Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s−1 and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.
Herein we report the synthesis, characterization, and electrocatalytic CO2 reduction activity of a series of PdII complexes supported by tetradentate pyridinophane ligands. In particular, we focus on the electrocatalytic CO2 reduction activity of a PdII complex supported by the mixed hard/soft 2,11-dithia[3.3](2,6)pyridinophane (N2S2) ligand . This is one of the few examples of a Pd complexes supported by a mixed hard-soft ligand which selectively produces CO from the electrocatalytic reduction of CO2. Notably, unlike previously reported molecular Pd complexes, selective CO2RR occurs in presence of weak proton sources such as 2, 2, 2 trifluoroethanol (TFE) and phenol, at mild overpotentials (~160 mV) and with high rates (kobs = 4.5 x 103 s-1, with phenol as proton source) and at Faradaic efficiencies of up to 70% for CO, without any H2 being detected. As the catalyst was not stable to long term electrolysis, we analyzed possible decomposition routes for this catalyst and, based on the characterization of its reaction with CO by UV-vis, NMR, and IR spectroscopy, we propose the intermediacy of a binuclear [(N2S2)PdI(η2-CO)]2 species toward the ultimate decomposition of the catalyst into free ligand and Pd0. Overall, these studies offer important insights into Pd catalyst decomposition and may explain the historically poor performance of related Pd molecular catalysts for CO2 reduction. In addition, the structurally-related hard N-donor diazapyridinophane (RN4)Pd complexes are shown to be unstable towards bulk electrolysis at cathodic potentials, suggesting that such compounds are ill-suited for CO2 electroreduction.
Photo-assisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein we report metal-specific studies using static and transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4-di-tert-butyl, o-tol = ortho-tolyl). We discovered that the ground state of this complex has a mixed-spin character of ~70/30% singlet/triplet. Furthermore, we confirm that the long-lived (~5 ns) excited state is a tetrahedral metal-centered triplet state. These results pave the way for the future design of Ni-bipyridine based photocatalysts by, for example, judiciously tuning the electronic and geometric properties of the ligands with the goal of increasing excited-state lifetimes and quantum yields of reactive species.
Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s-1 and a moderate overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.
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