Electrical Conductivity of Copper Hexamers Tuned by their Ground-State Valences.

A new design concept has been realized for the construction of molecular conductors, whereby the building unit contains a core reservoir of carriers made up of metal ions with controllable valence states and shelled by flat organic ligands having an extended π-system to promote supramolecular electronic communication. Therefore, reacting the conjugated multidentate ligand 5,5'-pyridyl-3,3'-bi-1 H-pyrazole with different copper salts solvothermally led to three interesting hexameric salts having different ground-state valences, [CuII6(L)4(NO3)(CH3OH)2](NO3)3·4CH3OH, [(CH3)2NH2][CuICuII5(L)4](SO4)2·4H2O, and [CuI2CuII4(L)4](NO3)2·2CH3OH. The monovalent CuII6 salt is an insulator, but the mixed-valent CuII5-CuI and CuII4-CuI2 salts are semiconductors. Magnetic exchange interactions up to JNN = -158 cm-1 dominate the susceptibilities and lead to ground-state spin ST = 1 (CuII6), 1/2 (CuII5-CuI), and 0 (CuII4-CuI2) at 40 K. Cyclic voltammetry shows the stepwise one-electron oxidation-reduction through all the possible valence states. The theoretical calculations of the electronic and band structures of the three compounds substantiate the experimentally observed physical properties.

Magnetostructural relationship for µ2-phenoxido bridged ferric dimers.

Three Fe(iii) dimers, [Fe2(L-H)2]·2CH3CN (1), [Fe2(L-OCH3)2] (2) and [Fe2(L-OC2H5)2]·2CH3CN (3), containing the pentadentate O,N,N,O,O-donor Schiff-base ligands with variable size pendants, were synthesized and structurally characterized. The three ligands were generated in situ from 2-(iminomethyl)phenol, 2-methoxy-6-(iminomethyl)phenol and 2-ethoxy-6-(iminomethyl)phenol, respectively. All three crystal structures contain centrosymmetric dimers of edge-sharing octahedra of Fe(iii) ions through a pair of µ2-phenoxido bridges. The exchange coupling is ferromagnetic for 1 (J = +0.47(1) cm-1, ∠Fe-O-Fe = 98.02°) and 2 (J = +0.86(1) cm-1, ∠Fe-O-Fe = 97.17°), but antiferromagnetic for 3 (J = -0.72(1) cm-1, ∠Fe-O-Fe = 98.53°), which are correlated by high-field electron paramagnetic resonance revealing moderate magneto-anisotropy of D = -0.24(3) cm-1, E = 0.08(1) cm-1 for 1, D = -0.38(1) cm-1, E = 0.11(1) cm-1 for 2, and D = 0.30(3) cm-1, E = 0.02(1) cm-1 for 3. The exchange couplings were further estimated by DFT calculations, which gave the finest Fe-O-Fe angle of 97.83° for the ferromagnetic-antiferromagnetic crossover.