A Test of the Transition-Metal Nanocluster Formation and Stabilization Ability of the Most Common Polymeric Stabilizer, Poly(vinylpyrrolidone), as Well as Four Other Polymeric Protectants^†

Following an introduction to the nanocluster stabilization literature and DLVO (Derjaugin−Landau−Verwey−Overbeek) theory of colloidal stability, the most common steric stabilizer of transition-metal nanoclusters, poly(vinylpyrrolidone) (PVP), has been examined for its efficacy in the formation, stabilization, and subsequent catalytic activity of prototype, test case Ir(0)_n nanoclusters. First, the five criteria established previously for ranking nanocluster protectants for their nanocluster formation and stabilization ability were evaluated for 1 monomer equiv of 10000 average molecular weight (MW_av) PVP in the absence, and then presence, of the traditionally weakly coordinating anion BF₄^- as well as the absence and presence of the strongly coordinating, superior anionic stabilizer P₂W₁₅Nb₃O₆₂^9-, all in propylene carbonate solvent. It is found that neither 1 equiv of BF₄^- in propylene carbonate nor 1 monomer equiv of (undried) PVP alone allows for isolable and redissolvable nanoclusters without bulk Ir(0)_n metal formation. Careful predrying of the PVP, and by implication other polymers, is shown to be necessary for the formation and stabilization of the nanoclusters. Next, 40 monomer equiv of 10000 MW_av PVP and 1 equiv of BF₄^- in propylene carbonate are shown to allow isolable, redissolvable nanoclusters. Control experiments reveal little difference on nanocluster stabilization by 3500 or 55000 (i.e., vs 10,000) MW_av PVP, but yield interesting effects on nanocluster nucleation by the 3500 MW_av PVP, as well as by the polymer poly(bis(ethoxy)phosphazene) (PBEP). Four other key polymers reported in the literature to be nanocluster stabilizers are tested by the five criteria method for their efficacy in the formation and stabilization of Ir(0)_n nanoclusters (now in acetone due to the polymers' solubility) and in comparison to each other, specifically, poly(methyl methacrylate) (PMMA), poly(styrene) (PS), poly(methylhydrosilane) (PMHS), and PBEP. Only 40 monomer equiv dried PMMA allows isolable and redissolvable nanoclusters in acetone. Control/reference point experiments show that the electrostatic stabilizer P₂W₁₅Nb₃O₆₂^9- is superior to each of the five polymeric stabilizers studied herein in both acetone and propylene carbonate, at least for the test case of Ir(0)_n nanoclusters. Further controls show that 40 monomer equiv of PVP added to P₂W₁₅Nb₃O₆₂^9--stabilized nanoclusters has no discernible effect on the five criteria other than to reduce by ∼50% the nanocluster catalytic activity and total catalytic lifetime for cyclohexene hydrogenation. The main finding of this work is that DLVO theory as applied to nanocluster stabilization is fully supported; that is, surface-bound anions in high dielectric constant solvents provide superior stabilization. The importance of even traditionally weakly coordinating anions such as BF₄^- in nanocluster stabilization is a second, important finding of this work. The fact that HPO₄^2- has been shown to be a simple, cheap, commercially available, thermally robust, and ³¹P-NMR-handle-containing analogue of the more esoteric P₂W₁₅Nb₃O₆₂^9- stabilizer is also discussed in the 14 total Conclusions from this first study ranking polymeric stabilizers of modern transition-metal nanoclusters.