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...

Exploring the Effect of Metal Ions and Counteranions on the Structure and Magnetic Properties of Five Dodecanuclear CoII and NiII Clusters

“…X-ray structural determinations revealed that nitrate anion still plays the template role and new counteranions (perchlorate) successfully incorporate the final structure to balance the charge in 4 and 5 ( Figure 1). Following the clues mentioned above, the reason why the perchlorate anions only act as counteranions in 4 and 5 can be concluded to be 1) the tetrahedral anion ClO 4 À other than the planar anion NO 3 À is too big to template these high nuclearity species, and 2) the discrete M 12 clusters in the crystal lattice pack upon each other to form sheets, and NO 3 À anions that occupy the space between M 12 clusters; as expected, a larger anion ClO 4 À can replace NO 3 À fit into the void space. These are facts that further underscore the importance of the anion size and shape in promoting a particular triangular structure.…”

“…[3,4] From the point of view of organic ligand design, it is desirable to synthesize bulky ligands that have the coordinating atoms pointing on one side of the molecule. This favors magnetic isolation of the clusters [12] and also changes the correlation of the magnetic clusters.…”

“…[3] Given the intricate structure-property relationships that govern the intramolecular interactions between the spin centers in a magnetic molecule, the synthesis of isostructural compounds that comprise different spin centers is of particular importance for the development of rational approaches to predefined magnetic properties. The similarity of the coordination chemistry of Ni II and Co II suits these spin centers for the production of common structural motifs.…”

“…[6] An important factor in the construction of such clusters is the choice of "key" ligands, which will dictate not only the symmetry, topAbstract: We present the synthesis, characterization of the structures, and magnetic properties of five isostructural dodecanuclear coordination clusters of Ni II and Co II : [Co 12 (bm) 12 6 A C H T U N G T R E N N U N G (EtOH) 6 ]A C H T U N G T R E N N U N G (NO 3 ) 5 (1), [Ni 12 (bm) 12 In particular, the larger perchlorate counterions in 4 further change the overall correlation interaction between clusters, thus leading to an enhanced blocking temperature for the less-symmetrical 4 (pseudo-C 3 ) relative to 1 and 3 (true C 3 ). Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that the three dodecanuclear clusters of 1-3 retain their compositions in solution.…”

“…[1][2][3][4] A key feature for the observation of SMM in these compounds is the native Ising (or easy-axis)-type anisotropy of the whole molecules and the segregation of the clusters in the solids. [5] In following the observations of SMMs in Mn III compounds, the field has developed much more rapidly for compounds that contain manganese than for other first-row transition metals.…”