A Phosphine-Coordinated Boron-Centered Gomberg-Type Radical

Trimethylsilyldiazomethane Disassembly at a Three-Fold Symmetric Iron Site

The reaction of equimolar trimethylsilyldiazomethyllithium (LiTMSD) with high spin (S = 2) PhB(AdIm)3FeCl (PhB(AdIm)3- = tris(3-adamantylimidazol-2-ylidene)phenylborate) affords the corresponding N-nitrilimido complex PhB(AdIm)3Fe-N═N═C(SiMe3). This complex can be converted to the thermodynamically more favorable C-isocyanoamido isomer PhB(AdIm)3Fe-C═N═N(SiMe3) by reaction with an additional equivalent of LiTMSD. While the iron(II) complexes are four-coordinate, the diazomethane is bound side-on in the iron(I) congener PhB(AdIm)3Fe(N,N'-κ2-N2C(H)Si(CH3)3). The latter complex adopts high spin (S = 3/2) ground state and features an unusually weak C-H bond. Photolysis of the iron(II) complexes induces N═N bond cleavage, with the iron(II) cyanide PhB(AdIm)3Fe-C≡N and iron(IV) nitride PhB(AdIm)3Fe≡N complexes being the major products of the reaction. The same products are obtained when the iron(I) complex is photolyzed or treated with a fluoride source. The trimethylsilyldiazomethane-derived ligand disassembly reactions are contrasted with those observed for related tris(carbene)amine complexes.

Dianionic and Neutral Diboron-Centered Classical Diradicaloids

Herein, we report the syntheses and electronic structures of crystalline dianionic as well as neutral diboron-centered classical diradicaloids as boron analogues of classical Thiele, Chichibabin, and Müller (this only for dianionic diradicaloids!) hydrocarbons. These are based on borane radical anion and NHC-stabilized boryl radical spin carriers, respectively. All these dianionic diboron-centered diradicaloids exhibit triplet population at room temperature regardless of the π-conjugated spacer: p-phenylene, p,p'-biphenylene, or p,p″-terphenylene. In the case of neutral diboron-centered diradicaloids, the employed π-conjugated spacer plays a crucial role for the triplet population at room temperature: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. The findings emphasize the importance of the spin carriers for the resulting ground-state: borane radical anion vs NHC-stabilized boryl radical along with the pivotal role of the π-conjugated spacer as spin-coupler between two spins. Notably, 100 years (a century) after the first report by Krause of the triphenyl borane radical-anion, being isoelectronic to the triphenylmethyl radical, we convey borane radical anion-based diradicaloids. Furthermore, while donor-stabilized boryl radicals were introduced in the 1980s by Giles and Roberts, said concept is herewith being extended to NHC-stabilized boryl radical-based diradicaloids.

A quantum-chemical insight into SOMO-HOMO conversion in phosphorus-boron cation radicals

Organic radicals exhibiting SOMO-HOMO conversion (SHC) electronic configurations have recently garnered increasing attention due to their exceptional stability and photophysical properties. In this study, we investigate two series of phosphorus-boron cation radicals based on 1,3,5-trimethylphenyl units substituted with P and B atoms, varying numbers of P-B moieties, and π-conjugation linkers. We perform quantum-chemical calculations to systematically assess the influence of chemical substituents on the SHC electronic structural features. Our computational results demonstrate that the SHC electronic configurations of the studied complexes are primarily determined by the number of P-B moieties, specifically, phosphorus-boron cation radicals with two P-B moieties as terminal groups in π-conjugation linkers, which efficiently arrange electrons to increase HOMO energies compared to corresponding radicals with only one P-B unit. Furthermore, spin density distributions change as the size of π-conjugation linkers increases. Natural bond orbital (NBO) and atoms-in-molecules (AIM) analyses reveal strong intramolecular charge transfer between P and B atoms along with other stabilized donor-acceptor interactions and significant covalent bonds between P and B atoms. Moreover, synergistic effects resulting from 1,3,5-trimethylphenyl substitutions and enlarged π-conjugation linkers containing P-B units confer excellent photophysical properties upon these studied radicals, making them potential stable radicals in optoelectronic applications.

B-substituted group 1 phosphides: synthesis and reactivity

1-Boryl-8-phosphinonaphthalenes 1-BCy2-8-PCl2-C10H6 (1) and 1-BCy2-8-PPhCl-C10H6 (2) were prepared and used as starting materials for the synthesis of B-substituted phosphides. The reduction of 1 and 2 by Mg provided neutral compounds [1-BCy-8-PCy-C10H6]2 (3) and [1-BCy2-8-PPh-C10H6]2 (4). Compound 3 represents the dimer of phosphinoborane 1-BCy-8-PCy-C10H6 while complex 4 is a rare example of a discrete B ← P coordinated diphosphine. The reduction of 2 by Na or K in THF yielded B-substituted group 1 phosphides [Na(THF)3]+[1-BCy2-8-PPh-C10H6]- (5) and {[K(THF)2]+[1-BCy2-8-PPh-C10H6]-}∞ (6), which structurally resembled bulky group 1 phosphides. Complex 5 showed easy activation of elemental chalcogens E (E = O, S, Se) to give B-substituted chalcogenophosphinites {[Na(THF)2]+[1-BCy2-8-P(E)Ph-C10H6]}2 (E = O (7), S (8), Se (9)) as the products of chalcogen insertion into the P-Na bond. Importantly no oxidation to dichalcogenophosphinates was observed. Compound 5 is tolerant of the CO polar bonds in organic substrates and the reactions of 5 with 2,3-butanedione or an acyl chloride provided {[Na(THF)2]+[1-BCy2-8-P{CHC(O)C(Me)O}Ph-C10H6]-}2 (10) and [1-BCy2-8-P{C(O)tBu}Ph-C10H6] (11). Finally, B-coordinated phosphatetrylenes [1-BCy2-8-P(SnL)Ph-C10H6] (12) and [1-BCy2-8-P(PbL)Ph-C10H6] (13) (L is {2,6-(Me2NCH2)C6H3}-) were also prepared by substitution reactions of 5.

Light-Induced Polymeric Frustrated Radical Pairs as Building Blocks for Materials and Photocatalysts

Polymeric frustrated Lewis pairs, or poly(FLP)s, have served to bridge the gap between functional polymer science and main group catalysis, pairing the uniqueness of sterically frustrated Lewis acids and bases with a polymer scaffold to create self-healing gels and recyclable catalysts. However, their utilization in radical chemistry is unprecedented. In this paper, we disclose the synthesis of polymeric frustrated radical pairs, or poly(FRP)s, by in situ photoinduction of FLP moieties, where their Lewis acidic and basic centers are tuned to promote single electron transfer (SET). Through systematic manipulation of the chemical structure, we demonstrate that inclusion of ortho-methyl groups on phosphine monomers is crucial to enable SET. The generation of radicals is evidenced by monitoring the stable polymeric phosphine radical cations via UV/vis and EPR spectroscopy. These new poly(FRP)s enable both catalytic hydrogenation and radical-mediated photocatalytic perfluoroalkylations. These polymeric radical systems open new avenues to design novel functional polymers for catalysis and photoelectrical chemistry.

Theoretical investigations on P-stabilized boryl cation radicals: from the Aufbau principle to SOMO-HOMO conversion

We report the characterization of a series of novel phosphinidene-stabilized (P-stabilized) boryl cation radicals, in which the phosphinidene and boryl are stabilised by ^iPrNHC (^iPrNHC[:C{N(iPr)C(H)}₂]), and the P-stabilized boryl (P → B) moieties are linked by 1,8-naphthalene (1PB-a), 1,10-biphenyl (1PB-b), 1,2-perylene (1PB-c), and 4,5-perylene (1PB-d), to form a series of 1PB compounds. The 2PB series is designed by adding another P-stabilized boryl (P → B) unit into the 1PB series, in which the two P-stabilized boryl (P → B) moieties for each 2PB compound are linked by 1,4,5,8-naphthalene, (2PB-a), 1,5,6,10-biphenyl (2PB-b), 1,2,7,8-perylene (2PB-c), and 4,5,10,11-perylene (2PB-d), respectively. Theoretical calculations demonstrate that for all the studied molecules, the spin density mainly locates on the B atoms. Interestingly, the series of 2PB(a-d) compounds possess SOMO-HOMO conversion properties, while 1PB(a-d) compounds obey the Aufbau principle, resulting from the difference in the number of the P-stabilized boryl (P → B) moieties and an increase of the π-conjugation bridge that lead to the significantly increased HOMO energy in 2PB(a-d) compounds, which should be responsible for the different structural properties of compounds 1PB(a-d) and 2PB(a-d). The natural bond orbital (NBO) and atoms in molecules (AIM) analysis reveal how the interactions contribute to the covalent bond between P and B atoms. Moreover, the absorption properties show that the spectra of the 2PB(a-d) compounds are red-shifted relative to those of the corresponding 1PB(a-d) compounds in the near infrared region. We hope this work can provide new insights into tuning the electronic structures of the well-defined forms of P-stabilized boryl cation radicals and expand their potential application in organic optoelectronics.

Neutral Boryl Radicals in Mixed-Valent B(III) Br-B(II) Adducts

The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B^(III) Br-B^(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.

Predicting Dinitrogen Activation by Five-Electron Boron-Centered Radicals

Due to the high bond dissociation energy (945 kJ mol^-1) and the large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV), dinitrogen activation under mild conditions is extremely challenging. On the other hand, the conventional Haber-Bosch ammonia synthesis under harsh conditions consumes more than 1% of the world's annual energy supply. Thus, it is important and urgent to develop an alternative approach for dinitrogen activation under mild conditions. In comparison with transition metals, main group compounds are less explored for nitrogen activation. Here, we carry out density functional theory calculation to screen boron radicals for dinitrogen activation. As a result, the experimentally available seven-electron boron-centered radicals are found to be inactive to N₂ activation, whereas some five-electron boron-centered radicals become favorable for dinitrogen activation, inviting experimental chemists' examination. The principal interacting spin-orbital analyses suggest that a five-electron boron-centered radical can mimic a transition metal on a synergic interaction with dinitrogen in the transition states.

Taking Advantage of Ortho- and Peri-Substitution to Design Nine-Membered P,O,Si-heterocycles*

A family of cyclic phosphine-disiloxane featuring peri-substituted naphthyl(Nap)/acenaphthyl(Ace) scaffolds has been prepared and fully characterized including X-ray structure, which enables a detailed structural analysis. This straightforward synthesis takes advantage of both ortho- and peri-substitution of Nap/Ace-substituted phosphine oxides. The synthetic method allows diversifying the polycyclic aromatic platform (Nap and Ace) as well as the Si substituents (Me and Ph). Despite a strong steric congestion, the P-atom remains reactive toward oxidation or coordination. In particular, Au(I) complex could be prepared. All the compounds display absorption/luminescence in the UV-Vis range. Surprisingly, the P-trivalent derivatives display unexpected luminescence in the green in solid-state.

Electron doping of single-walled carbon nanotubes using pyridine-boryl radicals

Pyridine-boryl (py-boryl) radicals serve as efficient electron-doping reagents for single-walled carbon nanotubes (SWCNTs). The doping mechanism comprises electron transfer from the py-boryl radical to the SWCNT. The formation of a stable py-boryl cation is essential for efficient doping; the captodative effect of the py-boryl cation is important to this process.

Bicyclic (alkyl)(amino)carbene stabilized zinc(0) complex with singlet biradicaloid ground state

A storable bicyclic (alkyl)(amino)carbene (BICAAC) stabilized two coordinate zinc(0) complex [(BICAAC)2Zn] (2) was synthesized. DFT calculations reveal that BICAAC plays a decisive role in imparting the stability to 2. This complex activates the C(sp3)-Cl bond of trityl chloride generating the Gomberg's free radical with greater efficiency than metallic Zn powder.

Lewis pairing and frustration of group 13/15 elements geometrically enforced by (ace)naphthalene, biphenylene and (thio)xanthene backbones

The synthesis, structure, and reactivity of mixed group 13/group 15 compounds (E¹³ = B, Al, Ga, In, Tl; E¹⁵ = N, P, Sb, Bi) featuring a rigid (ace)naphthalene or (thio)xanthene backbone are discussed in this review. The backbone may either enforce or prevent E¹⁵→E¹³ interactions, resulting in Lewis pairing or frustration. The formation of strong E¹⁵→E¹³ interactions is possible upon peri-substitution of (ace)naphthalenes. This gives the opportunity to access and study highly reactive species, as exemplified by P-stabilised borenium salts and boryl radicals. In turn, rigid expanded spacers such as biphenylenes, (thio)xanthenes and dibenzofurans impose long distances and geometrically prevent E¹⁵→E¹³ interactions. Such P-B derivatives display ambiphilic coordination properties and frustrated Lewis pair behaviour towards small molecules, their preorganised structure favouring reversible interaction/activation. Throughout the review, the importance of the scaffold in enforcing or preventing E¹⁵→E¹³ interactions is highlighted and discussed based on experimental data and theoretical calculations.

Isolable diboryl radicals acting as highly efficient reaction intermediates under mild conditions

Activation of the B–B bond in diborane with dimesitylpyridyl boranes afforded the stable diboryl radicals in moderate yields.

Transition-metal free C-C bond cleavage/borylation of cycloketone oxime esters

An efficient transition-metal free C-C bond cleavage/borylation of cycloketone oxime esters has been described. In this reaction, the B₂(OH)₄ reagent not only served as the boron source but also acted as an electron donor source through formation of a complex with a DMAc-like Lewis base. This complex could be used as an efficient single electron reductant in other ring-opening transformations of cycloketone oxime esters. Free-radical trapping, radical-clock, and DFT calculations all suggest a radical pathway for this transformation.

Destabilizing Character of a π-Conjugated Boron Center in Bisphenol Radicals

Although boron-containing radicals are promising materials for molecular electronic devices, the electronic effect of the σ-donating yet π-accepting boron center on the stability of open-shell species has been less discussed. In this work, the role of a tricoordinate boron center in π-conjugated radicals was explored through electron paramagnetic resonance measurement of several boron-linked bisphenol radicals and diradicals. Replacing the bridging methine fragment of a neutral Galvinoxyl radical with an arylboryl group led to the corresponding boron-linked radical anion that requires excessive steric protection at the boron center to be persistent in solution. Experimental and theoretical investigations revealed that the introduction of boron would diminish the quinoidal character of the phenoxyl radical and increase both the electrophilicity and nucleophilicity of the open-shell species. Therefore, it is important to consider the steric protection of the boron center in boron-containing π-conjugated organic radicals.

Deciphering Stability of Five-Membered Heterocyclic Radicals: Balancing Act Between Delocalization and Ring Strain

Computational studies on five-membered heterocycles with single heteroatom and their isomeric dehydro-borole 1a-1c, cyclopentadiene 2a-2c, pyrrole 3a-3c, furan 4b-4c, phosphole 5a-5c, and thiophene 6b-6c radicals have been carried out. Geometrical aspects through ground state electronic structures and stability aspects using bond dissociation energies (BDE) and radical stabilization energies (RSE) have been envisaged in this regard. Spin densities, electrostatic potentials (ESP), and natural bond orbital (NBO) analysis unveiled the extent of spin delocalization. The estimated nucleus-independent chemical shifts (NICS) values revealed the difference in aromaticity characteristics of radicals. Particularly the heteroatom centered radicals exhibit odd electron π-delocalized systems with a quasi-antiaromatic character. Various factors such as, the relative position of the radical center with respect to heteroatoms, resonance, ring strain and orbital interactions influence the stability that follows the order: heteroatom centered > β-centered > α-centered radicals. Among the influences of various factors, we confirmed the existence of a competition between delocalization and the ring strain, and the interplay of both decides the overall stability order.

Formal Nickelate(-I) Complexes Supported by Group 13 Ions

Formal nickelate(-I) complexes bearing Group 13 metalloligands (M=Al and Ga) were isolated. These 17 e^- complexes were synthesized by one-electron reduction of the corresponding Ni⁰ →M^III precursors, and were investigated by single-crystal X-ray diffraction, EPR spectroscopy, and quantum chemical calculations. Collectively, the experimental and computational data support: 1) the strengthening of the Ni-M bond upon one-electron reduction, and 2) the delocalization of the unpaired spin across the Ni and M atoms. An intriguing electronic configuration is revealed where three valence electrons occupy two σ-type bonding interactions: Ni(3dz2 )² →M and σ-(Ni-M)¹ . The latter is an unusual Ni-M σ-bonding molecular orbital that comprises primarily the Ni 4p_z and M np_z /ns atomic orbitals.

Organocatalytic reductive coupling of aldehydes with 1,1-diarylethylenes using an in situ generated pyridine-boryl radical

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established.

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established via a combination of computational and experimental studies. Density functional theory calculations and control experiments suggest that the ketyl radical from the addition of the pyridine-boryl radical to aldehydes is the key intermediate for this C–C bond formation reaction. This metal-free reductive coupling reaction features a broad substrate scope and good functional compatibility.

Dealkanative Main Group Couplings across the peri-Gap

Here, we highlight the ability of peri-substitution chemistry to promote a series of unique P-P/P-As coupling reactions, which proceed with concomitant C-H bond formation. This dealkanative reactivity represents an interesting and unexpected expansion to the established family of main-group dehydrocoupling reactions. These transformations are exceptionally clean, proceeding essentially quantitatively at relatively low temperatures (70-140 °C), with 100% diastereoselectivity in the products. The reaction appears to be radical in nature, with the addition of small quantities of a radical initiator (azobis(isobutyronitrile)) increasing the rate dramatically, as well as altering the apparent order of reaction. DFT calculations suggest that the reaction involves dissociation of a phosphorus centered radical (stabilized by the peri-backbone) to the P-P coupled product and a free propyl radical, which carries the chain. This unusual reaction demonstrates the powerful effect that geometric constraints, in this case a rigid scaffold, can have on the reactivity of main group species, an area of research that is gaining increasing prominence in recent years.

Organic Electron Acceptors Comprising a Dicyanomethylene-Bridged Acridophosphine Scaffold: The Impact of the Heteroatom

Stable two-electron acceptors comprising a dicyanomethylene-bridged acridophosphine scaffold were synthesized and their reversible reduction potentials were efficiently tuned through derivatization of the phosphorus center. X-ray crystallographic analysis combined with NMR, UV/Vis, IR spectroscopic, and electrochemical studies, supported by theoretical calculations, revealed the crucial role of the phosphorus atom for the unique redox, structural, and photophysical properties of these compounds. The results identify the potential of these electron deficient scaffolds for the development of functional n-type materials and redox active chromophores upon further functionalization.

Borylative Radical Cyclizations of Benzo[3,4]cyclodec-3-ene-1,5-diynes and N-Heterocyclic Carbene-Boranes

Borylative radical cyclization of benzo[3,4]cyclodec-3-ene-1,5-diynes to provide 5-borylated 6,7,8,9-tetrahydrobenzo[a]azulenes has been developed. The experimental results suggest that the reaction proceeds by a radical chain mechanism, in which di-tert-butyl hyponitrite (TBHN) works as a good radical initiator to form boryl radicals from N-heterocyclic carbene-boranes (NHC-boranes). The present reaction is a rare model that illustrates addition of boryl radicals to alkynes.

Electrophilicity and Nucleophilicity of Boryl Radicals

We carried out a survey of the relative reactivity of a collection of 91 neutral boryl radicals using density functional calculations. Their reactivities were characterized by four indices, i.e., the global electrophilicity, global nucleophilicity, local electrophilicity, and local nucleophilicity. Particularly, the global electrophilicity and nucleophilicity indices span over a moderately wider range than those of carbon radicals, indicating their potentially broader reactivity. Thus, boryl radicals may be utilized in electrophilic radical reactions, while traditionally they are only considered for nucleophilic radical reactions. In contrast, the local electrophilicity and nucleophilicity indices at the boron center show a different reactivity picture than their global counterparts. The inconsistency is rooted in the low and varying spin density on boron (for most radicals, less than 50%) in different boryl radicals, which is a combinative result of radical stabilization via electron delocalization and the low electronegativity of boron (compared to carbon). In short, the boron character in boryl radicals may be weak and their reactivity is not reflected by the local indices based on boron but by the global ones.

Planar-Chiral 1,1'-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene

The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes₂ (Mes = 2,4,6-trimethylphenyl) and ClBCy₂ as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tert-butylated boryl cyclopentadienides with MCl₂ (M = Fe, Co) selectively leads to 1,1'-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes₂ groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes₂ rings. There is only weak, if any, direct M···B interaction in 6 and 7.

Consecutive reduction, radical-cyclization, and oxidative-dehydrogenation reaction of ortho-substituted diboryl compounds

An elegant synthetic route to a ladder-type di-borate compound 2a was reported based on the reduction, radical-cyclization, and oxidative-dehydrogenation reaction of 1.