η^2 bonded Nickel(0) thiophene π-complexes - identifying the missing link in catalyst transfer polymerization

<p>We have structurally characterized a nickel(0) thiophene complex that represents a close analog to the proposed intermediates in the mechanism of catalyst transfer polycondensation (CTP) of thiophenes. Additional studies on this intermediate allow us to determine the bonding in such complexes using a combination of nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy, and DFT-based computations. Our structure of the catalyst agrees with previous postulates but provides important new structural insights into the species. Furthermore, our studies explain why these complexes actually exist and are stable enough to support living CTP.</p

A Terthiophene-Containing Alkynylplatinum Terpyridine Pacman Complex: Controllable Folding/Unfolding Modulated by Weak Intermolecular Interactions

Folded and unfolded solid-state structures of a bimetallic alkynylplatinum terpyridine complex with a flexible terthiophene linker have been obtained. Weak intermolecular interactions including π–π stacking and C–H···O and C–H···Cl interactions as well as Cl−π interactions stabilize the folded structure. In solution, folding is studied by electronic absorption spectroscopy and ¹H and NOESY NMR experiments

Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins

Thirty-two Ru­(porp)­L₂ complexes have been synthesized, where porp = the dianion of <i>meso</i>-tetramesitylporphyrin (TMP) or <i>meso</i>-tetrakis­(4-methylphenyl)­porphyrin (H₂T-<i>p</i>Me-<i>P</i>P), and L = a thiol, a sulfide, a disulfide, or a trisulfide. Species studied were with RSH [R = Me, Et, ^<i>n</i>Pr, ^<i>i</i>Pr, ^<i>t</i>Bu, Bn (benzyl), and Ph], RSR (R = Me, Bn), RSSR (R = Me, Et, ^<i>n</i>Pr, Bn) and MeSS^<i>t</i>Bu, and RSSSR (R = Me, Bn). All the species except two, which were the isolated Ru­(T-<i>p</i>Me-<i>P</i>P)­(^<i>t</i>BuSH)₂ and Ru­(TMP)­(MeSSMe)₂, were characterized in situ. The disulfide complex was characterized by X-ray analysis. ¹H NMR data for the coordinated thiols are the first reported within metalloporphyrin systems, and are especially informative because of the upfield shifts of the axial sulfur-containing ligands due to the porphyrin π-ring current effect, which is also present in the di- and trisulfide species. The disulfide in the solid state structure of Ru­(TMP)­(MeSSMe)₂ is η¹(end-on) coordinated, the first example of such bonding in a nontethered, acyclic dialkyl disulfide; ¹H–¹H EXSY NMR data in solution show that the species undergoes 1,2-S-metallotropic shifts. Stepwise formation of the bis­(disulfide) complex from Ru­(TMP)­(MeCN)₂ in solution occurs with a cooperativity effect, resembling behavior of Fe^II–porphyrin systems where crystal field effects dominate, but ligand trans-effects are more likely in the Ru system. The η¹(end-on) coordination mode is also favored for the trisulfide ligand. Discussed also are the remarkable linear correlations that exist between the ring-current shielding shifts for the axial ligand C¹ protons of Ru­(porp)­(RS_<i>x</i>R)₂ and <i>x</i> (the number of S atoms). The Introduction briefly reviews literature on Ru- and Fe porphyrins (including heme proteins) with sulfur-containing ligands or substrates, and relationships between our findings and this literature are discussed throughout the paper

Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins

Thirty-two Ru­(porp)­L₂ complexes have been synthesized, where porp = the dianion of <i>meso</i>-tetramesitylporphyrin (TMP) or <i>meso</i>-tetrakis­(4-methylphenyl)­porphyrin (H₂T-<i>p</i>Me-<i>P</i>P), and L = a thiol, a sulfide, a disulfide, or a trisulfide. Species studied were with RSH [R = Me, Et, ^<i>n</i>Pr, ^<i>i</i>Pr, ^<i>t</i>Bu, Bn (benzyl), and Ph], RSR (R = Me, Bn), RSSR (R = Me, Et, ^<i>n</i>Pr, Bn) and MeSS^<i>t</i>Bu, and RSSSR (R = Me, Bn). All the species except two, which were the isolated Ru­(T-<i>p</i>Me-<i>P</i>P)­(^<i>t</i>BuSH)₂ and Ru­(TMP)­(MeSSMe)₂, were characterized in situ. The disulfide complex was characterized by X-ray analysis. ¹H NMR data for the coordinated thiols are the first reported within metalloporphyrin systems, and are especially informative because of the upfield shifts of the axial sulfur-containing ligands due to the porphyrin π-ring current effect, which is also present in the di- and trisulfide species. The disulfide in the solid state structure of Ru­(TMP)­(MeSSMe)₂ is η¹(end-on) coordinated, the first example of such bonding in a nontethered, acyclic dialkyl disulfide; ¹H–¹H EXSY NMR data in solution show that the species undergoes 1,2-S-metallotropic shifts. Stepwise formation of the bis­(disulfide) complex from Ru­(TMP)­(MeCN)₂ in solution occurs with a cooperativity effect, resembling behavior of Fe^II–porphyrin systems where crystal field effects dominate, but ligand trans-effects are more likely in the Ru system. The η¹(end-on) coordination mode is also favored for the trisulfide ligand. Discussed also are the remarkable linear correlations that exist between the ring-current shielding shifts for the axial ligand C¹ protons of Ru­(porp)­(RS_<i>x</i>R)₂ and <i>x</i> (the number of S atoms). The Introduction briefly reviews literature on Ru- and Fe porphyrins (including heme proteins) with sulfur-containing ligands or substrates, and relationships between our findings and this literature are discussed throughout the paper

Intermolecular C–H Activations of Hydrocarbons Initiated by a Tungsten Trimethylsilylallyl Complex

Thermolysis of Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) [Cp* = η⁵-C₅Me₅; Npt = CH₂CMe₃] at 55 °C leads to the loss of neopentane and the formation of the 16-electron η²-allene intermediate Cp*W­(NO)­(η²-CH₂CCHSiMe₃), which activates hydrocarbons at their methyl groups. In the case of linear alkanes, only terminal C–H activation occurs. This selectivity persists in the presence of an ether functionality, but not with other oxygen-containing substrates such as aldehydes and alcohols. With these latter substrates, the organometallic complex is oxidized to Cp*W­(O)₂(Npt). The existence of the allene intermediate has been confirmed by its reaction with PMe₃ to form the 18-electron adduct and by its diagnostic reaction with cyclohexene. Carbonylation of Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) with CO (550 psig) at room temperature results in the clean formation of the corresponding Cp*W­(NO)­(η¹-C­(O)­Npt)­(η³-CH₂CHCHSiMe₃) complex, which exists as a mixture of two interconverting isomers differing in their modes of attachment of the (η³-CH₂CHCHSiMe₃) ligands to the tungsten centers. The congeneric molybdenum complex, Cp*Mo­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃), has also been synthesized, and although it generates the requisite η²-allene intermediate upon thermolysis, its preferred mode of reactivity is coupling of the allyl and alkyl ligands. Consequently, the molybdenum complex is inferior to the tungsten system for effecting C–H activations. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses

Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes

The thermolysis of Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) (<b>1</b>; Cp* = η⁵-C₅Me₅; Npt = CH₂CMe₃) in benzene at 55 °C generates three isomeric products having the composition Cp*W­(NO)­(H)­(η³-Me₃SiCHCHCHPh) (<b>2</b>). These are isomers of the expected Cp*W­(NO)­(Ph)­(η³-Me₃SiCHCHCH₂) compound and result from an intramolecular Ph/allyl H exchange. Thermolysis of <b>2</b> in the presence of pyridine produces the η²-olefin pyridine adduct Cp*W­(NO)­(η²-Me₃SiCH₂CHCHPh)­(C₅H₅N) (<b>3</b>). However, when the same reaction is carried out in deuterobenzene with 10 equiv of pyridine, NMR spectroscopic data suggest that the meso hydrogen of the allyl ligand is exchanged for a deuterium atom before pyridine trapping occurs. The activation of fluorobenzenes (i.e., pentafluorobenzene, <i>p</i>-difluorobenzene, and <i>o</i>-difluorobenzene) by Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) has also been studied, and for these substrates, C–H bond activation occurs exclusively. Selectivity for the activation of these C–H bonds appears to be determined by sterics. Intramolecular migration of the newly formed fluoroaryl ligands onto the allyl ligands does not occur when there is a fluorine atom in the position ortho to the newly formed W–C bond. This behavior is probably a manifestation of the fact that metal–<i>o</i>-fluoroaryl bonds tend to be stronger than metal–aryl linkages. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses

Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes

The thermolysis of Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) (<b>1</b>; Cp* = η⁵-C₅Me₅; Npt = CH₂CMe₃) in benzene at 55 °C generates three isomeric products having the composition Cp*W­(NO)­(H)­(η³-Me₃SiCHCHCHPh) (<b>2</b>). These are isomers of the expected Cp*W­(NO)­(Ph)­(η³-Me₃SiCHCHCH₂) compound and result from an intramolecular Ph/allyl H exchange. Thermolysis of <b>2</b> in the presence of pyridine produces the η²-olefin pyridine adduct Cp*W­(NO)­(η²-Me₃SiCH₂CHCHPh)­(C₅H₅N) (<b>3</b>). However, when the same reaction is carried out in deuterobenzene with 10 equiv of pyridine, NMR spectroscopic data suggest that the meso hydrogen of the allyl ligand is exchanged for a deuterium atom before pyridine trapping occurs. The activation of fluorobenzenes (i.e., pentafluorobenzene, <i>p</i>-difluorobenzene, and <i>o</i>-difluorobenzene) by Cp*W­(NO)­(Npt)­(η³-CH₂CHCHSiMe₃) has also been studied, and for these substrates, C–H bond activation occurs exclusively. Selectivity for the activation of these C–H bonds appears to be determined by sterics. Intramolecular migration of the newly formed fluoroaryl ligands onto the allyl ligands does not occur when there is a fluorine atom in the position ortho to the newly formed W–C bond. This behavior is probably a manifestation of the fact that metal–<i>o</i>-fluoroaryl bonds tend to be stronger than metal–aryl linkages. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses

Electrocatalytic Reduction of CO₂ with Palladium Bis-N-heterocyclic Carbene Pincer Complexes

A series of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC) palladium pincer complexes were electrochemically characterized and screened for CO₂ reduction capability with 2,2,2-trifluoroethanol, acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources. The lutidine-linked pincer complexes electrocatalytically reduce CO₂ to CO at potentials as low as −1.6 V versus Ag/AgNO₃ in the presence of TFA. The one-electron reduction of these complexes is shown to be chemically reversible, yielding a monometallic species, with density functional theory studies indicating charge storage on the redox-active ligand, thus addressing a major source of deactivation in earlier triphosphine electrocatalysts

Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO₂ Reduction

Phenanthro- and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated into lutidine-linked bis-NHC Pd pincer complexes to investigate the effect of these polyannulated NHCs on the ability of the complexes to electrochemically reduce CO₂ to CO in the presence of 2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources. These complexes are screened for their ability to reduce CO₂ and modeled using density functional theory calculations, where the annulated phenanthrene and pyrene moieties are shown to be additional sites for redox activity in the pincer ligand, enabling increased electron donation. Electrochemical and computational studies are used to gain an understanding of the chemical significance of redox events for complexes of this type, highlighting the importance of anion binding and dissociation

Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO₂ Reduction

Phenanthro- and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated into lutidine-linked bis-NHC Pd pincer complexes to investigate the effect of these polyannulated NHCs on the ability of the complexes to electrochemically reduce CO₂ to CO in the presence of 2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources. These complexes are screened for their ability to reduce CO₂ and modeled using density functional theory calculations, where the annulated phenanthrene and pyrene moieties are shown to be additional sites for redox activity in the pincer ligand, enabling increased electron donation. Electrochemical and computational studies are used to gain an understanding of the chemical significance of redox events for complexes of this type, highlighting the importance of anion binding and dissociation