Rational design of an argon-bindingsuperelectrophilic anionMartin Mayera,1, Valentin van Lessenb,1, Markus Rohdenburgc,1, Gao-Lei Houd, Zheng Yangd, Rüdiger M. Exnerb,Edoardo Apràe, Vladimir A. Azovf, Simon Grabowskyg, Sotiris S. Xantheash,i, Knut R. Asmisa, Xue-Bin Wangd,2,Carsten Jenneb,2, and Jonas Warneked,j,2

aWilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, 04103 Leipzig, Germany; bAnorganische Chemie, Fakultät fürMathematik und Naturwissenschaften, Bergische Universität Wuppertal, 42119 Wuppertal, Germany; cInstitut für Angewandte und Physikalische Chemie,Fachbereich 2-Biologie/Chemie, Universität Bremen, 28359 Bremen, Germany; dPhysical Sciences Division, Pacific Northwest National Laboratory, Richland,WA 99352; eEnvironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352; fDepartment of Chemistry,University of the Free State, 9300 Bloemfontein, South Africa; gInstitut für Anorganische Chemie und Kristallographie, Fachbereich 2-Biologie/Chemie,Universität Bremen, 28359 Bremen, Germany; hAdvanced Computing, Mathematics and Data Division, Pacific Northwest National Laboratory, Richland, WA99352; iDepartment of Chemistry, University of Washington, Seattle, WA 98195; and jDepartment of Chemistry, Purdue University, West Lafayette, IN 47907

Edited by R. Graham Cooks, Purdue University, West Lafayette, IN, and approved March 7, 2019 (received for review December 6, 2018)

Chemically binding to argon (Ar) at room temperature has remainedthe privilege of the most reactive electrophiles, all of which arecationic (or even dicationic) in nature. Herein, we report a conceptfor the rational design of anionic superelectrophiles that arecomposed of a strong electrophilic center firmly embedded in anegatively charged framework of exceptional stability. To validate ourconcept, we synthesized the percyano-dodecoborate [B12(CN)12]

2−, theelectronically most stable dianion ever investigated experimen-tally. It serves as a precursor for the generation of the monoanion[B12(CN)11]

−, which indeed spontaneously binds Ar at 298 K. Ourmass spectrometric and spectroscopic studies are accompanied byhigh-level computational investigations including a bonding analy-sis of the exceptional B-Ar bond. The detection and characterizationof this highly reactive, structurally stable anionic superelectrophilestarts another chapter in the metal-free activation of particularlyinert compounds and elements.

superelectrophilic anions | multiple-charged anions | Ar compounds |photoelectron spectroscopy | dodecaborates

Converting small saturated hydrocarbons or inert chemicalwaste into valuable functional molecules is of overarching

economic and environmental significance and represents one ofthe outstanding challenges of contemporary molecular science.This requires the design and realization of stable reactants that canactivate the most inert compounds (1). Canonically, free radicalsare employed to cleave or form chemical bonds homolytically, butsuch reactions tend to be difficult to control (2). An alternativeapproach involves electronically closed-shell molecules, which areusually less reactive than radicals, but provide access to a com-pletely different and potentially more controllable kind of chem-istry by either donating (nucleophiles) or accepting (electrophiles)an electron pair. Nucleophiles are naturally limited in their re-activity, since electrons that are very loosely bound tend to auto-detach. In contrast, the reactive strength of electrophilic centers isnot limited in such a way in principle.The binding of the argon atom (Ar) at room temperature

(RT) serves as a critical benchmark for novel superstrong elec-trophiles and is highly challenging. Argon, the most abundantnoble gas (NG) on Earth, received its name from the Greekword “ἀργόν” which means “lazy” or “inactive” (3). Accordingly,Ar binding to molecules is extremely rare with exceptions beingsimple bi- and triatomic species in outer space or in cold matricesnear the absolute zero degree, where very weak interactions withargon can hold the compound together. It is, therefore, notsurprising that only a single neutral Ar-containing compoundHArF, isolated in a low-temperature matrix, is presently known(4–7) and that the strongest interactions with Ar have beenreported in transient cationic species (8–13). The stabilization ofa sufficiently strong electrophilic binding site carrying a positive

charge within a larger molecular framework is extremely chal-lenging even in an inert environment, since such cationic speciestend to rearrange to more stable isomers. Therefore, the mainpathways leading to Ar compounds investigated at RT are SN2-type reactions involving either small radical cations or dications, inwhich the formation of long-lived unstable free binding sites can becircumvented. However, these product ions survive only sufficientlylong to be detected in the absence of competing nucleophiles suchas water. Therefore, room temperature Ar chemistry is currentlylimited to a small number of isolated cations, whose observationand characterization are rather challenging. Due to the negativeelectron affinity of Ar (14–16), the possibility of binding Ar directlyto an anion has never been seriously considered because anionsusually behave like nucleophiles. In this contribution, we report agroundbreaking concept for the rational design of highly reactivebut stable molecules, so-called superelectrophilic anions, a class ofcompounds that are able to bind rare gas atoms, like argon, cova-lently and at room temperature in the presence of competing strongnucleophiles like water. Our approach is based on a counterintuitiveidea that anions can, under well-defined circumstances, behave as

Significance

Strong electrophilicity usually goes hand in hand with positivecharge. In contrast, most negative ions behave like nucleophiles.Here we challenge this conventional wisdom by introducing anapparently counterintuitive idea that anions can, under well-defined circumstances, behave as superelectrophiles and evenshow superior binding strength and kinetic product stabilizationin comparison with typical superelectrophilic cations. Emanatingfrom the most stable gas-phase dianion [B12(CN)12]

2−, synthe-sized here, we generate its superelectrophilic fragment anion[B12(CN)11]

−, which binds Ar covalently at room temperature.This opens up additional directions for activation of inert com-pounds and elements.

Author contributions: S.G., S.S.X., K.R.A., X.-B.W., C.J., and J.W. designed research; M.M.,V.v.L., M.R., G.-L.H., Z.Y., R.M.E., E.A., X.-B.W., and J.W. performed research; V.v.L., R.M.E.,and C.J. contributed new reagents/analytic tools; M.M., V.v.L., M.R., G.-L.H., Z.Y., E.A.,V.A.A., S.G., S.S.X., K.R.A., X.-B.W., C.J., and J.W. analyzed data; M.M., M.R., E.A., V.A.A.,S.G., K.R.A., X.-B.W., C.J., and J.W. wrote the paper; and J.W. developed the principle ideaand coordinated the research.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1M.M., V.v.L., and M.R. contributed equally to this work.2To whom correspondence may be addressed. Email: xuebin.wang@pnnl.gov, carsten.jenne@uni-wuppertal.de, or jwarneke@purdue.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1820812116/-/DCSupplemental.

Published online April 5, 2019.

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superelectrophiles with advantages to form and stabilize productswith extremely weak nucleophiles like Ar.Recently, we reported on the exotic properties of anions (17)

generated by the abstraction of a halogenide substituent X− fromthe exceptionally stable, weakly coordinating closo-dodecaboratedianions [B12X12]

2− (Fig. 1A). The free boron coordination sitein [B12X11]

− carries a substantial positive partial charge and is,therefore, electrophilic, albeit the overall charge of the ion isnegative. In the following, we propose a concept based on theextraordinary combination of five characteristic properties thatrationalizes why anions of the type [B12X11]

− are advantageousin the formation and stabilization of chemical bonds with inertnoncharged species, such as NGs. These properties include thefollowing:

i) Strong electrophilicity. The free boron-binding site carries apartial positive charge (Fig. 1A) and the lowest unoccupiedmolecular orbital (LUMO) with negative energy is localizedon this boron center (details in SI Appendix, section S1).

ii) Structural stability. The rigid and highly stable dodecaboratescaffold prevents potential intramolecular rearrangementreactions. The highly reactive electrophilic center is pre-served and stays chemically available.

iii) Advantages in formation and stabilization of the collisioncomplex. The strongly electrophilic (positive) binding sitewithin the overall anionic framework generates an unusualelectric field in the vicinity of the reactive boron atom. Polarnucleophiles approaching the anion in the preferred orien-tation (Fig. 1B) have to overcome a barrier and need tochange orientation so that the nucleophilic site can reactwith the anion (17). This lowers the cross-section for reac-tive collisions. Nonpolar molecules like noble gases are not

prone to this effect. The large molecular framework of thesepolyatomic cage anions allows the efficient redistribution ofthe collision energy supplied by the reaction partner(s) overthe many, in particular, lower-energy vibrational degrees offreedom so that the collision complex lives long enough tobe stabilized by collisional cooling.

iv) Protection against substitution. After the NG bond to boronhas been formed, its substitution with a nucleophile througha typical SN2 mechanism is inhibited by the cage structure ofthe borate, since the NG-B bond is sterically protected(Fig. 1C).

v) Favorable electrostatics and dispersion forces. In addition tothe dative NG-B bond, dispersion interactions and electro-statics contribute to the stabilization of NG complexes. Theelectrophilic binding site is located within a “crater” definedby five partially negatively charged substituents X (Fig. 2A),providing a large interaction area for attractive London dis-persion forces (18). The bound NG atom, on the other hand,provides electron density to the adjacent boron atom andbecomes slightly positive, leading to a stabilizing attractiveelectrostatic interaction with the surrounding substituents(Fig. 1C).

These aspects rationalize our recent observation: [B12Cl11]−

binds Xe and even Kr at RT in the presence of the competingstrong nucleophile water. Highly electrophilic cations withcomparable thermodynamic binding strength toward NG (e.g.,C6H5

+) show no NG products under the same experimentalconditions (details in SI Appendix, section S2), but exclusivelyform water adducts (17). This may be explained by properties iiiand iv (see above), which account for kinetic advantages informing and stabilizing the NG product in the anion case,highlighting the potential of these anions to provide experi-mental access to exotic compounds. One prominent example isAr compounds for which no molecular anions are currentlyknown at RT. Therefore, we decided to explore the potential ofthis chemical tool by increasing the electrophilicity of [B12X11]

−

type anions, first identifying alternative substituents X thatshould yield improved properties.The thermodynamic stability of the precursor dianion [B12X12]

2−

plays a key role in obtaining highly electrophilic anions [B12X11]−

through gas-phase fragmentation. As a rule of thumb, an increasein electronic stability of the dianion correlates with an increase inelectrophilicity of the corresponding monoanion. Therefore, thepreparation of the most electrophilic anion requires synthesizingthe most stable precursor dianion. The search for small, stable

Fig. 1. (A) Fragmentation reaction that yields the electrophilic anion[B12X11]

− depicting the differently charged areas within the reactant andproduct. Note that the color coding is just for illustration purposes and isneither quantitative nor linear. (B) Visualization of the electric field resultingfrom the molecular charge distribution (see SI Appendix, section S1 for moredetailed information). Colored arrows depict the force on a negative pointcharge, which is large and attractive near the positively charged binding site(green arrows) and becomes repulsive (red arrows) at larger distances due tothe overall negative charge. The change in direction of the electric field closeto the binding site leads to reversal of the preferred orientation of a dipole.(C) The NG atom binds to the positive binding site, provides electron density(dative bond, dark gray), and is charged slightly positive, allowing for anattractive electrostatic interaction with the surrounding negatively chargedsubstituent shell. SN2-type substitution of the bound NG atom is preventedby the dodecaborate scaffold (reaction path crossed out in red). Addition-ally, a large contact surface is available for dispersion interactions (light blue;see Fig. 2A for a 3D image of the binding cavity).

Fig. 2. (A) Electrostatic potential (ESP) plotted on the molecular surface(electron density isosurface at ρ = 0.001 a.u.) of [B12(CN)11]

−. (B) Isolines ofequal ESP outside the molecular surface (gray). Black lines correspond tonegative whereas red lines correspond to positive ESP values (details in SIAppendix, section S1). Blue background marks the region of anionic field(force on a negative charge points away from the ion), while yellow back-ground marks regions of attraction. These regions were obtained from thederivative of the ESP.

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multiply charged anions (MCAs) represents a long-standingchallenge in chemistry and physics (19–22). Closo-dodecaborateshave been shown to exhibit particularly robust electronic struc-tures and several anionic fragments have been shown to be highlyreactive (17, 23, 24). Electronic stability critically depends on thechoice of the substituent X (25–29). Recently, the stability of thepercyano (CN) and perboronyl (BO) dianions, [B12(CN)12]

2− and[B12(BO)12]

2− was predicted to exceed all known MCA recordswith binding energies of the second electron exceeding 5 eV (27,28). The synthesis of such exceptionally stable dianions will openup additional perspectives in battery development and energystorage (27). To test whether these dianions represent promisingprecursors for the generation of exceptionally electrophilic anionswe computed the atomic charges of the free boron binding sites in[B12(CN)11]

− and [B12(BO)11]−. Natural population analysis

(NPA) assigns charges (QNPA) of +0.82 e (X = CN) and +0.75 e(X = BO) to the boron binding sites, suggesting an enhancedelectrophilicity of these anions in comparison with [B12Cl11]

−

(QNPA + 0.65 e), which binds Kr but not Ar. Fig. 2 visualizes theelectrostatic potential (ESP) of and the electric field around[B12(CN)11]

−. The comparison with [B12Cl11]− is shown in SI

Appendix, section S1.There are no established synthetic procedures for introducing

boronyl substituents into closo-borates (30). The cyanation ofdodecaborates was explored solely by Trofimenko and Cripps(31) and Trofimenko (32) in the 1960s, who reported the sub-stitution of halogens in [B12Cl12]

2− and [B12Br12]2− with cyanide

anions in an aqueous solution upon irradiation with a low-pressuremercury lamp. Up to seven and nine halogens were substituted byCN groups in [B12Cl12]

2− and [B12Br12]2−, respectively. The in-

crease in the substituent substitution efficiency with halogen sizewas attributed to weakening of the boron–halogen bond.We decided to further explore this synthetic strategy to obtain[B12(CN)12]

2−. Following the original idea, we irradiated an aque-ous solution of K2[B12I12] and KCN for 3 h with a 150-W medium-pressure mercury lamp (see experimental details in Methods andSI Appendix, section S3) and obtained a mixture containing[B12(CN)12-n(OH)n]

2− (n = 0–5) anions. After resalting withtetrabutylammonium bromide, electrospray ionization mass spec-trometry (ESI-MS) in the negative ion mode (Fig. 3A) producedthese dianions and the anionic [B12(CN)12-n(OH)n+N(C4H9)4]

− ionpairs in the gas phase. This constitutes conclusive experimentalevidence for the formation and observation of [B12(CN)12]

2−.The electronic properties of the [B12(CN)12-n(OH)n]

2− dia-nions isolated in the gas phase were characterized using negativeion photoelectron (PE) spectroscopy. The 157-nm PE spectraare shown in Fig. 3B (see SI Appendix, section S4 for details),which yield vertical (adiabatic) detachment energies for n = 0–2of 5.75 eV (5.55 eV), 5.23 eV (4.97 eV), and 4.95 eV (4.50 eV),respectively. These values, and in particular those of [B12(CN)12]

2−,substantially exceed the previous experimental record of 3.4 eV(2.9 eV) of [ZrF6]

2− for measured detachment energies of MCAs(33). Moreover, they are in satisfactory agreement with the pre-dicted values for these dianions (SI Appendix, section S5 and ref. 27).

Fig. 3. (A) (−) ESI mass spectrum of products after photochemically induced cyanation of [B12I12]2− in water. In the lower mass-to-charge ratio region, the free

dianions were detected while in the higher mass region anionic ion pairs with the counterion tetrabutylammonium were found. (B) Negative ion photo-electron spectra of [B12(CN)10(OH)2]

2− (green), [B12(CN)11OH]2− (blue), and [B12(CN)12]2− (red) measured at a fixed photodetchment wavelength of 157 nm.

Note that the photodetachment cross-section for [B12(CN)12]2− is smaller as the photoelectron signal is suppressed by the repulsive Coulomb barrier, resulting

in low signal intensity and relatively more pronounced spectral features originating from photofragmentation visible at slightly lower electron-bindingenergies than those of the parent dianion spectral band (details in SI Appendix, section S4). Vertical detachment energies are indicated by dotted lines. (C)Time-of-flight mass spectra after isolation of [B12(CN)11]

− and subsequent trapping at RT for up to 100 ms using helium (black) or argon (red) as a buffer gas.The water adducts are due to water vapor, which is present as an impurity (details in SI Appendix, section S7).

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These results provide experimental evidence of the remarkableelectronic stability of dodecaborate cyanides synthesized in thisstudy and establish the percyano-dodecaborate dianion as theelectronically most stable MCA investigated experimentally.Tandem mass spectrometry was used to isolate the detected

species and examine their fragmentation behavior with the goalto generate [B12(CN)11]

−. These experiments revealed that, incontrast to the halogenated dodecaborates, the target anioncould not be generated by direct substituent abstraction from[B12(CN)12]

2−. Rather, collisional activation produced severaldoubly charged fragments in low abundance, confirming theexceptional electronic and structural stability of [B12(CN)12]

2−

(SI Appendix, section S6). However, collisional activation of theion pair [B12(CN)12+N(C4H9)4]

− led to fragmentation of thecounter cation. Loss of tributylamine and subsequent loss ofbutene resulted in formation of [B12(CN)12H]−. This protonateddodecaborate subsequently fragmented by HCN loss yielding thetarget anion [B12(CN)11]

−, which was then isolated in the iontrap for spectroscopic as well as reactivity measurements.To quantify the electrophilicity of the free boron binding site

spectroscopically, we measured the site-induced vibrational fre-quency shift ΔνCO upon binding carbon monoxide to [B12(CN)11]

−

(see SI Appendix, section S7 for details). The CO stretchingfrequency is sensitive to the strength of the binding site andshows a red shift (ΔνCO < 0) for nucleophilic and a blue shift(ΔνCO > 0) for electrophilic sites (34). For [B12(CN)11CO]− wedetect a blue shift of +115 cm−1, a value that is roughly twice (!)as large as that for the corresponding Kr-binding perchlorocompound [B12Cl11]

− (+66 cm−1) and still more than 35% largerthan that of the strongly electrophilic phenyl cation (+84 cm−1).These measurements experimentally verify the exceptionalstrength of the electrophilic binding site in the [B12(CN)11]

−

monoanion, in accordance with our predictions from electronicstructure calculations (Fig. 2 and SI Appendix, section S1).To probe the reactivity of [B12(CN)11]

− toward smaller NGatoms we trapped the mass-selected anions at RT for up to100 ms using either helium (3.0·10−3 mbar) or argon (1.7·10−3 mbar)as a collision gas. Note that water vapor as well as N2 and O2 aretypically present at RT as trace impurities in such experiments.The corresponding time-of-flight mass spectra are shown in Fig.3C. For pure helium, predominantly H2O adduct formation isobserved, corresponding to the broad B12 isotopomer distribu-tions, which result from the natural isotope distribution of boron(80% 11B, 20% 10B), centered at +m/z 18 (+H2O) and, to lesserextent, at +m/z 36 (+2 H2O). No evidence for He binding isfound. For argon, on the other hand, an additional distribution ofpeaks is observed at +m/z 40, which we assign to the Ar adduct[B12(CN)11Ar]−. This first observation of an anion which is able tobind Ar at RT manifests the exceptional property of [B12(CN)11]

−

ions to create previously unobservable bonding motifs.Experimental observations of B-Ar bond formation at RT are

scarce. Koskinen and Cooks (8) observed the formation of B-Ar+

(Fig. 4A), while Levee et al. (35) reported the formation of F2B-Ar+ (Fig. 4B). In both cases, these ions were formed in an en-dothermic reaction following the loss of a halogen radical from aprecursor cation. A more recent study showed the exothermicbinding of Ar to cyclic boroxol cations, [B5O7]

+ (Fig. 4C), atcryogenic temperatures; theoretical investigations suggested thepresence of a strong boron–argon bond in [Ar-B5O7]

+ that mayexist even at RT (36). We calculated the 0-K enthalpy of Arbinding for these three cations at different levels of theory andcompared them to that for [B12(CN)11]

− (Fig. 4D). The resultsare listed in Table 1. Ar attachment to [B5O7]

+ and [B12(CN)11]−

exhibits similar enthalpies of around 60 kJ∙mol−1, making thecomparison of the boron–argon binding motif in these very dif-ferent ions especially interesting.A quantum theory of atoms in molecules (QTAIM) analysis

provides evidence for a bond path for all boron–argon interactions

in the four molecules with a bond critical point (bcp) and a sig-nificant accumulation of electron density inside the bond (seeelectron density ρ at the bcp in Table 1 and SI Appendix, sectionS8). The negative values of the total energy density (37) H at thebcp reveal a considerable amount of covalent interaction in allfour cases. While the Ar attachment energies correlate with the B-Ar bond lengths (Table 1), the electrophilicity of the binding site isrelated to the positive partial charge on the reactive boron. Re-markably, a comparison of the NPA charges QNPA in the free ionspoints to a considerably stronger electrophilic boron atom in[B5O7]

+ (1.50 e) than in [B12(CN)11]− (0.82 e), indicating that

other stabilizing parameters must be present in the anion to reachthe very similar attachment energy and to account for a compar-atively high positive partial charge of the Ar atom (0.41 e) in theanion. We attribute these additional stabilizing forces to electro-static and dispersion interactions between the Ar atom and thesurrounding substituent shell. This is confirmed by an energy de-composition analysis (EDA) (38, 39) showing that the relativecontributions of the orbital overlap vs. dispersion and electrostaticforces to the binding energy (Table 1) are substantially differentfor [B12(CN)11]

− compared with [B5O7]+. Electrostatic and dis-

persion forces are significantly larger in the anion compared withall cations in relation to the orbital overlap. The method and basisset dependence of binding energies and a complementary bondinganalysis are provided in SI Appendix, section S8 and underline thedifferent B-Ar bond character in [B5O7Ar]

+ and [B12(CN)11Ar]−.

In conclusion, we have developed an approach for the rationaldesign of superelectrophiles that is based on the incorporation ofan electrophilic center into an anionic framework and then dem-onstrated its feasibility using dodecaborate-based anions. In thecourse of this study, we developed the synthesis and performed acomprehensive spectroscopic characterization of [B12(CN)12]

2−, theelectronically most stable MCA available today. Furthermore, weused tandemmass spectrometry to abstract a substituent, generatingthe RT argon-binding anion [B12(CN)11]

−. Its strong electrophi-licity, which appears counterintuitive at first sight due to its anionicnature, can be traced back to the pronounced electronic stabilityof the parent dianion. Formation of a localized, structurally pro-tected but chemically accessible, positively charged binding site

Fig. 4. (A–D) Molecular structures of the experimentally observed ions thatcontain a B-Ar bond.

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embedded in a spherical anionic framework results in outstandingbinding properties. The chemically highly potent electrophilicanion [B12(CN)11]

− may open another chapter in the metal-freeactivation of particularly inert compounds and elements. In addi-tion, the formation of strong noble gas bonds with anions opensfundamentally more opportunities to stabilize them in the con-densed phase: Before our findings, the strongest interactions ofnoble gases were observed for transient cations. The existence ofsuch cations in the condensed phase is unlikely, because coun-teranions need to be present to form a condensed-phase com-pound. Even the weakest coordinating anions may successfullycompete with noble gases for the positively charged, electrophilicbinding site in such reactive cations, leading to substitution andrelease of noble gas atoms. In contrast, a noble gas bond in anelectrophilic anion avoids this problem because the requiredcounterion would be a cation. A cation should not compete withthe noble gas for the positive binding site within the electrophilicanion (cations are usually electrophiles themselves). Gaseous iondeposition on surfaces (ion soft landing) has recently been shownto generate condensed-phase material layers from mass selectedanions (40). [B12(CN)11]

− binds Xe with enthalpies substantiallyabove 100 kJ/mol (SI Appendix, section S2), suggesting that[B12(CN)11Xe]− may survive an ion soft-landing process. Jointdeposition with inert counter cations may enable the generation ofionic material. These exceptional anions may complement otheranionic clusters used for the construction of functional condensed-phase materials (41).

MethodsSynthesis of [NBu4][B12(CN)(12-n)(OH)n] (n = 0–4). Potassium cyanide (AnalRNORMAPUR) and tetrabutylammonium bromide (Alfa Aesar) were purchasedand used without further purification. K2[B12I12] (42) was prepared bya known procedure. The reaction was performed using a TQ 150(Heraeus; power supply from UV-Technik) 150-W mercury medium pres-sure immersion lamp.

In a quartz tube equipped with a stirring bar, K2[B12I12] (1.00 g, 0.58 mmol)and KCN (7.52 g, 115 mmol) were dissolved in water (70 mL). The reactionmixture was stirred under irradiation for 3 h at room temperature. The so-lution was cooled down to 4 °C and 14.4 mL (173 mmol) concentratedhydrochloric acid was added. Subsequently, the reaction mixture was heatedto 120 °C overnight to evaporate the solvent. The residue was dissolved in 40mL water and tetrabutylammonium bromide (0.93 g, 2.89 mmol) was addedto the solution and stirred for 30 min. The grayish precipitate (170 mg) wasfiltered off and dried at 120 °C overnight.

DetailedNMRdata and ESImass spectra of [NBu4][B12(CN)(12-n)(OH)n] (n= 0–4)can be found in SI Appendix, section S3.

Mass Spectrometry. MS/MS experiments to explore fragmentation pathwaysyielding [B12(CN)11]

− were conducted using a Bruker HCT Ion Trap and aThermo Hybrid Linear Ion Trap (LTQ)/Orbitrap. Samples were dissolved inacetonitrile at concentrations of ∼10−5–10−6 mol·L−1 and injected into themass spectrometer via a syringe pump at a flow rate of 3 μL·min−1.

The mass spectrometry of [B12(CN)11Ar]− and infrared photodissociation

(IRPD) spectroscopy, to determine the stretching vibration of CO bound tothe electrophilic binding site, were studied using the Leipzig cryogenic iontrap triple mass spectrometer described in detail elsewhere (43, 44). In brief(for more details, see SI Appendix, section S7.1) [B12(CN)11]

− anions wereproduced via skimmer collision-induced dissociation (sCID) from precursorions formed using a nanospray ion source with a 0.5 mmol/L solution of[B12(CN)12-n(OH)n][N(C4H9)4]2 (n = 0–5) in CH3OH/H2O (2:1, vol/vol). After theions passed two helium buffer gas-filled radio-frequency (RF) ion guides,they were selected according to their m/z ratios in a quadrupole mass filterand subsequently trapped in a temperature-controllable RF ring-electrodeion trap filled with either helium or argon at room temperature or a mixture ofcarbon monoxide in helium at 55 K to form [B12(CN)11Ar]

− or [B12(CN)11(CO)2]−

complexes. All ions were extracted from the ion trap after 100 ms and focusedinto the center of an orthogonally mounted reflectron TOF tandem photo-fragmentation spectrometer, where IRPD spectra were obtained by irradiatingthe anions with tunable IR radiation from an Nd:YAG laser pumped OPO/OPA/AgGaSe2 laser system.

Detailed data onmass spectrometry and IRPD spectroscopy can be found inSI Appendix, sections S6 and S7.

Negative Ion Photoelectron Spectroscopy. The negative ion photoelectronspectroscopy (NIPES) experiments were performed using the Pacific North-west National Laboratory cryogenic NIPES instrument for mass-selected an-ions, coupled with an electrospray ionization source (45). Electrospraying0.1-mM acetonitrile solutions of tetrabutylammonium (TBA) salts into vacuumafforded the corresponding [B12(CN)12-n(OH)n]

2− (n = 0, 1, 2) anions. Pho-toelectron spectra of the three resulting ions were obtained at 20 K, usingan F2 excimer laser at 157 nm (7.866 eV). Photoelectrons were collected withalmost 100% efficiency with a magnetic bottle and analyzed in a 5.2-m-longflight tube. The resulting time-of-flight photoelectron spectra data werecalibrated by recording the known spectra of I− (46) and Au(CN)2

− (47). In allinstances, the laser was operated at 20 Hz to enable shot-to-shot backgroundcorrection of the observed intensities, and the resulting spectra have a resolutionof ∼50 meV for electrons with kinetic energies of ∼2.5 eV.

Detailed descriptions of the conducted NIPES experiments can be found inSI Appendix, section S4.

Theoretical Investigations. Analysis of the B-Ar bond was accomplished bycomputing the 0-K attachment enthalpies of the molecules [BAr]+, [F2BAr]

+,[B5O7Ar]

+, and [B12(CN)11Ar]− using density-functional theory (DFT) (B3LYP

with D3 dispersion correction), spin-component-scaled Møller–Plessetperturbation theory (SCS-MP2), and coupled-cluster with single, double,and perturbative triple excitations [CCSD(T)] quantum chemistry methods.

Table 1. Computational values of bond parameters

Formula B-Ar F2B-Ar B5O7Ar B12(CN)11Ar

Charge state + + + −Ar attachment energy, 0-K enthalpy, kJ/mol −22 −40 −62 −61Bond length, Å 2.52 2.11 2.03 1.98ρbcp, e·Å−3 0.30 0.40 0.42 0.40Hbcp/ρbcp, Ha·e−1 −0.18 −0.52 −0.64 −0.60QNPA free B, e 1.00 1.67 1.50 0.82QNPA bound Ar, e 0.13 0.26 0.33 0.41Attractive energy components, EDA

Orbital interaction, % 77 82 80 65Electrostatics, % 20 15 17 25Dispersion, % 3 3 3 10

The observed trends are predominantly independent of the theoretical method. The given values for Arattachment enthalpies are CCSD(T)/cc-pVTZ results. The given values should be understood with an uncertaintyof ±5 kJ/mol based on a comprehensive method and basis set dependency analysis including basis set superpo-sition error (BSSE)-corrected SCS-MP2 and B3LYP with QZ and 5Z basis sets (details in SI Appendix, section S8). TheCCSD(T) binding energies are obtained at the MP2 optimized geometries, for which the B-Ar bond lengths arelisted. All other parameters are electron-density related and are derived fromDFT calculations [B3LYP/6-311++G(2d,2p)including GD3BJ dispersion correction]. ρbcp, electron density at the bcp; Hbcp, total energy density at the bcp; QNPA,atomic charge derived from NPA calculations.

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Hybrid DFT methods (B3LYP and PBE0) with dispersion correction were againused to compute adiabatic and vertical detachment energies. B3LYP wasused to compute harmonic frequencies, too.

Since chemical bonds are not quantum-mechanical observables, there is nosingle method to satisfactorily capture the nature of an interatomic in-teraction, which is especially problematic for medium-to-weak polar covalentbonds as found here (B-Ar). Therefore, we resort to the full potential of acomplementary bonding analysis that includes different bond analysistechniques from real, orbital, and energy space (48). Among others, frontiermolecular and natural bond orbital theories, electron density and electronlocalizability topologies, and energy decomposition analysis are utilized. Forreferences to all methods and the software employed, see SI Appendix,section S8. The analyses are based on fully relaxed geometries with a D3BJ-dispersion–corrected B3LYP functional and triple-zeta quality basis sets.

ACKNOWLEDGMENTS. J.W. thanks Prof. Julia Laskin for many helpfuldiscussions. K.R.A. acknowledges instrumental support from the Fritz-Haber-Institute of the Max-Planck-Society. S.G. acknowledges the German

Research Foundation for an Emmy Noether Fellowship funded within GR4451/1-1. E.A. and S.S.X. acknowledge support from the Center for ScalablePredictive Methods for Excitations and Correlated Phenomena (SPEC), whichis funded by the US Department of Energy (DOE), Office of Science, BasicEnergy Sciences, Chemical Sciences, Geosciences and Biosciences Division, aspart of the Computational Chemical Sciences Program at Pacific NorthwestNational Laboratory (PNNL). X.-B.W. acknowledges support from the USDOE, Office of Science, Office of Basic Energy Sciences, Division of ChemicalSciences, Geosciences and Biosciences at PNNL. Battelle operates the PNNLfor the US DOE. This research used computer resources provided by PNNLInstitutional Computing and by the National Energy Research ScientificComputing Center, which is supported by the Office of Science of the USDOE under Contract DE-AC02-05CH11231. A portion of this research (E.A.and X.-B.W.) was performed in Environmental Molecular Sciences Labora-tory (EMSL), a DOE Office of Science User Facility sponsored by the Office ofBiological and Environmental Research and located at PNNL. J.W. acknowl-edges a Feodor Lynen Fellowship from the Alexander von HumboldtFoundation.

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