Rational Design of Persistent Phosphorus-Centered Singlet Tetraradicals and Their Use in Small-Molecule Activation

Reaction of NHOs with Bisphosphanes - Designing Diradicaloids, Zwitterions and Radicals

The linkage of an imidazole-based N-heterocyclic olefin (NHO), containing a terminal CH2 donor group, with a phosphorus-centered diradical molecular fragment leads to an open-shell singlet diphospha-indenylide system, a new class of P-heterocycles, which can be interpreted both as a phosphorus-centered diradicaloid and as a zwitterion with a permanent, overall charge separation between the N- and P-heterocyclic ring systems. The rotation of the imidazole ring, which is thermally possible due to a central C-C bond with a weakened π-component, changes both the charge separation and diradical character depending on the dihedral angle, as quantum mechanical calculations indicate. By varying the bulkiness of substituents at the imidazole-based NHO, it was possible to obtain different diphospha-indenylide species with different rotation angles in the solid state and hence varying diradical character. Imidazolium-diphospha-indenylides represent a new class of NHO-based zwitterions with diradical character. Their synthesis, structure, and activation chemistry are described, as well as the quantum mechanical description of the electronic structure in these unusual heterocycles. In addition, along the synthesis route to diphospha-indenylide, we also succeeded in isolating a highly reactive monoradical anion, which was also fully characterized.

Recent advances in heavier group 15 (P, As, Sb, Bi) radical chemistry - frameworks, small molecule reactivity, and catalysis

Main group radical chemistry has been a targeted research area for several decades. With growing examples of phosphorus radicals, even heavier pnictogen radicals including arsenic, antimony, and bismuth have also become important targets. A diverse framework of group 15 radicals has been reported in the 21st century and is covered herein. Reactivity and applications of selected radicals and future directions for this field are discussed.

Isolation of a Staudinger-type Intermediate Utilizing a Five-Membered Phosphorus-Centered Biradicaloid

The Staudinger reaction provides chemists with a valuable tool for the reduction of azides, which are notoriously unstable and can decompose explosively. By providing a controlled method for the conversion of azides to amines, the reaction opened up new avenues for the synthesis of various amine-containing compounds that are widely used in natural products, pharmaceuticals and polymers. The Staudinger reaction begins with the nucleophilic attack of a trivalent phosphine (usually triphenylphosphine), leading to the formation of a triazenide intermediate, a highly reactive species. Here we report how a divalent phosphorus-centered biradicaloid reacts with covalent azides and show that it is possible to capture and fully characterize the transient intermediate. The experimental data is supported by quantum chemical calculations of the reaction paths and in terms of thermodynamics and chemical bonding.

Expanding the Landscape of Phosphorous-Based Open Shell Species: Stable Mono-, Di-, and Trianionic Radicals Based on a Contorted Triphosphaalkene

The stepwise reduction of the highly contorted truxene-based triphosphaalkene 1 using KC8 led to the isolation of mono-, di-, and tri-anionic species. The solid-state molecular structures of mono- and diradical anionic species were elucidated by single crystal X-ray diffractions, revealing elongated P-C bonds and a pronounced "indene" aromatization compared to the parent system. All three radical species displayed distinct Electron Paramagnetic Resonance (EPR) spectra, providing compelling evidence for the open-shell electronic configuration of both the diradical and triradical species-an observation unprecedented in any previously reported phosphorous-based anionic polyradicals. Mulliken spin density calculations revealed a dominant localization of radical spin on a single phosphorous atom in the monoanion. In the dianion, spin localization is observed on two phosphorous atoms (~34 % each), with a minor contribution from the third phosphorous (0.13 %), while the trianion demonstrates a uniform distribution of spin density (~30 %) across each phosphorous atom.

Structural Snapshots of Reversible Carbon Dioxide Capture and (De)oxygenation at Group 14 Diradicaloids

Although diradicals should exhibit a rather small reaction barrier as compared to closed-shell species for activating kinetically inert molecules, the activation and functionalization of carbon dioxide with stable main-group diradicals remain virtually unexplored. In this work, we present a thorough study on CO2 activation, reversible capture, and (de)oxygenation mediated by stable Group 14 singlet diradicals (i.e., diradicaloids) [(ADC)E]2 (E = Si, Ge, Sn) based on an anionic dicarbene (ADC) framework (ADC = PhC{N(Dipp)C}2; Dipp = 2,6-iPr2C6H3). [(ADC)E]2 readily undergo [4 + 2]-cycloadditions with CO2 to result in barrelene-type bis-metallylenes [(ADC)E]2(OC═O). The CO2 addition is reversible for E = Ge; thus, CO2 detaches under vacuum or at an elevated temperature and regenerates [(ADC)Ge]2. [(ADC)Sn]2(OC═O) is isolable but deoxygenates additional CO2 to form [(ADC)Sn]2(O2CO) and CO. [(ADC)Si]2(OC═O) is extremely reactive and could not be isolated or detected as it spontaneously reacts further with CO2 to yield elusive monomeric Si(IV) oxides [(ADC)Si(O)]2(COn) or carbonates [(ADC)Si(CO3)]2(COn) (n = 1 or 2) via the (de)oxygenation of CO2. The molecular structures of all isolated compounds have been established by X-ray diffraction, and a mechanistic insight of their formation has been suggested by DFT calculations.

Theoretical design of phosphorus-doped perylene derivatives as efficient singlet fission chromophores

Singlet fission (SF) is considered as a promising strategy to overcome the Shockley-Queisser limit of single-junction solar cells. However, only a handful of chromophores were observed to undergo SF to date. To broaden the number of SF chromophores, we designed a series of phosphorus-doped perylenes based on the diradical character strategy and examined their SF feasibility using theoretical calculations. By analysis of frontier orbitals, diradical character and aromaticity, SF-capable candidates were prescreened. These analyses reveal that the diradical character of perylene is effectively enhanced by P-doping at bay- and peri-positions of perylene, making SF more thermodynamically feasible. However, the diradical character remains nearly unchanged when P atoms are doped at ortho-positions because the spin center cannot be stabilized, leading to a more endothermic SF. This study shows how SF-related energies and diradical character of SF chromophores are altered by P doping, and extends the SF-capable molecular library.

Leap from Diradicals to Tetraradicals by Topological Control of π-Conjugation

In this work, we explore the series of diradical(oid)s based on 2,2'-(5,11-dihydroindolo[3,2-b]carbazole-3,9-diyl)dimalononitrile (further referred to as PH). Hydrogen atoms in the central benzenoid (CB) ring of PH are substituted by the series of substituents with various lengths of π-conjugated chain and electron-donating or electron-withdrawing properties to study how they modulate the diradical character of the parent compound. The diradical character of molecules increases up to 88-89% by two groups doubly bonded to both sides of the CB ring of PH in para relative positions. This breaks the direct π-conjugation between unpaired electrons that gives rise to two radical centers and restricts the minimal polyradical identity of the compound to diradical. We show that diradicals and tetraradicals can be designed, and their polyradical character can be modulated by controlling the topology of π-conjugation as long as there is sufficient aromatic stabilization. Henceforth, the bridge between diradicals and tetraradicals is established, leading to the tetraradical(oid) molecule, which has been predicted to have narrow low-spin to high-spin energy gaps in our recent Letter.

A Phosphorus-Centred, Zirconocene-Bridged Tetraradical: Synthesis, Structure and Application as Molecular Double Switch

Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi)=1,2-ethylene-1,10-bis(η5-tetrahydroindenyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zirconcene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570 nm), this tetraradical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5 h. Upon irradiation at 415 nm, the reverse reaction is observed, fully recovering tetraradical 5. Through single-crystal-to-single-crystal transformation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light.

Hydrogen splitting at a single phosphorus centre and its use for hydrogenation

Catalytic processes are largely dominated by transition-metal complexes. Main-group compounds that can mimic the behaviour of the transition-metal complexes are of great interest due to their potential to substitute or complement transition metals in catalysis. While a few main-group molecular centres were shown to activate dihydrogen via the oxidative addition process, catalytic hydrogenation using these species has remained challenging. Here we report the synthesis, isolation and full characterization of the geometrically constrained phosphenium cation with the 2,6-bis(o-carborano)pyridine pincer-type ligand. Notably, this cation can activate the H-H bond by oxidative addition to a single PIII cationic centre, producing a dihydrophosphonium cation. This phosphenium cation is also capable of catalysing hydrogenation reactions of C=C double bonds and fused aromatic systems, making it a main-group compound that can both activate H2 at a single molecular main-group centre and be used for catalytic hydrogenation. This finding shows the potential of main-group compounds, in particular phosphorus-based compounds, to serve as metallomimetic hydrogenation catalysts.

A Stable Chichibabin Diradicaloid with Near-Infrared Emission

Conjugated molecules with multiple radical centers such as the iconic Chichibabin diradicaloid hold promise as building blocks in materials for quantum sensing and quantum information processing. However, it is a considerable challenge to design simple analogues of the Chichibabin hydrocarbon that are chemically inert, exhibit high diradical character and emit light at a distinct wavelength that may offer an optical readout of the spin state in functional ensembles. Here we describe the serendipitous discovery of the stable TTM-TTM diradicaloid, which exhibits high diradical character, a striking sky-blue color and near-infrared (NIR) emission (in solution). This combination of properties is unique among related diradicaloids and is due to the presence of hydrogen and chlorine atoms in "just the right positions", allowing a perfectly planar, yet predominantly benzenoid bridge to connect the two sterically stabilized radical centers. In-depth studies of the optical and magnetic properties suggest that this structural motif could become a mainstay building block of organic spin materials.

Pathway to Polyradicals: A Planar and Fully π-Conjugated Organic Tetraradical(oid)

In this work, we provide a general strategy to stabilize the ground state of polyradical(oid)s and make higher spin states thermally accessible. As a proof of concept, we propose to merge two planar fully π-conjugated diradical(oid)s to obtain a planar and cross-conjugated tetraradical(oid). Using multireference quantum chemistry methods, we show that the designed tetraradical(oid) is stabilized by aromaticity and delozalization in the π-system and has six thermally accessible spin states within 1.72 kcal/mol. Analysis of the electronic structure of these six states of the tetraradical(oid) shows that its frontier π-system consists of two weakly interacting subsystems: aromatic cycles and four unpaired electrons. Conjugation between unpaired electrons, which favors closed-shell structures, is mitigated by delocalization and the aromaticity of the bridging groups, leading to the synergistic cross-coupling between two diradical(oid) subunits to stabilize the tetraradical(oid) electronic structure.

Isolation of an Anionic Dicarbene Embedded Sn2 P2 Cluster and Reversible CO2 Uptake

Decarbonylation of a cyclic bis-phosphaethynolatostannylene [(ADC)Sn(PCO)]2 based on an anionic dicarbene framework (ADC = PhC{N(Dipp)C}2 ; Dipp = 2,6-iPr2 C6 H3 ) under UV light results in the formation of a Sn2 P2 cluster compound [(ADC)SnP]2 as a green crystalline solid. The electronic structure of [(ADC)SnP]2 is analyzed by quantum-chemical calculations. At room temperature, [(ADC)SnP]2 reversibly binds with CO2 and forms [(ADC)2 {SnOC(O)P}SnP]. [(ADC)SnP]2 enables catalytic hydroboration of CO2 and reacts with elemental selenium and Fe2 (CO)9 to afford [(ADC)2 {Sn(Se)P2 }SnSe] and [(ADC)Sn{Fe(CO)4 }P]2 , respectively. All compounds are characterized by multinuclear NMR spectroscopy and their solid-state molecular structures are determined by single-crystal X-ray diffraction.

Activation of H2 using ansa-aminoboranes: solvent effects, dynamics, and spin hyperpolarization

Spin hyperpolarization generated upon activation of parahydrogen, the spin-0 isomer of H2, by ansa-aminoboranes (AABs) constitutes a rare but interesting example of applied metal-free catalysis in parahydrogen-induced polarization (PHIP). AAB molecular moieties made of light elements would be useful in important areas of NMR, such as chemosensing and the production of hyperpolarized substances, or generally in NMR sensitivity enhancement. At the same time, little is known about the detailed mechanistic aspects of underlying chemical processes. Herein, we present a joint experimental-computational study of the kinetic and thermodynamic aspects of H2 activation by AABs, for the first time providing molecular-level details and results of PHIP experiments with AABs in various solvents. Specifically, a large number of kinetic and thermodynamic parameters are measured experimentally for H2 activation by 2-aminophenylboranes of variable steric bulkiness of the boryl site. A clear correlation between the experimental and DFT-predicted thermochemical parameters is observed. PHIP effects in toluene, dichloromethane, and acetonitrile are characterized and rationalized based on the use of the kinetic and nuclear spin relaxation parameters. Altogether, the obtained results provide valuable information for the further rational design of efficient AAB catalysts for metal-free PHIP based on frustrated Lewis pair (FLP) chemistry.

Light-Driven Enantioselective Carbene-Catalyzed Radical-Radical Coupling

An enantioselective carbene-catalyzed radical-radical coupling of acyl imidazoles and racemic Hantzsch esters is disclosed. This method involves the coupling of an N-heterocyclic carbene-derived ketyl radical and a secondary sp3 -carbon radical and allows access to chiral α-aryl aliphatic ketones in moderate-to-good yields and enantioselectivities without any competitive epimerization. The utility of this protocol is highlighted by the late-stage functionalization of various pharmaceutical compounds and is further demonstrated by the transformation of the enantioenriched products to biologically relevant molecules. Computational investigations reveal the N-heterocyclic carbene controls the double-facial selectivity of the ketyl radical and the alkyl radicals, respectively.

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.