Abstract:The photochemical activation of methane over a substrate containing Rh'(C0)z species has been studied using transmission infrared spectroscopy. At 233 K, methane activation is achieved by the oxidative addition of the C-H bond to a postulated 16-electron surface species, Rh'(CO), which is produced by UV (325 nm) photodecomposition of the Rh1(CO)2 on the surface. The oxidative addition product, Rh(CO)(H)(CH3), further undergoes a migratory-insertion reaction in the presence of CO to form a proposed Rh(CO)(H)(CO… Show more
“…Methane Activation. The activation of methane on photoproduced Rh I (CO) sites was then examined by Wong and Yates . During this experiment, a lower temperature (233 K) was employed to increase the lifetime of the weakly adsorbed methane molecule on the surface and, thus, increase its probability of coordinating onto an unsaturated Rh I (CO) site.…”
Section: Heterogeneous-phase Photochemical Bond
Activationgeneral As...mentioning
confidence: 99%
“… 13 Comparison of the carbonyl stretching frequencies of acyl ligands of different Rh(III) organometallic complexes. (Table reproduced from ref . The references listed in the table may be found in ref .…”
Section: Heterogeneous-phase Photochemical Bond
Activationgeneral As...mentioning
confidence: 99%
“…It was recently discovered that interesting chemical processes can occur by exposing the Rh I (CO) 2 /Al 2 O 3 species to ultraviolet light at low temperatures (<300 K). − 9a When irradiated, one of the CO ligands is ejected, creating a coordinatively unsaturated Rh site. This site is then capable of activating strong chemical bonds such as the C−H bond in alkanes, − which would not be activated thermally at these temperatures on the unphotolyzed Rh I (CO) 2 species.…”
Section: Introductionmentioning
confidence: 99%
“…It was recently discovered that interesting chemical processes can occur by exposing the Rh I (CO) 2 /Al 2 O 3 species to ultraviolet light at low temperatures (<300 K). − 9a When irradiated, one of the CO ligands is ejected, creating a coordinatively unsaturated Rh site. This site is then capable of activating strong chemical bonds such as the C−H bond in alkanes, − which would not be activated thermally at these temperatures on the unphotolyzed Rh I (CO) 2 species. This photochemically generated site has been used extensively to cause a variety of molecular bond-breaking processes to occur, including the activation of the H−H bond in hydrogen, the OO bond in O 2 , and the CO bond in CO 2 .…”
The activation of chemical bonds in molecules on photochemically produced RhI sites is reviewed in this
article. The focus is primarily on the RhI(CO) species generated during the ultraviolet photolysis of the
well-known RhI(CO)2 species supported on an aluminum oxide surface (designated RhI(CO)2/Al2O3). The
RhI(CO)2 surface species is the heterogeneous analogue of the homogeneous phase CpRh(CO)2 and Cp*Rh(CO)2 (Cp* = η5-C5(CH3)5; Cp = η5-C5H5) known for its ability to activate the C−H bond in alkanes during
ultraviolet irradiation. Here we review some of the key studies that have led to the molecular understanding
of the activation of the RhI(CO)2/Al2O3 species by photochemical methods. Relevant examples of the
homogeneous-phase bond activation studies utilizing a rhodium center will be discussed as well as theoretical
studies that have provided valuable information regarding the active nature of the photoproduced Rh site.
This will be followed by a review of recent photochemical studies of surface-bound RhI(CO)2. The
coordinatively unsaturated RhI(CO) species produced photochemically can activate the C−H bond in C6H12
and CH4, the H−H bond in H2, the OO bond in O2, and the CO bond in CO2.
“…Methane Activation. The activation of methane on photoproduced Rh I (CO) sites was then examined by Wong and Yates . During this experiment, a lower temperature (233 K) was employed to increase the lifetime of the weakly adsorbed methane molecule on the surface and, thus, increase its probability of coordinating onto an unsaturated Rh I (CO) site.…”
Section: Heterogeneous-phase Photochemical Bond
Activationgeneral As...mentioning
confidence: 99%
“… 13 Comparison of the carbonyl stretching frequencies of acyl ligands of different Rh(III) organometallic complexes. (Table reproduced from ref . The references listed in the table may be found in ref .…”
Section: Heterogeneous-phase Photochemical Bond
Activationgeneral As...mentioning
confidence: 99%
“…It was recently discovered that interesting chemical processes can occur by exposing the Rh I (CO) 2 /Al 2 O 3 species to ultraviolet light at low temperatures (<300 K). − 9a When irradiated, one of the CO ligands is ejected, creating a coordinatively unsaturated Rh site. This site is then capable of activating strong chemical bonds such as the C−H bond in alkanes, − which would not be activated thermally at these temperatures on the unphotolyzed Rh I (CO) 2 species.…”
Section: Introductionmentioning
confidence: 99%
“…It was recently discovered that interesting chemical processes can occur by exposing the Rh I (CO) 2 /Al 2 O 3 species to ultraviolet light at low temperatures (<300 K). − 9a When irradiated, one of the CO ligands is ejected, creating a coordinatively unsaturated Rh site. This site is then capable of activating strong chemical bonds such as the C−H bond in alkanes, − which would not be activated thermally at these temperatures on the unphotolyzed Rh I (CO) 2 species. This photochemically generated site has been used extensively to cause a variety of molecular bond-breaking processes to occur, including the activation of the H−H bond in hydrogen, the OO bond in O 2 , and the CO bond in CO 2 .…”
The activation of chemical bonds in molecules on photochemically produced RhI sites is reviewed in this
article. The focus is primarily on the RhI(CO) species generated during the ultraviolet photolysis of the
well-known RhI(CO)2 species supported on an aluminum oxide surface (designated RhI(CO)2/Al2O3). The
RhI(CO)2 surface species is the heterogeneous analogue of the homogeneous phase CpRh(CO)2 and Cp*Rh(CO)2 (Cp* = η5-C5(CH3)5; Cp = η5-C5H5) known for its ability to activate the C−H bond in alkanes during
ultraviolet irradiation. Here we review some of the key studies that have led to the molecular understanding
of the activation of the RhI(CO)2/Al2O3 species by photochemical methods. Relevant examples of the
homogeneous-phase bond activation studies utilizing a rhodium center will be discussed as well as theoretical
studies that have provided valuable information regarding the active nature of the photoproduced Rh site.
This will be followed by a review of recent photochemical studies of surface-bound RhI(CO)2. The
coordinatively unsaturated RhI(CO) species produced photochemically can activate the C−H bond in C6H12
and CH4, the H−H bond in H2, the OO bond in O2, and the CO bond in CO2.
“…The ultraviolet photolysis of Rh I (CO) 2 species, supported on Al 2 O 3 (designated Rh I (CO) 2 /Al 2 O 3 ), produces an unstable and coordinatively unsaturated Rh(CO) intermediate which can activate the C−H bond in alkanes − and the H−H bond in hydrogen . The atomically-dispersed Rh I (CO) 2 supported on Al 2 O 3 is produced from disruption and oxidation of alumina-supported metallic Rh particles in a reaction with CO gas and surface hydroxyl groups. − A critical step for successful bond activation in alkanes and in H 2 is the temporary adsorption of the molecule on the photochemically-generated Rh(CO) centers.…”
The UV (3.8 eV) photolysis of atomically-dispersed Rh I (CO) 2 species supported on an Al 2 O 3 surface in the presence of N 2 at 175 K caused the replacement of CO by N 2 ligands. Therefore dinitrogen chemisorption effectively probes the coordinatively unsaturated Rh(CO) species generated during photodecomposition of Rh I (CO) 2 / Al 2 O 3 . Two infrared bands, observed at 2234 and 2048 cm -1 , are attributed respectively to ν N2 and ν CO in Rh(N 2 )(CO), and a band at 2188 cm -1 is assigned to Rh(N 2 ) 2 surface species. The assignments are based on frequencies and the relative rates of spectral development for the two N 2 -containing species. The Rh(N 2 ) 2 species on Al 2 O 3 exhibits identical N-N stretching modes to a similar species produced by matrix isolation methods. It is shown that both the Rh-CO bond and the Rh-N 2 bond may be broken by photolysis.
The recognition of the fundamental contributions by G. A. Olah on the elucidation of the structure of nonclassical carbocations, in the form of the award of the Nobel prize for chemistry, has recently emphasized the importance of electron-deficient bonds in the understanding of chemical bonding in organic chemistry. In the field of coordination chemistry, the formulation of electrondeficient bonds has been used for some time to describe nonclassical interactions between atoms. Traditional ligands in coordination chemistry such as amines and phosphanes bond to metal centers through their lone pair of electrons. Synergistic bonding effects dominate in the coordination of Jr-bonded ligands such as alkenes. In the mid-1980s the discovery of dihydrogen complexes having side-on coordination of H, gave fresh impetus to transition metal chemistry as well as to the understanding of the interaction of o-coordinating ligands with transition metals. In the meantime, transiton metal complexes can be obtained with a variety of ocoordinated X-H fragments, and their mode of bonding can be understood by a common and quite general model. The chemistry of o-bound silane ligands is particularly varied and well-investigated. These silane ligands enable the investigation of a large range of G-coordinated metal complex fragments up to complete oxidative addition with cleavage of the Si-H bond and formation of silyl(hydrid0) complexes, which has thus also widened our general understanding of the bonding of other o-bound ligands. Whilst there is a large range of isolable and stable H, and SiR, complexes available, there are no such alkane analogues known at present. Only when the C-H bond is part of a ligand that is already directly bonded to the transition metal center will the resulting chelate effect stabilize this agostic C-H-M interaction. The complexation of SiH, , the simplest heavier homologue of CH,, was achieved recently. This is a further step towards the understanding of the factors which govern o-complexation of ligands at transition metal centers.
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