A mechanistically unique, simultaneous activation of two C–H bonds of methane has been identified during the course of its reaction with the cationic copper carbide, [Cu–C]+. Detailed high-level quantum chemical calculations support the experimental findings obtained in the highly diluted gas phase using FT-ICR mass spectrometry. The behavior of [Cu–C]+/CH4 contrasts that of [Au–C]+/CH4, for which a stepwise bond-activation scenario prevails. An explanation for the distinct mechanistic differences of the two coinage metal complexes is given. It is demonstrated that the coupling of [Cu–C]+ with methane to form ethylene and Cu+ is modeled very well by the reaction of a carbon atom with methane mediated by an oriented external electric field of a positive point charge.

AbstractDie Bindung von Acetonitril an zweiatomiges [ZnO].+ führt bei der thermischen Aktivierung von Methan in der Gasphase zu einem unerwarteten Wechsel der Reaktionsmechanismen und einer Umverteilung der Produkte. Theoretischen Studien zufolge spielt die starke Metall-Kohlenstoff-Bindung bei der ligandenfreien [ZnO].+-Spezies eine bedeutende Rolle für die Rückbindung des CH3.-Radikals an das Metallzentrum; hierdurch wird die kompetitive Bildung von CH3., OH. und CH3OH ermöglicht. Diese Wechselwirkung wird durch einen CH3CN-Liganden drastisch reduziert, was mechanistisch zu einem Wechsel von der bei [ZnO].+/CH4 dominierenden protonengekoppelten Elektronenübertragung zum klassischen Wasserstoffatomtransfer, gefolgt von der ausschließlichen Abspaltung von CH3., führt. Dieser Ligandeneffekt kann durch das gerichtete externe elektrische Feld einer negativen Punktladung gut modelliert werden.

Nonselective methane activation can be changed to a highly selective process as a result of the dipole field of a single acetonitrile ligand attached to bare ZnO+. The ligand effect can be modeled well by an oriented external electric field of a negative point charge. In their Communication on page 10219 ff., H. Schwarz, S. Shaik, and co-workers provide more details.

AbstractAn unexpected mechanistic switch as well as a change of the product distribution in the thermal gas-phase activation of methane have been identified when diatomic [ZnO].+ is ligated with acetonitrile. Theoretical studies suggest that a strong metal–carbon attraction in the pristine [ZnO].+ species plays an important role in the rebound of the incipient CH3. radical to the metal center, thus permitting the competitive generation of CH3., OH., and CH3OH. This interaction is drastically weakened by a single CH3CN ligand. As a result, upon ligation the proton-coupled single electron transfer that prevails for [ZnO].+/CH4 switches to the classical hydrogen-atom-transfer process, thus giving rise to the exclusive expulsion of CH3.. This ligand effect can be modeled quite well by an oriented external electric field of a negative point charge.

Unselektive Methanaktivierung …… lässt sich dank des Dipolfelds eines einzelnen Acetonitrilliganden an ZnO+ in einen hochselektiven Prozess umwandeln. Der Ligandeneffekt kann gut modelliert werden kann, indem der Ligand durch das gerichtete externe elektrische Feld einer negativen Punktladung ersetzt wird. In ihrer Zuschrift auf S. 10353 liefern H. Schwarz, S. Shaik et al. weitere Details hierzu.

In this article, we present density functional theory (DFT) calculations on the iron(IV)-oxo catalyzed methane C–H activation reactions for complexes in which the FeIV═O core is surrounded by five negatively charged ligands. We found that it follows a hybrid pathway that mixes features of the classical σ- and π-pathways in quintet surfaces. These calculations show that the Fe–O–H arrangement in this hybrid pathway is bent in sharp contrast to the collinear character as observed for the classical quintet σ-pathways before. The calculations have also shown that it is the equatorial ligands that play key roles in tuning the reactivity of FeIV═O complexes. The strong π-donating equatorial ligands employed in the current study cause a weak π(FeO) bond and thereby shift the electronic accepting orbitals (EAO) from the vertically orientated O pz orbital to the horizontally orientated O px. In addition, all the equatorial ligands are small in size and would therefore be expected have small steric effects upon substrate horizontal approaching. Therefore, for the small and strong π-donating equatorial ligands, the collinear Fe–O–H arrangement is not the best choice for the quintet reactivity. This study adds new element to iron(IV)-oxo catalyzed C–H bond activation reactions.

We present a high-level computational study on methanol C–H and O–H bond cleavages by bare [FeIVO]2+, as well as benchmarks of various density functional theory (DFT) methods. We considered direct and concerted hydrogen transfer (DHT and CHT) pathways, respectively. The potential energy surfaces were constructed at the CCSD(T)/def2-TZVPP//B3LYP/def2-TZVP level of theory. Mechanistically, (1) the C–H bond cleavage is dominant and the O–H activation only plays minor role on the PESs; (2) the DHT from methyl should be the most practical channel; and (3) electronic structure analysis demonstrates the proton and electron transfer coupling behavior along the reaction coordinates. The solvent effect is evident and plays distinct roles in regulating the two bond activations in different mechanisms during the catalysis. The effect of optimizing the geometries using different density functionals was also studied, showing that it is not meaningful to discuss which DFT method could give the accurate prediction of the geometries, especially for transition structures. Furthermore, the gold-standard CCSD(T) method was used to benchmark 19 different density functionals with different Hartree–Fock exchange fractions. The results revealed that (i) the structural factor plays a minor role in the single point energy (SPE) calculations; (ii) reaction energy prediction is quite challenging for DFT methods; (iii) the mean absolute deviations (MADs) reflect the problematic description of the DFs when dealing with metal oxidation state change, giving a strong correlation on the HF exchange in the DFs. Knowledge from this study should be of great value for computational chemistry, especially for the de novo design of transition metal catalysts.

The conversion of benzene to phenol by high-valent bare FeO2+ was comprehensively explored using a density functional theory method. The conductor-like screen model (COSMO) was used to mimic the role of solvent effect with acetonitrile chosen as the solvent. Two radical mechanisms and one oxygen insertion mechanism were tested for this conversion. The first radical mechanism can also be named as the concerted mechanism in which the hydrogen-atom abstraction process is accomplished via a four-centered transition state. The second radical mechanism is initiated by a direct hydrogen-atom abstraction with a collinear C–H–O transition structure. It is actually the same as the well-accepted rebound mechanism for the C–H bond activation by heme and nonheme iron-oxo catalysts. The third is an oxygen insertion mechanism which is essentially an aromatic electrophilic attack leading to an arenium σ-complex intermediate. The formation of a precomplex with an η4 coordinate environment in the first radical mechanism is energetically more favorable. However, the relatively lower activation energy barrier of the oxygen insertion mechanism compared to the radical ones makes it highly competitive if the FeO2+ collides with benzene in the proper orientation. The detailed potential energy surfaces also indicate that the second radical mechanism, i.e., the benzene C–H bond activation through the rebound mechanism, is less favorable. This thorough theoretical study, especially the electronic structure analysis, may offer very important clues for understanding and studying C–H bond activation by iron-based catalysts and enzymatic reactions in protein active pockets.

Alkane C–H bond activation by various catalysts and enzymes has attracted considerable attention recently, but many issues are still unanswered. The conversion of ethane to ethanol and ethene by bare [FeIII═O]+ has been explored using density functional theory and coupled-cluster method comprehensively. Two possible reaction mechanisms are available for the entire reaction, the direct H-abstraction mechanism and the concerted mechanism. First, in the direct H-abstraction mechanism, a direct H-abstraction is encountered in the initial step, going through a collinear transition state C···H···O–Fe and then leading to the generation of an intermediate Fe–OH bound to the alkyl radical weakly. The final product of the direct H-abstraction mechanism is ethanol, which is produced by the hydroxyl group back transfer to the carbon radical. Second, in the concerted reaction mechanism, the H-abstraction process is characterized via overcoming four/five-centered transition states 6/4TSH_c5 or 4TSH_c4. The second step of the concerted mechanism can lead to either product ethanol or ethene. Moreover, the major product ethene can be obtained through two different pathways, the one-step pathway and the stepwise pathway. It is the first report that the former pathway starting from 6/4IM_c to the product can be better described as a proton-coupled electron transfer (PCET). It plays an important role in the product ethene generation according to the CCSD(T) results. The spin–orbital coupling (SOC) calculations demonstrate that the title reaction should proceed via a two-state reactivity (TSR) pattern and that the spin-forbidden transition could slightly lower the rate-determining energy barrier height. This thorough theoretical study, especially the explicit electronic structure analysis, may provide important clues for understanding and studying the C–H bond activation promoted by iron-based artificial catalysts.

The conversion of benzene to phenol by high-valent bare FeO2+ was comprehensively explored using a density functional theory method. The conductor-like screen model (COSMO) was used to mimic the role of solvent effect with acetonitrile chosen as the solvent. Two radical mechanisms and one oxygen insertion mechanism were tested for this conversion. The first radical mechanism can also be named as the concerted mechanism in which the hydrogen-atom abstraction process is accomplished via a four-centered transition state. The second radical mechanism is initiated by a direct hydrogen-atom abstraction with a collinear C–H–O transition structure. It is actually the same as the well-accepted rebound mechanism for the C–H bond activation by heme and nonheme iron-oxo catalysts. The third is an oxygen insertion mechanism which is essentially an aromatic electrophilic attack leading to an arenium σ-complex intermediate. The formation of a precomplex with an η4 coordinate environment in the first radical mechanism is energetically more favorable. However, the relatively lower activation energy barrier of the oxygen insertion mechanism compared to the radical ones makes it highly competitive if the FeO2+ collides with benzene in the proper orientation. The detailed potential energy surfaces also indicate that the second radical mechanism, i.e., the benzene C–H bond activation through the rebound mechanism, is less favorable. This thorough theoretical study, especially the electronic structure analysis, may offer very important clues for understanding and studying C–H bond activation by iron-based catalysts and enzymatic reactions in protein active pockets.