Ferromagnetic Cu8 pinwheel clusters as building blocks for larger magnetic networks: Long-range structural and magnetic ordering

The structural and magnetic properties of a series of Cu8 pinwheel clusters with the tritopic ligands 2POMP, 2POPP, Cl2POMP and SEt2POMP are described. Two different ligand binding modes lead to two different arrangements of Cu–Cu bridge connections within the pinwheel. In one case, all the Cu(II) centers are bridged by hydrazone oxygen atoms, and the Cu–Cu distances are all similar (∼4 Å). In the other case, the central Cu(II) centers are bridged by hydrazone oxygen atoms, while the peripheral Cu2+ cations are bridged to the core by diazine (N–N) groups. In both cases, the bridges are orbitally strictly orthogonal and so ferromagnetic exchange dominates. In the complex [(2POMP-2H)4Cu8Br6](CuBr4)(H2O)11 (1) bromide ions bridge the peripheral Cu(II) centers of adjacent Cu8 pinwheel cluster molecules forming a 2D network with additional CuBr42- ions occupying cavities in the lattice. In [(Cl2POMP-2H)4Cu8(N(CN)2)8](H2O)9(CH3CN) (3), nitrogen atoms of the coordinated dicyanamide (dca) ions bound to the peripheral Cu(II) centers interact with ligand bound chlorine atoms of adjacent molecules to form stacks of pinwheels. In [(Set2POMP-2H)4Cu8(H2O)8](PF6) 8(CH3OH)6(CH3CN)1.5 (4) relatively short S⋯S contacts exist between pinwheels and cause stacking in the extended structure. Surprisingly, there is evidence for long-range magnetic interactions in 3 via the intermolecular contacts, but not in 1, which has direct bonding between pinwheel subunits. Exchange integrals within the pinwheel clusters fall in the rangeJ = 4–7 cm−1.

Tritopic pyridine-2,6-bis-hydrazone ligands self-assemble with Cu2+ forming ferromagnetic Cu8 pinwheels. Individual pinwheels can be linked to form 2D networks using bridging ligands. [(2POMP-2H)4Cu8Br6](CuBr4) (H2O)11 (1) has pinwheels connected via Br− ions bound to peripheral Cu2+ centers. Cavities within the structure contain H-bonded CuBr42- guests. Dicyanamide leads to an extended pinwheel structure with N–Cl contacts, and weak long-range cooperative magnetic interactions.Figure optionsDownload as PowerPoint slide