ARTICLES PUBLISHED ONLINE: 13 OCTOBER 2013 | DOI: 10.1038/NCHEM.1776

A boron–boron coupling reaction between two ethyl cation analogues Sebastian Litters, Elisabeth Kaifer, Markus Enders and Hans-Jo¨rg Himmel* The design of larger architectures from smaller molecular building blocks by element–element coupling reactions is one of the key concerns of synthetic chemistry, so a number of strategies were developed for this bottom-up approach. A general scheme is the coupling of two elements with opposing polarity or that of two radicals. Here, we show that a B–B coupling reaction is possible between two boron analogues of the ethyl cation, resulting in the formation of an unprecedented dicationic tetraborane. The bonding properties in the rhomboid B4 core of the product can be described as two B–B units connected by three-centre, two-electron bonds, sharing the short diagonal. Our discovery might lead the way to the long sought-after boron chain polymers with a structure similar to the silicon chains in b-SiB3. Moreover, the reaction is a prime textbook example of the influence of multiple-centre bonding on reactivity.

R

eactions involving C–C coupling are of key importance for the synthesis of all sorts of chemical products starting with small Cn building blocks (especially n ¼ 1 or 2) such as those provided by steam cracking of crude oil or by catalytic reactions starting with synthesis gas. Consequently, a number of C–C coupling reactions have been developed1–5, which are either stoichiometric (such as the Grignard reactions) or catalytic (such as palladium-catalysed cross-coupling, which led to a Nobel prize in 2010 for Heck, Negishi and Suzuki). In general, C–C bonds are formed between positively and negatively polarized carbon atoms. In an alternative, although less practical, approach, a new C–C bond can be established by the combination of two carbon-centred radicals. In both approaches, two electrons form a new two-centre, two-electron bond. In contrast, a route that involves reaction between two positively or between two negatively polarized C atoms is not feasible. Boron is different as it is able to handle an electron deficit, and willingly engages in multiple-centre bonding6. Recently, we reported the synthesis of the doubly-base-stabilized diborane(4) [HB(m-hpp)]2 (hpp ¼ 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) (1), which is easily prepared7–9 by a rare example of an efficient (catalytic) dehydrocoupling reaction (65% yield) in boron chemistry10,11. It features two sp3-hybridized boron atoms and an electron-precise B–B bond. Each hpp unit contributes three electrons to the electron count, so [HB(m-hpp)]2 is isolobal to [B2H6]22 and ethane (Fig. 1a)12. Here we show that hydride abstraction from 1 leads to a boron–boron coupling reaction (Fig. 1b,c).

Results and discussion Hydride abstraction from 1 was achieved with the Lewis acid B(C6F5)3 (refs 13,14). NMR spectra recorded from the reaction mixture clearly demonstrated the formation of the borate anion [HB(C6F5)3]2 and hence signalled successful hydride abstraction. The reaction product crystallized from CH2Cl2 solution. To our surprise, the analytical data (see below) showed that instead of a salt of the monocation 2, a salt of its dimer, the dication [B4H2(m-hpp)4]2þ (3), was formed. Product 3[HB(C6F5)3]2 was obtained in a yield of 78% as a crystalline material, and turned out to be a remarkably stable compound. No decomposition was observed, even in boiling tetrahydrofuran (THF) solution. Moreover, the compound

is air-stable, and also stable in THF solutions saturated with H2O (it is insoluble in pure H2O). For comparison, the carbon analogue, the n-butane dication, can only be generated in the gas phase as a transient species with a very short lifetime by electron impact at an energy of 200 eV (ref. 15). The B–H stretching mode of 3[HB(C6F5)3]2 appears at 2,396 cm21 in the infrared spectrum. The high wavenumber in comparison to the B–H stretching modes, n(B–H), in 1 (2,272 and 2,249 cm21) can be explained by the positive charge. The Raman spectrum of 3[HB(C6F5)3]2 also shows a broad signal centred at 2,400 cm21. In both cases it was not possible to resolve clearly the in-phase (Raman active) and out-of-phase (infrared active) vibrations of the two B–H oscillators (Supplementary Fig. S1). With electrospray ionization (ESIþ) mass spectrometry, two intense signals centred at m/z ratios (m ¼ mass, z ¼ charge number) of 299.2 and 1,111.5 were found (Supplementary Fig. S2), which can be assigned to the dication 3 and the {3[HB(C6F5)3]}þ monocation on the basis of their m/z values and characteristic isotopic patterns. The isotopic pattern for the signal centred at m/z ¼ 299.2 points to the sole presence of the B4 unit and the absence of the monocation 2 (which would give rise to a signal at the same position, but with a distinctly different isotopic pattern; Fig. 2). Three signals, a singlet at a chemical shift of d ¼ 17.56 ppm and doublets at 28.51 and 225.43 ppm, appeared in the 11B NMR spectrum of 3[HB(C6F5)3]2 dissolved in CD2Cl2 (Supplementary Fig. S3 and Table S1). Of these, the doublet at d ¼ 225.43 ppm with coupling constant 1JBH ¼ 90 Hz belongs to [HB(C6F5)3]2. Furthermore, the 19F NMR spectrum showed signals assignable to the borate anion [HB(C6F5)3]2 at d ¼ 2133.93, 2164.53 and 2167.53 ppm. The 11B chemical shifts changed only very slightly to d ¼ 17.59, 28.45 and 225.46 ppm when the compound was dissolved in d8-THF in place of CD2Cl2. With d ¼ 17.37, 28.46 and 225.41 ppm, the 11B chemical shifts were also similar in dimethylsulfoxide (d6-DMSO). Diffusion ordered spectroscopy (DOSY) NMR experiments (carried out with similar results for several concentrations in d8-THF solutions; Supplementary Figs S4–S6) further confirmed the complete absence of the monocation 2 (ref. 16). A diffusion coefficient of 5.9 × 10210 m2 s21 was measured with 1 H NMR, and a similar diffusion coefficient (5.8 × 10210 m2 s21)

Institute of Inorganic Chemistry, Ruprecht-Karls-University Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. * e-mail: [email protected] NATURE CHEMISTRY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturechemistry

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DOI: 10.1038/NCHEM.1776

N

H

N

N

B

B

2–

H

B H

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C H

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H

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H

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b B + B

2+

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+ B

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+ B(C6F5)3 H

H –

[HB(C6F5)3]–

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N

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2

N N

N B

x2

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Figure 1 | Isolobal analogy and reaction sequence leading to 3. a, Isolobal analogy between 1, [B2H6]22 and C2H6. b, Spontaneous B–B coupling of two diborane cations to give a dication with a rhomboid B4 skeleton. The double dashed lines between boron atoms describe the situation whereby two 2e, s-aromatic B3 rings share the short diagonal. c, Hydride abstraction leading to the postulated monocationic intermediate 2, which is isolobal to the ethyl cation, and its dimerization to dication 3.

with 19F NMR. The values are independent of concentration (Supplementary Table S2). For comparison, compound 1 (which is in molecular weight and size similar to 2) had an experimentally determined much higher diffusion coefficient of 1.1 × 1029 m2 s21. Additional DOSY measurements for 3[HB(C6F5)3]2 in solvents of different polarity (d8-THF, CD2Cl2 and d6-DMSO) indicated the presence of solvent-separated ion pairs in solvents of high polarity (DMSO), and contact ion pair formation in solvents of lower polarity (Supplementary Tables S3 and S4)16,17. Most importantly, the DOSY experiments verified the absence of monomer 2 in solution. Furthermore, addition of strong bases (for example, pyridine) to the solution of 3[HB(C6F5)3]2 , which should shift a possible monomer–dimer equilibrium to the monomer side by formation of a donor–acceptor bond, had no measurable effect. Finally, as mentioned above, the 11B NMR chemical shifts are similar in solvents of different polarity (d8-THF, CD2Cl2 or d6-DMSO). Figure 3a presents the structure of dication 3 in the solid state, as determined by X-ray diffraction. There are no directed chemical interactions between the cations and anions in the solid state (and also in solution). All four boron atoms are located in one plane, and form a structure that could be described either as a distorted rhombus or a zigzag chain. With 1.703(4) Å, the central B–B bond is shorter than any other B–B bond in the dication. The B1–B2 bond is already 1.896(3) Å long, and distance B1–B2′ measures 1.949(3) Å. The small value of the B1–B2–B2′ angle (65.31(15)8) directly indicates B–B–B three-centre, two-electron (3c,2e) bonding. The B4 skeleton hosts four electrons. On the basis of the structure, the bonding could be described in terms of two B–B–B (3c,2e) bonds that share two boron atoms. Consequently, the distance between these two boron atoms (B2–B2′ ) is short. Some molecules with B4 skeletons have been prepared 2

previously. In B4[(B(NMe2)2]2(NMe2)2 (4)18, B4H2[C3H3(SiMe3)3]2 (5)19,20 and B4(BF2)4F4 (6)21, the B4 units contain eight, six and four electrons, respectively22. With respect to the electron count in the B4 skeleton, 6 and the dication 3 should be comparable. However, the properties of the two species are very different. Although the B4 unit is planar in 3, it is folded in 6. Moreover, 6 is thermally highly unstable, decomposing rapidly at 273 K. In contrast, the salt 3[HB(C6F5)3]2 is thermally robust, showing no sign of decomposition even in boiling THF solution. Trityl (triphenyl carbocation, [C(C6H5)3]þ) salts lead in similar high yield to the same hydride abstraction reaction and formation of salts of the dication 3. The use of trityl salts offers the possibility of replacing the [HB(C6F5)3]2 anion in an easy way by weakly coordinating anions. Me3Si NMe2 B

Me2N B Me2N

B

SiMe3

H

B

NMe2 B

B

Me3Si

B

B

NMe2

B B

NMe2

SiMe3

H

Me3Si SiMe3

4

5 F2B

BF2 B

F

B F F2B

B

B BF2

F F

6

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DOI: 10.1038/NCHEM.1776

a

299.23263

Exp. for 3 (dication)

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Sim. for 3 (dication)

Sim. for 2 (monocation)

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1,111

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1,115

1,116

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Figure 2 | Measured and simulated high-resolution mass spectrometry (HR-ESI1) peaks of 3[HB(C6F5)3]2 , showing the absence of monocation 2. a, Experimental isotopic pattern for dication 3 (top), and simulated isotopic patterns for both dication 3 (middle) and monocation 2 (bottom). The experimental and simulated patterns match for 3. b, Experimental (top) and simulated (bottom) isotopic patterns for cation {3[HB(C6F5)3]}þ, which are in good agreement and indicate contact ion pair formation.

To complement our experiments, quantum-chemical calculations (B3LYP/TZVP) were carried out for an isolated dication 3. The calculated energy minimum structure turned out to be in good agreement with the experimentally obtained structure (see Supplementary Fig. S7 and a comparison of some structural parameters in Supplementary Table S5). Figure 3b illustrates the calculated electron density distribution. Bond critical points (marked by circles) are points in space at which the gradient of the density vanishes and that are maxima in two directions of space and minima in the third direction. The pair of trajectories that start at the bond critical point and end at two different atoms defines the bond path. This is the line of maximal electron density between two connected atoms. The electron density distribution clearly confirms the presence of two B– B–B (3c,2e) bonds, sharing two boron atoms and therefore explaining the short central B–B bond distance. The bond path is highly curved (as is typical for three-centre bonding), leading to a situation in

which the bonding is close to changing to a T-shaped bonding23. The calculated 11B NMR chemical shifts (d ¼ 28.2 ppm for the boron bonded to hydrogen (B1) and d ¼ 16.5 ppm for B2) are in pleasing agreement with the experimental shifts of 28.51 and 17.56 ppm (Supplementary Table S1). The calculated chemical shifts for the monocation 2 (d ¼ 25.9 and 59.2 ppm) are significantly different, arguing again for the absence of the monocation in solution. The calculations predict the energy difference between the Raman-active inphase or symmetric (calculated at 2,524.8 cm21, unscaled value) and the infrared-active out-of-phase or asymmetric (calculated at 2,524.4 cm21, unscaled value) B–H stretching mode to be very small, in line with the experimental results. The gas-phase dimerization of two equivalents of the boron monocation 2 to dication 3 was calculated to be mildly exothermic with the BP (Becke, Perdew) functional in combination with the SVP (split-valence plus polarization) basis set (DH ¼ 22 kJ mol21 at 1 bar, 298 K), but endothermic with the

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DOI: 10.1038/NCHEM.1776

b

B2'

B1

N4

N1 B2'

B1

H H

H

H B1'

B2 N2

N5

B2

B1'

Figure 3 | Structure of tetraborane dication 3 in the salt 3[HB(C6F5)3]2 , as revealed by single-crystal XRD analysis and calculated charge density of 3. a, Structure of dication 3 from XRD analysis (hydrogen bound to carbon, as well as the two anions [HB(C6F5)3]2, omitted for clarity). Vibrational ellipsoids are drawn at the 50% probability level. Selected bond distances (in Å) and angles (in deg) in the B4 core: B1–B2, 1.896(3); B1–B2′ , 1.949(3); B2–B2′ , 1.703(4); B1–B2–B2′ , 65.31(15). The former B–B bond in 1 becomes the longest B–B bond (B1–B2’) in the product (see Supplementary Table S5 for more structural details). b, Topology of the electron density distribution of 3 (B3LYP/TZVP). Lines were drawn at 0.108, 0.216, 0.324, 0.432, 0.540, 0.648, 0.756, 0.864, 1.080, 1.296, 2.024, 3.037, 3.239, 6.411, 6.748, 16.871 and 33.742 eÅ23. Circles mark bond critical points. Electron density (average, in eÅ23) at these points: 1.188 (B–H), 0.884 (B2–B2′ ), 0.729 (B1–B2–B2′ ). The bond paths connecting the critical points with two atoms are also shown. The electron density distribution confirms multiple-centre bonding in the B4 core.

B3LYP (Becke, three-parameter, Lee–Yang–Parr) functional together with a TZVP (triple-zeta valence plus polarization) basis set (DH ¼ þ46 kJ mol21 at 1 bar, 298 K). It can be assumed that solvent effects and ion pairing favour the dimerization. It is unlikely that B–B coupling occurs in one step together with hydride abstraction, because such a process would involve four (sterically demanding) molecules at the same time. We cannot exclude a mechanism in which cation 2 first inserts into the B–H bond of a neutral molecule 1 followed by another hydride abstraction. According to quantum-chemical calculations, the product of such an insertion reaction (Supplementary Fig. S8 and Table S6) is [HB(m-hpp)2(m-H)B-B(m-hpp)2BH]þ, with one hydrogen adopting a bridging position between two boron atoms (similar to the structure of protonated 1, see ref. 8). The insertion is exothermic (calculated DH 0 at 298 K (BP/SVP) of 2108 kJ mol21). However, we obtained no experimental evidence for such a species (calculated 11B NMR shifts of d ¼ 0.50, 20.01, 11.94 and 6.04 ppm), even in experiments in which 1 was reacted with B(C6F5)3 in a 2:1 molar ratio at low temperatures. As already discussed, an equilibrium between the monocation and the dication can be excluded on the basis of the experimental results and also the comparison between calculated and experimental spectroscopic properties. Finally the structure of the hypothetical dication [B4H6(NH3)4]2þ was calculated to analyse the influence of the hpp bridges on the structure. The energy minimum structures of [B4H6(NH3)4]2þ and 3 turned out to be very similar (Supplementary Fig. S9 and Table S7). Hence, it could be expected that other double-base-stabilized diborane(4) species such as the diphosphino-stabilized species B2H2L2 (L ¼ PMe3 or L2 ¼ Me2PCH2PMe2)24 might give similar products. New dication 3 might exhibit unusual reactivity. Cationic boron compounds are interesting for several applications, especially in the field of catalysis25. Recently, the first monomeric boron compound in formal oxidation state þII, radical cation [HB(caac)2]†þ (7) (where the boron is three-coordinate, and caac is a special cyclic (alkyl)(amino)carbine shown as part of the structure of 7, in which dipp is 2,6-diisopropylphenyl), was reported26. Fully characterized dicationic BIII compounds are restricted to only a few 4

compounds27,28. So far, only one dicationic BII compound has been reported, namely the dinuclear compound [B2(m-hpp)2(NHMe2)2]2þ (8), in which both boron atoms are four-coordinate, and which could therefore be classified as a bis-boronium dication29. Dication 3 is the first example of a dicationic tetranuclear BII compound.

Dipp N

H B

N Dipp N Me2HN

N

N

B

B

N

N

NHMe2

N 8

7 L R R

B L

Boronium cation

There is an even more interesting possible application. The new compound might represent an ideal precursor for extended boron chain compounds and polymers with a structure different to polyolefins. It is not attached to a metal, is air-stable and is thermally extremely robust. The strongly bound guanidinate bridges prevent the chains from rearrangement into clusters. The reactive B–H functions are located at both ends of the compound, and the growth of the chains might be controllable by stepwise deprotonation. Catenation of boron compounds was attempted for a long time (in uncontrolled reductive coupling experiments B–B coupling products were obtained in poor yield and poor control of the geometry), culminating in the recently reported synthesis of a B4 chain compound coordinated to a transition-metal centre30,31. A prediction of the structure of a possible chain compound formed from the precursor presented in this work could be made on the basis of

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considerations made by Balakrishnarajan and Hoffmann22. They pointed out that rhomboid rings are not only found in boron chemistry, but also in silicon chemistry22. Hence, b-SiB3 (a semiconducting material) is built of layers of linked B12 clusters and chains of linked rhombic Si4 units. All silicon atoms are bound to boron atoms of adjacent B12 units. They identified [Si4H8F2]2þ as a molecular equivalent, and showed by calculations that this molecule also exhibits a rhomboid Si4 skeleton in its global energy minimum structure. The [Si4H8F2]2þ dication is isolobal to the dianion [B4H10]22 (as already noticed by Balakrishnarajan and Hoffmann) and also to the new dication 3. By successive two-electron reduction and hydride abstraction (or directly by deprotonation) one might achieve a step-by-step growth of oligomomeric {[B4(m-hpp)4]nH2}2þ chains on the way to the infinite neutral polymer [B4(m-hpp)4]1 , with a structure similar to the silicon chains in the semiconducting b-SiB3. In preliminary studies, we calculated the possible structure of the neutral B4(m-hpp)4 and also the deprotonation intermediate [B4H(m-hpp)4]þ. These calculations indeed argue for a rhombic structure of the B4 core in B4(m-hpp)4 (Supplementary Fig. S10 and Table S8), in analogy to the [Si4H8F2]2þ structure. The calculated absolute gas-phase proton affinities of B4(hpp)4 and [HB4(hpp)4]þ are very different for B4(hpp)4 , but 939 kJ mol21 for (1,456 kJ mol21 þ [HB4(hpp)4] ). As anticipated, abstraction of the first proton of 3 is much easier than abstraction of the second proton. For comparison, the absolute gas-phase proton affinity of pyridine was calculated to be 928 kJ mol21 and therefore the (gas-phase) basicity of pyridine (py) and [HB4(hpp)4]þ are comparable. Of course, the gas-phase proton affinities differ from the proton affinities in solution. Nevertheless, the results suggest that bases stronger than pyridine should be used in the deprotonation experiments with 3. Dication 3 should exhibit a rich and interesting chemistry, which is the topic of ongoing and future work.

Methods Synthetic work. The reactions were carried out under a dry argon atmosphere using standard Schlenk techniques. All solvents were rigorously dried by applying standard procedures and stored over molecular sieves (4 Å) before use. The synthesis of 1 was accomplished as reported previously8–10. B(C6F5)3 was delivered from Strem Chemicals, purified by sublimation, and stored in a glovebox (MBraun LABmaster dp, MB-20-G) under argon. Infrared spectra were recorded on a BIORAD Excalibur FTS 3000. A BRUKER Avance II 400 or BRUKER Avance III 600 machine was used for NMR spectroscopy. Elemental analyses were carried out at the Microanalytical Laboratory of the University of Heidelberg. ESI mass spectra were obtained on a Bruker ApexQe FT-ICR instrument. Synthesis of 3[HB(C6F5)3]2. A solution of B(C6F5)3 (347 mg, 0.68 mmol) in toluene (2 ml) was added to a solution of 1 (186 mg, 0.62 mmol) in toluene (10 ml). After stirring at room temperature for 1 h, the solvent was removed and the residue was washed several times with toluene (6 ml) to give, after recrystallization from dichloromethane, colourless crystals suitable for X-ray diffraction in 78% yield (0.39 g, 0.24 mmol). C, H, N analysis (%) for C64H52B6F30N12 (1,624.00 g mol21): calcd: C 47.33, H 3.23, N 10.35; found: C 47.67, H 3.41, N 10.36; 1H NMR (400 MHz, CD2Cl2): d ¼ 3.37–3.10 (m, 32H, CH2-N), 2.02–1.91 (m, 8H, CH2-N), 1.89–1.75 (m, 8H, CH2) ppm; 13C{1H} NMR (100 MHz, CD2Cl2): d ¼ 155.53 (4C, Cq), 128.62 (12C, o-C6F5), 127.81 (6C, p-C6F5), 124.88 (12C, m-C6F5), 47.08 (4C, CH2-N), 46.20 (4C, CH2-N), 44.44 (4C, CH2-N), 42.56 (4C, CH2-N), 21.75 (4C, CH2), 21.18 (4C, CH2) ppm; 11B NMR (128 MHz, CD2Cl2): d ¼ 17.56 (s, 2B, B), 28.51 (d, 1J(B,H) ¼ 84 Hz, 2B, BH), 225.43 (d, 1J(B,H) ¼ 90 Hz, 2B, BH) ppm; 19 F NMR (376 MHz, CD2Cl2): d ¼ 2133.93 (d, 12F, o-F), 2164.53 (t, 6F, p-F), 2167.53 (dt, 12F, m-F) ppm; IR (KBr): n ¼ 2,968 (m) (C–H val.), 2,871 (m) (C–H val.), 2,396 (m) (B–H val.), 2,224 (w), 2,026 (w), 1,609 (s) (C ¼ N val.), 1,569 (s) (C ¼ N val.), 1,509 (s), 1,462 (s), 1,403 (m), 1,373 (m), 1,323 (s), 1,273 (s), 1,236 (s), 1,187 (m), 1,104 (s) cm21; MS (ESIþ): m/z ¼ 299.2 (26%) [C28H50B4N12]2þ, 1,111.5 (100%) [C46H51B5F15N12]þ; MS (ESI2): m/z ¼ 513.0 (100%) [C18HBF15]2. Crystal data for [B4H2(m-hpp)4][HB(C6F5)3]2.3CH2Cl2: C67H58B6Cl6F30N12 , Mr ¼ 1,878.81, 0.45 × 0.35 × 0.20 mm3, triclinic, space group P 2 1, a ¼ 11.293(2), b ¼ 12.225(2), c ¼ 14.880(3) Å, a ¼ 95.68(3)8, b ¼ 97.34(3)8, g ¼ 111.40(3)8, V ¼ 1,873.30 Å3, Z ¼ 1, rcalcd ¼ 1.665 Mg m23, Mo-Ka radiation (graphitemonochromated, l ¼ 0.71073 Å), T ¼ 100 K, urange ¼ 4.2–55.98. Reflections measured 16,654, independent 8,890, Rint ¼ 0.0278. Final R indices [I . 2s(I)]: R1 ¼ 0.0527, wR2 ¼ 0.1444.

Quantum-chemical calculations. Density functional theory (DFT) calculations were carried out with the TURBOMOLE32 program using the BP or B3LYP functionals in combination with either SVP or TZVP basis sets. The program Multiwfn 2.6.133 was used to calculate the topology of the electron density distribution for 3. X-ray crystallographic study. Suitable crystals of 3[HB(C6F5)3]2 were taken directly out of the mother liquor, immersed in perfluorinated polyether oil, and fixed on top of a glass capillary. Measurements were made on a Nonius-Kappa chargecoupled device diffractometer with a low-temperature unit using graphitemonochromated Mo-Ka radiation. The temperature was set at 100 K. The data collected were processed using standard Nonius software34. All calculations were performed using the SHELXT-PLUS software package. Structures were solved by direct methods using the SHELXS-97 program and refined with the SHELXL-97 program35–37. Graphical handling of the structural data during solution and refinement was performed with XPMA38. Atomic coordinates and anisotropic thermal parameters of non-hydrogen atoms were refined by full-matrix least-squares calculations. Crystallographic data (excluding structure factors) for the structure reported in this Article have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 927270, and can be obtained free of charge.

Received 3 April 2013; accepted 4 September 2013; published online 13 October 2013

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Acknowledgements The authors thank W. Siebert for discussions and the Deutsche Forschungsgemeinschaft (DFG) for continuous financial support.

Author contributions H-J.H. conceived and supervised the study. S.L. performed the syntheses and the computational experiments. E.K. performed the X-ray crystallographic measurements. M.E. performed the DOSY-NMR measurements. S.L. and M.E. analysed the data and co-wrote the paper.

Additional information Supplementary information and chemical compound information are available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to H.J.H.

Competing financial interests The authors declare no competing financial interests.

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