Postsynthetic modification of dicarbene-derived metallacycles via photochemical [2 + 2] cycloaddition.

Molecular squares obtained from two olefin-bridged bis(NHC) ligands, NHC-Ar-C═C-Ar-NHC, and two Ag(+) or Au(+) ions undergo postsynthetic modifications via a UV-irradiation-initiated [2 + 2] cycloaddition reaction to yield the corresponding cyclobutane-bridged dinuclear tetrakis(NHC) complexes. The tetrakis(NHC) ligand can be liberated from the Ag(I) complexes as the tetraimidazolium salt. For the Au(I) complexes, the substituents at N3 and N3' of the dicarbene ligands determine the outcome of the reaction in the solid state.