No abstract

A series of tetranuclear copper(II) and nickel(II) complexes is described, all of which form by a strict self-assembly process involving just a single ligand and the metal salt. The ligands POAP, POAPZ, PZOAP, and 6POAP contain terminal pyridine and pyrazine residues bound to a central flexible diazine subunit (N−N) and contain two potentially bridging groups (alkoxo and diazine). In all cases but one the metals are linked by alkoxo oxygens alone, forming essentially square [M4(μ2-O)4] clusters. A rectangular copper(II) complex [Cu4(μ2-N2)2(μ2-O)2] involves a mixture of two alkoxo and two diazine bridges. The square Cu4O4 clusters exhibit ferromagnetic coupling between the metal centers, associated with the orthogonal arrangement of magnetic orbitals, while for the Ni4O4 clusters the nickel(II) centers are coupled antiferromagnetically. The [Cu4(μ2-N2)2(μ2-O)2] cluster exhibits strong antiferromagnetic coupling through a trans Cu(N-N)Cu bridging arrangement, typical for systems of this sort. X-ray structures are reported for [Cu4 (POAP-H)4(H2O)2](NO3)4·4H2O (2), [Cu4(POAPZ-H)4(H2O)](NO3)4·3H2O (3), [Cu4(6POAP-H)4](ClO4)4 (4), [Cu4(PZOAP-H)4](NO3)4·3H2O (5), [Ni4(POAP-H)4 (H2O)4](NO3)4·8H2O (6), and [Ni4(PZOAP-H)4(H2O)4](ClO4)4·5H2O (9). 2 crystallized in the monoclinic system, space group C2/c, with a = 20.479(3) Å, b = 14.920(2) Å, c = 19.671(3) Å, β = 90.591(4)°, and Z = 8. 3 crystallized in the monoclinic system, space group P21/c, with a = 14.12(1) Å, b = 14.116(3) Å, c = 29.043(4) Å, β = 94.50(3)°, and Z = 4. 4 crystallized in the monoclinic system, space group C2/c, with a = 22.646(4) Å, b = 25.842(5) Å, c = 12.349(5) Å, β = 110.34(2)°, and Z = 4. 5 crystallized in the monoclinic system, space group P2/n, with a = 14.3573(4) Å, b = 10.8910(6) Å, c = 20.5360(10) Å, β = 96.975(4)°, and Z = 4. 6 crystallized in the triclinic system, space group P1̄, with a = 12.0509(6) Å, b = 12.7498(6) Å, c = 23.1208 Å, α = 93.1050(10)°, β = 100.1500(10)°, γ = 108.5050(11)°, and Z = 2. 9 crystallized in the orthorhombic system, space group Pbcn, with a = 14.368(4) Å, b = 25.469(7) Å, c = 18.479(5) Å, and Z = 4.

The X-ray crystal structure and magnetic properties of a molecular crystal consisting of 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl radical and hydroquinone (pyvd:hq) are presented. The structure contains a two-dimensional network of hydrogen bonds involving the hydroquinones and the pyridine ring of the pyvd radical. The radicals adopt an unusual head-over-tail (antiparallel) pi-stacked array perpendicular to the hydrogen-bonded planes. The variable-temperature magnetic susceptibility data can be modeled using a one-dimensional antiferromagnetic chain model, with J = -58 cm(-1). The strength of the magnetic coupling is very unusual because there are no close intermolecular radical-radical contacts to provide conventional pathways for magnetic interactions. A pathway for coupling is proposed involving the mediation of magnetic exchange interactions between radical centers by the pyridine rings. Density functional calculations on the pyvd radical, as well as aggregates thereof based on the X-ray structure, have been employed in attempts to understand the possible mechanisms by which the strong magnetic interactions are achieved.