Richard D. Adams

Tetrarhena-heterocycle from the Palladium-Catalyzed Dimerization of Re2(CO)8(μ-SbPh2)(μ-H) Exhibits an Unusual Host–Guest Behavior

The six-membered heavy atom heterocycles [Re(2)(CO)(8)(μ-SbPh(2))(μ-H)](2), 5, and Pd[Re(2)(CO)(8)(μ-SbPh(2))(μ-H)](2), 7, have been prepared by the palladium-catalyzed ring-opening cyclo-dimerization of the three-membered heterocycle Re(2)(CO)(8)(μ-SbPh(2))(μ-H), 3. The palladium atom that lies in the center of the heterocycle 7 was removed to yield 5. The palladium removal was found to be partially reversible leading to an unusual example of host-guest behavior. A related dipalladium complex Pd(2)Re(4)(CO)(16)(μ(4)-SbPh)(μ(3)-SbPh(2))(μ-Ph)(μ-H)(2), 6, was also formed in these reactions of palladium with 3.

Unusual Structures and Reactivity of Mixed Metal Cluster Complexes Containing the Palladium/Platinum Tri-t-butylphosphine Grouping

Polynuclear metal carbonyl complexes have a range of applications in chemical research: for example, they can serve as surface models to probe features of heterogeneous catalysis and can perform novel transformations of organic molecules in solutions. Mixed metal complexes can demonstrate bimetallic cooperativity and synergism and can also serve as precursors to multimetallic heterogeneous catalysts that have superior activities and selectivities. This Account describes the results of our recent comprehensive study of the chemistry of mixed metal cluster complexes containing the sterically encumbered M(PBu(t)(3)), M = Pd or Pt, group. This grouping readily adds to the metal-metal bonds of metal carbonyl cluster complexes and modifies their reactivity. We have prepared new, highly electronically unsaturated mixed metal complexes that exhibit unusually high reactivity toward hydrogen. The platinum atom of the Pt(PBu(t)(3)) grouping can bond to as many as five metal atoms, and it can interconvert, sometimes rapidly, between the different bonding modes. The large steric effects of the PBu(t)(3) ligand allowed us to prepare highly unsaturated, stable, mixed-metal complexes, and these complexes react with hydrogen, sometimes reversibly, under very mild conditions to yield polyhydride complexes. Strong evidence suggests that the Pt(PBu(t)(3)) group can also activate metal-hydrogen bonds in other complexes. In the future, we expect that researchers will prepare a greater variety of mixed metal complexes containing the Pd/Pt(PBu(t)(3)) group or other similar bulky groups, and that some of these complexes will exhibit even more unusual chemistry than what we have observed so far.

Metal segregation in bimetallic clusters and its possible role in synergism and bifunctional catalysis

Abstract Aspects of the reactivity of segregated bimetallic compounds and their unusual catalytic properties are reviewed with an emphasis on the authors own studies of platinum—ruthenium mixed-metal cluster complexes and their ability to produce hydrogenation and hydrosilylation of alkynes, catalytically.

Structural and dynamic properties of dicyclopenta-dienylhexacarbonyldimolybdenum in various solvents

Abstract The structural and dynamical properties of [(h5-C5H5)Mo(CO)3]2 have been studied by infrared and nmr spectroscopy in various solvents. The infrared results indicate that as solvent dielectric constant increases an increasing proportion of the molecules rearrange from the trans (C2h) rotamer to a gauche (C2) rotamer. The free energy (25), enthalpy and entropy changes for the gauche to trans rearrangement in acetone are: −0.57±0.33 Kcal/mole, 0.44±0.15 Kcal/mole, and 3.38±0.61 cal/mole-degree. The rate of trans-gauche interconversions as a function of temperature was determined by fitting of computer-simulated to measured nmr spectra. The Arrhenius parameters of activation are Ea = 15.3±1 Kcal/mole, Log A = 13.0±1. The substantial energy for interconversion of rotamers found here implies that barriers to rotation may play critical roles in cis-trans interconversion in other systems, such as [(h5C-5H5)Fe(CO)2]2. A modification of the previously suggested diagram of energy vs molecular configuration of [h5-C5H5)Fe(CO)2]2 is proposed.

New catalytic liquid-phase ammoxidation approach to the preparation of niacin (vitamin B3).

New highly dispersed bimetallic nanoscale catalysts based on rhenium combined with antimony or bismuth have been shown to be highly effective for the ammoxidation of 3-picoline to nicotinonitrile (precursor for vitamin B3) under mild conditions in the liquid phase.

Synthesis, characterization, electronic structure and catalytic performance of bimetallic and trimetallic nanoparticles containing tin

When anchored on a high-area, siliceous supports, nanoparticle catalysts, consisting of two or three different metals, but totaling no more than twenty atoms in all, exhibit exceptional activities and selectivities in solvent-free, one-step hydrogenation reactions at low temperatures (< 420 K) and much lower pressures (e.g. 30 bar) than those required in current industrial manufacture. The two selective hydrogenations illustrated here are the conversion of (a) cyclododecatriene (CDT) to cyclododecene (CD) and (b) dimethyl terephthalate (DMT) to cyclohexane dimethanol (CHDM); each of these products is extensively used in the polymer industry. All our mixed-metal nanoparticles are derived from an appropriately chosen parent (precursor) mixed-metal carbonyl having phenyl-containing tin ligands, e.g. Ru4(mu4-SnPh)2(CO)12. Various techniques are used to characterize the denuded, anchored cluster catalysts; and it is expected that aberration-corrected high-resolution electron microscopy (and other techniques, which are outlined) will be invaluable in such characterization. Density functional theory has provided important insights into the structures and electronic properties of our catalysts and their precursors.

Ruthenium–tin cluster complexes and their applications as bimetallic nanoscale heterogeneous hydrogenation catalysts

This review describes recent studies on the synthesis and characterization of new polynuclear ruthenium–tin cluster complexes, their conversion to heterometallic nanoparticles, and some studies on their applications as catalysts for the hydrogenation of unsaturated organic molecules of commercial interest.

Reactions involving hydrogen transfer from triosmium clusters to an activated nitrile

Abstract The reactions of H2Os3(CO)10, Ia and H2Os3(CO)9PMe2Ph, Ib with CF3CN have been investigated. Both la and Ib react with CF3CN to give the products HOs3[μ-η2-(CF3)CNH](CO)9Land HOs3[μ-η1-NC(H)CF3](CO)9L, IIa, IIIa, L = CO; IIb and IIIb, L = PMe2Ph. IIb and IIIb have been characterized crystallographically. In each, one nitrile molecule was added to the cluster and one hydride ligand was transferred to the nitrile ligand, but in IIb the hydride was transferred to the nitrogen atom to form a CF3CNH ligand which bridges an edge of the cluster while in IIIb the hydride was transferred to the carbon atom to form a CF3(H)CN ligand which also bridges an edge of the cluster. On the basis of spectroscopic measurements IIa and IIIa are believed to have analogous structures. An isotope scrambling experiment established that the formation of Ilia occurs by an intramolecular process. IIa was decarbonylated to yield the compound HOs3[μ3-η2-(CF3)CNH](CO)9, which is believed to contain a triply-bridging iminyl ligand. Ilia reacts with PMe2Ph to give two mono-substitution products, one of which is IIIb.

Group 10 telluride and polytelluride complexes: Synthesis and structures of Pd(Te4)22− and (Ph3P)2Pt(μ-Te)2Pt(PPh3)2

Abstract PdCl 2 reacts with K 2 Te 4 in DMF to produce Pd(Te 4 ) 2 2− , which was isolated as a bis-PPh 4 + salt. The X-ray structure of this compound revealed a square-planar palladium atom with two chelated Te 4 2− ligands. Pt(PPh 3 ) 4 reacts with (Bu 4 N) 4 Hg 4 Te 12 in DMF to produce (Ph 3 P) 2 Pt(μ-Te) 2 Pt(PPh 3 ) 2 which was also characterized by single-crystal X-ray diffraction.

Cluster synthesis—XXIII. The synthesis, structure and bonding of Fe4(CO)10(μ-CO)(μ4-S)2

Abstract The compound Fe4(CO)10(μ-CO)(μ4-S)2 (1) was synthesized in 38% yield by the UV induced decarbonylation of Fe(CO)5 in the presence of Fe3(CO)9(μ3-S)2. Compound 1 was characterized by a single-crystal X-ray diffraction analysis. Space group: Pccn, a = 6.603(1), b = 15.429(3), c = 17.292(4) A, Z = 4. The structure was solved by direct methods and was refined (807 reflections) to the final values of the residuals R = 0.043 and Rw = 0.054. The molecule consists of a planar array of four iron atoms with a quadruply bridging sulphido ligand on each side of the plane. The shortest metal-metal bond, 2.489(3) A, contains a bridging carbonyl ligand. Semi-bridging carbonyl ligands bridge the two adjacent metal-metal bonds, 2.532(2) A. The longest metal-metal bond, 2.605(2) A, has no carbonyl bridge. Compound 1 is unsaturated (by EAN rule) by the amount of two electrons. The two semi-bridged carbonyl-metal bonds in 1 are significantly shorter than those in the saturated cluster Fe2CO2(CO)11(μ4-S)2. A molecular orbital description which explains the differences in bonding between the two compounds is proposed.