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									                   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                          658              or in combination with other elements, have
                                                                       been known for nearly a century, and
  Metal Ion Extraction                                662
                                                                       intensive studies of their structures, bonding,
  Anticrown Reagents:                                 662
  Host–Guest Recognition of Anions                                     and reactivity have been under way for more
                                                                       than half that period; yet interest and practical applications
  (Almost) Noncoordinating Anions                     664
                                                                       in this area continue to grow. Two main reasons can be
  Nonlinear Optics                                    664
                                                                       identified for this attention: the three-dimensional
  Homogeneous Catalysis                               665
                                                                       delocalized bonding that confers exceptional stability in
  Liquid Crystals                                     666              these clusters is now recognized to have broad significance
  Ion-Selective Electrodes                            666              for deepening our understanding of covalent bonding, with
  Chains, Rings, Rods, and Boxes:                     667              implications for both organic and inorganic chemistry;
  Boron Clusters As Building Blocks                                    secondly, many of the special properties of boron clusters
  Other Applications                                  668              are uniquely suited to specific applications. This article
  Future Directions                                   669              attempts to summarize the current state of the art, illustrated
                                                                       by examples selected to convey some of the excitement and
                                                                       possibilities for future exploitation of these remarkable

                                                                       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,                     673     Chemical Education Research: Major Sources                 725
Onions, and Tubes Andrew P. E. York                                      of Difficulty in Students’ Understanding
                                                                         of Basic Inorganic Qualitative Analysis
Products of Chemistry: Dentifrice Fluoride                       677
                                                                         Kim Chwee Daniel Tan, Ngoh Khang Goh,
Philip E. Rakita
                                                                         Lian Sai Chia, David F. Treagust
Syntheses and Characterization of Ruthenium(II)                  718
                                                                         A Unified Approach to Electron Counting                    733
Tetrakis(Pyridine) Complexes. An Advanced
                                                                         in Main-Group Clusters
Coordination Chemistry Experiment or Mini-Project
                                                                         John E. McGrady
Benjamin J. Coe
                                                                         The Trinity of Life: The Genome, the Proteome,             738
Lewis Acid–Base, Molecular Modeling,                             722
                                                                         and the Mineral Chemical Elements
and Isotopic Labeling in a Sophomore
                                                                         R. J. P. Williams and J. J. R. Fraústo da Silva
Inorganic Chemistry Laboratory
Chip Nataro, Michelle A. Ferguson, Katherine M. Bocage,                  Featured Molecules: Boron Clusters                         768
Brian J. Hess, Vincent J. Ross, Daniel T. Swarr                          William F. Coleman

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

                             •   Vol. 81 No. 5 May 2004      •   Journal of Chemical Education         657
   Viewpoints: Chemists on Chemistry

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

      Boron is the only element other than carbon that can          resistant: its dicesium salt survives temperatures above 810
build molecules of unlimited size by covalently bonding to           C without decomposition, making B12H122− arguably the
itself, a property known as catenation. In contrast to the          most stable molecule in all of chemistry (and most definitely
chains and rings favored by carbon, boron—owing to the              not “electron-deficient” in any rational sense). One aspect of
presence of only three electrons in its valence shell—adopts        its chemical inertness that relates to biomedical applications,
a cluster motif that is reflected in the various forms of the       discussed below, is its very low toxicity in humans. Theoreti-
pure element (all of which feature B12 icosahedra) and in the       cal analyses (13) show that the source of stability in B12H122−
huge area of polyhedral borane chemistry that has developed         is the highly delocalized bonding in the boron framework,
over the years. In combination with many other elements—            with 26 “skeletal electrons” occupying 13 bonding molecu-
indeed, most of the periodic table other than the noble gases       lar orbitals (MOs) on the polyhedral surface; such σ-aromatic
and actinides—boron clusters exhibit an astonishing variety         bonding may be regarded as a kind of 3-dimensional ana-
of stable architectures, as a few examples from the recent lit-     logue of benzene and has been labeled “superaromatic” (13,
erature (1–7) illustrate (Figure 1).                                14). As Figure 2 indicates, B12H122− is the largest member of
      To many students and teachers, such structures may ap-        a family of BnHn2− stable boron hydride anions where n = 6–
pear bizarre and disconnected from mainstream science; yet          12, whose molecular geometries are deltahedra (polyhedra
exotic molecules, especially those whose structures surprise        having only triangular faces, designated by the prefix closo);
and confound the theory of the day, play an important role          deltahedra with one missing vertex are labeled nido from the
in chemical education since they force us to rethink our no-        Greek word for nest.
tions of chemical bonding and reactivity. Students in par-                Analogous families of closo-carboranes exist in which one
ticular need to understand that chemistry is not a static           or more BH units are formally replaced by isoelectronic CH+
science with its basic ideas frozen in place; rather, our un-       units (Figure 3, top). For example, known carboranes corre-
derstanding of fundamental concepts of spectroscopy, bond-          sponding to B12H122− include the CB11H12− anion and neu-
ing, molecular structure, and chemical reactivity is continually    tral C2B10H12, the latter existing in the form of three isomers
evolving. In fact, entirely new fields of chemistry can come        having the carbon atoms in ortho, meta, or para locations in
into existence unpredicted by anyone or hinted at in any text-      the cage (see Figure 3). Neutral C2Bn-2Hn closo-carboranes cor-
book, and seemingly mined-out areas can be brought to life;         responding to all of the BnHn2− dianions are known, and in
sometimes a single discovery is all it takes.                       addition there are many non-closo-carboranes, that is, clusters
      For most novel compounds, it may require a long time,         having open-cage structures (Figure 3, bottom). Beyond this,
if ever, for useful applications to be found; thus benzene, to-     the ability of many other elements to occupy vertexes in stable,
day a leading industrial chemical, was little more than a labo-     isolable boron cluster frameworks, as illustrated in Figure 1, is
ratory curiosity for nearly three-quarters of a century following   a defining characteristic of this field and gives it a scope and
its discovery in 1825. Ferrocene caused a sensation fifty years     variety that is without parallel outside of organic chemistry.
ago when its unprecedented sandwich structure was revealed,               Reviews of many aspects of this field that provide de-
but direct commercial applications of metallocenes (metal–          tailed discussions of synthesis, structure, bonding, and chem-
hydrocarbon sandwich complexes) were slow to develop.               istry are available (9). The purpose of this article is to acquaint
      As molecular exotica go, the polyhedral boranes offer an      readers of this Journal with some of the contemporary uses
interesting, and in some ways unique, case history. Their dis-      that are emerging for boron clusters and to suggest a few pos-
covery in the early 1900s by Alfred Stock and coworkers (8)         sibilities for the future. This discussion is necessarily selec-
opened the door to an entirely new realm of structural chem-        tive and limited in scope.
istry—one that a century later is still unfolding and offering
new surprises. The stable existence of the neutral boranes,         Boron Clusters in Medicine
the polyhedral borane anions (Figure 2), and their various                In general, two characteristic properties of polyhedral
derivatives containing heteroatoms bound in the cage skel-          boranes make them attractive for medical and pharmacologi-
eton (9) such as carboranes,1 metallaboranes, and main-group        cal applications. Their low chemical reactivity and resistance
heteroboranes, has profoundly influenced other areas of in-         to breakdown in biological systems render them relatively
organic and even organic chemistry (consider the nonclassi-         nontoxic and, as will be shown, they can be tailored to spe-
cal hydrocarbons; ref 10), and helped to force a revolution         cific purposes. The second important attribute is the very
in the way chemists think2 about covalent bonding (11, 12).         large neutron cross-section (ability to absorb neutrons) of the
This alone, even if no practical uses for boron cluster com-           B isotope, which accounts for 20% of boron in nature, 11B
pounds existed, would justify continuing exploration in this        constituting the remainder. In 1935 (15) it was found that a
area. However, the fact is that there are diverse areas of ap-      collision between boron-10 nuclei and slow neutrons pro-
plication that exploit the remarkable properties of boron clus-     duces α particles (helium-4 nuclei) via the reaction:
ters. How remarkable? Consider the prototype borane, the
dodecahydrododecaborate (2 ) ion, B12H122− (Figure 2, lower          10
                                                                        B + 1n          4
                                                                                         He + 7Li + 2.31 MeV (94%)
right). This molecule, which possesses perfect icosahedral
symmetry, is both highly water soluble and amazingly heat-                                    + 2.79 MeV (6%) + γ 0.48 MeV

658      Journal of Chemical Education        •   Vol. 81 No. 5 May 2004        •
                                                                                                                     Boron Clusters Come of Age

                     CH, C-alkyl,
         B, BH                        N          P       Cl

            S        Mn         Fe    Ni         Zn

                                                                                      B 6H62              B 7H72                   B 8H82             B 9H92


                                                                                       B 10H102               B 11H112                  B 12H122
                                                                                  Figure 2. Structures of BnHn2− dianions.

     H                 H
 Me3N                  NMe3
            H                         B                                           representative closo -carboranes:                     BH           C, CH
     H                  H


                                                                                    1,5-C2B 3H5       2,4-C 2B 5H7        1,7-C2B 6H8            1-CB11H12

                                                                                     1,2-C 2B 10H12                1,7-C 2B 10H12                1,12-C2B 10H12

            E                                                                     representative open-cage carboranes:

                                                                                            H                              H
                                                                                                  H                  H         H                 H      H

                                                                                       1,2-C 2B 3H7                  2-CB5H9                 2,3-C2B 4H8

                                                                                                                                                 H           H
Figure 1. Molecular structures of:                                                                                                                           H

   (A) Mn3[(SiMe3)2C2B4H4]43− (1),             (B) P2B4Cl4 (2),
   (C) [(nido-C2B9H11)ZnNMe3]2 (3),
                                                                                      2,3,4-C3B 3H7                  2,3,4,5-C4B 2H6         4,5-C2B 7H13
   (D) [(Me2HC3B2Me2)Ni]n (4),
   (E) S2B16H16 (5),        (F) NB11H11 (6),
   (G) [(C6H6)Fe(Et2C2B4H3-7-C C]3C6H3 (7).                                      Figure 3. Structures of selected carboranes.

                                         •   Vol. 81 No. 5 May 2004          •    Journal of Chemical Education                          659
   Viewpoints: Chemists on Chemistry

The following year a physician, G. L. Locher, noted that if         a tetracarboranyl porphyrin (Figure 4) whose biological prop-
this reaction were conducted in tissue, the energy of the par-      erties show promise for BNCT application (22). Additional
ticles produced would destroy the immediate cell but not its        possibilities are created if the porphyrins are coordinated to
neighbors (16). Thus, if 10B could somehow be selectively in-       metal ions; for example, complexes incorporating radionu-
corporated into tumor cells in sufficient concentration, irra-      clides such as 67Cu can function as tracers to follow the dis-
diation of these cells with otherwise benign low-energy             tribution of boron in the system.
neutrons would constitute a novel noninvasive treatment of                A wide array of carborane- and borane-substituted BNCT
inoperable cancers. The problem with implementing boron             agents is under study, including polyamines, glucosides, car-
neutron capture therapy (BNCT) in Locher’s day was the lack         bohydrates, nucleosides, and immunoconjugates, as well as li-
of suitable boron compounds: most were toxic, low solubil-          posomes (20). Of particular interest are carriers containing
ity, monoboron species such as boric acid. Despite a few early      multiple C2B10 clusters that can deliver a hundred or more
clinical studies (17), not until the discovery of the BnHn2−        boron atoms per molecule into cancer cells; Hawthorne and
polyhedral borane anions and their carborane analogues (18)         coworkers have prepared liposomes containing large borane
did BNCT attract serious attention. The stability, solubility       or carborane cage species that are taken up very selectively by
in aqueous media, and high boron content of polyboron clus-         tumor tissue (23). In addition, small-molecule approaches to
ters such as B12H122− reignited interest in this approach, and      BNCT are being pursued: for example, electrically neutral
a Japanese physician, H. D. Hatanaka, who had access to a           water soluble azanonaboranes of the type nido-RNB8H11NHR′
neutron source, treated human brain tumor patients with             [R = HO(CH2)3, Me, MeO(CH2)3; R′ = H, (CH2)3OH] show
BNCT with some success over several years using the                 very low toxicity, although they are not selectively incorpo-
mercapto-substituted derivative B12H11SH2− (19) .                   rated into tumor cells (24). To date, clinical BNCT trials ap-
      In recent years BNCT has been undergoing clinical tri-        proved by the U.S. Food and Drug Administration have been
als in the United States, the Netherlands, and other coun-          limited almost exclusively to patients afflicted with the deadly
tries (20). In order to be effective, this therapy requires boron   brain tumor glioblastoma multiforme (GBM) and utilize just
concentrations of 20–35 µg, or about a billion (109) 10B at-        three boron compounds, all known for decades (20c): an
oms per cell (somewhat less if they are localized in the cell       arylboronic acid, 4-dihydroxyborylphenylalanine; a mercapto-
nucleus), with high selectivity in the tumor versus healthy         substituted derivative of B12H122−, Na2(B12H11SH); and the
tissue. One currently employed strategy is to attach boron          salt Na2(B10H10). More recent developments in boron cluster
clusters to tumor antibodies targeted to specific cell types.       synthesis have yet to be explored in BNCT studies.
Among those under study are glycosides (21) and porphy-                   Other therapeutic uses of the 10B-neutron interaction are
rins attached to several carborane cages, as in the example of      also under investigation; for example, in the treatment of rheu-

                                                      BH       C

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

                                                     R                          NH          N

                                                                                N       HN                        R
                                                                                                        H2 C




660      Journal of Chemical Education        •   Vol. 81 No. 5 May 2004        •
                                                                                                        Boron Clusters Come of Age

matoid arthritis, a procedure known as boron neutron cap-                    cluster geometry: the volume displaced by even the bulky 12-
ture synvectomy (BNCS) is being tested (25). In this proce-                  vertex C2B10H12 icosahedron is only moderately larger than
dure, arthritic tissue is ablated by introducing 10B via suitable            that of a phenyl group spinning on one of its 2-fold axes (Fig-
carrier compounds and then subjecting it to neutron bom-                     ure 6), and of course the steric requirements of the lower
bardment. In contrast to conventional radiation treatment for                carboranes are smaller yet. Consequently, up to a point it is
arthritis, BNCS is more localized in the body and hence mini-                feasible to replace C6H5 groups in organic molecules with
mizes damage to healthy tissue. In one approach, small uni-                  polyhedral borane cages without undue crowding, as in the
cellular liposomes were employed to incorporate boron into                   porphyrin derivative in Figure 4.
the tissues of arthritic laboratory rats, with encouraging re-                    Imaging techniques using carboranes labeled with radio-
sults (26); another method utilizes carborane derivatives hav-               active isotopes such as 99Tc, including positron emission to-
ing cortisone or related substituent groups (Figure 5A) (27).                mography (PET) and magnetic resonance imaging (MRI),
     Boron cluster compounds are also prime candidates for                   are currently under investigation (29). Radiolabeled “Venus
medical applications not involving neutron capture. Because                  flytrap” metallacarboranes (Figure 7) containing 57Co have
of the exceptional stability of hydrophobic polyhedral borane                been attached to a monoclonal antibody T84.66 that was used
cages under typical in vivo conditions, carborane- and                       to demonstrate localization in mouse tumors (30). BNCT-
metallacarborane-based carriers for pharmaceuticals and ra-                  candidate carborane derivatives designed to bind to the mi-
diotracers (20b, 28) tend to resist degradation better than con-             nor grooves of DNA (Figure 5B) have been prepared (31);
ventional organic reagents. Such applications effectively                    the 73Se-containing species allows the compound distribution
exploit a basic (if somewhat counterintuitive) fact of boron                 to be determined using PET. These approaches are especially

                                                    H               •

                                                                                                           C           BH
              Me       H                     O

                   H        H
                                                                         N                N
                                                                                          H                                       H
                                                            X = NMe3 , Se

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

                                                                                     CH       BH, B       trapped M3 ion X = O, S, SO2


                                                                                    CH2                                     CH2
                                                                                   X                                    X
                                                                                    CH2                                     CH2

                                                                                               A                                  B
  Figure 6. Volumes occupied by an icosahedral carborane (left)
  and a benzene ring spinning on a 2-fold axis (right), ignor-                 Figure 7. Venus flytrap biscarboranes, uncomplexed (A) and
  ing H atoms in both molecules.                                               with trapped metal ion (B) (30).

                       •   Vol. 81 No. 5 May 2004                 •      Journal of Chemical Education      661
   Viewpoints: Chemists on Chemistry

useful in determining where boron accumulates in the body,          dionuclides from nuclear waste (39) and are currently em-
which in turn helps in the design of better carriers for BNCT       ployed on a substantial scale for this purpose and as sensors
and BNCS treatments.                                                for Cs+ and Sr2+ in milk and body fluids.
      Boron cluster-containing pharmaceuticals are a focus of             A problem with the parent (unsubstituted) species (Fig-
intensive study in many laboratories, most (but not all) of         ure 9A) is its slow decomposition in concentrated nitric acid,
this work centering on icosahedral C2B10H12 carborane de-           which presents a serious disadvantage in metal ion extrac-
rivatives. These efforts, which have been reviewed recently         tion. However, this difficulty is overcome by replacement of
(20b), include carborane-based insect neuropeptides, antine-        some of the boron hydrogen atoms by methyl groups or halo-
oplastic and cytotoxic agents, estrogen agonists and antago-        gens; the hexachloro derivative shown in Figure 9B resists
nists, retinoids, protein kinase C modulators, and others. This     3 M nitric acid (38, 39b, 40), thereby meeting a critical re-
research area is a prime example of interdisciplinary effort,       quirement for use in the recovery of 137Cs and 90Sr from ra-
drawing on the expertise of workers in such disparate fields        dioactive wastes. The consequences of employing other metals
as biochemistry, radiology, organic synthesis, and boron clus-      (e.g., iron or nickel) in place of cobalt, using carborane ligands
ter chemistry, and has led to intriguing findings. For example,     with separated cage carbons, and introducing aromatic or oxy-
carbon-substituted phenolic derivatives of 1,12-C2B10H12            gen-donor substituents, have been explored by Teixidor et
(Figure 3, second row) have been shown to be potent estro-          al. (41). Derivatives containing ether groups on the cage car-
gen receptor agonists (32a) and a bis(carboranyl) organotin         bons show improved selectivity over the parent complex, and
compound is more active against seven human tumor cell              the tetra-C–phenyl species in Figure 9C is reported to ex-
lines than are 5-fluoroacil, cis-platin, and carboplatin (32b).     tract Cs+ from acid solution better than any other available
It is noteworthy that when carboranyl cages are introduced          reagent (41a).
to pharmacologically active organics, the receptor binding af-            Several groups have examined the extraction of alkali,
finity in many cases is improved, probably because of the hy-       alkaline-earth, and lanthanide metal cations with nitroben-
drophobic character of the carborane (20b, 33).                     zene solutions of the cobaltate anions together with crown
      Metallacarboranes (carboranes having one or more metal        ethers, polyethylene glycols, polyethers, and calix[n]arenes
atoms in the cage framework) and exo-polyhedral metallated          (42). Nitrobenzene promotes selectivity in metal extraction
carborane derivatives have shown significant anticancer ac-         (41) and is widely used for this reason; however, it is eco-
tivity; in some cases they are more effective against certain       logically unfriendly and eliminating it from the process is a
glioma and breast cancer cell lines than are standard drugs         long-range goal. In a different approach discussed later in this
used in screening. Figure 8 contains a sampling of MC3B7            article, Co(1,2-C2B9H11)2− ions and various derivatives are
(34) and MC2B4 (35) metallacarborane clusters that exhibit          incorporated as doping agents in conducting organic poly-
cytotoxic behavior against common tumor cell types, includ-         mers such as polypyrroles, creating “intelligent membranes”
ing L-1210 lymphoid leukemia, Tmolt3 leukemia, murine               that can capture cations with selectivity controlled by an ap-
P388 lymphocytic growth, Sk-2 melanoma, HeLa-S3 human               plied potential (41a, 43) (another example utilizes
uterine carcinoma, and lung broncogenic MB-9812. Studies            mercurocarborand complexes, discussed below). A hydrogen-
on selected compounds in this group have shown that they            sensitive microelectrode based on this principle has been de-
interfere with the synthesis of RNA and DNA in the tumor            veloped (43b).
cells (34a, 35a), suggesting some similarity with the mode                Extraction systems based on boron clusters other than
of action of ferrocenium salts and other metallocenes (metal-       metallacarboranes are also known. For example, derivatives
cyclopentadienyl sandwich compounds) that show antican-             of B12H122− having phosphorus oxide substituents selectively
cer activity (36).                                                  remove 241Am and 152Eu from nuclear waste (44). The met-
                                                                    als recovered from radioactive effluent are often valuable and
Metal Ion Extraction                                                can be reused, for example; 137Cs and 90Sr are employed as
                                                                    thermoelectric generators and in medical equipment steril-
     Electrically neutral boranes and carboranes typically dis-     ization (40). Applications of boron cluster chemistry to metal
solve in organic media but not in water. In contrast, polyhe-       extraction technology address important issues, and seem des-
dral borane anions (Figure 2) are very water soluble, as are        tined to grow in importance and scope.
many anionic metallacarboranes. Perhaps surprising is the fact
that many metallacarborane anions are also soluble in organic       Anticrown Reagents: Host–Guest Recognition
solvents, a property that is traceable to the negative (hydridic)   of Anions
character on the BH hydrogens resulting from charge delo-
calization over the cage framework. One metallacarborane                 Crown ethers and their ability to trap alkail metal cat-
type that has attracted considerable interest is the family of      ions have been known for some time (45), and the field of
bis(dicarbollyl)metallate(III) ions 3-MIII(1,2-C2B9H11)2−, first    host–guest chemistry is well established (46), centering on
prepared in 1965 (37), and their substituted derivatives (Fig-      the ability of Lewis base functionalities such as the oxygen
ure 9). Remarkably, although Na+ Co(C2B9H11)2− has elec-            atoms in 12-crown-4 (Figure 10, top left) to coordinate posi-
trolytic properties similar to NaCl, by shaking an 0.5 M            tively charged ions. Anticrowns—charge-reversed analogues
aqueous solution of this salt with an equal volume of diethyl       of crown ethers having acidic groups that can trap anions
ether one can effect a quantitative transfer of the                 (47)—were rare until Hawthorne’s group synthesized and
metallacarborane to the ether layer (38)! Compounds of this         investigated several families of Hgn(C2B10H10)n macrocycles
class, which are resistant to thermal degradation and to at-        or “mercurocarborands” whose electrophilic mercury atoms
tack by acids, bases, and radioactive substances, were first        cause them to function as efficient anticrowns, binding tightly
employed by Czech workers three decades ago to extract ra-          to halide ions (48). Two representative examples, a trimer (A)

662      Journal of Chemical Education        •   Vol. 81 No. 5 May 2004        •
                                                                                                                      Boron Clusters Come of Age

       C, CH                   BH, B               R = Me, Et, SiMe 3
                                                                                                     CH       BH, B    Co

                 Fe                  AsF6                       Fe                                                                          Cl


        R                                          R
                                Cl                                                                        A                             B
            R             M
                          Fe                               M

             M = Ta, Nb                                M = Fe, Co

   R                            R                           R
   R                             R                              R
            Fe                                Fe
                                              Fe                     Co

                                                           Me3P           PMe3

Figure 8. Metallacarboranes shown to be cytotoxic toward                                    Figure 9. CoIII(1,2-C2B9H11)2 ions and derivatives for extrac-
tumor cell strains and solid tumors (34, 35).                                               tion of metal ions.

                      O                  O

                      O                   O

                                                                                  BH   CH      Hg


                                A                                                      B                                            C

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).

                                                •   Vol. 81 No. 5 May 2004        •   Journal of Chemical Education         663
   Viewpoints: Chemists on Chemistry

and a tetramer (B), together with a chloride complex of the        Mes is 2,4,6-trimethylphenyl) and demonstrated that the
tetramer (C) in which the Cl− is trapped at the center of a        silylium ion is well-separated from the anions and solvent
square array of Hg atoms held in place by four carborane           molecules (55a). Arenium salts have been similarly charac-
clusters are depicted in Figure 10. This chemistry is rich and     terized, and C6H7+ CB11Br6Me5− was found to survive at
diverse (48, 49); for example, the tetrameric                      150 C (55b)! A very recent demonstration of the extraordi-
mercuracarborand (Figure 10B) binds not one but two io-            nary stabilizing power of the carborane monoanions is the
dide ions, while the trimer (Figure 10A) combines with wa-         synthesis and structural proof of the first known azafullerene
ter and benzene to form a novel π-sandwich complex,                salt, C59N+ Ag(CB11H6Cl6)2− (56), shown in Figure 11. This
[Hg3(C2B10H8Me2)3 H2O]2 C6H6 in which the benzene is               particular application, like others described in this article, il-
locked between two parallel Hg3C6 planes (50).                     lustrates how the unique capabilities of boron clusters can
     The complexation and stereochemical properties of these       be optimized for specific purposes by appropriate modifica-
reagents can be modified by replacing one or more of the           tion; in this case, the introduction of electron-withdrawing
boron hydrogens with organic or inorganic substituents; thus,      halogens as substituents on the cage generates the strongest
the trimeric mercurocarborand Hg3(C2B10H8Me2)3 has been            superacids yet isolated.
incorporated into a membrane that selectively traps iodide
ions at nanomolar concentrations (51). The development of          Nonlinear Optics
mercurocarborands is a noteworthy example of “designer
chemistry” that takes advantage of both the steric and the              Another area in which boron clusters are attracting at-
electronic properties of carboranes.                               tention is the study of nonlinear optical (NLO) materials,
                                                                   which have the ability to convert an applied electromagnetic
(Almost) Noncoordinating Anions                                    field to a new field with altered properties such as frequency
                                                                   and pulse. NLO materials are increasingly important in sev-
      For many years chemists have searched for anionic spe-       eral developing technologies including data storage and re-
cies that have the least possible affinity in solution for resi-   trieval, communications, and optical switching (57). While
dent cations, attempting to stabilize highly reactive cationic     the emphasis has been primarily on solid-state systems, mo-
species such as R3Si+, arenium ions, and organotransition          lecular NLO materials offer a number of advantages includ-
metal ions that are important in olefin polymerization ca-         ing very fast response times, lower dielectric constants, and
talysis. In recent years this quest has seen major advances        improved processabilty, among others (58). In the rational
based on the finding that the icosahedral CB11H12− ion, and        search for new and improved second-order NLO systems, a
especially its boron-halogenated derivatives, show extremely       basic criterion is the value of the first hyperpolarizability β
weak coordination to cations (52). The parent ion, shown in        as a measure of NLO response, and attention has centered
Figure 3, is isoelectronic with C2B10H12 and B12H122− and          on organic molecules that combine electron-donating and
like them is very resistant to heat and to cage degradation by     electron-withdrawing groups—so-called “push-pull” sys-
strong acids. The stability of CB11H12− is reflected in a huge     tems—that result in extremely high molecular polarities. Syn-
HOMO–LUMO energy gap that is far larger than that of               thetic and theoretical studies have shown that the
benzene and other arenes (52a), and is further enhanced by         incorporation of electron-withdrawing polyhedral borane
replacing several BH hydrogens with halogen or methyl              cages can result in dramatically increased hyperpolarizabilities.
groups to give species such as CB11X6R6−, HCB11X6R5−, and          Several different boron cluster systems have been investigated,
CB11Me12− where X is Cl or Br and R is H or Me.                    including derivatives of B10H102−, B12H122−, CB11H12−, and
      Tetra- and pentafluorinated derivatives of the 10-vertex     C2B10H12 isomers (58, 59); of these, 1,12-C2B10H10 (p-
CB9H10− anion as well as the perfluoro species B12F122− (53)       carborane) has proved particularly interesting, as in the ex-
have been explored by Strauss et al. (54). The latter ion is       amples depicted in Figure 12. The fullerene (C60)-carborane
remarkably inert, showing no reaction toward 98% sulfuric          system (A) is an excellent “push-pull” NLO molecule exhib-
acid, 70% nitric acid, Ce4+, or metallic sodium, and the           iting a very high β value, a perhaps surprising result if one
dilithium salt is unchanged on heating to 450 C (54b). De-         expects both the fullerene and carborane cages to be elec-
spite its dinegative charge, B12F122− is weakly coordinating       tron-withdrawing. However, the studies of Yamamoto et al.
and is far less basic (in a Lewis sense) than BF4−, as reflected   (60) indicate that the C60 unit in this case acts as an electron
in the BF C distances in crystal structures of salts formed        donor relative to the carborane, in contrast to other situa-
with CPh3+ and related cations.                                    tions where the boron cluster is the donor (61, 62).
      The CB11 clusters have so little affinity for protons or          Figure 12B shows a proposed tropylium-p-carborane-
other electrophiles that their conjugate acids (H+CB11X6R5 )       cyclopentadienyl system whose calculated β values (which
function as exceedingly powerful proton donors, far stron-         vary with R) are much higher than those of their fully or-
ger than most well-known superacids such as H2SO4 or               ganic analogues in which the central ring is benzene rather
HClO4, but without their oxidizing capacity (52). This com-        than carborane (58a). Interestingly, the effect of the carborane
bination of properties has allowed the stabilization and iso-      cage is to reduce the electronic communication between the
lation of salts of some heretofore very elusive species, among     end rings and consequently increase the charge polarization,
which are silylium ions (R 3 Si + ), hydrated protons              in comparison to the benzene-centered molecules. Ferrocenyl-
[(H2O)4H+], C6H7+ (benzenium), and other protonated                carborane derivatives with an imine spacer such as that in
arenes including C 6 H 6 + , C 6 MeH 6 + , C 6 Me 6 H + , and      Figure 12C with high hyperpolarizabilities have been pre-
C6Me3H4+ (55). Reed and coworkers have isolated and crys-          pared, and UV and other data indicate that the carborane
tallographically characterized (Mes)3Si+ CB11Br6Me5− (where        plays the role of electron acceptor (63).

664      Journal of Chemical Education       •   Vol. 81 No. 5 May 2004        •
                                                                                                         Boron Clusters Come of Age

       Homogeneous Catalysis                                                 = 1–3), are also active in catalyzing the hydrosilylation of sty-
                                                                             rene and phenylacetylene (65), with some differences in se-
             One of the earliest efforts to harness the special proper-      lectivity compared to the parent rhodacarborane. As this
       ties of polyhedral boranes was the exploration by Hawthorne           illustrates, there is considerable potential for electronic tai-
       and coworkers of hydridorhodium complexes of the formula              loring of such systems via the introduction of appropriate
       3,1,2-H(Ph3P)2RhIII(C2B9H11) as precatalysts for the hydro-           electron-attracting or electron-releasing substituents at bo-
       genation, hydroformylation, and hydrosilylation of alkenes            ron or carbon cage vertexes. This versatility underlines a sig-
       and alkynes (64). The catalytically active species is an exo-         nificant advantage of metal–boron cluster chemistry in
       nido-(Ph 3P) 2 Rh I-C 2B 9 H 12 tautomer whose 16-electron            catalyst design, since in most organometallic systems the pos-
       rhodium center is outside the carborane cage (Figure 13A),            sibilities for tailoring are considerably more limited in scope.
       which in solution exists in equilibrium with the 18-electron                Prompted by the industrial interest in metallocene-based
       closo-RhIIIC2B9 cluster, as shown. Boron-fluorinated deriva-          olefin polymerization catalysts of the type Cp2MIVR+ (where
       tives of this complex, as in H(Ph3P)2RhIII(C2B9H11-nFn) (n            Cp is η5-C5H5 and M is an early transition metal), isoelec-

                                     CH               BH, B   Cl                  Ph 3P PPh 3
                                                                                   H Rh
                                                                                                                         Ph 3P        H
                                                                                                                         Ph 3P        H




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



                                                                                                                                            B, BH
                                   BH         C                                                                                             C, CH


                                                                                           2                        Hf     H


                                                                              C                                             D


    H                                     N                                                R                                                     Me2
                                                       Fe                                        CH2                                  Ti         P
                                                                                    Cl                                     Me
                                                                                           Ti                                               P
                                                                                    Cl          NMe2

Figure 12. Selected NLO-active p-carborane derivatives (58–60).             Figure 13. Metallacarborane catalyst precursors for the hydroge-
                                                                            nation or polymerization of alkenes and alkynes.

                                  •   Vol. 81 No. 5 May 2004       •    Journal of Chemical Education                     665
   Viewpoints: Chemists on Chemistry

tronic, electrically neutral analogues having the general for-      or 1,12-C2B10H10 (p-carborane), shown in Figures 14C and
mula Cp(R)MIV(R′2C2B9H11) have been investigated by Jor-            14D. The combination of boron clusters with organic rings
dan et al. and found to be catalytically active in the              can be engineered to produce liquid crystalline materials hav-
hydrogenation of internal alkynes to cis-alkenes (66) (Figure       ing properties not otherwise attainable, such as UV-transpar-
13B); the active species is proposed to be the monomeric            ency combined with high polarity, and to favor, for example,
hydridohafnium complex. Zirconium metallacarboranes of              the formation of nematic versus smectic phases (73a).
the type Cp′2Zr(Me)(C2B9H12) (Cp′ is C5Me5 or C5Me4Et)
polymerize ethylene at 60 C and 300 psi (67), and con-              Ion-Selective Electrodes
strained-geometry Ti(IV) complexes of the type shown in Fig-               Chemical sensors based on ion-sensitive electrodes (ISEs)
ure 13C in the presence of methyl aluminum oxane (MAO)              are important analytical tools in clinical diagnostic applications
catalyze the formation of high molecular weight polymers            and environmental monitoring. Particularly important are those
from ethylene (68).                                                 employing liquid polymer membranes (usually polyvinyl chlo-
      Other applications of large-cage metallacarboranes in         rides or PVCs) containing ion-complexing agents (ionophores)
catalysis include the controlled radical polymerization of sty-     that selectively complex the ion of interest, together with lipo-
rene and n-butyl acrylate (69) and the formation of ethyl-          philic ionic additives for sensing anions or cations (74). For
ene-norbornene copolymers (70). In the small                        cation detection, tetraphenylborates have been employed for
metallacarborane area, our group working with M. G. Finn            decades (75) but are sensitive to hydrolysis induced by acids,
has shown that titanium complexes of the type                       oxidizing agents, and light (76a). Now, there is increasing in-
L 2 X 2 Ti(Et 2 C 2 B 4 H 4 ) (X is Cl or alkyl; L 2 is             terest in drawing upon the properties of boron clusters in this
2PR3,R2P(CH2)nPR2, et cetera.; R is Ph or alkyl) in combi-          area (41, 43, 51, 76). So-called “least-coordinating” halogenated
nation with MAO catalyze the polymerization of ethylene at          monocarbon carborane anions, CB11X11− where X is Cl, Br, or
room temperature and 1 atm pressure (71); the most effec-           I (see the discussion above), have found application as alterna-
tive of these found to date is the bis(dimethyl-                    tives to tetraphenylborates in ISEs, combining strong
phosphinopropane) complex depicted in Figure 13D, which             lipophilicity with chemical inertness; the CB11I11− ion in par-
requires only minimal concentrations of MAO and gener-              ticular shows significantly improved selectivity and lower de-
ates high molecular weight polyethylene. These small                tection limits compared to tetraphenylborates (76a). Similar
titanacarborane clusters exhibit unusually high thermal and         results have been found with the cobalt-dicarbollide complex
oxidative stability (for catalyst precursors) and are readily de-   3,1,2-Co(C2B9H11)2− (Figure 9A) (76b), and as mentioned ear-
rivatized by the introduction of organic or inorganic func-         lier, this complex is also employed as a dopant in cation-sensi-
tional groups to the carborane framework (71). Applications         tive microelectrodes based on polypyrrole conducting polymers
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             A                              B
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 or-
dered structure and exhibit birefringence, known as liquid
crystals, are of considerable importance in modern technol-
ogy, notably in devices for electro-optical display (72). In or-
der for molecules to adopt a nematic or smectic liquid
crystalline phase (Figure 14) they must be nonspherical (com-
monly disc- or rodlike, although other shapes are known).
Typical molecular rods contain rigid cores—for example, hy-              C
drocarbon rings such as phenyl, cyclohexyl, or
bicyclo[2.2.2]octyl—either directly linked or connected via             C7H13O             N                      N              OC7H13
organic groups such as alkyl, alkenyl, or azo. The actual physi-
cal 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 mo-
 B, BH
lecular core units. Boron clusters are especially attractive be-
cause of their thermal stability, delocalized bonding, and ease          C5H11                                        C5H11
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         Figure 14. Nematic (A) and smectic (B) liquid crystal phases; boron
bulk material to applied electric fields. Typical of the materi-       cluster-based (C and D) liquid crystalline molecules (73).
als under investigation are liquid crystals containing B10H102−

666      Journal of Chemical Education        •   Vol. 81 No. 5 May 2004         •
                                                                                                     Boron Clusters Come of Age

(43) or in PVC membrane electrodes (41a, 76b). Nanomolar                 complexes are virtually unknown. Not so, in boron chemis-
detection limits for iodide ion have been achieved with the anti-        try: following our original triple-decker syntheses 30 years
crown “mercurocarborand-3” (Figure 10A; ref 51), a species               ago (85), large families of isolable, electrically neutral
that when embedded in a membrane captures Cl− with high                  multidecker sandwiches having 3, 4, 5, or 6 decks and in-
selectivity (77).                                                        corporating C2B3 or C3B2 planar rings have been prepared
                                                                         (9c, 78b, 79b, 79c, 86), and metal stacking reactions are now
Chains, Rings, Rods, and Boxes:                                          a standard tool for assembling extended systems from small
Boron Clusters As Building Blocks                                        metallacarboranes.
      It has long been recognized that carboranes and other                   Multicluster systems that are directly connected or linked
boron cluster types are potentially excellent synthons for con-          by unsaturated entities such as alkenes, alkynes, and arenes
structing covalently bonded, architecturally novel macromol-             might function as conducting or semiconducting polymers.
ecules and polymers whose electronic (and other) properties              In fact, three characteristics of polyhedral boron clusters sug-
could be tailored to specific purposes (78); however, not un-            gest that there is real potential for these compounds as build-
til recent years have the synthetic tools been fully in place to         ing blocks in 21st century nanoscale electronics: their
build such systems in a rational, controlled manner (9c, 79).            electron-delocalized “super-aromatic” skeletal bonding, their
Carborane and metallacarborane clusters can be connected                 thermal and oxidative stability, and their tailorability via in-
via their cage carbon atoms (Figure 15, top; refs 78c, 80).              troduction of organic functional groups. Since these are not
Alternatively, linkage through boron-attached substituents has           inexpensive compounds, one is talking about small-scale,
been achieved by adapting organometallic techniques origi-               highly specialized polymeric materials. Nevertheless, it is clear
nally developed for C C binding, such as metal-catalyzed                 that boranes, carboranes, and their metallo derivatives can
coupling, as illustrated for a p-carborane system in Figure 15,          take us in directions that are simply not otherwise accessible.
bottom (81). This approach has also been applied to the con-             Studies of the electronic properties of C2B3- and C3B2-based
struction of polycluster compounds and extended arrays based             sandwich complexes using electrochemistry, ESR, UV-visible
on small metallacarboranes (Figure 16; refs 7, 82 ).                     spectroscopy, and multinuclear NMR show that the extent
      A different, and powerful, method for achieving cluster            and type of metal–metal interaction (electron delocalization)
linkage that is almost exclusive to boron compounds is metal–            depend upon molecular architecture, metal oxidation state
ligand stacking to create multidecker sandwich complexes                 and electron configuration, and the presence of attached sub-
(Figure 16, bottom). Although a few “all-hydrocarbon” triple-            stituents (62, 78b, 79b, 86–88). Upon electrochemical re-
decker complexes such as the classic Cp3Ni2+ ion are known               duction, fulvalene-linked tetradecker sandwich oligomers
(83), stable examples of these are rare (84) and higher-decker           such as that in Figure 17A (89) are extensively delocalized

                                  linkage via cage carbon atoms                  C    BH

                                                           1) n –BuLi                                                          1) n –BuLi
                                  H                   H                      H                                        H
                                                           2) CuCl 2                                                           2) R3SiCl
                                                           3) H 3O+
                                                                                                                                R = alkyl

                                  1) n –BuLi
                                               R3Si                                                       •
                             SiR 3
                                  2) CuCl 2

 Figure 15. Examples of
   metal-promoted C C
      and C B coupling
  of p-carborane cages.
                                   linkage via cage boron atoms                  CH    BH, B

                                                                                                                O2, CuI
                                        I                    I    +     HC       C                   C    CH



                                                  I                      C       C                    C   C    •


                         •   Vol. 81 No. 5 May 2004               •   Journal of Chemical Education                 667
   Viewpoints: Chemists on Chemistry

(90), whereas others having intervening phenyl groups link-                        ure 17D incorporates four (C5Me5)Co(Et2C2B4H2) clusters
ing the sandwich units are insulators (91); the C3B2-based                         linked by diacetylene chains and features a 24-atom planar
polydecker in Figure 1D is a semiconductor (4).                                    octagonal C16B8 ring (82b). Electrochemical reduction of this
     Conducting polymers based on nonmetal-containing                              tetracobalt complex generates a monoanion that exhibits elec-
boron clusters are also under investigation. Kaszynski and                         tronic communication between the metal centers (82b),
coworkers investigated the electronic interactions between                         which is of interest inasmuch as there is much current atten-
closo-boron clusters of 6, 10, and 12 vertices (see Figure 2)                      tion directed to alkyne linkers as “carbon wires” in organic
linked by alkynyl and other triple-bonded groups, using UV,                        and organometallic systems (95).
IR, and NMR spectroscopic data and theoretical calculations,
and concluded that the electronic conjugation between the                          Other Applications
clusters and the linkers is greatest for the 6-vertex cages (92).                        A number of actual or potential areas of utilization of
Similar studies demonstrated that the 1,12-C2B10H12 ( p-                           boron clusters exist beyond those discussed in this article (38),
carborane) polyhedron transmits electronic effects, although                       some of which may be surprising to some readers. For ex-
the mechanism was not established (93).                                            ample, a widely used air bag propellant system for automo-
     The macrocycles in Figures 17C and 17D illustrate the                         biles employs the dicesium salt of the B12H122− ion (see Figure
current state of the art in controlled synthesis of large sys-                     2) as a burning accelerant to ensure rapid but controlled in-
tems from monomeric boron clusters. The “big wheel” in Fig-                        flation of the bag (96). o-Carborane (1,2-C2B10H12) and other
ure 17C consists of alternating m-carboranyl (1,7-C2B10H10)                        boron cluster compounds are vaporized and fired at high tem-
cages and 1,3-phenylene rings (94) while the tetramer in Fig-                      perature to create boron films and wall coatings in tokamak

                                                        CR        BH       ,          transition metal ions           arene or Cp ring

                                 alkyne linkage via cage boron atoms

    Figure 16. Methods for
       construction of multi-                                     H
                                                                       I2, Et3N
    decker sandwiches and                                              PdII, CuI
       B-alkyne-linked small

                                                                                           SiMe 3                 H

                                       H   H
                                                   1) t–BuLi                                        n–Bu4NF               ClCH2C(O)Me
                                                   2)            B–C–C–SiMe3                           THF                      Et3 N

                                   decapping and metal stacking

                                                                          H       H
                                                    decap                                    deprotonate               halide

                                                    Lewis base                                  BuLi

668       Journal of Chemical Education        •    Vol. 81 No. 5 May 2004                       •
                                                                                                                   Boron Clusters Come of Age

reactors for nuclear fusion (97). A boron-carbon alloy has                               molecular structures, and to a lesser extent the reactivity, of
been fabricated from o-carborane enriched in the 10B isotope                             these once inscrutable molecules, while newer experimental
and employed as a solid-state neutron detector (98).                                     techniques and theoretical tools allow the controlled synthe-
Carborane-based stationary phases have been used in gas                                  sis of boron-containing compounds and materials that are tai-
chromatography for years, primarily because they allow op-                               lored for specific purposes. The objectives of this research are
erating temperatures (400 C or higher) that are not possible                             both practical (e.g., development of new materials and bio-
with conventional GC materials (99).                                                     medical applications) and basic, as in the designed synthesis
                                                                                         of new metallaboranes and metallacarboranes for studies of
Future Directions                                                                        electronic structure and other properties.
      Polyhedral boranes and their many families of derivatives                                Particularly noteworthy is the increasing interest in bo-
have been around for years, yet research in this area contin-                            ron clusters by researchers in areas once far removed from
ues to attract interest and funding. In parallel with the devel-                         this field—for example, nuclear medicine, pharmacology, ra-
oping uses for these compounds in modern technology as                                   dioactive waste treatment, polymer science, catalysis, fibers,
outlined in this article, fundamental investigations in this field                       and films—as they discover how the unique attributes of these
continue with a pace and energy that one might not expect                                species can be exploited. In essence, polyhedral boranes of-
in a field many decades old. General paradigms are now avail-                            fer a kind of alternative organic chemistry whose structural
able that enable chemists to understand the bonding and                                  principles and reaction modes are very different from con-

         A                                          B                                                   C











                                                                               n                        •




















                                                                                                                   C        C        C    C

                   C                                            C


                             C                                            C

                            C                                            C

                                                                                                        C                                            C

                                                                                                                   C        C        C    C

                                       BH, B-alkyl, B-halo         C –alkyl

                                                     Co             Co, Ni

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

                           •    Vol. 81 No. 5 May 2004                          •   Journal of Chemical Education                      669
   Viewpoints: Chemists on Chemistry

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                                •    Vol. 81 No. 5 May 2004           •   Journal of Chemical Education               671
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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 pio-
neered 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

672       Journal of Chemical Education            •   Vol. 81 No. 5 May 2004           •

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