Understanding the relative binding ability of hydroxyfullerene to divalent and trivalent metals

Abstract

Metal contamination of water is a serious challenge faced by environmental chemists, but there is also economic value in the removal of metals for recycling or extraction. Our prior observation that hydroxyfullerenes [C₆₀O_x(OH)_y]ⁿ⁻ act as chelate agents to Fe³⁺ suggests that these material, or derivatives, may be used for co-precipitation. We report the removal of main group (Al³⁺, Ag⁺, Ca²⁺, and Zn²⁺) as well transition metal (Fe³⁺, Co²⁺, Cu²⁺, Mn²⁺, and Ni²⁺) and lanthanide (La³⁺ and Nd³⁺) ions from solution. The resulting complexes have been characterized by XPS, SEM, TEM, and DLS. The competitive binding with Fe³⁺ shows that the binding affinity with hydroxyfullerenes follows the order: Zn²⁺ > Co²⁺ > Mn²⁺ > Ni²⁺ > La³⁺ > Nd³⁺ > Cd²⁺ > Cu²⁺ > Ag⁺ > Ca²⁺ > Fe³⁺ > Al³⁺. The relative binding ability is dependent on ionic radius; however, divergent trends are observed for M²⁺ and M³⁺, i.e., for divalent ions the binding is stronger for smaller ions, while the opposite trend is observed for trivalent ions. Previously the coordination of hydroxyfullerene to metals was assumed to be analogous to a 1,2-diol or catechol. However, while ab initio calculations using [M(catecholate)_n]ⁿ⁻ (n = 2, 3) provide an explanation of the observed trend for M²⁺, the use of cis–cis-1,3,5-cyclohexanetriol and cis-1,3-cyclohexanediol as model ligands provides insight into the relative binding efficiency for M³⁺.