Aluminium 8-Hydroxyquinolinate N-Oxide as a Precursor to Heterometallic Aluminium–Lanthanide Complexes

A reaction in anhydrous toluene between the formally unsaturated fragment [Ln(hfac)(3)] (Ln(3+) = Eu3+, Gd3+ and Er3+; Hhfac = hexafluoroacetylacetone) and [Al(qNO)(3)] (HqNO = 8-hydroxyquinoline N-oxide), here prepared for the first time from [Al((OBu)-Bu-t)(3)] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac)(3)Al(qNO)(3)] (Ln(3+) = Eu3+, Gd3+ and Er3+) in high yields. The molecular structures of these new compounds revealed a dinuclear species with three phenolic oxygen atoms bridging the two metal atoms. While the europium and gadolinium complexes show the coordination number (CN) 9 for the lanthanide centre, in the complex featuring the smaller erbium ion, only two oxygens bridge the two metal atoms for a resulting CN of 8. The reaction of [Eu(hfac)(3)] with [Alq(3)] (Hq = 8-hydroxyquinoline) in the same conditions yields a heterometallic product of composition [Eu(hfac)(3)Alq(3)]. A recrystallization attempt from hot heptane in air produced single crystals of two different morphologies and compositions: [Eu-2(hfac)(6)Al(2)q(4)(OH)(2)] and [Eu-2(hfac)(6)(mu -Hq)(2)]. The latter compound can be directly prepared from [Eu(hfac)(3)] and Hq at room temperature. Quantum mechanical calculations confirm (i) the higher stability of [Eu(hfac)(3)Al(qNO)(3)] vs. the corresponding [Eu(hfac)(3)Alq(3)] and (ii) the preference of the Er complexes for the CN 8, justifying the different behaviour in terms of the Lewis acidity of the metal centre.