New avenues for ligand-mediated processes: expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Redox-active ligands have evolved from being considered spectroscopic curiosities - creating ambiguity about formal oxidation states in metal complexes - to versatile and useful tools to expand on the reactivity of (transition) metals or to even go beyond what is generally perceived possible. This review focusses on metal complexes containing either catechol, o-aminophenol or o-phenylenediamine type ligands. These ligands have opened up a new area of chemistry for metals across the periodic table. The portfolio of ligand-based reactivity invoked by these redox-active entities will be discussed. This ranges from facilitating oxidative additions upon d(0) metals or cross coupling reactions with cobalt(III) without metal oxidation state changes - by functioning as an electron reservoir - to intramolecular ligand-to-substrate single-electron transfer to create a reactive substrate-centered radical on a Pd(II) platform. Although the current state-of-art research primarily consists of stoichiometric and exploratory reactions, several notable reports of catalysis facilitated by the redox-activity of the ligand will also be discussed. In conclusion, redox-active ligands containing catechol, o-aminophenol or o-phenylenediamine moieties show great potential to be exploited as reversible electron reservoirs, donating or accepting electrons to activate substrates and metal centers and to enable new reactivity with both early and late transition as well as main group metals

How to Control the Rate of Heterogeneous Electron Transfer across the Rim of M6L12 and M12L24 Nanospheres

Catalysis in confined spaces, such as those provided by supramolecular cages, is quickly gaining momentum. It allows for second coordination sphere strategies to control the selectivity and activity of transition metal catalysts, beyond the classical methods like fine-tuning the steric and electronic properties of the coordinating ligands. Only a few electrocatalytic reactions within cages have been reported, and there is no information regarding the electron transfer kinetics and thermodynamics of redox-active species encapsulated into supramolecular assemblies. This contribution revolves around the preparation of M6L12 and larger M12L24 (M = Pd or Pt) nanospheres functionalized with different numbers of redox-active probes encapsulated within their cavity, either in a covalent fashion via different types of linkers (flexible, rigid and conjugated or rigid and nonconjugated) or by supramolecular hydrogen bonding interactions. The redox probes can be addressed by electrochemical electron transfer across the rim of nanospheres, and the thermodynamics and kinetics of this process are described. Our study identifies that the linker type and the number of redox probes within the cage are useful handles to fine-tune the electron transfer rates, paving the way for the encapsulation of electroactive catalysts and electrocatalytic applications of such supramolecular assemblies

Redox-Active Supramolecular Heteroleptic M4L2L `(2)Assemblies with Tunable Interior Binding Site

Three Pt4L2L′2 heteroleptic rectangles ( 1–3 ), containing ditopic redox‐active bis‐pyridine functionalized perylene bisimide (PBI) ligands PBI‐pyr2 ( L ) are reported. Co‐ligand L′ is a dicarboxylate spacer of varying length, leading to modified overall size of the assemblies. 1H NMR spectroscopy reveals a trend in the splitting and upfield chemical shift of the PBI‐hydrogens in the rectangles with respect to free PBI, most pronounced with the largest strut length ( 3 ) and least with the smallest strut length ( 1 ). This is attributed to increased rotational freedom of the PBI‐pyr 2 ligand over its longitudinal axis (Npy‐Npy), due to increased distance between the PBI‐surfaces, which is corroborated by VT‐NMR measurements and DFT calculations. The intramolecular motion entails desymmetrization of the two PBI‐ligands, in line with cyclic voltammetry (CV) data. The first (overall two‐electron) reduction event and re‐oxidation for 1 display a subtle peak‐to‐peak splitting of 60 mV, whilst increased splitting of this event is observed for 2 and 3 . The binding of pyrene in 1 is probed to establish proof of concept of host‐guest chemistry enabled by the two PBI‐motifs. Fitting the binding curve obtained by 1H NMR titration with a 1:1 complex formation model led to a binding constant of 964±55 m−1. Pyrene binding is shown to directly influence the redox‐chemistry of 1 , resulting in a cathodic and anodic shift of approximately 46 mV on the first and second reduction event, respectively

Probing the influence of substrate binding on photocatalytic dehalogenation with a heteroleptic supramolecular [M₄L^a₂L^b₂] square containing PDI photosensitizers as ligands

Photoredox catalysis is a valuable tool in a large variety of chemical reactions. Main challenges still to be overcome are photodegradation of photocatalysts and substrates, short lifetimes of reactive intermediates, and selectivity issues due to unwanted side reactions. A potential solution to these challenges is the pre-organization of the photosensitizer, substrate and (co)-catalyst in supramolecular self-assembled structures. In such architectures, (organic) dyes can be stabilized, and higher selectivity could potentially be achieved through pre-organizing desired reaction partners via non-covalent interactions. Perylene diimide (PDI) is an organic dye, which can be readily reduced to its mono- and dianion. Excitation of both anions leads to highly reducing excited states, which are able to reduce a variety of substrates via single electron transfer. The incorporation of PDI into a heteroleptic [M4La2Lb2] supramolecular square has been recently demonstrated. Herein we investigate its photophysical properties and demonstrate that incorporated PDI indeed features photocatalytic activity. Initial results suggest that the pre-organisation by binding positively affects the outcome.</p

Reversible multi-electron storage in dual-site redox-active supramolecular cages

M6L412+ supramolecular cages  3a  and  3b  (M = Pd, Pt), soluble in organic solvents, contain two different ligand-centered redox sites that enable the reversible storage of up to 16 electrons, as probed by CV, UV/vis spectro-electrochemistry (SEC-UV/Vis), bulk electrolysis and EPR. Encapsulation of a B12F122− anion is confirmed by 1H, 19F NMR and 19F DOSY NMR spectroscopy and mass spectrometry