Hexahalodiborate Dianions: A New Family of Binary Boron Halides

Abstract The electron‐precise binary boron subhalide species [B2X6]2− X=F, Br, I) were synthesized and their structures confirmed by X‐ray crystallography. The existence of the previously claimed [B2Cl6]2−, which had been questioned, was also confirmed by X‐ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well‐known tetrahaloborate anions (BX4 −), and are rare examples of molecular electron‐precise binary boron species beyond B2X4, BX3, and [BX4]−.

Binary species are fundamental to the systematic understanding of an element and its chemistry.B inary halogen species, [1] in particular,a re among the most important, not only because,o wing to their high electronegativity,t hey can form compounds with most elements and are as such wellrepresented across the periodic table,b ut also because they are typically either very reactive (ClF 3 ,S bCl 5 ,N Cl 3 ,e tc.) or very stable ([PF 6 ] À , [BF 4 ] À ,etc.) Boron halides are,along with hydrides,a rguably the most important class of binary boron species,and include awide variety of cluster-type species such as B 4 Cl 4 ,B 9 X 9 ,[B 9 X 9 ] 2À ,etc. [1] Electron-precise boron halides (i.e., that possess only classical two-electron bonds), however, are limited to haloboranes (BX 3 ), tetrahaloborates (BX 4 À ), the four diboron subhalides B 2 X 4 , [2] thetransient species BX, and the claimed [3] subhalide [B 2 Cl 6 ] 2À ,f or which structural evidence is lacking (Figure 1). [B 2 F 6 ] 2À is also mentioned in the patent literature with little data available to support its existence. [4] Because of the instructional importance of the tetracoordinate borate anions [BX 4 ] À ,w hich demonstrate the Lewis acidic character of the corresponding boranes BX 3 ,w e wondered whether the analogous subhalide species [B 2 X 6 ] 2À could [3a,c] exist. Indeed, their Lewis acidic B 2 X 4 counterparts are all highly reactive and most of them are rather unstable and decompose to BX 3 and to larger boron subhalides at various rates at room temperature.T he fact that the two charges in [B 2 X 6 ] 2À are delocalized over only eight atoms (in contrast to the known, closely related pseudohalide derivatives [B 2 (CN) 6 ] 2À and [B 2 (NCS) 6 ] 2À ,w hich both bear polyatomic ligands) [5] could potentially make them prohibitively unstable.W eherein report on the successful isolation and full characterization of the four hexahalodiborate anions as organophosphonium and/or -ammonium salts.
Given the convenient synthetic route to all four B 2 X 4 precursors from B 2 Br 4 ,published recently by our group, [6] the direct nucleophilic addition of the X À group to the appropriate B 2 X 4 was open for exploration. When B 2 Cl 4 and B 2 Br 4 were treated with tetraphenylphosphonium chloride and bromide,r espectively,i nd ichloromethane solutions,w hite crystalline material precipitated after af ew minutes at room temperature.  [PPh 4 ]Br·CH 3 CN of sufficient quality for X-ray diffraction, which confirmed the existence of the hexacoordinate dianion 1 (Scheme 1).
Thet reatment of B 2 I 4 with TBAI in dichloromethane yielded solutions that displayed 11 BNMR signals that shifted to higher fields as the iodide content increased, indicative of complex equilibria in solution. Such behavior is also observed for the I À /BI 3 /[BI 4 ] À system. [7] At 10 equivalents of iodide,one major signal was observed at d( 11 B) = À29 ppm, which was assigned to 4.R elying on the expectedly lower solubility of salts of the dianionic 4 compared to that of the putative [B 2  Thec rystal structures of salts of 1-4 (Scheme 1) demonstrate that the B À Bbond remains intact in these species.T he [B 2 X 6 ] 2À dianions were found in astaggered conformation of approximate D 3d symmetry.I nt he solid state,t he [B 2 X 6 ] 2À species in salts of 2-4 possess ac rystallographic inversion center. BÀBb ond lengths were mostly unaffected by the halogen substituents and were found to be identical within standard deviations,int he 1.7 range (Scheme 1), in agreement with computationally predicted values (Table 1). This contrasts with predicted BÀBb ond lengths for the parent B 2 X 4 , [8] which steadily decrease from 1.725 (X = F) to 1.654 (X = I) ( Table 1). Theo bserved BÀBb ond lengths are also in stark contrast with predicted values for the more weakly bound hypothetical [B 2 X 6 ]C À radical anions (> 2.11 ). [9] TheB ÀXb ond lengths in 1-4 (Scheme 1) are all significantly larger than those predicted for B 2 X 4 ( Table 1). TheB -X bond lengths in 1-4 are comparable to,although noticeably longer than, the BÀXbond length in the respective [BX 4 ] À anion. TheB ÀXbond lengths within the BX 3 moieties of the asymmetric units of 1-4 vary to some degree (Scheme 1), reflecting varying degrees of interactions with the cations or cocrystallized solvent with each halogen substituent in the solid state.S imilar variations are observed in [BX 4 ] À (Table 1). [10][11][12] Solid-state 11 BNMR spectra for of salts of 2-4 were acquired. In addition, solution 11   [a] Predicted values for D 2d geometry. [8] See also ref. [14]. [b] Predicted values (this work) at the wb97xd/6-311 + g(d,p)-SMD/PCMlevel. [c] This work (see the SupportingInformation), see also ref. [11].
[d] This work (see the Supporting Information), see also ref. [12]. Thes trong shielding effect of Br À and I À found for [BBr 4 ] À (d( 11 B) = À24) [6] and [BI 4 ] À (d( 11 B) = À127) [14] have no equivalent in 3 and 4. Thev ibrational spectra (Raman and IR) of the [B 2 X 6 ] 2À salts are dominated by the intense and numerous bands belonging to the organic cations.N evertheless,t he observed spectra are consistent with the predicted vibrational spectra of [B 2 X 6 ] 2À (see the Supporting Information). Moreover,t he comparison of spectra with those reported [15] for known B n X n derivatives and [BX 4 ] À supports the assignment of the isolated materials as [B 2 X 6 ] 2À salts and not as decomposition products. Some intense characteristic vibrations for the [B 2 X 6 ] 2À anions could be unambiguously observed and tentatively assigned with the aid of quantum chemical calculations (see the Supporting Information for adetailed discussion). Thebands for the BÀXstretching modes are the least ambiguous and are listed in Table 3.
TheI Rb and observed for 2 (complex band centered at 588 cm À1 )isinqualitative agreement with the reported [3a] IR bands at 694, 665, and 600 cm À1 .T he bands at 665 and 694 cm À1 previously reported for 2 are problematic,a st hey coincide with known bands of [BCl 4 ] À , [3a, 15b] av ery common side-product of 2.H owever,t hey are also very close to the Raman-allowed in-phase B À Cl asymmetric stretching mode, which might have been IR-allowed in the solid state due to lower site symmetry.T he BÀBstretch could not be observed, presumably obscured by the cation bands in the 1000-1200 cm À1 region. Expectedly,B ÀXs tretching frequencies decrease as the size of the halogen increases,asisthe case for [BX 4 ] À ,a nd they are at lower frequency than for the parent B 2 X 4 (Table 3).
We conducted quantum thermochemical calculations to verify the stability of [B 2 X 5 ] À and [B 2 X 6 ] 2À with respect to the loss of ah alide ligand. TheG ibbs free energy change was estimated in dichloromethane solutions for the single halide addition to B 2 X 4 and to [B 2 X 5 ] À ,r espectively (Scheme 2). Difficulties in correctly accounting for specific solvation effects for our charged species are expected to lead to significant uncertainties.N evertheless,o ur computational estimates allow us to identify important qualitative trends. Interestingly,our computed Gibbs free energy change for the first halide addition (Scheme 2, top), yielding [B 2 X 5 ] À ,i s comparable to that of the halide addition to the monoboranes to yield [BX 4 ] À (Scheme 2, bottom). [21] Unsurprisingly,t he second halide addition to yield [B 2 X 6 ] 2À is always significantly less favorable than the first, to the point that the formation of [B 2 X 6 ] 2À from [X] À and [B 2 X 5 ] À is predicted to be (somewhat) endergonic for X = Cl, Br and, by extrapolation, I(Scheme 2, center). Qualitatively,t his suggests that the isolation of salts of 2,and in particular 3 and 4,should be strongly affected by subtle differences between the solvation and the lattice enthalpies of the salts,i na greement with our experimental observations in the case of 4,which could be isolated because because its salts are less soluble than salts of [B 2 I 5 ] À . 2 and 3 are more borderline cases and appear to be stable species in solution (see the Supporting Information), in agreement with conductimetric measurements on 2. [3a] Thed ianions 2-4 proved to be kinetically significantly more stable than their respective tetrahalodiborane counterparts at room temperature.B X 3 is the most common decomposition product of B 2 X 4 and the only one that is easily detectable by 11 [16] ;673r [17] 1375 [16] ;1368r [17] 886;6 26r 843;(850r) [a] ca. 780r [18] ca. 1100 [18] Cl 728;4 01r [19] 917 [19] 554/569; 354r 591/601;6 41r 405r [15b] 670 [15b] Br 592 [20] ;2 45r 777 [20] 498 [b] ;2 14r 498 [b] ;  [b] ;2 00r 464 [b] ;(486/517/534)r -5 17 [15a] [a] Not observed,p redicted value (wb97xd/6-311 + g(d,p)-SMD/PCM) supplied instead. [b] The broad band observed in the IR spectra likely corresponds to both n(BX 3 )s ym and n(BX 3 )a sym.  [5,22] and of an even more limited family of homoleptic dianionic diborane species. [3a, 5, 22n-p,23] Although conductimetric,IRspectroscopic, and elemental analysis data have been reported for salts of 2, [3a,b] no further data nor as tructural confirmation could be found in the literature.M oreover,a nother study has cast doubt on the results of the initial report (see the Supporting Information for further discussion). [ have not yet been made by simple reactions between X À and B 2 X 4 (where X = CN or NCS).
In conclusion, three rare examples of new electron-precise boron-halogen binary species were isolated and fully characterized, and the existence of the previously claimed [B 2 Cl 6 ] 2À was confirmed by X-ray crystallography and NMR and Raman spectroscopy.S pecies 1-4 are isoelectronic to the carbon binary halides C 2 X 6 and, in analogy to these,c ould potentially display interesting photodissociative and halogenation behaviors. [24] In the context of the renewed interest in electron-precise diborane chemistry [25] and of the likely involvement of diborane-based anions in metal-free borylation reactions, [22c,d,i, 26] species 1-4 add fundamental knowledge [27] to catalytically relevant systems in arapidly advancing field. [28] CCDC 1916763,1916764,1916765,1916766,1916767,1916768,1916769,1916770,1916771 and 1916772 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from TheC ambridge Crystallographic Data Centre.