Speciation of Ternary Cobalt(II) Phenanthroline-Silica Surface Complexes as a Function of pH and Ligand Steric Bulk

Co(II) solution species containing 1 equiv of phenanthroline (phen), 2-methyl-1,10-phenanthroline (MMP), or 2,9-dimethyl-1,10phenanthroline (DMP) ligand formed inner-sphere surface complexes when grafted on silica. The speciation on the silica surface depended on both the pH of the grafting solution and the steric bulk of the ligand. [Co(DMP)]2+ formed tetrahedral surface adducts exclusively, with a 1:1 ligand-Co ratio. These surface adducts were first detectable at pH values above 5.1. [Co(MMP)]2+ and [Co (phen)]2+ formed exclusively octahedral adducts on the surface with a 1:1 ligand-Co ratio at pH values below 5. The [Co(MMP)]2+ complex formed a tetrahedral adduct initially at pH 6 and increasingly as the pH was raised. The [Co(phen)]2+ complex did not produce a comparable tetrahedral surface species under any conditions. Instead, mixtures of octahedral surface species with both 1:1 and 2:1 ligand-Co ratios began to form at pH values above 6. Taken together, the results indicated that the development of tetrahedral stereochemistry was strongly influenced by steric factors in the presence of a nitrogen-donating ligand. All three phenanthroline derivatives promoted surface binding of the Co(II) ion adducts, so that maximal binding occurred at lower pH values than for binding of [Co(H2O)6]2+, which formed exclusively tetrahedral adducts.