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

ABSTRACT: 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.

SUBMITTER: Sigmund LM

PROVIDER: S-EPMC8161688 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Metal-Ligand Cooperativity of the Calix[4]pyrrolato Aluminate: Triggerable C-C Bond Formation and Rate Control in Catalysis.

Project description:Metal-ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [1]- , which features a square-planar AlIII , we transfer this concept into the p-block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal-based MLC (aromatization upon substrate binding), substrate binding in [1]- occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C-C and Al-O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C-C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal-ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti-van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.

| S-EPMC7540271 | 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 homolytic activation of water <i>via</i> metal-ligand cooperativity in a T-shaped Ni(ii) complex.

Project description:A T-shaped Ni(ii) complex [Tol,PhDHPy]Ni has been prepared and characterized. EPR spectra and DFT calculations of this complex suggest that the electronic structure is best described as a high-spin Ni(ii) center antiferromagnetically coupled with a ligand-based radical. This complex reacts with water at room temperature to generate the dimeric complex [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] which has been thoroughly characterized by SXRD, NMR, IR and deuterium-labeling experiments. Addition of simple ligands such as phosphines or pyridine displaces water and demonstrates the reversibility of water activation in this system. The water activation step has been examined by kinetic studies and DFT calculations which suggest an unusual homolytic reaction via a bimetallic mechanism. The ΔH ‡, ΔS ‡ and KIE (k H/k D) of the reaction are 5.5 kcal mol-1, -23.8 cal mol-1 K-1, and 2.4(1), respectively. In addition to the reversibility of water addition, this system is capable of activating water towards net O-atom transfer to substrates such as aromatic C-H bonds and phosphines. This reactivity is facilitated by the ability of the dihydrazonopyrrole ligand to accept H-atoms and illustrates the utility of metal ligand cooperation in activating O-H bonds with high bond dissociation energies.

| S-EPMC6354739 | biostudies-literature

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization.

Project description:We outline the generality and requirements for cooperative N2H4 capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH2 stabilization. While the Lewis acids are required to bind N2H4, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

| S-EPMC7340558 | biostudies-literature

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.

| S-EPMC9306640 | biostudies-literature

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

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes.

Project description:Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

| S-EPMC8849050 | biostudies-literature

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

Project description: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.

| S-EPMC8362023 | biostudies-literature

Manganese(I)-Catalyzed H-P Bond Activation via Metal-Ligand Cooperation.

Project description:Here we report that chiral Mn(I) complexes are capable of H-P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal-ligand cooperation, a strategy previously not considered for catalytic H-P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.

| S-EPMC8662621 | biostudies-literature

Facile C=O Bond Splitting of Carbon Dioxide Induced by Metal-Ligand Cooperativity in a Phosphinine Iron(0) Complex.

Project description:New iron complexes [Cp*FeL]- (1-σ and 1-π, Cp*=C5 Me5 ) containing the chelating phosphinine ligand 2-(2'-pyridyl)-4,6-diphenylphosphinine (L) have been prepared, and found to undergo facile reaction with CO2 under ambient conditions. The outcome of this reaction depends on the coordination mode of the versatile ligand L. Interaction of CO2 with the isomer 1-π, in which L binds to Fe through the phosphinine moiety in an η5 fashion, leads to the formation of 3-π, in which CO2 has undergone electrophilic addition to the phosphinine group. In contrast, interaction with 1-σ-in which L acts as a σ-chelating [P,N] ligand-leads to product 3-σ in which one C=O bond has been completely broken. Such CO2 cleavage reactions are extremely rare for late 3d metals, and this represents the first such example mediated by a single Fe centre.

| S-EPMC6856682 | biostudies-literature

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