Cristina Tiozzo

An Organometallic Approach to Gold Nanoparticles: Synthesis and X‐Ray Structure of CO‐Protected Au21Fe10, Au22Fe12, Au28Fe14, and Au34Fe14 Clusters

Ligand-stabilized quasi-molecular gold nanoparticles, consisting of chunks of cubic close-packed lattice featuring “magic numbers” of gold atoms, are being intensively investigated both experimentally and theoretically, owing to their potential use in miniaturized basic devices in electronics, models and precursors of metallic catalysts, stains of biological samples, and imaging nanoprobes for drug screening and diagnosis. The monodispersity of several of the above thiol/thiolate or phosphine monolayer protected gold nanoparticles has been disputed. Furthermore, STM experiments and DFT calculations of adsorption of methylthiolate on Au(111) showed formation of linear RS-Au-SR staple motives. This feature was later confirmed by calculations on an Au38 cluster of Oh symmetry, [8] and has been experimentally demonstrated by the first structural characterizations of gold–thiolate clusters, namely [Au25(SCH2CH2Ph)18] [9] and the giant [Au102(p-MBA)44] (pMBA = p-mercaptobenzoic acid) cluster. The only other structurally characterized gold particles exceeding 1 nm in at least one dimension are the ligand-protected [Au16(AsPh3)8Cl6], [11] [Au25(PPh3)10(SEt)5Cl2] , and [Au39(PPh3)14Cl6]Cl2 clusters, [13] which do not display such a feature. Herein, we report an organometallic approach to a new kind of molecular ligand-stabilized gold nanoparticle, consisting of the synthesis of Au–Fe colloidal nanoparticles, in which {Fe(CO)x} (x= 3, 4) moieties take the place of thiol or thiolate ligands in protecting and stabilizing the gold kernel. These iron carbonyl groups share and may also exceed the bonding versatility of thiols/thiolates. The synthesis of CO-protected Au–Fe clusters involves the oxidation of [Fe3(CO)11] 2 with [AuCl4] salts in acetone and under an inert atmosphere. After formation of the previously unknown yellow-orange [Au5{Fe(CO)4}4] 3 cluster, the reaction affords brown solutions of colloidal Au–Fe nanoparticles, which display broad and unresolved IR carbonyl absorptions shifting from 1960 to 2010 cm 1 as a function of the starting ratio of the reagents. The colloidal nature of these solutions was confirmed by dynamic light scattering (DLS) measurements in acetonitrile for two samples. The first sample (nCO at 1980 cm ) revealed the presence of two sets of nanoparticles displaying hydrodynamic diameters in the ranges 10–30 and 100–200 nm. The second sample (nCO at 1990 cm ) showed particles with nominal diameters of 35–60 and 110–300 nm. No evidence of smaller particles could be gathered. These results parallel recent measurements of solutions from which monodispersed [Au25(SCH2CH2Ph)x] was obtained in good yields. [15] The addition of Au salts in excess gives rise to separation of gold powder and the formation of the dark-green [Au{Fe2(CO)8}2] cluster, two isomers of which have previously been isolated and characterized by other routes. In our investigations of the above two samples we have so far isolated five molecular species, namely, [NEt4]3[Au5{Fe(CO)4}4] (nCO in CH3CN at 1945s, 1861s cm ), [NEt4]6[Au21{Fe(CO)4}10]·Cl (nCO in CH3CN at 1982s, 1937sh, 1889sh cm ), [NEt4]6[Au22{Fe(CO)4}12]·(CH3)2CO·0.5C6H14 (nCO in CH3CN at 1980s, 1925sh, 1880sh cm ), [NEt4]8[Au28{Fe(CO)3}4{Fe(CO)4}10]·6CH3CN (nCO in CH3CN at 1985s, 1927sh, 1887sh cm ) and [NEt4]10[Au34{Fe(CO)3}6{Fe(CO)4}8]·2Cl·7.6CH3CN (nCO in CH3CN at 1990s, 1932sh, 1900sh cm ). Their structures have been determined by single-crystal X-ray diffraction studies. The [Au5{Fe(CO)4}4] 3 cluster (1) is isostructural with the corresponding [Cu5{Fe(CO)4}4] 3 [18] and [Ag5{Fe(CO)4}4] 3 [14] species (see Figure S1 in the Supporting Information). As shown in Figure 1, [Au22{Fe(CO)4}12] 6 (2) may be formally envisioned to derive by sandwiching two [Au5] 3+ fragments between three [Au4{Fe(CO)4}4] 4 [19] moieties in a tripledecker fashion. The outer {Fe(CO)4} groups adopt C3v local symmetry of the carbonyl groups and behave as triply bridging (m3) ligands, whereas the central {Fe(CO)4} groups adopt C2v local symmetry of the carbonyl groups and behave as m4 ligands. The [Au21{Fe(CO)4}10] 5 structure (3) may be envisioned as a molecular model of LicurgoCs cup (Figure 2). The metal frame consists of an inner Au-centered pentagonal antiprism, at the top and bottom of which two pentagonal Au5Fe5 rings are condensed. As a result the whole metal frame may be described as deriving from two fused concave cups generated by a Au5Fe5-Au5-Au-Au5-Au5Fe5 sequence of layers, sharing the unique Au atom and with opposite orientations. In spite of [*] Dr. C. Femoni, Prof. M. C. Iapalucci, Prof. G. Longoni, C. Tiozzo, Dr. S. Zacchini Dipartimento di Chimica Fisica ed Inorganica Universit. di Bologna Viale Risorgimento 4, 40136 Bologna (Italy) Fax: (+39)051-209-3690 E-mail: femoni@ms.fci.unibo.it

Synthesis, Structure, and Spectroscopic Characterization of [H8−nRh22(CO)35]n− (n = 4, 5) and [H2Rh13(CO)24{Cu(MeCN)}2]− Clusters: Assessment of CV and DPV As Techniques to Circumstantiate the Presence of Elusive Hydride Atoms

The previously ill-characterized [H(x)Rh(22)(CO)(35)](4-/5-) carbonyl cluster has been obtained as a byproduct of the synthesis of [H(3)Rh(13)(CO)(24)](2-) and effectively separated by metathesis of their sodium salts with [NEt(4)]Cl. Although the yields are modest and never exceed 10-15% (based on Rh), this procedure affords spectroscopically pure [H(3)Rh(22)(CO)(35)](5-) anion. Formation of the latter in mixture with other Rh clusters was also observed by electrospray ionization-mass spectrometry (ESI-MS) in the oxidation of [H(2)Rh(13)(CO)(24)](3-) with Cu(2+) salts. The recovery of further amounts of [H(3)Rh(22)(CO)(35)](5-) was hampered by too similar solubility of the salts composing the mixture. Conversely, the reaction in CH(3)CN of [H(2)Rh(13)(CO)(24)](3-) with [Cu(MeCN)(4)](+)[BF(4)](-) leads to the [H(2)Rh(13)(CO)(24){Cu(MeCN)}(2)](-) bimetallic cluster. The X-ray crystal structures of [H(4)Rh(22)(CO)(35)](4-), [H(3)Rh(22)(CO)(35)](5-), and [H(2)Rh(13)(CO)(24){Cu(MeCN)}(2)](-) are reported. From a formal point of view, the metal frame of the former two species can be derived by interpenetration along two orthogonal axes of two moieties displaying the structure of the latter. The availability of [H(8-n)Rh(22)(CO)(35)](n-) salts prompted their detailed chemical, spectroscopic, and electrochemical characterization. The presence of hydride atoms has been directly proved both by ESI-MS and (1)H NMR. Moreover, both [H(4)Rh(22)(CO)(35)](4-) and [H(3)Rh(22)(CO)(35)](5-) undergo distinctive electrochemically reversible redox changes. This allows to assess electrochemical studies as indisputable though circumstantial evidence of the presence of (1)H NMR-silent hydride atoms in isostructural anions of different charge.

New high-nuclearity carbonyl and carbonyl-substituted rhodium clusters and their relationships with polyicosahedral carbonyl-substituted palladium- and gold-thiolates.

A reinvestigation of the synthesis of [H(5-n)Rh(13)(CO)(24)](n-) (n = 2, 3) led to isolation of a series of Rh(19), Rh(26), and Rh(33) high-nuclearity carbonyl and carbonyl-substituted rhodium clusters. The [Rh(19)(CO)(31)](5-) (1) is electronically equivalent with [Pt(19)(CO)(22)](4-), but poor crystal diffraction data of all salts obtained to date have prevented its geometrical analysis. The structures of Rh(26)(CO)(29)(CH(3)CN)(11) (2) as 2·2CH(3)CN and [Rh(33)(CO)(47)](5-) (3) as [NEt(4)](5)[3]·Me(2)CO were determined from complete X-ray diffraction determinations. The latter two species adopt polyicosahedral metal frameworks, and notably, [Rh(33)(CO)(47)](5-) represents the molecular group 9 metal carbonyl cluster of highest nuclearity so far reported.

An efficient epoxidation of terminal aliphatic alkenes over heterogeneous catalysts: when solvent matters

The epoxidation of unfunctionalized terminal aliphatic alkenes over heterogeneous catalysts is still a challenging task. Due to the tuning of a peculiar catalyst/oxidant/solvent combination, it was possible to attain good alkene conversions (73%) and excellent selectivity values (>98%) in the desired terminal 1,2-epoxide. Over the titanium–silica catalyst and in the presence of tert-butylhydroperoxide, the use of α,α,α-trifluorotoluene as an uncommon non-toxic solvent was the key factor for a marked enhancement of selectivity. The titanium–silica catalyst was efficiently recycled and reused after a gentle rinsing with fresh solvent.

Sn-centred icosahedral Rh carbonyl clusters: synthesis and structural characterization and 13C–{103Rh} HMQC NMR studies

The reaction of [Rh7(CO)16]3- with SnCl(2).2H2O in a 1 : 1 molar ratio under N2 results in the formation of the new heterometallic cluster, [Rh12Sn(CO)27]4-, in very high yield (ca. 86%). Further controlled additions of SnCl(2).2H2O, or solutions of HCl, or [RhCl(COD)]2, give [Rh12(mu-Cl)2Sn(CO)23]4-. Similarly, addition of HBr to [Rh12Sn(CO)27]4- gives the related cluster [Rh12(mu-Br)2Sn(CO)23]4-. Notably, if the addition of SnCl(2).2H2O to [Rh12Sn(CO)27]4- is carried out under a CO atmosphere, the reaction takes a different course and leads to the formation of the new cluster, [Rh12Sn(mu3-RhCl)(CO)27]4-. All the above clusters have been shown by single-crystal X-ray diffraction studies to have a metal framework based on an icosahedron, which is centred by the unique Sn atom. Their chemical reactivity and 13C-{103Rh} HMQC NMR spectroscopic characterization are also reported.

Tungsten oxide: a catalyst worth studying for the abatement and decontamination of chemical warfare agents

Abstract Tungsten(VI) oxide, WO3, was studied and used as a heterogeneous catalyst for the liquid-phase oxidative abatement and solid-phase decontamination of simulants of chemical warfare agents, CWAs. The catalytic performance of WO3 was compared to the one of a soluble W-containing model catalyst, W(IV)-heptaisobutyl polyhedral oligomeric silsesquioxane, W-POSS. In liquid-phase abatement tests, WO3 promoted a complete degradation of the toxic agent simulant within 24 h, in the presence of aqueous hydrogen peroxide, at room temperature. In solid-phase decontamination tests, when WO3 was mixed with sodium perborate as a solid oxidant, it was also tested in the decontamination of a cotton textile support from organosulfide and organophosphonate agents (simulants of blistering and nerve CWAs, respectively), showing promising performances comparable to, or sometimes better than, a nanostructured TiO2 catalyst, taken as a reference material. The environmental impact of the WO3 catalyst was assessed on bioluminescent Photobacterium leiognathi Sh1 bacteria, over which no acute nor chronic detrimental effects were recorded. Then, when in contact with a vegetable species such as Phaseolus vulgaris L. (common bean), WO3 did not cause damage to the photosynthetic apparatus of the plant, whereas a clear inhibition of the seed germination was evidenced.