# BF2 Complexes of N-arylated 1-amino-anthraquinone, -xanthone, -thioxanthone, and Related Compounds: Synthesis and Properties

Ahlem N. Khelf-maghraoui, Ghenia Bentabed-Ababsa, Nicolas Mast, William Erb, Jean-Pierre Hurvois, Thierry Roisnel, Nicolas Richy, Guillaume Calvez, Olivier Mongin, Charline Piroud, et al.

► To cite this version:

Ahlem N. Khelf-maghraoui, Ghenia Bentabed-Ababsa, Nicolas Mast, William Erb, Jean-Pierre Hurvois, et al.. BF2 Complexes of N-arylated 1-amino-anthraquinone, -xanthone, -thioxanthone, and Related Compounds: Synthesis and Properties. European Journal of Inorganic Chemistry, 2025, pp.e202500470. 10.1002/ejoc.202500470. hal-05408484

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Submitted on 10 Dec 2025

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# BF₂ Complexes of N-arylated 1-amino-anthraquinone, -xanthone, -thioxanthone, and Related Compounds: Synthesis and Properties

Ahlem N. Khelf-Maghraoui, Ghenia Bentabed-Ababsa,* Nicolas Mast, William Erb, Jean-Pierre Hurvois, Thierry J. Roisnel, Nicolas Richy, Guillaume Calvez, Olivier Mongin, Charline Piroud, Thomas Robert, Stéphane Bach, and Florence Mongin*

The physical properties of anthraquinones can be modulated by the introduction of arylamino groups next to the carbonyl function. Indeed, formation of an intramolecular hydrogen bond can enhance atom polarization and thus reduce their highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap, resulting in light absorption at longer wavelength. In this study, it successfully prepares different arylamino derivatives of anthraquinone, xanthone, and thioxanthone, compounds that are readily converted to their respective boracycles by

reaction with BF₃·Et₂O. The effect of the BF₂ moiety on the electronic properties is evaluated by electrochemical and photophysical studies. These studies respectively reveal for the complexes easier reduction and red-shifted absorption wavelengths. In addition, promising 8% fluorescence quantum yield and 52% singlet oxygen quantum yield are recorded for one of the diboron-anthraquinone complexes. Representative compounds are finally tested on a panel of disease-related protein kinases.

## 1. Introduction

Anthraquinones are a family of polycyclic compounds built around a quinone unit, widely distributed in nature, and endowed with biological activity. Chrysophanol (Figure 1, compound A), which has been shown to have antibacterial, antiviral, anti-inflammatory, antidiabetic, and antitumor properties, is a good example.[1]

The high affinity of anthraquinone derivatives for electrons has led to their use in many applications. Capable of undergoing reversible two-electron reduction, they have been widely

investigated as cathode materials in batteries.[2] Their various applications based on redox behavior also include their use as electron shuttle in the bioreduction of organic pollutants[3] and the carbon dioxide capture capacity of the quinone dianion.[4]

Due to their push-pull structure, anthraquinone derivatives such as alizarin and disperse blue 1 (hydroxy or amino groups; Figure 1, compounds B and C) have been known for decades for their dyeing properties.[5] The absorption spectra of these π-conjugated molecules can also be finely tuned; for example, the introduction of appropriate substituents can reduce their

A. N. Khelf-Maghraoui, N. Mast, W. Erb, J.-P. Hurvois, T. J. Roisnel, N. Richy, G. Calvez, O. Mongin, F. Mongin

Univ Rennes

INSA Rennes

CNRS

ISCR (Institut des Sciences Chimiques de Rennes)

UMR 6226

F-35000 Rennes, France

E-mail: florence.mongin@univ-rennes.fr

A. N. Khelf-Maghraoui, G. Bentabed-Ababsa

Laboratoire de Synthèse Organique Appliquée

Faculté des Sciences Exactes et Appliquées

Université Oran 1 Ahmed Ben Bella

BP 1524 El M'Naouer, 31000 Oran, Algeria

E-mail: bentabedg@gmail.com

C. Piroud, T. Robert, S. Bach

Sorbonne Université

CNRS

UMR8227

Integrative Biology of Marine Models Laboratory (LBI2M)

Station Biologique de Roscoff, F-29680 Roscoff, France

C. Piroud, T. Robert, S. Bach

Sorbonne Université

CNRS

FR2424

Plateforme de criblage KISSf (Kinase Inhibitor Specialized Screening facility)

Station Biologique de Roscoff, F-29680 Roscoff, France

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/ejoc.202500470

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© 2025 The Author(s). European Journal of Organic Chemistry published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Structures of chrysophanol (A), alizarin (B), disperse blue 1 (C) and anthraquinone (D). N-arylated 1,4-, 1,5- and 1,8-diaminoanthraquinones (E) studied previously, and BF₂ complexes of anthraquinone, xanthone and thioxanthone (F, G and H, respectively) investigated in the present study.</figcaption></figure>

HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap, resulting in the absorption of light at longer wavelengths.[6] This has led to the successful development of dyes for challenging applications such as light-driven production of hydrogen using a wider fraction of the solar spectrum.[7,8]

The anthraquinone electron-accepting ability can be further enhanced by atom polarization through weak interactions such as intramolecular hydrogen bonding.[9] This was nicely exploited by Takeda and coworkers who synthesized a library of N-arylated 1,4-, 1,5-, and 1,8-diaminoanthraquinones (Figure 1, compounds E). In this study, they found that it was possible to tune the color (absorption from the visible 350–650 nm to the near infrared 650–1000 nm) by controlling the N–H···O=C weak bond (and therefore the frontier orbitals) through an appropriate choice of substituents (e.g., absorption at λ<sub>max</sub> 559 nm in the case of 1,8-di(4-tolyl) vs. 434 nm for 1,8-di(4-tolylsulfonyl)).[10]

The incorporation of BF₂ into organic molecules can improve their photophysical properties by extending light absorption to a wider range of wavelengths,[11] which can be useful for applications in fields such as biology and medicine.[12] However, while such boron complexes have been documented for other aromatic ketones containing an adjacent electron-donating group (e.g., N-substituted 9-aminophalenones[13,14] and 1-hydroxy-10-phenylacridone),[15] examples in the anthraquinone series are surprisingly lacking.

As part of a project on the synthesis of polycyclic compounds, we reacted 1,8-di(arylamino)anthraquinones with BF₃·Et₂O[16] in order to access ceramidonines. Instead of the desired product, we observed the formation of complexes with BF₂. This unexpected result sparked our interest in this family of compounds, and here we report the synthesis and properties of BF₂ complexes in the anthraquinone, xanthone,

and thioxanthone series (Figure 1; compounds F, G and H, respectively).

## 2. Synthesis of the BF₂ Complexes

### 2.1. Preliminary Studies

As mentioned above, we initially attempted the cyclization of 1,8-di(4-anisylamino)- and 1,8-di(4-tolylamino)anthraquinones (1a, b) under the conditions reported for a similar reaction from 2-(arylamino)benzophenones.[16] However, by using BF₃·Et₂O in toluene at 85 °C for 0.5 h, as indicated, the expected 2-substituted ceramidonines were not obtained. Instead, BF₂ complexes F-1a (unambiguously identified by X-ray diffraction) and F-1b were isolated in respective yields of 51% and 50% (Scheme 1, top). Although reactive, the ketone function of anthraquinone is less suited to intramolecular cyclization because the presence of the quinone ring makes it more difficult for the molecule to adopt a suitable conformation. This is probably why ceramidonines are obtained from 2-(arylamino)benzophenones and BF₂ complexes from 1-(arylamino)anthraquinones.

At the time, the 1,8-di(arylamino)anthraquinones 1a, b required were prepared from commercially available 1,8-diaminoanthraquinone by copper-mediated double N-arylation using aryl iodides, in dimethylsulfoxide (DMSO) at high temperature. This protocol, inspired by the work of Hellwinkel and Ittemann,[17] is regularly used in our group to access related compounds.[18–23] The single N-methylation of 1a using NaH and iodonethane in tetrahydrofuran (THF) was also attempted, but only afforded the N-methylated and the N,N'-dimethylated derivatives 2a1 and 2a2 in modest yields (Scheme 1, bottom).

As shown in Figure 2, the introduction of arylamino substituents into the anthraquinone structure reduces the HOMO-LUMO
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Scheme 1.** Top: unsuccessful cyclization of the 1,8-di(4-anisylamino)- and 1,8-di(4-tolylamino)anthraquinones (1a, b) to 2-methoxylated and 2-methyl-ated 13-(arylamino)ceramidonines (left); unexpected formation of the BF₂ complexes F-1a and F-1b; ORTEP diagram (30% probability) of F-1a (selected distances [Å]: B1-F2 1.367, B1-F3 1.390, B1-N4 1.521, B1-O12 1.508, N4-C5 1.358, C11-O12 1.310, O12-H21 1.897, C13-O14 1.218, C19-N21 1.362, O12-H21 1.897, H21-N21 0.845; torsion angles [°]: B1-N4-C5-C10 -18.92, B1-O12-C11-C10 14.44) (right).<sup>[44]</sup> Bottom: copper-mediated synthesis of 1a, b from 1,8-diaminoanthraquinone; N-methylation of 1a (right); ORTEP diagram (30% probability) of 2a1 (selected distances [Å]: O1-C2 1.224, C3-O4 1.235, O4-H37 1.958, C36-N37 1.366, H37-N37 0.869) (left).<sup>[44]</sup></figcaption></figure>

Since complexation with BF₂ should modify the electronic properties of the anthraquinone core, we decided to prepare a small library of such complexes and evaluate their

properties. However, concerned by the low yields recorded for 1a, b so far, we decided to turn to Buchwald-type coupling to generate these (arylamino)anthraquinones.

## 2.2. Synthesis of the (arylamino)anthraquinone Precursors

Thus, both compounds were prepared from commercially available 1,8-dichloroanthraquinone by applying a protocol developed by Beletskaya, Guilard, and coworkers.[24] Using the required 4-substituted aniline (3 equiv) in the presence of Cs₂CO₃ (3 equiv), catalytic Pd(dba)₂ (dba = dibenzylideneacetone), and dppf (dppf = 1,1'-bis(diphenylphosphino)ferrocene), the expected products 1a, b were isolated in much higher yields (Scheme 2, top). We therefore decided to apply the same protocol for the high-yielding synthesis of the 1-(arylamino)anthraquinones 3a-c and 1,5-di(arylamino)

<figure><img src="image_9.png" /><figcaption>**Scheme 2.** Top: palladium-catalyzed synthesis of 1a, b from 1,8-dichloroanthraquinone. Bottom: conversion of 1-chloro- and 1,5-dichloroanthraquinone to the 1-(arylamino)anthraquinones 3a-c and 1,5-di(arylamino)anthraquinone 4b, respectively.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 2.** HOMO-LUMO energy gaps of D, 1-(4-tolylamino)-, 1,5-di(4-tolylamino)- and 1,4-di(4-tolylamino)anthraquinones (3b, 4b and 5b; Ar = 4-tolyl), all determined by Hückel theory calculations as implemented in HuLiS.[45]</figcaption></figure>
<figure><img src="image_11.png" /></figure>

anthraquinone 4b from 1-chloro- and 1,5-dichloroanthraquinone, respectively (Scheme 2, bottom).

### 2.3. Conversion of the Synthesized (arylamino) anthraquinones to BF₂ Complexes

In order to generate the desired BF₂ complexes of (arylamino) anthraquinones, we tested the protocol employed in the N-substituted 9-aminophenalenone series.[13,14] We therefore reacted the 1-(arylamino)anthraquinones 3a-c with BF₃·Et₂O (1.2 equiv) in toluene at rt for 2 h before refluxing for 12 h (Scheme 3, top). Under these conditions, the BF₂ complexes F-3a to F-3c were isolated in yields dependent on the electron richness of the aryl (63% for the 4-anisyl F-3a, 49% for the 4-tolyl F-3b and 33% for the 4-(trifluoromethyl)phenyl F-3c), the rest being mainly the starting 1-(arylamino)anthraquinone.

Compound F-3b was unambiguously identified by X-ray diffraction (Scheme 3, bottom). Unlike F-1a, which displays an out-of-the-plane BF₂ group in a tricyclic unit that is slightly curved due to the presence of the arylamino in position 8 (see Scheme 1, top, and Supporting Information), the tetracyclic core of F-3b is completely planar.

From the 1,5-di(arylamino)anthraquinone 4b, the single boracycle F-4b1 was mainly formed provided the amount of BF₃·Et₂O was reduced to 0.90 equivalent (53% yield), while the double boracycle F-4b2 was isolated in 66% yield in the presence of an excess of reagent (Scheme 4, top). Similarly, the commercially available 1,4-di(arylamino)anthraquinone 5b could afford either the single boracycle F-5b1 (91% yield) or the double boracycle F-5b2 (99% yield). Both products were unambiguously identified by X-ray diffraction (Scheme 4, middle). Compared with F-1a and its out of the plane BF₂ group of the tricyclic unit, and F-3b which is almost planar, these BF₂ complexes F-5b1 and F-5b2 show

<figure><img src="image_12.png" /><figcaption>**Scheme 3.** Top: conversion of 3a-c to the corresponding BF₂ complexes F-3a to F-3c. Bottom: ORTEP diagram (30% probability) of F-3b (selected distances [Å]: B1-F2 1.377, B1-F3 1.367, B1-O4 1.498, B1-N20 1.544, C5-O5 1.286, C12-O13 1.220, C18-N20 1.335; torsion angles [°]: B1-N20-C18-C19 0.92, B1-O4-C5-C19 -0.58).[44]</figcaption></figure>

intermediate tricyclic cores, therefore slightly curved with an out-of-plane boron unit. In contrast, the use of commercially available 1-hydroxy-4-(4-tolylamino)anthraquinone (6b) to reach

<figure><img src="image_13.png" /><figcaption>**Scheme 4.** Top: conversion of 4b to the corresponding BF₂ complexes F-4b1 and F-4b2. Middle: conversion of the 1,4-di(arylamino)anthraquinone 5b to the corresponding BF₂ complexes F-5b1 and F-5b2, and ORTEP diagrams (30% probability) of F-5b1 (selected distances [Å]: B1-F2 1.370, B1-F3 1.389, B1-N4 1.548, B1-O12 1.471, N4-C24 1.334, O12-C13 1.308, C20-O21 1.249, O21-H28 1.846, C27-N28 1.348; torsion angles [°]: B1-N4-C24-C23 -2.65, B1-O12-C13-C23 7.65) and F-5b2 (selected distances [Å]: B51-F72 1.368, B51-F73 1.375, B51-N54 1.570, B51-O76 1.475, N54-C62 1.319, C67-O77 1.322; torsions angles: B41-N54-C62-C75 -9.63, B51-O76-C74-C75 11.80, B78-O77-C67-C66 -10.43, B78-N81-C65-C66 6.72).[44] Bottom: conversion of 1-hydroxy-4-(4-tolylamino)anthraquinone (6b) to the corresponding BF₂ complexes F-6b2.</figcaption></figure>
<figure><img src="image_14.png" /></figure>

a single borcycle proved more tricky. As mentioned before, the use of an excess of BF₃·Et₂O favored the formation of the double boracycle **F-6b2** in an almost quantitative yield; however, it was found to be less stable during purification stages (Scheme 4, bottom).

## 2.4. Synthesis of 1-(arylamino)xanthones and- thioxanthones, as well as their BF₂ Complexes

Since instability has been shown to be a characteristic of certain anthraquinone-based borcycles, we sought to obtain more stable complexes by increasing the coordination of the organic fragment to BF₂. In this context, we became interested in more electron-rich cores. Thus, we reacted 1-iodo-xanthone and -thioxanthone with 4-anisidine, 4-toluidine and 4-(trifluoromethyl)aniline to give the 1-(arylamino) derivatives **7a-c** and **8a-c**, all isolated in high yields (Scheme 5, top).

We then attempted to prepare their respective BF₂ complexes using the same conditions as for the anthraquinone series (see Scheme 3). A general trend emerged from these experiments. Higher yields were invariably obtained from electron-enriched arylamino groups, both in the xanthone (products **G-7a** and **G-7b**) and in the thioxanthone series (products **H-8a** and **H-8b**), with yields even almost quantitative for the latter. On the other hand, the presence of a trifluoromethyl in position 4 of the arylamino led to low yields of **G-7c** and **H-8c** (Scheme 5, bottom), possibly indicating that both a coordinating C=O function and an electron-enriched arylamino group are required. With regard to the stability of the complexes, we also found that those from the 1-(arylamino)thioxanthones **8a** and **8b** were the most stable of all (superior to those based on the 1-(arylamino)xanthones **7a** and **7b**), along with those derived from 1,4-di(4-tolylamino)anthraquinone (**5b**).

## 3. Electrochemical Properties

Anthraquinone-based molecules are attractive structures for energy storage,[25] and also able to interact with biological structures (e.g., enabling electrochemical detection of DNA).[26] As complexation to BF₂ is expected to modify the electronic properties of the heterocyclic ketones, we next explored their behavior in electrochemical reduction and selected the complexes **F-1a**, **F-1b**, **F-3a**, **F-3b**, **F-3c**, **F-4b1**, **F-5b1**, **F-4b2**, and **F-5b2**. For comparison purposes, we also included the (arylamino)anthraquinones **1a**, **1b**, **3b**, **4b**, and **5b** in our study. These compounds were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in degassed DMF, using Ag/AgCl as the reference electrode and nBu₄NPF₆ as the supporting electrolyte. The main data are gathered in Table 1. All the first reduction peaks corresponding to the formation of the radical anion were found to be reversible, with an *i*<sub>pc</sub>/i<sub>pa</sub> ratio close to unity.

In a first round of experiments, the three borcycles **F-3a**, **F-3b**, and **F-3c**, bearing R groups with different electronic effects (OMe, Me, and CF₃), were compared. For the first and second

anthraquinone reduction peaks, anodic shifts were observed from methoxy to methyl and, above all, trifluoromethyl. The results obtained for **F-3b** were next compared with those of the BF₂-free compound **3b**. The redox potentials of this 1-(arylamino)anthraquinone **3b** were found close to those of bare anthraquinone (D). As might be expected given the electron-withdrawing nature of BF₂, moving from **3b** to the complex **F-3b** made first and second reduction easier, with potentials shifted by 0.56 and 0.48 V, respectively. A similar trend was observed in the 1,8-di(arylamino)anthraquinone series. Thus, from **1a** or **1b** to the BF₂ complexes **F-1a** and **F-1b**, the reduction potential values were shifted by 0.46–0.49 and 0.40–0.41 V.

<figure><img src="image_15.png" /></figure>

**7a** (X = O, R = OMe): 93%

**7b** (X = O, R = Me): 82%

**7c** (X = O, R = CF₃): 90%

**8a** (X = S, R = OMe): 90%

**8b** (X = S, R = Me): 96%

**8c** (X = S, R = CF₃): 95%

<figure><img src="image_16.png" /></figure>

**G-7a** (X = O, R = OMe): 60%

**G-7b** (X = O, R = Me): 51%

**G-7c** (X = O, R = CF₃): 18%

**H-8a** (X = S, R = OMe): 97%

**H-8b** (X = S, R = Me): 96%

**H-8c** (X = S, R = CF₃): 13%

<figure><img src="image_17.png" /></figure>

8a

8b

<figure><img src="image_18.png" /></figure>

H-8a

H-8b

**Scheme 5.** Top: palladium-catalyzed synthesis of the 1-(arylamino)xanthones **7a-c** and -thioxanthones **8a-c** from 1-iodoxanthone and -thioxanthone, respectively, and conversion to the corresponding BF₂ complexes **G-7a-c** and **H-8a-c**. Bottom: ORTEP diagrams (30% probability) of **8a** (selected distances [Å]: C7-O8 1.240, O8-H15 1.849, C14-N15 1.373, H15-N15 0.831), **8b** (selected distances [Å]: C7-O8 1.241, O8-H15 1.836, C14-N15 1.368, H15-N15 0.879), **H-8a** (selected distances [Å]: B1-F2 1.392, B1-F3 1.373, B1-O4 1.492, B1-N18 1.519, O4-C5 1.297, C17-N18 1.355) and **H-8b** (selected distances [Å]: B1-F2 1.383, B1-F3 1.379, B1-O12 1.485, B1-N4 1.524, N4-C24 1.353, O12-C13 1.292).[44]
<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>Table 1. Electrochemical data (in V) of the selected compounds.</figcaption></figure>

<table><thead><tr><th rowspan="2">Compound<sup>a)</sup></th><th colspan="4">1st reduction</th><th colspan="4">2nd reduction</th></tr><tr><th>E<sub>pc</sub><sup>b)</sup></th><th>E<sub>pa</sub><sup>b)</sup></th><th>i<sub>pc</sub>/i<sub>pa</sub><sup>b)</sup></th><th>E<sub>1/2</sub><sup>c)</sup></th><th>E<sub>pc</sub><sup>b)</sup></th><th>E<sub>pa</sub><sup>b)</sup></th><th>i<sub>pc</sub>/i<sub>pa</sub><sup>b)</sup></th><th>E<sub>1/2</sub><sup>c)</sup></th></tr></thead><tbody><tr><td>D</td><td>-0.79</td><td>-0.70</td><td>0.91</td><td><strong>-0.74</strong></td><td>-1.45</td><td>-1.35</td><td>0.98</td><td><strong>-1.40</strong></td></tr><tr><td>F-3a</td><td>-0.31</td><td>-0.22</td><td>0.97</td><td><strong>-0.26</strong></td><td>-1.00</td><td>-0.90</td><td>0.82</td><td><strong>-0.94</strong></td></tr><tr><td>F-3b</td><td>-0.38</td><td>-0.19</td><td>0.95</td><td><strong>-0.23</strong></td><td>-0.96</td><td>-0.87</td><td>0.76</td><td><strong>-0.90</strong></td></tr><tr><td>F-3c</td><td>-0.21</td><td>-0.14</td><td>0.92</td><td><strong>-0.16</strong></td><td>-0.90</td><td>-0.83</td><td>1.38</td><td><strong>-0.84</strong></td></tr><tr><td>3b</td><td>-0.85</td><td>-0.77</td><td>1.03</td><td><strong>-0.79</strong></td><td>-1.43</td><td>-1.34</td><td>1.15</td><td><strong>-1.38</strong></td></tr><tr><td>1a</td><td>-0.82</td><td>-0.72</td><td>1.09</td><td><strong>-0.77</strong></td><td>-1.37</td><td>-1.27</td><td>0.93</td><td><strong>-1.32</strong></td></tr><tr><td>F-1a</td><td>-0.36</td><td>-0.26</td><td>1.02</td><td><strong>-0.31</strong></td><td>-0.97</td><td>-0.88</td><td>1.15</td><td><strong>-0.92</strong></td></tr><tr><td>1b</td><td>-0.84</td><td>-0.74</td><td>1.07</td><td><strong>-0.79</strong></td><td>-1.38</td><td>-1.29</td><td>1.02</td><td><strong>-1.33</strong></td></tr><tr><td>F-1b</td><td>-0.34</td><td>-0.25</td><td>1.00</td><td><strong>-0.30</strong></td><td>-0.96</td><td>-0.87</td><td>1.12</td><td><strong>-0.92</strong></td></tr><tr><td>4b</td><td>-0.84</td><td>-0.76</td><td>1.65</td><td><strong>-0.76</strong></td><td>-1.39</td><td>-1.31</td><td>1.33</td><td><strong>-1.30</strong></td></tr><tr><td>5b</td><td>-0.88</td><td>-0.80</td><td>0.93</td><td><strong>-0.84</strong></td><td>-1.37</td><td>-1.29</td><td>1.00</td><td><strong>-1.33</strong></td></tr><tr><td>F-4b1</td><td>-0.24</td><td>-0.17</td><td>0.99</td><td><strong>-0.20</strong></td><td>-0.86</td><td>-0.79</td><td>0.84</td><td><strong>-0.80</strong></td></tr><tr><td>F-5b1</td><td>-0.47</td><td>-0.39</td><td>0.93</td><td><strong>-0.42</strong></td><td>-1.01</td><td>-0.92</td><td>0.96</td><td><strong>-0.95</strong></td></tr><tr><td>F-4b2</td><td>+0.08</td><td>+0.16</td><td>0.98</td><td><strong>+0.13</strong></td><td>-0.43</td><td>-0.36</td><td>0.97</td><td><strong>-0.38</strong></td></tr><tr><td>F-5b2</td><td>-0.16</td><td>-0.07</td><td>0.89</td><td><strong>-0.11</strong></td><td>-0.71</td><td>-0.62</td><td>0.96</td><td><strong>-0.65</strong></td></tr><tr><td>7b</td><td>-1.61</td><td>-1.51</td><td>0.88</td><td><strong>-1.56</strong></td><td>-2.30</td><td>—</td><td>—</td><td><strong>-2.23</strong></td></tr><tr><td>G-7b</td><td>-0.92</td><td>-0.84</td><td>0.81</td><td><strong>-0.87</strong></td><td>-1.89</td><td>—</td><td>—</td><td><strong>-1.84</strong></td></tr><tr><td>8b</td><td>-1.51</td><td>-1.42</td><td>0.85</td><td><strong>-1.45</strong></td><td>-2.11</td><td>—</td><td>—</td><td><strong>-2.04</strong></td></tr><tr><td>H-8b</td><td>-0.83</td><td>-0.74</td><td>0.81</td><td><strong>-0.78</strong></td><td>-1.75</td><td>—</td><td>—</td><td><strong>-1.69</strong></td></tr><tr><td>H-8a</td><td>-0.84</td><td>-0.75</td><td>0.81</td><td><strong>-0.79</strong></td><td>-1.76</td><td>—</td><td>—</td><td><strong>-1.70</strong></td></tr></tbody></table>

One can additionally note that the introduction of a second 4-tolylamino group at C8, C5, or C4 onto **3b** to respectively give **1b**, **4b**, or **5b** had only a small impact. A slightly more significant change in the redox potentials was noticed from the boracycle F-3b, when this 4-tolylamino was introduced at C8 (compound F-1b), at C5 (F-4b1) or at C4 (F-5b1).

Finally, as observed above by comparing **1a** with F-1a, **1b** with F-1b, and **3b** with F-3b, the formation of the BF₂ complexes from the 1,4-di(arylamino)anthraquinone **4b** made first/second reduction significantly easier, with redox potentials shifted by 0.56/0.50 V in the case of the single boracycle F-4b1, and by 0.89/0.92 V for the double boracycle F-4b2 (Figure 3). Similarly,

<footnote>a) Potentials values in DMF (3 mM for D, 1a, F-1a, 1b, F-1b, 5b, F-5b1, F-5b2, 7b, G-7b, 8b, H-8b and H-8a; 2 mM for 3b; 1.5 mM for F-3a and F-3b; 1 mM for F-4b1 and F-4b2; 0.5 mM for F-3c and 4b) given relative to Ag/AgCl, scan rate = 100 mV·s⁻¹. b) From CV experiments. c) From DPV experiments, except in the case of 1a, F-1a, 1b and F-1b.</footnote>
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /><figcaption>Figure 3. Plots of cyclic voltammograms of 4b, F-4b1 and F-4b2 in DMF.</figcaption></figure>

formation of single and double boracycles F-5b1 and F-5b2 from the 1,4-di(arylamino)anthraquinone 5b shifted its first/second reduction potentials by 0.42/0.38 V and 0.73/0.68 V, respectively (Figure 4).

As we also had a few 1-substituted (thio)xanthones as well as the corresponding boracycles, we selected the 4-tolylamino complexes G-7b and H-8b, as well as their precursors 7b and 8b to explore their behavior in electrochemical reduction (Figure 5). As expected, the corresponding xanthone 7b and thioxanthone 8b were found more difficult to reduce than 1-(4-tolylamino)anthraquinone (3b), with potentials for first/second reduction shifted by

0.77/0.85 and 0.66/0.66 V, respectively. Similar differences, related to the BF₂ complex F-3b, were also measured for G-7b and H-8b, with potentials shifted by 0.64/0.94 and 0.55/0.79 V, respectively.

As observed from 3b to F-3b, moving from the BF₂-free compounds 7b and 8b to the complexes G-7b and H-8b made first and second reduction easier, with potentials shifted by 0.69/0.39 and 0.67/0.35 V, respectively. Moreover, moving from the xanthone 7b to the thioxanthone 8b made first and second reduction easier, with potentials shifted by 0.11 and 0.19 V, respectively. A similar trend was observed from G-7b to H-8b, with reduction potential values shifted by 90 and 150 mV.

<figure><img src="image_23.png" /><figcaption>Figure 4. Plots of cyclic voltammograms of 5b, F-5b1 and F-5b2 in DMF.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>Figure 5. Plots of cyclic voltammograms of 7b, 8b, G-7b and H-8b in DMF.</figcaption></figure>
<figure><img src="image_25.png" /></figure>

Finally, replacing the tolyl group of **H-8b** with an anisyl (compound **H-8a**) did not modify significantly the electrochemical properties.

## 4. Photophysical Properties

The photophysical properties of the synthesized compounds, and notably the anthraquinone derivatives,[27,28] was next studied with the goal to identify new long-wavelength absorbing dyes. The experimental data obtained in CH₂Cl₂ are gathered in **Table 2**. Compared with anthraquinone (D; λmax value of 326 nm), the introduction of an arylamino group at the 1-position (compound **3b**) led to absorption at a λmax value of 511 nm (absorption red-shifted by 185 nm). This bathochromic effect has previously been explained by the combination of an electron-donating arylamino group and an electron-withdrawing anthraquinone, resulting in a favorable intramolecular N–H···O=C hydrogen bond that enhances charge transfer interaction.[9,10]

By switching from **3b** to its BF₂ complex **F-3b**, a similar bathochromic shift was observed, with an absorption at λmax value of 557 nm obtained for **F-3b**, which is red-shifted by 46 nm and 231 nm compared to **3b** and **D**, respectively. This shift can be modulated by varying the substituent at the 4-position of the arylamino. Thus, the presence of an electron-donating OMe group (compound **F-3a**) had a slightly stronger effect than Me (233 nm red-shifted absorption compared to **D**), and that of an electron-withdrawing CF₃ (compound **F-3c**) reduced this shift (226 nm red-shifted absorption compared to **D**). While the absorption band of free ligand **3b** was broad and unstructured, its rigidification by the formation of the BF₂ complex **F-3b** led to a more structured absorption band (**Figure 6**).

Compared with 1-(4-tolylamino)anthraquinone (**3b**; λmax value of 511 nm), its xanthone and thioxanthone analogs **7b** and **8b** showed hypsochromic absorptions at 421 nm (blue-shifted by 90 nm) and 448 nm (blue-shifted by 63 nm), respectively. After complexation of their parent xanthone **7b** and thioxanthone **8b** to BF₂, the absorptions of **G-7b** (λmax value of 516 nm) and **H-8b** (λmax value of 557 nm) were found to be red-shifted by 95 nm and 109 nm, respectively. Finally, as observed for the electrochemical properties, replacing the tolyl group of **H-8b** with an anisyl (compound **H-8a**) did not change significantly the absorption properties (**Figure 7**).

In data previously recorded in the same solvent (CH₂Cl₂), 1,8-di(4-tolylamino)anthraquinones (**1b**) absorbed with a λmax value of 559 nm,[10] red-shifted by 48 nm compared with **3b** (511 nm). As for 1,4- and 1,5-di(4-tolylamino)anthraquinone (**5b** and **4b**), λmax values of 643 and 539 nm were respectively determined, red-shifted by 132 and 28 nm compared with **3b**. While switching from **5b** to its monoboron and diboron complexes **F-5b1** (635 nm) and **F-5b2** (631 nm) caused a slight blue-shift (8-12 nm), significant red-shifts (101 to 157 nm) were noticed from **4b** to its monoboron and diboron complexes **F-4b1** (640 nm) and **F-4b2** (696 nm). As previously observed for **3b**, the lowest energy band of **4b** is not structured, whereas the corresponding boron complexes **F-4b1** and **F-4b2** exhibit a structure band (**Figure 8**).

<figure><img src="image_26.png" /><figcaption>**Table 2.** Photophysical properties of the selected compounds.</figcaption></figure>

<table><thead><tr><th>Compound<sup>a)</sup></th><th>λ<sub>abs</sub><sup>b)</sup><br>[nm]</th><th>ε<sub>max</sub><sup>c)</sup><br>[M<sup>-1</sup> cm<sup>-1</sup>]</th><th>λ<sub>em</sub><sup>d)</sup><br>[nm]</th><th>Φ<sub>Δ</sub><sup>e)</sup><br>[%]</th><th>Φ<sub>Δ</sub><sup>f)</sup><br>[%]</th></tr></thead><tbody><tr><td><strong>3b</strong></td><td>511</td><td>9700</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>F-3b</strong></td><td>557 (+46)</td><td>10 500</td><td>g)</td><td>g)</td><td>—</td></tr><tr><td><strong>F-3a</strong></td><td>559</td><td>13 000</td><td>g)</td><td>g)</td><td>—</td></tr><tr><td><strong>F-3c</strong></td><td>552</td><td>7200</td><td>g)</td><td>g)</td><td>—</td></tr><tr><td><strong>7b</strong></td><td>421</td><td>13 000</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>8b</strong></td><td>448</td><td>13 600</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>G-7b</strong></td><td>516 (+95)</td><td>9700</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>H-8b</strong></td><td>557 (+109)</td><td>13 200</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>G-7a</strong></td><td>421</td><td>9300</td><td>g)</td><td>g)</td><td>—</td></tr><tr><td><strong>H-8a</strong></td><td>558</td><td>18 100</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>H-8c</strong></td><td>552</td><td>10 000</td><td>693</td><td>2<sup>h)</sup></td><td>j)</td></tr><tr><td><strong>5b</strong></td><td>643</td><td>16 900</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>4b</strong></td><td>539</td><td>16 800</td><td>g)</td><td>g)</td><td>j)</td></tr><tr><td><strong>F-5b1</strong></td><td>635 (–8)</td><td>26 700</td><td>660</td><td>0.2</td><td>j)</td></tr><tr><td><strong>F-5b2</strong></td><td>631 (–12)</td><td>28 400</td><td>660</td><td>8<sup>i)</sup></td><td>52</td></tr><tr><td><strong>F-4b1</strong></td><td>640 (+101)</td><td>22 600</td><td>g)</td><td>g)</td><td>—</td></tr><tr><td><strong>F-4b2</strong></td><td>696 (+157)</td><td>34 300</td><td>g)</td><td>g)</td><td>—</td></tr></tbody></table>

In the 1,4-series, the absorption band of the parent compound **5b** is already slightly structured, and the resolution of the vibronic structure increases for its boron complexes **F-5b1** and **F-5b2** (**Figure 9**). High molar extinction coefficients (>2.10⁴ M⁻¹ cm⁻¹) were recorded for the BF₂ complexes derived from **4b** and **5b**.

Some amino-substituted anthraquinones can emit red fluorescence from an excited state possessing an intramolecular

<footnote><sup>a)</sup>In CH₂Cl₂ (≈5 µM); <sup>b)</sup>Maximum of the first absorption band (when possible, the difference with the boron-free parent compound is given into brackets); <sup>c)</sup>Molar extinction coefficient at λ<sub>abs</sub>; <sup>d)</sup>Emission maximum; <sup>e)</sup>Fluorescence quantum yield using quinine bisulfate in 0.5 M H₂SO₄ as a standard (Φ<sub>F</sub> = 54.6%); <sup>f)</sup>Singlet oxygen quantum yield obtained by direct measurement of the characteristic ¹O₂ luminescence at 1272 nm, using tetraphenylporphyrin in CH₂Cl₂ as a standard (Φ<sub>Δ</sub> = 60%); <sup>g)</sup>No fluorescence; <sup>h)</sup>HOMO-LUMO energy gap of 1.995 eV; <sup>i)</sup>HOMO-LUMO energy gap of 1.93 eV; <sup>j)</sup>No singlet oxygen production detected.</footnote>
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /><figcaption>**Figure 6.** Absorption spectra of the anthraquinone derivatives 3b, F-3a, F-3b and F-3c in CH₂Cl₂.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure 7.** Absorption spectra in CH₂Cl₂ of the xanthone derivatives 7b, G-7a and G-7b, and the thioxanthone derivatives 8b, H-8a and H-8b.</figcaption></figure>

charge transfer character.[29] Unfortunately, in our case, most of the products synthesized did not exhibit fluorescence. The fluorescence emission spectra were acquired in CH₂Cl₂ for the most promising compounds F-5b1, F-5b2, G-7c, and H-8c, and the higher quantum yield (8%) was recorded for the red emitter F-5b2 which is the diboron complex derived from 1,4-di(4-tolylamino)anthraquinone (Figure 10).

The photosensitizing properties were studied for most of the compounds of Table 2, by direct measurement of the characteristic phosphorescence of singlet oxygen at 1272 nm. No signal was detected, except for the diboron complex F-5b2, which exhibits a singlet oxygen quantum yield of 52%, meaning that for this compound, intersystem crossing (ISC) is the main

de-excitation pathway. Interestingly, F-5b2 also exhibits one of the highest molar extinction coefficients and the highest fluorescence quantum yield of the series (8%).

## 5. Activity as Protein Kinase Inhibitors

Anthraquinone derivatives are known to exhibit a variety of biological activities.[1] Moreover, among the four-coordinated organoboron compounds, adducts of the N,O-BF₂ type may be useful in disease-related applications.[12] We therefore chose to evaluate[18] some of the synthesized compounds against a panel of protein kinases often deregulated in diseases such as cancers
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>**Figure 8.** Absorption spectra of the anthraquinone derivatives 4b, F-4b1 and F-4b2 in CH₂Cl₂.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 9.** Absorption spectra of the anthraquinone derivatives 5b, F-5b1 and F-5b2 in CH₂Cl₂.</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Figure 10.** Absorption and emission spectra of the anthraquinone derivative F-5b2 in CH₂Cl₂.</figcaption></figure>
<figure><img src="image_34.png" /></figure>

<table><caption>**Table 3.** Inhibitory activities of synthesized compounds against a panel of cancer-related protein kinases. The table displays the percentages of remaining kinase activity, detected after treatment with 10 µM (or 1 µM) of the tested compounds. Results are expressed in % of maximal activity, i. e. measured in the absence of inhibitor but with an equivalent dose of DMSO (solvent of the tested compounds). ATP concentration used in the kinase assays was 10 µM. Kinases are from human origin except when it is specified (Rn: *Rattus norvegicus*; Mm: *Mus musculus*).</caption><thead><tr><th>Kinase/Cpd</th><th>1a</th><th>F-1a</th><th>3a</th><th>3b</th><th>3c</th></tr></thead><tbody><tr><td>CDK9/Cyclin T</td><td>94 <sup>(a)</sup></td><td>45 <sup>(a)</sup></td><td>95 <sup>(a)</sup></td><td>97 (99)</td><td>92 <sup>(a)</sup></td></tr><tr><td>PIM1</td><td><b>1 (61)</b></td><td><b>6 (71)</b></td><td>93 (79)</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(<sup>a</sup>)</td></tr><tr><td>PIM2</td><td>-</td><td>-</td><td>45 (39)</td><td>48 <sup>(a)</sup></td><td>48 (65)</td></tr><tr><td>DYRK1A<sup>b)</sup></td><td>80 <sup>(a)</sup></td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(90)</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(<sup>a</sup>)</td></tr><tr><td>CLK1<sup>c)</sup></td><td>93 <sup>(a)</sup></td><td>89 (97)</td><td>97 (96)</td><td>84 <sup>(a)</sup></td><td>96 <sup>(a)</sup></td></tr><tr><td>GSK-3α</td><td><b>11 (57)</b></td><td><b>9 (52)</b></td><td>60 (64)</td><td>43 (78)</td><td>42 (62)</td></tr><tr><td>GSK-3β</td><td><b>30 (60)</b></td><td><b>20 (52)</b></td><td>70 (83)</td><td>50 (89)</td><td>58 (76)</td></tr><tr><td>Haspin</td><td>89 (98)</td><td>37 <sup>(a)</sup></td><td>54 (73)</td><td>45 (97)</td><td>57 (71)</td></tr><tr><td>VEGFR2</td><td>24 (68)</td><td><b>19 (58)</b></td><td>-</td><td>-</td><td>-</td></tr><tr><td>Kinase/Cpd</td><td>F-3a</td><td>F-3b</td><td>F-3c</td><td>F-5b2</td><td>7a</td></tr><tr><td>CDK9/Cyclin T</td><td>90 (99)</td><td>87 <sup>(a)</sup></td><td>92 <sup>(a)</sup></td><td>78 (92)</td><td>93 (98)</td></tr><tr><td>PIM1</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup> (97)</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(79)</td><td><sup>a</sup>(83)</td></tr><tr><td>PIM2</td><td>84 <sup>(a)</sup></td><td>43 (43)</td><td>47 (69)</td><td>50 (84)</td><td><sup>a</sup>(97)</td></tr><tr><td>DYRK1A<sup>b)</sup></td><td><sup>a</sup>(<sup>a</sup>)</td><td>68 <sup>(a)</sup></td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(<sup>a</sup>)</td><td>98 (98)</td></tr><tr><td>CLK1<sup>c)</sup></td><td>89 (97)</td><td>86 (93)</td><td><sup>a</sup>(93)</td><td>79 (97)</td><td>87 (93)</td></tr><tr><td>GSK-3α</td><td>65 (99)</td><td>61 (60)</td><td>52 (79)</td><td><b>11 (58)</b></td><td>61 (91)</td></tr><tr><td>GSK-3β</td><td>81 (98)</td><td>67 (62)</td><td>67 (83)</td><td><b>14 (63)</b></td><td>66 (83)</td></tr><tr><td>Haspin</td><td>71 (96)</td><td><b>24 (52)</b></td><td>53 (55)</td><td>37 (80)</td><td>70 <sup>(a)</sup></td></tr><tr><td>VEGFR2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Kinase/Cpd</td><td>G-7a</td><td>8a</td><td>8b</td><td>8c</td><td>H-8a</td></tr><tr><td>CDK9/Cyclin T</td><td>88 (92)</td><td>98 <sup>(a)</sup></td><td><sup>a</sup>(97)</td><td><sup>a</sup>(<sup>a</sup>)</td><td>94 <sup>(a)</sup></td></tr><tr><td>PIM1</td><td><sup>a</sup>(77)</td><td><sup>a</sup>(93)</td><td>96 <sup>(a)</sup></td><td>70 <sup>(a)</sup></td><td>92 <sup>(a)</sup></td></tr><tr><td>PIM2</td><td>72 (92)</td><td>45 (75)</td><td>40 (70)</td><td><b>26 (57)</b></td><td>43 (81)</td></tr><tr><td>DYRK1A<sup>b)</sup></td><td>86 (73)</td><td><sup>a</sup>(69)</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(<sup>a</sup>)</td><td><sup>a</sup>(<sup>a</sup>)</td></tr><tr><td>CLK1<sup>c)</sup></td><td>84 (97)</td><td>91 <sup>(a)</sup></td><td>89 <sup>(a)</sup></td><td><sup>a</sup>(88)</td><td>96 (99)</td></tr><tr><td>GSK-3α</td><td>57 (63)</td><td>54 (73)</td><td>73 (71)</td><td>46 (83)</td><td>60 (92)</td></tr><tr><td>GSK-3β</td><td>77 (66)</td><td>68 (71)</td><td>72 (82)</td><td>55 (77)</td><td>73 (85)</td></tr><tr><td>Haspin</td><td>54 (77)</td><td><b>37 (55)</b></td><td>42 (59)</td><td>44 (63)</td><td>28 (74)</td></tr><tr><td>VEGFR2</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

and neurodegeneration.[30] cyclin-dependent kinase 9 (HsCDK9/Cyclin T), proto-oncogene kinases HsPIM1 and HsPIM2, dual specificity tyrosine phosphorylation regulated kinase 1A (RnDYRK1A), dual specificity cdc-like kinase 1 (MmCLK1), glycogen synthase kinase-3α and -3β (HsGSK-3α and -3β), mitotic kinase HsHaspin, and vascular endothelial growth factor receptor 2 tyrosine-protein kinase (HsVEGFR2) (Table 3).

While most of the tested compounds were found almost inactive, promising results were recorded for the 1,8-disubstituted products **1a** and **F-1a** toward PIM1, GSK-3, and VEGFR2. Compared with **3b**, its BF₂ complex **F-3b** seemed to be more efficient on Haspin. Moreover, the diboron complex **F-5b2** showed a moderate inhibiting activity against GSK-3. Finally, moderate activity was recorded for the thioxanthones **8a-c** against PIM2 and Haspin.

## 6. Conclusion

In this study, a series of 1-(arylamino) derivatives as well as 1,4-, 1,5-, and 1,8-di(arylamino) derivatives of anthraquinone, xanthone, and thioxanthone were prepared by Buchwald-type coupling between the 1-chlorinated/iodinated precursors and 4-substituted anilines. Their reaction with BF₃-Et₂O allowed the synthesis of a variety of original mono and di N,O-BF₂ complexes in moderate to excellent yields.

Due to the additional electron-withdrawing nature of the BF₂ moiety, the boron complexes (and especially the diboron ones) were found to be much easier to reduce than their precursors. As far as photophysical properties are concerned, the formation of BF₂ complexes from the arylamino precursors in most cases improved them by red-shifting the absorption wavelength of

<footnote><sup>a)</sup>≥100. <sup>b)</sup>RnDYRK1A. <sup>c)</sup>MmCLK1.</footnote>
<figure><img src="image_35.png" /></figure>

these dyes. Indeed, while the HOMO-LUMO energy gap for 1,4-di(4-tolyl)anthraquinone (5b) has been estimated at 2.28 eV, the gap obtained here for the corresponding diboron complex F-5b2 is 1.93 eV. Furthermore, although this red emitter F-5b2 has been identified as the most fluorescent in the series, it also exhibits interesting photosensitizing properties.

Although weak biological activities have been recorded for these new compounds, their electrochemical and physicochemical properties make them appealing for applications in fields such as sensors, phototherapy, electrochemical mediation, and bioinspired energy conversion.

In view of these results, it could be interesting to study the properties of 1,4,5,8-tetrasubstituted anthraquinones. This would not only confirm the trends observed in this work, but also enable a rational choice of substituents for fine-tuning the properties of these promising scaffolds.

## Supporting Information

Hückel molecular orbital calculations of the HOMO-LUMO energy gap; General information, crystallography and electrochemical measurements; Compound synthesis, analyses, and X-ray crystallographic details; ORTEP diagrams of F-1a, 2a1, 2a2, 3c, F-3b, F-5b1, F-5b2, 8a, 8b, 8c, H-8a and H-8b; NMR spectra of 1a, 2a1, 2a2, F-1a, F-1b, 3a-c, F-3a, F-3b, F-3c, F-4b1, F-4b2, F-5b1, F-5b2, F-6b2, 7a-c, 8a-c, G-7a, G-7b, G-7c, H-8a, H-8b and H-8c; Voltammograms for compounds D, F-3a, F-3b, F-3c, 3b, 1a, F-1a, 1b, F-1b, 4b, 5b, F-4b1, F-5b1, F-4b2, F-5b2, 7b, G-7b, 8b, H-8b and H-8a. The authors have cited additional references within the Supporting Information.[31-43]

## Acknowledgements

W.E. and F.M. thank the "Comité d'Ille et Vilaine 35" de la Ligue Contre le Cancer and the Centre National de la Recherche Scientifique for supporting this research. W.E., J.P.H., and F.M. gratefully acknowledge the financial support by the Université de Rennes (Ph.D. grant to N.M.). We also thank Rennes Métropole (Spectrum 100 Perkin Elmer spectrometer) and the Fonds Européen de Développement Régional (FEDER; D8 Venture Bruker-AXS diffractometer). C.P., T.R., and S.B. thank the Cancéropôle Grand Ouest ("Marine molecules, metabolism and cancer" network), IBiSA (French Infrastructures en sciences du vivant : biologie, santé et agronomie), and Biogenouest (Western France life science and environment core facility network supported by the Conseil Régional de Bretagne) for supporting the KISSf screening facility, a member of the French research infrastructure ChemBioFrance.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** anthraquinone · boracycle · dye · thioxanthone · xanthone

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Manuscript received: May 1, 2025

Revised manuscript received: September 22, 2025

Version of record online: