Cu(0) Nanoclusters Derived from Poly(propylene imine) Dendrimer Complexes of Cu(II)

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Abstract
A family of diaminobutane core, poly(propylene imine) dendrimers coordinated to Cu(II), DAB-Amn-Cu(II)x (n = 4, 8, 16, 32, 64, x = n/2), was studied by means of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies. The geometry of the dipropylene triamine (dpt)-Cu(II) end-group complexes for all dendrimer generations is reported for the first time and is found to be that of a square-based pyramid with each Cu ion bound to three nitrogen atoms (Cu−N distance ∼2.03 Å) of the dpt end group of the dendrimer. An oxygen atom residing 1.96 Å from the Cu ion also occupies the equatorial plane, and the pyramid is completed by an axial oxygen at ∼2.65 Å. In addition, we report for the first time that reduction of the Cu(II)-dendrimer complexes with NaBH4 yields DAB-Amn-Cu(0)cluster species. Transmission electron microscopy (TEM) studies of the reduced species demonstrate that there is a systematic decrease in the size of the generated Cu clusters with increasing dendrimer generation. Additionally, it was found that the size of the nanoclusters is a function of the n/x ratio of the DAB-Amn-Cu(II)x precursor, with highly monodisperse, extremely small nanoclusters (rcluster = 8.0 ± 1.6 Å) obtained with n = 64 and x = 16. EXAFS and XANES measurements on the reduced DAB-Amn-Cu(0)clustercorroborate the TEM data, and provide additional information on the possible encapsulation of the Cu nanoclusters by the dendrimers.

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