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Thermodynamic Characterization of the Self-Assembly Process of a Three Component Heterobimetallic Bisporphyrin Macrocycle ´ lvarez,‡ Antoni Frontera,‡ and Pablo Ballester*,†,‡ Almudena Gonza´lez-A Catalan Institution for Research and AdVanced Studies (ICREA), Pg. Lluı´s Companys 23, 08010-Barcelona, and Institute of Chemical Research of Catalonia (ICIQ), AV. Paı¨sos Catalans 16, 43007-Tarragona, Spain ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: June 30, 2009
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 × 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. Introduction Simple monoporphyrins have found applications in catalysis,1 chemical sensing,2 and molecular optoelectronic devices.3 Consequently, the incorporation of porphyrin building blocks into well-defined, self-assembled architectures represents a logical strategy for the construction of elaborate and functionalized multiporphyrin supramolecular assemblies4 and materials.5 In recent years, the structures of many discrete and kinetically labile metal-mediated assemblies that contain porphyrin units have been fully elucidated both in solution and in the solid state.6 Detailed understanding of the solution behavior of molecular self-assembly systems becomes especially important when trying to rationalize the results of their applications in different functions like molecular recognition, catalysis, optoelectronic molecular devices, and so forth. In general, accurate information on the extent of the self-assembly process and the composition of the mixture is required. In spite of these requirements, the detailed thermodynamic characterization of the multiporphyrin self-assembly process has only been undertaken in a limited number of examples.7 This is most likely due to the inherent complexity of these systems. In order to describe the solution behavior of a mixture of molecules capable of reversible association, it is necessary to know both their initial concentrations as well as the stoichiometry and overall stability constants of all the species (complexes) in which they are involved. Theoretical treatments of self-assembled macrocyclizations occurring under thermodynamic control were put forward by * To whom correspondence should be addressed. E-mail: pballester@ iciq.es. Tel: +34 977 920 206. Fax: +34 977 920 221. † ICREA. ‡ ICIQ.
Hunter7 and Ercolani.8 In particular, Ercolani has recently presented an interesting and exhaustive overview of the principles underlying the thermodynamics of self-assembly in solution using selected examples of metal-mediated porphyrin assemblies.9 Both treatments are based in two fundamental quantities, the effective molarity, EM,10 and the stability constant for the intermolecular interaction, Kinter. Hence, the knowledge of these two quantities, Kinter and EM, allows the modeling of the solution behavior of a mixture of components operating under thermodynamic equilibrium. Furthermore, the EM value can also be used in assessing the presence of cooperativity in self-assembly systems.11 It is worth noting that due to the dependence of the molecular self-assembly process on concentration (entropy), the quantitative formation of a cyclic aggregate will only take place at a specific concentration and appropriate monomer molar ratios if the condition EMKinter g 185n is satisfied.12 Often times, the system will not produce a single or major aggregate but an equilibrating mixture of varied cyclic aggregates (i.e., trimer and tetramer) and open aggregates. One of the less-explored strategies for the self-assembly of multiporphyrin cyclic assemblies involves the combination of three different but complementary ditopic molecular components, a metalated bisporphyrin unit (PorM1sM1Por), a ditopic ligand with two Lewis basic sites (LsL), and a metal center (M2) containing two free coordination sites. This strategy grants easy access to the preparation of tetrameric cyclic architectures containing two different types of metallic centers as structural units (Scheme 1). Different structures can be generated when one considers that the ditopic ligand can have two basic sites that can be identical (LsL) or not (LsL′).
10.1021/jp9053742 CCC: $40.75 2009 American Chemical Society Published on Web 07/27/2009
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SCHEME 1: Schematic Representations of Three-Component Self-Assemblies Producing Heterobimetallic Multiporphyrin Cyclic Structures
In fact, using the three component strategy, one of us reported convincing evidence for the quantitative self-assembly of a heterobimetallic macrocycle at millimolar concentrations.13 Almost simultaneously, van Leeuwen et al. described the use of the same methodology for the preparation of “supramolecular bidentate ligands”.14 It is likely that the experimental difficulties associated with the accurate determination of the overall stability constant for a three component assembly (AmBnCl) has restricted the discussion of the theoretical treatment of self-assembled macrocyclization to examples involving aggregates formed by just one or two different components with general formulas of Am or AmBn. We report here the first application of the theoretical treatment of self-assembled macrocyclization to the experimental thermodynamic characterization of the self-assembly process of the three-component heterobimetallic macrocycle 4. We experimentally calculated the overall stability constant of the cyclic tetrameric assembly 4 and derived its effective molarity value (EM). The microscopic stability constants of the two different intermolecular interactions that are involved in the cyclic structure (Kinter) are extracted from the study of simple reference models. Using the calculated values for Kinter, we statistically determined the overall stability constants of the plausible self-assembled complexes that may exist at equilibrium. Next, we applied numerical methods to investigate the effect of total concentration in the self-assembled equilibrium speciation of the three monomers. We showed that under strict stoichiometric control there is reasonably good agreement between the simulated speciation and the experimental data. This result serves to validate the application of the theoretical treatment of self-assembled macrocyclization to the three component strategy. Experimental Section Synthesis. Bisporphyrin 1-H4 was obtained by reaction of the monoamino porphyrin 5-(4-aminophenyl)-10,25,20-tris(4penthylphenyl)-21H,23H-porphine15 with 2,6-pyridinedicarboxylic acid chloride in dichloromethane under anhydrous conditions. Bisporhyrin 1-H4 was purified using column chromatography and isolated in 60% yield. Metalation was achieved by reaction of the free base form of porphyrin 1-H4 with Zn(II) acetate in a mixture of chloroform and methanol. The Zn-metalated porphyrin 1 was isolated in almost quantitative yield after purification with an alumina column. The ditopic ligand 4-pyridyldiphenylphosphine 316 and the cycloocta-1,5-diene methyl palladium(II) chloride 2 · COD17 complex were synthesized following procedures described in the literature. NMR Measurements and Titrations. 1H NMR spectra were recorded on Bruker Advance 400 and Bruker Advance 500 Ultra shield NMR spectrometers. 1H NMR titrations were carried out by running a spectrum of the titrand solution in a deuterated solvent at a mM concentration and adding to it incremental aliquots of a titrant solution that was 10 times more concentrated. To avoid the dilution of the titrand solution, the titrant solutions
were prepared using the solution of the titrand as solvent. After each addition, a new NMR spectrum was acquired. Deuterochloroform was previously deacidified by passing through a short column of Aluminum Oxide 60 active basic. UV-vis Spectroscopy and Titrations. UV-vis spectra were measured on a UV-vis spectrophotometer Shimadzu UV2401PC. The UV-vis titrations were carried out through a similar methodology as described above. In this case, the titrand concentration was around 1 µM and to cover a wide range of molar ratios it was necessary to prepare several titrant solutions with different concentrations. The data obtained from the UV-vis spectrophotometric titrations were analyzed by fitting the whole series of spectra at 1 nm interval using the software SPECFIT 3.0,18 which uses a global system with expanded factor analysis and Marquardt least-squares minimization to obtain globally optimized parameters. Isothermal Titration Calorimetry (ITC). Stability constants and enthalpies of complexation, ∆H, of the ligands 3 or 5 with the metal Zn(II) or Pd(II) were determined using a microcalorimeter Microcal VPITC. ITC titrations were performed by adding microliter injections of the titrant solution to a titrand solution 7-10-fold more diluted placed in the calorimetric cell. After each injection, the heat change was monitored. The titration data were fitted using the binding model implement in the Microcal ITC Data Analysis software or combining the speciation simulation module of the SPECFIT software with the equations for the calculation of the heat content (Q(i)) and for the heat release (∆Q(i)) after any injection i as described in the text. Results and Discussion General Considerations. The promiscuity in the complexation properties of a symmetric LsL type ligand, that is, 4,4′bipyridine, toward two metals ions, that is, M1 ) Zn(II) and M2 ) Pt(II), implicated in the structure of the self-assembled macrocycle (Scheme 1) hampered the accurate thermodynamic characterization of the three component assembly.13 On the contrary, the use of a dissymmetric ditopic LsL′ ligand, in which the two individual basic centers are selective in coordinating to one specific metal center of the assembly, simplifies the analysis of the titration data avoiding the production of competitive homometallic cyclic or acyclic architectures. van Leeuwen et al. have reported that the pyridylphosphine ligand 3 selectively binds Zn-porphyrins through its pyridyl nitrogen, leaving the phosphorus atom available for transition metal coordination.19 For this reason, we decided to use pyridylphosphine ligand 3 in tackling the thermodynamic characterization of the multicomponent equilibrium self-assembly of Zn-bisporphyrin 1 (previously used in our laboratory) and the Pd(II) complex 2 · COD (source of the external metal center) yielding the supramolecular macrocyle 4. Although the Pd(II) complex 2 · COD includes a cis coordinating cyclooctadiene (COD) ligand, it should not be considered a cis-protected metal center. The COD can easily be replaced by two phosphines yielding exclusively trans square-planar complexes.20 However, it is also documented that square planar complexes resulting from the substitution of COD with phosphine ligands can potentially exist as cis and trans isomers. In fact, for chelating diphosphines it is well known that the bite angle has a large influence on the cis-trans isomerization equilibrium.21 In short, the use of 2 · COD introduces a level of flexibility in the coordination geometry adopted by the external metal center in macrocyle 4 (cis or trans, see Figure 1).
Three Component Heterobimetallic Bisporphyrin Macrocycle
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Figure 1. Chemical structures of the molecular components used in the self-assembly of the heterobimetallic macrocycle 4 studied in this work. CAChe minimized structures of two possible cyclic assemblies with cis and trans coordination geometry in the external metal center (Pd). Nonpolar hydrogen atoms are removed for clarity. The Zn and Pd metal centers are shown as CPK spheres.
SCHEME 2: Schematic Representation of the Species Involved in the Binding Equilibria of 3 with Zn-Bisporphyrin 1a
a The stepwise binding constants K11 and K11T12 are indicated as well as their relationship with Km (the microscopic binding constant), R (cooperativity factor), and statistical correction factors.
Study of Reference Models. To assist in the study of the three component self-assembly process of Zn-bisporphyrin 1 with the Pd(II) metal center 2 · COD and 4-pyridyldiphenylphosphine ligands 3, we investigated in detail the following reference models: (a) the binding interaction of Zn-bisporphyrin 1 with 4-pyridyldiphenylphosphine ligand 3, and (b) the coordination properties of 4-pyridyldiphenylphosphine 3 and triphenylphosphine 5 to cycloocta-1,5-diene methyl Pd(II) chloride 2 · COD. The assembly process of the macrocyclic bimetallic architecture 4 based on the three molecular subunits (Zn-bisporphyrin 1, 4-pyridyldiphenylphosphine 3 and Pd(II) metal center 2 · COD) will be explained in detail in a forthcoming section. Binding of 1 with 3. The interaction between 4-pyridyldiphenylphosphine 3 and a simple Zn-monoporphyrin, 5,10,15,20tetraphenyl-21H,23H-porphine (TPP), leads to the exclusive formation of a 1:1 complex. The 3 · TPP complex has a stability constant of K11 ) 6.1 × 103 M-1 in toluene.22 When we probed the interaction of 3 and TPP using 1H NMR spectroscopy, we observed that the addition of 0.2 equiv of 3 to a toluene solution of TPP resulted in large upfield shifts of the pyridyl protons (∆δR ) -5.19 ppm, ∆δβ ) -1.57 ppm) due to the shielding effect of the porphyrin ring. The protons in the phenyl rings of 3 were observed as three different signals (δHo) 6.35 ppm, δHm ) 6.65 ppm, and δHp) 6.78 ppm) experiencing a moderate upfield shift with respect to the corresponding proton signals in free ligand (multiplet at δH) 7.29 ppm). The addition of incremental amounts of 3 caused downfield shifts of all the aforementioned proton signals. In agreement with the findings reported by van Leuween et al.,22 these observations support
that ligand 3 interacts exclusively through axial coordination of the nitrogen atom to the zinc porphyrin and that free and bound 3 are involved in a chemical exchange that is fast on the 1 H NMR time scale. Typically, Zn-porphyrins display a red shift in both their Soret and Q bands upon axial coordination to amine ligands.23,24 The association constants for these complexes are usually in the range of 103-105 M-1 and UV-vis spectroscopy titrations are appropriate to calculate such values with accuracy. In the case of the interaction between bisporphyrin 1 and pyridylphosphine 3, it should be expected to obtain species with higher stoichiometry than the simple 1:1 complex discussed above for the 3 · TPP complex. Thus, when the concentration of 3 is low with respect to that of the bisporphyrin, 1:1 complexes will be expected to form preferentially. Further addition of the amine will lead to the formation of complexes with 1:2 stoichiometry (bisporphyrin: pyridylphosphine) as the major species (Scheme 2). Our interest lies in determining the stepwise binding constants for coordination of 3 with 1 (K11 and K11T12) and in evaluating the extent of cooperativity in the binding process. Accordingly, we performed UV-vis titrations of bisporphyrin 1 ([1] ) 4 × 10-5 M) with pyridylphosphine 3 in toluene. We followed the spectroscopic changes experienced in the Q bands of 1 upon complexation with 3. The obtained titration data were fit, using the SPECFIT18 software, to a binding model consisting of three colored species, free Zn-bisporphyrin 1 and two complexes that involve 1 in different binding stoichiometry with respect to pyridylphosphine 3 (1:1 and 1:2). We determined the following
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values for the stepwise binding constants: K11 ) 6.3 × 103 M-1 and K11T12 ) 1.6 × 103 M-1. Considering the relationship K11 ) 2Km, we calculated the microscopic binding constant for the Zn-pyridine interaction as Km(N-Zn) ) 3.15 × 103 M-1. This value is in complete agreement with the one determined by van Leeuwen et al. for the same interaction using TPP.22 We used the relationship K11T12 ) KmR/2 to determine the cooperativity factor. We obtained a value of R ) 1.01 indicating that the porphyrin binding sites are independent. We also probed the complexation process of bisporphyrin 1 with pyridylphosphine 3 by means of isothermal titration calorimetry (ITC) experiments.25 ITC is a thermodynamic technique that directly measures the heat released or absorbed due to the interaction between any two molecules. The quantity of heat absorbed or released is directly proportional to the amount of binding. As the system reaches saturation, the heat signal diminishes until only heat of dilution of the titrant is observed. A binding isotherm can be obtained from a plot of the normalized integration heats from each addition of titrant against the ratio of titrant (ligand added) to titrand (binding partner in the cell). The visual analysis of the obtained binding curve usually gives a clear indication on the number of stepwise binding events (number of sets of binding sites in the binding partner) that are operative in the binding process and if all of them are identical or not. Thus, a binding isotherm with a shape of a single sigmoidal curve is indicative of the existence of “n” binding events between the interacting substances that should be almost identical in the case that n > 1. The value of the molar ratio of the ligand to the binding partner at the inflection point of the sigmoidal binding curve determines the number “n” of stepwise binding events of the system. The “n” value is also used to assign the stoichiometry of the complex or complexes that are formed during the binding process. The determination of n values
