Elucidation of Proton Assisted Fluxionality in Transition-Metal Oxide (TMO) Clusters Raghunath O. Ramabhadran Indiana University, Bloomington 1

Research Overview Collaborative study of metal-oxide clusters Electronic structure & Anion PES & Reaction mechanisms Mass spectrometry Recent focus on Mo/W oxide clusters & their reactivity with small molecules 2

Motivation: Why Clusters? Cluster Models – Surface defect sites Chemistry is local – adequate mechanistic insight from cluster models Metal (Mo ∕ W) oxides are used as industrial catalysts (fuel purification, methane activation). Catalysts’ structure ∕ behavior needs to be understood Newer chemical features – Better Catalysts 3

Some new features of TMO Clusters d-orbital aromaticity – Boldyrev et. al (2007), Wang et. al (2005) Cluster deposition/surface – Dohnaleck et. al (2006) Superoxide formation – Wang et. al (2006) Theoretical elucidation of Fluxionality – Raghavachari et. al (2010) 4

Introduction to Fluxionality Fluxionality – Rapid interchange of atoms leading to structural rearrangement Rearrangement – Degenerate or Non- degenerate Two types – Structural fluxionality Fluxionality in chemical reactions 5

Classic Example: Berry Pseudorotation 6 Image modified from Wikipedia Degenerate structural fluxionality TBP – SP – TBP conversion of Iron-pentacarbonyl Ax 1 2 Eq

Fluxionality in Clusters Structural fluxionality as well as fluxionality in the course of reaction – widely utilized in purely metallic clusters Heiz and co-workers: Top. Catal. 2007, 44, 145. Structural fluxionality – well documented Fluxionality in reactions – Not studied so far Fluxionality with metal oxide clusters? 7

Benefits of studying Fluxionality in the reactions of TMO Clusters Fluxionality affects the reactive site – implication for catalysis in industrially relevant processes E.g. H 2 liberation, C−H activation, desulfurization Larger clusters – more reactive sites – systematic language to understand mechanism 8

Proton assisted Fluxionality Model systems - chemisorbed water on the surface of W 3 O 6 − and Mo 3 O 6 − Proton assisted fluxionality – “proton hop” Apparent OH migration – actual “proton hop” 9

Why is proton assisted Fluxionality important? Too many oxygens from different sources!! Framework to keep track of the oxygens Industrial relevance – Ammoxidation with bismuth molybdates 10

Strategy to probe fluxionality a)Map out lowest energy isomer, ground spin state b)Obtain electrostatic complex c)Follow the reaction pathway d)Repeat a) through c) for higher energy spin states 11

Computational Methodology Geometries: B3LYP Stuttgart-Dresden relativistic pseudopotentials Augmented Double zeta basis sets – diffuse and polarization functions Single points: Triple zeta basis sets Additional exponents added Vibrational analyses IRC 12

Electronic Structure of W 3 O 6 − Ground state – doublet electronic state 4 kcal/mol more stable than quartet 3 bridging and 3 terminal oxygens Bridging oxygens more reactive than terminal 13

The Fluxionality Pathway Reaction Co-ordinate Reaction Energy (Kcal/mol) 14

Fluxanality pathway For different metals and different spin states Step 1 Step 2 Step 3 Step 4 Step 5 Reaction Co-ordinate Relative Energy(Kcal/mol) 15

Differences – Due to Metal Step 3 – preparation step for “proton hop” Higher Barrier for Mo oxide Metal-oxygen σ bond broken and metal-oxygen π bond formed ~ 5kcal/mol difference consistent with bond energy differences 16

Differences – Due to Spin States Rate Determining Step = step 3 for doublets = step 5 for quartets Proton Hop (Step 4) = exothermic for doublets = endothermic for quartets 17

Key Observations Proton hop – always lower barrier than step Preceding it - get it prepared Significance of the right conformation Absence of proton hop – No energetic drive to rearrange 18

Role of the non-metal in the reacting molecule Study chemisorbed H 2 S and NH 3 on the surface of W 3 O 6 − and Mo 3 O 6 − O vs S vs N Will appreciable differences be noticed? 19

Fluxionality Pathway with H 2 S Reaction Energy (Kcal/mol) Reaction Co-ordinate 20

Differences in H 2 S reactivity & H 2 O reactivity: Doublet states H 2 O – lower barrier for proton-hop H 2 S – higher barrier for proton-hop Explanation in the preparation step With H 2 S With H 2 O 21

H 2 S reactivity: Doublets vs Quartets Doublets – higher barrier for proton-hop Quartets – lower barrier for proton-hop Explanation in the preparation step Doublets Quartets 22

More to the story: Fluxionality pathway with NH 3 Reaction Energy (Kcal/mol) Reaction Co-ordinate 23

H 3 N reactivity: Doublet state Interesting Metal Dependence Proton-hop: lower barrier with Mo-oxide Explanation: Right conformation in the preparation step obtained only with Mo-oxide 24

H 3 N reactivity: Quartet states do not show Metal Dependence Proton-hop: low barrier for both Mo-oxide as well as W-oxide Preparation step leads to favorable conformation for proton-hop in the case of both Mo/W oxides 25

Summary: Factors affecting Fluxionality Metal-oxygen bond strengths Preparation energy: to get a favorable conformation for proton-hop Non-metal in reacting molecule: Scope for diverse chemistry Competition in hydrogen bonding Spin-states: Surprisingly quartet states appear to be relatively insensitive to the nature of the non- metal 26

More details about the work Raghunath O. Ramabhadran, Nicholas J. Mayhall, Krishnan Raghavachari, “A Proton Hop Paves the Way for Hydroxyl Migration: Theoretical Elucidation of Fluxionality in Transition Metal Oxide Clusters”, Journal of Physical Chemistry Letters, 2010, 1, Raghunath O. Ramabhadran, Edwin L. Becher III, Arefin Chowdhury, Krishnan Raghavachari, “Fluxionality in the Chemical Reactions of Transition Metal Oxide Clusters: The Role of Metal, Spin-State and the Reactant Molecule”, Journal of physical Chemistry A, (Accepted, doi: /jp303593d). 27

Scope and Prospectus Pathways leading to photocatalytic H 2 liberation: Off-shoot of fluxionality pathway (theory- experimental collaborative work in progress) Fluxionality pathway makes facile use of both the hydrogens in H 2 S: Applications in gas- sensing & Desulfurization catalysis Reactivity with NH 3 : Interesting structures, free hydrogen that can be used up in further reactivity 28

Acknowledgements The Raghavachari group members The C. C. Jarrold group members $$$ 29