Interaction and Activation of Carbon–Heteroatom π Bonds with a Zr/Co Heterobimetallic Complex

Single-electron transfer from the ZrIVCo–I heterobimetallic complex (THF)­Zr­(MesNPiPr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)­Zr­(MesNPiPr2)3Co-N2]2 (4) via coupling of two ketyl radicals. Thermolysis of 4 led to cleavage of the CO bond to generate a Zr/Co μ-oxo species featuring an unusual terminal CoCPh2 carbene linkage (3). In this work monomeric ketyl radical complexes have been synthesized, and the reactivity of these compounds has been explored. The electronic preference for the formation of a ketyl radical complex or a coordination complex has been investigated computationally. Furthermore, thione substrates were allowed to react with 1, generating new complexes that bind the thione to the Co rather than undergoing single-electron transfer (12, 14). The preference of thiones to coordinate to Co can be overridden if the Co is ligated by CO, in which case a thioketyl radical complex forms (13) analogous to 4. The reaction between 1 and imines resulted in N–H bond activation, affording a μ-methyleneamido Co–H complex (16) that can undergo cyclometalation and loss of H2 (15)

Interaction and Activation of Carbon–Heteroatom π Bonds with a Zr/Co Heterobimetallic Complex

Single-electron transfer from the ZrIVCo–I heterobimetallic complex (THF)­Zr­(MesNPiPr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)­Zr­(MesNPiPr2)3Co-N2]2 (4) via coupling of two ketyl radicals. Thermolysis of 4 led to cleavage of the CO bond to generate a Zr/Co μ-oxo species featuring an unusual terminal CoCPh2 carbene linkage (3). In this work monomeric ketyl radical complexes have been synthesized, and the reactivity of these compounds has been explored. The electronic preference for the formation of a ketyl radical complex or a coordination complex has been investigated computationally. Furthermore, thione substrates were allowed to react with 1, generating new complexes that bind the thione to the Co rather than undergoing single-electron transfer (12, 14). The preference of thiones to coordinate to Co can be overridden if the Co is ligated by CO, in which case a thioketyl radical complex forms (13) analogous to 4. The reaction between 1 and imines resulted in N–H bond activation, affording a μ-methyleneamido Co–H complex (16) that can undergo cyclometalation and loss of H2 (15)

Electric Fields Detected on Dye-Sensitized TiO₂ Interfaces: Influence of Electrolyte Composition and Ruthenium Polypyridyl Anchoring Group Type

Electric fields at the dye-sensitized interface of anatase TiO₂ nanocrystallites interconnected in a mesoporous thin film are reported using carboxylic acid-derivatized and phosphonic acid-derivatized ruthenium polypyridyl complexes. Systematic investigations with [Ru­(dtb)₂(dpb)]­(PF₆)₂, where dtb is 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine and dpb is 4,4′-bis-(PO₃H₂)-2,2′-bipyridine, were carried out in conjunction with its carboxylic acid structural analogue. Electric fields attributed to cation adsorption were measured from a bathochromic (red) shift of the sensitizer’s UV–visible absorption spectra upon replacement of neat acetonitrile solution with metal cation perchlorate acetonitrile electrolyte. Electric fields attributed to TiO₂ electrons were measured from the hypsochromic (blue) shift of the absorption spectra upon electrochemical reduction of the sensitized TiO₂ thin films. Electric fields, induced by either cation adsorption or electrochemically populated electrons, increase in magnitude following the same general cation-dependent trend (Na⁺ < Li⁺ < Ca²⁺ ≤ Mg²⁺ < Al³⁺), regardless of the sensitizer’s anchoring group type. For the first time, surface electric fields in the presence of trivalent cations (i.e., Al³⁺) were measured using [Ru­(dtb)₂(dpb)]­(PF₆)₂. The magnitude of electric fields detected by the carboxylic acid sensitizer was 3 times greater than that detected by the phosphonic acid structural analogue under the same experimental conditions. The influence of protons and water in the acetonitrile electrolyte was also quantified. The added water was found to decrease the electric field, whereas protons had a very similar influence as did metal cations

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex

Stoichiometric CO Bond Oxidative Addition of Benzophenone by a Discrete Radical Intermediate To Form a Cobalt(I) Carbene

Single electron transfer from the Zr^IIICo⁰ heterobimetallic complex (THF)­Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂ (<b>1</b>) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph₂CO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂]₂ (<b>2</b>) via coupling of two ketyl radicals. In this work, thermolysis of <b>2</b> in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C–O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal CoCPh₂ carbene linkage, (η²-MesNP^<i>i</i>Pr₂)­Zr­(μ-O)­(MesNP^<i>i</i>Pr₂)₂CoCPh₂ (<b>3</b>). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex <b>3</b> was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to <b>3</b> leads to PhSiH₂O–Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂ (<b>5</b>) via a proposed Co–alkyl bond homolysis route

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex

Interaction and Activation of Carbon–Heteroatom π Bonds with a Zr/Co Heterobimetallic Complex

Single-electron transfer from the Zr^IVCo^–I heterobimetallic complex (THF)­Zr­(MesNP^<i>i</i>Pr₂)₃Co-N₂ (<b>1</b>) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph₂CO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co-N₂]₂ (<b>4</b>) via coupling of two ketyl radicals. Thermolysis of <b>4</b> led to cleavage of the CO bond to generate a Zr/Co μ-oxo species featuring an unusual terminal CoCPh₂ carbene linkage (<b>3</b>). In this work monomeric ketyl radical complexes have been synthesized, and the reactivity of these compounds has been explored. The electronic preference for the formation of a ketyl radical complex or a coordination complex has been investigated computationally. Furthermore, thione substrates were allowed to react with <b>1</b>, generating new complexes that bind the thione to the Co rather than undergoing single-electron transfer (<b>12</b>,<b> 14</b>). The preference of thiones to coordinate to Co can be overridden if the Co is ligated by CO, in which case a thioketyl radical complex forms (<b>13</b>) analogous to <b>4</b>. The reaction between <b>1</b> and imines resulted in N–H bond activation, affording a μ-methyleneamido Co–H complex (<b>16</b>) that can undergo cyclometalation and loss of H₂ (<b>15</b>)

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex

Stoichiometric CO Bond Oxidative Addition of Benzophenone by a Discrete Radical Intermediate To Form a Cobalt(I) Carbene

Single electron transfer from the Zr^IIICo⁰ heterobimetallic complex (THF)­Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂ (<b>1</b>) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph₂CO)­Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂]₂ (<b>2</b>) via coupling of two ketyl radicals. In this work, thermolysis of <b>2</b> in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C–O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal CoCPh₂ carbene linkage, (η²-MesNP^<i>i</i>Pr₂)­Zr­(μ-O)­(MesNP^<i>i</i>Pr₂)₂CoCPh₂ (<b>3</b>). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex <b>3</b> was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to <b>3</b> leads to PhSiH₂O–Zr­(MesNP^<i>i</i>Pr₂)₃Co–N₂ (<b>5</b>) via a proposed Co–alkyl bond homolysis route

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex