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The molecular self-assembly of macrocycle 4 is induced by the simultaneous coordination of two molecules of 4-pyridyldiphenylphosphine 3, a highly selective ditopic ligand, to Zn-bisporphyrin 1 and a square-planar Pd(II) complex 2.COD. We report a detailed thermodynamic characterization of the assembly process based on the quantification of each one of the two metal(Zn,Pd)-ligand(N,P) pairwise binding interactions implicated in the supramolecular macrocycle and its effective molarity value (EM). The experimental values of the pairwise metal-ligand interactions have been derived from UV-vis, NMR titrations, and isothermal titration calorimetry experiments of reference model systems. In turn, an EM = 1 x 10(-2) M has been determined by relating the experimental overall stability constant determined for the cyclic assembly with the equation to evaluate the statistical (noncooperative) self-assembly equilibrium constant. We used numerical methods (SPECFIT program) to predict the solution behavior (speciation) of two mixtures of the three molecules 1, 3, and 2.COD in a 1:2:1 relative stoichiometry at two different overall concentrations. The method uses the overall stability constant values and the stoichiometries of eleven species (complexes) implicated in the multicomponent equilibrium self-assembly of 4. Estimated stability constants of some of the species were statistically determined. The agreement observed between the theoretical simulations and the experimental data validates the suitability of the theoretical treatment of self-assembly macrocyclization in a three component strategy.

During the last ten years, our research group has been applying metal-mediated self-assembly processes to the construction of multiporphyrin functional assemblies. The construction of well-defined and discrete supramolecular structures resulting from self-assembly requires the use of multiple and separated connections operating in one or more closed loops. Consequently, the great majority of the multiporphyrin assemblies that we have prepared are of cyclic nature. We have placed special emphasis not only on the characterization in solution of the formed assemblies but also on the thermodynamic characterization of the assembly process and in the assessment of cooperativity. Finally, we also present examples in which functionality has been derived from the three-dimensional structures of multicomponent assemblies.

The efficient use of a polyazamacrocycle as chiral solvating agent (CSA) for the determination of the enantiomeric excess of different carboxylic acids has been studied. All the data agree with the formation of multimolecular diastereomeric complexes in solution, which render good splitting of the NMR signals for the enantiomers of the acids (up to ΔΔ δ = 0.20 ppm) using a small amount (even 0.125 equiv) of the receptor.

The syntheses of new C 2 -symmetrical optically active macrocyclic polyazacyclophanes have been achieved from a common synthetic precursor, which has been prepared in enantiopure form by means of a chemoenzymatic pathway. The effect of the aromatic spacer on the flexibility of the systems is also discussed.

Two different macrocyclic hosts are amplified and expressed by metal guests from the same dynamic combinatorial library (DCL) of oligoimines. We studied the thermodynamic template effect and the structure of the final compounds by a combination of ESI-MS, UV and NMR spectroscopy, and X-ray crystallography techniques. The use of BaII or CdII metal salts allows the selective synthesis of dimeric [2+2] or trimeric [3+3] cyclic structures, respectively. We observed a cooperative molding effect for the Cd ion-templated process. A study of the DCL behavior in situ demonstrated a second level of molecular diversity, based on the imine/α-methoxyamine equilibrium. This process is probably promoted by metal ion coordination and shows an interesting diastereoselectivity in the formation of the new chiral centers. Semiempirical (PM3) theoretical calculations also support these experimental observations. Reduction of the imine bonds allows the selective preparation and isolation of each macrocyclic polyamine on a synthetic scale and in a one-pot procedure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

During the last ten years, our research group has been applying metal-mediated self-assembly processes to the construction of multiporphyrin functional assemblies. The construction of well-defined and discrete supramolecular structures resulting from self-assembly requires the use of multiple and separated connections operating in one or more closed loops. Consequently, the great majority of the multiporphyrin assemblies that we have prepared are of cyclic nature. We have placed special emphasis not only on the characterization in solution of the formed assemblies but also on the thermodynamic characterization of the assembly process and in the assessment of cooperativity. Finally, we also present examples in which functionality has been derived from the three-dimensional structures of multicomponent assemblies.

The interaction of a synthetic enantiopure azamacrocyclic receptor (L) with biologically important chiral dicarboxylates (A, 1-7) has been studied by means of potentiometric titrations in 0.15 M NaCl aqueous solution in a wide pH range. This macrocycle forms strong complexes of the type [HnLA](n-2) (with n = 0-5). As a general trend, the binding is much tighter at basic or neutral pH than in acidic medium. Interestingly, nonprotected excitatory amino acids (Asp and Glu) are strongly bound even at acidic pH. Regarding selectivity, the receptor showed stereoselective binding toward those substrates bearing an H-bonding donor at Calpha, being S-selective in most of the cases, except for glutamic acid. Thus, L displayed an excellent enantioselectivity for (S)-malate dianion (KS/KR = 11.50 at pH 10.0 and KS/KR = 6.86 at pH 7.0) and exhibited moderate enantiopreference for (S,S)-tartrate (KSS/KRR = 3.01 at pH 10 and KSS/KRR = 1.70 at pH 7.0). For this last anion, a very good diastereopreference was also observed (KSS/KRS = 8.46 at pH 10 and KSS/KRS = 4.99 at pH 7.0). On the contrary, L is smoothly R-selective toward (R)-Glu (KR/KS = 3.22 at pH 10 and KR/KS = 2.05 at pH 7.0) due to its longer and more flexible molecular structure. The stereoselectivity of the corresponding complexes decreased when decreasing pH values. For the hydroxy derivatives, mass spectrometry also reflected the trends observed by potentiometry and confirmed the receptor:dicarboxylate 1:1 stoichiometry of the supramolecular complexes. Additional experimental techniques were used to study the most stereoselective example. Solution studies by NMR suggested a good geometrical complementarity between the malate dianion and the receptor, which showed a predominant helical conformation in solution. Besides, self-diffusion rates (PGSE) of the diastereomeric complexes with malate also agree with binding data. Circular dichroism was also used in this case at different pH values, showing a very good correlation between the helical content of the receptor and the stereoselectivity of the molecular recognition process.

3 pages, 3 figures, 2 schemes.-- PMID: 16718310 [PubMed].-- Available online Apr 21, 2006., Cadmium promoted diastereoselective amplification of a single member from a dynamic combinatorial library of stereoisomeric oligoimines of different sizes allows the efficient preparation of a new macrocyclic polyamine., Financial support from the Spanish M.E.C. (CTQ-2004-04185) is gratefully acknowledged. We also thank personal support for A. G.-A. (III PRI, Principado de Asturias), and I. A. (Ramón y Cajal program, M.E.C.).

The highly enantioselective molecular recognition of the malate dianion by a synthetic receptor in aqueous solution has been studied by potentiometric titrations, mass spectrometry (ESI-MS), diffusion measurements (PGSE NMR) and molecular modeling. Garcia-España Monsonis, Enrique, Enrique.Garcia-Es@uv.es