Radical Ligand Research Articles

Direct Spectroscopic Confirmation of the Oxygen‐Centered Diradical Character of the Tetraoxidorhenium(VII) Cation [Re(O)4]+

AbstractMononuclear inorganic diradical species are scarce. Here, we confirm, via X‐ray absorption spectroscopy in the gas phase combined with computational studies, the oxygen‐centered diradical character of the tetraoxidorhenium(VII) cation. A dioxido‐superoxido isomer, close in energy to the diradical, is also found, where rhenium appears in its rare oxidation state of +6. Addition of one or two hydrogen atoms to [Re,O4]+ forms hydroxido ligands, and strongly disfavors isomers with any oxygen‐oxygen bond. This adds spectroscopic characterization of the rhenium oxidation state and the nature of ligands to the known ability of [Re,O4]+ to perform two consecutive hydrogen‐atom abstraction reactions from methane, and demonstrates that pentaatomic [Re,O4]+ combines a metal center in its highest oxidation state with two oxygen‐centered radical ligands in a highly reactive species.

N-Heterocyclic Carbene-Derived Carbon Disulfide Radical Ligands for Palladium Diradicals.

N-heterocyclic carbenes (NHCs) are recognized for their ability to stabilize various main group radicals; however, NHC-derived, sulfur-based radicals remain rare. In this study, we successfully synthesized and characterized a series of palladium diradical complexes that featured new sulfur-based radical ligands from NHC-carbon disulfide adducts. Spectroscopic and computational characterizations of the palladium complexes confirmed the open-shell singlet ground state, which resulted from the antiferromagnetic coupling of two unpaired electrons on each ligand. Proton nuclear magnetic resonance relaxometry was used to experimentally confirm the presence of these unpaired electrons. Moreover, the redox behavior of the complexes was localized on the ligand center, confirming the redox activity of the ligands. The discovery of this sulfur-based, redox-active radical ligand underscores the versatility and significance of NHC-derived radicals, thereby expanding the repertoire of radical ligands and opening new avenues for advanced material and catalytic systems.

Supramolecular Spin Chains via Radical-Radical Contacts Stabilizing Ferromagnetic Interactions between Heisenberg or Ising-like Spins.

A new paramagnetic ligand, 4-(2'-4-(2''-furyl)-pyrimidyl)-1,2,3,5-dithiadiazolyl (furylpymDTDA) and three transition metal coordination complexes, M(hfac)2(furylpymDTDA) M = Mn, Co, Ni; hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-), are reported. The solid-state structures are influenced by the geometry of the coordination sphere of the M(II) centers: trigonal (Mn) vs. octahedral (Co and Ni). While the hs-Mn(II) complex exhibits pairwise multi-centre 2-electron bonds (i.e., pancake bonds) between the planar π radical DTDA moieties, the hs-Co(II) and Ni(II) complexes crystallize with close contacts between coordinated furylpymDTDA radical ligands that define linear 1D arrays of molecular units. The magnetic data for the hs-Co(II) and Ni(II) complexes indicate ferromagnetic (FM) interactions between molecular units, apparently mediated by radical-radical contacts along the supramolecular chains. Computational analysis suggests proximity between regions of large α- and β-spin density on neighbouring molecular sites enabling FM exchange, in accordance with the McConnell I mechanism. The magnetic data for the Ni(II) complex are consistent with a Heisenberg spin chain, whereas the hs-Co(II) complex exhibits Ising-like spin chain behaviour and a magnetic phase transition to an FM ordered state at 4.6 K.

Building Catalytic Reactions One Electron at a Time.

ConspectusClassical education in organic chemistry and catalysis, not the least my own, has centered on two-electron transformations, from nucleophilic attack to oxidative addition. The focus on two-electron chemistry is well-founded, as this brand of chemistry has enabled incredible feats of synthesis, from the development of life-saving pharmaceuticals to the production of ubiquitous commodity chemicals. With that said, this approach is in many ways complementary to the approach of nature, where enzymes frequently make use of single-electron "radical" steps to achieve challenging reactions with exceptional selectivity, including light detection and C-H hydroxylation. While the power of radical elementary steps is undeniable, the fundamental understanding of─and ability to apply─these in catalysis remains underdeveloped, constraining the palette with which chemists can make new reactions.Motivation to remedy this traditional underemphasis on radical catalysis has been intensified by the runaway success of outer-sphere photoredox catalysis, not only confirming the versatility of radicals in anthropogenic catalysis but also teaching the value of robust and well-understood catalytic cycles for reaction design. Indeed, I would argue the success of outer-sphere photoredox catalysis has been fueled by strong fundamental understanding of its underlying radical elementary steps, with consideration of single-electron transfer (SET) energetics allowing new reactions to be designed de novo with enviable confidence. However, outer-sphere photoredox catalysis is an outlier in this regard, with other mechanistic approaches remaining underexplored.Our research group is part of a growing movement to expand the vocabulary of synthetic radical catalysis beyond the traditional outer-sphere photoredox SET manifold, assembling new cycles comprised of hydrogen atom transfer (HAT), light-induced homolysis (LIH), and radical ligand transfer (RLT) steps in new combinations to achieve challenging transformations. These efforts have been made possible by the ever-growing understanding of these radical elementary steps and discovery of catalyst systems with significant mechanistic flexibility, most recently iron/thiol (Fe/S) cocatalysis.In this Account, I will focus on our efforts applying HAT and LIH steps in Fe/S cocatalysis, sharing broad guidelines we have found helpful for using these steps and demonstrating how they can be combined to make new reactions using three case studies: radical hydrogenation (HAT + HAT), decarboxylative protonation (LIH + HAT), and alkene hydrofluoroalkylation (LIH + HAT, with an intervening radical alkene addition). These efforts have highlighted the importance of several key parameters, including bond dissociation energy (BDE) and radical polarity, and I hope our findings similarly provide a valuable framework to others designing new radical catalytic reactions.

Enhancement of Effective Energy Barrier and Magnetic Blocking Temperature in Tetraoxolene Radical Coupled Dinuclear Dysprosium Complex.

A well-judged combination of a high axial ligand field and a bridging radical ligand in a dinuclear lanthanide complex provides a single-molecule magnet with a higher effective energy barrier for magnetic relaxation and blocking temperature compared to its non-radical analog due to significant magnetic exchange coupling between radical and Ln(III) ions. In this work, we report two chloranilate (CA) bridged dinuclear dysprosium complexes, [{(bbpen)Dy(μ2-CA)Dy(bbpen)}] (1Dy) and [{(bbpen)Dy(μ2-CA•)Dy(bbpen)}-{CoCp2}+] (2Dy), where 2Dy is the radical bridged Dy-complex obtained via the chemical reduction of bridging CA moiety (H2bbpen = N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine). The presence of high electronegative phenoxide moiety enhances the axial anisotropy of pseudo-square antiprismatic Dy(III) ions. The diffused spin of radical is efficiently coupled with anisotropic Dy(III) centres and decreases the quantum tunnelling of magnetization (QTM) in the magnetic relaxation process. The magnetic relaxation of 1Dy follows Orbach, Raman, and QTM processes whereas for 2Dy it follows Orbach and Raman Processes. Due to less involvement of the QTM relaxation process, 2Dy shows a higher thermal energy barrier (Ueff = 700 K) and a high blocking temperature (6.7 K), compared to its non-radical analog. Remarkably, the radical coupled 2Dy complex shows the highest energy barrier among the radical bridged Dy(III)-based SMMs to date.

Complexes of a Noncyclic Carbazole-based N5-donor Schiff base: Structures, Redox, EPR and Poor Activity as Hydrogen Evolution Electrocatalysts.

A new noncyclic pentadentate N5-donor Schiff-base ligand, HL2Etpyr (1,1'-(3,6-ditert-butyl-9H-carbazole-1,8-diyl)bis(N-(2-(pyridin-2-yl)ethyl)methanimine)), prepared from 1,8-diformyl-3,6-ditertbutyl-carbazole (HUtBu) and two equivalents of 2-(2-pyridyl)ethylamine, along with four tetrafluoroborate complexes, [MIIL2Etpyr](BF4), where M = Co, Ni, Cu, and Zn, and two [CoIIL2EtPyr]·1/2[CoIIX4] complexes where X = NCS or Cl, isolated as solvates, are reported. All six complexes were structurally characterized, revealing the cations to be isostructural, with M(II) in a trigonal bipyramidal N5-donor environment. Only the Zn(II) complex is fluorescent. Cyclic voltammograms of [MIIL2Etpyr](BF4) in MeCN reveal reversible redox processes at positive potentials: 0.61 (Zn), 0.62 (Cu), 0.57 (Ni), and 0.25 V (Co), and for the cobalt complex a second quasi-reversible process occurs at 0.92 V vs Fc+/Fc. EPR data for the first oxidation product clearly demonstrate that the Zn complex undergoes a ligand centered oxidation, and support this being the case for the Ni and Cu complexes, although this is not definitively shown. After both oxidations the EPR data shows that the Co complex is best described as a low spin Co(III)-ligand radical. In the presence of 80 mM acetic acid, controlled potential electrolysis carried out on [MIIL2Etpyr](BF4) at -1.68 V in MeCN shows some electrocatalytic hydrogen evolution reaction (HER) performance in the order Ni(II) > Cu(II) > Co(II) - but the control, Ni(II) tetrafluoroborate, is more active than all three of the complexes.

Merging Iron-Mediated Radical Ligand Transfer (RLT) Catalysis and Mechanochemistry for Facile Dihalogenation of Alkenes

Merging Iron-Mediated Radical Ligand Transfer (RLT) Catalysis and Mechanochemistry for Facile Dihalogenation of Alkenes

Synthesis of a potassium capped terminal cobalt-oxido complex.

An unusual example of a potassium capped terminal cobalt-oxido complex has been isolated and crystallographically characterized. The synthesis of [tBu,TolDHP]CoOK proceeds from a previously reported parent compound, [tBu,TolDHP]CoOH, via deprotonation with KOtBu. Structural and electronic characterization suggest a weakly coupled dimer in a distinct seesaw geometry with a Co(III) oxidation state and a non-innocent radical ligand.

Cobalt Complex of an N-Linked Carbacorrole Analog: Axial Ligand Effect on Electronic Structure

Corrole, recognized as a contracted tetrapyrrolic macrocycle featuring an α,α'-linked bipyrrole moiety, has garnered attention for its unique trianionic N4 donor environment at the inner core. This characteristic has prompted the exploration of various high-valent metal species for potential catalytic applications. In our pursuit of uncovering new reactivity in metallo-corroles, we have previously synthesized a distinct carbacorrole ligand known as N-confused/N-linked corrole analogs.[1] The former variant incorporates an inverted pyrrole moiety connected at the α and β’-linking positions, while the latter exhibits a unique linkage between the α-pyrrole ring and the pyrrolic N site. The resulting [NNNC] coordination environment of carbacorroles stabilizes distinctive organometallic species, particularly with copper(III) ions.[2]In this study, we synthesized novel cobalt complexes featuring an N-linked carbacorrole analog with a benzo annulation, referred to as benzonorrole. The electronic structure of the ensuing complex is notably influenced by the axial ligands. The monodentate pyridine axial coordination to the cobalt norrole complex revealed unique reactivity, resulting in an oxygenated product at the inner carbon site. The oxygenated cobalt norrole complex can be characterized as a singlet Co(II) ligand radical at the ground state, as evidenced by various spectroscopic methods and magnetic susceptibility measurements. Upon substitution with a triphenylphosphine ligand, the resulting five-coordinated cobalt norrole complex was found to be stable and considered a diamagnetic cobalt(III) species, devoid of redox noninnocent character. This talk will delve into the detailed structure and electrochemical properties of these intriguing complexes.[1] Fujino, K.; Hirata, Y.; Kawabe, Y.; Morimoto, T.; Srinivasan, A.; Toganoh, M.; Miseki, Y.; Kudo, A.; Furuta, H. Angew. Chem. Int. Ed. 2011, 50, 6855-6859; Toganoh, M.; Kawabe, Y.; Uno, H.; Furuta, H. Angew. Chem. Int. Ed. 2012, 51, 8753-8756.[2] Maurya, Y.; Noda, K.; Yamasumi, K.; Mori, S.; Uchiyama, T.; Kamitani, K.; Hirai, T.; Ninomiya, K.; Nishibori, M.; Hori, Y.; Shiota, Y.; Yoshizawa, K.; Ishida, M.; Furuta, H. J. Am. Chem. Soc. 2018, 140, 6883-6892.

Robust free-base and metalated corrole radicals with reduction-induced emission

Preparing free-base porphyrinoid radicals that can function as coordination ligands is a challenging task. Here we report the synthesis of a stable, free-base benzocorrole (BC) radical containing only two inner NH protons via a retro-Diels-Alder conversion. The radical character of BC was fully supported by crystallographic analysis, spectroscopic evidence, and theoretical calculations. This neutral radical ligand allowed easy insertion of Zn(II), Ga(III), and Pd(II) ions to produce radical complexes. All these radicals exhibited luminescence-on responses under weak reducing atmosphere, corresponding to the conversion to their aromatic anions. The red fluorescence was observed for BC and its Zn(II) and Ga(III) complexes, and the near-infrared phosphorescence (> 900 nm) was detected for Pd(II) complex at room temperature. Furthermore, Ga(III) corrole exhibited a variation in fluorescence in response to axial coordination. Our findings provide a promising radical platform for coordination and developing novel functional materials with switchable spin and emission.

Mechanochemistry Drives Alkene Difunctionalization via Radical Ligand Transfer and Electron Catalysis.

A general and modular protocol is reported for olefin difunctionalization through mechanochemistry, facilitated by cooperative radical ligand transfer (RLT) and electron catalysis. Utilizing mechanochemical force and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), ferric nitrate can leverage nitryl radicals, transfer nitrooxy-functional group via RLT, and mediate an electron catalysis cycle under room temperature. A diverse range of activated and unactivated alkenes exhibited chemo- and regioselective 1,2-nitronitrooxylation under solvent-free or solvent-less conditions, showcasing excellent functional group tolerance. Mechanistic studies indicated a significant impact of mechanochemistry and highlighted the radical nature of this nitrative difunctionalization process.

Bis(acyl)phosphide Complexes of U(III)/U(IV): A Case of a Hidden Redox-Active Ligand.

The recently reported tris(bis(2,4,6-triisopropylbenzoyl)-phosphide)uranium (UIII(trippBAP)3, 2) complex (Inorg. Chem. 2022, 61 (32), 12508-12517) demonstrated a silent 31P NMR spectrum. This complex was described as a U(III) complex with an organic radical ligand fragment. Moreover, the EPR spectrum of 2 was indicative of an organic radical in the ligand framework complexed to uranium, in contrast to that of UIV(mesBAP)4, 1. Herein, with the help of relativistic density functional theory (DFT) calculations, the electronic structures of 1, 2, and U(mesBAP)3 (4) are examined in an effort to understand the unusual 31P NMR spectrum of 2. Results indicate the reduction of the carbonyl bonds and delocalization of the electrons over the ligands, indicative of U → L backbonding. Additionally, the reduced acyl carbons are found to exist as ketyl radicals [O═C• -] that are responsible for the silent 31P NMR spectra of 2. These findings demonstrate the redox noninnocent nature of BAP- in 2 and 4, causing uranium to exist in a formal oxidation state of +4.

All‐in‐One: Photo‐Responsive Lanthanide‐Organic Framework for Simultaneous Sensing, Adsorption, and Photocatalytic Reduction of Uranium

AbstractThe selective removal and immobilization of uranyl ions from aqueous solutions is essential for the sustainable development of nuclear energy. Herein, a robust lanthanide‐organic framework material (IHEP‐24) is developed for simultaneous fluorescence sensing, selective adsorption, and photocatalytic reduction of uranium, integrating three different functions in one material. The confined space formed by the coordination assembly of viologen derivative ligands and metal‐oxygen clusters can act as precise recognition sites for uranyl, allowing IHEP‐24 to efficiently detect and capture uranyl ions. In addition, the presence of a viologen‐based radical ligand enables IHEP‐24 to a further photocatalytic reduction of the adsorbed uranyl to amorphous UO2. The mechanisms of adsorption and photocatalysis are revealed by batch experiments, photoelectrochemical characterizations, and theoretical calculations. This study provides a reference for the construction of robust multi‐functional MOF materials and also provides support for the deep removal and immobilization of radionuclides from aqueous solution.

Chemical Repair of Radical Damage to the GC Base Pair by DNA-Bound Bisbenzimidazoles.

The migration of an electron-loss center (hole) in calf thymus DNA to bisbenzimidazole ligands bound in the minor groove is followed by pulse radiolysis combined with time-resolved spectrophotometry. The initially observed absorption spectrum upon oxidation of DNA by the selenite radical is consistent with spin on cytosine (C), as the GC• pair neutral radical, followed by the spectra of oxidized ligands. The rate of oxidation of bound ligands increased with an increase in the ratio (r) ligands per base pair from 0.005 to 0.04. Both the rate of ligand oxidation and the estimated range of hole transfer (up to 30 DNA base pairs) decrease with the decrease in one-electron reduction potential between the GC• pair neutral radical of ca. 1.54 V and that of the ligand radicals (E0', 0.90-0.99 V). Linear plots of log of the rate of hole transfer versus r give a common intercept at r = 0 and a free energy change of 12.2 ± 0.3 kcal mol-1, ascribed to the GC• pair neutral radical undergoing a structural change, which is in competition to the observed hole transfer along DNA. The rate of hole transfer to the ligands at distance, R, from the GC• pair radical, k2, is described by the relationship k2 = k0 exp(constant/R), where k0 includes the rate constant for surmounting a small barrier.

A novel functional nitronyl nitroxide and its copper and cobalt complexes: Synthesis, structures and magnetic properties

Four new metal complexes derived from a functional nitronyl nitroxide ligand, 2-(6-bromo-3-pyridyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (NIT-6Br-3Py) and M(hfac)2+ (M2+ = Cu2+, Co2+; hfac = hexafluoroacetylacetonate), [(Cu(hfac)2)3(NIT-6Br-3Py)2] (1), [Cu(hfac)2NIT-6Br-3Py]n (2), [Co(hfac)2(H2O)NIT-6Br-3Py] (3) and [(Co(hfac)2)2(NIT-6Br-3Py)2] (4) have been synthesized and characterized. In complex 1, the NIT-6Br-3Py ligand acts as bridging ligand to link three Cu(II) ions leading to {Cu3} complex while complex 2 displays a 1D-chain structure. Complex 3 possesses mono-nuclear structure and 4 is a four-spin complex with a dimeric structure, where the NIT-6Br-3Py ligand links Co ions via the oxygen atom of NO group and the nitrogen atom from pyridine ring. Magnetic studies show that the antiferromagnetic interactions are dominated in complexes 1, 3 and 4, while there exist strong ferromagnetic interactions between CuII and coordinated nitronyl nitroxide radical ligand in complex 2.

Luminescent Radicals.

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Spectroscopic and thermodynamic characterization of a cobalt-verdazyl valence tautomeric system. influence of crystal structure, solvent and counterion.

Crystallization of the verdazyl-based valence tautomeric ion [Co(dipyvd)2]2+ (where dipyvd is the radical ligand 1-isopropyl-3,5-di(2'-pyridyl)-6-oxoverdazyl) with a variety of different counterions results in materials that show varying degrees of valence tautomeric (VT) transition in the solid state. The X-ray structure of the SbF6 salt at 150 K reveals a localized structure for the S = 1/2 tautomer, with a Co3+ cation and distinct anionic and radical ligands. Comparison with the structure of the same material at 300 K reveals large structural changes in the ligand as a result of the valence tautomeric equilibrium. Data for the S = 3/2 form is less conclusive; X-ray spectroscopy on the PF6 salt suggests a degree of low spin Co2+ character for the S = 3/2 tautomer at very low temperature though this is inconsistent with EPR data at similar temperatures and structural information at 150 K. Magnetic measurements on the [BArF4]- and triflate salts in organic solvents show that the VT equilibrium is dependent on solvent and ion pairing effects.

What governs magnetic exchange couplings in radical-bridged dinuclear complexes?

Coupling transition metal or lanthanide ions through a radical bridging ligand is a promising route to increase performances in the area of single molecular magnets. A better understanding of the underlying physical mechanisms governing the magnetic exchange couplings is thus of valuable importance to design future compounds. Here, couplings in three series of metal-radical-metal compounds based on transition metal ions are investigated by means of the decomposition/recomposition methods. This work presents the generalisation and first application of the method to systems with an arbitrary number of magnetic centres featuring several unpaired electrons. Thanks to the decomposition into the three main contributions (direct exchange, kinetic exchange, and spin polarisation) as well as a description in terms of electron-electron interactions, we study the influence of the nature of the metal centre and the radical ligand on the couplings. We combine the energetic contributions extracted with orbital and charge population analysis to rationalise the results.

2p–4f one-dimensional chains and two-dimensional networks assembled by a multicoordinating nitronyl nitroxide radical ligand

Two series of 2p–4f complexes, 1D chains and unique 2D networks, were successfully constructed using a multicoordinating nitronyl nitroxide radical. The complex Dy exhibits slow relaxation of magnetization.

Simple, catalytic C(sp3)-H azidation using the C-H donor as the limiting reagent.

C-N bonds play a critical role in pharmaceutical, agrochemical, and materials sciences, necessitating ever-better methods to forge this linkage. Here we report a simple procedure for direct C(sp3)-H azidation using iron or manganese catalysis and a nucleophilic azide source. All reagents are commercially available, the experimental procedure is simple, and we can use the C-H donor substrate as the limiting reagent, a challenge for many C-H azidation methods. Preliminary experiments are consistent with a hydrogen atom transfer (HAT)/radical ligand transfer (RLT) radical cascade mechanism and a wide variety of substrates can be azidated in moderate to high yields.