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Single-molecule magnets (SMMs) [1][2][3][4][5][6] are chemically and physically interesting compounds that exhibit hitherto unobserved magnetic properties. To prevent reversal of the molecular magnetic moment, the use of heavy lanthanide ions is becoming popular because of their large spin multiplicity and large magnetic anisotropies in the ground state. [3][4][5][6] Lanthanide ions exhibit flexibility in magnetic anisotropy, which is another advantage of Ln III -based SMMs that is attributable to the flexible design and control of the ligandfield (LF) anisotropy. These anisotropies are correlated through Stevens factor q m as B [ We found that phenoxo oxygen donors have higher negative charges than other donor atoms.[5] To achieve an equatorial LF, we focused on the macrocyclic Schiff base and oxime ligands shown in Scheme 1, which provide a metallacrown coordination environment [3j, 8] for the central metal ion with six phenoxo oxygen donors and have a rigid and planar framework owing to the p-conjugated moieties. The ligands are formed by condensation of 2,3-dihydroxybenzene-1,4-dicarbaldehyde and a diamine in the presence of metal ions as templates. The six phenoxo oxygen atoms are in equatorial positions around a central Ln III ion, and hence an equatorial LF is produced. We have reported syntheses and structures of mixed-metal tetranuclear complexes constructed with L2 6À , [9] of which the oxime ligand showed a slight deviation from an ideal plane because of the longer NÀN distance of the diamine. Hence, we decided to employ an ethylenediamine derivative to achieve a more planar structure of the complex.The wheel-shaped tetranuclear complex [Er III Zn II 3 (L1)-(OAc)(NO 3 ) 2 (H 2 O) 1.5 (MeOH) 0.5 ] (1) was synthesized by reaction of Er(NO 3 ) 3 ·6 H 2 O, 2,3-dihydroxybenzene-1,4-dicarbaldehyde, (R,R)-1,2-diphenylethylenediamine, and Zn-(OAc) 2 ·2 H 2 O in 1:3:3:3 ratio (see Experimental Section; Scheme 1. Structures of macrocyclic ligands.

Lanthanide complexes have attracted considerable attention because of their electronic properties such as magnetism [1][2][3][4] and luminescence. [5] In the research area of molecule-based magnets, single-molecule magnets (SMMs), that are formed through the combination of a large spin multiplicity of the ground state and an easy-axis (or Ising-type) magnetic anisotropy of the entire molecule, have been extensively investigated. Heavy lanthanide ions are being used for designing SMMs because of their large angular momentum in the ground multiplet state and large magnetic anisotropy. In a previous study, [6] our research group has demonstrated a technique for designing the magnetic anisotropy of heavy lanthanideA C H T U N G T R E N N U N G (III) ions by controlling the ligand field (LF) anisotropy in a series of isostructural Ln III -Cu II complexes (Ln = Tb, Dy, Ho, and Er) as well as in a wheel shaped Er III -Zn II 3 complex. Although the inner 4f electrons are effectively shielded by the 5s and 5p electrons, the 4f electrons are, nonetheless, affected by the negative charges of donor atoms that form an anisotropic electrostatic field. This allows for flexibility in the control of magnetic anisotropy by designing the LF. For example, the Dy III ion with a f 9 configuration has an 6 H 15/2 ground state of j J, J z > = j 15/2, J z > (J z = AE 15/2, AE 13/2, .., AE 1/2), and the sub-levels with the largest j J z j value, j 15/2, AE 15/2 > , are known to have an oblate spheroidal electronic distribution.[7] The stabilization of these two sub-levels leads to stronger easy-axis anisotropy, which was achieved by locating strong negative charges at axial positions. The theoretical analysis of the Dy III -Cu II complex in the previous work predicted an extremely large LF splitting of the Stark sub-levels (> 400 K), which engages a very strong SMM feature of this complex; however, the observed barrier for magnetization reversal was approximately 32 K.[6a] Because of the exchange interaction with Cu II , the ground sub-level of Dy III is further split into two levels with a separation of 26 K, which may be related to the observed barrier. We theorized that the absence of Cu II would enhance the high potential of the Dy III ion for the construction of SMMs, and by changing Cu II to diamagnetic Zn II , we achieved a single-ion SMM with a huge magnetic anisotropy that is close to the theoretical value. Herein, we report the synthesis, structure, and detailed SMM behavior of a Dy III -Zn II dinuclear complex (1, Figure 1) in which the Dy III ion is the only paramagnetic center.Compound 1 was obtained by reacting dysprosiumA C H T U N G T R E N N U N G (III) nitrate, salicylaldehyde (Hsal), and bromide ion with the mono-zinc(II) complex [Zn(L)] (H 2 L denotes a Schiff-base ligand formed by a condensation reaction of o-vaniline and 2,2-dimethylpropanediamine in a 2:1 ratio) [8] in the presence of a base. X-ray analysis (Figure 1) Figure 1. An ORTEP drawing of complex 1 with selected atom numbering (50 % probability ellipsoids)...