A Self‐Assembled, Magnetically Coupled Square Cu₁₆ 4×[2×2] Grid

Abstract

Self-assembly strategies in coordination chemistry have reached a level of maturity such that predictable structures can be created by the simple expedient of designing and synthesizing a ligand with suitably encoded coordination information. With an appropriate balance between the coordinating features of the ligand and the coordination requirements of a metal, square [n n] grid structures have been achieved routinely for n = 2 and 3 (M 4 , M 9 ) with sixcoordinate transition-metal ions (M 4 Spin-exchange interactions occur between the paramagnetic metal ions through direct bridging (e.g. m-O, m-NN) connections. [1][2][3] The ability to concentrate spin-bearing sites in ordered "flat" arrays has created a broad interest in such systems within the chemistry and physics communities; [4][5][6][7] this interest has focused on their novel quantum-based magnetic properties, and in particular, in the case of [Mn II 5 MnIII 4 ] mixed-spin-state [3 3] grids, their potential to act as spin "qubits". [7] The coordination capacity of such ligands can be expanded to "tetratopic" by starting with a central dinucleating pyridazine fragment, rather than pyridine (Scheme 1), and using similar extension strategies. The tetratopic pyridazine bishydrazone ligand L1 was shown to produce a linear trinuclear Ni II complex, [8] with an empty potential coordination site (occupied by water), and a open, square-based, incomplete Cu 12 [n n] grid.[9] Other pyridazine-based polytopic ligands include tritopic and pentatopic examples, which produce a [3 3] Ag 9 grid complex [10] and a 2 [2 5] Ag 20 incomplete gridlike complex, respectively, [11] in which the pyridazine groups act as bridges, and the congruence of the bidentate ligand pockets creates four-coordinate (N 4 ) metal sites. A recent review article highlights a number of [n n] grid-based systems in this general class.

Self-Assembled Nanostructures of Oligopyridine Molecules

“…Thus, the first gridlike complex was reported in 1992, built up from a bispyridyl-pyridazyl ligand with Cu(I) ions in a tetrahedral surrounding (Figure a) . Five years later, a corresponding complex was described with Co(II) centers in octahedral coordination and a bis(terpyridine)-like ligand containing pyrimidine units (Figures and b). ,, Basically, both principles are maintained in most other gridlike complexes reported later on. , The coordinating nitrogen donor moieties were varied by pyrazol(at)e, ,, naphthyridine, or acyclic functional groups like Schiff bases, ,,, hydrazones, ,, amidines, ,,− ,, and carboxamidates ,,,,− (Figure ).…”

“…In order to use the highly ordered arrangement of the metal ions in the grids as addressable units in nanotechnology, the grids have to be enlarged or further two- and/or three-dimensional assembly has to follow (Figure ). Big effort has been undertaken for extending the [2 × 2] to the corresponding [3 × 3], ,− [4 × 4], ,,, and even [5 × 5] or [4 × 5], respectively, gridlike complexes. The drawback of poor solubility in the synthesis of the corresponding large ligands and the structural constraints in coordinating the metal ions by the ligands due to the relative rigidity of the aromatic backbone hampered further extension of this concept.…”

Formation of a Magnetically Coupled Neutral [4×4] Square Grid from a 2,6‐Pyridinedicarbaldehyde Bis(hydrazone) Ligand

“…Variable-temperature magnetic susceptibility data for 1 is shown in the form of χ m T (χ m = molar magnetic susceptibility) in Figure 4. A room-temperature χ m T value of Upon lowering the temperature, the value of χ m T gradually decreased down to 1.80 cm 3 mol -1 K at 20 K. With any further decrease in the temperature, the value of χ m T decreased rapidly to a final value of 1.01 cm 3 mol -1 K at 1.8 K. The susceptibility data were fitted according to the simplified spin-exchange model given in Figure 5 and as defined by the Hamiltonian in Equation (1). Considering the complexity of magnetic exchange, a nice fitting of the χ m T versus T plot was obtained from the model to give J 1 = 2.01 cm -1 , J 2 = -55.4 cm -1 , and TIP = 320 ϫ 10 -6 (10 2 R = 1.2) (TIP = temperature-independent paramagnetism) with a g value of 2.08 (obtained from EPR), in which the exchange pathways were proposed to be based on the Cu-OCu bridging angle.…”

“…[1] In this paper, we report the synthesis, structural analysis, and magnetic properties of a novel [4ϫ4] grid complex having the molecular formula [Cu 16 L 8 ](dmf) 3 (1). The ligand, H 4 L = {2, 6-bis[(3-methoxysalicylidene) [10] suggest that all of the copper ions are in the +2 oxidation state (Table S1, Supporting Information).…”

“…Predetermined polymetallic architectures can be developed if specifically designed ligands are coordinated to metal ions according to their coordination requirements. [1] Although significant contributions to polymetallic cages with polypyridyl ligands have already been documented, [2][3][4][5] logical approaches for the inclusion of large numbers of metal ions in a small, single molecular entity with interesting functional properties [6][7][8][9] is a synthetic challenge and is of current research interest. Often the topology of the assembly can be predicted in advance if relatively rigid organic ligands are combined with transition-metal ions that have a definite coordination preference.…”