DFT studies of the methyl exchange reaction between Cp2M–CH3or Cp*2M–CH3(Cp = C5H5, Cp* = C5Me5, M = Y, Sc, Ln) and CH4. Does M ionic radius control the reaction?

Electron Dynamics in Alkane C-H Activation Mediated by Transition Metal Complexes

Alkanes, ideal raw materials for industrial chemical production, typically exhibit limited reactivity due to their robust and weakly polarized C-H bonds. The challenge lies in selectively activating these C-H bonds under mild conditions. To address this challenge, various C-H activation mechanisms have been developed. Yet, classifying these mechanisms depends on the overall stoichiometry, which can be ambiguous and sometimes problematic. In this study, we utilized density functional theory calculations combined with intrinsic bond orbital (IBO) analysis to examine electron flow in the four primary alkane C-H activation mechanisms: oxidative addition, σ-bond metathesis, 1,2-addition, and electrophilic activation. Methane was selected as the representative alkane molecule to undergo C-H heterolytic cleavage in these reactions. Across all mechanisms studied, we find that the CH3 moiety in methane consistently uses an electron pair from the cleaved C-H bond to form a σ-bond with the metal. Yet, the electron pair that accepts the proton differs with each mechanism: in oxidative addition, it is derived from the d-orbitals; in σ-bond metathesis, it resulted from the metal-ligand σ-bonds; in 1,2-addition, it arose from the π-orbital of the metal-ligand multiple bonds; and in electrophilic activation, it came from the lone pairs on ligands. This detailed analysis not only provides a clear visual understanding of these reactions but also showcases the ability of the IBO method to differentiate between mechanisms. The electron flow discerned from IBO analysis is further corroborated by results from absolutely localized molecular orbital energy decomposition analysis, which also helps to quantify the two predominant interactions in each process. Our findings offer profound insights into the electron dynamics at play in alkane C-H activation, enhancing our understanding of these critical reactions.

Computational Study of Methane C-H Activation by Main Group and Mixed Main Group-Transition Metal Complexes

In the present density functional theory (DFT) research, nine different molecules, each with different combinations of A (triel) and E (divalent metal) elements, were reacted to effect methane C-H activation. The compounds modeled herein incorporated the triels A = B, Al, or Ga and the divalent metals E = Be, Mg, or Zn. The results show that changes in the divalent metal have a much bigger impact on the thermodynamics and methane activation barriers than changes in the triels. The activating molecules that contained beryllium were most likely to have the potential for activating methane, as their free energies of reaction and free energy barriers were close to reasonable experimental values (i.e., ΔG close to thermoneutral, ΔG^‡ ~30 kcal/mol). In contrast, the molecules that contained larger elements such as Zn and Ga had much higher ΔG^‡. The addition of various substituents to the A-E complexes did not seem to affect thermodynamics but had some effect on the kinetics when substituted closer to the active site.

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized.

A DFT study of the reactivity of actinidocenes (U, Np and Pu) with pyridine and pyridine N-oxide derivatives

The ortho C-H bond activation of several pyridine and pyridine N-oxide derivatives mediated by Cp(2)An(IV)(CH(3))(2) and Cp(2)An(III)(CH(3)) (Cp = C(5)H(5) and An = U, Np, Pu) have been investigated at the DFT level. For uranium(IV) complex, an excellent agreement with experimental observations is found, and particularly in the case of 2-picoline where only ortho sp(2) C-H activation is observed without any sp(3) C-H activation. These differences of reactivity can be explained by the charges distribution at the metathesis transition state level. A predictive study of the reactivity of neptunium and plutonium complexes with pyridine and pyridine N-oxide is reported. This study shows that neptunium and plutonium have almost the same reactivity. The C-H activation mediated by actinide (III) complexes have found to be more kinetically and thermodynamically favorable than for actinide (IV) complexes. The influence of 5f electrons on organoactinide reactivity is found to be small by comparing implicit and explicit treatment of 5f electrons.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Theoretical prediction of O–H, Si–H, and Si–C σ-bond activation reactions by titanium(IV)–imido complex

The O–H σ-bond activation of methanol, the Si–H σ-bond activation of silane, and the Si–C σ-bond activation of methylsilane by titanium(IV)–imido complex (Me3SiO)2Ti(NSiMe3) were theoretically investigated with DFT and MP2 to MP4(SDQ) methods. The O–H σ-bond activation of methanol occurs with small activation barrier (Ea) of 7.1 (14.6) kcal/mol and large exothermicity (Eexo) of 65.8 (61.4) kcal/mol to afford (Me3SiO)2Ti(OCH3)[NH(SiMe3)], indicating that the O–H σ-bond activation occurs easier than the C–H σ-bond activation (Ea = 14.6 (21.5) kcal/mol and Eexo = 22.7 (16.5) kcal/mol), where DFT- and MP4(SDQ)-calculated values are presented without and in parenthesis hereafter. Though the OCH3 group becomes anionic and the H atom becomes proton-like in this activation reaction, population changes more moderately occur than those of the C–H σ-bond activation. This is because the H–OCH3 bond is already polarized in methanol. In the Si–H σ-bond activation, two reaction courses were investigated; in one course, the product is (Me3SiO)2Ti(SiH3)[NH(SiMe3)] in which the H atom and the SiH3 group are bound to the N atom and the Ti center, respectively, while in the other course the product is (Me3SiO)2Ti(H)[N(SiH3)(SiMe3)] in which the H atom and the SiH3 group are bound to the Ti center and the imido N atom, respectively. Though the former reaction occurs with small Ea value and large exothermicity, the latter reaction occurs easier with further smaller Ea value of 2.6 (4.3) kcal/mol and larger Eexo value of 32.5 (34.1) kcal/mol than those of the former reaction. This is because the Ti–H bond energy is much larger than the Ti–SiH3 one. The Si–C σ-bond activation occurs with moderate activation barrier of 19.1 (18.6) kcal/mol and considerably large exothermicity of 33.9 (37.7) kcal/mol. Based on these results, we wish to propose the theoretical prediction that the titanium(IV)–imido complex is useful for O–H, Si–H, and Si–C σ-bond activation reactions.

Mechanisms of C-H bond activation: rich synergy between computation and experiment

Recent computational studies of C-H bond activation at late transition metal systems are discussed and processes where lone pair assistance via heteroatom co-ligands or carboxylates are highlighted as a particularly promising means of cleaving C-H bonds. The term 'ambiphilic metal ligand activation' (AMLA) is introduced to describe such reactions.

The sigma-CAM Mechanism: sigma complexes as the basis of sigma-bond metathesis at late-transition-metal centers

Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.

Reactions of Monomeric [1,2,4-(Me3C)3C5H2]2CeH and CO with or without H2: An Experimental and Computational Study

Addition of CO to [1,2,4-(Me3C)3C5H2]2CeH,Cp'2 CeH, in toluene yields the cis-(Cp'2Ce)2(mu-OCHCHO), in which the cis-enediolate group bridges the two metallocene fragments. The cis-enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100 degrees C over 7 months. When the solvent is pentane, Cp'2Ce(OCH2)CeCp'2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis-enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'2Ce(OCH2)CeCp'2, generating Cp'2CeOCH2COCeCp'2. The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H2) bond anti-periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H2. The oxomethylene complex reacts with CO to give the cis-enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H2 to give Cp2CeOMe is exoergic by 50 kcal mol-1. The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta2-CHO), is formed by further reaction with H2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H2 by CH4 increases the activation energy by 17 kcal mol-1.

Organolanthanide mediated catalytic cycles: a computational perspective

In this perspective the contribution of recent theoretical studies to our understanding of lanthanide (Ln) catalysis is explored. In general, the results of computational studies have proven consistent with available experimental evidence. Considerable success has been obtained in elucidating the mechanisms for C-H bond activation (sigma-bond metathesis in particular) and the addition of C-X bonds across an unsaturated functionality (and the hydroamination of alkenes in particular). Ln catalysts are computationally challenging because relativistic effects are important, and large ligands are required to restrict high coordination numbers, in addition to limiting facile redistribution processes. Thus, key technical issues relating to the computational investigation of organolanthanide complexes are discussed. Increasing computational resources have seen studies expand from the optimisation of simple molecules to the study of catalytic cycles where the Ln is coordinated by larger and more complex ligands. The ability of theoretical studies to complement experimental developments by supplying a deeper understanding of the mechanistic process is reviewed with emphasis on the elucidation of transition state structures, intermediates, spectator ligand coordination, and negative entropy steps. Recent computational investigations of the catalytic cycle for Ln mediated hydroamination are a focus, as these have provided substantial and detailed rationalisations for the regio- and stereo-selectivity of inter- and intra-molecular hydroamination. Examination of transition state geometries and electronic structure appears to offer insights that could be used to facilitate the rational design of new Ln-based catalysts.