Hydrogenation and Hydrosilylation of Nitrous Oxide Homogeneously Catalyzed by a Metal Complex

Due to its significant contribution to stratospheric ozone depletion and its potent greenhouse effect, nitrous oxide has stimulated much research interest regarding its reactivity modes and its transformations, which can lead to its abatement. We report the <i>homogeneously</i> catalyzed reaction of nitrous oxide (N₂O) with H₂. The reaction is catalyzed by a PNP pincer ruthenium complex, generating efficiently only dinitrogen and water, under mild conditions, thus providing a green, mild methodology for removal of nitrous oxide. The reaction proceeds through a sequence of dihydrogen activation, “O”-atom transfer, and dehydration, in which metal–ligand cooperation plays a central role. This approach was further developed to catalytic O-transfer from N₂O to Si–H bonds

Metal–Ligand Cooperation as Key in Formation of Dearomatized Ni^II–H Pincer Complexes and in Their Reactivity toward CO and CO₂

The unique synthesis and reactivity of [(^RPNP*)­NiH] complexes (<b>1a</b>,<b>b</b>), based on metal–ligand cooperation (MLC), are presented (^RPNP* = deprotonated PNP ligand, R = ⁱPr, ^tBu). Unexpectedly, the dearomatized complexes <b>1a</b>,<b>b</b> were obtained by reduction of the dicationic complexes [(^RPNP)­Ni­(MeCN)]­(BF₄)₂ with sodium amalgam or by reaction of the free ligand with Ni⁰(COD)₂. Complex <b>1b</b> reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex <b>3b</b>. Complexes <b>1a</b>,<b>b</b> also react with CO₂ via MLC to form a rare example of η¹ binding of CO₂ to nickel, complexes <b>4a</b>,<b>b</b>. An unusual CO₂ cleavage process by complex <b>4b</b>, involving C–O and C–P cleavage and C–C bond formation, led to the Ni–CO complex <b>3b</b> and to the new complex [(PⁱPr₂NC₂O₂)­Ni­(P­(O)ⁱPr₂)] (<b>5b</b>). All complexes have been fully characterized by NMR and X-ray crystallography

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

Direct Observation of Reductive Elimination of MeX (X = Cl, Br, I) from Rh^III Complexes: Mechanistic Insight and the Importance of Sterics

Rare cases of directly observed reductive elimination (RE) of methyl halides from Rh^III complexes are described. Treatment of the coordinatively unsaturated complexes [(^tBuPNP)­Rh­(CH₃)­X]­[BF₄] (<b>1</b>–<b>3</b>, X = I, Br, and Cl; ^tBuPNP = 2,6-bis-(di-<i>tert-</i>butylphosphinomethyl)­pyridine) with coordinating and noncoordinating compounds results in the formation of the corresponding free methyl halides and Rh^I complexes. The rate increase of CH₃I and CH₃Br RE in the presence of polar aprotic solvents argues in favor of an S_N2 RE mechanism. However, the RE of CH₃Cl is faster in polar protic solvents, which argues in favor of a concerted C–Cl RE. The RE of methyl halides from complexes <b>1</b>–<b>3</b> is induced by steric factors, as treatment of the less bulky complexes [(ⁱPrPNP)­Rh­(CH₃)­X]­[BF₄] (<b>19</b>–<b>21</b>; X = I, Br, Cl, respectively) with coordinating compounds leads to the formation of the adducts complexes rather than RE of the methyl halides. The accumulated evidence suggests that the RE process is nonassociative

N–H Activation by Rh(I) via Metal–Ligand Cooperation

In continuation of our studies on bond activation and catalysis by pincer complexes, based on metal–ligand cooperation, we present here a rare example of amine N–H activation by Rh­(I) complexes. The novel dearomatized pincer complexes [(PNN*)­RhL′] (PNN = 2-(CH₂-P^<i>t</i>Bu₂)-6-(CH₂-NEt₂)­C₅H₃N, PNN* = deprotonated PNN, L′ = N₂ (<b>5</b>), C₂H₄ (<b>6</b>)) and [(^<i>i</i>PrPNP*)­RhL′] (^<i>i</i>PrPNP = 2,6-(CH₂-P^<i>i</i>Pr₂)₂C₅H₃N, ^<i>i</i>PrPNP* = deprotonated ^<i>i</i>PrPNP, L′ = C₂H₄ (<b>7</b>), cyclooctene (<b>9</b>)) were prepared and fully characterized by NMR and X-ray analysis. Complexes <b>5</b>–<b>7</b> and <b>9</b> undergo facile N–H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal center to form stable anilide complexes [(PNN)­Rh­(NHAr)] and [(^<i>i</i>PrPNP)­Rh­(NHAr)] (Ar = C₆H₅, <i>o</i>-Br-C₆H₄, <i>m</i>-Cl-<i>p</i>-Cl-C₆H₃, <i>p</i>-NO₂-C₆H₄). Anilines possessing electron-withdrawing groups accelerate the N–H activation and yield more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the NH– protons of anilines prior to and after the N–H activation. The reverse N–H formation by metal–ligand cooperation from the anilide complexes was observed to give free anilines and dearomatized Rh­(I) complexes upon addition of CO or PEt₃. Deprotonation of complexes [(PNL)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)] (<b>13</b>, P = P^<i>t</i>Bu₂, L = NEt₂; <b>15</b>, P = L = P^<i>i</i>Pr₂) yields the dearomatized anionic complexes [(PNL*)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)]. An associative mechanism, involving N–H activation of an apically coordinated aniline in a pentacoordinated Rh­(I) complex, is suggested

Direct Observation of Reductive Elimination of MeX (X = Cl, Br, I) from Rh^III Complexes: Mechanistic Insight and the Importance of Sterics

Rare cases of directly observed reductive elimination (RE) of methyl halides from Rh^III complexes are described. Treatment of the coordinatively unsaturated complexes [(^tBuPNP)­Rh­(CH₃)­X]­[BF₄] (<b>1</b>–<b>3</b>, X = I, Br, and Cl; ^tBuPNP = 2,6-bis-(di-<i>tert-</i>butylphosphinomethyl)­pyridine) with coordinating and noncoordinating compounds results in the formation of the corresponding free methyl halides and Rh^I complexes. The rate increase of CH₃I and CH₃Br RE in the presence of polar aprotic solvents argues in favor of an S_N2 RE mechanism. However, the RE of CH₃Cl is faster in polar protic solvents, which argues in favor of a concerted C–Cl RE. The RE of methyl halides from complexes <b>1</b>–<b>3</b> is induced by steric factors, as treatment of the less bulky complexes [(ⁱPrPNP)­Rh­(CH₃)­X]­[BF₄] (<b>19</b>–<b>21</b>; X = I, Br, Cl, respectively) with coordinating compounds leads to the formation of the adducts complexes rather than RE of the methyl halides. The accumulated evidence suggests that the RE process is nonassociative

N–H Activation by Rh(I) via Metal–Ligand Cooperation

In continuation of our studies on bond activation and catalysis by pincer complexes, based on metal–ligand cooperation, we present here a rare example of amine N–H activation by Rh­(I) complexes. The novel dearomatized pincer complexes [(PNN*)­RhL′] (PNN = 2-(CH₂-P^<i>t</i>Bu₂)-6-(CH₂-NEt₂)­C₅H₃N, PNN* = deprotonated PNN, L′ = N₂ (<b>5</b>), C₂H₄ (<b>6</b>)) and [(^<i>i</i>PrPNP*)­RhL′] (^<i>i</i>PrPNP = 2,6-(CH₂-P^<i>i</i>Pr₂)₂C₅H₃N, ^<i>i</i>PrPNP* = deprotonated ^<i>i</i>PrPNP, L′ = C₂H₄ (<b>7</b>), cyclooctene (<b>9</b>)) were prepared and fully characterized by NMR and X-ray analysis. Complexes <b>5</b>–<b>7</b> and <b>9</b> undergo facile N–H activation of anilines involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal center to form stable anilide complexes [(PNN)­Rh­(NHAr)] and [(^<i>i</i>PrPNP)­Rh­(NHAr)] (Ar = C₆H₅, <i>o</i>-Br-C₆H₄, <i>m</i>-Cl-<i>p</i>-Cl-C₆H₃, <i>p</i>-NO₂-C₆H₄). Anilines possessing electron-withdrawing groups accelerate the N–H activation and yield more stable anilide complexes. The pincer and the ancillary ligands also affect the activation rate, which supports an associative mechanism. Spin saturation transfer experiments show chemical exchange between the pyridylic arm of the pincer ligand and the NH– protons of anilines prior to and after the N–H activation. The reverse N–H formation by metal–ligand cooperation from the anilide complexes was observed to give free anilines and dearomatized Rh­(I) complexes upon addition of CO or PEt₃. Deprotonation of complexes [(PNL)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)] (<b>13</b>, P = P^<i>t</i>Bu₂, L = NEt₂; <b>15</b>, P = L = P^<i>i</i>Pr₂) yields the dearomatized anionic complexes [(PNL*)­Rh­(<i>p</i>-NO₂-NH₂C₆H₄)]. An associative mechanism, involving N–H activation of an apically coordinated aniline in a pentacoordinated Rh­(I) complex, is suggested

O₂ Activation by Metal–Ligand Cooperation with Ir^I PNP Pincer Complexes

A unique mode of molecular oxygen activation, involving metal–ligand cooperation, is described. Ir pincer complexes [(^tBuPNP)­Ir­(R)] (R = C₆H₅ (<b>1</b>), CH₂COCH₃ (<b>2</b>)) react with O₂ to form the dearomatized hydroxo complexes [(^tBuPNP*)­Ir­(R)­(OH)] (^tBuPNP* = deprotonated ^tBuPNP ligand), in a process which utilizes both O-atoms. Experimental evidence, including NMR, EPR, and mass analyses, indicates a binuclear mechanism involving an O-atom transfer by a peroxo intermediate

CO Oxidation by N₂O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism

Both CO and N₂O are important, environmentally harmful industrial gases. The reaction of CO and N₂O to produce CO₂ and N₂ has stimulated much research interest aimed at degradation of these two gases in a single step. Herein, we report an efficient CO oxidation by N₂O catalyzed by a (PNN)­Ru–H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N₂O to the Ru–H bond to form a Ru–OH intermediate, followed by intramolecular OH attack on an adjacent CO ligand, forming CO₂ and N₂. Thus, the Ru–H bond of the catalyst plays a central role in facilitating the OAT from N₂O to CO, providing an efficient and novel protocol for CO oxidation

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