Molecular Wheels Research Articles

The route to highly stable MeBxNyCz molecular wheels. I. The features of preliminary results

By means of ab initio quantum chemical methods we have determined the energies and electronic structures of molecular wheels TiBn, TiBnN10−n, TiCnN10−n and TiCnB10−n (for n = 0–10). The ground state energies and the corresponding spin states of each atom, cluster and molecular wheel were calculated first in the framework of Hartree-Fock self-consistent-field (HF-SCF) using minimal and more accurate basis sets STO-3G and 6-31G. Computations at higher level and accuracy are processing in a follow-up study. The most stable wheel system is TiCnB10−n (for n = 5–10). Thereof particularly highly stable is the TiC5B5 molecular wheel followed by the TiC6B4. At the HF-SCF/6-31G level, however, we have calculated the wheel system MeC5B5 considering for Me, the first row of transition metal atoms Me = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. The molecular wheel MeC5B5 favours Sc atom at the centre, but also Ti and Fe are the next favoured atoms.

Observation of the Highest Coordination Number in Planar Species: Decacoordinated Ta©B10− and Nb©B10− Anions

Molecular wheels: Decacoordinated molecular wheels, Ta©B10− (see picture) and Nb©B10−, showing the highest coordination number for the central atom in a planar environment at present, were produced in a laser-vaporization supersonic molecular beam and characterized by photoelectron spectroscopy and ab initio calculations. The highly symmetric Ta©B10− and Nb©B10− anions are doubly aromatic with six delocalized π electrons and ten delocalized σ electrons.

Single‐Strand Molecular Wheels and Coordination Polymers in Copper(II) Benzoate Chemistry by the Employment of α‐Benzoin Oxime and Azides: Synthesis, Structures, and Magnetic Characterization

AbstractThe use of α‐benzoin oxime (bzoxH2) in copper(II) benzoate chemistry, in the absence or presence of ancillary azido ligands, is reported. The reaction of Cu(O2CPh)2·2H2O with one equivalent of bzoxH2 in N,N‐dimethylformamide (DMF) affords the decanuclear complex [Cu10(bzox)10(DMF)4] (1) in good yield. Dissolution of 1 in CH2Cl2 leads to the subsequent isolation of the solvent‐free complex [Cu10(bzox)10] (2) in moderate yields. Complexes 1 and 2 are isostructural and possess a loop or single‐strand molecular wheel topology. The bzox2– dianions behave as η1:η1:η2:μ3 ligands, which give rise to an overall [Cu10(μ‐ONR)10(μ‐OR′)10] core. Both 1 and 2 stack to form nanotubular columns with beautiful supramolecular architectures. The reaction of Cu(O2CPh)2·2H2O with bzoxH2 and NaN3 in a 1:1:1 molar ratio in MeOH gives the bzoxH2‐free complex [Cu(N3)(O2CPh)(MeOH)]n (3), which is a 1D chain. The CuII atoms in 3 are linked by a single, end‐on N3– group, a syn,syn‐η1:η1:μ PhCO2– ion, and an oxygen atom from the bridging MeOH ligand. The 1D chains are hydrogen bonded into 2D sheets through Nazide···H(OMeOH) interactions. Variable‐temperature, solid‐state direct‐current magnetic studies were carried out on 1–3. The data for 1 and 2 indicate very strong antiferromagnetic exchange interactions and a S = 0 ground state, which is expected for even‐membered loop arrays of CuII atoms. In contrast, 3 exhibits ferromagnetic exchange interactions; the data were fitted to the appropriate equation derived from the Hamiltonian H = –JΣ(Si·Si+1), which includes a zJ′ interchain interaction term. The best‐fit parameters were J = +49.6(4) cm–1, g = 2.067(3), and zJ′ = 2.3(1) K. The combined results demonstrate the ligating flexibility of both the bzoxH2 and azido groups and their usefulness in the synthesis of polynuclear CuII clusters and coordination polymers.

Geometrical requirements for transition-metal-centered aromatic boron wheels: the case of VB10−

A class of transition-metal-centered aromatic boron wheels (D(nh)-M©B(n)(q-)) have been recently produced and characterized according to an electronic design principle. Here we investigate the interplay between electronic and geometric requirements for the molecular wheels using the case of VB(10)(-), which is isoelectronic to the decacoordinated molecular wheels, Ta©B(10)(-) and Nb©B(10)(-). Photoelectron spectra of VB(10)(-) are observed to be broad and complicated with relatively low electron binding energies, in contrast to the simple and high electron binding energies observed for the molecular wheels of its heavier congeners. An unbiased global minimum search found the most stable isomer of VB(10)(-) to be a singlet "boat"-like structure (C(2)), in which the V atom is coordinated to a quasi-planar B(10) unit. A similar triplet C(2v) boat-like isomer is found to be almost degenerate to the C(2) structure, whereas the beautiful molecular wheel structure, D(10h)-V©B(10)(-), is significantly higher in energy on the potential energy surface. Therefore, even though the VB(10)(-) system fulfills the electronic requirement to form a D(10h)-M©B(10)(-) aromatic molecular wheel, the V atom is too small to stabilize the ten-membered boron ring.

An octanuclear helical ‘molecular wheel’ from hierarchical assembly of four dinuclear Cu2 units in a mixed-ligand array

Reaction of Cu(II) salts with a combination of two different types of ligand affords an unusual ‘molecular wheel’, based on a cyclic array of four pyrazolate-bridged dinuclear Cu(II) units which are interconnected by bis-bidentate bridging ligands, in a hierarchical self-assembly process.

Transition-Metal-Centered Nine-Membered Boron Rings: MⓒB9 and MⓒB9– (M = Rh, Ir)

We report the observation of two transition-metal-centered nine-atom boron rings, RhⓒB(9)(-) and IrⓒB(9)(-). These two doped-boron clusters are produced in a laser-vaporization supersonic molecular beam and characterized by photoelectron spectroscopy and ab initio calculations. Large HOMO-LUMO gaps are observed in the anion photoelectron spectra, suggesting that neutral RhⓒB(9) and IrⓒB(9) are highly stable, closed shell species. Theoretical calculations show that RhⓒB(9) and IrⓒB(9) are of D(9h) symmetry. Chemical bonding analyses reveal that these complexes are doubly aromatic, each with six completely delocalized π and σ electrons, which describe the bonding between the central metal atom and the boron ring. This work establishes firmly the metal-doped B rings as a new class of novel aromatic molecular wheels.

Back Cover: Molecular Wheels of Ruthenium and Osmium with Bridging Chalcogenolate Ligands: Edge-Shared-Octahedron Structures and Metal-Ion Binding (Angew. Chem. Int. Ed. 11/2012)

Wheel-like assemblies are fascinating and exquisite. Anderson-type molecular wheels (structures on the periphery first proposed in 1937 and right structure) each feature six skew-edge-shared octahedra. In their Communication on page 2614 ff., C. M. Che and co-workers report selective binding of Cu+ or Ag+ by isolated [{M(S-4-R-C6H4)2(CO)2}6] (right structure to bottom-right structure) and isolation of [{M(SPh)2(CO)2}8] (bottom left), thus demonstrating feasible expansion of Anderson-type wheels to octanuclear congeners.

Molecular Wheels of Ruthenium and Osmium with Bridging Chalcogenolate Ligands: Edge-Shared-Octahedron Structures and Metal-Ion Binding

Inventing new wheels: reaction of [M(3)(CO)(12) ] (M=Ru, Os) with 4-RC(6)H(4)SH afforded [{M(S-4-RC(6)H(4))(2)(CO)(2)}(8)] (R=H; I) or [{M(S-4-RC(6)H(4))(2)(CO)(2)}(6)] (R=Me, iPr; II; see scheme), all of which have been structurally characterized. The octamers I are unique metal molecular wheels featuring skew-edge-shared octahedra with a central planar M(8) octagon. [{Ru(S-4-iPrC(6)H(4))(2)(CO)(2)}(6)] selectively binds a Cu(+) or Ag(+) ion to form [M'{Ru(S(4-iPr-C(6)H(4)))(2)(CO)(2)}(6)](+) (III).

Molecular Wheels of Ruthenium and Osmium with Bridging Chalcogenolate Ligands: Edge‐Shared‐Octahedron Structures and Metal‐Ion Binding

Räder neu erfunden: Die Reaktion von [M3(CO)12] (M=Ru, Os) mit 4-RC6H4SH ergibt die Produkte [{M(S-4-R-C6H4)2(CO)2}8] (R=H; I) oder [{M(S-4-R-RC6H4)2(CO)2}6] (R=Me, iPr; II; siehe Bild). Die Oktamere I sind molekulare Räder aus verdrillt-kantenverknüpften Oktaedern um ein zentrales planares M8-Achteck. Durch die selektive Bindung eines Cu+- oder Ag+-Ions an [{Ru(S-4-iPr-C6H4)2(CO)2}6] entsteht [M′{Ru(S(4-iPr-C6H4))2(CO)2}6]+ (III). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Valence isoelectronic substitution in the B8− and B9− molecular wheels by an Al dopant atom: Umbrella-like structures of AlB7− and AlB8−

The structures and the electronic properties of two aluminum-doped boron clusters, AlB(7)(-) and AlB(8)(-), were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB(7)(-) and AlB(8)(-) are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B(8)(-) and B(9)(-) planar clusters in AlB(7)(-) and AlB(8)(-). Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B(7) (C(6v)) and B(8) (C(7v)) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B(7)(3-) and planar B(8)(2-) molecular wheels derived from double aromaticity.

Inside Cover: A [Mn32] Double-Decker Wheel (Angew. Chem. Int. Ed. 19/2011)

A mixed-valent [Mn32] cluster with a very unusual “double-decker” wheel topology is described by E. K. Brechin, A. J. Tasiopoulos et al. in their Communication on page 4441 ff. It is by far the highest-nuclearity example of its type and one of the largest known manganese clusters, and it displays single-molecule-magnet behavior with the largest effective barrier to magnetization relaxation of any molecular wheel.

Molecular wheel to monocyclic ring transition in boron–carbon mixed clusters C2B6− and C3B5−

In this joint experimental and theoretical work we present a novel type of structural transition occurring in the series of C(x)B(8-x)(-) (x=1-8) mixed clusters upon increase of the carbon content from x=2 to x=3. The wheel to ring transition is surprising because it is rather planar-to-linear type of transition to be expected in the series since B(8), B(8)(-), B(8)(2-) and CB(7)(-) are known to possess wheel-type global minimum structures while C(8) is linear.

Molecular iron clusters, wheels and cages: Syntheses, structural aesthetics and magnetic properties

Structural features and synthetic strategies of iron clusters, wheels and cages of nuclearities from 6 to 64 are discussed. Synthetic methodologies are categorized in a few groups. Magnetic behaviours of the polynuclear iron species are also highlighted. Structural features and synthetic strategies of iron clusters, wheels and cages of nuclearities from 6 to 64 are discussed. Synthetic methodologies are categorized in a few groups. Magnetic behaviours of the polynuclear iron species are also highlighted. ► The polynuclear iron compounds act as Single Molecule Magnets (SMMs). ► These sometimes mimic the active sites of different biomolecules. ► The syntheses and structures are of utmost importance to chemists and biologists.

On the Analogy of B−BO and B−Au Chemical Bonding in B11O− and B10Au− Clusters

During photoelectron spectroscopy experiments, the spectra of B(11)O(-) and B(10)Au(-) clusters are found to exhibit similar patterns except for a systematic spectral shift of ∼0.5 eV, hinting that they possess similar geometric structures. The electron affinities are measured to be 4.02 ± 0.04 eV for B(11)O and 3.55 ± 0.02 eV for B(10)Au. DFT calculations at the B3LYP level show that B(11)O(-) and B(10)Au(-) adopt similar C(1) ((1)A) ground states, which are based on the quasiplanar B(10) cluster interacting with a BO unit and Au, respectively. The B(11)O(-) and B(10)Au(-) clusters are thus valent isoelectronic because both BO and Au can be viewed as monovalent units, forming highly covalent B-BO and B-Au bonds analogous to the B-H bond in B(10)H(-). For B(10)Au(-), we also find a highly symmetric D(10h) ((1)A(1g)) planar molecular wheel as a minimum on the potential energy surface. However, it is 45 kcal/mol above the ground state at the B3LYP level and not viable for experimental observation. Natural bond orbital analyses reveal interesting covalent versus ionic B-Au bonding in the C(1) B(10)Au(-) and D(10h) B(10)Au(-) structures, respectively, providing insight for the design of D(nh) MB(n) molecular wheels.

High-Frequency Electron-Spin-Resonance Study of the Octanuclear Ferric Wheel CsFe8

High-frequency (f = 190 GHz) electron paramagnetic resonance (EPR) at magnetic fields up to 12 T and Q-band (f = 34.1 GHz) EPR were performed on single crystals of the molecular wheel CsFe(8). In this molecule, eight Fe(III) ions, which are coupled by nearest-neighbor antiferromagnetic (AF) Heisenberg exchange interactions, form a nearly perfect ring. The angle-dependent EPR data allow for the accurate determination of the spin Hamiltonian parameters of the lowest spin multiplets with S ≤ 4. Furthermore, the data can be well reproduced by a dimer model with a uniaxial anisotropy term, with only two free parameters J and D. A fit to the dimer model yields J = -15(2) cm(-1) and D = -0.3940(8) cm(-1). A rhombic anisotropy term is found to be negligibly small, E = 0.000(2) cm(-1). The results are in excellent agreement with previous inelastic neutron scattering and high-field torque measurements. They confirm that the CsFe(8) molecule is an excellent experimental model of an AF Heisenberg ring. These findings are also important within the scope of further investigations on this molecule such as the exploration of recently observed magnetoelastic instabilities.

ChemInform Abstract: Synthesis and Structure of (Fe(OMe)2(O2CCH2Cl))10, a Molecular Ferric Wheel.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Weak antiferromagnetic coupling in molecular ring is predicted correctly by density functional theory plus Hubbard U

We apply density functional theory with empirical Hubbard U parameter (DFT+U) to study Mn-based molecular magnets. Unlike most previous DFT+U studies, we calibrate U parameters for both metal and ligand atoms using five binuclear manganese complexes as the benchmarks. We note delocalization of the spin density onto acetate ligands due to pi-back bonding, inverting spin polarization of the acetate oxygen atoms relative to that predicted from superexchange mechanism. This inversion may affect the performance of the models that assume strict localization of the spins on magnetic centers for the complexes with bridging acetate ligands. Next, we apply DFT+U methodology to Mn(12) molecular wheel and find antiparallel spin alignment for the weakly interacting fragments Mn(6), in agreement with experimental observations. Using the optimized geometry of the ground spin state instead of less accurate experimental geometry was found to be crucial for this good agreement. The protocol tested in this study can be applied for the rational design of single molecule magnets for molecular spintronics and quantum computing applications.

Molecular Wheels as Nanoporous Materials: Differing Modes of Gas Diffusion through Ga10 and Ga18 Wheels Probed by Hyperpolarized 129Xe NMR Spectroscopy

The study of crystals of molecular wheels as nanoporous materials is reported. Hyperpolarized (129)Xe NMR spectroscopy has been used to characterize the mode of molecular diffusion and Xe interactions within the supramolecular nanochannels formed upon crystallization of the molecular wheels [Ga(10)(OMe)(20)(O(2)CMe)(10)] and [Ga(18)(pd)(12)(pdH)(12)(O(2)CMe)(6)(NO(3))(6)](NO(3))(6). In agreement with expectations based on the collision diameter of the Xe atom relative to the differing internal diameters of the two types of gallium wheels, single-file diffusion occurs in the Ga(10) channels, whereas in the Ga(18) system the data are consistent with normal, Fickian diffusion. Information about the electronic environment inside the channels was probed by the Xe chemical shift. The interaction of the gas with the channel walls is found to be substantially stronger than the interaction in organic nanotubes and zeolites. The results establish the ability of crystals of molecular wheel compounds to function as a new class of porous nanotubular materials, and ones of a known and variable diameter, for studying the channel diameter dependence of molecular exchange and unidirectional diffusion on the micrometer length scale.

Shape-persistent rings and wheels

Shape-persistent macrocycles based on the arylene-ethynylene backbone are synthetically challenging targets that can be obtained in good to high yields either by statistical or by template-supported oxidative Glaser coupling of the corresponding bisacetylenes. The macrocycles can be adsorbed on highly oriented pyrolytic graphite (HOPG) to form well-ordered monolayers as visualized by scanning tunneling microscopy (STM). Moreover, bithiophene-containing macrocycles are able to bind fullerenes at the electron-rich sites of the rings. One-dimensional tubular structures based on shape-persistent macrocycles can also be obtained by oxidative acetylene coupling. These contain intraannularly bound conjugated polymers and represent bichromophoric systems that allow an accumulation of excitation energy on the conjugated core. In the field of defined 2D objects we describe molecular spoked wheels with a lateral expansion of more than 5 nm. These compounds are highly rigid. In order to provide sufficient solubility, they contain peripheral substituents and additional substituents orthogonal to the plane of the molecules.

Large All-Hydrocarbon Spoked Wheels of High Symmetry: Modular Synthesis, Photophysical Properties, and Surface Assembly

In a convergent modular synthesis, a very efficient pathway to shape-persistent molecular spoked wheels has been developed and applied according to the covalent-template concept. The structurally defined two-dimensional (2D) oligo(phenylene-ethynylene-butadiynylene)s (OPEBs) presented here are about 8 nm sized hydrocarbons of high symmetry. 48 alkyl chains attached to the molecular plane (hexyl and hexadecyl, respectively) guarantee a high solubility of the compounds. The structure and uniformity of these defined, stable, D(6h) symmetrical compounds is verified by MALDI-MS, GPC analysis, and high-temperature (HT) (1)H and (13)C NMR. Detailed photophysical measurements of nonaggregated molecules in solution (as confirmed by dynamic light scattering (DLS)) focus on the identification of chromophores by comparison with suitable model compounds. Moreover, time-resolved measurements including fluorescence lifetime and depolarization support the chromophore assignment and reveal the occurrence of intramolecular energy transfer. Scanning tunneling microscope (STM) characterization at the solid/liquid interface demonstrates the efficient self-assembly of the OPEBs into hexagonal 2D crystalline layers with a periodicity determined by both the size of the OPEB backbone and the length of peripheral side chains. Atomic force microscope (AFM) studies show a very different assembly behavior of the two spoked wheel molecules, on both graphite and mica. While the hexyl-substituted wheel can form stacked superstructures, hexadecyl groups prevent any ordering in the film aside from the monolayer directly in contact with the surface.