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.