Hydrogen Bonding Drives Helical Chirality via 10-Membered Rings in Dipeptide Conjugates of Ferrocene-1,1'-Diamine.

Considering the enormous importance of protein turns as participants in various biological events, such as protein-protein interactions, great efforts have been made to develop their conformationally and proteolytically stable mimetics. Ferrocene-1,1'-diamine was previously shown to nucleate the stable turn structures in peptides prepared by conjugation with Ala (III) and Ala-Pro (VI). Here, we prepared the homochiral conjugates of ferrocene-1,1'-diamine with l-/d-Phe (32/35), l-/d-Val (33/36), and l-/d-Leu (34/37) to investigate (1) whether the organometallic template induces the turn structure upon conjugation with amino acids, and (2) whether the bulky or branched side chains of Phe, Val, and Leu affect hydrogen bonding. Detailed spectroscopic (IR, NMR, CD), X-ray, and DFT studies revealed the presence of two simultaneous 10-membered interstrand hydrogen bonds, i.e., two simultaneous ß-turns in goal compounds. A preliminary biological evaluation of d-Leu conjugate 37 showed its modest potential to induce cell cycle arrest in the G0/G1 phase in the HeLa cell line but these results need further investigation.

Conformational Preferences and Antiproliferative Activity of Peptidomimetics Containing Methyl 1'-Aminoferrocene-1-carboxylate and Turn-Forming Homo- and Heterochiral Pro-Ala Motifs.

The concept of peptidomimetics is based on structural modifications of natural peptides that aim not only to mimic their 3D shape and biological function, but also to reduce their limitations. The peptidomimetic approach is used in medicinal chemistry to develop drug-like compounds that are more active and selective than natural peptides and have fewer side effects. One of the synthetic strategies for obtaining peptidomimetics involves mimicking peptide α-helices, ß-sheets or turns. Turns are usually located on the protein surface where they interact with various receptors and are therefore involved in numerous biological events. Among the various synthetic tools for turn mimetic design reported so far, our group uses an approach based on the insertion of different ferrocene templates into the peptide backbone that both induce turn formation and reduce conformational flexibility. Here, we conjugated methyl 1'-aminoferrocene-carboxylate with homo- and heterochiral Pro-Ala dipeptides to investigate the turn formation potential and antiproliferative properties of the resulting peptidomimetics 2-5. Detailed spectroscopic (IR, NMR, CD), X-ray and DFT studies showed that the heterochiral conjugates 2 and 3 were more suitable for the formation of ß-turns. Cell viability study, clonogenic assay and cell death analysis showed the highest biological potential of homochiral peptide 4.

Mechanically Responsive Crystalline Coordination Polymers with Controllable Elasticity.

Crystalline coordination polymers tend to be brittle and inelastic, however, we now describe a family of such compounds that are capable of displaying mechanical elasticity in response to external pressure. The design approach successfully targets structural features that are critical for producing the desired mechanical output. The elastic crystals all comprise 1D cadmium(II) halide polymeric chains with adjacent metal centres bridged by two halide ions resulting in the required stacking interactions and short "4 Å" crystallographic axes. These polymeric chains (structural "spines") are further organized via hydrogen bonds and halogen bonds perpendicular to the direction of the chains. By carefully altering the strength and the geometry of these non-covalent interactions, we have demonstrated that it is possible to control the extent of elastic bending in crystalline coordination compounds.

Helically Chiral Peptides That Contain Ferrocene-1,1'-diamine Scaffolds as a Turn Inducer.

A series of peptides that contain homo- and heterochiral Ala-Pro sequences attached to the turn-inducing ferrocene-1,1'-diamine scaffold were synthesized. The effects of the backbone chirality and the N-terminal group (Boc/Ac) on the conformational properties of the novel peptidomimetics were thoroughly explored by IR, NMR, and CD spectroscopy and the experimental observations were corroborated by DFT studies in solution. The most stable conformers of the homochiral peptides adopted the interstrand hydrogen-bond patterns, realized through ten- and thirteen-membered rings. The common feature of the most stable conformers of the heterochiral peptides was the adoption of the turn-like structures that feature the simultaneous intra- (seven-membered) and interstrand (sixteen-membered) hydrogen-bonded rings. An exchange of two N-terminal groups had a somewhat larger influence on the distribution of the hydrogen-bond patterns in homochiral than in heterochiral derivatives. The homochiral peptides that contain pyridine moieties as metal coordination sites formed 1:1 complexes with divalent metal ions, which included Zn2+ , Cd2+ , Cu2+ and Fe2+ .

From Simple Palladium(II) Monomers to 2D Heterometallic Sodium-Palladium(II) Coordination Networks with 2-Halonicotinates.

The 2D heterometallic sodium-palladium(II) coordination polymers with 2-halonicotinates [2-chloropyridine-3-carboxylate (2-chloronicotinate), 2-Clnic- and 2-bromopyridine-3-carboxylate (2-bromonicotinate), 2-Brnic-], {[Na2(H2O)2(µ-H2O)4PdCl2(µ-2-Clnic-N:O')2]}n (1), and {[Na2(H2O)2(µ-H2O)4PdBr2(µ-2-Brnic-N:O')2]·2H2O}n (2) were prepared in aqueous solutions under the presence of NaHCO3, while palladium(II) monomers with the neutral 2-chloronicotinic and 2-bromonicotinic acid ligands, [PdCl2(2-ClnicH-N)2]·2DMF (3) and [PdCl2(2-BrnicH-N)2]·2DMF (4), were prepared in DMF/water mixtures (DMF = N,N'-dimethylformamide). The zigzag chains of water-bridged sodium ions are in turn bridged by [PdCl2(2-Clnic)2]2- moieties in 1 or by [PdBr2(2-Brnic)2]2- moieties in 2, leading to the formation of the infinite 2D coordination networks of 1 or 2. The DFT calculations showed the halosubstituents type (Cl vs Br) does not have an influence on the formation of either trans or cis isomers. The trans isomers were found in all reported compounds; being more stable for about 10 to 15 kJ mol-1. The 2D coordination networks 1 and 2 are more stabilized by the formation of Na-Ocarboxylate bonds, comparing to the stabilization of palladium(II) monomers 3 and 4 by hydrogen-bonding with DMF molecules. The difference in DFT calculated energy stabilization for 1 and 2 is ascribed to the type of halosubstituents and to the presence/absence of lattice water molecules in 1 and 2. The compounds show no antibacterial activity toward reference strains of Escherichia coli and Staphylococcus aureus bacteria and no antiproliferative activity toward bladder (T24) and lung (A549) cancer cell lines.

Synthesis and Characterization of Benzene- and Triazine-Based Azo-Bridged Porous Organic Polymers.

Porous organic polymers incorporating nitrogen-rich functionalities have recently emerged as promising materials for efficient and highly selective CO2 capture and separation. Herein, we report synthesis and characterization of new two-dimensional (2D) benzene- and triazine-based azo-bridged porous organic polymers. Different synthetic approaches towards the porous azo-bridged polymers were tested, including reductive homocoupling of aromatic nitro monomers, oxidative homocoupling of aromatic amino monomers and heterocoupling of aromatic nitro monomers and a series of aromatic diamines of different lengths and rigidity. IR spectroscopy, 13C CP/MAS NMR spectroscopy, powder X-ray diffraction, elemental analysis, thermogravimetric analysis, nitrogen adsorption-desorption experiments and computational study were used to characterize structures and properties of the resulting polymers. The synthesized azo-bridged polymers are all amorphous solids of good thermal stability, exhibiting various surface areas (up to 351 m2 g-1). The obtained results indicated that the synthetic methods and building units have a pronounced effect on the porosity of the final materials. Reductive and oxidative homocoupling of aromatic nitro and amino building units, respectively, lead to 2D azo-bridged polymers of substantially higher porosity when compared to those produced by heterocoupling reactions. Periodic DFT calculations and Grand-canonical Monte Carlo (GCMC) simulations suggested that, within the used approximations, linear linkers of different lengths do not significantly affect CO2 adsorption properties of model azo-bridged polymers.

Synthesis of aromatic polynitroso compounds: Towards functional azodioxy-linked porous polymers.

The polymerization property of aromatic polynitroso compounds could be used to create azodioxy porous networks with possible application for the adsorption of CO2, the main greenhouse gas. Herein, we report the synthesis and characterization of new aromatic polynitroso compounds, with para-nitroso groups attached to the triphenylbenzene, triphenylpyridine, triphenyltriazine and triphenylamine moiety. The synthesis of the pyridine-based trinitroso compound was performed by reduction of the corresponding trinitro derivative to N-arylhydroxylamine followed by oxidation to the trinitroso product. For the synthesis of the benzene- and triazine-based trinitroso compounds, a novel synthetic strategy was implemented, which included cyclotrimerization of the 4-nitrosoacetophenone and 4-nitrosobenzonitrile, respectively. Reduction of the trinitro compound with triphenylamine unit produced the dinitroso product. In a solid state, all synthesized compounds form E-azodioxy oligomers or polymers. While azodioxy polymer with triphenylbenzene moiety is an amorphous solid, other azodioxy oligomers and polymers displayed sharp diffraction peaks pointing to their crystalline nature. A computational study indicated that eclipsed AA configurations are preferred over staggered AB and inclined AA' configurations. The serrated layers may be the most likely outcome when/if 2D layers form an organized polymer network of azodioxy linked triphenyltriazine-based building blocks.

Exploring the Co-Crystallization Landscape of One-Dimensional Coordination Polymers Using a Molecular Electrostatic Potential-Driven Approach.

The ability of coordination polymers (CPs) to form multicomponent heteromeric materials, where the key structural features of the parent CP are retained, has been explored via molecular electrostatic potential-driven co-crystallization technologies. Thirteen co-formers presenting hydrogen-bond donors activated through a variety of electron-withdrawing functionalities were employed, and the extent of activation was evaluated using molecular electrostatic potential values. Attempted co-crystallizations of the seven most promising co-formers with a family of nine CPs ([CdX'2(X-pz)2]n; X' = I, Br, and Cl; X = I, Br, and Cl) resulted in six successful outcomes; all four of the structurally characterized compounds displayed the intended hydrogen bond. The rationalization of the main structural features revealed that strict structural and electrostatic requirements were imposed on effective co-formers; only co-formers with highly activated hydrogen-bond donors, and with a 1,4-orientation of electron-withdrawing moieties bearing effective acceptor sites, were successfully implemented into the three-dimensional architectures composed of one-dimensional building units of CPs.

Correction to Two-Dimensional Anisotropic Flexibility of Mechanically Responsive Crystalline Cadmium(II) Coordination Polymers.

[This corrects the article DOI: 10.1021/acs.chemmater.2c00062.].

Two-Dimensional Anisotropic Flexibility of Mechanically Responsive Crystalline Cadmium(II) Coordination Polymers.

Crystals of a family of six one-dimensional (1D) coordination polymers of cadmium(II) with cyanopyridines [[CdX2L2] n , where X = Cl, Br, or I and L = 3-cyanopyridine (3-CNpy) or 4-cyanopyridine (4-CNpy)] presented a variety of morphologies and mechanical responses with dominant two-dimensional (2D) anisotropic flexibility, which has not been previously reported. All mechanically adaptable crystals were 2D flexible and displayed a variety of direction-dependent responses; in addition to 2D isotropic flexibility observed for solely elastic materials, 2D anisotropic flexibility was noticed for both elastic and elastic → plastic crystals. The consequences of fine and controlled structural variations on mechanical behavior were additionally explored via microfocus single-crystal X-ray diffraction and complementary theoretical studies, revealing that the relative strength and direction of the hydrogen bonding interactions were the key parameters in delivering a specific mechanical response.

Novel ferrocene imide derivatives: synthesis, conformational analysis and X-ray structure.

The synthesis and structural characterization of the ferrocene imide derivatives Fc-CO-NH-CO-Me (4), Fc-CO-NH-CO-Fc (7) and Fc-CO-NH-CO-Fn-CO-NH-CO-Fc (8) have been reported. The mononuclear, dinuclear and trinuclear ferrocene imides were prepared by the reaction of ferrocenecarboxamide (3), with acetyl chloride, ferrocenecarbonyl chloride (2) and ferrocene-1,1'-(dicarbonyl chloride) (6), respectively. IR spectroscopic analysis revealed the absence of intramolecular hydrogen bonds in solutions of imides 4, 7 and 8. The crystal packing of N-acetylferrocenecarboxamide (4) is characterized by N-H⋯O hydrogen bonds forming centrosymmetric dimers, while the molecules of its homologue N-methylferrocenecarboxamide (5) are self-assembled by intermolecular N-H⋯O bonds into infinite chains. A detailed conformational analysis (DFT study) suggests the cis-trans configuration of ferrocene imide derivative 7 in solution. The effect of different substituents attached to bridged imide nitrogen on conformational properties of bis-ferrocenyl imides was further investigated and results compared to the existing experimental data.

Exploring and predicting intermolecular binding preferences in crystalline Cu(ii) coordination complexes.

A simple model focusing on electrostatic contributions to interaction energies was found to be very effective for rationalizing the appearance of specific supramolecular interactions in a series of Cu(ii) coordination compounds. The experimental space was provided by a combination of Cu(ii) cations with acac-based anions (hexafluoracetylacetonato and trifluoracetylacetonato) and a series of pyridine-oxime ligands (3-pyridinealdoxime, methyl 3-pyridyl ketoxime, 4-pyridinealdoxime, methyl 4-pyridyl ketoxime, phenyl 4-pyridyl ketoxime). The calculated molecular electrostatic potential (MEP) values at competing hydrogen-bond acceptor sites, for ten structurally characterized complexes, provided guidelines for predicting supramolecular connectivity in cases when the MEP difference exceeded certain cut-off values, while two different and well-defined outcomes are possible within the so called 'grey zone', delineated by a range of MEP differences. It was also shown that the structural outcome within this region is determined by the influence of relatively weak, but distinct, auxillary interactions.

Building inorganic supramolecular architectures using principles adopted from the organic solid state.

In order to develop transferable and practical avenues for the assembly of coordination complexes into architectures with specific dimensionality, a strategy utilizing ligands capable of simultaneous metal coordination and self-complementary hydrogen bonding is presented. The three ligands used, 2(1H)-pyrazinone, 4(3H)-pyrimidinone and 4(3H)-quinazolinone, consistently deliver the required synthetic vectors in a series of CdII coordination polymers, allowing for reproducible supramolecular synthesis that is insensitive to the different steric and geometric demands from potentially disruptive counterions. In all nine crystallographically characterized compounds presented here, directional intermolecular N-H⋯O hydrogen bonds between ligands on adjacent complex building blocks drive the assembly and orientation of discrete building blocks into largely predictable topologies. Furthermore, whether the solids are prepared from solution or through liquid-assisted grinding, the structural outcome is the same, thus emphasizing the robustness of the synthetic protocol. The details of the molecular recognition events that take place in this series of compounds have been clearly delineated and rationalized in the context of calculated molecular electrostatic potential surfaces.

The conjugates of ferrocene-1,1'-diamine and amino acids. A novel synthetic approach and conformational analysis.

A novel synthetic approach toward a poorly explored bioorganometallic consisting of ferrocene-1,1'-diamine bearing structurally and chirally diverse amino acid sequences is reported. Until now, ferrocene-1,1'-diamine was suitable for accommodating only identical amino acid sequences at its N-termini, leading to the symmetrically disubstituted homochiral products stabilized through a 14-membered intramolecular hydrogen-bonded ring as is seen in antiparallel ß-sheet peptides. The key step of the novel synthetic pathway is the transformation of Ac-Ala-NH-Fn-COOH (5) (Fn = 1,1'-ferrocenylene) to orthogonally protected Ac-Ala-NH-Fn-NHBoc (7). The spectroscopic analysis (IR, NMR, CD) of the novel compounds, corroborated with DFT studies, suggests the interesting feature of the ferrocene-1,1'-diamine scaffold. The same hydrogen-bonding pattern, i.e. a 14-membered hydrogen-bonded ring, was determined both in solution and in the solid state, thus making them promising, yet simple scaffolds capable of mimicking ß-sheet peptides. In vitro screening of potential anticancer activity in Hep G2 human liver carcinoma cells and Hs 578 T human breast cancer cells revealed a cytotoxic pattern for novel compounds (150-500 µM) with significantly decreased cell proliferation.

Vibrational spectroscopic and DFT calculation studies of cobalt(II) complexes with 3-hydroxypicolinic acid.

Two cobalt(II) complexes with 3-hydroxypicolinic acid (3-hydroxypyridine-2-carboxylic acid, 3-OHpicH), trans-[Co(3-OHpic)2(py)2] (2) and cis-[Co(3-OHpic)2(4-pic)2] (3) (py=pyridine; 4-pic=4-picoline or 4-methylpyridine), previously synthesized and characterized by X-ray diffraction, are here studied by Raman and mid-infrared spectroscopy with the help from the corresponding DFT vibrational calculations using B3LYP/6-311G(d,p) computational model. Intramolecular O-H⋯O hydrogen bond appears in both complexes 2 and 3, while weak C-H⋯O hydrogen bonds assemble molecules of 2 or 3 into 3D architecture. A complete presentation of all Raman, infrared and theoretical results is given for complex 3. The measured spectra are shown, relative intensities and bandwidths are discussed and the assignment of vibrational bands is given on the basis of the DFT calculations. The calculated spectra agree very well with the presented experimental findings, thanks to the suitable grouping of modes. The same vibrational calculations also reveal insignificant influence of H→CH3 substitution for the spectroscopic characterization of the complex. A careful study of differences between calculated and observed wavenumbers suggests that modified single-factor scaling is actually better than the classic multi-factor scaling approach.