Influence of H-bond strength on chelate cooperativity

Intermolecular complexes formed between metalloporphyrins and pyridine ligands equipped with multiple H-bond donors and acceptors have been used to measure the free energy contributions due to intramolecular ether-phenol H-bonding in the 24 different supramolecular architectures using chemical double mutant cycles in toluene. The ether-phenol interactions are relatively weak, and there are significant populations of partially bound states where between zero and four intramolecular H-bonds are made in addition to the porphyrin-ligand coordination interaction. The complexes were analyzed as ensembles of partially bound states to determine the effective molarities for the intramolecular interactions by comparison with the corresponding intermolecular ether-phenol H-bonds. The properties of the ether-phenol interactions were compared with phosphonate diester-phenol interactions in a closely related ligand system, which has more powerful H-bond acceptor oxygens positioned at the same location on the ligand framework. This provides a comparison of the properties of weak and strong H-bonds embedded in the same 24 supramolecular architectures. When the product of the intermolecular association constant and the effective molarity KEM > 1, there is a linear increase in the free energy contribution due to H-bonding with log EM, because the intramolecular interactions contribute fully to the stability of the complex. When KEM < 1, the H-bonded state is not significantly populated, and there is no impact on the overall stability of the complex. Intermolecular phosphonate diester-phenol H-bonds are 2 orders of magnitude stronger than ether-phenol H-bonds in toluene, so for the phosphonate diester ligand system, 23 of the 24 supramolecular architectures make intramolecular H-bonds. However, only 8 of these architectures lead to detectable H-bonding in the ether ligand system. The other 15 complexes have a suitable geometry for formation of H-bonds, but the ether-phenol interaction is not strong enough to overcome the reorganization costs associated with making intramolecular contacts, i.e., KEM < 1 for the ether ligands, and KEM > 1 for the phosphonate diester ligands. The values of EM measured for two different types of H-bond acceptor are linearly correlated, which suggests that EM is a property of the supramolecular acrchitecture. However, the absolute value of EM for an intramolecular phosphonate diester H-bond is about 4 times lower than the corresponding value for an intramolecular ether-phenol interaction embedded in the same supramolecular framework, which suggests that there may be some interplay of K and EM.     

Dissection of complex molecular recognition interfaces

The synthesis of a family of zinc porphyrins and pyridine ligands equipped with peripheral H-bonding functionality has provided access to a wide range of closely related supramolecular complexes featuring between zero and four intramolecular H-bonds. An automated UV/vis titration system was used to characterize 120 different complexes, and these data were used to construct a large of number of different chemical double mutant cycles to quantify the intramolecular H-bonding interactions. The results probe the quantitative structure-activity relationship that governs cooperativity in the assembly of complex molecular recognition interfaces. Specifically, variations in the chemical structures of the complexes have allowed us to change the supramolecular architecture, conformational flexibility, geometric complementarity, the number and nature of the H-bond interactions, and the overall stability of the complex. The free energy contributions from individual H-bonds are additive, and there is remarkably little variation with architecture in the effective molarity for the formation of intramolecular interactions. Intramolecular H-bonds are not observed in complexes where they are geometrically impossible, but there are no cases where excellent geometric complementarity leads to very high affinities. Similarly, changes in conformational flexibility seem to have limited impact on the values of effective molarity (EM). The major variation that was found for all of the 48 intramolecular interactions that were examined using double mutant cycles is that the values of EM for intramolecular carboxylate ester-phenol H-bonds (200 mM) are an order of magnitude larger than those found for phosphonate diester-phenol H-bonds (30 mM). The corresponding intermolecular phosphonate diester-phenol H-bonds are 2 orders of magnitude more stable than carboxylate ester-phenol H-bonds, and the large differences in EM may be due to some kind of compensation effect, where the stronger H-bond is harder to make, because it imposes tighter constraints on the geometry of the complex.     

Hydrogen bond-free flavin redox properties: managing flavins in extreme aprotic solvents

We report a simple, single step reaction that transforms riboflavin, which is insoluble in benzene, to tetraphenylacetyl riboflavin (TPARF), which is soluble in this solvent to over 250 mM. Electrochemical analysis of TPARF both alone and in complexes with two benzene-soluble flavin receptors: dibenzylamidopyridine (DBAP) and monobenzylamidopyridine (MBAP), prove that this model system behaves similarly to other previously studied flavin model systems which were soluble only in more polar solvents such as dichloromethane. Binding titrations in both benzene and dichloromethane show that the association constants of TPARF with its ligands are over an order of magnitude higher in benzene than dichloromethane, a consequence of the fact that benzene does not compete for H-bonds, but dichloromethane does. The alteration induced in the visible spectrum of TPARF in benzene upon the addition of DBAP is very similar to the difference produced by transfer to dichloromethane, further indicating that the flavin head group engages in a similar degree of hydrogen bonding with dichloromethane as with its ligands. This work underlines the importance of using a truly nonpolar solvent for the analysis of the effects of hydrogen bonding on the energetics of any biomimetic host-guest model system.     

Hydrogen-Bonding Interactions between Harmane and Pyridine in the Ground and Lowest Excited Singlet States

Abstract— A spectroscopic (UV-visible, Fourier transform IR, steady-state and time-resolved fluorescence) study of hydrogen-bonding interactions between harmane (1-meth-yl-9H-pyrido/3,4- b /indole) and pyridine in the ground and lowest excited singlet state is reported. In low polar and weakly or nonhydrogen-bonding solvents, such as cy-clohexane, chloroform, carbon tetrachloride, toluene and benzene, the analysis of the spectroscopic data indicates that harmane and pyridine form 1:1 stoichiometric hydrogen-bonded complexes in both the ground and singlet excited states. The formation constants of the complexes are greater in the excited than in the ground state. Hydrogen-bonding interaction in the excited state is essential for the quenching of the fluorescence of harmane by pyridine. The stabilities of the hydrogen-bonded complexes between harmane and pyridine diminish as the polarity and hydrogen-bonding ability of the solvent increase.     

Effects of polar protic solvents on dual emissions of 3-hydroxychromones

3-Hydroxychromones (3HC), exhibit dual emissions highly sensitive to solvent properties due to excited state intramolecular proton transfer (ESIPT). Therefore, 3HCs find wide applications as fluorescence probes in biological systems. Here, it is particularly important to understand the fluorescence behaviour of 3HCs in polar environments. Herein, we studied 3-hydroxyflavone, 2-(2-furyl)-3-hydroxychromone and 2-(2-benzofuryl)-3-hydroxychromone in high polarity solvents characterized by different H-bond donor abilities, donor concentrations and acceptor abilities. Our results show that the dual emissions of the dyes are insensitive to solvent basicity but strongly depend on the two other parameters. Moreover, furyl-and benzofuryl-substituted dyes were significantly more sensitive than the 3-hydroxyflavone to H-bond donor ability, while all three dyes showed roughly equivalent high sensitivity to H-bond donor concentration. These results can be explained by different mechanisms. Thus, the sensitivity of all three dyes to increasing concentrations of H-bond donors probably results from increase in the population of solvated dye with disrupted intramolecular H-bonds. Meantime, the sensitivity to H-bond donor ability of the solvent, observed mainly with furyl and benzofuryl dyes, is probably related to the strength of the H-bonds between the solvent and the 4-carbonyl group of the dye with intact intramolecular H-bonds. The present results provide new insights for further applications of 3HC derivatives as environment-sensitive probes and labels of biological molecules.     

The study of enantioselectivity in copper complexes with some 1,3-dicarbonyl ligands by circular dichroism spectroscopy

The circular dichroism spectra of copper(II) complexes with hydroxymethylene camphor, hydroxymethylene menthone, and trifluoroacetyl camphor as well as of mixed-ligand complexes with acetylacetone have been studied in different solvents at different temperatures. The changes observed in the spectra are attributed to a strong coordination of the solvent molecules in axial positions of square-planar complexes. It has been shown that the contributions of two ligands to the net spectrum of circular dichroism are additive and that the absorption spectra of the complexes do not depend on the optical configuration of the ligands. The conclusion has been drawn that no noticeable inter-ligand interactions exist in the complex and no enantioselective effects are exhibited.     

Diels-Alder Addition of Dicyclopentadiene with Cyclopentadiene in Polar Solvents

Diels-Alder addition of dicyclopentadiene and cyclopentadiene in polar solvents has been studied to produce tricyclopentadiene（TCPD） that is a potential high-density fuel precursor. GC and MS analysis shows that the adducts contain two isomers, namely exo- and endo-TCPD. Theoretical simulation shows that although the transition state of endo-TCPD has a lower activation energy, exo-TCPD is thermodynamically preferred. Polar solvents can accelerate the reaction rate and improve the exo/endo ratio of TCPD because the transition state of exo-TCPD has a higher polarity than that of endo-TCPD. The solvent effect follows the order of polarity： benzyl methanol〉cyclohexanone〉toluene. The conversion rises when the temperature ranges from 120 to 150 ℃, but the selectivity of TCPD slightly decreases. Increasing the pressure can improve the conversion but the exo/endo ratio of TCPD is unchanged. The apparent kinetics in different solvents was determined via nonlinear regression. The activation energies are 99.47, 101.15, and 107.32 kJ/mol for benzyl methanol, cyclohexanone, and toluene, respectively. The optimal reaction conditions are as follows： benzyl methanol as solvent, temperature 150 ℃, and pressure 900 kPa. After an 11-hour reaction, a conversion of 58.0%, a TCPD selectivity of 95.7%, and an exo/endo ratio of 1/5.3 has been obtained.     

Thallium-205 NMR study of ti(I) macrocyclic complexes in nonaqueous solvents

Thallium-205 and carbon-13 NMR measurements were used to determine, in nonaqueous solvents, the stabilities of thallium(I) complexes with macrocyclic ligands of different structures but very nearly the same cavity sizes. In a given solvent the complexing abilities of the ligand vary in the order DA18C6 > 18C6 > DC18C6 > DB18C6 > DT18C6. In all cases the stabilities of the complexes varied inversely with the Gutmann donicities of the solvents.     

Stepwise effective molarities in porphyrin oligomer complexes: preorganization results in exceptionally strong chelate cooperativity

Complexes of zinc porphyrin oligomers with multivalent ligands can be denatured by adding a large excess of a monodentate ligand, such as quinuclidine. We have used denaturation titrations to determine the stabilities of the complexes of a cyclic zinc-porphyrin hexamer with multidentate ligands with two to six pyridyl coordination sites. The corresponding complexes of linear porphyrin oligomers were also investigated. The results reveal that the stepwise effective molarities (EMs) for the third through sixth intramolecular coordination events with the cyclic hexamer are extremely high (EM = 10(2)-10(3) M), whereas the values for the linear porphyrin oligomers are modest (EM ≈ 0.05 M). The speciation profiles for the denaturation reactions demonstrate that intermediate species are not significantly populated and that these equilibria are well described by a highly cooperative two-state model.     

Palladium-catalyzed Heck coupling reactions using different fluorinated phosphine ligands in compressed carbon dioxide and conventional organic solvents

Palladium-catalyzed Heck reaction of iodobenzene and styrene was investigated in compressed CO₂ using different fluorinated phosphine compounds as ligands at a temperature of 70 °C. The reaction mixture is a single phase at 12 MPa but biphasic at 8 MPa, a little higher than the critical pressure of pure CO₂ under the reaction conditions used. Although the solubility of fluorinated ligands is very high in dense CO₂, they have marginal improvements in Heck conversion in this medium compared with a non-fluorinated ligand of triphenylphosphine. The activity of palladium complexes strongly depends on the kind of phosphine compound used, in the order of bis(pentafluorophenyl)phenylphosphine (III)>triphenylphosphine (I), tris(pentafluorophenyl)phosphine (IV)>diphenyl(pentafluorophenyl)phosphine (II), tris(p-fluorophenyl)phosphine (V)>tris(p-trifluoromethyl phenyl)phosphine (VI), 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (VII), for the homogeneous reaction at 12 MPa. This order of effectiveness of these ligands is different from those obtained in conventional organic solvents. Hexane, toluene, ethanol, and N-methylpyrrolidone (NMP) showed maximum conversions with the ligands VI, IV, V and VII, respectively. The conversion in CO₂ with the ligand III is comparable with those in polar solvents of ethanol and NMP, and larger than those in hexane and toluene in the presence of the best ligands. The dense CO₂ may affect the specific activity of palladium complex catalysts and/or the reactivity of reacting species. Small quantities of fluorinated products were observed to form at high pressure of CO₂ and this is direct evidence of P–C bond cleavage during Heck reaction in dense CO₂. The activity of palladium complexes with those ligands is higher in more polar solvent.     

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N−H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the –NHCO– functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.     

Solvent effect on SiH stretching bands of substituted silanes

Infrared frequencies of SiH stretching vibration bands of dimethyl naphthoxy silane and dimethyl quinoxy silane have been measured in a number of solvents. The results have been used to examine the importance of the bulk dielectric effect and specific solute—solvent interactions. The deviation from the Kirkwood—Bauer—Magat relationship produced by polar, as well as by non-polar, solvents is analysed in terms of the formation of molecular complexes.     

Influence of the solvent on the extraction of copper(II) from nitrate medium using salicylideneaniline

Different solvents including cyclohexane, dichloromethane, chloroform, toluene, 1-octanol, and methyl isobutyl ketone (MIBK) have been evaluated in extracting copper(II) from nitrate medium by salicylideneaniline. Extracted species differs from solvent to solvent: CuL₂ in cyclohexane, toluene, 1-octanol, and methyl isobutyl ketone. However, in dichloromethane or chloroform, there are two complexes of the type CuL₂ and CuL₂(HL). The extraction constants and percentage of extraction (%E) are calculated for different solvents. Solvent played an important role in recovering copper(II) from the aqueous solution, thus affecting the extraction equilibrium and extraction efficiency. The nonpolar solvent showed better performance than the polar solvent. The maximum extraction efficiency was 85.75% at pH?=?4.5, which was from cyclohexane.     

The complexation of the ammonium ion by 18-crown-6 in different solvents and by noncyclic ligands,crown ethers and cryptands in methanol

The complexation of the ammonium ion with the macrocyclic ligand 18-crown-6 was studied using calorimetric and potentiometric titrations in different solvents. In water and dimethyl sulphoxide the stability constants had the lowest values compared with all other solvents examined. No specific interactions between the ammonium ion and solvent molecules were observed. Crown ethers formed more stable complexes in methanol with the ammonium ion than diaza crown ethers. The most stable complexes were formed with cryptands. The highest values of the stability constants for the reaction with macrocyclic and macrobicyclic ligands were measured if the dimensions of the ammonium ion and of the cavities were nearly identical.     

Effect of solvent upon CH...O hydrogen bonds with implications for protein folding

The series of CH...O bonds formed between CF(n)H(4-n) (n = 0-3) and water are studied by quantum calculations under vacuum and in various solvents, including aqueous environment. The results are compared with the OH...O bond of the water dimer in the same solvents. Increasing polarity of the solvent leads in all cases to a lessening of the H-bond interaction energy, in a uniform fashion such that the CH...O bonds all remain weaker than OH...O in any solvent. These H-bond weakenings are coupled to a shortening of the inter-subunit separation. The contraction of the covalent CH bond to the bridging proton is reduced as the solvent becomes more polar, and the blue shift of its stretching vibration is likewise diminished. A process is considered that simulates protein folding by starting from a pair of noninteracting subunits in aqueous solvent and then goes to a H-bonded pair within the confines of a protein environment. This process is found to be energetically more favorable for some of the CH...O H-bonds than for the nominally stronger conventional OH...O H-bond. This finding suggests that CH...O bonds can make important energetic contributions to protein folding, on par with those made by traditional H-bonds.     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

Contribution des fonctions thermodynamiques en phase vapeur et en solution à l'étude de l'équilibre conformationnel de la chloro-4 cyclohexanone

The thermodynamic properties related to the conformational equilibrium of 4-chloro cyclohexanone have been determined by n.m.r. and i.r. with two unpolar solvents CCl₂ = CCl₂ and CS₂. The predominance of a conformation is discussed by means of the enthalpic and entropic contributions of the isolated and solvated molecules: intramolecular interactions of the isolated molecules and a quadrupolar effect at the time of the solvation explain that result.     

Modification of Phosphoryl Substituents of Phthalocyanines As a Route to Targeted Tuning of Lipophilic-Hydrophilic Properties

О′,О′-Diethyl-[2-(3,4-dicyanophenoxy)phenyl]phosphonate has been synthesized for the first time and introduced in template condensation on lithium and zinc cations to prepare a series of new neutral and acidic phthalocyanines containing four fragments of 2-oxyphenylphosphonic acid. A scheme of modification of the peripheral substituents at the phosphoryl group was developed allowing a targeted synthesis of phthalocyanine ligands and complexes soluble in organic solvents or aqueous media.     

Kinetics and mechanism of urethane reactions: Phenyl isocyanate–alcohol systems

Urethane reactions of phenyl isocyanate alcohol systems with toluene as solvent and various aprotic polar solvents (including tertiary amines) as additives were carried out at constant temperature of 10–40°C. Analysis of the variation of the second order rate constants of these systems and those available in the literature indicates that formation of the hydrogen bonding complexes (alcohol with phenyl isocyanate and with aprotic solvent) and electron donor number (DN) of the aprotic solvent are the two factors allowing satisfactory explanation of the catalysis and inhibition effects of the wide range of aprotic solvents (including amines, amides, etc.). Based on these considerations, an ion-pair mechanism and the resulting kinetic equation for the urethane reaction are proposed. Verification on the kinetic equation with experimental results for the systems of phenyl isocyanate with alcohol in toluene (for the self catalysis of the alcohol), with dimethyl formamide and dimethyl sulfoxide in toluene (for the catalysis of the aprotic solvents), and with triethylamine in toluene (for the catalysis of the tertiary amines) shows satisfactory. In the mechanism, the aprotic solvent is considered to solvate the complex of phenyl isocyanate/alcohol at the active hydrogen to form an ion-pair which can undergo the urethane reaction more easily.     

Solvent reorganization entropy of electron transfer in polar solvents

We report the results of molecular dynamics simulations of the solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile at varying temperatures. The simulations confirm the prediction of microscopic solvation theories of a positive reorganization entropy in polar solvents. The results of simulations are analyzed in terms of the splitting of the reorganization entropy into the contributions from the solute-solvent interaction and from the alteration of the solvent structure induced by the solute. These two contributions mutually cancel each other, resulting in the reorganization entropy amounting to only a fraction of each component.