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 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.