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Magnesium cobaltates (Arnacnac)MgCo(COD)2 (1–3) were synthesised by reacting (Arnacnac)MgI(OEt2) with K[Co(η4-COD)2] (COD = 1,5-cyclooctadiene) [Arnacnac = CH(ArNCMe)2; Ar = 2,4,6-Me3-C6H2 (Mes), 2,6-Et2-C6H3 (Dep), 2,6-iPr2-C6H3Mes (Dipp)]. Compounds 1–3 form contact ion-pairs in toluene, while solvent separated ion-pairs are formed in THF. The effect of ion-pairing on the reactivity is illustrated by reaction of 2 with tert-butylphosphaalkyne, which affords distinct 1,3-diphosphacyclobutadiene complexes. The heteroleptic sandwich complex [(Depnacnac)MgCo(P2C2tBu2)]2 (4) is selectively formed in toluene, while the homoleptic bis(1,3-diphosphacyclobutadiene) complex [(Depnacnac)Mg(THF)3][Co(P2C2tBu2)2] (5) is obtained in THF. Complex 4 is a precursor to further unusual phosphaorganometallic compounds. Substitution of the labile COD ligand in 4 by white phosphorus (P4) enabled the synthesis of the phosphorus-rich sandwich compound [(Depnacnac)MgCoP4(P2C2tBu2)]2 (6). The heterobimetallic complex (Cp*NiP2C2tBu2)Co(COD) (7) was isolated after treatment of 4 with Cp*Ni(acac) (Cp* = C5Me5, acac = acetylacetonate)., Ion-pairing strongly influence the reactivity of low-oxidation state transition metalate anions, enabling the synthesis of novel phosphaorganometallic compounds.

Sandwich-type clusters with the planar fragment containing a heterometallic sheet have remained elusive. In this work, we introduce the [K(2,2,2-crypt)]4{(Ge9)2[η6-Ge(PdPPh3)3]} complex that contains a heterometallic sandwich fragment. The title compound is structurally characterized by means of single-crystal X-ray diffraction, which reveals the presence of an unusual heteroatomic metal planar fragment Ge@Pd3. The planar fragment contains a rare formal zerovalent germanium core and a peculiar bonding mode of sp2-Ge@(PdPPh3)3 trigonal planar structure, whereas the nonagermanide fragments act as capping ligands. The chemical bonding pattern of the planar fragment consists of three 2c-2e Pd-Ge σ-bonds attaching Pd atoms to the core Ge atom, while the binding between the planar fragment and the aromatic Ge9 ligands is provided by six 2c-2e Pd-Ge σ-bonds and two delocalized 4c-2e σ-bonds. The synthesized cluster represents a rare example of a sandwich compound with the heteroatomic metal planar fragment and inorganic aromatic capping ligands.

A novel electrochemiluminescence (ECL) biosensor for the determination of lipopolysaccharide (LPS), a biomarker of endotoxemia, was designed. The nanoprobe (Ru1@SiO2-peptide) with a specific peptide (CIGKLHSAGK) labeled with a carboxyl functionalized Ru@SiO2 (Ru1@SiO2) was prepared by the acylation reaction. The ECL peptide biosensor was obtained by the strong gold-sulfur bond between Au on the gold electrode (AuE) surface and thiol group in the peptide. In the present of LPS, the peptide on the ECL biosensor can capture effectively LPS and the LPS/peptide/AuE was fabricated. When the Ru1@SiO2-peptide nanoprobe was bound, a sandwich compound of Ru1@SiO2-peptide/LPS/peptide was formed, and Ru(bpy)32+, in the sandwich compound, will do the ECL performance. The effective ECL amplification of Ru1@SiO2 exhibited excellent analytical performance to LPS with a wide detection linear range of 1.0~500 ng mL-1 and the detection limit was 0.3 ng mL-1. The biosensor could provide an evaluate of LPS expression on the bacteria surface and guide a new path to the clinical analysis of LPS related diseases.

The scandium Cp/COT hybrid sandwich compound [(η 5-C 5H 5)Sc(η 8-C 8H 8)] is resistant to metallation via conventional alkyllithium and lithium amide bases. In this work, clean, selective, stoichiometric and high-yielding mono- and dimetallation is accomplished using tandem trans-metal-trapping (TMT) involving LiTMP and iBu 2AlTMP with deprotonation occurring selectively at the Cp and Cp/COT rings respectively, providing the first examples of selective metallation of a sandwich complex featuring a group 3 element.

The known sandwich compound [η5-(CH2)3N2(BPh)2CMe]2Fe in which adjacent C2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B3N3H6, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me4B2N2CH)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me4C5H)2M. Low-energy sandwich structures having parallel B2N2C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η5-Me4B2N2CH)2M sandwich compounds relative to the corresponding metallocenes (η5-Me4C5H)2M. Thus (η5-Me4B2N2CH)2Cr has a quintet ground state in contrast to the triplet ground state of (η5-Me4C5H)2Cr. Similarly, the sextet ground state of (η5-Me4B2N2CH)2Mn lies ∼18 kcal mol−1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me4C5H)2Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η5-(CH2)3N2(BPh)2CMe]2Fe.

An improved, one-pot synthesis of the linear sandwich compound [Sc(η5 -C5 H5 )(η8 -C8 H8 )] is presented. The synthetic procedure is amenable to boryl- and silyl-substituted cyclopentadienyl and cyclooctatetraenyl ligands, thereby yielding the first functionalized derivatives. We found that the synthesis of the silyl-substituted mixed sandwich complexes produces higher yields when the ligand exchange is carried out stepwise, by isolating the intermediate trimethylsilylated half-sandwich complex [Sc(η8 -C8 H7 SiMe3 )Cl(THF)] (THF=tetrahydrofuran). The molecular structures of the parent complex, as well as of its mono-boryl-substituted derivatives, have been determined by single-crystal X-ray diffraction. In addition, the optical and electrochemical properties of the mixed sandwich complexes are reported.

Palladium catalyzed bis-arylations, -alkylations, and -allylations on the Cp ring of iron and cobalt sandwich compounds have been achieved using the bidentate picolinamide directing group. This directing group along with catalytic Pd(OAc)2 was found to be highly efficient for C–H functionalization, giving up to 87% yields. The palladacyclic intermediate for the C–H activation of the Cp ring has been isolated and structurally characterized for the cobalt sandwich compound [η5-C5H5]Co(η4-C4Ph4). Attempted C–H annulation reactions using picolinamide-derived sandwich compounds did not yield the expected annulated products and instead oxidized the Cp- and picolinamide-bound CH2 unit to aldehydes. Detailed studies on this novel and unprecedented oxidation indicated that this happens only with the assistance of the picolinamide directing group. We have also shown that the sp2 C–H functionalization and the sp3 C–H oxidation can be effectively carried out as a one-pot reaction.

A covalent organic helical cage (COHC) with D3 symmetry bearing two 1,3,5-trimethylphenyl cores and six di-tert-butyldiethynylallene moieties was synthesized and fully characterized. This molecular structure cage, unlike a previously reported one, favors inclusion-complex formation with organometallic sandwich compounds due to the presence of methyl groups on the aryl rings. The strong chiroptical responses of these COHCs, along with their ability to entrap guest molecules, enabled the detection of a ruthenium sandwich compound by means of electronic circular dichroism (ECD) spectroscopy.

We investigated the coordination behavior of AgI towards three polycondensed aromatic hydrocarbons (PAHs) in solution, in the solid state, and theoretically. Silver(I) trifluoromethanesulfonate (AgTfl) coordinates to fluorene to yield crystals of the composition {[(µ-η2:η2-fluorene)Ag2(Tfl)2]}n (1), the fluorene derivative 5,12-diphenyl-5,6;11,12-di-o-phenylene-5,12-dihydrotetracene (pseudorubrene) reacts with silver(I) perchlorate to yield crystals of the composition bis[(η2-pseudorubrene)(η2-toluene)AgClO4]·toluenesolv (2), and the formation of the crystalline dimeric AgI sandwich compound [(µ-η2:η2-truxene)2(η1-toluene)(µ-H2O)Ag2(ClO4)2] (3) is accomplished by π-coordination of the PAH truxene. In complexes 1–3, it is eminent from their crystal structures that the anionic silver moieties act as linking units between the PAH molecules. Theoretical calculations based on DFT methods of the aromatic bridging ring ligands substantiate the experimentally observed selective AgI coordination for each of the three PAH molecules fluorene, pseudorubrene, and truxene.

Molecular compounds of the rare-earth elements in which the metal is bound between two π-bonded aromatic ligands are termed sandwich complexes, with the description being extended to closely related systems containing one such ligand (half-sandwich complexes) and, frequently, more than two such ligands. Sandwich compounds have been a cornerstone of rare-earth organometallic chemistry for several decades, from the early pioneering studies describing their synthesis and basic physico-chemical properties to their contemporary applications in synthesis, catalysis and small-molecule activation. In this chapter, we focus on the electronic structure, oxidations states and magnetic properties of rare-earth sandwich compounds, setting the key advances in the context of earlier milestones in the field. While we place emphasis on rare-earth organometallic physics, the indispensable link with innovative synthetic chemistry is also highlighted. The pivotal role played by cyclopentadienyl (Cp) ligands in rare-earth organometallic chemistry is showcased through sandwich complexes that contain the full series of lanthanides in their divalent form. The coverage of unusual oxidations states incorporates the only rare-earth compounds to contain formally zerovalent metals. We consider compounds in which dysprosium is sandwiched between two cyclopentadienyl ligands, which represent the current state-of-the-art in single-molecule magnetism, a phenomenon in which magnetic hysteresis is observed in a molecular material. Extending the single-molecule magnet design principles to erbium shows that another well-established ligand in f-element chemistry—cyclooctatetraene (COT)—has adopted a new role in the design of magnetic materials. Finally, a new type of rare-earth sandwich compound based on cyclononatetraenyl (Cn) ligands is presented, along with its photophysical properties. The overarching vision of the chapter is to demonstrate that rare-earth sandwich chemistry has a remarkable ability to re-invent itself and, despite the maturity of the field, also has considerable potential for development in the future.

International audience; Salt metathesis between the anionic rhenium(I) compound, Na[Re(eta(5)-Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-beta-diketiminate), and the uranium(III) salt, UI3 (1,4-dioxane)(1.5), generated the triple inverse sandwich complex, U[(mu-eta(5):eta(5)-Cp)Re(BDI)(3), which was isolated and structurally characterized as the Lewis base adducts, (L)U[(mu-eta(5):eta(5)-Cp)Re(BDI)](3) (1.L, L = THF, 1,4-dioxane, DMAP). The assignment as one uranium(III) and three rhenium(I) centers was supported by X-ray crystallography, NMR and EPR spectroscopies, and computational studies. An unusual shortening of the rhenium-Cp bond distances in 1.L relative to Na[Re(eta(5)-Cp)(BDI)] was observed in the solid-state and reproduced in calculated structures of 1-THF and the anionic fragment, [Re(eta(5)-Cp)(BDI)](-). Calculations suggest that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron-electron repulsions in the rhenium-Cp moieties and thereby strengthening those interactions, while also making uranium-Cp bonding more favorable.

A comprehensive review on the synthesis, reactions and applications centered on the highly stable and sterically hindered cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) is presented. The derivatization of the cyclopentadienyl and cyclobutadiene rings, reactions, structural properties and applications, particularly in the field of asymmetric catalysis of (η5-Cp)Co(η4-C4Ph4) and its derivatives and analogues are discussed. The chemistry of this cobalt sandwich compound is also compared with analogous derivatives of ferrocene.

Recently an all-metal aromatic sandwich compound of a [Sb3Au3Sb3](3-) ion has been synthesized and characterized experimentally, which indicates that there might exist a variety of stable all-metal sandwich complexes. The intralayer and interlayer chemical bonding interaction in this system plays significant roles in their stability, chemical properties and functionalities. Here we report a systematic theoretical study on the geometries, electronic structures, and chemical bonding of the [Sb3Au3Sb3](3-) ion and its congeners of [X3Au3X3](3-) (X = N, P, As, Sb, Bi, Uup) as well as [X3M3X3](3-) (M, X = Cu, As; Ag, Bi; Au, Sb; Rg, Uup) to understand the special stabilities of these species. Additional studies are also performed on the oligomers [Sb3(Au3Sb3)n](3-) (n = 1-4) to explore whether the sandwich compound can form stable extended systems. Through extensive theoretical analyses, we have shown that among the [Au3X6](3-) (X = N, P, As, Sb, Bi, Uup) species, [Sb3Au3Sb3](3-) is most stable due to superb matching of Sb3 and Au3 in both geometric size and fragment orbital energies. The significant stability of the [Au3Sb6](3-) ion is determined by the interlayer (p-d-p)σ interactions between the vertical Au 5d6s hybrid orbitals of Au3 and Sb 5pπ orbitals of the Sb3 rings. Each Sb3 ring demonstrates unique σ aromaticity, which remains when the complex is extended to oligomers. The results suggest that it is likely that there might exist other stable [ApMpAp](x-) (M = transition metals, A = main group elements, p = 3, 4, 5, …) sandwich ions and oligomers.

DFT B3LYP/6-31G(d) calculations were performed to examine the feasibility of graphene-like C42H18 and starbenzene C6(BeH)6 (SBz) polymers as ligands of 3D-extensible sandwich compounds (3D-ESCs) with uninterrupted sandwich arrays. The results revealed that sandwich compounds with three or more C42H18 ligands were not feasible. The possible reason may be the localization of π electrons on certain C6 hexagons due to π-metal interactions, which makes the whole ligand lose its electronic structure basis (higher degree of π electron delocalization) to maintain the planar structure. For comparison, with the aid of benzene (Bz) molecules, the SBz polymers can be feasible ligands for designing 3D-ESCs because the C-Be interactions in individual SBz are largely ionic, which will deter the π electrons on one C6 ring from connecting to those on neighbouring C6 rings. This means that high degree of π electron delocalization is not necessary for maintaining the planarity of SBz polymers. Such a locally delocalized π electron structure is desirable for the ligands of 3D-ESCs. Remarkably, the formation of a sandwich compound with SBz is thermodynamically more favourable than that found for bis(Bz)chromium. The assembly of 3D-ESCs is largely exothermic, which will facilitate future experimental synthesis. The different variation trends on the HOMO-LUMO gaps in different directions (relative to the sandwich axes) suggest that they can be developed to form directional conductors or semiconductors, which may be useful in the production of electronic devices.

The ruthenocenecarbo­nitrile mol­ecule exhibits mirror symmetry in the solid state., The mol­ecular structure of ruthenocenecarbo­nitrile, [Ru(η5-C5H4C N)(η5-C5H5)], exhibits point group symmetry m, with the mirror plane bis­ecting the mol­ecule through the C N substituent. The RuII atom is slightly shifted from the η5-C5H4 centroid towards the C N substituent. In the crystal, mol­ecules are arranged in columns parallel to [100]. One-dimensional inter­molecular π–π inter­actions [3.363 (3) Å] between the C N carbon atom and one carbon of the cyclo­penta­dienyl ring of the overlaying mol­ecule are present.

The hexagonal and heptagonal holes of B-39(-) allow its complexation with a half sandwich complex ECp*+ (E=Si, Ge, Sn, Pb). Structure and the nature of bonding of (eta(6/7)-B-39)E(eta(5)-Cp-*) are explored through the density functional theory based computation. (eta(6)-B-39)E(eta(5)-Cp-*) isomers are more stable than (eta(7)-B-39)E(eta(5)-Cp*) and the energy difference between these two isomers decreases down the group from Si to Pb. The dissociation path, (eta(6/7)-B-39)E(eta(5)-Cp-*) -> B-39(-)+ ECp*+ is studied. For all E, (eta(6/7)-B-39)E(eta(5)-Cp-*) is formed exergonically at 298 K temperature as given by the Delta G values of dissociation path [60.1(Si) to 68.3(Pb) kcal/mol for (eta(6)-B-39)E(eta(5)-Cp-*) and 58.3(Si) to 67.8(Pb) kcal/mol for (eta(7)-B-39)E(eta(5)-Cp-*)]. The adduct becomes bent around the central E atom when B-39(-) gets attached to ECp*+ and the amount of bending increases gradually down the group from Si to Pb. Bonding analysis of the stable isomer, (eta(6)-B-39)E(eta 5-Cp*) has been done by natural bonding orbital (NBO) and energy decomposition analyses (EDA). The electron density from B-39(-) is transferred to the ECp*+ moiety as revealed by the NBO analysis. All the complexes are mainly stabilized by the electrostatic and orbital interactions between B-39(-) and ECp*+ fragments as highlighted by the EDA results.

Three ferrocene analogues, D 5h (η5-CB5H5)2M (M = Fe2−, Co−, and Ni), with triplet aromatic CB5H5 ligands have been predicted at TPSSh/6–311+G(d,p) level. We find that the M atom interacts drastically with the two CB5H5 ligands through a nearly fully-filled 3d subshell, which is different from (η5-C5H5)2Fe. The natural population analyses suggest that (η5-CB5H5)2M have an unconventional charge distribution, i.e., the M atom is negatively charged, while the two boron rings are positively charged. The analyses of the electronic and dynamic stabilities indicate that (η5-CB5H5)2Co− is the most stable among (η5-CB5H5)2M. Thus, we theoretically confirm that the triplet aromatic CB5H5 cluster can be regarded as a potential new ligand. Our theoretical predictions are awaiting future experimental verification.

The cocondensation of Mn atoms, I 2 and toluene at 77 K yields small quantities of [Mn(η-C 6 H 5 Me) 2 ] + I − . A single crystal X-ray structure determination shows that the manganese sits on a crystallographic inversion centre with a Mn ring steroid distance of 1.56(1) A.

The salt [Mo(η-C6H6)2][PF6] has been prepared by iodine oxidation of [Mo(η-C6H6)2] followed by metathesis with NaPF6. The cyclic voltammogram of [Mo(η-C6H6)2][PF6] in MeCN exhibits a reversible redox couple (E½) at –730 mV vs. a saturated calomel electrode (SCE). The solid-state room-temperature EPR spectrum exhibits an isotropic resonance centred at g= 1.993 which is consistent with an assignment of a 2A1g ground state for the 17-electron [Mo(η-C6H6)2]+ radical cation. The salt [Mo(η-C6H6)2][FeBr4] has been synthesised from [NEt4][FeBr4] and [Mo(η-C6H6)2][PF6]. It crystallises in the orthorhombic space group Pbca, a= 18.527(17), b= 12.521(5), c= 14.915(6)A; R and R′= 0.051 and 0.059. The salt [Mo(η-C6H6)2][FeBr4] consists of segregated chains of cations and anions. The solid-state room-temperature EPR spectrum exhibits two resonances centred at g= 1.987 and 2.008 assignable to the cation and anion respectively. The solid-state magnetic susceptibility data for [Mo(η-C6H6)2][FeBr4] can be fitted to the Curie–Weiss law between 40 and 300 K with C= 3.93 and θ=–5.4 K. Below 15 K the [FeBr4]– anion sub-lattice orders antiferromagnetically leaving a magnetic susceptibility which is dominated by the [Mo(η-C6H6)2]+ cations.

The lithium-cyclopropylethynylamidinates Li[c-C3H5-C[triple bond, length as m-dash]C-C(NR)2] (1a: R = (i)Pr, 1b: R = cyclohexyl (Cy)) have been used as precursors for the preparation of five new series of half-sandwich complexes. These complexes contain the large flat cyclooctatetraenyl ligand (C8H8(2-), commonly abbreviated as COT), and were isolated as solvated, unsolvated and inverse sandwich complexes. Treatment of the halide precursors [(COT)Pr(μ-Cl)(THF)2]2 with 1b and [(COT)Nd(μ-Cl)(THF)2]2 with 1a and 1b in THF in a 1 : 2 molar ratio, respectively, afforded (COT)Ln[μ-c-C3H5-C[triple bond, length as m-dash]C-C(NR)2]2Li(L) (2: Ln = Pr, R = Cy, L = Et2O; 3: Ln = Nd, R = (i)Pr, L = THF; 4: Ln = Nd, R = Cy, L = THF). Treatment of the dimeric cerium(iii) bis(cyclopropylethynylamidinate) complexes [{c-C3H5-C[triple bond, length as m-dash]C-C(NR)2}2Ce(μ-Cl)(THF)]2 (5: R = (i)Pr; 6: R = Cy) in situ with K2C8H8 in a 1 : 1 molar ratio in THF at room temperature afforded the inverse-sandwich complexes (μ-η(8):η(8)-COT)[Ce{c-C3H5-C[triple bond, length as m-dash]C-C(NR)2}2]2 (7: R = (i)Pr; 8: R = Cy). This reaction represents a new method for encapsulation of a planar (C8H8)(2-) ring in lanthanide complexes containing amidinate ligands in the outer decks. Novel unsolvated dinuclear lanthanide half-sandwich complexes were prepared by using the precursors 1a, 1b and COT(2-). Unlike the complexes 2-4, the reaction of [(COT)Pr(μ-Cl)(THF)2]2 with 1a afforded the unsolvated centrosymmetric complex [(COT)Pr(μ-c-C3H5-C[triple bond, length as m-dash]C-C(N(i)Pr)2)]2 (9). These dimeric structures could be also accessed by reaction of LnCl3 (Ln = Ce or Nd) with 1a or 1b and K2COT in a 1 : 1 : 1 molar ratio as a one-pot reaction to give novel [(COT)Ln(μ-c-C3H5-C[triple bond, length as m-dash]C-C(NR)2)]2 complexes (10: Ln = Ce, R = (i)Pr; 11: Ln = Ce, R = Cy; 12: Ln = Nd, R = (i)Pr). Similar treatment of HoCl3 with 1a or 1b and K2COT as three-component reactions in a 1 : 1 : 1 molar ratio afforded the solvated half-sandwich complexes (COT)Ho(c-C3H5-C[triple bond, length as m-dash]C-C(NR)2)(THF) (13: R = (i)Pr; 14: R = Cy). A unique multidecker sandwich complex [(μ-η(8):η(8)-COT){Nd(c-C3H5-C[triple bond, length as m-dash]C-C(NCy)2)(μ-Cl)}2]4 (15) was prepared by reaction of anhydrous NdCl3 with K2COT and 1b in a one-pot reaction. The solid state structure of 15 revealed the presence of an unprecedented macrocyclic sandwich compound ("giant neodymium wheel") consisting of four COT rings sandwiched between eight Nd(3+) ions, and each Nd(3+) ion is bonded to one amidinate ligand and bridged by two chlorine atoms with the neighbouring Nd(3+) ion.

A comprehensive review on the synthesis, reactions and applications centered on the highly stable and sterically hindered cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) is presented. The derivatization of the cyclopentadienyl and cyclobutadiene rings, reactions, structural properties and applications, particularly in the field of asymmetric catalysis of (η5-Cp)Co(η4-C4Ph4) and its derivatives and analogues are discussed. The chemistry of this cobalt sandwich compound is also compared with analogous derivatives of ferrocene.

The electronic properties of nine different hexaarylbenzene molecules substituted by sandwich compounds have been studied by applying density functional theory. Different structures and the particular electron donor power of these systems have been considered in order to analyze their oxidant capacity, using bis(ciclopentadienyl) scandium, ferrocene, and bis(benzene)chromium as sandwich compounds. Both monometallic and bimetallic combinations are investigated. According to the ionization energies and electron affinities, compounds with Cr are nucleophiles and represent the best electron donors, whereas compounds with Sc are electrophiles and represent the best electron acceptors. The worse electron donor or acceptor is hexakis(4-ferrocenyl phenyl) benzene. This is very significant, as it implies that the very well-known electronic properties of hexakis(4-ferrocenyl phenyl) benzene can be improved by substituting with other metals, such as Sc and Cr. This suggests several possible applications for these compounds.