AmericanChemicalSociety.com

Group 11 metals such as copper have recently been demonstrated in catalytic C-N and C-O coupling reactions.  Numerous precatalysts have been used, with the most often reported being homogeneous Cu(I) and Cu(II) metal-ligand complexes.  There are very few examples of heterogeneous catalysts (e.g. Cu(0) and mixed-valent copper nanoparticles or supported metal-ligand complexes).  Although hypothesized mechanisms have been published for homogeneous metal-ligand complexes, no study has addressed whether truly heterogeneous catalysts can be developed.  In this project, we hypothesize that systematic application of numerous catalytic, kinetic, and poisoning studies to newly designed heterogeneous copper precatalysts in parallel with their homogeneous analogues will allow for the consolidation of potential reactions paths and potential true catalytic species.  In addition, the development of heterogeneous, supported catalysts, if successful, will open a new heterogeneous avenue for synthesis of C-N and C-O coupled products.  Application of the same probes to selected set of gold precatalysts will allow the extension of the results to other Group 11 metals.  Knowledge of what types of active species and reaction paths are feasible will help researchers in the both the homogeneous and heterogeneous communities focus their catalyst design efforts on fruitful paths and lead to additional research on Group 11 metals in catalytic coupling reactions.

A new Ph.D student, Ms. Linda Al-Hmoud, has taken on this project for her PhD thesis.  A student with a strong background in chemical engineering, this project has substantially strengthened Ms. Al-Hmoud's knowledge of synthetic chemistry, nanoscience and catalysis, and is allowing her to develop skills at the science/engineering interface.

The project has been broken into two phases.  In this first phase, assorted copper oxide nanopartcles of different crystal shapes are being prepared and evaluated as catalysts for catalytic C-N coupling reactions using amines and aromatic halides.  The overall goal is to assess whether solid nanoparticles surface are catalytically active, as heterogeneous catalysts, or whether the nanoparticles must dissolve for soluble, molecular copper ions to act as homogeneous catalysts.  A hypothesis is that by synthesizing copper oxide nanoparticles of different shapes, whioch prevent different crystal surfaces, one can assess whether the nature of the solid copper surface affects the reaction, consistent with a heterogeneous catalytic mechanism.  The results obtained thus far suggest that there are differences in reactivity when different sources of copper (II) oxide are used.  For instance, CuO nanoparticles with a nanorod shape have proven to be less active than commercial CuO that presents an ill-defined, globular shape.  Whether this difference is associated with different rates of dissolution under reaction conditions or surface catalysis continues to be a subject of investigation.

The second phase of the project has just begun.  In this phase, we are assessing the potential for creation of heterogeneous copper catalysts by ion-exchange of aluminosilicate zeolites with Cu(I) and Cu(II) ions.  With this approach, the heterogeneity of the reactions can be assessed by careful choice of reaction substrates while exploiting the shape-selective properties of different zeolite frameworks.  If this approach allows for heterogeneous catalysis, the approach may be extended to solid supports with larger pores such as mesoporous aluminosilicates, allowing for a wide variety of C-N and C-O coupling reactions to be carried out heterogeneously.

To date, this grant has initiated a new research activity in the group targeting the scientific questions, catalysts and reactions described above.  It is anticipated that the last year of this project will result in publications on each of the subtopics above and enough information to assess whether either the copper oxide nanoparticle or zeolite-supported copper research paths offer promise for development of heterogeneous copper catalysts for important C-N and C-O coupling reactions.