Giant molecular spoked wheels in giant voids: two-dimensional molecular self-assembly goes big

We present here the formation of giant pores in surface-confined molecular networks of a triangular-shaped dehydrobenzo-[12]annulene derivative: the diameter of the pores reaches over 7 nm and the giant pores are used as templates to accommodate a giant molecular spoked wheel, which allows us to observe rotational and adsorption-desorption dynamics of single guest molecules.     

Molecular Spoked Wheels: Synthesis and Self‐Assembly Studies on Rigid Nanoscale 2D Objects

We present the efficient synthesis of a new molecular spoked‐wheel structure ( MSW‐3 ). Two derivatives with diameters of approximately 4 nm have been prepared. By highlighting the importance of pseudo‐high‐dilution conditions during cyclization, we were able to access the compounds on a several hundred milligram scale. In addition to the standard characterization (NMR spectroscopy, MS), we describe a detailed investigation of the optical properties of the fluorescent MSWs by comparison with appropriate model chromophores. Furthermore, a comprehensive study of the structure in solution by means of light‐ and X‐ray scattering experiments has been conducted. Scanning tunneling microscopy (STM) revealed the two‐dimensional organization of the molecules on highly oriented pyrolytic graphite and emphasized the spoked‐wheel structure. The diameter of these molecules measured by small‐angle X‐ray scattering is in very good agreement with that obtained from STM and matches the results of molecular modeling. This confirms the rigidifying effect of the spokes, which results in highly shape‐persistent nanometer‐sized oblate organic compounds.     

Covalent Template-Directed Synthesis of a Spoked 18-Porphyrin Nanoring**

Rings of porphyrins mimic natural light-harvesting chlorophyll arrays and offer insights into electronic delocalization, providing a motivation for creating larger nanorings with closely spaced porphyrin units. Here, we demonstrate the first synthesis of a macrocycle consisting entirely of 5,15-linked porphyrins. This porphyrin octadecamer was constructed using a covalent six-armed template, made by cobalt-catalyzed cyclotrimerization of an H-shaped tolan with porphyrin trimer ends. The porphyrins around the circumference of the nanoring were linked together by intramolecular oxidative meso-meso coupling and partial β-β fusion, to give a nanoring consisting of six edge-fused zinc(II) porphyrin dimer units and six un-fused nickel(II) porphyrins. STM imaging on a gold surface confirms the size and shape of the spoked 18-porphyrin nanoring (calculated diameter: 4.7 nm).     

Synthetic and magnetic studies of a dodecanuclear cobalt wheel

The synthesis, structural characterisation and preliminary magnetic studies of a Co12 wheel are reported; the magnetic investigations reveal that the electronic ground state has a spin S = 6, which corresponds to ferromagnetic interactions between the twelve Co(II) ions.     

Excitation energy migration in a dodecameric porphyrin wheel

Intramolecular excitation energy hopping (EEH) time within a dodecameric porphyrin wheel C6ZA, in which six meso-meso linked zinc(II) diporphyrin (Z2) subunits are bridged by 1,3-phenylene spacers, is deduced by a F?rster energy hopping model based on S(1)-S(1) exciton-exciton annihilation and anisotropy depolarization. Under the assumption that the energy hopping sites are six Z2 subunits, two different observables (e.g., exciton-exciton annihilation and anisotropy depolarization times) consistently give the EEH time of 4.0 +/- 0.4 ps via 1,3-phenylene spacer of C6ZA, which is faster than 9.4 ps of linear 2Z2 (1,3-phenylene-linked zinc(II) tetraporphyrin). As a consequence, C6ZA serves as a well-defined two-dimensional model for a light-harvesting complex.     

Mercury(II) Site-Selective Binding to a DNA Hairpin. Relationship of Sequence-Dependent Intra- and Interstrand Cross-Linking to the Hairpin-Duplex Conformational Transition

Hg(II) interacted site selectively with only one of three deoxyribooligonucleotides examined; these "oligos" each had a different number of unmatched T residues. Thus, Hg(II) formed an intrastrand T-Hg-T cross-link between the first and fourth T residues of the hairpin, d(GCGCTTTTGCGC) (T4). The DNA strand formed a loop around the Hg, as if the Hg atom had been lassoed. The interactions of Hg(II) with two other oligos, d(ATGGGTTCCCAT) (T2) and d(GCGCTTTGCGC) (T3), were less specific. Previously, we found that at high DNA and salt concentrations, T2 was a mixture of hairpin and duplex forms while T3 and T4 had the hairpin form; modeling studies showed that in the free T4 hairpin the two T's at the ends of the (T)(4) loop form a T.T wobble base pair. Only in T4 are the T residues positioned to form an intrastrand cross-link readily. The Hg(II)-oligo adducts formed as a function of added Hg(II) were investigated by titrations monitored by UV, CD, and (1)H NMR spectroscopy. The appearance of a new set of (1)H signals with the concomitant decay of the free oligo (1)H signals indicated that 1:1 Hg(II):T2, 1.5:1 Hg(II):T3, and 1:1 Hg(II):T4 adducts were formed with Hg(NO(3))(2). In H(2)O, these adducts all had spectra with very downfield signals for the exchangeable TN(3)H and GN(1)H groups, a characteristic of base-paired regions. All upfield N(3)H signals from the (T)(2) and (T)(3) sequences of the free oligo disappeared in the spectra of the 1:1 Hg(II):T2 and 1.5:1 Hg(II):T3 adducts. The disappearance of the NH signals, the UV spectral changes, and the stoichiometries (1:1 Hg(II):T2 and 1.5:1 Hg(II):T3) indicate that these adducts are duplexes containing two and three T-Hg-T interstrand cross-links for T2 and T3, respectively. The (1)H and (13)C signals of the 1:1 Hg(II):T4 adduct in D(2)O were nearly completely assigned by 2D NMR spectroscopy. The spectrum of the adduct in H(2)O had only two of the four original TN(3)H signals from the (T)(4) sequence present in the spectrum of T4; this result is consistent with the presence of a TN3-Hg-TN3 cross-link. The (13)C chemical shift changes upon Hg(II) binding indicated that the TN3-Hg-TN3 cross-link was between the T's at each end of the (T)(4) loop. The NOESY, CD, and UV spectra were all consistent with a hairpin conformation for the 1:1 Hg(II):T4 adduct. A hairpin conformation also appeared reasonable from molecular modeling calculations. In conclusion, the length of the central (T)(n)() sequence influenced the type of T-Hg-T cross-link formed and, in turn, the conformation of the adducts. For (T)(2) and (T)(3), interstrand T-Hg-T cross-linking favored the duplex form. In contrast, for (T)(4), intrastrand T-Hg-T cross-linking stabilized the hairpin form.     

Towards technomimetic spoked wheels: dynamic hexakis-heteroleptic coordination at a hexakis-terpyridine scaffold

The synthesis of hexakis-terpyridine and an expedient approach to its dynamic hexakis-heteroleptic complexes are elaborated, the latter being readily accessible precursors for the construction of technomimetic molecular spoked wheels.     

Linking [2]rotaxane wheels to create a new type of metal organic rotaxane framework

Three new [2]rotaxanes with aromatic nitrogen donors appended to the crown ether wheel have been synthesized and used as ligands to coordinate Cd(II) ions. One of these yields a new type of 2-periodic, metal organic rotaxane framework in which the wheel rather than the axle is used to link the metal nodes.     

Spin transition at the mesophase transition temperature in a cobalt(II) compound with branched alkyl chains

A cobalt(II) compound, [Co(C5C12C10-terpy)2](BF4)2 [C5C12C10-terpy = 4',5' '-decyl-1' '-(heptadecyloxy)-2,2':6',2' '-terpyridine] with branched alkyl chains, based on a terpyridine frame, was synthesized. The cobalt(II) compound exhibits a spin transition between low-spin and high-spin with a thermal hysteresis loop (T(1/2) upward arrow = 288 K and T(1/2) downward arrow = 284 K) at the liquid-crystal transition temperature. It is the first example in the cobalt(II) compounds in which the spin transition occurs at the crystal-liquid crystal transition temperature.     

Hg(II) ion specifically binds with T:T mismatched base pair in duplex DNA

Metal-mediated base pair formation, resulting from the interaction between metal ions and artificial bases in oligonucleotides, has been developed for its potential application in nanotechnology. We have recently found that the T:T mismatched base pair binds with Hg(II) ions to generate a novel metal-mediated base pair in duplex DNA. The thermal stability of the duplex with the T-Hg-T base pair was comparable to that of the corresponding T:A or A:T. The novel T-Hg-T base pair involving the natural base thymine is more convenient than the metal-mediated base pairs involving artificial bases due to the lack of time-consuming synthesis. Here, we examine the specificity and thermodynamic properties of the binding between Hg(II) ions and the T:T mismatched base pair. Only the melting temperature of the duplex with T:T and not of the perfectly matched or other mismatched base pairs was found to specifically increase in the presence of Hg(II) ions. Hg(II) specifically bound with the T:T mismatched base pair at a molar ratio of 1:1 with a binding constant of 10(6) M(-1), which is significantly higher than that for nonspecific metal ion-DNA interactions. Furthermore, the higher-order structure of the duplex was not significantly distorted by the Hg(II) ion binding. Our results support the idea that the T-Hg-T base pair could eventually lead to progress in potential applications of metal-mediated base pairs in nanotechnology.     

A novel isopolytungstate functionalized by ruthenium: [HW(9)O(33)Ru(II)(2)(dmso)(6)](7-)

The ruthenium-supported isopolyanion [HW(9)O(33)Ru(II)(2)(dmso)(6)](7-) (1) is composed of a nonatungstate wheel stabilized by two Ru(dmso)(3) groups, representing the first structurally characterized Ru-coordinated polyoxotungstate and a novel class of isopolyanions supporting photochromic moieties.     

Dual nanomolar and picomolar Zn(II) binding properties of metallothionein

Each of the seven Zn(II) ions in the Zn(3)S(9) and Zn(4)S(11) clusters of human metallothionein is in a tetrathiolate coordination environment. Yet analysis of Zn(II) association with thionein, the apoprotein, and analysis of Zn(II) dissociation from metallothionein using the fluorescent chelating agents FluoZin-3 and RhodZin-3 reveal at least three classes of sites with affinities that differ by 4 orders of magnitude. Four Zn(II) ions are bound with an apparent average log K of 11.8, and with the methods employed, their binding is indistinguishable. This binding property makes thionein a strong chelating agent. One Zn(II) ion is relatively weakly bound, with a log K of 7.7, making metallothionein a zinc donor in the absence of thionein. The binding data demonstrate that Zn(II) binds with at least four species: Zn(4)T, Zn(5)T, Zn(6)T, and Zn(7)T. Zn(5)T and Zn(6)T bind Zn(II) with a log K of approximately 10 and are the predominant species at micromolar concentrations of metallothionein in cells. Central to the function of the protein is the reactivity of its cysteine side chains in the absence and presence of Zn(II). Chelating agents, such as physiological ligands with moderate affinities for Zn(II), cause dissociation of Zn(II) ions from metallothionein at pH 7.4 (Zn(7)T <==> Zn(7-n)T + nZn(2+)), thereby affecting the reactivity of its thiols. Thus, the rate of thiol oxidation increases in the presence of Zn(II) acceptors but decreases if more free Zn(II) becomes available. Thionein is such an acceptor. It regulates the reactivity and availability of free Zn(II) from metallothionein. At thionein/metallothionein ratios > 0.75, free Zn(II) ions are below a pZn (-log[Zn(2+)](free)) of 11.8, and at ratios < 0.75, relatively large fluctuations of free Zn(II) ions are possible (pZn between 7 and 11). These chemical characteristics match cellular requirements for Zn(II) and suggest how the molecular structures and redox chemistries of metallothionein and thionein determine Zn(II) availability for biological processes.     

Solvatomagnetism-induced Faraday effect in a cobalt hexacyanochromate-based magnet

Solvent exchange caused reversible variations in color, magnetic properties, and the Faraday spectra of Co(II)(1.5)[Cr(III)(CN)(6)].7.5H2O (1) prepared in water. Compound 1 turned from peach to deep blue, which was due to a change in the coordination geometry on Co(II) ion from six-coordinate pseudo-octahedral (OhCo(II)) to four-coordinate pseudo-tetrahedral (TdCo(II)) geometries, when it was immersed in EtOH. The confirmed formula for the deep blue powder was Co(II)(1.5)[Cr(III)(CN)(6)].2.5H2O.2.0EtOH. The magnetic properties also changed; that is, the magnetic critical temperature, saturation magnetization, and coercive field went from 25 to 18 K, from 7.0 to 5.5 micro(B), and from 240 to 120 G, respectively. This solvatomagnetism is because the ferromagnetic magnetic coupling between OhCo(II) (S = 3/2) and Cr(III) (S = 3/2) is replaced by the antiferromagnetic coupling between TdCo(II) (S = 3/2) and Cr(III) (S = 3/2). Accompanying the solvatochromism and solvatomagnetism, the Faraday spectra drastically changed. The Faraday ellipticity (FE) spectrum of 1 had a distorted dispersive peak (A), which is due to the 4T1g --> 4T1g, 2T1g transitions of OhCo(II) ion, around 480 nm, but the FE spectra of 2 showed a new dispersive-shaped band (B) at 580 nm. The observed B band was assigned to the 4A2 --> 4T2 transition of the TdCo(II) ion. The Faraday spectra were well reproduced by a simulation that considers the ligand field splitting, spin-orbital coupling, and the ferromagnetic ordering. These solvatochromic effects were repeatedly observed.     

Cation-controlled self-assembly of a hexameric europium wheel

The simple asymmetric tetradentate ligand 2,2':6',2' '-terpyridine-6-carboxylic acid leads to the self-assembly of the first europium nanowheel containing europium ions in two different coordination environments. Moreover the self-assembly of bis(terpyridinecarboxylate) europium species to form a hexameric wheel capable of strongly binding LnIII cations is controlled by the ligand/cation ratio.     

Effects of DNA Probe Length on the Performance of a Dynamics‐based Electrochemical Hg(II) Sensor

We investigated the effects of DNA probe length on the performance of a dynamics‐based electrochemical metal ion (E‐ION) Hg(II) sensor. A systematic comparison of three versions of the Hg(II) sensor fabricated using oligothymine (oligo‐T) probes of different lengths (6, 12, and 18 bases) is presented here. Independent of the probe length, the sensing mechanism of the sensor remains the same. It is based on the specific interactions between Hg(II) and thymine (T), formation of the T‐Hg(II)‐T complexes rigidifies the methylene blue (MB)‐modified oligo‐T probes, resulting in a concentration‐dependent reduction in the MB signal. Although there are noted differences in sensor characteristics such as the limit of detection and dynamic range, all three sensors have demonstrated to be specific and selective. Thus, depending on the specific sensor properties that are required for the analysis, a shorter or longer oligo‐T probe should be employed. With further optimization, this sensor could find applications in real time detection of Hg(II) in environmental samples.     

A large thermal hysteresis loop produced by a charge-transfer phase transition in a rubidium manganese hexacyanoferrate

In a rubidium manganese hexacyanoferrate, RbMn[Fe(CN)(6)], the magnetic susceptibility (chi(M)) decreased at 225 K (=T(1/2)decreasing) and abruptly increased at 300 K (=T(1/2)increasing) in the cooling and warming processes, respectively. X-ray photoelectron spectroscopy and infrared spectroscopy indicated that the high-temperature (HT) and low-temperature (LT) phases were composed of Mn(II)-NC-Fe(III) and Mn(III)-NC-Fe(II), respectively. A structural change from cubic (F43m, a = 10.533 A) to tetragonal (I4m2, a = b = 7.090 A, c = 10.520 A) accompanied the phase transition, and, on the basis of these results, the HT and LT phases were assigned to Mn(II)(t(2g)(3)e(g)(2), (6)A(1g); S = (5)/(2))-NC-Fe(III) (t(2g)(5), (2)T(2g); S = (1)/(2)) and Mn(III)(e(g)(2)b(2g)(1)a(1g)(1), (5)B(1g); S = 2)-NC-Fe(II) (b(2g)(2)e(g)(4), (1)A(1g); S = 0), respectively. This phenomenon is caused by a metal-to-metal charge transfer from Mn(II) to Fe(III) and a Jahn-Teller distortion of the produced Mn(III) ion. The reaction mechanism is discussed, considering the entropy difference between the HT and LT phases.     

Solvent induced cooperativity of Zn(II) complexes cleaving a phosphate diester RNA analog in methanol

The kinetics of cyclization of 2-hydroxypropyl p-nitrophenyl phosphate (1) promoted by two mononuclear Zn(II) catalytic complexes of bis(2-pyridylmethyl)benzylamine (4) and bis(2-methyl 6-pyridylmethyl)benzylamine (5) in methanol were studied under (s)(s)pH-controlled conditions (where (s)(s)pH refers to [H(+)] activity in methanol). Potentiometric titrations of the ligands in the absence and presence of Zn(2+) and a non-reactive model for 1 (2-hydroxylpropyl isopropyl phosphate (HPIPP, 6)) indicate that the phosphate is bound tightly to the 4:Zn(II) and 5:Zn(II) complexes as L:Zn(II):6(-), and that each of these undergoes an additional ionization to produce L:Zn(II):6(-):((-)OCH(3)) or a bound deprotonated form of the phosphate, L:Zn(II):6(2-). Kinetic studies as a function of [L:Zn(II)] indicate that the rate is linear in [L:Zn(II)] at concentrations well above those required for complete binding of the substrate. Plots of the second order rate constants (defined as the gradient of the rate constant vs. [complex] plot) vs. (s)(s)pH in methanol are bell-shaped with rate maxima of 23 dm mol(-1) s(-1) and 146 dm mol(-1) s(-1) for 4:Zn(II) and 5:Zn(II), respectively, at their (s)(s)pH maxima of 10.5 and 10. A mechanism is proposed that involves binding of one molecule of complex to the phosphate to yield a poorly reactive 1 : 1 complex, which associates with a second molecule of complex to produce a transient cooperative 2 : 1 complex within which the cyclization of 1 is rapid. The observations support an effect of the reduced polarity solvent that encourages the cooperative association of phosphate and two independent mononuclear complexes to give a reactive entity.     

Synthesis, redox, and magnetic properties of a neutral, mixed-valent heptanuclear manganese wheel with S = 27/2 high-spin ground state

Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads to the formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {MnII subset[MnII2MnIII4Cl6(L3)6]} (4). The manganese wheel crystallizes in the triclinic space group P as 4.2CHCl3 or 4.3THF when either diethyl ether or n-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of each manganese ion in 4.2CHCl3 or 4.3THF were assigned on the basis of detailed symmetry, bond length, and charge considerations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and were further supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4.2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagnetic exchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higher than 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements on single crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and by high-frequency electron spin resonance measurements on a polycrystalline powder of 4.2CHCl3 (D = -0.068 K, E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U approximately 10 K. Finally, 2DEG Hall magnetization measurements revealed that 4.2CHCl3 shows single-molecule magnet behavior up to the blocking temperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of the magnetization.     

Synthesis and spectroscopic and magnetic characterization of tris(3,5-dimethylpyrazol-1-yl)borate iron tricyanide building blocks, a cluster, and a one-dimensional chain of squares

The synthesis and spectroscopic and magnetic characterization of several hydridotris(3,5-dimethylpyrazol-1yl)borate (Tp*) iron(II) and iron(III) tricyanide complexes, a rectangular cluster, and a one-dimensional chain of squares are described. Treatment of [NEt4][(Tp*)Fe(III)(CN)3] (3) with manganese(II) triflate in dimethylformamide (DMF) affords rectangular clusters (6, {[(Tp)Fe(CN)2(mu-CN)Mn(DMF)4]2[OTf]2}.2DMF), while tosylate salts afford one-dimensional networks (5, {Mn(II)(DMF)2(mu-OTs)(mu-NC)2(NC)Fe(III)(Tp*)}n) containing embedded [(Tp*)2Fe(III)2Mn(II)2(CN)6]2+ clusters via in situ trapping; the cluster and network crystallize in the monoclinic (6, P2(1)/n) and triclinic (5, P1) space groups, respectively. The 1-D network (5) appears to be derived from {cis-(mu-O3SC6H4Me)2Mn(II)(DMF)4}n (4, P2(1)/n), which is obtained via crystallization of Mn(OTs)2 from DMF/Et2O mixtures. For 4, magnetic studies indicate that the Mn(II) centers are magnetically isolated, with calculated J, g, and theta constants of 6.7 x 10(-3) cm(-1), 2.03, and -0.52 K. Additional magnetic studies of 5 and 6 indicate that the [(Tp*)Fe(III)(CN)3]- centers are highly anisotropic (g = 2.9) and are antiferromagnetically coupled to adjacent Mn(II) centers. For 5 and 6, fitting of the chiT vs T data via the Curie-Weiss expression affords Curie (6.25 and 10.8 cm(3) K mol(-1)) and Weiss (-14.37 and -8.80 K) constants that are consistent with antiferromagnetically coupled low-spin Fe(III) and high-spin Mn(II) centers; least-squares fitting of the chiT vs T data using molecular field theory affords g(avg.), J1, J2, and J' values of 2.25, -1.72, -0.58, and -0.12 cm(-1) for 5. Overall, bridging tosylates appear to be poor communicators of spin information. For 6, the g, J1, and J2 (2.15, -2.02, and -0.78 cm(-1)) values were obtained via least-squares fitting of the chiT vs T data using an expression derived using the Kambe vector coupling method; simulations of the data via MAGPACK afford g(avg.) and J(iso) values of 2.1 and -2.1 cm(-1).