Probing the Boundaries between Lewis‐Basic and Redox Behavior of a Parent Borylene

Abstract The parent borylene (CAAC)(Me3P)BH, 1 (CAAC=cyclic alkyl(amino)carbene), acts both as a Lewis base and one‐electron reducing agent towards group 13 trichlorides (ECl3, E=B, Al, Ga, In), yielding the adducts 1‐ECl3 and increasing proportions of the radical cation [1]•+ for the heavier group 13 analogues. With boron trihalides (BX3, X=F, Cl, Br, I) 1 undergoes sequential adduct formation and halide abstraction reactions to yield borylboronium cations and shows an increasing tendency towards redox processes for the heavier halides. Calculations confirm that 1 acts as a strong Lewis base towards EX3 and show a marked increase in the B−E bond dissociation energies down both group 13 and the halide group.


Introduction
With their formal lone pair at boron, boryl anions and borylenes are strong boron-based nucleophiles, while their formally empty p orbital(s) also make them highly electrophilic (Figure 1). Since the isolation of the first boryl anion, [I] À , by Yamashita in 2006 [1] and the first metal-free doubly basestabilized borylene, II, by Bertrand in 2010 [2] (Figure 2) significant progress has been made in the targeted synthesis and the exploration of the reactivity of these unusually electron-rich boron(I) compounds.
To date, however, there has been no systematic study of Lewis-basic versus redox reactivity of boron(I) compounds. In this work we present a highly reactive phosphine-stabilized parent borylene and systematically investigate its reactivity towards the series of group 13 trichlorides (ECl 3 , E = B, Al, Ga, In) and of boron trihalides (BX 3 , X = F, Cl, Br, I). We show that trends in the selectivity of these reactions for either Lewis adduct formation and/or redox chemistry can be correlated to both the nature of the group 13 element and that of the halide. Computational investigations provide insights into the nature of the B-E bond in a series of borylene-EX 3 adducts.

Synthesis and NMR-spectroscopic characterization of adducts with group 13 trichlorides
We therefore set out to investigate the adduct formation of 1 with Lewis acidic group 13 trihalides. The room-temperature reaction of borylene 1 with one equiv. (Me 2 S)BCl 3 in benzene resulted in the crystallization of the colorless borylene-borane adduct 1-BCl 3 in 74 % yield over the course of 30 min at room temperature (Scheme 2a). [15] The 11 B NMR spectrum of 1-BCl 3 displays two broad resonances at 11.4 and À 21.4 ppm corresponding to the BCl 3 and borylene moieties, respectively ( Table 1). Given that the 11 B NMR shift of Lewis base adducts of boranes is dependent on the overall electron-donor strength of the Lewis base, a comparison with the 11 B NMR shifts of literature-known donor complexes of BCl 3 (Figure 4) [16] shows that borylene 1 is a comparatively weak Lewis base, similar to dimethyl ether and dimethyl sulfide. The 31 P NMR shift of 1-BCl 3 at À 10.9 ppm is significantly downfield-shifted from that of 1 (δ 11B = À 25.4 ppm), and comparable to that of the [(CAAC)(PMe 3 )BH 2 ] + cation, [1-H] + (δ 31P = À 10.6 ppm). [17] In the 1 H{ 11 B} NMR spectrum the BH resonance appears at 1.80 ppm as a broad doublet coupling to the neighboring phosphorus nucleus ( 2 J 1H-31P = 12.3 Hz), while the CAAC ligand resonances are all split due to the presence of the chiral borylene center and the hindered rotation around the BÀ C CAAC bond.

Reactivity of borylene 1 towards boron trihalides
Having determined these trends in the reactivity of ECl 3 with borylene 1 (E = B, Al, Ga, In), we studied variations of the halide to identify further trends. Independent of the reaction conditions, combining 1 with (Et 2 O)BF 3 resulted in a rapid 1 : 2 reaction, [30] 4 ] as the sole NMR-active by-product (Scheme 4ac). Furthermore, the radical species 1 * + was detected by EPR spectroscopy. The formation of [1-BF 2 ][BF 4 ] can be rationalized by fluoride ion abstraction from an initial 1-BF 3 adduct by a second BF 3 equivalent (Scheme 4a,b). The fact that 1-BF 3 was never observed implies that fluoride abstraction occurs significantly faster than adduct formation in this case, presumably due to the much lower Lewis acidity of BF 3 compared to BCl 3 [31] and its high fluoride ion affinity. [32] The 11

1-InCl 3
[d]  Independent of the reaction conditions the reaction of 1 with (Me 2 S)BBr 3 or BBr 3 proved highly unselective. [35] While the formation of 1-BBr 3 (δ 11B = À 4.9 (br, BBr 3 ), À 20.1 (br, BH); δ 31P = À 12.1 (m) ppm, Scheme 4a) was observed when working with substoichiometric amounts of BBr 3 at À 70°C, this adduct could not be isolated cleanly. [36] As with (Et 2 O)BF 3 the room temperature reaction always consumed two equiv. BBr 3 and also resulted in a complex mixture of at least five boron-containing species. Over the course of one day at room temperature, however, this mixture resolved into two major products, formed in a 1 : 1 ratio: (Me 3 P)BBr 3 (δ 11B = À 4.4 (d, J 11B-31P = 150 Hz) ppm; δ 31P = À 7.9 (m) ppm) and the known compound (CAAC)BHBr 2 , 3-Br (Scheme 4e). [37] In order to elucidate the mechanism of this reaction, our attention turned to the analogous 1 :  Figure S42 in the Supporting Information,  4 ] converted overnight to a 1 : 1 mixture of the neutral sp 2 -sp 3 diborane (CAAC)BHCl(BCl 2 ) (2-Cl: δ 11B = 75.8 (br), À 13.0 (br) ppm) and (Me 3 P)BCl 3 (δ 11B = 3.1 (d, 1 J 11B-31P = 164 Hz) ppm, Scheme 4d). [38] The 11 B NMR shifts of 2-Cl resemble those of the singly NHC-stabilized adducts of B 2 Cl 4 (δ 11B = + 69, À 5 ppm), which are formed at low temperature and decompose upon warming. [39] Diborane 2-Cl was also unstable both in solution and in the solid state, undergoing a BÀ B bond-cleaving intramolecular chloride migration to yield the known compound (CAAC)BHCl 2 , 3-Cl, [35] as the sole isolable product (Scheme 4e). The comparison of the 11  The only other crystalline product that was consistently isolated from the reaction of 1 with (Me 2 S)BBr 3 , albeit not in quantities sufficient for full characterization, was the unsymmetrical doubly base-stabilized diborane 4 (δ 11B = À 4.8 (BBr 2 PMe 3 ) and À 15.3 (BHBr) ppm; δ 31P = À 11.7 (m) ppm), which results from the phosphine-bromide rearrangement of 1-BBr 3 (Scheme 4f). [40] The solid-state structure of 4 ( Figure 8, Table 2) confirms the migration of the PMe 3 ligand to B2 and of one bromide to B1. The B-B bond length of 1.736(5) Å is significantly shorter than in 1-BCl 3 (1.784(4) Å). The phosphine and CAAC ligands are in an anti conformation, with a (P1À B2À B1À C1) torsion angle of 169.9(2)°. Compound 4, which proved indefinitely stable at room temperature in solution, is the first structurally characterized example of a neutral (trihalo) hydrodiborane and a rare example of a neutral diborane stabilized by two different Lewis bases. [41] It is structurally very similar to the carbene-and PMe 3 -stabilized tetrabromodiborane reported by Kinjo and co-workers. [41b] Finally, the 1 : 1 reaction of 1 with BI 3 in DFB proceeded very selectively, and independent of reaction temperature, to a single product displaying a broad 11 B NMR resonance at À 28.3 ppm and a 31 P NMR multiplet at À 14.3 ppm (Scheme 4g). After filtration from a small amount of intractable brown byproduct, [42] recrystallization yielded single crystals of the twoelectron oxidation product [1-I]I (Figure 8, Table 2). The fact that only one equivalent of BI 3 is required and the PMe 3 ligand remains bound to the boron center suggests a different reaction pathway from that of 1 with BBr 3 . Assuming that, here too, the Lewis adduct 1-BI 3 is formed first as an intermediate, the latter may be decomposing directly to

Computational analysis of B-E bonding in 1-EX 3
In order to gain a deeper understanding of the bonding situation in the borylene-EX 3 adducts, BÀ E bond dissociation energies (BDEs) for the isolated 1-ECl 3 adducts (E = B, Al, Ga, In) and the putative 1-BX 3 adducts (X = F, Br, I) were calculated at the BP86-D3BJ/TZ2P//PBEh-3c and the improved double hybrid RI-DSD-BLYPÀ D3BJ/def2-QZVPP//PBEh-3c levels of theory (see details in the Supporting Information). The calculated BÀ E bond lengths match those of the solid-state structures closely (within 1.5 to 2 %), including BÀ Al being slightly longer than BÀ Ga (Tables 2 and 3). The BÀ E BDEs at both levels of theory show similar trends, notwithstanding the typical overbinding by the BP86 functional. The comparison of the BÀ B BDEs of the putative 1-BX 3 adducts with the BÀ E BDEs of the isolated 1-ECl 3 adducts shows that the former would theoretically be stable enough for isolation if subsequent halide abstraction and/or redox processes could be prevented. In line with the general trend in Lewis acidities of the boron trihalides, [43] calculated B-B BDEs in 1-BX 3 nearly double between 1-BF 3 and 1-BCl 3 , then increase more slowly upon descending the halide group further. The B-E BDEs of 1-ECl 3 increase substantially from BCl 3 to InCl 3 , nearly doubling upon going from 1-BCl 3 and 1-AlCl 3 , then increasing more slowly upon descending group 13 further. Energy decomposition analysis (EDA) shows that this trend goes back to substantially diminished preparation energies E Prep (the energy necessary to deform the fragments from their individual equilibrium structures to the structures they assume in the respective adduct), whereas the corresponding interaction energies are essentially constant for all four 1-ECl 3 adducts. Deformation of the ECl 3 fragments in particular dominates~E Prep . While the structural deformation, as measured by the sum of angles about the group 13 atom, is almost identical in all four cases, the bending potentials flatten substantially from BCl 3 to InCl 3 (see Figure S43 in the Supporting Information). Furthermore, while BÀ B bonding in 1-BX 3 is dominated by orbital interactions (54-60 %), the contribution of which increases upon descending the halide group, BÀ E bonding in 1-ECl 3 (E = Al, Ga, In) is dominated by electrostatic interactions (51-57 %), the contribution of which increases upon descending group 13 (see Tables S4 and S5 in the  Supporting Information). This is in line with the increasing polarization of the BÀ B bond in 1-BX 3 for the lighter halides and of the BÀ E bond in 1-ECl 3 for the heavier group 13 elements.

Conclusion
In this study we have shown that the (CAAC,PMe 3 )-stabilized hydroborylene 1 offers an easily accessible, versatile platform for the systematic assessment of reactivity patterns of borylenes towards Lewis-acidic group 13 trihalides, EX 3 . Depending on the nature of E and X the reactivity can be tuned either in favor of neutral Lewis adduct formation or one-and/or two-electron redox processes.
With all group 13 trichlorides the 1 : 1 reaction yields the corresponding Lewis adduct 1-ECl 3 (E = B, Al, Ga, In) as the major product. The proportion of the radical cation by-product 1 * + , resulting from the one-electron oxidation of 1 by ECl 3 , increases upon descending group 13, as the corresponding reduction potential of ECl 3 becomes more positive. The influence of the halide in these reactions becomes apparent in the reactions of 1 with BF 3 , BBr 3 and BI 3 sources. While it appears reasonable to assume initial formation of 1-BX 3 adducts, these species are too reactive to isolate. For X = F, fluoride abstraction by a second equivalent BF 3 is significantly more rapid than 1-BF 3 adduct formation, leading to the stable borylboronium species [1-BF 2 ][BF 4 ]. For X = Br, 1-BBr 3 also converts instantly to [1-BBr 2 ][BBr 4 ] which is, however, highly unstable towards intramolecular ligand exchange and redox processes, ultimately resulting in the two-electron oxidation of 1. Finally, for X = I, only the product of the two-electron redox reaction between 1 and BI 3 is observed, thereby confirming the trend for increased redox processes down the group, as the BÀ X bond weakens. [44] Based on the calculated BDEs, 1 acts as a typical strong Lewis base towards BX 3 . For the adducts of 1 with ECl 3 the BÀ E BDEs increase down the group owing to successively weaker bending potentials of the ECl 3 groups, which facilitates geometric distortion in the course of adduct formation.
Beyond the fundamental interest in the reactivity patterns of a Lewis-basic borylene towards group 13 Lewis acids, and the study of borylene-group 13 Lewis adduct bond enthalpies, the 1 : 2 reaction of 1 and BCl 3 also provides a novel synthetic route towards an otherwise inaccessible, electron-precise, unsymmetrical diborane, 2-Cl. Such species have become highly sought after as they display an intrinsic polarization of the BÀ B bond, [45] making them significantly more reactive than commercially available symmetrical diboron reagents, in particular for uncatalyzed borylation and diboration reactions. [46] Furthermore, the presence of halide substituents in the cationic