C15 H11 PO3 )(μ2 -AcO)3 (AcO)2 (CH3 OH)2 }·2CHCl3 ( II ) has also been isolated with closely related Dy coordination and it exhibits similar SMM behaviour in zero field.