Activation of C-H, C-C and C-I bonds by Pd and cis-Pd(CO)2I2. Catalyst-substrate adaptation

We have studied the oxidative addition reactions of methane and ethane C-H, ethane C-C and iodomethane C-I bonds to Pd and cis-Pd(CO)₂I₂ at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (DFT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal-ligand coordination complex. For both Pd and cis-Pd(CO)₂I₂, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (S_N2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from Pd to cis-Pd(CO)₂I₂, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of Pd for C-I over C-H activation. Obviously, cis-Pd(CO)₂I₂ is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C-X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO)₂I₂ complex, e.g., C-X or X-C along an I-Pd-CO axis, or in between two I-Pd-CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst-substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies ΔE^≠ are decomposed into the activation strain of and the stabilizing TS interaction between the reactants in the activated complex: .     

Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX₃)₂ and Pd(X₂PCH₂CH₂PX₂) where X = Me, H, Cl. It was shown that the C_ipso-Br activation energy by Pd(PX₃)₂ correlates well with the rigidity of the X₃P-Pd-PX₃ angle and increases via the trend X = Cl < H < Me. The more rigid the X₃P-Pd-PX₃ angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX₃ bond strength increasing faster than the Pd(0)-PX₃ bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX₃)₂, the C_ipso-Br activation energy by Pd(X₂PCH₂CH₂PX₂) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d_π orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X₂PCH₂CH₂PX₂ than for the case of PX₃. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl).     

A density functional theory study of the oxidative addition of methyl iodide to square planar [Rh(acac)(P(OPh)3)2] complex and simplified model systems

The transition state for the oxidative addition reaction [Rh(acac)(P(OPh)₃)₂] + CH₃I, as well as two simplified models viz. [Rh(acac)(P(OCH₃)₃)₂] and [Rh(acac)(P(OH)₃)₂], are calculated with the density functional theory (DFT) at the PW91/TZP level of theory. The full experimental model, as well as the simplified model systems, gives a good account of the experimental Rh-ligand bond lengths of both the rhodium(I) and rhodium(III) β-diketonatobis(triphenylphosphite) complexes. The relative stability of the four possible rhodium(III) reaction products is the same for all the models, with trans-[Rh(acac)(P(OPh)₃)₂(CH₃)(I)] (in agreement with experimental data) as the most stable reaction product. The best agreement between the theoretical and experimental activation parameters was obtained for the full experimental system.     

Orbital-band interactions and the reactivity of molecules on oxide surfaces: from explanations to predictions

 The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides, systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces. Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect, theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and reducing the waste of time, energy and materials characteristic of an empirical design. Received: 21 June 2001 / Accepted: 8 October 2001 / Published online: 1 February 2002