NANOMETER-SIZE CLUSTERS AS EXPERIMENTAL MODELS--REACTION KINETICS AT BRUCITE-LIKE STRUCTURES

Geochemist have long used a few dozen atoms to answer questions about mineral reactivities, but almost always in computer studies. However, questions about Earth materials can often be answered directly through the use of real nanometer-size aqueous clusters that are stable in solution and that can be spectroscopically probed in ways that colloids and minerals cannot.

Here we review recent work on aqueous clusters having a brucite-like structure. These Co(II)-centered materials have attracted enormous attention because they efficiently catalyze the electrolytic splitting of water into H₂ and O₂ fuels at near-neutral pH. Our interest in these materials is also geochemical---because these structures are so well representated in clays, we can isolate the key functional groups using large tungstate ligands and probe the kinetics using high-pressure ¹⁷O-NMR and EPR. With these tools we show that the kinetics of ligand exchanges at the edges of these brucite-like structures fall into the same range as for smaller monomer ions.

The compounds were: [Co₄(H₂O)₂(B-α-PW₉O₃₄)₂]^10- and the larger [Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16- ions. Using a range of methods, we find that the molecules are unstable in acidic solutions, but not equally so -- for the smaller molecule, evidence suggests that it forms a similar structure with partial vacancy of one or more of the Co(II) metals in the sandwich, possibly releasing Co(H₂O)₆²⁺, at pH<5. No such Co(II) monomer, however was detected at pH~5-7 for either sandwich compound, leading to reliable estimates of the rate parameters for exchange of a water molecule using ¹⁷O-NMR. At pH=5, we estimate: k²⁹⁸ = 1-2•10⁶ s^-1, ΔH = 30-38 kJ mol^-1 and ΔS = 3 to -30 J mol^-1 K^-1. The largest source of uncertainty is precise knowledge of the molecular weight of the ions. These rates compare well with those for the [Co(H₂O)₆]²⁺ ion: k²⁹⁸ = 1 - 3.2•10⁶ s^-1, ΔH = 47 kJ mol^-1 and ΔS = 37 J mol^-1 K^-1. The kinetics of ligand substitution are relatively unaffected by coordination into a brucite-like layer.