Cu(0) nanoclusters derived from poly(propylene imine) dendrimer complexes of Cu(II)

A family of diaminobutane core, poly(propylene imine) dendrimers coordinated to Cu(II), DAB-Am(n)-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 approximately 2.03 A) of the dpt end group of the dendrimer. An oxygen atom residing 1.96 A from the Cu ion also occupies the equatorial plane, and the pyramid is completed by an axial oxygen at approximately 2.65 A. In addition, we report for the first time that reduction of the Cu(II)-dendrimer complexes with NaBH4 yields DAB-Am(n)-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-Am(n)-Cu(II)x precursor, with highly monodisperse, extremely small nanoclusters (r(cluster) = 8.0 +/- 1.6 A) obtained with n = 64 and x = 16. EXAFS and XANES measurements on the reduced DAB-Am(n)-Cu(0)(cluster) corroborate the TEM data, and provide additional information on the possible encapsulation of the Cu nanoclusters by the dendrimers.