Bimetallic nanocluster with atomic precision has gained widespread attention due to its unique synergism. The coreless Au4Cu5 bimetallic nanoclusters were selected as models to explore the relationship between their microstructure and performance, and compare with the coreless monometallic nanoclusters, core–shell nanoclusters, and single atom catalyst (SAC). The experimental results show that the coreless bimetallic nanocluster catalyst Au4Cu5/activated carbon (AC) exhibits high activity and stability in the Ullmann C–O coupling reaction, much higher than coreless monometallic nanoclusters (Au11/AC and Cu11/AC), core–shell nanoclusters (Au25/AC, Cu25/AC, and Au1Cu24/AC), and single atom catalysts (Au SAC and Cu SAC). Moreover, the coreless Au4Cu5/AC catalyst efficiently catalyzed the Ullmann C–O coupling of benzyl alcohol for the first time. This structure–activity relationship was successfully extended to other coreless bimetallic systems, such as Au4Cu4/AC nanocluster, and it is expected to provide new insights for the design of bimetallic catalysts with well-defined performance.