Selective and Sequential Heterometallic Assembly in Amine/Imine Dendrimers

The use of dendrimers for metal cluster synthesis is an attractive approach for alloying multiple metal atoms into metal clusters because of the nanosized spaces and wide variety of metal salts that can be coordinated by dendrimers. However, the repeating structure of the dendrimers is generally not suitable for the precise assembly of different kinds of metal salts in them. Here, an amine/imine hybrid dendrimer was synthesized as a template for bimetallic assembly via the partial reduction of dendritic polyphenylazomethines (DPAs), which exhibit atomically precise metal salt assembly via 1:1 coordination between metal ions and their imine sites. The partial reduction of 12-imine DPA gave an 8-amine-4-imine hybrid dendrimer. The difference in the reactivity of the amine and imine sites enabled selective and sequential coordination to different metal salts. At first, Ir(III) was selectively coordinated to imine sites via the cyclometalation reaction. X-ray analysis demonstrated the selective assembly of Ir(III) at the imine sites. Then, Fe(III) or Ga(III) complexes were formed at the remaining amine sites. The stepwise metal ion assembly was also confirmed by theoretical calculations based on density functional theory. Therefore, our amine/imine hybrid dendrimer can form multinuclear metal complexes with atomic precision.