Identifying Multinuclear Organometallic Intermediates in On-Surface [2+2] Cycloaddition Reactions

We investigate the on-surface [2+2] cycloaddition reaction of 2,3,6,7,10,11-hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu-dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo-silver framework on Ag(111) and an organo-copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo-silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo-copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo-gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo-copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.