Synthesis and VT-NMR studies of cationic hydride platinum clusters

The synthesis of three trinuclear platinum hydrides [Pt₃(L-L)₃(H)₃]⁺ (L-L = 1,2-bis(diphenylphosphino)ethane, dppe, l; 1-diphenylphosphino-2-diphenylarsinoethane, dppae. 2; 1,2-bis(diphenylarsino)ethane, dpae, 3) is reported. The complexes were characterized by IR, FAB-MS, and NMR (¹H,³¹P and¹⁹⁵Pt) spectroscopic techniques. The fast exchange of the hydride ligands, observed at ambient temperature, is frozen out at low temperature. The low-temperature¹H and³¹P NMR spectra are consistent with an open array of Pt atoms in the clusters, in keeping with a 16-electron configuration on each platinum atom. Two of the hydride ligands are terminally bonded to two metal centers, whereas the third one is µ₃-coordinated, interacting more tightly with the unique platinum atom.     

Molecular orbital calculations on transition metal clusters

Transition metal clusters have been subject of experimental and theoretical interest due to their catalytic activity, as well as their unusual physical properties. Semi-empirical extended Hückel molecular orbital calculations are presented for a series of small metal clusters with nuclearity ranging from 3 to 6. Naked and carbonylated clusters of Fe, Ru, and Os are studied. The charge transfer between ligands shell and metal core is found to be a function of nuclearity, CO coordination and the chemical species forming the bare cluster. The observed magnetic properties of these systems are discussed in terms of their electronic structure and CO-metal charge transfer.     

Molecular simulation of oxygen on supported platinum clusters

Molecular dynamics simulations are performed to study oxygen adsorption on platinum clusters supported on a graphite surface. The Sutton–Chen many-body potential is used for the Pt–Pt interaction, whereas a Steele potential was used to represent the carbon surface. The oxygen–oxygen intramolecular force is modeled by a harmonic oscillator model and other interactions are described by the Lennard–Jones potential. The results indicate an optimum loading of platinum for maximum specific adsorption of oxygen. Adsorption isotherms are constructed and the energies and orientation of adsorbed oxygen are reported. The relevance of this study to electrode processes is discussed.     

Molecular dynamics simulations of the interactions between platinum clusters and carbon platelets

Molecular dynamics simulations have been performed with two reactive force fields to investigate the structure of a Pt100 cluster adsorbed on the three distinct sides of a carbon platelet. A revised Reax force field for the carbon-platinum system is presented. In the simulations, carbon platelet edges both with and without hydrogen termination have been studied. It is found that the initial mismatch between the atomic structure of the platelet egde and the adsorbed face of the Pt100 cluster leads to a desorption of a few platinum atoms from the cluster and the subsequent restructuring of the cluster. Consequently, the average Pt-Pt bond length is enlarged in agreement with experimental results. This change in the bond length is supposed to play an important role in the enhancement of the catalytic activity, which is demonstrated by studying the changes in the bond order of the platinum atoms. We found an overall shift to lower values as well as a loss of the well-defined peak structure in the bond-order distribution.     

Ruthenium hydride complexes of the hindered phosphine ligand tris(3-diisopropylphosphinopropyl)phosphine

The synthesis and characterization of the novel hindered tripodal phosphine ligand P(CH(2)CH(2)CH(2)P(i)Pr(2))(3) (P(3)P(3)(iPr)) (1) are reported, along with the synthesis and characterization of ruthenium chloro and hydrido complexes of 1. Complexes [RuCl(P(3)P(3)(i)Pr)][BPh(4)] (2[BPh(4)]), RuH(2)(P(3)P(3)(i)Pr) (3), and [Ru(H(2))(H)(P(3)P(3)(iPr))][BPh(4)] (4[BPh(4)]) were characterized by crystallography. Complex 2 is fluxional in solution, and low-temperature NMR spectroscopy of the complex correlates well with two dynamic processes, an exchange between stereoisomers and a faster turnstile-type exchange within one of the stereoisomers.     

Molecular orbital analysis of chemical carcinogens

Simple molecular orbital calculations are employed in searching electronic parameters which may characterize the chemical carcinogens. Using frontier orbitals, the carcinogen-DNA bond formation is described as an electron transfer from the highest occupied molecular orbital (HOMO) of DNA to the lowest unoccupied molecular orbital (LUMO) of the carcinogen. Analysis of the DNA bases units shows that the electron donation occurs preferentially at the guanine site. The calculated low LUMO energy of several carcinogens indicate correctly the electrophilic character of these compounds. The difference between the carcinogen and the ultimate carcinogen is analyzed. Epoxides, free radicals, alkylating agents, and other metabolite forms are studied. A reasonable correlation is found between the LUMO energy and the carcinogenic function. © 1997 John Wiley & Sons, Inc.     

Decomposition of electron-excited molecules of molybdenum and tungsten phosphine hydride and phosphine nitrogen complexes

The kinetics and mechanism of photodehydrogenation of the phosphine hydride complexes MH₄L₄ (M = Mo, W; L are phosphine ligands) and the formation of coordinatively unsaturated species M_L4 were studied by the absorbance of long-wavelength bands with λ_max at 450–460 nm appeared in the absorption spectra of the photoproducts. The rate constants of the reactions of the coordinatively unsaturated M(DPPE)₂ species (M = Mo, W; DPPE = Ph₂PCH₂CH₂PPh₂) with molecular nitrogen in benzene were determined (k _W = 200 s⁻¹, k _Mo = 8700 s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 282–284, February, 2008.     

Stereochemistry of heterometallic platinum clusters

Heterometallic platinum complexes cover a huge field, as shown by a recent survey covering the crystallographic and structural data of almost 1500 examples. About 5% of those complexes exists as isomers and are summarized in this review; except one cis-trans example, the remainder are distortion isomers. These are discussed in terms of the coordination about the platinum atom, and correlations are drawn between donor atom, bond lengths and interbond angles, with attention to trans effect and metal-metal bonds. Distortion isomers, differing only by degree of distortion in Pt–L and Pt–M distances and L–M–L bond angles, spread over a wide range of oxidation states of platinum: zero, +1, +2 (most common) and +4. The mean Pt–Pt bond distance elongate with increase in oxidation state of platinum: 2.705 Å (Pt(0)–Pt(0)) < 2.720 Å (Pt(I)–Pt(I)) < 2.773 Å (Pt(II)–Pt(II)). The shortest mean Pt–M bond distances are: Pt(0)–Ga = 2.37 Å; Pt(I) = Au 2.697 Å, Pt(II)–Fe = 2.625 Å and Pt(IV)–Sn, 2.580 Å.     

Molecular orbital analysis of the bonding in high nuclearity gold cluster compounds

Extended Hückel molecular orbital calculations on high nuclearity gold clusters of the general type [Au(AuPH₃)_n]^x+ have demonstrated that they can be classified into two broad topological classes according to the three-dimensional disposition of the peripheral gold atoms. If they lie approximately on a sphere they are characterised by a total of 12n + 18 valence electrons, but if they adopt a toroidal or eliptical arrangement the total electron count is 12n + 16. The computed energy differences between alternative polyhedral geometries is generally small and accounts for the stereochemical non-rigidity of the gold cluster compounds in solution. Detailed aspects of the structures of the high nuclearity gold cluster compounds have been interpreted in terms of molecular orbital calculations on clusters derived from the centred chair [Au₇(PH₃)₆]⁺ by edge- and face-capping with Au(PH₃)⁺ fragments.     

Sulfide aggregates and clusters of platinum

The homometallic and lteterometallic chemistry of platinum sulfide polynuclear complexes with or without metal-metal bonds is reviewed. The synthetic strategies, structural and bonding characteristics, and chemical properties of these materials are emphasized. Other dinuclear platinum sulfide complexes which are potential precursors to larger aggregates (non-M-M bonded) or clusters (M-M bonded) are also identified.     

Structural evolution of subnano platinum clusters

We present a theoretical study of the structural evolution of small minimum energy platinum clusters, using density functional theory (DFT). Three growth pathways were identified. At the subnanoscale, clusters with triangular packing are energetically most favorable. At a cluster size of approximately n = 19, a structural transition from triangular clusters to icosahedral clusters occurs. A less energetically favorable transition from triangular clusters to fcc‐like clusters takes place at around n = 38. Ionization potentials, electron affinities, and magnetic moments of the triangular clusters were also calculated. Understanding the structures and properties will facilitate studies of the chemical reactivity of Pt nanoclusters toward small molecules. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Reactivity and flexibility in platinum metal clusters

The synthesis, structural properties, and fluxional behaviour of platinum-triosmium and platinum-triruthenium clusters derived from Os₃Pt(-H)₂ (CO)₁₀(PR₃) and Ru₃Pt(-H)(-CC ^t Bu)(CO)₉ (dppe) and related species are described.     

Molecular orbital treatment of gas adsorption on 3d-metal clusters III. Adsorption of nitrogen

This third part of the series presents results for nitrogen atomic and molecular adsorption on 3d-metal clusters, received with the Hückel method in the frontier molecular orbitals approximation. Non-dissociative adsorption of nitrogen above the clusters of the metals from Sc to Cr proves energetically preferred. Above the clusters of Fe, Co and Ni the process is expected to be activated and endothermic. The activation barrier for dissociation in adsorbed state is however lower than that for the rupture of the N-N bond in an isolated nitrogen molecule. Dissociative adsorption above the clusters of Mn and Cu is activated. Reference is made to the experimental results, concerning 3d-metal adsorption and catalytic properties in the reaction of ammonia synthesis, and a satisfactory agreement with the numerical data is found.     

Reduction of phosphorus in organic compounds to phosphine under the influence of lithium aluminum hydride

Conclusions The reaction for the reduction of phosphorus-containing organic compounds using lithium aluminum hydride is selective and cannot be used to create a general method for the analysis of phosphorus.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2314–2316, October, 1971.     

A new series of luminescent phosphine stabilised platinum ethynyl complexes

A series of cis-platinum ethynyl complexes with the general formula cis-[Pt(dppe)(C[triple bond]CR)2](dppe = 1,2-bis(diphenylphosphino)ethane; R = C6H4-p-NO2 1, C6H4-p-CH3 2, C6H4-p-C[triple bond]CH 3 and C6H4-p-C6H4-p-C[triple bond]CH 4) have been prepared by the coupling reaction of cis-[Pt(dppe)Cl2] with two equivalents of the appropriate alkyne. The new complexes have been fully characterized by spectroscopic techniques, and the cis square planar arrangement at the platinum centre has been confirmed by single-crystal X-ray diffraction studies of complexes 1, 2 and 4. The absorption spectra of the complexes 1-4 are dominated by a pi-->pi* band that contains some platinum (n + 1) p orbital character. The position of the band is dependent on the electron donating or withdrawing properties of the ethynyl substituents, R. Complex 1 displays a triplet emission in the green, at room temperature, while complexes 2-4, display singlet emissions in the blue. Again, the difference can be attributed to the nature of the R substituents.     

The indirect location of hydride ligands in metal carbonyl clusters

A semi-empirical method of predicting the location of hydride ligands in metal carbonyl cluster compounds using X-ray structural information is described. The hydride positions thus derived are compared with those obtained from neutron diffraction experiments and predictions made for previously ambiguous cases.     

Molecular orbital studies of the thienopyrazines

The calculated reactivity indicies and bond lengths for thieno[2,3-b]- and thieno[3,4-b]-pyrazine are reported.