Lead(II) complexes of lateral macrobicyclic receptors that incorporate a crown moiety and a pyridine head unit

[Abstract] The coordinative properties towards lead(II) of two lateral macrobicyclic receptors that incorporate either a 1,10‐diaza‐[15]crown‐5 (L7) or a 4,13‐diaza‐[18]crown‐6 (L8) fragment are reported. Spectrophotometric titrations performed in acetonitrile solution indicate only the formation of mononuclear complexes in solution. The X‐ray crystal structures of the two receptors show that the conformation adopted by the ligand is imposed by the presence of intramolecular hydrogen‐bonding interactions that involve the secondary amine groups and the pivotal nitrogen atoms. The solid‐state structure of [Pb(L7)(NCS)](SCN)·0.5H2O shows that the metal ion is asymmetrically coordinated inside the macrobicyclic cavity. The PbII ion is coordinated to the nitrogen atom of the pyridine unit, the two secondary amine atoms, two oxygen atoms of the crown moiety, and a nitrogen atom of an isothiocyanate group. The distances between the PbII ion and the two pivotal nitrogen atoms as well as one of the oxygen atoms of the crown moiety are too long (>2.92 Å) to be considered unequivocal bonds, and should be regarded only as weak interactions. The protonation constants of L7 and L8 as well as the stability constants of their PbII complexes were investigated by using potentiometric titrations in 95 % methanol (I = 0.1 M, nBu4NClO4, 25 °C). The two receptors undergo two protonation processes in the pH range investigated (2.0 < pH < 12.0), which correspond to the protonation of the nitrogen atoms of the oxa–aza moiety. The log KPbL value obtained for L7 [9.906(1)] is approximately 1.1 log K units higher than the one determined for L8 [8.75(1)].Xunta de Galicia; PGIDIT06TAM10301PRXunta de Galicia; INCITE09E1R103013E

Symmetry in copper and silver cryptates

The cryptand ligands imBT (1,4,7,10,13,16,21,24-Octaaza-bicyclo[8.8.8]hexacosa-4,6,13,15,21,23-hexaene) and amBT (1,4,7,10,13,16,21,24-Octaazabicycl0[8.8.8]hexacosane) form interesting disilver(I) and trisilver(I) cryptates, as well as a dicopper(I) and a well studied average valence dicopper(1.5) cryptate. Detailed structural and spectroscopic studies of the silver cryptates show that complex equilibria exist in solution, and the trisilver form appears to be thermodynamically favoured, the additional stabilisation apparently being due to argentophilic interactions. An optically pure form of the dicopper(I) imBT cryptate was successfully obtained, and is undergoing X-ray diffraction studies aimed at determining whether a copper-copper interaction exists, by direct examination of the electron density. Synthetic studies aimed at introducing substitution to the imBT and amBT ligands were complicated by competing reactions. A strategy to modify cryptand cavity size by incorporating asymmetric tetraamine caps succeeded, yielding dicopper(I) and disilver(I) cryptates with properties intermediate between cryptates incorporating the related symmetric caps. Manganese(II) cryptates of imBT and amBT were investigated as potential MRl contrast agents, the iminocryptate showing surprisingly high relaxivity, despite the fact that no water molecules were located in the crystal structure. The observation of high mass peaks in the FAB mass spectra of imBT and amBT cryptates suggests the presence of 6+4 Schiff base condensation products, as well as the more abundant 3+2 products. It has not proved possible to isolate these molecules as yet, however initial studies aimed at a rational synthesis of the 6+4 condensation products were made, as these ligands could be of great interest for modelling the recently reported CUz site

"Macrocycles, macrobicycles: a study"

A template directed [2+2] Schiff base condensation of 2,6-diformyl pyridine and 1,3 diaminopropane afforded the 20 membered Ns macrocycle "VT". The X- ray structure of the barium templated VT has been determined <M. G.B. Drew, at the University of Reading). Complexes of first transition series ions, obtained by transmetallation, have been characterized using electronic, infrared and esr spectroscopy. Results indicate that VT provides a cavity suitable for single atom bridged binuclear assemblies. The Schiff-base strategy was further used to yield macrobicyclic cyclophanes from the [2+3] condensation of tris(amino ethyl) amine ie tren, with a series of aromatic dicarbonyls. X-ray structures and 'Hnmr. studies show that these macrobicycles have very different conformations in their complexed and uncomplexed forms

A merged experimental and theoretical conformational study on alkaline-earth complexes with lariat ethers derived from 4,13-diaza-18-crown-6

[Abstract] Herein, we report the synthesis and structural characterization of alkaline-earth complexes with the bibracchial lariat ethers N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L2) and N,N′-bis(benzimidazol-2ylmethyl)-4,13-diaza-18-crown-6 (L4). The X-ray crystal structures of the Ca(II) and Sr(II) complexes of L2 show the pendant arms of the ligand disposed on opposite sides of the macrocyclic mean plane, which results in an anti conformation in the solid state. A similar anti conformation is also observed for the Mg(II) complex of L4, whereas the Ca(II), Sr(II) and Ba(II) complexes of L4 adopt a syn conformation in the solid state, with the two pendant arms pointing at the same side of the crown moiety. However, a different behavior is observed in solution. Indeed, 1H and 13C NMR spectroscopy, in combination with density functional theory (DFT) calculations performed at the B3LYP level, suggests that the [M(L2)]2+ and [M(L4)]2+ (M = Ca, Sr or Ba) complexes exist in solution as a mixture of syn and anti isomers involved in a dynamic equilibrium. Our results also show that the relative abundance of the syn conformation increases as the ionic radius of the metal ion increases and, furthermore, for a given metal ion the proportion of syn isomer is always higher for L4 complexes than for L2 ones.Xunta de Galicia; PGIDIT06TAM10301PRXunta de Galicia; INCITE09E1R103013E

Ditopic molecular architectures for the recognition of anionic species

Dissertation presented to obtain the Ph.D degree in ChemistryAnions are ubiquitous and very important species in biological, medicinal, industrial and environmental processes. In biology, anions are essential for normal metabolic functions, where their specific recognition, transport and detection play a very important role. On the other hand, the uncontrolled release of anions into the environment poses a significant threat. The development of synthetic receptors capable of sequestering anions should, therefore, provide solutions to a number of problems of current interest. To be suitable for real-life applications, as for instance the detection and quantification of biologically active anions in clinical laboratories or the environmental monitoring and/or removal of pollutants, synthetic receptors need to be able to function in aqueous solution. However, due to the high dielectric constant and good hydrogen bond donor and acceptor capabilities, water is the most challenging medium for anion recognition.(...)The work described in this thesis was entirely carried out in Instituto de Tecnologia Química e Biológica, Oeiras, Portugal. Pedro Mateus has received financial support from Fundação para a Ciência e Tecnologia, ref. SFRH/BD/36159/2007

Steroid diversification by multicomponent reactions

Reports on structural diversification of steroids by means of multicomponent reactions (MCRs) have significantly increased over the last decade. This review covers the most relevant strategies dealing with the use of steroidal substrates in MCRs, including the synthesis of steroidal heterocycles and macrocycles as well as the conjugation of steroids to amino acids, peptides and carbohydrates. We demonstrate that steroids are available with almost all types of MCR reactive functionalities, e.g., carbonyl, carboxylic acid, alkyne, amine, isocyanide, boronic acid, etc., and that steroids are suitable starting materials for relevant MCRs such as those based on imine and isocyanide. The focus is mainly posed on proving the amenability of MCRs for the diversityoriented derivatization of naturally occurring steroids and the construction of complex steroid-based platforms for drug discovery, chemical biology and supramolecular chemistry applications.Fil: Reguera, Leslie. Universidad de La Habana; CubaFil: Attorresi, Cecilia Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad de Microanálisis y Métodos Físicos en Química Orgánica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Unidad de Microanálisis y Métodos Físicos en Química Orgánica; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Orgánica; ArgentinaFil: Ramirez, Javier Alberto. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad de Microanálisis y Métodos Físicos en Química Orgánica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Unidad de Microanálisis y Métodos Físicos en Química Orgánica; ArgentinaFil: Rivera, Daniel G.. Universidad de La Habana; Cub

2,2'-(2,2'-oxybis(ethane-2,1-diyl)bis(oxy))bis(N-(2,2'-bithiophen-5-ylmethylene)aniline)

We would like to thank Xunta de Galiza (Spain) for project 09CSA043383PR and the University of Vigo, Vicou for projects INOU UVIGO/VICOU/K914-122P64702/2009 and UVIGO/VICOU/K912-122P64702/2009. Thanks to the FCT-MCTES/FEDER (Portugal) through national projects POCI/QUI/55519/2004 and PDTC/QUI/66250/2006. B. P thanks FCT/Portugal for the PhD Grant SFRH/BD/27786/2006. C.L. and J.L. thank Xunta de Galicia for the Isidro Parga Pondal Research Program.A new flexible fluorescent compound L derived from 1,5-bis(2-aminophenoxy)-3-oxapentane (A) has been synthesized by classical Schiff-base reaction between (A) and 2,2 ́-bithiophene carbaldehyde (B). The same synthesis was reproduced by a green methodology using an ultrasonication reaction in a classical sonication bath. The structure of the new compound was confirmed by elemental analysis, IR, 1H-NMR, MALDI-TOF-MS and EI-MS-spectra, UV-vis and fluorescence emission spectroscopy.publishersversionpublishe

Supramolecular Explorations: Exhibiting the Extent of Extended Cationic Cyclophanes

Acting as hosts, cationic cyclophanes, consisting of π-electron-poor bipyridinium units, are capable of entering into strong donor–acceptor interactions to form host–guest complexes with various guests when the size and electronic constitution are appropriately matched. A synthetic protocol has been developed that utilizes catalytic quantities of tetrabutylammonium iodide to make a wide variety of cationic pyridinium-based cyclophanes in a quick and easy manner. Members of this class of cationic cyclophanes with  box like geometries, dubbed  Ex n Box m 4+ for short, have been prepared by altering a number of variables: (i)  n , the number of “horizontal”  p -phenylene spacers between adjoining pyridinium units, to modulate the “length” of the cavity; (ii)  m , the number of “vertical”  p -phenylene spacers, to modulate the “width” of the cavity; and (iii) the aromatic linkers, namely, 1,4-di- and 1,3,5-trisubstituted units for the construction of macrocycles ( ExBoxes ) and macrobicycles ( ExCages ), respectively. This Account serves as an exploration of the properties that emerge from these structural modifications of the pyridinium-based hosts, coupled with a call for further investigation into the wealth of properties inherent in this class of compounds. By variation of only the aforementioned components, the role of these cationic receptors covers ground that spans (i) synthetic methodology, (ii) extraction and sequestration, (iii) catalysis, (iv) molecular electronics, (v) physical organic chemistry, and (vi) supramolecular chemistry.  Ex 1 Box 4+ (or simply  ExBox 4+ ) has been shown to be a multipurpose receptor capable of binding a wide range of polycyclic aromatic hydrocarbons (PAHs), while also being a suitable component in switchable mechanically interlocked molecules. Additionally, the electronic properties of some host–guest complexes allow the development of artificial photosystems.  Ex 2 Box 4+ boasts the ability to bind both π-electron-rich and -poor aromatic guests in different binding sites located within the same cavity.  ExBox 2 4+ forms complexes with C 60 in which discrete arrays of aligned fullerenes result in single cocrystals, leading to improved material conductivities. When the substitution pattern of the  Ex n Box 4+ series is changed to 1,3,5-trisubstituted benzenoid cores, the hexacationic  cage like compound, termed  ExCage 6+ , exhibits different kinetics of complexation with guests of varying sizes—a veritable playground for physical organic chemists. The organization of functionality with respect to structure becomes valuable as the number of analogues continues to grow. With each of these minor structural modifications, a wealth of properties emerge, begging the question as to what discoveries await and what properties will be realized with the continued exploration of this area of supramolecular chemistry based on a unique class of receptor molecules

Self-Assembly, Adaptive Response, and in,out-Stereoisomerism of Large Orthoformate Cryptands

© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. We report on triethylene glycol-based orthoformate cryptands, which adapt their bridgehead configurations in response to metal templates and intramolecular hydrogen bonding in a complex manner. In contrast to smaller 1.1.1-orthoformate cryptands, the inversion from out,out-2.2.2 to in,in-2.2.2 occurs spontaneously by thermal homeomorphic isomerization, i. e., without bond breakage. The global thermodynamic minimum of the entire network, which includes an unprecedented third isomer (in,out-2.2.2), could only be reached under conditions that facilitate dynamic covalent exchange. Both inversion processes were studied in detail, including DFT calculations and MD simulations, which were particularly helpful for explaining differences between equilibrium compositions in solvents chloroform and acetonitrile. Unexpectedly, the system could be driven to the in,out-2.2.2 state by using a metal template with a size mismatch with respect to the out,out-2.2.2 cage

Development of Bi-metallic Catalysts for Multi-electron Chemistry: of Consequence to Sustainable Energy

The new ligand L1 (21), 1-N,1-N-bis(pyridine-2-ylmethyl)-3-N-(pyridine-2-ylmethylidene)benzene-1,3- diamine, was synthesised as a platform to study bi-metallic first row transition complexes containing redox-active ligands. The asymmetric ligand L1 (21) contains a redox-active α-iminopyridine unit bridged to redox-inert or “innocent” bis(2-pyridylmethyl)amino counterpart and offers two distinct coordination sites. The co-ordination chemistry of L1 (21) with Fe2+, Cu2+, and Zn2+ was examined. Reaction with Zn2+ afforded the asymmetric binuclear complex [(L1)Zn2Cl4] (C1), whereas the symmetric [(L1)2Fe2(OTf)2](OTf)2 (C2), [(L1)2Fe2(CH3CN)2](PF6)4 (C3) and [(L1)2Cu2](OTf)4 (C4) complexes were isolated in reactions with iron and copper salts. Both metal- and ligand-centered redox processes are available to complexes C2, C3 and C4 and were investigated by cyclic voltammetry. Complexes C2 and C3 favor ligand-centered reduction while C4 favors metal-centered reduction. EPR, Mössbauer spectroscopy and magnetic susceptibility studies establish that complexes C2, C3 and C4 are paramagnetic. Ferrous complexes C2 and C3 were studied as potential C-H oxidation catalysts. Difficult C-H oxidation transformations are known to be facilitated by synthetic and biomimetic iron species. The C2 and C3 complexes provide a unique contrast non-heme enzyme active sites and biomimetic complexes due to the large Fe…Fe inter-nuclear distances of > 7Å. Complexes C2 and C3 provided evidence of moderate catalytic activity toward a range of substrates of varying bond strengths including cyclohexene (80 kcal mol-1), 9,10-dihydroanthracene (76 kcal mol-1), xanthene (75.5 kcal mol-1) and triphenylmethane (81 kcal mol-1). The oxidising strength of complexes C2 and C3 toward C-H bonds were found to be limited by bond strength with both C2 and C3 proving to be incapable of oxidising cyclohexane (99 kcal mol-1) and adamantane (96 kcal mol-1). The oxidation pathway was investigated by employing deuterated substrates in place of cyclohexene, DHA and xanthene. The increased strength of the C-D bond over a CH bond (1.4 kcal mol-1) should induce decreased reactivity and hence product formation if the hydrogen abstraction pathway is being accessed. Decreased reactivity was observed when the deuterated versions of cyclohexene, DHA and xanthene were employed illustrating the hydrogen abstraction pathway is indeed the mechanistic means by which complexes C2 and C3 oxidise C-H bonds. The structural identity of the active species formed by C3 during C-H oxidation, was investigated in the absence of substrate which exposed the formation of a transient species identified by λmax 430 nm in the UV-Vis spectrum - labelled C7. Complex C7 decays to the di-ferric species C8. Spectroscopic studies in the presence of substrate uncovered the existence of two active species with two distinct rates of oxidation. Exposing complexes C7 and C8 to DHA unveiled complex C7 as the lesser active species, while complex C8 was unreactive toward DHA oxidation. The structural identity of the more reactive species has been elusive to the studies presented herein. A new bi-macrocyclic ligand scaffold which might more readily favour the formation of two-electron mixed valence species was designed. A series of five structures with varying M…M inter-nuclear distances were designed and the synthetic routes toward their formation are discussed accompanied by calculated geometry optimised structures as full characterisation of the final complexes are yet to be obtained