π‑Hole Interactions Involving Nitro Compounds: Directionality of Nitrate Esters

The MEPs of a variety of nitro compounds (R–NO₂) suggest the existence of a π-hole with a potential of up to +54 kcal/mol in <b>10</b> (R = CF₃). Several of these nitro compounds were considered as partners for anions (F^–, Cl^–, NC^–) and the electron rich molecules acetonitrile and dimethyl ether. In most cases a π-hole complex was obtained with calculated binding energies of up to 20 kcal/mol with anions and 5 kcal/mol with the neural molecules. A thorough analysis of the CSD revealed that nitrate esters are highly directional π-holes in the solid state, for at least sp² O atoms. This was further illustrated by highlighting several crystal structures where more than 0.2 Å van der Waals overlap was observed between the N atom of the nitrate ester and an electron rich atom like oxygen

Homogeneous Hydrogenation and Isomerization of 1‑Octene Catalyzed by Nickel(II) Complexes with Bidentate Diarylphosphane Ligands

A systematic library of 24 nickel­(II) complexes with bidentate diphosphane ligands was synthesized, and the solid-state structures of five of them were determined with X-ray crystallography. The compounds <b>C1</b>–<b>C3</b> are common P₂Ni^IIX₂-type complexes, while <b>C4</b> contains a unique [P₂Ni^II(NH₃)­(OAc)]⁺ square-planar structure with a P₂NO donor set and <b>C5</b> constitutes a rare [(P₂Ni^II)₂(μ-OH)₂]²⁺ dinuclear compound. The catalytic activity of all complexes was tested in the hydrogenation and/or isomerization of 1-octene in a CH₂Cl₂/CH₃OH reaction medium. Catalyst precursors bearing ligands with <i>o</i>-alkoxy aryl rings selectively hydrogentate 1-octene to <i>n</i>-octane, while catalytic systems comprising ligands without the <i>o</i>-alkoxy functionality selectively isomerize the substrate to a mixture of internal alkenes, mostly <i>cis</i>- and <i>trans</i>-2-octene. The conversion is enhanced by equipping the ligand aryl rings with electron-donating alkoxy groups, by increasing the steric bulk of the backbone and/or the aryl rings, by employing relatively noncoordinating anions, and by adding a base as the cocatalyst. Using the compound [Ni­(L3X)­I₂] as the catalyst precursor and upon application of standard hydrogenation conditions, full conversion of the substrate was achieved in 1 h to isomerization products only (TON = 1940). When a catalytic amount of the base is added, a similar result is obtained even in the absence of H₂. A maximum TON of 4500 in 1 h with 96% selectivity for <i>n</i>-octane was achieved by employing [Ni­(oMeO-L3X)­(NH₃)­(OAc)]­PF₆ as the catalyst precursor

Homogeneous Hydrogenation and Isomerization of 1‑Octene Catalyzed by Nickel(II) Complexes with Bidentate Diarylphosphane Ligands

A systematic library of 24 nickel­(II) complexes with bidentate diphosphane ligands was synthesized, and the solid-state structures of five of them were determined with X-ray crystallography. The compounds <b>C1</b>–<b>C3</b> are common P₂Ni^IIX₂-type complexes, while <b>C4</b> contains a unique [P₂Ni^II(NH₃)­(OAc)]⁺ square-planar structure with a P₂NO donor set and <b>C5</b> constitutes a rare [(P₂Ni^II)₂(μ-OH)₂]²⁺ dinuclear compound. The catalytic activity of all complexes was tested in the hydrogenation and/or isomerization of 1-octene in a CH₂Cl₂/CH₃OH reaction medium. Catalyst precursors bearing ligands with <i>o</i>-alkoxy aryl rings selectively hydrogentate 1-octene to <i>n</i>-octane, while catalytic systems comprising ligands without the <i>o</i>-alkoxy functionality selectively isomerize the substrate to a mixture of internal alkenes, mostly <i>cis</i>- and <i>trans</i>-2-octene. The conversion is enhanced by equipping the ligand aryl rings with electron-donating alkoxy groups, by increasing the steric bulk of the backbone and/or the aryl rings, by employing relatively noncoordinating anions, and by adding a base as the cocatalyst. Using the compound [Ni­(L3X)­I₂] as the catalyst precursor and upon application of standard hydrogenation conditions, full conversion of the substrate was achieved in 1 h to isomerization products only (TON = 1940). When a catalytic amount of the base is added, a similar result is obtained even in the absence of H₂. A maximum TON of 4500 in 1 h with 96% selectivity for <i>n</i>-octane was achieved by employing [Ni­(oMeO-L3X)­(NH₃)­(OAc)]­PF₆ as the catalyst precursor

Homogeneous Hydrogenation and Isomerization of 1‑Octene Catalyzed by Nickel(II) Complexes with Bidentate Diarylphosphane Ligands

A systematic library of 24 nickel­(II) complexes with bidentate diphosphane ligands was synthesized, and the solid-state structures of five of them were determined with X-ray crystallography. The compounds <b>C1</b>–<b>C3</b> are common P₂Ni^IIX₂-type complexes, while <b>C4</b> contains a unique [P₂Ni^II(NH₃)­(OAc)]⁺ square-planar structure with a P₂NO donor set and <b>C5</b> constitutes a rare [(P₂Ni^II)₂(μ-OH)₂]²⁺ dinuclear compound. The catalytic activity of all complexes was tested in the hydrogenation and/or isomerization of 1-octene in a CH₂Cl₂/CH₃OH reaction medium. Catalyst precursors bearing ligands with <i>o</i>-alkoxy aryl rings selectively hydrogentate 1-octene to <i>n</i>-octane, while catalytic systems comprising ligands without the <i>o</i>-alkoxy functionality selectively isomerize the substrate to a mixture of internal alkenes, mostly <i>cis</i>- and <i>trans</i>-2-octene. The conversion is enhanced by equipping the ligand aryl rings with electron-donating alkoxy groups, by increasing the steric bulk of the backbone and/or the aryl rings, by employing relatively noncoordinating anions, and by adding a base as the cocatalyst. Using the compound [Ni­(L3X)­I₂] as the catalyst precursor and upon application of standard hydrogenation conditions, full conversion of the substrate was achieved in 1 h to isomerization products only (TON = 1940). When a catalytic amount of the base is added, a similar result is obtained even in the absence of H₂. A maximum TON of 4500 in 1 h with 96% selectivity for <i>n</i>-octane was achieved by employing [Ni­(oMeO-L3X)­(NH₃)­(OAc)]­PF₆ as the catalyst precursor

Disaggregation is a Mechanism for Emission Turn-On of <i>ortho</i>-Aminomethylphenylboronic Acid-Based Saccharide Sensors

<i>ortho</i>-Aminomethylphenylboronic acid-based receptors with appended fluorophores are commonly used as molecular sensors for saccharides in aqueous media. The mechanism for fluorescence modulation in these sensors has been attributed to some form of photoinduced electron transfer (PET) quenching, which is diminished in the presence of saccharides. Using a well-known boronic acid-based saccharide sensor (<b>3</b>), this work reveals a new mechanism for fluorescence turn-on in these types of sensors. Compound <b>3</b> exhibits an excimer, and the associated ground-state aggregation is responsible for fluorescence modulation under certain conditions. When fructose was titrated into a solution of <b>3</b> in 2:1 water/methanol with NaCl, the fluorescence intensity increased. Yet, when the same titration was repeated in pure methanol, a solvent in which the sensor does not aggregate, no fluorescence response to fructose was observed. This reveals that the fluorescence increase is not fully associated with fructose binding, but instead disaggregation of the sensor in the presence of fructose. Further, an analogue of the sensor that does not contain a boronic acid (<b>4</b>) responded nearly identically to <b>3</b> in the presence of fructose, despite having no functional group with which to bind the saccharide. This further supports the claim that fluorescence modulation is not primarily a result of binding, but of disaggregation. Using an indicator displacement assay and isothermal titration calorimetry, it was confirmed that fructose does indeed bind to the sensor. Thus, our evidence reveals that while binding occurs with fructose in the aqueous solvent system used, it is not related to the majority of the fluorescence modulation. Instead, disaggregation dominates the signal turn-on, and is thus a mechanism that should be investigated in other <i>ortho</i>-aminomethylphenylboronic acid-based sensors