Macrocyclic Effect Research Articles

ChemInform Abstract: Supramolecular Assistance to Regioselectivity in the Reactions of Chlorocyclophosphazenes with Sodium Oxyanions: Macrocyclic Effect and Anion Dependence.

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A Macrocyclic Effect on the Reduction of p-Quinones

A macrocyclic compound that has four phenolic and four aromatic amino groups is synthesized. This compound reacts with p-quinones to give regiospecific oxidation products. During the reaction, the quinone is reduced to its hydroquinone form. The rate constant for the macrocycle is significantly large compared to its acyclic analogues. Analysis of the rate constants for several p-quinones indicates that the rate determining step involves an electron transfer process. Cyclic voltammetry measurement suggests that the complexation of the macrocycle with the quinone is related to lowering of the activation barrier of electron transfer. The role of the phenolic groups in the complexation is discussed. Based on the experimental results, the macrocyclic effect is evaluated to be 27 kJ mol−1 as the difference in Gibbs energy change necessary for the complexation.

Supramolecular assistance to regioselectivity in the reactions of chlorocyclophosphazenes with sodium oxyanions: macrocyclic effect and anion dependence.

Comparative studies of the naphtholysis of crown-bearing tetrachlorocyclotriphosphazene 2 and its acyclic analogue, nongem-diethoxytetrachlorocyclotriphosphazene 5 (which was synthesized and isolated in the pure cis form for the purpose of this study), reveal a significant macrocyclic effect of the substituent on both the kinetics and regiochemistry of chlorine substitution in the N(3)P(3) ring. Whereas substitution of 5 with sodium naphthoxide provided moderate yields of the respective sterically and electronically favored nongem-naphthoxydiethoxy derivative as the major product, isolated as the mixture of cis and trans isomers 8 + 9, the corresponding reaction of crown substrate 2 resulted in highly regioselective formation of the gem-to-macrocycle substituted mononaphthoxy PNP-crown derivative 11a. The importance of the PNP-crown related effects for the regio- and stereocontrol of the substitution of chlorine atoms in the N(3)P(3) ring is discussed in terms of supramolecular sodium cation-assisted interactions between the ring oxy substituent(s) and incoming oxyanions. To approach the problem of the anion-dependence of the regioselectivity, a comparison has been drawn between the reactions of the PNP-crown substrate 2 with sodium arylates derived from beta-naphthol (10a) and phenol (10b), carried out with equimolar or 100% excess sodium oxyanion with respect to 2, and the corresponding reactions of 2 with sodium monoenolates derived from the beta-dicarbonyl compounds: acetylacetone (10c) and ethyl acetylacetate (10d). The effectiveness of supramolecularly assisted transition state stabilization is found to be dependent not only on the dimensional complementarity between the cation and the macrocycle but also on the properties of the four interacting ionic centers according to the HSAB principle and on the nature of the electronic interaction of the PNP-crown substituent with the N(3)P(3) ring, the latter determining the extent of regiocontrol to either one or two macrocyclic chloride functions. The increase of the extent of regioselectivity of gem-substitution at the PNP-macrocycle when passing from arylate to beta-dicarbonyl enolate oxy anions is related to the observed trends in the (31)P NMR chemical shifts and refers to the electronic structure of the oxy substituent.

Stability of 2‐naphthalenediazonium ion and its complexation with crown ethers and acyclic polyethers in solution

AbstractThe effects of solvent, temperature and pH (in aqueous solution) on the rate of decomposition of 2‐naphthalenediazonium tetrafluoroborate were studied by UV spectrometry. The host–guest complexation and the kinetics of thermal dediazoniation of this model naphthalenediazonium ion, in the presence of four crown ethers and three acyclic polyethers, were studied in 1,2‐dichloroethane (DCE) at 40 °C by UV spectrometry. Fast atom bombardment mass spectrometry (FAB‐MS) was used for the gas phase complexation and characterization.The values of the activation enthalpy ΔH# for the thermal dediazoniation of the uncomplexed salt were observed to be high, and the corresponding values of activation entropy ΔS# were clearly positive. All hosts, except short 12‐crown‐4, formed 1:1 complexes under FAB conditions. The complexation equilibrium constant K and the values of the stabilizing ability of the complexation (k2/k1) in DCE were calculated using a kinetic method. In accordance with earlier studies of arene‐ and 1‐naphthalene diazonium ions, the thermodynamic and kinetic stabilities were observed to be greater for the inclusion complex—the largest for 21‐crown‐7 and its derivatives, formed with crown ethers containing at least six oxygen atoms—than for the non‐specific adduct formation formed with 15‐crown‐5. The values of the thermodynamic and kinetic macrocyclic effect were discussed. The results are consistent with a heterolytic SN1‐like mechanism involving the decomposition of the uncomplexed and complexed naphthalenediazonium ion into a highly reactive naphthyl cation, followed by fast product‐determining reactions with nucleophiles to give the products. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of crown ethers on structure, stability, activity, and enantioselectivity of subtilisin Carlsberg in organic solvents

Colyophilization or codrying of subtilisin Carlsberg with the crown ethers 18-crown-6, 15-crown-5, and 12-crown-4 substantially improved enzyme activity in THF, acetonitrile, and 1,4-dioxane in the transesterification reactions of N-acetyl-L-phenylalanine ethylester and 1-propanol and that of (+/-)-1-phenylethanol and vinylbutyrate. The acceleration of the initial rate, V(0), ranged from less than 10-fold to more than 100-fold. All crown ethers activated subtilisin substantially, which excludes a specific macrocyclic effect from being responsible. The secondary structure of subtilisin was studied by Fourier-transform infrared (FTIR) spectroscopy. 18-Crown-6 and 15-crown-5 led to a more nativelike structure of subtilisin in the organic solvents employed when compared with that of the dehydrated enzyme obtained from buffer alone. However, the high level of activation with 12-crown-4 where this effect was not observed excluded overall structural preservation from being the primary cause of the observed enzyme activation. The conformational mobility of subtilisin was investigated by performing thermal denaturation experiments in 1,4-dioxane. Although only a small effect of temperature on subtilisin structure was observed for the samples prepared with or without 12-crown-4, both 18-crown-6 and 15-crown-5 caused the enzyme to denature at quite low temperatures (38 degrees C and 56 degrees C, respectively). No relationship between this property and V(0) was evident, but increased conformational mobility of the protein decreased its storage stability. The possibility of a "molecular imprinting" effect was also tested by removing 18-crown-6 from the subtilisin-18-crown-6 colyophilizate by washing. V(0) was only halved as a result of this procedure, an effect insignificant compared with the ca. 80-fold rate enhancement observed prior to washing in THF. This suggests that molecular imprinting is likely the primary cause of subtilisin activation by crown ethers, as recently suggested.

Effect of crown ethers on structure, stability, activity, and enantioselectivity of subtilisin Carlsberg in organic solvents

AbstractColyophilization or codrying of subtilisin Carlsberg with the crown ethers 18‐crown‐6, 15‐crown‐5, and 12‐crown‐4 substantially improved enzyme activity in THF, acetonitrile, and 1,4‐dioxane in the transesterification reactions of N‐acetyl‐L‐phenylalanine ethylester and 1‐propanol and that of (±)‐1‐phenylethanol and vinylbutyrate. The acceleration of the initial rate, V0, ranged from less than 10‐fold to more than 100‐fold. All crown ethers activated subtilisin substantially, which excludes a specific macrocyclic effect from being responsible. The secondary structure of subtilisin was studied by Fourier‐transform infrared (FTIR) spectroscopy. 18‐Crown‐6 and 15‐crown‐5 led to a more nativelike structure of subtilisin in the organic solvents employed when compared with that of the dehydrated enzyme obtained from buffer alone. However, the high level of activation with 12‐crown‐4 where this effect was not observed excluded overall structural preservation from being the primary cause of the observed enzyme activation. The conformational mobility of subtilisin was investigated by performing thermal denaturation experiments in 1,4‐dioxane. Although only a small effect of temperature on subtilisin structure was observed for the samples prepared with or without 12‐crown‐4, both 18‐crown‐6 and 15‐crown‐5 caused the enzyme to denature at quite low temperatures (38°C and 56°C, respectively). No relationship between this property and V0 was evident, but increased conformational mobility of the protein decreased its storage stability. The possibility of a “molecular imprinting” effect was also tested by removing 18‐crown‐6 from the subtilisin‐18‐crown‐6 colyophilizate by washing. V0 was only halved as a result of this procedure, an effect insignificant compared with the ca. 80‐fold rate enhancement observed prior to washing in THF. This suggests that molecular imprinting is likely the primary cause of subtilisin activation by crown ethers, as recently suggested. © 2001 John Wiley & Sons, Inc. Biotechnol Bioeng 74: 295–308, 2001.

Macrocyclic effects upon isomeric CuIIMII and MIICuII cores. Formation with unsymmetric phenol-based macrocyclic ligands

This paper discusses coordination-position isomeric MIICuII and CuIIMII complexes, using unsymmetric dinucleating macrocycles (Lm;n)2− ((L2;2)2−, (L2;3)2− and (L2;4)2−) that comprise two 2-(N-methyl)-aminomethyl-6-iminomethyl-4-bromo-phenonate entities, combined through the ethylene chain (m = 2) between the two amine nitrogens and through the ethylene, trimethylene or tetramethylene chain(n = 2, 3 or 4) between the two imine nitrogens. The macrocycles have dissimilar N(amine)2O2 and N(imine)2O2 metal-binding sites sharing the phenolic oxygens. The reaction of the mononuclear CuII precursors, [Cu(L2;2)], [Cu(L2;2)] and [Cu(L2;2)], with a MII perchlorate and a MII acetate salt formed (acetato)MIICuII complexes: [CoCu(L2;2)(AcO)]ClO4·0.5H2O] (1), [NiCu(L2;2) (AcO)]ClO4 (2), [ZnCu(L2;2) (AcO)]ClO4 (3), [CoCu(L2;3)(AcO)]ClO4·0.5H2O (4), [NiCu(L2;3)(AcO)]ClO4 (5), [ZnCu(L2;3)(AcO)]ClO4·0.5H2O (6), [CoCu(L2;4)(AcO)(DMF)]ClO4 (7), [NiCu(L2;4) (AcO)]ClO4·2DMF (8) and [ZnCu(L2;4)(AcO)]ClO4 (9) (the formulation [MaMb (Lm;n)]2+ means that Ma resides in the aminic site and Mb in the iminic site). The site selectivity of the metal ions is demonstrated by X-ray crystallographic studies for 2·MeOH,3,5,7, and9. An (acetato)CuIIZnII complex, [CuZn(L2;3)(AcO)]ClO4 (10), was obtained by the reaction of [PbCu(L2;3)]-(ClO4)2 with ZnSO4·4H2O, in the presence of sodium acetate. Other complexes of the CuIIMII type were thermodynamically unstable to cause a scrambling of metal ions. The Cu migration from the iminic site to the aminic site in the synthesis of10 is explained by the ‘kinetic macrocyclic effect’. The coordination-position isomers,6 and10, are differentiated by physicochemical properties.

Macrocyclic effect upon site-selective CuIIMII or MIICuII core formation with unsymmetric phenol-based macrocyclic ligands †

Complexes of phenol-based dinucleating macrocycles, (L2;2)2− and (L2;4)2−, comprised of two 4-bromo-2-iminomethyl-6-methylaminomethylphenolate moieties combined by an ethylene chain at the amino nitrogens and by an ethylene or a 1,4-tetramethylene chain at the imino nitrogens, have been prepared: mononuclear [Cu(L2;2)] and [Cu(L2;4)], and dinuclear [PbCu(L2;2)][ClO4]2 and [PbCu(L2;4)][ClO4]2. The reaction of [Cu(L2;2)] and [Cu(L2;4)] with MCl2 salts formed dinuclear [MCu(L2;2)Cl2] and [MCu(L2;4)Cl2] (M = Mn, Co, Ni, Cu or Zn), respectively. A crystallographic study of [NiCu(L2;2)Cl2] indicates the NiII to reside in the aminic site and the CuII in the iminic site. The complexes, except for that of M = ZnII, show a significant antiferromagnetic interaction within each dinuclear unit. The reaction of [PbCu(L2;2)][ClO4]2 with MSO4 salts formed dinuclear CuIIMII perchlorate complexes [CuM(L2;2)][ClO4]2 (M = Co, Ni, Cu or Zn). A similar transmetallation for [PbCu(L2;4)][ClO4]2 caused a scrambling of the metal ions. A crystallographic study of [CuNi(L2;2)(dmf)][ClO4]2·MeOH indicates that the CuII migrates from the iminic to the aminic site to accommodate the NiII in the iminic site. Kinetic and thermodynamic effects of the macrocycles upon the site selectivity for the metal ions are discussed.

The complex formation of non-cyclic polyethers and crown ethers with Ag+ in acetone and propylene carbonate studied by potentiometric and calorimetric methods

Potentiometric and calorimetric titrations are performed to study the complexation reactions of non-cyclic polyethers and crown ethers with Ag+ in acetone and propylene carbonate. With increasing chain length of the non-cyclic ligands the complex stability increases due to favourable enthalpic contributions. The macrocyclic polyethers already possess a preformed cavity for the complexation of the Ag+ ion. The cavity size and substituents influence the stability constants of the complexes formed. Only the smallest crown ether examined, 12-crown-4, forms 2:1 complexes in acetone and propylene carbonate solutions. The highest values of the stability constants for the reaction of Ag+ with crown ethers are measured if the dimensions of the silver ion and of the cavities are nearly identical. The results clearly demonstrate the entropic origin of the macrocyclic effect in both solvents.

An Optical Probe Detection Method to Study Photolysis of Methylpyridinocobaloxime in Aqueous Solution

A novel optical probe detection method consisting of a laser heterodyne interferometer has been adapted to study photolysis of methylpyridinocobaloxime (CH3Co(DH)2py) aqueous solution in the temperature range 10 ∼ 30 °C. The quantum yield and reaction volume change for the photolysis of anaerated CH3Co(DH)2py solution at 355nm are determined to be 0.011 and 0.5 ml/mol, respectively. The resulting quantum yield is close to the value obtained by Schrauzer et. al. at 440nm, and the relatively small reaction volume change suggests that the contributions due to the conformational change of macrocyclic ligand and solvation effect accompanying Co-C bond cleavage are relatively unaffected.

Stabilities in water and Gibbs free energies of transfer from water to nonaqueous solvents of alkali metal ion complexes with 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (acyclic polyether)

Stability constants of 1:1 complexes, ML+ (M+ and L being a univalent metal ion and a ligand, respectively), of Na+ and K+ with 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (AC · B18C6), a linear counterpart of benzo-18-crown-6 (B18C6), were determined in water at 25°C by conductometry. The stability of ML+ is lower for AC · B18C6 than for B18C6 because of the macrocyclic effect, and AC · B18C6 is selective for Na+ in contrast to B18C6. Also determined were Gibbs free energies of transfer (ΔGH2O→S°) from water to nonaqueous solvent S (S=acetonitrile, propylene carbonate and methanol) of AC · B18C6 and B18C6 at 25°C through measurements of the distribution constants between tetradecane and the polar solvent. From these data as well as the literature values of KML in the nonaqueous solvents and of ΔGH2O→S° of M+, the ΔGH2O→S° values of ML+ complexes of the alkali-metal ions with AC · B18C6 and B18C6 were calculated. The ΔGH2O→S° values of M+ are positive but those of ML+ and L are negative in all the systems. This indicates that, although the M+ ions are more soluble in water than in the nonaqueous solvents, when the M+ forms a complex with the lipophilic L, the complex becomes more soluble in the nonaqueous solvent than in water. The lipophilicity of AC · B18C6 is higher than that of B18C6, but the reverse holds for the ML+ complexes. It was concluded that AC · B18C6 shields the M+ ions less effectively in the complexes from the surrounding solvents than B18C6.

Electrochemistry of nitrite reductase model compounds 6. Voltammetric and spectroelectrochemical studies of iron(II) nitrosyl complexes with porphyrins, hydroprophyrins and porphinones

Abstract The effect of hydroporphine and porphinone macrocycles on the voltammetry and spectroscopy of Fe(P)(NO) was examined. Two oxidation and two reduction waves were observed for all the complexes examined. The variation in the half-wave potentials fell into two categories: those where the E 1 2 of the complex varied linearly with the E 1 2 of the free-based, and those where the relationship was non-linear. Linear relationships were observed where the electron transfer was primarily macrocycle-centered, while non-linear relationships were observed for metal-centered processes. Of the four waves observed, the first reduction and second oxidation of Fe(P)(NO) was non-linear, while the second reduction and first oxidation waves were linear. Comparison with the voltammetry of Fe(P)(Cl) showed the same relationships. The first reduction and second oxidation waves were non-linear while the second and third reductions and the first oxidation waves were linear. For the linear relationships, where the E 1 2 of the complex was proportional to the E 1 2 of the free-based, the energy of the LUMO or HOMO controlled the redox potential so that the porphyrin molecular orbital was involved substantially in the redox product, if not itself oxidized or reduced. This would indicate that the first reduction of Fe(P)(NO) was centered on the Fe(NO) moiety, while the second reduction led to the formation of a porphyrin radical anion. The variation in oxidation redox potentials was consistent with previous formulations of Fe(P)(NO)+ as a π-cation radical (except for Fe(OEP)(NO)+). The variations in the E 1 2 of the second oxidation wave would indicate that the oxidation was centered on the Fe(NO) moiety. The variation in the half-wave potentials of Fe(X-TPP)(NO), where X was a substituent at the three- or four-position on the phenyl ring, also confirmed this interpretation. The slope of the E 1 2 values for the first wave was 0.045 V, while a value of 0.072 V was observed for the second wave. It has been previously shown that slopes of about 0.068 V were indicative of porphyrin reduction, while lower values were generally observed for metal reduction. The formation constants for pyridine with Fe(P)(NO) were also determined. For the porphyrins and hydroporphyrins, the values were small (about 0.3 to 1.1), which were consistent with other iron nitrosyl porphyrins and chlorins. The porphinone and porphinedione complexes, though, showed a much greater affinity for pyridine, with formation constants of 4.8±0.2 and 88±3. These values are one to two orders of magnitude larger than the values observed for other iron-porphyrins nitrosyl complexes, and indicate a chemical advantage for the porphinedione (heme d1) in dissimilatory nitrite reductases, where the heme is coordinated by an imidazole in the fifth position. A weak complex (K = 9) was observed between Fe(2,4-OEiBC)(NO)+ and pyridine.

Chelate ring geometry, and the metal ion selectivity of macrocyclic ligands. Some recent developments

Abstract The idea (Hancock, 1992) that the dominant architectural feature in controlling metal ion selectivity in both open-chain and macro-cyclic ligands is the size of the chelate ring is pursued further. It is shown that when more than one or two six-membered chelate rings are present in the complex of a nitrogen donor macrocycle, the steric requirements of the six-membered chelate ring of a M-N bond length of 1.6 Å and N-M-N angle of 109.5° become particularly severe, and can only be met by a small tetrahedral metal ion. Thus, the ligand 16-aneN4 (1,5,9,13-tetraazacyclohexadecane) forms complexes of low stability with all metal ions studied to date, but a conformer of 16-aneN4 is identified by MM calculation which is predicted to form complexes of high stability with very small tetrahedral metal ions. The question of the M-O bond length and O-M-O angles that will produce minimum strain in chelate rings containing neutral oxygen donor is addressed. The observation (Hay, 1993) that the geometry around an ethereal oxygen coordinated to a metal ion approximates to trigonal planar rather than tetrahedral leads to ideal M-O-C angles of about 126°, which leads to minimum strain energy with much longer M-L lengths in chelate rings containing neutral oxygen donors than neutral nitrogen donors. It is suggested that this fact accounts for the general tendency of crown ethers to form their most stable complexes with potassium out of the alkali metal ions, and also accounts for the very small macrocyclic effect observed in complexes of macrocycles containing mixed nitrogen and oxygen donor groups. The preferred geometry of four-membered chelate rings is discussed, and it is shown that higher coordination numbers of metal ions are associated with four membered chelate rings, and that four membered chelate rings may be used to engineer preference for larger metal ions. Very rigid reinforced chelate rings are discussed, and it is shown that open-chain ligands with reinforced bridges between the donor atoms can display all the thermodynamic and kinetic aspects associated with macrocyclic ligands.

Polydentate Amine and Ether Solvates of Lithium Hexamethyldisilazide (LiHMDS): Relationship of Ligand Structure, Relative Solvation Energy, and Aggregation State

6Li, 15N, and 13C NMR spectroscopic investigations of [6Li,15N]lithium hexamethyldisilazide ([6Li,15N]LiHMDS) coordinated by 29 polyamines, polyethers, and aminoethers reveal a range of structural types including η1-coordinated mono- and disolvated dimers, η2-coordinated (3-coordinate) monomers, η1,η2-coordinated (4-coordinate) monomers, η2,η2-coordinated (5-coordinate) monomers, polymers (linked dimers), triple ions, and solvent-separated ion pairs. Ligand binding constants on the LiHMDS monomers shed light on chelate ring size and steric effects, aza- and oxaphilicity, mechanisms and rates of ligand substitution, and the “macrocyclic effect.”

Silver(I) Complexes of the Derivatized Crown Thioether Ligands 3,6,9,12,15,18-Hexathianonadecanol and 3,6,9,13,16,19-Hexathiaicosanol. Determination of Stability Constants and the Crystal Structures of [Ag(19-aneS6-OH)][CF(3)SO(3)] and [Ag(20-aneS6-OH)][BF(4)

The derivatized 19- and 20-membered macrocyclic thio crowns 3,6,9,12,15,18-hexathianonadecanol C(13)H(26)OS(6) (19-aneS6-OH) (1) and 3,6,9,13,16,19-hexathiacycloicosanol C(14)H(28)OS(6) (20-aneS6-OH) (2) have been synthesized by [1 + 1] cyclization in about 30% yield. The ligands 1 and 2 react readily at room temperature with different silver(I) salts in water and in organic solvents to form in quantitative yields the complexes [Ag(19-aneS6-OH)](+) (3) and [Ag(20-aneS6-OH)](+) (4) for which crystals of X-ray quality were grown by slow diffusion of diethylether into methanol. [Ag(19-aneS6-OH)][CF(3)SO(3)] crystallizes in the triclinic space group P&onemacr; with Z = 2, a = 10.760(1), b = 10.853(2) and c = 11.326(2)Å, and alpha = 78.73(1), beta = 73.47(1), and gamma = 74.99(1) degrees. [Ag(20-aneS6-OH)][BF(4)] also crystallizes in the triclinic space group P&onemacr; with Z = 4. The unit cell constants were determined with a = 10.076(4), b = 10.525(3), and c = 22.135(8)Å, alpha = 93.32(2), beta = 102.43(2), and gamma = 100.32(2) degrees. The complex cations [Ag(19-aneS6-OH)](+) and [Ag(20-aneS6-OH)](+) are coordinated through only four sulfur atoms; thus, a distorted tetrahedral coordination geometry is exhibited. In addition we found a highly asymmetric Ag-S bond lengths distribution throughout all complex cations. The stability constants of [Ag](+) with 1 and 2 and, for comparison with [18-aneS6] (5), have been determined in methanol by potentiometric [Ag](+) measurements. Log K values for the formation of 3, 4, and [Ag(18-aneS6](+) (6) are 12.04 +/- 0.19, 11.49 +/- 0.15, and 12.67 +/- 0.13 respectively. Owing to a comparable macrocyclic effect, the similar log K values are reasonable but, since 6 coordinates octahedrally, not expected. (1)H and (13)C NMR investigations at various temperatures give evidence for fluxional coordinative behavior between all six sulfur atoms in solution. Consequently [Ag(19-aneS6-OH)](+), [Ag(20-aneS6-OH)](+), and [Ag(18-aneS6](+) seem to exhibit principally the same solution structures although the solid structures are very different.

Fluorescent Sensors for Transition Metals Based on Electron‐Transfer and Energy‐Transfer Mechanisms

AbstractFluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal‐receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one‐electron oxidation of the metal, quenching takes place through a photoinduced metal‐to‐fluorophore electron‐transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy‐transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam‐containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.

Crown thioethers and the kinetic macrocyclic effect. Solvolysis of molybdenum(0) tricarbonyl complexes of cyclic and acyclic trithioethers

Solvolysis of the macrocyclic trithioether complexes fac-[Mo(CO)3([9]aneS3)] ([9]aneS3 = 1,4,7-trithiacyclononane) and fac-[Mo(CO)3(ttob)] (ttob = 2,5,8-trithia[9]-o-benzenophane), and the acyclic trithioether complex fac-[Mo(CO)3(ttn)] (ttn = 2,5,8-trithiononane) in dimethylsulfoxide (DMSO) solution gives the free trithioether in each case. The kinetics of these reactions were studied by 1H-n.m.r. spectroscopy. In the absence of acid or base, the rate of solvolysis of [Mo(CO)3([9]aneS3)] is at least 4.6 × 105 slower than that of [Mo(CO)3(ttn)]; larger macrocycles give intermediate rates. [Mo(CO)3-([9]aneS3) and [Mo(CO)3(ttob)] undergo stoichiometric reactions with acid and base in DMSO which in all cases lead to more rapid loss of the intact macrocycle than in DMSO alone. The results are discussed in terms of the structure and bonding of likely intermediate complexes.

Quantitation and Nature of the Macrocyclic Effect: K+ Complexation with 18-Crown-6 and Pentaglyme

Quantitation and Nature of the Macrocyclic Effect: K+ Complexation with 18-Crown-6 and Pentaglyme

Factors affecting stabilities of chelate, macrocyclic and macrobicyclic complexes in solution

The factors which contribute to the chelate, macrocyclic and cryptate effects are described. These include the dilution effect, translational entropy, intrinsic basicities of donor atoms, coulombic attractions and repulsions of charged ions and groups, and covalent character of the coordinate bonds. The reduction in hydration/solvation energies and the related coulombic repulsions of the donor atoms of multidentate ligands are related to preorganization of the ligands for complexation of metal ions. A not insignificant part of the chelate and macrocyclic effects is due to increase in the basicities of the donor atoms that occurs on ring formation, as well as to reduction of the steric repulsions of alkyl groups. The effects of ring size on stabilities are described. These factors are illustrated with stabilities of chelates, macrocyclic complexes and cryptates taken from the literature.

Comparison of the macrocyclic effect for ether hosts in aqueous and organic solvents

Equilibrium constants and enthalpies of host-guest complexation have been determined by calorimetry for cyclic and acyclic polyether hosts with a variety of guests and solvents. The thermodynamic parameters for complexation of group I cation guests by crown ethers in water solvent may be correlated to the hydration enthalpy and relative sizes of cation and ether. The macrocyclic effect is observed for cation-ether systems in water solvent but not for the complexation of malononitrile by ethers in nonpolar aromatic or alcohol solvent