Three Single-Ion Magnet Candidates from Erbium(III) Squarate Complexes in Relation with a Square-Antiprismatic Coordination Structure

Abstract

Mononuclear [Er(sq)(Hsq)(H₂O)₆]•(H₂O) (sq = squarate, C₄O₄²⁻) and heteroleptically bridged 3D polymer [Er₂(sq)₂(ox)(H₂O)₄]_n (ox = oxalate, C₂O₄²⁻) were newly prepared. The Arrhenius plot for the alternating-current magnetic susceptibility data clarified the effective energy barriers for magnetization reversal, U_eff/k_B = 42.0(7) K at a direct-current bias field of 1000 Oe for the former and 21.7(8) K at 800 Oe for the latter. They are greater than that of known 2D [Er₂(sq)₃(H₂O)₈]_n. All of the erbium(III) coordination structures investigated here can be described approximately as a square antiprism with a slight variation in axial compression within a D_4d symmetry framework. To elucidate a suitable degree of D_4d compression, the complete-active-space self-consistent-field calculation was carried out on the discrete neutral molecule [Er(sq)(Hsq)(H₂O)₆] and its modifications after anisotropically upto 10% elongation or compression along the principal axis. The calculated energy gap between the ground and the first excited states in the Stark sublevels is 151.5 cm⁻¹ for the original [Er(sq)(Hsq)(H₂O)₆] molecule, and the gap became wider with more compressed as well as elongated ErO₈ structures. The gap is highly sensitive to the compressed/elongated modification, justifying the variety and diversity of the single-ion magnet characteristics observed.