Electrochemistry and Photoluminescence of Icosahedral

Review pubs.acs.org/CR

Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives Rosario Núñez,† Màrius Tarrés,†,‡ Albert Ferrer-Ugalde,†,∥ Fabrizia Fabrizi de Biani,*,§ and Francesc Teixidor*,† †

Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain § Dipartimento di Biotecnologie, Chimica e Farmacia, Universita degli Studi di Siena, via Aldo Moro, 2, 53100 Siena, Italy ABSTRACT: Icosahedral boranes, carboranes, and metallacarboranes are extraordinarily robust compounds with desirable properties such as thermal and redox stability, chemical inertness, low nucleophilicity, and high hydrophobicity, making them attractive for several applications such as medicine, nanomaterials, molecular electronics, energy, catalysis, environmental chemistry, and other areas. The hydrogen atoms in these clusters can be replaced by convenient groups that open the way to a chemical alternative to conventional “organic” or “organometallic” realms. Icosahedral boron cluster derivatives have been reviewed from different perspectives; however, there is a need for a review dedicated to the redox and photophysical characteristics of easily accessible borane and carborane derivatives, which are excellent materials for a wide range of applications. This review deals with the redox properties and photoluminescence behavior of this collection of compounds, as well as their influence on the properties of materials and devices whose working principles are related to electron-transfer or electron-promotion phenomena. We hope that this review will be of great value to boron cluster scientists and researchers working in the photoluminescence and electrochemistry fields who are interested in exploring the possibilities of these unique and promising systems.

CONTENTS 1. Introduction 2. Redox Behavior of Boranes, Carboranes, and Metallacarboranes 2.1. Effect of Carbon Number and Position on Redox Behavior 2.2. Effect of Number and Position of Cage Substituents 2.3. Effect of the Coordinated Metal 2.4. Redox Behavior of Boron Cluster Oligomers and Polymers 2.5. Properties of Bridging Boron Clusters 2.6. Summary and Outlook 3. Photoluminescence (PL) Behavior of CarboraneContaining Small Molecules 3.1. Photophysical Properties of Carborane Derivatives Containing Fluorophores Not Directly Bonded to the Ccluster (Cc) 3.2. PL of Donor−Acceptor Systems (dyads and triads) Based on Aryl−Carbazole, Dinaphthyl, Bodipy, Diazaborolyl, or Triphenylamine Bonded to o-, m-, and p-Carboranes 3.3. Carboranes as Linkers for Spatial Energy Transfer in Donor−Acceptor (D−A) Dyads 3.4. Carborane-Containing Fluorophores Showing Energy Transfer from AggregationInduced Emission (AIE)

© 2016 American Chemical Society

3.5. Photophysical Properties of Thiophene, Biphenyl, and p-Terphenyl Fused o-Carboranes 3.6. Summary and Outlook 4. Photoluminescence (PL) Behavior of Carborane and Metallacarborane-Containing Large Molecules 4.1. Photophysical Properties of Carborane and Metallacarborane-Containing Porphyrins and Phthalocyanines 4.2. Photophysical Properties of Carborane and Metallacarborane-Containing Star-Shaped Molecules, Dendrimers, and Octasilsesquioxanes 4.3. Photophysical Properties of p-CarboraneContaining Nanocars 4.4. Summary and Outlook 5. Carborane-Containing Polymers 5.1. Electropolymerization of Boron-Cluster-Containing Materials 5.1.1. Electropolymerization with Boron Clusters as Doping Agents 5.1.2. Electropolymerization with Covalently Linked Boron Clusters as Self-Doping Agents or as Decorating Units

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Chemical Reviews 5.2. Photoluminescence of Carborane-Containing Polymers 5.2.1. Photophysical Properties of CarboraneContaining p-Phenylene-ethynylene Polymers 5.2.2. Photophysical Properties of CarboraneContaining Polyfluorene Polymers 5.3. Summary and Outlook 6. Applications 6.1. Redox-Driven Applications of Boron-ClusterBased Materials 6.1.1. Ion-Selective Electrodes 6.1.2. Electrochromism 6.1.3. Electronic Motion 6.1.4. Dye-Sensitized Solar Cells 6.2. Applications of Photoluminescent Carborane-Containing Compounds 6.2.1. Materials Containing o-Carborane Moieties for Sensing Nucleophilic Anions 6.2.2. Dyads Containing Carborane Moieties in Organic Field-Effect Transistors (OFETs) 6.2.3. Dyads Containing Carborane Moieties in Phosphorescent Organic Light-Emitting Diodes (PHOLEDs) 6.2.4. Carborane-Containing Macromolecules for Fluorescence Microscopy Imaging 6.3. Summary and Outlook Author Information Corresponding Authors Present Addresses Notes Biographies Acknowledgments Abbreviations References

Review

More closely related to the contents of this review, both the electronic excitation and the electron-transfer processes involving boron clusters have briefly been tackled.50−53 Because of this, it is our intention in this review to merge the current advances in the electrochemistry (EC) and photoluminescence (PL) properties of carboranes, boranes, and metallacarboranes icosahedral clusters, with emphasis on the most stable and probably the easiest to be applied: C2B10H12, [B12H12]2−, [3,3′Co(1,2-C2B9H11)2]−, and [3,3′-Fe(1,2-C2B9H11)2]−. We decided to combine EC and PL of boron clusters because both processes are related in the sense that promotion of one electron to an excited state occurs in both cases, although the origin of the electron promotion is different (voltage or a photon). Also, a hole is generated in both cases, although it is usually intramolecular in PL and intermolecular in EC. As we shall see throughout the review, there are several characteristics that sustain the merging of both properties, EC and PL, into a single review. However, there exists a key feature which, from our point of view, stands out above the rest, that being the Ccluster---Ccluster (Cc---Cc) connections in carboranes, either adjacent in ocarborane or separated as in m- and p-carborane.

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2. REDOX BEHAVIOR OF BORANES, CARBORANES, AND METALLACARBORANES To the best of our knowledge, a complete review of the redox behavior of boranes, carboranes, and metallacarboranes has not been published since 1985.54 A more recent review by Kaim and co-workers, dedicated to the paramagnetic properties of boronrich two- and three-dimensional systems, only briefly summarizes their electrochemistry.55 Here we will supply tables of their redox potential to be used as future reference and will attempt to summarize their general redox behavior. In an effort to make uniform tables, the redox potential values, if they were not originally reported as such, have all been converted as referenced to the couple Fc+/Fc, of which the redox potential is known under many different experimental conditions.56 For unclear reasons, a search through the published data has very often had to face difficulties arising from the lack of exhaustively described experimental information and, above all, from the unclear definition of the reference potential used. Sometimes, unsolvable incongruities between the published data have emerged, and these will be outlined throughout the text. Compounds in which the redox process is undoubtedly centered on an attached substituent have been omitted in our discussion and tables, except in those cases in which the bridging role of the carborane moieties is discussed. Measurements of the redox potentials have been performed under many different experimental conditions, and apparently, a non-negligible (but not always easily predictable) effect of the solvent comes into view. For this reason, the solvent has also been reported in the tables, except in the case of metallacarboranes, which have been almost invariably measured in acetonitrile. The material of the working electrode has also been found to have an effect, as clearly shown in Figure 1 for 1-Ph-2-B(Mes)2-1,2-C2B10H10.57 In addition, an influence of ferrocene, often added as internal reference, has also been reported.58 Even if a discussion about these phenomena is beyond the scope of this review, it seems reasonable to advise researchers to keep in mind these often neglected aspects. To conclude, information about the reversibility and the number of electrons involved in the redox process has been indicated in the tables only when explicitly declared in the original paper.

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1. INTRODUCTION Icosahedral carborane clusters were first reported in the 1960s, and since then this area has experienced enormous growth. Many new synthetic procedures have been developed1−7 with improved yields that have allowed accurate measurements of their physicochemical properties, which have revealed that often they exhibit a very different behavior to electronically similar organic compounds.8,9 The synthetic advances and bonding theories of boron clusters, and more specifically icosahedral carborane derivatives, have been utilized10,11 in such diverse areas12 as medicine (including boron neutron capture therapy and drug delivery), catalysis, nonlinear optical materials, liquid crystals, metal-ion extraction (particularly nuclear waste remediation), superacid chemistry,13 conducting organic polymers, coordination polymers, and others. The density of research in the different areas is apparent from the number of reviews specifically dedicated to these areas. Carborane compounds have been extensively reviewed in relation to their applications in medicine,14−35 as well as in catalysis,36−40 polymers,41,42 selfassembly via C−H···X and C−H···H−X dihydrogen bonding interactions,43 optoelectronic applications,44−48 and metal-ion extraction (particularly nuclear waste remediation).49 The number of reviews in each area reasonably reflects the current application tendencies of the boron clusters, or alternatively, upon publication of a thematic review this leads to the development of practical applications for the particular materials. 14308

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Figure 3. Molecular structure of [B24H23]3−.

magnitude between [B12H12]2− and [CB11H12]− has been hypothesized but never demonstrated. Only recently, the full characterization of the permethylated [B 12 Me 12 ] 2− and [CB11Me12]− has offered the opportunity to compare the redox potential of a couple of icosahedral analogues. In MeCN, these species are oxidized at +0.03 and +1.15 V, respectively, thus ratifying that the introduction of the carbon atom induces an anodic shift of more than 1 V. Following the same reasoning, it is not unexpected that the replacement of two B−H fragments by two C−H fragments to generate dicarbaboranes (or simply dicarboranes through the text) as in 1,2-, 1,7-, and 1,12-C2B10H12, or ortho- (o-), meta- (m-), and para- (p-) carboranes, prevents the oxidation of the cluster, irrespective of the nature of the peripheral substituents. On the other hand, while 1,2-C2B10H12 and its derivatives can be reduced, accepting two electrons either sequentially or in a single step, reductions are never observed for either [B12H12]2− or [CB11H12]−. Computational studies indicate that either the removal of one or two electrons from [B12H12]2− and [CB11H12]− or the addition of electrons to 1,2-C2B10H12 leads to Jahn−Teller distortions.7,61−63 The reduction process of the unsubstituted 1,7-C2B10H12 and 1,12-C2B10H12 has also never been observed, and in general, o-carboranes are reduced more easily than m-carboranes, which in turn are reduced more easily than p-carboranes. Thus, in spite of the fact that a huge amount of data is available for o-carboranes, the same is not true for the meta and para analogues and there are not many complete ortho, meta, and para collections. Moreover, the few existing complete sets all have one or more halogen as carbon substituent, but the peculiar nature of the redox processes in these species, in which the halogen itself is usually the center of the electron addition, makes the redox pattern less straightforward and the meaning of redox values more uncertain. Nonetheless, by comparing the redox potential values in the set 1,2-, 1,7-, and 1,12-C2XRB10H10 (X = Cl, Br, I; R = H, Me),54 we may observe that the ortho → meta and the ortho → para cathodic shifts are ∼500 and ∼650 mV, respectively, when the halogen is Cl and decrease to ∼200 and ∼350 mV when the halogen is either Br or I. This is in agreement with the fact that the peripheral shell of Br or I provides a major contribution to the redox orbital, as further demonstrated by the strongest influence of their presence on the redox potential value also in the series 1,2-X2-1,2-C2B10H10 (E = −1.74, −0.94, and −0.59 V for X = Cl, Br, and I, respectively; to be compared with E = −2.96 V for 1,2-C2B10H12). Thus, being mostly centered on the peripheral ligands, the reduction is less affected by the position of Ccluster (Cc) atoms in the case of Br and I. By comparison, for 1-Ph-1,2-C2B10H11 the ortho → meta cathodic shift of the reduction is 500 mV (Figure 4). In the case of 9-I-1,2-C2B10H11, for which the absolute influence on the redox potential of the B-linked iodine is much less prominent (E = −2.58 V for 9-I-1,2-C2B10H11), the ortho → meta cathodic shift for the reduction, fully centered on the C2B10 cage, is high again (580 mV). Apparently, compounds with two halogens as carbon substituents do not exhibit the same straightforward trend.

Figure 1. CV plots of 1-Ph-2-B(Mes)2-1,2-C2B10H10 with different solvents and working electrodes. Adapted from ref 57. Copyright 2015 The Royal Society of Chemistry.

The numbering scheme for boranes, monocarbaboranes, and dicarbaboranes is reported in Figure 2, where one of the carbon atoms, if present, always occupies position 1.

Figure 2. Numbering of icosaedral boron-based clusters.

2.1. Effect of Carbon Number and Position on Redox Behavior

The cluster [B12H12]2− undergoes an irreversible one-electron oxidation with the formation of [B24H23]3−, a dimer in which two B12 cages are linked by H (Figure 3).59,60 The introduction of carbon atoms in the 12-vertex cage suddenly increases the oxidation redox potential, so that it has never been possible to observe the oxidation of [CB11H12]− under any experimental conditions. In fact, based on what had been observed for the analogues [B10H10]2− and [CB9H10]−, in which the C-substituted species is oxidized ∼1.4 V more anodically than the unsubstituted counterpart, a shift of similar 14309

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electrochemical oxidation of the perhalogenated dianions [B12X12]2− (X = F, Cl, Br, I) has been achieved for the whole series of halogens (see Table 1).71,72 Due to their superior kinetic stability, the radical anions are usually stable, at least on the time scale of cyclic voltammetry, and the second oxidation to the neutral species is also observed. For periodinated cluster [B12I12]2−, the electrons are removed mainly from the iodine shell and the redox profile is a complicated superposition of irreversible processes. The observed trend reveals that moving down the halogen group, the first oxidation is more difficult while the second is almost unaffected. As a result, the separation between the first and the second oxidation potentials (ΔE2−1) diminishes along the series. In the closomers group, both the first and the second oxidations are dramatically shifted toward the cathodic region, so that the purple anionic and the orange neutral hypercloso clusters are stable and easily obtained.67,68 This phenomenon is particularly enhanced in the case of the giant molecule B12{O-1-Hex-2-R-1,2-C2B10H10}12 (R = H, Me), in which the central B12 cage is surrounded by 12 carborane clusters, thus suggesting that the exceptional stability of the oxidized species is mainly due to steric hindrance effects (Table 1).73 Inspection of the redox potential values of monocarboranes reveals that the amount of information has remained almost unchanged with respect to that reviewed in 2013.62,7 In fact, more than 250 monocarbaborane compounds have been described, but only for a few of them has electrochemistry been reported. This may be explained by the particularly high unresponsiveness of this class of compounds toward the redox reactions. Indeed, the oxidation process seems to be obtained only in the case of heavily methylated compounds, while only for those with methyl groups in the hemisphere antipodal to the carbon vertex the oxidation also exhibits reversibility features. This latter aspect again suggests that steric factors are almost as important as inductive effects in the stabilization of the oxidized species. Thus, most of the compounds for which the potentials have been reported fit into the class of [1-X-12-Y-CB11Me10]−, depicted in Figure 5, and with only the C and B(12) positions free for substitutions, their redox potentials fall in the relatively short range from +1.1 to +1.5 V. All of the redox potentials of [CB11H12]− and its derivatives are collected in Table 2. The redox potential of the dodecamethylated anion [CB11Me12]− in SO2 is +1.08 V,74 while under the same conditions [CB11H12]− is not observed. This suggests that the effect of permethylation may be more pronounced in the case of monocarbaborane than in the case of boranes, since in this latter case the full substitution of peripheral hydrogens by methyl groups causes a cathodic shift of ∼1 V (cf. Ep = +1.05 V for [B12H12]2− vs E° = +0.03 V for [B12Me12]2− in MeCN). The effect of multiple methyl substituents is additive: the cathodic shift caused by each inequivalent cage position depends on its different sensitivity to substitution and was evaluated to be 52, ∼70, and 9 mV for positions 1, 2−11, and 12, respectively.58 Once established that the presence of one, at least partial, cumbersome shell is necessary, the effect of the other substituents appears to follow the behavior expected on the basis of their electron donor/acceptor ability and has been previously subjected to a detailed study.58 In the [1-X-12-YCB11Me10]− series, a nice correlation of the redox potential vs the σp Hammet constant has been found and indicates that the sensitivity of the redox potential to substituent effects is approximately twice as large in position 12 (slope ≈ 0.55 V) than in position 1 (∼0.31 V). On the basis of these findings, the additive increment for the 1-Me and 12-Me substituent has been

Figure 4. Molecular structure and redox potentials of ortho and meta isomers of Ph-C2B10H11 and I-C2B10H11.

2.2. Effect of Number and Position of Cage Substituents

All of the redox potentials of [B12H12]2− and its derivatives are collected in Table 1. Since the last review, nearly 30 years ago,54 only a few new borane compounds have been electrochemically characterized. Conversely, improvements in the electrochemistry of the previously reported compounds are many, mainly because of the use of liquid SO2, the experimental window of which extends to +4.3 V vs Fc+/Fc64 and which has demonstrated to be an excellent solvent for characterization of these compounds. Most of the compounds are fully substituted, and when this is not the case, the lack of precise information about the position of the substituents, together with the use of different experimental conditions, makes it very difficult to soundly evaluate the effect on the redox potentials of either the number or the position of substituents. As an example, while it would be reasonable to expect that hydroxyl substitution has an additive effect in the shift of the potential, currently a comparison of the redox values found for [B12(OH)xH12−x]2−, with x = 0, 1, 2, and 12, inexplicably does not fully support this hypothesis. A non-negligible dependence of the redox potential of [B12(OH)H11]2− from pH has been claimed by Fojt et al., who found that increasing the pH value by one unit makes the oxidation cathodically shift by ∼15 mV.65 The same team also highlighted the influence of the analyte concentration on the redox potential and tentatively ascribed it to supramolecular cation−anion aggregation phenomena. At variance with what has been observed with [B12H12]2−, the oxidation of the cage-substituted [B12YnH12−n]2− clusters (Y being any electron-donating group) is often chemically reversible, and the blue radical anions, or even the blue hypercloso neutral species, have in some cases not only been observed but also been isolated. The generally observed extra stability of the oxidized species is ascribed to a combination of electron-donating effects and steric encumbrance of the substituents. As it is shown in Chart 1, the experimental redox potentials for the first electron removal of substituted 12-vertex boron cages fall in two well-separated groups. As a whole, the redox potential of [B12YnH12−n]2− covers ∼3 V. In the first group, positive with respect to the Fc+/Fc couple, are the halogensubstituted clusters in the more anodic region and hydroxyl- or methyl-substituted clusters in the less anodic one. The experimental observation that the introduction of hydroxyl groups as cage substituents makes the oxidation of the cluster easier, whereas the introduction of halogen groups makes it more difficult, dates back to 1964.66 Reasonably, as a general rule, the oxidation of [B12YnH12−n]2− is easier whenever electron-donating substituents are present, but the effect is remarkable when the cage is substituted with esters and ethers to form the so-called “closomers”.67−70 These all make the second group of redox potential values negative with respect to the Fc+/ Fc couple. In spite of its highly positive redox potential, the 14310

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Table 1. Redox Potential Values (in volts vs Fc+/Fc) of Borane [B12H12]2− and Its Derivativesa E

compound X

Y

H12

H11

OH NH3 Br I

H10

H6 H2 {OH}12

(OC2H4)2-OPh {OH}2 1,7-Cl2 1,7-Br2 1,7-I2 Br6 Cl10

F12 Cl12 Br12 Br10 I12 Me12 {OCH2Ph}12 {OHex}12 {Oi-Pe}12 {OPe}12 {OBu}12 {OEt}12 {O-1-buten-4-yl}12 {OMe}12 {OAll}12 {OCH2-4-MePh}12 {OCH2-4-MeOPh}12 {OCH2-4-FPh}12 {OCH2-4-ClPh}12 {OCH2-4-BrPh}12 {OCH2-3-FPh}12 {OCH2-3-BrPh}12 {OCH2-4-CF3Ph}12 {OCH2-3,5-(CF3)2Ph}12 {OCH2C6F5}12 {O-1-Hex-1,2-C2B10H11}12 {O-1-Hex-2-Me-1,2-C2B10H10}12

{OH}2

2−/− +1.05b >+1.2c +1.53b,d +1.66b +0.82c +1.06b,d +1.49b,d,e +1.44b,d +1.50b +1.13b,d +0.60b +0.60b,c +1.5b,d,g +1.3b,d,g +1.1b,d,g +1.62b >+1.8 +0.45 +0.75i +1.45 +1.68k +2.10 +2.11k >+1.8b +2.27k >+1.09c ∼+2.1b,k +0.03 −0.41 −0.74 −1.18 −1.17 −1.16 −1.16 −1.08 −0.90 −0.78 −0.74 −0.90 −0.87 −0.66 −0.56 −0.53 −0.52 −0.51 ∼−0.05n −0.05 ∼−0.45n −1.31 −1.22

−/0

+2.71j,k +2.67k +2.67k ∼+2.2b,k +0.05 −0.23 −0.66 −0.68 −0.63 −0.63 −0.61 −0.44 −0.31 −0.25 −0.34 −0.30 −0.18 +0.02 +0.03 +0.03 +0.04 +0.56 +0.68 ∼+0.30n −0.54 −0.47

solvent

ref

MeCN H2O H2O SO2 H2O H2O H2O H2O MeCN H2O DMSO H2O H2O H2O H2O MeCN MeCN MeCN H2O MeCN SO2 MeCN SO2 MeCN SO2 H2O SO2 MeCN MeCN DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM

59 66 65 72 66 65 65 65 59 65 f 66 65 65 65 59 59 h h 71 71, 72 71 71, 72 59 71, 72 66 71 l, m 67 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 69 69 70 73 73

a Entries are indicated by the type and number of cage substituents. bChemically irreversible. cObtained by polarography, working electrode: graphite-nujol paste, KH2PO4(0.1 M)/H2O, pH = 4.5. dIn phosphate buffer, pH = 8. Rotating GCE. eCharge compensated. fBirsöz, B.; Nar, I.; Gül, A. Synthesis, Characterization and Electrochemical Investigation of Phthalocyanines Carrying 96 Boron Atoms. J. Organomet. Chem. 2014, 755, 64− 71. gValue measured, but not reported in the original paper, has been extracted from Figure 3 of ref 65. hVan, N. D.; Tiritiris, I.; Winter, R. F.; Sarkar, B.; Singh, P.; Duboc, C.; Muoz-Castro, A.; Arratia-Prez, R.; Kaim, W.; Schleid, T. Oxidative Perhydroxylation of [closo-B12H12]2− to the Stable Inorganic Cluster Redox System [B12(OH)12]2−/•− Experiment and Theory. Chem. Eur. J. 2010, 16, 11242−11245. iT ≈ 75 °C, reversible above 1 V s−1. jObtained by Osteryoung square-wave voltammetry. kT ≈ −60 °C. lPeymann, T.; Knobler, C. B.; Hawthorne, M. F. An Unpaired Electron Incarcerated within an Icosahedral Borane Cage: Synthesis and Crystal Structure of the Blue, Air-Stable {[closo-B12(CH3)12]•}− Radical. Chem. Commun. 1999, 2039−2040. mPeymann, T.; Knobler, C. B.; Khan, A. I.; Hawthorne, M. F. Dodecamethyl-closo-dodecaborate(2−). Inorg. Chem.

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Table 1. continued 2001, 40, 1291−1294. nNot explicitly reported in the original paper; this datum has been extracted from the cyclic voltammogram in the Supporting Information of ref 69.

pattern is also usually greatly affected by the solvent. A glance at Table 3 reveals that, with very few exceptions, two different redox potential values for the first and second electron addition are only reported for those cases in which both Cc atoms are substituted with bulky substituents. Reasonably, the presence of two bulky substituents at both adjacent Cc atoms makes the compound electrochemically more robust, since reactive sites are hampered. In this situation, the reduction is split into two one-electron steps and reversibility can, in some case, be achieved. The quasireversible behavior of the monosubstituted 1-BMes2-1,2C2B10H11 (BMes2 = dimesitylboryl), which carries a particularly bulky substituent (Figure 7), should also be noted. The previously mentioned exceptions are the 1-halogensubstituted compounds (1-X-2-R-1,2-C2B10H10), which also undergo two equivalent redox processes. Both are two-electron processes, since the addition of the first two electrons causes the cleavage of the Cc−X bond and parallel formation of a new carborane species, which is further reduced (Scheme 1).78 Similar behavior is also found for 1-p-XPh-1,2-C2B10H11.80 The set of 1,2-Ar2-1,2-C2B10H10 has been extensively studied since their members always exhibit two subsequent one-electron additions. This was previously explained by hypothesizing that the extra electron was received by the Ar2 system. In fact, computational chemistry shows that the frontier orbitals of the neutral clusters are mainly centered on the aryl rings. Nevertheless, calculations on the optimized geometry of the radical anions disclose a relaxed electronic structure, in which the frontier orbitals are centered on the C2B10 cage, which now has an elongated Cc−Cc distance. Once isolated, the structural features of the anions exhibit a remarkable Cc−Cc elongation (∼0.65 Å), while the aryl shell suffers no deformation, as in the dimeric dianion shown in Figure 8.81 The second electron is added to the C2B10 cage of this structurally rearranged system, and this explains why the potential of the first reduction is highly sensitive to the nature of the aryl group, while the second one is much less so.82 On the basis of these findings, the neutral and anionic forms have been defined as “essentially independent species”83,84 and the cathodic/anodic redox couple may be regarded essentially as an ECE process (ECE = electrochemical reaction−chemical reaction−electrochemical reaction), in which the intermediate “chemical reaction” is thermodynamically reversible. In fact, it has been found that the elongation of the Cc−Cc bond in neutral species facilitates the stability of the monoanionic radicals.79 This would assign a further role to the bulky substituents, since the more encumbering they are the more the Cc atoms are forced to stay apart. The primary role of the Cc−Cc bond length determining the electronic structure of o-carboranes was first suggested in 2001 on the basis of a detailed NMR analysis85 and successively confirmed by theoretical calculations86,87 by Teixidor and co-workers, who observed that the LUMO is mostly situated on this couple of heteroatoms. While the shape of the redox pattern is mainly affected by steric factors, the redox potential values are also sensitive to the electronic nature of the substituents. In fact, the redox potentials for the reduction of differently substituted o-carboranes span a remarkable range of ∼2.5 V (see Table 3). The huge amount of data available for o-carboranes is better handled with the help of

Chart 1. Distribution of the Redox Potential for the First Oxidation of [B12YnH12−n]2−

Figure 5. Molecular structure of [1-X-12-Y-CB11Me10]−.

reconsidered to be 30 mV in position 1 and 68 mV in position 12 (Table 2), i.e., the order is reversed with respect to its previous evaluation.58 Reduction of 1,2-C2B10H12 by alkali metals leads to the formation of nido-m-carborane ([nido-7,9-C2B10H12]2−) (Figure 6) with the complete cleavage of the cage C−C bond.75−77 The dianion then protonates to yield a stable monoanion.78 Neither nido-o-carborane nor nido-p-carborane has been reported during this reductive process. Depending on the experimental conditions, the highly reactive [nido-7,9-C2B10H12]2− can undergo different chemical reactions.79 As a rule, substituted o-carboranes can always accept two electrons. Depending on the stability of the intermediate radical anion, the addition of the two electrons will be simultaneous or will occur in two separate steps. In any case, even stable anionic species often undergo a deep structural rearrangement, mainly forming nido structures, and electrochemical reversibility seldom occurs. Chemical reactions, typically cleavage of the carbon ligands and/or the formation of dimers, go along with the reduction, thus affecting the chemical reversibility of the redox reaction. After this description, it is not surprising that the redox 14312

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Table 2. Redox Potential Values (in volts vs Fc+/Fc) of Monocarbaborane ([CB11H12]−) and Its Derivativesa E

compound C

B(12)

B(2−11)

H

H

H10

H H H H H H

Me H Me H Me H

H10 2−6 Me5, H5 2−6Me5, H5 7−11Me5, H5 7−11Me5, H5 Me10

H

Me

Me10

H H H H H

F Cl Cl Br I

Me10 Me10 Cl10 Me10 Me10

Me Me Me Me Me Me Me

H Me H Me H Me H

H10 H10 2−6Me5, H5 2−6Me5, H5 7−11Me5, H5 7−11Me5, H5 Me10

Me

Me

Me10

Me Me Et Et Et Pr Pr Bu Bu Bu Hex Hex Hex CH2CH(C2H5)C4H9 OMe B(OH)2 F Br I COOH COOH

F I H Me I Me I H Me I H Me I H Me Me Me Me Me H Me

Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 Me10 14313

−/0

solvent

ref

>+2.0 >+2.5c >+2.5c +1.77c,d +1.49c +1.63c,d +1.52c +1.09c +1.26e +1.21 +1.04 +0.98 +1.12c +1.19e +1.17 +0.98 +0.91 +1.34e +1.39e >+1.6 +1.42e +1.43d,e +1.31d +1.13d +1.08d >+2.5c +2.46c +1.74c +1.41c +1.60c +1.37c +1.14c +1.23e +1.15 +1.08c +1.16e +0.95 +0.89 +1.31e +1.39c,e +1.24e +1.16e +1.40c,e +1.17e +1.41c,e +1.24e +1.16e +1.38c,e +1.23e +1.20e +1.39c,e +1.22e +1.20e +1.32e +1.34e +1.31e +1.31e +1.40e +1.32e

MeCN SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 MeCN DCM isoflurane SO2 SO2 MeCN DCM isoflurane SO2 SO2 MeCN SO2 SO2 MeCN DCM isoflurane SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 MeCN SO2 SO2 DCM isoflurane SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2 SO2

b 74 74 74 74 74 74 74 58,f f f f 74 58,f f f f 58 58 g 58 58,f f f f 74 74 74 74 74 74 74 58 f, h 74 58,f f f 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58

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Table 2. continued E

compound C COOMe COOMe

B(12) Me I

B(2−11)

−/0 +1.35e +1.52c,e

Me10 Me10

solvent SO2 SO2

ref 58 58

a Entries are indicated by the type, number, and position of cage substituents. bWiersema, R. J.; Hawthorne, M. F. Electrochemistry and Boron-11 Nuclear Magnetic Resonance Spectra of Monocarbon Carboranes. Inorg. Chem. 1973, 12, 785−788. cT = 0 °C. dChemically irreversible. eT = −65 ́ ́ J. Isoflurane as a Solvent for Electrochemistry. Electrooxidation Study of Icosahedral J.; Michl, J.; Ludvik, °C. fWahab, A.; Kvapilová, H.; Klima, Carborane Anions in four Different Solvents. J. Electroanal. Chem. 2013, 689, 257−261. gXie, Z.; Tsang, C.; Xue, F.; Mak, T. C. W. Chlorination of Icosahedral Carborane Anions. X-ray Crystal Structure of [Me3NH][CH3-1-CB11Cl11]. Inorg. Chem. 1997, 36, 2246−2247. hKing, B. T.; Janousek, Z.; Grüner, B.; Trammell, M.; Noll, B. C.; Michl, J. Dodecamethylcarba-closo-dodecaborate(−) Anion, CB11Me12− J. Am. Chem. Soc. 1996, 118, 3313−3314.

With the exception of this interesting piece of information, thus far the electrochemistry of compounds with Cc---Cc linkages has not proven to be equally informative. In fact, to the best of our knowledge, electrochemical characterization is available only for the limited set of compounds listed in Table 4. In the case of dithio derivatives, the ligand cleavage has been observed to accompany the electron addition, followed by the formation of 1,2-C2B10H12.89 These findings, suggested by the redox potential values as well as by the cyclic voltammogram shape, have been safely confirmed by bulk electrolysis experiments. On the other side, compounds with an aliphatic/aromatic bridge, for which electrochemical data are available, have a ring with n = 5 or 6, and their chemical reduction invariably leads to the carbon-adjacent [nido-1,2-μ-L-7,8-C2B10H10]2−.94 It is difficult to draw unambiguous conclusions concerning the redox mechanism from the available data. A remarkable case is that of 1,2-μ-(1,2,3,4-Et4Butadiene)-1,2-C2B10H10,94 in which the conjugated linker generates a fused aromatic ring with the carbon atoms of the dicarbaborane cage. This compound is reversibly reduced at the exceptionally low potential of −0.98 V to give what has been assumed to be the radical anion, which might be stabilized by the butadienediyl bridge (Figure 10).

Figure 6. Molecular structure of [nido-7,9-C2B10H12]2−.

some statistics: the histograms in Chart 2 may be used to identify some trends. The blue large curve (H, Y) shows that a single generic substituent Y on one of the Cc atoms is enough to span almost the whole achievable potential range. The same is true if one of the substituents is methyl (purple large curve; Me, Y), but a generally modest anodic shift of the group is also observed. If one of the carbon substituents is a phenyl group, the accessible potential values are less extended and the curve (pale pink; Ph, Y) becomes narrower and is anodically shifted. A similar and even more pronounced effect is observed in the case wherein one of the carbons is halogen substituted (magenta curve; X, Y). Alternatively, if substituents are on the cage, while the C−H groups remain unchanged (black curve; H, H), the potential values move toward that of the parent 1,2-C2B10H12, which is the more resistant to reduction in the series. All of the redox potentials for o-, m-, and p-carboranes and their derivatives are shown in Table 3. As seen, the reduction of dicarba-closo-dodecaboranes is often a quite intricate process in which both electronic and steric factors play a decisive role. Some attempts to force the direction of the chemistry associated with the subsequent addition of electrons have been made by introducing a linkage between the two cage carbons in o-carboranes. If sufficiently short and rigid, the linker should force the carbon atoms to remain adjacent during the reduction process. In fact, this is what has been observed when alkali metals were used to reduce the series 1,2-μ[1′,2′-C6H4(CH2)2]-1,2-C2B10H10, 1,2-μ-[1′,8′-C10H6(CH2)2]1,2-C 2 B 10 H 10 , and 1,2-μ-[1,1′-(C 6 H 4 ) 2 -2,2′-(CH 2 ) 2 ]-1,2C2B10H10, in which the ring C−L−C has n members (n = 6, 7, and 8, respectively).93 During the reactions with Na or K, the cage carbon atom adjacency is maintained if n = 6 and 7, Figure 9, and salts of the [nido-7,8-C2B10H12]2− are obtained. Moreover, an excess of alkali metal further reduces the o-carborane to carbon-adjacent arachno-o-carborane. This is an interesting feature, since the tetra-anion can be obtained only starting from the carbon adjacent dianion, because the carbon-apart dianions are stronger reducers than alkali metals. On the contrary, when the carborane with n = 8 is reduced under the same conditions, the [nido-7,10-C2B10H12]2− is obtained.

2.3. Effect of the Coordinated Metal

Boranes and carboranes form both σ and π complexes with almost all members of the transition metal series. Examples of the former coordination mode are known in which either the carbon or the boron atoms, or both, can act as a σ donor, even in a chelating mode. An interesting example is reported in Figure 11.95 σ-Bonded complexes are not rare, but π complexes are immeasurably more abundant, and since the 1960s, when [Fe(C2B9H11)2]− was first prepared and characterized, hundreds of sandwich and mixed-sandwich complexes have appeared and the term “carbollide” was coined to indicate the [C2B9H11]2− bowl. π sandwich and mixed-sandwich complexes exist for borane, monocarbaborane, and biscarbaborane cages, but again, the number of the latter examples is immensely superior. In particular, π complexes of boranes are quite rare: only a couple of Ni and Cu 1,1′-metalla-commo-bis(undecaborane) sandwich complexes96 and only a couple of Ni97,98and one Pd99,100 mixed-sandwich closo-1-metalladodecaborane complexes have been structurally characterized (Figure 12). Monocarbollide metal complexes with structures shown in Figure 12 have been described,101 while to the best of our knowledge, none of the other possible isomers have been structurally described. A search of the Cambridge Structural Database returns 591 mixed-sandwich closo-3-metalla-1,2-dicarbadodecaborane and 304 3,3′-metalla-commo-bis(1,2-dicarbadodecaborane) complexes. In 75% of the latter the sandwiched metal is Co, while 14314

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Table 3. Redox Potential Values (in volts vs Fc+/Fc) of o-, m-, and p-Carboranes and Their Derivativesa E

compound C

B

H2 H, Me H, Cl H, Br H, I H, CH2Cl H, CH2Br H, Ph

H10 H10 H10 H10 H10 H10 H10 H10

H, H, H, H, H,

H10 H10 H10 H10 H10

4-Tol 4-BrPh 4-IPh vinyl B(Mes)2

H, Fe(CO)2Cp Me2 Me, Cl Me, Br Me, I Me, Bz Me, PPh2 Me, Fe(CO)2Cp Me, CH2Fe(CO)2Cp Me, bdzb Cl2 Cl, CH2Cl Br2 Br, CH2Cl Br, Ph I2 I, CH2Cl I, Ph Ph2

Ph, Ph, Ph, Ph, Ph, Ph, Ph, Ph, Ph, Ph, Ph,

4-FC6H4 4-NH2C6H4 4-NMeC6H4 4-OMeC6H4 4-OHC6H4 C6F5 4-CF3C6H4 4-MeC6F4 4-CF3C6F4 Fe(CO)2Cp B(Mes)2

H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10

H10 H10 H10 H10 H10 H10 H10 H10 H10 H10 H10

(1,3-Et2-bdzb)2

H10

1,3-Et2-bdzb, H 1,3-Et2-bdzb, Me 1,3-Et2-bdzb, Ph

H10 H10 H10

1,3-Et2-bdzb, tBu

H10

1,3-Et2-bdzb, SiMe3

H10

o

m

−2.96b,c −2.89b,c −1.57,b,c −2.98b,c −0.94,b,c −2.91b,c −0.50,b,c −0.77b,c −2.41b,c −1.59b,c −2.40b,c −2.25b,c −2.50 −2.63b,c −2.07,c −2.38c −2.27 −2.23c,d −1.99c,d

E°C. The distribution centered at ca. −1.0 V contains at the more negative side the data for the [7,9-C2B9H11]2− (m-carborane) complexes and at the less negative side the data for the carbon alkyl-substituted complexes. The isolated datum close to 0 V corresponds to a chargecompensated complex. In the case of the Fe metallacarboranes, the more cathodic group contains, at the more negative side of the distribution bell, the data for complexes with both carbon atoms carrying an alkyl substituent and, at the less negative side, the complexes with one boron carrying a halide substituent. Not unexpectedly, the more anodic group contains the chargecompensated complexes. This is a limited set of data which does not allow one to draw strong conclusions about the effect of the cage substituents, if not supported by the analysis of the data set of the Co complexes, which is much more numerous. In this case, the more cathodic group contains, at the more negative side, complexes wherein one or more cage boron atoms carry an alkyl

Figure 15. Feasible thermal isomerizations of Ni(1,2-μ-C2H6-1,2C2B9H9)2]n−.

Chart 3. Plot of the E° Values (in volts vs Fc+/Fc) of Both MCb2 and CpMCb vs the E° values of MCp2 for Redox Processes Involving the Same Metal Oxidation State Changesa

a

Slope 1 is also shown (black line) for comparison. 14322

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possibly serve as a guide to estimate the potential of as-yet unknown complexes.

substituent, and at the less negative side, complexes with one or more chloride-substituted boron atoms. The more anodic groups contain both the charge-compensated cages and the complexes with one or more iodide-substituted boron atoms. To resume, a few general features for the MIII/II couple can be extracted: trivially, charge-compensated complexes are easier to reduce by ∼1 V; halogens act as electron-withdrawing units and anodically shift the compound’s potential; iodide is a more efficient electron withdrawer than chloride; and the effect of multiple iodides is additive, reaching the region of charge-compensated complexes. A detailed picture has not yet been achieved. In fact, alkyl groups (specifically methyl groups) in all cases act as electron donors when bonded to boron, but they act as electron-withdrawing groups when linked to carbon in the case of Co and Ni complexes, as demonstrated by Teixidor and co-workers,116 and as electron-donors in the case of Fe complexes. Also, exchanging [7,8-C2B9H11]2− with adjacent carbon atoms for [7,9-C2B9H11]2− with a boron between the two carbon atoms causes the potential to anodically shift in the case of the Co complex and to cathodically shift in the case of the Ni complex. The weakness of this kind of statistical approach lies in the fact that the metal electron content has been disregarded in this reasoning. When this is taken into account, we may notice that the difference ΔE°(n−/(n + 1)−) between complexes of the same metal remains almost constant, for example, E°(CoIV/III) − E°(CoIII/II) ≈ 2.9 ± 0.1 V, regardless of the ligand. Moreover, E°(CoIII/II) − E°(CoII/I) = E°(NiIV/III) − E°(NiIII/II) ≈ 1.0 ± 0.2 V consistently. Also, for the same set of ligands, the difference ΔE°(dn/dn+1) between the redox potential values of the redox process involving the same change of the d electron count (i.e., FeIII/II vs CoIV/III or CoIII/II vs NiIV/III and so on, for both the first and second transition series), remains almost constantly ∼1.7 V (16 entries, median = 1.68 V, σ = 0.16), as graphically shown in Chart 5. As an example, [3,3′-NiIV(1,2-C2B9H11)2] (isomer A) is

Chart 6. Graphical Representation of the dn/dn+1 Redox Potential Values of Pristine Cu, Ni, Co, and Fe Metallacarboranes, Regarding Their Corresponding Redox Processesa

a

Bold black lines correspond to the experimentally observed redox processes.

A small collection of redox data for ansa-metallacarboranes is also available and reported in Table 6. The redox behavior of these complexes, in which a bridge connects the two coordinating bowls, bound to either two boron or two carbon atoms or both (diansa-metallacarborane), cannot be discussed in great detail because of the limited extent of available data. Many of them are charge compensated and show the usual anodic shift.119,122 An interesting class of compounds is represented by the family of the diansa-1-phospha-2-benzene-cobaltabisdicarbollidephanes, in which the synergic effect of the donor, D (the PR bridge), and the acceptor, A (the benzene bridge), can be appreciated without the complexes having to suffer the perturbation of a strain-induced distortion.123

Chart 5. For Each Set of Ligands, the Potential E° of Isolectronic Redox Changes vs the Charge of the Complex Has a Constant Slope ≈ 1.7 V

2.4. Redox Behavior of Boron Cluster Oligomers and Polymers

Polymers/oligomers of boranes are unquestionably uncommon. To the best of our knowledge, the few halogenated derivatives of [B 24 H 23 ]3− are the only examples of electrochemically characterized dimeric boranes. The parent compound [B24H23]3− has recently been characterized124,125 by X-ray diffraction, and its structure is congruent with that conjectured by Wiersema and Middaugh.59 The unsubstituted dimer is more difficult to oxidize than the parent monomer. In any case, a clear redox pattern due to the presence of halogens as substituents cannot be achieved. The set of data is undoubtedly too small to allow any sound speculation, also because there is an irregular trend observed upon the progressive substitution of H with Br, which initially causes the potential to shift toward more positive values and then move back in the negative direction (Table 7). This phenomenon could be possibly due to the position of the bromo units on the cages, but in the absence of structural data, speculation is futile. Polymers/oligomers of monocarbaborane are less unusual than those of boranes; nevertheless, in this case the known redox data are even less extensive. We are aware of a single pair of data in which either an ethynyl or a vinyl bridge connects two undecamethyl monocarbaboranes through a Cc---Cc connection (Table 8). This single example is highly interesting since, even if the linkers do not allow the electronic communication between

reduced at −0.17 V, while [3,3′-CoIII(1,2-C2B9H11)2]− is reduced at −1.75 V, i.e., its potential is 1.68 V more negative than that of the neutral Ni complex. This can be reasonably taken as merely due to the increasing negative charge of the Co complex and is the same effect that explains the trend in Chart 5. Summarizing all these data, we obtain a sketch in which the d metal orbitals are split by a similar ligand field and then the set of orbitals is stabilized/destabilized according to the charge of the complex, like in Chart 6. This picture needs certainly to be consolidated by a flux of new experimental data, but it may 14323

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Table 6. Redox Potential Values (in volts vs Fc+/Fc) of Ansa and Diansa Metallacarboranesa E

compound M

Cb

B,B′-LB b

Fe

1,2-C2B9H10 7,9-C2B9H10b 1,2-C2B9H10 1,2-C2B9H10

Co

MIV/MIII

dppe μ-Pyr μ-(CH2OCH2) μ-Ph PPh POPh PSPh PBu PPh POPh PBu POBu

μ-Ph μ-Ph μ-Ph μ-Ph μ-Pyr μ-Pyr μ-Pyr

7,9-C2B9H10b 7,9-C2B9H10b 7,9-C2B9H10b

Ni Cu

C,C′-LB

+1.16

MIII/MII +0.10 −0.10 −1.62 −2.11 −1.64 −1.43 −1.47 −1.66 −1.58 −1.44 −1.85 −1.71 −0.99 −0.59 −0.48

MII/MI

ref

−2.20 −2.16 −1.12

122 119 c 123 123 123 123 123 123 123 123 123 119 119 119

a

Entries are indicated by the metal M, the ligand (carborane semi-cage) Cb, and the bridging ligand connecting either boron atoms B,B′-LB or carbon atoms C,C′-LB. bCharge compensated. cHarwell, D. E.; Nabakka, J.; Knobler, C. B.; Hawthorne, M. F. Synthesis and Structural Characterization of an Ether-Bridged Cobalta-Bis(Dicarbollide) - A Model for Venus Flytrap Cluster Reagents. Can. J. Chem. 1995, 73, 1044−1049.

Table 7. Redox Potential Values (in volts vs Fc+/Fc) of Dimeric Boranes in MeCN Solutiona

Redox data of dicarbaboranes are more abundant than those of boranes and monocarbaboranes, and the same is true for their oligomers. Dimers directly linked through a C−C bond exist for both o- and m-unsubstituted dicarbaboranes, and both are redox active, being reduced at a potential less negative than the parent compounds.54 This is in agreement with the electron-withdrawing character of carboranes, which is also evident in the anodic shift of the redox potential values of the redox-active bridges, in some other entries of Table 9.127,128 As expected, phenyl and biphenyl bridges make the reduction more accessible, but only in the case of the 1,4-phenylene-bridged species, the cyclic voltammetry manifests the signs of electronic communication between the two clusters. In fact, usually only a single 4e− process is observed, whereas for the case of the 1,4-phenylenebridged compound two nearly overlapped 2e− reduction processes are observed.81,88 In fact, when 1,3-phenylene links together two 1-Ph-1,2-C2B10H10 moieties, four reduction peaks are also detected by cyclic voltammetry. This has been ascribed to an inductive effect rather than to the presence of electronic communication between the cages, as further supported by the fact that the potential exhibits only a minor shift with respect to the parent 1,2-Ph2-1,2-C2B10H10. A small set of dimeric metallacarboranes all linked at B(8) either by the viologen-containing bridge ((OC2H4)2-MV2+(C2H4O)2)117 or by a porphyrin121 only demonstrates the inertness of these bridges in promoting any electronic communication between the connected moieties (Table 10). Recently, a set of phthalocyanines carrying four cobaltabisdicarbollide units has been synthesized with the aim to obtain highly boronated photoactive systems, which could be suitable for an attractive mix of boron neutron capture therapy (BNCT) and photodynamic therapy (PDT) of cancer (Figure 17).129 Considering the nonconjugated nature of the ethereal arms linking the cobaltacarborane moieties to the phtahlocyanine, unsurprisingly, the redox behavior of the first is unaltered and both a 4 e− oxidation and a 4 e− reduction are observed (Table 11).

E

compound B1

B2

LB

3−/2−

B12H11 7-I-B12H10 Brx-B12H10 Brx-B12H10 Brx-B12H10

B12H11 7-I-B11H10 Bry-B12H10 Bry-B12H10 Bry-B12H10

H H H H H

+1.34b +0.79b +0.38b +0.48b +0.74b

notes

ref

x+y=7 x + y = 10 x + y = 11

59 59 59 59 59

a

Entries are indicated by the borane cages B1 and B2 and by the bridging ligand LB. bIrreversible process.

Table 8. Redox Potential Values (in volts vs Fc+/Fc) of Dimeric Monocarbaboranes in MeCN Solutiona E

compound Cb

LB

2−/−

−/0

ref

CB11Me11 CB11Me11

ethinyl vinyl

+1.30b +1.30b

+1.54b +1.56b

126 126

a

Entries are indicated by the carborane cage Cb and the bridging ligand LB. bIrreversible process.

the two cages, it was possible to isolate the first examples of monocarbaborane-based neutral biradical systems (Figure 16). The black biradicals, whose stability was proven by cyclic voltammetry, have been successively prepared and isolated by chemical oxidation with PbO2/CF3COOH/CH3CN.126

Figure 16. Two examples of bridged monocarbaborane-based neutral biradicals. 14324

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Table 9. Redox Potential Values (in volts vs Fc+/Fc) of Dimeric and Trimeric Dicarbaboranesa E

compound n

LB

1,2-C2B10H11 7,9-C2B10H11 1,2-C2B10H11 1,2-C2B10H11 1,2-C2B10H11 1-Ph-1,2-C2B10H10 1-Ph-1,2-C2B10H10 1-Ph-1,2-C2B10H10

2 2 2 2 2 2 2 2

1,2-C2B10H10 1-Me-1,2-C2B10H10 1-Et-1,2-C2B10H10 1-Bu-1,2-C2B10H10

2 2 2 2

1-Me-1,2-C2B10H11 1-Me-1,2-C2B10H11 1-SiBuMe2-1,2-C2B10H10 1-Ph-1,2-C2B10H10

2 2 2 3

direct CC bond direct CC bond 1,4-Ph 1,3-Ph 2,2′-BPh 1,3-Ph 1,4-Ph 1,4-C6F4 TIPS-PEN 6,13-bis-ethynylPEN 6,13-bis-ethynylPEN 6,13-bis-ethynylPEN 6,13-bis-ethynylPEN (ppy)Ir(acac) 4-4′(ppy)Ir(acac) 5-5′(ppy)Ir(acac) (6,13-bis(p-Me-Ph)PEN-2,3,9,10-tetrayl)tetrakis(SiMe3) 4,4′,4″-B(2,6-Me2Ph)3

Cb

bridge

+0.37 +0.64 +0.67 +0.69 +0.69 +0.4 +0.56 +0.55

Cb

ref

−1.89 −3.07 −1.76,b −1.91b −1.77c −1.72c −1.60, −1.75, −1.86, −1.95 −1.56,b −1.94b −1.20,b −1.79b

54 54 88 88 88 81 81 81 d 127 127 127 127 128 128 128 e f

−2.60 −2.12 −2.19 −2.06 −1.6

−1.35

Entries are indicated by the carborane cage (Cb), the bridging ligand LB, and the number of connected units n. b2e− process. c4e− redox process. d Liang, Z.; Tang, Q.; Mao, R.; Liu, D.; Xu, J.; Miao, Q. The Position of Nitrogen in N-Heteropentacenes Matters. Adv. Mater. 2011, 23, 5514−5518. e Wang, Y. M.; Fu, N. Y.; Chan, S. H.; Lee, H. K.; Wong, H. N. C. Synthesis, Characterization, And Reactions of 6,13-Disubstituted 2,3,9,10Tetrakis(trimethylsilyl) Pentacene Derivatives. Tetrahedron 2007, 63, 8586−8597. fLee, K. M.; Huh, J. O.; Kim, T.; Do, Y.; Lee, M. H. A Highly Lewis Acidic Triarylborane Bearing Peripheral O-Carborane Cages. Dalton Trans. 2011, 40, 11758−11764. a

Table 10. Redox Potential Values (in volts vs Fc+/Fc) of Dimeric Metallacarboranesa E

compound MCb Fe(1,2-C2B9H10)2 Co(1,2-C2B9H10)2 Co(1,2-C2B9H10), (8-I-1,2-C2B9H9) Co(1,2-C2B9H10)2 Co(1,2-C2B9H10)2 Co(1,2-C2B9H10)2

LB

M

IV/IIIb

2+

8,8-(OC2H4)2MV (C2H4O)2 8,8-(OC2H4)2MV2+(C2H4O)2 8,8′-(OC2H4)2MV2+(C2H4O)2 8,8-(C2H4O)2-N,N′-H2TPP-(C2H4O)2 8,8-(C2H4O)2-N,N′-H2OEP-(C2H4O)2 8,8-NH[(C2H4O)2-H2TPP-NH[(C2H4O)2

+1.07c,d, +1.14c,d +1.14c,d

MIII/IIb

ref

−0.72 −1.75 −1.53 −1.90d −1.89d −1.85d

117 117 117 121 121 121

a Entries are indicated by the metallacarborane (MCb) and the bridging ligand LB. b2e− process. cChemically irreversible. dMeasured in PhCN. A correction has been applied based on the fact that the Fc+/Fc redox potential is anodically shifted by ∼100 mV in PhCN with respect to MeCN. Siclovan, O. P.; Zappi, G.; Soloveichik, G. L. High-Temperature Cyclic Voltammetry in Non-Aqueous Solvents. ECS Electrochem. Lett. 2014, 3, H41−H43.

2.5. Properties of Bridging Boron Clusters

Fc-1,2-C2B10H11, 1,2-Fc2-1,2-C2B10H10, and 1-Fc-2-Fc-ethynyl1,2-C2B10H10 (Figure 19) perfectly reflects this behavior;130 in this case, the bridging o-dicarborane splits the oxidation processes of the two connected ferrocene units into two different waves, separated by 140 mV, and this separation reduces to ∼80 mV when an ethynyl linker is interposed.

In spite of the large amount of literature on substituted boron clusters, compounds in which the cluster plays the role of a bridging ligand linked to two redox units have only been seldomly electrochemically characterized. Nevertheless, some examples exist in which o-, m-, or p-carborane connects directly or via a further bridge (LB) to two symmetrical or nonsymmetrical A, B redox units (Figure 18). The separation ΔE between the redox processes of two equivalent units is a measure of the efficiency of the dicarbaborane to facilitate the electronic communication. By looking at the meagre collection of data in Table 12, it emerges that directly linked o- and p-carboranes seem to allow electronic communication between the redox units. Data for the m-dicarbaborane linker are not available, but it is reasonable to expect a similar behavior, which is the effect of the highly conjugated bonding in boron clusters. The presence of one or two additional bridging groups between the dicarbaborane and the two redox units strongly attenuates the electronic communication between them, eventually leading to its annihilation. The set of compounds 1-

2.6. Summary and Outlook

In this section, the updated electrochemical properties of the larger icosahedra borane, monocarbaborane, and dicarbaborane clusters together with their metal complexes have been reviewed. This review has shown the wide diversity of redox potential values possible that span almost every available solvent electrochemical window. Directions for how to tune the redox potential have been given. The electrochemical properties are mainly directed by the charge of the cluster, usually −2, −1, or 0. When boron clusters are studied, reduction of dinegative species is usually unattainable, whereas their oxidation is accessible. On the other side, neutral species (closo-carboranes) can be reduced but only oxidized with difficulty. Intermediate situations can be generated by substituting the B−H units by B-donor or B14325

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range of E° values that can also be modified by the nature of the sandwiched metal. We conclude this survey on the redox properties of boron clusters by underlining their very rich redox behavior, a detailed overview of which has now been provided. However, the effectiveness of the many possibilities for tuning the systems promises to reveal new features in the future. Thus, it is the opinion of the writers of this review that this field of research is far from being exhausted.

3. PHOTOLUMINESCENCE (PL) BEHAVIOR OF CARBORANE-CONTAINING SMALL MOLECULES Geometrical rigidity and optical transparency to UV−vis light below 200 nm,131−134 among others, are important features that carboranes (C2B10H12) can offer to scientists in order to customize the final optical properties of a particular material. In addition, it has been demonstrated that the influence of the position of the two carbon atoms within the carborane cluster (ortho, meta, and para), the nature and the geometry arrangement of the substituents of the cluster, and the electronic nature of the cage (closo or nido) have critical effects on the photoluminescence (PL) behavior of the final molecule. It was proven by electrochemical and theoretical calculations that the electronic acceptor capability of the different isomers increases when the symmetry of the cage is lower in the following order: ortho ≫ meta > para.135,136 These differences between the different isomers can be explained as a result of a larger distortion of the cluster in the case of the ortho isomer, which leads to a minor electronic density delocalization within the cage.137 Over the last 10 years these facts have attracted the interest of many scientists in order to understand the PL of systems containing these boron cages. In this section, the photophysical properties of discrete molecules containing o-, m-, or p-carboranes will be mainly discussed. Other fluorescent systems that contain boron clusters different to C2B10H12, such as B12(OR)12,70 have also been reported; nevertheless, they are not be included in this review.

Figure 17. Phthalocyanines functionalized with four cobaltabidicarbollide units (M = Zn, Co, MnCl).

Table 11. Redox Potential Values (in volts vs Fc+/Fc) of Tetrameric Cobaltabisdicarbollides Bridged by Metallophthalocyanine Unitsa M Zn Co Co MnCl a

CoIV/III +1.17

CoIII/II

solvent

ref

−1.74 −1.71 −1.75 −1.73

DMF MeCN DMF DMF

129 129 129 129

Entries are indicated by the phthalocyanine-bound central metal.

Figure 18. Bridging modes of dicaboranes.

3.1. Photophysical Properties of Carborane Derivatives Containing Fluorophores Not Directly Bonded to the Ccluster (Cc)

acceptor groups or C−H units by C-donor or C-acceptor groups. Remarkably, there are many B−H units that can be stepwise substituted, thus opening a wide range of possibilities. In the case of dicarbaboranes the electrochemistry is very much dependent on the isomer studied, i.e., o-, m-, or p-carborane; this phenomenon is recurrent in this section as well as in the sections that follow. For o-carboranes the LUMO is mostly located on the two carbon atoms (Cc) and is relatively low in energy, a situation that facilitates its reduction, but this does not occur with the meta and para isomers. This relatively accessible reduction determines the electrochemical reduction of ocarboranes and many of their photoluminescent properties (PL), as will be described in the following sections. Therefore, a great difference in chemical reactivity and properties related to electron transfer between the o-carborane and its isomers m- and p-carboranes is foreseeable. In what concerns metallacarboranes the most widely studied are the metallabisdicarbollides, which have strong similarities to metallocenes. Similarly to the latter, they usually display electrochemical reversibility but are negatively charged. As with the boranes and carboranes their E° values can be tuned by substituting the hydrogen atoms in B− H units by donor or acceptor groups, thus again providing a wide

The importance of the cluster’s electronic nature on PL properties was observed by Núñez and co-workers in 2012 when a series of styrenyl derivatives that included both closo (1− 4) and nido o-carborane clusters (Figure 20) was studied.138 It was found that closo compounds exhibited much higher fluorescence quantum yields (ΦF), up to 40% in the case of 3 (Table 13), over their nido derivatives in a polar solvent such as MeCN. As expected, both absorption and emission spectra were dominated by the styrene fluorophores of the molecules, showing strong emission bands at 312 nm for 2−4. However, Ph-o-carborane derivative 1 exhibited very different behavior, not only experiencing an almost total lack of fluorescence in MeCN but also showing a very small emission band at 409 nm, which was attributed to a charge transfer process (CT) from the electron-donor styrene moiety to the electron-acceptor Ph-ocarborane unit. To confirm this point, cyclic voltammetry and TD-DFT calculations were carried out. CVs indicated that the reduction potential in 1 was 0.9 V more positive than that of 3, indicating the improved electron-acceptor capacity of the Ph-ocarborane unit compared with the Me-o-carborane and ocarborane units. Theoretical calculation results confirmed that the HOMO and LUMO are always localized on the styrene moiety in the fluorescent compounds 2−4, whereas the LUMO 14326

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Table 12. Redox Potential Values (in volts vs Fc+/Fc) and ΔE Separations (in mV) of Compounds with Bridging Dicarboranesa E

compound Cb p-C2B10 p-C2B10 p-C2B10 Hg(p-C2B10)2 o-C2B10 o-C2B10 m-C2B10 p-C2B10 o-C2B10 o-C2B10 o-C2B10 o-C2B10 p-C2B10 p-C2B10

LB1

LB2

CC

CC

p-Ph p-Ph p-Ph p-Ph p-BiPh

p-Ph p-Ph p-Ph p-BiPh

CC CCPhCC CCPhCC

CCPhCC CCPhCC

A

B

EA

η-CpFe(CO)2 η-CpFe(CO)2 η-CpFe(CO)2 η-CpFe(CO)2 4-(N-carbazole) 4-(N-carbazole) 4-(N-carbazole) 4-(N-carbazole) 4-(N-carbazole) Fc Fc Fc BiPy−Ir(dfppy)2 BiPy−Ir(dfppy)2

H η-CpFe(CO)2 η-CpFe(CO)2 η-CpFe(CO)2 H 4-(N-carbazole) 4-(N-carbazole) 4-(N-carbazole) 4-(N-carbazole) H Fc Fc BiPy−Ir(dfppy)2 BiPy−Ir(dbpz)

+1.14 +1.04 +1.05 +1.09 +0.33 +0.32 +0.34 +0.24 +0.31 +0.26 +0.20 +0.19e −1.53 −1.55

E EB

ΔE

+1.18 +1.05 +1.09

140 0 0

+0.32 +0.34 +0.24 +0.31

0 0 0 0

+0.34 +0.27e −1.53 −1.55

140 80 0 0

Cb

−2.28

−2.19

ref b,c b,c b,c b,c 92,d 92,d 92,d 92,d 92,d 130 130 130 f f

a

Entries are indicated by carborane clusters (Cb), intermediate bridging groups (LB1‑2), and redox units (A and B). bWedge, T. J.; Herzog, A.; Huertas, R.; Lee, M. W.; Knobler, C. B.; Hawthorne M. F. Metal−Metal Communication through Carborane Cages Supporting Electroactive [η5CpFe(CO)2] substituents. Organometallics 2004, 23, 482−489. cBitner, T. W.; Wedge, T. J.; Hawthorne, M. F.; Zink, J. I. Synthesis and Luminescence Spectroscopy of a Series of [η5-CpFe(CO)2] Complexes Containing 1,12-Dicarba-closo-dodecaboranyl and -ylene Ligands. Inorg. Chem. 2001, 40, 5428−5433. dLi, X. Q.; Wang, C. H.; Zhang, M. Y.; Zou, H. Y.; Ma, N. N.; Qiu, Y. Q. Tuning Second-Order Nonlinear Optical Properties of the Two-Dimensional Benzene/Carborane Compounds with Phenyl Carbazoles: Substituent Effect and Redox Switch J. Organomet. Chem. 2014, 749, 327−334. eValues estimated from the square wave voltammogram taken from ref 130. fIndelli, M. T.; Bura, T.; Ziessel, R. pCarborane-Bridged Bipyridine Ligands for Energy Transfer between Two Iridium Centers. Inorg. Chem. 2013, 52, 2918−2926. Other redox reactions are also present, centered on the peripheral Ir ligands, which are not reported here.

With the aim of providing some guidance on how to tune the solution PL of different o-carborane derivatives and relating this to the Cc−Cc bond distance, the same authors reported the synthesis of two sets of closo-carborane derivatives bearing fluorene and anthracene as fluorophore groups (Figure 21).141 In the first place, for all fluorene derivatives 5−8, in which the fluorophore group is distant from the o-carborane cage, an intense emission band in the fluorescence spectra with maxima between 345 and 352 nm was observed, independent of the electronic nature of the substituent at the adjacent Cc (Me or Ph). Indeed, the ΦF of 5−8 was quite high (27−37%, Table 13). Conversely, the highly emissive fluorene group is totally quenched in CH2Cl2 when it is directly attached to the cage, confirming the presence of a new CT process from the fluorene to the antibonding orbital on the Cc−Cc bond of the cage. This process was investigated in more depth by attaching a second fluorophore group to the adjacent Cc (9−10). Both compounds showed bands corresponding to both fluorophore groups in their absorption spectra. However, in the fluorescence spectra the emission of 9 is absent, and only a very low emission was observed for 10 (ΦF = 5%). This can be explained by considering the direct bond of the fluorene to the Cc, which makes the fluorene−carboranyl unit an extremely efficient electron-acceptor group. A correlation between the cluster Cc−Cc distance and the emission intensity displayed by the anthracene derivatives 11−15 was also reported therein. In these cases, the importance of the electronic nature of the cluster substituent becomes extremely noticeable, due to the very high efficiencies of the Meo-carborane derivatives 11 and 14 (ΦF = 82% and 94%) and the total lack of emission of their counterparts with Ph-o-carboranes 12 and 15. In parallel, 11 and 14 presented significantly shorter Cc−Cc distances than 12 and 15. Therefore, a generally good correlation between the Cc−Cc distance and the fluorescence intensity can be made: the longer the distance, the more chances to quench the emission.

Figure 19. Cyclic voltammograms of 1-Fc-1,2-C2B10H11 (top), 1,2-Fc21,2-C2B10H10 (middle), and 1-Fc-2-Fc-ethynyl-1,2-C2B10H10 (bottom). Adapted from ref 130. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

is localized on the phenyl unit of the Ph-o-carboranyl derivative 1, supporting the existence of a CT process from the styrene moiety to the Ph-o-carborane cluster, due to the electron-withdrawing character of the latter (Figure 20). A key feature also observed was the lengthening of the Cc−Cc bond distance of compound 1 in comparison with 2−4 (Table 13). This phenomenon was reported earlier by the same group, when it was observed that the Cc−Cc distance in o-carborane could be tuned with groups linked to the Cc that possess lone pairs. The explanation provided was the partial filling of the carborane LUMO that was predominantly centered on the cage Cc−Cc unit (σ* Cc−Cc), leading to a C−C bond elongation.86,87,139,140 14327

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Figure 20. closo-o-Carboranyl-styrenyl derivatives 1−4 and the partial density of states spectra (PDOS) of (a) 1 and (b) 3, showing their MO contributions. Reprinted with permission from ref 138. Copyright 2012 Wiley.

Table 13. Key Data for Selected o-Carborane Derivatives Containing Fluorophores Not Directly Attached to the o-Carborane Cluster though the Carbon Atoms compound

a

no.

fluorophore

Cc-X

close in space

d(Cc-Cc, Å)

λabs (nm)

λem (nm)

ΦF

ref

1a 2a 3a 4a 5b 6b 7b 8b 9b 10b 11b 12b 13b 14b 15b 16c 17c

CH2-styrene CH2-styrene CH2-styrene CH2-styrene (CH2)3Si(CH2)2-fluorene (CH2)3Si(CH2)2-fluorene CH2C6H4(CH2)2Si(CH2)2-fluorene CH2C6H4(CH2)2Si(CH2)2-fluorene CH2-styrene CH2-anthracene CH2-anthracene CH2-anthracene CH2-anthracene CH2-anthracene CH2-anthracene amino benzo[g,h,i]perylene amino benzo[g,h,i]perylene

Ph Me H CH2-styrene Me Ph Me Ph (C8H17)2-fluorene (C8H17)2-fluorene Me Ph CH2-anthracene Me Ph Me Me

yes yes yes yes no no no no yes yes yes yes yes yes yes yes yes

1.712 1.684 1.645 1.688 1.667 N/R N/R N/R N/R N/R 1.674 1.702 1.686 1.674 1.694 N/R N/R

254 254 254 254 287 285 288 287 259 252 251 251 251 255 255 316 314

409 312 312 312 352 352 345 347 N/R 419 418 N/R 419 427 N/R 453 449

[B(3,5-(CF3)2Ar)4]− ≈ [1-HCB11Cl11]− ≫ [1-HCB11H5Br6]−. Electrodes containing a Pb2+-selective ion carrier were also used to evaluate the functionality of each cation exchanger. An evaluation of the response characteristics such as slope and selectivity indicated that the undecaiodinated and the undecabrominated derivatives were quite comparable to the behavior of B(3,5-(CF3)2Ar)4−. Interestingly, both showed a marked selectivity improvement over Cd2+. With its excellent stability and very low binding affinity, [1-HCB11I11]− was stated by the authors to be the most promising exchanger. Ion-selective membranes of conventional composition containing calix[6]arene hexaacetic acid hexaethylester (Cs−I) as ion carrier and [1-HCB11I11]− as ion exchanger exhibited excellent performance with respect to their selectivity over alkali and alkaline earth metal cations with detection limits of 1 ppb for Cs+.233 On the other hand, [CB11H6Br6]− was used as an anionic additive in Ag+ ISEs,234 and their performance was compared to those of [B(4-ClAr)4]−, observing a reduction of the Hg2+ interference of 6 orders of magnitude. The synthesis of one carbon-substituted derivative of the closomonocarborane anion with a polymerizable olefinic group demonstrated that the covalent grafting of the cation exchanger onto the polymeric backbone of the sensing material may be a viable way to circumvent its leakage.235 This new derivative was copolymerized with methyl and decyl methacrylate to fabricate a plasticizer-free polymer with cation-exchange properties. The soobtained ISE was evaluated in terms of response function, response time, and selectivity. In all cases, the new material exhibited behavior similar to the free [BPh4]−-derivative-based membranes together with a greatly improved stability of the ionexchanger retention. In addition to monocarboranes, metallacarboranes have also been widely used in this field. Teixidor and co-workers produced the first glassy carbon electrode containing a polypyrrole film doped with the [Co(C2B9H11)]− anion.236 This electrode has been demonstrated to be appropriate for pH measurements and for acid/base titrations. The long response time, due to the film thickness, which would have prevented the use of this electrode for multiprotic titrations, has been significantly improved by obtaining PPy wires using the templating effect of a non-

with high selectivity, low detection limit, and fast response time is a very active field of research. Liquid membranes have received much interest because of their versatility. In this case, the membrane itself is set up by a liquid immiscible in the (aqueous) solutions to be analyzed and usually stabilized by a polymeric matrix. The crossing of the membrane barrier by selected ions is effected by ion exchangers and/or specific ion carriers dissolved in the liquid membrane. These latter catch and release a specific ion, according to the concentration gradient. This way the duty of selectivity is assigned to the ion carrier, and this is the main source of the versatility of liquid membranes, since many different complexing agents can be designed and tested. The transport mechanism, based upon the formation of a complex,

Figure 76. Molecular structures of (a) 9-crown-3 ether, (b) [9]mercuracarborand-3 (MC3) macrocycle, and (c) its hexamethylated derivative.

together with the lipophilic nature of the membrane (necessary to avoid its dissolution in water) make the development of hydrophilic-anion-selective membranes a formidable task. In recent decades, the incorporation of boron cluster compounds as ion carriers into the membranes has attracted the attention of many researchers. Bachas and co-workers228 prepared and tested a chloride-selective electrode by using the so-called “[9]mercuracarborand-3” (MC3, Figure 76), a neutral macrocyclic Lewis acid, as ion carrier, which can be considered the chargereversed analogue of the well-known 9-crown-3 ether. Mercuracarborands are air stable, lipophilic enough to be dissolved in the membrane, and have a cavity, the dimensions of which can be conveniently preorganized. Moreover, the presence of the dicarborane offers the opportunity for fine functionalization of the skeleton, which can be made, for example, more or less lipophilic and more or less electron rich. Membranes loaded with MC3 turned out to be highly sensitive and selective for chloride. The ligand acts as a neutral carrier, and accordingly, anionic response is observed only in the presence of tridodecylmethylammonium chloride (TDMAC) as lipophilic cationic site in the membrane. MC3/TDMAC-based electrodes have a near-Nerstian response to chloride concentration over a wide concentration range and with micromolar detection limits. Moreover, they have a good selectivity for chloride over other anions, including highly lipophilic anions such as perchlorate, nitrate, and thiocyanate, and the selectivity coefficients meet the requirement for clinical applications. In addition, these chloride sensors show fast response times (in the order of few seconds) as well as short recovery times, being able to resist pH changes over the range of 2.5−7.0 and to maintain their response characteristics after 2 months of aging. An equally efficient optode has been built by using 4′,5′-dibromofluorescein octadecyl ester as the revealing proton chromoionophore.229 Successively, it was found by the same authors229 that the addition of two peripheral methyl groups to each carborane of the macrocycle (Figure 76) resulted in a less selective ion carrier, because of the weakening of 14363

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Figure 77. Sequential color changes following the redox reactions of cobalt (a) and iron (b) viologen derivatives.

and oxidized states, changing from dark orange to light blue, passing through orange, yellow-green, faded green, green-blue, and blue.214 Furthermore, an electrochromic device based on films has been prepared that is stable and robust. In the case of PEDOTn[(m-6Cb(T)2]m copolymers, with different EDOT:m6Cb(T)2 ratios, the color range is more limited, ranging from purple (neutral) to blue (oxidized) films.215 Very recently,117 dimeric iron and cobalt metallacarboranes have been prepared in which the clusters are coupled either ionically or covalently to viologen (see section 1). These compounds showed impressive electrochromic behavior through sequential electrolysis, each step being perfectly reversible, the limit being the addition of two electrons to viologen (Figure 77). In the case of the cobalt derivative, the solution switched from a bright yellow color (CoIII and viologen) to navy blue (CoIII and viologen radical cation) and finally to orange (CoIII and diradical viologen). In the case of the iron compounds, four different colors are obtained: the initial brown (FeIII and viologen), pale pink (FeII and viologen), navy blue (FeII and viologen radical), and orange (FeII and diradical viologen). As can be seen, little has been published in the field of electrochromism of boron clusters, and with the notable exception of [Fe(C2B9H10I)2]2MV, the redox changes of boron clusters are not directly involved in the color change phenomenon. This may be due to the electrochemical instability of several oxidation states of the boron-based compounds; nevertheless, the existence of a brightly colored accessible and stable redox state, as in the case of the closomers, suggests that this is a research field which deserves to be explored in deeper detail. 6.1.3. Electronic Motion. As discussed in section 1, metallabisdicarbollides are able to adopt different conformations (cis or transoid) by changing the oxidation state of the sandwiched metal. Conformational changes have been mostly studied for nickelabisdicarbollide and its derivatives. A review analyzing this property of nickelabisdicarbollide appeared in 2000 by Sivaev and Bregadze;243 this part of the review will briefly go over the most recent updates and advances in this field.

conducting polymer (PVC) cast over the graphite surface of the electrode before the polypyrrole electrodeposition. This electrode can be used in organic media for long periods without significant loss of response. In addition, as discussed in section 4, this anion provides enhanced ORL to PPy and, at the same time, the use of such a high-volume anion with low charge density prevents the dopant−anion exchange usually observed in alkaline media.237 These novel [Co(C2B9H11)]− electrodes were further improved, realizing the fabrication of hydrogen-selective microelectrodes on silicon needle-shaped substrates.238,239 The devices operate satisfactorily, with a response showing good sensitivity and selectivity against common interfering cations in background solutions. In 2008, Teixidor and co-workers240 went a step further and designed a specific ISE containing [Co(C2B9H11)]− to detect and quantify Tuberculosis antibiotics. The metallacarborane is bound together via dihydrogen bonds with both the protonated amines of the drugs and the plasticizer (PVC), preventing the leakage of the electroactive component. The similar but still somewhat different responses of the electrodes containing different drugs in the electroactive ion-pair complexes allows discrimination between the drugs themselves. This principle has also been used to generate electrodes able to quantify dopamine, nicotinic acid, nicotinamide, histamine, and metformin in aqueous solutions.241 Moreover, the incorporation of enantiomerically pure protonated amino acids in the electroactive ion-pair complexes (tryptophan, histidine, and arginine) allowed the authors to easily obtain an enantiomer-selective membrane electrode.242 These studies established the first evidence that detection of enantiomers is possible without chiral receptors. 6.1.2. Electrochromism. Electrochromism is a new application/property of boron cluster compounds that has appeared in recent years. This property relies on the chromatic change of compounds at different oxidation states. The polymer Pm-6Cb(T)2 and the copolymer PEDOTn[(m-6Cb(T)2]m discussed in section 4 are recent examples of the electrochromic ability of boron-cluster-containing materials. Pm-6Cb(T)2 exhibits a multicolor electrochromic behavior between its neutral 14364

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As discussed in section 1, NiIII metallacarborane has a transoid sandwich conformation with two pairs of carbon vertices reflected through a center of symmetry, but NiIV species have instead a cisoid conformation (see Figure 14). The interconver-

have been characterized by NMR and X-ray structural determination. Three years later, in 2012, Teixidor and Viñas245 envisaged a cobaltabisdicarbollide derivative as an electrochemically driven molecular clutch or molecular switch. Bearing in mind the steric hindrance of the outcoming beams of the two carboranyl ligands and the distance between them, they proposed an engagement/ disengagement/engagement system. Taking as an example the pristine cobaltabis(dicarbollide), the outcoming beams of the upper [7,8-C2B9H11]2− ligand merge with the lower partner in the CoIII compound. At this point there is no possibility of rotation at room temperature, due to the merging and the steric hindrance of the beams (engaged form). Alternatively, when reducing from CoIII to CoII, the distance between the two carboranyl ligands elongates from 2.25 to2.50 Å (due to the increased CoII radius). This fact allows the free rotation of the ligands in the CoII isomer (molecular clutch disengaged). Finally, a reoxidation of the metallacarborane leads again to a shortening of the distance between the ligands, recovering an engaged conformation at its most energetically favored conformation (depending on the substituents of the carboranyl ligands in each case). This molecular clutch system is clearly illustrated in Figure 80. Finally, a very recent paper from Su and co-workers246 predicted the clockwise or anticlockwise rotation of the Ni(C2B9H11)2 conformer (and its B and C derivatives) during the NiIV/III redox change, based on the relative energies obtained by DFT calculations. The authors focused mainly on the stable states of each conformer before and after the rotation, controlled by simple electron transfer. Some of the allowed and forbidden rotations are depicted in Figure 81, depending on the substituent, the oxidation state of the metal, and the optimized geometry and relative stability of each conformer. 6.1.4. Dye-Sensitized Solar Cells. Boron clusters have had not a high impact in the field of dye-sensitized solar cells (DSSCs); however, a couple of relevant papers were published in 2010 by Mirkin, Marks, Hupp, and co-workers.247 Nickelbisdicarbollide was used as a platform to produce a set of derivatives with different electron-withdrawing and -donating groups (see Figure 82) to be tested as an alternative to the typical I−/I3− redox couple as electrolytes in DSSCs. These analogues exhibit extraordinary chemical stability, noncorrosive behavior, and good solubility, readily undergoing multiple redox transformations involving net charges of −2, −1, and 0 with NiII, NiIII,

Figure 78. Calculated energies as a function of rotation angle for the pristine nickelabis(dicarbollide) for both NiIII and NiIV compounds, confirming the transoid and cisoid stability, respectively. Reprinted with permission from ref 108. Copyright 2006 IUPAC and Walter de Gruyter GmbH.

sion between the two provides the basis for controlled rotational and oscillatory motion, which is able to deliver useful work to nanodevices under the control of electrochemical or photochemical power sources. In fact, in 2004, Hawthorne and coworkers were able to control the rotatory motion around the molecular axis by simple electron-transfer processes and by photoexcitation.106 The energies of the NiIII and NiIV species were calculated as a function of the rotation angle, as depicted in Figure 78, demonstrating that the minimum energy for the pristine NiIII metallacarborane appears at 0° (fully transoid conformation), while the minimum energy for the Ni IV compound appears at 144° (cisoid conformation).108 The Xray structure of two conformers is shown in Figure 79. The same fact has been confirmed again by Hawthorne and coworkers in 2009244 with another couple of nickel compounds. Two diastereoisomers of nickel bis(C-monomethyldicarbollide) complexes, derived from the racemic [nido-7-CH 3 -7,8C2B9H11]− anion, with a cisoid and transoid conformation depending on the oxidation state of the central metallic atom,

Figure 79. Crystal structures of [(3-Ph-7,8-C2B9H10)2NiIV] (left) and [(3-Ph-7,8-C2B9H10)2NiIII]− (right). Reprinted with permission from ref 108. Copyright 2006 IUPAC and Walter de Gruyter GmbH. 14365

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Figure 80. Engagement/disengagement process of [3,3′-Co(1,2-C2B9H11)2]−. The negative charge corresponding to each cluster has been omitted to emphasize the motif of interest. Adapted with permission from ref 245. Copyright 2012 IUPAC and Walter de Gruyter GmbH.

and NiIV oxidation states, respectively. These systems provided an impressive DSSC open-circuit voltage of up to 770 mV, in contrast to the 200 mV obtained by using the Fc/Fc+ couple. This significant enhancement was attributed to the large rotational barrier of the dicarbollide ligands.248 6.2. Applications of Photoluminescent Carborane-Containing Compounds

In this part the applications of fluorescent compounds containing borane and carborane moieties will be compiled. One of the most thoroughly explored applications is the use of these materials as sensors for nucleophilic anions,249−251 exploiting the possibility of easily changing the electronic nature of the o-carborane cluster from closo to nido. Other important applications tested for these fluorescent materials are their use as dyads in optoelectronic devices, such as organic field-effect transistors (OFETs)91 or phosphorescent organic light-emitting diodes (PHOLEDs)143 and their use in microscopy imaging and especially in twophoton absorption (TPA) for biomedicine purposes.253 6.2.1. Materials Containing o-Carborane Moieties for Sensing Nucleophilic Anions. In 2011 Lee and co-workers began exploring the possibility of incorporating side-chain methyl-o-carborane moieties to polyethylene in order to retain the properties of the carboranyl monomer (Figure 83).249 These polymers when bearing Me-o-carborane clusters (i.e., P211) showed very low emission in solution but strong purple emission in the solid state, showing two clear emission bands at 347 and 363 nm upon excitation at 292 nm, which were very similar values to the starting carboranyl monomer. After partial degradation of the closo clusters to form nido clusters, a complete quenching of the fluorescence is observed in the solid state, due to the change of the electronic nature of the cage. In addition, the lack of emission in the nido-carboranyl monomer was also confirmed. Although the results of this work demonstrated the validity of these carboranyl polymers as chemodosimeters for sensing anions such as OH− or F−, they presented two important

Figure 81. Uni- or bidirectional rotation of the cage ligands in nickel− metallacarboranes, depending on its substituents, its oxidation state, and the stability of each conformer. Reprinted with permission from ref 246. Copyright 2013 Royal Society Chemistry.

Figure 82. Chemical structure of six different nickelabisdicarbollide derivatives, with their NiIV/III redox change.

Figure 83. Partial degradation of Me-o-carborane in polymer P211. 14366

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observing a conversion from 213 to 214 and a color change in the emission from blue to green, which suggests the possibility of using 213 as a chemodosimeter for F−. 6.2.2. Dyads Containing Carborane Moieties in Organic Field-Effect Transistors (OFETs). One of the most sought-after applications for materials is their use as semiconductors for organic field-effect transistors. Although Koezuka and co-workers reported the first OFETs in 1987,254 their commercialization by the global high-tech industry to produce low-cost, large-area electronic devices is quite recent. One of the most important features of these materials is the inclusion of aromatic systems, which possess high π-electron delocalization. When an electron-withdrawing or -donating group is attached to the aromatic system, the electron mobility is usually improved, enabling the production of improved devices. Buckminsterfuller-

drawbacks: (i) low incorporation of carborane groups into the polymer and (ii) low solubility of the polyethylene backbone in organic solvents. In a second study, Lee et al. improved both features by synthesizing polynorbornene copolymers with pendant Ph- or Me-o-carboranes and carbazoles.251 Both the absorption and the emission spectra were found to be controlled by the carbazole group. Whereas for the Me-o-carborane polymer the emission intensity followed a similar pattern as in the previous work, for the Ph-o-carborane polymers the opposite effect was observed. CV studies demonstrated that the fluorescence quenching of the carbazole group in the polymers bearing Ph-closo-o-carboranes is due to a PET process from the carbazole to the o-carborane. When the latter becomes a nido cage, the process is blocked, restoring the original emission of the carbazole. The degradation of the closo clusters of the polymer that contain the highest Ph-o-carborane to carbazole ratio led to an emission color change from light yellow to blue in the solid

Figure 85. Molecular structure of compounds 215−219 including an ocarborane unit attached to PCBM.

Figure 84. Molecular structure of the triarylborane dyad 214 containing nido-Me-o-carborane.

ene and its derivatives, especially [6,6]-phenyl-C61-butyric acid methyl ester (PCMB), have been widely studied for this application due to their excellent electron-transport properties. With the aim of further improving the electron mobility of PCMB, Lee and co-workers synthesized a new set of PCMB derivatives including o-carborane clusters as electron-withdrawing groups (215−219 in Figure 85) via trans-esterification.91 Different sized linkers (Ph, (CH2)n, where n = 1, 3, 6, 11) between the fullerene and the carborane were used to investigate the importance of the distance between both moieties in their electronic properties. As expected, absorption spectra and cyclic voltammetries showed that all of the optical and electrochemical properties were heavily dominated by the methanofullerene moiety, with very small or no influence by the o-carborane cage. For the testing of the dyads as OFET devices, it was seen that the shorter the distance between the cluster and the C60, the higher the electron mobility of the FET, obtaining the best result for the compound with the shortest linker with an electron mobility value of 1.72 × 10−2 cm2·V−1·s−1, which is similar to that of PCBM.255 6.2.3. Dyads Containing Carborane Moieties in Phosphorescent Organic Light-Emitting Diodes (PHOLEDs). For the development of new PHOLEDs, materials with high triplet energies are required to exhibit efficient blue phosphorescence. For this purpose, the organic functional groups must be electronically isolated and their original phosphorescence preserved. In addition, these materials have to possess high glass transition temperatures (Tg) and low volatilities. These properties were achieved by Kang and coworkers, who used m- and p-carborane moieties to isolate two carbazolyl phenyl units as well as to increase their thermal stability (Figure 86).252 In their work they compared materials

state, making these compounds suitable for sensing nucleophilic anions. In parallel, the same group synthesized a triarylborane dyad in which a closo-o-carborane is attached to a Ph-dimesitylborane (213)250 with the aim of enhancing the Lewis acidity and making it a suitable candidate for F− sensing. The corresponding nido-ocarborane derivative (214 in Figure 84) was also synthesized, and the PL properties of both compounds were measured. The absorption spectra of 213 in different solvents have the same maxima positions, whereas 214 is blue shifted with increasing polarity of the solvent. On the other hand, in the emission spectra both 213 and 214 are red shifted when increasing the polarity of the solvent. Remarkably, 214 fluoresces at much lower energy (about 70 nm) and with much more intensity (up to 20-fold in acetone) than 213. The solvatochromic behavior of both compounds in the excited state implies that the latter has a polar character, suggesting an intramolecular CT process, a fact that was confirmed by TD-DFT calculations. The F− affinity of both compounds was tested by the addition of TBAF in two different media, pure THF and THF−H2O mixture (9:1 vol), and examined by UV−vis titrations. Whereas the quenching of both the absorption and the emission bands of 213 is noticeable, implying very high binding constants (K up to 107 M−1), for 214 the response to F− addition was much lower. These results confirmed that the fluorophilicity of the B atom increases when 213 is introduced, making the triarylborane a better Lewis acid. However, the low quantum efficiency of 213 may be an important drawback for fluorescence sensing. For this reason, the addition of TBAF in a THF−H2O mixture (9:1 vol) at different temperatures was also investigated by fluorescence spectroscopy, 14367

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Figure 86. Molecular structure of carborane-containing dyads 220−222.

Figure 87. Fluorescence micrographs of cells after 24 h incubation with 10 μM porhyrin 87. (b and c) Dotted red fluorescence distributed throughout the cytoplasm (excitation at 420 nm and fluorescence detection between 650 and 700 nm); (e and f) fluorescence distribution observed after incubation with 10 mM Lucifer Yellow (excitation at 425 nm and fluorescence detection between 500 and 550 nm). (a and d) Bright field images, (b and e) fluorescence images, and (c and f) overlaid bright field and fluorescence images. Reprinted with permission from ref 256. Copyright 2007 Elsevier.

has become one of the most important tools in modern biomedicine, as some fluorochromes are able to stain cells, tissue components, or bacteria. As they can be monitored by microscopy, they can provide fundamental information to scientists on how pharmacophores can reach specific targets. Appropriate molecules carrying carborane clusters can be followed by fluorescence microscopy and can be useful for BNCT. Fluorescence microscopy was used to examine the intracellular distribution pattern of carboranyl-containing porphyrin 87168 in the B16F1 cell line.256 The images thus obtained showed that melanoma cells incubated for 24 h with the porphyrin exhibited a dotted red fluorescence distributed throughout the cytoplasm. A large number of fluorescent spots are visible, especially in the pseudopodia, which are a prominent feature of actively motile cells in vitro. The fluorescence images of 87-loaded cells are shown in Figure 87. A very similar fluorescence distribution was found for cells stained in parallel with the endosome probe Lucifer Yellow, suggesting an essentially identical localization with the porphyrin. Conjugates 90−94 showed red-shifted (1−5 nm) and broadened absorption bands in HEPES buffer (20 mm, pH

containing carboranyl units with their benzyl analogues in order to understand the role of the carborane in the final optoelectronic properties. UV−vis and emission spectra revealed small Stokes shifts for the m- and p-carborane derivatives (221, 222) compared with the other compounds, particularly with the ocarborane derivative (220), which exhibits an intramolecular CT process. The experimental energy gap between the excited triplet and the ground states (T1−S0) for 221 and 222 are the highest (3.05 eV) of all of the compounds, whereas the Tg values of the carboranyl derivatives were also the highest and increased in the order 220 < 221 < 222 (164 °C). In addition, 222 was the only compound that exhibited electron mobility (2.5 × 10−4 cm2 V−1 s−1) and the compound showing the highest hole mobility (1.1 × 10−3 cm2 V−1 s−1) within the carboranyl derivatives. Taking these results into account, the authors tested 221 and 222 as host materials in PHOLED devices. As expected, 222 has a better performance and was proven to be the best material with quantum efficiencies of 15.3%. This result opens the way to further research on the use of p-carborane linkers as insulators for optoelectronic devices. 6.2.4. Carborane-Containing Macromolecules for Fluorescence Microscopy Imaging. Fluorescence microscopy 14368

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7.4) containing 1% DMSO compared with the DMSO solutions as well as fluorescence quenching, indicating aggregation. As for previous conjugates, their subcellular localization was followed by fluorescence microscopy to observe aggregates of all conjugates within the cells giving weak fluorescence signals. The intracellular localization of porphyrines 93a and 94a was studied in human carcinoma HEp2 cells by fluorescence microscopy, which showed that the preferential sites of intracellular localization were the cell lysosomes.171 In-vitro studies using HEp2 and human glioblastoma T98G cells show that these porphyrins are nontoxic in the dark up to 100 μM concentrations. Due to the poor water solubility of conjugates 99−104,174 their intracellular localization in HEp2 cells was performed by using Lipofectamine liposomes as the delivery vehicle, since in

Figure 89. Molecular structure of Miele’s dodecaborate derivative 223, exhibiting σmax at 1040 GM.

Table 30. Selected Data of Compounds for TPA Bearing Boron Clusters compound a

222 224b 225b 226b 227c 228c 229c 230c a

λabs (nm)

λem (nm)

ΦF

σmax (GM)

ref

403 395 363 360 394 398 408 403

N/R 496 458 438 466 470

0.49 0.58 0.59 0.69 0.55 0.29 0.004 0.33

1040 470 300 200 142 240

257 260 260 260 253 253 253 253

476

511

ACN. bCH2Cl2, cTHF

were relatively low (σmax = 1.6−50 GM), a bifluorene derivative with a σmax = 1040 GM value was achieved, 223 (Figure 89, Table 30), confirming the possibility of using boron clusters for TPM. A set of p-carborane-containing fluorophores for TPA microscopy was synthesized by Nicoud et al. (Figure 90).260,261 All three compounds showed intense absorption in the 300−400 nm range, with ε values between 10 and 12 × 104 M−1 cm−1. Fluorophores 225 and 226 showed blue emission, whereas 224 showed green emission in CH2Cl2 with high ΦF values (Table 30). Interestingly, compound 9 showed an intense TPA band with σmax= 470 GM at 810 nm and 200−300 GM at 700 nm for 7 and 8, as indicated in Table 30. These values make the compounds attractive as most lasers used in two-photon microscopes have their optimal efficiency around 800 nm. TPEM images of HeLa cells incubated with compounds 224 and 226 indicated that both compounds had penetrated into the cells. In the case of 9, heterogeneous distribution with visible vesicles was observed. A more recent advance in fluorescence microscopy has been published by Huang and co-workers, who synthesized starshaped fluorophores incorporating o- and m-carboranes into the fluorescent core of tris(4-stilbene)amine,253 through both carbon and boron substitution yielding four different compounds 227, 228, 229, and 230 (Figure 91). These carborane-bearing compounds were designed based on their three-dimensional structure and the unique electronic properties of the clusters in order to tune the TPA properties of the core. Photophysical studies revealed that, as expected, the absorption and emission bands were typical of the luminescent core, although meta derivatives were 8−10 nm red shifted compared with their ortho counterparts. Another expected feature was the fluorescence quenching of o-carborane, due to the variability of the Cc−Cc bond. The two-photon excitation spectra determined that the carboranyl units induced a significant enhancement in TPA values when compared with the starting organic core in the range of 680−780 nm. Compound m-carborane, which has the highest σmax value (Table 30), was chosen to be tested for two-photon

Figure 88. Subcellular localization of porphyrin 100 in HEp2 cells at 1 μM in liposomes for 18 h. (a) Phase contrast, (b) overlay of 100 fluorescence and phase contrast, (c) LysoSensor Green fluorescence, (e) MitoTracker Green fluorescence, (d and f) overlays of organelle tracers with 100 fluorescence. Scale bar: 10 μm. Reprinted with permission from ref 174. Copyright 2005 American Chemical Society.

the absence of liposomes the fluorescent signal was too weak to be clearly detected above the background autofluorescence. Using liposomes as the delivery vehicle, a very punctuated fluorescence was observed for all conjugates, indicating that they all localize within vesicles (Figure 88). The use of compounds with high TPA cross sections is essential for two-photon excitation microscopy (TPM), which is a noninvasive live-cell imaging technique that offers higher depth and spatial resolution as well as less photobleaching and phototoxicity than classical one-photon fluorescence microscopy.253 A few years ago, Miele and co-workers published the first examples of boron clusters as candidates for TPA processes.257−259 There electron-rich π-conjugated systems bearing closo-dodecaborate clusters were synthesized and their photophysical properties studied. Although the majority of the maximum TPA cross-section values (σmax) of these compounds 14369

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Figure 90. Molecular structures of Nicoud’s p-carborane derivatives 7−9.

Figure 91. Molecular structure of Huang’s star-shaped compounds 227−230.

fluorescence imaging in HepG2 liver cancer cells. The results showed no significant cytoxicity, even at high doses, nor photobleaching, thus providing high-quality images demonstrating that m-carborane is taken up by the cancer cells and internalized into their cytoplasm.

groups at the boron-based cluster and presumably leading to new chromatic combinations. The electronic motion of metallacarboranes is also an underexplored field. Just from a theoretical point of view, one could envisage these compounds as robust platforms or nanodevices able to generate motion by the simple addition/ subtraction of a single electron. This fact gives such molecular engines tremendous possibilities in a variety of fields such as chemical engineering, transport phenomena, and on/off nanodevices or switches, among others. In addition, initial studies into the application of these compounds in DSSCs, substituting the classical I−/I3− couple, have been carried out. The first results are quite promising, but there is still room to improve the resulting devices. As this field is hot topic in contemporary science, boron clusters should be kept in mind when trying to optimize the characteristics of solar cells. Regarding the PL applications of carboranes and other borane clusters, their integration into large structures to achieve materials for different kinds of applications is steadily increasing. The change of the electronic nature of the cluster, and therefore its PL behavior, has been proven to be useful for selective basic anion sensors, especially for F− and OH−. Another relevant field in which the these clusters are becoming more commonplace is in optoelectronics. Their chemical and thermal stability has already been exploited in the fabrication of various devices such as OFETs and PHOLEDs, exhibiting high

6.3. Summary and Outlook

Some of the redox applications disclosed in this section have already found their place and caught the attention of the general scientific community. As a clear example, several ISEs have been produced to quantify a range of different analytes, providing good performance and selectivities as well as a great robustness of the electrode itself. Notably, the observed B−H···H−C interactions have opened a new field of research in this area, providing new possibilities to create novel devices based on this technology. In addition, a new set of enantioselective electrodes sensitive to different enantiopure amino derivatives have been produced, enlarging the usage of boron-based materials in ISEs. The electrochromic behavior of carboranes and metallacarboranes is still an unexplored property, but initial studies have revealed the great potential of these compounds to be used as dyes or electron cumulative molecules. The fact that most of their redox transitions are reversible could enable their use as easily tunable electrochromic materials. Besides the described compounds, one could envisage a large variety of derivatives of these compounds, incorporating many different functional 14370

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electron mobilities, which in some cases are competitive with those of commercially available devices. Finally, fluorescence microscopy and particularly TPM have been regarded as some of the most promising potential applications of this type of compound, due to their selective cell uptake and low toxicity. Research on novel molecules containing carboranes with large TPA cross sections remains a major challenge for fluorescent compounds bearing boron clusters.

as a postdoctoral research associate at Queen’s University of Belfast, United Kingdom, on confidential industrial projects. Fabrizia Fabrizi de Biani graduated with a dregree in Chemistry from the University of Florence (with Sandro Bencini, 1994), obtained her Ph.D. degree from the consorted Universities of Siena and Perugia (with Piero Zanello, 1998), and did postdoctoral research at the University of Barcelona with Santiago Á lvarez and Eliseo Ruiz and at the University Pierre et Marie Curie Paris VI with Michel Verdaguer before joining the faculty at the University of Siena in 2001. Her main interest is in the use of electrochemistry and spectroelectrochemistry to probe the molecular and electronic structures of inorganic and organometallic compounds. She is also interested in computational chemistry and strongly believes in multidisciplinarity.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *Fax: (+34) 935805729. E-mail: [email protected].

Francesc Teixidor obtained his Ph.D. degree in Chemistry in 1979 from Universitat Autònoma de Barcelona under the supervision of Prof. Heribert Barrera and pursued postdoctoral research with Prof. Ralph Rudoph at the University of Michigan. He was appointed Assistant Professor at Universitat Autònoma de Barcelona in 1982. In 1987 he obtained an Associate Professor position at the Spanish Council for Scientific Research (CSIC) at the Materials Science Institute (ICMAB). Since 1999 he has held a full professor position at the same institute. His research interests are in chemistry, particularly in the formation of B−C and B−P bonds, and the application of boron cluster compounds in molecular materials with a particular emphasis on energy.

Present Addresses ‡

M.T.: Grup de Quı ́mica Bioinspirada, Supramolecular i Catàlisi (QBIS-CAT), Institut de Quı ́mica Computacional i Catàlisi (IQCC), Departament de Quı ́mica, Universitat de Girona, Campus Montilivi, E17017 Girona, Catalonia, Spain. ∥ A.F.-U.: School of Chemistry and Chemical Engineering, Queen’s University of Belfast, David Keir Building, Belfast BT9 5AG, United Kingdom. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been supported by the Ministerio de Economiá y Competitividad, MINECO (CTQ2013-44670-R, CTQ201458801) and by the Generalitat de Cataluya (2014/SGR/149). ICMAB acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV2015-0496).

Biographies Rosario Núñez was born in Loja, Granada, Spain, in 1968. In 1991 she graduated with a degree in Chemistry from Universidad de Granada. In 1994 she received her M.Sc. degree in Chemistry from Universitat Autònoma de Barcelona (UAB). She carried out her thesis in the Inorganic Materials & Catalysis Laboratory (LMI) of the Material Science Institute of Barcelona (ICMAB-CSIC) under the supervision of Prof. Clara Viñas and received her Ph.D. degree from UAB in 1996. After that she worked as a postdoctoral fellow with Prof. Bruno Boury and Prof. Robert Corriu at Université de Montpellier II. In 1999, she joined the ICMAB as a fellow staff doctor to work in the LMI managed by Prof. Francesc Teixidor. Since 2001 she has held a permanent position as a tenured scientist. Her research interests are the synthesis and study of the properties of boron cluster derivatives and the preparation of carboranyl-containing molecular and hybrid materials.

ABBREVIATIONS 1-Terthioph− terthiophenyl −3BrBz 3-bromobenzyl −3FBz 3-fluorobenzyl −4BrBz 4-bromobenzyl −4ClBz 4-chlorobenzyl −4FBz 4-fluorobenzyl −4MeBz 4-methylbenzyl −4MeOBz 4-methoxybenzyl All allyl Ant anthracene bdzb benzodiazaborolyl B(Mes)2 dimesitylboryl Bz benzoyl dA 2′-deoxyadenosine dA-dThd dinucleoside 2′-deoxyadenosine, thymidine dC 2′-deoxycytidine dG 2′-deoxyguanosine DPPE 1,2-bis(diphenylphosphino)ethane dThd thymidine H2OEP H2-octaethylporphyrin H2TPP H2-tetraphenylporphyrin Hex hexyl i-Pe isopentyl Ir(dbpz)2 Ir(dibenzo[a,c]phenazine) Ir(dfppy)2 Ir(2-(2,4-difluorophenyl)-pyridine)2 Isoflurane 1-chloro-2,2,2-trifluoroethyl difluoromethyl ether

Màrius Tarrés was born in Sabadell, Catalonia, Spain, in 1987. He obtained his B.Sc. degree in Chemistry from the Universitat Autònoma de Barcelona in 2009. In 2010 he received his M.Sc. degree from the same university. He completed his Ph.D. studies in the Inorganic Materials & Catalysis Laboratory at the Material Science Institute of Barcelona (ICMAB-CSIC) in 2014 under the supervision of Prof. Francesc Teixidor. In 2015 he was offered a position as a Postdoctoral Research Associate at the University of Strathclyde, in Glasgow, Scotland, United Kingdom, under the guidance of Prof. Eva Hevia. In 2016, he moved back to Catalonia to start his second postdoctoral experience at the University of Girona, at the QBIS-CAT group under the tutelage of Dr. Xavi Ribas. He is currently working on cobaltcatalyzed C−H bond activation. Albert Ferrer-Ugalde was born in Barcelona, Spain, in 1980. In 2005 he graduated with a degree in Chemistry from Universitat de Barcelona. In 2008 he received his M.Sc. degree in Chemical Sciences and Technologies from the Universitat Autonòma de Barcelona. In May 2013 he was awarded his Ph.D. degree in Chemistry under the supervision of Dr. Rosario Núñez from the Material Science Institute of Barcelona (ICMAB-CSIC). Since September 2013 he has been working 14371

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Methyl Viologen O-allyl p-anisole butylphosphanyl [6,6]-phenyl-C61-butyric acid phenyl ester 1-pentyl pentacene butylphosphinothioic phenylphosphoryl phenylphosphanyl polypyrrole phenylphosphinothioic triisopropylsilylethynyl pentacene dicarbaborane or carborane cluster dimethyl sulfoxide acetonitrile dichloromethane dimethylformamide nucleus-independent chemical shift

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DOI: 10.1021/acs.chemrev.6b00198 Chem. Rev. 2016, 116, 14307−14378