Covalent organic frameworks with designable periodic skeletons and ordered nanopores have attracted increasing attention as promising cathode materials for rechargeable batteries. However, the reported cathodes are plagued by limited capacity and unsatisfying rate performance. Here we report a honeycomb-like nitrogen-rich covalent organic framework with multiple carbonyls. The sodium storage ability of pyrazines and carbonyls and the up-to twelve sodium-ion redox chemistry mechanism for each repetitive unit have been demonstrated by in/ex-situ Fourier transform infrared spectra and density functional theory calculations. The insoluble electrode exhibits a remarkably high specific capacity of 452.0 mAh g−1, excellent cycling stability (~96% capacity retention after 1000 cycles) and high rate performance (134.3 mAh g−1 at 10.0 A g−1). Furthermore, a pouch-type battery is assembled, displaying the gravimetric and volumetric energy density of 101.1 Wh kg−1cell and 78.5 Wh L−1cell, respectively, indicating potentially practical applications of conjugated polymers in rechargeable batteries. Covalent organic frameworks are receiving increasing attention as promising cathode materials for rechargeable batteries. Here the authors report a honeycomb-like nitrogen-rich COF design in which the pyrazines and carbonyls enable favorable redox chemistry and remarkable Na-ion storage performance.

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Nitrogen-rich covalent organic frameworks with multiple carbonyls for high-performance sodium batteries.

Photogeneration of Ar 2 H + , Kr 2 H + and Xe 2 H + and their deuterated counterparts is reported in H(D)Cl doped Ar, and H(D)X (X = Cl, Br or I) doped Kr and Xe matrices, respectively. The formation of these species is ascribed to optical access of the delocalized X/rare gas charge transfer states, and subsequent charge trapping processes. Once generated, the absorptions of the cations bleach thermally at low temperatures ( 12 K). The observed second-order decay kinetics is discussed in terms of charge-recombination involving a distribution of barrier heights. The deuterated cations bleach on a significantly longer time-scale. Bihalide ions, HX − 2 , of D ∞h symmetry are the other products formed in the photolysis. Annealing of the extensively irradiated HCl, HBr and HI doped xenon and HCl doped krypton yields strong infrared absorptions in the 1200–1600 cm −1 spectral region. These bands are tentatively ascribed to linear asymmetric (X-H—X) − ions perturbed by a nearby positive charge, or to (XeHX) + and (KrHX) + species.

Photogeneration of ionic species in Ar, Kr and Xe matrices doped with HCl, HBr and HI

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of -2 at a transition-metal center. For a series of formal high-valent Ni-IV complexes, aryl-CF3 bond-forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015, 137, 8034-8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni-IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal-ligand bonds, a phenomenon attributable to sigma-noninnocence. A direct consequence is that the elimination of aryl-CF3 products occurs in an essentially redox-neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF3 group is transferred to an electrophilic aryl group. The uncovered role of sigma-noninnocence in metal-ligand bonding, and of an essentially redox-neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.

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σ‐Noninnocence Masked Phenyl Cation Transfer at Formal Ni(IV)

Terminal Imido Complexes of the Groups 9–11: Electronic Structure and Developments in the Last Decade

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including ‘click chemistry’, photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity — or, rather, ‘charge frustration’ — as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor–acceptor ligand systems for reversible 1e− chemistry or switching. Charge-separated organic molecules find diverse applications as functional materials. This Review describes zwitterionicity as a general design principle for ‘smart’ coordination chemistry and the activation of strong bonds.

Charge frustration in ligand design and functional group transfer

Binary transition metal fluorides are textbook examples combining complex electronic features with most fundamental molecular structures. High-valent nickel fluorides are among the strongest known fluorinating and oxidizing agents, but there is a lack of experimental structural and spectroscopic investigations on molecular NiF3 or NiF4 . Apart from their demanding synthesis, also their quantum-chemical description is difficult due to their open shell nature and low-lying excited electronic states. Distorted tetrahedral NiF4 (D2d ) and trigonal planar NiF3 (D3h ) molecules were produced by thermal evaporation and laser ablation of nickel atoms in a fluorine/noble gas mixture and spectroscopically identified by a joint matrix-isolation and quantum-chemical study. Their vibrational band positions provide detailed insights into their molecular structures.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.202015501

Searching for Monomeric Nickel Tetrafluoride: Unravelling Infrared Matrix Isolation Spectra of Higher Nickel Fluorides.

Transition metal complexes bearing terminal oxido ligands are quite common, yet group 11 terminal oxo complexes remain elusive. Here we show that excited coinage metal atoms M (M = Au, Ag, Cu) react with OF2 to form hypofluorites FOMF and group 11 oxygen metal fluorides OMF2, OAuF and OAgF. These compounds have been characterized by IR matrix-isolation spectroscopy in conjunction with state-of-the-art quantum-chemical calculations. The oxygen fluorides are formed by photolysis of the initially prepared hypofluorites. The linear molecules OAgF and OAuF have a 3Σ − ground state with a biradical character. Two unpaired electrons are located mainly at the oxygen ligand in antibonding O−M π* orbitals. For the 2B2 ground state of the OMIIIF2 compounds only an O−M single bond arises and a significant spin-density contribution was found at the oxygen atom as well. While transition metal complexes bearing terminal oxido ligands are common, those of group 11 elements have yet to be experimentally observed. Here, Riedel and colleagues synthesise molecular oxygen fluorides of copper, silver and gold, and show that the oxo ligands possess radical character.

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Oxygen radical character in group 11 oxygen fluorides.

Terminal oxo compounds of high-valent late transition metals attracted attention because of their peculiar metal–oxygen bond in which the oxo ligand exhibits an electrophilic and distinct radical oxyl (O˙−) rather than the commonly expected nucleophilic (O2−) character. These properties confer the late transition metal oxo compounds their promising high reactivity in oxidation reactions, as well as H-atom transfer (HAT) and O-atom transfer (OAT) reactions. On the other hand, because of their highly reactive nature, direct spectroscopic evidence especially for high-valent nickel oxo compounds is rare. In this work, we report on linear oxo monofluorides OMF (C∞v) and difluorides OMF2 (C2v) of the group 10 metal atoms M = Ni, Pd and Pt, as well as on the hypofluorite FOPdF (Cs) and the oxo trifluoride OPtF3 (C2v). These molecules were prepared by the reaction of the metal atoms with gaseous OF2 and isolated in solid Ne and Ar matrices. They have been investigated by a joint analysis of IR matrix isolation spectroscopy and electronic structure calculations at the DFT, CCSD(T), as well as CASPT2 and MRCI levels of theory. The linear OMF molecules have 4Σ− ground states and bear a terminal oxyl radical ligand. The oxo difluorides have planar T-shaped structures in the 3A2 electronic states. The terminal oxo ligand in these difluorides also carries large spin densities of ≥1.0. A contradiction about the electronic ground state for the recently reported ONiF2 [Wei et al., Inorg. Chem., 2019, 58, 9796–9810] could be solved experimentally by the observation of a more complete vibrational spectrum. The relationship between the electron and spin population on the oxo ligand in the group 10 transition metal oxofluorides and related compounds will be discussed. Remarkable differences were found for the metal–oxygen bonds in ONiF2 and OCuF2. The continuous transition from a predominantly ionic to an inverted ligand field is shown by comparing the linear oxo monofluorides of group 10 (O[10]F), with the difluorides of group 9 (F[9]F) and the isoelectronic dioxides of group 11 (O[11]O).

Molecular oxofluorides OMFn of nickel, palladium and platinum: oxyl radicals with moderate ligand field inversion

Uranium and thorium hydrides are known as functional groups for ligand stabilized complexes and as isolated molecules under matrix isolation conditions. Here, the new molecular products of the reactions of laser-ablated U and Th atoms with HCl and with HBr, namely HUCl, HUBr and HThCl, HThBr, based on their mid and far infrared spectra in solid argon, are reported. The assignment of these species is based on the close agreement between observed and calculated vibrational frequencies. The H-U and U-35 Cl stretching modes of HUCl were observed at 1404.6 and 323.8 cm-1 , respectively. Using DCl instead to form DUCl gives absorption bands at 1003.1 and 314.7 cm-1 . The corresponding bands of HThCl are 1483.8 (H-Th) and 1058.0 (D-Th), as well as 340.3 and 335.8 cm-1 (Th-35 Cl), respectively. HUBr is observed at 1410.6 cm-1 and the BP86 computed shift from HUCl is 6.2 cm-1 in excellent agreement. The U-H stretching frequency increases from 1383.1 (HUF), 1404.6 (HUCl), 1410.6 (HUBr) to 1423.6 cm-1 (UH) as less electronic charge is removed from the U-H bond by the less electronegative substituent. These U-H stretching frequencies follow the Mayer bond orders calculated for the three HUX molecules. A similar trend is found for the Th counterparts. Additional absorptions are assigned to the H2 AnX2 molecules (An=U, Th, X=Cl, Br) formed by the exothermic reaction of a second HX molecule with the above primary products.

Lin Li

Papers

Searching for Monomeric Nickel Tetrafluoride: Unravelling Infrared Matrix Isolation Spectra of Higher Nickel Fluorides.

Oxygen radical character in group 11 oxygen fluorides.

Molecular oxofluorides OMFn of nickel, palladium and platinum: oxyl radicals with moderate ligand field inversion

Infrared Spectra of the HAnX and H2AnX2 Molecules (An=Th and U, X=Cl and Br) in Argon Matrices Supported by Electronic Structure Calculations

Reactions of titanium, zirconium and hafnium atoms with hydrogen selenide: A matrix-isolation study