[M^III₂M^II₃]ⁿ⁺trigonal bipyramidal cages based on diamagnetic and paramagnetic metalloligands

Abstract

A family of [MIII2MII3]n+ trigonal bipyramidal cages are characterised in the solution and solid state. A family of five [MIII2MII3]n+ trigonal bipyramidal cages (MIII = Fe, Cr and Al; MII = Co, Zn and Pd; n = 0 for 1–3 and n = 6 for 4–5) of formulae [Fe2Co3L6Cl6] (1), [Fe2Zn3L6Br6] (2), [Cr2Zn3L6Br6] (3), [Cr2Pd3L6(dppp)3](OTf)6 (4) and [Al2Pd3L6(dppp)3](OTf)6 (5) (where HL is 1-(4-pyridyl)butane-1,3-dione and dppp is 1,3-bis(diphenylphosphino)propane) are reported. Neutral cages 1–3 were synthesised using the tritopic [MIIIL3 ] metalloligand in combination with the salts CoIICl2 and ZnIIBr2 which both act as tetrahedral linkers. The assembly of the cis -protected [PdII(dppp)(OTf)2] with [MIIIL3 ] afforded the anionic cages 4–5 of general formula [MIII2PdII3](OTf)6 . The metallic skeleton of all cages describes a trigonal bipyramid with the MIII ions occupying the two axial sites and the MII ions sitting in the three equatorial positions. Direct current (DC) magnetic susceptibility, magnetisation and heat capacity measurements on 1 reveal weak antiferromagnetic exchange between the FeIII and CoII ions. EPR spectroscopy demonstrates that the distortion imposed on the {MO6} coordination sphere of [MIIIL3] by complexation in the MIII2MII3 supramolecules results in a small, but measurable, increase of the zero field splitting at MIII . Complete active space self-consistent field (CASSCF) calculations on the three unique CoII sites of 1 suggest DCo ≈ −14 cm−1 and E/D ≈ 0.1, consistent with the magnetothermal and spectroscopic data.