Search

Copper-catalyzed electrochemical atom transfer radical addition ( e ATRA) is a new method for the creation of new C-C bonds under mild conditions. In this work, we have explored the reactivity of an analogous series of N 4 macrocyclic Cu II complexes as e ATRA precatalysts, which are primed by reduction to their monovalent oxidation state. These complexes were fully characterized structurally, spectroscopically, and electrochemically. A spectrum of radical activation reactivity was found across the series [Cu I (Me 4 cyclen)(NCMe)] + (Me 4 cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), [Cu I (Me 4 cyclam)(NCMe)] + (Me 4 cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [Cu I (Me 2 py 2 clen)(NCMe)] + (Me 2 py 2 clen = 3,7-dimethyl-3,7-diaza-1,5(2,6)-dipyridinacyclo-octaphane). The rate of radical production by [Cu(Me 2 py 2 clen)(NCMe)] + was modest, but rapid radical capture to form the organocopper complex [Cu I (Me 2 py 2 clen)(CH 2 CN)] led to a dramatic acceleration in catalysis, greater than seen in any comparable Cu complex, but this led to rapid radical self-termination instead of radical addition.

The utility and scope of Cu-catalyzed halogen atom transfer chemistry have been exploited in the fields of atom transfer radical polymerization and atom transfer radical addition, where the metal plays a key role in radical formation and minimizing unwanted side reactions. We have shown that electrochemistry can be employed to modulate the reactivity of the Cu catalyst between its active (Cu I ) and dormant (Cu II ) states in a variety of ligand systems. In this work, a macrocyclic pyridinophane ligand (L1) was utilized, which can break the C-Br bond of BrCH 2 CN to release • CH 2 CN radicals when in complex with Cu I . Moreover, the [Cu I (L1)] + complex can capture the • CH 2 CN radical to form a new species [Cu II (L1)(CH 2 CN)] + in situ that, on reduction, exhibits halogen atom transfer reactivity 3 orders of magnitude greater than its parent complex [Cu I (L1)] + . This unprecedented rate acceleration has been identified by electrochemistry, successfully reproduced by simulation, and exploited in a Cu-catalyzed bulk electrosynthesis where [Cu II (L1)(CH 2 CN)] + participates as a radical donor in the atom transfer radical addition of BrCH 2 CN to a selection of styrenes. The formation of these turbocharged catalysts in situ during electrosynthesis offers a new approach to the Cu-catalyzed organic reaction methodology.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C} 2 (μ-1,3-C 6 H 4 )] + have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH 2 Cl 2 )) on ([{Cp*(dppe)RuC≡C} 2 (μ-1,3-C 6 H 3 -n-OMe)] + (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the β-HOSO and β-LUSO, whilst the relative phases of the β-HOSO and β-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

Together with the more intuitive and commonly recognized conductance mechanisms of charge-hopping and tunneling, quantum-interference (QI) phenomena have been identified as important factors affecting charge transport through molecules. Consequently, establishing simple and flexible molecular-design strategies to understand, control, and exploit QI in molecular junctions poses an exciting challenge. Here we demonstrate that destructive quantum interference (DQI) in meta-substituted phenylene ethylene-type oligomers (m-OPE) can be tuned by changing the position and conformation of methoxy (OMe) substituents at the central phenylene ring. These substituents play the role of molecular-scale taps, which can be switched on or off to control the current flow through a molecule. Our experimental results conclusively verify recently postulated magic-ratio and orbital-product rules, and highlight a novel chemical design strategy for tuning and gating DQI features to create single-molecule devices with desirable electronic functions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)