Nickel and cobalt hexamethylentetramine complexes (NO3)2Me(H2O)6(HMTA)2.4H2O (Me = Co2+, Ni2+): new molecular precursors for the preparation of metal dispersions

Nickel and cobalt hexamethylenetetramine (HMTA) complexes (NO3)2Me(H2O)6(HMTA)2.4H2O were prepared and characterized structurally by single-crystal X-ray diffraction. Both compounds crystallize in the triclinic P1 space group with the same structure. The structures are three-dimensional hydrogen-bonded supramolecular frameworks containing two-dimensional cationic assemblies connected with proton acceptors, which are noncoordinated anionic species (nitrate) and neutral HMTA molecules. Thermal decomposition of these compounds under an inert atmosphere leads to the high-surface-area metal-carbon foams containing nickel and cobalt nanoparticles embedded within onionlike carbon shells. The decomposition process as studied by thermal analysis and in situ X-ray absorption spectroscopy (XAS) measurements occurs as a rapid loss of oxygen in the form of CO, beginning at temperatures as low as 323 K. As the in situ XAS study demonstrated, the evolution of nickel and cobalt coordination spheres occurs with intermediate formation of amorphous metal carbides, which subsequently decompose to the metal nanoparticles covered with carbon shells.