Copper
and Silver Complexes of Tris(triazole)amine
and Tris(benzimidazole)amine Ligands: Evidence that Catalysis of an
Azide–Alkyne Cycloaddition (“Click”) Reaction
by a Silver Tris(triazole)amine Complex Arises from Copper Impurities
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Abstract
The
synthesis and characterization of a silver complex of the tripodal
triazole ligand, tris(benzyltriazolylmethyl)amine (TBTA, <b>L</b><sup><b>1</b></sup>), that is used as promoter to enhance Cu<sup>I</sup>-catalyzed azide–alkyne cycloaddition (CuAAC) reactions
is reported. X-ray analysis of the silver(I) complex with <b>L</b><sup>1</sup> reveals a dinuclear cation, [Ag<sub>2</sub>(<b>L</b><sup><b>1</b></sup>)<sub>2</sub>]<sup>2+</sup>, that is essentially
isostructural to the copper(I) analogue. While the [Ag<sub>2</sub>(<b>L</b><sup><b>1</b></sup>)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub> complex provides catalysis for the azide–alkyne
cycloaddition process, evidence is presented that this arises from
trace copper contamination. The synthesis of silver(I), copper(II),
and copper(I) complexes of a second tripodal ligand, tris(2-benzimidazolymethyl)amine
(<b>L</b><sup><b>2</b></sup>), which is used to enhance
the rate of CuAAC reactions, is also reported. X-ray crystallography
of the Cu<sup>I</sup> complex [Cu<sup>I</sup><sub>3</sub>(<b>L</b><sup><b>2</b></sup>)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>](BF<sub>4</sub>)<sub>3</sub> offers structural insight into previous
mechanistic speculation about the role of this ligand in the CuAAC
reaction
