Boron-chalcogen heterocycles and linear tetraboranes from a cyclic tetra(amino)tetraborane

Small ring systems play a crucial role in chemical synthesis, serving as fundamental scaffolds in molecular architectures and as key components in pharmacologically active compounds^{1,2,3,4,5,6,7,8,9,10,11}. Their unique structural characteristics, and their inherent ring strain in particular, allow them to function as versatile synthetic intermediates, enabling diverse chemical transformations, including ring expansions, desymmetrizations, and ring-opening processes critical for constructing complex molecular structures^{1,2,3,4,5,6,7}. While these traits are well-established for three- and four-membered rings based on carbon, boron-containing analogs have largely remained unexplored, only recently gaining more prominence in scientific research^{12,13,14,15,16,17}.

The synthesis of small boron rings presents intricate challenges stemming from both structural and electronic characteristics. With only three covalent bonds, boron leaves an empty p orbital that creates an incomplete octet, thus rendering the atom inherently electron-deficient. This electronic configuration drives boron’s propensity for multicenter bonding, which fundamentally destabilizes conventional ring structures abundantly found for cyclic hydrocarbons^18,19. A cyclic triborane of the form B₃R₃ with σ-bonds between the boron atoms, i.e., a structural analog of cyclopropane, still remains an elusive target. Yamamoto’s reported triaminotriborane(3) exemplifies these challenges²⁰: despite employing electron-donating amino substituents to alleviate electron-deficiency and promote classical bonding, the compound adopts a bent chain structure in the solid state with delocalized 3c-2e π-bonds rather than a closed three-membered B₃ ring. Density functional theory (DFT) analysis suggests that the bent chain configuration primarily stems from the steric hindrance of the bulky TMP ligands (TMP = 2,2,6,6-tetramethyl-piperidino), as the sterically less encumbered dimethylamino derivative is calculated to prefer a three-membered ring structure. Similar complexities emerged in attempts to create a neutral tricycloborane(3) through two-electron oxidation of the 2π-aromatic triboracyclopropenyl dianion (A, Fig. 1), in which case the B₃ ring, stabilized by dicyclohexylamino substituents, proved unstable and engaged in ring-opening reactions²¹. While a neutral cyclotriborane(3) remains uncharacterized, two closely related tetraborane(4) derivatives with B₄ rings have been successfully isolated: a diisopropylamino-substituted cyclotetraborane from the group of Siebert²², and a dicyclohexylamino derivative from our own research (B, Fig. 1)^23,24. The former was generated by reductive homocoupling of a diborane(4) precursor, while the latter emerged from the reduction of a dihaloborane that led to the catenation of four boron atoms. Both boranes (B) adopt a folded structure similar to that of the hydrocarbon cyclobutane. Derivatives with different ring substituents, such as TMP groups or alkyl and halogen substituents, illustrate the delicate balance governing ring stabilization and the inherent constraints on functional group diversity in boron rings^22,25,26,27. In each of these derivatives, the boron atoms reorganize into a tetrahedral structure characterized by electron-deficient multicenter bonding. With mixed boryl and amino substituents, yet another structural motif emerges in a neutral tetraborane(4)²⁸. Derived from the dianionic 2π-aromatic compound C (Fig. 1)^29,30, this configuration adopts a non-classical planar rhomboid structure with delocalized σ- and π-bonding that preserves its aromatic character²⁸.

Examples of small homocyclic boron rings featuring classical bonding patterns known in the literature: the triboracyclopropenyl dianion A; cyclotetraboranes B; the cyclotetraborane dianion C.

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Considering the intricate effects of substituents on structural preferences, tetraboranes(4) with an electron-precise four-membered B₄ ring are rare, leaving their chemistry largely unexplored. In a recent communication, we detailed the fundamental redox chemistry of compound 1, which highlighted its unique ability to undergo both reduction and oxidation while preserving its distinctive folded ring structure²³. Here we report structurally confirmed ring-expansion and ring-opening transformations of tetraborane 1, thus elucidating two nuances of small boron cycles’ basic chemistry in great detail. Chalcogen atom insertion generates five-membered B₄E rings (E = S, Se, Te), while halogenating agents induce ring scission to form linear (pseudo)halide-functionalized tetraboranes.

Ring-expansion reactivity

Given its small ring structure, we anticipated that tetraborane 1 would readily engage in a variety of ring-expansion and ring-opening reactions. We first examined its reactivity with elemental chalcogens (S and Se), and found that compound 1 underwent largely unselective reactions after their initiation at elevated temperatures. In addition to the expected chalcogen atom insertions producing five-membered B₄E rings (E = S and Se), several other products were identified through ¹¹B NMR spectroscopy. These included 1,3-dichalcogeno-2,4-diboretanes with the general formula [Cy₂NBE]₂ (E = S and Se), as will be discussed later in this section.

In an effort to achieve a more selective insertion process, we explored alternative chalcogen sources. Here, we found that diphenyl disulfide (Ph₂S₂), diphenyl diselenide (Ph₂Se₂), and diphenyl ditelluride (Ph₂Te₂) generated the corresponding ring-enlarged products, i.e. 1-thia-, 1-selena-, and 1-tellura-2,3,4,5-tetraborolanes 2S, 2Se, and 2Te, with high selectivity in good to excellent isolated yields (Fig. 2). Sulfur insertion into 1 using diphenyl disulfide was most effectively accomplished via UV irradiation (210–600 nm) of a benzene solution at room temperature for 2 d. The resulting five-membered product 2S was collected as a colorless solid in a yield of 81%. 2S displays two distinct ¹¹B NMR signals at δ = 55.5, 45.9 ppm. Crystals suitable for X-ray diffraction analysis were readily obtained by cooling a saturated hexane solution to −30 °C. In the solid state, the B₄S ring adopts a twisted conformation, in which only the B − S − B atoms lie in one plane, while the other two boron atoms are positioned above and below this plane, respectively (Fig. 3). The bond angle at sulfur is 95.56(6)°. The boron-sulfur bond distances of 1.871(1) and 1.876(1) Å are consistent with single bonds, as are the B − B bond lengths, which show values between 1.702(2) and 1.727(2) Å. The boron atom that deviates most from ideal trigonal planarity (B3, Fig. 3), with a sum of bond angles of 353.8°, exhibits a slightly longer boron-nitrogen bond (1.430(8) Å) than those of the other three B atoms (1.408(2), 1.405(2), 1.40(1) Å). While examples of five-membered rings containing only boron and sulfur atoms are known^{31,32,33,34,35}, a B₄S ring featuring a chain of four boron atoms, as seen in 2S, has remained absent in the literature thus far.

Reactions of B₄(NCy₂)₄ (1) with diphenyl dichalcogenides selectively produce B₄E (E = S, Se, Te) ring systems 2S, 2Se, and 2Te via chalcogen atom insertion and ring expansion.

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Molecular structures of 2S, 2Se, and 2Te are presented from both top (top) and side views (bottom). Displacement ellipsoids shown at the 50% probability level; hydrogen atoms and ellipsoids of the cyclohexyl groups are omitted for clarity.

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We observed that half an equivalent of diphenyl disulfide was sufficient to fully convert 1 into 2S, which suggests that both of its sulfur atoms are incorporated into the product. The elimination of one sulfur atom would formally produce diphenyl sulfide (Ph₂S), which might subsequently act as a secondary chalcogen source. Indeed, when 1 was reacted with diphenyl sulfide under UV irradiation, 2S was isolated in 74% yield. Both sulfide and disulfide reagents can generate phenylthiyl radicals under these conditions^36,37; however, their involvement in product formation remains uncertain and warrants further investigation. It is important to highlight that no additional insertion of chalcogen atoms was observed, even when an excess of the diphenyl dichalcogenide reagent was used.

The heavier chalcogen analogs of 2S are readily available by reaction of 1 with diphenyl diselenide and diphenyl ditelluride under thermal or photolytic conditions, respectively (Fig. 2). Similar to the sulfur derivative, 2Te is best prepared by UV irradiation of a mixture of 1 and Ph₂Te₂ in a benzene solution for 1 d. By contrast, the selenium derivative 2Se is most efficiently generated by heating 1 with Ph₂Se₂ at 80 °C for 2 d. Both compounds were isolated as colorless solids, with ¹¹B NMR chemical shifts of δ = 55.4, 46.8 ppm (2Se), and δ = 58.7, 44.5 ppm (2Te), placing them in a region similar to that of 2S. The chalcogen atoms are observed with chemical shifts of δ(⁷⁷Se) = 254 ppm, and δ(¹²⁵Te) = 15.6 ppm. In the solid state, the B₄E rings of both 2Se and 2Te adopt the same twisted conformation as observed for 2S, with chalcogen bond angles near 90° (2Se 92.6(4)°; 2Te 88.43(6)°; Fig. 3). The boron-boron and boron-nitrogen distances are comparable to those in 2S but are more uniform. Alongside 2S, these boron-chalcogen heterocycles featuring four boron atoms and a single chalcogen atom represent an intriguing class of inorganic ring compounds that was previously unknown. The four boron atoms in the ring’s backbone are interconnected by classical B − B single bonds, a structural feature presumably attributed to efficient electron donation from the amino substituents that inhibits the boron atoms from participating in nonclassical bonding^22,23,24.

As mentioned at the beginning of this section, the reactions of 1 with elemental chalcogens lack selectivity, yielding a variety of products (Fig. 4). For instance, treating 1 with sulfur in a toluene solution at 85 °C for one week resulted in the appearance of several new resonances in the ¹¹B NMR spectrum. In addition to some unidentified species (δ(¹¹B) = 43.3, 40.5, 30.8, 28.5 ppm), the boron-sulfur heterocycle 2S and an additional compound with a shift of δ = 37.5 ppm were identified. Mass spectrometry and X-ray crystallography confirmed this latter compound as [Cy₂NBS]₂ (3S), a 1,3-dithia-2,4-diboretane with structural precedent in the literature (see Supplementary Information for details)³⁸. It could be isolated from the mixture of products in 15% yield. Other products eluded accurate quantification, as only very low amounts of crystalline material were obtained. The same dithiadiboretane product was identified in reactions of 1 with trimethylphosphine sulfide (Me₃PS) as the chalcogen source. Similar results were found with elemental selenium, for which a range of ¹¹B NMR signals emerged from the reaction mixture. Only the five-membered ring product (2Se) and 1,3-diselena-2,4-diboretane 3Se (δ(¹¹B) = 35.5 ppm) were identified unambiguously, although the latter could not be characterized by X-ray crystallography. Reaction of 1 with trimethylphosphine selenide (Me₃PSe) proved to be unselective as well. For tellurium, ring expansion proved unsuccessful entirely; 1 was inert toward elemental tellurium, and its reaction with tri-n-butylphosphine telluride (nBu₃PTe) produced inseparable product mixtures.

Attempts aiming at ring expansion of tetraborane 1 via chalcogen atom insertion into B−B bonds using elemental chalcogens as reagents are unselective.

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In our pursuit of the lightest chalcogen derivative, i.e., 2 O, we tried several approaches to incorporate an oxygen atom into 1 through ring expansion. Surprisingly, tetraborane 1 was found to be remarkably stable under ambient conditions and does not readily react with molecular oxygen or moisture. Both in the solid state and in solution, 1 remains unaffected for several days without any signs of decomposition. In an effort to induce a reaction with oxygen, we allowed a solution of 1 in benzene to react for 4 d at room temperature in a 1 bar oxygen atmosphere. Monitoring the reaction by NMR spectroscopy revealed the emergence of several ¹¹B NMR resonances, suggesting an unselective reaction. In the reaction mixture, we identified the signal for the boroxine [Cy₂NBO]₃ (δ(¹¹B) = 20.9 ppm), which was further characterized by X-ray crystallography, as its structural data was previously lacking in the literature³⁹. To promote a more selective and controlled oxidation reaction of 1, we also used other reactive oxidizing agents such as trimethylamine N-oxide (Me₃NO), nitrous oxide (N₂O), and meta-chloroperbenzoic acid (mCPBA). While an unselective oxidation reaction was induced with mCPBA at room temperature, Me₃NO and N₂O showed no conversion under the same conditions. However, heating these solutions at 80 °C led to the formation of the corresponding boroxine alongside other unidentified products.

Chalcogen atom insertions into B − B single bonds, while rare, have been documented for selected diborane(4) systems^40,41,42. Himmel et al. achieved sulfur insertion using both elemental sulfur and diphenyl disulfides⁴⁰, while Ghosh and coworkers demonstrated both sulfur and selenium insertions⁴¹. Our group reported oxygen atom insertion into a dialkynyl-substituted diborane(4) using Me₃NO⁴². Similar to the reactivity of cyclic tetraborane 1 towards elemental chalcogens described here, we have previously also observed rather unselective chalcogen insertions into an amino-substituted triborane(5), yielding a complex mixture of insertion products that proved difficult to isolate⁴³.

Redox chemistry of the B₄E rings

As a new class of boron-chalcogen heterocycles, 2E (where E = S, Se, and Te), we explored their fundamental chemical and redox properties. Our findings reveal that these compounds exhibit remarkable thermal stability and resistance to chemical attack. The heterocycles 2E are stable under an oxygen atmosphere, even upon heating at 80 °C for several days, and towards other oxygenation reagents such as Me₃NO. They also demonstrate high stability toward strong Brønsted acids ([H(OEt₂)₂][BAr^F₄], Ar^F = 3,5-(CF₃)₂C₆H₃)^44,45, as well as reduction and the coordination of Lewis acids (e.g. AlCl₃, GaCl₃, AlMe₃). Their robust nature is further exemplified by their inertness towards phosphines (PMe₃, PPh₃), which failed to abstract the chalcogen atom to reform 1. However, our studies showed that these heterocycles are susceptible to oxidative B−B bond cleavage, yielding ring-opened, difunctionalized borane derivatives.

Electrochemical studies of 2S in THF showed a reversible oxidation process (E_1/2 = 0.23 V vs. Fc^+/0) in its cyclic voltammogram, indicating the formation of a stable radical cation species. Hence, treatment of a 1,2-difluorobenzene solution of 2S with one equivalent of [Ag][Al{OC(CF₃)₃}₄]⁴⁶ produced a dark green solution, from which the radical cation [2S][Al{OC(CF₃)₃}₄] was isolated in 79% yield (Fig. 5a). The EPR spectrum of this species in a mixture of toluene and 1,2-difluorobenzene features a broad, poorly resolved signal at a g factor of ca. 2.0022. Quantum chemical calculations at the SMD-ωB97xd/def2-TZVP//ωB97xd/def2SVPP level of theory⁴⁷ suggest that the spin density is delocalized predominantly along the σ-framework of the B₄ entity within the ring; a spin density plot of [2S]^•+ is depicted in Fig. 3c. This effect is also illustrated nicely by the topology of the singly occupied molecular orbital (SOMO) of [2S]^•+, which shows significant contributions from all three B − B σ-bonds (Fig. 5d). We note that similar results were recently obtained with the radical cation of 1, [1][Al{OC(CF₃)₃}₄], in parallel studies²³. X-ray diffraction analysis confirmed the structure to be the radical cation of 2, with the five-membered ring remaining intact in the +1 oxidation state (Fig. 5b). This single-electron oxidation of 2S induces notable structural changes: B − N bonds (1.366(4)−1.388(3) Å) contract, B − B (1.737(4)−1.785(4) Å) and B − S bonds (1.843(2)−1.846(3) Å) elongate, and the B − S − B angle widens slightly (97.1(1)°). We also note that all our efforts to generate a dication of 2S by applying two or more equivalents of [Ag][Al{OC(CF₃)₃}₄] failed thus far, and only [2S][Al{OC(CF₃)₃}₄] could be detected.

a One-electron oxidation of 2S to afford [2S][Al{OC(CF₃)₃}₄]. b Molecular structure of [2S][Al{OC(CF₃)₃}₄] in the solid state. Displacement ellipsoids shown at the 50% probability level; hydrogen atoms and ellipsoids of the cyclohexyl groups are omitted for clarity. c Spin density plot of [2S]^•+ (isosurface plot at 0.004 a.u.), and d SOMO of [2S]^•+ (isosurface plot at 0.04 a.u.), illustrating the importance of σ-electron delocalization effects (SMD-ωB97xd/def2-TZVP).

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No radical species was detected for the reaction of 2S with silver trifluoromethanesulfonate (AgOTf). Instead, we observed ring opening caused by nucleophilic attack of the triflate anion to the boron ring atoms. When treated with two equivalents of AgOTf in a benzene solution at 50 °C, we were able to isolate a colorless solid product with a yield of 33% (Fig. 6). Characterization of single crystals of the product, 4S, by X-ray crystallography indicated the cleavage of the B − B bond at the ring site opposite the sulfur center, and the formation of a ring-opened product with triflate groups at each end. In this linear chain form, the bond angle at sulfur is widened to nearly 119.1(1)°, while the B − S bond distances (1.876(3), 1.860(3) Å) are unaltered from those in the B₄S ring of 2S. The boron-boron bond distances in 4S (1.718(4), 1.707(4) Å) also fall within the range observed in 2S. The B − N bonds involving the triflate-bearing terminal boron atoms are slightly longer than the internal ones, which remain similar to those in 2S. The B − O bond length of 1.466(3) Å aligns well with those observed in other triflate-containing boranes featuring three-coordinate, planar boron centers^48,49. Variable-temperature NMR studies of 4S in toluene solution are reminiscent of dynamic behavior, thus suggesting the presence of multiple conformations. While two distinct ¹¹B NMR resonances are observed from room temperature down to 193 K, the ¹⁹F NMR spectra show increasing complexity with the emergence of additional signals, indicating the presence of distinct conformational species in slow exchange. Heating the toluene solution initially decreased the linewidths of the ¹¹B NMR signals, but ultimately led to rapid product decomposition. While numerous cyclic compounds containing a B − S − B moiety are documented in the literature, acyclic analogs featuring a structure such as 4S, remain rare. Only one example, a diboryl sulfide reported by Nöth as a side product from chloroborane reduction, has been previously characterized⁵⁰. A related structure featuring a six-membered B₄S₂ ring with B − B − S − B − B connectivity was also reported by the same group³¹. Interestingly, analogous oxidation reactions of the higher homologues 2Se and 2Te with AgOTf (2 eq.) did not yield any reaction under the same conditions (Fig. 6), which is particularly noteworthy given that cyclic voltammetry indicated similar first oxidation potentials (2Se E_1/2 = 0.20 V, 2Te E_1/2 = 0.16 V; vs. Fc^+/0). However, 2Se could be forced to react with AgOTf by applying higher temperatures (80 °C) and significantly longer reaction times (6 days), but proved highly unselective and produced no tractable materials. 2Te, by contrast, showed no reactivity toward AgOTf at all, even at 100 °C. The reason for these differences is unknown to us so far.

Synthesis of linear 4S via selective oxidative ring-opening of 2S by AgOTf, along with its molecular structure (displacement ellipsoids at 50% probability level; hydrogen atoms and ellipsoids of the cyclohexyl groups omitted for clarity). For E = Se, Te, experiments targeting similar ring-opening events were either unselective or failed at all.

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Ring-opening reactivity of 1

The successful B−B bond cleavage of 2S upon pseudohalogenation to form 4S led us to explore whether cyclic tetraborane 1 can also be used to generate 1,4-difunctionalized tetraboranes. Initial investigations focused on AgOTf, followed by studies with conventional halogenating agents such as bromine (Br₂), hexachloroethane (C₂Cl₆), and antimony trifluoride (SbF₃). It should be noted that, similar to 2S, cyclic tetraborane 1 undergoes one-electron oxidation to form a radical cation, which we previously isolated using the weakly coordinating polyfluoro-alkoxyaluminate [Al{OC(CF₃)₃}₄]⁻ counterion²³. While reactions with other silver salts of different anions were generally unselective, we discovered that treatment of 1 with two equivalents of AgOTf in benzene yields the ring-opened 1,4-bistriflate derivative 5OTf as a colorless solid in 92% yield (Fig. 7). Although X-ray diffraction analysis confirmed the connectivity of 5OTf, extensive disorder in the crystal structure precluded detailed structural analysis. Solution NMR studies suggested conformational dynamics similar to those observed for 4S, as evidenced by four distinct ¹⁹F NMR signals for the CF₃ groups, which coalesce into three signals at lower temperatures (see Supplementary Information for details). Its ¹¹B NMR spectrum features two broad signals at δ = 55.5 and 36.2 ppm for the inner and outer boron atoms, respectively.

Selective ring-opening processes of 1 – formation of linear tetraboranes 5F, 5Cl, and 5Br by reaction with halogenating agents.

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Using SbF₃, C₂Cl₆, and Br₂ as halogenation agents, we successfully transferred this reactivity to the respective halide-substituted tetraboranes 5F, 5Cl, and 5Br, which were isolated in good yields ranging from 47 to 78% (Fig. 7). Again, each of these compounds show two distinct ¹¹B NMR signals for the four boron atoms; the outer two boron atoms at lower chemical shifts (δ(¹¹B) = 37.9–43.4 ppm), the inner two boron atoms in a narrow region between δ = 56.9 and 57.5 ppm. The presence of fluoride served as a reliable spectroscopic probe for verifying conformational dynamics in solution for 5F similar to 4S and 5OTf. Thus, 5F shows five distinct ¹⁹F NMR signals in a range between δ = –76.18 and –88.53 ppm. While these dynamics are directly detectable only in fluorine-containing species, it seems reasonable to conclude that analogous conformational behavior exists for the other halide derivatives as well. To gain better insights into these dynamics, we exemplarily studied the thermodynamics of four hypothetical conformers of 5F by theoretical methods, all of them derived from its “gauche”-type X-ray structure (see below). To this end, we optimized 5 F (5F^conf1), and generated conformers by 180° rotations around the B2 − B3 bond (5F^conf2), the B3 − B4 bond (5F^conf3), or a combination of both operations (5F^conf4; see Supplementary Information for details). As expected, the conformers are close in energy, with the experimentally verified “gauche”-type arrangement 5F^conf1 being thermodynamically slightly favored. The other conformers display only marginally higher energy levels: 5F^conf4 ΔE = +0.3 kcal/mol; 5F^conf3 ΔE = +2.1 kcal/mol; 5F^conf2 ΔE = +5.4 kcal/mol. While we have not investigated the barriers between conformers, their comparable thermodynamic stability supports our hypothesis of conformational dynamics. We also note that the calculated ¹⁹F NMR chemical shifts of the conformers are found in a similar range as the experimental signals (see Supplementary Information for details). The molecular structures of all derivatives confirm their open-chain structure (Fig. 8). All species adopt a “gauche” conformation, characterized by dihedral angles of around 60° between the BX(NCy₂) groups (X = F, Cl, Br). Specifically, the angles measured are 46.9(1)° for the fluorine derivative 5 F, 74.9(3)° and 72.5(3)° for the chlorine derivative 5Cl, and 63.6(2)° for the bromine derivative 5Br. The nearly perpendicular arrangement of the trigonal planes of the four boron atoms results in either a syn or anti relationship between the halogen substituents. While the fluoride and chloride derivatives adopt an anti configuration, the bromides in 5Br show a syn arrangement. This orientation induces statistically significant differences in the B − N and B − B bonds. In 5Br, the B − N bonds on the terminal boron atoms are marginally shorter than the central B − N bonds, while the outer B − B bonds are measurably shorter than the central B − B bond, reflecting the influence of the electronegative halide substituents.

Molecular structures of the linear tetraboranes 5 F, 5Cl, and 5Br with displacement ellipsoids at 50% probability level (hydrogen atoms and ellipsoids of the cyclohexyl groups omitted for clarity).

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Recently, we generated related 1,4-difunctionalized tetraboranes by halogenation of linear hexakis(dimethylamino)tetraborane(6), B₄(NMe₂)₆⁵¹. Despite the overall similarity in spectroscopic profiles, a notable difference is evident in the NMR data of its OTf derivative, which displays a single ¹⁹F NMR resonance at δ = −77.43 ppm. This observation suggests a higher degree of symmetry in solution, presumably due to a dynamic equilibrium of conformers. The rapid interconversion of these conformers is likely facilitated by the low steric hindrance associated with the NMe₂ substituents, minimizing any steric interactions hindering free rotation around the boron-boron bonds. It is also worth noting that 5 F represents the first example of a fluoride-substituted derivative.

By contrast to the reactivity described above, 1,2-dibromo-1,2-bis(dicyclohexyl-amino)diborane(4) 6 is formed instead of the linear tetraborane 5Br, when 1 is reacted with an excess of elemental bromine Br₂ (Fig. 9). After work-up, 6 is isolated in a moderate yield of 56%, easily identified by a ¹¹B NMR chemical shift of δ = 37.8 ppm, which is nearly identical to that of the related NMe₂-substitued diborane(4) (δ(¹¹B) = 37.7 ppm)⁵². In addition to multinuclear NMR spectroscopy, 6 was characterized by solid-state X-ray diffraction and mass spectrometry. Similar to the dimethylamino derivative, the angles between the planes of the boron atoms are nearly perpendicular, forming an angle of 88.6°. Also, bond lengths for B − N (1.382(2), 1.384(2) Å), B − B (1.691(3) Å), and B − Br (1.978(2), 1.982(2) Å) are comparable. No further oxidation of 6 to the corresponding amino(dibromo)borane (Cy₂N)BBr₂ was observed regardless the amount of Br₂ used in the bromination of 1. Also, no related overchlorination reactivity was observed during the reaction of 1 with hexachloroethane, and we did not detect any evidence for the generation of the corresponding 1,2-dichlorodiborane(4). However, we note that selective oxidation of 1 to 5Cl was also achieved when 1 was reacted with one equivalent of AlCl₃ and GaCl₃. With its confirmed ring-opening reactivity towards halogenating agents, cyclotetraborane 1 thus exhibits typical behavior associated with small ring systems. This reactivity makes 1 a valuable building block for the synthesis of 1,4-difunctionalized tetraboranes, which are otherwise challenging to produce, thereby opening new avenues for developing complex boron-chain-containing compounds.

Synthesis of diborane B₂(NCy₂)₂Br₂ (6) by ring-opening reaction of 1 with excess Br₂.

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We have demonstrated that tetrakis(dicyclohexylamino)cyclotetraborane, B₄(NCy₂)₄, readily participates in ring-expansion reactions with diphenyl dichalcogenides to form non-planar five-membered B₄E heterocycles (E = S, Se, Te). The sulfur derivative shows interesting reactivity patterns, including oxidation to a radical cation and ring opening with silver triflate, which generates a rare diboryl sulfide. Similar reactivitiy was observed with the cyclic tetraborane precursor, which reacted with various halogenating reagents to produce synthetically valuable, linear tetraboranes functionalized at the termini with triflate, fluoride, chloride, and bromide. The steric hindrance of the dicyclohexyl groups restricts rotation around the boron-boron bonds, leading to the formation of various conformers in solution, as suggested by ¹⁹F NMR spectroscopy and DFT calculations. Further investigations into the reactivity of the cyclic tetraborane are currently underway in our laboratory, with a particular focus on enhancing our understanding of boron-rich compounds that exhibit classical bonding between boron atoms.

General

All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated solvents were dried over molecular sieves and degassed by three freeze-pump-thaw cycles prior to use. All other solvents were distilled and degassed from appropriate drying agents. Both deuterated and non-deuterated solvents were stored under argon over activated 4 Å molecular sieves. Liquid-phase NMR spectra were acquired on a Bruker Avance 400 (¹H: 400.1 MHz, ¹¹B:128.5 MHz, ¹³C: 100.7 MHz), Bruker Avance 500 (¹¹B: 160.5 MHz, ¹³C: 125.7 MHz), or Bruker Avance 600 (¹H: 600.1 MHz, ¹¹B: 192.6 MHz, ¹³C: 150.9 MHz, ¹⁹F: 564.7 MHz, ⁷⁷Se: 114.5 MHz, ¹²⁵Te: 189.3 MHz) spectrometers. Chemical shifts (δ) are reported in ppm and internally referenced to the carbon nuclei (¹³C{¹H}) or residual protons (¹H) of the solvent. Heteronuclear NMR spectra are referenced to external standards (¹¹B: BF₃ ∙ OEt₂, ¹⁹F: CCl₃F, ⁷⁷Se: Me₂Se, ¹²⁵Te: Me₂Te). Resonances are given as singlet (s), multiplet (m) or broad (br). High-resolution mass spectrometry (HRMS) data were obtained from a Thermo Scientific Exactive Plus spectrometer. EPR measurements at X-band (9.86 GHz) were carried out using a Bruker ELEXSYS E580 CW EPR spectrometer. The spectral simulations were performed using MATLAB 9.12.0.1884302 (R2022a) and the EasySpin 5.2.33 toolbox⁵³. Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell configuration was employed using a platinum disk working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the reference electrode. Formal redox potentials are referenced to the ferrocene/ferrocenium ([Cp₂Fe]^+/0) redox couple by using decamethylferrocene ([Cp*₂Fe]; E_1/2 = −0.427 V in THF) as an internal standard. Tetra(n-butyl)ammonium hexafluorophosphate ([nBu₄N][PF₆]) was employed as the supporting electrolyte. Compensation for resistive losses (iR drop) was employed for all measurements. Photolysis reactions were carried out on an Hg/Xe arc lamp (I = 19 A, U = 26 V, P = 500 W, λ = 210–600 nm) from the company LOT-QuantumDesign. Solvents, hexamethyldisilazane (HMDS), Ph₂S₂, Ph₂Se₂, Ph₂Te₂, SbF₃, C₂Cl₆, Br₂, and AgOTf (OTf = trifluoromethanesulfonate) were purchased from Sigma-Aldrich, ABCR or Alfa Aesar. [Ag][Al{OC(CF₃)₃}₄] was provided by the group of Prof. Ingo Krossing (University of Freiburg, Germany).

Single-crystal X-ray diffraction

The crystallographic data of 2S, 2Se, 2Te, [Cy₂NBO]₃, [2S][Al(OC(CF₃)₃)₄], 4S, 5 F, 5Cl, 5Br, and 6 were collected on a XtaLAB Synergy Dualflex HyPix diffractometer with a Hybrid Pixel array detector and multi-layer mirror monochromated Cu_Kα or Mo_Kα radiation. The structures were solved using the intrinsic phasing method⁵⁴, refined with the ShelXL program⁵⁵ and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions.

Computational details

All computations were performed using the Gaussian16 (Revision C.01) package⁴⁷. All structures were fully optimized without symmetry constraints at the ωB97xd level of theory employing def2-SVPP basis sets for all atoms^56,57. Zero-point vibrational energies and thermal corrections were computed from frequency calculations with a standard state of 298 K and 1 atm; thermal free energies (ΔE₂₉₈) were obtained from these single-point frequency calculations. The presence of true energy minima on the potential energy surface was verified for all optimized species by the absence of imaginary frequencies. For EPR, NMR, and orbital calculations def2-TZVP basis sets were used, combined with the SMD solvation model (scrf=smd) for inclusion of tetrahydrofuran solvent effects⁵⁸. Calculated ¹⁹F NMR chemical shifts were referenced to CCl₃F, the commonly used external standard in NMR spectroscopy, by first referencing to the calculated chemical shift of optimized C₆F₆ and putting the values into relation to its experimentally observed value of δ = –164.9 ppm. Illustrations of optimized structures, as well as orbital and spin density plots were prepared with IQmol 3.1.3⁵⁹.

Synthesis of B₄(NCy₂)₄ (1)

B₄(NCy₂)₄ (1) was prepared by the following procedure²³: (Cy₂N)BCl₂ (200 mg, 760 μmol) and sodium sand (69.9 mg, 3.04 mmol, 4.0 equiv.) were combined in pentane (10 mL) and stirred for 3 d at room temperature, during which time a purple color gradually developed, accompanied by the precipitation of a purple solid. After filtration, all volatile components were removed in vacuo, and the resulting residue was washed with DME (3 × 5 mL) and dried under reduced pressure. Thus, 1 was isolated as a blue solid (60.0 mg, 78.4 μmol, 41%). ¹H{¹¹B} NMR (400.1 MHz, C₆D₆, 297 K): δ = 3.39–3.27 (m, 8H, CH), 2.04–1.94 (m, 16H, o-CH₂), 1.88–1.80 (m, 16H, m-CH₂), 1.71–1.58 (m, 24H, o-/p-CH₂), 1.46–1.32 (m, 16H, m-CH₂), 1.25–1.12 (m, 8H, p-CH₂) ppm. ¹³C{¹H} NMR (100.7 MHz, C₆D₆, 297 K): δ = 63.9 (CH), 35.7 (o-CH₂), 26.9 (m-CH₂), 26.2 (p-CH₂) ppm. ¹¹B NMR (128.5 MHz, C₆D₆, 297 K): δ = 68.0 (br s) ppm. HRMS LIFDI for [C₄₈H₈₈N₄B₄]⁺ = [M]⁺: calc. 764.7376, found 764.7372.

Synthesis of B₄(NCy₂)₄S (2S)

B₄(NCy₂)₄ (1, 100 mg, 131 μmol) and diphenyl disulfide (14.3 mg, 65.4 μmol, 0.5 equiv.) were combined in benzene (6 mL) and stirred for 2 d under UV irradiation, resulting in the decolorization of the blue solution. After removing all volatiles from the reaction mixture under vacuum, the residue was washed with DME (4 × 5 mL), filtered, and then dried under reduced pressure. This yielded product 2S as a colorless solid (85.0 mg, 106 μmol, 81%). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from hexane at –30 °C. Note: When 1 is reacted with diphenyl sulfide, 2S is obtained in 74% yield. ¹H{¹¹B} NMR (600.1 MHz, C₆D₆, 297 K): δ = 3.39–3.30 (m, 4H, CH), 3.26–3.14 (m, 2H, CH₂), 3.07–3.00 (m, 2H, CH), 2.97–2.76 (m, 2H, CH), 2.02–1.63 (m, 40H, CH₂), 1.61–1.47 (m, 12H, CH₂), 1.44–1.18 (m, 22H, CH₂), 1.13–1.00 (m, 4H, CH₂) ppm. ¹³C{¹H} NMR (125.7 MHz, C₆D₆, 297 K): δ = 66.3 (CH), 66.1 (CH), 60.5 (CH), 57.0 (CH), 36.5 (CH₂), 36.0 (CH₂), 34.8 (CH₂), 34.0 (CH₂), 33.8 (CH₂), 33.5 (CH₂), 33.3 (CH₂), 32.7 (CH₂), 27.6 (CH₂), 27.5 (CH₂), 27.2 (CH₂), 27.1 (CH₂), 27.1 (CH₂), 26.6 (CH₂), 26.5 (CH₂), 26.3 (CH₂), 26.3 (CH₂), 26.2 (CH₂), 26.0 (CH₂) ppm. ¹¹B NMR (160.5 MHz, C₆D₆, 297 K): δ = 55.5 (br s), 45.9 (br s) ppm. HRMS LIFDI for [C₄₈H₈₈B₄N₄S]⁺ = [M]⁺: calc. 796.7092; found 796.7096.

Synthesis of B₄(NCy₂)₄Se (2Se)

B₄(NCy₂)₄ (1, 100 mg, 131 μmol) and diphenyl diselenide (20.4 mg, 65.4 μmol, 0.5 equiv.) were combined in benzene (5 mL) and stirred for 2 d at 80 °C, resulting in a color change of the reaction mixture from blue to yellow. After removing all volatiles from the reaction mixture under vacuum, the residue was washed with DME (4 × 5 mL), filtered, and dried under reduced pressure. This yielded product 2Se as a colorless solid (66.3 mg, 78.6 μmol, 60%). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from hexane at –30 °C. ¹H{¹¹B} NMR (600.1 MHz, C₆D₆, 297 K): δ = 3.40–3.32 (m, 4H, CH), 3.08–3.00 (m, 2H, CH), 2.99–2.91 (m, 1H, CH), 2.88–2.78 (m, 2H, CH), 2.10–2.04 (m, 4H, CH₂), 1.99–1.63 (m, 38H, CH₂), 1.61–1.46 (m, 12H, CH₂), 1.43–1.17 (m, 22H, CH₂), 1.13–0.98 (m, 4H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, C₆D₆, 297 K): δ = 67.3 (CH), 66.6 (CH), 60.8 (CH), 57.5 (CH), 36.7 (CH₂), 36.3 (CH₂), 35.0 (CH₂), 33.8 (CH₂), 33.2 (CH₂), 32.6 (CH₂), 27.5 (CH₂), 27.4 (CH₂), 27.3 (CH₂), 27.2 (CH₂), 27.2 (CH₂), 27.1 (CH₂), 26.6 (CH₂), 26.5 (CH₂), 26.3 (CH₂), 26.3 (CH₂), 26.2 (CH₂), 25.9 (CH₂) ppm. ¹¹B NMR (192.6 MHz, C₆D₆, 297 K): δ = 55.4 (br s), 46.8 (br s) ppm. ⁷⁷Se{¹¹B,¹H} NMR (114.5 MHz, C₆D₆, 297 K): δ = 254 ppm. HRMS LIFDI for [C₄₈H₈₈B₄N₄Se]⁺ = [M]⁺: calc. 843.6569; found 843.6577.

Synthesis of B₄(NCy₂)₄Te (2Te)

B₄(NCy₂)₄ (1, 60.0 mg, 78.5 μmol) and diphenyl ditelluride (16.1 mg, 39.2 μmol, 0.5 equiv.) were combined in benzene (5 mL) and stirred for 1 d under UV irradiation, resulting in a color change of the reaction mixture from blue to brown. After removal of all volatiles from the reaction mixture in vacuo, the residue was washed with DME (3 × 5 mL), filtered, and dried under reduced pressure. This yielded product 2Te as a colorless solid (34.6 mg, 38.8 μmol, 49%). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from THF at –30 °C. ¹H{¹¹B} NMR (600.1 MHz, C₆D₆, 297 K): δ = 3.56–3.35 (m, 6H, CH and CH₂), 3.11–2.95 (m, 4H, CH), 2.86–2.75 (m, 2H, CH₂), 2.28–2.20 (m, 2H, CH₂), 2.01–1.46 (m, 52H, CH₂), 1.40–1.15 (m, 18H, CH₂), 1.12–0.98 (m, 4H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, C₆D₆, 297 K): δ = 68.7 (CH), 66.9 (CH), 61.1 (CH), 57.6 (CH), 36.5 (CH₂), 36.5 (CH₂), 35.0 (CH₂), 33.6 (CH₂), 33.2 (CH₂), 32.8 (CH₂), 32.7 (CH₂), 32.6 (CH₂), 27.1 (CH₂), 27.0 (CH₂), 27.0 (CH₂), 26.9 (CH₂), 26.2 (CH₂), 26.0 (CH₂), 25.9 (CH₂) ppm. ¹¹B NMR (192.6 MHz, C₆D₆, 297 K): δ = 58.7 (br s), 44.5 (br s) ppm. ¹²⁵Te NMR (189.3 MHz, C₆D₆, 297 K): δ = 15.6 ppm. HRMS LIFDI for [C₄₈H₈₈B₄N₄Te]⁺ = [M]⁺: calc. 893.6443; found 893.6481.

Synthesis of B₂(NCy₂)₂S₂ (3S)

B₄(NCy₂)₄ (1, 30.0 mg, 39.2 μmol) and S₈ (5.03 mg, 19.6 μmol, 0.5 equiv.) were combined in benzene (3 mL) and stirred for 5 d at 85 °C, resulting in a color change of the reaction mixture from blue to yellow. After removing all volatiles from the reaction mixture under vacuum, the residue was washed with pentane (2 × 2 mL), filtered, and dried under reduced pressure. This yielded 3S as a colorless solid (5.30 mg, 11.9 μmol, 15%). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from benzene at room temperature. ¹H{¹¹B} NMR (400.1 MHz, C₆D₆, 297 K): δ = 3.39–3.23 (m, 4H, CH), 1.95–1.70 (m, 16H, o-CH₂), 1.68–1.59 (m, 8H, m-CH or p-CH₂), 1.51–1.40 (m, 4H, m-CH or p-CH₂), 1.24–1.10 (m, 8H, m-CH or p-CH₂), 1.05–0.91 (m, 4H, m-CH or p-CH₂ ppm. ¹³C{¹H} NMR (100.7 MHz, C₆D₆, 297 K): δ = 58.2 (CH), 34.0 (o-CH₂), 26.7 (CH₂), 25.8 (CH₂) ppm. ¹¹B NMR (128.5 MHz, C₆D₆, 297 K): δ = 37.5 (br s) ppm. HRMS LIFDI for [C₂₄H₄₄B₂N₂S₂]⁺ = [M]⁺: calc. 446.3127; found 446.3119.

Synthesis of [B₄(NCy₂)₄S][Al{OC(CF₃)₃}₄] ([2S][Al{OC(CF₃)₃}₄])

To a solution of 2S (100 mg, 125 μmol) in 1,2-difluorobenzene (8 mL), [Ag][Al(OC(CF₃)₃)₄] (134 mg, 125 μmol, 1.0 equiv.) was added and stirred for 20 min at room temperature under exclusion of light, resulting in the reaction mixture turning dark green. The insoluble materials in the reaction mixture were subsequently removed by filtration, and the solvent from the green filtrate was evaporated under reduced pressure. The residue was washed with benzene (2 × 5 mL) and dried under reduced pressure, yielding [2S][Al{OC(CF₃)₃}₄] as a dark green solid (175 mg, 99.2 μmol, 79%). Single crystals suitable for X-ray diffraction analysis were obtained by crystallization in a mixture of toluene and DME at –30 °C. HRMS LIFDI for [C₄₈H₈₈B₄N₄S]⁺ = [M]⁺: calc. 796.7096; found 796.7089. UV-vis (1,2-difluorobenzene): λ₁ = 422 nm, λ₂ = 673 nm (shoulder).

Synthesis of B₄(NCy₂)₄S(OTf)₂ (4S)

B₄(NCy₂)₄S (2S, 50 mg, 62.8 μmol) and AgOTf (32.3 mg, 125.5 μmol, 2.0 equiv.) were combined in benzene (4 mL) and stirred for 17 h at 50 °C, causing the reaction mixture to change from colorless to yellow. The insoluble materials in the reaction mixture were separated by filtration. After the removal of all volatiles from the reaction mixture in vacuo, the residue was extracted with cold pentane (4 × 1 mL, –30 °C), filtered again, and the solvent from the filtrate was removed under reduced pressure. The extraction in cold pentane was repeated until the filtrate became colorless, yielding 4S as a colorless solid (23.0 mg, 21.0 μmol, 33%). Single crystals suitable for X-ray diffraction analysis were obtained by crystallization in toluene at –30 °C. Note: In solution, a mixture of temperature-dependent conformational isomers is present, which cannot be distinguished from each other. ¹H{¹¹B} NMR (600.1 MHz, d₈-toluene, 298.15 K): δ = 4.17–4.02 (m, 1H, CH), 3.52–3.39 (m, 2H, CH), 3.23–3.07 (m, 2H, CH), 2.82–2.60 (m, 2H, CH), 2.45–2.33 (m, 1H, CH), 2.06–0.82 (m, 80H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, d₈-toluene, 293.15 K): δ = 131.4 (CF₃), 122.4 (CF₃), 120.3 (CF₃), 118.2 (CF₃), 116.1 (CF₃), 71.1 (CH), 68.4 (CH), 66.3 (CH), 61.0 (CH), 60.8 (CH), 60.4 (CH), 59.8 (CH), 59.4 (CH), 57.0 (CH), 56.0 (CH), 56.0 (CH), 55.8 (CH), 55.2 (CH), 54.3 (CH), 54.1 (CH), 38.2 (CH₂), 37.8 (CH₂), 37.1 (CH₂), 36.6 (CH₂), 36.0 (CH₂), 35.1 (CH₂), 34.7 (CH₂), 34.5 (CH₂), 34.1 (CH₂), 33.9 (CH₂), 33.8 (CH₂), 33.8 (CH₂), 33.7 (CH₂), 33.6 (CH₂), 33.5 (CH₂), 33.2 (CH₂), 32.9 (CH₂), 32.5 (CH₂), 32.1 (CH₂), 31.4 (CH₂), 29.1 (CH₂), 28.0 (CH₂), 27.7 (CH₂), 27.6 (CH₂), 27.6 (CH₂), 27.4 (CH₂), 27.3 (CH₂), 27.2 (CH₂), 27.2 (CH₂), 27.2 (CH₂), 27.1 (CH₂), 27.1 (CH₂), 27.1 (CH₂), 27.0 (CH₂), 26.9 (CH₂), 26.7 (CH₂), 26.6 (CH₂), 26.6 (CH₂), 26.6 (CH₂), 26.5 (CH₂), 26.5 (CH₂), 26.4 (CH₂), 26.3 (CH₂), 26.3 (CH₂), 26.3 (CH₂), 26.2 (CH₂), 26.2 (CH₂), 26.1 (CH₂), 26.0 (CH₂), 25.7 (CH₂), 25.6 (CH₂), 25.4 (CH₂), 25.3 (CH₂), 25.1 (CH₂), 25.0 (CH₂), 24.8 (CH₂), 24.6 (CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 293.15 K): δ = 42.1 (br s), 34.4 (br s) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 193.15 K): δ = 36.3 (br s), 26.2 (br s) ppm. ¹⁹F NMR (564.7 MHz, d₈-toluene, 293.15 K): δ = –74.99, –76.29, –76.61, –76.93, –77.08, –78.20 ppm. ¹⁹F NMR (564.7 MHz, d₈-toluene, 193.15 K): δ = –75.41, –75.98, –76.16, –76.37, –76.48, –76.66, –76.76, –76.83, –77.17, –77.39, –78.01 ppm. HRMS LIFDI for [C₄₉H₈₈B₄F₃N₄O₃S₂]⁺ = [M—CSF₃O₃]⁺: calc. 945.6617; found 945.6616.

Synthesis of B₄(NCy₂)₄F₂ (5 F)

B₄(NCy₂)₄ (1, 50.0 mg, 65.4 μmol) and SbF₃ (23.4 mg, 131 μmol, 2.0 equiv.) were combined in a flask (silanized with HMDS) and dissolved in benzene (5 mL). After stirring for 20 h at 85 °C, the initially blue solution underwent decolorization. The insoluble materials in the reaction mixture were subsequently removed by filtration. After removal of all volatiles from the reaction mixture in vacuo, the residue was washed with DME (4 × 2 mL), filtered, and dried under reduced pressure. This yielded product 5 F as a colorless solid (25.0 mg, 31.2 μmol, 47%). Single crystals suitable for X-ray diffraction analysis were obtained by crystallization in benzene at room temperature. Note: In solution, a mixture of temperature-dependent conformational isomers is present, which cannot be distinguished from each other. ¹H{¹¹B} NMR (600.1 MHz, d₈-toluene, 383.15 K): δ = 3.41–3.33 (m, 2H, CH), 3.30–3.19 (m, 2H, CH), 3.16–3.08 (m, 2H, CH), 2.72–2.63 (m, 2H, CH₂), 2.20–2.09 (m, 2H, CH₂), 2.01–0.99 (m, 76H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, d₈-toluene, 383.15 K): δ = 68.1 (CH), 62.9 (CH), 58.6 (CH), 58.6 (CH), 55.5 (CH), 37.5 (CH₂), 35.4 (CH₂), 33.7 (CH₂), 28.0 (CH₂), 27.7 (CH₂), 27.5 (CH₂), 27.1 (CH₂), 26.8 (CH₂), 26.7 (CH₂), 26.6 (CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 383.15 K): δ = 56.9 (br s), 37.9 (br s) ppm. ¹⁹F NMR (564.7 MHz, d₈-toluene, 403.15 K): δ = –79.81 ppm. ¹⁹F NMR (564.7 MHz, d₈-toluene, 298.15 K): δ = –76.18, –77.04, –84.96, –87.05, –88.53 ppm. HRMS LIFDI for [C₄₈H₈₈B₄F₂N₄]⁺ = [M]⁺: calc. 802.7344; found 802.7329.

Synthesis of B₄(NCy₂)₄Cl₂ (5Cl)

B₄(NCy₂)₄ (1, 50 mg, 65.4 μmol) and hexachloroethane (15.5 mg, 65.4 μmol, 1.0 equiv.) were combined in benzene (3 mL) and stirred for 3 h at 60 °C, resulting in the decolorization of the blue solution. The insoluble materials in the reaction mixture were subsequently removed by filtration, and the solvent of the filtrate was removed under reduced pressure. This produced 5Cl as a colorless solid (43.0 mg, 51.5 μmol, 78%). Single crystals suitable for X-ray diffraction analysis were obtained by crystallization in DME at –30 °C. Note: Selective oxidation of 1 to 5Cl was also observed when 1 was reacted with one equivalent of AlCl₃ or GaCl₃. ¹H{¹¹B} NMR (600.1 MHz, C₆D₆, 297 K): δ = 3.97–3.88 (m, 2H, CH), 3.79–3.67 (m, 2H, CH), 3.60–3.52 (m, 2H, CH), 3.06–2.54 (m, 4H, CH and CH₂), 2.43–0.93 (m, 78H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, d₈-toluene, 383.15 K): δ = 68.0 (CH), 67.1 (CH), 63.7 (CH), 57.8 (CH), 39.2 (CH₂), 38.7 (CH₂), 35.8 (CH₂), 35.4 (CH₂), 34.5 (CH₂), 34.1 (CH₂), 33.6 (CH₂), 28.0 (CH₂), 28.0 (CH₂), 27.9 (CH₂), 27.9 (CH₂), 27.8 (CH₂), 27.2 (CH₂), 27.1 (CH₂), 27.0 (CH₂), 26.7 (CH₂), 26.5 (CH₂), 26.4 (CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 383.15 K): δ = 57.5 (br s), 43.4 (br s) ppm. HRMS LIFDI for [C₄₈H₈₈B₄Cl₂N₄]⁺ = [M]⁺: calc. 834.6751; found 834.6753.

Synthesis of B₄(NCy₂)₄Br₂ (5Br)

To a solution of B₄(NCy₂)₄ (1, 50 mg, 65.4 μmol) in pentane (20 mL) cooled to –60 °C, a pentane solution (5 mL) of Br₂ (10.5 mg, 0.20 mL of 0.325 M in benzene 65.4 μmol, 1.0 equiv.) at –60 °C was added dropwise. The reaction mixture was stirred for 24 h while slowly warming to room temperature, resulting in the decolorization of the blue solution. After removal of all volatiles from the reaction mixture in vacuo, the residue was washed with cold hexane (2 mL, –60 °C) and dried under reduced pressure. This yielded 5Br as a colorless solid (43.0 mg, 46.5 μmol, 71%). Single crystals for X-ray diffraction analysis were obtained by crystallization in pentane at –30 °C. ¹H{¹¹B} NMR (600.1 MHz, d₈-toluene, 383.15 K): δ = 3.94–3.88 (m, 2H, CH), 3.65–3.56 (m, 4H, CH), 3.51–3.27 (m, 2H, CH), 3.63–2.44 (m, 2H, CH₂), 3.43–2.35 (m, 2H, CH₂), 2.28–2.20 (m, 2H, CH₂), 2.15–2.09 (m, 2H, CH₂), 1.94–0.99 (m, 72H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, d₈-toluene, 383.15 K): δ = 68.6 (CH), 66.8 (CH), 64.8 (CH), 59.3 (CH), 39.0 (CH₂), 38.9 (CH₂), 35.9 (CH₂), 35.4 (CH₂), 34.5 (CH₂), 34.1 (CH₂), 33.8 (CH₂), 28.0 (CH₂), 27.9 (CH₂), 27.8 (CH₂), 27.7(CH₂), 27.1 (CH₂), 27.0 (CH₂), 26.9 (CH₂), 26.7 (CH₂), 26.5 (CH₂), 26.4 (CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 383.15 K): δ = 57.0 (br s), 43.3 (br s) ppm. HRMS LIFDI for [C₄₈H₈₈B₄BrN₄]⁺ = [M—Br]⁺: calc. 844.6575; found 844.6564.

Synthesis of B₄(NCy₂)₄(OTf)₂ (5OTf)

B₄(NCy₂)₄ (1, 50 mg, 65.4 μmol) and AgOTf (33.6 mg, 130.8 μmol, 2.0 equiv.) were combined in benzene (4 mL) and stirred for 1 h at room temperature, resulting in the decolorization of the blue solution. The insoluble materials in the reaction mixture were subsequently separated by filtration, and the solvent of the filtrate was removed under reduced pressure, yielding 5OTf as a colorless solid (64.0 mg, 65.4 μmol, 92%). Note: Single crystals of 5OTf suitable for X-ray diffraction analysis could not be obtained despite numerous crystallization attempts. Attempts involved systematic variations of solvents, temperatures, and crystallization techniques, yet the resulting crystals consistently exhibited excessive disorder, preventing reliable X-ray diffraction analysis. Note: In solution, a mixture of temperature-dependent conformational isomers is present, which cannot be distinguished from one another. ¹H{¹¹B} NMR (600.1 MHz, C₆D₆, 393.15 K): δ = 4.01–3.94 (m, 1H, CH), 3.76–3.55 (m, 4H, CH), 3.29–3.22 (m, 1H, CH), 3.16–3.05 (m, 1H, CH), 3.00–2.92 (m, 1H, CH), 2.54–2.43 (m, 3H, CH₂), 2.37–2.31 (m, 1H, CH₂), 2.25–2.16 (m, 4H, CH₂), 2.06–1.02 (m, 72H, CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 393.15 K): δ = 55.5 (br), 36.2 (br) ppm. ¹³C{¹H,¹¹B} NMR (150.9 MHz, d₈-toluene, 383.15 K): δ = 120.7 (CF₃), 118.6 (CF₃), 71.8 (CH), 67.6 (CH), 67.5 (CH), 60.1 (CH), 60.1 (CH), 57.4 (CH), 56.8 (CH), 39.0 (CH₂), 38.7 (CH₂), 38.1 (CH₂), 37.5 (CH₂), 36.3 (CH₂), 36.1 (CH₂), 35.8 (CH₂), 34.2 (CH₂), 33.6 (CH₂), 31.2 (CH₂), 30.2 (CH₂), 28.5 (CH₂), 28.4 (CH₂), 28.2 (CH₂), 27.9 (CH₂), 27.8 (CH₂), 27.6 (CH₂), 27.2 (CH₂), 27.0 (CH₂), 26.9 (CH₂), 26.7 (CH₂), 26.3 (CH₂), 26.2 (CH₂), 26.0 (CH₂), 25.9 (CH₂), 25.9 (CH₂) ppm. ¹⁹F NMR (470.6 MHz, d₈-toluene, 297.15 K): δ = –75.66, –75.83, –75.95, –76.20, –77.26, –78.10 ppm. ¹⁹F NMR (564.7 MHz, d₈-toluene, 173.15 K): δ = –76.20, –77.62, –77.81 ppm. HRMS LIFDI for [C₄₉H₈₈B₄N₄F₃O₃S]⁺ = [M—CSF₃O₃]⁺: calc. 913.6896; found 913.6889.

Synthesis of B₂(NCy₂)₂Br₂ (6)

To a solution of B₄(NCy₂)₄ (1, 50 mg, 65.4 μmol) in benzene (10 mL), a chilled solution of Br₂ (20.9 mg, 0.40 mL of 0.235 M in benzene, 130 μmol, 2.0 equiv.) in benzene at 0 °C was added and stirred for 2 h. During this time, decolorization of the blue reaction mixture was observed. After removal of all volatiles from the reaction mixture in vacuo, the residue was washed with cold hexane (2 mL, –30 °C) and dried under reduced pressure, yielding 6 as a colorless solid (39.8 mg, 73.4 μmol, 56%). Single crystals suitable for X-ray diffraction analysis were obtained by crystallization in hexane at –30 °C. ¹H{¹¹B} NMR (600.1 MHz, d₈-toluene, 383.15 K): δ = 3.36–3.30 (m, 2H, CH), 3.28–3.21 (m, 2H, CH), 2.44–2.33 (m, 4H, CH₂), 1.99–1.93 (m, 2H, CH₂), 1.72–1.60 (m, 16H, CH₂), 1.52–1.41 (m, 6H, CH₂), 1.29–1.06 (m, 10H, CH₂)_, 1.02–0.91 (m, 2H, CH₂) ppm. ¹³C{¹H} NMR (150.9 MHz, d₈-toluene, 383.15 K): δ = 65.5 (CH), 58.8 (CH), 33.6 (CH₂), 33.5 (CH₂), 27.1 (CH₂), 27.0 (CH₂), 26.7 (CH₂), 26.5 (CH₂), 25.9 (CH₂), 25.8 (CH₂) ppm. ¹¹B NMR (192.6 MHz, d₈-toluene, 383.15 K): δ = 37.8 (s) ppm. HRMS LIFDI for [C₂₄H₄₄B₂Br₂N₂]⁺ = [M]⁺: calc. 542.2031; found 542.2018.