Low-spin Potential Energy Surfaces Research Articles

CH4 activation by PtX+ (X = F, Cl, Br, I).

Reactions of PtX+ (X = F, Cl, Br, I) with methane have been investigated at the density functional theory (DFT) level. These reactions take place more easily along the low-spin potential energy surface. For HX (X = F, Cl, Br, I) elimination, the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreases in going as the sequence: X = F, Cl, Br, I. A reversed trend is observed in the loss of H2 for X = F, Cl, Br, while it is not favorable for PtI+ in the loss of either HI or H2. For HX eliminations, the transfer form of H is from proton to atom, last to hydride, and the mechanisms are from PCET to HAT, last to HT for the sequence of X = F, Cl, Br, I. One reason is mainly due to the electronegativity of halogens. Otherwise, the mechanisms of HX eliminations also can be explained by the analysis of Frontier Molecular Orbitals. While for the loss of H2, the transfer of H is in the form of hydride for all the X ligands. Noncovalent interactions analysis also can be explained the reaction mechanisms.

Computational Analysis of Two-State Reactivity in β-Hydride Elimination Mechanisms of Fe(II)– and Co(II)–Alkyl Complexes Supported by β-Diketiminate Ligand

β-Hydride elimination in Fe(II)– and Co(II)–alkyl complexes is known to occur through two different spin multiplicity states and is thus a good example of two-state reactivity. In this study, the automated reaction path search method combined with the mixed-spin effective Hamiltonian approach has been applied to understand the detailed reaction mechanisms including the characterization of the spin-inversion points between the high-spin and low-spin potential energy surfaces for the Fe(II)–C2H5 and Co(II)–C2H5 complexes supported by a β-diketiminate ligand. Density functional theory (DFT) with different exchange-correlation functionals has been used in the reaction path search calculations. We found that the β-hydride elimination process for these complexes consists of multiple steps including two spin-inversion points. We have also investigated the substituent effect in the β-diketiminate ligand to understand the steric and electronic effects on the spin-inversion process. The efficiency of the spin-inver...

Theoretical investigation on cyclohexane dehydrogenation catalyzed by V2+ in gas-phase

The dehydrogenation reaction mechanism of cyclohexane catalyzed by dimer transition metal cluster V2+ has been investigated at the B3LYP/6-31G (d, p) level of density functional theory. Density of states (DOS) graph is used to understand more deeply the roles of the front molecular orbital of the initial complexes. After the first molecular dehydrogenation, the reaction mainly consists of two competition mechanisms. First, the C-H bonds of cyclohexane can be effectively activated by the V2+ cation, yielding the same-face dehydrogenation products. Second, the C-C bonds are activated, forming the different-face dehydrogenation products. Our calculations indicate that the reaction takes place more easily along the low-spin potential energy surface on the same-face and is a low-barrier or even barrier-free transformation. Carbon-carbon single bonds are nonpolar and generally far less reactive. A comparison of the reaction mechanism of V2+ and congener Ti2+ with cyclohexane has been presented. The bond dissociation energies (BDEs) of V2+ are greater than that of Ti2+, leading to difficulties in forming sandwich complexes in the different-face dehydrogenation of cyclohexane, and the same-face dehydrogenation is an important reaction channel.

Geometries of low spin states of multi-centre transition metal complexes through extended broken symmetry variational Monte Carlo.

The correct description of the ground state electronic and geometrical properties of multi-centre transition metal complexes necessitates of a high-level description of both dynamical and static correlation effects. In di-metallic complexes, the ground state low spin properties can be computed starting from single-determinants High-Spin (HS) and Broken Symmetry (BS) states by reconstructing an approximated low spin potential energy surface through the extended broken symmetry approach, based on the Heisenberg Hamiltonian. In the present work, we first apply this approach within the variational Monte Carlo method to tackle the geometry optimization of a Fe2S2(SH)42- model complex. To describe the HS and BS wavefunctions, we use a fully optimized unrestricted single determinant with a correlated Jastrow factor able to recover a large amount of dynamical correlation. We compared our results with those obtained by density functional theory and other multiconfigurational approaches, discussing the role of the nodal surface on the structural parameters.

A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides

The reaction paths for the conversion of chlorobenzene to p-chlorophenol are presented in detail using iron and manganese monoxides via the hydroxo insertion intermediate, HO–M–C6H4Cl (M=FeO, MnO). The molecular geometries and electronic structures for the reactants, intermediates, transition states, and products were optimized and analyzed in detail by density functional methods. The reaction potential surface profiles indicate that the metaloxo species can activate the para C–H bond of the chlorobenzene to lead to the p-chlorophenol via the successive formation and the dissociation of the metal carbon bond, followed by removal of the metal atom (Fe or Mn). The intrinsic reaction co-ordinate (IRC) analyses indicated that no crossover point was searched for between the high-spin and low-spin potential energy surfaces; thus, no spin crossing was found between these two states potential energy surfaces. The low-spin potential energy surface lies above the high-spin one for the entire reaction pathway. Our theoretical study on the possible reaction pathways for the conversion of chlorobenzene to p-chlorophenol will also be useful for analyzing the catalytic functions of C–H bond activation and metal–carbon bond formation by transition metal complexes.

Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal–Oxo Species

Abstract Quantum chemical studies by the author and co-workers on dioxygen activation and methane hydroxylation by diiron and dicopper enzyme species as well as related metal–oxo species are reviewed. The activation of the O–O bond of dioxygen at the mono- and dimetal sites of metalloenzymes is an essential process in the initial catalytic stages of naturally occurring oxygenation reactions. A way of orbital thinking about dioxygen activation at diiron and dicopper enzymes is developed at the extended Hückel level of theory. Two different reaction pathways that lead from dimetal peroxo complexes with µ-η1:η1-O2 and µ-η2:η2-O2 modes to the corresponding dioxo complexes are discussed in detail. In considering the mechanism of methane hydroxylation, special attention to FeO+-mediated methane hydroxylation that occurs under ion cyclotron resonance (ICR) conditions is given. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ions ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+ are systematically analyzed on the basis of density functional theory (DFT) calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process. The spin inversion from the high-spin state to the low-spin state effectively decreases the barrier height of C–H activation. Another feature in the reaction is that no radical species is involved in the course of the hydroxylation because the methyl species formed as a result of the C–H bond cleavage can directly coordinate to the metal active site. The coupling of the OH and CH3 ligands occurs to produce methanol at the metal active center. The reaction profiles obtained from DFT calculations are similar to those of the Gif chemistry proposed by D. H. R. Barton, in particular the involvement of the HO–M–CH3 species as an intermediate in the hydroxylation of methane. The nonradical mechanism is extended to methane hydroxylation by the diiron and dicopper species of methane monooxygenase (MMO). This mechanism is possible when the metal active center is coordinatively unsaturated to have a space for the coordination of the OH and CH3 groups as ligands. Although compelling discussion to support the involvement of oxygen- and carbon-centered radicals is provided, the nonradical mechanism still seems to be applicable for methane hydroxylation from the viewpoint of reaction selectivity. Kinetic isotope effects (KIEs) in the C–H activation process by the bare FeO+ complex and diiron and dicopper enzyme models are compared with respect to the radical and nonradical mechanisms by using transition state theory.

Constrained spin-density dynamics of an iron-sulfur complex: Ferredoxin cofactor

The computation of antiferromagnetic exchange coupling constants J by means of efficient density-based approaches requires in practice to take care of both spin projection to approximate the low spin ground state and proper localization of the magnetic orbitals at the transition metal centers. This is demonstrated here by a combined approach where the extended broken-symmetry (EBS) technique is employed to include the former aspect, while spin density constraints are applied to ensure the latter. This constrained EBS (CEBS) approach allows us to carry out ab initio molecular dynamics on a spin-projected low spin potential energy surface that is generated on-the-fly by propagating two coupled determinants and thereby accessing the antiferromagnetic coupling along the trajectory. When applied to the prototypical model of the oxidized [2Fe-2S] cofactor in Ferredoxins, [Fe(2)S(2)(SH)(4)](2-), at room temperature, CEBS leads to remarkably good results for geometrical structures and coupling constants J.

Methane Activation by MH+ (M = Os, Ir, and Pt) and Comparisons to the Congeners of MH+ (M = Fe, Co, Ni and Ru, Rh, Pd)

The mechanism of ligated-transition-metal- [MH(+) (M = Os, Ir, and Pt)] catalyzed methane activation has been computed at the B3LYP level of density functional theory. The B3LYP energies of important species on the potential energy surfaces were compared to CCSD(T) single-point energy calculations. Newer kinetic and dispersion-corrected methods such as M05-2X provide significantly better descriptions of the bonding interactions. The reactions take place more easily along the low-spin potential energy surface. The minimum-energy pathway proceeds as MH(+) + CH(4) → M(H)(2)(CH(3))(+) → TS → MH(CH(2))(H(2))(+) → MH(CH(2))(+) + H(2). The ground states are (5)Π, (4)Σ(-), and (1)Σ(+) for OsH(+), IrH(+), and PtH(+), respectively. The energy level differences of the reactants between the high- and low-spin states gradually become smaller from OsH(+) to PtH(+), being 30.66, 9.17, and 0.09 kcal/mol, respectively. The C-H bond can be readily activated by MH(+) (M = Os, Ir, and Pt) with a negligible barrier in the low-spin state; thus, OsH(+), IrH(+), and PtH(+) are likely to be excellent mediators for the activition of the C-H bond of methane. H(2) elimination is quite facile without barriers in the presence of excess reactants. The products of the reactions of MH(+) (M = Os, Ir, and Pt) + methane are all carbene complexes MH(CH(2))(+). The exothermicities of the reactions are 3.99, 15.66, and 12.14 kcal/mol, respectively. The results for MH(+) (M = Os, Ir, and Pt) are compared with those for the first- and second-row congeners, and the differences in behavior and mechanism are discussed.

The small metal cluster IrAu+ mediated C–N coupling: A density functional theory study

A detailed investigation of the reaction mechanism underlying the small metal cluster IrAu+ mediated C–N coupling of methane and ammonia has been performed by the density functional theory. At first, both high-spin and low-spin potential energy surfaces are characterized in activation of methane by IrAu+ in detail. Our calculations indicate that the step reaction proceeds on the low-spin potential energy surface although it may involve crossing of potential energy surfaces. The C–H bond of methane could be effectively activated by the cation IrAu+, yielding the bimetallic cation carbene IrAuCH2+. Second, the reaction of the product 2IrAuCH2+ and ammonia may result in C–N bond coupling, though loss of H2′ in methylene is the most favorable among the three paths dynamically.

Density Functional Theory Studies of Thermal Activation of Methane by MH+ (M = Ru, Rh, and Pd)

The dehydrogenation reaction mechanisms of methane catalyzed by a ligated transition metal MH(+) (M = Ru, Rh, and Pd) have been investigated theoretically. Activation of methane by MH(+) complexes is proposed to proceed in a one-step manner via one transition state: MH(+) + CH(4) --> MH(+)CH(4) --> [TS] --> (MCH(3)(+))H(2) -->MCH(3)(+) + H(2). Both high-spin and low-spin potential energy surfaces are characterized in detail. Our calculations indicate that the ground-states species have low electron spin and a dominant 4d(n) configuration for RuH(+), RhH(+), and PdH(+), and the whole reaction proceeds on the ground-states potential energy surfaces with a spin-allowed manner. The MH(+) (M = Ru, Rh, and Pd) complexes are expected from the general energy profiles of the reaction pathways to efficiently convert methane to metal methyl, thus RuH(+), RhH(+), and PdH(+) are likely to be excellent mediators for the activity of methane. In the reactions of MH(+) with methane, the H(2) elimination from the dihydrogen complex is quite facile without barriers. The exothermicities of the reactions are close for Ru, Rh, and Pd: 11.1, 1.2, and 5.2 kcal/mol, respectively.

DFT studies of thermal activation of methane by [Ru(H)(OH)] +

The dehydrogenation reaction mechanism of methane catalyzed by ligated transition-metal [Ru(H)(OH)] + has been investigated theoretically. Activation of methane by [Ru(H)(OH)] + complex is proposed to proceed in a one-step manner via one transition state: [Ru(H)(OH)] + + CH 4 → [Ru(H)(OH)(CH 4)] + → [TS] → [Ru(CH 3)(OH)] +H 2 → [Ru(CH 3)(OH)] + + H 2. Both high-spin and low-spin potential energy surfaces are characterized in detail. The DFT calculations indicate the reactant and the product prefer to have a high-spin electron configuration for their metal center but a low-spin configuration for the metal center in its transition state. Consequently, a crossing between high-spin and low-spin potential energy surfaces takes place at both the entrance and the exit channels of the reaction. The [Ru(H)(OH)] + complex is expected from the general energy profile of the reaction pathway to efficiently convert methane to methyl. Therefore, it is likely to be an excellent mediator for the activity of methane. The overall hydrogenation reaction catalyzed by the cationic [Ru(H)(OH)] + is exothermic.

Cobalt-Mediated Cyclic and Linear 2:1 Cooligomerization of Alkynes with Alkenes: A DFT Study

The mechanism of the cobalt-mediated [2 + 2 + 2] cycloaddition of two alkynes to one alkene to give CpCo-complexed 1,3-cyclohexadienes (cyclic oligomerization) has been studied by means of DFT computations. In contrast to the mechanism of alkyne cyclotrimerization, in which final alkyne inclusion into the common cobaltacyclopentadiene features a direct "collapse" pathway to the complexed arene, alkene incorporation proceeds via insertion into a Co-C sigma-bond rather than inter- or intramolecular [4 + 2] cycloaddition. The resulting seven-membered metallacycle 7 is a key intermediate which leads to either CpCo-complexed cyclohexadiene 5 or hexatriene 13. The latter transformation, particularly favorable for ethene, accounts, in part, for the linear oligomerization observed occasionally in these reactions. With aromatic double bonds, a C-H activation mechanism by the cobaltacyclopentadiene seems more advantageous in hexatriene product formation. Detailed investigations of high- and low-spin potential energy surfaces are presented. The reactivity of triplet cobalt species was found kinetically disfavored over that of their singlet counterparts. Moreover, it could not account for the formation of CpCo-complexed hexatrienes. However, triplet cobalt complexes cannot be ruled out since all unsaturated species appearing in this study were found to exhibit triplet ground states. Consequently, a reaction pathway that involves a mixing of both spin-state energy surfaces is also described (two-state reactivity). Support for such a pathway comes from the location of several low-lying minimum-energy crossing points (MECPs) of the two surfaces.

Methane-to-Methanol Conversion by First-Row Transition-Metal Oxide Ions: ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+

The reaction pathway and energetics for methane-to-methanol conversion by first-row transition-metal oxide ions (MO+s) are discussed from density functional theory (DFT) B3LYP calculations, where M is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The methane-to-methanol conversion by these MO+ complexes is proposed to proceed in a two-step manner via two transition states: MO+ + CH4 → OM+(CH4) → [TS] → OH−M+−CH3 → [TS] → M+(CH3OH) → M+ + CH3OH. Both high-spin and low-spin potential energy surfaces are characterized in detail. A crossing between the high-spin and the low-spin potential energy surfaces occurs once near the exit channel for ScO+, TiO+, VO+, CrO+, and MnO+, but it occurs twice in the entrance and exit channels for FeO+, CoO+, and NiO+. Our calculations strongly suggest that spin inversion can occur near a crossing region of potential energy surfaces and that it can play a significant role in decreasing the barrier heights of these transition states. The reaction pathway from methane to methanol is ...

Methane−Methanol Conversion by MnO+, FeO+, and CoO+: A Theoretical Study of Catalytic Selectivity

The entire reaction pathway for the gas-phase methane−methanol conversion by late transition-metal-oxide ions, MnO+, FeO+, and CoO+, is studied using an ab initio hybrid (Hartree−Fock/density-functional) method. For these oxo complexes, the methane−methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO+ + CH4 → OM+(CH4) → [TS1] → HO−M+−CH3 → [TS2] → M+(CH3OH) → M+ + CH3OH, where M is Mn, Fe, and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO+ and CoO+, but it occurs only once near TS2 for MnO+. The activation energy from OMn+(CH4) to HO−Mn+−CH3 via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO+ and CoO+, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates that HO−M+−CH3 plays a central role as an intermediate in the reaction betwe...

Periodic Trends for Transition Metal Dihydrides MH2, Dihydride Dihydrogen Complexes MH2·H2, and Tetrahydrides MH4 (M = Ti, V, and Cr)

Ab initio quantum mechanical methods were employed to study the periodic trends of transition metal (M = Ti, V, and Cr) hydrides MH{sub 2}, dihydride dihydrogen complexes MH{sub 2}.H{sub 2}, and tetrahydrides MH{sub 4}. All three MH{sub 2}.H{sub 2} complexes are in the high-spin ground state. The low-spin dihydrides interact with the H{sub 2} moiety more strongly than do the high-spin species. The d {yields} {sigma}{sup *} back donation was so strong for the low-spin TiH{sub 2} that H{sub 2} dissociates without barrier upon contact with singlet TiH{sub 2} to form TiH{sub 4}. Due to the Jahn-Teller distortion the ground state of VH{sub 4} is the {sup 2}A{sub 1} electronic state having D{sub 2d} symmetry. TiH{sub 4} is predicted to lie 9 kcal mol{sup -1} lower in energy than its ground state MH{sub 2}.H{sub 2} isomer, whereas VH{sub 4} and CrH{sub 4} are higher in energy by 22 and 39 kcal mol{sup -1}, respectively, at the TZP CCSD level of theory. However, comparing MH{sub 4} and MH{sub 2}.H{sub 2} in the same spin state, MH{sub 4} is always lower in energy than its dihydrogen complex isomer, MH{sub 2}.H{sub 2}, on the low-spin potential energy surface. Comparison between the present workmore » and experimental IR spectra from the matrix isolation of the cocondensation of transition metal atoms (Ti, V, and Cr) with H{sub 2} molecules confirmed the existence of CrH{sub 2}.H{sub 2} by identifying a strong unique absorption at 1510 cm{sup -1}. 17 refs., 4 figs., 8 tabs.« less