Strategies for the anchoring of metal complexes, clusters, and colloids inside nanoporous alumina membranes

Two complementary strategies are presented for the anchoring of molecular p alladium complexes, of cobalt or platinum clusters or of gold colloids insi de the nanopores of alumina membranes. The first consists in the one step c ondensation of an alkoxysilyl functional group carried by the metal complex with the hydroxy groups covering the surface of the membrane pores. Thus, using the short-bite alkoxysilyl-functionalized diphosphane ligands (Ph2P)( 2)N(CH2)(3)Si (OMe)(3) (1) and (Ph2P)(2)N(CH2)(4)SiMe2(OMe)} (2) derived fr om (Ph2P)(2)NH (dppa) (dppa = bis(diphenylphosphanyl)amine), the palladium complexes [Pd(dmba){kappa (2)-P,P-(Ph2P)(2)N(CH2)(3)Si(OMe)(3)}] Cl (3) and [Pd(dmba){kappa (2)-P,P-(Ph2P)(2)N (CH2)(4)SiMe2(OMe)}]Cl (4) (dmba-H=dime thylbenzylamine). respectively, were tethered to the pore walls. After cont rolled thermal treatment, confined and highly dispersed palladium nanoparti cles were formed and characterized by transmission electron microscopy (TEM ), This method could not be applied to the cobalt cluster [Co-4(CO)(8)(mu - dppa){mu -P,P-(Ph2P)(2)N(CH2)(4)SiMe2(OMe)}] (7) owing to its too limited s olubility. However, its anchoring was achieved by using the second method w hich consisted of first derivatizing the pore walls with 1 or 2. The covale nt attachment of the diphosphane ligands provides a molecular anchor that a llows subsequent reaction with the cluster [Co-4(CO)(10)(mu -dppa)] 6 to ge nerate anchored 7 and this step was monitored by UV/Vis spectroscopy. In ad dition, the presence of carbonyl ligands in the cluster provides for the fi rst time a very sensitive spectroscopic probe in the IR region which confir ms both cluster incorporation and the retaining of its molecular nature ins ide the membrane. The presence of the bridging dppa ligand in 6 provides ad ditional stabilization and accounts for the selectivity of the procedure. U sing this method, platinum clusters (diameter ca. 2 nm) and gold colloids ( diameter ca. 13 nm) were immobilized after passing their solution through t he functionalized membrane pores. The resulting membranes were characterize d by TEM which demonstrated the efficiency of the complexation and showed t he high dispersion of the metal loading. The successful application of thes e methods has demonstrated that nanoporous alumina membranes are not only u nique supports to incorporate metal complexes, clusters, or colloids but ca n also be regarded as functional matrices or microreactors, thus opening ne w fields for applications.