[MIII2MII3]ⁿ⁺ trigonal bipyramidal cages based on diamagnetic and paramagnetic metalloligands

Abstract

A family of five [MIII2MII3] ^<i>n</i>+ trigonal bipyramidal cages (M^III = Fe, Cr and Al; M^II = Co, Zn and Pd; <i>n</i> = 0 for <b>1-3</b> and <i>n</i> = 6 for <b>4-5</b>) of formulae [Fe₂Co₃L₆Cl₆] (<b>1</b>), [Fe₂Zn₃L₆Br₆] (<b>2</b>), [Cr₂Zn₃L₆Br₆] (<b>3</b>), [Cr₂Pd₃L₆(dppp)₃](OTf)₆ (<b>4</b>) and [Al₂Pd₃L₆(dppp)₃](OTf)₆ (<b>5</b>) (where HL is 1-(4-pyridyl)butane-1,3-dione and dppp is 1,3-bis(diphenylphosphino)propane) are reported. Neutral cages <b>1-3</b> 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 <i>cis</i>-protected [Pd^II(dppp)(OTf)₂] with [M^IIIL₃] afforded the anionic cages <b>4-5</b> 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 <b>1</b> 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 <b>1</b> suggest <i>D</i>_Co ≈ -14 cm^-1 and <i>E</i>/<i>D</i> ≈ 0.1, consistent with the magnetothermal and spectroscopic data.