Photochemical Modification of Carbene-Based Metallacycles: A Facile Route to Polycarbene Complexes and Their Derivatives

The modification of dinuclear silver metallcycles via photochemical [2 + 2] cycloaddition is described. Reaction of benzimidazole with trans-4,4′-dibromostilbene gives trans-4,4′-bis­(1H-benzo­[d]­imidazolyl)­stilbene (L), which after subsequent dialkylation with alkyl bromides and anion exchange yields the corresponding dibenzimidazolium salts H2-1(PF6)2 and H2-2(PF6)2. The dibenzimidazolium salts react with Ag2O to give the dinuclear silver­(I) tetracarbene metallacycles Ag2(1)2(PF6)2 and Ag2(2)2(PF6)2 in high yield. Irradiation (UV light, λ 365 nm) of Ag2(1)2(PF6)2 and Ag2(2)2(PF6)2 in [D6]­DMSO resulted in rapid conversion into the corresponding dinuclear cyclobutane-carbene complexes Ag2(3)­(PF6)2 and Ag2(4)­(PF6)2 quantitatively. The cyclobutane-bridged polycarbene precursors were isolated in good yields as their tetrabenzimidazolium salts.