Transition metal polyhydride complexes: a theoretical view

Heptanuclear Silver Hydride Clusters as Catalytic Precursors for the Reduction of 4-Nitrophenol

We report on the design, synthesis, and characterization of the first silver hydride clusters solely protected and stabilized by dithiophosphonate ligands and their application for the in situ generation of silver nanoparticles towards the catalytic reduction of 4-nitrophenol in an aqueous system. The synthesis of the silver monohydride cluster involves the incorporation of an interstitial hydride using sodium borohydride. Poly-nuclear magnetic resonance and mass spectrometry were used to establish the structural properties. The structural properties were then confirmed with a single-crystal X-ray diffraction analysis, which showed a distorted tetracapped tetrahedron core with one hydride ion encapsulated within the core of the silver framework. Additionally, the synthesized heptanuclear silver hydride was utilized as a precursor for the in situ generation of silver nanoparticles, which simultaneously catalyzed the reduction of 4-nitrophenol. The mechanism of the catalytic activity was investigated by first synthesizing AgNPs, which was subsequently used as a catalyst. The kinetic study showed that the pseudo-first constant obtained using the cluster (2.43 × 10−2 s−1) was higher than that obtained using the synthesized AgNPs (2.43 × 10−2 s−1). This indicated that the silver monohydride cluster was more active owing to the release of the encapsulated hydride ion and greater reaction surface prior to aggregation.

Hydride-encapsulated bimetallic clusters supported by 1,1-dithiolates

A four-coordinated hydride lying at the center of heptanuclear bimetallic clusters was anisotropically refined to convergence by X-ray crystallography.

Synthesis of di- and trinuclear iridium polyhydride complexes surrounded by light-absorbing ligands

New di- and trinuclear iridium (Ir) penta- and hexahydride complexes containing light-absorbing diphosphine ligands have been developed. The trinuclear Ir complex possessed a higher absorption coefficient (standardized per Ir) than that of the parent mononuclear Ir complex, indicating the assembly effect. The trinuclear complex showed high reactivity toward catalytic hydrogenation of diphenylacetylene under mild conditions, and the reaction was considerably accelerated under photoirradiation.

Including Memory Friction in Single- and Two-State Quantum Dynamics Simulations

We present a simple computational algorithm that allows for the inclusion of memory friction in a quantum dynamics simulation of a small, quantum, primary system coupled to many atoms in the surroundings. We show how including a memory friction operator, F̂, in the primary quantum system's Hamiltonian operator builds memory friction into the dynamics of the primary quantum system. We show that, in the harmonic, semi-classical limit, this friction operator causes the classical phase-space centers of a wavepacket to evolve exactly as if it were a classical particle experiencing memory friction. We also show that this friction operator can be used to include memory friction in the quantum dynamics of an anharmonic primary system. We then generalize the algorithm so that it can be used to treat a primary quantum system that is evolving, non-adiabatically on two coupled potential energy surfaces, i.e., a model that can be used to model H atom transfer, for example. We demonstrate this approach's computational ease and flexibility by showing numerical results for both harmonic and anharmonic primary quantum systems in the single surface case. Finally, we present numerical results for a model of non-adiabatic H atom transfer between a reactant and product state that includes memory friction on one or both of the non-adiabatic potential energy surfaces and uncover some interesting dynamical effects of non-memory friction on the H atom transfer process.

Hydrodefluorination of pentafluoropyridine at rhodium using dihydrogen: detection of unusual rhodium hydrido complexes

The pentafluoropyridyl complex [Rh(4-C5NF4)(PEt3)3] (3) reacts with H2 to give initially the dihydrido complex cis-mer-[Rh(H)2(4-C5NF4)(PEt3)3] (6). Within a few hours 2,3,5,6-tetrafluoropyridine as well as two rhodium(III) complexes mer-[Rh(H)3(PEt3)3] (mer-) and fac-[Rh(H)3(PEt3)3] (fac-) are formed. A catalytic C-F activation process for the formation of 2,3,5,6-tetrafluoropyridine starting from pentafluoropyridine and dihydrogen using 3 as a catalyst has been developed. Reaction of [RhH(PEt3)3] (1) with hydrogen affords fac-[Rh(H)3(PEt3)3] (fac-7) and mer-[Rh(H)3(PEt3)3] (mer-7) in a ratio of 1 : 7.25 at 193 K. The latter complex represents the first mononuclear rhodium compound bearing trans-hydrides.

Performance of Molecular Orbital Methods and Density Functional Theory in the Computation of Geometries and Energies of Metal Aqua Ions

Geometries and energies of various metal aqua ions have been computed with Hartree-Fock (HF), single-reference second-order perturbation theory according to Møller and Plesset (MP2), complete active space self-consistent field (CAS-SCF) methods, multi-configurational quasi-degenerate second-order perturbation theory (MCQDPT2), and density functional theory (DFT), whereby for the latter the most widely used functionals BLYP, B3LYP, LDA ("local density approximation"), and another functional (SOP) exhibiting Slater exchange, have been investigated. The geometries as well as the energies are sensitive to the computational method, results of which can lead to different conclusions. The geometries of certain reactants, products, intermediates, or transition states cannot be obtained with all of the above-mentioned computational methods: in some cases, the structures are very poor, exhibiting large errors in the bond parameters, and in others, the targeted species cannot be calculated at all. The different computational techniques do not always predict the same coordination number of a given aqua ion or the same preferred water exchange mechanism. Inaccuracies arising from inadequate basis sets are also discussed.

The hydride anion in an extended transition metal oxide array: LaSrCoO3H0.7

We present the synthesis and structural characterization of a transition metal oxide hydride, LaSrCoO3H0.7, which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering at temperatures up to at least 350 kelvin. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal--containing electronic and magnetic materials.

Synthesis and characterization of ReCl(H(2))(AsMePh(2))(4), a classical hydride complex; reexamination of ReCl(H(2))(PMePh(2))(4) and theoretical calculations on model compounds

The synthesis of ReCl(H(2))(AsMePh(2))(4) is reported and verified on the basis of (1)H NMR and FAB-MS. The minimum T(1) time for the metal-bonded hydrogen atoms was determined to be 44 ms (-78 degrees C) and 89 (-48 degrees C) ms at 200 and 400 MHz, respectively. From this minimum T(1) time, and, allowing for contributions from other nuclei to the relaxation, a metal-bonded H to H atom distance of 1.57 A can be assessed. The complex is reversibly oxidized at a potential E(1/2)(ox) = +0.07 V. The J(HD) coupling in the related ReCl(HD)(AsMePh(2))(4) compound was at 3.3 Hz. These data for ReCl(H(2))(AsMePh(2))(4) suggest that this molecule contains classical hydrides, and interpretations on T(1) and X-ray data published previously for ReCl(H(2))(PMePh(2))(4) suggest that this molecule contains a very long metal-bonded H-H interaction of 1.39 A. Theoretical calculations on model compounds ReCl(H(2))(XR(3))(4) (X = P, As; R = H, Me) give virtually identical average d(H-H) values of 1.5 (XH(3)) and 1.63 (XMe(3)) A.

Structural distortions in mer-M(H)3(NO)L2 (M = Ru, Os) and their influence on intramolecular fluxionality and quantum exchange coupling

Molecules of the type mer-M(H)3(NO)L2 [M = Ru (1), Os (2); L = PR3] are characterized on the basis of 1H NMR T1min values and IR spectra as pseudo-octahedral trihydrides significantly distorted by compression of the cis H-M-H angles to approximately 75 degrees. The distortion, uncharacteristic of six-coordinate d6 complexes, is rationalized with DFT (B3LYP) calculations as being driven by increased H-to-M sigma donation and by the exceptional pi-accepting ability of linear NO+. In both 1 and 2, hydrides undergo intramolecular site exchange with delta HHH++(1) = 10-11 kcal/mol and delta HHH++(2) = 16-20 kcal/mol, depending on L, whereas for mer-Ru(H)3(NO)(PtBu2Me)2 (1b), moderate exchange couplings (up to 77 Hz) are featured in the low-temperature 1H NMR spectra, in addition to chemical exchange. On the basis of experimental and theoretical results, a dihydrogen intermediate is suggested to mediate hydride site exchange in 1. The cis H-M-H distortion shortens the tunneling path for the exchanging hydrides in 1, thereby increasing the tunneling rate; diminishes the "conflict" between trans hydrides in the mer geometry; and decreases the nucleophilicity of the hydrides. The generality of the observed structural distortion and its dependence on the ligand environment in late transition metal tri- and dihydrides are discussed. A less reducing metal center is generally characterized by greater distortion.