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Nanocluster Formation and Stabilization Fundamental Studies. 2. Proton Sponge as an Effective H+ Scavenger and Expansion of the Anion Stabilization Ability Series

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posted on 2002-09-06, 00:00 authored by Saim Özkar, Richard G. Finke
The two main goals of the current work are:  (i) to test the effects of Proton Sponge as a H+ scavenger and (ii) to test and rank the relative efficacy of the anions (Y-) listed below for their relative ability to allow the formation, stabilization (including isolability), high catalytic activity, and long catalytic lifetime in the following, more generalizable transition metal nanocluster formation reaction:  1.0[Bu4N]qY + 1.0[(1,5-COD)Ir(I)(CH3CN)2]BF4 + 2.5H2 → 1.0cyclooctane + 1/n{[Bu4N]nq[Ir(0)n·Y]} + H+BF4- + 2CH3CN. The anions investigated, Y-, are the tri-Nb(V)-substituted polyoxoanion SiW9Nb3O407-, the tri-Ti(IV)-substituted polyoxoanion ([P2W15(TiOH)3O59]9-)n (n = 1, 2), citrate trianion (C6H5O73-), acetate (OAc-), trimetaphosphate (P3O93-), chloride (Cl-), and hydroxide (OH-). The five criteria we developed recently (Özkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796) are used to determine the effects of Proton Sponge (in comparison to control experiments employing Bu4N+OH-) and to rank the Y- anions. The results reveal that Proton Sponge is an effective, weakly coordinating, and generally preferred Brønsted base in comparison to the more basic and more coordinating OH-, at least for the formation and catalytic properties of Ir(0) nanoclusters in acetone with Bu4N+ and for other conditions examined. The results also yield an expanded anion series of the relative ability of anions to promote the kinetically controlled formation, stabilization, and good catalytic properties of Ir(0) nanoclusters in acetone with Bu4N+ cations:  P2W15Nb3O629- ∼ [(P2W15Nb3O61)2O]16- ∼ SiW9Nb3O407- ∼ ([P2W15(TiOH)3O59]9-)n (n = 1, 2) > C6H5O73- > [−CH2−CH(CO2-)−]nn- ∼ OAc- ∼ P3O93- ∼ Cl- ∼ OH-. The essence of this series, the first of its kind, is:  Brønsted basic polyoxoanions > citrate3- > other common anions used in nanocluster syntheses. The results allow three other (five total) conclusions, results that should assist others in picking the best anions for the formation and stabilization of their own transition metal nanoclusters.

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