Superparamagnetic and spin glass behavior in mesoporous niobium oxide bis(cyclopentadienyl)nickel composites

Herein we report the synthesis and characterization of a new family of mesoporous niobium oxide bis(cyclopentadienyl)nickel composites which exhibit superparamagnetic or spin glass behavior depending on the different loading levels of the organometallic in the pores. XRD and nitrogen adsorption studies confirm the retention of the mesostructure in these new materials upon introduction of the guest species. The Nb 3d region of the XPS provides evidence for the slight reduction of the Nb(V) oxide walls while the Ni 2p region shows the presence of both neutral and cationic bis-cyclopentadienyl nickel species with peaks at 856.3 eV and 861.9 eV respectively, as well as two more highly oxidized species at 865.5 eV and 867.3 eV. The presence of neutral and cationic bis(cyclopentadienyl)nickel is further confirmed by EPR. SQUID magnetic measurements show superparamagnetic behavior for the material with a Ni∶Nb ratio of 0.07∶1, while at a ratio of 0.1∶1 the material shows spin glass behavior, a phenomenon normally associated with amorphous metal alloys. The difference in magnetic behavior is rationalized by an increased density of unpaired spins in the pore structure.