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

Abstract

A series of tetranuclear nickel(II) complexes [Ni^II₄L¹₄(H₂O)₄] (1), [Ni^II₄L²₄(MeOH)₃(H₂O)] (2), [Ni^II₄L²₄(H₂O)₄] (3), [Ni^II₄L³₄(MeOH)₄] (4), [Ni^II₄L⁴₄(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 Ni^II₄ 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.