Kejuan Chen

Two-State Reactivity in Low-Valent Iron-Mediated C–H Activation and the Implications for Other First-Row Transition Metals

C-H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C-H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C-H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C-C coupling and finalizing the C-H functionalization. Regeneration of the Fe(II) catalyst for the next round of C-H activation involves SET oxidation of the Fe(I) species generated after the C-C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C-H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C-H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C-H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C-H activation reactions.

Probing Ligand Effects on O–O Bond Formation of Ru-Catalyzed Water Oxidation: A Computational Survey

Ligand effects of some representative monomeric Ru-based water oxidation catalysts on the key O-O formation step are revealed in this work. Three effects, namely, cis-effect, net charge effect, and steric hindrance effect, are identified, which can exert sizable modulation on the O-O formation barriers for the two widely accepted O-O formation mechanisms of WNA (water nucleophilic attack) and I2M (direct coupling of two high-valent metal oxo units). The study demonstrates that, through the way of ligand design, there remains a large space for improving O-O bond formation reactivity.

Spin-Orbit Coupling and Outer-Core Correlation Effects in Ir- and Pt-Catalyzed C-H Activation.

The transition metal-dependent spin-orbit coupling (SOC) and outer-core (5s5p) correlation effects in Ir- and Pt-catalyzed C-H activation processes are studied here using high level ab initio computations. The catalysts involve complexes with oxidation states: Ir(I), Ir(III), Pt(0), and Pt(II). It is demonstrated that for these heavy 5d transition metal-containing systems, the SOC effect and outer-core correlation effect on C-H activation are up to the order of ∼1 kcal/mol, and should be included if chemical accuracy is aimed. The interesting trends in our studied systems are: (1) the SOC effect consistently increases the C-H activation barriers and is apparently larger in higher oxidation states (Pt(II) and Ir(III)) than in low-oxidation states (Pt(0) and Ir(I)); and (2) the magnitude of outer-core (5s5p) correlation effects is larger in less coordinate-saturated system. The effect of basis set on the outer-core correlation correction is significant; larger basis sets tend to increase the C-H activation barriers.

What factors control O2 binding and release thermodynamics in mononuclear ruthenium water oxidation catalysts? A theoretical exploration.

Mononuclear Ru-based water oxidation catalysts (WOCs) constitute an important class of WOCs for water splitting. This work constitutes a systematic study of Ru-O2 complexes of mononuclear ruthenium WOCs, with a focus on the thermodynamics of water-assisted O2 release in various electronic states and conformations. Our extensive DFT study reveals several factors that affect the O2 release thermodynamics: (1) steric effect from the ligand sphere of Ru; (2) trans effect of ligands trans to O2; (3) oxygen cis coordinating effect; (4) carbon coordinating effect; and (5) Ru coordination strength. Some of these effects could selectively stabilize/destabilize some states/conformations of the Ru-O2 complexes relative to Ru-OH2 complexes, and affect thereby the O2 release thermodynamics. The identification and rationalization of factors for O2 release thermodynamics, as in this work, could be helpful toward a better understanding of this final step of the ruthenium-catalyzed water oxidation.

Stereocontrolled Synthesis of Benzo[k]fluoranthenes—An Unexpected Isomerization Mediated by Rhodacyclopentadiene

Herein, a Rh(III) -catalyzed stereocontrolled synthesis of benzo[k]fluoranthenes is reported. It was found that the unexpected E/Z isomerization was highly sensitive to the electronic effects of the substituents on the aryl groups. Theoretical calculations revealed that this controllable stereochemistry originates from the mediation of rhodacyclopentadiene intermediates during the isomerization. The fact that similar stereochemistry was observed when using an Ir(III) catalyst further suggests a certain generality of this discovery toward some other transition metals.

First results obtained from the soft x-ray pulse height analyzer on experimental advanced superconducting tokamak

An assembly of soft x-ray pulse height analyzer system, based on silicon drift detector (SDD), has been successfully established on the experimental advanced superconducting tokamak (EAST) to measure the spectrum of soft x-ray emission (E=1-20 keV). The system, including one 15-channel SDD linear array, is installed on EAST horizontal port C. The time-resolved radial profiles of electron temperature and K(alpha) intensities of metallic impurities have been obtained with a spatial resolution of around 7 cm during a single discharge. It was found that the electron temperatures derived from the system are in good agreement with the values from Thomson scattering measurements. The system can also be applied to the measurement of the long pulse discharge for EAST. The diagnostic system is introduced and some typical experimental results obtained from the system are also presented.

Modeling σ-Bond Activations by Nickel(0) Beyond Common Approximations: How Accurately Can We Describe Closed-Shell Oxidative Addition Reactions Mediated by Low-Valent Late 3d Transition Metal?

Accurate modelings of reactions involving 3d transition metals (TMs) are very challenging to both ab initio and DFT approaches. To gain more knowledge in this field, we herein explored typical σ-bond activations of H-H, C-H, C-Cl, and C-C bonds promoted by nickel(0), a low-valent late 3d TM. For the key parameters of activation energy (ΔE‡) and reaction energy (ΔER) for these reactions, various issues related to the computational accuracy were systematically investigated. From the scrutiny of convergence issue with one-electron basis set, augmented (A) basis functions are found to be important, and the CCSD(T)/CBS level with complete basis set (CBS) limit extrapolation based on augmented double-ζ and triple-ζ basis pair (ADZ and ATZ), which produces deviations below 1 kcal/mol from the reference, is recommended for larger systems. As an alternative, the explicitly correlated F12 method can accelerate the basis set convergence further, especially after its CBS extrapolations. Thus, the CCSD(T)-F12/CBS(ADZ-ATZ) level with computational cost comparable to the conventional CCSD(T)/CBS(ADZ-ATZ) level, is found to reach the accuracy of the conventional CCSD(T)/A5Z level, which produces deviations below 0.5 kcal/mol from the reference, and is also highly recommendable. Scalar relativistic effects and 3s3p core-valence correlation are non-negligible for achieving chemical accuracy of around 1 kcal/mol. From the scrutiny of convergence issue with the N-electron basis set, in comparison with the reference CCSDTQ result, CCSD(T) is found to be able to calculate ΔE‡ quite accurately, which is not true for the ΔER calculations. Using highest-level CCSD(T) results of ΔE‡ in this work as references, we tested 18 DFT methods and found that PBE0 and CAM-B3LYP are among the three best performing functionals, irrespective of DFT empirical dispersion correction. With empirical dispersion correction included, ωB97XD is also recommendable due to its improved performance.