Louis H. Pignolet

Phosphine-stabilized, platinum-gold and palladium-gold cluster compounds and applications in catalysis

Abstract This is a comprehensive review on the chemistry and spectroscopic properties of phosphinestabilized, M-centered Au cluster compounds where M=platinum or palladium. The nuclearity of the clusters ranges from 3 (e.g. [Pt(NO 3 )(PPh 3 ) 2 (AuPPh 3 ) 2 ] + ) to 25 (e.g. Pt 2 Ag 13 (AuPPh 3 ) 10 Cl 7 ). The latter consists of two Pt-centered icosahedral units sharing a common vertex. Many of the clusters contain additional metals such as Ag, Cu, or Hg in their periphery. Preparative methods are reviewed and organized by reaction type. Emphasis has been placed on reactions and transformations of preformed clusters so that the reader will acquire a useful knowledge of the reaction chemistry. Structural properties of the clusters are also described. The reactivity and structural properties of the clusters are rationalized in terms of an electron counting model and this is used throughout the review. Spectroscopic properties (e.g. nuclear magnetic resonance (NMR), fast atom bombardment mass, UV-visible, and X-ray photoelectron spectroscopy) of the clusters are described primarily for characterization; however, general trends are pointed out. Several applications in catalysis are also reviewed. Many of the clusters are good catalysts for H 2 activation and can serve as potential models for bimetallic surfaces. Kinetic data for the H 2 D 2 equilibration reaction (H 2 +D 2 ⇄ HD) are given and discussed in terms of cluster composition and structure. The results provide some insight into the well-known synergism observed for PtAu and PdAu heterogeneous catalysts. The mechanism of catalytic H 2 D 2 equilibration under homogeneous conditions is also discussed. The cluster [Pt(AuPPh 3 ) 8 ] 2+ is an excellent catalyst for D 2 g)H 2 O(1) isotope exchange in pyridine solution. This interesting and important reaction is described and kinetic results are related to other isotope exchange systems. A tentative mechanism is proposed based on NMR analysis of the reaction mixture. Finally, prospects for future studies are given. The recent discovery of the catalytic applications of these clusters provides renewed interest in this challenging field.

DRIFTS studies of carbon monoxide coverage on highly dispersed bimetallic Pt-Cu and Pt-Au catalysts

Abstract Silica supported platinum-gold and platinum-copper catalysts prepared from organometallic cluster precursors were characterized with diffuse reflectance Fourier-transform spectroscopy (DRIFTS) of adsorbed CO. Because of the effects that dipole coupling of adsorbed carbon monoxide molecules can have on the infrared spectrum, coverage studies were employed to evaluate the interactions between CO and the catalyst surfaces. DRIFTS spectra of the catalysts prepared from bimetallic molecular cluster precursors showed significantly lower CO stretching frequencies relative to a traditionally prepared platinum catalyst. The lowering of the CO stretching frequency may be the result of the formation of unique alloys and particle morphologies. In the case of the catalyst prepared from the platinum-copper cluster, two peaks were identified in the PtCO stretching region, one of which is postulated to be a bridging or semi-bridging mode between Pt and Cu. Infrared spectroscopy results are also discussed in relation to kinetic data from the n-hexane conversion reaction. Differences in catalyst activities and selectivities are concluded to be primarily due to geometric (i.e. structural or morphological) effects.

Mass spectrometry data for tris- and bis(N,N-dialkyldithiocarbomato) complexes of chromium, iron, cobalt, ruthenium, rhodium and thallium

Abstract The mass spectra of a series of tris(N,N-dialkyldithiocarbamato)metal complexes of chromium, iron, cobalt, ruthenium, rhodium and thallium have been investigated. The fragmentation patterns of these and related compounds are discussed. The major decomposition pathways involve loss of the dithiocarbamate ligand radical and loss of S, S2 and SCNR2 groups from tris- and bis(dithiocarbamato)metal complex ions. The parent molecular ions are present in the mass spectra of most of these complexes but with relative abundances which vary greatly. In complexes of the type LFe(S2CNR2)2 where L = Cl or S2C2(CF3)2, neutral dithiolene and chlorine atom elimination are the major decomposition reactions under electron impact. The parent molecular ions for these complexes are virtually absent and dithiocarbamate ligand radical loss from the parent molecular ion is not observed.

Gold clusters: synthesis and characterization of [Au8(PPh3)7(CNR)]2+, [Au9(PPh36(CNR2]3+ and [Au11(PPH37(CNR)2O]2+ and their reactivity towards amines. The crystal structure of [Au11(PPh3)7(CN-i-Pr)2I](PF6)2

Abstract In the reactions of [Au8(PPh3)7]2+, [Au8(PPh3)8]2+ and [Au9(PPh3)8]3+ with RNC (R = isopropyl and t-butyl) in dichloromethane [Au8(PPh3)7CNR]2+ is initially, and is then converted into [Au9(PPh3)6(CNR)2]3+ via various intermediates. [Au9(PPh3)6(CNR)2]3+ reacts with I− at low temperature (−78°C) in methanol to yield [Au11(PPh3)7(CNR)2I]2+, but when the reaction is carried out at room temperature Au11 (PPh3)6(CNR)I3 is formed. The cluster compounds have been characterised by elemental analysis, 31P{1H} NMR, conductivity measurements, IR and 197Au Mossbauer spectroscopy. The reactions of the clusters with amines to form carbene clusters are very slow, and the reasons for this are considered. The structure of [Au11C134H112IN2P7](PF6 was determined by X-ray diffraction. Mr = 3796.39 cubic, space group 143d, a 37.955(12) A, V 54677.2 A3, Z = 16, Dc = 2.21 Mg m−3, Mo-Kα radiation (graphite crystal monochromator, λ 0.71069 A), μ(Mo-Kα) 125.2 cm−1, F(000) = 33510.3, T 293 K. Final conventional R-factor = 0.048, Rw = 0.062 ofr 1867 unique reflections and 198 variables. The Au-skeleton is the same as in Au11(PPh3)8I3 having C3v symmetry with one central and 10 peripheral Au atoms.

Oxidative addition of acid chlorides to cationic rhodium(I) complexes of chelating diphosphines

Abstract The reaction of benzoyl chloride with [Rh(dppp) 2 ]Cl at 190°C and with [Rh(dppp)Cl] 1 or 2 at 25°C where dppp  1,3-bis(diphenylphosphino)propane has been examined. In both cases the five coordinate compound RhCl 2 (COPh)-(dppp) was rapidly formed and isolated in high yield. This compound does not undergo phenyl migration to RhCl 2 (CO)(Ph)(dppp) even upon warming to 190°C in benzoyl chloride solution and no decarbonylation products are observed. This is in marked contrast to the reaction of RhCl(PPh 3 ) 3 with benzoyl chloride where the migrated product RhCl 2 (CO)(Ph)(PPh 3 ) 2 is formed with the eventual reductive elimination of chlorobenzene. The single crystal X-ray analysis of RhCl 2 (COPh)(dppp) has been carried out ( R  0.036). The compound is square pyramidal with the COPh group in the apical position. The Rh—C bond distance of 1.992(3) A is short for a Rh III —Cσ bond and indicates d π → π ★ back bonding.

Alkane dehydrogenation with silica supported platinum and platinum–gold catalysts derived from phosphine ligated precursors

Abstract In this paper we describe the preparation, catalysis of hexane and propane conversion, and characterization results of several phosphorus containing Pt and Pt–Au catalysts on silica supports. The catalysts were prepared by adsorption from solution of mono- and bi-metallic molecular precursors that are ligated by triphenylphosphine. The effects of phosphorus in the calcined and activated catalysts are much greater than that of gold and lead to remarkable changes in selectivity and stability compared with conventional, non-phosphorus containing Pt/SiO2 and co-deposited Pt–Au/SiO2 catalysts. All catalysts derived from triphenylphosphine precursors showed high selectivity and activity towards dehydrogenation products. Other reactions, namely cyclization, isomerization and cracking, were severely inhibited. Preliminary results for propane conversion show that phosphorus containing catalysts are excellent catalysts for propane dehydrogenation (90% selectivity to propylene at 35% conversion, 550°C, SV=0.5 h−1). The phosphorus containing catalysts also have much greater stability on stream than the non-phosphorus containing catalysts. DRIFTS data on adsorbed CO show that the presence of phosphorus in the catalysts has a significant effect on the CO stretching frequency and causes an upward (blue) shift of about 10 cm−1. TPD of adsorbed CO showed that desorption of CO from a phosphorus containing Pt catalyst had a maximum at ca. 140°C while CO desorption from a conventional Pt/SiO2 catalyst peaked between 240–270°C. These results are consistent with a phosphorus ligand effect on Pt. Preliminary TEM data with a phosphorus containing Pt catalyst showed very small Pt particles estimated to be less than 1 nm.

Halide derivatives of palladium-gold cluster compounds. X-ray crystal and molecular structure of (I)Pd(AuPPh3)7(AuI)2

The reaction chemistry of [Pd(AuPPh3)8](NO3)2 with the halide ions Cl−, Br− and I− and with the pseudohalide CN− has been studied. Acetone solutions of [Pd(AuPPh3)8](NO3)2 reacted with these ions to give the new PdAu clusters (I)Pd(AuPPh3)7(AuI)2, [Pd(AuPPh3)7(AuBr)3]+, Pd(AuPPh3)8(AuCl)3 and Pd(AuPPh3)8(AuCN)3, respectively. In the presence of added acid the reaction with Cl− yielded the cationic cluster [Pd(AuPPh3)7(AuCl)3]+. The structure of (I)Pd(AuPPh3)7(AuI)2 has been determined by X-ray diffraction (unit cell: P21/n, a=37.29, b=22.69, c=16.639 A, β=114.61° , Z=4, V=12 800 A3; refinement: R=0.069). The geometry of this cluster is that of a Pd-centered icosahedral fragment in accord with its eighteen-electron configuration. The coordinated iodine anion of this cluster can be displaced by reaction with CO to give [(CO)Pd(AuPPh3)7(AuI)2]+, while the sixteen-electron clusters [Pd(AuPPh3)7(AuBr)3]+ and [Pd(AuPPh3)7(AuCl)3]+ reacted cleanly with CO to give simple adducts. The eighteen-electron clusters Pd(AuPPh3)8(AuCl)3 and Pd(AuPPh3)8(AuCN)3 showed no reaction with CO. The mechanism of these interesting cluster fragmentation and growth reactions is discussed.

Synthesis and characterization of a series of diphosphine ligated PtAu cluster compounds

Abstract A series of diphosphine Ph2P(CH2)nPPh2, n = 2–4, abbreviated P-n-P ligated platinum-gold cluster compounds has been synthesized in high yield by ligand exchange reactions with PPh3 ligated, hydrido PtAu clusters. The exchange reaction between [(H)(PPh3)Pt(AuPPh3)7]2+ and the P-2-P ligands gives the clusters [(PPh3PtAu8(P-n-P)4]2+ (1, 2), and with P-4-P the cluster [(P-4-P)PtAu6(P-4-P)3]2+ (3) is formed. The higher nuclearity cluster [PtAu10(P-3-P)5]2+ (8) was prepared by reaction of [(H)Pt(AuPPh3)9]2+ with P-3-P. All of these clusters have the 18-electron configuration and thus are unreactive with small electron donor molecules such as CO, Hg and H2. Clusters 2, 3 and 8 react with the Lewis acids MX (MX = CuCl and AgNO3) giving tri-metallic compounds of higher nuclearity. The products of the MX addition to 1 and 2 are the four new clusters [(PPh3)Pt(MCl)Au8(P-n-P)4]2+ where n = 2 and 3 and M = Cu and Ag (4–7). The reaction of MX with 8 gives only the double addition products [Pt(MCl)2Au10(P-3-P)5]2+ (9, 10). The molecular structures of clusters 2, 3, 5, 7 and 8 were determined by single crystal X-ray diffraction ( Mo K α, λ = 0.71037 A , 173 K ; crystal data for 2 : P 1 , a = 17.1484(4), b = 20.8146(5), c = 23.1098(5), α = 71.284(1) A , β = 70.717(1), γ = 71.632(1)°, V = 7169.6(3) A 3 , Z = 2, R = 0.10; 3: P 1 , a = 14.833(1), b = 15.905(2), c = 27.068(3) A , α = 81.175(7), β = 77.088(12), γ = 67.742(6)°, V = 5743.7(10) A 3 , Z = 2, R = 0.051; 5: P2 1 /c, a = 24.353(3), b = 26.326(3), c = 20.554(2) A , β = 108.082(6)°, V = 12526(2) A 3 , Z = 4, R = 0.067; 7: P 1 , a = 17.4401(3), b = 18.4505(2), c = 23.1111(3) A , α = 97.615(1), β = 104.731(1), γ = 104.275(1)°, V = 6817.3(2) A 3 , Z = 2, R = 0.087; 8: P 1 , a = 16.048(3), b = 16.015(3), c = 30.790(5) A , α = 83.408(12), β = 89.677(12), γ = 65.135(13)°, V = 7124(2) A 3 , Z = 2, R = 0.096) . All have Pt-centered, icosahedral fragment geometries for their PtMAu8 metal frames with the diphosphine ligands bridging adjacent Au atoms and also chelated to Pt in 3. The diphosphine ligated clusters are less labile and less fluxional than their PPh3 analogs due to constraints on the number of skeletal rearrangement pathways imposed by the bridging diphosphine ligands.

Carbonyl adducts of bis(1,4-bis(diphenylphosphino)butane)rhodium(I) cation. Crystal and molecular structure of [Rh2(Ph2P(CH2)4PPh2)3(CO)4](PF6)2

Abstract The reaction of CO with CH2Cl2 solutions of [Rh(dppb)2]+X−, where X = BF4 or PF6 and dppb = Ph2P(CH2)4PPh2, yields the binuclear complex [Rh2(dppb)3(CO)4]2+ and uncoordinated dppb. This complex was isolated in the solid state as the PF6 salt and characterized by single crystal X-ray diffraction. The structure consists of two trigonal bipyramidal RhP3(CO)2 cores bridged by a dppb ligand via axial coordination sites. The CO ligands are cis and occupy equatorial sites and the chelating dppb ligands span equatorial and axial sites of their respective coordination cores. One binuclear molecule (Rh2(dppb)3)(CO)4](PF6)2 is contained in the P 1 unit cell such that an inversion center exists in the center of the backbone of the bridging dppb ligand. The unit cell dimensions are a = 12.385(2), b = 15.286(2), c = 12.353(4) A, α = 99.77(2), β = 107.03(2), γ = 103.66(1)°, and V = 2100 A3. Atotal of 5780 observed reflections were used to solve the structure and the final R value was 0.060. The formation of the binuclear complex in acetone solution was monitored by 31P{1H} NMR. The complex is fluxional at 25°C but exhibits a complicated pattern at −65°C in addition to a sharp singlet due to uncoordinated dppb ligand.

Catalytic Decarbonylation of Aldehydes Using Cationic Complexes of Rhodium(I) with Chelating Diphosphine Ligands

The decarbonylation of aldehydes, acyl halides, aroyl halides and alcohols is a useful and important reaction in organic synthesis.1,2 Although several methods not utilizing transition metals are known (including various deformylation reactions, thermal and photochemical decarbonylations ),3,4 they are not general and not usually applicable under mild conditions where undesirable side reactions are minimized. Stoichiometric homogeneous decarbonylation of aldehydes and acid chlorides using transition metal complexes of monodentate tertiary phosphine ligands such as RhCl(PPh3)3 5−7, [Rh−(PPh3)2(solvent)n]+ 8, and [Ru2Cl3(PEt2Ph)+ 9 is now well established. Of these, RhCl(PPt3)3 I has received the most study and has been proven useful as a reagent in organic synthesis.1,10 I decarbonylates aldehydes and acid chlorides under mild thermal conditions (<100°C) homogeneously in solution. The reactions are stoichiometric and are summarized by eq 1 and 2 where X = H or C1.6,11−13 Reaction Open image in new window 2 where X = CI occurs when a β-H is present while reaction 2 where X = H is of minor importance (e. g., decarbonylation of heptanal using I yields 86% hexane and 14% 1-hexene) .6 Hence, complex I is useful for the stoichiometric conversion of aldehydes into alkanes and of acid halides into alkyl or aryl halides and olefins (if β-H is present).