[M₂ ^IIIM₃ ^II]ⁿ⁺ trigonal bipyramidal cages based on diamagnetic and paramagnetic metalloligands

A family of five [MIII2MII3]ⁿ⁺ trigonal bipyramidal cages (M^III = Fe, Cr and Al; M^II = 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 [M^IIIL₃] metalloligand in combination with the salts Co^IICl₂ and Zn^IIBr₂, which both act as tetrahedral linkers. The assembly of the cis-protected [Pd^II(dppp)(OTf)₂] with [M^IIIL₃] afforded the anionic cages 4-5 of general formula [MIII2PdII3](OTf)₆. The metallic skeleton of all cages describes a trigonal bipyramid with the M^III ions occupying the two axial sites and the M^II 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 Fe^III and Co^II ions. EPR spectroscopy demonstrates that the distortion imposed on the {MO₆} coordination sphere of [M^IIIL₃] by complexation in the {MIII2MII3} supramolecules results in a small, but measurable, increase of the zero field splitting at M^III. Complete active space self-consistent field (CASSCF) calculations on the three unique Co^II sites of 1 suggest D_Co ≈ -14 cm^-1 and E/D ≈ 0.1, consistent with the magnetothermal and spectroscopic data.