A Single‐Molecule‐Magnetic, Cubane‐Based, Triangular Co₁₂ Supercluster

Abstract

For single-molecule magnet (SMM) behavior, molecular clusters require Ising magnetoanisotropy (D mol < 0) and a large ground-state spin multiplicity.[1] These requirements result in a high thermal barrier (U) to magnetization relaxation. Such clusters elicit intense interest in both chemistry and physics owing to behavior that is intermediate between that of a simple paramagnet (or classical bulk magnet) and that of a quantum/classical system, [2] which heralds applications in quantum devices.[3] The development of SMMs is often determined by serendipitous assembly. [2,4] However, the synthesis of molecular clusters with SMM-type behaviors presents a formidable challenge because of the difficulties in assembling predetermined structures with predictable magnetic properties. [4] The Mn 12 and Fe 8 clusters and their derivatives [2,4] are the most widely studied of homo-and heterometallic SMM systems, [4][5][6] and only very few cobalt-based SMMs are known. [7] One reason may be the difficulty in synthesizing cobalt clusters that display a large spin and molecular anisotropy. Because Co II has an exceptionally large D ion > 0 (a characteristic known to dominate molecular anisotropy), [8] only a limited number of high-nuclearity clusters are known for cobalt compared with manganese and iron. [4, 7c] Previous investigations implied that an alkoxido-bridged metal-cubane structure probably favors ferromagnetic coupling through m 3 -O bridges, in which orthogonal hard-axis alignment of four single-ion spins (D ion > 0) may result in a negative D mol value for the clusters. [7a, 8] As 2-(hydroxymethyl)pyridine (hmp) can furnish a cubane-like M 4 O 4 motif, [9] we chose the analogous (1H-benzimidazol-2-yl)methanol (Hbm) to serve as a chelating/bridging ligand to bring (Figure 1).[10] The cubane subunit is constructed by four Co II ions and four alkoxido oxygen atoms of m 3 -h 1 :h 3 -bm ligands.The Co1 and Co2 atoms in distorted octahedral N 2 O 4 environments are complexed by two O,N-chelating sites, one bm m 3 -alkoxido oxygen atom, and one EtOH ligand. The Co3 and Co4 atoms both form distorted CoO 6 octahedra with three alkoxido groups, two acetate groups, and one nitrate group. The cuboidal core is distorted with all the Co-O-Co angles in the range (98 AE 4)8 (Table S1 in the Supporting Information) for Co···Co dominant ferromagnetic exchange. [7a,b] Similar to the hmp analogue, the Co···Co distances vary from 3.034 to 3.243 . A Jahn-Teller distortion of the four d 7 Co 2+ ions lowers the symmetry of the cubes to S 4 . In addition to the central m 6 -NO 3 À bridge, six syn-syn acetate ligands also interconnect neighboring cubane units in the supercluster. Five discrete NO 3 À ions balance the overall positive charge of the cluster. There are no significant intermolecular interactions. A space-filling plot of 1 reveals that the cluster has an approximate trefoil shape with a diameter of about 2.0 nm ( Figure S1 in the Supporting Information). High coordination numbers of nitrate, a weak ligand, to transition met...

Synthesis, crystal structures, magnetic and luminescent properties of nickel(II) and cadmium(II) coordination polymers bearing 5-(2′-carboxylphenyl) nicotate ligands

“…The most useful strategy to construct such supramolecular structures is to employ appropriate bridging ligands. Many multi-carboxylate or heterocyclic carboxylic acids have been used for this purpose [1][2][3][4][5][6][7][8]. In order to extend our investigations in this field, we chose 5-(2 0 -carboxylphenyl) nicotic acid (H 2 cpna) as a functional ligand, based on the following considerations.…”

“…The design and synthesis of transition metal coordination polymers with interesting luminescence and magnetic properties have received enormous attention in recent decades [1][2][3][4][5][6]. The most useful strategy to construct such supramolecular structures is to employ appropriate bridging ligands.…”

Utilization of mixed ligands to construct two new coordination polymers: Syntheses, structures and properties

“…The value of χ Μ slowly increases with decreasing temperature (almost constant values), from 0.018 emu mol À 1 at 300 K to 0.022 emu mol -1 at 13 K, and then exponentially, reaching a maximum of 0.043 emu mol -1 at 2 K. At 300 K, the χ Μ T value of 5.54 emu K mol -1 is much higher than the spin-only value of 3.78 emu K mol -1 expected for two noninteracting high-spin Co II ions (S¼3/2, g¼2.0). This magnetic behavior indicates the presence of a large orbital contribution arising from the 4 T 1g ground state of Co(II) ions, which is known to be significant in an octahedral or trigonal bipyramidal environment [58][59][60]. Upon cooling, the χ Μ T value gradually decreases to a minimum of 0.086 emu K mol -1 at 2 K, which is a typical behavior of the antiferromagnetic exchange coupling between the cobalt ions.…”

Evolvement of molecular nanomagnets in China

“…The compound, [Co 7 (μ 3 -OH) 2 (bhqe) 3 (H 2 O) 6 ] (6, H 4 bhqe = 1,2-bis(8-hydroxyquinolin-2-yl)ethane-1,2-diol), was reported by Zeng et al [27]. The Co 1 centre is coordinated by six μ 3 -alkoxo oxygen atoms from three bhqe 4 c-axis, sandwiching the Co 1 (figure 2).…”