On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

Direct Access to Palladium(II) Complexes Based on Anionic C,C,C-Phosphonium Ylide Core Pincer Ligand

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH₂)₃PPh₃](OTf)₂ with PdCl₂ in the presence of an excess of Cs₂CO₃ afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX₂-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively.

Panchromatic dirhodium photocatalysts for dihydrogen generation with red light

A series of three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂ (1, DPhB = diphenylbenzamidine; bncn = benzocinnoline), cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ (2, DPhTA = diphenyltriazenide), and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ (3, DPhF = N,N′-diphenylformamidinate) shown to act as single-molecule photocatalysts for H₂ production was evaluated. Complexes 1–3 are able to generate H₂ in the absence of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-reduced, activated form of the catalyst that is able to generate H₂. The present work shows the effect of substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes 1–3 operate following a double reductive quenching mechanism and, importantly, are active with red light. This work lays the foundation for the design of single-molecule photocatalysts that operate from the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed and utilized for solar energy conversion.

Three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂, cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ are shown to act as single-molecule photocatalysts for H₂ production.