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

The tris-benzimidazolium cyclophane cage [2,4,6-Me3C6((CH2)2C7H5N2)3C6Me3-2,4,6]3+ (LH33+), in MeCN soln., in the presence of OH-, forms with Cu(I) and Ag(I) ions complexes [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 ∼30° each. The two [M(I)(LH)]2+ complexes show high thermodn. stability and are inert with respect to metal substitution, due to the mech. constraints imposed by the ligating framework. Complexation with Cu(I) and Ag(I) with the ref. unidentate carbene ligand N,N'-dibenzylbenzimidazolylidene (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 equil. Q complexes of both metals are labile with respect to metal substitution and those of Ag(I) are more stable than those of Cu(I). A significant cooperative effect was obsd. with the formation of the [Ag(I)Q2]+ complex. The mol. structures of LH3Br3, [Cu(LH)](ClO4)2 and [Ag(LH)](NO3)2 were detd. by x-ray crystallog.