Superparamagnetic ceramic nanocomposites: Synthesis and pyrolysis of ring-opened poly(ferrocenylsilanes) inside periodic mesoporous silica

[1]Silaferrocenophanes fcSiMe(2) (1) and fcSi(CH2)(3) (3) (fc = Fe(eta(5)-C5H4)(2)) were incorporated into the well-ordered, hexagonal channels of mesoporous silica (MCM-41). Characterization of the composite materials indicates the presence of ring-opened monomer, oligomer, and polymer, and, in the case of 1, also free monomer. Polymerization of monomer 1 inside the channels of MCM-41 was demonstrated by DSC. However, in the case of 3 only the ferrocenophane ring was completely opened, and the silacyclobutane ring was mostly intact. When pyrolyzed at 900 degrees C in a nitrogen atmosphere, the mesoporous silica/ferrocenylsilane composites formed Fe nanoparticles in the ca. 30-40 Angstrom channels of MCM-41. Characterization of the magnetic properties of the product formed when 1 is pyrolyzed in the channels of mesoporous silica indicated that the clusters were superparamagnetic. Magnetization versus field data at 300 K were fit to a sum of two Langevin functions, implying the presence of two distinct magnetic phases. From the magnetization data, the two phases are best described as Fe nanoparticles with diameters of 50-64 Angstrom coated with a thin (ca. 4-6 Angstrom) oxide layer. The product obtained from the pyrolysis of poly(ferrocenylsilane) 2 in the channels of MCM-41 contains much smaller Fe particles than those observed in a pyrolysis of bulk 2, indicating that the silica channels affect the nucleation and growth of the Fe nanoparticles.