Reversing the Tradeoff between Rate and Overpotential in Molecular Electrocatalysts for H₂ Production

A longstanding challenge in molecular electrocatalysis is to design catalysts that break away from the tradeoff between rate and overpotential arising from electronic scaling relationships. Here we report an inversion of the rate–overpotential correlation through system-level design of [Ni­(P^R₂N^{R^′}₂)₂]²⁺ electrocatalysts for the production of H₂. The overpotential is lowered by an electron-withdrawing ligand, while the turnover frequency is increased by controlling the catalyst structural dynamics, using both ligand design and solvent viscosity. The cumulative effect of controlling each of these system components is an electrocatalyst with a turnover frequency of 70000 s^–1 and an overpotential of 230 mV, corresponding to a 100-fold rate enhancement and a 170 mV reduction in overpotential in comparison to the parent nickel catalyst. Molecular Tafel plot analysis reveals that the new catalysts reported here are substantially more efficient than other leading molecular electrocatalysts for the production of H₂

A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>‑Nitrosothiol

<i>S</i>-Nitrosothiols RSNO represent circulating reservoirs of nitric oxide activity in the plasma and play intricate roles in protein function control in health and disease. While nitric oxide has been shown to reductively nitrosylate copper­(II) centers to form copper­(I) complexes and ENO species (E = R₂N, RO), well-characterized examples of the reverse reaction are rare. Employing the copper­(I) β-diketiminate [Me₂NN]­Cu, we illustrate a clear example in which an RS–NO bond is cleaved to release NO_gas with formation of a discrete copper­(II) thiolate. The addition of Ph₃CSNO to [Me₂NN]Cu generates the three-coordinate copper­(II) thiolate [Me₂NN]­CuSCPh₃, which is unstable toward free NO

A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>‑Nitrosothiol

<i>S</i>-Nitrosothiols RSNO represent circulating reservoirs of nitric oxide activity in the plasma and play intricate roles in protein function control in health and disease. While nitric oxide has been shown to reductively nitrosylate copper­(II) centers to form copper­(I) complexes and ENO species (E = R₂N, RO), well-characterized examples of the reverse reaction are rare. Employing the copper­(I) β-diketiminate [Me₂NN]­Cu, we illustrate a clear example in which an RS–NO bond is cleaved to release NO_gas with formation of a discrete copper­(II) thiolate. The addition of Ph₃CSNO to [Me₂NN]Cu generates the three-coordinate copper­(II) thiolate [Me₂NN]­CuSCPh₃, which is unstable toward free NO

Noninteracting, Vicinal Frustrated P/B-Lewis Pair at the Norbornane Framework: Synthesis, Characterization, and Reactions

Hydroboration of dimesitylnorbornenylphosphane with Piers’ borane [HB­(C₆F₅)₂] gave the frustrated Lewis pair (FLP) <b>4</b> in good yield. It has the −PMes₂ Lewis base attached at the 2-endo position and the −B­(C₆F₅)₂ group 3-exo oriented at the norbornane framework. The vicinal FLP <b>4</b> was shown by X-ray diffraction and by spectroscopy to be a rare example of an intramolecular noninteracting pair of a Lewis acid and Lewis base functionality. The FLP <b>4</b> rapidly splits dihydrogen heterolytically at ambient temperature to yield the phosphonium/hydrido borate zwitterion <b>5</b>. It adds to the carbonyl group of benzaldehyde and to carbon dioxide to yield the adducts <b>6</b> and <b>7</b>, respectively. Compounds <b>5–7</b> were characterized by X-ray diffraction. Compound <b>4</b> adds to the SO function of sulfur dioxide to give a pair of diastereomeric heterocyclic six-membered ring products due to the newly formed sulfur chirality center, annulated with the norbornane skeleton, which were investigated by ³¹P/¹¹B single and double resonance solid state NMR experiments. Compound <b>8</b> was also characterized by X-ray diffraction. The FLP <b>4</b> undergoes a clean <i>N</i>,<i>N</i>-addition to nitric oxide (NO) to give a norbornane annulated five-membered heterocyclic persistent FLP-NO aminoxyl radical <b>12</b> (characterized, e.g., by X-ray diffraction and EPR spectroscopy). Additionally, the FLP radical was characterized by ¹H solid state NMR spectroscopy. The radical <b>12</b> undergoes a H-atom abstraction reaction with 1,4-cyclohexadiene to yield the respective diamagnetic FLP-NOH product <b>13</b>, which was also characterized by X-ray diffraction and solid state NMR spectroscopy

Noninteracting, Vicinal Frustrated P/B-Lewis Pair at the Norbornane Framework: Synthesis, Characterization, and Reactions

Hydroboration of dimesitylnorbornenylphosphane with Piers’ borane [HB­(C₆F₅)₂] gave the frustrated Lewis pair (FLP) <b>4</b> in good yield. It has the −PMes₂ Lewis base attached at the 2-endo position and the −B­(C₆F₅)₂ group 3-exo oriented at the norbornane framework. The vicinal FLP <b>4</b> was shown by X-ray diffraction and by spectroscopy to be a rare example of an intramolecular noninteracting pair of a Lewis acid and Lewis base functionality. The FLP <b>4</b> rapidly splits dihydrogen heterolytically at ambient temperature to yield the phosphonium/hydrido borate zwitterion <b>5</b>. It adds to the carbonyl group of benzaldehyde and to carbon dioxide to yield the adducts <b>6</b> and <b>7</b>, respectively. Compounds <b>5–7</b> were characterized by X-ray diffraction. Compound <b>4</b> adds to the SO function of sulfur dioxide to give a pair of diastereomeric heterocyclic six-membered ring products due to the newly formed sulfur chirality center, annulated with the norbornane skeleton, which were investigated by ³¹P/¹¹B single and double resonance solid state NMR experiments. Compound <b>8</b> was also characterized by X-ray diffraction. The FLP <b>4</b> undergoes a clean <i>N</i>,<i>N</i>-addition to nitric oxide (NO) to give a norbornane annulated five-membered heterocyclic persistent FLP-NO aminoxyl radical <b>12</b> (characterized, e.g., by X-ray diffraction and EPR spectroscopy). Additionally, the FLP radical was characterized by ¹H solid state NMR spectroscopy. The radical <b>12</b> undergoes a H-atom abstraction reaction with 1,4-cyclohexadiene to yield the respective diamagnetic FLP-NOH product <b>13</b>, which was also characterized by X-ray diffraction and solid state NMR spectroscopy