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Ranking the Lacunary (Bu4N)9{H[r2-P2W17O61]} Polyoxometalate’s Stabilizing Ability for Ir(0)n Nanocluster Formation and Stabilization Using the Five-Criteria Method Plus Necessary Control Experiments Christopher R. Graham, Lisa Starkey Ott, and Richard G. Finke* Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523 ReceiVed August 4, 2008. ReVised Manuscript ReceiVed October 7, 2008 The primary goal of the present studies is to rank the monoprotonated, lacunary Wells-Dawson-type polyoxometalate {H[R2-P2W17O61]}9- as a stabilizing anion for the formation and subsequent stabilization of Ir(0)n nanoclusters in ¨ zkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, acetone using the five-criteria method developed previously (O 5796). A related goal is to compare this potentially tetradentate, three W-O- plus one W-OH ligand {H[R2P2W17O61]}9- system to the nanocluster formation and stabilization abilities of the present “gold standard” polyoxometalate, [P2W15Nb3O62]9-, with its established tridentate, three Nb-O-Nb ligating system. A comprehensive table of 54 references at present examining polyoxometalates (POMs) as additives/stabilizers of nanoclusters is also provided as Table S1 of the Supporting Information. To accomplish the above-noted two main goals, the organicsolvent-soluble tetrabutylammonium salts, (Bu4N)8.4{H1.6[R2-P2W17O61]} · 1.4H2O, (Bu4N)9{H[R2-P2W17O61]}, and (Bu4N)9[P2W15Nb3O62] were prepared and their basic structures (by IR) and purity (by 31P NMR, plus elemental analysis where appropriate) were determined. The parent Wells-Dawson POM (Bu4N)6[R-P2W18O62] was also prepared, characterized, and then used as a control of a polyoxometalate with little surface anionic charge density, one therefore expected to be a poor stabilizer for at least metal(0)n, overall neutral core, nanoclusters. Also prepared and characterized was (Bu4N)4{H3[PW11O39]}, but its attempted deprotonation by (Bu4N)OH with direct 31P NMR monitoring did not yield a clean product by 31P NMR, so studies with this second lacunary POM were deemphasized. The resultant POMs of known composition, protonation state, and thus overall charge were then evaluated by the five criteriasthe one presently available methodsfor their ability to promote the kinetically controlled formation, stabilization, and subsequent catalytic activity of prototype Ir(0)n nanoclusters. A number of additional control experiments necessary to provide confidence in the results are also reported. One main finding is that the efficacy of the POMs studied herein, as stabilizers for Ir(0)n nanoclusters in acetone solvent, is [P2W15Nb3O62]9- > {H[R2-P2W17O61]}9- . [R-P2W18O62]6∼ {H3[PW11O39]}4-, the potentially tetradentate, lacunary {H[R2-P2W17O61]}9-proving somewhat less efficacious as a stabilizer than the tridentate Nb-O-Nb containing [P2W15Nb3O62]9-POM. Another finding is that the degree of protonation and the overall charge of the POM matter, the more highly charged {H[R2-P2W17O61]}9- POM being a better stabilizer then the more protonated, less charged {H2[R2-P2W17O61]}8-. Two additional important findings are better insights into the inherent errors underlying three of the five criteria, and thus the use of the five-criteria method itself, and further evidence supporting the hypothesis that future studies of direct measurements of nanocluster agglomeration rate constants k3 and k4 (Ott, L. S.; Finke, R. G. Chem. Mater. 2008, 20, 2592-2601) should prove valuable.
Introduction Nanocluster chemistry1 is of current interest2-8 for applications including quantum dots,9 chemical sensors,10 components in * To whom correspondence should be addressed. E-mail: rfinke@lamar. colostate.edu. (1) Reviews of nanoclusters (see also): (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (c) Green, M. Chem. Commun. 2005, 24, 3002. (d) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (e) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (f) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (g) Kra´lik, M.; Biffis, A. J. Mol. Catal. A: Chem. 2001, 177, 113. (h) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. (i) Fendler, J. H., Ed. Nanoclusters and Nanostructured Films; Wiley-VCH: Weinheim, Germany, 1998. (j) Fu¨rstner, A., Ed. ActiVe Metals: Preparation, Characterization, and Applications; VCH: Weinheim, Germany, 1996. (k) Bradley, J. S. In Clusters and Colloids. From Theory to Applications; Schmid, G., Ed.; VCH: New York, 1994; pp 459-544. (l) Schmid, G. Chem. ReV. 1992, 92, 1709. (m) A superb series of papers is available in: Faraday Discussions 1991, 92, 1-300. (n) Schmid, G. In Aspects of Homogeneous Catalysis; Ugo, R., Ed.; Kluwer: Dordrecht, The Netherlands, 1990; Chapter 1. (o) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A., III; Kaldor, A.; Louie, S. G.; Moscovits, M.; Peercy, P. S.; Riley, S. J.; Siegel, R. W.; Spaepen, F.; Wang, Y. J. Mater. Res. 1989, 4, 704. (p) Henglein, A. Chem. ReV. 1989, 89, 1861. (q) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517. (r) Jena, P.; RaoB. K.; Khanna, S. N. Physics and Chemistry of Small Clusters; Plenum: New York, 1987.
industrial lithography,11 and catalysis.7,12-14 A central issue underpinning the whole nanocluster literature, however, is that a very large assortment of anions,15-17 solvents,18-20 polymers,21-23 dendrimers,24-26 siloxanes,27 and other additives (2) Schmid, G.; Chi, L. AdV. Mater. 1998, 10, 515. (3) Schmid, G.; Baumle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. ReV. 1999, 28, 179. (4) Aiken, J. D., III; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1. (5) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (6) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (7) Astruc, D.; Lu, F.; Aranzes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (8) Ott, L. S.; Finke, R. G. Coord. Chem. ReV. 2007, 251, 1075. (9) Simon, U.; Scho¨n, G.; Schmid, G. Angew. Chem., Int. Ed. 1993, 32, 250. (10) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (11) Reetz, M. T.; Winter, M.; Dumpich, G.; Lohau, J.; Friedrichowski, S. J. Am. Chem. Soc. 1997, 199, 4539. (12) Lin, Y.; Finke, R. G. J. Am. Chem. Soc. 1994, 116, 8335. (13) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891. (14) Bo¨nnemann, H.; Nagabhushana, K. S. Colloidal Nanoparticles in Catalysis. In Surface and Nanomolecular Catalysis; Richards, R., Ed.; CRC: Boca Raton, FL, 2006; pp 63-93. ¨ zkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796. (15) O ¨ zkar, S.; Finke, R. G. Langmuir 2002, 18, 7653. (16) O (17) Turkevich, J.; Kim, G. Science 1970, 169, 873. (18) Chaudret, B.; Vidoni, O.; Philippot, K.; Amiens, C.; Balmes, O.; Malm, J.-O.; Bovin, J.-O.; Senocq, F.; Casanove, M.-J. Angew. Chem., Int. Ed. 1999, 38, 3736.
10.1021/la8025254 CCC: $40.75 2009 American Chemical Society Published on Web 01/09/2009
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(including, for just a few more examples, reverse micelles,28 ionic liquids,29-31 and resorcinarene molecules32,33) are all often explicitly or implicitly claimed to be superior stabilizers of at least transition-metal nanoclusters. This myriad of purportedly nanocluster-stabilizing additives has been referred to in the literature as a “dizzying variety”6 of putative stabilizers. Claims of transition-metal nanocluster stabilization are often made based primarily on a TEM (transmission electron micrograph) showing anywhere from a few to a few hundred nanoclusters. Hence, an important question for the whole field of transition-metal nanocluster chemistry is which among this “dizzying variety”6 is truly the bestsor even an adequatesnanocluster stabilizer34 in solution, for a given application, and based on what evidence? With the goal of answering the above question in at least an initial way and with a focus toward catalytically active transitionmetal nanoclusters (vide infra), in 2002 we developed a method for determining which nanocluster stabilizers yield the best control over the nanocluster nucleation, growth, and subsequent stability for both isolation and catalytic activity, the so-called five-criteria method.15 The five criteria are (i) the level of kinetic control during the nanocluster formation reaction as measured by the k2/k1ratio of autocatalytic surface-growth (k2) to nucleation (k1)slarger values indicating a higher level of kinetic control in the nanocluster formation reaction,15,35 (ii) the nanoclusters’ size distribution as determined by TEM (( 15% having been defined previously as “near-monodisperse” nanoclusters4), (iii) the ability to isolate from solution, and ideally bottle, the nanoclusters for future use without detectable agglomeration to bulk metal once redispersed, (iv) the catalytic hydrogenation activity of the isolated nanoclusters once redissolved in solution with fresh cyclohexene substrate and hydrogen, and (v) the total catalytic lifetime for the test reaction of cyclohexene hydrogenation using freshly prepared, in situ generated nanoclusters. The five-criteria method improves considerably upon past qualitatiVe literature methods for ranking colloidal stabilizers (then in water36,37), and at present (19) Collier, P. J.; Iggo, J. A.; Whyman, R. J. Mol. Catal. A: Chem. 1999, 146, 149. (20) Shiraishi, Y.; Arakawa, D.; Toshima, N. Eur. Phys. J. E 2002, 8, 377. (21) Romero-Cano, M. S.; Martin-Rodriguez, A.; de las Nieves, F. J. Langmuir 2001, 17, 3505. (22) Kim, A. Y.; Berg, J. C. Langmuir 2002, 18, 3418. (23) Fritz, G.; Schadler, V.; Willenbacher, N.; Wagner, N. J. Langmuir 2002, 18, 6381. (24) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (25) Dagani, R. Chem. Eng. News 1999, 77, 33. (26) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (27) Chauhan, B. P. S.; Rathore, J. S.; Chauhan, M.; Krawichz, A. J. Am. Chem. Soc. 2003, 125, 2876. (28) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (29) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R J. Am. Chem. Soc. 2002, 124, 4228. (30) Itoh, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 3026. (31) Cassol, C. C.; Umpierre, A. P.; Machado, G.; Wolke, S. I.; Dupont, J. J. Am. Chem. Soc. 2005, 127, 3298. (32) Balasubramanian, R.; Beomseok, K.; Tripp, S. L.; Wang, X.; Lieberman, M.; Wei, A. Langmuir 2002, 18, 3676. (33) Tripp, S. L.; Pusztay, S. V.; Ribbe, A. E.; Wei, A. J. Am. Chem. Soc. 2002, 124, 7914. (34) Colloidal/nanocluser stability is often discussed at least initially in terms of DLVO (Derjaugin-Landau-Verwey-Overbeek) theory in which that stabilization is achieved through a delicate balance of interparticle Coulombic repulsion opposing van der Waals attraction. Hence, DLVO theory predicts that anions adsorbed on the coordinatively unsaturated, electrophilic nanocluster surface are a key to stability, providing the Coulombic repulsion component opposing van der Waals attraction between the nanoclusters (accordingly, DLVO-type stabilization is also commonly referred to as “electrostatic” stabilization). In addition, anions are a component of the diffuse layer of ions surrounding and stabilizing nanoclusters. This diffuse layer is the Debye layer (denoted by 1/κ; values are typically in nm). Steric, as well as “electrosteric” stabilization of colloidal/ nanocluster particles is also known (ref 8). (b) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids, 2nd ed.; Dover Publications, Inc.: Mineola, NY, 1999. (35) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382.
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is the only more general method for ranking modern transitionmetal nanocluster stabilizers in organic solvents. However, the five-criteria method is still relatively untested versus anions or other stabilizers that provide, for example, very different observables for those five criteria. In addition, the five-criteria method is not well-tested enough so that one knows what level of differences (i.e., inherent errors) in each of the five observables is truly needed to ensure a reliable ranking by the five criteria. Hence, another important part of the present studies is to further calibrate and evaluate the five-criteria method itself. With the use of the five-criteria method the first “anion stabilizer series” was also measured for prototype Ir(0)n nanoclusters under what have come to be “standard conditions” (3.6 µmol of Ir in the precatalyst, 2.5 mL of acetone, 0.5 mL of cyclohexene, initial pressure of 40 psig H2, 22 °C).15 The resultant anion series for the stabilization of Ir(0)n and related (vide infra) nanoclusters at present is ([P2W15Nb3O62]9- ≈ [(P2W15Nb3O61)2O]16- ≈ {[P2W15(TiOH)3O59]9-}n (n ) 1, 2) > [SiW9Nb3O40]7- > HPO42> H2PO4- > [C6H5O7]3- . [P3O9]3- > [-CH2-CH2(CO2)-]nn> Cl- > OH-). Several additional general insights which have resulted from application of the five-criteria method38-40 are summarized in a footnote for the interested reader.41 For the anion-to-metal lattice size-matching reasons summarized elsewhere,42 the above stabilizer abilities are expected to extend in at least a general way to other metal nanoparticles besides Ir(0)n, specifically metal(0)n nanoparticles of Fe, Ru, Os, Re, Co, Rh, Ni, Pd, and Pt. The reader not familiar with DLVO theory of surface-bound anion stabilization of nanoclusters, and the contribution of the charge-mirror of such bound anions to the resultant nanocluster stability, will want to examine those topics to more fully appreciate the importance of the questions asked and results obtained herein.8,34 This is not to say that many issues do not remain to be resolved. Specifically, there is a desperate need for the measurement of nanocluster M(0)n-ligand bond energies for nanoclusters of known composition, charge, and size, all as a function of changing the metal, the nanocluster core’s overall charge, and for neutral as well as charged ligands. Such badly needed fundamental thermodynamic information, along with appropriate surface spectroscopic and associated electronic structure computational and other studies, promises to sharpen greatly how the community thinks about nanocluster “stability” and aggregation pathways. Indeed, such studies are needed to increase the understanding of all other nanocluster properties where the ligands on the nanocluster surface, or ligand dissociation from the nanocluster surface (thereby providing access to the nanocluster’s surface), are directly relevant to the nanocluster physical properties of interest. As the above anion series makes apparent, Brønsted basic, strongly ligating polyoxometalates (hereafter POMs) are the top 4 anions of 11 total nanocluster-stabilizing anions evaluated to (36) Zsigmondy, R. Z. Anal. Chem. 1901, 697. (37) Thiele, H.; von Levern, H. S. J. Colloid Sci. 1965, 20, 679. ¨ zkar, S.; Finke, R. G. Langmuir 2003, 19, 6247. (38) O (39) Ott, L. S.; Finke, R. G. Inorg. Chem. 2006, 45, 8382. (40) Ott, L. S.; Hornstein, B. J.; Finke, R. G. Langmuir 2006, 22, 9357. (41) A few insights afforded previously by the application of the five-criteria method include (i) that proton sponge (PS) is of value as one preferred scavenger for the H+ generated during the nanocluster syntheses under H2 (ref 16); (ii) that HPO42- is a simple, effective, readily available, robust, and previously unappreciated nanocluster stabilizer (ref 38); (iii) that a tridentate array of POM surface oxygens in POMs such as [P2W15Nb3O62]9- appear to be preferred and lead to superior stabilization (ref 46); (iv) that the traditionally weakly coordinating anion BF4- provides for considerable nanocluster stability in high dielectric constant solvents (ref 39), even in the presence of the steric stabilizer poly(vinylpyrrolidone) (PVP) (ref 40); (v) that the observed nanocluster stabilization by tridentate species should translate in at least a general way to other tridentate anions or ligand systems as well as to other metals including Rh, Pt, Au, and Cu (ref 40). ¨ zkar, S. Coord. Chem. ReV. 2004, 248, 135. (42) Finke, R. G.; O
POM Stabilizers for Ir(0)n Nanocluster Formation
date. The literature has grown considerably43,44 since the report of the first well-characterized, highly stabilized transition-metal nanoclusters stabilized by the basic, tridentate [P2W15Nb3O62]9POM.12,13 A comprehensive table of the literature to date (54 references at present) of POM stabilized transition-metal nanoclusters is provided as Table S1 of the Supporting Information for the interested reader. Several reports have investigated lacunary (i.e., metal-oxo-removed, and hence open or defect site) POMs such as [R2-P2W17O61]10-, [PW11O39]7-, or [SiW11O39]8- as potential nanocluster stabilizers.44 However, no lacunary POMs have been investigated by the five-criteria method, nor for the prototype Ir(0)n nanocluster system, nor have the precise protonation statesand thus overall chargesnor the purity of the true lacunary POM stabilizer been clearly established previously. In short, lacunary POMs have not been previously ranked versus the POMs or other anionic stabilizers listed in the above anion series. Outstanding questions of interest here include (i) how do the lacunary anions [R2-P2W17O61]10-, [PW11O39]7with their tetradentate, four W-O-, terminal oxygen binding sites45 (or three W-O- and one W-OH in the case of the {H[R2P2W17O61]}9-, vide infra) compare to the prior “gold standard” POM stabilizer [P2W15Nb3O62]9- with its three Nb-O-Nb bridging oxygen binding sites,46 and (ii) is it important, as we expect, to know the full composition of the POMs and their level of protonation, and to use POMs with 31P NMR handles (a) so that they can be deprotonated as far as possible to yield the most anionic (and thus expected to be best) stabilizers, and (b) so that the level of deprotonation, purity, and stability of the resultant deprotonated POM can be verifiedsthat is, so that reliable data on known composition and known oVerall charge POMs can be obtained? Also, (iii) is the parent POM [R-P2W18O62]6- ineffective as at least an Ir(0)n nanocluster stabilizer in acetone and under our standard conditions, as we expect and based on past experience with such polyoxometalates with relatively little surface-oxygen basicity or anionic charge density?8,12 (This lack of surface charge density can be seen by rewriting the POM formula as it exists structurally, [P2W18O62]6- ) {[(PO4)2]6-(W18O56)0}6-, so that the phosphate core has much of the charge with, at least formally, zero charge density on the surface oxygens.) The latter is an important question since [R-P2W18O62]6- has been reported to be a stabilizer for Pt(0)n nanoparticles,47 but this claim has not been verified by any ranking method such as the five-criteria method. A final, fourth question of interest is (iv) when is poor (43) (a) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911. (b) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440. (c) Keita, B.; Mbomekalle, I.; Nadjo, L.; Haut, C. Electrochem. Commun. 2004, 6, 978. (d) Zhang, J.; Keita, B.; Nadjo, L.; Mbomekalle, I. M.; Liu, T. Langmuir 2008, 24, 5277. (e) Kang, Z.; Tsang, C. H. A.; Zhang, Z.; Zhang, M.; Womg, N.; Zapien, J. A.; Shan, Y.; Lee, S. J. Am. Chem. Soc. 2007, 129, 5326. (f) Shanmugam, S.; Viswanathan, B.; Varadarajan, T. K. J. Mol. Catal. A: Chem. 2005, 241, 52. (g) Maayan, G.; Neumann, R. Chem. Commun. 2005, 4595. (h) Izumi, Y.; Konishi, K.; Tsukahara, M.; Obaid, D. M.; Aika, K. J. Phys. Chem. C 2007, 111, 10073. (i) Boujday, S.; Blanchard, J.; Villanneau, R.; Krafft, M.; Geantet, C.; Louis, C.; Breysse, M.; Proust, A. ChemPhysChem 2007, 8, 2636. (j) Qi, W.; Li, H.; Wu, L. J. Phys. Chem. B 2008, 112, 8257. (44) (a) Kogan, V.; Aizenshtat, Z.; Neumann, R. New J. Chem. 2002, 26, 272. (b) Lica, G. C.; Browne, K. P.; Tong, Y. J. Cluster Sci. 2006, 17, 349. (c) Gordeev, A. V.; Ershov, B. G. High Energy Chem. 1999, 33, 218. (d) Maksimov, G. M.; Zaikovskii, V. I.; Mateev, K. I.; Likholobov, V. A. Kinet. Catal. 2000, 41, 844. (e) Kogan, V.; Aizenshtat, Z.; Popovitz-Biro, R.; Neumann, R. Org. Lett. 2002, 4, 3529. (f) Gordeev, A. V.; Kartashev, N. I.; Ershov, B. G. High Energy Chem. 2002, 36, 102. (g) Maksimova, G. M.; Chuvilin, A. L.; Moroz, E. M.; Likholobov, V. A.; Matveev, K. I. Kinet. Catal. 2004, 45, 870. (45) Pope, M. T. Polyoxo Anions: Synthesis and Structure. In ComprehensiVe Coordination Chemistry; McCleverty, J., Meyer, T. J., Eds.; Pergamon Press: Oxford, U.K., 2004; Vol. 4, pp 635-678. (46) Elsewhere the necessary details are discussed regarding the number of surface oxygens that can serve as ligands in polyoxometalates such as P2W15Nb3O629- and how they can match with different metal lattices (ref 48). (47) Martel, D.; Kuhn, A.; Kulesza, P. J.; Galkowski, M. T.; Malik, M. A. Electrochim. Acta 2001, 46, 4197.
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apparent nanocluster stability really an artifact of poor control over the nucleation and growth? We have recently shown in our nanocluster formation mechanistic work12,13,35,48,49 that very small nucleation rate constants (k1)swith resultant formation of larger nanoclusters where the autocatalytic growth rate constant (k2) is also largerscan actually be what underlies excessive agglomeration.48 In addition, nanocluster growth can also be supersensitive49 to the precise reaction conditions, especially how much of the easily reducible (1,5-COD)Ir(solvent)2+ (1,5-COD ) 1,5cyclooctadiene) is released into solution by the nanocluster precursor complex, [(1,5-COD)Ir · POM]x-. It is just these questions plus the needed control and other experiments, along with the resultant conclusions and take-home messages, that are the focus of the present contribution. The present studies constitute just our part of some of the necessary initial studies8,15,16,38-40,42,49-51 sthat is, our initial foraysinto the factors that stabilize especially isolable, bottleable transition-metal nanoclusters so that the needed subsequent M(0)n-ligand bond energy, computational, surface spectroscopic, lattice-matching verification or refutation42 as well as other needed studies alluded to earlier can be performed. Such studies will be a key to better understanding the stability and catalytic and other reactivity of transition-metal nanoclusters at the same level we understand the stability, catalytic activity, and other desired properties of, for example, small molecule inorganics and organometallics.
Results and Discussion Synthesis and Characterization of the Needed, Known Protonation and Hydration State Polyoxometalates. Although there are several reports of P2W17O6110-, PW11O397-, and SiW11O398- polyoxometalates as nanocluster stabilizers in the literature,44 unknown typically are the true protonation states particularly in nonaqueous solvent44dsand, therefore, the true overall anionic charge and thus what is actually the true POM stabilizer. Hence, our initial goal was to prepare the needed POMs as their mixed Bu4N+/H+ salts by following literature procedures where available,52,53 with an emphasis on the unequivocal characterization of the resultant POM’s degree of deprotonation, hydration state, as well as the resultant deprotonated POM’s stability. Even the level of water present via the POM’s waters of hydration is important, since water is known to have an effect on the nucleation and growth process of Ir(0)n, and very probably other transition-metal, nanoclusters formed in organic solvents.13 We began by preparing (Bu4N)8.4{H1.6[R2-P2W17O61]} · 1.4H2O by Bartis et al.’s procedure52 and characterized the resultant material by IR (Figure S1 of the Supporting Information), 31P NMR (Figure S2 of the Supporting Information), and elemental analysis as detailed in the Experimental Section. The elemental analysis of our compound agrees with Bartis et al.’s analysis, showing a weighted average between eight and nine (Bu4N)+ groups. We then deprotonated the material in CD3CN using (Bu4N)OH to give the monoprotonated salt, (Bu4N)9{H[R2P2W17O61]}, in ca. 80% purity by 31P NMR (Figure S3 of the Supporting Information), all while following the deprotonation directly by 31P NMR and subsequently drying the resultant material. (48) (a) Finney, E. E.; Finke, R. G. Chem. Mater. 2008, 20, 1956–1970. (b) Besson, C.; Finney, E. E.; Finke, R. G. J. Am. Chem. Soc. 2005, 127, 8179. (c) Besson, C.; Finney, E. E.; Finke, R. G. Chem. Mater. 2005, 17, 4925. (49) Ott, L. S.; Finke, R. G. J. Nanosci. Nanotechnol. 2008, 8, 1551–1556. (50) Aiken, J. D., III; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 9545. (51) Ott, L. S.; Finke, R. G. Chem. Mater. 2008, 20, 2592–2601. (52) Bartis, J.; Sukal, S.; Dankova, M.; Kraft, E.; Kronzon, R.; Blumenstein, M.; Francesconi, L. C. J. Chem. Soc., Dalton Trans. 1997, 1937. (53) Radkov, E.; Beer, R. H. Tetrahedron 1995, 14, 2139.
1330 Langmuir, Vol. 25, No. 3, 2009
Although there are no reports in the literature describing the synthesisandcharacterizationofthefullydeprotonated“(Bu4N)10[R2P2W17O61]”, we attempted its preparation since it would have a full 10- charge and would therefore be expected to be a better stabilizer than the monoprotonated (Bu4N)9{H[R2-P2W17O61]}. What we found is that the fully deprotonated “(Bu4N)10[R2P2W17O61]” could not be prepared in pure form, at least in our hands. Adding 1.6 equiv of (Bu4N)OH to a solution of (Bu4N)8.4{H1.6[R2-P2W17O61]} results in a blue powder (the blue color due, apparently, to trace reduction of W(IV) to W(V) · by the basic organic solution) but with the important point being that the resultant material was only ∼60% pure by 31P NMR, Figure S4 of the Supporting Information. No further attempts were made to synthesize the hence still unknown “(Bu4N)10[R2P2W17O61]”, although we cannot rule out that the use of lower temperatures, slower base addition, or milder bases (or a combination of these approaches) would not yield the desired material if it is crucial for some other study. Next we prepared the known53-55 (Bu4N)4{H3[PW11O39]} POM and characterized it by IR, 31P NMR (Figures S5 and S6, respectively, in the Supporting Information), plus elemental analysis. Attempts to deprotonate (Bu4N)4{H3[PW11O39]} using (Bu4N)OH to produce pure, less protonated {Hx[PW11O39]}(7-x)(x < 3) were only partially successful. The deprotonation of (Bu4N)4{H3[PW11O39]} in CH3CN by 1 equiv of (Bu4N)OH produced what is primarily (Bu4N)5{H2[PW11O39]} by IR (the IR spectrum of (Bu4N)5{H2[PW11O39]} shows no change to the POM structure when compared with the original IR spectrum of (Bu4N)4{H3[PW11O39]}, see Figure S5 in the Supporting Information), but the resultant material is {H[R2-P2W17O61]}9- . [R-P2W18O62]6- ∼ {H3[PW11O39]}4for their ability to support the formation, stabilization, and catalytic activity of Ir(0)n nanoclusters in acetone under the stated standard conditions. • The prior anion stabilizer series15,16 can, therefore, now be expanded to the following, with the reminder that this series was obtained for the formation and stabilization of Ir(0)n nanoclusters in acetone under our standard conditions: [P2W15Nb3O62]9- ≈ [(P2W15Nb3O61)2O]16- ≈ {[P2W15(TiOH)3O59]9-}n (n ) 1, 2) > [SiW9Nb3O40]7- > HPO42- > {H[R2-P2W17O61]}9- > H2PO4> [C6H5O7]3- . [P3O9]3- > [-CH2-CH2(CO2)-]nn- > Cl- > OH- > {H3[PW11O39]}4- ∼ [R-P2W18O62]6-. For the anion-tometal lattice size-matching reasons summarized elsewhere,42 the above stabilizer abilities are expected to extend at least generally to other metal nanoparticles besides Ir(0)n, specifically metal(0)n nanoparticles of Fe, Ru, Os, Re, Co, Rh, Ni, Pd, and Pt. However, this latter hypothesis remains to be tested experimentally so that the electronic and other factors, not yet taken into account in extending the above series to different metals, can be elucidated. • The fact that the tridentate, three Nb-O-Nb containing [P2W15Nb3O62]9- is better for the formation and stabilization of Ir(0)n nanoclusters, than is the equal charge {H[R2-P2W17O61]}9with its three W-O- and one W-OH binding sites, is an interesting finding. This result is consistent with, and supportive of, the stabilizer-to-metal surface lattice size-matching hypothesis provided elsewhere.42 • The additional five-criteria data collected in Table 1 (e.g., for entries 1-4) are valuable in more clearly showing the error limits inherent in the five criteria. In particular, differences greater than factors of 2-3 are desirable in criteria 1, 3, and 5 (k2/k1, -d[H2]/ dt after redissolving the nanoclusters, and the TTOs, respectively) before using those three criteria to achieve reliable stabilizer rankings. In this sense, the present study shows that criteria 2 and 4 (the diameter and dispersity of the nanoclusters, and their complete redispersibility or not, respectively) are among the most straightforward of the five criteria to interpret. The k2/k1 ratio is, however, very valuable as well since without it one cannot judge if, for example, agglomerated nanoclusters were just poorly formed or agglomerated subsequently due to a less effective stabilizer (i.e., and as appears to be the case for {H[R2P2W17O61]}9- studied herein). • The present study also, therefore, focuses attention on use of measurements of k3 (i.e., B + B f C) and k4 (i.e., B + C f 1.5C) agglomeration steps48,50 as a direct way to assay nanocluster stability,8,50 assuming that the hurdles in making such kinetic measurements can be worked out in the future (hurdles which include the requirement of having either isolable or at least kinetically well-formed, solution-stable nanoclusters as starting materials).50 With k3 and k4 measurements, then one could either (a) use k3 and k4 alone, (b) use a “three-criteria method” consisting of k3 and k4 along with criteria 2 and 4 (a method in principle not limited to catalytically active nanoclusters), or (c) use a revised, “four-criteria method” for ranking nanoclusters consisting of k3
Graham et al.
and k4 along with criteria 1, 2, and 4 (a method also in principle not limited to catalytically active nanoclusters). One would also then have the option of (d) using a “six-criteria method” consisting of the present five criteria along with k3 and k4 as a powerful sixth criteria. As is the present case with the five-criteria method, any of the above three to six criteria methods is of fundamental value since no other more generally applicable methods exist at present for ranking transition-metal nanocluster formation and subsequent stability.8 The above seven conclusions are additional, previously unavailable insights41 resulting from studies employing the fivecriteria method.
Experimental Section Since the general types of experiments performed herein are ones where we have several prior publications,15,16,38-40,49 and in the interest of saving precious journal space, the full, detailed Experimental Section for the present work is presented in the Supporting Information.
Acknowledgment. We thank Joe Mondloch and Eric Finney for their help fitting kinetic data with MacKinetics. We also thank Dr. JoAn Hudson and Amar Kumbhar from Clemson University for the analysis of TEM samples. This work was supported by DOE Grant DE-FG02-03ER15453. Supporting Information Available: Table S1, table of the previous literature for POM stabilized transition-metal nanoclusters; Figure S1, IR spectra of (Bu4N)8.4{H1.6[R2-P2W17O61]} · 1.4H2O prepared in this work, an IR spectrum from the literature, IR spectrum of authentic K10[R-P2W17O61] · 15H2O, and IR spectrum of authentic K6[RP2W18O62] · 14H2O, all prepared in KBr pellets; Figure S2, 31P NMR of (Bu4N)8.4{H1.6[R2-P2W17O61]} · 1.4H2O in CD3CN; Figure S3, 31P NMR of titration in CD3CN of (Bu4N)8.4{H1.6[R2-P2W17O61]} · 1.4H2O with (Bu4N)OH, attempted complete deprotonation of (Bu4N)8.4{H1.6[R2P2W17O61]} · 1.4H2O; Figure S4, 31P NMR of (Bu4N)10[R-P2W17O61] in CD3CN; Figure S5, IR spectrum of (Bu4N)4{H3[PW11O39]} in a KBr pellet; Figure S6, 31P NMR of (Bu4N)4{H3[PW11O39]} in CD3CN, attempted deprotonation of (Bu4N)4{H3[PW11O39]} to give (Bu4N)5{H2[PW11O39]}; Figure S7, 31P NMR of (Bu4N)5{H2[PW11O39]} in CD3CN, attempted complete deprotonation of (Bu4N)4{H3[PW11O39]} to give (Bu4N)7[PW11O39]; Figure S8, IR spectrum of (Bu4N)7[PW11O39] in a KBr pellet; Figure S9, 31P NMR of (Bu4N)7[PW11O39] in CD3CN, synthesis of (Bu4N)6[R-P2W18O62] · H2O; Figure S10, IR spectrum of (Bu4N)6[R-P2W18O62] and authentic K6[R-P2W18O62] · 14H2O, both in KBr pellets; Figure S11, 31P NMR of (Bu4N)6[R-P2W18O62] in CD3CN, control experiment demonstrating that reduced, heteropoly blue [R-P2W18O62]-6-x formed in solution does not mask the presence of Ir(0)n nanoclusters after a standard conditions hydrogenation reaction; Figure S12, cyclohexene hydrogenation curve fits for a solution of (a) [(1,5-COD)Ir(CH3CN)2][BF4] and PS added to (Bu4N)9[P2W15Nb3O62] and (b) (Bu4N)9[P2W15Nb3O62] added to [(1,5-COD)Ir(CH3CN)2][BF4] and PS; Figure S13, cyclohexene hydrogenation curve fit by the threestep mechanism for a solution of [Bu4N]9[P2W15Nb3O62] added to a solution of [(1,5-COD)Ir(CH3CN)2][BF4] and PS; Figure S14, histogram of the size distribution of Ir(0)n nanoclusters stabilized by {H[R2P2W17O61]}10-; Figure S15, standard conditions hydrogenation of 3.6 µmol of [(1,5-COD)Ir(CH3CN)2][BF4], 1 equiv of (Bu4N)8.4{H1.6[R2P2W17O61]} · 1.4H2O, and 1 equiv of PS; Figure S16, standard conditions hydrogenation reaction with [(1,5-COD)Ir(CH3CN)2], PS, and (Bu4N)8.4{H1.6[R2-P2W17O61]} fit to the four-step mechanism; Figure S17, hydrogenation curve of 3.6 µmol of [(1,5-COD)Ir(NCCH3)2][BF4], 1 equiv of (Bu4N)6[R-P2W18O62] · H2O, and 1 equiv of PS in acetone under standard conditions; Figure S18, hydrogenation curve of 3.6 µmol of [(1,5-COD)Ir(NCCH3)2][BF4], 1 equiv of (Bu4N)4{H3[PW11O39]}, and 1 equiv of PS in acetone under standard conditions; Figure S19, standard conditions hydrogenation reaction with [(1,5-COD)Ir(CH3CN)2][BF4], PS, and 1 equiv of ∼80% pure (Bu4N)5{H2[PW11O39]} fit to the two-step and four-step mechanisms. This material is available free of charge via the Internet at http://pubs.acs.org. LA8025254
