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

A reaction in anhydrous toluene between the formally unsaturated fragment [Ln(hfac)] (Ln = Eu, Gd and Er; Hhfac = hexafluoroacetylacetone) and [Al(qNO)] (HqNO = 8-hydroxyquinoline -oxide), here prepared for the first time from [Al(OBu)] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac)Al(qNO)] (Ln = Eu, Gd and Er) 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)] with [Alq] (Hq = 8-hydroxyquinoline) in the same conditions yields a heterometallic product of composition [Eu(hfac)Alq]. A recrystallization attempt from hot heptane in air produced single crystals of two different morphologies and compositions: [Eu(hfac)Alq(OH)] and [Eu(hfac)(µ-Hq)]. The latter compound can be directly prepared from [Eu(hfac)] and Hq at room temperature. Quantum mechanical calculations confirm (i) the higher stability of [Eu(hfac)Al(qNO)] vs. the corresponding [Eu(hfac)Alq] 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.