Copper(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a μ 1,3formate-bridged dicopper(II) complex (1 H ) to a hexacopper(I) hydride cluster (2 H ) upon reduction. Isotopic labeling studies revealed that both the H − and CO 2 originate from the bound μ 1,3 -formate in 1 H , which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1 H to 2 H is established. The structure of 2 H features two Cu 3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2 H is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands − CCPh (3) while retaining the hexacopper(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2 H and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2 H and 3) predicted by DFT calculations.
Herein, we report on a facile and selective one-pot synthetic route to silicon–boron radicals. Reduction of Br2BTip (Tip = 2,4,6- i PrC6H2) with KC8 in the presence of LSi-R affords LSi( t Bu)-B(Br)Tip (1) and LSi(N(TMS)2)-B(Br)Tip (2) [L = PhC(N t Bu)2]. These first examples of silicon–boron isolated radical species feature spin density on the silicon and boron atoms. 1 and 2 exhibit extraordinary stability to high temperatures under inert conditions in solution and air stability in the solid state. Both radicals have been isolated and fully characterized by electron paramagnetic resonance spectroscopy, SQUID magnetometry, mass spectrometry, cyclic voltammetry, single-crystal X-ray structure analysis, and density functional theory calculations. Moreover, compound 1 exhibits one-electron transfer when treated with 1 equiv of AgSO3CF3 and [Ph3C]+[B(C6F5)4]−, respectively, resulting in the corresponding cations [LSi( t Bu)-B(Br)Tip]+[CF3SO3]− (3) and [LSi( t Bu)-B(Br)Tip]+[B(C6F5)4]− (4). Compounds 3 and 4 have been characterized with multinuclear NMR and mass spectrometry.
Molecular systems combining light harvesting and charge storage are receiving great attention in the context of, for example, artificial photosynthesis and solar fuel generation. As part of ongoing efforts to develop new concepts for photoinduced proton-coupled electron transfer (PCET) reactivities, we report a cyclometallated iridium(III) complex [Ir(ppy) 2 ( S−S bpy)](PF 6 ) ([1]PF 6 ) equipped with our previously developed sulfurated bipyridine ligand S−S bpy. A new one-step synthetic protocol for S−S bpy is developed, starting from commercially available 2,2′bipyridine, which significantly facilitates the use of this ligand. [1] + features a two-electron reduction with potential inversion (|E 1 | > | E 2 |) at moderate potentials (E 1 = −1.12, E 2 = −1.11 V versus. Fc +/0 at 253 K), leading to a dithiolate species [1] − . Protonation with weak acids allows for determination of pK a = 23.5 in MeCN for the S−H•••S − unit of [1H]. The driving forces for both the H atom and the hydride transfer are calculated to be ∼60 kcal mol −1 and verified experimentally by reaction with a suitable H atom and a hydride acceptor, demonstrating the ability of [1] + to serve as a versatile PCET reagent, albeit with limited thermal stability. In MeCN solution, an orange emission for [1]PF 6 from a triplet-excited state was found. Density functional calculations and ultrafast absorption spectroscopy are used to give insight into the excited-state dynamics of the complex and suggest a significantly stretched S−S bond for the lowest triplet-state T 1 . The structural responsiveness of the disulfide unit is proposed to open an effective relaxation channel toward the ground state, explaining the unexpectedly short lifetime of [1] + . These insights as well as the quantitative ground-state thermochemistry data provide valuable information for the use of S−S bpy-functionalized complexes and their disulfide-/dithiol-directed PCET reactivity.
Two mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N4} ligands provide a constrained equatorial coordination with one large N−Ru−N angle, and additional non‐coordinating N atoms in case of 2. Their redox properties have been investigated (spectro)electrochemically, and their potential to serve as water oxidation catalysts has been probed using cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged and exhibits 80 % efficiency in the CeIV‐driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07×10−2 s−1 (at 100 equiv. CAN). The initial rate of O2 evolution exhibits 1st order dependence on catalyst concentration, suggesting a water nucleophilic attack mechanism. Repeated addition of CAN and control experiments show that high ionic strength conditions (both NO3− and CeIII) significantly decrease the TOF.
The urgent need for the development of carbon-neutral energy conversion schemes has inspired chemists worldwide to investigate catalytic water splitting using sunlight as the energy source. A key component for the preparation of devices employing this process is the water oxidation catalyst (WOC), and understanding the factors that govern its catalytic performance is among the major challenges. Beyond the well-established WOCs based on ruthenium, catalysts containing abundant first-row transition metals are a desirable target but require a deeper understanding before their large-scale application is viable. Along these lines, this work focusses on both the in-depth analysis of diruthenium WOCs, as well as the search for new catalysts based on copper and cobalt. A glance is also cast at dioxygen activation by dicopper complexes, which is not only of interest on its own but can additionally provide insight into intermediates that are relevant to water oxidation. emphasizes the necessity for oxidatively robust ligand systems. In addition to the analysis of the electrocatalytic water oxidation ability, magnetic susceptibility measurements were conducted for several of the dicopper and dicobalt complexes bearing the pyrazolate/tacn hybrid ligands, viz. 7 MeCN , 5 HCO2 and its acetate-bridged congener 5 OAc as well as the (L 2b ) − -ligated dicobalt analogs 8 HCO2 and 8 OAc . Interestingly, 8 HCO2 and 8 OAc feature slow relaxation of magnetization with relaxation times τ0 of 7.7•10 −6 s −1 and 3.7•10 −6 s −1 , as well as energy barriers Ueff of 8.9 and 6.0 cm −1 , respectively, and thus belong to a very small circle of homodinuclear cobalt(II) complexes with SMM behavior.5 OAc and 5 HCO2 were further investigated in regard to dioxygen activation following the strategy: (i) reduction of the dicopper(II) complex for the in situ generation of a dicopper(I) species (5 I/I ) and (ii) addition of dry O2 to 5 I/I for copper-oxygen adduct formation. Surprisingly, reduction of 5 OAc to 5 I/I affords a mixed-valent Cu I Cu II intermediate (5 I/II ), characterized both by UV/vis and EPR spectroscopy. This observation is unprecedented for dinuclear copper complexes bearing pyrazolate/tacn hybrid ligands and is attributed to the acetate unit in the bimetallic pocket, which is proposed to change its coordination mode from bridging to chelating upon one-electron reduction in order to stabilize the Cu II site. Addition of molecular oxygen to 5 I/II monitored by time-resolved UV/vis spectroscopy in a stopped-flow setup at −40 °C results in a superoxo species (5 S ) as primary Cu II 2O2 adduct, in line with a 1e − reduction of the dioxygen. The generated 5 S rapidly converts to a µ-1,2-peroxo compound (5 P ), distinguished by its characteristic UV/vis-spectroscopic features, by reduction with either an additional 5 I/II species or a second 5 S compound. On the other hand, 5 P is directly formed when exposing 5 I/I to O2, and can be electrochemically oxidized at −0.55 V vs. Fc +/0 (O2 2− /O2 •− redox couple) at −45 °C in MeCN. The low st...
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