PAPER www.rsc.org/dalton | Dalton Transactions

The nature of the chlorination reaction in [1-C6H5-1-CB9H9]- boron clusters†

Pau Farras,a Clara Vinas,a Reijo Sillanpaa,b Francesc Teixidor*a and Milagros Reya

Received 6th May 2010, Accepted 21st May 2010First published as an Advance Article on the web 21st July 2010DOI: 10.1039/c0dt00433b

Preferential chlorination sites resulting from sequential radical substitution reactions in carboraneanions have been studied combining experimental and computational methods. Results have beenobtained experimentally by mixing the substrate with incremental ratios of N-chlorosuccinimide andanalysing the resulting samples by negative MALDI-TOF-MS. The theoretical results have beenobtained calculating the 2a-NPA charges on the starting material and computing the most energeticallyfavourable reaction pathway.

1. Introduction

closo-Carborane anions are finding applications in ionic liquids,1

in catalysis,2 metathesis, oxidation chemistry as well as in sta-bilizing highly reactive cations,3 strong Brønsted acids4 and asdoping agents.5 Halogenated derivatives have attracted particularinterest because of their robustness that make them suitable forapplications. However, despite their relevance little is knownabout the halogenation process: how many halogen atoms canbe introduced?, is the process dominated by thermodynamic orkinetic factors?, is it radical or ionic? In this paper we will refer inparticular to chlorine substituents as these have, excluding B–F,the largest B–X bond enthalpies,6 and again with the exceptionof fluorine, are the least polarizable and reactive of the halogensproviding the highest protection against degradation.7 The mostinvestigated monoanionic closo deltahedral clusters are [CB11H12]-,[CB9H10]- and [3,3¢-Co(1,2-C2B9H11)2]-, they are also the onesmore readily available. Electrophilic radical fluorination of [1-CB9H9]- was reported years ago.8 Recently, the stepwise redoxpotential modulation of [3,3¢-Co(1,2-C2B9H11)2]- by chlorinationhas been reported by us.9 For this work, we focused on [CB9H10]-

because there are penta- and hexacoordinated boron atoms in themolecule and because there existed 6 crystal structures available onCSD of halogenated derivatives of [1-C6H5-1-CB9H9]- [1]-,10 andin particular [1-(4¢-BrC6H4)-1-CB9Br5H4]-.10a The latter resultsfrom the bromination of [1]-, and has the aryl ring brominatedbefore the cluster C adjacent boron atoms suffered bromination.Another reason to study [1]- was that it has in its molecularstructure two fragments of different reactivity, the aryl ring andthe boron cluster. Therefore a reliable theory that explained thehalogenation had to account for the order of halogenation in thesetwo fragments of such great different reactivity.

In this study, we combine experimental work with calculationsto cover the former questions. We show here that all of the B–H positions can be substituted, that there are preferential sites,and that the latter are not the result of thermodynamics butkinetics. Also, our studies evidence the radical nature of the

aInstitut de Ciencia de Materials de Barcelona, Campus U.A.B., 08193,Bellaterra, SpainbDepartment of Chemistry. University of Jyvaskyla, FIN-40014, Finland† On the occasion of the 70th anniversary of Prof. B. Stibr for hisoutstanding contribution to the development of boron cluster chemistry.

chlorination reaction. This contradicts earlier reports and appearsto be opposite to aromatic halogen substitution.4e

2. Experimental section

General details

All carborane anions prepared are air and moisture stable; how-ever, some reagents used are moisture-sensitive. Therefore, Schlenkand high-vacuum techniques were employed whenever necessary.N-Chlorosuccinimide (NCS), TEMPO and VAZO R© catalyst 88were purchased from Aldrich and used as received. K[1-C6H5-1-CB9H9] was prepared according to literature methods.11 IR spectrawere recorded from KBr pellets on a Shimadzu FTIR-8300 spec-trophotometer. The mass spectra were recorded in the negative ionmode using Bruker Biflex MALDI-TOF-MS [N2 laser; lexc 337 nm(0.5 ns pulses); voltage ion source 20.00 kV. The 1H, 1H{11B}-NMR(300.13 MHz), 11B-NMR (96.29 MHz) and 13C{1H}-NMR (75.47MHz) spectra were recorded on a Bruker ARX 300 spectrometer.All NMR spectra were recorded from acetone-d6 solutions at25 ◦C. Chemical shift values for 11B-NMR spectra were referencedto external BF3·OEt2, and those for 1H, 1H{11B}, and 13C{1H}-NMR spectra were referenced to Si(CH3)4. Chemical shifts arereported in units of parts per million downfield from the reference,and all coupling constants are reported in Hertz.

General synthetic procedure for the preparation of [1-C6H5-1-CB9H9]- chlorinated derivatives

K[1-C6H5-1-CB9H9] (20 mg, 0.085 mmol) and the correspondingmolar quantity of N-chlorosuccinimide (NCS), were crushedtogether in a glove box until a homogeneous powder was obtained.A thick-walled Pyrex tube (20 cm long, 5 mm id and 8 mm od)was charged with the powder. The lower part of the tube wasthen cooled with liquid N2, evacuated and sealed under vacuumwith an energetic flame. Afterwards, the tube was placed inside aprotective iron cylinder and put inside a preheated tubular furnace.The temperature of the furnace was maintained for 2 h at 194 ±6 ◦C. The protective cylinder containing the tube was then carefullyremoved and the whole cooled down to room temperature. Thetube was then opened and THF was added in order to performthe characterization by negative MALDI-TOF-MS. Molar K[1-C6H5-1-CB9H9] to NCS ratios utilized are indicated in Table 1.

7684 | Dalton Trans., 2010, 39, 7684–7691 This journal is © The Royal Society of Chemistry 2010

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Table 1 MALDI-TOF-MS compositions after the reaction of [1-C6H5-1-CB9H9]- and NCS. ry represents the initial molar ratio of [1-C6H5-1-CB9H9]- to NCS from 1 : 1 to 1 : 20. Experiments were done at 194 ± 6 ◦Cfor 2 h. In the column headed by y, the species observed by MALDI-TOF-MS are indicated in bold numbers, indicative of the number ofinserted chlorine atoms. In parenthesis is indicated the molar fractionof the different chlorinated species

ry y

1 0 (0.13), 1 (0.67), 2 (0.20)2 0 (0.05), 1 (0.14), 2 (0.41), 3 (0.34), 4 (0.05)3 2 (0.10), 3 (0.41), 4 (0.39), 5 (0.10)4 3 (0.20), 4 (0.34), 5 (0.29), 6 (0.17)5 3 (0.02), 4 (0.32), 5 (0.40), 6 (0.26)6 6 (0.90), 7 (0.10)7 6 (0.63), 7 (0.37)8 5 (0.04), 6 (0.46), 7 (0.32), 8 (0.14), 9 (0.03)9 6 (0.24), 7 (0.47), 8 (0.25), 9 (0.04)10 6 (0.20), 7 (0.46), 8 (0.28), 9 (0.06)11 6 (0.14), 7 (0.36), 8 (0.33), 9 (0.17), 10 (0.02)12 6 (0.11), 7 (0.33), 8 (0.34), 9 (0.17), 10 (0.04)13 6 (0.05), 7 (0.34), 8 (0.37), 9 (0.19), 10 (0.04)14 6 (0.02), 7 (0.24), 8 (0.36), 9 (0.27), 10 (0.11)15 6 (0.01), 7 (0.22), 8 (0.35), 9 (0.28), 10 (0.11), 11 (0.02)16 6 (0.02), 7 (0.24), 8 (0.36), 9 (0.24), 10 (0.08), 11 (0.03), 12 (0.01)17 6 (0.02), 7 (0.21), 8 (0.36), 9 (0.26), 10 (0.10), 11 (0.03), 12 (0.01)18 6 (0.02), 7 (0.19), 8 (0.30), 9 (0.26), 10 (0.14), 11 (0.04), 12 (0.03),

13 (0.02)19 6 (0.01), 7 (0.19), 8 (0.29), 9 (0.27), 10 (0.13), 11 (0.05), 12 (0.04),

13 (0.02), 14 (0.01)20 7 (0.16), 8 (0.35), 9 (0.34), 10 (0.10), 11 (0.03), 12 (0.02), 13 (0.01),

14 (0.01)

Synthesis of [N(CH3)4][1-(4¢-Cl–C6H4)-6,7,8,9,10-Cl5-1-CB9H4]

K[1-C6H5-1-CB9H9] (20 mg, 0.085 mmol) and NCS (68.10 mg,0.51 mmol) were crushed with a pestle and mortar in a glove boxuntil an homogeneous powder was obtained. The mixture wastransferred to a Pyrex tube and the general procedure was followedas described above. Diethyl ether was added and succinimidewas removed by filtration. Then the diethyl ether was removedunder vacuum leaving a solid, which was treated with excess ofan aqueous solution of [N(CH3)4]Cl to yield a white powder. Thewhite precipitate was filtered off, washed with distilled water, anddried under vacuum. Yield: 34.11 mg (91%). IR: n/cm-1 = 3580(B–H), 3020 (Caryl–H), 817,6 (C–Cl). 1H NMR, d : 7.78 (d, 3J(H,H)= 8.4, 2H, Caryl–H), 7.47 (d, 3J(H,H) = 8.4, 2H, Caryl–H). 1H{11B}NMR, d : 7.78 (d, 3J(H,H) = 8.4, 2H, Caryl–H), 7.47 (d, 3J(H,H) =8.4, 2H, Caryl–H), 2.31 (br s, 4B, B–H). 13C{1H} NMR, d : 131.24(br s, p-C(aryl)–Cl), 129.48 (br s, C(aryl)), 128.89 (br s, C(aryl)), 128.19(br s, C(aryl)). 11B NMR, d : 26.23 (s, 1B, B(10)), -4.72 (s, 4B, B(6–9)–Cl), -8.06 (d, 1J(B–H) = 162.1, 4B, B(2–5)). MALDI-TOF-MSm/z: calcd for C7H8B9Cl6 400.15; found 400.8.

X-ray crystallographic study of Cs[1-(4¢-ClC6H4)-6,7,8,9,10-Cl5-1-CB9H4]

C7H8B9Cl6Cs, Mr = 535.03, colorless plate, 0.28 ¥ 0.24 ¥ 0.12 mm,tetragonal, space group P4/n, a = b = 7.7582(6) A, c = 16.4850(15)A, V = 992.23(14) A3, Z = 2, Dc = 1.791 gcm-3, T = 173 K withMo-Ka (0.71073 A), m(Mo-Ka) = 2.659 mm-1, F(000) = 504, qmax

= 27.00◦, 6484 reflections, 1093 independent (Rint = 0.0622), R1 =0.0623, wR2 = 0.1277 for 65 parameters and 745 reflections withI > 2s(I). CCDC 729737.

The crystals were obtained in dichloromethane–acetonitrile.Crystallographic data were collected at 173 K with a Nonius-Kappa CCD area detector diffractometer, using graphite-monochromatized Mo-Ka radiation (l = 0.71073 A). The datasets were corrected for absorption using SADABS12a The structurewas solved by direct methods by use of the SHELXS-97 program12b

in the space group P4/n (in P4/nmm the structure did not form asensible structure). The full-matrix, least-squares refinements onF 2 were performed using SHELXL-97 program.12b The CH andBH hydrogen atoms were included at fixed distances with the fixeddisplacement parameters from their host atoms.

Computational details

Calculations were performed with the Gaussian 98/03 suites ofprograms.13 Geometries of [1-C6H5-1-CB9H9]- and their chlori-nated derivatives were fully optimized and NPA (natural pop-ulation analysis) charges14 calculated at the B3LYP/6-31+G*level of theory,15 as well as their thermochemical properties.All stationary points were found to be true minima (numberof imaginary frequencies, Nimag = 0). Thermodynamic quantityH0 (at 298 K) was calculated using standard procedures takinginto account zero-point energies, finite temperature (0 to 298 K)correction and the pressure–volume work term pV . Calculationson the mechanism for both ground and excited states were doneusing B3LYP/3-21G* level of theory. The potential minima arecharacterized by all positive frequencies and the transition statesare characterized by a single imaginary frequency.

Thermodynamic calculations

Thermodynamic details can be obtained by using a modificationof the group increment method described by Schleyer and co.16

Taking this procedure as a model, we have examined the energeticrelationships between two consecutive chlorinated [1-C6H5-1-CB9H9]- anions. The equation employed, namely eqn (1), isbalanced with regard to the negative charges.

[ ] [ ]Cl H - -C - -CB Cl Cl H - -C - -CB HCy yH

y y14 6 9 2 1 13 6 91 1 1 1−−

+ −−+ ⎯ →⎯ +D ll

(1)

The reaction energies of each step, DH, and the cumulative totalenergy, DHadd, (based on [1-C6H5-1-CB9H9]- as the reference zero)are shown in Table 2. In Fig. 1, DHadd, are plotted as a function ofnumber of chlorinated vertexes.

3. Results and discussion

To learn from the preferential halogenation numbers and thepreferential halogenation sites, we planned experiments eachleading to the synthesis of a set of compounds that couldeasily be quantified. For the quantification, we have used theMALDI-TOF-MS technique that is quantitative for mixtures ofmonoanionic boron clusters (Fig. 2).17 Recognizing that MALDI-TOF-MS determines only elemental composition, we refer to eachpeak in the MS corresponding to a halogenated derivative of[1]- as a ‘compound’ as it may consist of positional isomers. Wehave done the synthesis of mixtures of chlorinated derivatives of[1]- by reacting it with N-chlorosuccinimide (NCS) in the solidstate at 194 ± 6 ◦C. The experimental results for the reaction of

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Fig. 1 Plot based on DHadd (cumulative B–Cl addition energy in kcal mol-1 from Table 2) for the [ClyH14-y-1-C6-1-CB9]- (for which y = 0–14), vs. numberof chlorine atoms (y).

Fig. 2 MALDI-TOF-MS spectra of a mixture obtained by the reaction of NCS/[1]- = 1.

Table 2 Data for anions [ClyH14-y-1-C6-1-CB9]- (for which y = 0–14);total energies in au; zero-point energies (ZPE, in kcal mol-1); reactionenergies from eqn (1) (DH, in kcal mol-1); cumulative B–Cl additionenergies (DHadd, in kcal mol-1)

y B3LYP/6-31+G* ZPE DH DHadd

0 -288428.703 139.28 0.00 0.001 -288426.620 134.70 -59.20 -59.202 -288426.578 129.84 -56.71 -115.913 -288424.227 125.03 -56.15 -172.074 -288424.209 120.17 -54.53 -226.605 -288411.155 116.46 -53.15 -279.756 -288434.700 110.26 -54.53 -334.287 -288421.939 105.19 -52.39 -386.678 -288420.863 100.14 -52.19 -438.869 -288398.236 95.14 -51.11 -489.9710 -3197283.171 89.08 -29.64 -519.6111 -3485782.255 82.93 -25.18 -544.7912 -3774175.107 76.80 -24.37 -569.1613 -4062554.729 69.99 -6.55 -575.7014 -4350925.984 63.97 -8.10 -583.80

NCS with [1]- are summarized in Table 1. The ratio of reagentsutilized is shown in column ry, so ry = 10 means a ratio ofNCS/[1]- = 10 and ry = 19 a ratio of NCS/[1]- = 19. Column yindicates the compounds observed in each reaction. In parenthesisis indicated the molar fraction of the different chlorinated species.Each number indicates the number of Cl on [1]-. So 7 meansthat 7 H in [1-C6H5-1-CB9H9]- have been substituted by 7 Cl.Alternatively, it could be named [Cl7-1]-.

It is to be seen in Table 1 that from r1 to r6 the dominantchlorinated product parallels ry. e.g. at a ratio NCS/[1]- = 5the major product is [Cl5-1]-. The observed discontinuity from[Cl6-1]- to [Cl7-1]-, that breaks the correspondence between ry

and the major compound in y, and that requires a reagent ratioNCS/[1]- equal to 9 to produce [Cl7-1]- as the major productsuggests that the 7th chlorination will take place on a differentplane in the framework of [1]- than the former substitutions.Conversely it implies that [Cl6-1]- has completely filled a set ofequivalent positions in pristine [1]-. Inspecting the structure of

7686 | Dalton Trans., 2010, 39, 7684–7691 This journal is © The Royal Society of Chemistry 2010

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[1]- the only way to fill six positions having a 7th in another setof non-equivalent positions is by filling the platform as in thecrystal structure of [Cl6-1]- shown in Fig. 3a. The discontinuityfrom [Cl7-1]- to [Cl8-1]- is even more pronounced than this from[Cl6-1]- to [Cl7-1]-. [Cl8-1]- is produced as the major productfor ry = 12, thereafter it is dominant. This implies that [Cl6-1]-

and [Cl8-1]- are very stable compounds that very likely could beobtained almost pure directly from reaction. To prove this, weconducted reactions at slightly different conditions that permitted[Cl6-1]- to be obtained pure enough to get crystals as its Cs(I) salt.Fig. 3a shows the structure of the anion. Therefore albeit [Cl14-1]-, corresponding to perchlorination, has been achieved, somepreferential numbers of chlorination exist, [Cl6-1]- and [Cl8-1]-,the latter shown in Fig. 4.

Fig. 3 (a) The structure of [1-(4¢-ClC6H4)-6,7,8,9,10-Cl5-1-CB9H4]- an-ion. The phenyl group is disordered over two positions in a 1 : 1 ratio.Selected bond lengths [A]: Cl1–B3 1.810(8), Cl2–B4 1.80(2), Cl3–C81.73(1), C1–B2 1.580(14). Symmetry codes: i = -x+ 1

2, -y+ 1

2, z, ii = -y+ 1

2,

x, z, iii = y, -x+ 12, z. (b) A coordination sphere of Cs+ cation. The Cs–Cl1

distances are 3.635(2) A and Cs–Cl2 distances are 3.885(4) A.

Fig. 4 Computed structure of [Cl8-1]-anion at B3LYP/6-31G*.

The diatomic bond enthalpies for B–Cl, B–H, C–Cl and C–Hare 535.04, 338.58, 397.1 ± 29.26 and 338.58 kJ mol-1, respectively.6

Therefore, total substitution of B–H by B–Cl and of C–H by C–Clshould produce a net enthalpy gain in [1]-. If only thermodynamicconsiderations are contemplated it would be expected that theobserved chlorination regioselectivity in [1]- would imply largerenthalpy gains in the more favoured cluster sites and the fullchlorination of the boron cluster, in preference to the aromaticring. The plot of DHadd computed at the DFT level (Fig. 1 andTable 2) vs. number of added Cl on B, according to eqn (1), is astraight line with no singular points.

The slope is altered when substitution takes place on Caryl. Thus,there is no site related thermodynamic stabilization and, givensuitable conditions, all B–H and Caryl–H could be chlorinated.Indeed, perchlorination of the simpler [1-CB9H10]- has beendemonstrated by Xie et al.,18 and in the former experiments where[Cl14-1]- has been obtained.

Despite experimental evidence given above that preferentialchlorination sites exist in [1]-, it is unlikely that adequate protocolsof synthesis for the highly chlorinated derivatives, except perhapsfor [Cl14-1]- can be found. This situation is not unique, andin a sense is reminiscent of electrophilic aromatic substitutionwhere the mixtures are likewise generated for kinetic reasons. Theproportions of ortho-, meta- and para-isomers in a substitutedbenzene reflect the relative rates at each of these sites and thedetermining step is related to the accumulation of negative chargeon the aromatic ring induced by the substituent. To interpret theformer experimental results, we studied the charges on [1]-. TheNPA charges on B and Caryl atoms shown in Fig. 5a suggest thatelectrophilic substitution would take place first on p-Caryl, then onm-Caryl on the benzene ring. As shown by the NCS chlorinationexperiments, and by the crystal structures available, the NPAcharges do not provide the correct order of halogenation. A precisecorrelation is obtained, however, by considering the two atomsNatural Population Analysis method (2a-NPA) charges on B–Hor Caryl–H shown in Fig. 5b (where 2a-NPA charges are defined asthe sum of the NPA charges of the two bonded atoms). Accordingto the 2a-NPA, the sequence of substitution would be first onB(10), next on B(6–9), then on p-Caryl and then on m-Caryl. Ourexperimental evidence ends at this point, but we could predict

Fig. 5 (a) NPA charges on boron and carbon calculated in this workfor [1-C6H5-1-CB9H9]-, displaying the numbering of the boron atoms.(b) 2a-NPA charges on B–H and C–H bonds calculated in this work for[1-C6H5-1-CB9H9]- at the B3LYP/6-31+G* level of theory.

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from the 2a-NPA that next step would be on o-Caryl to finalize withchlorination at B(2–5). To show the consistency of the method,NPA and 2a-NPA charges have been calculated for [Cl5-1]-. Thesubsequent halogenations for the latter would, according to 2a-NPA charges, follow the same sequence as for [1]-. Details for[Cl5-1]- are given in Fig. 6.

Fig. 6 2a-NPA charges on B–H and C–H bonds calculated in this work for[1-C6H5-6,7,8,9,10-Cl5-1-CB9H4]- at the B3LYP/6-31+G* level of theory.

It is known experimentally that B(6) is the first position to bechlorinated in [1]- despite 2a-NPA calculations indicating thatB(10) should be the first targeted boron. The energy differencebetween isomers [6-Cl-1-C6H5-1-CB9H8]- and [10-Cl-1-C6H5-1-CB9H8]- in the ground state, calculated at the B3LYP/6-31+G*level of theory is low: the 10-Cl isomer is 1.4 kcal mol-1 morestable than the 6-Cl isomer. The discrepancy between experimentand theory could be due to the simplicity of the computationalmethod, as it only refers to the fundamental state of the molecule.To get a more precise view of the first step of chlorination, we needto learn more about the transition state and to calculate activationenergy barriers. Thus, following atom relaxation on a transitionstate calculation it is found that Cl+ has a preference for B(6),while the former H(6) has moved closer to the aryl ring in anadjacent triangular face of the cluster. However, in the search fora transition state derived from the interaction of Cl+ and [1-C6H5-1-CB9H9]- assuming that the attack takes place first on B(6)–H,unusual high energy barriers of 283.2 kcal mol-1 were calculated.In Fig. 7, the best calculated pathway for the monochlorinationof [1]- is shown. These high energy barriers would prevent thereaction happening, but experiments show that chlorination doestake place, therefore the reaction is not electrophilic. Then what isit? We considered the possibility that it could be a radical reactionas had been suggested earlier.19

Fukui indexes,20 used to predict the susceptibility of a certainposition to be attacked by a nucleophile, an electrophile or aradical, were calculated for [1]-. The results are found in Table 3,but the most important point is that in B(6) we find that the radicalindex is 0.05 higher than the electrophilic one, suggesting that,actually, the reaction could be of a radical origin. The mechanism,shown in Fig. 8, was then recalculated as a radical attack to theboron cluster. The simple anion [1-CB9H10]- was taken as a modelto simplify the calculations. By replacing Cl+ by Cl∑ in the reactionmechanism, the activation energy of the reaction has becomereasonable with a value of 6.21 kcal mol-1. The transition stateis very similar to that found in the electrophilic pathway in which

Table 3 Fukui indexes for boron and carbon vertices calculated by usingthe equations found in the literature.20

Atom Nucleophilic Electrophilic Radical HOMO charges

C(1) -0.0702 -0.0746 0.2911 -0.16399B(2) -0.0636 -0.0147 -0.0703 -0.03435B(3) -0.0627 -0.0147 -0.0698 -0.03424B(4) -0.0636 -0.0147 -0.0703 -0.03436B(5) -0.0627 -0.0147 -0.0698 -0.03424B(6) -0.0838 -0.0125 0.0648 -0.28489B(7) -0.0347 -0,1478 -0.0364 0.0419B(8) -0.0838 -0.0125 -0.0648 -0.28489B(9) -0.0347 -0.1478 -0.0364 0.0419B(10) -0.1388 -0.1611 -0.1377 -0.31127

the initial H in B(6)–H is shifted to a triangular face next to the C inthe boron cluster. We shall see that the position of the substitutedhydrogen atom does not differ much either in the 6-Cl or 10-Cl activated complex. But, why does the substitution take placepreferentially at B(6) and not at B(10)? The answer can be found inthe pre-reaction complexes. The pre-reaction 6-substituted adductis 1.72 kcal mol-1 more stable than the pre-reaction 10-substitutedadduct.

Therefore, theory indicates that chlorination on [1]- occurs ina radical and not an electrophilic mechanism. To support sucha mechanism and following our experiment↔theory interdepen-dence, we sought for one experiment that validated the mechanism.For that purpose, we chose as radical scavenger TEMPO andas radical initiator VAZO R© catalyst 88. If the mechanism wasionic the addition of TEMPO or VAZO R©-88 would not alter thereaction. On the contrary the reaction with VAZO R©-88 would bemore efficient than with TEMPO. Indeed, the last situation is theone that occurred. For a specific experiment with a particular ry,we noticed that the chlorination degree on the dominant productvaried 4 Cl depending on the addition of VAZO R©-88 or TEMPO(Table 4).

The correlation between calculated NPA charges on chlorineatoms and the crystal structure of Cs[1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4] has also been studied. The calculated NPA chargesof Cs[1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4] on chlorine atomsby using Gaussian 03 program show that the apical chlorine atomin the para position to the monocarbaborane C–Ph group possessa weaker charge than the meta chlorine sites, -0.126 vs. -0.151, andin fact corroborates the preference of Cs+ cation to interact with

Table 4 ry represents the initial molar ratio of [1-C6H5-1-CB9H9]- toNCS. In the column headed by y, the species observed by MALDI-TOF-MS are indicated in bold numbers, indicative of the number of insertedchlorine atoms. In parenthesis indicates the molar fraction of the differentchlorinated species

ry y

7 6 (0.63), 7 (0.37)7a 1 (0.35), 2 (0.46), 3 (0.15), 4 (0.04)7b 2 (0.02), 3 (0.04), 4 (0.10), 5 (0.22), 6 (0.27), 7

(0.20), 8 (0.11), 9 (0.04)14 6 (0.02), 7 (0.24), 8 (0.36), 9 (0.27), 10 (0.11)14a 2 (0.10), 3 (0.35), 4 (0.30), 5 (0.15), 6 (0.09)14b 3 (0.01), 4 (0.01), 5 (0.02), 6 (0.21), 7 (0.29), 8

(0.26), 9 (0.15), 10 (0.05)

a Adding 5 equivalents of TEMPO in the reaction mixtures. b Adding 20 mgof radical initiator VAZO R© catalyst 88.

7688 | Dalton Trans., 2010, 39, 7684–7691 This journal is © The Royal Society of Chemistry 2010

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Fig. 7 Reaction pathway for the chlorination of [1-C6H5-1-CB9H9]- by a Cl+. Calculations performed at the B3LYP/3-21G* level of theory. Hydrogenatoms are not shown except for H(6).

Fig. 8 Reaction pathway for the chlorination of [1-CB9H10]- by a Cl∑. Calculations performed at the B3LYP/3-21G* level of theory. Hydrogen atomsare not shown except for H(6) and H(10).

those meta chlorine atoms of the anion [1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4]-. Moreover, the charge on the chlorine atom bondedto the aromatic ring is even lower, -0.030, indicating that this atomhas not enough electron density to interact with the cation, as isseen in the X-ray structure.

The solid state structure of Cs[1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4] is formed of [1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4]-

anions (Fig. 3a) and the Cs+ cations, which are surrounded bytwelve chlorine atoms (Fig. 3b) with a distorted cuboctahedralcoordination sphere; eight Cs–Cl distances are of 3.635(2) Aand the remaining four are of 3.885(4) A. These Cs–Cl bonddistances are comparable to the bond distances found in Cs2B12Cl12

salt,21 in which the Cs–Cl bond distances are from 3.644 to3.782 A in the 12-coordinated Cs+-ions. The packing diagramfor Cs[1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4] (Fig. 9) reveals thatthe structure is formed of polymeric layers of Cs+ cations bondedto the Cl atoms. Each Cs+ cation is surrounded by four [1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4]- anions, whose Cl atoms forma distorted cuboctahedral coordination sphere for Cs(I) ions.The CsCl12 layers are formed of [1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4]- anion rows in which the anions are asymmetrically “aboveand below” the layer. So in Cs[1-(4¢-Cl-C6H4)-6,7,8,9,10-Cl5-1-CB9H4] there are two types of chlorine atom at the boron clusterthat interact with the Cs+ cation. The apical chlorine atom para to

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Table 5 Calculated NPA charges (e)a on the carbon, boron, chlorineand hydrogen atoms calculated in this work for [1-C6H5-6,7,8,9,10-Cl5-1-CB9H4]- at the B3LYP/6-31+G* level of theory

Atom NPA charge Atom NPA charge

Boron cluster Aromatic ring

C1 -0.621 C5 0.005B2 0.050 C6¢ -0.199H2 0.064 H6i 0.255B2i 0.050 C7i -0.251H2i 0.064 H7i 0.252B2ii 0.050 C8 -0.061H2ii 0.064 Cl3 -0.030B2iii 0.050 C7 -0.251H2iii 0.064 H7 0.252B3 -0.004 C6 -0.199Cl3 -0.149 H6 0.255B3i -0.009Cl1i -0.151B3ii -0.004Cl1ii -0.149B3iii -0.009Cl1iii -0.151B4 -0.122Cl2 -0.126

the monocarbaborane C–Ph group exhibits interactions with fourCs+ cation sites in a square plane, whereas the meta chlorine sitesbridge two Cs+ cations in a bent arrangement, with Cs+–Cl–Cs+

angles of 97.96(6). The found B–Cl ◊ ◊ ◊ Cs+ interactions indicate,that the compound is an ionic salt like Cs2B12Cl12.21 The same wasobserved with the structures of the Cs+ salts of the polybrominated[1-HCB9Br9]-18 and [1-HCB9H4Br5]-22 anions.

Fig. 9 A part of the crystal packing in Cs[1-(4¢-ClC6H4)-6,7,8,9,10-Cl5-1-CB9H4]. The Cs+ cations are marked using a violet colour,the carbons atoms grey, boron atoms brown and chlorines green.

4. Conclusions

Summarizing, [1-C6H5-1-CB9H9]- has been taken as a modelcompound to study chlorination in monoanionic boron clusters.It has been shown experimentally that there exist preferentialchlorination sites in this cluster. Theoretical calculations haveshown that these preferential sites are of a kinetic origin andthat the attack is not ionic but by radicals. The substitutionreaction rates have been rationalised by considering 2a-NPA onthe starting material unperturbed by chlorine substituents. The2a-NPA charges reproduce the chlorination order of attack muchmore accurately than simple NPA.

Acknowledgements

We acknowledge the financial support from CSIC (I3P grantto P.F.) and the Generalitat de Catalunya 2009 SGR 279.Access to the computational facilities to the CESCA Centre deSupercomputacio de Catalunya and the CSIC computing centreis also gratefully acknowledged.

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