Co-existence of ferro- and antiferromagnetic interactions in a hexanuclear mixed-valence Co III2Mn II2Mn IV2 cluster sustained by a multidentate Schiff base ligand

Abstract

The successful utilization of the “direct synthesis” approach yielded the unprecedented hexanuclear complex of formula [Co₂Mn^II₂Mn^IV₂(L¹)₄Cl₂(μ₃-O)₂(dmf)₄]·2dmf (1) (H₃L is the Schiff base derived from the condensation of salicylaldehyde and 3-aminopropane-1,2-diol). Single crystal X-ray analysis revealed that 1 crystallizes in the monoclinic system P2₁/c and it contains a rare mixed-valence {Co^III₂Mn^II₂Mn^IV₂(μ₂-O)₈(μ₃-O)₂} core where all metal ions are linked through the phenolato and alkoxo groups of the L³⁻ ligand. Besides the charge balance resulting from the X-ray structure, the oxidation state of the metal ions has been confirmed by XPS spectroscopy. Cryomagnetic studies indicate the coexistence of ferro- (Mn^IV–Mn^II, J₂ = +1.10(3) cm⁻¹, J₃ = +2.19(3) cm⁻¹; Mn^II–Mn^II, j = +0.283(3) cm⁻¹) and antiferromagnetic interactions (Mn^IV–Mn^IV, J₁ = −17.31(4) cm⁻¹), with the six-coordinate Co^III ions being diamagnetic. DFT type calculations were carried out to substantiate these values. The energy diagram for the different spin states using the best-fit parameters shows the occurrence of six low-lying spin states (S = 0–5) which are close in energy but clearly separated from the remaining ones, with the ground spin state being S = 5. Complex 1 is found to be the first example where weak ferromagnetic exchange between Mn^II ions through the long –O–Mn^IV–O– pathway takes place.