The solution stability of copper(I) and silver(I) complexes with N-heterocyclic carbenes

The tris-benzimidazolium cage LH33+, in MeCN solution, in the presence of OH-, forms with Cu-I and Ag-I ions complexes of formula [M-I(LH)](2+), in which each metal is linearly coordinated by two carbenes and one imidazolium N-H fragment remains intact. To achieve two-coordination, the two N-heterocyclic moieties of the cage make a saloon-door type motion, with a conformationally costless rotation of ca. 30 degrees each. The two [MI(LH)](2+) complexes show high thermodynamic stability and are inert with respect to metal substitution, due to the mechanical constraints imposed by the ligating framework. Complexation with Cu-I and Ag-I with the reference unidentate carbene ligand Q, derived from the benzimidazolium precursor QH(+), was studied for comparison. Both metals in MeCN form 1:1 and 1:2 complexes with the carbene ligand Q according to two stepwise equilibria. Q complexes of both metals are labile with respect to metal substitution and those of AgI are more stable than those of CuI. A significant cooperative effect has been observed with the formation of the [Ag(I)Q(2)](+) complex.