Synthesis, Electronic Properties and Reactivity of [B12X11(NO2)]2− (X=F–I) Dianions

Abstract Nitro‐functionalized undecahalogenated closo‐dodecaborates [B12X11(NO2)]2− were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X‐diffraction, mass spectrometry, and gas‐phase ion vibrational spectroscopy. The NO2 substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B12X12]2− (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum‐chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO.). The homolytic NO. elimination from the dianion under very soft collisional excitation in gas‐phase ion experiments results in the formation of the radical [B12X11O]2−.. Theoretical investigations suggest that the loss of NO. proceeds via the rearrangement product [B12X11(ONO)]2−. The O‐bonded nitrosooxy structure is thermodynamically more stable than the N‐bonded nitro structure and its formation by radical recombination of [B12X11O]2−. and NO. is demonstrated.


Results and Discussion
Synthesis and characterization of [B 12 X 11 (NO 2 )] 2À À (X = F-I) Startingf rom the parent closo-dodecaborate [B 12 H 12 ] 2À the ammonio-substituted perhalogenated clusters [B 12 X 11 (NH 3 )] À (X = F-I) were obtained in two steps by known procedures. [13a,e, 16] Subsequently,t he ammonio group was oxidized with H 2 O 2 following ap rocedure used before for the synthesis of [B 12 (OH) 11 (NO 2 )] 2À . [15,17] For this purpose, the potassium salts of [B 12 X 11 (NH 3 )] À were dissolved in an aqueous solution of hydrogen peroxide (30 %). Subsequently,p otassium hydroxide was added to adjust the pH value to 8-10 (Caution:As trong gas evolutionc an occur due to decomposition of H 2 O 2 .T he use of a burst shield to protect from possible explosion is recommended.). Aliquots of H 2 O 2 werea dded severalt imes ad ay and the reaction progress wasm onitored by 11 BNMR spectroscopy. Scheme 1i llustrates the 3-step synthesis starting from [B 12 H 12 ] 2À and full details are given in sectionS2o ft he Supporting Information.
The progress of the oxidation of the halogenated closo-dodecaborates was monitored over time by NMR spectroscopy. As an example, the 11 Ba nd 19 FNMR spectra for the reactiono f K[B 12 F 11 (NH 3 )] À with H 2 O 2 are showni nF igure 1. The oxidation of the other derivatives was monitored in the same way anda related diagram for [B 12 Cl 11 (NO 2 )] 2À is given in section S2.5 of the Supporting information. At the beginning, the 11 BNMR spectrums hows three signals in 1:10:1 ratio. After some time, as econd set of three signals in 1:10:1 ratio appearsi nt he 11 BNMR spectrum,w hich can be assigned to [B 12 F 11 (NO 2 )] 2À . 11 B-11 BC OSY NMR spectra helped to assign the resonances to the respective compounds. In the 19 FNMR spectrum the integral ratio of the startingm aterial is 6:5a nd splits into a1 :5:5 ratio in the product. Obviously,t he chemical shift of the fluorine atom attached to the antipodal boron atom is significantly influenced by the nitro group.
The oxidation of [B 12 F 11 (NH 3 )] À requires approximately two days. In the case of X = Cl and X = Br the reactiont ime increases to approximately ten days and two weeks, respectively.F or [B 12 I 11 (NH 3 )] À ac omplete conversion could not be reached even after two weeks. There is as ignificanti ncreaseofreaction time by going from X = Ft oX= I. We calculated reactione nergies for the oxidation of [B 12 X 11 (NH 3 )] À according to Equation (1), which are highly exergonic (about 700 kJ mol À1 for all halogens) and only slightly decreases (by 2%)f rom X = Ft o X = I( Ta ble S11o ft he Supporting Information).T herefore, different activation energies for the elementary reaction steps are more likely responsible for the observed change in reactivity along the halogen series than the overall reaction enthalpy.
An attack of the nitrogen atom lone electron pairo ft he amine (the ammonio group is deprotonated under basic conditions) on the H 2 O 2 molecule resultingi nt he formation of an amine oxide may be as implified representation. The actual mechanism is unknown, but presumably proceeds via different steps involving cyclic transition states. [18] Al arge halogena tom Xn eighbored to the NH 2 group shields the nitrogen atom and certainly causes kinetic barriers due to steric hindrance( see Figure 2). The BÀXb ond is shorter than the BÀNb ond for X = F, but longerf or X = Cl-I. In addition, basicity of the NH 2 group Scheme1.Reactionsequence to produce [B 12 X 11 (NO 2 )] 2À (X = F-I).
decreases from X = Ft oX= I( see Table S10 of the Supporting Information), which mayalso influencet he kinetics of this reaction.

Influence of the nitro group on the electronic properties
In organic chemistry, it is well known that the electron-withdrawing nitro group deactivates aromaticsa gainst electrophilic attack while the electron donating amino group leads to activation. The same trend is expected for halogenated closo-dodecaborate anions.T oe valuatet he difference in electronic stability between [B 12 X 11 (NH 2 )] 2À ,[ B 12 X 12 ] 2À and[ B 12 X 11 (NO 2 )] 2À (X = F-I) we used photoelectrons pectroscopy (PES) in the gas phase compared with quantum-chemical calculations and cyclic voltammetry in solution.
The photoelectron spectra measured at al aser wavelength of 157 nm for X = Cl are shown in Figure 4. All other spectra can be found in sectionS 7o ft he Supporting Information. In general, the electron donating amino group lowers the electron binding energy of the dianion in the gas phase, whilet he electronw ithdrawing nitro group leads to an increasec ompared to the perhalogenated dianions ( Figure 5), in accord with calculated vertical (VDE) and adiabatic detachmente ner-gies (ADE) ( Table 1). Hartree-Fock-orbitale nergies( eigenvalues) are plottedb elow the spectra and shiftedi ne nergy so that the energy of the HOMO matches the VDE (Figure 4). The shift compensates fort he deviation from Koopman's theorem.     (NH 2 )] 2À , the orbital of the free electron pair of the NH 2 group overlaps with only one of the four highest lying clustero rbitals, significantly destabilizing it energetically.I nc ontrast, the energy of the four highest orbitals are affected by the NO 2 group. While two orbitals are only slightly stabilized and constitute the twofold degenerate HOMO of [B 12 Cl 11 (NO 2 )] 2À ,asignificant contribution of the oxygen orbitals is presentf or HOMO-1 and HOMO-2, which are strongly stabilized.
The electronic stabilization caused by the nitro group is strongest for X = F( + 0.45 eV). Since [B 12 X 12 ] 2À becomes electronically more stable from X = Ft oX= Br,t he additional stabilization affected by substituting one Xw ith NO 2 decreases along the halogens eries (X = Cl (+ 0.21 eV)), X = Br (+ 0.17 eV)). X = Ic onstitutes as pecial case. While the HOMO for X = FÀBr has as ignificant boron contributiona nd is influenced by the electronic effect of the substituents Yi n[ B 12 X 11 Y] 2À ,t he HOMO for X = Ii sa lmost exclusively localized on the iodine atoms. [4a] Therefore, the electronic stabilities of [B 12 I 11 NH 2 ] 2À ,[ B 12 I 12 ] 2À and [B 12 I 11 NO 2 ] 2À are almost identical, see Figure 5.
Complementary to the determinationo ft he electronic stabilitiesi nt he gas phase, cyclovoltammetric measurements were performed in liquid sulfur dioxide solution in order to estimatet he electrochemical potentials. In the past, liquid sulfur dioxide has been shown to be as uitable solvent for similar closo-borate anions with very high oxidation potentials. [4b,c, 19] The cyclic voltammograms for [B 12 X 11 (NO 2 )] 2À (X = F-Cl) are shown in sectionS3o ft he Supporting Information. The [B 12 X 11 (NO 2 )] 2À anions for X = Cl and Br show aq uasi-reversible redox process, while it is not reversible for [B 12 F 11 (NO 2 )] 2À .Compared to [B 12 X 12 ] 2À ,t he oxidation potentials increasei nt he presence of the nitro group (see Ta ble 1).
These results demonstrate that the electronic properties of [B 12 X 11 Y] 2À are determined by the choice of Y. NO 2 constitutes a functional group which allows for furtherc hemical modification of the anion.

Reactivity of [B 12 X 11 (NO 2 )] 2À À anions
Closo-borate anions are three-dimensional aromatics. [20] Therefore, it is reasonable to assumethat the NO 2 group maybefurther functionalized by established procedures known for nitrobenzene derivatives. For instance, the reaction with nascent hydrogen obtained by acidifying ah eterogenic zinc solution, which is knownt or educe aromatic NO 2 groups, generates [B 12 X 11 (NH 3 )] À in good yields (section S5 of the Supporting Information). Surprisingly,w eo bserved at hermally induced reaction for [B 12 X 11 (NO 2 )] 2À ,w hich is known for nitrobenzeneo nly as as ide reaction:I nas imultaneoust hermogravimetric (TG) and differential scanning calorimetric (DSC) measurement of [N(nBu) 4 ] 2 [B 12 X 11 (NO 2 )] (X = F-Br) in the temperature range from 25 to 350 8C, as tep in the TG analysis with onset temperatures between 214 and2 48 8Cw as observed ( Figure 6). The detected mass losses are in accord with NOC loss from the anion (see Ta ble 2f or experimental and calculated values). The onset tem- Figure 5. Visualization of the averaged,e xperimental, adiabatic electron detachmentenergies of the different substituted closo-dodecaboratea nions in the gas phase. Table 1. Experimental and calculated electron detachment energies in the gas phase and oxidation potentials in SO 2 solution. Energy differencesw ith respect to [B 12 X 12 ] 2À are given in brackets. [a,b,c] AnionExp. ADE [a] [eV] average The loss of the radicalN O C as an exclusive reactiona tf airly low temperatures from an even electron molecule with an N-bondedN O 2 group is surprising. Quantum-chemical calculations suggestt hat the N-bonded NO 2 group is transformed via an h 2 -N,O-bonded transition state (+ 157 kJ mol À1 )i nto an Obondedn itrosooxy moiety (R-ONO), which lies several kJ mol À1 lower in energyt han the N-bonded isomer,a ccording to DFT analysis. Subsequently,t he ONO-substituted clustere asily eliminates an NOC molecule (Figure 7).
The change in transition state and reactione nthalpies along the halogen series are qualitativelyi na greement with the trend observed in thermogravimetry.Adrivingf orce for the rearrangement may be the formation of the strong boronoxygen bond. For comparison, for nitrobenzene the corresponding transition statei s1 00 kJ mol À1 higheri ne nergy and the nitrosooxy isomer C 6 H 5 (ONO) is less energetically stable than nitrobenzene [21] ( Figure S41 of the SupportingI nformation). The remaining [B 12 X 11 O] 2À C radicalp ossesses an oxygenlocalized unpaired electron (sectionS 8a nd Figure S44o ft he Supporting Information) and must be highly reactive. To generate this intermediate, as ample of Cs 2 [B 12 F 11 (NO 2 )] was heated in an open vessel to 300 8Cf or 40 min and the residue was analyzed by IR, NMR and MS methods and compared to the starting material (spectra are shown in section S4 of the Supporting Information). The 11 BNMR spectrum changed from as ignal in 1:10:1 ratio to as ignificantly different pattern in 11:1 ratio (Figure S25 of the Supporting Information), indicating that the B 12 unit is still present, but has been chemically modified.I nt he IR spectrum( Figure S24 of the Supporting Information), the signals assigned to the antisymmetric NO 2 stretcha nd to the BÀN stretch have disappearedand abroad OÀHb and becomes visible. This is confirmed by the detection of [B 12 F 11 (OH)] 2À in the mass spectrometric analysis( Figure S26 of the Supporting Information). Since the heatedv essel was not protected from ambient molecules like H 2 O, [B 12 X 11 (OH)] 2À is formed by HC abstraction.
For ad irect evidence of the proposed NOC loss, we aimed for the observation of [B 12 X 11 O] 2À C.D ue to the ions' high reactivity,p otential reaction partnersn eed to be eliminated. Usually,t he observation of isolated radical dianionsi sc hallenging, because autodetachment of an electron occurs. However, the exceptional electronic stability of halogenated closo-borate anions can prevent the dianionic radical from electron emission in the gas phase. Electrospray ionization (ESI) was used to transfer [B 12 X 11 (NO 2 )] 2À into the gas phase of our mass spectrometers. Even at very "soft" conditions (low collisional excitation, see parameters in section S2 of the Supporting Information), ions with m/z 15 smaller than [B 12 X 11 (NO 2 Cl 11 O] 2À C were obtained from vibrationals pectra using infraredp hotodissociation spectroscopy (IRPD). [22] The comparison of the IRPD spectrum of [B 12 Cl 11 (NO 2 )] 2À with harmonic IR spectra (Figure 8a) from DFT calculations shows that the BÀN( and not the BÀO) boundi somer is present in the gas phase. The symmetric and antisymmetric nitro stretchingb ands are observed at 1388 cm À1 and 1478 cm À1 ,r espectively. In contrast, the IRPD spectrum of [B 12 Cl 11 O] 2À C reveals no IR-active modes of significant intensity above the dominant absorption band at 1032 cm À1 (Figure 8b), which is associated with BÀCl stretching Figure 6. Thermogravimetric analyses of [N(nBu) 4 ] 2 [B 12 X 11 (NO 2 )] (X = F, Cl, Br) with ah eating rate of 5Kmin À1 .  vibrations, coupled to B 12 -cage deformation modes. [23] The absence of any NH 2 signals ( Figure S46 Figure 8c,i sd ifferent from the other two and exhibits ac haracteristic free N = Os tretching band at 1600 cm À1 . The formation and reactivity of [B 12 Cl 11 O] 2À C was furtheri nvestigated by gas-phase experiments in the presence of coun-terions. Following an established procedure, alkyl cations [C n H 2n + 1 ] + (n = 3, 4) were bound to the dianion by fragmentation of at etraalkylammonium counterion. [11a] For the fully perhalogenated clusters, CID of the anionic ion pair[ C n H 2n + 1 ] + [B 12 X 12 ] 2À resultsi nt he loss of an alkene and formation of the strong acid H + [B 12 X 12 ] 2À (see Figure 9a for af ragmentation scheme). In contrast, the equivalent reaction is only observed as as ide-reaction for [C n H 2n + 1 ] + [B 12 X 11 (NO 2 )] 2À .T he loss of NOC from the ion pair is more pronounced. The resulting radical dianion appearst oa ttack the cation. Then ext fragmentation step is the loss of the radical[ C n-1 H 2n-1 ]C,w hich transforms the anion into the even electron system [B 12 X 11 (OCH 2 )] À (Figure 9b). The pronounced differences in the fragmentation behavior of [C n H 2n + 1 ] + [B 12 X 12 ] 2À and [C n H 2n + 1 ] + [B 12 X 11 (NO 2 )] 2À underline the strong tendency for the loss of NOC and reveali nteresting perspectivesf or reactions with the [B 12 X 11 O] 2À C dianions.

Conclusions
The halogenated [B 12 X 11 (NO 2 )] 2À dodecaborates were obtained in high purity and yield by oxidation of the corresponding amines.T his new class of halogenated closo-dodecaborates showsi mprovede lectrochemical stability compared to the  (NO 2 )] 2À ions due to incomplete fragmentation(for further information seesection S2 of the supporting Information)c ould not be avoided and are responsible for the spectral band marked with an asterisk.S ee Ta ble S13 of the Supporting Information for band positions and their assignment. Figure 9. a) Fragmentation scheme of the ion pairs [C n H 2n + 1 ] + [B 12 X 11 (NO 2 )] 2À and [C n H 2n + 1 ] + [B 12 X 12 ] 2À .While CID of the latter exclusively results in the loss of C n H 2n ,thisiso nly as ide reaction for [C n H 2n + 1 ] + [B 12 X 11 (NO 2 )] 2À (indicated by the dashed arrow) and the loss of the NOC is morepronounced. The subsequent loss of [C n-1 H 2n-1 ]C transforms the fragmenti nto the even electron ion [B 12 X 11 (OCH 2 )] À .The fragmentation of both ionpairs finally resultsi n [B 12 X 11 ] À (for detailed MS n analysisconfirmingt he shownpathway for X = Br and for exemplary spectraf or X = Cl, seesection S9 of the SupportingInformation). b) CID mass spectrumo fthe isolated [C 4 H 9 ] + [B 12 Br 11 (NO 2 )] 2À ion pair showing the NOC and C 3 H 7 C loss as main fragmentationpathways and the C 4 H 8 lossasas ide reaction (dashed arrow). common [B 12 X 12 ] 2À anions. Their ability to releasen itric oxide by thermal treatment gives access to the dianionic oxygenbound radical[B 12 X 11 O] 2À C,which might be avery useful precursor for the chemical modification of the halogenated closo-dodecaborates.

Experimental Section
Numbering scheme, experimental details and spectroscopic data, cyclic voltammetry,t hermal NOC cleavage, reduction of the nitro group by nascent hydrogen, crystal structures, photoelectron spectroscopy,q uantum-chemical calculations, gas phase chemistry are reported in the Supporting Information. Deposition numbers 2009135, 2009136, 2009137, 2009138, 2009139, 2009140, 2009141, 2009142, and 2009143 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.