Formation of Formic Acid Derivatives through Activation and Hydroboration of CO2 by Low-Valent Group 14 (Si, Ge, Sn, Pb) Catalysts

Insights into the variations of kinetic and potential energies in a multi-bond reaction: the reaction electronic flux perspective

CONTEXT

The debate over whether kinetic energy (KE) or potential energy (PE) are the fundamental energy components that contribute to forming covalent bonds has been enduring and stimulating over time. However, the supremacy of these energy components in reactions where multiple bonds are simultaneously formed or broken has yet to be explored. In this study, we use the reaction electronic flux (REF), an effective tool for investigating changes in driving electronic activity when bond formation or dissociation occurs in a chemical reaction, to examine the fluctuations in the KE and PE in a multi-bond reaction. To that end, the activation of CO2 by low-valent group 14 catalysts through a concerted σ -bond metathesis mechanism is analyzed. The findings of this preliminary study suggest that the REF can be utilized as a tool to rationalize alterations in the KE and PE in a multi-bond reaction. Specifically, analyses across the reaction coordinate reveal that changes in the KE and PE precede activation in the REF, stimulating the electronic activity where bond formation or dissociation processes dominate.

METHODS

The activation of CO2 by the low-valent LEH catalysts (L = N,N'-bis(2,6-diisopropyl phenyl)- β -diketiminate; E = Si, Ge, Sn, and Pb) was studied along the reaction coordinate at the M06-2X/6-31 G(d,p)-LANL2DZ(E) level of theory. The respective minimum energy path calculations were obtained using the intrinsic reaction coordinate (IRC) procedure. The reaction electronic flux (REF) was calculated through the computation of the electronic chemical potential using the frontier molecular orbital approximation. Mayer bond orders along the reaction coordinate have been determined using the NBO 3.1 program in Gaussian16. Most of the reaction coordinate quantities reported in this study (REF, KE, PE, among others) have been determined using the Kudi program and custom Python scripts.

Mononuclear organogermanium(IV) catalysts for a [3 + 2] cycloaddition reaction

Three mononuclear Ge(IV) compounds, [(C6H5)2Ge(C13H8N2O4)] (1), [(C6H5)2Ge(C14H10N2O5)] (2), and [(C6H5)2Ge(C14H11NO3)] (3), have been synthesized by the reaction of pro-ligands H2L1 (C13H10N2O4), H2L2 (C14H12N2O5), and H2L3 (C14H13NO3) with (C6H5)2GeCl2 in the presence of triethylamine. All compounds were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal X-ray diffraction analysis shows that the germanium(IV) atom exhibits a five-coordinated geometry in compounds 1 and 2. All compounds were screened as Lewis acid catalysts in the [3 + 2] cycloaddition reaction between sodium azide and various nitriles. The reactions resulted in the formation of 5-substituted 1H-tetrazoles with yields of up to 96%. Based on the experimental findings and DFT calculations, a plausible mechanism is proposed for the [3 + 2] cycloaddition reaction.

Heterobimetallic Hydrides with a Germanium(II)-Zinc Bond

In the presence of TMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine), zinc dihydride reacted with germanium(II) compounds (BDI-H)Ge (1) and [(BDI)Ge][B(3,5-(CF3 )2 C6 H3 )4 ] (3) (BDI-H = HC{(C=CH2 )(CMe)(NAr)2 }, BDI = [HC(CMeNAr)2 ]; Ar = 2,6-i Pr2 C6 H3 ) by formal insertion of the germanium(II) center into the Zn-H bond of polymeric [ZnH2 ]n to give neutral and cationic zincagermane with a H-Ge-Zn-H core [(BDI-H)Ge(H)-(H)Zn(tmeda)] (2) and [(BDI)Ge(H)-(H)Zn(tmeda)][B(3,5-(CF3 )2 C6 H3 )4 ] (4), respectively. Compound 2 eliminated [ZnH2 ] giving diamido germylene 1 at 60 °C. Compound 2 and deuterated analogue 2-d2 exchanged with [ZnH2 ]n and [ZnD2 ]n in the presence of TMEDA to give a mixture of 2 and 2-d2 . Compounds 2 and 4 reacted with carbon dioxide (1 bar) at room temperature to form zincagermane diformate [(BDI-H)Ge(OCHO)-(OCHO)Zn(tmeda)] (5) and formate bridged digermylene [({BDI}Ge)2 (μ-OCHO)]+ [B(C6 H3 (CF3 )2 )4 ] (6) along with zinc formate [(tmeda)Zn(μ-OCHO)3 Zn(tmeda)][B(C6 H3 (CF3 )2 )4 ] (7), respectively. The hydridic nature of the Ge-H and Zn-H bonds in 2 and 4 was probed by reactions with Brönsted and Lewis acids.

How Does Electronic Activity Drive Chemical Reactions? Insights from the Reaction Electronic Flux for the Conversion of Dopamine into Norepinephrine

Hydrogen atom transfer (HAT) is a crucial step in the physiological conversion of dopamine into norepinephrine catalyzed by dopamine β-monooxygenase. The way the reaction takes place is unclear, and a rational explanation on how the electronic activity drives the HAT seems to be necessary. In this work, we answer this question using the reaction electronic flux (REF), a DFT-based descriptor of electronic activity. Two reaction mechanisms will be analyzed using the REF's decomposition in polarization and electron transfer effects. Results show that both mechanisms proceed as follows: (1) polarization effects initiate the reactions producing structural distortions; (2) electron transfer processes take over near the transition states, triggering specific chemical events such as bond forming and breaking which are responsible to push the reactions toward the products; (3) after passing the transition state, polarization shows up again and drives the relaxation process toward the product. Similar polarization effects were observed in both reactions, but they present an opposite behavior of the electronic transfer flux disclosing the fact that electron transfer phenomena govern the reaction mechanisms.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

Bis(catecholato)germane: An Effective Catalyst for Friedel-Crafts Alkylation Reaction

Bis(catecholato)germane, [Ge(C6H4O2)2(H2O)2] (1) was synthesized by the reaction of catechol and germanium oxide in water according to a reported method. Complex 1 was characterized by FT-IR spectroscopy, NMR spectroscopy and...

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Low-Valent Germanylidene Anions: Efficient Single-Site Nucleophiles for Activation of Small Molecules

Rare mononuclear and helical chain low-valent germanylidene anions supported by cyclic (alkyl)(amino)carbene and hypermetallyl ligands were synthesised by stepwise reduction from corresponding germylene precursors via stable and isolable germanium radicals. The electronic structures of the anions can be described with ylidene and ylidone resonance forms with the Ge-C π-electrons capable of binding even weak electrophiles. The germanylidene anions reacted with CO2 to give μ-CO2 -κC:κO complexes, a rare coordination mode for low-valent germanium and inaccessible for the related neutral germylones. These results implicate low-valent germanylidene anions as efficient single-site nucleophiles for activation of small molecules.

Hydrostannylation of carbon dioxide by a hydridostannylene molybdenum complex

Reaction of the aryltin(II) hydrides {Ar^iPr₄Sn(μ-H)}₂ or {Ar^iPr₆Sn(μ-H)}₂ (Ar^iPr₄ = -C₆H₃-2,6-(C₆H₃-2,6-ⁱPr₂)₂, Ar^iPr₆ = -C₆H₃-2,6-(C₆H₂-2,4,6-ⁱPr₃)₂) with two equivalents of the molybdenum carbonyl [Mo(CO)₅(THF)] afforded the divalent tin hydride transition metal complexes, Mo(CO)₅{Sn(Ar^iPr₆)H}, (1), or Mo(CO)₅{Sn(Ar^iPr₄)(THF)H} (2), respectively. Complex 1 effects the facile hydrostannylation of carbon dioxide, to yield Mo(CO)₅{Sn(Ar^iPr₆)(κ²-O,O'-O₂CH)}, (3), which features a bidentate formate ligand coordinating the tin atom. Reaction of 3 with the pinacolborane, HBpin (pin = pinacolato) in benzene regenerated 1 in quantitative yield. All complexes were characterized by X-ray crystallography, as well as UV-visible, IR, and multinuclear NMR spectroscopies. The isolation of 1 and 2 is consistent with the existence of monomeric forms of {Ar^iPr₄Sn(μ-H)}₂ and {Ar^iPr₆Sn(μ-H)}₂ in solution. Regeneration of 1 from 3via reaction with pinacolborane as the hydrogen source shows the catalytic potential of 1 in the hydrogenation of CO₂.

Theoretical insights on boron reducing agent for the reduction of carbonyl compounds

In this perspective, we present computational progress in the reduction of carbonyl compounds using boron reducing agents, such as L·BH₃, HBcat, HBpin, and 9-BBN. For the catalytic reduction reactions, establishing a catalytic mechanism will provide an important theoretical basis for the improvement of a more efficient combination of reducing agents and catalysts. Current computational studies reveal that the mechanisms of reactions are different due to the various combinations of electrophilic boron reducing agents and catalysts (transition-metal catalyst, main group metal catalysts, and metal-free frustrated Lewis pair). We discuss the role of boron reducing agents on the efficiency of reactions and believe that possible Lewis acid-base interaction between B^δ+, M^δ+ and O^δ-, H^δ- existing in boron reducing agent, unsaturated substances, and catalyst should be considered fully. A tentative outlook on future opportunities of this research field is proposed.

Aluminum Amidinates: Insights into Alkyne Hydroboration

The mechanism of the aluminum-mediated hydroboration of terminal alkynes was investigated using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. The new aluminum complexes were fully characterized and found to facilitate the formation of the (E)-vinylboronate hydroboration product, with rates and orders of reaction linked to complex size and stability. Kinetic analysis and stoichiometric reactions were used to elucidate the mechanism, which we propose to proceed via the initial formation of an Al-borane adduct. Additionally, the most unstable complex was found to promote decomposition of the pinacolborane substrate to borane (BH3), which can then proceed to catalyze the reaction. This mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in the catalytic cycle.

CO2 Derivatives of Molecular Tin Compounds. Part 2: Carbamato, Formato, Phosphinoformato and Metallocarboxylato Complexes

Single-crystal X-ray diffraction structures of organotin compounds bearing hemicarbonate and carbonate ligands were recently reviewed by us—“CO2 Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes”, Inorganics 2020, 8, 31—based on crystallographic data available from the Cambridge Structural Database. Interestingly, this first collection revealed that most of the compounds listed were isolated in the context of studies devoted to the reactivity of tin precursors towards carbon dioxide, at atmospheric pressure or under pressure, thus highlighting the suitable disposition of Sn to fix CO2. In the frame of a second part, the present review carries on to explore CO2 derivatives of molecular tin compounds by describing successively the complexes with carbamato, formato, and phosphinoformato ligands, and obtained from insertion reactions of carbon dioxide into Sn–X bonds (X = N, H, P, respectively). The last chapter is devoted to X-ray structures of transition metal/tin CO2 complexes exhibiting metallocarboxylato ligands. As in Part 1, for each tin compound reported and when described in the original study, the structural descriptions are supplemented by synthetic conditions and spectroscopic data.

Structural Characterization, DFT Calculation, NCI, Scan-Rate Analysis and Antifungal Activity against Botrytis cinerea of (E)-2-{[(2-Aminopyridin-2-yl)imino]-methyl}-4,6-di-tert-butylphenol (Pyridine Schiff Base)

Botrytis cinerea is a ubiquitous necrotrophic filamentous fungal phytopathogen that lacks host specificity and can affect more than 1000 different plant species. In this work, we explored L1 [(E)-2-{[(2-aminopyridin-2-yl)imino]-methyl}-4,6-di-tert-butylphenol], a pyridine Schiff base harboring an intramolecular bond (IHB), regarding their antifungal activity against Botrytis cinerea. Moreover, we present a full characterization of the L1 by NMR and powder diffraction, as well as UV–vis, in the presence of previously untested different organic solvents. Complementary time-dependent density functional theory (TD-DFT) calculations were performed, and the noncovalent interaction (NCI) index was determined. Moreover, we obtained a scan-rate study on cyclic voltammetry of L1. Finally, we tested the antifungal activity of L1 against two strains of Botrytis cinerea (B05.10, a standard laboratory strain; and A1, a wild type strains isolated from Chilean blueberries). We found that L1 acts as an efficient antifungal agent against Botrytis cinerea at 26 °C, even better than the commercial antifungal agent fenhexamid. Although the antifungal activity was also observed at 4 °C, the effect was less pronounced. These results show the high versatility of this kind of pyridine Schiff bases in biological applications.