Polynuclear coordination compounds: a magnetostructural study of ferromagnetically coupled Ni4O4 cubane core motif

摘要

A series of tetranuclear nickel(II) complexes [Ni4~(II)L4~1(H2O)4] (1), [Ni4~(II)L4~2(MeOH)3(H2O)] (2), [Ni4~(II)L4~2(H2O)4] (3), [Ni4~(II)L4~3(MeOH)4] (4), [Ni4~(II)L4~4(MeOH)4] (5) have been synthesized by employing alkoxo-phenoxo ligands. All these compounds are characterized structurally and magnetochemically. Single-crystal X-ray diffraction studies suggest all the compounds contain a [Ni4(μ3-OR)4] cubane core motif with the alkoxo oxygen atoms of the ligands and octahedrally coordinated nickel(II) atoms placed alternatively at the corners of the cube. Magnetic susceptibility studies indicate a combination of ferromagnetic and antiferromagnetic exchange interactions between the four metal centers, resulting in an S_T = 4 spin ground state in all the Ni4~(II) complexes, which is confirmed by an isothermal magnetization measurement. These data were simulated using the Heisenberg-Dirac-Van Vleck (HDvV) spin Hamiltonian H = -2J1(S1·S2 + S3·S4) - 2J2(S1·S3 + S1·S4 + S2·S3 + S2·S4), affording the parameters J1 = -3.9 cm~(-1), J2 = +7.55 cm~(-1), g = 2.06 for 1, J1 = -2.45 cm~(-1), J2 = +7.3 cm~(-1), g = 2.055 for 2 and J1 = -3.2 cm~(-1), J2 = +7.8 cm~(-1), g = 2.045 for 3 and J1 = -3.9 cm~(-1), J2 = +8.4 cm~(-1), g = 2.13 for 4. The differences in the J values result from the differences in the Ni-O-Ni angles in the Ni4O4 cores. It is worth noting that small structural variations were found for the Ni-O-Ni angles of the Ni4O4 cubane cores of compounds 2 and 3 due to the change of terminal MeOH coordination in 2 to terminal H2O coordination in 3. In spite of these slight structural variations, both compounds 2 and 3 posses a S_T = 4 spin ground state. Spin ladder calculations based on the magnetostructural data clearly indicate the |J2/J1| ratio is larger than 1.5 and in accord with a nonet spin ground state.