Hydrogenation of Dimethyl Carbonate to Methanol by <i>trans</i>-[Ru(H)₂(PNN)(CO)] Catalysts: DFT Evidence for Ion-Pair-Mediated Metathesis Paths for C–OMe Bond Cleavage

Milstein and co-workers have reported that the pincer complexes <i>trans</i>-[Ru­(H)₂(PNN)­(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru–H bonds to produce the six-coordinate <i>trans</i>-[Ru­(OCH­(OMe)₂)­(H)­(PNN)­(CO)] intermediate and (ii) a metal–ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH­(OMe)₂]⁻ anion along with cleavage of a C–OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru­(H)­(PNN)­(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru­(H)₂(PNN)­(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH­(OMe)₂]⁻ anion in which the C–H bond is facing the metal center, (ii) reorientation of the [OCH­(OMe)₂]⁻ anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C–OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and <i>trans</i>-[Ru­(H)­(OMe)­(PNN)­(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal–ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru­(H)₂(PNN)­(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru­(H)­(OCH­(OMe)₂)­(PNN)­(CO)] and [Ru­(H)­(OCH₂OMe)­(PNN)­(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study

Hydrogenation of Dimethyl Carbonate to Methanol by <i>trans</i>-[Ru(H)₂(PNN)(CO)] Catalysts: DFT Evidence for Ion-Pair-Mediated Metathesis Paths for C–OMe Bond Cleavage

Milstein and co-workers have reported that the pincer complexes <i>trans</i>-[Ru­(H)₂(PNN)­(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru–H bonds to produce the six-coordinate <i>trans</i>-[Ru­(OCH­(OMe)₂)­(H)­(PNN)­(CO)] intermediate and (ii) a metal–ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH­(OMe)₂]⁻ anion along with cleavage of a C–OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru­(H)­(PNN)­(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru­(H)₂(PNN)­(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH­(OMe)₂]⁻ anion in which the C–H bond is facing the metal center, (ii) reorientation of the [OCH­(OMe)₂]⁻ anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C–OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and <i>trans</i>-[Ru­(H)­(OMe)­(PNN)­(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal–ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru­(H)₂(PNN)­(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru­(H)­(OCH­(OMe)₂)­(PNN)­(CO)] and [Ru­(H)­(OCH₂OMe)­(PNN)­(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study

Hydrogenation of Dimethyl Carbonate to Methanol by <i>trans</i>-[Ru(H)₂(PNN)(CO)] Catalysts: DFT Evidence for Ion-Pair-Mediated Metathesis Paths for C–OMe Bond Cleavage

Milstein and co-workers have reported that the pincer complexes <i>trans</i>-[Ru­(H)₂(PNN)­(CO)] catalyze the unprecedented homogeneous hydrogenation of dimethyl carbonate to methanol. A mechanism for this reaction was proposed on the basis of (i) carbonyl group insertion into one of the Ru–H bonds to produce the six-coordinate <i>trans</i>-[Ru­(OCH­(OMe)₂)­(H)­(PNN)­(CO)] intermediate and (ii) a metal–ligand cooperative transformation, involving proton transfer from the phosphine arm of the PNN ligand to a methoxy group of the Ru-coordinated [OCH­(OMe)₂]⁻ anion along with cleavage of a C–OMe bond, to produce methanol and an O-bound methyl formate complex of the dearomatized square-pyramidal form of the catalyst, [Ru­(H)­(PNN)­(CO)]. We investigate herein the possibility of an alternative reaction pathway proceeding as (i) an outer-sphere hydride transfer from [Ru­(H)₂(PNN)­(CO)] to the carbonyl of dimethyl carbonate to give an ion pair of the cationic metal fragment and the [OCH­(OMe)₂]⁻ anion in which the C–H bond is facing the metal center, (ii) reorientation of the [OCH­(OMe)₂]⁻ anion within the intact ion pair to coordinate a methoxy group to the metal, and (iii) C–OMe bond cleavage (methoxide abstraction by the cationic ruthenium center) to yield methyl formate and <i>trans</i>-[Ru­(H)­(OMe)­(PNN)­(CO)]. Using DFT calculations applied at the M06 and ωB97X-D levels with a polarizable continuum representing THF as solvent, we calculate the energy profile of this pathway to be significantly lower than the metal–ligand cooperative pathway. The analogous pathway is also favored for the reaction of [Ru­(H)₂(PNN)­(CO)] with methyl formate. The new mechanism corresponds to a direct metathesis transformation in which a hydride and an alkoxide are exchanged between a metal center and a carbonyl group via an outer sphere ion pair formation and reorientation of the alkoxide anion. The calculations also indicate that the metathesis can proceed indirectly via outer sphere ion pair mediated carbonyl insertion of dimethyl carbonate and methyl formate to give [Ru­(H)­(OCH­(OMe)₂)­(PNN)­(CO)] and [Ru­(H)­(OCH₂OMe)­(PNN)­(CO)], respectively, as intermediates, followed by ion pair mediated deinsertion of methyl formate or formaldehyde. Inclusion of one methanol molecule as an explicit H-bond donor solvent does not change the main conclusions of the study

Continuous-Flow Alkane Dehydrogenation by Supported Pincer-Ligated Iridium Catalysts at Elevated Temperatures

Pincer-ligated iridium complexes of the form [Ir­(^R4PCP)­L] (^R4PCP = κ³-C₆H₃-2,6-(XPR₂)₂; X = CH₂, O; R = <i>t</i>Bu, <i>i</i>Pr) are efficient homogeneous alkane dehydrogenation catalysts that have been reported to be highly active at temperatures of 240 °C or below. In this work, silica-supported [Ir­(C₂H₄)­(<i>p</i>-^<i>t</i>Bu₂PO-^<i>t</i>Bu4POCOP)] (<b>1/SiO</b>_<b>2</b>) was used to study a model continuous-flow gas-phase acceptorless alkane dehydrogenation system. This particular supported framework is thermally stable at temperatures up to 340 °C, 100 °C above the highest temperature at which analogous homogeneous complexes have been reported to show stable activity, with observed butane dehydrogenation rates of ca. 80 mol_butenes mol_cat.^–1 h^–1. Solid-state ³¹P MAS NMR and ATR IR are used to demonstrate that the backbone pincer ligand remains intact and coordinated at 340 °C. The complex is fully converted to [Ir­(CO)­(<i>p</i>-^<i>t</i>Bu₂PO-^<i>t</i>Bu4POCOP)] (<b>3/SiO</b>_<b>2</b>) above 300 °C. <b>3/SiO</b>_<b>2</b> is observed to be catalytically active at the higher temperatures tested, and reaction rates are comparable to those of <b>1/SiO</b>_<b>2</b>. <b>3/SiO</b>_<b>2</b> and <b>1/SiO</b>_<b>2</b> act as resting states for the active 14-electron fragment, through dissociation of the CO or olefin ligand, respectively. Given that <b>3/SiO</b>_<b>2</b> is air resistant at ambient temperature and is structurally stable and catalytically active at elevated temperatures, it is a suitable candidate as a catalyst for the highly endothermic acceptorless dehydrogenation of alkanes

Alkane Dehydrogenation by C–H Activation at Iridium(III)

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>), has been described. The reaction between <b>1a</b> and octane resulted in quantitative formation of (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>3a</b>) and octene. At early reaction times 1-octene is the major product, indicative of terminal C–H activation by <b>1a</b>. In contrast to prior reports of alkane dehydrogenation with Ir, C–H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water

Selective Dehydrogenative Coupling of Ethylene to Butadiene via an Iridacyclopentane Complex

An iridium complex is found to catalyze the selective dehydrogenative coupling of ethylene to 1,3-butadiene. The key intermediate, and a major resting state, is an iridacyclopentane that undergoes a surprisingly facile β-H elimination, enabled by a partial dechelation (κ³–κ²) of the supporting 3,5-dimethylphenyl-2,6-bis­(oxazolinyl) ligand

Selective Dehydrogenative Coupling of Ethylene to Butadiene via an Iridacyclopentane Complex

An iridium complex is found to catalyze the selective dehydrogenative coupling of ethylene to 1,3-butadiene. The key intermediate, and a major resting state, is an iridacyclopentane that undergoes a surprisingly facile β-H elimination, enabled by a partial dechelation (κ³–κ²) of the supporting 3,5-dimethylphenyl-2,6-bis­(oxazolinyl) ligand

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems

Alkane Dehydrogenation by C–H Activation at Iridium(III)

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>), has been described. The reaction between <b>1a</b> and octane resulted in quantitative formation of (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>3a</b>) and octene. At early reaction times 1-octene is the major product, indicative of terminal C–H activation by <b>1a</b>. In contrast to prior reports of alkane dehydrogenation with Ir, C–H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems