We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and single-crystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable σ-donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP–Rh complexes catalyze C2-selective monoalkylation of pyridine with alkenes.

Rhodium Complexes Bearing PAlP Pincer Ligands.

The unexpectedly facile insertion of Rh or Ir into a B–Ph bond (reversible for Rh) converts a borane/bis(phosphine) precursor into a boryl/bis(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen.

Facile Insertion of Rh and Ir into a Boron-Phenyl Bond, Leading to Boryl/Bis(phosphine) PBP Pincer Complexes.

Iron– and cobalt–dinitrogen complexes bearing PSiP-type pincer ligands are newly designed and prepared, on the basis of our previous proposal that iron– and cobalt–dinitrogen complexes bearing trimethylsilyl ligands are key reactive intermediates in the catalytic formation of silylamine from nitrogen gas under ambient reaction conditions. These iron– and cobalt–dinitrogen complexes are found to work as effective catalysts toward the catalytic transformation of nitrogen gas into silylamine.

Synthesis and Reactivity of Iron– and Cobalt–Dinitrogen Complexes Bearing PSiP‐Type Pincer Ligands toward Nitrogen Fixation

Bond activation and catalysis are central to the development of a sustainable energy system. Frustrated Lewis Pairs have conceptually revolutionized the activation of inert chemical bonds. Far less...

Implementation of Cooperative Designs in Polarized Transition Metal Systems—Significance for Bond Activation and Catalysis

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C–H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

/pdf/resurgence-and-advancement-of-photochemical-hydrogen-atom-3mpr1ito.pdf

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

Formation of Chlorosilyl Pincer-Type Rhodium Complexes by Multiple Si–H Activations of Bis(phosphine)/Dihydrosilyl Ligands

Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C-H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, E° (Cl•/-), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH•+ by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH•+*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M-1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C(sp3)-H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant.

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine.

A bis(phosphine)/triflatosilyl pincer-type Rh(i) complex can reversibly store one equivalent of H2 across the Si-Rh bond upon triflate migration from silicon to rhodium. The triflatosilyl complex serves as an effective precatalyst for norbornene hydrogenation, but Si-OTf bond cleavage is not implicated in the major catalytic pathway. The combined findings suggest possible strategies for M/Si cooperation in catalytic processes.

Examining the role of Rh/Si cooperation in alkene hydrogenation by a pincer-type [P2Si]Rh complex

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Factors that Impact Photochemical Cage Escape Yields.

Stabilization of ions and radicals often determines reaction kinetics and thermodynamics, but experimental determination of the stabilization magnitude remains difficult, especially when the species is short-lived. Herein, a competitive kinetic approach to quantify the stabilization of a halide ion toward oxidation imparted by specific stabilizing groups relative to a solvated halide ion is reported. This approach provides the increase in the formal reduction potential, ΔE°′(Χ•/–), where X = Br and I, that results from the noncovalent interaction with stabilizing groups. The [Ir(dF-(CF3)-ppy)2(tmam)]3+ photocatalyst features a dicationic ligand tmam [4,4′-bis[(trimethylamino)methyl]-2,2′-bipyridine]2+ that is shown by 1H NMR spectroscopy to associate a single halide ion, Keq = 7 × 104 M–1 (Br–) and Keq = 1 × 104 M–1 (I–). Light excitation of the photocatalyst in halide-containing acetonitrile solutions results in competitive quenching by the stabilized halide and the more easily oxidized diffusing halide ion. Marcus theory is used to relate the rate constants to the electron-transfer driving forces for oxidation of the stabilized and unstabilized halide, the difference of which provides the increase in reduction potentials of ΔE°′(Br•/–) = 150 ± 24 meV and ΔE°′(I•/–) = 67 ± 13 meV. The data reveal that Keq is a poor indicator of these reduction potential shifts. Furthermore, the historic and widely used assumption that Coulombic interactions alone are responsible for stabilization must be reconsidered, at least for polarizable halogens.

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers

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