BF2 Complexes Research Articles

Synthesis of Naphthalene- and Phenanthrene-Fused Smaragdyrins and Their BF2 Complexes.

Naphthalene- and phenanthrene-fused [22]smaragdyrin BF2 -complexes were synthesized by 1) Suzuki-Miyaura coupling of β-brominated [22]smaragdyrin BF2 complexes with 2-formylarylboronates, 2) Witting-type methoxymethylenation of the formyl group, and 3) methanesulfonic acid-catalyzed cyclization reaction. Subsequently these BF2 complexes were deboronized and oxidized to the corresponding antiaromatic [20]smaragdyrin free bases. The installed fused structures led to decrease of the aromatic characters of the [22]smaragdyrin BF2 complexes and the antiaromatic characters of the [20]smaragdyrin free bases.

A novel central seven-membered BOPYOs: Synthesis, optical properities and optimization of BF2 removal

Many efforts have been made to enrich the variety of BF2 complexes because of their excellent optical properties. However, the investigation on seven-membered ring N, O-chelated BF2 complexes is rare due to their instability with the removal of BF2 unit. Herein, a novel seven-membered ring N, O-chelated BF2 complexes (BOPYOs) with dual-state emission has been synthesized via a facile method. The results of optical properties showed that the fluorescence quantum yield of BOPYO-2 with donor group on 1 and 2-position of 1-indanone unit is much higher than that of BOPYO-1, 3–5 in toluene. The emission spectra of BOPYO-6 or 7 have redshift phenomenon compared with BOPYO-1–5 with weak fluorescence intensity due to their highly distorted structure or intramolecular charge transfer (ICT) effect. BOPYOs show relatively moderate solid emission from orange to deep red color with 596 nm to 686 nm. On the contrary, fluorescence quantum yield of BOPYO-2 in solid is the lowest. The optical properties in solution and solid states are further supported by the single-crystal structure and DFT calculation. Furthermore, the investigation on optimization of BF2 removal shows that the corresponding precursors of BOPYOs could be obtained in protic solvents without adding other catalysts.

Near‐IR Electrochromic Dihydroindolo(2,3a)carbazole‐Based Macrocycles and Their [b]‐Annulated BODIPY Complexes

AbstractA new class of redox‐active dihydroindolo(2,3a)carbazole(InC)‐based porphyrin‐like macrocycles (thiophene embedded: InCT and p–phenylene embedded: InCB) and the corresponding [b]‐annulated BODIPY complexes (InCTBF2 and InCBBF2) are reported. A facile synthetic methodology is adopted to accomplish the new expanded‐porphyrin‐like macrocyclic systems using the key precursor 11 based on 11,12‐dihydroindolo(2,3a)carbazole building block 4. In these macrocyclic systems, the subunit 4 is deemed to be a donor and an azafulvene‐extended with π‐conjugated fragment (thiophene or p‐phenylene) acts as an acceptor. Single crystal X‐ray structures are obtained for both InCT and InCB. Spectroelectrochemical studies on these macrocycles and their BF2 complexes display intriguing near‐IR electrochromic properties with reversible multi‐color switching upon cathodic scans at room temperature. InCTBF2 shows high stability with a radical comproportionation constant (Kc) of 1.22 × 106. Thiophene and p‐phenylene subunits in InCT and InCB exhibit ring rotation at room temperature as evident from NMR studies. Such ring rotation is restricted upon BF2 complexation, and they show unusual stability in acidic conditions.

Synthesis of N,O-bidentate organic difluoroboron complexes and their photophysical studies

We disclose a novel boron trifluoride induced C–H activation and difluoroboronation at room temperature, thus providing a straightforward gateway to a series of N,O-bidentate organic BF2 complexes. The scope of the method is demonstrated with 24 examples. All the synthesized compounds exhibit fluorescence and some of them have large Stokes shifts.Graphical

Design and facile synthesis of quinazolinone-based full color-tunable boron difluoride dyes: Efficient solvent/solid-state emission and bioimaging application

To develop the construction of small organic fluorophores with full color-tunable luminescence through facile synthesis is highly imperative for applications. Herein, novel quinazolinone based BF2 complexes (2a-2g) have been designed and synthesized. By modifying the structure of quinazolinone ligand, the synthesized boron difluoride dyes exhibited fluorescence in the almost full-color range of the visible spectrum (∼450–750 nm) both in solution and solid-state with good to excellent quantum yield. The influence of three electron donating substituents, such as methyl, methoxy and N,N-dimethylamino group, in quinazolinone core on the photophysical properties of BF2 compounds was discussed. The position of substituent in quinazolinone has some influence on the emission efficiency, but has little influence on the absorption and emission of BF2 compounds. Finally, 2g showed good cell compatibility and living cell imaging property.

Facile Formation of Stable Neutral Radicals and Cations from [22]Smaragdyrin BF2 Complexes.

meso-Trimesityl-substituted [20]smaragdyrin freebase was synthesized by p-toluenesulfonic acid catalyzed reaction of 5-mesityldipyrromethane and 2,14-dibromodipyrrin in an improved yield of 63 %. Unexpectedly, treatment of the [20]smaragdyrin freebase with BF3 ⋅ OEt2 and triethylamine (TEA) gave a stable radical species, in which the BF2 unit is coordinated at the tripyrrin site, probably by ready release of a hydrogen atom of a [22]smaragdyrin BF2 complex. Similar treatment of [22]smaragdyrin free base produced another [22]smaragdyrin BF2 complex, in which the BF2 unit is coordinated at the dipyrrin site. The tripyrrin site coordinated neutral radical was oxidized with AgSbF6 to give a stable antiaromatic cation; this was reduced with NaBH4 to its 22π congener, which was easily oxidized back to the neutral radical in the air and rearranged to thermodynamically stable dipyrrin site coordinated [22]smaragdyrin BF2 complex upon treatment with BF3 ⋅ OEt2 and TEA. Further, the dipyrrin site coordinated [22]smaragdyrin BF2 complex was similarly oxidized to a stable neutral radical and a stable cation in a stepwise manner. This work demonstrates a rare ability of smaragdyrin BF2 complexes to exist in multiple redox states, particularly forming a stable neutral radical by facile release of a hydrogen atom.

A series of boron difluoride complexes of azinylcarbazoles: synthesis and structure-property relationships.

A series of boron difluoride (BF2) complexes of azinylcarbazoles 1b-1h were synthesized, and the effects of the structure of azine moieties on the photophysical and electrochemical properties of the BF2 complexes were clarified. UV-vis analysis of 1b with quinoline, 1c with isoquinoline, and fully fused 1d revealed that fusion with a benzene ring to a pyridylcarbazole BF2 complex (1a) resulted in red shifts of longest-maximum absorption wavelengths (λmax). UV-vis analysis of 1e and 1f with pyrimidine, 1g with pyridazine, and 1h with pyrazine revealed that substitution of a carbon atom to a nitrogen atom in 1a also resulted in red shifts of λmax. The fluorescence quantum yields (Φf) decreased from 1a to 1b-1h, and especially, the fluorescence of 1e, 1g, and 1h was quenched in solution. At 77 K, the emission intensities of 1b-1h were significantly increased compared with those at ambient temperature, and they also exhibited phosphorescence with relatively narrow energy gaps between the singlet and triplet excited states. These results on the emission at 77 K indicate that the quench of fluorescence from 1e, 1g, and 1h at ambient temperature originates from both internal conversions and intersystem crossing. In the solid state, all of the complexes including 1e, 1g, and 1h exhibited emission. Distinctive aggregation-induced emission properties were observed for 1e-1h. Electrochemical measurements revealed that the replacement of the pyridine moiety in 1a with azine moieties reduced electrochemical gaps mainly due to a decrease in the LUMO levels. The effects of azine moieties on electronic structures were also discussed based on theoretical calculations.

Boron-Mediated Regioselective Aromatic C-H Functionalization via an Aryl BF2 Complex.

An efficient regioselective functionalization of 2-aryl-heteroarenes and aryl aldehydes via an azaaryl BF2 complex has been developed. Mechanistically the reaction comprises fluoride to bromide ligand exchange on an aryl boron species and consecutive C-B bond cleavage to deliver a broad range of functionalized products. The reaction is high yielding, has a broad substrate scope where several different heteroarenes can be functionalized with chloro, bromo, iodo, hydroxyl, amine and BF2 in a highly regioselective fashion. The method can be applied for late-stage functionalization or for rapid skeleton remodeling with for instance cross-couplings.

Fused Hybrids of a [22]Smaragdyrin BF2 Complex with a NiII Porphyrin.

[22]Smaragdyrin BF2 complex (S)-NiII porphyrin (P) fused hybrids were synthesized through Suzuki-Miyaura coupling of brominated S with borylated P and subsequent oxidative fusion reaction with FeCl3 as the first example of directly fused porphyrinoids bearing cycloheptatriene linkage. These fused hybrids have been fully characterized, including their X-ray structural analysis. Owing to the cycloheptatriene linkage, these fused hybrids adopt waving structures. These fused hybrids display red-shifted and enhanced Q-like bands and characteristic redox potentials, depending upon the constitution and the linking position.

Fused Hybrids of a [22]Smaragdyrin BF2 Complex with a NiII Porphyrin

Abstract[22]Smaragdyrin BF2 complex (S)‐NiII porphyrin (P) fused hybrids were synthesized through Suzuki–Miyaura coupling of brominated S with borylated P and subsequent oxidative fusion reaction with FeCl3 as the first example of directly fused porphyrinoids bearing cycloheptatriene linkage. These fused hybrids have been fully characterized, including their X‐ray structural analysis. Owing to the cycloheptatriene linkage, these fused hybrids adopt waving structures. These fused hybrids display red‐shifted and enhanced Q‐like bands and characteristic redox potentials, depending upon the constitution and the linking position.

Asymmetric Core-Expanded BOPYOs: Facile Synthesis and Optical Properties.

N,O-bidentate BF2 complexes with five- and six-membered core rings have been thoroughly investigated. However, the development of seven-membered N,O-boron complexes is still an area to be explored. We have developed BF3 ⋅ OEt2 -induced self-condensation and coordination reactions based on a single starting material, which had been elucidated by experiment and calculation. This parent asymmetric core-expanded borondifluoride-(Z)-1,3-di(1H-pyrrol-2-yl)but-2-en-1-one (BOPYO) showed reactivity with a wide range of aldehydes, thus providing a series of conjugation BOPYOs. Moreover, a BOPYO derivative with a dimethylamino group was developed as a new NIR dye that responds to acid with favorable photophysical properties based on intramolecular charge transfer effect.

BF3 Mediated [1,5]‐Hydride Shift Triggered Cyclization: Thioethers Join the Game

Abstract2‐(2‐(Benzylthio)benzylidene)malonates can undergo the 1,5‐hydride shift triggered cyclization resulting in thiachromanes in 45–84 % yield. Boron trifluoride as a reaction promotor is the key to success. DFT calculations revealed that the reaction proceeds via a chelate BF2 complex, which was confirmed by NMR‐analysis. Experimental and theoretical comparison of nitrogen, sulfur and carbon analogues revealed that sulfur derivatives have the highest activation barrier for the hydride transfer due to the unfavorable transition state geometry, which explains the lack of previous reports on 1,5‐hydride shifts in (alkylthio)styrenes.

3,6-Carbazoylene Octaphyrin (1.0.0.0.1.0.0.0) and Its Bis-BF2 Complex.

3,6-Carbazole precursors were used to prepare an octaphyrin. The conformation and electronic structure of the system could be modulated through trifluoroacetate (TFA) protonation and BF2 complexation. The resulting nonaromatic macrocyclic complexes, 2-2TFA and 2-2BF2, displayed noteworthy photophysical properties. For instance, the diprotonated species 2-2TFA showed a strong panchromic absorption up to 800 nm, while the bis-BF2-chelated dipyrromethene (BODIPY)-like complex 2-2BF2 exhibited an intense visible absorption feature (ε535nm = 2.1 × 105 M-1 cm-1), as well as a relatively red-shifted emission at 640 nm characterized by a large Stokes shift. It was found that 2-2BF2 could be used to construct a high-quality organic microlaser that functions under optical pumping. The present study highlights the potential utility of expanded porphyrins as possible laser dyes.

Deconstructive Cycloaromatization Strategy toward N,O-Bidentate Ligands from Indolizines and Cyclopropenones.

The innovative construction of novel N,O-bidentate ligands represents a long-standing challenge for chemists. Here, we report an unprecedented approach for the construction of N,O-bidentate derivatives via the merging of ring deconstruction with cycloaromatization of indolizines and cyclopropenones. Without any catalysts, our method can deliver a series of polyaryl 2-(pyridin-2-yl)phenols in excellent yields. In addition, N,O-bidentate organic BF2 complexes can also be constructed via this one-pot protocol.

Meso‐Monoaryl‐Substituted Neo‐confused Calixsmaragdyrins and Meso‐monoaryl Substituted Smaragdyrins: Synthesis, Structures and Properties

AbstractWhile SNAr reaction of 3,5‐dibromo‐8‐mesityl BODIPY 1 with a 5,10‐diaryltripyrrane with Cs2CO3 was reported to provide 5,10,19‐triaryl[22]smaragdyrin as the first synthetic method of all‐aza smaragdyrin, the reaction of 1 with meso‐free tripyrrane 6 took a different course to give a doubly Neo‐confused calixsmaragdyrin 7. On the other hand, the SNAr reaction of 2,3,5,6‐tetrabromo‐8‐mesityl BODIPY 9 with 6 provided 16,22‐dibromo‐19‐mesitylsmaragdyrin BF2 complex 10. In addition, calixsmaragdyirn 7 has been shown to serve as a ligand for RhI and BIII metals, and oxidation of 10 with MnO2 gave meso‐oxidized smaragdyrin 11 that was shown to be weakly antiaromatic.

Conformationally Distinct [26]Heterorubyrin(1.1.0.1.1.0) Macrocycles and Their Bis-BODIPYs: Synthesis, Structure, and Optical Properties.

Two conformationally different [26]rubyrin(1.1.0.1.1.0) macrocycles with varying heteroatoms (S/O) and their bis-BODIPYs are reported. The solid-state structure confirms O2N4 with fairly planar pyrrole-inverted conformation, whereas a thiophene-inverted structure for S2N4 is observed. Such conformational differences can also be clearly realized from their spectral and optical features. Upon BF2 complexation, both rubyrins led to their respective bis-BODIPYs where S2N4-BOD displayed a perfectly planar conformation as evident from its X-ray structure.

Synthesis, structure, and properties of palladium(ii) complex of α-formyl pyrrolyl dipyrromethene

A simple α-formyl pyrrolyl dipyrromethene ligand was synthesized by deboronation of a BF2 complex of α-formyl pyrrolyl dipyrrin under Lewis acid catalyzed conditions.

Donor-Acceptor Boron-Ketoiminate Complexes with Pendent N-Heterocyclic Arms: Switched-on Luminescence through N-Heterocycle Methylation.

A series of intramolecular, donor-stabilized BF2 complexes supported by phenanthridinyl-decorated, β-ketoiminate chelating ligand scaffolds is described, along with their characterization by spectroscopy and X-ray diffraction. In solution, the relative orientation of the pendent phenanthridinyl arm is fixed despite not coordinating to the boron center, and a well-resolved through-space interaction between a phenanthridinyl C-H and a single fluorine atom can be observed by 19F-1H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to nonradiative decay pathways enabled by rotation of the N-heterocyclic unit. Methylation of the phenanthridinyl nitrogen restricts this rotation, "switching on" comparably strong emission in solution. Modeling by density functional theory (DFT) and time-dependent DFT (TDDFT) indicates that the character of the lowest energy excitation changes upon methylation, with shallow calculated potential energy surfaces of the neutral complexes consistent with their lack of significant radiative decay.

Photophysical Properties and Electronic Structure of Symmetrical Curcumin Analogues and Their BF2 Complexes, Including a Phenothiazine Substituted Derivative

Symmetrically substituted curcumin analogue compounds possess electron donor moieties at both ends of the conjugated systems; their difluoroboron complexes were synthesized, and their structures were fully characterized. A novel compound with enhanced photophysical properties bearing phenothiazine moieties is reported. The introduction of BF2 into the molecular structures resulted in bathochromic shifts both in the absorption and emission spectra, indicating that the π-conjugation was more extended than the one in the initial compounds. The solvatochromic effects were studied, which in case of the phenothiazinyl-curcumin BF2 complex was the most notable. Theoretical study of the investigated compounds was carried out using DFT and TD-DFT methods to evaluate the ground state geometries and vertical excitation energies.

Engineering a tumor-specific and mitochondria targeted fluorescent probe for modulated autophagy and exploited anti-cancer therapy

Autophagy induced by mitophagy is related to numerous physiological and pathological processes, which also could represent a tumor-suppressor mechanism. The induction of apoptosis by persistent autophagy provides a new opportunity for cancer treatment. Herein, in this work, a biotinylated fluorescence probe (BO-biotin) based BF2 complex of curcumin analogues was designed and prepared successfully. The biotin-connected compound could target mitochondria in live cancer cells. We could visualize the complete mitophagy-related autophagy process induced by the curcumin analogues BO-biotin with the prolonged time. In addition, taking advantage of the analysis of the vital rate and expression of related proteins, the apoptotic assay results and mechanism investigation further verified that this compound could induce apoptosis of cancer cells by autophagy modulation. These findings demonstrate for the first time that the tumor-specific, mitochondria targeted and autophagy-modulating fluorescent probe can be employed as a new anticancer therapy.