Catalytic Solar Water Splitting Inspired by Photosynthesis. Homogeneous Catalysts with a Mechanical ( Machine-Like ) Action.

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1 Catalytic Solar Water Splitting Inspired by Photosynthesis. Homogeneous Catalysts with a Mechanical ( Machine-Like ) Action Gerry Swiegers Australian Research Council Centre of Excellence for Electromaterials Science and Intelligent Polymer Research Institute University of Wollongong Wollongong, SW 2522 Australia

2 Sub-theme: Molecular Machines Much interest in developing molecular machines that drive chemical reactions But, what is the mechanical action that must occur within a molecular catalyst to turn it into a molecular l machine??

3 Two General Methods of Inducing Change Energy Gradient ( Thermodynamics ) Change driven by an overall release of energy e.g. a ball falling to Earth under gravity Mechanical Interaction ( Mechanics ) Change driven by a physical collision i (action-reaction i sequence that plays out over time) e.g. two billiard balls physically colliding

4 Chemistry: Collision Theory STEP (1) Reactant collision controlled by the collision frequency (A) mechanics STEP (2) Product formation controlled by the activation energy (E a ) thermodynamics reactants A products B Mechanical reaction: transition state H + + H - H 2 k = s ential Energ gy Pot E a reactants products Reaction Coordinate

5 What Happens in Catalysis? STEP (1) Reactant collision controlled by the collision frequency (A) mechanics STEP (2) Product formation controlled by the activation energy (E a ) thermodynamics A time: t TS energy: E a catalyst B reactants catalyst binds & activates reactants t transition state formed products bind catalyst products released Pot tential Energ gy reactants E a E a uncatalyzed catalyzed products Reaction Coordinate

6 Mechanical Catalysts: H 2 generation Dynamic proton binding Fe Fe.. +2H + + Fe H H Fe + -2H + 2. Fe Ḣ H Fe e - [2] TSC + Fe Fe + 3 H 2 2(g) + Fe H H Fe + produces 5 molecules H 2 s -1 catalyst -1 over at least 5 days of continuous operation -Two dynamic processes (catalyst flexing and proton binding) which are only synchronized if the catalyst flexes rapidly about a structure that complements the transition state Chem Commun 2004, 308 Chem Eur J 2009, 15, 4746

7 Mechanical Catalysts: 2 reduction y 2 Co Co 3 Co Co Pac-Man Catalyst H 2 (4 e - process) 4 2 H 2 (4 e - process) Chem Commun 2007, 3352 Chem Eur J 2009, 15, 4746

8 Some Common Features of Mechanical Molecular Catalysts (1) Reaction controlled by the Catalyst-Mediated Collision Frequency (low Activation Energy) (2) The maximum catalytic rate depends on the rate of conformational flexing (conformational flexing = the mechanical impetus ) (3) Catalyst typically flexes rapidly about a shape that complements the transition ii state (4) Highly efficient and selective form of catalysis (like a machine) (5) Michaelis-Menten Menten kinetics Chem Eur J 2009, 15, 4746 These features also found in many enzymes QUESTI: Are enzymes mechanical catalysts?

9 Mechanical Catalysis: Methods of Enzymatic, Homogeneous, and Heterogeneous Catalysis Swiegers, G. F. John Wiley & Sons, ew York, 2008

10 Can we Mimic an Enzyme? CRE: (p-me-c 6 H 4 ) 2 P 2 Bio-inspired -oxo Cubane Model Complex G. C. Dismukes, Princeton University Water-xidizing Complex of Photosystem II (PSII-WC) Cubane: - Shape and structure similar to enzyme active site - Dynamically self-assembles -Flexible - Low activation energy for 2 release (photolytic)

11 Cubane forms 2 when illuminated 2? + 1 ligand + 2 H 2-4H e distance lengthens hν + 1 ligand released - distances shorten Corner 's collide Peroxo ( 2-2 ) forms Superoxo ( 2 - ) forms DeAngelis, Carr "butterfly" 9 + 2

12 Cubane in afion layer on GC electrode Electrode biased at 1.0 V (vs. Ag/AgCl) Illuminated at nm 2.0 Current (µa) A Time (sec) Curr rent (µa) B >295nm >395nm >455nm >495nm >570nm >275nm Time (sec) Peak turnover frequency: 270 molecules 2 h -1 catalyst -1 Total turnovers: >1000 readily achieved Angew Chem Int Ed Engl 2008, 47, 7335

13 Cubane + ion-exchanged into afion: CV in afion 8 2+ in afion Current (µa) Potential (V vs Ag/AgCl) (V vs Ag/AgCl) 1.2

14 Proposed Mechanism Catalyst dis-assembles and re-assembles under turnover conditions Phys Chem Chem Phys 2009, 11, 6441 Inorg Chem 2009, 48, 7269

15 CV 8 + in afion 2+/3+ Ru - - Ru 2+/3+ rent (µa) Curr A Potential V (vs Ag/AgCl) hν Ru(bpy) 2+ 3 Ru(bpy) e -

16 Water-Splitting Graetzel Cell L. Spiccia, Monash University

17 Water-Splitting Graetzel Cell Uses H 2 as electrolyte even impure H 2 (seawater). Eliminates the need for acetonitrile electrolyte which bedevils existing Graetzel cells J Am Chem Soc 2010, 132, 2892 System splits seawater into pure oxygen (no chlorine formation) Spin-off company: Cube Catalytics LLC (funded by ew Energy Ventures, ew Jersey)

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19 Acknowledgements Princeton University / Rutgers University, USA Prof Chuck Dismukes Dr Greg Felton Monash University, Australia Prof Leone Spiccia Dr Rob Brimblecombe Dr Rosalie Hocking Dr Annette Koo University i of Wollongong, Australia Prof Gordon Wallace Dr Jun Chen Dr Chee Too CSIR Molecular and Health Technologies, Australia Dr Junhua Huang Funding Australian Research Council Australian Academy of Science University of Wollongong CSIR