Synthesis and Electronic Spectra of Fluorinated Aryloxide and Alkoxide [NiX₄]²⁻ Anions

Five new homoleptic [NiX4]2− compounds have been prepared with the fluorinated ligands OC6F5 (OArF), OC6H3(CF3)2 (OAr′), and OC4F9 (ORF) and characterized with X-ray crystallography, magnetic susceptibility, and elemental analysis. Electronic spectral studies show that these ligands engender a ligand-field environment similar to that of fluoride and thus act electronically like fluoride, but with none of the drawbacks of F− as a transition-metal ligand

Nickel Catalysts for the Dehydrative Decarbonylation of Carboxylic Acids to Alkenes

Combining high-throughput experimentation with conventional experiments expedited discovery of new first-row nickel catalysts for the dehydrative decarbonylation of the bioderived substrates hydrocinnamic acid and fatty acids to their corresponding alkenes. Conventional experiments using a continuous distillation process (180 °C) revealed that catalysts composed of Ni^II or Ni⁰ precursors (NiI₂, Ni­(PPh₃)₄) and simple aryl phosphine ligands were the most active. In the reactions with fatty acids, the nature of the added phosphine influenced the selectivity for α-alkene, which reached a maximum value of 94%. Mechanistic studies of the hydrocinnamic reaction using Ni­(PPh₃)₄ as catalyst implicate a facile first turnover to produce styrene at room temperature, but deactivation of the Ni(0) catalyst by CO poisoning occurs subsequently, as evidenced by the formation of Ni­(CO)­(PPh₃)₃, which was isolated and structurally characterized. Styrene dimerization is a major side reaction. Analysis of the reaction mechanism using density functional theory supported catalyst regeneration along with alkene formation as the most energetically demanding reaction steps

Roles of Monomer Binding and Alkoxide Nucleophilicity in Aluminum-Catalyzed Polymerization of ε<b>-</b>Caprolactone

The kinetics of polymerization of ε-caprolactone (CL) initiated by aluminum-alkoxide complexes supported by the dianionic forms of N,N-bis­[methyl-(2-hydroxy-3-tert-butyl-5-R-phenyl)]-N,N-dimethylethylenediamines, (LR)­Al­(Oi-Pr) (R = OMe, Br, NO2) were studied. The ligands are sterically similar but have variable electron donating characteristics due to the differing remote (para) ligand substituents R. Saturation kinetics were observed using [CL]0 = 2–2.6 M and [complex]0 = 7 mM, enabling independent determination of the substrate coordination (Keq) and insertion (k2) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both Keq and k2 yielded thermodynamic parameters for these steps. The rate constant k2, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter Keq is invariant as a function of ligand electronic properties. Density functional calculations provide atomic-level detail for the structures of key reaction intermediates and their associated thermochemistries

Mechanistic Studies of ε‑Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations

Aluminum alkoxide complexes (<b>2</b>) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO₂, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (<b>1</b>). While <b>1</b> and <b>2</b> exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant <i>K</i>_eq, similar Hammett ρ = +1.4(1) and 1.2(1) for <i>k</i>₂, respectively), ROP by <b>2</b> was significantly faster than for <b>1</b>. Theoretical calculations confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design

Roles of Monomer Binding and Alkoxide Nucleophilicity in Aluminum-Catalyzed Polymerization of ε<b>-</b>Caprolactone

The kinetics of polymerization of ε-caprolactone (CL) initiated by aluminum-alkoxide complexes supported by the dianionic forms of <i>N</i>,<i>N</i>-bis­[methyl-(2-hydroxy-3-<i>tert</i>-butyl-5-R-phenyl)]-<i>N</i>,<i>N-</i>dimethylethylenediamines, (L^R)­Al­(O<i>i</i>-Pr) (R = OMe, Br, NO₂) were studied. The ligands are sterically similar but have variable electron donating characteristics due to the differing remote (<i>para</i>) ligand substituents R. Saturation kinetics were observed using [CL]₀ = 2–2.6 M and [complex]₀ = 7 mM, enabling independent determination of the substrate coordination (<i>K</i>_eq) and insertion (<i>k</i>₂) events in the ring-opening polymerization process. Analysis of the effects of the substituent R as a function of temperature on both <i>K</i>_eq and <i>k</i>₂ yielded thermodynamic parameters for these steps. The rate constant <i>k</i>₂, related to alkoxide nucleophilicity, was strongly enhanced by electron-donating R substituents, but the binding parameter <i>K</i>_eq is invariant as a function of ligand electronic properties. Density functional calculations provide atomic-level detail for the structures of key reaction intermediates and their associated thermochemistries

Mechanistic Studies of ε‑Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations

Aluminum alkoxide complexes (<b>2</b>) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO₂, Br, OMe) were prepared, and the kinetics of their ring-opening polymerization (ROP) of ε-caprolactone (CL) were investigated as a function of temperature, with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (<b>1</b>). While <b>1</b> and <b>2</b> exhibit saturation kinetics and similar dependences of their ROP rates on substituents X (invariant <i>K</i>_eq, similar Hammett ρ = +1.4(1) and 1.2(1) for <i>k</i>₂, respectively), ROP by <b>2</b> was significantly faster than for <b>1</b>. Theoretical calculations confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design