Synthesis and Properties of 4,5,6-Triphospha[3]radialene

[3]Radialenes: 4,5,6-Tris(2,4,6-tri-tert-butylphenyl)phospha[3]radialene has been synthesized. The compound can be easily handled in air under ambient conditions, despite the [3]radialene moiety containing P?C bonds, and exhibits red-shifted absorption as well as high electron-accepting ability. The unique electronic properties are brought about by the synergistic effect of the [3]radialene framework and the phosphorus substitution.     

Novel pi-expanded radialene macrocycles with inner cavity

[structure: see text] Polyenyne macrocycles with pi-extended [9]- and [12]radialene frameworks have been synthesized. These radialenes exhibit restricted rotation of the aromatic rings, and the D3- and D4-symmetric structures in solutions have been determined by dynamic NMR. The macrocyclic radialenes bear small to medium inner cavities, and the small cavity of the pi-extended [9]radialene can incorporate a silver cation.     

Aromaticity and electron affinity of Carbo(k)-[3]radialenes, k=0, 1, 2

Aromaticity enhancement is a possible driving force for the low reduction potentials of buta-1,3-diynediyl-expanded [N]radialenes: this hypothesis is theoretically analyzed for the expanded [3]radialene prototype. This study is undertaken within a more general prospect, namely the evaluation of the variation of aromaticity with endocyclic and peripheral carbomeric expansion of [3]radialene and its mono- and dianions. The structures, denoted as [C-H](6) (h)[C-C](3) (k)carbo-[3]radialene(q) (h=0, 1; k=0, 1, 2; q=0, -1, -2), were optimized in relevant singlet, doublet, or triplet spin states at the B3PW91/6-31G** level. They were found to be all planar. The structural aromaticity was measured through the average bond length d(av) over the [C-C](3) (k)carbo-[3]radialene core, and by the corresponding bond-length equalization parameter sigma(d), related to Krygowski's GEO. The magnetic aromaticity was measured by Schleyer's NICS values at the center of the rings. Regarding the relative variation of NICS and sigma(d), two classes of species can be distinguished according to their endocyclic expansion level. The species with a nonexpanded (k=0) or doubly expanded (k=2) ring constitute the first class: they exhibit D(3h) symmetry and a strong correlation of NICS with sigma(d). The species with a singly expanded ring (k=1) fall far from the correlation line, and constitute the second class. This class distinction is related to the degeneracy scheme of the frontier orbitals of the neutral representative. A finer appraisal of the electron (de)localization is brought by the ELF (Electron Localization Function) analysis of the electron density. It allows for a weighting of relevant resonance forms. Unsubstituted species are well described by the superimposition of two or three resonance forms. For (doublet spin state) monoanionic species, their respective weights are validated by comparison with AIM spin density. The weighted mean, n, of the formal numbers of paired pi(z) electrons in the resonance forms was calculated and compared with the closest even integer of either forms 4m+2 or 4m. A density-based continuous generalization of the orbital-based discrete Hückel rule is then heuristically proposed through an analytical correlation of NICS versus sigma(d), n, and S, the spin of the species. The frontier-orbital-degeneracy pattern of neutral species is discussed with respect to structural and magnetic aromaticity criteria. A decreasing HOMO-LUMO gap versus endocyclic expansion is obtained, but [C-C](3) (1)carbo-[3]radialene possesses the highest HOMO and LUMO energies. Vertical and adiabatic electron affinities of neutral and monoanionic species were also computed and compared with related experimental data.     

Einelektronen-Redox-Reaktionen von Octamethyl[4]radialen: Nachweis der Alternanz geradzahliger Radialene

One-Electron Redox Reactions of Octamethyl[4]radialene: Proof of the Alternancy of Even-Membered Radialenes The photoelectron spectrum of octamethyl[4]radialene ( = tetrakis(dimethylmethylidene)cyclobutane) yields the rather low first vertical ionization of only 7.30 eV and, therefore, the molecule can be oxidized by AlCl₃/CH₂Cl₂ to its radical cation. The ESR and ENDOR signal patterns recorded are almost identical with those of the radical anion, generated in a [2.2.2]cryptand THF solution by reduction at a potassium metal mirror. Their near congruency is in accord with the alternant π molecular properties of even-membered radialenes as predicted by HMO theory, and larger changes of the D_2d structure of C₈(CH₃)₈ during the redox reactions are presumably hindered by the steric overcrowding in persubstituted [4]radialenes.     

Heteroatom-substituted expanded radialenes: one-pot synthesis and characterization of expanded 1,3-dithiolane[n]radialenes

The one-pot synthesis, crystal structure, and electronic absorption properties of expanded 1,3-dithiolan[5]radialene (6a), the first heteroatom-substituted expanded radialene molecule with dithiolanylidene groups as strongly donor termini, has been developed. Catalyzed by Ni(PPh3)2Cl2-PdCl2-CuI, radialene 6a was assembled by a highly efficient one-pot and protection-free oxidative coupling reaction of five alpha-enediyne units starting from the ready available alpha,alpha-diethynyl ketene dithioacetal (13) under mild conditions. The crystal of 6a, obtained by slow evaporation of DMSO and acetone solutions, shows that the inclusion complex of DMSO molecules in the cavity of radialene 6a molecule is formed. More importantly, owing to the introduction of the strong electron-donating alkylthio groups, per(alkylthiolated) expanded radialene 6a shows an extremely strong intramolecular push-pull interaction and enhanced macrocyclic cross-conjugation.     

Description of carbo-oxocarbons and assessment of exchange-correlation functionals for the DFT description of carbo-mers

Mono- and polycyclic valence isomers of carbo-[3]oxocarbon C(9)O(3) and carbo-[5]oxocarbon C(15)O(5) have been characterized on the singlet spin state potential energy surface. By contrast to their geometry, their relative stability is highly sensitive to the calculation level. The performance of LDA, GGA, meta-GGA, and hybrid functionals is compared to that of HF, post-HF, and multiconfigurational calculations. The results obtained for C(9)O(3) are compared to those obtained for hydrocarbon analogues such as &[3]pericyclyne C9(H2)3 and carbo-[3]radialene C9(CH2)3 and are analyzed on the basis of an energy decomposition scheme. The respective role of the exchange and correlation counterparts of the functional in the discrepancy of the results is discussed.     

Ruthenium complexes of hexakis(cyanophenyl)[3]radialenes and their di(cyanophenyl)methane precursors: synthesis,photophysical, and electrochemical properties

The coordination chemistry of cross-conjugated ligands and the effect of cross-conjugation on the nature of metal–metal and metal–ligand interactions have received limited attention. To explore the effects of cross-conjugation eight ruthenium complexes were synthesized, mononuclear complexes of two isomeric cross-conjugated [3]radialenes [RuCp(PPh₃)₂(L)]PF₆ and [{RuCp*(dppe)}(L)]PF₆ (L?=?hexakis(4-cyanophenyl)[3]radialene, 2; hexakis(3-cyanophenyl)[3]radialene, 3), and dinuclear complexes [{RuCp(PPh₃)₂}₂(L)](PF₆)₂ and [{RuCp*(dppe)}₂(L)](PF₆)₂ of the diarylmethane precursors (L?=?4,4′-dicyanodiphenylmethane, 4; 3,3′-dicyanodiphenylmethane, 5) to the [3]radialenes. Considerable synthetic challenges allowed only clean isolation of mononuclear complexes of the multidentate radialenes 2 and 3. As expected, coordinating a positively charged metal induces a red shift for the π–π* transition in complexes of ligand 2, but unexpectedly a blue shift for the same transition in complexes of 3 was observed. This points to conformational differences for the [3]radialene in the ruthenium complexes of the para- (2) versus meta- (3) substituted hexaaryl[3]radialenes. Cyclic voltammetry indicates that the methylene spacer in 4 and 5 does not enable any interaction between metal centers and the absorption behavior is essentially as observed for [Ru(NCPh)(PPh₃)₂Cp]PF₆ and [Ru(NCPh)(dppe)Cp*]PF₆ but generally with a slight red shift in absorbance maxima.     

Radical Cations of Dithia- and Tetraselenaradialenes with Restricted Delocalization of the Unpaired Electron

The radical cations of the permethylated dithia[6]radialene 1 and the tetraselena[8]radialene 2 have been investigated by ESR- and ENDOR-spectroscopy. It is shown that in the radical cation of 1 the unpaired electron is delocalized only in one half of the molecule. The magnitude of the (33)S coupling constant suggests a restricted localization within the 2,3-dithiatetramethyl-butadiene unit. We ascribe the slow electron transfer from one part of the molecule to the other to steric effects. The analysis of the ESR spectrum of 2(*)()(+)() shows that only two of the four selenium atoms are involved. The delocalization of the unpaired electron is restricted to only one of the divinyl diselenide moieties with a slow electron transfer rate to the other one.     

Hexakis (methylidene)-cyclohexane (“[6]Radialene”). Chemical and spectral properties

Upon flash pyrolysis between 580 to 660°, 2,4,6-tris(chloromethyl)mesitylene ( 2 ) leads to products formed through the loss of one, two and three molecules of HCl. The highly reactive chlorine free product is shown to be [6]radialene ( 1 ) both by its chemical reactions and by its spectral properties.     

α-Cyano Triaryl[3]radialene: Unsymmetrical Stereo-configuration,Clustering-enhanced Excimer Emission,and Radical-involved Multimodal Information Switching

[3]Radialene has a peculiar topology and cross-conjugation system, representing a unique molecular scaffold in organic materials. Herein, we report a special class of stereoisomeric α-cyano triaryl[3]radialenes ( CTRs ) that show concentration-caused quenching in solution but emit red-shifted and enhanced luminescence in the crystalline state. Clustering of multiple cyano groups and their through-space interactions with the [3]radialene ring significantly extend π-electron communication meanwhile rigidifying the propeller conformation multivalently, thus playing a key role behind the state-dependent luminescence. These radialenes with a substantial electron affinity undergo a reversible electron transfer transition to anionic radicals with good stability, showing switching of photoabsorption, photoluminescence and electron spin resonance (ESR) signal. We also established proof-of-concept applications of CTRs for multimodal information encryption and chemical sensing.     

Synthesis and Characterization of the Dilithium Salt of the Octasilyl[4]radialene Dianion: Evidence for a Lithium Walk on the Eight-Center,Ten-Electron π-Electron System

A highly symmetric structure is displayed in toluene by 1 , which was obtained by reduction of octasilyl[4]radialene with lithium in THF. The two Li⁺ ions of 1 are not fixed to the π-electron system in toluene, but are fluxional, giving rise to a bis-contact ion pair. In a solvating medium such as THF, one of the Li⁺ ions dissociates in such a way that 1 exists as a contact ion pair and a solvent-separated ion pair.     

Hexa(2-pyridyl)[3]radialene: self-assembly of a hexanuclear silver array

Reaction of the new ligand hexa(2-pyridyl)[3]radialene with silver tetrafluoroborate results in the formation of a M6L2 cage with an encapsulated mu 3-fluorido anion.     

Unexpected Formation of a [4]Radialene and Dendralenes by Addition of Tetracyanoethylene to a Tetraaryl[5]cumulene

The use of cumulenes in synthetic transformations offers the possibility to form structurally interesting and potentially useful conjugated molecules. The cycloaddition reaction of a tetraaryl[5]cumulene with the electron‐deficient olefin tetracyanoethylene affords unusual products, including functionalized dendralenes and alkylidene cyclobutanes, as well as a symmetric [4]radialene that shows unique solvatochromism, with λ_max values approaching the near‐IR region. These carbon‐rich products have been investigated spectroscopically and by X‐ray crystallographic analysis (five structures). The cycloaddition reaction sequence has also been explored by mechanistic and theoretical studies. The obtained results clearly demonstrate the potential of [5]cumulenes to serve as precursors for unprecedented conjugated structures.     

Expanded radialenes with bicyclo[4.3.1]decatriene units: new precursors to cyclo[n]carbons

A new method for the formation of conjugated polyynes has been developed based on both the rearrangement of vinylidenes to alkynes and the [2+1] cheletropic fragmentation of dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene derivatives. A model study of the photolysis of simple dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-trienes resulted in cheletropic fragmentation followed by 1,2-migration to give the corresponding linear polyynes, although undesired isomerization to methylenebicyclo[5.3.0]triene derivatives took place concurrently. Expanded [3]-, [4]-, [5]-, and [6]radialene derivatives with exocyclic bicyclo[4.3.1]decatriene units were prepared by oxidative coupling of the monomeric units as precursors to the corresponding cyclo[n]carbons, monocyclic forms of carbon clusters. The spectroscopic properties of the expanded radialenes were investigated in connection with cross conjugation of the core pi system and with its perturbation by the extraannular bicyclic pi system. In negative-mode laser-desorption time-of-flight (LD-TOF) mass spectra, the expanded radialenes exhibited peaks due to the corresponding cyclo[n]carbon anions (n = 18, 24, 30, and 36) formed by the stepwise loss of the aromatic indane fragments.     

Tetrakis (methylidene)cyclobutane (‘[4]Radialene’): Electronic states of the molecular ion

The photoelectron spectrum of tetrakis (methylidene)cyclobutane ( 1 , ‘[4]radialene’) is reported. The electronic states of 1 ⁺ are assigned on the basis of model calculations and with reference to related systems. Jahn-Teller activity in the degenerate states is discussed. A failure of the simple LCBO-model for the π(e_g)-orbital of 1 is noted and traced to the fact that this orbital, though having a symmetry-equivalent π*-counterpart, does not interact with it. This feature is confined to [4n]radialenes; their total π-energies are therefore higher than those of the other members. It is shown that radialenes, in principle, do not constitute a class analogous to that of the linear polyenes as inferred earlier.     

Thermal and Photochemical Isomerization of Tetraaryl Tetrakis(trifluoromethyl)

The isomerization of tetraaryl tetrakis(trifluoromethyl)[4]radialenes was studied. When type II (all-Z) isomers of 5,6,7,8-tetraaryl-5,6,7,8-tetrakis(trifluoromethyl)[4]radialenes were heated in tetralin at 170-200 degrees C, isomerization occurred to give mixtures of four [4]radialenes in a ratio of ca. I:II:III:IV = 1:10:5:1. However, when the isomeric mixtures were heated in the solid state at the same temperature, selective isomerization took place to give type II isomers in good selectivity (>91%). Upon irradiation with light, the type II isomers first isomerized to mixtures of the four [4]radialene isomers (I:II:III:IV = 2:2:48:48) and then rearranged to cyclobuta[b]naphthalenes via a 6pi-electrocyclic reaction followed by 1,3-hydrogen migration.     

Synthesis of a Tetrabenzotetraaza[8]circulene by a “Fold‐In” Oxidative Fusion Reaction

Tetrabenzotetraaza[8]circulene ( 1 ) has been synthesized in good yield by a “fold‐in” oxidative fusion reaction of a 1,2‐phenylene‐bridged cyclic tetrapyrrole. X‐ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctatetraene (COT) core that shows little alternation of the bond lengths. Despite these structural features, 1 shows aromatic‐like character, such as sharp absorption bands, high fluorescence quantum yields (Φ_F=0.55 in THF), and a single exponential fluorescence decay with τ_F=3.8 ns. These observations indicate a dominant contribution of an [8]radialene‐like π conjugation and hence aromatic character of the local aromatic segments in 1 .     

Einelektronen-Redox-Reaktionen von Octaphenyl[4]radialen: Erzeugung und ESR/ENDOR-Charakterisierung seines Radikal-Anions und Radikal-Kations

One-Electron Redox Reactions of Octaphenyl[4]radialene: Generation and ESR/ENDOR Characterisation of Its Radical Anion and Radical Cation The cyclovoltammograms of octaphenyl[4]radialene in DMF or THF at room temperature disclose each two quasireversible reduction and oxidation potentials at ?1.4 V/?1.7 V and +0.7 V/+0.9 V. Accordingly, both the radical anion and the radical cation can be generated: Ph₈C by K metal mirror reduction of a [2.2.2]cryptand containing THF solution, and Ph₈C by TI^3⊕(^?OOCCF₃)₃ oxidation in H₂CCl₂. Their ESR/ENDOR and General Triple spectra differ considerably in the number of resolved ¹H couplings (M^·?: 5 and M^·⊕: 3) as well as in their spectral widths (M^·?: a_1H 0.090 to 0.017 mT; M^·⊕: a_1H 0.066 to 0.023 mT) suggesting different changes in the D_2d structure of the neutral molecule on electron uptake or extrusion.     

Oligomerization of early transition metal [M(OH)3]2(mu-C2) acetylides toward the formation of [(OH)3MC]n (n = 4, 6) metalla carbides: a theoretical study by density functional theory

Density functional calculations were performed on a series of M(OH3)-substituted (M = Ti,V) cyclopolyenes as simple models is metal carbides. We studied the oligomerization of the metal acetylide complexes [MLn]2 (mu-C2) as a possible precursor of these [(OH)3MC]n (n = 4,6) hypothetical species. Special emphasis was placed on the comparison of the main properties of these metal substituted cyclopolyenes with those of the corresponding cyclopolyenes in an attempt to study the effects of the metal substituents on the organic C4 and C6 cyclic moieties. Whereas for the titanium species, the pi system of the polyene moiety is slightly perturbed, the electronic structures and molecular geometries found for the vanadium species suggest a metalla radialene nature for these compounds, with the endocyclic conjugation of carbon-carbon double bonds replaced by an exocyclic arrangement of carbon-metal double bonds.