Evidence for Extensive Single‐Electron‐Transfer Chemistry in Boryl Anions: Isolation and Reactivity of a Neutral Borole Radical

Despite the synthesis of a boryl anion by Yamashita et al. in 2006, compounds that show boron‐centered nucleophilicity are still rare and sought‐after synthetic goals. A number of such boryl anions have since been prepared, two of which were reported to react with methyl iodide in apparent nucleophilic substitution reactions. One of these, a borolyl anion based on the borole framework, has now been found to display single‐electron‐transfer (SET) reactivity in its reaction with triorganotetrel halides, which was confirmed by the isolation of the first neutral borole‐based radical. The radical was characterized by elemental analysis, single‐crystal X‐ray crystallography, and EPR spectroscopy, and has implications for the understanding of boron‐based nucleophilic behavior and the emergent role of boron radicals in synthesis. This radical reactivity was also exploited in the synthesis of compounds with rare B? Sn and B? Pb bonds, the latter of which was the first isolated and structurally characterized compound with a “noncluster” B? Pb bond.     

Synthesis and Structure of an o-Carboranyl-Substituted Three-Coordinate Borane Radical Anion

Bis(1-(4-tolyl)-carboran-2-yl)-(4-tolyl)-borane [(1-(4-MeC₆H₄)-closo-1,2-C₂B₁₀H₁₀-2-)₂(4-MeC₆H₄)B] ( 1 ), a new bis(o-carboranyl)-(R)-borane was synthesised by lithiation of the o-carboranyl precursor and subsequent salt metathesis reaction with (4-tolyl)BBr₂. Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one-electron first reduction. In a selective reduction experiment the corresponding paramagnetic radical anion 1^.− was isolated and characterized. Single-crystal structure analyses allow an in-depth comparison of 1 , 1^.− , their calculated geometries, and the S₁ excited state of 1 . Photophysical studies of 1 show a charge transfer (CT) emission with low quantum yield in solution but a strong increase in the solid state. TD-DFT calculations were used to identify transition-relevant orbitals.     

Synthesis,Structure, and Reactivity of Borole‐Functionalized Ferrocenes

Herein, we report on the synthesis of ferrocenylborole [Fc(BC₄Ph₄)₂] featuring two borole moieties in the 1,1′‐positions. The results of NMR and UV/Vis spectroscopy and X‐ray diffraction studies provided conclusive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc(BC₄Ph₄)₂] and the 4‐Me‐NC₅H₄ adduct of monoborole [Fc(BC₄Ph₄)], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in general was nicely demonstrated by the reaction of [Fc(BC₄Ph₄)] with 4,4′‐bipyridine. Unlike common borane derivatives such as [FcBMe₂], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4′‐bipyridine remaining uncoordinated. In addition, the reduction chemistry of ferrocenylboroles [Fc(BC₄Ph₄)] and [Fc(BC₄Ph₄)₂] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC₄Ph₄)] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either η⁵‐coordinated C₅H₄ or η⁵‐coordinated BC₄Ph₄ moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC₄Ph₄)₂], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to further enhance the borole ratio in ferrocenylboroles aiming at the synthesis of [Fc(BC₄Ph₄)₄] failed and, instead, generated an uncommon ansa‐ferrocene containing two borole fragments in the 1,1′‐positions and a B₂C₄ ansa‐bridge.     

Hydroboration of Alkynes with Zwitterionic Ruthenium–Borate Complexes: Novel Vinylborane Complexes

Building upon previous studies on the synthesis of bis(sigma)borate and agostic complexes of ruthenium, the chemistry of nido‐[(Cp*Ru)₂B₃H₉] ( 1 ) with other ligand systems was explored. In this regard, mild thermolysis of nido‐ 1 with 2‐mercaptobenzothiazole (2‐mbzt), 2‐mercaptobenzoxazole (2‐mbzo) and 2‐mercaptobenzimidazole (2‐mbzi) ligands were performed which led to the isolation of bis(sigma)borate complexes [Cp*RuBH₃L] ( 2 a – c ) and β‐agostic complexes [Cp*RuBH₂L₂] ( 3 a – c ; 2 a , 3 a : L=C₇H₄NS₂; 2 b , 3 b : L=C₇H₄NSO; 2 c , 3 c : L=C₇H₅N₂S). Further, the chemistry of these novel complexes towards various diphosphine ligands was investigated. Room temperature treatment of 3 a with [PPh₂(CH₂)_nPPh₂] (n=1–3) yielded [Cp*Ru(PPh₂(CH₂)_nPPh₂)‐BH₂(L₂)] ( 4 a – c ; 4 a : n=1; 4 b : n=2; 4 c : n=3; L=C₇H₄NS₂). Mild thermolysis of 2 a with [PPh₂(CH₂)_nPPh₂] (n=1–3) led to the isolation of [Cp*Ru(PPh₂(CH₂)_nPPh₂)(L)] (L=C₇H₄NS₂ 5 a – c ; 5 a : n=1; 5 b : n=2; 5 c : n=3). Treatment of 4 a with terminal alkynes causes a hydroboration reaction to generate vinylborane complexes [Cp*Ru(R?C?CH₂)BH(L₂)] ( 6 and 7 ; 6 : R=Ph; 7 : R=COOCH₃; L=C₇H₄NS₂). Complexes 6 and 7 can also be viewed as η‐alkene complexes of ruthenium that feature a dative bond to the ruthenium centre from the vinylinic double bond. In addition, DFT computations were performed to shed light on the bonding and electronic structures of the new compounds.     

Synthesis,Structure, and Radical Anion of the First Stable p‐Phosphaquinone

Significant localization of the unpaired electron on the phosphorus atom of phosphasemiquinone radical anion 1 was revealed by EPR spectroscopy. This species was generated by reduction of 1 , the first stable p‐phosphaquinone. Employment of the 3,5‐di‐tert‐butyl‐4‐oxocyclohexa‐2,5‐dien‐1‐ylidene moiety was essential for the synthesis as well as the effective kinetic protection of 1 .     

Chemical Reduction and Dimerization of 1‐Chloro‐2,3,4,5‐tetraphenylborole

As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π‐nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1‐chloro‐2,3,4,5‐tetraphenylborole ( 1 ) and its corresponding borole dianion resulting from the two‐electron reduction of 1 with KC₈. The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro‐bicyclic compound and the resulting dimer was fully characterized including a single‐crystal X‐ray analysis.     

Zwitterionic Base‐Stabilized Digermadistannacyclobutadiene and Tetragermacyclobutadiene

The syntheses of a zwitterionic base‐stabilized digermadistannacyclobutadiene and tetragermacyclobutadiene supported by amidinates and low‐valent germanium amidinate substituents are described. The reaction of the amidinate Ge^I dimer, [LGe:]₂ ( 1 , L=PhC(NtBu)₂), with two equivalents of the amidinate tin(II) chloride, [LSnCl] ( 2 ), and KC₈ in tetrahydrofuran (THF) at room temperature afforded a mixture of the zwitterionic base‐stabilized digermadistannacyclobutadiene, [L₂Ge₂Sn₂L′₂] ( 3 ; L′=LGe:), and the bis(amidinate) tin(II) compound, [L₂Sn:] ( 4 ). Compound 3 can also be prepared by the reaction of 1 with [L^ArSnCl] ( 5 , L^Ar=tBuC(NAr)₂, Ar=2,6‐iPr₂C₆H₃) in THF at room temperature. Moreover, the reaction of 1 with the “onio‐substituent transfer” reagent [4‐NMe₂‐C₅H₄NSiMe₃]OTf ( 8 ) in THF and 4‐(N,N‐dimethylamino)pyridine (DMAP) at room temperature afforded a mixture of the zwitterionic base‐stabilized tetragermacyclobutadiene, [L₄Ge₆] ( 9 ), the amidinium triflate, [PhC(NHtBu)₂]OTf ( 10 ), and Me₃SiSiMe₃ ( 11 ). X‐ray structural data and theoretical studies show conclusively that compounds 3 and 9 have a planar and rhombic charge‐separated structure. They are also nonaromatic.     

The Radical Anion of Cyclopentasilane‐Fused Hexasilabenzvalene

The radical anion of cyclopentasilane‐fused hexasilabenzvalene was synthesized by the reduction of the corresponding neutral compound. X‐ray crystallographic analysis showed a more trans‐bent structure of the disilene moiety than the neutral compound. Theoretical calculations showed that the highly trans‐bent structure is attributed to the hexasilabenzvalene structure. The EPR spectrum showed that an unpaired electron exists mainly at the disilene moiety. In the UV/Vis spectrum, a large bathochromic shift was observed compared with the neutral compound.     

Synthesis and Application of a Perfluorinated Ammoniumyl Radical Cation as a Very Strong Deelectronator

The perfluorinated dihydrophenazine derivative (perfluoro‐5,10‐bis(perfluorophenyl)‐5,10‐dihydrophenazine) (“phenazine^F”) can be easily transformed to a stable and weighable radical cation salt by deelectronation (i.e. oxidation) with Ag[Al(OR^F)₄]/ Br₂ mixtures (R^F=C(CF₃)₃). As an innocent deelectronator it has a strong and fully reversible half‐wave potential versus Fc⁺/Fc in the coordinating solvent MeCN (E°′=1.21 V), but also in almost non‐coordinating oDFB (=1,2‐F₂C₆H₄; E°′=1.29 V). It allows for the deelectronation of [Fe^IIICp*₂]⁺ to [Fe^IV(CO)Cp*₂]²⁺ and [Fe^IV(CN‐^tBu)Cp*₂]²⁺ in common laboratory solvents and is compatible with good σ‐donor ligands, such as L=trispyrazolylmethane, to generate novel [M(L)_x]ⁿ⁺ complex salts from the respective elemental metals.     

Rapid Synthesis of Zwitterionic Phosphonium Benzoates by a Three‐Component Coupling Involving Phosphines,Arynes and CO2

A mild and easy to perform multicomponent coupling involving phosphines, arynes generated from 2‐(trimethylsilyl)aryl triflates, and CO₂ allowing the transition‐metal‐free synthesis of zwitterionic phosphonium benzoates has been developed. The reaction proceeds via the generation of 1 : 1 zwitterionic intermediates from phosphines and arynes followed by the interception with CO₂ to deliver the carboxylates in moderate to good yields instead of the anticipated benzooxaphosphol‐3(1H)‐ones.     

Alkali‐Metal‐Ion‐Assisted Hydrogen Atom Transfer in the Homocysteine Radical

Intramolecular hydrogen atom transfer (HAT) was examined in homocysteine (Hcy) thiyl radical/alkali metal ion complexes in the gas phase by combination of experimental techniques (ion‐molecule reactions and infrared multiple photon dissociation spectroscopy) and theoretical calculations. The experimental results unequivocally show that metal ion complexation (as opposed to protonation) of the regiospecifically generated Hcy thiyl radical promotes its rapid isomerisation into an α‐carbon radical via HAT. Theoretical calculations were employed to calculate the most probable HAT pathway and found that in alkali metal ion complexes the activation barrier is significantly lower, in full agreement with the experimental data. This is, to our knowledge, the first example of a gas‐phase thiyl radical thermal rearrangement into an α‐carbon species within the same amino acid residue and is consistent with the solution phase behaviour of Hcy radical.     

Thermal and Photophysical Properties of Highly Quadrupolar Liquid-Crystalline Derivatives of the [closo-B12H12]2− Anion

Two series of 1,12-bis-zwitterionic derivatives of the [closo-B₁₂H₁₂]²⁻ anion ( B ), containing either two 4-alkoxypyridinium groups ( 1B[n]-p ) or one 4-alkoxypyridinium and one 4-pentylthianium groups ( 2B[n]-p ), were prepared and their structural (XRD, DFT), thermal, and photophysical properties were compared with those of the analogous derivatives of the [closo-B₁₀H₁₀]²⁻ anion ( 1A[n]-p and 2A[n]-p ). Some 1,7-derivatives of B were isolated and investigated. Both series 1[n] and 2[n] exhibit nematic and crystalline polymorphism; the 12-vertex derivatives ( B ) have higher transition temperatures than those of the 10-vertex analogues ( A ). All compounds fluoresce with quantum yields higher for 1B (Φ_F=0.37 for 1B[7]-p and Φ_F=0.27 for 2B[7]-p ) than those for the 10-vertex analogues (Φ_F=0.04 for 2A[5]-p ). DFT calculations demonstrate an order of magnitude lower first hyperpolarizability, β_(−ω,ω,0), for 2B[7]-p than that for the 10-vertex analogue 2A[7]-p (1.7×10⁻³⁰ vs. 18.9×10⁻³⁰ esu at ω=0).