Synthesis, properties, and catalysis of p-block complexes supported by bis(arylimino)acenaphthene ligands

Bis(arylimino)acenaphthene (Ar-BIAN) ligands have been recognized as robust scaffolds for metal complexes since the 1990 s and most of their coordination chemistry was developed with transition metals. Notably, there have been relatively few reports on complexes comprising main group elements, especially those capitalizing on the redox non-innocence of Ar-BIAN ligands supporting p-block elements. Here we present an overview of synthetic approaches to Ar-BIAN ligands and their p-block complexes using conventional solution-based methodologies and environmentally-benign mechanochemical routes. This is followed by a discussion on their catalytic properties, including comparisons to transition metal counterparts, as well as key structural and electronic properties of p-block Ar-BIAN complexes.

D uring the last decade, the development of metal complexes supported by redox-active ligands has attracted considerable interest in both academic and industrial settings, owing to their versatile electronic structures and potential applications in catalysis. The demonstrated ability of these ligands to display multiple consecutive oxidation states, together with their coordination to metal centers, induces both radical reactivity and electron-reservoir behavior, making them ideal ligands to support a wide range of catalytic transformations, examples of which are illustrated in Fig. 1.
Bis(imino)acenaphthene (BIAN) is a family of diimine ligands that can be considered as a fusion product between 1,4-diaza-1,3-butadiene (DAB) and a naphthalene unit with great potential as redox noninnocent ligands. Although they have been known since the 1960s, interest in the use of BIAN derivatives as ligands only intensified after the 1990s 1 . The BIAN compounds with aryl substituents on the diimine nitrogen atoms are named bis(arylimino)acenaphthene (Ar-BIAN), whereas those with alkyl substituents are bis(alkylimino)acenaphthene (R-BIAN). The two imine functionalities are often orthogonal to the naphthalene unit and therefore, the arylimines are usually not conjugated with the π-accepting framework. In contrast to the R-DAB ligands in which the imine nitrogen atoms preferentially adopt the s-trans conformation, the https://doi.org/10.1038/s42004-020-00359-0 OPEN rigid naphthalene backbone in Ar-BIAN ligands prevents rotation around the diimine C-C bond, thus locking them in s-cis conformations that enable them to coordinate readily to metal centers (Fig. 2a) 2 . An essential feature of BIAN ligands is their ability to accept electrons. Their strong electron-accepting properties are evident from (i) the ability of the α-diimines to stabilize a range of photochemically 3-5 and electrochemically [6][7][8] generated open-shell complexes through delocalization of the electron density into the antibonding orbitals, and (ii) the ready reduction of the naphthalene unit by alkali metals 9 . As illustrated in Fig. 2b, reduction of Ar-BIAN by a one-electron process first produces a radical anion that is delocalized over the NCCN fragment. This is followed by a second electron reduction, which results in the formation of a dianionic ene-diamide. The third and fourth

Charge-localized structures on Ar-BIAN (a) (b)
electron reductions will then be delocalized into the naphthalene backbone, which disrupts the aromaticity 1 . Accordingly, Fedushkin et al. had demonstrated that Ar-BIAN ligands can accept up to four electrons to form stable mono-, di-, tri-, and tetraanions by conducting a systematic synthetic study on the reduction of Dipp-BIAN (Dipp = 2,6-diisopropylphenyl) with sodium metal 10 . Owing to this facile electron-accepting property, Ar-BIAN ligands are widely recognized as redox noninnocent ligands able to support multiple formal metal redox states. The stabilities of metal complexes formed with Ar-BIAN ligands can be controlled by the diimine substituents, with more electron-rich substituents likely to facilitate stronger binding to higher oxidation-state metal centers 11 . To evaluate the coordination abilities, Gasperini et al. compared the binding constants among a series of Ar-BIAN complexes with palladium(0) and (II), and proposed that the chelating strengths of the bidentate Ar-BIAN ligands fall between the acyclic Ph-DAB ligands (Ph-DAB = diphenyldiazabutadiene) and the more popular π-accepting ligands, such as phenanthroline and bipyridine 11 .
To date, transition-metal BIAN complexes have been extensively studied and applied as catalysts for important chemical transformations [12][13][14][15][16] , pioneered by the coordination chemistry of late-transition metals by the Benedix 2 and Templeton groups 17 . In contrast, there have been few reports devoted to the chemistry of BIAN ligands with the main group, especially p-block elements, despite their tantalizing prospects as transition-metal-like, redox-active systems. In this review, we first examine selected examples of the latest breakthroughs, since the authoritative work by Cowley et al. in 2009 18 , in the chemistry of Ar-BIAN ligands with main-group elements. Subsequently, we summarize the field on the solid-state syntheses of main-group Ar-BIAN complexes. Finally, we highlight the photophysical and electrochemical properties of main-group Ar-BIAN complexes, and the potential implications for catalysis. Together with the emergence of solidstate mechanochemical syntheses of these systems, we envision bright prospects in the adoption of Ar-BIAN main-group complexes for catalysis and optoelectronic applications that exploit their redox versatility.
Synthesis of Ar-BIAN ligands. Traditional solution-based synthesis of Ar-BIAN ligands and complexes: Ar-BIAN ligands have typically been synthesized via condensation reactions between acenaphthoquinone and the corresponding aniline under acidic conditions. For example, Dipp-BIAN was synthesized by heating acenaphthoquinone with 2,6-diisopropylaniline in acetic acid for 1 h at reflux 2 , while o-CF3Ar-BIAN (o-CF3 = o-trifluoromethyl) was formed by refluxing acenaphthoquinone with o-trifluoromethylaniline in a mixture of toluene and sulfuric acid for 3 days using a Dean-Stark trap to isolate the water that resulted 19 . In many other cases, templating with either zinc chloride (ZnCl 2 ) or nickel bromide (NiBr 2 ) was necessary before removal of the metal ion to furnish the desired ligand (Fig. 3a) 2 . Accordingly, a number of Ar-BIAN ligands, such as p-MeOAr-BIAN (p-methoxyphenyl), p-NMe2Ar-BIAN (p-dimethylaminophenyl), p-MeAr-BIAN (p-methylphenyl), p-BrAr-BIAN (p-bromophenyl), and p-ClAr-BIAN (p-chlorophenyl) were synthesized by refluxing acenaphthoquinone with two equivalents or small excesses of the corresponding aniline in acetic acid in the presence of excess ZnCl 2 or NiBr 2 , followed by demetallation using potassium carbonate (K 2 CO 3 ) or sodium oxalate (Na 2 C 2 O 4 ) 20 . Besides serving as a template around which the Ar-BIAN is formed 21,22 , Ragaini et al. suggested that another driving force for the condensation with metal halides is the precipitation of the resulting metal Ar-BIAN complexes due to their lower solubilities in the reaction media compared with the starting materials 21 .
Beyond the simple, symmetric Ar-BIAN ligands, the synthesis of asymmetric variants from two different anilines had been reported by Ragaini et al. in 2004 23 . Two strategies were described, the first involving a transimination starting from a symmetric Ar-BIAN Zn complex bearing electron-withdrawing CF 3 groups on the diimine moiety 23 . In the second approach, a two-step process starting by condensation of acenaphthenequinone with 3,5-bis(trifluoromethyl)aniline to form the monosubstituted intermediate, was followed by a second ZnCl 2 -templated condensation with an aniline-possessing electron-donating group (s) (Fig. 3b) 23 . These synthetic procedures rely on the large electronic differences and hence disparities in the kinetics of condensation between the two anilines with the acenaphthenequinone. Even more sophisticated tridentate ligands based on BIAN with additional pendant O, P, and S donor atoms were also synthesized through a stepwise procedure that involved initial formation of a monoimine using a bulky aniline, followed by reaction of the intermediate with another amine tethered to the remaining donor group (Fig. 3c) 24 .
Mechanochemical synthesis of Ar-BIAN: In recent years, mechanochemical synthesis has been gaining momentum for the preparation of a variety of organic, organometallic, polymeric, nano-, and alloyed compounds and materials, which have been successfully applied to numerous areas, such as the synthesis of pharmaceutical ingredients, catalysis, mineralogy, and even geology. Consequently, several insightful reviews and books that discuss the history and development of mechanochemistry and its application in these areas have been published [25][26][27][28][29] ; further indepth discussions on these established fields will not be included in this review.
Similar to the trajectory of Ar-BIAN complexes, mechanochemistry in the fields of synthesis and catalysis has predominantly been focused on transition-metal systems [30][31][32] . Nonetheless, the mechanochemical synthesis of main-group inorganic and organometallic compounds is a nascent and growing area. The first few examples of crystallographically characterized main-group complexes synthesized by mechanochemistry were a tris(allyl)aluminum complex 33 and a bis(n-propyltetramethylcyclopentadienyl)strontium complex 34 reported by Rheingold and Blair, respectively. In addition to organometallic small molecules, mechanochemical synthesis has also been successfully employed for the construction of non-carbon frameworks such as the tert-butyl-substituted adamantoid phosph (III)azane P 4 (N t Bu) 6 , which was previously believed to be inaccessible due to the bulkiness of the tert-butyl group 35 . Other recent advances in the application of mechanochemical methods to main-group compounds have been summarized in recent reviews 27,28,36 .
Specific to Ar-BIAN systems, mechanochemistry was utilized for the direct one-pot, two-step access to indium(III) complexes by milling acenaphthoquinone, the corresponding aniline, and InCl 3 in stainless-steel grinder jars for 30 min-2 h (Fig. 4a, b) 37 . Notably, this procedure bypassed the use of transition-metal halide-templating agents and could be generally used with anilines possessing both electron-donating and electronwithdrawing groups. Moreover, mechanochemistry facilitated the introduction of ester and carboxylate functionalities at the bay region of Ar-BIAN ligands (Fig. 4c) 38 . Based on the DFT calculations performed for five heteroleptic iridium complexes and supported by the Ar-BIAN ligands, the HOMO and LUMO of such complexes reside on the N,N-dimethylaniline and the acenaphthylene core of the Ar-BIAN ligands, respectively (Fig. 4d). Previously, Ar-BIAN compounds derived from 5carboxymethylacenaphthoquinone via solution methods with ZnCl 2 -templated condensation only proceeded in low yields owing to hydrolysis of the imines during removal of the metal template 11 , which hindered purification of the ligand. On the contrary, an acid-catalyzed mechanochemical condensation successfully generated the desired ligand, highlighting how mechanochemical synthesis through ball milling is a powerful tool for the solid-state synthesis of Ar-BIAN ligands with facile purification steps 37 .
Ar-BIAN-supported catalysis. In the arena of developing highly selective and efficient catalysts for industrially and pharmaceutically relevant chemical transformations, redox noninnocent ligands are especially popular since they offer additional chargetransfer capabilities 39 . However, the majority of catalysis supported by Ar-BIAN consists of transition-metal complexes, with some main-group counterparts being reported. Therefore, for completeness of the review and to best illustrate the potential of Ar-BIAN-supported catalytic processes, we briefly discuss the catalytic chemistry of the transition-metal Ar-BIAN complexes in addition to the p-block complexes.
Transition-metal complexes supported by redox noninnocent ligands have drawn considerable interest for the multielectron activation of small molecules and for catalytic reactions [39][40][41] . For example, Clark has recently reported the synthesis of [V(dmp-BIAN) 3 ](PF 6 )] (dmp = 3,5-dimethylphenylimino), which was revealed to consist of two discrete redox isomers due to valence tautomerism 42 3 , Cl) have also been synthesized, and their catalytic activity toward olefin oxidation reactions was demonstrated 43 . Reaction of Dipp-BIAN with VCl 3 resulted in an oxidovanadium(IV) complex [(Dipp-BIAN) VOCl 2 ] with one unpaired electron. The electrochemical study on this complex revealed two quasi-reversible reductions at -0.32 and -1.05 V (Fig. 5a), followed by an irreversible reduction at -1.5 V (vs. Ag/AgCl). The first two reductions were assigned to the reduction of Dipp-BIAN, whereas the last reduction was presumably due to conversion of V(IV) into V(III). The EPR spectrum of [(Dipp-BIAN)VOCl 2 ] showed an eight-line isotropic signal characteristic of a d 1 electronic configuration, and the hyperfine interaction values fall in the typical range for oxidovanadium(IV) complexes (Fig. 5b). As shown from the magnetic susceptibility measurements, the µ eff at 300 K is 1.67 µ B , which does not change with temperature. This consistent value of µ eff together with a small Weiss constant suggested the absence of significant exchange interactions in the complex (Fig. 5c). The catalytic activity of this complex toward oxygenation of linear, cyclic, and branched alkanes by H 2 O 2 was recently studied by Fomenko et al. 44 . Efficient oxidation was observed, especially in the presence of PCA (pyrazine-2-carboxylic acid), as revealed by the kinetic curves obtained for the formation of oxygenates from cyclohexane. A plausible mechanism was proposed by the authors, which involved the formation of an "activating complexation" system commonly observed for complexes of flavins and quinones. According to the DFT calculations, it was L = NR 2 , PR 2 , OR, SR   38 , under CC BY. The symbol for mechanical milling was proposed by Hanusa et al. 98 .

(a) SoluƟon-based syntheƟc route to Ar-BIAN compounds
In Toluene proposed that the generation of HO • was associated with the redox-active nature of the BIAN ligand, and did not require a change in the metal oxidation state.
Another earth-abundant first-row transition metal that has been supported by sterically bulky BIAN ligands is Fe, as reported by Lei's group in 2017 15 . It had previously been shown that Ar-BIAN Fe(II) complexes catalyze the hydrosilylation of aldehydes and ketones at 70°C under solvent-free condition 45 . To improve the efficiency of this catalytic reaction, increasing the steric bulk of the diimine substituents would be expected to help stabilize the Fe hydride intermediate formed. However, unlike palladium diimine complexes that showed significant variations in their catalytic activity with different sterically hindering groups 46 , the differences in electronic and steric properties of the Ar-BIAN Fe (II) complexes did not appear to affect the catalytic reactivity for the hydrosilylation of various aldehydes and ketones 15  Along with the high yield, a high E selectivity has also been demonstrated for these pincer complexes, which was attributed to the fast isomerization of Z alkenes 48 .
Moving down the period, it had been shown that Ar-BIAN Cu (I) complexes could be employed as light harvesters in both dyesensitized solar cells and photoredox catalysis 16,49 . Owing to the remarkable π-accepting properties of the Ar-BIAN ligands, some of the Cu(I) complexes are panchromatic and absorb by metal-toligand charge transfer up to 1400 nm in the near-infrared region 49 . Later versions of these Cu(I) photosensitizers were reported by Soo's group, in which three Ar-BIAN-Cu(I) complexes were synthesized and their spectroelectrochemical properties were studied. Cyclic voltammetry (CV) measurements were performed to determine the feasibility of the Ar-BIAN-Cu(I) for mediating photoredox catalysis. During the cathodic scan, a quasi-reversible wave was observed at -0.99 V, suggesting possible complex regeneration after photocatalysis (Fig. 6a). To access the absorption and emission properties of the Cu(I) dyes, UV-vis and photoluminescence spectroscopic measurements were performed. As shown in Fig. 6b, the optical absorptions extended into the NIR region up to~1000 nm, and a strong emission band was observed at 510 nm when the complexes were excited between 430 nm and 460 nm (Fig. 6c). Insights into their excited-state lifetimes were further obtained by time-correlated single-photon-counting spectroscopy (TCSPC), and recombination lifetimes of up to 11 ns were observed, which is longer than the rate of diffusion-controlled reactions and alludes to possible applications in photoredox catalysis (Fig. 6d) 16 . The Cu(I) complexes mediated the Karasch addition with C-C bond formation between styrene and CBr 3 radicals derived from CBr 4 in respectable yields via a radical chain propagation cycle 16 .
In 2014, Lahiri reported the formation of five Ru-BIAN-based compounds [Ru(acac) 2 (Ar-BIAN)] (Ar = Ph, 4-MeC 6 H 4 , 4-MeOC 6 H 4 , 4-ClC 6 H 4 , and 4-NO 2 C 6 H 4 ), which have been structurally, electrochemically, spectroscopically, and computationally characterized 50 . The strained carbon framework in the BIAN ligands offers a sensitive diagnosis for assessing the metalto-ligand charge transfer in the resulting chelating compound. In a more recent article, the same group examined the synthesis and characterization of BIAN-based redox-active complexes [Ru(trpy) (Ar-BIAN)Cl][ClO 4 ] (trpy = 2,2′:6′,2′′-terpyridine). These complexes were investigated for catalytic epoxidation reactions. It was found that electron-donating and -withdrawing groups on the para-position of the arylimino moiety of BIAN had little influence on the catalytic epoxidation process 51 . In addition, the catalytic activity of the newly synthesized Ru-BIAN complexes was explored in the oxidation of primary and secondary alcohols with H 2 O 2 as the oxidant. These complexes demonstrated high selectivity toward the desired aldehydes and ketones since no overoxidized by-products were observed 51 .
Examples of f-block complexes supported by Ar-BIAN ligands were first reported in 2007, and remained uncommon in the field of coordination chemistry [52][53][54][55] . In recent years, Niklas et al. described the synthesis of a modified BIAN ligand, namely Phen-BIAN, which integrates the acenaphthene backbone with a tetradentate mixed-donor O − N − N − O salophen-type binding motif 56 . Further treatment of the Phen-BIAN ligand with U (OAc) 2 resulted in the corresponding U(VI) complex in 84% yield. Structural characterization revealed that each metal center binds to one tetradentate ligand, and the complex adopts a dimeric aryloxide-bridged structure. Although catalytic studies have not been carried out, a wealth of redox activity and accessible oxidation states were suggested by the electrochemical studies, which portends future investigations of these complexes toward plausible catalytic reactions 56 .
In contrast to the extensive work on transition-metal complexes, the diversity of catalytic systems mediated by main-group complexes has been comparatively small, and very few studies include Ar-BIAN ligands, despite their redox versatility. However, the number of examples of the main catalytic systems is rapidly increasing. In 2006, Harder introduced a calcium hydride complex that exhibited catalytic activity toward alkene hydrosilylation and hydrogenation 57 . More lately, examples of calcium benzyl and borate complexes have also been reported to be catalytically active in the hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) 58 . These constitute clear demonstrations that main-group organometallics are not limited to Lewis acidic and basic catalysis. In addition, Power in 2010 highlighted how heavier main-group elements can mimic transition metals in smallmolecule (e.g., H 2 and NH 3 ) activation 59 . The new structural and bonding insights that heavier main-group elements can potentially exhibit multiple oxidation states and versatile coordination environments, make them attractive alternatives to conventional transition-metal-based catalysts.
With the emerging studies on many s-and p-block systems as effective catalysts in a multitude of reactions (e.g., cyclization, hydrogenation, and hydroaminaton), more attention has been turned to control enantioselectivity in addition to obtaining reasonable conversions. A recent review article by Wilkins and Melen described the competitive yields and enantioselectivities of catalytic C-C, C-H, C-N, C-O, and C-P bond-formation reactions shown by a new generation of catalysts composed of main-group elements 60 . These studies exploited the wide range of new opportunities offered by main-group chemistry in catalysis, an area previously dominated by heavy transition metals.
Nonetheless, we observed that the above-mentioned catalytic transformations involving noninnocent Ar-BIAN ligands within the main-group arena are still underexploited. In a recent example, the influence of Ar-BIAN's configurational rigidity and electronic lability on the chemical properties of the resulting complex can be illustrated through a digallane species [(Dipp-BIAN)Ga-Ga(Dipp-BIAN)], which is capable of activating alkyne triple bonds. Furthermore, the resulting complex is reactive toward electron-rich reagents, similar to olefins upon coordination to transition metals 61 . A plausible mechanism of the alkyne addition to [(Dipp-BIAN)Ga-Ga(Dipp-BIAN)] was proposed to involve a concerted interaction between the LUMO (π*) of the alkyne with the HOMO (π) of the gallium complex. Alkyne molecules are added across the Ga-N-C fragment with regioselectivity controlled by electronic factors. To test for its catalytic reactivity toward new C-N bond-formation reactions, phenylacetylene was mixed with various anilines in the presence of [(Dipp-BIAN)Ga-Ga(Dipp-BIAN)]. It was found that this gallium complex serves well as a Markovnikov-selective catalyst for the hydroamination of PhC≡CH, with reaction rates comparable to other catalytic systems based on transition metals.
Overall, despite being clearly capable of controlling and finetuning catalytic processes, the incorporation of Ar-BIAN ligands in main-group catalysis remains a largely unexplored territory.
Structural and electronic properties of p-block complexes of Ar-BIAN. Group 13 complexes: The synthesis of 2-bromo-N,N ′-bis(2′,6′-diisopropylphenyl)acenaphtho-1,3,2-diazaborole, appearing as burgundy red crystalline material, was achieved through the reaction of a diazaborolium salt with fivefold excess of 1% sodium amalgam in toluene 62 . The UV-vis spectrum of the resulting compound in CH 2 Cl 2 showed a broad absorption band ranging from 450 to 600 nm, but exhibited no fluorescence. In contrast to 2-bromo-1,3,2-diazaboroles, which are prone to substitution of the Br by a wide range of nucleophiles like lithium aryls, the resulting diazaborole is reluctant to undergo such processes. DFT calculations suggest that the HOMO of this compound corresponds to the antibonding interaction of the acenaphthylene and the diazaborole framework, while the LUMO is mainly localized on the acenaphtho fragment. The electrostatic potential (ESP) map suggests a significant negative charge at the acenaphtho framework, which helps to explain the observed lack of reactivity toward even strong nucleophiles.  3 ], the Ga fragment acts as an anionic ligand toward a Mo(II) center (Fig. 7a). This adaptive behavior of the Ga carbenoids is enabled by the redox noninnocent Dipp-BIAN, which allows the reactivity at both Ga and the transition-metal center to be tuned 70 . The related complex [(Dipp-BIAN)Ga-Ga (Dipp-BIAN)] undergoes facile cycloaddition with alkynes, resulting in the formation of C-C and C-Ga bonds 71,72 . This unique reactivity was extended to the catalytic hydroamination of alkynes 61 with anilines with comparable activity to those of transition-metal-based systems 73,74 (vide supra). However, a similar reaction with dmp-BIAN as the ligand did not afford a compound with a metal-metal bond-like that formed with Dipp-BIAN. Instead, [(dmp-BIAN) 2 Ga] with two ligands coordinated was generated, in which one dmp-BIAN is nominally dianionic, while the other one is a radical anion 75 . Addition of phenylacetylene to [(dmp-BIAN) 2 Ga] was studied. The X-ray crystallographic data suggest that addition of the phenylacetylene occurred at the Ga-N-C group of the dianionic ligand. The regioand stereoselectivity of addition is proposed to be controlled by both electronic and steric effects 75 .
Although the complexation of Ar-BIAN ligands to heavier group 13 elements is less commonly reported compared with the lighter analogs, several crystallographically characterized examples of Ar-BIAN In(III) complexes have been recently synthesized 76 . Four different Ar-BIAN ligands were mixed with InCl 3 in THF to give the corresponding complexes. The ligands with parasubstituents generally formed distorted octahedral complexes with coordinating solvents bound to In, while the ligands with ortho-substituents resulted in distorted square pyramidal In complexes. This behavior was attributed to the increased steric hindrance around In induced by the substituents at the ortho positions of the aniline.
Progressing to the next period, Mes-BIAN reacted with TlPF 6 to form [Tl(Mes-BIAN) 2 ][PF 6 ], which was the first example of a structurally authenticated Tl(I) complex of a neutral α-diimine ligand. It consists of a Tl atom linked to four imine N atoms from two Mes-BIAN ligands, forming a distorted square pyramidal geometry with the Tl atom at the vertex of the square pyramid 77 . ligand forms a radical anion 80 . The EPR spectra for both complexes revealed coupling between the unpaired electron and the Ge, Cl, and N nuclei. However, despite their structural similarity, the 73 Ge, 35 Cl, and 37 Cl hyperfine coupling constants in [(Dipp-BIAN)GeCl] and [(dtb-BIAN)GeCl] are significantly different. This disparity was attributed to the difference in contributions of the germanium s and p orbitals to the Ge-Cl bond. The larger the contribution of the s orbital to the Ge-Cl bond, the larger are the Ge and Cl hyperfine coupling constants.
With the aim of synthesizing tin analogs of the germylenes, metathesis of SnCl 2 with the disodium salt of Dipp-BIAN ligand [(dipp-BIAN)Na 2 ] was performed 80 77 . In contrast to the Mes-BIAN ligand, the reaction of SnCl 2 with the dtb-BIAN ligand did not lead to disproportionation of the metal center, but gave the corresponding Sn(II) complex [(dtb-BIAN) SnCl 2 ] instead 80 . X-ray crystallographic analysis on [(dtb-BIAN) SnCl 2 ] revealed that the SnCl 2 is coordinated to only one N atom from the dtb-BIAN ligand (Fig. 7b). However, its 1 H NMR spectrum in C 6 D 6 suggested the presence of a mirror plane bisecting the N-C-C-N framework in solution since the tertbutyl groups on each diimine moiety appeared to be magnetically equivalent. This suggests that the SnCl 2 fragment exchanges rapidly between the two diimine N atoms in solution on the timescale of the experiment.
Group 15 and 16 complexes: Reeske et al. reported the first application of Ar-BIAN ligands in the context of group 15 chemistry, which focused on the reaction of PCl 3 or AsCl 3 with Dipp-BIAN 82 (Fig. 7b). It was demonstrated that the reduction of PCl 3 with SnCl 2 in the presence of bis(phosphine) ligands resulted in the formation of cyclic triphosphenium ions 83 . In this reaction, an initial formation of "PCl" and SnCl 4 occurred via reduction of PCl 3 with SnCl 2 . The "PCl" species was then coordinated to the bis(phosphine) ligand concomitant with or prior to a halide abstraction by SnCl 4 . Inspired by these findings, Reeske et al. investigated the result of trapping the "PCl" species with ligands other than phosphines, e.g., the Ar-BIAN ligands 82  ·THF] could also be obtained with AsCl 3 using the same protocol. X-ray structural analysis of these complexes revealed a PN 2 C 2 ring with the C-C and C-N bond distances corresponding to a C = C double bond and C-N single bond, respectively. Hence, the Dipp-BIAN ligand appeared to be reduced by two electrons, while the oxidation state of the P or As center remained as +3. The authors also studied the reaction of Dipp-BIAN with PI 3 without SnCl 2 as the reducing agent. This reaction resulted in quantitative formation of the phosphenium triiodide salt [(Dipp-BIAN)P][I 3 ], which also possessed a phosphenium cation as indicated by the 31 P NMR data.
The antimony (Sb) and bismuth (Bi) complexes supported by Ar-BIAN ligands were obtained from the reaction of Ar-BIAN directly with the halide salts of the corresponding pnictogen. Besides the heavier pnictogens, there has also been some exploratory work on the chemistry of group 16 BIAN complexes. The reaction of TeI 4 with Mes-BIAN and Dipp-BIAN resulted in two-electron reduction of the metal center and formation of (Mes-BIAN)TeI 2 and (Dipp-BIAN)TeI 2 , respectively 86 . The bond distances obtained from the X-ray crystallographic analysis each revealed a neutral BIAN ligand coordinating to a Te(II) center, forming a square planar geometry around Te. In contrast, spectroscopic evidence suggested that treatment of TeCl 4 with Dipp-BIAN led to the formation of a 1:1 complex without reduction of the Te(IV).
Physicochemical properties and application of noninnocent ligands in catalysis. To date, the literature on main-group Ar-BIAN complexes has focused on synthetic and structural studies with no major efforts devoted to the investigation of their physicochemical properties. In-depth studies were first described in 2016, where the spectroscopic and electrochemical properties of two Ar-BIAN indium(III) complexes were presented 37 . In general, the extinction coefficient is lower for complexes formed with an electron-withdrawing group (e.g., p-BrAr-BIAN-InCl 3 ) than that with an electron-donating group (e.g., p-MeOAr-BIAN-InCl 3 ) (Fig. 8a). The same trend was observed when comparing the ligands p-BrAr-BIAN and p-MeOAr-BIAN, which was consistent with similar observations reported by Hasan et al. 20 . The higher-energy bands below 350 nm and the lower-energy bands above 400 nm may be assigned to the π-π* ligand-centered transitions from within both diimine and acenaphthene, and between diimine and acenaphthene, respectively, similar to the assignments reported in the literature for related Ar-BIAN systems 87,88 . These UV-vis spectral band assignments were also supported by the electron-density distribution, which showed that the HOMO mainly consists of the arylimino fragments from Ar-BIAN, whereas the LUMO is delocalized over the acenaphthene moiety (Fig. 8b).
The electrochemical behavior of the indium(III) Ar-BIAN complexes was also studied by cyclic voltammetry. During the cathodic scan of p-MeOAr-BIAN-InCl 3 , a total of six reduction waves were observed. The first reduction at E p red = -0.80 V is chemically reversible because ΔE p = 60 mV, and the ratio of the cathodic and anodic peak currents stays around one, regardless of the scan rate (Fig. 8c). This was further confirmed by a linear fit of the oxidation peak current (where E p oxi = +0.74 V) plotted against the square root of the scan rate (Fig. 8d). In contrast, no chemically reversible processes were identified in the case of p-BrAr-BIAN-InCl 3 , with only two quasi-reversible reduction waves observed between E p red = −1.0 V and E p red = −2.2 V. It was proposed that these two reduction waves arise from the reduction of the In(III) metal center in p-BrAr-BIAN-InCl 3 37 . When the metal is reduced, one of the Cl − dissociates, which will undoubtedly result in structural changes to the molecule. This in turn leads to a reoxidation process at a different potential. Subsequent reduction processes observed in p-BrAr-BIAN-InCl 3 are probably due to ligand reduction. The fact that reduction waves attributed to the reduction of In(III) metal center appeared to be electrochemically reversible for p-MeOAr-BIAN-InCl 3 , but not in the case of p-BrAr-BIAN-InCl 3 , suggested an increased stability of the putative [(p-MeOAr-BIAN) 2  Similar studies on the absorption and electrochemical behavior have been expanded to indium(III) complexes with a larger variety of Ar-BIAN ligands in a later report 76 . It was concluded that all the indium(III) Ar-BIAN complexes discussed absorb light in the UV and visible regions of the electromagnetic spectrum, with complexes bearing strongly electron-donating substituents absorbing further to the red region due to the reduced HOMO-LUMO gap of the complexes. Also, chemically reversible reduction waves were only observed for complexes with electron-donating substituents on the arylimino fragment, suggesting an increased stability of the putative reduced species in these complexes as compared with those having electronwithdrawing substituents.
Beyond fundamental studies and characterization, the application of noninnocent ligands in catalysis can be classified into four different strategies, as summarized by de Bruin 89 . The first involves oxidation and reduction of the ligand to accommodate the electronic changes to the metal center. In the second approach, the ligand serves as an electron reservoir, which allows for multiple electron transformations in metal complexes where the metal itself is reluctant to undergo redox changes. The third approach involves the generation of ligand radicals that participate in bond formation during catalysis. The last strategy requires the activation of the substrate in cases where the substrate binds as a redox noninnocent ligand as well 89 . The first approach can be illustrated by the oxidation of H 2 using Cp*Ir ( t BA F Ph), where H 2 t BA F Ph is 2-(2-trifluoromethyl)anilino-4,6di-tert-butylphenol. This cationic complex contains a oneelectron oxidized ligand radical cation, which results in increased Lewis acidity of the metal center 90 . The second approach is more often observed in homogeneous catalysis where multiple electron transfers occur between the complex and the substrate. While this process seems trivial for late-transition metals, it is more difficult to achieve with early transition metals due to the lack of easily accessible, contiguous oxidation states. This highlights how redox noninnocent ligands can serve as electron reservoirs. In these first two approaches, the catalytic reactivity occurs at the metal center, meaning that each redox noninnocent ligand plays the role of a "spectator". In contrast, the ligands play more "active" roles in the last two approaches, where bond formation and breakage with the substrate are expected.
In this context, one of the well-known applications exhibited by Ar-BIAN complexes is in ring-opening polymerization (ROP). activity for ε-caprolactone with excellent molecular weight control 91 . Among main-group complexes, promising ROP of cyclic esters was also shown by In(III) complexes, as illustrated recently by a cationic salen-type In complex 92 . Cationic Ga and Al complexes supported by Dipp-BIAN were lately reported by Fedushkin to show similar catalytic activity 93 . An electrondeficient gallylene [(Dipp-BIAN)Ga]B(C 6 F 5 ) 3 complex was formed through the reaction of [(Dipp-BIAN)Ga−Ga(Dipp-BIAN)] with B(C 6 F 5 ) 3 , and it was shown to serve as an initiator for the ROP of ε-caprolactone 59,93 .
Outlook. Transition-metal catalysis has been the workhorse in organic and organometallic chemistry, owing to the accessibility of multiple oxidation states and coordination environments of the complexes. Lately, however, more insights into the bonding, structural, and electronic properties of main-group complexes have been obtained, leading to greater efforts being devoted to the study of catalytic reactions facilitated by main-group complexes 59 . For example, Power highlighted the appearance of nonbonded electron density and concomitant geometrical distortions in heavy main-group metal complexes, which led to the previously unknown small-molecule activation reactions of H 2 , NH 3 , CO, and C 2 H 4 59 . Other redox noninnocent ligands, such as N-heterocyclic carbenes, have further enriched the landscape of main-group coordination chemistry and catalytic applications 12,14,94 .
Very recently, Bertrand's group reported the isolation of monosubstituted carbenes, which are the first examples of monosubstituted carbenes isolable at room temperature, and link the rich chemistry of disubstituted triplet carbenes with the transient parent carbene 95 . In addition, six-membered cyclic (alkyl)(amino)carbenes have also been reported and demonstrated superior reactivity in the arylation of ketones with aryl chlorides relative to the traditional five-membered N-heterocyclic carbenes 96 . Jones et al. had also reviewed the redox noninnocence of heavy main-group antimony complexes supported by ambiphilic ligands 97 , with the intention of applying them toward transition-metal-halide-bond activation reactions to expose active sites for further catalysis. Thus, the synergy of maingroup elements with redox noninnocent ligands remains a very active area of exploration.