Amine mediated proton transfer reaction and C–Cl bond activation of solventchloroform by a trinuclear copper(ii) complex of a glucopyranosylamine derived ligand

Abstract

An amine mediated C–Cl bond activation process of the solvent chloroform has been explored by a coordinatively labile trinuclear Cu(II) complex, [Cu₃(L¹)₂(MeOH)(H₂O)] (1), derived from N-(3-tert-butyl-2-hydroxybenzylidene)-4,6-O-ethylidene-β-D-glucopyranosylamine (H₃L¹). The effect of activation is extremely high with methylamine, resulting in the formation of [Cu(MeNH₂)₅]Cl₂ (2) and [Cu(L²)₂] (3; HL² = 2-tert-butyl-6-[(methylimino)methyl]phenol), however, under identical conditions it is moderate with ethylamine resulting in the isolation of crystals of the intermediate amine bound trinuclear copper(II) complex, [Cu₃(L¹)₂(EtNH₂)₂(MeOH)₂] (5), which was further converted into the mononuclear complex, [Cu(HL¹)(EtNH₂)] (6), in a novel crystal-to-crystal transformation. The successive isolation of the ethylamine-bound tri- and mononuclear complexes, 5 and 6, supported the occurrence of proton transfer reactions, which might be a key step in C–Cl bond activation. The primary and secondary amines, 2-aminomethylpyridine, N,N′-dimethylethylenediamine, and 1,4,7-triazacyclononane, also having chelating features further enhance the rate of activation. No activation has been noted in the case of triethylamine and N,N,N′,N′-tetramethylethylenediamine. Formation of a carbene-trapped compound, 2,6-xylyl isocyanide, was confirmed in the reaction of complex 1 with 1,4,7-triazacyclononane and 2,6-xylidine in CHCl₃, suggesting that the C–Cl bond cleavage led to the generation of dichlorocarbene. In addition, the mononuclear complex 6 has been transformed into a homotrinuclear complex [Cu₃(L¹)₂(MeOH)₂] by treatment with Cu(II) ions in MeOH/CHCl₃, suggesting the possibility that the former could be regarded as a suitable metalloligand for heterotrimetallic complex synthesis.