59th Annual Report on Research 2014

Figure 1. ORTEPs of representative, unique PDI complexes with reaction directors located in the secondary coordination sphere synthesized and outlined in this progress report. One of the major goals outlined in the original proposal was to utilize the secondary coordination sphere to stabilize the formation of CO-derived iron formyls (formed from the addition of hydride to the reduced dicarbonyl species).  However, reduced dicarbonyl species formed from the complexes in Figure 1 do not display any formyl formation.  Instead, deprotonation of the –CH₃ groups on the 2,6-diacetylpyrindine core occurs upon the addition of hydride due to the acidity of the acetyl protons in these complexes.  In order to circumvent these unwanted side reactions, we have begun to synthesize complexes based on the 2,6-dibenzoylpyridine core instead.  Examples of these complexes are shown in Figure 2.

Figure 1. ORTEPs of representative FePDI complexes highlighting the acidic –CH₃ (left) and the nonacidic –C₆H₅ groups (middle and right) in the PDI backbone. We are currently exploring the reduction chemistry of these compounds by producing reduced iron dicarbonyls. We are also in the process of synthesizing tripodal tetraamine-based ligands with pendant Lewis/Bronsted acids/bases and redox-active sites within the ligand scaffold.  The tripodal tetraamine fragment is a common building block in inorganic chemistry due to its ability to stabilize metal complexes with uncommon geometries and hence diverse catalytic capabilities.  As shown in Figure 3, we are investigating the tripodal systems based on the tren backbone and also the rigid o-phenylamine backbone.  We are integrating redox active sites into these systems by synthesizing the Schiff base analogs of the tren and o-phenylamine backbones.  These systems have potential advantages over the PDI systems described above.  It is likely that in the CO reduction reaction that multiple electron equivalents may be stored within these scaffolds while still only chelating one CO molecule (instead of two in the PDI system).  The o-phenylamine backbone has been shown to form ligand-based radicals upon oxidation but the reduction chemistry is largely unexplored.  We have successfully synthesized the zinc triflate analog of the tris-tert-butylimine complex based on the tren core, as shown in Figure 3 (right).

Figure 3. Synthesis of tripodal systems with reaction directors and redox active sites incorporated into the ligand scaffold.

  In the next year we are planning to publish our new complexes, as well as continue to investigate the reactivity of these complexes in the C-H and C-C bond forming steps in the F-T reactions of CO.