Syntheses and investigation of metal complexes with macrocyclic polythioether ligands

Copper(I) complexes with the macrocyclic thioether ligands 1,4,8,11-tetrathiacyclotetradecane (tetrathiacyclam, 14-S₄) and 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam, 14-N₂S₂) were synthesised and structurally characterised. While the copper(I) complexes showed no reactivity towards dioxygen, the formation of "dioxygen adduct complexes" could be spectroscopically detected with ozone using low temperature stopped-flow techniques. Furthermore, it was possible to synthesise and characterise iron(II) and cobalt(II) complexes with the tetrathiacyclam ligand. No "dioxygen adduct" intermediates were observed when these complexes were reacted with dioxygen or ozone. Depending on the reaction conditions, the coordination of the metal ions could be controlled (endo- vs. exo-coordination and cis- vs. trans-coordination) and in addition to mononuclear complexes, also coordination polymers were obtained.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

Impact of Intramolecular Hydrogen Bonding on the Reactivity of Cupric Superoxide Complexes with O-H and C-H Substrates

The dioxygen reactivity of a series of TMPA-based copper(I) complexes (TMPA=tris(2-pyridylmethyl)amine), with and without secondary-coordination-sphere hydrogen-bonding moieties, was studied at -135 °C in 2-methyltetrahydrofuran (MeTHF). Kinetic stabilization of the H-bonded [( ( X 1 ) ( X 2 ) TMPA)Cu^II (O₂ ^.- )]⁺ cupric superoxide species was achieved, and they were characterized by resonance Raman (rR) spectroscopy. The structures and physical properties of [( ( X 1 ) ( X 2 ) TMPA)Cu^II (N₃ ^- )]⁺ azido analogues were compared, and the O₂ ^.- reactivity of ligand-Cu^I complexes when an H-bonding moiety is replaced by a methyl group was contrasted. A drastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C-H bond-dissociation energies is observed, correlating with the number and strength of the H-bonding groups.

A Comprehensive Study of Copper Guanidine Quinoline Complexes: Predicting the Activity of Catalysts in ATRP with DFT

Copper complexes of the hybrid guanidine ligands 1,3-dimethyl-N-(quinolin-8-yl)-imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)-guanidine (TMGqu) have been studied comprehensively with regard to their structural and electrochemical properties and their activity in atom transfer radical polymerization (ATRP). A simple analysis of the molecular structures of the complexes gives no indication about their activity in ATRP; however, with the help of DFT and NBO analysis the influence of particular coordinating donors on the electrochemical properties could be fully elucidated. With an adequate DFT methodology and newly applied theoretical isodesmic reactions it was possible to predict the relative position of the redox potentials of copper complexes containing DMEGqu and TMGqu ligands. In addition, predictions could be made as to whether the complexes of DMEGqu or TMGqu are more active in ATRP. Four new Cu(I) complexes were tested in standard ATRP reactions and kinetically investigated both in bulk and in solution. It could be proven that complexes featuring DMEGqu possess a lower redox potential and are more active in ATRP, although the tetramethylguanidine moiety represents the stronger donor.

Tetraguanidino-functionalized phenazine and fluorene dyes: synthesis, optical properties and metal coordination

Functionalization of phenazine with guanidino groups leads to new highly fluorescent dyes and ligands, and metal coordination is shown to alter significantly the optical properties.

Counter-ligand control of the electronic structure in dinuclear copper-tetrakisguanidine complexes

Decision-making counter-ligands: a bridging redox-active ligand in a dinuclear copper complex could be either neutral (complex type [Cu^II-GFA-Cu^II]) or dicationic (complex type [Cu^I-GFA-Cu^I]), depending on the nature of the counter-ligands X.

Redox-controlled hydrogen bonding: turning a superbase into a strong hydrogen-bond donor

Herein the synthesis, structures and properties of hydrogen-bonded aggregates involving redox-active guanidine superbases are reported. Reversible hydrogen bonding is switched on by oxidation of the hydrogen-donor unit, and leads to formation of aggregates in which the hydrogen-bond donor unit is sandwiched by two hydrogen-bond acceptor units. Further oxidation (of the acceptor units) leads again to deaggregation. Aggregate formation is associated with a distinct color change, and the electronic situation could be described as a frozen stage on the way to hydrogen transfer. A further increase in the basicity of the hydrogen-bond acceptor leads to deprotonation reactions.

Stabilization of Complexes of Redox-Active Guanidino-Functionalized Aromatic Compounds (GFAs) by Hydrogen-Bonding

The guanidino-functionalized 1,2,4,5-tetrakis(N,N′-diisopropylguanidino)benzene could act as a redox-active switch, and reversibly forms hydrogen-bond aggregates upon two-electron oxidation. Herein the influence of hydrogen bonding on the structure and electronic properties of the first transition metal complexes of the neutral and oxidized compound are studied. Reaction with CuCl2 leads by coupled redox- and coordination processes to a dinuclear CuII complex of the dicationic guanidine, in which CuCl2– counterions are locked through strong hydrogen-bonds in positions above and below the C6 ring plane. The electronic situation in the electronic ground and excited states of this complex were analysed by quantum chemical calculations.