Synthetic, structural, spectroscopic and theoretical study of a Mn(iii)-Cu(ii) dimer containing a Jahn-Teller compressed Mn ion

The heterobimetallic complex [Cu(ii)Mn(iii)(L)₂(py) ₄](ClO₄)·EtOH (1) built using the pro-ligand 2,2′-biphenol (LH₂), contains a rare example of a Jahn-Teller compressed Mn(iii) centre. Dc magnetic susceptibility measurements on 1 reveal a strong antiferromagnetic exchange between the Cu(ii) and Mn(iii) ions mediated through the phenolate O-atoms (J = -33.4 cm^-1), with magnetisation measurements at low temperatures and high fields suggesting significant anisotropy. Simulations of high-field and high frequency powder EPR data suggest a single-ion anisotropy D_Mn(III) = +4.45 cm^-1. DFT calculations also yield an antiferromagnetic exchange for 1, though the magnitude is overestimated (J_DFT = -71 cm^-1). Calculations reveal that the antiferromagnetic interaction essentially stems from the Mn(d_x²-y²)-Cu(d_x²-y ²) interaction. The computed single-ion anisotropy and cluster anisotropy also correlates well with experiment. A larger cluster anisotropy for the S = 3/2 state compared to the single-ion anisotropy of Mn(iii) is rationalised on the basis of orbital mixing and various contributions that arise due to the spin-orbit interaction.