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Crystals 2016, 6(5), 50; doi:10.3390/cryst6050050

Review
N,O-Type Carborane-Based Materials
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus U.A.B., 08193 Bellaterra, Spain
*
Author to whom correspondence should be addressed.
Academic Editor: Umit B. Demirci
Received: 29 March 2016 / Accepted: 27 April 2016 / Published: 4 May 2016

Abstract

: This review summarizes the synthesis and coordination chemistry of a series of carboranyl ligands containing N,O donors. Such carborane-based ligands are scarcely reported in the literature when compared to other heteroatom-containing donors. The synthetic routes for metal complexes of these N,O-type carborane ligands are summarized and the properties of such complexes are described in detail. Particular attention is paid to the effect that the incorporation of carboranes has into the coordination chemistry of the otherwise carbon-based ligands and the properties of such materials. The reported complexes show a variety of properties such as those used in magnetic, chiroptical, nonlinear optical, catalytic and biomedical applications.
Keywords:
metal complexes; carboranes; functional materials; N,O-donors; reactivity; chirality

1. Introduction

The icosahedral closo carboranes (dicarba-closo-dodecaboranes; C2B10H12) are an interesting class of exceptionally stable boron-rich clusters with high thermal and chemical stability, hydrophobicity and acceptor character [1,2,3]. Carborane chemistry has experienced a major surge of interest across a wide spectrum of technologies, fueled by developing applications in diverse areas such as in catalysis, materials science and medicine [1,4,5,6,7,8,9,10,11]. There are three isomers of carborane that differ in the relative position of both carbon atoms in the clusters (ortho-, meta- and para-, or o-, m- and p-; Figure 1). Although the volume of the three isomers of carborane is roughly the same, they show very different dipole moments as a consequence of the different arrangement of the carbon atoms in the cluster (4.53 D, 2.85 D and zero D for o-, m- and p-, respectively) [8]. The average size of the three isomers of carborane (141–148 Å3) is comparable to that of adamantane (136 Å3), significantly larger (40%) than the phenyl ring rotation envelope (102 Å3) and slightly smaller (10%) than C60 (160 Å3) [12]. The presence of ten hydridic hydrogens at the boron atoms of the clusters makes them extremely hydrophobic, surpassing that for adamantane [13]. The hydrophobicity of carboranes has been extensively used to trigger desired biological actions [7,8]. Concerning the electronic effect, all cluster carbon atoms exert an electron-withdrawing effect on attached substituents, which decreases in the order o- to m- to p-carborane. For example, when bonded by a cluster carbon atom, o-carborane exhibits an electron-withdrawing substituent effect similar to that of a fluorinated aryl group. Experimental evidence shows that the electron-withdrawing character of the carborane isomers has a clear impact on the acidity of substituents at carbon, the acidity decreasing in the same order (o-, m-, p-), and all being more acidic than the related phenyl moiety [3]. Thus, the C–H bonds of the icosahedral closo carboranes can be deprotonated with strong bases (e.g., alkyllithium) and the generated carboranyl nucleophile can react with a wide variety of electrophiles (e.g., alkylhalides, carbonyl derivatives, etc.) producing C–functionalized carboranes. Monosubstitution of the carboranes is not trivial because the monolithiation of the o-carborane moiety is complicated by the tendency of the monolithio o-carborane to disproportionate into o-carborane and its dianion [14]. Several strategies have been followed to overcome this problem, for example, by using protecting/deprotecting methodologies, using dimethoxyethane as the solvent, or by doing the reaction at high dilution [15,16]. We recently revealed that mono and disubstitution of carboranes can be conveniently done in ethereal solvents at a very low temperature [17]. Such nucleophilic substitution methodology is perhaps the more general route for functionalizing carboranes as it can be applied to all carborane isomers.
Over the years, our group and others have been interested in the synthesis of new carborane-based ligands containing a variety of donor centers (N, P, S, N/C, N/S, N/P, P/C, P/P P/Si, P/S, S/C or S/S donors) and their metal complexes an applications [2,6,18,19,20,21,22,23,24,25,26,27,28]. Carborane ligands containing N,O donors are scarce in the literature. This is somewhat surprising when considering the importance of classical N,O-ligands in metal complexes and their properties [29,30,31,32,33,34]. One of the main objectives of our research in the last few years was to study the chemistry of carboranylmethylalcohols, particularly of those containing a heteroatom such as nitrogen, and exploring their properties. Our interest in N/O-functionalized carboranes primarily stems from our rationale that introducing a carborane moiety in the place of a conventional carbon-based moiety would strongly influence the coordination chemistry of such compounds, in addition to other relevant properties, such as higher stability, hydrophobicity, etc. Integration of carboranes in place of organic ring systems (typically benzene) is a very popular strategy to trigger desirable properties in (bio)medicine [7,8] but is much less exploited in chemistry or materials science [35].
In the present review, we summarize our results and the results of others on the synthesis, structure and reactivity of carboranyl ligands containing N,O-donor atoms and their metal complexes and properties. Metallacarborane complexes, incorporating one or more metal atoms within a polyhedral carborane cage structure, are excluded of the present review. For some recent reviews on metallacarboranes see references [36,37,38,39,40,41].

2. Carboranyl Compounds with N,O-Donor Functionalities and Properties

2.1. Closo-Carboranylmethylalcohols with Nitrogenated Aromatic Rings

Reported pyridine-type containing carboranyl-based N,O-donor compounds are summarized in Chart 1. Carboranyl methanols are easily available by the addition of lithiocarboranes to aldehydes or ketones. Using this methodology, a wide variety of mono substituted carboranyl methanol derivatives have been synthesized [42,43]. Following a similar procedure we [44,45,46,47,48,49] and others [50,51] have prepared an extensive series of new monosubstituted o-, m- and p-carboranylmethylalcohols bearing nitrogenated aromatic rings, by the addition of lithiocarboranes to the corresponding pyridylaldehydes (14, Chart 1 and Scheme 1). The addition of dilithiocarboranes to two equivalents of the corresponding aldehydes, under the same reaction conditions, provided a new series of disubstituted o- and m-carboranylmethylalcohols (56, Chart 1) [52,53]. This synthetic methodology allows the preparation of the compounds in good yields in gram quantities from one-pot reactions, starting from commercially available materials.
This family of carboranylmethylalcohols contains one (14; Chart 1) or two (56; Chart 1) chiral carbon centers. The monosubstituted compounds are therefore obtained as racemic mixtures, and they can be easily resolved into the R and S enantiomers by using HPLC over a chiral stationary phase [49,54], or by diastereomers formation with (1S)-(−)-camphanic acid chloride [50,51]. In the case of the disubstituted compounds (56), the situation is more complex (Scheme 2). These compounds contain two chiral centers that can adopt either R or S configuration and, therefore, lead to the formation of two diastereoisomers (Scheme 2), a meso compound (RS; OH groups in a syn orientation) and a racemic compound (mixture of SS and RR; OH groups in an anti orientation). The enantiopure compounds can be exploited in coordination chemistry, as will be described in the following sections. Separation of the syn- and anti-isomers in the disubstituted series of compounds has been carried out in the case of o-carborane derivatives 5a and 5b [53,55].
Both mono and disubstituted carboranylmethylalcohols mentioned above possess hydroxyl (OH) groups as hydrogen bond donors and nitrogen atoms that act as hydrogen bond acceptors. Indeed, the supramolecular chemistry of such compounds is dominated by moderate O–H∙∙∙N hydrogen bonding. In the case of 2-pyridyl derivatives, 2a, 3a and 4b (both in racemic and enantiopure forms), they all form homochiral helical networks and it has been shown that a correlation exists between the OCCN torsion angles of the molecules in the solid state and the handedness of the supramolecular helices [49]. Regarding the disubstituted derivatives, 5af, it was observed that syn and anti stereoisomers crystallized separately from their mixtures and the detailed analysis of their supramolecular structures revealed that homochiral recognition seems to operate also in these molecular systems [53].

2.2. Other Closo-Carboranes Incorporating N and O Functionalities

Other reported non pyridine-type containing carboranyl-based N,O-donor compounds are summarized in Chart 2. A related family of compounds to that of 14 and 6 (Chart 1) is that of chiral carboranylpyrroles 711 (Chart 2) [56]. In these molecules, the pyridyl moieties in the former ones are replaced by a pyrrol moiety. These carboranylpyrroles were prepared by the reactions of mono or dialdehydes derivatives of o-, m- and p-carborane with pyrroles in the presence of acid catalysts (Scheme 3). Provided that the pyrrol moieties could be deprotonated, these compounds might provide rich coordination chemistry.
Reaction of o-carboranylmethyl ammonium salt with commercially available phenyl aldehyde provided the phenyl(carboranylmethyl)imine 12 (Chart 2 and Scheme 3) in good yield [57]. Another interesting series of compounds is that of chiral bis(oxazolilnyl)-m-carboranes 1314 that were synthesized via a multistep synthesis [58]. Briefly, m-carborane dicarboxylic acid was transformed to the acyl chloride with SOCl2 and further condensed with two equivalents of the corresponding resolved amino alcohols to provide the uncycled bis(hydroxyamide)-m-carborane intermediates. Double cyclization reaction by diethylaminosulfur trifluoride (DAST) afforded enantiopure compounds 1314 in very high yields (Scheme 3).

3. Synthesis of Coordination Complexes and Properties

In the following, it will be shown the effect that the incorporation of carboranes has into the coordination chemistry of the otherwise carbon-based ligands (whenever possible) and the properties of such materials.

3.1. Complexes of Monosubstituted 14 and 12

Conventional N,O ligands such as (hydroxymethyl)pyridines (hmpH; Scheme 4) have proved to be successful building blocks for the self-assembly of metallosupramolecular architectures with exciting physical properties [54] (and references therein). Carborane compounds 14 can be regarded as hmpH ligands where one of the H atoms at the –CH2– position of the methylalcohol moiety has been replaced by a carboranyl fragment (Scheme 4). The introduction of carborane into the hmpH backbone provokes a bigger decrease of the alcohol pKa value, with respect to the related phenyl-hmpH (phhmpH) derivative (Scheme 4), in addition to an increase of the size and hydrophobicity of 14 with respect to phhmpH.
Furthermore, the possibility for placement of the methylalcohol moiety at the 2-, 3- or 4-position with respect to the pyridine (or quinoline) nitrogen, which usually coordinates to the metal center, is a key feature that allows these ligands to support a whole family of supramolecules of a different nature. Heterobidentate ligands of this type offer several advantages over traditional symmetrical bidentate ligands by creating steric, electronic, asymmetry and chirality at the metal centers [59].

3.1.1. Cobalt

The 2-pyridyl derivatives 1a and 2a reacted with CoCl2·6H2O in a 2:1 ratio under aerobic conditions to provide the corresponding CoII complexes 15a and 16a, respectively (Scheme 5) [60]. X-ray diffraction studies confirmed that 2a acts as a bidentate N,O-ligand, giving an octahedral-coordinated CoII complex. We showed experimentally (both in solution and solid state) that the Co–OH(R) bonds in 16a are labile and that the coordination strength of the alcohol function can be modulated by solvent-assisted intermolecular hydrogen bonding. We also showed that full deprotonation of both alcohol hydrogens in the octahedral cobalt complex 16a afforded a rare square-planar CoII complex 19a that was characterized by single crystal X-ray diffraction (XRD). The square-planar geometry in this complex seemed to be induced by the steric hindrance generated by the carborane moiety on the ligand. Complex 17a seems to enable O2 activation, followed by transformation of the ligands and metal oxidation states affording a CoIII carborane complex 18.
The 3- and 4-pyridyl or quinolyl derivatives 1bc, 1e and 2bc, 2e also reacted with CoCl2·6H2O under the same reaction conditions, providing, in this case, the corresponding tetrahedral CoII complexes 15a and 16a, respectively (Scheme 6) [60]. Octahedral complex 21 was, however, formed in the presence of excess of 2c. The structure for complexes 19c, 19e, 20b, 20e and 21 were confirmed by XRD.
It is interesting that even though the above CoII complexes are paramagnetic, we were able to perform and characterize most of the complexes by NMR spectroscopy. The solid-state, variable-temperature (2–300 K) magnetic susceptibility data were collected on polycrystalline samples of 16a, 20b and 20c (Figure 2) and the data agree well with their crystallographic data and stress the relevance of intermolecular interactions among neighboring molecules providing well-organized supramolecular 1 D systems (vide infra).
In the solid state, all the above CoII complexes show intermolecular O–H∙∙∙Cl/O hydrogen bonds. From those, unsolvated structures show exclusively O–H∙∙∙Cl hydrogen bonds giving supramolecular chains. Those are, however, interrupted whenever an oxygen-containing solvent is included in the structures. In that case, O–H∙∙∙O hydrogen bonds are also formed, interfering partially (21) or totally (16a) with the O–H∙∙∙Cl hydrogen bonds. The supramolecular chemistry of 16a serves as an example of this phenomena and of how the carborane moieties can have an influence on the solid structure and properties of the molecular complex. A comparison of the molecular and supramolecular structures of the octahedral complex 16a with that of related (not containing carborane) cobalt complexes revealed that the chirality of 2a in conjunction with the bulky carborane favors RR/SS alternation as a more economic packing arrangement. As shown at the top of Figure 3, the unsolvated form of 16a gives chains, alternating RR and SS enantiomeric complexes, along the c axis via the O–H∙∙∙Cl hydrogen bond interactions (Figure 3, top left). The proximity of the complexes (Co∙∙∙Co: 5.722 Å) forces the carborane cages of consecutive molecules to be staggered (Figure 3, top middle). The solid structure significantly changed when 16a was recrystallized from methanol. The methanol solvate of the latter, 16a·2MeOH, also shows chains of alternating RR and SS enantiomeric complexes (Figure 3, bottom). However, two methanol molecules are inserted now in the hydrogen bonding network, resulting in a longer distance between consecutive Co centers (Co∙∙∙Co: 7.281 Å) than in 16a. As a consequence, the molecules in 16a·2MeOH are not staggered but eclipsed (Figure 3, bottom). This has important consequences in the three-dimensional (3D) structures of these two complexes, as shown in Figure 3 (right column). The eclipsed chains in 16a·2MeOH are more closely packed than the staggered chains in 16a and as a consequence, the packing of stagger chains of 16a creates defined channels running along the c axis parallel to hydrogen bonded chains (Figure 3, right). The solvation process from 16a to 16a·2MeOH has been demonstrated experimentally by exposing 16a to liquid methanol, or even vapors.

3.1.2. Iron

The reaction of 2a with FeCl2 in a 1.5:1 ratio afforded the FeIII complex Fe2Cl3(2a)3 (22) in nearly quantitative yield (Scheme 7) [54]. When the same reaction was carried out with the phenyl-modified ligand phhmpH, initial formation of the mononuclear FeII complex FeCl2(phhmpH)2 (23) was observed, followed by its conversion to the trinuclear FeIII complex [Fe3Cl4(phhmp)4][FeCl4] (24). Structures for complexes 2224 have been solved by XRD. It was observed that deprotonation occurred spontaneously in the reactions of 2a with iron, but this was faster in the case of the carborane-based ligand 2a than with the phenyl-based one phhmpH, in agreement with a higher acidity for 2a.
These results clearly showed how the introduction of the bulky o-carborane into the 2-(hydroxymethyl)pyridine (hmpH) architecture significantly alters the coordination of the simple or arylsubstituted 2-hmpH. The comparison of 22 with all other FeIII complexes in the literature having arylsubstituted 2-hmpH ligands revealed that the latter always show two alkoxide pyridylalcohol ligands bridging two close FeIII ions (Scheme 8), whereas the dinuclear complex 22 contains three alkoxide bridges. This unusual architecture seems to be triggered by the poor nucleophilicity of the alkoxide ligand (2a).
The presence of three alkoxide bridges in 22 is rather surprising, owing to the size of the carborane cages, and it has important structural consequences. Each of the pyridylalcohol ligands can adopt an R or S configuration, so that RRR, SSS, RRS and SSR could all be expected in complex 22. However only RRS and/or SSR combinations are possible due to the steric hindrance imposed by the same hardness of the ligands (22·acetone, Figure 4). This was confirmed by synthesizing the enantiopure complexes of 22 from pure R and S enantiomers of 2a. The chirality of the enantiopure ligands ((R)-(+)-2a/((S)-(−)-2a) and corresponding complexes (S,S)AFe(R)AFe-(+)-22 and (R,R)CFe(S)CFe-(−)-22 was confirmed by circular dichroism (CD) measurements in solution and by second-harmonic generation (SHG) measurements in the solid state (Figure 5).
The crystalline powder obtained during the synthesis of 22 was identified as a racemic mixture of (S,S)AFe(R)AFe-(+)22 and (R,R)CFe(S)CFe-(−)22 by Powder X-ray Diffraction (PXRD). This racemic mixture (rac-22) showed a very rare case of spontaneous resolution that takes place on precipitation or exposition to vapors giving a conglomerate compound (co-22), as shown in Figure 6.
Complex 22 constitutes the first dinuclear FeIII system containing three alkoxide bridges that displays an antiferromagnetic behavior. DFT calculations have corroborated the latter and show that the Fe–O distance is the main parameter that controls the magnetic behavior. Overall, complex 22 represents an interesting class of multifunctional molecular materials that combine magnetic, chiroptical and second-order optical properties.

3.1.3. Platinum

Contrary to the above metals, when a Group 10 metal such as Platinum was employed, only N-coordination was observed. Reactions of racemic 2c and/or enantiopure 3bc/4bc with [Pt(MeCN)terpy](NO3)2 or [PtI2(phen)] provided the platinum complexes 25bc, 26bc, 27bc or 28bc (Scheme 9) [51,61]. Recrystallization of the complexes 25bc, 26bc and 27bc from hot water was necessary in order to remove the byproduct [Pt(OH)terpy]NO3. Under such conditions, the o-carborane ligands 2bc were deboronated to the corresponding 7,8-nido-carboran-7-yl)pyridylmethanol complexes 25bc. Deboronation was not observed in any of the m- or p-carborane derivatives, consistent with their higher stability. The deboronation of o-carborane-containing ligands is known to be enhanced when coordinated to metal centers [62,63,64,65,66]. The phenomenon is attributed to the electron density being withdrawn from the boron cluster upon metal complexation. There are, however, some reports, mainly dealing with PtII complexes where deboronation seems to occur prior to metal complexation and most probably due to the nucleophilicity of the ligand itself [67,68].
The use of β-cyclodextrin (CD) as biodelivery agents for carborane clusters is of particular relevance to their exploitation as unique hydrophobic pharmacophores in medicinal chemistry [7,8]. Chiral complexes 26bc and 27c form water-soluble supramolecular 1:1 host-guest β-CD adducts [51]. The nature of the carborane cage itself (i.e., the positional isomer and the overall charge) and the chirality and nature of the substituent on the cage each contribute to its molecular recognition by β-CD. S27c forms a remarkably stable ternary system, involving, simultaneously, DNA metallointercalation and β-CD encapsulation (Figure 7). Complexes 28bc, containing two closo-carborane clusters, were also treated with β-CD to provide the corresponding series of water-soluble 2:1 host-guest adducts [61]. DNA-binding studies demonstrated the avid binding affinity of these complexes for calf thymus DNA.

3.1.4. Titanium

In situ deprotonation of 12 followed by reaction with TiCl4 afforded complex 29 (Scheme 10) [57]. This complex is an efficient catalyst for α-olefin polymerization to produce high molecular weight polyethylene and poly(ethylene/methyl-10-undecannoate). Catalytic activity of complex 29 is clearly superior to that of I (bottom Scheme 10) [69] and comparable to one of the most potent phenoxy–imine Ti complexes (Ti-FI catalysts) [70].

3.2. Complexes of Disubstituted 56 and 1314

The 2-pyridyl disubstituted closo-carboranylmethyl alcohols 56 (Chart 1) or 1314 (Chart 2), constitute a second generation of ligands, where two pyridyl/quinolylmethylalcohol or oxazolinyl chiral moieties radiate out of the cluster carbon atoms. The presence of two chiral carbons and the different positional isomers offer enough molecular diversity to explore the coordination chemistry of such ligands.

3.2.1. Cobalt

Racemic anti-5a (see Scheme 2 for nomenclature) formed the octahedral cobaltII complex 30 upon reaction with CoCl2 (Scheme 11) [71]. The X-ray structure of 30 revealed a distorted geometry where each cobaltII center is coordinated by all nitrogen and oxygen atoms of an anti-diastereomer of 5a that is acting as a tetradentate N2O2-ligand. Crystals for the CoII complex are formed by a racemic mixture of Δ-30 and Λ-30 units.
The OH groups from anti-5a remained intact in complex 30 and therefore they can act as proton donors for hydrogen bonding and were also observed in complex 16a (Scheme 3 and Figure 3). Complex 30 forms homochiral ribbons (Δ- or Λ-enantiomeric complexes), along the b axis via O–H∙∙∙Cl hydrogen bond interactions (Figure 8). Thus, homochiral recognition seems to be happening in the CoII complexes of anti-5a.
We showed that anti-5a is an unprecedented and distinct tetradentate N2O2-type ligand and represents a new type of C2-symmetric chiral building block. Reported tetradentate N2O2 ligands are mainly reduced to Schiff-base backbone ligands [72,73,74]. These ligands and their derivatives coordinate predominantly in a planar arrangement to various metal ions giving trans geometries in octahedral complexes (Figure 9). An increased propensity to form cis structures has been achieved in some cases by increasing the backbone chain length. Carborane-based anti-5a ligand adopts preferentially a cis-α configuration around the CoII center (Figure 9) and it is, therefore, able to produce chiral-at-metal complexes.

3.2.2. Iron

The reaction of anti-5a with FeCl2 provided the dinuclear ironIII complex 31 (Scheme 12) [55]. Anti-5a behaves also as a distinct tetradentate N2O2 ligand, as already observed in the previous Co complex 30 and confirms this ligand as a new type of C2-symmetric chiral building block. X-ray structural analysis reveals that the alcohol groups are deprotonated, as already observed also in the ironIII complex 22 (Scheme 5, Figure 4). As in the case of complex 30, the crystallization of homochiral dimers in 31 suggests that enantiomeric forms of racemic anti-5a self-recognize to form exclusively stereospecific, homochiral dinuclear complexes.
The coordination chemistry of the meso form of 5a (syn-5a; see Scheme 2 for nomenclature) resulted in being more complicated than that of the chiral anti-form, probably as a result of the syn-arrangement of the OH groups. After several attempts, the dinuclear ironIII complex 33 crystallized and showed an unusual dinuclear ironIII complex with a mixed octahedral and square pyramidal geometry (Figure 10).
Complex 31 could be obtained in the solid state as a single phase, and the solid-state, variable-temperature (2–300 K) magnetic susceptibility data using 0.03 and 0.5 T fields were measured. It was found that an exchange coupling for both FeIII atoms in 31 was strongly antiferromagnetic.

3.2.3. Nickel, Palladium and Rhodium Complexes

Chiral NBN pincer complexes of disubstituted enantiopure oxazolinyl m-carboranes 1314 (Chart 2), were prepared by their reaction with RhCl3·3H2O, [Ni(COD)2] or [Pd(MeCN)4](BF4)2 under heating conditions (Scheme 13) [58]. Chiral rhodium complexes 32 (chloride form) and 35 (acetate form) were found to be an effective catalyst (1 mol%) for asymmetric conjugate reaction of α,β-unsaturated esters, giving both a high enantiomeric excess (93%–94% ee). Lower ee were obtained with complexes 36 and 37. The enantioselectivities were similar to those obtained with the Phebox pincer complex, having a phenyl ring in place of the m-carborane (Scheme 13) [75]. Complexes 3537 were also found to be active catalysts for the asymmetric reductive aldol reaction of benzaldehyde, tert-butyl acrylate and (EtO)2MeSiH. In this case, the ee was sensibly higher (91% ee) than the corresponding Phebox pincer complexes (77%–87% ee) [76].
Reaction of the disubstituted 2-pyridyl closo-carboranylmethyl alcohols 5a, 5f and 6a (Chart 1) with [PdCl2(MeCN)2] provided the pincer palladium complexes 4446, respectively, under mild conditions (Scheme 14) [52]. XRD of these complexes show unambiguously B–H activation of the carborane cages at B(3/6)H in o-carborane or B(2/3)H in the m-carborane-based ligands. The structures of the three complexes displayed exceptionally long Pd–Cl distances in the solid state (2.49–2.51 Å), suggesting a strong trans influence of the carborane moieties and comparable with that for alkyl-based pincer Pd complexes (2.49–2.52 Å). However, a combined study of experimental and calculated bond distances reveals that two effects are operative in modulating the Pd–Cl distance in the crystal structures. One is the trans influence of the carborane moieties, the other being the intermolecular moderate H-bonding interactions among neighboring complexes in the solid state. Thus, it can be inferred that there is a stronger trans influence of the meta-carborane than the ortho-carborane moieties in the pincer complexes, as expected.
Catalytic applications of 44 and 46 have shown the complexes are good catalyst precursors in Suzuki coupling reactions in water, and with remarkably low amounts of catalyst loadings (0.0001 mol %) and good functional group tolerance for the substrates. Complex 44 shows a better catalytic profile than 46 and with excellent conversions and TON values ranging from 770,000 to 990,000, thus showing a very high catalytic activity which rivals previous reports on Suzuki coupling performed by very low amounts of palladium catalysts, even with other pincer complexes [52].

4. Conclusions and Perspectives

The coordination chemistry of N,O-type carborane-based ligands with TiIV, FeIII, CoII, RhIII, NiII, PdII, PtII and ZnII has been summarized, along with the properties and applications of these metal complexes. The above results nicely expand the already rich carborane chemistry and show how introduction of the carborane framework into the otherwise conventional carbon-based ligands, opens up new avenues in coordination chemistry with exciting metal-mediated reactivity and properties. The convenient preparation of N,O-type carborane-based compounds, many of them in one-pot reaction and from commercially available starting materials, make of these carborane derivatives valuable ligands for coordination chemistry. The diverse coordination modes of such ligands towards a variety of metals and their properties are all advantageous. The pyridine-containing o-carboranylmethyl alcohol ligands 14 (Chart 1) are analogous to the (hydroxymethyl)pyridines (hmpH; Scheme 2), or other derivatives of the latter. It has been, however, demonstrated that the replacement of an H atom or a phenyl ring by a carboranyl moiety in these systems has an enormous influence on the final metal complexes and properties. This led to the formation of a dinuclear chiral iron complex combining magnetic, chiroptical and second-order optical nonlinear properties. The same complex showed a fascinating case of spontaneous resolution on precipitation or exposition to vapors. It has been described how the carborane moieties triggered the porosity of an antiferromagnetic CoII complex. Platinum complexes incorporating o-carboranylmethyl alcohol ligands formed supramolecular host-guest β-CD and/or DNA adducts. Titanium, nickel, palladium and rhodium complexes provided active catalysts for a variety of chemical transformations such as polymerization, enantioselective asymmetric conjugate reaction of α,β-unsaturated esters or aldol reactions, and Suzuki coupling reactions in water and with very low catalytic loadings.
Another area of interest is that of chirality, as some of the present N,O-ligands are chiral and can be easily obtained in enantiopure forms. This will certainly facilitate the use of such chiral ligands and their corresponding complexes in NLO, ferroelectric or multifunctional materials. This review highlights the versatility of carboranes as alternatives to carbon-based ligands in metal complexes for solving problems that might spoil their applicability, such as, e.g., thermal or water stability, or just by improving the activity or selectivity of catalysts. Boron chemistry in general, and carborane chemistry in particular, is nowadays a very mature and established area of research. New developments are appearing constantly and are limited only by our imagination.

Acknowledgments

We thank MEC grants CTQ2013-44670-R, MAT2013-47869-C4-2-P and Generalitat de Catalunya 2014/SGR/00149 for financial support. We would like to thank and dedicate this review to the people and key players in the papers of our group; their work has wonderfully influenced our research in the past few years.

Author Contributions

Francesc Teixidor and Clara Viñas contributed to the critical reading and discussion of the review contents; José Giner Planas wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Graphical representation of the carborane isomers (closo-C2B10H12) with vertex numbering.
Figure 1. Graphical representation of the carborane isomers (closo-C2B10H12) with vertex numbering.
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Chart 1. Synthesized carboranylmethylalcohols with nitrogenated aromatic rings. o-, m- and p-CB refers to ortho-, meta- and para-carborane.
Chart 1. Synthesized carboranylmethylalcohols with nitrogenated aromatic rings. o-, m- and p-CB refers to ortho-, meta- and para-carborane.
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Scheme 1. General procedure for the syntheses of carboranylmethylalcohols 16 (see Chart 1 for nomenclature). Conditions: (a) 1eq. n-BuLi, Ether/THF (0/−78 °C); 1eq. pyridine/quinolinecarboxaldehyde (−84/−63 °C); (b) 2eq. n-BuLi, Ether/THF (0 °C); 2eq. pyridinecarboxaldehyde (−94 °C for oCB or −63 °C for mCB).
Scheme 1. General procedure for the syntheses of carboranylmethylalcohols 16 (see Chart 1 for nomenclature). Conditions: (a) 1eq. n-BuLi, Ether/THF (0/−78 °C); 1eq. pyridine/quinolinecarboxaldehyde (−84/−63 °C); (b) 2eq. n-BuLi, Ether/THF (0 °C); 2eq. pyridinecarboxaldehyde (−94 °C for oCB or −63 °C for mCB).
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Scheme 2. Stereoisomers for chiral disubstituted carboranylmethylalcohols.
Scheme 2. Stereoisomers for chiral disubstituted carboranylmethylalcohols.
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Chart 2. Other synthesized carboranes incorporating N and O functionalities. o-, m- and p-CB refers to ortho-, meta- and para-carborane.
Chart 2. Other synthesized carboranes incorporating N and O functionalities. o-, m- and p-CB refers to ortho-, meta- and para-carborane.
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Scheme 3. General procedure for the syntheses of carborane derivatives in Chart 2. Conditions: (a) pyrrole (excess), TFA or InCl3 (0 °C), 1 h; (b) NaHCO3, 2-hydroxybenzaldehyde, toluene (reflux), two days; (c) n-BuLi, Ether, CO2, HCl; (d) SOCl2 (reflux), 24 h; (e) 3 eq. DAST, CH2Cl2 (−20 °C), 12 h.
Scheme 3. General procedure for the syntheses of carborane derivatives in Chart 2. Conditions: (a) pyrrole (excess), TFA or InCl3 (0 °C), 1 h; (b) NaHCO3, 2-hydroxybenzaldehyde, toluene (reflux), two days; (c) n-BuLi, Ether, CO2, HCl; (d) SOCl2 (reflux), 24 h; (e) 3 eq. DAST, CH2Cl2 (−20 °C), 12 h.
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Scheme 4. Comparison of various pyridylmethyalcohol derivatives.
Scheme 4. Comparison of various pyridylmethyalcohol derivatives.
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Scheme 5. Syntheses of complexes 1518.
Scheme 5. Syntheses of complexes 1518.
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Scheme 6. Syntheses of complexes 1921.
Scheme 6. Syntheses of complexes 1921.
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Figure 2. χM Tvs T plots for compound 16a (black squares), 20b (white circles) and 20c (white rhombs) between 2.0 and 300.0 K. Inset: Increased section of the graph containing all three compounds from 2 to 50 K. Solid lines in χMT vs. T plots are for eye guide.
Figure 2. χM Tvs T plots for compound 16a (black squares), 20b (white circles) and 20c (white rhombs) between 2.0 and 300.0 K. Inset: Increased section of the graph containing all three compounds from 2 to 50 K. Solid lines in χMT vs. T plots are for eye guide.
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Figure 3. Supramolecular assemblies of 16a and 16a·2MeOH. Left column: Projections showing four molecules of each compound forming hydrogen-bonded chains. Middle column: Projections along the hydrogen-bonded chains showing a staggered arrangement of the carboranyl fragments in 16a (top) versus an eclipsed arrangement in 16a·2MeOH (bottom). Right column: A comparison of the 3D supramolecular assemblies of 16a (left) and 16a·2MeOH (right) showing the well-defined channels (yellow-orange) running along the c axis in the former and the absence of voids in the latter. All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Color code: B pink; C grey; H white; O red; N light blue; Cl green; Co blue.
Figure 3. Supramolecular assemblies of 16a and 16a·2MeOH. Left column: Projections showing four molecules of each compound forming hydrogen-bonded chains. Middle column: Projections along the hydrogen-bonded chains showing a staggered arrangement of the carboranyl fragments in 16a (top) versus an eclipsed arrangement in 16a·2MeOH (bottom). Right column: A comparison of the 3D supramolecular assemblies of 16a (left) and 16a·2MeOH (right) showing the well-defined channels (yellow-orange) running along the c axis in the former and the absence of voids in the latter. All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Color code: B pink; C grey; H white; O red; N light blue; Cl green; Co blue.
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Scheme 7. Syntheses of complexes 2224.
Scheme 7. Syntheses of complexes 2224.
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Scheme 8. μ2–O versus μ3–O bridging of hmpH in Fe complexes.
Scheme 8. μ2–O versus μ3–O bridging of hmpH in Fe complexes.
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Figure 4. Ball and stick representation of the molecular structure of 22·acetone showing both enantiomers in the racemate; All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Blue = N, pink = B, dark Grey = C, Orange = Fe.
Figure 4. Ball and stick representation of the molecular structure of 22·acetone showing both enantiomers in the racemate; All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Blue = N, pink = B, dark Grey = C, Orange = Fe.
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Figure 5. (A) CD spectra of R(+)2a (blue dotted lines), S(–)2b (red dotted lines), (R,R)CFe(S)CFe-(−)22 (blue plain lines) and (S,S)AFe(R)AFe-(+)22 (red plain lines); (B) SHG Intensity of a mixture of (R,R)CFe(S)CFe-(−)22 and (S,S)AFe(R)AFe-(+)22 versus temperature between −200 °C and 200 °C.
Figure 5. (A) CD spectra of R(+)2a (blue dotted lines), S(–)2b (red dotted lines), (R,R)CFe(S)CFe-(−)22 (blue plain lines) and (S,S)AFe(R)AFe-(+)22 (red plain lines); (B) SHG Intensity of a mixture of (R,R)CFe(S)CFe-(−)22 and (S,S)AFe(R)AFe-(+)22 versus temperature between −200 °C and 200 °C.
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Figure 6. Schematic representation of racemic (rac-22) and conglomerate (co-22) formation for bulk samples of 22.
Figure 6. Schematic representation of racemic (rac-22) and conglomerate (co-22) formation for bulk samples of 22.
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Scheme 9. Syntheses of complexes 2528.
Scheme 9. Syntheses of complexes 2528.
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Figure 7. Model showing the ternary structure with intercalation of S27c·β-CD from the major groove of the hexanucleotide. The d(GTCGAC)2 residues are depicted in green and the phosphodiester/ribose backbones in purple. The platinum(II)-terpy complex is depicted in red, the carborane cage is white, and the β-CD is yellow. Reproduced from Reference [50] with permission of The Royal Society of Chemistry.
Figure 7. Model showing the ternary structure with intercalation of S27c·β-CD from the major groove of the hexanucleotide. The d(GTCGAC)2 residues are depicted in green and the phosphodiester/ribose backbones in purple. The platinum(II)-terpy complex is depicted in red, the carborane cage is white, and the β-CD is yellow. Reproduced from Reference [50] with permission of The Royal Society of Chemistry.
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Scheme 10. Synthesis of complex 29.
Scheme 10. Synthesis of complex 29.
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Scheme 11. Synthesis of complex 30.
Scheme 11. Synthesis of complex 30.
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Figure 8. Supramolecular assembly of 30 showing two hydrogen-bonded homochiral ribbons (enantiomers indicated with arrows). All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Blue = N, red = O, pink = B, dark grey = C, violet = Co, green = Cl.
Figure 8. Supramolecular assembly of 30 showing two hydrogen-bonded homochiral ribbons (enantiomers indicated with arrows). All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Blue = N, red = O, pink = B, dark grey = C, violet = Co, green = Cl.
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Figure 9. Octahedral-based geometric isomers for not branched tetradentate ligand.
Figure 9. Octahedral-based geometric isomers for not branched tetradentate ligand.
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Scheme 12. Synthesis of complex 31.
Scheme 12. Synthesis of complex 31.
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Figure 10. Molecular structure of Fe2Cl3(syn-5a2)(EtO)(H2O) (31). All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Pink = B, dark Grey = C.
Figure 10. Molecular structure of Fe2Cl3(syn-5a2)(EtO)(H2O) (31). All hydrogen atoms, except those for the CHOH group, are omitted for clarity. Pink = B, dark Grey = C.
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Scheme 13. Syntheses of complexes 3242.
Scheme 13. Syntheses of complexes 3242.
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Scheme 14. Syntheses of complexes 4446.
Scheme 14. Syntheses of complexes 4446.
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