Multielectronic Light Driven Water Activation for Oxygen Atom Transfer Reactions under Mild Conditions – MULTIPLET

i. Elaboration of dinuclear sensitizer/catalyst dyads as building units for light/induced water activation. We prepare molecular ensembles corresponding to Mn-based and Fe-based catalytic units. In both cases the sensitizer unit is a Ru polypyridyl complex functionalized for the covalent coupling to the catalytic unit.
ii. Screening of sensitizer/laccase hybrids dyads as photocatalytic building unit for the four/e- reduction of O2. By grafting the sensitizer at the surface of the enzyme we intend to enhance the vectorial ET to the catalytic centre of the laccase thereby limiting any deactivation pathway through energy transfer to O2.
iii. Assembly of the catalytic cores through the Ru sensitizer to form the final triads. Two strategies: either a non-covalent docking or a covalent linkage of the sensitizer catalyst unit to the laccase. Comparison of these binding modes is essential to gain information on the ET pathways and coupling force between the synthetic and biological catalytic units.
iv. Examination of the light/induced activation of the photocatalytic units using Laser Flash Spectroscopy: a) in the dyads in the presence of sacrificial electron donors/acceptors (insights into i)- the rate constants for ET between the chromophore and the catalyst in its different oxidation states, ii) the electronic spectra of fugacious intermediates, iii) the influence of pH in generating the highly oxidised metal (Mn or Fe) oxo species); b) in the final triads.
v. Performance of photocatalytic OATR runs: a) with targeted sensitizer/catalyst assemblies (focus on TN, TOF (e.g. for styrene oxygenation) and on the chemical state of the photocatalytic units to identify possible deleterious pathways); b) with the final triads. The laccase plays the role of the electron acceptor and the oxidation catalyst plays the role of the electron donor. Investigatation of whether efficient directional light driven ET can be achieved for all redox states of the catalytic parts.

Photocatalytic Fe Complex/Ru diads
Characterization of metal species with a high degree of oxidation
Characterization of photo-induced charge transfer in the Ru-Catox and Ru-Catred diads.
The importance of a relay in the electron transfer between the sensitizer and the catalytic entities is demonstrated
Photoinduced oxidation of olefins in the presence of air by Ru-laccase diads

Mn complex/Ru photocatalytic diads
Integration of the electronic relay in the Ru-Catox and Ru-Catred diades
Improvement of the photo-induced oxidation of olefins in the presence of an electronic relay
Catox-Ru-Catred photo-catalytic triads

Probing the Surface of a Laccase for Clues towards the Design of Chemo-Enzymatic Catalysts. V. Robert, E. Monza, L. Tarrago, F. Sancho, A. De Falco, L. Schneider, E. Npetgat Ngoutane, Y. Mekmouche, P. Rousselot Pailley, A. J. Simaan, V. Guallar, T. Tron, Chem Plus Chem, 2017, 82, 607.

Perhaps the most important fundamental question in chemical research resides in how we can convert abundant stable molecules such as H2O, CO2, O2 and N2 into an energy carrier and provide valuable chemical synthons for industrial usage so as to mitigate the environmental and industrial constraints from extensive use of fossil resources. Nature provides us with main clues on how to do so through the process of photosynthesis where energy from sunlight is used to drive multielectron catalysis, first oxidising water in a four-electron and four-proton process and then reducing CO2 to sugars or protons to H2. The coupling of different catalytic modules performing on one side an oxidation process and on the other a reductive one is a source of inspiration for chemists. Performing light driven multielectronic catalytic chemical transformations are targeted objectives for chemists. For instance, Oxygen Atom Transfer Reactions (OATR) can be achieved through the photo generation of metal-oxo species with water as the sole source of oxygen atom. However, these systems are intrinsically limited by the requirement of a sacrificial electron acceptor. If we want to deploy controlled light driven multielectron chemical transformation in viable applications it is urgent to exclude sacrificial electron acceptors in OATR.
With MULTIPLET our objective is to build a self-sufficient system for OATR by grafting a synthetic sensitizer-catalyst (Mn or Fe complexes "clicked" to a chromophore) able to photoactivate H2O (2e-+2H+, eq. 1) to the surface of an active robust multi-copper oxidase (laccase) that performs O2 reduction (4e-+4H+, eq. 3). The resulting hybrids should then couple the substrate oxygenation reaction (eq. 2) to dioxygen reduction.
Mn+-OH2 ? [M(n+2)+-O] + 2e- + 2H+ (1)
[M(n+2)+-O] + Substrate ? Substrate(O) + Mn+ (2)
4e- + 4H+ + O2 ? 2H2O (3)
The challenging and ambitious goal of this project is to couple the light triggered oxidative process to a photo-driven catalytic reduction of dioxygen at the enzyme laccase. This proposal is based on our continuing effort to investigate on alternative routes for solar energy conversion. Taking profit of a versatile synthetic route based on the Huisgen cycloaddition (click chemistry) we have established in our labs we will develop original molecular assemblies consisting of our designed molecular catalysts based on Mn or Fe complexes and a photoactive chromophore in order to drive the light activation of a metal bound H2O molecule. Targeting OATR from water to an organic substrate, we should recover two electrons and two protons per water molecule. On the other hand, we have recently published on the photocatalytic reduction of O2 into H2O by an active biological catalyst, a laccase. Now, in MULTIPLET, we will covalently link the OATR photocatalytic unit to a laccase to form a photocatalytic triad able to synergistically couple the two processes.
Providing an unprecedented way to use light, water and O2 to oxidise organic substrates with water as sole by-product, this strategy will be beneficial on several grounds, i) circumventing the use of non-sustainable electron acceptors, and ii) with the help of a laccase to recover electrons and protons issued from a synthetic catalytic unit, providing a positive feedback to drive the initial energetically uphill oxidation process. Thus, successes in this research project would open new realms in the development of unique photocatalytic transformations of value-added compounds.
With MULTIPLET, we seek to stimulate discussion on important phenomena that may be pertinent to multiple electron transfer processes. We believe that the goals of MULTIPLET are within reach in our consortium and the outcome of this project will contribute to set experimental as well as fundamental grounds to further development in photocatalysis using abundant renewable sources for the production of a fuel or for chemical conversion.