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+ = Eu 3+ , Gd 3+ and Er 3+ ; Hhfac = hexafluoroacetylacetone) and [Al(qNO) 3 ] (HqNO = 8-hydroxyquinoline N -oxide), here prepared for the first time from [Al(O t Bu) 3 ] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac) 3 Al(qNO) 3 ] (Ln 3+ = Eu 3+ , Gd 3+ and Er 3+ ) 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 (µ-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.