Mechanistic Investigations of the Catalytic Formation of Lactams from Amines and Water with Liberation of H₂

The mechanism of the unique lactam formation from amines and water with concomitant H₂ liberation with no added oxidant, catalyzed by a well-defined acridine-based ruthenium pincer complex was investigated in detail by both experiment and DFT calculations. The results show that a dearomatized form of the initial complex is the active catalyst. Furthermore, reversible imine formation was shown to be part of the catalytic cycle. Water is not only the oxygen atom source but also acts as a cocatalyst for the H₂ liberation, enabled by conformational flexibility of the acridine-based pincer ligand

Reversible Aromaticity Transfer in a Bora-Cycle: Boron–Ligand Cooperation

Aromaticity is a central concept in chemistry. Reaction pathways involving reversible ligand dearomatization sequences emerged as a powerful tool for bond activation by metal complexes. Exploring this concept with a metal-free system, we have synthesized a pyridine-coordinated aminoborane which undergoes a temperature-induced formal dearomatization of the pyridine ring. NMR studies and DFT calculations revealed that this formal dearomatization sequence led to an aromaticity switch and the formation of a six-π-electron boron-containing heteroaromatic system. Disrupting this aromatic system by coordination of an amine or a carboxylic acid to the boron center enabled N–H activation and O–H cleavage, leading to an unprecedented reversal aromaticity switch

Reductive Cleavage of CO₂ by Metal–Ligand-Cooperation Mediated by an Iridium Pincer Complex

A unique mode of stoichiometric CO₂ activation and reductive splitting based on metal–ligand-cooperation is described. The novel Ir hydride complexes [(^<i>t</i>Bu-PNP*)­Ir­(H)₂] (<b>2</b>) (^<i>t</i>Bu-PNP*, deprotonated ^<i>t</i>Bu-PNP ligand) and [(^<i>t</i>Bu-PNP)­Ir­(H)] (<b>3</b>) react with CO₂ to give the dearomatized complex [(^<i>t</i>Bu-PNP*)­Ir­(CO)] (<b>4</b>) and water. Mechanistic studies have identified an adduct in which CO₂ is bound to the ligand and metal, [(^<i>t</i>Bu-PNP-COO)­Ir­(H)₂] (<b>5</b>), and a di-CO₂ iridacycle [(^<i>t</i>Bu-PNP)­Ir­(H)­(C₂O₄-κ_C,O)] (<b>6</b>). DFT calculations confirm the formation of <b>5</b> and <b>6</b> as reversibly formed side products, and suggest an η¹-CO₂ intermediate leading to the thermodynamic product <b>4</b>. The calculations support a metal–ligand-cooperation pathway in which an internal deprotonation of the benzylic position by the η¹-CO₂ ligand leads to a carboxylate intermediate, which further reacts with the hydride ligand to give complex <b>4</b> and water

The Ferraquinone–Ferrahydroquinone Couple: Combining Quinonic and Metal-Based Reactivity

A ferraquinone–ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal–ligand cooperation

Hydrogen-Bond and Solvent Dynamics in Transition Metal Complexes: A Combined Simulation and NMR-Investigation

Self-assembling ligands through complementary hydrogen-bonding in the coordination sphere of a transition metal provides catalysts with unique properties for carbon–carbon and carbon–heteroatom formation. Their most distinguishing chemical bonding pattern is a double-hydrogen-bonded motif, which determines much of the chemical functionality. Here, we discuss the possibility of double proton transfer (DPT) along this motif using computational and experimental methods. The infrared and NMR spectral signatures for the double-hydrogen-bonded motif are analyzed. Atomistic simulations and experiments suggest that the dynamics of the catalyst is surprisingly complex and displays at least three different dynamical regimes which can be distinguished with NMR spectroscopy and analyzed from computation. The two hydrogen bonds are kept intact and in rapid tautomeric exchange down to 125 K, which provides an estimate of 5 kcal/mol for the barrier for DPT. This is confirmed by the simulations which predict 5.8 kcal/mol for double proton transfer. A mechanistic interpretation is provided and the distribution of the solvent shell surrounding the catalyst is characterized from extensive simulations

Template Catalysis by Metal–Ligand Cooperation. C–C Bond Formation via Conjugate Addition of Non-activated Nitriles under Mild, Base-free Conditions Catalyzed by a Manganese Pincer Complex

The first example of a catalytic Michael addition reaction of non-activated aliphatic nitriles to α,β-unsaturated carbonyl compounds under mild, neutral conditions is reported. A new de-aromatized pyridine-based PNP pincer complex of the Earth-abundant, first-row transition metal manganese serves as the catalyst. The reaction tolerates a variety of nitriles and Michael acceptors with different steric features and acceptor strengths. Mechanistic investigations including temperature-dependent NMR spectroscopy and DFT calculations reveal that the cooperative activation of alkyl nitriles, which leads to the generation of metalated nitrile nucleophile species (α-cyano carbanion analogues), is a key step of the mechanism. The metal center is not directly involved in the catalytic bond formation but rather serves, cooperatively with the ligand, as a template for the substrate activation. This approach of “template catalysis” expands the scope of potential donors for conjugate addition reactions

Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, <i>in situ</i>-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid. A σ-allyl complex was identified as the resting state of the catalytic transformation and characterized by X-ray crystallographic analysis. By means of ESI-MS investigations we were able to detect a reactive intermediate of the catalytic cycle