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Inorg Chem. 2006 Oct 2;45(20):8382-93.

Nanocluster formation and stabilization fundamental studies: investigating "solvent-only" stabilization en route to discovering stabilization by the traditionally weakly coordinating anion BF4- plus high dielectric constant solvents.

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Colorado State University, Department of Chemistry, Fort Collins, Colorado 80523, USA.


The nanocluster literature contains a wide variety of nanocluster stabilizing agents. In addition to the plethora of putative stabilizing additives, 12 claims appear of "solvent-only" stabilization of transition-metal nanoclusters-a hypothesis that is tested for the first time as part of the present studies. When the two main modes of nanocluster stabilization, electrostatic and steric are considered, "solvent-only" stabilization can only be steric (i.e., is not electrostatic). Solvent-only stabilization would, therefore, require that a strongly coordinated, perhaps even kinetically nonlabile, solvent be present on the nanocluster surface. Hence, an investigation has been conducted into potential sources for the stabilization of prototype Ir(0)n transition-metal nanoclusters prepared from [(1,5-COD)Ir(CH3CN)2][BF4] in five different solvents, with a special focus on the formulation and testing of alternative hypotheses regarding the true source of the nanocluster stabilization in putative solvent-only stabilization conditions. Seven total hypotheses are tested with five being initially ruled out; they are, namely, stabilization by (i) trace chloride (ii), surface hydrides, (iii) scavenged charge, (iv) solvent oxidative addition reactions with the nanocluster surface, or (v) polymerized solvent. This led in turn to two additional main alternative hypotheses: (vi) nanocluster surface ligation by high-donor number solvents (i.e., in the absence of anions) and (vii) nanocluster stabilization by surface-coordination of the traditionally weakly coordinating anion BF(4-). Our results reveal a significant contribution to nanocluster stability from the traditionally weakly coordinating BF(4-) in high dielectric constant solvents, such as propylene carbonate. Literature claims of solvent-only nanocluster stabilization are not supported by our findings. Overall, DLVO (Derjaugin-Landau-Verwey-Overbeek) theory of colloidal stability is supported and found to apply to even traditionally weakly coordinating anions.

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