The rise of functional organoarsenic chemistry

Abstract

Organoarsenic compounds have long been viewed through the lens of toxicity, yet recent advances in their synthesis, reactivity, and molecular design have begun to redefine their role in modern chemistry. This review highlights the emerging field of functional organoarsenic chemistry, which spans molecular frameworks, polymeric materials, metal coordination complexes, and reactive intermediates. Key developments include the design of π-conjugated arsole-based polymers with tunable optoelectronic properties, the construction of coordination and redox-active frameworks by harnessing the soft Lewis basicity of arsenic, and the exploitation of As(III)/As(V) redox cycles for catalysis. Furthermore, the development of arsonium-based ionic liquids and hypervalent arsenic-centered dications demonstrates the expanding versatility of arsenic across diverse chemical contexts. Collectively, these findings illustrate how arsenic's unique electronic and structural attributes—distinct from its phosphorus congener—enable new functionalities that enrich the toolbox of molecular and materials science. The rise of functional organoarsenic chemistry represents both a revival and a reimagination of this historically overlooked element, paving the way for future applications in catalysis, photonics, sensing, and soft materials.

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Hiroaki Imoto

Prof. Hiroaki Imoto received his Ph.D. from Kyoto University in 2012. From 2009 to 2012, he was a JSPS Research Fellow in the group of Prof. Yoshiki Chujo at Kyoto University. In 2012, he joined JSR Corporation as a researcher. He began his academic career at the Kyoto Institute of Technology (KIT) as an Assistant Professor in 2014 and was promoted to Associate Professor in 2019. Since 2023, he has also served as a Research Fellow of the JST FOREST Program. His research interests include organic–inorganic hybrid molecules, polymers, and supramolecular materials.

Chihiro Okochi

Chihiro Okochi received his Bachelor of Engineering degree from the Kyoto Institute of Technology (KIT), where he is currently pursuing a Master's degree in Materials Synthesis under the supervision of Prof. Kensuke Naka. His research focuses on the development of synthetic methodologies for novel arsenic compounds and the exploration of their unique reactivity.

Kazuma Kikuchi

Kazuma Kikuchi received his B.Eng. degree in 2021 and his M.Eng. degree in 2023 from the Kyoto Institute of Technology (KIT), where he is currently pursuing his Ph.D. under the supervision of Prof. Kensuke Naka. Since 2023, he has been a recipient of a JST Fellowship under the program “The Establishment of University Fellowships Toward the Creation of Science Technology Innovation”. His research interests include the design and construction of supramolecular architectures based on arsenic complexes.

Kensuke Naka

Prof. Kensuke Naka received his Ph.D. from Kyoto University in 1991. He began his academic career as a Research Assistant at Kagoshima University in 1990 and later moved to Kyoto University in 1996, where he became an Associate Professor in 2000. In 2007, he was promoted to Full Professor at the Kyoto Institute of Technology (KIT). He received the SPSJ Wiley Award in 2007 and the Award of The Adhesion Society of Japan in 2020. He currently serves as Director of the Materials Innovation Laboratory at KIT.

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1. Background

The integration of inorganic elements into organic frameworks has emerged as a powerful strategy to surpass the inherent limitations of traditional hydrocarbon-based materials.^1–5 In the pursuit of novel functionalities, researchers have progressively incorporated heavier main-group elements, including those from the late p-block. Notably, even bismuth—regarded as the heaviest non-radioactive element—has found applications in phosphorescent materials, X-ray shielding agents, and high-refractive-index polymers.^6–9 However, despite this expanding scope, arsenic remains conspicuously underexplored in the context of functional materials based on organo-element hybrids.

Arsenic, named after the yellow mineral arsenikon, is an abundant element in the Earth's crust and has been widely employed in the field of inorganic materials chemistry.¹⁰ During the 18th and 19th centuries, arsenic-containing pigments such as Paris Green (Cu(C₂H₃O₂)₂·3Cu(As₂)₂) and Scheele's Green (AsCuHO₃) were developed and widely used. In the 1950s, gallium arsenide (GaAs) was identified as a compound with semiconducting properties, a discovery that laid the foundation for its subsequent role as a key material in modern electronics. On the other hand, the notorious toxicity of arsenic compounds has long shaped their societal perception. In France, arsenic trioxide (As₂O₃) was colloquially termed poudre de succession (“inheritance powder”) owing to its use as a poison in clandestine murders.

In contrast to inorganic arsenic chemistry, the history of organoarsenic compounds is relatively recent (Fig. 1).^11–16 The first known organoarsenic compound, cacodyl (Me₂AsAsMe₂), was synthesized by Louis Claude Cadet de Gassicourt in 1760, with its molecular structure elucidated in the 19th century. A landmark development in medicinal chemistry was the synthesis of Salvarsan in 1907,¹⁷ the first effective treatment for syphilis—the exact structure of Salvarsan remained ambiguous for nearly a century, until its cyclic nature was finally confirmed in 2005.¹⁸ Unfortunately, the early promise of organoarsenic chemistry was overshadowed by its misuse in warfare: compounds such as Adamsite (diphenylaminechlorarsine), Lewisite (2-chlorovinyldichloroarsine), and Sneezing Gas (chlorodiphenylarsine) were deployed as chemical weapons during World War I.^19–21 The long-lasting environmental damage caused by these arsenic-based munitions continues to pose serious ecological threats. These historical misapplications have strongly shaped the negative perception of organoarsenic chemistry, leading to a significant decline in research interest, primarily owing to toxicity concerns.

Fig. 1 Chemical structures of historically representative organoarsenic compounds.

As a result, organoarsenic chemistry has increasingly shifted toward computational investigations, which provide insights into bonding, reactivity, and electronic structure without the inherent hazards of experimental work. Nevertheless, one of the central challenges in experimental organoarsenic research remains the safe synthesis of these compounds. Historically, highly volatile and toxic arsenic precursors have been employed, placing researchers at substantial risk of exposure. Therefore, the development of safer and more accessible synthetic methodologies is a crucial prerequisite for revitalizing the field.

This review begins by highlighting recent advances in practical and safer synthetic methods for organoarsenic compounds. It then explores the functional potential of these molecules across diverse domains, including π-conjugated systems, polymeric materials, metal-coordinating ligands, reactive intermediates, and ionic species.

2. Synthetic strategy

2.1. Preparation of electrophiles

To construct As–C bonds, conventional and recently developed methods have been comprehensively summarized by Tay and Pullarkat.²² Traditionally, arsenic chlorides such as trichloroarsine (AsCl₃), dichlorophenylarsine (PhAsCl₂), and chlorodiphenylarsine (Ph₂AsCl) have served as standard electrophilic arsenic sources (Scheme 1a). However, these compounds are highly volatile—for instance, AsCl₃ exhibits a vapor pressure of 10 mmHg at 23.5 °C—posing substantial safety risks, although its volatility is lower than that of the phosphorus analogue PCl₃ (120 mmHg at 25 °C). Moreover, the synthesis of PhAsCl₂ is particularly cumbersome, requiring the reaction of phenylarsonic acid (PhAsO₃H₂), hydrochloric acid (HCl), and sulfur dioxide (SO₂), followed by vacuum distillation for purification (Scheme 1b).²³

Scheme 1 Preparation methods for arsenic electrophiles. (a) Traditional electrophiles, (b) synthesis of PhAsCl₂, (c) As–As bond cleavage by I₂, (d) in situ generation of RAsI₂ (R = Me, Ph) from cyclooligoarsines, (e) generation of AsX₃ (X = Br, I), (f) synthesis of dithiaarsoles, and (g) As–As bond cleavage by IX (X = Cl, I).

An efficient alternative involves the oxidative cleavage of As–As bonds to generate arsenic halides. For example, treatment of Me₂AsAsMe₂ with iodine (I₂) yields iododimethylarsine (Me₂AsI) (Scheme 1c).²⁴ In this context, cyclooligoarsines such as pentamethylpentaarsine (Me₅As₅) and hexaphenylhexaarsine (Ph₆As₆) have emerged as practical precursors for generating arsenic electrophiles without resorting to volatile reagents.^25,26 Upon treatment with I₂, these compounds undergo selective As–As bond cleavage to afford diiodoarsines in situ: Me₅As₅ yields diiodomethylarsine (MeAsI₂), while Ph₆A₆ gives diiodophenylarsine (PhAsI₂) (Scheme 1d).²⁷ Notably, these electrophiles are generated cleanly, without byproducts, and can be used directly in subsequent reactions without isolation.

As safer and more manageable alternatives to AsCl₃, we proposed the use of tribromoarsine (AsBr₃) and triiodoarsine (AsI₃), both of which are solids at room temperature. These compounds can be synthesized directly from As₂O₃ using aqueous hydrobromic acid (HBr) or hydroiodic acid (HI), respectively (Scheme 1e).²⁸ AsBr₃ is conveniently extracted using n-hexane, affording an isolated yield of 92%. By contrast, owing to the poor solubility of AsI₃, filtration is required to isolate the solid product, resulting in a lower yield (33%). Importantly, AsBr₃ is readily soluble in common organic solvents such as n-hexane, diethyl ether, tetrahydrofuran, and toluene, making it a particularly convenient and non-volatile precursor for organoarsenic synthesis. This reagent is particularly suited for the preparation of A₃-type (C₃-symmetric) compounds bearing three identical substituents.

The selective synthesis of AB₂-type (C_s-symmetric) arsines bearing two different substituents presents a greater challenge. In the case of phosphines, stepwise substitution of trichlorophosphine (PCl₃) is often effective; however, for arsenic, the longer As–C bond reduces steric hindrance and favors over-substitution to yield A₃-type products. A novel strategy has been developed to circumvent this limitation by exploiting the high affinity of arsenic halides for thiols. For instance, AsBr₃ reacts with 1,2-benzenedithiol to form a brominated dithiaarsole intermediate (Scheme 1f).²⁹ This species can undergo sequential nucleophilic substitutions: first at the As–Br bond, then via displacement of the chelating dithiol unit by a second nucleophile. This strategy enables precise control over substitution, affording AB₂-type arsines with high selectivity—including chiral arsa-Buchwald-type ligands (see section 5.3 for details).

Another useful approach involves the oxidative cleavage of As–C bonds to generate arsenic halides.^30–32 In 1989, Pazik et al. transformed trineopentylarsine into arsenic bromide via reaction with bromine (Br₂). This method was later extended to other organoarsenic species such as 9-methyl-9-arsafluorene.³³ Treatment with I₂ or iodine monochloride (ICl) results in As–C bond cleavage, forming As–I or As–Cl bonds with the concurrent release of iodomethane (MeI), as illustrated in Scheme 1g. The resulting arsenic halides readily undergo nucleophilic substitution with Grignard or organolithium reagents to furnish various 9-arsafluorene derivatives. This strategy is also applicable to methyldiphenylarsine (AsMePh₂), providing access to iododiphenylarsine (Ph₂AsI).

2.2. Preparation of nucleophiles

Organoarsenic nucleophiles can be generated by deprotonation of As–H bonds, as well as by cleavage of As–C and As–As bonds. Traditionally, alkali metals such as lithium (Li), sodium (Na), and potassium(K) have been employed to generate arsenic nucleophiles (Scheme 2a). Tzschach et al. reported a method for obtaining diphenylarsenyllithium (Ph₂AsLi) by reacting diphenylarsine (Ph₂AsH) with phenyllithium (PhLi).³⁴ Furthermore, reacting triphenylarsine (AsPh₃) with an alkali metal (e.g., Li, Na, or K) has been reported to generate the corresponding arsenic nucleophiles.^35,36 Martín and colleagues reported the transformation of triphenylarsine (AsPh₃) into sodium diphenylarsenide (NaAsPh₂), which subsequently reacts with tributyltin chloride (Bu₃SnCl) to afford stannyldiphenylarsine (Bu₃Sn–AsPh₂) (Scheme 2b).^37–42 This stannylated species serves as a valuable precursor for Pd-catalyzed cross-coupling with aryl iodides (ArI), enabling the formation of unsymmetrical triarylarsines (AsArPh₂).

Scheme 2 Preparation methods for arsenic nucleophiles. (a) Deprotonation of Ph₂AsH, (b) As–C bond cleavage to generate arsenosodium species, (c) As–As bond cleavage of diarsane by phenyllithium, and (d) As–As bond cleavage of Ph₆As₆ by organolithium reagents.

An alternative approach involves the cleavage of As–As bonds to access nucleophilic arsenic species. Kauffmann and colleagues demonstrated that diarsane compounds undergo selective bond scission upon treatment with phenyllithium (PhLi), leading to the formation of arsenolithium intermediates (Scheme 2c).^43,44 One phenyl group is introduced onto one arsenic center, while the other is converted into an organolithium species. This strategy can be extended to cyclic oligoarsines. For example, sequential cleavage of the As–As bonds in hexaphenylhexaarsine (Ph₆As₆) by PhLi generates diphenylarsenyllithium (Ph₂AsLi) (Scheme 2d), which can be subsequently alkylated with benzyl bromide to furnish benzyldiphenylarsine.⁴⁵

2.3. Other arsination methods

Hydroarsination of alkenes and alkynes remains a foundational method for synthesizing organoarsenic compounds.^46–55 The required arsenic hydride precursors can be obtained by reducing arsenic halides or arsonic acids.^56,57 One classic example is the hydroarsination of 1,3-diynes with phenylarsine (PhAsH₂), which yields arsole derivatives (Scheme 3a).⁵⁵ Leung and colleagues developed a broad range of hydroarsination protocols, particularly those involving asymmetric induction.^{48–51,53,54} Using chiral palladium complexes as templates, they enantioselectively synthesized arsenic-containing ligands (Scheme 3b),⁴⁸ including arsenic–nitrogen and arsenic–phosphorus bidentate scaffolds. In addition, they established nickel-catalyzed asymmetric hydroarsination of nitrostyrenes (Scheme 3c).⁵¹ As a complementary approach, phosphine-promoted hydroarsination offers a metal-free route: triphenylphosphine (PPh₃) facilitates the addition of diphenylarsine (Ph₂AsH) across C=C double bonds.⁵³

Scheme 3 Arsination reactions. (a) Synthesis of arsole via hydroarsination and (b) chiral template for hydroarsination. (c) Asymmetric hydroarsination, (d) bisarsination using Me₅As₅, and (e) Pd-mediated aryl group scrambling reaction between aryl triflates and AsPh₃.

Diarsanes bearing As–As bonds also serve as useful precursors for bisarsination reactions, enabling the simultaneous incorporation of two arsenic units. For example, reaction of tetraphenyldiarsane (Ph₂AsAsPh₂) with phenylacetylene under azobis(isobutyronitrile) (AIBN) initiation and photoirradiation yields 1,2-bis(arsino)ethylenes.⁵⁸ Cyclooligoarsines such as Me₅As₅ can also undergo efficient bisarsination. When irradiated in the presence of 2,3-dimethyl-1,3-butadiene, Me₅As₅ furnishes 1,2,4,5-tetramethyl-tetrahydrodiarsinine in 95% yield (Scheme 3d).⁵⁹ Related polymerization and cycloaddition reactions involving cyclooligoarsines and alkynes are discussed in sections 4.1 and 5.1, respectively.

Chan and colleagues reported a unique Pd-mediated aryl group scrambling reaction, wherein aryl groups from Pd–Ar species are exchanged with phenyl groups from AsPh₃. This process generates C_s-symmetric AsArPh₂ via Pd–Ar/As–Ph transmetalation (Scheme 3e).^60,61 Notably, this strategy is compatible with various aryl triflates and has been employed to synthesize chiral arsenic–nitrogen bidentate ligands and their corresponding metal complexes. The Friedel–Crafts reaction is one of the few methods available for the direct introduction of arsenic into aromatic rings.⁶² While several examples of intramolecular Friedel–Crafts reactions have been reported, intermolecular cases remain rare. Lewis and colleagues obtained phenoxarsine by refluxing diphenyl ether with trihaloarsine and subsequently adding aluminum chloride.

Recently, yellow arsenic (As₄) has been employed as a precursor and converted into a variety of arsenic compounds incorporating transition metal complexes.⁶³ The As–As bond rearrangements give rise to oligomeric arsenic species, further expanding the scope of organoarsenic chemistry. These advances highlight the unique reactivity of As₄ compared with its lighter homologue white phosphorus (P₄). Moreover, the development of storage materials and transfer reagents for As₄ has opened new opportunities for safer handling and broader synthetic applications.

3. Conjugated molecules

3.1. Arsoles

Arsoles were first reported in the 1920s.^64,65 Traditionally, their synthesis has relied on volatile and toxic arsenic chlorides or hydrides (Scheme 4a).^55,66–70 Owing to serious safety concerns, many studies in this area remained computational, focusing on estimating structural and electronic properties. Consequently, experimental investigations into arsole derivatives were scarce until our group developed a synthetic strategy in 2015 that avoided the use of hazardous precursors.²⁷ This method utilizes diiodoarsines generated in situ from cyclooligoarsines, which react with organometallic reagents to afford a variety of arsole derivatives (Scheme 4b).

Scheme 4 Synthesis of arsoles: (a) conventional and (b) practical methods.

One notable class of arsoles is arsafluorenes, which are arsenic analogues of carbazole. These compounds are synthesized from RAsI₂ (R = Me, Ph) and 2,2′-dilithiobiphenyl.²⁷ Importantly, access to 9-phenylarsafluorene enabled the first direct experimental comparison across group 15 analogs of carbazole (Fig. 2a)—namely carbazole (N), phosphafluorene (P), arsafluorene (As), stibafluorene (Sb), and bismafluorene (Bi).⁷¹ While the nitrogen atom in carbazole exhibits a delocalized lone pair that enforces a trigonal planar geometry, the heavier pnictogen analogs adopt trigonal pyramidal geometries owing to limited lone pair delocalization. Reactivity studies using oxidants such as O₂, I₂, and AuCl revealed contrasting behaviors among the group 15 elements. Carbazole was unreactive owing to the inertness of the C–N bond and lone-pair delocalization. Phosphafluorene easily oxidized in air while maintaining its molecular framework. By contrast, arsafluorene demonstrated excellent oxidative stability and strong coordination ability, readily forming complexes with AuCl. The heavier congeners displayed increased reactivity: 9-phenylstibafluorene lost the phenyl group, while 9-phenylbismafluorene underwent complete cleavage of the Bi–C bond. These studies revealed that arsafluorene uniquely balances structural stability with coordination reactivity. Photophysical measurements further elucidated group-specific behavior. All derivatives—carbazole, arsafluorene, stibafluorene, and bismafluorene—showed dual emission (fluorescence and phosphorescence), arising from lone-pair or heavy-atom-mediated intersystem crossing (ISC). We also demonstrated that bridging arsafluorene with silicon or germanium enhances phosphorescence via increased spin–orbit coupling.⁷²

Fig. 2 Arsole derivatives. (a) Group 15 analogs of carbazole. (b–e) Emissions of (b) 1,2,5-triphenylarsole and 1,2,5-triphenylphosphole, (c) 1,2,3,4,5-pentaphenylarsole, (d) 4-phenyldithieno[3,2-b:2′,3′-d]arsoles, and (e) 4-phenyldithieno[3,4-b:3′,4′-d]arsole. (f) Selective transformation of dipyridinoarsole. (g–i) Chemical structures of (g) As-heteropentacenes, (h) benzoarsole, and (i) dithiazoloarsole.

Additional arsole derivatives were synthesized by reacting PhAsI₂ with organometallic precursors such as titanacyclopentadienes, which enabled regioselective access to 1,2,5-triarylarsoles.^73,74 Unlike arsafluorene, these 1,2,5-triarylarsoles exhibit strong room-temperature emission. For instance, 1,2,5-triphenylarsole emits at 458 nm with a quantum yield (Φ) of 0.59, comparable to that of 1,2,5-triphenylphosphole (emission maximum: λ_em = 465 nm, Φ = 0.56) (Fig. 2b).

Solid-state luminescence properties of arsoles have also been explored. 1,2,3,4,5-Pentaarylarsoles show aggregation-induced emission.⁷⁵ While the Φ of 1,2,3,4,5-pentaphenylarsole in solution is low (0.01) owing to phenyl group rotation, restricted motion in the solid state boosts Φ to 0.61 (Fig. 2c)—comparable to that of pentaphenylsilole. The stability of cycloalka[c]arsoles, synthesized via reactions of PhAsI₂ with zirconacyclopentadienes, is highly dependent on ring size.⁷⁶ For example, cyclopentane-fused derivatives are stable under ambient conditions, while cyclohexane-fused derivatives decompose in solution via photoinduced reaction with singlet oxygen (¹O₂), regenerating phenylarsonic acid.⁷⁷ This unique degradation pathway enables chemical recycling of arsenic units back into arsoles.

π-Extended arsoles such as 4-phenyldithieno[3,2-b:2′,3′-d]arsoles were developed by Heeney et al. in 2016.⁷⁸ Dibromination using N-bromosuccinimide, followed by Pd-catalyzed coupling, provided access to π-extended dithienoarsoles and their polymers (see section 4.1).⁷⁹ These compounds exhibit strong emission, tunable via oxidation or coordination at the As center. Computational analyses revealed arsenic contributions to the lowest unoccupied molecular orbital (LUMO) through σ*–π* interactions, similar to phospholes.⁸⁰ Oxidation or metal coordination lowers the LUMO energy, resulting in bathochromic shifts in absorption and emission (Fig. 2d). Substituent effects were also significant: sterically bulky groups distorted the π-system, producing red-shifted emission owing to structural relaxation in the excited state via As–C bond elongation.⁸¹

Dithieno[3,4-b:3′,4′-d]arsoles, isomers of dithieno[3,2-b:2′,3′-d]arsoles, displayed different photophysical behavior:⁸² while dithieno[3,2-b:2′,3′-d]arsoles showed phosphorescence at 77 K, dithieno[3,2-b:2′,3′-d]arsoles exhibited fluorescence only (Fig. 2e). Dipyridinoarsoles demonstrated chemoselective redox and coordination chemistry—arsenic centers selectively oxidize or coordinate, while pyridyl nitrogens are quaternized to form viologen-type structures (Fig. 2f).⁸³ These species exhibit reversible electrochromism. This behavior is likely associated with the oxidative resistance of As(III), which contributes to electrochemical stability. Notably, Baumgartner and co-workers have reported reversible electrochromism in phosphaviologens, specifically in the oxidized forms of phosphoryl-bridged viologens.^84,85 In addition, the hard basicity of nitrogen further enables selective coordination to Zn²⁺, allowing metal–organic framework (MOF) construction without involving the arsenic center (see section 4.3).⁸⁶

Additional arsole derivatives include benzothiophene-fused As-heteropentacenes (Fig. 2g), which show higher charge mobility than their benzofuran analogs owing to intermolecular As⋯S interactions in thin films.⁸⁷ Substituent-dependent fluorescence behavior was also observed in 2-arylbenzo[b]arsoles (Fig. 2h): electron-donating groups promote ISC, populating T₁ states that deactivate non-radiatively.⁸⁸ Heeney and colleagues synthesized dithiazoloarsole from PhAsCl₂ and studied its aromaticity, air stability, and photophysics (Fig. 2i).⁸⁹ Comparative studies of arene-fused benzoarsoles with silole, germole, and phosphole analogs revealed that the trivalent arsenic atom enhances ISC through its lone pair and heavy atom effect, contributing to relatively efficient phosphorescence.⁹⁰ Notably, the corresponding arsole oxide, in which the arsenic lone pair is absent, displayed efficient fluorescence rather than phosphorescence, underscoring the critical role of the lone pair in facilitating ISC.

In response to toxicity concerns, the cytotoxicity of π-conjugated organoarsenic compounds was evaluated using HCT-116 human colon cancer cells.⁹¹ Nanoparticle formulations of these compounds exhibited low-to-negligible cytotoxicity (IC₅₀ > 10 μM), in stark contrast to the well-known toxin phenylarsine oxide (IC₅₀ = 2.8 μM). Notably, phenylarsonic acid—a key synthetic precursor—also showed minimal cytotoxicity, further supporting the safety of these advanced materials.

3.2. Excited state dynamics

Luminescent dyes that undergo substantial structural relaxation upon photoexcitation have attracted considerable attention owing to their large Stokes shifts and environmentally responsive emission behavior. A representative example is cyclooctatetraene, in which excited-state planarization is driven by Baird aromaticity—an electronic phenomenon in which 4n π-electron systems become aromatic in the excited state, in contrast to Hückel's rule governing ground-state aromaticity.^92,93 In this context, the seven-membered ring system of arsepin is particularly intriguing, as it possesses an 8π-electron configuration suitable for Baird aromaticity in the excited state. By contrast, arsoles, with their 6π-electron systems, follow conventional Hückel-type aromaticity.⁹⁴ Arsepins adopt bent geometries in the ground state in accordance with Hückel's rule but planarize upon photoexcitation owing to the onset of Baird aromaticity. Dibenzoarsepin and its gold(I) chloride complex have been synthesized and studied in detail. Dibenzoarsepin displays dual fluorescence at 371 nm and 557 nm, corresponding to emission from the Franck–Condon geometry and the planarized excited state, respectively (Fig. 3a). By contrast, the gold(I) chloride complex exhibits only the shorter-wavelength emission (375 nm), attributed to coordination of the arsenic lone pair, which suppresses planarization and thus precludes long-wavelength emission. Temperature-dependent photoluminescence measurements revealed that the energy barrier between the two excited-state conformers of dibenzoarsepin is 3.2 kJ mol⁻¹. Following this discovery, various heteropins, including oxepins and azepins, have been developed to explore excited-state aromaticity phenomena.^95,96

Fig. 3 Excited state dynamics and photoluminescence spectra of (a) dibenzoarsepin and (b) arsenic-bridged diphenyl sulfone.

Arsenic-bridged diphenyl sulfones present another example of unusual excited-state dynamics.⁹⁷ Like triarylphosphines, triarylarsines undergo planarization upon excitation owing to a reduction in electron density around the pnictogen atom.⁹⁸ This effect enhances the Lewis acidity of the pnictogen center, enabling photo-induced Lewis pair formation when an internal Lewis base is present. In arsenic-bridged diphenyl sulfone, the As⋯O distance shortens from 3.18 Å in the ground state to 2.27 Å in the singlet excited state, leading to significant bending of the central six-membered ring (Fig. 3b). An energy barrier between the Franck–Condon and relaxed geometries results in dual fluorescence, with emission properties sensitive to environmental factors such as temperature and solvent viscosity. The excited-state relaxation behavior of other pnictogen-bridged systems—such as diphenyl ethers, diphenyl sulfides, and diphenyl sulfoximines—has also been examined.^99,100 Both the identity and oxidation state of the bridging pnictogen atom strongly influence structural changes upon excitation. In the case of pnictogen-bridged sulfoximines, substituents on the sulfoximine nitrogen also play a critical role in tuning excited-state dynamics.

3.3. Other conjugated molecules

Orthaber and colleagues reported the synthesis of an arsaalkene featuring an exocyclic arsenic atom on a cyclopentadithiophene core (Fig. 4a).¹⁰¹ Compared to its phosphaalkene analog, the arsaalkene displays a more stabilized LUMO, highlighting the electronic influence of arsenic relative to phosphorus. Our group recently synthesized a series of arene-substituted arsines, in which the phenyl groups of AsPh₃ were replaced by expanded π-systems such as naphthyl, phenanthryl, and pyrenyl groups (Fig. 4b).¹⁰² The naphthyl and phenanthryl derivatives phosphoresced at 77 K, whereas the pyrenyl analogs primarily fluoresced. (Diphenylarsino)quinolines also phosphoresced at low temperature, and their solid-state emission properties were found to depend critically on the substitution pattern of the diphenylarsino group on the quinoline ring (Fig. 4c).¹⁰³

Fig. 4 (a) Synthesis of alsaalkene. (b and c) Chemical structures of (b) triarylarsines and (c) AsPh₂-substituted quinoline derivatives.

Designing planar π-conjugated molecules incorporating arsenic is particularly challenging owing to the inherently pyramidal geometry of tertiary arsines. One strategy to achieve planarity involves the development of heavier analogs of pyridine, such as phosphinines and arsinines. However, these heavier heterocycles often suffer from instability and dimerization, which necessitate the use of bulky substituents that can, in turn, compromise planarity.^104,105 To circumvent this, Yamada and Matsuo introduced benzothiophene-fused phosphinines, which demonstrated high stability without the need for steric protection.¹⁰⁶ Inspired by this, we designed and synthesized thiophene- and benzothiophene-fused arsinines (Fig. 5a), whose planarity was confirmed by structural analysis.¹⁰⁷ Among them, benzothiophene-fused arsinines exhibited remarkable ambient stability. Their emission properties vary depending on the fused ring and fusion mode. Notably, α-type benzothiophene-fused arsinines exhibit intermolecular As⋯As interactions in the solid state—interactions not observed in their phosphorus analogs—owing to the larger atomic radius of arsenic. These interactions offer novel design opportunities for arsenic-based optoelectronic materials.

Fig. 5 Chemical structures and emission behaviors of (a) arsinines and (b) arsaborines.

The incorporation of other heteroatoms into the π-conjugated framework is another promising strategy to diversify arsenic-containing materials. Dibenzoheteraborins are particularly attractive owing to their rigid, planar architectures, which promote electronic communication between the heteroatoms and the π-system. Shuford reported theoretical studies on the aromaticity of arsaborins.¹⁰⁸ We synthesized the first dibenzoarsaborins and evaluated their photophysical properties (Fig. 5b).¹⁰⁹ At 298 K, these compounds fluoresced with large Stokes shifts, attributed to significant structural relaxation. At 77 K, strong phosphorescence was observed, driven by the heavy-atom effect of arsenic. A thiophene-fused arsaborin derivative displayed temperature-dependent emission color changes in the solid state, resulting from shifts in the contributions of monomer fluorescence, excimer fluorescence, and phosphorescence.

4. Polymer materials

4.1. Conjugated polymers

The first structurally characterized arsenic-containing conjugated polymer is poly(vinylene arsine).^110–113 This polymer is obtained by radical-initiated copolymerization of cyclooligoarsines (As₅Me₅ or As₆Ph₆) with terminal alkynes in the presence of AIBN. The resulting poly(vinylene arsine)s feature well-defined alternating sequences owing to a mechanism involving radical chain ring-opening alternating copolymerization (RCRAC) (Scheme 5). The AIBN-derived radicals cleave As–As bonds in the cyclic oligomers, forming reactive arsenic radicals. Ring opening leads to further homolytic bond cleavage, and these arsenic radicals add to terminal alkynes, generating vinyl radicals that propagate the copolymerization. Notably, the resulting polymers are soluble in common organic solvents, enabling full structural characterization by nuclear magnetic resonance spectroscopy, size exclusion chromatography, and other methods.

Scheme 5 Mechanism of RCRAC.

Following these early reports, arsenic-containing conjugated polymers remained largely unexplored until 2016, when three independent studies, including our own, reignited interest in the field.^78,79,114 Heeney and colleagues synthesized a dithienoarsole dibromide monomer and polymerized it via Stille coupling with trans-1,2-bis(tributylstannyl)ethene to produce a dithienoarsole-based polymer (Scheme 6a).⁷⁸ This polymer exhibited promising hole mobility (0.08 cm² V⁻¹ s⁻¹) owing to efficient solid-state aggregation and crystallinity, along with notable ambient stability. Simultaneously, our group developed a dithienoarsole polymer via Suzuki–Miyaura polycondensation of dithienoarsole dibromide with fluorene diboronic acid (Scheme 6b).⁷⁹ The resulting polymer displayed strong yellow fluorescence in solution with high quantum yield (Φ = 0.44).

Scheme 6 Synthesis of arsole polymers reported in 2016. (a) Microwave-assisted Stille coupling, (b) Suzuki–Miyaura coupling, and (c) post-polymerization functionalization with titanocyclopentadiene polymer.

We also collaborated with Tomita to synthesize another type of arsole polymer via post-polymerization functionalization.¹¹⁴ A polymer bearing titanocyclopentadiene units was reacted with PhAsI₂ to afford an arsole polymer (Scheme 6c). The same method successfully produced a phosphole analog for comparison.¹¹⁵ Cyclic voltammetry revealed that the arsole polymer exhibited quasi-reversible redox behavior, while the corresponding phosphole polymer irreversibly degraded, highlighting the greater oxidative robustness of the As(III) center.

In 2017, Kuehne and Orthaber independently expanded the family of arsenic-containing π-conjugated polymers.^116,117 Kuehne and colleagues synthesized heterofluorene–fluorene copolymers incorporating various heteroatoms (Si, Ge, N, As, Se, and Te) into the central fluorene unit (Scheme 7a).¹¹⁶ The arsafluorene-based polymer, synthesized from arsafluorene dibromide, emitted blue light at 458 nm with a moderate quantum yield (Φ = 0.11). Remarkably, among the series, only the arsafluorene and germafluorene copolymers demonstrated amplified spontaneous emission (ASE) upon nanosecond laser excitation, positioning arsenic-based systems as strong candidates for photonic and laser applications. Orthaber and colleagues further synthesized a thiophene-substituted arsaalkene monomer and electropolymerized it (Scheme 7b).¹¹⁷ Chronoamperometry of the resulting 2 μm thick film revealed a reversible electrochromic response at 600 nm with a high contrast ratio of 58%, maintained over multiple cycles. The doped polymer remained stable for several days under inert conditions.

Scheme 7 Synthesis of (a) arsafluorene polymer, (b) arsaalkene polymer via electropolymerization, (c) dithienoarsole polymers (TBAB = tetrabutylammonium bromide), (d) dithienoarsole polymer via electropolymerization, (e) dithienoarsole polymer for OPV, and (f) NIR-emissive polymers.

A key advantage of arsenic-based π-conjugated polymers lies in their low propensity for metal contamination. In typical transition-metal-catalyzed polycondensations (e.g., Suzuki–Miyaura, Sonogashira–Hagihara), residual metal catalysts can be problematic, particularly owing to the coordinating ability of As(III). However, we found that palladium and copper residues can be effectively removed using scavenger resins post-polymerization (Scheme 7c).¹¹⁸ X-ray fluorescence spectroscopy confirmed negligible metal content, indicating that As(III) exhibits weaker coordination than phosphorus, facilitating purification.

The electrochemical stability of arsenic-containing conjugated units has also been validated. Hayashi and our group demonstrated that while dithienoarsole is electrochemically inert in the presence of tetraethylammonium tetrafluoroborate (Et₄NBF₄), boron trifluoride diethyl etherate (BF₃·OEt₃) lowers its oxidation potential, enabling electropolymerization to produce a uniform homopolymer film on an ITO substrate (Scheme 7d).¹¹⁹

Dithienoarsole–benzodithiophene copolymers have been evaluated as donor materials in organic photovoltaic (OPV) devices (Scheme 7e).¹²⁰ Synthesized via Stille coupling between dithienoarsole dibromide and stannylated benzodithiophene, these polymers exhibited good ambient stability—an essential property for OPV materials. The power conversion efficiency (PCE) reached 3.90%, with open-circuit voltages as high as 0.91 V. Variations in alkyl chain length influenced PCE, solubility, and morphology. Further functionalization of dithienoarsole monomers with long alkyl chains enhanced solubility and backbone planarity.¹²¹ Copolymerization with electron-deficient units such as benzothiadiazole or benzoxadiazole afforded donor–acceptor polymers, exhibiting near-infrared emission (Scheme 7e). The benzoxadiazole copolymer emitted at 886 nm.

Singlet oxygen (¹O₂) photosensitizers are a particularly noteworthy application of dithienoarsole–fluorene copolymers (Fig. 6).¹²² The polymer showed an exceptional ¹O₂ quantum yield of 0.54, the highest reported for a single-component π-conjugated polymer, and outperformed arsenic-free analogs (e.g., 0.14 for bithiophene–fluorene copolymer). The ¹O₂-generation efficiency is attributed to the heavy atom effect of arsenic. Importantly, the polymer backbone remains photostable under ¹O₂-generating conditions. Under continuous pulsed laser excitation, two dithienoarsole polymers exhibited sustained ASE for over 15 hours at pump energies up to 28.9 μJ.¹²³ Under similar conditions, rhodamine 6G rapidly degraded, highlighting the superior photostability of the arsenic-based systems.

Fig. 6 Singlet oxygen generation using dithienoarsole polymer as a photosensitizer.

Additionally, we synthesized a novel arsole monomer, 2,3-diarylbenzoarsole dibromide, via a zirconacycle intermediate.¹²⁴ This monomer was copolymerized with fluorene diboronic acid using Suzuki–Miyaura coupling to yield a main-chain polymer (Scheme 8a). A side-chain polymer analog was also synthesized via ring-opening metathesis polymerization of a norbornyl-substituted monomer using Grubbs’ third-generation catalyst (Scheme 8b). In the solid-state, the main-chain polymer exhibited aggregation-caused quenching owing to π–π stacking, while the side-chain polymer showed aggregation-induced emission enhancement owing to restricted intramolecular motion. Furthermore, 2,3-diarylbenzoarsoles possess As-stereogenic centers, and optical resolution of 2,3-diphenylbenzoarsole was achieved, indicating that their chiroptical properties warrant further exploration in polymeric systems.

Scheme 8 Synthesis and quantum yields of (a) main-chain- and (b) side-chain-type 2,3-diphenylbenzoarsole polymers.

4.2. Reactive polymers

Polymers bearing arsonic acid functionalities have been studied since the 1970s, primarily as metal-binding agents and polyelectrolytes owing to their ionic character.^125–127 In this minireview, we highlight recent developments in side-chain-type polymers that exploit the chemical reactivity of arsenic atoms. Work in this area has been pioneered by Wilson and colleagues.^128–131 They synthesized diblock copolymers comprising poly(ethylene glycol) methyl acrylate (PEGA), N-isopropylacrylamide (NIPAM), and phenylarsonic acid-functionalized acrylamide (AsAm) segments via living radical polymerization. Upon reduction of the arsonic acid groups with phosphinic acid (H₃PO₂), cross-linking occurs between NIPAM and AsAm segments through As–As bond formation, resulting in the formation of polymeric nanoparticles (Scheme 9a).¹²⁹ A similar approach was applied to prepare hydrogels: random copolymers of NIPAM and AsAm were synthesized by free-radical polymerization, and their reduction with H₃PO₂ generated As–As-linked networks, producing hydrogels.¹³⁰ Furthermore, p-arsanilic acid could be incorporated into the hydrogel matrices and released under oxidative conditions, demonstrating potential as a redox-responsive drug delivery system.¹³¹

Scheme 9 Reactive polymers having arsenic side chains for (a) cross-linking and (b) catalytic arsa-Wittig reaction. VA-044: 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride; TCP = tetra(p-chlorophenyl)porphyrinate; PMHS = polymethylhydrosiloxane.

Tang and colleagues developed a different class of reactive polymers by introducing diphenylarsine (AsPh₂) moieties into polymer side chain for use in catalytic arsa-Wittig reactions (see section 6.1 and Scheme 9b).¹³² While free AsPh₃ is weakly nucleophilic and requires activation with Fe(TCP)Cl (TCP = tetra(p-chlorophenyl)porphyrinate) for ylide generation from diazo compounds, immobilizing the AsPh₂ units on a polymer backbone enhances recyclability. Ethylene–diphenyl(undec-10-enyl)arsine copolymers were synthesized by coordination polymerization and exhibited high catalytic activity and E-selectivity in the arsa-Wittig reaction of ketones with diazo compounds in the presence of Fe(TCP)Cl and polymethylhydrosiloxane (PMHS). These polymer-supported catalysts retained their activities over at least five cycles.

4.3. Coordination polymers

MOFs are a class of crystalline porous materials with wide-ranging applications in gas storage, separation, catalysis, and sensing. Post-synthetic functionalization (PSF) is a powerful strategy to enhance MOF functionality and complexity.^133,134 MOFs that feature coordination-free donor sites within their pores are particularly attractive for incorporating metal catalysts. However, protecting groups are often required during MOF synthesis to prevent premature coordination. Since deprotection post-assembly can be challenging, protection-free strategies are highly desirable. Phosphines have been explored for PSF owing to their soft Lewis basicity and limited interaction with hard metal nodes. However, their air sensitivity and tendency to oxidize during MOF synthesis necessitate stringent anaerobic conditions. By contrast, arsenic offers a promising alternative: it has soft Lewis basicity like phosphorus but exhibits significantly lower oxophilicity, making it more compatible with aerobic MOF construction.

To demonstrate this concept, we designed a novel ligand, cis-1,4-dihydro-1,4-diarsinine (cis-DHDA) tetracarboxylic acid, which bears four carboxylate groups for MOF construction and two arsenic centers reserved for post-synthetic metal coordination (Scheme 10a).¹³⁵ Although a copper(II)-based MOF was successfully formed from this ligand, its crystalline structure collapsed upon solvent removal, likely owing to insufficient framework rigidity. More successful implementation of arsenic-centered PSF was reported by Humphrey and colleagues.¹³⁶ They synthesized ACM-1, a coordination polymer formed from a pyridyl-functionalized triarylarsine ligand and Ni(II) cations (Scheme 10b). In this framework, two Ni(II) cations are bridged by carboxylate anions and a μ₂-OH, while the N-donor sites of the arsenic-containing ligand coordinate to the Ni(II) centers. Notably, the uncoordinated arsenic atoms within the framework remained accessible and were successfully functionalized with gold(I) chloride in a post-synthetic modification step.

Scheme 10 Synthesis and crystal structures of MOFs bearing (a) DHDA, (b) pyridyl-functionalized triarylarsine, and (c) dipyridinoarsole ligands (guest: solvent (DMF) or gas (N₂ and CO₂) molecules). The hydrogen atoms are omitted for clarity in the crystal structures.

In another example, Zn(II)-based MOFs incorporating dipyridinoarsole ligands were constructed using the two pyridyl groups for framework formation,⁸⁶ leaving the arsenic atoms uncoordinated and available for PSF (Scheme 10c). This study revealed notable structural flexibility and gas adsorption behavior. Remarkably, a phenyl-substituted dipyridinoarsole formed a “breathing” MOF, exhibiting reversible pore opening and closing in response to guest molecules adsorption/desorption. By contrast, the MOFs using methyl-substituted dipyridinoarsole or phenyl-substituted dipyridinophosphole molecules partially amorphized upon solvent removal and did not exhibit breathing behavior. These results indicate that the steric bulk of the phenyl group at the arsenic atom plays a critical role in stabilizing the closed-pore conformation.

5. Ligands for metal complexes

5.1. Ligand library

Unlike the broad availability and commercial accessibility of phosphine ligands, structurally diverse arsine ligands remain scarce. Beyond AsPh₃, most arsine ligands are synthetically challenging to access, limiting the systematic investigation of structure–property relationships in arsine-ligated metal complexes. As a result, the design of functional coordination compounds leveraging the unique electronic and steric properties of arsenic has long been underexplored. However, the recent development of practical and modular synthetic methods for organoarsenic compounds has significantly advanced the field.

Cyclooligoarsines Me₅As₅ and Ph₆As₆ serve as key precursors for the synthesis of structurally diverse arsine ligands. During the RCRAC process, vinyl radicals can be stabilized by electron-withdrawing substituents (e.g., esters), promoting Z-to-E isomerization.^137,138 Subsequent dimerization of the E-form radicals yields cyclic diarsenic compounds known as cis-1,4-dihydro-1,4-diarsinines (cis-DHDAs). These cyclic ligands have been coordinated with a variety of transition metals, including Pt(II), Pd(II), Au(I), and Cu(I) (Scheme 11).^139–146 The cis-DHDA framework holds two arsenic donor atoms at a fixed short distance (∼3 Å), offering a rigid template for the construction of di- or multinuclear metal complexes.

Scheme 11 Synthesis of cis-DHDA-ligated transition metal complexes.

The arsenic electrophile PhAsI₂, which is readily generated from Ph₆As₆, has been used to prepare diarylphenylarsines (Fig. 7a).¹⁴⁷ Moreover, dithiaarsole can provide further versatile C_s-symmetric ligands such as arsa-Buchwald ligands (Fig. 7b).²⁹ In turn, nucleophilic substitution using organolithium reagents (RLi) with Ph₆As₆ furnishes RPhAsLi species, which can be elaborated into C₁-symmetric arsines bearing a stereogenic arsenic center (Fig. 7c).⁴⁵ These nucleophiles are also suitable for preparing bidentate arsine ligands. Additionally, Ph₂AsI, accessible via oxidative cleavage of Ph₂AsMe, provides a route to chelating diphenylarsine-type ligands. These synthetic platforms have enabled access to arsenic analogs of well-established phosphine ligands such as dppe, xantphos, and dppf, thereby extending the structural diversity of arsine ligands available for coordination chemistry (Fig. 7d).¹⁴⁸

Fig. 7 Arsenic ligand library. (a) Diarylphenylarsines, (b) arsa-Buchwald ligands, (c) C₁-symmetric ligand, (d) bidentate ligands, (e) NAN-ligand, (f) arsafluorene dimer, and (g) arsanylborane.

A new class of chelating ligands has been established through the synthesis of phenyldiquinolinylarsine, a tridentate N–As–N (NAN) pincer-type ligand (Fig. 7e).¹⁴⁹ Copper(I) halide complexes formed with this ligand display highly distorted geometries. The steric strain typically associated with tridentate ligand binding is alleviated around the arsenic center, which exhibits less directional coordination behavior than phosphorus, reflecting the softer and more diffuse lone pair of As(III).

Orthaber and colleagues also reported arsafluorene dimers linked via phenylene bridges on the arsenic centers (Fig. 7f).¹⁵⁰ The flexibility of these ligand backbones permits the formation of both mono- and dinuclear metal complexes. Remarkably, a unique rearrangement of an allylic intermediate was observed during complexation, leading to the formation of an arsapalladacycle—a structural motif rarely seen in organoarsenic chemistry.

Scheer and co-workers demonstrated the versatility of arsanylborane complexes (Fig. 7g) through selective functionalization and coordination. For instance, Lewis acid/base-stabilized phosphanyl- and arsanylboranes undergo selective dihalogenation at the pnictogen atom with CX₄ (X = Cl, Br), affording novel complexes comprehensively characterized by spectroscopy and X-ray crystallography.¹⁵¹ Moreover, gold(I) complexes bearing phosphanyl- and arsanylborane ligands exhibit unsupported Au⋯Au interactions and one-dimensional chain formation in the solid state, leading to intriguing luminescence properties such as strong temperature-dependent red shifts correlated with decreasing Au⋯Au distances.¹⁵² In addition, a tert-butyl-substituted arsanylborane has been developed as a versatile building block, showing diverse reactivity toward Lewis acids and transition metals, and enabling the first synthesis of oligomeric arsanylboranes under thermal conditions.¹⁵³

Functionally tailored ligands have also been developed using cyclooligoarsine-based methodologies. In particular, luminescent metal complexes and transition-metal catalysts derived from these ligands are discussed in subsequent sections. We designed dibenzoarsacrowns, in which an oxygen atom in conventional crown ethers is replaced with an arsenic atom (Fig. 8a).¹⁵⁴ The soft Lewis basicity of arsenic allows for selective coordination to late transition metals, while the oxygen atoms retain their strong binding affinity for alkali metal cations (Fig. 8b). Intriguingly, pre-coordination of the arsenic atom to AuCl enhances the alkali metal binding constant, demonstrating a positive allosteric effect. Moreover, weak secondary interactions between the arsenic atom and the encapsulated alkali metal modulate the redox behavior of the crown structure, enabling electrochemical sensing of alkali metal ions.¹⁵⁵ Additionally, arsafluorene dimers linked via oligo(ethylene glycol) chains can form enchained-type metallacrown architectures through coordination to Pt(II) dihalides (Fig. 8c).¹⁵⁶ The geometric structure around the Pt(II) center and its phosphorescent properties were found to be strongly influenced by the coordinated halide anions.

Fig. 8 (a) Chemical structures of dibenzoarsacrowns. (b) Coordination behaviors of 21-dibenzoarsacrown-7 and crystalline structures of the resultant complexes. (c) Construction of enchained-type metallacrown ethers with arsafluorene-dimer.

5.2. Luminescent materials

Luminescent transition metal complexes bearing arsine or arsine oxide ligands have been extensively investigated. Recent advances in arsine ligand synthesis have enabled systematic studies on the relationship between ligand structure and photophysical properties.¹⁵⁷ Complexes of platinum(II), gold(I), silver(I), and copper(I) with structurally diverse arsine ligands have been synthesized, and in some cases, they exhibit highly efficient room-temperature phosphorescence or thermally activated delayed fluorescence.^{144,145,158–168} This is likely attributed to the heavy-atom effect of arsenic, which enhances spin–orbit coupling and promotes ISC from the singlet to the triplet excited state. For example, the platinum(II) complex trans-[PtI₂(9-phenyl-9-arsafluorene)₂] has a phosphorescence quantum yield of 0.52 in the solid state at room temperature.¹⁶³ Substituent variation on the arsenic center via a straightforward substitution route (Scheme 1g) revealed that both the emissive properties and the cis/trans geometry of the complexes are strongly affected.¹⁵⁸ Notably, only trans-isomers with specific ligand/halogen combinations exhibited significant emission, whereas all cis-isomers were non-emissive.

Oligomeric and polymeric architectures have also been proven to exhibit strong phosphorescence. A tetranuclear complex incorporating Au(I)⋯Cu(I) double salts, stabilized by homo- and heterometallophilic interactions, was constructed using the bidentate ligand dpam (for dpam, please see Fig. 7d).¹⁶⁷ This complex formed three distinct crystalline polymorphs depending on the recrystallization solvent. All polymorphs exhibited remarkably strong room-temperature phosphorescence, with quantum yields reaching up to 0.97, and demonstrated a broad range of emission colors, including near-infrared and white-light emission. Interestingly, these polymorphs could be reversibly interconverted by external stimuli. Additionally, coordination polymers provide a valuable strategy for achieving intense luminescence. Dinuclear rhomboid Cu₂I₂ clusters coordinated by AsPh₃ were linked through various bidentate N-heteroaromatic ligands, yielding strongly emissive coordination polymers with quantum yields up to 0.60.¹⁵⁹ The emission color could be finely tuned across the green-to-red spectrum by altering the bridging ligands.

Beyond emission efficiency, arsine-ligated metal complexes have also demonstrated stimui-responsive luminescence, particularly in crystalline states. The relatively weak and less directional coordination ability of arsine ligands, compared to phosphines, is thought to impart greater structural flexibility, which facilitates crystal-to-crystal transformations. This behavior is particularly pronounced in solvent vapor-responsive crystals, as exemplified below. Crystals of trans-[PtBr₂(9-phenyl-9-arsafluorene)₂] form nonporous molecular crystals (NMCs) without apparent 1D channels.¹⁶⁹ Crystals grown from chlorobenzene (PhCl) are non-emissive owing to encapsulation of PhCl, which acts as a quencher. Exposure to volatile organic compound (VOC) vapors triggers crystal-to-crystal transformation, releasing the PhCl molecules and restoring phosphorescence (Fig. 9a). Remarkably, the NMC exhibits shape-selective molecular recognition: VOCs with smaller minimum cross-sectional diameters induce emission recovery, while bulkier VOCs do not. A broad range of VOCs—including alcohols, ethers, haloalkanes, and alkanes—are applicable.

Fig. 9 Stimuli-responsive crystals. (a) Molecular shape recognition by NMC, (b) switching between porous and non-porous molecular crystals, and (c) MeOH vapor sensing.

A structurally distinct example involves the compound trans-[PtI₂(9-pentafluorophenyl-9-arsafluorene)₂].¹⁷⁰ When crystallized from o-dichlorobenzene, it forms a molecular porous crystal (MPC), while an NMC is obtained from a CH₂Cl₂/hexane mixture. The MPC is non-emissive, whereas the NMC is emissive. These two forms are reversibly interconvertible via solvent vapor exposure, enabling visible luminescence switching (Fig. 9b). Notably, the porous framework of the MPC remains intact even after complete solvent removal, meaning that reversible pore opening and closing is possible without structural collapse.

Another notable case is trans-[PtCl₂(21-dibenzoarsacrown-7)₂], which functions as a turn-on luminescent methanol (MeOH) sensor.¹⁷¹ Crystals obtained from CH₂Cl₂/n-hexane mixtures are non-emissive, while those crystallized from CH₂Cl₂/MeOH are emissive. X-ray crystallography indicates that MeOH molecules are incorporated via hydrogen bonding with the crown ether moieties, restricting molecular motions and thereby suppressing non-radiative decay. The crystal shows fast and reversible luminescence switching upon exposure to MeOH vapor (Fig. 9c): emission turns on within ∼30 s and off within ∼90 s. This MeOH selectivity is exceptionally high, and the sensing performance remains stable over at least five cycles.

In addition to As(III) ligands, arsine oxides (As(V)) have also been explored as metal-coordinating ligands. Phosphine oxides are widely recognized as effective antenna ligands for sensitizing lanthanide (Ln³⁺) emission owing to their low vibrational frequencies (∼1150–1160 cm⁻¹), which reduce non-radiative decay. Furthermore, they induce asymmetric coordination environments, promoting radiative f–f transitions. In this context, arsine oxides (As=O) are attractive alternatives. Their vibrational frequencies (900–920 cm⁻¹) are comparable to that of P=O, and their higher polarizabilities, stemming from the lower electronegativity of arsenic, enhances ligand-to-metal charge transfer according to the dynamic coupling theory.

We synthesized europium(III) complexes bearing arsine oxide ligands.^172–175 For instance, a Eu(NO₃)₃ complex with triphenylarsine oxide (O=AsPh₃) exhibited a 7.9-fold enhancement in photosensitized energy-transfer efficiency compared with its O=PPh₃ analog.¹⁷² Photophysical and computational studies revealed that the T₁ state of O=AsPh₃ dominates the energy-transfer process, supported by the heavy-atom effect of arsenic. These complexes also show exceptional thermal durability, maintaining luminescence at elevated temperatures. In parallel, Layfield reported the synthesis and structural characterization of yttrium complexes bearing arsine, arsenide, and the first rare-earth arsinidene ligand, obtained through stepwise deprotonation of a primary arsine precursor.¹⁷⁶

Bidentate arsine oxide ligands have also been applied to lanthanide coordination polymers (Fig. 10).¹⁷⁵ Eu(hfa)₃ (hfa = hexafluoroacetylacetonate) polymers bridged by bisarsine oxide linkers displayed high quantum yields, enhanced energy transfer efficiency, and excellent thermal stability. Emission intensity increased upon heating from 300 K to 400 K, indicating heat-induced luminescence enhancement—a rare phenomenon attributable to the robust polymeric structure and favorable coordination environment supported by the arsine oxide ligands. Notably, intense luminescence was sustained, even at 550 K.

Fig. 10 Thermally durable emission of bisarsine oxide-bridged Eu(hfa)₃ complex.

5.3. Transition metal catalysts

In transition metal catalysis, arsine ligands have been anticipated to offer significant advantages over traditional phosphine-based systems, potentially enabling breakthroughs in reactivity and selectivity. A landmark study by Farina demonstrated that a palladium catalyst ligated with AsPh₃ accelerated the Stille coupling of iodobenzene and tributylvinyltin by three orders of magnitude compared with its PPh₃ counterpart (Scheme 12a).¹⁷⁷ This dramatic rate enhancement is likely due to the inherently weaker σ-donating ability of arsine ligands, which facilitates more efficient reductive elimination. In the field of hydroformylation, van Leeuwen and co-workers reported that newly designed wide-bite-angle arsine ligands based on the xantphos scaffold exhibited unprecedented activity and selectivity in platinum/tin-catalyzed hydroformylation of terminal alkenes (Scheme 12b).¹⁷⁸ This study provided the first clear demonstration that arsine-based ligands can outperform their phosphine counterparts in industrially relevant hydroformylation processes. Shibasaki et al. developed an axially chiral arsine ligand, BINAAs, the arsenic analog of BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl).¹⁷⁹ In the intramolecular asymmetric Heck reaction, BINAAs provided both higher yields and enantioselectivity compared with phosphine analogs, highlighting the potential of chiral arsine ligands in asymmetric catalysis (Scheme 12c). Nishibayashi reported an arsenic–nitrogen–arsenic (ANA) pincer ligand as a structural analog of phosphorus-based PNP pincers.¹⁸⁰ Ruthenium complexes bearing the ANA framework showed high efficiency and selectivity in dehydrogenation reactions (Scheme 12d), expanding the scope of pincer-type catalyst design.¹⁸¹ Dong and colleagues, known for developing Pd/norbornene cooperative catalysis platforms, discovered that AsPh₃ was essential to promoting vicinal difunctionalization of thiophenes (Scheme 12e). Again, this underscores the unique electronic contributions of arsine ligands in catalytic cycles that require delicate control over elementary steps such as oxidative addition and reductive elimination.

Scheme 12 Reactions with arsine-ligated transition metal catalysts. (a) Stille coupling reaction, (b) hydroformylation, (c) asymmetric Heck reaction, (d) dehydrogenative coupling reaction of alcohol and amine, and (e) C–H difunctionalization of thiophene.

The expanding arsine ligand library, encompassing monodentate, bidentate, and even tridentate structures, provides a valuable platform for investigating the structure–reactivity relationships in transition metal catalysis. Numerous C–C bond-forming reactions—such as Mizoroki–Heck, Stille, Suzuki–Miyaura, and C–H activation—have been explored using structurally diverse arsine ligands.^{28,29,147,148,182–184} A particularly illustrative example is the development of arsa-Buchwald ligands, arsenic analogs of the widely used Buchwald-type phosphines. In the Suzuki–Miyaura coupling of sterically hindered substituents, these ligands exhibited excellent performance (Scheme 13).¹⁸² This behavior can be rationalized in terms of steric parameters, particularly the percent buried volume (%V_bur), which quantifies the steric congestion around the metal center. The %V_bur of SArs—a structural analog of SPhos—is 27.9%, significantly smaller than that of SPhos (35.1%). This indicates that arsine ligands afford a more sterically open catalytic site, which can facilitate the approach and transformation of bulky substrates.

Scheme 13 Suzuki–Miyaura coupling reaction using arsa-Buchwald ligand.

In summary, the incorporation of arsine ligands into transition metal catalytic systems introduces a distinct steric and electronic landscape, enabling reactivity and selectivity patterns that are challenging to achieve using conventional phosphine ligands. As the arsine ligand toolbox continues to expand, new applications in homogeneous catalysis are expected.

6. Reactive species

6.1. Catalytic Wittig reaction

The Wittig olefination is a cornerstone transformation in organic synthesis, enabling the construction of C=C bonds to access highly functionalized or structurally complex molecules. Since the development of the catalytic variant by O'Brien in 2009, which relies on the reductive regeneration of phosphine oxides, extensive efforts have been made to design efficient phosphine catalysts.¹⁸⁵ However, notably, the arsa-Wittig reaction—based on As(III)/As(V) redox cycling—was explored prior to O'Brien's report. Early systems employed simple aryl and alkyl arsines such as AsPh₃ (Scheme 14a)¹⁸⁶ and tributylarsine (AsBu₃) (Scheme 14b),¹⁸⁷ but these were limited by low reactivity and poor stability. Owing to their weak nucleophilicity, arsines like AsPh₃ often require iron co-catalysts, and AsBu₃ suffers from volatility and air sensitivity, hindering practical applications.

Scheme 14 Catalytic arsa-Wittig reactions using (a) AsPh₃, (b) AsBu₃, and (c) tris(p-(dimethylamino)phenyl)arsine as arsine catalysts.

Recent advances have revitalized interest in arsine-mediated olefination. We designed synthetically accessible arsines that enable the development of electron-rich cyclic and acyclic arsine catalysts.^188,189 These new catalysts promote arsa-Wittig reactions under mild, metal-free conditions, even at room temperature. Among them, tris(p-(dimethylamino)phenyl)arsine demonstrated high catalytic efficiency and excellent E-selectivity, offering a promising platform for arsine-based catalytic systems (Scheme 14c).

6.2. Redox behavior

Phosphine-mediated redox reactions, such as the Wittig, Appel, Staudinger, and Mitsunobu reactions, are fundamental to synthetic organic chemistry.^190–192 These transformations rely on the high oxophilicity of phosphorus, which facilitates the formation of phosphine oxides during the oxidative step. However, the reduction of phosphine oxides is often energetically demanding and typically requires strong reductants (e.g., silanes). Cyclic phosphine designs help overcome this barrier by enhancing nucleophilicity and introducing ring strain that destabilizes the phosphine oxide.

By contrast, arsenic exhibits significantly lower oxophilicity, making it a compelling alternative for redox-mediated transformations. In 1986, Abalonin reported that O=AsPh₃ could oxidize benzyl bromide to benzaldehyde, concomitantly regenerating AsPh₃¹⁹³—behavior not observed for O=PPh₃. Inspired by this precedent, we developed an arsine-based oxygen-transfer system involving complementary reactions (Scheme 15): the arsa-Appel reaction, where As(III) is oxidized to As(V), and benzyl bromide oxidation, which reduces As(V) back to As(III).¹⁹⁴ Although the process currently exhibits modest efficiency, it provides a conceptual framework for future arsine-mediated redox catalysis.

Scheme 15 Sequential oxygen atom transfer using arsines.

6.3. Frustrated Lewis pairs

Lewis acid–base chemistry is classically categorized into classical Lewis adducts (CLAs) and frustrated Lewis pairs (FLPs).^195,196 In 1942, Brown reported that steric hindrance between lutidine and trimethylborane prevented adduct formation—a phenomenon later generalized by Stephan in 2006, who showed that sterically encumbered FLPs could activate dihydrogen (H₂). This seminal discovery catalyzed rapid growth in FLP research, particularly in catalysis, small-molecule activation, and CO₂ capture. While most FLP systems utilize phosphines and boranes, FLPs involving arsines as the Lewis base are much less explored. Ketkov et al. reported the crystal structure and theoretical analysis of the CLA formed between AsPh₃ and tris(pentafluorophenyl)borane (B(C₆F₅)₃)—one of the few well-characterized arsine-containing adducts.¹⁹⁷

We synthesized a series of tertiary arsines with varying steric and electronic properties to systematically investigate their interactions with B(C₆F₅)₃.¹⁹⁸ Depending on the ligand structure, the resulting Lewis pairs exhibited behavior consistent with either CLAs or FLPs (Scheme 16a). For example, trimesitylarsine and B(C₆F₅)₃ form an FLP-like encounter complex in solution. These systems were reactive toward small molecules such as phenylacetylene and diphenyl disulfide, yielding arsonium borate and thioarsonium thioborate salts, respectively (Scheme 16b and c). It should be emphasized that the relatively weak Lewis basicity of arsines generally precludes H₂ activation, which is a hallmark reaction of FLP chemistry. Nevertheless, this limitation also opens opportunities: the modest reactivity of arsenic-based FLPs may enable controlled, selective transformations that would be unachievable with more reactive phosphorus analogs. Further exploration of reactivity modulation through ligand design and cooperative effects is currently underway.

Scheme 16 (a) Lewis pair formation behavior between arsines and B(C₆F₅)₃. Reactions of tricyclohexylarsine (AsCy₃) with (b) phenylacetylene and (c) diphenyl disulfide in the presence of B(C₆F₅)₃.

7. Ionic species

7.1. Arsonium cations

Quaternary arsonium cations occur naturally in compounds such as arsenobetaine, a well-known non-toxic organoarsenic species.¹⁹⁹ However, synthetic arsonium cations have received relatively little attention in the field of materials science, such as ionic liquids (ILs)^200–202 and conjugated molecules.²⁰³ Phosphonium cations tend to reduce the viscosity of ILs relative to their ammonium counterparts owing to the larger atomic radius of phosphorus.^204–206 By extension, arsonium cations, which are even larger and less directional, are expected to further lower the viscosity and improve fluidity in IL systems. May et al. reported the electrochemical properties of tetramethylarsonium bis(trifluoromethylsulfonyl)amide (TFSA), although the salt exhibited a relatively high melting point (∼140 °C), limiting its applicability.²⁰⁷

To expand on this concept, we synthesized a novel arsonium-based IL, trihexylmethylarsonium TFSA, and compared its physicochemical properties with the corresponding trihexylmethylphosphonium IL.²⁰⁸ The arsonium IL showed a lower glass transition temperature, reduced viscosity, and higher ionic conductivity than the phosphonium analog (Fig. 11). These enhancements are attributed to the greater conformational flexibility of the arsonium cation, which reduces lattice energy and promotes ion mobility. Although the arsonium IL exhibited a slightly narrower electrochemical window, its thermal stability and alkali resistance were comparable to those of the phosphonium counterpart. These findings highlight the potential of arsonium cations as promising building blocks for high-performance, low-viscosity ILs.

Fig. 11 Properties of ionic liquids containing arsonium and phosphonium cations. (a) Variable-temperature viscosity. (b) Arrhenius plots of ionic conductivities.

7.2. Dications

Main-group-element polycations have gained increasing interest owing to their applications in synthesis, Lewis acid catalysis, and organocatalysis. A common stabilization strategy involves coordination by neutral donor ligands, which facilitate hypercoordination at the polycationic center. While polycations of heavier pnictogens, such as Sb and Bi, have been extensively studied—exemplified by donor-stabilized species such as [Ph₃Sb]²⁺ and [Ph₃Bi]²⁺, reported by Burford^209,210—the corresponding arsenic-centered dications have remained underexplored owing to the smaller atomic radius of arsenic, which limits its ability to accept additional ligands.

Weigand and colleagues addressed this challenge by synthesizing [Ph₃As]²⁺ species stabilized through tetracoordination with DMAP and IDipp (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (Fig. 12a).²¹¹ These arsenic dications pentacoordinated through two triflate (TfO⁻) anions. Pentacoordinated arsenic-centered dications were isolated using neutral donor ligands such as DMAP and pyridine (py), marking the first examples of such species stabilized without anionic frameworks (Fig. 12b).²¹² Furthermore, Stephan synthesized a toluene-coordinated pentamethylcyclopentadienyl arsenic dication exhibiting η⁵- and π-coordination (Fig. 12c).²¹³

Fig. 12 Arsenic dications with (a) tetracoordination, (b) pentacoordination, (c) η⁵- and π-coordination, (d) divinyldiarsene motif, and (e) hexacoordination.

Ghadwal and co-workers developed the first crystalline diarsene radical cations and dications (Fig. 12d) through the stepwise one-electron oxidation of NHC-stabilized divinyldiarsenes with gallium trichloride (GaCl₃).²¹⁴ The radical cations [{(NHC)C(Ph)}As]₂(GaCl₄) and the corresponding dications [{(NHC)C(Ph)}As]₂(GaCl₄)₂ were isolated as stable crystalline solids. Structural analyses revealed sequential elongation of the As=As bond and contraction of the C–As bonds upon oxidation, accompanied by distinct red-shifted absorption bands and characteristic EPR signals. Computational studies indicated that the unpaired electron in the radical cations is delocalized over the conjugated CAs₂C-framework, with spin density mainly located on the arsenic and vinylic carbon atoms. These findings establish diarsene radical cations and dications as rare and well-defined main-group radical species, expanding the frontier of arsenic-centered redox chemistry.

For hexacoordinated arsenic dictations, an intramolecular chelation strategy was employed using three 2-(2-pyridyl)phenyl (ppy) ligands (Fig. 12e). Sakabe, Sato, and our group successfully synthesized hexacoordinated arsenic-centered dication salts [(ppy)₃As]Cl₂ and [(ppy)₃As](SbCl₆)₂.²¹⁵ These compounds exhibit exceptional thermal and hydrolytic stabilities, remaining intact at elevated temperatures and in aqueous media. Both experimental characterization and computational analyses indicate that the remarkable stability originates from strong As–N coordination, which effectively rigidifies the complex. Surprisingly, these As-centered dications were more thermally stable than their heavier analogs, such as [(ppy)₃Sb](SbCl₆)₂,²¹⁶ despite antimony's larger size and common association with hypercoordination. These findings reveal that with appropriate ligand design, arsenic-centered dications can match or even outperform their heavier congeners in terms of stability and structural robustness, broadening the scope of polycationic main-group chemistry.

8. Final remarks

Over the past decades, organoarsenic chemistry has undergone a profound transformation—from a field once stigmatized by its association with toxicity and warfare to a rapidly evolving area of functional molecular science. The recent development of practical, safe, and modular synthetic methods has opened the door to a broad range of structurally and electronically diverse organoarsenic compounds. As highlighted in this review, these advances have given rise to functional organoarsenic materials with emerging applications in catalysis, photophysics, redox chemistry, molecular recognition, and ionic materials. Arsenic's distinctive chemical features—including its moderate Lewis basicity, low oxophilicity, heavy-atom character, and flexible coordination geometry—are not merely alternatives to those of phosphorus but offer orthogonal design elements that expand the chemical space of molecular and materials chemistry. Notably, arsine-based ligands are now recognized for enabling catalytic transformations that are inaccessible or inefficient with traditional phosphines. In photofunctional materials, arsenic contributes to tunable and stimuli-responsive emission behaviors, as well as to thermal robustness. Furthermore, the development of arsonium-based ionic species and hypervalent arsenic-centered dications illustrates that the versatility of arsenic spans all oxidation states and structural types.

The rise of functional organoarsenic chemistry thus reflects more than a technical progression—it represents a paradigm shift in how we view the element arsenic: not solely as a hazard to be avoided, but as a resourceful and adaptable component of next-generation molecular design. With growing interest in element diversity, sustainable catalysis, and functional hybrid materials, arsenic is poised to occupy a strategic position in the design of innovative molecular systems. It is anticipated that future progress will be driven by interdisciplinary collaborations that integrate synthetic, computational, and application-oriented approaches. As new arsenic-based platforms continue to emerge, we expect the functional potential of this historically underutilized element to be further unlocked, advancing not only fundamental science but also real-world technologies.

Author contributions

H. Imoto: conceptualization, visualization, writing – original draft, writing – review and editing, project administration, supervision; C. Okochi: visualization, writing – original draft; K. Kikuchi: visualization, writing – original draft; K. Naka: writing – original draft, writing – review and editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Acknowledgements

The project on functional organoarsenic chemistry has been supported by JST FOREST Program, Grant Number JPMJFR221K to HI. In addition, a part of this project has been supported by the establishment of university fellowships towards the creation of science technology innovation, Grant Number JPMJFS2124 to KK.

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