Control of Relative Direction and Amplitude in Extension/Contraction Motions of Molecular Strands Induced by Ion Binding

The shape of ligand strands composed of six‐membered aza‐heterocycles (het) connected at the α and α′ positions by hydrazone (hyz) units is determined in a predictable fashion by the nature of the heterocyclic groups (pyridine, pyrimidine, pyrazine etc.), and covers the range from extended linear to compact helical structures. The binding of metal ions to the coordination subunits, defined by the het‐hyz sequences, leads to marked shape changes by inter‐converting bent and linear conformations of the subunits, thus inducing relative motions of strand domains either in the same (con‐sense, “twirling”) or in opposite (dis‐sense, “flapping”) directions. The amplitude of the motion induced by metal‐ion binding and release and the relative directions of the formal motions can be controlled by the nature of the heterocyclic units. Thus, motions around a central 4,6‐disubstituted pyrimidine are dis‐sense motions, whereas there are con‐sense motions around a central 2,5‐disubstituted pyrazine unit, as illustrated by model ligands 1 and 2 , respectively. The more extended helical 3 and undulating (zigzag shape) 4 ligands undergo larger‐amplitude motions combining the relative displacements displayed by 1 and 2 . Ligands 3 and 4 form linear tetranuclear Pb^II and Zn^II complexes, thus producing an extension motion. The same holds for [Ru( 4 )(terpy)₄](PF₆)₈ (terpy=terpyridine). Reversible acid–base‐triggered molecular motions have been generated with [Zn₄( 4 )(OTf)₈] (TfOH=triflic acid).     

Five coordination modes of 4-aminopyrimidine with N-hydroxy-ethylethylenediaminetriacetatoruthenate(II)

Summary The complex [Ru^II(hedta)(4NH₂pym)]⁻, hedta³⁻ = N-hydroxyethylethylenediaminetriacetate, 4NH₂pym = 4-aminopyrimidine, exists at pH 7 as five different coordination isomers, which are most readily distinguished by their electrochemical waves in comparison with the 2-aminopyridine (2NH₂py) complex. The 2NH₂py complex exhibits N(1) (pyridine bound), exo-NH₂ (amine bound) and N(1), NH₂-chelated species. The 4NH₂pym complex forms N(1), exo-amine and N(3), NH₂-chelated isomers analogues to the 2NH₂py species, but also engages in η² (olefin bound) coordination of the dearomatized 4NH₂pym ring in C(5)–C(6), and another η² type of complex involving electron density between N(1) and N(3) of the ring (η³ form). N(1), η² and η³ isomers have also been detected for unsubstituted pyrimidine (pym), 4-methylprimidine (4CH₃pym) and 2-aminopyrimidine (2NH₂pym). Electrochemical waves (V versus NHE) for the five isomers are assigned as follows: (Ru^II/III) exo-NH₂ (0.06 V), N(1) (0.29 V), η² (0.49 V); (Ru^II/III) η³ (0.76 V); N(3), NH₂-chelated (1.09 V).     

Solid-state self-assembly of polymeric double helicates leading to linear arrays of silver(I) ions and reversible strand/double helix interconversion in solution

Reaction of a bent py-hyz-pym-hyz-pym 1 and of a linear py-hyz-py-hyz-pym 3 (py=pyridine; pym=pyrimidine; hyz=hydrazone) ligand strands with silver(I) tetrafluoroborate in CH(3)NO(2) generates double-helical dinuclear 2 and trinuclear 4 complexes. These complexes form polymeric, highly ordered solid-state structures, with wirelike, linear continuous or discontinuous polycationic Ag(n) (+) arrays with Ag--Ag distances of 2.78 to 4.42 A. Ligand 5, an isomer of 1, is found to yield a [2x2] grid-type complex 6. Titration experiments reveal the formation of linear rack-type dinuclear species from 1 and 5. Acid-base modulated, reversible interconversion between strand 1 and double helicate 2 may be achieved by using tren as a competing complexing agent (tren=N(CH(2)CH(2)NH(2))(3)). Progressive addition of silver(I) ions to a 1:1 mixture of 1 and 5 leads to the preferential formation of the double helicate 2 over the grid complex 6, illustrating a process of self-organisation with selection of the correct ligand.     

Structural Basis for the Identification of an i‐Motif Tetraplex Core with a Parallel‐Duplex Junction as a Structural Motif in CCG Triplet Repeats

CCG triplet repeats can fold into tetraplex structures, which are associated with the expansion of (CCG)_n trinucleotide sequences in certain neurological diseases. These structures are stabilized by intertwining i‐motifs. However, the structural basis for tetraplex i‐motif formation in CCG triplet repeats remains largely unknown. We report the first crystal structure of a CCG‐repeat sequence, which shows that two dT(CCG)₃A strands can associate to form a tetraplex structure with an i‐motif core containing four C:C⁺ pairs flanked by two G:G homopurine base pairs as a structural motif. The tetraplex core is attached to a short parallel‐stranded duplex. Each hairpin itself contains a central CCG loop in which the nucleotides are flipped out and stabilized by stacking interactions. The helical twists between adjacent cytosine residues of this structure in the i‐motif core have an average value of 30°, which is greater than those previously reported for i‐motif structures.     

Lanthanoid‐Peroxo‐Komplexe mit μ3‐η2:η2:η2‐(O22—)‐Koordination. Die Kristallstrukturen von [Ln4(O2)2Cl8(Py)10] · Py mit Ln = Sm,Eu, Gd

Lanthanoid Peroxo Complexes with μ₃‐η²:η²:η²‐(O₂^2—) Coordination. Crystal Structures of [Ln₄(O₂)₂Cl₈(Py)₁₀] · Py mit Ln = Sm, Eu, Gd The four‐nuclear peroxo complexes [Ln₄(O₂)₂Cl₈(Py)₁₀]·py (py = pyridine) with Ln = Sm ( 1 ·py), Eu ( 2 ·py) und Gd ( 3 ·py) are formed as pale yellow ( 1 ·py) and colourless ( 2 ·py and 3 ·py) crystals by action of atmospheric oxygen on heated solutions of the anhydrous trichlorides LnCl₃ in pyridine/ diacetone alcohol (4‐hydroxy‐4‐methyl‐2‐pentanone). According to the X‐ray structural analyses the three complexes crystallize isostructural in the triclinic space group PP1¯ with two formula units per unit cell. 1—3 form centrosymmetrical molecular structures, in which the four lanthanoid atoms in coplanar array are linked via the two peroxo groups in a hitherto unobserved μ₃‐η²:η²:η² coordination. Additionally, they are bonded by four �μchloro bridges. Two of the Ln atoms complete their coordination sphere by three pyridine molecules each, the other two by two chlorine atoms and two pyridine molecules. The gadolinium compound is additionally characterized by its complete vibrational spectrum (i.r. and Raman).     

Fe–Hg Interactions and P Chemical Shifts in [Fe(CO)3 (PPh2R)2(HgCl2)] (R=pym, fur, py, thi)

In order to study the effects of R group on Fe–Hg interactions and 31P chemical shifts, the structures of mononuclear complexes Fe(CO)₃(PPh₂R)₂ (R=pym:1, fur: 2, py: 3,thi: 4; pym=pyrimidine, fur=furyl, py=pyridine, thi=thiazole) and binuclear complexes [Fe(CO)₃(PPh₂R)₂(HgCl₂)] (R=pym: 5, fur: 6, py: 7, thi: 8) were studied using the density functional theory (DFT) PBE0 method. The 31P chemical shifts were calculated by PBE0-GIAO method. Nature bond orbital (NBO) analyses were also performed to explain the nature of the Fe–Hg interactions. The conclusions can be drawn as follows: (1) The complexes with nitrogen donor atoms are more stable than those with O or S atoms. The more N atoms there are, the higher is the stabilility of the complex. (2) The Fe–Hg interactions play a dominant role in the stabilities of the complexes. In 5 or 6, thereisa σ-bond between Fe and Hg atoms. However, in 7 and 8, the Fe–Hg interactions act as σ_P–Fe→n_Hg and σ_C–Fe→n_Hg delocalization. (3) Through Fe→Hg interactions, there is charge transfer from R groups towards the P, Fe, and Hg atoms, which increases the electron density on P nucleus in binuclear complexes. As a result, compared with their mononuclear complexes, the 31P chemical shifts in binuclear complexes show some reduction.     

Unusual NH Activation of 2‐Aminopyrimidine: Supramolecular Assembly into an AgI Metal–Organic Framework

A rare example of coordination at the amino group of NH₂pym (2‐aminopyrimidine) relevant to N? H activation is described that leads to a novel Ag^I–imide 3D metal–organic framework (MOF). The coordination of Ag^I to NH₂pym produced an electron‐withdrawing effect and thus increased its acidity, which facilitated the N? H activation and the subsequent formation of the Ag–imide bond. A cooperative metalation/deprotonation process for the N? H activation of NH₂pym is suggested. Interestingly, photoluminescence of 1 is switched on at the low temperature of 77 K.     

Synthesis of S,N-chelating heterocyclic thionates of palladium(II). Crystal structure of cis-[Pd(pym2S)(PPh3)2] (pym2S = pyrimidine-2-thionate)

The synthesis of mononuclear palladium(II) complexes containing chelating heterocyclic thionates is described. The new compounds of general formula cis-[Pd(RS-N)(L) _x ](ClO₄) [x = 2, L = PPh₃, RS-N = pyridine-2-thionate (py2S) (1), pyrimidine-2-thionate (pym2S) (2), imidazolidine-2-thionate (imzdS) (3), 1-methylimidazoline-2 thionate (mimzS) (4), 1,3-thiazoline-2-thionate (tzdS) (5); x = 1, L = dppe, RS-N = pyridine-2-thionate (py2S) (6), pyrimidine-2-thionate (pym2S) (7), imidazolidine-2-thionate (imzdS) (8), 1-methylimidazole-2 thionate (mimzS) (9) and 1,3-thiazoline-2-thionate (tzdS) (10)] were prepared by directly reacting the hydroxo-complexes [{Pd(PPh₃)₂(-OH) }₂](ClO₄)₂ and [ {Pd(dppe)(-OH) }₂](ClO₄)₂ with the corresponding heterocyclic thiones (RS-N)H. The complexes have been characterized by partial elemental analyses, conductance measurements and spectroscopic methods (I.r., FAB, ¹H- and ³¹P-n.m.r.). No evidence for monomer-dimer equilibrium was found in solution. The crystal structure of (2) has been determined by X-ray diffraction analysis.     

"Directed" assembly of metallacalix[n]arenes with pyrimidine nucleobase ligands of low symmetry: metallacalix[n]arene derivatives of cis-[a2M(cytosine-N3)2]2+ (M=Pt(II), Pd(II); n=4 and 6)

Pyrimidine (pym) ligands with their two endocyclic N‐donor atoms provide 120° angles for molecular constructs, which, with the 90° angle metal fragments cis‐a₂M^II (M=Pt, Pd; a=NH₃ or a₂=diamine), form cyclic complexes known as metallacalix[n]arenes (with n=3, 4, 6, 8, …?). The number of possible isomers of these species depends on the symmetry of the pym ligand. Although highly symmetrical (C_2v) pym ligands form a single linkage isomer for any n and can adopt different conformations (e.g., cone, partial cone, 1,3‐alternate, and 1,2‐alternate in the case of n=4), low‐symmetry pym ligands (C_s) can produce a higher number of linkage isomers (e.g., four in the case of n=4) and a large number of different conformers. In the absence of any self‐sorting bias, the number of possible species derived from a self‐assembly process between cis‐a₂M^II and a C_s‐symmetrical pym ligand can thus be very high. By using the C_s‐symmetric pym nucleobase cytosine, we have demonstrated that the number of feasible isomers for n=4 can be reduced to one by applying preformed building blocks such as cis‐[a₂M(cytosine‐N3)₂]ⁿ⁺ or cis‐[a₂M(cytosinate‐N1)₂] (for the latter, see the accompanying paper: A. Khutia, P. J. Sanz Miguel, B. Lippert, Chem. Eur. J. 2011 , 17, DOI: 10.1002/chem.2010002723) and treating them with additional cis‐a₂M^II. Moreover, intramolecular hydrogen‐bonding interactions between the O2 and N4H₂ sites of the cytosine ligands reduce the number of possible rotamers to one. This approach of the “directed” assembly of a defined metallacalix[4]arene is demonstrated.     

Chirality Control of Quadruple Helixes of Metal Strings by Peripheral Chiral Ligands

Chirality control of helixes with the Δ (P) or Λ (M) form is interesting in various fields such as extended metal atom chains (EMACs), in which the metal backbones are helically wrapped by four ligands. Herein, we report two EMACs, Δ‐[Ni₅((?)camnpda)₄] and Λ‐[Ni₅((+)camnpda)₄], whose chiralities are controlled by chiral ligands with naphthyridine and camphorsulfonyl groups. There is a large energy difference (108 kcal mol^?1) between the two helical structures with one chiral ligand. Furthermore, the electron communication between [Ni₂]³⁺ units is more pronounced than in [Ni₅(bna)₄Cl₂]²⁺ (bna=binaphthyridylamido). The results demonstrate control of small‐scale helical structure and set the stage for future development of chiral controlled base and nanoelectronic devices.     

Double versus single helical structures of oligopyridine-dicarboxamide strands. Part 1: Effect of oligomer length

Oligoamides of 2,6-diaminopyridine and 2,6-pyridinedicarboxylic acid were previously shown to fold into single helical monomers and to hybridize into double helical dimers. A new series of these oligomers comprising 5 to 15 pyridine units, 4-decyloxy residues, and benzylcarbamate end groups were synthesized using a new convergent scheme that involves an early disymmetrization of the diamine and of the diacid. The hybridization of these compounds into double helices was studied by ¹H NMR spectroscopy in chloroform solutions at various temperatures. Somewhat unexpectedly, these studies revealed that dimerization increases with oligomer length up to a certain point, and then decreases down to undetectable levels for the longest strands. NMR studies show that both double helices and single helices become more stable when strand length increases. The measured values of enthalpy and entropy of hybridization for oligomers of various length show that the enthalpic gain constantly decreases with strand length. This can be interpreted as being the result of an increasing enthalpic price of the spring-like extension that the strand undergoes upon hybridization as its length increases. On the other hand, the entropic loss of hybridization also constantly decreases with strand length. Presumably, the helical preorganization of the monomers increases with strand length, which allows the longer strands to hybridize with a minimal loss of motional freedom, that is to say at a low entropic price. The competiton between these two factors results in a maximum of hybridization for the strands having an intermediate length.     

Two-dimensional supramolecular networks generated from weak AgCpy interactions: Synthesis, structural, thermal and fluorescence studies of silver(I) complexes of 5,5-diethylbarbiturato with pyridine-2-ylmethanol and 2,6-dimethoxypyridine

Two new silver(I) complexes [Ag(barb)(pym)]·H₂O (1) and [Ag(barb)(dmpy)]·1.5H₂O (2) (barb = 5,5-diethylbarbiturate, pym = pyridine-2-ylmethanol and dmpy = 2,6-dimethoxypyridine) have been synthesized and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. In both complexes, the silver(I) ions are linearly coordinated by the N atoms of a barb anion and a pym or a dmpy ligand, forming mononuclear species. The molecules of 1 and 2 are doubly bridged by N–HO hydrogen bonds involving the barb moieties and these hydrogen-bonded dimers are assembled into two-dimensional layered networks through weak AgC_py (η¹) interactions of ca. 3.3 Å. Additionally, the thermal and fluorescent properties of these complexes are also investigated.     

Generalizing the Aromatic δ-Amino Acid Foldamer Helix

A series of aromatic oligoamide foldamer sequences containing different proportions of three δ-amino acids derived from quinoline, pyridine, and benzene and possessing varying flexibility, for example due to methylene bridges, were synthesized. Crystallographic structures of two key sequences and ¹H NMR data in water concur to show that a canonical aromatic helix fold prevails in almost all cases and that helix stability critically depends on the ratio between rigid and flexible units. Notwithstanding subtle variations of curvature, i. e. the numbers of units per turn, the aromatic δ-peptide helix is therefore shown to be general and tolerant of a great number of sp³ centers. We also demonstrate canonical helical folding upon alternating two monomers that do not promote folding when taken separately: folding occurs with two methylenes between every other unit, not with one methylene between every unit. These findings highlight that a fine-tuning of helix handedness inversion kinetics, curvature, and side chain positioning in aromatic δ-peptidic foldamers can be realized by systematically combining different yet compatible δ-amino acids.     

Synthesis, Characterization and Crystal Structure of a One-dimensional Diamagnetic Metal-radical Complex: [Zn(NIT4py)2(DTB)2(H2O)2]

A novel one-dimensional complex [Zn(NIT4py)₂(DTB)₂(H₂O)₂] (1), with mixed ligands [where NIT4py is 2-(4′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and DTB is 3,5-dinitrobenzoate] has been synthesized and characterized by elemental analyses, i.r., u.v.–vis spectra, thermogravimetric analysis, X-ray single crystal diffraction and magnetic measurements. The complex crystallizes in the triclinic crystal system and space group Pî. The Zn ^II ion is in a distorted octahedral environment: two nitrogen atoms from two NIT4py entities, two oxygen atoms from two DTB units in the basal plane; and two oxygen atoms from the two water molecules in the axial position. The [Zn(NIT4py)₂(DTB)₂(H₂O)₂] units are connected as a one dimension chain by the intermolecular hydrogen bonds. The complex exhibits intramolecular antiferromagnetic interactions between the two radicals.     

Supramolecular Interaction of Fullerenes with a Curved π‐Surface of a Monomeric Quadruply Ring‐Fused Porphyrin

Molecular binding of fullerenes, C₆₀ and C₇₀, with the Zn^II complex of a monomeric ring‐fused porphyrin derivative ( 2 ‐py) as a host molecule, which has a concave π‐conjugated surface, has been confirmed spectroscopically. The structures of associated complexes composed of fullerenes and 2 ‐py were explicitly established by X‐ray diffraction analysis. The fullerenes in the 2:1 complexes, which consist of two 2 ‐py molecules and one fullerene molecule, are fully covered by the concave surfaces of the two 2 ‐py molecules in the crystal structure. In contrast, in the crystal structure of the 1:1 complex consisting of one 2 ‐py molecule and one C₆₀ molecule, the C₆₀ molecule formed a π–π stacked pair with a C₆₀ molecule in the neighboring complex using a partial surface, which was uncovered by the 2 ‐py molecule. Additionally, the molecular size of fullerene adopted significantly affects the ¹H NMR spectral changes and the redox properties of 2 ‐py upon the molecular binding.     

Self‐Assembly of Five 8‐Hydroxyquinolinate‐Based Complexes: Tunable Core,Supramolecular Structure,and Photoluminescence Properties

Five new Zn^II complexes, namely [Zn₃(L)₆] ( 1 ), [Zn₂(Cl)₂(L)₂(py)₂] ( 2 ), [Zn₂(Br)₂(L)₂(py)₂] ( 3 ), [Zn(L)₂(py)] ( 4 ), and [Zn₂(OAc)₂(L)₂(py)₂] ( 5 ), were prepared by the solvothermal reaction of ZnX₂ (X^?=Cl^?, Br^?, F^?, and OAc^?) salts with a 8‐hydroxyquinolinate ligand (HL) that contained a trifluorophenyl group. All of the complexes were characterized by elemental analysis, IR spectroscopy, and powder and single‐crystal X‐ray crystallography. The building blocks exhibited unprecedented structural diversification and their self‐assembly afforded one mononuclear, three binuclear, and one trinuclear Zn^II structures in response to different anions and solvent systems. Complexes 1 – 5 featured four types of supramolecular network controlled by non‐covalent interactions, such as π???π‐stacking, C? H???π, hydrogen‐bonding, and halogen‐related interactions. Investigation of their photoluminescence properties exhibited disparate emission wavelengths, lifetimes, and quantum yields in the solid state.     

Folding a nonbiological polymer into a compact multihelical structure

The only molecules that are currently known to fold into unique three-dimensional conformations and perform sophisticated functions are biological polymers - proteins and some RNA molecules. Our aim is to create a nonbiological sequence-specific polymer that folds in aqueous solution. Toward that end, we synthesized sequence-specific 30mer, 45mer, and 60mer peptoid oligomers (N-substituted glycine polymers) consisting of 15mer units we chained together by disulfide and oxime linkages to mimic the helical bundle structures commonly found in proteins. Because these 15mer sequences were previously shown to form defined helical structures that aggregate together at submillimolar concentrations, we expected that by covalently linking multiple 15mers together, they might fold as helical bundles. To probe whether they folded, we used fluorescence resonance energy transfer (FRET) reporter groups. We found that certain constructs fold up with a hydrophobic core and have cooperative folding transitions. Such molecules may ultimately provide a platform for designing specific functions resembling those of proteins.     

Coordination of di- and triimine ligands at ruthenium(II) and ruthenium(III) centers: structural,electrochemical and radical scavenging studies

Herein, we explore the coordination of di- and triimine chelators at ruthenium(II) and ruthenium(III) centers. The reactions of 2,6-bis-((4-tetrahydropyranimino)methyl)pyridine (thppy), N¹,N²-bis((3-chromone)methylene)benzene-1,2-diamine (chb), and tris-((1H-pyrrol-2-ylmethylene)ethane)amine (H₃pym) with trans-[Ru^IICl₂(PPh₃)₃] afforded the diamagnetic ruthenium(II) complex cis-[RuCl₂(thppy)(PPh₃)] (1) and the paramagnetic complexes [mer-Ru₂(μ-chb)Cl₆(PPh₃)₂] (2), and [Ru(pym)] (3), respectively. The complexes were characterized by IR, NMR, and UV–vis spectroscopy and molar conductivity measurements. The structures were confirmed by single crystal X-ray diffraction studies. The redox properties of the metal complexes were probed via cyclic- and squarewave voltammetry. Finally, the radical scavenging capabilities of the metal complexes towards the NO and 2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH) radicals were investigated     

Synthesen und Strukturen der Lithiumtitanate(III) und ‐(IV), (py)2Li[(py)2Ti(OPh)4] und (py)2Li[(py)Ti(OPh)5]

Syntheses and Structures of the Lithiumtitanates(III)/(IV) (py)₂Li[(py)₂Ti(OPh)₄] and (py)₂Li[(py)Ti(OPh)₅] The new lithiumtitanates (py)₂Li[(py)₂Ti(OPh)₄] ( 1 ) and (py)₂Li[(py)Ti(OPh)₅] ( 2 ) have been obtained from the reaction of titaniumtrichloride (respectively titaniumtetrachloride 2 ) with LiOPh in the presence of the base pyridine (py). The crystal structures of both compounds show that the titanium atoms are in the centres of distorted octahedral coordination figures. In compound 1 , four oxygen and two nitrogen atoms (in cis orientation) are bonded to titanium, whereas in 2 , five oxygen and one nitrogen atom form the coordination polyeder around titanium. In both compounds, the lithium atoms are attached through phenolate bridges to the octahedra. The titanate (py)₂Li[(py)₂Ti(OPh)₄] ( 1 ) has a single absorption band in the visible region of the UV‐spectrum showing a shoulder shifted to the bathochromic region, due to the Jahn‐Teller‐effect for d¹‐systems.     

Multiple Fused Anthracenes as Helical Polycyclic Aromatic Hydrocarbon Motif for Chiroptical Performance Enhancement

Polycyclic aromatic hydrocarbons consisting of three fused anthracene units were designed as new π‐conjugated compounds having helical structures. These expanded helicenes were synthesized by Pt‐catalyzed cycloisomerization of the corresponding ethynyl‐substituted precursors. The nonplanar and helical structure was confirmed by X‐ray analysis and DFT calculations, and the barrier to helical inversion was estimated to be 34 kJ mol^?1. The enantiomers of the diphenyl derivative were successfully resolved by chiral HPLC. Enantiopure samples showed good chiroptical performance in the CD (|Δ?| 1380 L mol^?1 cm^?1) and CPL (|g_lum| 0.013) spectra, and these values were considerably large for simple organic molecules. The unique chiroptical properties are discussed on the basis of the molecular structure and the electronic state with the aid of time‐dependent DFT calculations.