A Cationic NHC‐Supported Borole

Abstract This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3)3]4} from the NHC‐chloroborole adduct yields the first stable NHC‐supported 1‐(MeNHC)‐2,5‐(SiMe3)2‐3,4‐(Ph*)2‐borole cation. Electronically, it features both a five‐membered cyclic conjugated 4 π‐electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass‐spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1‐mesityl borole reveals strong Lewis acidity, reduced HOMO–LUMO gaps, and increased anti‐aromatic character.

the frontier orbitals ituation of cyclopentadienyl cations to the one in borolest hrough variation of inductively actives ubstituents in [(RC) 4 CR'] + ,s table derivativesm ay be accessible. [6] The opposite approach to stable electronic mimics of cyclopentadienylc ationsw ould be to electronically disguise ab oron atom in boroles as ac arbon atom. In our attemptst od os o, we were able to isolate ab orole-derived borenium cation with at hree-coordinateb oron atom that still features cyclic conjugationof the four-electron p-system.I tt hus represents both a cationic,b orole-derived electronic mimic to cyclopentadienyl cationa sw ell as an extensiono faseries of formal single electron additionst o( PhC) 4 BR' (R' = Aryl or NHC) (Scheme1). [7] Ab orole-derived boronium cation with at etra-coordinated borona tom and thus without any cyclic delocalization of the p-systemw as observed by Braunschweig, when two molecules of 4-methylpyridine replaceachloridei n( PhC) 4 BCl. [8] Donorsupported 9-borafluorene cations have been described by Nçth and most recently by Gilliard. [9] However the central (C 4 B) 4 p-systemi nb orafluorenes is conjugated with benzenem oieties and delocalized, significantly reducing the anti-aromatic character and reactivity. [10] Therefore borafluorenes are not suitably comparable to actual free boroles.
We have recently found access to at hermally stable chloroborole A-Cl. [11] Addition of 1equiv of 1,3-(R) 2 -4,5-dimethylimidazol-2-ylidenes (R = Me, Me NHC;R= iPr, iPr NHC;N HC = N-heterocyclic carbene) to solutionso fA-Cl,g ives crystalline colourless NHC-adducts 1a and 1b in ca. 70 %y ield (Scheme 2). The 1 HNMR spectrum of 1a,b reveals four signals sets for the four alkyl groups of the NHC in line with no rotation of the NHC aroundt he BÀCb ond within the NMR timescale. The 11 BNMR resonance shifts from d 11B = 70.8 ppm (A-Cl)t od 11B = 0.5 (1a) and 1.3 (1b)p pm fully in line with at etracoordinate boron atom. The crystal structureo f1a is documented in the Supporting Information.
Af ew examples of NHC-adductst oh aloboroles have previously been reported, [7a, 12] however usually applyingb ulky Naryl imidazole-2-ylidenes.B raunschweig and co-workers obtained ab orole monoanion with B-centred nucleophilicity by reducing such an adduct of (PhC) 4 BCl. [7a] When solutions of 1a in dichloromethane are treated with Na[B(Xyl F ) 4 ](Xyl F = 3,5-(CF 3 ) 2 (C 6 H 3 )), immediate quantitative consumption of 1a yields dark yellow-green solutionsa nd sodium chloridep recipitates.N MR-spectroscopic examination of these samples kept below À15 8Cr eveal clean formation of as ingle new product with one signal set for the N-Me groups of the NHC, but the new compound decomposess wiftly at room temperature. Dehalogenation with Krossing'ss uperbly inert and weakly coordinating anion salt Li[Al{OC(CF 3 ) 3 } 4 ] [13] in CH 2 Cl 2 leads to the same immediate formation of as ingle new compound with no sign of decomposition. Similar dehalogenation approaches previouslya llowed for the synthesis of, for example, highly reactive borinium cations. [14] Single-crystal diffraction confirmed the formation of the cationic borole [2a] + + (Figure 1) being an unprecedentedt ype of NHC-supported boreniumc ations. [15] Compound [2a][Al{OC(CF 3 ) 3 } 4 ]c an be isolated as ad ark yellow-green crystalline material quantitatively (98 %) and is stable in solution for severald ays at room temperatureb ut is extremely sensitive to oxygen and water and immediately decomposes in ambient atmosphere.
Within the C 4 Bc ycle,t he structure features two localized double bonds as to be expected for boroles and Jahn-Te ller distorted singlet cyclopentadienyl cations.Afairly large torsion angle of 70(2)8 between the NHC-plane and the C 4 B-planei s observed. Likely due to steric reasons,t he two silyl groups both are mildly tilted out above and below the C 4 Bp lane. The structuralf eatures of [2a] + + are virtually identical to its most related neutrald erivative featuring ab oron-bound mesityl (Mes; 2,4,6-Me 3 (C 6 H 2 )) group (A-Mes)( Ta ble1). [11] Me NHC and Mes feature as imilar steric profile. NMR-spectroscopy reveals for [2a] + + a 11 BNMR resonance at d 11B = 73.9 ppm pronouncedly less lowfield shiftedt han comparable neutral aryl boroles [11,16] including A-Mes or [R 2 B(NHC)] + borenium cations (Table 1). [15d,e] The 13 CNMR resonances within the C 4 B-cyclea re found at d 13C = 135.5 ppm (C a )a nd 190.3 ppm (C b )w ith C b being significantly low-field shiftedc ompared to A-Mes.P articularly the combination of relatively highfield-shifted 11 Ba nd low-field shiftedC b 13 Cresonances are in line with an increased donation of p-electron density from the butadiene-system into the energetically very low-lying empty p-orbital at boron in this cationic borole. This can be rationalized by the contribution of resonance Scheme2.Synthesis of precursors 1a,b and borole cation [2 a] + + . Table 1. Structural, spectroscopic and computational characteristics of borole cation [2 a] + + andtheir comparison to mesityl-borole A-Mes.  (0) (8), C5ÀN1 1.332 (7), C5ÀN2 1.347 (5), see also Table 1.
Af urther feature is the remarkably enhanced Lewis acidity of the cationic borole [2a] + as assessed by the Gutmann-Becket (GB) method. Addition of OPEt 3 to solutionso f[ 2a] + leads to immediate decolorization and formation of the adduct [2a(OPEt 3 )] + with as trongly low-field shifted 31 P-NMR resonance of d 31P = 83.4 ppm (CD 2 Cl 2 )a nd aG utmann-Beckett acceptor number (AN) of 93.7. Similarf ree boroles A-Ar including A-C 6 F 5 revealed only AN of 73(AE 1). [11] These observed AN valuesf or [2a] + are significantly higher than, with respect to the electronegativity of three boron-bound carbon atoms, comparable planar pentacyclic [NHC-B(Ar) 2 ] + (AN 84) or (m)NHC-(9-BBN)b orenium cations (AN 78-79) reported by Crudden and Stephan. [15d,e,g] Other borenium cationsw ere found more Lewis-acidic according to GB and it was shown that direct comparisons with landmark neutral Lewis acids such as B(C 6 F 5 ) 3 are to be questioned. [24] Scheme3.Mesomeric contributions to the electronic structure of am odel cationic NHC-supported borole (HC) 4 B( Me NHC) including the dominant contributions accordingt oN RT calculations. [19]   The adduct formation is particularly remarkable when compared to mesityl borole A-Mes (AN 11.3) where almost anyi nteraction of OPEt 3 is ruled out due to the steric shielding of the two methyl groups in ortho-position.T his steric protection reduces the reactivity of mesityl boroles.S imply changing the B-bound residue from as ix-(A-Mes)t oafive-membered ring ([2a] + )d rastically alters the accessibility of the reactive C 4 B cycle for other reagents such as diphenylacetylene (Scheme4). While A-Mes reveals no reaction under the same conditions, [2a] + cleanly forms the colourless Diels-Alder product [3] + within af ew hours at room temperature. [25] [3] + is characterized spectroscopically and its structural assignment is corroborated computationally (see Supporting Information).
Ah igh-field shifted 11 BNMR resonancea tÀ11.4 ppm is in line with interactions of the Lewis-acidic boron atom with an adjacent CC p-bond as previously observed. [26] Reactions of alkynes with boroles are known to form as eries of products including Diels-Alder adducts, ring expanded borepins, or more complicated rearrangements, depending on the electronics and sterics of substituents. [4c, 25a, 26, 27] When [2a] + is exposed to an atmosphere of CO, immediate decolourization and quantitative conversion to cationicC Oadduct [4] + is observed by NMR-spectroscopy.U nder an atmosphere of CO, [4] + can be kept at room temperature in solution for ad ay allowing characterization of this kinetic product [4] + beforev ery slow follow-up reactions are observed. In an (inert)o pen atmosphere, CO is readily liberated at room temperature and the free cation is rapidlyr ecovered.
[4] + features as trongly high-field shifted 11 Br esonance at À18.8 ppm, indicative for tetra-coordinate boron. Other than previously reported borane carbonyls, [4e, 28] the CO stretching frequency in [4] + at 2128 cm À1 is even ab it lower than in free CO at 2143 cm À1 .C rystals of [4][Al{OC(CF 3 ) 3 } 4 ]a re obtained from solutions in dichloromethane layered with pentane at À40 8C, but they eventually liberate CO and despite great care, only am ediocre dataset was obtained by X-ray diffractionc onfirming and corroborating the computationalstructure (see Supporting Information).
The observed IR stretching frequency in [4] + hints at ac onsiderable degree of p-back donation from the borole p-system. Such interactions were proposed to cause the lability of the pentaphenyl borole CO adduct. [4e] An ETS-NOCV [29] analysis indeed revealed as trong (443 kJ mol À1 ) s-donation from the CO fragment into the boron centered LUMO and as ignificant p-backdonation component (109 kJ mol À1 )( Figure3).
The observation of the labile, yet isolable rare organoborane CO complex [4] + is noteworthy with regards to CO adducts of neutralb oroles. [28b,c, 31] Piers and co-workersd escribed the only example of as table CO-complexo fp erfluorinated pentaphenyl borole with aC Os tretching frequency at 2199 cm À1 indicative of little to no p-backbonding. [4e] The non-fluorinated pentaphenylb orole likely only forms the adduct as an intermediate at very low temperatures and undergoes insertions/rearrangements below À10 8C. [4e, 5b] AC 4 Bb ased HOMO accessible for pinteraction was considered to promote furtherr eactivity and electronw ithdrawing groups favour isolation of the CO-complex. [4e, 32] Erker's related2 ,5-disilylborole even adds the CO to a C a atom and gives, after rearrangements, ak etene derivative. [26] Despitet he low CO-stretching frequency,t he CO-borole complex [4] + more resembles Piers' (Ph F C) 4 BPh F -CO complex.
Since the kinetic product [4] + can be isolated, the Lewis-acidic boron atom of the cation seems to bind CO strong enough while the C 4 B p-systemd oes not provide pathways of sufficiently low-barriers that would preventfrom its preliminary isolation.
Lastly,the application of the small Me NHCfor successful isolation of this stable borole cation system seems mandatory since analogoust reatment of the sterically more demanding iPr NHC derivative 1b with Li[Al{OC(CF 3 ) 3 } 4 ]p utativelyl eads to initial formation of [2b] + as indicated by intense brown-yellow colourization and suitable NMR signatures, but even at lowt emperatures (À40 8C) progressive decomposition of putative [2b] + to severaly et unidentified pale yellow to colourless follow-upp roducts is observed (see Supporting Information).
In summary we presented as table cationic borole extending the series of true free boroles to cationic systems. The central C 4 Bm oiety featuringa nN HC-stabilized boreniumc ation within ac yclic 4 p-electron system is both isoelectronic to and bears the same overall charge as the elusive cyclopentadienyl cation. However, the antiaromaticc haracteri sn ot drastically increased to levels expected forc yclopentadienyl cations.T his work sheds light on the implications of imidazolium substituents on the (opto)-electronic properties of boroles and highlights the impact of formally cationicb oron-bound substituents, particularly compared to traditional strong electron withdrawing groups sucha s-(C 6 F 5 ). Thea ccessibility of the reactive [C 4 B] moiety in the presented examples for small molecules such as CO and acetylenes highlight its exceptionalr eactivity stemming from ac ombination of the frontier p-orbitalsa nd ac ationic charge.

Experimental Section
Crystallographic data:D eposition numbers 1982736, 1982737, and 1986857 contain the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.