o-C2B10H12 isomerizes to m-C2B10H12 upon heating at 400 ºC. Deboronation in o-C2B10H12 is a relatively easy process, whereas it is more difficult in m-C2B10H12. These two experimental facts indicate that m-C2B10H12 is thermodynamically more stable than o-C2B10H12. On the other hand, it is widely accepted that closo boranes and carboranes are aromatic compounds. In this work, we relate difficulty in the deboronation of the carboranes with stability and aromaticity. We do this by combining lab work and by means of DFT calculations. Computationally, our results show that the higher thermodynamic stability of m-C2B10H12 is not related to aromaticity differences but to the location of the C atoms in the carborane structure. It is also demonstrated that the aromaticity observed in closo boranes and carboranes is also present in their nido counterparts and, consequently, we conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g. ferrocene) and sandwich metallabis(dicarbollides) (e.g.[Co(C2B9H11)2] -) have traditionally been considered similar. Here it is shown that they are not. Metallabis(dicarbollides) display global aromaticity whereas metallocenes present local aromaticity in the ligands. Remarkable and unique is the double probe given by 1 H-and 11 B-NMR tracing the reciprocally antipodal endocyclic open face Hec and B1. These magnetic studies have permitted to correlate both nuclei and relate them to a diatropic current in the plane at the middle of the nido [C2B9H12] -. This observation is the first and unique data that proves experimentally the existence of diatropic currents, thence aromaticity, in clusters and is comparable to the existence of diatropic currents in planar aromatic compounds.Additionally, heteroboranes with two carbon atoms have been compared to heterocycles with two nitrogen or boron atoms, C2B10H12 carboranes against planar N2C4H4 diazines or [B2C4H4] 2diboratabenzenes, proving the higher persistence of the aromaticity of the tri-dimensional compounds in heteroatom substituted species. This research accounts very well for the "Paradigm for the Electron Requirements of Clusters" in which a closo-cluster that is aromatic upon addition of 2ebecomes also an aromatic nido species and explains the nice schemes by R.W. Rudolph and R. E. Williams.
The reductive amination process under hydrogen at high pressure catalyzed by iron complexes is of great synthetic interest. In this work, we report density functional theory (DFT) studies on the reductive amination catalyzed by a Knolker-type iron complex. Different modifications of the catalyst are explored to improve the efficiency and guide experiments toward milder conditions. DFT calculations in conjunction with analysis of the chemical structure in terms of geometry, fragment partial charges, effective oxidation states (EOS), and aromaticity allows us to conclude that the presence of electron-withdrawing substituents on the cyclopentadienone ring induces a decrease of the activation barriers of most relevant steps, leading to a more efficient catalysis. The present work is a clear example that predictive catalysis can have a fundamental role in sustainable catalytic transformations.
A detailed computational investigation of the mechanistic aspects of the water oxidation catalysis (WOC) for ruthenium‐based catalysts [Ru(bda)(isoq)2] (H2bda = 2,2′‐bipyridine‐6,6′‐dicarboxylic acid; isoq = isoquinoline). Density functional theory (DFT) calculations describe the kinetics by means of the energy barrier that leads to the O–O coupling, the rate‐limiting step for WOC. To test the effect of the axial ligand environment, we investigated bda complexes with para‐substituted isoquinolines, phthalazines and pyridines. Since previous screening analyses of the potential energy surface revealed a face to face orientation of metal‐oxo species to be best for facilitating the O–O bond formation, this paper probes the deeper role of the axial ligands that saturate the Ru(bda) based complexes from a computational point of view.
The present study aims to provide a solid ground for the identification, characterization and controlled design of pro-aromatic quinoidal organic compounds holding conjugated rings with Hückel and/or Baird (singlet and triplet) excited state aromatic character, and expects to become a reference work for future studies on Baird-aromaticity. Concretely, we explore a wide range of compounds with a central conjugated ring of different sizes and symmetric exocyclic substitutions. We employ a combination of different computational tools and we also introduce a new energy-based approach for the quantification of the Baird-aromatic character. The key findings of this study indicate that Baird aromaticity is achieved in pro-aromatic quinoidal compounds having small anionic rings or in electron deficient rings with the proper exocyclic substitution. Low-lying states of these systems show very strong hole/electron overlaps, implying that their aromaticity cannot be related to intramolecular charge transfer. File list (2) download file view on ChemRxiv manuscript.pdf (5.62 MiB) download file view on ChemRxiv supp_info.pdf (38.90 MiB)
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