Single-Molecule Magnet Behavior of Confined Dy(III) in a Mixed Heteroatom-Substituted Polyoxotungstate
Sandhya Kapurwan, Pradip Kumar Sahu, Sanjit Konar
Abstract
Two heteroatom-templated Dy(III)-confined polyoxotungstates [H 2 N(CH 3 ) 2 ] 7 Na 7 [Dy 2 (H 2 O) 7 (W 4 O 9 )(HPSeW 15 O 54 )(α-SeW 9 O 33 ) 2 ]·31H 2 O ( 1 ) and [H 2 N(CH 3 ) 2 ] 14 K 2 Na 18 {[Dy 2 (H 2 O) 13 W 14 O 40 ] 2 [α-SeW 9 O 33 ] 4 [HPSeW 15 O 54 ] 2 }·44H 2 O ( 2 ) were synthesized by a one-pot aqueous reaction and structurally characterized. The most distinctive structural feature of complexes 1 & 2 is the simultaneous presence of both trivacant Keggin [α-SeW 9 O 33 ] 8– and Dawson [HPSeW 15 O 54 ] 10– building blocks containing P(III)–Se(IV) heteroatoms. The trimeric polyanion of 1 can be represented as a fusion of two trivacant Keggin [α-SeW 9 O 33 ] 8– and Dawson [HPSeW 15 O 54 ] 10– building units encapsulating the [Dy 2 (H 2 O) 7 (W 4 O 9 )] 12+ cluster. On the other hand, hexameric polyoxoanions of 2 are described as four trivacant Keggin [α-SeW 9 O 33 ] 8– and two Dawson [HPSeW 15 O 54 ] 10–, building units anchoring a [Dy 4 (H 2 O) 26 W 28 O 80 ] 20+ cluster. The magnetic investigation revealed the presence of significant magnetic anisotropy and slow relaxation of magnetization behavior for complex 1 with a phenomenological energy barrier, U eff = 13.58 K in the absence of an external magnetic field, and U eff = 24.57 K in the presence of a 500 Oe external dc magnetic field. On the other hand, complex 2 favors the QTM relaxation process in the absence of an external magnetic field and shows field-induced slow relaxation of magnetization with U eff = 11.11 K at 1500 Oe applied dc field. The in-depth analysis of magnetic relaxation dynamics shows that the relaxation process follows the Orbach as well as Raman relaxation pathways. Further, the ab initio calculation of the studied complexes confirms that the highly axial ground and first excited energy states (containing pure highest m J states) are responsible for the observed single-molecule magnet (SMM) behavior. Remarkably, this is the first example of a mixed heteroatom-based Dy(III)-substituted polyoxotungstate with both trimeric Keggin [α-SeW 9 O 33 ] 8– and Dawson [HPSeW 15 O 54 ] 10– building units showing SMM behavior.