Molecular insights for how preferred oxoanions bind to and stabilize transition-metal nanoclusters: a tridentate, C 3 symmetry, lattice size-matching binding model

Abstract

The recent discovery of an anion efficacy series for the formation and stabilization of transition-metal Ir(0) n nanoclusters, specifically P 2W 15Nb 3O 62 9− ∼ SiW 9Nb 3O 40 7− > C 6H 5O 7 3− > [–CH 2–CH(CO 2 −)–] n n− ∼ OAc − ∼ P 3O 9 3− ∼ Cl − ∼ OH −—that is, polyoxoanions > citrate 3− > other commonly employed nanocluster stabilizing anions, raises the question of what are the underlying factors behind this preferred order of stabilizers? A brief discussion of three relevant nanocluster papers in the literature, plus a concise summary of the relevant interfacial electrochemistry and surface science literature of C 3 symmetry SO 4 2− binding to Ir(1 1 1) (as well as to Rh(1 1 1), Pt(1 1 1), Au(1 1 1) and Cu(1 1 1 1)), are presented first as key background for the lattice size-matching model which follows in which tridentate anions coordinate to transition-metal nanocluster surfaces. A table of nanocluster formation and stabilization data for tridentate oxoanion stabilizers is presented, results which allow two fundamental, previously unavailable, important insights (out of 10 total insights): (i) the premier anionic stabilizers of transition-metal(0) nanoclusters present a tridentate, facial array of oxygen atoms for coordination to the metal(0) surface; and (ii) the preferred tridentate oxoanion stabilizers of nanoclusters are those that have the best match between the ligand O–O and surface Ir–Ir distances, all other factors being equal—that is, there is a previously unappreciated, geometric, anion-to-surface-metal lattice-size-matching component to the best anionic stabilizers of transition-metal nanoclusters. These are the first molecular-level insights for how the to-date premier tridentate, anionic stabilizers of transition-metal nanoclusters achieve their higher level of stabilization—a non-trivial advance since there was a lack previously of molecular-level insights into how transition-metal nanoclusters are stabilized. Four experimentally testable predictions of the C 3 symmetry, lattice size-matching model for nanocluster M(1 1 1) surfaces are presented and briefly discussed. One key prediction is that HPO 4 2− is a heretofore unappreciated simple, effective and readily available stabilizer of Ir(0) and other transition-metal nanoclusters where there is a lattice-size match between the O–O and the surface M–M distances. Recent experimental evidence is summarized revealing that this prediction is, in fact, true—that is, the third key, new finding of this work is (iii) the first rational design of a new nanocluster stabilizer, HPO 4 2−, one shown to be as good a stabilizer as the common nanocluster stabilizer citrate 3−. The C 3 symmetry, lattice size-matching model is significant in seven additional ways which are detailed in the text and summary which follows.