Theoretical study of the magnetic behavior of ferric wheels.

Calculations of exchange coupling constants (J) based on density functional theory for eight complete, nonmodeled ferric wheels have been performed, and a comparison with the values obtained from magnetic susceptibility data is presented. The calculated J values obtained with a generalized-gradient approximation (GGA) functional are in good agreement with the experiment probably because the inclusion of pseudopotentials partially compensates the overestimation of the spin delocalization. The magnetostructural correlation obtained shows a strong dependence of the exchange coupling on both the Fe-O-Fe bond angle and the Fe-O bond distance. This correlation holds for both the ferric wheels and the alkoxo-bridged Fe(III) dinuclear complexes reported in the literature.     

Informatics and magnetic behavior of [Fe6S6]3+ superclusters

Theoretical study on informational and magnetic behavior of a trivalent iron ion in the iron–sulfur supercluster with six Fe centers was performed based on Heisenberg exchange and double exchange (delocalisation) model. The three-valent supercluster can be modeled by three Fe(III) and three Fe(II) ions arranged within an experimental structure of prismane. Energy spectrum of the system has been calculated numerically, depending on the Heisenberg exchange J _i and double exchange b parameters. Such knowledge allows us to obtain an entropy function of the considered supercluster that makes possible effective analysis from the informatics point of view. Values of von Neumann entropy and total spin energy have been calculated for the case of fully antiferromagnetic as well as partially ferromagnetic spin interactions. On the ground of the theoretical studies, possible interpretations of available experimental EPR data for the (Et₄N)₃Fe₆S₆Cl₆CH₃Cl compound are proposed.     

Synthesis and magnetic properties of wheel-shaped [Mn12] and [Fe6] complexes

The reactions of the ligands N-methyldiethanol amine and N-ethyldiethanol amine (abbreviated H(2)mdea and H(2)edea, respectively) with [Mn(12)O(12)(O(2)CCH(3))(16)(H(2)O)(4)] yield novel dodecanuclear wheel-shaped products. The capability of the ligands H(2)mdea and H(2)edea to support wheel structures in metals other than Mn is demonstrated with the crystal structure of a new hexanuclear ferric wheel.     

2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane frameworks containing tetra-substituted dibenzo[24]crown-8 wheels

[2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane framework materials that utilise DB24C8 as the wheel component are well known and structural variations based on changing the axle component are common. Studies in which the DB24C8 wheel is structurally modified are much more limited. Herein, is described the synthesis of symmetrical DB24C8 analogues containing four CH(2)OR (R = CH(2)CH(2)CH(3), CH(2)(C(6)H(5)), C(6)H(5) and C(6)H(4)(4-COOEt)) substituents on the 4 and 5 positions of the aromatic rings. The effect of these molecular appendages on the stability and structures of the interpenetrated and interlocked molecules derived from these new wheels is described. The major effects are an increase in association constants for the formation of [2]pseudorotaxanes relative to DB24C8, the crystal packing of [2]rotaxanes and a change on the internal structure of a 2D MORF (R = C(6)H(5)) compared to DB24C8.     

Syntheses,structures and magnetic properties of Fe6 and Fe12 ferric wheels

Starting from the same Fe3O(RCO2)6 complex and using a similar synthesis strategy, the Fe6 and Fe12 wheels were obtained by utilizing two designed ligands, 2-amino-2-methyl-1,3-propanediol(L1) and 1,3-propanediol(L2), respectively. A biological buffer reagent, Bis-Tris(2-[bis-(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol) was first introduced in the synthesis of the Fe12 wheel, playing a vital role in adjusting the acidity of reaction environment. Magnetic studies on both Fe6 and Fe12 wheels revealed strong antiferromagnetic coupling between the spins on FeIII ions.     

Theoretical study of the [10]-annulenes geometry and stability

Five structures of [10]-annulene, proposed in the literature, namely the naphthalenic, planar, boat and helix forms have been studied by EHT, CNDO/2, MINDO/2 methods and the empirical method of Dashevskyi. Optimum geometries of the single structures were determined either from energy curves or from the coplanarity condition for bond groups of the ethylenic type. The relative stabilities of the structures were established by comparing their heats of formation with the values of the other stable cyclic polyenes. The most favourable boat form appears as an antiaromatic inverse to benzene.     

Theoretical study of the second-order nonlinear optical properties of [N]helicenes and [N]phenylenes

The second-order nonlinear optical properties of helicenes and phenylenes have been theoretically investigated at the time-dependent Hartree-Fock level using the Austin model 1 semiempirical Hamiltonian. Both the antisymmetric isotropic component of the first hyperpolarizability (beta) and its projection on the dipole moment (beta(parallel)) have been determined for increasingly large helical systems as well as for their analogs substituted by donor/acceptor pairs. It is found that (i) in nonsubstituted helicenes and phenylenes, beta increases monotonically with the size of the system and slightly depends on the nature of the helix; (ii) the corresponding beta(parallel) is mostly determined by the radial component of the first hyperpolarizability vector; (iii) in helicenes, beta(parallel) is positive and presents quasiperiodic oscillations with the helix; (iv) in phenylenes, beta(parallel) depends upon the size of the helix and it can be either positive or negative as a result of the differences in evolution with N of the radial components of the dipole moment and first hyperpolarizability. Substituting the helicenes and phenylenes by the prototypical NH2/NO2 donor/acceptor pair provides a diversity of effects on beta and beta(parallel) that encompasses decrease, increase, and change in sign.     

High-yield syntheses and reactivity studies of Fe10 "ferric wheels": structural, magnetic, and computational characterization of a star-shaped Fe8 complex

Convenient, high-yield routes have been developed to [Fe 10(OMe) 20(O 2CR) 10] ( 1) "ferric wheels" involving the alcoholysis of [Fe 3O(O 2CR) 6(H 2O) 3] (+) salts in MeOH in the presence of NEt 3. Reactivity studies have established [Fe 10(OMe) 20(O 2CMe) 10] ( 1a) to undergo clean carboxylate substitution with a variety of other RCO 2H groups to the corresponding [Fe 10(OMe) 20(O 2CR) 10] product. In contrast, the reaction with phenol causes a nuclearity change to give a smaller [Fe 8(OH) 4(OPh) 8(O 2CR) 12] ( 2) wheel. Similarly, reactions of [Fe 10(OMe) 20(O 2CR) 10] with the bidentate chelate ethylenediamine (en) cause a structural change to give either [Fe 8O 5(O 2CMe) 8(en) 8](ClO 4) 6 ( 3) or [Fe 2O(O 2CBu (t))(en) 4](NO 3) 3 ( 4), depending on conditions. Complex 3 possesses a "Christmas-star" Fe 8 topology comprising a central planar [Fe 4(mu 4-O)] (10+) square subunit edge-fused to four oxide-centered [Fe 3(mu 3-O)] (7+) triangular units. Variable-temperature, solid-state dc and ac magnetization studies on complexes 1a- 4 in the 5.0-300 K range established that all the complexes possess an S = 0 ground state. The magnetic susceptibility data for 4 were fit to the theoretical chi M versus T expression derived by the use of an isotropic Heisenberg spin Hamiltonian and the Van Vleck equation, and this revealed an antiferromagnetic exchange parameter with a value of J = -107.7(5) cm (-1). This value is consistent with that predicted by a previously published magnetostructural relationship. Theoretically computed values of the exchange constants in 3 were obtained with the ZILSH method, and the pattern of spin frustration within its core and the origin of its S = 0 ground state have been analyzed in detail.     

Theoretical study on the protonation of bambus[6]uril

Abstract  

Quantum mechanical density functional theory (DFT) calculations were used to derive the most probable structures of the bambus[6]uril·H₃O⁺ and bambus[6]uril·(H₃O⁺)₂ cationic complex species. In these two complexes, each of the considered H₃O⁺ ions is bound by three strong linear hydrogen bonds to the three corresponding carbonyl oxygens of the parent macrocyclic receptor.     

Spin densities in dialkoxy-[16]annulene anion radicals: dimerization of alkoxy-[8]annulenes

The anion radicals of alkoxy-substituted cyclooctatetraenes in hexamethylphosphoramide spontaneously dimerize to form the dianions of dialkoxy-[16]annulenes. The dianions reveal the expected high-field NMR resonance for the internal protons. After electron transfer, the EPR spectra of the corresponding anion radicals reveal that only the 1,5-dialkoxy systems are formed. Further, the measured proton and (13)C spin densities show that the odd electron resides in a molecular orbital with six hydrogens in "deep" nodal positions that completely hide them from EPR detection. This MO corresponds to the nonbonding (singly occupied) MO of higher energy after splitting of the degenerate nonbonding MOs by the two-electron-withdrawing substituents. The surprising electron-withdrawing nature of the alkoxy substituents is attributed to a rather strong mixing of the sigma and pi systems in [16]annulene.     

Computational study on the electrocyclic reactions of [16]annulene

The electrocyclic reactions of [16]annulene have been investigated by ab initio and DFT calculations. Among the six conformers of [16]annulene identified, 3, with Cs symmetry, is taken as the starting reactant for the cyclization reactions of [16]annulene, even though it is 31.4 kJ mol(-1) less stable than the most stable conformer, 1. The pathways of the electrocyclic reactions from reactant 3 to two tricyclic products, 5 and 6, have been found. All pathways identified are stepwise, i.e., the two ring closure processes occur one after the other. Among the pathways found, the ones with the lowest overall barrier for reactions 3 --> 5 and 3 --> 6 have the same rate-determining step and hence the same overall barrier, 131.0 kJ mol(-1). Thus, based on the barriers calculated, it is not possible to determine whether 5 or 6 is the dominant product in the cyclization reaction of 3.     

Theoretical study on the protonation of cucurbit[6]uril

Abstract  

The most probable structures of the cucurbit[6]uril·H₃O⁺ and cucurbit[6]uril·(H₃O⁺)₂ cationic complex species have been derived by quantum mechanical DFT calculations. In these two complexes, each of the H₃O⁺ ions is bound by three strong linear hydrogen bonds to three carbonyl oxygen atoms of the parent macrocycle.     

Excitonically Coupled Cyclic BF2 Arrays of Calix[8]- and Calix[16]phyrin as Near-IR-Chromophores

Two giant calix[n]phyrin derivatives namely calix[8]- ( 4 ) and calix[16]phyrin ( 5 ), involving two and four BF₂ units, respectively, were prepared through the condensation of the bis-naphthobipyrrolylmethene-BF₂ complex ( 3 ) with pentafluorobenzaldehyde. Calix[n]phyrins 4 and 5 display extremely high extinction coefficients (3.67 and 4.82×10⁵ m ⁻¹ cm⁻¹, respectively) in the near-IR region, which was taken as initial evidence for strong excitonic coupling within these cyclic multi-chromophoric systems. Detailed insights into the effect of excitonic coupling dynamics on the electronic structure and photophysical properties of the macrocycles came from fluorescence, time-correlated single-photon counting (TCSPC) and transient absorption (TA) measurements. Support for these experimental findings came from theoretical studies. Theory and experiment confirmed that the coupling between the excitons depends on the specifics of the calix[n]phyrin structure, not just its size.     

Self-Assembly of an [M8L24]16+ Intertwined Cube and a Giant [M12L16]24+ Orthobicupola

Through coordination-driven self-assembly, aesthetically captivating structures can be formed by tuning the length or flexibility of various components. The self-assembly of an elongated rigid terphenyl-based tetra-pyridyl ligand ( L1 ) with a cis-Pd(II) acceptor produces an [M₁₂ L1 ₆]²⁴⁺ triangular orthobicupola structure ( 1 ). When flexibility is introduced into the ligand by the incorporation of a -CH₂- group between the dipyridylamine and terphenyl rings in the ligand ( L2 ), anunique [M₈ L2 ₄]¹⁶⁺ water-soluble ‘intertwined cubic structure’ ( 2 ) results. The inherent flexibility of ligand L2 might be the key factor behind the formation of the thermodynamically stable and ′intertwined cubic structure′ in this scenario. This research showcases the ability to design and fabricate novel, topologically distinctive molecular structures by a straightforward and efficient approach.     

Trisdehydro[14]annuleno[16]annulene

Trisdehydro[14]annuleno[16]annulene consisting of an aromatic bisdehydro[14]annulene and an antiaromatic bisdehydro[16]annulene has been synthesized. The strong paratropicity was observed in the [16]annulene moiety being comparable with that of extremely unstable parent bisdehydro[16]annulene.     

Tetrakisdehydro[16]annuleno[18]annulene

Tetrakisdehydro[16]annuleno consisting of trisdehydro[16]annulene and bisdehydro[18] annulene has been synthesized. Induction of para- and diamagnetic ring currents in 16- and 18-membered rings, respectively, was clearly recognized by the ¹H NMR spectroscopy. A marked suppresion of the diatropicity in the 18π moiety was observed in the same trend as observed in tetrakisdehydro[14]annuleno[16]annulene.