| Three general routes to binucleating bis(amidinate) ligands containing dibenzofuran and 9,9-dimethylxanthene backbones are reported. The free-base form of one of the ligands, (MesPh)2DBF(H2), forms a 1:1 adduct with acetone. Single crystal X-ray diffraction of this adduct reveals bidentate H-bonding of the bis(amidine) to the ketone oxygen. A variety of dialuminium complexes were synthesized by reaction of the bis(amidines) with organoaluminium reagents (AIMe3, AliBu3, AlEt2Cl) in toluene solution. These complexes are of the general formulation L(AlR2)2, solid-state structures of four of the dialuminium complexes are presented. These show the aluminium centers have pseudo-tetrahedral geometry with a range of Al-Al distances of 5.9303(9)-7.216(1) A. 1H NMR spectroscopic studies indicate the dialuminium complexes interact weakly with certain Lewis bases (e.g. DMF, DMSO) to effect the exchange of the aluminium-bound groups. In the presence of one equivalent of DMF the DeltaG‡ for the Al-Me exchange of ( iPr4)DBF(AlMe2)2, (iPr 4)Xan(AlMe2)2 and (Cy4)Xan(AlMe 2)2 was found to be 57.7, 59.7 and 62.5 kJ·mol -1, respectively. The observed exchange rate also showed a dependence on the concentration and the identity of the Lewis base, (DMSO>DMF>TMEDA>THF), with DMSO causing the most rapid exchange. The dialuminium complexes also successfully catalyze the addition of TMSCN to benzaldehyde. The rate of this addition reaction responded to structural and electronic changes of the added dialuminium complexes, the following dialuminium catalysts were rated from most to least effective; (iPr4)Xan(AlEtCl) 2, (iPr4)Xan(AlMe2)2, (iPr4)DBF(AlEtCl)2, (iPr 4)Xan(AliBu2)2, (iPr 4)DBF(AlMe2)2, (iPr4)DBF(Al iBu2)2. Mechanisms for both the exchange and catalytic processes are proposed. |
