Anionic Oligomerization of Li2[B12H12] and Li[CB11H12]: An

Article pubs.acs.org/JPCC

Anionic Oligomerization of Li2[B12H12] and Li[CB11H12]: An Experimental and Computational Study Juan Z. Dávalos,*,† Javier González,† Andrés Guerrero,† Drahomír Hnyk,‡ Josef Holub,‡ and Josep M. Oliva*,† †

Instituto de Química-Física Rocasolano, CSIC, C/Serrano, 119, E-28006 Madrid, Spain Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Husinec-Ř ež, Czech Republic

‡

S Supporting Information *

ABSTRACT: We present an experimental and computational study of the oligomerization in icosahedral closo-(car)boranes Li2[B12H12] (I) and Li[CB11H12] (II). The experiments were performed on a hybrid ESI-TQ-FT-ICR mass spectrometer equipped with a 7.0 T superconducting magnet. The computational study consisted of finding energy minimum structures for the molecules I, II and the derived anions [B12H12]2−, [CB11H12]−, [I2−Li]−, and [II2−Li]−, as well as an estimate for the dissociation energy in the processes [X2−Li]− → X + [X−Li]− with X = I, II. Comparison of experiments and computations shows an excellent agreement for the bond dissociation energy in the process [II2−Li]− → II + [II−Li]− with ΔE = 1.5 eV.

1. INTRODUCTION Polyhedral heteroboranes show a rich variety of molecular architectures with a wide variety of applications ranging from biosciences1 and medicine2 (boron neutron capture therapy, BNCT) to materials sciences.3 The large stability of some polyhedral closed (closo) heteroborane cages, particularly icosahedral,4−7 make them ideal candidates for experimental and computational studies of “soft” interactions with biomolecules.8 Synthesis and structural characterization of alkali metal complexes derived from carbanions,9 or the (quantum-chemical studied) interaction between Li+ and icosahedral B12H122− and CB11H12− cage anions, may be relevant for understanding alkali metal reaction mechanisms inside and outside heteroborane cages.10,11 Controlled Li-release inside an heteroborane molecular architecture might also be relevant for medical and new materials sciences, such as nonlinear optics (NLO), given the remarkable enhanced computed electro-optical response of the endohedral complex Li@ B10H14.12 Such systems may evoke the possibility to explore a new thriving area, that is, alkali-metal-boranes for NLO applications. In this work, we study experimentally (using MS spectrometry), and by means of computational modeling, the energies and modes of interaction in icosahedral (car)borane cages Li2[B12H12] and Li[CB11H12] in the anionic mode.

with an electrospray ionization source (ESI) and coupled to a Fourier transform ion cyclotron resonance spectrometer (FT-ICR) Agilent/Varian 920 provided with a 7.0 T superconducting magnet. Sample solutions were directly infused into the ESI source at 20 μL/min of flow rate in the negative mode. The ESI needle voltage was maintained at −3.5 kV, and the capillary voltage was kept at −72 V. Nitrogen was used as the drying gas, and compressed air was used as the nebulizer gas. The ion source and drying gas temperatures were kept at 42 and 140 °C, respectively. 2.2. CID Experiments. The dissociation experiments were performed in TQ-spectrometer by isolating the desired anions in the first quadrupole (Q1) with a peak width including the signal of the main boron ionic isotopes. The isolated ions were allowed to undergo collision induced dissociation (CID) with argon gas leaked into a curved (90°) collision chamber (Q2) at a pressure of 1.4 mTorr. The geometry of Q2 reduces the background noise and increases the signal-to-noise ratio. The dissociation product ions were finally analyzed with the third quadrupole (Q3). The CID experiments were performed at 16 different collision energies, corresponding to the center-of-mass energies (Ecm) of 0.25−4 eV in 0.25 eV steps. The center-of-mass energy was calculated using the equation Ecm = Elab[m/(M + m)], where Elab is the ion kinetic energy in the laboratory frame, m is the mass of the collision gas (argon), and M is the mass of the anion. The final intensities of the dissociation product ions were determined as the average of 100 measurements of one second of detection. 2.3. High-Resolution Experiments. High-resolution mass spectrometry measurements of Li2[B12H12] and Li[CB11H12] were performed by FT-ICR. The anions, produced in the

2. EXPERIMENTAL SECTION 2.1. Materials and Instrumentation. Samples of Li2[B12H12] and Li[CB11H12] were provided by Dr. D. Ellis and Prof. A. J. Welch of Heriot-Watt University, UK. Li2B12H12 (I) and LiCB11H12 (II) were dissolved in acetonitrile (50−250 μM). The manipulation of the samples was performed inside of an argon drybox to minimize potential decomposition processes. The experiments were carried out on a Triple Quadrupole mass spectrometer (TQ) Agilent/Varian MS-320 equipped © 2012 American Chemical Society

Received: October 16, 2012 Revised: December 17, 2012 Published: December 20, 2012 1495

dx.doi.org/10.1021/jp3102354 | J. Phys. Chem. C 2013, 117, 1495−1501

The Journal of Physical Chemistry C

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ESI source under the conditions already described, were allowed to cross the TQ without filtering them, and were diverted to the FT-ICR. The FT-ICR conditions were optimized in order to measure within the 50−800 m/z range. Commercially beer malto-oligosaccharides were used as mass calibrants and tuning standards in the negative ion mode.13

3. COMPUTATIONAL DETAILS The electronic structure computations included in this work were carried out with hybrid Hartree−Fock/Density Functional Theory methods (B3LYP),14−17 which provide good accuracy for geometries and energies in ground-state many-electron systems.18 The quantum chemical modeling of the molecules was carried out with the suite of programs Gaussian 09,19 and using the model chemistry B3LYP/ 6-311++G(d,p), which includes a triple-ζ basis set with diffuse and polarization functions for all atoms. All reported molecular geometries correspond to energy minima, checked through analytical second derivatives of the energy versus nuclear displacements. 4. RESULTS 4.1. FT-ICR Results. The obtained high-resolution FT-ICR mass spectra of Li2[B12H12] (I) and Li[CB11H12] (II) samples recorded at different m/z ranges are shown in Figures 1 and 2, respectively.

Figure 2. ESI-FT-ICR mass spectra of LiCB11H12 (II) in the negative mode, optimized for (a) m/z < 400 (the harmonic signal of the peak at m/z 143.204 is marked with an asterisk (*)) and (b) around m/z = 293.4 in the 100 < m/z < 350 range.

Figure 3. L-CID simulation of σ versus Ecm for reaction 1.

identified as the doubly charged anion [B12H12]2− or [I−2Li]2− (Figure 1, top); and (ii) peaks of the dimer [I2−Li]− and trimer [I3−Li]− monocharged anions are overlapping with those from the tetramer [I4−2Li]2− and hexamer [I6−2Li]2− dicharged anions, respectively. The singly and doubly charged signals can easily be distinguished by an extended mass spectrum, inasmuch as each signal shows its characteristic isotope distribution. The inset mass window of Figure 1b displays a detailed view of very well-resolved peaks of the singly charged dimer, [I2−Li]− (○), and the nonoverlapped peaks of the doubly charged tetramer, [I4−2Li]2−. Unlike the previous case, the overall mass spectrum of II (Figure 2a) shows anionic peaks of three different species, without any overlapping: (i) monomer II without Li, singly charged at m/z 143.204 ([II−Li]−), and (ii) two different

Figure 1. ESI-FT-ICR mass spectra of Li2[B11H12] (I) in the negative mode, optimized for (a) m/z < 160 and (b) 100 < m/z < 650 range. The inset in (b) shows the isotopic distribution peaks of dimer [I2−Li]− (○) and those from the tetramer [I4−2Li]2− (including its nonoverlapping peaks).

The spectrum of I sample contains two types of peaks, both of them formed by adding successively this species: mono- and dicharged anions (by one or two losses of Li), which were assigned to oligomer anions [In−Li]− (n = 1, 2, 3, 4) and [In−2Li]2− (n = 4, 5, 6, 7), respectively. We should also emphasize the following results: (i) the intensive peak at m/z 71.107 was 1496



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Figure 4. Optimized geometries, energy minima, for (a) B12H122−, (b) [I−Li]− (LiB12H12−), and (c) I (Li2B12H12) clusters with Ih, C3v, and D3d point-group symmetry, respectively. The 3-fold axis of rotation Ĉ 3 is shown for clarity. Computations used the B3LYP/6-311++G(d,p) model chemistry.

dimers, coordinating Li (m/z 293.423, [II2−Li]−) and Na (m/z 309.397, [Na(CB11H12)2]−), the last one an impurity coming from the synthesis process. The signal of dimer [II2−Li]− optimized in the range between m/z 100 and 350 is depicted in Figure 2b. In the mass scaleexpanded spectrum of this dimer, we can clearly observe the peaks of its characteristic isotope distribution. 4.2. Energy Resolved CID Experiments. The CID experiments were carried out only for the anionic dimer of the sample II, because for sample I, it is not possible to separate the overlapped signals of its dimer and tetramer (as we saw before). The bond dissociation energy (BDE) of the dimer [II2−Li]−, or [CB11H12···Li···CB11H12]−, was obtained from the corresponding dissociation energy threshold, E0: [II 2−Li]− → [II−Li]− + II

E01

during the dissociation are detected and that the effect of multiple collisions is not relevant. In Figure 3 we show a simulation of σ for reaction 1 as a function of Ecm obtained with L-CID operational simpler method developed by Chen and co-workers.20 The threshold energy extracted was E01 = 1.5 ± 0.2 eV. This energy corresponds to the highest energy along the reaction coordinate for dissociation without reverse activation barrier in a “loose transition state”.21,22 The corresponding uncertainty was obtained by checking the flexibility of the fit upon variations in the adjustable parameters, such as fwhm (full-width at halfmaximun) of kinetic ion distribution, temperature, and number of ions in the simulation. The error limits in E01 were then established by noting its value when the fit to the data became significantly worse. Our experimental value of E01 is in good agreement with the computed change of total energy at 0 K for reaction 1, ΔE0K = 1.54 eV (vide infra). 4.3. Quantum-Chemical Computations. a. Li2[B12H12] (I). Figure 4 shows the optimized geometries of the B12H122−, [I−Li]− (Li[B12H12]−), and I (Li2[B12H12]) clusters, all corresponding to energy minima. Tables with the Cartesian coordinates of the

(1)

The experimental total reaction cross section, σ, was determined using the relation IR = Itot·e−σ·ρ·l, where Itot and IR are the total ion intensity and fragment ion intensity, respectively, both measured after the dissociation, ρ is the collision gas density, and S is the path length (18.5 cm) of the collision cell. It was assumed both that all fragment ions formed 1497



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Figure 5. Two different views of the optimized geometry, energy minimum, for the complex anion [I2−Li]− with D3d point-group symmetry. The molecule in view (b) is rotated 60° with respect to view (a) around the Ĉ 3 rotation axis connecting the three lithium atoms. Computations used the B3LYP/6-311++G(d,p) model chemistry.

geometry for the complex anion [I2−Li]−, corresponding to an energy minimum with D3d symmetry, is displayed in Figure 5. The central Li atom is coordinated to six hydrogen atoms. In a previous work, we proved by means of quantumchemical computations that no two Li atoms can be found above two contiguous triangular faces of the neutral complex I as an energy minimum.23 Therefore, any combination of three Li atoms with two B12H12 cages leads to additional isomers, provided the above restrictions apply. Quantum-chemical computations on all of these additional isomers proved that they are all above the energy minimum structure from Figure 5, the maximum energy difference being ∼10 kcal/mol. Figure 6 shows the simplified structure from Figure 5 and three additional isomers of the dimeric anion [I2−Li]−; further isomers can be derived using the results from ref 23. We study the following dissociation process from the computational viewpoint:

Figure 6. Some energy minimum isomers derived from three Li atoms and two B12H12 cages ([I2−Li]−). Isomer (a) corresponds to the structure from Figure 5.

Table 1. Energies (in atomic units, au) of Clusters B12H122−, [I−Li]−, I, and [I2−Li]−, and Estimated ΔE0K2 (in eV) from Equation 2a cluster/sym B12H122−/Ih −

[I−Li] = [LiB12H12]−/C3v I = Li2B12H12/D3d [I2−Li]− = [Li3(B12H12)2]−/D3d

energy (au)

ΔE0K (eV)

−305.762851 −313.382273 −320.885764 −634.319812

1.41

[I 2−Li]− → [I−Li]− + I

a

B3LYP/6-311++G(d,p) computations. All structures correspond to energy minima.

E02

(2)

Table 1 gathers the electronic energy corresponding to the three structures from eq 2, all energy minima, with a quantumchemical result of the bond dissociation energy of E02 = ΔE0K = 1.41 eV. b. Li[CB11H12] (II). Figure 7 displays the optimized geometry of the CB11H12− ([II−Li]−) anion and the II neutral complex using the B3LYP/6-311++G(d,p) model chemistry. Quantum-chemical

optimized geometries of the clusters can be found in the Supporting Information. The experimental measurements show the formation of the anions B12H122−, [I−Li]−, and [I2−Li]−. The optimized 1498



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Figure 7. Optimized geometries, energy minima, for (a) CB11H12− ([II−Li]−) and (b) II (LiCB11H12) systems with C5v and Cs point-group symmetry, respectively. Symmetry operators Ĉ 5 and σh are shown for clarity. Computations used the B3LYP/6-311++G(d,p) model chemistry.

in Figure 9, using the B3LYP/6-311++G(d,p) method, as with anionic and neutral monomers of II. A frequency analysis shows an unphysical residual imaginary frequency of ν1 = 25i cm−1, which corresponds to a rotation of each cage, clockwise and anticlockwise, respectively, around the x axis (a frequency computation in C1 symmetry for the structure from Figure 9 shows no imaginary frequencies). This is confirmed by a Hartree− Fock/6-311++G(d,p) geometry optimization of [II2−Li]−, which shows zero imaginary frequencies with C2h symmetry, the lowest positive frequency being ν1 = 23 cm−1 with exactly the same normal mode vibration ν1 as with the stationary point found with the B3LYP method. We can thus consider the C2h homodimer [II2−Li]− as an energy minimum. A further analysis on the frequencies of this dimer shows six frequencies below 100 cm−1, which correspond to cage normal modes; only the cages move as a whole, following normal modes of the different irreducible representations in C2h: ag, bg, au, and bu. We should also highlight the normal modes ν7 = 135 cm−1, ν8 = 135 cm−1, and ν9 = 409 cm−1, which correspond, respectively, to harmonic motion of the Li+ cation along y, z, and x axes; see Figure 9. The largest frequencies correspond to the practically degenerate in-phase and out-of-phase C−H stretchings, with ν = 3200 cm−1; only the bu inphase stretching can be detected in the IR spectrum. As shown in Figure 9, the Li+ cation is coordinated to six hydrogen atoms forming an octahedron, which is slightly elongated (Δd = 0.1 Å) in the equatorial plane, deq(Li−H) = 2.160 Å. The optimized geometry of the dimer [II2−Li]− is included in the Supporting Information. The energies of [II−Li]−, II, and [II2−Li]− are displayed in Table 2. The computed change of total energy at 0 K, for reaction 1, E01 = ΔE0K = 1.54 eV is in good agreement with experimental results. We also carried out a Natural Population Analysis (NPA)25 on the species described in Table 2. As for the dimer, the crucial NPA charges are displayed in Figure 9, showing negatively charged boron atoms surrounding the central Li atom. In the neutral monomer II, Li occupies 100% its s-orbital, and therefore there is no formal indication of any classical “chemical” bonding, its charge being 0.90, the boron atoms surrounding it being also negatively charged with similar values as compared to the dimer [II2−Li]−.

Figure 8. Mulliken charges in CB11H12−. B3LYP/6-311++G(d,p) computations. Red = −0.43|e|, black = 0|e|, green = +0.43|e|.

computations on II showed that the energy minimum geometry corresponds to the Li atom placed above a triangular face farthest from the carbon atom,24 as shown in Figure 7. The optimized geometries of CB11H12− and II are tabulated in the Supporting Information. Although the carbon atom in the cage of CB 11 H 12 − ([II−Li]−) is more electronegative than the boron cage atoms, one can readily observe that if we plot the Mulliken charges in this anion, Figure 8, the hydrogen atom bound to carbon has the largest positive charge (q = +0.32|e|) in the cluster, and the hydrogen atoms surrounding it below and bound to the boron atoms below carbon, forming the B5 pentagon in black color (black = zero charge, almost zero charge on each B atom), have negative charge. Further below, the hydrogen atoms are even more negatively charged, and therefore the Li+ cation prefers to anchor above a face where the three hydrogen atoms are positively charged, as indicated in Figure 8. The positive charge from this hydrogen atom on the top, attached to the carbon atom, thus pushes the Li+ cation away from the neigboring CBB triangular faces of the icosahedron. The C−B, C−H, B−B, and B−H distances hardly differ from [II−Li]− to II, with |Δ| ≤ 0.01 Å, except for the B−H bonds attached to the Li+ cation, which increase in II by ∼0.02 Å. The Cartesian coordinates of the optimized geometries for these molecules are gathered in the Supporting Information. As regards to the homodimer anion [II 2 −Li] − ([CB11H12···Li···CB11H12]−) detected in the experiments, one can assume, as in the neutral complex II, that the carbon atoms will be farthest away from the Li+ cation, and hence an optimized structure was found with C2h symmetry, as displayed 1499



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Figure 9. Two different views of the C2h optimized geometry of the homodimer anion [II2−Li]− ([CB11H12···Li···CB11H12]−), which corresponds to an energy minimum. Computations with the B3LYP/6-311++G(d,p) method. In (a), the geometry is projected perpendicular to the σh symmetry plane of the molecule; hence the Li atom in the center is coordinated to six hydrogen atoms. View (b) is a rotation of 60° around the x axis from view (a).

Table 2. Energies (in atomic units, au) of Monomeric and Dimeric Clusters ([II−Li]−, II, and [II2−Li]−) and Estimated ΔE0K (in eV) from Equation 2a

a

cluster/sym

energy (au)

ΔE0K (eV)

[II−Li]− = CB11H12− II = LiCB11H12 [II2−Li]− = [Li(CB11H12)2]−

−319.057255 −326.541077 −645.654768

1.54

B3LYP/6-311++G(d,p) computations. Figure 10. (a) Projection of CB11H12 perpendicular to the B−C axis with C at the bottom, behind the central B atom. The figure shows the five equivalent faces of the pentagon. A rotation of 90° following a rotation of 30° leads to (b), showing five equivalent triangular faces; the other five equivalent faces are behind the plane. Hydrogen atoms are not shown for clarity.

The carbon atom is negatively charged in all three systems with practically identical values: qC([II−Li]−) = −0.66|e|, qC(II) = −0.65|e|, and qC([II2−Li]−) = −0.66|e|. Stationary Points of the Homodimer Anion [II2−Li]− ([CB11H12···Li···CB11H12]−). Given the C2h structure from Figure 9, a question put forward is the orbital motion of the Li+ cation around the two CB11H12− cages in the homodimer anion [II2− Li]−. We know from quantum-chemical computations that in the neutral system II, the Li+ cation is located above a triangular face of the icosahedron farthest away from the carbon atom24 and never above a face containing this carbon atom; see also Figure 8. Thus, if we take a CB11H12− cage, the Li+ cation will be anchored above one of the five equivalent triangular faces opposite to the CB5 pentagon with the carbon atom in the center, as displayed in Figure 10a, where the hidden carbon

atom is located below the central boron atom. The Li+ cation can also be anchored in one of the 10 faces shown in Figure 10b. Given the possible anchoring points of the Li atom on the triangular faces shown in Figure 10, we proceeded to the exploration of the potential energy surface, starting off with the a priori most favorable energy minimum, the C2h structure shown in Figure 9. After optimization of all possible symmetryunique orientations of the two CB11H12 cages around the central Li, we confirmed that the global energy minimum is the 1500



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(3) Prokop, A.; Vacek, J.; Michl, J. ACS Nano 2012, 6, 1901−1914. (4) Sivaev, I. B.; Bregadze, V. I.; Sjöberg, S. Collect. Czech. Chem. Commun. 2002, 67, 679−727. (5) Körbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106, 5208− 5249. (6) Garroni, S.; Milanese, C.; Pottmaier, D.; Mulas, G.; Nolis, P.; Girella, A.; Caputo, R.; Olid, D.; Teixidor, F.; Baricco, M.; Marini, A.; Suriñach, S.; Dolors Baró, M. J. Phys. Chem. C 2011, 115, 16664− 16671. (7) Oliva, J. M.; Schleyer, P. v. R.; Aullón, G.; Burgos, J. I.; Fernández-Barbero, A.; Alkorta, I. Phys. Chem. Chem. Phys. 2010, 12, 5101−5108. (8) Wyttenbach, T.; Bowers, M. T. Annu. Rev. Phys. Chem. 2007, 58, 511−533. (9) Izod, K.; Bowman, L. J.; Wills, C.; Clegg, W.; Harrington, R. W. Dalton Trans. 2009, 17, 3340−3347. (10) Serrano-Andrés, L.; Oliva, J. M. Chem. Phys. Lett. 2006, 432, 235−239. (11) Jemmis, E. D.; Balakrishnarajan, M. M. J. Am. Chem. Soc. 2000, 122, 7392−7393. (12) Muhammad, S.; Xu, H.; Liao, Y.; Kan, Y.; Su, Z. J. Am. Chem. Soc. 2009, 131, 11833−11840. (13) Clowers, B. H.; Dodds, E. D.; Seipert, R. R.; Lebrilla, C. B. Anal. Biochem. 2008, 381, 205−213. (14) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (15) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (16) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200− 1211. (17) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627. (18) Jensen, F. Introduction to Computational Chemistry, 2nd ed.; John Wiley & Sons: Chichester, 2007. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (20) Narancic, S.; Bach, A.; Chen, P. J. Phys. Chem. A 2007, 111, 7006−7013. (21) Armentrout, P. In Threshold Collision-Induced Dissociations for the Determination of Accurate Gas-Phase Binding Energies and Reaction Barriers. Modern Mass Spectrometry; Schalley, C., Ed.; Springer: Berlin, Heidelberg, 2003; pp 233−262. (22) Baer, T.; Mayer, P. M. J. Am. Soc. Mass. Spectrom. 1997, 8, 103− 115. (23) Oliva, J. M.; Fernández-Barbero, A.; Serrano-Andrés, L.; CanleL., M.; Santaballa, J. A.; Fernández, M. I. Chem. Phys. Lett. 2010, 497, 172−177. (24) Manero, V.; Oliva, J. M.; Serrano-Andrés, L.; Klein, D. J. J. Chem. Theory Comput. 2007, 3, 1399−1404. (25) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985, 83, 735−746.

one displayed in Figure 9, as we previously suggested. All other minima with different positions of the carbon atoms are lower in energy, the maximum difference being ∼5 kcal/mol. We also included, as energy minimum, the possibility of a charge-transfer complex, by locating the Li atom followed by the two CB11H12 cages, thus [Li···CB11H12···CB11H12]−. The energy minimization of this complex led to an optimized structure (energy minimum) 40 kcal/mol higher than the [II2− Li]−/C2h complex displayed in Figure 9. The optimized structure of this charge-transfer complex can also be found in the Supporting Information.

5. CONCLUSIONS A hybrid ESI-TQ-FT-ICR mass spectrometer combines the highest available mass accuracy with advanced MS/MS capabilities and is perfectly suited to study the interactions of icosahedral closo-(car)boranes lithium-based clusters Li2[B12H12] (I) and Li[CB11H12] (II). The experimental results are explained and rationalized by quantum-chemical studies, finding the most stable structures of I, II, and their derived cluster anions. According to the experimental results, the dianion B12H122− shows a bigger affinity for lithium than the monoanion CB11H12−. Signals of oligomer anions of I singly and doubly charged have been detected (depending on the loss of one or two Li+ ions), which were identified, respectively, as [In−Li]− (n = 1, 2, 3, 4) and [In−2Li]2− (n = 1, 4, 5, 6, 7). As for II, only monomer and dimer singly charged anions [IIn−Li]− (n = 1, 2) were detected. We have estimated experimentally the bond dissociation energy (BDE) between Li and the icosahedral borane cages of the dimeric anion [II2−Li]−, in good accordance with quantum-chemical results, as ΔE = 1.5 ± 0.2 eV. The corresponding BDE for [I2−Li]−, was only determined computationally as ΔE = 1.4 eV, a very close result as compared to the previous complex anion [II2−Li]−, which should imply similar phenomena for the oligomerization process on both species I and II in the anionic mode.

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 91 5619400. Fax: +34 91 564247. E-mail: jdavalos@ iqfr.csic.es (J.Z.D.); [email protected] (J.M.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. D. Ellis and Prof. A. J. Welch (HeriotWatt University, UK) for providing the title compound samples used in the experiments. This work has been financed by the Spanish project MICINN (CTQ 2009-13652), Czech Science Foundation project No P208/10/2269, and joint project CSIC/CAS 2010Cz0008. We are grateful to Margarita Martín (Madrid) for helpful discussions.

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REFERENCES

(1) Scholz, M.; Hey-Hawkins, E. Chem. Rev. 2011, 111, 7035−7062. (2) Campo, F.; Mossotti, M.; Panza, L. Synlett 2012, 23, 120−122. 1501


Anionic Oligomerization of Li2[B12H12] and Li[CB11H12]: An

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