Understanding the stabilization of metal carbide endohedral fullerenes m(2)c(2)@c(82) and related systems

We analyze the electronic structure of carbide endohedral metallofullerenes of the type Sc(2)C(2)@C(82) and study the possibility of rotation of the encapsulated Sc(2)C(2) moiety in the interior of the cage. Moreover, we rationalize the higher abundance of M(2)C(2)@C(82) (M = Sc, Y) in which the metal-carbide cluster is encapsulated in the C(3v)-C(82):8 carbon cage with respect to other carbides of the same family on the basis of the formal transfer of four electrons from the cluster to the cage and sizeable (LUMO-3)-(LUMO-2) gap in the empty cages. This rule also applies to all those endohedral metallofullerenes in which the encapsulated cluster transfers four electrons to the carbon cage as, for example, the reduced [M@C(82)](-) systems (M = group 3 or lanthanide metal ion).     

Expanding the world of endohedral fullerenes--the Tm3N@C2n (39< or =n< or =43) clusterfullerene family

A new family of endohedral fullerenes, based on an encaged trithulium nitride (Tm(3)N) cluster, was synthesised, isolated and characterised by HPLC, mass spectrometry, and visible-NIR and FTIR spectroscopy. Tm(3)N clusterfullerenes with cages as small as C(76) and as large as C(88) were prepared and six of them were isolated. Tm(3)N@C(78) is a small clusterfullerene. The two isomers of Tm(3)N@C(80) (I and II) were the most abundant structures in the fullerene soot. Tm(3)N@C(82), Tm(3)N@C(84), and Tm(3)N@C(86) represent a new series of higher clusterfullerenes. All six isolated Tm(3)N clusterfullerenes were classified as large energy-gap structures with optical energy gaps between approximately 1.2 and approximately 1.75 eV. Tm(3)N@C(80) (I) and Tm(3)N@C(80) (II) were assigned to the C(80) cages C(80):7 (I(h)) and C(80):6 (D(5h)). For Tm(3)N@C(78), the analysis pointed to an elliptical carbon cage with C(78):1 (D(3)) or C(78):4 (D(3h)) being the probable structures.     

A "sea urchin" family of boranes and carboranes: the 6m + 2n electron rule

A new family of related borane and carborane cages has been designed computationally. These compounds obey a new electron counting rule (6m + 2n rule) rather than Wade's rule. The structures of these cages can be conceived by combining m aromatic pyramidal and n aromatic triangular units. The interstitial electrons from the m pyramids (six electrons for each unit) and the n triangles (two electrons for each unit) constitute the total 6m + 2n skeletal electrons. The greater number of skeletal electron pairs in large closo-borane cages (e.g., B32H328- or C8B24H32) achieves stabilization through the optimal occupancy of all bonding orbitals. The favorable electronic structure, the large HOMO-LUMO gaps, the large lowest positive frequencies, and the local aromaticity of the pyramidal and triangular units (as demonstrated by the large negative NICS values) of the new large closo-cages auger well for their eventual experimental realization.     

Molecular dynamics simulation of NMR powder lineshapes of linear guests in structure I clathrate hydrates

We perform molecular dynamics simulations (up to 6 ns) for the structure I clathrate hydrates of linear molecules CS, CS(2), OCS, and C(2)H(2) in large cages at different temperatures in the stability range to determine the angular distribution and dynamics of the guests in the large cages. The long axes of linear guest molecules in the oblate large structure I clathrate hydrate cages are primarily confined near the equatorial plane of the cage rather than axial regions. This non-uniform spatial distribution leads to well-known anisotropic lineshapes in the solid-state NMR spectra of the guest species. We use the dynamic distribution of guest orientations in the cages during the MD simulations at different temperatures to predict the (13)C NMR powder lineshapes of the guests in the large cages. The length of the guests and intermolecular interactions of the guests in the water cages determine the angular distribution and the mobility of the guests in the sI large cages at different temperatures. At low temperatures the range of motion of the guests in the cages are limited and this is reflected in the skew of the predicted (13)C lineshapes. As the guest molecules reach the fast motion limit at higher temperatures, the lineshapes for CS, OCS, and C(2)H(2) are predicted to have the "standard" powder lineshapes of guest molecules.     

Coordination cages of rhodium and iridium as exoreceptors for alkali metal ions

Hexanuclear coordination cages of the formula [(C5Me4R)M(C7H3NO4)]6 (M = Rh, Ir; R = Me, H) were obtained by stepwise reaction of [(C5Me4R)MCl2]2 with, first, AgOAc and, then, pyridine-3,5-dicarboxylic acid. Crystallographic analyses show that the cages adopt a distorted octahedral geometry with the pyridine-3,5-dicarboxylates functioning as dianionic, bridging ligands, each of which connects three different (C5Me4R)M fragments. The cages act as exoreceptors for the large alkali metal ions K(+) and Cs(+) but show low affinity for Na(+). Crystallographic and NMR spectroscopic analyses indicate that two metal ions can be coordinated to the surface of the cages. The respective binding sites comprise three carbonyl O-atoms from the bridging pyridine-3,5-dicarboxylate ligand.     

The first fullerene-metal sandwich complex: an unusually strong electronic communication between two C(60) cages

Reaction of Rh(6)(CO)(9)(dppm)(2)(mu(3)-eta(2),eta(2),eta(2)-C(60)) (1) with C(60) in refluxing chlorobenzene followed by treatment with CNR (R = CH(2)C(6)H(5)) at room temperature affords the first fullerene-metal sandwich complex Rh(6)(CO)(5)(dppm)(2)(CNR)(mu(3)-eta(2),eta(2),eta(2)-C(60))(2) (2). Compound 2 has been characterized by an X-ray diffraction study. Electrochemical study of 2 reveals six well-separated reversible redox couples localized at C(60) cages due to a strong electronic communication between the two C(60) centers via the Rh(6) cluster spacer.     

Endohedral and external through-space shieldings of the fullerenes c50, c60, c60(-6), c70, and c70(-6)-visualization of (anti)aromaticity and their effects on the chemical shifts of encapsulated nuclei

Endohedral and external through-space NMR shieldings (TSNMRS) and the magnetic susceptibilities of the fullerene carbon cages of C50, C60, C60(-6), C70, and C70(-6) were assessed by ab initio molecular orbital calculations. Employing the nucleus-independent chemical shift (NICS) concept, these TSNMRS were visualized as isochemical shielding surfaces (ICSS) and were applied to quantitatively estimate either the aromaticity or the anti-aromaticity on the fullerene surface pertaining to the five- or six-membered ring moieties and the shielding of any nuclei enclosed within the carbon cages. Differences between the NICSs calculated at the center of the fullerene carbon cages and the experimental chemical shifts of encapsulated NMR-active nuclei as well as experimental shieldings observed for different encapsulated nuclei were able to be understood readily for the first time.     

C~1~2~0O的分子结构和光谱性质的理论研究

用INDO系列方法对双笼氧化物C~1~2~0O进行了理论研究,结果表明:双笼氧化物C~1~2~0O的形成缓解了C~6~0O中环氧三元环的角张力,并形成了呋喃型五元环将两碳笼连接在一起。两碳笼的直接键连使两笼距离较近,有较弱的相互作用,但仍各自表现一定的独立性,C~1~2~0O可发生分解生成新的化合物,C~1~2~0O的电子光谱与母体分子C~6~0相似。     

C~6~0SiH~2的结构和电子光谱的量子化学研究

用INDO系列方法研究了C~6~0SiH~2的两种结构: 一是SiH~2加在两个六元环之间的键上形成C~2~v构型; 另一是SiH~2加在一个五元环和一个六元环之间的键上形成C~s构型。从总能量和LUMO-HOMO能级差看, C~6~0SiH~2的稳定结构应是C~2~v构型, 其中桥C(15)-C(30)的键长为0.1508nm, 键序为0.9369, 说明不开环, 形成类环丙烷结构。文中计算了两种构型的电子吸收光谱和NMR谱, 此类计算是基于对C~6~0SiH~2的等电子体C~6~0O和C~6~0CH~2的研究之上, 且后两者的研究结果与实验相一致。     

A highly symmetric diamond-like assembly of cyclotricatechylene-based tetrahedral cages

The combination of six Cu(II) centres with four cyclotricatechylene (ctc(6-)) ligands generates large (Cu(6)ctc(4))(12-) cages with the topology of the tetrahedron. The anionic tetrahedral cages are cemented together, triangular face to triangular face, by Na(+) cations together with water molecules to generate a highly symmetric diamond-like assembly (space group Fd-3m, the same as that of diamond itself) whose adamantane-like units have an iodide ion at the centre of every cyclohexane-like face.     

Octanuclear cubic coordination cages

Two new bis-bidentate bridging ligands have been prepared, L (naph) and L (anth), which contain two chelating pyrazolyl-pyridine units connected to an aromatic spacer (naphthalene-1,5-diyl and anthracene-9,10-diyl respectively) via methylene connectors. Each of these reacts with transition metal dications having a preference for octahedral coordination geometry to afford {M 8L 12} (16+) cages (for L (anth), M = Cu, Zn; for L (naph), M = Co, Ni, Cd) which have an approximately cubic arrangement of metal ions with a bridging ligand spanning each of the twelve edges, and a large central cavity containing a mixture of anions and/or solvent molecules. The cages based on L (anth) have two cyclic helical {M 4L 4} faces, of opposite chirality, connected by four additional L (anth) ligands as "pillars"; all metal centers have a meridional tris-chelate configuration. In contrast the cages based on L (naph) have (noncrystallographic) S 6 symmetry, with a diagonally opposite pair of corners having a facial tris-chelate configuration with the other six being meridional. An additional significant difference between the two types of structure is that the cubes containing L (anth) do not show significant interligand aromatic stacking interactions. However, in the cages based on L (naph), there are six five-membered stacks of aromatic ligand fragments around the periphery, each based on an alternating array of electron-rich (naphthyl) and electron-deficient (pyrazolyl-pyridine, coordinated to M (2+)) aromatic units. A consequence of this is that the cages {M 8(L (naph)) 12} (16+) retain their structural integrity in polar solvents, in contrast to the cages {M 8(L (anth)) 12} (16+) which dissociate in polar solvents. Consequently, the cages {M 8(L (naph)) 12} (16+) give NMR spectra in agreement with the symmetry observed in the solid state, and their fluorescence spectra (for M = Cd) display (in addition to the normal naphthalene-based pi-pi* fluorescence) a lower-energy exciplex-like emission feature associated with a naphthyl --> pyrazolyl-pyridine charge-transfer excited state arising from the pi-stacking between ligands around the cage periphery.     

Face-driven corner-linked octahedral nanocages: M6L8 cages formed by C3-symmetric triangular facial ligands linked via C4-symmetric square tetratopic Pd(II) ions at truncated octahedron corners

The face-driven corner-linked truncated octahedral nanocages, [Pd6L8]12+ (1, L1 = N,N',N' '-tris(3-pyridinyl)-1,3,5-benzenetricarboxamide; 2, L2 = N,N',N' '-tris(4-pyridinylmethyl)-1,3,5-benzenetricarboxamide), were prepared with eight C3-symmetric tridentate ligands and six square planar tetratopic palladium(II) ions. The combination of the nitrogen donor atom at a approximately 120 degrees kink position of the carboxamido pyridinyl group and the tilted pyridyl versus the facial plane of the ligands can provide the needed curvature for the formation of octahedral cages. The nitrogen atoms can coordinate to the square planar palladium(II) ions to form kinks with approximately 120 degrees angles at the C4-symmetric square planar corners of the truncated octahedron. Depending on the conformation of the ligand, L1, two different truncated octahedral cages of around 2.4 nm in diameters were formed. The major form of 1 with syn-conformational ligands has a cavity volume of approximately 1600 A3. The cage has 12 ports (3.4 x 3.5 A2) at all edges of the octahedron. The minor form of cage 1 with anti-conformational ligands has a slightly increased cavity volume ( approximately 1900 A3) and port size (3.3 x 8.0 A2). The insertion of a methylene group in L2 has not only increased the cavity volume of 2 to approximately 2200 A3 but also enlarged the port size to 4.1 x 8.0 A2. However, an atomic force microscopy (AFM) study of cage 2 showed that the cages had a height of 1.8 +/- 0.1 nm. This value is about 30% smaller than the calculated size of 2.6 nm from the crystal structure. This tip-induced decrease in height in cage 2 suggests the nonrigidity of cage 2.     

Subcomponent self-assembly and guest-binding properties of face-capped Fe4L4(8+) capsules

A general method for preparing Fe(4)L(4) face-capped tetrahedral cages through subcomponent self-assembly was developed and has been demonstrated using four different C(3)-symmetric triamines, 2-formylpyridine, and iron(II). Three of the triamines were shown also to form Fe(2)L(3) helicates when the appropriate stoichiometry of subcomponents was used. Two of the cages were observed to have nearly identical Fe-Fe distances in the solid state, which enabled their ligands to be coincorporated into a collection of mixed cages. Only one of the cages combined a sufficiently large cavity with the sufficiently small pores required for guest binding, taking up a wide variety of guest species in size- and shape-selective fashion.     

Selection and amplification of mixed-metal porphyrin cages from dynamic combinatorial libraries

Mixed metallo-porphyrin cages were selected and amplified from dynamic combinatorial libraries (DCLs) by using appropriate templates. The cages are composed of two bisphosphine substituted zinc(II) porphyrins as ligand donors and two rhodium(III) or ruthenium(II) porphyrins as ligand acceptors, and are connected through metal-phosphorus coordination. Ru and Rh porphyrins that display a large structural diversity were employed. The templating was achieved by using 4,4'-bpy, 3,3'-dimethyl-4,4'-bipyridine and benzo[lmn]-3,8-phenanthroline, and acts through zinc-nitrogen coordination. The absolute amount of amplification from the DCLs is strongly dependent on the combination of the Ru/Rh porphyrin and the template; cages with sterically demanding porphyrins can only form with smaller templates. In the case of tert-butyl-substituted TPP (TPP=tetraphenylporphyrin), cages are not formed at all. The formation of the cages is usually complete within 24 h at an ambient temperature; in the case of the cage containing Rh(III)OEP (OEP=octaethylporphyrin) and bpy, the pseudo-first-order rate constant of cage formation was determined to be 2.1+/-0.1x10(-4) s(-1) (CDCl(3), 25 degrees C). Alternatively, heating the mixtures to 65 degrees C and cooling to room temperature yields the cages within minutes. The (1)H NMR chemical shifts of several characteristic protons show large differences upon changing the identity of the Ru/Rh porphyrin and the central metal; this is most likely to arise from variations in the geometry of the cages. The X-ray crystal structure of a cage, which contains Rh(III)OEP as a porphyrin acceptor and bpy as template, demonstrates that the cages can adopt severely distorted conformations to accommodate the relatively short templates. An extension to mixed DCLs showed that only limited selectivity is displayed by the various templates. Formation of mixed cages that contain two different rhodium porphyrins prevents effective selection, although the kinetic lability of the systems allows for some amplification. This lability, however, also prevents isolation of the individual cages. Removal of the template leads to re-equilibration, thus the templates act as scaffolds to keep the structures intact.     

Spatial imaging of individual vibronic states in the interior of single molecules

Selective excitations of specific vibronic modes in position space are realized in single naphthalocyanine molecules adsorbed on an ultrathin alumina film by a scanning tunneling microscope at low temperature. Distinct spatial distributions are imaged for the different vibronic modes, which are in accordance with spectra recorded over different points of the molecule and its orbital structure. These distinct vibronic images, together with the differential conductance images and calculated molecular orbitals, lead to vibrational excitations that are associated with the doubly degenerate lowest unoccupied molecular orbitals (LUMO)--LUMO-α and LUMO-β. These results reveal the presence of different molecular conformations on the surface and the nature of the electron-vibrational coupling.     

Synthesis of ether- and carbon-linked polycarboranyl porphyrin dimers for cancer therapies

Porphyrin dimers bearing multiple carborane cages for potential use as sensitizers in boron neutron capture therapy (BNCT) and photodynamic therapy (PDT) were synthesized from protoporphyrin dimethyl ester and characterized. Diastereomeric ether-linked dimers bearing four closo carborane cages (40 boron atoms) were found to be unstable to the acidic conditions necessary for conversion into water-soluble salts. In contrast, the carbon---carbon-linked dimers bearing six icosahedral carboranes (60 boron atoms) were stable to acid and could be isolated as water-soluble sodium salts. In vitro and in vivo studies of these novel molecules are currently under investigation.     

Electronic interactions in a new fullerene dimer: C(122)H(4), with two methylene bridges

The isolation of a new fullerene dimer, C(122)H(4), and its structural characterization by (13)C NMR and (1)H NMR spectroscopy and by UV/vis and IR spectroscopy are reported. The structure of this dimer consists of two fullerene cages, which are directly connected through two C-C bonds and two methylene bridges. Consequently, adjacent hexagonal faces of the two fullerene cages are arranged in a face to face manner. Molecular orbital calculations indicate that the proximity of the fullerene cages results in significant through space overlap in both the HOMO and LUMO. As a consequence of this overlap, the electrochemistry of the dimer shows electronic communication with stepwise reduction of each cage.     

Valence electronic charge density of distorted C60- monomers in polymerized KC60 and RbC60

We investigate the valence electronic charge density of the C(60) (-) monomers in (C(60) (-))(n) polymer chains in K- and RbC(60) by means of a nonorthogonal tight-binding formalism using experimental data on the positions of the carbon atoms. Various configurations of the C(60) cages are considered. Starting from the ideal icosahedral C(60) structure and moving to the realistic, experimentally determined spatial configuration of the C(60) cages in K- and RbC(60), we observe a systematic increase of the electric quadrupole moments on the C(60) (-) monomers. We also confirm the validity of factorizing the charge density of a C(60) (-) monomer into an angular and a radial part.     

A family of polynuclear cobalt and nickel complexes stabilised by 2-pyridonate and carboxylate ligands

The synthesis and structural characterisation of a series of cobalt and nickel cages are reported. Eight of these structures contain a [M10(mu3-OH)6(eta2, mu3-xhp),(eta2, mu2-O2CR)6]2+ core (where M = Co or Ni; xhp = 6-chloro- or 6-methyl-2-pyridonate: R = Me, Ph, CHMe2, CH2Cl, CHPh2 or CMe3), where the ten metal atoms describe a centred-tricapped-trigonal prism (ttp). The cage contains six hydroxide ligands around the central metal, and the exterior is coated with pyridonate and carboxylate ligands. For four of the cages additional metal centres are found attached to the upper and/or lower triangular faces of the trigonal prism, generating dodeca- and undecanuclear cages. Three further cages are reported that contain a metal core based on an incomplete centred-tetraicosahedron. These cages involve trimethylacetate as a ligand in company with either 6-methyl-2-pyridonate or 6-chloro-2-pyridonate. Comparison of these latter structures with the trigonal prisms reveal that they can be described as a pentacapped-trigonal prism missing one edge. Magnetic studies of three of the nickel cages with trigonal prismatic cores show spin ground states of S = 8, 4 and 2 for Ni12, Ni11 and Ni10 cages, respectively.     

Molecular dynamics simulations of binary structure H hydrogen and methyl-tert-butylether clathrate hydrates

Binary structure H (sH) hydrogen and methyl-tert-butylether (MTBE) clathrate hydrates are studied with molecular dynamics simulations. Simulations on a 3 x 3 x 3 sH unit cell with up to 4.7 mass % hydrogen gas are run at pressures of 100 bars and 2 kbars at 100 and 273 K. For the small and medium cages of the sH unit cell, H2 guest molecule occupancies of 0, 1 (single occupancy), and 2 (double occupancy) are considered with the MTBE molecule occupying all of the large cages. An increase of the small and medium cage occupancies from 1 to 2 leads to a jump in the unit cell volume and configurational energy. Calculations are also set up with 13, 23, and 89 of the MTBE molecules in the large cages replaced by sets of three to six H2 molecules, and the effects on the configurational energy and volume of the simulation cell are determined. As MTBE molecules are replaced with sets of H2 guests in the large cages, the configurational energy of the unit cell increases. At the lower temperature, the energy and volume of the clathrate are not sensitive to the number of hydrogen guests in the large cages; however, at higher temperatures the repulsions among the H2 guest molecules in the large cages cause an increase in the system energy and volume.