Theoretical Study of Oxidative Addition to Platinum Metal Complexes. III. Activation of Methane, Ethane, and Propane with 16-Electron Palladium(II) and Platinum(II) Chlorocyclopentadienyl Complexes

Nonempirical MO methods were used to calculate the structure of the palladium(II) and platinum(II) cyclopentadienyl complexes CpMCl (M = Pd, Pt) and the reactivity of the complexes toward σ-donors (water, HCN) and alkanes R-H (methane, ethane, and propane). The 16-electron complexes, unlike isoelectronic square-planar derivatives of these metals, form strong complexes with water and hydrogen cyanide, as well as σ complexes with methane. The results of MP2 and B3LYP calculations are compared. The oxidative addition of methane to chlorocyclopentadienylpalladium(II) is a synchronous process with a low activation energy, while the reaction with the platinum analog occurs with heat evolution. The much higher exothermicity and rate of the addition reactions of these complexes compared with planar palladium and platinum complexes is explained by the fact that in the former case the coordination entity should not strongly deform during the reaction. According to MP2 and B3LYP calculations, the exothermicity of addition of small alkanes to chlorocyclopentadienylplatinum(II) falls in the order R = n-Pr ≈ i-Pr > Et > Me

Theoretical study of oxidative addition to platinum metal complexes: VI. Complex formation of Rh(I), Pd(II), Ir(I), and Pt(II) with tridentate N- and P-donor fac-chelating ligands as a tool for controlling activity of metal complexes in oxidative methane addition

An approach was formulated to designing tridentate chelating ligands L ensuring stabilization of high-valence platinum metal hydridoalkyl complexes [(Me)M(L)(H)Cl] (M = Rh, Pd+, Ir, and Pt+) and thermodynamically more favorable oxidative methane addition to the corresponding low-valence complexes [M(L)Cl]. The approach is based on diminishing strain in ligand framework and in chelate rings formed by the ligand, on using strong-field ligands, and also on minimizing deformation of metal coordination entity in the course of the reaction with methane. Within the framework of a nonempirical molecular-orbital method full geometry optimization was performed for the [M(L)Cl] and [(Me)M(L)(H)Cl] species containing eight N- and P-donor ligands L fitting to a greater or lesser extent the above-mentioned requirements: tris(iminomethyl)-methane, cyclopropane-1,2,3-triamine, 1,3,5-triazacyclohexane, cyclohexane-1,3,5-triamine, 1,4,7-triazacyclonona-2,5,8-triene, cyclonona-1,4,7-triene-3,6,9-triamine, tris(2-phosphavinyl)methane, and cyclonona-1,4,7-triene-3,6,9-triphosphine. By calculations at the MP2 and B3LYP levels with allowance for electron correlation it was found that the energies of the reactions of the [M(L)Cl] complexes with methane vary by almost 38 kcal/mol (for example, from -19 to +19 kcal/mol for M = Pd+). A relationship between the deformation of chelate rings, the dentacity of ligand L in the [M(L)Cl] species, and the efficiency of the ligand in stabilization of high-valence methal hydridoalkyl complexes was established. The existence of complexes [(Me)M(L) · (H)Cl] with L = cyclohexane-1,3,5-triamine and 1,4,7-triazacyclonona-2,5,8-triene, moderately stable at M = Pd(IV) and stable at M = Pt(IV) and Ir(III), and also of stable alkylhydridorhodium(III) complexes was predicted for the first time. Characteristics of the frontier orbitals of complexes [M(L)CI]p, [CpMCl]q, and [MCl2(PH3)2]q (M = Rh, Ir, q = -1, p = 0; M = Pd, Pt, q = 0, p = +1) with distorted and undistorted coordination entities were compared

Theoretical Study of Oxidative Addition of Methane and Propane to Dihalobis(phosphine)palladium(II) Complexes PdX2(PH3)2 (X = F, Cl, Br, and I)

The possible routes of methane reactions with dihalobis(phosphine)palladium(II) complexes PdX2(PH3)2 (X = F, Cl, Br, and I), namely, electrophilic replacement of hydrogen, oxidative halogenation of methane, and oxidative addition of methane CH bonds to the palladium atom were considered. All the gas-phase reactions are thermodynamically unfavored and have an almost equal energy. The oxidative addition reaction was studied in detail: The geometry and relative stability of six diastereomeric reaction products, hydridomethyl palladium(IV) complexes CH3Pd(H)X2(PH3)2, were calculated. The most substantial structural features of the complexes and their relative stability are determined by relative arrangement of the hydrido and methyl ligands, having the strongest trans-effect, around the palladium atom. The endothermic effect of methane oxidative addition to the palladium complex increases in the order X = F n-Pr > Me. The kinetic barriers of the methane reactions were calculated. The oxidative addition path was studied by the internal coordinate method. The oxidative addition is a concerted process with a late transition state. The results of the calculations are in qualitative agreement with experimental data

Theoretical study of oxidative additions to platinum metal complexes: VII. Mechanisms of methane activation by 16-and 14-electron platinum(II) and palladium(II) chlorophosphine complexes

Stationary points of the MP2//MP2 and B3LYP//B3LYP potential energy surfaces were studied for two model systems: methane-trans-dichlorobisphosphineplatinum(II) and methane-trans-dichlorobisphosphinepalladium(II). The points correspond to the products and transition states of the oxidative addition reactions and of metathesis of C-H bonds of methane and M-Cl bonds of the complexes with their participation, i.e., to 14-electron MCl2(PH3) species and their η3 complexes with methane, and also to the products and transition states of oxidative additions and metathesis of the C-H and M-Cl bonds involving [MCl2(PH3)] complexes. The electron correlation for all the complexes was considered within the framework of Møller-Plesset (MP2) and coupled cluster [CCD, CCSD(T)] methods and density functional (B3LYP) theory. Corrections for the energy of zero vibrations of the complexes and enthalpies and entropies of the reactions were calculated. The complexes [PdCl2(PH3)] and especially [PtCl2(PH3)] tend to activate methane by the scheme of oxidative addition to a greater extent as compared to the metathesis of the C-H and M-Cl bonds

Theoretical Study of Oxidative Addition to Platinum Metal Complexes. IV. Thermodynamic Substrate and Regioselectivities of 16-Electron Planar Complexes of Rhodium(I) and Palladium(II) and Cyclopentadienyl Complexes of Platinum(II) and Iridium(I) in Reactions with Small Alkanes

Nonempirical MO calculations by the GAMESS program were used to perform full geometry optimization of products of methane, ethane, and propane oxidative addition to the square-planar complexes RhCl(PH3)3 and PdCl2(PH3)2 and to the cyclopentadienyl derivatives CpMCl (M = Pt, Ir-) and CpIr(PH3). The most stable conformers formed by rotation about the metal-alkyl bond were determined. The energies of the reactions were calculated with allowance for electron correlation by the MP2 and B3LYP methods using the GAUSSIAN 94 package. Substrate and regioselectivities of the metal complexes were determined. The "classical" selectivity order i-Pr-H ≫ n-Pr-H ≥ Et-H > Me-H for platinum complexes was shown to correspond to prevailing contribution of electronic factors to the energy of the metal-alkyl bonds, whereas increased energy of steric alkyl-ligand interactions produces partial or complete inversion of this order, resulting in "nonclassical" selectivity. The difference in the MP2 and B3LYP selectivites is explained by the fact that the first method overestimates the electronic contribution to the energy of the metal-alkyl bond

Structure and vibrational spectra of mononitroalkanes

Structures and force fields for several mononitroalkane molecules were determined by ab initio quantum-chemical methods. The data obtained were used for calculation of the frequencies and modes of normal vibrations. Potentialities of different methods (RHF, MP2, and B3LYP) and basis sets for estimation of the structures and spectra were studied

Theoretical Study of Oxidative Addition to Platinum Metal Complexes. II. New Approach to Thermodynamics of Methane Reaction with 16-Electron Square-Planar Complexes of Rhodium(I), Palladium(II), Iridium(I), and Platinum(II)

The geometries and thermodynamic parameters of reagents and diastereomeric products of oxidative addition of methane to square-planar complexes of rhodium(I), palladium(II), iridium(I), and platinum(II) [MXnL4-n]q (where M is Rh or Ir, X is Cl, L is PH3, n = 1-3, and q = 2 - n; M is Pd and Pt, X is Cl, L is PH3, n = 1-3, and q = 2 - n; M is Pd, X is F, Cl, Br, I, Me, SiH3, and SnH3, L is PH3, NH3, and AsH3, n = 2, and q = 0) were calculated by the nonempirical MO method. The trans effect order of the ligands, similar for all metals under study was refined: H- > Me- ≈ SH- 3 ≈ SnH- 3 > Cl- > AsH3 ≈ PH3 > NH3. The M-Y bond under the influence of trans hydride or methyl ligand becomes weaker in the following order: M-H, M-C, M-Si, M-Sn ≫ M-P, M-As > M-Cl > M-N. A simple procedure to predict the relative stability of diastereomeric products of oxidative addition of alkanes to the planar complexes of the corresponding metal was developed. The energy of methane addition to the planar metal complexes was calculated by the general procedure involving three components: (a) the deformation energy of the planar complex into a butterfly-like carbene complex by bending one of the ligands under the coordination plane (Edef), (b) the energy of singlet-triplet rearrangement of metal carbene (Es-t), and (c) the algebraic sum of dissociation energies of the Me-H, M-H, and M-Me bonds (ΣEb). The relationship between Edef and structural parameters of the metal complex was established. The relationship of Es-t with the ligand composition and the nature of complex-forming metal is qualitatively described by the crystal field theory. The spectrochemical series of studied ligands was refined: I- Rh ≫ Pt > Pd. The theoretical and experimental data on the oxidative addition of alkanes to the platinum metal complexes were generalized and, as a result, the composition and structure of new metal complexes, which are effective activators of alkanes, were predicted

Exclusive diffractive photoproduction of dileptons by timelike Compton scattering

We derive the forward photoproduction amplitude for the diffractive $\gamma p \to l^+ l^- p$ reaction in the momentum space. within the formalism of $k_\perp$- factorization. Predictions for the $\gamma p \to l^+ l^- p$ reaction are given using unintegrated gluon distribution from the literature. We calculate the total cross section as a function of photon-proton center of mass energy and the invariant mass distribution of the lepton pair. We also discuss whether the production of timelike virtual photons can be approximated by continuing to the spacelike domain $q^2 < 0$. The present calculation provides an input for future predictions for exclusive hadroproduction in the $p p \to p l^+ l^- p$ reaction.Comment: 11 pages, 7 figure