[M2IIIM3II]n+ trigonal bipyramidal cages based on diamagnetic and paramagnetic metalloligands

A family of five [MIII2MII3] trigonal bipyramidal cages (M = Fe, Cr and Al; M = Co, Zn and Pd; n = 0 for 1-3 and n = 6 for 4-5) of formulae [FeCoLCl] (1), [FeZnLBr] (2), [CrZnLBr] (3), [CrPdL(dppp)](OTf) (4) and [AlPdL(dppp)](OTf) (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 [ML] metalloligand in combination with the salts CoCl and ZnBr, which both act as tetrahedral linkers. The assembly of the cis-protected [Pd(dppp)(OTf)] with [ML] afforded the anionic cages 4-5 of general formula [MIII2PdII3](OTf). The metallic skeleton of all cages describes a trigonal bipyramid with the M ions occupying the two axial sites and the M 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 and Co ions. EPR spectroscopy demonstrates that the distortion imposed on the {MO} coordination sphere of [ML] by complexation in the {MIII2MII3} supramolecules results in a small, but measurable, increase of the zero field splitting at M. Complete active space self-consistent field (CASSCF) calculations on the three unique Co sites of 1 suggest D ? -14 cm and E/D ? 0.1, consistent with the magnetothermal and spectroscopic data.