Alkylated carborane anions and radicals
New tools for chemists: Novel weakly nucleophilic anions whose salts conduct electricity well in nonpolar solvents and which form strongly oxidizing, stable radicals.
Weakly coordinating anions can stabilize highly electrophilic or oxidizing cations. These anions are useful in fundamental research, but they can also play a variety of practical roles, such as counterions to cationic catalysts for olefin polymerization, electrolytes in lithium ion batteries, nonlinear optical materials, and solvents for extracting radioactive cations from wastes.
Weakly nucleophilic anionsthose that minimally influence their cationshave been widely studied in the past decade (1, 2). Ideally, these anions should possess the following qualities:
The icosahedral CB11 cluster is an ideal core for weakly coordinating anions. It is large (~3.4 Å average BB diam) and very stable, with no lone pairs. The long-known closo-CB11H12 anion 1 (3) serves as a starting material for the preparation of many weakly nucleophilic anions. Replacing the relatively nucleophilic BH vertices with the more inert Bhalogen vertices has been the most common strategy, and a large family of promising halogenated anions has been developed (49).
Developing weakly coordinating and lipophilic anions is not the only reason to study the CB11 cluster. Understanding reactivity and electronic properties in simple electronic models is a hallmark of modern chemistry, but cluster chemistry lags behind many other areas in this respect. The arrow-pushing formalisms of organic chemistry cannot be used with these clusters because they are not easily described by simple valence bond models. Mechanisms are proposed infrequently for cluster reactions, and trends in reactivity are described by empirical rules. Elegant models have been developed that successfully predict the coarse properties of clusters, such as topology and electron count, but they are less useful in describing more subtle properties. Current understanding of clusters is somewhat similar to that of conjugated -systems half a century ago, when Hückel theory was known but laborious to apply, and the perturbational molecular orbital method and WoodwardHoffman rules were not yet developed.
The Frontier MO theory of chemical reactivity relates the sensitivity of a position to electrophilic attack to the amplitude of the HOMO at that position. In this case, the theory actually needs to take into consideration both the HOMO and the HOMO1 orbital pairs. This accounts for
The relative reactivity of the vertices 711 and vertex 12 should be sensitive to HOMO and HOMO1 energies, which in turn will depend on the substituents present on the cage. This dependence has been examined using a series of methylated derivatives whose anodic oxidation potentials correlate with MO energies in a way that is nicely rationalized by the inspection of the MO coefficients in 1 at the position of substitution (11).
Preparation of the parent CB11 anions
Our original conversion of 1 to CB11Me12 (2) by methylation with methyl trifluoromethanesulfonate (triflate) required an expensive base, 2,6-di-tert-butylpyridine, to quench the triflic acid generated in the reaction (15). We have developed an improved synthesis that uses inexpensive calcium hydride as the only base (16). The overall procedure, starting with 1, provides a 95% yield of >99% pure Cs+2. In these alkylation reactions, alkyl triflates can sometimes be replaced by the cheaper bromides (17), but not in the synthesis of 2.
The anion 2 can be oxizided electrochemically (1.16 V vs ferrocene) to give the stable isostructural free radical CB11 Me12 (2). This result can also be accomplished with Pb(CF3CO2)4, providing 2 in 74% isolated yield (Figure 3) (18). Unlike its ionic precursor, 2 dissolves in oxidation-resistant nonpolar solvents (e.g., pentane, CCl4) to give deep blue solutions. The stability of 2 can be attributed to steric protection of the delocalized free-valencecarrying centers in the carborane icosahedron by a sheath of methyl groups.
Properties of the CB11Me12 anion
The CB11Me12 anion is remarkably lipophilic. Although all of the salts we studied are soluble in polar organic solvents (e.g., Et2O, CH2Cl2), their solubility in less polar organic solvents depends on the cation. Lithium salts are the most soluble, and 300 g/L (1.0 M) saturated benzene solutions of Li+2 can be prepared. A detectable solubility was observed even in alkanes, and the replacement of a single methyl substituent with hexyl in the anion suffices to render its lithium salt very soluble in these solvents. Salts of the heavier alkali metals and of di- and trivalent cations are less soluble; nonetheless Cs+2 partitions from water into toluene with a partition coefficient of 0.4. Anion 2 also solubilizes many organic cations (e.g., polyaryl-substituted pyrylium and pyridinium) in low-polarity organic solvents.
A saturated solution of Li+2 in benzene is electrically conductive. The conductivity strongly depends on the water content and increases from 1.8 × 105 S/cm for the dry solution to 6.2 × 103 S/cm for a solution containing 1.75 H2O molecules per Li+ ion (the lithium salt can be dried easily by heating to 180 °C under reduced pressure). This conductivity implies the presence of benzene-solvated dissociated Li+ ions. The nature of the solvation is hinted at by the -face interaction between Li+ and an aromatic ring in the crystal of LiCB11Me12 toluene (10).
Other CB11 derivatives
Substitution at boron requires differentiation among the three inequivalent vertices. This can be provided by prior substitution, and the Pd-catalyzed Kumada cross-coupling of 12-I-CB11H11 with RMgX (R = Me, Et, n-Bu, n-hexyl, and Ph) yields 12-alkylated and 12-arylated derivatives (22). This coupling is sensitive to steric hindrance and fails when five methyl groups are present in positions 711.
All the boron vertices of 1 react with electrophiles, but as indicated in the earlier discussion of the HOMO and HOMO1 orbitals, they do so at different rates. This is best illustrated for halogenation (9). The antipodal position 12 is the most reactive and requires the mildest conditions for selective substitution. The distal positions 711 are less reactive but can be halogenated under harsher conditions. The proximate positions 26 are less reactive still, and their halogenation requires forcing conditions. Permethylation with methylating agents can be achieved under mild conditions because incorporating the methyl groups activates the cluster for further methylation (15). Other alkylations are possible, for instance, the introduction of tropylium as a substituent (22).
The mechanism of these electrophilic substitutions has not been studied in detail. A tentative way of rationalizing the observed processes is to imagine that an electrophile E, such as a proton, is transferred to a BX bond, forming a three-center two-electron bond (in reality, the protonation may be occurring on an edge or a triangular face of the cluster). Then an electrophile is lost with the re-formation of a two-electron exocyclic bond.
An example of such a process is the electrophile-promoted nucleophilic substitution reaction (23), in which an acid HY induces nucleophilic substitution on a boron vertex of the CB11 anion, converting BH or BMe to BY, presumably with the formation of molecular hydrogen or methane (X = H or Me, E = H+, Nu = Y). Thus, 2 reacts with HF to yield 70% 12-F-CB11Me11 and 30% 7-F-CB11Me11 (18). Also, a base must be present during the electrophilic methylation of 1 with MeOTf (15) to avoid a similar reaction of the TfOH byproduct with 2.
Protecting group strategies combined with an understanding of intrinsic reactivity differences among the vertices permit access to a variety of substitution patterns (11). A bulky i-Pr3Si-group can be easily introduced into position 1 to protect adjacent positions 26 from substitution. It can be later removed with F. Similar protection is provided by 1-F substitution. An iodine substituent protects its boron vertex from methylation and can be removed later by dissolving-metal reduction.
In our laboratories, the fundamental reactions described in Figure 4 have been combined in various ways to prepare ~50 variously substituted CB11 anions with fivefold symmetry in the substitution pattern. The cyclic voltammetry of many of these compounds has been studied; most undergo reversible one-electron oxidation.
Substitution that breaks fivefold symmetry is harder to control. A 2-Br derivative is easily available from the carbene insertion reaction (10), and some reactions that introduce bulky substituents can be constrained to monosubstitution. A curious example is the replacement of a methyl group by an aryl, which is accomplished by heating a lithium salt of a methylated CB11 anion in an aromatic solvent; the substitution can occur in position 7 or 12. This reaction possibly proceeds by the lithium cation performing a second-order electrophilic substitution (SE2) on the carbon of a methyl group to give methyllithium and a boronium ylide, which then performs an electrophilic substitution on the arene present.
Electrochemical uses of CB11Me12
Moisture has a dramatic effect on the rate constant of these isomerizations. The very dry solution is a poor catalyst, the addition of up to 0.1 mol of water/mol of Li+ greatly enhances catalysis, and additional water attenuates it. Solvation of Li+ by water apparently favors the dissociation of the LiCB11Me12 tight ion pairs and enhances conductivity, but excessive hydration of dissociated Li+ makes it a poorer catalyst. Only in the narrow region where there is sufficient water to favor the dissociation of the ion pairs, but not enough to attenuate the catalytic activity of Li+, is LiCB11Me12 optimally active, and this ideal situation appears to be approximated by standard benchtop conditions.
On the basis of these results, we wanted to examine other additives and solvents that might enhance ion-pair dissociation but not bind the Li+ ion strongly. A cursory survey of scrupulously dry solvents showed that solutions in 1,2-dichloroethane were especially effective. In an attempt to find out whether catalytic amounts of Li+2 would be sufficient, we conducted a series of reactions with ~0.03 M Li+2 in C2D4Cl2 under anhydrous conditions. In these dry and dilute solutions, the catalysis is less pronounced than in the saturated benchtop solutions in benzene, but it is still very significant. For instance, at 25 °C, the quadricyclane to norbornadiene rearrangement, which normally requires heating to 150 °C, proceeds with a half-life of 4.2 h (vs 11 min in a saturated benchtop solution of Li+2 in benzene).
We have now observed catalysis of other reactions as well, such as DielsAlder additions and allyl and silyl ether solvolyses. Much optimization still remains to be done, but judging by these preliminary observations and given the now low cost of 2, the CB11Me12 salts of Li+ and other cations, possibly including chiral ones, promise to become practical catalysts.
Nonaqueous solvents are used widely in studies of redox reactions of organic and coordination compounds (26). There are many reasons to perform voltammetry and other electrochemical measurements in such solvents: increased solubility of nonpolar compounds, elimination of proton transfer reactions, suppression of analyte adsorption onto the electrode, and effects of solvent ligation. Until now, aromatic hydrocarbons have found limited use as solvents in electrochemistry because they do not dissolve the supporting electrolytes required for achieving a reasonable electrical conductivity. Whereas considerable effort has been made to use aromatic solvents or polyaromatic melts in the electrodeposition of various metals (27), routine electrochemical experiments with conventional electrodes, concentrations, and scan rates have rarely been attempted. The electrical conductivity of Li+2C6H6 is comparable with that of common nonaqueous electrochemical media and offers an attractive alternative for performing electro chemical reactions. We therefore decided to examine the pos sible utility of Li+2 as a supporting electrolyte for electrochemical measurements in nonpolar solvents (28).
The range of potentials in which a gold electrode and 0.5 M Li+2 in benzene can be used is conveniently wide: It is limited by oxidation of 2 at +0.7 V to the neutral radical 2 and by reduction of Li+ at 2.0 V. Ferrocene oxidation proceeds reversibly at a half-wave potential of +0.04 V. The separation of anodic and cathodic peaks is 59 mV at a scan rate up to 4 V/s, as expected for a reversible one-electron redox process. Even a saturated solution of Li+2 in silicone oil [(PhMeSiO)n] gives a good ferrocene wave, but only at elevated temperatures (the viscosity is too high at room temperature).
A solution of Ag+2 in 0.5 M Li+2 in benzene yields a well-developed quasireversible cathodic peak and a correspon ding anodic dissolution of deposited silver metal. When the voltage scan is reversed at a value more negative than 1.55 V, the observed anodic wave indicates the formation of LiAg alloys. Forming amalgams of alkali metals is an industrially important process that permits reduction of alkali metals at potentials 1 V less negative than their equilibrium potentials (29).
Uses of CB11Me12 in SPEs
Lithium rechargeable batteries based on SPE technologies have been proposed to replace electrolytes in a wide variety of applications. Thus far, however, the SPE-based batteries have exhibited several important performance limitations (31). Most SPE materials development has focused on modifications in polymers and additives, and relatively few salts have been investigated. Yet the nature and concentration of the incorporated salt may have a major influence on the properties of an SPE. In particular, the anion strongly affects phase composition and conductivity, as has been shown for such commonly used anions as perchlorate and triflate (32).
Highly alkylated or peralkylated carboranyl anions may improve some major characteristics of SPE materials. These anions will have low mobility in the polymer solutions, increasing cation transference numbers by decreasing concentration gradients. Incorporating 2 into polymeric structures would maximize the cation transference number and could greatly improve conductivities because of the very low nucleophilicity of this anion and thus a significantly weaker anioncation interaction. Finally, 2 and its analogues could provide high thermal, chemical, and electrochemical stability to SPE materials. These ideas were confirmed when we found that Li+2 is soluble in polymers such as poly(dimethylsiloxane) and poly(methylphenylsiloxane) at concentrations up to 20% (33). Even before any optimization, the conductivity of the very viscous high-concentration solutions was 5 × 105 S/cm at 25 °C, which is comparable to that of other SPE materials produced to date (34).
We started by making a permethylated anion 4 (Figure 5) with a polymerizable 5-hexenyl substituent instead of a methyl group in position 1 of 2. We also incorporated 2 directly into phenyl-containing polymers by heating Li+2 with polystyrene or poly(phenylmethylsiloxane) under vacuum for 3 days at 160 °C to yield 5, relying on the phenylmethyl substitution reaction described above.
Because direct methylation of the 5-hexenyl derivative of 1 with methyl triflate methylates the double bond as well as the cage, 4 was prepared by introducing a 6-chlorohexyl group into position 1 of 1, permethylating, and dehydrochlorinating with a Schwesinger base. Anion 4- was polymerized with a zirconocene-based catalyst to a mixture of low molecular-weight oligomers with a conductivity of 2.2 × 103 S/cm. The 2-doped poly(phenylmethylsiloxane) showed a conductivity of 1.62 × 104 S/cm. These preliminary results show that SPE materials based on 2 can be made relatively easily and that they promise to provide solid polymer electrolytes with superior properties.
Additional uses of the anion
Yet another possible use for 2 is in nonlinear optical materials. The ylides obtained by placing a tropylium substituent at the 12-position of 1 (22) and 2 (11) have higher nonlinear activity than the p-nitroaniline standard. These ylides strong absorption also occurs at shorter wavelengths than that of the standard, perhaps making carborane anions generally useful in this context.
Uses of the CB11Me12 radical
To illustrate the utility of 2 in this respect, we will describe the preparation of R3E+ salts of 2, where E is an atom of a group 14 element (35). Normally, it is difficult to prepare and crystallize group-14 R3E+ salts because adducts form even with weakly nucleophilic solvents, and the solubility is often too low in sufficiently inert ones. Reaction of 2 with a neutral R3E-containing precursor in an inert solvent to produce the R3E+ salt of the solubilizing and only weakly nucleophilic anion 2 suggests a general solution to this problem.
The white solid n-Bu3Sn+2 (6) was prepared this way in dry pentane from n-Bu6Sn2 and two equivalents of 2.
Whereas the chemical shift is much higher than that of other known trialkylstannyl cations, confirming the weakness of the coordination, it is still far lower than the value expected for an isolated cation, >1500 ppm. Metal cationalkane interactions are of considerable research interest (37), and crystal structures with a cationic transition metal atom coordinated to a methyl group have been known for more than a decade (38).
In a similar fashion, the oxidation of Me3E-containing precursors, Me3EEMe3, Me4E, or (t-Bu3E)2Hg (E = Ge, Sn, and Pb), with 2 yielded the salts Me3E+2 (35). Using NMR and extended X-ray absorption fine structure (EXAFS) measurements and ab initio and density functional theory (DFT) calculations, we found that the coordination of the Me3E+ cations with the methyl group of the 2 anion grows stronger with increasing exothermicity of the SE2 reaction, in which the methyl is transferred from boron to the metal atom as one goes up column 14 of the periodic table. Along the series, the equilibrium geometry changes as would be expected for the path of such a backside SE2 reaction, and the results provide a nice example of the BürgiDunitz analysis of reaction paths (39). In the case of silicon, such a displacement apparently actually takes place, because the oxidation reaction yields a tetraalkyl silane and a novel internally charge-compensated boronium ylide CB11Me11, stable only up to 60 °C. This result would be expected if the now even more exothermic substitution of the methyl group by the R3Si+ cation proceeds to completion. An attempt to generate the salt t-Bu+2 led to similar results.
The boronium ylide CB11Me11 is an extremely potent electrophile. It reacts rapidly at low temperatures with alcohols, ethers, aromatics, and other nucleophiles to yield 12- substituted derivatives of 2. As indicated earlier, we suspect that it is a general intermediate in the acid-promoted nucleophilic substitution on 2. The oxidation of the stannylene [(Me3Si)2N]2Sn with 2 or 1-n-Bu-CB11Me11 radicals produces salts of a cyclic stannylene with a positively charged quaternary nitrogen atom (35).
The scope of this new approach to crystalline salts of highly reactive cations is limited by the salts solubility in inert solvents and by the coordinating ability and chemical reactivity of the anion 2. In the following section, we describe an attempt to overcome the reactivity limitation.
Preparation and properties of CB11(CF3)12
Fluorination in CFCl3 of Cs+2 adsorbed on silica gel using an excess of 10% F2 in N2 gave a mixture of incompletely fluorinated anions. Repeated attempts at perfluorination using fluorine with and without irradiation, at various pressures, temperatures, and stirring rates were unsuccessful and provided only incompletely fluorinated mixtures. Unlike 2, the partially fluo rinated mixture is unaffected by anhydrous hydrogen fluoride; hence treatment with Bartletts reagent (K2NiF6) in liquid HF was possible. It provided the perflate salt in 25% overall isolated yield (96% for each of 36 successive substitutions).
The anion 7 meets all of the usual requirements for a useful weakly coordinating anion.
Two shortcomings make 7 less than ideal. The first is its tendency to be disordered in the crystalline state. Numerous attempts at single-crystal X-ray diffraction analysis with a variety of salts failed. However, electron density maps did reveal that 7 is essentially spherical, with an ~8.0-Å outer F-sphere diameter and a 3.38-Å inner CB11-sphere diameter, in agreement with the 8.12-Å and 3.42-Å average diameters in a DFT-optimized structure. Paradoxically, the criteria for a good weakly coordinating anion are also the criteria for anions that will tend to be disordered: large anions that do not exhibit any specific interactions with their cations.
The second significant shortcoming of 7 is that it is explosive and unsafe. When ignited, its Cs+ salt burns vigorously. Scratching 300 mg of Cs+7 contained in a Pyrex flask with a metal spatula caused an explosion that broke glassware 2 m away. We believe it is likely that other salts of 7 will also be explosive and they should be treated with extreme care. Principal products of the explosive decomposition of Cs+7 in O2 include BF3, BF4, CO2, and soot. The calculated heat of explosion of 7 is 1272 kcal/mol, or 1.32 kcal/g (TNT is 1.05 kcal/g). The high energy content is mainly due to the greater strength of the BF bond (154 kcal/mol in BF3) compared with the CF bond (116 kcal/mol).
According to calculations, the perfluorinated analogue 7 of the stable neutral radical 2 is stable, with a structure very similar to that of 7. It is also calculated to be an unusually potent chemical oxidant, 2.9 V above the 2F → F2 couple (40). It remains to be seen whether it can be made and used to produce extremely highly oxidized states of matter.
To sum up
Benjamin T. King is a National Institutes of Health postdoctoral fellow at the University of California, Berkeley (email@example.com), working with Robert G. Bergman on molecular recognition in organometallic systems. He received his B.S. in chemistry from Northeastern University, Boston. After 2 years in industry, he obtained his Ph.D. in chemistry with Josef Michl at the University of Colorado, Boulder. He was the 1997 recipient of an ACS Graduate Fellowship.
Ilya Zharov is a Beckman Fellow at the University of Illinois, UrbanaChampaign (firstname.lastname@example.org), working with Steven C. Zimmerman on the preparation of dendrimers and hyperbranched polymers for molecular recognition. He received his Dipl. Chem. (Honors) from Chelyabinsk State University, Chelyabinsk, Russia, and his M.Sc. from TechnionIsrael Institute of Technology, Haifa. He obtained his Ph.D. in chemistry with Josef Michl at the University of Colorado. He was the 1999 recipient of a Link Foundation Energy Fellowship.
Josef Michl is a professor of chemistry at the University of Colorado (Department of Chemistry and Biochemistry, Boulder, CO 80309-0215; email@example.com). He received his M.S. in chemistry at Charles University, Prague, Czechoslovakia, and his Ph.D. with Rudolf Zahradník at the Czechoslovak Academy of Sciences, also in Prague. He left Czechoslovakia in 1968. He did postdoctoral work at the University of Houston, TX; the University of Texas, Austin; Aarhus University, Denmark; and the University of Utah, Salt Lake City, where he stayed, became a full professor in 1975, and served as chairman from 1979 to 1984. From 1986 to 1990 he held the M. K. Collie-Welch Regents Chair in Chemistry at the University of Texas, Austin, then moved to the University of Colorado. He has been the editor of Chemical Reviews since 1984. He has held many visiting professorships and named lectureships and has won numerous awards. He is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, and the International Academy of Quantum Molecular Science. His current research interests are the development of a molecular-size Tinkertoy construction set for molecular electronics, photochemistry, chemistry of silicon and boron, preparation and study of reactive organic and main-group organometallic molecules, and the use of quantum chemical and experimental methods for better understanding of electronic excited states.