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

Abstract

A series of tetranuclear nickel(II) complexes [NiII4L14(H2O)4] (1), [NiII4L24(MeOH)3(H2O)] (2), [NiII4L24(H2O)4] (3), [NiII4L34(MeOH)4] (4), [NiII4L44(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 ST = 4 spin ground state in all the NiII4 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 ST = 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.