Synthesis and reactivity of Li and TaMe3 complexes supported by N,N'-bis(2,6-diisopropylphenyl)-o-phenylenediamido ligands.

Abstract

The dilithium complex of N,N'-bis(2,6-diisopropylphenyl)-o-phenylenediamide, [Li2L(thf)3], reacts with TaMe3Cl2 in THF/Et2O to yield [Li(Et2O)(thf)LTaMe3Cl] in which the phenylene backbone of (2-) is bound η(4) to the Ta centre. This dinuclear species reacts with MeLi to yield the tetramethyltantalum complex [Li(Et2O)(thf)LTaMe4]. Double deprotonation of N,N'-bis(2,6-diisopropylphenyl)(4,5-dimethyl)-o-phenylenediamine (H2L') in Et2O yielded the dilithium complex [Li2L'(OEt2)2]. The two additional methyl groups on L'(2-) change the observed reactivity towards TaMe3Cl2: rather than bridging between Ta and Li, ligand oxidation occurs to afford mononuclear [LiL'(OEt2)]. This monolithium radical species, which was characterized by EPR spectroscopy, can also be synthesized using the more conventional oxidant AgBF4. Double deprotonation of H2L with KCH2Ph in toluene followed by reaction with TaMe3Cl2 furnished [TaLMe3]. Preliminary reactivity studies show [TaLMe3] reacts with unsaturated substrates N,N'-dicyclohexylcarbodiimide and mesityl azide to undergo migratory insertion into one of the Ta-C bonds: the corresponding amidinate and triazenido ligands are generated. When subjected to UV irradiation, [TaLMe3] undergoes reduction accompanied by loss of a methyl group to yield the dimeric species [TaLMe2]2.