Phosphole Ring Research Articles

Synthesis, Structures, and Properties for PIII-Doped Hetero-Buckybowls and Their Phosphonium Salts.

Doping polycyclic aromatic hydrocarbons with heteroatoms enables manipulation of their electronic structures. Herein, the structures and properties of phosphorus (P) doped heterosumanenes (HSEs) are regulated by varying the valence states of P-dopant. The phosphine sulfide (PV) and chalcogens (S, Se, Te) co-doped HSEs (1-3) are reduced to trivalent phosphorus (PIII) doped analogues 4-6. Then, the PIII-dopants on 4-6 are converted to phosphonium salts (R4P+), giving 7-9. The valence states of P-dopant show great influence on molecular geometries and electronic structures. Taking P and S co-doped HSEs as example, bowl-depths increase in the order of 1 (PV)<7 (R4P+)<4 (PIII), and the HOMO energy levels and HOMO-LUMO gaps increase to be 7<1<4. Consistent with the theoretical calculation, the first oxidation potentials decrease and the absorption/emission bands show blue shift from 7 to 1 to 4. The transformation of PV to PIII leads to large variations on the coordination with Ag+, owing to the alteration of coordination site from P=S to PIII. The phosphonium salts show ring-opening of phosphole rings under electrochemical reduction. It is found that chalcogen atoms play pivotal roles on coordination patterns of coordination complexes and the conversion rates of ring-opening reactions.

2-Aryl-3H-1,3-Benzazaphosphole Oxides: Synthesis, Optical Properties, and Excited State Intramolecular Proton Transfer.

Inclusion of a heteroatom to the phosphole ring is a promising strategy to intrinsically modulate the optical properties of phosphole derivatives. We report on a series of 2-aryl-3H-1,3-benzazaphosphole oxides that were efficiently prepared via sequential C-P cross-coupling, dehydrative [3+2] cycloaddition, and ring-oxidation reactions. The inclusion of one nitrogen atom into the benzophosphole framework caused red shifting of the absorption and emission maxima, reflecting the greater stabilization of the LUMO level. 2-(2-Hydroxyphenyl)benzazaphosphole oxide underwent excited state intramolecular proton transfer and emitted a weak fluorescence from the excited state of the N-H tautomer.

The Synthesis and Properties of Ladder-Type π-Conjugated Compounds with Pyrrole and Phosphole Rings.

The phosphole ring is known as a useful building block for constructing π-conjugated organic materials. Here, we report ladder-type benzophospholo[3,2-b]indole (BPI) derivatives where the phosphole and the pyrrole rings are directly fused. Compounds 8a-8d with different aryl groups on the phosphorous center were successfully synthesized, and the solid-state structure of 8a was confirmed using X-ray crystallographic analysis. The BPIs exhibit relatively high fluorescence quantum yield (Φ 0.50-0.72) and demonstrate a larger Stokes shift compared with a series of benzophospholo[3,2-b]benzoheteroles. The benzophospholo[3,2-b]carbazole derivative 9, which possesses a benzene ring between the phosphole and the pyrrole rings of the BPI, was also synthesized, and its solid-state structure was confirmed using X-ray crystallographic analysis. Compound 9 was found to show a smaller Stokes shift compared with the BPI.

Anomeric‐Schleyer hyperconjugative interaction as a convenient avenue for aromaticity enhancement of phospholes

AbstractDue to the insufficient interaction of the phosphorus lone pair with the butadiene moiety, the aromaticity of the phosphole ring is lower than that of its counterpart's pyrrole, furan, and thiophene. Considering the high importance of phosphole core in organic chemistry, increasing its stability through reinforcement its aromaticity can be very valuable. In the present work, the aromaticity of the phosphole on the anomeric carbon in both the axial and equatorial conformers of the unsaturated six‐membered heterocycles, using structural, electronic, energetic, and magnetic indices were investigated by the DFT‐B3LYP/6‐311++G(d,p) computational method. Electron pumping through anomeric and then Schleyer hyperconjugative interaction increase the aromaticity of the phosphole ring in axial conformer of compounds 1–11. Based on various types of aromaticity indices, the results showed that the phosphole ring in the axial position has far higher aromaticity than the equatorial position. The phosphole ring containing cyano groups shows an efficient anomeric effect and, as a result, higher aromaticity. Excellent correlations were observed between aromaticity indices with different backgrounds.

Pentasubstituted Phospholes with extended π-conjugated Arm – Synthesis, electrochemistry, spectra and quantum chemical calculations

λ3–1H-Phosphole is a unique heterocyclic compound characterized by low aromaticity with a limited number of cross-coupling reactions to allow side-chain modification. This severely limits the use of pentasubstituted phospholes in materials chemistry, including organic light-emitting diodes (OLEDs), organic solar cells (OSCs) or probes. Therefore, we developed a procedure for simple side-chain modification by Suzuki reactions. The synthesized pentasubstituted phospholes with a π-conjugated arm in position 2 of the phosphole ring were investigated electrochemically. In addition to the electrochemical study, we measured also the UV–VIS and fluorescence spectra for the prepared 1H-phospholes. The experimentally acquired data were correlated with quantum chemical calculations. The oxidation and reduction of all derivatives in acetonitrile at various electrodes is (quasi)reversible, the respective potentials as well as the extent of intramolecular electron communication and π-delocalization depend on the combination of electron withdrawing and donating substituents. The difference between the first oxidation and the first reduction potential (ΔE1) can be correlated with excitation energy.

Phosphole aromaticity enhancement by electron pumping through Schleyer hyperconjugative aromaticity: A comprehensive DFT study

Combination of aromaticity and hyperconjugation as two fundamental concepts in organic chemistry results hyperconjugative aromaticity. Due to trigonal pyramidal P (III) atom in phosphole ring, it is only slightly more aromatic than cyclopentadiene. In the present work, the hyperconjugative aromaticity of phosphole derivatives, bearing an electron donating group, have been studied at the DFT-B3LYP/6–311++G(d, p) level of theory using various aromaticity indices such as geometric, magnetic, electronic and energetic criteria. Our DFT calculations reveal that an electropositive substituent can enhance phosphole aromaticity through Schleyer hyperconjugative aromaticity. This hyperconjugative electron donation by the substituents results in a partially anionic phosphole ring. Among the electropositive groups, silyl and germanium substituents have the best performance. The excellent correlations are observed between all aromaticity indices. Our findings could help chemists to design novel hyperconjugative aromatic phospholes.

Substituent effects of fused Hammick silylenes via density functional theory survey

AbstractWe compared and contrasted the substituent effects on ΔΕs‐t (singlet‐triplet energy difference), band gap (ΔΕHOMO‐LUMO), aromaticity, atomic polar tensor (APT) charge distribution and reactivity of 54 singlet and triplet fused Hammick silylenes derived from silabenzopyridine‐4‐ylidene (1s‐27s, and 1t‐27t), and the synthesized dialkylsilylene by Kira, respectively. In this investigation we found the following: (1) The most and the least thermodynamic stability compared to Kira's silylene is demonstrated by fusion of two thiophene rings in para‐position and two phosphole rings in ortho‐position, respectively (ΔΕs‐t = 331.0 and 115.1 vs. 138.6 kJ/mol, respectively). (2) Although fusion of benzene, pyrrole, furan, thiophene, and phosphole ring kinetically stabilizes the corresponding silylenes, fusion of pyridine and phosphinine ring destabilizes their silylenes versus the isolated silylene's Kira (with the range of ΔEHOMO‐LUMO from 312.8 to 410.2 vs. 351.7 kJ/mol). (3) The trend of substituent effects per number and size of rings is two five‐membered rings &gt; one five‐membered rings &gt; one six‐membered rings &gt; two six‐membered rings. (4) Almost all cases, singlet Hammick silylenes reveal greater nucleophilicity, lesser electrophilicity, lower chemical potential, and greater global hardness than the scrutinized silylene by Kira. (5) We are waiting for experimental testing and verifications that place the heteroatom in the silapyridine's plane in a favorable spatial position to act not only as π electron donor but also acceptor of σ electron density.

Substituent effects of fused Hammick germylenes: Estimating the stability and reactivity using density functional theory

AbstractIn this theoretical survey, we compared and contrasted the substituent influences on stability, aromaticity, band gap, atomic charge distribution, and reactivity of 54 singlet and triplet fused Hammick germylenes derived from germabenzopyridine‐4‐ylidene as well as the synthesized five‐ and six‐membered ones by Herrmann and Driess, respectively. Investigations show (1) the lowest and the highest singlet–triplet energy difference (ΔΕs–t) is demonstrated by fusion of two phosphole rings and two furan rings in ortho‐positions (35.6 and 253.6 kJ/mol, respectively) compared with those of the synthesized germylenes (208.8 and 179.9 kJ/mol, respectively). (2) Whereas fusion of benzene, pyridine, phosphinine, and phosphole rings thermodynamically destabilizes the corresponding germylenes, fusion of pyrrole, furan, and thiophene rings stabilizes the corresponding germylenes. (3) The tendency of substituent influences in number and size of rings is arranged by two five‐membered rings &gt; one five‐membered rings &gt; one six‐membered rings &gt; two six‐membered rings. (4) While pyrrole, furan, and thiophene rings kinetically stabilize the corresponding germylenes, other rings destabilize their germylenes with respect to germapyridine‐4‐ylidene and the isolated germylene's Driess. (5) All singlet Hammick germylenes reveal lower nucleophilicity and softness, also higher electrophilicity, chemical potential, and global hardness than their scrutinized triplet states. (6) We are waiting for experimental testing and verifications to fuse heterocyclic rings to the germapyridine‐4‐ylidene's plane in a favorable spatial position to act not only as π‐electron giver but also σ‐electron receiver.

Control of Physicochemical Properties for Thiophene-Fused Naphthodiphospholes By Precise Fusion of Heterole Rings

Fused polycyclic aromatics have been vigorously pursued over the years because of their attracting structural, electronic, and photophysical features associated with the rigid and planar π-conjugated frameworks. For polycyclic aromatics with heterole-fused structures, the orientation of fused heterole rings as well as the geometry of their fused structures has a large impact on the physicochemical properties. In this regard, we expected that introducing two phosphorus atoms into polycyclic aromatics would provide an assortment of isomers originated from not only the position and orientation of the phosphole rings, but also the orientation of substituents on the phosphorus atoms (trans/cis configurations). Herein, we designed and synthesized a series of six isomers of thiophene-fused naphthodiphospholes based on our envision. Systematic investigations into the physicochemical properties revealed that the positions of heterole rings are of utmost importance to tune their electronic properties, and the orientations of heterole rings and substituents on the phosphorus atoms play critical roles in the molecular arrangements in the solid states.

Synthesis of triphenylene-fused phosphole oxides via C-H functionalizations.

The synthesis of triphenylene-fused phosphole oxides has been achieved through two distinct C–H functionalization reactions as key steps. The phosphole ring was constructed by a three-component coupling of 3-(methoxymethoxy)phenylzinc chloride, an alkyne, and dichlorophenylphosphine, involving the regioselective C–H activation of the C2 position of the arylzinc intermediate via 1,4-cobalt migration. The resulting 7-hydroxybenzo[b]phosphole derivative was used for further π-extension through Suzuki–Miyaura couplings and a Scholl reaction, the latter closing the triphenylene ring. The absorption and emission spectra of the thus-synthesized compounds illustrated their nature as hybrids of triphenylene and benzo[b]phosphole.

High variety of coordination modes of π-conjugated phospholes in dinuclear rhenium carbonyls. Fluxional behavior of σ,π-complexes

A variety of dirhenium carbonyl complexes containing π-conjugated phosphole derivatives were obtained from reaction of [Re2(CO)8(CH3CN)2] with each of the following phospholes: 2,5-bis(2-thienyl)-1-phenylphosphole (btpp), 2,5-bis(2-pyridyl)-1-phenylphosphole (bpypp) and 1,2,5-triphenylphosphole (tpp). The π-conjugated phospholes were found to behave as two-, four- or six-electron donor ligands via σ or σ-π interactions with the metal centers, presenting bridging or chelating coordination modes as determined by spectroscopic methods and single crystal X-ray diffraction analysis. Metal-metal bond cleavage was evidenced when btpp was used, leading to a mono-substituted mononuclear complex. Variable-temperature 1H NMR studies for σ,π-complexes showed a fluxional behavior due to the restricted rotation around the PC and CC bonds of the 1,2,5-trisubstituted phosphole ring.

​4+2] versus [​2+2] Homodimerization in P(V) Derivatives of 2,4-Disubstituted Phospholes

Phosphole P(V) derivatives are interesting building blocks for various applications from ligand synthesis to material sciences. We herein describe the preparation and characterisation of new 2,4-disubstituted oxo-, thiooxo-, and selenooxophospholes. The nature of the substituents on the phosphole ring determines the reactivity of these compounds towards homodimerization reactions. Aryl and trimethylsilyl substituted oxophospholes undergo selective [4+2] dimerization, whereas, for thiooxo- and selenooxophospholes, light-induced, selective [2+2] head-to-head dimerization occurs in the case of aryl substituents. DFT calculations provide some insights on these differences in reactivity.

Conjugated Metallo-Supramolecular Polymers Containing a Phosphole Unit

A new building block, TPT, composed of the substituted phosphole ring surrounded by two thiophene rings with 2,2′:6′,2″-terpyridine-4′-yl (tpy) end-groups, is prepared and assembled with metal ions (Co2+, Cu2+, Fe2+, Ni2+, and Zn2+) into metallo-supramolecular polymers (MSPs), and properties of both TPT and the MSPs are compared with those of their counterparts with terthiophene central blocks. A distinct red-shifting of the UV/vis band about by 60–100 nm proves the decrease in the bandgap energy due to replacing the thiophene-2,5-diyl with a phosphole-2,5-diyl central unit, which is due to the lowered aromaticity of phosphole ring compared to the aromaticity of thiophene ring. Assembling TPT with metal ions gives oligomeric chains comprising up to 10 unimeric units in dilute solutions. MSPs with Fe2+ and Ni2+ ion couplers exhibit very slow constitutional dynamics, while those with Cu2+ and Zn2+ ion couplers quite fast constitutional dynamics. A metal–ligand charge transfer is observed only for Fe2+-MSPs,...

ChemInform Abstract: Versatile Synthesis of Phospholides from Open‐Chain Precursors. Application to Anellated Pyrrole— and Silole—Phosphole Rings.

AbstractThe first general synthesis of phospholides and annelated phospholides starting from open‐chain precursors is presented.

Reaction of Phospholes with Aldimines: A One-Step Synthesis of Chelating, Alpha-C2-Bridged Biphospholes.

Phospholes react with aldimines at 170 °C in the presence of mild Lewis acids to give C2-bridged biphospholes in good yields. The mechanism includes a series of [1,5] shifts of the P-substituents around the phosphole ring, a P-H + aldimine condensation, and the formation of a transient three-membered ring that dimerizes.

Versatile synthesis of phospholides from open-chain precursors. Application to annelated pyrrole- and silole-phosphole rings.

Phospholides are easily obtained by treatment of the open-chain acetylenic phosphines shown by an excess of lithium at room temperature in THF (12 examples).

Helicene-like chiral auxiliaries in asymmetric catalysis.

This literature overview demonstrates that helically chiral ligands and organocatalysts have been largely neglected so far. However, a few recent studies on helical pyridine, the corresponding ammonium salts and N-oxides have highlighted the significant potential of these compounds as organocatalysts for Michael type additions, aldehyde propargylations, epoxide openings, and others. In addition, helicenes displaying a fused phosphole ring at the end of their polyaromatic structures, have been used as ligands in enantioselective gold promoted cycloisomerization reactions, giving both excellent catalytic activity and high enantiomeric excesses. These recent results are expected to stimulate further research on the catalytic applications of helically chiral auxiliaries in the next few years.

Insertion of a phospholide unit into a metal–metal bond: Synthesis and X-ray structure of [Ru3(CO)9(μ:η1:η5-PC4H2Me2)(η1-C6H5)

The reaction of [Ru3(CO)12] with 3,4-dimethyl-1-phenylphosphole under mild conditions affords three derivatives which were identified by means of spectroscopic methods as the substitution compounds [Ru3(CO)12−x(PhPC4H2Me2)x] (x=1, 2, 3) in which the phosphole ligand acts as a tertiary phosphine bonded through the phosphorus atom. Thermolysis of the monosubstituted [Ru3(CO)11(PhPC4H2Me2)] in cyclohexane affords two new clusters, characterised as [Ru3(CO)9(μ3:η2:η2:η2-PhPC4H2Me2)] and [Ru3(CO)9(μ:η1:η5-PC4H2Me2)(η1-C6H5)], where the phosphole ring behaves as a six- and seven-electron donor ligand respectively. The reaction with [Os3H2(CO)10] gives the new species [Os3(μ-H)(CO)9(μ3:η1:η1:η2-PhPC4H3Me2)] in which the ligand behaves as a five-electron donor.

Syntheses and coordination chemistry of bis(4‐pyridyl)‐ and mixed (4‐pyridyl) (2‐pyridyl)‐phospholes

AbstractThe syntheses, spectroscopic characterizations, and X‐ray structures of two new N,P,N‐ligands 1 and 2 are described. These ligands are based on a central 2,5‐substituted phosphole ring and have, respectively, two 4‐pyridyl moieties, or one 4‐pyridyl moiety and one 2‐pyridyl moiety as substituents. The coordination chemistry of these ligands has been investigated. A Au(I)‐complex 5 bearing ligand 1 as a P‐donor and a Pt(II) complex 6 featuring ligand 2 as a P,N‐chelate were obtained. Complex 6 spontaneously evolves to complex 7 bearing a 2‐phospholene ring. Spectroscopic characterizations for complexes 5–7 as well as the X‐ray crystal structure of 7 are described. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:339–347, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20687

Towards Spontaneous Heterolysis of the Homonuclear PP Bond in Diphosphines: The Case of Diazaphospholeniumtriphospholides

Computational studies on a series of polyphospholyl-substituted N-heterocyclic phosphines (CH)(2)(NR)(2) P-P(n)(CH)(5-n) (R=Me, n=1-5) disclosed that increasing formal replacement of CH units in the phosphole ring by phosphorus atoms is associated with an increase in P-P distances and charge separation, and a decrease in covalent bond orders. Altogether, these trends imply that the CH versus P substitution enhances ionic P-P bond polarization in these compounds. Experimental verification of this hypothesis was obtained for the triphospholyl diazaphospholenes (CR)(2)(NR')(2)P-P(3)(CtBu)(2) (8a: R=H, R'=tBu; 8b: R=Me, R'=Mesityl [Mes]), which were prepared through metathesis reactions from suitable precursors and identified by solution and solid-state NMR data and a single-crystal X-ray diffraction study of 8a. Analysis of J(PP) coupling patterns suggested that both species are characterized by the absence of a strong covalent P-P bond connecting both rings. This interpretation was confirmed by the finding of a unique P-P distance of 2.79 A for crystalline 8 a, and further supported by computational studies, which led to the conclusion that both species are better described as diazaphospholenium-triphospholide contact ion pairs rather than covalent molecules. Variable-temperature (VT) NMR spectra of 8b showed a collapse of J(PP) couplings between atoms in different rings, which indicates scrambling of the diazaphospholenium and triphospholide units between different molecules in solution, and further substantiates the proposed view on the molecular structure.