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

Study of Precatalyst Degradation Leading to the Discovery of a New Ru⁰ Precatalyst for Hydrogenation and Dehydrogenation

The complex Ru-MACHO (<b>1</b>) is a widely used precatalyst for hydrogenation and dehydrogenation reactions under basic conditions. In an attempt to identify the active catalyst form, <b>1</b> was reacted with a strong base. The formation of previously unreported species was observed by NMR and mass spectrometry. This observation indicated that complex <b>1</b> quickly degraded under basic conditions when no substrate was present. X-ray crystallography enabled the identification of three complexes as products of this degradation of complex <b>1</b>. These complexes suggested degradation pathways which included ligand cleavage and reassembly, along with reduction of the ruthenium atom. One of the decomposition products, the Ru⁰ complex [Ru­(N­(CH₂CH₂PPh₂)₃)­CO] (<b>5</b>), was prepared independently and studied. <b>5</b> was found to be active, entirely additive-free, in the acceptorless dehydrogenation of aliphatic alcohols to esters. The hydrogenation of esters catalyzed by <b>5</b> was also demonstrated under base-free conditions with methanol as an additive. Protic substrates were shown to add reversibly to complex <b>5</b>, generating Ru^II–hydrido species, thus presenting a rare example of reversible oxidative addition from Ru⁰ to Ru^II and reductive elimination from Ru^II to Ru⁰

B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes

Organic derivatives of boronic acid are widely used reagents useful in various synthetic applications. A fundamental understanding and the exploration of new reaction pathways of boronic reagents with organometallic systems hold promise for useful advancement in chemical catalysis. Herein we present the reactions of simple boranes with dearomatized ruthenium pincer complexes based on PNP (2,6-bis­(di-<i>tert</i>-butylphosphinomethyl)­pyridine) or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)­pyridine) ligands. NMR studies revealed dehydrogenative addition of the borane B–H bond across the metal center and the ligand. Remarkably, new complexes were observed, which contain the boryl moiety at the benzylic carbon of the pincer ligand arm. X-ray crystal structures of new dearomatized boryl pincer complexes were obtained, and DFT calculations revealed mechanistic details of the adduct formation process through a dehydrogenative pathway. In addition, catalytic aryl–boron coupling reactions were explored. The new boryl pincer systems may possibly be useful in future postmodification techniques for ruthenium pincer complexes, as well as in catalytic B–B and B–C coupling reactions

B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes

Organic derivatives of boronic acid are widely used reagents useful in various synthetic applications. A fundamental understanding and the exploration of new reaction pathways of boronic reagents with organometallic systems hold promise for useful advancement in chemical catalysis. Herein we present the reactions of simple boranes with dearomatized ruthenium pincer complexes based on PNP (2,6-bis­(di-<i>tert</i>-butylphosphinomethyl)­pyridine) or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)­pyridine) ligands. NMR studies revealed dehydrogenative addition of the borane B–H bond across the metal center and the ligand. Remarkably, new complexes were observed, which contain the boryl moiety at the benzylic carbon of the pincer ligand arm. X-ray crystal structures of new dearomatized boryl pincer complexes were obtained, and DFT calculations revealed mechanistic details of the adduct formation process through a dehydrogenative pathway. In addition, catalytic aryl–boron coupling reactions were explored. The new boryl pincer systems may possibly be useful in future postmodification techniques for ruthenium pincer complexes, as well as in catalytic B–B and B–C coupling reactions

B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes

Organic derivatives of boronic acid are widely used reagents useful in various synthetic applications. A fundamental understanding and the exploration of new reaction pathways of boronic reagents with organometallic systems hold promise for useful advancement in chemical catalysis. Herein we present the reactions of simple boranes with dearomatized ruthenium pincer complexes based on PNP (2,6-bis­(di-<i>tert</i>-butylphosphinomethyl)­pyridine) or PNN (2-(di-<i>tert</i>-butylphosphinomethyl)-6-(diethylaminomethyl)­pyridine) ligands. NMR studies revealed dehydrogenative addition of the borane B–H bond across the metal center and the ligand. Remarkably, new complexes were observed, which contain the boryl moiety at the benzylic carbon of the pincer ligand arm. X-ray crystal structures of new dearomatized boryl pincer complexes were obtained, and DFT calculations revealed mechanistic details of the adduct formation process through a dehydrogenative pathway. In addition, catalytic aryl–boron coupling reactions were explored. The new boryl pincer systems may possibly be useful in future postmodification techniques for ruthenium pincer complexes, as well as in catalytic B–B and B–C coupling reactions

Bottom-Up Construction of a CO₂‑Based Cycle for the Photocarbonylation of Benzene, Promoted by a Rhodium(I) Pincer Complex

The use of carbon dioxide for synthetic applications presents a major goal in modern homogeneous catalysis. Rhodium–hydride PNP pincer complex <b>1</b> is shown to add CO₂ in two disparate pathways: one is the expected insertion of CO₂ into the metal–hydride bond, and the other leads to reductive cleavage of CO₂, involving metal–ligand cooperation. The resultant rhodium–carbonyl complex was found to be photoactive, enabling the activation of benzene and formation of a new benzoyl complex. Organometallic intermediate species were observed and characterized by NMR spectroscopy and X-ray crystallography. Based on the series of individual transformations, a sequence for the photocarbonylation of benzene using CO₂ as the feedstock was constructed and demonstrated for the production of benzaldehyde from benzene

Bottom-Up Construction of a CO₂‑Based Cycle for the Photocarbonylation of Benzene, Promoted by a Rhodium(I) Pincer Complex

The use of carbon dioxide for synthetic applications presents a major goal in modern homogeneous catalysis. Rhodium–hydride PNP pincer complex <b>1</b> is shown to add CO₂ in two disparate pathways: one is the expected insertion of CO₂ into the metal–hydride bond, and the other leads to reductive cleavage of CO₂, involving metal–ligand cooperation. The resultant rhodium–carbonyl complex was found to be photoactive, enabling the activation of benzene and formation of a new benzoyl complex. Organometallic intermediate species were observed and characterized by NMR spectroscopy and X-ray crystallography. Based on the series of individual transformations, a sequence for the photocarbonylation of benzene using CO₂ as the feedstock was constructed and demonstrated for the production of benzaldehyde from benzene