Tomás Martín

The Construction of Open GdIII Metal–Organic Frameworks Based on Methanetriacetic Acid: New Objects with an Old Ligand

The preparation, X-ray crystallography and magnetic investigation of the first examples of methanetriacetate (mta)-containing lanthanide(III) complexes of formulae [Gd(mta)(H(2)O)(3)](n)4 n H(2)O (1) [Gd(mta)(H(2)O)(3)](n)2 n H(2)O (2) and [Gd(2)(mta)(2)(H(2)O)(2)](n)2 n H(2)O (3) are described herein. This tripodal ligand promotes the formation of 6(3) networks; thus 1 consists of a honeycomb structure, whereas in 2 two of these layers are condensed to form a rare five-connected two-dimensional (4(8)6(2)) network. Compound 3 can be seen as an aggregation of 6(3) layers leading to a three-dimensional (6,6)-connected binodal (4(12)6(3))(4(9)6(6))-nia net, in which the gadolinium(III) ions and the mta ligands act as octahedral and as trigonal prismatic nodes, respectively. The magnetic properties of 1-3 were investigated in the temperature range 1.9-300 K. A close fit to the Curie law (1) and weak either antiferro- [J=-0.0063(1) cm(-1) (2)] or ferromagnetic [J=+0.0264(6) cm(-1) (3)] interactions between the Gd(III) ions are observed; the different exchange pathways involved [extended tris-bidentate mta (1) and mu-O(1);kappa(2)O(1),O(2) (2 and 3) plus single syn-syn carboxylate-mta (3)] accounting for these magnetic features. The nature and magnitude of the magnetic interactions, between the Gd(III) ions in 1-3, agree with the small amount of data existing in the literature for these kind of bridges.

A Novel Approach for the Evaluation of Positive Cooperative Guest Binding: Kinetic Consequences of Structural Tightening

Cooperativity is one of the most relevant features displayed by biomolecules. Thus, one of the challenges in the field of supramolecular chemistry is to understand the mechanisms underlying cooperative binding effects. Traditionally, cooperativity has been related to multivalent receptors, but Williams et al. have proposed a different interpretation based on the strengthening of noncovalent interactions within receptors upon binding. According to such an interpretation, positive cooperative binding operates through structural tightening. Hence, a quite counterintuitive kinetic behavior for positively cooperative bound complexes may be postulated: the more stable the complex, the slower it is formed. Such a hypothesis was tested in a synthetic system in which positive cooperative binding was previously confirmed by calorimetric experiments. Indeed, a linear correlation between the thermodynamics (ΔG°) and the kinetics (ΔG(≠)) of guest binding confirmed the expected behavior. These distinctive kinetics provide solid evidence of positive cooperative guest binding, which is particularly useful bearing in mind that kinetic experiments are frequently and accurately carried out in both synthetic and biological systems.

Synthesis and conformational analysis of cyclic homooligomers from pyranoid ε-sugar amino acids.

New pyranoid ε-sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution-phase coupling procedures, cyclic homooligomers containing pyranoid ε-sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X-ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U-shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five-membered hydrogen-bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides.

Expedient synthesis of C3-symmetric hexasubstituted benzenes via Nicholas reaction/[2 + 2 + 2] cycloaddition. new platforms for molecular recognition.

An expedient methodology to synthesize macrocyclic compounds in one step based on the Nicholas reaction is disclosed. The key step features two intermolecular reactions followed by an intramolecular reaction from the starting dicobalt hexacarbonyl-propargylic complex. The macrocycles obtained were modified through [2 + 2 + 2] cycloaddition, generating two new C3-symmetric hexasubstituted benzene structures suitable for molecular recognition purposes.

Three new europium(III) methanetriacetate metal‐organic frameworks: the influence of synthesis on the product topology

Three new metal-organic framework structures containing Eu(III) and the little explored methanetriacetate (C7H7O6(3-), mta(3-)) ligand have been synthesized. Gel synthesis yields a two-dimensional framework with the formula [Eu(mta)(H2O)3]n·2nH2O, (I), while two polymorphs of the three-dimensional framework material [Eu(mta)(H2O)]n·nH2O, (II) and (III), are obtained through hydrothermal synthesis at either 423 or 443u2005K. Compounds (I) and (II) are isomorphous with previously reported Gd(III) compounds, but compound (III) constitutes a new phase. Compound (I) can be described in terms of dinuclear [Eu2(H2O)4](6+) units bonded through mta(3-) ligands to form a two-dimensional framework with topology corresponding to a (6,3)-connected binodal (4(3))(4(6)6(6)8(3))-kgd net, where the dinuclear [Eu2(H2O)4](6+) units are considered as a single node. Compounds (II) and (III) have distinct three-dimensional topologies, namely a (4(12)6(3))(4(9)6(6))-nia net for (II) and a (4(10)6(5))(4(11)6(4))-K2O2; 36641 net for (III). The crystal density of (III) is greater than that of (II), consistent with the increase of temperature, and thereby autogeneous pressure, in the hydrothermal synthesis.

Fluorescent β-Blockers as Tools to Study Presynaptic Mechanisms of Neurosecretion

Several, if not all adrenergic β-blockers (β-Bs), accumulate progressively inside secretory vesicles in a time- and concentration-dependent manner, and could be considered to be false neurotransmitters. This transmitter effect is most likely unrelated to their ability to block adrenergic receptors, but it could explain the delay in lowering arterial pressure in hypertensive patients. We have developed a new drug to monitor the accumulation of β-Bs inside living cells, RCTM-3, which fluoresces in the visible spectrum. Here we describe the procedure to synthesize this new compound, as well as its fluorescent properties, pharmacological profile and its accumulation inside the secretory vesicles of PC12 cells.

Tailor-made copper(II) coordination polymers based on the C3 symmetric methanetriacetate as a ligand

The construction of coordination polymers based on flexible ligands in a pre-designed manner is a difficult task which needs deep knowledge of the ligands involved. With this objective in mind, we have investigated the complex formation between methanetriacetic acid (H3mta) and copper(II) in the presence of 2,2′-bipyridine (bpy) as a blocking coligand to limit the number of coordination sites available at the metal ion. In this primary target, five complexes of formulae [Cu3(bpy)6(mta)](ClO4)3·2H2O (1), [Cu4(bpy)6(mta)2(H2O)2](ClO4)2·20H2O (2), [Cu(bpy)(Hmta)]n (3), {[Cu4(bpy)4(mta)2](ClO4)2·3H2O}n (4) and {[Cu3(mta)2(bpy)3(H2O)]3·3dmf·13H2O}n (5) were synthesized and their crystal structures were solved by single crystal X-ray diffraction. Compounds 1 and 2 are tri- and tetranuclear species, respectively, whereas 3 and 4 are one-dimensional compounds and 5 is a three-dimensional coordination polymer. This series of complexes reveals that methanetriacetate behaves as a tritopic ligand which keeps its C3 symmetry despite its flexibility. On the basis of the structural knowledge of 1–5, a (6,3)-honeycomb network of formula {[Cu12(tppz)6(mta)4(H2O)24](NO3)12·66H2O}n (6) [tppz = tetrakis(2-pyridyl)pyrazine] was prepared by design where each methanetriacetate ligand acts as a three-connected node. Magnetic susceptibility measurements in the temperature range 2.0–300 K for 1–6 show the occurrence of very weak antiferro- (1 and 2) and ferromagnetic (3–5) interactions, whereas an intermediate antiferromagnetic coupling (J = −43 cm−1, the spin Hamiltonian being defined as H = −JS1·S2) is observed for 6. The different size and nature of the magnetic interactions in 1–6 are analyzed and discussed in the light of the respective exchange pathways involved.