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Designing catalyst to achieve ammonia synthesis at mild conditions is a meaningful challenge in catalysis community. Defective g-C3N4 nanosheet supported single-cluster ruthenium and iron catalysts were investigated for their ammonia synthesis performance. Based on density functional theory (DFT) calculations and microkinetic simulations, Ru3 single-cluster anchored on defective g-C3N4 nanosheet (Ru3/Nv-g-C3N4) has a turnover frequency (TOF) 5.8 times higher than the Ru(0001) step surface at industrial reaction conditions of 673 K and 100 bar for ammonia synthesis. In other words, similar TOFs could be achieved on Ru3/Nv-g-C3N4 at much milder conditions (623 K, 30 bar) than on Ru(0001) (673 K, 100 bar). Our computations reveal the reaction proceeds parallelly on Ru3/Nv-g-C3N4 through both dissociative and alternative associative mechanisms at typical reaction conditions (600–700 K, 10–100 bar); N-N bond cleavage of *N2 and *NNH from the two respective pathways controls the reaction collectively. With increasing temperatures or decreasing pressures, the dissociative mechanism gradually prevails and associative mechanism recedes. In comparison, Fe3/Nv-g-C3N4 catalyst shows a much lower catalytic activity than Ru3/Nv-g-C3N4 by two orders of magnitude and the reaction occurs solely through the dissociative pathway. The finding provides a prospective candidate and deepens the mechanistic understanding for ammonia synthesis catalyzed by single-cluster catalysts (SCCs). [ABSTRACT FROM AUTHOR]

A series of cyclometalated diruthenium complexes with a redox-active amine bridge were synthesized. Depending on the terminal ligands of the ruthenium components and the substituent on the amine unit, the one-electron-oxidized state can be either in the form of a weakly or strongly coupled mixed-valence diruthenium complex, a fully charge-delocalized three-center system, or a bridge-biased electrophore. This transition among different electronic forms was supported by electrochemistry, near-infrared absorption, electron paramagnetic resonance, and density functional theory analysis. [ABSTRACT FROM AUTHOR]

Bis-triarylamine 2 and cyclometalated diruthenium 6(PF6)2 with a linear trans, trans-urea bridge have been prepared, together with the bis-triarylamine 3 and cyclometalated diruthenium 8(PF6)2 with a folded cis, cis- N, N-dimethylurea bridge. The linear or folded conformations of these molecules are supported by single-crystal X-ray structures of 2, 3, and other related compounds. These compounds display two consecutive anodic redox waves (N .+/0 or RuIII/II processes) with a potential separation of 110-170 mV. This suggests that an efficient electronic coupling is present between two redox termini through the cross-conjugated urea bridge. The degree of electronic coupling has been investigated by using spectroelectrochemical measurements. Distinct intervalence charge-transfer (IVCT) transitions have been observed for mixed-valent (MV) compounds with a linear conformation. The IVCT transitions can also be identified for the folded MV compounds, albeit with a much weaker intensity. DFT results support that the electronic communication occurs by a through-bond and through-space pathway for the linear and folded compounds, respectively. The IVCT transitions of the MV compounds have been reproduced by TDDFT calculations. For the purpose of comparison, a bistriarylamine and a diruthenium complex with an imidazolidin-2-one bridge and a urea-containing mono-triarylamine and monoruthenium complex have been synthesized and studied. [ABSTRACT FROM AUTHOR]