65 research outputs found

    Unexpected Electronic Process of H<sub>2</sub> Activation by a New Nickel Borane Complex: Comparison with the Usual Homolytic and Heterolytic Activations

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    H–H σ-bond activation promoted by Ni­[MesB­(<i>o</i>-Ph<sub>2</sub>PC<sub>6</sub>H<sub>4</sub>)<sub>2</sub>] (<b>1</b><sup><b>Mes</b></sup>) was theoretically investigated with the density functional theory method. In <b>1</b><sup><b>Mes</b></sup>, the nickel 3d, 4s, and 4p orbital populations are similar to those of the typical nickel­(II) complex. First, one H<sub>2</sub> molecule coordinates with the nickel center to form a dihydrogen complex, <b>2</b>, which induces an increase in the nickel 3d and 4p orbital populations and thus a decrease in the nickel oxidation state. Then, the H–H σ-bond is cleaved under the unusual cooperation of the electron-rich nickel center and the electron-deficient borane ligand in a polarized manner, leading to an unprecedented <i>trans</i>-nickel­(II) hydridoborohydrido complex, <b>3</b>. In the transition state, charge transfer (CT) occuring from the H<sub>2</sub> moiety to the <b>1</b><sup><b>Mes</b></sup> moiety (0.683 e) is much larger than the reverse CT (0.284 e). As a result, cleavage of the H–H σ-bond affords two positively charged hydrogen atoms. In this process, the boron atomic population and the nickel 4p orbital population increase, but the nickel 3d orbital population decreases. After cleavage of the H–H σ-bond, CT from the nickel 4p orbital to these positively charged hydrogen atoms occurs to afford <b>3</b>, where the oxidation state of the nickel center increases to +2. These electronic processes are different from those of the usual homolytic and heterolytic H–H σ-bond activations. Regeneration of <b>1</b><sup><b>Mes</b></sup> and the role of the borane ligand in these reactions are also discussed in detail

    Embedded Cluster Model for Al<sub>2</sub>O<sub>3</sub> and AlPO<sub>4</sub> Surfaces Using Point Charges and Periodic Electrostatic Potential

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    An embedded cluster model with either a large number of point charges (PCs) or periodic electrostatic (PE) potential was proposed to incorporate the electrostatic effects by the bulk surface and applied to non-transition-metal oxide supports such as Al<sub>2</sub>O<sub>3</sub> and AlPO<sub>4</sub>. A large number of PCs are placed on several layers of surface. The PC values were taken to be the same as the Bader charges obtained from periodic DFT calculation of slab model. The PE potential was derived so as to consider the infinite three-dimensional PC distribution obtained by the calculation of slab model. One electron integral of the PE potential in the Gaussian basis function was evaluated using Poisson’s equation, Fourier transformation within a supercell approach, and the Ewald summation method. These embedded cluster models were applied to Rh<sub>2</sub>-adsorbed Al<sub>2</sub>O<sub>3</sub> and AlPO<sub>4</sub>. A bare cluster model with neither PC nor the PE potential presented very poor computational results for the interaction energies of Rh<sub>2</sub> with Al<sub>2</sub>O<sub>3</sub> and AlPO<sub>4</sub> surfaces, density of states, projected density of states, frontier orbital features, and spin density distribution. In contrast, the embedded cluster model successfully reproduced those properties when either a large number of PCs or the PE potential was employed. These results indicate that the embedded cluster models proposed here are useful for investigating theoretically non-transition-metal oxide surface using the hybrid DFT functional

    Theoretical Study of Reactivity of Ge(II)-hydride Compound: Comparison with Rh(I)-Hydride Complex and Prediction of Full Catalytic Cycle by Ge(II)-hydride

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    The reaction of a Ge­(II) hydride compound HC­{CMeArN}<sub>2</sub>GeH (Ar = 2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) <b>1</b> with 2,2,2-trifluoroacetophenone (CF<sub>3</sub>PhCO) is theoretically investigated with density functional theory and spin-component-scaled second-order Møller–Plesset methods. This reaction easily occurs with moderate activation barrier and considerably large exothermicity, to afford a Ge­(II) alkoxide <b>2</b> through a four-membered transition state. In the transition state, the charge transfer from the Ge–H σ-bonding molecular orbital (MO) to the CO π*-antibonding MO of CF<sub>3</sub>PhCO plays an important role. Acetone ((CH<sub>3</sub>)<sub>2</sub>CO) and benzophenone (Ph<sub>2</sub>CO) are not reactive for <b>1</b>, because their π*-antibonding MOs exist at higher energy than that of CF<sub>3</sub>PhCO. Though <b>2</b> is easily formed, the catalytic hydrogenation of CF<sub>3</sub>PhCO by <b>1</b> is difficult because the reaction of <b>2</b> with a dihydrogen molecule needs a large activation energy. On the other hand, our calculations clearly show that the catalytic hydrogenation of ketone by <i>cis</i>-RhH­(PPh<sub>3</sub>)<sub>2</sub> <b>4</b> easily occurs, as expected. The comparison of catalytic cycle between <b>1</b> and <b>4</b> suggests that the strong Ge–O bond of <b>2</b> is the reason of the very large activation energy for the hydrogenation by <b>1</b>. To overcome this defect, we investigated various reagents and found that the catalytic cycle can be completed with the use of SiF<sub>3</sub>H. The product is silylether CF<sub>3</sub>PhCHOSiF<sub>3</sub>, which is equivalent to alcohol because it easily undergoes hydrolysis to afford CF<sub>3</sub>PhCHOH. The similar catalytic cycles are also theoretically predicted for hydrosilylations of CO<sub>2</sub> and imine. This is the first theoretical prediction of the full catalytic cycle with a heavier main-group element compound

    Mo–Mo Quintuple Bond is Highly Reactive in H–H, C–H, and O–H σ‑Bond Cleavages Because of the Polarized Electronic Structure in Transition State

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    The recently reported high reactivity of the Mo–Mo quintuple bond of Mo<sub>2</sub>(N<sup>∧</sup>N)<sub>2</sub> (<b>1</b>) {N<sup>∧</sup>N = μ-κ<sup>2</sup>-CH­[N­(2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)]<sub>2</sub>} in the H–H σ-bond cleavage was investigated. DFT calculations disclosed that the H–H σ-bond cleavage by <b>1</b> occurs with nearly no barrier to afford the <i>cis</i>-dihydride species followed by cis–trans isomerization to form the <i>trans</i>-dihydride product, which is consistent with the experimental result. The O–H and C–H bond cleavages by <b>1</b> were computationally predicted to occur with moderate (Δ<i>G</i>°<sup>⧧</sup> = 9.0 kcal/mol) and acceptable activation energies (Δ<i>G</i>°<sup>⧧</sup> = 22.5 kcal/mol), respectively, suggesting that the Mo–Mo quintuple bond can be applied to various σ-bond cleavages. In these σ-bond cleavage reactions, the charge-transfer (CT<sub>Mo→XH</sub>) from the Mo–Mo quintuple bond to the X–H (X = H, C, or O) bond and that (CT<sub>XH→Mo</sub>) from the X–H bond to the Mo–Mo bond play crucial roles. Though the HOMO (dδ-MO) of <b>1</b> is at lower energy and the LUMO + 2 (dδ*-MO) of <b>1</b> is at higher energy than those of RhCl­(PMe<sub>3</sub>)<sub>2</sub> (LUMO and LUMO + 1 of <b>1</b> are not frontier MO), the H–H σ-bond cleavage by <b>1</b> more easily occurs than that by the Rh complex. Hence, the frontier MO energies are not the reason for the high reactivity of <b>1</b>. The high reactivity of <b>1</b> arises from the polarization of dδ-type MOs of the Mo–Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the dδ-MO of the Mo–Mo bond and the σ*-antibonding MO of the X–H bond to facilitate the CT<sub>Mo→XH</sub> and reduce the exchange repulsion between the Mo–Mo bond and the X–H bond. This polarized electronic structure of the transition state is similar to that of a frustrated Lewis pair. The easy polarization of the dδ-type MOs is one of the advantages of the metal–metal multiple bond, because such polarization is impossible in the mononuclear metal complex

    A Theoretical Study of an Unusual Y-Shaped Three-Coordinate Pt Complex: Pt(0) σ-Disilane Complex or Pt(II) Disilyl Complex?

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    The unusual Y-shaped structure of the recently reported three-coordinate Pt complex Pt­[NHC­(Dip)<sub>2</sub>]­(SiMe<sub>2</sub>Ph)<sub>2</sub> (NHC = N-heterocyclic carbene; Dip = 2,6-diisopropylphenyl) was considered a snapshot of the reductive elimination of disilane. A density functional theory study indicates that this structure arises from the strong trans influence of the extremely σ-donating carbene and silyl ligands. Though this complex can be understood to be a Pt­(II) disilyl complex bearing a distorted geometry due to the Jahn–Teller effect, its <sup>195</sup>Pt NMR chemical shift is considerably different from those of Pt­(II) complexes but close to those of typical Pt(0) complexes. Its Si···Si bonding interaction is ∼50% of the usual energy of a Si–Si single bond. The interaction between the Pt center and the (SiMe<sub>2</sub>Ph)<sub>2</sub> moiety can be understood in terms of donation and back-donation interactions of the Si–Si σ-bonding and σ*-antibonding molecular orbitals with the Pt center. Thus, we conclude that this is likely a Pt(0) σ-disilane complex and thus a snapshot after a considerable amount of the charge transfer from disilane to the Pt center has occurred. Phenyl anion (Ph<sup>–</sup>) and [R–Ar]<sup>−</sup> [R–Ar = 2,6-(2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>] as well as the divalent carbon(0) ligand C­(NHC)<sub>2</sub> also provide similar unusual Y-shaped structures. Three-coordinate digermyl, diboryl, and silyl–boryl complexes of Pt and a disilyl complex of Pd are theoretically predicted to have similar unusual Y-shaped structures when a strongly donating ligand coordinates to the metal center. In a trigonal-bipyramidal Ir disilyl complex [Ir{NHC(Dip)<sub>2</sub>}(PH<sub>3</sub>)<sub>2</sub>(SiMe<sub>3</sub>)<sub>2</sub>]<sup>+</sup>, the equatorial plane has a similar unusual Y-shaped structure. These results suggest that various snapshots can be shown for the reductive eliminations of the Ge–Ge, B–B, and B–Si σ-bonds

    Mo–Mo Quintuple Bond is Highly Reactive in H–H, C–H, and O–H σ‑Bond Cleavages Because of the Polarized Electronic Structure in Transition State

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    The recently reported high reactivity of the Mo–Mo quintuple bond of Mo<sub>2</sub>(N<sup>∧</sup>N)<sub>2</sub> (<b>1</b>) {N<sup>∧</sup>N = μ-κ<sup>2</sup>-CH­[N­(2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)]<sub>2</sub>} in the H–H σ-bond cleavage was investigated. DFT calculations disclosed that the H–H σ-bond cleavage by <b>1</b> occurs with nearly no barrier to afford the <i>cis</i>-dihydride species followed by cis–trans isomerization to form the <i>trans</i>-dihydride product, which is consistent with the experimental result. The O–H and C–H bond cleavages by <b>1</b> were computationally predicted to occur with moderate (Δ<i>G</i>°<sup>⧧</sup> = 9.0 kcal/mol) and acceptable activation energies (Δ<i>G</i>°<sup>⧧</sup> = 22.5 kcal/mol), respectively, suggesting that the Mo–Mo quintuple bond can be applied to various σ-bond cleavages. In these σ-bond cleavage reactions, the charge-transfer (CT<sub>Mo→XH</sub>) from the Mo–Mo quintuple bond to the X–H (X = H, C, or O) bond and that (CT<sub>XH→Mo</sub>) from the X–H bond to the Mo–Mo bond play crucial roles. Though the HOMO (dδ-MO) of <b>1</b> is at lower energy and the LUMO + 2 (dδ*-MO) of <b>1</b> is at higher energy than those of RhCl­(PMe<sub>3</sub>)<sub>2</sub> (LUMO and LUMO + 1 of <b>1</b> are not frontier MO), the H–H σ-bond cleavage by <b>1</b> more easily occurs than that by the Rh complex. Hence, the frontier MO energies are not the reason for the high reactivity of <b>1</b>. The high reactivity of <b>1</b> arises from the polarization of dδ-type MOs of the Mo–Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the dδ-MO of the Mo–Mo bond and the σ*-antibonding MO of the X–H bond to facilitate the CT<sub>Mo→XH</sub> and reduce the exchange repulsion between the Mo–Mo bond and the X–H bond. This polarized electronic structure of the transition state is similar to that of a frustrated Lewis pair. The easy polarization of the dδ-type MOs is one of the advantages of the metal–metal multiple bond, because such polarization is impossible in the mononuclear metal complex

    CASPT2 Study of Inverse Sandwich-Type Dinuclear Cr(I) and Fe(I) Complexes of the Dinitrogen Molecule: Significant Differences in Spin Multiplicity and Coordination Structure between These Two Complexes

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    Inverse sandwich-type complexes (ISTCs), (μ-N<sub>2</sub>)­[M­(AIP)]<sub>2</sub> (AIPH = (<i>Z</i>)-1-amino-3-imino-prop-1-ene; M = Cr and Fe), were investigated with the CASPT2 method. In the ISTC of Cr, the ground state takes a singlet spin multiplicity. However, the singlet to nonet spin states are close in energy to each other. The thermal average of effective magnetic moments (μ<sub>eff</sub>) of these spin multiplicities is close to the experimental value. The η<sup>2</sup>-side-on coordination structure of N<sub>2</sub> is calculated to be more stable than the η<sup>1</sup>-end-on coordination one. This is because the d-orbital of Cr forms a strong d<sub>π</sub>–π* bonding interaction with the π* orbital of N<sub>2</sub> in molecular plane. In the ISTC of Fe, on the other hand, the ground state takes a septet spin multiplicity, which agrees well with the experimentally reported μ<sub>eff</sub> value. The η<sup>1</sup>-end-on structure of N<sub>2</sub> is more stable than the η<sup>2</sup>-side-on structure. In the η<sup>1</sup>-end-on structure, two doubly occupied d-orbitals of Fe can form two d<sub>π</sub>–π* bonding interactions. The negative spin density is found on the bridging N<sub>2</sub> ligand in the Fe complex but is not in the Cr complex. All these interesting differences between ISTCs of Cr and Fe are discussed on the basis of the electronic structure and bonding nature

    Theoretical Study on the Transition-Metal Oxoboryl Complex: M–BO Bonding Nature, Mechanism of the Formation Reaction, and Prediction of a New Oxoboryl Complex

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    The Pt–BO bonding nature and the formation reaction of the experimentally reported platinum­(II) oxoboryl complex, simplified to PtBr­(BO)­(PMe<sub>3</sub>)<sub>2</sub>, were theoretically investigated with the density functional theory method. The BO<sup>–</sup> ligand was quantitatively demonstrated to have extremely strong σ-donation but very weak d<sub>π</sub>-electron-accepting abilities. Therefore, it exhibits a strong trans influence. The formation reaction occurs through a four-center transition state, in which the B<sup>δ+</sup>–Br<sup>δ−</sup> polarization and the Br → Si and O p<sub>π</sub> → B p<sub>π</sub> charge-transfer interactions play key roles. The Gibbs activation energy (Δ<i>G</i>°<sup>⧧</sup>) and Gibbs reaction energy (Δ<i>G</i>°) of the formation reaction are 32.2 and −6.1 kcal/mol, respectively. The electron-donating bulky phosphine ligand is found to be favorable for lowering both Δ<i>G</i>°<sup>⧧</sup> and Δ<i>G</i>°. In addition, the metal effect is examined with the nickel and palladium analogues and MBrCl­[BBr­(OSiMe<sub>3</sub>)]­(CO)­(PR<sub>3</sub>)<sub>2</sub> (M = Ir and Rh). By a comparison of the Δ<i>G</i>°<sup>⧧</sup> and Δ<i>G</i>° values, the M–BO (M = Ni, Pd, Ir, and Rh) bonding nature, and the interaction energy between [MBrCl­(CO)­(PR<sub>3</sub>)<sub>2</sub>]<sup>+</sup> and BO<sup>–</sup> with those of the platinum system, MBrCl­(BO)­(CO)­(PR<sub>3</sub>)<sub>2</sub> (M = Ir and Rh) is predicted to be a good candidate for a stable oxoboryl complex

    Theoretical Study of Mononuclear Nickel(I), Nickel(0), Copper(I), and Cobalt(I) Dioxygen Complexes: New Insight into Differences and Similarities in Geometry and Bonding Nature

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    Geometries, bonding nature, and electronic structures of (N<sup>∧</sup>N)­Ni­(O<sub>2</sub>) (N<sup>∧</sup>N = β-diketiminate), its cobalt­(I) and copper­(I) analogues, and (Ph<sub>3</sub>P)<sub>2</sub>Ni­(O<sub>2</sub>) were investigated by density functional theory (DFT) and multistate restricted active space multiconfigurational second-order perturbation (MS-RASPT2) methods. Only (N<sup>∧</sup>N)­Ni­(O<sub>2</sub>) takes a <i>C</i><sub>S</sub> symmetry structure, because of the pseudo-Jahn–Teller effect, while all other complexes take a <i>C</i><sub>2V</sub> structure. The symmetry lowering in (N<sup>∧</sup>N)­Ni­(O<sub>2</sub>) is induced by the presence of the singly occupied δ<sub>d<sub><i>xy</i></sub>–π<sub><i>x</i></sub><sup>*</sup></sub> orbital. In all of these complexes, significant superoxo (O<sub>2</sub><sup>–</sup>) character is found from the occupation numbers of natural orbitals and the O–O π* bond order, which is independent of the number of d electrons and the oxidation state of metal center. However, this is not a typical superoxo species, because the spin density is not found on the O<sub>2</sub> moiety, even in open-shell complexes, (N<sup>∧</sup>N)­Ni­(O<sub>2</sub>) and (N<sup>∧</sup>N)­Co­(O<sub>2</sub>). The M–O and O–O distances are considerably different from each other, despite the similar superoxo character. The M–O distance and the interaction energy between the metal and O<sub>2</sub> moieties are determined by the d<sub><i>yz</i></sub> orbital energy of the metal moiety taking the valence state. The binding energy of the O<sub>2</sub> moiety is understood in terms of the d<sub><i>yz</i></sub> orbital energy in the valence state and the promotion energy of the metal moiety from the ground state to the valence state. Because of the participations of various charge transfer (CT) interactions between the metal and O<sub>2</sub> moieties, neither the d<sub><i>yz</i></sub> orbital energy nor the electron population of the O<sub>2</sub> moiety are clearly related to the O–O bond length. Here, the π bond order of the O<sub>2</sub> moiety is proposed as a good measure for discussing the O–O bond length. Because the d electron configuration is different among these complexes, the CT interactions are different, leading to the differences in the π bond order and, hence, the O–O distance among these complexes. The reactivity of dioxygen complex is discussed with the d<sub><i>yz</i></sub> orbital energy

    Evaluation Procedure of Electrostatic Potential in 3D-RISM-SCF Method and Its Application to Hydrolyses of Cis- and Transplatin Complexes

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    In the three-dimensional reference interaction site model self-consistent field (3D-RISM-SCF) method, a switching function was introduced to evaluate the electrostatic potential (ESP) around the solute to smoothly connect the ESP directly calculated with the solute electronic wave function and that approximately calculated with solute point charges. Hydrolyses of cis- and transplatins, <i>cis</i>- and <i>trans</i>-PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>, were investigated with this method. Solute geometries were optimized at the DFT level with the M06-2X functional, and free energy changes were calculated at the CCSD­(T) level. In the first hydrolysis, the calculated activation free energy is 20.8 kcal/mol for cisplatin and 20.3 kcal/mol for transplatin, which agrees with the experimental and recently reported theoretical results. A Cl anion, which is formed by the first hydrolysis, somehow favorably exists in the first solvation shell as a counteranion. The second hydrolysis occurs with a similar activation free energy (20.9 kcal/mol) for cisplatin but a somewhat larger energy (23.2 kcal/mol) for transplatin to afford <i>cis</i>- and <i>trans</i>-diaqua complexes. The Cl counteranion in the first solvation shell little influences the activation free energy but somewhat decreases the endothermicity in both cis- and transplatins. The present 3D-RISM-SCF method clearly displays the microscopic solvation structure and its changes in the hydrolysis, which are discussed in detail
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