Erin M. Duffy, Brett M. Marsh, Jonathan M. Voss, Etienne Garand University of Wisconsin, Madison International Symposium on Molecular Spectroscopy June 22, 2015 Spectroscopic Investigation of Proton- Coupled Electron Transfer (PCET) in Water Oxidation Catalyzed by a Ruthenium Complex, [Ru(tpy)(bpy)(OH 2 )] 2+ 1

 Role of Solvent (Water) in Facilitating PCET?  Make water clusters of catalyst & intermediates!  Role of Solvent (Water) in Facilitating PCET?  Make water clusters of catalyst & intermediates! Water Oxidation 2 4H 2 O 2O 2 Δ E = 1.23 V -4e -, -4H + catalyst [Ru(tpy)(bpy)(OH 2 )] 2+ tpy = 2,2’:6,2”-terpyridine bpy = 2,2’-bipyridine D.E. Polyansky, et al. J. Am. Chem. Soc. 2011, 133,

Experimental Methods 3 Ion SourceCryogenic Ion Trap Mass Selection Laser Fragmentation Detector

Experimental Methods 4 Ion Source J.A. Fournier, A.B. Wolk, and M.A. Johnson. Anal. Chem. 2013, 85, 7339 − To Potentiostat Outer Housing Held at ESI Voltage Generating [Ru—OH] 2+ Generating [Ru—OH 2 ] 2+

Experimental Methods Ion D 2 or H 2 O He  + 0 H 2 O : Pulse He/D 2 gas to “tag” [ Ru -OH 2 ] 2+ ions (10 K)  Low T needed to tag w/ D 2  H 2 O : Pulse He gas seeded w/ H 2 O to solvate ions (185 K) Cryogenic Ion Trap

 Mass-select species of interest  Intersect ion beam with infrared laser  IR transition causes loss of D 2 tag or a single H 2 O molecule  Predissociation yield vs. IR wavenumber = IR spectrum Experimental Methods 6 Mass Selection Laser Fragmentation Detector

7 “free” H 2 O Bare Catalyst: [ Ru -OH 2 ] 2+ (D 2 ) 2 Experiment (10 K) Calculation CAM-B3LYP G(d,p)/SDD [C H N O / Ru]

Solvated Catalyst: [ Ru -OH 2 ] 2+ (H 2 O) 1-2 8

9 ~3200 cm -1  Compare this ligand O-H stretch to other H-bonded H 2 O stretches:  M 2+ -H 2 O-H 2 O: ~ 3400 cm -1  H 3 O + -H 2 O-H 2 O: ~ cm -1

[ Ru -OH 2 ] 2+ (H 2 O) 3 10 C.J. Johnson, et al. J. Phys. Chem. A 2014, 118,

Charge Transfer 11 n=0 n=1 n=2 n=3 - electron density + electron density 2.21 Å 2.19 Å 2.17 Å2.15 Å

First Intermediate: [ Ru -OH] 2+ (D 2 ) 2 12  Ru—O distance: ~1.93 Å  Calc. OH ~75 cm -1 too blue

13 Solvated Intermediate: [ Ru -OH] 2+ (H 2 O) 2  Ru—O distance: ~1.91 Å  Broad feature at ~3100 cm -1 unaccounted for in calculation  Both donating O-H stretches calculated at ~3350 cm-1  Ru-OH may be mistreated, as before  Other isomer with stronger H bonding?

[ Ru -OH 2 ] 2+ (H 2 O) 2 vs. [ Ru -OH] 2+ (H 2 O) 2 14 Ru -OH (intermediate) Ru -OH 2 (initial catalyst) ~165 cm -1 ~50 cm -1

 IR spectra of water clusters of [Ru(tpy)(bpy)(OH 2 )] 2+ and [Ru(tpy)(bpy)(OH)] 2+ provide insight into role of solvent in facilitating the first step in catalytic water oxidation cycle  Finish set of Ru -OH water clusters  Extended hydrogen-bonding network strengthens Ru—O interaction  Evidence of charge transfer in [ Ru —OH 2 ] 2+ water clusters suggests move toward formation of [ Ru —OH] 2+, in support of PCET mechanism Summary & Future Work 15

Acknowledgments  Garand Group Brett Marsh Prof. Etienne Garand Jon Voss 16  Funding University of Wisconsin National Science Foundation