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Dioxygen Activation and Pyrrole α-Cleavage with Calix[4]pyrrolato Aluminates: Enzyme Model by Structural Constraint.

ABSTRACT: The present work describes the reaction of triplet dioxygen with the porphyrinogenic calix[4]pyrrolato aluminates to alkylperoxido aluminates in high selectivity. Multiconfigurational quantum chemical computations disclose the mechanism for this spin-forbidden process. Despite a negligible spin-orbit coupling constant, the intersystem crossing (ISC) is facilitated by singlet and triplet state degeneracy and spin-vibronic coupling. The formed peroxides are stable toward external substrates but undergo an unprecedented oxidative pyrrole α-cleavage by ligand aromatization/dearomatization-initiated O-O σ-bond scission. A detailed comparison of the calix[4]pyrrolato aluminates with dioxygen-related enzymology provides insights into the ISC of metal- or cofactor-free enzymes. It substantiates the importance of structural constraint and element-ligand cooperativity for the functions of aerobic life.

SUBMITTER: Sigmund LM

PROVIDER: S-EPMC8362023 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Calix[4]pyrrolato Aluminates: The Effect of Ligand Modification on the Reactivity of Square-Planar Aluminum Anions.

Project description:Structural constraint represents an attractive tool to modify p-block element properties without the need for unusual oxidation or valence states. The recently reported methyl-calix[4]pyrrolato aluminate established the effect of forcing a tetrahedral aluminum anion into a square-planar coordination mode. However, the generality of this structural motif and any consequence of ligand modification remained open. Herein, a systematic ligand screening was launched, and the class of square-planar aluminum anions was extended by two derivatives that differ in the meso-substitution at the calix[4]pyrrolato ligand. Strikingly, this modification provoked opposing trends in the preference for a Lewis acidic binding mode with σ-donors versus the aluminum-ligand cooperative binding mode with carbonyls. Insights into the origin of these counterintuitive experimental observations were provided by computation and bond analysis. Importantly, this rationale might allow to exploit mode-selective binding for catalytic rate control.

| S-EPMC8048585 | biostudies-literature

A Bis-calix[4]pyrrole Enzyme Mimic That Constrains Two Oxoanions in Close Proximity.

Project description:Herein we describe a large capsule-like bis-calix[4]pyrrole 1, which is able to host concurrently two dihydrogen phosphate anions within a relatively large internal cavity. Evidence for the concurrent, dual recognition of the encapsulated anions came from 1H NMR and UV-vis spectroscopies and ITC titrations carried out in CD2Cl2/CD3OD (9/1, v/v) or dichloroethane (DCE), as well as single crystal X-ray diffraction analyses. Receptor 1 was also found to bind two dianionic sulfate anions bridged by two water molecules in the solid state. The resulting sulfate dimer was retained in DCE solution, as evidenced by spectroscopic analyses. Finally, receptor 1 was found capable of accommodating two trianionic pyrophosphate anions in the cavity. The present experimental findings are supported by DFT calculations along with 1H NMR and UV-vis spectroscopies, ITC studies, and single crystal X-ray diffraction analyses.

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Cesium Halide Ion Pair Recognition by a Pyrrole Strapped Calix[4]pyrrole.

Project description:The binding properties of the pyrrole-strapped calix[4]pyrrole 2 for cesium halide ion pairs were studied via 1H NMR spectroscopic and single crystal X-ray diffraction analyses. Receptor 2 was found to bind CsF, CsCl, and CsBr in the solid state and in chloroform/methanol (4/1, v/v) solution with relatively high affinity as compared with the parent calix[4]pyrrole 1. It was also revealed by solid-liquid extraction experiments that receptor 2 was capable of solubilizing CsF in CDCl3, a medium in which this salt is otherwise insoluble. Single crystal X-ray diffraction analyses and 1H NMR spectroscopic data recorded in 20% CD3OD in CDCl3 provide support for the suggestion that the strap pyrrolic NH proton of 2, as well as those of the calix[4]pyrrole framework, contribute to anion recognition, thus increasing affinity for cesium halide salts relative to the parent system 1. In the solid state, receptor 2 interacts with CsF to form a two dimensional coordination polymer in the presence of methanol. A linear coordination polymer is observed in the case of CsCl and CsBr. Receptor 2 was also found to form a complex with CsF in chloroform/methanol (4/1, v/v) solution, albeit with a different binding mode than is seen in the solid state.

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A chiral "Siamese-Twin" calix[4]pyrrole tetramer.

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4-Chloro-benzoyl-meso-octa-methyl-calix[2]pyrrolidino[2]pyrrole: an acyl chloride derivative of a partially reduced calix[4] pyrrole.

Project description:In the title compound, C(35)H(47)ClN(4)O, the two pyrrolidine rings have envelope conformations. The conformation of the macrocycle is stabilized by N-H⋯N hydrogen bonds and a C-H⋯N inter-action. The benzoyl ring is inclined to the adjacent pyrrole ring by 11.66 (11)°, with a centroid-centroid distance of 3.7488 (13) Å. In the crystal, molecules are linked by N-H⋯O hydrogen bonds into helical chains propagating in [010] and C-H⋯O and C-H⋯π interactions are also observed.

| S-EPMC3343950 | biostudies-literature

Calix[4]pyrrolato Aluminate Catalyzes the Dehydrocoupling of Phenylphosphine Borane to High Molar Weight Polymers.

Project description:High molar weight polyphosphinoboranes represent materials with auspicious properties, but their preparation requires transition metal-based catalysts. Here, calix[4]pyrrolato aluminate is shown to induce the dehydropolymerization of phosphine boranes to high molar mass polyphosphinoboranes (up to Mn =43 000 Da). Combined GPC and 31 P DOSY NMR spectroscopic analyses, quantum chemical computations, and stoichiometric reactions disclose a P-H bond activation by the cooperative action of the square-planar aluminate and the electron-rich ligand framework. This first transition metal-free catalyst for P-B dehydrocoupling overcomes the problem of residual d-block metal impurities in the resulting polymers that might interfere with the reproducibility of the properties for this emerging class of inorganic materials.

| S-EPMC9313825 | biostudies-literature

Reversible OH-bond activation and amphoterism by metal-ligand cooperativity of calix[4]pyrrolato aluminate.

Project description:Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal-ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal-ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al-O bond by a dative Al-N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.

| S-EPMC8161688 | biostudies-literature

4-Meth-oxy-benzoyl-meso-octa-methyl-calix[2]pyrrolidino[2]pyrrole: an acyl chloride derivative of a partially reduced calix[4]pyrrole.

Project description:In the title compound, C(36)H(50)N(4)O(2), the two pyrrolidine rings have envelope conformations. The conformation of the macrocycle is stabilized by N-H⋯N hydrogen bonds and a C-H⋯N inter-action. The benzoyl ring is inclined to an adjacent pyrrole ring by 6.76 (9)°, with a centroid-to-centroid distance of 3.6285 (10) Å. In the crystal, apart from a C-H⋯O and a C-H⋯π inter-action, mol-ecules are linked via an N-H⋯O hydrogen bond, leading to the formation of helical chains propagating along [010].

| S-EPMC3343912 | biostudies-literature

Structural Modulation of Chromic Response: Effects of Binding-Site Blocking in a Conjugated Calix[4]pyrrole Chromophore.

Project description:Herein, we modulate the chromic response of a highly colored tetrapyrrole macrocycle, namely, tetrakis(3,5-di-tert-butyl-4-oxocyclohexadien-2,5-yl)porphyrinogen (OxP) by structural modification. N-Benzylation at the macrocyclic nitrogen atoms leads to stepwise elimination of the two calix[4]pyrrole-type binding sites of OxP and serial variation of the chromic properties of the products, double N-benzylated Bz2OxP and tetra N-benzylated Bz4OxP. The halochromic (response to acidity) and solvatochromic (response to solvent polarity) properties were studied by using UV/Vis spectroscopy and NMR spectroscopy in nonpolar organic solvents. Titration experiments were used to generate binding isotherms to elucidate their binding properties with difluoroacetic acid. Differences in the halochromic properties of the compounds allowed construction of a colorimetric scale of acidity in nonpolar solvents, as the compounds in the series OxP, Bz2OxP, and Bz4OxP are increasingly difficult to protonate but maintain their propensity to change color upon protonation. The concurrent effects of binding-site blocking and modulation of acidity sensitivity are important new aspects for the development of colorimetric indicators.

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Calix[4]pyrrolato Stannate(II): A Tetraamido Tin(II) Dianion and Strong Metal-Centered σ-Donor.

Project description:Anionic, metal-centered nucleophiles are emerging compounds with unique reactivities. Here, we describe the isolation and full characterization of the first tetraamido tin(II) dianion, its behavior as ligand towards transition metals, and its reactivity as a tin-centered nucleophile. Experimental values such as the Tolman electronic parameter (TEP) and computations attest tin-located σ-donor ability exceeding that of carbenes or electron-rich phosphines. Against transition metals, the stannate(II) can act as η1 - or η5 -type ligand. With aldehydes, it reacts by hydride substitution to give valuable acyl stannates. The reductive dehalogenation of iodobenzene indicates facile redox pathways mediated by halogen bond interaction. Calix[4]pyrrolato stannate(II) represents the first example of this macrocyclic ligand in low-valent p-block element chemistry.

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