Balance between Metal and Ligand Reduction in Diiminepyridine Complexes of Ti

Reaction of the diiminepyridine ligand ^EtDIP (2,6-Et₂-C₆H₃NCMe)₂C₅H₃N) with TiCl₃(THF)₃ gave the corresponding Ti­(III) complex (^EtDIP)­TiCl₃ (<b>1</b>). Reduction of <b>1</b> with 1 equiv of KC₈ produced the formally Ti­(II) complex (^EtDIP)­TiCl₂ (<b>2</b>). From this, (^EtDIP)­TiClR complexes (R = Me (<b>3a</b>), Me₃SiCH₂ (<b>3b</b>), Ph (<b>3c</b>)) were obtained by addition of 1 equiv of RLi. Similarly, dialkyl complexes (^EtDIP)­TiR₂ (R = Me (<b>4a</b>), Me₃SiCH₂ (<b>4b</b>)) were obtained with 2 equiv of RLi. All new complexes except <b>3b</b> were characterized by single-crystal X-ray diffraction. EPR studies indicate that complex <b>1</b> is best regarded as a true Ti­(III) complex with an “innocent” DIP ligand. Complexes <b>2</b>–<b>4</b> are all diamagnetic. In contrast to DIP complexes of the late transition metals Fe and Co, the new complexes <b>2</b>–<b>4</b> show strong upfield ¹H NMR shifts for the pyridine β and γ protons caused by transfer of negative charge to the DIP ligand. On the basis of this and the CN and C_imine–C_Py bond lengths, a description involving Ti­(IV) and a dianionic ligand seems most appropriate, and DFT calculations support this interpretation. This means that reduction of Ti­(III) complex <b>1</b> results in oxidation of the metal center to Ti­(IV). VT-NMR studies of <b>4a</b> suggest a small and temperature-dependent thermal population of a triplet state, and indeed calculations indicate that <b>4a</b> has the lowest singlet–triplet energy difference of the systems studied

Balance between Metal and Ligand Reduction in Diiminepyridine Complexes of Ti

Reaction of the diiminepyridine ligand ^EtDIP (2,6-Et₂-C₆H₃NCMe)₂C₅H₃N) with TiCl₃(THF)₃ gave the corresponding Ti­(III) complex (^EtDIP)­TiCl₃ (<b>1</b>). Reduction of <b>1</b> with 1 equiv of KC₈ produced the formally Ti­(II) complex (^EtDIP)­TiCl₂ (<b>2</b>). From this, (^EtDIP)­TiClR complexes (R = Me (<b>3a</b>), Me₃SiCH₂ (<b>3b</b>), Ph (<b>3c</b>)) were obtained by addition of 1 equiv of RLi. Similarly, dialkyl complexes (^EtDIP)­TiR₂ (R = Me (<b>4a</b>), Me₃SiCH₂ (<b>4b</b>)) were obtained with 2 equiv of RLi. All new complexes except <b>3b</b> were characterized by single-crystal X-ray diffraction. EPR studies indicate that complex <b>1</b> is best regarded as a true Ti­(III) complex with an “innocent” DIP ligand. Complexes <b>2</b>–<b>4</b> are all diamagnetic. In contrast to DIP complexes of the late transition metals Fe and Co, the new complexes <b>2</b>–<b>4</b> show strong upfield ¹H NMR shifts for the pyridine β and γ protons caused by transfer of negative charge to the DIP ligand. On the basis of this and the CN and C_imine–C_Py bond lengths, a description involving Ti­(IV) and a dianionic ligand seems most appropriate, and DFT calculations support this interpretation. This means that reduction of Ti­(III) complex <b>1</b> results in oxidation of the metal center to Ti­(IV). VT-NMR studies of <b>4a</b> suggest a small and temperature-dependent thermal population of a triplet state, and indeed calculations indicate that <b>4a</b> has the lowest singlet–triplet energy difference of the systems studied

How Solvent Affects C–H Activation and Hydrogen Production Pathways in Homogeneous Ru-Catalyzed Methanol Dehydrogenation Reactions

Insights into the mechanism of the catalytic cycle for methanol dehydrogenation catalyzed by a highly active PNP pincer ruthenium complex in methanol solvent are presented, using DFT-based molecular dynamics with an explicit description of the solvent, as well as static DFT calculations using microsolvation models. In contrast to previous results, we find the amido moiety of the catalyst to be permanently protonated under catalytic conditions. Solvent molecules actively participate in crucial reaction steps and significantly affect the reaction barriers when compared to pure gas-phase models, which is a direct result of the enhanced solvent stabilization of methoxide anion intermediates. Further, the calculations reveal that this system does not operate via the commonly assumed Noyori-type outer-sphere metal–ligand cooperative pathway. Our results show the importance of incorporating a molecular description of the solvent to gain a deeper and accurate understanding of the reaction pathways, and stress on the need to involve explicit solvent molecules to model complex catalytic processes in a realistic manner

Charge-Delocalized κ²<i>C</i>,<i>N</i>‑NHC-Amine Complexes of Rhodium, Iridium, and Ruthenium

The development of a novel set of complexes bearing an NHC-amine ligand (C^NHC-NH₂) is described. M­(cod) complexes (M = Ir, Rh) and a Ru complex have been synthesized in which three different coordination modes of the ligand were established: monodentate, neutral bidentate, and anionic bidentate. The anionic bidentate coordination mode of the anionic C^NHC-NH^– ligand arises from deprotonation of the amine moiety of the neutral C^NHC-NH₂ ligand. Ligand deprotonation proved to be reversible for the Rh and Ir complexes, as was shown by subsequent treatment of the complexes with base and acid. The structural parameters of these differently coordinated ligands were examined, and it was shown that the conjugation of the aniline ring plays a major role in determining the ligand properties. Structural parameters derived from DFT calculations confirm delocalization of the anionic charge over the ligand framework, as is clear from a comparison of the (hypothetical) neutral bidentate complexes [M­(cod)­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH₂})]⁺ with those of the (synthesized) monoanionic complexes [M­(cod)­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH})] (M = Rh, Ir). A similar trend in the structure and bond lengths of the aniline rings was found in the solid-state structure of the novel dimeric complex [(Ru­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH})­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH₂})­Cl)₂(μ-Cl)]­(PF₆). The octahedral d⁵ ruthenium­(III) centers in this complex both contain a neutral bidentate C^NHC-NH₂ ligand as well as an anionic bidentate C^NHC-NH^– ligand. Quite remarkably, the complex is diamagnetic, arising from antiferromagnetic coupling of the two low-spin ruthenium­(III) centers over the chloride linker. DFT calculations indeed confirm that the open-shell singlet electronic structure is most stable

Charge-Delocalized κ²<i>C</i>,<i>N</i>‑NHC-Amine Complexes of Rhodium, Iridium, and Ruthenium

The development of a novel set of complexes bearing an NHC-amine ligand (C^NHC-NH₂) is described. M­(cod) complexes (M = Ir, Rh) and a Ru complex have been synthesized in which three different coordination modes of the ligand were established: monodentate, neutral bidentate, and anionic bidentate. The anionic bidentate coordination mode of the anionic C^NHC-NH^– ligand arises from deprotonation of the amine moiety of the neutral C^NHC-NH₂ ligand. Ligand deprotonation proved to be reversible for the Rh and Ir complexes, as was shown by subsequent treatment of the complexes with base and acid. The structural parameters of these differently coordinated ligands were examined, and it was shown that the conjugation of the aniline ring plays a major role in determining the ligand properties. Structural parameters derived from DFT calculations confirm delocalization of the anionic charge over the ligand framework, as is clear from a comparison of the (hypothetical) neutral bidentate complexes [M­(cod)­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH₂})]⁺ with those of the (synthesized) monoanionic complexes [M­(cod)­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH})] (M = Rh, Ir). A similar trend in the structure and bond lengths of the aniline rings was found in the solid-state structure of the novel dimeric complex [(Ru­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH})­(κ²<i>C</i>,<i>N</i>-{C^NHC-NH₂})­Cl)₂(μ-Cl)]­(PF₆). The octahedral d⁵ ruthenium­(III) centers in this complex both contain a neutral bidentate C^NHC-NH₂ ligand as well as an anionic bidentate C^NHC-NH^– ligand. Quite remarkably, the complex is diamagnetic, arising from antiferromagnetic coupling of the two low-spin ruthenium­(III) centers over the chloride linker. DFT calculations indeed confirm that the open-shell singlet electronic structure is most stable

Computed Propagation and Termination Steps in [(Cycloocta-2,6-dien-1-yl)Rh^III(polymeryl)]⁺ Catalyzed Carbene Polymerization Reactions

This paper discloses the DFT-computed pathways for chain propagation, chain transfer, and chain termination during carbene polymerization catalyzed by cationic [(cycloocta-2,6-dien-1-yl)­Rh^III(alkyl)]⁺ species. In contrast to carbene polymerization calculated for neutral [(cod)­Rh^I(alkyl)]⁺ catalysts, chain propagation at the cationic [(cycloocta-2,6-dien-1-yl)­Rh^III(alkyl)]⁺ species is clearly competitive with β-hydride elimination, thus explaining the formation of high molecular weight polymers. Computed chain-end-controlled chain propagation reveals a clear preference for syndiotactic polymerization. Chain transfer involving alcohol-mediated protonolysis is computed to be a more favorable pathway than β-hydride elimination. These results are all in agreement with experimental observations. Chain propagation from species with a stereoerror at the α-carbon atom of the growing chain is substantially slower compared to propagation from syndiotactic species without stereoerrors, providing a possible explanation for the experimentally observed low initiation efficiencies of the Rh catalysts in carbene polymerization reactions. These new computational insights, combined with experimental results disclosed in earlier reports, largely clarify the mechanism of Rh-mediated carbene polymerization reactions

How Solvent Affects C–H Activation and Hydrogen Production Pathways in Homogeneous Ru-Catalyzed Methanol Dehydrogenation Reactions

Insights into the mechanism of the catalytic cycle for methanol dehydrogenation catalyzed by a highly active PNP pincer ruthenium complex in methanol solvent are presented, using DFT-based molecular dynamics with an explicit description of the solvent, as well as static DFT calculations using microsolvation models. In contrast to previous results, we find the amido moiety of the catalyst to be permanently protonated under catalytic conditions. Solvent molecules actively participate in crucial reaction steps and significantly affect the reaction barriers when compared to pure gas-phase models, which is a direct result of the enhanced solvent stabilization of methoxide anion intermediates. Further, the calculations reveal that this system does not operate via the commonly assumed Noyori-type outer-sphere metal–ligand cooperative pathway. Our results show the importance of incorporating a molecular description of the solvent to gain a deeper and accurate understanding of the reaction pathways, and stress on the need to involve explicit solvent molecules to model complex catalytic processes in a realistic manner

Photo- and Thermal Isomerization of (TP)Fe(CO)Cl₂ [TP = Bis(2-diphenylphosphinophenyl)­phenylphosphine]

The title complex displayed structural flexibility via photo- and thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)­Fe­(CO)­Cl₂ (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)­Fe­(CO)­Cl₂ (<b>B</b>), and <i>sym</i>-(<i>fac-</i>TP)­Fe­(CO)­Cl₂ (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis spectroscopy combined with TD-DFT calculations revealed the nature of the photoisomerization process. Kinetics of the thermal isomerization of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with ³¹P NMR spectroscopy in CD₂Cl₂, and activation parameters were determined. Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically characterized

Photo- and Thermal Isomerization of (TP)Fe(CO)Cl₂ [TP = Bis(2-diphenylphosphinophenyl)­phenylphosphine]

The title complex displayed structural flexibility via photo- and thermal-isomerization reactions between three isomers: (<i>mer</i>-TP)­Fe­(CO)­Cl₂ (<b>A</b>), <i>unsym</i>-(<i>fac</i>-TP)­Fe­(CO)­Cl₂ (<b>B</b>), and <i>sym</i>-(<i>fac-</i>TP)­Fe­(CO)­Cl₂ (<b>C</b>). Irradiation of <b>A</b> at RT with 525 nm light selectively produces <b>B</b>, while at 0 °C isomer <b>C</b> is formed with the intermediacy of <b>B</b>. UV–vis spectroscopy combined with TD-DFT calculations revealed the nature of the photoisomerization process. Kinetics of the thermal isomerization of <b>C</b> to <b>B</b> and <b>B</b> to <b>A</b> have been studied with ³¹P NMR spectroscopy in CD₂Cl₂, and activation parameters were determined. Isomers <b>A</b> and <b>B</b> have been isolated and crystallographically characterized

Co^III–Carbene Radical Approach to Substituted 1<i>H</i>‑Indenes

A new strategy for the catalytic synthesis of substituted 1<i>H</i>-indenes via metalloradical activation of <i>o</i>-cinnamyl <i>N</i>-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co^III carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1<i>H</i>-indene derivatives. The cheap and easy to prepare low spin cobalt­(II) complex [Co^II(MeTAA)] (MeTAA = tetramethyltetraaza[14]­annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt­(II) complex, and for the first time catalytic activity of [Co^II(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1<i>H</i>-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C­(sp²)–H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co^III-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt­(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl <i>N</i>-<i>tert</i>-butylnitrone (PBN) reveal the radical nature of the reaction