BF2 Complexes Research Articles

The Role of Spacers as a Probe in Variation of Photoluminescence Properties of Mono- and Bi-Nuclear Boron Compounds.

A series of N,O donor-based mono- and binuclear four-coordinated boron complexes were synthesized. Depending on the substitution and spacer, these complexes exhibit intense blue, green and yellow emission in solution states. Notably, the fluorescence quantum yields (ΦF) and fluorescence decay (lifetime, τ) of mononuclear boron complexes (2 a-2 e) were higher than the binuclear boron complexes (2 f-2 k). The lowest lifetime and quantum yield in binuclear boron complexes were due to intramolecular rotation induced non radiative processes. The disulphide spacer-based boron complexes 2 i-2 k showed aggregation-caused quenching in the THF/H2O mixture whereas no other complexes were ACQ responsive. These complexes show large Stokes shift, one of them i. e. 2 e has the highest Stokes shift of 130 nm. Further, the electrochemical study suggests the presence of two redox incidences. Theoretical studies show close corroboration between the TD-DFT computed and experimentally measured absorption maxima as well as DFT (GIAO) calculated and experimentally measured 11B NMR values. This complements the appropriate selection of the theoretical methods to shed light on the electronic transitions in the mono- and binuclear BF2 complexes.

1-Azaazulene: A Ligand Moiety for Boron Difluoride (BF2) Complexes To Achieve Long-Wavelength Absorption.

1-Azaazulene was adopted as a new ligand moiety for boron difluoride (BF2) complexes, resulting in large red shifts of the absorption bands up to 103 nm compared to their isomeric quinoline-based complexes. Such change was attributed to the nonalternant nature of 1-azaazulene, as the complexes maintained the intrinsic 10π-peripheral electronic structure. 1a exhibited the most red-shifted absorption among reported BF2 complexes containing a phenolic backbone, and the absorption edge of 1b reached the red region.

Synthesis, Photophysical Properties and Cytotoxic Activities of Isoxazole‐Containing Difluoroboron Complexes

ABSTRACTA large series of previously unknown isoxazole containing difluoroboron β‐diketonate has been designed and synthesized from the available starting compounds. The photophysical properties of obtained compounds were studied, and the effects of the substituents in aromatic and isoxazole cycles of the BF2 complexes on the fluorescence were investigated. The effect of solvatochromism were demonstrated. Cytotoxic activity against MCF‐7 (breast carcinoma), HCT‐116 (colon carcinoma), A549 (pulmonary carcinoma) and WI38 (fibroblasts from lung tissue) cell lines was tested and moderate toxicity in the concentration range of 10–100 μM was found for several compounds.

5,18‐Dimesitylorangarin: a Stable Antiaromatic [20]Pentaphyrin(1.0.1.0.0) Displaying Remarkable Oxidative Self‐Coupling Reactions

Abstract5,18‐Dimesitylorangarin and its BF2 complex were synthesized by double SNAr reaction of 3,5‐dibromo‐BODIPY with 2‐pyrrolydipyrrin as the first examples of meso‐aryl‐substituted orangarin. These orangarins, delineated as [20]pentaphyrin(1.0.1.0.0), are strongly antiaromatic but rather stable. The free base orangarin was coupled by oxidation with MnO2 to give a 11,11′‐linked dimer, a cyclooctatetraene(COT)‐centered trimer, and a spiro‐trimer. Fused COT‐centered 3H‐orangarin dimer was oxidized to the corresponding 2H‐orangarin dimer, which was further coupled to give a triply COT‐centered 2H‐orangarin tetramer. 3H‐Orangarin oligomers are all antiaromatic as evinced by extremely low‐field‐shifted 1H NMR signals of the inner NH and ill‐defined absorption spectra with broad tails. In contrast, COT‐centered 2H‐orangarin dimer and tetramer show moderately low‐field‐shifted NH signals and intense NIR absorbance over 900 nm, suggesting effective π‐conjugation through the COT bridge and almost non‐antiaromatic character. These orangarin oligomers exhibit many reversible redox potentials owing to the intramolecular electronic interactions. Regardless of the different aromatic characters, all the orangarin monomers and oligomers exhibit very rapid excited‐state decays.

5,18-Dimesitylorangarin: a Stable Antiaromatic [20]Pentaphyrin(1.0.1.0.0) Displaying Remarkable Oxidative Self-Coupling Reactions.

5,18-Dimesitylorangarin and its BF2 complex were synthesized by double SNAr reaction of 3,5-dibromo-BODIPY with 2-pyrrolydipyrrin as the first examples of meso-aryl-substituted orangarin. These orangarins, delineated as [20]pentaphyrin(1.0.1.0.0), are strongly antiaromatic but rather stable. The free base orangarin was coupled by oxidation with MnO2 to give a 11,11'-linked dimer, a cyclooctatetraene(COT)-centered trimer, and a spiro-trimer. Fused COT-centered 3H-orangarin dimer was oxidized to the corresponding 2H-orangarin dimer, which was further coupled to give a triply COT-centered 2H-orangarin tetramer. 3H-Orangarin oligomers are all antiaromatic as evinced by extremely low-field-shifted 1H NMR signals of the inner NH and ill-defined absorption spectra with broad tails. In contrast, COT-centered 2H-orangarin dimer and tetramer show moderately low-field-shifted NH signals and intense NIR absorbance over 900 nm, suggesting effective π-conjugation through the COT bridge and almost non-antiaromatic character. These orangarin oligomers exhibit many reversible redox potentials owing to the intramolecular electronic interactions. Regardless of the different aromatic characters, all the orangarin monomers and oligomers exhibit very rapid excited-state decays.

Construction of Boron Difluoride Complexes with Asymmetric N,N'-Bidentate Ligands.

Boron difluoride (BF2) complexes with asymmetrical N,N'-bidentate ligands have received increasing attention due to their fascinating properties and broad applications. They are generally constructed in two steps: ligand formation, followed by boron complexation. This review focuses on categorizing these BF2 complexes based on the key synthetic strategies that have been applied in the ligand formation steps. The post-functionalization, properties and applications of different types of BF2 complexes are presented. Their challenges and opportunities are also discussed. This should help the future rational design and synthesis of BF2 complexes with intriguing properties and practical applications.

Ion-Pairing Assemblies of Anion-Responsive π-Electronic Systems That Have Noncovalently Assisted Expanded Planar Region.

Arylethynyl-substituted dipyrrolyldiketone BF2 complexes as anion-responsive π-electronic molecules exhibited characteristic electronic properties derived from conformation changes upon anion binding, which caused an increase in UV/vis absorption and associated two-photon absorption. The anion complexes showed expanded planar regions assisted by intramolecular interactions, resulting in charge-by-charge ion-pairing assemblies in the solid state.

Fused Aromatic and Antiaromatic Smaragdyrin Dimers

AbstractSingly‐linked aromatic [22]smaragdyrin BF2 complex dimer was synthesized by the reductive coupling of 16‐brominated [22]smaragdyrin BF2 complex, which was oxidized to a stable diradical with PbO2. As the first example of fused smaragdyrin dimer, a fused [22]smaragdyrin BF2 complex dimer was synthesized by the oxidation of a CuCl‐BF2 complex dimer with FeCl3 and subsequent reduction with NaBH4. After removal of the BF2 group, the singly‐linked and fused aromatic dimers were oxidized to the corresponding antiaromatic [20]smaragdyrin free base dimers. The first oxidation and reduction potentials of these dimers are split depending upon the intramolecular electronic interactions, which are larger for the fused dimers. Despite the large electronic interactions, the aromatic and antiaromatic characters are well preserved in the fused dimers.

Fused Aromatic and Antiaromatic Smaragdyrin Dimers.

Singly-linked aromatic [22]smaragdyrin BF2 complex dimer was synthesized by the reductive coupling of 16-brominated [22]smaragdyrin BF2 complex, which was oxidized to a stable diradical with PbO2. As the first example of fused smaragdyrin dimer, a fused [22]smaragdyrin BF2 complex dimer was synthesized by the oxidation of a CuCl-BF2 complex dimer with FeCl3 and subsequent reduction with NaBH4. After removal of the BF2 group, the singly-linked and fused aromatic dimers were oxidized to the corresponding antiaromatic [20]smaragdyrin free base dimers. The first oxidation and reduction potentials of these dimers are split depending upon the intramolecular electronic interactions, which are larger for the fused dimers. Despite the large electronic interactions, the aromatic and antiaromatic characters are well preserved in the fused dimers.

Fluorescent BF2 Complexes Inspired by Ageladine A

AbstractThe pH‐dependent UV/Vis absorption and fluorescence of several analogs of the marine natural product ageladine A were investigated. Besides ageladine A itself, its BF2 complex and the corresponding debrominated analogs were included, as were the chloro analog of the debromoageladine‐BF2 complex, an imidazolone analog, and a triazole derivative, which lack the free amino group. Importantly, the pH‐dependence of the fluorescence of the chloro analog closely resembles that of the natural product ageladine A, but with about 25‐fold higher fluorescence intensity and a maximum between pH 3 and 6. The imidazolone derivative behaves in a complementary manner and shows fluorescence only above pH 6, but with less intensity than ageladine A. The calculated Stokes shifts agree well with the experiment, which underlines the accuracy of the DFT calculation method (ωB97XD/TApr‐cc‐pVDZ, PCM). The conformational fixation of arylated imidazo[4,5‐c]pyridines by BF2 units seems to be promising with regard to the development of novel, pH‐dependent fluorescent probes.

BF2 complexes of pyridyl‐isoindoline‐1‐ones as an efficient corrosion inhibitor for mild steel in 1 M HCl and the application for surgical instruments anticorrosion

AbstractIn this study, BF2 complexes of pyridyl‐isoindoline‐1‐ones (BPIO) has been firstly designed and investigated as an excellent corrosion inhibitor for the mild steel in 1 M HCl. The corrosion protection properties of BPIO were studied by using a series of experiments. The results indicated BPIO has excellent inhibition performance, and inhibition efficiency of BPIO reached up to 96.6%. The effects of immersion temperature and time were investigated by weight loss experiments to evaluate the stability of adsorbed BPIO film in protecting steel surface. Based on potentiodynamic polarization studies, BPIO acted as one mixed‐type corrosion inhibitor with predominant anodic effectiveness, and its adsorption on the mild steel follows the Langmuir adsorption isotherm. The values of ΔGads suggested the adsorption of BPIO on the mild steel surface was through a combination of chemisorption and physisorption. The adsorption of BPIO molecules on the steel surface was further identified by the techniques of scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS). The quantum chemical calculations based on density functional theory (DFT) and molecular dynamics (MD) simulations were used to optimize the BPIO molecular structure and investigate the inhibitive properties on the theoretical level, which agreed well with the experimental results. Besides, BPIO has been used for the development of water soluble metal antirusting agent.

A-Frame-Templated High-Coordinate Platinum(IV) cis-Bis(boryl) Complexes.

The addition of Et2O·BF3 or Me2S·BCl3 to the BNBN-cumulene-bridged Pt(II) A-frame complexes [(μ-1,1-BNBN(TMS)2)(μ-dmpm)2Pt2X2] (TMS = SiMe3, dmpm = CH2(PMe2)2, X = Br 1Br, I 1I) resulted in the oxidative addition of one B-F or B-Cl bond, respectively, to the internal BN bond of the bridging, iminoborane-like B-N≡B-N moiety, and coordination of one Pt(II) center to the resulting adjacent BF2 (complex 2Br-F) or BCl2 (complexes 2Br-Cl and 2I-Cl) moiety, respectively. X-ray crystallographic and multinuclear NMR-spectroscopic data show that the Pt→BF2 interaction in 2Br-F is very weak and merely electrostatic, while the Pt→BCl2 interaction in 2Br-Cl and 2I-Cl is a stronger donor-acceptor bond. In contrast, the reaction of Me2S·BBr3 with 1Br yielded a ca. 3:2 mixture of the analogous B-Br addition product to the iminoborane, 2Br-Br, and the product of a subsequent oxidative addition of one B-Br bond of the chelating BBr2 moiety to the adjacent platinum center, the mixed-valence boranediyl-bridged, Pt(II)-Pt(IV)-bromoboryl complex 3-Br5. The analogous reactions of Me2S·BI3 with 1Br and Me2S·BBr3 with 1I yielded complex product mixtures of Pt(II)-Pt(II)-borane (2Br-I and 2I-Br, respectively) and Pt(II)-Pt(IV)-boryl complexes (3-BrnI5-n, n = 1-3) analogous to 2X-Y and 3-Br5, respectively, the proportion of the latter increasing with the proportion of iodide in the precursor mixture. Both multinuclear NMR-spectroscopic and X-ray crystallographic data show evidence of complex and extensive inter- and intramolecular bromide-iodide exchanges between the soft, iodide-affine platinum centers and the harder, more bromide-affine boron centers. A clue to the mechanism of these halide exchanges is provided by the reactions of BBr2Ar (Ar = 2,4,6-Me3C6H2 (Mes), 2,3,5,6-Me4C6H (Dur)) with 1Br, which yielded the cationic Pt(II)-Pt(II)-borenium analogues of 2Br-Br, the complexes 4Br-Ar, generated by the sterics-induced displacement of the bromide substituent from the chelating Pt→BBrAr moiety, and displaying a rare metal→borenium donor-acceptor bond.

Stabilization of the Neutral [25]Hexaphyrin(1.0.1.0.1.0) Radical by Hetero-Bimetal-Coordination.

Stabilization of hexaphyrin(1.0.1.0.1.0) (named "rosarin") in its 25π radical state is achieved using a hetero-bimetal-coordination strategy. The antiaromatic BF2 complex B-1 was first synthesized, and then rhodium ion was inserted into B-1 to produce the BF2/Rh(CO)2 mixed complex Rh-B-1 as a highly air-stable radical. The structures of B-1 and Rh-B-1 were determined by single-crystal X-ray diffractions, and the antiaromatic or radical character was identified by various spectroscopy evidence and theoretical calculations. Rh-B-1 exhibits excellent redox properties, enabling amphoteric aromatic-antiaromatic conversion to their 24/26π states. Compared to the 24/26π conjugation systems on the same skeleton, Rh-B-1 has the narrowest electrochemical and optical band gaps, with the longest absorption band at 1010 nm. The ring-current analysis reveals intense paratropic currents for B-1 and co-existing diatropic-paratropic currents for Rh-B-1. This hetero-bimetal-coordination system provides a novel platform for organic radical stabilization on porphyrinoids, showing the prospect of modulating ligand oxidation states through rational coordination design.

Styryl-extended Azopyrroles and their boron difluoride complexes: Synthesises, structures, spectra, and DFT calculation

Three styryl-extended azopyrrole compounds (2–4) with different substituents (-H, -NO2, and -N(CH3)2) have been synthesized by Knoevenagel condensation reaction between azopyrrole and substituted benzaldehyde. Their BF2 complexes (5–7) were also synthesized. The planar structure of compound 3 was characterized by X-ray crystallographic study. Hirshfeld surface analysis reveals that van der Waals interactions and hydrogen-bonding are the dominant interactions in the crystal packing. All compounds have λmax absorption greatly red-shift even to deep-red light region (646 nm) due to the extended π-conjugation by styryl group and the introduction of BF2 group. The structures, frontier molecular orbitals (FMOs), energy gaps (ΔE) and UV–Vis spectra of these compounds were evaluated by DFT calculations. The results reveal that the styryl, electron donating and withdrawing groups, and BF2 group all can reduce the energy gap (ΔE) of the compounds, and finally cause their long-wavelength absorption.

N,O-Bidentated difluoroboron complexes based on pyridine-ester enolates: Facile synthesis, post-complexation modification, optical properties, and applications

An array of pyridine-ester enolate based organoboron complexes has been designed and synthesized via a one-pot cascade of Pd-catalyzed α-arylation and BF2 complexation. The rapid structure-activity relationship (SAR) studies indicated that unsymmetrical N,O-chelated BF2 complexes were highly fluorescent in solid state, and exhibited large Stokes shifts, excellent photostability, along with insensitivity to pH. The α-aryl group could not only modulate the electronic effect but also inhibit the intermolecular π-π stacking to promote the aggregation-induced emission (AIE) effect. DFT calculations and experiments identified that the intramolecular charge transfer properties of these N,O-chelates could be switched by the modification of substituents, resulting tunable fluorescence wavelengths. Furthermore, post-complexation modification was accomplished, including Suzuki-Miyaura cross-coupling, Buchwald-Hartwig amination, oxidative cleavage, along with a unique triple substitution reaction involving propargyl Grignard reagents. The exemplificative application of dimethylamine substituted boron complex as a reversible acidic vapor sensor was also demonstrated.

Mechanistic insights into the AIE properties of BF2 complexes of N-acetyl 2-aminobenzothiazoles as rotorless polyheteroaromatics

A series of four novel BF2 complexes of N-acetyl 2-aminobenzothiazoles featuring rigid rotorless polyheteroaromatic structures have been designed and synthesized. All of these compounds exhibited AIE properties with excellent solid-state fluorescence quantum yields up to 81.7%. Theoretical calculations revealed the characteristics of their ground-state structures and excitation processes. Their seemingly abnormal nonradiative decay in solutions was investigated from both direct vibrational relaxation and S0/S1 surface crossing perspectives by the two-channel model. The minimum energy conical intersection (MECI) located by SF-TDDFT method indicated that the CI is mainly induced by the deformation of the BF2 complex ring, leads to elevated energy level, and thus is not responsible for the fluorescence quenching. The potential energy curve (PEC) constructed along the distortion angle θ of BF2 complex ring showed that when θ reaches 50° (the dark-state region approaching MECI), the energy barrier is only 0.11 eV, suggesting that the deformed dark state could serve as an effective deactivation channel via ultrafast internal conversion (IC). Crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle could effectively prohibit detrimental π-π stacking and enhance intermolecular interactions, thereby resulting in efficient solid-state emission.

Harnessing reductive BF2-complexation via Ru(II)-catalyzed N-O cleavage of isoxazoles.

Developed here is a highly fluorescent organic N,O bidentate BF2 complex from isoxazole in the presence of a Ru(II) catalyst. Herein, the complexation proceeds via a selective N-O cleavage of the isoxazole ring. The complex shows absorption (λmax,abs) in the range of 352-363 nm with an extinction coefficient (ε) in the range of 8000-64 000 M-1 cm-1, and fluorescence emission (λmax,em) in the range of 413-485 nm with a Stokes shift of 61-125 nm having quantum yield up to 33%. Apart from the solution state, the solid BF2 complex 2u exhibits absorption at 405 nm and strong fluorescence emission at 550 nm with a quantum yield of 26.9%.

The unprecedented strong paratropic ring current of a bis-PdII complex of 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0).

Acid-catalyzed Friedel-Crafts-type cyclization of tetrapyrrolic BF2 complex 1 and α,α'-dibromotripyrrin 2 gave 5,10,23-trimesityl [28]heptaphyrin(1.1.0.0.1.0.0) BF2 complex 3BF2 as a stable and moderate antiaromatic macrocycle. Demetalation of 3BF2 with methanesulfonic acid followed by treatment with HCl gave free-base salt 3HCl that holds a chloride anion at the core. This salt displays a planar structure with an inverted pyrrole and a stronger paratropic ring current. Metalation of neutral free-base 3 with PdCl2 gave bis-PdII complex 3Pd2 as a stable antiaromatic molecule. The 1H NMR spectrum of 3Pd2 displays signals due to pyrrolic β-protons in the range of -1.06 ∼ -1.90 ppm, indicating the unprecedented strong paratropic ring current.

Mechanistic insights into diversified photoluminescence behaviours of BF2 complexes of N-benzoyl 2-aminobenzothiazoles.

Many BF2 complexes of heteroaromatics are well known for their dual-state emission (DSE) properties. However, AIE and ACQ effects have also been observed in certain cases. To date, no rational explanations have been proposed for these uncommon photoluminescence (PL) behaviours. The current research prepared four BF2 complexes of N-benzoyl 2-aminobenzothiazoles with diversified photoluminescence (PL) properties as model compounds and utilized quantum chemical calculation tools to address this issue. Theoretical calculations revealed that the electron-donating groups (EDGs) at the para-position of the exocyclic phenyl ring exert significant influence on their ground-state electronic structures and vertical excitation features. Potential energy curve (PEC) analysis showed that the exocyclic phenyl ring and NMe2 could not function as effective rotors due to elevated energy barriers. Only the NPh2 of BFBB-3 could spontaneously rotate ∼60° to induce the formation of an emissive twisted intramolecular charge transfer (TICT) state. The two-channel model involving both vibronic relaxation and S0/S1 surface crossing revealed that the drastic narrowing of the S1/S0 energy gap in the region approaching minimun energy conical intersection (MECI) led to the generation of a dark state in BFBB-1. The small energy barrier to access the dark-state region makes the resulting fast internal conversion a competitive channel for excited-state deactivation. In contrast, the presence of EDGs in BFBB-2 and 4 inhibits this pathway, thereby resulting in intense fluorescence emissions in solution. In addition, crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle promoted a slipped face-to-face packing mode and enhanced intermolecular interactions. The efficiencies of their solid-state emissions are mainly affected by the degree of π-π overlaps.

Nanographene-Fused Expanded Carbaporphyrin Tweezers.

A nanographene-fused expanded carbaporphyrin (5) and its BF2 complex (6) were synthesized. Single-crystal X-ray structures revealed that 5 and 6 are connected by two hexa-peri-hexabenzocoronene (HBC) units and two dipyrromethene or BODIPY units, respectively. As prepared, 5 and 6 both show nonaromatic character with figure-of-eight carbaoctaphyrin (1.1.1.0.1.1.1.0) cores and adopt tweezers-like conformations characterized by a partially confined space between the two constituent HBC units. The distance between the HBC centers is >10 Å, while the dihedral angles between the two HBC planes are 30.5 and 35.2° for 5 and 6, respectively. The interactions between 5 and 6 and fullerene C60 were studied both in organic media and in the solid state. Proton NMR spectral titrations of 5 and 6 with C60 revealed a 1:1 binding mode for both macrocycles. In toluene-d8, the corresponding binding constants were determined to be 1141 ± 17 and 994 ± 10 M-1 for 5 and 6, respectively. Single-crystal X-ray diffraction structural analyses confirmed the formation of 1:1 fullerene inclusion complexes in the solid state. The C60 guests in both complexes are found within triangular pockets composed of two HBC units from the tweezers-like receptor most closely associated with the bound fullerene, as well as an HBC unit from an adjacent host. Femtosecond transient absorption measurements revealed subpicosecond ultrafast charge separation between 5 (and 6) and C60 in the complexes. To the best of our knowledge, the present report provides the first example wherein a nanographene building block is incorporated into the core of a porphyrinic framework.