Chini Clusters Research Articles

Further insights into platinum carbonyl Chini clusters

The oxidation of [Ph3P(CH2)12PPh3][Pt15(CO)30] with CF3COOH in THF afforded [Ph3P(CH2)12PPh3][Pt18(CO)36] as a precipitate which was re-crystallized from dmf/isopropanol. This salt self-assembles in the solid state adopting an unprecedented morphology which consists of infinite chains of [Pt9(CO)18]− units. The solid state structure of [Ph3P(CH2)12PPh3][Pt18(CO)36] may be viewed as a snapshot in which [Pt9(CO)18]− units are approaching and ready to exchange outer Pt3(CO)6 fragments. The reactions of Chini clusters with isonitriles proceed via redox-fragmentation, at difference with those involving phosphines that may occur both via non-redox substitution and redox fragmentation, depending on the experimental conditions. Thus, the reaction of [Pt6(CO)12]2− with CNXyl afforded Pt5(CNXyl)10, whereas Pt9(CNXyl)13(CO) was obtained from the reaction of [Pt15(CO)30]2− with CNXyl. These two new neutral clusters have been structurally characterized as their Pt5(CNXyl)10·2toluene and Pt9(CNXyl)13(CO)·solv solvates. DFT studies on the CO exchange of [Pt6(CO)12]2− suggest an associative interchange mechanism, which may be extended also to larger Chini clusters and the initial steps of their reactions with other soft nucleophiles.

Generation and Stabilization of Small Platinum Clusters Pt12±x Inside a Metal-Organic Framework.

The generation and matrix stabilization of ligand-free, small platinum nanoclusters (NCs) Pt12±x is presented. The metal-organic framework-template approach is based on encapsulating CO-ligated, atom-precise Pt9 Chini clusters [{Pt3(CO)6}3]2- into the zeolitic imidazolate framework ZIF-8. The selective formation of the air-stable inclusion compound [NBu4]2[{Pt3(CO)6}4]@ZIF-8 of defined atomicity Pt12 and with Pt loadings of 1-20 wt % was monitored by UV/vis and IR spectroscopy and was confirmed by high-resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD). Thermally induced decarbonylation at 200 °C yields the composite material Ptn@ZIF-8 with a cluster atomicity n close to 12, irrespective of the Pt loading. The PtNCs retain their size even during annealing at 300 °C for 24 h and during catalytic hydrogenation of 1-hexene at 25 °C in the liquid phase. The Ptn@ZIF-8 material can conveniently be used for storing small PtNCs and their further processing. Removal of the protective ZIF-8 matrix under acidic conditions and transfer of the PtNCs to carbon substrates yields defined aggregation to small Pt nanoparticles (1.14 ± 0.35 nm, HR-TEM), which have previously shown exceptional performance in the electrocatalytic oxygen reduction reaction (ORR).

Ni3(iPr2Im)3(µ2‐CO)3(µ3‐CO)] – A CO‐Stabilized NHC Analogue of the Parent Neutral Nickel Chini‐Type Cluster

The reactivity of nickel tetracarbonyl [Ni(CO)4] with the N‐Heterocyclic Carbenes (NHCs) 1,3‐diisopropylimidazolin‐2‐ylidene (iPr2Im) and 1,3‐di(isopropyl)‐4,5‐di(methyl)imidazolin‐2‐ylidene (iPr2ImMe) is reported. The reaction of [Ni(CO)4] with two equiv. iPr2Im and iPr2ImMe and one equiv. iPr2ImMe leads to the formation of the complexes [Ni(iPr2Im)2(CO)2] 2, [Ni(iPr2ImMe)2(CO)2] 3, and [Ni(iPr2ImMe)(CO)3] 4. The reaction of [Ni(CO)4] with one equiv. (or less) iPr2Im affords the triangular 44 VE (valence electron) cluster complex [Ni3(iPr2Im)3(µ2‐CO)3(µ3‐CO)] 1, a NHC‐stabilized analogue of the parent neutral nickel Chini cluster [Ni3(CO)3(µ2‐CO)3].

Functionalization, Modification, and Transformation of Platinum Chini Clusters

In this microreview, the authors summarize the reactions of [Pt3n(CO)6n]2– (n = 1–10) Chini clusters aimed at their functionalization, modification and transformation. The results are organized on the basis of the fact that the final products do or do not retain the structure of the parent Chini clusters. The first category comprises the redox reactions of homoleptic Chini clusters, CO/phosphine substitution affording heteroleptic Chini‐type clusters and the formation of Lewis acid‐base adducts by capping the external Pt3‐triangular faces with Lewis acids. The second category consists of globular molecular platinum nanoclusters (platinum browns) obtained by thermal decomposition of Chini clusters, as well as surface decorated platinum carbonyl clusters, composed of a low valent Pt‐CO core decorated by cationic metal fragments. The relevance of such species to molecular cluster chemistry and metal nanoclusters is outlined.

Reactions of Platinum Carbonyl Chini Clusters with Ag(NHC)Cl Complexes: Formation of Acid-Base Lewis Adducts and Heteroleptic Clusters.

The reactions of anionic platinum carbonyl Chini clusters [Pt3n(CO)6n]2- [n = 2 (1), 3 (2), 4 (3)] with Ag(IPr)Cl [IPr = C3N2H2(C6H3iPr2)2] afford the neutral acid-base Lewis adducts [Pt9(CO)18(AgIPr)2] (4) and [Pt6(CO)12(AgIPr)2] (5). These are thermally transformed into the homometallic heteroleptic neutral cluster [Pt3(CO)4(IPr)2] (6). Alternatively, 6 can be obtained from the reactions of 1-3 with an excess of the free IPr carbene ligand. The formation of 6 is sometimes accompanied by trace amounts of [Pt4(CO)4(IPr)3] (7). The reaction of 6 with free IPr affords the closely related [Pt3(CO)3(IPr)3] (8) heteroleptic cluster by substitution of the unique terminal CO ligand with a third IPr ligand. The reactions of 1-3 with Ag(IMes)Cl [IMes = C3N2H2(C6H2Me3)2] proceed differently from those involving Ag(IPr)Cl. Indeed, the only product isolated after workup is the bimetallic tetranuclear cluster [Pt3(CO)3(IMes)3(AgCl)] (9). 9 slowly reacts under a CO atmosphere, resulting in the pentanuclear [Pt5(CO)7(IMes)3] (10) complex. All of the new clusters 4-10 have been spectroscopically characterized and their molecular structures determined by X-ray crystallography. 4 and 5 retain the original trigonal-prismatic structures of the parent anionic Chini clusters, which are capped by two [Ag(IPr)]+ moieties. Conversely, 6-9 are based on a Pt3 triangular core decorated by CO and N-heterocyclic carbene ligands as well as Pt(CO) (in the case of 7) and AgCl (9) moieties. 10 displays an edge-bridged tetrahedral geometry.

Globular molecular platinum carbonyl nanoclusters: Synthesis and molecular structures of the [Pt26(CO)32]− and [Pt14+x(CO)18+x]4− anions and their comparison to related platinum “browns”

The thermal decomposition of [Pt3n(CO)6n]2− (n=2–10) Chini clusters results in the formation of globular molecular platinum carbonyl clusters, whose nature depends of the nuclearity of the parent cluster as well as the counter-ion and solvent employed. Among these, the new [Pt14+x(CO)18+x]4− (x=0,1) and [Pt44(CO)45]n−, as well as the previously reported [Pt15(CO)19]4−, [Pt19(CO)22]4−, [Pt24(CO)30]2−, [Pt26(CO)32]2−, [Pt33(CO)38]2− and [Pt38(CO)44]2− have been identified. Oxidation of [Pt14+x(CO)18+x]4− (x=0,1) with HBF4·Et2O affords [Pt26(CO)32]2−, which is further oxidized to the related [Pt26(CO)32]− mono-anion. Oxidation of [Pt19(CO)22]4−, under similar conditions, affords the unstable [Pt19(CO)22]2− di-anion, that rearranges during crystallization into the new [Pt36(CO)44]2−. Conversely, the reduction of [Pt19(CO)22]4− with Na/naphthalene results in the [Pt19(CO)22]5− penta-anion which, after work-up, is transformed into the new [Pt23(CO)27]2−. The new clusters [Pt14+x(CO)18+x]4− (x=0,1) and [Pt26(CO)32]− have been fully characterized by means of single crystal X-ray diffractometry as their [NEt4]4[Pt14+x(CO)18+x]·MeCN (x=0.18) and [NEt4][Pt26(CO)32]·2.12thf·0.38C6H14 salts. Conversely, in the case of the new [Pt23(CO)27]2−, [Pt36(CO)44]2− and [Pt44(CO)45]n−, due to the poor quality of their crystals, only a partial structural determination has been possible. [Pt14+x(CO)18+x]4− (x=0,1) displays a distorted bcc structure, [Pt26(CO)32]− and [Pt23(CO)27]2− adopt hcp structures, [Pt36(CO)44]2− presents a distorted ccp metal core, whereas [Pt44(CO)45]n− possesses a twinned hcp/ccp ABCBA structure. The structures of the metal cores of these clusters is compared to previously reported globular platinum carbonyl nanoclusters (“platinum browns”).

Heteroleptic Chini-Type Platinum Clusters: Synthesis and Characterization of Bis-Phospine Derivatives of [Pt3n(CO)6n]2- (n = 2-4).

The reactions of [Pt3n(CO)6n]2- (n = 2-4) homoleptic Chini-type clusters with stoichiometric amounts of Ph2PCH2CH2PPh2 (dppe) result in the heteroleptic Chini-type clusters [Pt6(CO)10(dppe)]2-, [Pt9(CO)16(dppe)]2-, and [Pt12(CO)20(dppe)2]2-. Their formation is accompanied by slight amounts of neutral species such as Pt4(CO)4(dppe)2, Pt6(CO)6(dppe)3, and Pt(dppe)2. A similar behavior was observed with the chiral ligand R-Ph2PCH(Me)CH2PPh2 (R-dppp), and two isomers of [Pt9(CO)16(R-dppp)]2- were identified. All the new species were spectroscopically characterized by means of IR and 31P NMR, and their structures were determined by single-crystal X-ray diffraction. The results obtained are compared to those previously reported for monodentate phosphines, that is, PPh3, as well as more rigid bidentate ligands, that is, CH2═C(PPh2)2 (P^P), CH2(PPh2)2 (dppm), and o-C6H4(PPh2)2 (dppb). From a structural point of view, functionalization of anionic platinum Chini clusters preserves their triangular Pt3 units, whereas the overall trigonal prismatic structures present in the homoleptic clusters are readily deformed and transformed upon functionalization. Such transformations may be just local deformations, as found in [Pt9(CO)16(dppe)]2-, [Pt9(CO)16(R-dppp)]2-, [Pt12(CO)22(PPh3)2]2-, and [Pt9(CO)16(PPh3)2]2-; an inversion of the cage from trigonal prismatic to octahedral, as observed in [Pt6(CO)10(dppe)]2- and [Pt6(CO)10(PPh3)2]2-; the reciprocal rotation of two trigonal prismatic units with the loss of a Pt-Pt contact as found in [Pt12(CO)20(dppe)2]2-.

Monomeric Chini‐Type Triplatinum Clusters Featuring Dianionic and Radical‐Anionic π*‐Systems

Owing to their unique topologies and abilities to self-assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt3 (CO)6 ]n (2-) ("Chini clusters") continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron-transfer series [Pt3 (μ-CO)3 (CNAr(Dipp2) )3 ](n-) (n=0, 1, 2), which represents a unique set of monomeric Pt3 clusters supported by π-acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest-occupied molecular orbitals of the mono- and dianionic clusters consist of a combined π*-framework of the CO and CNAr(Dipp2) ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non-innocent manner.

Monomeric Chini‐Type Triplatinum Clusters Featuring Dianionic and Radical‐Anionic π*‐Systems

AbstractOwing to their unique topologies and abilities to self‐assemble into a variety of extended and aggregated structures, the binary platinum carbonyl clusters [Pt3(CO)6]n2− (“Chini clusters”) continue to draw significant interest. Herein, we report the isolation and structural characterization of the trinuclear electron‐transfer series [Pt3(μ‐CO)3(CNArDipp2)3]n− (n=0, 1, 2), which represents a unique set of monomeric Pt3 clusters supported by π‐acidic ligands. Spectroscopic, computational, and synthetic investigations demonstrate that the highest‐occupied molecular orbitals of the mono‐ and dianionic clusters consist of a combined π*‐framework of the CO and CNArDipp2 ligands, with negligible Pt character. Accordingly, this study provides precedent for an ensemble of carbonyl and isocyanide ligands to function in a redox non‐innocent manner.

Redox Properties of the Chini Clusters [Pt3(CO)6]n2– (n = 1–7) in Solution: A Hybrid DFT Study. Application to their Oxidation by O2

The Chini clusters (Pt3(CO)6)n2– display fascinating size-dependent redox properties. These redox properties are poorly described by ordinary density functional theory (DFT) calculations. In particular, the functionals PBE, B3LYP, and MPW1K yield very poor values of the reduction potentials of the couples (Pt3(CO)6)n–/(Pt3(CO)6)n2– in solution, suggesting that the clusters resist oxidation by O2 in contradiction to the experimental evidence. We propose an original hybrid DFT method, using PBE for the metal and MPW1K for the CO ligands, which yields much more realistic values of the reduction potentials. As an application we show that the growth of the Chini clusters subsequent to their oxidation is due to a transfer of the neutral Pt3(CO)6 unit from the oxidized to a nonoxidized cluster.

PPh3-Derivatives of [Pt3n(CO)6n]2–(n= 2–6) Chini’s Clusters: Syntheses, Structures, and31P NMR Studies

The reaction of the [Pt3n(CO)6n](2-) (n = 2-6) Chini's clusters with increasing amounts of PPh3 has been investigated in detail by combined FT-IR, (31)P{(1)H} NMR, and electrospray ionization-mass spectrometry (ESI-MS) studies, showing that up to three CO ligands are gradually substituted by PPh3, resulting in isonuclear phosphine-substituted anionic clusters of general formula [Pt3n(CO)(6n-x)(PPh3)(x)](2-) (n = 2-6; x = 1-3). Further addition of PPh3 results in the elimination of the neutral Pt3(CO)3(PPh3)3 species and formation of lower nuclearity anionic clusters. [Pt12(CO)22(PPh3)2](2-) and [Pt9(CO)16(PPh3)2](2-) have been structurally characterized, and they maintain the trigonal prismatic structures of the parent homoleptic clusters, with the two PPh3 ligands bonded to different external Pt3-triangles in relative cis-position. Conversely, the crystal structure of [Pt6(CO)10(PPh3)2](2-) shows that its metal cage is transformed from trigonal prismatic to trigonal antiprismatic after CO/PPh3 exchange.

Is a naked platinum nanocatalyst better than the analogous supported catalysts?

The performance of naked nanocatalyst 1, derived from Chini cluster ([Pt15(CO)30]2−), has been evaluated with special reference to the analogous catalysts 2 and 3 which were derived from the same [Pt15(CO)30]2− but supported by MCM-41 and the water soluble polymer PDADMAC, respectively.

UV−Visible Absorption Spectra of Small Platinum Carbonyl Complexes and Particles: A Density Functional Theory Study

We investigate the influence of a carbonyl coating on the UV-visible absorption of platinum complexes and particles with the TDDFT method. We first investigate the Chini clusters [Pt(3)(CO)(6)](n)(2-), n = 1-4, for which the absorption spectra are known. We show that PBE is realistic but displays convergence issues and that B3LYP overestimates intertriangle distances and absorption intensities. We discuss the structure of the spectra and the parallel vs perpendicular character of the transition dipole with respect to the stacking axis. We then investigate the spectra of model carbonylated platinum particles: [Pt(13)(CO)(12)](2-) (with only terminal carbonyls) and [Pt(13)(CO)(24)](2-) (with only bridging carbonyls). B3LYP proves much more realistic than in the case of Chini clusters. The influence of the morphology of the platinum particle and of the carbonyl coating on the absorption spectra is discussed. The results suggest an interpretation of a spectrum, recorded in a recent synthesis.

MCM-41-supported ruthenium carbonyl cluster-derived catalysts for asymmetric hydrogenation reactions

Abstract The anionic ruthenium carbonyl cluster [Ru 4 (μ-H) 3 (CO) 12 ] − has been ion-paired with (3-chloropropyl)trimethoxysilyl-cinchonidium or sparteinium groups chemically bound to the surfaces of MCM-41 [(MCM-41-)(-O) 3 SiCH 2 CH 2 CH 2 NR 3 + Cl − , NR 3 = cinchonidine or sparteine]. The resultant materials have been characterized before and after use as hydrogenation catalysts by IR, XPS, and TEM. The molecular identity of the cluster is retained in the fresh catalysts, but under hydrogen pressure (≥30 bar) the carbonyl groups are lost. Ruthenium (0) and a small amount of Ru 2+ can be observed on the fresh catalyst by XPS, while the used catalyst shows only Ru(0). TEM micrographs show retention of the MCM structure and no observable aggregation of metal. Under optimal conditions, good (≤75%) and observable (≤30%) enantioselectivities are obtained for the hydrogenations of methyl pyruvate and acetophenone, respectively. The enantioselectivity of the catalyst towards methylpyruvate hydrogenation is retained even with relatively high turnovers. This behavior is in sharp contrast to that of the Chini cluster [Pt 12 (CO) 24 ] 2− , derived analogous catalyst reported earlier by us.

A water soluble nanostructured platinum (0) metal catalyst from platinum carbonyl molecular clusters: Synthesis, characterization and application in the selective hydrogenations of olefins, ketones and aldehydes

High nuclearity platinum carbonyl cluster anions (Chini's clusters) have been used as precursors to prepare a platinum nanocatalyst. The ionic polyelectrolyte poly(diallyldimethylammonium chloride) has been used as the support material for anchoring [Pt 30(CO) 60] 2− via ion-pairing and subsequent stabilization of the nanoparticles. The polymer-supported material has been studied by spectroscopy (NIR, 13C NMR, and IR) and TEM before and after its use as a water soluble hydrogenation catalyst. The nanocatalyst is found to be effective for the chemoselective hydrogenation of olefinic, aldehydic and ketonic double bonds. For most of the substrates isolation of the product and reuse of the catalyst are extremely easy due to the automatic phase separation of the products from the catalyst. The spectral features of the fresh catalyst show retention of the carbonyl ligands and molecular identity of the parent cluster, but after use the carbonyl ligands appear to be lost. TEM of the supported material before and after use as a catalyst shows the presence of platinum nanoparticles with majority (≥70%) of the particles in the range of 2–6 nm. Smaller particles are dominant in the used catalyst and this observation is rationalized on the basis of the known reactivity of Chini's clusters with dihydrogen.

Reduction of nitrite to NO in an organised triphasic medium by platinum carbonyl clusters and redox active dyes as electron carriers

Artificial electron donors such as leuco methylene blue and leuco safranin O reduce nitrite ion to nitric oxide. The reaction is effected in a U-tube where nitrite ion and dye in two aqueous layers are separated by a layer of dichloromethane (a close model for a biological liquid membrane) that contains the platinum carbonyl cluster ([Bu(4)N]2[Pt12(CO)24], Chini cluster). On passing dihydrogen an electron transfer chain involving dihydrogen, the dye, the clusters and the nitrite ion is initiated. The cluster catalytically reduces the dye in the presence of dihydrogen, the reduced dye migrates across the phase boundaries and in turn reduces the nitrite ions. The resultant nitric oxide in the effluent gas has been identified by its reactions with cobalamine and myoglobin. When safranin O is the dye, an adduct is formed between the reduced dye and NO. It has been identified by spectroscopic techniques and its probable structure investigated by DFT calculations.

Activity evaluation of carbon paste electrodes loaded with pt nanoparticles prepared in different radiolytic conditions

Three Pt-based catalysts prepared in different radiolytic conditions and supported on graphite powder were packed into a carbon paste electrode configuration. They were compared to each other, to the commercial (Pt) deposited on activated carbon powder (Johnson Matthey) and to pure Vulcan XC-72 for their respective abilities toward the hydrogen evolution reaction (HER). The Tafel parameters were determined for all these electrodes. From the I–V curves and their quantitative treatment, the following order of activity emerged unambiguously and reads: (PtCO)2 (fcc structure) > (PtCO)1 (Chini cluster) > (Pt)neat > (Pt)JM (Johnson Matthey) ≫ (Vulcan XC-72). As expected, all the Pt-loaded electrodes were more efficient than Vulcan XC-72. The classification appears to be linked with the mean nanoparticle size, and for comparable sizes, with the surface morphology of the materials. The results and the stability of the electrodes suggest that the small particle sizes and the good dispersity on the carbon support were maintained during the HER.

Platinum nanowires and layers obtained by self assembly of Chini clusters deposited on supports

Chini [ Pt 3 ( CO 6 ) ] n 2 - ( n = 6) clusters were prepared by γ-irradiation of a Pt(II) salt under CO atmosphere and deposited on HOPG, mica and Au. The deposits were imaged by AFM and STM. A clear support effect was detected on the surface morphology of the few first layers, which are primarily constituted of nanowires formed by a self assembly of Chini clusters. Such a self assembly was not described previously for this system. In the mean areas between nanowires, where only Pt monolayers are seen, STM imaging on HOPG indicates that the structure of initial Chini clusters has been transformed into a planar structure keeping the triangular units with the same intra-triangular Pt–Pt distance of 2.66 Å.

Studies on the Reactions of Dihydrogen with Salts of Platinum Carbonyl Cluster Anions (Chini Clusters) and Redox Active Countercations

Reactions of An[Pt12(CO)24] (A = methylene blue (MB+), safranine O (Saf+), nicotinamide benzyl chloride (BNA+), n = 2; A = methylviologen (MV2+), n = 1) with dihydrogen in DMF and DMSO have been st...

Platinum carbonyl derived catalysts on inorganic and organic supports: a comparative study

Fumed silica, silica gel, silica–alumina and cross-linked (5.5%) polystyrene have been functionalized with quaternary ammonium groups and the Chini cluster [Pt 12(CO) 24] 2− has been anchored onto these functionalized materials by ion pairing. A catalyst has also been prepared by the adsorption of Na 2[Pt 12(CO) 24] on unfunctionalized fumed silica. The catalytic activities of the resultant materials, and that of commercially purchased 5% platinum on alumina have been studied for the hydrogenation of a variety of unsaturated compounds. The substrates studied are: α-acetamidocinnamic acid, cyclohexanone, acetophenone, methyl pyruvate, ethyl acetoacetate, nitrobenzene and benzonitrile. Compared to the polystyrene supported catalyst, the inorganic oxide supported catalysts have higher surface areas and for most of the substrates have notably higher activities. The functionalized fumed silica-based catalyst gives higher conversions than functionalized silica gel and silica–alumina-based catalysts. In the hydrogenation of acetophenone and ethyl acetoacetate, the functionalized fumed silica-based catalyst show superior activity compared to the commercial platinum catalyst, and the catalyst made by conventional adsorption method. In benzonitrile hydrogenation with all the cluster-derived catalysts a hydrazine derivative is selectively formed, but when the commercial platinum catalyst is used benzyl amine is the main product.