Faraday Discussions最新文献

Ga was identified earlier as one of the “overlooked” metals for catalyzing the electrochemical nitrogen reduction reaction (ENRR). We investigate here the electrocatalytic activity of Ga towards the nitrogen reduction reaction. We used a combination of molecular modelling and simulations using periodic density functional theory calculations (DFT), and experimental ENRR measurements. The ENRR was found to proceed via an associative mechanism where the first PCET to dinitrogen forming the surface adsorbed N₂H* species is the overpotential limiting step. The bare Ga cathode has a high overpotential (>2 V (SHE)) for the ENRR. We also investigated the effect of a water-in-salt electrolyte (WISE) on the rate of ammonia formation. The addition of an Li salt lowers the overpotential to 1.88 V (SHE). DFT calculations revealed that the H-adatom was more favorably bound than the N-adatom, and the hydrogen evolution reaction (HER) is expected to dominate at high cathodic potentials. Experimental ENRR tests corroborate our results wherein no significant NH₃ formation was detected. The low electrochemical activity of Ga is attributed to poor binding and activation of N₂ which originates from an electropositive surface charge distribution.

We present a charge density study of two linkage isomer photoswitches, [Pd(Bu₄dien)(NO₂)]BPh₄·THF (1) and [Ni(Et₄dien)(NO₂)₂] (2) using Hirshfeld Atom Refinement (HAR) methods implemented via the NoSpherA2 interface in Olex2. HAR is used to explore the electron density distribution in the photoswitchable molecules of 1 and 2, to gain an in-depth understanding of key bonding features and their influence on the single-crystal-to-single-crystal reaction. HAR analysis is also combined with ab initio calculations to explore the non-covalent interactions that influence physical properties of the photoswitches, such as the stability of the excited state nitrito-(η¹-ONO) isomer. This insight can be fed back into the crystal engineering process to develop new and improved photoswitches that can be optimised towards specific applications.

Catalyst confinement within microporous media provides the opportunity to site isolate reactive intermediates, enforce intermolecular functionalization chemistry by co-localizing reactive intermediates and substrates in molecular-scale interstices, and harness non-covalent host–guest interactions to achieve selectivities that are complementary to those accessible in solution. As part of an ongoing program to develop synthetically useful nitrogen-atom transfer (NAT) catalysts, we have demonstrated intermolecular benzylic amination of toluene at a Ru₂ nitride intermediate confined within the interstices of a Ru₂-based metal–organic framework (MOF), Ru₃(btc)₂X₃ (btc = 1,3,5-benzenetricarboxylate, i.e., Ru-HKUST-1 for X = Cl). Nitride confinement within the extended MOF lattice enabled intermolecular C–H functionalization of benzylic C–H bonds in preference to nitride dimerization, which was encountered with soluble molecular analogues. Detailed study of the kinetic isotope effects (KIEs, i.e., k_H/k_D) of C–H amination, assayed both as intramolecular effects using partially labeled toluene and as intermolecular effects using a mixture of per-labeled and unlabeled toluene, provided evidence for restricted substrate mobility on the time scale of interstitial NAT. Analysis of these KIEs as a function of material mesoporosity provided approximate experimental values for functionalization in the absence of mass transport barriers. Here, we disclose a combined experimental and computational investigation of the mechanism of NAT from a Ru₂ nitride to the C–H bond of toluene. Computed kinetic isotope effects for a H-atom abstraction (HAA)/radical rebound (RR) mechanism are in good agreement with experimental data obtained for C–H amination at the rapid diffusion limit. These results provide the first detailed analysis of the mechanism of intermolecular NAT to a C–H bond, bolster the use of KIEs as a probe of confinement effects on NAT within MOF lattices, and provide mechanistic insights unavailable by experiment because rate-determining mass transport obscured the underlying chemical kinetics.

Nickel fluoride complexes of the type [Ni(F)(L)₂(Ar^F)] (L = phosphine, Ar^F = fluorinated arene) are well-known to form strong halogen and hydrogen bonds in solution and in the solid state. A comprehensive study of such non-covalent interactions using bis(carbene) complexes as acceptors and suitable halogen and hydrogen bond donors is presented. In solution, the complex [Ni(F)(iPr₂Im)₂(C₆F₅)] forms halogen and hydrogen bonds with iodopentafluorobenzene and indole, respectively, which have formation constants (K₃₀₀) an order of magnitude greater than those of structurally related phosphine supported nickel fluorides. Co-crystallisation of this complex and its backbone-methylated analogue [Ni(F)(iPr₂Me₂Im)₂(C₆F₅)] with 1,4-diiodotetrafluorobenzene produces halogen bonding adducts which were characterised by X-ray analysis and ¹⁹F MAS solid state NMR analysis. Differences in the chemical shifts between the nickel fluoride and its halogen bonding adduct are well in line with data that were obtained from titration studies in solution.