Boron Clusters Come of Age - Journal of Chemical Education (ACS

Viewpoints: Chemists on Chemistry Boron Clusters Come of Age Russell N. Grimes

Boron Clusters Come of Age

658–672

Polyhedral clusters containing boron, alone

Boron Clusters in Medicine

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or in combination with other elements, have

Metal Ion Extraction

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Anticrown Reagents: Host–Guest Recognition of Anions

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(Almost) Noncoordinating Anions

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Nonlinear Optics

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Homogeneous Catalysis

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Liquid Crystals

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these clusters is now recognized to have broad significance

Ion-Selective Electrodes

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for deepening our understanding of covalent bonding, with

Chains, Rings, Rods, and Boxes: Boron Clusters As Building Blocks

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implications for both organic and inorganic chemistry;

Other Applications

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are uniquely suited to specific applications. This article

Future Directions

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attempts to summarize the current state of the art, illustrated

been known for nearly a century, and intensive studies of their structures, bonding, and reactivity have been under way for more than half that period; yet interest and practical applications in this area continue to grow. Two main reasons can be identified for this attention: the three-dimensional delocalized bonding that confers exceptional stability in

secondly, many of the special properties of boron clusters

by examples selected to convey some of the excitement and possibilities for future exploitation of these remarkable compounds. The structures of a number of the molecules discussed in this article are available in fully manipulable Chime format as

JCE Featured Molecules in JCE Online (see page 768).

Other Articles on Inorganic Chemistry in This Issue Products of Chemistry: Inorganic Fullerenes, Onions, and Tubes Andrew P. E. York

673

Products of Chemistry: Dentifrice Fluoride Philip E. Rakita

677

Syntheses and Characterization of Ruthenium(II) Tetrakis(Pyridine) Complexes. An Advanced Coordination Chemistry Experiment or Mini-Project Benjamin J. Coe

718

Lewis Acid–Base, Molecular Modeling, and Isotopic Labeling in a Sophomore Inorganic Chemistry Laboratory Chip Nataro, Michelle A. Ferguson, Katherine M. Bocage, Brian J. Hess, Vincent J. Ross, Daniel T. Swarr

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Chemical Education Research: Major Sources of Difficulty in Students’ Understanding of Basic Inorganic Qualitative Analysis Kim Chwee Daniel Tan, Ngoh Khang Goh, Lian Sai Chia, David F. Treagust

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A Unified Approach to Electron Counting in Main-Group Clusters John E. McGrady

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The Trinity of Life: The Genome, the Proteome, and the Mineral Chemical Elements R. J. P. Williams and J. J. R. Fraústo da Silva

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Featured Molecules: Boron Clusters William F. Coleman

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Viewpoints: Chemists on Chemistry is supported by a grant from The Camille and Henry Dreyfus Foundation, Inc.

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Journal of Chemical Education

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Viewpoints: Chemists on Chemistry

Boron Clusters Come of Age Russell N. Grimes Department of Chemistry, University of Virginia, Charlottesville, VA 22904-4319; [email protected]

Boron is the only element other than carbon that can build molecules of unlimited size by covalently bonding to itself, a property known as catenation. In contrast to the chains and rings favored by carbon, boron—owing to the presence of only three electrons in its valence shell—adopts a cluster motif that is reflected in the various forms of the pure element (all of which feature B12 icosahedra) and in the huge area of polyhedral borane chemistry that has developed over the years. In combination with many other elements— indeed, most of the periodic table other than the noble gases and actinides—boron clusters exhibit an astonishing variety of stable architectures, as a few examples from the recent literature (1–7) illustrate (Figure 1). To many students and teachers, such structures may appear bizarre and disconnected from mainstream science; yet exotic molecules, especially those whose structures surprise and confound the theory of the day, play an important role in chemical education since they force us to rethink our notions of chemical bonding and reactivity. Students in particular need to understand that chemistry is not a static science with its basic ideas frozen in place; rather, our understanding of fundamental concepts of spectroscopy, bonding, molecular structure, and chemical reactivity is continually evolving. In fact, entirely new fields of chemistry can come into existence unpredicted by anyone or hinted at in any textbook, and seemingly mined-out areas can be brought to life; sometimes a single discovery is all it takes. For most novel compounds, it may require a long time, if ever, for useful applications to be found; thus benzene, today a leading industrial chemical, was little more than a laboratory curiosity for nearly three-quarters of a century following its discovery in 1825. Ferrocene caused a sensation fifty years ago when its unprecedented sandwich structure was revealed, but direct commercial applications of metallocenes (metal– hydrocarbon sandwich complexes) were slow to develop. As molecular exotica go, the polyhedral boranes offer an interesting, and in some ways unique, case history. Their discovery in the early 1900s by Alfred Stock and coworkers (8) opened the door to an entirely new realm of structural chemistry—one that a century later is still unfolding and offering new surprises. The stable existence of the neutral boranes, the polyhedral borane anions (Figure 2), and their various derivatives containing heteroatoms bound in the cage skeleton (9) such as carboranes,1 metallaboranes, and main-group heteroboranes, has profoundly influenced other areas of inorganic and even organic chemistry (consider the nonclassical hydrocarbons; ref 10), and helped to force a revolution in the way chemists think2 about covalent bonding (11, 12). This alone, even if no practical uses for boron cluster compounds existed, would justify continuing exploration in this area. However, the fact is that there are diverse areas of application that exploit the remarkable properties of boron clusters. How remarkable? Consider the prototype borane, the dodecahydrododecaborate (2) ion, B12H122− (Figure 2, lower right). This molecule, which possesses perfect icosahedral symmetry, is both highly water soluble and amazingly heat658


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resistant: its dicesium salt survives temperatures above 810 C without decomposition, making B12H122− arguably the most stable molecule in all of chemistry (and most definitely not “electron-deficient” in any rational sense). One aspect of its chemical inertness that relates to biomedical applications, discussed below, is its very low toxicity in humans. Theoretical analyses (13) show that the source of stability in B12H122− is the highly delocalized bonding in the boron framework, with 26 “skeletal electrons” occupying 13 bonding molecular orbitals (MOs) on the polyhedral surface; such σ-aromatic bonding may be regarded as a kind of 3-dimensional analogue of benzene and has been labeled “superaromatic” (13, 14). As Figure 2 indicates, B12H122− is the largest member of a family of BnHn2− stable boron hydride anions where n = 6– 12, whose molecular geometries are deltahedra (polyhedra having only triangular faces, designated by the prefix closo); deltahedra with one missing vertex are labeled nido from the Greek word for nest. Analogous families of closo-carboranes exist in which one or more BH units are formally replaced by isoelectronic CH+ units (Figure 3, top). For example, known carboranes corresponding to B12H122− include the CB11H12− anion and neutral C2B10H12, the latter existing in the form of three isomers having the carbon atoms in ortho, meta, or para locations in the cage (see Figure 3). Neutral C2Bn-2Hn closo-carboranes corresponding to all of the BnHn2− dianions are known, and in addition there are many non-closo-carboranes, that is, clusters having open-cage structures (Figure 3, bottom). Beyond this, the ability of many other elements to occupy vertexes in stable, isolable boron cluster frameworks, as illustrated in Figure 1, is a defining characteristic of this field and gives it a scope and variety that is without parallel outside of organic chemistry. Reviews of many aspects of this field that provide detailed discussions of synthesis, structure, bonding, and chemistry are available (9). The purpose of this article is to acquaint readers of this Journal with some of the contemporary uses that are emerging for boron clusters and to suggest a few possibilities for the future. This discussion is necessarily selective and limited in scope. Boron Clusters in Medicine In general, two characteristic properties of polyhedral boranes make them attractive for medical and pharmacological applications. Their low chemical reactivity and resistance to breakdown in biological systems render them relatively nontoxic and, as will be shown, they can be tailored to specific purposes. The second important attribute is the very large neutron cross-section (ability to absorb neutrons) of the 10 B isotope, which accounts for 20% of boron in nature, 11B constituting the remainder. In 1935 (15) it was found that a collision between boron-10 nuclei and slow neutrons produces α particles (helium-4 nuclei) via the reaction: 10

B + 1n

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He + 7Li + 2.31 MeV (94%) + 2.79 MeV (6%) + γ 0.48 MeV

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B, BH S

CH, C-alkyl, C-SiMe3

N

P

Mn

Ni

Zn

Fe

Cl

B 7H72ⴚ

B 6H62ⴚ

B 9H92ⴚ

B 8H82ⴚ

3ⴚ

BH

B 11H112ⴚ

B 10H102ⴚ

A

H

B 12H122ⴚ

Figure 2. Structures of BnHn2− dianions.

H

H

NMe3

Me3N

B

H H

H

BH

representative closo -carboranes:

C, CH −

C 1,5-C2B 3H5

D

1,2-C 2B 10H12

−

1-CB11H12

1,7-C2B 6H8

2,4-C 2B 5H7

1,12-C2B 10H12

1,7-C 2B 10H12

F representative open-cage carboranes:

E G

H

H

H

H

1,2-C 2B 3H7

H

2-CB5H9

H

H H

Figure 1. Molecular structures of: (A) Mn3[(SiMe3)2C2B4H4]43− (1),

H

H

H H

(B) P2B4Cl4 (2),

H

(C) [(nido-C2B9H11)ZnNMe3]2 (3),

2,3,4,5-C4B 2H6

2,3,4-C3B 3H7

(D) [(Me2HC3B2Me2)Ni]n (4), (E) S2B16H16 (5),

H

2,3-C2B 4H8

4,5-C2B 7H13

−

(F) NB11H11 (6), Figure 3. Structures of selected carboranes.

(G) [(C6H6)Fe(Et2C2B4H3-7-C⬅C]3C6H3 (7).

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The following year a physician, G. L. Locher, noted that if this reaction were conducted in tissue, the energy of the particles produced would destroy the immediate cell but not its neighbors (16). Thus, if 10B could somehow be selectively incorporated into tumor cells in sufficient concentration, irradiation of these cells with otherwise benign low-energy neutrons would constitute a novel noninvasive treatment of inoperable cancers. The problem with implementing boron neutron capture therapy (BNCT) in Locher’s day was the lack of suitable boron compounds: most were toxic, low solubility, monoboron species such as boric acid. Despite a few early clinical studies (17), not until the discovery of the BnHn2− polyhedral borane anions and their carborane analogues (18) did BNCT attract serious attention. The stability, solubility in aqueous media, and high boron content of polyboron clusters such as B12H122− reignited interest in this approach, and a Japanese physician, H. D. Hatanaka, who had access to a neutron source, treated human brain tumor patients with BNCT with some success over several years using the mercapto-substituted derivative B12H11SH2− (19) . In recent years BNCT has been undergoing clinical trials in the United States, the Netherlands, and other countries (20). In order to be effective, this therapy requires boron concentrations of 20–35 µg, or about a billion (109) 10B atoms per cell (somewhat less if they are localized in the cell nucleus), with high selectivity in the tumor versus healthy tissue. One currently employed strategy is to attach boron clusters to tumor antibodies targeted to specific cell types. Among those under study are glycosides (21) and porphyrins attached to several carborane cages, as in the example of

a tetracarboranyl porphyrin (Figure 4) whose biological properties show promise for BNCT application (22). Additional possibilities are created if the porphyrins are coordinated to metal ions; for example, complexes incorporating radionuclides such as 67Cu can function as tracers to follow the distribution of boron in the system. A wide array of carborane- and borane-substituted BNCT agents is under study, including polyamines, glucosides, carbohydrates, nucleosides, and immunoconjugates, as well as liposomes (20). Of particular interest are carriers containing multiple C2B10 clusters that can deliver a hundred or more boron atoms per molecule into cancer cells; Hawthorne and coworkers have prepared liposomes containing large borane or carborane cage species that are taken up very selectively by tumor tissue (23). In addition, small-molecule approaches to BNCT are being pursued: for example, electrically neutral water soluble azanonaboranes of the type nido-RNB8H11NHR′ [R = HO(CH2)3, Me, MeO(CH2)3; R′ = H, (CH2)3OH] show very low toxicity, although they are not selectively incorporated into tumor cells (24). To date, clinical BNCT trials approved by the U.S. Food and Drug Administration have been limited almost exclusively to patients afflicted with the deadly brain tumor glioblastoma multiforme (GBM) and utilize just three boron compounds, all known for decades (20c): an arylboronic acid, 4-dihydroxyborylphenylalanine; a mercaptosubstituted derivative of B12H122−, Na2(B12H11SH); and the salt Na2(B10H10). More recent developments in boron cluster synthesis have yet to be explored in BNCT studies. Other therapeutic uses of the 10B-neutron interaction are also under investigation; for example, in the treatment of rheu-

H

BH

C R

Figure 4. A porphyrin containing four attached nido -C 2 B 9 cage substituents (22a).

CH 2

CH2 H

N

NH

R

HN

N

R

H2 C

H2C R

H

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H


matoid arthritis, a procedure known as boron neutron capture synvectomy (BNCS) is being tested (25). In this procedure, arthritic tissue is ablated by introducing 10B via suitable carrier compounds and then subjecting it to neutron bombardment. In contrast to conventional radiation treatment for arthritis, BNCS is more localized in the body and hence minimizes damage to healthy tissue. In one approach, small unicellular liposomes were employed to incorporate boron into the tissues of arthritic laboratory rats, with encouraging results (26); another method utilizes carborane derivatives having cortisone or related substituent groups (Figure 5A) (27). Boron cluster compounds are also prime candidates for medical applications not involving neutron capture. Because of the exceptional stability of hydrophobic polyhedral borane cages under typical in vivo conditions, carborane- and metallacarborane-based carriers for pharmaceuticals and radiotracers (20b, 28) tend to resist degradation better than conventional organic reagents. Such applications effectively exploit a basic (if somewhat counterintuitive) fact of boron

A

•

H

O OH

cluster geometry: the volume displaced by even the bulky 12vertex C2B10H12 icosahedron is only moderately larger than that of a phenyl group spinning on one of its 2-fold axes (Figure 6), and of course the steric requirements of the lower carboranes are smaller yet. Consequently, up to a point it is feasible to replace C6H5 groups in organic molecules with polyhedral borane cages without undue crowding, as in the porphyrin derivative in Figure 4. Imaging techniques using carboranes labeled with radioactive isotopes such as 99Tc, including positron emission tomography (PET) and magnetic resonance imaging (MRI), are currently under investigation (29). Radiolabeled “Venus flytrap” metallacarboranes (Figure 7) containing 57Co have been attached to a monoclonal antibody T84.66 that was used to demonstrate localization in mouse tumors (30). BNCTcandidate carborane derivatives designed to bind to the minor grooves of DNA (Figure 5B) have been prepared (31); the 73Se-containing species allows the compound distribution to be determined using PET. These approaches are especially

O

HO Me H

C

O

H

BH

H

B

O

X N

N N N H

H

N H

X = NMe3 , Se

O

Figure 5. (A) Cortisone-substituted 1,2-C2B10H12 (27). (B) Imino derivatives of 1,2-C2B10H12 for DNA binding (31).

CH

BH, B

trapped M3ⴙ ion X = O, S, SO2 4

CH2

CH2

X

X

CH2

Figure 6. Volumes occupied by an icosahedral carborane (left) and a benzene ring spinning on a 2-fold axis (right), ignoring H atoms in both molecules.

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CH2

A

B

Figure 7. Venus flytrap biscarboranes, uncomplexed (A) and with trapped metal ion (B) (30).

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useful in determining where boron accumulates in the body, which in turn helps in the design of better carriers for BNCT and BNCS treatments. Boron cluster-containing pharmaceuticals are a focus of intensive study in many laboratories, most (but not all) of this work centering on icosahedral C2B10H12 carborane derivatives. These efforts, which have been reviewed recently (20b), include carborane-based insect neuropeptides, antineoplastic and cytotoxic agents, estrogen agonists and antagonists, retinoids, protein kinase C modulators, and others. This research area is a prime example of interdisciplinary effort, drawing on the expertise of workers in such disparate fields as biochemistry, radiology, organic synthesis, and boron cluster chemistry, and has led to intriguing findings. For example, carbon-substituted phenolic derivatives of 1,12-C2B10H12 (Figure 3, second row) have been shown to be potent estrogen receptor agonists (32a) and a bis(carboranyl) organotin compound is more active against seven human tumor cell lines than are 5-fluoroacil, cis-platin, and carboplatin (32b). It is noteworthy that when carboranyl cages are introduced to pharmacologically active organics, the receptor binding affinity in many cases is improved, probably because of the hydrophobic character of the carborane (20b, 33). Metallacarboranes (carboranes having one or more metal atoms in the cage framework) and exo-polyhedral metallated carborane derivatives have shown significant anticancer activity; in some cases they are more effective against certain glioma and breast cancer cell lines than are standard drugs used in screening. Figure 8 contains a sampling of MC3B7 (34) and MC2B4 (35) metallacarborane clusters that exhibit cytotoxic behavior against common tumor cell types, including L-1210 lymphoid leukemia, Tmolt3 leukemia, murine P388 lymphocytic growth, Sk-2 melanoma, HeLa-S3 human uterine carcinoma, and lung broncogenic MB-9812. Studies on selected compounds in this group have shown that they interfere with the synthesis of RNA and DNA in the tumor cells (34a, 35a), suggesting some similarity with the mode of action of ferrocenium salts and other metallocenes (metalcyclopentadienyl sandwich compounds) that show anticancer activity (36). Metal Ion Extraction Electrically neutral boranes and carboranes typically dissolve in organic media but not in water. In contrast, polyhedral borane anions (Figure 2) are very water soluble, as are many anionic metallacarboranes. Perhaps surprising is the fact that many metallacarborane anions are also soluble in organic solvents, a property that is traceable to the negative (hydridic) character on the BH hydrogens resulting from charge delocalization over the cage framework. One metallacarborane type that has attracted considerable interest is the family of bis(dicarbollyl)metallate(III) ions 3-MIII(1,2-C2B9H11)2−, first prepared in 1965 (37), and their substituted derivatives (Figure 9). Remarkably, although Na+ Co(C2B9H11)2− has electrolytic properties similar to NaCl, by shaking an 0.5 M aqueous solution of this salt with an equal volume of diethyl ether one can effect a quantitative transfer of the metallacarborane to the ether layer (38)! Compounds of this class, which are resistant to thermal degradation and to attack by acids, bases, and radioactive substances, were first employed by Czech workers three decades ago to extract ra662


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dionuclides from nuclear waste (39) and are currently employed on a substantial scale for this purpose and as sensors for Cs+ and Sr2+ in milk and body fluids. A problem with the parent (unsubstituted) species (Figure 9A) is its slow decomposition in concentrated nitric acid, which presents a serious disadvantage in metal ion extraction. However, this difficulty is overcome by replacement of some of the boron hydrogen atoms by methyl groups or halogens; the hexachloro derivative shown in Figure 9B resists 3 M nitric acid (38, 39b, 40), thereby meeting a critical requirement for use in the recovery of 137Cs and 90Sr from radioactive wastes. The consequences of employing other metals (e.g., iron or nickel) in place of cobalt, using carborane ligands with separated cage carbons, and introducing aromatic or oxygen-donor substituents, have been explored by Teixidor et al. (41). Derivatives containing ether groups on the cage carbons show improved selectivity over the parent complex, and the tetra-C–phenyl species in Figure 9C is reported to extract Cs+ from acid solution better than any other available reagent (41a). Several groups have examined the extraction of alkali, alkaline-earth, and lanthanide metal cations with nitrobenzene solutions of the cobaltate anions together with crown ethers, polyethylene glycols, polyethers, and calix[n]arenes (42). Nitrobenzene promotes selectivity in metal extraction (41) and is widely used for this reason; however, it is ecologically unfriendly and eliminating it from the process is a long-range goal. In a different approach discussed later in this article, Co(1,2-C2B9H11)2− ions and various derivatives are incorporated as doping agents in conducting organic polymers such as polypyrroles, creating “intelligent membranes” that can capture cations with selectivity controlled by an applied potential (41a, 43) (another example utilizes mercurocarborand complexes, discussed below). A hydrogensensitive microelectrode based on this principle has been developed (43b). Extraction systems based on boron clusters other than metallacarboranes are also known. For example, derivatives of B12H122− having phosphorus oxide substituents selectively remove 241Am and 152Eu from nuclear waste (44). The metals recovered from radioactive effluent are often valuable and can be reused, for example; 137Cs and 90Sr are employed as thermoelectric generators and in medical equipment sterilization (40). Applications of boron cluster chemistry to metal extraction technology address important issues, and seem destined to grow in importance and scope. Anticrown Reagents: Host–Guest Recognition of Anions Crown ethers and their ability to trap alkail metal cations have been known for some time (45), and the field of host–guest chemistry is well established (46), centering on the ability of Lewis base functionalities such as the oxygen atoms in 12-crown-4 (Figure 10, top left) to coordinate positively charged ions. Anticrowns—charge-reversed analogues of crown ethers having acidic groups that can trap anions (47)—were rare until Hawthorne’s group synthesized and investigated several families of Hgn(C2B10H10)n macrocycles or “mercurocarborands” whose electrophilic mercury atoms cause them to function as efficient anticrowns, binding tightly to halide ions (48). Two representative examples, a trimer (A)

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C, CH

BH, B

R = Me, Et, SiMe 3

CH

ⴙ

Co ⴚ

ⴚ

AsF6

Fe

BH, B

ⴚ

Fe

Cl

or

Cl

SbF6ⴚ

Cl Cl Cl Cl

R R

R

Cl

R

Fe M

ⴚ

M = Ta, Nb

R

M = Fe, Co

R

R

B

A

Fe M

Cl

R

R

R Fe

Fe Fe

Fe Co Fe

Me3P

PMe3

C

Cl

−

Figure 9. CoIII(1,2-C2B9H11)2 ions and derivatives for extraction of metal ions.

Figure 8. Metallacarboranes shown to be cytotoxic toward tumor cell strains and solid tumors (34, 35).

O

O

O

O BH

CH

Hg

12-crown-4

Cl

A

C

B

Figure 10. Structures of a crown ether, Hg3 and Hg4 anticrown “mercurocarborands” (A and B), and an Hg4 anticrown with a trapped chloride ion (C)(48).

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and a tetramer (B), together with a chloride complex of the tetramer (C) in which the Cl− is trapped at the center of a square array of Hg atoms held in place by four carborane clusters are depicted in Figure 10. This chemistry is rich and diverse (48, 49); for example, the tetrameric mercuracarborand (Figure 10B) binds not one but two iodide ions, while the trimer (Figure 10A) combines with water and benzene to form a novel π-sandwich complex, [Hg3(C2B10H8Me2)3H2O]2C6H6 in which the benzene is locked between two parallel Hg3C6 planes (50). The complexation and stereochemical properties of these reagents can be modified by replacing one or more of the boron hydrogens with organic or inorganic substituents; thus, the trimeric mercurocarborand Hg3(C2B10H8Me2)3 has been incorporated into a membrane that selectively traps iodide ions at nanomolar concentrations (51). The development of mercurocarborands is a noteworthy example of “designer chemistry” that takes advantage of both the steric and the electronic properties of carboranes. (Almost) Noncoordinating Anions For many years chemists have searched for anionic species that have the least possible affinity in solution for resident cations, attempting to stabilize highly reactive cationic species such as R3Si+, arenium ions, and organotransition metal ions that are important in olefin polymerization catalysis. In recent years this quest has seen major advances based on the finding that the icosahedral CB11H12− ion, and especially its boron-halogenated derivatives, show extremely weak coordination to cations (52). The parent ion, shown in Figure 3, is isoelectronic with C2B10H12 and B12H122− and like them is very resistant to heat and to cage degradation by strong acids. The stability of CB11H12− is reflected in a huge HOMO–LUMO energy gap that is far larger than that of benzene and other arenes (52a), and is further enhanced by replacing several BH hydrogens with halogen or methyl groups to give species such as CB11X6R6−, HCB11X6R5−, and CB11Me12− where X is Cl or Br and R is H or Me. Tetra- and pentafluorinated derivatives of the 10-vertex CB9H10− anion as well as the perfluoro species B12F122− (53) have been explored by Strauss et al. (54). The latter ion is remarkably inert, showing no reaction toward 98% sulfuric acid, 70% nitric acid, Ce4+, or metallic sodium, and the dilithium salt is unchanged on heating to 450 C (54b). Despite its dinegative charge, B12F122− is weakly coordinating and is far less basic (in a Lewis sense) than BF4−, as reflected in the BFC distances in crystal structures of salts formed with CPh3+ and related cations. The CB11 clusters have so little affinity for protons or other electrophiles that their conjugate acids (H+CB11X6R5) function as exceedingly powerful proton donors, far stronger than most well-known superacids such as H2SO4 or HClO4, but without their oxidizing capacity (52). This combination of properties has allowed the stabilization and isolation of salts of some heretofore very elusive species, among which are silylium ions (R 3 Si + ), hydrated protons [(H2O)4H+], C6H7+ (benzenium), and other protonated arenes including C 6 H 6 + , C 6 MeH 6 + , C 6 Me 6 H + , and C6Me3H4+ (55). Reed and coworkers have isolated and crystallographically characterized (Mes)3Si+ CB11Br6Me5− (where 664


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Mes is 2,4,6-trimethylphenyl) and demonstrated that the silylium ion is well-separated from the anions and solvent molecules (55a). Arenium salts have been similarly characterized, and C6H7+ CB11Br6Me5− was found to survive at 150 C (55b)! A very recent demonstration of the extraordinary stabilizing power of the carborane monoanions is the synthesis and structural proof of the first known azafullerene salt, C59N+ Ag(CB11H6Cl6)2− (56), shown in Figure 11. This particular application, like others described in this article, illustrates how the unique capabilities of boron clusters can be optimized for specific purposes by appropriate modification; in this case, the introduction of electron-withdrawing halogens as substituents on the cage generates the strongest superacids yet isolated. Nonlinear Optics Another area in which boron clusters are attracting attention is the study of nonlinear optical (NLO) materials, which have the ability to convert an applied electromagnetic field to a new field with altered properties such as frequency and pulse. NLO materials are increasingly important in several developing technologies including data storage and retrieval, communications, and optical switching (57). While the emphasis has been primarily on solid-state systems, molecular NLO materials offer a number of advantages including very fast response times, lower dielectric constants, and improved processabilty, among others (58). In the rational search for new and improved second-order NLO systems, a basic criterion is the value of the first hyperpolarizability β as a measure of NLO response, and attention has centered on organic molecules that combine electron-donating and electron-withdrawing groups—so-called “push-pull” systems—that result in extremely high molecular polarities. Synthetic and theoretical studies have shown that the incorporation of electron-withdrawing polyhedral borane cages can result in dramatically increased hyperpolarizabilities. Several different boron cluster systems have been investigated, including derivatives of B10H102−, B12H122−, CB11H12−, and C2B10H12 isomers (58, 59); of these, 1,12-C2B10H10 (pcarborane) has proved particularly interesting, as in the examples depicted in Figure 12. The fullerene (C60)-carborane system (A) is an excellent “push-pull” NLO molecule exhibiting a very high β value, a perhaps surprising result if one expects both the fullerene and carborane cages to be electron-withdrawing. However, the studies of Yamamoto et al. (60) indicate that the C60 unit in this case acts as an electron donor relative to the carborane, in contrast to other situations where the boron cluster is the donor (61, 62). Figure 12B shows a proposed tropylium-p-carboranecyclopentadienyl system whose calculated β values (which vary with R) are much higher than those of their fully organic analogues in which the central ring is benzene rather than carborane (58a). Interestingly, the effect of the carborane cage is to reduce the electronic communication between the end rings and consequently increase the charge polarization, in comparison to the benzene-centered molecules. Ferrocenylcarborane derivatives with an imine spacer such as that in Figure 12C with high hyperpolarizabilities have been prepared, and UV and other data indicate that the carborane plays the role of electron acceptor (63).

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Homogeneous Catalysis One of the earliest efforts to harness the special properties of polyhedral boranes was the exploration by Hawthorne and coworkers of hydridorhodium complexes of the formula 3,1,2-H(Ph3P)2RhIII(C2B9H11) as precatalysts for the hydrogenation, hydroformylation, and hydrosilylation of alkenes and alkynes (64). The catalytically active species is an exonido-(Ph 3P) 2 Rh I-C 2B 9 H 12 tautomer whose 16-electron rhodium center is outside the carborane cage (Figure 13A), which in solution exists in equilibrium with the 18-electron closo-RhIIIC2B9 cluster, as shown. Boron-fluorinated derivatives of this complex, as in H(Ph3P)2RhIII(C2B9H11-nFn) (n

= 1–3), are also active in catalyzing the hydrosilylation of styrene and phenylacetylene (65), with some differences in selectivity compared to the parent rhodacarborane. As this illustrates, there is considerable potential for electronic tailoring of such systems via the introduction of appropriate electron-attracting or electron-releasing substituents at boron or carbon cage vertexes. This versatility underlines a significant advantage of metal–boron cluster chemistry in catalyst design, since in most organometallic systems the possibilities for tailoring are considerably more limited in scope. Prompted by the industrial interest in metallocene-based olefin polymerization catalysts of the type Cp2MIVR+ (where Cp is η5-C5H5 and M is an early transition metal), isoelec-

A BH, B

CH

PPh 3 Ph 3P H Rh

Cl

H

Ph 3P Rh

N

H

Ph 3P

Ag

B H

Figure 11. Structure of a bis(carboranyl)silver azafullerene salt (56).

Hf

Hf

H

H2 −H2

B, BH

C

BH

C, CH

A

CH3

2

Hf

H

R

B

R

ⴙ

ⴚ R R

C

D

C H

N

R

Fe

Cl

CH2

TiTi

Cl

Figure 12. Selected NLO-active p-carborane derivatives (58–60).

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Me Me

NMe2

Me2 P

Ti

P Me2

Figure 13. Metallacarborane catalyst precursors for the hydrogenation or polymerization of alkenes and alkynes.

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tronic, electrically neutral analogues having the general formula Cp(R)MIV(R′2C2B9H11) have been investigated by Jordan et al. and found to be catalytically active in the hydrogenation of internal alkynes to cis-alkenes (66) (Figure 13B); the active species is proposed to be the monomeric hydridohafnium complex. Zirconium metallacarboranes of the type Cp′2Zr(Me)(C2B9H12) (Cp′ is C5Me5 or C5Me4Et) polymerize ethylene at 60 C and 300 psi (67), and constrained-geometry Ti(IV) complexes of the type shown in Figure 13C in the presence of methyl aluminum oxane (MAO) catalyze the formation of high molecular weight polymers from ethylene (68). Other applications of large-cage metallacarboranes in catalysis include the controlled radical polymerization of styrene and n-butyl acrylate (69) and the formation of ethylene-norbornene copolymers (70). In the small metallacarborane area, our group working with M. G. Finn has shown that titanium complexes of the type L 2 X 2 Ti(Et 2 C 2 B 4 H 4 ) (X is Cl or alkyl; L 2 is 2PR3,R2P(CH2)nPR2, et cetera.; R is Ph or alkyl) in combination with MAO catalyze the polymerization of ethylene at room temperature and 1 atm pressure (71); the most effective of these found to date is the bis(dimethylphosphinopropane) complex depicted in Figure 13D, which requires only minimal concentrations of MAO and generates high molecular weight polyethylene. These small titanacarborane clusters exhibit unusually high thermal and oxidative stability (for catalyst precursors) and are readily derivatized by the introduction of organic or inorganic functional groups to the carborane framework (71). Applications of metallacarboranes in catalysis are still in the exploratory stage but may ultimately find specialized roles in industry. Liquid Crystals An area of application that takes advantage of both the three-dimensional cage geometry and the special electronic properties of boron clusters is in the development of new types of liquid crystalline materials. Substances that exhibit fluid behavior at room temperature but have a partially ordered structure and exhibit birefringence, known as liquid crystals, are of considerable importance in modern technology, notably in devices for electro-optical display (72). In order for molecules to adopt a nematic or smectic liquid crystalline phase (Figure 14) they must be nonspherical (commonly disc- or rodlike, although other shapes are known). Typical molecular rods contain rigid cores—for example, hydrocarbon rings such as phenyl, cyclohexyl, or bicyclo[2.2.2]octyl—either directly linked or connected via organic groups such as alkyl, alkenyl, or azo. The actual physical behavior of a liquid crystal system is closely correlated with its molecular structure, and the continuing development of this technology has sparked interest in novel kinds of molecular core units. Boron clusters are especially attractive because of their thermal stability, delocalized bonding, and ease of derivatization, which allows them to be used as modules for constructing molecular rods that show distinctive liquid crystalline behavior (73). In many cases the introduction of borane or carborane cluster units into the chain significantly affects its electronic structure and hence the response of the bulk material to applied electric fields. Typical of the materials under investigation are liquid crystals containing B10H102− 666


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or 1,12-C2B10H10 (p-carborane), shown in Figures 14C and 14D. The combination of boron clusters with organic rings can be engineered to produce liquid crystalline materials having properties not otherwise attainable, such as UV-transparency combined with high polarity, and to favor, for example, the formation of nematic versus smectic phases (73a). Ion-Selective Electrodes Chemical sensors based on ion-sensitive electrodes (ISEs) are important analytical tools in clinical diagnostic applications and environmental monitoring. Particularly important are those employing liquid polymer membranes (usually polyvinyl chlorides or PVCs) containing ion-complexing agents (ionophores) that selectively complex the ion of interest, together with lipophilic ionic additives for sensing anions or cations (74). For cation detection, tetraphenylborates have been employed for decades (75) but are sensitive to hydrolysis induced by acids, oxidizing agents, and light (76a). Now, there is increasing interest in drawing upon the properties of boron clusters in this area (41, 43, 51, 76). So-called “least-coordinating” halogenated monocarbon carborane anions, CB11X11− where X is Cl, Br, or I (see the discussion above), have found application as alternatives to tetraphenylborates in ISEs, combining strong lipophilicity with chemical inertness; the CB11I11− ion in particular shows significantly improved selectivity and lower detection limits compared to tetraphenylborates (76a). Similar results have been found with the cobalt-dicarbollide complex 3,1,2-Co(C2B9H11)2− (Figure 9A) (76b), and as mentioned earlier, this complex is also employed as a dopant in cation-sensitive microelectrodes based on polypyrrole conducting polymers

A

B

C C7H13O

N

N

OC7H13

•

B, BH

C

D C5H11

C5H11

Figure 14. Nematic (A) and smectic (B) liquid crystal phases; boron cluster-based (C and D) liquid crystalline molecules (73).

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(43) or in PVC membrane electrodes (41a, 76b). Nanomolar detection limits for iodide ion have been achieved with the anticrown “mercurocarborand-3” (Figure 10A; ref 51), a species that when embedded in a membrane captures Cl− with high selectivity (77). Chains, Rings, Rods, and Boxes: Boron Clusters As Building Blocks It has long been recognized that carboranes and other boron cluster types are potentially excellent synthons for constructing covalently bonded, architecturally novel macromolecules and polymers whose electronic (and other) properties could be tailored to specific purposes (78); however, not until recent years have the synthetic tools been fully in place to build such systems in a rational, controlled manner (9c, 79). Carborane and metallacarborane clusters can be connected via their cage carbon atoms (Figure 15, top; refs 78c, 80). Alternatively, linkage through boron-attached substituents has been achieved by adapting organometallic techniques originally developed for CC binding, such as metal-catalyzed coupling, as illustrated for a p-carborane system in Figure 15, bottom (81). This approach has also been applied to the construction of polycluster compounds and extended arrays based on small metallacarboranes (Figure 16; refs 7, 82 ). A different, and powerful, method for achieving cluster linkage that is almost exclusive to boron compounds is metal– ligand stacking to create multidecker sandwich complexes (Figure 16, bottom). Although a few “all-hydrocarbon” tripledecker complexes such as the classic Cp3Ni2+ ion are known (83), stable examples of these are rare (84) and higher-decker

complexes are virtually unknown. Not so, in boron chemistry: following our original triple-decker syntheses 30 years ago (85), large families of isolable, electrically neutral multidecker sandwiches having 3, 4, 5, or 6 decks and incorporating C2B3 or C3B2 planar rings have been prepared (9c, 78b, 79b, 79c, 86), and metal stacking reactions are now a standard tool for assembling extended systems from small metallacarboranes. Multicluster systems that are directly connected or linked by unsaturated entities such as alkenes, alkynes, and arenes might function as conducting or semiconducting polymers. In fact, three characteristics of polyhedral boron clusters suggest that there is real potential for these compounds as building blocks in 21st century nanoscale electronics: their electron-delocalized “super-aromatic” skeletal bonding, their thermal and oxidative stability, and their tailorability via introduction of organic functional groups. Since these are not inexpensive compounds, one is talking about small-scale, highly specialized polymeric materials. Nevertheless, it is clear that boranes, carboranes, and their metallo derivatives can take us in directions that are simply not otherwise accessible. Studies of the electronic properties of C2B3- and C3B2-based sandwich complexes using electrochemistry, ESR, UV-visible spectroscopy, and multinuclear NMR show that the extent and type of metal–metal interaction (electron delocalization) depend upon molecular architecture, metal oxidation state and electron configuration, and the presence of attached substituents (62, 78b, 79b, 86–88). Upon electrochemical reduction, fulvalene-linked tetradecker sandwich oligomers such as that in Figure 17A (89) are extensively delocalized

linkage via cage carbon atoms

C

BH

1) n –BuLi

H

H

1) n –BuLi

H

H

2) CuCl 2 3) H 3O+

1) n –BuLi 2) CuCl 2

Figure 15. Examples of metal-promoted CC and CB coupling of p-carborane cages.

2) R3SiCl R = alkyl

R3Si

•

linkage via cage boron atoms

I

I

+

CH

HC

SiR 3

BH, B

C

C

CH

O2, CuI pyridine, toluene reflux

I

C

C

C

C

•

• •

•

•

•

•

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(90), whereas others having intervening phenyl groups linking the sandwich units are insulators (91); the C3B2-based polydecker in Figure 1D is a semiconductor (4). Conducting polymers based on nonmetal-containing boron clusters are also under investigation. Kaszynski and coworkers investigated the electronic interactions between closo-boron clusters of 6, 10, and 12 vertices (see Figure 2) linked by alkynyl and other triple-bonded groups, using UV, IR, and NMR spectroscopic data and theoretical calculations, and concluded that the electronic conjugation between the clusters and the linkers is greatest for the 6-vertex cages (92). Similar studies demonstrated that the 1,12-C2B10H12 ( pcarborane) polyhedron transmits electronic effects, although the mechanism was not established (93). The macrocycles in Figures 17C and 17D illustrate the current state of the art in controlled synthesis of large systems from monomeric boron clusters. The “big wheel” in Figure 17C consists of alternating m-carboranyl (1,7-C2B10H10) cages and 1,3-phenylene rings (94) while the tetramer in Fig-

CR

BH

ure 17D incorporates four (C5Me5)Co(Et2C2B4H2) clusters linked by diacetylene chains and features a 24-atom planar octagonal C16B8 ring (82b). Electrochemical reduction of this tetracobalt complex generates a monoanion that exhibits electronic communication between the metal centers (82b), which is of interest inasmuch as there is much current attention directed to alkyne linkers as “carbon wires” in organic and organometallic systems (95). Other Applications A number of actual or potential areas of utilization of boron clusters exist beyond those discussed in this article (38), some of which may be surprising to some readers. For example, a widely used air bag propellant system for automobiles employs the dicesium salt of the B12H122− ion (see Figure 2) as a burning accelerant to ensure rapid but controlled inflation of the bag (96). o-Carborane (1,2-C2B10H12) and other boron cluster compounds are vaporized and fired at high temperature to create boron films and wall coatings in tokamak

transition metal ions

,

arene or Cp ring

alkyne linkage via cage boron atoms Figure 16. Methods for construction of multidecker sandwiches and B-alkyne-linked small metalla-carboranes.

H

I2, Et3N PdII, CuI

H

SiMe 3

H

H

1) t–BuLi O

2)

O

n–Bu4NF

ClCH2C(O)Me

THF

Et3 N

B–C–C–SiMe3

decapping and metal stacking H decap

2ⴚ

H deprotonate

Lewis base

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BuLi

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molecular structures, and to a lesser extent the reactivity, of these once inscrutable molecules, while newer experimental techniques and theoretical tools allow the controlled synthesis of boron-containing compounds and materials that are tailored for specific purposes. The objectives of this research are both practical (e.g., development of new materials and biomedical applications) and basic, as in the designed synthesis of new metallaboranes and metallacarboranes for studies of electronic structure and other properties. Particularly noteworthy is the increasing interest in boron clusters by researchers in areas once far removed from this field—for example, nuclear medicine, pharmacology, radioactive waste treatment, polymer science, catalysis, fibers, and films—as they discover how the unique attributes of these species can be exploited. In essence, polyhedral boranes offer a kind of alternative organic chemistry whose structural principles and reaction modes are very different from con-

reactors for nuclear fusion (97). A boron-carbon alloy has been fabricated from o-carborane enriched in the 10B isotope and employed as a solid-state neutron detector (98). Carborane-based stationary phases have been used in gas chromatography for years, primarily because they allow operating temperatures (400 C or higher) that are not possible with conventional GC materials (99). Future Directions Polyhedral boranes and their many families of derivatives have been around for years, yet research in this area continues to attract interest and funding. In parallel with the developing uses for these compounds in modern technology as outlined in this article, fundamental investigations in this field continue with a pace and energy that one might not expect in a field many decades old. General paradigms are now available that enable chemists to understand the bonding and

A

B

C

•

• •

• •

•

•

•

•

• • •

•

•

•

•

•

•

• • •

• C C

•

• •

•

n

•

• • •

•

•

•

• • • • • • • •

•

• •

•

•

•

• • • •

•

•

•

• C C

• •

•

n

D

•

•

• • • •

•

• C

C C

• •

•

C

C

C

n

•

•

• • • •

•

•

C

C

C

C

C

C

C

C

n C

BH, B-alkyl, B-halo Co

C

C

C

C –alkyl Co, Ni

Figure 17. (A) Fulvalene-bridged poly(tetradecker) chain (89); (B) alkynyl-linked poly(p-carboranyl) chain (81b); (C) tris(m-carboranylphenylene) macrocycle (94); (D) tetra(cobaltacarboranyl-B-diethynyl) macrocycle (82b).

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ventional organics and hence open the way to new possibilities. Perhaps the most interesting emerging applications are those that go beyond the exploitation of basic physical properties such as thermal stability, and make use of the special architectures and electronics of electron-delocalized “superaromatic” polyhedral frameworks. Chemistry, like other branches of science, is most exciting when it is expanding into uncharted territory. Acknowledgments I am deeply grateful to my student, postdoctoral, and faculty colleagues and other collaborators over many years for their dedicated efforts and imaginative insights. Many thanks also to the four reviewers of this manuscript, who provided some excellent suggestions for improvements and pointed out a number of needed corrections. Notes 1. The term “carborane”, a condensation of the IUPAC “carbaborane”, designates a borane cluster containing one or more carbon atoms in the polyhedral framework. Similarly, “metallaboranes” and “heteroboranes” refer to clusters containing metal or main-group heteroatoms. 2. The breakthrough concept, originally elucidated by H. C. Longuet-Higgins (11), W. N. Lipscomb (12), and others, was that of the two-electron, three-center bond, wherein a single pair of electrons firmly binds three atoms, a situation that deviates from the standard Lewis model and is labeled “nonclassical” by inorganic and organic chemists.

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36.

37. 38. 39.

40. 41.

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43.

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88. (a) Stephan, M.; Müller, P.; Zenneck, U.; Pritzkow, H.; Siebert, W.; Grimes, R. N. Inorg. Chem. 1995, 34, 2058. (b) Stephan, M.; Hauss, J.; Zenneck, U.; Siebert, W., Grimes, R. N. Inorg. Chem. 1994, 33, 4211. 89. Meng, X.; Sabat, M.; Grimes, R. N. J. Am. Chem. Soc. 1993, 115, 6143. 90. Geiger, W. E., unpublished data. 91. (a) Pipal, J. R.; Grimes, R. N. Organometallics 1993, 12, 4459. (b) Pipal, J. R.; Grimes, R. N. In Current Topics in the Chemistry of Boron; Kabalka, G. W., Ed., Royal Society of Chemistry: Cambridge, United Kingdom, 1994; p 318. 92. Kaszynski, P.; Pakhomov, S.; Young, V. G. Collect. Czech. Chem. Commun. 2002, 67, 1061. 93. Fox, M. A.; MacBride, J. A. H.; Peace, R. J.; Wade, K. J. Chem. Soc., Dalton Trans. 1998, 401. 94. Clegg, W.; Gill, W. R.; MacBride, J. A. H.; Wade, K. Angew. Chem., Int. Ed. 1993, 32, 1328. 95. Selected recent references: (a) Stang, S. L.; Paul, F.; Lapinte, C. Organometallics 2000, 19, 1035. (b) Schimanke, H.; Gleiter, R. Organometallics 1998, 17, 275. (c) Mayr, A.; Yu, M. P. Y.; Yam, V. W.-W. J. Am. Chem. Soc. 1999, 121, 1760. (d) Meyer, W. E.; Amoroso, A. J.; Horn, C. R.; Jaeger, M.; Gladysz, J. A. Organometallics, 2001, 20, 1115. 96. (a) Thiokol Corp. U.S. Pat. 4,358,998, Nov. 16, 1982. (b) Thiokol Corp. U.S. Pat. 5,401,340, March 28, 1995. 97. (a) Li, J. G.; Zhao, Y. P.; Gu, X. M.; Li, C. F.; Wan, B. N.; Zhang, X. D.; Luo, J. R.; Gong, X. Z.; Xie, J. E.; Wan, Y. X.; Qin, P. J.; Wang, X. M.; Meng, Y. D.; Li, S. F.; Gao, X.; Yang, Y.; Xue, D. Y.; Mao, Y. Z.; Den, X.; Chen, L.; Fang, Y. C.; Yin, F. X.; Liu, S. X.; Yang, X. K.; Xu, D. Z.; Ding, J. Y.; Jie, Y. X.; Zhao, Q. C.; Mao, J. S.; Zhang, S. Y.; Zhao, J. Y.; Hu, J. S.; Fan, H. Y.; Wei, M. S.; Lin, B. L.; Wang, G. X.; Fang, Y. D.; Shen, W. C. Nuclear Fusion 1999, 39, 973. (b) Buzhinsky, O. I.; Ostroshchenko, V. G.; Whyte, D. G.; Baldwin, M.; Conn, R. W.; Doerner, R. P.; Seraydarian, R.; Luckhardt, S.; Kugel, H.; West, W. P. J. Nucl. Mater. 2003, 313, 214. (c) Tafalla, D.; Tabares, F. L. Vacuum 2002, 67, 393 98. Robertson, B. W.; Adenwalla, S.; Harken, A.; Welsch, P.; Brand, J. I.; Dowben, P. A.; Claassen, J. P. Appl. Phys. Lett. 2002, 80, 3644. 99. de Zeeuv, J.; Luong J. TRAC - Trends in Analytical Chemistry 2002, 21, 594, and references therein.

Viewpoints: Chemists on Chemistry Boron Clusters Come of Age Russell N. Grimes Department of Chemistry, University of Virginia, Charlottesville, VA B.S. 1957, Chemistry, Lafayette College Ph.D. Chemistry, 1962, University of Minnesota Postdoctoral Work, 1962–63, Harvard University and University of California, Riverside Russell N. Grimes is Professor Emeritus of Chemistry at the University of Virginia, Charlottesville. His research contributions with his coworkers over four decades include both serendipitous discoveries and designed syntheses of novel boranes, carboranes, and metallaboron clusters. These include the discovery of metal-promoted oxidative cluster fusion; air-stable transition-metal and main-group metallaboranes; air-stable triple-decker and multidecker sandwiches; tricarbon and large tetracarbon carboranes; four-carbon supra-icosahedral 13- and 14-vertex metallacarboranes; and, in recent years, the designed assembly of macromolecular oligomers and polymers from small monomeric clusters. His group has also pioneered the application of new NMR techniques to boron chemistry, including 11B–11B and 11B–1H two-dimensional COSY spectroscopy, first described in 1980 and 1981. He is currently working on a second edition of a monograph on carborane chemistry.

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