# Presentation: 133811

## DOE Basic Energy Sciences Contractors’ Meeting

**Selectivity Breakout Session – May 25, 2004**
**Report**
- Examples of recent advances and future prospects, for specific topics selected from those below, are given in the
- appendix.
**The role of mechanistic understanding in controlling selectivity at catalyst active sites**
-      a) Using mechanism-based understanding (especially good kinetic information) to understand and predict
-          selectivity
-      b) Better methods for using kinetics and steric and electronic effects to understand the relationship between structure and selectivity in well-defined heterogeneous catalysts
-      c) How to apply what is known about selectivity control in homogeneous systems to heterogeneous systems.  Example: how to transfer information from enantioselective homogeneous systems to enantioselective heterogeneous systems.  Other types of selectivity should be explored as well.
-      d) Start looking at surfaces as ligands.  Most supported catalysts are oxides—need to look at other supports; e.g. MgCl2, TiCl3, understand how structure of the catalyst influences the selectivity—e.g., chiral crystals

## 2) Understanding and utilizing novel methods for controlling selectivity at active sites of 

**    ****catalysts (e.g., spatial and temporal control by compartmentalization, constrained volume, **
**    ****reaction sequence, shape control, orientation control, phase separation)**.
-      a) How to employ the mechanisms used by enzymes in synthetic catalytic systems (e.g., hydrogen
-          bonding, electrostatic effects, shape selectivity, etc.)
-      b) Examining size effects on selectivity issues at the nanoscale: i. e., synthesis of different sized particles
-          and examination of how their size affects catalytic reactivity and selectivity (e.g., Pt-catalyzed alkane
-          hydrogenation)
-      c) How to stabilize nanoparticles on heterogeneous supports; new anchors to avoid sintering during
-          catalytic reactions
-      d) Use of membranes (for reactant selection/product separation, or as a catalyst support) to control catalyst
-          selectivity
-      e) Immobilized enzymes: how to anchor them to surfaces and control and understand their behavior
-      f) Spin control of reactivity
-      g) Understanding selectivity of naturally anchored enzymes (e.g., those bound to membranes, inorganic
-          surfaces)

## 3) New non-thermal methods of catalyst activation (e.g., photochemical, microwave, plasmas, 

-          reactions
-      i) How to control selectivity between one-electron and multi-electron reactivity
-      j) Cascade reactions; chemoselective consecutive reactions: how to control multiple catalysts in the same solution;
-         improving selectivity by coordination of tandem reactions: kinetics and transport issues
-     k) Methods are needed to develop new supramolecular and “nanovessel” catalysts, as well as heterogeneous catalysts
-         with a wider range of pore and binding site sizes with the goal of moving from binding studies to the development of
-         well-defined materials with specifically tailored “active sites”.  These should not just recognize molecules, but also
-         accelerate their reactions.
**3) New non-thermal methods of catalyst activation (e.g., photochemical, microwave, plasmas, **
**    ****phase transition)**
-     a) Use of alternate activation methods to change selectivity: e.g., microwave irradiation, high-energy plasmas,
-          photochemistry, etc., and understanding the physical basis of these phenomena; controlling selectivity in highly
-          energetic species by specific bond activation
-     b) Channeling of energy released in exothermic steps into the next endothermic steps to lower the energy required
-           for the overall reaction.

## 4) Determining the structure of the active site at the molecular level

-      a) How to define the “target size” in both homogeneous and heterogeneous catalysis?
-      b) Better methods for depositing nanoclusters on solid supports and understanding their structure and function
-      c) How to design active sites of heterogeneous catalysts to control selectivity.  Need more structure-activity
-          relationships for heterogeneous systems
-      d) Developing surface chemistry: e.g., cutting crystals at certain angles to develop enantioselective chemistry
-      e) Need to control redox properties and coordination properties to control reactivity
-      f) How to deal with the problem of how many sites on a surface are actually active, and how many are inactive?
-         Need ways to focus on the actually active sites: use of isotope labeling, poisoning experiments to sort this out
-      g) Ways to tie structure/function to synthesis/characterization
**5) Polymerization and oligomerization selectivity (stereochemical control, molecular weight control)**
-      a) Modification of polymers, e.g. with chiral ligands
-      b) Use of constrained binding to affect non-statistical oligomerization of alkenes, or multiple activation of substrates
-      c) Selectivity in sequencing polymers
-      d) Controlling molecular weight and measurement of kprop vs kct, the rate constants involved in determining this,
-          especially with regard to structure-activity relationships for different ligands, metals, etc.  Also for copolymerization,
-          there is a need for fundamental kprop vs kct’s for different co-monomers.
-      e) What is required to efficiently incorporate polar co-monomers?  New strategies are needed for instilling metal
-          preference for C=C vs heteroatom lone electron pairs, as has been accomplished for Ru metathesis catalysts.

## Selectivity Breakout SessionAppendix: Examples of Recent Accomplishments and Future Prospects 

## How Does Understanding Mechanism in Homogeneous Catalysis Serve as a Guide to Selectivity?

*Tacticity control in olefin polymerization* can be achieved by understanding the mechanism and structural characteristics of the metal-olefin complexes:
- Ansa linkage permits control of side-to-side and top-to-bottom coordination.

- Example:
- Alkane -complexes lead to selective activation at the thermodynamically preferred position on an alkane:
- Metal can migrate up and down alkyl chain until it finds the ‘best’ position for insertion, leading to *selective activation of terminal methyl groups* with _many_ metal complexes.

- What is needed:
- Further studies of mechanism are likely to lead to new advances in alkane functionalizations, polymerizations, reductions, hydrogenations, and green chemical reactions.  How can reactions in alternative media/geometries be used to control selectivity?  Calculations can provide insight into relative barriers for processes that cannot be directly observed.

## Catalyst Support Surfaces:  Ligands for Selectivity Control

**Rationale:**
** ****Selectivity Controlled by Relative Rates of Competing**
**   ****Pathways**
** ****Relative Rates Influenced by Local Molecular**
**   ****Structures and Electronic Interactions**
** ****Support Ligands Affect Local Molecular Structures**
**   ****and/or Electronic Interactions**
** ****Optimizing Selectivity Requires Controlling the Local**
**   ****Molecular Structures and/or Electronic Interactions**

**Future Work:**
**Characterize bonding between Catalysts and**
**   ****Supports Experimentally & Theoretically**** **
**Establish Relationship Between Local Molecular**
**   ****Structures/Electronic Interactions and Support**
**   ****Ligands**
** ****Establish Relationship Between Local Molecular**
**   ****Structures/Electronic Interactions and Kinetics of**
**   ****Competing Pathways**

**Example:**

**Redox**

**Redox**

**Acid**

**Acid**

**Acid**

**Redox**

** ****Support Ligands Control Electron Density/Cluster Size of Active Surface Site**

## Known: Single-Site Catalysis on “Super” Brønsted Acids  

-   **97 ******** 2 % Sites Active!**

**87 ******** 2 % Sites Active!**

**Close Proximity ******** Multiple Coupled Transformations ?**

**Ziegler Site**
**Oligomerization Site**
**ROMP Site**
**Chain Transfer Site**

**Cationic Site**
**Anionic Site**
**Second Ziegler Site**
**Hydrogenation Site**

***Possible*****: Tandem Processes**

## Stereocontrol in Alkene Polymerization Reactions

**Stereocontrol in Alkene Polymerization Reactions**

## How to take advantage of enzymes and their sophisticated control 

- mechanisms in synthetic catalytic systems?

**Why enzymes?**
- Enzymes embed and use all what we know as “**smart**” features in catalysis (self-regulation, on-off switches, etc.)
-  Understanding these features at the molecular and atomic levels is critical for successful **integration** in synthetic catalytic systems.
-  The structure and dynamics of enzymes control very tightly *nonproductive* competing pathways and therefore **selectivity**.
- Emerging concepts in catalysis point to a critical role of **dynamics** -even for heterogeneous systems *traditionally thought as “rigid”*- in the control of both** selectivity **and** activity **at the atomic level.
- Possibilities for immobilization of enzymes -or **bioengineered** functionally-active subdomains- are expanding and will open invaluable opportunities at the frontiers of **biomimetics** and **heterogeneous** catalysis.

**Areas to be  targeted:**
-  Determine paradigm mechanisms used by enzymes, coupling selectivity to high activity at the molecular and atomic levels.
-  Investigate the role of “soft” tuning of prosthetic catalytic sites in enzymes allowing the same “core catalyst” (e.g. heme) perform very different reactions (e.g. reductions, oxidations, mono- or di-oxygenation..).
-  Integrate “smart” biocatalytic features in heterogeneous and homogeneous model systems.
-  Develop efficient immobilization procedures for bioengineered catalytic components in synthetic catalytic systems.

**Examples:** Control features in NO catalytic biosynthesis by
- NOS enzymes:

- Cofactor/substrate-driven dynamic redox modulation of the active site:

- Shell/protein-assisted control of electronic properties of the active site by long range H-bonding/-stacking:

**Fe**

*Static H-bonding/****-stacking of the shell ensures the right electronic properties of the core iron-heme*

*Dynamic binding of cofactor and/or substrate assists in fine-tuning redox properties of iron to facilitate electron transfers required for oxygen activation.*

*Dynamic active site *
*of NOS enzymes*